Article

# Electronic structure of GaN and Ga investigated by soft x-ray spectroscopy and first-principles methods

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## Abstract

The electronic structure and chemical bonding of wurtzite-GaN investigated by N 1s soft x-ray absorption spectroscopy and N K, Ga M1, and Ga M2,3 emission spectroscopy is compared to that of pure Ga. The measurements are interpreted by calculated spectra using first-principles density-functional theory (DFT) including dipole transition matrix elements and additional on-site Coulomb interaction (WC-GGA+U). The Ga 4p - N 2p and Ga 4s - N 2p hybridization and chemical bond regions are identified at the top of the valence band between -1.0 and -2.0 and further down between -5.5 and -6.5 eV, respectively. In addition, N 2s - N 2p - Ga 4s and N 2s - N 2p - Ga 3d hybridization regions occur at the bottom of the valence band between -13 and -15 eV, and between -17.0 and -18.0 eV, respectively. A band-like satellite feature is also found around -10 eV in the Ga M1 and Ga M2,3 emission from GaN, but is absent in pure Ga and the calculated ground state spectra. The difference between the identified spectroscopic features of GaN and Ga are discussed in relation to the various hybridization regions calculated within band-structure methods.

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... The sensitivity of x-ray spectroscopy enables us to probe the orbital directional occupations to investigate the bonding structure at the internal interfaces that determines the bond characteristics (symmetry and orbital directions at interfaces between cubic and hexagonal crystals) and local chemistry that affects conductivity and hardness [13]. The cubic-AlN phase that may form has a band gap of about 6 eV [14] while it is 3 eV for ZrN [15]. Theoretically, it has been found that for increasing x in Zr x Al 1−x N, the Fermi level (E F ) is shifted into the conduction band [16,17]. ...
... This scaling is necessary due to insufficient hybridization between the Zr 4s-N 2s orbitals at the bottom of the valence band that is inherent in the density functional theory (DFT). A similar bond situation with strong interaction at the bottom of the valence band has previously been observed in XES for shallow core levels of Ge [36] and Ga [15]. ...
... The increased hardness with increasing Al content was suggested to be due to an increasing amount of strain at the interfaces between the ZrN and the AlN precipitates, which we have identified in the N 1s XAS spectra in Fig. 5 as a gradual shift of the N 2p-e g * states away from the E F supported by the calculations. Experiments have shown that the Young's modulus increases from 250 GPa in c-ZrN to 300 GPa in Zr 0.57 Al 0.43 N consistent with calculations [44] as well as XRD of ZrAlN multilayers [15]. Furthermore, wide-angle x-ray scattering [45] has shown that the ZrAlN system exhibits high thermal phase stability [46,47] and hardness [48] during annealing [41,[49][50][51]. ...
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The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr 1−x Al x N (x = 0.0, 0.12, 0.26, 0.40) are investigated by resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to first principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first principles all-electron full-potential calculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity and element selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develops from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... The de-convolution of the VB spectra was performed to understand the nature of the hybridization states after reviewing the available literature reports. [29][30][31][32][33] The de-convoluted VB spectra for the samples revealed five identified peaks labelled as P I , P II , P III , P IV and P V . The positions and the corresponding hybridization states of the mentioned peaks are tabulated in Table 2. ...
... The positions and the corresponding hybridization states of the mentioned peaks are tabulated in Table 2. These peaks denote the N 2p-Ga 4p [31][32][33] and N 2p-Ga 4s* 24 mixed orbitals 31,32 (or surface adsorbates) and N 2p-Ga 4s 31-33 hybridization states along with a small satellite peak. 24,33 The N 2p-Ga 4s* corresponds to a slightly higher energy, but due to the non-zero density of states, was ascribed to this energy. ...
... These peaks denote the N 2p-Ga 4p [31][32][33] and N 2p-Ga 4s* 24 mixed orbitals 31,32 (or surface adsorbates) and N 2p-Ga 4s 31-33 hybridization states along with a small satellite peak. 24,33 The N 2p-Ga 4s* corresponds to a slightly higher energy, but due to the non-zero density of states, was ascribed to this energy. The better explanation for locating the peak at the above mentioned position can be found in earlier reports. ...
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A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-Molecular Beam Epitaxy is carried out. Photoemission (XPS & UPS) measurements were performed to investigate the nature of surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperature lead to lower amount of surface oxide, however vice versa was perceived for higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) position shifted towards higher binding energy (BE) with oxide coverage and revealed a downwards band bending. XPS valence band spectra were convoluted to understand the nature of hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements using (FESEM & AFM). FESEM and AFM analysis revealed that the surface was covered with hexagonal pits which played significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. The pit density and pit depth were observed to be important parameters governing the surface oxide coverage. The contribution of surface oxide increased with increase in average pit density as well as pit depth.
... This phase has high thermal, mechanical, and chemical stability according to the experimental studies and exhibits outstanding optoelectronic properties with a wide direct band gap of 3.4-3.5 eV. [50][51][52][53][54][55][56][57][58] The advanced epitaxial growth techniques and efficient use of GaN in shortwavelength blue and ultraviolet LEDs, high-power, and high-frequency devices 6,59 have led to a great potential for its optoelectronic applications, corroborated with the Nobel prize for physics awarded in 2014 for the use of GaN in efficient blue LEDs. 9 AlN also crystallizes in hexagonal wurtzite structure (wz-AlN) under ambient conditions. ...
... eV with respect to the reported experimental values. [50][51][52][53][54][55][56][57][58][75][76][77] Fig. 2(b) shows the optimized atomic configurations and calculated energy bands of 3D AlN in wz, zb, and rs structures for the sake of comparison. The direct band gaps of 3D wz-AlN, zb-AlN, and rs-AlN are 4.2, 3.3, and 4.6 eV, respectively. ...
... Expt. 50 distribution around Ga-N bonds is shown in Fig. 3(a). Three of each Ga-sp 2 and N-sp 2 hybrid orbitals form ionic r-bonds along Ga-N bonds arranged as a hexagon and provide the strength of h-GaN. ...
Article
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Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed.
... This fact and the large deviation in calculated L 3 /L 2 and t 2g /e g branching ratios beyond one-electron theory has to be treated as many-body effects including extended exchange and mixed terms between the core states [47]. A related issue is the large discrepancy between theory and experiment for the energy positions and intensities of the shallow 3d core levels in Ga [71] and Ge [26,25], where additional on-site Coulomb interaction is needed to obtain physical agreement. A similar problem has been found in the Cr-containing MAX-phases, such as Cr 2 GeC, where the magnetic Cr d-states must be carefully handled either within an ad hoc + U potential [17] or under a hybrid functional formalism [72]. ...
... A particularly interesting feature is the isotropic 4s states observed at −12.5 eV in the Ge M 2,3 XES data. The Ge 4s states exhibit significant intensity that is not reproduced in ground-state DFT calculations at 0 K. Generally, the 4s/3d intensity ratio of Ge and Ga is not in agreement between experiment and DFT calculations [23,26,71]. A complicating factor may be the strong electron-phonon coupling with a Ge oscillation along the c-axis that has a higher frequency than 13. ...
... However, this disagreement cannot be accounted only by the effect of the rather large electron-phonon coupling, and it was found that the redistribution of intensity from the shallow 3d core levels to the 4s valence band provides large DOS at E F . A similar disagreement between experiment and DFT results is known for Ge in Ti 3 GeC 2 [23] and V 2 GeC [26] as well as for Ga in GaN [71]. ...
Article
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This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... This fact and the large deviation in calculated L 3 /L 2 and t 2g /e g branching ratios beyond one-electron theory has to be treated as many-body effects including extended exchange and mixed terms between the core states [47]. A related issue is the large discrepancy between theory and experiment for the energy positions and intensities of the shallow 3d core levels in Ga [71] and Ge [26,25], where additional on-site Coulomb interaction is needed to obtain physical agreement. A similar problem has been found in the Cr-containing MAX-phases, such as Cr 2 GeC, where the magnetic Cr d-states must be carefully handled either within an ad hoc + U potential [17] or under a hybrid functional formalism [72]. ...
... A particularly interesting feature is the isotropic 4s states observed at −12.5 eV in the Ge M 2,3 XES data. The Ge 4s states exhibit significant intensity that is not reproduced in ground-state DFT calculations at 0 K. Generally, the 4s/3d intensity ratio of Ge and Ga is not in agreement between experiment and DFT calculations [23,26,71]. A complicating factor may be the strong electron-phonon coupling with a Ge oscillation along the c-axis that has a higher frequency than 13. ...
... However, this disagreement cannot be accounted only by the effect of the rather large electron-phonon coupling, and it was found that the redistribution of intensity from the shallow 3d core levels to the 4s valence band provides large DOS at E F . A similar disagreement between experiment and DFT results is known for Ge in Ti 3 GeC 2 [23] and V 2 GeC [26] as well as for Ga in GaN [71]. ...
Article
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The peak positions for all the samples are tabulated in Table 1. Comparisons with theoretical calculations and experimental measurements previously reported [19] for GaN reveal interesting changes in the valence band for the three morphologies. For sample 2, a peak appears at 2.32 eV, which is close to the energy range of 1-2 eV below the Fermi level for the N 2p-Ga 4p hybridization as attributed in the above reference. ...
... A prominent peak appears at 4.02 eV for the epilayer and at 4.10 and 4.55 eV for sample 1 and sample 2, respectively; and is attributed again to the hybridization between N 2p and Ga 4p levels. Although this peak lies outside the energy range mentioned earlier for this hybridization level, the theoretically obtained curve shows the presence of a non-zero density of states in this energy region [19] and hence such an attribution is reasonable. A small peak due to the hybridization between the N 2p-Ga 4s energy levels appears at 5.79 and 5.99 eV for the epilayer and sample 1 respectively. ...
... c 5 125 Å). The density of states for pure GaN (Fig. 4) shows the top valence band dominated by N 2p states, as well as the chemical bond regions and hybridization of the Ga 4p and N 2p states between −1.0 and −2.0 eV and of the Ga 4s and N 2p states between −5.5 and −6.5 eV, in accordance with earlier works 16 . The main contribution from the Ga 3d states occurs deeper in the valence band (between −11.3 eV and −13.3 eV, not shown here) but a small contribution is also present at the top valence band. ...
... A band gap of 3.46 eV was obtained, which is in very good agreement with the experimental value of 3.503(5) eV 5 , having an accuracy comparable to simulations using the OEPx(cLDA) + G 0 W 0 method (3.24 eV) 17 and a much better agreement with experiment than previous simulations using the WC-GGA + U functional (2.40 eV) 16 . The bandstructure is characterised by a direct band gap at the Γ point, as expected for the wurtzite structure of GaN (Fig. 5). ...
