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Dependence of electronic properties on nitrogen concentration in GaAs 1− x N x dilute alloys
Abstract
Based on the pseudopotential scheme, the effect of nitrogen concentration on electronic properties of zinc-blende GaAs1−xNx alloys has been investigated for small amounts of N. The agreement between our calculated electronic band parameters and the available experimental data is generally reasonable. In agreement with recent experiment, we find that the incorporation of a few percent of N in the material of interest reduces substantially the fundamental band-gap energy and narrows the full valence band width. The electron and heavy hole effective masses are found to decrease rapidly when adding a concentration of nitrogen less than 0.005 in GaAs. This may increase the mobility of electrons and heavy holes providing new opportunities regarding the transport properties. The information derived from the present study shows that GaAs1−xNx (0⩽x⩽0.05) properties may have an important optoelectronic applications in infrared and mid-infrared spectral regions.
... III-V dilute nitride alloys such as GaAs 1−x N x , GaSb 1−x N x , etc. obtained by incorporating a small fraction (generally less than 5%) of nitrogen into III-V binary compounds such as GaAs, GaSb, InAs, etc. are highly important for the fabrication of near-infrared optoelectronic devices [1][2][3][4][5][6]. These dilute systems have sparked a great interest and have been extensively studied over the two last decades [7][8][9][10][11]. This is because of their uncommon physical properties. ...
... This is because of their uncommon physical properties. In particular, their band gap energy is drastically reduced as more N-atoms are inserted [7,8] into the host material. The strong reduction of the gap is the consequence of the strong parameter of curvature (optical bowing parameter) that has been experimentally observed for some dilute nitrides such as GaPN [7,12,13], InPN [14,15] and GaAsN [7,[16][17][18]. ...
... It is worth noting that the N composition x = 0.03 is an experimentally realistic value as reported in Refs. [7,8]. Besides, our quantum well is a stained one because of the difference between the lattice parameters of the well layer (GaAs 0.97 N 0.03 ) and the barrier layer (GaAs) respectively. ...
The optical gain of a strained GaAs1−xNx/GaAs quantum-well laser has been calculated for a nitrogen concentration of 0.03 corresponding to a so-called dilute alloy. The effect of the density of carriers along with that of the quantum well width on the optical gain of the considered laser have been investigated and analyzed. Besides, the emitted wavelength has been also derived as a function of the quantum well width. Numerical results clearly show that by increasing the density of carriers and the quantum well width the optical gain is increased. The emitted wavelength is also enhanced as the quantum well width is augmented. The laser diode being studied here is shown to emit in the infrared-red region of the electromagnetic spectrum.
... For a better understanding and control of both material and device properties, theoretical investigation turns out to be very important. GaP 1−x N x , GaAs 1−x N x , GaSb 1−x N x and InP 1−x N x dilute nitrides have already been investigated [21][22][23][24]. Regarding InAs 1−x N x dilute alloys, only limited data are available in the literature despite their potential application in several optoelectronic devices. ...
... The behavior of the direct gap at and indirect gaps at X and L points indicates clearly that the absorption at the optical gaps in InAs 1−x N x is direct within a whole range of the nitrogen content x (0 x 0.05). It should be noted that the introduction of only 1% of N into InAs 1−x N x reduces the band-gap energy approximately by 20 meV, whereas the insertion of the same amount of N into GaAs and GaSb is found to result in a diminution of 130 and 230 meV, respectively [22,23]. Hence, the band-gap narrowing in InAs 1−x N x demonstrates the possibility of using this material for mid-infrared optoelectronic applications. ...
The energy band gaps, the electron and heavy hole effective masses and dielectric constants of ternary dilute alloys InAs1−xNx are calculated using the pseudopotential method. The study was undertaken for small amounts of N. The disorder in these alloys is modeled by an improved virtual crystal approximation approach. The numerical results thus obtained are compared where possible with experimental and previous theoretical data. It is found that the incorporation of a few per cent of N in InAs reduces substantially the fundamental band-gap energy, which is a manifestation of the huge band-gap bowing commonly reported in the III–V–N-type alloys. The behavior of all studied quantities with respect to the nitrogen concentration x is shown to be nonlinear. The information gathered by the present study may be useful for mid-infrared applications.
