Te trilayer thin films

Accepted Manuscript Swift Heavy Ion irradiation induced nanocrystallisation in Te/Cd/ Te trilayer thin films

Smita Survase, Madhavi Thakurdesai, I. Sulania, D. Kanjilal PII: DOI: Reference:

S0040-6090(17)30475-3 doi: 10.1016/j.tsf.2017.06.040 TSF 36047

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

17 November 2016 18 June 2017 19 June 2017

Please cite this article as: Smita Survase, Madhavi Thakurdesai, I. Sulania, D. Kanjilal , Swift Heavy Ion irradiation induced nanocrystallisation in Te/Cd/Te trilayer thin films, Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.06.040

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ACCEPTED MANUSCRIPT Swift Heavy Ion irradiation induced nanocrystallisation in Te/Cd/Te trilayer thin films Smita Survase1, Madhavi Thakurdesai1, I. Sulania2, D.Kanjilal2 1

Thin Film Research Laboratory, Department of Physics, Birla College (Affiliated to University Of Mumbai), Kalyan 421 304, India 2 Inter University Accelerator Center, New Delhi- 110-067, India.

Abstract:

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In this paper, we report Swift Heavy Ion (SHI) irradiation induced nanocrystallisation

trilayer thin films were irradiated by 100 MeV

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in Te/Cd/Te trilayer thin films. In the present investigation, thermally evaporated Te/Cd/Te Ag ion beam at varying fluence. The

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structural characterization of Te/Cd/Te trilayer thin films was carried out using X-ray

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Diffraction (XRD) technique. XRD studies revealed that SHI irradiation results in Te/Cd/Te layer mixing leading to formation of CdTe nanocrystalline phase. Scanning Electron

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Microscopy (SEM) and Atomic Force Microscopy (AFM) were employed for surface studies. SEM studies indicate ion beam induced surface modifications. Further, AFM pictures

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indicate nano grain formation in the irradiated films. The elemental analysis of trilayer films

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before and after irradiation was carried out using Energy Dispersive X-ray Spectroscopy (EDX). EDX analysis indicates that percentage ratio of Cd and Te remains unaffected after

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SHI irradiation. Optical characterization was done using UV-Vis spectroscopy. Band gap values determined on the basis of UV-Vis spectroscopy of all the irradiated films were found

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to be more than the reported bulk value of 1.5 eV. The increase in bandgap value is attributed to the combined effect of ion induced strain and quantum confinement effect. SHI irradiation induced formation of nanocrystallisation in Te/Cd/Te trilayer films is explained mainly in the framework of Ion Beam Mixing (IBM).

Keywords:Nanocrystalline CdTe, SHI, XRD, SEM, AFM, UV-VIS Spectroscopy.

ACCEPTED MANUSCRIPT Introduction: Cadmium telluride (CdTe) is one of the important II–VI compound semiconductor materials. Nanocrystalline thin films of CdTe are widely used in the field of opto-electronic devices such as Light Emitting Diodes (LED), solar cells, photo detectors, biosensors, etc [1]. Currently many methods are used to synthesize nanocrystalline CdTe thin films. However, in

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most of the synthesis methods, the process temperature is generally above room temperature [2] and also in many cases the post-deposition high temperature thermal annealing is an

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unavoidable step [3]. This may lead to change in particle size and degrade the performance of

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the devices based on the thin films [4].

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Another difficulty in synthesizing thin films of compound semiconductors like CdTe is to achieve desired stoichiometric phase [5]. One of the ways to overcome this problem is to

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deposit CdTe thin films by Stack Elemental Layer (SEL) method [6]. In this method a stack of bilayer, trilayer or multilayer is deposited and subsequently annealed at suitable

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temperature. This method is particularly advantageous as the desired stoichiometry can be

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achieved by controlling the individual layer thickness and annealing temperature [7]. However, this method is not suitable for preparation of nanocrystalline thin films, as the post

distribution [8].

