Air-suspended TiO₂-based HCG reflectors for visible spectral range

Abstract

For GaN-based microcavity light emitters, such as vertical-cavity surface-emitting lasers (VCSELs) and resonantcavity light emitting diodes (RCLEDs) in the blue-green wavelength regime, achieving a high reflectivity wide bandwidth feedback mirror is truly challenging. The material properties of the III-nitride alloys are hardly compatible with the conventional distributed Bragg reflectors (DBRs) and the newly proposed high-contrast gratings (HCGs). Alternatively, at least for the top outcoupling mirror, dielectric materials offer more suitable material combinations not only for the DBRs but also for the HCGs. HCGs may offer advantages such as transverse mode and polarization control, a broader reflectivity spectrum than epitaxially grown DBRs, and the possibility to set the resonance wavelength after epitaxial growth by the grating parameters. In this work we have realized an air-suspended TiO 2 grating with the help of a SiO 2 sacrificial layer. The deposition processes for the dielectric layers were fine-tuned to minimize the residual stress. To achieve an accurate control of the grating duty cycle, a newly developed lift-off process, using hydrogen silesquioxan (HSQ) and sacrificial polymethyl-methacrylate (PMMA) resists, was applied to deposit the hard mask, providing sub-10 nm resolution. The finally obtained TiO 2 /air HCGs were characterized in a micro-reflectance measurement setup. A peak power reflectivity in excess of 95% was achieved for TM polarization at the center wavelength of 435 nm, with a reflectivity stopband width of about 80 nm (FWHM). The measured HCG reflectance spectra were compared to corresponding simulations obtained from rigorous coupled-wave analysis and very good agreement was found.

All‐Dielectric Heterogeneous Metasurface as an Efficient Ultra‐Broadband Reflector

“…Alternative materials such as TiO 2 have been reported to replace Si for visible range applications, [34][35][36] but the reflection characteristics are not as good due to its lower refractive index, compared to that of Si or GaAs in IR range.…”

Progress and challenges in electrically pumped GaN-based VCSELs

“…More details about the characterization method and constraints to be able to estimate the absolute reflectance of these HCG structures can be found in Ref. 82 . Simulations: normal incidence Fig.…”

“…We proposed a new approach to achieve HCGs for GaN-based VCSELs using dielectric materials instead 82 . TiO2 is a dielectric material which has a refractive index similar to that of GaN and has a very low optical absorption for wavelengths above 400 nm.…”

“…81 This concept offers a more mechanically rigid structure, but the lower index contrast results in a much smaller fabrication window to achieve a reflectivity above 99%. An alternative approach for an HCG for IIInitride-based light-emitters is a free-standing dielectric HCG in TiO2 realized by a sacrificial SiO2 layer 82 . It offers approximately the same high index contrast as that of free-standing GaN HCGs, since the refractive index of TiO2 is about 2.6 at a wavelength of 450 nm, with a negligible absorption for wavelengths above 400 nm.…”