Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects
Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects
复制标题
真空沉积稳定 a-Se 薄膜中的频率和时间分辨光电流：价态交替缺陷的作用
DOI：
10.1007/s10854-020-04111-1
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Jacobs J
中科院分区：
文献类型：
--
作者：
Jacobs J
稳定的非晶硒（a-Se）由于其X射线光电导性和其通过常规真空沉积均匀沉积在大面积TFT基板上的能力，目前用于大多数直接转换乳房X射线成像检测器中。我们报告的实验结果光电流光谱（频率分辨光谱（FRS）和单次瞬态），真空沉积的α-硒薄膜。我们表明，所有测量的光电流关键取决于所花费的相对时间的材料在光和在黑暗中。我们确定，所观察到的光学响应的显着变化取决于200 nm的表面内的捕获（光学注入）电荷的密度，并表明它是控制这种电荷的密度的黑暗和光照时间的比例。我们的数据证实，局部电荷从根本上影响的光电流瞬态形状，由于在200 nm的表面内的有效屏蔽的应用领域。光场的改变发生在消光深度范围内，改变了光生过程和载流子的漂移。我们的数据的许多方面进行的签名的已知属性的价交替对（VAP）的缺陷，控制的许多属性的α-硒。在时域中的建模表明，光生成的VAP随后由光学触发的VAP缺陷转换可以导致近表面电荷不平衡，表明VAP缺陷可以占不寻常的光学响应。稳定的a-Se膜在高于具有a-Se：0.3%As掺杂ppm Cl的组成的合金的玻璃化转变温度下沉积。电子顺磁共振测量在温度下降到5 K没有检测到任何自旋活性缺陷，即使在光激发带隙以上。
Stabilised amorphous selenium (a-Se) is currently used in the majority of direct conversion mammographic X-ray imaging detectors due to its X-ray photoconductivity and its ability to be uniformly deposited over large area TFT substrates by conventional vacuum deposition. We report experimental results on photocurrent spectroscopy (frequency-resolved spectroscopy (FRS) and single-time transients), on vacuum-deposited a-Se films. We show that all measured photocurrents depend critically on the relative time spent by the material in the light and in the dark. We identify that the observed pronounced variation in optical response depends on the density of trapped (optically injected) charge within 200 nm of the surface and show that it is the ratio of dark and light exposure time that controls the density of such charge. Our data confirm that the localised charge radically influences the photocurrent transient shape due to the effective screening of the applied field within 200 nm of the surface. The field modification occurs over the optical extinction depth and changes both the photogeneration process and the drift of carriers. Many aspects of our data carry the signature of known properties of valence alternation pair (VAP) defects, which control many properties of a-Se. Modelling in the time domain shows that light generation of VAPs followed by optically triggered VAP defect conversion can lead to near-surface charge imbalance, demonstrating that VAP defects can account for the unusual optical response. The stabilised a-Se films were deposited above the glass transition temperature of the alloy with composition a-Se:0.3% As doped with ppm Cl. Electron paramagnetic resonance measurements at temperatures down to 5 K did not detect any spin active defects, even under photoexcitation above band gap.