Infrared Optical Studies of Semiconductors at Large Hydrostatic Pressures
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E.E. HALLER AND M.D. McCLUSKEY Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720 USA
ABSTRACT Among the various external disturbances used in the study of semiconductors, including electric and magnetic fields as well as uniaxial pressure, large hydrostatic pressures can be employed to induce dramatic changes in host lattice, dopant and defect properties. Diamond anvil cells with an appropriate pressure medium (e.g. liquid N 2 or alcohol mixtures) allow the application of pressures up to hundreds of kbar. In this pressure range the global conduction band minimum of a semiconductor can become a local minimum. GaAs for example changes near 45 kbar from a direct (F-band) to an indirect (X-band) semiconductor. Donors in GaAs and InP transform from their shallow, hydrogenic state to the DX configuration at hydrostatic pressures near 23 and 82 kbar, respectively. This donor configuration change has been studied using local vibrational mode (LVM) spectroscopy in the far infrared region of the electromagnetic spectrum. Recently we have investigated several LVM's of H-containing complexes in GaAs as a function of hydrostatic pressure at liquid He temperatures. Depending on the specific complex we find the LVM frequencies to vary either linearly, sub- or superlinearly with hydrostatic pressure. In the case of 0 in Si the vibrational mode changes its character from that of a harmonic oscillator to a rotor as pressure is applied. The implications of the pressure dependences of LVM's are discussed. INTRODUCTION Local vibrational mode (LVM) spectroscopy is a useful technique for determining the microscopic structure of impurities and defects in semiconductors [1,2]. The application of hydrostatic pressure is an excellent tool for probing the electronic and vibrational properties of defects in semiconductors. The necessary large, time independent hydrostatic pressures are conveniently produced by diamond anvil cells (DACs) [3]. In the particular case of high pressure semiconductor studies the DAC offers a range of advantages as well as limitations. The optical properties of diamonds allow access to a sample inside the DAC over an extremely wide range of the electromagnetic spectrum, the only limitations being imposed by the diamond multiphonon absorption band near 2100 cm"' and, depending on the quality of the diamonds, absorption bands due to impurities [type I (Nitrogen): 700-1500 cm"1, type Ila (pure): 1900-2400 cm"1 and type lib (Boron): 2400-3100 cm' 1 ]. In contrast to this almost ideal situation regarding optical studies of semiconductors in DACs, few studies using electrical measurements have been reported [4]. The introduction of electrical leads has only been achieved with powder pressure media which may lead to non-uniform pressure distributions. current address: Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 USA 371 Mat. Res. Soc. Symp. Proc. Vol. 499 c 1998 Materials Research Society
In the case of far infrared optical spectroscopy the limit
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