| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 106 === In this thesis, ZnGa2O4 thin films were prepared on c-plane sapphire substrates by RF sputtering. Due to its wide bandgap, ZnGa2O4 films can be used for fabricating the metal-semiconductor-metal (MSM) deep ultraviolet photodetectors (DUVPDs). The sputtered Ga2O3 films were used as the reference samples for comparison. By adjusting the process parameters such as substrate temperature, gas atmosphere for growth, annealed temperature and annealed atmosphere, the characteristics of ZnGa2O4 films and optoelectronic performances of DUVPDs can be optimized.
Firstly, ZnGa2O4 thin films were deposited in Ar atmosphere at various substrate temperatures (RT to 600 °C). Based on the x-ray diffraction (XRD) results, when the substrate temperature was higher than 400 °C, the crystal structure of ZnGa2O4 film was transformed from amorphous to crystalline. These ZnGa2O4 films deposited at various substrate temperatures were then used as the active layers to fabricate the PDs. Among these devices, the PD prepared with the 400 °C-grown ZnGa2O4 film possessed better optoelectronic performances. At a bias voltage of 5 V and 240-nm irradiation, this PD possessed a larger photocurrent of 5.69×10-8 A, a smaller dark current of 5.77×10-12 A, and a higher responsivity of 0.7 A/W.
More oxygen vacancies may be generated in the film when the pure argon atmosphere was used during the growth process. Subsequently, the oxygen gas was properly introduced in the growth process, and the effect of argon/oxygen ratio on the film’s properties was investigated. XRD analysis shows that the crystallinity of the film is lowered by introducing the oxygen gas. Additionally, after adding the oxygen gas in the growth process, the PDs with these films have both lower photocurrents and photoresponses in comparison to those with the film grown in pure argon atmosphere.
Based on the above results, the substrate temperature of 400 °C and the pure argon atmosphere are the optimum growth conditions for fabricating the ZnGa2O4 PD. To further improve the device characteristics, the thermal annealing treatment was performed on the as-deposited ZnGa2O4 film. After thermal annealing at suitable temperatures in air atmosphere, the film’s crystallinity can be improved. After fabricating the PD with the 700 C-annealed film, the device performances are enhanced, which has a photocurrent of 2.02×10-7A, a dark current of 5.35×10-12A, and a responsivity of 2.53 A/W (@5 V and 240 nm).
Finally, via the adjustment of annealing atmosphere, it can be found that the device performance is mainly affected by the oxygen atmosphere. When the film was annealed in vacuum, the content of oxygen vacancy was increased, leading to both increments in the photocurrent and dark current of fabricated PD. Moreover, as the pure oxygen atmosphere was used in the annealing process, the content of oxygen vacancy of the film was reduced, which resulted in an effective decrease in the dark current of fabricated PD. However, the photocurrent and responsivity of this device both reduced. When the annealing process was performed in air atmosphere, the fabricated PD had the optimum optoelectronic performances including the photo/dark current ratio and responsivity. Moreover, the spectral response peak showed a blue shift to 240 nm and higher responsivity data as compared with those of the Ga2O3 photo detectors.
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