Front and back side processed unintentionally doped GaAs Schottky detectors for X-ray detection

Abstract

ABSTRACT We have studied the current voltage and X-ray detection using front and back side processed, unintentionally doped bulk GaAs Schottky detectors. GaAs detectors with large enough thickness and low enough doping could be used for X-ray imaging, especially for medical applications. GaAs Schottky detectors were fabricated using front and back side photolithographic processing with Ti/Au for Schottky and Ge/Au/Ni/Au for Ohmic contacts. A number of detectors of size 2 mm 2 were tested. The breakdown voltage reached 600- 800 V in semi insulated (SI) GaAs Schottky front and back side processed detectors. For these detectors the dark current was found to be between 2- 90 nA. These detectors were also characterized with 150 keV, 3mA X-ray radiation and they responded well by showing more than a hundred fold increase in photocurrent due to production of electron hole pairs by the ionization processes. The processing of the detectors and the I-V and X-ray characterization is presented in this report. Key Words: SI-GaAs, Schottky detectors, I-V characteristics, X-ray Detection 1. INTRODUCTION Ideally for X-ray detection, one require materials of high atomic number Z with a bandgap of about 1.5 eV which is large enough to lower the electronic noise and small enough to allow charge carriers to recombine and produce a maximum number of electron - hole pairs. Semiconductor materials with high mobility and lifetime are preferable. Among a few materials that fulfill these requirements are GaAs, and CdZnTe (CZT). They are viable alternative to Si or Ge for the detection of X-rays above a few keV. They are two of the compound semiconductors with bandgap ~1.5 eV high enough to permit room temperature operation. The atomic number of their constituents are greater than or equal to that of germanium, and have the advantages of a Ge Detector - such as high detection efficiency and robustness. Another requirement for X-ray detection is having thick absorber layer with a low enough free carrier concentration. CZT detectors exhibit a charge collection efficiency closer to 1 with a good energy resolution even for photons of low energies.