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Effect of composition on the electrical and structural properties of As–Te–Ga thin films
Applied Surface Science 185 (2001) 1±10
Effect of composition on the electrical and structural properties of As±Te±Ga thin ®lms M. Dongola,*, M.M. Ha®zb, M. Abou-Zieda, A.F. Elhadya a
b
Faculty of Science, Physics Department, South Valley University, Qena, Egypt Faculty of Science, Physics Department, UAE University, United Arab Emirates Received 12 May 2001; accepted 18 June 2001
Abstract Ê were As30 Te70 x Gax (x 0:5, 1, 3, 6 and 10 at.%) chalcogenide thin ®lms were studied. Specimens of thickness 2500 A used for resistivity (r) measurements as a function of temperature (T) in the temperature range from 300 to 443 K. The resistivity (r) exhibits an activated temperature dependence in accordance with the relation r T r0 exp DE=kT. It was found that the activation energy for conduction (DE) and room temperature resistivity (r300) decrease with increasing Ga content up to 3 at.%. For x greater than 3 at.%, it was found that DE and r300 increase with increasing Ga content. The results were discussed according to the valence alternation pair (VAP) model and the alloying effect. Thermal annealing above Tg was found to decrease r and DE. The decrease of r and DE after annealing at T > Tg was attributed to the amorphous±crystalline transformation. XRD, SAED, TEM and DSC were used to study the structure of the as-deposited and annealed ®lms. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrical conduction; Chalcogenide; Semiconductors; Thin ®lms; Transmission electron microscopy (TEM)
1. Introduction In the As±Te system, the vitreous region which formed depends mainly on the quenching rate of the molten materials. When the melt was quenched in air, glasses with a tellurium content ranging from 45 to 55 at.% were obtained [1]. Increasing the cooling rate extends the vitreous region. A glass with tellurium content up to 60 at.% was obtained by Cornet and Rossier [2] after a rapid cooling of the As±Te melt. The addition of a third component, such as copper, thallium or silver, to arsenic telluride led to relative extensive vitri®cation region [3,4].
* Corresponding author. E-mail address: [email protected] (M. Dongol).
Dunaev and Mikhailov [5] measured DC conductivity of glasses of the system As±Te±Ga in the temperature range from 190 to 300 K. It was found that the addition of up to 5 at.% of Ga to the AsTex glasses causes about 0.5 order of magnitude increase in conductivity. The energy of chemical bonds in these glasses was shown to correlate with the energy of ionization. The dimensions of the glass forming regions of As±Te±Ga and As±Te±In systems and their positions in the concentration triangle as well as the character of the in¯uence of gallium and indium on the properties of glasses point to a substantial analogy between the behavior of gallium and indium in the glassy arsenic telluride. However, just as in the case of selenides, more gallium than indium could be introduced into arsenic tellurides (gallium up to 20 at.% and indium up to 15 at.%) [6].
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 9 4 - 4
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
In the present study, the aim is to examine the in¯uence of Ga content on the electrical resistivity, the activation energy for conduction and the structural properties of As30 Te70 x Gax thin ®lms, where x 0:5, 1, 3, 6, and 10 at.%. 2. Experimental Appropriate amounts of high purity (99.999%) Te, As and Ga (from Aldrich, UK) were weighed (5 g total weight ) according to their atomic percentage. The materials were added to clean fused silica ampoules and sealed under vacuum of 10 4 Torr 1 Torr 133:22 Pa. The sealed ampoules were then heated in Heraus programmable tube furnace (type RO7115) for 25 h at 1100 K. Then they were quenched in cold water. Thin ®lms were prepared by evaporation under a vacuum of 10 7 Torr using Edwards Coating Unit Ed 306A coater provided with an FTM5 quartz crystal monitor for ®lm thickness determination. Ê with evaporated Al Films of thickness 2500 A electrodes were used for the electrical resistance measurements, over the temperature range from 300
to 443 K, using a Keithly 610C electrometer. The effect of annealing temperature on the electrical conduction of As30 Te70 x Gax ®lms was studied. The ®lms were annealed at different annealing temperatures in the temperature range from 353 to 403 K for 2 h under vacuum. Metal electrodes (Al) were deposited after annealing. The measurements were carried out under vacuum and in dark. The composition of the as-prepared a-As30 Te70 x Gax ®lms was investigated using the energy dispersive spectral (EDS), and was corrected up to 0.5 at.%. X-ray diffractograms (XRDs) for the As±Te±Ga ®lms were obtained using Philips chart diffractometer. The radiation source used was Cu Ka of wavelength 0.154 nm. The X-ray investigation was carried out on the as-prepared ®lms as well as on ®lms annealed at certain temperatures. A Shimadzu TA-50 differential scanning calorimeter (DSC) was used for thermal analysis at a heating rate of 10 K/min. The morphology as well as the electron diffraction measurements were investigated for as-prepared and Ê annealed As30 Te70 x Gax ®lms of thickness 300 A using JEOL 100CXII transmission electron microscope. The microscope is provided with selected area
Fig. 1. The ternary phase diagram shows the region of glass formation As±Te±Ga. The compositions of the as-prepared As±Te±Ga glasses of the present work are shown in the ®gure.
