Electrochemical Atomic Layer Deposition of a CdTe/PbTe Superlattice for the Absorber Layer of a Solar Cell

Pritpal Singh and Amal Kabalan

Villanova University, Villanova, PA, 19085, USA

Abstract — Electrochemical Atomic Layer Deposition

(ECALD) has been used to deposit CdTe and PbTe thin films. CdTe and PbTe are of interest in forming a superlattice structure to be used in the absorber layer of a solar cell. Cyclic voltammetry was used to study the Underpotential Deposition (UPD) of the constituent compounds. Pb was grown at a constant -0.35V vs. Ag/AgCl reference electrode.. Te deposition potential was varied from -0.55V to -0.4V. Cd was deposited at -0.55V and Te was deposited at -0.8V. The coverage of the films was estimated using the coulometry method. The chemical composition of the films was characterized using Energy-Dispersive X-ray Spectroscopy (EDS) on a Scanning Electron Microscope (SEM). The crystallinity of the films was studied using a glancing angle x-ray diffractometer. The bandgaps of the films were determined using optical reflection data. Index Terms — electrochemical process, superlattice,

photovoltaic cells, cadmium, tellurium, lead.

I. INTRODUCTION II-VI compound semiconductors are an important group

of materials that have been used in a wide variety of optoelectronics devices such as: photodetectors, light emitting diodes and solar cells [1]. In order to obtain high quality thin films for such applications, high temperature vacuum methods have been used. Molecular beam epitaxy (MBE) [2], Chemical Vapor Deposition (CVD) [3] and Metal Organic Vapor Phase Epitaxy (MOVPE)[4] are some of the methods that have been used. The primary film control in those methods is the film deposition temperature (200-500°C) [5]. Electrochemical Atomic Layer Deposition is a simple

and low cost thin film deposition technique that can be used to deposit metals and chalcogenide thin films at room temperature[6]. During the past few years a research group led by Dr. John Stickney at the University of Georgia developed this technique and has been using it to deposit films for different applications[7-10].

Electrochemical ALD uses surface limited reactions in order to control the growth of the film. Surface limited reactions are generally referred to as under potential deposits (UPD). That is, in the deposition of one element on a second, frequently the first element will form an atomic layer at a potential under, prior to, that needed to deposit the element on itself [11]. EC-ALD is the combination of Underpotential Deposition (UPD) and Atomic Layer Epitaxy (ALE) [12]. Atomic layers of a compound's component elements are deposited at under-potentials in a cycle, to directly form a compound. It is generally a more complex procedure than most of the compound electrodeposition methods, requiring a cycle to form each monolayer of the compound. However, it is layer-by-layer growth, avoiding 3-D nucleation, and offering increased degrees of freedom, atomic level control, and promoting of epitaxy[13]. This paper presents a study on depositing a PbTe/CdTe

superlattice to be used as the absorber layer of a solar cell. As a first step, the deposition of PbTe and CdTe was carried separately on glass gold-coated substrates. Cyclic voltammetry was used to study the oxidation and reduction of the compounds involved in the deposition. Then a CdTe/PbTe superlattice was deposited. The films were deposited using the ECALD system. The films were characterized using a scanning electron microscope to determine the chemical composition and film uniformity. A glancing angle x-ray diffractometer was used to determine the crystallinity of the films. Reflection measurements were done to calculate the bandgap of each of the deposited films.

II. EXPERIMENTAL Thin films of PbTe and CdTe were grown in this study

using an automated thin layer flow electrodeposition system consisting of a series of solution reservoirs, computer controlled pumps, valves, a thin layer electrochemical cell (TLEC) and a potentiostat[14]. The electrochemical flow cell was designed to promote

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laminar flow. The working electrode was Au on glass, the auxiliary was a Au on wire, and the reference electrode was Ag/AgCl (3M NaCl). The cell volume was 0.3 mL and the solutions were pumped at 50 mLmin-1. The system was contained within a nitrogen purged Plexiglas box to reduce the influence of oxygen during electrodeposition. Substrates used in the study consisted of 300 nm thick

vapor deposited gold on glass microscope slides. Prior to insertion in the TLEC the slides were cleaned with 60 % concentrated nitric acid, rinsed with deionized water and dried with nitrogen gas. PbTe solutions were prepared using reagent grade or

