pubs.acs.org/Langmuir © 2010 American Chemical Society
Area-Selective Atomic Layer Deposition of Lead Sulfide: Nanoscale Patterning and DFT Simulations Wonyoung Lee,*,† Neil P. Dasgupta,† Orlando Trejo,‡ Jung-Rok Lee,† Jaeeun Hwang,† Takane Usui,† and Fritz B. Prinz†,§ †
Department of Mechanical Engineering, ‡Department of Chemical Engineering, and §Department of Materials and Science Engineering, Stanford University, California 94305 Received October 29, 2009. Revised Manuscript Received January 8, 2010
Area-selective atomic layer deposition (ALD) of lead sulfide (PbS) was achieved on octadecyltrichlorosilane (ODTS)patterned silicon substrates. We investigated the capability of ODTS self-assembled monolayers (SAMs) to deactivate the ALD PbS surface reactions as a function of dipping time in ODTS solution. The reaction mechanism was investigated using density functional theory (DFT), which showed that the initial ALD half-reaction is energetically unfavorable on a methyl-terminated SAM surface. Conventional photolithography was used to create oxide patterns on which ODTS SAMs were selectively grown. Consequently, PbS thin films were grown selectively only where ODTS was not present, whereas deposition was blocked in regions where ODTS was grown. The resulting fabricated patterns were characterized by scanning electron microscopy and Auger electron spectroscopy, which demonstrated that ALD PbS was well confined to defined patterns with high selectivity by ODTS SAMs. In addition, AFM lithography was employed to create nanoscale PbS patterns. Our results show that this method can be applied to various devicefabrication processes, presenting new opportunities for various nanofabrication schemes and manifesting the benefits of self-assembly.
Introduction Lead chalcogenide materials such as lead sulfide (PbS), lead selenide (PbSe), and lead telluride (PbTe) have received increasing attention in their single-crystalline and polycrystalline forms because of their use in infrared lasers, photovoltaics, and infrared detectors. Among semiconductors, these lead salts exhibit unique properties, including a small direct band gap at the L point of the Brillouin zone and a large static dielectric constant.1 In addition, the use of devices with nanoscale dimensions made of these materials has attracted much interest because of their unique properties that allow the investigation of quantum confinement effects. In this study, we focused on PbS because it has properties favorable to quantum confinement applications, including a small band gap (∼0.41 eV), a relatively large Bohr radius (∼18 nm),2,3 and small relative effective mass of charge carriers (∼0.08m0).4 PbS nanocrystals have also demonstrated multiple exciton generation (MEG), making PbS a potentially attractive material for next-generation solar cells.5 Of several deposition techniques for fabricating high-quality ultrathin films of PbS, atomic layer deposition (ALD) is one of the most promising.6,7 The self-limiting adsorption reactions in *Corresponding author. E-mail:
[email protected]. Tel: 1-650-3531673. Fax: 1-650-723-5034. (1) Kumar, S.; Sharma, T. P.; Zulfequar, M.; Husain, M. Physica B 2003, 325, 8– 16. (2) Wise, F. W. Acc. Chem. Res. 2000, 33, 773–780. (3) Dutta, A. K.; Ho, T.; Zhang, L.; Stroeve, P. Chem. Mater. 2000, 12, 1042– 1048. (4) Torimoto, T.; Takabayashi, S.; Mori, H.; Kuwabata, S. J. Electroanal. Chem. 2002, 522, 33–39. (5) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Nano Lett. 2005, 5, 865–871. (6) Ritala, M; Leskel€a, M. Handbook of Thin Film Materials. In Deposition and Processing of Thin Films; Nalwa, H. S., Ed.; Academic: New York, 2002; Vol. 1, Chapter 2. (7) Ritala, M.; Kukli, K.; Rahtu, A.; R€als€anen, P. I.; Leskl€a, M.; Sajavaara, T.; Keinonen, J. Science 2000, 288, 319–321.
