Laser-Thermal Impurity Pumping of Shallow Donors in Ultrapure Germanium
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LASER-THERMAL IMPURITY PUMPING OF SHALLOW DONORS IN ULTRAPURE GERMANIUM T. THEILER, F. KEILMANN* AND E. E. HALLER"* Max-Planck-Institut fiir Festk~rperforschung, Heisenbergstr. 1,7000 Stuttgart 80, FRG University of California at Berkeley and Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 ABSTRACT Extremely narrow far-infrared lines [1] of OH-donors in ultrapure germanium are used to probe the fundamental dynamic processes of impurities. We examine the I s-2p transition as a function of laser intensity, using photothermal ionization spectroscopy (PTIS) modified by Zeeman tuning with frequency-fixed lasers. We observe a change of the resonance line shape in the intensity region near 1 0 4 W/cm 2.This effect can be quantitatively understood in a rate equation model which shows that at the critical intensity the ground state becomes depleted and the dependence of the recombination on the degree of ionization becomes important. Therefore the critical intensity depends also on compensation. EXPERIMENT An ultrapure Ge single crystal with the dimensions 1 x 3 x 7 mm3 grown at the Lawrence Berkeley Laboratory is studied. The long axis is orientated in the [111] direction, the 7 x 3 mm 2 faces are
Us
reference magnetic field
Fig. 1 Experimental setup Mat. Res. Soc. Symp. Proc. Vol. 163. ©1990 Materials Research Society
134
2 normal to the [112] direction. The two 7 x 1 mm faces are ion-implanted with P (25keV, 4 x 1014 cm-2) and annealed at 300°C for 2 h in Ar to form good ohmic contacts. The sample has an OHconcentration of about 1011 cm-3 and a P concentration of 30 - 60% of that value. The concentration of the residual acceptors is of the order of 1010 cm 3 . The crystal is mounted in a liquid He bath cryostat at 4.2K with the electric field in [111] direction and the magnetic field in [112] direction. The Is-2p-A transition of the D(OH) centre is Zeeman-tuned and excited with a CO2 pumped alcohol laser at 117.227 gm. The intensity of the c.w. laser is varied between 1046 and 10-2 W/cm 2. Figure 1 shows the experimental setup: The photothermal current of the sample is measured with and without radiation using a lock-in technique. The difference signal divided by the actual laser intensity, which is measured by a Golay cell via a beam splitter, is plotted versus the magnetic field.
In this way small variations of the laser intensity are eliminated. A typical line shape is shown in
fig. 2. The decreasing background photoconductivity arises from the magnetoresistance. I= 0.06 mW/cm2
E= 4.3 V/cm
T = 4.2K C
(cc V (U
.0 00~ 0.,, 0 0
0 0
12
3
4
Magnetic field (kG) Fig.2 Line shape of the ls-2p-A transition asfunction of the magneticfield RESULTS
Figure 3 shows the ls-2 p A-resonance width at 4.2K measured at half maximum (FWHM) as a function of laser intensity for two different applied fields (E = 1.86 V/cm and E = 2.86 V/cm). The frequency shift of the ls-2 p.A-resonance is directly proportional to the applied magnetic field up to 5 kG [2] and therefore the linewidth can be calculated dire
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