- Email: [email protected]
Universal behaviour of the normalized photoconductivity at low temperatures in amorphous semiconductors
274
Journal of Non-Crystalline Solids 114 (1989) 274-276 North-Holland
Section 9: Opticalproperties and recombination UNIVERSAL BEHAVIOR OF THE NORMALIZED PHOTOCONDUCTIVITY AT L O W TEMPERATURES IN AMORPHOUS SEMICONDUCTORS Robert E. JOHANSON*, H. FRI'IZSCHE* and A. VOMVASt * The James Franck Institute, The University of Chicago, Chicago, IL 60637, USA 1" Physics Department of the University of Patras, Patras 26110, Greece We have measured the photoconductivity ap between 4K and 300K for a variety of amorphous semiconductors including Si with and without hydrogen, Ge, Se, and several chalcogenide alloys. Below a certain temperature .(rpof each sample becomes asymptotically constant and cr,~G r with 0.93<~1.0 where G is the electron-hole pair generauon rate. At 4.2K the value of(rp/eG lies between 4x10 -1~ and 3x10 -11 cm2/V. At higher temperatures ap increases rapidly and ~,fails to around 0.5. The data test recently proposed theories of (rp at low temperature.
1. INTRODUCTION
and
Several experiments have shown that the normalized photoconductivity of a-Si:H, that is the photoconductivity
GeSe2
and a sputtered film of the alloy
As35Te28S21GeI5Sel. NiCr evaporated onto the surface formed electrodes 2mm apart. The light used in the
divided by the volume generation rate trp/eG =rll.tV, becomes temperature independent below about 50K with a
measurements came from a focused 75W Xe arc lamp or a
value that does not much depend on doping or sample history.l,2, 3 In their pioneering work, Hoheisel et al.
a 540nm low pass filter.
photons with energies from 1.7eV to 2.3eV of sufficient
proposed that the photocurrent is due to the drift of
flux to produce easily measured currents. Owing to its
tungsten-halogen lamp and passed through heat filters and This arrangement produced
photoexcited carriers during thermalization to the mobility
larger band gap the GeSe2 sample required extending the
edge. 1 Based partly on low temperature drift mobility measurements Spear et al. proposed that the predominant
range to 3.2eV. In order to determine G the absorbance spectrum of
contribution to the photocurrent is hopping among the
each sample was measured at room temperature and shifted
localized band tail states though their model relies on a
appropriately to take into account the temperature
specific form of the density of tail states)
dependence of the optical gap.
Recently
The product of the
Shklovskii et al. developed a simple but general theory of
absorbance and the light's spectrum was numerically
transport during hopping through a distribution of localized states that is relatively independent of the exact form of the
integrated to obtain the flux of absorbed photons. Provided
density of states.4 This leads to the possibility that the behavior of the photoconductivity seen with a-Si:H might
absorbed flux divided by the sample thickness yields the
that the photoconductivity is linear with light intensity the generation rate G.
be a general property of all amorphous semiconductors.
Figure 1 shows the temperature dependence of the
We report here measurements of trp/eG of a variety of non-
normalized photoconductivity of all the samples. The
crystalline semiconductors between 4K and 300K.
curves are remarkably similar.
At a low enough
temperature trp/eG is independent of temperature although 2. EXPERIMENTAL DETAILS AND RESULTS
the value is different for each sample. Then at a higher temperature, again different for each sample, the
The samples include tetrahedral material such as undoped a-Si:H and a-Si:H doped in the gas phase with
photoconductivity rapidly rises. As trp/eG begins to rise
either 800ppm PH3 or 100ppm B2H6, chemical vapor deposited (CVD) a-Si and evaporated Ge as well as
with temperature the exponent ~describing the dependence trp~G r drops from a value between 0.93 and 1.0 to about
evaporated films of the chalcogenide glasses Se, As2Se3,
0.5 as shown in figure 2 for As2Se3. Chalcogenide glasses
0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
R.E. Johanson et M. / Universal behavior of the normalized photoconductivity
-7
i
i
i
i
i¢
275
/~
a-Si:H also undergoes a metastable change when
/tZ
exposed to light, namely the creation of Staebler-Wronski defects, which decreases O'p and Crp/eG but only in the rising part of ap(T) and not at lower temperature. 6 The data of the a-Si:H samples shown in figure 1 were taken after light exposure.
-9 ~
-9
........
I
........
I
.......
~E
0
p I o-o
--,.
,/i
°
.....o
1.0
i
E 0.5E 0.0
-14
]__As I~e
n
.........................
