Photoconductivity of InSb/GaAs heterostructures at low temperature

Abstract

We have studied the low-temperature coplanar photoconductivity under pulsed and continuous illumination of n-InSb/GaAs heterostructures grown by metalorganic magnetron sputtering. The large lattice mismatch and the large difference in band gaps give rise respectively to a highly defective region at the interface and a high built-in field. The effect of these factors on the photoconductivity is then a function of the InSb film thickness. For photon energies above the InSb band gap, up to about 0.4 eV, we observe positive photoconductivity due to an increase in the electron density coupled with trapping of the excess minority carriers. As the film thickness is decreased from 2 to 0.2 μm, the photoconductivity gain per absorbed photon decreases and the decay time increases, indicating the presence of trapping or recombination centers near the interface. Illumination with photons of energy greater than 0.4 eV leads to the coexistence of a positive photoconductivity and a very slow transient negative photoconductivity which gives rise to high gains at low modulation frequencies. The negative photoconductivity increases with increasing photon energy and decreasing film thickness. Illumination at 0.9 eV from both sides of the heterostructure shows that the effect is related to diffusion and trapping of hot electrons at the interface. The onset of the saturation of the effect indicates a concentration of electron traps of about 1014 cm−2 at the interface, a value similar to the concentration of edge-type dislocations that predominantly accommodate the lattice mismatch in this heterostructure.