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The effect of the Sb content on the physical properties of amorphous Se0.75Ge0.25−y thin films
Vacuum 52 (1999) 277 — 284
The effect of the Sb content on the physical properties of amorphous Se0.75Ge0.25!y thin films M. Fadel Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Received 23 June 1998
Abstract The effect of the addition of varying amounts of antimony in concentrations from 0.01 to 0.20 at% to Se Ge glass on its dc \W conductivity is analysed. The electrical conductivity of amorphous thin films of vacuum evaporated Se Ge Sb were \W W determined during and after light exposure and at different temperatures. The time dependence of the electrical conductivity measured Ge Sb (y"0.01, 0.05, in darkness or when exposed to light at about room temperature has been studied for amorphous Se \W W 0.10, 0.15, 0.18 and 0.20) thin films of different compositions and different thicknesses. The conduction activation energy *E and the pre-exponential factor p (0, ¹) which appear in the dc conductivities are found to decrease with increasing Sb content. The mean value of the threshold voltage, was measured either in darkness, » M , and after exposure of light » M , for different compositions and temperatures. The pronounced glass-forming tendencies of alloys of Se and Ge with Sb were discussed topologically in terms of the chemical bonds expected to be present in these materials. These chemical bonds have been used to estimate the cohesive energies (CE) of the glasses. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Chalcogenide glasses have been recognized as promising materials for infrared optical elements [1], infrared optical fibres [2], and for the transfer of information [3]. They have also found applications in Xerography [4], switching and memory devices [5], photolithography [5], and in the fabrication of inexpensive solar cells [6], and, more recently, for reversible phase change optical records [7]. Thus, it is important to have an insight into their electronic properties. The addition of an impurity has a pronounced effect on the conduction mechanism and the structure of the amorphous glass and this effect can be widely different for different impurities. The study of doped amorphous semiconductors has become of great interest to scientists. It was believed [8] that impurities have little effect on the properties of amorphous semiconductors as each impurity atom can satisfy its valence requirements by adjusting its nearest-neighbour environment. However, it has been shown [9] that the effect of charged additives in lone-pair semiconductors depend on whether the charged additives equilibrate or not with valence-alternation defects. In our earlier communication preliminary results on the electrical properties of
antimony-doped Se Ge glassy alloy with Sb \W concentration form 0.01 at% were explained on the basis of Sb acting as a Ge-chain terminator causing decreased disorder. Much effort has been devoted towards the preparation and properties of chalcogenide glasses of Se—Ge—Sb system in several papers [10—14]. The aims of the present work were: (i) to obtain the density of the system Se Ge Sb (y"0.01, 0.05, \W W 0.10, 0.15, 0.18 and 0.20) by Archimedes method, (ii) to study the effect of illumination on the electrical properties for the given compositions as a function of: time, temperature and film thickness to obtain the conduction activation energy and the pre-exponential factor; for dark E , p , for illuminated conductivity E , p and for photoconductivity behaviour *E and *p , (iii) to inves tigate the effect of light on the switching phenomenon, and (iv) to correlate the experimental data with the chemical bond expected to be present.
2. Experimental technique The bulk glasses under study were prepared by direct synthesis from selenium (4 N), germanium and antimony
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 2 9 8 - X
M. Fadel / Vacuum 52 (1999) 277 — 284
278
(5 N) purity in the way described before [10, 11, 13]. In all cases a total weight of elements of 10 g was placed in silica ampoules, of length 20 cm and inner diameter 12 mm under a vacuum of the order of 10\ Torr. Each sample was kept inside a rocking oscillatory furnace whose temperature was raised step by step to the melting point of each element (Se, Sb and Ge) at rate of 100°C/h and was kept at this value for 2 h, then maintained at 1000°C for 15 h. Thereafter, the samples were quenched in icy water to form the glasses. Density measurements for bulk compositions of the system Se Ge Sb (x"0.75 and y"0.01, 0.05, 0.10, V \W W 0.15, 0.18 and 0.20), were made by the Archimedes method using the hydrostatic weighing in toluene. A single crystal of germanium was used as a reference material for characterizing the toluene. Thin films of the six compositions were prepared on the clean glass substrate by the thermal evaporation technique [11, 15] in high vacuum held at a pressure of the order of 1;10\ Torr (coating unit Edward’s E 306A). The film thickness was measured during deposition using an hf crystal monitor (Edward FTMS). The amorphicity of the samples was confirmed by the absence of any sharp peak in the X-ray diffraction pattern [15, 16]. For electrical measurements, the film resistance was measured with a digital electrometer (Keithley type E 616A) in a coplanar configuration with Al electrodes evaporated onto a clean glass substrate at an interval of width 1—2 mm and length 3 mm, before evaporation of samples under test. For the conductivity—time dependence measurements samples of different compositions and thicknesses were measured at room temperature directly after film preparation either in darkness (curves a), or during exposure to a tungsten lamp, 10 cm apart from the sample such that the rating was 60 W which gives an output of 700 lm. The luminous intensity is 56 lumen per solid angle produced at the surface of the sample (curves b) and in all cases was maintained constant. 3. The results 3.1. Density measurements Density measurements were made on compositions under test by the Archimedes method using the hydro-
