Effects of Initial Thermal Cleaning Treatment of a Sapphire Substrate Surface on The GaN Epilayer

The effects of the in situ thermalcleaning treatment of a sapphire substrate surface in hydrogenambient on the structural, optical, and electrical properties ofepitaxial GaN films grown by metalorganic chemical vapor deposition(MOCVD). Hall effect, X-ray diffraction (XRD), and photoluminescencemeasurements of GaN films grown at 1040°C clearly indicatethat the film quality is strongly affected by the thermal cleaningtreatment of the substrate surface. GaN films under the optimizedthermal cleaning treatment at 1070°C for 10 min showedminimum full-widths at half maximum (FWHMs) of 273 arcsec and728 arcsec for XRD peaks, respectively. In addition,the FWHM of the band-edge emission peak was as narrow as 28.3 meV,and the intensity ratio between the band edge emission and the yellowband emission was as high as 100 at room temperature. It was alsofound that the roughness of sapphire surface was reduced after thethermal treatment.

GaN micro-light-emitting diodes (micro-LEDs) with monolithically integrated microlenses have been demonstrated. Microlenses, with a focal length of 44 μm and a root mean square roughness of ∼1 nm, have been fabricated on the polished back surface of a sapphire substrate of an array of micro-LEDs by resist thermal reflow and plasma etching. The optical properties of the microlenses have been demonstrated to alter the emission pattern of the LED emitters. The cone of light emitted from this hybrid device is significantly less divergent than a conventional broad-area device. This combination of micro-LED and microlens technologies offers the potential for further improvement in the overall efficiency of GaN-based light emitters.


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