Original Ruasen l.l Copyrigh: 9 2003 i ebo/sin, Shrchetinited / rom Neorgonicheskie Maternal). loz. 32, No. 9,2003. pp. Role of surface energy in the vapor-Liquid-Solid growth of silicon v. A Nebolsin and A.A. shchetinin Voronezh State Technical University, Moskovskii pr: 14, Voronezh, 394026 Russia Received December 3. 2002 Abstract-The conditions of vapor-phase Si whisker growth are examined, and the role of the surface Gibbs nergy in the vapor-liquid-solid proc s evaluated The mechanism responsible for the catalytic activity of the driving force acting on the three-phase line orcoted. Experimental surface tension data are used to estimate contact upon a displacement of the liquid droplet in the course of whisker growth INTRODUCTION ture. no nutrient was fed to the reaction zone. so that The unique properties of silicon whiskers are due to whisker growth was stopped. The process was restarted the vapor-liquid-solid (VLS) growth mechanism-the only after the temperature was fully stabilized at a new only mechanism involving more than two phases in level equilibrium with one another According to the existing concepts [2], a key feature RESULTS AND DISCUSSION of the VLS mechanism is the catalytic activity of the liquid phase, that is, the large accommodation coeffi When the substrate is heated to 1300 K and the cient of atoms adsorbed on the melt surface and the nutrient gas mixture is fed to the reactor, the solid-liq- reduced activation energy for nucleation at the crystal- uid interface beneath the Si-metal melt droplet melt interface. Unfortunately, the processes underlying becomes the growth front, the volume of the etch pit this "physical catalysis" are not yet fully understood. decreases, the liquid droplet rises above the substrate The mechanisms of Si whisker growth and sha surface, its shape changes, and the wetting perimeter were considered in [3, 4] from the viewpoint of the decreases. These processes are associated with the for- thermodynamic equilibrium of a liquid droplet on the mation of interfacial regions parallel to the ill) sub- tip of the growing whisker, which can be characterized strate surface and roof-shaped protrusions at the wet whisker radius during unsteady-state growth, the for- ing crystal. The rise of the droplet over the substrate mation of a whisker pedestal, and the stability of cylin- surface is accompanied by a reduction in the area of the drical crystal growth. At the same time, a number of liquid-solid interface. As a result, the diameter of the effects could not be understood in the framework of this growing crystal decreases, and the crystal takes a coni- approach. A question of major importance is the origin cal shape. The resulting monohedral solidification front of unstable whisker growth The objective of this work was to gain detailed ng polished sections of whiskers and is also evidenced insight into the role of interfacial energy in the filamen by the sharp boundaries of impurity bands resulting from tary growth of silicon in VLS systems special vapor-phase doping of whiskers [6] In addition to the flatness of the (111 growth front, EXPERIMENTAL which points to the layer-by-layer growth mechanism the following features of experimental data warrant Silicon whiskers were grown on ( 111) single-crys- attention tal substrates in a standard chloride-hydrogen system as described in [2], using gold, copper, and other metal 1. During cylindrical crystal growth by the VLs articles as growth leaders. To examine the morphol- mechanism, Si whiskers typically have a circular cross ogy of etch pits and determine the position of the solid- section owing to the circular wetting perimeter. Lateral liquid interface at different stages of whisker growth, mechanism [4] faceting develops in later stages, by the vapor-solid we used polished axial sections of the whiskers and substrates. The axial growth rate was determined by the 2. It follows from the morphology of crystals having time marker" method [2]. During changes in tempera- a circular cross section that the liquid droplet rests on 0020-1685/03/3909-0899$2500C 2003 MAIK"Nauka/Interperiodica0020-1685/03/3909- $25.00 © 2003 0899 MAIK “Nauka/Interperiodica” Inorganic Materials, Vol. 39, No. 9, 2003, pp. 899–903. Translated from Neorganicheskie Materialy, Vol. 39, No. 9, 2003, pp. 1050–1055. Original Russian