In this work bulk sulphur-doped (ND>1017 cm-3) GaSb crystals have been investigated by performing Hall and photo-Hall measurements at different intensities of subgap illumination in the temperature range 14-300 K. At low temperature (T<100 K) a persistent photoconductivity (PPC) effect, similar to that observed for the DX centre in AlxGa1-xAs, has been found. Low-temperature conductivity exhibits the light-induced Mott transition, indicating the existence of sulphur-related shallow states linked to the Gamma minimum. In contrast to AlxGa1-xAs, the low-temperature saturated PPC electron concentration ( approximately 4*1015 cm-3) is much smaller than that expected from high-temperature measurements. Starting from the PPC condition, transient Hall measurements in the temperature range 80-100 K, where thermally activated electron capture is observed, have revealed a quenching of the photosensitivity, which depends on the total number of incident photons. On this basis a phenomenological model of the deep donors in GaSb:S is proposed, invoking the existence of two sulphur-related deep states with different photosensitivity and thermally activated capture cross section. Hall data are also supported by low-temperature capacitance-voltage measurements performed in the dark and after white-light illumination.

Hall and photo-Hall effect measurements on sulphur-doped GaSb

Hall and photo-Hall data have been taken in Te-doped Al x Ga 1 − x Sb epitaxial layers of different AlSb molar fractions and doping densities in the 10 17 –10 18 cm −3 range. The evidence of the persistent photoconductivity effect at low temperatures reveals the presence of the DX center, whose occupancy level is as deep as the AlSb molar fraction increases. The temperature dependences of the Hall carrier density and mobility, n H ( T ) and μ H ( T ), have been carefully investigated by varying the density of the photoionized DX centers between the dark value and the saturation one. At low temperatures the electrical data are dominated by the occupancy of a Te-donor level in thermal equilibrium with the conduction band states, responsible of a semiconductor-to-metal transition when the density of the photoexcited electrons becomes sufficiently high. The isothermal μ H ( n ) curve is a single valued function, independent of the experimental procedure. Possible explanations of this result have been briefly discussed.

Control of the n-type doping in AlxGa1 − xSb: DX-center behavior of the Te impurity

Due to the vanishing of both electron emission and capture rates, the DX centre shows a non-equilibrium occupancy at low temperature. It is shown that this effect results in a non-uniform free-electron density profile that can be controlled by biasing the junction during cooling. Preliminary results are reported showing that the I-V characteristics of Schottky barriers on AlGaAs (x=0.25) are affected by the DX centre occupation.

E. Gombia

Papers

Evidence for non-equilibrium free electron density in AlGaAs at low temperatures