InP quantum dot (QDs) lasers grown on GaAs substrates have potential applications in photodynamic therapies, as multi-wavelength sources and for biophotonic sensing. However, to make these devices practical, further improvements are required in threshold current at elevated temperature. The main reason for this study is to identify the factors in the improved performance of lasers with respect to lowering the threshold current density and lowering the temperature dependence of threshold current density for samples with different Ga composition in the upper confining layer (UCL). A new way of determining the mode loss per unit length (αi) was introduced by extracting the peak net modal gain (G - αi) value of 6 cm-1 for a 2-mm-long laser from the averaged value of the modal gain (G), which is more accurate and significant than determining αi just at the value which loss (at net modal absorption or A + αi) and gain (at net modal gain or G - αi) spectra tend to at low photon energy. The highest αi value is 2.30 cm-1 for Ga = 0.54, 1.10 cm- 1 for Ga = 0.52, and Ga = 0.56 and 0.58 have almost zero αi values. I show that to maintain the same peak modal gain at 300 K at a higher temperature, for instance 360 K, one will need to compensate for two situations. First, increasing the current density to achieve 300 K inversion level (or the difference between the quasi-Fermi level separation and the absorption edge) to compensate for the increased nonradiative recombination processes and secondly adding more current density on top of that to compensate for the carrier spreading to higher energy states, in order to reach the peak net modal gain required at 360 K. Spontaneous emission rate spectra measured at J6 cm -1 show more filled QW states for Ga = 0.54 compared to Ga = 0.58 and compared with data taken at constant inversion level indicates that more carriers are supplied to the Ga = 0.54 to compensate for its high optical mode loss (αi), when compared to Ga = 0.58. As the temperature increases, some of the energetic carriers from the QW escape to the lower confining layer (LCL) and spontaneous emission measurements show this happens more in the Ga = 0.58 than in the Ga = 0.54. Absorption measurements indicate this is because QD and QW states move closer to the LCL states as the Ga composition in the well increases. Three series of samples grown at different times but with similar designs were compared in the study. Lowering the Al composition in the cladding layer, tends to lower the optical confinement factor (Γ), which causes the threshold current density to be increased in Series 1. The results show that αi plays the dominant role, not only in lowering the Jth but also lowering the threshold current temperature-dependence of these series.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:633569 |
| Date | January 2014 |
| Creators | Awg Hj Kasim, Awg Makarimi |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/68908/ |
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