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Optical properties of tin-selenid films
ELSEVIER
Physica A 216 (1995) 77-84
Optical properties of tin-selenid films H.S. S o l i m a n a'*, D.A. A b d e l H a d y b, K . F . A b d e l R a h m a n a, S.B. Y o u s s e f a, A.A. E1-Shazly a aDepartment of Physics, Faculty of Education, Ain Shams University, Cairo, Eqypt bDepartment of Enyineeriny Physics, Faculty qf Enqineerin9, Ain Shams University, Cairo, Eclypt
Received 15 July 1994
Abstract SnSe thin films of different thicknesses were prepared by the thermal evaporation technique in vacuum of 10 4 Pa. The structure analysis of the films as determined from the electron diffraction pattern and X-ray diffraction indicate that the films were polycrystalline of orthorhombic structure. The transmittance and reflectance of SnSe films were measured at normal incidence in the wavelength range of 760 2200 nm. It was found that the refractive index n and the absorption index k are independent of the film thickness. Graphical representation of log(a) as a function of (1/2) shows two distinct linear parts indicating the existence of two optical transitions. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region revealed an indirect and direct allowed transition with energy gaps E~nd= 0.895 eV and E~ = 1.27, respectively.
1. Introduction The surveying of the literature concerned with the optical and electrical properties of SnSe films revealed that: (i) these films have the potential to be used in different applications such as solar cells [ 1 4 ] and m e m o r y switching devices [-5]; (ii) SnSe films were already employed in holographic-recording system [6]. The direct band gap of SnSe films was determined via different approaches, namely electroreflectance and thermoreflectance measurements [7,8] and absorption measurements [9,10]. On the other hand, indirect transitions in SnSe films were also observed [9-15]. However, considerable scatter is found a m o n g the proposed values for electronic transitions threshold. Regarding the structure of SnSe films it was reported by Miklaichok et al. [16,17] and Palatnik et al. [18] that these films were composed of an orthorhombic phase
* Corresponding author. 0378-4371/95/$09.50 (C' 1995 Elsevier Science BN. All rights reserved SSDI 0 3 7 8 - 4 3 7 1 ( 9 4 ) 0 0 2 9 8 - 3
78
H.S. Soliman et al. / Physica A 216 (1995) 77 84
besides a cubic modification with NaC1 structure. On the other hand, the investigation of Avilov et al. [ 19] on the structure of SnSe films could not prove the existence of the cubic phase in the films. Hoffman et al. 1-20] and Okazaki [21] Okazaki et al. [22] reported that SnSe in the bulk form crystallizes in an orthorhombic structure with lattice parameters a = 0.446 nm, b = 0.419 nm, c = 1.157 nm. The appeal of SnSe films for different applications, as illustrated before, and the controversy about both the scatter mentioned above and the structure of the films persuaded us to investigate the optical properties and structure of SnSe films.
2. Experimental procedure SnSe thin films were prepared by the conventional thermal evaporation technique in vacuum. SnSe powder was supplied by M o r t o n Mhiokol Inc. "Alfa"-England and claimed by the supplier to be 99.999% pure. It was located in a silica bottle which was surrounded by a tungsten spiral filament to supply the heat energy necessary for the
T
film
%
Q
Powder
o
I
I
I
I
I
I
I
I
90
80
70
60
50
40
30
20
Fig. 1. X-ray diffraction patterns of SnSe (bottom) in powder form, (top) in thin film form.
H.S. Soliman et al. / Physica A 216 (1995) 77 84
79
Fig. 2. (top) Transmission Electron Micrograph for SnSe film 30 nm thick. (bottom) Electron diffraction pattern for SnSe film 30 n m thick.
evaporation. Well cleaned quartz slides were used as substrates for the films prepared for optical measurements while glass slides were used as substrates for the structure study. The substrate holder was rotating, during deposition, with a speed of 240 r.p.m. to get SnSe films with uniform thickness. The pressure inside the vacuum chamber was pumped down to 2 x 1 0 - 4 p a . before starting the evaporation process. A mechanical shutter was used to avoid any contamination on the substrates which were at ambient temperature. The thickness and the deposition rate (1 nm/sec.) of the prepared films were controlled by a quartz crystal monitor. The exact thicknesses of the films were measured, after breaking the vacuum, interferometrically [23]. Six groups of SnSe films were prepared and each group was characterized by a particular thickness. All factors effecting the structure and quality of film such as degree of vacuum, deposition rate, substrate kind and temperature were kept constant. The films thicknesses ranged from 51.6 nm to 318.2 nm. For optical measurement, the transmittance T and the reflectance R of the films deposited on quartz substrates were determined at normal incidence in the
80
H.S. Soliman et al. / Physica A 216 (1995) 77-84
0.6 ."
