Preparation And Characterization Of Gallium Oxide Thin Films

As a novel semiconductor material,gallium oxide（Ga₂O₃）is widely used in various power devices due to its good thermal stability,stable chemical property and high breakdown voltage.Meanwhile,it has high transmittance to visible light with a band gap of 4.5 eV-4.9eV.Compared with other materials such as AlGaN and ZnMgO,the deposition of Ga₂O₃ is more simple which results in a good application prospect in solar cells and photodetectors.In this thesis,gallium oxide thin films with a thickness of 164 nm were deposited on c-plane sapphire（Al₂O₃）substrates by magnetron sputtering.The samples were post annealed at different temperatures in different atmospheres using a tube furnace.Afterwards,the samples were analyzed by X-ray diffraction（XRD）,atomic force microscopy（AFM）,ultraviolet-visible spectroscopy（UV-Vis）to study the effects of post annealing temperature（PAT）on crystal structure,surface morphology and optical properties of Ga₂O₃ thin films.Ti/Ni（30nm/80 nm）was deposited on the samples by electron beam evaporation,and the Ti/Ni interdigital electrodes were fabricated into photodetectors with a structure of metal-semiconductor-metal（MSM）by lift-off process.The ultraviolet photoelectric properties of the photodetectors were investigated.From the perspective of device structure,this thesis studied the scaling characteristics of the devices by changing the size of the devices.The research content can be divided into the following sections:（1）Ga₂O₃ films were deposited by magnetron sputtering at 400℃,500℃and 600℃respectively.According to XRD,higher diffraction peak intensities and better crystal qualities of the Ga₂O₃ thin films were obtained with the increase of deposition temperature.（2）The Ga₂O₃ films were post annealed at 800℃,900℃,1000℃and 1100℃in air and nitrogen atmospheres,respectively.The properties of the films were characterized.The results show that the samples annealing in different atmospheres have no obvious changes in the crystal orientation.All of them are polycrystalline Ga₂O₃ thin films withβphase.With the increase of PAT,the grain size of the thin films gets lager with the increase of PAT,but it gets smaller with further increase of PAT.The cut-off wavelength of the film is aroundλ=260nm with a PAT of 1000℃.Samples annealed in air atmosphere show better crystal qualities.As the PAT increase,the optical band gap of the film increases from 4.65 eV to 5.10 eV,and the optical absorption edge are blue shifted significantly.（3）Based on the Ga₂O₃ thin films annealed in air atmosphere,MSM type solar blind UV photodetectors were fabricated.The effects of PAT on the photoelectric properties of Ga₂O₃thin films were investigated.The intensities of the photodetectors’response to ultraviolet light are blue shifted from 267 nm to 235 nm,and the full width at half maximum（FWHM）of the spectral response peak decreases along with the increase of PAT,indicating that the annealing treatment improves the selectivity of the devices to ultraviolet light.The photocurrent and dark current of the samples after annealing are significantly increased with a PAT below 1100℃.The annealed samples possess larger photo to dark current ratio（PDCR）and faster response time than the unannealed samples.When PAT is 1000℃,the PDCR of the devices are larger than other samples.The phase of response peak of ultraviolet light isλ=247 nm.All of the results show that the annealed films have good photoelectric properties.Thus,the experiment provides an effective and convenient method for preparing wide band gap deep ultraviolet materials.（4）In this experiment,different sizes of solar-blind UV photodetectors were fabricated on same substrate by changing the width of the interdigital fingers and scaling the dimensions of the devices under same fabrication process.The current response,spectral response,and time response of the photodetectors were studied.The results show that the change in device size does not affect the UV response range of the devices.The phases of spectral response peaks of the devices are very close.Under the condition that the light receiving area is constant,the denser the fingers,the greater the light response of the devices.When the device is scaled down,the photocurrent,PDCR and the response time does not change significantly.