Vacuum/volume 47lnumber 3lpages 265 to 26911996 Copyright 6 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X196 $15.00+.00 Pergamon 0042-207x(95)00199-9 The switching phenomenon films M A Afifi, N A Hegab and A E Bekheet, Semiconductor Ain Shams University, Heliopolis, Cairo, Egypt in amorphous In,Te, thin Laboratory, Physics Departments Faculty of Education received 13 July 7995 Investigation of the switching phenomenon in amorphous In,Te, films revealed that it is typical for a memory switch. The-thickness dependence of the mean value of the switching voltage Vt,, was linear in the investigated range and Vt,, decreased exponentially with a temperature rise from 298 to 373 K. The switching voltage activation energy (6) calculated from the temperature dependence of vlrhis about 0.25 eV. The conduction activation energy (E,) obtained from the temperature dependence of film resistance was found to be 0.51 eV. The agreement between the obtained value of the ratio &,(0.49) and those of the temperature difference between that inside the film and that of its surface with their values obtained before suggests that the switching phenomenon in the investigated In,Te, films may be explained according to an electrothermal model for the switching process. 1. Introduction The switching effect is a reversible transformation’ between a highly resistive and a conductive state under the effect of an applied electric field. The switching effect can be regenerative and nondestructive.’ Various models have been proposed to explain the switching process based on thermal and mixed electrothermal mechanisms.‘A In,Te, is a A2n1BSV’compound with very interesting electrical and optical properties.%’ The switching effect was reported for Ga,Te,” single crystal, In,Se,” In,Te,_,” and In,Te, films.‘3.‘4However, the latter study was insufficient for the investigation of the switching phenomenon in In,Te, films. The aim of this work is to study the switching phenomenon in amorphous In,Te,, as well as, parameters affecting the switching voltage. The switching mechanism is also investigated. 2. Experimental Amorphous In,Te, was synthesised15 by direct fusion of stoichometric amounts of the constituent elements In and Te (purity 99.999%) in an evacuated sealed silica tube (lop5 Torr). The temperature of the tube was raised at a rate of 5O’Cjh to 800°C at which it was kept constant for three days, then quenched in icy water. Thin film samples of the synthesized amorphous In,Te, were obtained at room temperature by thermal evaporation technique under vacuum using highly polished pyrographite and glass substrates. X-ray diffractometer of the type Philips (PM8203) was used to investigate the structure of the obtained sample in bulk and thin film form. The thickness of the film samples was measured during deposition using an hf crystal monitor (Edward FTMS) and confirmed after deposition by Tolansky’s interferometric method.16 The current-voltage (Z-V) characteristics at room temperature as well as at elevated temperatures were obtained. Z-V characteristics were measured in the usual way using a high impedance digital electrometer (Keithley 616) for potential drop measurements and a digital multimeter (TE924) for current measurements. For this purpose a copper sample holder was used for point contact construction, the upper electrode of which was a platinum electrode of circular end of diameter 0.2 mm (Figure 1). 3. Results and discussion X-ray diffraction patterns carried out for the investigated compound in powder (a) and the as-deposited thin film forms (b) are shown in Figure 2. It is clear that both powder and thin film samples are in the amorphous state. Room temperature Z-I/ characteristics for the investigated compound were studied for samples of different thicknesses (212654 nm). The obtained Z-V curves are typical for the memory switching phenomenon. As a representative example, the Z-V curve for a film sample of thickness 461 nm is given in Figure 3. The OFF-State of the obtained Z-V (oa region) can be divided into three subregions o-e, e-f andf-a where the first subregion (ae) is linear (Figure 4). In the second subregion (ef), the current increases exponentially with the square root of the voltage (Figure 5(a)) according to the relation”~‘* Z = Z, exp( V/ VJ”) (1) where Z, is the intercept along the current axis, I’ the potential drop across the sample, V, = 4T2k2d2/&,f where k is Boltzmann constant, T the absolute temperature, d the film thickness and &r 265 MA Afifi et al: In,Te, thin films n d = 461 nm Figure 3. I-V characteristic curve for InzTe, film of thickness 461 nm. Figure 1. Schematic representation of the cell used for I-V measurements 1--co-axial cable, 2 -a brass plate, 3 -teflon plated 4 -two brass rods, 5 -base of the cell made of brass, 6 -holder of platinum electrode, 7 slight spring, 8 - the platinum electrode, 9 -lower copper brass electrode, IO -the substrate and 11-the thin film layer. &/@ A0 5 l 10 I 15 I 20 I 25 I 30 v (volts) Figure 4. OFF state I-V characteristic curve for In,Te, film of thickness 461 nm. - 80 70 60 50 40 30 20 Figure 2. X-ray diffraction patterns for In,Te, (a) Powder form (b) Asdeposited films. the Poole-Frenkel coefficient. It was found that values of Z, and V,, in eqn (1) are 2 x lo-’ A and 1.07 V, respectively as obtained from Figure 5(a). In the third subregion (fu) the current is an exponential function of voltage (Figure 5(b)) according to the relation:“,” I = Z, exp( V/V,) (2) where values of I, and V, are 1.7 x lo-’ A and 8.89 V, respectively. The switching voltage was measured for every film sample at different points uniformly distributed throughout the surface of the film and their mean value was calculated. The mean value of switching voltage Ythwas determined for film samples of different thicknesses in the range (212-654 nm) at room temperature. Z-V characteristic curves for which Vth, is equal to p,;h, were obtained and are illustrated in Figure 6 for all the investigated thicknesses. 