SUPPLEMENTARY INFORMATION

Transcription

1 SUPPLEMENTARY INFORMATION Investigating and Understanding the Initial Growth Mechanisms of Catalyst-Free Growth of 1D SiC Nanostructures Yoo Youl Choi and Doo Jin Choi *, Department of Materials Science and Engineering, Yonsei University, 262 Seongsanno, Sudaemun-Gu, Seoul , Republic of Korea *Corresponding author. Tel: , Fax: Supplementary Information Table S1. Deposition conditions for catalyst-free growth of SiC NWs. Diluent gas species Deposition temp. ( C) Deposition time (min) H 2, N 2, Ar Input gas ratio (α) MTS flow rate (sccm) Total pressure (Torr) Table S1. Deposition conditions for catalyst-free growth of SiC NWs. Each factor was carefully controlled by fixing the other factors to a default condition. The default conditions were as follows: 1200 C, α = 30, 5 Torr, MTS = 10 sccm, H 2 ambient, and total flow rate = 300 sccm. 1

2 Supplementary Information Figure S1. Microanalysis of an NW was performed using an FIB/FE-SEM dual-beam system to obtain a crosssectional view of NW. First, an NW that grew vertically from the surface was selected (Figure S1a). To prevent any structural collapse of the NW and prevent any damage caused by the Ga + ion beam during the TEM sampling process, Pt deposition was performed in the entire region near the selected NW (Figure S1b). Then, the center of the Pt-covered NW was sliced down to a constant thickness and shifted to a copper TEM half-grid. Finally, the TEM sample with the exposed cross-section of the NW was carefully thinned to less than 40 nm using FIB milling, to allow for a clear analysis in the TEM observation (Figure S1c). Figure S1d shows the final cross-sectional view of the sample after the FIB process was carried out. Figure S1. TEM sampling process using FIB. (a) Selection of a SiC NW. (b) Observation of the area around the NW before Pt deposition. (c) Slicing the Pt-covered NW. (d) Final side view of the NW sample attached on a TEM grid. 2

3 Supplementary Information Figures S2 S4. Figures S2 and S3 display many images of the NWs in the initial stages of catalyst-free growth under different deposition conditions. To investigate the various aspects of the initial growth of the NWs, the deposition time was set to only 3 min. The controlled deposition factors were the temperature, input gas ratio (α), pressure, total flow rate, and diluent gas. To understand the exclusive effect of each deposition factor, other conditions were kept fixed when a particular factor was being controlled. The default conditions were as follows: 1200 C, α = 30, 5 Torr, MTS = 10 sccm, H 2 ambient atmosphere, and total flow rate = 300 sccm. In Figure S4, the measured growth densities of NWs and the deposited film area coverage are displayed. To measure the density of NWs and the deposited film area, Imagejpc software (IF 1.45m, Tiago Ferreira & Wayne Rasband) was used. First, as shown in Figure S2a d, as the deposition temperature increased, the thickness of the NWs increased and the density of the NWs increased up to 1200 C. In addition, the surface area coverage increased as the deposition temperature increased. This result corresponds relatively well to the finding that the source decomposition and surface diffusion increase at higher temperatures, inducing higher deposition rates. 1 The surface morphologies of the structures are observed to change from granular to facet forms. When the input gas ratio (α) is changed, since the amount of MTS gas and the total pressure are maintained, the total flow rate must be increased. This causes the density of NWs to rapidly decrease as the surface area is covered with films. Figure S2e h shows that the NW density decreases and area coverage increases as α increases. In general, if the total flow rate increases when the flow rate of the source gas MTS is fixed, the deposition rate can decrease because of source dilution. However, in this case, the amount of film deposition increased as α increased. This phenomenon can be explained in terms of the boundary layer thickness. In our previous study, the total deposition rate at 1200 C is controlled by the mass transfer rate. 1 Comparison of the thickness of each boundary layer in Figure S2e h shows that the thickness decreases as α increases, because the boundary layer thickness is inversely proportional to the flow rate factor. 2 Therefore, gas molecules travelling through the boundary layer to reach the surface arrive earlier when the boundary layer is thinner. Thus, the concentration of adatoms on the surface increases, which causes film growth to predominate instead of NW growth as α increases. However, the amount of deposition slightly decreased when α increased from 50 to 100. This result proves that the source dilution effect slowly becomes important after α = 50. In addition, the results obtained when α was controlled correlate well with the experimental results obtained when the total flow rate was controlled (Figure S3a-c): when the total flow rate is increased and the α ratio is fixed at 30, the density of NWs rapidly decreases and the deposited structure changes NWs to films. Therefore, it is evident that the main growth factor determining whether NW growth or film growth occurs is the boundary layer thickness rather than gas source dilution. 3

4 When the pressure conditions are controlled, the density of NWs abruptly increases when the pressure is increased, whereas the area coverage decreases, as shown in Figure S2i l. In particular, a large quantity of striation-patterned NWs was synthesized when the pressure was over 50 Torr. This tendency can also be explained by the changes in boundary layer thickness. If the total pressure in the reactor increases, the total flow rate spontaneously decreases, and the increased boundary layer thickness induces lower deposition rate conditions. Thus, NW growth is preferred. Finally, when the diluent gas was changed from H 2 to N 2 or Ar, the deposited structure completely changed from NWs to films (Figure S3d-f). Because H 2, N 2, and Ar do not prefer to bond with SiC, this sudden morphology change is necessarily related to the kinetic boundary layer thickness. Consequently, the experimental result shows that when the deposition temperature is fixed and the degree of source decomposition and surface diffusion are equal, catalyst-free growth NWs occurs under conditions with relatively large boundary layers. Figure S2. Typical SEM images of SiC NWs grown on a graphite substrate for 3 min with different deposition factors. Each controlled factor is varied from fixed default conditions defined as follows: 1200 C, α = 30, 5 Torr, MTS = 10 sccm, total flow rate = 300 sccm, and H 2 ambient. (a-d) Temperatures were increased from 1000, to 1100, to 1200, to 1300 C, respectively. (e-h) Input gas ratios (α) were increased from 5, to 30, to 50, to 100, respectively. (i-l) Total pressures were increased from 2.5, to 5, to 50, to 100 Torr, respectively. The white lines indicate 1 µm. 4

