Preparation And Photoelectric Properties Of Ga₂O₃ Thin Films

Ga₂O₃（Gallium oxide） is a new type of semiconductor material. In particular, the β-Ga₂O₃, as a direct band gap oxide semiconductor material, has a wide band gap（4.5⁴.9 e V） at room temperature, responding to solar-blind ultraviolet wavelengths region（200-280 nm）, the characteristics of high melting point, thermal conductivity and critical breakdown voltage. Therefore, it has caused more and more attention in the field of semiconductor optoelectronic materials research, such as power electronic devices, light-emitting diodes, blind ultraviolet detector, and other fields has extensive application value.Presently, the materials for solar-blind detection are mainly based on wide bandgap semiconductors such as Al Ga N, Zn Mg O, diamond. However, for realizing solar-blind detection, Al Ga N needs high Al components, which will cause degradation of the crystallization quality. For Zn Mg O, a large amount of Mg doping deteriorates the quality of thin film as well. Diamond is expensive and unable to realize the whole solar-blind ultraviolet detection, because its band gap is too large to achieve the detection of the wavelength more than 220 nm. However, as a kind of binary oxide material, β-Ga₂O₃ with stable physico-chemical properties and simple preparation method has been considered to be one of the most ideal materials for solar-blind photodetector. In recent years, The nanowires, core-shell structures, nanobelts, and nano films based on the material of Ga₂O₃ are all reported to be used in preparation of deep uv detector. while, in many aspects, such as the stability, reliability and repeatability of process, the actual value of thin films detector is the highest.Preparation of β-Ga₂O₃ film has been reported by various methods such as molecular beam epitaxy（MBE）, pulsed laser deposition（PLD）, metal organic chemical vapor deposition（MOCVD） and Atom Layer Deposition（ALD）. While, β-Ga₂O₃ thin films prepared by Radio Frequency Sputtering were rarely reported. In this paper, β-Ga₂O₃ thin films were deposited by RF magnetron sputtering on different substrates with a range of substrate temperatures and annealed in a laboratory tube furnace with different temperatures. The influences of deposition and annealing temperature on the quality of the morphological characteristics, optical and electrical properties of the grown thin films were investigated, which proved that the c-plane sapphire is a better substrate for sputtering depositing β-Ga₂O₃ thin film. The β-Ga₂O₃（（2|—）01） plane is parallel to the c plane of the sapphire substrate [Al₂O₃（0001） plane].With the growth of substrate temperature, the amorphous phase film changed into β-Ga₂O₃, preferentially oriented along the {（2|—）01} family of crystal planes, grain size and roughness of surface of thin films increased, the optical band gap enlarged. As the film was deposited at 750℃, the visible light transmittance was over 90%, optical absorption edge was located at 260 nm, and the band gap was about 4.85 e V. In the study of film annealing treatment, a process of recrystallization occured at high temperature and significantly improved the crystal quality of thin films. An obvious blue shift of optical absorption edge occurred because of Al3+ diffusion from sapphire.The MSM structure based on β-Ga₂O₃ thin films was fabricarted by lift-off technology and electron beam evaporation. The interdigital Ti/Al electrode fingers were 15μm wide,500μm long and with distance of 15μm. Thin film annealed at 850℃ exhibited a rapid response to DUV light at 10 V bias, about 1 n A of the dark current and 800 n A of photocurrent. The maximum response of the device was 0.4A/W at 255 nm, 100 times as much as the responsivity at 290 nm. With the mercury lamps witched, the rising time and decay time were about 60 ms, 40 ms, respectively. Turning on/off mercury lamp every 10 s, photodetector still showed stable electro-optical properties after multiple loop switches.