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Switching phenomenon in TlGaSe2 layered semiconductor
Solid-State Electronics 94 (2014) 39–43
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Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Switching phenomenon in TlGaSe2 layered semiconductor MirHasanYu. Seyidov a,b,⇑, Rauf A. Suleymanov a,b, Ertan Balaban c, Yasin Sß ale a a
Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey Institute of Physics Azerbaijan National Academy of Sciences, AZ – 1143 Baku, Azerbaijan c TUBITAK-BILGEM, Scientific and Technical Research Council of Turkey, Gebze, Kocaeli 41470, Turkey b
a r t i c l e
i n f o
Article history: Received 31 May 2013 Received in revised form 13 January 2014 Accepted 2 February 2014 The review of this paper was arranged by Prof. S. Cristoloveanu Keywords: Switching effect Disordered semiconductors Ferroelectric–semiconductor Incommensurate phase
a b s t r a c t Electrical switching phenomenon was observed in TlGaSe2 layered ferroelectric–semiconductor applying different types of electrodes on different TlGaSe2 samples in both directions parallel and perpendicular to the pane of the layers. The non-linear current–voltage (CV) characteristics were measured by sweeping the current while measuring the voltage drop and could be classified as current-controlled S-type negative resistance phenomenon. The effects of temperature, illumination and as well as long time annealing within the incommensurate phase on the switching characteristics were also been studied. The switching phenomenon is discussed on the basis of the models widely used for disordered semiconductors. It was shown that TlGaSe2 crystal demonstrates the peculiar behavior that is typical to chalcogenide glassy semiconductors (CGS). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The current-controlled electrical switching from the high resistance semiconducting off-state to the conducting on-state in CGS is a well-known phenomenon discussed in various review articles (see, for example [1–5]). Different mechanisms have been proposed to explain the electrical switching in CGS. These include pure electronic, thermal and mixed electronic-thermal mechanisms. It is believed that switching may be purely thermal only in bulk samples whereas in thin-film samples, the role of electronic processes is crucial. It was shown in recent investigations [2,3] that ionization of the so called U-centers which is characteristic of CGS in strong electric fields increases the free-carrier concentration and causes a strong nonlinearity in the current–voltage characteristics. Joule heating increases the temperature of the specimen which increases the concentration of the free-carriers and leads to switching to the conductive state. The family of Tl-based ternary dichalcogenide semiconductors, which have layered or chain structure has been in the focus of researchers for decades, which are mainly because of the presence of structural phase transformations. Several examples of low-dimensional dichalcogenide semiconductors, such as quasione-dimensional chain TlInSe2 [6], TlInTe2 [7], TlGaTe2 [8] and ⇑ Corresponding author at: Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey. Tel.: +90 262 605 1329; fax: +90 262 653 8490. E-mail address: [email protected] (M. Seyidov). http://dx.doi.org/10.1016/j.sse.2014.02.001 0038-1101/Ó 2014 Elsevier Ltd. All rights reserved.
quasi-two-dimensional layered TlGaS2 [9] have been found to exhibit electrical switching. Ternary thallium dichalcogenide TlGaSe2 is known as a ferroelectric–semiconductor which has a layered crystalline structure and undergoes the thermally stimulated phase transitions [10–12] at temperatures lower than 120 K. It is a native p-type semiconductor and has band gap energy 2.2 eV at the room temperature. Several reports discussed the transport mechanisms in TlGaSe2 layered semiconductor [13–15]. The considerable interest remains regarding the mechanism of the current flow in the low-temperature region, especially below 220 K, where hopping conductivity is the main transport mechanism. Recent investigations on these crystals revealed some unusual physical properties [13,16–19]. One of the most interesting properties among these is the existence of large internal electric fields. These fields appear through the specific disorder phase which is observed within the wide temperature range below 200–220 K [12,16–19]. It is suggested that deep impurity states are responsible for such disorder, which strongly affects the electrical properties of this compound. The role of internal electric fields is extremely important as they can influence even thermal expansion processes, thus making TlGaSe2 crystals very sensitive to external perturbations such as illumination, external electric fields and annealing procedure [16,17]. Note that the effects of internal electric fields which were outlined above are mostly pronounced in the temperature region T > 140 K that is out of the temperature range of known phase transitions in this crystal.