... The conversion efficiency of a solar cell can be improved by making a judicious choice of the number of junctions in the cell and the materials whose energy gap can be controlled. This control can be easily obtained by alloying binary compounds to obtain ternary semiconductor alloys such as GaAs 1-x N x , GaSb 1x N x , or InAs 1-x N x [36][37][38][39]. Other alloys that contain low concentration magnetic elements, called dilute magnetic semiconductors, can also be considered [40][41][42][43]. ...
This study presents a numerical investigation of the reflectivity of a Single Anti-Reflective Layer (SARL) and a stack of antireflective layers made of porous silicon. The stack consists of a certain number of periods, and each period contains two layers with different porosity. The simulations were conducted using the well-known Stratified Medium Theory (SMT) framework and the effect of porosity was studied. The optimal value was determined at 60% for the SARL and 65/55% for the stack of 12 periods and 6 layers. The angle of incidence was found to have more influence on the stack reflection than on the SARL reflection. The results of this investigation show that porous silicon can be used as an effective anti-reflective coating for silicon solar cells.
... The hydrogen storage capacities calculated for LiH and NaH were 22.5% and 8.06%, respectively. The electronic and optical properties of the materials are classically described by their electronic band structure [30][31][32][33]. Accurate knowledge of the band parameters of a given material provides precious information regarding its synthesis and devices fabrication [34][35][36][37]. ...
This paper presents ab initio calculations within the Density Functional Theory (DFT) for the structural and optoelectronic properties of the alkali metal hydrides LiH and NaH in rocksalt structure (B1). This study used the Generalized Gradient Approximation (GGA) of Wu-Cohen to consider the electronic exchange and correlation interactions. In addition, the Tran-Blaha modified Becke-Johnson exchange potential was used with the GGA approach (GGA-TBmBJ) to calculate the band structure with high accuracy. The structural properties, namely the lattice parameter, the bulk modulus, and the pressure derivative of the bulk modulus were determined and found to be generally in good agreement with other research findings. Furthermore, the energy band gaps, the Density Of States (DOS), the static and high-frequency dielectric constant, along the refractive index were addressed and analyzed. These results could be useful for hydrogen storage purposes.
... More details about the method used can be found in Refs. [14,[22][23][24]. ...
The electronic and optical features of InSb spherical quantum dots have been investigated by a pseudopotential approach as a function of their radius taken in the range 1-10 nm. The direct- and indirect band gaps along with the electron and heavy hole effective masses are all found to be diminished as the quantum dot radius is increased. However, the refractive index, the static- and high frequency dielectric constant as well as the transverse effective charge are shown to be augmented by increasing the quantum dot radius. It is noted that the quantum confinement is of great impact on all the studied quantities for quantum dot radius below 6 nm. This could result in more opportunities to obtain desired optoelectronic properties that cannot be obtained in the bulk InSb materials.
... More details about the method used can be found in Refs. [14,[22][23][24]. ...
The size-dependent energy gaps, electron and heavy hole effective masses, phonon frequencies and polaron related parameters in InSb spherical quantum dots are investigated. The calculations are performed using a pseudopotential approach. Good agreement is obtained between our results and those of the literature for bulk InSb. When proceeding from bulk to nano-scale, all studied properties are found to be changed significantly. This is attributed to the quantum confinement effect.
... This is performed using a non-linear least-squares fitting procedure (Kobayasi and Nara 1993). More details about the method are reported in Gueddim et al. (2006Gueddim et al. ( , 2007Gueddim et al. ( , 2015, and Gueddim and Bouarissa (2009). ...
