Modifications on CdS thin films due to low-energy ion bombardment

Radiation Effects & Defects in SolidsVol. 167, No. 1, January 2012, 59–68

Modifications on CdS thin films due to low-energy ionbombardment

Indra Sulaniaa,b*, Dinesh Agarwalb, Surya K. Tripathic and Mushahid Husaina

aDepartment of Physics, Jamia Millia Islamiya University, New Delhi 110025, India; bInter UniversityAccelerator Centre, Material Science Division, New Delhi 110067, India; cDepartment of Physics, Centre

of Advanced Study in Physics, Panjab University, Chandigarh 160014, India

(Received 18 October 2010; final version received 3 March 2011 )

In the present study, thin films of cadmium sulfide (CdS) of thickness ∼300 nm were used to bombardwith 350 keV argon (Ar4+) ions. The films were deposited on glass by thermal evaporation method. Theirradiation was performed for ion fluences 1 × 1015, 3 × 1015 and 1 × 1016 ions/cm2 at normal incidenceto study the modification in surface nanostructures as well as structural and optical properties of the films.The pristine and irradiated films were characterized by X-ray diffractometer, UV–visible spectroscopy andatomic force microscopy (AFM). The bombarded surface shows the evolution of structures from circulargrains of wider size distribution to smaller grains (with a narrow distribution of sizes) with an increasein the ion fluence. X-ray diffraction of the pristine film shows the polycrystalline nature of the film withmost intense peak at 26.49◦ along the (002) plane in a hexagonal phase. The crystallite size was foundto be between 29 and 35 nm. The band gap of the pristine film was observed as 2.24 eV. The low-energybombardment has resulted in the relaxation of strain and in an improvement in the crystallinity of the CdSfilms.

Keywords: AFM; cadmium sulfide; ion beam; XRD

PACS: 68.37.Ps, 81.05.Dz, 41.75.Ak, 61.05.cp

1. Introduction

The bombardment of surfaces with low-energy ions can lead to the development of variousmicron and nano-sized surface structures. These structures include pits, cones, dots, ripples, etc.The phenomena responsible for production of such features are sputtering (1, 2), re-deposition ofsputtered material and surface diffusion (3). It can be understood by looking at the interaction ofenergetic ions with matter. When low-energy ions pass through a material, they lose their energythrough a series of collisions with the nuclei of different atoms. In this process, the ions createmany defects in the material through a series of collision cascades (1, 4). The ion can fully transferits energy to the atom and displace it from its position or partially transfer its energy, creating thesecondary collision cascades. In this manner, ions can play a crucial role in altering the structural

*Corresponding author. Email: [email protected]

ISSN 1042-0150 print/ISSN 1029-4953 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/10420150.2011.569715http://www.tandfonline.com

60 I. Sulania et al.

as well as optical properties of the material. It also modifies the surface of the material giving riseto ordered structures such as nanoripples or nanodots.

The evolution of nanostructures induced by ion bombardment was first explained by Bradleyand Harper (5) in 1988 with the linear equation. This surface evolution is caused due to aninterplay between sputtering-induced roughening and diffusion-induced smoothing. The Bradleyand Harper theory (BH theory) and its refinements (6–9), based on the surface instability causedby local surface curvature dependent sputtering, are used frequently to explain the formation ofordered nanostructures on the surface of the material due to ion bombardment (10). A lot of workhas been carried out in the field of nanoscale patterning of the surfaces with low-energy ions ofdifferent species and ion energies, especially on bulk materials (11–14). Recently, few reportshave come on the formation of ordered dots on metallic thin films (15). In the present work, wehave studied the formation of ordered nanodots on cadmium sulfide thin films. Cadmium sulfide(CdS) is a II–VI compound semiconductor. Thin films of CdS have attracted many researchers fortheir importance in electrochemical cells (16), gas sensors (17) and most importantly in solar cellheterostructure (18). CdS does not play a direct role though; it acts as an optical window (19–22)due to its large band gap (Eg). It is preferred for coating over other materials of wider Eg due toits compact crystallographic cell. It has been used in high-efficiency solar cells formed with Cu2S(23), CuSe2 (24), etc. The Eg of bulk CdS is 2.5 eV (25); however, for CdS thin films, it variesdepending upon the technique used for deposition. For thin films prepared by evaporation, it wasfound to be 2.2 eV (26) and 2.6 eV for the films deposited by chemical methods (27). The stressat the CdS film–substrate interface is another factor that influences the value of Eg (28). To obtaina good-quality film, annealing is done usually at different temperatures (29). But annealing dueto ion beams is different from conventional annealing. Irradiation leads to the creation of a widevariety of defects as well as annealing of defects that change the physical, chemical and opticalproperties of the material.

