Central to the LED lighting revolution, optoelectronics based on III-nitride materials have far-reaching applications in consumer displays, visible light communication, industrial, defense, and scientific markets. Development of (Al,In,Ga)N material growth and fabrication has matured yielding blue LEDs and laser diodes with outstanding efficiency. New technologies are needed to expand the applicable wavelength of GaN emitters. The unique optical and mechanical properties of porous GaN enable potential to disrupt traditional µLEDs, edge emitting laser diodes, and vertical cavity surface emitting lasers.
Blue laser diodes rely on index contrast provided by AlGaN or InGaN in lower cladding, but defects introduced through lattice mismatch limit the thickness and composition. Porous GaN is not only lattice matched to GaN, but has a high index contrast, making it an ideal candidate for optical confinement. Several batches of edge emitting lasers were fabricated with porous GaN. Lateral mode confinement was observed by a tapering 20% porous layer, allowing estimates for the electrical and thermal properties. Lasers with porous cladding reached threshold at 4 kA/cm2 under pulsed operation at a wavelength of 454 nm. The low slope efficiency of 0.24 W/A and considerable excess loss over 60 cm-1 was attributed to scattering in parasitic porous layers. Laser action was observed under continuous wave operation with thermal rollover due to internal heating. Green lasers with porous cladding at 510 nm were fabricated fixing the etch selectivity with slope efficiency of 0.13 W/A and threshold current density of 14 kA/cm2. The much lower voltage kept the peak wall plug efficiency of 0.9% comparable to typical research lasers at these wavelengths, but the combination of low injection efficiency and high loss cause performance to lag behind state-of-the-art.
The dearth of high index contrast III-nitride material similarly affects visible wavelength vertical cavity surface emitting lasers. GaN VCSELs have complex fabrication schemes or excessively long difficult growths. Instead, porous GaN can be used to make epitaxial high reflectivity mirrors by electrochemical etching. Porous GaN mirrors were fabricated exhibiting more than 95% reflectivity at the target wavelength. Hybrid VCSEL cavities constructed with porous GaN and dielectric mirrors demonstrated electroluminescent spectra with narrow emission modes, but no laser action.
Improving longer wavelength efficiency also requires high quality active material. Porous GaN enables a novel method to mechanically relax InGaN and incorporate higher indium content at higher growth temperature. While this has been achieved on sapphire substrates, there are benefits and challenges associated with heterogeneous integration on silicon substrates. In0.07Ga0.93N layers were grown on silicon with relaxation levels over 65%. Micro-LEDs grown and fabricated on top of the relaxed porous GaN templates had a 20 nm wavelength red shift in the porous devices, but with low emission power and significant leakage current.
