Preparation and characterization of indium

Applied Surface Science 449 (2018) 55–67

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Preparation and characterization of indium chalcogenide thin films: A material for phase change memory M. Pandian a , P. Matheswaran a,∗ , B. Gokul a,b , R. Sathyamoorthy a , K. Asokan c a b c

Department of Physics, Kongunadu Arts and Science College, Coimbatore, Tamilnadu, 641029, India Department of Physics, Dr. N.G.P. Institute of Technology, Coimbatore, Tamilnadu, 641048, India Materials Science Division, Inter University Accelerator Centre, New Delhi, 110067, India

a r t i c l e

i n f o

Article history: Received 29 September 2017 Received in revised form 20 December 2017 Accepted 4 January 2018 Available online 3 February 2018 Keywords: Indium chalcogenide Phase change memory Band gap energy Resistivity Carrier concentration Carrier mobility Activation energy

a b s t r a c t The versatile resistive switching behavior of metal chalcogenide thin films is highly desired for the fabrication of phase change memory (PCM) - electronic devices. The In2 (Te1-x Sex )3 thin films might be one of the prime candidates for the application of PCM devices. In this paper, the effects of Se-Te in the phase change characteristics of In-Se-Te material are investigated by X-ray diffraction, Field emission scanning electron microscopy, the I–V measurement and the optical transmittance. The resistance ratio between the amorphous and crystalline states of In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 and In2 (Te0.90 Se0.1 )3 thin films are about two orders of magnitude. The stoichiometric phase change from In2 (Te1-x Sex )3 to In2 Se3 and In2 Te3 phases in all the samples have been observed from the temperature dependent I–V characteristics. The electrical switching occurs at 80 ◦ C, Further rise in temperature leads no change in the threshold voltage after switching. For the In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 and In2 (Te0.90 Se0.1 )3 films, the forward biased current/threshold voltages are found to be about 8 ␮A/4.2 V, 6 ␮A/3.00 V and 4 ␮A/2.8 V respectively. The In2 (Te1-x Sex )3 thin film showed the high resistance state at low voltage region. However, when it reaches the threshold voltage, the resistance drastically reduced through the formation of conducting path. From these studies, it can be concluded that the effective way to enhance the comprehensive performance of In-Se-Te system is by varying the concentrations of the Te and Se and it may find potential application in switching volatile PCM devices. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Metal chalcogenide materials have attracted the attention in Phase Change Memory (PCM) to meet the growing need for nonvolatile applications for next generation [1]. The PCM based hybrid main memory systems have been investigated by few researchers [2–4], and using this architecture level model of PCM offers more lifetime, hence benefits of DRAM. In last few years, III–IV semiconductor compounds find much attention due to their potential applications in electronics, opto electronic devices, photovoltaics and solid state devices [5]. The fast growing demands for semiconductor memories with high speed and low power consumption have motivated the search of new and alternative memory technologies. More importantly, the transport and phase change properties of Indium chalcogenides thin films have been widely investigated in the past years [6–8]. The phase transition between

∗ Corresponding author. E-mail address: [email protected] (P. Matheswaran). https://doi.org/10.1016/j.apsusc.2018.01.027 0169-4332/© 2018 Elsevier B.V. All rights reserved.

the amorphous and crystalline states in Te based alloys explained by applying high resistance (RESET) and low resistance (SET) in early 1970s by R.R. Ovshinsky [9]. The materials employed in PCM are mainly Te-Se based chalcogenide and therefore most studies have been focused mainly on Ge/Sb-Te and In2 Se3 as a potential candidate in PCM [7,10–13]. The term chalcogenide describes two polymorphic states, namely amorphous and crystalline. The amorphous state has high resistance compared to crystalline state for its low free electron density, high activation energy and short atomic range order. Whereas, the crystalline state, the material holds long range atomic order, low activation energy and high free electron density which results in low resistance [14]. The switching behavior of these two states are important operations for phase change memory. In order to succeed the phase transition from amorphous to crystalline state, a low voltage and long pulse (SET pulse) are applied. While a high voltage and short pulse (RESET pulse) are applied to cause the crystalline state to amorphous state. Recent literature survey suggests that the InSeTe thin films were prepared by several techniques by number of researchers [15–18] and these methods include chemical vapour deposition (CVD), pulsed

