Properties of CdO Thin Films Produced by Chemical Vapor Deposition

The properties of CdO films formed by low-pressure chemical vapor deposition were investigated as a function of deposition temperatures from 150-450°C. Atomic force microscopy, X-ray diffraction, and transmission electron microscopy revealed that the grain size, bulk crystallinity, and intragain quality all improved with increasing temperature. Spectrophotometry showed that the optical bandgap decreased from 3.10 to 2.35 eV with increasing deposition temperature, whereas Hall measurements indicated that carrier concentration decreased from mid 10²⁰ to low 10¹⁹ cm^-3. Over the same range of temperature, the mobility increased from ∼6 to 216 cm² V^-1 s^-1, the latter being the highest value ever achieved for CdO in any form. The high mobility achieved at increased deposition temperature was due partly to a reduction in the effective mass of carriers but mainly to an increase in their relaxation time associated with improved crystallinity. The optical and Hall measurements suggest that the higher optical bandgap for low deposition temperature is likely due to a Moss-Burstein shift, from which the reduced effective mass was estimated as approximately 0.11 m_e. A novel charge-transport method was used to determine the density-of-states effective mass of electrons. This mass increased from 0.14-0.18 m_e with carrier concentration. Using this range of values and the reduced effective mass, we calculated that the approximate value of the density-of-states valence-band effective mass is 0.28-0.51 m_e.