Epitaxy Case

1. What is epitaxy?

The word epitaxy is derived from the greek words for surface and axis and relates to the atomic growth of a substance on to the surface of a substrate in a way that ensures the atoms are aligned in a perfect crystal lattice. Also, epitaxy can be defined as the process of depositing a thin layer of single crystal material over a single crystal substrate.

There are 2 types of epitaxy:

i) Homoepitaxy

- The film and the substrate are the same material.For instance, Si epitaxy layer on Si substrate. [1]Normally, epitaxial grown layers are purer than the substrate and can be doped independently of it.

ii) Heteroepitaxy

- The film and substrate are different materials. For example, AlGaAsepitaxy layer on GaAs substrate.[1]Heteroepitaxy allows for optoelectronic structures and band gap engineered devices.

Figure 1.1 shows a SEM micrograph of a GaN layer exhibiting columnar structure in the form of whisker-like nanocrystals, grown directly on Si(111) surface.

Figure 1.1 SEM micrograph of a GaN layer grown directly on Si(111) surface, exhibiting columnar structure in the form of whisker-like nanocrystals, [2]

2. Why epitaxial layer is important in semiconductor devices?

Modern devices require very sophisticated structure, which are composed of thin layers with various compositions. Quality, performance and lifetime of these devices are determined by the purity, structural perfection and homogeneity of the epitaxial layers.[3] For example, when a lightly doped crystalline layer is grown over a substrate that is heavily doped, then a higher breakdown voltage is achieved, and at the same time the collector resistance is kept low. This basically provides for a higher operating speed and improved bipolar performance. Also, silicon epitaxycan be used to produce JBTs and CMOS integrated circuits, as well as for the production of compound semiconductors. It has made it affordable to create high quality crystal growth, particularly for materials that are important technologically. With certain CMOS circuits, epitaxy helps to minimize the occurrence of latch-up. In addition, better control of the concentrations of doping on the devices can be gained. Also, the performance of RAM devices can be improved. Epitaxial layer improve a lot of semiconductor devices performance. This make it become so important in semiconductor industries.[4]

3. Briefly describe:

I) Three applications of epitaxial layers

1. Discrete and power devices

Normally, power discrete devices are built on an epitaxial wafer because it provides the device fab with a great controllability of series resistance (or conductivity) in the base of diodes, rectifiers, thyristors, or bipolar junction transistors (BJT), or in the drift zone of MOSFETs and IGBTs. Typically, thin base or drift is preferred, which is achievable and cost-effective using epi wafers. Additionally, an epi wafer's inherently uniform resistivity distribution, both from wafer to wafer and within the wafer, provides additional benefit. [5]Silicon epitaxy is done to improve the performance of discrete bipolar devices. By growing a lightly doped epitaxy layer over a heavily-doped silicon substrate, a higher breakdown voltage across the collector-substrate junction is achieved while maintaining low collector resistance. Lower collector resistance allows a higher operating speed with the same current.[6]

2. Integrated circuits (IC)

By using epitaxial substrates for IC designs allows them to have less of a problem with latch-up than when bulk wafer material is used as a starting point. In order to get a low noise performance, there must be an excellent amount of isolation between analog circuits. With the low resistivity in the substrate material,