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Table 1a Structural parameters of In2 (Te1-x Sex )3 thin film (x = 2). Sample (x = 2)

2 (degree)

Phase & Orientation (hkl)

Crystalline Size (nm)

Dislocation Density Lines/m2 (1015 )

Strain (␧) dyn/cm2

FWHM (␤) (degree)

Obs. dhkl (Å)

Cal. dhkl (Å)

Pristine

23.12 27.6 40.54 25.09 27.62 40.70

¯ In2 (Se.Te)3 (21 5) In2 Se3 (105) ¯ In2 (Se.Tex )3 (21 1¯ 4) In2 Te3 (333) In2 Se3 (105) ¯ In2 (Se.Tex )3 (21 1¯ 4)

25.22 21.39 22.59 12.86 10.30 11.57

0.0015 0.0021 0.0019 0.006 0.009 0.007

1.43 1.69 1.60 2.81 3.51 3.12

0.336 0.400 0.391 0.65 0.82 0.76

3.84 3.22 2.22 3.54 3.22 2.21

3.84 3.23 2.23 3.55 3.21 2.21

After switching

Table 1b Structural parameters of In2 (Te1-x Sex )3 thin film (x = 6). Sample (x = 6)

2 (degree) Phase & Orientation (hkl)

Pristine

23.75 27.10 40.75 25.07 27.84 40.70

After switching

¯ In2 (Se.Te)3 (21 5) In2 Se3 (105) ¯ In2 (Se.Tex )3 (21 1¯ 4) In2 Te3 (333) In2 Se3 (105) ¯ In2 (Se.Tex )3 (21 1¯ 4)

Crystalline Size (nm)

Dislocation Density Lines/m2 (1015 )

Strain (␧) dyn/cm2

FWHM (␤) (degree)

Obs. dhkl (Å)

Cal. dhkl (Å)

31.40 25.87 30.54 20.19 17.82 33.41

0.0014 0.0014 0.001 0.002 0.004 0.001

1.15 1.39 1.18 1.79 2.03 0.89

0.27 0.33 0.19 0.42 0.48 0.26

3.74 3.31 2.21 3.57 3.22 3.81

3.75 3.31 2.21 3.56 3.21 3.82

Crystalline Size (nm)

Dislocation Density Lines/m2 (1015 )

Strain (␧) dyn/cm2

FWHM (␤) (degree)

Obs. dhkl (Å)

Cal. dhkl (Å)

8.77 12.89 18.71 10.59 13.36 16.39

0.013 0.006 0.002 0.008 0.005 0.003

4.12 2.80 1.93 3.40 2.70 2.20

0.966 0.663 0.473 0.82 0.64 0.54

3.81 3.22 2.22 3.52 3.20 2.21

3.82 3.22 2.23 3.51 3.21 2.22

Table 1c Structural parameters of In2 (Te1-x Sex )3 thin film (x = 10). Sample (x = 10)

2 (degree) Phase & Orientation (hkl)

Pristine

23.30 27.60 40.52 24.69 27.77 40.65

After switching

¯ In2 (Se.Te)3 (21 5) In2 Se3 (105) ¯ In2 (Se.Tex )3 (21 1¯ 4) In2 Te3 (333) In2 Se3 (105) ¯ In2 (Se.Tex )3 (21 1¯ 4)

laser deposition (PLD), physical vapour deposition (PVD). In spite of effective studies on the growth and characterization of InSeTe thin films prepared by various techniques, there is lack of understanding -about the structural, morphological and electrical properties which greatly depend on the performance of devices in PCM. The materials change from amorphous to crystalline nature with temperature. In this article, we report the PCM properties of In2 (Te1-x Sex )3 thin films in connection with the structural, surface topography and most importantly, the electrical properties. 2. Experimental 2.1. Film deposition In the present work, the elements In, Te and Se (SigmaAldrich chemicals – 99.99% purity) with desired compositions In2 (Te1-x Sex )3 (x = 2, 6 and 10) were taken in a quartz tube and vacuum shielded at 1 × 10−5 mbar. Using rotating furnace, the sealed quartz tube was annealed at 900 ◦ C for 12 h and then quenched to room temperature to form the stoichiometric compound. The ingots were grained well to make a fine powder. Using this fine powder, different compositions of In2 (Te1-x Sex )3 thin films were deposited on the glass substrates by thermal evaporation technique in Ar atmosphere and followed by post annealing at 200 ◦ C for 30 min under Ar atmosphere. The annealed 200 ◦ C thin films are considered as pristine (before switching) samples. 2.2. Film characterizations The structural properties of thin films performed by X-ray diffraction (XRD) using Shimadzu 6000 facility, X-ray diffraction was generated by directing an X-ray (monochromatic Cu-K␣ radi-

