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Optical and electrical properties of thermally evaporated Se90Sb10 thin film
Materials Science & Engineering B xxx (xxxx) xxx–xxx
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Optical and electrical properties of thermally evaporated Se90Sb10 thin film ⁎
M.I. Abd-Elrahman , A.A. Abu-Sehly, Sherouk Sh. El-sonbaty, M.M. Hafiz Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: Chalcogenide Thin films Optical constants Electrical conductivity
Chalcogenide Se90Sb10 thin films are deposited by thermal evaporation from the bulk alloy. X-ray diffraction examination for the annealed films shows the amorphous-crystalline transformation. This is beneficial for optical disk data storage technology. The crystallinity is improved by increasing the annealing temperature. The films annealed at relatively low temperatures exhibit highly transparence reaching to about 90% at incident light of wavelength of 900 nm. The as-prepared and annealed Se90Sb10 films reveal an indirect allowed optical transition. The annealed film at 473 K has an optical band gap of 1.676 eV which is suitable value for solar cell as photovoltaic application. Both the indirect optical energy band gap (Eg) and the oscillator energy (Eo) decrease whereas the oscillator strength (Ed) increases with increasing the annealing temperature. The annealing increases the conductivity and decreases the activation energy for conduction resulting in enhancement of film properties for adapting to solar cells.
1. Introduction Chalcogenide material alloys are the subject of considerable studies due to their applications in high infrared transmission, photovoltaics and solar energy conversion [1–3]. The various film growth techniques have strong influence on stoichiometric fidelity, structural, optical, electrical, photosensitive properties etc. can be affected by the deposition methods. Most of the chalcogenide glasses behave as p-type semiconductors because the position of Fermi level rests slightly below the middle of the gap. These glasses have potential applications in industry like optical data storage, grating and sensors [4–6]. Based on the property of reversibly switched between amorphous and the crystalline states, these material are appliances in phase change memory devices beside in the thermal sensor materials [7]. Selenium as chalcogenide element has low photosensitivity and thermal stability. To improve these properties, additives like Ge, Te, Bi, Sb and As for alloying Se have been attempted [8–11]. However, the adding of Sb was found to be greater than Te in encouraging crystallization of Se [12]. These additives also strongly affect the electric transport in chalcogenide materials [13]. The alloys belonging to Sb–Se binary system exhibits thermoelectric properties, switching effect, photovoltaic and photoconductivity for their potential applications as absorber layer in the optoelectronic devices and solar cells etc. due to enhancement in the properties of Se upon alloying [14,15]. The heat treatment by thermal annealing is one of essential treatment to enhance the optical and electrical properties of thin films. The
⁎
annealing induces homogenization, phase transitions and relocation of defects. The changes in physical properties as result of annealing depend considerably on the rate of heating and the time of annealing. In this work, the transmission and reflection spectra in NIR-VIS-UV are used to characterize optical constants and parameters of dispersion of the refractive index. We also examine the effect of thermal annealing at different temperatures on optical and electrical properties of Se90Sb10 films to adapt the film properties to optical filters and solar cells as photovoltaic application. 2. Experimental The chalcogenide Se90Sb10 bulk was prepared from a mixture of Se and Sb elements with purity 99.99% (Aldich Chem. Co., USA) by the melt-quench technique. The experimental details for preparation of bulk as well as the deposition of thin film samples by thermal evaporation were discussed in our pervious work [16,17]. The thermal behavior of the bulk sample was examined using the Differential Scanning Calorimetery (DSC) of model DU Pont 1090. An X-Ray Diffractometer (Philips, PW 1710) was used to record the X-ray diffraction patterns of the thin films. A Scanning Electron Microscopy (SEM) apparatus, Jeol (JSM)-T200 type (Japan), was used to investigate the surface morphology of the as-prepared and thermally annealed samples. A double beam spectrophotometer of type (Shimadzu 2101, Japan) was employed to measure the transmittance and reflectance spectra of the thin films in the wavelength range 250–2500 nm. The
Corresponding author. E-mail address: [email protected] (M.I. Abd-Elrahman).
https://doi.org/10.1016/j.mseb.2018.10.018 Received 21 August 2017; Received in revised form 22 April 2018; Accepted 29 October 2018 0921-5107/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Abd-Elrahman, M.I., Materials Science & Engineering B, https://doi.org/10.1016/j.mseb.2018.10.018
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Fig. 1. DSC traces for Se90Sb10 at heating rate of 10 K/min. Fig. 2. X-ray diffraction patterns for as-prepared and annealed Se90Sb10 thin films.
dark electrical conductivity of the as-prepared and annealed Se90Sb10 thin films were measured in the temperature range from 300 to 500 K using high internal impedance electrometer (Keithely 175A). The ohmic contacts were made by evaporating aluminum electrodes. In order to study the effect of temperature annealing, four deposited films were individually heated at four temperatures 323, 373, 423 and 473 K in vacuum for one hour using dark oven and then cooled to room temperature.
Table 1 Measured and standard JCPDS values for diffraction angles (2θ) and interplanar distances (d) and hkl for the annealed Se90Sb10 thin films at different annealing temperatures for 1 h.
3. Results and discussion 3.1. Thermal analysis Fig. 1 shows the DSC trace of Se90Sb10 bulk sample in the temperature range from 300 to 600 K at heating rate of 10 K/min. It is observed four characteristic temperatures representing four thermal events along the DSC curve. The first is the glass transition temperature (Tg) at 325 K. The second and third events are the temperatures of the onset (To) and peak (Tp) of crystallization process at 384 and 404 K, respectively. The last thermal event corresponds the peak temperature of melting (Tm) at 492 K. The existence of only one melting peak indicates the homogeny of the material content. The property of thermal transformation from amorphous phase to crystalline one is considered as an important requirement of those needed for the chalcogenide materials for encoding binary information and constructs the rewritable optical discs and of memory devices.
