Misfit and threading dislocations in HgCdTe epitaxy

Abstract

A new theory relating substrate, misfit, and threading dislocations is presented for abrupt and graded heterojunctions based on the assumption of complete local strain relief. We have for the first time been able to treat two- and three-dimensional misfit dislocation structures in a unified way and experimentally verify the distinction in epitaxial HgCdTe. We confirm earlier findings that the areal density of interfacial dislocations nA in a cross section of the interface is proportional to the lattice constant gradient. If the interface width w is narrower than a theoretically predicted critical value w0, the misfit structure becomes two-dimensional and a linear dislocation density nL applies. The maximum of nA (attained at w=w0) is equal to n2L. Another new result of the theory is that the average misfit segment length is inversely proportional to the substrate dislocation density. The threading dislocation density is predicted to be low when the average segment length is comparable to the substrate lateral dimensions. For cases of low epitaxial dislocation content, the calculated substrate densities corresponding to such segments compare favorably with measured values. Compositional profiles and misfit dislocation densities were obtained on the same angle lapped HgCdTe epilayers by visible reflection spectroscopy and preferential etching, respectively. The former monitored the E1 interband transition as a function of position with a microspectrophotometer while the latter employed a modified Polisar No. 2 etchant. Both linear and areal distributions were observed and their occurrence was correctly predicted by the theory.