Study on the structural, morphological and optical properties of RF sputtered gallium doped zinc oxide thin films

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 4 (2017) 4417–4433

www.materialstoday.com/proceedings

ISPAN-2015

Study on the structural, morphological and optical properties of RF sputtered gallium doped zinc oxide thin films V.M. Vimuna, R. Sreeja Sreedharan, R. Resmi Krishnan, V.S. Kavitha, S.R. Chalana, S. Suresh and V.P. Mahadevan Pillai* Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram -695581, Kerala, India.

Abstract

Ga doped ZnO films with various gallium oxide doping concentrations (0, 0.5, 1, 3, and 5 wt%) are prepared using radio frequency (RF) magnetron sputtering technique. The structural, morphological, and optical properties of the films are studied. XRD analysis shows that all the films exhibit hexagonal wurtzite structure with preferred orientation along <002> direction. The moderate doping of Ga in ZnO lattice is found to improve the crystallinity of the films. Micro-Raman spectra also reveal the formation of hexagonal wurtzite ZnO phase in the films. AFM and SEM images of the films present uniform dense distribution of grains. All the Ga doped films show higher values of transmittance compared to the undoped film. Band gap energies were calculated using Tauc plot. The photoluminescence spectra show both UV and visible emissions. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Symposium on Photonics Applications and Nanomaterials (ISPAN-2015). Keywords:Zinc oxide; gallium oxide; radio frequency magnetron sputtering technique; XRD; Micro-Raman spectra; AFM; SEM; UV-Visible spectra; photoluminescence spectra; highly textured films; transparent conductors. *Corresponding author. E-mail address: [email protected]

2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Symposium on Photonics Applications and Nanomaterials (ISPAN-2015).

4418

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

1. Introduction ZnO is one of the most versatile II-VI material semiconductors and have long been the subject of investigation. The material ZnO is known since the Bronze Age [1] and is an important topic of research in the 21st century. This is also one of the most important materials that we come across in our day-to-day lives. Zinc Oxide is used as a pigment in paints and in coatings for paper. Some of the favorable aspects of ZnO include its radiation hardness, abundance in nature and nontoxicity, biocompatibility, excellent piezoelectric and semiconducting properties among many others [2-10]. Such multi-functional properties of ZnO make it suitable for applications in electronic and optoelectronic devices. The present work is an attempt to study the influence ofGa3+dopingon the structural, morphological and optical properties of ZnO thin films. Several techniques have been used for the preparation of high-quality ZnO thin films. Some of them are metal organic chemical vapour deposition (CVD), evaporation, magnetron sputtering, sol–gel, plasma assisted molecular beam epitaxy (MBE), pulsed laser deposition(PLD), spin coating, spray pyrolysis etc [11-16]. Most of these techniques need to use moderate temperatures to obtain films with low values of resistivity. Among various deposition techniques to grow doped ZnO thin films, magnetron sputtering has emerged as a popular choice because of high deposition rate, good adhesion, and easy control of growth parameters. However, films grown by sputtering technique exhibit large stress due to continuous bombardment by energetic particles during the growth [16]. The sputtering process is one of the promising techniques for depositing ZnO films due to high deposition rate in the low temperature and relative simplicity. Therefore, in this work RF magnetron sputtering is used for the deposition of ZnO films with various Ga doping concentrations. The effect of dopant Ga3+ on the structural, morphological, optical, and luminescence properties of the ZnO nanostructures are investigated in detail. 2. Experimental Procedure The targets for preparing RF sputtered ZnO films were prepared from commercially available ZnO powder (Sigma Aldrich purity 99.99%) and gallium oxide powder (Sigma Aldrich purity 99.99%). The ZnO powder was mixed with Ga2O3 powder (doping concentrations 0, 0.5, 1,3 and 5 wt %) and the mixer was ground well. The thoroughly mixed powders were pressed well and were used as the target for the preparation of films. The sputter chamber was initially evacuated to abase pressure of 5x10-6 mbar. Argon gas was then admitted into the chamber and argon pressure was maintained at 0.02 mbar. The quartz substrates were fixed at a distance of 5 cm from the target. The target was powered through a magnetron power supply (Advanced Energy MDX 500, Colorado). The sputtering was carried out under constant RF power of 150 watts for a period of 30 min. The as-prepared ZnO films with Ga doping concentrations 0, 0.5, 1, 3, and 5 wt% are designated as G0, G0.5, G1, G3, and G5 respectively. The crystalline structure and crystallographic orientation of the as-deposited films were investigated by X-ray diffraction (XRD) measurements (Bruker Kappa Apex II). Diffraction patterns of the samples were recorded in the 2θ range 20 to 70˚ with a step-size of 0.01671˚ at a scan speed of 2˚ per min in the Bragg Brentano geometry.

