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].

Cubic InGaN grown by MOCVD

We report on the growth of high-quality cubic phase InGaN on GaAs by MOCVD. The cubic InGaN layers are grown on cubic GaN buffer layers on GaAs (001) substrates. The surface morphology of the films are mirror-like. The cubic nature of the InGaN films is obtained by Xray diffraction (XRD) measurements. The InGaN layers show strong photoluminescence (PL) at room temperature. Neither emission peak from wurtzite GaN nor yellow luminescence is observed in our films. The highest In content as determined by XRD is about 17% with an PL emission wavelength of 450 nm. The FWHM of the cubic InGaN PL peak are 153 meV and 216 meV for 427 nm and 450 nm emissions, respectively. It is found that the In compositions determined from XRD are not in agreement with those estimated from PL measurements. The reasons for this disagreement are discussed.

p-type co-doping study of GaN by photoluminescence

Photoluminescence measurements were performed on p-type co-doping effects of C, As, and Mg in GaN. The dopants were incorporated into GaN by ion implantation performed at 77 K. We find that the 3.42 eV luminescence line is sensitive to hole concentration, and propose that after cartful calibration the 3.42 eV line may be used as a probe to measure hole concentration in GaN. Simply doping one kind of accepters will not result in holes, while co-doping can substantially improve p-type doping efficiency. As + C and As + Mg co-doping induce an acceptor level of 180 meV above the valence band. Mg + C co-doping is the most promising method for p-type doping, the related acceptor level is determined to be as shallow as 130 meV. The improvement of the doping efficiency by co-doping is probably due to the decrease of the acceptor ionization energy. (C) 1999 Elsevier Science B.V. All rights reserved.

The growth of an AlGaN/GaN modulation-doped heterostructure by NH3 source molecular beam epitaxy

Using NH3 cracked on the growing surface as the nitrogen precursor, an AlGaN/GaN modulation-doped (MD) heterostructure without a buffer layer was grown on a nitridated sapphire substrate in a home-made molecular beam epitaxy (MBE) system. Though the Al composition is as low as 0.036, as deduced from photoluminescence (PL) measurements, the AlGaN barrier layer can be an efficient carrier supplier for the formation of a two-dimensional electron gas (2DEG) at the heterointerface. The 2DEG characteristics are verified by the variable temperature Hall measurements down to 7 K. Using a parallel conduction model, we estimate the actual mobility of the 2DEG to be 1100 cm(2)/V s as the sheet carrier density to be 1.0 x 10(12) cm(-2). Our results show that the AlGaN/GaN system is very suitable for the fabrication of high electron mobility transistors (HEMTs). (C) 1998 Elsevier Science B.V. All rights reserved.

High-concentration hydrogen in unintentionally doped GaN

We use nuclear reaction analysis to study hydrogen in unintentionally doped GaN, and high-concentration hydrogen, nearly 10(21) cm(-3), is detected. Accordingly, a broad but intense infrared absorption zone with a peak at 2962 cm(-1) is reported, which is tentatively assigned to the stretch mode of NH: Ga complex. The complex is assumed to be one candidate answering for background electrons in unintentionally doped GaN. (C) 1998 Elsevier Science B.V. All rights reserved.

Properties of GaN epilayers with various growth conditions grown by gas source molecular beam epitaxy

GaN epilayers on sapphire (0001) substrates were grown by the gas source molecular beam epitaxy (GSMBE) method using ammonia (NH,) gas as the nitrogen source. Properties of gallium nitride (GaN) epilayers grown under various growth conditions were investigated. The growth rate is up to 0.6 mu m/h in our experiments. Cathodoluminescence, photoluminescence and Hall measurements were used to characterize the films. It was shown that the growth parameters have a significant influence on the GaN properties. The yellow luminescence was enhanced at higher growth temperature. And a blue emission which maybe related to defects or impurity was observed. Although the emission at 3.31 eV can be suppressed by a low-temperature buffer layer, a high-quality GaN epilayer can be obtained without the buffer layer. (C) 1998 Elsevier Science B.V. All rights reserved.

Structural identification of a cubic phase in hexagonal GaN films grown on sapphire by gas-source molecular beam epitaxy

The structural characteristics of gallium nitride (GaN) films grown on sapphire(0001) substrates by gas source molecular beam epitaxy (GSMBE) have been investigated using high-resolution synchrotron irradiation X-ray diffraction and cathodoluminescence with a variable energy electron beam. Besides the well-known GaN hexagonal structure, a small portion of cubic phase GaN was observed. The X-ray measurements provide an essential means for the structural identification of the GaN layers. Arising from the variable penetration depth of the electron beam in the cathodoluminescence measurements, it was found that the fraction of the GaN cubic-phase typically increased as the probing depth was increased. The results suggest that the GaN cubic phase is mostly located near the interface between the substrate and GaN layer due to the initial nucleation.

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.

Pockels effect in GaN/Al-x,Gal(1-x)N superlattice with different quantum structures - art. no. 69841G

The linear electro-optic (Pockels) effect of wurtzite gallium nitride (GaN) films and six-period GaN/AlxGa1-xN superlattices with different quantum structures were demonstrated by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The samples were prepared on (0001) sapphire substrate by low-temperature metalorganic chemical vapor deposition (MOCVD). The measured coefficients of the GaN/AlxGa1-xN superlattices are much larger than those of bulk material. Taking advantage of the strong field localization due to resonances, GaN/AlxGa1-xN SL can be proposed to engineer the nonlinear responses.

