Electrodeposition and Characterization of CuTe and Cu 2 Te Thin Films

An electrodeposition method for fabrication of CuTe and Cu 2 Te thin films is presented. The films’ growth is based on the epitaxial electrodeposition of Cu and Te alternately with different electrochemical parameter, respectively. The deposited thin films were characterized by X-ray diffraction (XRD), field emission scanning electronic microscopy (FE-SEM) with an energy dispersive Xray (EDX) analyzer, and FTIR studies. The results suggest that the epitaxial electrodeposition is an ideal method for deposition of compound semiconductor films for photoelectric applications.


Introduction
Semiconducting compounds such as I-VI copper chalcogenides are widely used in the fabrication of photoconductive and photovoltaic devices [1].Copper based chalcogenides exhibited the characteristics of a p-type semiconductor for the vacancies of copper and are potential materials for wide applications.Thin films of copper chalcogenides especially have been a subject of interest for many years mainly because of their wide range of applications in solar cells [2], superionic conductors [3], photodetectors, photothermal [4] converters [5], electroconductive electrodes [6], and so forth.
Of these copper chalcogenides, copper telluride compounds have gained great interest owing to its superionic conductivity [7], direct band gap between 1.1 and 1.5 eV [8], and large thermoelectric power.In the literature, a number of methods for preparation of Cu x Se [9] and Cu x S [10,11] thin films have been reported.However, fabrication of CuTe thin films is much less studied to data.Copper telluride compounds (Cu x Te, where  = 1, 2 or between 1 and 2) were known to exist in a wide range of compositions and phases whose properties are controlled by the Cu : Te ratio [12] and can be grown by chemical bath deposition, coevaporation, and fusion method [13].
Electrochemical atomic layer deposition is considered as a controllable and simple deposition technique [14] for homogeneous compound semiconductors on conductive substrates without annealing [15].The electrochemical atomic layer deposition was based on the alternated underpotential deposition which was a phenomenon of surface limited [16] so that the resulting deposit was generally limited to one atomic layer [17].Thus, each deposition cycle formed a single layer of the compound [18,19], and the number of deposition cycles controls the thickness of deposits [20].In this paper, an epitaxial electrodeposition method for preparation of CuTe and Cu 2 Te thin films on ITO substrates by controlling the solution conditions in contact with the deposit and the potential of the electrode is reported.The crystallographic structures of the obtained films are discussed on the basis of X-ray diffraction data.Field emission scanning electronic microscopy (FE-SEM) with an energy dispersive X-ray (EDX) analyzer shows investigation of morphology.Optical characteristics of the films are studied by FTIR.

Experimental
Electrochemical experiments were carried out using a CHI 660A electrochemical workstation (CH Instrument, USA).
The deposition was performed in a three-electrode cell with a platinum wire as counter electrode and Ag/AgCl/sat.KCl as reference electrode.Indium doped tin oxide (ITO) glass slide (≈20 Ω/cm) was used as a working electrode.Prior to electrodeposition, the ITO substrate was ultrasonic cleaned with acetone, ethanol, and water sequentially.
All solutions were prepared with nanopure water purified by the Milli-Q system (Millipore Inc., nominal resistivity 18.2 MΩ cm), and all chemicals were of analytical reagent grade.The oxygen was removed by blowing purified N 2 before each measurement, and the whole experiments were conducted at room temperature.
The crystallographic structures of the thin films obtained were determined by XRD (Rigaku D/max-2400).The morphology is investigated by FE-SEM (Kevex JSM-6701F, Japan) equipped with an EDX analyzer.Glancing angle absorption measurements were performed using an FTIR spectrophotometer (Nicolet Nexus 670, USA).