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4419

CuKα1 radiation with wavelength1.5406 A˚ was used for recording the X-ray diffraction patterns. Micro-Raman spectra of the films were recorded using micro-Raman spectrometer (Labram HR-800, Horiba JobinYvon, Germany) using a laser radiation of wavelength 514.5 nm from an argon ion laser. The spectra were recorded with a spectral resolution of ~ 1 cm-1. The surface morphology of the films was investigated using atomic force microscopy (AFM) analysis using Digital Instruments Nanoscope III, USA (Si3N4 tip, 100μ cantilever, 0.58N/m force constant) in the contact mode. The FESEM measurements were carried out using Nova NanoSEM-450 (Model No.1027647, FEI, USA) equipped with XFlash detector 6/10 (Bruker). Transmittance spectra of the as-deposited films were recorded using UV-VIS double beam spectrophotometer (JASCO V -550) in the spectral range of 200 – 900 nm. Photoluminescence spectra of the as-deposited films were recorded using HoribaJobinYvonFlourolog III modular spectroflourometer equipped with Xe-flash lamp using an excitation wavelength of 325 nm. Thicknesses of the films were measured using lateral FESEM measurements. 3. Results and Discussion 3.1.XRD Analysis The X-ray diffraction patterns of Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% are shown in Fig 1(a). The diffraction peaks obtained for all the films can be indexed to hexagonal wurtzite structure of ZnO phase [JCPDS -75-0576]. The XRD patterns of the undoped films present a sharp intense peak at 2θ value ~ 34.30˚ and two weak peaks at 2θ values ~ 47.29˚ and 62.73˚. These peaks can be indexed to lattice reflection planes (002), (102) and (103) respectively of hexagonal wurtzite phase of ZnO. the XRD patterns of the Ga3+doped films also show the presents of hexagonal wurtzite phase with peaks corresponding to reflection planes (002), (102) and (103). The preferred orientation in all the films is along <002> direction indicating that the films are highly textured alongc axis. The c-axis orientation of Ga3+ doped ZnO thin films may be explained in terms of lowest surface free energy of (002) plane in ZnO[17]. Quantitative information regarding the preferential orientation of crystallographic planes can be obtained from the texture coefficient, TC, defined as, [18] (

(ℎ ) =

) (

(

)

(1)

) (

)

Where I (hkl ) the intensity of the X-Ray diffraction peak and n is the number of diffraction peaks considered.

I0(hkl) is the standard intensity of the (hkl) plane taken from the JCPDS data. The X-Ray diffraction patterns of all the films present three diffraction peaks, viz (002), (102) and (103). Hence the maximum value of TC (hkl possible is 3.The TC (hkl

)

) values for the three diffraction peaks corresponding to different lattice reflection planes

of the films are given in Table 1. It is found that the highest TC (hkl ) value is obtained for (002) lattice reflection plane for all the films indicating that (002) is the preferential orientation of crystal growth in them [18].

4420

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

Fig 1(a).X-ray diffraction patterns of as deposited RF sputtered Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates and (b) Variations of FWHM and intensity of the (002) diffraction peak of these films as a function of doping concentrations.

The intensity of (002) diffraction peak in ZnO is found to be increasing with Ga3+ doping concentration up to 3wt% and thereafter shows a reduction with further increase in Ga3+ doping concentration (Table 1). The FWHM of the Ga3+ doped films are found to be lower than that of undoped film except for 5wt% Ga3+ doped ZnO film (Table 1). This indicates that moderate doping of Ga3+ in ZnO lattice improves the crystalline quality of the ZnO films. The variations of FWHM and intensity of the (002) diffraction peak in the ZnO films with various Ga3+ doping concentrations are shown in Fig 1(b).It is found that the5wt% Ga3+ doped ZnO film (G5) shows the lowest value for intensity and highest value for FWHM of the (002) diffraction peak among all the films. This indicates that

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4421

heavy doping of Ga3+ in the ZnO lattice deteriorates the crystalline quality of the film. No diffraction peaks of any secondary or impurity phases were found within the detection limit in any of the films.