Ta K
2θexp. deg.
dexp. Ǻ
dstand. Ǻ
hkl
D nm
323
23.04 40.96
3.860 2.203
3.790 2.195
(0 1 1) (4 1 2)
21 12
373
23.20 41.04
3.834 2.201
3.790 2.196
(0 1 1) (4 1 2)
16 12
423
16.48 23.16 33.84 41.00
5.379 3.840 2.649 2.201
5.230 3.790 2.680 2.190
(2 0 1) (0 1 1) (1 1 3) (2 1 4)
43 24 50 20
473
16.48 23.20 33.72 41.00
5.379 3.830 2.658 2.201
5.230 3.790 2.680 2.190
(2 0 1) (0 1 1) (1 1 3) (2 1 4)
33 29 25 29
crystallinity by increasing the annealing temperature. The crystallite size (D) are calculated using the well known Scherer’s formula
D=
0.94λ , γ cos θ
(1)
where λ in nm is the X-ray wavelength, γ in radian is the width of the diffraction peak and θ is the diffraction angle in radian. The calculated D values for observed crystalline phases are listed in Table 1. The particle size of the crystalline phases for the samples annealed at 323 and 373 K is smaller than that annealed at 423 and 473 K.
3.2. X-ray diffraction X-ray diffraction patterns of the thermally evaporated film of thickness 200 nm for the as-prepared and those annealed at four different temperatures (Ta) are shown in Fig. 2. For the as-prepared film, the absence of any sharp diffraction peak reflects the amorphous nature of the material of this film. The observed diffraction peaks in all annealed films indicate the amorphous to crystalline transformation. The crystalline phase can be identified according to the best agreement between the observed of inter-planar spacing and those of Joint Committee on Powder Diffraction Standards (JCPDS). Accordingly, the crystallized phase is Sb2Se3 phase with different preferred orientations as presented in Table 1. The films annealed at Ta = 323 K and 373 K exhibit characteristic peaks of Se crystal plane (0 1 1) and (4 1 2). For the films annealed at 423 K and 473 K, other characteristic peaks of Se crystallized plane (2 0 1) and (1 1 3) appear. The increasing of peaks number and their intensities are evidences for the enhancement of
3.3. Surface morphology The surface morphology of the films was observed using scanning electron microscopy (SEM-Jeol T200). It was observed in SEM images shown in Fig. 3a that the as-prepared film reveals the formation of a heterogeneous cluster in the glassy matrix. For the film annealed at 423 K, Fig. 3b shows crystallization growth resulting in the form of spheroids and rods. Similar shapes of crystallization are observed for the films annealed at the other annealing temperatures. The observation of the amorphous structure for the as-prepared and the poly crystallization for those annealed is confirmed in the XRD (Fig. 2). 2
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Fig. 3. SEM micrographs showing the surface microstructure of the as-prepared sample (A) and annealed (B, C and D) at 423 K for Se90Sb10 thin film.
Fig. 4. Transmittance spectra for as-prepared and annealed Se90Sb10 thin films.
Fig. 5. Reflectance spectra as a function of the wavelength for as-prepared and annealed Se90Sb10 thin films.
3.4. Transmission and reflection spectra 3.5. Absorption coefficient and optical energy gap The transmission spectra of Se90Sb10 of the as-prepared and annealed films are shown in Fig. 4. It is observed that the increase of the annealing temperatures shifts the optical transmittance to the lower energies suggesting the decrease of the optical band gap. For all films, the transmittance (T) of the film increases as the wavelength increases up to a certain value depending on Ta. Then the transmittance decreases and almost keeps constant for higher wavelengths. On the other hand, T decreases as Ta increases. For as-prepared film and those annealed 323 K and 373 K, the films exhibit highly transparence reaching to about 90% at 900 nm (infrared region). Highly transparent of Se90Sb10 film candidates it for the application in color filters. The transmittance decreases as Ta increases. This may be attributed to the light scattering effect due to grain-boundary scattering. Taking into account the increase of crystallite size with Ta is extracted from XRD and presented in Table 1. The reflection spectra of Se90Sb10 film of the as-prepared and annealed films are shown in Fig. 5. The reflectance shows successive up and down curvatures for incident wavelength up to 1200 nm. These oscillations are due to the interference by the front and back film surfaces. At higher wavelengths, the observed upward curvature in the transmittance together with the corresponding downward one in the reflectance is attributed to the interaction of light waves with the free carrier absorption [18].
It can calculate the absorption coefficient (α) using the measured transmittance, T, and the reflectance, R, by the following Eq. [19]
α=
1 (1 − R)2 ln , d T
(2)
where d is the thickness of the film in cm. The dependence of α on the incident photon energy (hν) for as-prepared and annealed films of Se90Sb10 film is shown Fig. 6. The film absorption increases with increase of both photons energy of the incident light and Ta. The observed increasing behavior in α with increasing annealing temperature may be due to the increase of crystallinity in films with rising annealing temperatures as evidenced from XRD in Fig. 2. The optical energy band gap (Eg) can be determined using the fundamental absorption which corresponds to electron excitation from the valence band to conduction band. The nature of electronic transition for Se90Sb10 thin film and the value of Eg’s of studied films are obtained using the Tauc relation [20]:
α=
B (hν − Eg )m hν
,
(3)
where B is the absorption constant and m is an index that characterizes the optical absorption process. The value of exponent m is theoretically equal to ½ and 2 for direct and indirect allowed transitions. The plot 3
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as the film annealing temperature increases as presented in Table 2. The observed decrease in bandgaps coincides with the behavior of the absorption band edges which shift to lower energy with increasing of Ta. The behavior of Eg can be explained as follows. It have been reported by several researchers that the annealing process above the glass transition temperature causes formation of surface dangling bonds around crystallites [21–25]. After further raising the annealing temperature, the crystallites break down into a number of small crystals. An increase in the number of dangling bonds produces further dangling bonds in polycrystalline solids. The concentration of localized states is also increased. Therefore, an increase in the width of tails in the band gap (Ee) occurs. Consequently, the optical band gap decreases [22–24]. 3.6. Refractive index and extinction coefficient The optical constants such as refractive index (n) and extinction coefficient (k) are important constants for recommendation a film for applications in optical devices. The values of n and k relate to the experimentally measured reflectance (R) by the following Eq. [26]: Fig. 6. Absorption coefficient (α) as a function of photon energy (hν) for asprepared and annealed Se90Sb10 thin films.
n=
1+R 4R 1 +( − k2 ) 2 , 1−R 1 − R2
k = α λ /4π ,
(4) (5)
where λ is the wavelength. Fig. 8 shows the annealing temperature dependence of refractive index, n, as a function of the wavelength, λ. It can be observed that the refractive index shows anomalous dispersion in the low wavelength range (λ < 1360 nm). This behavior is characterized by independency of n on Ta and λ. The gradual decrease in n with λ in high region of wavelength represents the normal dispersion manner. The anomalous and normal behaviors of n have been observed for GeSeSn thin film [27]. The value of n in the normal dispersion range increases as Ta increases expect for annealing at 473 K which partially evaporates the material film. The observed increase in n with heat treatment is consistent with the reduced optical band gap. The extinction coefficient (k) is known as absorption index and is defined as the amount of light loses due to scattering and absorption per unit distance. Fig. 9 shows k as a function of wavelength λ for Se90Sb10 of as-prepared and annealed films with different Ta. For all films, k increases as λ increases up to a certain value and then k decreases. The decrease of extinction coefficient or absorption index with increase in wavelength is an obvious phenomenon due to commencement of strong electronic absorption transition at band gaps.
