Using Fourier transform infrared grazing incidence reflectivity to study local vibrational modes in GaN

Both Fourier transform infrared (FTIR) grazing incidence reflectivity and FTIR transmission methods have been used to study GaN films grown on alpha-Al2O3 (0001) substrates by atmospheric pressure metal-organic chemical vapor deposition and low pressure metal-organic chemical vapor deposition. The results show that in the frequency range from 400 to 3500 cm(-1) the signal-to-noise ratio of the FTIR grazing incidence measurement is far higher than that of the FTIR transmission measurement. Some new vibrational structures appearing in the former measurement have been discussed. The features around 1460 and 1300 cm(-1) are tentatively assigned to scissoring and wagging local vibrational modes of CH2 in GaN, respectively. (C) 1999 American Institute of Physics. [S0021-8979(99)06509-3].

Cubic-phase GaN light-emitting diodes

The feasibility of growing device-quality cubic GaN/GaAs(001) films by metal organic chemical vapor deposition has been demonstrated. The optical quality of the GaN films was characterized by room-temperature photoluminescence measurements, which shows a full width at half maximum of 46 meV. The structural quality of the films was investigated by transmission electron microscopy. There are submicron-size grains free from threading dislocations and stacking faults. More importantly, a cubic-phase GaN blue light-emitting diode has been fabricated. The device process, which is very simple and compatible with current GaAs technology, indicates a promising future for the blue light-emitting diode. (C) 1999 American Institute of Physics. [S0003-6951(99)01416-3].

Microstructure evolution of GaN buffer layer on MgAl2O4 substrate

Microstructure of GaN buffer layer grown on (111)MgAl2O4 substrate by metalorganic vapor phase epitaxy (MOVPE) was studied by transmission electron microscopy (TEM). It has been observed that the early deposition of GaN buffer layer on the substrate at a relatively low temperature formed a continual island-sublayer (5 nm thick) with hexagonal crystallographic structure, and the subsequent GaN buffer deposition led to crystal columns which are composed of nano-crystal slices with mixed cubic and hexagonal phases. After high-temperature annealing, the crystallinity of nano-crystal slices and island-sublayer in the buffer layer have been improved. The formation of threading dislocations in the GaN him is attributed not only to the lattice mismatch of GaN/MgAl2O4 interface, but also to the stacking mismatches at the crystal column boundaries. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

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.

Rapid thermal annealing processing of GaN epilayer on sapphire(0 0 0 1)

The effect of rapid thermal annealing (RTA) in a Nz ambient up to 900 degrees C has been investigated for GaN films grown on sapphire(0 0 0 1) substrates. Raman spectra, X-ray diffractometry and Hall-effect studies were performed for this purpose. The Raman spectra show the presence of the E-2 (high) mode and a shift in the wave number of this mode with respect to the annealing processing. This result suggests the presence and relaxation of residual stress due to thermal expansion misfit in the films which are confirmed by X-ray measurements and the structure quality of GaN epilayer was improved. Furthermore, the electron mobility increased at room temperature with respect to decrease of background electron concentration after RTA. (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.

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.

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.

Strong circular photogalvanic effect in ZnO epitaxial films

We report a strong circular photogalvanic effect (CPGE) in ZnO epitaxial films under interband excitation. It is observed that CPGE current is as large as 100 nA/W in ZnO, which is about one order in magnitude higher than that in InN film while the CPGE currents in GaN films are not detectable. The possible reasons for the above observations are the strong spin orbit coupling in ZnO or the inversed valence band structure of ZnO.

Abnormal photoabsorption in high resistance GaN epilayer

Unintentionally doped GaN epilayers are grown by the metalorganic chemical vapor deposition (MOCVD). Photovoltaic (PV) spectroscopy shows that there appears an abnormal photoabsorption in some undoped GaN films with high resistance. The peak energy of the absorption spectrum is smaller than the intrinsic energy band gap of GaN. This phenomenon may be related to exciton absorption. Then metal-semiconductor-metal (MSM) Schottky photodetectors are fabricated on these high resistance epilayers. The photo spectrum responses are different when the light individually irradiates each of the two electrodes with the photodetector which are differently biased. When the excitation light irradiates around the reverse biased Schottky junction, the responsivity is almost one order of magnitude larger than that around the forward biased junction. Furthermore, when the excitation light irradiates the reverse biased Schottky junction, the peak energy of the spectrum has a prominent red-shift compared with the peak energy of the spectrum measured with the excitation light irradiating the forward biased Schottky junction. The shift value is about 28 meV, and it is found to be insensitive to temperature. According to the analyses of the distribution of the electric field within the MSM device and the different dependences of the response on the electric field intensity between the free carriers and excitons, a reliable explanation for the different response among various areas is proposed.

Some properties of gallium nitride films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of this mode with respect to the GaN epilayer thickness. The Raman scattering results suggest the presence of stress due to lattice and thermal expansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. The residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an increase in the mobility due probably to stress relaxation as suggested by an observed shift in the E-2(high) mode in the Raman spectra after annealing.

GaN m-i-n LED grown by MOVPE

Undoped and Zinc-doped GaN films have been grown using TMGa, DEZn and Ammonia by MOVPE. The GaN blue-green LEDs of m-i-n structure have been fabricated. They can be operated at forward bias less than 5 volts. The EL peak wavelengths was from 455 nm to 504 nm.

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.

METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION GROWTH OF GAN

Single-crystal GaN films have been deposited on (01 (1) over bar 2) sapphire substrates using trimethylgallium (TMGa) and NH3 as sources. The morphological, crystalline, electrical and optical characterizations of GaN film are investigated. The carrier concentration ofundoped GaN increases with decreasing input NH3-to-TMGa molar flow ratio.

Structural and Optical Performance of GaN Thick Film Grown by HVPE

Thick GaN films were grown on GaN/sapphire template in a vertical HVPE reactor. Various material characterization techniques,including AFM, SEM, XRD, RBS/Channeling, CL, PL, and XPS, were used to characterize these GaN epitaxial films. It was found that stepped/terraced structures appeared on the film surface,which were indicative of a nearly step-flow mode of growth for the HVPE GaN despite the high growth rate. A few hexagonal pits appeared on the surface, which have strong light emission. After being etched in molten KOH, the wavy steps disappeared and hexagonal pits with {1010} facets appeared on the surface. An EPD of only 8 ×10~6cm~(-2) shows that the GaN film has few dislocations. Both XRD and RBS channeling indicate the high quality of the GaN thick films. Sharp band-edge emission with a full width at half maximum（FWHM）of 67meV was observed, while the yellow and infrared emissions were also found. These emissions are likely caused by native defects and C and O impurities.