;,-'-/---.~/ - "
/I
~
....
~
0A
0.2
.\. "~'\'$'\~7//'A\-\. ,/' \\
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I
.,.bo,
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....
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.,'°-~o<
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=.=~ ,,,°=.~=3Q0 th " ck
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:::
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I
. " ~
.
/
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o
/
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/of'~
•
/
./
- ~--~;~~~
--~
0.2
~~:,"/ 1000 Wavelength,
,
,
1500
2000
X (nm)
Fig. 3. The spectral dependence of T(2) (bottom) and R(2) (top) for SnSe films of different thicknesses.
wavelength of 760-2200 nm by means of a double-beam spectrophotometer (Cary 2390, Varian) attached with reflectance stage with a V-W configuration. If Ict is the intensity of the transmitted light through the film/quartz combination, while Iq is that through the quartz reference only, then the measured transmittance of the SnSe film is given by: T = (Ic,/Iq) (1 -- Rq),
(1)
where Rq is the reflectance of quartz. In addition, if the intensity of light reflected from the sample mirror reaching the detector is Irr and that reflected from the reference mirror is lm, then R = ( I f r / l m ) 1/2 - - T
2Rq.
(2)
H.S. Soliman et aL / Physica A 216 (1995) 7 7 84
81
2.0 Ref. ( 12 ) Ref. ( 1 5 ) 1.5
13
E
A
1.0
at~
o
k
0.5
I 1000
"'-,,--.,.
I
I
1500
2000
Wavelength, A ( n m )
Fig. 4. The spectral dependence of refractive index n(2) and absorption index k(2) for SnSe thin films.
From the measured T, R and the film thickness d, the values of the refractive index n and absorption index k were computed by a computational method [26] based on Murmann's formulas 1-27]. Taking into account the experimental error in measuring the film thickness to be _+ 2.5% and that in T and R to be ± 1%, the error in the calculated values of n and k was estimated to be 3% and 2.5%, respectively. Structural studies of SnSe in the powder and thin film forms were carried out using an X-ray diffractometer (Philips Pwl410) with C o - K s radiation and a transmission electron microscope (type J E O L / J E M 100S) operating at 60 kV. The microscope is attached to a electron diffraction stage.
3. Results and discussion Fig. 1 shows the X-ray diffractograms of the SnSe in powder form as well as in thin-film form. The film thickness is 318.2 nm and the deposition rate is 1 nm/sec. Analysis of the diffraction patterns indicated that the SnSe in the powder form or thin-film form has polycrystalline nature with an orthorhombic structure with lattice parameters a = 1.147 nm, b = 0.414 nm and c = 0.444 nm in case of the powder form and a = 1.142 nm, b = 0.420 nm, c = 0.442 nm in thin film form, belonging to the D~ 6 Pnma space group. The mean uncertainty in calculating the lattice parameters was 1%. The obtained results for SnSe films were confirmed by the electron diffraction patterns carried out for SnSe thin film 30 nm thick deposited at the same conditions.
H.S. Soliman et al. / Physica A 216 (1995) 77-84
82
10 5
/ io fo it i
10 4
"7 E La
J
10 3
10 2
3.8
I
I
3.9
4.0
Log ( I I ~ ) ,
Fig. 5. The variation of log(s) with log(l/A}.
