Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in $n$ -type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as $n_e^{2∕3}$ for $n_e\le 4.2\times {10}^{19}{\text{cm}}^{-3}$ , which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at $5.4-8.4\times {10}^{19}{\text{cm}}^{-3}$ , consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widening and the latter an offsetting narrowing. The band gap narrowing $\left(\Delta E^{\text{BGN}}\right)$ derived from the band gap renormalization effect did not show a good $n_e^{1∕3}$ dependence predicated by a weakly interacting electron-gas model, but it was in excellent agreement with a perturbation theory considering different many-body effects. Based on this theory a simple expression, $\Delta E^{\text{BGN}}=A{n}_e^{1∕3}+B{n}_e^{1∕4}+C{n}_e^{1∕2}$ , was deduced for $n$ -type ZnO, as well as $p$ -type ZnO, with detailed values of A, B, and C coefficients. An empirical relation once proposed for heavily doped Si could also be used to describe well this gap narrowing in AZO.

(Zn,Al)O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of $0–50\mathrm{at.}\%$ . The structural, optical, and electrical properties of (Zn,Al)O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region $(<10\mathrm{at.}\%)$ , Al-doped ZnO (AZO) thin films could be obtained at $400°\mathrm{C}$ in an $\mathrm{Ar}–{\mathrm{O}}_2$ ambient with good properties. The optimal results of $n$ -type AZO films were obtained at an Al content of $4\mathrm{at.}\%$ , with low resistivity $\sim {10}^{-4}\Omega \mathrm{cm}$ , high transmittance $\sim 90\%$ in the visible region, and acceptable crystal quality with a high $c$ -axis orientation. The band gap could be widened to $3.52\mathrm{eV}$ at $4\mathrm{at.}\%$ Al due to the Burstein-Moss shift [E. Burstein, Phys. Rev. 93, 632 (1954)] modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the residual, compressive stress in film, which may be the reason for the improvement in film stability and the increment in grain size as well. In a medium Al content region $(10–30\mathrm{at.}\%)$ , however, the film quality was degraded, which was presumably due to the formation of clusters or precipitates in the grains and boundaries. Besides the (002) plane, other diffraction peaks such as (100) and (101) planes of ZnO were observed, but the (Zn,Al)O films still exhibited a single-phase wurtzite ZnO structure. An intragrain cluster scattering mechanism was proposed to interpret the reduction of carrier mobility in films with the Al contents in the $7–20\mathrm{at.}\%$ region. The solubility limit of Al in ZnO film was identified to be in the $20–30\mathrm{at.}\%$ range, much higher than the thermodynamic solubility limit of $2–3\mathrm{at.}\%$ in ZnO. In a high Al content region $(⩾30\mathrm{at.}\%)$ , there were distinct observations for (Zn,Al)O films. As the Al content was $30\mathrm{at.}\%$ , the film appeared in a two-phase nature with ZnO hexagonal and ${\mathrm{Al}}_2{\mathrm{O}}_3$ rhombohedral structures. At the $50\mathrm{at.}\%$ Al content, the matrix of the (Zn,Al)O film was ${\mathrm{Al}}_2{\mathrm{O}}_3$ , and no evident trace of wurtzite ZnO was observed. The electrical and optical properties for both cases were also very different from those at the Al contents below $30\mathrm{at.}\%$ .