Comparative study on the broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers

Linewidth broadening of exciton luminescence in wurtzite and zinc-blende GaN epilayers was investigated as a function of temperature with photoluminescence. A widely accepted theoretical model was used to fit the experimental data, so that the coupling parameters between exciton and acoustic and longitudinal optical phonons were obtained for both structures. It was found that the coupling constants of both exciton-acoustic optical phonon coupling and exciton-longitudinal optical phonon coupling for zinc-blende GaN are almost twice as much as the corresponding values of wurtzite GaN. These results show that the relatively strong exciton-phonon scattering seems to be characteristic to zinc-blende GaN film. (C) 2002 American Institute of Physics.

Statistical investigation on morphology development of gallium nitride in initial growth stage

Morphology of Gallium Nitride (GaN) in initial growth stage was observed with atomic force microscopy (AFM) and scanning electron microscopy (SEM), It was found that the epilayer developed from islands to coalesced film. Statistics based on AFM observation was carried out to investigate the morphology characteristics. It was found that the evolution of height distribution could be used to describe morphology development. Statistics also clearly revealed variation of top-face growth rate among islands. Indium-doping effect on morphology development was also statistically studied. The roughening and smoothing behavior in morphology development was explained. (C) 2002 Elsevier Science B.V. All rights reserved.

High-quality GaN grown by gas-source MBE

High-quality GaN epilayers were consistently obtained using a home-made gas-sourer MBE system on sapphire substrates. Room-temperature electron mobility of the grown GaN film is 300 cm(2)/V s with a background electron concentration as low as 2 x 10(17) cm(-3) The full-width at half-maximum of the GaN (0 0 0 2) double-crystal X-ray rocking curve is 6 arcmin. At low temperature (3.5 K), the FWHM of the: near-band-edge photoluminescence emission line is 10 meV. Furthermore, using piezoelectric effect alone with the high-quality films, two-dimensional electron gas was formed in a GaN/AlN/GaN/sapphire structure. Its room-temperature and low-temperature (77 K) electron mobility is 680 cm(2)/V s and 1700 cm(2)/V s, and the corresponding sheet electron density is 3.2 x 10(13) and 2.6 x 10(13) cm(-2), respectively. (C) 2001 Published by Elsevier Science.

Epitaxial growth of SiC on complex substrates

Epitaxial growth of SiC on complex substrates was carried out at substrate temperature from 1200 degreesC to 1400 degreesC. Three kinds of new complex substrates, c-plane sapphire, AlN/sapphire, and GaN/AlN/sapphire, were used in this study. We obtained a growth rate in the range of 1-6 mum/h. Thick (6 mum) SIC epitaxial layers with no cracks were successfully obtained on AlN/sapphire and GaN/AlN/sapphire substrates. X-ray diffraction patterns have confirmed that single-crystal SiC was obtained on these complex substrates. Analysis of optical transmission spectra of the SIC grown on sapphire substrates shows the lowest-energy gap near 2.2 eV, which is the value for cubic SiC. The undoped SIC showed n-type electrical conductivity. (C) 2001 Elsevier Science B.V. All rights reserved.

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Energy bands and acceptor binding energies of GaN

The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are-given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these accepters is deviated from that given by the effective mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of accepters. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. [S0163-1829(99)15915-0].

The growth and characterization of GaN grown on an Al2O3 coated (001)Si substrate by metalorganic vapor phase epitaxy

Single crystal GaN films have been grown on to an Al2O3 coated (001)Si substrate in a horizontal-type low-pressure MOVPE system. A thin Al2O3 layer is an intermediate layer for the growth of single crystal GaN on to Si although it is only an oriented polycrystal him as shown by reflection high electron diffraction. Moreover, the oxide was not yet converted to a fully single crystal film, even at the stage of high temperature for the GaN overlayer as studied by transmission electron microscopy. Double crystal X-ray diffraction showed that the linewidth of (0002) peak of the X-ray rocking curve of the 1.3 mu m sample was 54 arcmin and the films had heavy mosaic structures. A near band edge peaking at 3.4 eV at room temperature was observed by photoluminescence spectroscopy. (C) 1998 Elsevier Science B.V. All rights reserved.

Wurtzite GaN epitaxial growth on a Si(001) substrate using gamma-Al2O3 as an intermediate layer

Wurtzite GaN films have been grown on (001) Si substrates using gamma-Al2O3 as an intermediate layer by low pressure (similar to 76 Torr) metalorganic chemical vapor deposition. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements revealed that the thin gamma-Al2O3 layer of "compliant" character was an effective intermediate layer for the GaN film grown epitaxially on Si. The narrowest linewidth of the x-ray rocking curve for (0002) diffraction of the 1.3 mu m GaN sample was 54 arcmin. The orientation relationship of GaN/gamma-Al2O3/Si was (0001) GaN parallel to(001) gamma-Al2O3 parallel to(001) Si, [11-20] GaN parallel to[110] gamma-Al2O3 parallel to[110] Si. The photoluminescence measurement for GaN at room temperature exhibited a near band-edge peak of 365 nm (3.4 eV). (C) 1998 American Institute of Physics.

