Search

We investigate the continuous annealing of orthorhombic κ-Ga2O3 films on AlN/c-plane sapphire templates grown by Mist Chemical Vapor Deposition (mist-CVD) using in situ high-temperature x-ray diffraction (HT-XRD). Increasing the annealing temperature from 400 to 1100 °C in both vacuum and ambient air reveals a phase transition onset at 825 °C. High-resolution transmission electron microscopy and XRD demonstrated that annealing within the stability window of 650 to 775 °C effectively improves the crystal quality of the κ-Ga2O3 thin film. Optical transmittance and low-loss electron energy loss spectroscopy (EELS) show the pristine film’s bandgaps to be 4.96 and 4.67 eV, respectively, with reduced bandgaps in annealed films due to increased defect density. EELS-derived optical joint density of states indicates that air-annealing fosters sub-bandgap radiative processes, while vacuum annealing suppresses them, qualitatively correlated with the observed photoluminescence intensity variations. The results of this comprehensive high-temperature annealing study offer crucial insight into the influence of annealing ambient conditions on the crystallographic properties of κ-Ga2O3 films and the associated evolution of extended sub-bandgap states.

This study reveals the significant role of the pre-melting process in growing high-quality (100) β-Ga2O3 single crystals from 4N powder (99.995% purity) using the edge-defined film-fed growth (EFG) method. Among various bulk melt growth methods, the EFG method boasts a fast growth rate and the capability of growing multiple crystals simultaneously, thus offering high productivity. The pre-melting process notably enhanced the structural, optical, and electrical properties of the crystals by effectively eliminating impurities such as Si and Fe. Specifically, employing a 100% CO2 atmosphere during pre-melting proved to be highly effective, reducing impurity concentrations and carrier scattering, which resulted in a decreased carrier concentration and an increased electron mobility in the grown Ga2O3 single crystals. These results demonstrate that pre-melting is a crucial technique for substantially improving crystal quality, thereby promising better performance in β-Ga2O3-based device applications.

Double buffer layers composed of (AlxGa1−x)2O3/Ga2O3 structures were employed to grow a Sn-doped α-Ga2O3 epitaxial thin film on a sapphire substrate using mist chemical vapor deposition. The insertion of double buffer layers improved the crystal quality of the upper-grown Sn-doped α-Ga2O3 thin films by blocking dislocation generated by the substrates. Rapid thermal annealing was conducted for the double buffer layers at phase transition temperatures of 700–800 °C. The slight mixing of κ and β phases further improved the crystallinity of the grown Sn-Ga2O3 thin film through local lateral overgrowth. The electron mobility of the Sn-Ga2O3 thin films was also significantly improved due to the smoothened interface and the diffusion of Al. Therefore, rapid thermal annealing with the double buffer layer proved advantageous in achieving strong electrical properties for Ga2O3 semiconductor devices within a shorter processing time.

In this study, we explored the effect of an active afterheater on the growth of gallium oxide single crystals using the EFG method. We analyzed the temperature distribution of the crystal under the growing process through multiphysics simulations of the models with and without an active afterheater and investigated the morphology of crystals by applying each model to real experimental growths. The afterheater is a component in the growing furnace that activates radiant heat transfer, and its performance depends on its location, size, material, and shape. The simulation results showed that the afterheater applied in this study was found to be effective in obtaining good temperature distribution in the reactor. Through experimental crystal growth corresponding to the simulation approaches, it was confirmed that an appropriate afterheater reduces thermal stress at the growth front and provides a thermal annealing effect on the post-grown crystals during the growing process to improve crystal quality.

Bi2O2Se has an intrinsically low thermal conductivity and a high Seebeck coefficient, which suggests as potential thermoelectric materials. However, pristine Bi2O2Se polycrystalline exhibits very low electrical conductivity. In this study, we investigated the influence of I doping in enhancing electrical conductivity of Bi2O2Se by synthesizing a series of Bi2O2Se1-xIx polycrystalline samples (x = 0, 0.0025, 0.005, and 0.0075). By I doping, the electrical conductivity was enhanced by significant increase in carrier concentration. The carrier concentration was increased more than 10 times 2.1–4.6 × 1019 cm-3 from 2.1 × 1018 of the undoped sample. As results, the maximum power factor was enhanced to 0.24 mW/mK2 for x = 0.0025 from 0.086 mW/mK2 of undoped Bi2O2Se at 300 K. The maximum power factor of 0.42 mW/mK2 was achieved for x = 0.05 sample at 790 K. These increases are discussed in terms of the increase in effective mass and weighted mobility. Additionally, the moderate reduction of thermal conductivity was observed in the I doped samples due to the point defect scattering. As a result, a maximum zT value was enhanced to 0.32, 0.35, and 0.33 for x = 0.0025, 0.005, and 0.0075) at 790 K, representing an improvement of more than 50% compared to 0.21 of the undoped Bi2O2Se (x = 0).

In this paper, we demonstrate a scalable process for the precise position-controlled selective growth of GaN nanowire arrays by metalorganic chemical vapor deposition (MOCVD) using a pulsed-mode growth technique. The location, orientation, length, and diameter of each GaN nanowire are controlled via pulsed-mode growth parameters such as growth temperature and precursor injection and interruption durations. The diameter and length of each GaN nanowire are in the ranges of more than 240 nm and 250-1250 nm, respectively, with different vertical-to-lateral aspect ratios that depend on the growth temperature. Also, it is found that a higher growth temperature helps increase the vertical growth rate and reduces the lateral growth rate of GaN nanowire arrays. Furthermore, in the case of longer TMGa injection duration, the Ga-rich region allows the higher lateral growth rate of GaN nanostructures, which leads to a transition in the morphology from nanowires to a thin film, while in the case of longer NH3 injection duration, the surface morphology changes from nanowires to pyramidal structures. In addition, the surface structure can also be controlled by varying the precursor interruption duration. Finally, we report and discuss a growth model for GaN nanowire arrays under pulsed-mode MOCVD growth. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the influence of parameters such as type of carrier gas and NH3/HCl flow ratio on the growth of vertical GaN microstructures by selective-area growth (SAG) hydride vapor phase epitaxy (HVPE). On various strain-induced templates such as GaN/sapphire, GaN/Si, and AlN/Si, regular arrays of Ga-polar GaN microrods were properly achieved by adjusting the growth parameters. The photoluminescence and micro-Raman measurements reveal not only the crystal quality of the GaN microrods but also strain distribution. These results will give insight into the control of the morphology of GaN microrods in terms of the strain induced from templates in SAG-HVPE. The precisely controlled arrays of GaN microrods can be used for next-generation light-emitting diodes (LEDs) by realizing InGaN/GaN multi–quantum wells (MQWs) with a radial structure. [ABSTRACT FROM AUTHOR]

We report on the material and optical properties of core–shell InGaN layers grown on GaN nanorod arrays. The core–shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core–shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer. [ABSTRACT FROM AUTHOR]

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances. [ABSTRACT FROM AUTHOR]