Thin Film Deposition
3.1.1.CuTe Thin Film Deposition. Figure 1 shows the cyclic voltammograms of ITO electrode in blank and Cu solution, respectively.For CuTe film growth, H 2 SO 4 was used as supporting electrolyte.From Figure 1(b), only one pair of redox peaks was observed at −0.34 V (C1) and 0.30 V (A1), corresponding to Cu 2+ reduction to Cu, as reaction (1) shows Figure 2 shows the cyclic voltammograms of Cu-covered ITO electrode in 0.1 M H 2 SO 4 and in 5 mM H 2 TeO 3 + 0.1 M H 2 SO 4 solutions.In these experiments, the potential scanning was started at 0 V to avoid the oxidative stripping of Cu.Similar to most literatures, two reduction peaks are seen: peak C2 at about −0.21 V based upon the four-electron process for Te reduction shown in reaction (1) and peak C3 at about −0.46 V, which should be corresponded to bulk Te (0) reduction to Te 2− , as reaction (2) shows Te Therefore, we applied −0.30V as the electrodeposition potentials for Cu and −0.20 V for Te.Repeat electrodepositing Cu at −0.30 V and Te at −0.20 V for 15 s alternately as many times as desired to grow epitaxial nanofilms of CuTe on ITO substrate.

Cu 2
Te Thin Film Deposition.For Cu 2 Te film growth, KNO 3 was used as supporting electrolyte because Cu + ions cannot exist in a strong acid solution like 0.1 M H 2 SO 4 .Figure 3 shows the cyclic voltammograms of ITO electrode in blank KNO 3 and Cu solution, respectively.In Figure 3 and reduction of Cu on the ITO substrate, as reaction ( 4) and (1) show [14]: Figure 4 shows the cyclic voltammograms of Cu Therefore, we applied −0.20 V as the electrodeposition potentials for Cu and −0.60 V for Te.Repeat electrodepositing Cu at −0.20 V and Te at −0.60 V for 15 s alternately as many times as desired to grow epitaxial nanofilms of Cu 2 Te on ITO substrate.

X-Ray Investigations.
Identification of the obtained thin films was carried out using the X-ray diffraction method.The recorded XRD patterns of deposited CuTe and Cu 2 Te are presented in Figure 5. Figure 5(a) shows the XRD patterns of deposited CuTe film.The observed peak positions of the deposited CuTe film are in well agreement with those due to reflection from (0 1 1), (1 0 1), and (1 1 2) planes of the reported CuTe data with an orthorhombic structure (JCPDS 22-0252).The XRD pattern of deposited Cu 2 Te film is presented in Figure 5(b).As can be seen, the analysis indicates that the deposited Cu 2 Te film is in hexagonal structure, with the preferential orientation of (0 0 6) plane (JCPDS 49-1411).
The average crystal size was estimated using the wellknown Debye-Scherrer relationship: where  is the Bragg angle,  is the X-ray wavelength, and  is the full width at half-maximum.It was found that the average crystal size of the deposited CuTe film is 92.11 nm and Cu 2 Te film was found to be about 36.84 nm, which are consistent with the SEM observation.
where  is the constant,   is the band gap, and ℎ] is the photon energy.Figure 7 shows the variation of (ℎ]) 2 with ℎ] for deposited CuTe and Cu 2 Te.By extrapolating straight line portion of (ℎ]) 2 against ℎ] plot to  = 0, the optical band gap energy was found to be 1.51 eV for CuTe and 1.12 eV for Cu 2 Te films, comparable with the value reported earlier for CuTe and Cu 2 Te thin film [1,15].

Conclusion
In this work, the Cu/Te ratio has been successfully controlled to prepare crystalline CuTe and Cu 2 Te thin films on the ITO electrode via electrodeposition.The copper-tellurium films were epitaxial electrodeposited under layer-by-layer, potentiostatic conditions.XRD, SEM, and IR studies of the deposited CuTe and Cu 2 Te thin films confirm the high quality of the deposits and demonstrate that the epitaxial electrodeposition is applicable to the deposition of stoichiometric nanofilms of copper-tellurium films of good quality.

5 )Figure 3 :Figure 4 :
Figure 4 shows the cyclic voltammograms of Cu 2 Ocovered ITO electrode in 0.1 M KNO 3 and in 5 mM H 2 TeO 3 + 0.1 M KNO 3 solutions.From Figure 4(b), two reduction peaks are also seen: peak C6 at about −0.35 V based upon the H 2 TeO 3 reduction to Te and peak C7 at about −0.60 V corresponding to Te reduction to H 2 Te, which immediately react with the underlying Cu 2 O layer to form Cu 2 Te, as reaction (5) shows Cu 2 O + H 2 Te ←→ Cu 2 Te + H 2 O (5)