A typical electron diffraction pattern is shown in Fig. 2. The observed dhk I spacings were determined from the relation dhk I = 2L2/R, where L is the distance between the specimen and the fluorescent screen of the microscope, 2 is the electronic wavelength corresponding to the operating voltage of the electron microscope, and R is the ring diameter. The spectral distribution T(2) and R(2) for six SnSe films deposited onto quartz substrates in the thickness range of 51.6-318.2 nm is illustrated in Fig. 3. At longer wavelengths, with respect to the absorption edge, T + R = 1 indicating that SnSe thin
83
H.S. Soliman et al. / Physica A 216 (1995) 77 84
300
150 xlO 9
o
250
200
lO0
C'4 c~
E
E u
>
>
150
100
50
50 ° / o
Ol o/
t
/
o o ° I
0.8
/
1.0
~nol °°°
/ I
1.2
/ I
I
1.4
I
I
16
hz/(eV),
Fig.6. The spectral dependence of (2hT} ~'2 and (~h't')2 for SnSe films.
films exhibit no scattering or absorption. Knowing the film thickness d, the refractive index n and the absorption index k for SnSe films were determined from the absolute values of the measured transmittance and reflectance. The spectral distributions n(2) and k(2) for SnSe films for different thicknesses are illustrated in Fig. 4. From this figure, one can conclude that the discrepancy of both n and k lies within the limits of the experimental error. Thus both n and k are practically independent of the film thickness in the range 51.6--318.2 rim. Also the dispersion of the refractive index n(2) shows anomalous dispersion in the spectral range of 1200 2500 nm. The absorption coefficient ~ of SnSe thin films was calculated using the relation = ( 4 ~ k / 2 ) , where k is the mean value of the absorption index at the given
84
H.S. Soliman et al. / Physica A 216 (1995) 77 84
wavelength 2. Fig. 5 illustrates the dependence oflog(~) as a function of log(I/2). It can be observed that the obtained dependence yields two distinct parts of different slopes indicating the existence of two optical transitions. It is well known that the absorption coefficient ~ is proportional to ( h 7 - Eg) x, where x = 1/2 for the direct optical transition and x = 2 for the indirect one. Fig. 6 represents the dependence of both (0~hj 2 and (~hT) 1/2 o n the photon energy h?. It shows that both (0~hj 2 and (0¢hy) 1/2 vary linearly with hT, indicating the existence of both direct and indirect transitions. Extrapolating the straight parts of these relations towards the h7 axis yields the values of the corresponding energy gap. It was found that the direct and indirect energy gaps for SnSe films are 1.27 eV and 0.895 eV respectively. These values can be compared with those reported by Dan Tran Quan [28,29] which were 0.96 eV for indirect optical transition and 1.21 eV for direct optical transition calculated at the center of the Brillouin zone or 1.19 eV for SnSe thin films calculated from transmittance and reflectance measurements.
References [1] [2] [3] [4] [5] [6] [7]
J.J. Loferski, J. Appl. Phys. 27 (1956) 777. J.J. Loferski, Proc. IEEE 51 (1975) 345. M. Rodot, Acta Electronica 15 (1975) 345. M. Rodot, Rev. Phys. Appl. 12 (1977) 411. C.R. Dongwoo, G.Walser and T. Courtney, Appl. Phys. Lett., 24 (1974) 479. G.R. Valiukonis, D.A. Guseinova, G. Krivat and A. Shileika, Phys. Status Solidi (B) 135 (1986) 299. G.R. Valiukonis, F.M.G. Gashimzade, D.A. Guseinova, G. Krivaite, M.M. Mmedov and A. Sileika, Phys. Status Solidi (B) 122 (1984) 623. [8] F. Lukes, J. Humlicek and E. Schmidt, Solid State Commun. 45 (1983) 445. I-9] F. Lukes, E. Schmidt, J. Humlicek, P. Dub and F. Kosek, Phys. Status Solidi (B) 137 (1986) 569. 1-10] D.A. Guseinova, G.Z. Krivaite and M.M. Mamedov, Sov. Phys. Semicond. 19 (1985) 923. [11] A.P. Lambros, D. Geeraleas and N. Economou, J. Phys. Chem. Solids 35 (1974) 537. [12] T. Arai, K. Takahashi and K. Kudo, Sci. Light (Tokyo) 21 (1972) 131. [13] A.M. Ekorashy, Phys. Chem. Solids 27 (1986) 497. [14] Y. Mochida, Sci. Light (Tokyo) 17 (1968) 57. [15] A.K. Garg, A.K. Jain and G.P. Agnihotri, Ind. J. Pure Appl. Phys. 21 (1983) 276. [16] A.G. Mikolaicuk, Ya.I. Dutchak and D.M. Freik, Sov. Phys. Crystallogr. 