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Title: Efficiency limiting processes in novel laser materials for optical computing and communications applications
Author: Hossain, Nadir
ISNI:       0000 0004 2745 5822
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 μm 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, Jth 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 temperature, T0 (T1 is measured to be ~104K (~99K) around 200K, which drops to ~58K (~37K) around RT. 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, T0 and T1 at RT. The band-structure properties of novel BxGa1-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 BxGa1-x P layers grown on silicon as a function of Boron composition from which we derive the properties of freestanding BxGa1-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 a Jth of 1.6 kA/cm2 at the lasing wavelength of 860 nm at 165 K. The T0 (T1) 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 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 pressure 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, T0 (T1) 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