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Chemical bath deposition of PbS nanocrystals: Effect of substrate
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Thin Solid Films 516 (2008) 3791 – 3795 www.elsevier.com/locate/tsf
Chemical bath deposition of PbS nanocrystals: Effect of substrate Alex P. Gaiduk a,⁎, Peter I. Gaiduk b , Arne Nylandsted Larsen c a
b
Chemistry Department, Belarusian State University, prosp. Nezavisimosti, 4, Minsk 220030, Belarus Physical Electronic Department, Belarusian State University, prosp. Nezavisimosti, 4, Minsk 220030 Belarus c Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark Received 8 November 2006; received in revised form 2 May 2007; accepted 13 June 2007 Available online 20 June 2007
Abstract Nanocrystalline PbS layers have been deposited chemically on Si, Ge and GaAs substrates from alkaline solutions containing 0.05 mol l− 1 of Pb(NO3)2, 0.04 mol l− 1 of thiourea, 0.05 mol l− 1 of triethanolamine, and 0.15 mol l− 1 of NaOH. Rutherford backscattering spectroscopy, transmission electron and atomic force microscopy reveal that the chemical nature of the substrate has a profound influence on the structure and thickness of the deposited layers. It is found that a large lattice mismatch between the substrate and PbS results in formation of coarse-grained layers with a small effective thickness (e.g. PbS on Si). On the other hand, close matching of lattice constants leads to deposition of thicker layers with smaller grain size (e.g. PbS on Ge, GaAs). © 2007 Elsevier B.V. All rights reserved. Keywords: PbS; Nanocrystals; Chemical bath deposition; Structural properties
1. Introduction Narrow energy gap semiconductors are of great interest due to their unique properties and applications. Lead sulfide (PbS) has a relatively small band gap (0.41 eV at 300 K) and therefore can be used for fabrication of mid-infrared detectors (3 b λ b 30 μm) [1]. Moreover, the band gap of PbS can be easily adjusted up to a few electronvolts when in the form of nanometer sized dots. Such a significant widening of the band gap is associated with small effective masses of electrons and holes (me = mh = 0.09 m0) as well as with a relatively large exciton Bohr radius (20 nm) of PbS [2,3]. Thus, it is possible to vary optical and electrical properties of nanocrystalline PbS by adjusting the grain size. A great deal of research efforts has been devoted to the synthesis of PbS nanoparticles of varying size in a controlled manner. Both “wet” and “dry” methods of deposition have been used. Vacuum evaporation [3], hot-wall epitaxy [3] and molecular-beam epitaxy [3,4] are among the most successful “dry” methods for PbS synthesis. The frequently used “wet” methods include spray pyrolysis [5], chemical bath deposition ⁎ Corresponding author. Tel.: +375 29 2741017. E-mail address: [email protected] (A.P. Gaiduk). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.122
[5–8] and electrochemical deposition [9]. Chemical bath deposition is of special interest as it is a simple, but highly efficient method. It provides a powerful and versatile control of the size and surface density of nanoparticles and therefore can be used for preparation of high quality nanocrystalline PbS films. The characteristics of the deposited layers depend strongly on growth conditions (i.e. composition and temperature of solution, duration of deposition process) as well as on the chemical and topographical nature of the substrate [5]. Chemical bath deposition of thin PbS nanocrystalline films has been investigated thoroughly for numerous substrates e.g. glasses [5–7], polymers, [10], sol–gel materials [11,12] and semiconductors [13–15]. Still, there has not been comparative study of the size and density distribution of PbS nanocrystals deposited on different substrates. The aim of this work was to investigate and compare the characteristics of nanocrystalline PbS layers deposited chemically on Si, Ge and GaAs substrates. 2. Experimental details In this work, layers of PbS were deposited on p-type, (100) oriented single-crystalline Si, Ge or GaAs substrates. Small chips of the wafers (5 × 5 mm2) were used. Prior to deposition,
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A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795
they were cleaned with acetone in an ultrasonic bath (10 min) followed by chemical polishing (10–20 s) in a mixture of HF and HNO3 [15]. Subsequently, the samples were washed in clean deionized water and stuck to the 75 × 25 × 1 mm3 commercial glass slides (supplied by Marienfeld) so that one slide contained substrates of different types at a time. Finally, the glass slides were placed at an angle of ca 20° off normal in a container filled with a fresh deposition solution. The solution contained 0.05 mol l− 1 of Pb(NO3)2, 0.04 mol l− 1 of thiourea (NH2)2CS, 0.05 mol l− 1 of triethanolamine N(C2H4OH)3, and 0.15 mol l− 1 of NaOH of analytic-grade purity. Using a thermostat, a deposition procedure was carried out at different temperatures (10–50 °C) during 1–180 min at a moderate stirring. After deposition, the samples were rinsed in deionized