influence of substrate, process conditions, and post

Influence of substrate, process conditions, and post-1

annealing temperature on the properties of ZnO thin films 2

grown by SILAR method 3

4

Bijoy Chandra Ghos1**, 2†, Syed Farid Uddin Farhad1†, *, Md Abdul Majed Patwary2,3,, Shanta 5

Majumder1**, 2, Md. Alauddin Hossain2, Nazmul Islam Tanvir1, Mohammad Atiqur Rahman2, 6

Tooru Tanaka3, and Qixin Guo3 7

8

1 Energy Conversion and Storage Research Section, Industrial Physics Division, BCSIR Labs, Dhaka 9

1205, Bangladesh Council of Scientific and Industrial Research (BCSIR), Bangladesh; 10

[email protected] (N.I.T.) 11

12

2 Physical Chemistry Research Laboratory, Department of Chemistry, Comilla University, Cumilla 13

3506, Bangladesh; [email protected] (B.C.G.), [email protected] (M.A.H.), 14

[email protected] (S.M.) 15

3 Department of Electrical and Electronic Engineering, Saga University, Honjo, Saga 840-8502, Japan 16

[email protected] (T.T), [email protected] (Q.G) 17

†Contributed equally to this work 18

** Student during postgraduate research in IPD, BCSIR. 19

*Correspondence: [email protected] (S.F.U. Farhad). 20

Abstract: Here we report the effect of substrate, sonication process, and post-annealing on the 21

structural, morphological, and optical properties of ZnO thin films grown in the presence of 22

isopropyl alcohol (IPA) at temperature 30 – 65 ℃ by successive ionic layer adsorption and reaction 23

(SILAR) method on both soda lime glass (SLG) and Cu foil. The X-ray diffraction (XRD) patterns 24

confirmed the preferential growth (002) and (101) plane of wurtzite ZnO structure while grown on 25

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SLG and Cu foil substrate respectively. Both XRD and Raman spectra confirmed the ZnO and Cu-26

oxide phases of the deposited films. Scanning electron microscope (SEM) image of the deposited films 27

shows compact and uniformly distributed grains for samples grown without sonication while using 28

IPA at temperature 50 and 65 ℃. The post-annealing treatment improves the crystallinity of the films, 29

further evident by XRD and transmission and reflection results. The estimated optical bandgaps are 30

in the range of 3.37-3.48 eV for as-grown samples. Results revealed that high-quality ZnO thin films 31

could be grown without sonication using IPA dispersant at 50 ℃, which is much lower than the 32

reported results using the SILAR method. This study suggests that in the presence of IPA, the SLG 33

substrate results in better c-axis oriented ZnO thin films than that of DI water, ethylene glycol, 34

propylene glycol at the optimum temperature of 50 ℃. Air-annealing of the samples grown on Cu 35

foils induced the formation of CuxO/ZnO junctions which is evident from the characteristic I-V curve 36

including the structural and optical data. 37

Keywords: ZnO thin films, SILAR, IPA dispersant, copper oxide, Post-annealing, c-axis orientation 38

39

40

1. INTRODUCTION 41

ZnO is amongst the most widely used n-type metal oxide semiconductor materials because of its 42

unique structural, optical and electrical properties in conjugation with cheap, non-toxic nature, and 43

natural abundance [1-3]. It has distinctive optoelectronic and physical properties such as tunable 44

direct wide bandgap of about 3.37 eV, high transparency (>80 %) in the visible region, and large 45

exciton binding energy (60 meV) at room temperature [4-12], optimum refractive index (n ≈ 2.0), 46


notable electron mobility (as large as 155 cm2/V.s) [13-15]. Moreover, ZnO are chemically and 47

thermodynamically stable and basically crystallizes in the hexagonal wurtzite structure [16]. All the 48

above unique features make ZnO a suitable material for diverse applications including anti-reflective 49

coating (ARC), solar cells, and transparent conductive oxide for flat panel displays [17,18], 50

photodiodes [19], gas sensors [20,21], light emitting diodes [22], surface acoustic waves [23], 51

