Spectroscopic and electrical analysis of vacuum co-evaporated CdxZn1-xTe thin films
Graphical abstract
Introduction
Polycrystalline CdxZn1-xTe (CZT) is a versatile ternary chalcogenide, which is well suited for optoelectronic device applications. Apart from tunability of the optical band gap, CZT preserves some of the promising properties like high absorption coefficient [1], high binding energy [2], high average atomic number [3], good electron-transport properties and higher quantum efficiency compared to CdTe [4]. Due to these properties, CZT ternary chalcogenide thin films are widely used in opto-electronic devices such as solar cells [5], photo-detectors [6], gamma and X-ray detectors [7] etc. In most of the CdTe based solar cells, p-type CdTe is used as absorbing layer with different back contact methods, which suffer from long-term instabilities due to the in-diffusion of contaminants or impurities. To overcome this drawback, as an alternative, p-type window layer with n-type CdTe absorber can be used. Reports on n-type CdTe absorber layer used in graphitic carbon Schottky - type heterojunction [8], reduced grapheme oxide (rGO) solar cells [9] and diamond/CdTe inverted heterojunction solar cells [10] are available in literature. CZT films with n-type conductivity can improve the performance of these type of solar cells due to their favourable optoelectronic properties mentioned in the beginning.
Several physical and chemical methods were reported on the preparation of CZT films such as molecular beam epitaxy [11], pulsed laser deposition [12], sputtering [13], thermal vacuum evaporation [14], chemical bath deposition [15], closed space sublimation method [16] etc. Substrate temperature [17], thickness of the films [18], types of substrates [19] and post deposition heat treatment [11] are the important factors that influence the structural and optical properties of CZT thin films. Current research in renewable energy is focussed on low cost, excellent stability, high efficiency in solar cells. Solar cell with high efficiency can be achieved by using wide band gap window and tandem solar cell in which structure consists of top cell and bottom cell which are connected in series. Applications like radiation detectors and solar cell require high quality, cost effective CZT films as absorber layer with large grains, low defect concentration, high lattice ordering and uniform distribution of the elements.
In the present work, thermal co-evaporation technique is adopted for synthesis of the CZT films with CdTe and ZnTe as the source materials. Because of advantages low cost, like high deposition rate, uniform deposition, less material consumption and most productive over other methods, thermal co-evaporation method is preferred over other techniques. The objective of this work is to study the structural, morphological, optical and electrical properties; analyse defects states present in the compositionally varied CZT films.
Section snippets
Experimental details
A series of CdxZn1-xTe (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) thin films of thickness 450 ± 5 nm were grown on cleaned borosilicate glass substrates by thermal co-evaporation method under a base pressure of 3 × 10−6 mbar. CZT films were grown by taking stoichiometric ratio of CdTe and ZnTe (Alfa Aesar, 99.999% purity) as source material which were weighed in appropriate proportion and placed in molybdenum boats separately. Substrate cleaning is the prior step in the film deposition as it affects
Morphological analysis
All films with different compositions were found to be homogeneous, uniform and free from cracks. Fig. 1 (a), (b) and (c) are representative SEM topology of the CZT films with x = 0, 0.4, 1. SEM images show that grains in the films are relatively dense and have greater grain size with less grain boundaries as compared with previous reports on CZT films [23,24]. These are essential properties to reduce the trapping effect in the boundaries and improve the collection of carriers which qualifies
Conclusions
This research project has demonstrated that thermal co-evaporation can be a suitable technique to grow CZT films with required stoichiometry. Structural and optical characterizations have confirmed the device quality of 450 nm thick films grown by this technique. Minor deviation in the Zn composition is attributed to lower sticking coefficient of Zn compared to Cd. Optical bad gap engineering has been successfully achieved and films with x = 0.8 found to be more suitable for absorber layer
CRediT author statement
Sahana Nagappa Moger: Experiment – growth, optical, electrical and graphical conversion of the data. Writing the manuscript. Deepika U Shanubhogue; Structural study - XRD, Rashmitha Keshav: Data analysis – Optical and structural. Mahesha M G: Building the concept, data analysis, reviewing and editing.
Declaration of competing interest
Authors have no conflict of interest to declare.
Acknowledgement
The authors are grateful to UGC DAE CSR, Indore, Govt. of India (CSR-IC-MSRSR-11/CRS-219/2017–18/1300) for financial assistance. Also, authors are grateful to Dr. Vasanth Sathe, Raman Lab, CSR Indore, India, for extending the Raman Spectroscopy facility.
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