GaN-based optical resonant cavities

Abstract

The bandgap energies of the III-nitride semiconductors can potentially cover the spectral range from deep ultraviolets to the near-infrared, making them excellent candidates for optoelectronic applications. As a matter of fact, a range of devices including light-emitting diodes and laser diodes based on III-nitride semiconductors have been commercialized. Nevertheless, the III-nitride laser diodes are predominantly of the edge-emitting types, leaving plenty of room to explore alternative laser configurations. On the other hand, commercial GaN films grown epitaxially on sapphire substrates enjoys low dislocation density, however, the weak reflection provided by the GaN / sapphire interface poses a challenge to the formation of the optical resonant cavities. In this work, we study different optical resonant cavities fabricated on GaN thin-film substrate, which comprises a GaN film eutectically bonded to a Si substrate with the conductive n-GaN exposed. The eutectic bonding layer functions as a mirror to promote optical confinement, which can also support vertical conduction across the GaN film. GaN-based microdisks are whispering-gallery mode (WGM) optical resonant cavities that offer excellent optical confinement. However, emission from a microdisk is in-plane isotropic due to its circularity and thus is difficult to be extracted efficiently for practical applications. A common approach is to place a waveguide in close proximity to the circumference of the microdisk so as to allow evanescent wave coupling. The air-gap requires fine dimensional control which greatly limits the choice of fabrication method. An alternative scheme is to form a direct junction between the microdisk and the waveguide for the non-evanescent coupling. We demonstrate the non-evanescent coupling of the WGMs to a 0.16 µm-wide waveguide connected directly to the microdisk circumference on the GaN thin-film substrate based on microsphere and nanowire lithography. Vertical-cavity-surface-emitting-lasers (VCSEL) are attractive laser geometries which offer arraying capability and can potentially be operated with low thresholds. The development of III-nitride VCSELs has been hindered by the fabrication of effective reflectors. Conventionally, III-nitride distributed Bragg reflectors (DBRs) are employed to provide the required reflectivities. The growth of these multiple layers faces substantial challenges including thickness uniformity and interlayer strain. Due to the diffraction effect, a near-wavelength grating with suitably designed dimensions exhibits strong internal reflection. The grating reflectors are very compact as compared with DBRs. We demonstrate a vertical optical resonant cavity comprising a grating reflector on the GaN thin-film substrate. The enhanced internal reflection was due to the near-wavelength grating and the eutectic bonding layer. Our work paves the way towards further development of GaN VCSELs based on the thin-film architecture. Further analysis also shows that the grating reflector exhibits distributed feedback (DFB) lasing.