# Quick Fabrication VCSELs for Characterisation of Epitaxial Material
## 1 Introduction  
Vertical cavity surface emitting lasers (VCSELs) have become ubiquitous in recent years due to their use as light sources in a wide range of applications from 3D imaging to optical fibre data transmission and, with the emerging technology of LiDAR, the market is rapidly expanding [1, 2, 3, 4]. This expansion has led wafer manufacturers to scale up production to larger substrates; currently 6-inch (152 mm) [5] and beyond [6], whilst work is also being done to maximise the uniformity over individual wafers, as well as between wafers from the same and separate growth runs. This relies on material characterisation methods that are compatible with a commercial manufacturing process, that is, rapid time-to-result with representative information output. This is typically managed with non-destructive wafer characterisation techniques such as defect scanning [7] and measurement of the reflectivity spectrum for information on the resonant wavelength of the VCSEL stack [8]. Destructive techniques such as capacitance-voltage measurements on etchedVCSEL material to validate doping levels [9] and secondary ion mass spectroscopy to measure material compositions [10] are employed on sacrificial wafers. Photoluminescence spectroscopy is also often used to obtain information on quantum well (QW) layers [11], but this involves growth of a test structure or removal of the top DBR from the full VCSEL material, which is less representative of the full structure. Ultimately, these methods of wafer characterisation aim to provide information that is used to predict yield and the final performance of fabricated VCSEL devices. Often, some VCSELs are fabricated from calibration material for light-current-voltage (L-I-V) and wavelength (\(\lambda\)) measurements to supplement the material characterisation. The information revealed is extremely valuable, but this can prove to be a time-consuming process. Here, we report on the development of a Quick Fabrication (QF) VCSEL test structure, that will enable epitaxial material to be characterised based on laser performance. Key figures of merit from the laser characterisation can be mapped across a wafer in the same way as commercial-standard measurements, but here we seek to assess the physical impact of a simplified (and therefore more rapid) fabrication process on performance for devices relative to typical VCSELs. In this work, we focus on VCSELs produced on AlGaAs-based epitaxial material designed for 940 nm emission wavelengths. Infrared emitting VCSELs have been widely shown to have low threshold currents resulting from a combination of advances in epitaxial design and improved processing [12; 13; 14; 15; 16], but the threshold current of a VCSEL is highly dependent on the size of the active area [17] and the particular device structure. Thus, we assess the size-dependent threshold current for the devices produced in this study and use this as a useful way to compare different design variations. Additionally, improved output powers have been achieved by rethinking the design and packaging of the VCSELs [18] and high single-mode output powers have been achieved at this emission wavelength by employing Zn-diffusion in the fabrication process [19; 20]. However, here, we are concerned only with top-emitting oxide-confined VCSELs. The optical power of VCSELs has been shown to increase with increasing active diameter [21; 22], but this has also been seen to saturate for large devices [23] and with increasing internal heating [24]. The optical power of the QF devices and the dependence on device size is examined in this study and a comparison of the relative efficiencies of the different designs is made. Further, VCSELs inherently operate at a single longitudinal mode due to the standing wave pattern formed by the epitaxial structure [25; 26] and, unlike edge-emitting lasers, the emission wavelength of a VCSEL is determined by the material cavity resonance, as opposed to the wavelength corresponding to peak gain.