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
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electron diffraction (SAED), Micro-beam electron diffraction where the beam spot could be less than 4 mm in diameter and high dispersion electron diffraction. The analysis of the SAED determines the most probable crystalline phases accompanied in the transformation. The calibrated camera constant, obtained from the diffraction rings of standard element gold, was used to calculate the d-spacing corresponding to the different radii of the diffraction patterns of the annealed ®lms. 3. Results The distribution of the prepared compositions in the As±Te±Ga system is shown in Fig. 1. It is clear that it lies within the glass formation region in the data of Dunaev et al. [6]. This gives an indication that the composition of the glasses used in the present study is well de®ned and reproducible. In Fig. 2, we presented data of the DSC analysis of the samples with different compositions at a heating rate of 10 K/min. The thermograms are characterized by an endothermic effect of glass transition at T Tg followed by one or two according to the composition, exothermic effects of crystallization at T Tc and then by endothermic effects of melting of the crystalline phase so formed. Several interesting features are observed. The glass transition temperature Tg and the ®rst exothermic peak Tc were increased with increasing Ga content. The results are given in Table 1. As-deposited ®lms of all compositions were found to be amorphous. The X-ray diffraction pro®les of typical As30Te69.5Ga0.5 as-deposited and annealed (at 423 K for 2 h) ®lms are shown in Fig. 3a and b, respectively. The analysis of the X-ray diffraction data for crystallized ®lms showed the coexistence of AsTe and Ga2Te5 phases in the crystallized ®lms. The morphology as well as electron diffraction were investigated for as-prepared and annealed Ê . TEM invesAs30 Te70 x Gax ®lms of thickness 300 A tigations and SAED indicated that the as-prepared ®lms were structureless and have uniform contrast as it would be expected for amorphous ®lms. The diffuse rings was observed in the electron diffraction pattern, e.g. As30Te60Ga10 diffraction pattern shown in Fig. 4a, also con®rmed the amorphous nature of the asprepared ®lms. The SAED of annealed As30Te60Ga10
Fig. 2. DSC thermograms for: (a) As30Te69Ga1; (b) As30Te67Ga3; (c) As30Te64Ga6; (d) As30Te60Ga10.
Table 1 The transition temperature (Tg) and the crystallization temperature (Tc) for different glass composition Composition (at.%) As
Te
Ga
30 30 30 30
69 67 64 60
1 3 6 10
Tg (8C)
Tc (8C)
Tc
103 110 117 131
163 182 190 218
60 72 73 87
Tg (8C)
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Fig. 3. X-ray diffraction pro®les of: (a) as-deposited As30Te60Ga10 ®lms; (b) annealed As30Te60Ga10 for 1 h.
®lm is characterized by diffraction rings as shown in Fig. 4b. The microstructure obtained for the As30Te60Ga10 ®lms annealed at 473 K for 2 h is shown in Fig. 4c. The whole ®lm was transformed to polycrystalline phase. The d-spacing in selected area diffraction for annealed As30Te60Ga10 ®lms are reported in Table 2. The calculated d-spacings of this annealed ®lm are fairly in good agreement with d-spacing of AsTe, Ga2Te3, GaTe and GaAs [7±10]. The temperature dependence of the electrical resistivity (r) for amorphous ®lms of the As30 Te70 x Gax system is shown in Fig. 5. It was observed that log r varied linearly with 1/T. The values of the activation energy for conduction, DE were calculated from the
relation r T r0 exp DE=kT, where r T is the resistivity at temperature T, r0 the pre-exponential factor and k the Boltzmann constant. Fig. 6 shows the dependence of the activation energy for conduction, DE (eV), and room temperature resistivity (r300) as a function of Ga content for as-deposited As30 Te70 x Gax ®lms. It is observed that DE and r300 decrease with increasing Ga content up to 3 at.%. For x greater than 3 at.%, it was found that DE and r300 increase with increasing Ga content. The dependence of the room temperature resistivity and the activation energy for conductance are listed in Table 3. Reversible resistivity changes with heat treatment were observed in this system. The results of the variation of the electrical resistivity versus 1/T for
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
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Fig. 4. The electron diffraction pattern and TEM images for: (a) as-prepared As30Te60Ga10 ®lm; (b) annealed As30Te60Ga10 ®lms at 473 K for 2 h; (c) TEM micrograph for As30Te60Ga10 ®lms annealed at 473 K for 2 h.