Article
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The potential use of combined e−-γ vs γ-γ Perturbed Angular Correlations (PAC) experiments as a possible alternative to study electronic properties of materials and/or samples where Hall effect measurements are difficult to perform due to low-quality ohmic contacts is here demonstrated using Si- and Zn-doped GaN samples as a showcase example. To do so, the lattice site of implanted 181Hf/181Ta and the recombination of Ta ionized and excited electronic states were studied as a function of temperature and sample doping in GaN. By combining the γ-γ and e−-γ PAC results with Density Functional Theory simulations, it was possible to assign a single stable site with a double-donor character for Ta in GaN. A metastable charge state was also identified at particular temperatures using e−-γ PAC. A thermally activated process was observed for the electronic recombination at high temperatures with activation energies of 15(2) meV and 12(1) meV for the Si- and Zn-doped samples, respectively, and attributed to Si shallow donors present in both samples. A reduced number of available electrons was observed in the Zn-doped sample due to donor compensation by the Zn acceptors. At low temperatures, it is suggested that the recombination process occurs via Variable Range Hopping. The doping characteristics of both samples were successfully distinguished.
... The predicted bandgap of GaN is 3.273 eV, which is in good consistency with present calculations and experiments, 3.33 eV by mBJ [40], 3.261, 3.23 eV by HSE06 [39,46], and 3.40-3.50 eV by experiments [47][48][49]. As observed in In y Ga 1−y N, our DFT results confirm that E g values of In y Ga 1−y N continuously decrease as y is increased from 0 to 50%. ...
... The contour plot for the bandgap of quaternary In y Ga 1−y N 1−x Bi x alloys is shown in Fig. 4. The bandgaps [39,40,46] and experimental [47][48][49][50][51] results are also plotted Compared with InGaN, the incorporation of Bi induces a sharper bandgap reduction. But beyond that, a significant increase in SO is obtained due to the strong SOC effect of bismuth where the advanced interaction between the electron spin and orbital angular momentum decreases the SO band energy. ...
Article
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To realize feasible band structure engineering and hence enhanced luminescence efficiency, InGaNBi is an attractive alloy which may be exploited in photonic devices of visible light and mid-infrared. In present study, the structural, electronic properties such as bandgap, spin-orbit splitting energy, and substrate strain of InGaNBi versus In and Bi compositions are studied by using first-principles calculations. The lattice parameters increase almost linearly with increasing In and Bi compositions. By bismuth doping, the quaternary InGaNBi bandgap could cover a wide energy range from 3.273 to 0.651 eV for Bi up to 9.375% and In up to 50%, corresponding to the wavelength range from 0.38-1.9 µm. The calculated spin-orbit splitting energy are about 0.220 eV for 3.125%, 0.360 eV for 6.25%, and 0.600 eV for 9.375% Bi, respectively. We have also shown the strain of InGaNBi on GaN; it indicates that through adjusting In and Bi compositions, InGaNBi can be designed on GaN with an acceptable strain.
... Table 3.6 summarizes the position and the corresponding hybridization states obtained from the deconvoluted VB spectra. These peaks are attributed to the N2p-Ga4p [48][49][50], N2p-Ga4s* [41], mixed orbitals [48,49] (or surface adsorbates) and N2p-Ga4s [48][49][50] 33 hybridization states along with a small satellite peak [41]. The de-convolution revealed that the valence band hybridization states originate from the interaction of Ga 4s and Ga 4p orbitals with N 2p orbital whose intensity varies with oxide coverage. ...
... Table 3.6 summarizes the position and the corresponding hybridization states obtained from the deconvoluted VB spectra. These peaks are attributed to the N2p-Ga4p [48][49][50], N2p-Ga4s* [41], mixed orbitals [48,49] (or surface adsorbates) and N2p-Ga4s [48][49][50] 33 hybridization states along with a small satellite peak [41]. The de-convolution revealed that the valence band hybridization states originate from the interaction of Ga 4s and Ga 4p orbitals with N 2p orbital whose intensity varies with oxide coverage. ...
... Theoretically, the spin-orbit splitting is larger (4.4 eV) and the states are also about 4.5 eV closer to the E F than in the experiment. The enhanced Ge 4s intensity observed experimentally may be attributed to electron-correlation, many-body effects or phonon vibrations, but more extensively to hybridization and charge-transfer effects, as in the case of Ga in GaN [39]. ...
... In the DFT calculations, the Ge 3d states are overestimated and not enough intensity is redistributed to the 4s valence band states. The same type of deficiency in DFT calculations is known also for Ge in Ti 3 GeC 2 [37] and V 2 GeC [38] as well as for Ga in GaN [39]. We clearly observe that the calculated 4s states at the bottom of figure 4 are too low in intensity, in particular at the E F . ...
Article
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
... However, the bare minimum shift of 0.1 eV can also be neglected as fitting errors and the separation was assumed to be identical. The de-convolution of the VB spectra was carried out after reviewing the available theoretical and experimental reports [26][27][28][29]. The spectra were de-convoluted into four major peaks labelled as P I , P II , P III and P IV . ...
... The N2p-Ga4s ⁄ usually observed at slightly higher binding energy, but since it possess a non-zero density of states over a wide range of energy, it can also be ascribed to lower BE. The better explanation for locating the peak at above mentioned position can be found in previous reports [26,29]. It was observed that the contribution of N2p-Ga4p and N2p-Ga4s ⁄ varied from S1 to S2 which indicates that the substrate orientation can also influence the VB hybridization states. ...
... This is because that since AlN displays larger elastic constants, thereby larger Bulk and Young's modula, than GaN, as listed in Table I, the u 1 shows less sensitive to the interaction between different nitrides in ternary alloys than the u 2 . When 33 < À0.17 and 11 > 0.13, c/a ' 0.60, u 1 ¼ u 2 ¼ 0. 50. This indicates that the wurtziteto-graphitelike phase transition can be obtained in Al 0.5 Ga 0.5 N alloy for uniaxial and biaxial , as shown in the panels (b) and (d) of Figure 1, which is similar to the results of AlN and GaN: 11,12 c/a ' 1.20 and u ¼ 0.50 for graphitelike phase. ...
... 11,12 On the other hand, the orbital hybridizations between N s and Al(Ga) s are observed near the CBM, which is located at C. The hybridization has been examined by soft x-ray spectroscopy and first-principle simulation for ground-state GaN. 50 Furthermore, the hybridization between N s and Ga s plays the dominant role in the CBM state, which is due to that the Ga s level is lower than that of Al s. Figure 10 shows the HSE06 E g as a function of uniaxial and biaxial . The maximum values of 4.17 eV and 4.32 eV appear at 33 ¼À0.02 and 11 ¼À0.04, ...
Article
Structural phase transition, band structure, and piezoelectric response of Al 0.5Ga0.5N alloy under uniaxial and biaxial strains are systematically investigated using first-principle calculations. The main findings are summarized as follows: (I) Although the wurtzite structure transforms to an intermediate graphite-like structure for both uniaxial and biaxial strains, the second-order phase transition is found for uniaxial strain and the first-order transition for biaxial strain. The transition is driven by the mechanical and dynamical instabilities for uniaxial strain, and by the mechanical instability for biaxial strain. (II) The wurtzite phase always remains the direct band structure, whereas the band gap of graphite-like phase is always indirect. The band gaps of wurtzite and graphite-like phases are greatly reduced by internal strains. (III) The drastic enhancements in piezoelectric response are observed near phase transition, which is attributed to the flat and shallow local energy minima associated with two different phases. Our calculated results are compared with the available experimental and other theoretical data, and good agreements are obtained.
... The difference in the wavelength and color of the emitted light is due to the materials themselves from which the diode laser is produced. GaN has a higher band gap, approximately 3.4 eV, where as GaAs based diode lasers have a comparatively smaller band gap around 1.43 eV at 300K [10,11]. This band gap difference means that when photons in the GaN semiconductor are generated, they contain more energy, higher frequencies, and thus, smaller wavelengths closer to the blue end of the visible electromagnetic spectrum. ...
... The value of T o is highly dependent on the active layer used for the diode laser [17]. Specifically, larger values of the threshold current can be expected for lasers that use semiconductors with wider band gaps, and higher energy gaps, such as GaN as previously mentioned [8,11,19]. ...
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The temperature dependence of laser properties was explored using a diode laser and Peltier cooler. Threshold currents were calculated at various temperatures and comparable to expected results. L-I characteristic plots for seven temperatures were produced to investigate threshold current, slope efficiency, and characteristic temperature. The output beam spatial cross-sectional power distribution was plotted to investigate the beam profile and beam divergence.
... At room temperature, GaN has a direct band gap of 3.42 eV [1] and an exciton binding energy of 25 meV [2]. GaN has unipolar structure with high electron mobility [3][4][5]. ...
... Known data are substituted in Eqs. (1)(2)(3)(4), and the results are summarized in Table 1. The calculated magnetic moments of the doped systems are shown in Table 1. ...
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The effects of Be/Mg/Ca doping on the physical properties of GaN have been widely investigated experimentally and theoretically. However, the effects of Be/Mg/Ca and interstitial H coexistence with different valence states of VGa/VN on the photocatalytic performance of GaN are rarely reported. This study examines the structure and stability of Ga34MHiN36(VGa³⁻/VGa²⁻/VGa¹⁻/VGa⁰) (M = Be/Mg/Ga) and Ga35MHiN35(VN³⁺/VN¹⁺/VN⁰) (M = Be/Mg/Ga) systems and the main factors affecting photocatalytic performance by using the generalized gradient approximation plane wave ultrasoft pseudopotential + U method within the framework of density functional theory. Results show that the Ga34MHiN36(VGa³⁻/VGa²⁻/VGa¹⁻/VGa⁰) (M = Be/Mg/Ga) and Ga35MHiN35(VN³⁺/VN¹⁺/VN⁰) (M = Be/Mg/Ga) systems are more readily formed and have a more stable structure under N-rich conditions compared with other conditions. The visible-light effect, electric dipole moment, effective mass, and oxidation reduction reaction affecting the photocatalytic performance of doped systems are analyzed. The Ga34CaHiN36(VGa³⁻) system shows the best visible-light effect, best carrier activity, longest carrier lifetime, and strongest oxidation reduction ability. These results suggest that the Ga34CaHiN36(VGa³⁻) system is an excellent photocatalyst that can be utilized in designing novel GaN photocatalysts.
