Effect of Zn/Cd ratio on the optical constants and photoconductive gain of ZnO–CdO crystalline thin films

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Abstract

Optical and photo-electrical properties of ZnO–CdO films with the ratio of Zn/Cd=1:0, 3:1, 1:1, 1:3 and 0:1 has been studied. XRD study confirms the combination of hexagonal ZnO and cubic CdO phase present in the polycrystalline sample. Atomic force microscopy results indicate that the crystal grains are agglomerated and surface roughness enhanced due to higher Cd concentration in ZnO. From optical studies, it is found that the transmittance and the band gap decreased as Cd content increased. Photoluminescence studies on ZnO–CdO films showed intense near-band edge emissions at room temperature and is attributed to recombination of excitons localized within band tail states likely caused by non-uniform Cd distribution in ZnO–CdO matrix. The dispersion of refractive index was analyzed by the Wemple–DiDomenico single-oscillator model. The third-order nonlinear polarizability is found high with higher concentration of cadmium at higher photon energies. Some other optical parameters such as dissipation factor, optical conductivity, interband transition strength, surface and volume energy loss have been calculated depending on dielectric constant evaluated from optical data. Finally, photoconductive gain and carrier lifetime have been calculated and found dependent on Zn/Cd ratio.

Introduction

Thin film of transparent conducting oxide (TCO) which has high transparency through the visible spectrum attracts a lot of research interest due to their numerous applications such as light transparent electrodes, solar cells, thin-film photovoltaic and many other opto-electronic devices [1], [2], [3], [4], [5]. Among several of n-type TCOs, ZnO is regarded as a promising optoelectronic material due to its direct wide band gap (3.37 eV) and highest value of free exciton binding energy (60 meV) at room temperature (RT). This allows the excitonic transitions which leads higher radiative recombination efficiency for spontaneous emissions [6]. Also ZnO absorbs larger fractions of solar radiation [7] and the high thermal energy of ZnO (25 meV) leads to the extreme stability of excitons at RT to high temperatures [8]. CdO is also an n-type degenerate semiconductor of cubic structure with a direct band gap of 2.3 eV that combines many beneficial characteristics of both CdO and ZnO. So the band gap of ZnO can be modulated by substituting CdO which is one of the major requirements for the design of opto-electronics devices. Moreover, it is the fundamental and practical interest to study the Zn–Cd–O ternary system, since Zn and Cd belong to the same group in the periodic table and the ratio of Zn and Cd cations becomes important for obtaining a TCO film. These two attractive materials along with their derivatives have been fabricated by various methods, including molecular beam epitaxy [9], sol–gel [10], electrodeposition [11], thermal decomposition technique [12], pulsed laser deposition [13], sputtering [14], chemical vapor deposition [15] and spray pyrolysis [16]. Among these techniques, spray pyrolysis offers many advantages like low cost, easy handling, large area and nano-structured film productions.

Many researchers concentrate on the preparation of alloy films made of ZnO and CdO and study the physical properties such as optical constants of these films [17]. The optical constants of ZnO–CdO films are key parameters to use these materials in optoelectronic devices. Also, in the field of material science, the energy loss process of single electron in the surface/bulk of the solid has attracted much attention. When the primary electron impinges and penetrates or single electron escapes it suffers certain energy loss through inelastic scattering process inside the solid. In general the energy loss function results from dielectric constant of the materials and the loss energy function is formed by the inter-band transitions [18]. The inter-band transitions involve two kinds of carriers and take place between the conduction band and the valence band. The process of intra-band transitions happen inside either the conduction or the valence band and involve only one type of carrier.

In this paper, we report the preparation and characterization of pure ZnO, CdO and a series of ZnO–CdO films. The main interest of the current work is to study the effect of Zn/Cd ratios on the structural, morphological, optical and photo-electrical properties of these films to use opto-electrical devices.

Section snippets

Experimental

ZnO, CdO and ZnO–CdO films were deposited on glass substrate by spray pyrolysis technique [19] at 360 °C in air ambient. The glass substrates were cleaned by using lukewarm aqueous solution of sodium carbonate, nitric acid and distilled water. The spray solution was prepared by mixing 0.1 M of zinc acetate dehydrate [Zn(COOCH3)2·2H2O] and cadmium acetate dehydrate [Cd(COOCH3)2·2H2O] diluted in ethanol and de-ionized water at 1:1 ratio. Then to obtain ZnO, ZnO–CdO and CdO thin films, the ratios of

Result and discussion

The XRD patterns of the films are shown in Fig. 1. From XRD patterns, it is clear that the films are polycrystalline in nature and is a combination of hexagonal ZnO and cubic CdO phases. The lattice parameters estimated from most oriented planes are tabulated in Table 1. The micro-structural details depending on Zn/Cd are given in [20], [21]. The texture coefficient, Tc(hkl), of a film is a parameter that solely governed the preferential orientation of a crystal has been calculated for (100),

Conclusions

In this work, a series of ZnO–CdO thin films successfully prepared by a very simple cost effective spray pyrolysis method. XRD results show that the synthesized ZnO–CdO samples are polycrystalline with combination of hexagonal ZnO and cubic CdO phases. The lattice parameters a increased and c decreased with increasing Cd content in ZnO. The AFM study reveals that the crystal grains are agglomerated in the c-axis. The surface of the films changes with Zn/Cd ratio and observed to increase

Acknowledgments

One of the authors A.M.M. Tanveer Karim is thankful to the Ministry of Science and Information & Communication Technology (MOSICT) (Grant No. 39.012.002.01.30.014.2010-93(101)) of the Peoples Republic of Bangladesh for providing financial support. We gratefully acknowledge Dr. M. Faruk Hossain, Department of Electrical and Electronic Engineering, Rajshahi University of Engineering and Technology, Bangladesh for providing photo-electrical measurement.

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