static weight in toluene. Six separate determinations were made on each sample and the average of these was corrected for the buoyancy of air. The precision of measurement was $0.05, the obtained results are given in Table 1. It is clear from this table that the density increases with increasing Sb content. As is known, the change of the density is related to the change in the atomic weight and the atomic volume of the elements constituting the system. The atomic weights of Se (4S4P), Ge (4S4P) and Sb (5S5P), are 78.7, 72.5 and 121.75, respectively, and their respective atomic radii are 1.14, 1.22 and 1.36°A. 3.2. DC conductivity 3.2.1. Time dependence Figs. 1 and 2 depict the time dependence of electrical conductivity for amorphous thin-film samples, Fig. 1 for the system Se Ge Sb (x"0.75 and y"0.01, 0.05, V \W W 0.10, 0.15, 0.18 and 0.20), and Fig. 2 for the composition Se Ge Sb as a representative example. The time-conductivity measurements were measured directly after preparation (curves a) when kept in darkness, this is the first stage. In the second stage, measurements were made for the same samples, which achieved saturation in darkness, with their surface now illuminated by a tungsten lamp for periods exceeding the new saturation attained under illumination (curves b). From these Figs. 1 and 2, it is observed that: (a) The photoconductivity increased rapidly in the first 12 s (¹ &at 12 s), then
slowly decreased with the increasing exposure time (to about 30 s) and reached saturation at 180 s. When the lamp is switched off, the photoconductivity decreases rapidly approximately to the value of the saturated dark conductivity. This means that ¹ , depending on the
exposure time, and this change in the illuminated values, is not a sudden change of state, but may be due to opposite effects of light and also to the recombination process of electrons and holes generated by light. (b) The samples with lower amount of Sb have lower conductivities in both stages investigated. (c) Photoconductivity starts at higher values than dark conductivities. The decreased conductivity with increasing film thicknesses for Se Ge Sb as in Fig. 2 can be understood through the assumption that the films are heterogeneous [17].
Table 1 Composition
Thickness (nm)
Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb
77.60 32.49 1073.0 982.2 1004.5 250.0
Density (gm\)
*E KE (eV)
p +p () cm)\
*E (eV)
p () cm)\
*E (eV)
p () cm\)
4.49 4.50 4.62 4.65 4.66 4.68
0.842 0.821 0.803 0.780 0.730 0.628
3.85;10 3.16;10 3.25;10 2.85;10 1.22;10 3.66;10
0.46 0.46 0.51 0.46 — —
0.179 0.48 0.48 1.47 — —
0.13 0.14 — 0.32 — —
9.08;10\ 7.49;10\ 1.85;10\ — — —
M. Fadel / Vacuum 52 (1999) 277 — 284
Fig. 1. The time dependence of electrical conductivity for Se Ge Sb (0.014y40.20) thin films (a) without illumination, and \W W (b) illuminated samples.
3.2.2. Temperature dependence The electrical conductivity for these compositions has been measured before [10, 11] without any effects. But these new measurements study the effect of light on the samples with respect to the electrical conductivity. Before any new measurements, we exposed the samples under test to a tungsten lamp for about 200 s at room temperature (&20°C), then stopped the illumination and started to record the resistivity reading immediately after switching off the lamp directly. Fig. 3 depicts this variation of dc conductivity (which deduced from resistivity measurements) with inverse temperature (10/¹) for Se Ge Sb glasses with V \W W x"0.75 and y"0.01, 0.05, 0.10, 0.15, 0.18 and 0.20 at%. The dark conductivity p over the entire temperature range (294—455 K) can be expressed as a sum of the activated processes of the form p (0, ¹)"p exp
!*E1 !*E2 #p exp k¹ k¹
#p exp
!*E3 k¹
(1)
279
Fig. 2. The time dependence of electrical conductivity for Se Ge Sb thin films with different thickness (30—250 nm), (a) without illumination and (b) with illumination.
where the symbols have their usual meaning. Furthermore, the graph shows a prominent kink. It is observed that p (0, ¹) data cannot be approximated by single activation energy over a wide temperature range. We suggest that the plot shows that three or two types of conduction channels contribute to the conductivity according to the amount of Sb and perhaps due to the effect of light and the low temperature of the lamp. The linearity of p (0, ¹) against 1/¹ indicates that p (0, ¹) indicates that p (0, ¹) in this region exhibits activated behaviour in accordance with the relation p (0, ¹)"p exp
!*E1 k¹
Supposing, *E KE and p "p , where E can be calculated from the slopes of the straight lines of the compositions under investigation (Fig. 3). Table 1 summarizes the electrical conductivity parameters over the entire temperature. Fig. 4 depicts the temperature dependence of the dark electrical conductivity for Se Ge Sb thin films as a representative example of different thicknesses (30, 100, 150 and 250 nm), and in the temperature range 294—417 K. As one can readily see, the obtained relations of p"F(1/¹), are parallel straight lines. This indicates
280
M. Fadel / Vacuum 52 (1999) 277 — 284
Fig. 4. p as a function of 1000/¹ for Se Ge Sb films.
