Optimising terahertz time-domain spectroscopy for III-V semiconductors

Thesis

Terahertz (THz) time-domain spectroscopy and optical-pump THz-probe (OPTP) spectroscopy are powerful, non-contact methods for probing charge transport in semiconductors with sub-picosecond time resolution. This thesis advances both the instrumentation and analysis of THz spectroscopy, enabling accurate evaluation of material parameters. These refined tools are then applied to the characterisation and optimisation of technologically important III–V semiconductors.

First, I develop spintronic THz emitters with specially designed dielectric coatings: a high-reflectivity stack that suppresses residual transmission of the 800 nm pump by over 99.9%, and an anti-reflective coating that increases the excitation pump intensity inside the emitter. The coated emitters block transmission of the excitation pump, preventing undesirable photoexcitation of the sample and protecting sensitive detection optics. Furthermore, the emitted THz electric field is increased by roughly 40% compared to uncoated emitters, improving the measurement’s signal-to-noise ratio.

Next, using the coated emitters, I investigate the relationship between geometric resonances in THz photoconductivity spectra and the size of features in micro/nanostructured GaAs. An inverse relationship between resonant frequency and feature size is established through measurements of nanowire ensembles and well-defined microsquare arrays. The surface plasmon model is validated as a method to separate the resonance from the intrinsic material properties, thus enabling precise measurement of the intrinsic chargetransport parameters.

Finally, OPTP spectroscopy is utilised to characterise and optimise In0.53Ga0.47As via two different tuning mechanisms: incorporation of Fe during growth and application of uniaxial strain. Fe doping reduces electron lifetimes to less than a picosecond while maintaining high mobility, whereas uniaxial strain has little impact on the electronic properties of In0.53Ga0.47As. These results inform practical device design of high-speed optoelectronics operating at telecommunication wavelengths.

Authors: Wagner, FM

• DOI: 10.5287/ora-4r9k90j2k
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: far-infrared; terahertz spectroscopy; time-domain spectroscopy; surface plasmon; THz; spintronic thz emitters; terahertz; spectroscopy; III-V semiconductors
• Subjects: Terahertz spectroscopy