... Gallium nitride (GaN) 16) and gallium phosphide (GaP) 17) also have p electrons in their outer shell. Although GaN has been applied as a photoanode, [18][19][20][21][22][23][24][25][26] the electrochemical or photo-electrochemical reduction of CO 2 using GaN as a cathode has not yet been reported. ...
Article
We report on the complex catalytic behavior of Ga2O3 for the electrochemical reduction of CO2 to formic acid (HCOOH). Although the experiments were reproducible, the behavior observed during the reaction was complex. A characteristic feature of the reaction was that Faradaic efficiency was strongly dependent on the electric charge during electrolysis. This result implied that the produced HCOOH affected the CO2 reduction reaction on the surface of the electrode, which was confirmed by experiments with initially added acid. The Faradaic efficiency for HCOOH production (η_HCOOH) increased with electric charge, and was further increased by the presence of initially added acid. We also show electrochemical CO2 reduction over other Ga compounds such as GaN and GaP, for which it can be assumed that p electrons and the Ga–Ga distance on the surface of the catalyst have important roles in selective HCOOH production as in the case of Ga2O3.
... The peak at 3.8 eV is assigned to Ga 4s N 2p hybridized orbitals. [37] The peak P 3 , as mentioned before, is generally attributed to be either adsorbate related or due to mixed hybrid orbitals. Since this peak intensity did not change with sputtering, we discount the role of adsorbates, and attribute it to mixed hybrid orbitals. ...
Article
The effect of film morphology on its surface chemistry and band structure has been analyzed for gallium nitride epitaxial films grown by molecular beam epitaxy. The film morphology has been studied using scanning electron microscopy and atomic force microscopy, and the bandstructure, defect and emission properties have been studied by X ray photoelectron spectroscopy and cathodoluminescence spectroscopy. It was found that the highly porous GaN nanowall network shows the highest relative conductivity and does not have defect related luminescence. The flatter films were more resistive and showed yellow luminescence, due to Ga vacancies. GaN nanowall network exhibited a Fermi level pinning at (1.8 $\pm$ 0.2) eV above valence band maximum, suggesting the presence of a Ga adlayer on the surface of GaN nanowall network. Ar ion sputtering was found to preferentially sputter N atoms leading to surface metallization.
... We point out that the predominance of N p-character in the valence band and the intermixing of N s and p orbitals in the conduction band is similar to what has been reported for bulk wurtzite GaN. 19 The same pattern applies to the valence and conduction bands of an InGaN MLQW in the presence of C. However, as shown in Fig. 2(b), a shallow state close to E V and due to the p orbital of C can be found. ...
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InGaN alloys and, in particular, InGaN monolayer quantum wells (MLQWs) are attracting an increasing amount of interest for opto-electronic applications. Impurities, incorporated during growth, can introduce electronic states that can degrade the performance of such devices. For this reason, we present a density functional and group theoretical study of the electronic properties of C, H, or O impurities in an InGaN MLQW. Analysis of the formation energy and symmetry reveals that these impurities are mostly donors and can be held accountable for the reported degradation of InGaN-based devices
... Gallium nitride (GaN) can be found in cubic zincblende or wurtzite crystal structures. Both of them have a wide band gap in the range of 3.30 -3.50 eV [1,2]. Owing to their band gap, GaN structures are the most commonly used semiconductors in optoelectronic device applications, e.g., fabrication of light-emitting diodes (LED) to operate in blue-light and ultraviolet regions, room temperature laser diodes, high temperature/high power electrical devices [3,4]. ...
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In this study, we examined the adsorption and diffusion of platinum (Pt) adatom on two-dimensional hexagonal gallium nitride (h-GaN), by using first-principles plane-wave calculations. Two different levels of platinum coverage ratio (θ=1/8 and θ=1/32) were considered and the changes in the electronic structure for high-level platinum coverage ratio (θ=1/8) were examined. Low-level coverage ratio (θ=1/32) is used to calculate the diffusion barrier energy of Pt adatom on GaN monolayer. Our theoretical calculations have shown that Pt atom strongly binds on the top of nitrogen atoms in GaN monolayer and high energy is required for its diffusion. While GaN monolayer has 2.1 eV indirect band gap (Γ→K), this band gap reduces to 1.3 eV with Pt adsorption. These results may lead to further investigations on forming Pt nanoparticles or Pt coating on GaN sheet.
... These features can also be seen in Density of States's (DOS) behavior of materials. Where DOS, as a function of energy, exhibits non-zero density at Fermi level for metallic elements and especially transition metals [26,69,150], semiconductor's DOS reveals gap at Fermi level [65,96]. For the Heusler-structure it is typical to have a hybrid behavior of DOS for minority and majority electrons. ...
Thesis
In dieser Arbeit haben wir ultraschnelle Entmagnetisierung an einer Heusler-Legierung untersucht. Es handelt sich um ein Halbmetall, das sich in einer ferromagnetischen Phase befindet. Die Besonderheit dieses Materials besteht im Aufbau einer Bandstruktur. Diese bildet Zustandsdichten, in der die Majoritätselektronen eine metallische Bänderbildung aufweisen und die Minoritätselektronen eine Bandlücke in der Nähe des Fermi-Niveaus aufweisen, das dem Aufbau eines Halbleiters entspricht. Mit Hilfe der Pump-Probe-Experimente haben wir zeitaufgelöste Messungen durchgeführt. Für das Pumpen wurden ultrakurze Laserpulse mit einer Pulsdauer von 100 fs benutzt. Wir haben dabei zwei verschiedene Wellenlängen mit 400 nm und 1240 nm benutzt, um den Effekt der Primäranregung und der Bandlücke in den Minoritätszuständen zu untersuchen. Dabei wurde zum ersten Mal OPA (Optical Parametrical Amplifier) für die Erzeugung der langwelligen Pulse an der FEMTOSPEX-Beamline getestet und erfolgreich bei den Experimenten verwendet. Wir haben Wellenlängen bedingte Unterschiede in der Entmagnetisierungszeit gemessen. Mit der Erhöhung der Photonenenergie ist der Prozess der Entmagnetisierung deutlich schneller als bei einer niedrigeren Photonenenergie. Wir verknüpften diese Ergebnisse mit der Existenz der Energielücke für Minoritätselektronen. Mit Hilfe lokaler Elliot-Yafet-Streuprozesse können die beobachteten Zeiten gut erklärt werden. Wir haben in dieser Arbeit auch eine neue Probe-Methode für die Magnetisierung angewandt und somit experimentell deren Effektivität, nämlich XMCD in Refletiongeometry, bestätigen können. Statische Experimente liefern somit deutliche Indizien dafür, dass eine magnetische von einer rein elektronischen Antwort des Systems getrennt werden kann. Unter der Voraussetzung, dass die Photonenenergie der Röntgenstrahlung auf die L3 Kante des entsprechenden Elements eingestellt, ein geeigneter Einfallswinkel gewählt und die zirkulare Polarisation fixiert wird, ist es möglich, diese Methode zur Analyse magnetischer und elektronischer Respons anzuwenden.
... GaN is a binary direct band gap semiconductor that usually can be of two main crystalline structures: wurtzite (Wz) or hexagonal and zinc blende or cubic, however, under ambient conditions, the thermodynamical stable structure is the Wz. GaN has a relatively large band gap (3.4 eV), exhibit piezoelectricity, high thermal conductivity, mechanical strength and low electron affinity [23]. Extensive research efforts have been directed toward the fabrication of GaN-based NWs owing to their applications in electronic, optoelectronic, and photonic devices. ...
Article
Photoelectrochemical (PEC) water splitting using semiconductor materials as light absorbers have been extensively studied. Several semiconducting materials have been proposed, such as TiO 2 , ZnO, and GaN. Because the efficiency of PEC water splitting is dependent on visible light absorption, the ability to tune the bandgap of GaN by alloying with In makes it advantageous over other wide bandgap semiconductors. The fabrication of GaN-based materials with nanoscale geometry offers more merit for their use in PEC water splitting. In this review, we provide an overview of the recent progress made in the synthesis and application of GaN-based nanomaterials in PEC water splitting. The outstanding challenges and the future prospects of this field will also be addressed.
... Compared with traditional indirect bandgap semiconductors such as Si (~1.1 eV) and Ge (~0.74 eV), wurtzite gallium nitride (wz-GaN) has a wide and direct bandgap of~3.4 eV [1][2][3][4], suggesting that it is a suitable material for optoelectronic applications in the near-ultraviolet part of spectrum. In the last two decades, the wz-GaN has become a key material of optoelectronic devices such as light emitting diodes (LEDs) and laser diodes [5][6][7]. ...
Article
Employing density-functional theory, G 0 W 0 approach, and solving Bethe–Salpter equation (BSE), we investigate the effects of strain and surface modification (hydrogenation or fluorination) on stability, electronic and optical properties of monolayer GaN. Monolayer GaN is predicted to be an indirect semiconductor with wide gap of 4.44 eV, whose bandgap can be effectively modulated by biaxial strain, ranging from 5.49 eV (δ = −6.5%) to 2.27 eV (δ = +8%). In particular, the compressive strain arouses the structural instability of GaN monolayer. Hydrogenation or fluorination not only eliminates its instability, but also tunes the bandgap from indirect to direct. More attractively, tensile strain can significantly redshift the optical spectra of GaN monolayer into the visible-light region, broadening light harvesting. Hydrogenation or fluorination shifts optical activity to the ultra-violet region. The possibility of tuning the optoelectronic properties via strain and surface modification opens doors to novel application of the promising material.
... This started with the independent-particle approximation (IPA) within density functional theory (DFT) [8][9][10][11][12]. An improvement was obtained by including electronhole interactions still relying on the core-hole approximation [13,14]. This does not capture the full picture, because electron-hole correlation plays a crucial role in RIXS even in weakly correlated materials. ...
Article
Full-text available
Resonant inelastic x-ray scattering (RIXS) is a powerful spectroscopic technique that offers an elemental- and orbital-selective probe of the electronic excitations over a huge energy range. We present a many-body approach to determine RIXS spectra in solids, yielding an intuitive expression for the RIXS cross section in terms of pathways between intermediate many-body states containing a core hole, and final many-body states containing a valence hole. Explicit excited many-body states are obtained from the diagonalization of the Bethe-Salpeter equation in an all-electron framework. For the paradigmatic example of the fluorine K edge of LiF, we show how the excitation pathways determine the spectral shape of the emission, and demonstrate the nontrivial role of electron-hole correlation in the RIXS spectra.
