Interfacial Structure and Defects in GaN/AlGaN Heterojunction Epitaxially Grown on LiGa02 Substrate by Molecular Beam Epitaxy

The performance of III-Nitride based Light Emitting Diodes (LEDs), LASERs, GaN/AlGaN MODFETs (Modulation-doped Field Effect Transistors) and HEMTs (High Electron Mobility Transistors) have been improved dramatically over the past few years [1,2], despite the relatively poor material quality of GaN, as compared to GaAs, for example. The intrinsic properties of the materials system make it extremely well suited to both optoelectronic and microwave power transistor applications. Typically, GaN is grown on substrates such as GaAs, Al2O3 (sapphire) or SiC with large lattice mismatch. This has usually resulted in an extremely high defect density in the GaN layer. The growth of GaN on lithium gallate LiGaO2 (LGO) affords many advantages compared to all other available substrates. LGO offers the smallest average lattice mismatch of any available substrate for the Ill-nitrides. This facilitates the growth of high quality GaN directly on Lithium Gallate without the need for a defective buffer to decouple the strain associated with the large lattice mismatch of other substrates [3].

Compliant Substrate Processes

ABSTRACTRecent results indicate that compliant substrates offer significant promise as a new approach for strain management in semiconductors. The potential applications include 1) the growth of device-quality highly mismatched materials on dissimilar substrates, and 2) the lateral control of material properties resulting from the effects of strain on bandstructure and/or growth dynamics. A significant amount of research in this area is dedicated to the reduction of extrinsic processing effects resulting from compliant substrate fabrication, and the development of simple models for understanding the observed reduction in defect density and/or strain in the epitaxial films grown on compliant substrates. A recent focus in our work has been on the growth of GaN on a novel and easily removable substrate -lithium gallate- for the regrowth on a bonded GaN template. The first step in this approach is the optimization of the growth of GaN on lithium gallate. In addition, this approach requires the use of an appropriate bonding layer to reduce the strain or defect production during growth due to coefficient of thermal expansion mismatches between the GaN sample and the handle wafer. Our work in this area will be highlighted in the context of an overview of various compliant substrate approaches and current results that indicate their efficacy.