Growth process

Up to this point this text has focused primarily on materials themselves and not how to produce them. A major aspect of materials science is the control of the kinetic and thermodynamic conditions under which materials are produced to yield specific properties. This chapter and the ones that follow describe some of the ways semiconductor electronic materials are created as thin films. For comparison, the most popular method of production of bulk materials was covered in Chapter 4. Bulk wafers are useful as substrates but are impractical for many applications, especially where alloys are needed. In current technology, thin films constitute most of the active and passive layers that are used in electronic devices. 

This chapter covers the common features of all vapor phase thin fihn growth techniques - the processes by which atoms land on surfaces, move about, leave the surface, and how surface atoms go on to produce complete films. As with other chapters in this book, whole texts have been written on the subject so this treatment reviews only the highlights. Following chapters will cover specific classes of processes. Subjects of this chapter and include adsorption, desorption, surface structure and energy and how they are related to surface diffusion and the evolution of morphology, and adhesion. 

In many of the earlier chapters we have assumed the ability to create a desired series of thin layers on a thick substrate. Usually, when semiconducting, these layers are grown as high-quality single erystals beeause of the problems with defeets deseribed in Chapter 7. As one can tell from the diseussion of the devices in Chapter 3, the 

Solid phase growth is used where only structural phase transformations occur and long-range transport of atoms or removal of product species is not required. This is necessary because in solids atoms caimot move very far and the solid material must serve as its own atom source and sink. Solid phase growth is only applied significantly in current semiconductor processing for crystallization of amorphous materials or solid phase reactions, as in formation of silicides by reaction of a metal with silicon. 

With so many silicides to choose from one might then ask which forms first. An obvious possibility is that the most stable silicide (with the highest free energy of 

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Attached growth processes are capable of removing up to 90 percent of BOD and are thus less effective than suspended growth methods. 

With the development of multichaimel spectroscopic ellipsometry, it is possible now to use real-time spectroscopic ellipsometers, for example, to establish the optimum substrate temperature in a film growth process [44, 42]. 

The requirements of thin-film ferroelectrics are stoichiometry, phase formation, crystallization, and microstmctural development for the various device appHcations. As of this writing multimagnetron sputtering (MMS) (56), multiion beam-reactive sputter (MIBERS) deposition (57), uv-excimer laser ablation (58), and electron cyclotron resonance (ECR) plasma-assisted growth (59) are the latest ferroelectric thin-film growth processes to satisfy the requirements. 

J. C. Brice, Crystal Growth Processes, John Wiley Sons, Inc., New York, 1986. 

To confirm that the matrix is amorphous following primary solidification, isothermal dsc experiments can be performed. The character of the isothermal transformation kinetics makes it possible to distinguish a microcrystalline stmcture from an amorphous stmcture assuming that the rate of heat released, dH/dt in an exothermic transformation is proportional to the transformation rate, dxjdt where H is the enthalpy and x(t) is the transformed volume fraction at time t. If microcrystals do exist in a grain growth process, the isothermal calorimetric signal dUldt s proportional to, where ris... 

In an amorphous material, the aUoy, when heated to a constant isothermal temperature and maintained there, shows a dsc trace as in Figure 10 (74). This trace is not a characteristic of microcrystalline growth, but rather can be well described by an isothermal nucleation and growth process based on the Johnson-Mehl-Avrami (JMA) transformation theory (75). The transformed volume fraction at time /can be written as... 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

Molecular beam epitaxy (MBE) is a radically different growth process which utilizes a very high vacuum growth chamber and sources which are evaporated from controlled ovens (15,16). This technique is well suited to growing thin multilayer stmctures as a result of very low growth rates and the abihty to abmpdy switch source materials in the reactor chamber. The former has impeded the use of MBE for the growth of high volume LEDs. 

Fig. 14. Phase diagrams of HgCdTe used to defiae the Hquid-phase epitaxial growth process where composition is ia mole fractioa, X, and the numbers represent temperatures ia °C (a) Te-rich corner where the dotted Haes A—F correspoad to values of of 0.1, 0.2, 0.3, 0.5, 0.8, and 0.9, respectively, and (b) Hg-rich corner where A—F correspond to values of X of 0.9, 0.8, 0.6, 0.4, 0.2, and 0.1, respectively.

Fig. 15. Excess carrier concentration in HgCdTe in a saturated Hg vapor as a function of temperature where the dashed line represents Hg vacancies. The extrinsic impurity concentration can be adjusted in the growth process from low 10 up to mid-10. Low temperature annealing reduces Hg vacancy...

Nutritional Requirements. The nutrient requirements of mammalian cells are many, varied, and complex. In addition to typical metaboHc requirements such as sugars, amino acids (qv), vitamins (qv), and minerals, cells also need growth factors and other proteins. Some of the proteins are not consumed, but play a catalytic role in the cell growth process. Historically, fetal calf semm of 1—20 vol % of the medium has been used as a rich source of all these complex protein requirements. However, the composition of semm varies from lot to lot, introducing significant variabiUty in manufacture of products from the mammalian cells. 

Chain Polymerization The growth process of a polymer postulates a three-step mechanism ... 

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