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deposition thermal annealing may lead to grain agglomeration and result in wide size

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All the above mentioned problems can be eliminated if nanocrystalline CdTe thin films are synthesized by irradiating multilayer by energetic ion beams at energies 1 MeV/u known as Swift Heavy Ion (SHI) irradiation. SHI irradiation is a unique technique of athermal annealing. Effect of SHI irradiation on various multilayer systems is studied rigorously by various research groups and phenomena such as ion beam mixing [9], changes in optical, magnetic, electrical properties [10] etc. are reported. However, not much work is

ACCEPTED MANUSCRIPT done on SHI induced nanocrystallisation in multilayer films. Therefore it is important to explore the possibility of SHI induced nanocrystallisation in multilayer films. SHI loses energy as they traverse through the target which is either spent in displacing atoms by elastic collisions or in exciting or ionizing the atoms by inelastic collision [11]. When the energy transferred to the target atoms is large, the atoms are pushed out of their

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lattice positions which may collide with other target atoms. Thus a single ion is responsible

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for relocating thousands of atoms. If the recoil cascade overlaps with the interface of a

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layered system, it results in atomic mixing in the vicinity of interface [12]. During SHI, irradiation latent tracks of nanometric dimensions are formed in the wake of ion beam if

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deposited energy is more than a threshold value. Therefore interface mixing can be confined to narrow dimensions and formation of nanocrystallline phase can be expected.

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In SHI induced nanocrystallisation in multilayers, desired phase can be achieved by controlling individual layer thickness and grain size can be controlled by controlling

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irradiation parameters such as ion beam energy, ion fluence etc [13-18]. This process is

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particularly advantageous for synthesis of nanocrystalline thin films because of its spatial selectivity. Also in this process, nanocrystallisation can be achieved at room temperature

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without any further processing. Thus wide size distribution due to post -deposition thermal annealing can be eliminated [19].

100 MeV

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Therefore, in the present investigation, Te/Cd/Te trilayer films are irradiated by 107

Ag ion beam at various fluence of 1 1012 ions/ cm2, 5 1012 ions/ cm2 and

11013 ions/ cm2. SHI induced nanocrystallisation in Te/Cd/Te trilayer films is studied in detail. The possible mechanism behind SHI induced nanocrystallisation process is discussed in brief.

ACCEPTED MANUSCRIPT 2. Experimental: In the present investigation, Te/Cd/Te trilayer films are deposited by SEL method and subsequently irradiated by 100 MeV

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Ag ion beam at various fluence of 1 1012 ions/ cm2,

5 1012 ions/ cm2 and 1 1013 ions/ cm2. 2.1. Te/Cd/Te trilayer deposition by thermal evaporation method:

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The Te/Cd/Te trilayer thin films prepared for the present study were deposited on

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glass substrates of dimensions 10 mm  10 mm using thermal evaporation technique. The vacuum inside the chamber was ~ 10-6 torr and the substrates were kept at room temperature.

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The Te/Cd/Te stack of 200 nm thickness was prepared by following method. Commercially

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available 99.95 % pure tellurium (Te) was evaporated and a layer of thickness 50 nm was deposited first. Later, 100 nm thick layer of cadmium (Cd) was deposited by evaporating

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commercially available 99.95 % pure cadmium and then again a 50 nm thick layer of tellurium (Te) was deposited. The trilayer film thickness was monitored in situ by a quartz

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2.2 SHI irradiation:

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crystal monitor.

The Te/Cd/Te trilayer thin films were irradiated with 100 MeV Ag ion beam.

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Irradiation was performed at room temperature using 16 MV Pelletron facility at IUAC, New Delhi, India. The films were irradiated at a fluence of 1 1012 ions/ cm2, 5  1012 ions/ cm2

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and 11013 ions/ cm2. The charge state of Ag ions was 8+. The beam current was maintained ½ pna for all the ions. The electronic energy loss (Se) and the nuclear energy loss (Sn) of the 100 MeV Ag ions were calculated using SRIM code [20]. They were found to be 1.823 keV/ Å and 0.0121 keV/Å respectively. Hence it can be asserted that, the electronic energy loss is the dominant mechanism through which the silver ions lose their energy to the target. The range of 100 MeV Ag8+ ion beams, i.e. the distance over which an ion dissipates its energy completely via elastic and inelastic collisions is calculated from the SRIM code

ACCEPTED MANUSCRIPT [20]. The range of ions is 10.07 µm which is much more than the thickness (200 nm) of Te/Cd/Te trilayer films. As-deposited as well as irradiated films are systematically characterized. Structural phase formation is studied by X-ray diffraction (XRD) using Phillips Pananalytical Xpert Pro MPD spectrometer with CuKα radiation = 1.5418 Å. Surface morphology of the films was investigated using Zeiss Scanning Electron Microscope

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(SEM)and by Nanoscope IV Atomic Force Microscope (AFM). The elemental analysis was

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carried out using Energy-dispersive X-ray microanalysis (EDX) with INCA Oxford, at ICON

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laboratories; Mumbai. UV–Vis absorption analysis is performed using Varian CARRY 500 UV-VIS-NIR Spectrometer in the range 300 to 800 nm to study the optical properties.

