A Study of GaSb Quantum Ring Vertical-Cavity Surface-Emitting Devices at Telecommunications Wavelengths
Creators: name: Jones, Samuel
Publisher: name: Lancaster University
Year: 2025

description: 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.