arXiv subjects

Kuang-Hui Li

Publications and source records attributed to Kuang-Hui Li.

7 recordsLinked to original sources

arXiv2020

Epitaxial Growth of $β$-Ga$_2$O$_3$ Coated Wide Bandgap Semiconductor Tape for Flexible UV Photodetector

The epitaxial growth of technically-important $β$-Ga$_2$O$_3$ semiconductor thin films have not been realized on flexible substrates due to limitations by the high-temperature crystallization conditions and the lattice-matching requirements. In this report, for the first time single crystal $β$-Ga$_2$O$_3$(-201) thin films is epitaxially grown on the flexible CeO2 (001)-buffered hastelloy tape. The results indicate that CeO$_2$ (001) has a small bi-axial lattice mismatch with $β$-Ga$_2$O$_3$ (-201), thus inducing a simultaneous double-domain epitaxial growth. Flexible photodetectors are fabricated based on the epitaxial $β$-Ga$_2$O$_3$ coated tapes. Measurements show that the obtained photodetectors have a responsivity of 40 mA/W, with an on/off ratio reaching 1000 under 250 nm incident light and 5 V bias voltage. Such photoelectrical performance is already within the mainstream level of the $β$-Ga$_2$O$_3$ based photodetectors by using the conventional rigid single crystal substrates; and more importantly remained robust against more than 1000 cycles of bending tests. In addition, the epitaxy technique described in the report also paves the way for the fabrication of a wide range of flexible epitaxial film devices that utilize the materials with lattice parameters similar to $β$-Ga$_2$O$_3$, including GaN, AlN and SiC.

physics.app-ph↗

arXiv2020

\b{eta}-phase (AlxGa1-x)2O3 thin film with Al composition more than 70%

In this work, we have demonstrated wide-composition-range \b{eta}-(AlxGa1-x)2O3 thin films with record-high Al compositions up to 77% for \b{eta}-(AlxGa1-x)2O3 covering bandgaps from 4.9 to 6.4 eV. With optimized thermal annealing conditions, the \b{eta}-Ga2O3 binary thin films on sapphire substrates transformed to the \b{eta}-(AlGa)2O3 ternary thin films with different compositions. The binary to ternary transformation resulted from the Al atom diffusion from sapphire into the oxide layers; meanwhile, the Ga atoms diffused into sapphire leading to thicker thin films than the original thicknesses. The interdiffusion processes were confirmed by transmission electron microscopy, which enhanced in proportion to the annealing temperature. The strain states of the \b{eta}-(AlGa)2O3 films have been analyzed showing reduced in-plane compressive strain with higher annealing temperature; and the film eventually became strain-free when the temperature was 1400 oC corresponding to the Al composition of 77%. The proposed method is promising for the preparation of the \b{eta}-(AlGa)2O3 thin films without employing sophisticated direct-growth techniques for alloys.

physics.app-ph↗

arXiv2020

Optical properties of BAlN and BGaN for applications in latticematched UV optical structures

The optical properties of BAlN and BGaN ternary alloys are investigated using first-principle calculation. Hybrid density functional theory is applied to determine the refractive indices of different alloys. A peculiar non-linear behavior of the static refractive index as a function of boron composition is found. The results of this calculation are interpolated to generate a three dimensional dataset, which could be used for designing a myriad of strained and strain-free optoelectronic and photonic devices. This is then used to find a lattice-matched heterostructure optimized for DBR applications (B0.108Ga0.892N/AlN). A DBR design with 25 pairs at a wavelength of 375 nm is found to have peak reflectivity of 99.8% and a bandwidth of 26 nm.

cond-mat.mtrl-sci↗

arXiv2019

Improvement of temperature uniformity of induction-heated T-shape susceptor for high-temperature MOVPE

The induction heating is a common method applied in metalorganic vapor phase epitaxy (MOVPE) especially for higher-temperature growth conditions. However, compared to the susceptor heated by the multiple-zone resistant heater, the inductive-heated susceptor could suffer from severe thermal non-uniformity issue. In this simulation study, we propose to employ a T-shape susceptor design with various geometric modifications to significantly improve the substrate temperature uniformity by manipulating thermal transfer. Specifically, the thermal profile can be tailored by horizontal expansion and vertical elongation of the susceptor, or forming a cylindrical hollow structure at the susceptor bottom cylinder. Three optimized designs are shown with different temperature uniformity as well as various induction heating efficiencies. The temperature variation of the entire substrate surface can be less than 5 °C at ~1900 °C with high induction heating efficiency after applying the proposed techniques.

physics.app-ph↗

arXiv2019

Conduction and valence band offsets of Ga2O3/h-BN heterojunction

h-BN and Ga2O3 are two promising semiconductor materials. However, the band alignment of the Ga2O3/h-BN heterojunction has not been identified, hindering device development. In this study, the heterojunction was prepared by metalorganic chemical vapor deposition and pulsed laser deposition. Transmission electron microscopy confirmed sharp heterointerface and revealed structural evolution as amorphous-Ga2O3 grew thicker on lattice mismatched h-BN. The valence and conduction band offsets were determined by high-resolution X-ray photoemission spectroscopy to be 1.75 and 3.35-3.65 eV, respectively, corresponding to a type-II heterojunction. The extremely large type-II band offsets along with indirect bandgap of Ga2O3 may be leveraged for exceptional electron confinement and storage.

physics.app-ph↗

arXiv2019

Determination of band offsets of Ga2O3/FTO heterojunction for current spreading for high temperature and UV applications

Because of relatively low electron mobility of Ga2O3, it is important to identify proper current spreading materials. Fluorine-doped SnO2 (FTO) offers superior properties to those of indium tin oxide (ITO) including higher thermal stability, larger bandgap, and lower cost. However, the Ga2O3/FTO heterojunction including the important band offset and the I-V characteristics have not been reported. In this work, we have grown the Ga2O3/FTO heterojunction and performed X-ray photoelectron spectroscopy (XPS) measurement. The conduction and valence band offsets were determined to be 0.11 and 0.42 eV, indicating a minor barrier for electron transport and type-I characteristics. The subsequent I-V measurement of the Ga2O3/FTO heterojunction exhibited ohmic behavior. The results of this work manifests excellent candidacy of FTO for current spreading layers of Ga2O3 devices for high temperature and UV applications.

physics.app-ph↗

arXiv2018

High-Temperature Photocurrent Mechanism of \b{eta}-Ga2O3 Based MSM Solar-Blind Photodetectors

High-temperature operation of metal-semiconductor-metal (MSM) UV photodetectors fabricated on pulsed laser deposited \b{eta}-Ga2O3 thin films has been investigated. These photodetectors were operated up to 250 °C temperature under 255 nm illumination. The photo current to dark current (PDCR) ratio of about 7100 was observed at room temperature (RT) while it had a value 2.3 at 250 °C at 10 V applied bias. A decline in photocurrent was observed from RT to 150 °C and then it increased with temperature up to 250 °C. The suppression of the blue band was also observed from 150 °C temperature which indicated that self-trapped holes in Ga2O3 became unstable. Temperature-dependent rise and decay times of carriers were analyzed to understand the photocurrent mechanism and persistence photocurrent at high temperatures. Coupled electron-phonon interaction with holes was found to influence the photoresponse in the devices. The obtained results are encouraging and significant for high-temperature applications of \b{eta}-Ga2O3 MSM deep UV photodetectors.

physics.app-ph↗