- Email: [email protected]
Photoconductivity in RbI and KI
J. Phys. Chm. Solids
Pergamon
Press 1961. Vol. 22, pp. 327331.
PHOTOCONDUCTIVITY Y. NAKAIt
Printed in Great Britain.
IN RbI AND KI*
and K. TEEGARDEN:
Institute of Optics, University
of Rochester
Abstract-A threshold for intrinsic photoconductivity has been observed in RbI and KI at room temperature and lower temperatures. These experiments were carried out with an electrode arrangement quite different from that employed previously by TEEGARDENand in a considerably cleaner vacuum. Photoconductivity occurs on the short wavelength side of the first exciton band in the region of the shoulder previously ascribed to the onset of band-to-band transitions. Photoconductivity also can occur on the long wavelength side of the first exciton peak when F-centers or other kinds of electron-surplus centers, formed by irradiation with ultraviolet light, are present in the crystals.
past twenty years to detect intrinsic photoconductivity in the alkali halides.(l) Three years have passed since one of the authors started preliminary experiments on this subject.(s) Subsequent work has been devoted to extensive improvement of the experimental technique and reliability of the data. The results described in this paper are in good agreement with the results of the preliminary work.
1. INTRODUCTION
TI-~E PURPOSE of this experiment was to determine the band gap of KI and RbI by measuring the threshold for intrinsic photoconductivity. There are several reasons why this experiment is of interest. (1) The relationship of the bandgap to the various features of the optical absorption spectrum of the alkali halides, i.e. the “exciton” bands, is of importance in determining the true nature of these features. (2) The question of whether or not photoconductivity occurs at the position of the first exciton band cannot be answered unless the threshold for photoconductivity is actually determined. That is, one cannot say that the absence of photoconductivity in the exciton band is significant unless one demonstrates that photoconductivity in other regions can actually be detected. (3) Energy transfer mechanisms, such as those thought to be responsible for host-sensitized luminescence in the alkali halides, include the production of free holes and electrons and the transfer of energy cannot be attributed to other mechanisms until photoconductivity has been ruled out. Several attempts have been made during the * Work supported in part by the U.S. Air Force through the Office of Scientific Research of the Air Research and Development Command. t On leave of absence from the University Kyoto, Japan. ; Alfred P. Sloan Foundation
Research
of Kyoto,
Fellow.
2. EXPERIMENTAL Crystals used in this experiment were cleaved from boules grown by Dr. K. KORTH of Kiel, Germany, by the Kyropoulos technique. The crystals were mounted in a cryostat so that they could be cooled to low temperature. The cryostat was provided with an evaporator so that films of KC1 could be deposited on to the iodide crystals and electrodes after they had been mounted in the crystal holder. Several electrode arrangements were used in the experiment. Two of these are shown in Fig. 1. The second arrangement provided the most homogeneous electric field distribution in the crystal and therefore provided the most quantitative information. Thin sheets of Mylar were placed between the crystal and the electrodes to prevent charge from entering or leaving the crystal. A film of KC1 approximately 1000 A thick was evaporated on to the crystal and electrodes to prevent photoemission from these surfaces. Currents were measured with an Applied Physics Laboratory vibrating reed electrometer. The “rate of charge” method was used in detecting the small currents involved. Dry cells were used to produce the required fields in the crystals. A Bausch and Lomb quartz prism monochromator was used with a hot-cathode low-pressure hydrogen discharge tube as a source. The light from the monochromator was focussed on to the crystal with a quartz lens. The intensity of the light was monitored by means of
327
Y.
328
NAKAI
and
a lP21 photomultiplier and a sodium salycilate phosphor. An absolute measure of the light intensity was obtained with a previously calibrated 6199 end-on photomultiplier with sodium salycilate on the outside of the end window. The end-on tube was used in order to insure that all the light incident on the crystals was intercepted by the detector. It is estimated that absolute measurements of the light intensity could be made to about 10 per cent.
K.
