Dopant Profiles in Heavily Doped ZnO

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X-ray photoelectron spectroscopy (XPS) is used to compare the composition as a function of depth of as-grown zinc oxide (ZnO) films heavily doped with Ga and similar samples annealed in air for 10 min at 600°C, with particular attention given to the near-surface region. Films are grown by pulsed laser deposition (PLD) using a ZnO target containing 3 wt% Ga2O3. Electrical properties of these samples are determined from temperature-dependent Hall-effect measurements. The as-grown film has the following characteristics: (1) ∼1∶1 Zn:O ratio with a Ga concentration of ∼3.3 at %; (2) no excess Ga in the near-surface region; and (3) excellent electrical characteristics: ρ=2.42×10−4  Ω-cm, n=8.05×1020  cm−3, and μ=32.1   cm2/V-s-s at 300 K. For the annealed sample: (1) the Zn:O ratio remains ∼1∶1, but the Ga concentration is ∼3 at %, which is ∼10% lower than in the as-grown film; (2) ∼7 at % Ga is measured in the near-surface region; and (3) a significant increase in resistivity to ρ=0.99  Ω-cm, n=1.97×1018  cm−3, and μ=3.2  cm2/V-s at 300 K. Analysis of the O chemical shift suggests formation of a mixed ZnO/Ga2O3 surface layer ≤5-nm-thick accounts for the observed changes in the Ga profile after annealing.