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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.

For the realization of highly efficient deep-ultraviolet light-emitting diodes (DUV-LEDs) based on III-nitride semiconductors, it is essential to improve the crystalline quality of the AlN templates for crystal growth of AlGaN. Our group has suggested sputtering deposition and post-deposition high-temperature face-to-face annealing (FFA) as a fabrication method of AlN films with low threading dislocation density (TDD) on sapphire substrates. Although the FFA enables reduction of TDDs, it possibly causes a cracking for AlN films due to a large thermal expansion coefficient mismatch between AlN and sapphire. In this work, we controlled the residual stress in AlN films by modifying the sputtering conditions. Consequently, we achieved crack-free AlN films with low TDDs.

A combination of the sputtering deposition and high-temperature annealing is a promising technique for preparing low-dislocation-density AlN templates. In this talk, MOVPE growth behavior of AlGaN films grown on the annealed AlN templates and on conventional MOVPE-grown AlN templates was comprehensively discussed. The low density of screw- and mixed-type dislocations of the annealed AlN templates invoked the formation of hillock structures. By adjusting the MOVPE growth conditions and utilizing sapphire substrates with appropriate surface off-cut, dislocation-induced hillock structures were suppressed. Improved surface flatness resulted in higher EQE and better wavelength uniformity of the DUV-LED fabricated on the annealed AlN templates.

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.

AlN templates fabricated via a combination of sputtering deposition and post-deposition high-temperature face-to-face annealing can be applied to deep-ultraviolet (DUV) light-emitting devices because of their extremely low threading dislocation density (TDD) despite their low-cost and simple fabrication process. First, this paper summarizes the overview and essential crystalline characteristics of the face-to-face annealed sputter-deposited AlN template (FFA Sp-AlN). Thereafter, recent progress in the TDD reduction of the FFA Sp-AlN and the metalorganic vapor phase epitaxy of AlN and Al x Ga1−x N on the FFA Sp-AlN have been reviewed. The TDD of approximately 4 × 107 cm−2 was obtained on the sapphire substrates with an AlN film thickness of 1.2 μm. Finally, the potential of the low-TDD FFA Sp-AlN has been discussed by demonstrating the performances of DUV light-emitting diodes fabricated on the FFA Sp-AlN.

This study investigated the influence of high-quality AlN templates on the internal quantum efficiency (IQE) of AlGaN-based multiple quantum wells (MQWs) using photoluminescence spectroscopy. An extremely high IQE of 90% at room temperature was obtained from MQWs on face-to-face annealed sputter-deposited AlN/sapphire templates. The dependence of efficiency curves on temperature indicated that nonradiative recombination centers were almost fully saturated under higher excitation power densities even at 400 K. Moreover, analysis of the efficiency curves using a rate equation model based on exciton recombination processes suggested that radiative recombination was the dominant recombination process even at 400 K.

c-plane sapphire (c-Sap) is most popular substrate for polar (Al, Ga)N growth, where it is known that the m-axes of sapphire and (Al, Ga)N are rotated by 30° to minimize lattice mismatch. On the other hand, it has been reported that the m-axes of sapphire and nitrides can be parallel with each other when grown on an a-plane sapphire. This epitaxial relation is beneficial for the laser cavity formation by cleavage. In this work, we demonstrated c-plane AlN/a-plane sapphire (c-AlN/a-Sap) templates by means of sputtering and high-temperature annealing [4]. X-ray diffraction (XRD) measurements revealed the epitaxial relation of a-Sap [1–100] //c-AlN [1–100] as expected from previous works. As the film thickness increased, the rocking curve width of (10–12) XRD improved, and 214 arcsec was obtained at the crack-free film thickness of 300 nm. These characteristics are attractive as an AlN template applicable for AlGaN-based DUV optical devices.

Diamond is an attractive widegap semiconductor for power devices due to the bandgap of 5.5 eV and high hole conductivity. Quantum sensing by nitrogen-vacancy (NV) centers has also been focused on in recent years. On the other hand, (Al, Ga)N has been extensively studied for deep ultraviolet LEDs and high electron mobility transistors, whereas the poor p-type conductance limits the performance of bipolar devices. Hence, the heterogeneous integration of diamond and (Al, Ga)N is expected to realize efficient carrier injection into widegap bipolar devices.

