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The strain relaxation of sputter-deposited AlN (sp-AlN) films on c-plane sapphire substrates before and after face-to-face annealing (FFA) was evaluated. After FFA, the AlN films consisted of a layer with compressive strain and an extremely low dislocation density, which was ascribed to solid-state growth during FFA. In addition, the crystallinity was further improved after the homoepitaxial growth of AlN by metal-organic vapor phase expitaxy (MOVPE). The full widths at half maximum of the X-ray rocking curves of AlN (0 0 0 2) and (1 0 −1 2) for the MOVPE-grown AlN layer on the FFA-sp-AlN film were 15 and 240 arcsec, respectively. The surface morphology of the MOVPE-grown AlN layer on the FFA-sp-AlN film was covered with an atomically flat step-and-terrace structure. The compressive strain of the MOVPE-grown AlN layer was inherited from the underlying FFA-sp-AlN film.

Post-growth high-temperature annealing of sputtered AlN films is a promising approach to realize high-quality AlN templates for deep-ultraviolet light-emitting diode applications. This paper proposes the double-sided sputtering of AlN on double-sided polished sapphire to suppress the wafer bowing caused by high-temperature annealing. Similar to the case of single-sided sputtering, the twist component of the X-ray rocking curve was markedly improved after annealing, yielding rocking curve widths of 256–321 arcsec in (10–12). By varying the backside thickness, the curvature was controlled from −27 km−1 to 29 km−1, while a crack-free surface was maintained on the front side of AlN. The experimentally obtained curvature and in-plane stress agreed well with the calculation based on thermal stress analysis in the multilayer system.

High-quality AlN templates fabricated by sputtering-deposition and post-deposition high-temperature annealing have great potential for deep ultraviolet light-emitting device applications. In this work, we fabricated AlN films on 6H-SiC substrates by sputtering and face-to-face annealing and characterized the structural quality of the AlN films before and after annealing. As reported in previous studies, to accomplish high-quality AlN films on SiC substrates using conventional methods, such as molecular beam epitaxy or metalorganic vapor phase epitaxy (MOVPE), it is important to grow the AlN on the SiC coherently. However, in this work, although the annealed AlN films were fully relaxed from the SiC substrates, or even had tensile strain, the AlN films indicated high crystallinity. The X-ray rocking curve full width at half maximum (XRC-FWHM) values of the 200-nm-thick annealed AlN film were 17 and 246 arcsec for the AlN (0002) and (10–12) diffraction, respectively. Though the annealed AlN film indicated rough surfaces with bunched step structures, the surface morphology was remarkably improved by MOVPE growth and clear atomic step-and-terrace structures were formed. The XRC-FWHM values of the MOVPE-grown AlN were 90 and 239 arcsec for the AlN (0002) and (10–12) diffraction, respectively.

N-polar face-to-face annealed sputtered AlN (FFA Sp-AlN) was fabricated by sputtering with an Al metal target and high-temperature annealing in a face-to-face configuration. The polarities of the FFA Sp-AlN samples fabricated with different sputtering targets (i.e., Al metal or sintered AlN targets) were checked by KOH etching and cross-sectional scanning transmission electron microscope images. As a result, samples sputtered with a sintered AlN target and an Al metal target resulted in Al-polar and N-polar FFA Sp-AlN, respectively. Then, we fabricated FFA Sp-AlN with N-polar AlN on the top-most layers (N-polar FFA Sp-AlN) with different total film thicknesses. The threading dislocation densities (TDDs) of N-polar FFA Sp-AlN were estimated from the full width at half maximum values of the X-ray rocking curves. Consequently, the TDD of N-polar FFA Sp-AlN decreased with increasing AlN film thickness, which was the same trend as that of Al-polar FFA Sp-AlN. The minimum TDD of 1.7 × 108 cm−2 was obtained from N-polar FFA Sp-AlN with a total thickness of 730 nm.

The metalorganic vapor phase epitaxial (MOVPE) growth of AlN films was performed on face-to-face annealed sputtered AlN templates (FFA Sp-AlN) having a nano-striped pattern with a period of 300 nm and depth of 120 nm. Then, the effect of MOVPE growth conditions on the crystallinity was elucidated. The faceted structure clearly changed with the growth temperature (Tg), following the same trend as the epitaxial lateral overgrowth of GaN film on micro-patterned GaN. High Tg levels enhanced the lateral growth, and thus with a high Tg of 1300 °C, the coalescence in the early growth stage resulted in AlN films with coalesced surfaces and a thickness of 1 µm. For the AlN film grown at a Tg of 1300 °C, the threading dislocation density (TDD) was estimated to be 6.0 × 108 cm−2. This value proved that coalesced AlN films on nano-patterned FFA Sp-AlN templates with low TDDs could be achieved with the appropriate MOVPE growth conditions. The compressive strain in the AlN film on nano-patterned FFA Sp-AlN was lower than that of the film grown on FFA Sp-AlN without a pattern.

