This is the first demonstration of continuous-wave (CW) operation of nonpolar GaN-based VCSELs. These devices had a dual-dielectric distributed Bragg reflector (DBR) design with ion implanted apertures and III-nitride tunnel junction (TJ) intracavity contacts. Unlike c-plane devices, nonpolar GaN-based VCSELs have anisotropic gain that leads to a 100% polarization ratio and polarization-locked VCSEL arrays. Previous nonpolar devices were unable to lase under CW operation, notably due to the thermally-insulating bottom dielectric DBR. Based on thermal modeling using COMSOL, the main thermal pathway was restricted to a thin p-side metal contact that goes around the bottom DBR to the submount. Heat flow was further impaired as the Au-Au thermocompression flip-chip bond created cracks and voids in the p-side metal. The thermal performance was improved in our latest VCSELs by increasing the cavity length to 23λ and utilizing Au-In solid liquid interdiffusion bonding to create a more robust pathway for heat transport. This led to stable CW VCSEL operation for over 20 minutes. The peak output powers for a 6 μm aperture VCSEL under CW and pulsed operation were 150 μW and 700 μW, respectively. Lasing wavelengths were observed at 406 nm, 412 nm, and 419 nm. The fundamental transverse mode was observed without the presence of filamentary lasing.
III-nitride light emitters, such as light-emitting diodes (LEDs) and laser diodes (LDs), have been demonstrated and studied for solid-state lighting (SSL) and visible-light communication (VLC) applications. However, for III-nitride LEDbased SSL-VLC system, its efficiency is limited by the “efficiency droop” effect and the high-speed performance is limited by a relatively small -3 dB modulation bandwidth (<100 MHz). InGaN-based LDs were recently studied as a droop-free, high-speed emitter; yet it is associated with speckle-noise and safety concerns. In this paper, we presented the semipolar InGaN-based violet-blue emitting superluminescent diodes (SLDs) as a high-brightness and high-speed light source, combining the advantages of LEDs and LDs. Utilizing the integrated passive absorber configuration, an InGaN/GaN quantum well (QW) based SLD was fabricated on semipolar GaN substrate. Using SLD to excite a YAG:Ce phosphor, white light can be generated, exhibiting a color rendering index of 68.9 and a color temperature of 4340 K. Besides, the opto-electrical properties of the SLD, the emission pattern of the phosphor-converted white light, and the high-speed (Gb/s) visible light communication link using SLD as the transmitter have been presented and discussed in this paper.
We report on the lasing of III-nitride nonpolar, violet, vertical-cavity surface-emitting lasers (VCSELs) with IIInitride tunnel-junction (TJ) intracavity contacts and ion implanted apertures (IIAs). The TJ VCSELs are compared to similar VCSELs with tin-doped indium oxide (ITO) intracavity contacts. Prior to analyzing device results, we consider the relative advantages of III-nitride TJs for blue and green emitting VCSELs. The TJs are shown to be most advantageous for violet and UV VCSELs, operating near or above the absorption edge for ITO, as they significantly reduce the total internal loss in the cavity. However, for longer wavelength III-nitride VCSELs, TJs primarily offer the advantage of improved cavity design flexibility, allowing one to make the p-side thicker using a thick n-type III-nitride TJ intracavity contact. This offers improved lateral current spreading and lower loss, compare to using ITO and p-GaN, respectively. These aspects are particularly important for achieving high-power CW VCSELs, making TJs the ideal intracavity contact for any III-nitride VCSEL. A brief overview of III-nitride TJ growth methods is also given, highlighting the molecular-beam epitaxy (MBE) technique used here. Following this overview, we compare 12 μm aperture diameter, violet emitting, TJ and ITO VCSEL experimental results, which demonstrate the significant improvement in differential efficiency and peak power resulting from the reduced loss in the TJ design. Specifically, the TJ VCSEL shows a peak power of ~550 μW with a threshold current density of ~3.5 kA/cm2, while the ITO VCSELs show peak powers of ~80 μW and threshold current densities of ~7 kA/cm2.