Fig. 1. (a–c) XRD pattern pristine and after switching In2 (Te1-x Sex )3 thin films.

ation with wavelength of 1.541 Å excited at 45 kV and 40 mA) on a small flat surface of the sample. The surface morphology of the samples was characterized by Field emission scanning electron microscopy (FESEM) along with Energy dispersive X-ray analysis (EDX) using CARLZEISS SIGMA version. Transmittance spectra were recorded using JASCO-670 dual beam spectrophotometer with the

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Fig. 2. FESEM image of In2 (Te1-x Sex )3 thin films of various compositions (x = 2, 6, 10), pristine (a–c) and after switching (d–f).

wavelength range from 400 to 2000 nm. I–V characteristics of the samples analyzed using Keithley 2612A instrument in the temperature range of 301 K–370 K. The Hall measurements were carried out using HMS-3000 (ECOPIA HALL EFFECT MEASUREMENT SYSTEM) to measure the carrier concentration, mobility, conductivity and hall co-efficient of the films. The temperature dependence of the electrical properties were analyzed by four probe method in the temperature range 301 K–473 K.

3. Result and discussion 3.1. Structural analysis Fig. 1(a–c) shows the XRD pattern of pristine and after switching In2 (Te1-x Sex )3 thin films of various compositions. The observed

peaks from all the pristine samples clearly confirms the formation of In2 (Te1-x Sex )3 as well as In2 Se3 phase and exhibits amorphous/crystalline in nature. The low and high intensity peaks at ¯ and ¯ (21 1¯ 4) 2␪ = 23.12◦ , 40.54◦ and 41.27◦ correspond to the (2 1 5), (152) plane of ternary In2 (Se·Te)3 phase for all the pristine samples (JCPDS: 71–1182). In addition to that the diffraction peak at 2␪ = 27.60◦ corresponds to (105) plane of In2 Se3 (JCPDS: 71–0250). After switching, the XRD pattern were recorded and it shows only the presence of In2 Se3 and In2 Te3 phases and there is no peak corresponding to In2 (Te. Se)3 phase. It clearly confirms the transformation of In2 (Se·Te)3 phase to In2 Se3 and In2 Te3 phases. The diffraction peaks at peak at 2␪ = 27.60◦ corresponds to (105) plane of In2 Se3 and the peak at 2␪ = 24.97◦ to (333) plane of In2 Te3 (JCPDS: 89-3978) for all the samples. -

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Fig. 3. EDX spectra of In2 (Te1-x Sex )3 thin films of various compositions (x = 2, 6, 10), pristine (a–c) and after switching (d–f).

The structural parameters like grain size (D) [19], Dislocation density (␦) [20], Strain (␧) [21] were calculated using the following equations, D=

k (nm) ␤ cos 

␦ = 1/D2 Lines/m2

␧ = ˇcos/4(dyn/cm2 ) Where, the constant ‘k’ is the shape factor = 0.94, ‘␭’ is the wavelength of X-rays (1.5406 nm for CuK␣), ‘␪’ is the Bragg’s angle and ‘␤’ is the full width at half maximum of diffraction peak measured in radians. The evaluated values are represented in Tables 1a–1c respectively. The experimentally observed peak positions and d(hkl) values are within the standard JCPDS values. The average crystallite size is found to be 19.18 nm for all the compositions with slight variations. This shows that the nature of the prepared sample depends on several factors like source temperature, evaporation rate and vapour pressure. The average crystalline size is 15.03 nm and found to decrease significantly after switching. It may be due to the re-crystallization of the materials, thus clearly confirms the phase change behaviors.

Fig. 4. Transmittance spectra and Tau’s plot of pristine In2 (Te1-x Sex )3 thin films.