Table 1.Structural parameters of the undoped and Ga3+ doped ZnO films. Sample

2θ

FWHM

dhkl

Lattice

Strain

Code

(˚)

(˚)

(A˚)

Constant

Stress (GPa)

Average

Texture Coefficient

Size of

c

Crystallite

(A˚)

s (nm)

(002)

(102)

(103)

G0

34.30

0.4553

2.6199

5.2399

0.8659

-3.9277

21

2.515

0.259

0.226

G0.5

34.42

0.35428

2.6113

5.2227

0.5352

-2.4278

18

2.25

0.2584

0.4926

G1

34.44

0.41796

2.6097

5.2195

0.4752

-2.1553

17

2.325

0.1965

0.4784

G3

34.48

0.35535

2.6066

5.2133

0.3557

-1.6134

27

2.354

0.1105

0.5358

G5

34.60

0.49979

2.5976

5.1951

0.0063

-0.0284

21

2.303

0.189

0.5081

The regular shift in the (002) diffraction peak towards higher 2θ values indicates that the Ga3+doped ZnO films are in a state of uniform strain. The inter-planar spacing (dhkl) in the films is calculated using Bragg’s relation [19].

nλ = 2dhkl sinθhkl

(2)

Where dhkl is the inter planar distance of the (hkl) plane, n is an integer and its value is one for first order reflection and

θhkl is the angle of diffraction. The dhkl

given in Table 1. The

values of the undoped and Ga3+ doped films are calculated and are

dhkl values of the films are found to be decreasing with increase in Ga3+doping concentration.

The reduction in the inter-planar distance with Ga3+doping indicates the contraction of the ZnO lattice with Ga3+incorporation. The ionic radius of Ga3+ ion (0.62A˚) is less than that of Zn2+ ion (0.74A˚) [20]. Hence, the substitution of Ga3+ ion in to the Zn2+ sites may result in the contraction of the lattice. This leads to a decrease in the value of

dhkl

with increase in Ga3+doping concentration [20]. Hence the value of c lattice parameter of the

Ga3+doped films is expected to be lower than that of the undoped film. The lattice parameters of the hexagonal crystal lattice can be calculated by knowing the inter-planar distance dhkl [19].

1 4  h 2 + hk + k 2  = 2 d hkl 3  a2

 l2  + 2  c

(3)

4422

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

Where h, k and

l are the miller indices of the plane, a, b and c are the lattice parameters along x, y and z directions

respectively. The value of the lattice parameter c is calculated for all the films using (002) diffraction peak (Table 1). The c lattice constant of the Ga3+doped films are found to be decreasing with increase in Ga3+doping concentration. The strain along c-axis is calculated (Table 1) using the expression [21]

ε (%)=

c film − cbulk cbulk

×100

(4)

Where ε is the strain along the c-axis perpendicular to the substrate surface, c film is the c lattice constant of the strained film calculated from XRD data and cbulk is the c lattice constant of the bulk ZnO obtained from JCPDS data card. The stress in the films is calculated (Table 1) using the following equation [19]:

σ = −453 .6

c film − cbulk cbulk

GPa

(5)

The positive value of stress indicates that the crystallites are in a state of tensile stress and negative value indicates that the crystallites are in a state of compressive stress (22). The average size of the crystallites in the films is evaluated (Table 1)using the following Debye Scherrer equation [19]

Dhkl =

0.9λ βhkl cos(θhkl )

where Dhkl is the average size of the crystallites,

(6)

λ

diffraction angle of the selected diffraction peak and

is the wavelength of Cu kα1radiation,

βhkl is

θ hkl

is the Bragg

the full width at half- maximum (FWHM) of the

diffraction peak in radian.

3.2. Micro-Raman Spectroscopy The influence of Ga3+ doping on the structural and vibrational properties of ZnO thin films was investigated by micro Raman spectroscopy. Group theoretical analysis yields nine optical modes [23] (excluding the three acoustic modes A1+E1) at the center of Brillouin zone for ZnO and are given by

Γopt = A1 + 2 B1 + E1 + 2 E2

(7)

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4423

Fig 2. Micro-Raman spectra of RF sputtered Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates.

