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Photoconductivity of some alkali halide crystals in the fundamental absorption range
J. Phys.
Chem. Solids
Pergamon
Press 1961. Vol. 22, pp. 333-338.
Printed in Great Britain.
PHOTOCONDUCTIVITY OF SOME ALKALI HALIDE CRYSTALS IN THE FUNDAMENTAL ABSORPTION RANGE G. KUWABARA* Department
and
of Physics, University
K. AOYAGI of Tokyo, Tokyo, Japan
Abstract-The photoconductivity of KI, KBr and KC1 in the fundamental absorption range was studied between -70” and 110°C. The spectral response curve consists of three characteristic regions. The first is a bell-shaped structure in the low energy tail of the intrinsic absorption band, which is temperature sensitive and presumed to be due to imperfections; the second is a very faint photoconduction at the first exciton peak, and the third is the temperature-insensitive photoconduction above the higher energy tail of the first absorption peak due to direct photoionization in the undisturbed crvstal. The direct ionization starts at ca. 5.8, 6.8 and 7.5 eV in photon energy for KI KBr and KCi, respectively.
1. INTRODUCTION
THERE are several structures in the fundamental absorption range of alkali halide crystals. One group in the lower energy side has been interpreted as transitions creating electron-hole pairs coupled with each other, and another in the higher energy side as those creating free carriers. Recent experimentsuy 3) have shown that there are much more complicated structures than those expected by simple considerations, especially in crystals composed of heavy halogens. Excitons do not cause conductivity unless they dissociate by absorbing thermal energy or ionize other impurity centers by transfering their energy. Therefore the study of the photoconductivity in this region may afford an important clue to the interpretation of the absorption band or behavior of excitons in these crystals. The external photoelectric effect has been measured,(39 4) while the detection of the internal photoelectric effect has been unsuccessful as far as we know.(4* 5) In this paper the spectral response of the photoconductivity and some discussions thereof are given. 2. EXPERIMENTAL
PROCEDURES
The vacuum monochromator used is the same as that reported previously.@) The light intensity * Now at Argonne National Laboratory, Argonne, Illinois, on leave from the University of Tokyo. Z
333
in the photoconductivity measurement was about 1010 photons/cm3 set for the pass band of 3 rnp at 200 mp. The current was measured with a d.c. amplifier by the charging-up method or with a vibrating-reed electrometer. The noise level of the former is lo-13 C, and that of the latter is 2 x lo-16 A in the measuring condition. In this experiment, it is essential to reduce the external photoelectric current as far as possible, because most of the ordinary materials emit photoelectrons below 200 rnp, and the internal photocurrent will be completely masked.(4* 3) In order to reduce the external photocurrent from the electrode or from the crystal, various electrode arrangements and materials were tested. The best one so far obtained is shown in Fig. l(a). The crystal is pressed between two electrodes, on which LiF films are carefully evaporated. The light falls on the crystal through many fine holes drilled on the front electrode, i. e. the incident light is parallel to the applied electric field. In this arrangement the forward or reverse direction denotes the case in which the negative or positive potential is applied to the front electrode. The photoconductivity was measured with the forward direction, in which the created photoelectrons are drawn into the interior of the crystal. In some cases a third positive grid is placed before the front electrode to draw up the external photoelectrons from the front electrode. Another arrangement, as in Fig.
334
G. KUWABARA
l(b) was also tested, in which the background current due to the external photoeffect of the crystal was larger than that in the former, especially below 200 rn,~ in KI. The crystals used were those purchased from Harshaw and grown in our Laboratory by the Bridgeman method. They were cleaved to about 10 x 10 x 1.3 mm, The voltage applied was 450 V, i.e. the electric field was ca. 3500 V/cm.
and K. AOYAGI 3. RESULTS AND DISCUSSIONS The spectral response of the photocurrent in K-1 for the forward direction at room temperature is shown in Fig. 2(a). The ordinate is a photocurrent normalized to incident photon numbers. The curve consists of three regions. One is a sharp hv,
eV
- IOOOV
_rl-
+45 v -I --
2 4
3. VI-:
‘+ov
ib)
FIG. 1. Electrode arrangements. Electric field is parallel (a) and perpendicular (b) to the incident light. (a) 1. Dewar, 2. crystal, 3. lead to the electrometer, 4. rear electrode, 5. heater, 6. spring, 7, front electrode and 8. radiation shield. (b) 1. electrode, 2. crystal, 3. electrode and 4. shielding case.