TEEGARDEN to minimize both these effects. It was found that a film of KC1 on the crystal and electrodes greatly suppressed photoemission. The spectral distribution of the photoemission from the crystal and electrodes was actually measured so that this phenomenon could be recognized if it occurred during the course of the photoconductivity measurements. Blank experiments, in which the KI or RbI crystals were replaced with KCl, were performed to be sure that the currents observed were not due to
MYLAR SHEET
@
-72’C
@
-176’C
SAMPLE MYLAR SHEET AI ELECTRODE COPPER HOLDER
TEFLON KCI (EVAR) TEFLON T 6 EV
(2)
5
FIG. 1. Schematic diagrams of the electrode arrangement. The copper holder was attached to the bottom of the inner Dewar of a cryostat.
FIG. 2. Photoconductivity in a single crystal of RbI with the electrode arrangement shown in (1) of Fig. 1.
It was found that a very clean vacuum was necessary to avoid contamination of the crystal surfaces at low temperature. A Varian Vat-Ion pump was finally used to evacuate the cryostat used in this experiment. If an oil-diffusion pump was used, the crystals became contaminated to the extent that photoemission could not be observed at low temperature. The greatest experimental difficulties encountered in this work were photoemission from the crystal and surrounding surfaces and polarization of the crystals due to the motion of the electrons.(s) Great care was taken
some extraneous source. It was found that it was necessary to use very low intensities of light to avoid polarization of the crystals and the build-up of various types of color centers. Also, a certain relaxation cycle was used at room temperature to reduce further the effects of polarization. The crystals were exposed to ultraviolet radiation with the field in one direction and the current measured. Subsequently they were irradiated with both visible and ultraviolet light with the field reduced to zero. Then the current was measured at the same wavelength with the field in the opposite direction. This
(1). At 20°C.
(2). At -72°C.
(3). At -176°C.
PHOTOCONDUCTIVITY cycle gave very reproducible results and the currents with both field directions were identical within a very small error. Curves 1 and 2 of Fig. 4 show the difference in photoconductivity which occurred when the crystal was not irradiated at zero field. 3. DISCUSSION
OF IWXJLTS
The primary results of this experiment are shown in Figs. 2,3 and 4. Fig. 2 shows the spectral distribution of the photocurrent in a crystal of
IN
Rbf
and
329
IS1
small minima or inflection points occur at photon energies near the position of the first and second exciton bands in RbI. (4) These features are shifted to somewhat higher photon energies in KI, as would be expected. The temperature dependence of the photocurrents shown in Figs. 2 and 3 has not been satisfactorily explained as yet, although it appears to be reproducible with a given electrode arrangement.
T
1
Rbl
KI
12)
(1)
1o-‘3 -
0 0 0
20* G -729
c
-176*
C
B 5 0. a
,(j”
_
d5
-
z c B & 3 E ::
----k-G EV
PIG. 4. Photoconducti~~ in a single crystal of RbI with the electrode arrangement shown in (2) of Fig. 1.
7
FIG. 3. Photoeonductivity in a single crystal of KI with the electrode arrangement shown in (1) of Fig. 1. (1). At 20°C.
(2) At -72°C.
(3). At
-176°C.
RbI at three temperatures. This data was obtained using the electrode arrangement number 1 in Fig. 1. It should be remembered that these curves have been corrected for changes in incident light intensity. They are not corrected for changes in the reflectivity of the crystals. In all cases the main feature of the curves is the relatively abrupt increase in the photocurrent in the vicinity of 6.0 eV. Either
(1). At 20°C. The crystal was irradiated with both unfiltered tungsten light and ultraviolet light of the wavelength used for the measurement with the electric field reduced to zero after every single reading for both polarities of electric field. (2). At 20°C. No irradiation at zero field. (3). At -66°C. No irradiation at zero field. (4). At -130°C. No irradiation at zero field.
