Hydrogen impurities and shallow donors in SnO2 studied by infrared spectroscopy
Hydrogen has been found to be an important source of n-type conductivity in the transparent conducting oxide SnO2. We have studied the properties of H in SnO2 single crystals with infrared spectroscopy. When H or D is introduced into SnO2 by annealing in an H-2 or D-2 ambient at elevated temperature, several O-H and O-D vibrational lines are produced, along with the low-frequency absorption characteristic of free carriers. To probe the relationship between H and the free carriers it introduces, the thermal stability of free carrier absorption and its relationship to the thermal stabilities of O-H lines have been examined. Two H-related donors are found: one is stable at room temperature on a time scale of weeks, and a second is stable up to 600 degrees C. These electrically active defects are found to interact with other O-H centers and can be converted from one to another by thermal treatments. The vibrational modes have been found to have distinctive polarization properties that provide an important test of microscopic defect models for the several O-H and (O-H)(2) centers we have observed.