Infrared Near-Field Spectroscopy of AlGaN/GaN Heterostructures for Probing Two-Dimensional Electron Gas

Infrared near-field spectroscopy, or nano-FTIR, offers nanoscale resolution in three dimensions to probe the chemical and physical properties of samples, making it a unique characterization tool. This nanoscopic resolution in three dimensions is particularly suitable to probe a two-dimensional electron gas (2DEG) where a 2DEG has an effective thickness of a few nanometers and exists a few tens of nanometers below the capping layer. This work employs nano-FTIR spectroscopy to noninvasively probe the 2DEG of AlGaN/GaN heterostructures, which are crucial for high-power electronic devices and sensing applications. Higher harmonic amplitude and phase of the nano-FTIR spectra are sensitive enough to the carrier concentration of the 2DEGs, which is supported by analytical calculations based on the finite dipole model. A comparative analysis confirms that incorporating the 2DEG layer into the model is essential to matching spectral features with experimental observations. Furthermore, hyperspectral imaging of a cross-sectional sample provides a visual representation of the 2DEG. The findings demonstrate that nano-FTIR enables the characterization of 2DEG in AlGaN/GaN heterostructures with nanometric resolution under ambient conditions, hence expanding its applicability in the study of such systems.