With the electrodes arranged as in part (2) of Fig. 1, the dependence on temperature was somewhat different, as can be seen from Fig. 4. It was found that the photocurrents increased linearly with increasing field at a photon energy of
330
Y. NAKAI
and
6.2eV, for both RI and RbI. This was true for both directions of applied field and the slope of the straight line was the same for both field directions. No saturation was observed for either field direction up to fields of approximately 6000 V/cm. The currents also increased linearly with increasing light intensity at 6.2 eV. Figure 4 shows the results obtained with a crystal of RbI using the second electrode arrangement shown in Fig. 1. In this case the electric field was probably quite homogeneous in the crystal and an attempt has been made to give the results in terms of 7~s (1 -R), where 77 is the quantum efficiency, ws is the range in unit field, and R is the reflectivity of the crystal. The value of qwe (1 -R} was obtained by using the formula: quo (1 -ri) = ~(~)2~e~o~, where e is the charge of an electron, no the number of photons incident per second, V the applied voltage, i the photocurrent, and d the effective electrode distance or the actuai distance corrected for the presence of the two Mylar sheets. Values of these parameters are shown in Fig. 4. 4. CONCLUSIONS The results presented above indicate a value of about 6.0 eV for the band gap of RbI and KI. The threshold for photoconductivity seems to correspond to the shoulder appearing in the optical absorption spectrum of these crystals which has been previously attributed to the onset of band-to-band transitions.(5) Photocurrents were found to be smaller by a factor of at least 30 at photon energies corresponding to the peak of the first fundamental band, relative to the maximum photocurrents in KI and RbI. There are two explanations for this result. The one generally accepted is that the first absorption peak is due to the excitation of a nonphotoconducting bound state of holes and electrons. Another is that the holes and electrons produced by photons of energy corresponding to the peak of the first fundamental band are created near the surface because of the high absorption coefficient in this region. It is to be noted that the absorption coefficient at the band peak is of the order of 10s per cm, while it is of the order of 105 per cm at 6.2 eV. The data indicate only a reduction by a factor of about 2 or 3 near the peak of the second exciton band where the absorption
K. TEEGARDEN coefficient is also of the order of 10s per cm. This result means that the yield for free holes and electrons is indeed smaller at photon energies near the peak of the first fund~ental band and supports the hypothesis that the band is due to a non-photoconducting transition. It is to be noted, however, that photoconductivity does appear at the position of the second exciton peak at about 6.5 eV in RbI and about 6.7 eV in KI. This seems to indicate that these bands lie about O-5 eV above the bottom of the conduction band. It still has not been demonstrated to the satisfaction of either of the authors that light absorbed in the first fundamental band gives rise to mobile excitons, however. In this regard the small but measureable yield for photoconducti~ty at photon energies lower than the position of the first fundamental band is of significance. While it was found that the currents stimulated with 6.2 eV photons increased linearly with increasing light intensity, no such relationship was found for the currents stimulated with photons of 5.6 eV and lower energies. There was some indication that these rise as the square of the intensity. Therefore it is believed that these currents may depend upon the photoproduction and destruction of color centers in the crystals. The fact that F-centers can be ionized with photons of these energies has been demonstrated by INCHAUSP~). Because of this result, recent experiments which interpret host-sensitized impurity luminescence produced in alkali halide crystals by irradiation in the long wavelength tail of the first fundamental band as being due to the diffusion of excitons are open to criticism. Excitons are only one of the energy transfer mechanisms which can give rise to such luminescence. Another is the holes and electrons which are produced by irradiation in the fundamental band tail. It has been demonstrated that free-charge carriers can indeed cause luminescence of impurities in the crystals(7) Future experiments will be devoted to a study of the photoconductivity of these and other alkali halides at lower temperatures and higher photon energies. Since these materials have such a high absorption coefficient, the state of the crystal surfaces may play an important role in the photoconductive process, It would therefore be interesting to study the effect of different surface treatments on the photoconductivity.
PHOTOCONDUCTIVITY authors wish to thank Professor D. B. DUTTON for his extremely helpful criticism and advice during the course of this work. We also wish to thank Professor D. L. DEXTER for his many helpful suggestions and support.
