| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系奈米科技碩博士班 === 104 === In this thesis, combining two dimensional material-based (Graphene or MoS2) with conventional Si-based photo sensors, photodiode or phototransistor, are demonstrated and well examined successfully. We provide low leakage (nA), high conversion capability of optical-to-electrical characteristic, directly photo voltage output in graphene/n-Si photodiode without peripheral complex circuit like CMOS image sensor, and ultrahigh current gain of 11,583 of graphene-based heterojunction transistor.
In the graphene-based photodiode, few-layer graphene with optical transmittance of 90% and high mobility benefits to be the conducting electrode, except forming the Schottky junction (optical detection region) between few-layer graphene and n-type Si. Therefore, the few-layer graphene plays an important role for multi-functions no matter what in electrical or optical performance. We demonstrate the high photoresponsivity of 95 mA/W under zero bias and directly conversion capability of current-to-voltage output by inserting 14 nm SiO2 between few-layer graphene and n-type Si simply to provide photo voltage of 90 mV under intensity of 189.5 Lux in this work.
In conventional bipolar junction transistor (BJT), the narrower base width is, the higher current gain becomes because of lower recombination possibility during carrier diffusion in neutral base region, in addition to device punch through issue. In this study, we provide the first two dimensional material (graphene and MoS2) heterojunction phototransistor to achieve ultrahigh current gain of 11,583 under low bias (current gain of ~200 in conventional BJT under 1 V). Critically, device punch through due to narrow base (~4 Å) can be depressed by arranging the energy band diagram among n++-Si (emitter)/graphene (base)/n-Si (collector) properly. According to the designed band diagram, the distribution of depletion region will locate at n++-Si (emitter) and n-Si (collector) because the interface between graphene and n++-Si and that between graphene and n-Si are Schottky junctions. Therefore, the device design concept benefits ultrahigh current gain without punching through. Moreover, the electrical properties of current gain and I-V current get agreement with measurement results.
In MoS2-based heterojunction transistor (phototransistor) case, the structure Au (emitter)/MoS2 (base)/n-Si (collector) indicate a Schottky junction and a p-n junction among Au-MoS2 and MoS2-n-type Si, respectively. According to the analysis of band diagram, the higher barrier height between Au and MoS2 profit to suppress device dark current. Additionally, we deposit gold film as thin as 20 nm which light can penetrate into both detection junctions, the interface between gold and MoS2 and that between MoS2 and n-Si, respectively, to achieve higher photo current output. Besides, photo generation electrons from interface between gold and MoS2 can move to MoS2 and raise the fermi level of MoS2 leading to lower barrier height. Owing to 4 nm of MoS2, electrons can quickly pass through MoS2 to n-Si resulting in current amplification. In this way, it can reach 5 orders ratio between dark current and photo current under illumination as low as 59 Lux without bias.
In this study, we demonstrate three kinds of photodetectors which are graphene-based photodiode, graphene-based heterojunction phototransistor (transistor), and MoS2-based heterojunction phototransistor (transistor), respectively. These devices can be provided to the demanded applications in electronics or optoelectronics.
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