protective surface coatings [24], piezoelectric transducers [25] etc. These potential applications have 52

boosted research related to the development of better quality ZnO thin films over the span of ongoing 53

decades. 54

Both physical and chemical methods have been used for the synthesis of ZnO thin films for instances, 55

successive ionic layer adsorption and reaction (SILAR) [26,27], chemical bath deposition (CBD) [28], 56

pulsed laser deposition (PLD) [29], RF magnetron sputtering [30], metal organic chemical vapour 57

deposition (MOCVD) [31], sol-gel derived dip coating [32], spray pyrolysis [16], hydrothermal [33], 58

molecular beam epitaxy [34], drop casting [35], and different sol-gel derived spin coating [36] 59

techniques etc. Among them, SILAR is one of the simplest and economically favorable chemical 60

methods because it produces durable and adherent thin films comparatively at low processing 61

temperatures and it does not need any sophisticated and modern instruments [36,37]. Furthermore, 62

this technique allows bulk region deposition on various substrates such as soda lime glass (SLG) 63

microscopy slides, fluorine doped tin oxides (FTO), and Cu foil substrates [16,38,39] etc. The 64

deposition technique relies on wide range of processing parameters such as bath temperature, 65

solution pH, complexing and dispersant agents, and rinsing procedures [40-44] etc. 66

To our best knowledge, only a few reports have been published so far regarding the deposition of 67


ZnO thin films using Cu foil. Raidou et al. [45] have grown ZnO thin films on three kinds of substrates 68

such as Cu, Si, and glass by the SILAR method. They have showed that the structure of the film 69

depends strongly on the nature of the substrate, for instance, ZnO particles deposited on Cu substrate 70

formed hexagonal structure whereas spindles shape was formed on the Si substrate and for glass 71

substrate the film was in the form of small flowers and prisms [45]. 72

Gao et al. [27] first reported ZnO thin films deposition by incorporating an ultrasonic rinsing step in 73

the SILAR method. Subsequently, Shei et al. improved the process and investigated the effects of 74

deionized water (DI), ethylene glycol, and propylene glycol between the rinsing steps as well as 75

rinsing temperature on the structural and optical properties of ZnO thin films. In those cases, ethylene 76

glycol imposes environmental risks [5]. They reported that higher growth temperature is necessary 77

to produce highly c-axis oriented ZnO thin films when using ethylene glycol and propylene glycol 78

during the rinsing processes [5,46,47]. To address above issues, this study aims to investigate the 79

influence of sonication, usage of isopropyl alcohol (IPA) dispersant, and thereafter post-annealing 80

effect on the structural, morphological and optical properties of SILAR deposited ZnO thin films on 81

SLG and Cu foil substrates. Cu foils were chosen mainly to investigate the copper oxide forming 82

conditions near the ZnO nucleation cite, as well as the formation of Cu2O/ZnO or CuO/ZnO junctions 83

depending on IPA and post-annealing temperature. The use of IPA showed a strong effect as 84

dispersing agent over other conventional dispersants and the post-annealing was found to be 85

beneficial for making CuxO/ZnO (x = 1, 2) heterojunction. The experimental results are presented and 86

discussed below. 87

88

2. MATERIALS AND METHODS 89


2.1. Materials 90

In this work, zinc chloride (ZnCl2, purity~ 98 %, Scharlau), isopropyl alcohol ((CH3)2CHOH, 91

purity ~99.70 %, Active Fine Chemicals), and concentrated ammonia (NH4OH, ~28% solution, Merck 92

Millipore) were used. All the reagents were of analytical grade and used without further purification. 93

Both non-conducting SLG (40×25×1 mm3) and conducting thin Cu foil (40×20×0.2 mm3) were used to 94

deposit ZnO thin films. 95

96

2.2. Synthesis of ZnO Thin Films 97

ZnO thin films were deposited simultaneously both on SLG and Cu foil substrates by using a 98

similar SILAR method describe elsewhere [41]. Briefly, the SLG were cleaned with detergent followed 99

by successive cleaning in an ultrasonic bath using DI water, toluene, acetone, isopropyl alcohol and 100

again DI water each for 15 minutes. On the other hand, Cu foils were treated with cottonwood 101

soaked in 10M HNO3 solution and finally dried in air. Prior to the film deposition, zinc complex 102

