1
3
Structural, morphological and electrical properties of pulsed
4
electrodeposited CdIn
2Se
4thin films
5 6
7
D. Sudha
a, S. Dhanapandian
b,⇑, C. Manoharan
b, A. Arunachalam
b8 aManonmaniam Sundaranar University, Tirunelveli, Tamil Nadu, India
9 bDepartment of Physics, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India
1011 12
1 4 a r t i c l e i n f o
15 Article history:
16 Received 24 July 2016 17 Accepted 5 September 2016 18 Available online xxxx
19 Keywords:
20 Structural studies
21 XPS
22 AFM
23 Optical properties and photosensitivity 24
2 5
a b s t r a c t
Semiconducting CdIn2Se4thin films have been deposited on to the conducting glass substrates using 26 pulsed electrodeposition technique. X-ray diffraction (XRD) shows the films are polycrystalline in nature 27 having cubic crystal structure. Energy dispersive analysis (EDAX) spectrum of the surface composition 28 confirms the nearly stoichiometric CdIn2S4nature of the film. Atomic force microscope (AFM) shows that 29 the deposited films are well adherent and grains are uniformly distributed over the surface of substrate. 30 The surface roughness and grain size increased from 0.2 nm and 12 nm to 0.7 nm and 20 nm respectively 31 with increase of duty cycle. The films are highly transparent in the visible region with an average trans- 32 mittance reaching up to 80%, which indicates the better crystalline nature with less defects. The CdIn2Se4 33 thin films show band gap energies of the films decreased from 3.12 eV to 3.77 eV. 34
Ó2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// 35 creativecommons.org/licenses/by-nc-nd/4.0/). 36
37 38
39 Introduction
40 Recently compounds of ternary II–III–VI group are a focus for 41 many researchers due to their applications in solar cells and opto- 42 electronic devices. Cadmium indium selenide (CdIn2Se4) is a semi- 43 conducting ternary chalcogenide of the type AIIBIIIX4where A = Cd, 44 Zn or Hg, B = Ga or In and X = S, Se or Te. The ternary chalcogenides 45 have potential applications in solar energy conversion due to their 46 interesting tailored properties[1–3], Among II–III–IV ternary group 47 compounds; CdIn2Se4is one of the interesting semiconductors due 48 to its absorption property with narrow band gap and low resis- 49 tance[4,5]. Because of its narrow band gap, this compound is also 50 widely used in many applications such as in nonlinear optics[6], in 51 optoelectronic devices, semiconducting devices, radiation detec- 52 tors, laser materials, thermoelectric devices, solar energy convert- 53 ers, etc.[7,8]. The material in thin film form can be obtained by 54 electrodeposition, slurry pasting technique, vacuum evaporation 55 and spray pyrolysis[10]. The properties of Cd-Chalcogenide thin 56 films have been studied by others[11,12]. In this report, results 57 obtained on pulse electrodeposited CdIn2Se4 films are presented 58 and discussed.
59 In the present study, it is aimed to enhance the optical and elec- 60 trical properties of CdIn2Se4 using pulsed electrodeposition 61 method for photovoltaic application.
Experimental procedure 62
CdIn2Se4 films were pulse electrodeposited at different duty 63 cycles in the range of 6–50%. The deposition potential was main- 64 tained at0.95 V(SCE). Titanium and conducting glass substrates 65 were used. The films were deposited at 80°C. The precursors used 66 were 0.1 M CdSO4, 0.25 M In2(SO4)2and 5 mM SeO2in diethylene 67 glycol. Thickness of the films measured by Mitutoyo surface pro- 68 filometer increased from 600 to 950 nm with increase of duty 69 cycle. The preparative parameters, such as substrate temperature 70 and concentration of precursor solution have been optimized and 71 are found to be 280°C and 0.0125 M, respectively. The pH of the 72 bath was adjusted to 2 by adding H2SO4. The total deposition time 73 was 60 min. Thickness of the films measured by Mitutoyo surface 74 profilometer was in the range of 500–900 nm with increase of duty 75 cycle. 76
The structural characterization of the deposited films was car- 77 ried out by X-ray diffraction technique on SHIMADZU-6000 78 (monochromatic Cu-K
a
radiation,k= 1.5406 Å). The surface topo- 79 logical studies were carried out using Atomic force Microscope 80 (Nano surf Easy scan2) AGILENT-N9410A-5500. Optical absorption 81 spectrum was recorded in the range of 300–1200 nm using JASCO 82 V-670 spectrophotometer. The electrical resistivity, carrier concen- 83 tration and mobility were measured by automated Hall Effect mea- 84 surement (ECOPIA HMS – 3000) at room temperature in a van der 85 Pauw (VDP) four – point probe configuration. 86http://dx.doi.org/10.1016/j.rinp.2016.09.004 2211-3797/Ó2016 Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
⇑ Corresponding author.
