Structural, optical, and electrical properties of (Zn,Al)O films over a wide range of compositions

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(Zn,Al)O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0–50at.%. 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 (<10at.%), Al-doped ZnO (AZO) thin films could be obtained at 400°C in an Ar–O2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4at.%, with low resistivity ∼10−4Ωcm, high transmittance ∼90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52eV at 4at.% 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 (10–30at.%), 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 7–20at.% region. The solubility limit of Al in ZnO film was identified to be in the 20–30at.% range, much higher than the thermodynamic solubility limit of 2–3at.% in ZnO. In a high Al content region (⩾30at.%), there were distinct observations for (Zn,Al)O films. As the Al content was 30at.%, the film appeared in a two-phase nature with ZnO hexagonal and Al2O3 rhombohedral structures. At the 50at.% Al content, the matrix of the (Zn,Al)O film was Al2O3, 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 30at.%.