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Table 2 Ê corresponding to rings of Fig. 4b and those observed GaTe, Ga2Te3, GaAs and AsTe Comparison of d-space in A Ê ), present work d (A
hkl
d(AsTe) [7]
d(Ga2Te3) [8]
d(GaTe) [9]
d(GaAs) [10]
2.523 2.141 1.962 1.442 1.333 1.261 1.158 1.039 0.942 0.883
310 912 112 400 331 532 326 310 600 600
± ± ± 1.440 1.325 ± ± ± ± 0.963
2.502 ± ± ± ± 1.261 1.581 1.038 ± ±
± 2.151 1.960 ± ± ± ± ± ± ±
± ± ± ± ± ± ± ± ± 0.824
as-prepared As30Te60Ga10 thin ®lms is shown in Fig. 7. A linear decrease of log r with 1/T was observed in the temperature range (302±409 K) during heating. A sharp drop in the ®lm resistivity was noticed during heating at temperature higher than 409 K. The arrow (in Fig. 7) indicates the glass transition temperature Tg,
which was determined from the DSC thermograms. During the cooling process, the ®lm resistivity was slightly increased. The dependence of the ®lms resistivity on the annealing temperature for As30Te64Ga6 ®lms, as an example for As30 Te70 x Gax system, is shown in Fig. 8.
Fig. 5. Logarithm of resistivity versus 1000/T (K 1) for as-deposited As30 Te70 x Gax thin ®lms.
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Fig. 6. The room temperature resistivity plotted as log r and activation energy for conduction plotted as DE as a function of Ga content for as-deposited As30 Te70 x Gax ®lms. Table 3 The effect of composition of As30 Te70 x Gax on the activation energy for conduction (DE) and the room temperature resistivity (r300) Ga (at.%)
DE (eV)
r300 103 Ocm
0.5 1 3 6 10
0.365 0.356 0.354 0.367 0.389
7.94 6.50 6.66 8.00 10.59
The ®lm resistivity slightly increased for ®lms annealed at 373 K, then decreased continuously with increasing annealing temperature. Fig. 9 shows the relation between the activation energy for conduction DE (eV) and the annealing temperature for As30Te64Ga6 ®lms. It is noticed that the activation energy for conduction decreases with increasing annealing temperature and a sharp drop can be observed for ®lms annealed at 393 K. 4. Discussion The structural examination (XRD, TEM, and SAED) showed that the structure of all as-prepared
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Fig. 7. Logarithm resistivity versus 1000/T for as-prepared As30Te60Ga10 thin ®lm.
As30 Te70 x Gax (x 0:5, 1, 3, 6, and 10 at.%) thin ®lms is amorphous. With increasing Ga content, the glass transition temperature Tg increases smoothly from 376 to 404 K and the crystallization temperature, Tc, increases from 436 to 491 K (Table 1). The difference between TC and Tg DT Tc Tg is used to indicate the characteristic glass crystallization ability. The greater this difference, the lower the crystallization ability of the glass. The present measurements showed that this difference increased with increasing Ga content in the As30 Te70 x Gax alloys which means that the crystallizing ability decreases with increasing Ga content. Our DSC results are in good agreement with the results of Rykova et al. [11] on AsTe1:5 x Agx glasses. Phase separation of crystalline phases was proved after annealing at T > Tg , as shown in Figs. 3±5. Crystalline phases of AsTe and Ga2Te5 appeared in the annealed ®lms of low Ga content (<3 at.%), while crystalline phases of GaTe and As2Te3 appeared in the annealed ®lms of high Ga content (>3 at.%). The separated crystalline phases are found to depend on the original composition as well as the annealing temperature. The dependence of the resistivity as well as the activation energy for conduction (DE) on the Ga
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Fig. 8. Logarithm resistivity versus 1000/T for as-prepared and annealed As30Te64Ga6 thin ®lm. The annealing time was 2 h.
