Pulsed terahertz (THz) systems for spectroscopy and imaging are traditionally based on Ti:Sapphire femtosecond laser excitation, with low-temperature (LT)-GaAs photoconductive devices. A number of designs of GaAs photoconductive antennae were characterised in this thesis using 800 nm laser excitation. The THz power emitted was investigated as a function of specific antenna geometry, laser excitation position, laser power, and applied bias. The electric field profile of the devices under bias was investigated by electron beam induced current. These measurements give an indication of the device electric field profile, and can be applied to both pulsed and continuous-wave THz (cw-THz) antennas. There are, however, considerable advantages in moving to longer wavelength excitation. These include a step towards the optimum wavelengths for fibre-optic laser delivery, and potential reductions in the cost of the laser technology. In this thesis, the operation of a pulsed THz imaging system that uses, for the first time, a 1.06 <i>m</i>m femtosecond laser, and photoconductive devices fabricated from LT-In<sub>0.3</sub>Ga<sub>0.7</sub>As is demonstrated. Biological and non-biological images are presented to show the many potential applications of THz radiation, and the technology compared to existing 800 nm based systems. It is found that <i>ex-situ</i> post growth annealing plays a critical role in determining the properties of LT-InGaAs, and that material with a sub-500 fs carrier lifetime, and superior resistivity can be obtained. The dependence of the carrier lifetime and resistivity on growth and anneal temperature are discussed in detail. To characterise the material, x-ray diffraction is used to identify the presence of defects or traps. Time-resolved photoreflectance shows how the carrier lifetime varies, and electrical measurements are used to give the resistivity of the material. It was demonstrated that upon a mid-temperature anneal (400-600°C) the carrier lifetime actually falls, whilst the resistivity increases.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:596274 |
| Date | January 2004 |
| Creators | Baker, C. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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