[en] A low-resistance Ohmic contact of Pd/Pt/Au metallization scheme is formed on a p-type GaN film grown by metal-organic chemical vapor deposition (MOCVD). Two samples are prepared to explore the effect of contaminations at the metal and GaN interface on the properties of the Ohmic contact. One directly transfers the fresh GaN film from MOCVD to the metal deposition chamber through an ultrahigh vacuum (UHV) tube, without exposing the GaN surface to air; the other transfers the GaN film from MOCVD to metal deposition through the atmosphere. The results show a lower specific contact resistance (6.2 ± 0.9) × 10${}^{-<!--\; ???\; -->5}$ Ω cm${}^2$ for the directly UHV-transferred sample than the air-transferred sample (2.1 ± 0.9) × 10${}^{-<!--\; ???\; -->4}$ Ω cm${}^2$. Once exposed to air, carbonaceous particles and an amorphous oxide layer easily adhere to the p-type GaN surface, which not only increase the Schottky barrier height but also generate more grain boundaries in the Pd/Pt/Au stacks. The grain boundaries, acting as diffusion channels, make Au atoms diffuse easily into the Pd layer and propagate to the p-GaN surface layer after annealing. Therefore, the Ohmic contact fabricated in UHV ambience with lower oxygen and carbon contaminations at the interface shows superior characteristics than the samples prepared by the traditional method. (© 2020 Wiley‐VCH GmbH)

[en] Growth mechanism and temperature of monoclinic gallium oxide (β‐Ga${}_2$O${}_3$) films grown by ozone molecular beam epitaxy (MBE) are investigated herein. Phase-pure ($\stackrel{¯<!--\; ??\; -->}{2}$01) β‐Ga${}_2$O${}_3$ films can be grown on c-plane sapphire substrates when the growth temperature exceeds 500 °C, and the grain size increases with the increase in the growth temperature. Sixfold rotational domains are obtained based on the epitaxial relationships (Ga${}_2$O${}_3$<010>//Al${}_2$O${}_3$<1$\stackrel{¯<!--\; ??\; -->}{1}$00> and Ga${}_2$O${}_3$<102>//Al${}_2$O${}_3$<11$\stackrel{¯<!--\; ??\; -->}{2}$0>). For the film grown at 700 °C, the threefold facets are observed for the first time, which are resulted from the easy-cleaved (100) plane and rotational domains. In addition, the cathodoluminescence spectra of the β‐Ga${}_2$O${}_3$ films show that the proportion of ultraviolet emission increases with the increase in the growth temperature, which is attributed to the modulation of defect. Finally, the growth rate can be modulated by the ratio of Ga flux and ozone pressure. A reference is provided for heteroepitaxy of β‐Ga${}_2$O${}_3$ films and better understanding of the ozone MBE growth mechanism of oxides. (© 2020 Wiley‐VCH GmbH)

[en] An abnormal aging phenomenon is reported for GaN-based near-ultraviolet laser diodes (LDs). Under an electrical stress for several minutes, the threshold current of the LDs decreased, while the light output power and the operation voltage increased. The amplitude of the abnormal aging phenomena was found to be mainly related to the excess Mg concentration in the p-AlGaN electron blocking layer (EBL). It almost disappeared when the Mg concentration in the AlGaN EBL was reduced to 2  ×  10¹⁹ cm⁻³. We propose that this phenomenon was related to the Mg–V_N–H complex defects formed in highly doped AlGaN EBL ([Mg]  >  3  ×  10¹⁹ cm⁻³). (paper)

[en] Interface-enhanced high-temperature superconductivity in one unit-cell FeSe films on SrTiO_3(001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. By combined in situ scanning tunneling microscopy/spectroscopy and ex situ scanning transmission electron microscopy studies, we report on atomically resolved structure including both lattice constants and actual atomic positions of the FeSe/STO interface under both non-superconducting and superconducting states. We observed TiO_2 double layers and significant atomic displacements in the top two layers of STO, lattice compression of the Se–Fe–Se triple layer, and relative shift between bottom Se and topmost Ti atoms. By imaging the interface structures under various superconducting states, we unveil a close correlation between interface structure and superconductivity. Our atomic-scale identification of FeSe/STO interface structure provides insight on investigating the pairing mechanism of this interface-enhanced high-temperature superconducting system. (paper)

[en] Herein, bandgap tuning of monoclinic (-201)-oriented β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ thin films is achieved through a β-Ga${}_2$O${}_3$/Al${}_2$O${}_3$ heterojunction by a feasible annealing process with an O${}_2$ atmosphere. During the annealing process, Al atoms of the Al${}_2$O${}_3$ substrate outdiffuse easily into the β-Ga${}_2$O${}_3$ thin film deposited by ozone-assisted molecular beam epitaxy (OMBE). The Al compositions in the β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ samples are tuned through adjusting the annealing temperature from 800 to 1300 °C and experimentally determined from the result of X-ray photoelectron spectroscopy (XPS) measurements combined with Vegard's law. Successive Al-composition-gradient β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ thin films with controlled bandgap are constructed. On these bases, β-Ga${}_2$O${}_3$ thin films are deposited on β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ (0 ≤ x ≤ 0.65) substrates through OMBE, yielding β-Ga${}_2$O${}_3$/β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ (0 ≤ x ≤ 0.65) heterojunction structures, and the band offsets of this heterojunction are determined by XPS accordingly. This methodology to achieve high-quality β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ thin films with adjustable Al composition and tunable band offsets of the β-Ga${}_2$O${}_3$/β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$ interface will provide guidance for potential strategies to develop and fabricate β-(Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$)${}_2$O${}_3$-based deep-UV photodetectors and power devices. (© 2021 Wiley‐VCH GmbH)