Investigation of the Switching Phenomena in TlGaSe 2 Single Crystal

Transcription

1 JKAU: Sci., Vol. 21 No. 1, pp: (09 A.D. / 1430 A.H.) Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal S.A. Hussein, G. Attia 1, S.R. Al harbi 2, A.A. Al Ghamdi 3, F.S. Al Hazmi 3 and S.E. Al Garni 2 Physics Department, Faculty of Science, Sou Valley University, Fayoum University 1, Egypt, and Girls Colleges of Education 2, Faculty of Science 3, King Abdulaziz University, Saudi Arabia Abstract. An Investigation was made of switching in TlGaSe 2 single crystals under static condition. Current-controlled negative resistance (CCNR) in TlGaSe 2 single crystals have been observed for e first time. It has been found at Thallium gallium diselenide single crystals exhibit bistable or memory switching. The results strongly indicated at e phenomenon in our sample is very sensitive to temperature, light intensity and sample ickness. The current-voltage characteristics is symmetrical wi respect to e reverse of e applied voltage and current. The switching parameters were checked under e influence of different factors of e ambient condition. 1. Introduction In recent years, The ІІІ-ІІІ-VI 2 family layer-structured Thallium chalcogenides such as TlGaS 2, TlInS 2 and TlGaSe 2 have been studied extensively [1]. The semiconducting compound TlGaSe 2 belongs to a new type of highly anisotropic crystals and have been increasingly interesting due to eir structural properties and potential applications [2]. The structure of TlGaSe 2 was reported as monoclinic [3,4] structure wi lattice parameters of a= Å, b= Å, c= Å and β=99.92º. Defects in TlGaSe 2 single crystals have been studied [5]. Recombination processes in TlGaSe 2 crystals were investigated [6] at 77K. The Ramman spectra of TlGaS 2,TlGaSe 2 and TlInS 2 have been investigated [7,8]. Recently electrical conductivity [9], and trapping center parameters of TlGaSe 2 layered crystal were discussed [10]. The results of measurements 27

2 28 S. A. Hussein, et al. of time dependences of e dielectric constant of TlGaSe 2 were reported [11,12]. Optical, photoluminescence spectra and photoelectric properties of TlGaSe 2 were studied by many auors [13-16]. Previously [17] we have studied e ermoelectric power TEP, electrical conductivity σ and Hall effect R H. To e present time, e main physical properties of is compound are not enough deeply investigated. Alough ere are many valuable works on is compound, no information about switching phenomena has been established. In e present work, single crystals of TlGaSe 2 were grown, and negative resistance region was observed in e CVC. The present auors are e first who present ese results. 2. Experimental Procedure Details of e experimental equipment for crystal grow by e Bridgman meod have been reported elsewhere [18]. The materials were supplied from Aldrich wi a purity of % for Tl, Ga, and Se, g of allium ( %), g of gallium ( %) and g of selenium ( %) were used as starting materials in e experiment. The single crystals were grown in an evacuated quartz ampoule at 10 6 Torr. The ampoule wi its charge was introduced into a ree zone tube furnace [18]. At e beginning of e grow run, e ampoule was held in e first zone of e furnace (at T=1090K) for about 10h. Melt homogenization was done in is zone where e temperature is higher an e melting point. Via a very slow rate of movement, e ampoule wi its charge was made to enter e second zone of e furnace where e temperature equals at of e crystallization point [19]. Finally solidification occurs in e ird zone. Such processes need about 14 days for one to have single crystals of e TlGaSe 2 compound. The resulting ingot did not have cracks and voids on eir surface. The single crystallinity of e product crystal was confirmed by means of X-ray diffraction analysis and DTA investigation. Specimens of TlGaSe 2 wi plane-parallel mirror surfaces were prepared from a large ingot. Their typical dimensions are mm 3. The samples are symmetric sandwich type structure in which single crystal samples are placed between two metal electrodes. The sample wi its holder was positioned in a special system to allow temperature control in e investigated range. Details of e sample holder was described early []. The system was attached to a vacuum pump giving e possibility of measurements under vacuum. The environment temperature of e specimen under test was