Thermal cycle annealing (TCA) is introduced to accelerate the dislocation annihilation in sputter-deposited AlN films on sapphire. Compared with constant temperature annealing, AlN films processed by TCA showed lower dislocation densities, smoother surface morphology, and fewer defects generated from the AlN/sapphire interface. After optimizing the film thickness, AlN films with a thickness of 800 nm, and X-ray rocking curve full widths at half maximum of 10–20 arcsec (0002) and 80–90 arcsec (10-12) were demonstrated, providing a simple and low-cost way to prepare high-quality AlN/sapphire templates for high-performance ultraviolet light-emitting diodes.

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

GaN-based multijunction light-emitting diodes (MJ-LEDs) capable of operating at high voltage and high power were fabricated. The device structure of the MJ-LEDs was based on a thin-film flip-chip LED and consisted of several junctions connected in series. High-voltage operation and uniform electroluminescence were achieved in the MJ-LEDs whose chip size was from 1.5 to 2.5-mm-square. Under a driving power of 5.8 W, the light output power of a 12-junction LED was over 3.1 W with encapsulation. A white lamp based on the MJ-LED and a YAG:Ce phosphor exhibited a luminous efficacy of 111 lm W−1 in 6.1 W operations when the stage temperature was 85 °C.

InGaAsN on Ge (001) is proposed to be a part of the InGaP(N)/InGaAs/InGaAsN/Ge four-junction solar cell to increase a conversion efficiency over 40%. In this work, InGaAsN lattice-matched film and GaAs buffer layer grown on Ge (001) substrate by metal organic vapor phase epitaxy (MOVPE) were examined by transmission electron microscopy (TEM). Electron diffraction pattern of InGaAsN taken along the [110]-zone axis illustrates single diffracted spots, which represent a layer with a uniformity of alloy composition. Cross-sectional bright field TEM image showed line contrasts generated at the GaAs/Ge interface and propagated to the InGaAsN layer. Dark field TEM images of the same area showed the presence of boundary-like planar defects lying parallel to the growth direction in the InGaAsN film and GaAs buffer layer but not in the Ge substrate. TEM images with the (002) and (00-2) reflections and the four visible {111} planes reflections illustrated planar defects which are expected to attribute to antiphase boundaries (APBs). Moreover, the results observed from atomic force microscopy (AFM) and field emission electron microscopy (FE-SEM) demonstrated the surface morphology of InGaAsN film with submicron-sized domains, which is a characteristic of the APBs.

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.

AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes.

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 impact of a face-to-face annealed sputtered AlN/sapphire (FFA Sp-AlN) template with threading-dislocation densities (TDDs) of 2 × 108 cm−2 and an n-type AlGaN (n-AlGaN) underlayer on optical properties of AlGaN multiple quantum wells (MQWs) with an ultraviolet C (UVC) emission is investigated comprehensively. For comparison of the FFA Sp-AlN template with low TDDs, a conventional MOVPE (metalorganic vapor phase epitaxially)-grown AlN/sapphire (MOVPE-AlN) template with TDDs of 1 × 109 cm−2 was prepared. Consequently, cathodoluminescence (CL), temperature-dependent photoluminescence (PL), and time-resolved PL (TR-PL) measurements verified that both the FFA Sp-AlN template and n-AlGaN underlayer are indispensable for obtaining MQWs with high internal quantum efficiencies, which decrease the TDDs and point defect (PD) densities. Our results revealed that 10-period quantum wells (10QWs)/n-AlGaN/AlN grown on the FFA Sp-AlN template exhibit a lower dark spot density in CL panchromatic intensity maps, a higher integrated emission intensity ratio from the temperature-dependent PL (from 15 to 300 K), and a longer nonradiative lifetime from the TR-PL measurements at 300 K compared with those grown on the MOVPE-AlN template. Moreover, we found that the optical properties of 10QWs/AlN in FFA Sp-AlN and MOVPE-AlN templates do not exhibit a significant difference because of the existence of numerous PDs. Our experimental results demonstrate the favorable impact of the FFA Sp-AlN template for low-TDDs and the n-AlGaN underlayer for low-PDs, which holds promise for highly efficient AlGaN deep-ultraviolet light-emitting devices.

