Free-standing GaN Films Research Articles

Radiative recombination and ultralong exciton photoluminescence lifetime in GaN freestanding film via two-photon excitation

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one-photon and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultralong exciton PL lifetime of 17.2ns at 295K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission wavelength show good agreement with the theoretical predictions, indicating that radiative recombination dominates for bulk excited state relaxation processes.

Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy

The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.

High pressure annealing of HVPE GaN free-standing films: redistribution of defects and stress

ABSTRACTThe effect of high temperature, high pressure annealing on morphology, optical and structural properties of free-standing GaN films grown by hydride vapor phase epitaxy is studied. The annealing is found to change the intensities of the photoluminescence peaks as a result of a redistribution of the impurities and native defects in the thick GaN films. A positron annihilation study shows a decrease of the Ga vacancy-related defects below the detection limit after the annealing. The defect redistribution is correlated with a flattening of the stress distribution across the thickness, as revealed by micro Raman study, and with a decrease of the curvature of the annealed free-standing films.

Freestanding GaN‐substrates and devices

Various physical aspects and potential applications of the laser-induced separation of GaN epilayers from their sapphire substrate are reviewed. The effect of short laser pulses on the thermal decomposition of GaN and possible applications of the laser-induced dissociation of GaN for fast etching of this material is discussed. Particular emphasis is placed on the defect-free delamination of large area GaN films with thicknesses ranging from 3 to 300 μm from sapphire substrates. The use of the resulting freestanding GaN films in device technology and homoepitaxy of III-nitrides are outlined. Specific examples are the flip-chip bonding of freestanding InGaN/GaN LEDs to a silicon submount and the production of pseudosubstrates for the homoepitaxy of high quality GaN epilayers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and separation related properties of HVPE-GaN free-standing films

Hydride vapour phase epitaxial GaN layers with thicknesses in the range 10–150 μm grown directly on sapphire or using metalorganic vapour phase deposited GaN templates have been separated by laser-induced lift-off technique. Both faces of the free-standing films have been studied by photoluminescence and high-resolution X-ray measurements and stress analysis has been performed. A comparison with as-grown films reveals the changes in the properties of the material after the separation process. The separation conditions are found to be responsible for the bowing in the free-standing GaN films while the type and intensity of emission bands, as well as defect and impurity distributions are related only to the growth conditions. The residual strain in the free-standing layers is attributed to both non-optimized separation conditions and non-uniform defect density in the films.

Low-etch-pit-density GaN substrates by regrowth on free-standing GaN films

In this study, GaN substrates with low-density etch pits were obtained by regrowth on free-standing GaN films (two steps) by hydride vapor-phase epitaxy (HVPE). The etch-pit density was lower than 4×104 cm−2 by atomic-force microscopy. The density is significantly lower than that of the HVPE-grown (one-step) GaN films (HVPE GaN), using sapphire as a substrate. The optical and electrical properties of the two-step HVPE-grown GaN substrates are superior to those of HVPE GaN. Temperature-dependent photoluminescence measurements reveal that thermal quenching behavior of the 2.9 eV band is possibly attributed to a shallow acceptor level at about 118±5 meV above the valence band.

Overgrowth of GaN layer on GaN nano-columns by RF-molecular beam epitaxy

The growth behavior of GaN layers overgrown on nano-columns on sapphire substrates by RF-plasma molecular beam epitaxy was investigated. Free-standing GaN films were obtained through the coalescence of nano-columns, showing the layered structure supported by underlying columnar GaN. Overgrown GaN layers were characterized with respect to the residual strain. As a result, it was found that overgrown GaN films with layer thickness of 2.7 μm possessed a stress-free property such as Raman frequency of high-frequency E 2 mode of 568.1 cm −1.

Defect Reduction in HVPE Growth of GaN and Related Optical Spectra

GaN technology is still based on highly mismatched heteroepitaxial growth on foreign substrates, and therefore needs to overcome a high defect density and a high level of stress in the epitaxial layers. Various attempts have been made to reduce the defects and stress in thick GaN layers. We here report a reduction of the defect density in thick GaN layers grown by hydride vapour phase epitaxy, using regrowth on free-standing GaN films, as well as introducing an AlN buffer and AlN interlayer in the growth sequence. Special focus is put on the optical properties of the material.

Structural and chemical characterization of free-standing GaN films separated from sapphire substrates by laser lift-off

Laser lift-off of GaN heteroepitaxial layers from sapphire substrates is a promising method for electronic device integration and GaN substrate creation. Of critical importance is the structural and chemical quality of the GaN layers following laser processing. In this letter, transmission electron microscopy techniques are used to characterize the modifications that occur at the resulting GaN surfaces. Structural alteration and chemical intermixing following lift-off are confined to approximately the first 50 nm. These results indicate that laser lift-off is a viable route for GaN substrate creation, as well as for electronic device integration.