A study of the performance of terahertz quantum cascade devices

• Main Author: Fowler, J. C.
• Published: University of Cambridge 2005
• Subjects: 535
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599148

This thesis investigates the performance of terahertz Quantum Cascade (QC) devices. The main emphasis is on the reproducibility and uniformity of Molecular Beam Epitaxy (MBE) growth and the optimization of semiconductor processing. By exploring performance with reference to fabrication, this work aims to assist the physicist in characterizing new Quantum Cascade Lasers (QCLs) by yielding knowledge of performance variations unrelated to design. Work on QCLs and their forerunners, Quantum Cascade Emitters (QCEs), is described. Preliminary research investigated the ohmic contacts used in the fabrication of QC structures. It was found that traditional recipes such as AuGe or AuGeNi are unsuitable because the ohmic material can diffuse excessively and penetrate into the laser active region. Instead, PdGe ohmic contacts were identified as the most appropriate. Different types and lateral configurations of top contact were tested on a QCL structure, in which it was observed that for a shallow ohmic, complete coverage of the laser surface is preferred for uniformity of current injection. For penetrating contacts such as AuGe, it was found that a stripped configuration along the edges of the laser stripe could be used. To investigate MBE growth reproducibility and uniformity, four nominally identical structures of the first terahertz QCL were grown, across two growth campaigns, at varying chamber ‘cleanliness’. MBE chamber conditions were assessed from lateral mobility measurements of high-electron-mobility-transistor test samples. Sets of lasers were characterized from the middles and edges of all the wafers, all processed at the same time for maximum comparability. Samples were subjected to electrical, optical, magnetic and structural characterization. Substantial differences in lasing properties were observed between wafers. Lasers fabricated from high-mobility wafers emitted up to five times more power than those from low-mobility wafers and the threshold current density in devices from poor quality material increased by up to 90%. These results are attributed to variations in the lifetime of the upper state of the lasing transition. It is argued that the mechanism of this change is the degree of non-radiative scattering, affected by the quality of the heterostructure interfaces.