water. Then, the non-adherent outer PbS layer was removed during 15 min treatment in an ultrasonic bath (filled in series with deionized water, ethanol and acetone), and the compact inner layer was dried in a CO2 flow. The layers obtained were homogeneous and strongly adherent to the substrate. Rutherford backscattering spectroscopy (RBS) of 1.5 MeV He+ ions was used to determine a composition and to estimate the effective thickness of the deposited layers. The XRUMP computer code was used for evaluation and computer simulation of the RBS spectra. Phase composition and microstructure of the layers were studied with a Philips CM 20 transmission electron microscope (TEM) operating at 120 kV. Both plan-view (PV-TEM) and cross-section (XTEM) techniques for the sample preparation were applied. Size and surface distribution of PbS nanoparticles were measured by atomic force microscopy (AFM) using a Rasterscope 3000 AFM instrument. 3. Experimental results A typical bright-field PV-TEM image of a PbS layer deposited on an Si substrate during 15 min is shown in Fig. 1a. The black spots which are clearly seen in the TEM image consist of crystalline
Fig. 1. Bright field PV-TEM (a) and X-TEM (c) micrographs of PbS particles deposited on Si at 22 °C for 15 min. The inset in (a) contains an enlarged image of the particle indicated by an arrow. Electron diffraction pattern (b) refers to the PV-TEM image.
Fig. 2. AFM images of PbS layers deposited on different substrates: (a) Si, (b) Ge and (c) GaAs. Deposition was carried out at 22 °C for 15 min. The insets represent the profile of the surface between A and B points.
A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795
PbS with a rock-salt cubic structure, as it can be seen from the electron diffraction pattern (Fig. 1b). The typical size of the PbS particles varies between 40 and 60 nm. Most of the particles are spherical in shape; some of them have facets (see inset in the Fig. 1a). Similar results are also obtained by the X-TEM investigations (Fig. 1c). The PbS nanocrystals appear in the XTEM image as dark spots which have a sharp interface to the substrate. The average size of the particles is about 50 nm, which agrees well with the results of the PV-TEM measurements (40–60 nm). It is found by TEM that the crystalline structure and shape of the PbS nanocrystals deposited on Ge and GaAs substrates do
3793
Table 1 The effective thickness of PbS layers grown on Si, Ge or GaAs substrates and corresponding size and surface density of the nanocrystals Substrate Lattice Effective mismatch with thickness of PbS, % [18] PbS layers, nm
Mean size of Surface density PbS particles, of PbS particles, nm cm− 2
Si Ge GaAs
30–50 20–30 20–30
8.5 4.7 4.8
1–2 4–6 4–6
1·1010 2·1011 2·1011
not differ significantly from those deposited on Si. However, the morphology and the thickness of the PbS layers strongly depend on the chemical nature of a substrate. In Fig. 2, typical AFM images of PbS layers deposited on Si (Fig. 2a), Ge (Fig. 2b) and GaAs (Fig. 2c) are collected. It can be seen from Fig. 2a that after deposition, the Si substrate contains spatially separated PbS nanocrystals, which fill about 20–30% of a total area of the substrate. The nanocrystals are relatively large, their typical size varies between 30 and 50 nm, and the average surface concentration of the particles is about 1·1010 cm− 2. In contrast the Ge and GaAs substrates are covered by compact PbS layers with clear polycrystalline structure (Fig. 2b and c). The typical size of the PbS particles varies between 20 and 30 nm, and their surface concentration exceeds 2 · 1011 cm− 2. Fig. 3 presents RBS spectra of the PbS layers deposited on Si (Fig. 3a), Ge (Fig. 3b) and GaAs (Fig. 3c) substrates. All the spectra contain a strong single peak in the high-energy region, which corresponds to backscattering from Pb atoms. As the backscattering yield is directly proportional to the effective thickness of thin film [16], it is possible to compare the thickness of the PbS layers deposited on different substrates. The values of effective thicknesses calculated from RBS spectra are given in Table 1 together with the results of AFM measurements. It can be seen from the table that the effective thickness of the PbS layers correlates well with the size and surface concentration of PbS crystals. That is, PbS films deposited on Ge and GaAs possess an effective thickness of 4– 6 nm but a relatively small grain size. On the contrary, the Si substrate is characterized by a quite small effective thickness of the layer (1–2 nm) whereas the average size of the PbS crystals is large. 4. Discussion The mechanism of PbS deposition is not well understood, although many papers have been devoted to this subject [5–7]. The main chemical reactions leading to the formation of PbS in an alkaline media can be presented as follows. First, the addition of NaOH to the solution containing Pb2+ in the form of [Pb (TEA)]2+ ions results in precipitation of Pb(OH)x:
Fig. 3. RBS spectra of (a) Si, (b) Ge and (c) GaAs substrates after deposition at 22 °C for 15 min. The energy resolution is 2.83 keV/channel.
