Mechanism of carrier transport across the junction of narrow band-gap planar n+p HgCdTe photodiodes grown by liquid-phase epitaxy

The dark current of n+p Hg1−xCdxTe (x∼0.22) photodiodes fabricated on layers grown by liquid-phase epitaxy was studied systematically as a function of applied voltage (reverse and forward), temperature, and junction dimensions. The carrier transport mechanisms in different ranges of bias voltage and temperature have been identified and interpreted using the conventional models for carrier transport mechanisms in p-n junction devices. It is found that the forward current is dominated by a combination of bulk- and surface-related tunneling currents. In large-area diodes bulk tunneling current dominates the diode current and in small-area diodes surface tunneling dominates the diode forward current. Most probably, the tunneling mechanism across the n+p junction is via energy levels located within the diode depletion region. Modification of the existing theoretical model for forward-bias tunneling in p-n junction devices is needed in order to account for the experimental findings. Above 90K, the reverse current is found to be dominated by diffusion carrier transport mechanism. Below 90K, band-to-band reverse tunneling currents become dominant. No clear evidence for generation-recombination currents was found over the entire bias voltage and temperature range measured. The results of the present work demonstrate that the measurement of the temperature dependence of the zero-bias dynamic resistance is insufficient for determining the mechanism of carrier transport across the Hg1−xCdxTe junction and a detailed measurement of the current-voltage characteristics at various temperatures should be carried out.