氫化非晶矽p-i-n光感測器之沉積製程對光電特性影響研究

Abstract

本論文研究氫化非晶矽(a-Si:H) p-i-n光感測器結構，它應用於醫療用數位X光影像系統，在醫療影像應用方面，必須同時面對影像品質，及如何使病人承受最低輻射劑量的兩個重大問題。因為影像必須能夠準確地把疾病或不正常的地方顯示出來，同時病人所承受的輻射劑量也是愈低愈好。因此在醫療影像方面，氫化非晶矽p-i-n光二極體就必須有很低的暗電流、高的光靈敏度，本研究利用電漿輔助化學汽相沉積系統 (plasma enhanced chemical vapor deposition , PECVD) 之方式在玻璃基板上沉積氫化非晶矽p-i-n層，分析光、電特性，藉由調整H2/SiH4氣體流量比，以提高本質層intrinsic layer 非晶矽薄膜(a-Si:H)品質，良好電性品質的氫化非晶矽薄膜要求，其暗電導需小於1x10-10 (Ω-cm)-1，光電導需大於1x10-5 (Ω-cm)-1，光靈敏度大於1x105，氫含量(CH)原子百分比在9~11%，微結構因子(R*)小於0.1，即氫化非晶矽薄膜的矽氫鍵結以單矽氫鍵結Si(H)為主，薄膜較為緻密，並具有良好的電性品質，此研究以H2/SiH4氣體流量比例為10沉積的氫化非晶矽薄膜，可得到暗電導1.2x10-11 (Ω-cm)-1，光電導為2.0x10-5 (Ω-cm)-1，其光靈敏度可達2.1x106，FTIR量測得到，氫含量(CH)為10.2 at%，微結構因子(R*)為0.19。 在n+層及p+層的研究中，我們藉由調整PH3/SiH4及TMB/SiH4的氣體流量比例，改善其光電特性，增加氣體流量比例、增加摻雜濃度，進而降低阻值，以提升內建電場，可得到較佳的光電流8 x10-9 A及明暗電流比為3922，並藉由降低p-i-n 元件p層+厚度，使大部份的光能進入到本質層(intrinsic layer)以提升元件明暗電流比，可得到最佳明暗電流比為6210。

In this study, we fabricated hydrogenated amorphous silicon (a-Si:H) p-i-n photodiodes based x-ray detectors for digitized medical imaging applications. We investigated the influence of process conditions of the p-i-n photodiodes on the photovoltaic properties of the detectors. The x-ray image contrast and the least exposure of patients to x-ray are mostly concerned for medical imaging applications. Therefore, the a-Si:H p-i-n photodiodes must be very sensitive to x-ray with a very low leakage current. We fabricated the a-Si:H p-i-n photodiode on glass substrates by plasma-enhanced chemical vapor deposition (PECVD). By optimizing the H2/SiH4 gas ratio, we significantly improved the film quality of the intrinsic (i) a-Si:H layer. An i-a-Si:H thin film with desired electrical characteristics requires a dark conductivity smaller than 1x10-10(Ω-cm)-1, a photoconductivity greater than 1x10-5 (Ω-cm)-1, a photosensitivity greater than 1x105. In order to obtain i-a-Si:H thin film with the above quality, the hydrogen content (CH) should be from 9 to 11 atomic % and the microstructure factor (R*, the density ratio of the Si(H2) unit to the Si(H) unit) must be smaller than 0.1. Under the deposition condition of the H2/SiH4 gas flow ratio of 10, the deposited i-a-Si:H thin films, which have a CH = 10.2 at% and R* = 0.19, have the photoconductivity of 2.0x10-5 (Ω-cm)-1, a photosensitivity of 2.1x106 and a dark conductivity of 1.2x10-11 (Ω-cm)-1. To fabricate a-Si:H p-i-n photodiodes of good photovoltaic performance, we optimized the PH3/SiH4 and TMB/SiH4 gas flow ratios for the deposition of the n-layer and p-layer, respectively, to increase the doping concentration so that the electrical resistivity of the active layers is decreased and the built-in potential is increased. A photo current as high as 8 x10-9A and a Iph/Idark ratio of 3922 were obtained for the photoconductor of which the p, i and n layers have a thickness of 40nm, 1m and 50 nm, respectively. When the thickness of the p+ layer of the p-i-n device is reduced to 20 nm, more light can penetrate into intrinsic layer through the p-layer, significantly improving the Iph/Idark ratio to 6210.