½PbðTEAÞ2þ ↔Pb2þ þ TEA
ð1Þ
Pb2þ þ xOH− →PbðOHÞx ↓:
ð2Þ
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A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795
due to a depletion of reagents in the deposition mixture [5,8].
PbðOHÞx þ HS− →PbS þ ðx−1ÞOH− þ H2 O
ð4Þ
A crucial stage of the above mechanism is the formation of PbS nuclei on the substrate. The rate of nucleation is governed primarily by the physical and chemical state of the surface, i.e. the number and orientation of dangling bonds, concentration of defects on the surface, contamination of the surface with dopants, etc. The surface of all samples was treated in the same way and thus was similar with respect to defects or contaminations. Another important characteristic which strongly influences the nucleation process is the lattice mismatch between the substrate and deposited material. When the lattice parameters of PbS and the substrate match well, the Gibbs energy change of nucleation is small, which facilitates nucleation [5]. In this case, a huge number of nucleation sites is provided, which leads to a high density of small PbS crystallites deposited on the substrate. Intensive nucleation makes then possible a high rate of PbS growth resulting in thicker layers as compared to ‘energetically unfavorable’ substrates with a large lattice mismatch. This conclusion is in a good agreement with the data presented in Table 1. It can be seen in particular that thicker layers with smaller crystalline size are obtained on the Ge and GaAs substrates which have a relatively small lattice mismatch with PbS (4.7–4.8%). Increase in the lattice mismatch (e.g. 8.5% for PbS on Si substrate) results in decreasing of the thickness of deposited layers as well as in increasing of the size of PbS crystallites.
Pb2þ þ HS− þ OH− →PbS þ H2 O:
ð5Þ
5. Conclusion
Fig. 4. The thickness–time dependence of PbS layers deposited on an Si substrate at different temperatures (12, 22, 35 and 45 °C). The thickness is given as an integrated RBS yield from the Pb atoms.
Simultaneously, excess of OH− ions initiates the metathesis and desulfuration of thiourea: ðH2 NÞ2 CS þ OH− →ðH2 NÞ2 CO þ HS− :
ð3Þ
Then, the reaction of HS− with Pb(OH)x or Pb2+ ions leads to PbS deposition:
Thus, deposition includes different limiting physical and chemical processes which determine the kinetic behavior of the PbS thickness. The layers of PbS were deposited for different time at 12, 22, 35 or 45 °C and the final structures were inspected by RBS to extract the effective thickness. Fig. 4 represents the kinetic dependencies of the PbS deposition on a (100) Si substrate; similar curves were obtained also in the cases of Ge and GaAs substrates. The kinetic dependencies appear to follow a typical sigmoidal profile, similar to those observed for autocatalytic reactions [17]. The sigmoidal profile of the curves indicates that the deposition is governed by the following main stages [8]:
PbS nanocrystalline layers have been deposited chemically on single-crystalline (100) wafers of Si, Ge or GaAs. It is found by TEM investigations that the layers deposited on all the above substrates consist of PbS nanocrystals of a rock-salt cubic crystalline structure. On the other hand, AFM and RBS investigations show that the morphology and the thickness of the layers strongly depend on the chemical nature of the substrate. That is, the layers of PbS deposited on Ge and GaAs are thicker than those deposited on Si but have a relatively small grain size. The results are interpreted by the different lattice mismatch of the substrates and PbS phase. Acknowledgements
(i) Nucleation stage. It is the initial stage requiring a high activation energy in which reactive centers (nuclei) are formed on the surface of the substrate. The rate of nucleation is rather small, however, it rises with the increase in the temperature of the bath (compare curves for 12 and 45 °C in Fig. 4). (ii) Growth stage. It is the second stage, which is characterized by an enhanced rate of PbS deposition. The high rate of deposition is associated with the accelerated growth of PbS nuclei formed on the substrate during the nucleation stage. (iii) Termination stage. During this stage, the rate of deposition gradually slows down. This probably is