([Zn(NH3)4]2+) precursor solution was prepared by mixing 0.1 M ZnCl2 and concentrated (~28 %) 103

ammonia solution (NH4OH). NH4OH was added to adjust the solution pH 10 [46,47]. Subsequently, 104

both SLG and Cu foil (tied back to back) [41] were immersed together into the precursor zinc-complex 105

solution and then dipped into unheated deionized water each for 20 seconds which results in the 106

formation of Zn(OH)2 precipitate. 107

Additionally, counter ion (Cl-) and coarsely adhered Zn(OH)2 grains were removed from the 108

substrate by immersing it into ultrasonic-assisted DI water for 30 seconds. Furthermore, the 109

substrates were treated with IPA for 20 seconds to form ZnO. IPA acted as a dispersing agent which 110

reduced the ZnO agglomeration and enhance the decomposition capability of Zn(OH)2 to ZnO when 111


the temperature was increased from room temperature (~30 ℃) to 65 ℃. Finally, the substrates were 112

dipped into ultrasonic-assisted DI water (see Fig. 1). The above steps were repeated up to 20 times 113

for preparing the sample with IPA at 30, 50, and 65 ℃ and labeled the as-deposited samples as IPA30, 114

IPA50, and IPA65 respectively. The same deposition procedure was repeated by eliminating the 115

sonication step. The most important deposition parameters and processing conditions are 116

summarized in Table 1. 117

118

TABLE 1. Sampling details for the deposition of ZnO thin films with IPA at different 119

temperature. 120

Deposition

temperature (0C)

Glass substrate Copper foil substrate

Sonication Without sonication Sonication Without sonication

30 G1 G4 C1 C4

50 G2 G5 C2 C5

65 G3 G6 C3 C6

121

122


FIG. 1. A schematic diagram showing the SILAR deposition of ZnO thin film simultaneously on 123

SLG and Cu foil substrates using IPA dispersant. The optimized deposition process facilitates to 124

eliminate the ultrasonication steps. 125

126

The overall reactions involved in the ZnO film deposition are given below [5]: 127

128

129

After deposition, the samples were thoroughly rinsed by DI water, and then dried under laboratory 130

atmosphere and safely stored into the sample boxes for characterization. Some of the samples were 131

cut into equal pieces for subsequent characterizations as well as 1 h air-annealing at 250 ℃; while one 132

piece of each batch was kept as as-deposited sample for reference. 133

134

2.3. Characterization techniques 135

The structural properties and phase of the deposited thin films were investigated by XRD 136

(Philips PANalytical X’Pert MRD) with a CuKα (λ = 0.15406 nm) radiation source in θ-2θ coupled 137

mode and Raman spectroscopy (Horiba HR800) with 488 nm laser excitation (P≤ 5 mW). Surface 138

morphologies of the samples were imaged by scanning electron microscope (SEM) (Philips XL30 EEG 139

SEM). The optical properties were examined by using a double-beam UV-Visible-NIR 140

spectrophotometer (Shimadzu UV 2600 ISR plus) in the range of 220-1200 nm. Both diffuse reflection 141

and transmission spectra were taken to eliminate substrate contributions [35] form the thin film 142

where necessary. Dark and LED (white) illuminated I-V curve of the SILAR grown CuxO/ZnO 143

Zn2+

+ 2OH

-

Zn(OH)2(s)

Zn2+

+ 4NH4

+ + 4OH

-

Zn(OH)2(s)

ZnO(s) + H2O

[Zn(NH3)

4]

2+ + 4H

2O


junctions including ohmic contact nature on the ZnO layer were recorded by a Keithley 2450 source 144

measure unit (SMU) coupled with a homemade multiprobe workstation (IPD, BCSIR). 145

