45 keV N5+ ions induced spikes on CdS thin films: Morphological, structural and optical properties

Accepted Manuscript Title: 45nullkeV N5+ ions induced spikes on CdS thin films: Morphological, structural and optical properties Authors: G. Bakiyaraj, J.B.M. Krishna, G.S. Taki, K. Selvaraju, R. Dhanasekaran PII: DOI: Reference:

S0169-4332(18)30569-5 https://doi.org/10.1016/j.apsusc.2018.02.208 APSUSC 38666

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APSUSC

Received date: Revised date: Accepted date:

6-10-2017 15-2-2018 21-2-2018

Please cite this article as: G.Bakiyaraj, J.B.M.Krishna, G.S.Taki, K.Selvaraju, R.Dhanasekaran, 45x202f;keV N5+ ions induced spikes on CdS thin films: Morphological, structural and optical properties, Applied Surface Science https://doi.org/10.1016/j.apsusc.2018.02.208 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

45 keV N5+ ions induced spikes on CdS thin films: morphological, structural and optical properties G. Bakiyaraja,*, J.B.M. Krishnab, G.S. Takic, K. Selvarajud and R. Dhanasekarane a

Department of Physics and Nanotechnology, SRM Institute of Science and Technology

(formerly known as SRM University), Kattankulathur- 603203, India UGC-DAE Consortium for Scientific Research, Kolkata Centre, III/LB-8 Bidhannagar,

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b

Kolkata-700 098, India

Variable Energy Cyclotron Centre, 1/AF- Bidhannagar, Kolkata-700 064, India

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PG & Research Department of Physics, Government Arts College, Ariyalur, Tamil Nadu,

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c

India

Crystal Growth Centre, Anna University, Chennai – 600 025, India

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*Corresponding author: Tel.: +91-44-27417835

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e

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E-mail address: [email protected] (G. Bakiyaraj)

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Graphical abstract

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Highlights    

45 keV N5+ ion beam implanted on CdS thin films with different fluences. No phase transforms (hexagonal structure) occurred for ion implanted films. Optical band gap decreases with increase of implantation fluences. Red, Yellow, Green and Band edge emission is discussed in luminescence spectrum.

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Abstract CBD grown CdS thin films were implanted using 45 keV N5+ ions of different fluences from 1×1015 to 5×1016 ions/cm2 by electron cyclotron resonance (ECR) ion source. Structural properties of pristine and ion beam implanted films were studied by glancing angle X-ray diffraction (GAXRD) and it becomes a hexagonal structure of polycrystalline thin films. High resolution scanning electron microscopy (HRSEM) studies

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shows that the pristine sample forms agglomerated particles of CdS. After ion implantation of higher fluence 5×1016 ions/cm2, numerous defects created spikes on the surface of the film. The spike formation explained with the help of nuclear elastic collisions. The optical

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band gap energy of N5+ ion implanted thin films was reduced with increases of ion fluences.

This is probably due to lattice disorder produced band-tailing and/or creation of impurity states. The red, yellow, green and band edge emissions were studied in correspondence

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with photoluminescence spectrum.

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Keywords: Cadmium sulfide, Ion implantation, Structural properties, Optical properties.

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Postal adress: Department of Physics and Nanotechnology, SRM Institute of Science and

Introduction

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Technology (formerly known as SRM University), Kattankulathur – 603203, India.

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CdS semiconductor became a vital material among the II-VI compound semiconductor with 2.4 eV of band gap at room temperature. Over a past few decades, CdS

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has been identified as a prominent one for its enormous advantages in the conversion of photoelectric charges in solar cells, light emitting capacity in diodes and transistors [1,2]. Numerous methods have been developed to produce CdS thin films via vapor/liquid to

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solid phase transformation as matrix-assisted pulsed laser evaporation (MAPLE) [3], closespaced sublimation (CSS) [4], close spaced vapor transport (CVST), radio-frequency

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planar magnetron sputtering, laser ablation and chemical bath deposition (CBD) [5,6]. Among all these methods CBD is relatively simple, economical, deposition in the large area under very low temperature with the wide range of substrate material. In addition, it limits the oxidation as well as corrosion of metallic substrate when deposition carried out at low temperature. The properties of the CdS thin films which are prepared by CBD are

