Sukkaneste Tungasmita

Interface structure of hydride vapor phase epitaxial GaN grown with high-temperature reactively sputtered AlN buffer

Thick hydride vapor phase epitaxy GaN layers have been grown on a-plane sapphire using high-temperature ion-assisted reactively sputtered AlN as a buffer layer. Transmission electron microscopy and atomic force microscopy were carried out to study the formation of the two interfaces sapphire/AlN and AlN/GaN, and their influence on the microstructure of both the buffer layer and the main GaN layer. It was demonstrated that the high-temperature reactively sputtered buffer layer provides a good alternative for hydride vapor phase epitaxy growth of GaN layers. In particular, the buffer promotes a specific interface ordering mechanism different from that observed on low-temperature buffers.

Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.

Enhanced quality of epitaxial AlN thin films on 6H–SiC by ultra-high-vacuum ion-assisted reactive dc magnetron sputter deposition

Epitaxial AlN thin films have been grown on 6H–SiC substrates by ultra-high-vacuum (UHV) ion-assisted reactive dc magnetron sputtering. The low-energy ion-assisted growth (Ei=17–27 eV) results in an increasing surface mobility, promoting domain-boundary annihilation and epitaxial growth. Domain widths increased from 42 to 135 nm and strained-layer epitaxy was observed in this energy range. For Ei>52 eV, an amorphous interfacial layer of AlN was formed on the SiC, which inhibited epitaxial growth. Using UHV condition and very pure nitrogen sputtering gas yielded reduced impurity levels in the films (O: 3.5×1018 cm−3). Analysis techniques used in this study are in situ reflection high-energy electron diffraction, secondary-ion-mass spectroscopy, atomic-force microscopy, x-ray diffraction, and cross-section high-resolution electron microscopy.

Deformation potentials of the E1(TO) mode in AlN

The deformation potentials of the E-1(TO) mode in AlN are experimentally determined by combining infrared reflection spectroscopy and x-ray diffraction measurements and using a reported value of th ...

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.

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.

Epitaxial growth and orientation of AlN thin films on Si(001) substrates deposited by reactive magnetron sputtering

Epitaxial domain formation and textured growth in AlN thin films deposited on Si(001) substrates by reactive magnetron sputtering was studied by transmission electron microscopy and x-ray diffraction. The films have a wurtzite type structure with a crystallographic orientation relationship to the silicon substrate of AlN(0001)‖Si(001). The AlN film is observed to nucleate randomly on the Si surface and grows three dimensionally, forming columnar domains. The in-plane orientation reveals four domains with their a axes rotated by 15° with respect to each other: AlN⟨112¯0⟩‖Si[110], AlN⟨011¯0⟩‖Si[110], AlN⟨112¯0⟩‖Si[100], and AlN⟨011¯0⟩‖Si[100] An explanation of the growth mode based on the large lattice mismatch and the topology of the substrate surface is proposed.

Pulsed low-energy ion-assisted growth of epitaxial aluminum nitride layer on 6H-silicon carbide by reactive magnetron sputtering

Epitaxial aluminum nitride thin films have been grown on silicon carbide (6H-SiC) substrates by pulsed low-energy ion-assisted reactive magnetron sputter deposition (+5/-20 V of bias pulses), with ion-assisted energy (Ei)?22eV, under ultrahigh-vacuum conditions. Surface ion interactions during the negative bias pulse gave rise to enhanced surface mobility of adatoms with beneficial effects, which extended over the limit of ion repelling in the positive pulse as the film thickness increased. High-resolution electron microscopy shows that a large (>90 nm) AlN domain width can form on the substrate. Domain-boundary annihilation and domain suppression during film growth have been observed. The growth rate also increased by a factor of ~4 compared to growth conditions with no ion assistance (Ei=2eV) and by a factor of 2 from dc ion-assisted growth. This indicates that the supply of nitrogen is a limiting factor for AlN formation and that the reactivity of nitrogen is increased on the growing AlN film surface for pulse ion-assisted deposition. High-resolution x-ray diffraction shows a reduction in the full width at half maximum of the rocking curve from 1490 to 1180 arcsec when pulsed ions are used. The cathodoluminescence shows high intensity of near-band edge emissions at wavelengths of 206 (6.02 eV) and 212 nm (5.84 eV) at a measured temperature of 5 K, with relatively low defect and oxygen and carbon impurity related emission, which is indicative of a high quality electronic material.

Effect of Si doping of metalorganic chemical vapor deposition-GaN templates on the defect arrangement in hydride vapor phase epitaxy-GaN overgrown layers

Two different types of dislocation arrangements have been observed in hydride vapor-phase epitaxial GaN films grown on sapphire substrates using both undoped and Si-doped GaN templates grown by metalorganic chemical vapor deposition: (i) predominantly straight threading dislocations parallel to the [0001] direction in the layer grown on an undoped template, and (ii) a network of interacting dislocations of edge, screw, and mixed character in the layer grown on a Si-doped template. The two types of defect distribution result in essentially different surface morphologies, respectively: (i) low-angle grain boundaries formed by pure edge dislocations around spiral grown hillocks, and (ii) smooth surface intersected by randomly distributed dislocations. The Si doping of the GaN templates was found to enhance defect interaction in the templates and to enable a reduction of the dislocation density in the overgrown thick GaN films, although it does not lead to an improvement of the overall structural properties o...

Strain evolution in high temperature AlN buffer layers for HVPE-GaN growth

High temperature AlN buffer layers are deposited on a-plane sapphire by reactive magnetron sputtering. The effect of the buffer thickness on the AlN structural properties and surface morphology are ...