Electrodeposition of CuInSe2 (CIS) via Electrochemical Atomic Layer

Jan 2, 2012 - Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and...
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Electrodeposition of CuInSe2 (CIS) via Electrochemical Atomic Layer Deposition (E-ALD) Dhego Banga,†,‡ Nagarajan Jarayaju,‡ Leah Sheridan,‡ Youn-Geun Kim,‡ Brian Perdue,‡ Xin Zhang,§ Qinghui Zhang,‡ and John Stickney*,‡ †

Materials Physics Department, Sandia National Laboratories, Livermore, California 94550, United States Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, United States § Institute of Analytical Science, Northwest University, Xi’an, Shaanxi 710069, Peopole's Republic of China ‡

ABSTRACT: The growth of stoichiometric CuInSe2 (CIS) on Au substrates using electrochemical atomic layer deposition (EALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu2Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu2Se. The deposits were grown using 0.14, −0.7, and −0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.

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INTRODUCTION Photovoltaics represents one of the most sustainable and carbon neutral alternatives for future large scale energy production. CuInSe2 (CIS) is a compound semiconductor belonging to I−III−VI2 chalcopyrite family, and is of interest as an advanced absorber material in single junction thin film solar cells. The advantages of CIS involve its direct band gap (1.01 eV), high absorption coefficient (105 cm−1), extraordinary stability,1 and high conversion efficiency (15%).2 In addition, the properties of CIS can be improved by replacement of some indium with gallium, or some selenium with sulfur, resulting in a family of Cu(In,Ga)(Se,S)2 variations,3 for which conversion efficiencies as high as 19.9%4 have been reported in the laboratory. The high absorbance and direct band gap of CIS based materials are particularly attractive as they result in the need for layers only a micrometer thick to form a solar cell, limiting consumption of the less abundant materials. Techniques used to deposit CuInSe2 included coevaporation,5 flash evaporation,6 chemical vapor deposition,7 chemical spray pyrolysis,8,9 chemical synthesis,10 electrochemical codeposition,11−13 and pulse electrodeposition.14,15 Selenization of sputtered,16 evaporated,5 or electrodeposited17 Cu and In stacked layers, or alloys, have also been carried out. The © 2012 American Chemical Society

methodology used here is the electrochemical version of atomic layer deposition (ALD). ALD describes the formation of materials one atomic layer (AL) at a time using surface limited reactions, for layer by layer growth.18−24 The vast majority of ALD work is performed using gas or vacuum phase deposition. However, E-ALD is based on the use of electrochemical surface limited reactions, generally referred to as under potential deposits (UPD). UPD is a phenomenon where an atomic layer of one element deposits on a second at a potential prior to (under) that needed to deposit the element on itself.25−29 EALD is use of UPD in an ALD cycle for the conformal growth of materials with atomic level control, where deposit thickness is proportional to the number of cycles performed. This report describes the sequence of steps (potentials, times, and solutions) making up E-ALD cycles for the formation of CuInSe2. The cycle is repeated to produce nanofilms of the desired thickness. A recent publication by Wang et al. has made use of the E-ALD method to grow CIS on a conductive polymer film.30 They describe development of a Received: September 12, 2011 Revised: December 15, 2011 Published: January 2, 2012 3024

