Optical characterization of Mg-doped ZnO thin films ...

Optical characterization of Mg-doped ZnO thin films deposited by RF magnetron sputtering technique Satyendra Kumar Singh, Purnima Hazra, Shweta Tripathi, and P. Chakrabarti Citation: AIP Conference Proceedings 1728, 020168 (2016); doi: 10.1063/1.4946219 View online: http://dx.doi.org/10.1063/1.4946219 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characterization of ZnO:SnO2 (50:50) thin film deposited by RF magnetron sputtering technique AIP Conf. Proc. 1728, 020567 (2016); 10.1063/1.4946618 Optical and local structural study of Gd doped ZrO2 thin films deposited by RF magnetron sputtering technique AIP Conf. Proc. 1665, 080056 (2015); 10.1063/1.4917960 Resistive switching behavior of RF magnetron sputtered ZnO thin films AIP Conf. Proc. 1665, 080051 (2015); 10.1063/1.4917955 Compositional study of vacuum annealed Al doped ZnO thin films obtained by RF magnetron sputtering J. Vac. Sci. Technol. A 29, 051514 (2011); 10.1116/1.3624787 Growth of heteroepitaxial ZnO thin film and Zn O ∕ ( Mg , Zn ) O nanomultilayer by off-axis rf magnetron sputtering J. Vac. Sci. Technol. A 23, 1 (2005); 10.1116/1.1814105

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Optical Characterization of Mg-doped ZnO Thin Films Deposited by RF Magnetron Sputtering Technique Satyendra Kumar Singh1, Purnima Hazra2, Shweta Tripathi1 and P. Chakrabarti1,3,a* 1

Department of Electronics and Communication Engineering, Motilal Nehru National Institute of Technology Allahabad-211004 2 School of Electronics and Communication Engineering, Shri Mata Vaishno Devi University, Katra-182320 3 Department of Electronics Engineering, Indian Institute of Technology (BHU), Varanasi-221005 a* Corresponding author: [email protected]

Abstract. This paper reports the in-depth analysis on optical characteristics of magnesium (Mg) doped zinc oxide (ZnO) thin films grown on p-silicon (Si) substrates by RF magnetron sputtering technique. The variable angle ellipsometer is used for the optical characterization of as-deposited thin films. The optical reflectance, transmission spectra and thickness of as- deposited thin films are measured in the spectral range of 300- 800 nm with the help of the spectroscopic ellipsometer. The effect of Mg-doping on optical parameters such as optical bandgap, absorption coefficient, absorbance, extinction coefficient, refractive Index and dielectric constant for as-deposited thin films are extracted to show its application in optoelectronic and photonic devices.

INTRODUCTION Zinc oxide (ZnO) is an inherently n-type compound semiconductor with a direct band gap of 3.37 eV and large exciton binding energy (60 meV) at room temperature [1-2]. Since last few years, ZnO has gained much interest because of its potential use in optoelectronic and photonic applications in ultra-violet wavelength region such as transparent electrodes in solar cells, light emitting diodes, nanolasers, UV photodetectors, antireflection coating due to its wide bandgap [1]. However, some limitations are there in the application of ZnO thin films in optoelectronic Integrated circuits, operated in deep UV region as the band gap of ZnO is not wide enough. In these cases, doping of metallic dopants in ZnO is a facile and effective method to modify the optical, electrical and structural properties of ZnO material in a wide range of wavelength [3]. Recently, Mg-doped ZnO thin film has received much attention because its bandgap can be tuned from 3.37 eV to 6.7 eV depending upon the Mg concentration in ZnO (0 x 1) [4]. Although Mg-doping is important to tailor the optical band gap of ZnO thin film, but simultaneously crucial optical constants like reflectance, abrorbance, extinction coefficient are also changed due to the change in band gap wavelength [3]. Therefore, in-depth study of significant optical parameters of Mg-doped ZnO films is necessary to demonstrate its application in optoelectronic devices. In the present work, Mg-doped ZnO films are deposited on Si substrate to fabricate ZnO/Si heterostructures using RF magnetron sputtering as this technique is more versatile. A wide range of deposition parameters, like RF power, substrate temperature, pressure, deposition rate, deposition time and controlled gas flow rate can be controlled manually in this technique [1].

