Polytetrafluoroethylene Metals and insulators Measured resistivities range over more than 30 orders of magnitude Material Resistivity (Ωm) (295K) Resistivity (Ωm) (4K) 10-12 “Pure”Metals Copper 10-5   Semi-Conductors Ge (pure) 5  102 1012 Insulators Diamond 1014 Polytetrafluoroethylene (P.T.F.E) 1020 Potassium 2  10-6 10-10

Metals, insulators & semiconductors? 1020- At low temperatures all materials are insulators or metals. Diamond 1010- Resistivity (Ωm) Germanium Pure metals: resistivity increases rapidly with increasing temperature. 100 - Copper 10-10- 100 200 300 Temperature (K) Semiconductors: resistivity decreases rapidly with increasing temperature. Semiconductors have resistivities intermediate between metals and insulators at room temperature.

Bound States in atoms V(r) r Electrons in isolated atoms occupy discrete allowed energy levels E0, E1, E2 etc. . The potential energy of an electron a distance r from a positively charge nucleus of charge q is V(r) E2 E1 E0 r Increasing Binding Energy

Bound and “free” states in solids The 1D potential energy of an electron due to an array of nuclei of charge q separated by a distance R is Where n = 0, +/-1, +/-2 etc. This is shown as the black line in the figure. V(r) E2 E1 E0 V(r) Solid V(r) lower in solid (work function). r + R Nuclear positions

Energy Levels and Bands In solids the electron states of tightly bound (high binding energy) electrons are very similar to those of the isolated atoms. Lower binding electron states become bands of allowed states. We will find that only partially filled bands conduct Band of allowed energy states. E Electron level similar to that of an isolated atom + + + + + position

Reasonable for “simple metals” (Alkali Li,Na,K,Cs,Rb) Band Theory The calculation of the allowed electron states in a solid is referred to as band theory or band structure theory. Free electron model: U(r) U(r) Neglect periodic potential & scattering (Pauli) Reasonable for “simple metals” (Alkali Li,Na,K,Cs,Rb)

Energy band theory Solid state N~1023 atoms/cm3 2 atoms 6 atoms

Metal – energy band theory

Band theory ctd. To obtain the full band structure, we need to solve Schrödinger’s equation for the full lattice potential. This cannot be done exactly and various approximation schemes are used. We will introduce two very different models, the nearly free electron and tight binding models. We will continue to treat the electrons as independent, i.e. neglect the electron-electron interaction.

Influence of the lattice periodicity In the free electron model, the allowed energy states are   where for periodic boundary conditions nx , ny and ny positive or negative integers. L- crystal dimension Periodic potential Exact form of potential is complicated Has property V(r+ R) = V(r) where R = m1a + m2b + m3c where m1, m2, m3 are integers and a ,b ,c are the primitive lattice vectors. E

Tight Binding Approximation Tight Binding Model: construct wavefunction as a linear combination of atomic orbitals of the atoms comprising the crystal. Where f(r) is a wavefunction of the isolated atom rj are the positions of the atom in the crystal.

The tight binding approximation for s states Solution leads to the E(k) dependence!! 1D: + a Nuclear positions

E(k) for a 3D lattice Simple cubic: nearest neighbour atoms at   So E(k) = - a -2g(coskxa + coskya + coskza) Minimum E(k) = - a -6g for kx=ky=kz=0 Maximum E(k) = - a +6g for kx=ky=kz=+/-p/2 Bandwidth = Emav- Emin = 12g For k << p/a cos(kxx) ~ 1- (kxx)2/2 etc. E(k) ~ constant + (ak)2g/2 c.f. E = (hk)2/me k [111] direction p/a -p/a = 10 g = 1 E(k) Behave like free electrons with “effective mass” h/a2g

Each atomic orbital leads to a band of allowed states in the solid Gap: no allowed states

Reduced Brillouin zone scheme The only independent values of k are those in the first Brillouin zone. Discard for |k| > p/a Results of tight binding calculation

The number of states in a band Independent k-states in the first Brillouin zone, i.e. kx < /a etc. Finite crystal: only discrete k-states allowed Monatomic simple cubic crystal, lattice constant a, and volume V. One allowed k state per volume (2)3/V in k-space. Volume of first BZ is (2/a)3 Total number of allowed k-states in a band is therefore Precisely N allowed k-states i.e. 2N electron states (Pauli) per band This result is true for any lattice: each primitive unit cell contributes exactly one k-state to each band.

Metals and insulators In full band containing 2N electrons all states within the first B. Z. are occupied. The sum of all the k-vectors in the band = 0. A partially filled band can carry current, a filled band cannot Insulators have an even integer number of electrons per primitive unit cell. With an even number of electrons per unit cell can still have metallic behaviour due to band overlap. Overlap in energy need not occur in the same k direction EF Metal due to overlapping bands

EF Full Band Empty Band Energy Gap Part Filled Band Full Band Partially Filled Band Energy Gap EF INSULATOR METAL METAL or SEMICONDUCTOR or SEMI-METAL

Insulator -energy band theory

Covalent bonding Atoms in group III, IV,V,&VI tend to form Filling factor T. :0.34 F.C.C :0.74

Covalent bonding Crystals: C, Si, Ge Covalent bond is formed by two electrons, one from each atom, localised in the region between the atoms (spins of electrons are anti-parallel ) Example: Carbon 1S2 2S2 2p2 C C 2D 3D Diamond: tetrahedron, cohesive energy 7.3eV

Covalent Bonding in Silicon Silicon [Ne]3s23p2 has four electrons in its outermost shell Outer electrons are shared with the surrounding nearest neighbor atoms in a silicon crystalline lattice Sharing results from quantum mechanical bonding – same QM state except for paired, opposite spins (+/- ½ ħ)

diamond

semiconductors

Intrinsic conductivity ln(s) 1/T

Extrinsic conductivity – n – type semiconductor ln(s) 1/T

Extrinsic conductivity – p – type semiconductor

Conductivity vs temperature ln(s) 1/T