The study was supported in part by the Belarusian Research Foundation (grant No.T05-020), the Danish National Scientific Research Council and by the NATO grant (CBP.EAP.CLG 982384). References [1] D.E. Bode, Phys. Thin Films 3 (1966) 275. [2] K.K. Nanda, F.E. Kruis, H. Fissan, M. Acet, J. Appl. Phys. 91 (2002) 2315. [3] D.L. Partin, J. Heremans, in: T.S. Moss, S. Mahajan (Eds.), Handbook on Semiconductors, vol. 3, Elsevier, Amsterdam, 1994, p. 369.
A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795 [4] H. Zogg, W. Vogt, H. Melchior, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 253 (1987) 418. [5] K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, Phys. Thin Films 12 (1982) 167. [6] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 65 (2000) 1. [7] P.K. Nair, M.T.S. Nair, Semicond. Sci. Technol. 4 (1989) 807. [8] P. O'Brien, J. McAleese, J. Mater. Chem. 8 (1998) 2309. [9] E.A. Streltsov, N.P. Osipovich, A.S. Lyakhov, L.S. Ivashkevich, Inorg. Mater. 33 (1997) 442. [10] A.K. Dutta, T. Ho, L. Zhang, P. Stroeve, Chem. Mater. 12 (2000) 1024. [11] T. Saraidarov, R. Reisfeld, A. Sashchiuk, E. Lifshitz, J. Sol–Gel Sci. Tech. 34 (2005) 137.
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[12] A. Sashchiuk, E. Lifshitz, R. Reisfeld, T. Saraidarov, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 67. [13] J. Puišo, S. Tamulevicius, G. Laukaitis, S. Lindroos, M. Leskelä, V. Snitka, Thin Solid Films 403/404 (2002) 457. [14] A.B. Preobrajenski, K. Barucki, T. Chassé, Phys. Rev. Lett. 85 (2000) 4337. [15] J.L. Davis, M.K. Norr, J. Appl. Phys. 37 (1966) 1670. [16] J.W. Mayer, E. Rimini, Ion Beam Handbook for Material Analysis, Academic Press, New York, 1977. [17] A.A. Frost, R.G. Pearson, Kinetics and Mechanism, 2nd edn., Wiley, Chichester, 1965, p. 19. [18] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.
Thin Solid Films 516 (2008) 3791 – 3795 www.elsevier.com/locate/tsf
Chemical bath deposition of PbS nanocrystals: Effect of substrate Alex P. Gaiduk a,⁎, Peter I. Gaiduk b , Arne Nylandsted Larsen c a
b
Chemistry Department, Belarusian State University, prosp. Nezavisimosti, 4, Minsk 220030, Belarus Physical Electronic Department, Belarusian State University, prosp. Nezavisimosti, 4, Minsk 220030 Belarus c Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark Received 8 November 2006; received in revised form 2 May 2007; accepted 13 June 2007 Available online 20 June 2007
Abstract Nanocrystalline PbS layers have been deposited chemically on Si, Ge and GaAs substrates from alkaline solutions containing 0.05 mol l− 1 of Pb(NO3)2, 0.04 mol l− 1 of thiourea, 0.05 mol l− 1 of triethanolamine, and 0.15 mol l− 1 of NaOH. Rutherford backscattering spectroscopy, transmission electron and atomic force microscopy reveal that the chemical nature of the substrate has a profound influence on the structure and thickness of the deposited layers. It is found that a large lattice mismatch between the substrate and PbS results in formation of coarse-grained layers with a small effective thickness (e.g. PbS on Si). On the other hand, close matching of lattice constants leads to deposition of thicker layers with smaller grain size (e.g. PbS on Ge, GaAs). © 2007 Elsevier B.V. All rights reserved. Keywords: PbS; Nanocrystals; Chemical bath deposition; Structural properties
1. Introduction Narrow energy gap semiconductors are of great interest due to their unique properties and applications. Lead sulfide (PbS) has a relatively small band gap (0.41 eV at 300 K) and therefore can be used for fabrication of mid-infrared detectors (3 b λ b 30 μm) [1]. Moreover, the band gap of PbS can be easily adjusted up to a few electronvolts when in the form of nanometer sized dots. Such a significant widening of the band gap is associated with small effective masses of electrons and holes (me = mh = 0.09 m0) as well as with a relatively large exciton Bohr radius (20 nm) of PbS [2,3]. Thus, it is possible to vary optical and electrical properties of nanocrystalline PbS by adjusting the grain size. A great deal of research efforts has been devoted to the synthesis of PbS nanoparticles of varying size in a controlled manner. Both “wet” and “dry” methods of deposition have been used. Vacuum evaporation [3], hot-wall epitaxy [3] and molecular-beam epitaxy [3,4] are among the most successful “dry” methods for PbS synthesis. The frequently used “wet” methods include spray pyrolysis [5], chemical bath deposition ⁎ Corresponding author. Tel.: +375 29 2741017. E-mail address: [email protected] (A.P. Gaiduk). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.122
