Effect of thickness on the microstructural, optoelectronic and morphological properties of electron beam evaporated ZnTe films

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Abstract

ZnTe films of different thicknesses were deposited on glass substrates at a substrate temperature of 300 °C. The thickness of the films varied in the range of 110–320 nm. The films exhibited cubic structure with preferential orientation in the (1 1 1) direction. A very high value of absorption co-efficient (104 cm−1) is observed. Band gap values in the range between 2.23 and 2.38 eV are observed when the film thickness was varied between 320 and 110 nm, respectively. The refractive index values are found to vary between 2.68 and 2.90 for the films of different thicknesses. It has been observed that the conductivity increases continuously with temperature as well as with thickness. Laser Raman spectra showed the presence of peaks at 206.8 and 411.2 cm−1corresponding to the first order and second order LO phonon of monophase ZnTe films. PL analysis confirmed the formation of monophase ZnTe films with nano grains.

Introduction

ZnTe is a promising material for pure-green light emitting diode, windows layer in tandom solar cells, and transparent conductive thin films [1]. ZnTe can be used for back contact layer (BCL) on p-CdTe absorber layer in CdTe based solar cells before its metallization because the valence-band offset between p-ZnTe and p-CdTe is less than 0.05 eV [2].

This direct bandgap nature of ZnTe with a value of 2.26 eV makes it a potential candidate for the fabrication of pure-green LED devices [3], [4], [5], [6]. Because of its high electro-optic coefficient, ZnTe also promises to be useful in the production and detection of terahertz (THz) radiation [3], [7]. Since there is only a small valence-band offset of 0.05 eV between ZnTe and CdTe, ZnTe can be used as a back contact material to obtain higher solar energy conversion efficiency in CdTe based solar cells [8]. Though some research groups have reported the fabrication of ZnTe based devices like LEDs and terahertz detectors, most of them have preferred highly sophisticated techniques like molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), etc. to obtain ZnTe films [3], [4], [5], [6], [7], [8], electrodeposition from aqueous solutions [9], [10]. ZnTe is a very attractive host for optoelectronic device realizations because of its sensitivity in the green spectral region (2.26 eV). Specifically, for bright light emitting diodes (LEDs), ZnTe is a technologically important material since the emission wavelength matches well with the maximum sensitivity of the human eye. Utilization of ZnTe thin films in device development were reported by the realization of LED prototypes [11], [12], high-efficiency multi-junction solar cells [13], and terahertz (THz) devices [14]. In some reports the concept of laser crossing and all-optical laser transmission digitizing with GaAs [15], [16], CdS [17] and InP has been successfully demonstrated [18]. In an earlier report on electron beam evaporated ZnTe films, the optical and dielectric properties were discussed [19]. The exploration of novel thin film materials and simple technique based technologies for future light-based communication systems, such as all-optical switches and hybrid device structures [20], have been the driving motivation for the present work. In this study, structural, optical, morphological and electrical properties of ZnTe films deposited by the electron beam (EB) evaporation technique are presented in detail.

Section snippets

Experimental

ZnTe films of varying thickness were deposited on glass substrates by keeping the substrate temperature constant at 300 °C. Commercial ZnTe powder (99.99% purity) was used as the source. The substrate temperature was fixed well below the decomposition temperature observed from the DTA results which indicated an exothermic peak at 437 °C due to the partial decomposition of ZnTe. The films were characterized by X-ray diffraction (XRD) studies using CuKα radiation form an Xpert PANanalytical XRD

Results and discussion

The X-ray diffraction pattern (Fig. 1) shows that the ZnTe films of different thicknesses deposited at 300 °C substrate temperature possess cubic structure with an average lattice constant ‘a = 6.093 Ǻ. The different peaks were indexed and the corresponding interplanar spacing ‘d’ were calculated and compared with the standard ASTM values. Preferential orientation in the (1 1 1) direction is also observed. The films deposited at room temperature indicate amorphous structure. As the deposition

Conclusion

The ZnTe thin films with nano grains were deposited onto the glass substrates kept at 300 °C. The results of this study indicate that uniform and device quality ZnTe films with cubic structure can easily be deposited by the EB evaporation technique. Optical studies showed direct band gap value in the range of 2.23–2.38 eV which are found decreasing with the increase of film thickness. The grain size was in the range 70–150 nm as observed from AFM studies. Raman scattering and PL results confirmed

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