The electron energy levels, direct energy band gaps, electron and hole effective masses as well as the transverse effective charge of InAs spherically shaped quantum dots have been studied as a function of the quantum dot radius considered as varying from 1 to 10 nm. The direct energy band-gap as well as the electron and heavy hole effective masses decrease non-linearly with increasing the quantum dot radius. Nevertheless, the transverse effective charge is found to increase with increasing the quantum dot radius. It is concluded that the quantum confinement has a strong influence on all the studied physical quantities for quantum dot radius below 6 nm. The results of the present contribution show that more opportunities can be offered to tailor desired optoelectronic properties surpassing those presented by bulk InAs materials. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
... Noteworthy is that GaNyAs1−y (x = 0) and AlNyAs1−y (x = 1) converted their transition types between direct and indirect by variation of y even though the transition types of their two constituent binaries (GaN and GaAs for GaNyAs1−y, and AlN and AlAs for AlNyAs1−y) are consistent as shown in table I or in figure 2. A similar conversion was calculated by Gueddim et al. for GaNyAs1−y by using the empirical pseudopotential method under the virtual crystal approximation even though the calculated bandgap energy values stayed positive [12]. ...
Bandgap energies of the group III-V quaternary alloy semiconductor, cubic Al
x
Ga1−x
N
y
As1−y
, were calculated by means of the dielectric method. While only GaN and GaAs are considered to be direct transition type among the four constituent binary compounds of this quaternary alloy system, the calculation results show that the bandgap energy range covered in the direct transition regime of this alloy system was further extended to the higher energy side of GaN as well as to the lower energy sides of GaAs. The extension to the higher energy side was attributed to the larger direct bandgap of AlN. On the other hand, the extension to the lower energy side was caused by the large bowing in the bandgap energy between group III nitrides and arsenides. Calculations under lattice matching to Si and GaAs are also presented.
... More details about the procedure for bulk semiconductors are reported in Refs. [28][29][30]. ...
... C 11 and C 12 are the GaAs 1Àx N x bulk material elastic constants. These parameters have been determined as a function of the composition x in our previous work [19] and found to be, when taking into account the effect of the compositional disorder. It is noted that: ...
Theoretical investigations of the conduction band offset (CBO) and valence band offset (VBO) of the relaxed and pseudo-morphically strained GaAs1−xNx/GaAs1−yNy heterointerfaces at various nitrogen concentrations (x and y) within the range 0–0.05 and along the [001] direction are performed by means of the model-solid theory combined with the empirical pseudopotential method under the virtual crystal approximation that takes into account the effects of the compositional disorder. It has been found that for yx. The band gap of the GaAs1−xNx over layer falls completely inside the band gap of the substrate GaAs1−yNy and thus the alignment is of type I (straddling) for yx, the alignment remains of type I but in this case it is the band gap of the substrate GaAs1−yNy which is fully inside the band gap of the GaAs1−xNx over layer. Besides the CBO, the VBO and the relaxed/strained band gap of two particular cases: GaAs1−xNx/GaAs and GaAs1−xNx/GaAs0.98N0.02 heterointerfaces have been determined.
The feasibility of a distributed Bragg reflector (DBR) based on wide band gap II-VI semiconducting alloys is reported. The reflectivity spectra of a Zn0.66Mg0.34Se/Zn0.74Cd0.26Se DBR are calculated within the stratified medium theory using matrix representation. The composition x for each alloy is taken in such a manner to obtain a convenient difference between the respective refractive indices of each constituent material. Both normal and oblique incidence are considered. For both cases the effect of the number of pairs has been studied. We found out that the reflectivity maximum is enhanced as the number of pairs increases. The DBR bandwidth is found to be dependent on the refractive indices difference. The reflectivity spectrum is shifted towards smaller wavelengths, and the Bragg wavelength is diminished as the angle of incidence is augmented.
Abstract
Using the empirical pseudopotential method, we have undertaken a detailed study of the electronic and optical properties of zinc-blende InAs and InSb spherical quantum dots as a function of their diameter taken as varying in the range 1–10 nm. Features such as energy levels, energy band gaps, electron and heavy hole effective masses, refractive index, dielectric constants and transverse effective charge have been investigated.
The quantum confinement is found to be of major effect on all the studied quantities for quantum dot diameter below 6 nm. Beyond 6 nm, all properties of interest tend towards retrieving the bulk properties.
