Gallium arsenide fast photodetectors: Selection criteria and predicted response

The main characterislics (sensitivity, response time, dynamic range) of a photoconductive detector (PCD) depend on the intrinsic physical (lattice parameter, extended and local defects) and electrical properties (resistivity, mobility, carrier lifetime) of the material. Selection criteria for semi-insulating undoped gallium arsenide (GaAs) were thoroughly examined for their capacity to select the more suitable material for the fabrication of fast PCDs, used in gamma-ray pulse (E ∼ 1.2 MeV, τ_FWHM = 30 ns) metrology. An analytical model, which assumes that carrier recombination takes place through EL2 type recombination centers [M. O. Manasreh and B. C. Covington, Phys. Rev. B 35, 2524 (1987)], was developed. Taking into account the initial physical and electrical properties of the GaAs material, the model accurately predicts the response of the PCDs after a neutron preirradiation step (dose range: 5 × 10¹⁴ to 1 × 10¹⁶ neutrons/cm²), which was used to optimize the PCD characteristics and in particular to reduce the response time to values lower than 100 ps. This led to the identification and validation of key parameters of the initial properties of single crystal GaAs materials influencing the PCDs final characteristics. Thus, depending on the user defined specifications, GaAs material initial properties and neutron preirradiation dose can be fixed to fabricate GaAs PCDs exhibiting the desired response time, sensitivity, and dynamic range. The study established that the PCDs characteristics are controlled only by the initial GaAs material properties when neutron preirradiation doses remain below 1 × 10¹⁵ neutrons/cm², whereas they are mainly controlled by neutron induced crystal defects above this dose.