13 (1968) 490. [17] A.G. Mikolaichuk and D.M. Freik, Sov. Phys. Solid State 11 (1970) 2033. [18] L.S. Palatnik and V.V. Levitin, Dokl. Akad. Nauk SSSR 96 (1954) 975. [19] A.S. Avilov, R.M. Lmamov and S.N. Navasadyan, Soy. Phys. Crystallogr. 24 (4) (1979) 504. [20] W. Hoffman, Z. Kristallogr. 92 (1935) 161. 1-21] A. Okazaki, J. Phys. Soc. Jap. 13 (1958) 1151. [22] A. Okazaki and I. Ueda, J. Phys. Soc. Jap. 11 (1956) 470. [23] S. Tolansky, Multiple-Beam Interferometry of Metals (Academic, London, 1970) p. 55. [24] I.N. Shklyareveskii, T.I. Korneeva and K.N. Zozula, Opt. Spect. 27 (1969) 174. [25] L.N. Hadley and D.M. Dennison, J. Opt. Soc. Am. 37 (1974) 451. [26] H.S. Soliman, N. E1-Kadry, O. Gammjoun, M.M. EI-Nahass and H.B. Darwish, Ind. J. Optics 17 (2) (1988) 47. [27] H.M. Liddell, Computer Aided Techniques for the Design of Multilayer Films (Adam Hilger Ltd., Bristol, 19811 p. 118. [28] Dan Tran Quan, Phys. Stat. Sol. (A) 86 (1984) 421. [29] Dan Tran Quan, Thin Solid Films 149 (1987) 197.
Physica A 216 (1995) 77-84
Optical properties of tin-selenid films H.S. S o l i m a n a'*, D.A. A b d e l H a d y b, K . F . A b d e l R a h m a n a, S.B. Y o u s s e f a, A.A. E1-Shazly a aDepartment of Physics, Faculty of Education, Ain Shams University, Cairo, Eqypt bDepartment of Enyineeriny Physics, Faculty qf Enqineerin9, Ain Shams University, Cairo, Eclypt
Received 15 July 1994
Abstract SnSe thin films of different thicknesses were prepared by the thermal evaporation technique in vacuum of 10 4 Pa. The structure analysis of the films as determined from the electron diffraction pattern and X-ray diffraction indicate that the films were polycrystalline of orthorhombic structure. The transmittance and reflectance of SnSe films were measured at normal incidence in the wavelength range of 760 2200 nm. It was found that the refractive index n and the absorption index k are independent of the film thickness. Graphical representation of log(a) as a function of (1/2) shows two distinct linear parts indicating the existence of two optical transitions. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region revealed an indirect and direct allowed transition with energy gaps E~nd= 0.895 eV and E~ = 1.27, respectively.
1. Introduction The surveying of the literature concerned with the optical and electrical properties of SnSe films revealed that: (i) these films have the potential to be used in different applications such as solar cells [ 1 4 ] and m e m o r y switching devices [-5]; (ii) SnSe films were already employed in holographic-recording system [6]. The direct band gap of SnSe films was determined via different approaches, namely electroreflectance and thermoreflectance measurements [7,8] and absorption measurements [9,10]. On the other hand, indirect transitions in SnSe films were also observed [9-15]. However, considerable scatter is found a m o n g the proposed values for electronic transitions threshold. Regarding the structure of SnSe films it was reported by Miklaichok et al. [16,17] and Palatnik et al. [18] that these films were composed of an orthorhombic phase
* Corresponding author. 0378-4371/95/$09.50 (C' 1995 Elsevier Science BN. All rights reserved SSDI 0 3 7 8 - 4 3 7 1 ( 9 4 ) 0 0 2 9 8 - 3
78
H.S. Soliman et al. / Physica A 216 (1995) 77 84
besides a cubic modification with NaC1 structure. On the other hand, the investigation of Avilov et al. [ 19] on the structure of SnSe films could not prove the existence of the cubic phase in the films. Hoffman et al. 1-20] and Okazaki [21] Okazaki et al. [22] reported that SnSe in the bulk form crystallizes in an orthorhombic structure with lattice parameters a = 0.446 nm, b = 0.419 nm, c = 1.157 nm. The appeal of SnSe films for different applications, as illustrated before, and the controversy about both the scatter mentioned above and the structure of the films persuaded us to investigate the optical properties and structure of SnSe films.