146

3. RESULTS AND DISCUSSION 147

3.1. Structural Characterization 148

The phase and crystal structure of both as-deposited and annealed samples were examined by 149

XRD in the range of 2θ = 25-45° and the relating XRD patterns are shown in Fig. 2. The deposited 150

samples grown on SLG exhibited three intense peaks at 2θ ≈31.74°, 34.40° and 36.21° which could be 151

assigned respectively to (100), (002) and (101) planes of ZnO with hexagonal wurtzite structure [35]. 152

No diffraction peaks of Zn(OH)2 are discernible in the XRD patterns (see Fig. 2a). In the case of 153

samples deposited on Cu foil, thin films are preferably deposited along the (101) plane of ZnO, where 154

the diffraction peak at 2θ ≈ 43.5° corresponds to the Cu (111) plane arising from the underlying 155

substrate (see Fig. 2b). All of the Cu foil samples produced CuxO/ZnO (x = 1, 2) structure after post-156

annealing at 250 ℃ irrespective of the growth temperature with IPA and sonication process (see top 157

panels in Fig. 2). This may be due to the oxidation of Cu foil substrate as can be seen from both XRD 158

and Raman spectra depicted in Fig. 2b and Fig. 3b. These results suggest the formation of Cu-159

oxide/ZnO heterojunction depend only on the post-annealing but not on the sonication process and 160

IPA dispersant. Thus, the post-annealing is beneficial for the formation of CuxO/ZnO heterojunctions 161

[16,45,48]. It is evident from the Fig. 2a, the strong preferential growth along (002) plane of ZnO 162

observed for samples grown on SLG suggesting highly c-axis oriented films [35] while preferred 163

orientation along (101) plane for samples deposited on Cu foil [45] as can be seen in Fig. 2b. Thus, the 164

crystal growth is strongly influenced by the substrate types. The strong peak along (002) plane for 165


the as-deposited G5(IPA50) and G6(IPA65) samples signifies highly c-axis oriented ZnO films [35] 166

which is absent for G4(IPA30) samples, suggesting that temperature of IPA promotes crystallinity of 167

the as-deposited ZnO film. The intensity of the concerned diffraction peak is seen to increase further 168

after post-annealing. Therefore, increasing the IPA temperature and post-annealing improved the 169

crystallinity of the deposited thin films [35,46]. The same trend is also observed for samples deposited 170

on Cu foil. In both cases, without sonication, samples exhibited better crystallinity as shown in Fig. 2 171

and the good quality films can be formed using IPA with a minimum of 50 ℃. Shei et al. [5,46] have 172

reported no film growth below 75 and 95 ℃ respectively by using ethylene glycol and propylene 173

glycol. 174

175

176

FIG. 2. XRD patterns of both as-made and post-annealed samples grown on (a) SLG and (b) Cu foil. 177

The diffraction peaks of corresponding materials are shown by arrow sign for clarity. 178


179

It is also evident from Fig. 2a that highly textured films can be prepared for samples G5(IPA50) and 180

G6(IPA65) without sonication steps. This may be due to the fact that IPA acts as a better dispersant 181

compared to ethylene glycol, propylene glycol and DI water reported in refs. [5,46,47] and results in 182

depositing better quality ZnO thin films. 183

The important structural parameters and mean crystallite sizes (D) of SLG-samples were calculated 184

by using Scherrer equation [49] to the 002 diffraction peak of ZnO and summarized in Table 2: 185

D = 𝑘𝜆

𝛽𝑐𝑜𝑠𝜃 (1) 186

Where, λ= wavelength of X-ray (0.15406 nm for CuKα radiation source), k= constant which is 0.94, β 187

= Full width at half maxima (FWHM), θ= Diffracted angle (FWHM and 2θ are in degrees). 188

189

TABLE 2. Mean crystallite size and lattice strain of as-made and annealed samples 190

deposited on SLG using IPA at different temperatures. 191

Sample 2θ

(deg.)

FWHM

(deg.)

