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Thermally stimulated currents in trigonal selenium
Volume 26A. number 5
PHYSICS
LETTERS
trons can be determined from the measured Fermi level and the condition of compensation as shown by the solid circles. The experimental points do not agree either with curve A or with B predicted from the modified two-band model, where A and B are standing for the spin splitting smaller and larger than the orbital splitting, respectively. Taking into account the effect of other bands upon the two bands under consideration, Baraff [4] has shown that the lowest Landau level of the electrons moves linearly with the magnetic field as e(n=O, s=-1) = aH, where a is a constant connected with matrix elements between the conduction band and other bands. For higher Landau levels, the modified two-band model has been shown to be a good approximation. Our result shows that a = -4.4 X 10e20 erg/gauss.
*
THERMALLY
STIMULATED
29 January 1968
The mass parameters used in the present analysis are those of Kao [5] and the energy gap for electron band and the zero field carrier density were assumed to be 15 meV and 2.85 x X 1017cm-3? respectively.
References 1. G. E. Smith, G.A. Baraff and J. M. Rowell, Phys. Rev. 135 (1964) A1118. 2. G.A. Williams and G. E. Smith, IBM J. Res. Dev. 8 (1964) 276. 3. R. T. Bate and N.G. Einspruch, J. Phys. Sot. Japan, 21 supplement (1966) 673. 4. G.A.Baraff, Phys. Rev. 137 (1965) A842. 5. Y.H.Kao. Phys. Rev. 129 (1963) 1122. **4
c *
CURRENTS
IN
TRIGONAL
SELENIUM
H. K. HENISCH and M. H. ENGINEER Materials
Research Laboratoq, University Park,
The Pennsylvania Pennsylvania,
State
University,
USA
Received 16 November 1967
Upon U.V. irradiation of trigonal selenium. thermally stimulated currents have been observed between lOOoK and 320’K. from which two trapping levels. 0.45 and 0.54’V below the conduction band, have been derived
Recent studies [l] have indicated the probable existence of electron traps in crystals of trigonal selenium grown from the vapour phase. We have observed thermally stimulated currents in crystals of trigonal selenium grown by Harrison [2] under high pressure, in the temperature range lOOoK to 320’K. In an apparatus of conventional design, the selenium crystal was mounted on a quartz slide at the end of the “cold finger” and U.V. radiation was provided by a Blak-Ray UVL-22 lamp. The chamber was evacuated to about 10e4 mm of Hg. The specimens (10 X 6.5 x 1.5mm3) were cut with a wire saw from a large boule mounted in epoxy resin, hand polished using a grit of aluminium oxide (< 0.1 micron grain size) and etched with sodium sulphite solution and, finally, rinsed in de-ionised water. Evaporated gold was 188
used for current contacts to the specimen. These contacts are known to be ohmic and of considerably lower resistance than the specimen used, which was of the order of 1.5 megohms at room temperature. Power was supplied by a stabilized voltage source (- 75 V) and the specimen current measured by a Keithley 600 A electrometer, the output of which was fed into the X-channel of a Moseley Model 2D-2A X-Y recorder. The current direction was roughly perpendicular to the c-axis. The Y-channel was made to record specimen temperature, increasing at an approximately linear rate of 14’C/sec near the major peaks. Two processes can be envisaged to occur during radiation: (i) electrons may be raised from the valence band directly into traps, leaving behind free holes, and (ii) electrons are raised
Volume 26A. number
5
PHYSICS
LETTERS
29 January 1968
-/ TIME
Fig. 1. Initial photoconductive decay at excitation perature.