In the measurements above room temperature, the detection chamber was filled with dry nitrogen of 0.8 atmospheric pressure. With these procedures, the background current or the external photoelectric current for the forward direction could be reduced below the noise level for light above 160 ml”. The current was kept small as far as possible, i.e. order of lo-15 A. Even with this small current the space charge effect appeared for light below 220 rnp in the case of ICI, because of small light penetration depth.
FIG. 2. Photocurrent and reflectivity of KI at room temperature : electrode arrangement (a), forward direction. (a) photocurrent of KI in O-8 atm Nz, (b) reflectance of KI single crystal, (c) absorption of KI film (after EBY et al.), (d) photocurrent of mechanically treated KI, i.e. the surface was rubbed with emery paper and polished.
structure of a bell shape, whose maximum lies at about 234 q extending between 210 and 225 rnp (denoted as the first peak). The second is a very faint photoconduction* around the first absorption band (denoted as the second region). There is a sharp dip between these two regions. The third is * This is iust a little above the noise level and is not shown in the figure. A detailed study of this region with higher sensitivity is in progress and will be reported in another paper.
PHOTOCONDUCTIVITY
OF ALKALI
HALIDE CRYSTALS
the broad structure which rises from the higher energy side of the first absorption band (third region). In the case of the forward direction an almost linear relation existed between the applied voltage and the amount of photocurrent for light of 185 rnp as well as that of 230 ny~ up to a field of 6,000 V/cm. This result shows that the photocurrent is due to the internal photoeffect. The curves (a) and (b) in Fig, 3 are the photoresponses
IN FUNDAMENTAL
335
Curve (d) in Fig. 1 is the response of a mechanically treated crystal, i.e. the surface is rubbed with emery paper and then polished with a fine cloth to reduce the light scattering. With this procedure, which is supposed to cause a large amount of strain or imperfections near the surface, the current in the third region decreased to less than onetenth of its original value, while that in the first peak remains almost the same except for a slight
eV
hy
hv, 8 Cl
50
ABSORPTION
’
6
I
I
I
250
200
150 Wavelength,
eV
7
mp
FIG. 3. Photocurrent of KI at room temperature, reverse direction. (a) reverse direction in 0.8 atm Na, (b) in vacuum, (c) electrode arrangement (b) in 0.8 atm Nz. for the reverse direction obtained in Ns of 0.8 atm and in vacuum, respectively, and (c) is for the electrode arrangement (b). These curves show that the external photocurrent from the crystal rapidly increases below 200 rnp. The temperature dependence of the photoresponse between 110” and - 70°C is shown in Fig. 4. As the temperature is decreased, the height of the first peak decreases remarkably compared with that of the third region, and its peak position shifts towards higher energy with a rate of 1.0 x 10-s eV/deg, which is almost the same rate as that of the exciton peak.
FIG. 4.
250
200 Wavelength,
m,u
Temperature dependence of photocurrent of KI.
change in the form of the higher energy side cutoff. By dissolving the surface layer with water, the photocurrent recovered almost the same value as that before treatment. The vacuum evaporation of a LiF film on to the surface also caused a similar effect. The current in the third region also decreases conspicuously with continuous irradiation by light in this band, while that of the first peak remains almost unchanged by irradiation with light in the first peak or in the third region. This is because in the former case a higher local space charge builds up near the surface than in the latter for an equal amount of current in the outer circuit. The above two facts show that the photoeffect in the first peak occurs almost uniformly
336
G.