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The main goal of the present study was to investigate in more detail the role of the above mentioned internal electric fields in TlGaSe2 crystals in current transport mechanism. We investigated switching effects in different TlGaSe2 samples, which had different contacts, in different directions namely parallel and perpendicular to the plane of the layers of the crystals. Experiments were conducted under different external effects like temperature, illumination and annealing. The observed switching phenomenon is discussed on the basis of the models that are widely used for disordered semiconductors. We demonstrate that TlGaSe2 single crystal behaves similarly to known chalcogenide glassy semiconductors. This finding helps understanding the extreme sensitivity of TlGaSe2 and probably other crystals from the same family to any external perturbation that influence specific defect centers in this type of crystals. Thus, TlGaSe2 type crystals which are usually considered single crystals demonstrate numerous peculiar properties that are characteristic of CGS. 2. Experiment TlGaSe2 single crystal was prepared using highly pure elements (at least 99.999%) taken in stoichiometric proportions. A single crystal was grown from the melt by the modified Bridgman–Stockberger method. Initially undoped p-type TlGaSe2 single crystals, which were picked from two different technological batches were used. The crystals had a high dark electrical resistivity (P106 X cm at 300 K). Four samples were prepared by splitting along the cleavage plane. The thickness of the samples was between 500 and 750 lm. Au and In contacts were formed on both sides of the samples by thermal evaporation under vacuum conditions. The distances between the electrodes of the samples that the current was measured in the direction parallel to the layers were 2 mm. To study the effect of the illumination, one of the electrodes in sandwich geometry made semitransparent. Below the text, the technological batches which the crystals were taken from are labeled by B and F, and the samples are abbreviated according to the technological batch, electrode type and geometry. For example, ‘‘sample B Au–In perpendicular’’ refers to the sample taken from the B technological batch with Au and In contacts in the direction perpendicular to the plane of the layers. The current–voltage measurements were taken over the temperature range 10–300 K. Electrical switching behavior of TlGaSe2 was studied by means of a source-measurement unit (SMU) with incremental current steps. A constant current was passed through the sample, and the voltage across the sample was measured by the SMU. The measurements were taken over a current range of 10 nA to 0.5 lA, where the voltage ranged from 0 to 30 V. A Janis closed cycle refrigeration system was used as the lowtemperature cooling device. A control sensor (diode DT-470) and a resistive control heater were mounted under the base and used to control the temperature to an accuracy of less than 0.1 K by using a Lake Shore-340 auto tuning temperature controller. The following annealing procedure was employed. Starting from the room temperature the sample was cooled down to 10 K and held at this temperature for 30 min and then it was heated up to Tann 113 K and annealed at this fixed temperature, which is within the incommensurate (INC) phase of the crystal for, about 5 h. Then the sample was cooled again down to 10 K and the I–V characteristics of TlGaSe2 were recorded at different stabilized temperatures over the temperature range of 10–300 K in the heating regime.
in Fig. 1. A switching behavior for all samples is clearly seen. At the threshold point, Vth, the conductivity of the samples changed drastically from the low-conductivity off-state to the high-conductivity on-state. It is well known [1] that in switching phenomena Vth correlates with the dark resistivity of samples, which means, the decrease in the resistivity of the sample leads to the lower value of Vth. For the same reason, the value of Vth decreases when temperature increases. Fig. 2a demonstrates the expected behavior of Vth for the sample F Au–In perpendicular. As shown in Fig. 1 the threshold voltage Vth increases sequentially among samples B Au–In perpendicular, B In–In perpendicular, F Au–In perpendicular and F Au–In parallel. The highest value of Vth for the sample F Au–In parallel which has the lowest resistivity (large slanting angle on I–V characteristics, Fig. 1) contradicts to the expected behavior. Another expectation was related to the effect of illumination of the absorbed light on the value of Vth. Fig. 2b demonstrates the expected behavior for the same sample F Au–In perpendicular: illumination with the light of k = 630 nm wavelength which is close to the band edge of TlGaSe2 crystal decreases notably the switching voltage. The striking difference with respect to the described behavior was observed while investigating the effect of illumination on the switching characteristic of the sample B, In–In perpendicular within the temperature range T 6 180 K (see Fig. 3a and b). For this sample, within this temperature range and with no illumination, the only observed state was the off-state. But for the same sample, under illumination condition switching behavior was observed. But, contrary to expectations (see Fig. 3b), illumination at k = 630 nm wavelength, substantially increased the switching voltage. Besides, there was an inverse correlation between the switching voltage Vth and temperature. The value of Vth decreased from 30 to 12.5 V while temperature decreased from 180 to 80 K (see Fig. 3b). Another series of electrical characteristics were measured after the annealing of the samples within the INC-phase at Tann = 113 K for about 5 h. Such annealing procedure is very specific for the crystals with INC-phase. According to the widely known model [20] the long time annealing within the INC-phase leads to the substantial rearrangement of the defects in the field of the modulated INC-structure. As a result, such rearrangement substantially modifies the impurity subsystem of the crystals and leads to the dramatic increase of the resistivity in all of the investigated samples. The effect of annealing on switching phenomenon is also
3. Experimental results I–V characteristics of all investigated samples which were measured by sweeping the current at 290 K, in the dark are compared
Fig. 1. I–V characteristics measured by sweeping the current at 290 K for all investigated samples.
