Te bilayer thin films

Materials Science in Semiconductor Processing 15 (2012) 486–491

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Thickness dependent structural and optical properties of In/Te bilayer thin films P. Matheswaran, B. Gokul, K.M. Abhirami, R. Sathyamoorthy n PG and Research Department of Physics, Kongunadu Arts and Science College, Coimbatore 641029, Tamilnadu, India

a r t i c l e in f o

abstract

Available online 2 June 2012

InxTey thin films have been prepared from In/Te bilayer by sequential thermal evaporation. The samples were analyzed by x-ray diffraction (XRD), optical transmittance spectra and scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) in order to investigate structural, optical properties, surface morphology and elemental composition of the prepared films. XRD spectra reveal that all films exhibit mixed phases of In2Te3 and In2Te5. Increase in grain size with film increase in thickness was observed. The surface was highly porous as observed by SEM analysis. Band gap energy of InxTey system is found to decrease with increase in film thickness. & 2012 Elsevier Ltd. All rights reserved.

Keywords: InTe bilayer Structural properties Optical properties

1. Introduction Indium telluride (InxTey) is a layered III–VI semiconductor compound with direct band gap energy which finds applications in gas sensors, pressure transducers and switching devices [1–3]. The bonding within the layer is typically covalent and much stronger than the van der Waals forces between the layers. There are numerous stable InxTey phases including InTe, In2Te3, In2Te5, In3Te4, In4Te3, In7Te10 and In9Te11. Among these, many phases have structural modifications such as a and b. It is very difficult to produce single phase indium telluride due to the narrow formation range of single phase in In/Te phase diagram. Few reports are available on structural, optical, electrical properties of indium telluride in bulk and thin film form [2–6]. However not much work has been carried out in In/Te bilayer thin film in connection with the thickness dependent properties. Hence an attempt has been made to prepare InxTey thin films with different

n

Corresponding author. E-mail addresses: [email protected], [email protected] (R. Sathyamoorthy). 1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.03.006

thickness by sequential thermal evaporation and to analyze the thickness dependant structural and optical properties. 2. Experimental details In/Te bilayer thin film with different thickness were deposited onto well cleaned glass substrate by sequential thermal evaporation of In and Te (99.999% purity) at room temperature. Total thickness of bilayers films were chosen ˚ 2000 A, ˚ 3000 A˚ and 4000 A˚ in order to study as 1000 A, the thickness dependant properties of the film. Thicknesses of the films were monitored by quartz crystal thickness monitor. All the experiments were carried out at a reduced pressure of 2  10  5 mbar in constant argon (Ar) gas flow atmosphere. The prepared films were annealed at 250 1C for 30 min in Ar atmosphere at a reduced pressure of 1  10  5 mbar. Ar gas is used to prevent the oxidation of the films during nucleation and growth. XRD analysis was carried out using Rigaku Ultima-3 instrument in order to find out the structural informations. Transmittance spectra were recorded by using Jasco spectrophotometer with the wavelength range from 400 nm to 2500 nm. SEM (S3400) and EDX were

P. Matheswaran et al. / Materials Science in Semiconductor Processing 15 (2012) 486–491

used to get the information about the surface morphology and elemental compositions of the InxTey thin films respectively.

3. Results and discussion 3.1. Structural analysis

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individual elements by solid state reaction. Electronegativity difference between the participating elements determines the stability of the compound. In the case of In (1.7 pauling) and Te (2.1 pauling), the electronegativity difference (0.4) is very small, which may lead to form a compound with poor stability or compound with mixed phases [7]. According to XRD spectra, we calculated the grain size (D) [8], the strain (e) [9] and dislocation density (d) [10]

Fig. 1(a–d) represents the XRD spectra of InTe thin ˚ 2000 A, ˚ 3000 A˚ and films with the thickness of 1000 A, 4000 A˚ respectively. From the Fig. 1(a–c), it is observed that the structure of the film contains the mixed phase of In2Te3 and In2Te5 (JCPDS 16-0445 and 71-0109). In2Te3 phase of the film exhibits preferential orientation along (511) plane and (660) plane at 2y ¼231 and 40.31 respectively, whereas In2Te5 phase exhibits the preferential orientation along (113) plane and (622) plane at 2y ¼ 271 and 331 respectively. Fig. 1(d) clearly indicates that In2Te5, phase is dominant than In2Te3. The formation of InxTey alloy results from the annealing treatment. Annealing (250 1C) leads to the formation of compound from

Table 1 Structural parameters of InxTey thin films. Film thickness ˚ (A)

FWHM Strain (  10  2)

Dislocation density (  1018 lines/m2)

Crystalline size (nm)

1000 2000 3000 4000

0.388 0.343 0.312 0.204

6.89 5.39 4.45 1.91

38.0 43.0 47.4 72.2

9.51 8.41 7.64 5.01

˚ (b) 2000 A, ˚ (c) 3000 A˚ and (d) 4000 A. ˚ Fig. 1. (a–d) XRD Spectra of InxTey thin films of various thicknesses (a) 1000 A,

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P. Matheswaran et al. / Materials Science in Semiconductor Processing 15 (2012) 486–491

˚ (b) 2000 A, ˚ (c) 3000 A˚ and (d) 4000 A. ˚ Fig. 2. (a–d) SEM images of InxTey thin films of various thicknesses (a) 1000 A,

Fig. 3. EDX spectra of InxTey thin films.

