| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 105 === This thesis describes the growth and characterization of (AlxGa1-x)2O3 thin films on c-plane sapphire substrates by pulsed laser deposition (PLD). The photodetectors based on these samples were fabricated to evaluate their deep ultraviolet response. It is well known that the performance of metal-semiconductor-metal (MSM) photodetectors based on wide bandgap semiconductors will be significantly affected by their material crystallinity. In this study, the crystallinity of (AlxGa1-x)2O3 films was optimized by adjusting the growth parameters such as gas atmosphere, repetition rate of pulsed laser, working pressure and substrate temperature.
Based on the x-ray diffraction (XRD) results, (AlxGa1-x)2O3 thin films prepared in O2 atmosphere showed single crystalline with three diffraction peaks of (-2 0 1), (-4 0 2), and (-6 0 3) planes. However, when the films were deposited in Ar atmosphere, they exhibited the amorphous structure. On the other hand, with increasing the repetition rate of pulsed laser from 1 to 10 Hz, the full width at half maximum value of XRD rocking curve at (-4 0 2) peak becomes smaller. Via the observations by transmission electron microscopy (TEM), the influence of working pressure on the film’s crystal structure can be demonstrated. It can be found that the surfaces of (AlxGa1-x)2O3 films prepared in high and low working pressures approached to (AlGa)2O3 and Ga2O3 phases, respectively. Additionally, for the substrate temperature higher than 700°C, the crystal structure of (AlxGa1-x)2O3 film was transformed from amorphous to crystalline.
Finally, the (AlxGa1-x)2O3 films deposited at substrate temperatures of 500-800 °C were selected to fabricate MSM photodetectors. Among these devices, the photodetector with the 800 ?C-grown (AlxGa1-x)2O3 film possessed better optoelectronic performances. When the wavelength of 240 nm and bias voltage of 5 V were applied, it exhibited a larger photocurrent of 7.39×10-8 A, a smaller dark current of 7.28 × 10-13 A, and a higher responsibility of 0.5 A/W. Moreover, the rise time and decay time of the photoresponse for this device were 3.28, and 0.08 seconds, respectively. It can be observed that the decay time is smaller than the rise time, revealing the recombination rate of the photogenerated electrons and holes is faster than the generation rate. These results indicate that the as-deposited (AlxGa1-x)2O3 films have high potential in deep-ultraviolet photodetector applications.
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