Surface photovoltage spectroscopy characterization of varied-doping GaAs

Summary form only given. The quantum efficiency of negative electron affinity (NEA) GaAs photocathodes mainly depends on the performance of GaAs material. Usually, the uniform-doping structure are used in the photoemissive layer of cathodes. To increase quantum efficiency of cathodes, the varied-doping structure are used. The quantum efficiency can be increased by using the varied-doping structure in GaAs material. The electric field exists in the varied-doping GaAs material, that improves the movement of photoexcited electron toward the surface. To evaluate the performance of GaAs material before they are activated, the measurement system of surface photovoltage spectroscopy (SPS) of GaAs is developed. The system consists of the light source, optical chopper, monochromator, the quartz fiber team, photovoltage test fixture, Lock-In Amplifier, computer and software. The system can measure the SPS of the GaAs material with range of 500~1000nm. The SPS analyses offer the possibility of performing nondestructive evaluation for GaAs material. The diagram of measurement system is shown in Fig.1. The reflection mode uniform-doping and gradient-doping GaAs material are grown by molecular beam epitaxy(MBE) with p-type beryllium doping. The detailed material doping structure of grown samples is shown in Fig.2. Sample 1 is a uniform-doping structure. The top layer is the photoemissive layer, p-GaN. The thickness is about 2.6um. The dopant concentration is about 1.0x10¹⁹cm^-3 . The second layer is the substrate, p-GaAs(100). Sample 2 is a gradient-doping structure. The epitaxial layers of samples 2 are divided into four sections. The total thickness is also 2.6μm. The dopant concentration of epitaxial layers varies from 1.0x10¹⁸ cm^-3 to 1.0x10¹⁹ cm^-3. SPS of the sample 1 and sample 2 are measured by developed measurement system. The measurement results are shown in Fig.3. The reflections of GaAs material - nder the different incident wavelength are measured by spectrophotometer. The electron diffusion length of GaAs material are obtained by equal light intensity method analysis of SPS[2,3,4]. Sample 1 and sample 2, respectively, the electron diffusion length is about 2.6 μm and 3.2 μm. The results show that the electron diffusion length of gradient-doping GaAs material is increased compared with uniform-doping GaAs material.