KUWABARA
throughout the crystal, while that in the third region occurs within a quite thin layer near the surface, owing to the extremely small penetration depth of light, and is liable to be affected greatly by the crystal imperfection near the surface. After irradiation in the first peak for several minutes, the light above 600 rnp (designated as infrared light) causes the photocurrent, which decreases to zero with time. Moreover, the first peak almost coincides with the absorption peak of the p-band. Accordingly, it may be suggested that the peak is correlated with the F-center which is formed by an exciton and successively ionized by another exciton or directly by absorbing light in the p-band. This mechanism, however, is denied by the following reasons. The first peak appears even in the freshly cleaved crystal and the amount of current neither increases by prolonged irradiation with ultraviolet light nor decreases after sufficient irradiation with infrared light which includes the F-band of KI. If the c(- or /!-band is the origin of the photocurrent, we expect a large amount of negative-ion vacancies and accordingly a higher first peak for the quenched crystal. However, the peak height is nearly the same irrespective of the various treatments. If we assume that the creation and successive ionization of the F-center is the origin of the photocurrent, we should have a superlinear relation between the amount of current and the light intensity. This also contradicts the experiment. There are two possible mechanisms which cause the first peak: (1) Localized exciton absorption, i.e. the excitation of the halogen ion near an imperfection, or the absorption band due to an impurity is hidden under the tail of the first absorption band. (2) Excitons created by light at normal lattice points transfer their energy to the electron trapping impurity centers, ionizing them. At the first peak, 2 x 109 photons/set are absorbed by the crystal, and we get 10-s ems/V as 7~ at room temperature, where v is the quantum efficiency per incident photon and w the schubweg per unit applied voltage. If we assume 2 per cent of the 234mp light is directly absorbed by the hidden band or 2 per cent of created excitons transfer their energy to impurities, the above value of w seems to be reasonable. In the former, the
and
K. AOYAGI
spectral sensitivity curve at around 234 q is interpreted to correspond to the shape of the impurity absorption affected by the tail of the fundamental absorption band, and the impurity concentration is estimated as 101s cm-s assuming the absorption coefficient of 0.8 cm-l (ca. 2 per cent absorption at 234 mp). In the latter, the decrease of the higher energy side may be interpreted to arise because excitons are formed too densely near the surface so that the most of them annihilate before transferring their energy effectively to the uniformly distributed impurity centers of low concentration. The building up of the infrared photoconductivity by U.V. radiation shows that the original impurity centers are destroyed and that ejected electrons are trapped by shallow traps. Nevertheless, the peak height remains unaltered. This seems to favor mechanism (2) because newly developed centers may also be ionized by excitons. The hidden impurity absorption band is not the p-band, because the peak height does not decrease even after photoconduction by i.r. light has completely disappeared. It cannot be decided at present which of the two mechanisms is responsible for the first region. This will be discussed in another paper together with the photocurrent at the exciton peak. The temperature dependence of the photoconductivity in the additively colored KI(7) shows that the schubweg is nearly constant between - 60” and 110°C. Therefore, the strong temperature dependence of the current in the first peak might be ascribed to the change of ionizing efficiency. Assuming the temperature dependence to have the form exp (-E/AT), we get E = O-11eV. Comparing the photoconductivity below 220 rnp with the reflection spectra, it may be seen that the photoconductivity arises from the high energy tail of the first absorption band or near the absorption shoulder and shows a peak around the low energy side of the second absorption band. The decrease at the second peak is not as steep as that at the first absorption peak and the structure below 200 rnp corresponds well with the reflection curve. The slight temperature dependence of the photocurrent in this region is mainly due to that of the schubweg, and ionization occurs with almost constant quantum efficiency irrespective of photon energy or temperature. These features show that above around 5.8 eV, the free carriers are directly
PHOTOCONDUCTIVITY
OF ALKALI
HALIDE
CRYSTALS
created by photons contrary to the region below 5.8 eV. Figure 5 and Fig. 6 are the photoresponses for KBr and KCI, respectively. The main features are nearly the same with those of KI, i.e. a bell-shape peak, which is probably due to imperfections,
IN FUNDAMENTAL
The spectral responses consist of three characteristic regions: a bell-shaped peak in the low energy side of the fundamental absorption band which is probably caused by impurities, a faint photoconduction at the first absorption peak and a temperahv,
hv,
eV IO
A# IO:_ \ -
KBr
i
337
ABSORPTION
9
8
eV
7 I
6 i
(a) ( b) --o(c) ---
z3 02L
-
2:, z
-
, \ l a \i , 0
Wavelength,
Wavelength,
m,,u
FIG. 5. Photocurrent of KBr at room temperature.