In this paper, we have studied the thin film of CdS instead of bulk material to study the creationof surface nanostructures and modification in other properties by low-energy ion bombardment.The latest technologies for the large production of solar cells are based on thin film geometry, asthese techniques are of low costs due to lesser material consumption and are easily available (16).

2. Experimental

In the present experiment, thermally evaporated CdS films have been used. These films wereprepared on glass by evaporating 99.99% CdS powder in a molybdenum (Mo) boat using resistiveheating method. The thickness of the films was around 300 nm. These samples were bombardedwith 350 keV Ar4+ ions in Low Energy Ion Beam (LEIB) facility at Inter University AcceleratorCentre (IUAC), New Delhi. The samples were bombarded at normal incidence with respectto the surface normal. The time of bombardment was varied for three fluences, viz. 1 × 1015,3 × 1015 and 1 × 1016 ions/cm2. The irradiation was carried out at a pressure of 10−6 Torr at roomtemperature. The beam current was ∼6 μA/cm2. The nuclear and electronic energy losses of Arions inside CdS were found to be 387.5 and 645.2 keV/μm, respectively, and range of the ionsinside CdS was around 253 nm as calculated using the SRIM code (30). It has been known fromthe literature that sputtering yield is a function of nuclear energy loss and incidence angle of theincoming ion. Although electronic energy loss (Se) dominates over nuclear energy loss (Sn), Se

is still below the threshold of tracks creation inside the sample. So the dominating process isSn which leads to the surface modifications. Since the two competing processes, sputtering andsurface diffusion, are responsible for the surface nanostructuring the value of Sn plays a significantrole in the tuning shape and size of the structures. The bombarded and pristine samples werecharacterised using a Multimode IIIa atomic force microscope (AFM), from Digital Instruments.

Radiation Effects & Defects in Solids 61

The AFM was used to observe changes in surface morphology of the films with the increase in ionfluence. TheAFM characterisation was performed using single-crystal Si probes, fromVeeco, witha radius of curvature ∼10 nm. The measurements were carried out in tapping mode, in air. Grazingincidence X-ray diffraction measurements of the thin film samples (pristine and irradiated) wereperformed using a X-ray diffractometer, from Bruker (AXS D8 Advance). The scans were taken atan incidence angle of 2◦ to see the structural changes inside the samples as a result of irradiation.The scanning range (2θ ) was 20◦–60◦. Step size of 0.02◦ and step time of 3 s were used for themeasurements. The source of X-rays was Cu Kα radiation having a wavelength λ = 0.154 nm.The identification of peak positions and crystalline phases were accomplished using PCPDFWINsoftware files. The optical absorption spectra were recorded with the conventional two-beammethod using U-3300 UV-visible spectrophotometer from Hitachi. All the characterisations wereperformed at IUAC, New Delhi.

3. Results and discussions

3.1. AFM analysis

The AFM micrographs of the pristine and bombarded films are shown in Figure 1. All the imagesare in 1 × 1 μm. The data scale (z) is shown inside the images in black letters. The AFM imageof the pristine sample (Figure 1(a)) show circular dots/grains with a wide distribution of sizes.The mean size of the dots was ∼60 nm with a root mean square (rms) roughness of 6.5 nm. Afterbombardment with energetic ions with a fluence of 1 × 1015 ions/cm2, the dots become irregularin shape with a wider distribution of size (Figure 1(b)). The roughness of the sample increasesto 8.8 nm. The mean size of the nano dots is ∼75 nm. At 3 × 1015 ions/cm2, the structures were

Figure 1. AFM micrographs of the pristine (a) and irradiated ((b), (c) and (d) for 1 × 1015, 3 × 1015 and1 × 1016 ions/cm2, respectively) CdS films. All the images are in 1 μm × 1 μm.


