TECHNOLOGY VCSELs
monolithic structure presents a viable path toward high-volume manufacture of GaN-based VCSELs.
The EPFL team has also pioneered a simple method for forming a current aperture – passivating portions of the p-GaN with reactive-ion-etching, plasma treatment. With ITO acting as the current spreading layer, uniform light emission is possible throughout the aperture. Devices lased at 420 nm, produced a pulsed output of more than 300 µW and had a 70 mA threshold current. Researchers blamed the high threshold current on ITO absorption loss and insuffi cient refl ectivity in the top DBR. Options for cutting current are to trim the thickness of the ITO and tune its position.
Which way forward?
Over the last few years, progress of the GaN VCSEL has been signifi cant, but key challenges must be overcome before this device can enjoy commercial success. Researchers must pursue either new DBR technologies, or develop methods to generate stand-alone cavities that have well-controlled lengths and can be attached to dielectric DBRs. If air- gap bottom DBRs could be created by selective removal of every other
λ/4-layer using band-gap-selective PEC etching, only several periods would
be needed to produce extremely high refl ectivities. However, fabrication of such structures is very tough. So it may be that further optimisation of the AlInN DBR scheme developed at EPFL leads to the fi rst manufacturable GaN-based VCSEL.
What is clear is that if approaches involving substrate removal via backside wafer thinning are to become commercially viable, they will require more precise control of cavity length. This might be possible with a combination of thinning and band-gap- selective PEC etching, which could planarize the cavity with an embedded stop-etch layer placed at the desired cavity thickness. Full substrate removal using PEC etching also presents challenges. However, this approach does provide cavity length control and it enables the recycling of expensive, free- standing GaN substrates.
Another area requiring improvement is that of the current spreading layers. Addressing this is essential for the realisation of larger-area devices that will deliver higher output powers. The ITO
Figure 5. The emission polarization is aligned along the [1210] (a-direction) for the non-polar GaN- based VCSEL. Polarization ratio is close to one
technology that is widely used today has been pushed to its performance limits, so further progress will hinge on the introduction of novel injection schemes and materials. This could involve the introduction of reliable, low-resistance tunnel junctions in GaN. Armed with this refi nement, engineers would not have to concern themselves with the current spreading issue, because p-GaN could be replaced with higher conductivity n-GaN. Alternatively, developers of GaN VCSELs could increase current spreading by employing novel, two-dimensional materials with high transparency and high lateral conductivity.
Lastly, efforts must be directed at a systematic examination of the non-polar and semi-polar orientations, to uncover their potential for increasing optical gain and reducing threshold current. Increasing the per-pass-gain would permit DBRs with lower refl ectivity, relaxing the design space. Success of green edge-emitters built on semi-polar planes motivates exploration of this orientation for green VCSELs. Non-polar and semi- polar orientations might also enable high-power VCSEL arrays with uniformly polarized emission characteristics.
© 2014 Angel Business Communications. Permission required.
Further reading T. C. Lu et. al. Appl. Phys. Lett. 92 141102 (2008) Y. Higuchi et. al. Appl. Phys. Express 1 121102 (2008) K. Omae et. al. Appl. Phys. Express 2 052101 (2009) T. C. Lu et. al. Appl. Phys. Lett. 97 071114 (2010) D. Kasahara et. al. Appl. Phys. Express 4 072103 (2011) T. Onishi et. al. IEEE J. Quant. Electron. 48 1107 (2012) C. Holder et. al. Appl. Phys. Express 5 092104 (2012) G. Cosendey et. al. Appl. Phys. Lett. 101 151113 (2012).
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