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Title: Terahertz intersubband electroluminescence from quantum cascade heterostructures
Author: Dhillon, S. S.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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Mid-infrared quantum cascade lasers (QCLs) have been extensively developed since their realisation in 1994, with a spectral range covered from 3.4μm (88THz) to 24μm (12.5THz). This is a direct result of advances in molecular beam epitaxy and band-structure engineering. QCLs are fabricated from multi-quantum well semiconductor heterostructures in which an appropriate engineering of the thickness and composition of the semiconductor layers adjusts the intersubband transition energies, offering considerable design flexibility of the band profile. By application of a suitable electric field and stacking together successive active regions, each injected electron cascades through the device, generating a number of photons. QCLs have shown considerable advances in performance with high powers and room temperature operation demonstrated. Extension of this quantum cascade scheme to the far-infrared, or terahertz (THz) range, is now being investigated, where the lack of sources remains acute. Specifically, operation is sought at energies smaller than the characteristic LO phonon energy of the semiconductor material, where currently no lasing has been shown (<36meV, 9 THz). The dynamics of this spectral range, however, are considerably different to those in the mid-infrared. LO phonon emission is effectively forbidden for subband spacings less than the phonon energy but increases in electron-electron scattering are expected to dominate. Although THz electroluminescence has been shown from cascade structures, systematic investigations into key parameters have not been reported. This dissertation reports a comprehensive study of THz electroluminescence from n-type A1GaAs/GaAs quantum cascade emitters as a basis for understanding the radiative and scattering mechanisms that occur in this spectral range, forming the foundations of the development of a THz semiconductor laser. The electroluminescence was correlated thoroughly with band structure calculations, along with the structural and electrical properties of the samples. The many features observed in the far-infrared were characterised, with the intersubband peaks investigated extensively to confirm their origin.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available