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Figure 1. Time-potential−current graph illustrating one E-ALD cycle of CIS. Reagent grade chemicals were used to make 0.5 mM Cu(SO4)2 (Alfa Aesar, Ward Hill, MA), pH 4.5, with 0.1 M NaClO4 (Fischer Scientific, Pittsburgh, PA) as a supporting electrolyte; 0.5 mM In2(SO4)3·xH2O (Alfa Aesar, Ward Hill, MA), pH 3, with 0.1 M NaClO4 (Fischer Scientific, Pittsburgh, PA); 0.5 mM SeO2 (Alfa Aesar, Ward Hill, MA), pH 5, buffered with 50.0 mM CH3COONa·3H2O and 0.1 M NaClO4 (Fischer Scientific, Pittsburgh, PA); and pH 3 0.1 M NaClO4 (Fischer Scientific, Pittsburgh, PA) as a blank solution. NaOH and HClO4 (Fischer Scientific, Pittsburgh, PA) were used to adjust pH for all solutions. Current and potential time graphs illustrating a ternary E-ALD cycle for CIS are shown in Figure 1. The cycle consisted of the following steps: First, Se solution was flowed into the cell for 5 s at the chosen potential for Se deposition, and then held quiescent for 15 s for deposition. The blank was then flowed at the same potential for 5 s, rinsing out excess HSeO3− ions. The In solution was then flowed for 5 s at the potential chosen for its deposition, and then held quiescent for 15 s, followed by a blank rinse at the same potential to remove excess In2+ ions. Se solution was then flowed again for 5 s at the Se potential and held quiescent for 15 s. Next, at the same potential, blank was flowed for 5 s rising out excess HSeO3− ions. The Cu solution was flowed for 5 s at the chosen Cu deposition potential, and held quiescent for 15 s, followed by a final blank rinse for 5 s to remove excess Cu2+ ions. This process is referred to here as one CIS E-ALD cycle, and was repeated to form CIS deposits. Cu2Se deposits were formed using the same cycle without the InSe steps, and InSe deposits were formed using it without the Cu2Se steps. Electron probe microanalysis (EPMA) was performed using a Joel JXA-8600 electron probe. Glancing angle X-ray diffraction patterns were obtained on a Scintag PAD V diffractometer using CuKα radiation (λ = 1.514 Ǻ ). A 6′′ set of Soller slits on the detector was used to improve resolution in the asymmetric diffraction configuration. Deposit morphology was characterized using a Molecular Imaging PicoPlus AFM instrument.

successful ternary cycle for the formation of a CIS thin film. In this report, however, two different deposition sequences were investigated: use of a ternary cycle to grow the compound directly, and use of a superlattice sequence where cycles for InSe and Cu2Se were alternated. E-ALD is based on breaking the deposition process into a sequence of controllable steps where pH, electrolyte, precursor, additives (complexing agents, surfactants, and so forth), deposition potential, timing and solution flow can be optimized individually for the deposition of each element. Experience in this group has found that even for room temperature deposited compounds, X-ray diffraction (XRD) patterns are frequently observed without annealing.31−35 E-ALD formation of the binary compounds (Cu2Se and InSe) is discussed first, followed by discussion of the formation of the ternary CuInSe2.

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EXPERIMENTAL SECTION

The automated flow cell electrodeposition system (Electrochemical ALD L.C., Athens, GA) used to grow the CIS films consisted of a series of solution reservoirs, computer controlled pumps and valves, an electrochemical flow cell (V-cell), potentiostat, and specialized “Sequencer” software. The reference electrode was Ag/AgCl (3 M NaCl) (Bioanalytical systems, Inc., West Lafayette, IN), while the auxiliary electrode was a Au wire embedded into the flow cell wall. The electrochemical flow cell was designed to promote laminar flow, with the substrate and auxiliary electrodes directly across from each other for a simple primary current distribution, and a cell volume of 0.3 mL. Solutions were pumped at 50 mL min−1. Pumps and plumbing were contained in a nitrogen purged Plexiglas box in order to minimize oxygen. Substrates were Au vapor deposited on glass microscope slides. Prior to insertion into the vapor deposition chamber, the glass slides were cleaned in 15% HF (by volume) and rinsed with deionized water from a Nanopure water filtration system (Barnstead, Dubuque, IA) fed from the house distilled water system. Deposition began with a 3 nm adhesion layer of Ti, followed by 300 nm of Au deposition at around 250 °C. Slides were then annealed at 400 °C and 10−6 Torr for 12 h.

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RESULTS AND DISCUSSION Development of the E-ALD cycles began with cyclic voltammetry (CV) of Au on glass substrates in Cu, In, and Se precursor solutions, to provide an indication of range of 3025

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Figure 2. (_____) Cyclic voltammetry in 0.5 mM Cu(SO4)2 (pH 4.5) for a Au electrode and (-----) a Au electrode covered with Se (electrode area: 4 cm2, scan rate: 10 mV s−1).

