On the Growth and Properties of InAs/Ga1-xInxSb Superlattices and Related Materials

Doctoral thesis, 1997

InAs/Ga1-xInxSb semiconductor superlattices and their constituent materials have been studied theoretically, grown by molecular-beam epitaxy and characterised by various techniques. InAs/Ga1-xInxSb superlattices are interesting for use in far-infrared detectors because of their narrow band gaps. Numerical calculations were performed using a three-band envelope function theory with the purpose of studying how the far-infrared absorption depends on the superlattice parameters. The calculations led to the establishment of certain rules of thumb for the design of far-infrared superlattices, e.g. that the band gap of the InAs/Ga1-xInxSb superlattice can be reduced almost without losing the absorption by increasing the In molar fraction of the Ga1-xInxSb layer. The theoretically predicted band gaps were seen to agree well with experimental band gaps of InAs/GaSb superlattices. Two extensions of the InAs/Ga1-xInxSb system were suggested based on calculated predictions: Ga1-xInxSb/InAs1-ySby superlattices having a stronger absorption of far-infrared radiation than other type-II superlattices and Ga1-xInxSb/Ga1-yInyAs superlattices, which can be used for detector structures sensitive for two-different parts of the infrared spectrum. Furthermore, special attention was focused on the growth of Ga1-xInxSb, since this material has received comparatively little attention in the literature. It was seen that Ga1-xInxSb is grown with the best crystalline properties at temperatures slightly below 500° C. For lower growth temperatures the surface is rough and covered by various square defects. Silicon can be used in the 1017 cm-3 range as an acceptor in Ga1-xInxSb for In molar fractions x < 0.85. The doping efficiency improves for low x and for high growth temperatures with an optimum of approximately 510° C. Finally, the implications of using high In molar fractions in the Ga1-xInxSb alloy on the structural quality of highly lattice-mismatched InAs/Ga1-xInxSb superlattices were studied. For In concentrations x > 0.4 in the Ga1-xInxSb layers, it becomes difficult to grow the superlattices even at low growth temperatures, and the InAs and Ga1-xInxSb layers relax almost completely to their equilibrium lattice constants.