content are shown in Fig. 6. According to the Ga concentration, this dependence is divided into two regions. In the ®rst region, DE decreased by 0.011 eV with the addition of 3 at.% Ga. In the second region, where the Ga content is greater than 3 at.%, DE increases continuously with increasing Ga content. The present results are in good agreement with that reported by different authors [12±14], where they showed that the activation energy for conduction (DE) has a minimum at a concentration of 2±5 at.% of group III elements. Zope et al. [15] also reported a decrease in the activation energy for conduction for Ge±Se±Tl glasses with increasing thallium content. The decrease of the activation energy in the ®rst region 0 x 3 at:% may indicate a shift of the Fermi level with the addition of Ga content, which is supposed to form states different from those formed according to the (8-N) role. The composition
modi®cation of the chalcogenide glasses through the addition of Al, Ga, and in support the assumption that the coordination number of Ga in the chalcogenide does not correspond to the number of its valence electrons [12±14]. The decrease in the activation energy due to the addition of Ga acts to create new charged centers in the mobility gap, which modi®es the energy band diagram of As±Te. In the case of undoped glasses, the Fermi level is located midway between the deep line levels of the C 3 and C1 charged centers. Since the addition of Ga to As±Te produces holes, the Fermi level will shift towards the valence band. Kastner model [16] proposed that the charge centers C 3 and C1 in chalcogenides are created in equal concentrations. The addition of Ga to As±Te may disturb the balance of the characteristic charged defects, which can affect the electronic conduction, i.e. the density of the charged
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
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Fig. 10. (a) Energy band diagram of As±Te with C1 and C 3 centers. (b) Energy band of As±Te±Ga with C1 and Ga4 C02 3 centers.
Fig. 9. The activation energy for conduction (DE) as a function of annealing temperature for As30Te64Ga6 thin ®lm.
states is modi®ed and some trap states can appear in the gap of the semiconductor. Our results concerning the decrease in the activation energy for conduction on the addition of Ga can be illustrated in Fig. 11 in the framework of the valence alternation pairs (VAPs) model [16]. The increase in the value of the activation energy for conduction with increasing Ga content (above 3 at.%) could introduce a modi®cation in the energy region above EC and below EV. With increasing concentration of Ga, the formation of Ga±C bonds increases which results in a decrease of other bonds in the glass. The value of DE at higher concentrations of Ga will be determined by the relation between the energy characteristics of the bonds in the glass and their relative concentration. Therefore, at high concentrations of Ga, we deal with a general three-component glass. The transition to a new distribution of the states in the gap depends on the concentration of the initial glass. The dependence of the activation energy for conduction of As30 Te70 x Gax ®lms with different Ga content on the annealing temperature is shown in Fig. 10. The experimental results showed that the thermal annealing at temperatures T > Tg decreases
the activation energy, and enough vibrational energy is present to break some of the weaker bonds, thus introducing some translational degree of freedom to the system. These additional degrees of freedom results in an increase in the ®lm heat capacity. So, crystallization via nucleation and growth becomes possible and the amount of crystalline phases depends on the annealing temperature [17]. TEM investigations indicated the amorphous±crystalline transformations for thermally annealed As30Te64Ga6 ®lms (Fig. 5). The amount and type of the separated polycrystalline phases depend on the original composition of the ®lm and annealing temperature. The continuous decrease of the activation energy with increasing annealing temperature could be attributed to the phase separation of crystalline phases. 5. Conclusion The variation of r and DE with composition and temperature is reported and discussed for As30 Te70 x Gax glasses. The addition of Ga content to As±Te glasses results in a decrease of r and DE, followed by an increase for x > 3 at:%. The decrease of the activation energy for conduction was discussed according to the VAP model introduced by Kastner et al. [16] where Ga acts as a dopant up to 3 at.%. The increase of the activation energy for conduction with increasing Ga content above 3 at.% was attributed to the alloying of the three-component glass. The decrease of r after annealing at T > Tg was attributed to the amorphous±crystalline transformation and the phase separation of polycrystalline phases.
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References [1] Z.U. Borisova, V.R. Panus, T.S. Pykova, V.Ya. Vaninov, Izv. Akad. Nauk SSSR, Neorg. Mater. 13 (1977) 1959. [2] J. Cornet, D. Rossier, J. Non-cryst. Solids 12 (1973) 16. [3] A.R. Hilton, M. Boru, Infrared Phys. 3 (1963) 69. [4] V.R. Panus, Z.U. Borisova, Zh. Prikl. Khim. 39 (1966) 987. [5] A.A. Dunaev, M.D. Mikhailov, Vestn. Lenin. Univ. Fiz. Khim. (USSR) 4 (1979) 81. [6] A.A. Dunaev, Z.U. Borisova, M.D. Mikhailov, I.V. Privalova, Fiz. Khim., Stekla 4 (3) (1978) 346. [7] R. Quinn, Mater. Res. Bull. 9 (1974) 803. [8] M. Julien-Pouzal, Acta Crystallogr. Sci. B 33 (1977) 2770. [9] T. Wadsten, Chem. Commun., Univ. Stockholm 10 (1985) 1. [10] A. Baublitz Jr., M. Rouff, J. Appl. Phys. 35 (1982) 6179.