Crystallite size

(nm)

Lattice strain (ε)

× 𝟏𝟎−𝟑

G4(IPA30) 34.37 0.73 12 10.3

G4 Ann. 250 ℃ 34.51 0.63 14 8.9

G5(IPA50) 34.41 0.37 23 5.2

G5 Ann. 250 ℃ 34.48 0.45 19 6.3

G6(IPA65) 34.48 0.48 18 6.7

G6 Ann. 250 ℃ 34.51 0.41 21 5.8

192

It is evident from the Table 2 that the mean crystallite sizes were 12-23 nm and 14-21 nm respectively 193

for as-deposited and annealed samples grown on SLG. The crystallite size shows an increasing trend 194

and the lattice strain decreases with post-annealing at 250 ℃ (G4 Ann. 250 ℃ and G6 Ann.250 ℃) 195

which signifies the improvement of the crystallinity [5] of the films as can be seen from Figure 2a. 196


The sample deposited on SLG at 50 ℃ (G5) in the absence of sonication exhibited the highest 197

crystallinity (D = 23 nm) and minimum lattice strain among all samples with (002) preferential 198

growth. Therefore, these observations indicated the optimum temperature in presence of IPA should 199

be 50 ℃ for growing better quality c-axis oriented ZnO thin films without sonication steps. 200

201

3.2. Raman Analysis 202

Room temperature Raman measurements of samples deposited on both SLG and Cu foil were 203

carried out to identify the phase purity of Zn- and Cu-oxides as well as to investigate the effect of 204

processing conditions on their crystalline structure. Raman spectroscopy is an effective tool 205

to analyze the small changes as the vibrational signals are very sensitive to the local environment of 206

the molecule, crystal structure, chemical bond, etc. [50]. The Raman spectra of the samples deposited 207

on both SLG and Cu foil are shown in Fig. 3a and Fig. 3b respectively. 208

209

210


FIG. 3. Raman spectra of the samples grown on (a) SLG and (b) Cu foil. The reference Raman shift 211

values are indicated by different symbols in the figure. 212

213

It is clear from Fig. 3a that the post-annealing exhibited a broad Raman signal approximately at 574 214

cm-1 which could be attributed to ZnO [51] for samples G1 Ann 250 ℃ and G2 Ann.250 ℃ for which 215

XRD peaks were not clear in Fig. 2a. In contrast, the samples G5(IPA50) and G6(IPA65) (without 216

sonication) showed two distinguishable peaks centering at ~440 and ~574 cm-1 which have been 217

attributed to highly crystalline c-axis oriented ZnO films due to a decrease of defects in the interior 218

of the crystal [51]. 219

In Fig. 3b, the films on Cu foil show Raman peaks centering at 97, 98, 405, 407, 410, 428, 434, 494, 496, 220

569 and 574 cm-1 correspond to ZnO phase. Moreover, the Raman signals for copper oxide (Cu2O + 221

CuO) mixture phases were evidenced for all annealed samples (C1 Ann. 250 ℃ to C6 Ann. 250 ℃). 222

In addition, peaks at around 147, 214, 644 cm-1 appeared for samples grown on the Cu foil. The new 223

Raman shift appeared close to 145, 216, 284, 493 cm-1 confirmed the existence of Cu2O phase while 224

those close to 298, 330, 346, 626 cm-1 are attributed to CuO phase [50-59]. These observations indicate 225

the possibility of the facile CuxO/ZnO formation by post-annealing at a temperature as low as 250 ℃, 226

and are also in consistent with the observed XRD pattern shown in Fig. 2b. From both XRD and 227

Raman analyses, it can be concluded that for depositing single phase highly crystalline c-axis oriented 228

ZnO films, it may be better to deposit on SLG and post-annealing at 250 ℃ for 1 h results in the 229

formation of Cu-oxide/ZnO heterojunction for films grown on Cu foil. 230

231


3.3. Morphological characterization 232

Fig. 4 compares the surface morphologies of thin films grown on both SLG and Cu foil. From 233

Fig. 4a and Fig. 4d, it is clearly seen that the samples deposited using IPA at 30 ℃ (labeled as IPA30) 234

exhibits cotton-like amorphous morphology (see also XRD patterns in Fig. 2). In contrast, compact 235

and uniformly distributed spherical grains were observed both for pristine IPA50 and IPA65 samples. 236

