Donor and Acceptor Concentrations from a Single Mobility Measurement in Degenerate Semiconductors: ZnO

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Transparent conductive electrodes are an important part of LCD displays, solar cells, and lightemitting diodes, but a replacement must be found for indium tin oxide (ITO), which has become very expensive. One of the prime candidates is ZnO doped with Al, Ga, or In. Here we present a convenient analytical mobility model that yields donor ND and acceptor NA concentrations from Hall-effect measurements at a single temperature, including room temperature (RT). This model includes scattering from phonons and boundary imperfections as well as from ionized impurities and point defects. We apply it to films grown by pulsed laser deposition at 200 °C in 10 mTorr of pure Ar. For a film of thickness 275 nm, the Hall-effect measurements yield a RT resistivity ρ = 1.9 x 10-4 Ω-cm, mobility μ = 28 cm2/V-s, and carrier concentration n = 1.1 x 1021 cm-3, and the mobility model gives ND = 1.6 x 1021 cm-3 and NA = 4.9 x 1020 cm-3. Also, the transmittance is above 90% in the visible range. Annealing in forming gas gives ρ = 1.5 x 10-4 Ω-cm, mobility μ = 42 cm2/V-s, carrier concentration n = 1.0 x 1021 cm-3, ND = 1.1 x 1021 cm-3 and NA = 1.0 x 1020 cm-3. If the compensation ratio K = NA/ND can be maintained at about 0.1, and ND increased to about 3 x 1021 cm-3, then the model predicts a RT resistivity of less than 7 x 10-5 Ω-cm, well below present ITO values.


Presented at the Proceedings of SPIE: Oxide-Based Materials and Devices II, San Francisco, CA.

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