[5–8] and electrochemical deposition [9]. Chemical bath deposition is of special interest as it is a simple, but highly efficient method. It provides a powerful and versatile control of the size and surface density of nanoparticles and therefore can be used for preparation of high quality nanocrystalline PbS films. The characteristics of the deposited layers depend strongly on growth conditions (i.e. composition and temperature of solution, duration of deposition process) as well as on the chemical and topographical nature of the substrate [5]. Chemical bath deposition of thin PbS nanocrystalline films has been investigated thoroughly for numerous substrates e.g. glasses [5–7], polymers, [10], sol–gel materials [11,12] and semiconductors [13–15]. Still, there has not been comparative study of the size and density distribution of PbS nanocrystals deposited on different substrates. The aim of this work was to investigate and compare the characteristics of nanocrystalline PbS layers deposited chemically on Si, Ge and GaAs substrates. 2. Experimental details In this work, layers of PbS were deposited on p-type, (100) oriented single-crystalline Si, Ge or GaAs substrates. Small chips of the wafers (5 × 5 mm2) were used. Prior to deposition,
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A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795
they were cleaned with acetone in an ultrasonic bath (10 min) followed by chemical polishing (10–20 s) in a mixture of HF and HNO3 [15]. Subsequently, the samples were washed in clean deionized water and stuck to the 75 × 25 × 1 mm3 commercial glass slides (supplied by Marienfeld) so that one slide contained substrates of different types at a time. Finally, the glass slides were placed at an angle of ca 20° off normal in a container filled with a fresh deposition solution. The solution contained 0.05 mol l− 1 of Pb(NO3)2, 0.04 mol l− 1 of thiourea (NH2)2CS, 0.05 mol l− 1 of triethanolamine N(C2H4OH)3, and 0.15 mol l− 1 of NaOH of analytic-grade purity. Using a thermostat, a deposition procedure was carried out at different temperatures (10–50 °C) during 1–180 min at a moderate stirring. After deposition, the samples were rinsed in deionized water. Then, the non-adherent outer PbS layer was removed during 15 min treatment in an ultrasonic bath (filled in series with deionized water, ethanol and acetone), and the compact inner layer was dried in a CO2 flow. The layers obtained were homogeneous and strongly adherent to the substrate. Rutherford backscattering spectroscopy (RBS) of 1.5 MeV He+ ions was used to determine a composition and to estimate the effective thickness of the deposited layers. The XRUMP computer code was used for evaluation and computer simulation of the RBS spectra. Phase composition and microstructure of the layers were studied with a Philips CM 20 transmission electron microscope (TEM) operating at 120 kV. Both plan-view (PV-TEM) and cross-section (XTEM) techniques for the sample preparation were applied. Size and surface distribution of PbS nanoparticles were measured by atomic force microscopy (AFM) using a Rasterscope 3000 AFM instrument. 3. Experimental results A typical bright-field PV-TEM image of a PbS layer deposited on an Si substrate during 15 min is shown in Fig. 1a. The black spots which are clearly seen in the TEM image consist of crystalline
Fig. 1. Bright field PV-TEM (a) and X-TEM (c) micrographs of PbS particles deposited on Si at 22 °C for 15 min. The inset in (a) contains an enlarged image of the particle indicated by an arrow. Electron diffraction pattern (b) refers to the PV-TEM image.