Fig. 3. The dependence of dark electrical conductivity, p , of the system Se Ge Sb films on temperature and after exposing the samples \W W to white light at a certain time for a certain duration.
that the activation energy E (E K0.63 eV), which cal culated from the slope of these lines, shows a value independent of the film thickness. 3.2.3. Effect of temperature on dc conductivity under illumination Fig. 5a and b depicts the temperature dependence of dark electrical conductivity p (curves a), under illumina tion p (curves b), with white light as a function of 10000/¹. Photoconductivity *p was calculated by subGJ tracting p from p and is represented in Fig. 6 for the GJ compositions of the considered system. The conduction activation energy values for photoconductivity *E which calculated from the slopes of the obtained lines of Fig. 6 are given in Table 2 together with electrical parameters for dark conductivity. It is obvious that *E is less than E and E for the studied composi tions, which may be explained by the presence of a charge carrier trapping level in the pseudogap of the studied glass [18].
3.2.4. Effect of light and heat on the switching phenomenon Both dynamic and static characteristic curves were obtained on a pyrographite substrate for thin-film samples. The dynamic curve was displayed on the screen of a cathode-ray oscilloscope, using an ac source, and the static curves were measured with a dc source. Both dynamic and static I—» characteristic curves have three states, off state with high resistance, negative Resistance State beyond the threshold voltage and the on state with low resistance. These facts indicates that the system is characterized by possessing negative resistance and memory phenomena. Fig. 7 illustrates the static current—voltage characteristic curves for evaporated films of Se Ge Sb V \W W (x"0.75 and y"0.01, 0.05, 0.10, 0.15, 0.18 and 0.20), measured at room temperature (&25°C). It is also observed from Fig. 7 that the recorded mean value of the threshold voltage, » decreases with increasing Sb con tent. Measurements of » under illumination were made at different temperatures for thin-film samples with thickness K450 nm for the investigated compositions, to obtain improved switching as shown in Fig. 8a and b, where the sample was exposed to light at RT from a tungsten lamp for about 12 s, and during the exposure, the value of » , was recorded. The lamp was switched off and the temperature increased. The sample was then exposed to light and the value of » recorded. This was repeated for all values of temperature shown in Fig. 8.
M. Fadel / Vacuum 52 (1999) 277 — 284
281
Fig. 5. Temperature dependence of electrical conductivity in dark, p , and in illumination p for the system Se—Ge—Sb.
Table 2 Composition
Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb
From Figure 3 E (eV)
p () cm)\
0.842 0.821 0.803 0.780 0.730 0.628
3.85;10 3.16;10 3.25;10 2.85;10 1.22;10 3.66;10
It is observed from Fig. 8 and Table 3 that » are less than » , especially, at room temperature, where » decreases rapidly in the first second, this is attributed to the effect of the illumination, and then onwards the decrease becomes normal due to the effect of temperature. This behaviour is similar to the behaviour of the experimental results of Figs. 1 and 2. Also, the exposure with temperature produces a change very similar to that produced by variations in doping concentration, where » decreases with increasing Sb content and temper ature.
From Figure 5 E (eV) 0.79 0.730 0.713 0.690 0.677 0.639
p () cm)\ 2.02;10 3.09;10 2.37;10 1.74;10 1.37;10 9.99;10
From Figure 6 *E (eV)
p *il () cm)\
0.719 0.70 0.69 0.639 0.628 0.595
4.64;10 3.24;10 4.26;10 7.11 6.55 2.39
4. The discussion and the conclusion From Tables 1 and 2, it is observed that the photoconductivity values of pre-exponential factor p and activa tion energy *E are less than the values for dark con ductivity p (from Fig. 3) and that E and E (from Fig. 5) decreases with increasing Sb content. Also, the observed decrease of p (0, ¹), with the addition of Sb, in the present work indicated that the conduction in the linear part is thermally assisted tunnelling of charge carriers in the extended states present in the energy band
M. Fadel / Vacuum 52 (1999) 277 — 284
282 Table 3 Composition
Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb
»M 25°C, l (V)
»M 25°C, (V)
¹ Ref. 17 (°C)
84.14 63.68 50.0 40.20 38.00 36.20
