E.M. Goldys

Crystal size and oxygen segregation for polycrystalline GaN

The authors would like to acknowledge the support of a U. S. NICOP Contract, No. N00014-99-1-GO17 sponsored through the U. S. Office of Naval Research. One of the authors (K.S.A.B.) would like to further acknowledge the support of a Macquarie University Research Fellowship.

INFLUENCE OF THE SURFACE MORPHOLOGY ON THE YELLOW AND EDGE EMISSIONS IN WURTZITE GAN

In this letter we examine an influence of surface morphology on yellow and edge emissions in wurtzite phase GaN. Our cathodoluminescence measurements show that the yellow emission does not correlate with the surface morphology, but simultaneously the “edge” emission shows very strong spatial fluctuations. The observed effect is attributed to granular structures in GaN films and enhancement of the yellow emission in the interface region.

Influence of growth rate on the structure of thick GaN layers grown by HVPE

Thick GaN films grown by hydride vapour phase epitaxy have been investigated by cathodoluminecsence, X-ray diffraction, and photoluminescence. Cross-sectional studies of thick GaN layers grown on sapphire without buffers reveal three zones: a highly disordered interface region; a columnar defective region and a good quality main region of the layer. The influence of the highly doped columnar region on the surface morphology and crystal structure of the layers has been studied. We show that the columnar region influences the material quality more strongly in thinner films. Thicker layers exhibit improved morphology with lower surface pit density and better crystal quality shown in photoluminescence and X-ray diffraction spectra. The relationship between the near-interface columnar structures and surface pits is revealed. A strong effect of the growth rate on the structure of thick layers is found. The results suggest that GaN layers with optimum crystalline quality may be obtained by varying the growth rate during growth.

Contribution of free-electron recombination to the luminescence spectra of thick GaN films grown by hydride vapor phase epitaxy

Emission spectra of thick unintentionally doped GaN layers grown on sapphire by hydride vapor phase epitaxy have been studied. In addition to sharp exciton emission lines, a broad asymmetric band extending to energies above the band gap is observed in the near band-gap regions of the photoluminescence and cathodoluminescence spectra. We explain this feature as a free-electron recombination band. Using spatially resolved cathodoluminescence we were able to separately record the spectra of lightly doped regions which contain only sharp bound exciton, the spectra of highly doped regions which show the broadband only, as well as the spectra of the areas containing surface defects which show both spectral features simultaneously. Using a model in which the electrons are degenerate and the holes are localized together with the results of Hall-effect measurements we can simulate the spectral shape of the broadband. Raman scattering spectroscopy confirms the coexistence of lightly doped (∼1×1017 cm−3) and heavily...

Intersubband optical absorption in strained double barrier quantum well infrared photodetectors

A systematic theoretical investigation of intersubband optical absorption in AlGaAs-AlAs-InGaAs strained double barrier quantum well is presented for the first time. Electron states are calculated within the effective mass approximation which includes the effects of subband nonparabolicity and strain, and found to be in good agreement with experiments. Intersubband optical absorption is investigated using the density matrix formalism with the intrasubband relaxation taken into account. Analytical formulas are given for electron energies, absorption coefficient, and responsivity. Subband nonparabolicity and elastic strain are found to significantly influence both electron states and intersubband optical absorption. The peak absorption wavelength is found to decrease linearly if the In composition is increased, and an approximate formula is given. Electron states and optical absorption are affected by the inner barrier thickness if it is less than 40 /spl Aring/. The results are useful for design and improvement of the performance of quantum well infrared photodetectors operating in the important wavelength region between 1.5 and 4 /spl mu/m.

Direct observation of large-scale nonuniformities in hydride vapor-phase epitaxy-grown gallium nitride by cathodoluminescence

Hydride vapor phase epitaxial GaN films grown on sapphire without a buffer are found to contain large-scale regions with high electron concentration located close to the interface. These regions are composed of individual columns forming an irregular but quasicontinuous layer, while the rest of the film has a much lower carrier concentration. The highly doped regions are easily visualized using cathodoluminescence. The coexistence of regions with low and high electron concentration allows us to explain the concurrent evidence of high film quality in photoluminescence, Raman spectroscopy and x-ray diffraction, and a high electron concentration measured in transport studies.

Thick Hydride Vapour Phase Epitaxial GaN Layers Grown on Sapphire with Different Buffers

We report a comparative study of the crystalline quality of thick GaN layers grown by hydride vapour phase epitaxy, using a nitridation and a GaCl pretreatment of the sapphire as well as a reactive sputtered AlN buffer and metalorganic chemical vapour deposition grown GaN ‘template’ layers. The structure quality was investigated using X-ray diffraction measurement and cathodoluminescence spectroscopy and imaging of cross-section of the films. The morphology of the layers was revealed by optical and atomic force microscopy. A distinct reduction of both the columnar near-interface region and the domain formation were observed in layers grown on AlN and GaN ‘template’ buffers resulting in improved bulk quality and significant smoother film surfaces.

Recrystallization prospects for freestanding low-temperature GaN grown using ZnO buffer layers

Remote plasma enhanced-laser-induced chemical vapor deposition was used to grow gallium nitride films on zinc oxide buffer layers deposited by atomic layer epitaxy on soda lime glass. Freestanding layers of gallium nitride were processed by etching away the substrate and ZnO buffer layer. The n-type carrier mobility for the GaN on ZnO/soda lime glass was found to be similar to the highest values achieved on pure silica, and was accompanied by high carrier concentration. As-grown polycrystalline materials were recrystallized at low temperature (below the 570°C gallium nitride growth temperature). This recrystallization process greatly improved the film structure with a self-assembled multilayer structure evident in the oxygen-rich surface layer of the films that had undergone the process.

Cathodoluminescence depth-profiling studies of GaN/AlGaN quantum-well structures

In this paper we evaluate the in-depth homogeneity of GaN epilayers and the influence of electric field present in strained GaN/AlGaN heterostructures and quantum wells on the yellow and “edge” emission in GaN and AlGaN. Our depth-profiling cathodoluminescence measurements show an increased accumulation of defects at the interface. Inhomogeneities in the doping level are reflected by the enhancement of the yellow emission in the interface region. The piezoelectric effect is found to strongly reduce the emission from the strained AlGaN quantum-well barriers. We also show that Ga droplets, commonly found on surfaces of samples grown in Ga-rich conditions, screen the internal electric field in a structure and thus result in a local enhancement of the edge emission intensity.

Surface morphology of cubic and wurtzite GaN films

Abstract We describe a comparative study of surfaces of gallium nitride films grown by a variety of techniques at low growth temperatures (molecular beam epitaxy and laser-assisted chemical vapour deposition) as well as by metalorganic chemical vapour deposition. The cubic, wurtzite and mixed phase cubic–wurtzite films were grown on buffers, these included ultrathin (4 nm) SiC as well as more commonly used AlN. We find that the surface morphology of GaN films grown by MBE shows micrometer-scale structures which reflect the symmetry of the film. Surface topography may thus be used as an identification measure of film symmetry. Monochromatic cathodoluminescence images taken at the maximum of the band edge emission show granular structures reflecting surface morphologies, whereas similar structures are only very weakly visible in the red/yellow band.