Review of Semiconductor Physics Solidstate physics The daunting

Review of Semiconductor Physics Solid-state physics The daunting task of solid state physics • Quantum mechanics gives us the fundamental equation • The equations are only analytically solvable for a handful of special cases • One cannot solve the equations for more than two bodies! • Solid-state physics is about many-body problems There are 5 × 1022 atoms/cm 3 in Si Si atom: 1 s 22 p 63 s 23 p 2 Core: Nueclear + 1 s 22 p 6, Valence electrons: 3 s 23 p 2 We’ll come back to this later Each particle is in the potential of all the other particles, which depends on their positions, which must be solved from the equation… You have an equation with ~1023 unknowns to solve. Mission impossible! • Solid state physic is all about approximations.

Review of Semiconductor Physics Crystal structures If we assume the atomic cores have known and fixed positions, we only need to solve the equations for the valence electrons. Life much easier! Static lattice approximation • Justification • Related/similar approximation: Born-Oppenheimer Crystal structures If you shine X-ray on a piece of solid, very likely you’ll have a diffraction pattern. Remember Bragg? That means periodicity in the structure.

Review of Semiconductor Physics Crystal structures Bravais Lattices A mathematical concept: • No boundary or surface • No real (physical) thing – just points, hence no defects • No motion Unit cells (or primitive unit cells) -- The smallest unit that repeats itself. Fig. 4. 1

Fig. 4. 2 Crystal structure = lattice + basis Honeycomb Simple cubic From Geim & Mc. Donald, Phys Today Aug 2007, 35.

Lattices Conventional & primitive unit cells BCC How many atoms in the conventional unit cell? BCC & FCC are Bravais Lattices. FCC

U. K. Mishra & J. Singh, Semiconductor Device Physics and Design E-book available on line thru UT Lib. Fast production of e-books. The caption is NOT for this figure. Try not to be confused when reading fast generated books/papers nowadays.

Bragg refraction and the reciprocal lattice • Bragg refraction • Definition of the reciprocal lattice • 1 D, 2 D, and 3 D The 1 D & 2 D situations are not just mathematical practice or fun, they can be real in this nano age…

• BCC & FCC are reciprocal lattices of each other 4 4

• Miller indices Referring to the origin of the reciprocal lattice’s definition, i. e, Bragg refraction, a reciprocal lattice vector G actually represents a plane in the real space z y x (100) (200) Easier way to get the indices: Reciprocals of the intercepts

• Wigner-Seitz primitive unit cell and first Brillouin zone The Wigner–Seitz cell around a lattice point is defined as the locus of points in space that are closer to that lattice point than to any of the other lattice points. The cell may be chosen by first picking a lattice point. Then, lines are drawn to all nearby (closest) lattice points. At the midpoint of each line, another line (or a plane, in 3 D) is drawn normal to each of the first set of lines. 1 D case 2 D case 3 D case: BCC

The first Brillouin zone is the Wigner-Seitz cell of the reciprocal lattice 1 D 2 D

3 D: Recall that the reciprocal lattice of FCC is BCC. 4 4 4 /a Why is FCC so important? 4

Why is FCC so important? It’s the lattice of Si and many III-V semiconductors. Si: diamond, a = 5. 4 Å Ga. As: zincblende Crystal structure = lattice + basis Modern VLSI technology uses the (100) surface of Si. Which plane is (100)? Which is (111)?