74 55 35 31 29 26
245 230 190 185 150 120
E Ref 17 (eV) 2.08 1.98 1.90 1.80 1.75 1.72
Excess of Se—Se bond
CE (eV/atom)
Bond
Bond energy (kcal/mol)
51 55 60 65 68 70
2.718 2.936 2.797 2.658 2.575 2.519
Se—Se Ge—Ge Sb—Sb Se—Ge Se—Sb
44 37.6 30 59.87 51.15
As reported by Haisty et al. [20], the addition of a few percent Sb is sufficient to cause crystallization of Se unless some Ge is also present, in which case glasses are formed. In fact, 5% Ge is sufficient to ensure glass formation with up to about 30% Sb. This is because Ge is capable of forming covalent bonds with Se atoms and connects chains mutually to form a strong three-dimensional network structure. The role of Sb in promoting cyrstallization of Se was explained [21] in connection with the Sb—Se system. It is mainly due to the fact that Sb forms three weaker bonds in addition to the three strong bonds. The weaker bonds direct themselves towards the Se atoms of the neighbouring chains or rings, the Se—Se bond of these chains are thereby weakened. The present stoichiometric glasses (Se Ge Sb \W W where 0.014y40.20) exhibit a decrease of ¹ as Sb is increased. As Sb is progressively increased, more and more Sb Se SU with weaker Sb—Se bonds replace GeSe SU with stronger bonds. The resulting weakening of the lattice structure accounts for the observed decrease of ¹ . As a mixed network derives its ¹ from its weakest bonds [20] the addition of a small concentration of relatively weaker Sb—Se bond to a network of Ge—Sb bonds has a large effect in initially reducing the ¹ of GeSe glass. Therefore, one can estimate the bond energies E for \ heteronuclear bonds by using the relation [22] Fig. 6. Temperature dependence of *p "p !p for the Se Ge Sb (0.014y40.20) amorphous thin films. W
where p (0, ¹)'10 [19]. But around room temper ature (ranges 293—333 K) Figs. 3 and 4, the conduction occurs via variable-range hopping of the charge carriers in the localized state near the Fermi level, where p , p (1, see Table 1. For discussion of the mechanism of the change in conduction type, information on the glass structure is required. The structure of the glasses studied is predominantly composed of structural units (SU) tetrahedual GeSe , trigonal Sb Se and some connecting Se chain. Only Ge—Se, Sb—Se and Se—Se bonds exist in this system.
E "[E !E ] #30(X !X ) \ \ \ where E and E are the energies of the homonuclear \ \ bonds, and X and X are the electronegativity of the atoms. The values of X are 2.6 for Se, 1.8 for Ge and 1.9 for Sb. The types of bonds expected to occur in the investigated system are listed in Table 3. After all these bonds are formed, there are still unsatisfied Se valences ‘‘excess bonds’’ which must be satisfied by the formation of the Se—Se bond. The number of excess bonds are also given in Table 3. Thus, Sb increases the weak bond density in the system, while Ge forms covalent bonds with Se-atoms Therefore, the observed increase in p (0, ¹) and decrease in *E (see Tables 1 and 2) may be due to the decrease of covalent bond density and the increase of weak bond density.
M. Fadel / Vacuum 52 (1999) 277 — 284
Fig. 7. Room temperature state I—» curves for Se
Ge Sb (0.014y40.20) thin film of thicknessK450 nm. \W W
Fig. 8. Static I—» characteristic curve for Se—Ge—Sb films of thickness 450 nm at different elevated temperatures (298—423 K).
283
M. Fadel / Vacuum 52 (1999) 277 — 284
284
However, the apparent correspondence between CE and ¹ should not be used carelessly, since the transition temperature depends on the product of the transition. The chemical bond approach can also help us to understand why Sb which is present in such a small amount (0.124Sb40.26) plays a crucial role in the memory glasses, by enhancing the reversibility of these film in memory processes [23].
References
Fig. 8. (Continued).
If we now further assume that the bond energies are additive, we can estimate the cohesive energy (CE) i.e. the stabilization energy of an infinitely large cluster of the material per atom by summing the band energies expected in the material. This is equivalent to assuming a simplified model consisting of noninteracting electron pair bonds highly localized between adjacent pair of atoms The result of CE of compositions under test are listed in Table 3. These data reveal that the addition of Sb leads to change in the considered properties. The increase of Sb leads to a decrease of CE, » , ¹ and E respectively.
