# Quick Fabrication VCSELs for Characterisation of Epitaxial Material
## 2 Materials and Methods  
The epitaxial structure used in this study consists of a multiple quantum well active region sandwiched between upper \(p\)-doped and lower \(n\)-doped GaAs/AlGaAs distributed Bragg reflector (DBR) mirror pairs and is designed for 940 nm emission wavelength. A high aluminium content layer was included between the upper DBR and above the confinement layers of the active region for definition of the VCSEL aperture. The epi-structures were grown via MOCVD on n+ GaAs substrates and provided by IQE plc. Devices from several variations on a quick fabrication process were compared with the performances of typical commercial-standard VCSELs, which we refer to as standard VCSELs. The QF structures were based on an etched-trench bridge-mesa design, whereby the central cylindrical VCSEL cavity is not fully isolated from the surrounding planar material. The remaining material is referred to as the bridge, as this facilitates deposition of a bond pad in the same plane as the p-metal ring contact without the need for a planarisation process step. A SEM image of a QF VCSEL mesa is shown in Figure 1a, with that of a standard VCSEL in Figure 1b. Fabrication followed common processing techniques which, for the standard VCSEL structures was as follows: definition of the mesa by inductively coupled plasma (ICP) etch to just below the active layers, a depth of \(\sim\)3 \(\upmu\)m. Wet thermal oxidation of the samples at 400 \({}^{\circ}\)C to form the oxide apertures with an oxidation depth of 5.5 \(\pm\) 0.5 \(\upmu\)m. The samples were spin-coated with benzocyclobutane (BCB) and subsequently etched-back to planarise the material to the height of the mesas. Ti/Au (10/500 nm) \(p\)-metal ring contacts and bond pads were deposited simultaneously on the top of the samples, followed by standard AuGe/Ni/Au deposition for the global substrate-side \(n\)-metal contact. The process for the Quick Fabrication VCSELs was as follows: a blanket-coat of 200 nm SiN was deposited and used as the etch mask for definition of the mesa by inductively coupled plasma (ICP) etch, as was done for the standard VCSELs. Here, though, the SiN was patterned to be used as an insulating layer to prevent electrical pumping underneath the bond pad, given that the mesas were not fully isolated. The samples were oxidised in the same conditions as for the standard VCSELS, but for a total oxidation length of 12 \(\upmu\)m. The deposition for the top p-metal and substrate-side \(n\)-metal layers were the same as for the standard VCSELs. However, prior to the deposition of the \(n\)-contact, the substrate of some samples was mechanically lapped, thinning the substrate from 600 \(\upmu\)m to approximately 150 \(\upmu\)m, followed by a polishing process. While substrate thickness is usually reduced for ease of die singulation, for the purpose of testing QF VCSELs this step is not a necessary requirement; here, the effect of removing this step is carried out to understand the impact on device performance. Light-current-voltage measurements were performed on a range of device sizes using a CW current source with an integrating sphere for light collection and power measurement. Spectral (\(\lambda\)) measurements were done by coupling light into an optical spectrum analyser (resolution 0.1 nm) using a focusing lens and multimode fibre connect. The VCSELs were mounted on a temperature-controlled stage to facilitate temperature dependent measurements between 23 and 80 \({}^{\circ}\)C. Near-field images for the devices were obtained using a focusing lens to collect light which was then detected with a CCD camera. More time-consuming measurements, such as wavelength variation with current and temperature, were performed on a reduced number of devices based on L-I-V-A screening. Additionally, surface photovoltage spectroscopy (SPVS) [31; 32; 33] was used to measure the cavity mode wavelength of the epi-material. SPVS involves shining wavelength-varied  
Figure 1: (a) SEM Image of a Quick Fabrication (QF) VCSEL; (b) SEM image of a standard VCSEL.