Scheme 3.1 Reaction steps to form tubular SiC on Si wafer
3.4 Field Emission Property Studies
Field emission property of the tubes is evaluated.25 As shown in Figure 3.10A, a sample grown on Si(100) at 873 K followed by heat treatment at 1273 K shows an excellent emission property Eto (turn-on field) 2.8 V/μm and Eth (threshold field) exceeding 16 V/μm. Another sample, deposited at 923 K followed by heat treatment at 1273 K, showed an Eto 2.5 V/μm and Eth less than 7 V/μm in Figure 3.10B. Clearly, the tubes grown at 923 K performed better than the ones grown at 873K, even though they were heat-treated at the same temperature of 1273 K. For both samples, the Fowler-Nordheim plots of the curves showed linear sections above Eto. This suggests that their field emission mechanism is conventional.26 It is known that the field enhancement factor, β, is strongly dependent on the sample geometry.27-28 Generally, samples with high one dimensional aspect ratio structures showed low turn on values.29,30 In this study, employing a work function (Φ) value of 4.4 eV for SiC and slopes of the F-N plots in the insets in Figure 3.8, the β values are estimated to be 1800 and 2900 for the samples grown at 873 K and 923 K, respectively.31 The reason why that the tubes grown at 923 K performed better, emitting higher current at lower field, is attributed to the higher content of crystallized β-SiC in the product. As far as we know, the results discussed above are the first field emission property data for SiC tubes. The data are much lower than most of the reported data of other SiC nanostructures.29-32
Figure 3.10 Field emission J-E curve and Fowler-Nordheim (FN) plot (inset). (A) a tubular SiC material on Si(100) at 873 K and heat treated at 1273 K. (B) a tubular SiC material on Si(100) at 923 K and heat treated at 1273 K.
3.5 Conclusions
In this study, we have synthesized SiC tubes via a VSRG pathway employing the vapor of MeSiHCl2 to react with Ca deposited on Si. The reaction is a solvent-free Yajima-type process that takes place at the vapor-solid interface. For the reaction, the products CaCl2 and SiCxHy phase-segregate and undergo transformation into a cable-like radial heterostructure.
After heat treatment, which converts the preceramic SiCxHy shell material into SiC and removes the CaCl2 core, the layer of SiC tubes is fabricated on Si. From the tubes, emission of
electrons with a current 10 μA/cm2 can be obtained at an applied field as low as 2.5 V/μm.
We suggest that the high performance is not only due to the high aspect ratio of the one dimensional tubular morphology but also to the large field enhancing factor β. This excellent result indicates that the SiC tubes may have promising field-emitting applications for vacuum microelectronic devices.
3.6 References
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3.7 Appendix
Composition Analysis
We assume that the samples may contain a-C, Si, a-SiC, and β-SiC. In the curve (A) in Figure 3.9, we observe a maximum 5% weight loss at 873 K. It is proposed that this is due to the oxidation of C atoms in a-C. From 873 K to 1273 K, the weight increase is 8%. Then, the weight increases 2% at 1273 K. From 1273 K to 425 K, the weight increases 3%. From 873 K to 1273 K then to 425 K, the weight increases 13 % in total. We attribute this to the oxidation of Si atoms in a-SiC and Si to form SiO2. The final weight is 108% of the original one. The weight loss at 873 K is assumed to be all contributed by the oxidation of C. The assumptions, equations, and results are shown below.
1. The weight % of -SiC is x. Assume it is resistant to oxidation and the weight % does not change.
2. The weight % of a-SiC is y. Assume it can be fully oxidized to form SiO2 and CO2. The weight % after the oxidation is [M.W.(SiO2)/ M.W.(a-SiC)]*y, (60/40)y = 1.5y.
3. The weight % of Si is z. Assume it can be fully oxidized to form SiO2. The weight % after the oxidation is [M.W.(SiO2)/ A.W.(Si)]*z, (60/28)z
4. The weight % of a-C is w, which is the weight loss at 873 K. Assuming it can be fully oxidized to form CO2, the weight % after the oxidation reduces to 0.
Two equations can be established for the initial and final weight %.
Initial weight %: x + y + z + w = 100
Final weight %: x + 1.5 y + (60/28)z + 0 = Total final weight % found in TGA For (A), w is 5. The total final weight % found in TGA is 108.