E-mail address:[email protected](S. Dhanapandian).
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Results in Physics
j o u r n a l h o m e p a g e : w w w . j o u r n a l s . e l s e v i e r . c o m / r e s u l t s - i n - p h y s i c s
87 Results and discussion
88 Structural studies
89 X-ray diffraction pattern recorded for CdIn2Se4thin films pre- 90 pared on titanium substrates with bath composition of 50 mM 91 CdSO4, 25 mM In2(SO4)3and 5 mM SeO2at900 mV versus SCE 92 at 80°C and at different duty cycles is shown inFig. 1. The total 93 deposition time was 50 min in all cases. X-ray diffraction patterns 94 revealed that the deposited films are polycrystalline with tetrago- 95 nal structure with lattice constants (a= 5.77 Å; c= 11.59 Å), the 96 diffraction peaks of tetragonal CdIn2Se4 with the lattice planes 97 (1 0 1), (1 1 1), (1 1 2), (3 0 0), (3 1 1) and (1 0 4) . The different peaks 98 in the diffractogram are indexed and the corresponding values of 99 interplanar spacing ‘d’ are calculated and compared with standard 100 values JCPDS card no – 74-0216. All the peaks identified are from 101 CdIn2Se4and no additional lines corresponding to Cd, In and Se 102 are present. The height of (1 1 1) peak increases with duty cycle, 103 which indicates that the deposited films exhibit preferential orien- 104 tation along (1 1 1) planes. It is also observed that the height of 105 preferential peak is found to increase with duty cycle. The crystal- 106 lite size of the deposited films is calculated using FWHM data and 107 Debye Scherrer formula given below[13].
108
D¼0:95k=ðbcoshÞ ð1Þ
110 110
111 wherekis the wavelength of Cu K
a
target used (k= 1.540 Å),his 112 Bragg’s diffraction angle at peak position in degrees andbis Full 113 Width at Half Maximum of the peak in radian. The crystallite size 114 increased from 15 nm to 40 nm with increase of duty cycle. The dis- 115 location densityd, defined as the length of dislocation lines per unit 116 volume of the crystal has been evaluated using the formula[14]117 Fig. 1.XRD pattern of CdIn2Se4films deposited at different duty cycles.
Table 1
Microstructural parameters of CdIn2Se4films deposited at different duty cycle.
Duty cycle (%) Thickness (nm) Lattice parameter (Å)
Crystal size (nm) Strain (104) Dislocation density (1015cm3)
a c
6 600 5.76 11.58 12 2.51 6.94
9 680 5.76 11.58 20 2.45 2.50
15 760 5.77 11.59 25 2.24 1.60
33 825 5.77 11.59 30 2.12 1.11
50 950 5.77 11.59 34 1.87 0.87
Table 2
Chemical composition of CdIn2Se4films deposited at different duty cycles.
Duty cycle (%) Cd (at.%) In (at.%) Se (at.%)
6 14.75 28.56 56.69
9 14.80 29.00 56.20
15 14.84 29.24 55.92
33 14.90 29.35 55.75
50 15.10 29.60 55.30
Fig. 2.XRD pattern of CdIn2Se4films deposited at 50% duty cycle and post heat treated at different temperatures (a) 400°C, (b) 475°C and (c) 525°C.
Fig. 3.EDS spectrum of CdIn2Se4films deposited at 50% duty cycle.