Keywords
Quantum dots • Nanomaterials • Electronic structure • Optical properties • quantum confinement . InAs •InSb
Résumé
Les propriétés électroniques et optiques des quantum dots sphériques d’InAs et d’InSb dans la phase zinc-blende ont été étudiées, en fonction de leur diamètre pris dans la gamme 1–10 nm, en utilisant la méthode empirique du pseudopotentiel. Les niveaux énergétiques, les différents gaps d’énergie, les masses effectives des électrons et des trous, les constantes diélectriques, l’indice de réfraction et la charge effective transverse ont tous été explorés.
Le confinement quantique a été trouvé d’un grand effet sur toutes les grandeurs étudiées pour un diamètre des quantum dots en déçà de 6nm. Au-delà de 6 nm, toutes les propriétés examinées tendent à retrouver celles du matériau massif.
Mots clés
Quantum dots • Nanomatériaux • Structure électronique • propriétés électroniques. confinement quantique •InAs•InSb
خلاصة البحث
نتناول في هذا البحث بالدراسة الخصائص الإلكترونية و الضوئية للنقط الكمومية الكروية InAs وInSb في طور كبريتيد الزنك بدلالة قطر هذه النقط الكمومية المتغير في المجال 1–10 nm ، و ذلك باستخدام طريقة الكمون الكاذب الامبيريقية. وقد اهتممنا بتحديد سويات الطاقة و مختلف الفواصل الطاقوية و الكتل الفعالة للحوامل إضافة إلى ثابتي العزل السكوني و عالي التواتر و قرينة الانكسار و الشحنة الفعالة المستعرضة .
و قد وجدنا أن أثر الحصر الكمومي مهم جدا سيّما حينما يكون قطر النقط الكمومية أقل من 6nm. و لأجل القيم القطرية الأعلى من 6nm فإن كل الخصائص تؤول إلى نظائرها الحجمية .
كلمات مفتاحيه
النقط الكمومية . موادالنانومترية .البنية الالكترونية .الخصائص الضوئية. الحصر الكمومي . InAs . InSb
Using first-principles total energy calculations, we have studied the structural, mechanical and electronic properties of GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (with x from 0 to 1) within density functional theory (DFT) framework. To obtain the ideal band gap, we employ the semi-empirical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U). The calculated results illustrate the varying lattice constants and band gap in GaAs1-xNx alloys as functions of the nitrogen concentration x. According to the pressure dependence of the lattice constants and volume, the higher N concentration alloy exhibits the better anti-compressibility. In addition, an increasing band gap is predicted under 20 GPa pressure for GaAs1-xNx alloys.
The composition dependence of direct and indirect band gap energies, anti-symmetric gap, valence band width, refractive index and high-frequency dielectric constant has been investigated for GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (x from 0 to 1). The calculations are mainly based on the pseudopotential approach within the virtual crystal approximation. The variation of band-gaps versus nitrogen content show important bowing parameters. Trends in ionicity have been discussed in terms of the anti-symmetric gap. The alloy concentration dependence of the optical parameters of interest is found to be highly nonlinear.
The results are presented of an ab initio total energy study of the high-pressure phases of GaN x As1− x dilute alloys calculated within the full-potential linearized augmented plane wave method in the generalized gradient approximation. Three candidate structures, namely zinc-blende, rocksalt and CsCl, have been considered. In each case, the structural parameters and transition pressures of dilute alloys of interest are calculated. In agreement with earlier calculations of Stenuit and Fahy [Phys. Rev. B 76 (2007) 035201], the deviation of the lattice constants from Vegard's law for zinc-blende GaN x As1− x alloys is found to be negligible.