UPD potentials that might be used to grow the compound. Figure 2 (_____) is a CV of a clean Au substrate in the 0.5 mM Cu2+ solution which displays a broad UPD region beginning with peak C1 at 0.28 V and ending with the onset of bulk Cu deposition prior to peak C2, about 0.0 V. Bulk Cu deposition begins at −0.06 V, the formal potential (E0′) for Cu in this solution, and is evident as peak C2 (−0.17 V), and continues to lower potentials. In the subsequent positive scan, peak A2 (0.05 V) corresponds to bulk oxidation and is followed by a broad UPD oxidation feature between 0.1 and 0.3 V, finishing with peak A1. Above E0′ (−0.06 V) for Cu2+, only surface limited Cu deposition takes place. Similar features have been previously reported.36 Figure 2 (-----) is a CV for a Se UPD coated Au substrate in the Cu2+ solution. The Cu UPD peak, C1′, was shifted negative of C1 by 0.17 V on clean Au, indicating the Cu was not as stable on Se as on Au. However, the coverage of Cu UPD was greater in the presence of Se than without. It may be that Cu atoms deposit both on top of the Se, as well as between the Se and Au, as has been proposed for Cd on Se and Te coated Au.37−45 Figure 3 displays “window opening” cyclic voltammetry for a Au electrode in 0.5 mM In2(SO4)3, pH 3, to successively lower potentials. Two reductive features are evident in the negative going scan, a broad peak around −0.35 V and the increased reduction current below −0.58 V. From the CVs, the formal potential appears to be about −0.55 V, and there are two oxidative features, a large peak at −0.4 V and a smaller feature around −0.25 V. It appears that the reduction near −0.35 V and the oxidation near −0.25 V correspond to In UPD and its stripping, respectively. The large oxidation near −0.40 V is consistent with bulk In oxidation. The reduction feature below −0.5 V is a convolution of bulk In deposition, hydrogen evolution, and some oxygen reduction (though the solution was

Figure 3. Cyclic voltammetry of Au electrode in 0.5 mM In2(SO4)3.xH2O, pH 3, (electrode area: 4 cm2, scan rate: 10 mV s−1).

sparged with N2), as the charge for reduction is significantly greater than the subsequent In oxidation peak. Figure 4 is a voltammetry for a Au electrode in 0.5 mM HSeO3− and is consistent with previous reports.34,35 The voltammetry of selenite in aqueous solutions is known to display slow and complex kinetics.46−48 Starting negative from 0.25 V, there is a peak at about 0.0 V, continuing negative to −0.4 V. The charges for peaks 2 and 3 do not change appreciably in size as a function of scan rate, indicating they are surface limited reactions. However, those deposits cannot technically be referred to as UPD, the formal potential for Se being closer to 0.35 V, and in fact should be categorized as overpotential deposits.35,49 In the positive going scan, there are four features. The right most (1′) is the beginning of Au oxidation, while the right most reductive feature (1) is Au oxide reduction. The middle two oxidation features are associated with the dissolution of surface limited deposits of Se. Generally, 3026

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conditions on Au initially were not those needed subsequently to grow the film.50−54 This can be seen in the difference in voltammetry for Cu on Au and Cu on Se coated Au (Figure 2). A way to counteract the change in amounts of Se deposited from cycle to cycle would be to change the potential from cycle to cycle over the course of the transition from Au to the bulk compound. The deposition software allowed for systematic potential changes from cycle to cycle, which is referred to as “stepping” the potential in this group. In the present case, deposits were attempted where the potential for Cu or the potential for Se was stepped for the first few cycles without success, achieving only deposits where the Cu/Se ratios were other than 2. However, a ratio of 2 was achieved using a cycle program, where the deposition potentials of Se and Cu were both stepped, although in opposite directions, over the first 20 cycles. After 20 cycles, the potentials were held constant, at “steady state”, for all remaining cycles. The Se potential was stepped negatively from −0.2 to −0.35 V, while the Cu potential was stepped positively from 0.05 to 0.1 V. A 200 cycle deposit formed using this program appeared homogeneous, viewed with 1000× optical microscopy. Electron probe microanalysis (EPMA) atomic % data (Table 1) indicated a

Figure 4. Cyclic voltammetry of Au electrode in 0.5 mM HSeO3+, pH 5, (electrode area: 4 cm2, scan rate: 10 mV s−1).

