Mechanisms of reactive sputtering of indium II: Growth of indium oxynitride in mixed N2-O2 discharges

The reactive sputtering of indium in mixed N₂-O₂ discharges was investigated using in situ optical absorption and emission spectroscopic analyses of plasma processes. All measured discharge parameters including the target sputtering rate, the target current and the sputtered indium emission intensity and neutral atom density exhibited abrupt changes with oxygen concentration at a critical value C¹. For a total pressure of 50 mTorr C¹ was approximately 2.5 mol.%. The abrupt change was caused by the formation of an indium oxide layer on the target surface over a very narrow range of O₂ partial pressures. The nitrogen coverage on the target was small at all gas compositions because of rapid preferential sputtering.X-ray photoelectron spectroscopy, Auger electron spectroscopy and X-ray diffraction were used to characterize the deposited film chemistry and structure as functions of the growth conditions. All films were n-type semiconductors and could be classified into the following three groups: (1) films grown at 0<C₀₂<2.5 mol.% were InN-based alloys with the wurtzite structure containing oxygen in solid solution; (2) films grown at 2.5 mol.% <C_O2<6 mol.% were two-phase mixtures; (3) films grown at C_O2>6 mol.% were cubic In₂O₃-based alloys with nitrogen in solid solution. Optical band gap measurements showed that E_g ranged from 1.7 to 2.4 eV for films of type (1) and from 2.4 to 3.6 eV for films of type (3).