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B. Liu,
Z. Zhang,
R. Zhang,
D. Y. Fu,
Z. L. Xie,
H. Lu,
W. J. Schaff,
L. H. Song,
Y. C. Cui,
X. M. Hua,
P. Han,
Y. D. Zheng,
Y. H. Chen,
Z. G. Wang
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ABSTRACT: InN films with electron concentration ranging from n∼1017 to 1020cm−3 grown by metal–organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature
photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of
these InN samples with electron concentration above 1018cm−3 show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions,
the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low
temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature.
The exciton localization energy σ
loc is found to follow the n
5/12 power relation, which testifies to the observed strong localization effects in InN with high electron concentrations.
Applied Physics A 05/2012; 99(1):139-143. · 1.63 Impact Factor
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ABSTRACT: We report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/α-Al2O3(0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al0.4Ga0.6N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.
Applied Physics Letters 06/2011; 98(26):261916-261916-3. · 3.84 Impact Factor
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ABSTRACT: The influences of biaxial and uniaxial strain on the ultraviolet emission efficiencies of both c- and m-plane AlxGa1−xN films with different Al concentrations are investigated under the framework of k⋅p perturbation theory. The optimal high efficiency windows, for ultraviolet light emissions are quantitatively estimated. c-plane AlxGa1−xN modified by uniaxial strain, shows more advantages over biaxial-strained AlxGa1−xN. This is due to the relatively more flexible tuning range and the advantage of obtaining pure linear polarization, which can be utilized to design polarized emission devices. For m-plane AlxGa1−xN, there are always in-plane polarized emissions under both biaxial and uniaxial strain conditions, thus, it is more likely to obtain high surface emission efficiency.
Journal of Applied Physics 11/2010; 108(10):103107-103107-5. · 2.17 Impact Factor
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ABSTRACT: In this paper, considerable magnitude of spin splitting for the conduction subband at the Fermi energy is obtained in AlGaN quantum wells (QWs) grown along the c-axis. We have analyzed how the magnitude of spin splitting of the first electron subband in AlGaN QWs with different sheet carrier concentration changes as a function of applied gate voltage, well width, and Al content in the barrier. It is also found that the contribution to spin splitting from Dresselhaus term is much larger than that from Rashba term, the contribution of Dresselhaus term to the total spin splitting depends greatly on the carrier concentrations, the change of well width has little effect on total spin splitting, and the magnitude of spin splitting can be greatly modulated by Al content in the barrier, gate voltage, and sheet carrier concentration. The internal polarized electric field is crucial for considerable spin splitting in III-nitride QWs. Moreover, the magnitude of total spin splitting calculated here is comparable with other theoretical and experimental values observed in III-nitride heterostructures.
physica status solidi (b) 06/2010; 248(1):187 - 192. · 1.32 Impact Factor
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B. Liu,
J. Y. Kong,
R. Zhang,
Z. L. Xie,
D. Y. Fu,
X. Q. Xiu,
P. Chen,
H. Lu,
P. Han,
Y. D. Zheng,
S. M. Zhou
[show abstract]
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ABSTRACT: We investigated the polarization and temperature dependence of photoluminescence (PL) of m-plane GaN grown on γ-LiAlO2 (100) substrate. The calculated electronic band structure with k•p Hamiltonian points out the energy splitting as well as polarization selection originate from the m-plane GaN epilayer under anisotropic strain. The polarization-angle dependence PL spectra are found to be selected from in-plane x- and z-polarized emission, corresponding to T1 and T2 transition. And the intensity distribution of the fitting peaks satisfies the Malus’ law. An S-shape energy evolution of near band edge peak on temperatures is observed, which originates from the transition between the localized holes and electrons in triangular potentials induced by basal stacking faults.
Applied Physics Letters 08/2009; 95(6):061905-061905-3. · 3.84 Impact Factor
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ABSTRACT: The effects of anisotropic strain on wurtzite GaN valence subbands are investigated both theoretically and experimentally. k•p perturbation theory reveals that the in-plane asymmetric strain not only affects the transition energies, but also determines the polarization properties, which is analyzed to be the essential cause of the optical anisotropy. Considerable in-plane anisotropy of strained C -plane GaN in polarized photoluminescence is reported. The experimental result in good agreement with theoretical study directly proves the strain effects on the transitions polarization states. The fine accordance of observed and simulated photoluminescence dependences on strain asymmetry degree demonstrates a primary realization of strain controlled optical anisotropy, and such modulation indicates the great potential of utilizing GaN-based semiconductors in polarization-sensitive optoelectronics.