2. Experimental procedure SnSe thin films were prepared by the conventional thermal evaporation technique in vacuum. SnSe powder was supplied by M o r t o n Mhiokol Inc. "Alfa"-England and claimed by the supplier to be 99.999% pure. It was located in a silica bottle which was surrounded by a tungsten spiral filament to supply the heat energy necessary for the
T
film
%
Q
Powder
o
I
I
I
I
I
I
I
I
90
80
70
60
50
40
30
20
Fig. 1. X-ray diffraction patterns of SnSe (bottom) in powder form, (top) in thin film form.
H.S. Soliman et al. / Physica A 216 (1995) 77 84
79
Fig. 2. (top) Transmission Electron Micrograph for SnSe film 30 nm thick. (bottom) Electron diffraction pattern for SnSe film 30 n m thick.
evaporation. Well cleaned quartz slides were used as substrates for the films prepared for optical measurements while glass slides were used as substrates for the structure study. The substrate holder was rotating, during deposition, with a speed of 240 r.p.m. to get SnSe films with uniform thickness. The pressure inside the vacuum chamber was pumped down to 2 x 1 0 - 4 p a . before starting the evaporation process. A mechanical shutter was used to avoid any contamination on the substrates which were at ambient temperature. The thickness and the deposition rate (1 nm/sec.) of the prepared films were controlled by a quartz crystal monitor. The exact thicknesses of the films were measured, after breaking the vacuum, interferometrically [23]. Six groups of SnSe films were prepared and each group was characterized by a particular thickness. All factors effecting the structure and quality of film such as degree of vacuum, deposition rate, substrate kind and temperature were kept constant. The films thicknesses ranged from 51.6 nm to 318.2 nm. For optical measurement, the transmittance T and the reflectance R of the films deposited on quartz substrates were determined at normal incidence in the
80
H.S. Soliman et al. / Physica A 216 (1995) 77-84
0.6 ."
;,-'-/---.~/ - "
/I
~
....
~
0A
0.2
.\. "~'\'$'\~7//'A\-\. ,/' \\
1
I
.,.bo,
0.8
....
0.6
0.4
.,'°-~o<
/ /.~---~\
=.=~ ,,,°=.~=3Q0 th " ck
x
86.6
_ _ _
,o,
:::
iii:i
/
7
I
. " ~
.
/
".
x
/ f \.\\ ! l ~
o
/
\ /
/of'~
•
/
./
- ~--~;~~~
--~
0.2
~~:,"/ 1000 Wavelength,
,
,
1500
2000
X (nm)
Fig. 3. The spectral dependence of T(2) (bottom) and R(2) (top) for SnSe films of different thicknesses.
wavelength of 760-2200 nm by means of a double-beam spectrophotometer (Cary 2390, Varian) attached with reflectance stage with a V-W configuration. If Ict is the intensity of the transmitted light through the film/quartz combination, while Iq is that through the quartz reference only, then the measured transmittance of the SnSe film is given by: T = (Ic,/Iq) (1 -- Rq),
(1)
where Rq is the reflectance of quartz. In addition, if the intensity of light reflected from the sample mirror reaching the detector is Irr and that reflected from the reference mirror is lm, then R = ( I f r / l m ) 1/2 - - T
2Rq.
(2)
H.S. Soliman et aL / Physica A 216 (1995) 7 7 84
81
2.0 Ref. ( 12 ) Ref. ( 1 5 ) 1.5
13
E
A
1.0
at~
o
k
0.5
I 1000
"'-,,--.,.
I
I
1500
2000
Wavelength, A ( n m )
Fig. 4. The spectral dependence of refractive index n(2) and absorption index k(2) for SnSe thin films.
From the measured T, R and the film thickness d, the values of the refractive index n and absorption index k were computed by a computational method [26] based on Murmann's formulas 1-27]. Taking into account the experimental error in measuring the film thickness to be _+ 2.5% and that in T and R to be ± 1%, the error in the calculated values of n and k was estimated to be 3% and 2.5%, respectively. Structural studies of SnSe in the powder and thin film forms were carried out using an X-ray diffractometer (Philips Pwl410) with C o - K s radiation and a transmission electron microscope (type J E O L / J E M 100S) operating at 60 kV. The microscope is attached to a electron diffraction stage.