Fig. 7. The variation of (αhν)1/2 as a function of the photon energy (hν) for asprepared and annealed Se90Sb10 thin films. Table 2 Variation energies of optical band gap (Eg), dispersion (Eo) single-oscillator (Ed) and the index m of Eq. (3) for as-prepared and annealed Se90Sb10 thin films. Ta K
Eg eV
Eo eV
Ed eV
m
As-pre. 323 373 423 473
2.065 2.054 1.899 1.882 1.676
0.84 0.83 0.79 0.77 0.82
7.69 8.46 11.64 15.20 11.17
1.8 1.9 2.2 2.0 2.2
that covers the widest range of data and intercept the hν-axis is obtained for the (αhν)2 = f (hν) dependence as shown in Fig. 7. The obtained value of m = 2, characterizes an indirect allowed optical transition. For conformation, the value of m can be theoretically calculated by plotting a relation between 1/(d(lnαhν)/dhν) on y-axis and hν on xaxis and the slope of the straight line will give 1/m. The obtained values of m for as-prepared and annealed films are listed in Table 2. Eg’s are obtained from the intercept of the resulting straight lines with the energy axis at (αhν)2 = 0 (Fig. 7). The value of Eg is found to be decreased
Fig. 8. Refractive index as a function of the wavelength for as-prepared and annealed Se90Sb10 thin films. 4
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Fig. 9. The extinction coefficient (k) as a function of the wavelength for asprepared and annealed Se90Sb10 thin films.
Fig. 11. Temperature dependence of the dark electrical conductivity (σ) for asprepared and annealed Se90Sb10 thin films.
3.7. Dispersion of refractive index
behavior of Eg.
The dispersion of refractive index is a significant factor for both a fundamental and technological viewpoints. The dispersion data in semiconductor can be analyzed using the single-oscillator Wemple–DiDomenico (WDD) model [28,29]. According to this model, the oscillator energy or dispersion energy (Eo) and oscillator strength (Ed) are relates to n by the following relation:
n2 = 1 +
Eo Ed . Eo2 − (hν )2
3.8. Electrical conductivity The measurements of electrical conductivity (σ) of the as-prepared and annealed films of Se90Sb10 as function of temperature in the range 300–490 K are given in Fig. 11. It can be noticed that σ increases as Ta increase expect for 473 K-annealed sample which undergoes partial evaporation due to the relatively high temperature annealing. The increase of σ with Ta is attributed to the decrease of electrical activation energy due to the increase of energy width of localized states because of the increase of dangling bonds and defects in the band structure. As the conductivity increases by annealing, thus Se90Sb10 material is suitable for electrical applications of multilevel resistivity by adopting different resistivity values. That is, the amorphous state and the crystalline one are characterized by difference in the electrical resistivity. This difference between the two states allows non-volatile bilevel storage and can in principle be exploited for multilevel. To utilize the multilevel approach in the case of phase change memories, it is necessary to employ a chalcogenide of different electrical resistance in the range from the maximum to the minimum available value [31]. Fig. 11 shows an exponential increase of conductivity with temperature indicating the semiconductor nature of the studied films. That is, the observed increase in σ can be attributed to the increase of the number of thermally excited electrons as the temperature increases. Therefore, the conductivity process in the Se90Sb10 films is expected to be due to the activated electrons in the extended states above the conduction band edge [32]. The temperature dependence of conductivity yields the activation energy for conduction (ΔE) according to the relation [33]:
(6)
The dispersion parameters Ed and Eo can be obtained by plot of (n2 − 1)−1 versus (hν)2 as shown in Fig. 10. The obtained values of Eo and Ed from the slope and intercept are listed in Table 2. It is noticed that Ed increases while Eo decreases as Ta increases except for the annealing at highest temperature (Ta = 473 K). The increase of Ed with the increase of Ta is attributed to the increase of the diffusion rate of atoms in the films giving more number of atoms at interstitial sites. This leads to impurity type scattering centers, thereby Ed increases [30]. The decrease of Eo with increasing of Ta seems to be in agreement with the
ΔE ⎤ σ = σo exp ⎡− ⎢ ⎦ ⎣ kB T ⎥
(7)
where σo is the pre-exponential factor for conductivity, kB is the Boltzmann's constant and T is absolute temperature. The plot of ln (σ) against 103/T presented in Fig. 12 shows two linear parts of two slopes corresponding activation energies ΔE1 and ΔE2. This means the conductivity is arising from carriers being dominant at high temperature region and the others at low one. Therefore, the conduction can be represented in the two regions based on Eq. (7) as
Fig. 10. A plot of (n2 − 1)−1 against (hν)2 for as-prepared and annealed Se90Sb10 thin films. 5
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Fig. 13. Plots of ln(σT1/2) versus 1000/T1/4 for as-prepared and annealed Se90Sb10 thin films in the low temperature region.
wave function and is assumed to be 0.125 Å−1 [21] and N(EF) is the density of the trap states near the Fermi level. Fig. 13 shows the plots of ln(σT1/2) versus 100/T 1/4 for the as-prepared and annealed films in the low temperature region. The validity of hopping conduction mechanism is indicated by the liner relations. The deduced N(EF) values from the slopes of a straight lines and those of T0 are listed in Table 3. The observed decrease in T0 with increasing Ta indicates the decrease of disorder in the material as confirmed by XRD in Fig. 2.
4. Conclusion
Fig. 12. Logarithmic plots of dark electrical conductivity (σ) against 1000/T for as-prepared and annealed Se90Sb10 thin films.
