A method of growing lattice-mismatched InxGa1-xAs epitaxial layers on InP substrates using intermediate buffer layers of InAsyP1-yhas been invented to improve the performance of InxGa1-xAs thermophotovoltaic devices. The use of buffer layers is required to minimize the density of threading dislocations generated because of the lattice mismatch between low-bandgap InxGa1-xAs and InP. These defects degrade the electrical performance of the InGaAs device by acting as recombination centers for minority carriers.

The figure depicts the buffer-layer structure of a typical InxGa1-xAs thermophotovoltaic device fabricated by the present method. In this case, xis chosen to be 0.68 to obtain a bandgap of 0.6 eV. Only two InAsyP1-y buffer layers are needed: For the first buffer layer, y is chosen to be 0.16 to obtain a lattice mismatch of 0.58 percent with the substrate. For the second buffer layer, y is chosen to be 0.32 to obtain both a lattice mismatch of 0.51 percent with the first buffer layer and a lattice match with the first InxGa1-xAs layer.

The traditional approach to buffer layer design strives to accommodate the stress developed by the lattice mismatch through generation of misfit dislocations that are confined to the substrate/epilayer interface, while minimizing the generation of threading dislocations that propagate through the epitaxial layer(s). Often, many buffer layers, strained layer superlattices (SLS) or thermal cycle growth techniques are used to increase the interaction of threading dislocations, thereby reducing the overall dislocation density. The method described here utilizes a different phenomenon first observed in InGaAs grown on GaAs, whereby the strain of lattice mismatch is accommodated by dislocation formation in the substrate and underlying buffer layers rather than the top device epilayers. The success of this method depends on the selection of the composition of each buffer layer according to several criteria, most notably the following:

  • The yield strength of each buffer layer must exceed that of the adjacent lower layer (including that of the substrate) so that dislocations are preferentially generated in the softer, lower layers.
  • The buffer layers must be in compression, relative to the substrate.
  • The compositions of the buffer layers must be chosen to make the lattice mismatch between any two adjacent layers less than a critical value, below which few or no dislocations propagate up through the layers to the overlying InxGa1-xAs. This translates to making the compositions of the adjacent buffer layers differ by less than a corresponding critical amount.

It has been seen that the yield strength of an alloy of two materials varies with composition, with the maximum occurring at a 50/50 mixture. For example, it has been suggested that the yield strength of InGaAs has a maximum at a composition of In0.5Ga0.5As (at elevated temperatures characteristic of epitaxial growth). The use of InGaAs buffer layers for the growth of low bandgap (i.e., 0.6 eV) In0.68Ga0.32As on InP may begin with a In0.53Ga0.47As buffer layer lattice-matched to the InP substrate and be comprised of InGaAs layers with increasing In content and lower yield strength. Thus, the buffer layer structure begins with a strong material followed by successively weaker materials. Stress tends to be relieved by dislocation formation in the weaker overlying layers. The opposite is true for buffer layers composed of InAsyP1-y. The buffer structure begins with InP and proceeds with successively higher yield strength material, thereby encouraging the formation of threading dislocations in the underlying materials. Cross-sectional TEM (transmission electron microscopy) analysis has verified this behavior of InAsyP1-y buffer layers on InP. This technique allows buffer layers to be produced with fewer and thinner layers, providing cost and operational benefits.

This work was done by David Wilt of Glenn Research Center and Richard W. Hoffman of Essential Research, Inc.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Cleveland
Ohio 44135

Refer to LEW-16776.