Of these, the A1and E1modes are both Raman and IR active, whereas E2 modes are only Raman active and B1modes are inactive in both the spectra. The A1and E1 modes are polar and split into transverse optical (TO) andlongitudinal optical (LO) modes. The E2 mode is non-polar with two frequencies; E2 (high) and E2 (low) [24]. The micro Raman spectra of the undoped and Ga3+doped ZnO films recorded in the spectral range 80–700 cm-1 using an excitation radiation of wavelength 514.5 nm of Ar ion laser are shown in Fig 2. The Raman spectra of films present a broad spectral feature. The broad nature of Raman bands in the spectra of the films can be due to the residual stress in the films. The medium intense band around 490 cm-1 observed in all the films can have the contribution from the quartz substrate. In the Raman spectra of the films E2(low) mode is observed ~100 cm-1. In the undoped film, E2 (high) modes are observed ~440 cm-1. This band shows a systematic shift towards lower wave numbers with increase in Ga3+doping concentration up to 3 wt%. The medium intense Raman band observed ~ 590 cm-1 may be attributed to

4424

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

E1(LO) mode. This band becomes much stronger at higher doping concentrations. The origin of this band is generally attributed to the defects in the films such as oxygen vacancies, zinc interstitials etc. This indicates the incorporation of defects in the ZnO lattice due to Ga3+incorporation. The A1(TO) mode appears as a shoulder ~ 390 cm-1. The weak band observed ~332 cm-1 in the films can be attributed to E2(high)- E2(low) [24]. The micro Raman spectra of the undoped and Ga3+ doped ZnO films recorded in the spectral range 80–700 cm-1 using an excitation radiation of wavelength 514.5 nm of Ar ion laser are shown in Fig. 2. The Raman spectra of films present a broad spectral feature. The broad nature of Raman bands in the spectra of the films can be due to the residual stress in the films. The medium intense band around 490 cm-1 observed in all the films can have the contribution from the quartz substrate. In the Raman spectra of the films E2 (low) mode is observed ~100 cm-1. In the undoped film, E2 (high) modes are observed ~440 cm-1. This band shows a systematic shift towards lower wave numbers with increase in Ga3+doping concentration up to 3 wt%. The medium intense Raman band observed ~ 590 cm-1 may be attributed to E1 (LO) mode. This band becomes much stronger at higher doping concentrations. The origin of this band is generally attributed to the defects in the films such as oxygen vacancies, zinc interstitials etc. This indicates the incorporation of defects in the ZnO lattice due to Ga3+incorporation. The A1 (TO) mode appears as a shoulder ~ 390 cm-1. The weak band observed ~332 cm-1 in the films can be attributed to E2 (high) - E2 (low) [24]. Compared to the Raman spectra of other films, the Raman spectrum of 5 wt% Ga3+doped film presents broader spectral feature. This can be attributed to the decline in crystallinity due to the heavy doping as evident from the XRD analysis.

3.3. AFM analysis Fig 3(a) show AFM images of undoped and Ga3+ doped ZnO films. AFM images of all the films present uniform dense distribution of grains of more or less uniform size (around 45 nm) with well-defined grain boundary. The undoped film shows an rms surface roughness of 4.06 nm (Table 2). The G1 and G3 films present relatively smooth surface compared to the other films. The undoped and Ga3+doped films show that ZnO nucleated as individual, pyramidal islands. This kind of surface structure has a potential application in enhanced light trapping [26].

3.4 FESEM analysis Fig 4 shows the FESEM images of the undoped and Ga3+doped ZnO films. All the films present dense distribution of grains. As observed by AFM analysis the SEM images also suggest a dense distribution of grains in the films. The thicknesses of the films are estimated from the lateral SEM images (Fig 5)and the value of thickness obtained for undoped and Ga3+doped ZnO filmsare given in Table 2. The thicknesses of the doped films are found to be higher than that of the undoped film.

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4425

3.5. UV-Visible spectra analysis The optical transmission spectra of the undoped and Ga3+doped films recorded in the wavelength range of 300- 900 nm is shown in Fig 6. All the films exhibit good transmittance (>80%) in the visible region. All the films show a sharp reduction in transmittance ~380 nm corresponding to the fundamental absorption edge of ZnO. The sharp absorption onset in the UV- region and high transmittance in the visible region indicates the good crystalline and optical quality and direct band gap nature of the films. The average transmittances of the films in the wavelength range 400- 900 nm are given in Table 2. Table 2. Optical parameters of the undoped and Ga3+ doped ZnO films.