Most studies of the anomalous electronic properties of the GaAs1-Nx alloy have focused on near-edge states, but x-ray spectroscopic experiments [V. N. Strocov et al., Phys. Status Solidi B 233, R1 (2002)] have now revealed anomalous properties deep inside the valence and conduction bands. Indeed, whereas N p character is found in GaN in the energy region near the valence-band maximum (VBM), when ˜3% N is introduced into GaAs one finds that N p character exists about 2 3 eV below the VBM and as two narrow peaks just above the conduction-band minimum. First-principles calculations of the N p character in Ga32As31N and GaN show that the valence resonances are due the fact that the VBM of GaAs0.97N0.03 itself lies >2 eV above that of GaN. Thus, there is no need to involve an N-->As charge transfer to explain the data. This conclusion is further confirmed by our calculated valence-->conduction-band absorption spectra. We also show that the broken-symmetry core-hole calculations are necessary to explain, within the local density approximation (LDA), the energy of the N 1s
Using the empirical pseudopotential method and large atomistically relaxed supercells, we have systematically studied the evolution of the electronic structure of GaP1-xNx and GaAs1-xNx, from the dilute nitrogen impurity regime to the nascent nitride alloy. We show how substitutional nitrogen forms perturbed host states (PHS) inside the conduction band, whereas small nitrogen aggregates form localized cluster states (CS) in the band gap. By following the evolution of these states and the “perturbed host states” with increasing nitrogen composition, we propose a new model for low-nitrogen-content GaAs1-xNx and GaP1-xNx alloys: As the nitrogen composition increases, the energy of the CS is pinned while the energy of the PHS plunges down as the nitrogen composition increases. The impurity limit (PHS above CS) is characterized by strongly localized wave functions, low pressure coefficients, and sharp emission lines from the CS. The amalgamation limit (PHS overtake the CS) is characterized by a coexistence of localized states (leading to high effective mass, exciton localization, Stokes shift in emission versus absorption) overlapping delocalized PHS (leading to asymmetrically broadened states, low temperature coefficeint, delocalized E+ band at higher energies). The alloy limit (PHS well below CS) may not have been reached experimentally, but is predicted to be characterized by conventional extended states. Our theory shows that these alloy systems require a polymorphous description, permitting the coexistence of many different local environments, rather than an isomorphous model that focuses on few impurity-host motifs.
The ab initio pseudopotential method within the local-density approximation and the quasiparticle approach have been used to investigate the electronic properties of AlN and GaN in the wurtzite and zinc-blende structures. The quasiparticle band-structure energies are calculated using a model dielectric matrix for the evaluation of the electron self-energy. For this calculation, good agreement with the experimental results for the minimum band gaps in the wurtzite structure is obtained. In the zinc-blende structure we predict that AlN will be an indirect (Γ to X) wide band-gap semiconductor (4.9 eV) and that GaN will have a direct gap of 3.1 eV at Γ in good agreement with recent absorption experiments on cubic GaN (3.2–3.3 eV). A discussion of the direct versus indirect gap as well as other differences in electronic structure between the wurtzite and zinc-blende phases is presented. Other properties of quasiparticle excitations are predicted in this work and remain to be confirmed by experiment.
We present a comprehensive, up-to-date compilation of band parameters for the technologically important III{endash}V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other. {copyright} 2001 American Institute of Physics.
We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III-V semiconductors that have been investigated to date. The two main classes are: (1) ``conventional'' nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) ``dilute'' nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III-V material, e.g., GaAsN and GaInAsN). As in our more general review of III-V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature and alloy-composition dependences are also recommended wherever they are available. The ``band anticrossing'' model is employed to parameterize the fundamental band gap and conduction band properties of the dilute nitride materials.
GaNAs films grown on GaAs(001) substrates by metalorganic molecular beam epitaxy were studied by high-resolution x-ray diffraction (XRD) mapping measurements. The lattice constants of epitaxial films are usually estimated from symmetric and asymmetric XRD 2θ–θ measurements. In this study, it is pointed out that the consideration of the tilt angle between the GaAs(115) and GaNAs(115) planes caused by elastic deformation of the films is crucial to determine the lattice constants of the GaNAs films coherently grown on GaAs substrates. Mapping measurements of (115) XRD (2θ–θ ) –Δω were performed for this purpose. The band gap energy of the films was determined by Fourier transform absorption spectroscopy measurements. The band gap energy bowing measured up to the N composition of 4.5% will be discussed by comparing with other measurements and theoretical calculations.
We propose a novel material, GaInNAs, that can be formed on GaAs to drastically improve the temperature characteristics (T-0) in long-wavelength-range laser diodes. The feasibility of our proposal is demonstrated experimentally.
