Investigation of optical and structural characteristics of ...

Journal of Materials and Electronic Devices 1(2016) 50-56

Investigation of optical and structural characteristics of ZnO:Cd nanocrystalline thin films B. Arifa, Dilawar Alib a b

Department of Physics, Faculty of Sciences, Firat University, 23169, Elaziğ, Turkey Department of Physics, GC University, 54000, Lahore, Pakistan

Nanostructured cadmium (Cd) doped ZnO thin films were deposited on glass substrate by sol-gel spin coating method and characterized by X-ray diffraction (XRD), scanning electron microscope and UV-VIS-NIR spectrophotometer. The XRD analysis showed that thin films were crystalline with hexagonal wurtzite structure. The preferred orientation of un-doped and Cd-doped ZnO thin films lies along (002) plane. FESEM images revealed that morphology consist of spherical, non-spherical and partly cylindrical structures. The results of EDX showed that Cd was doped into ZnO structure. The UV-Vis transmittance spectra showed that substitution of Cd into ZnO leads to bandgap narrowing. The urbach energy increases with the increase in Cd concentration.

Keywords: Zinc oxide, Bandgap, Structural properties and Surface morphology

Submission date: 1 January 2016 Acceptance date:1 January 2016

Corresponding authors: [email protected] (B. Arif)

1. Introduction Metal oxide semiconductors have been extensively explored due their wide variety of applications and their interesting structural, electrical and optical properties. Among these ZnO has gained considerable attention because of its extraordinary properties, in recent years and being studied for applications such as, thin film transistors, photodiodes, solar cells, chemical and gas sensors, Liquid crystal displays, and more [1-5]. Beside various fascinating nanostructures like nanorods, nanobelts, nanoneedles, nonofibers, nonotubes have been grown using chemical vapour deposition, molecular beam epitaxy, sputtering and sol-gel method. we can also tune the electrical and optical properties of these nanostructures by doping. Lee et al. [6] has studied the effect of dopants (Ga, In, Zr and Sn) on the structural, electrical and optical properties of ZnO polycrystalline films derived from sol-gel method. All the zinc oxide doped thin films showed >90% transparency in the visible region and among all Ga doped film exhibited highest Hall mobility 12.87 cm2/Vs and lowest resistivity

1.41×102 Ωcm. Xuebin et al. [7] has reported the influence of Cd doping and annealing effect on the ZnO thin films. M. Souissi and co-workers [8] have grown ZnO:Cd doped thin films by MOCVD and investigated the ferromagnetism effect at room temperature. The efficiency of the dopant element is attributed to its electronegativity and the difference between its ionic radius and ionic radius of zinc [9]. Cd+2 ion with ionic radius 0.097 nm can occupy the lattice site or interstitial site in the matrix of ZnO ( Zn+2 ionic radius 0.074 nm) and thus may causes to produce lattice distortions. S. Ventakaraj et al. [10] deposited zinc oxide film with different Cd content using spray pyrolysis. According to X-ray diffraction, they reported that the films have (002) prefer orientation. Also with increase in Cd content the crystallinity degrades and absorption edge red shift was observed. Fahrettin et al. [11] prepared cd doped thin ZnO films using sol-gel method. They found that the films are polycrystalline and don’t exhıbit any prefer growth orientation. The SEM images of all the films have a wrinkle network effect obtained by cylindrical structures of 1-2 µm width.


51 2. Experimental details

3. Results and discussion

Intensity (a.u.)

(004)

Cd-3%

Cd-1%

ZnO

20

25

30

35

40

45

50

55

60

65

70

2(deg.) Fig.1: X-ray diffraction spectra for Cd-doped ZnO thin films.

next layer will be coated on it and this procedure from coating to heating will be repeated several times. If the former layer forms an ideal crystal structure, the probability of getting c-axis oriented thin by this method will be more. One more interesting thing in these XRD patterns is that the intensity of (002) plane increases with the increase in Cd content which might be due to the fact that a moderate amount of Cd atoms exists as interstitial that share the oxygen with Zn atom thereby improving (002) orientation [13]. The peak position of 2θ of (002) plane shifts progressively towards lower angle on increase in Cd doping concentration. This shifting of peak clearly shows the presence of compressive micro strain in the thin film due to lattice mismatch of Zn and Cd ions. The value of lattice strain has been determined using the relation given below [14]:

3.1. Structural Analysis Figure 1 shows the XRD pattern of undoped and Cd-doped ZnO thin films. These XRD patterns can be indexed to ZnO hexagonal wurtzite structure JCPDS card file no. 36-1451 and no other impurity phase is found, which indicates that Cd ion occupies lattice site position rather than interstitial position. The XRD pattern clearly shows that preferred orientation of Cd un-doped and Cd-doped ZnO thin films lies along (002) plane which indicates that thin film grown along c-axis perpendicular to the glass substrate. The reason for c-axis oriented growth is that (002) plane of ZnO has minimum surface energy [12]. The growth process of ZnO thin films by sol-gel spin coating method is multiple coating process. After heating the former layer, the

(002)

Cd-5%

ZnO and Cd doped ZnO films were prepared using sol-gel method and deposited on the glass substrate by spin coater. Zinc acetate dihydrate Zn(CH3COO)2.2H2O (Carlo Erba, Analytical grade) and cadmium acetate dihydrate Cd(CH3COO)2.2H2O (Carlo Erba, Analytical grade) were used as a precursor and dopant. The zinc acetate was first dissolved in 2-methoxethanol and Monoethanolamine (MEA) was added further followed as a stabilizer. The molar ratio between MEA to zinc acetate was maintained at 1:1. The concentration of the zinc acetate solution was 0.5 M. In order to obtain thin films with different Cd doping concentrations, the solutions with different Cd/Zn ratio varied as 1%, 3% and at.5% were prepared by adding cadmium acetate into the zinc acetate precursor solution. The prepared solutions were stirred at 60 C for 2 h to obtain a clear solution. Prior to the film deposition, the glass substrates were cleaned with ethanol and deionized water each for 5 min using ultrasonic cleaner. The films were spin coated at 1000 rpm for 10 s. After each coating the films were dried at 250 C for 10 min and this process was repeated five times to get the significant thickness of the films. Finally, the films were annealed at 450 C for one hour. The crystal phase of the prepared films was investigated using Rigaku-Ultima-IV X-ray diffractometer, utilizing Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV, 30 mA. The thickness of films was examined by park systems XE - 100E atomic force microscope (AFM). The optical spectra were measured by UV-VIS-NIR spectrophotometer (Shimadzu -3600PC). JEOL JSM-7001F scanning electron microscopy (SEM) were employed to study the morphology of the films.



 cos 4

(1)

where β is the full width (FWHM). The value of lattice strain obtained in this manner have been given Table 1. With the increase in Cd content the lattice strain increases. The value of crystallite size can be evaluated from Scherrer formula [14]:

D

k  cos

(2)

where k is the shape factor, λ is the wavelength of x-rays, and θ is the diffracting angle. There is slight decrease in the value of crystallite size on increasing the doping concentration.

75

80


55 OnZ

%1-dC

%3-dC

%5-dC

Fig.2: FESEM images of un-doped and Cd-doped ZnO films.

The surface morphology of un-doped and Cd-doped ZnO thin was studied using FESEM (Fig. 2). The morphology consists of spherical, nonspherical and partly cylindrical structures. The cylindrical structures might be developed due to sintering necks between spherical particles. The compositional analysis of thin film was confirmed by EDX. The EDX spectra of un-doped and Cd-doped ZnO thin films are shown in Fig.3. The spectra clearly show the presence of Zn, O, and Cd elements along with peaks of Au and Si. The presence of Si peak may be probably coming from the glass substrate and Au peak is due to the coating of Au film on the thin film before FESEM/EDX. The transmittance spectra of un-doped and Cd-doped ZnO thin have been shown in Fig. 4. The average transparency in the visible region of all films is greater than 85%. The band absorption edge of undoped and Cd-doped ZnO thin film can be determined from the first derivative of transmittance. The maximum peak position in dT/dλ vs wavelength (as shown in the inset of Fig. 4) plot will give us absorption edge. On increase in Cd concentration the

absorption edge shifts towards longer wavelength indicating red shift.


53 2.0 ZnO 1% 3% 5%

(Abs.h)2. (eV/m)2

1.6 1.2 0.8 0.4

Fig.3: EDX spectra for Cd-doped ZnO thin films.

0.0 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

The optical bandgap can be determined using Tau relation [15]:

Energy (eV)

h  E 

n

  Bo

g

(3)

h

where α is the absorption coefficient, Bo is the transition probability, hv is the energy of the photons, Eg is the optical band gap energy, and exponent n characterize the electron transition process. For direct allowed, direct forbidden electronic transitions the value of n is 1/2, 3/2 and for indirect allowed, indirect forbidden electronic transitions the value of n is 2, 3, respectively. Since ZnO is direct bandgap semiconductor therefore we take n = ½. Fig. 5 shows a plot of (𝛼ℎ𝑣)2 versus photon energy (hv). The value of Eg can be determined using by extrapolation of linear regions of plots to zero absorption ((𝛼ℎ𝑣)2=0) (Table 2). The value of bandgap energy decreases with the increase in Cd content. This narrowing of bandgap might be due to merging of donor level and conduction band.

