PbTe

Publication Date (Web): September 21, 2006. Copyright © 2006 American ... AFM suggested conformal superlattice growth on the Au on glass substrate. B...
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Langmuir 2006, 22, 10590-10595

Preliminary Studies in the Electrodeposition of PbSe/PbTe Superlattice Thin Films via Electrochemical Atomic Layer Deposition (ALD)† Raman Vaidyanathan,‡ Steven M. Cox,§ Uwe Happek,§ Dhego Banga,‡ Mkhulu K. Mathe,‡ and John L. Stickney*,‡ Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602-2556, and Department of Physics, UniVersity of Georgia, Athens, Georgia 30602-2556 ReceiVed June 6, 2006. In Final Form: August 15, 2006 This paper concerns the electrochemical growth of compound semiconductor thin film superlattice structures using electrochemical atomic layer deposition (ALD). Electrochemical ALD is the electrochemical analogue of atomic layer epitaxy (ALE) and ALD, methods based on nanofilm formation an atomic layer at a time, using surface-limited reactions. Underpotential deposition (UPD) is a type of electrochemical surfaced-limited reaction used in the present studies for the formation of PbSe/PbTe superlattices via electrochemical ALD. PbSe/PbTe thin-film superlattices with modulation wavelengths (periods) of 4.2 and 7.0 nm are reported here. These films were characterized using electron probe microanalysis, X- ray diffraction, atomic force microscopy (AFM), and infrared reflection absorption measurements. The 4.2 nm period superlattice was grown after deposition of 10 PbSe cycles, as a prelayer, resulting in an overall composition of PbSe0.52Te0.48. The 7.0 nm period superlattice was grown after deposition of 100 PbTe cycle prelayer, resulting for an overall composition of PbSe0.44Te0.56. The primary Bragg diffraction peak position, 2θ, for the 4.2 superlattice was consistent with the average (111) angles for PbSe and PbTe. First-order satellite peaks, as well as a second, were observed, indicating a high-quality superlattice film. For the 7.0 nm superlattice, Bragg peaks for both the (200) and (111) planes of the PbSe/PbTe superlattice were observed, with satellite peaks shifted 1° closer to the (111), consistent with the larger period of the superlattice. AFM suggested conformal superlattice growth on the Au on glass substrate. Band gaps for the 4.2 and 7.0 nm period superlattices were measured as 0.48 and 0.38 eV, respectively.

Introduction Superlattices are materials where the unit cell has been artificially nanostructured in one dimension.1,2 In a sense, they are the alternated deposition of nanofilms of two compounds (films with thicknesses between 1 to tens of nanometers in thickness) to create a new material, a new unit cell. They are defined by their “period”, the combined thickness of the two nanofilms. Changing the superlattice period by even a few atomic layers can change the material’s optical and electrical properties. Interfacial sharpness, lattice mismatch, and stoichiometric modulations through the superlattice period all have substantial effects on the optical and electronic properties of the superlattice. Nanostructuring of IV-VI compound semiconductors are promising for thermoelectric applications,3-5 as well as infrared sensors.6 In addition, current transport properties of IV-VI compound semiconductor superlattices3-5,7-11 are known to change with periodicity. These IV-VI compounds have large †

Part of the Electrochemistry special issue. * To whom correspondence should be addressed. ‡ Department of Chemistry. § Department of Physics. (1) Switzer, J. A.; Shane, M. J.; Phillips, R. J. Electrodeposited Ceramic Superlattices. Science 1990, 247, 444. (2) Switzer, J. A. Electrodeposition of superlattices and multilayers. In Electrochemistry of Nanomaterials; Hodes, G., Ed.; Wiley-VCH: Weinheim, 2001; p 67. (3) Harman, T. C.; Taylor, P. J.; Spears, D. L.; Walsh, M. P. Thermoelectric quantum-dot superlattices with high ZT. 2000, 29 (1), L1-L4. (4) Beyer, H.; Nurnus, J.; Bottner, H.; Lambrecht, A.; Roch, T.; Bauer, G. PbTe based superlattice structures with high thermoelectric efficiency. Appl. Phys. Lett. 2002, 80 (7), 1216-1218. (5) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Quantum dot superlattice thermoelectric materials and devices. Science 2002, 297 (5590), 22292232. (6) Ueta, A. Y.; Abramof, E.; Boschetti, C.; Closs, H.; Motisuke, P.; Rappl, P. H. O.; Bandeira, I. N.; Ferreira, S. O. IV-VI Compound heterostructures grown by molecular beam epitaxy. Microelectron. J. 2002, 33 (4), 331-335.