Acknowledgements-The
REFERENCJXS 1. FEIKXJSONJ. N., Phys. Rev. 66,220 (1944); TAYLOR J. W. and HARTMANP. L., F%ys. Rev. 113, 1421 (1959). 2. TEEGARDEN K., AFOSR TN-59303, ASTIA Document No. A.D.-213087. 3. Apxxa L. and TAFT E., Phys. Rev. 79, 964 (1950) ;
IN
RbI
and
Kl
331
81, 698 (1951); 82, 814 (1951); PHILIPP H. R. and TAFT E. A., Phys. Rev. 106,671 (1957). 4. MARTIEN~~EN W., J. Phys. Chem. Solids 2, 257 (1957); EBY J. E., TEEGARDENK. J. and DUTTON D. B., Phys. Rev. 116,1099 (1959); MARTIEN~~EN W., J. Phys. Chem. Solids 8, 294 (1959); HARTMAN P. L., SIEGFRIED J. G. and NELSON J. R.,
Phys. Rev. 105,23 (1957). 5. TAFT E. A. and PHILIPP H. R., J. Phys. Chem. Solids 3, 1 (1957); PHILIPP H., TAFT E. A. and APKER L., Phys. Rev. 120,49 (1960). 6. INCHAUSP~N., Phys. Rev. 106,898 (1957). 7. TEEGARDENK. and WEEKS R., J. Phys. Chem. Solids 10, 211 (1959); TEEGARDENK. and EDGERTONR. to be published.
DISCUSSION A. ROSE: What is the response time (i.e. rise and decay time) for the photocurrents? Y. NAKAI: In the intrinsic region, i.e. in the region of photon energy above 5.8 eV, we did not observe any delay in response as far as the detector could follow. In other words, both the rise and the decay times are smaller than 0.5 sec. However, in the region of lower photon energy, we observed that photocurrents increased with time of irradiation, starting from a very small initial
value. With the U.V. light turned off, photocurrents decrease abruptly, that is, the decay time was shorter than 0.5 sec. If we repeated the same measurement without irradiating the crystal with visible light, the photocurrents started with an initial value approximately the same as the final value of the previous measurement. We believe that this type of response was due to the build-up of color cet_.tirs produced by ultraviolet light in the region of photon energy lower than 5.8 eV.
Pergamon
Press 1961. Vol. 22, pp. 327331.
PHOTOCONDUCTIVITY Y. NAKAIt
Printed in Great Britain.
IN RbI AND KI*
and K. TEEGARDEN:
Institute of Optics, University
of Rochester
Abstract-A threshold for intrinsic photoconductivity has been observed in RbI and KI at room temperature and lower temperatures. These experiments were carried out with an electrode arrangement quite different from that employed previously by TEEGARDENand in a considerably cleaner vacuum. Photoconductivity occurs on the short wavelength side of the first exciton band in the region of the shoulder previously ascribed to the onset of band-to-band transitions. Photoconductivity also can occur on the long wavelength side of the first exciton peak when F-centers or other kinds of electron-surplus centers, formed by irradiation with ultraviolet light, are present in the crystals.
past twenty years to detect intrinsic photoconductivity in the alkali halides.(l) Three years have passed since one of the authors started preliminary experiments on this subject.(s) Subsequent work has been devoted to extensive improvement of the experimental technique and reliability of the data. The results described in this paper are in good agreement with the results of the preliminary work.
1. INTRODUCTION
TI-~E PURPOSE of this experiment was to determine the band gap of KI and RbI by measuring the threshold for intrinsic photoconductivity. There are several reasons why this experiment is of interest. (1) The relationship of the bandgap to the various features of the optical absorption spectrum of the alkali halides, i.e. the “exciton” bands, is of importance in determining the true nature of these features. (2) The question of whether or not photoconductivity occurs at the position of the first exciton band cannot be answered unless the threshold for photoconductivity is actually determined. That is, one cannot say that the absence of photoconductivity in the exciton band is significant unless one demonstrates that photoconductivity in other regions can actually be detected. (3) Energy transfer mechanisms, such as those thought to be responsible for host-sensitized luminescence in the alkali halides, include the production of free holes and electrons and the transfer of energy cannot be attributed to other mechanisms until photoconductivity has been ruled out. Several attempts have been made during the * Work supported in part by the U.S. Air Force through the Office of Scientific Research of the Air Research and Development Command. t On leave of absence from the University Kyoto, Japan. ; Alfred P. Sloan Foundation
Research
of Kyoto,
Fellow.