M. Seyidov et al. / Solid-State Electronics 94 (2014) 39–43
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1. Two of the investigated samples, B Au–In perpendicular and F Au–In perpendicular demonstrate the expected behavior characteristics for switching phenomenon, i.e. higher conductivity leads to the lower switching voltage. The effect of illumination and annealing also can be understood if the decreasing or increasing resistance after the illumination and annealing, respectively, are taken into account. 2. At first glance, two samples, F Au–In parallel and B In–In perpendicular behave unusually. F Au–In parallel sample having the lowest resistance has the highest value of Vth. On the other hand, sample B In–In perpendicular exhibited an unusual behavior under the illumination, where Vth increases under the illumination, and Vth(T) dependence had the ‘‘inverse’’ character, i.e. Vth increases with temperature.
Fig. 2. The temperature dependence of the switching effect in the sample F Au–In perpendicular: (a) in dark; (b) under the illumination with the light k = 630 nm.
To clarify the non-standard behavior of these two samples, we further investigated the effect of illumination and annealing on both crystals electric characteristics in off-state. Fig. 5 demonstrates the distinctive behavior of B In–In perpendicular sample under the illumination. This sample had the highest photosensitivity in 630 nm wavelength range and photocurrent substantially decreases while increasing the temperature. This is reasonable because 630 nm wavelength range is very close to exciton transitions in TlGaSe2 crystals, which are very well pronounced in this sample. As a rule, the intensity of exciton lines in semiconductors substantially decreases with increase in the temperature, thus leading to the lower photo-conductivities in this spectral range. Effect of annealing on F Au–In parallel sample needs special consideration. As it was already shown annealing leads to increase of the resistivity in all of the investigated samples and it is the main reason explaining the substantial increase of Vth in all samples. The sample F Au–In parallel demonstrates the opposite behavior of Vth although it acquires the highest resistance after the annealing procedure. So, one can come to the conclusion, that the highest value of the conductivity in this sample is not the only deciding factor in switching phenomenon.
Fig. 3. The temperature dependence of the switching effect in the sample B In–In perpendicular: (a) in dark; (b) under the illumination with the light k = 630 nm. The scales in figures ‘‘a’’ and ‘‘b’’ are different.
appeared to be rather impressive: the annealing leads to a substantial increase of Vth. As a result, switching effect began to register only at sufficiently high temperatures close to 300 K. The only exception was the sample F, Au–In parallel with the highest Vth at usual conditions (see Fig. 1). This sample started to switch at lower voltages and even at lower temperatures, Fig. 4a and b. Below, we summarize obtained results for samples demonstrating the nontrivial behavior.
Fig. 4. The temperature dependence of the switching effect in the sample F Au–In parallel: (a) in dark under normal conditions; (b) in dark after the sample annealing within the INC-phase at 113 K for 5 h. The scales in figures (a) and (b) are different.
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data and analysis of different transport mechanisms in chalcogenide glasses [2] shows that it is difficult to identify a particular mechanism. That further studies are required to discriminate between the different mechanisms. Among the most available mechanisms of switching effect in chalcogenide glasses following models exist: Poole–Frenkel ionization of the impurity states, field-induced delocalization of tail states, space-charge limited current, transport through crystalline inclusions, etc. Much work needs to be done before a full explanation of the electrical switching in TlGaSe2 can be provided. In the present paper, we have focused only on experimental observations of the resistance switching phenomena in TlGaSe2 without a comparison to theoretical models. However, some important points can be addressed.
Fig. 5. Photoconductivity spectra of sample B In–In measured perpendicular to layers at different temperatures.
