Carrier transport in undoped CdO films grown by atmospheric-pressure chemical vapor deposition

• Undoped CdO films were grown on c- and r-plane sapphire substrates by CVD.
• Hall effect measurements were performed for the CdO films at 83–343 K.
• For many CdO films, the carrier concentration n was independent of temperature.
• The grain boundary scattering plays a minor role in the CdO films.
• The dominant intra-grain scattering exhibited the continuous change with n.

Temperature dependent Hall effect measurements were performed for the undoped CdO films with carrier concentrations (n) ranging from 2.4 × 1019 to 2.0 × 1020 cm− 3 grown on c- and r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using Cd powder and H2O as source materials. The n dependence of the optical gap energy (Eopt) could be explained by the combination of the band gap widening due to Burstein–Moss shift and the band gap shrinkages due to the electron–electron and electron–impurity interactions. For all the films, the carrier concentrations (n) were independent of measurement temperature (T), indicating that these films were n-type degenerate semiconductors. The barrier heights at grain boundaries determined from the 1000/T-ln(μT) curves were smaller than the thermal energy at 300 K, suggesting that the grain boundary scattering plays a minor role on the carrier transport in comparison with the intra-grain scattering. The n dependence of the gradient of the μ–T curve revealed the continuous transformation of the dominant intra-grain scattering mechanism from the phonon scattering to the ionized impurity scattering with increasing n.