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Optical absorption band edge of single crystal β-GeSe2
Volume 110A, number 7,8
PHYSICS LETTERS
12 August 1985
OPTICAL A B S O R P T I O N BAND EDGE OF SINGLE CRYSTAL 13-GeSez Zoran V. P O P O V I C Institute of Physics, P.O. Box 57, 11000 Belgrade, Yugoslavia
and A. B R E I T S C H W E R D T Max - Planck- Institut fftr Festk$rperforschung, 7000 Stuttgart 80, Fed. Rep. Germany
Received 27 February 1985; accepted for publication 28 May 1985
The absorption spectra of single crystal fl-GeSe2 were investigated in the range from 400 to 600 nm at temperatures of 300, 77 and 4.2 K for E IIa and E IIb polarization of light. The values of the direct energy gap are obtained and compared with earlier results.
1. Introduction. A number of papers on various novel phenomena in GeSe 2 have been published lately. These papers have dealt with the anomalous structure properties [1 ] and exciton absorption [2] of GeSe 2 and the photo-conductivity of Ag doped GexSel_ x among other topics [3]. Despite this rise in interest in GeSe2, the structure of single-crystal GeSe 2 energy bands is still unknown. Very few of the papers have dealt with the optical properties of singlecrystal GeSe 2 in the neighborhood of the absorption edge. Moreover, the presented experimental data are not in agreement. The first GeSe 2 single-crystal direct transition band gap values of Eg(0 K) = 2.690 eV and Eg(300 K) = 2.485 eV [4] were obtained using unpolarized light. In later studies, the analysis of the absorption edge by using Urbach's rule in the a < 103 cm -1 range yielded Eg = 2.772 eV and Eg = 2.782 eV at 0 K for E 11a and E IIb, respectively [5]. An exciton transition o f E = 2.854 eV for E IIa at 4.2 K has been discovered [2] in single-crystal germanium diselenide. Reflection peaks were observed in the reflection spectra [6], that correspond to energy band gaps of 2.47 eV and 2.53 eV (300 K) and 2.69 eV and 2.74 eV (77 K) for E IIa and E IIb, respectively. In the course of the work presented in this paper
426
on single-crystal/3-GeSe 2 absorption spectra in the neighborhood of the absorption edge, the energy gap was determined for Ella a n d E I I b at 4.2 K, 77 K and 300 K. The obtained results are compared with earlier results. 2. E x p e r i m e n t a l and results. The GeSe 2 single-crystals used were obtained by the Bridgeman method. Samples used to observe tranmaission spectra were obtained by "peeling" from the cleavage plane using sticking tape. Six to fivehundred micrometer thick samples were obtained in this manner. The samples were mounted in a cryostat. The transmission coefficient was determined at 4.2 K, 77 K and 300 K with a Cary 17 spectrophotometer. The absorption spectra in fig. 1 were calculated from the observed transmission spectra using relation:
T = (1 - R ) 2 e - ~ d / ( 1 - R 2 e - 2 ~ d ) ,
(1)
where T and R are the reflection and transmission coefficients, respectively, and d is the sample thickness. A constant value o f R = 0.20 [7] for the reflection coefficient was assumed to obtain the absorption spectra. 3. Discussion. The absorption spectra in fig. 1 were
0.375-9601/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 110A, number 7,8 I
I
PHYSICS LETTERS
I
I
I
I
12 August 1985
"--T--T------I"
I
1
1
T
t3- GeSe2 ....
Ella
E IIb(--)
EIIb
10h
il 42K l
E IIa (--,)
GcSe 2
/
;/~
,,/ //
3"
E
/; :/,/ /, ///
Z
u_J 103
77K
A t%#
'E U
300K
X
"6
I.L LL. LLJ
5O[
o (,..)
,/I
pa.. 0 m <
Yi°y
300K/// 10
2J
2.2
23
2./.