Thus, at relatively high temperatures good quality films are produced as it provides sufficient energy 237

for complete conversion of Zn(OH)2 to ZnO [5]. The grain sizes of the films grown on SLG were 238

slightly larger (~260-300 nm) than those grow on Cu foil (~200-230 nm) further indicating better 239

quality films corroborating the XRD results. Some overgrown clusters for Cu foil-samples can be seen 240

which might be detrimental for device applications. Thus, the film quality is not affected only by the 241

temperature of the IPA but also the types of substrate. Previous studies reported that relatively higher 242

temperature ( ≥ 95 ℃ ) was required to decrease agglomeration for ethylene glycol and propylene 243

glycol used as dispersing agent [5,46]. Since isopropyl alcohol is a monohydric alcohol, it forms only 244

inter-molecular hydrogen bond [60] and affects the deposition process, free from releasing thermal 245

energy due to the breaking of intra-molecular H-bonding. Consequently, Zn(OH)2 species are easily 246

removed through H-bonding which are loosely adsorbed on the ZnO surface. This property makes 247

IPA to act as a better dispersant than ethylene glycol and propylene glycol at relatively low 248

temperatures [61]. Intriguingly, the IPA50 sample grown without sonication exhibited a compact 249

microstructural morphology together with appreciable crystallite size (D = 23 nm) and optical band 250

gap (Eg = 3.37 eV). These observations assert that the surface morphologies can be controlled by 251

controlling the IPA temperature and by selecting a suitable substrate. 252

253

254


255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

FIG. 4. SEM micrographs of the samples deposited using IPA dispersant at various temperatures on 284

SLG (a) G4(30 ℃), (b) G5(50 ℃), (c) G6(65 ℃); and on Cu foil (d) C4(30 ℃), (e) C5(50 ℃), (f) C6(65 285

℃). 286

287

3.4. Optical characterization 288

To elucidate the optical properties of the as-grown and annealed samples on SLG, both 289

transmission and diffuse reflection spectra (normalized using same illumination area of dia~6 mm 290

for all samples) were recorded. In case of the samples deposited on Cu foil, only the reflection spectra 291

f

e

d

c

b

a


were taken. The diffuse reflection spectra of samples grown without the sonication process at 292

different temperatures with IPA have been included in Fig. 5. 293

294

295

FIG. 5. Normalized Transmission (a) and Diffuse reflection (b) spectra of the samples deposited on 296

SLG substrate, and Normalized diffuse reflection spectra of the samples deposited on Cu foil (c). 297

Both as-deposited and annealed samples have been included in the same graph for comparison. The 298

vertical lines are indicating the approximate absorption edge of ZnO, Cu2O and CuO. 299

300

From Fig. 5a, it is clearly seen that the samples grown on SLG shows roughly 35 - 65% transparency 301

in the visible region and the transparency of the films is seen to improve upon annealing except for 302

G6 Ann.250 0C. Upon annealing, the absorption edge is seen to shifted from the lower wavelength 303


(λ ≈ 350 nm, marked by solid line) to higher wavelength (λ ≈ 380 nm, marked by dotted line) region 304

(Fig. 5a and Fig. 5b) and such red shift of the absorption edges can be attributed to the crystalline 305

improvement of the ZnO thin films [35]. From Fig. 5b, samples grown with IPA at higher 306

temperatures (50 and 65 ℃) exhibit red-shifted absorption edges which further confirmed the better 307

crystallinity and film growth with fewer defects in the interior crystal [5] and supports the XRD data. 308

In contrast, for the samples grown on Cu foil (Fig. 5c), sharp absorption edges near λ ≈380, 580 and 309

780 nm can be clearly seen (see fade vertical lines) which could be attributed to ZnO, Cu2O and CuO 310

phase respectively [41,50,62], The presence of mixed (Cu2O + CuO) phases formed on Cu foil were 311

also confirmed from the XRD and Raman spectra shown in Fig. 2b and Fig. 3b. 312

The optical bandgap was estimated from the Tauc plot generated by using the reflection data and the 313

