X-Ray Diffraction Studies of Low Temperature GaAs
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X-RAY DIFFRACTION STUDIES OF LOW TEMPERATURE GaAs 1
, M.LESZCJYNSKI 2 , A.KURPIEWSKI 1 , M.KAMINSKA 1 , . E.R.WEBER T.SUSKI ±Institu e of Experimental Physics, Warsaw University, Warsaw, Poland, High Pressure Res arch Center, Polish Academy of Sciences, Warsaw, Poland, Department of Materials Science, University of California, Berkeley, USA. G.KOWALJKI ,
ABSTRACT GaAs layers grown by molecular beam epitaxy (MBE) at low substrate temperatures (LT GaAs) were studied in a novel purpose designed X-ray experiment. It combines X-ray double crystal rocking curve measurements with some elements usually found in optical setups like light illumination at liquid nitrogen temperatures applied to transfer EL2 type defects into metastable state. Ability to record such transfers with the X-ray experiment as well as large lattice relaxation accompanying this process is presented.
Introduction The LT GaAs grown by molecular beam epitaxy at temperatures 1900 to 300 0 C exhibit high resistivity thus making it perfect for other technological purposes like metal-semiconductor field-effect transistor (MESFETs) structures or high electron mobility transistors (HEMTs). The recent growth of interest in those materials is stimulated by still unresolved crystal structure as well as electronic properties. Some preliminary results concerning crystal structure are already available [e.g.l,2] and show first of all that the layers of LT GaAs are crystalline and (2] that there is a mismatch of the lattice constant between substrate and the layer. Lattice mismatch was established [2] to be about• a/a= 10and was connected with the excess arsenic present in the layer. The mismatch can be eliminated by annealing the crystals above 600 0 C. One of the main structural properties of the LT GaAs is the presence of large amounts of excess (up to 1%) arsenic as recorded for example by auger electron spectroscopy [3]. EPR (electron paramagnetic resonance) studies (2] show that arsenic ar~tisite As ? defects with the concentrations around 10 cmwere de ectable in as grown LT GaAS. The optical absorption measurements [4,5] confirmed some similarities between defects in LT GaAs and EL2 defects in semi-insulating (SI) melt grown GaAs e.g. comparable absorption spectra, metastability, thermal dependence of the centers recoverable from the metastable state at around 120 K. But there are significant differences between EL2 in SI GaAs (in both charged EL2 + and neutral EL2 0 states) and EL2-like defects in LT GaAs [2,4,5] : i) EPR defect signal for LT GaAs is not optically quenchable, ii) maximal efficiency of the metastable transfer is wavelength dependent-900 nm for LT GaAs and 1050 nm for SI GaAs, iii) absorption spectrum of LT GaAs is not fully quenchable when transferring defects into metastable state, iiii) recovery from the metatsable state can be induced by light of the wavelength 1350 nm in LT GaAs but not in bulk SI GaAs. Mat. Res. Soc. Symp. Proc. Vol. 241. 61992 Materials Research Society
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Method The X-ray double crystal spectrometer is already w
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