Strain relief in epitaxial HgCdTe by growth on a reticulated substrate |
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Authors: | David R Rhiger Sanghamitra Sen Eli E Gordon |
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Affiliation: | (1) Raytheon IRCOE, 75 Coromar Dr., 93117 Goleta, CA |
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Abstract: | 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 precise 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 near the interface, (2) as threading dislocations, (3) as surface topographical
textures, and (4) as cross-hatch lines seen by x-ray topography. We have found that much of the strain can be relieved by
growing on a reticulated substrate. Specifically, when the substrate has been etched to form mesas prior to growth of the
layer, the resulting layer on the tops of the mesas shows evidence of significantly reduced strain. CdZnTe substrates oriented
(111)A were prepared with two sets of mesas on 125 μm centers and 60 μm centers, and with other planar areas remaining for
comparison. From a Hg melt, a layer of LWIR HgCdTe was grown about 16 μm 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 showed
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. Samples were cleaved and examined
in cross section. The linear density of etch pits seen in the cross section near the substrate, which represent misfit dislocations,
was three times lower in the layer on the mesas than in the layer in the unpatterned region, although both regions have the
same layer/substrate lattice mismatch. When an epilayer is grown on an unpatterned wafer (the conventional approach), the
growth in any small region is confined laterally by the growing layer in the neighboring regions. However, when growth occurs
on a reticulated surface, the lateral confinement is removed, providing strain relief and fewer defects. |
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Keywords: | Cross hatch defects growth area HgCdTe lateral growth lattice mismatch liquid phase epitaxy (LPE) misfit dislocations residual strain strain relief |
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