Investigation of crystal properties of TmP/GaAs and GaAs/TmP/GaAs heterostructures grown by molecular beam epitaxy
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R.J. Hwu and L.P. Sadwick Department of Materials Science and Engineering, and Department of Electrical Engineering, University of Utah, Salt Lake City, Utah 84112 (Received 29 January 2001; accepted 4 September 2001)
Single-crystal thulium phosphide (TmP) was grown heteroepitaxially on (001) GaAs substrates by molecular beam epitaxy with the orientation relationship [100]TmP//[100]GaAs and {001}TmP//{001}GaAs. The crystal properties and the defects in TmP/GaAs, GaAs/TmP/GaAs heterostructure were characterized through x-ray diffraction, atomic force microscopy, and transmission electron microscopy. TmP was found to have a huge difference in thermal expansion coefficient compared GaAs, which produced high tensile residual stress and may result in the formation of defects. The major defects in the top GaAs layer are stacking faults or microtwins, and they directly correlated with the islandlike surface morphology of the GaAs overlayer. The composition profiles of the TmP/GaAs heterostructure were measured by secondary ion mass spectrometry. The reason for surface segregation of Tm and Ga atoms is discussed and is primarily due to their higher diffusion coefficient near the surface as compared to that in the TmP epilayer bulk. The thermally stable characters of the TmP/GaAs heterostructures allow them to be promising candidates in various device applications.
I. INTRODUCTION
The ability to grow thermally stable Schottky contacts and buried, epitaxial metallic or semi-metallic layers on semiconductors have many applications in novel device structures. Based on their potential applications in electronic or photonic devices, studies of metal-semiconductor contacts and stable thin heteroepitaxial buried metal layers have gained increased attention in recent years. Solidstate electronic devices employing metal–semiconductor contacts and thin geteroepitaxial buried metal layers include metal base transistors, resonant tunneling devices, and metal–semiconductor field effect transistors (MESFETs). The number of candidates for good epitaxial growth and overgrowth is limited. Metal– semiconductor heterostructures reported to date are either thermodynamically unstable and diffuse into GaAs at elevated temperatures or they have a relatively large lattice mismatch with GaAs. When these metal– semiconductor heterostructures are exposed to high temperatures during fabrication or operation, interface reactions between the metal and semiconductor can take place that result in degradation in their structural and electrical properties. Also, long-term device reliability 3266
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J. Mater. Res., Vol. 16, No. 11, Nov 2001 Downloaded: 13 Mar 2015
considerations require that these heterostructures must be able to withstand relatively harsh environments and high temperatures for a long period of time. Furthermore, the lower melting points of some metals result in higher diffusivities and possible agglomeration of ultrathin metallic films at elevated temperatures. This agglomeration will limit their application
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