better chemicals and deionized water. The lead solution consisted of 0.5 mM of PbClO4, pH 3, with 0.1 M of NaClO4 as the supporting electrolyte. The tellurium solution was made of 0.5 mM TeO2, pH 9, buffered with 50 mM sodium borate and 0.1 NaClO4 as the supporting electrolyte. A pH 3 blank solution of 0.1 NaClO4 was used as well. The pH values of all the solutions were adjusted with HClO4 and NaOH. CdTe solutions were prepared using reagent grade or

better chemicals and deionized water. The cadmium solution consisted of 0.5 mM of CdSO4, pH 3, with 0.1 M of NaClO4 as the supporting electrolyte. The tellurium solution was made of 0.5mM TeO2, pH 9, buffered with 50mM sodium borate and 0.1 NaClO4 as the supporting electrolyte. A pH 3 blank solution of 0.1 NaClO4 was used as well. The pH values of all the solutions were adjusted with HClO4 and NaOH. Cyclic voltammetry was performed to obtain the Under

Potential Deposition (UPD) for the PbTe and CdTe films on gold. The study was detailed in another paper [14, 15]. We will present the results here. The deposition potentials were as follows: Te was deposited at -800 mV vs. Ag/AgCl reference electrode. and Cd at -550 mV for the CdTe film. Pb was deposited at -350 mV and the potential for Te was ramped up from -550 mV to -400 mV for the first 20 cycles and kept at -550 mV for the rest of the cycles. The above potentials insured that the films were deposited at a rate of 0.5 monolayer (ML)/cycle for the constituent compounds. When the same potentials were used to deposit the

superlattice a non-uniform film was being deposited. This was revealed in the uneven deposition of the constituent compounds at a rate higher than the desired 0.5 ML/cycle. Fig. 1 shows the ML/cycle of Cd and Te deposited on the CdTe prelayer for the first five cycles. The ML/cycle for Cd starts at 1.4 and decreases to 1.1 at fifth cycle . The

ML/cycle of Te starts at 2 and decreases to 1.5 by the fifth cycle. Fig. 2 shows the ML/cycle of Pb and Te deposited on the CdTe film for the first five cycles. The ML/cycle for Te starts at 2 and increases abruptly to 2.7 in the second cycle and keeps increasing till 3.5 ML/cycle at the fifth cycle. The ML/cycle for Pb starts at 2 and increases abruptly to 4.5 ML/cycle in the second cycle and keeps a steady value at 4.5 ML/cycle further on. There are two major problems with the above results. In Fig. 1, the ML/cycle of Cd and Te is not 0.5 which means that during each cycle more than 1 monolayer of CdTe is being deposited. The other problem is illustrated in Fig. 2 where the monolayers of Pb and Te starts at higher than 0.5 and increases abruptly to 4.5 and 2.5 respectively. The sudden increase is an indication of the roughness of the surface. When the surface is not uniform the film starts forming rapidly. In order to solve the above problems a closer study of the UPD of PbTe on CdTe will be presented in the following section. Fig. 3 is a cyclic voltammogram of Te on 10

monolayers of CdTe. The CV shows that the film has a major reduction peak at -800 mV and a major oxidation peak at 150 mV. The film deposit very little between -400 to -600 mV. This means that this range can be used in order to reduce the over deposition of the film per cycle. Fig. 4 shows the cyclic voltammogram of Pb on a 10 monolayers as-deposited Pb film. The CV was done by sweeping the voltage over two ranges: 300 à -400 mV and 300 à -600 mV. The first cycle is blue and the second is red. In the first, it is noticed that the film has a strong oxidation peak at -300 mV and it decreases gradually. In the second cycle, the film has a strong reduction at -200 mV and another one at -400 mV. Between -200 mV and -400 mV the film seems to deposit the least. In the region between 0 and -150 mV the film does not deposit at all. In the region between -200 and -400 mV the film seems to deposit uniformly. Based on the above potential ranges, experiments were

conducted in order to determine the ML/cycle deposited at the indicated potentials. Fig. 5 shows the range of potentials used to deposit Pb and Te on 10 monolayers of as-deposited CdTe film. Fig. 6 shows the monolayers deposited at the different potentials. It is seen that the ML/cycle drops significantly below -500 mV and below -300 mV for Te and Pb respectively. This indicates that the potential range for depositing a uniform Pb and Te are: -550à -500 mV for Te and -350à -300 mV for Pb. Figs. 7-10 show that applying the above potentials results in a uniform deposition of the PbTe and CdTe films. This is indicated by a deposition rate of approximately 0.5 ML/cycle throughout the first 15 cycles.