2.5
2.6
27
2.8
2.9
3.0
ENERGY (¢V) 1
2.0
I
21
I
I
I
I
2.2 23 24 25 E N E R G Y , EIeV]
26
Fig. 2. a 2 as function of photon energy at 300 K, 77 K and 4.2 K for Ella and Eli b polarization of light.
I
27
28
Fig. 1. The absorption coefficient of #-GeS¢2 for E IIa and Ell b polarization at 300 K, 77 K and 4.2 K.
obtained from transmission measurements on 6 to 500 thick samples. Thus absorption spectra were obtained in the wide range from 5 cm - 1 to 1.5 X 104 cm - 1 . Large absorption coefficient values above 1000 cm - 1 indicate a direct energy gap for both polarizations. The ~2 versus hv dependence in fig. 2 was used to obtain the direct energy gap values. This was done by extending the linear portion o f the curve to intersect with the hu axis, resulting in the values presented in table 1 for both polarizations. The direct gap values obtained are comparable to published values at room temperature and at liquid nitrogen temperature. Namely, the values obtained from reflection coefficient measurements [6] at 300 K o f 2.47 eV and 2.5 3 eV and at 80 K o f 2.69 eV and 2.74 eV, for E IIa and E IIb, respectively, are in agree-
ment with corresponding energy gap values given in table 1. Our values for Eg at 4.2 K are greater than the values at 0 K obtained by analysis using Urbach's rule [5 ]. We are o f the opinion that the discrepancy is due to the fact that Urbach's rule was used on GeSe 2 absorption spectra in the o~< 103 cm - 1 indirect transition range. Our absorption spectra were for a > 103 cm - 1 where the absorption coefficient does not change exTable 1 Energy gap values of fl-GeSe2 single crystal at different temperatures and polarizations. T (K)
300 77 4.2
Eg (eV) Ella
Ellb
2.50 2.675 2.725
2.49 2.685 2.735
427
Volume 110A, number 7,8
PHYSICS LETTERS
ponentially with hv (especially at 4.2 K) precluding the use o f Urbach's rule to obtain energy gap. This indicates that the zone structure is more complex than the parabolic that is requisite for the application of Urbach' rule. Recent studies [2] have substantiated an excitonic transition in single-crystal germanium diselenide of E = 2.854 eV for EII a at 4.2 K. Thus, Eg at 0 K has to exceed the exciton energy at 4.2 K. An exciton absorption was not observed at 2.854 eV in the absorption spectra in fig. 1 at 4.2 K. Possibly due to the thickness of the samples ( > 6 / ~ m ) , we did not enter the spectral region in which excitons can be observed. At 4.2 K, a plateau from 2.7 eV to 2.75 eV was observed in fig. 1. This plateau vanishes with an increase in temperature, disappearing at 77 K. This can be attributed to a complex E(k) in GeSe 2 which is a layer
428
12 August 1985
crystal. Similar effects have also been observed in the absorption spectra o f the GaTe layer crystal at 4.2 K [8].
References [ 1] J.C. Philips, Comm. Solid State Phys. 10 (1982) 165. [2] S.A. Boiko, D.I. Bletskan and S.F. Terekhova, Phys. Stat. Sol. 90b (1978) K49. [3] C.H. Chem and K.L. Tai, Appl. Phys. Lett. 37 (1980) 1075. [4] N.P. Gavaleshko, M.V. Kurik and A.I. Savchuk, Fiz. Tekh. Poluprovodn. 1 (1967) 1099. [5] V.S. Blazhkiv, M.V. Kurik and I.S. Zhmurko, Opt. Spektrosk. 29 (1970) 554. [6] V.V. Sobolev, V.M. Kramer and Z.D. Kovaljuk, Zh. Prikl. Spektrosk. 39 (1983) 52. [7] Z.V. Popovi6, Fizika 14 (1982) 21. [8] J.L. Brebner and G. Fischer, Proc. Int. Conf. on Physics of semiconductors (Exeter, 1962) p. 760.
