Researchers developed a high-precision terahertz time-domain ellipsometry for evaluating wide bandgap semiconductors.
In semiconductor devices, there is an increasing demand for semiconductors with high carrier density and carrier mobility. These two properties primarily determine the device performance. Hence, characterizing carrier density and mobility accurately in a semiconductor device is critical.
Terahertz waves with wavelengths of around 300 µm and frequency of about 1 THz are widely used for nondestructive testing of semiconductors. The carriers in a semiconductor material absorb Hz radiation, making it possible to estimate the electrical properties of semiconductors using THz waves.
In THz ellipsometry, linearly polarized THz pulses are incident on a sample and the electric field strength of the reflected THz waves as a function of time is measured. The ratio of the parallel and perpendicular polarization components yields information on the electric permittivity of the sample, allowing for the evaluation of the carrier density and mobility.
Researchers at Osaka University and Nippo Precision Co., Ltd. have developed a THz time-domain ellipsometry system (Tera Evaluator) that extends the range of carrier concentrations measurable by THz waves up to ~10^20 cm^-3 and higher.
The researchers used multi-angle technique, wherein the analyzer angular orientation is varied from 0° to 360° in 15° increments. This eliminates systematic errors from the amplitude and phase of the detected THz electric field in the analysis. For validation of this technique, researchers evaluated gallium nitride semiconductors.
The work is described in the journal Scientific Reports.