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Preillumination – Induced change of electronic transport properties of TlGaSe2 semiconductor
Accepted Manuscript Preillumination – Induced Change of Electronic Transport Properties of TlGaSe2 Semiconductor Mir HasanYu. Seyidov, Rauf A. Suleymanov, Ertan Balaban, Yasin Şale PII:
S1293-2558(14)00102-2
DOI:
10.1016/j.solidstatesciences.2014.04.009
Reference:
SSSCIE 4932
To appear in:
Solid State Sciences
Received Date: 17 December 2013 Revised Date:
17 March 2014
Accepted Date: 20 April 2014
Please cite this article as: M.H. Seyidov, R.A. Suleymanov, E. Balaban, Y. Şale, Preillumination – Induced Change of Electronic Transport Properties of TlGaSe2 Semiconductor, Solid State Sciences (2014), doi: 10.1016/j.solidstatesciences.2014.04.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preillumination – Induced Change of Electronic Transport Properties of TlGaSe2 Semiconductor
1 2
Department of Physics, Gebze Institute of Technology, 41400, Gebze, Kocaeli, Turkey
Institute of Physics Azerbaijan National Academy of Sciences, AZ - 1143 Baku, Azerbaijan
TUBITAK-BILGEM, Scientific and Technical Research Council of Turkey, Gebze, Kocaeli 41470, Turkey
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Abstract
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3
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Мir HasanYu. Seyidov 1, 2*, Rauf A. Suleymanov 1, 2, Ertan Balaban1, 3, and Yasin Şale1
The effect of the pre-illumination on the dark and the photo - conductivity of the TlGaSe2 layered semiconductor is investigated within the temperature range of 80 – 300 K. After the illumination predominantly at high temperatures, a substantial decrease in both dark and photo - conductivities
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was observed. Observed phenomena resemble the Staebler - Wronski effect, which is typical for the amorphous semiconductors. The main contribution of this work is to show that the TlGaSe2 single crystals with well apparent crystalline structure can demonstrate certain characteristics peculiar to
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amorphous semiconductors.
Keywords: Amophous semiconductor; Chalcogenide semiconductors; The Staebler - Wronski effect; Photoconductivity;
Corresponding author address: Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey. Tel.: +90 262 605 1329; fax: +90 262 605 1305 E-mail address: [email protected] (MH.Yu.Seyidov)
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1. Introduction Ternary thallium gallium dichalcogenide (TlGaSe2) single crystal, which belongs to a group of
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layered semiconductors, displays a number of unique physical properties, such as photo – induced changes of dielectric and thermal properties [1 - 3]. According to [2, 3], the linear thermal expansion coefficients of TlGaSe2 are significantly affected by a light exposure treatment. This
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motivates us to study transport properties of TlGaSe2 after the light illumination treatment of
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crystals.
Indeed, our previous research demonstrated the electric field induced changes in electronic transport properties and an unusual Urbach tail behavior near the absorption edge of TlGaSe2 crystals [4 - 5]. It was shown that observed effects are very similar to those observed in amorphous semiconductors. One of the most frequently mentioned effects inherent to amorphous
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semiconductor is the Staebler - Wronski effect, i.e. the reduction of dark conductivity and photoconductivity of amorphous silicon upon extended exposure to light [6 - 8].
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The objective of this study was to investigate the role of preillumination on the current transport mechanism of TlGaSe2 semiconductors. Various samples were picked from several technological
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batches and with various pairs of metallic contacts, which were formed by evaporating on the cleaved surfaces. The parameters regarding the transport properties were measured both parallel and perpendicular to the plane of layers of the crystals. We study the role of the illumination by exposing the samples to white light while cooling them from room temperature down to 80 K, after which the measurements regarding the electronic transport properties are carried out. The experimental findings unambiguously show that the
ACCEPTED MANUSCRIPT 3 preillumination had substantially decreased the electrical conductivity and the photo – conductivity, which is similar to the Staebler – Wronski effect [5 - 6]. As a result, we show that the TlGaSe2
semiconductors. 2. Experiment
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single bulk semiconductor demonstrates peculiar properties, which are characteristic to amorphous
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Investigated TlGaSe2 single crystals were grown using elements taken in stoichiometric proportions. A single crystal was grown from the melt by the modified Bridgman – Stockberger
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method. Undoped p – type TlGaSe2 single crystals that have a high dark electrical resistivity (> 106 Ω·cm at 300 K) were cleaved from different technological batches and then used throughout experiments. The composition of the samples was examined by a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) facilities. It was found [7] that samples had a
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negligible amount of native impurities, such as carbon, oxygen and silicon, which are usually contained in undoped TlGaSe2 crystals. EDX data demonstrate further that investigated samples have an excess of Tl atoms and deficit of Se atoms: Tl (28.87 at%), Ga (26.23 at%) and Se (44.90
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at%). It is well known that impurities do not determine the p - type conductivity in most of chalcogenide semiconductors and it is the deviation from the stoichiometry that probably leads to p
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- type conductivity regardless of the doping. Under vacuum conditions, Au, In and Cu - metallic contacts were prepared by evaporation on both sides of the high – quality cleavage surface of the TlGaSe2 crystals. Then these contacts were used to measure the conductivity in directions parallel and perpendicular to the plane of the layers of TlGaSe2 crystals.
ACCEPTED MANUSCRIPT 4 The experiments were performed over the temperature range of 80 – 300 K. A Janis closed cycle helium refrigerator, which is equipped with glass windows for optical measurements, was used as the cooling device. A control sensor (diode DT - 470) and a resistive control heater were
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mounted under the base and used to control the temperature within accuracy of ∼ 0.1 K by using a Lake Shore - 340 auto tuning temperature controller. All of the measurements were made within a
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running vacuum of the order of 10−3 mbar.
The measurements were performed on three different TlGaSe2 bulk samples which had a
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thickness of ~ 500 - 750 µm. When the current was measured in the direction parallel to the layers, the distances between the electrodes of the samples, were within ~ 2 mm range. Below the investigated samples are referred as F, U and Y.
A 300 W Thermo Oriel Xenon Lamp was used as the light source. The radiation from this light
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source was passed through a Triax 550 - type UV - visible spectrophotometer to form the monochromatic light within the wavelength range of 300 - 900 nm with 2 nm steps. Prepared TlGaSe2 samples were exposed to this light for photoconductivity studies. The light beam was
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directed perpendicular to the surface of the crystal, on which the contact was made. The photocurrent signal corresponding to different wavelengths was measured by means of a
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programmable electrometer (Keithley 6517A) from which the data were automatically sampled and recorded. The bias voltage of 15 V which was applied to the samples through photocurrent measurements was also generated by the electrometer. Both the spectrophotometer and the electrometer were controlled via GPIB connection by software running on a personal computer. The whole measurement system was shielded against external interference using cooper sheets. The temperature fluctuation at which the measurements were carried out was within ± 0.05 limits.
ACCEPTED MANUSCRIPT 5 Through the photocurrent calculations the spectral distribution of the light source was taken into account. The change of electronic transport properties of TlGaSe2 induced by the preillumination was
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determined using the following procedure. Prior to the measurements, each TlGaSe2 sample was subjected to the white light illumination while it was cooled from room temperature down to 80 K during ~1 hour. Excitation of a sample was provided by a commercial high power Light Emitting
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Diode (LED) that was placed near the sample’s surface and had an approximate power of 0.8 mW/cm2. After temperature reached to 80 K the illumination source was turned off. Then, the
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electronic transport properties of the sample were investigated by photoconductivity measurements carried out in the heating regime within 80 – 300 K temperature range and 10 K increments. Each set of experiments took approximately 5 hours. After turning the exposit light off, the electronic transport properties of TlGaSe2 are retained for a long time and in the whole temperature range.
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During this time interval, the effect of the illumination was clearly detected. So, the relaxation time of the observed effects can be assumed as 5 hours or more. It is well known that annealing of the amorphous semiconductor at high temperatures (usually higher than 300K) for a few hours reverses
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the Staebler – Wronski effect. Annealing can also take place at lower temperatures, however the process takes a longer amount of time. For that reason we undertake the long time annealing of
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TlGaSe2 samples at and above the room temperature. As a result the original transport properties of the samples were fully recovered. 3. Experimental Results
For the sample Y with Au and Cu contacts, the dependence of the photocurrent to the temperature that is measured perpendicular to the plane of layers is depicted on the Fig 1. The measurements were carried out for 3 selected wavelengths (namely 310 nm, 608 nm and 824 nm) as
ACCEPTED MANUSCRIPT 6 well as without the preillumination. Additionally, the dependence of dark current to the temperature is also depicted. Based on the Fig.1 following conclusions can be made. Starting from 200 K, both dark and photocurrent increase exponentially with respect to the temperature. Cooling under the
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illumination significantly decreases both dark and photocurrent in the low temperature region, T < 200 K. Besides, the activation energies obtained from the exponentially increasing part of the dark conductivities of normal and preilluminated cases appear to be different (see Fig.1a).
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Preillumination increases the activation energy and makes it two times higher than that in normal case (0.150 eV). Indeed the substantial decrease of the conductivity and photoconductivity after the
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preillumination is a well known property of amorphous semiconductors [6, 8 - 10]. It is interesting to note that a similar behavior of the dark and photocurrent relative to the temperature is also a well known characteristic of the conduction mechanism of amorphous semiconductors [11].
Fig. 2 depicts the photo – conductivity spectra for F sample with Au and In contacts for both
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directions parallel and perpendicular to the plane of the layers with and without preillumination. As it can be seen, the effect of illumination is clearly manifested in both directions. Note also that the
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effect of preillumination is more pronounced in the direction parallel to the layers of the crystal.
As it can be seen from the Fig.2 the well pronounced peak near the 606 nm is observed which is
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usually attributed to the excitonic state in TlGaSe2 crystals [12]. The influence of preillumination on this peak is relatively weak, whereas the effect of preillumination is particularly noticed in the short wavelengths region, λ < 350 nm. This finding together with the more effective influence of the preillumination in the direction parallel to the plane of layers indicates the important role of the surface in the observed phenomena.
ACCEPTED MANUSCRIPT 7 While detailed information is provided in Discussion section, it is suggested that some type of photochemical reaction is realized during the illumination, which leads to the observed phenomenon. For the realization of the photochemical reaction, it is essential to illuminate the
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samples at sufficiently high temperatures that are close to 300 K. The measurements carried out after cooling the samples from room temperature down to 80 K in dark and then exposing them to the same exciting white light source revealed completely different results. In this case, the effect of
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illumination is the same as in the case of well known thermally stimulated current measurements: after illumination only at 80 K, the current measured at low temperatures increases due to the filling
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of some trap states. This difference in the characteristics of current transport is an important feature which allows us to distinguishing the effect of photochemical reaction from other photo induced phenomena. Fig. 3 depicts the importance of the illumination which starts from the room temperature and the illumination performed only at 80 K.
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In conclusion, preillumination of the TlGaSe2 samples has a great impact on both the dark and the photoconductivity of TlGaSe2 crystals. At low temperatures (~ 80K), the decrease in the conductivity can be more than an order of the magnitude. The activation energy, which is obtained
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from the exponentially increasing part of the conductivity, doubles after the preillumination. Mostly
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the effect becomes apparent when measuring the conductivity parallel to the plane of the layers, which is presumably due to the predominant transformation of the surface conductivity.
4. Discussion
We start our discussion by analyzing the dependences of the dark and the photocurrent to the temperature, which are shown on Fig.1. It is important to note that temperature dependences of the dark and photocurrent have similar character. Both of them are only slightly depend to the temperature in the low temperature region (T < 200 K), and start to increase exponentially at the
ACCEPTED MANUSCRIPT 8 higher temperatures. Such type of dark current behavior has been reported for TlGaSe2 crystals and it is suggested that in the low temperature region conductivity of TlGaSe2 crystals has the hopping character [13 - 14].
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The similar behavior of the dark and photocurrent relative to the temperature is the characteristic feature of the conduction mechanism that is observed in amorphous semiconductors. Charge transport in such semiconductors can be explained by means of specific localized states (the so
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called “band tails”) in the forbidden gap. For the same reason, the hopping conductivity that occurs in a wide temperature range and the peculiar sensitivity of such conductivity to external
of the amorphous semiconductors [15].
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perturbations (electric field, degree of doping and compensation etc.) are also the integral features
The main question which arises in the context of above argumentations is: to what extent TlGaSe2 crystals can be considered as amorphous semiconductors? To answer the question it is
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necessary to remind some important characteristics of TlGaSe2 semiconductors. TlGaSe2 is known as a native p - type semiconductor without special doping (this peculiarity is also one of the integral features of the amorphous semiconductors). The structural investigations of
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TlGaSe2 including our own data [16] show that this material has a well apparent crystal structure [17 - 19]. The only difference may arise due to the stacking faults between layers, which are
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characteristic to all of the crystals with the layered structure. These specific defects are the main reason of the anisotropy of conductivity in layered crystals. At the same time, a number of recent investigations, including the present ones, indicate that the disorder in the direction perpendicular to the plane of the layers is not the only type of disorder in TlGaSe2 crystals. For example, an analysis of temperature behavior of an optical absorption edge in TlGaSe2 reveals an unusual behavior of the Urbach tail, which is characteristic to glassy type semiconductors [5]. The metastable disordered state in TlGaSe2 was also predicted from non - structural experiments [20, 21]. According to these
ACCEPTED MANUSCRIPT 9 findings, it was concluded that some glassy phase is formed while cooling the crystal below the 200 K. We conclude that deep charged defects are the main reason of the observed effects. Such types of charged defects are usually observed in strongly compensated semiconductor materials [21]. The
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high values of resistivity of the investigated samples of TlGaSe2 allow assuming that these samples are highly compensated. Besides, it was shown in our previous investigations of thermally stimulated current in TlGaSe2 crystals [7] that shallow donor states present in undoped samples
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which greatly influence the current transport in these crystals. In [22] temperature - dependent electrical conductivity and carrier concentration measurements in undoped TlGaSe2 crystals reveals
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the extrinsic p - type of conduction with an acceptor and donor states with concentrations of 9.0 x 1015 and 1.3 x 1016 cm-3, respectively, with a donor to acceptor compensating ratio of 0.69. According to [15, 23], in the case of large degrees of compensation the carriers are distributed very inhomogeneously in the crystal lattice, thus highly compensated semiconductors can be considered
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as the model of an amorphous semiconductor [23].
5. Conclusion
In conclusion, the effect of the preillumination on the conductivity and photoconductivity was
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observed in TlGaSe2 layered semiconductor in both directions parallel and perpendicular to the plane of layers. Preillumination significantly changes the electrical properties of the samples and
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leads to a substantial decrease in both dark and photo - conductivities at low temperatures, where hopping is the dominant transport mechanism. Such behavior is similar to the Staebler - Wronski effect, which is observed in the amorphous semiconductor thin films. Thus, we conclude that bulk semiconductor crystal TlGaSe2 can demonstrate the peculiar behavior, which is typical to the amorphous semiconductors. 6. References
ACCEPTED MANUSCRIPT 10 [1] M-H. Yu. Seyidov, R. A. Suleymanov, S. S. Babaev, T. G. Mamedov, G. M. Sharifov, Physics of the Solid State. 50 (2008) 108 – 117.
of Physics and Chemistry of Solids. 69 (2008) 2544 – 2547/
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[2] M-H. Yu. Seyidov, R. A. Suleymanov, E. Yakar, N. A. Abdullayev, T. G. Mammadov, Journal
[3] M-H. Yu. Seyidov, R. A. Suleymanov, E. Yakar, N. A. Abdullayev, T. G. Mammadov, J. Appl. Phys. 106 (2009) 063529
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[4] М-H. Yu. Seyidov, R. Suleymanov, E. Balaban, Y. Şale, J. Appl. Phys. 114 (2013) 093706
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[5] M-H. Yu. Seyidov, R. A. Suleymanov, Y. Şale, J. Appl. Phys. 112 (2012) 103106. [6] D. L. Staebler, C. R. Wronski, Appl. Phys. Lett. 31 (1977) 292 - 294. [7] M-H.Yu Seyidov, Y Sahin, M H Aslan and R A Suleymanov, Semicond. Sci. Technol. 21 (2006) 1633–1638
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[8] D. L. Staebler, C. R. Wronski, J. Appl. Phys. 51 (1980) 3262 - 3268. [9] K. Shimakawa, A. Kolobov, S.R Elliott, Adv. Phys. 44 (1995) 475 – 588. [10] T. Kruger, A. F. Sax, Physica B. 353 (2004) 263 – 277
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[11] V.Grivickas, V. Bikbajevas, and P.Grivickas, Phys. Stat. Sol. (b) 243(2006), R31–R33
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[12] R. H. Bube, Photoelectronic properties of semiconductors, Cambridge Univ. Press, 1992. [13] S. N. Mustafaeva, V. A. Aliyev, M. M. Asadov, Physics of the Solid State. 40 (1998) 41 – 44. [14] S. N. Mustafayeva, M. M. Asadov, A. A. Ismailov, Physics of the Solid State. 50 (2008) 2040 – 2043.
[15] S. Baranovskii, Charge transport in disordered solids, Wiley, 2006.
ACCEPTED MANUSCRIPT 11 [16] M-H. Yu. Seyidov, R. A. Suleymanov, E. Yakar and M. Açikgöz, Solid State Sciences 14 (2012) 311 - 316
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[17] D. F. Mc. Moorrow, R. A. Cowley, P. D. Hatton, J. Banys, J. Phys. - Condens. Mat. 2 (1990) 3699 – 3712.
[18] S. Kashida, Y. Kobayashi, J. Korean Phys. Soc. 32 (1998) Suppl.S Part 1 S40 – S43.
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[19] W. Henkel, H. D. Hochheimer, C. Carlone, A. Werner, S. Ves, H. G. Vonschnering, Phys. Rev.
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B 26 (1982) 3211 – 3221/
[20] F. Salehli, Y. Bakis, M-H. Yu. Seyidov, R. A. Suleymanov, Semicond. Sci. Tech. 22 (2007) 843 – 850.
[21] A. A. Anikyev, V. M. Burlakov, M. P. Yakheev, Sov. Phys. Solid State. 32 (1990) 2131 –
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2133.
[22] A.F. Qasrawi and N.M. Gasanly, Materials Research Bulletin 39 (2004) 1353–1359
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[23] B. I. Shklovskii, A. L. Efros, Soviet Physics JETP 35 (1972) 610 – 614.
ACCEPTED MANUSCRIPT 12 7. Figure captions. Fig. 1 – The effect of preliminary light exposure (pre-illumination) on the temperature dependences
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of the dark (a), and photo - current (b - d) for Y, Au - Cu, perpendicular, sample. Fig.1a demonstrates the exponential increase of the dark conductivity with activation energies 0.150 eV and 0.312 eV for no pre-illuminated and pre-illuminated sample, respectively.
parallel (a) and perpendicular (b) direction at 80 K.
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Fig. 2 - The effect of pre-illumination on the photoconductivity spectra of F Au – In sample in
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Fig. 3 – The effect of illumination on the temperature dependence of the dark current for U sample with Au and In contacts in the direction parallel to the layers plane: crystal illuminated by white light at cooling from 300 to 80 K (pre-illumination) and crystal illuminated by white light only at 80
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ACCEPTED MANUSCRIPT Highlights
1. The effect of the preillumination on transport properties of TlGaSe2 was investigated.
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2. Results are discussed on the basis of the Staebler - Wronski effect. 3. The Staebler - Wronski effect is detected in bulk TlGaSe2 single semiconductor.
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4. TlGaSe2 single crystal has properties that are typical to the amorphous semiconductors.
S1293-2558(14)00102-2
DOI:
10.1016/j.solidstatesciences.2014.04.009
Reference:
SSSCIE 4932
To appear in:
Solid State Sciences
Received Date: 17 December 2013 Revised Date:
17 March 2014
Accepted Date: 20 April 2014
Please cite this article as: M.H. Seyidov, R.A. Suleymanov, E. Balaban, Y. Şale, Preillumination – Induced Change of Electronic Transport Properties of TlGaSe2 Semiconductor, Solid State Sciences (2014), doi: 10.1016/j.solidstatesciences.2014.04.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT 1
Preillumination – Induced Change of Electronic Transport Properties of TlGaSe2 Semiconductor
1 2
Department of Physics, Gebze Institute of Technology, 41400, Gebze, Kocaeli, Turkey
Institute of Physics Azerbaijan National Academy of Sciences, AZ - 1143 Baku, Azerbaijan
TUBITAK-BILGEM, Scientific and Technical Research Council of Turkey, Gebze, Kocaeli 41470, Turkey
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Abstract
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3
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Мir HasanYu. Seyidov 1, 2*, Rauf A. Suleymanov 1, 2, Ertan Balaban1, 3, and Yasin Şale1
The effect of the pre-illumination on the dark and the photo - conductivity of the TlGaSe2 layered semiconductor is investigated within the temperature range of 80 – 300 K. After the illumination predominantly at high temperatures, a substantial decrease in both dark and photo - conductivities
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was observed. Observed phenomena resemble the Staebler - Wronski effect, which is typical for the amorphous semiconductors. The main contribution of this work is to show that the TlGaSe2 single crystals with well apparent crystalline structure can demonstrate certain characteristics peculiar to
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amorphous semiconductors.
Keywords: Amophous semiconductor; Chalcogenide semiconductors; The Staebler - Wronski effect; Photoconductivity;
Corresponding author address: Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey. Tel.: +90 262 605 1329; fax: +90 262 605 1305 E-mail address: [email protected] (MH.Yu.Seyidov)
ACCEPTED MANUSCRIPT 2
1. Introduction Ternary thallium gallium dichalcogenide (TlGaSe2) single crystal, which belongs to a group of
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layered semiconductors, displays a number of unique physical properties, such as photo – induced changes of dielectric and thermal properties [1 - 3]. According to [2, 3], the linear thermal expansion coefficients of TlGaSe2 are significantly affected by a light exposure treatment. This
SC
motivates us to study transport properties of TlGaSe2 after the light illumination treatment of
M AN U
crystals.
Indeed, our previous research demonstrated the electric field induced changes in electronic transport properties and an unusual Urbach tail behavior near the absorption edge of TlGaSe2 crystals [4 - 5]. It was shown that observed effects are very similar to those observed in amorphous semiconductors. One of the most frequently mentioned effects inherent to amorphous
TE D
semiconductor is the Staebler - Wronski effect, i.e. the reduction of dark conductivity and photoconductivity of amorphous silicon upon extended exposure to light [6 - 8].
EP
The objective of this study was to investigate the role of preillumination on the current transport mechanism of TlGaSe2 semiconductors. Various samples were picked from several technological
AC C
batches and with various pairs of metallic contacts, which were formed by evaporating on the cleaved surfaces. The parameters regarding the transport properties were measured both parallel and perpendicular to the plane of layers of the crystals. We study the role of the illumination by exposing the samples to white light while cooling them from room temperature down to 80 K, after which the measurements regarding the electronic transport properties are carried out. The experimental findings unambiguously show that the
ACCEPTED MANUSCRIPT 3 preillumination had substantially decreased the electrical conductivity and the photo – conductivity, which is similar to the Staebler – Wronski effect [5 - 6]. As a result, we show that the TlGaSe2
semiconductors. 2. Experiment
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single bulk semiconductor demonstrates peculiar properties, which are characteristic to amorphous
SC
Investigated TlGaSe2 single crystals were grown using elements taken in stoichiometric proportions. A single crystal was grown from the melt by the modified Bridgman – Stockberger
M AN U
method. Undoped p – type TlGaSe2 single crystals that have a high dark electrical resistivity (> 106 Ω·cm at 300 K) were cleaved from different technological batches and then used throughout experiments. The composition of the samples was examined by a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) facilities. It was found [7] that samples had a
TE D
negligible amount of native impurities, such as carbon, oxygen and silicon, which are usually contained in undoped TlGaSe2 crystals. EDX data demonstrate further that investigated samples have an excess of Tl atoms and deficit of Se atoms: Tl (28.87 at%), Ga (26.23 at%) and Se (44.90
EP
at%). It is well known that impurities do not determine the p - type conductivity in most of chalcogenide semiconductors and it is the deviation from the stoichiometry that probably leads to p
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- type conductivity regardless of the doping. Under vacuum conditions, Au, In and Cu - metallic contacts were prepared by evaporation on both sides of the high – quality cleavage surface of the TlGaSe2 crystals. Then these contacts were used to measure the conductivity in directions parallel and perpendicular to the plane of the layers of TlGaSe2 crystals.
ACCEPTED MANUSCRIPT 4 The experiments were performed over the temperature range of 80 – 300 K. A Janis closed cycle helium refrigerator, which is equipped with glass windows for optical measurements, was used as the cooling device. A control sensor (diode DT - 470) and a resistive control heater were
RI PT
mounted under the base and used to control the temperature within accuracy of ∼ 0.1 K by using a Lake Shore - 340 auto tuning temperature controller. All of the measurements were made within a
SC
running vacuum of the order of 10−3 mbar.
The measurements were performed on three different TlGaSe2 bulk samples which had a
M AN U
thickness of ~ 500 - 750 µm. When the current was measured in the direction parallel to the layers, the distances between the electrodes of the samples, were within ~ 2 mm range. Below the investigated samples are referred as F, U and Y.
A 300 W Thermo Oriel Xenon Lamp was used as the light source. The radiation from this light
TE D
source was passed through a Triax 550 - type UV - visible spectrophotometer to form the monochromatic light within the wavelength range of 300 - 900 nm with 2 nm steps. Prepared TlGaSe2 samples were exposed to this light for photoconductivity studies. The light beam was
EP
directed perpendicular to the surface of the crystal, on which the contact was made. The photocurrent signal corresponding to different wavelengths was measured by means of a
AC C
programmable electrometer (Keithley 6517A) from which the data were automatically sampled and recorded. The bias voltage of 15 V which was applied to the samples through photocurrent measurements was also generated by the electrometer. Both the spectrophotometer and the electrometer were controlled via GPIB connection by software running on a personal computer. The whole measurement system was shielded against external interference using cooper sheets. The temperature fluctuation at which the measurements were carried out was within ± 0.05 limits.
ACCEPTED MANUSCRIPT 5 Through the photocurrent calculations the spectral distribution of the light source was taken into account. The change of electronic transport properties of TlGaSe2 induced by the preillumination was
RI PT
determined using the following procedure. Prior to the measurements, each TlGaSe2 sample was subjected to the white light illumination while it was cooled from room temperature down to 80 K during ~1 hour. Excitation of a sample was provided by a commercial high power Light Emitting
SC
Diode (LED) that was placed near the sample’s surface and had an approximate power of 0.8 mW/cm2. After temperature reached to 80 K the illumination source was turned off. Then, the
M AN U
electronic transport properties of the sample were investigated by photoconductivity measurements carried out in the heating regime within 80 – 300 K temperature range and 10 K increments. Each set of experiments took approximately 5 hours. After turning the exposit light off, the electronic transport properties of TlGaSe2 are retained for a long time and in the whole temperature range.
TE D
During this time interval, the effect of the illumination was clearly detected. So, the relaxation time of the observed effects can be assumed as 5 hours or more. It is well known that annealing of the amorphous semiconductor at high temperatures (usually higher than 300K) for a few hours reverses
EP
the Staebler – Wronski effect. Annealing can also take place at lower temperatures, however the process takes a longer amount of time. For that reason we undertake the long time annealing of
AC C
TlGaSe2 samples at and above the room temperature. As a result the original transport properties of the samples were fully recovered. 3. Experimental Results
For the sample Y with Au and Cu contacts, the dependence of the photocurrent to the temperature that is measured perpendicular to the plane of layers is depicted on the Fig 1. The measurements were carried out for 3 selected wavelengths (namely 310 nm, 608 nm and 824 nm) as
ACCEPTED MANUSCRIPT 6 well as without the preillumination. Additionally, the dependence of dark current to the temperature is also depicted. Based on the Fig.1 following conclusions can be made. Starting from 200 K, both dark and photocurrent increase exponentially with respect to the temperature. Cooling under the
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illumination significantly decreases both dark and photocurrent in the low temperature region, T < 200 K. Besides, the activation energies obtained from the exponentially increasing part of the dark conductivities of normal and preilluminated cases appear to be different (see Fig.1a).
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Preillumination increases the activation energy and makes it two times higher than that in normal case (0.150 eV). Indeed the substantial decrease of the conductivity and photoconductivity after the
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preillumination is a well known property of amorphous semiconductors [6, 8 - 10]. It is interesting to note that a similar behavior of the dark and photocurrent relative to the temperature is also a well known characteristic of the conduction mechanism of amorphous semiconductors [11].
Fig. 2 depicts the photo – conductivity spectra for F sample with Au and In contacts for both
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directions parallel and perpendicular to the plane of the layers with and without preillumination. As it can be seen, the effect of illumination is clearly manifested in both directions. Note also that the
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effect of preillumination is more pronounced in the direction parallel to the layers of the crystal.
As it can be seen from the Fig.2 the well pronounced peak near the 606 nm is observed which is
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usually attributed to the excitonic state in TlGaSe2 crystals [12]. The influence of preillumination on this peak is relatively weak, whereas the effect of preillumination is particularly noticed in the short wavelengths region, λ < 350 nm. This finding together with the more effective influence of the preillumination in the direction parallel to the plane of layers indicates the important role of the surface in the observed phenomena.
ACCEPTED MANUSCRIPT 7 While detailed information is provided in Discussion section, it is suggested that some type of photochemical reaction is realized during the illumination, which leads to the observed phenomenon. For the realization of the photochemical reaction, it is essential to illuminate the
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samples at sufficiently high temperatures that are close to 300 K. The measurements carried out after cooling the samples from room temperature down to 80 K in dark and then exposing them to the same exciting white light source revealed completely different results. In this case, the effect of
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illumination is the same as in the case of well known thermally stimulated current measurements: after illumination only at 80 K, the current measured at low temperatures increases due to the filling
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of some trap states. This difference in the characteristics of current transport is an important feature which allows us to distinguishing the effect of photochemical reaction from other photo induced phenomena. Fig. 3 depicts the importance of the illumination which starts from the room temperature and the illumination performed only at 80 K.
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In conclusion, preillumination of the TlGaSe2 samples has a great impact on both the dark and the photoconductivity of TlGaSe2 crystals. At low temperatures (~ 80K), the decrease in the conductivity can be more than an order of the magnitude. The activation energy, which is obtained
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from the exponentially increasing part of the conductivity, doubles after the preillumination. Mostly
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the effect becomes apparent when measuring the conductivity parallel to the plane of the layers, which is presumably due to the predominant transformation of the surface conductivity.
4. Discussion
We start our discussion by analyzing the dependences of the dark and the photocurrent to the temperature, which are shown on Fig.1. It is important to note that temperature dependences of the dark and photocurrent have similar character. Both of them are only slightly depend to the temperature in the low temperature region (T < 200 K), and start to increase exponentially at the
ACCEPTED MANUSCRIPT 8 higher temperatures. Such type of dark current behavior has been reported for TlGaSe2 crystals and it is suggested that in the low temperature region conductivity of TlGaSe2 crystals has the hopping character [13 - 14].
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The similar behavior of the dark and photocurrent relative to the temperature is the characteristic feature of the conduction mechanism that is observed in amorphous semiconductors. Charge transport in such semiconductors can be explained by means of specific localized states (the so
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called “band tails”) in the forbidden gap. For the same reason, the hopping conductivity that occurs in a wide temperature range and the peculiar sensitivity of such conductivity to external
of the amorphous semiconductors [15].
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perturbations (electric field, degree of doping and compensation etc.) are also the integral features
The main question which arises in the context of above argumentations is: to what extent TlGaSe2 crystals can be considered as amorphous semiconductors? To answer the question it is
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necessary to remind some important characteristics of TlGaSe2 semiconductors. TlGaSe2 is known as a native p - type semiconductor without special doping (this peculiarity is also one of the integral features of the amorphous semiconductors). The structural investigations of
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TlGaSe2 including our own data [16] show that this material has a well apparent crystal structure [17 - 19]. The only difference may arise due to the stacking faults between layers, which are
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characteristic to all of the crystals with the layered structure. These specific defects are the main reason of the anisotropy of conductivity in layered crystals. At the same time, a number of recent investigations, including the present ones, indicate that the disorder in the direction perpendicular to the plane of the layers is not the only type of disorder in TlGaSe2 crystals. For example, an analysis of temperature behavior of an optical absorption edge in TlGaSe2 reveals an unusual behavior of the Urbach tail, which is characteristic to glassy type semiconductors [5]. The metastable disordered state in TlGaSe2 was also predicted from non - structural experiments [20, 21]. According to these
ACCEPTED MANUSCRIPT 9 findings, it was concluded that some glassy phase is formed while cooling the crystal below the 200 K. We conclude that deep charged defects are the main reason of the observed effects. Such types of charged defects are usually observed in strongly compensated semiconductor materials [21]. The
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high values of resistivity of the investigated samples of TlGaSe2 allow assuming that these samples are highly compensated. Besides, it was shown in our previous investigations of thermally stimulated current in TlGaSe2 crystals [7] that shallow donor states present in undoped samples
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which greatly influence the current transport in these crystals. In [22] temperature - dependent electrical conductivity and carrier concentration measurements in undoped TlGaSe2 crystals reveals
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the extrinsic p - type of conduction with an acceptor and donor states with concentrations of 9.0 x 1015 and 1.3 x 1016 cm-3, respectively, with a donor to acceptor compensating ratio of 0.69. According to [15, 23], in the case of large degrees of compensation the carriers are distributed very inhomogeneously in the crystal lattice, thus highly compensated semiconductors can be considered
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as the model of an amorphous semiconductor [23].
5. Conclusion
In conclusion, the effect of the preillumination on the conductivity and photoconductivity was
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observed in TlGaSe2 layered semiconductor in both directions parallel and perpendicular to the plane of layers. Preillumination significantly changes the electrical properties of the samples and
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leads to a substantial decrease in both dark and photo - conductivities at low temperatures, where hopping is the dominant transport mechanism. Such behavior is similar to the Staebler - Wronski effect, which is observed in the amorphous semiconductor thin films. Thus, we conclude that bulk semiconductor crystal TlGaSe2 can demonstrate the peculiar behavior, which is typical to the amorphous semiconductors. 6. References
ACCEPTED MANUSCRIPT 10 [1] M-H. Yu. Seyidov, R. A. Suleymanov, S. S. Babaev, T. G. Mamedov, G. M. Sharifov, Physics of the Solid State. 50 (2008) 108 – 117.
of Physics and Chemistry of Solids. 69 (2008) 2544 – 2547/
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[2] M-H. Yu. Seyidov, R. A. Suleymanov, E. Yakar, N. A. Abdullayev, T. G. Mammadov, Journal
[3] M-H. Yu. Seyidov, R. A. Suleymanov, E. Yakar, N. A. Abdullayev, T. G. Mammadov, J. Appl. Phys. 106 (2009) 063529
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[4] М-H. Yu. Seyidov, R. Suleymanov, E. Balaban, Y. Şale, J. Appl. Phys. 114 (2013) 093706
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[5] M-H. Yu. Seyidov, R. A. Suleymanov, Y. Şale, J. Appl. Phys. 112 (2012) 103106. [6] D. L. Staebler, C. R. Wronski, Appl. Phys. Lett. 31 (1977) 292 - 294. [7] M-H.Yu Seyidov, Y Sahin, M H Aslan and R A Suleymanov, Semicond. Sci. Technol. 21 (2006) 1633–1638
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[8] D. L. Staebler, C. R. Wronski, J. Appl. Phys. 51 (1980) 3262 - 3268. [9] K. Shimakawa, A. Kolobov, S.R Elliott, Adv. Phys. 44 (1995) 475 – 588. [10] T. Kruger, A. F. Sax, Physica B. 353 (2004) 263 – 277
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[11] V.Grivickas, V. Bikbajevas, and P.Grivickas, Phys. Stat. Sol. (b) 243(2006), R31–R33
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[12] R. H. Bube, Photoelectronic properties of semiconductors, Cambridge Univ. Press, 1992. [13] S. N. Mustafaeva, V. A. Aliyev, M. M. Asadov, Physics of the Solid State. 40 (1998) 41 – 44. [14] S. N. Mustafayeva, M. M. Asadov, A. A. Ismailov, Physics of the Solid State. 50 (2008) 2040 – 2043.
[15] S. Baranovskii, Charge transport in disordered solids, Wiley, 2006.
ACCEPTED MANUSCRIPT 11 [16] M-H. Yu. Seyidov, R. A. Suleymanov, E. Yakar and M. Açikgöz, Solid State Sciences 14 (2012) 311 - 316
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[17] D. F. Mc. Moorrow, R. A. Cowley, P. D. Hatton, J. Banys, J. Phys. - Condens. Mat. 2 (1990) 3699 – 3712.
[18] S. Kashida, Y. Kobayashi, J. Korean Phys. Soc. 32 (1998) Suppl.S Part 1 S40 – S43.
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[19] W. Henkel, H. D. Hochheimer, C. Carlone, A. Werner, S. Ves, H. G. Vonschnering, Phys. Rev.
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B 26 (1982) 3211 – 3221/
[20] F. Salehli, Y. Bakis, M-H. Yu. Seyidov, R. A. Suleymanov, Semicond. Sci. Tech. 22 (2007) 843 – 850.
[21] A. A. Anikyev, V. M. Burlakov, M. P. Yakheev, Sov. Phys. Solid State. 32 (1990) 2131 –
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2133.
[22] A.F. Qasrawi and N.M. Gasanly, Materials Research Bulletin 39 (2004) 1353–1359
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[23] B. I. Shklovskii, A. L. Efros, Soviet Physics JETP 35 (1972) 610 – 614.
ACCEPTED MANUSCRIPT 12 7. Figure captions. Fig. 1 – The effect of preliminary light exposure (pre-illumination) on the temperature dependences
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of the dark (a), and photo - current (b - d) for Y, Au - Cu, perpendicular, sample. Fig.1a demonstrates the exponential increase of the dark conductivity with activation energies 0.150 eV and 0.312 eV for no pre-illuminated and pre-illuminated sample, respectively.
parallel (a) and perpendicular (b) direction at 80 K.
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Fig. 2 - The effect of pre-illumination on the photoconductivity spectra of F Au – In sample in
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Fig. 3 – The effect of illumination on the temperature dependence of the dark current for U sample with Au and In contacts in the direction parallel to the layers plane: crystal illuminated by white light at cooling from 300 to 80 K (pre-illumination) and crystal illuminated by white light only at 80
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1. The effect of the preillumination on transport properties of TlGaSe2 was investigated.
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2. Results are discussed on the basis of the Staebler - Wronski effect. 3. The Staebler - Wronski effect is detected in bulk TlGaSe2 single semiconductor.
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4. TlGaSe2 single crystal has properties that are typical to the amorphous semiconductors.