Hall effect analysis of bulk ZnO comparing different crystal growth techniques

The relaxation time approximation was used to interpret Hall effect data from n-type ZnO bulk samples grown using the high-pressure melt, seeded-chemical-vapor transport, and hydrothermal techniques. These samples represent a range of free-carrier concentrations due to different amounts of donors and compensating acceptors. Treatment of intrinsic mechanisms includes polar-optical phonon scattering using an effective Tpo=750 K, piezoelectric potential scattering using P⊥=0.25, and deformation potential scattering using E1=3.8 eV. Intrinsic mobilities from 60 to 400 K for electrons and for holes in ZnO are predicted. For extrinsic behaviors, ionized and neutral impurities are included. Donor ionization energies for dilute concentrations were determined. Shallow group III donors (Al, Ga) are responsible for the free carriers in the high-pressure melt and seeded-chemical-vapor transport crystals. The hydrothermally grown sample is closely compensated and exhibits hopping conduction below 200 K. Free carriers in the hydrothermal ZnO crystal are generated by thermal activation of deep nickel donors with the Ni2+/3+ level at about 270 meV below the conduction band.