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Báo cáo hóa học: " Study of the formation processes of gold droplet arrays on Si substrates by high temperature anneals"

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  1. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 http://www.nanoscalereslett.com/content/6/1/151 NANO EXPRESS Open Access Study of the formation processes of gold droplet arrays on Si substrates by high temperature anneals Alla Klimovskaya, Andrey Sarikov*, Yury Pedchenko, Andrey Voroshchenko, Oksana Lytvyn, Alexandr Stadnik Abstract In this study, the peculiarities of the transformations of gold films deposited on the Si wafer surfaces as a result of high temperature anneals are investigated experimentally depending on the conditions of wafer surface preparation and the annealing regimes. The morphology and the distribution functions of the crystallites of gold films as well as the gold droplets formed as a result of anneals are studied as functions of annealing temperature, type of annealing (rapid thermal or rapid furnace annealing), and the state of the surface of Si wafers. The results obtained can be used for the controlled preparation of the arrays of catalytic gold droplets for subsequent growth of Si wire-like crystals. Introduction of catalytically active droplets can be formed by different techniques such as patterned metal deposition [4,5], self- Semiconductor Si wire-like crystals grown on Si sub- aggregation in the droplets during metal deposition [6], strates using the catalytic gold droplets have been stu- or temperature-stimulated disjoining of a solid metal died since 1960 as prospective structures for the film deposited onto Si substrate [3,7]. The metal catalyst development of micro- and nano-electronic devices [1]. can undergo additional argon plasma etching to assist In the typical schema of the experiment, the gold dro- the disjoining of metal film and the formation of cataly- plets are first formed on the Si substrates. The growth tic islands [8,9]. The regime of thermal treatment before process proceeds with the inlet flow of reactive gas that the wire-like crystal growth determines the evolution consists of Si-containing molecules (monosilane is a kinetics of droplet ensemble and, hence, the properties typical example) into the growth chamber [2,3]. The of the subsequent process of Si wire growth. preferential decomposition of reactive gas molecules and This article presents the results of an experimental the silicon incorporation in the positions of droplets investigation of the peculiarities of the formation of the take place, which cause the growth of elongated wire- arrays of gold islands in the course of high temperature like crystals, diameters of which are determined by the anneals of Si wafers with gold films deposited on their diameters of droplets. The droplet caps remain on the surfaces depending on the conditions of wafer surface top of wires to enable the continuous catalytic process preparation and annealing regimes. of the decomposition of Si-containing reactive species from the gas phase, the preferential Si incorporation Experimental into the droplets, transportation within them and/or on The experiments were carried out on 500- μ m-thick, the cap surface, and incorporation in the wires growing at their interfaces with the droplets. (111)-oriented, boron-doped Cz-Si wafers with resistivity of 10 Ω cm obtained from two sources, namely, The initial system for the Si wire growth before the inlet flow of active gas mixture is the catalytic gold dro- Wacker-Chemitronic GmbH, Germany, and Silicon Ltd., plet array on the surface of Si substrate. The ensemble Ukraine. Before the deposition of gold films, the surfaces of Si wafers were made to undergo one of the two treat- ments, namely, (i) degreasing in acetone vapour without * Correspondence: andrey.sarikov@gmx.de removal of native oxide or (ii) growing a uniform stoi- 1 V. Lashkarev Institute of Semiconductor Physics, National Academy of chiometric silicon oxide film by thermal oxidation in Sciences of Ukraine, 41 Nauki Avenue, 03028 Kiev, Ukraine © 2011 Klimovskaya et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
  2. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 2 of 8 http://www.nanoscalereslett.com/content/6/1/151 Short-time furnace anneals were carried out in Ar gas vacuum (the native oxide was removed in 5% HF with atmosphere by rapid insertion of samples in the heated subsequent 5-10 min. rinsing in deionised water before zone in the central part of furnace reactor. The anneal- this procedure and the wafers were subsequently ing temperature was in the range of 900-1050°C, the annealed in hydrogen atmosphere during 40 min at duration in the range of 10-20 s, and the pressure of Ar 450°C). The thicknesses of oxide films were monitored in the gas chamber corresponded to 1 atmosphere. ellipsometrically. The control over the thicknesses of deposited gold Thin films of gold (3 and 5 nm thicknesses) were films, their morphology as well as monitoring of changes deposited on Si wafer surfaces by vacuum sputtering at a pressure below 2 × 10 -6 Torr. Prepared structures produced by annealing were done by scanning atomic force microscopy (AFM) (NanoScope IIIa). were annealed to initiate the formation of the arrays of golden islands on the surface of Si substrates. Two Results and discussion methods of annealing were applied, namely, rapid ther- mal anneals (RTAs) and quick furnace anneals. The development of the structure of gold films depos- The RTA treatments were realised by the illumination ited onto the Si substrates takes place already during the of structures under investigation by linear halogen stage of film deposition. Gold films acquire different lamps whose