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Internal photoemission of holes and electrons into KN3
Volume 39A, number 2
PHYSICS LETTERS
24 April 1972
INTERNAL PHOTOEMISSION OF HOLES AND ELECTRONS INTO KN3* A. De PANAFIEU and B.S.H. ROYCE Solid State and Materials Laboratory, Princeton University, Princeton, N.J., USA Received 6 March 1972
Holes and electrons have been photoinjected into single crystals of KN3 from both gold and silver electrodes. For gold the injection thresholds for electrons and holes were 4.4 ± 0.1 and 3.92 ± 0.15 eV respectively and for silver electrodes 4.0 ± 0.1 eV and 4.44 ± 0.15 eV. From these values a band gap of 8.44 ± 0.25 eV is obtained for KN3. The
m"product for electrons at room temperature was found to be (1.8 ± 0.2)X 10"4 em2/V. For holes ttr ,~ (6 ± 2) X I0-s cm2/V.
Potassium azide is a wide band gap ionic crystal that may be caused to decompose by light having a wavelength on the order of the band gap, by ionizing radiation such as X-rays or even thermally at temperatures near its melting point. This paper is concerned with experiments designed to provide additional information about the band structure and the mobility of charge carriers within the crystal. Both holes and electrons have been photoinjected into single crystals of KN 3 from gold and silver vacuum deposited electrodes using the technique of internal photoemission [1]. Single crystal specimens were cut from boules of melt grown [2] KN 3 kindly supplied by Dr. J. Sharma of Picatinny Arsenal. This material had a total impurity content of circa 300 ppm. Specimens of approximately 0.25 cm 2 and 0.1 cm thickness had their principal faces normal to the crystalline c-axis and a semi-transparent electrode of either silver or gold was evaporated onto one of these. The sample was placed on the cold finger of a vacuum cryostat and was illuminated with light from a Bausch and Lomb grating monochromator (Model 33-86-01) with a pass band of about 9.5 nm. The light source was a xenon lamp and the incident intensity was measured with a Hewlett Packard model 8334A calibrated thermopile. The photon flux at the metal/crystal interface was corrected for the absorption of the electrode by measuring the optical density of a film deposited on a thin KBr crystal during the same evapopartially supported by AROD Grant C-0049.
* Work
# DAHC-04-69-
ration. The photoinjection current in the KN 3 was measured using a model 816A EKCO vibrating reed electrometer. With an applied field of circa 3× 103
Q ELECTRONEMISSION 450 V HOLEEMISSION I00 V / ~10 AG ELECTRODE
/
/
'7"
0.6
z
./
.~
0.4
~2 t'3
a-
!
4
h
EeV.
=
5
Fig. 1. Normalized electron and hole photoinjection currents as a function of photon energy.
77
Volume 39A, number 2
PHYSICS LETTERS
0.30
d
031
~.70
•
&32
,Hrr,,Ir (a)
844 ~
H,r,,,i,
irr,1,,r, (b)
Fig. 2. Band gap information for KN3. a) Thresholds for injection from gold electrodes, b) Thresholds for injection from silver electrodes. V/cm and an incident photon flux of circa 1 mW/cm 2 currents on the order of 10 -11 A were obtained for electron injection and 10 -14 A for hole injection. The dark current was circa 10 -15 A for both polarities of the injection electrode with respect to the crystal. The dark current was allowed to reach a steady state at the beginning of each measurement and the crystal then subjected to alternate periods of illumination and dark. The wavelength of the incident fight was varied in 5nm increments and the steady state photocurrent at each wavelength was measured. Fig. 1 shows a plot of the square root of the normalized photocurrent per incident photon as a function of photon energy. In accordance with Fowler's [3] theory this is found to be linear and the curve may be extrapolated to zero current to obtain the threshold energy for electron or hole injection. For gold electrodes these threshold values are 4.40 + 0.10eV and 3.92 + 0.15 eV respectively. For the silver electrodes the thresholds are 4.00 + 0.10eV and 4.44 ± 0.15 eV respectively. The main source of error in evaluating these thresholds are the large pass band of the monochromator and the uncertainty in the optical absorption of the semitransparent electrode, which was found to be dependent on the substrate onto which it was evaporated. The shift in the threshold energies in going from an
