Chih-Chiang Kao

CW lasing of current injection blue GaN-based vertical cavity surface emitting laser

Here, we report the cw laser operation of electrically pumped GaN-based vertical cavity surface emitting laser (VCSEL). The GaN-based VCSEL has a ten-pair InGaN∕GaN multiple quantum well active layer embedded in a GaN hybrid microcavity of 5λ optical thickness with two high reflectivity mirrors provided by an epitaxially grown AlN∕GaN distributed Bragg reflector (DBR) and a Ta2O5∕SiO2 dielectric DBR. cw laser action was achieved at a threshold injection current of 1.4mA at 77K. The laser emitted a blue wavelength at 462nm with a narrow linewidth of about 0.15nm. The laser beam has a divergence angle of about 11.7° with a polarization ratio of 80%. A very strong spontaneous coupling efficiency of 7.5×10−2 was measured.

Improvement of InGaN-GaN light-emitting diode performance with a nano-roughened p-GaN surface

This investigation describes the development of InGaN-GaN light-emitting diode (LED) with a nano-roughened top p-GaN surface which uses Ni nano-mask and wet etching. The light output of the InGaN-GaN LED with a nano-roughened top p-GaN surface is 1.4 times that of a conventional LED, and wall-plug efficiency is 45% higher. The operating voltage of InGaN-GaN LED was reduced from 3.65 to 3.5 V at 20 mA and the series resistance was reduced by 20%. The light output is increased by the nano-roughening of the top p-GaN surface. The reduction in the series resistance can be attributed to the increase in the contact area of nano-roughened surface.

Fabrication of InGaN/GaN nanorod light-emitting diodes with self-assembled Ni metal islands

We report the fabrication of InGaN/GaN nanorod light-emitting diodes (LEDs) using inductively coupled plasma reactive-ion etching (ICP-RIE) and a photo-enhanced chemical (PEC) wet oxidation process via self-assembled Ni nanomasks. An enhancement by a factor of six times in photoluminescence (PL) intensities of nanorods made with the PEC process was achieved in comparison to that of the as-grown structure. The peak wavelength observed from PL measurement showed a blue shift of 3.8 nm for the nanorods made without the PEC oxidation process and 8.6 nm for the nanorods made with the PEC oxidation process from that of the as-grown LED sample. In addition, we have demonstrated electrically pumped nanorod LEDs with the electroluminescence spectrum showing more efficiency and a 10.5 nm blue-shifted peak with respect to the as-grown LED sample.

GaN-based two-dimensional surface-emitting photonic crystal lasers with AlN∕GaN distributed Bragg reflector

GaN-based two-dimensional (2D) surface-emitting photonic crystal (PC) lasers with AlN∕GaN distributed Bragg reflectors are fabricated and demonstrated. The lasing threshold energy density is about 3.5mJ∕cm2 per pulse under optical pumping at room temperature. Only one dominant emission wavelength of 424.3nm with a narrow linewidth of 1.1A above the threshold is observed. The laser emission covers whole circularly 2D PC patterns (50μm in diameter) with a small divergence angle. The lasing wavelength emitted from 2D PC lasers with different lattice constants occurs at the calculated band-edges provided by the PC patterns. The characteristics of large area, small divergence angle, and single mode emission from the GaN-based 2D surface-emitting PC lasers should be promising in high power blue-violet emitter applications.

Light-output enhancement in a nitride-based light-emitting diode with 22° undercut sidewalls

We successfully fabricated nitride-based light-emitting diodes (LEDs) with ∼22° undercut sidewalls. The ∼22° etching undercut sidewalls were achieved by controllable inductively coupled plasma reactive ion etching. With a 20-mA current injection, the output powers of the LED with ∼22° undercut sidewalls and standard LED were 5.1 and 3 mW, respectively - a factor of 1.7 times enhancement. It was found that such undercut sidewalls could enhance the probability of escaping the photons outside from the LED in the near horizontal and in-plane directions. This simple and controllable method is beneficial to fabricate brighter LEDs.