3.2. Surface morphology and compositional analysis The surface morphology of pristine and after switching In2 (Te1-x Sex )3 thin film was analyzed by recording SEM images

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with different magnifications. The pristine (Fig. 2a–c) sample shows the presence of spherical shaped grains with uniform distribution. In conclusion, the SEM image reveals that the pristine film has homogeneous surface, highly crystalline and thoroughly dense morphology of In- Se- Te without any voids in all the samples. The surface morphological change in samples can be seen at the place of switching occurs (Fig. 2d–f) i.e the image contrast can be explained later in connection with I–V studies. The chemical compositions of In2 (Te1-x Sex )3 thin films were examined by EDX analysis, as shown in Fig. 3(a–c). The sharp peaks at different positions clearly show the presence of In, Se and Te elements. The elemental compositions of the films are presented in Table 2. It is noticed that the chemical composition of the samples remains similar even after switching (Fig. 3d–f) for the different composition of In2 (Te1-x Sex )3 thin films.

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Table 2 Elemental composition of In2 (Te1-x Sex )3 thin films. In2 (Te1-x Sex )3

Elemental composition (wt.%) In

Te

Se

(x = 2)

Pristine After switching Pristine After switching Pristine After switching

31.75 31.22 31.42 31.30 30.89 30.26

62.33 62.73 61.44 61.55 59.01 59.01

(x = 6) (x = 10)

5.92 5.70 7.18 7.15 10.10 9.97

3.3. Optical properties The influence of Se concentration in In2 (Te1-x Sex )3 thin films on the optical properties was studied extensively. Fig. 4 illustrates the

Fig. 5. I–V characteristics (a) In2 (Te1-x Sex )3 (x = 2), (b) In2 (Te1-x Sex )3 (x = 6), (c) In2 (Te1-x Sex )3 (x = 10) thin films. (d) Resistance Vs Voltage characteristics of In2 (Te1-x Sex )3 (x = 2, 6, 10) thin films, (e) Temperature dependent resistivity of In2 (Te1-x Sex )3 (x = 2, 6, 10) thin films and (f) Variability in the amorphization and crystallization process of In2 (Te1-x Sex )3 (x = 2) thin films.

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Fig. 5. (Continued)

Table 3 Bandgap energy of In2 (Te1-x Sex )3 thin films. Sample

Pristine

Bandgap Energy (eV) x=4

x=6

x = 10

0.80

0.82

0.83

variations in the transmittance in the range 200 nm–2600 nm. It is clear that transmission increase with increasing wave length. The lower and higher intensity in transmittance peaks show the optical interference pattern, which explains the formation of layered structure. The different compositions of transmittance spectra are found to be similar in nature. Fig. 4 shows the tau plot of the as grown In2 (Te1-x Sex )3 thin films. The band gap energy of the pristine are estimated and it is found to 0.80, 0.82 and 0.81 eV for the various composition of In2 (Te1-x Sex )3

(x = 2, 6 and 10) thin films respectively (Table 3). The estimated band gap energy values are agree with the earlier report [22]. The band gap energy decreases at higher concentration of Se (x = 10). It indicates that the addition of Se in In and Te results in formation of Se-In bonds, which enhances the carrier concentration. The presence of high density of localized states in the band structure is responsible for the lower energy band gap. Hence, the density of localized states increase as the band gap decreases [23]. The optical properties are very essential to understand the behavior of phase change materials in order to control the switching threshold current [24]. The energy band gap of pristine thin film is about 0.80 eV which is higher than that of Ge-Sb-Te (GST) (0.70 eV). Therefore the threshold current for In2 (Te1-x Sex )3 thin film is expected to be smaller compared to that of GST. It may be due to the following reasons; the melting point of In is (156.6 ◦ C) lower compared to other present element namely Se (220.8 ◦ C) and Te (449.5 ◦ C) in the In-Se-Te system. Thus, the overall melting tem-

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61

Fig. 5. (Continued)

perature of the system will be reduced with the presence of In, and the needed heating will be smaller. For thit reason, the increase in the temperature caused by joule heating in In2 (Te1-x Sex )3 thin film will be smaller compared to Ge2 Sb2 Te5 thin film. Both require lower temperature (Fig. 5a–d) and hence could contribute to the lower current. Refractive index (n) of the In2 (Te1-x Sex )3 thin films was estimated from the transmission spectra using the well known Manifacier’s envelope method [25].



