優化電洞阻障層以提升高增益崩潰式光導元件之光電特性

Abstract

本研究開發應用於醫療X光檢測器之高倍增崩潰式放大非晶質光導元件(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。本研究利用濺鍍之方式沉積電洞阻障層(CeO2 & ZnO)，並控制氧流量來改變氧空缺，而後利用熱蒸鍍沉積非晶硒薄膜於電洞阻障層上，試片沉積溫度低於40°C，接著塗佈分散電阻層提高失效電場，提升元件明暗電流比，並觀察其暗電流的變化。

This research fabricated high-gain avalanche rushing amorphous photoconductors (HARP) for medical x-ray detector applications, using amorphous selenium (a-Se) as the photoconductive material because of its high photoconversion efficiency. Under a strong electric field, photogenerated holes travel in the photoconductive 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 low 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 at temperatures below 40 °C. The CeO2 & ZnO hole blocking layers, which sandwiched between the ITO substrate and the a-Se layer, were prepared by sputter deposition. In order to observe the correlation between the dark current and traps inside the CeO2 & ZnO thin film, we controlled the flow rate of oxygen when sputtering CeO2 & ZnO. In addition, we also deposited distributed resistive layer on the a-Se layer to improve the electrical breakdown voltage for the HARP structure.