Vapor crystal growth

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

In most cases, the activator impurity must be incorporated during crystal growth. An appropriate amount of impurity element is dissolved in the molten Ge and, as crystal growth proceeds, enters the crystal at a concentration that depends on the magnitude of the distribution coefficient. For volatile impurities, eg, Zn, Cd, and Hg, special precautions must be taken to maintain a constant impurity concentration in the melt. Growth occurs either in a sealed tube to prevent escape of the impurity vapor or in a flow system in which loss caused by vaporization from the melt is replenished from an upstream reservoir. 

For crystal growth from the vapor phase, one better chooses the transition probability appropriate to the physical situation. The adsorption occurs ballistically with its rate dependent only on the chemical potential difference Aj.1, while the desorption rate contains all the information of local conformation on the surface [35,48]. As long as the system is close to equilibrium, the specific choice of the transition probability is not of crucial importance. 

We have so far assumed that the atoms deposited from the vapor phase or from dilute solution strike randomly and balHstically on the crystal surface. However, the material to be crystallized would normally be transported through another medium. Even if this is achieved by hydrodynamic convection, it must nevertheless overcome the last displacement for incorporation by a random diffusion process. Therefore, diffusion of material (as well as of heat) is the most important transport mechanism during crystal growth. An exception, to some extent, is molecular beam epitaxy (MBE) (see [3,12-14] and [15-19]) where the atoms may arrive non-thermalized at supersonic speeds on the crystal surface. But again, after their deposition, surface diffusion then comes into play. 

Balog, M., Schieber, M., Patai, S., andMichman, M., Thin Films of Metal Oxides on Silicon by Chemical Vapor Deposition with Organometallic Compounds, J. of Crystal Growth, 17 298-301 (1972)... 

XII. Antimony Of various new papers on the crystal growth of SbSI, the one by Ishikawa et al. (425) is worth mentioning, since it describes a new device especially designed to avoid formation of hollows in vapor-grown crystals. 

The gas-phase methods usually applied to the crystal growth of borides are two chemical vapor deposition (CVD) and chemical vapor transport (CVT). 

Chemical vapor deposition processes are complex. Chemical thermodynamics, mass transfer, reaction kinetics and crystal growth all play important roles. Equilibrium thermodynamic analysis is the first step in understanding any CVD process. Thermodynamic calculations are useful in predicting limiting deposition rates and condensed phases in the systems which can deposit under the limiting equilibrium state. These calculations are made for CVD of titanium - - and tantalum diborides, but in dynamic CVD systems equilibrium is rarely achieved and kinetic factors often govern the deposition rate behavior. 

Crystal Growth of Borides by Chemical Vapor Transport. 

We have seen that the deposition of crystals from the vapor is much too slow to model by MD techniques. Most laboratory equipment for producing thin films involves relatively slow crystal growth processes, and is not suitable for direct simulation. Information on the stability and properties of thin films can be obtained by similar modeling techniques, however. We describe below some of our results that provide necessary data to find the equilibrium configuration of thin films at low temperatures. 

If the material whose single crystal we want is volatile or sublimable, then we may choose a vapor-method of crystal growth. These methods have been used for a variety of crystals including ZnS and CdS. In this method, a carrier- gas is most often used for material transport and for the sulfides, H2S is the gas of choice. The following shows a simple apparatus ... 

E)ven though it is evacuated, capsules have been known to explode because the quartz (metal) walls could not contain the internal vapor pressure of the material being grown as single crystal. Care must be exercised not to handle the hot ciy>sule before and after crystal growth. 

In addition to sublimation or vaporization, we can also use chemical tremsport as a method of single crystal growth. For example, we could use either of the apparati of 6.12.1. or 6.12.4. to grow a crystal of ZnCl2 by the following reactions ... 

Vapor Methods Used for Single Crystal Growth 292... 

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