... On the other hand, bulk group-III nitrides, are essential materials for both solid-state lighting [16] and solar cells [17]. Usually, the threedimensional (3D) GaN crystal with a hexagonal wurtzite (wz-GaN) is a semiconductor that exhibits a large direct bandgap of roughly 3.4 eV [18,19]. It is chemically, thermally, and mechanically stable, making it an excellent material for visible and ultraviolet optoelectronic applications such as photodetectors [20,21], solar cells [22][23][24], lasers [25], and light-emitting diodes [26,27]. ...
Article
Full-text available
Two-dimensional materials with a wide bandgap characteristic are critical to developing novel nanoscaled electronic devices. This study investigates the layer, in-plane biaxial strain, and vertical external electric field effects on the stability and electronic properties of multilayered GaN employing the density functional theory. Using structural optimization and phonon-mode calculations, the GaN structures with one to five layers are found to be stable and refer to the Born–Oppenheimer local minima. Under dimensionality effect, GaN unveils a semiconducting feature with an indirect bandgap at the HSE06 level that changes from 3.37 eV for a monolayer to 2.79 eV for a five-layered structure as the quantum confinement effect is reduced. The calculations demonstrate that the indirect bandgap of multilayered GaN decreases linearly when increasing the tensile strain. However, compressive strain engineering would induce an indirect-to-direct gap transition. Additionally, when the strength of the applied perpendicular external electric field to the multilayered GaN surface is increased, the GaN bandgap rapidly disappears due to the field-induced near free electronic gas. Calculated carrier effective mass indicate that both external effects impact charge carrier mobility as well. Such significant changes in the electronic properties under external excitations show that multilayered GaN shows promise in configurable nanoelectronic devices.
... [7][8][9][10][11] These materials have similar graphene structural properties, but they present different electronic features due to a non-zero bandgap. The electronic structure of homogeneous monolayers composed by graphene-like materials has been widely investigated [12][13][14][15][16][17][18][19][20] and two-dimensional heterostructures of group-III nitride compounds are still being theoretically and experimentally studied, with important results already obtained. [21][22][23][24][25][26] Among these structures, single layers of AlN and GaN have already been synthesized, showing matched lattices and tunable bandgap values. ...
Preprint
Due to the wide range of possible applications, atomically thin two-dimensional heterostructures have attracted much attention. In this work, using first-principles calculations, we investigated the structural and electronic properties of planar AlN/GaN hybrid heterojunctions with the presence of vacancies at their interfaces. Our results reveal that a single vacant site, produced by the removal of Aluminum or Gallium atom, produces similar electronic band structures with localized states within the bandgap. We have also observed a robust magnetic behavior. A nitrogen-vacancy, on the other hand, induces the formation of midgap states with reduced overall magnetization. We have also investigated nanotubes formed by rolling up these heterojunctions. We observed that tube curvature does not substantially affect the electronic and magnetic properties of their parent AlN/GaN heterojunctions. For armchair-like tubes, a transition from direct to indirect bandgap was observed as a consequence of changing the system geometry from 2D towards a quasi-one-dimensional one. The magnetic features presented by the AlN/GaN defective lattices make them good candidates for developing new spintronic technologies.
Thesis
Annealing-induced solid phase crystallization of In₂O₃:H leads to a significantly improved electron mobility, which is confirmed by Hall measurements. Indium hydroxide dehydroxylation occurs in In₂O₃:H during annealing, which is well responsible for the structural transformation and a high electron mobility with a decreased carrier concentration in crystallized In₂O₃:H. A significant decrease in the intensity of occupied gap states is observed in crystallized In₂O₃:H, possibly due to a decrease in carrier concentration. Doped In₂O₃ variants have been found to have a quite deeper allowed transition level below the valence-band edge than undoped In₂O₃, which in particular applies to crystallized In₂O₃:H, but most likely attributed to a change of the crystal structure upon annealing and/or a different O 2p-In 4d coupling near the VBM compared to amorphous In₂O₃:H. To well understand the interface properties of Ag/In₂O₃:H upon annealing, a thin Ag film was grown on the In₂O₃:H substrate and annealed in vacuum up to 300 °C. During annealing, the potential Ag diffusion into the bulk In₂O₃:H and/or a change of an annealing-induced Ag topography (i.e., cluster formation) occurs, with a small Ag oxidation (i.e., Ag₂O and AgO). With Ag deposition, an initial downward band bending of (0.11±0.05) eV was present in In₂O₃:H, attributed to a Schottky contact formed at the Ag/In₂O₃:H interface. Upon annealing, the downward band bending reduces gradually, and the Schottky-barrier height at the Ag/In₂O₃:H interface also decreases. A thickness series of the individual materials on the respective “substrate” (i.e., MnS/Si, GaN/MnS, and ZnO/GaN) was epitaxially grown on Si (100) wafer, and the interfacial chemistry and energy-level alignment at the respective interfaces are examined using photoelectron spectroscopy. At the MnS/Si interface, an interface-induced band bending (IIBB) appears in Si, which of values are found to be (0.15±0.07) and (0.23±0.07) eV for 4 and 15 nm MnS/Si stacks, respectively. The MnS/Si heterointerface shows a type-II (staggered) band lineup with a VBO of (-0.37±0.10) eV and the corresponding CBO of (2.27±0.10) eV. For the GaN/MnS interface, a significant diffusion of Mn into the GaN layer takes place during GaN deposition. In addition, an interface-induced band bending (IIBB) by ~0.30 eV is observed in MnS. The GaN/MnS interface shows a type-II (staggered) band lineup with a VBO of (1.46±0.10) eV and the corresponding CBO of (-1.09±0.10) eV. At the ZnO/GaN interface, a significant N diffusion from GaN into ZsnO takes place, i.e., Zn-N bonds, when ZnO is grown on the GaN layer. Also, an interfacial oxide (GaOx) layer was formed during ZnO deposited on GaN films. The ZnO/GaN heterointerface shows a type-II (staggered) band lineup with a VBO of (2.48±0.10) eV and the corresponding CBO of (-2.50±0.10) eV, respectively.
Article
Magnetic properties of Ga vacancies VGa and vacancy pairs in GaN are analyzed by employing the Generalized Gradient Approximation with the +U corrections. Strong spin polarization stabilizes high spin configurations of VGa, and leads to its negative-Ueff character. Both features are reflected in the magnetic properties of vacancies pairs. Because of electron transfer between two vacancies induced by negative-Ueff, both vacancies can be in different charge and spin states, and thus the spin ground states of a pair can be ferrimagnetic rather than ferro- or antiferromagnetic. Magnetic coupling of VGa-VGa pairs as a function of separation between the defects, their relative orientation, and of the charge state, was calculated. The strength of magnetic coupling is reduced by the U-induced localization of the wave functions. The obtained results show that gallium vacancies can lead to the observed ferromagnetism in irradiated GaN samples.
Article
Doping of isovalent (S, Se, and Te) elements in ZnO is a new doping method. However, the factors affecting the photocatalytic performance of a doped system by triaxial strain are often ignored. In this study, we have applied strain on model and performed first-principle calculation to investigate the effect of triaxial strain on the stability of the doped system, red shift of the absorption spectrum, electric dipole moment, and carrier lifetime. Calculation results showed that all doped systems exhibited high binding energy and stability under unstrained conditions. However, when the applied strain was increased, the energy of all the systems increased, and the stability decreased. The stability, red shift of absorption spectrum, electric dipole moment, and carrier lifetime of all doped systems were studied. When the tensile strain was 5%, the red shift of the absorption spectrum and the electric dipole moment of the doped system (Zn36SO35) were the largest. Moreover, the carrier lifetime of the doped system (Zn36SO35) was the longest. Considering the red shift of the absorption spectrum, electric dipole moment, and carrier lifetime, the photocatalytic performance of the doped system (Zn36SO35) was the best, when the tensile strain was 5%.
Article
Exploring new polymorphs of groups III to V compounds of evolved physical properties has recently received substantial interest from researchers. Accordingly, we explored new pressure‐driven polymorphs of gallium nitride (GaN) and investigated their physical properties using density functional theory (DFT)‐based full‐potential (FP) linearized‐augmented‐plus‐local‐orbital (L[APW + lo]) approach. Our analysis shows the transition of ground‐state wurtzite (wz) structure to beryllium oxide (β‐BeO)‐type structure at a tensile stress of ~−6.82 GPa and to silicon carbide (SiC)‐type structure by applying moderate compressive stress of magnitude 0.27 GPa. Similarly, the transition of wz‐GaN to nickle arsenide (NiAs) and titanium arsenide (TiAs)‐type structures has been realized at 43.78 and 45.72 GPa, respectively. These new polymorphs of GaN exhibited comparable cohesive energies with wz‐structure and the phonon dispersions free of imaginary frequencies, which indicate them as stable as the ground‐state wz‐phase. Investigations of the electronic structures show the wz‐, β‐BeO‐, and SiC‐phases of GaN as semiconductors of direct bandgap of energy 3.10, 3.15, and 2.97 eV, whereas the NiAs‐ and TiAs‐ phases of GaN exhibited indirect bandgap of energy 2.59 and 2.82 eV. All the GaN polymorphs demonstrated transparent behavior for the incident light photon of energy less than 13 eV. They exhibited optical absorption as high as 3.02 × 106 cm−1, 2.23 × 106 cm−1, 2.62 × 106 cm−1, 2.67 × 106 cm−1, and 2.67 × 106 cm−1 in the case of wz‐, β‐BeO‐, NiAs‐, SiC‐, and TiAs‐structured GaN, respectively. These interesting features of the novel polymorphs of GaN indicate them promising for electronic and optoelectronic applications.
Article
The gate and drain bias dependence of hot electron-induced degradation in GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) was investigated in this work. Devices exhibit an abnormal increase in peak transconductance ( $G_{m,max}$ ) during hot carrier stress (HCS) and a partially quick recovery of that after removing the electrical stress. A physical model is proposed to explain the abnormal electrical characteristics caused by HCS. By using density functional theory (DFT), we calculated the energy for electrons to dehydrogenate preexisting [N $_{Ga}$ H $_{3}$ ]⁻¹ complexes in GaN layer during stress. The dehydrogenation of defects affects the $G_{{m,max}}$ of devices. Meanwhile, the neutralization of donor traps in AlGaN barrier layer also plays a significant role in the increase of $G_{{m,max}}$ and the detrapping effect of electrons from these traps after removing the electrical stress accounts for the partially quick recovery of $G_{m, max}$ .