[11] T.S. Rykova, V.L. Vaninov, Z.U. Borisova, V.R. Panus, Inorg. Mater. 18 (1977) 1569. [12] A.A. Dunaev, Z.U. Borisova, M.D. Mikhailov, A.V. Bratov, Phys. Chem. Glass (USSR) 6 (1980) 174. [13] N.N. Apikhtin, M.D. Mikhailov, V.R. Panus, T.N. Salamatova, Phys. Chem. Glass (USSR) 6 (1980) 383. [14] M.D. Mikhailov, V.R. Panus, N.N. Apikhtin, A.A. Dunaev, Proceedings of the Conference on Amorphous Semiconductors'80, Kishinev, 1980. [15] M. Zope, B.D. Muragi, J.K. Zope, J. Non-cryst. Solids 103 (1988) 195. [16] M. Kastner, D. Adler, H. Fritzsche, Phys. Rev. Lett. 37 (1976) 1504. [17] M.M. Ha®z, A.H. Moharram, A.A. Abu-Sehly, Appl. Surf. Sci. 115 (1997) 203.
Effect of composition on the electrical and structural properties of As±Te±Ga thin ®lms M. Dongola,*, M.M. Ha®zb, M. Abou-Zieda, A.F. Elhadya a
b
Faculty of Science, Physics Department, South Valley University, Qena, Egypt Faculty of Science, Physics Department, UAE University, United Arab Emirates Received 12 May 2001; accepted 18 June 2001
Abstract Ê were As30 Te70 x Gax (x 0:5, 1, 3, 6 and 10 at.%) chalcogenide thin ®lms were studied. Specimens of thickness 2500 A used for resistivity (r) measurements as a function of temperature (T) in the temperature range from 300 to 443 K. The resistivity (r) exhibits an activated temperature dependence in accordance with the relation r T r0 exp DE=kT. It was found that the activation energy for conduction (DE) and room temperature resistivity (r300) decrease with increasing Ga content up to 3 at.%. For x greater than 3 at.%, it was found that DE and r300 increase with increasing Ga content. The results were discussed according to the valence alternation pair (VAP) model and the alloying effect. Thermal annealing above Tg was found to decrease r and DE. The decrease of r and DE after annealing at T > Tg was attributed to the amorphous±crystalline transformation. XRD, SAED, TEM and DSC were used to study the structure of the as-deposited and annealed ®lms. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrical conduction; Chalcogenide; Semiconductors; Thin ®lms; Transmission electron microscopy (TEM)
1. Introduction In the As±Te system, the vitreous region which formed depends mainly on the quenching rate of the molten materials. When the melt was quenched in air, glasses with a tellurium content ranging from 45 to 55 at.% were obtained [1]. Increasing the cooling rate extends the vitreous region. A glass with tellurium content up to 60 at.% was obtained by Cornet and Rossier [2] after a rapid cooling of the As±Te melt. The addition of a third component, such as copper, thallium or silver, to arsenic telluride led to relative extensive vitri®cation region [3,4].
* Corresponding author. E-mail address: [email protected] (M. Dongol).
Dunaev and Mikhailov [5] measured DC conductivity of glasses of the system As±Te±Ga in the temperature range from 190 to 300 K. It was found that the addition of up to 5 at.% of Ga to the AsTex glasses causes about 0.5 order of magnitude increase in conductivity. The energy of chemical bonds in these glasses was shown to correlate with the energy of ionization. The dimensions of the glass forming regions of As±Te±Ga and As±Te±In systems and their positions in the concentration triangle as well as the character of the in¯uence of gallium and indium on the properties of glasses point to a substantial analogy between the behavior of gallium and indium in the glassy arsenic telluride. However, just as in the case of selenides, more gallium than indium could be introduced into arsenic tellurides (gallium up to 20 at.% and indium up to 15 at.%) [6].