Kubelka-Munk function (F(𝑅∞) [35] represented by equation: 314

(hυF(𝑅∞))𝑛 = A(hυ - 𝐸𝑔) (2) 315

Where, 𝐸𝑔= Bandgap energy, R∞= Diffuse reflection, h = Planck’s constant and υ = Frequency of the 316

incident light. ZnO is a direct bandgap material (n=2) which showed direct forbidden transition at 317

wavelength λ ≈380 nm. Therefore, putting n = 2 in the above equation, the 𝐸𝑔 values obtained by 318

plotting (hυF(𝑅∞))2 vs hυ, where the quantity (hυF(𝑅∞))2 extrapolated to zero [35,41]. The bandgap 319

plots are shown in Fig. 6 and 𝐸𝑔 values together with XRD and Raman data are summarized in Table 320

3. 321

322


323

FIG. 6. Tauc plots of the samples grown on (a) SLG (b) Cu foil using diffuse reflection data. The Eg 324

values are calculated by extrapolating the quantity (hυF(R∞))2 = 0 325

326

TABLE 3. Optical Bandgap energy and phase identification evidenced from XRD and Raman 327

spectra for as-deposited and annealed samples deposited at different temperature using IPA 328

*Crystallite sizes of the samples grown on SLG substrate are shown in Table 2 329

330

From Table 3, it is clear that the Eg values are in the range of 3.37-3.47 eV for as-deposited samples 331

and 2.98-3.28 eV those for post-annealed samples which could be attributed to the ZnO. However, in 332

case of Cu foil samples post-annealing treatment exhibited additional Eg in the range of 1.65 – 2.00 333

eV(see dotted line in Fig. 6b) which could be attributed to the CuxO phase [50]. Notice that samples 334

Glass Samples Band gap,

Eg(eV) ± 0.01

Cu foil samples Bandgap,

Eg(eV) ± 0.01

Phase composition

(XRD and Raman)

G4(IPA30) 3.48 C4(IPA30) 3.47 ZnO

G5(IPA50) 3.37 C5(IPA50) 3.43 ZnO

G6(IPA65) 3.40 C6(IPA65) 3.45 ZnO

G4 Ann. 250 ℃ 3.28 C4 Ann. 250 ℃ 1.65 & 3.21 CuxO/ZnO




deposited on SLG exhibited a reduction in bandgap with increasing IPA temperatures due to improve 335

crystallinity of the ZnO and corroborates the calculated crystallite sizes given in Table 2. However, 336

post-annealing treatment induced a significant reduction of Eg from ~3.4 eV to ~3.2 eV is due to the 337

improvement of crystallinity [5] with diminished lattice strain (see Table 2). These observations imply 338

that post-annealing treatment at 250 ℃ affected films largely compared to IPA temperatures. It is 339

worth noting that samples grown on Cu-foil indicating the possibility of CuxO/ZnO junction 340

formation by post-annealing (see Table 3). As a proof-of-concept, a preliminary heterojunction with 341

Cu/CuxO/ZnO/Au structure was formed by Cu contact with CuxO layer (a part of thin films from the 342

cell were scratched off) and a spring-loaded gold (Au) coated pin contact with ZnO layer (See top-343

left inset in Fig. 7a). The I-V characteristic curve of this cell under dark and white LED illumination 344

confirms the photo-response of CuxO/ZnO junction, which can be more conspicuously seen in the 345

zoomed area of I-V curve near zero bias voltage (bottom-right inset in Fig. 7a). Though a downward 346

shift of the LED illuminated I-V curve is quite expected for CuxO/ZnO based solar, however, a similar 347

but smaller downward shift of the dark I-V curve may suggest a non-ideal probing contact of the 348

present cell structure. Even though metallic aluminum (Al) would be a good ohmic contact for ZnO 349

layer, spring-loaded Au coated pin probe formed reasonably good ohmic contact as can be seen from 350