Article
In this work, molecular dynamics (MD) approach was performed in order to study the vacancy defect and temperature effects of GaN under Ar atomic bombardment (AB). In our computational study, the interatomic forces of nanostructures are based on Lennard-Jones and Tersoff force-fields. The results show that the vacancy defect of atomic structures is an important factor in AB procedure and the ideal matrix shows the maximum mechanical strength after Ar AB. In this structure, the final number of Ga and N atoms missing is smaller than defected matrix at T = 300 K. Furthermore, the effects of matrix temperature on dynamical manner of samples were investigated. MD calculations show that by temperature increasing from T = 300 K to T = 350 K, the GaN structure atom missing increases and so the atomic mechanical stability decreases in this procedure. Numerically, by a 50 K increase in the temperature of GaN atoms, the number of atoms missing in this structure increases by 54.
Article
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The density of localized tail states in amorphous ZnON (a-ZnON) thin film transistors (TFTs) is deduced from the measured current-voltage characteristics. The extracted values of tail state density at the conduction band minima (Ntc) and its characteristic energy (kTt) are about 2 × 10(20) cm(-3)eV(-1) and 29 meV, respectively, suggesting trap-limited conduction prevails at room temperature. Based on trap-limited conduction theory where these tail state parameters are considered, electron mobility is accurately retrieved using a self-consistent extraction method along with the scaling factor '1/(α + 1)' associated with trapping events at the localized tail states. Additionally, it is found that defects, e.g. oxygen and/or nitrogen vacancies, can be ionized under illumination with hv ≫ Eg, leading to very mild persistent photoconductivity (PPC) in a-ZnON TFTs.
Article
Wide band gap semiconductors find applications in optoelectronics and in the fabrication of high-power and high-frequency microelectronic devices. This chapter reviews the main applications of XAFS for the characterization of the group-III nitrides, SiC, diamond and other II–VI and III–V wide band gap semiconductors. Some aspects, as for example the effect of ion implantation, temperature, pressure and alloying are discussed more extensively in other chapters of the book.
Article
Ab initio studies of a GaN(0001)-Ga system with various thicknesses of a metallic Ga layer were undertaken. The studied systems extend from a GaN(0001) surface with a fractional coverage of gallium atoms to a Ga-GaN metal-semiconductor (m-s) contact. Electronic properties of the system are simulated using density functional theory calculations for different doping of the bulk semiconductor. It is shown that during transition from a bare GaN(0001) surface to a m-s heterostructure, the Fermi level stays pinned at a Ga-broken bond highly dispersive surface state to Ga-Ga states at the m-s interface. Adsorption of gallium leads to an energy gain of about 4 eV for a clean GaN(0001) surface and the energy decreases to 3.2 eV for a thickly Ga-covered surface. The transition to the m-s interface is observed. For a thick Ga overlayer such interface corresponds to a Schottky contact with a barrier equal to 0.9 and 0.6 eV for n- and p-type, respectively. Bond polarization-related dipole layer occurring due to an electron transfer to the metal leads to a potential energy jump of 1.5 eV, independent on the semiconductor doping. Additionally high electron density in the Ga-Ga bond region leads to an energy barrier about 1.2 eV high and 4 Å wide. This feature may adversely affect the conductivity of the n-type m-s system.
Article
By applying the on-site Coulomb interaction corrections on the anion:2p and the cation:3d electrons, we find that the GGA + U approach can completely compensate the underestimation of band gap of ZnO and GaN, two wide band gap semiconductors. Based on such approach, we investigate the electronic structure of ZnO/GaN (0001) heterostructure, particularly for the two dimensional electron gas formed near the polar interface. The polarization difference between ZnO and GaN induces the surface charge, resulting in the accumulation of band electrons on the N-polar interface.
Article
The graphitic sheet of group III-nitrides are a subject of interest due to their novel technological applications. In this paper, we focus on GaN graphitic sheet, investigating its electronic and optical properties in the framework of density functional theory. The optical properties of the GaN graphitic sheet such as dielectric function, refraction index, electron energy loss function, reflectivity, absorption coefficient, optical conductivity and extinction index are calculated for both parallel and perpendicular electric field polarizations. The results show that the optical spectra are anisotropic along these two polarizations. Optical conductivity for (E‖x) and (E‖z) is zero when the energy is <2.14 and 3.98 eV, respectively; which confirms that GaN graphitic sheet has semiconductor property.
Article
Obtaining a reliable positive‐type (p‐type) GaN semiconductor is difficult because of the unipolarity of GaN. This difficulty is one of the bottlenecks restricting the development of GaN‐based optoelectronic devices. To address this problem, this paper adopted the method of generalized gradient approximation (GGA) plane wave ultrasoft pseudopotential based on the framework of density functional theory to construct Ga35MN36, Ga34MN36, and Ga34MHiN36 (M = Be/Mg/Ca; Hi = interstitial hydrogen) models. Ga35MN35 and Ga35MHiN35 (M = Be/Mg/Ca) models were also constructed. Results of our calculations indicated that the Ga35MN35 and Ga35MHiN35 (M = Be/Mg/Ca) models cannot achieve a p‐type doping system. Furthermore, the formation energy of Ga34MN36 and Ga34MHiN36 (M = Be/Mg/Ca) systems was greater under Ga‐rich conditions than that under N‐rich conditions, indicating that both doping systems more readily formed and had a more stable structure under N‐rich conditions. Moreover, the formation energy of Ga34MHiN36 (M = Be/Mg/Ca) system was lower than that of Ga34MN36 (M = Be/Mg/Ca) system, and the existence of interstitial H proved to be beneficial to the improvement in system stability. The Ga34CaHiN36 system had the largest hole mobility and the best conductivity. Therefore, the Ga34CaHiN36 system is an ideal material for the application of conductive GaN devices. This study provides guidance into the preparation of p‐type conductive GaN materials.
Article
Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN (g−GaN) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that g−GaN can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, g−GaN in two dimensions has a relatively wider indirect band gap. Moreover, 2D g−GaN displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding g−GaN are preserved when g−GaN is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to g−GaN, is found to be an excellent substrate for growing g−GaN. Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of g−GaN, and they can display electronic and optical properties that can be controlled by the number of g−GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of g−GaN layers.
Article
The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades. In the present article, the bandgap tuning and its influence on optical properties of In 1– x Ga x N/P, where ( x = 0.0, 0.25, 0.50, 0.75, and 1.0) alloys are comprehensively analyzed by density functional theory based on full-potential linearized augmented plane wave method (FP-LAPW) and modified Becke and Johnson potentials (TB-mBJ). The direct bandgaps turn from 0.7 eV to 3.44 eV, and 1.41 eV to 2.32 eV for In 1– x Ga x N/P alloys, which increases their potentials for optoelectronic devices. The optical properties are discussed such as dielectric constants, refraction, absorption, optical conductivity, and reflection. The light is polarized in the low energy region with minimum reflection. The absorption and optical conduction are maxima in the visible region, and they are shifted into the ultraviolet region by Ga doping. Moreover, static dielectric constant ε 1 (0) is in line with the bandgap from Penn’s model.
Article
This study investigates how the prediction of the gallium nitride (GaN) bandgap is affected by treating semi-core d-electrons as either valence or core states in the pseudopotentials, which correspond to small-core and large-core approximations, respectively. To distinguish the effect of semi-core treatment from another bandgap problem recognized in density functional theory (DFT), that is, the underestimation related to the self-interaction problem, we perform diffusion Monte Carlo (DMC) evaluations under the fixed-node approximation and the optical gap scheme (where the evaluation uses N-electron many-body wavefunctions). A comparison to experimental measurements of bandgap energies indicates that DMC predictions are overestimated, whereas DFT simulations, which are used as a guiding function (DFT → DMC), are typically underestimated. This agrees with the trend reported in previous DMC studies on bandgap estimates. The large-core approximation results in a greater overestimation than the small-core treatment in both DFT and DMC. The bias in the overestimation is ∼30% for the DFT → DMC operation. Several possible causes of this bias are considered, such as pd-hybridization, core-polarization, and electronic screening effects. However, although these factors could qualitatively account for the overestimation caused by the large-core treatment, the estimated magnitude of the bias is too small to explain the evaluated difference between small-core and large-core approximations of the bandgap.
Article
The effects of Be/Mg/Ca doping on the optical properties of GaN have been widely investigated experimentally and theoretically. However, metalorganic chemical vapor deposition and vacuum coating methods are used in the experiments, and interstitial H is difficult to remove in GaN. The effects of Be/Mg/Ca doping and interstitial H coexistence with Ga vacancy on the photocatalytic performance of GaN are rarely explored in a vacuum environment. This study examines the formation energy and electronic structure of Ga34MN36 (M = Be/Mg/Ca) and Ga34MHiN36 (M = Be/Mg/Ca) systems and the main factors affecting their photocatalytic performance by using the generalized gradient approximation plane wave ultrasoft pseudopotential + U method within the framework of density functional theory. Results show that the formation energy of the Ga34MN36 (M = Be/Mg/Ca) and Ga34MHiN36 (M = Be/Mg/Ca) systems are greater under Ga-rich conditions than under N-rich conditions, indicating that both doping systems are more readily formed and have a more stable structure under N-rich conditions than under Ga-rich conditions. The visible light effect, electric dipole moment, effective mass, and oxidation–reduction reaction affecting the photocatalytic performance of the doping systems were analyzed. Compared with the Ga34MN36 (M = Be/Mg/Ca) system, the Ga34CaHiN36 system shows a more obvious red-shift in the absorption spectrum, a larger absorption spectrum intensity, a better carrier activity, a faster carrier separation rate, a longer carrier lifetime, and a stronger oxidation ability. These results suggest that the Ga34CaHiN36 system is an excellent photocatalyst that can be utilized in designing and preparing novel GaN photocatalysts.
Article
GaN is demonstrated to be an ideal anode for Li-ion batteries (LIBs) for the first time. Amorphous GaN@Cu nanorods (a-GaN@Cu) freestanding electrode is designed via a low-temperature pulsed laser deposition method, which exhibits prominent rate capability and untralong lifespan as an anode for LIBs. With porous interconnected metal nanorods substrate to improve the structure integrity and electronic conductivity, the a-GaN@Cu electrode delivers a capacity recovery of 980 mAh g−1 after 150 cycles from 0.25 to 6.25 A g−1 and a high discharge capacity of 509 mAh g−1 after 3000 cycles at 10.0 A g−1. The lithium storage in the a-GaN is also systematically studied, which suggests a redox reaction mechanism.