266 The obtained thickness dependence of vL;his linear in the investigated range as is clear from Figure 7. The slope of the obtained line of thickness dependence represents the mean value of the threshold field &,, (6.67 x lo7 V/m). The observed relation of thickness dependence of rC,, agrees with previous observations for different amorphous systems. “Jo” The variation of PC,,with thickness is investigated also at elevated temperatures. The obtained results are also illustrated in Figure 7. It is clear that vt,, increased also linearly with thickness at elevated temperatures in the investigated range. Values of &, decreased from 6.67 x 10’ to 9.1 x lo6 V/m with increasing temperature from 298 to 373 K. The temperature dependence of the Z-V characteristics of the investigated In,Te, compound was studied in the range 298-378 K for films of different thicknesses in the range 212-654 nm. As a representative example the obtained Z-V curves for a film sample of thickness 461 nm are shown in Figure 8. The obtained parallel straight lines for the relation of In vt,, vs l/r (Figure 9) indicates that vrh decreases exponentially in the investigated ranges of temperature and thickness satisfying the following relation: MA Afifi et al: InZTe, thin films -12 r (b) I 0 I I 200 400 Thickness I 600 d (nm) Figure 7. Thickness dependence of the mean value of the switching for In,Te, films at different temperatures. -13 voltage 5 I-50 - -14 . 10 t P-2 I I I I l 15 20 25 30 + o A 0 v I.oo (volts) Figure 5. (a) Dependence of In I on V ‘P for In,Te, film of thickness 461 nm. (b) Dependence of In I on V for In,Te, film of thickness 461 nm. - ? 5% 25 40 60 80 105 50- l 212 A 255 + 315 0 390 o 461 0 523 x 654 3 10 20 30 L 0 v 40 v (volts) Figure 6. I-V characteristic nesses. Fth = V,exp(~/kT) (volts) Figure 8. I-V characteristic curves for film In*Te, films of thickness nm at different temperatures. curves for film In*Te, films of different thick- 461 of the obtained lines of Figure 9 (0.255 eV) is thickness independent in the investigated range. The mean value of the film resistance R was measured as a function of temperature in the range 298-378 K for different film thicknesses in the range 212-654 nm simply by dividing the potential drop across the sample by the current passing through it within the linear part of the OFF-State of the corresponding I-V characteristic curves for different points uniformly distributed on the whole surface of the film and taking their mean value. The results obtained for the temperature dependence of sample resistance are illustrated in Figure 10 as In R vs 1/T which yields straight lines satisfying the following relation: (3) Where E is the switching voltage activation energy, and 1 the absolute temperature. The value of E calculated from the slopes i? = Cexp(E,/kT) Where E,, is the electrical (4) conduction activation energy, and C a 267 MA AMi et al: In2Te3 thin films Table 1. Values of ATbrea,.dawn for In,Te, films at different temperatures d W) 212 A 255 + 315 0 390 0 461 0 523 x 654 l I- T W) ATbreakdown W) 298 313 333 353 373 14.7 16.9 19.1 21.5 24.0 can be understood in terms of an electrothermal model which can be solved to a certain extent by finding a stationary state solution for the heat transport equation. = aE2 + V(uVT) Cg the charge conservation !.5 I I I I 2.7 2.9 3.1 3.3 ldp-_= -idE 1000/T (K-l) Figure 9. Plots of In vt;hvs lOOO/Tfor In,Te, films of different thicknesses. d @ml (6) equation: v ‘E where C is the heat capacity, u the thermal conductivity, E the electric field, p the charge density and cr the electrical conductivity which is given by: o = (T, exp(E,/kT) 212 A 255 + 315 o 390 0 461 (7) (8) l In the case of steady state breakdown, the time derivative of temperature (dT/dt) can be neglected for the solution of eqn (6). Hence, the heat conduction equation for a small difference (AT = T,,,- TS) between the temperature at the middle of the specimen T,,, and that of the surface T, gives? lp: 8r~+aE2=0 9 (9) where d is the thickness of the sample. The steady state breakdown occurs when the amount of heat generated by Joule-heating cannot be removed by thermal conduction and the temperature difference necessary for breakdown can be obtained from the equation:24,2s A Tbreakdown= T21(-Wk) I I I I 2.7 2.9 3.1 3.3 1000/T (K-t) Figure 10. Plots of In R vs lOOO/Tfor In,Te, films of different thicknesses. constant. The value of E,, calculated from the slopes of the parallel lines obtained (0.51 eV) is independent of sample thickness in the investigated ranges. The calculated value of the ratio c/E, (0.49) agrees well with that obtained previously”S’9-25 for other amorphous semiconducting films. It is also in good agreement with the value of (t/E,) derived theoretically on the basis of an electro-thermal model23 for the switching process. From eqns (3) and (4) and taking into account that ~/E,,=0.5, we obtain the following equation: V&/R = constant (5) This implies that the power dissipated during the switching process is constant. The observed temperature dependence of switching voltage 268 (10) According to this equation and using the value of E,, ATbreakdown was calculated for In,Te, films at different temperatures and the obtained values are given in Table 1. It is clear that the obtained with previous values of ATbreakdownare in good agreement for amorphous semiconducting films. Also taking results ‘9-22,2G26 into account the good agreement of the obtained value of (c/E,,=O.49) mentioned above with both values obtained earlier ‘S22.24--26 and its value derived theoretically for the breakdown process, it can be concluded that the observed memory type switching in In2Te, films can be satisfactorily explained according to electrothermal breakdown process. 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