A crack-free aluminum nitride (AlN) layer of 9 ± 1 μm thickness was grown on a nanopatterned sapphire substrate (NPSS) with a sputter-deposited AlN buffer layer. The buffer layer was thermally annealed and AlN was regrown by hydride vapor-phase epitaxy (HVPE). The dependence of the crystallinity of HVPE-grown AlN layers on the growth temperature was investigated. It was found that undesired misaligned AlN growth can be prevented by choosing an appropriate growth temperature. The full widths at half maximum of the (0002)- and (10–12)-plane X-ray rocking curves were improved to as low as 102 and 219 arcsec, respectively, by applying an NPSS with a sputter-deposited annealed AlN film. Compared with substrates without nanopatterning, the NPSS was also effective for suppressing cracks owing to the formation of voids at the interface of the HVPE-grown AlN layer and NPSS template.

A GaAs/InGaAsN p-i-n solar-cell structure on a Ge substrate has been fabricated intending incorporation in high-efficiency InGaP/InGaAs/InGaAsN/Ge four-junction-structure solar cells. An improved InGaAsN MOVPE growth process is adopted in which the Ge(001) vicinal substrates and a two-step-growth GaAs buffer layer combined with post-growth rapid thermal annealing (RTA) are the keys. With a prototype p-i-n photo-cell, the photovoltaic effect and a photo-current edge shift to the longer wavelength of InGaAsN with 1 eV bandgap are demonstrated. After RTA, however, the I-V characteristics are degraded with a significant reverse-bias current leakage. Electrical measurements showed a conduction-type conversion from n-type to p-type and significant increase of the hole concentration. The N-H-VGa complexes acting as shallow acceptors formed through RTA may be the most feasible cause of the drastic change of the electrical property of the InGaAsN film after RTA. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated an effect of N incorporation on InGaAsN on Ge (001), which is proposed to be a part of the InGaP(N)/InGaAs/InGaAsN/Ge four-junction solar cell, and on its growth behavior. Results obtained from high resolution X-ray diffraction and Raman scattering demonstrated that high quality In0.11Ga0.89As1-yNy films with N (y) contents up to 5% were successfully grown on n-type doped Ge (001) substrate by metalorganic vapor phase epitaxy using low-temperature (500°C) GaAs buffer layer. As expectation, the In0.11Ga0.89As0.96N0.04 film is examined to be under lattice-matching condition. Anti-phase domains were observed for the film without N incorporation, which exhibits submicron-size domains oriented along the [110] direction on the grown surface. With increasing N content, the domains become less orientation, and present in a larger domain size. Based on results of transmission electron microscopy, a high density of anti-phase domains was clearly observed at the interface of low-temperature GaAs buffer layer and Ge substrate. On the other hand, it is found to drastically reduce within the N-contained InGaAsN region. Furthermore, the lattice-matched In0.11Ga0.89As0.96N0.04 film is well developed to reduce the density of anti-phase domains.

We have grown blue light-emitting diodes (LEDs) with low threading dislocation densities (TDDs) by using SiN interlayers on Si (111) substrates. Our growth technique using SiN layers makes it possible to decrease twist components (edge-type threading dislocation components). The edge-type TDDs are almost the same values as those of LEDs grown on Al2O3 (0001) substrates. EQE of LEDs grown on Si (111) substrates increases with decreasing edge-type dislocation in the low-current-density region, and the EQE of the sample with low TDD is almost as high as that of the LED grown on an Al2O3 (0001) substrate at room temperature. It is found that the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region, but are less than those of an LED grown on an Al2O3 (0001) substrate. Time-resolved photoluminescence (TRPL) shows that the dominant origin of the thermal quenching is edge-type dislocations in our samples, but other defects such as screw-type dislocations also contribute to it. We also found the fluctuated emission patterns consisting of bright and dark areas originated from the difference of Shockley–Read–Hall (SRH) type defect densities in the multi-quantum wells (MQWs) grown on Si (111) substrates. The bright areas spread, and the configurations of the bright areas change into ring-like patterns with reducing edge-type TDDs. We suggest that the internal quantum efficiency (IQE) of dark areas should be promoted to improve the performance of the MQWs grown on Si (111) substrates.

We have grown blue light-emitting diodes (LEDs) having InGaN/GaN multi-quantum wells (MQWs) with thin AlyGa1−yN (0