tem-
from the valence to the conduction band creating free electron-hole pairs. After irradiation, while the specimen is being heated, the observed current depends on the total number of free carriers i.e., those ordinarily present in the valence and conduction bands during their lifetime, augmented by those thermally released from traps. A typical current decay curve after irradiation at 120°K is shown in fig. 1. The initial sharp drop is probably due to direct recombination of electron-hole pairs. Thereafter, the rate of decay is much slower and is presumably governed by the rate of thermal electron release from traps followed, in turn, by recombination. In general, the time constants of these decays are functions of both u.v-dosage and temperature. The heating part of the experimental cycle was, in all instances, begun at a stated time after the sharp initial decay in the photocurrent. Typical experimental results on thermally stimulated currents are shown in fig. 2. There are two peaks which may be identified with trapping levels 0.45 and 0.54 eV below the conduction band, using Urbach’s approximate formula [3] E = T*/500, where T* is the temperature of the peak in degrees Kelvin. The deep electron trap at 0.9 eV below the conductive band found by Stuke [l] could not be observed in the present experiment as it corresponds to a peak temperature of approximately 450°K. The curve for zero dose was well reproducible (within 2%) after a two-day rest period following irradiation, showing that the system was essentially stable. The remaining curves on fig. 1 do not refer to changes from the equilibriated state but were taken in much more rapid succession. The position of the peaks is nevertheless sensibly constant. The dependence of properties on defects, including those introduced into selenium by mechanical handling and deformation, is now well recognized [4], and this leads to a demand for ad-
Fig. 2. Thermally stimulated currents in trigonal nium after U.V. irradiation. Curve
U.V. dose arb. units
Delay minutes
1 2 3 4 5 6 7
0 2 4 8 12 16 20
0 12 15 15 18 20 20
sele-
ditional and reliable characterization procedures. Measurements of the type here described are simple and though the peak magnitudes are not easily interpreted, the corresponding peak temperatures lead to trap-depth estimates in a simple and familiar way. It is thus plausible that the method might be used for differentiating between specimens of equal impurity content but different concentrations of native defects.
References 1. J. Stuke, Proc. Recent advances in selenium physics, (Pergamon Press,Oxford, 1964) pp. 35-51. 2. D. E. Harrison, Proc. Recent advances in selenium physics (Pergamon Press,Oxford, 1964) pp. 67. 3. F.Urbach, Symp. on Solid luminescent materials, Cornell University (Wiley, New York, 1946) p. 115. 4. V. Prosser and H.K.Henisch, Mat. Res. Bull. 1 (1966) 283.
*****
189
PHYSICS
LETTERS
trons can be determined from the measured Fermi level and the condition of compensation as shown by the solid circles. The experimental points do not agree either with curve A or with B predicted from the modified two-band model, where A and B are standing for the spin splitting smaller and larger than the orbital splitting, respectively. Taking into account the effect of other bands upon the two bands under consideration, Baraff [4] has shown that the lowest Landau level of the electrons moves linearly with the magnetic field as e(n=O, s=-1) = aH, where a is a constant connected with matrix elements between the conduction band and other bands. For higher Landau levels, the modified two-band model has been shown to be a good approximation. Our result shows that a = -4.4 X 10e20 erg/gauss.
*
THERMALLY
STIMULATED
29 January 1968
The mass parameters used in the present analysis are those of Kao [5] and the energy gap for electron band and the zero field carrier density were assumed to be 15 meV and 2.85 x X 1017cm-3? respectively.