smaller in size with a mean size of ∼50 nm and rms roughness was 7.5 nm (Figure 1(c)). At1 × 1016 ions/cm2, the structures are circular in shape with smaller sizes. The structures have anarrow distribution of size (Figure 1(d)).The mean size of the structures was found to be 45 nm witha roughness of 4.1 nm. This is the most ordered surface with minimum roughness and narrowerdistribution (Figure 2). The roughness alone gives insufficient information about the change inmorphology as it is 2-dimensional (2D) information in x and y planes only. It is a numericalparameter and depends on the scan size. To get the complete information, power spectral density(PSD) was studied to understand the changes observed in the morphology due to irradiation. PSDcan provide quantitative information about the surface roughness in both the vertical and lateraldirections and is independent of the scan size. The 2D PSD function is defined as (31)

PSD(y) = 1

area

[∫∫d2r

2πe−iqr〈h(r)t 〉

]2

, r = (x, y),

where q is the spatial frequency (nm−1) and h(r) is the surface height at a point r .

Figure 2. Size distribution of the particles of pristine and irradiated CdS films (calculated by the section analysis fromAFM).


Figure 3. PSD versus spatial frequency plots of the pristine and irradiated CdS films.

The scan sizes of 5 μm × 5 μm and 512 × 512 data points were used to scan the samples inAFM. The scaling behaviour of the thin film samples was analysed using PSD curves obtainedfrom AFM images by extracting the values of roughness exponent, α, and growth exponent, β.The exponents α and β give spatial and temporal evolution of the structures, respectively. Thelog plot of 2D PSD function versus spatial frequency is shown in Figure 3. The horizontal low-frequency part shows the uncorrelated white noise that arises due to random arrival of ions onthe surface. There is an increase in the intensity of the plateau for a fluence of 1 × 1015 ions/cm2

which indicates an increase in roughness. The intensity falls off at higher fluences. The high-frequency linear part shows correlated surface features. The curves do not show any significantpeaks corresponding to the nanodot structure which tells us that the structures lacks the selection ofparticular wavelength (32) and follows power law dependence (33). The slopes (n) for the differentcurves of PSD function was calculated to be in the range of 3.6–3.9 and α was determined using therelation, α = (n − d)/2, where n is the slope obtained and d is the dimension of PSD taken fromAFM images. The value of α was found to be between 0.7 and 0.9. This high value of α indicatesthat as the ions are bombarded on the surface, the ion-enhanced surface diffusion term dominates.The surface becomes smoother for higher fluences. The growth parameter, β, was determined byplotting rms surface roughness of the thin film samples versus ion fluence in log scale (Figure 4).The slope gives the value of β which was found to be 0.32 ± 0.09. This value of β also suggests thatthe diffusion induced due to ion bombardment dominates. The Edward–Wilkinson (EW) equation(34) shows the same scaling properties; in this regime, non-linear effects eventually stabilises thesurface and surface relaxation takes place by diffusion with β = 0.3. The scaling parameter, z =α/β, was found to be 0.8/0.34 = 2.34 The CdS thin film, after bombardment with the ion beam,follows the scaling behavior as mentioned in the KPZ equation, given by Kardar et al. (35), themodified BH equation. This equation gives information about the evolution of growing interfaces.

3.2. XRD analysis

Figure 5 shows the XRD spectra of the pristine as well as bombarded samples. The XRD spectrareveal that CdS films were polycrystalline in nature with the hexagonal phase. The main peak


Figure 4. Roughness (rms) versus fluence plot.

Figure 5. XRD spectra of the pristine and irradiated CdS films (inset shows the shift in the peaks).

with the highest intensity was evolved along (002) at a 2θ value of 26.49◦. The peak was shiftedto a lower value compared with its original value of 26.53◦. This indicates that the pristine filmwas under an expansive strain. The energetic ion bombardment leads to strain relaxation. Theother significant peaks were along planes (102), (103), (112) and (004) with corresponding 2θ

values of 36.73◦, 47.96◦, 51.96◦ and 54.66◦, respectively. There is an increase in the intensityof the peaks with an increase in the ion fluence. This shows an improvement in the crystallinity


of the samples due to annealing of defects (34, 36) after bombardment. However, the full-widthat half-maximum (FWHM) of the peaks also increases. Moreover, peaks also shifted slightlytowards the higher 2θ value. These observations suggest that ion irradiation relaxes the strain thatis already present in the film. The average crystallite sizes of the CdS particles were calculatedusing Scherer’s formula (37):

