Whispering-gallery mode lasing in GaN microdisks

The wide range of wavelengths covered by the Gallium nitride (GaN) group of materials is probably the major reason for its growing use by the semiconductor industry. GaN light emitting diodes are used extensively for illumination and display purposes,¹ while UV emitters have been employed in a wide range of biomedical applications.² High-performance GaN transistors have also been demonstrated. The laser diodes emitting at 405nm and found in the latest high-density DVD players are also GaN-based. Shorter wavelengths allow data to be packed more tightly, so it becomes possible to fit more data on a disc even though it is the same size as a CD/DVD.

GaN laser diodes are currently fabricated in the conventional edge-emitting geometry, where the laser light propagates parallel to the wafer surface of the semiconductor. However, the newer surface-emitting versions, where the light propagates in the direction perpendicular to the semiconductor surface, offer more advantages. The first is their lower fabrication cost since the chips can be tested on-wafer, unlike edge-emitters that can only be tested at the end of the fabrication process. They also offer beams with lower divergence due to large output apertures, low power consumption, and wavelength tuning.

However, the transition from edge to surface emission is not trivial in GaN structures. In principle, a surface-emitting laser requires a vertical cavity with top and bottom mirrors. In practice, high quality GaN materials are grown on sapphire substrates, which makes the insertion of bottom mirrors inherently difficult. One common solution is the deposition of AlGaN/GaN distributed Bragg reflector stacks between the device structure and the sapphire substrate.³ Alternatively, undercut microdisk lasers that address mirror insertion challenges have also been fabricated using selective photoelectrochemical etching techniques.⁴

Recently, we have also developed a microdisk fabrication method based on GaN grown on Si substrates. Pivoted GaN undercut microdisks were fabricated on GaN/Si templates using a combination of dry and wet etching processes, enabling the exposure of the bottom GaN surface.⁵ In our approach, 20μm-microdisks are first defined by photolithography and subsequently transferred to a 1μm-thick GaN layer by inductively coupled plasma etching. The sample is dipped into an isotropic wet etchant which creates the undercut as it removes the Si surrounding the GaN microdisks. The wet-etching process is allowed to proceed until only a tiny Si pedestal remains to support the microdisk while providing a pathway for current injection. This is illustrated in Figure 1(a), showing the cross-sectional SEM image of a microdisk. A microdisk array is also shown in Figure 1(b).

Figure 1. Scanning-electron-microscope images showing (a) the cross section of a single pivoted microdisk and (b) a microdisk array.

The photoluminescence spectra of as-grown and microdisk samples at low excitation energies are shown in Figure 2(a). Two peaks can be seen at ∼360 and ∼400nm, corresponding to emission from GaN and InGaN/GaN multiple quantum wells respectively. Additionally, periodic resonant modes can be observed in microdisk spectra, with a mode spacing of about 1.1nm, assigned to whispering gallery mode oscillations, as predicted by the relation Δλ_WG=λ² 2π Rn.⁶ With increasing excitation power, non-linear increases—indicative of stimulated emission—are also observed in the intensities of the 365nm-peak beyond threshold conditions, as depicted in Figure 2(b).

Figure 2. (a) Luminescence spectrum of As-grown and microdisk samples. (b) Luminescence spectrum of the microdisk at increasing excitation powers. A plot of integrated emission intensity vs excitation energy is shown in the inset.

To conclude, a micro-cavity structure consisting of microdisks with undercuts fabricated on a GaN/Si substrate can enable optical confinement via whispering gallery modes. Besides the observation of resonant modes, lasing action is also achieved by optical pumping. Current-injection lasing also seems promising since both microdisk surfaces are accessible for mirror coating and since the complete structure is also vertically conducting. We anticipate that our technique will translate into a wide range of possibilities for the fabrication of GaN-based resonant cavity devices, and may even represent a significant step towards realizing the elusive GaN vertical cavity surface-emitting laser.