Deep levels in high resistivity GaN epilayers grown by MOCVD

Undoped high resistivity (HR) GaN epilayers were grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Thermally stimulated current (TSC) and resistivity measurements have been carried out to investigate deep level traps. Deep levels with activation energies of 1.06eV and 0.85eV were measured in sample 1. Gaussian fitting of TSC spectra showed five deep levels in different samples. (c) 2006 WILEY VCH Vertag GmbH & Co. KGaA, Weinheim

Growth of GaN layers on GaAs and GaP (111) and (001) substrates by molecular beam epitaxy

Films of GaN have been grown using a modified MBE technique in which the active nitrogen is supplied from an RF plasma source. Wurtzite films grown on (001) oriented GaAs substrates show highly defective, ordered polycrystalline growth with a columnar structure, the (0001) planes of the layers being parallel to the (001) planes of the GaAs substrate. Films grown using a coincident As flux, however, have a single crystal zinc-blende growth mode. They have better structural and optical properties. To improve the properties of the wurtzite films we have studied the growth of such films on (111) oriented GaAs and GaP substrates. The improved structural properties of such films, assessed using X-ray and TEM method, correlate with better low-temperature FL.

Properties of cubic GaN grown by MBE

We review our investigation of cubic GaN films on (001) GaAs, focusing on the structural, optical, and electrical properties of these films. Cubic GaN films grown epitaxially on GaAs suffer from the large lattice mismatch between these two materials in that they contain extremely high densities of structural defects. Surprisingly, the optical quality of these films does not seem much affected by the presence of defects, as intense photoluminescence is detected a? room temperature and above. Finally, the rather high background electron concentrations in our films is shown to be a consequence of contamination with O and not to be an intrinsic property of cubic phase GaN. (C) 1997 Elsevier Science S.A.

Valence hole subbands and optical gain spectra of GaN/Ga1-xAlxN strained quantum wells

The valence hole subbands, TE and TM mode optical gains, transparency carrier density, and radiative current density of the zinc-blende GaN/Ga0.85Al0.15N strained quantum well (100 Angstrom well width) have been investigated using a 6 X 6 Hamiltonian model including the heavy hole, Light hole, and spin-orbit split-off bands. At the k = 0 point, it is found that the light hole strongly couples with the spin-orbit split-off hole, resulting in the so+lh hybrid states. The heavy hole does not couple with the light hole and the spin-orbit split-off hole. Optical transitions between the valence subbands and the conduction subbands obey the Delta n=0 selection rule. At the k not equal 0 points, there is strong band mixing among the heavy hole, light hole, and spin-orbit split-off hole. The optical transitions do not obey the Delta n=0 selection rule. The compressive strain in the GaN well region increases the energy separation between the so1+lh1 energy level and the hh1 energy level. Consequently, the compressive strain enhances the TE mode optical gain, and strongly depresses the TM mode optical gain. Even when the carrier density is as large as 10(19) cm(-3), there is no positive TM mode optical gain. The TE mode optical gain spectrum has a peak at around 3.26 eV. The transparency carrier density is 6.5 X 10(18) cm(-3), which is larger than that of GaAs quantum well. The compressive strain overall reduces the transparency carrier density. The J(rad) is 0.53 kA/cm(2) for the zero optical gain. The results obtained in this work will be useful in designing quantum well GaN laser diodes and detectors. (C) 1996 American Institute of Physics.

Electronic structures of the zinc-blende GaN/Ga1-xAlxN compressively strained superlattices and quantum wells

The electronic structures of the zinc-blende GaN/Ga0.85Al0.15N compressively strained superlattices and quantum wells are investigated using a 6 x 6 Hamiltonian model (including the heavy hole, light hole and spin-orbit splitting band). The energy bands, wavefunctions and optical transition matrix elements are calculated. It is found that the light hole couples with the spin-orbit splitting state even at the k=0 point, resulting in the hybrid states. The heavy hole remains a pure heavy hole state at k=0. The optical transitions from the hybrid valence states to the conduction states are determined by the transitions of the light hole and spin-orbit splitting states to the conduction states. The transitions from the heavy hole, light hole and spin-orbit splitting states to the conduction states obey the selection rule Delta n=0. The band structures obtained in this work will be valuable in designing GaN/GaAlN based optoelectronic devices. (C) 1996 Academic Press Limited

Electronic properties of zinc-blende GaN, AlN, and their alloys Ga1-xAlxN

The electronic properties of wide-energy gap zinc-blende structure GaN, A1N, and their alloys Ga(1-x)A1(x)N are investigated using the empirical pseudopotential method. Electron and hole effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Gamma and those of the conduction band at Gamma and X are obtained for GaN and AIN, respectively. The energies of Gamma, X, L conduction valleys of Ga(1-x)A1(x)N alloy versus Al fraction x are also calculated. The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices based on these materials in the blue light range application. (C) 1995 American Institute of Physics.