Bohr radii, 50 nm for PbTe and 46 nm for PbSe.12 When semiconductor dimensions fall below the Bohr radii, excitons formed in the material become confined, and optical, electronic, and/or magnetic properties of the semiconducting material change due to this quantum confinement.12 IV-VI compound semiconductors, such as PbSe, PbTe, and PbS, have small and relatively equal electron and hole masses, compared with III-V or II-VI compound semiconductors, resulting in relatively larger confinement effects, compared with III-V or II-VI superlattice systems with similar dimensions.12 The primary methodologies for forming superlattices with atomic-level control are molecular beam epitaxy (MBE),3,6,13-16 (7) Fedorov, A.; G, S. I. A.; Sipatov, A.Y.; Kaidalova, E. V. X-ray diffraction investigation of diffusion in PbTe-PbSe superlattices. J. Cryst. Growth 1999, 198, 1211-1215. (8) Broido, D. A.; Reinecke, T. L. Thermoelectric power factor in superlattice systems. Appl. Phys. Lett. 2000, 77 (5), 705-707. (9) Broido, D. A.; Reinecke, T. L. Theory of thermoelectric power factor in quantum well and quantum wire superlattices. Phys. ReV. B 2001, 6404 (4), 45324. (10) Dresselhaus, M. S.; Lin, Y. M.; Cronin, S. B.; Rabin, O.; Black, M. R.; Dresselhaus, G. Quantum wells and quantum wires for potential thermoelectric applications. Semicond. Semimet. 2001, 71 (Recent Trends in Thermoelectric Materials Research III), 1-121. (11) Nicic, I.; Shannon, C.; Bozack, M. J.; Braun, M.; Link, S.; El-Sayed, M. In Electrodeposition and characteriazation of CdTe/PbTe superlattices: A preliminary inVestigation; National Meeting of the Electrochemical Society; Washington DC, 2001; Andricacos, P. C., Searson, P. C., Reidsema-Simpson, C., Allongue, P., Stickney, J. L., Oleszek, G. M., Eds. ECS: Washington DC, 2001. (12) Wise, F. W. Lead salt quantum dots: the limit of strong quantum confinement. Acc. Chem. Res. 2000, 2000 (33), 773-780. (13) Guldner, Y.; Bastard, G.; Vieren, J. P.; Voos, M.; Faurie, J. P.; Million, A. Magneto-Optical Investigations of a Novel Superlattice: HgTe-CdTe. Phys. ReV. Lett. 1983, 51, 907. (14) Osbourn, G. C. Strained-layer superlattices: a brief review. IEEE J. Quantum Electron. 1986, QE-22, 1677. (15) Sakuma, Y.; Ozeki, M.; Kodama, K.; Ohtsuka, N. Inas/Inp Short-Period Strained-Layer Superlattices Grown By Atomic Layer Epitaxy. J. Cryst. Growth 1991, 115 (1-4), 324.