4. Discussion We start our discussion by analyzing the role of conductivity in switching phenomena. As it became clear from the above considerations, most of the findings can be explained on the basis of the simple principle: the higher the conductivity (dark conductivity, photoconductivity, photoconductivity after the annealing procedure) the lower the value of the switching voltage Vth. Almost all discussed theoretical models of switching effect [1–5] are not contrary to this conclusion. Certainly, this is true within the framework of specific band structure and especially specific defect states, which are characteristic of disordered semiconductors. So, the main question which arises within the context of our investigations is: in what extent TlGaSe2 crystals can be considered as semiconductors with disordered structure with all of the consequences that come out of such assumption? Structural investigations of TlGaSe2 crystals as well as other crystals of the same family show that these materials grown by a usual Bridgman technique have well apparent crystal structure. As it was already mentioned, a number of recent investigations, including the present one indicate that the disorder is one of the characteristic features of TlGaSe2 crystals. For example, an analysis of temperature dependence of an optical absorption edge in TlGaSe2 reveals an unusual behavior of the Urbach tail, which is characteristic to glassy type semiconductors [19]. In series of investigations of the current transport mechanisms in TlGaSe2 crystals [13,18,19] it was shown that these crystals can be considered as inhomogeneous semiconductors. The metastable disordered state in TlGaSe2 was predicted from non-structural experiments [13,18,19,21,22]. From those experiments, it was concluded that the disorder phase is formed either after cooling the crystal below the so-called «glass» transition temperature 200 K or by isothermal long time annealing of crystal inside the INC-phase. In this regard, the effect of annealing is of special interest. It was shown in [19] that annealing within the INC-phase cardinally transforms the Urbach tail in TlGaSe2 crystals: long time annealing within the INC-phase leads to the glassy type Urbach rule. In that case, it becomes possible to explain the unusual behavior of Vth in F In–In parallel sample after the annealing. Annealing that leads to the glassy like structure and decreases the conductivity creates also favorable conditions for easy switching analogous to that in disordered semiconductor. The mechanism of switching in even known disordered semiconductors is not yet clear. An overview of the experimental
1. The electrical switching effect refers to reversible changes between two metastable resistance states of the materials induced by an external voltage. It is possible, that one of the resistive phases of TlGaSe2 crystals is associated with the perfect crystal structure and another one with a disorder of it. The specific feature of such heterogeneous structure is the peculiar sensitivity to temperature, illumination and annealing within the INC-phase. Illumination and/or annealing within the INC-phase may greatly affect the disordered structure network; thus transform the electronic subsystem of TlGaSe2. So, we base on the electronic origin of the switching effect in TlGaSe2 crystals. 2. By considering the experimental results obtained from other materials, which exhibit electrical switching behavior and theoretical models discussed we come at the conclusion that the most acceptable mechanism of the switching effect in TlGaSe2-type crystals which takes into account the whole set of existing experimental findings is the mechanism of electrical transport through the crystalline structure incorporating the disordered inclusions. The model of disordered semiconductor reveals also other peculiar properties of TlGaSe2 crystals that we observed before. For example, the great sensitivity of thermal expansion processes in TlGaSe2 to external perturbations like illumination, external electric fields and annealing becomes clear because such sensitivity is the peculiar property of chalcogenide glasses [23,24]. 5. Conclusion In conclusion, the resistance switching phenomenon was investigated in TlGaSe2 single crystals. These studies indicate that a current-controlled S-type switching is characteristic for these crystals in both directions parallel and perpendicular to the plane of layers. The effects of temperature, light illumination and annealing within the INC-phase on the switching characteristics were studied. The switching phenomenon is discussed based on the models widely used for disordered chalcogenide semiconductors. The most appropriate mechanism of the switching effect behind TlGaSe2-type crystals is the mechanism of electrical transport through the crystalline structure incorporating the disordered inclusions. Thus, it was shown that TlGaSe2 crystals behave in many respects as chalcogenide glassy semiconductors and demonstrate the peculiar behavior which is typical for such type semiconductors. References [1] [2] [3] [4] [5]
Adler D, Henisch HK, Mott SN. Rev Mod Phys 1978;50:209. Bogoslowsky NA, Tsendin KD. Semiconductors 2012;46:577. Nardone M, Simon M, Karpov IV, Karpov VG. J Appl Phys 2012;112:071101. Karpov VG, Nardone M. Appl Phys Lett 2013;102:236101. Redaelli A, Pirovano A, Benvenuti A, Lacaita AL. J Appl Phys 2008;103:111101.
M. Seyidov et al. / Solid-State Electronics 94 (2014) 39–43 [6] Karakotsou Ch, Anagnastopoulos AN. Physica D 1996;93:157. [7] Al-Ghamdi AA, Nagat AT, Bahabri FS, Al-Orainy RH, Al-Harbi SR, Al-Hazmi FS. J Alloy Compd 2009;484:561. [8] Hanias MP, Anagnastopoulos AN. Phys Rev B 1993;47:4261. [9] Al Ghamdi AA, Nagat AT, Bahabri FS, Al Orainy RH, Al Garni SE. Appl Surf Sci 2011;257:3205. [10] Moorrow DFMc, Cowley RA, Hatton PD, Banys J. J Phys: Condens Mater 1990;2:3699. [11] Kashida S, Kobayashi Y. J Korean Phys Soc 1998;32(Part 1 Suppl. S):40. [12] Henkel W, Hochheimer HD, Carlone C, Werner A, Ves S, Vonschnering HG. Phys Rev B 1982;26:3211. [13] Seyidov M-HYu, Suleymanov RA, Sahin Y. Solid State Sci 2008;10:1666. [14] Mustavaeva SN, Aliyev VA, Asadov MM. Phys Solid State 1998;40:48.