3 Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal 29 measured by means of a calibrated spot-welded chromel-alumel ermocouple. The investigation was carried out in wide range of temperature in order to show e influence of ambient temperature on switching behavior. The current-voltage characteristic was measured using DC stabilized and regulated voltage supplied by means of digital programmable power supply ermo EC type. The current was measured by means of digital keiley 617 electrometer. The current passing rough e sample can easily be reversed or cut-off by appling ree-pole double stage reversing switch. In order to investigate e effect of light intensity on e switching phenomena at 300K, samples wi appropriate ickness were mounted in a cryostat equipped wi suitable windows and clamped in its holder provided wi apertures to allow e passage of e radiation. Details of e apparatus and cryostat as well as e using circuit are described in ref [21]. The sample was illuminated at normal incidence. Luxmeter (fisher scientific mark) was used for measuring light intensity. The current and e potential drop across e sample as a function of intensity of illumination were registered directly. The effect of sample ickness on e CVC was also studied. The specimen wi initial ickness equal to 0.14cm was first tested for e current-voltage characteristic, and en its ickness was successively reduced. Samples wi ickness varying from ( ) were used to investigate e influence of e sample ickness on e switching characteristics. 3. Results and Discussion In is work we studied e memory switching phenomenon of bulk p-type TlGaSe 2 single crystal in sandwich form of structure Ag-TlGaSe 2 - Ag. Investigation of e effect of temperature, light intensity, and sample ickness on e switching behavior were observed. Figure 1 represents e DC current-voltage characteristics (CVC) of TlGaSe 2 in e temperature range from 90 to 300K. when e current flows parallel to e layers (to c-axis). It is seen at e i-v characteristics are strongly influenced by surrounding temperature. Wi increase in temperature, e CVC as a whole is shifted toward e lower potentials. It is clear from e curves in Fig.1 at at low DC voltage, e i-v characteristics is nearly close to a linear one. Wi increase in applied voltage, e dependence of current on voltage gradually becomes non-linear. Increasing e applied voltage to a certain value (reshold voltage V ) e crystal goes into a negative resistance state (ON state) in which e series resistor limits e

4 30 S. A. Hussein, et al. voltage applied to prevent destruction of e crystal. When e voltage decreased e ON state may be maintained and e material retains back its original state in few seconds. This effect is termed switching wi memory. The memory state persists if e current is decreased slowly to its zero value. However, if current is forced to decay suddenly, e specimen returns to e high resistance state. Switching behavior takes place at electric field value V/cm at room temperature. As seen from ese curves, for e i-v behavior, ere are two distinct regions; one is e off-state region and e oer e NDR region. As is evident from e experimental curve of Fig. 1, as well as predicted by e 90 K 1 K 150 K 0 K 300 K 60 ix10 (amp.) V (Volt) Fig. 1. Current-voltage characteristics at different values of temperature for TlGaSe 2 single crystal. electroermal model [22], e ambient temperature greatly influences bo e form of e i-v curves and e reshold voltage V, at is, ere is a weaker appearance of e NDR region of e i-v characteristics at higher temperature. There exist some fluctuation in e value of V, when e first ree or four switching cycles were measured. After ree or four sets of measurements, e device becomes more stable and V sets to a constant value. Also we observe from e curves in Fig.1, a marked increase in e holding current i h wi decreasing in temperature, while

5 Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal 31 e holding voltage, gradually increases wi decreasing in temperature. The switching process takes place wi bo polarities on crystal and e CVC of TlGaSe 2 are symmetric relative to e polarity of e applied field as shown in Fig. 2. The effect of e ambient temperature on e switching parameters V and i is illustrated in Fig. 3. It is clear from is figure at when e temperature increases, e reshold voltage decreases, and reasonably e reshold current increases. This indicates at an electroermal mechanism works in e switching process (23). The reshold power depends also on e ambient temperature. So is result is quite logical, since e power necessary to initiate switching decreases when temperature increases as shown in Fig. 4. The dependence of V on T was analyzed on e basis of e ermal-field Frenkel effect. Allowance for is effect in reference (24), e relation between V and T is described by expression π ε d V = o ε ( ct ) 2 e ϕ V (Volt) ix10 (amp.) Fig. 2. Symmetrical of e CVC of TlGaSe 2 relative to e polarity.

6 32 S. A. Hussein, et al. V i 25 V (Volt) i X10 (amp.) T ((K) k ) Fig. 3. Ambient temperature effect on reshold current and voltage for TlGaSe 2 single crystal P X10 (watt) T ((K) k ) Fig. 4. Relation between P and temperature for TlGaSe 2 sample.