emission maximum wavelength was 1 μm. nanocrystalline structures depending on the state of the The treatment temperatures were in the range of 300- oxide on the Si wafer. The AFM images of gold films 1100° С . Linear halogen lamps were arranged in two deposited on the Si surfaces with different states of parallel rows on both sides of samples to enable faster oxide coverage are shown in Figure 1. The size distribu- temperature growth (up to 70°С/s) and reduced thermal tions of the crystallites formed are shown in Figure 2. gradients that produce thermal-mechanical stresses. To As can be seen from these figures, the gold films formed avoid uncontrolled oxidation of the sample during the crystallites with a typical size of 12 nm on substrates RTA treatments, the RTA chamber was refilled with with native oxide after cleaning in neutral solution argon at atmospheric pressure before each treatment (Figure 2a). The nanocrystal sizes increased with the cycle. increase of annealing temperature for RTA-treated Si Figure 1 AFM images of 3-nm-thick gold films deposited on Silicon Ltd. Si substrates with different surface oxide layer states: (a) initial substrate with 2.4-nm-thick natural oxide layer; (b) substrate with an oxide layer modified by RTA (650°C, 15 s, oxide thickness after RTA is 1.7 nm); and (c) substrate after RTA (950°C, 15 s) with a 3.3-nm-thick modified oxide layer. The maps of heights are shown on the left-hand side; the same maps with distinguished grain boundaries are shown on the right-hand side. Numerical grain parameters are shown in Figure 2.
  3. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 3 of 8 http://www.nanoscalereslett.com/content/6/1/151 Diam #20 Diam #12 200 150 150 100 Counts Counts 100 50 50 0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Diameter [nm] Diameter [nm] Diam #14 5 4 #14 40 #12 3 #20 Height [nm] 2 Counts 1 20 0 -1 -2 0 0 100 200 300 400 500 0 5 10 15 20 25 30 35 Surface position [nm] Diameter [nm] Figure 2 Histograms of the distributions of the characteristics of gold films presented in Figure 1: grain diameters (a-c) and the surface profiles (d). For a good layout, the distribution (a) is superimposed on distributions (b,c). seen that the formation of separated gold islands on the w afers (to 15 and 18 nm for RTA at 650 and 950°C, Silicon Ltd. wafers (high surface roughness, RMS = 1.83 respectively, see Figure 2b,c). Such behaviour was mainly nm) begins already at 900°C (Figure 3). For the Wacker- caused by the increase in surface homogeneity of native Chemitronic Si wafers (RMS = 0.25 nm), the formation silicon oxide coverage that is supported by the surface of individual gold islands is not observed at any rate up profiles shown in Figure 2d. The gold films grown to the temperature of 950°C (see Figure 5). Instead, the on initial Si surfaces had the most developed surfaces individual intergrain boundaries often form joints 120°j (RMS = 0.6 nm). RTA treatments led to the decrease of for the mentioned temperatures, indicating a steady- this value down to 0.4 nm. The histograms in Figure 2a, state gold film recrystallisation process. b,c demonstrate additionally the increase in the mean Increase in the RTA temperature resulted in the for- diameter of gold crystallites as a result of the increase of mation of gold nanoislands on the surface of Si wafers. RTA temperature and the decrease of gold film rough- The size and the density of islands were determined by ness. It follows therefore that through modifying the the annealing temperature (Figures 4 and 6). For both oxide coverage on the surface of Si substrates, one can types of substrates used, the typical sizes of nanoislands control the deposited gold film structure and subse- were in the range of 15-30 nm. quently, the process of the formation of catalytically It is worthy to note that the gold evaporation from the active nanoislands for the growth of Si wire-like crystals. Si substrate surface was the accompanying process to Fast anneals at high temperatures of the structures of the formation of gold island arrays during the high tem- Si substrates with gold films deposited on them both in perature anneals. Figure 7 shows the contents of oxygen furnace and RTA equipment result in the disjoining of and gold in the subsurface layers of Au/Si structures gold films and the formation of the arrays of separated after 15-s RTA treatments at different temperatures for gold islands. This process is strongly dependent on the the Wacker-Chemitronic Si wafers. The contents of quality of the surface of Si wafers. The results on gold both gold and oxygen were determined from the results island formation by RTA on the Si wafers procured of X-ray energy-dispersive analysis of the scanning elec- from Silicon Ltd., and Wacker-Chemitronic are pre- tron microscope, Zeiss Evo-50. The integration was sented in Figures 3, 4, and 5, 6, respectively. It can be
  4. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 4 of 8 http://www.nanoscalereslett.com/content/6/1/151 Figure 3 AFM images of the surfaces of 3-nm-thick gold film deposited onto Silicon Ltd. Si substrates with the natural oxide layer after RTA: (a) 900°C, 15 s; (b) 1000°C, 20 s; and (c) 1050°C, 20 s. The maps of heights are shown on the left-hand side; the same maps with distinguished grain boundaries are shown on the right-hand side. For quantitative parameters see Figure 4. Figure 4 Histograms of the diameter distributions of gold droplets (left) and the same histograms weighted on droplet volumes (right) (ordinate axis shows the total droplet volume) corresponding to the structures shown in Figure 3. The density (part of covered surface) of droplets amounts to 912 μm-2 (22.2%), 336 μm-2 (12.7%), and 96 μm-2 (7.1%), respectively for Figure 3 (a-c).
