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Title: GaInSb quantum wells grown on metamorphic buffer layers for mid-infrared lasers
Author: Thompson, Michael Dermot
ISNI:       0000 0004 7232 8988
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2014
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This work studies the use of Ga0.12-0.i6In0.88-0.84Sb quantum wells grown by molec­ular beam epitaxy (MBE) on a highly mismatched substrate for use in light emit­ting diodes (LEDs) and lasers emitting in the 3-4 /μm spectral range. Quantum well samples were grown at Lancaster which had abrupt interfaces and showed room temperature photoluminescence (PL) emission between 3.6 and 4.0 /μm. Trans­mission electron microscope (TEM) imaging revealed a very high defect density of more than 10[10]10 cm-2 in the buffer layer and due to this, Shockley-Read-Hall (SRH) recombination was found to dominate the temperature quenching of the PL emission. Despite the structural problems with the material, the PL quenching performance compared well with other materials designed for this spectral range. Modelling of the quantum wells found that a conduction band offset ratio of 80% gave the best agreement with the experimentally determined transition energies. LEDs fabricated at Lancaster emitted in pulsed mode up to room temperature at 3.6 μm and with an efficiency of 34%. At room temperature SRH recombination was found to dominate the total recombination up to 350 mA drive current and this was also reflected in the temperature quenching of the LED output. From the fitting of the temperature dependence of the LED efficiency the SRH recombination centres were calculated to be 30-50 meV from the centre of the band gap. Fitting the room temperature LED emission spectra revealed that the emission comprised of transitions involving the first two heavy hole states as well as holes in the valence band of the barrier. The emission spectra from the edge of the LED mesa contained amplified spontaneous emission modes which were attributed to radial modes formed due to current crowding under the LED top contact. Lasers fabricated by QinetiQ were examined and from the gain spectra the internal loss was found to be -94 cm- 1 . This was attributed to the high defect density in the structure. The devices emitted at 3.2 /μm at 130 K and from po­larisation measurements it was found that the emission was completely polarised in the TE mode corresponding to emission from the heavy hole band. The lasers tested failed to reach operating temperatures above 130 K due to a sharp increase in the threshold current. An analysis of the temperature dependence of the thresh­old current provided evidence for hole current leakage as the cause of the increase in threshold current between 80 and 130 K.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available