Applied Physics Letters 08/2009; · 3.84 Impact Factor
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ABSTRACT: A pronounced hump structure at about −5 V in the high-frequency capacitance-voltage (C-V) curve of an undoped InGaN/GaN heterostructure is observed and this hump weakens gradually with decreasing measurement frequency, indicating the occurrence of an inversion behavior in the InGaN/GaN heterostructure. The inversion behavior in the C-V curve is attributed to hole accumulation at the heterointerface where a hole well is formed due to the strong piezoelectric polarization effect in the InGaN/GaN heterostructure. The acceptor traps related to Ga vacancies in the InGaN layer are thought to be the source of the minority carriers. The theoretical calculation of band diagram of the InGaN/GaN heterostructure confirms the formation of the hole well at the heterointerface and supports the behavior of hole accumulation under negative bias voltage.
Applied Physics Letters 07/2009; 95(1):012112-012112-3. · 3.84 Impact Factor
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ABSTRACT: The performances of InGaN Schottky photodetectors with varied fabrication processes were investigated. The photoresponse and dark current of InGaN Schottky photodetectors can be obviously improved by inserting a thin Si<sub>3</sub>N<sub>4</sub> passivation layer between the InGaN layer and the Schottky metal. Furthermore, a mesa process gives not only a further increase in the photoresponse but also a pronounced reduction in the reverse leakage current of about two orders of magnitude. A lateral surface leakage current mechanism associated with the 2-D variable-range hopping conduction through high-density surface states in InGaN is proposed to explain the reduction of the reverse leakage current after etching the mesa.
IEEE Electron Device Letters 07/2009; · 2.85 Impact Factor
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ABSTRACT: The k ∙ p perturbation theory is adopted to calculate the strain-modulated excitonic transition energies and their polarization properties in c - and m -plane AlN. The two topmost valence subbands exchange their band characteristics at the degenerate point where ε<sub>zz</sub>=0.98% and ε<sub>xx</sub>=ε<sub>yy</sub>=-1.70% . The surface emission efficiency of c -plane AlN films can be dramatically enhanced with ε<sub>zz</sub>≫0.98% (ε<sub>xx</sub>=ε<sub>yy</sub>≪-1.70%) , where the lowest excitonic transition is predominantly z -polarized. Besides, nonpolar plane ( m - or a -plane) AlN experiencing anisotropic in-plane strain can be chosen as a candidate for enhancing the surface emission efficiency by proper strain manipulation.
Applied Physics Letters 06/2009; · 3.84 Impact Factor
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ABSTRACT: Large-area Au / Pt /n- In <sub>0.2</sub> Ga <sub>0.8</sub> N Schottky contacts have been fabricated for photovoltaic devices. The current transport mechanisms of the Schottky contacts to n- In <sub>0.2</sub> Ga <sub>0.8</sub> N with different background carrier concentrations are investigated. The thermionic emission is a dominating current transport mechanism at the Pt /n- In Ga N interface in a low background carrier concentration sample, while the defect-assisted tunneling current and trap-related recombination current play important roles in high background carrier concentration samples. The Schottky diode fabricated using the low background carrier concentration sample gives much better Schottky barrier characteristics and exhibits a three to four order of magnitude higher spectral responsivity and a larger rejection ratio in comparison with those fabricated using the high background carrier concentration samples.
Journal of Applied Physics 04/2009; · 2.17 Impact Factor
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B. Liu,
R. Zhang,
Z. L. Xie,
J. Y. Kong,
J. Yao,
Q. J. Liu,
Z. Zhang,
D. Y. Fu,
X. Q. Xiu,
P. Chen,
P. Han,
Y. Shi,
Y. D. Zheng,
S. M. Zhou,
G. Edwards
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ABSTRACT: The m-plane GaN films grown on LiAlO2(100) by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson–Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN[0001] in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress εxx = −0.79% and εzz = −0.14% with an out-of-plane dilatation εyy = 0.38%. This anisotropic strain further separates the energy levels of top valence band at Γ point. The energy splitting as 37 meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature.