References 1. G. E. Smith, G.A. Baraff and J. M. Rowell, Phys. Rev. 135 (1964) A1118. 2. G.A. Williams and G. E. Smith, IBM J. Res. Dev. 8 (1964) 276. 3. R. T. Bate and N.G. Einspruch, J. Phys. Sot. Japan, 21 supplement (1966) 673. 4. G.A.Baraff, Phys. Rev. 137 (1965) A842. 5. Y.H.Kao. Phys. Rev. 129 (1963) 1122. **4
c *
CURRENTS
IN
TRIGONAL
SELENIUM
H. K. HENISCH and M. H. ENGINEER Materials
Research Laboratoq, University Park,
The Pennsylvania Pennsylvania,
State
University,
USA
Received 16 November 1967
Upon U.V. irradiation of trigonal selenium. thermally stimulated currents have been observed between lOOoK and 320’K. from which two trapping levels. 0.45 and 0.54’V below the conduction band, have been derived
Recent studies [l] have indicated the probable existence of electron traps in crystals of trigonal selenium grown from the vapour phase. We have observed thermally stimulated currents in crystals of trigonal selenium grown by Harrison [2] under high pressure, in the temperature range lOOoK to 320’K. In an apparatus of conventional design, the selenium crystal was mounted on a quartz slide at the end of the “cold finger” and U.V. radiation was provided by a Blak-Ray UVL-22 lamp. The chamber was evacuated to about 10e4 mm of Hg. The specimens (10 X 6.5 x 1.5mm3) were cut with a wire saw from a large boule mounted in epoxy resin, hand polished using a grit of aluminium oxide (< 0.1 micron grain size) and etched with sodium sulphite solution and, finally, rinsed in de-ionised water. Evaporated gold was 188
used for current contacts to the specimen. These contacts are known to be ohmic and of considerably lower resistance than the specimen used, which was of the order of 1.5 megohms at room temperature. Power was supplied by a stabilized voltage source (- 75 V) and the specimen current measured by a Keithley 600 A electrometer, the output of which was fed into the X-channel of a Moseley Model 2D-2A X-Y recorder. The current direction was roughly perpendicular to the c-axis. The Y-channel was made to record specimen temperature, increasing at an approximately linear rate of 14’C/sec near the major peaks. Two processes can be envisaged to occur during radiation: (i) electrons may be raised from the valence band directly into traps, leaving behind free holes, and (ii) electrons are raised
Volume 26A. number
5
PHYSICS
LETTERS
29 January 1968
-/ TIME
Fig. 1. Initial photoconductive decay at excitation perature.
tem-
from the valence to the conduction band creating free electron-hole pairs. After irradiation, while the specimen is being heated, the observed current depends on the total number of free carriers i.e., those ordinarily present in the valence and conduction bands during their lifetime, augmented by those thermally released from traps. A typical current decay curve after irradiation at 120°K is shown in fig. 1. The initial sharp drop is probably due to direct recombination of electron-hole pairs. Thereafter, the rate of decay is much slower and is presumably governed by the rate of thermal electron release from traps followed, in turn, by recombination. In general, the time constants of these decays are functions of both u.v-dosage and temperature. The heating part of the experimental cycle was, in all instances, begun at a stated time after the sharp initial decay in the photocurrent. Typical experimental results on thermally stimulated currents are shown in fig. 2. There are two peaks which may be identified with trapping levels 0.45 and 0.54 eV below the conduction band, using Urbach’s approximate formula [3] E = T*/500, where T* is the temperature of the peak in degrees Kelvin. The deep electron trap at 0.9 eV below the conductive band found by Stuke [l] could not be observed in the present experiment as it corresponds to a peak temperature of approximately 450°K. The curve for zero dose was well reproducible (within 2%) after a two-day rest period following irradiation, showing that the system was essentially stable. The remaining curves on fig. 1 do not refer to changes from the equilibriated state but were taken in much more rapid succession. The position of the peaks is nevertheless sensibly constant. The dependence of properties on defects, including those introduced into selenium by mechanical handling and deformation, is now well recognized [4], and this leads to a demand for ad-
Fig. 2. Thermally stimulated currents in trigonal nium after U.V. irradiation. Curve
U.V. dose arb. units
Delay minutes
1 2 3 4 5 6 7
0 2 4 8 12 16 20
0 12 15 15 18 20 20
sele-
ditional and reliable characterization procedures. Measurements of the type here described are simple and though the peak magnitudes are not easily interpreted, the corresponding peak temperatures lead to trap-depth estimates in a simple and familiar way. It is thus plausible that the method might be used for differentiating between specimens of equal impurity content but different concentrations of native defects.
References 1. J. Stuke, Proc. Recent advances in selenium physics, (Pergamon Press,Oxford, 1964) pp. 35-51. 2. D. E. Harrison, Proc. Recent advances in selenium physics (Pergamon Press,Oxford, 1964) pp. 67. 3. F.Urbach, Symp. on Solid luminescent materials, Cornell University (Wiley, New York, 1946) p. 115. 4. V. Prosser and H.K.Henisch, Mat. Res. Bull. 1 (1966) 283.
*****
189