3. Results and discussion Fig. 1 shows the X-ray diffractograms of the SnSe in powder form as well as in thin-film form. The film thickness is 318.2 nm and the deposition rate is 1 nm/sec. Analysis of the diffraction patterns indicated that the SnSe in the powder form or thin-film form has polycrystalline nature with an orthorhombic structure with lattice parameters a = 1.147 nm, b = 0.414 nm and c = 0.444 nm in case of the powder form and a = 1.142 nm, b = 0.420 nm, c = 0.442 nm in thin film form, belonging to the D~ 6 Pnma space group. The mean uncertainty in calculating the lattice parameters was 1%. The obtained results for SnSe films were confirmed by the electron diffraction patterns carried out for SnSe thin film 30 nm thick deposited at the same conditions.
H.S. Soliman et al. / Physica A 216 (1995) 77-84
82
10 5
/ io fo it i
10 4
"7 E La
J
10 3
10 2
3.8
I
I
3.9
4.0
Log ( I I ~ ) ,
Fig. 5. The variation of log(s) with log(l/A}.
A typical electron diffraction pattern is shown in Fig. 2. The observed dhk I spacings were determined from the relation dhk I = 2L2/R, where L is the distance between the specimen and the fluorescent screen of the microscope, 2 is the electronic wavelength corresponding to the operating voltage of the electron microscope, and R is the ring diameter. The spectral distribution T(2) and R(2) for six SnSe films deposited onto quartz substrates in the thickness range of 51.6-318.2 nm is illustrated in Fig. 3. At longer wavelengths, with respect to the absorption edge, T + R = 1 indicating that SnSe thin
83
H.S. Soliman et al. / Physica A 216 (1995) 77 84
300
150 xlO 9
o
250
200
lO0
C'4 c~
E
E u
>
>
150
100
50
50 ° / o
Ol o/
t
/
o o ° I
0.8
/
1.0
~nol °°°
/ I
1.2
/ I
I
1.4
I
I
16
hz/(eV),
Fig.6. The spectral dependence of (2hT} ~'2 and (~h't')2 for SnSe films.
films exhibit no scattering or absorption. Knowing the film thickness d, the refractive index n and the absorption index k for SnSe films were determined from the absolute values of the measured transmittance and reflectance. The spectral distributions n(2) and k(2) for SnSe films for different thicknesses are illustrated in Fig. 4. From this figure, one can conclude that the discrepancy of both n and k lies within the limits of the experimental error. Thus both n and k are practically independent of the film thickness in the range 51.6--318.2 rim. Also the dispersion of the refractive index n(2) shows anomalous dispersion in the spectral range of 1200 2500 nm. The absorption coefficient ~ of SnSe thin films was calculated using the relation = ( 4 ~ k / 2 ) , where k is the mean value of the absorption index at the given
84
H.S. Soliman et al. / Physica A 216 (1995) 77 84
wavelength 2. Fig. 5 illustrates the dependence oflog(~) as a function of log(I/2). It can be observed that the obtained dependence yields two distinct parts of different slopes indicating the existence of two optical transitions. It is well known that the absorption coefficient ~ is proportional to ( h 7 - Eg) x, where x = 1/2 for the direct optical transition and x = 2 for the indirect one. Fig. 6 represents the dependence of both (0~hj 2 and (~hT) 1/2 o n the photon energy h?. It shows that both (0~hj 2 and (0¢hy) 1/2 vary linearly with hT, indicating the existence of both direct and indirect transitions. Extrapolating the straight parts of these relations towards the h7 axis yields the values of the corresponding energy gap. It was found that the direct and indirect energy gaps for SnSe films are 1.27 eV and 0.895 eV respectively. These values can be compared with those reported by Dan Tran Quan [28,29] which were 0.96 eV for indirect optical transition and 1.21 eV for direct optical transition calculated at the center of the Brillouin zone or 1.19 eV for SnSe thin films calculated from transmittance and reflectance measurements.
References [1] [2] [3] [4] [5] [6] [7]
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