n=

N+

 N2 + n20 n21

‘n0 ’ is refractive index of air, ‘n1 ’ the refractive index of the substrate. The transmission with respect to wavelength spectra show a large difference for all the as grown thin film. Hence, the refractive index (n) is calculated for all the samples and it is found to be 1.87 for all the samples. This clearly indicates the uniform nature

of the prepared samples as evident from the transmittance versus wavelength spectra. 3.4. Electrical properties The electrical properties, by the Hall Effect measurement, current-voltage characteristics and four point probe have been measured to understand the effective transport mechanism of the charge carriers in In2 (Te1-x Sex )3 thin films. More evidently, the phase change mechanism of In2 (Te1-x Sex )3 thin films have been studied from the I–V characteristics. Furthermore, the influence of Te-Se on the electrical properties of In2 (Te1-x Sex )3 thin films have been studied (Table 4). 3.4.1. I–V measurements From the I–V measurements (Fig. 5a–c), the phase change behaviors of In2 (Te1-x Sex )3 thin films are explained in connection

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Fig. 5. (Continued) Table 4 Hall effect measurement of the In2 (Te1-x Sex )3 thin films. x percentage of Se

Resistivity (-cm)

Hall mobility ␮H (cm3 /V s)

NH (cm−3 )

␴at R.T. ( cm)−1

RH (cm3 /coul.)

x=2 x=6 x = 10

0.8624 0.5121 1.908

211.4 17.09 0.796

3.42 *16 7.01 *17 4.10*18

1.6 0.19 0.52

182.3 8.89 1.51

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Fig. 5. (Continued)

with the XRD pattern and FESEM images. It is evident that the current switches from nanoampere (nA) to microampere (␮A), when the samples measured at 80 ◦ C, 90 ◦ C, 100 ◦ C respectively. With the applied voltage, the current remains constant at 4.20 V and switches from nA to 8 ␮A for the lower Se concentration (x = 2). The current switches up to 6 ␮A for the Se concentration of x = 6.The current switches up to 4 ␮A at 2.80 V for the higher Se concentration (x = 10). It is noted that current varies linearly with increase in voltage for all the samples. The current increases with respect to temperature up to 70 ◦ C, indicating semiconducting nature of all the samples [26]. This conducting behavior might be the conduction of ternary In2 (TeSe)3 phase. The conduction above threshold voltage may be due to the conduction of In2 Te3 phase as compared to In2 Se3 phase (high resistance). This switching behavior is caused by phase transition from amorphous to crystalline phase due to the joule heating. It can be explained by the following reason, a small amount current flows at the low voltage region and it is keep on increasing with increase in the voltage. When the voltage reaches the critical (i.e threshold voltage) value (i.e 4.20 V, 3.00 V, 2.80 V), the current increases significantly and then is continuously increases with the applied voltage. The XRD pattern is recorded to confirm the phase transition, i.e. after performing I–V characteristics (Fig. 5a–c). It can be seen that there is no significant peaks that correspond to In(TeSe) phase and only show the presence of In2 Te3 and In2 Se3 phases as evident from Fig. 1(a–c). In order to retain the phase formation, thesamples quenched to room temperature from 100 ◦ C [27]. It is concluded

that the ternary In2 (TeSe)3 recrystallizes at 80 ◦ C into In2 Se3 and In2 Te3 phases. The conducting path can be explained on the basis of the bond energies of the elements. Thus, the bond energy of Se–Se higher than the Te–Te [28]is the reason and also the In-Se bond energy is higher compared to In-Te system. Thus, the In2 Te3 phase can respond to the electric field easier than the In2 Se3 and hence the conductivity of In2 Se3 is lower than the In2 Te3 system. Consequently, the difference in the bond energy between the elements leads to the phase transition from ternary In2 (TeSe)3 phase recrystallize at 80 ◦ C into In2 Se3 phase and In2 Te3 phase. The observed phase change mechanism is well inline with the results obtained from XRD pattern. It was known that at the threshold voltage, the resistivity of the system was drastically reduced through the formation of the electrically conducting path. The oriented crystalline domains or crystalline defects can be the reason for the formation of conducting path. This conducting path defines the current flow in the device. Since current flow through the conducting path results joule heating, it crystallizes the surrounding zone by nucleation and lead to grain growth. The surface morphology of the sample is recorded after switching, to examine the phase transition. The conducting path may form through the spherical shaped grains in the entire surface (i.e. see Fig. 2d–f), where the applied thermal energy promotes local melting and recrystallization of the conductive materials into crystalline state which in turn leads to change in the image contrast in SEM image. During switching, the electrons are accelerated by applied