78
24 April 1972
Au to Ag electrode is consistent with published data on the work function of similarly evaporated toms for both polarities of the injection electrode. The value obtained for the band gap of KN 3 (fig. 2) is 8.44 + 0.25 eV and is consistent with the earlier value of 8.55 eV obtained by Deb [4] from optical absorption measurements and also in agreement with the value of 8.6 eV obtained from the band structure calculations of Young [5]. The data also indicates an electron affinity for KN 3 of circa 0.3 eV. The voltage depedence of the photoinjection current was found to take the form: ~I/Io) = (V/Vo) [1 - e x p ( - V o / V ) ] which is consistent with the model developed by Lampert [6] for injection with blocking electrodes. Here Io and Vo = (12/#T) arq constants, l is the sample thickness, bt the carrier mobility and T its mean free time. Vo was obtained from a fit of experimental data to the above expression and, by combining this with the sample thickness the ~¢ product evaluated. At room temperature/aT = (1.8 + 0.2)X 10-4cm2/V for electrons. The data for holes was less reliable but gT was estimated to be (6 + 2)X 10-Scm2/V. Preliminary measurements indicate that the hole conductivity may be thermally activated. It is interesting to note that no photo-decomposition of the specimens was observed during these measurements. This may indicate that the creation of N3~bY the injection of holes into the valence band is not sufficient to initiate decomposition.
References [ 1] R. Williams, Semiconductors and semimetals, Vol.6, eds. R.K. Willardson and A. Beer (Academic Press, ~970) p. 97. [2] J.N. Appleton and J. Sharma, Mat. Res. Bull. 5'(1970) 227. [3] A.L. Hughes and L.A. DuBridge, Photoelectric phenomena (McGraw-Hill,New York, 1932) p. 243. [4] S.K. Deb, J. Chem. Phys. 35 (1961) 2122. [5] D.A. Young, Progress in Solid State Chemistry 5 (Pergamon Press, 1970) 401. [6] M.A. Lampert and P. Mark, Current injection in solids (Academic Press, 1970) p. 150.
PHYSICS LETTERS
24 April 1972
INTERNAL PHOTOEMISSION OF HOLES AND ELECTRONS INTO KN3* A. De PANAFIEU and B.S.H. ROYCE Solid State and Materials Laboratory, Princeton University, Princeton, N.J., USA Received 6 March 1972
Holes and electrons have been photoinjected into single crystals of KN3 from both gold and silver electrodes. For gold the injection thresholds for electrons and holes were 4.4 ± 0.1 and 3.92 ± 0.15 eV respectively and for silver electrodes 4.0 ± 0.1 eV and 4.44 ± 0.15 eV. From these values a band gap of 8.44 ± 0.25 eV is obtained for KN3. The
m"product for electrons at room temperature was found to be (1.8 ± 0.2)X 10"4 em2/V. For holes ttr ,~ (6 ± 2) X I0-s cm2/V.
Potassium azide is a wide band gap ionic crystal that may be caused to decompose by light having a wavelength on the order of the band gap, by ionizing radiation such as X-rays or even thermally at temperatures near its melting point. This paper is concerned with experiments designed to provide additional information about the band structure and the mobility of charge carriers within the crystal. Both holes and electrons have been photoinjected into single crystals of KN 3 from gold and silver vacuum deposited electrodes using the technique of internal photoemission [1]. Single crystal specimens were cut from boules of melt grown [2] KN 3 kindly supplied by Dr. J. Sharma of Picatinny Arsenal. This material had a total impurity content of circa 300 ppm. Specimens of approximately 0.25 cm 2 and 0.1 cm thickness had their principal faces normal to the crystalline c-axis and a semi-transparent electrode of either silver or gold was evaporated onto one of these. The sample was placed on the cold finger of a vacuum cryostat and was illuminated with light from a Bausch and Lomb grating monochromator (Model 33-86-01) with a pass band of about 9.5 nm. The light source was a xenon lamp and the incident intensity was measured with a Hewlett Packard model 8334A calibrated thermopile. The photon flux at the metal/crystal interface was corrected for the absorption of the electrode by measuring the optical density of a film deposited on a thin KBr crystal during the same evapopartially supported by AROD Grant C-0049.
