Making highly conductive ZnO: creating donors and destroying acceptors

We obtain room-temperature resistivities as low as rho = 1.40 x 10(-4) Omega-cm in transparent Ga-doped ZnO grown on Al2O3 by pulsed laser deposition (PLD) at 200 C in 10 mTorr of pure Ar and then annealed in a Zn environment. Donor N-D and acceptor N-A concentrations are calculated from a recently developed scattering theory that is valid for any degenerate semiconductor material and requires only two input parameters, mobility mu and carrier concentration n measured at any temperature in the range 5 - 300 K. By comparison with SIMS and positron annihilation measurements, it has been shown that the donors in these samples are mostly Ga-Zn, as expected, but that the acceptors are point defects, Zn vacancies V-Zn. PLD growth in Ar at 200 degrees C produces a high concentration of donors [Ga-Zn] = 1.4 x 10(21) cm(-3), but V-Zn acceptors are produced at the same time, due to self-compensation. Fortunately, a large fraction of the V-Zn can be eliminated by annealing in a Zn environment. The theory gives N-D and N-A, and thus [Ga-Zn] and [V-Zn], at each step of the growth and annealing process. For convenience, the theory is presented graphically, as plots of mu vs n at various values of compensation ratio K = N-A/N-D. From the value of K corresponding to the experimental values of mu and n, it is possible to calculate N-D = n/(1 - 2K) and N-A = nK/(1 - 2K).