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3. Results and discussion: 3.1. X-ray diffraction (XRD) study:

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In the present investigation, Te/Cd/Te trilayer films were irradiated with 100 MeV Ag ion beam at a fluence of 1 1012 ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2. To

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investigate the phase formed after irradiation, the thin films were examined by XRD

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technique. The XRD spectra for the as-deposited and irradiated thin films are shown in Figure 1(A)–(D). The XRD spectrum for the as-deposited film exhibits polycrystalline nature

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of the film and consists of elemental Te and Cd peaks [JCPDS data 36-1452 05-0671]. No peak corresponding to CdTe is observed. When the Te/Cd/Te trilayer film was irradiated by

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100 MeV Ag ion beam at a fluence of 1  1012 ions/ cm2 the polycrystalline nature of the film was retained. However, XRD spectrum of this film reveals the onset of trilayer mixing as an effect of irradiation. The XRD spectrum of this film exhibits two peaks at 2θ = 23.730 and 32.740 indicating CdTe phase along with one peak corresponding to elemental Cd at 2θ =38.330 and two peaks corresponding to elemental Te at 2θ =27.560 and2θ =40.440. Further, when the irradiation was carried out at a fluence of 5 1012 ions/ cm2, the position of all the peaks remained almost the same as in the earlier case except from the fact that peak corresponding to CdTe is slightly broadened.

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Figure 1: XRD of trilayer Te/Cd/Te thin films (A) as-deposited (B) irradiated at a fluence of 1  1012 ions/ cm2 (C) irradiated at a fluence of 5  1012 ions/ cm2 (D) irradiated at a fluence of 1  1013 ions/ cm2

Finally, when the irradiation was carried out at a fluence of 1 1013 ions/ cm2, a single

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peak corresponding to hexagonal phase of CdTe was detected at 2 = 23.730. The single XRD peak suggests (110) orientation of the film. No peaks corresponding to elemental Cd and Te were seen. This clearly indicates that in a film irradiated at a fluence of 11013 ions/cm2, complete Te/Cd/Te trilayer mixing leading to stoichiometrically correct CdTe hexagonal phase formation. It has been reported that the mixing rate strongly depends on value of Se in case of SHI induced mixing. The layer mixing is possible if Se reaches a certain threshold value [21]. In the present case, it is possible that this threshold value was attained

ACCEPTED MANUSCRIPT only at the irradiation fluence of 1 1013 ions/ cm2. Thus Te/Cd/Te trilayer mixing was observed for this value of fluence. Table 1 lists all the peak positions and hkl parameters of the as-deposited and irradiated Te/Cd/Te trilayer thin films. Peak

(hkl)

27.560

Te (H)

(100)

34.740

Cd(H)

(100)

0

Cd (H)

(101)

40.440

Te (H)

As-Deposited

38.33

49.6

0

Te (H)

23.730

32.740

1 10 ions/ cm

2

38.33

0

40.440 23.730

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27.56

32.740

Irradiated by 5 10 ions/ cm

2

Irradiated by

(100)

Te (H)

(100)

CdTe (H)

(102)

Cd (H)

(101)

Te (H)

(102)

CdTe (H)

(100)

Te (H)

(100)

CdTe (H)

(102)

Cd (H)

(101)

40.440

Te (H)

(102)

CdTe (H)

(100)

38.33

23.73

0

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1 1013 ions/ cm2

(110)

0

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12

0

(102)

CdTe (H)

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Irradiated by 12

0

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27.56

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Angle (2θ)

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Thin film

Table 1: XRD data for the Te/Cd/Te trilayer as-deposited and irradiated films

The XRD peak indicating CdTe phase in case of the film irradiated at a fluence of

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11013 ions/cm2 was found to be quite broadened in comparison with other peaks seen in rest

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of the films. This peak broadening can be assigned to formation of nano grains in this film. SHI irradiation is known to induce strain [22] thus the grain size and strain are calculated by Williamson –Hall method [23]. It should be noted that XRD spectrum of the film irradiated at 1 1013 ions/ cm2 exhibits only one peak therefore the grain size and the strain induced in this film cannot be calculated using Williamson –Hall method. Therefore grain size and strain is also calculated using Debye Scherer’s formula for all the films. Grain size and strain calculated by both the methods quite match with each other. Table 2 lists the values of grain size and strain calculated by both the methods.

