Effects of varying the oxide composition on the terahertz emission of n-Si/CuxO heterostructures
We investigated how the oxide composition affects the terahertz (THz) emission from n-doped silicon/copper oxide (n-Si/CuxO) heterostructures through THz-time domain spectroscopy (THz-TDS). The n-Si/CuxO heterostructures were grown by thermal evaporation of Cu on a (100) n-Si substrate followed by ex-situ direct thermal oxidation. The Cu-coated n-Si was oxidized in ambient air at different temperatures to control the amount of cuprous oxide (Cu2O) and cupric oxide (CuO) in the CuxO films. Raman spectroscopy showed that at 150°C, an n-Si/Cu/Cu2O stack forms due to the incomplete oxidation of Cu, while an n-Si/Cu2O/CuO stack forms at 250°C. The Cu film was completely oxidized to CuO at 350°C, resulting in an n-Si/CuO stack. Reflectance measurements showed that the CuxO film increases the absorbance of n-Si by more than 8%. THz-TDS measurements showed that the n-Si/Cu2O/CuO heterostructure has the strongest THz emission compared to n-Si/CuO and bare n-Si. The strong THz emission is attributed to (1) the increase in absorption due to Cu2O and CuO, and (2) the built-in electric field at the n-Si/Cu2O interface as explained by band diagram and electric field calculations.