The composition dependence of energy gaps and refractive indices in the AlxGa1−xAs alloy system in the zincblende structure are reported within the pseudopotential framework under the virtual crystal approximation in which the effect of composition disorder is involved.The calculated energy gaps and refractive indices are in good agreement with experiments for the parent compounds, whereas for the AlxGa1−xAs it appears necessary to include the disorder effect. The variation of the refractive index as a function of the photon energy for different concentrations x shows that this later behaves differently for different Al mole fractions x and that the disorder effect on the refractive index depends strongly on the alloy composition x.
A comparative survey of known experimental and theoretical values of heavy-hole and electron effective mass in GaAs, InAs, and A1As is presented in this work. Recommended room-temperature values of these parameters are given for the above binary solutions and for their ternary compounds.
Resolved critical point structures in Schottky-barrier electroreflectance spectra of Ga 3dV-sp3 conduction band transitions in the 20-22-eV range provide a direct proof that the L6C equivalent minima lie approximately 170+/-30 meV below the X6C minima in GaAs. This ordering, opposite to that assumed and apparently supported by previous experiments, is in fact consistent with these experiments and provides natural explanations for many formerly puzzling features of GaAs.
Dilute GaAs1−xNx alloys were grown by molecular beam epitaxy on (001)-oriented GaAs substrates and misoriented 2° towards (110) with two Ga and As atoms, in the same plane terminated with single Ga and As bonds, respectively. Photoluminescence spectra analysis reveals several features we have attributed to excitons bound to isoelectronic traps. The dependence of these features on substrate misorientation, growth temperature, rapid thermal annealing (RTA) and thermal annealing is presented and studied. We have shown that these features are very sensitive to these parameters and can affect the statistical distribution of nitrogen atoms, explaining the striking difference in photoluminescence (PL) spectra.
The band structures of some zincblende III–V semiconductor alloys are calculated beyond the virtual crystal approximation (VCA) using the empirical pseudopotential method (EPM). Our results show that the calculated optical bandgap bowing parameter agree with the experiment only when going beyond the VCA. We also found that the direct bandgap (Γ→Γ) bowing factor becomes generally larger on going from alloys with small lattice mismatches to those with larger ones.
Dilute GaAs1−xNx alloys (x<0.3%) were grown by metalorganic vapor phase epitaxy to investigate their photoluminescence and photoluminescence excitation characteristics. Photoluminescence excitation spectra show clear excitonic absorption peaks at low temperatures and their peak energy drastically decreases with increasing nitrogen concentration due to the band-gap bowing in the GaAsN system. This result indicates that the band-gap bowing starts at a nitrogen concentration as low as 1018 cm−3, and its bowing parameter is −22 eV. According to this band-gap bowing, the GaAsN alloys show two photoluminescence lines whose peak energy decreases with increasing nitrogen concentration. Their dependence on the nitrogen concentration suggests that these lines correspond to excitonic and carbon-related transitions in the GaAsN alloy. © 1997 American Institute of Physics.
The design, growth by metalorganic chemical vapor deposition, and processing of an In0.07Ga0.93As0.98N0.02 solar cell, with 1.0 eV band gap, lattice matched to GaAs is described. The hole diffusion length in annealed, n-type InGaAsN is 0.6- 0.8 mu m, and solar cell internal quantum efficiencies >70% are obtained. Optical studies indicate that defects or impurities, from InGaAsN doping and nitrogen incorporation, limit solar cell performance. (C) 1999 American Institute of Physics. [S0003-6951(99)01805-7].
In this paper we review the basic theoretical aspects as well as some important experimental results of the band anticrossing effects in highly electronegativity-mismatched semiconductor alloys, such as GaAs1−xNx and InyGa1−yAs1−xNx. The many-impurity Anderson model treated in the coherent potential approximation is applied to these semiconductor alloys, in which metallic anion atoms are partially substituted by a highly electronegative element at low concentrations. Analytical solutions of the Green's function provide dispersion relations and state broadenings for the restructured conduction bands. The solutions also lead to the physically intuitive and widely used two-level band anticrossing model. Significant experimental observations, including large bandgap reduction, great electron effective mass enhancement and unusual pressure behaviour of the bandgap, are compared with the predictions of the band anticrossing model. The band anticrossing model is extended over the entire Brillouin zone to explain the pressure behaviour of the lowest conduction band minimum in GaP1−xNx. Finally, we show that the band anticrossing can also account for the large bandgap bowing parameters observed in GaAsxSb1−x, InAsySb1−y and GaPxSb1−x alloys.
