Kappa-phase gallium oxide (κ-Ga2O3) is an emerging piezoelectric semiconductor with potential applications in radio-frequency devices. However, heteroepitaxial growth of κ-Ga2O3 on silicon substrates remains challenging owing to large lattice mismatch and interfacial oxidation. This study demonstrates the growth of polycrystalline κ-Ga2O3 thin films deposited by mist chemical vapor deposition (mist CVD) on Si(100) substrates using Ga-doped ZnO (GZO) buffer layers. Structural characterization via X-ray diffraction, scanning electron microscopy, and transmission electron microscopy reveals that κ-Ga2O3 films exhibit a c-axis oriented polycrystalline structure with random in-plane orientations, which yields isotropic properties that are advantageous for device applications. A ZnGa2O4 intermediate layer is identified at the κ-Ga2O3/GZO interface, which plays a critical role in phase stabilization. These findings indicate that polycrystalline κ-Ga2O3 on Si substrates represents a promising platform for piezoelectric semiconductor devices.

Towards the 6th generation mobile communications system (6G), it is considered to expand a frequency band higher than that for 5G, 95 GHz–3 THz, and electromagnetic waves up to approximately 300 GHz are under consideration[1]. Therefore, developing and realizing devices that can operate in these frequency bands is an urgent issue. It is essential to elucidate the dielectric, magnetic, and electrical properties of semiconductors and other electronic materials, which constitute the development and manufacturing processes, in these frequency bands. It is said that 6G will be in operation in 2030, the demand for material property measurement systems for the frequency range from the high-frequency band of millimeter waves to the THz wave region (300 GHz up to 3 THz) is expected to significantly increase in the next decade. The authors are focusing particularly on dilute bismide (Bi) III-V compound semiconductors grown at low temperatures as a candidate material for photoconductive antenna (PCA) that is a device often used for generating and detecting THz wave in these frequency bands as a drive source of ultrashort laser pulses in the THz time-domain spectroscopy system.

We have structurally evaluated GaAs1_xBix obtained by solid-phase epitaxial growth of amorphous GaAs1_ xBix thin films via transmission electron microscopy and related techniques. Amorphous GaAs1_xBix layers are obtained using a molecular beam epitaxy apparatus at 180 degrees C. When the amorphous GaAs1_ xBix layer is annealed for 1 h at a medium temperature of 350 degrees C, an epitaxial GaAs1_xBix layer is grown on the GaAs substrate, forming a thick polycrystalline layer between the region above the epi-layer and the GaAs cap layer. In addition, three types of defects are generated in the region above the interface between the epi-layer and the substrate: small As precipitates (approximately 5 nm in diameter) and large zincblende GaAs1_ xBix and rhombohedral Bi precipitates (10-15 nm in diameter). On the other hand, we found that a precipitate-free GaAs1_ xBix epitaxial layer is grown after annealing for 1 h at 600 degrees C, forming only a small amount of zincblende GaAs1_xBix and rhombohedral Bi precipitates (no As precipitates) in the region above the epi/sub interface. Furthermore, the difference between the above results and in the case of low-temperature-grown molecular beam epitaxy GaAsBi crystals after annealing is discussed. We also observed varying features and distributions of the defects in LTG GaAs1_ xBix layers (Tg = 250 degrees C) after annealing for 20 min at 600 degrees C from those of the same in a previous study: 1) rh-As precipitates are generated only at the epi/sub-interface; 2) Type-A (V-shaped) and Type-B (?-shaped) rhBi precipitates are generated in the upper and lower regions of the GaAs1_ xBix layer, respectively. Based on these results, we discussed the origins of the above differences and factors affecting the generation of these precipitates.

We have characterized defects in κ -(In x Ga 1– x ) 2 O 3 thin films grown on (001) FZ-grown ε -GaFeO 3 substrates by mist CVD using TEM. We found two types of defects: dislocation half-loops and microdefects. The half-loops are U-shaped and lie on the (100) plane. From contrast experiment, their Burgers vector was determined to be parallel to 〈010〉. While the microdefects were observed just above the interface between the κ -(In x Ga 1– x ) 2 O 3 film and the ε -GaFeO 3 substrate. They are 5–15 nm in size and accompany strong strain field. From (010) high-resolution transmission electron microscopic observation, it has been found that they are planar defects lying on the (001) plane. From these results, generation mechanisms of these defects are discussed.