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current and thereby increases in the temperature of the material due to joule heating and leads to melt the conducting path and later crystallized by cooling in between the contact electrodes and it is evident for memory switching [29]. The switching behavior of Te based chalcogenide occur because of the presence of long Te chains in the samples (compared to In and Se) and leads to easy occurrence of atomic rearrangement and recrystallization as the glassy telluride have higher electrical conductance compared to that of sulphides and selenides. The increase in the Se concentration leads reduction in the dangling bonds which are available during the crystallization process. At this instance, the In-Se bonds are stronger than In-Te, leading to oppose the crystallization. But the presence of high concentration of localized state in the band structure is responsible for higher conductivity in the case of In2 Te3 amorphous thin films [23]. Based on the above facts it is concluded that the observed switching behavior is due to molten state of In2 Te3 phase because of the poor binding energy, which leads to electrically conductive paths in the In2 (Te1-x Sex )3 system. Further rise in temperature to 90 ◦ C, and 100 ◦ C result in increase in current after switching and there is no change in threshold voltage. It is well known that in PCM device the single phase chalcogenide materials used to switch from its amorphous to crystalline by applying SET (lower amplitude) pulse to make it conductive and vice verse RESET (higher amplitude) pulse applied to make it electrically insulating amorphous phase [30]. In the present I–V studies, the obtained results are irreversible characteristics of In2 (Te1-x Sex )3 thin films. Since the reported results are obtained by forward bias only so we are working on to get it reverse bias for better understanding of switching memories. Fig. 5d shows the R-V characteristics of In2 (Te1-x Sex )3 (x = 2, 6, 10) thin film. The resistance ratio between the amorphous and crystalline states of In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 and In2 (Te0.90 Se0.1 )3 thin films are up to two or more orders of magnitude. The resistance of both amorphous and crystalline state for In2 (Te0.90 Se0.1 )3 sample is lower than that of In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 thin films, which is consistent with the I–V characteristics as shown in Fig. 5a–c. We have also investigated the temperature dependent resistivity using four probe method as shown in Fig. 5e, where the decrease of resistivity with increasing in temperature corresponds to an amorphous to crystalline transition in the films. The resistivity of the of In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 and In2 (Te0.90 Se0.1 )3 films is approximately 3.9 × 10−5 -cm, 6.4 × 10−5 -cm and 1.6 × 10−4 -cm. Among all these films, In2 (Te0.90 Se0.1 )3 thin film exhibits a higher amorphous/crystalline resistivity ratio of about few orders of 1011 . Moreover, it also shows a higher resistance in crystalline state compared with In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 films, which means that it could lead to a low current consumption in the PRAM operation of Non Volatile Memory. Four samples of In2 (Te0.98 Se0.02 )3 were measured to understand the repeated switching behavior and shown in Fig. 5f. It depicts the variability in the amorphization and crystallization process. The slight variations in threshold voltage as well as switching current have been observed. Similar behaviors (not shown here) have been observed for the thin films of In2 (Te0.94 Se0.06 )3 and In2 (Te0.90 Se0.1 )3 . This is due to the variation in resistance for both amorphous and crystalline phases. Further investigation on the data retention measurement for In2 (Te1-x Sex )3 system will be carried out in near future for the better understanding of PCM materials. 3.4.2. Four probe measurement The resistivity of In2 (Te1-x Sex )3 thin film was measured using four probe technique. The current versus voltage characteristics of In2 (Te1-x Sex )3 thin films was measured in the temperature range

Fig. 6. Current vs Voltage plot of pristine In2 (Te1-x Sex )3 thin films.