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highly sensitive to the mode of preparation like agitation, duration, concentration, and type of reactants [7-9]. In general, CBD grown CdS thin films exhibit n-type conductivity which arises from a non-stoichiometric ratio of Cd and S, specifically due to S deficiency. Practically the production of p-type CdS is considered to be very difficult because of selfcompensation effects due to sulfur vacancies. This can be overcome by the use of various acceptor dopants such as nitrogen, phosphorus, copper, and bismuth ion species is

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implanted in CdS lattice atoms. Ion implantation has seemed as one of the powerful

technique for introducing impurities into semiconductors [10-13]. The major essential of

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these are (i) ease of lattice introduction with a single impurity type alone (ii) temperature flexibility (iii) possibility to define the accuracy of area and depth of the implanted region. Narayanan et al. [11] have been reported the formation of acceptor level by N ion irradiation causes for the type conversion (p-type conductivity) in CdS thin films with an

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energy of 130 keV. Also, many others reported the modification of structural and optical

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properties using different ion (N, Mn, Ar, Cu and Ni) implantation on CdS thin films are

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shown in Table 1 [14-20]. So that ion implantation is a very good method for introducing these dopants into CdS thin films in a well-controlled manner. The controlling process

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depends on the incident ions energy and fluence (dose) applied for implantation experiments. In the present work, low energy of 45 keV N5+ ions is used for whole

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experiments with different fluences of 1×1015, 5×1015, 1×1016, 5×1016 ions/cm2 at room

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temperature and the target material as CdS thin films. After ion beam implantation the properties of pristine and implanted CdS thin films were characterized with the support of

Experimental

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GAXRD, HRSEM, UV-visible, and PL.

Before deposition, the glass substrate was rinsed with a soft soap solution, and then

the glass substrate was etched for 30 min with 5% of HCl and further cleaned using

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ultrasonically with de-ionized water. After cleaning, the CdS thin film were prepared by a basic chemical bath solution consists of 40 ml of 1 M cadmium acetate, 40 ml of 2 M thiourea, 13 M ammonia solution and 10 ml of 7.2 M triethanolamine solution. The total volume of the solution is completed to 150 ml by addition of deionized water. The resultant pH of the final solution was 11 ± 0.1.The films are obtained at 90 °C for 15 minutes. The grown film was ultrasonically cleaned for removal of loosely adherent particle on the

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substrate. Finally, the sample was cut into a size of 1×1 cm2. The species of the sample were implanted with N5+ ions energy of 45 keV ion beam with the dose ranging from 1×1015 to 5×1016 ions/cm2. The electron cyclotron resonance (ECR) facility used to carry out the low energy ion implantation and this facility utilized in variable energy cyclotron centre (VECC), Kolkata, India. For even distribution of irradiation over on the thin films, the ion beam was used as a raster scan. The chamber vacuum of 1.6×10-6 torr was maintained for the entire ion beam implantation experiment. A schematic setup used for the

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N5+ ion implantation is given in Fig. 1. The Monte Carlo Stopping and Range of Ions in

Matter (SRIM-2010) [21] computer simulation was used to calculate the projected range, energies for 45 keV N5+ ions in CdS are shown in Table 2.

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longitudinal straggling, lateral straggling, electronic stopping and nuclear stopping

The films crystalline structure was characterized using glancing angle X-ray diffraction (GAXRD) measurements utilized by Rigaku X-ray diffractometer in the 2θ

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range of 10°- 80° with CuKα radiation. The morphological surface of the sample was

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observed by FEI Quanta FEG 200 with energy dispersive X-ray spectrometry (EDX).

