Scattering mechanisms and electronic behavior in transparent conducting Zn _xIn ₂O _x+3 indium-zinc oxide thin films

Thin films of Zn _xIn ₂O _x+3 (x=2, 3, 4, 5, 7, 9, 11, 13, and 15) were grown by pulsed laser deposition and their electronic properties characterized. Through resistivity, Hall mobility and carrier concentration measurements as a function of temperature, we found that whatever the composition, these films behave as n-type degenerate semiconductors exhibiting a metal-like behavior. Transport properties are driven by two types of electronic scattering, namely ionized impurities (oxygen vacancies) at low temperature and optical phonons at high temperature. Modeling of optical transmittance spectra in the visible-infrared region yields two different trends among the different compositions: for (x=2, 3, 4, 5, 7, and 9) values the electronic behavior can be described by applying the single Drude model on a plasma of nearly free carriers, whereas for zinc-rich (x=11, 13, and 15) compounds a supplementary Lorentz oscillator is necessary to simulate the spectroscopic data, revealing the presence of bound optically active electrons. Good agreement is found in every case between the experimental Hall mobility and the "optical" mobility calculated from relaxation time and effective mass fitted parameters. We show that the mobility drop can be explained primarily by a drastic increase in the effective mass for the highest x values.