Fig. 7b (the inset shows the photograph of Au/ZnO/Au probing arrangement). The diffuse reflection 351

of the same cell at two different areas confirmed the formation CuxO layer (see Fig. 5c) in the 352

CuxO/ZnO based heterojunction (cf. Fig. 7c and cf. Fig. 5c). However, further experimental 353

investigations are in progress to optimize the cell structure as well as to assess photovoltaic 354

performance of SILAR grown CuxO/ZnO junctions. 355

356


357

358

FIG. 7. A typical SILAR Grown ZnO layer (IPA50) grown on Cu Foil followed by the annealing at 359

250 0C for 1 h in air for the formation of CuxO/ZnO junction: (a) I-V characteristic curve of a typical 360

Cu/CuxO/ZnO/Au structure under dark and white LED illumination (top-left inset shows the 361

photograph of cell contact, bottom-right inset shows the zoomed area of the I-V curve marked by 362

fade circle; (b) I-V characteristic curve of the ZnO layer with Au contact showing ohmic behavior 363

(inset shows the photograph of the ohmic contact with Au coated probes), (c) Diffuse reflection 364

measured at two different area of the same cell showing the formation of CuxO layer. 365

366


4. CONCLUSIONS 367

In this work, ZnO thin films have been synthesized on both SLG and Cu foil and the effect of substrate, 368

sonication process, and post-annealing on the properties of the deposited films were systematically 369

investigated. XRD analysis revealed the growth of highly crystalline c-axis oriented ZnO thin films 370

deposited on SLG with (002) preferred orientation while (101) preferential growth on Cu foil. XRD 371

and Raman spectroscopy confirmed that all of the post-annealing samples produced CuxO/ZnO 372

heterojunctions irrespective to the growth temperature using IPA and sonication process. Samples 373

excluding sonication steps exhibited compact and uniformly distributed grain surface morphologies 374

in the presence of IPA at 50 and 65 ℃ observed from SEM micrographs. The estimated Eg values 375

were 3.47-3.37 eV for as-deposited samples and bandgaps were found to be decreased significantly 376

with increasing annealing temperature due to crystallinity improvement of the ZnO thin films. The 377

sample grown at 50 ℃ revealed the best quality film grown in this work with the Eg value of 3.37 eV. 378

This study proposed that for highly c-axis oriented ZnO thin films, it may be better to deposit the 379

films on SLG in presence of IPA as a dispersing agent. We hope that this study will open up a new 380

approach for growing ZnO thin film with less processing steps as well as solution processable Cu-381

oxide/ZnO heterojunctions for diverse applications. 382

383

Author Contributions: B.C. Ghos performed the sample synthesis, data analysis and wrote the 384

preliminary manuscript. S.F.U. Farhad conceived the project idea and designed experimental works, 385

conceptualization, edited the manuscript, and led the overall research works. M.A.M. Patwary and 386

N.I. Tanvir helped with the experimental methodology and characterization setups. S. Majumder, 387

M.A. Hossain, and M.A. Rahman contributed to the formal data analyses and scientific discussion of 388


the results. Q. Guo helped with the Raman setup and T. Tanaka helped with the XRD and SEM setups 389

and discussions. All authors contributed to the writing, reviewing and editing of the final manuscript. 390

391

Acknowledgments: S.F.U. Farhad and N.I. Tanvir gratefully acknowledge the experimental 392

support of the Energy Conversion and Storage Research (ECSR) Section, Industrial Physics Division 393

(IPD), BCSIR Laboratories, Dhaka 1205, Bangladesh Council of Scientific and Industrial Research 394

(BCSIR) under the scope of R&D project#100-FY2017-2020 and S.F.U. Farhad also acknowledges the 395

TWAS grant#20-143 RG/PHYS/AS_I for ECSR, IPD. M.A.M. Patwary, T. Tanaka, and Q. Guo 396

acknowledge the characterization support of Department of Electrical and Electronic Engineering, 397

Saga University, Japan. 398

399

Conflicts of Interest: The authors declare no conflicts of interest. 400

401

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