There are 3 variables to be solved but we can set up only 2 equations. Thus, we use the trial and error method to derive possible compositions of (A). Possible compositions should offer between 7 and 8 weight % increase relative to the initial weight %.
Possible compositions of (A):
Trial 1 β-SiC a-SiC Si a-C Total
estimated initial weight % 84 0 11 5 100
weight % after oxidation 84 0 23.6 0 107.6
estimated atomic % 72 0 14 14 100
Trial 2 β-SiC a-SiC Si a-C Total
estimated initial weight % 83 2 10 5 100
weight % after oxidation 83 3 21.4 0 107.4
estimated atomic % 72 2 12 14 100
Trial 3 β-SiC a-SiC Si a-C Total
estimated initial weight % 79 8 8 5 100
weight % after oxidation 79 12 17.1 0 108.1
estimated atomic % 69 7 10 14 100
In the curve (B), we observe 7% weight loss at 873 K for the oxidation of a-C. From 873 K to 1273 K then to 425 K, the weight increase is 20 % for the oxidation of a-SiC and/or Si to form SiO2. The final weight is 113 % of the original one. The trial and error method is used again to derive the possible compositions of (B) offering an overall 13 – 14 weight % increase.
Since the product was prepared at low temperature, we assume that the a-SiC content should be high while the Si content is low. Thus, even though Trial 4 can offer satisfactory composition estimation, the result is unlikely because the amount of a-SiC in the trial is assumed to be 0.
Possible compositions of (B):
Trial 1 β-SiC a-SiC Si a-C Total
estimated initial weight % 53 40 0 7 100
weight % after oxidation 53 60 0 0 113
estimated atomic % 46 34 0 20 100
Trial 2 β-SiC a-SiC Si a-C Total
estimated initial weight % 58 30 5 7 100
weight % after oxidation 58 45 10.7 0 113.7
estimated atomic % 49 25 6 20 100
Trial 3 β-SiC a-SiC Si a-C Total
estimated initial weight % 71 8 14 7 100
weight % after oxidation 71 12 30 0 113
estimated atomic % 58 7 16 19 100
Trial 4 β-SiC a-SiC Si a-C Total
estimated initial weight % 75 0 18 7 100
weight % after oxidation 75 0 38.6 0 113.6
estimated atomic % 60 0 21 19 100
Chapter 4
Synthesis of Pseudo Thin Plate Silicon Carbide Nanostructure- Another Case of Yajima-Type Reaction at the Vapor-Solid Interface
4.1 Introduction
In Chapters 2 and 3, we fabricated cubical nanocages and tubular one-dimensional SiC nanostructures via VSRG methods. NaCl and CaCl2 acted as in-situ generated templates to assist the SiC morphology formation.1-2 Here, we present another example showing in-situ generated MgCl2 can act as a template to assit the formation of large area pseudo thin plate SiC nanostructure by employing vapors of organochlorosilanes, MeSiHCl2 and Me2SiCl2 to react with Mg metal at relatively low temperatures.
4.2 Experimental Section
A hot-wall reactor composed of a Lindberg HTF55122A tube furnace and a 27 mm diameter quartz tube was used. RF power (13.56 MHz) was applied to the reactor through two pieces of copper foil electrodes outside the furnace. Mg powder (Showa 99 %, 0.30 g, 12.5 mmole) in a 10 cm quartz boat was placed in the plasma region. The powder was treated with hydrogen plasma (10 W, H2 20 sccm) under the assistance of a constant flow of H2 20 sccm at room temperature for 1 h to remove surface oxide. Then, under 1 atm of Ar, the treated powders were pushed into the reactor center. MeHSiCl2 was vaporized at 255 K under the assistance of a constant flow of Ar (1 - 2 sccm) into the reactor to react with Mg metal at 823 K 1 atm for 14 h. Yellow powder was obtained after heat treatment at 1273 K for 1 h. In the other case, black powder was obtained from Me2SiCl2 reacting with Mg powder at 923 K for 14 h and then heat treated at 1273 K for 1 h.
SEM and EDX data were collected using a JEOL JSM-6330F at 15 kV. TEM and ED images were obtained on a JEOL JEM-2010 at 200 kV. HRTEM images were acquired on a JEOL JEM-4000EX at 400 kV. XRD studies were carried out using a BRUKER AXS D8 ADVANCE diffractometer using Cu Kα1 radiation. FT-IR spectra were collected using a Perkin Elmer Spectrum One.