PHYSICS LETTERS
12 August 1985
OPTICAL A B S O R P T I O N BAND EDGE OF SINGLE CRYSTAL 13-GeSez Zoran V. P O P O V I C Institute of Physics, P.O. Box 57, 11000 Belgrade, Yugoslavia
and A. B R E I T S C H W E R D T Max - Planck- Institut fftr Festk$rperforschung, 7000 Stuttgart 80, Fed. Rep. Germany
Received 27 February 1985; accepted for publication 28 May 1985
The absorption spectra of single crystal fl-GeSe2 were investigated in the range from 400 to 600 nm at temperatures of 300, 77 and 4.2 K for E IIa and E IIb polarization of light. The values of the direct energy gap are obtained and compared with earlier results.
1. Introduction. A number of papers on various novel phenomena in GeSe 2 have been published lately. These papers have dealt with the anomalous structure properties [1 ] and exciton absorption [2] of GeSe 2 and the photo-conductivity of Ag doped GexSel_ x among other topics [3]. Despite this rise in interest in GeSe2, the structure of single-crystal GeSe 2 energy bands is still unknown. Very few of the papers have dealt with the optical properties of singlecrystal GeSe 2 in the neighborhood of the absorption edge. Moreover, the presented experimental data are not in agreement. The first GeSe 2 single-crystal direct transition band gap values of Eg(0 K) = 2.690 eV and Eg(300 K) = 2.485 eV [4] were obtained using unpolarized light. In later studies, the analysis of the absorption edge by using Urbach's rule in the a < 103 cm -1 range yielded Eg = 2.772 eV and Eg = 2.782 eV at 0 K for E 11a and E IIb, respectively [5]. An exciton transition o f E = 2.854 eV for E IIa at 4.2 K has been discovered [2] in single-crystal germanium diselenide. Reflection peaks were observed in the reflection spectra [6], that correspond to energy band gaps of 2.47 eV and 2.53 eV (300 K) and 2.69 eV and 2.74 eV (77 K) for E IIa and E IIb, respectively. In the course of the work presented in this paper
426
on single-crystal/3-GeSe 2 absorption spectra in the neighborhood of the absorption edge, the energy gap was determined for Ella a n d E I I b at 4.2 K, 77 K and 300 K. The obtained results are compared with earlier results. 2. E x p e r i m e n t a l and results. The GeSe 2 single-crystals used were obtained by the Bridgeman method. Samples used to observe tranmaission spectra were obtained by "peeling" from the cleavage plane using sticking tape. Six to fivehundred micrometer thick samples were obtained in this manner. The samples were mounted in a cryostat. The transmission coefficient was determined at 4.2 K, 77 K and 300 K with a Cary 17 spectrophotometer. The absorption spectra in fig. 1 were calculated from the observed transmission spectra using relation:
T = (1 - R ) 2 e - ~ d / ( 1 - R 2 e - 2 ~ d ) ,
(1)
where T and R are the reflection and transmission coefficients, respectively, and d is the sample thickness. A constant value o f R = 0.20 [7] for the reflection coefficient was assumed to obtain the absorption spectra. 3. Discussion. The absorption spectra in fig. 1 were
0.375-9601/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 110A, number 7,8 I
I
PHYSICS LETTERS
I
I
I
I
12 August 1985
"--T--T------I"
I
1
1
T
t3- GeSe2 ....
Ella
E IIb(--)
EIIb
10h
il 42K l
E IIa (--,)
GcSe 2
/
;/~
,,/ //
3"
E
/; :/,/ /, ///
Z
u_J 103
77K
A t%#
'E U
300K
X
"6
I.L LL. LLJ
5O[
o (,..)
,/I
pa.. 0 m <
Yi°y
300K/// 10
2J
2.2
23
2./.