2. EXPERIMENTAL Crystals used in this experiment were cleaved from boules grown by Dr. K. KORTH of Kiel, Germany, by the Kyropoulos technique. The crystals were mounted in a cryostat so that they could be cooled to low temperature. The cryostat was provided with an evaporator so that films of KC1 could be deposited on to the iodide crystals and electrodes after they had been mounted in the crystal holder. Several electrode arrangements were used in the experiment. Two of these are shown in Fig. 1. The second arrangement provided the most homogeneous electric field distribution in the crystal and therefore provided the most quantitative information. Thin sheets of Mylar were placed between the crystal and the electrodes to prevent charge from entering or leaving the crystal. A film of KC1 approximately 1000 A thick was evaporated on to the crystal and electrodes to prevent photoemission from these surfaces. Currents were measured with an Applied Physics Laboratory vibrating reed electrometer. The “rate of charge” method was used in detecting the small currents involved. Dry cells were used to produce the required fields in the crystals. A Bausch and Lomb quartz prism monochromator was used with a hot-cathode low-pressure hydrogen discharge tube as a source. The light from the monochromator was focussed on to the crystal with a quartz lens. The intensity of the light was monitored by means of
327
Y.
328
NAKAI
and
a lP21 photomultiplier and a sodium salycilate phosphor. An absolute measure of the light intensity was obtained with a previously calibrated 6199 end-on photomultiplier with sodium salycilate on the outside of the end window. The end-on tube was used in order to insure that all the light incident on the crystals was intercepted by the detector. It is estimated that absolute measurements of the light intensity could be made to about 10 per cent.
K.
TEEGARDEN to minimize both these effects. It was found that a film of KC1 on the crystal and electrodes greatly suppressed photoemission. The spectral distribution of the photoemission from the crystal and electrodes was actually measured so that this phenomenon could be recognized if it occurred during the course of the photoconductivity measurements. Blank experiments, in which the KI or RbI crystals were replaced with KCl, were performed to be sure that the currents observed were not due to
MYLAR SHEET
@
-72’C
@
-176’C
SAMPLE MYLAR SHEET AI ELECTRODE COPPER HOLDER
TEFLON KCI (EVAR) TEFLON T 6 EV
(2)
5
FIG. 1. Schematic diagrams of the electrode arrangement. The copper holder was attached to the bottom of the inner Dewar of a cryostat.
FIG. 2. Photoconductivity in a single crystal of RbI with the electrode arrangement shown in (1) of Fig. 1.
It was found that a very clean vacuum was necessary to avoid contamination of the crystal surfaces at low temperature. A Varian Vat-Ion pump was finally used to evacuate the cryostat used in this experiment. If an oil-diffusion pump was used, the crystals became contaminated to the extent that photoemission could not be observed at low temperature. The greatest experimental difficulties encountered in this work were photoemission from the crystal and surrounding surfaces and polarization of the crystals due to the motion of the electrons.(s) Great care was taken
some extraneous source. It was found that it was necessary to use very low intensities of light to avoid polarization of the crystals and the build-up of various types of color centers. Also, a certain relaxation cycle was used at room temperature to reduce further the effects of polarization. The crystals were exposed to ultraviolet radiation with the field in one direction and the current measured. Subsequently they were irradiated with both visible and ultraviolet light with the field reduced to zero. Then the current was measured at the same wavelength with the field in the opposite direction. This
(1). At 20°C.
(2). At -72°C.
(3). At -176°C.