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d¼1=D2 ð2Þ 119
119
120 The microstructural parameters are presented inTable 2. From 121 Table 1, it is observed that the crystallite size increases with the 122 decrease of strain and dislocation density which indicate better 123 crystallinity. Information on particle size and strain for the films 124 was obtained from the full-width at half-maximum of the diffrac- 125 tion peaks. The full-width at half-maximumbcan be expressed as 126 a linear combination of the contributions from the particle size,D 127 and strain,
e
through the relation[15]128
bcosh=k¼1=Dþ
e
sinh=k ð3Þ130 130
131 The plot ofbcosh/kvssinh/kallows us to determine both strain 132 and particles size from slope and intercept of the graph. The esti- 133 mated values for films deposited at different duty cycles are listed 134 inTable 1. The deviation in the lattice parameter values from the 135 bulk value observed in the present case clearly suggests that the 136 grains in the films are under stress. Such behavior can be attributed 137 to the change of nature, deposition conditions and the concentra- 138 tion of the native imperfections developed in thin films. This 139 results in either elongation or compression of the lattice and the 140 structural parameters. The density of the film is therefore found 141 to change considerably in accordance with the variations observed 142 with the lattice constant values. The defects have a probability to 143 migrate parallel to the substrate surface so that the films will have 144 a tendency to expand and develop an internal tensile stress. This 145 type of change in internal stress is always predominant by the 146 observed recrystallization process in polycrystalline films. The 147 stress relaxation is mainly considered as due to dislocation glides 148 formed in the films. The decrease of internal stress may be attrib-
uted to a decrease in dislocation density. From Table 1, it is 149 observed that along with the increase of Cd and Se content, the 150 strain and dislocation density increases which is because of the dif- 151 ference in the ionic radius of Cd, In and Se. The stress of the films 152 was found to be compressive stress and is indicated with a nega- 153 tive sign, which is slightly increased due to the change in the mor- 154 phology of the film while doping. The reduction in the strain and 155 dislocation density with increase of duty cycle may be due to the 156 reduction in concentration of lattice imperfections. The films were 157 post heat treated in argon atmosphere at different temperatures in 158 the range of 400–525°C for 20 min in order to induce photoactivity 159 (seeFig. 2). 160
Composition analysis 161
Compositional analysis of the film Cd1.01In1.96Se4.03deposited at 162 50% duty cycle indicates the nearly stoichiometric composition. 163 Fig. 3 shows the EDAX spectrum of CdIn2Se4 films deposited at 164 50% duty cycle. With decrease of duty cycle, slight excess of Se is 165 observed. This is due to the more noble character of Se compared 166 to Cd and In, at duty cycles less than 33%, the concentration of Se 167 ions is higher than that at higher duty cycles. The composition is 168 shown inTable 2. 169
XPS spectral analysis 170
In order to investigate the valence states of elements in the 171 films, XPS spectral analyses were conducted (as shown inFig. 4). 172 The core levels of Cd 3d, In 3d, and Se 3d were observed. The Cd 173
Fig. 4.XPS spectrum of CdIn2Se4films deposited at 50% duty cycle.
D. Sudha et al. / Results in Physics xxx (2016) xxx–xxx 3
3d core splits into 3d5/2(405.0 eV) and 3d3/2(411.8 eV) peaks, 174 which are all in good agreement with the reported values for 175 Cd2+. The peaks centered at binding energy of 444.7 and 452.3 eV 176 coincide well with In 3d5/2and In 3d3/2. The Se 3d peak is located 177 at 53.06 eV, as deduced from the intensities of the relevant XPS 178 peaks, the stoichiometry of Cd/In/Se is 1.00:2.03:4.05. All these 179 results indicate that only Cd2+, In3+, Se2are present in the product. 180
Morphological studies 181
Surface morphology of the films studies by atomic force micro- 182 scope indicated that the grain size increased with duty cycle. The 183 surface roughness also increased with duty cycle due to increase 184 in grain size.Fig. 5shows the Atomic force micrographs of CdIn2- 185 Se4films deposited at different duty cycles. The surface roughness 186 and grain size increased from 0.2 nm and 12 nm to 0.7 nm and 187 20 nm respectively with increase of duty cycle. 188
Optical studies 189
Transmission spectra of the films deposited at different duty 190 cycles are shown in Fig. 6. The spectra exhibited interference 191 fringes. 192
Fig. 5.Atomic force micrographs of CdIn2Se4films deposited at different duty cycles (a) 9%, (b) 33% and (c) 50%.
Fig. 6.Transmission spectra of CdIn2Se4films deposited at different duty cycles (a) 6%, (b) 15%, (c) 33% and (d) 50%.
4 D. Sudha et al. / Results in Physics xxx (2016) xxx–xxx
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The optical absorption of the films was obtained from the trans- 193 mission spectra and studied in the range 300–1200 nm. The trans- 194 mission spectra of the films show high transparency in the visible 195 region with an average transmittance reaching values up to 80% 196 indicate better crystal structure with less defects obtained at 197 higher substrate temperatures. The high transparency obtained 198 for the film in the visible region shows its potential application 199 in the photovoltaic device applications. 200
The variation of optical density with wavelength is analyzed to 201 find out the nature of transition involved and the optical band gap. 202 The nature of the transition is determined using the relation: 203 Fig. 7.Tauc’s plot of CdIn2Se4films deposited at different duty cycles (a) 50%, (b)
33%, (c) 15% and (d) 6%.
