Slow transient response of Ge:Ga far-infrared photoconductors for space applications

Two slow transient times in the response of Ge:Ga photoconductors to photon influx are found to fit well to τ_a=τ_o(N_d/p₁) and τ_m=τ_o(S₁/p₁), derived by analyzing a two-region model, where τ_o is hole lifetime, N_d is the donor concentration, and p₁ and S₁ are concentrations of hole and space charge, respectively in the region near the metal–p⁺ ohmic contact. Both τ_a and τ_m are consistent with the response times observed in Ge:Ga photoconductors with Ga concentrations of 2×10¹⁴ cm⁻³ and 1×10¹⁴ cm⁻³ as well as in a stressed Ge:Ga photoconductor under low background photon influx at low temperatures: Those characteristic times are mainly determined by the compensation and carrier density in the crystal. We discuss the relation between the characteristic times proposed in this study and those theoretically derived by Westervelt and Teitsworth R.M. Westervelt, S.W. Teitsworth, J. Appl. Phys. 57 (1985) 5457–5469], by Fouks B.I. Fouks, Proc. ESA Symp. Photon Detectors for Space Instrumentation (ESA SP-356), 1992, pp. 167–174], and by Haegel et al. N.M. Haegel, C.A. Latasa, A.M. White, Appl. Phys. A 56 (1993) 15–21; N.M. Haegel, C.R. Brennan, A.M. White, J. Appl. Phys. 80 (1996) 1510–1514]. As the compensation of Ge:Ga photoconductors decreases, responsivity to step change in photon influx and the ratio of slow response in the total response increases. These results also indicate that the slow response characteristics depend mainly on crystal qualities such as compensation if the metal–p⁺ contact is good enough.