PHOTOCONDUCTIVITY cycle gave very reproducible results and the currents with both field directions were identical within a very small error. Curves 1 and 2 of Fig. 4 show the difference in photoconductivity which occurred when the crystal was not irradiated at zero field. 3. DISCUSSION
OF IWXJLTS
The primary results of this experiment are shown in Figs. 2,3 and 4. Fig. 2 shows the spectral distribution of the photocurrent in a crystal of
IN
Rbf
and
329
IS1
small minima or inflection points occur at photon energies near the position of the first and second exciton bands in RbI. (4) These features are shifted to somewhat higher photon energies in KI, as would be expected. The temperature dependence of the photocurrents shown in Figs. 2 and 3 has not been satisfactorily explained as yet, although it appears to be reproducible with a given electrode arrangement.
T
1
Rbl
KI
12)
(1)
1o-‘3 -
0 0 0
20* G -729
c
-176*
C
B 5 0. a
,(j”
_
d5
-
z c B & 3 E ::
----k-G EV
PIG. 4. Photoconducti~~ in a single crystal of RbI with the electrode arrangement shown in (2) of Fig. 1.
7
FIG. 3. Photoeonductivity in a single crystal of KI with the electrode arrangement shown in (1) of Fig. 1. (1). At 20°C.
(2) At -72°C.
(3). At
-176°C.
RbI at three temperatures. This data was obtained using the electrode arrangement number 1 in Fig. 1. It should be remembered that these curves have been corrected for changes in incident light intensity. They are not corrected for changes in the reflectivity of the crystals. In all cases the main feature of the curves is the relatively abrupt increase in the photocurrent in the vicinity of 6.0 eV. Either
(1). At 20°C. The crystal was irradiated with both unfiltered tungsten light and ultraviolet light of the wavelength used for the measurement with the electric field reduced to zero after every single reading for both polarities of electric field. (2). At 20°C. No irradiation at zero field. (3). At -66°C. No irradiation at zero field. (4). At -130°C. No irradiation at zero field.
With the electrodes arranged as in part (2) of Fig. 1, the dependence on temperature was somewhat different, as can be seen from Fig. 4. It was found that the photocurrents increased linearly with increasing field at a photon energy of
330
Y. NAKAI
and
6.2eV, for both RI and RbI. This was true for both directions of applied field and the slope of the straight line was the same for both field directions. No saturation was observed for either field direction up to fields of approximately 6000 V/cm. The currents also increased linearly with increasing light intensity at 6.2 eV. Figure 4 shows the results obtained with a crystal of RbI using the second electrode arrangement shown in Fig. 1. In this case the electric field was probably quite homogeneous in the crystal and an attempt has been made to give the results in terms of 7~s (1 -R), where 77 is the quantum efficiency, ws is the range in unit field, and R is the reflectivity of the crystal. The value of qwe (1 -R} was obtained by using the formula: quo (1 -ri) = ~(~)2~e~o~, where e is the charge of an electron, no the number of photons incident per second, V the applied voltage, i the photocurrent, and d the effective electrode distance or the actuai distance corrected for the presence of the two Mylar sheets. Values of these parameters are shown in Fig. 4. 4. CONCLUSIONS The results presented above indicate a value of about 6.0 eV for the band gap of RbI and KI. The threshold for photoconductivity seems to correspond to the shoulder appearing in the optical absorption spectrum of these crystals which has been previously attributed to the onset of band-to-band transitions.(5) Photocurrents were found to be smaller by a factor of at least 30 at photon energies corresponding to the peak of the first fundamental band, relative to the maximum photocurrents in KI and RbI. There are two explanations for this result. The one generally accepted is that the first absorption peak is due to the excitation of a nonphotoconducting bound state of holes and electrons. Another is that the holes and electrons produced by photons of energy corresponding to the peak of the first fundamental band are created near the surface because of the high absorption coefficient in this region. It is to be noted that the absorption coefficient at the band peak is of the order of 10s per cm, while it is of the order of 105 per cm at 6.2 eV. The data indicate only a reduction by a factor of about 2 or 3 near the peak of the second exciton band where the absorption
