An alternative to chemical mechanical polishing for III-nitride nonpolar VCSELs

20th August 2014

Photoelectrochemical etching produces VCSELS with polarisation ratio of 100 percent 

Researchers from the University of California, Santa Barbara, have demonstrated an alternative to chemical-mechanical polishing and laser-lift off for removing the substrate of III-nitride VCSELs.

Called photoelectrochemical (PEC) band gap selective undercut etching, the technique has also been applied by the team to top down etching as a way to achieve a high degree of cavity length control.

The researchers described their use of PEC etching for fabricating 405nm-wavelength m-plane VCSELs based on InGaN multiple quantum wells, in Applied Physics Letters.

Nonpolar III-nitride optoelectronic devices have created a lot of research interest due to their potential to overcome disadvantages in c-plane III-nitride devices. In c-plane devices, the piezoelectric and spontaneous polarisation fields lie at right angles to the plane of the quantum wells (QWs), leading to the quantum confined Stark effect (QCSE).

Additionally, nonpolar m-plane lasers have been shown to have linear gain vs current characteristics, lower transparency carrier and current densities, higher peak material gain, and anisotropic gain characteristics, making the m-plane of particular interest for III-nitride lasers.

Compared to edge-emitting lasers, III-nitride VCSELs offer the advantage of a potential circular output beam, single longitudinal mode operation, low output beam divergence, reduced threshold current, reduced device footprint, high modulation frequency, and characteristic vertical emission normal to the substrate, enabling the fabrication of high density 2D arrays.

These characteristics suit them to 2D/3D displays, high density optical storage, pico-projectors, laser printing, laser-based lighting, bio-sensing, and Li-Fi communication. A number of groups have achieved room-temperature continuous-wave (CW) lasing with violet and blue c-plane VCSELs and pulsed lasing with green c-plane VCSELs and violet m-plane VCSELs.

But III-nitride VCSELs are tricky to make. They have commonly employed hybrid epitaxial/dielectric distributed Bragg reflector (DBR) design or a dual dielectric DBR design -  and both methods have problems.  

The dual dielectric DBR design, for example, which uses dielectric DBRs on the n-type and p-type sides of the VCSEL, requires flip-chip bonding and substrate removal  to deposit the DBR on the n-type side of the VCSEL. Typically, this has been done with laser lift-off and/or chemical-mechanical polishing (CMP), which can make cavity thickness control difficult, resulting in a misalignment of the Fabry-Perot (FP) wavelength (FP) and the peak gain wavelength (gain).

Aside from wafer uniformity issues, it is also difficult to fabricate single longitudinal mode cavities. 

PEC etching on m-plane VCSELs is a promising alternative method to substrate removal. According to the researchers, this technique enables epitaxially defined cavity-length control and epitaxial-like roughness on the n-type side of the flip-chipped VCSEL, effectively minimising cavity length uncertainties and scattering loss at the DBR on the n-type side of the VCSEL. Additionally, the relatively non-destructive nature of PEC undercut etching offers the potential for substrate reuse.

When they analysed the temperature-dependent lasing characteristics of m-plane VCSELs made using PEC etching techniques, measurements of multiple VCSELs from the same wafer indicated that the entire array was uniformly polarized along the a-direction with a polarization ratio of 100 percent, resulting from the intrinsic anisotropic gain of m-plane InGaN/GaN QWs.

'Nonpolar III-nitride vertical-cavity surface emitting lasers with a polarization ratio of 100% fabricated using photoelectrochemical etching' by CP Holder et al  is in Appl. Phys. Lett. 105, 031111 (2014) 

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