Fabi Zhang

Wide bandgap engineering of (AlGa)2O3 films

Bandgap tunable (AlGa)2O3 films were deposited on sapphire substrates by pulsed laser deposition (PLD). The deposited films are of high transmittance as measured by spectrophotometer. The Al content in films is almost the same as that in targets. The measurement of bandgap energies by examining the onset of inelastic energy loss in core-level atomic spectra using X-ray photoelectron spectroscopy is proved to be valid for determining the bandgap of (AlGa)2O3 films as it is in good agreement with the bandgap values from transmittance spectra. The measured bandgap of (AlGa)2O3 films increases continuously with the Al content covering the whole Al content range from about 5 to 7 eV, indicating PLD is a promising growth technology for growing bandgap tunable (AlGa)2O3 films.

Observation of low voltage driven green emission from erbium doped Ga2O3 light-emitting devices

Erbium doped Ga2O3 thin films were deposited on Si substrate by pulsed laser deposition method. Bright green emission (∼548 nm) can be observed by naked eye from Ga2O3:Er/Si light-emitting devices (LEDs). The driven voltage of this LEDs is 6.2 V which is lower than that of ZnO:Er/Si or GaN:Er/Si devices. Since the wide bandgap of Ga2O3 contain more defect-related level which will enhance the effects of recombination between electrons in the defect-related level and the holes in the valence band, resulting in the improvement of the energy transfer to Er ions. We believe that this work paves the way for the development of Si-based green LEDs by using wide bandgap Ga2O3 as the host materials for Er3+ ions.

Temperature dependence of Raman scattering in β-(AlGa)2O3 thin films

We report a detailed investigation on temperature-dependent Raman scattering of β-(AlGa)2O3 thin films with different Al content (0-0.72) under the temperature range of 77-300 K. The temperature-dependent Raman shifts and linewidths of the phonon modes were obtained by employing Lorentz fitting. The linewidths broadening of phonon modes with the temperature can be well explained by a model involving the effects of thermal expansion, lattice-mismatch-induced strain, and decay of optical phonon into two and three phonons. It is clearly demonstrated dependence of the linewidths and decay process on the Al content in β-(AlGa)2O3 thin films, which can provide an experimental basis for realization of (AlGa)2O3-based optoelectronic device applications.

Toward controlling the carrier density of Si doped Ga2O3 films by pulsed laser deposition

Precise control of dopant composition is critical for the production of semiconductor films with desired properties. Here, we present results on the electrical properties for Si doped Ga2O3 films grown by pulsed laser deposition technique (PLD). The Si composition in the films can be controlled by changing the target composition as observed from the secondary ion mass spectroscopy measurement. The carrier density of the films is varied from the order of 1015 to 1020 cm−3 while the conductivity from 10−4 to 1 S cm−1 as measured by Hall equipment. The carrier density of the films has been verified by Kelvin force microscopy, which shows an increased surface work function with the increase of carrier density. The results suggest that the carrier density of β-Ga2O3 films is controllable by Si doping by PLD, paving a way to develop the Ga2O3 film-based electronic devices.

Electronic structure of β-Ga2O3 single crystals investigated by hard X-ray photoelectron spectroscopy

By combination of hard X-ray photoelectron spectroscopy (HAXPES) and first-principles band structure calculations, the electronic states of β-Ga2O3 were investigated to deepen the understanding of bulk information for this compound. The valence band spectra of HAXPES presented the main contribution from Ga 4sp, which are well represented by photoionization cross section weighted partial density of states. The experimental data complemented with the theoretical study yield a realistic picture of the electronic structure for β-Ga2O3.

Evolution of optical properties and band structure from amorphous to crystalline Ga2O3 films

The optical properties and band structure evolution from amorphous to crystalline Ga2O3 films was investigated in this work. Amorphous and crystalline Ga2O3 films were obtained by changing the growth substrate temperatures of pulsed laser deposition and the crystallinity increase with the rising of substrate temperature. The bandgap value and ultraviolet emission intensity of the films increase with the rising of crystallinity as observed by means of spectrophotometer and cathodoluminescence spectroscopy. Abrupt bandgap value and CL emission variations were observed when amorphous to crystalline transition took place. X-ray photoelectron spectroscopy core level spectra reveal that more oxygen vacancies and disorders exist in amorphous Ga2O3 film grown at lower substrate temperature. The valence band spectra of hard X-ray photoelectron spectroscopy present the main contribution from Ga 4sp for crystalline film deposited at substrate temperature of 500 oC, while extra subgap states has been observed in amor...

Ultraviolet detectors based on (GaIn)2O3 films

We demonstrated a (GaIn)2O3 films based UV photodetector with a planar photoconductor structure, and investigated the photoresponse properties of the fabricated devices. The (GaIn)2O3 films are of ohmic contact with a Au/Ti electrode. The fabricated photodetectors show relatively high photoresponsivity of more than 0.1 A/W. The turn-on wavelength of the photodetectors varied from 285 to 315 nm with the increase of the indium content from 0.25 to 0.49. The properties of the films were also further investigated. The films are of a (−201) oriented monoclinic phase with a high transmittance of more than 90% in the visible region and smooth surfaces without phase separation. The absorption edges of the films shift toward a longer UV wavelength region with the increase of indium content. The above results suggest that wavelength selective UV detectors can be realized based on these films.

Characterization of epitaxial ZnTe layers grown on GaAs substrates by transmission electron microscopy and photoluminescence

The authors have investigated the effects of a low-temperature ZnTe buffer layer on structural and optical properties of ZnTe epilayers grown on (100) GaAs substrates by transmission electron microscopy and photoluminescence measurements. It has been found that the low-temperature ZnTe buffer layer can reduce the defects such as disordered region in the ZnTe buffer layer and suppress the dislocations at the ZnTe surface region, resulting in the formation of a high quality ZnTe epilayer grown on the low-temperature buffer layer.