K. TEEGARDEN coefficient is also of the order of 10s per cm. This result means that the yield for free holes and electrons is indeed smaller at photon energies near the peak of the first fund~ental band and supports the hypothesis that the band is due to a non-photoconducting transition. It is to be noted, however, that photoconductivity does appear at the position of the second exciton peak at about 6.5 eV in RbI and about 6.7 eV in KI. This seems to indicate that these bands lie about O-5 eV above the bottom of the conduction band. It still has not been demonstrated to the satisfaction of either of the authors that light absorbed in the first fundamental band gives rise to mobile excitons, however. In this regard the small but measureable yield for photoconducti~ty at photon energies lower than the position of the first fundamental band is of significance. While it was found that the currents stimulated with 6.2 eV photons increased linearly with increasing light intensity, no such relationship was found for the currents stimulated with photons of 5.6 eV and lower energies. There was some indication that these rise as the square of the intensity. Therefore it is believed that these currents may depend upon the photoproduction and destruction of color centers in the crystals. The fact that F-centers can be ionized with photons of these energies has been demonstrated by INCHAUSP~). Because of this result, recent experiments which interpret host-sensitized impurity luminescence produced in alkali halide crystals by irradiation in the long wavelength tail of the first fundamental band as being due to the diffusion of excitons are open to criticism. Excitons are only one of the energy transfer mechanisms which can give rise to such luminescence. Another is the holes and electrons which are produced by irradiation in the fundamental band tail. It has been demonstrated that free-charge carriers can indeed cause luminescence of impurities in the crystals(7) Future experiments will be devoted to a study of the photoconductivity of these and other alkali halides at lower temperatures and higher photon energies. Since these materials have such a high absorption coefficient, the state of the crystal surfaces may play an important role in the photoconductive process, It would therefore be interesting to study the effect of different surface treatments on the photoconductivity.
PHOTOCONDUCTIVITY authors wish to thank Professor D. B. DUTTON for his extremely helpful criticism and advice during the course of this work. We also wish to thank Professor D. L. DEXTER for his many helpful suggestions and support.
Acknowledgements-The
REFERENCJXS 1. FEIKXJSONJ. N., Phys. Rev. 66,220 (1944); TAYLOR J. W. and HARTMANP. L., F%ys. Rev. 113, 1421 (1959). 2. TEEGARDEN K., AFOSR TN-59303, ASTIA Document No. A.D.-213087. 3. Apxxa L. and TAFT E., Phys. Rev. 79, 964 (1950) ;
IN
RbI
and
Kl
331
81, 698 (1951); 82, 814 (1951); PHILIPP H. R. and TAFT E. A., Phys. Rev. 106,671 (1957). 4. MARTIEN~~EN W., J. Phys. Chem. Solids 2, 257 (1957); EBY J. E., TEEGARDENK. J. and DUTTON D. B., Phys. Rev. 116,1099 (1959); MARTIEN~~EN W., J. Phys. Chem. Solids 8, 294 (1959); HARTMAN P. L., SIEGFRIED J. G. and NELSON J. R.,
Phys. Rev. 105,23 (1957). 5. TAFT E. A. and PHILIPP H. R., J. Phys. Chem. Solids 3, 1 (1957); PHILIPP H., TAFT E. A. and APKER L., Phys. Rev. 120,49 (1960). 6. INCHAUSP~N., Phys. Rev. 106,898 (1957). 7. TEEGARDENK. and WEEKS R., J. Phys. Chem. Solids 10, 211 (1959); TEEGARDENK. and EDGERTONR. to be published.
DISCUSSION A. ROSE: What is the response time (i.e. rise and decay time) for the photocurrents? Y. NAKAI: In the intrinsic region, i.e. in the region of photon energy above 5.8 eV, we did not observe any delay in response as far as the detector could follow. In other words, both the rise and the decay times are smaller than 0.5 sec. However, in the region of lower photon energy, we observed that photocurrents increased with time of irradiation, starting from a very small initial
value. With the U.V. light turned off, photocurrents decrease abruptly, that is, the decay time was shorter than 0.5 sec. If we repeated the same measurement without irradiating the crystal with visible light, the photocurrents started with an initial value approximately the same as the final value of the previous measurement. We believe that this type of response was due to the build-up of color cet_.tirs produced by ultraviolet light in the region of photon energy lower than 5.8 eV.