with surface limited features, peaks 2 and 2′ would correspond to deposition and oxidation, respectively, of an atomic layer (AL) of Se, as would 3 and 3′. However, that is not the case here. In a window opening to more negative potentials, reversing after peak 2, the oxidation peak 3′ appears, and reversing past peak 3, oxidation peak 3′ grows larger, suggesting that Se deposited in peaks 2 and 3 strip in 3′. Peak 2′ only grows in slowly as the window is opened more to potentials negative of peak 3, after scanning through peaks 2 and 3. Ordinarily, deposits formed at more negative potentials are formed on top of deposits formed at more positive potentials, and thus would be expected to strip first. This raises the question of why Se deposited negative of peaks 2 and 3 is still on the surface after Se corresponding to peaks 2 and 3 has been stripped in peak 3′? These observations suggest Se stripped in peak 2′ was formed on parts of the polycrystalline Au substrate not coated with Se deposited in peaks 2 and 3, and formed with very slow kinetics. When the window opening scans35 were reversed at potentials more negative than −0.3 V, the oxidation peak 4′ appeared, and was attributed to bulk Se stripping. Normally, bulk deposits would increase at still more negative potentials. However, the reductive shoulder (4) results from a Se reduction to Se2− (a soluble species), as well as some bulk Se deposition and extensive hydrogen evolution.46−48 Thus bulk Se did not accumulate; it was converted to selenide ions which diffused away. Given the studies in Figures 2, 3, and 4, initial E-ALD cycle programs were devised to form Cu2Se, InSe, and CuInSe2 thin films. Cu2Se Thin Film Deposition. On the basis of the CVs in Figures 2 and 4 for the Cu2+ and HSeO3− solutions, potentials of 0.05 and −0.2 V were chosen as starting potentials for Cu and Se, respectively, as they corresponded to the deposition of surface-limited amounts on Au. The initial Cu2Se cycle consisted of filling the cell with the HSeO3− solution for 5 s at −0.2 V, holding quiescent and depositing for 15 s. The blank solution was then flushed through for 5 s at the same potential to remove extra HSeO3− ions. Cu2+ solution was then flushed for 5 s at 0.05 V and held quiescent for 15 s for deposition. The cell was then flushed with blank at the same potential for 5 s to remove excess Cu2+ ions from the cell. This cycle was intended to form one monolayer of Cu2Se, and was to be repeated in order to form compound films of the desired thickness. However, Se deposition charges decreased over the first few cycles, with no film growth evident past the fifth cycle. Similar behavior had previously been observed, where the optimal

Table 1. Electron Probe Microanalysis (EPMA) Data of a 200 ALD Cycle Cu2Se Sample Grown on Au on Glass When Se and Cu Were Ramped from −0.2 and 0.05 V to −0.35 and 0.1 V, Respectivelya

a

sample

Cu2Se thin film

Cu atomic % Se atomic % Au atomic Cu/Se ratio stoichiometry

44 21 35 2.1 Cu2.1Se

Electrode area: 4 cm2.

stoichiometric deposit Cu2.1Se. The current time trace after stepping the potentials for the first 20 cycles showed that the currents remained steady throughout the remaining 180 deposition cycles. Figure 5 is the XRD pattern for the 200

Figure 5. X-ray diffraction pattern of the 200 ALD-cycle Cu2Se sample grown on Au on glass when Se and Cu were ramped from −0.2 and 0.05 V to −0.35 and 0.1 V, respectively. Angle of incidence is 0.1°, Cu Kα source. Electrode area: 4 cm2.

cycle-deposit, displaying three peaks, (111), (220), and (311), for the face-centered cubic Cu2Se lattice, along with Au (111) and (200) reflections. The Au (111) peak is prominent as it 3027

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was for the Au substrate, while the intensity of Cu2Se (220) reflection is convoluted with that for the Au (200) reflection. No elemental reflections for Se or Cu are evident, consistent with a stoichiometric deposit. InSe Thin Film Deposition. As with Cu2Se, constant deposition potentials were investigated first. In contrast to Cu2Se, a working cycle for InSe was devised based on constant potentials and makes use of −0.75 V for In3+ and −0.6 for Se. The −0.6 V used for Se in InSe deposition was significantly lower than the −0.22 V used for Se in Cu2Se. However, as discussed for the CV in Figure 4, use of a potential of −0.6 should be low enough that any extra, bulk, Se would be converted into selenide ions (Se2−) and diffuse away. The result was that just the surface limited amount Se was left on the surface. Using −0.6 V made it easier to keep the In on the surface while Se was depositing. The cycle used for InSe started with flushing the HSeO3− solution for 5 s at −0.6 V, and holding the potential quiescent for 15 s for deposition. The blank was then flushed at the same potential for 5 s, to rinse out excess HSeO3− ions. The In3+ solution was flushed for 5 s at −0.75 V and held for 15 s for deposition. Finally, the blank solution was flushed at the same potential to remove excess In2+ ions. It was intended that this cycle would form one monolayer of InSe and be repeated to form deposits. The current/potential time trace for a 100 cycleInSe deposit showed that the deposition currents remained steady through the whole deposition program. Again, the deposit appears homogeneous with optical microscope, and EPMA results (Table 2) indicated a stoichiometric InSe1.1