(a) forward direction in 0.8 atm Na, (b) reverse direction in vacuum, (c) mechanically treated, forward 0-S atm Na.
direction
in
appears in the low energy tail of the first absorption band, the current drops sharply or is hardly detectable at the absorption peak and again increases as the photon energy exceeds the higher energy tail of the first peak. However, there are some differences in the details, the study of which are now in progress. The approximate photon energy for the onset of direct photoionization in the undisturbed lattices of KI, KBr and KC1 at room temperature are listed in Table 1. 4. CONC!LUSIONS The photoconducti~~ of KI, KBr and KC1 were studied in the fundamental absorption range.
mp
FIG. 6. Photocurrent of KC1 at room temperature. (a) forward direction in 0.8 atm Na, (b) mechanically treated, forward direction in 0.8 atm N2.
Onset of direct photoCrystal ionization (eV), room temperature 5.8 KI KBr 6.8 KC1 7.5
The first peak of photocurrent (eV), room temperature s-25 S-85 6.81
p---p ture-insensitive photoconduction due to direct photoionization in the undisturbed lattice which occurs above the high energy tail of the first peak with nearly constant quantum efficiency irrespective of the photon energy.
G. KUWABARA
338 REFxRENcEs
1. EBY J. E., TEEGARDENK. J. and DUTTON D. B., Phys. Rev. 116; 1099 (1959). 2. FISCHER F. and HILXH R., N&r. &ad. Wiss. Gdttingen 8, 241 (1959). 3. TAFT E. A. and PHILIPP H. R., J. Phys. Chem. Solids 3, 1 (1957).
and
K.
AOYAGI
4. TAYLOR W. J. and HARTMANP. L., Phys. Rev. 113, 1421 (1959). 5. FERGUSONJ. N., Phys. Rev. 66, 220 (1944). 6. AOYAGI K. and K~WABAFZA G., J. Phys. Sot. Japan 15, 2334 (1960). 7. GLMER G., N&r. Math-Phys. Gb’ttingen 3, 31 (1937).
DISCUSSION A. ROSE: If you assume that each photon excites one free electron, can you estimate the lifetime of a free electron?
G. KUWABARA: No, we cannot, because the mobility of an electron in KI has not been determined.
Chem. Solids
Pergamon
Press 1961. Vol. 22, pp. 333-338.
Printed in Great Britain.
PHOTOCONDUCTIVITY OF SOME ALKALI HALIDE CRYSTALS IN THE FUNDAMENTAL ABSORPTION RANGE G. KUWABARA* Department
and
of Physics, University
K. AOYAGI of Tokyo, Tokyo, Japan
Abstract-The photoconductivity of KI, KBr and KC1 in the fundamental absorption range was studied between -70” and 110°C. The spectral response curve consists of three characteristic regions. The first is a bell-shaped structure in the low energy tail of the intrinsic absorption band, which is temperature sensitive and presumed to be due to imperfections; the second is a very faint photoconduction at the first exciton peak, and the third is the temperature-insensitive photoconduction above the higher energy tail of the first absorption peak due to direct photoionization in the undisturbed crvstal. The direct ionization starts at ca. 5.8, 6.8 and 7.5 eV in photon energy for KI KBr and KCi, respectively.