Sample Code

r.m.s Thickness Of The Films Roughness (Nm) (nm) (From Lateral SEM Images)

Average Transmittance (%)

Band Gap (eV)

G0

4.06

80

82

3.28

G0.5

4.19

94

87

3.28

G1

1.98

86

91

3.29

G3

1.78

96

88

3.30

G5

3.93

106

86

3.31

The undoped film shows an average transmittance of 81%. All the Ga3+doped films show higher values of transmittance compared to the undoped film. The higher values of transmittance exhibited by G1 and G3 films may be attributed to the enhanced crystalline quality of these films as indicated by XRD analysis and reduced surface roughness as revealed from the AFM analysis [27]. Among the doped films 5wt% Ga3+doped ZnO films shows least value of transmittance. The XRD analysis already revealed deterioration in crystalline quality for this film. The AFM analysis shows a relatively higher value of rms surface roughness for this film (G5) compared to other doped films. The oscillations observed in the transmission spectra of the undoped and Ga3+doped films can be due to the interference of light arising from the difference in refractive indices of the film and the substrate and the interference of multiple reflections arising from the film and substrate surface. These oscillations in the spectra indicate that smooth films are formed on quartz substrate [28-29]. The band gap of the films can be calculated by plotting (αhν) n vs photon energy (hν). The optical absorption coefficient α can be calculated from the transmittance spectra using the following relation [30],

1 1 t T 

α = ln 

(8)

4426

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

Fig 3 (a). 3D AFM micrographs of RF sputtered Ga doped ZnO films and (b) variation of Rms surface roughness of the films with Ga3+doping concentration.

Where t is the thickness of the film and T is the transmittance of the film. The optical band gaps of the films are calculated using the relation [30],

αhν = A(hν − Eg )n Where ncan have values

(9)

1

2

,

3

2

,

2or 3respectively

for direct allowed, direct forbidden, indirect allowed and

indirect forbidden transitions, h is the Plank’s constant ν is the frequency of the incident photon and A is the band edge constant depending on electron – hole mobility. The band gap Eg can be obtained by extrapolating the linear region of (αhν) 1/n vshν plot to hν=0. The best fit for Tauc relation is observed for n = 1 indicating direct allowed 2 transition in these films.

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4427

Fig 4. FESEM micrographs of RF sputtered Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates.

The Tauc-plots of undoped and Ga3+doped ZnO films are shown in Fig 7. The undoped film and G0.5 film shows a band gap of 3.28eV.Other doped films show slightly higher valuesofband gap. There are several reasons for the shift of band gap in the films such as improvement or reduction in crystallinity, modification in barrier height due to the change in crystallite dimension, quantum size effect, and change in the density of impurities, tensile or compressive strain in the films Burstein-Moss effect etc. [31].

4428

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

Fig 5. Lateral FESEM micrographs of RF sputtered Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates showing the thickness of the films.

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4429

Fig 6. Transmittance spectra of RF sputtered Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates.

The Burstein–Moss effect explains the broadening of band gap energy with the increase in carrier concentration. The blue shift of the absorption edge of the Ga3+doped ZnO films may be attributed to the combined effects of stress in the films and Burstein-Moss effect [32].

3.6. PL Studies Fig 8 shows the photoluminescence spectra of the undoped and Ga3+ doped ZnO films recorded at room temperature using an excitation radiation of wavelength 325 nm. The undoped film shows an intense PL emission in the 380-550 nm regions. The emission in the UV regioncanbe attributed to the near band edge emission related to free-excitonrecombinationand the visible emission can be attributed to the defect oriented deep level emission. The deep level emission in the visible region is attributed to structural defects such as oxygen vacancies and interstitial zinc [33, 34]. The intensity of PL emissions enhances withGa3+doping. The visible emission shows appreciable enhancement in intensity with Ga3+doped films except for G3 film. The green luminescence observed ~ 540 nm due to intrinsic defects such as oxygen vacancies shows higher intensity in G0.5, G1 and G5 films [35]. The intensity of this peak is relatively less in G3 film, but higher thanthat in the undoped film. There are still no satisfactory explanations for the origins of the defect related deep level PL emission due to the complexity of the micro structure of ZnO.

4430

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

In G1 film the intense UV emission centered ~382 nm. The visible region shows a broad nature. In G3 film UV emission centered ~397 nm and has blue emission centered around 425,470 and 495 nm. This film has a green emission ~542 nm. In G5 film UV emission centered ~386 nm and blue emissions centered ~ 417, 471 and 493 nm.

