New Mechanism Used to Grow Crystals Vapor-liquid-solid mechanism can be applied to most crystalline substances
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A new mechanism of crystal growth has been discovered by metallurgists at Bell Telephone Laboratories, Mur ray Hill, N.J. (C&EN, March 9, page 3 9 ) . Called the VLS mechanism—for vapor-liquid-solid—it gives nearly perfect crystals, according to Bell's Dr. Richard S. Wagner and Dr. William C. Ellis. Crystal growth via the VLS mech anism takes place when a droplet, consisting of a saturated solution of a crystalline material, plus an impurity, receives atoms from a vapor and de posits these atoms at the interface between the droplet and a crystalline substrate. The droplet possibly serves as a catalyst for the reaction, the Bell workers say. The mechanism [AppL Phijs. Letters, 4, 89 (1964)] can be applied to most crystalline substances, they add. It can be used to grow a variety of crystal forms, ranging from whiskers to epitaxial layers. The nearly perfect crystals, accord ing to Dr. Wagner and Dr. Ellis, can be grown at considerably lower tem peratures than those needed when crystals are made by the vapor-phase crystal growth process, a method cur rently in use. For example, the VLS
technique is being used for silicon at about 750° C , compared to 1200° C. for conventional vapor-phase growth. Rate of crystal growth is about the same for both techniques. Filamentary Crystals. The VLS mechanism stems from studies of the growth of "whiskers." These are hair like filaments (about 1 / 8 millionthinch thick) that grow on some metals. For example, tin plating produces whiskers (gold plating prevents them). Whiskers also grow on zinc. Research on whisker crystals started during the late 1940's when engineers found that filamentary growths on metal surfaces of some electronic equipment were causing short circuits. To grow whiskers or rod-like crys tals of silicon by the new mechanism, small particles (filings) of liquidforming impurities such as gold (rang ing in size from 10 microns to a milli meter) are placed on a silicon sub strate crystal. The substrate crystal is heated in a furnace to form droplets of a gold-silicon alloy saturated with sili con. A vapor of silicon tetrachloride and hydrogen (which can react to form elemental silicon) is then passed over the droplets. The droplets re-
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For your free copy of Bulletin FM A, write to:
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European ReDresentative: Etablissements A. Jowa, Brussels, Belgium
48
C&EN
MAR.
16,
1964
VLS MECHANISM. The vapor-liquid-solid mechanism of crystal growth repre sented here occurs when a droplet of a saturated solution of a crystalline material and an impurity (gold in this case) receives atoms from a vapor and deposits them at the interface between the droplet and a crystalline substrate
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ceive silicon atoms from the vapor; they possibly act as catalysts, too. The entering silicon atoms super saturate the liquid, causing silicon and minute (about 10ir> atoms per cubic centimeter) quantities of gold to solid ify—"freeze out"—on the crystal sub strate. As the process continues, the alloy droplet is displaced from the substrate crystal and rides on top of the growing whisker. The deliberate making of the droplet containing an impurity is the main difference be tween the VLS technique and the vapor-phase crystal growth process. When they started their research, Dr. Wagner and Dr. Ellis found that screw dislocations weren't present in their silicon whiskers. They also knew that impurities played some role. This role was clarified when they found that a liquid droplet containing an impurity existed at the top of a growing whisker. The conclusion reached by the metallurgists is that VLS growth depends on the presence of a critical impurity, but does not need a dislocation or other crystal imperfection.
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