[1] Ilibton AR, Hayes D, Rechtin M. J Non-Cryst Solids 1965; 17:319. [2] Ganon JR. Proc SPIE 1981;266:62. [3] Marquez E, Villars P, Jimenez R-G. J Mater Res 1988;3:314. [4] Dessaur JH, Clarke HE. Xerography and Related Processes. Local, London, 1965. [5] Ovshinsky R. Phys Rev Lett 1986;21:1450. [6] Carlson DE, Wronski CR. Appl Phys Lett 1976;28:671. [7] Fusong J, Okuda M. Jpn J Appl Phys 1991;30:97. [8] Mott NF. Adv Phys 1967;16:49; Philos Mag 1976;34:1101. [9] Fritzsche H, Kastner M. Philos Mag 1978;B 37:285. [10] Fadel M, El-Shair HT. Vacuum 1992;43:253. [11] Afifi MA, Labib HH, El-Fazary MH, Fadel M. Appl Phys 1992; A55:167. [12] Mahadevan S, Giridhar A, Singh AK. J Non-Cryst Solids 1983;57:423. [13] Afifi MA, Hegab NA, Labib HH, Fadel M. Ind J Pure Appl Phys 1992;30:211. [14] Mehra RM, Ganjoo A, Mathur PC. J Appl Phys 1994;75:7334. [15] Fadel M. Vacuum 1997;48:73. [16] El-Samanoudy MM, Fadel M. J Mater Sci 1992, 72, 646. [17] Lyubin VM, Madzinskii VS. Fiz Tv Tela 1964;6:3740 [Sov Phys Solids St., 1965;6:3091]. [18] Guriin NT, Semak DG, Fedek VV. Sov Phys Semicond 1975; 9:499. [19] Mott NF, Davis EA. Electronic Processes in Non-Cryst Mater, Clarendon, Oxford, 1971. [20] Haisty RW, Krebs HJ. Non-Cryst Solids 1969;1:399. [21] Mehra RM, Kumar H, Mathur PC. Thin Solid Films 1989; 170:15. [22] Pauling L. The nature of the Chemical Bond, 3rd edn. Cornell University Press, Ithca, NY, 1960, p. 91. [23] Moss SC, de Neuvill JP. Mat Res Bull 1972;7:423.
The effect of the Sb content on the physical properties of amorphous Se0.75Ge0.25!y thin films M. Fadel Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Received 23 June 1998
Abstract The effect of the addition of varying amounts of antimony in concentrations from 0.01 to 0.20 at% to Se Ge glass on its dc \W conductivity is analysed. The electrical conductivity of amorphous thin films of vacuum evaporated Se Ge Sb were \W W determined during and after light exposure and at different temperatures. The time dependence of the electrical conductivity measured Ge Sb (y"0.01, 0.05, in darkness or when exposed to light at about room temperature has been studied for amorphous Se \W W 0.10, 0.15, 0.18 and 0.20) thin films of different compositions and different thicknesses. The conduction activation energy *E and the pre-exponential factor p (0, ¹) which appear in the dc conductivities are found to decrease with increasing Sb content. The mean value of the threshold voltage, was measured either in darkness, » M , and after exposure of light » M , for different compositions and temperatures. The pronounced glass-forming tendencies of alloys of Se and Ge with Sb were discussed topologically in terms of the chemical bonds expected to be present in these materials. These chemical bonds have been used to estimate the cohesive energies (CE) of the glasses. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Chalcogenide glasses have been recognized as promising materials for infrared optical elements [1], infrared optical fibres [2], and for the transfer of information [3]. They have also found applications in Xerography [4], switching and memory devices [5], photolithography [5], and in the fabrication of inexpensive solar cells [6], and, more recently, for reversible phase change optical records [7]. Thus, it is important to have an insight into their electronic properties. The addition of an impurity has a pronounced effect on the conduction mechanism and the structure of the amorphous glass and this effect can be widely different for different impurities. The study of doped amorphous semiconductors has become of great interest to scientists. It was believed [8] that impurities have little effect on the properties of amorphous semiconductors as each impurity atom can satisfy its valence requirements by adjusting its nearest-neighbour environment. However, it has been shown [9] that the effect of charged additives in lone-pair semiconductors depend on whether the charged additives equilibrate or not with valence-alternation defects. In our earlier communication preliminary results on the electrical properties of
antimony-doped Se Ge glassy alloy with Sb \W concentration form 0.01 at% were explained on the basis of Sb acting as a Ge-chain terminator causing decreased disorder. Much effort has been devoted towards the preparation and properties of chalcogenide glasses of Se—Ge—Sb system in several papers [10—14]. The aims of the present work were: (i) to obtain the density of the system Se Ge Sb (y"0.01, 0.05, \W W 0.10, 0.15, 0.18 and 0.20) by Archimedes method, (ii) to study the effect of illumination on the electrical properties for the given compositions as a function of: time, temperature and film thickness to obtain the conduction activation energy and the pre-exponential factor; for dark E , p , for illuminated conductivity E , p and for photoconductivity behaviour *E and *p , (iii) to inves tigate the effect of light on the switching phenomenon, and (iv) to correlate the experimental data with the chemical bond expected to be present.