Article
Lateral and vertical heterostructures constructed of two-dimensional (2D) single-layer h-GaN and h-AlN display novel electronic and optical properties and diverse quantum structures to be utilized in 2D device applications. Lateral heterostructures formed by periodically repeating narrow h-GaN and h-AlN stripes, which are joined commensurately along their armchair edges, behave as composite semiconducting materials. Direct–indirect characters of the fundamental band gaps and their values vary with the widths of these stripes. However, for relatively wider stripes, electronic states are confined in different stripes and make a semiconductor–semiconductor junction with normal band alignment. This way one-dimensinonal multiple quantum well structures can be generated with electrons and holes confined to h-GaN stripes. Vertical heterostructures formed by thin stacks of h-GaN and h-AlN are composite semiconductors with a tunable fundamental band gap. However, depending on the stacking sequence and number of constituent sheets in the stacks, the vertical heterostructure can transform into a junction, which displays staggered band alignment with electrons and holes separated in different stacks. The weak bonds between the cations and anions in adjacent layers distinguish these heterostructures from those fabricated using thin films of GaN and AlN thin films in wurtzite structure, as well as from van der Waals solids. Despite the complexities due to confinement effects and charge transfer across the interface, the band diagram of the heterostructures in the direct space and band lineup are conveniently revealed from the electronic structure projected to the atoms or layers. Prominent features in the optical spectra of the lateral composite structures are observed within the limits of those of 2D parent constituents; however, significant deviations from pristine 2D constituents are observed for vertical heterostructures. Important dimensionality effects are revealed in the lateral and vertical heterostructures.
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The present work investigates the less explored thermoelectric properties of n-type GaN semiconductors by combined both the experimental and computational tools. Seebeck coefficients of epitaxial thin films of GaN were experimentally measured in the wide temperature range from 77 K to 650 K in steps of ~10 K covering both low and high-temperature regimes as a function of carrier concentration 2 x1016, 2 x 1017, 4 x 1017 and 8 x 1017 cm-3. The measured Seebeck coefficient at room temperature was found to be highest, -374 microV/K, at the lowest concentration of 4 x 1016 cm-3 and decreases in magnitude monotonically (-327.6 microV/K, -295 microV/K, -246 microV/K for 2 x 1017, 4 x 1017, 8 x 1017 cm-3, respectively) as the carrier concentration of samples increases. Seebeck coefficient remains negative in the entire temperature range under study indicate that electrons are dominant carriers. To understand the temperature-dependent behavior, we have also carried out the electronic structure, and transport coefficients calculations by using Tran-Blaha modified Becke-Johnson (TB-mBJ) potential, and semiclassical Boltzmann transport theory implemented in WIEN2k and BoltzTraP code, respectively. The experimentally observed carrier concentrations were used in the calculations. The estimated results obtained under constant relaxation time approximations provide a very good agreement between theoretical and experimental data of Seebeck coefficients in the temperature range of 260 to 625 K.
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Structural, optical and surface properties of epitaxially grown 2D GaN nanowall network using Molecular Beam Epitaxy have been compared to those of non-epitaxially grown single-crystalline 1D GaN nanowires using Chemical Vapour Deposition. The kinetics of growth mechanisms and formed morphology are shown to significantly influence the respective band structures and consequently their luminescence properties. X-ray diffraction and Raman spectroscopy reveal that the epitaxial 2D nanowall network experiences a hydrostatic strain in addition to a compressive strain, whereas non-epitaxial 1D nanowires possess a morphology-dependent tensile/compressive strain and a negligible hydrostatic strain. Slightly blue-shifted photoluminescence emission from both these nanostructures is markedly enhanced compared to that from an epilayer. The epitaxial nanowall network exhibits the highest enhancement in the band edge emission among them. X-ray photoelectron spectroscopy spectra show shifts in the valence band features and in the hybridization of shallow core levels. Using the XRD, Raman, PL and XPS data, a variation in the band structure of these differently kinetically formed GaN nanostructures is also sketched.
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Electronic structure of a defect center containing the gallium vacancy and substitutional oxygen atom at nitrogen site (VGaON) in zinc blende and wurtzite GaN was analyzed within GGA+U approach. The +U term was applied to d(Ga), p(N), p(O), and d(In). Neutral VGaON is in the stable high spin state with spin S = 1. The defect structure is strongly dependent on geometry of the defect and the charge state. Two spin structures, which arise due to two different configurations in VGaON, with ON either along the c-axis or in one of three equivalent tetrahedral positions in wurtzite structure were analyzed. The weak ferromagnetic coupling between centers was found. The strength of magnetic coupling is increased when there is a complex containing VGaON with additional substitutional indium atom at the second neighbor to vacancy gallium site (VGaONInGa). Magnetic coupling between VGaONInGa is ferromagnetic due to strong spin polarization of p electrons of the nearest and distant nitrogen atoms.
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In recent years, constructing van der Waals (vdW) heterostructures based on different two‐dimensional (2D) materials has been a highly effective way to obtain desired electronic and optical properties. Herein, on the basis of first‐principles calculations, we construct a 2D vdW heterostructure by vertically stacking the indirect‐band‐gap GaN and SiH monolayers. For the formed heterostructure, the structual, electronic, and optical properties are all computed. The obtained results show that the heterostructure is a direct‐band‐gap semiconductor with typical type‐II band alignment. Furthermore, the linearly tunable band gap, together with the robust type‐II band alignment, can be realized by applying vertical strains or external electric fields. It is also found that both monolayers exhibit the same UV absorption range and the notably enhanced UV absorption is obtained after forming the heterostructure. In particular, the maximum absorptivity can reach to 21.6%. These results are expected to provide useful references for the design of ultraviolet optoelectronic devices based on the 2D GaN/SiH vdW heterostructure. This article is protected by copyright. All rights reserved.
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Due to the wide range of possible applications, atomically thin two-dimensional heterostructures have attracted much attention. In this work, using first-principles calculations, we investigated the structural and electronic properties of planar AlN/GaN hybrid heterojunctions with the presence of vacancies at their interfaces. Our results reveal that a single vacancy site, produced by the removal of Aluminum or Gallium atom, yields similar electronic band structures with localized states within the bandgap. We have also observed a robust magnetic behavior. A nitrogen-vacancy, on the other hand, induces the formation of midgap states with reduced overall magnetization. We have also studied nanotubes formed by rolling up these heterojunctions. The results showed that nanotube curvature does not substantially affect the electronic and magnetic properties of their corresponding planar AlN/GaN heterojunctions. For armchair-like nanotubes, a transition from direct to indirect bandgap was observed as a consequence of changing the system geometry from 2D to a quasi-one-dimensional one.
Article
1-dimensional GaN nanowires (NW) and nanorods (NR) grown using catalyst assisted Chemical Vapour Deposition are investigated for their structural and optical properties, and their electronic band structure. X-ray Diffraction (XRD) studies reveal that biaxial strain increases with aspect ratio, which is corroborated by Raman studies wherein phonon related confinement effects are observed for NR due to the crystal strain. Photoluminescence studies show 3 and 6 times enhancement in the intensity of band edge emission of NW and NR, respectively, signifying the presence of a large number of band tail states. A small red shift in NW (3.43 eV) and large red-shift in NR (3.28eV) is reported, which are attributed to the change in the band gap and band structure of these morphologies. Valence band studies by X-ray Photoelectron Spectroscopy reveal the presence of band tail states for NR and surface bond contraction for NW. The effect of crystal strain on the electronic band structure on optical properties of 1-D GaN nanostructures with different aspect ratios is elucidated.
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Impurity doping is one of the important means to control the physical properties of intrinsic semiconductors. By combining data mining and first principles calculation, a series of potential dopable elements in gallium nitride (GaN) were predicted by Shannon radius and probability model methods in this study. The Shannon radius difference and replacement probability were used as the criterion to screen the dopable elements and a total of 36 dopants were predicted. Then the structural stability and electronic structure of these doped structures were systematically studied by the first principles calculations. Among these dopants, the defect formation energy of 14 kinds is less than 3 eV, namely O N , Zr G a , Ti G a , Si G a , F N , Ge G a , Nb G a , S N , Se N for n -type and Be G a , Mg G a , Ca G a , Zn G a and Mn G a for p -type. In these potential doping systems, the changes of electronic structure can be divided into two types according to whether the impurity energy level is introduced or not. The doping of Ti G a , Nb G a , F N and Mn G a can introduce significant impurity levels in the band gap, the electrons of which are directly involved in the carrier transition. For other dopants, no obvious impurity energy level is introduced in the band gap. Compared with the undoped GaN, the electronic density of states of the doped systems are significantly enhanced at the conduction band minimum. These results are expected to provide an effective theoretical reference for the experimental screening of appropriate GaN dopants
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The electronic structure of wurtzite GaN, Al0.5Ga0.5N, and AlN has been studied using synchrotron radiation excited soft x-ray emission spectroscopy. In particular, the elementally resolved partial densities of states has been measured and found to agree well with calculations. The shift in energy of the valence band maximum as x varies from 0 to 1 in AlxGa1-xN was measured by recording N K-emission spectra, and found to be linear. Furthermore, N K-emission spectra revealed resonantlike hybridization of N 2p and Ga 3d states at 19 eV below the GaN valence band maximum. The spectral intensity of this feature is proportional to Ga content. © 1998 American Vacuum Society.
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The electronic structure of the wurtzite AlxGa1-xN alloy system has been studied for numerous values of Al concentration x ranging from 0 (pure GaN) to 1 (pure AlN). The occupied and unoccupied partial density of states was measured for each alloy using synchrotron radiation excited soft x-ray absorption and emission spectroscopies. High-resolution x-ray emission spectroscopy allowed the motion of the elementally resolved bulk valence-band maximum to be measured as a function of Al concentration. Using this technique we estimate that the value of the band-gap bowing parameter for AlxGa1-xN is zero. Furthermore, the x-ray emission spectra revealed resonantlike emission at approximately 19 eV below the GaN valence-band maximum. By measuring the intensity of this feature as a function of Ga content we prove conclusively that this emission arises from hybridization of N 2p and Ga 3d states. Finally, we find that the N K- and Al K-absorption spectra depend strongly on the photon angle of incidence with respect to the surface normal. We explain this in terms of orbital anisotropy in AlxGa1-xN.
