Materials Chemistry and Physics 56(1998)256-262 ics of VLS-grown whiskers, as affected by the growth the present study. a heating rate of about 10.C/min was conditions, can be accomplished by proper selection of the employed to reach the desired temperature and a total pressure chemical vapor deposition(CVD)parameters and liquid- of 100 torr was chosen for each deposition run. After an forming agents. In particular, the liquid solution, in its usual appropriate duration of the vapor-phase reaction, the depos form of a droplet on the top of the growing whisker, may ited Sic whiskers were then examined by seM to determine play a decisive role in the determination of the whisker their nucleation and growth characteristics The whisker diameter, being one of the important whisker characteristics, has been studied to some extent. It was gen- 3. Results and discussion erally reported in the literature that the diameter of whiskers was determined by the size or the diameter of liquid droplets The presence of solidified alloy droplets on whisker tips [3, 6-10] during VLS whisker growth. However, it is com- when they are cooled from the deposition temperature is monly found that the diameter of whiskers is variable during commonly regarded as evidence for the successful operation vapor-phase growth, particularly at the root of the whisker of the VLS mechanism. As discussed in our previous report where growth starts. The conditions at which the proportion- [12] and in the SEM micrograph shown later, the VLS mech- ality between the whisker diameter and that of the anism was responsible for Sic whisker growth in the present droplet prevails and the underlying factors that control the Ni-activated CVD experiment diameter of whiskers should be clearly elucidated among the The deposition of VLS-grown whiskers proceeds by the various process parameters involved precipitation of crystalline materials at the solid-liquid inter- In this study the factors that determine the diameter of face of the supersaturated liquid droplet. The area of the First, an estimation regarding the variation of liquida ated. interface for precipitation will then become the cross section liquid droplet on the solid substrate or crystalline Sic is deter- tionship between liquid droplet and whisker will be given. mined by two major factors, i.e., the volume of the liquid Then the validity of the estimated value and the effect of droplet and the interfacial equilibrium at the VLS triple-phase vapor-phase deposition on the thickening of the whisker will Inction. For two liquids having the same volume but with be experimentally verified. Growth of Sic whiskers was car- different wetting characteristics on a substrate, the liquid that ried out by thermal decomposition of methyltrichlorosilane wets the substrate better will cover the substrate with a larger (MTS)in a hot-wall reactor in the temperature range 1100 area. This phenomenon can be explained in terms of Fig. I to 1300"C. Elemental nickel as a liquid-forming agent for in which the cross sections taken through the center of the roplet formation was chosen to activate whisker growth by spherical liquid droplets(or cap-shaped droplets)are shown the VLs mechanism. The deposited whiskers were examined with respective areas covered being expressed by the diam- by scanning electron microscopy(SEM) and their growth eters of circular areas d, and d2. If liquid A and liquid B are characteristics were determined of the same volume, but the former wets the substrate better than the latter, a smaller liquid/ solid contact angle would be obtained for liquid A(a<B)and a correspondingly larger 2. Experimental area of the substrate would be covered, i. e, d, is larger than d2. Similar reasoning can be applied to liquid droplets of Growth of Sic whiskers by CVD was conducted in an different volumes with the same wetting behavior on the extemally heated reactor using Sic heating elements. A substrate, as represented by liquids a and C in Fig.1.The detailed description of the apparatus and related specifica tions can be found in a previous report [11]. High-density tropic graphite plates with dimensions of 20 mm X20 nm x2 mm were used as substrates. Elemental Ni as the liquid B liquid-forming agent for whisker growth was applied to the liquid A graphite substrates by electroplating with a coating thickness of 2.5 um. The source reactant MTS was fed into the reactor by bubbling the mts saturator held at oC with purified hydrogen. Hydrogen was also used as the main gas to adjust the reactants appropriately to the desired concentration. To evaluate the thickening kinetics of whiskers, CVD of Sic whiskers was performed in the temperature interval 1100 to 1300@C for constructing the arrhenius plot of the growth rate in the radial direction. The flow rates of H2 and MTS vapor Fig. 1 Schematic cross sections taken were metered at 1360 and 3. 4 sccm(standard cubic centi- with different wetting characteristics vS liquid B, d,>d2) and meter per minute), respectively, for the various CVD runs in with different volumes(liquid A vs liquid C, d, >d3)I.