HgCdZnTe quaternary materials for lattice-matched two-color detectors

As the number of bands and the complexity of HgCdTe multicolor structures increases, it is desirable to minimize the lattice mismatch at growth interfaces within the device structure in order to reduce or eliminate mismatch dislocations at these interfaces and potential threading dislocations that can degrade device performance. To achieve this we are investigating the use of Hg1-x-yCdxZnyTe quaternary alloys which have an independently tunable lattice constant and bandgap. Lattice matching in Hg1-x-yCdxZnyTe structures can be achieved using small additions of Zn (y < 0.015) to HgCdTe ternary alloys. We have investigated some of the basic properties of Hg1-x-yCdyZn,Te materials with x approximate to 0.31 and 0 less than or equal to y less than or equal to 0.015. The quaternary layers were grown on (112)CdZnTe substrates using MBE and the amount of Zn in the layers was determined from calibrated SIMS measurements. As expected, the lattice constant decreased and the bandgap increased as Zn was added to HgCdTe to form Hg1-x-yCdxZnyTe. Hall-effect results for both n-type (In) and p-type (As) Hg1-x-yCdxZnyTe layers were very similar to HgCdTe control samples. We have also utilized x-ray rocking curve measurements with (246) asymmetric reflections as a novel sensitive technique to determine the correct amount of Zn needed to achieve lattice matching at an interface. MWIR/LWIR n-p-n two-color triple-layer heterojunction structures were grown to evaluate the effects of minimizing the lattice mismatch between the widest bandgap p-type collector layer, using Hg1-x-yCdxZnyTe, and the HgCdTe MWIR and LWIR collector layers and compared to structures that did not incorporate the quaternary. Sequential mode two-color detectors were fabricated using a 256 x 256, 30 mu m unit cell design. There were several interesting findings. Macro defects predominantly affected the LWIR band (Band 2) operability and had little effect on the MWIR band (Band 1). The incorporation of Hg1-x-yCdxZnyTe p-type collector layers had little effect on MWIR detector performance, but overall the LWIR performance was generally better. These initial detector results indicate that the use of Hg1-x-yCdxZnyTe alloys in multicolor detector structures are potentially promising and should be pursued further.