EXPERIMENTAL TECHNIQUE Undoped ZnO and Mg-doped ZnO thin films were deposited on p-type Silicon substrate using RF magnetron sputtering technique (equipment model Hind HIVAC) using 2ʺ diameter target to prepare ZnO/Si heterostructures. To measure transmission spectra of the as-deposited films, these films are simultaneously grown on glass substrates also. The Mg-doped ZnO targets were prepared by mixing appropriate weight ratios of ZnO and MgO powders. The targets were made up of 99.99% pure ZnO and 99.99% pure MgO powder. The powder was

International Conference on Condensed Matter and Applied Physics (ICC 2015) AIP Conf. Proc. 1728, 020168-1–020168-4; doi: 10.1063/1.4946219 Published by AIP Publishing. 978-0-7354-1375-7/$30.00

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mixed by ball milling machine at 100 rpm for 2h duration. The powder was pressed into target by applying hydraulic pressure of 68.95 MPa (5 ton-force). All targets were sintered at 1000˚C for 8h. Prior to deposition, the Si substrate was cleaned by standard RCA-1and RCA-2 cleaning process while glass was cleaned in acetone medium by ultrasonicator. The zinc oxide and Mg-doped ZnO thin films were deposited on Si substrates at room temperature using sintered targets (2-inch diameter). During deposition of thin films by RF magnetron sputtering, the optimum deposition parameters were applied as follows: substrate temperature→27°C, deposition pressure→5.5×10-6 mbar, deposition rate→1.2 Å/s to 2 Å/s and deposition time→20 min and argon gas flow rate→25 sccm.

EXPERIMENTAL TECHNIQUE The surface morphology of as-fabricated thin films is studied by atomic force microscopy (AFM). The AFM 2-D images for the post-annealed RF magnetron derived Mg doped ZnO samples (scanning area→2μm×2μm) are shown in fig. 1 (a-c). AFM images show that as-fabricated thin films are packed homogeneously and there is no major porosity or voids. The root mean square value of roughness is increases by increasing the doping concentration into ZnO. The root mean square values of roughness were obtained as 1.45 nm, 1.77 nm and 3.79 nm for ZnO, 5% Mg doped ZnO and 10% Mg doped ZnO respectively. Further, it can be obtained from AFM images that the thin films are having variable grain size in the range of 21–33.75 nm with almost round gain structures. It is obtained that the grain size of thin films is decreased with increase in the doping concentration of Mg into ZnO.

FIGURE 1. Two dimentional AFM images of as-fabricated Mg-doped thin films: (a) undoped ZnO, (b) 5% Mgdoped ZnO and (c) 10% Mg-doped ZnO The optical transmittance, reflectance spectra and thickness of as-fabricated undoped and Mg-doped ZnO thin films are measured by spectroscopic ellipsometer (V-VASE, J. A. Woollam.Co.Inc.) to investigate optical characteristics of the films. The transmittance and reflectance spectra of the thin films in the wavelength region of 300–800 nm are shown in fig. 2. (a). The thin films show 80% to 95% transmittance in the visible range. When Mg doping concentration increases, the optical absorption edge presents a blue shift for higher photon energy [3]. When Mg is introduced in ZnO lattice, new defect levels are created due to the electro negativity and ionic radii difference between Zn and Mg atoms. Due to lower electron affinity of MgO as compared to ZnO, more electrons contributed by Mg dopants and these electrons shift the energy levels in the bottom of the conduction band. Therefore, the radiative recombination of excitons may lead to a blue shift [4]. The absorbance ( A) and absorption coefficient (α ) of the as-fabricated thin films is calculated using Beer– 1 1 2.303 (1) ln A d T d where d is thickness and T is transmittance coefficient of thin films. The variation of absorbance ( A) and absorption coefficient (α ) of the as-fabricated thin films in the wavelength region of 300-800 nm are shown in fig. 2 (b) respectively. In the direct bandgap semiconductor, the dependence of optical band-gap on the absorption

Lambert’s law [5]:

α

coefficient is obtained by the following equation [5]: (αhυ ) 2 B(hυ E g )

(2)