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The electronic structure of wurtzite GaN, Al{sub 0.5}Ga{sub 0.5}N, and AlN has been studied using synchrotron radiation excited soft x-ray emission spectroscopy. In particular, the elementally resolved partial densities of states has been measured and found to agree well with calculations. The shift in energy of the valence band maximum as {ital x} varies from 0 to 1 in AlâGa{sub 1-x}N was measured by recording N {ital K}-emission spectra, and found to be linear. Furthermore, N {ital K}-emission spectra revealed resonantlike hybridization of Nthinsp2p and Gathinsp3d states at 19 eV below the GaN valence band maximum. The spectral intensity of this feature is proportional to Ga content. {copyright} {ital 1998 American Vacuum Society.}
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We present the results of Kohn-Sham calculations on molecules, surfaces, and solids which were obtained using a recently proposed exchange-correlation energy functional [ Z. Wu and R. E. Cohen Phys. Rev. B 73 235116 (2006)]. The Wu-Cohen (WC) functional, like the well-known PBE functional [ J. P. Perdew et al. Phys. Rev. Lett. 77 3865 (1996)], is of the generalized gradient approximation form and was derived from the homogeneous electron gas and mathematical relations obeyed by the exact functional. The results on molecular systems show that among the functionals we tested, PBE remains superior for the energetics of covalent and noncovalent bonds. While this is not too surprising for noncovalent bonds due to the very good performance of PBE, unfortunately this holds also for covalent bonds, where PBE is a functional of rather poor quality. Calculations on transition-metal surfaces show that WC improves over local-density approximation (LDA) and PBE for the surface formation energy of 3d elements, while LDA is the best for heavier elements. In most cases, the lattice constant of solids as determined by the WC functional is in between the LDA and PBE results and on average closer to experiment. We show for each group of compounds which functional performs best and provide trends. In the particular case of lattice constants whose values are determined by weak interactions (e.g., the interlayer distance in graphite), the LDA functional is more accurate than the generalized gradient approximation functionals.
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All-electron band-structure calculations and photoemission experiments on II-VI semiconductors both exhibit a metal d subband inside the main valence band. It has nevertheless been customary in pseudopotential and tight-binding approaches to neglect the metal d band by choosing Hamiltonian parameters which place this band inside the chemically inert atomic cores. Using all-electron self-consistent electronic-structure techniques (which treat the outermost d electrons on the same footing as other valence electrons) and comparing the results to those obtained by methods which remove the d band from the valence spectrum, we study their effects on valence properties. For II-VI semiconductors we find that p-d repulsion and hybridization (i) lower the band gaps, (ii) reduce the cohesive energy, (iii) increase the equilibrium lattice parameters, (iv) reduce the spin-orbit splitting, (v) alter the sign of the crystal-field splitting, (vi) increase the valence-band offset between common-anion II-VI semiconductors, and (vii) modify the charge distributions of various II-VI systems and their alloys. p-d repulsion is also shown to be responsible for the occurrence of deep Cu acceptor levels in II-VI semiconductors (compared with shallow acceptors of Zn in III-V), for the anomalously small band gaps in chalcopyrites, and for the negative exchange splitting in ferromagnetic MnTe.
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The effects of the Ga 3d semicore levels on the electronic structure of GaN are discussed. While band-structure theory using the local-density approximation predicts these states to overlap with the N 2s band and to have important effects on the total energy, x-ray photoelectron spectroscopy (XPS) shows that they occur ∼3 eV below the N 2s band. This apparent discrepancy is resolved by means of a so-called Δ SCF or difference of self-consistent-fields calculation, in which the binding energy is calculated as a total-energy difference including solid state screening effects by means of the excited-atom model. The calculated valence-band densities of states are found to be in good agreement with the XPS spectrum. The differences between zinc blende and wurtzite GaN are discussed.
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The electronic structure of thin-film wurtzite GaN has been studied using a combination of soft-x-ray absorption and emission spectroscopies. We have measured the elementally and orbitally resolved GaN valence and conduction bands by recording Ga L and N K spectra. We compare the x-ray spectra to the partial density of states from a recent ab initio calculation and find good overall agreement. The x-ray emission spectra confirm that the top of the valence band is dominated by N 2p states whereas they reveal that there is only weak hybridization between Ga 4s and N 2p states. Surprisingly, we found a weak feature in the N K emission at approximately 19.5 eV below the valence-band maximum that arises from hybridization between N 2p and Ga 3d states. X-ray absorption spectra show that the bottom of the conduction band is a mixture of Ga 4s and N 2p states, again in very good agreement with the theory.
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We report ab initio calculations of the lattice constants and the electronic structure of the hexagonal wurtzite semiconductors CdS and CdSe. The calculations have been carried out self-consistently in the local-density approximation employing nonlocal, separable, and norm-conserving pseudopotentials. We use Cd12+ ionic pseudopotentials so that the Cd 4d electrons are explicitly taken into account as valence electrons. The calculated electronic structure is compared with photoemission data. Calculated and measured bands show good agreement in the energy region of the mostly anion-derived s-p valence bands. The calculated Cd 4d bands result for both compounds roughly 3 eV too high in energy as compared to the measured data. This rigid shift of the narrow 4d bands is related to correlation effects which are not fully taken into account in the local-density approximation.
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A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si and Ge emerges from an analysis based on all-electron, self-consistent, GW calculations. The deep-core electrons are shown to play a key role via the exchange diagram-if this effect is neglected, Si becomes a semimetal. Contrary to current lore, the Ge 3d semicore states (e.g., their polarization) have no impact on the GW gap. Self-consistency improves the calculated gaps-a first clear-cut success story for the Baym-Kadanoff method in the study of real-materials spectroscopy; it also has a significant impact on the QP lifetimes. Our results embody a new paradigm for ab initio QP theory.
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High-resolution soft X-ray emission and absorption spectra near the N K-edge of wurtzite GaN are presented. The experimental data are interpreted in terms of band structure based full-potential electronic structure calculations. The absorption spectra, compared with calculations including core hole screening, indicate partial core hole screening in the absorption process. The resonant emission spectra demonstrate pronounced dispersions of the spectral structures, identifying effects of momentum conservation due to resonant inelastic X-ray scattering (RIXS) with anisotropic electronic structure of GaN. In view of a wide range of optoelectronic applications of GaN, our findings on the momentum selectivity in RIXS can be utilized in development of GaN based nanoelectronics devices by controlling direct vs indirect band gap character of GaN nanostructures. Comment: 9 pages, 7 figures, submitted to Phys. Rev. B
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Absolute lattice parameter methods are useful for determining alloy composition, understanding point defects and dopants in semiconductor substrate materials and for the evaluation of lattice relaxation in heteroepitaxial layers. This paper reviews the techniques available. The assumptions and uncertainties of each technique are discussed.
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This paper deals with the ground state of an interacting electron gas in an external potential v(r). It is proved that there exists a universal functional of the density, Fn(r), independent of v(r), such that the expression Ev(r)n(r)dr+Fn(r) has as its minimum value the correct ground-state energy associated with v(r). The functional Fn(r) is then discussed for two situations: (1) n(r)=n0+n(r), n/n01, and (2) n(r)= (r/r0) with arbitrary and r0. In both cases F can be expressed entirely in terms of the correlation energy and linear and higher order electronic polarizabilities of a uniform electron gas. This approach also sheds some light on generalized Thomas-Fermi methods and their limitations. Some new extensions of these methods are presented.
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The inherent advantages of the hot-wall metal organic chemical vapor deposition (MOCVD) reactor (low temperature gradients, less bowing of the wafer during growth, efficient precursor cracking) compared to a cold-wall reactor make it easier to obtain uniform growth. However, arcing may occur in the growth chamber during growth, which deteriorates the properties of the grown material. By inserting insulating pyrolytic BN (PBN) stripes in the growth chamber we have completely eliminated this problem. Using this novel approach we have grown highly uniform, advanced high electron mobility transistor (HEMT) structures on 4″ semi-insulating (SI) SiC substrates with gas-foil rotation of the substrate. The nonuniformities of sheet resistance and epilayer thickness are typically less than 3% over the wafer. The room temperature hall mobility of the 2DEG is well above 2000cm2/Vs and the sheet resistance about 270Ω/sqr.
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Murnaghan's theory of finite strain is developed for a medium of cubic symmetry subjected to finite hydrostatic compression, plus an arbitrary homogeneous infinitesimal strain. The free energy is developed for cubic symmetry to include terms of the third order in the strain components. The effect of pressure upon the second-order elastic constants is found and compared with experiment, with particular reference to the compressibility; the pressure-volume relation in several approximations is compared with the measurements to 100,000 kg/cm2. The simplest approximation is shown to give a satisfactory account of most of the experimental data. The results are also compared with some of the calculations based on Born's lattice theory.
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The positions of the semicore Ga d levels in GaX semiconductors (X=N,P, and As) are underestimated in density functional calculations using either the local density approximation LDA or the generalized gradient approximation GGA for the exchange functional. Correcting for this inaccuracy within LDA+U calculations with an on-site Coulumb interaction U on the semicore d-states results in a modest enhancement of the band gap. We show that this modest enhancement of the band-gap energy comes from the movement of the valence-band maximum alone, thus not affecting the conduction-band states. Further, the localization of the charge on Ga d states with U leads to a regulation of charge on Ga. This yields a shift of 1–2 eV of the core levels on the Ga atom while the anion core levels remain unchanged.
Article
We present a nonempirical density functional generalized gradient approximation (GGA) that gives significant improvements for lattice constants, crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA. The functional is based on a diffuse radial cutoff for the exchange hole in real space, and the analytic gradient expansion of the exchange energy for small gradients. There are no adjustable parameters, the constraining conditions of PBE are maintained, and the functional is easily implemented in existing codes.
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The optical absorption and excitonic properties of wurtzite AlN are investigated by means of an ab initio approach taking into account electron-hole correlations. This is done by solving the Bethe-Salpeter equation, using the results of density functional theory calculations as a starting point. The main focus is on the calculation of excitonic spectra near the conduction-band edge. The response is dominated by the exciton A formed out of excitations from valence Γ7 band. The n−2 quantum-number dependence of the energies in Elliott’s model fits rather well the ab initio calculations whereas the n−3 decay of the intensities is less obvious for the calculated oscillator strengths.
Article
The anisotropy of the electronic structure of ternary nanolaminate V2GeC is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of V L2,3, C K, Ge M1, and Ge M2,3 in V2GeC are compared with those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed: V 3d-C 2p bonding at −3.8 eV, Ge 4p-C 2p bonding at −6 eV, and Ge 4p-C 2s interaction mediated via the V 3d orbitals at −11 eV below the Fermi level. We find that the anisotropic effects are high for the 4p valence states and the shallow 3d core levels of Ge, while relatively small anisotropy is detected for the V 3d states. The macroscopic properties of the V2GeC nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.
