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Introduction
Thin-film materials science is wafer-based and flux-driven. Up to now, most thin-film applications occur on devices built on semiconductor wafers. To process a microelectronic or opto-electronic device, the basic step consists of adding a monolayer of atoms on or subtracting it from a wafer surface. In these processes, we are not dealing with equilibrium states of materials; rather, we deal with kinetic states of a flux of atoms. Furthermore, for example, a p-n junction in a semiconductor is not at an equilibrium state. If we anneal the junction at a high temperature for a long time, it will disappear by interdiffusion of the p-type and n-type dopants. At device operation near room temperature, the dopants are supersaturated and frozen in place in the semiconductor to produce the electrical potentials, the built-in potentials, needed to guide the transport of charges. In doping a semiconductor, we need to diffuse or to implant a flux of atoms into the semiconductor to obtain the desired concentration profile of dopant. In device operation based on field effects, we pass an electric current or a flow of charge particles through the device to turn on or turn off the FETs. Thus, we consider flux-driven processes.
Generally speaking, we can have a flux or a flow of matter, a flow of energy (heat), or a flow of charge particles in a system. Indeed, in electronic devices, the operation can have all three kinds of flow coexist in the devices.