  5. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 5 of 8 http://www.nanoscalereslett.com/content/6/1/151 Figure 5 AFM images of the surfaces of 3-nmthick gold films deposited onto the Wacker-Chemitronic Si substrates with the natural oxide layer after 15 s RTA: (a) 400°C; (b) 700°C; (c) 950°C; and (d) 1050°C. The maps of heights are shown on the left hand side; the same maps with distinguished grain boundaries are shown on the right hand side. For quantitative parameters see Figure 6. carried out over the area of 10 × 10 μm2 with a collec- The thickness of oxide layer on the surface of Si wafer had a great effect on the formation of gold nanoisland tion time of 150 s. As can be noted from the data pre- arrays during the RTA treatments (Figures 8 and 9). sented, a sharp decrease (by about 1/3) of gold contents One can see that the gold films formed islands more in the structures under investigation took place after the efficiently on the artificial oxide coverage at the RTA threshold temperature of about 800°C. Besides, a non- temperature of 950°C than on the native oxide coverage. linear increase in the oxygen contents in the subsurface A gradual increase in the oxide thickness promoted the layers was observed with the increase of RTA tempera- increase in the free space between the grains, in contrast ture (by a factor of 4 for the temperature range of 300- to the closely packed grains on the substrates covered 1050°C). This effect can be due to the presence of with native oxide. At the same time, the size distribution oxygen traces in the atmosphere of experimental setup of grains and its maximum practically did not change and possible diffusion of oxygen from the substrate bulk with the oxide thickness (Figure 9). to the hetero-boundary in the course of annealing.
  6. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 6 of 8 http://www.nanoscalereslett.com/content/6/1/151 Figure 6 Histograms of the diameter distributions of gold droplets (left) and the same histograms weighted on droplet volumes (right) (ordinate axis shows the total droplet volume) corresponding to the structures shown in Figure 5. Droplet density amounts to 2460, 2520, 1940, and 264 μm-2, respectively, for (a-d). moment, and needs to be addressed to in more detail in S hort furnace anneals of the structures under study future studies. The authors believe that it can be related under the same conditions as for RTA treatments to the transition processes during sample heating. resulted in the formation of structured gold films with smaller mean grain sizes: 18 nm against 35 nm for the Conclusions RTA-treated samples. Even at 1024°C, the formation of islands did not take place, although in some films, the In this study, the detailed investigations of the peculiari- regions containing nanopits, from which gold had evapo- ties of the formation of the arrays of gold islands in the rated, were observed. This difference is unclear at the course of high temperature anneals of Si wafers with
  7. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 7 of 8 http://www.nanoscalereslett.com/content/6/1/151 gold films deposited on their surfaces depending on the 1.2 conditions of wafer surface preparation and annealing regimes are carried out. RTA of Si wafers before the 1.0 gold deposition was found to smoothen the native oxide Content [weight %] layer on their surfaces and stimulate the formation of 0.8 gold films with bigger crystalline grain structures. RTAs of Au/Si structures at the temperatures 900°C and 0.6 higher were shown to produce the separate Au droplets on the Si wafer surfaces. Increase of the oxide film 0.4 thickness on the surface of Si wafers promotes the for- O mation of isolated gold droplets compared to the closely Au 0.2 packed droplets formed on the Si surfaces covered with native oxide. Rapid furnace anneals of Au/Si structures 0.0 300 400 500 600 700 800 900 1000 1100 were demonstrated not to result in the gold droplet for- RTA Temperature [C] mation but only in the gold film recrystallisation. The Figure 7 Dependence of oxygen and gold contents in the results obtained are valuable for the choice of the tech- subsurface layers of Au/Si structures on the RTA temperature. nological regimes for obtaining the required properties Figure 8 AFM images of the surface of 5-nm-thick gold film deposited on Wacker-Chemitronic Si substrates with different thicknesses of grown oxide and subjected to 15 s RTA at 950°C: (a) oxide thickness is 1.8 nm; (b) oxide thickness is 1.9 nm; (c) oxide thickness is 2.0 nm. The maps of heights are shown on the left-hand side; the same maps with distinguished grain boundaries are shown on the right-hand side. For quantitative parameters see Figure 9.