Temperature (K)
Ise
*,
I
I
Temperature (K)
I#4#
I
FIGURE
:2
Normalized photoconductivity crpleG and exponent ~,of As2Se3.
,oo
HGURE 1 Normalized photoconductivity crp/eG of different samples as a function of temperature. The asterisks refer to calculated O'pvalues as mentioned in reference 7.
........
7.-~ E 0
-10
u
........
i
.......
Se 0 0
E
.C
experience photodarkening which is a photo-induced
o v
-11
• • ,OOOoo# ~
metastable structural change that decreases the optical gap. 5 This decrease is reversed by annealing at a temperature higher than the temperature during the exposure. Figure 3 shows this effect in an evaporated selenium film. The low
I
-(--PC
v
o
1.0~ 0.5~ 0.0 ~
0
_go - 1 2
¢rp value at 4.2K was measured immediately after cooling ........
the annealed sample.
After a 60 min. exposure to
G=1020 cm-3s-1 light at 4.2K (rp had increased by nearly a
i
........
!
......
10 100 T e m p e r a t u r e (K)
factor of 2 because of the increased absorption and thus larger G due to the decreased optical gap.
The
photodarkening effect anneals with increasing T and the smaller T=ll0K.
value of the annealed state was restored at
FIGURE 3 Photoconductivity Crpand the exponent ~,of Se. The lower value of Crp at 4.2K corresponds to the annealed state. The Crp(T) curve was measured while heating after strong exposure to light at 4.2K.
R.E. Johanson et al./ Universal behavior of the normalized pbotoconductivity
276
GeSe2, As2Se3 and Se. This disagreement may be caused A I 2 2.5 3...5 3.3
-8 - x t~ v ,, ,~
>
B I 2 3.5 14 140
sample n,a-Si:H p,a-Si: H o-Ge alloy glass As2Se 3
~ o
by the fact that a stronger Coulomb attraction of the electron hole pairs in these chalcogenide glasses, which was not included in the theory, enhances geminate recombination thereby decreasing crp. The Coulomb attraction is stronger in these materials because of their
-9
%
v
smaller dielectric constants.
d %
We normalized the vertical scale of figure 1 by a factor B and the horizontal scale by a factor 1/A so that the
x
various trp(T) curves superimpose as shown in figure 4.
-I0
The normalization constants of each material are listed in
o
the figure. Although the data of the samples plotted fall on
x
top of one another the data of GeSe2 and Se do not fit well which suggests that this simple scaling, which we do
-II
without a theoretical basis, may not lead to a universal I
I
I
I
I I Illl[
I
I0
I
i , rll,I
CUl'VC.
I00
TIA (K)
ACKNOWLEDGEMENTS FIGURE 4 Normalized photoconductivity of several samples after scaling the axes. The scaling factors of each sample are listed in the table.
This work was supported by NSF DMR 8806197 and by the Materials Research Laboratory of the University of Chicago, funded by NSF. One of us (A. V.) thanks the Fulbright Foundation for a fellowship.
3. DISCUSSION Low temperature mobility measurementsT,8 by the traveling wave method indicate that trp at low T is due to energy loss hopping among localized states 4 and not due to conduction in extended states. 1
Moreover, electron
conduction dominates trp at low T in both n-type and p-type a-Si:H.
Although we have no evidence for excluding
extended state conduction in the other materials, we will compare our results with the predictions of the hopping theory. 4 In this theory the low T limit of trp/eG is proportional to a2/e0 and equal to about 3x10-12 cm2/V using a localization radius a=lnm and an exponential decay parameter of the tail state density e0=0.025eV.
This
REFERENCES 1. M. Hoheisel, R. Carius and W. Fuhs, J. Non-Cryst. Solids 63 (1984) 313. 2. M. Vanecek, J. Stuchlik, J. Kocka and A. Triska, J. Non-Crystalline Solids 77&78 (1985) 299. 3. W.E. Spear and Carolyn S. Cloude, Phil. Mag. Lett. 55 (1987) 271. 4. B.I. Shklovskii, H. Fritzsche and S. D. Baranovskii, Phys. Rev. Lett. 62 (1989) 2989. 5. R. Chang, Mat. Res. Bull. 2 (1967) 145. 6. A. Vomvas and H. Fritzsche, J. Non-Cryst. Solids 97&98 (1987) 823.