7 Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal 33 Where ε o is e permittivity of vacuum, ε is e electron component of permittivity, d is e distance between e electrodes, c is a constant, e is e electron charge, φ is e dep of potential well and T is e absolute temperature. Variation of V wi temperature is plotted in Fig. 5 on e /2 V (volt) T ( k ) Fig. 5. Temperature dependence of T (K) basis of e above equation, using e coordination (V ) 1/2 and T. It is seen at in e whole temperature range of investigation, e reshold voltage decreases from 5.19 to 2.85 (volt) wi temperature increasing as expectant from e above equation. Clearly, wiin limits of e experimental error, e result fits a single straight line. This shows at e switching in M- TlGaSe 2 -M structures from a high- to a low-resistivity state occurs under e simultaneous action of an electric field and temperature. This is supported by e dependence of e reshold field on ickness of e active region. The effect of surrounding temperature on e sample resistance ratio was also determined. The OFF and ON state resistance ratio (R OFF /R ON ) depends on temperature, decreases as temperature increases. The effect of light intensity on e CVC is represented in Fig. 6 which shows e i-v characteristics of TlGaSe 2 at room temperature. As observed from e curves in e figure e behavior of e CVC has e general form of switching wi S-shape. It is evident from is figure at e i-v characteristics as a whole are shifted toward lower potentials wi an increase in e intensity of e incident light. Consequently e reshold voltage V decreases, also reshold current i decreases wi increasing of light intensity as shown in Fig. 7. The relation between 1/ V 2 for TlGaSe 2 single crystal.

8 34 S. A. Hussein, et al. Lux 0 Lux 400 Lux 1100Lux 1800Lux 30-3 ix10 (amp.) V (Volt) Fig. 6. The effect of light intensity on i-v characteristics of TlGaSe 2 specimen. 90 V i V (Volt) i X10 (amp.) I (Lux) Fig. 7. Dependence of i and V on light illummation for TlGaSe 2 compound.

9 Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal 35 reshold power P wi light intensity is presented graphically in Fig. 8. As we notice, P decreases linearly wi increasing e incident light intensity. This may be due to photogeneration processes which take place under illumination of e sample and lead to low power for switching as P X10 (watt) I (Lux) Fig. 8. Effect of light intensity on reshold power P for TlGaSe 2. e intensity dose increases. The dependence of e resistance ratio R OFF /R ON on e illumation intensity was also determined. This ratio decreases as e light intensity increases. Investigation of e effect of e sample ickness on switching phenomena is useful for choosing of a specimen whose resistance is changed from high value (OFF state) to a very low value (ON state) by lowest switching power. Figure 9 shows e effect of sample ickness on switching phenomena of p-type TlGaSe 2 at room temperature. The figure indicates at e reshold potential and current changes wi e ickness of e active region and e wid of e dashed lines which represent e variation from OFF to ON state decreases wi ickness. This result indicates at e switching can be easily controlled wi sample ickness. It is also observed from e curve at e holding voltage (V h ) increases wi increasing of sample ickness and its value lies between ( volt). Also e holding

10 36 S. A. Hussein, et al. 80 d=0.02 cm d=0.036 cm d=0.056 cm d=0.072 cm d=0.102 cm d=0.125 cm d=0.14 cm -3 ix10 (amp.) V (Volt) Fig. 9. The effect of e specimen ickness on switching phenomena for TlGaSe 2 single crystal. current i h increases as e sample ickness increases and its values lies between ( amp.). It is clear from e curves in Fig. 10 at e reshold voltage decreases rapidly wi increasing e sample ickness, while e reshold current increases exponentially wi sample ickness. Figure 11 shows e dependence of e reshold field on e ickness of e sample. It is clear at E decreases wi sample ickness. This indicates, e electric field has a profound influence on e ability of e samples to undergo a transition from e OFF state to an effective region of negative differential resistivity. The variation of e reshold power P wi sample ickness is plotted in Fig. 12. It is seen at e reshold power required for switching increase as e sample ickness increases reaching a maximum value of P at d equal to 1 mm, after is P decrease as e ickness of e sample increases. This result indicates at e switching can be easily controlled wi specimen ickness. The ratio R OFF /R ON depends on e sample ickness. Decreasing e ickness leads to a sampler value of is ratio.

11 Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal V i V X10 (Volt) i X10 (amp.) d (cm) Fig. 10. Variation of i and V wi TlGaSe 2 sample ickness E (volt/cm) d(cm) Fig. 11. The dependence of e reshold field E on e ickness of e TlGaSe 2 sample.