Fig.5: Plot between (𝛼ℎ𝑣)2 against photon energy (hv) and showing the bandgap values with Cd doping variation.

Other possible explanation of this redshift is due to increase in carrier concentration on increase in doping content. Yakuphanoglue et al [11], Shan et al [16] and Maiti et al [17] also reported the redshift in Eg on doping with Cd in ZnO thin films. The reflectance spectra of Cd doped thin films has been shown in fig.6. The average reflectance of the films decreases as we increase the cd content. The peak in visible region shows the interaction of photons near the band gap energy of the films. The peaks in reflectance spectra are clearly shifting towards the higher wavelength. Which also indicates the decrease in the bad gap of Cd doped ZnO thin films. 8

ZnO 1% 3% 5%

ZnO 1% 3% 5%

60 4

40

3

dT/d

Transmittance (%)

80

6

4

2

2

1

20

0 350

360

370

380

390

400

410

wavelength(nm)

0 200

Reflectance (%)

100

400

600

800

1000

1200

0 200

400

600

wavelength(nm)

wavelength(nm) Fig.4: Transmittance spectra and the inset is first derivative of transmittance dT/dλ of undepd ZnO and Cddoped ZnO.

Fig.6: Reflectance spectra for ZnO with different Cd doping content.

800


55 1.8

ZnO 1% 3% 5%

16.5 16.0

Ln (m-1)

15.5

ZnO 1% 3% 5%

Refractive index (n)

17.0

1.6

1.4

15.0 14.5 14.0

1.2

13.5 13.0

1.0 200

12.5 3.0

3.2

3.4

3.6

Energy (eV)

3.8

600

800

-a0.6

Fig.7: Urbach plots of Cd doped ZnO films.

The absorption coefficient (α) near the band edge in the energy region hν < Eg empirically follows the exponential law i.e. Urbach tail expression [18]:

 h  ( )   o exp  Eu

400

wavelength(nm)

4.0

Extinction Coefficient (K)

12.0 2.8

  

(4)

ZnO 1% 3% 5%

0.4

0.2

0.0 400

600

800

wavelength(nm)

where h is Plank’s constant, v is the -bfrequency of radiation, αo is a constant and Eu is the Urbach energy that describe the width of localized Fig.8 (a-b): Refractive index and extinction coefficient state in the bandgap and is consider as parameter that for ZnO with different Cd doping content. includes the effect of all possible defects. The Eu of un-doped and Cd-doped ZnO thin can determined by plotting log-linear graph between α and hν. The 4R  1 R  n  K 2 (5) reciprocal of the slope in the linear region will give  2 1  R   1  R  us the value of Eu and are given in Table 2. The atomic structural disorder content increase with where R is the reflectance and k is the increase in the Cd content. The optical extinction coefficient. The reflectance spectra of unmeasurements are correlated very well with XRD doped and Cd doped ZnO thin films have been measurements. presented in Fig. 6. Some other optical parameters such as refractive index and extinction coefficient of thin films were also determined using the relations given below [19]: Table 1: d-spacing, FWHM, dislocation, strain and crystallite size of un-doped and Cd-doped ZnO thin films. Sample

2θ(◦)

d(Ao)

FWHM (◦) -

Crystallite size (nm) -

Dislocation δ (* 10-4 nm)2 -

Strain (*10-3) -

ZnO std

34.422

2.60332

Cd 1%

34.357

2.60813

0.3389

24.547

16.6

1.412

Cd 3%

34.321

2.61071

0.3502

23.753

17.7

1.459

Cd 5%

34.301

2.61219

0.3605

23.073

18.8

1.502


55 Table 1: Optical and morphological parameters of un-doped and Cd-doped ZnO thin films. Sample

Bandgap (eV)

Eo (eV)

Ed (eV)

EU (meV)

β

ZnO

3.263

6.619

10.052

160

0.162

Cd 1%

3.163

6.232

6.901

183

0.142

Cd 3%

3.152

6.089

6.577

186

0.139

Cd 5%

3.064

5.960

6.238

209

0.124

real part of dielectric constant

The refractive index dispersion below interband absorption edge was studied using single oscillator model [20]:

3.0

n2  1 

2.5 2.0 1.5 1.0 200

400

600

800

wavelength(nm)

imaginary part of dielectric constant

-a-

0.8 ZnO 1% 3% 5%

0.6

0.4

0.2

0.0 200

400

600

wavelength(nm)

800

-bFig.9 (a-b): Dependence of real part ε1 and imaginary part ε2 of refractive index on the wavelength for ZnO with different Cd doping content.