1. INTRODUCTION
THERE are several structures in the fundamental absorption range of alkali halide crystals. One group in the lower energy side has been interpreted as transitions creating electron-hole pairs coupled with each other, and another in the higher energy side as those creating free carriers. Recent experimentsuy 3) have shown that there are much more complicated structures than those expected by simple considerations, especially in crystals composed of heavy halogens. Excitons do not cause conductivity unless they dissociate by absorbing thermal energy or ionize other impurity centers by transfering their energy. Therefore the study of the photoconductivity in this region may afford an important clue to the interpretation of the absorption band or behavior of excitons in these crystals. The external photoelectric effect has been measured,(39 4) while the detection of the internal photoelectric effect has been unsuccessful as far as we know.(4* 5) In this paper the spectral response of the photoconductivity and some discussions thereof are given. 2. EXPERIMENTAL
PROCEDURES
The vacuum monochromator used is the same as that reported previously.@) The light intensity * Now at Argonne National Laboratory, Argonne, Illinois, on leave from the University of Tokyo. Z
333
in the photoconductivity measurement was about 1010 photons/cm3 set for the pass band of 3 rnp at 200 mp. The current was measured with a d.c. amplifier by the charging-up method or with a vibrating-reed electrometer. The noise level of the former is lo-13 C, and that of the latter is 2 x lo-16 A in the measuring condition. In this experiment, it is essential to reduce the external photoelectric current as far as possible, because most of the ordinary materials emit photoelectrons below 200 rnp, and the internal photocurrent will be completely masked.(4* 3) In order to reduce the external photocurrent from the electrode or from the crystal, various electrode arrangements and materials were tested. The best one so far obtained is shown in Fig. l(a). The crystal is pressed between two electrodes, on which LiF films are carefully evaporated. The light falls on the crystal through many fine holes drilled on the front electrode, i. e. the incident light is parallel to the applied electric field. In this arrangement the forward or reverse direction denotes the case in which the negative or positive potential is applied to the front electrode. The photoconductivity was measured with the forward direction, in which the created photoelectrons are drawn into the interior of the crystal. In some cases a third positive grid is placed before the front electrode to draw up the external photoelectrons from the front electrode. Another arrangement, as in Fig.
334
G. KUWABARA
l(b) was also tested, in which the background current due to the external photoeffect of the crystal was larger than that in the former, especially below 200 rn,~ in KI. The crystals used were those purchased from Harshaw and grown in our Laboratory by the Bridgeman method. They were cleaved to about 10 x 10 x 1.3 mm, The voltage applied was 450 V, i.e. the electric field was ca. 3500 V/cm.
and K. AOYAGI 3. RESULTS AND DISCUSSIONS The spectral response of the photocurrent in K-1 for the forward direction at room temperature is shown in Fig. 2(a). The ordinate is a photocurrent normalized to incident photon numbers. The curve consists of three regions. One is a sharp hv,
eV
- IOOOV
_rl-
+45 v -I --
2 4
3. VI-:
‘+ov
ib)
FIG. 1. Electrode arrangements. Electric field is parallel (a) and perpendicular (b) to the incident light. (a) 1. Dewar, 2. crystal, 3. lead to the electrometer, 4. rear electrode, 5. heater, 6. spring, 7, front electrode and 8. radiation shield. (b) 1. electrode, 2. crystal, 3. electrode and 4. shielding case.
In the measurements above room temperature, the detection chamber was filled with dry nitrogen of 0.8 atmospheric pressure. With these procedures, the background current or the external photoelectric current for the forward direction could be reduced below the noise level for light above 160 ml”. The current was kept small as far as possible, i.e. order of lo-15 A. Even with this small current the space charge effect appeared for light below 220 rnp in the case of ICI, because of small light penetration depth.
FIG. 2. Photocurrent and reflectivity of KI at room temperature : electrode arrangement (a), forward direction. (a) photocurrent of KI in O-8 atm Nz, (b) reflectance of KI single crystal, (c) absorption of KI film (after EBY et al.), (d) photocurrent of mechanically treated KI, i.e. the surface was rubbed with emery paper and polished.
structure of a bell shape, whose maximum lies at about 234 q extending between 210 and 225 rnp (denoted as the first peak). The second is a very faint photoconduction* around the first absorption band (denoted as the second region). There is a sharp dip between these two regions. The third is * This is iust a little above the noise level and is not shown in the figure. A detailed study of this region with higher sensitivity is in progress and will be reported in another paper.