10.1021/la061625z CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006

Electrodeposition of PbSe/PbTe Superlattice Thin Films

vapor phase epitaxy (VPE),17 and a number of other derivative vacuum-based techniques.4,5,18,19 Those methods control growth rates via reactant fluxes and substrate temperature. Growth temperatures in MBE and VPE are important parameters, with 200 °C being considered low temperature. However, 200 °C can still result in interdiffusion of component elements, and a small amount can have a disproportionate effect on the properties of a superlattice. Interdiffusion results in blurring of the interfaces and formation of materials better described as alloys, than a superlattice. Junction integrity frequently determines device quality. Electrodeposition is a low-temperature technique, minimizing interdiffusion. It is also less costly than vacuum-based methodologies and is thus appealing for the formation of superlattices.2,11,20,21 There have been a significant number of studies using pulse plating, usually for the formation of metal superlattices, with variable results.22-24 Pulse plating has the advantage of simplicity, a single solution, and application of a simple waveform in current or potential. The disadvantage is that, at the more positive potential, a pure element or compound is deposited, while during the pulse, the second element or compound formed is inevitably contaminated with the first. There are reports of the use of two bath methods, where the deposit is shuttled between baths, resulting in pure films of both elements or compounds. Electrochemical formation of compound superlattices has been pioneered by Switzer et al.2,25 The standard method for compound electrodeposition has been co-deposition, where a set potential or current density is applied to a single solution containing precursors for all the elements in a compound. Co-deposition of PbSeTe multilayer periodic structures have been reported by Strelsov et al.26-28 Atomic layer epitaxy (ALE) and atomic layer deposition (ALD) are nanofilm formation methodologies29-32 based on the deposi(16) Mailhiot, C.; Smith, D. L. Strained-Layer Semiconductor Superlattices. Crit. ReV. Sol. State Mater. Sci. 1990, 16, 131. (17) Booker, G. R.; Klipstein, P. V.; Lakrimi, M.; Lyapin, S.; Mason, N.; Murgatroyd, I. J.; Nicholas, R.; Seong, T. Y.; Symons, D. M.; Walker, P. J. Growth of Inas/Gasb Strained-Layer Superlattices 0.2. J. Cryst. Growth 1995, 146, (1-4), 495. (18) Falco, C. M.; Bennett, W. R.; Boufelfel, A. Metal-metal superlattices. Springer Series in Surface Sciences 1985, 3 (Dyn. Phenom. Surf., Interfaces Superlattices), 35-47. (19) Fedorov, A. G.; Shneiderman, I. A.; Sipatov, A. Y.; Kaidalova, E. V. X-ray diffraction investigation of diffusion in PbTe-PbSe superlattices. J. Cryst. Growth 1999, 199, 1211-1215. (20) Streltsov, E. A.; Osipovich, N. P.; Lyakhov, A. S.; Ivashkevich, L. S. Electrodeposition of metal sulfide superlattices. Inorg. Mater. 1997, 33 (5), 442446. (21) Switzer, J. A.; Hung, C.-J.; Breyfogle, B. E.; Shumsky, M. G.; Leeuwen, R. V.; Golden, T. D. Electrodeposited Defect Chemistry Superlattices. Science 1994, 264, 1573. (22) Lashmore, D. S.; Dariel, M. P. Electrodeposited Cu-Ni Textured Superlattices. J. Electrochem. Soc. 1988, 135, 1219. (23) Moffat, T. P. Electrochemical production of single-crystal cu-ni strainedlayer superlattices on Cu(100). J. Electrochem. Soc. 1995, 142 (11), 3767-3770. (24) Simunovich, D.; Schlesinger, M.; Snyder, D. D. Electrochemically Layered Copper-Nickel Nanocomposites with Enhanced Hardness. J. Electrochem. Soc. 1994, 141, L10. (25) Switzer, J. A.; Shane, M. J.; Phillips, R. J. Electrodeposition of nanomodulated ceramic thin films. Mater. Res. Soc. Symp. Proc. 1990, 180 (Better Ceram. Chem. 4), 1053-9. (26) Streltsov, E.; Osipovich, N. P.; Ivashkevich, L. S.; Lyakhov, A. S. Electrochemical deposition of multilayer periodic structure in Pb-Se-Te system. Dokl. Akad. Nauk Bel. 1997, 41 (5), 62-65. (27) Streltsov, E. A.; Osipovich, N. P.; Ivashkevich, L. S.; Lyakhov, A. S.; Sviridov, V. V. Electrochemical deposition of PbSe, PbTe, and PbSe1-xTex films. Russ. J. Appl. Chem. 1997, 70 (10), 1651-1653. (28) Streltsov, E. A.; Osipovich, N. P.; Ivashkevich, L. S.; Lyakhov, A. S. Electrochemical deposition of PbSe(1-x)Te(x) solid solutions. Electrochim. Acta 1998, 44, 407-413. (29) Herman, F.; Kortum, R. L.; Kuglin, C. D.; Van Dyke, J. P. Electronic structure of tetrahedrally bonded semiconductors: Empirically adjusted OPW energy band calculations. In Methods in Computational Physics; Alder, B., Fernbach, S., Rotenberg, M., Eds.; Acamdemic Press: New York, 1968; Vol. 8, p 193.