P. Matheswaran et al. / Materials Science in Semiconductor Processing 15 (2012) 486–491

3.2. Surface and composition analysis

by using the following equations: D ¼ kl=b cos y

ð1Þ

e ¼ ðb cos yÞ=4

ð2Þ

d ¼ 1=D2

ð3Þ

where, the constant ‘k’—shape factor (0.94), ‘l’—wavelength of x-ray (1.5406 nm for CuKa), ‘y’—Bragg’s angle and ‘b’—Full width at half maximum (FWHM) of diffraction peak measured in radians from XRD meter. The calculated results are listed in Table 1. It is observed that FWHM decrease with increase in film thickness, which implies the improvement in crystallanity in both the phases. Improvement in crystallite size was observed with increase in film thickness [11]; it may be due to reduction in stoichiometric vacancy. As film thickness increases, the strain and dislocation density decreases progressively. This may be due to the movement of interstitial atoms from its grain boundary to the crystallites, which may be leading to reduction in the concentration of lattice imperfections [12].

Table 2 Elemental composition of InxTey thin films. Film thickness ˚ (A)

Element

Composition (At%)

1000

In Te In Te In Te In Te

38.0 62.0 37.7 62.3 36.5 63.5 33.9 66.1

2000 3000 4000

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Fig. 2(a–d) represents the SEM image of InTe thin films ˚ 2000 A, ˚ 3000 A˚ and 4000 A˚ with the thickness of 1000 A, respectively. SEM images with same magnifications clearly denote that all the films have pinholes. As thickness increases, the grains tend to agglomerate with each other to form bigger grains [11,13], which support the XRD result. Spherical geometry of the grains was vanished at higher film thicknesses due to agglomeration of grains, thereby reducing grain boundaries. Elemental compositions of the films were analyzed by using EDX techniques (Fig. 3) and the composition values are tabulated in Table 2. The composition of the film with the thickness of 1000 A˚ is found to be slightly differing from the composition of In2Te3 phase. As thickness increases the elemental composition In and Te is differ from In2Te3 phase, which confirms there is a possibility of existence of other phases. Excess Te content at higher film thickness implies that presence of In2Te5 phase along with In2Te3 phase. 3.3. Optical properties Fig. 4 shows the optical transmittance spectra of InxTey thin films. It can be observed that the transmittance decreases rapidly with the increases in film thickness, it may be due to the increase in opaqueness, indicating the cation vacancies [14], and surface roughness [15]. Transmittance continuously decreases with wavelength from near infrared through visible region due to the increase of scattering loss at the porous surface [16]. Band gap energy of InxTey system is estimated from the Fig. 5. Scattering originated from grain boundaries plays a major role in determining the optical band gap of the system. In the case of lower film thickness, the crystalline size is lower, so that the crystal lattice may have high grain boundaries. As a result of scattering at

Fig. 4. Optical transmittance spectra of InxTey thin films.

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P. Matheswaran et al. / Materials Science in Semiconductor Processing 15 (2012) 486–491

high grain boundaries may be the reason for higher band gap value. Fig. 6 shows the thickness dependant band gap ˚ 2000 A, ˚ energy of the films with the thickness of 1000 A, ˚ In the present work, band gap energy 3000 A˚ and 4000 A. is found to be decrease progressively with increase in film thickness. It may be due to reduction of grain boundaries, thereby decreasing the scattering phenomena. As evident from XRD analysis, the grain size increases with increase in thickness, which results the reduction of grain boundaries and thus decreasing the scattering phenomena and corresponds with the band gap. Vacancies and anti-site defects are likely to play an important role in this defective crystal structure. Stoichiometric cation vacancies present are themselves neutral but their presence is responsible for easy restoration of radiation ejected atoms into lattice sites across low energy barriers. Localized phonons due to high density of stoichiometric vacancies/neutral defects are expected to have an additional contribution on optical band gap of this system. Similar result was observed by Emziane et al. and Sen et al. for

Fig. 6. Dependence of band gap energy with film thicknesses.

˚ (b) 2000 A, ˚ (c) 3000 A˚ and (d) 4000 A. ˚ Fig. 5. (a–d) (ahn)2 vs photon energy plot of InxTey thin films of various thicknesses (a) 1000 A,

P. Matheswaran et al. / Materials Science in Semiconductor Processing 15 (2012) 486–491

In2Te3 phase [11,16]. In addition, SEM analysis shows the high porous/pinhole nature at low thickness which results high band gap energy of InxTey system. The estimated band gap energy of the film of thickness 2000 A˚ (1.15 eV) is found to be nearer to the band gap energy of In2Te3 phase (1.10 eV), which corroborates the dominant nature of In2Te3 phase than In2Te5 phase.

4. Conclusion In/Te bilayer thin film with different thickness were deposited by sequential thermal evaporation in a reduced pressure of 2  10  5 mbar at Ar atmosphere and subsequently annealed at 250 1C for 30 min in a reduced pressure of 1  10  5 mbar at Ar atmosphere. XRD studies reveal that all the film exhibits mixed phase of In2Te3 and In2Te5, whereas In2Te3 phase is dominant at lower thickness. Improvement in crystallite size with film thickness was confirmed. SEM analysis infers that agglomeration of grains with increase in thickness. Band gap energy of InxTey system decreases with film thickness which may be due to increase in grain size, thereby reduction in grain boundary scattering. Localized phonons due to high density of stoichiometric vacancies/neutral defects are expected to have an additional contribution on optical band gap of this system.

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