Hydrogen sensors based on Pt-AlGN/AIN/GaN Schottky diode - art. no. 68291R

Pt/AlGaN/AlN/GaN Schottky diodes have been fabricated and characterized for H-2 sensing. Platinum (Pt) with a thickness of 20nm was evaporated on the sample to form the Schottky contact. The ohmic contact, formed by evaporated Ti/Al/Ni/Au metals, was subsequently annealed by a rapid thermal treatment at 860 degrees C for 30 s in N-2 ambience. Both the forward and reverse current of the device increased greatly when exposed to H-2 gas. The sensor's responses under different hydrogen concentrations from 500ppm to 10% H-2 in N-2 at 300K were investigated. A shift of 0.45V at 297K is obtained at a fixed forward current for switching from N-2 to 10% H-2 in N-2. Time response of the sensor at a fixed bias of 0.5 V was also measured. The turn-on response of the device was rapid, while the recovery of the sensor at N-2 atmosphere was rather slow. But it recovered quickly when the device was exposed to the air. The decrease in the barrier height of the diode was calculated to be about 160meV upon introduction of 10% H-2 into the ambient. The sensitivity of the sensor is also calculated. Some thermodynamics analyses have been done according to the Langmuir isotherm equation.

High responsivity ultraviolet photodetector based on crack-free GaN on Si (111)

A Schottky-based metal-semiconductor-metal photodetector is fabricated on 1 mu m-thick, crack-free GaN on Si (I 11) substrate using an optimized AlxGal-xN/AlN complex buffer layer. It exhibits a high responsivity of 4600A/W at 366nm which may be due to both a crack-free sample and high internal gain. The relationship between responsivity and bias voltage is also investigated. The experiment results indicate that the responsivity increases with the bias voltage and shows a tendency to saturate. (c) 2007 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim.

Room temperature mobility above 2100 cm2/Vs in Al0.3Ga0.7N/AIN/GaN heterostructures grown on sapphire substrates by MOCVD

Al0.3Ga0.7N/AlN/GaN HEMT structures with significantly high mobility have been grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. At room temperature (RT) a Hall mobility of 2104 cm(2)/Vs and a two-dimensional electron gas (2DEG) density of 1.1x10(13) cm(-2) are achieved, corresponding to a sheet resistance of 277.8 Omega/sq. The elimination of V-shaped defects were observed on Al0.3Ga0.7N/AlN/GaN HEMT structures and correlated with the increase of 2DEG mobility. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

Molecular beam epitaxial growth of GaN on 3c-SiC/Si(111) substrates using a thick AIN buffer layer

Hexagonal GaN films (similar to 3 mu m) were grown on 3c-SiC/Si(111) and carbonized Si(111) substrates using a thick AlN buffer Cracks are observed on the surface of the GaN film grown on the carbonized Si(111), while no cracks are visible on the 3c-SiC/Si(111). XRD exhibits polycrystalline nature of the GaN film grown on the carbonized Si(111) due to poorer crystalline quality of this substrate. Raman spectra reveal that all GaN layers are under tensile stress, and the GaN layer grown on 3c-SiC/Si(111) shows a very low stress value of sigma(xx) = 0.65 Gpa. In low-temperature Photoluminescence spectra the remarkable donor-acceptor-pair recombination and yellow band can be attributed to the incorporation of Si impurities from the decomposition of SiC.

The difference of Si doping efficiency in GaN and AlGaN in GaN-based HBT structure

An AlGaN/GaN HBT structure was grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on sapphire substrate. From the high-resolution x-ray diffraction and transmission electron microscopy (TEM) measurements, it was indicated that the structure is of good quality and the AlGaN/GaN interfaces are abrupt and smooth. In order to obtain the values of Si doping and electronic concentrations in the AlGaN emitter and GaN emitter cap layers, Secondary Ion Mass Spectroscopy (SIMS) and electrochemical CV measurements were carried out. The results showed that though the flow rate of silane (SiH4) in growing the AlGaN emitter was about a quarter of that in growing GaN emitter cap and subcollector layer, the Si sputtering yield in GaN cap layer was much smaller than that in the AlGaN emitter layer. The electronic concentration in GaN was about half of that in the AlGaN emitter layer. It is proposed that the Si, Al co-doping in growing the AlGaN emitter layer greatly enhances the Si dopant efficiency in the AlGaN alloy. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

Influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy

The influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy (XPS) is discussed, and a modification method based on a modified self-consistent calculation is proposed to eliminate the influence and thus increasing the precision of XPS. Considering the spontaneous polarization at the surfaces and interfaces and the different positions of Fermi levels at the surfaces, we compare the energy band structures of Al/Ga-polar AlN/GaN and N-polar GaN/AlN heterojunctions, and give corrections to the XPS-measured valence band offsets. Other AlN/GaN heterojunctions and the piezoelectric polarization are also introduced and discussed in this paper.

Measurement of w-InN/h-BN Heterojunction Band Offsets by X-Ray Photoemission Spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) of the w-InN/h-BN heterojunction. We find that it is a type-II heterojunction with the VBO being -0.30 +/- A 0.09 eV and the corresponding conduction band offset (CBO) being 4.99 +/- A 0.09 eV. The accurate determination of VBO and CBO is important for designing the w-InN/h-BN-based electronic devices.