Ga2O3 has attracted significant attention for various applications such as power-switching applications and deep-ultraviolet optoelectronics. In this study, we demonstrated a lattice-matching κ-(In1−xGax)2O3 thin film grown on an ε-GaFeO3 substrate via the mist chemical vapor deposition process. The X-ray diffraction peak of the thin film was almost coincident with that of the substrate and exhibited Laue oscillations. Atomic force microscopy revealed that the surface of the κ-(In1−xGax)2O3 thin film exhibited a step-terrace morphology and was atomically flat. The selected area electron diffraction of transmission electron microscopy showed that the diffraction spots of the thin film and substrate overlapped, indicating that the thin film was almost lattice-matched with the substrate. We believe that the lattice-matched κ-(In1−xGax)2O3 thin film with an ε-GaFeO3 substrate will contribute significantly to the demonstration of ferroelectric κ-Ga2O3 based high electron mobility transistors.

•GaAs1-xBix was successfully grown by SPE at 600 °C.•GaAs1-xBix epitaxial and poly-crystalline layers were formed by SPE at 350 °C.•As-precipitates, GaAsBi- and Bi-precipitates were generated by SPE at 350 °C.•SPE at 600 °C formed an epitaxial layer, generating GaAsBi- and Bi-precipitates.•Novel features of precipitates were obtained in annealed LTG-GaAsBi crystals.

We demonstrated the growth of a single-domain κ-Ga2O3 thin film on ε-GaFeO3 by using an organic-free compound as a precursor for mist chemical vapor deposition. X-ray diffraction analysis revealed that an 86 nm thick κ-Ga2O3 thin film was grown almost coherently with slight lattice relaxation. The surface morphology of the κ-Ga2O3 thin film exhibited a step-terrace structure without island growth. Furthermore, plan-view TEM observations revealed that the κ-Ga2O3 thin film grown on ε-GaFeO3 had a single domain, whereas the previously reported κ-Ga2O3 thin film grown on AlN template had a domain structure.

This paper reviews the status of characterization of defects in β -Ga 2 O 3 crystals grown by edge-defined film-fed growth and hydride vapor phase epitaxy using chemical etching, scanning electron microscopy, focused ion beam scanning ion microscopy, X-ray topography (XRT), and transmission electron microscopy (TEM). The observed defects are classified into four types: dislocations, stacking faults (SFs), twins, and plate-like nanovoids (PNVs). First, we present the detailed characterization of dislocations in the crystal by chemical etching, XRT, and TEM, and discuss possible slip systems. Next, we describe XRT analyses of two types of SFs: SFs 1 lying on the ( 2 ¯ 01 ) plane and SFs 2 on the (111) and ( 1 1 ¯ 1 ) planes. We describe the results for twins found in crystals via high-resolution TEM and electron diffraction analysis, and PNVs corresponding to etch pits on the (010) plane. Finally, we discuss possible generation mechanisms of the defects and their influence on device characteristics.

In this chapter, we describe defects generated during various thin-film crystal growth techniques: liquid phase epitaxy, metalorganic vapor phase epitaxy, and molecular beam epitaxy. The defects are classified into two types: interface defects and bulk defects. We also describe two major materials issues due to thermal instability of alloy semiconductors during growth: composition modulation and atomic ordering. We compare these defects and thermal instability in III–Vs and III-nitrides and discuss their generation mechanisms. Based on these results, the influence of the defects and thermally unstable structures on the reliability of lasers and methods for their suppression are also presented.

•Segregation of As precipitates at epi/sub interface in undoped LTG-InGaAs.•Absence of precipitates in Be-doped LTG-InGaAs after annealing.•Absence of precipitates at dislocation core in LTG-InGaAs after annealing.•Detection of surface defects due to evaporation of As atomsduring annealing.•Physical origin for absence of precipitates in Be-doped InGaAs after annealing.

Through transmission electron microscopy and related techniques, we performed structural evaluation of low temperature grown (LTG) InGaAs crystals on (001) InP substrates. These crystals were grown at a substrate temperature of 200-250 degrees C. The results revealed that As precipitates (diameter: 7-15 nm) are generated in undoped InGaAs crystals (mainly near the interface between the epi-layer and the substrate) after annealing at 550 degrees C for 1 h. However, no precipitates were observed in Be-doped LTG InGaAs crystals before and after annealing. Moreover, in the case of Be-doped LTG-InGaAs crystals highly mismatched with the InP substrate, we observed no preferential generation of precipitates at the core of the misfit dislocations. Based on these findings, the generation mechanism of the precipitates in undoped LTG-InGaAs crystals during the annealing process and physical origin for the absence of precipitates in Be-doped LTG-InGaAs crystals after the annealing are discussed. Apart from the precipitates, we have also found that surface defects elongated in the [110] direction are formed in the InGaAs crystal, which are presumably generated by the evaporation of As-atoms from the surface during annealing.