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Qasrawi AF, Gasanly NM. Semicond Sci Technol 2004;19:505. Seyidov M-HYu, Suleymanov RA, Yakar E. J Appl Phys 2009;106:023532. Seyidov M-HYu, Suleymanov RA. J Appl Phys 2010;108:1. Seyidov M-HYu, Suleymanov RA, Bakis Y, Salehli F. J Appl Phys 2010;108:074114. Seyidov M-HYu, Suleymanov RA, S ß ale Y. J Appl Phys 2012;112:103106. Jamet JP. Phase Transitions 1988;11:335. Abdullaeva SG, Aliev VA, Mamedov NT, Sheynkman MK. Semiconductors 1983;17:1787. Anikyev AA, Burlakov VM, Yakheev MP. Sov Phys Solid State 1990;32:2131. Shimakawa K, Kolobov A, Elliot SR. Adv Phys 1995;44:475. Tanaka K, Shimakawa K. Amorphous Chalcogenide Semiconductors and Related Materials. Springer; 2011.
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Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Switching phenomenon in TlGaSe2 layered semiconductor MirHasanYu. Seyidov a,b,⇑, Rauf A. Suleymanov a,b, Ertan Balaban c, Yasin Sß ale a a
Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey Institute of Physics Azerbaijan National Academy of Sciences, AZ – 1143 Baku, Azerbaijan c TUBITAK-BILGEM, Scientific and Technical Research Council of Turkey, Gebze, Kocaeli 41470, Turkey b
a r t i c l e
i n f o
Article history: Received 31 May 2013 Received in revised form 13 January 2014 Accepted 2 February 2014 The review of this paper was arranged by Prof. S. Cristoloveanu Keywords: Switching effect Disordered semiconductors Ferroelectric–semiconductor Incommensurate phase
a b s t r a c t Electrical switching phenomenon was observed in TlGaSe2 layered ferroelectric–semiconductor applying different types of electrodes on different TlGaSe2 samples in both directions parallel and perpendicular to the pane of the layers. The non-linear current–voltage (CV) characteristics were measured by sweeping the current while measuring the voltage drop and could be classified as current-controlled S-type negative resistance phenomenon. The effects of temperature, illumination and as well as long time annealing within the incommensurate phase on the switching characteristics were also been studied. The switching phenomenon is discussed on the basis of the models widely used for disordered semiconductors. It was shown that TlGaSe2 crystal demonstrates the peculiar behavior that is typical to chalcogenide glassy semiconductors (CGS). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The current-controlled electrical switching from the high resistance semiconducting off-state to the conducting on-state in CGS is a well-known phenomenon discussed in various review articles (see, for example [1–5]). Different mechanisms have been proposed to explain the electrical switching in CGS. These include pure electronic, thermal and mixed electronic-thermal mechanisms. It is believed that switching may be purely thermal only in bulk samples whereas in thin-film samples, the role of electronic processes is crucial. It was shown in recent investigations [2,3] that ionization of the so called U-centers which is characteristic of CGS in strong electric fields increases the free-carrier concentration and causes a strong nonlinearity in the current–voltage characteristics. Joule heating increases the temperature of the specimen which increases the concentration of the free-carriers and leads to switching to the conductive state. The family of Tl-based ternary dichalcogenide semiconductors, which have layered or chain structure has been in the focus of researchers for decades, which are mainly because of the presence of structural phase transformations. Several examples of low-dimensional dichalcogenide semiconductors, such as quasione-dimensional chain TlInSe2 [6], TlInTe2 [7], TlGaTe2 [8] and ⇑ Corresponding author at: Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey. Tel.: +90 262 605 1329; fax: +90 262 653 8490. E-mail address: [email protected] (M. Seyidov). http://dx.doi.org/10.1016/j.sse.2014.02.001 0038-1101/Ó 2014 Elsevier Ltd. All rights reserved.