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Av. grain size D (nm)

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Te/Cd/Te trilayer irradiated at fluence

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Figure 2: W-H Plot of trilayer Te/Cd/Te thin films (a) as-deposited (b) irradiated at a fluence of 1  1012 ions/ cm2 (c) irradiated at a fluence of 5  1012 ions/ cm2

56 22 20 13

BY WH plot 48 24 20 ---

1.42 1.64 1.81 2.78

1.20 1.68 1.98

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As-deposited 1 1012 ions/ cm2 5 1012 ions/ cm2 1 1013 ions/ cm2

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By XRD

Strain   10–3(lin–2.m–4) By XRD BY WH plot

Table 2: Variation of average grain size and strain after irradiation from XRD results and W-H plot

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XRD results indicate that the SHI irradiation affects the crystallinity of films. During irradiation, SHI transfers energy to the target by the processes of ionization and electronic excitation and provokes the atomic rearrangements [24]. SHI deposits large amount of energy to the target and as a result, a displacement cascade is produced consisting of highly localized interstitials and vacancies. The atoms are displaced from their original sites because of impact of the heavy ion. This can cause change in the preferential orientation of the target material [25]. Therefore in the present work, initially at the irradiation fluence of 1 1012 ions/ cm2

ACCEPTED MANUSCRIPT and 5 1012 ions/ cm2, the intensity of the strong Te (110) peak is reduced slightly and the preferred orientation of elemental Te and Cd is changed. Further due to the energy deposited by the SHI beam some diffusion of Te/Cd atoms can occur at the interface resulting in CdTe phase formation. As a result The CdTe peaks are observed in XRD pattern along with elemental Te and Cd peaks.

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With increasing fluence more amount of energy is deposited by SHI beam. This

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causes intense heating due to energy and momentum transfer of excited electrons to the target

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and latent tracks are formed along the ion trajectory. The latent tracks consist of damaged zones. Formation of latent track thus introduces amorphous fraction in the lattice. As a result

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crystalline quality of the target degrades and generally an amorphous phase is formed. However, in the present work, XRD studies indicate polycrystalline to nanocrystalline

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phase transition at irradiation fluence of 11013 ions/ cm2 instead of formation of an amorphous phase. The observed nanocrystalline phase can be attributed to nanograin

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formation due ion induced grain fragmentation. The process of nanograin formation is

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explained elaborately further in section 3.6. 3.2. Scanning Electron Microscopy (SEM) studies:

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Surface morphology of as-deposited and irradiated Te/Cd/Te trilayer films is studied using Scanning Electron Microscopy (SEM). SEM micrograph of Te/Cd/Te trilayer as-

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deposited film shows irregular features on the surface and the surface is found to be quite rough. These trilayer films are irradiated with 100 MeV Ag ion beam at a fluence of 1 1012 ions/ cm2, 5 1012 ions/ cm2 and 1 1013 ions/ cm2. SEM micrographs of all the irradiated films suggest ion beam induced surface modifications. SEM micrograph of the film irradiated at a fluence of 1 1012 ions/ cm2 is shown in Figure 3 (b). (The magnification of SEM images for as-deposited film is 5m and for irradiated films it is 200 nm).It is observed that after irradiation, islands of rod shaped

ACCEPTED MANUSCRIPT structures are formed on the surface. Some very small circular clusters and few bigger irregular shaped clusters are also formed. SHI beam deposits a large amount of energy to the target as a result the surface grains may get thermally activated and can assemble in various minimum energy configurations. The islands comprising rod like structures, spherical and

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irregular shaped clusters etc. are encircled in Figure 3 (b).

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Figure 3: SEM micrograohs of trilayer Te/Cd/Te thin films (a) as-deposited (b) irradiated at a fluence of 1  1012 ions/ cm2 (c) irradiated at a fluence of 5  1012 ions/ cm2 (d) irradiated at a fluence of 1  1013 ions/ cm2

Further when irradiation fluence is increased to 5 1012 ions/ cm2 the islands seem to be fragmented into thinner rod like structures due to heavy ion impact. The irregular clusters are also seemed to fragment into rods. During SHI irradiation grain fragmentation may occur due to ion induced strain. The reduction in grain size after irradiation is also suggested by XRD studies. Therefore the diameter of the rod made of these grains is probably reduced.