theoretical value agrees reasonably well with the low
7. H. Fritzsche, this volume.
temperature values of some materials but not with those of
8. R.E. Johanson, to be published.
Journal of Non-Crystalline Solids 114 (1989) 274-276 North-Holland
Section 9: Opticalproperties and recombination UNIVERSAL BEHAVIOR OF THE NORMALIZED PHOTOCONDUCTIVITY AT L O W TEMPERATURES IN AMORPHOUS SEMICONDUCTORS Robert E. JOHANSON*, H. FRI'IZSCHE* and A. VOMVASt * The James Franck Institute, The University of Chicago, Chicago, IL 60637, USA 1" Physics Department of the University of Patras, Patras 26110, Greece We have measured the photoconductivity ap between 4K and 300K for a variety of amorphous semiconductors including Si with and without hydrogen, Ge, Se, and several chalcogenide alloys. Below a certain temperature .(rpof each sample becomes asymptotically constant and cr,~G r with 0.93<~1.0 where G is the electron-hole pair generauon rate. At 4.2K the value of(rp/eG lies between 4x10 -1~ and 3x10 -11 cm2/V. At higher temperatures ap increases rapidly and ~,fails to around 0.5. The data test recently proposed theories of (rp at low temperature.
1. INTRODUCTION
and
Several experiments have shown that the normalized photoconductivity of a-Si:H, that is the photoconductivity
GeSe2
and a sputtered film of the alloy
As35Te28S21GeI5Sel. NiCr evaporated onto the surface formed electrodes 2mm apart. The light used in the
divided by the volume generation rate trp/eG =rll.tV, becomes temperature independent below about 50K with a
measurements came from a focused 75W Xe arc lamp or a
value that does not much depend on doping or sample history.l,2, 3 In their pioneering work, Hoheisel et al.
a 540nm low pass filter.
photons with energies from 1.7eV to 2.3eV of sufficient
proposed that the photocurrent is due to the drift of
flux to produce easily measured currents. Owing to its
tungsten-halogen lamp and passed through heat filters and This arrangement produced
photoexcited carriers during thermalization to the mobility
larger band gap the GeSe2 sample required extending the
edge. 1 Based partly on low temperature drift mobility measurements Spear et al. proposed that the predominant
range to 3.2eV. In order to determine G the absorbance spectrum of
contribution to the photocurrent is hopping among the
each sample was measured at room temperature and shifted
localized band tail states though their model relies on a
appropriately to take into account the temperature
specific form of the density of tail states)
dependence of the optical gap.
Recently
The product of the
Shklovskii et al. developed a simple but general theory of
absorbance and the light's spectrum was numerically
transport during hopping through a distribution of localized states that is relatively independent of the exact form of the
integrated to obtain the flux of absorbed photons. Provided
density of states.4 This leads to the possibility that the behavior of the photoconductivity seen with a-Si:H might
absorbed flux divided by the sample thickness yields the
that the photoconductivity is linear with light intensity the generation rate G.
be a general property of all amorphous semiconductors.
Figure 1 shows the temperature dependence of the
We report here measurements of trp/eG of a variety of non-
normalized photoconductivity of all the samples. The
crystalline semiconductors between 4K and 300K.
curves are remarkably similar.
At a low enough
temperature trp/eG is independent of temperature although 2. EXPERIMENTAL DETAILS AND RESULTS
the value is different for each sample. Then at a higher temperature, again different for each sample, the
The samples include tetrahedral material such as undoped a-Si:H and a-Si:H doped in the gas phase with
photoconductivity rapidly rises. As trp/eG begins to rise
either 800ppm PH3 or 100ppm B2H6, chemical vapor deposited (CVD) a-Si and evaporated Ge as well as
with temperature the exponent ~describing the dependence trp~G r drops from a value between 0.93 and 1.0 to about
evaporated films of the chalcogenide glasses Se, As2Se3,
0.5 as shown in figure 2 for As2Se3. Chalcogenide glasses
0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
R.E. Johanson et M. / Universal behavior of the normalized photoconductivity
-7
i
i
i
i
i¢
275
/~
a-Si:H also undergoes a metastable change when
/tZ
exposed to light, namely the creation of Staebler-Wronski defects, which decreases O'p and Crp/eG but only in the rising part of ap(T) and not at lower temperature. 6 The data of the a-Si:H samples shown in figure 1 were taken after light exposure.
-9 ~
-9
........
I
........
I
.......
~E
0
p I o-o
--,.
,/i
°
.....o
1.0
i
E 0.5E 0.0
-14
]__As I~e
n
.........................