2. Experimental technique The bulk glasses under study were prepared by direct synthesis from selenium (4 N), germanium and antimony
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 2 9 8 - X
M. Fadel / Vacuum 52 (1999) 277 — 284
278
(5 N) purity in the way described before [10, 11, 13]. In all cases a total weight of elements of 10 g was placed in silica ampoules, of length 20 cm and inner diameter 12 mm under a vacuum of the order of 10\ Torr. Each sample was kept inside a rocking oscillatory furnace whose temperature was raised step by step to the melting point of each element (Se, Sb and Ge) at rate of 100°C/h and was kept at this value for 2 h, then maintained at 1000°C for 15 h. Thereafter, the samples were quenched in icy water to form the glasses. Density measurements for bulk compositions of the system Se Ge Sb (x"0.75 and y"0.01, 0.05, 0.10, V \W W 0.15, 0.18 and 0.20), were made by the Archimedes method using the hydrostatic weighing in toluene. A single crystal of germanium was used as a reference material for characterizing the toluene. Thin films of the six compositions were prepared on the clean glass substrate by the thermal evaporation technique [11, 15] in high vacuum held at a pressure of the order of 1;10\ Torr (coating unit Edward’s E 306A). The film thickness was measured during deposition using an hf crystal monitor (Edward FTMS). The amorphicity of the samples was confirmed by the absence of any sharp peak in the X-ray diffraction pattern [15, 16]. For electrical measurements, the film resistance was measured with a digital electrometer (Keithley type E 616A) in a coplanar configuration with Al electrodes evaporated onto a clean glass substrate at an interval of width 1—2 mm and length 3 mm, before evaporation of samples under test. For the conductivity—time dependence measurements samples of different compositions and thicknesses were measured at room temperature directly after film preparation either in darkness (curves a), or during exposure to a tungsten lamp, 10 cm apart from the sample such that the rating was 60 W which gives an output of 700 lm. The luminous intensity is 56 lumen per solid angle produced at the surface of the sample (curves b) and in all cases was maintained constant. 3. The results 3.1. Density measurements Density measurements were made on compositions under test by the Archimedes method using the hydro-
static weight in toluene. Six separate determinations were made on each sample and the average of these was corrected for the buoyancy of air. The precision of measurement was $0.05, the obtained results are given in Table 1. It is clear from this table that the density increases with increasing Sb content. As is known, the change of the density is related to the change in the atomic weight and the atomic volume of the elements constituting the system. The atomic weights of Se (4S4P), Ge (4S4P) and Sb (5S5P), are 78.7, 72.5 and 121.75, respectively, and their respective atomic radii are 1.14, 1.22 and 1.36°A. 3.2. DC conductivity 3.2.1. Time dependence Figs. 1 and 2 depict the time dependence of electrical conductivity for amorphous thin-film samples, Fig. 1 for the system Se Ge Sb (x"0.75 and y"0.01, 0.05, V \W W 0.10, 0.15, 0.18 and 0.20), and Fig. 2 for the composition Se Ge Sb as a representative example. The time-conductivity measurements were measured directly after preparation (curves a) when kept in darkness, this is the first stage. In the second stage, measurements were made for the same samples, which achieved saturation in darkness, with their surface now illuminated by a tungsten lamp for periods exceeding the new saturation attained under illumination (curves b). From these Figs. 1 and 2, it is observed that: (a) The photoconductivity increased rapidly in the first 12 s (¹ &at 12 s), then
slowly decreased with the increasing exposure time (to about 30 s) and reached saturation at 180 s. When the lamp is switched off, the photoconductivity decreases rapidly approximately to the value of the saturated dark conductivity. This means that ¹ , depending on the
exposure time, and this change in the illuminated values, is not a sudden change of state, but may be due to opposite effects of light and also to the recombination process of electrons and holes generated by light. (b) The samples with lower amount of Sb have lower conductivities in both stages investigated. (c) Photoconductivity starts at higher values than dark conductivities. The decreased conductivity with increasing film thicknesses for Se Ge Sb as in Fig. 2 can be understood through the assumption that the films are heterogeneous [17].
Table 1 Composition
Thickness (nm)
Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb
77.60 32.49 1073.0 982.2 1004.5 250.0
Density (gm\)
*E KE (eV)
p +p () cm)\
*E (eV)
p () cm)\
*E (eV)
p () cm\)
4.49 4.50 4.62 4.65 4.66 4.68
0.842 0.821 0.803 0.780 0.730 0.628
3.85;10 3.16;10 3.25;10 2.85;10 1.22;10 3.66;10
0.46 0.46 0.51 0.46 — —
0.179 0.48 0.48 1.47 — —
0.13 0.14 — 0.32 — —
9.08;10\ 7.49;10\ 1.85;10\ — — —
M. Fadel / Vacuum 52 (1999) 277 — 284
Fig. 1. The time dependence of electrical conductivity for Se Ge Sb (0.014y40.20) thin films (a) without illumination, and \W W (b) illuminated samples.
3.2.2. Temperature dependence The electrical conductivity for these compositions has been measured before [10, 11] without any effects. But these new measurements study the effect of light on the samples with respect to the electrical conductivity. Before any new measurements, we exposed the samples under test to a tungsten lamp for about 200 s at room temperature (&20°C), then stopped the illumination and started to record the resistivity reading immediately after switching off the lamp directly. Fig. 3 depicts this variation of dc conductivity (which deduced from resistivity measurements) with inverse temperature (10/¹) for Se Ge Sb glasses with V \W W x"0.75 and y"0.01, 0.05, 0.10, 0.15, 0.18 and 0.20 at%. The dark conductivity p over the entire temperature range (294—455 K) can be expressed as a sum of the activated processes of the form p (0, ¹)"p exp
!*E1 !*E2 #p exp k¹ k¹
#p exp
!*E3 k¹
(1)
279
Fig. 2. The time dependence of electrical conductivity for Se Ge Sb thin films with different thickness (30—250 nm), (a) without illumination and (b) with illumination.