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tion of elements one atomic layer at a time. Surface-limited reactions are used to deposit each atomic layer. The use of surfacelimited reactions is intended to improve morphology and facilitate monolayer control of the growth rate. ALD offers greater control over deposit structure and thickness than methods based on the control of reactant fluxes for all elements simultaneously. By limiting growth to an atomic layer at a time, layer-by-layer (2D) growth is promoted, as well as epitaxy. This group has been developing the electrochemical analogue of ALD and ALE, electrochemical ALD.33,34 Underpotential deposition (UPD) is the most general example of an electrochemical surface-limited reaction and was the basis for the cycles used to deposit the superlattices described here. UPD35-39 involves the deposition of an atomic layer of one element on a second at a potential prior to (under) that needed to deposit the element on itself. UPD results from the free energy of formation of a surface compound, where the depositing atomic layer is stabilized by bonding with the terminal element on the deposit surface. The electrochemical ALD cycles, used for deposition of the compounds in the present studies, started with exposure of the deposit to a precursor solution of the first element at its underpotential. The cell was then rinsed, a precursor solution for the second element was introduced at its underpotential, and the cell was rinsed with blank again. This process was intended to form a monolayer of the desired compound. The number of cycles performed determining the nanofilm thickness. II-VI compounds such as CdTe,40-44 CdS,34,42,45-47 ZnSe,48 CdS/CdSe superlattices,49 and CdS/HgS junctions46 have been (30) Suntola, T.; Antson, J. Method for producing compound thin films. U.S. Patent 4,058,430, 1977. (31) Goodman, C. H. L.; Pessa, M. V. Atomic Layer Epitaxy. J. Appl. Phys. 1986, 60, R65. (32) Sitter, H.; W., F. Atomic-Layer Epitaxy of Ii-Vi Compound Semiconductors. Fertkorperprobleme-AdV. Solid State Phys. 1990, 30, 219. (33) Stickney, J. L. Electrochemical atomic layer epitaxy (EC-ALE): nanoscale control in the electrodeposition of compound semiconductors. AdV. Electrochem. Sci. Eng. 2002, 7, 1-105. (34) Gregory, B. W.; Stickney, J. L. Electrochemical atomic layer epitaxy (ECALE). J. Electroanal. Chem. 1991, 300 (1-2), 543-561. (35) Kolb, D. M.; Przasnyski, M.; Gerisher, H. Underpotential Deposition of metals and work function differences. J. Electroanal. Chem. 1974, 54, 25-38. (36) Adzic, R. R.; Simic, D. N.; Despic, A. R.; Drazic, D. M. Electrocatalysis by foreign metal monolayers: oxidation of formic acid on platinum. J. Electroanal. Chem. 1975, 65, 587-601. (37) Kolb, D. M.; Gerisher, H. Further aspects concerning the correlation between underpotential depsoition and work function defferences. Surf. Sci. 1975, 51, 323. (38) Kolb, D. M. Physical and Electrochemical Properties of Metal Monolayers on Metallic Substrates. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978; Vol. 11, p 125. (39) Adzic, R. R. Electrocatalytic Properties of the Surfaces Modified by Foreign Metal Ad Atoms. In AdVances in Electrochemistry and Electrochemical Engineering; Gerishcher, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; Vol. 13, p 159. (40) Forni, F.; Innocenti, M.; Pezzatini, G.; Foresti, M. L. Electrochemical aspects of CdTe growth on the face of (111) of silver by EC-ALE. Electrochem. Acta 2000, 45 (20), 3225-3231. (41) Gregory, B. W.; Suggs, D. W.; Stickney, J. L. Conditions for the deposition of cadmium telluride (CdTe) by electrochemical atomic layer epitaxy. J. Electrochem. Soc. 1991, 138 (5), 1279-84. (42) Streltsov, E. S.; Labarevich, I. I.; Talapin, D. V. Electrochemical Formation of Monolayer Films of Cadmium-Sulfide On the Au Surface. Dokl. Akad. Nauk Bel. 1994, 38 (5), 64. (43) Flowers, J., Billy H.; Wade, T. L.; Garvey, J. W.; Lay, M.; Happek, U.; Stickney, J. L. Atomic layer epitaxy of CdTe using an automated electrochemical thin-layer flow deposition reactor. J. Electroanal. Chem. 2002, 524-525, 273285. (44) Varazo, K.; Lay, M. D.; Sorenson, T. A.; Stickney, J. L. Formation of the first monolayers of CdTe on Au(111) by electrochemical atomic layer epitaxy (EC-ALE): studied by LEED, Auger, XPS, and in-situ STM. J. Electroanal. Chem. 2002, 522 (1), 104-114. (45) Boone, B. E.; Shannon, C. Optical Properties of Ultrathin Electrodeposited CdS films Probed by Resonance Raman Spectroscopy and Photoluminescence. J. Phys. Chem. 1996, 100 (22), 9480-9484. (46) Gichuhi, A.; Boone, B. E.; Shannon, C. Electrosynthesized CdS/HgS heterojunctions. Langmuir 1999, 15 (3), 763-766.