* Work
# DAHC-04-69-
ration. The photoinjection current in the KN 3 was measured using a model 816A EKCO vibrating reed electrometer. With an applied field of circa 3× 103
Q ELECTRONEMISSION 450 V HOLEEMISSION I00 V / ~10 AG ELECTRODE
/
/
'7"
0.6
z
./
.~
0.4
~2 t'3
a-
!
4
h
EeV.
=
5
Fig. 1. Normalized electron and hole photoinjection currents as a function of photon energy.
77
Volume 39A, number 2
PHYSICS LETTERS
0.30
d
031
~.70
•
&32
,Hrr,,Ir (a)
844 ~
H,r,,,i,
irr,1,,r, (b)
Fig. 2. Band gap information for KN3. a) Thresholds for injection from gold electrodes, b) Thresholds for injection from silver electrodes. V/cm and an incident photon flux of circa 1 mW/cm 2 currents on the order of 10 -11 A were obtained for electron injection and 10 -14 A for hole injection. The dark current was circa 10 -15 A for both polarities of the injection electrode with respect to the crystal. The dark current was allowed to reach a steady state at the beginning of each measurement and the crystal then subjected to alternate periods of illumination and dark. The wavelength of the incident fight was varied in 5nm increments and the steady state photocurrent at each wavelength was measured. Fig. 1 shows a plot of the square root of the normalized photocurrent per incident photon as a function of photon energy. In accordance with Fowler's [3] theory this is found to be linear and the curve may be extrapolated to zero current to obtain the threshold energy for electron or hole injection. For gold electrodes these threshold values are 4.40 + 0.10eV and 3.92 + 0.15 eV respectively. For the silver electrodes the thresholds are 4.00 + 0.10eV and 4.44 ± 0.15 eV respectively. The main source of error in evaluating these thresholds are the large pass band of the monochromator and the uncertainty in the optical absorption of the semitransparent electrode, which was found to be dependent on the substrate onto which it was evaporated. The shift in the threshold energies in going from an
78
24 April 1972
Au to Ag electrode is consistent with published data on the work function of similarly evaporated toms for both polarities of the injection electrode. The value obtained for the band gap of KN 3 (fig. 2) is 8.44 + 0.25 eV and is consistent with the earlier value of 8.55 eV obtained by Deb [4] from optical absorption measurements and also in agreement with the value of 8.6 eV obtained from the band structure calculations of Young [5]. The data also indicates an electron affinity for KN 3 of circa 0.3 eV. The voltage depedence of the photoinjection current was found to take the form: ~I/Io) = (V/Vo) [1 - e x p ( - V o / V ) ] which is consistent with the model developed by Lampert [6] for injection with blocking electrodes. Here Io and Vo = (12/#T) arq constants, l is the sample thickness, bt the carrier mobility and T its mean free time. Vo was obtained from a fit of experimental data to the above expression and, by combining this with the sample thickness the ~¢ product evaluated. At room temperature/aT = (1.8 + 0.2)X 10-4cm2/V for electrons. The data for holes was less reliable but gT was estimated to be (6 + 2)X 10-Scm2/V. Preliminary measurements indicate that the hole conductivity may be thermally activated. It is interesting to note that no photo-decomposition of the specimens was observed during these measurements. This may indicate that the creation of N3~bY the injection of holes into the valence band is not sufficient to initiate decomposition.
References [ 1] R. Williams, Semiconductors and semimetals, Vol.6, eds. R.K. Willardson and A. Beer (Academic Press, ~970) p. 97. [2] J.N. Appleton and J. Sharma, Mat. Res. Bull. 5'(1970) 227. [3] A.L. Hughes and L.A. DuBridge, Photoelectric phenomena (McGraw-Hill,New York, 1932) p. 243. [4] S.K. Deb, J. Chem. Phys. 35 (1961) 2122. [5] D.A. Young, Progress in Solid State Chemistry 5 (Pergamon Press, 1970) 401. [6] M.A. Lampert and P. Mark, Current injection in solids (Academic Press, 1970) p. 150.