from 30 ◦ C–150 ◦ C is shown in Fig. 6. The logarithm of resistivity is plotted against the reciprocal of absolute temperature for the given current: 50 ␮A, 100 ␮A and 150 ␮A (Fig. 7). It is observed that, the resistivity found to increase with temperature for all the In2 (Te1-x Sex )3 thin films. Activation energy is calculated by taking there the slope and using corresponding equation, Loge ␳ Eg = 2K 1/T Where, Eg – Band gap energy ␳ – Resistivity K – Boltzmann constant (8.6 × 10−5 eV/atom-K) T – Temperature From Fig. 8, plot of loge ␳ vs 1000/T the activation energy (for 50 ␮A) is found to be 0.020 eV, 0.029 eV and 0.029 eV for various

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Fig. 7. loge ␳ vs 1000/T of In2 (Te1-x Sex )3 thin films.

composition of In2 (Te1-x Sex )3 (x = 2, 6 and 10) as grown films. For 100 ␮A, the activation energy is found to be 0.020 eV, 0.023 eV and 0.029 eV for various composition of In2 (Te1-x Sex )3 as grown films. From these values, it can be seen that the activation energy decreases with current. The activation energy is found to increase with the Se concentration for all the samples. It clearly indicates the increased carrier concentration of In2 (Te1-x Sex )3 thin films when the concentration of Se increases and thereby reduces the energy band gap of the prepared samples as evident from the optical band gap. It may be the reason for reduced energy band gap from 0.82 eV to 0.81 eV as obtained from the optical properties of the films. 3.4.3. Hall effect measurement The dependence of the electrical properties, such as carrier concentration, mobility, resistivity and conductivity type of the In2 (Te1-x Sex )3 thin films were measured by the Hall effect measurements as shown in Fig. 9. The In2 (Te1-x Sex )3 (x = 4) thin film shows p-type conductivity in the order of magnitude of 1016 cm−3 of excessive holes. With increase in the Se concentration, the carrier

concentration changes gradually and these samples show excessive electrons in the order of 1016 –1018 cm−3 , indicating n-type conductivity. It is observed that conductivity (␴) decreases with increase in the Se concentration, it may be due to the relationship between the carrier concentration and the electrical conductivity. It is significantly affected by the structural and crystal defects and also the carrier concentration, carrier mobility and Hall co-efficient which agree with the earlier report [31]. These characteristics are due to the change in the composition, structure and localized states of thin films as well as the re-arrangement of atoms which produce more defects. The resistivity of In2 (Te1-x Sex )3 thin film is directly proportional to the reciprocal of the product of carrier concentration N and carrier mobility ␮, ␳ = 1/Ne ␮ Both the carrier concentration and mobility contribute to the conductivity. Any increase in the Se concentration leads to decrease

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Fig. 9. Hall effect measurement of the In2 (Te1-x Sex )3 thin films as a function of Se concentration (a) x = 2, (b) = 6, (c) = 10.

concentration (x = 10) and increase in carrier concentration due to the excessive electrons which denotes n-type conductivity of the samples. 4. Conclusion

Fig. 8. Arrhenius plots of the resistivity of In2 (Te1-x Sex )3 thin films.

in mobility. It may be due to the decrease in the concentration of Te as In tends to bind to Se chains. The increase in the electron scattering in amorphous/crystalline In-Se-Te promotes to further decrease in carrier mobility with the increase in the Se concentration while the Te gradually decrease. The resistivity for all the samples ranged from 10−1  cm–1002  cm. However, the resistivity of the In2 (Te1-x Sex )3 thin films shows the tendency to increase with Se concentration. The resistivity slightly decreases for the Se

In2 (Te1-x Sex )3 thin films have been deposited on clean class substrate by thermal evaporation under Ar gas atmosphere. From the I–V characteristics, the materials showed switching behavior at 80 ◦ C for all the samples. The threshold voltages of switching are at 4.20 V, 3.00 V and 2.80 V for the various compositions and the respective phase change behavior are explained in connection with the structural, surface morphology and EDS studies. The resistance ratio between the amorphous and crystalline states of In2 (Te0.98 Se0.02 )3 , In2 (Te0.94 Se0.06 )3 and In2 (Te0.90 Se0.1 )3 thin films are up to two or more orders of magnitude. In2 (Te0.90 Se0.1 )3 thin film exhibits a higher amorphous/crystalline resistivity ratio about few orders of 10−4 compared to In2 (Te0.98 Se0.02 )3 and In2 (Te0.94 Se0.06 )3 films. Hence, it could lead to a low current consumption in the PRAM operation of Non Volatile Memory. A small

