Investigation of Scattering Losses in a Buried Tunnel Junction 4 um GaSb VCSEL

The aim of this work is to investigate the scattering losses induced by the buried tunnel junction (BTJ) in a VCSEL operating at 4 ?m for gas spectroscopy [1]. In this device, losses are even more important than in a standard VCSEL, due to the presence of an active region (AR) of type II which displays lower gain compared to type I. This is due to a reduced carrier wavefunction overlapped integral. Therefore a robust optical design should guarantee minimal losses. In fact, the presence of the BTJ inside the cavity enables scattering losses, which is a very well known problem in long wavelength VCSEL; in [2] the effect of the BTJ size in a VCSEL emitting at 2:3 ?m was investigated by keeping the etching height constant and varying the step aperture and smoothness. In our work, scattering losses are analyzed by mean of a full 3D optical solver, VELM (Vcsel ELectroMagnetic) [3], and a parametrical analysis of the height and radius of the BTJ and top dielectric DBR size is performed in order to estimate the diffraction losses contribution in our device. In order to evaluate scattering losses, we assume lossless materials and compute the 1D optical threshold in 80 cm-1, which correspond to DBR radiation losses. In Fig. 1 we report a skecth of the simulated VCSEL with the material refractive index and the electric field map related to two different BTJ depth with same aperture size. In this analysis the AR size has been assumed to be large as the BTJ aperture plus the carrier diffusion length effect that has been estimated to contribute about 4 ?m on each side [1]. Comparing the two threshold gains, it can be grasped that a larger contribution of diffraction losses is given by the thickness of the BTJ: 43 cm?1 for the shallow TJ (22 nm) and as much as 270 cm-1 for the thicker one (53 nm). This open up perspective of improved second generation devices. The higher losses in the thicker BTJ are also visible by the skew rays appearing in the maps. Finally, the mode discrimination between fundamental mode (FM) and first order mode (FOM) is also optimized vs. BTJ aperture and thickness, so that to achieve stable single mode emission.