D = 0.9λ

β cos θ,

where λ is the X-ray wavelength (0.154 nm), β is the FWHM of the most intense peak, i.e.(002) peak and θ is the Bragg diffraction angle. However, the FWHM of the Bragg peaks atdifferent fluences can have contributions from the rms value of strain fluctuation and the finitecrystallite size. The average crystallite size of the pristine film was found to be 33.6 nm. Theaverage crystallite size decreases to 29.4 nm after irradiation with a fluence of 1 × 1015 ions/cm2

and the crystallite sizes were found as 31.5 and 32.6 nm, respectively, for 3 × 1015 and 1 ×1016 ions/cm2. The effect of strain is calculated using Williamson–Hall (WH) analysis (38) ofthe FWHM (β) of various Bragg peaks as shown in Figure 6. The graph was plotted betweenβ cos θ/λ on the y-axis and 2 sin θ/λ on the x-axis. The slope of the linear line fitted on the data

Figure 6. WH plot for pristine and irradiated samples.


Table 1. Strain and crystallite size as calculated by the WH plot and average grain size ascalculated by AFM section analysis.

Fluence Crystallite size (nm) Strain Particle size(ions/cm2) (from Scherer’s formula) (from WH plot) (nm) (from AFM)

0 33.6 0.031 601 × 1015 29.4 0.029 753 × 1015 31.5 0.028 501 × 1016 32.6 0.025 45

gives the value of the strain for each of the sample and the intercept on the y-axis gives the valueof the crystallite size. The calculated values of strain and crystallite size are tabulated in Table 1.The strain is maximum for the pristine sample and is minimum for the sample irradiated with thehighest fluence. This indicates strain relaxation and more ordering (also seen in the AFM imagein Figure 1(d)) after ion bombardment.

3.3. Optical absorption analysis

The dependence of the absorption of CdS films deposited on glass on the wavelength is shown inFigure 7. The optical absorption spectra of the samples were recorded in the wavelength rangingfrom 300 to 900 nm. The spectra show the absorption edge around 550 nm. The absorption edgebecomes clearer for the irradiated samples, which indicate an increase in the crystallinity of thesamples. There is an increase in absorption due to irradiation. The band gap of the pristine andthe bombarded films was calculated using Tauc plots by plotting the (αhν)2 versus (hν) andextrapolating the linear part of the absorption edge to the energy axis. There is a slight changein the band gap observed as calculated using Tauc plots from pristine to bombarded films. Theband gap of the pristine film was found to be 2.24 eV, and for the samples irradiated at 1 × 1015,

Figure 7. Absorption spectra of pristine and irradiated CdS films.


3 × 1015 and 1 × 1016 ions/cm2, it was found as 2.26, 2.27 and 2.28 eV, respectively. There is nosignificant change observed in the band gap of the CdS films after irradiation.

4. Conclusion

The effect of low-energy ion bombardment on the CdS thin films has been analysed. The filmsformed were polycrystalline in nature with a hexagonal phase and with a band gap of 2.24 eV. Theabsorption of the films increases with ion irradiation. There is an improvement in the crystallinityobserved with the increase in the ion fluence. The average crystallite size was found to be ∼30 nm.There was an inherent strain observed in the as-grown samples. The strain got relaxed as a result ofbombardment. AFM analysis shows that the surface is becoming smoother with the increase in theion fluence. The nanodots observed were uniform in shape and size for the highest ion fluence.This surface ordering is due to the dominating diffusion term in the modified Bradley–Harperequation. The roughness and growth parameter found were consistent with the KPZ model. Theseparameters indicate that non-linear effects eventually stabilize the surface and surface relaxationtakes place through ion-induced surface diffusion.

Acknowledgements

The authors would like to thank Dr Praveen Kumar for delivering a stable beam during the experiment. The authors wouldalso like to acknowledge Mr Pawan Kulriya for his help in XRD measurements. The authors would also like to thank DSTfor providing the AFM and XRD facilities at IUAC, New Delhi.

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Modifications on CdS thin films due to low-energy ion bombardment

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