Fig. 8.Variation of room temperature resistivity with duty cycle for CdIn2Se4films.
Fig. 9.Variation of room temperature mobility with duty cycle for CdIn2Se4films.
Fig. 10.Variation of room temperature mobility with duty cycle for CdIn2Se4films.
Fig. 11.Photocurrent – Intensity characteristics of CdIn2Se4films deposited at different duty cycles (a) 6%, (b) 9%, (c) 15%, (d) 33% and (e) 50%.
D. Sudha et al. / Results in Physics xxx (2016) xxx–xxx 5
204
a
¼Aðhm
EgÞn=hm
ð4Þ206 206
207 where
a
is the optical absorption coefficient, hm
is the photon 208 energy,Ais a constant andnis the number that depends on the nat- 209 ure of transition (1/2 and 2 for direct and indirect transitions 210 respectively). For allowed direct transition, n= 1/2, the value of 211 absorption coefficient is of the order of 104cm1 supporting the 212 direct band gap nature of the semiconductor. The plot of (a
hm
)2ver-213 sus h
m
for a typical sample deposited at optimized preparative 214 parameters is depicted inFig. 7It is linear indicating the presence 215 of direct transition. The linear portion is extrapolated toa
= 0, at216 energy axis giving the band gap energy of CdIn2Se4varied in the 217 range of 3.12–3.77 eV as the duty cycle decreased. This variation 218 is due to quantum size effects. The grain size of the films deposited 219 at the lower duty cycle is lower compared to the films deposited at 220 higher duty cycles. This is supported by XRD results (grain size 221 decreases from 34 to 12 nm as the duty cycle decreases. The value 222 of the band gap is higher than an earlier report (Eg1.79 to 1.84 eV 223 (Dir)) [16]. It is observed that the band gap is increased with 224 increase in grain size. The variation of band gap of thin films is gen- 225 erally due to individual or combined effect of some factors like band 226 filling effect, quantum size confinement causing increase in band 227 gap with increase of grain size, charge impurities at the grain 228 boundaries, lattice strain present in the films and the extent of 229 structural disorder. Similar trend is observed by Preetam Singh 230 et al., 2007 and the author states that the increase in the band 231 gap with the increase of the deposition temperature, could be due 232 to lesser defects and better crystal structure[17].
233 Electrical studies
234 Hall Effect measurements are performed in order to investigate 235 the electrical properties (resistivity, carrier concentration and the 236 Hall mobility) of CdIn2Se4 thin films deposited at different sub- 237 strate temperatures. Room temperature transport parameters 238 were determined for the films deposited at different duty cycles.
239 Indium contacts were provided on the edges of the top surface of 240 the film.Fig. 8shows the variation of room temperature resistivity 241 with duty cycle. It is observed that as the duty cycle increased, the 242 resistivity decreased due to the increase of grain size. The resistiv- 243 ity value is lower than the earlier report[18].
244 Room temperature mobility decreased from 20 cm2V1s1to 245 5 cm2V1s1 as the duty cycle increased from 6% to 50%.Fig. 9 246 shows the variation of mobility with duty cycle.
247 Room temperature carrier concentration increased from 248 3.91015cm3 to 3.121016cm3 as the duty cycle increased.
249 Fig. 10shows the plot. The increase in carrier density with duty 250 cycle is the reason for the decrease of resistivity with duty cycle.
251 The decrease of resistivity with increase of substrate tempera- 252 ture is attributed to the reduction in the volume of grain bound- 253 aries. The grain boundary conduction plays an important role in 254 the conductivity of thin films. At higher substrate temperatures 255 the resistivity is increased, which is due to chemisorptions of oxy- 256 gen at grain boundaries[19].
257 Various crystalline imperfections in the film, such as vacancies, 258 dislocation and grain boundaries act as trapping or recombination 259 centers of the carriers and play an important role in photoconduc- 260 tion. These traps act as localized positive potential centers for elec- 261 trons and negative potential centers for holes. Therefore some 262 localized discrete energy levels are formed in the band gap, in 263 the vicinity of the conduction and valence bands respectively.
