Investigation on ZnTe/CdxZn1-xTe heterostructure for photodetector applications
Graphical abstract
Introduction
In past decades, the research on photodetectors was focussed on the low dimensional nano-structured materials developed by multidisciplinary approaches which exhibit excellent photoelectric properties ranging from ultra-violet to tetra hertz frequency [1]. Numerous II-VI chalcogenide semiconductor materials and their hetero-structures were used to fabricate photodetector with high sensitivity, large photocurrent gain, good reliability and low response time such as ZnO/ZnSxSe1-x/ZnS [2], CdS [3], ZnO/ZnSe [4], PbTe/ZnTe [5], CdSe/ZnTe [6], CdTe/ZnTe [7], p-CdTe/n-CdS [8] etc. Among these, CdTe and ZnTe chalcogenide semiconductors having a direct band gap of around 1.50 eV and 2.17 eV, which covers the visible and near IR regions, became promising candidates for photodetector applications. CdTe and CdxZn1-xTe with favourable properties such as band gap, high detective quantum efficiency and large atomic number have actively developed for room temperature X-ray or gamma ray solid state radiation detectors [[9], [10], [11]]. Also, CdZnTe/AlN composite structure was used for UV photodetector for which, significant low leakage current was observed in the range of 10-9 A and sufficiently high (about 23) Iphoto/Ileakage was obtained [12]. Mohd. Shkir et al [13] studied CdZnTe and InCdZnTe single crystals for visible light photodetectors. These crystals exhibit a sensitive, fast and stable photoresponse to the laser light 633 nm. However, it appears that in CdZnTe detectors the photo to dark current ratio has found to be low, therefore suitable interface engineering is needed to obtain high photoresponse.
Depending on the atomic % of constituent elements and type of native defects created, CdTe can be prepared with both n-type and p-type conductivity. In CdTe, the formation of Te vacancies (VTe) and/or Cd interstitials (Cdi) result in n-type conductivity. p-ZnTe/n-CdTe heterostructures prepared by thermal evaporation method have shown barrier height around 0.80 eV, which is higher than the theoretical value and this differences may be due to quantum size effect, defects states and lattice mismatches [14]. Tamotsu Oleamoto et. al [15] have investigated the effect of ZnTe layer on CdZnTe layer grown by Close Space Sublimation (CSS) method and results showed that additional ZnTe layer over graphite substrate enhances the nucleation and growth of CdZnTe films. In case of CdZnTe films grown by electrodeposition, the detector performance had improved with the addition of Zn to CdTe. In addition, inclusion of Zn layer between electrode and CdZnTe resulted in reduced electrode roughness and better ohmic contact with lower barrier height [16,17]. It is established that the addition of ZnTe layer on CdZnTe will reduce the barrier height and enhance the detection performance.
Even though there are several reports on ZnTe/CdZnTe heterostructure, to the best of authors’ knowledge, ZnTe/CdxZn1-xTe heterostructure grown by thermal co-evaporation for photodetector applications remains less explored. In our previous work [18], CdxZn1-xTe films were grown on glass substrate by thermal co-evaporation method by varying the composition parameter ‘x’. Pure ZnTe (x = 0) showed p-type conductivity and carrier type switched from p to n at x = 0.4 for CdxZn1-xTe films. Here, we synthesized p-ZnTe/CdxZn1-xTe heterostructure by thermal evaporation method aiming with lower barrier height and high photo response. Fabricated heterostructures were subjected to various structural, optical and electrical studies to evaluate the ultimate device performance.
Section snippets
Experimental details
The ZnTe/CdxZn1-xTe (0.2 ≤ x ≤ 1.0) bilayer thin films were deposited on cleaned glass substrates by thermal co - evaporation method under high vacuum (3 × 10-6) at room temperature. The glass substrates were initially cleaned in acidic medium. After that, they were washed with distilled water followed by isopropyl alcohol. To prepare CdxZn1-xTe layer, the source materials CdTe and ZnTe (Alfa Aesar, 99.999% purity) were taken in required quantity in two separate molybdenum boats (200 A) that
Structural analysis
XRD patterns (Fig. 1(a)) are analysed to study the structural properties of ZnTe/CdxZn1-xTe hetero-structures. Double diffraction peaks observed in all the samples has confirmed the bilayer growth with no or minimum diffusion. Crystallite size and crystal defects such as dislocations in the films are two key factors, which decide the XRD pattern including position and width of the peaks. In present case, the position of prominent peak of ZnTe and CdxZn1-xTe (0.2 ≤ x ≤ 1.0) have shifted to lower
Conclusion
In a summary, double diffraction peaks in XRD patterns of all the samples show that bilayer has been grown with no or less diffusion at the interface. Among the series of heterostructures ZnTe/CdxZn1-xTe (0.2 ≤ x ≤ 1.0) grown by vacuum deposition, sample with x = 0.8 has shown good rectification. Lower barrier height with maximum optical absorption are favourable factors for device applications on optoelectronics. Raman and XPS analysis have confirmed the formation of ternary CdxZn1-xTe
CRediT authorship contribution statement
Sahana Nagappa Moger: Investigation, Formal analysis, Methodology, Visualization, Writing - original draft. Mahesha MG: Conceptualization, Funding acquisition, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
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.
Ms. Sahana Nagappa Moger has obtained her MSc degree from Manipal Academy of Higher Education (MAHE), India and currently pursuing her PhD in Department of Physics, Manipal Institute of Technology.
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Cited by (0)
Ms. Sahana Nagappa Moger has obtained her MSc degree from Manipal Academy of Higher Education (MAHE), India and currently pursuing her PhD in Department of Physics, Manipal Institute of Technology.
Dr. Mahesha M G has obtained his PhD degree from National Institute of Technology Karnataka (NITK), India. He is Assistant Professor in Department of Physics, Manipal Institute of Technology. His research interests are characterization of nano-crystalline thin films for device applications like solar cells, photo-sensors, TFTs and memory devices.