A Study of GaSb Quantum Ring Vertical-Cavity Surface-Emitting Devices at Telecommunications Wavelengths

Jones, Samuel and Hayne, Manus and Davies, J. Iwan (2025) A Study of GaSb Quantum Ring Vertical-Cavity Surface-Emitting Devices at Telecommunications Wavelengths. PhD thesis, Lancaster University.

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Abstract

This work presents the results of studies that were conducted in the context of gallium antimonide (GaSb) quantum ring (QR) vertical-cavity surface-emitting lasers (VCSELs). The aim of this project was to produce GaSb QR VCSELs operating at telecomswavelengths for applications in data transfer and in mobile device sensing. Simulations of the optical and electrical properties of distributed Bragg reflectors (DBRs) and VCSELs were performed in order to inform and optimise the design of several generations of QR VCSELs. Numerous experimental techniques were used to characterise and provide insight into the optical, electrical and material properties of samples produced throughout the duration of this project, and to enable comparison of designs, models and experiments. Characterisation techniques included optical transmittance and reflectance measurements, electrical characterisation, photoluminescence and time-resolved photoluminescence (PL and TRPL), electroluminescence (EL), scanning electron and transmission electron microscopy (SEM and TEM), energy-dispersive Xray spectroscopy (EDS), beam-exit cross-sectional polishing in conjunction with atomic force microscopy (BEXP+AFM) and X-ray photoelectron spectroscopy (XPS). Epitaxial growth was performed in-house using molecular beam epitaxy (MBE), and externally using an industrial metal organic chemical vapour deposition (MOCVD) reactor. Device fabrication and processing studies were conducted in the Lancaster Quantum Technology Centre (QTC). A study into the annealing of Au/Ni/Au contacts to p-type GaAs yielded a new ohmic contact scheme with specific contact resistance as low as (1.6 ± 0.2) × 10−5 Ω cm2. An in-house reflectance monitoring setup was shown to provide accurate measurements of layer thickness for GaAs and AlAs layers grown by MBE and demonstrated that the approach is suitable for the calibration of group-III growth rates for future GaSb QR VCSEL growth runs. DBRs with a step-graded interface between GaAs/Al0.9Ga0.1As layers and grown by MBE show improved electrical resistance when compared to ungraded structures. The step-graded structures are p-doped with beryllium as highly as 5 × 1018 cm−3 without detriment due to the low optical absorption at 1300 nm. A set of continuously-graded DBRs were grown by IQE using MOCVD and demonstrated comparable electrical performance to conventional AlxGa1−xAs-based DBRs. The peak reflectance of such structures approached that of the conventional DBRs; however, the conventional structures exhibited a broader stopband. TRPL measurements provided new insight into the carrier recombination lifetimes of QR structures in the presence of an optical cavity and stimulate further interest in the area. QR VCSEL devices are shown to operate within the telecoms ‘O’-band but are plagued by background emission. Low temperature (77 K) operation of the devices reveals interesting device physics associated with the quenching of the output emission when driven at high biases. Improvements to the QR VCSELs are presented as well as avenues in need of further investigation regarding the fundamental device physics and the measurement of device performance.