ΔE ΔE σ = σo1 exp ⎡− 1 ⎤ + σo2 exp ⎡− 2 ⎤, ⎢ ⎥ ⎢ k T k ⎦ ⎣ B ⎦ ⎣ BT ⎥
The thermal induced amorphous-crystalline phase change in Se90Sb10 thin film benefices for optical disk data storage technology. The crystallinity of Se90Sb10 thin film is improved by increasing the annealing temperature. The observed highly transparent at 900 nm candidates this film for infrared transmission. The optical absorption data of as-prepared and annealed films reveal the mechanism of electronic transition to be indirect optical transition. The reducing of the optical energy gap by elevate the thermal annealing is assigned to the formation of localized states in the band structure. The optical constants (the refractive index and extinction coefficient) and the dispersion parameters (the oscillator energy and oscillator strength) are strongly dependant on the annealing temperature. The electrical conductivity of the film is caused by the band and hopping conductions. Annealing causes an increase in the electrical conductivity of the films and corresponding decrease in the activation energy for conduction. This is assigned to the amorphous-crystalline transformation. For future study, the addition of third element to Se–Sb binary system is expected to cause structural changes that modifies the glass forming region, creates compositional and configurational disorder, band structure and various physical parameters.
(8)
where σ01 and σ02 are the pre-exponential factors. The activation energies ΔE1 and ΔE2 as well as σ01 and σ02 of studied film are obtained and listed in Table 3. With increasing of Ta, the decrease of both ΔE and the increase of both σ0 consent with the trend of the conductivity. As shown in Fig. 12, the existence of two linear parts means the electrical conductivity in high and low temperature regions is caused by the band and the hopping conductions, respectively [34]. The latter occurs as a result of the excitation of the carriers into localized states at band edges and characterized by Mott’s variable-range hopping relation [35]:
ln(
ρ hop T
) = ln(ρo ) + (
To 1 )4 , T
(9)
where To is the measure of disorder in the material and given as
To = 18α3/[kB N (EF )],
(10)
where α is coefficient of the exponential decay of the localized state Table 3 The density of states at the Fermi level N(EF) and hopping parameters for the as-prepared and annealed Se90S10 thin films for 1 h. Ta K
ΔE1 eV
σ01 (Ω.cm)−1
ΔE2 eV
σ02 (Ω.cm)−1
To × 1010 K
N(Ef)×10−14 eV−1 cm−3
As-pre. 323 373 423 473
0.895 0.875 0.856 0.704 0.602
37.7 × 104 38.7 × 104 45.5 × 104 64.8 × 105 25.5 × 107
0.893 0.711 0.696 0.439 0.425
36.4 × 104 11.9 × 105 58.8 × 106 14.9 × 109 10.9 × 1010
4.79 4.43 3.77 0.80 0.61
5.19 5.61 6.59 0.31 0.40
6
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] V.M. García, L. Guerrero, M.T.S. Nair, P.K. Nair, Superficies y Vacío 9 (1999) 213. [19] J.I. Pankove, Optical Processes in Semiconductors, Dover New York 1971 Publ. (p. 103). [20] J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974, p. 159. [21] A.S. Soltan, Appl. Phys. A 80 (2005) 117. [22] N.M. Zulfequar, J. Alloys Compd. 576 (2013) 103. [23] M.M. Hafiz, A.A. Othman, M.M. El-Nahass, A.T. Al-Motasem, Phys. B 390 (2007) 348. [24] M.M.A. Imran, O.A. Lafi, M. Abu-Samak, Vacuum 86 (2012) 1589. [25] H.E. Atyia, A.S. Farid, J. Electron. Mater. 45 (2016) 357. [26] Y. Chen, D.M. Bagnall, H.J. Koh, K.T. Park, K. Hiraga, Z.Q. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [27] A.A.A. Darwish, H.A.M. Ali, J. Alloys Compd. 710 (2017) 431. [28] S.H. Wemple, M. DiDomenico, Phys. Rev. B 3 (1971) 1338. [29] S.H. Wemple, Phys. Rev. B 7 (1973) 3767. [30] M.A. Alvi, Opt. Commun. 295 (2013) 21. [31] Z. Li, R.G.D. Jeyasingh, J. Lee, M. Asheghi, H.-S.P. Wong, IEEE Trans. Electron. Devices 59 (2012) 3651. [32] E.A. Davis, N. Mott, Phil. Mag. 22 (1970) 903. [33] A.K. Hassan, R.D. Gould, A.K. Ray, Phys. Status Solidi A 158 (1996) K23. [34] M.L. Theye, Proceedings of the Fifth International Conference on Amorphous and Liquid Semiconductors, Garmisch-Partenkirchen, Germany, 1973, p. 479. [35] N.F. Mott, Philos. Mag. 32 (1975) 961.
V. Weidenhof, P. Franz, M. Wuttig, J. Appl. Phys. 87 (2000) 9. J.S. Sanghera, J. Heo, J.D.J. Machkenzie, J. Non-Cryst. Solid 103 (1988) 155. F.I. Ezema, R.U. Osuji, Chalcogenide Lett. 4 (2007) 69. P. Boolchand, W.J. Bresser, Nature 410 (2001) 1070. M. Frumar, T. Wagner, Curr. Opin. Solid State Mater. Sci. 7 (2003) 117. C.C. Huang, D.W. Hewak, Thin Solid Films 500 (2006) 247. D. Wamwangi, W.K. Njoroge, M. Wuttig, Thin Solid Films 408 (2002) 310. M.M. Hafiz, A.H. Moharram, M.A. Abdel-Rahim, A.A. Abu-Sehly, Thin Solid Films 292 (1997) 7. M.A. Majeed Kan, M. Zulfequar, M. Husain, J. Opt. Mater. 22 (2003) 21. J.Y. Shim, S.W. Park, H.K. Baik, Thin Solid Films 292 (1997) 31. J. Vazquez, C. Wagner, P. Villares, J. Non-Cryst. Solids 235 (1998) 548. N.F. Mott, E.A. Davis, Electronic processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1971. T.K. Kim, S.S. Han, B.S. Bae, Met. Mater. Int. 5 (1999) 33. S.K. Srivastava, P.K. Dwivedi, A. Kumar, Phys. B 183 (1993) 409. J. Hautala, S. Yamasaki, P.C. Taylor, J. Non-cryst. Solids 114 (1989) 85. M.I. Abd-Elrahman, Rasha M. Khafagy, Noha Younis, M.M. Hafiz, Phys. B 449 (2014) 155. M.I. Abd-Elrahman, M.M. Hafiz, M.A. Ammar Qasem, Abd Abdelraheem, Appl. Phys. A 122 (2016) 772.
