Atomic layer deposition of tin oxide and zinc tin oxide using tetraethyltin and ozone

Silicon or glass substrates exposed to sequential pulses of tetraethyltin (TET) and ozone (O₃) were coated with thin films of SnO₂. Self-limiting deposition was found using 8 s pulse times, and a uniform thickness per cycle (TPC) of 0.2 nm/cycle was observed in a small, yet reproducible, temperature window from 290 to 320 °C. The as-deposited, stoichiometric SnO₂ films were amorphous and transparent above 400 nm. Interspersing pulses of diethylzinc and O₃ among the TET:O₃ pulses resulted in deposition of zinc tin oxide films, where the fraction of tin, defined as [at. % Sn/(at. % Sn + at. % Zn)], was controlled by the ratio of TET pulses, specifically n_TET:(n_TET + n_DEZ) where n_TET and n_DEZ are the number of precursor/O₃ subcycles within each atomic layer deposition (ALD) supercycle. Based on film thickness and composition measurements, the TET pulse time required to reach saturation in the TPC of SnO₂ on ZnO surfaces was increased to >30 s. Under these conditions, film stoichiometry as a function of the TET pulse ratio was consistent with the model devised by Elliott and Nilsen. The as-deposited zinc tin oxide (ZTO) films were amorphous and remained so even after annealing at 450 °C in air for 1 h. The optical bandgap of the transparent ZTO films increased as the tin concentration increased. Hall measurements established that the n-type ZTO carrier concentration was 3 × 10¹⁷ and 4 × 10¹⁸ cm^-3 for fractional tin concentrations of 0.28 and 0.63, respectively. The carrier mobility decreased as the concentration of tin increased. A broken gap pn junction was fabricated using ALD-deposited ZTO and a sputtered layer of cuprous oxide. The junction demonstrated ohmic behavior and low resistance consistent with similar junctions prepared using sputter-deposited ZTO.