Boosting hole mobility in p-type SnOx (x < 2) film through Ge incorporation via co-sputtering

Electrical characteristics of SnOx (x < 2) films are investigated, accounting for germanium (Ge) incorporation in tin oxide. It was found that by tailoring the Ge atomic percentage from 0 to 13.77% in SnOx (x < 2), which was deposited at 5 mTorr by maintaining an O2/(Ar + O2) SCCM ratio to 4.33%, p-type characteristics were obtained. Postdeposition annealed (PDA) Ge incorporated SnOx, which underwent 450 °C annealing in N2 for 1 h, exhibited increased hole density from 1.36 × 1015 to 2.36 × 1017 cm−3, diminished resistivity from 4056 to 2.95 Ω cm, and increased Hall mobility from 1.14 cm2/V s (undoped SnOx) to 8.94 cm2/V s (13.77% Ge incorporated SnOx). Films were stable showing 10.79%–6.37% change in their resistivity followed by three months of air exposure. P-type character of Ge incorporated SnOx (SnOx:Ge) films was further established from UV-visible (UV-Vis) and UV photoemission spectroscopies, which revealed 780 meV down shift of the Fermi energy level (EF) for 13.77% Ge incorporated SnOx with respect to EF of its undoped counterpart. Furthermore, the impact of Ge incorporation was comprehended through XPS of Ge 3d and Sn 3d peaks, which corroborated PDA assisted sublimation of lower formation enthalpy (−41.4 kJ/mol) GeO into Ge and O2 and further oxidation of Ge into Ge2+, while the latter eventually substituted Sn vacant sites (VSn2−). Electrons released from the same oxidation reaction reduced available Sn2+ at the interstitial sites into Sn, which might impact orbital delocalization near the valence band, leading to a significant change in the Hall mobility of holes.