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Optical and electrical properties of thermally evaporated Se90Sb10 thin film ⁎
M.I. Abd-Elrahman , A.A. Abu-Sehly, Sherouk Sh. El-sonbaty, M.M. Hafiz Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: Chalcogenide Thin films Optical constants Electrical conductivity
Chalcogenide Se90Sb10 thin films are deposited by thermal evaporation from the bulk alloy. X-ray diffraction examination for the annealed films shows the amorphous-crystalline transformation. This is beneficial for optical disk data storage technology. The crystallinity is improved by increasing the annealing temperature. The films annealed at relatively low temperatures exhibit highly transparence reaching to about 90% at incident light of wavelength of 900 nm. The as-prepared and annealed Se90Sb10 films reveal an indirect allowed optical transition. The annealed film at 473 K has an optical band gap of 1.676 eV which is suitable value for solar cell as photovoltaic application. Both the indirect optical energy band gap (Eg) and the oscillator energy (Eo) decrease whereas the oscillator strength (Ed) increases with increasing the annealing temperature. The annealing increases the conductivity and decreases the activation energy for conduction resulting in enhancement of film properties for adapting to solar cells.
1. Introduction Chalcogenide material alloys are the subject of considerable studies due to their applications in high infrared transmission, photovoltaics and solar energy conversion [1–3]. The various film growth techniques have strong influence on stoichiometric fidelity, structural, optical, electrical, photosensitive properties etc. can be affected by the deposition methods. Most of the chalcogenide glasses behave as p-type semiconductors because the position of Fermi level rests slightly below the middle of the gap. These glasses have potential applications in industry like optical data storage, grating and sensors [4–6]. Based on the property of reversibly switched between amorphous and the crystalline states, these material are appliances in phase change memory devices beside in the thermal sensor materials [7]. Selenium as chalcogenide element has low photosensitivity and thermal stability. To improve these properties, additives like Ge, Te, Bi, Sb and As for alloying Se have been attempted [8–11]. However, the adding of Sb was found to be greater than Te in encouraging crystallization of Se [12]. These additives also strongly affect the electric transport in chalcogenide materials [13]. The alloys belonging to Sb–Se binary system exhibits thermoelectric properties, switching effect, photovoltaic and photoconductivity for their potential applications as absorber layer in the optoelectronic devices and solar cells etc. due to enhancement in the properties of Se upon alloying [14,15]. The heat treatment by thermal annealing is one of essential treatment to enhance the optical and electrical properties of thin films. The
⁎
annealing induces homogenization, phase transitions and relocation of defects. The changes in physical properties as result of annealing depend considerably on the rate of heating and the time of annealing. In this work, the transmission and reflection spectra in NIR-VIS-UV are used to characterize optical constants and parameters of dispersion of the refractive index. We also examine the effect of thermal annealing at different temperatures on optical and electrical properties of Se90Sb10 films to adapt the film properties to optical filters and solar cells as photovoltaic application. 2. Experimental The chalcogenide Se90Sb10 bulk was prepared from a mixture of Se and Sb elements with purity 99.99% (Aldich Chem. Co., USA) by the melt-quench technique. The experimental details for preparation of bulk as well as the deposition of thin film samples by thermal evaporation were discussed in our pervious work [16,17]. The thermal behavior of the bulk sample was examined using the Differential Scanning Calorimetery (DSC) of model DU Pont 1090. An X-Ray Diffractometer (Philips, PW 1710) was used to record the X-ray diffraction patterns of the thin films. A Scanning Electron Microscopy (SEM) apparatus, Jeol (JSM)-T200 type (Japan), was used to investigate the surface morphology of the as-prepared and thermally annealed samples. A double beam spectrophotometer of type (Shimadzu 2101, Japan) was employed to measure the transmittance and reflectance spectra of the thin films in the wavelength range 250–2500 nm. The
Corresponding author. E-mail address: [email protected] (M.I. Abd-Elrahman).
https://doi.org/10.1016/j.mseb.2018.10.018 Received 21 August 2017; Received in revised form 22 April 2018; Accepted 29 October 2018 0921-5107/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Abd-Elrahman, M.I., Materials Science & Engineering B, https://doi.org/10.1016/j.mseb.2018.10.018
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Fig. 1. DSC traces for Se90Sb10 at heating rate of 10 K/min. Fig. 2. X-ray diffraction patterns for as-prepared and annealed Se90Sb10 thin films.
dark electrical conductivity of the as-prepared and annealed Se90Sb10 thin films were measured in the temperature range from 300 to 500 K using high internal impedance electrometer (Keithely 175A). The ohmic contacts were made by evaporating aluminum electrodes. In order to study the effect of temperature annealing, four deposited films were individually heated at four temperatures 323, 373, 423 and 473 K in vacuum for one hour using dark oven and then cooled to room temperature.
Table 1 Measured and standard JCPDS values for diffraction angles (2θ) and interplanar distances (d) and hkl for the annealed Se90Sb10 thin films at different annealing temperatures for 1 h.
3. Results and discussion 3.1. Thermal analysis Fig. 1 shows the DSC trace of Se90Sb10 bulk sample in the temperature range from 300 to 600 K at heating rate of 10 K/min. It is observed four characteristic temperatures representing four thermal events along the DSC curve. The first is the glass transition temperature (Tg) at 325 K. The second and third events are the temperatures of the onset (To) and peak (Tp) of crystallization process at 384 and 404 K, respectively. The last thermal event corresponds the peak temperature of melting (Tm) at 492 K. The existence of only one melting peak indicates the homogeny of the material content. The property of thermal transformation from amorphous phase to crystalline one is considered as an important requirement of those needed for the chalcogenide materials for encoding binary information and constructs the rewritable optical discs and of memory devices.