ACCEPTED MANUSCRIPT When irradiation fluence is further increased to 1 1013 ions/ cm2, the rod like structures is seemed to be distributed more uniformly on the surface. The diameter of these rods is reduced further indicating further decrease in grain size with increasing ion fluence. The ion fluence dependent variation in surface configuration of Te/Cd/Te trilayer films can be attributed to SHI irradiation induced processes.

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During irradiation, SHI beam loses all its energy to the target mainly via electronic

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energy loss (Se). This energy transfer may initiate various processes such as (i) sputtering of

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the material [26] (ii) generation of additional adatoms and surface energy clusters (iii) ionassisted enhancement of surface diffusion [27-31]. All these processes along with ion beam

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induced strain lead to observed change in surface morphology. 3.3. Atomic Force Microscopy (AFM) studies:

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SEM studies revealed that the ion irradiation causes surface modifications. However, the grain size could not be estimated by SEM studies. Therefore AFM was employed to

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estimate grain size and to study the grain size distribution. The surface topography of the as-

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deposited and irradiated Te/Cd/Te thin films was studied using AFM in the contact mode. Figure 4 shows the two-dimensional (2D) AFM pictures of as-deposited and irradiated

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Te/Cd/Te thin films.

Figure 4 (a) shows the AFM image of the Te/Cd/Te as-deposited trilayer film. The

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average grain size calculated using Image J software is estimated to be 48 nm. The surface roughness value for this film is estimated to be 216.4 nm. SEM image of this film also suggested high surface roughness value. After irradiation, surface morphology changes as indicated by SEM studies. No pin holes or cracks are seen in any of the irradiated films. SEM images show formation of rod like structures after irradiation. In AFM images, the upper and lower tips of these rods are seen as grains (as illustrated in Figure 5).

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Figure 4: AFM images of trilayer Te/Cd/Te trilayer films (a) as-deposited (b) irradiated at a fluence of 1  1012 ions/ cm2 (c) irradiated at a fluence of 5  1012 ions/ cm2 and (d) irradiated at a fluence of 1  1013 ions/ cm2

Figure 5: AFM images of trilayer Te/Cd/Te thin irradiated at a fluence of 5  1012 ions/ illustrating the rod like formation

ACCEPTED MANUSCRIPT The average grain size estimated on the basis of Image J software is 26 nm, 20 nm and 12 nm respectively for the films irradiated at a fluence of 1 1012 ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2. The grain size estimated from the AFM images and surface roughness are listed in table 3. Surface Roughness (nm) 216.4 63.39 51.36 42.4

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As-deposited 1  1012 ions/ cm2 5  1012 ions/ cm2 1  1013ions/ cm2

Grain size from AFM images (nm) 48 26 20 12

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Samples

images

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Table 3: Variation of average grain size and surface roughness after irradiation estimated from AFM

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Further, analysis of AFM pictures indicates that the grains have size distribution. Thus to have an understating of size distribution histograms are plotted for all the films using

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Image J software. The grain size distribution is presented graphically by taking grain size ranges along X-axis and the frequencies of respective ranges on Y- axis. The grain size

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distribution is shown in Figure 6. The histogram indicates that the as-deposited film has

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maximum grains in the range 41-50 nm and after irradiation the grain size gradually decreases with increasing fluence. Number of grain having size < 20 nm is found to be

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maximum in case of the film irradiated at a fluence of 1  1013 ions/ cm2. The histograms suggest that the grain size decreases after irradiation. Further, the decrease in grain size with

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increasing irradiation fluence also justifies overall decrease in diameter of rod like structures seen in SEM micrographs of irradiated films decreases.

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Figure 6: Histogram of AFM images of trilayer Te/Cd/Te thin films (a) as-deposited (b) irradiated at a fluence of 1  1012 ions/ cm2 (c) irradiated at a fluence of 5  1012 ions/ cm2 and (d) irradiated at a fluence of 1  1013 ions/ cm2

3.4 Energy Dispersive X-ray Spectroscopy (EDX):

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EDX is an analytical technique used for the elemental analysis or chemical characterization of a thin film. The EDX spectrum of the as-deposited and irradiated films is

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shown in Figure7 (a)–(d). The EDX spectrum of all the films shows that the thin films mainly consists of Cd and Te atoms. The additional peaks corresponding to Si, O and C originate from the glass substrate. The qualitative analysis made on the spectra of as-deposited film gives Cd/Te ratio as 1.26. The composition of the films after irradiation has also been studied and the value of Cd/Te ratio for films irradiated with different ion fluence 1  1012 ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2 is found to be 1.22, 1.20 and 1.20 respectively.