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 9 4 - 4
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
In the present study, the aim is to examine the in¯uence of Ga content on the electrical resistivity, the activation energy for conduction and the structural properties of As30 Te70 x Gax thin ®lms, where x 0:5, 1, 3, 6, and 10 at.%. 2. Experimental Appropriate amounts of high purity (99.999%) Te, As and Ga (from Aldrich, UK) were weighed (5 g total weight ) according to their atomic percentage. The materials were added to clean fused silica ampoules and sealed under vacuum of 10 4 Torr 1 Torr 133:22 Pa. The sealed ampoules were then heated in Heraus programmable tube furnace (type RO7115) for 25 h at 1100 K. Then they were quenched in cold water. Thin ®lms were prepared by evaporation under a vacuum of 10 7 Torr using Edwards Coating Unit Ed 306A coater provided with an FTM5 quartz crystal monitor for ®lm thickness determination. Ê with evaporated Al Films of thickness 2500 A electrodes were used for the electrical resistance measurements, over the temperature range from 300
to 443 K, using a Keithly 610C electrometer. The effect of annealing temperature on the electrical conduction of As30 Te70 x Gax ®lms was studied. The ®lms were annealed at different annealing temperatures in the temperature range from 353 to 403 K for 2 h under vacuum. Metal electrodes (Al) were deposited after annealing. The measurements were carried out under vacuum and in dark. The composition of the as-prepared a-As30 Te70 x Gax ®lms was investigated using the energy dispersive spectral (EDS), and was corrected up to 0.5 at.%. X-ray diffractograms (XRDs) for the As±Te±Ga ®lms were obtained using Philips chart diffractometer. The radiation source used was Cu Ka of wavelength 0.154 nm. The X-ray investigation was carried out on the as-prepared ®lms as well as on ®lms annealed at certain temperatures. A Shimadzu TA-50 differential scanning calorimeter (DSC) was used for thermal analysis at a heating rate of 10 K/min. The morphology as well as the electron diffraction measurements were investigated for as-prepared and Ê annealed As30 Te70 x Gax ®lms of thickness 300 A using JEOL 100CXII transmission electron microscope. The microscope is provided with selected area
Fig. 1. The ternary phase diagram shows the region of glass formation As±Te±Ga. The compositions of the as-prepared As±Te±Ga glasses of the present work are shown in the ®gure.
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
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electron diffraction (SAED), Micro-beam electron diffraction where the beam spot could be less than 4 mm in diameter and high dispersion electron diffraction. The analysis of the SAED determines the most probable crystalline phases accompanied in the transformation. The calibrated camera constant, obtained from the diffraction rings of standard element gold, was used to calculate the d-spacing corresponding to the different radii of the diffraction patterns of the annealed ®lms. 3. Results The distribution of the prepared compositions in the As±Te±Ga system is shown in Fig. 1. It is clear that it lies within the glass formation region in the data of Dunaev et al. [6]. This gives an indication that the composition of the glasses used in the present study is well de®ned and reproducible. In Fig. 2, we presented data of the DSC analysis of the samples with different compositions at a heating rate of 10 K/min. The thermograms are characterized by an endothermic effect of glass transition at T Tg followed by one or two according to the composition, exothermic effects of crystallization at T Tc and then by endothermic effects of melting of the crystalline phase so formed. Several interesting features are observed. The glass transition temperature Tg and the ®rst exothermic peak Tc were increased with increasing Ga content. The results are given in Table 1. As-deposited ®lms of all compositions were found to be amorphous. The X-ray diffraction pro®les of typical As30Te69.5Ga0.5 as-deposited and annealed (at 423 K for 2 h) ®lms are shown in Fig. 3a and b, respectively. The analysis of the X-ray diffraction data for crystallized ®lms showed the coexistence of AsTe and Ga2Te5 phases in the crystallized ®lms. The morphology as well as electron diffraction were investigated for as-prepared and annealed Ê . TEM invesAs30 Te70 x Gax ®lms of thickness 300 A tigations and SAED indicated that the as-prepared ®lms were structureless and have uniform contrast as it would be expected for amorphous ®lms. The diffuse rings was observed in the electron diffraction pattern, e.g. As30Te60Ga10 diffraction pattern shown in Fig. 4a, also con®rmed the amorphous nature of the asprepared ®lms. The SAED of annealed As30Te60Ga10
Fig. 2. DSC thermograms for: (a) As30Te69Ga1; (b) As30Te67Ga3; (c) As30Te64Ga6; (d) As30Te60Ga10.
Table 1 The transition temperature (Tg) and the crystallization temperature (Tc) for different glass composition Composition (at.%) As
Te
Ga
30 30 30 30
69 67 64 60
1 3 6 10
Tg (8C)
Tc (8C)
Tc
103 110 117 131
163 182 190 218
60 72 73 87
Tg (8C)
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Fig. 3. X-ray diffraction pro®les of: (a) as-deposited As30Te60Ga10 ®lms; (b) annealed As30Te60Ga10 for 1 h.
