Characterisation of low-pressure VPE GaAs diodes before and after 24 GeV/c proton irradiation

GaAs Schottky diode particle detectors have been fabricated upon low-pressure vapour-phase epitaxial GaAs. The devices were characterised with both electrical and charge collection techniques. The height of the Ti-GaAs barrier used was determined via two electrical methods to be (0.81 ± 0.005) and (0.85 ± 0.01) eV. The current density was greater than that expected for an ideal Schottky barrier and the excess current was attributed to generation current in the bulk of the material. A space charge density of (2.8 ± 0.2) × 10¹⁴cm^-3 was determined from capacitance voltage characterisation. The charge collection efficiency was determined from front alpha illumination and 60keV gamma irradiation to be greater than 95% at a reverse bias of 50V. The diodes were characterised after an exposure to a radiation fluence of 1.25 × 10¹⁴ 24 GeV/c protons cm^-2. The reverse current measured at 20°C increased from 90 to 1500 nA at an applied reverse bias of 200 V due to the radiation induced creation of extra generation centres. The capacitance measurements showed a dependence upon the test signal frequency which is a characteristic of deep levels. The capacitance measured at 5 V reverse bias with a test frequency of 100 Hz fell with radiation from 300 to 40 pF due to the removal of measurable free carriers. The charge collection of the device determined from front alpha illumination also fell to (32 ± 5)% at a reverse bias of 200 V.