Fig 7. The Tauc-plots of RF sputtered Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates.

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4431

The origin of blue emission observed ~ 470 nm in the ZnO films is generally attributed to the transition between the levels of interstitial zinc to the valence band or may be due to the impurities and defect structures such as oxygen vacancies and zinc interstitials [36].

Fig 8. Photoluminescence spectra of the undoped and Ga3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% deposited on quartz substrates.

4432

Vimuna V.M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4. Conclusions The structural, morphological and optical properties of Ga3+ doped ZnO films prepared using RF magnetron sputtering are studied. XRD analysis reveals the formation of nanostructured films exhibiting hexagonal wurtzite structure with preferred orientation along <002> direction. The moderate doping of Ga3+ is found toimprove the crystalline quality of the ZnOfilms.The presents of E2high modes in the micro-Raman spectra also reveal the hexagonal wurtzite structure. AFM and SEM images of the films present uniform dense distribution of grains of more or less same size with well-defined grain boundaries. All the Ga3+doped films show higher values of transmittance compared to the undoped film. The band gap energy calculations using Tauc plot shows an increase in band gap with increase in Ga3+doping concentration. The photoluminescence spectra show UV emission due to near band edge emission in ZnO and defect related deep level visible emissions. References [1]

Brown HE. Zinc Oxide Rediscovered. New Jersey Zinc Co.NewYork; 1957.

[2]

Look DC. Recent advances in ZnO materials and devices. J Mater SciEng B 2001; 80:383–387.

[3]

Reynolds DC, Look DC, Jogai B. Optically pumped ultraviolet lasing from ZnO. Solid State Commun 1996;873:99.

[4]

Kamalasanan MN, Chandra S.Sol- gel synthesis of ZnO thin films. J Thin Solid Films 1996; 112: 288.

[5]

Shionoya S, Yen WH.(ed) Phosphor Handbook by Phosphor Research Society,Boca Raton, FL: CRC Press; 1997.

[6]

Florescu DI, Mourokh LG, Pollak FH, Look DC, Cantwell, Li X. High Spatial Resolution Thermal Conductivity of Bulk ZnO (0001). J Appl Phys2002; 890: 91.

[7]

Tuomisto F, Saarinen K, Look DC, Farlow GC. Introduction and Recovery of Point Defects in Electron-Irradiated ZnO. J PhysRevB 2005; 085206: 72.

[8] [9]

Look DC, Hemsky JW and Sizelove JR. Residual Native Shallow Donor in ZnO. J Phys RevLett1999; 2552: 82. Schmidt O, Kiesel P, Van de Walle CG, Johnson NM, Jand DN, JapanGH. Effects of an Electrically Conducting Layer at the Zinc Oxide SurfaJ. J Appl Phys2005; Part 1;7271: 44.

[10] Drapak IT. Visible luminescence of a ZnO-Cu2O heterojunction. Semiconductors 1968; 624: 2. [11] de Posada E, Tobin G, Mcglynn E, Lunney JG. Pulsed laser deposition of ZnO and Mn-doped ZnO thin films. Appl Surf Sci 2003; 208-9: 589- 93. [12] Heo YW, Ip K, Pearton SJ, Norton DP, Budai JD, Growth of ZnO thin films on c-plane Al 2O3 by molecular beam epitaxy using ozone as an oxygen source. Appl Surf Sci 2006; 252: 7442-8. [13] Smirnov M, Baban C, RusuGI.Structural and optical characteristics of spin-coated ZnO thin films. Appl Surf Sci 2010; 256: 2405-8. [14] Tan ST, Chen BJ, Sun XW, Fan WJ, Kwok HS, Zhang XH, et al. Blue shift of optical band gap in ZnO thin films grown by metal organic chemical-vapor deposition. J ApplPhys 2005; 98: 013505. doi:10.1063/1.1940137. [15] Lehraki N, Aida MS, Abed S, Attaf N, Attaf A, Poulain M. ZnO thin films deposition by spray pyrolysis: influence of precursor solution properties. CurrApplPhys 2012; 12: 1283e1287. doi: 10.1016/j.cap. 2012.03.012. [16] Bouderbala M, Hamzaoui S, Amrani B, Reshak AH, Adnane T, Sahraoui M, et al. Thickness dependence of structural, electrical and optical behavior of undopedZnO thin films. Physica B 2008; 403:3326-30. [17] Deng H, Ressel JJ, Lamb RN, Jiang B, Li Y, Zhou XY. Microstructure control of ZnO thin films prepared by single source chemical vapor deposition. Thin Solid Films 2004; 458: 43-6. doi: 10.1016/j.tsf.2003.11.288 [18] PauporteT.Electrochemical synthesis of ZnO in the presence of Dye or polymer additives. Cryst Growth Des.7 2007; 2310.