quasi-two-dimensional layered TlGaS2 [9] have been found to exhibit electrical switching. Ternary thallium dichalcogenide TlGaSe2 is known as a ferroelectric–semiconductor which has a layered crystalline structure and undergoes the thermally stimulated phase transitions [10–12] at temperatures lower than 120 K. It is a native p-type semiconductor and has band gap energy 2.2 eV at the room temperature. Several reports discussed the transport mechanisms in TlGaSe2 layered semiconductor [13–15]. The considerable interest remains regarding the mechanism of the current flow in the low-temperature region, especially below 220 K, where hopping conductivity is the main transport mechanism. Recent investigations on these crystals revealed some unusual physical properties [13,16–19]. One of the most interesting properties among these is the existence of large internal electric fields. These fields appear through the specific disorder phase which is observed within the wide temperature range below 200–220 K [12,16–19]. It is suggested that deep impurity states are responsible for such disorder, which strongly affects the electrical properties of this compound. The role of internal electric fields is extremely important as they can influence even thermal expansion processes, thus making TlGaSe2 crystals very sensitive to external perturbations such as illumination, external electric fields and annealing procedure [16,17]. Note that the effects of internal electric fields which were outlined above are mostly pronounced in the temperature region T > 140 K that is out of the temperature range of known phase transitions in this crystal.
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The main goal of the present study was to investigate in more detail the role of the above mentioned internal electric fields in TlGaSe2 crystals in current transport mechanism. We investigated switching effects in different TlGaSe2 samples, which had different contacts, in different directions namely parallel and perpendicular to the plane of the layers of the crystals. Experiments were conducted under different external effects like temperature, illumination and annealing. The observed switching phenomenon is discussed on the basis of the models that are widely used for disordered semiconductors. We demonstrate that TlGaSe2 single crystal behaves similarly to known chalcogenide glassy semiconductors. This finding helps understanding the extreme sensitivity of TlGaSe2 and probably other crystals from the same family to any external perturbation that influence specific defect centers in this type of crystals. Thus, TlGaSe2 type crystals which are usually considered single crystals demonstrate numerous peculiar properties that are characteristic of CGS. 2. Experiment TlGaSe2 single crystal was prepared using highly pure elements (at least 99.999%) taken in stoichiometric proportions. A single crystal was grown from the melt by the modified Bridgman–Stockberger method. Initially undoped p-type TlGaSe2 single crystals, which were picked from two different technological batches were used. The crystals had a high dark electrical resistivity (P106 X cm at 300 K). Four samples were prepared by splitting along the cleavage plane. The thickness of the samples was between 500 and 750 lm. Au and In contacts were formed on both sides of the samples by thermal evaporation under vacuum conditions. The distances between the electrodes of the samples that the current was measured in the direction parallel to the layers were 2 mm. To study the effect of the illumination, one of the electrodes in sandwich geometry made semitransparent. Below the text, the technological batches which the crystals were taken from are labeled by B and F, and the samples are abbreviated according to the technological batch, electrode type and geometry. For example, ‘‘sample B Au–In perpendicular’’ refers to the sample taken from the B technological batch with Au and In contacts in the direction perpendicular to the plane of the layers. The current–voltage measurements were taken over the temperature range 10–300 K. Electrical switching behavior of TlGaSe2 was studied by means of a source-measurement unit (SMU) with incremental current steps. A constant current was passed through the sample, and the voltage across the sample was measured by the SMU. The measurements were taken over a current range of 10 nA to 0.5 lA, where the voltage ranged from 0 to 30 V. A Janis closed cycle refrigeration system was used as the lowtemperature cooling device. A control sensor (diode DT-470) and a resistive control heater were mounted under the base and used to control the temperature to an accuracy of less than 0.1 K by using a Lake Shore-340 auto tuning temperature controller. The following annealing procedure was employed. Starting from the room temperature the sample was cooled down to 10 K and held at this temperature for 30 min and then it was heated up to Tann 113 K and annealed at this fixed temperature, which is within the incommensurate (INC) phase of the crystal for, about 5 h. Then the sample was cooled again down to 10 K and the I–V characteristics of TlGaSe2 were recorded at different stabilized temperatures over the temperature range of 10–300 K in the heating regime.