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Fig. 1: Average deposition currents for CdTe over the 4 periods used to form the 4CdTe:4PbTe superlattice

Fig. 2: Average deposition currents for PbTe over the 4 periods used to form the 4CdTe:4PbTe superlattice

Fig. 3: Cyclic voltammogram of Te on 10 monolayers of CdTe, 0.5 mM TeO20.1 M NaClO4, pH 3 (electrode area: 2.5cm x1.5cm, scan rate 5mV/s)

Fig. 4: Cyclic voltammogram of Pb on a 10 CdTe monolayers in 0.5 mM PbClO4, 0.1 M NaClO4, pH 3 (electrode area: 2.5cm x1.5cm, scan rate 5mV/s)

Fig. 5: The possible potentials obtained from the CV analysis for the deposition of PbTe on CdTe

Fig. 6: Average ML/cycle for PbTe over different potentials

Fig. 7: Monolayers per cycle for Cadmium in a CdTe film

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Fig. 8: Monolayers per cycle for Lead in a PbTe film (first period)

Fig. 9: Monolayers per cycle for Tellurium in a CdTe film

Fig. 10: Monolayers per cycle for Tellurium in a PbTe film (first period)

III. RESULTS

A photograph of the deposited CdTe/PbTe superlattice is shown in The superlattice is comprised of alternating layers of CdTe (4nm) and PbTe (4nm) repeated 5 times. The total thickness of the superlattice is 80 nm ((4 nm CdTe + 4 nm PbTe)*5). The film covers the gold substrate uniformly. The film has a dark blue/grey color. The stoichiometry of the film was analyzed using a scanning electron microscope. Four points were analyzed on the sample. As indicated in TABLE the ratio of Pb+Cd:Te is approximately 1:1 throughout the sample.

The crystallinity of the structure was also studies. Fig. 12 shows that the CdTe/PbTe superlattice is polycrystalline with PbTe peaks at (111), (220), (311), (222) and with CdTe peaks at (111). Three CdTe/PbTe superlattices with different

thicknesses were also fabricated in order to demonstrate the bandgap shift. The first superlattice is comprised of CdTe and PbTe with a thickness of 4 nm each repeated 5 times. The total thickness of the first superlattice is 40 nm. The second superlattice is made up of 5 nm CdTe and 20 nm PbTe films repeated 5 times. The total thickness of the second superlattice is 125 nm. The third superlattice is made up of 16 nm PbTe and 16 nm CdTe repeated 5 times. The total thickness of film is 160 nm. The bandgaps of the 40nm, 125nm and 160 nm superlattices are 0.92 eV, 0.74 eV and 0.65 eV respectively as shown in Fig. 13.

Fig. 11: Example of readable plot using different colors and line styles for clarity.

TABLE I

CHEMICAL COMPOSITION OF A CDTE/PBTE SUPERLATTICE WITH 100nm PRE-LAYER OF CDTE

Pt# Pb Te Cd Au (Cd+Pb)/Te 1 7.01 18.7 11.3 63.0 0.98 2 6.95 18.5 10.2 64.3 0.93 3 7.33 20.4 11.1 61.2 0.90 4 5.37 19.9 13.6 61.1 0.95

Average 6.66 19.39 11.54 62.41 0.94

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Fig. 12: XRD graph of a CdTe/PbTe superlattice with incident angle 1°

Fig. 13: Bandgap Shift of Three Superlattices of Different Thicknesses.

IV. CONCLUSION A PbTe/CdTe superlattice was fabricated using the

Electrochemical Atomic Layer Deposition system. The ratio of Pb+Cd:Te is approximately 1:1 throughout the sample. The structure is polycrystalline with PbTe peaks at (111), (220), (311), (222) and with CdTe peaks at (111). The bandgap shift was also demonstrated by fabricating three CdTe/PbTe superlattices with different thicknesses. Optical measurements showed that the superlattices had different bandgaps.

ACKNOWLEDGMENTS

The authors gratefully acknowledge support from Drs. Robert Birkmire and Kevin Dobson of the University of Delaware for help with making the optical absorption measurements. The authors also thank Dr. John Stickney and Brian Perdue of University of Georgia for making the XRD measurements. Finally, the authors wish to thank Dr. Ross Lee of Villanova University for useful discussions.

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