Quantum - Engineered Semiconductor Photomixers at Long Wavelength Illumination (1.55 micro metre) for THz Generation and Detection

  • Ioannis Kostakis

Student thesis: Phd


This thesis is concerned with the characterisation, fabrication and testing of devices capable of generating and detecting terahertz (THz) radiation. Such devices are based on semiconductor photoconductors grown under low temperature (LT) conditions using the technique of Molecular Beam Epitaxy (MBE). The absorption of a pulsed or continuous wave (CW) signal by these structures in conjunction with the presence of an electric field generates photocurrent, which is fed into an antenna structure fabricated on the surface of the active layers. As a result of such a sequence, a THz signal is generated and radiated from the substrate side into free space. Therefore, the efficiency of the devices is determined by the characteristics of the photoconductors and the geometry of the designed antenna structures. The desired material characteristics are high absorption at the corresponding illumination wavelength, high dark resistivity, high electron mobility and sub picosecond carrier lifetime. The determination of these characteristics for all the structures grown in this work composes the characterisation part of the thesis. The fabrication part comprises of the design of several antenna structures with various geometrical characteristics, while the testing part consists of their evaluation as THz sources and detectors in a time-domain spectroscopy (TDS) system under pulsed excitation. To date, THz devices based on low temperature grown GaAs (LT-GaAs) photoconductors have been reported to be the most efficient. However, their operational wavelength, at 800 nm, requires very expensive and complex components spurring interests in solutions consisting of devices operating at longer wavelengths, where cheaper and simpler components exist. The most desirable and practical operational wavelength is the telecommunication one at 1.55 micro metre. Thus, the biggest challenge is the development of efficient devices operating at this illumination wavelength. In this work, devices operating at the very important wavelength of 1.55 micro metre as well as at the wavelengths of 1 micro metre and 800 nm are presented. The key findings for the long wavelength devices (1.55 micro metre) demonstrate photoconductors with ultrafast carrier lifetimes (~ 120 fs), high resistivity (> 105 Ω / sq), high mobility (> 1000 cm2 / Vs) and system responses with spectral range up to 3 THz and power-to-noise ratio of 60 dB. These characteristics are among the best ever reported for such material systems, making them efficient THz devices for various optoelectronic applications, especially for telecommunication laser-driven CW THz systems.
Date of Award1 Aug 2014
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMohamed Missous (Supervisor)


  • THz emitters and detectors
  • low temperature grown photoconductor semiconductors

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