5 Figure S3. Typical SEM images of SiC NWs grown on a graphite substrate for 3 min with different deposition factors. Each controlled factor varied from fixed default conditions defined as follows: 1200 C, α = 30, 5 Torr, MTS = 10 sccm, total flow rate = 300 sccm, and H 2 ambient (a-c) Total flow rates were changed to 150, 300, and 600 sccm, respectively. (d-f) Diluent gases were chosen as H 2, N 2, and Ar, respectively. The white lines indicate 1 µm. Figure S4. Behavior of the density of NWs and area coverage of deposited films obtained under each condition. The data correspond to the conditions in (a) Figure S2a-d, (b) Figure S2e-h, (c) Figure S2i-l, and (d) Figure S3a-c. 5

6 Supplementary Information Figure S5. The boundary layer is a region close to the substrate surface where the gas flux, concentration of gas species, and temperature are different from those in the major gas stream. 3 The thickness of this boundary layer can be predicted based on the boundary layer model. 4 This model assumes that no gas flux exists on the substrate surface, which is defined as a stagnant layer (boundary layer), and that the deposition reaction occurs via atomic diffusion through this boundary layer. Here, the thickness of the boundary layer is given as follows: 5 where a is the proportionality constant, η is the viscosity of the gas, υ is the linear velocity of the gas, and ρ is the density of the gas. If deposition rate is determined by the mass transfer rate, the boundary layer thickness (δ) becomes a critical factor, since the total deposition rate depends on the rate at which reactants are transported to the substrate surface. In this study, the variation of the thickness of the boundary layers in the reactor was investigated for different diluent gases, gas flow rates, and temperatures. Figure S5 shows the calculated boundary layer thickness obtained when the gas flow rate and temperature are increased in each considered diluent gas (H 2, N 2, and Ar). The result in Figure S5a shows that the boundary layer thickness increases slightly as the deposition temperature increases. In addition, the boundary layer thickness in H 2 is two times larger than in N 2 or Ar. Therefore, at the same deposition temperature, the diluent gases of N 2 or Ar produce much shorter boundary layer thicknesses than does H 2, in which mass transfer is faster. 1,6 Thus a higher deposition rate is induced. Likewise, Figure S5b shows the boundary layer thickness as a function of the flow rate. The thickness decreases exponentially as the gas flow rate increases. Moreover, the boundary layer thickness is obviously short when N 2 or Ar is used instead of H 2 as the diluent gas. According to Eversteyn, the boundary layer thickness and mean velocity of the gas flow are related as follows: 7 Thus, as the velocity of the gas flow increases (V T ), the boundary layer thickness (δ) decreases. If the temperature and pressure used during deposition are fixed, the gas flow rate will increase and the boundary layer thickness will decrease as the amount of input gas increases. In addition, the gas densities of N 2 and Ar are higher than that of H 2 ; thus, the gas flow rate increases when they are used instead. Therefore, the boundary layer thicknesses are smaller when N 2 or Ar is used compared to when H 2 is used. Consequently, because of these differences between gas flow rates and diluent gas densities, the deposition rate of SiC NWs can be critically influenced by the boundary layer thickness. 6

7 Figure S5. Comparison of boundary layer thicknesses obtained with different diluent gases for different (a) deposition temperatures and (b) total flow rates. The practical deposition conditions used for the calculation are given at the top right of each figure. 7

8 Supplementary Information Figure S6. Figure S6 shows the branching NWs grown under the conditions of 1300 C temperature and with 7.5 sccm of MTS for 3 min. The result proves that if the NW growth is preferred over island nucleation because the surface energy of the CP-IBs region is much higher than the normal surface energy, continuous growth of NWs at the root region dominates instead of new island nucleation. This can be explained when surface diffusion is sufficient for adatoms on the surface to be transported toward the root of the NW such that they participate in the NW growth instead of forming a new island. This phenomenon is similar with Frank s explanation that at lower supersaturation stage, instead of few nucleation initiates, perfect seeds actually grow. 8 Thus, these branch-like NWs occur at higher temperatures under conditions wherein a relatively low amount of the source is provided. Figure S6. Typical SEM image of branched NWs grown at 1300 C with MTS = 7.5 sccm for 3 min. The critical island sizes and nucleation points for NW growth are indicated in the figure. 8

9 References (1) Y. Lee, D. Choi, J. Park, G. Hong, J. Mater. Sci., 2000, 5, (2) S. K. Ghandhi, R. J. Field, J. Cryst. Growth, 1984, 69, 619. (3) Bunshah, Handbook of deposition technologies for films and coatings 2nd Edit. Noyes Publications, 1994, 402. (4) J. L. Vossen, W. Kern, Thin Films Processes Ⅱ. Academic Press, 1992, 281. (5) A. S. Grove, Physics and Technology of Semiconductor Devices. John Wiley and Sons, Inc., 1967, chap. 1. (6) H. S, Kim, D. J. Choi, J. Am. Ceram. Soc., 1999, 82, 331. (7) F. C. Eversteyn, P. J. W. Severin, C. H. J. v. d. Brekel, H. L. Peek, J. Electrochem. Soc., 1970, 117, 925. (8) F. C Frank, Discuss. Faraday Soc., 1949, 5, 48. 9