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successfully formed using electrochemical ALD, as have some III-V compounds: GaAs,50,51 InAs,52 InSb,53 and superlattices of InAs/InSb.53 Torimoto et al. reported quantum confinement in thin films of ZnS,54 CdS,55 and PbS56 and superlattices of ZnS/CdS57,58 grown by electrochemical ALD. Quantum confinement of PbSe59 and PbTe thin films grown by electrochemical ALD have been reported as well. Ideally, two lattice-matched compounds are chosen to form a superlattice.2 PbSe and PbTe, however, have a significant lattice mismatch, 6%, and are thus considered to form strained-layer superlattice structures,60 where dislocations and island growth may occur. In this paper, the formation of PbSe/PbTe superlattice films is reported, electrodeposited using electrochemical ALD for the first time. Given the lattice match, these deposits are expected to fall in the category of strained-layer superlattices.14,61,62 Experimental Section An automated electrochemical thin-layer flow deposition system was used for the formation of the superlattices. The system consisted of a series of solution reservoirs, computer-controlled pumps, valves, and a potentiostat. Most of the hardware used has been described in previous articles.33,53,63 The system was contained within a nitrogen(47) Innocenti, M.; Pezzatini, G.; Forni, F.; Foresti, M. L. CdS and ZnS deposition on Ag(111) by electrochemcial atomic layer epitaxy. J. Electrochem. Soc. 2001, 148, c357. (48) Pezzatini, G.; Caporali, S.; Innocenti, M.; Foresti, M. L. Formation of ZnSe on Au(111) by electrochemcial atomic layer epitaxy. J. Electroanal. Chem. 1999, 475 (2), 164-170. (49) Zou, S.; Weaver, M. J. Surface-enhanced Raman Spectroscopic Characterization of Cadmium Sulfide/Cadmium Selenide Superlattices Formed on Gold by Electrochemical Atomic-Layer Epitaxy. Chem. Phys. Lett. 1999, 312, 101-107. (50) I. Villegas, J. L. S. Preliminary studies of GaAs deposition on Au(100), (110), and (111) surfaces by electrochemcial atomic layer eptiaxy. J. Electrochem. Soc. 1992, 139, 686. (51) I. Villegas, J. L. S. GaAs deposition on the (100) and (110) planes of Au by electrochemical atomic layer epitaxy: A low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy study. J. Vac. Sci. Technol. A 1992, 10, 3032. (52) Wade, T. L.; Ward, L. C.; Maddox, C. B.; Happek, U.; Stickney, J. L. Electrodepostion of InAs. Electrochem. Sol. State Lett. 1999, 2, (12), 616. (53) Wade, T. L.; Vaidyanathan, R.; Happek, U.; Stickney, J. L. Electrochemical Formation of a III-V Compound Semiconductor Superlattice: InAs/InSb. J. Electroanal. Chem. 2001, 500, 322-332. (54) Torimoto, T.; Obayashi, A.; Kuwabata, S.; Yasuda, H.; Mori, H.; Yoneyama, H. Preparation of size-quantized ZnS thin films using electrochemical atomic layer epitaxy and their photoelectrochemical properties. Langmuir 2000, 16 (13), 5820-5824. (55) Torimoto, T.; Nagakubo, S.; Nishizawa, M.; Yoneyama, H. Photoelectrochemical properties of size-quantized CdS thin films prepared by an electrochemical method. Langmuir 1998, 14 (25), 7077. (56) Torimoto, T.; Takabayashi, S.; Mori, H.; Kuwabata, S. Photoelectrochemical activities of untrathin lead sulfide films prepared by electrochemcial atomic layer eptiaxy. J. Electroanal. Chem. 2002, 522, 33-39. (57) Yoneyama, H.; Obayashi, A.; Nagakubo, S.; Torimoto, T. Electrochemical Preparation of superlattices composed of CdS and ZnS and Its photelectrochemical properties. Abstr. Electrochem. Soc. Meet. 1999, 99-2, 2138. (58) Torimoto, T.; Obayashi, A.; Kuwabata, S.; Yoneyama, H. Electrochemcal preparation of ZnS/CdS superlattice and Its photoelectrochemcial properties. Electrochem. Commun. 2000, 2, 359-362. (59) Vaidyanathan, R.; Stickney, J. L.; Happek, U. Quantum confinement in PbSe thin films electrodeposited by electrochemical atomic layer epitaxy (ECALE). Electrochim. Acta 2004, 49 (8), 1321-1326. (60) Springholz, G.; Wiesauer, K. nanoscale dislocation patterning in PbTe/ PbSe (100) lattice-mismatched heteroepitaxy. Phys. ReV. Lett. 2002, 88 (1), 015507-1. (61) Picraux, S. T.; Doyle, B. L.; Fuoss, J. Y.; Lamelas, F. J.; Brennan, S.; Imperatori, P. Strained layer superlattices. Willardson and Beer: San Diego, 1991; Vol. 33, p 170. (62) Parbrook, P. J.; Henderson, B.; O’Donnell, K. P.; Wright, P. J.; Cockayne, B. Critical thickness of common-anion II-VI strained layer superlattices (SLSs). J. Cryst. Growth 1992, 117 (1-4), 492-496. (63) Wade, T. L.; Flowers, B. H., Jr.; Vaidyanathan, R.; Mathe, K.; Maddox, C. B.; Happek, U.; Stickney, J. L. Electrochemical atomic-layer epitaxy: electrodeposition of III-V and II-VI compound semiconductors. Mater. Res. Soc. Symp. Proc. 2000, 581, (Nanophase and Nanocomposite Materials III), 145150.