®lm is characterized by diffraction rings as shown in Fig. 4b. The microstructure obtained for the As30Te60Ga10 ®lms annealed at 473 K for 2 h is shown in Fig. 4c. The whole ®lm was transformed to polycrystalline phase. The d-spacing in selected area diffraction for annealed As30Te60Ga10 ®lms are reported in Table 2. The calculated d-spacings of this annealed ®lm are fairly in good agreement with d-spacing of AsTe, Ga2Te3, GaTe and GaAs [7±10]. The temperature dependence of the electrical resistivity (r) for amorphous ®lms of the As30 Te70 x Gax system is shown in Fig. 5. It was observed that log r varied linearly with 1/T. The values of the activation energy for conduction, DE were calculated from the
relation r T r0 exp DE=kT, where r T is the resistivity at temperature T, r0 the pre-exponential factor and k the Boltzmann constant. Fig. 6 shows the dependence of the activation energy for conduction, DE (eV), and room temperature resistivity (r300) as a function of Ga content for as-deposited As30 Te70 x Gax ®lms. It is observed that DE and r300 decrease with increasing Ga content up to 3 at.%. For x greater than 3 at.%, it was found that DE and r300 increase with increasing Ga content. The dependence of the room temperature resistivity and the activation energy for conductance are listed in Table 3. Reversible resistivity changes with heat treatment were observed in this system. The results of the variation of the electrical resistivity versus 1/T for
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
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Fig. 4. The electron diffraction pattern and TEM images for: (a) as-prepared As30Te60Ga10 ®lm; (b) annealed As30Te60Ga10 ®lms at 473 K for 2 h; (c) TEM micrograph for As30Te60Ga10 ®lms annealed at 473 K for 2 h.
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Table 2 Ê corresponding to rings of Fig. 4b and those observed GaTe, Ga2Te3, GaAs and AsTe Comparison of d-space in A Ê ), present work d (A
hkl
d(AsTe) [7]
d(Ga2Te3) [8]
d(GaTe) [9]
d(GaAs) [10]
2.523 2.141 1.962 1.442 1.333 1.261 1.158 1.039 0.942 0.883
310 912 112 400 331 532 326 310 600 600
± ± ± 1.440 1.325 ± ± ± ± 0.963
2.502 ± ± ± ± 1.261 1.581 1.038 ± ±
± 2.151 1.960 ± ± ± ± ± ± ±
± ± ± ± ± ± ± ± ± 0.824
as-prepared As30Te60Ga10 thin ®lms is shown in Fig. 7. A linear decrease of log r with 1/T was observed in the temperature range (302±409 K) during heating. A sharp drop in the ®lm resistivity was noticed during heating at temperature higher than 409 K. The arrow (in Fig. 7) indicates the glass transition temperature Tg,
which was determined from the DSC thermograms. During the cooling process, the ®lm resistivity was slightly increased. The dependence of the ®lms resistivity on the annealing temperature for As30Te64Ga6 ®lms, as an example for As30 Te70 x Gax system, is shown in Fig. 8.
Fig. 5. Logarithm of resistivity versus 1000/T (K 1) for as-deposited As30 Te70 x Gax thin ®lms.
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Fig. 6. The room temperature resistivity plotted as log r and activation energy for conduction plotted as DE as a function of Ga content for as-deposited As30 Te70 x Gax ®lms. Table 3 The effect of composition of As30 Te70 x Gax on the activation energy for conduction (DE) and the room temperature resistivity (r300) Ga (at.%)
DE (eV)
r300 103 Ocm
0.5 1 3 6 10
0.365 0.356 0.354 0.367 0.389
7.94 6.50 6.66 8.00 10.59
The ®lm resistivity slightly increased for ®lms annealed at 373 K, then decreased continuously with increasing annealing temperature. Fig. 9 shows the relation between the activation energy for conduction DE (eV) and the annealing temperature for As30Te64Ga6 ®lms. It is noticed that the activation energy for conduction decreases with increasing annealing temperature and a sharp drop can be observed for ®lms annealed at 393 K. 4. Discussion The structural examination (XRD, TEM, and SAED) showed that the structure of all as-prepared
7
Fig. 7. Logarithm resistivity versus 1000/T for as-prepared As30Te60Ga10 thin ®lm.
As30 Te70 x Gax (x 0:5, 1, 3, 6, and 10 at.%) thin ®lms is amorphous. With increasing Ga content, the glass transition temperature Tg increases smoothly from 376 to 404 K and the crystallization temperature, Tc, increases from 436 to 491 K (Table 1). The difference between TC and Tg DT Tc Tg is used to indicate the characteristic glass crystallization ability. The greater this difference, the lower the crystallization ability of the glass. The present measurements showed that this difference increased with increasing Ga content in the As30 Te70 x Gax alloys which means that the crystallizing ability decreases with increasing Ga content. Our DSC results are in good agreement with the results of Rykova et al. [11] on AsTe1:5 x Agx glasses. Phase separation of crystalline phases was proved after annealing at T > Tg , as shown in Figs. 3±5. Crystalline phases of AsTe and Ga2Te5 appeared in the annealed ®lms of low Ga content (<3 at.%), while crystalline phases of GaTe and As2Te3 appeared in the annealed ®lms of high Ga content (>3 at.%). The separated crystalline phases are found to depend on the original composition as well as the annealing temperature. The dependence of the resistivity as well as the activation energy for conduction (DE) on the Ga
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
Fig. 8. Logarithm resistivity versus 1000/T for as-prepared and annealed As30Te64Ga6 thin ®lm. The annealing time was 2 h.
