Contact-free charge carrier density determination of doped sub-μm thin cubic GaN layers with terahertz time-domain spectroscopy

As cubic GaN is a promising material for optoelectronic applications, its layers need to be precisely doped to realize the desired properties. While contact-based methods are established to measure the realized charge carrier density, contact-free techniques are more advantageous for this purpose. We show terahertz time-domain spectroscopy measurements of cubic GaN layers with thicknesses between 0.6 $μ$m and 2 $μ$m with doping densities in the range from $10^{18}$ cm$^{-3}$ to $10^{20}$ cm$^{-3}$. From these measurements, we determine the charge carrier densities and verify the results with time-of-flight secondary-ion mass spectrometry measurements. The results are in accordance with simulations of the doping ranges which the technique is expected to be sensitive in. In preparation for the terahertz time-domain spectroscopy measurements, we validated the layer thicknesses by Fourier-transform infrared spectroscopy measurements. With this, a wide range of doped cubic GaN samples can be addressed by optical methods. In the future, these techniques could allow for a fast and non-destructive characterization of GaN layers, right after or even during the growth process.