12 38 S. A. Hussein, et al P X10 (watt) d(cm) Fig. 12. The variation of reshold power P wi sample ickness of TlGaSe 2 single crystal. 4. Conclusion Switching effect has been studied in our best p-type TlGaSe 2 single crystal samples. The switching effect observed in such crystals shows memory. A critical field of Vcm 1 is necessary for switching phenomena to appear at room temperature. The switching effect is sensitive to temperature light intensity, and sample ickness as well. Switching parameters (i, V, P, E, and R OFF /R ON ) are found to depend on e surrounding condition as well as sample ickness. It is found at e i-v behavior indicates two regions: The OFF state wi a very high resistance and a negative differential resistivety state (NDR) region. The TlGaSe 2 wi such properties can be used as switching elements in switching devices. References [1] Gasanly, N.M., Serpenguzel, A., Gurlü O., Aydinli, A. and Yilmaz, I., Solid State Commun., 108: 525 (1998). [2] Abay, B., Güder, H.S., Efeoglu, H. and Yogurtcu, T.K., Turk. J. Phys., 25: 543 (01). [3] Müller, V.D. and Hahn, H.Z., Anorg. All. Chem., 438: 258 (1978).

13 Investigation of e Switching Phenomena in TlGaSe 2 Single Crystal 39 [4] Morrow, D.F., Cowley, R.A., Hatton P.D. and Banys, J., J. Physics: Condens. Matter, 2: 3699 (1996). [5] Guseinov, G.D., Aliyev, V.A. and Bagirzade, E.F., Materials Chem. and Phys., 13: 541 (1985). [6] Aliev, V.A. and Guseinov, G.D., Sov. Phys. Semicond., 19: 1195 (1985). [7] Vinogradov, E.A., Zhighin, G.N., Melnik, N.N., Subbotion, S.I., Panfilov, V.V., Allakhverdiev, K.R., Salaev E. Ya. and Nani, R. Kh., Phys. Stat. Sol., (b) 95: 383 (1979). [8] Henkel, W., Hochheimer, H. D., Carlone, C., Wernes, A., Ves, S. and Schnering, H.G. V., Phys. Rev., B (condensed matter) 26: 3211 (1982). [9] Qasrawi, A.F. and Gasanly, N.M., Mater. Rech. Bull., 39: 1353 (04). [10] Yuksek, N.S., Kavas, H., Gasanly, N.M. and Ozkan, H., Physica B: Conden. Matt., 344: 249 (04). [11] Alekperov, O.Z., J. Phys. and Chemist. of Solids, 64: 1707 (03). [12] Mikailov, E.A., Basaran, E., Sentürk, E., Tümbek, L., Mammadov, T.G. and Aliev, V.P., Sol. Stat. Commun., 129: 761(04) 1:147 (07). [13] Ibragimov, T.D., J. Low Temp. Phys. [14] Gasanly, N.M., Serpengüzel, A., Aydinli, A. and Baten, S.M. A., J. Luminescence, 86: 39 (00). [15] Samedov, S.R. and Baykan, O., Int. Infrared Millimeter Wave, 24: 231 (03). [16] Ashraf, I.M., Abdel-Rahman, M.M. and Badr, A.M., J. Phys. D. Appl. Phys., 36: 109 (03). [17] Nagat, A.T., Al Garni, S.E., Bahabri, F.S., Attia, G., Al Harbi, S.R. and Al Ghamdi, A.A., JKAU: Science (08) Under Publication. [18] Hussein, S.A. and Nagat, A.T., Crystal Res. Technol., 24: 283 (1989). [19] Gürbulak, B., Physica B, 293: 289 (01). [] Nagat, A.T., Hussein, S.A., Gameel, Y.H. and Gamal, G.A., Phys. Stat. Sol., (a) 121: 1(1990). [21] Al Ghamdi, A.A., Hussein, S.A. and Nassary, M.M., Material Sci. Res. India, 2: 107(04). [22] Abay, B., Gurbulak, B., Yidirim, M., Efeoglu, H. and Yogurtcu, Y.K., Phys. Stat. Sol. (a), 153: 145 (1996). [23] Böre, K.W. and Ovshinsky, S.R., J. Appl. Phys., 41: 2675 (1970). [24] Aliev, S.I., Niftiev, G.M., Pliev, F.I. and Tagiev, B.G., Sov. Phys. Semicond., 13: 340 (1979).\

14 40 S. A. Hussein, et al