Capacitance and conductance of semiconductor heterojunctions with continuous energy distribution of interface states

Abstract

Electrical conduction in semiconductor heterojunctions with interface states presenting a continuous density of states distribution inside the gap is studied theoretically. The defect states are given with their density of states function and capture cross sections for transitions to the valence and conduction band. Conduction is considered under steady-state dc applied voltage V and under small-signal ac conditions. The formal developments are based on the resolution of Poisson equation and the continuity equations for electrons, holes, and occupied defect states. The numerical integration yields the position dependent dc and ac components of the concentrations of electrons, holes, occupied defect states, and of the current densities and recombination rates. For any value of x, one obtains the occupation function ft as function of the defect energy Et and the transition rates to the valence and conduction band. This allows a detailed analysis of the response of the interface state system to the external applied voltage. The main electrical characteristics obtained in the final analysis are the dc current–voltage and the ac capacitance–frequency and capacitance–voltage curves. The method is applied to a InGaAs/InP heterojunction with interface DOS functions being either constant inside the gap or having gaussian shapes with a given mean value and standard deviation. Discrete levels are treated as a particular case. The I(V) curve has an ideality factor n different from one, reaching the theoretical value of 2 for discrete midgap states. Comparison of C(f ) curves for different DOS functions allows us to show the appearance of characteristic features depending on the explicit form of the DOS function and different from those of the discrete level case. The C(f ) curves, computed for different values of applied voltage V, show cutoff frequencies whose values increase with V. It is shown that this is due to the fact that different defect states are involved in the dynamical response of the system. The low frequency C−2(V) curves are not linear, as for the ideal junction case, and present structures which are correlated with the slope of the interface charge density are represented as a function of applied voltage V.