Text Copyright © 2003 by Nebol’sin, Shchetinin. INTRODUCTION The unique properties of silicon whiskers are due to the vapor–liquid–solid (VLS) growth mechanism—the only mechanism involving more than two phases in equilibrium with one another [1]. According to the existing concepts [2], a key feature of the VLS mechanism is the catalytic activity of the liquid phase, that is, the large accommodation coeffi- cient of atoms adsorbed on the melt surface and the reduced activation energy for nucleation at the crystal– melt interface. Unfortunately, the processes underlying this “physical catalysis” are not yet fully understood. The mechanisms of Si whisker growth and shaping were considered in [3, 4] from the viewpoint of the thermodynamic equilibrium of a liquid droplet on the tip of the growing whisker, which can be characterized by the growth angle. This approach made it possible to account for many effects, such as the variation in the whisker radius during unsteady-state growth, the for￾mation of a whisker pedestal, and the stability of cylin￾drical crystal growth. At the same time, a number of effects could not be understood in the framework of this approach. A question of major importance is the origin of unstable whisker growth. The objective of this work was to gain detailed insight into the role of interfacial energy in the filamen￾tary growth of silicon in VLS systems. EXPERIMENTAL Silicon whiskers were grown on {111} single-crys￾tal substrates in a standard chloride–hydrogen system as described in [2], using gold, copper, and other metal particles as growth leaders. To examine the morphol￾ogy of etch pits and determine the position of the solid– liquid interface at different stages of whisker growth, we used polished axial sections of the whiskers and substrates. The axial growth rate was determined by the “time marker” method [2]. During changes in tempera￾ture, no nutrient was fed to the reaction zone, so that whisker growth was stopped. The process was restarted only after the temperature was fully stabilized at a new level. RESULTS AND DISCUSSION When the substrate is heated to 1300 K and the nutrient gas mixture is fed to the reactor, the solid–liq￾uid interface beneath the Si–metal melt droplet becomes the growth front, the volume of the etch pit decreases, the liquid droplet rises above the substrate surface, its shape changes, and the wetting perimeter decreases. These processes are associated with the for￾mation of interfacial regions parallel to the {111} sub￾strate surface and roof-shaped protrusions at the wet￾ting perimeter, which rapidly merge into a continuous flat front [5]. This leads to melt entrapment in the grow￾ing crystal. The rise of the droplet over the substrate surface is accompanied by a reduction in the area of the liquid–solid interface. As a result, the diameter of the growing crystal decreases, and the crystal takes a coni￾cal shape. The resulting monohedral solidification front is very flat, which can be established by directly examin￾ing polished sections of whiskers and is also evidenced by the sharp boundaries of impurity bands resulting from special vapor-phase doping of whiskers [6]. In addition to the flatness of the {111} growth front, which points to the layer-by-layer growth mechanism, the following features of experimental data warrant attention: 1. During cylindrical crystal growth by the VLS mechanism, Si whiskers typically have a circular cross section owing to the circular wetting perimeter. Lateral faceting develops in later stages, by the vapor–solid mechanism [4]. 2. It follows from the morphology of crystals having a circular cross section that the liquid droplet rests on Role of Surface Energy in the Vapor–Liquid–Solid Growth of Silicon V. A. Nebol’sin and A. A. Shchetinin Voronezh State Technical University, Moskovskii pr. 14, Voronezh, 394026 Russia Received December 3, 2002 Abstract—The conditions of vapor-phase Si whisker growth are examined, and the role of the surface Gibbs energy in the vapor–liquid–solid process is evaluated. The mechanism responsible for the catalytic activity of the liquid phase on the tip of Si whiskers is elucidated. Experimental surface tension data are used to estimate the driving force acting on the three-phase line of contact upon a displacement of the liquid droplet in the course of whisker growth