Applied Physics Letters 06/2008; 92(26):261906-261906-3. · 3.84 Impact Factor
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ABSTRACT: A thirty-pair AlGaN/AlN distributed Bragg reflector (DBR) targeted at a center wavelength of 320 nm was grown on a 2-inch sapphire substrate by metalorganic chemical vapor deposition (MOCVD). It is free of cracks in the main area of the wafer except for a ∼4 mm periphery observed under optical microscope. The measured reflectance spectrum shows a peak reflectivity of 93% at 313 nm and a bandwidth of 13 nm. Further investigation into the interfaces was performed using Auger electron spectroscopy (AES), scanning transmission electron microscope (STEM) and energy dispersive X-ray fluorescence spectrometer (EDX). It was found that the constitutional change is abrupt when AlGaN is deposited on an AlN layer but gradual in reverse, which resulted in a stack of quasi-three-layer periods and high Al composition and therefore both the center reflectivity and the bandwidth were depressed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 05/2008; 205(7):1572 - 1574. · 1.46 Impact Factor
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B. Liu,
R. Zhang,
Z. L. Xie,
H. Lu,
Q. J. Liu,
Z. Zhang,
Y. Li,
X. Q. Xiu,
P. Chen,
P. Han,
S. L. Gu,
Y. Shi,
Y. D. Zheng,
W. J. Schaff
[show abstract]
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ABSTRACT: This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.
Journal of Applied Physics 01/2008; 103(2):023504-023504-4. · 2.17 Impact Factor
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B. Liu,
R. Zhang,
Z. L. Xie,
C. X. Liu,
J. Y. Kong,
J. Yao,
Q. J. Liu,
Z. Zhang,
D. Y. Fu,
X. Q. Xiu,
H. Lu,
P. Chen,
P. Han,
S. L. Gu,
Y. Shi,
Y. D. Zheng,
J. Zhou,
S. M. Zhou
[show abstract]
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ABSTRACT: The nonpolar m -plane (1 1 00) thin film GaN and In Ga N / Ga N light-emitting diodes (LEDs) grown by metal-organic chemical vapor deposition on Li Al O <sub>2</sub> (100) substrates are reported. The LEDs emit green light with output power of 80 μ W under a direct current of 20 mA for a 400×400 μ m <sup>2</sup> device. The current versus voltage (I-V) characteristic of the diode shows soft rectifying properties caused by defects and impurities in the p-n junction. The electroluminescence peak wavelength dependence on injection current, for currents in excess of 20 mA , saturates at 515–516 nm . This proves the absence of polarization fields in the active region present in c -plane structures. The light output intensity versus current (L-I) characteristic of the diode exhibits a superlinear relation at low injection current caused by nonradiative centers providing a shunt path and a linear light emission zone at high current level when these centers are saturated.
Applied Physics Letters 01/2008; · 3.84 Impact Factor
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ABSTRACT: The technique of indium droplets catalyst is used for the synthesis of InN nanodots on (0001) sapphire substrates via metalorganic chemical vapor deposition (MOCVD) technique using trimethyindium (TMIn: (CH<sub>3</sub>)<sub>3</sub>In) and NH<sub>3</sub>, The morphology evolution of the InN nanodots are studied by AFM. The structure of the prepared sample is characterized by X-ray diffraction (XRD) and Raman scattering. And the XPS has been used to investigate the elemental composition of InN nanodots. Our results demonstrate that In droplets catalyst MOCVD can be a very effective way to grow InN nanodots.