Ta K
2θexp. deg.
dexp. Ǻ
dstand. Ǻ
hkl
D nm
323
23.04 40.96
3.860 2.203
3.790 2.195
(0 1 1) (4 1 2)
21 12
373
23.20 41.04
3.834 2.201
3.790 2.196
(0 1 1) (4 1 2)
16 12
423
16.48 23.16 33.84 41.00
5.379 3.840 2.649 2.201
5.230 3.790 2.680 2.190
(2 0 1) (0 1 1) (1 1 3) (2 1 4)
43 24 50 20
473
16.48 23.20 33.72 41.00
5.379 3.830 2.658 2.201
5.230 3.790 2.680 2.190
(2 0 1) (0 1 1) (1 1 3) (2 1 4)
33 29 25 29
crystallinity by increasing the annealing temperature. The crystallite size (D) are calculated using the well known Scherer’s formula
D=
0.94λ , γ cos θ
(1)
where λ in nm is the X-ray wavelength, γ in radian is the width of the diffraction peak and θ is the diffraction angle in radian. The calculated D values for observed crystalline phases are listed in Table 1. The particle size of the crystalline phases for the samples annealed at 323 and 373 K is smaller than that annealed at 423 and 473 K.
3.2. X-ray diffraction X-ray diffraction patterns of the thermally evaporated film of thickness 200 nm for the as-prepared and those annealed at four different temperatures (Ta) are shown in Fig. 2. For the as-prepared film, the absence of any sharp diffraction peak reflects the amorphous nature of the material of this film. The observed diffraction peaks in all annealed films indicate the amorphous to crystalline transformation. The crystalline phase can be identified according to the best agreement between the observed of inter-planar spacing and those of Joint Committee on Powder Diffraction Standards (JCPDS). Accordingly, the crystallized phase is Sb2Se3 phase with different preferred orientations as presented in Table 1. The films annealed at Ta = 323 K and 373 K exhibit characteristic peaks of Se crystal plane (0 1 1) and (4 1 2). For the films annealed at 423 K and 473 K, other characteristic peaks of Se crystallized plane (2 0 1) and (1 1 3) appear. The increasing of peaks number and their intensities are evidences for the enhancement of
3.3. Surface morphology The surface morphology of the films was observed using scanning electron microscopy (SEM-Jeol T200). It was observed in SEM images shown in Fig. 3a that the as-prepared film reveals the formation of a heterogeneous cluster in the glassy matrix. For the film annealed at 423 K, Fig. 3b shows crystallization growth resulting in the form of spheroids and rods. Similar shapes of crystallization are observed for the films annealed at the other annealing temperatures. The observation of the amorphous structure for the as-prepared and the poly crystallization for those annealed is confirmed in the XRD (Fig. 2). 2
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Fig. 3. SEM micrographs showing the surface microstructure of the as-prepared sample (A) and annealed (B, C and D) at 423 K for Se90Sb10 thin film.
Fig. 4. Transmittance spectra for as-prepared and annealed Se90Sb10 thin films.
Fig. 5. Reflectance spectra as a function of the wavelength for as-prepared and annealed Se90Sb10 thin films.
3.4. Transmission and reflection spectra 3.5. Absorption coefficient and optical energy gap The transmission spectra of Se90Sb10 of the as-prepared and annealed films are shown in Fig. 4. It is observed that the increase of the annealing temperatures shifts the optical transmittance to the lower energies suggesting the decrease of the optical band gap. For all films, the transmittance (T) of the film increases as the wavelength increases up to a certain value depending on Ta. Then the transmittance decreases and almost keeps constant for higher wavelengths. On the other hand, T decreases as Ta increases. For as-prepared film and those annealed 323 K and 373 K, the films exhibit highly transparence reaching to about 90% at 900 nm (infrared region). Highly transparent of Se90Sb10 film candidates it for the application in color filters. The transmittance decreases as Ta increases. This may be attributed to the light scattering effect due to grain-boundary scattering. Taking into account the increase of crystallite size with Ta is extracted from XRD and presented in Table 1. The reflection spectra of Se90Sb10 film of the as-prepared and annealed films are shown in Fig. 5. The reflectance shows successive up and down curvatures for incident wavelength up to 1200 nm. These oscillations are due to the interference by the front and back film surfaces. At higher wavelengths, the observed upward curvature in the transmittance together with the corresponding downward one in the reflectance is attributed to the interaction of light waves with the free carrier absorption [18].
It can calculate the absorption coefficient (α) using the measured transmittance, T, and the reflectance, R, by the following Eq. [19]
α=
1 (1 − R)2 ln , d T
(2)
where d is the thickness of the film in cm. The dependence of α on the incident photon energy (hν) for as-prepared and annealed films of Se90Sb10 film is shown Fig. 6. The film absorption increases with increase of both photons energy of the incident light and Ta. The observed increasing behavior in α with increasing annealing temperature may be due to the increase of crystallinity in films with rising annealing temperatures as evidenced from XRD in Fig. 2. The optical energy band gap (Eg) can be determined using the fundamental absorption which corresponds to electron excitation from the valence band to conduction band. The nature of electronic transition for Se90Sb10 thin film and the value of Eg’s of studied films are obtained using the Tauc relation [20]:
α=
B (hν − Eg )m hν
,
(3)
where B is the absorption constant and m is an index that characterizes the optical absorption process. The value of exponent m is theoretically equal to ½ and 2 for direct and indirect allowed transitions. The plot 3
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as the film annealing temperature increases as presented in Table 2. The observed decrease in bandgaps coincides with the behavior of the absorption band edges which shift to lower energy with increasing of Ta. The behavior of Eg can be explained as follows. It have been reported by several researchers that the annealing process above the glass transition temperature causes formation of surface dangling bonds around crystallites [21–25]. After further raising the annealing temperature, the crystallites break down into a number of small crystals. An increase in the number of dangling bonds produces further dangling bonds in polycrystalline solids. The concentration of localized states is also increased. Therefore, an increase in the width of tails in the band gap (Ee) occurs. Consequently, the optical band gap decreases [22–24]. 3.6. Refractive index and extinction coefficient The optical constants such as refractive index (n) and extinction coefficient (k) are important constants for recommendation a film for applications in optical devices. The values of n and k relate to the experimentally measured reflectance (R) by the following Eq. [26]: Fig. 6. Absorption coefficient (α) as a function of photon energy (hν) for asprepared and annealed Se90Sb10 thin films.
n=
1+R 4R 1 +( − k2 ) 2 , 1−R 1 − R2
k = α λ /4π ,
(4) (5)
where λ is the wavelength. Fig. 8 shows the annealing temperature dependence of refractive index, n, as a function of the wavelength, λ. It can be observed that the refractive index shows anomalous dispersion in the low wavelength range (λ < 1360 nm). This behavior is characterized by independency of n on Ta and λ. The gradual decrease in n with λ in high region of wavelength represents the normal dispersion manner. The anomalous and normal behaviors of n have been observed for GeSeSn thin film [27]. The value of n in the normal dispersion range increases as Ta increases expect for annealing at 473 K which partially evaporates the material film. The observed increase in n with heat treatment is consistent with the reduced optical band gap. The extinction coefficient (k) is known as absorption index and is defined as the amount of light loses due to scattering and absorption per unit distance. Fig. 9 shows k as a function of wavelength λ for Se90Sb10 of as-prepared and annealed films with different Ta. For all films, k increases as λ increases up to a certain value and then k decreases. The decrease of extinction coefficient or absorption index with increase in wavelength is an obvious phenomenon due to commencement of strong electronic absorption transition at band gaps.
