Cu3N is the parent binary of an exciting new class of materials based on the formula Cu-M-N, where M = Earth-abundant metal ion. This class of materials is predicted to display defect-tolerance, meaning that material properties such as conductivity and minority carrier lifetime are not significantly affected by the presence of point defects, high-angle grain boundaries, or surfaces. Cu3N has provided experimental evidence of this predicted defect-tolerance, in that it can be doped either n-type or p-type based solely on growth conditions.
In this presentation, the control of bipolar doping behavior as a function of growth conditions in Cu3N is demonstrated, and hypotheses as to the underlying physics of this behavior are explored. Thin films of Cu3N were deposited from a metallic copper target using reactive RF-magnetron sputtering and an atomic nitrogen source. In one set of experiments, growth temperature was varied combinatorially from 150°C to 50°C. All depositions in this set of experiments were performed with a target power density of 1.5 Wcm-2.
In another set of experiments, target power density was varied from 1.0 Wcm-2 to 3.0 Wcm-2. In this set of experiments, all depositions were performed at a nominal temperature of 50°C, i.e. no active heating. For both experiment sets, Hall effect and Seebeck coefficient measurements were used to characterize carrier type with respect to deposition conditions. Crystallographic profiles of each sample were obtained via X-ray diffraction to confirm phase purity. Samples were also characterized for absorption onset and coefficient using UV-Vis spectrophotometry. Sheet resistance was determined using 4-point probe coupled with thickness measurements obtained via Dektak profilometry. Finally, Near Edge X-ray Absorption Fine Structure (NEXAFS) measurements were performed to investigate the possibility that bipolar doping in Cu3N arises from fundamental differences in structure brought on by varying growth conditions.
It was found that Cu3N grown under copper-rich conditions in which the activity of nitrogen was low (T > 120°C or T < 70°C, or target power density >1.5 Wcm-2) exhibited n-type conductivity with Seebeck coefficients on the order of -10 μVK-1. However, films grown under copper-poor conditions in which the activity of nitrogen was high exhibited p-type conductivity with Seebeck coefficients on the order of +300 μVK-1. NEXAFS measurements revealed the presence of mixed Cu valency (both Cu+1 and Cu+2), and this discovery helps to shed light on the underlying reasons behind Cu3N bipolar doping behavior. This research is supported by the U.S. Department of Energy, office of Energy Efficiency and Renewable Energy.