Herein, single-domain κ-Ga2O3 thin films were grown on FZ-grown ε-GaFeO3 substrates via a step-flow growth mode. The ε-GaFeO3 possessing the same crystal structure and similar lattice parameters as those of the orthorhombic κ-Ga2O3 facilitated the growth of κ-Ga2O3 thin films, as observed by the X-ray diffraction (XRD) analysis. Furthermore, the surface morphologies of the κ-Ga2O3 thin films exhibited a step-terrace and atomically flat structure. XRD φ-scan and transmission electron microscopy with selected area electron diffraction revealed that there is no occurrence of in-plane rotational domains in the κ-Ga2O3 thin films on ε-GaFeO3 substrates and that the κ-Ga2O3 thin film comprised a single domain. TEM analysis revealed that there were no clear dislocations in the observation area. Moreover, high-resolution TEM observation showed that the atomic arrangements of the film and the substrate were continuous without the presence of an intermediate layer along the growth direction and were well-aligned in the in-plane direction.

We report on the crystalline quality of low-temperature-grown (LTG) InxGa1−xAs coherently grown on InP(0 0 1) substrate using molecular beam epitaxy at a substrate temperature of 250 °C. The InxGa1−xAs was characterized using high-resolution X-ray diffraction (XRD) measurement, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry (RBS). Results of 2θ/ω scan and Q-scan XRD revealed the same in-plane lattice constants for the as-grown LTG-In0.54Ga0.46As layer and the InP substrate. The results of the Q-scan indicated that the periodicity of lateral correlation length of LTG-In0.54Ga0.46As layer, which is defined by the inverse of the full width at half maximum of horizontal scan of Q-scan XRD curves, was down to around 700 nm from over 1.5 μm after annealing at 550 °C. However, in cross-sectional TEM observation, no misfit dislocations were observed in either the as-grown or annealed samples. This revealed that relatively low density of dislocations formed in both the as-grown and the annealed LTG-In0.54Ga0.46As layers. Angular RBS scans of the In signals in the as-grown and annealed LTG In0.54Ga0.46As aligned toward [1 0 0] and [1 1 0] observed deep dip curves, indicating that thermal annealing did not cause the crystalline quality of LTG In0.54Ga0.46As to noticeably deteriorate.

Here, we investigate the dislocations in beta-Ga2O3 single crystals grown by edge-defined film-fed growth (EFG) and halide vapor-phase epitaxy (HVPE) using synchrotron X-ray topography. The (001)- and ((2) over bar 01)-oriented crystals grown in the [010] direction by EFG exhibited dislocations along the < 010 > direction with some dislocations oriented in a line; in addition, wandering dislocations were observed on the (001) surface. Based on the invisibility criterion, the Burgers vector of some dislocations was determined to be < 010 >. On the other hand, in the (001) film grown by HVPE over the EFG substrate, threading dislocations propagating in the [001] direction were observed. Furthermore, it was found that the dislocations on the substrate grown by EFG were inherited by the film formed by HVPE: a dislocation was generated in the film grown by HVPE at both ends of the void defects in the substrate grown by EFG. (C) 2019 The Japan Society of Applied Physics

The electrical properties of Schottky barrier diodes (SBDs) on a (001) beta-Ga2O3 substrate were characterized and correlated with wet etchingrevealed crystal defects below the corresponding Schottky contacts. The etching process revealed etched grooves and etched pits, indicating the presence of line-shaped voids and small defects near the surface, respectively. The electrical properties (i.e., leakage currents, ideality factor, and barrier height) exhibited almost no correlation with the density of the line-shaped voids. This very weak correlation was reasonable considering the parallel positional relation between the line-shaped voids extending along the [010] direction and the (001) basal plane in which the voids are rarely exposed on the initial surface in contact with the Schottky metals. The distribution of small defects and SBDs with unusually large leakage currents showed similar patterns on the substrate, suggesting that these defects were responsible for the onset of fatal leak paths. These results will encourage studies on crystal defect management of (001) beta-Ga2O3 substrates for the fabrication of devices with enhanced performance using these substrates. (C) 2017 The Japan Society of Applied Physics