砷化镓光电阴极光谱响应与吸收率关系分析

摘要

为了研究砷化镓(GaAs)光电阴极光谱响应与吸收率曲线间的关系,采用分子束外延法(MBE)和金属有机化合物化学气相沉积法(MOCVD)制备了两类GaAs光电阴极,并测试得到了样品吸收率和光谱响应实验曲线.对每个样品的这两条曲线在同一坐标系中做最大值归一化处理,将归一的光谱响应曲线与归一的吸收率曲线做除法,得到了类似光电阴极表面势垒的形状.结果表明,两种方法制备的光电阴极光谱响应曲线相比吸收率曲线都发生了红移,MBE样品偏移量稍大于MOCVD样品.短波吸收率不截止,光谱响应截止于500 nm左右;可见光波段上,光谱响应曲线的峰值位置相比吸收率曲线红移了几百meV;近红外区域,光谱响应曲线的截止位置相比吸收率曲线红移了几个meV.MOCVD样品中杂质对带隙的影响更小,光谱响应相比吸收率发生的能量偏移更小.这些结论对提高GaAs光电阴极光电发射性能有指导意义.%In order to study the relation between spectral response and absorptivity of GaAs photocathode,two kinds of GaAs photocathodes are prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD),respectively.The samples grown by the MBE include varying doping GaAs photocathodes with different values of emission layer thickness from A to E.The thickness of GaAs emission layer is 1.6 μm or 2 μm.The Al component is 0.5 or 0.63.The samples grown by the MOCVD include varying doping or various component GaAs photocathodes with different values of emission layer thickness and different window layer components from F to J.The thickness values of GaAs emission layer are 1.4 μm,1.6 μm or 1.8 μm,respectively.The Al component is 0.7 or varies from 0.9 to 0.The doping concentration of the GaAs emission layer is divided into 8 sections between 1 × 1018 cm-3 and 1 × 1019 cm-3.The experimental spectral response curves for all samples are obtained by the optical spectrum analyzer.And the experimental reflectivity and transmittivity curves are measured by the ultraviolet visible near infrared spectrohootometer.Based on the law of energy conservation,the absorptivity curves are obtained according to the experimental refiectivity and transmittivity.In the same coordinate system,both the curves are obtained by unitary processing according to the max.A similar surface barrier can be given by dividing the normalized absorptivity by the normalized spectral response,and those are termed the similar Ⅰ barrier and the similar Ⅱ barrier,respectively.The results indicate that for both the GaAs photocathodes,the experimental spectral response curves both tend to move to the infrared band compared with the experimental absorptivity curves.The average energy differences between absorptivity and spectral response are calculated to be 0.3101 eV for the MBE sample,and 0.3025 eV for the MOCVD sample,respectively.The red-shifts of the photocathodes grown by MBE are a bit bigger than those of the photocathodes grown by MOCVD.In the shortwave region,the absorptivity is very large,but the spectral response cuts off nearby 500 nm.In the visible wavelength region,the peak position of the spectral response curve shifts toward the infrared band for several hundred meV in comparison with the absorptivity curve.In the near infrared region,a red shift of several meV appears at the cut-off position of the spectral response curve in comparison with the absorptivity curve.The results have the guiding significance for improving the photoemission performance of wide-spectrum GaAs photocathode by optimizing the optical performance.