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Optical properties of InP/AlGaInP quantum dot laser heterostructures

Optical characterisation of InP/AlGaInP quantum dot laser structures, involving laser threshold and wavelength, optical modal gain and absorption spectra, and radiative efficiency are presented. The samples were grown by MOVPE in Sheffield University on (100) 10 off and (211)B GaAs substrates, and consist of 5 layers of self-assembled InP dots, with each layer grown on Alo.3Gao.7InP and placed in a GaInP quantum well. A record low threshold current density of 290A/cm2 at a wavelength of 740nm for a 1.6 mm-long device with uncoated facets is obtained from one of the sample grown on (100) 10 off substrate at 690 C. This sample has an internal optical mode loss of 4 1 cm"1 and an internal quantum spontaneous emission efficiency of 30% for current densities corresponding to the threshold. The ground state modal gain is shown to saturate at 17cm"1 at room temperature, which is about lA of the full population inversion limit, and the saturation level increases with decreasing temperature. A sample grown at a lower temperature of 650 C has higher optical mode loss (7.5 1 cm"1) and quantum efficiency under 15%. A sample grown on (211)B substrate exhibits optical transitions at higher photon energies, consistent with smaller dot sizes. The thesis also presents an analysis of the segmented contact technique used to measure gain and absorption spectra, determining criteria for the excitation and detection geometries required to ensure accurate measurement. It is shown that the collection angle and device nearfield should be limited so that rays that intercept the stripe edges are not collected. If this is not satisfied, the measurement underestimates the modal gain. An exponential variation of the measured ASE upon stripe length cannot be taken as evidence for a correct collection geometry.
Date January 2005
CreatorsLutti, Julie
PublisherCardiff University
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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