where the symbols have their usual meaning. Furthermore, the graph shows a prominent kink. It is observed that p (0, ¹) data cannot be approximated by single activation energy over a wide temperature range. We suggest that the plot shows that three or two types of conduction channels contribute to the conductivity according to the amount of Sb and perhaps due to the effect of light and the low temperature of the lamp. The linearity of p (0, ¹) against 1/¹ indicates that p (0, ¹) indicates that p (0, ¹) in this region exhibits activated behaviour in accordance with the relation p (0, ¹)"p exp
!*E1 k¹
Supposing, *E KE and p "p , where E can be calculated from the slopes of the straight lines of the compositions under investigation (Fig. 3). Table 1 summarizes the electrical conductivity parameters over the entire temperature. Fig. 4 depicts the temperature dependence of the dark electrical conductivity for Se Ge Sb thin films as a representative example of different thicknesses (30, 100, 150 and 250 nm), and in the temperature range 294—417 K. As one can readily see, the obtained relations of p"F(1/¹), are parallel straight lines. This indicates
280
M. Fadel / Vacuum 52 (1999) 277 — 284
Fig. 4. p as a function of 1000/¹ for Se Ge Sb films.
Fig. 3. The dependence of dark electrical conductivity, p , of the system Se Ge Sb films on temperature and after exposing the samples \W W to white light at a certain time for a certain duration.
that the activation energy E (E K0.63 eV), which cal culated from the slope of these lines, shows a value independent of the film thickness. 3.2.3. Effect of temperature on dc conductivity under illumination Fig. 5a and b depicts the temperature dependence of dark electrical conductivity p (curves a), under illumina tion p (curves b), with white light as a function of 10000/¹. Photoconductivity *p was calculated by subGJ tracting p from p and is represented in Fig. 6 for the GJ compositions of the considered system. The conduction activation energy values for photoconductivity *E which calculated from the slopes of the obtained lines of Fig. 6 are given in Table 2 together with electrical parameters for dark conductivity. It is obvious that *E is less than E and E for the studied composi tions, which may be explained by the presence of a charge carrier trapping level in the pseudogap of the studied glass [18].
3.2.4. Effect of light and heat on the switching phenomenon Both dynamic and static characteristic curves were obtained on a pyrographite substrate for thin-film samples. The dynamic curve was displayed on the screen of a cathode-ray oscilloscope, using an ac source, and the static curves were measured with a dc source. Both dynamic and static I—» characteristic curves have three states, off state with high resistance, negative Resistance State beyond the threshold voltage and the on state with low resistance. These facts indicates that the system is characterized by possessing negative resistance and memory phenomena. Fig. 7 illustrates the static current—voltage characteristic curves for evaporated films of Se Ge Sb V \W W (x"0.75 and y"0.01, 0.05, 0.10, 0.15, 0.18 and 0.20), measured at room temperature (&25°C). It is also observed from Fig. 7 that the recorded mean value of the threshold voltage, » decreases with increasing Sb con tent. Measurements of » under illumination were made at different temperatures for thin-film samples with thickness K450 nm for the investigated compositions, to obtain improved switching as shown in Fig. 8a and b, where the sample was exposed to light at RT from a tungsten lamp for about 12 s, and during the exposure, the value of » , was recorded. The lamp was switched off and the temperature increased. The sample was then exposed to light and the value of » recorded. This was repeated for all values of temperature shown in Fig. 8.
M. Fadel / Vacuum 52 (1999) 277 — 284
281
Fig. 5. Temperature dependence of electrical conductivity in dark, p , and in illumination p for the system Se—Ge—Sb.
Table 2 Composition
Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb
From Figure 3 E (eV)
p () cm)\
0.842 0.821 0.803 0.780 0.730 0.628
3.85;10 3.16;10 3.25;10 2.85;10 1.22;10 3.66;10
It is observed from Fig. 8 and Table 3 that » are less than » , especially, at room temperature, where » decreases rapidly in the first second, this is attributed to the effect of the illumination, and then onwards the decrease becomes normal due to the effect of temperature. This behaviour is similar to the behaviour of the experimental results of Figs. 1 and 2. Also, the exposure with temperature produces a change very similar to that produced by variations in doping concentration, where » decreases with increasing Sb content and temper ature.