Vaidyanathan et al. purged Plexiglas box to reduce oxygen exposure during electrodeposition. A thin-layer electrochemical flow cell, designed to promote laminar flow, was used for the depositions and consisted of a Au working electrode, a Au-coated indium tin oxide (ITO) auxiliary electrode, and a Ag/AgCl (3M NaCl) reference electrode (Bioanalytical systems, Inc., West Lafayette, IN). Solutions used include 0.2 mM Pb(ClO4)2 (Alfa Aesar, Ward Hill, MA), pH 5.5, buffered with 50.0 mM CH3COONa‚3H2O (J. T. Baker, Phillipsburg, NJ); 0.2 mM TeO2 (Alfa Aesar), pH 9.2, buffered with 50.0 mM sodium borate; and 0.2 mM SeO2 (Alfa Aesar), pH 5.5, buffered with 50.0 mM CH3COONa‚3H2O (J. T. Baker). A pH 7 rinse solution was used as well. The pH values of all solutions were adjusted with CH3COOH and KOH (Fischer Scientific, Pittsburgh, PA). Supporting electrolyte, 0.1 M NaClO4 (Fischer Scientific, Pittsburgh, PA), was added to each solution. Solutions were made with water from a Nanopure water filtration system (Barnstead, Dubuque, IA) fed from the house distilled water system. All chemicals were reagent grade or better. Substrates were glass microscope slides (Gold Seal products), etched in HF and briefly rinsed with HNO3, prior to insertion into the vapor deposition chamber. The substrates were annealed in the turbo pumped deposition chamber at 400 °C for 12 h before vapor deposition. Thin, 3 nm, films of Ti were first vapor deposited, followed by 600 nm of Au, while the substrates were held at 400 °C. The substrates, removed from the chamber, were dipped in nitric acid and rinsed with Nanopure water. Prior to use, the substrates were annealed using a H2 flame (to a dull orange glow in the dark), cleaned again in hot nitric acid, and rinsed with Nanopure water. Absorption measurements were performed using a variable-angle reflection rig in conjunction with a Bruker 66v FTIR spectrometer equipped with a Si detector. Glancing-angle X-ray diffraction patterns were acquired on a Scintag PAD V diffractometer, equipped with a 6 in. long set of Sola slits on the detector, to improve resolution in the asymmetric diffraction configuration used. Electron probe microanalysis (EPMA) studies were performed using a Joel JXA8600 super probe.

Results and Discussion 4:4 Superlattice. The starting potentials for the deposition program were determined from cyclic voltammetry for the elements on Au substrates. Previous results have shown optimal potentials, those resulting in ML/cycle deposition, tend to shift as the deposit grows. Over the first 10-30 cycles, depending on the compound, potentials have frequently been adjusted before reaching steady-state potentials that can be used for all subsequent cycles. Steady-state potentials of -0.3 V for Pb, -0.3 V for Se, and -0.4 V for Te were previously reported in studies of the deposition of PbSe and PbTe nanofilms59,64,65 using electrochemical ALD. For the 4:4 superlattice, each period was formed using four ALD cycles of PbSe followed by four cycles of PbTe. The deposit consisted of 81 periods, formed on an initial prelayer of 10 atomic layers of PbSe. The prelayer was intended to allow attainment of steady-state potentials before compound alternation to form the superlattice was initiated. The period program was performed as follows. The flow cell was first filled with the Pb2+ solution for 2 s at a potential of -0.3 V. The potential was then held, without solution flow (static), for 15 s to allow Pb atomic layer deposition. It was then rinsed with blank for 2 s, filled with the Te precursor (HTeO3-) solution, at -0.4 V, and then deposited (64) Stickney, J. L.; Wade, T. L.; Flowers, B. H.; Vaidyanathan, R.; Happek, U. Electrodeposition of compound semiconductors by electrochemical atomic layer epitaxy (EC-ALE). Encycl. Electrochem. 2003, 1, 513-560. (65) Cox, S. M.; Mathe, M. K.; Stickney, J. L.; Happek, U. Quantum confinement effects of lead selenide thin films formed using EC-ALE. Proc. Electrochem. Soc. 2004, 2004-02, (State-of-the-Art Program on Compound Semiconductors XL and Narrow Band gap Optoelectronic Materials and Devices II), 234-240.

Electrodeposition of PbSe/PbTe Superlattice Thin Films

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Figure 1. X-ray diffraction of 81 period 4PbTe/4PbSe superlattice. Buffer layer is 10 cycle PbSe. Angle of incidence is 1°, Cu KR source.

for 15 s under static conditions. Excess HTeO3- ions were flushed from the cell with blank for 2 s to complete the cycle. This cycle was intended to form a ML of the IV-VI semiconductor PbTe and was repeated four times, forming a PbTe nanofilm. The complimentary PbSe nanofilm was grown similarly: the cell was filled with the Pb2+ solution at a -0.3 V for 2 s and then held static for 15 s. Excess Pb2+ was flushed from the cell by pumping the blank for 2 s. The cell was then filled for 2 s with the Se precursor (HSeO3-) solution, at -0.3 V, and held static for 15 s. Again, the cycle was completed by flushing with blank for 2 s. Each cycle was intended to result in the deposition of one ML of PbSe, and was repeated four times to form a PbSe nanofilm. The superlattice period consisted of four cycles of PbTe and four cycles of PbSe and was repeated 81 times. Figure 1 shows the X-ray diffraction pattern for the 81 period, 4:4 PbSe/PbTe superlattice. The (111) diffraction peak, along with both ( first-order satellite peaks, and one second-order peak, are evident and indicative of the formation of a superlattice. The satellite peaks were equidistant from the (111) diffraction peak. The odd second-order satellite peak observed for this superlattice is indicative of a square-wave modulation of the lattice and uniform composition through the superlattice,2 which is very encouraging. It must be noted, however, that the odd second-order peak is very close to the expected (200) position for this material. The superlattice period, H, can be calculated using the following equation,19 from the angular distance, ∆(2θ), between the satellite and the (111) Bragg peak:

H ) 57.3 λ/∆(2θ) cos θ

(1)

In the present study, the period thickness was found to be 4.23 nm. If a ML of the compound is thought of as one atomic layer each of Pb and Te or Se, essentially a (111) compound monolayer of PbSe or PbTe (rock salt structure), the thickness of this period would be 2.1 nm, rather than the 4.23 nm determined from XRD. The assumption that one compound monolayer would be the natural result of one ALD cycle is just that, an assumption. It may be that one cycle of deposition naturally resulted in the growth of two ML of the compound. Alternatively, the cycles for PbTe and PbSe were probably not optimal. These studies were performed using PbSe and PbTe cycle programs that had

Figure 2. Reflection absorption measurements for 81 period 4PbTe/ 4PbSe superlattice.

Figure 3. X-ray diffraction of 40 period 6PbTe/6PbSe superlattice. Buffer layer is 100 cycle PbTe. Angle of incidence is 1°, Cu KR source.

not been optimized. To answer questions concerning the optimal coverages/cycle for an ALD cycle requires studies of coverages, stoichiometry, and structure as a function of the potentials used for Pb, Te, and Se deposition. The present results were simply a first attempt, preliminary studies, to form superlattices using electrochemical ALD of PbTe and PbSe. EPMA of the deposits indicated a composition of PbSe0.52Te0.48, slightly richer in Se, as expected, given the initial prelayer of 10 cycles of PbSe. Infrared absorption measurements (Figure 2) suggested a band gap for the superlattice of 0.48 eV, blueshifted from the band gaps for both of the component compounds, indicating some quantum confinement in the deposit. Previous work with PbSe nanofilm formation using electrochemical ALD on Au-on-glass substrates resulted in even larger band gap shifts for films grown with 10-50 electrochemical ALD cycles. In that case, the dependence of band gap on deposit thickness was consistent with a hyperbolic band model.65 The decreased confinement evident in the superlattice (Figure 2) compared with the nanofilm65 is probably a function of confinement by air in the nanofilm case, while in the superlattice case, the two compounds have very similar band gaps, limiting confinement. 6:6 Superlattice. Figure 3 shows the X-ray diffraction pattern for a 40 period 6:6 PbSe/PbTe superlattice film. Each period of the superlattice was composed of six cycles of PbSe and six cycles of PbTe. This time the superlattice was deposited on a prelayer of 100 cycles of PbTe. There were two reasons to try the 100 cycle PbTe prelayer; one was as with the 4:4 superlattice,

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Figure 4. Reflection absorption measurements for 40 period 6PbTe/ 6PbSe superlattice.

that it allowed any initial changes in the potentials used for deposition to be completed, so that steady-state potentials could be used to deposit the superlattice. In addition, previous results64 in the deposition of PbTe showed a strong preferential (200) orientation for deposits formed using electrochemical ALD. It was felt that such a preferential orientation might be beneficial to deposition of the superlattice. In Figure 3, both (111) and (200) Bragg diffraction peaks, along with first-order satellite peaks, were evident in a relatively noisy XRD pattern. Satellite peaks were positioned equidistant about the (111) Bragg diffraction peak, similarly to those in Figure 1. In addition, minimal satellites can be detected equally spaced about the (200) peak. It is interesting to note the presence of a much stronger (200) peak, relative to (111), in Figure 3 compared with Figure 1. The (200) peak may come predominantly from the 100 cycle PbTe prelayer, rather then the superlattice, which would be consistent with the minimal satellites observed around the (200) in Figure 3. From the spacing of the (111) satellite peaks, a superlattice period, H, of 7.05 nm was suggested. This is, again, about twice the anticipated thickness, H ) 3.1 nm, expected if each cycle were to form a single (111) compound monolayer. Comparing the two superlattices, the relative increase in thickness/ period between the 4:4 and 6:6 superlattices was as expected, given that each cycle again resulted in two compound monolayers instead of one. EPMA suggested a stoichiometry of PbSe0.44Te0.56 for the overall deposit, heavy in Te, consistent with the 100 cycle PbTe prelayer. Figure 4 shows the infrared absorption data for this 6:6 superlattice deposit, which suggests a band gap of 0.38 eV. Increasing the superlattice period, from 4.23 to 7.05 nm, resulted in a blue-shift relative the individual compounds, but the shift was lower than observed for the shorter period superlattice, 0.48 eV, again consistent with quantum confinement in the deposits. An AFM image (Figure 5) of the PbSe0.52Te0.48 superlattice displayed some ∼30 nm white dots, apparently growing along grain boundaries. This suggests a small amount of 3-D growth. The scan size of the image is 5µm × 5µm, and the Z scale was 50 nm. The thin-film deposit consisted of crystallites apparently 300 nm in diameter, essentially conformal with the morphology of the Au substrate. The small amount of 3-D growth is consistent with using nonoptimal cycle programs for PbSe and PbTe. Some 3-D growth generally occurs when the deposition conditions are excessive, resulting in more then a monolayer/cycle using electrochemical ALD.

