This GOALI project represents a collaborative effort between researchers at MIT and Hewlett-Packard (Optoelectronics Division) to address fundamental materials problems in heteroepitaxy of lattice mismatched materials that prevent the fabrication of low threading dislocation density, relaxed InAlGaP on GaP substrates. Graded composition InAlGaP and InGaP on GaP substrates grown by MOVPE may lead to low threading dislocation density, relaxed layers. This project aims to develop a generic view of relaxed lattice-mismatched graded layers. Preliminary data show that extrapolation from the more ideal SiGe alloy system to the InGaP alloy system will not result in high quality buffers. Specifically, threading dislocation flow during graded buffer relaxation is inhibited by previously unexamined branch defects, resulting in higher dislocation nucleation rates which lead to higher threading dislocation densities in the final layer. Identifying the structure of the branch defects, their origin, and their formation as a function of MOVPE growth parameters are key aspects of the project directed toward better understanding and the ability to fabricate low threading dislocation density, relaxed InGaP buffers on GaP. Such buffers are technologically important in visible LED applications, since GaP substrates and graded InGaP buffers do not absorb visible light. The current process for achieving a transparent substrate LED technology uses a wafer bonding process, due to the lattice- mismatch problem between transparent GaP substrates and InAlGaP lattice-matched LED structures. A successful epitaxial solution to the lattice-mismatch problem is anticipated to be a more cost-effective solution, and therefore can have significant impact on the high brightness LED manufacturing process. In addition, a new range of direct band gap III-V materials may become accessible. Current binary substrates are not lattice -matched to many InAIGaP alloy concentrations that have direct band gaps. %%% The project addresses basic research issues in a topical area of materials science having high potential technological relevance. The research will contribute basic materials science knowledge at a fundamental level to important fabrication aspects of electronic/photonic devices. New experimental materials science and optical tools are now available to allow more detailed observation of elementary surface and optical processes which when better understood allow advances in fundamental science and technology. The basic knowledge and understanding gained from the research is expected to contribute to improving the perform-ance and stability of advanced devices and circuits for computing and communications. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. The project is co-supported by the DMR/EM, ECS/PFET, and the MPS OMA(Office of Multidisciplinary Activities). ***