Semiconducting and Insulating Materials, 2004. SIMC-XIII-2004. 13th International Conference on; 10/2004
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B. Liu,
R. Zhang,
Z.X. Bi,
Z.L. Xie,
X.Q. Xiu,
S.W. Fu,
Y.D. Ye,
S.L. Gu,
B. Shen,
Y. Shi,
Y.D. Zheng
[show abstract]
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ABSTRACT: The oxidation of InN and In-rich InN films grown on (0001) sapphires by low-pressure MOCVD has been investigated. X-ray diffraction (XRD) measurements show that stoichiometric InN is difficult to be oxidized at the temperature lower than 400°C. However, nonstoichiometric In-rich InN can be oxidized at low temperature even as low as 300°C. And In-rich InN films are fully oxidized at 450°C. According to the scanning electron microscope (SEM) images, the oxidization of metal In grains on the surface of In-rich InN film is the main process at the temperature lower than 400°C. Above 400°C, metal In and InN begin to be oxidized simultaneously, which also indicates that stoichiometric InN is difficult to be oxidized at the temperature less than 400°C.
Semiconducting and Insulating Materials, 2004. SIMC-XIII-2004. 13th International Conference on; 10/2004
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Z. Zhang,
R. Zhang,
Z.L. Xie, B. Liu,
M. Li,
D.Y. Fu,
H.N. Fang,
X.Q. Xiu,
H. Lu,
Y.D. Zheng,
Y.H. Chen,
C.G. Tang,
Z.G. Wang
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ABSTRACT: A sizable spin-dependent photocurrent related to the interband transition in InN films is observed. The surface charge accumulation layer is suggested to be the origin of the circular photogalvanic current, which is consistent with the result of uniaxial strain experiments and the comparison of front and back incidence. The homogeneous photocurrent demonstrates the existence of spin splitting in the InN surface layer, and the structure inversion asymmetry (SIA)-dominant mechanism indicates a great possibility for the manipulation of spin splitting, which would undoubtedly benefit further research and applications of spintronics.
Solid State Communications.
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B. Liu,
R. Zhang,
Z.L. Xie,
Q.J. Liu,
Z. Zhang,
Y. Li,
X.Q. Xiu,
J. Yao,
Q. Mei,
H. Zhao,
P. Han,
H. Lu,
P. Chen,
S.L. Gu,
Y. Shi,
Y.D. Zheng,
W.Y. Cheung,
N. Ke,
J.B. Xu
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ABSTRACT: The alloy AlxGa1−xN was grown by metal-organic chemical vapor deposition (MOCVD) using a high-temperature AlN interlayer on a thick GaN template. The Al composition (x) of the AlxGa1−xN was varied in the range 0.13⩽x⩽0.8. The in-situ reflectance spectra indicate that the growth process of AlGaN alloys is dominated by trimethylgallium (TMGa) molar flux when the molar flux of trimethylaluminium (TMAl) is kept constant. The Al compositions and growth rates of AlGaN alloys were determined by Rutherford backscattering, which indicates that the incorporation efficiency of TMAl is improved remarkably by decreasing the TMGa molar flux. The crystalline quality of these AlGaN alloys is evaluated by measuring the symmetric (0 0 2) and asymmetric (1 0 2) ω-scan X-ray diffraction peak widths. The best crystalline quality, among these AlxGa1−xN alloys, is for an Al composition of x=0.54 where the full-width at half-maximums of the AlGaN (0 0 2) and (1 0 2) diffraction peaks are 265 and 797 arcsec, respectively. This conclusion is consistent with the surface morphology of the AlGaN alloys probed by atomic force microscopy. Room temperature cathodoluminescence spectra show pronounced near band edge emission from these AlGaN alloys. The optical band gaps (Eg) are found to deviate from linear interpolation between EgGaN and EgAlN with a bowing parameter b=0.89.
Journal of Crystal Growth.
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Z. Zhang,
R. Zhang, B. Liu,
Z.L. Xie,
X.Q. Xiu,
P. Han,
H. Lu,
Y.D. Zheng,
Y.H. Chen,
C.G. Tang,
Z.G. Wang
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ABSTRACT: The circular photogalvanic effect (CPGE) is observed in InN at inter-band excitation. The function of the CPGE induced current on laser helicity is experimentally demonstrated and illustrated with the microscopic model. A spin-dependent current obtained in InN is one order larger than in the AlGaN/GaN heterostructures at inter-band excitation. The dependence of CPGE current amplitude on light power and incident angle can be well evaluated with phenomenological theory. This sizeable spin-dependent current not only provides an opportunity to realize spin polarized current at room temperature, but also can be utilized as a reliable tool of spin splitting investigation in semiconductors.
Solid State Communications.