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Thickness of the films was measured using Flimetrics method. Optical absorption spectrum was studied using Shimadzu UV-1601PC spectrophotometer over the wavelength range

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400-800 nm. Photoluminescence (PL) measurement was analyzed using a Perkin-Elmer LS-55 luminescence spectrometer with an excitation wavelength of 450 nm. Results and Discussion

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X-ray Diffraction Study

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GAXRD patterns of pristine and ion implanted CdS thin films as a role of ion implanted fluences is shown in Fig. 2. The pristine CdS sample has the diffraction peaks

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values at 2θ (degree) of 24.9º, 26.3º, 36.8º, 43.9º and 54.9º corresponding to (100), (002), (102), (110) and (004) planes of hexagonal/wurtzite structure of CdS thin films. The observed XRD patterns of the samples have been compared with standard PCPDF (PCPDF

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80-0006) files of CdS. After ion implantation all the samples has two extra peaks at 48.2º and 53.0º can be indexed as (103) and (201) planes of hexagonal phase. However, the XRD spectra of 5×1016 ions/cm2 fluence sample have one peak split up of (002) plane at 28.3º. This can be also indexed to (101) reflection of hexagonal CdS. To carry out the systematic study about the consequences of ion beam implantation on the degree of structural order in the films, the peak intensity of the (002) plane was observed. The peak intensity of the

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(002) diffraction suppressed and its full width at half maximum (FWHM) enlarged on increasing ion implantation fluence. It indicates decreases of crystallinity (or) increase of defect in CdS the films. The average crystallite size (d) was determined from the (002) peak using the Scherrer equation d=0.9λ/Dcosθ, where λ is the source of X-ray wavelength and D (radians) is the FWHM of X-ray diffracted peak at the diffraction angle θ. The estimated crystallite size of the pristine sample is about 19.62 nm. Whereas the film implanted at higher fluence (5×1016 ions/cm2), it is found to be 6.49 nm as shown in Table

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3. The average crystallite size of the CdS thin films decreased on increase in ion fluence. According to the kinematical theory of diffraction, if the crystallite peak broadening

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indicates either crystallite size smallness and/or if crystal lattice defects exist in a very large quantity [18]. In fact, similar observation of crystallinity reduction has been observed in Mn-implanted CdS thin films and metal ion doping in ZnO thin films [15, 22]. Morphological analyses

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The surface morphology of pristine and implanted CdS thin films was observed

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using HRSEM and are shown in Fig. 3. The pristine sample (Fig. 3(a)) shows the

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agglomerated particle present on the surface of the substrate. The formation of

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agglomeration particle depends on the growth mechanism of CdS thin films. There are two different possible mechanisms proposed for the CBD grown CdS thin films. Such as (1)

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ion-by-ion deposition (heterogeneous) via Cd and S ionic product grown on the substrate by one another and (2) clusters-by-clusters growth (homogeneous) through CdS colloidal

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atoms initiated in the solution then deposited on the substrate. These two mechanisms are depending on the volume concentrations of Cd2+, S2-, and CdS. While the concentrations

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of CdS atoms is higher than that of Cd2+ and S2-, clusters-by-clusters mechanism is more controlling the growth process: as in reverse, the ion-by-ion mechanism dominates the

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growth process of thin films [23]. In CBD, the as-grown thin film particle agglomerated by a homogeneous reaction. In the present case, the formation of agglomerated particle thin

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films on the substrate is confirmed by cluster-by-cluster growth. Fig. 3(b) shows the HRSEM image of N5+ ion implanted sample with a fluence of

5×1016 ions cm-2. This micrograph displays a surface with irregular spikes. Such a spike formations due to low energy N5+ ion implanted in a solid, it decelerate predominantly ascribed to nuclear elastic collisions with constituent lattice atoms of Cd and S. At initial stage, the incident ion energies of 45 keV N ions, the energy transfer exceeds the threshold

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displacement energy (Ed) of both the constituent lattice atoms of Cd and S (The amount of Ed values for Cd and S is 7.3 and 8.1 eV respectively) [24]. Then the several lattice atoms are relocated from their equilibrium lattice position by the bombardment of each single N ion initiated the collision cascade (recoil cascade). The dense cascade produces more amount of defect in target material is referred to as displacement spike. The formation of displacement spikes continues until the moving displaced atoms lost their energy for further displacement and assemble in a point. Energy transfers will be at subthreshold

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levels. At this point, the energy will be shared between neighboring atoms and will be dissipated as lattice vibrations or heat. This period of lattice heating is known as elastic or

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nuclear thermal spike and may exist for several picoseconds before being quenching to

ambient temperature [25-27]. This is causes for the considerable morphological changes in implanted CdS thin films. Fig. 4 shows the EDX spectrum of as-deposited CdS thin films, which displays cadmium Lα1 (3.14 keV) and Lβ1 (3.34 keV) peaks and sulfur Kα1 and Kβ1

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(2.32 keV) peaks. The other elements such as O, Na, Mg, Al and Si are from the glass

Optical absorption studies

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substrates.