Figure 5. AFM image of 81 period 4PbTe/4PbSe superlattice electrodeposited on annealed Au on glass substrate.

Reasons for the decrease in XRD pattern quality from the 4:4 (Figure 1) to the 6:6 (Figure 3) are not clear. It may result from the difference in prelayers used, as discussed above, or from depositing only 40 periods of the 6:6, instead of the 81 for the 4:4. Other possibilities include variability in the nature of the Au-on-glass substrates, as the morphology and crystal orientation of the Au was a function of the annealing process. These XRD studies were performed using incident angles of near 1° from the surface to maximize surface sensitivity. Small changes in that angle, as well as the presences of small amounts of surface roughness, can have striking effects on the resulting diffraction pattern quality. Finally, the lattice mismatch between the two compounds is significant, at 6%. Mismatches generally result in strain in the deposit and in the formation of superlattice, what is known as a strained-layer superlattice. Strain builds as a film grows on a mismatched substrate until the critical thickness, at which point dislocations and defects results. The principle of a strained layer superlattice is that, by switching back to the other compound before the critical thickness, the strain should relax, preventing the formation of the defects. In the present case, it may be that the critical thickness is above 4 nm and below 7 nm, so that by increasing the period from the 4:4 to the 6:6, a significant increase in the defect density resulted. The IV-VI compounds PbSe and PbTe have band gaps of 0.26 and 0.29 eV, respectively. Previous studies of lead chalcogenide superlattices have suggested that they have type II66-70 band alignment, which usually results in superlattice band gaps less than either of the constituent semiconducting compounds. In the present study, the band gaps were blue-shifted from those of the constituent compounds. (66) Jantsch, W.; Bauer, G.; Pichler, P.; Clemens, H. Anomalous Transport in Pbte Doping Superlattices. Appl. Phys. Lett. 1985, 47 (7), 738-740. (67) Bondarenko, V. V.; Zabudskii, V. V.; Sizov, F. F. Electron-phonon interaction and electron mobility in quantum- well type-II PbTe/PbS structures. Semiconductors 1998, 32 (6), 665-667. (68) Zabudsky, V. V.; Sizov, F. F.; Tetyorkin, V. V. Semiconductor Multilayer Structure and Type-Ii Superlattice Electrical Characteristic Simulation. Model. Simul. Mater. Sci. Eng. 1995, 3 (4), 575-582. (69) Sizov, F. F.; Gumenjuksichevskaya, J. V.; Tetyorkin, V. V.; Zabudsky, V. V. Band-Offset and Electronic-Properties of Type-Ii Pbte/Pbs Superlattices. Acta Phys. Pol. A 1995, 87 (2), 441-444. (70) Sizov, F. F.; Rogalski, A. Semicond. Superlattices Quantum-Wells Infrared Optoelectron. 1993, 17 (2), 93-164.

Electrodeposition of PbSe/PbTe Superlattice Thin Films

Conclusions PbSe/PbTe superlattices have been successfully formed using electrochemical ALD, with periods of 4:4 and 6:6. From XRD, the presence of satellite diffraction peaks clearly indicates the deposition of superlattices. While the presence of a second-order satellite peak for the 4:4 was indicative of the formation of a very good quality superlattice, suggesting a square-wave modulation of the lattice and uniform composition modulation throughout the thin film. Increasing the superlattice period resulted in a lower blue-shift in the adsorption spectrum, consistent with a decrease in the degree of quantum confinement. The literature suggests these materials should form type II superlattices, which tend to result in redshifts, not blue. The blue-shifts experienced were considerably less than expected from studies of individual compound nanofilms, however.

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Overall, these preliminary results were very encouraging, providing clear proof of the applicability of electrochemical ALD to the formation of superlattices. However, a number of questions have been raised, and further studies suggested. First, optimization studies for the deposition of PbSe and PbTe are required and are underway. Given such deposition programs, more superlattices should be formed were the dependence on thickness, period, period symmetry, and the need for a prelayer can all be addressed. In addition, the absences of type II behavior will need to be further investigated. Acknowledgment. Support from the National Science Foundation, divisions of Materials and Chemistry, as well as a NSF DMR NER grant from the Nanoscale Experimental Research program, is gratefully acknowledged. LA061625Z