From Figure 5 E (eV) 0.79 0.730 0.713 0.690 0.677 0.639
p () cm)\ 2.02;10 3.09;10 2.37;10 1.74;10 1.37;10 9.99;10
From Figure 6 *E (eV)
p *il () cm)\
0.719 0.70 0.69 0.639 0.628 0.595
4.64;10 3.24;10 4.26;10 7.11 6.55 2.39
4. The discussion and the conclusion From Tables 1 and 2, it is observed that the photoconductivity values of pre-exponential factor p and activa tion energy *E are less than the values for dark con ductivity p (from Fig. 3) and that E and E (from Fig. 5) decreases with increasing Sb content. Also, the observed decrease of p (0, ¹), with the addition of Sb, in the present work indicated that the conduction in the linear part is thermally assisted tunnelling of charge carriers in the extended states present in the energy band
M. Fadel / Vacuum 52 (1999) 277 — 284
282 Table 3 Composition
Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb Se Ge Sb
»M 25°C, l (V)
»M 25°C, (V)
¹ Ref. 17 (°C)
84.14 63.68 50.0 40.20 38.00 36.20
74 55 35 31 29 26
245 230 190 185 150 120
E Ref 17 (eV) 2.08 1.98 1.90 1.80 1.75 1.72
Excess of Se—Se bond
CE (eV/atom)
Bond
Bond energy (kcal/mol)
51 55 60 65 68 70
2.718 2.936 2.797 2.658 2.575 2.519
Se—Se Ge—Ge Sb—Sb Se—Ge Se—Sb
44 37.6 30 59.87 51.15
As reported by Haisty et al. [20], the addition of a few percent Sb is sufficient to cause crystallization of Se unless some Ge is also present, in which case glasses are formed. In fact, 5% Ge is sufficient to ensure glass formation with up to about 30% Sb. This is because Ge is capable of forming covalent bonds with Se atoms and connects chains mutually to form a strong three-dimensional network structure. The role of Sb in promoting cyrstallization of Se was explained [21] in connection with the Sb—Se system. It is mainly due to the fact that Sb forms three weaker bonds in addition to the three strong bonds. The weaker bonds direct themselves towards the Se atoms of the neighbouring chains or rings, the Se—Se bond of these chains are thereby weakened. The present stoichiometric glasses (Se Ge Sb \W W where 0.014y40.20) exhibit a decrease of ¹ as Sb is increased. As Sb is progressively increased, more and more Sb Se SU with weaker Sb—Se bonds replace GeSe SU with stronger bonds. The resulting weakening of the lattice structure accounts for the observed decrease of ¹ . As a mixed network derives its ¹ from its weakest bonds [20] the addition of a small concentration of relatively weaker Sb—Se bond to a network of Ge—Sb bonds has a large effect in initially reducing the ¹ of GeSe glass. Therefore, one can estimate the bond energies E for \ heteronuclear bonds by using the relation [22] Fig. 6. Temperature dependence of *p "p !p for the Se Ge Sb (0.014y40.20) amorphous thin films. W
where p (0, ¹)'10 [19]. But around room temper ature (ranges 293—333 K) Figs. 3 and 4, the conduction occurs via variable-range hopping of the charge carriers in the localized state near the Fermi level, where p , p (1, see Table 1. For discussion of the mechanism of the change in conduction type, information on the glass structure is required. The structure of the glasses studied is predominantly composed of structural units (SU) tetrahedual GeSe , trigonal Sb Se and some connecting Se chain. Only Ge—Se, Sb—Se and Se—Se bonds exist in this system.
E "[E !E ] #30(X !X ) \ \ \ where E and E are the energies of the homonuclear \ \ bonds, and X and X are the electronegativity of the atoms. The values of X are 2.6 for Se, 1.8 for Ge and 1.9 for Sb. The types of bonds expected to occur in the investigated system are listed in Table 3. After all these bonds are formed, there are still unsatisfied Se valences ‘‘excess bonds’’ which must be satisfied by the formation of the Se—Se bond. The number of excess bonds are also given in Table 3. Thus, Sb increases the weak bond density in the system, while Ge forms covalent bonds with Se-atoms Therefore, the observed increase in p (0, ¹) and decrease in *E (see Tables 1 and 2) may be due to the decrease of covalent bond density and the increase of weak bond density.
M. Fadel / Vacuum 52 (1999) 277 — 284
Fig. 7. Room temperature state I—» curves for Se
Ge Sb (0.014y40.20) thin film of thicknessK450 nm. \W W
Fig. 8. Static I—» characteristic curve for Se—Ge—Sb films of thickness 450 nm at different elevated temperatures (298—423 K).
283
M. Fadel / Vacuum 52 (1999) 277 — 284
284
However, the apparent correspondence between CE and ¹ should not be used carelessly, since the transition temperature depends on the product of the transition. The chemical bond approach can also help us to understand why Sb which is present in such a small amount (0.124Sb40.26) plays a crucial role in the memory glasses, by enhancing the reversibility of these film in memory processes [23].
References
Fig. 8. (Continued).
If we now further assume that the bond energies are additive, we can estimate the cohesive energy (CE) i.e. the stabilization energy of an infinitely large cluster of the material per atom by summing the band energies expected in the material. This is equivalent to assuming a simplified model consisting of noninteracting electron pair bonds highly localized between adjacent pair of atoms The result of CE of compositions under test are listed in Table 3. These data reveal that the addition of Sb leads to change in the considered properties. The increase of Sb leads to a decrease of CE, » , ¹ and E respectively.
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