# Quick Fabrication VCSELs for Characterisation of Epitaxial Material
## 3 Results and Discussion  
### Threshold Current  
We begin by analysing the performance of the full VCSEL structures, which is used as a baseline from which we compare the performance of the less complex QF device structures. Threshold currents were found to be sub-mA for devices <10 \(\upmu\)m diameter oxide aperture. Threshold currents for the complete set of devices ranged between 0.37 and 1.46 \(\pm\) 0.01 mA for 6-12 \(\upmu\)m apertures. The threshold currents quoted in this work were extracted from L-I curves by the second-derivative method and the uncertainty quoted is determined by the resolution of the current source and applies to all threshold current values presented here. It is well known that the threshold current of a diode laser scales with increasing active volume [17], and this is shown in Figure 2 for these VCSEL structures, where the uncertainty in the aperture is due to the resolution of the measurement of the oxidation extent under an infrared camera. The rate of oxidation increases as the device size reduces, hence there is a greater uncertainty in the aperture diameter for the smallest mesas. Additionally, for a 4 \(\upmu\)m aperture device, we see an increase in threshold current to 0.60 mA. This has been explained previously by an increased internal optical loss due to scattering at the oxide layers for small apertures [34; 35; 36], greater sidewall recombination as device diameter is reduced [17; 37; 38] and an increased internal temperature [28; 39; 40] driven by the increased series resistance of small aperture devices. The threshold currents for oxidised QF VCSELs are also plotted in Figure 2, and here we observe a minimum threshold current for a 6 \(\upmu\)m aperture QF device, which increases to 2.9 and 4.6 mA for 4 and 18 \(\upmu\)m aperture QF devices, respectively. The QF bridge-mesa design facilitates the deposition of a bond pad without the need for BCB planarisation and simplifies the feed metal deposition, which results in a 37% reduction in processing time. This design does, however, create a parallel current leakage path away from the cylindrical VCSEL cavity which results in the offset in threshold current that we observe for the QF devices. The effect of the leakage path is seen in the near-field imaging as light emission along the bridge, which is shown in Figure 3a, where this is not present in the standard VCSEL device (Figure 3b). Figure 2: Threshold currents of standard VCSELs and oxidised QF VCSELs. The increase in threshold current for the 4 \(\upmu\)m aperture QF device in comparison to the 6 \(\upmu\)m aperture QF device is due in part to the loss mechanisms previously described, however, there is also a proportionally greater leakage current along the bridge due to the high series resistance of the VCSEL in comparison to the larger aperture devices. This explains why the size of the increase with respect to the 6 \(\upmu\)m aperture threshold current minimum is greater for the QF devices (0.5 mA) than for the standard VCSELs (0.2 mA), the magnitude of which lies outside the margin of uncertainty determined by the threshold current value calculations. An estimate of the leakage current associated with the bridge-mesa design was made by evaluating the offset in the linear plots of threshold current versus active area, defined here as the area of the oxide aperture. These plots are shown in Figure 4. We exclude outlying high threshold current devices for the evaluation of the leakage offset. By comparing intercepts of the fits, we find a 1.5 \(\pm\) 0.02 mA offset, which gives the lower limit to the bridge leakage current. There is also an enhancement of surface recombination given that the QF VCSEL mesa sidewalls are exposed relative to the BCB-passivated standard VCSEL mesa, however, this can be treated as another contribution to the leakage current. The leakage current value is calculated from a difference in threshold current, hence the quoted uncertainty is determined by the error in both in threshold current values.