in Fig. 1. A switching behavior for all samples is clearly seen. At the threshold point, Vth, the conductivity of the samples changed drastically from the low-conductivity off-state to the high-conductivity on-state. It is well known [1] that in switching phenomena Vth correlates with the dark resistivity of samples, which means, the decrease in the resistivity of the sample leads to the lower value of Vth. For the same reason, the value of Vth decreases when temperature increases. Fig. 2a demonstrates the expected behavior of Vth for the sample F Au–In perpendicular. As shown in Fig. 1 the threshold voltage Vth increases sequentially among samples B Au–In perpendicular, B In–In perpendicular, F Au–In perpendicular and F Au–In parallel. The highest value of Vth for the sample F Au–In parallel which has the lowest resistivity (large slanting angle on I–V characteristics, Fig. 1) contradicts to the expected behavior. Another expectation was related to the effect of illumination of the absorbed light on the value of Vth. Fig. 2b demonstrates the expected behavior for the same sample F Au–In perpendicular: illumination with the light of k = 630 nm wavelength which is close to the band edge of TlGaSe2 crystal decreases notably the switching voltage. The striking difference with respect to the described behavior was observed while investigating the effect of illumination on the switching characteristic of the sample B, In–In perpendicular within the temperature range T 6 180 K (see Fig. 3a and b). For this sample, within this temperature range and with no illumination, the only observed state was the off-state. But for the same sample, under illumination condition switching behavior was observed. But, contrary to expectations (see Fig. 3b), illumination at k = 630 nm wavelength, substantially increased the switching voltage. Besides, there was an inverse correlation between the switching voltage Vth and temperature. The value of Vth decreased from 30 to 12.5 V while temperature decreased from 180 to 80 K (see Fig. 3b). Another series of electrical characteristics were measured after the annealing of the samples within the INC-phase at Tann = 113 K for about 5 h. Such annealing procedure is very specific for the crystals with INC-phase. According to the widely known model [20] the long time annealing within the INC-phase leads to the substantial rearrangement of the defects in the field of the modulated INC-structure. As a result, such rearrangement substantially modifies the impurity subsystem of the crystals and leads to the dramatic increase of the resistivity in all of the investigated samples. The effect of annealing on switching phenomenon is also
3. Experimental results I–V characteristics of all investigated samples which were measured by sweeping the current at 290 K, in the dark are compared
Fig. 1. I–V characteristics measured by sweeping the current at 290 K for all investigated samples.
M. Seyidov et al. / Solid-State Electronics 94 (2014) 39–43
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1. Two of the investigated samples, B Au–In perpendicular and F Au–In perpendicular demonstrate the expected behavior characteristics for switching phenomenon, i.e. higher conductivity leads to the lower switching voltage. The effect of illumination and annealing also can be understood if the decreasing or increasing resistance after the illumination and annealing, respectively, are taken into account. 2. At first glance, two samples, F Au–In parallel and B In–In perpendicular behave unusually. F Au–In parallel sample having the lowest resistance has the highest value of Vth. On the other hand, sample B In–In perpendicular exhibited an unusual behavior under the illumination, where Vth increases under the illumination, and Vth(T) dependence had the ‘‘inverse’’ character, i.e. Vth increases with temperature.
Fig. 2. The temperature dependence of the switching effect in the sample F Au–In perpendicular: (a) in dark; (b) under the illumination with the light k = 630 nm.
To clarify the non-standard behavior of these two samples, we further investigated the effect of illumination and annealing on both crystals electric characteristics in off-state. Fig. 5 demonstrates the distinctive behavior of B In–In perpendicular sample under the illumination. This sample had the highest photosensitivity in 630 nm wavelength range and photocurrent substantially decreases while increasing the temperature. This is reasonable because 630 nm wavelength range is very close to exciton transitions in TlGaSe2 crystals, which are very well pronounced in this sample. As a rule, the intensity of exciton lines in semiconductors substantially decreases with increase in the temperature, thus leading to the lower photo-conductivities in this spectral range. Effect of annealing on F Au–In parallel sample needs special consideration. As it was already shown annealing leads to increase of the resistivity in all of the investigated samples and it is the main reason explaining the substantial increase of Vth in all samples. The sample F Au–In parallel demonstrates the opposite behavior of Vth although it acquires the highest resistance after the annealing procedure. So, one can come to the conclusion, that the highest value of the conductivity in this sample is not the only deciding factor in switching phenomenon.
Fig. 3. The temperature dependence of the switching effect in the sample B In–In perpendicular: (a) in dark; (b) under the illumination with the light k = 630 nm. The scales in figures ‘‘a’’ and ‘‘b’’ are different.
appeared to be rather impressive: the annealing leads to a substantial increase of Vth. As a result, switching effect began to register only at sufficiently high temperatures close to 300 K. The only exception was the sample F, Au–In parallel with the highest Vth at usual conditions (see Fig. 1). This sample started to switch at lower voltages and even at lower temperatures, Fig. 4a and b. Below, we summarize obtained results for samples demonstrating the nontrivial behavior.
Fig. 4. The temperature dependence of the switching effect in the sample F Au–In parallel: (a) in dark under normal conditions; (b) in dark after the sample annealing within the INC-phase at 113 K for 5 h. The scales in figures (a) and (b) are different.