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The optical absorbance spectrum of pristine and implanted samples is shown in Fig. 5. The processes of optical absorption, which correlates with the electron, excite from the

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valence band to conduction band and it can be applied to calculate the amount of optical

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band gap (Eg). Absorption coefficient (α) is calculated using Beer-Lambert relation α = 2.303A/t, where A is absorbance and t is film thickness [28]. The Eg value has been

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determined by Tauc’s relation [29]: α = (A/hν) (hν-Eg)n, where A is a constant, hν is energy of the incident photon and the superscript n expect the values 3, 3/2, 2 and 1/2 for forbidden indirect, forbidden direct, allowed indirect and direct transitions respectively. The value of

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n is 1/2 for CdS because of allowed direct band gap material and αhν = A(hν-Eg)1/2. The Eg of material has been found by extrapolating the straight line of the plot (αhν)2 vs. hν on the

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energy axis, as shown in Fig. 6. The measured Eg value for the CdS thin films is found to be 2.54 eV. The optical band gap is decreased gradually with increases of the ion implantation fluence (Table 3). At the current moment, the remarkable amount of reduction in Eg can be ascribed to disorder influence band tailing and formation of impurity levels in between valance and conduction band. Diminishing of Eg due to the formation of impurity

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states as acceptors or donors has been described for CdS thin films doped/implanted with other metal foreign bodies (e.g. Sn and Ni) [20,30]. 3.4

PL measurements Fig. 7 shows the photoluminescence spectra of pristine and N ion implanted CdS

thin films with different fluence were recorded at room temperature. The spectral features

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appearing 1.54-2.00 eV, 2.00-2.07 eV, 2.07-2.18 eV and 2.18-2.54 eV are usually mentioned to as “infrared/red”, “orange”, “yellow” and “green” bands emission respectively. To recognize the possible transitions responsible for the photoluminescence

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process, an energy-level diagram of the present work are pictured in the Fig. 8.

The pristine sample spectrum has two dominant peaks at 1.83 eV and 2.34 eV which corresponds to red and green band emission respectively. The red emission (RE) is commonly due to sulfur states to valence band transitions in particular sulfur vacancies

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(Vs). The basis of green emission (GE) is due to recombination of donor-acceptor (D-A)

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pairs. In addition, the pristine sample spectrum also has two weak peaks at 2.07 eV

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(shoulder) and 2.55 eV is corresponding to yellow and band edge emission (BEE) respectively. The yellow emission (YE) is described by the transitions between interstitial

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cadmium Icd to valence band. The main reason for the very weak band edge emission is due to more Vs dominate the red emission and other deep defects exist in pristine samples

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[31]. The major cause for all emission in pristine sample spectrum is due to non-

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stoichiometry of CdS thin films grown by CBD technique [32]. In the case of all ion beam implanted samples observed the new weak peak at 2.47 eV can be believed the transitions

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between exciton bound to neutral-donor (D0-X) or exciton bound to neutral acceptor (A0X), both are accountable for such transitions. In practice, the D0-X and A0-X transitions

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values are nearly close to the band gap (Eg), i.e. VB to CB [33,34]. Another interesting feature is that with increasing the ion implantation fluences, the intensity of PL peaks tends to decrease and the band edge emission is observed red shift at lower energy regions. These

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are likely to be originated from the enhancement of non-radiative recombination via additional deep levels introduced by ion implantation. This is in the best concurrence with the optical band gap results wherein we found orderly lowering band gap with increasing ion fluences. 4

Conclusion

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Ion beam induced spike on CdS thin films were characterized by GAXRD, HRSEM, UV-visible and PL studies. GAXRD analyzed that the all samples have a hexagonal structure and it confirms the ion implantation does not change the crystal structure. Scanning electron microscopy studies of ion implantation fluence at 5×1016 ions/cm2 showed spike formation due to defects induced disorder. The red shift obtained for the band gap with the increases of ion implantation fluence in the CdS thin films. The PL spectra of pristine CdS films show three defect level emission peaks located at 1.87 eV,

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2.07 eV, and 2.34 eV is corresponding to red, yellow and green emission respectively and

the band edge emission obtained at 2.55 eV. All implanted sample observed an additional

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peak at 2.47 eV due to transitions involving excitonic (D0-X) or (A0-X). The red shifting

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of band edge emission when increasing ion fluences were discussed with optical band gap.

