Non-contact detection of a naturally formed oxide layer on copper metal surface using terahertz time-domain spectroscopy

We demonstrate the use of the terahertz time-domain spectroscopy (THz-TDS) as a complementary tool in detecting the naturally formed oxide (NFO) layer on a copper metal surface. The THz emission from Cu-metal sheets having NFO were compared with Cu-sheets that were rapidly oxidized between temperatures ranges from 100 degrees C to 500 degrees C for 2 h. Raman spectroscopy and scanning electron microscopy were initially utilized as standard techniques for characterizing the oxide layer. However, the two performed standard techniques could not provide distinguishable results between the NFO and the thin oxide layer grown at temperatures below 200 degrees C. The THz-TDS provides clear results for non-destructive detection of the NFO on the Cu-surface. Excitation-power dependence and magnetic field dependence measurements revealed that the dominant THz emission mechanism is diffusion for CuxO formed below 300 degrees C and drift for thicker oxides grown at temperatures greater than 400 degrees C. The carrier diffusion and carrier drift mechanism are also dependent on the absorption efficiency and structural properties of the oxide layer that was formed. No THz radiation mechanism originating form nonlinear optical process was observed.