## Semiconductor crystals ### Crystal structures Silicon, Germanium have diamond structure; while II-V semiconductors typically have zinc blende structure. Zinc blende structure is just replacing two 4 with one another atoms. See image below. ![[Drawing 2024-09-13 02.21.28.excalidraw.svg|300]] Source: https://fr.wikipedia.org/wiki/Structure_blende#/media/Fichier:Sphalerite-unit-cell-depth-fade-3D-balls.png People also use [Miller index](https://en.wikipedia.org/wiki/Miller_index) to describe lattice planes and directions, get familiar with these notations. ### Fabrication of crystals and wafers General procedures include: reduction of (Si, Ge) → chemical purification → production of single crystal ingot. ([CZ](https://en.wikipedia.org/wiki/Czochralski_method), zone melting) → Grinding, slicing, polishing (surface typically $(100)$, $(111)$ directions). For GaAs, it requires pure materials (Ga, As) compounding at high temp & pressure. And liquid-encapsulated CZ method is used to eliminate volatile anionic vapor (i.e., As) by coating with $\rm B_{2}O_{3}$. GaAs gets lower purity compared to Si. Impurity includes: Si, Cr, C, typically from graphite heater and silicon crucible. If $\rm B_{2}O_{3}$​ crucible gets applied, so-called undoped GaAs can be produced. Then it goes through grinding, slicing and polishing to get final product. ### Layer by layer growth (MBE) [Molecular beam epitaxy](https://en.wikipedia.org/wiki/Molecular-beam_epitaxy), operating in UHV, may achieve single layer growth of material. Typical growth temperature 500–600°C. In-situ observation may be done by [RHEED (reflected high energy electron diffraction)](https://en.wikipedia.org/wiki/Reflection_high-energy_electron_diffraction). To improve quality, one may grow a thick layer of some material, like GaAs; or repeatedly grow monolayers, as buffer layers. Other growing methods include vapor phase epitaxy (VPE), [MOCVD (metal-organic)](https://en.wikipedia.org/wiki/Metalorganic_vapour-phase_epitaxy), liquid phase epitaxy (LPE). >[!Info] >See more about epitaxy on wiki: https://en.wikipedia.org/wiki/Epitaxy and related book chapters.