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data and analysis of different transport mechanisms in chalcogenide glasses [2] shows that it is difficult to identify a particular mechanism. That further studies are required to discriminate between the different mechanisms. Among the most available mechanisms of switching effect in chalcogenide glasses following models exist: Poole–Frenkel ionization of the impurity states, field-induced delocalization of tail states, space-charge limited current, transport through crystalline inclusions, etc. Much work needs to be done before a full explanation of the electrical switching in TlGaSe2 can be provided. In the present paper, we have focused only on experimental observations of the resistance switching phenomena in TlGaSe2 without a comparison to theoretical models. However, some important points can be addressed.
Fig. 5. Photoconductivity spectra of sample B In–In measured perpendicular to layers at different temperatures.
4. Discussion We start our discussion by analyzing the role of conductivity in switching phenomena. As it became clear from the above considerations, most of the findings can be explained on the basis of the simple principle: the higher the conductivity (dark conductivity, photoconductivity, photoconductivity after the annealing procedure) the lower the value of the switching voltage Vth. Almost all discussed theoretical models of switching effect [1–5] are not contrary to this conclusion. Certainly, this is true within the framework of specific band structure and especially specific defect states, which are characteristic of disordered semiconductors. So, the main question which arises within the context of our investigations is: in what extent TlGaSe2 crystals can be considered as semiconductors with disordered structure with all of the consequences that come out of such assumption? Structural investigations of TlGaSe2 crystals as well as other crystals of the same family show that these materials grown by a usual Bridgman technique have well apparent crystal structure. As it was already mentioned, a number of recent investigations, including the present one indicate that the disorder is one of the characteristic features of TlGaSe2 crystals. For example, an analysis of temperature dependence of an optical absorption edge in TlGaSe2 reveals an unusual behavior of the Urbach tail, which is characteristic to glassy type semiconductors [19]. In series of investigations of the current transport mechanisms in TlGaSe2 crystals [13,18,19] it was shown that these crystals can be considered as inhomogeneous semiconductors. The metastable disordered state in TlGaSe2 was predicted from non-structural experiments [13,18,19,21,22]. From those experiments, it was concluded that the disorder phase is formed either after cooling the crystal below the so-called «glass» transition temperature 200 K or by isothermal long time annealing of crystal inside the INC-phase. In this regard, the effect of annealing is of special interest. It was shown in [19] that annealing within the INC-phase cardinally transforms the Urbach tail in TlGaSe2 crystals: long time annealing within the INC-phase leads to the glassy type Urbach rule. In that case, it becomes possible to explain the unusual behavior of Vth in F In–In parallel sample after the annealing. Annealing that leads to the glassy like structure and decreases the conductivity creates also favorable conditions for easy switching analogous to that in disordered semiconductor. The mechanism of switching in even known disordered semiconductors is not yet clear. An overview of the experimental
1. The electrical switching effect refers to reversible changes between two metastable resistance states of the materials induced by an external voltage. It is possible, that one of the resistive phases of TlGaSe2 crystals is associated with the perfect crystal structure and another one with a disorder of it. The specific feature of such heterogeneous structure is the peculiar sensitivity to temperature, illumination and annealing within the INC-phase. Illumination and/or annealing within the INC-phase may greatly affect the disordered structure network; thus transform the electronic subsystem of TlGaSe2. So, we base on the electronic origin of the switching effect in TlGaSe2 crystals. 2. By considering the experimental results obtained from other materials, which exhibit electrical switching behavior and theoretical models discussed we come at the conclusion that the most acceptable mechanism of the switching effect in TlGaSe2-type crystals which takes into account the whole set of existing experimental findings is the mechanism of electrical transport through the crystalline structure incorporating the disordered inclusions. The model of disordered semiconductor reveals also other peculiar properties of TlGaSe2 crystals that we observed before. For example, the great sensitivity of thermal expansion processes in TlGaSe2 to external perturbations like illumination, external electric fields and annealing becomes clear because such sensitivity is the peculiar property of chalcogenide glasses [23,24]. 5. Conclusion In conclusion, the resistance switching phenomenon was investigated in TlGaSe2 single crystals. These studies indicate that a current-controlled S-type switching is characteristic for these crystals in both directions parallel and perpendicular to the plane of layers. The effects of temperature, light illumination and annealing within the INC-phase on the switching characteristics were studied. The switching phenomenon is discussed based on the models widely used for disordered chalcogenide semiconductors. The most appropriate mechanism of the switching effect behind TlGaSe2-type crystals is the mechanism of electrical transport through the crystalline structure incorporating the disordered inclusions. Thus, it was shown that TlGaSe2 crystals behave in many respects as chalcogenide glassy semiconductors and demonstrate the peculiar behavior which is typical for such type semiconductors. References [1] [2] [3] [4] [5]
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