FIGURE 2. (a) Optical transmittance and (a-inset) Reflectance spectra; (b) Absorbance and (b-inset) Absorption coefficient of the undoped and Mg-doped ZnO thin films in the wavelength region of 300-800 nm where hυ , B and E g are the photonic energy, constant and optical band gap respectively. The optical bandgap can be determined by plotting the curve of (αhυ ) 2 versus hυ as shown in fig. 3 (a) and extrapolating the linear portion of the curve to hυ -axis. The obtained values of bandgap are 3.31 eV, 3.52 eV and 3.68 eV for ZnO, 5% Mg doped ZnO and 10% Mg doped ZnO thin film respectively. The refractive index is an important parameter for optoelectronic materials and their applications. The values of the extinction coefficient (k ) and refractive index (n) were evaluated by equation (3) and (4) [3]. αλ 4R (1 R ) k (3) and (4) n k2 2 4π (1 R ) (1 R )

FIGURE 3. (a) Band gap, (b) extinction coefficient and (b-inset) refractive index of undoped and Mg-doped ZnO thin films in the wavelength region of 300-800 nm. where λ is given wavelength in nm. The variation of extinction coefficient and refractive index for different doping concentration of Mg into ZnO thin films in the wavelength region of 300–800 nm are shown in fig. 3 (b). The refractive index and extinction coefficient of the Mg doped ZnO thin films has started decreasing with Mg incorporation into ZnO due to an increase in the carrier concentration in the Mg doped ZnO thin films [6]. However, when doping concentration is increased further, it has been started increasing again may be due to collision of excess carriers inside ZnO film. It can be observed from fig. 2 (b), as wavelength increases, due to scattering of light, absorbance is decreased and simultaneously the extinction coefficient (fig. 3 (b)) is also decreased [3]. The complex dielectric constant is a fundamental intrinsic property of the material for optoelectronic applications. The real part (ε1 ) and imaginary part (ε 2 ) of the dielectric constant are estimated by the extinction coefficient and the refractive index of the thin films by equation (5) and (6) respectively [3].


(5) and ε 2 nk (6) ε1 n 2 k 2 The imaginary part the dielectric constant shows how material absorbs energy from an electric field. The physical significance of the real part of the dielectric constant is that how it slows down the speed of light in the material. The real and imaginary part of dielectric constant in the wavelength region of 300-800 nm is shown in fig. 4 (a).

FIGURE 4. (a) Real part, (a-inset) Imaginary part of dielectric constant and (b) Real part, (b-inset) Imaginary part of optical conductivity of undoped and Mg-doped ZnO thin films in the wavelength region 300-800 nm. Further, the optical conductivity of the thin films depends on dielectric constant. The real part (σ 1 ) and imaginary part (σ 2 ) of the optical conductivity are obtained by equation (7) and (8) respectively [6]. (7) and (8) σ1 ωε 2ε 0 σ 2 ωε 1ε 0 where ω is the angular frequency, λ is operating wavelength region and ε 0 is dielectric constant of the free space. The wavelength dependence real part and imaginary part of the optical conductivity are shown in fig. 4 (b). Real and imaginary parts of optical conductivity were affected by the Mg incorporation into ZnO. Both real and imaginary parts of dielectric constant decreased for the wavelength higher than 450 nm and this may be because of the absorption coefficient [6].

CONCLUSION The effect of magnesium doping on the optical properties of RF sputtered undoped and Mg-doped ZnO thin films has been investigated using variable angle ellipsometer. AFM images are exhibited to show the surface morphology of as-deposited films. It has been observed that the grain size of the thin films is decreased with increase in doping concentration. Further, the optical transmittance and the optical band gap are increased with increase in the doping concentration while the reflectance decreases in visible range. The undoped and Mg-doped ZnO thin film shows 80–95% optical transmission in visible region. The optical band gap of 3.31 eV, 3.52 eV and 3.68 eV are estimated for ZnO, 5% Mg-doped ZnO and 10% Mg-doped ZnO thin films respectively. The optical parameters such as absorbance, absorption coefficients, refractive index, extinction coefficient, dielectric constants and optical conductivity are evaluated using the transmission spectra, optical reflectance and thickness of the asfabricated thin films.

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