改善非晶硒光感測器暗電流與熱穩定性之研究

Abstract

非晶硒光感測器是以非晶硒(a-Se)做為光導電材料(photoconductor)。非晶硒在光感測器應用上，因需具備高的光轉換效能，必須要有極小的暗電流與高的熱穩定性，以提升影像對比。本實驗研究利用一個新的電洞阻障層材料應用在單層與多層結構的非晶硒光感測器上來降低元件暗電流與熱穩定性，在非晶硒層與正電極之間利用反應性濺鍍的方式製備氧化鋅(ZnO)電洞阻障層來降低元件的暗電流。利用蒸鍍機並使載台旋轉的方式沉積非晶硒和砷化硒 (a-Se/AsxSe1-x)多層膜結構，並應用在非晶硒光感測元件來提升元件的熱穩定性。 實驗方法是沉積ZnO電洞阻障層時，藉由改變氧流量的不同，利用XPS, Raman 和PL觀察氧空缺在氧化鋅薄膜中的變化，並由DLTS分析得知ZnO有兩個深層電洞缺陷分別在價帶上方0.94和0.24 eV。電性結果發現氧空缺的多寡對於a-Se光感測元件有很大的影響，當a-Se光感測元件中的電洞阻障層ZnO氧空缺越多有較低的暗電流與較高的崩壞電場。 在多層膜方面，利用雙舟蒸鍍機並使載台旋轉的方式沉積a-Se/AsxSe1-x多層膜結構作為a-Se光感測元件，用來改善元件熱穩定性，在AsxSe1-x層中的As原子濃度和各層間的厚度與蒸發舟溫度有關。由XRD、RAMAN與 SIMS分析發現，在沉積過程中，As會擴散進入a-Se層中並改善多層膜之熱穩定性。儘管As的加入會讓a-Se中的載子缺陷增加，降低了藍光的轉換效率，但它有效的降低暗電流，比單層a-Se低了兩個數量級。此外a-Se/AsxSe1-x 光感測元件因為有高的熱穩定性，所以有較高的崩壞電場。這低的暗電流以及高的崩壞電場讓a-Se/AsxSe1-x多層膜元件有高的影像對比，並適合應用在光感測元件上。

This research fabricated amorphous photosensors using amorphous selenium (a-Se) as the photoconductive layer. For photosensor application, a-Se should have a high photoconversion gain with a very low dark current and high thermal stability so that a high image contrast can be obtained. We have proposed a new hole blocking layer for dark current reduction in single and multilayer structure a-Se based photoconductor for photosensor applications. ZnO thin films as a hole blocking layer (HBL) between a-Se and anode electrode were deposited by applying a reactive sputter deposition technique to suppress the dark current in the a-Se photosensor. And then the multilayer structure a-Se based film was fabricated using an alternating multilayer structure of a-Se and AsxSe1-x by applying a rotational thermal evaporation deposition technique to enhance the thermal stability in the a-Se photosensor. The ZnO HBL layers prepared at various oxygen flow rates were characterized using X-ray photoelectron spectroscopy, Raman scattering analysis, and photoluminescence spectroscopy. The results indicated that the oxygen flow rate considerably influenced the density of oxygen vacancies in the ZnO thin films. Deep-level transient spectroscopy measurement was conducted, reveals two hole trap levels in the ZnO thin films deposited using a reactive sputter deposition process; one of the levels was located at 0.94 eV and the other was located at 0.24 eV, above the valence band edge. The number of oxygen vacancies in the ZnO thin film considerably influenced the electrical performance of the a-Se photosensor. The a-Se photosensor containing the ZnO HBL that comprised the most oxygen vacancies exhibited the lowest dark current and highest breakdown field. The multilayer a-Se based photosensors with an alternating multilayer structure comprising a-Se and AsxSe1-x were fabricated using a rotational thermal evaporation deposition process. The atomic concentration of As in the amorphous AsxSe1-x layer and the thickness of each a-Se and AsxSe1-x layer depended on the evaporation temperature. The thermal stability of the multilayer thin film was examined through X-ray diffractometry, Raman spectroscopy analyses, and time-of-flight secondary ion mass spectrometry. During the deposition of the amorphous AsxSe1-x layers, As diffused into the underlying a-Se layers, improving the thermal stability of the multilayer photosensor. Although the As doping introduced carrier traps in the a-Se layers, the multilayer photosensors demonstrated an effective quantum efficiency comparable to that of the single layered a-Se sensor under blue light illumination but with a lower dark current density by two orders of magnitude. Moreover, the a-Se/AsxSe1-x sensor was robust to a higher breakdown field because of its high thermal stability. The improved thermal stability and low dark current density enabled using the a-Se/AsxSe1-x multilayer structure as a sensitive photosensor with a high image contrast.