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Title: Influence of electronic band structure on the efficiency of mid-infrared photonic devices
Author: Eales, Timothy
ISNI:       0000 0004 7967 3338
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2019
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The advances in semiconductor technology coupled with the potential of lab-on-chip spectroscopy, long wavelength telecommunications and small-scale optical interconnects has invigorated academic and commercial interest in mid-infrared optoelectronics. However, significant challenges remain. This thesis explores two approaches for mid-infrared optoelectronic lasers. Type-I GaSb quantum well lasers exhibit some of the highest performance metrics of any semiconductor laser system in the 2 μm - 3 μm wavelength range. However, the threshold current density increases substantially with increasing wavelength and temperature, impacting component reliability and the overall efficiency of a laser-based optoelectronic system. Through a combination of temperature and hydrostatic pressure techniques, Auger recombination is identified as the dominant cause for the performance degradation of type-I mid-infrared laser with increasing wavelength and temperature. Using hydrostatic pressure measurements, the wavelength dependence of the Auger coefficient over the 2 μm - 3 μm range is constructed, revealing two important regimes. At wavelengths < 2 μm the CHSH Auger process dominates the room temperature non-radiative threshold current. For wavelength > 2 μm, the CHSH process is effectively suppressed due to the energetic separation between the lasing energy and the spin-orbit split-off band. In this regime another Auger process, such as CHCC or CHLH recombination begins to dominate, increasing exponentially with wavelength. The temperature dependence of the radiative and non-radiative threshold current density indicates that this Auger process has an activated character and is sensitive to the intrinsic properties of the quantum well band structure. To leverage the advanced manufacturing capabilities and high yields of the Si-microelectronics industry, there is intense research activity to realise CMOS compatible optoelectronics. One emerging strategy is to augment the optical properties of group-IV materials through band structure engineering, such as incorporating Sn into the Ge lattice. Hydrostatic pressure measurements of the GeSn absorption edge exhibit an intermediate pressure coefficient between that of the Γ and L conduction band critical points. This is indicative of strong band mixing effects in the GeSn alloy. In the presence of band mixing the conventional distinction between the indirect and direct band gap breaks down. Instead it is more appropriate to discuss the nature of the band gap in terms of the fractional Γ-character of the conduction band states at the band edge. The pressure coefficient of the absorption edge for samples with Sn content between 6% - 10% reveal a continuous evolution in the Γ-character with increasing Sn-concentration. High Γ-character is observed even at low Sn concentrations of 6%, when the GeSn alloy is expected to exhibit a fundamentally indirect band gap. These band mixing effects have important implications for designing efficient photonic and electronic devices utilizing GeSn and related material systems.
Supervisor: Sweeney, Stephen Sponsor: RCUK
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral