TSC measurements in a-Ge22Se78−xBix thin films

Physica B 405 (2010) 4982–4985

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TSC measurements in a-Ge22Se78  xBix thin films S. Yadav a, D. Kumar b, R.K. Pal a, S.K. Sharma a, A. Kumar a,n a b

Department of Physics, Harcourt Butler Technological Institute, Kanpur, India Department of Physics, J.S.S Academy of Technical Education, Noida, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 June 2010 Received in revised form 28 August 2010 Accepted 3 October 2010

In the present paper thermally stimulated current measurements have been made in a-Ge22Se78  xBix (x ¼0, 10) thin films in order to determine trap parameters such as trap depth and trap density. Trap depth has been calculated using the initial rise method proposed by Garlick and Gibson. The trap depth is found to be 0.41 and 0.30 eV for x ¼ 0 and 10, respectively. The trap density (Nt) was also calculated, which was found to be 2.08  1017 and 1.48  1016 cm  3 for x ¼ 0 and 10, respectively. The decrease in trap density on Bi addition is explained in terms of the structure of the ternary alloy. & 2010 Elsevier B.V. All rights reserved.

Keywords: Chalcogenide glasses Trap parameters Thin films

1. Introduction Chalcogenide glassy semiconductors are one of the widely used amorphous semiconductors for a variety of applications in optics, electronics and optoelectronics, such as ultrafast optical switches, waveguides, optical memories, gratings, optical sensors, holography, infrared lasers, ionic sensors etc. These glasses generally exhibit p-type electrical conduction due to pinning of the Fermi level arising from the trapping of the charge carriers at localized gap states [1,2]. But in 1979, Tohge et al. [3,4] reported for the first time p- to n-type transition in Bi doped Ge–Se glasses at higher concentration of Bi (more than 7 at%). Due to this reason, Bi doping in Ge–Se glasses became important. Many physical properties, particularly electrical transport and photoconductive behavior, are governed by the traps, which are due to inherent defects in the chalcogenide glasses. The study of the density of defect states has, therefore, been a subject of importance in these glasses for better applications in solid state devices. There are so many methods for the determination of trap depth and concentration of traps. The thermally stimulated current method is one of them. According to this method, traps are filled by the photoexcitation of the semiconductor at a sufficiently low temperature such that upon ceasing the illumination the trapped carriers cannot be freed by the thermal energy available at that temperature. The temperature is then raised at a constant rate. The liberated carriers contribute, in an applied field, to an excess current until they recombine with carriers of the opposite type or join the equilibrium carrier distribution. This excess current,

n

Corresponding author. Tel.: +91 512 2294851; fax: + 91 512 2533812. E-mail address: [email protected] (A. Kumar).

0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.10.003

measured as a function of temperature during heating, is called a TSC curve. In previous years, this technique was extended to amorphous semiconductors. Several experiments have been carried out in hydrogenated amorphous silicon (a-Si:H) [5–7]. The present paper reports TSC measurements in amorphous thin films of Ge22Se78  xBix (x¼0, 10).

2. Experimental details 2.1. Material synthesis Glassy alloys of Ge22Se78  xBix (x ¼0, 10) were prepared by the quenching technique. The exact proportions of high purity (99.999%; Se, Ge and Bi) elements, in accordance with their atomic percentages, were weighed using an electronic balance (LIBROR, AEG-120) with the least count of 10  4 g. The material was then sealed in an evacuated ( 10  5 Torr) quartz ampoule (length  5 cm and internal diameter  8 mm). The ampoule containing the material was heated to 800 1C and was held at that temperature for 12 h. The temperature of the furnace was raised slowly at a rate of 3–4 1C/min. During heating, the ampoule was constantly rocked by rotating a ceramic rod to which the ampoule was tucked away in the furnace. This was done to obtain a homogeneous glassy alloy. After rocking for about 12 hours, the obtained melt was rapidly quenched in ice-cooled water. The quenched sample was then taken out by breaking the quartz ampoule. The glassy nature of the alloy was ascertained by X-ray diffraction. For this, X-ray diffraction (XRD) patterns of samples were taken at room temperature using an X-ray diffractometer (Philips, PW 1140/09). The X-ray diffraction pattern of binary Ge22Se78 glass is shown in Fig. 1. A similar pattern was also observed in the Ge22Se68Bi10 glass.

S. Yadav et al. / Physica B 405 (2010) 4982–4985

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a calibrated copper–constantan thermocouple mounted very near to the films. A voltage of 10 V was applied across the films and the resulting current was measured by a digital Pico-Ammeter. Before measurements, the films were first annealed at 450 K for 1 h in a vacuum  10  2 Torr. TSC measurements were made on a-Ge22Se78Bix (x ¼0, 10) at constant heating rates. In the present work, measurements were made in two states (I and II) at each heating rate. In state I, the sample is heated from room temperature 300 to 415 K without any light exposure and in state II, light is incident on the sample for 2 min through the transparent window at room temperature. After switching off the light, the decay of photoconductivity was allowed for 10 min. Now, the sample is again heated up to 415 K at the same heating rate. From the above measurements we have observed that the current in state II is higher than in state I in the case of a-Ge22Se78Bix (x¼0, 10), which is clear from Figs. 3 and 4. The difference of current in these two states is called thermally stimulated current (TSC). Figs. 5 and 6 show the temperature dependence of the TSC curve for a-Ge22Se78Bix (x¼0, 10) thin films. A maxima in TSC was observed at a particular temperature for both the samples. The values of peak temperature Tm and peak current I (Tm) are given in Table 1.

Absence of sharp diffraction peaks indicates that the prepared alloys are glassy in nature. 2.2. Thin film preparation Thin films of these materials were prepared by the vacuum evaporation technique keeping glass substrates at room temperature. Vacuum evaporated indium electrodes at the bottom were used for the electrical contact. The thickness of the films was  500 nm. The co-planar structure (length  1.2 cm and electrode separation 0.5 mm) was used for the present measurements (see Fig. 2). The Thin films were kept in the deposition chamber in the dark for 24 hours before mounting them on a sample holder. This was done to allow sufficient annealing at room temperature so that metastable thermodynamic equilibrium is attained in the sample. The deposition parameters were kept almost the same for both samples so that a comparison could be made for these samples.

3. Results and discussion For TSC measurements, thin films were mounted on a specially designed sample holder that had a transparent window to shine light. A vacuum 10  2 Torr was maintained throughout the measurements. The temperature of the films was controlled by mounting a heater inside the sample holder and measured by

Ge22Se78

-18 -19

State ( I ) State ( II )

ln I (T) A

-20 -21 -22 -23 -24 2.3

2.5

2.7

2.9 1000 / T (K-1)

3.3

3.5

Fig. 3. Temperature dependence of current in state I and state II for a-Ge22Se78 thin films.

Fig. 1. XRD pattern of glassy Ge22Se78.

0.5mm

1.2cm

Electrodes deposition on Glass Substrate

3.1

Film deposition

glass substrate Fig. 2. Co-planar structure of thin films.

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S. Yadav et al. / Physica B 405 (2010) 4982–4985

Ge22Se68Bi10

-19.5

3.1. Calculation of trap depth

-20 State I

ln I (T) A

-20.5

State II

-21 -21.5 -22 -22.5 -23 2.5

2.6

2.7

2.8 2.9 1000 / T (K-1)

3

3.1

3.2

Fig. 4. Temperature dependence of current in state I and state II for a-Ge22Se68Bi10 thin films.

Ge22Se78

3.00E-09

We have calculated the trap depth (Et) by means of the initial rise method. This method, proposed by Garlick and Gibson [8], is valid for all types of recombination kinetics [9], which states that in the rising part of the TSC curve the current is proportional to exp( Et/kT). Therefore, the plot of the logarithmic of the current as a function of 1/kT gives directly a straight line of slope Et The progressive shifting from linear behavior at high currents is likely due to the overcoming of critical temperature Tc [10], above which the Garlick–Gibson method is no longer valid. The advantage of this method lies essentially in the fact that it is independent of recombination kinetics, i.e. in this method only the rising part of the TSC curve has to be considered. Figs. 7 and 8 show a plot of the logarithm of thermally stimulated current (ln TSC) as a function of 1000/T for the initial rise part of TSC peak in amorphous Ge22Se78Bix (x¼0, 10) thin films. A linear dependence is observed in both the cases. From the slope of these curves Et was calculated for both the samples. The values of Et and Io (pre-exponential factor) are also given in Table 1. It is clear from this table that trap depth is found to be smaller in the Bi doped sample as compared to that in the binary Ge22Se78 system.

2.50E-09

TSC (A)

2.00E-09

Ge22Se78

-20 ln TSC (A)

1.50E-09 1.00E-09 5.00E-10 0.00E+00 340

350

360

370

380 T (K)

390

400

410

-22 -24 -26

420

2.8

2.5

3.1 1000 / T (K

Fig. 5. Experimental–TSC curves for a-Ge22Se78 thin films.

3.4

-1)

Fig. 7. ln TSC (A) versus 1000/T plot for the initial rise part of TSC peak for a-Ge22Se78 thin films.

Ge22Se68Bi10

1.8E-10 1.6E-10 1.4E-10

Ge22Se68Bi10

-22 ln TSC (A)

TSC (A)

1.2E-10 1E-10 8E-11 6E-11

-23

4E-11

-24

2E-11 0 290 300 310 320 330 340 350 360 370 380 390 400 T (K)

2.7

2.8

2.9 3 1000 / T (K-1)

3.1

3.2

Fig. 8. ln TSC (A) versus 1000/T plot for the initial rise part of TSC peak for a-Ge22Se68Bi10 thin films.

Fig. 6. Experimental–TSC curves for a-Ge22Se68Bi10 thin films.

Table 1 trap parameters in a-Ge22Se78Bix thin films. Bi (at%)

Tm (K) 0 10

Trap depth Et (eV)

Peak current and peak temperature

402 365

I(Tm) (A)

Pre-exponential factor (Io)

Initial rise method 9

2.88  10 1.63  10  10

0.41 0.30

2.36  10  4 3.11  10  6

S. Yadav et al. / Physica B 405 (2010) 4982–4985

Table 2 Calculated trap density in a-Ge22Se78Bix thin films. Bi (at%)

Trap density Nt (cm  3)

0 10

2.08  1017 1.48  1016

Storiopoulos and Fuhs [11] have explained their results of drastic increase in conductivity as due to a drastic decrease in band gap induced by Bi addition. In the present case, trap depth is found to be smaller in the Ge22Se68Bi10 system as compared to that in the binary Ge22Se78 system, which may be understood in terms of a decrease in band gap on Bi addition. 3.2. Calculation of trap density The concentration of the traps (Nt) was estimated using the relation [12] Nt ¼

Q ALeG

Here, Q is the quantity of the charge released during a TSC experiment and can be calculated from the area under the TSC peak, A and L the area and thickness of the sample, respectively, e the electronic charge and G the photoconductivity gain. The latter was calculated using the expression [13] G ¼ t=ttr where t is the carrier life time and ttr is the carrier transit time between the electrodes. The trap density in each alloy was calculated by the above method and the values are inserted in Table 2. Here it should be pointed out that Nt is calculated using G ¼1, because it is not possible to measure G under the same TSC conditions with the necessary accuracy. It is clear from Table 2 that trap density is also found to be smaller in Bi doped samples as compared to that in the binary Ge22Se78 system. According to Philip’s model [14] of medium range order for Ge chalcogenide glasses, ordered clusters of (Se)n chains, corner sharing tetrahedral Ge(Se1/2)4 and (Ge2Se1/2)6 ethane like structures should be some of the building blocks of Ge–Se glasses. In Se rich glasses, as in the present case, the first two clusters may be predominant. Our X-ray spectroscopic results indicate [15] that Bi makes bonds with Ge in the Ge22Se68Bi10 glass as well as in amorphous thin films. Following Philip’s model [14], one can argue that Bi enters into the Ge(Se1/2)4 tetrahedral structure. Pazin et al. [16] have also suggested that addition of Bi at high concentration (10 at%) modifies the Ge(Se1/2)4 clusters, penetrating into them to form units containing all three elements, Ge, Bi and Se. The

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chemically ordered clusters may favour these component clusters when a high concentration of Bi is added to the Ge–Se system. The results of infrared spectroscopy [17,18] also confirm the above statements as Bi–Ge modes were found to be infrared active in Ge–Se–Bi alloys at high concentration of Bi (10%). Bhatia et al. [19] also emphasized that at higher concentrations of Bi, the system becomes more rigid due to Ge–Se bonds. From the above discussion it is clear that the defects states in the Ge–Se–Bi system at higher concentration (10% Bi) should be smaller as Bi enters into the Ge–Se tetrahedral units instead of entering into Se chains. The decrease in trap density in the present case in Ge22Se68Bi10 as compared to that in the Ge22Se78 system may be due to the above structural arrangement on Bi addition.

4. Conclusion Glassy alloys of Ge22Se78  xBix (x ¼0, 10) were prepared by the quenching technique. TSC measurements were carried out on amorphous thin films of these glassy alloys, prepared by vacuum evaporation techniques. A maxima in TSC is observed at a particular temperature for each case. The trap depth is calculated by the initial rise method. The trap depth is found to be smaller in the Bi doped alloy as compared to that in the binary Ge22Se78 alloy. The trap density was also calculated for each alloy. The trap density is also found to be smaller in the Bi doped alloy as compared to that in the binary Ge22Se78 alloy. The decrease in trap density on Bi addition is explained in terms of the structure of the ternary alloy.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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