Temperature (K)
Ise
*,
I
I
Temperature (K)
I#4#
I
FIGURE
:2
Normalized photoconductivity crpleG and exponent ~,of As2Se3.
,oo
HGURE 1 Normalized photoconductivity crp/eG of different samples as a function of temperature. The asterisks refer to calculated O'pvalues as mentioned in reference 7.
........
7.-~ E 0
-10
u
........
i
.......
Se 0 0
E
.C
experience photodarkening which is a photo-induced
o v
-11
• • ,OOOoo# ~
metastable structural change that decreases the optical gap. 5 This decrease is reversed by annealing at a temperature higher than the temperature during the exposure. Figure 3 shows this effect in an evaporated selenium film. The low
I
-(--PC
v
o
1.0~ 0.5~ 0.0 ~
0
_go - 1 2
¢rp value at 4.2K was measured immediately after cooling ........
the annealed sample.
After a 60 min. exposure to
G=1020 cm-3s-1 light at 4.2K (rp had increased by nearly a
i
........
!
......
10 100 T e m p e r a t u r e (K)
factor of 2 because of the increased absorption and thus larger G due to the decreased optical gap.
The
photodarkening effect anneals with increasing T and the smaller T=ll0K.
value of the annealed state was restored at
FIGURE 3 Photoconductivity Crpand the exponent ~,of Se. The lower value of Crp at 4.2K corresponds to the annealed state. The Crp(T) curve was measured while heating after strong exposure to light at 4.2K.
R.E. Johanson et al./ Universal behavior of the normalized pbotoconductivity
276
GeSe2, As2Se3 and Se. This disagreement may be caused A I 2 2.5 3...5 3.3
-8 - x t~ v ,, ,~
>
B I 2 3.5 14 140
sample n,a-Si:H p,a-Si: H o-Ge alloy glass As2Se 3
~ o
by the fact that a stronger Coulomb attraction of the electron hole pairs in these chalcogenide glasses, which was not included in the theory, enhances geminate recombination thereby decreasing crp. The Coulomb attraction is stronger in these materials because of their
-9
%
v
smaller dielectric constants.
d %
We normalized the vertical scale of figure 1 by a factor B and the horizontal scale by a factor 1/A so that the
x
various trp(T) curves superimpose as shown in figure 4.
-I0
The normalization constants of each material are listed in
o
the figure. Although the data of the samples plotted fall on
x
top of one another the data of GeSe2 and Se do not fit well which suggests that this simple scaling, which we do
-II
without a theoretical basis, may not lead to a universal I
I
I
I
I I Illl[
I
I0
I
i , rll,I
CUl'VC.
I00
TIA (K)
ACKNOWLEDGEMENTS FIGURE 4 Normalized photoconductivity of several samples after scaling the axes. The scaling factors of each sample are listed in the table.
This work was supported by NSF DMR 8806197 and by the Materials Research Laboratory of the University of Chicago, funded by NSF. One of us (A. V.) thanks the Fulbright Foundation for a fellowship.
3. DISCUSSION Low temperature mobility measurementsT,8 by the traveling wave method indicate that trp at low T is due to energy loss hopping among localized states 4 and not due to conduction in extended states. 1
Moreover, electron
conduction dominates trp at low T in both n-type and p-type a-Si:H.
Although we have no evidence for excluding
extended state conduction in the other materials, we will compare our results with the predictions of the hopping theory. 4 In this theory the low T limit of trp/eG is proportional to a2/e0 and equal to about 3x10-12 cm2/V using a localization radius a=lnm and an exponential decay parameter of the tail state density e0=0.025eV.
This
REFERENCES 1. M. Hoheisel, R. Carius and W. Fuhs, J. Non-Cryst. Solids 63 (1984) 313. 2. M. Vanecek, J. Stuchlik, J. Kocka and A. Triska, J. Non-Crystalline Solids 77&78 (1985) 299. 3. W.E. Spear and Carolyn S. Cloude, Phil. Mag. Lett. 55 (1987) 271. 4. B.I. Shklovskii, H. Fritzsche and S. D. Baranovskii, Phys. Rev. Lett. 62 (1989) 2989. 5. R. Chang, Mat. Res. Bull. 2 (1967) 145. 6. A. Vomvas and H. Fritzsche, J. Non-Cryst. Solids 97&98 (1987) 823.
theoretical value agrees reasonably well with the low
7. H. Fritzsche, this volume.
temperature values of some materials but not with those of
8. R.E. Johanson, to be published.