高增益雪崩式非晶硒疊層膜光感測器研究

Abstract

本研究開發應用於醫療X光檢測器之高增益雪崩放大非晶質光電導元件(high-gain avalanche rushing amorphous photoconductors, HARP)，以具有很高的光電轉換效率之非晶質硒(a-Se)為光電導材料，在高電場條件下光激發產生的電洞，在a-Se材料內高速運動，與運動路徑上的原子發生碰撞，形成碰撞游離(impact ionization)作用，造成更多載子形成，此種連續性碰撞游離過程即所謂的雪崩倍增(avalanche multiplication)電流訊號；在低曝光劑量的X光顯影應用上，HARP元件需有很高的光電轉換增益和極小的雜訊，以提升影像對比，而經由a-Se光電轉換層前、後電極接觸界面注入的電洞與電子所產生的暗電流是劣化影像品質的主要因素之一。本研究熱蒸鍍沉積非晶硒薄膜厚達25 μm且基板溫度低於40oC，以濺鍍CeO2電洞阻障與Sb2S3電子阻障層(blocking layer)有效抑制暗電流(dark current)，並塗佈分散電阻層(distributed resistive layer)成功地在高電場70 V /μm 下，明暗電流比(ratio)高達70倍。

This research fabricated high-gain avalanche rushing amorphous photoconductors (HARP) for medical x-ray detector applications, using amorphous selenium (a-Se) as the photoconductor material because of its high photoconversion efficiency. Under a strong electrical field, photogenerated holes travel in the photoconductor layer with a high velocity and collide with atoms on the drift path, resulting in more photogenerated carriers. The successive impact ionization process induces the so-called “avalanche multiplication” of the electrical signal. For medical x-ray imagers of low irradiation exposure, HARP devices should have a high photoconversion gain with a very small noise so that a high image contrast can be obtained. The injection of holes and electrons into the a-Se layer via the top and bottom contact electrodes is the primary noise source. We used thermal evaporation to deposit a-Se thin films with a thickness up to 25 um at temperatures below 40oC. The CeO2 hole blocking and Sb2S3 electron blocking layers, which sandwiched the a-Se layer, were prepared by sputter deposition to minimize the dark current induced by the noise so that the ratio of the light current to the dark current can be enhanced. In addition, we also deposited a distributed resistive layer on the Sb2S3 layer to successfully improve the electrical breakdown voltage for the HARP structure. HARP devices fabricated in the study had a bright/dark current ratio as high as 70 at a bias field of 70 V/□m.