A simple pseudopotential scheme, which incorporates compositional disorder as an effective potential, is proposed for calculation of the band structure of ternary compound semiconductors. It is shown that the present theory, which is free from any additional parameter, satisfactorily produces the band-gap bowings of ternary compounds when the lattice mismatch is small.
Band-structure calculations are performed for cubic Al1−xGaxN using the empirical pseudopotential method. The band gaps at Γ, X and L points and the electron effective masses of Γ and X valleys are calculated as a function of the gallium fraction x. It is found that there is no significant change in these electronic band parameters on taking into account the alloy disorder. On the basis of a model solid theory, we have calculated the band discontinuities for heterointerfaces between strained Al1 −xGaxN and relaxed Al1−yGayN. The latter calculations are extended to the whole range of compositions x and y. The information derived from this investigation will be useful in the design of lattice-mismatched heterostructures in blue-light optoelectronics applications.
We have performed spectroscopic measurements in order to investigate the exciton localization mechanism and the bandgap energies of GaAsN in three regimes: (i) doped; (ii) intermediate doped-alloy; and (iii) alloy and the transition energies in strained GaInAsN/GaAs quantum wells laser structures.Low temperature photoluminescence spectrum of GaAsN layer in doped regime shows several features of excitons bound to nitrogen complexes. In the intermediate doped-alloy regime, these bound states are tightly coupled to form a wide band below the GaAsN bandgap energy. We have used the band anticrossing model to simulate the evolution of the GaAsN bandgap energies versus nitrogen composition. We have found that incorporation of 1% nitrogen shifts the bandgap energy of about 225 meV. The interband transitions in GaInAsN/GaAs quantum wells (QWs) structures are investigated using photovoltage measurements and can be identified using the envelope function formalism taking into account the effects of strain and the bandgap lowering due to the presence of nitrogen.
The empirical pseudopotential method within the virtual crystal approximation for the entire range of alloy concentrations of cubic GaxIn1−xAs is presented. The atomic form factors have been deduced empirically by fitting the band structure of parent compounds to experimental data available from the literature. To make allowance for the compositional disorder, a correction to the alloy potential has been introduced. Illustrative results of calculated electronic and lattice properties indicate that the contribution of the compositional disorder plays an important role and must be included to obtain a meaningful agreement with the experiment.
We demonstrate working prototypes of a GaInNAs-based solar cell lattice-matched to GaAs with photoresponse down to 1 eV. This device is intended for use as the third junction of future-generation ultrahigh-efficiency three- and four-junction devices. Under the AM1.5 direct spectrum with all the light higher in energy than the GaAs band gap filtered out, the prototypes have open-circuit voltages ranging from 0.35 to 0.44 V, short-circuit currents of 1.8 mA/cm2, and fill factors from 61% to 66%. The short-circuit currents are of principal concern: the internal quantum efficiencies rise only to about 0.2. We discuss the short diffusion lengths which are the reason for this low photocurrent. As a partial workaround for the poor diffusion lengths, we demonstrate a depletion-width-enhanced variation of one of the prototype devices that trades off decreased voltage for increased photocurrent, with a short-circuit current of 6.5 mA/cm2 and an open-circuit voltage of 0.29 V.