ACCEPTED MANUSCRIPT Table 4 summarizes the EDX results. EDX studies indicate that SHI irradiation does not

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affect Cd/Te ratio even though the CdTe layer mixing occurs after irradiation.

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Figure 7: EDX spectrum of trilayer Te/Cd/Te thin films (a) as-deposited (b) irradiated at a fluence of 1  1012 ions/ cm2 (c) irradiated at a fluence of 5  1012 ions/ cm2 (d) irradiated at a fluence of 1 1013 ions/ cm2

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Thin film

C

O

Si

Te

Cd

Cd/Te mass ratio

As-deposited

11.19

14.59

19.49

24.13

30.60

1.26

1  1012 ions/ cm2

12.09

14.75

19.51

24.09

29.56

1.22

5  1012 ions/ cm2

12.46

14.24

19.43

24.45

29.42

1.20

1  1013 ions/ cm2

11.90

14.57

19.65

24.49

29.39

1.20

Table 4: Weight (%) of elements found in the as-deposited and irradiated films

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3.5 Uv–Vis absorption spectroscopy:

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Figure 8: Tauc’s plot of (𝛼ℎ𝜈)2 versus ℎ𝜈 of Te/Cd/Te thin films thin films (a) irradiated at a fluence of 1 1012 ions/ cm2 (b) irradiated at a fluence of 5 1012 ions/ cm2(c) irradiated at a fluence of 1  1013ions/ cm2

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The optical absorption of as-deposited Te/Cd/Te trilayer film and films irradiated by Ag ion beam at various fluences is studied in the wavelength range of 300 nm to 900 nm. The optical absorption spectra of irradiated films are shown in Fig. 8 (a, b, c). CdTe is a direct bandgap semiconductor. In a direct band gap semiconductor, the absorption coefficient α is correlated with the optical band gap ‘Eg’ by the Tauc’s equation [32]. 1/2

𝛼ℎ𝜈 = 𝐴(ℎ𝜈 − 𝐸𝑔 )

Where ‘A’ is a constant and ‘h’ is photon energy.

(1)

ACCEPTED MANUSCRIPT The value of Eg is determined from an intercept of the tangent drawn on the linear portion of the (𝛼ℎ𝜈)

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versus ℎ𝜈 plot as shown in Figure 8 (a, b, c). The optical band gaps for the

Te/Cd/Te trilayer thin films irradiated with 100 MeV Ag ion beam are found to be 1.54 eV, 1.59 eV and 1.67 eV, respectively for the irradiation fluence of 1 1012ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2. An error of ± 0.02 eV is estimated in the bandgap

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values due positioning of tangent in the linear portion of the (𝛼ℎ𝜈) 2 versus ℎ𝜈 plot. Size of

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the grains can be calculated from the band gap shift (E g–E g (bulk)) with respect to bulk band

𝐸𝑔 = 𝐸𝑏𝑢𝑙𝑘 +

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gap value of CdTe, using Brus effective mass approximation [33] equation 2ℏ2 𝜋 2 𝜇𝑟 2

(2)

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Where E g is the band gap values obtained from experiment, E g (bulk) is the band gap of bulk CdTe, ‘r’ is the radius of the grains and μ is the effective mass of electron–hole pair given

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by,

1

1

1

=𝜇 +𝜇 𝑒

(3)

𝑒

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𝜇

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Where ‘e’ and ‘h’ are the effective masses of electron and hole respectively. The average size of the nanograins calculated using Brus effective mass approximation Equation is ~23 nm, 18 nm and 12 nm for the Te/Cd/Te thin films irradiated at a fluence of

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1  1012 ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2respectively. The grain size

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calculated on the basis of XRD and AFM analysis matches quite well with the grain size calculated, using Brus effective mass approximation equation. Table 5 lists the comparison of crystallite size obtained from XRD, AFM results and Brus effective mass approximation equation. Te/Cd/Te trilayer irradiated at fluence 1 1012 ions/ cm2 5 1012 ions/ cm2 1 1013 ions/ cm2

Debye Scherer formula 22 20 13

Average grain size D (nm) AFM UV-Vis spectroscopy 26 20 12

23 18 12

Table 5: Comparison of average grain size estimated from XRD, AFM and UV-Vis spectroscopy.