Fig. 7. The variation of (αhν)1/2 as a function of the photon energy (hν) for asprepared and annealed Se90Sb10 thin films. Table 2 Variation energies of optical band gap (Eg), dispersion (Eo) single-oscillator (Ed) and the index m of Eq. (3) for as-prepared and annealed Se90Sb10 thin films. Ta K
Eg eV
Eo eV
Ed eV
m
As-pre. 323 373 423 473
2.065 2.054 1.899 1.882 1.676
0.84 0.83 0.79 0.77 0.82
7.69 8.46 11.64 15.20 11.17
1.8 1.9 2.2 2.0 2.2
that covers the widest range of data and intercept the hν-axis is obtained for the (αhν)2 = f (hν) dependence as shown in Fig. 7. The obtained value of m = 2, characterizes an indirect allowed optical transition. For conformation, the value of m can be theoretically calculated by plotting a relation between 1/(d(lnαhν)/dhν) on y-axis and hν on xaxis and the slope of the straight line will give 1/m. The obtained values of m for as-prepared and annealed films are listed in Table 2. Eg’s are obtained from the intercept of the resulting straight lines with the energy axis at (αhν)2 = 0 (Fig. 7). The value of Eg is found to be decreased
Fig. 8. Refractive index as a function of the wavelength for as-prepared and annealed Se90Sb10 thin films. 4
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Fig. 9. The extinction coefficient (k) as a function of the wavelength for asprepared and annealed Se90Sb10 thin films.
Fig. 11. Temperature dependence of the dark electrical conductivity (σ) for asprepared and annealed Se90Sb10 thin films.
3.7. Dispersion of refractive index
behavior of Eg.
The dispersion of refractive index is a significant factor for both a fundamental and technological viewpoints. The dispersion data in semiconductor can be analyzed using the single-oscillator Wemple–DiDomenico (WDD) model [28,29]. According to this model, the oscillator energy or dispersion energy (Eo) and oscillator strength (Ed) are relates to n by the following relation:
n2 = 1 +
Eo Ed . Eo2 − (hν )2
3.8. Electrical conductivity The measurements of electrical conductivity (σ) of the as-prepared and annealed films of Se90Sb10 as function of temperature in the range 300–490 K are given in Fig. 11. It can be noticed that σ increases as Ta increase expect for 473 K-annealed sample which undergoes partial evaporation due to the relatively high temperature annealing. The increase of σ with Ta is attributed to the decrease of electrical activation energy due to the increase of energy width of localized states because of the increase of dangling bonds and defects in the band structure. As the conductivity increases by annealing, thus Se90Sb10 material is suitable for electrical applications of multilevel resistivity by adopting different resistivity values. That is, the amorphous state and the crystalline one are characterized by difference in the electrical resistivity. This difference between the two states allows non-volatile bilevel storage and can in principle be exploited for multilevel. To utilize the multilevel approach in the case of phase change memories, it is necessary to employ a chalcogenide of different electrical resistance in the range from the maximum to the minimum available value [31]. Fig. 11 shows an exponential increase of conductivity with temperature indicating the semiconductor nature of the studied films. That is, the observed increase in σ can be attributed to the increase of the number of thermally excited electrons as the temperature increases. Therefore, the conductivity process in the Se90Sb10 films is expected to be due to the activated electrons in the extended states above the conduction band edge [32]. The temperature dependence of conductivity yields the activation energy for conduction (ΔE) according to the relation [33]:
(6)
The dispersion parameters Ed and Eo can be obtained by plot of (n2 − 1)−1 versus (hν)2 as shown in Fig. 10. The obtained values of Eo and Ed from the slope and intercept are listed in Table 2. It is noticed that Ed increases while Eo decreases as Ta increases except for the annealing at highest temperature (Ta = 473 K). The increase of Ed with the increase of Ta is attributed to the increase of the diffusion rate of atoms in the films giving more number of atoms at interstitial sites. This leads to impurity type scattering centers, thereby Ed increases [30]. The decrease of Eo with increasing of Ta seems to be in agreement with the
ΔE ⎤ σ = σo exp ⎡− ⎢ ⎦ ⎣ kB T ⎥
(7)
where σo is the pre-exponential factor for conductivity, kB is the Boltzmann's constant and T is absolute temperature. The plot of ln (σ) against 103/T presented in Fig. 12 shows two linear parts of two slopes corresponding activation energies ΔE1 and ΔE2. This means the conductivity is arising from carriers being dominant at high temperature region and the others at low one. Therefore, the conduction can be represented in the two regions based on Eq. (7) as
Fig. 10. A plot of (n2 − 1)−1 against (hν)2 for as-prepared and annealed Se90Sb10 thin films. 5
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Fig. 13. Plots of ln(σT1/2) versus 1000/T1/4 for as-prepared and annealed Se90Sb10 thin films in the low temperature region.
wave function and is assumed to be 0.125 Å−1 [21] and N(EF) is the density of the trap states near the Fermi level. Fig. 13 shows the plots of ln(σT1/2) versus 100/T 1/4 for the as-prepared and annealed films in the low temperature region. The validity of hopping conduction mechanism is indicated by the liner relations. The deduced N(EF) values from the slopes of a straight lines and those of T0 are listed in Table 3. The observed decrease in T0 with increasing Ta indicates the decrease of disorder in the material as confirmed by XRD in Fig. 2.
4. Conclusion
Fig. 12. Logarithmic plots of dark electrical conductivity (σ) against 1000/T for as-prepared and annealed Se90Sb10 thin films.