  8. Klimovskaya et al. Nanoscale Research Letters 2011, 6:151 Page 8 of 8 http://www.nanoscalereslett.com/content/6/1/151 Diam. #10-1 1min Diam. #10-7 2min 30 30 20 20 Counts Counts 10 10 0 0 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Diameter [nm] Diameter [nm] Diam. #10-4 3min 40 # 10-1 1min #10-7 2min #10-4 3min 30 20 20 Height [nm] Counts 10 10 0 -10 -20 0 0 10 20 30 40 50 60 70 80 0 200 400 600 800 1000 Diameter [nm] Surface position [nm] Figure 9 Histograms of the distributions of the characteristics of structures shown in Figure 8: gold droplet diameters (a-c) and gold film surface profiles (d). For better layout, the distribution (a) is superimposed onto the distributions (b,c). of catalytic gold layers (gold nanodroplet arrays) on the 5. Liu ZQ, Xie SS, Zhou WY, Sun LF, Li YB, Tang DS, Zou XP, Wang CY, Wang G: Catalytic Synthesis of Straight Silicon Nanowires Over Fe surface of Si substrates for the subsequent growth of Si Containing Silica Gel Substrates by Chemical Vapor Deposition. J Cryst wire-like crystals. Growth 2001, 224:230. 6. Kramer A, Boeck T, Schramm P, Fornari R: Investigation of Au and In as Solvents for the Growth of Silicon Nanowires on Si(1 1 1). Physica E 2008, Authors’ contributions 40:2462. 7. Martinez-Gil A, Rota A, Maroutian T, Bartenlian B, Beauvillain P, Moyen E, AK planned the experiments, took major part in the interpretation of results, Hanbücken M: Nano-Patterned Silicon Surfaces for the Self-Organised participated in the manuscript preparation, AS took part in the interpretation Growth of Metallic Nanostructures. Superlatt Microstruct 2004, 36:235. of results and participated in the manuscript preparation, YP and AV made 8. Ren ZF, Huang ZP, Xu JW, Wang JH, Bush P, Siegal MP, Provencio PN: substrate pre-treatments and carried out annealing experiments, OL made Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass. AFM investigations, AS made gold film deposition. All authors have read and Science 1998, 282:1105. approved the final manuscript. 9. Griffiths H, Xu C, Barrass T, Cooke M, Iacopi F, Vereecken P, Esconjauregui S: Plasma Assisted Growth of Nanotubes and Nanowires. Surf Coat Technol Competing interests 2007, 201:9215. The authors declare that they have no competing interests. doi:10.1186/1556-276X-6-151 Received: 20 September 2010 Accepted: 16 February 2011 Cite this article as: Klimovskaya et al.: Study of the formation processes Published: 16 February 2011 of gold droplet arrays on Si substrates by high temperature anneals. Nanoscale Research Letters 2011 6:151. References 1. Wagner RS, Ellis WC: Vapor-Liquid-Solid Mechanism of Single Crystal Griowth. Appl Phys Lett 1964, 4:89. 2. Kwak DW, Cho HY, Yang W-C: Dimensional Evolution of Silicon Nanowires Synthesized by Au-Si Island-Catalyzed Chemical Vapor Deposition. Physica E 2007, 37:153. 3. Ozaki N, Ohno Y, Takeda S: Silicon Nanowhiskers Grown on a Hydrogen- Terminated Silicon {111} Surface. Appl Phys Lett 1998, 73:3700. 4. Hibino H, Watanabe Y: Arrangement of Au-Si Alloy Islands at Atomic Steps. Surf Sci 2005, 588:L233.
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