M. Pandian et al. / Applied Surface Science 449 (2018) 55–67

deviation in threshold voltage as well as switching current have been observed for In2 (Te0.98 Se0.02 )3 film. The band gap values are in the range from 0.80 eV to 1.01 eV, which depend on the Indium chalcogenide phase formation. The refractive index (n) is calculated for all the samples and it is found to be ∼1.87. The carrier concentration, mobility and Hall co-efficients change with respect to the variation in the Se concentrations. The resistivity for all the samples range from 10−1  cm–1002  cm. It is concluded that the phase change properties of In2 (Te1-x Sex )3 thin films open wide scope on PCM devices. Further, the investigation of data retention measurement for In2 (Te1-x Sex )3 system will be carried out in near future for the better understanding of PCM materials. Acknowledgement The authors greatly acknowledge the Inter University Accelerator Centre (IUAC) New Delhi, for providing financial support through the project UFUP-57306. References [1] M.H.R. Lankhorst, B.W.S.M.M. Ketelaars, R.A.M. Wolters, Low-cost and nanoscale non-volatile memory concept for future silicon chips, Nat. Mater. 4 (2005) 347–352, http://dx.doi.org/10.1038/nmat1350. [2] M.K. Qureshi, V. Srinivasan, J.A. Rivers, Scalable high performance main memory system using phase-change memory technology, ISCA vol. 37 (2009) 24–33, http://dx.doi.org/10.1145/1555815.1555760. [3] C. Zambelli, G. Navarro, V. Sousa, I.L. Prejbeanu, L. Perniola, Phase change and magnetic memories for solid-state drive applications, Proc. IEEE 105 (2017) 1790–1811, http://dx.doi.org/10.1109/JPROC.2017.2710217. [4] B. Lee, E. Ipek, O. Mutlu, D. Burger, Architecting phase change memory as a scalable DRAM alternative, ISCA vol. 37 (2009) 2–13, http://dx.doi.org/10. 1145/1555815.1555758. [5] A.A.A. Darwish, M.M. El-Nahass, M.H. Bahlol, Structural and electrical studies on nanostructured InSe thin films, Appl. Surf. Sci. 276 (2013) 210–216, http:// dx.doi.org/10.1016/j.apsusc.2013.03.068. [6] H. Lee, D.H. Kang, L. Tran, Indium selenide (In2 Se3 ) thin film for phase-change memory, Mater. Sci. Eng. B Solid–State Mater. Adv. Technol. 119 (2005) 196–201, http://dx.doi.org/10.1016/j.mseb.2005.02.060. [7] Y.T. Huang, C.W. Huang, J.Y. Chen, Y.H. Ting, K.C. Lu, Y.L. Chueh, W.W. Wu, Dynamic observation of phase transformation behaviors in indium (iii) selenide nanowire based phase change memory, ACS Nano 8 (2014) 9457–9462, http://dx.doi.org/10.1021/nn503576x. [8] E. Mafi, A. Soudi, Y. Gu, Electronically driven amorphization in phase-change In2 Se3 nanowires, J. Phys. Chem. C 116 (2012) 22539–22544, http://dx.doi. org/10.1021/jp305696w. [9] S.R. Ovshinsky, Reversible electrical switching phenomena in disordered structures, Phys. Rev. Lett. 21 (1968) 1450–1453, http://dx.doi.org/10.1103/ PhysRevLett.21.1450. [10] H. Lee, Y.K. Kim, D. Kim, D.H. Kang, Switching behavior of indium selenide-based phase-change memory cell, IEEE Trans. Magn. 41 (2005) 1034–1036, http://dx.doi.org/10.1109/TMAG.2004.842032. [11] D.D. Yu, S. Brittman, J.S. Lee, A.L. Falk, P. Hongkun, Minimum voltage for threshold switching in nanoscale Phase-change memory, Nano Lett. 8 (2008) 3429–3433, http://dx.doi.org/10.1021/nl802261s.

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