Fig. 2. AFM images of PbS layers deposited on different substrates: (a) Si, (b) Ge and (c) GaAs. Deposition was carried out at 22 °C for 15 min. The insets represent the profile of the surface between A and B points.
A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795
PbS with a rock-salt cubic structure, as it can be seen from the electron diffraction pattern (Fig. 1b). The typical size of the PbS particles varies between 40 and 60 nm. Most of the particles are spherical in shape; some of them have facets (see inset in the Fig. 1a). Similar results are also obtained by the X-TEM investigations (Fig. 1c). The PbS nanocrystals appear in the XTEM image as dark spots which have a sharp interface to the substrate. The average size of the particles is about 50 nm, which agrees well with the results of the PV-TEM measurements (40–60 nm). It is found by TEM that the crystalline structure and shape of the PbS nanocrystals deposited on Ge and GaAs substrates do
3793
Table 1 The effective thickness of PbS layers grown on Si, Ge or GaAs substrates and corresponding size and surface density of the nanocrystals Substrate Lattice Effective mismatch with thickness of PbS, % [18] PbS layers, nm
Mean size of Surface density PbS particles, of PbS particles, nm cm− 2
Si Ge GaAs
30–50 20–30 20–30
8.5 4.7 4.8
1–2 4–6 4–6
1·1010 2·1011 2·1011
not differ significantly from those deposited on Si. However, the morphology and the thickness of the PbS layers strongly depend on the chemical nature of a substrate. In Fig. 2, typical AFM images of PbS layers deposited on Si (Fig. 2a), Ge (Fig. 2b) and GaAs (Fig. 2c) are collected. It can be seen from Fig. 2a that after deposition, the Si substrate contains spatially separated PbS nanocrystals, which fill about 20–30% of a total area of the substrate. The nanocrystals are relatively large, their typical size varies between 30 and 50 nm, and the average surface concentration of the particles is about 1·1010 cm− 2. In contrast the Ge and GaAs substrates are covered by compact PbS layers with clear polycrystalline structure (Fig. 2b and c). The typical size of the PbS particles varies between 20 and 30 nm, and their surface concentration exceeds 2 · 1011 cm− 2. Fig. 3 presents RBS spectra of the PbS layers deposited on Si (Fig. 3a), Ge (Fig. 3b) and GaAs (Fig. 3c) substrates. All the spectra contain a strong single peak in the high-energy region, which corresponds to backscattering from Pb atoms. As the backscattering yield is directly proportional to the effective thickness of thin film [16], it is possible to compare the thickness of the PbS layers deposited on different substrates. The values of effective thicknesses calculated from RBS spectra are given in Table 1 together with the results of AFM measurements. It can be seen from the table that the effective thickness of the PbS layers correlates well with the size and surface concentration of PbS crystals. That is, PbS films deposited on Ge and GaAs possess an effective thickness of 4– 6 nm but a relatively small grain size. On the contrary, the Si substrate is characterized by a quite small effective thickness of the layer (1–2 nm) whereas the average size of the PbS crystals is large. 4. Discussion The mechanism of PbS deposition is not well understood, although many papers have been devoted to this subject [5–7]. The main chemical reactions leading to the formation of PbS in an alkaline media can be presented as follows. First, the addition of NaOH to the solution containing Pb2+ in the form of [Pb (TEA)]2+ ions results in precipitation of Pb(OH)x:
Fig. 3. RBS spectra of (a) Si, (b) Ge and (c) GaAs substrates after deposition at 22 °C for 15 min. The energy resolution is 2.83 keV/channel.
½PbðTEAÞ2þ ↔Pb2þ þ TEA
ð1Þ
Pb2þ þ xOH− →PbðOHÞx ↓:
ð2Þ
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A.P. Gaiduk et al. / Thin Solid Films 516 (2008) 3791–3795
due to a depletion of reagents in the deposition mixture [5,8].