264 Fig. 11 shows the variation of photocurrent with light intensity 265 of CdIn2Se4films deposited at different duty cycles. The photocur- 266 rent is found to increase with an increase of duty cycle due to 267 increase in film thickness and light intensity. As the thickness of 268 the film increases the crystalline nature increases and this helps
in the improvement of photocurrent. The increase in photocurrent 269 is attributed to an increase in the majority carrier concentration 270 and/or an increase in impurity centers acting as traps for minority 271 carriers. The variation of photocurrent with applied voltage in 272 CdIn2Se4films is shown inFig. 12. The photocurrent increases with 273 an increase in voltage. 274
Photosensitivity is the ratio of the increase in conductivity of 275 the material in the presence of light to the conductivity in darkness 276 and is given by the relation: 277
278 Photosensitivity¼D
r
=r
¼ ðILIdÞ=Id 280280whereILandIdrepresent the current under illumination and in the 281 dark respectively. It seems that some transitions that create addi- 282 tional free carriers effectively increase the free life time increasing 283 the photosensitivity of the material.Fig. 13shows a plot of photo- 284 sensitivity versus light intensity of CdIn2Se4 thin films. Thinner 285 films exhibit moderate photosensitivity, whereas thicker films are 286 found to exhibit higher photosensitivity. Crystallographical 287 Fig. 12.Photocurrent – Voltage characteristics of CdIn2Se4 films deposited at different duty cycles (a) 6%, (b) 9%, (c) 15%, (d) 33% and (e) 50%.
Fig. 13.Photosensitivity vs intensity of CdIn2Se4films deposited at different duty cycles (a) 6%, (b) 9%, (c) 15%, (d) 33% and (e) 50%.
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288 imperfections acting as trapping centers will enhance the 289 photosensitivity, whereas the recombination centers decrease the 290 photosensitivity.
291 Photoelectrochemical (PEC) cells were prepared using the films 292 deposited on titanium substrates heat treated at different temper- 293 atures. The films were lacquered with polystyrene in order to pre- 294 vent the metal substrate portions from being exposed to the redox 295 electrolyte. These films were used as the working electrode. The 296 electrolyte was 1 M polysulphide (1 M NaOH, 1 M Na2S, 1 M S).
297 The light source used for illumination was an ORIEL 250 W Tung- 298 sten halogen lamp. A water filter was introduced between the light 299 source and the PEC cell to cut off the IR portion. The intensity of 300 illumination was measured with a CEL suryamapi, whose readings 301 are directly calibrated in mW cm2. The intensity of illumination 302 was varied changing the distance between the source and the cell.
303 The power output characteristics of the cells were measured by 304 connecting the resistance box and an ammeter in series and the 305 voltage output was measured across the load resistance. The pho- 306 tocurrent, dark current and output voltage were measured with a 307 HIL digital multimeter.
308 The CdIn2S4photoelectrodes were dipped in the electrolyte and 309 allowed to attain equilibrium under dark conditions for about 310 10 min. The dark current and voltage values were noted. The cells 311 were then illuminated by the light source and the current and volt- 312 age were measured for each setting of the resistance box. The pho- 313 tocurrent and photovoltage were calculated as the difference 314 between the current under illumination and the dark current, 315 and voltage under illumination and dark voltage respectively.
316 Conclusion
317 CdIn2Se4 films have been deposited for the first time by the 318 pulse technique using non aqueous electrolyte. This method is an 319 easily reproducible one and is economically viable. X-ray diffrac- 320 togram of the films exhibit cubic spinal structure. EDAX studies 321 indicated the ratio of the weight percentage of In to Cd is nearly 322 2 and Se to Cd is nearly 4, which is required stoichiometric ratio 323 of CdIn2Se4films. XPS studies indicated the presence of Cd 3d, In 324 3d and Se 3d in the ratio of the starting composition. The films 325 are highly transparent in the visible region with an average trans- 326 mittance reaching up to 80%, which indicates the better crystalline
nature with less defects. Optical absorption measurements indi- 327 cated the band gap to vary from 3.12 eV to 3.77 eV as the duty 328 cycle decreased. Electrical resistivity was found to vary from 329 80.0Xcm to 40.0Xcm as the duty cycle increased. Mobility of 330 the films was found to decrease from 20 cm2V1s1to 5 cm2V1- 331 s1 with increase of duty cycle. Carrier concentration increases 332 from 3.91015cm3 to 3.121016cm3 as the duty cycle 333 increased. AFM studies indicated that the grain size increases from 334 10 to 30 nm as the duty cycle increases. The films exhibited 335 photoconductivity. 336
Uncited reference 337
[9]. 338
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