-C, Leu et al, /Materials Chemist©' and Physics 56 (1998) 256-26] 257 istics of VLS-grown whiskers, as affected by the growth conditions, can be accomplished by proper selection of the chemical vapor deposition (CVD) parameters and liquid￾forming agents. In particular, the liquid solution, in its usual form of a droplet on the top of the growing whisker, may play a decisive role in the determination of the whisker characteristics. The whisker diameter, being one of the important whisker characteristics, has been studied to some extent. It was gen￾erally reported in the literature that the diameter of whiskers was determined by the size or the diameter of liquid droplets [3,6-10] during VLS whisker growth. However, it is com￾monly found that the diameter of whiskers is variable during vapor-phase growth, particularly at the root of the whisker where growth starts. The conditions at which the proportion￾afity between the whisker diameter and that of the liquid droplet prevails and the underlying factors that control the diameter of whiskers should be clearly elucidated among the various process parameters involved. In this study the factors that determine the diameter of VLS-grown SiC whiskers will be thoroughly investigated. First, an estimation regarding the variation of liquid droplet diameter at different growth stages and the dimensional rela￾tionship between liquid droplet and whisker will be given. Then the validity of the estimated value and the effect of vapor-phase deposition on the thickening of the whisker will be experimentally verified. Growth of SiC whiskers was car￾ried out by thermal decomposition of methyltrichlorosilane (MTS) in a hot-wall reactor in the temperature range 1100 to 1300°C. Elemental nickel as a liquid-forming agent for droplet formation was chosen to activate whisker growth by the VLS mechanism. The deposited whiskers were examined by scanning electron microscopy (SEM) and their growth characteristics were determined. 2. Experimental Growth of SiC whiskers by CVD was conducted in an externally heated reactor using SiC heating elements. A detailed description of the apparatus and related specifica￾tions can be found in a previous report [ 11 ]. High-density isotropic graphite plates with dimensions of 20 ram× 20 mm× 2 mm were used as substrates. Elemental Ni as the liquid-forming agent for whisker growth was applied to the graphite substrates by electroplating with a coating thickness of 2.5 ~xm. The source reactant MTS was fed into the reactor by bubbling the MTS saturator held at 0°C with purified hydrogen. Hydrogen was also used as the main gas to adjust the reactants appropriately to the desired concentration. To evaluate the thickening kinetics of whiskers, CVD of SiC whiskers was performed in the temperature interval 1100 to 1300°C for constructing the Arrhenius plot of the growth rate in the radial direction. The flow rates of H2 and MTS vapor were metered at 1360 and 3.4 sccm (standard cubic centi￾meter per minute), respectively, for the various CVD runs in the present study. A heating rate of about 10°C/rain was employed to reach the desired temperature and a total pressure of 100 torr was chosen for each deposition run. After an appropriate duration of the vapor-phase reaction, the depos￾ited SiC whiskers were then examined by SEM to determine their nucleation and growth characteristics. 3. Results and discussion The presence of solidified alloy droplets on whisker tips when they are cooled from the deposition temperature is commonly regarded as evidence for the successful operation of the VLS mechanism. As discussed in our previous report [ 12] and in the SEM micrograph shown later, the VLS mech￾anism was responsible for SiC whisker growth in the present Ni-activated CVD experiment. The deposition of VLS-grown whiskers proceeds by the precipitation of crystalline materials at the solid-liquid inter￾face of the supersaturated liquid droplet. The area of the interface for precipitation will then become the cross section of the accompanying VLS whisker. The contact area of the liquid droplet on the solid substrate or crystalline SiC is deter￾mined by two major factors, i.e., the volume of the liquid droplet and the inteffacial equilibrium at the VLS triple-phase junction. For two liquids having the same volume but with different wetting characteristics on a substrate, the liquid that wets the substrate better will cover the substrate with a larger area. This phenomenon can be explained in terms of Fig. 1 in which the cross sections taken through the center of the spherical liquid droplets (or cap-shaped droplets) are shown with respective areas covered being expressed by the diam￾eters of circular areas dl and d> If liquid A and liquid B are of the same volume, but the former wets the substrate better than the latter, a smaller liquid/solid contact angle would be obtained for liquid A ( a </3) and a correspondingly larger area of the substrate would be covered, i.e., dl is larger than d2. Similar reasoning can be applied to liquid droplets of different volumes with the same wetting behavior on the substrate, as represented by liquids A and C in Fig. 1. The I' L I bs ato Fig. i. Schematic cross sections taken through the center of liquid droplets with different wetting characteristics (liquid A vs. liquid B, d1 >d2) and with different volumes (liquid A vs. liquid C, dl> d3)