Model for thickness dependence of mobility and concentration in highly conductive ZnO

摘要

The dependences of the 294-K and 10-K mobility u and volume carrier concentration n on thickness (d = 25 - 147 nm) were examined in Al-doped ZnO (AZO) layers grown in Ar ambient at 200 ℃ on quartz-glass substrates. Two AZO layers were grown at each thickness, one with and one without a 20-nm-thick ZnON buffer layer grown at 300 ℃ in Ar/N_2 ambient. Plots of the 10-K sheet concentration n_s vs d for buffered (B) and unbuffered (UB) samples give straight lines of similar slope, n = 8.36 × 10~(20) and 8.32 × 10~(20) cm~(-3), but different x-axis intercepts, 8d = -4 and +13 nm, respectively. Thus, the electrical thicknesses are d - 5d = d + 4 and d - 13 nm, respectively. Plots of n_s vs d at 294 K produced substantially the same results. Plots of μ vs d can be well fitted with the equation μ(d) = μ(∞)/[1 + d~*/(d-δd)], where d~* is the thickness for which μ(∞) is reduced by a factor 2. For the B and UB samples, d~* = 7 and 23 nm, respectively, showing the efficacy of the ZnON buffer. Finally, from n and μ(∞) we can use degenerate electron scattering theory to calculate bulk donor and acceptor concentrations of 1.23 × 10~(21) cm~(-3) and 1.95 × 10~(20) cm~(-3), respectively, and Drude theory to predict a plasmonic resonance at 1.3 4 μm. The latter is confirmed by reflectance measurements.