content are shown in Fig. 6. According to the Ga concentration, this dependence is divided into two regions. In the ®rst region, DE decreased by 0.011 eV with the addition of 3 at.% Ga. In the second region, where the Ga content is greater than 3 at.%, DE increases continuously with increasing Ga content. The present results are in good agreement with that reported by different authors [12±14], where they showed that the activation energy for conduction (DE) has a minimum at a concentration of 2±5 at.% of group III elements. Zope et al. [15] also reported a decrease in the activation energy for conduction for Ge±Se±Tl glasses with increasing thallium content. The decrease of the activation energy in the ®rst region 0 x 3 at:% may indicate a shift of the Fermi level with the addition of Ga content, which is supposed to form states different from those formed according to the (8-N) role. The composition
modi®cation of the chalcogenide glasses through the addition of Al, Ga, and in support the assumption that the coordination number of Ga in the chalcogenide does not correspond to the number of its valence electrons [12±14]. The decrease in the activation energy due to the addition of Ga acts to create new charged centers in the mobility gap, which modi®es the energy band diagram of As±Te. In the case of undoped glasses, the Fermi level is located midway between the deep line levels of the C 3 and C1 charged centers. Since the addition of Ga to As±Te produces holes, the Fermi level will shift towards the valence band. Kastner model [16] proposed that the charge centers C 3 and C1 in chalcogenides are created in equal concentrations. The addition of Ga to As±Te may disturb the balance of the characteristic charged defects, which can affect the electronic conduction, i.e. the density of the charged
M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
9
Fig. 10. (a) Energy band diagram of As±Te with C1 and C 3 centers. (b) Energy band of As±Te±Ga with C1 and Ga4 C02 3 centers.
Fig. 9. The activation energy for conduction (DE) as a function of annealing temperature for As30Te64Ga6 thin ®lm.
states is modi®ed and some trap states can appear in the gap of the semiconductor. Our results concerning the decrease in the activation energy for conduction on the addition of Ga can be illustrated in Fig. 11 in the framework of the valence alternation pairs (VAPs) model [16]. The increase in the value of the activation energy for conduction with increasing Ga content (above 3 at.%) could introduce a modi®cation in the energy region above EC and below EV. With increasing concentration of Ga, the formation of Ga±C bonds increases which results in a decrease of other bonds in the glass. The value of DE at higher concentrations of Ga will be determined by the relation between the energy characteristics of the bonds in the glass and their relative concentration. Therefore, at high concentrations of Ga, we deal with a general three-component glass. The transition to a new distribution of the states in the gap depends on the concentration of the initial glass. The dependence of the activation energy for conduction of As30 Te70 x Gax ®lms with different Ga content on the annealing temperature is shown in Fig. 10. The experimental results showed that the thermal annealing at temperatures T > Tg decreases
the activation energy, and enough vibrational energy is present to break some of the weaker bonds, thus introducing some translational degree of freedom to the system. These additional degrees of freedom results in an increase in the ®lm heat capacity. So, crystallization via nucleation and growth becomes possible and the amount of crystalline phases depends on the annealing temperature [17]. TEM investigations indicated the amorphous±crystalline transformations for thermally annealed As30Te64Ga6 ®lms (Fig. 5). The amount and type of the separated polycrystalline phases depend on the original composition of the ®lm and annealing temperature. The continuous decrease of the activation energy with increasing annealing temperature could be attributed to the phase separation of crystalline phases. 5. Conclusion The variation of r and DE with composition and temperature is reported and discussed for As30 Te70 x Gax glasses. The addition of Ga content to As±Te glasses results in a decrease of r and DE, followed by an increase for x > 3 at:%. The decrease of the activation energy for conduction was discussed according to the VAP model introduced by Kastner et al. [16] where Ga acts as a dopant up to 3 at.%. The increase of the activation energy for conduction with increasing Ga content above 3 at.% was attributed to the alloying of the three-component glass. The decrease of r after annealing at T > Tg was attributed to the amorphous±crystalline transformation and the phase separation of polycrystalline phases.
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M. Dongol et al. / Applied Surface Science 185 (2001) 1±10
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