We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.

The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated, and a sub-layer model of the degree of strain and related defects for all films is suggested. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sub-layer model has been successfully applied to the analysis of the ellipsometry data trough model calculations of the infrared dielectric function. © 2003 Elsevier B.V. All rights reserved.

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 the Raman-stress factor for hydrostatically stressed bulk AlN. The deformation potentials are found to strongly depend on published stiffness constants of AlN. A comparison with earlier theoretically calculated values of the deformation potentials is made. (C) 2002 American Institute of Physics.

Ternary Al1-xInxN thin films were grown by dual target direct current (DC) reactive magnetron sputtering under UHV conditions. The film compositions were determined to range from 0.30

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(SiC)(X)(AIN)(1-X) thin films have been grown epitaxially on vicinal 6H-SiC (0001) by low-energy ion assisted dual magnetron sputtering in UHV conditions. AES showed a decreasing Si and C content for an increasing magnetron power ratio, (P-Al/P-SiC). The epitaxial quality of the films was improved as the SiC fraction increased. Films containing less than 5% of Si and C show an evolution of domain width similar to the growth of pure AIN. HRXRD show a decreased c-axis lattice parameter for a film with composition of AINC(X) (0less than or equal toxless than or equal to0.1), indicating carbon substitution in AIN. CL spectra show defect-related peaks of similar to3.87 and similar to4.70 eV, corresponding to O and C impurities respectively as well as on un-identified peak at similar to3.40 eV.

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. © 2002 American Institute of Physics.

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 studied in correlation with the subsequent hydride vapour phase epitaxy of GaN. A minimum degree of mosaicity and screw dislocation density is determined for a 50 nm thick AlN buffer. With increasing the AlN thickness, a strain relaxation occurs as a result of misfit dislocation generation and higher degree of mosaicity. A blue shift of the E-1(TO) frequency evaluated by means of infrared reflection spectroscopy is linearly correlated with an increase in biaxial compressive stress in the films through the IR stress factor k(E1)(b) = 2.57 +/- 0.26 cm(-1) GPa(-1).

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. © 2001 Elsevier Science B.V. All rights reserved.

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.

The research presented in this thesis is focused on thin film synthesis of epitaxial wurtzite structure aluminum nitride (AlN) and related alloy, (SiC)_x(AlN)_1-x,by ultra-high-vacuum (UHV) reactive magnetron sputter deposition, on silicon carbide (6H-SiC) substrates. The emphasis of the work is on controlling the growth and quality of the films to be able to use the materials in electronic device applications. The quality of epitaxial AlN films is significantly improved by usin glow-energy ion assistance (E_i = 17-27 eV), during growth. The ion-assisted growth gives an increased surface mobility, which promotes domain boundary annihilation and epitaxial growth. This results in lateral expansion of column width (100 nm-wide at film thickness above 100 nm). Structural characterization by high-resolution XRD and electron microscopy reveal a very good crystal quality. The measured concentrations of O, C, and Si impurities in the films are at 3.5x10¹⁸, 1.3x 10x18 andl 1.0 x10¹⁷ cm^-3, respectively, which are among the purest AlN material that has been reported. The appearance of near band-edge CL emission (6.02 eV at 4K) is also an evidence of a high quality material. For metal-insulator-semiconductor devices (MIS) fabricated using AlN as the dielectric layer, electrical characterization by C-V measurement shows hysteresis with 1.2 V of flat-band voltage shift due to fixed charge and interface states, and a current leakage due to domain boundary formation in the AlN layer. Pulsed low-energy ion-assisted reactive magnetron sputtering was also applied to the growth of AlN in order to overcome the thickness limitation of DC ion-assisted growth due to a surface charging effect. AlN films with very large domain widths can thus be realized. The structural evolution resulting from increasing adatom mobility can be extended up to twice the thickness, compared to DC ionassisted deposition. The growth rate also increased by a factor of ∼4, compared to growth conditions with no ion assistance (E_i = 2 eV), and by a factor of ∼2 from the DC ion-assisted growth. Finally, solid solution (SiC)_x(AlN)_1-x thin films have beengrown epitaxially on 6H-SiC substrates. The composition of the films was controlled by varying the power of each magnetron during co-sputtering from A1 and SiC targets in a gas mixture of Ar and N₂. Compositional investigation from AES showed a decreasing Si and C content for an increasing magnetron power ratio, (P_A1/P_SiC). The microstructure of the films was improved as the P_A1/P_SiC increases. Films grown at P_A1/P_SiC 3. 6 show structural evolution of domain width similar to the growth of pure AlN. High-resolution XRD shows a minimum in c-axis lattice parameter as P_A1/P_SiC =3.6. CL spectra show defect-related peaks corresponding to O and C impurities with some shifts due to structural defects and concentration of the impurities.