The group III nitrides (AlN, GaN and InN) represent an important trio
of semiconductors because of their direct band gaps which span the
range 1.95-6.2 eV, including the whole of the visible region and
extending well out into the ultraviolet (UV) range. They form a
complete series of ternary alloys which, in principle, makes available
any band gap within this range and the fact that they also generate
efficient luminescence has been the main driving force for their
recent technological development. High brightness visible
light-emitting diodes (LEDs) are now commercially available, a
development which has transformed the market for LED-based full colour
displays and which has opened the way to many other applications, such
as in traffic lights and efficient low voltage, flat panel white light
sources. Continuously operating UV laser diodes have also been
demonstrated in the laboratory, exciting tremendous interest for
high-density optical storage systems, UV lithography and projection
displays. In a remarkably short space of time, the nitrides have
therefore caught up with and, in some ways, surpassed the wide band
gap II-VI compounds (ZnCdSSe) as materials for short wavelength
optoelectronic devices. The purpose of this paper is to review these
developments and to provide essential background material in the form
of the structural, electronic and optical properties of the nitrides,
relevant to these applications. We have been guided by the fact that
the devices so far available are based on the binary compound GaN
(which is relatively well developed at the present time), together
with the ternary alloys AlGaN and InGaN, containing modest amounts of
Al or In. We therefore concentrate, to a considerable extent, on the
properties of GaN, then introduce those of the alloys as appropriate,
emphasizing their use in the formation of the heterostructures
employed in devices. The nitrides crystallize preferentially in the
hexagonal wurtzite structure and devices have so far been based on
this material so the majority of our paper is concerned with it,
however, the cubic, zinc blende form is known for all three compounds,
and cubic GaN has been the subject of sufficient work to merit a brief
account in its own right. There is significant interest based on
possible technological advantages, such as easier doping, easier
cleaving (for laser facets) and easier contacting. It also appears, at
present, that the cubic form gives higher electron and hole mobilities
than the hexagonal form. The dominant hexagonal structure is similar
to that found in a number of II-VI compounds such as CdS and they can
therefore be taken as role models. In particular, the lower symmetry
gives rise to three separate valence bands at the zone centre and
exciton spectra associated with each of these have been reported by
many workers for GaN. Interpretation is complicated by the presence of
strain in many samples due to the fact that most material consists of
epitaxial thin films grown on non-lattice-matched substrates (bulk GaN
crystals not being widely available). However, much progress has been
made in understanding the physics of these films and we discuss the
current position with regard to band gaps, effective masses, exciton
binding energies, phonon energies, dielectric constants, etc. Apart
from a lack of knowledge of the anticipated valence band anisotropy,
it can be said that GaN is now rather well documented. Less detail is
available for AlN or InN and we make no attempt to provide similar
data for them. The structure of the paper is based on a historical
introduction, followed by a brief account of the various crystal
growth methods used to produce bulk GaN and epitaxial films of GaN and
the ternary alloys. This is then followed by an account of the
structural properties of hexagonal GaN as measured by x-ray
diffraction and electron microscopy, phonon properties from infrared
and Raman spectroscopy, electrical properties, with emphasis on n- and
p-type doping, and optical properties, measured mainly by
photoluminescence. A brief comparative account of cubic GaN properties
follows. Discussion of alloy properties in the context of their use in
quantum well and superlattice structures forms an introduction to the
device sections which close the paper. These include details of the
technology necessary for etching, contacting and forming laser facets,
as an introduction to recent results on LEDs and laser diodes. Having
described the current position, we speculate briefly on likely future
developments.
During the last few years the developments in the field of III-nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calculated spectra. Strain and its effect on the optical properties of the III-nitride layers have been studied both theoretically and experimentally. Both n and p conductivity have been achieved. InGaN quantum wells with GaN and AlGaN barriers and cladding layers have been investigated. PL of the quantum wells is affected by confinement effects, band filling, quantum confined Stark effect, and strain. This work has led to the fabrication of advanced optoelectronic and electronic devices. The light-emitting decodes emitting in the blue and green regions of the spectrum have been commercialized. The work leading to these developments is reviewed in this article. The device processing methods and actual devices are not discussed.
Self-consistent linear muffin-tin-orbital band-structure calculations are used to investigate the optical and structural properties of III-V semiconducting nitrides under hydrostatic pressure. The pressure behavior of the energy band structures is discussed in the context of the postulated chemical trends in III-V semiconductors. The regions in k space of dominant interband contributions to the elements of structure in the dielectric functions are identified. The total-energy calculations suggest that all the nitrides under pressure transform to the semiconducting rocksalt phase. The calculated transition pressures are 21.6 GPA (InN), 51.8 GPa (GaN), 16.6 GPa (AlN), and 850 GPa (BN). Experimental values that agree well with this have been found for the first three compounds. The fact that GaN and AlN have such different transition pressures in spite of their very similar ionicities is explained by the presence of 3d states on Ga.
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