Strain relief in epitaxial HgCdTe by growth on a reticulated substrate

In nearly all cases when an epitaxial layer of HgCdTe is grown on a CdZnTe substrate, there will be a finite lattice mismatch due to the lack of preci se control over the ZnTe mole fraction. This leads to strains in the layer, which can be manifested in one or more ways: (1) as misfit dislocations ne ar the interface, (2) as threading dislocations, (3) as surface topographic al textures, and (4) as crosshatch lines seen by x-ray topography. We have found that much of the strain can be relieved by growing on a reticulated s ubstrate. Specifically, when the substrate has been etched to form mesas pr ior to growth of the layer, the resulting layer on the tops of the mesas sh ows evidence of significantly reduced strain. CdZnTe substrates oriented (1 11)A were prepared with two sets of mes as on 125 mu m centers and 60 mu m centers, and with other planar areas remaining for comparison. From a Hg me lt, a layer of LWIR HgCdTe was grown about 16 mu m thick on each substrate. Nomarski microscopy showed that the layers on the mesa tops were extremely flat, showing no sign of curvature or surface texture. X-ray topography sh owed no cross hatch on the mesa tops, while the usual cross hatch appeared in the planar regions. The LPE layer extended laterally beyond the edges of the original mesa because of faster growth in non-(111) directions. Sample s were cleaved and examined in cross section. The linear density of etch pi ts seen in the cross section near the substrate, which represent misfit dis locations, was three times lower in the layer on the mesas than in the laye r in the unpatterned region, although both regions have the same layer/subs trate lattice mismatch. When an epilayer is grown on an unpatterned wafer ( the conventional approach), the growth in any small region is confined late rally by the growing layer in the neighboring regions. However, when growth occurs on a reticulated surface, the lateral confinement is removed, provi ding strain relief and fewer defects.