PHOTOCONDUCTIVITY
OF ALKALI
HALIDE CRYSTALS
the broad structure which rises from the higher energy side of the first absorption band (third region). In the case of the forward direction an almost linear relation existed between the applied voltage and the amount of photocurrent for light of 185 rnp as well as that of 230 ny~ up to a field of 6,000 V/cm. This result shows that the photocurrent is due to the internal photoeffect. The curves (a) and (b) in Fig, 3 are the photoresponses
IN FUNDAMENTAL
335
Curve (d) in Fig. 1 is the response of a mechanically treated crystal, i.e. the surface is rubbed with emery paper and then polished with a fine cloth to reduce the light scattering. With this procedure, which is supposed to cause a large amount of strain or imperfections near the surface, the current in the third region decreased to less than onetenth of its original value, while that in the first peak remains almost the same except for a slight
eV
hy
hv, 8 Cl
50
ABSORPTION
’
6
I
I
I
250
200
150 Wavelength,
eV
7
mp
FIG. 3. Photocurrent of KI at room temperature, reverse direction. (a) reverse direction in 0.8 atm Na, (b) in vacuum, (c) electrode arrangement (b) in 0.8 atm Nz. for the reverse direction obtained in Ns of 0.8 atm and in vacuum, respectively, and (c) is for the electrode arrangement (b). These curves show that the external photocurrent from the crystal rapidly increases below 200 rnp. The temperature dependence of the photoresponse between 110” and - 70°C is shown in Fig. 4. As the temperature is decreased, the height of the first peak decreases remarkably compared with that of the third region, and its peak position shifts towards higher energy with a rate of 1.0 x 10-s eV/deg, which is almost the same rate as that of the exciton peak.
FIG. 4.
250
200 Wavelength,
m,u
Temperature dependence of photocurrent of KI.
change in the form of the higher energy side cutoff. By dissolving the surface layer with water, the photocurrent recovered almost the same value as that before treatment. The vacuum evaporation of a LiF film on to the surface also caused a similar effect. The current in the third region also decreases conspicuously with continuous irradiation by light in this band, while that of the first peak remains almost unchanged by irradiation with light in the first peak or in the third region. This is because in the former case a higher local space charge builds up near the surface than in the latter for an equal amount of current in the outer circuit. The above two facts show that the photoeffect in the first peak occurs almost uniformly
336
G.
KUWABARA
throughout the crystal, while that in the third region occurs within a quite thin layer near the surface, owing to the extremely small penetration depth of light, and is liable to be affected greatly by the crystal imperfection near the surface. After irradiation in the first peak for several minutes, the light above 600 rnp (designated as infrared light) causes the photocurrent, which decreases to zero with time. Moreover, the first peak almost coincides with the absorption peak of the p-band. Accordingly, it may be suggested that the peak is correlated with the F-center which is formed by an exciton and successively ionized by another exciton or directly by absorbing light in the p-band. This mechanism, however, is denied by the following reasons. The first peak appears even in the freshly cleaved crystal and the amount of current neither increases by prolonged irradiation with ultraviolet light nor decreases after sufficient irradiation with infrared light which includes the F-band of KI. If the c(- or /!-band is the origin of the photocurrent, we expect a large amount of negative-ion vacancies and accordingly a higher first peak for the quenched crystal. However, the peak height is nearly the same irrespective of the various treatments. If we assume that the creation and successive ionization of the F-center is the origin of the photocurrent, we should have a superlinear relation between the amount of current and the light intensity. This also contradicts the experiment. There are two possible mechanisms which cause the first peak: (1) Localized exciton absorption, i.e. the excitation of the halogen ion near an imperfection, or the absorption band due to an impurity is hidden under the tail of the first absorption band. (2) Excitons created by light at normal lattice points transfer their energy to the electron trapping impurity centers, ionizing them. At the first peak, 2 x 109 photons/set are absorbed by the crystal, and we get 10-s ems/V as 7~ at room temperature, where v is the quantum efficiency per incident photon and w the schubweg per unit applied voltage. If we assume 2 per cent of the 234mp light is directly absorbed by the hidden band or 2 per cent of created excitons transfer their energy to impurities, the above value of w seems to be reasonable. In the former, the
and
K. AOYAGI
