ABSTRACT

Modern semiconductor devices require materials with a diversity of lattice constants, thermal expansion coefficients, and even crystal orientation or structure. Some of these materials are shown in the “bandgap engineering” diagrams of Figures 1.1 and 1.2. Because of the wide range of semiconductor materials used in devices, and the limited choices of single-crystal substrates on which to fabricate them, their manufacture almost always requires lattice-mismatched heteroepitaxial growth.1 Initial efforts were concentrated on pseudomorphic growth,2 in which the epitaxial materials are coherently strained and absent of misfit dislocations. However, the requirement to avoid lattice relaxation places considerable restrictions on the choices of compositions and layer thicknesses.3 In general, the thickness mismatch product is restricted to less than ~0.04 nm for individual layers within pseudomorphic structures.4 Mismatched epitaxy brings with it a host of challenges, including strain,5 misfit dislocations,2 and the associated threading dislocations that propagate through device regions,6 crystallographic tilt induced during relaxation,7 and the possible degradation of morphology due to three-dimensional nucleation8 or stressinduced surface roughening.9 Foremost among these problems is the density of threading dislocations, which can exceed 109 cm−2 and strongly influences the performance and reliability of devices utilizing the defected material.10