III-V compositionally graded buffers for heterostructure integration

Abstract

InyGa₁-yAs alloys are critical in commercial applications such as high speed transistors, light emitting diodes, solid state lasers, photovoltaics, and photo-detectors. However, the range of compositions used in these applications is often limited to the range of InyGa₁-yAs compositions which are lattice matched to elementary or binary semiconductor substrates. Additionally, the integration of InyGa₁-yAs based devices on silicon substrates has been limited by complicated processing procedures. In order to resolve these issues we developed two compositionally graded buffer systems to integrate InyGa₁-yAs devices on InP and Si substrates. The development of InyGa₁-yAs devices on Si substrates also used the direct growth of Ge on Si offcut substrates. InAsxP₁-x compositionally graded buffers were investigated for the growth of InyGa₁-yAs compositions with lattice constants greater than InP. We report the effects of strain gradient, growth temperature, and strain initiation sequence (gradual or abrupt strain introduction) on threading dislocation density, surface roughness, epi-layer relaxation, and tilt. We find that gradual introduction of strain causes increased dislocation densities (>10⁶ cm-²) and tilt of the epi-layer (> 0.10°). A method of abrupt strain initiation is proposed which can result in dislocation densities as low as 1.0x10⁵ cm-² for films graded from the InP lattice constant to InAs₀.₁₅P₀.₈₅. A model for a two-energy level dislocation nucleation system is proposed based on our results. We demonstrate a method for the growth of InyGa₁-yAs devices on Si substrates in a single process run. Two epitaxial layers were used to change the lattice constant from the Si substrate to the InyGa₁-yAs lattice constant. The first layer was a Ge layer grown directly on Si. To reduce the threading dislocation density to < 10⁸ cm-² we investigated the most efficient thermal cycle annealing procedure. The second layer was an InvAl₁-v,As compositionally graded buffer. In₀.₃Ga₀.₇As quantum well devices grown on InvAl₁-v,As/Ge/Si virtual substrates had threading dislocation density of 2x 10' cm-², mobility of 6400 cm2/Vs, and sheet carrier concentration of 1.1x 10¹² cm-²