Enhanced Output Power of GaN-Based LEDs With Nano-Patterned Sapphire Substrates

GaN-based light-emitting diodes (LEDs) with an emitting wavelength of 450 nm were grown on nano-patterned sapphire substrates (NPSS) fabricated by nanosphere lithography. The crystalline quality of the epitaxial film could be improved by using the NPSS technique. The output power of LED grown on NPSS was 1.3 and 1.11 times higher than those of LEDs grown on conventional and patterned sapphire substrates at the injection current of 20 mA, respectively. The enhancement in output power could be contributed to the efficiently scattering by NPSS. But some voids formed at the GaN/NPSS interface cause a thermal dissipation problem of NPSS LED operated at high injection current.

Nitride-based LEDs with nano-scale textured sidewalls using natural lithography

This investigation describes the development of a InGaN/GaN light-emitting diode (LED) with textured sidewalls using natural lithography with polystyrene spheres (PSs) as the etching mask and dry etching the epitaxial layers of LEDs to achieve nano-scale textured sidewalls. The LED with textured sidewalls increased the output power of the InGaN?GaN multiple quantum well (MQW) LEDs by a factor of 1.3, indicating that the LED with nano-scale textured sidewalls had larger light extraction efficiency. The wall-plug efficiency of nitride-based LEDs was increased by 30% using textured sidewalls.

Optically Pumped GaN-based Vertical Cavity Surface Emitting Lasers: Technology and Characteristics

We review the fabrication technology and performance characteristics of optically pumped GaN-based vertical cavity surface emitting lasers (VCSELs). Two types of VCSELs with different microcavity structures are described. First type of VCSEL has a hybrid microcavity structure that consists of an epitaxially grown AlN/GaN distributed Bragg reflector (DBR), a GaN active layer with InGaN/GaN multiple quantum wells (MQWs), and a Ta2O5/SiO2 dielectric DBR. Second type of VCSEL has a dielectric DBR microcavity structure that has a similar InGaN/GaN MQWs active layer sandwiched in two dielectric DBRs formed by Ta2O5/SiO2 and TiO2/SiO2. Both types of VCSELs achieved laser action under optical pumping at room temperature with emission wavelength of 448 and 414 nm for hybrid DBR VCSEL and dielectric DBR VCSEL, respectively. Both lasers showed narrow emission linewidth with high degree of polarization and large spontaneous emission coupling factors of about 10-2. In addition, a high characteristic temperature of over 240 K was measured, and a distinct spatially inhomogeneous emission pattern was observed.

Room-Temperature Operation of Optically Pumped Blue-Violet GaN-Based Vertical-Cavity Surface-Emitting Lasers Fabricated by Laser Lift-Off

Room-temperature optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) were demonstrated by laser lift-off. A VCSEL was fabricated by combining a GaN-based cavity with two dielectric distributed Bragg reflectors: SiO2/TiO2 and SiO2/Ta2O5. The Q factor of the VCSEL is 518 indicating a good interfacial layer quality of the structure. The laser emits blue-violet wavelength light at 414 nm under optical pumping at room temperature with a threshold pumping energy of 270 nJ. The laser emission has a narrow linewidth of 0.25 nm and a degree of polarization of 70%. The laser emission patterns clearly indicate a vertical lasing action of the VCSEL.

Characterization of InGaN/GaN Multiple Quantum Well Nanorods Fabricated by Plasma Etching with Self-Assembled Nickel Metal Nanomasks

High-density (3.0×1010 cm-2) InGaN/GaN multiple quantum well (MQW) nanorods were fabricated from an as-grown bulk light-emitting diode structure by inductively coupled plasma dry etching with self-assembled nickel metal nanomasks. The self-assembled nickel metal nanomasks were formed by rapid thermal annealing of a nickel metal film at 850°C for 1 min. The influence of the thicknesses of the Ni metal film on the dimensions and density of the nanorods was also investigated. The structural and optical properties of the InGaN/GaN MQW nanorods were established using field emission scanning electron microscopy, transmission electron microscopy and photoluminescence measurements. The diameters and heights of nanorods were estimated to be 60 to 100 nm and more than 0.28 µm, respectively. The peak emission wavelength of the nanorods showed a blue shift of 5.1 nm from that of the as-grown bulk. An enhancement by a factor of 5 in photoluminescence intensity of the nanorods compared with that of the as-grown bulk was observed. The blue shift is attributed to strain relaxation in the wells after dry etching, the quantum confinement effect, or a combination of the two, which results in the enhancement of emission intensity.