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Title: Efficiency limiting processes in novel laser materials for optical computing and communications applications
Author: Hossain, Nadir
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2012
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The novel Ga(NAsP)-based semiconductors have recently grown in popularity due to applications such as development of energy efficient long-term stable semiconductor lasers on silicon substrates for optical computing applications. GaAsSb-based active materials have also recently been extensively investigated for the development of temperature stable uncooled semiconductor lasers for 1.3 !-lm optical communications applications. Electrical injection lasing operation at room temperature (RT) is demonstrated in Ga(NAsP) / GaP quantum well (QW) lasers with a threshold current density, J th of 4 kA/ cm2 at the lasing wavelength of 981 nm: From temperature dependence measurements we find that the threshold current is dominated by non-radiative recombination process(es), which account for at least 92% of Jth at RT. The characteristic temp_erature, To (TI) is measured to be ~104K (~99K) around 200K, which drops to ~58K (~37K) aroundRT. Hydrostatic pressure measurements reveal a strong increase in threshold current with increasing pressure. This . implies that current leakage dominates carrier recombination, which is also responsible for their low characteristic temperatures, To and TI at RT. The band-structure properties of novel BxGal_xP alloys are also investigated. These layers are utilized as strain-compensating layers for the lattice-matched integration of Ga(NAsP) . quantum well lasers on an exact (001) silicon substrate. Experimental and-theoretical studies reveal the dependence of the direct and indirect band gaps for strained BxGal_xP layers grown on silicon as a function of Boron composition from which we derive the properties of free- standing BxGal_xP' For Boron fractions up to 6%. We find that the bowing parameter for the lowest (indirect) band gap is -6.2±0.2 eV. High crystalline quality and promising optical material properties are demonstrated and applied to monolithically integrated Ga(NAsP) / (BGa)P multi-quantum well heterostructures on (001) silicon substrates. Electrical injection lasing operation is demonstrated for the first time up to 165K in Ga(NAsP) / (BGa)P QW lasers monolithically integrated on a (001) silicon substrate. The devices show aJth of 1.6 kA/ cm' at the lasing wavelength of 860 nm at 165 K. The To (TI) in the devices is ~198K (~99K) at 100 K;decreasing to ~73K (~35 K) at 165 K. Temperature dependence of the "Z" analysis shows that Zth increases from 1.7 at 40 K to 2.3 at 165 K. The value of Zth < 2 at low temperatures signifies that the monomolecular (defect) current contribution at threshold in these devices is significant. The non-radiative contribution accounts for ~ 83% (of which at least ~40% is monomolecular recombination) of Jth even at a low temperature of 165K. It is proposed that defects originate due to the non-optimized miscut angle of the silicon substrate and due to diffusion of Nitrogen from active region to the barrier regions. A strong increase in Jth with increasing pressure at 165K suggests the presence of, carrier leakage. The temperature and press~e dependence of Jth for GaAsSb/GaAs QW lasers with different device characteristics are investigated. Thermally activated carrier leakage via defects is observed in the GaAsSb/GaAs QW devices. Devices grown under optimal conditions reduce the nonradiative recombination mechanism from 93% to 76% at RT, compared with a device grown under non-optimized conditions. This improvement in carrier recombination mechanisms leads to a large improvement in the Jth from 533 Acm-2/QW to 138 Acm-2/QW and the characteristic temperature, To (TI) from ~51K (~104K) to ~62K (~138K) near RT.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available