Figure 6. X-ray diffraction pattern of the 100 ALD-cycle InSe sample grown on Au on glass when both Se and In were held steady at −0.6 and −0.75 V, respectively, throughout the deposition run. Angle of incidence is 0.1°, Cu Kα source. Electrode area: 4 cm2.

limited deposition regions. Figure 7 shows a section of the current (dots) and potential (solid) time traces for the ternary

Table 2. Electron Probe Microanalysis (EPMA) Data of a 100 ALD Cycle InSe Sample Grown on Au on Glass When Both Se and in Were Held Constant at −0.6 and −0.75 V, Respectivelya

a

sample

InSe thin film

In atomic % Se atomic % Au atomic Se/In ratio stoichiometry

14.0 15.9 70.1 1.1 InSe1.1

Figure 7. A section of current and potential time traces of the ternary CIS with Se, In and Cu deposited at −0.22, −0.7, and 0.14 V and the resulting EPMA data of the deposit. Electrode area: 4 cm2.

CIS cycle. Oxidative current spikes are notable after In and Se, when solutions and potentials were stepped positive (Figure 7). Given the difference in potential between In (−0.7 V) and Se (−0.22 V) a positive current spike is expected, same for the step from Se (−0.22 V) to Cu (0.14 V), simply considering charging. However, it is entirely reasonable that some In will strip at both potentials. Se should not as the CV in Figure 4 shows that it does not oxidatively strip until 0.4 V. The sample again appeared homogeneous with optical microscopy. However, its composition from EPMA was nonstoichiometric Cu2.4InSe2.4 with Cu clearly in excess, consistent with loss of In during the positive going potential steps. To increase the In content, the superlattice program was used with ratios of InSe cycles to Cu2Se cycles greater than one. One idea was that InSe layers would be buried by subsequent InSe cycles, possibly limiting In corrosion during the Cu2Se cycles. Potentials of 0.14, −0.7, and −0.65 V were applied for Cu, In, and Se, respectively, in the superlattice program. In addition, the time for Se deposition was increased from 15 to 30 s, knowing that Se deposition kinetics was slow,46−48 as is evident in the cyclic voltammetry in Figure 4. Table 3 displays compositions for three deposits runs using different number of

Electrode area: 4 cm2.

deposit. Two low grade reflections for InSe, (101) and (004), were evident in the XRD pattern of the as grown deposit (Figure 6), and no elemental peaks for In or Se were observed. CuInSe2 Thin Film Deposition. Two different E-ALD program models were investigated for CIS. The first involved the use of a ternary E-ALD cycle for CIS deposition, consisting of one cycle of InSe followed by one cycle of Cu2Se. This CIS cycle was then repeated to form deposits. The second program model involved the use of a superlattice program, where x cycles of InSe were performed followed by y cycles of Cu2Se, creating one period. That period was then repeated to form deposits. This allowed the relative ratio of In to Cu in the deposit to be optimized by adjusting the relative ratio of x/y cycles in the period. Other deposition parameters were also investigated in an attempt to form the desired deposit stoichiometry. To grow the ternary CIS sample, potentials of 0.14, −0.7, and −0.22 V were selected based on the CVs (Figures 2−4) for Cu, In, and Se, respectively. No 3-D growth was expected using this deposition program, as all the potentials were in surface 3028

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compared with samples grown using 15 s for Cu deposition, though it did increase using 3 s for Cu deposition. The CIS sample with the best stoichiometry (Cu1.1InSe1.9) was grown with 3 s for Cu deposition. Cu deposition time of 3 s appears to be sufficient as Cu reduction current is still evident in Figure 9

Table 3. Electron Probe Microanalysis (EPMA) Data of Superlattice CIS Samples Grown on Au on Glass Electrode with Se, In, and Cu Deposited at −0.65, −0.7, and 0.14 V, Respectivelya superlattice CIS samples

Cu at.%

In at.%

Se at.%

stoichiometry

34P:2InSe:1CuSe 25P:3InSe:1CuSe 34P:2InSe:1CuSe

33 35 35

22 22 22

44 43 43

Cu1.5InSe2 Cu1.5InSe1.9 Cu1.6InSe2

a

Se deposition time was doubled from 15 to 30 s. Electrode area: 4 cm2.