PbðOHÞx þ HS− →PbS þ ðx−1ÞOH− þ H2 O
ð4Þ
A crucial stage of the above mechanism is the formation of PbS nuclei on the substrate. The rate of nucleation is governed primarily by the physical and chemical state of the surface, i.e. the number and orientation of dangling bonds, concentration of defects on the surface, contamination of the surface with dopants, etc. The surface of all samples was treated in the same way and thus was similar with respect to defects or contaminations. Another important characteristic which strongly influences the nucleation process is the lattice mismatch between the substrate and deposited material. When the lattice parameters of PbS and the substrate match well, the Gibbs energy change of nucleation is small, which facilitates nucleation [5]. In this case, a huge number of nucleation sites is provided, which leads to a high density of small PbS crystallites deposited on the substrate. Intensive nucleation makes then possible a high rate of PbS growth resulting in thicker layers as compared to ‘energetically unfavorable’ substrates with a large lattice mismatch. This conclusion is in a good agreement with the data presented in Table 1. It can be seen in particular that thicker layers with smaller crystalline size are obtained on the Ge and GaAs substrates which have a relatively small lattice mismatch with PbS (4.7–4.8%). Increase in the lattice mismatch (e.g. 8.5% for PbS on Si substrate) results in decreasing of the thickness of deposited layers as well as in increasing of the size of PbS crystallites.
Pb2þ þ HS− þ OH− →PbS þ H2 O:
ð5Þ
5. Conclusion
Fig. 4. The thickness–time dependence of PbS layers deposited on an Si substrate at different temperatures (12, 22, 35 and 45 °C). The thickness is given as an integrated RBS yield from the Pb atoms.
Simultaneously, excess of OH− ions initiates the metathesis and desulfuration of thiourea: ðH2 NÞ2 CS þ OH− →ðH2 NÞ2 CO þ HS− :
ð3Þ
Then, the reaction of HS− with Pb(OH)x or Pb2+ ions leads to PbS deposition:
Thus, deposition includes different limiting physical and chemical processes which determine the kinetic behavior of the PbS thickness. The layers of PbS were deposited for different time at 12, 22, 35 or 45 °C and the final structures were inspected by RBS to extract the effective thickness. Fig. 4 represents the kinetic dependencies of the PbS deposition on a (100) Si substrate; similar curves were obtained also in the cases of Ge and GaAs substrates. The kinetic dependencies appear to follow a typical sigmoidal profile, similar to those observed for autocatalytic reactions [17]. The sigmoidal profile of the curves indicates that the deposition is governed by the following main stages [8]:
PbS nanocrystalline layers have been deposited chemically on single-crystalline (100) wafers of Si, Ge or GaAs. It is found by TEM investigations that the layers deposited on all the above substrates consist of PbS nanocrystals of a rock-salt cubic crystalline structure. On the other hand, AFM and RBS investigations show that the morphology and the thickness of the layers strongly depend on the chemical nature of the substrate. That is, the layers of PbS deposited on Ge and GaAs are thicker than those deposited on Si but have a relatively small grain size. The results are interpreted by the different lattice mismatch of the substrates and PbS phase. Acknowledgements
(i) Nucleation stage. It is the initial stage requiring a high activation energy in which reactive centers (nuclei) are formed on the surface of the substrate. The rate of nucleation is rather small, however, it rises with the increase in the temperature of the bath (compare curves for 12 and 45 °C in Fig. 4). (ii) Growth stage. It is the second stage, which is characterized by an enhanced rate of PbS deposition. The high rate of deposition is associated with the accelerated growth of PbS nuclei formed on the substrate during the nucleation stage. (iii) Termination stage. During this stage, the rate of deposition gradually slows down. This probably is
The study was supported in part by the Belarusian Research Foundation (grant No.T05-020), the Danish National Scientific Research Council and by the NATO grant (CBP.EAP.CLG 982384). References [1] D.E. Bode, Phys. Thin Films 3 (1966) 275. [2] K.K. Nanda, F.E. Kruis, H. Fissan, M. Acet, J. Appl. Phys. 91 (2002) 2315. [3] D.L. Partin, J. Heremans, in: T.S. Moss, S. Mahajan (Eds.), Handbook on Semiconductors, vol. 3, Elsevier, Amsterdam, 1994, p. 369.
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