ΔE ΔE σ = σo1 exp ⎡− 1 ⎤ + σo2 exp ⎡− 2 ⎤, ⎢ ⎥ ⎢ k T k ⎦ ⎣ B ⎦ ⎣ BT ⎥
The thermal induced amorphous-crystalline phase change in Se90Sb10 thin film benefices for optical disk data storage technology. The crystallinity of Se90Sb10 thin film is improved by increasing the annealing temperature. The observed highly transparent at 900 nm candidates this film for infrared transmission. The optical absorption data of as-prepared and annealed films reveal the mechanism of electronic transition to be indirect optical transition. The reducing of the optical energy gap by elevate the thermal annealing is assigned to the formation of localized states in the band structure. The optical constants (the refractive index and extinction coefficient) and the dispersion parameters (the oscillator energy and oscillator strength) are strongly dependant on the annealing temperature. The electrical conductivity of the film is caused by the band and hopping conductions. Annealing causes an increase in the electrical conductivity of the films and corresponding decrease in the activation energy for conduction. This is assigned to the amorphous-crystalline transformation. For future study, the addition of third element to Se–Sb binary system is expected to cause structural changes that modifies the glass forming region, creates compositional and configurational disorder, band structure and various physical parameters.
(8)
where σ01 and σ02 are the pre-exponential factors. The activation energies ΔE1 and ΔE2 as well as σ01 and σ02 of studied film are obtained and listed in Table 3. With increasing of Ta, the decrease of both ΔE and the increase of both σ0 consent with the trend of the conductivity. As shown in Fig. 12, the existence of two linear parts means the electrical conductivity in high and low temperature regions is caused by the band and the hopping conductions, respectively [34]. The latter occurs as a result of the excitation of the carriers into localized states at band edges and characterized by Mott’s variable-range hopping relation [35]:
ln(
ρ hop T
) = ln(ρo ) + (
To 1 )4 , T
(9)
where To is the measure of disorder in the material and given as
To = 18α3/[kB N (EF )],
(10)
where α is coefficient of the exponential decay of the localized state Table 3 The density of states at the Fermi level N(EF) and hopping parameters for the as-prepared and annealed Se90S10 thin films for 1 h. Ta K
ΔE1 eV
σ01 (Ω.cm)−1
ΔE2 eV
σ02 (Ω.cm)−1
To × 1010 K
N(Ef)×10−14 eV−1 cm−3
As-pre. 323 373 423 473
0.895 0.875 0.856 0.704 0.602
37.7 × 104 38.7 × 104 45.5 × 104 64.8 × 105 25.5 × 107
0.893 0.711 0.696 0.439 0.425
36.4 × 104 11.9 × 105 58.8 × 106 14.9 × 109 10.9 × 1010
4.79 4.43 3.77 0.80 0.61
5.19 5.61 6.59 0.31 0.40
6
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] V.M. García, L. Guerrero, M.T.S. Nair, P.K. Nair, Superficies y Vacío 9 (1999) 213. [19] J.I. Pankove, Optical Processes in Semiconductors, Dover New York 1971 Publ. (p. 103). [20] J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974, p. 159. [21] A.S. Soltan, Appl. Phys. A 80 (2005) 117. [22] N.M. Zulfequar, J. Alloys Compd. 576 (2013) 103. [23] M.M. Hafiz, A.A. Othman, M.M. El-Nahass, A.T. Al-Motasem, Phys. B 390 (2007) 348. [24] M.M.A. Imran, O.A. Lafi, M. Abu-Samak, Vacuum 86 (2012) 1589. [25] H.E. Atyia, A.S. Farid, J. Electron. Mater. 45 (2016) 357. [26] Y. Chen, D.M. Bagnall, H.J. Koh, K.T. Park, K. Hiraga, Z.Q. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [27] A.A.A. Darwish, H.A.M. Ali, J. Alloys Compd. 710 (2017) 431. [28] S.H. Wemple, M. DiDomenico, Phys. Rev. B 3 (1971) 1338. [29] S.H. Wemple, Phys. Rev. B 7 (1973) 3767. [30] M.A. Alvi, Opt. Commun. 295 (2013) 21. [31] Z. Li, R.G.D. Jeyasingh, J. Lee, M. Asheghi, H.-S.P. Wong, IEEE Trans. Electron. Devices 59 (2012) 3651. [32] E.A. Davis, N. Mott, Phil. Mag. 22 (1970) 903. [33] A.K. Hassan, R.D. Gould, A.K. Ray, Phys. Status Solidi A 158 (1996) K23. [34] M.L. Theye, Proceedings of the Fifth International Conference on Amorphous and Liquid Semiconductors, Garmisch-Partenkirchen, Germany, 1973, p. 479. [35] N.F. Mott, Philos. Mag. 32 (1975) 961.
V. Weidenhof, P. Franz, M. Wuttig, J. Appl. Phys. 87 (2000) 9. J.S. Sanghera, J. Heo, J.D.J. Machkenzie, J. Non-Cryst. Solid 103 (1988) 155. F.I. Ezema, R.U. Osuji, Chalcogenide Lett. 4 (2007) 69. P. Boolchand, W.J. Bresser, Nature 410 (2001) 1070. M. Frumar, T. Wagner, Curr. Opin. Solid State Mater. Sci. 7 (2003) 117. C.C. Huang, D.W. Hewak, Thin Solid Films 500 (2006) 247. D. Wamwangi, W.K. Njoroge, M. Wuttig, Thin Solid Films 408 (2002) 310. M.M. Hafiz, A.H. Moharram, M.A. Abdel-Rahim, A.A. Abu-Sehly, Thin Solid Films 292 (1997) 7. M.A. Majeed Kan, M. Zulfequar, M. Husain, J. Opt. Mater. 22 (2003) 21. J.Y. Shim, S.W. Park, H.K. Baik, Thin Solid Films 292 (1997) 31. J. Vazquez, C. Wagner, P. Villares, J. Non-Cryst. Solids 235 (1998) 548. N.F. Mott, E.A. Davis, Electronic processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1971. T.K. Kim, S.S. Han, B.S. Bae, Met. Mater. Int. 5 (1999) 33. S.K. Srivastava, P.K. Dwivedi, A. Kumar, Phys. B 183 (1993) 409. J. Hautala, S. Yamasaki, P.C. Taylor, J. Non-cryst. Solids 114 (1989) 85. M.I. Abd-Elrahman, Rasha M. Khafagy, Noha Younis, M.M. Hafiz, Phys. B 449 (2014) 155. M.I. Abd-Elrahman, M.M. Hafiz, M.A. Ammar Qasem, Abd Abdelraheem, Appl. Phys. A 122 (2016) 772.
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