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ALD ensure precise control of film thickness and uniformity over large areas. PbS has been deposited by ALD with a variety of precursors, including Pb(tmhd)2 and H2S, which were used in this study.8-10 Recently, our group demonstrated successful thin film deposition of PbS by ALD and measured quantum confinement effects on the band gap of films with thicknesses below 10 nm.11 Although ALD provides perhaps the best available control of material thickness in the z direction, fabricating precise patterns in the lateral directions by ALD is very challenging. Recently, many groups have developed area-selective ALD processes to achieve lateral structures; most of these groups have employed selfassembled monolayers (SAMs), which are thin organic films that form spontaneously on solid surfaces and modify the surface physical, chemical, and electrical properties. A variety of SAMs are stable at temperatures of up to a few hundred degrees centigrade, unlike the resist layers used for photolithography and electron beam lithography. Because ALD relies on surface reactions, its deposition characteristics can be effectively controlled by the proper use of SAMs. Several groups have used SAMs as a chemical resist to (8) Leskel€a, M.; Niinist€o, L.; Niemela, P.; Nyk€anen, E.; Soininen, P.; Tiita, M.; V€ah€akangas J. Vacuum 1990, 41, 1457–1459. (9) Nyk€anen, E.; Laine-Ylijoki, J.; Soininen, P.; Niinist€o, L.; Leskel€a, M.; Hubert-Pfalzgraf, G. J. Mater. Chem. 1994, 4, 1409–1412. (10) Nyk€anen E.; Lehto S.; Leskel€a M.; Niinist€o L.; Soininen P. In Electroluminescence: Proceedings of the 6th International Conference on Electroluminescence; Singh V. P., McClure J. C., Eds.; Cinco Puntos Press: El Paso, TX, 1992; pp 199-204. (11) Dasgupta, N. P.; Lee, W.; Prinz, F. B. Chem. Mater. 2009, 21, 3973–3978. (12) Yan, M.; Koide, Y.; Babcock, J. R.; Markworth, P. R.; Belot, J. A.; Marks, T. J.; Chang, R. P. H. Appl. Phys. Lett. 2001, 79, 1709–1711. (13) Yang, P.; Zou, S.; Yang, Y. Small 2008, 4, 1527–1536. (14) Ras, R. H. A.; Sahramo, E.; Malm, J.; Raula, J.; Karppinen, M. J. Am. Ceram. Soc. 2008, 130, 11252–11253. (15) Park, M. H.; Jang, Y. J.; Sung-Suh, H. M.; Sung, M. M. Langmuir 2004, 20, 2257–2260. (16) Sinha, A.; Hess, D. W.; Henderson, C. L. J. Electrochem. Soc. 2006, 153, G465–G469. (17) F€arm, E.; Kemell, M.; Ritala, M.; Leskel€a, M. Thin Solid Films 2008, 517, 972–975.
Published on Web 01/25/2010
DOI: 10.1021/la904122e
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block various ALD precursors, including ZnO,12-14 TiO2,15-17 HfO2,18-21 ZrO2,18,22 Ir,17 Ru,23 and Pt.19,20,24 In this article, we demonstrate the capability of SAMs to block the deposition of PbS thin films by ALD. Octadecyltrichlorosilane (ODTS) SAMs were chosen to modify the surface termination because of their ability to deactivate ALD reactions as well as their good chemical and thermal stability. We also investigated the capability of ODTS to deactivate ALD PbS as a function of dipping time in ODTS solution by X-ray photoemission spectroscopy (XPS). Moreover, we characterized the thermal stability of ODTS at elevated temperatures and in the presence of precursors by XPS analysis. Density functional theory (DFT) simulations were carried out to investigate the reaction mechanism of the Pb(tmhd)2 precursor with different surface functional termination groups. Microscale patterns of ALD PbS with high spatial and chemical selectivity were fabricated on ODTSpatterned Si/SiO2 substrates. Auger electron spectroscopy (AES) analyses show that ODTS was selectively grown only on the oxide patterns defined by the photolithography and deactivated PbS deposition during the ALD process. Hence, materials were selectively deposited by the ALD process only where ODTS was not present. Additionally, nanoscale patterning of PbS was demonstrated using atomic force microscopy (AFM) lithography to pattern ODTS surfaces, suggesting its ability to create higherorder quantum confinement structures such as quantum wires or quantum dots.
