Piezoelectric polarization ( polarization caused by strain)

1. Spontaneous and piezoelectric polarization aid each other for tensile strain and oppose for compressive strain Piezoelectric polarization (polarization caused by strain)   strain in x, y, or z directions e  piezoelectric coefficients Therefore, Also, is always –ve, as e31 is always –ve, and others are +ve.

2. Polarization in heterostructures Interface charge is equal to negative divergence of Polarization ( in cm-3) (int incm-2) Quantum well is formed at the interface Ga-face epilayers -s b Tensile strain AlGaN PSP PPE Ec +int EF E0 GaN Buffer d PSP 2DEG comp int + ve GaN AlGaN c-plane sapphire 2DEG surf

3. Calculation of Polarization charge and effect of strain Problem: What would be the polarization charge at the interface of AlGaN/GaN with 35% Al composition?

4. Piezoresistive AlGaN/GaN microcantilevers • Cantilever material: GaN • AlGaN/GaN HFET embedded on the base of cantilever • Cantilever bending changes the 2DEG at the interface of HFET • Utilizes high piezoelectric polarization of AlGaN-GaN system

5. 2DEG carrier concentration (ns) of AlGaN/GaN HFET Fermi Energy, x = 0.26, d1 = 17.5 nm, d2 = 2 nm Channel Resistance, Rch = (1/qns)(Lch/wch) ns = 1.0086  1013 cm-2 Using above equations, ns = 9.903  1012 cm-2 Using 1D Poisson program,

6. Strain variation at AlGaN/GaN interface Strain obtained from COMSOL simulations Strain at AlGaN/GaN interface P/int DOS m*(x) Cond. m*(x) 35 0 ns  Rch

7. Effect of cantilever bending 20 m DOS m*(x) P/int Cond. m*(x) ns 