Article
The local electronic structure of a material can be determined from the energy-loss spectrum of a swift electron beam scattered through it. When the electron beam is focused down to the width of an atomic column, the electronic density of states (DOS) at an interface, grain boundary, or impurity site can be decomposed by site, chemical species and angular momentum. Here we discuss the use of electron-energy-loss spectroscopy (EELS) fine structure to provide insight into the origin of grain boundary and interfacial properties reported earlier [D. A. Muller et al., Phys. Rev. Lett. 75, 4744 (1995)] for Ni3Al. We examine the electronic structure trends in Ni-Al compounds, both experimentally with the EELS measurements and theoretically, using ab initio band-structure calculations. The conditions under which the band-structure calculations can quantitatively reproduce the EELS measurements (and in particular, the question of just which local DOS is being measured) are addressed. Cyrot-Lackmann’s moments theorem provides a framework to explain the systematic changes in the local DOS on alloying. The shape changes in the near-edge fine structure of both the Ni and Al L edges are readily understood by the sensitivity of the fourth moment of the local DOS to the angular character of the Ni-Al bonding. The language of bond-order potentials proved useful in linking shape changes in the DOS to changes in cohesion. The consequences for formation energies and ordering trends in the transition-metal–aluminum alloys are also discussed.
Article
The dynamical theory of x-ray spectra due to Nozières and De Dominicis (ND) is evaluated here numerically for numerous model systems including cases where the corehole potential possesses a bound state. It is shown that the resulting emission spectra obey the final-state rule rather accurately. An approximate but analytical derivation of this rule is given which provides insight into the mechanisms leading to the final-state rule. The evaluations are performed with the use of two methods, one based on an integral equation together with a separable core-hole potential and the other based on determinantal wave functions for a finite number (N) of electrons in a box. The equivalence of the two methods is demonstrated both formally and numerically. By comparing them we prove the finite- N approach to be accurate already for rather small N. We also show that a separable potential does not give rise to any spurious results but can actually be chosen to yield the same ND spectrum as a local potential. The ND theory of x-ray photoemission spectra is discussed and form calculations of the exponent function α(ω) for several model systems closely corresponding to simple metals we conclude the equivalence of this theory and its asymptotic approximation as far as the extraction of asymmetry indices is concerned. Recent criticism of the major conclusions reached here and in previous work is refuted.
Article
The energy distribution of the nitrogen antibonding electron states in the hexagonal epitaxial layers of AlN, GaN, and InN and cubic epitaxial layers of GaN and InN along pxy plane and pz direction is reported. The study was performed by the polarized x-ray absorption at the K edge of N. A strong polarization dependence of the absorption spectra indicating the significant anisotropy of the conduction band was found in the case of hexagonal samples. Very weak polarization dependencies observed in cubic samples correspond well with the defect distribution anisotropy. Qualitatively different and cation dependent antibonding states distribution point out the role played by a contribution of hybridized cation-nitrogen electronic states to the individual conduction band structures of AlN, GaN, and InN. © 1997 American Institute of Physics.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
The composition, surface structure, and electronic structure of zinc blende–GaN films grown on GaAs (100) and (110) by plasma‐assisted molecular beam epitaxy were investigated by means of core and valence level photoemission. Angle‐resolved photoelectron spectra (photon energy 30–110 eV) exhibited emission from the Ga 3d and N 2s levels, as well as a clear peak structure in the valence band region. These peaks were found to shift with photon energy, indicative of direct transitions between occupied and unoccupied GaN bands. By using a free electron final band, we are able to derive the course of the bands along the Γ‐X and Γ‐K‐X directions of the Brillouin zone and to determine the energy of critical points at the X point. The relative energies of the Ga 3d and nitrogen 2s bands were also studied, and a small amount of dispersion was detected in the latter. The resulting band structure is discussed in relation to existing band structure calculations. © 1996 American Vacuum Society
Article
Basic mechanical properties of single crystal gallium nitride are measured. A Vickers (diamond) indentation method was used to determine the hardness and fracture toughness under an applied load of 2N. The average hardness was measured as 12±2 GPa and the average fracture toughness was measured as 0.79±0.10 MPa√m. These values are consistent with the properties of brittle ceramic materials and about twice the values for GaAs. A methodology for examining fracture problems in GaN is discussed. © 1996 American Institute of Physics.
Article
The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The performance of the undulator agrees very well with the specifications, as shown by measurements using a photodiode. The energy resolution of the monochromator has been checked using absorption measurements in a gas cell. It was found to meet the expectations and exceeds a resolving power of 10 000 at 244 eV. The photon flux as a function of energy has been recorded as well and gives a maximum flux of 3×1013 photons/s/100 mA/0.1% BW. Beamlines I511 and I411 will be the first synchrotron beamlines making use of a so-called beam waist phenomenon, known from laser physics. We show results of ray-tracing calculations to determine the ultimate spot size on the sample location. The endstations to be used at this new beamline and their capabilities will be discussed as an example of the future use of this facility.
Article
The hot-wall metalorganic chemical vapor deposition (MOCVD) concept has been applied to the growth of AlxGa1−xN/GaN high electron mobility transistor (HEMT) device heterostructures on 2 inch 4H-SiC wafers. Due to the small vertical and horizontal temperature gradients inherent to the hot-wall MOCVD concept the variations of all properties of a typical HEMT heterostructure are very small over the wafer: GaN buffer layer thickness of 1.83 μm±1%, Al content of the AlxGa1−xN barrier of 27.7±0.1%, AlxGa1−xN barrier thickness of 25 nm±4%, sheet carrier density of 1.05×1013 cm−2±4%, pinch-off voltage of −5.3 V±3%, and sheet resistance of 449 Ω±1%.
Article
Using first-principles methods based on density functional theory within the local density approximation (LDA) we calculate the structural and electronic properties of wurtzite MgO, ZnO, and CdO, and discuss their similarities and dissimilarities with the corresponding Group-III nitrides AlN, GaN, and InN. We treat the semicore d states of Zn, Cd, Ga, and In explicitly as valence states in a pseudopotential approach, investigate the effects of including on-site Coulomb interaction for Zn, Cd, Ga, and In semicore d states within the LDA+U method, and propose a novel approach to calculate the parameter U. Our results show that the LDA+U approach systematically improves the LDA band gap by indirectly acting on both the valence-band maximum and conduction-band minimum. We also discuss the effects of the on-site Coulomb interaction on structural parameters and absolute deformation potentials of ZnO, CdO, GaN, and InN.
Article
We have refined the structure parameters of AlN and GaN using X-ray intensities from single crystals collected with an automatic single crystal diffractometer. The lattice constants and the u values are a = 3.110 Å, c = 4.980Å, u = 0.3821 for AlN and a = 3.190Å, c = 5.189 Å, u = 0.377 for GaN. The final R-values for anisotropic temperature factors are equal to 0.015 for AlN and 0.026 for GaN. The effective atomic charges in these compounds are estimated.
Article
A new basis set for a full potential treatment of crystal electronic structures is presented and compared to that of the well-known linearized augmented plane-wave (LAPW) method. The basis set consists of energy-independent augmented plane-wave functions combined with local orbitals. Each basis function is continuous over the whole unit cell but it may have a discontinuous slope at the muffin-tin boundaries, i.e. at the surfaces of atomic centered, non-overlapping spheres. This alternative way to linearize the augmented plane-wave method is shown to reproduce the accurate results of the LAPW method, but using a smaller basis set size. The reduction in number of basis functions is most significant for open structures.
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A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
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The local-density approximation for the exchange-correlation potential understimates the fundamental band gaps of semiconductors and insulators by about 40%. It is argued here that underestimation of the gap width is also to be expected from the unknown exact potential of Kohn-Sham density-functional theory, because of derivative discontinuities of the exchange-correlation energy. The need for an energy-dependent potential in band theory is emphasized. The center of the gap, however, is predicted exactly by the Kohn-Sham band structure.
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The Hohenberg-Kohn theorem is extended to fractional electron number N, for an isolated open system described by a statistical mixture. The curve of lowest average energy EN versus N is found to be a series of straight line segments with slope discontinuities at integral N. As N increases through an integer M, the chemical potential and the highest occupied Kohn-Sham orbital energy both jump from EM-EM-1 to EM+1-EM. The exchange-correlation potential Excn(r) jumps by the same constant, and limr Excn(r)>~0.
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The electronic structure and the anisotropy of the Al - N π and σ chemical bonding of wurtzite AlN has been investigated by bulk-sensitive total fluorescence yield absorption and soft x-ray emission spectroscopies. The measured N K, Al L1, and Al L2,3 x-ray emission and N 1s x-ray absorption spectra are compared with calculated spectra using first principles density-functional theory including dipole transition matrix elements. The main N 2p - Al 3p hybridization regions are identified at -1.0 to -1.8 eV and -5.0 to -5.5 eV below the top of the valence band. In addition, N 2s - Al 3p and N 2s - Al 3s hybridization regions are found at the bottom of the valence band around -13.5 eV and -15 eV, respectively. A strongly modified spectral shape of Al 3s states in the Al L2,3 emission from AlN in comparison to Al metal is found, which is also reflected in the N 2p - Al 3p hybridization observed in the Al L1 emission. The differences between the electronic structure and chemical bonding of AlN and Al metal are discussed in relation to the position of the hybridization regions and the valence band edge influencing the magnitude of the large band gap.
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The excitation energy dependence of the three-hole satellites in the L3-M4,5M4,5 and L2-M4,5M4,5 Auger spectra of nickel metal has been measured using synchrotron radiation. The satellite behavior in the non-radiative emission spectra at the L3 and L2 thresholds is compared and the influence of the Coster-Kronig channel explored. The three-hole satellite intensity at the L3 Auger emission line reveals a peak structure at 5 eV above the L3 threshold attributed to resonant processes at the 2p53d9 shake-up threshold. This is discussed in connection with the 6-eV feature in the x-ray absorption spectrum.
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From a theory of Hohenberg and Kohn, approximation methods for treating an inhomogeneous system of interacting electrons are developed. These methods are exact for systems of slowly varying or high density. For the ground state, they lead to self-consistent equations analogous to the Hartree and Hartree-Fock equations, respectively. In these equations the exchange and correlation portions of the chemical potential of a uniform electron gas appear as additional effective potentials. (The exchange portion of our effective potential differs from that due to Slater by a factor of 23.) Electronic systems at finite temperatures and in magnetic fields are also treated by similar methods. An appendix deals with a further correction for systems with short-wavelength density oscillations.