Experimental Details Preparation of ODTS SAMs. All chemicals, including ODTS (97%), toluene (anhydrous, 99.8%), and chloroform (99%), used to form SAMs were purchased from Aldrich (Milwaukee, WI) and were used as received. All silicon pieces were cut from Si(100) wafers (p type with boron dopant; resistivity of 0.1-0.9 Ω cm) before cleaning. The silicon pieces were cleaned by sonication in chloroform, acetone, and ethanol. This was followed by DI water rinsing and a piranha etching (7:3 H2SO4/ H2O2 with no external heating). After additional sonication in chloroform, acetone, and ethanol was conducted, the silicon pieces were rinsed with DI water and blown dry with nitrogen. The growth of the SAM was performed in a dry air-purged glovebox at room temperature. These cleaned silicon pieces were immersed in 10 mM ODTS solutions in toluene. After the desired dipping time elapsed, the samples were quickly immersed in toluene, acetone, and chloroform and blown dry with nitrogen before ALD processing.25 PbS ALD Conditions. The samples were loaded into a customized flow-type ALD system designed for the deposition of PbS thin films. The base pressure of the ALD chamber was 50 mTorr. The substrate temperature was maintained at 160 °C, and the precursor was sublimated at 140 °C.11 Bis(2,2,6,6-tetramethyl-3,5-heptanedionato)lead(II) (Pb(tmhd)2) (Strem Chemicals, Inc.) and a gas mixture of 3.5% H2S in N2 were used as precursors with an argon carrier gas at a flow rate of 10 sccm used to deposit PbS films. Patterning Process on the Microscale and Nanoscale. For microscale patterning, a 100-nm-thick SiO2 layer was fabricated by thermal oxidation at 800 °C for 3 h. Standard photolithography, (18) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F. Appl. Phys. Lett. 2004, 84, 4017–4019. (19) Chen, R.; Bent, S. F. Adv. Mater. 2006, 18, 1086–1090. (20) Chen, R.; Bent, S. F. Chem. Mater. 2006, 18, 3733–3741. (21) Liu, J.; Mao, Y.; Lan, E.; Banatao, D. R.; Forse, G. J.; Lu, J.; Blom, H.-O.; Yeates, T. O.; Dunn, B.; Chang, J. P. J. Am. Ceram. Soc. 2008, 130, 16908–16913. (22) Lee, J. P.; Sung, M. M. J. Am. Chem. Soc. 2004, 126, 28–29. (23) Park, K. J.; Doub, J. M.; Gougousi, T.; Parsons, G. N. Appl. Phys. Lett. 2005, 86, 051903. (24) Jiang, X.; Huang, H.; Prinz, F. B.; Bent, S. F. Chem. Mater. 2008, 20, 3897– 3905. (25) Wang, Y.; Lieberman, M. Langmuir 2003, 19, 1159–1167.
6846 DOI: 10.1021/la904122e
including photoresist coating, UV exposure, and developing, was used to create line patterns with a separation of 3 μm. Subsequent oxygen plasma etching for 5 min removed the exposed thermal oxide. Residual photoresist was removed with piranha (9:1 H2SO4/H2O2) etching at 120 °C, followed by DI water rinsing. Hydrofluoric (HF) acid etching (50:1) was performed immediately before immersion into ODTS solution to remove the native oxide layer between oxide patterns, replacing it with hydride termination (Si-H) while leaving a thick layer of thermal oxide patterns. The samples were then placed for a given time in a 10 mM ODTS solution to form a densely packed ODTS layer. For nanoscale patterning, a commercial AFM system (JSPM 5200, JEOL) was used for AFM lithography in contact mode with additional circuits to perform oxidation. The tips used were Pt-coated silicon tips (PPP-NCHPt, Nanosensors) with a radius of ∼40 nm. The relative humidity (RH) was controlled within a range of 60-70%. The electric pulse was controlled by the AFM system and an external circuit with 0-10 V (the AFM tip was always grounded) and 0.05-10 ms in magnitude and duration, respectively. When an electric field is applied through a conductive AFM tip, an anodic bias can induce local oxidation in an ODTS-grown silicon substrate, forming silicon oxide patterns and simultaneously removing SAMs that are located on top of the created oxide patterns.26,27 Subsequent HF etching locally removes oxide patterns, exposing the silicon substrate underneath, and SAMs remain undamaged.28 This patterned substrate can now be used as a template for further ALD processing. In this study, the ODTS and SiO2 layers were not removed after the ALD PbS deposition was completed. However, there are several methods to remove them. First, residual ODTS can be removed by oxygen plasma, ozone exposure, or a piranha solution. Alternatively, exposing the sample to a high temperature after the desired ALD process was completed can also remove the ODTS layer by breaking the ODTS-OH bonds. To remove SiO2, typical HF etching can be used after the ODTS layers are removed. However, the removal process should be carefully decided because the PbS layer can be affected by those chemical steps; in particular, oxygen plasma or ozone can oxidize the surface of the PbS layer. Analysis Techniques. For unpatterned substrates, ODTS SAMs were characterized before the ALD process with an ellipsometer (L116C, Gaertner Scientific Corporation) and a contact angle analyzer (FTA 2000, First Ten Angstroms, Inc.) to measure the ODTS film thickness and water contact angle, respectively. The chemical composition on sample surfaces after PbS deposition was measured by X-ray photoelectron spectroscopy (PHI VersaProbe, Physical Electronics). For the patterned substrate, Auger electron spectroscopy (PHI 700, Physical Electronics) was used for elemental analysis, line scans, and elemental mapping. All of the spectra shown in this article have a detection sensitivity of