InSe cycles. All three films looked homogeneous with optical microscope. However, varying the number of InSe cycles from 1 to 4 did not result in the expected increase in In. All three samples appeared to have similar amounts of In, and essentially the same CIS stoichiometry. Overall, however, deposits formed with the superlattice program showed a slight increase in In and better CIS stoichiometry (Cu1.5InSe2) compared to CIS deposits formed with the ternary program (Cu2.4InSe2.4). Another idea on how to decrease the Cu, or increase the In, was to decrease the Cu deposition time, and thus decrease the time available for In stripping. Cu UPD being a surface-limited process should not be affected by the amount of time for deposition. With that in mind, reducing the Cu deposition time should not greatly affect the amount of Cu deposited, as it is a relatively fast reaction.55,56 Stripping of In, however, should be slowed, it being below (or covered by) both Se and some Cu, and spending less time at 0.14 V for Cu deposition should result in less In corrosion each cycle. Four deposits were formed using the superlattice program. Each deposit consisted of 25 periods of 3 InSe cycles and 1 Cu2Se cycle, grown with Cu deposition time of 15, 10, 5, and 3 s, and with potentials of 0.14, −0.7, and −0.65 V for Cu, In, and Se, respectively. Figure

Figure 9. A section of current and potential time traces and the resulting EPMA data of a 25P/3InSe/CuSe CIS sample grown on Au on glass electrode with Cu deposition time of 3 s and constant deposition potentials of −0.65, −0.7, and 0.14 V for Se, In, and Cu respectively. Electrode area: 4 cm2.

that shows a section of the current/potential time trace of the resulting film. The XRD pattern for the as-grown CIS deposit, formed using a superlattice program with the 3 s Cu deposition time, is shown in Figure 10, where (112), (204/220), and

Figure 10. XRD pattern of a 25P/3InSe/1CuSe superlattice CIS sample with Cu deposition time of 3 s and constant deposition potentials of −0.65, −0.7, and 0.14 V for Se, In, and Cu, respectively. Electrode area: 4 cm2.

(116/312) peaks for a chalcopyrite structure are noted. Morphological studies of this sample were carried out by intermittent contact mode atomic force microscopy (Figure 11). Images of a Au on glass substrate and the CIS sample, with a stoichiometry of Cu1.1InSe1.9, are displayed and suggest a conformal deposit since the crystallite size for the deposit was not significantly different than that for the substrate. EPMA at various positions across the sample also indicated homogeneous deposition.

Figure 8. The effects of varying Cu deposition time on the atomic percentage values of constituent elements of 25P/3InSe/CuSe CIS samples grown on Au on glass electrode at varying Cu deposition time. Electrode area: 4 cm2.

8 displays EPMA atomic % as a function of the Cu deposition time for all three elements. The amount of In did tend to increase at shorter times, while Cu went down with decreases in the Cu deposition time. Se remained relatively steady, 3029

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Figure 11. AFM images of (a) Au on glass electrode and (b) 25P/3InSe/1CuSe superlattice CIS sample with Cu deposition time of 3 s and constant deposition potentials of −0.65, −0.7, and 0.14 V for Se, In, and Cu, respectively. Electrode area: 4 cm2.

