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 tau(a) = tau(o)(N-d/p(1)) and tau(m) = tau (o)(S-1/p(1)), derived by analyzing a two-region model, where tau(o) is hol e lifetime, N-d is the donor concentration, and p(1) and S-1 are concentrat ions of hole and space charge, respectively in the region near the metal-p( +) ohmic contact. Both tau(a) and tau(m) are consistent with the response t imes observed in Ge:Ga photoconductors with Ga concentrations of 2 x 10(14) cm(-3) and 1 x 10(14) cm(-3) as well as in a stressed Ge:Ga photoconductor under low background photon influx at low temperatures: Those characterist ic times are mainly determined by the compensation and carrier density in t he crystal. We discuss the relation between the characteristic times propos ed in this study and those theoretically derived by Westervelt and Teitswor th [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 instrument ation (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. B rennan, A.M. White, J. Appl. Phys. 80 (1996) 1510-1514]. As the compensatio n of Ge:Ga photoconductors decreases, responsivity to step change in photon influx and the ratio of slow response in the total response increases. The se results also indicate that the slow response characteristics depend main ly on crystal qualities such as compensation if the metal-p(+) contact is g ood enough. (C) 1999 Elsevier Science B.V. All rights reserved.