Ultrafast Properties and Applications of GaAs and InP Based Materials Grown by MBE at Low Temperatures
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ULTRAFAST PROPERTIES AND APPLICATIONS OF GaAs AND InP BASED MATERIALS GROWN BY MBE AT LOW TEMPERATURES
S. GUPTA *, G. MOUROU*, F. W. SMITH** AND A. R. CALAWA** * Ultrafast Science Laboratory, Rm. 1006 IST Bldg., 2200 Bonisteel Blvd., University of Michigan, Ann Arbor, MI 48109 ** Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA
ABSTRACT The ultrafast carrier dynamics in GaAs, Ino.52 A10 .48As on InP, and In0.53Ga0.47As on InP, grown by molecular-beam-epitaxy (MBE) at low substrate temperatures, are investigated. A reduction in the carrier lifetime is observed with decreasing growth temperatures. The shortest carrier lifetimes of typically a picosecond (ps) are obtained at the lowest growth temperature range of 150 - 200 'C. Femtosecond optical absorption and reflectance measurements have been used to verify the sub-picosecond carrier lifetimes. Photoconductive switching measurements on these materials, measured using the technique of electro-optic sampling have further confirmed the sub-picosecond carrier lifetimes, and have also resulted in the generation of subpicosecond electrical signals. These short electrical pulses have been used for a variety of ultrafast optoelectronic applications. LOW TEMPERATURE MBE GROWTH AND MATERIAL PROPERTIES RELATING TO CARRIER DYNAMICS Introduction For conventional, high optical and electrical quality epitaxial growth by MBE, the substrate is heated to -600"C and -500'C for GaAs and InP substrates, respectively. The substrate temperature (Ts), the group V/I11 flux ratio and the growth rate are important parameters in determining the growth mechanism and the quality of the epitaxial layers. Smith et al. [1] were the first to demonstrate the interesting properties of GaAs grown by MBE at Ts -200 0 C at the usual growth rates of -I.tm/hr, where it was used as a high resistivity buffer layer, completely eliminating the side-gating and back-gating effects in GaAs MESFETs. Since then a number of detailed characterization of the material properties of the low-temperature (LT) MBE grown layers has been undertaken [2]-[6]. Some of these are summarized below, which are pertinent to the understanding of the ultrafast carrier dynamics. Low-temperature MBE of GaAs For the LT-MBE grown GaAs layers, Ga and As 4 beam fluxes were used at a V/I1 beam equivalent pressure ratio of 10. The growth rate is 1.0 pgm/hr on (001)-oriented semi-insulating GaAs substrates. First an undoped buffer layer of GaAs is grown at a temperature of 600'C to smoothen the growth front. Next the substrate temperature is lowered down to the desired value, and typically 1-2 pgm thick LT-GaAs layer is grown. A range of growth temperatures was investigated: 400, 350, 300, 260, 200 and 190"C. Some of the layers were then annealed inside the growth chamber under the As-overpressure by raising the substrate tempeature to about 600'C for a period of 3-10 min. All the layers studied here (1-2 ptm thick) were of high
Mat. Res. Soc. Symp. Proc. Vol. 241. @1992 Materials Research Society
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