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In case of the films irradiated at a fluence of 1 1012ions/ cm2 and 5  1012ions/ cm2, along with CdTe phase some traces of elemental Cd and Te were identified by XRD studies. Thus the band gap values are slightly different than the reported CdTe bulk band gap value of 1.5 eV. Another reason for broadening of band gap could be ion induced strain. In case of the

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film irradiated at a fluence of 1 1013ions/ cm2, value of band gap is found to be 1.67 eV

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which is much higher than the bulk value of bandgap of CdTe. It is well known that in nano-

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sized semiconducting materials, when the grain size is of nanometric dimensions the bandgap increases with decreasing grain size, due to quantum confinement [20]. In case of the film

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irradiated at a fluence of 1  1013 ions/ cm2 the grain size estimated on the basis of XRD and AFM and UV-Vis spectroscopy is 121 nm . Therefore the observed increase in band gap is

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attributed to quantum confinement arising due to nanosized grain formation. 3.6 SHI induced nanocrystalline CdTe phase formation in Te/Cd/Te trilayer films.

Ag ion beam at fluence of 1  1012 ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2. All

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In the present investigation Te/Cd/Te trilayer thin films were irradiated by 100 MeV

the above discussed XRD, AFM and UV-Vis spectroscopy results indicate that SHI

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irradiation induces nanocrystallisation in Te/Cd/Te trilayer and the grain size depends on the

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irradiation fluence. Nanograins of approximate size of 121 nm are formed in the film irradiated at a fluence of 1 1013 ions/ cm2. The observed SHI induced nanocrystalline CdTe phase formation can be understood in the framework of Ion Beam Mixing (IBM) of Te/Cd/Te trilayer. During SHI irradiation the energy is deposited to the target primarily due to electronic energy loss (Se). Ion beam mixing in the electronic energy regime can be explained using two different models, namely (i) Coulomb-explosion model (ii) thermal- spike model [34].

ACCEPTED MANUSCRIPT According to Coulomb-explosion model, during the passage of SHI through the material, a highly ionized zone of positive ions is produced by electronic excitation. This highly ionized volume, along the ion trajectory leads to Coulomb- explosion [35]. The traversing ion transfers its energy to electrons by inelastic collisions, causing their ejection from the atoms. This coulomb explosion lasts for only 10-17s after the ion has passed, and

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occurs in a cylindrical region. Thus a cylindrical region containing charged ions is produced

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during the passage of the ion through the target. The violent electrostatic energy thus generated is connected to radial atomic movement under Coulomb forces. These atomic

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impulses favors inter atomic mixing across the interface [36]. This “Coulomb- spike” phase

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continues until the ions are finally screened by conduction electrons. Due to the resulting cylindrical shock wave, ion tracks may be formed along the trajectory of the ion. The

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complete process of Coulomb- explosion induces strain in the material inside and around the path of the incident ion, which may lead to the fragmentation of crystallites and grains [37].

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In case of metals, the conduction electrons smear out the excitation and ionization of atoms

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and therefore possibility of Coulomb-explosion is generally ruled out. However, for sufficiently high values of Se Coulomb- explosion can be considered as early stage of track

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formation [38].

According to the thermal -spike model, during SHI irradiation, energy is deposited

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by the projectile ions in the electronic sub-system of the target. This energy is shared amongst the target electrons by electron–electron coupling and transferred subsequently to the lattice atoms via electron–phonon coupling. This results in generation of thermal spike and a high temperature cylindrical zone along the ion trajectory is created [39]. Generally, the temperature during the “thermal- spike” phase reaches as high as 2000 0C and the molten zone is created along the ion track. If the interface of the two materials in contact melt, then the inter diffusion of atoms leads to mixing of layers [40]. In the present case, the melting

ACCEPTED MANUSCRIPT temperature of both Te and Cd is of the order of 450 0C and the eutectic temperature of CdTe is quite moderate [41]. Thus Te/Cd/Te trilayer mixing resulting in CdTe phase formation can be expected to occur during this thermal- spike phase. The molten zone is quenched rapidly within 10-12 to 10-11s as the melt cooling rate is very high (1015–1014 Ks−1) and material along the ion track re- solidifies. This re- solidified

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zone called as ‘latent track’ is completely in different structure than the surrounding region

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[42]. Materials undergoing latent track formation during SHI irradiation are called as Se

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sensitive materials [43]. For IBM to occur in multilayer, at least one of the layers should be Se sensitive [44]. Te is reported to be Se sensitive by Srashti et al [44]. Thus IBM of Te/Cd/Te

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trilayer can be expected in the present case.