spectral sensitivity curve at around 234 q is interpreted to correspond to the shape of the impurity absorption affected by the tail of the fundamental absorption band, and the impurity concentration is estimated as 101s cm-s assuming the absorption coefficient of 0.8 cm-l (ca. 2 per cent absorption at 234 mp). In the latter, the decrease of the higher energy side may be interpreted to arise because excitons are formed too densely near the surface so that the most of them annihilate before transferring their energy effectively to the uniformly distributed impurity centers of low concentration. The building up of the infrared photoconductivity by U.V. radiation shows that the original impurity centers are destroyed and that ejected electrons are trapped by shallow traps. Nevertheless, the peak height remains unaltered. This seems to favor mechanism (2) because newly developed centers may also be ionized by excitons. The hidden impurity absorption band is not the p-band, because the peak height does not decrease even after photoconduction by i.r. light has completely disappeared. It cannot be decided at present which of the two mechanisms is responsible for the first region. This will be discussed in another paper together with the photocurrent at the exciton peak. The temperature dependence of the photoconductivity in the additively colored KI(7) shows that the schubweg is nearly constant between - 60” and 110°C. Therefore, the strong temperature dependence of the current in the first peak might be ascribed to the change of ionizing efficiency. Assuming the temperature dependence to have the form exp (-E/AT), we get E = O-11eV. Comparing the photoconductivity below 220 rnp with the reflection spectra, it may be seen that the photoconductivity arises from the high energy tail of the first absorption band or near the absorption shoulder and shows a peak around the low energy side of the second absorption band. The decrease at the second peak is not as steep as that at the first absorption peak and the structure below 200 rnp corresponds well with the reflection curve. The slight temperature dependence of the photocurrent in this region is mainly due to that of the schubweg, and ionization occurs with almost constant quantum efficiency irrespective of photon energy or temperature. These features show that above around 5.8 eV, the free carriers are directly
PHOTOCONDUCTIVITY
OF ALKALI
HALIDE
CRYSTALS
created by photons contrary to the region below 5.8 eV. Figure 5 and Fig. 6 are the photoresponses for KBr and KCI, respectively. The main features are nearly the same with those of KI, i.e. a bell-shape peak, which is probably due to imperfections,
IN FUNDAMENTAL
The spectral responses consist of three characteristic regions: a bell-shaped peak in the low energy side of the fundamental absorption band which is probably caused by impurities, a faint photoconduction at the first absorption peak and a temperahv,
hv,
eV IO
A# IO:_ \ -
KBr
i
337
ABSORPTION
9
8
eV
7 I
6 i
(a) ( b) --o(c) ---
z3 02L
-
2:, z
-
, \ l a \i , 0
Wavelength,
Wavelength,
m,,u
FIG. 5. Photocurrent of KBr at room temperature.
(a) forward direction in 0.8 atm Na, (b) reverse direction in vacuum, (c) mechanically treated, forward 0-S atm Na.
direction
in
appears in the low energy tail of the first absorption band, the current drops sharply or is hardly detectable at the absorption peak and again increases as the photon energy exceeds the higher energy tail of the first peak. However, there are some differences in the details, the study of which are now in progress. The approximate photon energy for the onset of direct photoionization in the undisturbed lattices of KI, KBr and KC1 at room temperature are listed in Table 1. 4. CONC!LUSIONS The photoconducti~~ of KI, KBr and KC1 were studied in the fundamental absorption range.
mp
FIG. 6. Photocurrent of KC1 at room temperature. (a) forward direction in 0.8 atm Na, (b) mechanically treated, forward direction in 0.8 atm N2.
Onset of direct photoCrystal ionization (eV), room temperature 5.8 KI KBr 6.8 KC1 7.5
The first peak of photocurrent (eV), room temperature s-25 S-85 6.81
p---p ture-insensitive photoconduction due to direct photoionization in the undisturbed lattice which occurs above the high energy tail of the first peak with nearly constant quantum efficiency irrespective of the photon energy.
G. KUWABARA
338 REFxRENcEs
1. EBY J. E., TEEGARDENK. J. and DUTTON D. B., Phys. Rev. 116; 1099 (1959). 2. FISCHER F. and HILXH R., N&r. &ad. Wiss. Gdttingen 8, 241 (1959). 3. TAFT E. A. and PHILIPP H. R., J. Phys. Chem. Solids 3, 1 (1957).
and
K.
AOYAGI
4. TAYLOR W. J. and HARTMANP. L., Phys. Rev. 113, 1421 (1959). 5. FERGUSONJ. N., Phys. Rev. 66, 220 (1944). 6. AOYAGI K. and K~WABAFZA G., J. Phys. Sot. Japan 15, 2334 (1960). 7. GLMER G., N&r. Math-Phys. Gb’ttingen 3, 31 (1937).
DISCUSSION A. ROSE: If you assume that each photon excites one free electron, can you estimate the lifetime of a free electron?
G. KUWABARA: No, we cannot, because the mobility of an electron in KI has not been determined.