Face-to-face annealed metalorganic vapor phase epitaxy (MOVPE)- grown AlN templates with sputtered AlN nucleation layers (FFA MOSp-AlN templates) were fabricated on vicinal sapphire substrates. The sputtered thin AlN layer supplies AlN seeds that are well aligned to the c-axis. The MOVPE growth with a relatively thick AlN enlarges crystal grains with a low concentration of impurities. Face-to-face annealing at high temperatures reduces the twist component of AlN films due to recrystallization, and AlN films with low threading dislocation densities are formed. The full width at half maximum (FWHM) of the (10-12) X-ray rocking curves (XRCs) of AlN templates decreased as the thickness of the MOVPE-grown AlN layer increased. The FWHMs for the (0002) and (10-12) XRCs of the 300-nm-thick AlN templates with an off-cut angle of 0.6° were 10 and 337 arcsec, respectively. The concentrations of O, Si, and C impurities in the FFA MOSp-AlN templates were lower than those in the face-to-face annealed sputtered AlN templates. Si doped Al0.7Ga0.3N films, which are typically used as n-type AlGaN layers for UVC LEDs, were grown on the FFA MOSp-AlN templates with various off-cut angles. The AlGaN films with smooth flat surfaces were obtained using the FFA MOSp-AlN template with larger off-cut angles due to the suppression of the large hillock formation

GaAs buffer layers were grown on off-angle Ge substrates with a two-step growth process by metalorganic vapor phase epitaxy in order to prepare a high quality buffer layer for the InGaAsN p–i–n solar-cell structure. In our contribution, we present results from the optimized two-step growth of GaAs buffer layers, namely with a low-temperature step at 470 °C and a high-temperature step at 580 °C, combined with the use of Ge substrates misoriented by 4° and 6° towards [1 1 0]. HRXRD results showed that by using the off-angle substrates, the FWHM of (0 0 4) rocking curves was decreased to 6.7 sec, which is about 4 times the FWHM of the GaAs commercial substrate. A decreased FWHM indicates a narrower distribution of crystal orientation. Furthermore, a smooth surface with a RMS roughness of 0.7 nm was clearly observed by AFM. Cross-sectional dark-field TEM images showed the GaAs buffer layer with 20–30 nm diamond-shaped anti-phase domains (APDs) at the GaAs/Ge interface, followed by APDs-free GaAs regions on the 6° off-angle Ge substrate. However, anti-phase boundaries were generated along the [0 0 1] direction for the buffer layer on the on-axis substrate. Our results demonstrated that the use of 6° off-angle Ge substrate and a two-step growth process allow the suppression of APDs in the GaAs buffer layer.

AlN films with low-dislocation density and flat surfaces were grown by metalorganic vapor phase epitaxy (MOVPE) on nano-patterned sapphire substrates (NPSSs) with face-to-face annealed sputtered AlN (FFA Sp-AlN) as an initial layer. Dislocation annihilation mechanisms and coalescences of AlN layers were studied by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray rocking curve (XRC) measurements. From cross-sectional TEM images, the coalescence thickness was obtained to be 1.2 µm. This thickness is thinner than that for MOVPE-grown AlN films on NPSS without FFA Sp-AlN. AFM images revealed that atomically flat surfaces can be obtained from the samples with thicknesses above 3 μm. From XRC full width at half maximum values, the screw- and edge-dislocation densities of the 5-μm-thick AlN film on FFA Sp-AlN/NPSS were calculated to be 4 × 107 cm−2 and 6 × 108 cm−2, respectively. This edge-dislocation density, i.e., the dominant threading dislocation density, is approximately the same to that of AlN films grown on FFA Sp-AlN/flat-sapphire substrates (FSSs). These results show promise that FFA Sp-AlN is applicable not only for FSSs but also for NPSSs as the method for growing high-crystalline-quality AlN film.

The InGaAsN films lattice-matched to Ge substrates and having an about 1 eV bandgap energy have been grown on Ge(001) vicinal substrates by metal organic vapor phase epitaxy (MOVPE) varying the group-V precursor flow rates, and their photoluminescence (PL) properties have been investigated. Rapid thermal annealing (RTA) with suitable conditions has improved the luminescence properties. The PL spectral shifts with temperature and excitation intensity indicate that a larger flow of the group-V precursors enhances the carrier localization due to the potential fluctuations caused by the compositional non-uniformity and other localized states which originate from impurities or crystal defects. The huge amount of 1, 1-dimethylhydrazine (DMHy), which can efficiently incorporate N, may also introduce a significant amount of carbon species which can act as impurities, or may suppress the surface migration of the group-III atoms resulting in compositional non-uniformity and crystal defects.