Vimuna V. M et al. / Materials Today: Proceedings 4 (2017) 4417–4433

4433

[19] Cullity BD. Elements of X-ray Diffractions Addison-Wesley. Reading, MA: Addison-Weseley; 1978, p. 102. [20] Kim KH,Park KC, Ma DY. Structural, electrical and optical properties of Aluminiumdoped Zink oxide films prepared by radio frequency magnetron sputtering. J Applphy1997; 7764: 81. [21] Ghosh R, Basak D, FujiharaS.Effect of substrate induced strain on structural, electrical and optical properties of polycrystalline. J Appl Phys2004; 2689: 96. [22] Lupan O, Pauporte T, Chow L, Viana B, Pelle F, Ono LK, et al. Effects of annealing on properties of ZnO thin films prepared by electrochemical deposition in chloride medium. J Appl Surf Sci 2010; 256: 1895-907. [23] Fateley WG, Dollish FR, McDevitt NT, Bentley FF. Infrared and Raman selection rules for molecular and Lattice vibrations - the correlation method. New York: Wiley-Interscience; 1972. [24] Sreedharan SR, Ganesan V, Sudarsanakumar C, Bhavsar K, Prabhu R, MahadevanPillai VP. Highly textured and transparent RF sputtered Eu2O3 doped ZnO films. Nanoreviews. March 2015. [25] Scepanovic M, Srec´kovic T, Vojisavljevic K, Ristic MM. Modification of the structural and optical properties of commercial ZnO powder by mechanical activation. Sci Sinter 2006; 38: 169-75. doi: 10.2298/SOS0602169S. [26] Verghese PM, Clarke DR. Surface textured ZnO films. J Mater Res1999; 1039: 14-3. [27] Tauc J, The optical properties of Solids, ed F. Abeles, North-Holland:Amsterdam; 1996. [28] Bundesmann C, Ashkenov N, Schubert M, Spemann D, Butz T, Kaidashev EM, et al. Raman scattering in ZnO thin films doped with Fe, Sb, Al, Ga, and Li. J ApplPhys Lett2003; 83: 1974. Doi: 10.1063/1.1609251. [29] Bose RJ, Kumar RV, Sudheer SK, Reddy VR, Ganesan V, MahadevanPillai VP. Effect of silver incorporation in phase formation and band gap tuning of tungsten oxide thin films. J ApplPhys 2012; 112: 114311. doi: 10.1063/1.4768206. [30] Tauc J. Optical properties of amorphous semiconductors. Amorphous and liquid semiconductors. London: Plenum; 1974. [31] Suchea M, Christoulakis S, Kartharakis M, Vidakis N, Koudoumas E. Influence of thickness and growth temperature on the optical and electrical properties of ZnO thin films. Thin Solid Films 2009; 517: 4303-6. [32] Burstein E. Anomalous optical absorption limit in InSb. Phys Rev 1954; 93: 632. [33] Dutta S, Chattopadhyay S, Sutradhar M, Sarkar A, Chakrabarti M, Sanyaland D, Jana D. Controlled defects in ZnO by low energy Ar irradiation J. Phys. Condens. Matter2007; 236218: 19. [34] Peng X, Xu J, Zang H, Wang B, Wang Z. Structural and Pl properties of Cu doped ZnO films. J Lumin 2008; 128: 297-300. [35] Vinodkumar R, Lethy KJ, Beena D, Detty AP, Navas I, Nayar UV, et al. Effect of ITO buffer layers on the structural, opticaland electrical properties of ZnO multilayer thin films prepared by pulsed laser deposition technique. Sol Energ Mater Sol Cell 2010; 94: 68-74. [36] Vanheusden K, Seager CH, Warren WL, TallantDR,VoigetJA.Correlation between photoluminescence and oxygen vacancies in ZnO phosphors. ApplPhys Lett1996; 403: 68.