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Acknowledgements

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The authors thank all the ECR ion source staff, Mr. D.K.Chakraborty, Mr. Kamal Dev, Mr. K. Dutta, Mrs. Sumitra for helping us in ion implantation experiments. The

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authors wish thankful to Dr. Abhijit Saha, Director, UGC-DAE Consortium, Kolkata centre

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for providing the UV-Vis and PL measurements.

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Figure Captions Fig. 1 Schematic diagram of ECR ion source for N5+ ion implantation Fig. 2 XRD patterns of pristine and N5+ ion implanted CdS thin films with the fluences of 5×1015 ions/cm2, 1×1016 ions/cm2 and 5×1016 ions/cm2 Fig. 3 HRSEM picture: (a) Pristine and (b) N5+ ion implanted with the fluence of 5×1016 ions/cm2

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Fig. 4 EDX spectrum of as-deposited CdS thin film Fig. 5 Absorbance spectra: (a) Pristine, (b) 1×1015 ions/cm2, (c) 5×1015 ions/cm2, (d) 1× 1016 ions/cm2 and (e) 5×1016 ions/cm2

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Fig. 6 Plots of (αһν)2 and һν: (a) Pristine, (b) 1×1015 ions/cm2, (c) 5×1015 ions/cm2, (d) 1× 1016 ions/cm2 and (e) 5×1016 ions/cm2

Fig. 7 PL spectra: (a) Pristine, (b) 1×1015 ions/cm2, (c) 5×1015 ions/cm2, (d) 1×1016

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ions/cm2 and (e) 5×1016 ions/cm2

Fig. 8 Energy level diagram corresponding to different transitions (eV) of CdS thin

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RE GE BEE

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2.47 eV

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Table Captions Table 1 Ion beam implantation induced modification properties on CdS thin films Ion source for implantation

Fluences (ions cm-2)

Remarkable properties with reference

150 keV N+

1×1014-5×1015

Thermal evaporation

120 keV Mn

1×1013-1×1016

Vacuum evaporation CBD

140 keV Ar+

1×1015-1×1017

80 keV Ar+

5×1016

Thermal evaporation

90 keV Co+

0.6-3.6×1016

Vacuum evaporation

100 keV Ar+

1-1×1016

Thermal evaporation

90 keV Ni+

Decrease in band gap with fluences is due to implantation-induced lattice disorder [14]. Ion implantation does not lead to formation of any secondary phase [15]. Formation of metallic cluster was observed by XRD [16]. Post implantation annealing suggests the removal of defects and strain during annealing [17]. Implantation created grain growth, enhanced roughness and large degree of disorder [18]. FWHM of A1(LO) mode increase with implantation dose was attributed to implantation induced surface roughness and lattice damage [19]. Implantation induced grain growth and modifies the luminescence properties by creating shallow acceptor states [20]. HRSEM shows large number of defects created a spike on the surface of film and red shift observed in band gap with increasing fluence also correlated with photoluminescence studies [in this work].

M

SC R

U

N

A

EP

1-5×1016

A

CC

0.5-3×1016

D

TE 45 keV N5+

CBD

IP T

Thin film deposition methods CBD

21

Data calculated using SRIM for CdS thin film

Ion energy

electronic stopping (eV/Å)

nuclear stopping) (eV/Å)

Projected range (Å)

45 keV N5+

4056

1853

813

Longitudinal straggling (Å) 575

Table 3 Crystallite size and Band gap energy (Eg) of CdS thin films Dose (002) peak Crystallite size Eg 2 (ions/cm ) (FWHM in radians) (nm) (eV) 0.45

19.62

2.54

5x1015

0.87

10.15

2.44

1x1016

0.99

08.91

5x1016

1.36

06.49

SC R

As-grow

Lateral straggling (Å) 450

IP T

Table 2

2.41

A

CC

EP

TE

D

M

A

N

U

2.40

22