2.5
2.6
27
2.8
2.9
3.0
ENERGY (¢V) 1
2.0
I
21
I
I
I
I
2.2 23 24 25 E N E R G Y , EIeV]
26
Fig. 2. a 2 as function of photon energy at 300 K, 77 K and 4.2 K for Ella and Eli b polarization of light.
I
27
28
Fig. 1. The absorption coefficient of #-GeS¢2 for E IIa and Ell b polarization at 300 K, 77 K and 4.2 K.
obtained from transmission measurements on 6 to 500 thick samples. Thus absorption spectra were obtained in the wide range from 5 cm - 1 to 1.5 X 104 cm - 1 . Large absorption coefficient values above 1000 cm - 1 indicate a direct energy gap for both polarizations. The ~2 versus hv dependence in fig. 2 was used to obtain the direct energy gap values. This was done by extending the linear portion o f the curve to intersect with the hu axis, resulting in the values presented in table 1 for both polarizations. The direct gap values obtained are comparable to published values at room temperature and at liquid nitrogen temperature. Namely, the values obtained from reflection coefficient measurements [6] at 300 K o f 2.47 eV and 2.5 3 eV and at 80 K o f 2.69 eV and 2.74 eV, for E IIa and E IIb, respectively, are in agree-
ment with corresponding energy gap values given in table 1. Our values for Eg at 4.2 K are greater than the values at 0 K obtained by analysis using Urbach's rule [5 ]. We are o f the opinion that the discrepancy is due to the fact that Urbach's rule was used on GeSe 2 absorption spectra in the o~< 103 cm - 1 indirect transition range. Our absorption spectra were for a > 103 cm - 1 where the absorption coefficient does not change exTable 1 Energy gap values of fl-GeSe2 single crystal at different temperatures and polarizations. T (K)
300 77 4.2
Eg (eV) Ella
Ellb
2.50 2.675 2.725
2.49 2.685 2.735
427
Volume 110A, number 7,8
PHYSICS LETTERS
ponentially with hv (especially at 4.2 K) precluding the use o f Urbach's rule to obtain energy gap. This indicates that the zone structure is more complex than the parabolic that is requisite for the application of Urbach' rule. Recent studies [2] have substantiated an excitonic transition in single-crystal germanium diselenide of E = 2.854 eV for EII a at 4.2 K. Thus, Eg at 0 K has to exceed the exciton energy at 4.2 K. An exciton absorption was not observed at 2.854 eV in the absorption spectra in fig. 1 at 4.2 K. Possibly due to the thickness of the samples ( > 6 / ~ m ) , we did not enter the spectral region in which excitons can be observed. At 4.2 K, a plateau from 2.7 eV to 2.75 eV was observed in fig. 1. This plateau vanishes with an increase in temperature, disappearing at 77 K. This can be attributed to a complex E(k) in GeSe 2 which is a layer
428
12 August 1985
crystal. Similar effects have also been observed in the absorption spectra o f the GaTe layer crystal at 4.2 K [8].
References [ 1] J.C. Philips, Comm. Solid State Phys. 10 (1982) 165. [2] S.A. Boiko, D.I. Bletskan and S.F. Terekhova, Phys. Stat. Sol. 90b (1978) K49. [3] C.H. Chem and K.L. Tai, Appl. Phys. Lett. 37 (1980) 1075. [4] N.P. Gavaleshko, M.V. Kurik and A.I. Savchuk, Fiz. Tekh. Poluprovodn. 1 (1967) 1099. [5] V.S. Blazhkiv, M.V. Kurik and I.S. Zhmurko, Opt. Spektrosk. 29 (1970) 554. [6] V.V. Sobolev, V.M. Kramer and Z.D. Kovaljuk, Zh. Prikl. Spektrosk. 39 (1983) 52. [7] Z.V. Popovi6, Fizika 14 (1982) 21. [8] J.L. Brebner and G. Fischer, Proc. Int. Conf. on Physics of semiconductors (Exeter, 1962) p. 760.