Eo Ed

E   h  2 o

2

(6)

The parameter Eo is oscillator energy and Ed is oscillator strength and is related to interband optical transition. Eo and Ed values are obtained from the slope (EdEo)-1 and intercept (Eo/Ed) of the graph between (n2-1)-1 vs (hν)2. The estimated values of Eo and Ed are given in table 2. The oscillator energy Eo is an average energy gap and it is in close approximation Eo ≈ 1 .5Eg with the optical bandgap, as suggested by single oscillator model. The polarizability of un-doped and Cddoped ZnO thin films can be studied by measuring the dielectric constant. The real and imaginary part of dielectric constant can be determined using the expression given below [19]:

1   n 2  k 2 

(7)

 2  2nk

(8)

where 𝜖1 is the real part of the dielectric constant and 𝜖2 is the imaginary part of the dielectric constant. The dependence of 𝜖1 and 𝜖2 on wavelength have been shown in Fig. 9. The real and imaginary part of dielectric constant changes with the wavelength which suggests that interaction between photons and electrons in the thin films are taking place.

Conclusions The investigation carried out in present work demonstrate that pure and Cd doped ZnO thin films have been synthesized onto glass substrate using sol-gel spin coating method. The XRD results


55 showed that all thin films exhibits hexagonal wurtzite structure and the preferred orientation lies along (002) plane. FESEM micrographs shows that that thin films consist of spherical, non-spherical and partly cylindrical structures. EDX analysis confirms the Cd element was doped into the ZnO thin film. The transparency of all the films is more than 85%. Bandgap energy decreases with Urbach energy increases with the increase in Cd concentration. The Cd doping effects the dispersion parameters calculated using the single oscillator model. References: [1] S.Y. Park, B.J. Kim, K. Kim, M.S. Kang, K.H. Lim, T.I. Lee, J.M. Myoung, H.K. Baik, J.H. Cho, Y.S. Kim, Adv. Mater. 24 (2012) 834. [2] S.S. Shinde, K.Y. Rajpure, Mater. Res. Bull. 46 (2011) 1734-1737. [3] A. Chen, Q. Yuan and K. Zhu, App. Surf. Sci. 360, (2016) 693. [4] A. Mirzaei, S. Park, H. Kheel, S. Lee and C. Lee, Ceramic. Inter. 42 (2016) 6187. [5] N. Yamamoto, H. Makino, S. Osone, A. Ujihara, T. Ito, H. Hokari, T. Maruyama, T. Yamamoto, Thin Solid Films 520 (2012) 4131. [6] T. Chien-Yie and L. Wen-Che,Curr. App. Phys. 13, (2013) 60. [7] L. Gang, Z. Xuebin, T. Xianwu, S. Wenhai, S. Yuping and X. Pan, J. Alloy. Comp. 509 (2011) 4816.

[8] M. Souissi, A. Fouzri and G. Schmerber, Solid State Com. 218, (2015) 40. [9] P. Nunes, E. Fortunato, P. Tonello, F.B. Fernandes, P. Vilarinho, R. Martins, Vacuum 64 (2002) 281. [10] S. Vijayalakshmi, S. Venkataraj and R. Jayavel, J. Phys. D: Appl. Phys. 41 (2008) 245403. [11] F. Yakuphanoglu, S. Ilican, M. Caglar and Y. Caglar, Superlatt. Microstruc. 47 (2010) 732. [12] W. Chebil, A. Fouzri, A. Fargi, B. Azeza, Z. Zaaboud and V. Sallet, Mater. Reas. Bull. 70 (2015) 719. [13] B. J. Lee, J. H. Le, S. H. Seo and J. S. Park, Thin Solid Films 398 – 399 (2001) 641. [14] H. P. Klug, and Alexander, X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials 2nd edn. ( Wiley, NewYork, 1974). [15] J Tauc, Optical Properties of Solids,NorthHolland, Amsterdam,1970. [16] F. K. Shan, G. X. Liu, W. J. Lee and B. C. Shin, J. Cryst. Growth 291 (2006) 328. [17] U. N. Maiti, P. K. Ghosh, Sk. F. Ahmed, M. K. Mitra and K.K. Chattopadhyay, J. Sol. Gel Sci. Tech. 41 (2007) 87. [18] F. Urbach, Phys. Rev. 92 (1953) 1324. [19] F. Abeles, Optical Properties of Solids, NorthHolland, Publishing Company, London, UK, 1972. [20] M. DiDomenico, S.H. Wemple, J. Appl. Phys. 40 (1969) 720.