Due to very high cooling rate of the molten zone, the material along the latent track

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does not get sufficient time to re-crystallize and that would result in formation of nanocrystallites along the ion track [45]. However when irradiation fluence is as high as 1 

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1013 ions/ cm2, the ion induced damage is very severe. The nanocrystallites formed along ion

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track may undergo fragmentation and further decrease in size [24]. Thus formation of CdTe nanograins can be expected when Te/Cd/Te trilayer film is irradiated at a fluence of 1  1013

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ions/ cm2.

Though the exact dynamics of ion-matter interaction for the present investigation

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cannot be captured accurately, it can be concluded that the observed SHI irradiation induced nanocrystallisation in Te/Cd/Te trilayer films is a cumulative effect of two ongoing process i) production of new CdTe nanocrystallites through IBM process ii) fragmentation of existing nanocrystallites at higher fluence.

ACCEPTED MANUSCRIPT Conclusion: In the present investigation, Te/Cd/Te trilayer films are irradiated with 100 MeV 107

Ag ion beam at a fluence of 1 1012 ions/ cm2, 5  1012 ions/ cm2 and 1  1013 ions/ cm2.

X-ray Diffraction (XRD) studies carried out for structural characterization reveal that though the trilayer mixing initiates at irradiation fluence of 1 1012 ions/ cm2, 5  1012 ions/ cm2, the

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stoichiometrically correct hexagonal nanocrystalline CdTe phase is formed in a film

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irradiated at a fluence 1  1013 ions/ cm2. The grain size decreases with increasing fluence.

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The grain size estimated for this film on the basis of XRD studies is found to be 121 nm. Scanning Electron Microscopy (SEM) carried out for morphological studies indicate

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ion beam induced surface modifications. Islands of rod like structures composed of nanograins seemed to be formed after irradiation. With increasing ion fluence, the grain size

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decreases and thus the diameter of rod like structures also decreases. The reduction in grain size is further confirmed by AFM studies.

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Energy Dispersive X-ray Spectroscopy (EDX) indicates that SHI irradiation does not

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affect Cd/Te ratio even though the CdTe layer mixing occurs after irradiation. Uv-Vis spectroscopy carried out for optical characterization indicates increase in band gap value after

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irradiation. Grain size calculated using Brus effective mass approximation equation matches well with grain size estimated on the basis of XRD and AFM results. The band gap change

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for the film irradiated at a fluence of 1  1013 ions/ cm2 is 0.17 eV. The observed band gap widening is attributed to quantum confinement arising due to nanocrystalline nature of the film. Observed SHI irradiation induced nanocrystallization in Te/Cd/Te trilayer films can be due to cumulative effect of two ongoing process i) production of new CdTe nanocrystallites through IBM process ii) fragmentation of existing nanocrystallites at higher fluence. Finally, it can be said that, during SHI induced nanocrystallisation in Te/Cd/Te trilayer films though there is limited size distribution, the grain agglomeration as seen in

ACCEPTED MANUSCRIPT thermal processing can be avoided and grain size can be controlled by controlling irradiation parameters. Acknowledgement: Authors are grateful to IUAC, New Delhi for making SHI irradiation facility available. Thanks are due to Dr. Sandeep Ghosh, TIFR, Mumbai,India for allowing the

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use of UV-Vis spectrophotometer. Help rendered by Ms. Bhagyashree Chalke, TIFR,

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Mumbai, India and Dr. L. Ajith DeSilva, UWG, USA is acknowledged.

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Phys. Res. B. 387 (2016) 1-9.

ACCEPTED MANUSCRIPT Highlights 

Swift Heavy Ion irradiation induced nanocrystallisation in Te/Cd/Te trilayer thin films is reported.

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Ion-matter interaction responsible for nanocrystallisation is discussed.

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CdTe phase formation and grain size is found to be dependent on irradiation

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parameters.