Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thin films Fikry El Akkad and Yaser Abdulraheem Citation: Journal of Applied Physics 114, 183501 (2013); doi: 10.1063/1.4829453 View online: http://dx.doi.org/10.1063/1.4829453 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/18?ver=pdfcov Published by the AIP Publishing
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JOURNAL OF APPLIED PHYSICS 114, 183501 (2013)
Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thin films Fikry El Akkad1,a) and Yaser Abdulraheem2 1
Physics Department, College of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait Department of Electrical Engineering, College of Engineering and Petroleum, Kuwait University, Safat 13060 Kuwait 2
(Received 29 July 2013; accepted 22 October 2013; published online 8 November 2013) We report on a study of the physical properties of ZnTe:Cu films with Cu content up to 12 at. % prepared using rf magnetron sputtering. The composition and lateral homogeneities are studied using X-ray photoelectron spectroscopy (XPS). Atomic force microscopy measurements on films deposited at different substrate temperatures (up to 325 C) yielded activation energy of 12 kJ/mole for the grains growth. The results of XPS and electrical and optical measurements provide evidence for the formation of the ternary zinc copper telluride alloy in films containing Cu concentration above 4 at. %. The XPS results suggest that copper is incorporated in the alloy with oxidation state Cu1þ so that the alloy formula can be written Zn1yCuy Te with y ¼ 2x, where x is a parameter measuring the stoichiometry in the Cu site. The formation of this alloy causes appreciable shift in the binding energies of the XPS peaks besides an IR shift in the energy band gap. Detailed analysis of the optical absorption data revealed the presence of two additional transitions, besides the band gap one, originating from the C8 and C7 (spin-orbit) valence bands to a donor level at 0.34 eV below the C6 conduction band. This interpretation yields a value for the valence band splitting energy D ffi 0.87 eV independent of copper concentration. On the other hand, the mechanism of formation of the alloy is tentatively explained in terms of a point defect reaction in which substitutional Cu defect CuZn is also created. Assuming that substitutional Cu is the dominant acceptor in the Zn rich alloy as in ZnTe, its formation energy was determined to be 1.7 eV close to the theoretical value (1.41 eV) in C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4829453] ZnTe. V
I. INTRODUCTION
The II-VI compound ZnTe is considered as a potential candidate for applications in the field of optoelectronic devices particularly green light emitting diodes (LED’s) and solar cells.1–4 Its incorporation as part of hybrid systems such as, for example, HgZnTe,5 MgZnTe,6 and CdZnTe7 widens its range of applications to include infrared detectors, blue LED’s, and tandem solar cells, respectively. Moreover, the feasibility of photovoltaic devices using ZnTe homojunctions has recently been demonstrated.8 The conductivity of the nonintentionally doped ZnTe is p-type due to self doping by intrinsic defects (Zn vacancies).9 The conductivity can further be enhanced through doping with substitutional acceptors of group I (Cu, Au, Li) or group V (P, N) elements.10 Previous investigations showed that Cu is a suitable dopant for several applications because it produces a relatively shallow acceptor defect (EA ¼ 0.12–0.15 eV) that can be introduced either during growth or by post- preparation diffusion.11–13 Among the important possible applications of Cu-doped ZnTe is its use as ohmic contact to CdTe in the CdS/CdTe solar cell, which is a potential candidate for wide scale photovoltaic applications.14,15 All these applications have stimulated a large amount of work on ZnTe:Cu thin films prepared by various techniques including electro-deposition,16 metalorgnic vapor phase epitaxy,17 molecular beem epitaxy,18 vacuum evaporation,19 and rf sputtering.15,20–22 a)
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Despite the numerous publications on the physical properties of rf sputtered ZnTe:Cu thin films, little is known about the electrical and optical properties of films containing Cu concentration beyond 5 at. % (Ref. 13) and even less is known about the morphology of this type of films. Although at such high Cu concentration, the possibility of formation of the ternary ZnCuTe alloy was recognized by previous authors13–15 yet the signature of the alloy on the electrical and optical properties has been given relatively little attention. The identification of the alloy is made difficult by the close similarity between the lattice constants of ZnTe and Cu2Te in the cubic phase, which makes difficult the identification of the alloy using X-ray diffraction measurements.23 Further difficulty comes about from the scarce information on the ZnCuTe alloy particularly in the form of thin films. To our knowledge, the only report on the preparation and properties of ZnCuTe thin films is the work of Pistone et al.23 on films prepared by electrodeposition. Some of the electrical and optical properties of these films have been reported. In the present work, we use Atomic Force Microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and electrical and optical measurements for the study of some physical properties of ZnTe:Cu films containing Cu concentration up to 12 at. %. The formation of the alloy has been confirmed through a close investigation of the XPS core levels spectra and detailed analysis of the optical absorption spectra. The results of electrical measurements are analyzed on
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the basis of a proposed point defect model, which leads to the determination of the formation energy of the substitutional Cu acceptor (CuZn). Also the activation energy of the grain growth has been estimated from AFM results on films deposited in the range 60–325 C. II. EXPERIMENTAL
ZnTe thin films were prepared in the chamber of Edwards Auto 360 rf magnetron sputtering unit. The chamber was evacuated to a pressure of about 105 Torr (measured by a Penning gauge) using an oil diffusion pump and a liquid nitrogen trap. A high purity Argon gas (99.999% purity) is then admitted with a flow rate adjusted to maintain a pressure of 7.5 103 Torr using a pneumatic valve. The substrates were soda lime glass of dimension 2 cm 2 cm placed 5.0 cm above the center of the target. The substrate was cleaned by soaking in a solution of 5% detergent (Tide) in de-ionized water which was placed in an ultrasonic cleaner for 30 min. It was then sequentially rinsed in two beakers of clean de-ionized water. A hot pressed ZnTe target (delivered by E-Vac company) of diameter 6.4 cm and purity 99.999% was used. Before each deposition, the target was pre-sputtered for 10 min while covering the substrate with a shutter in order to remove any contamination and to eliminate any preferential sputtering effects. All the films were prepared using an rf power density of 1 W/cm2. The substrate temperature was measured using a chromel-alumel thermocouple and controlled in the range 30 Ts 325 C using an IR heater (quartz halogen lamp). For the preparation of Cu-doped films, a number of Cu strips (99.999% purity) each of dimension 2 mm 4 mm (measured by a traveling microscope) were uniformly distributed on the ZnTe target along the circumference of a circle of radius equals to half the radius (3.2 cm) of the target. The strips had a total surface area S up to 3% of the total target area (32 cm2). The Cu concentration in the 2 2 cm2 films was found to be uniform to 613% using XPS measurements (Sec. III B). The films’ thicknesses were measured using a Tencor instrument profiler type Alpha-step 200. AFM was used to study the samples morphology. The measurements system (type Agilent 5420 AFM) utilizes the AC-AFM scanning mode to ensure the highest possible resolution without damaging the samples. Silicon cantilevers for non-contact mode AFM imaging were used (Nanosensors) with a typical tip radius