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(4) Repins, I.; Contreras, M. A.; Egaas, B.; DeHart, C.; Scharf, J.; Perkins, C. L.; To, B.; Noufi, R. Prog. Photovolt.: Res. Appl. 2008, 16, 235. (5) Guillen, C.; Herrero, J. Sol. Energy Mater. Sol. Cells 2002, 73, 141. (6) Ashida, A.; Hachiuma, Y.; Yamamoto, N.; Ito, T.; Cho, Y. J. Mater. Sci. Lett. 1994, 13, 1181. (7) Jones, P. A.; Jackson, A. D.; Lickiss, P. D.; Pilkington, R. D.; Tomlinson, R. D. Thin Solid Films 1994, 238, 4. (8) Terasako, T.; Shirakata, S.; Isomura, S. Japn. J. Appl. Phys. Part 1 1999, 38, 4656. (9) Shirakata, S.; Murakami, T.; Kariya, T.; Isomura, S. Japn. J. Appl. Phys. 1996, 35, 191. (10) de Kergommeaux, A.; Fiore, A.; Bruyant, N.; Chandezon, F.; Reiss, P.; Pron, A.; de Bettignies, R.; Faure-Vincent, J. Sol. Energy Mater. Sol. Cells 2011, 95, S39. (11) Kaupmees, L.; Altosaar, M.; Volubujeva, O.; Mellikov, E. Thin Solid Films 2007, 515, 5891. (12) Hernandez-Pagan, E. A.; Wang, W.; Mallouk, T. E. Acs Nano 2011, 5, 3237. (13) Bhattacharya, R. N. J. Electrochem. Soc. 1983, 130, 2040. (14) Shao-Yu, H.; Wen-Hsi, L.; Shih-Chieh, C.; Yi-Lung, C.; YingLang, W. J. Electrochem. Soc. 2011, 158. (15) Palacios-Padros, A.; Caballero-Briones, F.; Sanz, F. Electrochem. Commun. 2010, 12, 1025. (16) Guillen, C.; Martinez, M. A.; Herrero, J. Vacuum 2000, 58, 594. (17) Guillen, C.; Herrero, J. JECS 1996, 143, 493. (18) Shen, C.; Liu, X.-y.; Huang, G.-z. Zhenkong 2006, 43, 1. (19) Ritala, M.; Leskela, M. Handbook of Thin Film Materials 2002, 1, 103. (20) Ritala, M. High-k Gate Dielectrics 2004, 17. (21) Niinisto, L. Proceedings of the Estonian Academy of Sciences, Physics, Mathematics; 2003, 52, 266. (22) Lavoie, A. R.; Dubin, V. Proceedings of SPIE-The International Society for Optical Engineering 2005, 6002, 60020J/1. (23) Gordon, R. G. PMSE Preprints 2004, 90, 726. (24) George, S. M.; Ferguson, J. D.; Klaus, J. W. Materials Research Society Symposium Proceedings 2000, 616, 93. (25) Kolb, D. M.; Przasnys., M; Gerische., H J. Electroanal. Chem. 1974, 54, 25. (26) Kolb, D. M.; Gerisher, H. Surf. Sci. 1975, 51, 323. (27) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978; Vol. 11, p 125. (28) Adzic, R. R.; Simic, D. N.; Drazic, D. M.; Despic, A. R. 1975, 61, 117. (29) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerishcher, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; Vol. 13, p 159.

CONCLUSIONS This report describes the electrodeposition of Cu2Se, InSe, and CuInSe2 thin films using E-ALD. Nearly stoichiometric binaries compounds, Cu2.1Se and InSe1.1, were formed. XRD patterns displayed expected reflections. CIS samples were grown using a ternary cycle program, as well as a superlattice programs. Using steady state potentials throughout the deposition, formation of a conformal stoichiometric CIS deposit (Cu1.1InSe1.9) was obtained by depositing 25 periods of a superlattice program composed of 3 InSe cycles followed by 1 Cu2Se cycle. The Cu deposition time was reduced to 3 s, to limit the time for In oxidative dissolution. The film appeared homogeneous when observed with optical microscope, which was confirmed using EPMA. XRD results in combination with stoichiometry from EPMA indicated that the resulting sample was chalcopyrite in nature. AFM images of the Au substrate and the CIS sample indicated a conformal deposit. Limiting the deposition of Cu was clearly an issue in this study. Use of a complexing agent in the Cu solution, as done by Wang et al.,30 should shift the deposition potential negative, allowing better control over stoichiometry. The XRD pattern was not well resolved, and could have indicated the presence of more than one chalcopyrite phases. However, all of the deposits were formed at room temperature, with no annealing. Subsequent studies will investigate the importance of low temperature annealing.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS Support from the National Science Foundation, division of Materials Science is gratefully acknowledged.

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