Research overview
This PhD thesis describes the development of the first time-resolved terahertz spectroscopy (TRTS) set-up in Spain and its use to study charge carriers in silicon. THz light is highly sensitive to free charge carriers, so it can measure conductivity optically, without contacts, and follow carrier dynamics with sub-picosecond resolution. The thesis measures how carrier mobility depends on injection level and temperature, finding quantitative agreement with classic contact-based measurements once diffusion is taken into account. In N-type silicon at 4 K it resolves a 100% yield of transient free carriers lasting up to about 300 ps, attributed to holes being trapped at and released from the SiO₂/Si interface. Measurements near the absorption edge reveal a bottleneck in free-carrier generation, attributed to a phonon bottleneck. With colleagues at the Universidad Politécnica de Madrid, the thesis also shows that phosphorus diffusion gettering greatly improves carrier lifetime and mobility in upgraded-metallurgical-grade silicon for photovoltaics.
Key findings
- First time-resolved THz spectroscopy set-up in Spain, built for contact-free conductivity measurements
- THz mobility measurements in silicon match classic electrical results when diffusion is included
- Free carriers lasting up to about 300 ps at 4 K, linked to the SiO₂/Si interface
- Phonon bottleneck in free-carrier generation near the silicon band edge
