Optimisation of the Heteroepitaxy of Ge on GaAs for Minority-Carrier Lifetime
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Optimisation Of The Heteroepitaxy Of Ge On GaAs For Minority-Carrier Lifetime
R. Venkatasubramanian ahd M.L. Timmons, Research Triangle Institute, Research Triangle Park, NC 27709; S. Bothra and J.M. Borrego, Rensselaer Polytechnic Institute, Troy, NY 12180.
Abstract: Growth of Ge on GaAs at reasonably high temperatures, which produce better crystallinity in the Ge, presents serious difficulties due to the dissociation of the GaAs substrate. In this paper, we describe the growth of a lowtemperature buffer layer of Ge on GaAs that prevents decomposition of the GaAs during high-temperature growth of Ge. Using this approach, we present the first report of highly specular, mass-transport-limited high-temperature growth of Ge on GaAs that is comparable to the homoepitaxy of Ge. The factors affecting the minority-carrier lifetime of Ge on GaAs, using such an epitaxial growth technique, were studied with a non-invasive microwave technique. Lifetime variations, from very low values to about 0.45 psec, were obtained as a function of the growth conditions. Significantly, the removal of the surface oxide on the GaAs substrate prior to low-temperature buffer-layer growth, terminating the flow of germane(GeH 4 ) during the ramp to high growth temperatures, thinner buffer layers, and high-temperature growth of Ge were found to be important for obtaining long lifetimes.
INTRODUCTION Germanium, although an indirect-gap semiconductor, has a significant absorption coefficient in the 1.0-1.5 um range for potential solar-cell applications in tandem with GaAs and related materials. Also, the high-quality growth of Ge on GaAs opens up the possibility for integration of photonic devices in the 1.3-1.5 pim range in Ge with electronic functions in GaAs. Also, the high intrinsic hole mobility in Ge may be of importance in a useful integration of certain Ge devices on GaAs. It is desirable to grow defect-free Ge on GaAs with high minority-carrier lifetime for many of these applications. Further, there is very little literature on lifetime measurements in Ge grown by the pyrolysis of GeH 4, much less as a function of growth conditions. These measurements will be useful for optimising Ge homoepitaxial stuctures. Ge does not luminesce, being an indirect-gap material, and unlike GaAs, PLdecay measurements are not useful to obtain minority-carrier lifetimes. Also, Ge substrates are not available as "semi-insulating", so that photoconductivitytransient measurements for lifetime are difficult because the parallel
Mat. Res. Soc. Symp. Proc. Vol. 198. @1990 Materials Research Society
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1.10
1.14 1.16
Figure2. Growth rate of Ge on GaAs between 675 and 775° C for a constant GeH 4 molefraction; also shown are data of other authors
Figurel. Single crystal x-ray diffraction trace from Ge/GaAs structures with various growth procedures described in text
TABLE I Sample No.
Deoxidation At 800"C
1( 1 2 3 4 5 0 8 9
10 11 12
7
NO No No No No No Yes y es Yes
Yes Yes Yes
Buffer-Layer ell4 Ramp Flow During Thickness (A
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