In amorphous and hexagonal selenium optical absorption and photoconductivity were studied in their dependence on temperature. The absorption edge shifts for amorphous selenium toward shorter wavelengths with decreasing temperature. For a temperature change from 300° to 90°K and a layer thickness of 98μ, for example, a shift from 6600 to 6100A is observed at an optical density of 3; for higher densities (shorter wavelengths) the shift is smaller. It can be shown that this shift is related to the thermal excitation of vibrational levels. At higher temperatures the population of the higher vibration levels increases and the separation of these from the excited state diminishes.

At low temperature (ca 90°K) photoconductivity is observed only after the light quanta exceed 2.5 ev and the absorption coefficient has reached 105 cm−1. Increase of temperature brings about a shift of the photoconductive threshold toward longer wavelengths, paralleling the absorption‐edge shift. The absorption coefficient has been followed over the wavelength range 6900 to 2100A; its values at the end points of this range are 1.6×102 and 5.2×105 cm−1, respectively; there is a slight maximum at 2600A where the absorption coefficient reaches 5.6×105 cm−1.

The transformation of amorphous into hexagonal selenium by heat brings about an increase of the optical absorption in the 5000 to 7200A region and the appearance of a distinct hump in the absorption edge at 5200A. Decrease of temperature sharpens the hump and causes a decrease of absorption at all wavelengths in the 4000 to 7200A range except in the vicinity of the hump. In hexagonal selenium photoconductivity is observed at wavelengths as long as 7500A for films 0.5μ thick. A peak in the photoresponse is found where the optical density of these films reaches ca 1 (10 percent transmission). It can be shown that this behavior is to be expected for a bimolecular law of recombination of electrons and holes when their inhomogeneous distribution is taken into account. With decreasing temperature the peak response shifts consequently toward shorter wavelengths with the absorption edge.

1.
A.
von Hippel
,
J. Chem. Phys.
16
,
372
(
1948
).
2.
See, for example, G. P. Barnard, The Selenium Cell (Richard R. Smith, Inc., New York, 1930),
and J. W. Mellor, Inorganic and Theoretical Chemistry (Longmans, Green and Co., New York, 1940).
3.
R. D.
Burbank
,
Acta Cryst.
4
,
140
(
1951
).
4.
R. D. Burbank, Tech. Rept. 43, ONR Contract N5ori‐07801, Laboratory of Insulation Research, Massachusetts Institute of Technology (April, 1951).
5.
A. J.
Bradley
,
Phil. Mag.
48
,
477
(
1924
);
M. K.
Slattery
,
Phys. Rev.
25
,
333
(
1925
).
6.
See, for example,
H.
Hendus
,
Z. Physik
119
,
265
(
1942
).
7.
P. K.
Weimer
,
Phys. Rev.
79
,
171
(
1950
).
8.
F.
de Boer
,
Philips Research Rept.
2
,
352
(
1947
);
a conductivity ratio for directions parallel and perpendicular to the c axis of 30 has been found by W. Sigrist, Progress Rept. III, ONR Contract N5ori‐78, Laboratory for Insulation Research, Massachusetts Institute of Technology (May, 1948).
9.
See, for example,
R. W.
Wood
,
Phil. Mag.
3
,
607
(
1902
);
A.
Becker
and
I.
Schaper
,
Z. Physik
122
,
49
(
1944
);
K.
Foersterling
and
V.
Fréedericksz
,
Ann. Physik
43
,
1227
(
1914
);
G. P. Barnard, see reference 2.
10.
G.
Mönch
,
Physik Z.
40
,
487
(
1939
);
A. Becker and I. Schaper, see reference 9.
11.
See G. P. Barnard, reference 2; A. L. Hughes and L. A. Du‐Bridge, Photoelectric Phenomena (McGraw‐Hill Book Company, New York, 1932).
B.
Gudden
and
R.
Pohl
,
Z. Physik
35
,
243
(
1925
), showed the red monoclinic form to be photoconductive in contradiction to prior claims.
12.
G. P. Barnard, see reference 2, A. L. Hughes and L. A. DuBridge, see reference 11; B. Gudden, Lichtelektrische Erscheinungen (Verlag. Julius Springer, Berlin, 1928).
13.
D. S.
Elliott
,
Phys. Rev.
5
,
53
(
1915
) observed a shift of 600A of the peak of response toward short wavelengths for a temperature drop from room temperature to −190 °C;
E. O.
Dieterich
,
Phys. Rev.
8
,
191
(
1916
), observed a shift of the peak toward long wavelengths with temperature rise above room temperature;
A.
Becker
,
Z. Physik
112
,
629
(
1939
)
and
A.
Becker
,
114
,
342
(
1939
) ,
Z. Phys.
and
B.
Lange
,
Phys. Z.
32
,
850
(
1931
), among others, have made observations of temperature‐dependent behavior of spectral response in barrier‐layer‐type selenium photocells. This type of cell behaves in quite a complicated manner which cannot be readily interpreted.
14.
A. H.
Pfund
,
Phys. Rev.
28
,
324
(
1909
);
A. Becker and I. Schaper (see reference 9);
K. Foersterling and V. Fréedericksz, see reference 9 (reflection measurements).
15.
A.
von Hippel
and
M. C.
Bloom
,
J. Chem. Phys.
18
,
1243
(
1950
).
16.
L. C. Pauling, The Nature of the Chemical Bond (Cornell University Press, Ithaca, N.Y., 1940).
17.
R. W. Wood, see reference 9;
and K. Foersterling and V. Fréedericksz, see reference 9, used cast and quenched Se;
A. Becker and I. Schaper, see reference 9, used sputtered layers, etc.
18.
F.
Möglich
and
R.
Rompe
,
Z. Physik
119
,
472
(
1942
).
19.
J.
Bardeen
and
W.
Shockley
,
Phys. Rev.
80
,
72
(
1950
).
20.
A.
Radkowsky
,
Phys. Rev.
73
,
749
(
1948
).
21.
H. Y.
Fan
,
Phys. Rev.
78
,
808
(
1950
).
22.
A.
von Hippel
and
E. S.
Rittner
,
J. Chem. Phys.
14
,
370
(
1946
).
23.
For a discussion of this integral and its application see the valuable, original paper by
Gibson
,
Rice
, and
Bayliss
,
Phys. Rev.
44
,
193
(
1933
).
Also see G. Herzberg, Molecular Spectra and Molecular Structure, Volume I: Spectra of Diatomic Molecules, (D. van Nostrand Company, Inc., New York, 1950), pp. 200 ff., 390 ff.
24.
H.
Gerding
and
R.
Westrik
,
Rec. Trav. Chim.
62
,
68
(
1943
), found lines of 269, 190, 164 cm−1 separation in the Raman spectrum for Se in CS2. The 269 cm−1 frequency (hν≃0.033 ev) presumably corresponds to the vibration of Se against Se in the chains and is assumed to be important to the temperature effects being discussed.
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