K. H. Kim

III-Nitride Blue and Ultraviolet Photonic Crystal Light Emitting Diodes

We present results on enhancement of 460 nm blue and 340 nm UV optical power output in III-nitride light emitting diodes (LEDs) using photonic crystals (PCs) under current injection. Triangular arrays of the PCs with diameter/periodicity of 300/700 nm were patterned using electron-beam lithography and inductively coupled plasma dry etching. The total power at 20 mA of 300×300 μm2 unpackaged LED chips revealed an increase by 63% and 95% for the blue and UV LEDs, respectively, as a result of the PC formation. Possible ways for further improving enhancement of light extraction using PCs are discussed.

Nitride deep-ultraviolet light-emitting diodes with microlens array

We report on the fabrication of 280-nm AlGaN-based deep-ultraviolet light-emitting diodes (UV LEDs) on sapphire substrates with an integrated microlens array. Microlenses with a diameter of 12μm were fabricated on the sapphire substrate by resist thermal reflow and plasma dry etching. LED devices were flip-chip bonded on high thermal conductive AlN ceramic submounts to improve the thermal dissipation, and the emitted UV light was extracted through the sapphire substrates. With the integrated microlens array, a 55% enhancement in the output power at 20-mA dc driving was achieved compared with the same LED without microlens. The light extraction enhancement is the result of the reduced internal reflections of the light caused by the microlens surface profile.

Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes

III-nitride photonic crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were fabricated. Triangular arrays of the PCs with different diameters∕periodicities were patterned using electron-beam lithography and inductively coupled plasma dry etching. The optical power output of LEDs was enhanced by a factor of 2.5 due to PC formation. It was observed that the optical enhancement factor depends strongly on the lattice constant and hole size of the PCs. The achievement of nitride PCs is expected to benefit many applications of III-nitride optoelectronics, particularly for the improvement of extraction efficiency in III-nitride deep-UV emitters (λ<340nm), which are crucial for many important applications, but presently have a very low quantum efficiency.

Polarization of III-nitride blue and ultraviolet light-emitting diodes

Polarization-resolved electroluminescence studies of III-nitride blue and ultraviolet (UV) light-emitting diodes (LEDs) were performed. The LEDs were fabricated on nitride materials grown by metalorganic chemical vapor deposition on sapphire substrates (0001). Transverse electric (TE) polarization dominates in the InGaN∕GaN quantum-well (QW) blue LEDs (λ′=458nm), whereas transverse magnetic (TM) polarization is dominant in the AlInGaN QW UV LEDs (λ=333nm). For the case of edge emission in blue LEDs, a ratio (r=I⊥∕I‖) of about 1.8:1 was observed between the EL intensities with polarization E⊥c (TE mode) and E‖c (TM mode), which corresponds to a degree of polarization ∼0.29. The UV LEDs exhibit a ratio r of about 1:2.3, corresponding to a degree of polarization ∼0.4. This is due to the fact that the degree of polarization of the bandedge emission of the AlxInyGa1−x−yN active layer changes with Al concentration. The low emission efficiency of nitride UV LEDs is partly related to this polarization property. P...

III-nitride ultraviolet light-emitting diodes with delta doping

We present the results on the fabrication and characterization of 340 nm UV light-emitting diodes (LEDs) based on InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition. By employing δ doping in the n- and p-type layers, we have demonstrated enhanced LED structural quality and emission efficiency. Combining with our interconnected microdisk LED architecture, the output power of a 300×300 μm2 bare LED chip measured from the sapphire side reached 50 μW under a standard dc operation condition (20 mA) at 4.6 V and 1.6 mW under a pulsed driving current.

Electrical and optical properties of Mg-doped Al0.7Ga0.3N alloys

Mg-doped Al0.7Ga0.3N epilayers (∼1μm) were grown on an AlN/sapphire template by metalorganic chemical vapor deposition and the electrical and optical properties of these epilayers were studied. For optimized Mg-doped Al0.7Ga0.3N epilayers, we have obtained a resistivity around 105Ωcm at room temperature and confirmed p-type conduction at elevated temperatures(>700K) with a resistivity of about 40Ωcm at 800 K. From the temperature dependent Hall effect measurement, the activation energy of Mg acceptor is found to be around 400 meV for Al0.7Ga0.3N alloy. The optimized Mg-doped Al0.7Ga0.3N epilayers have been incorporated into the deep-ultraviolet (UV) (λ<300nm) light-emitting diode (LED) structures as an electron blocking layer. An enhancement in the performance of the UV LEDs was obtained. LEDs with peak emission wavelengths at 280 nm were fabricated with a circular geometry (300 μm disk diameter). Output power reached 0.35 mW at 20 mA and 1.1 mW at 150 mA dc current. The importance of Mg-doped Al0.7Ga0.3N...

Transport properties of highly conductive n-type Al-rich AlxGa1-xN (x≥0.7)

We report here the growth and transport studies of conductive n-type AlxGa1−xN alloys with high Al contents (x⩾0.7). Si-doped AlxGa1−xN alloys were grown by metalorganic chemical vapor deposition on AlN-epilayer∕sapphire substrates with very smooth surface. Low n-type resistivities have been obtained for Al-rich AlxGa1−xN alloys. The resistivity was observed to increase rapidly with increasing x due to the deepening of the Si donor energy level. Transport measurements have indicated that we have achieved n-type conduction in pure AlN. From the temperature dependence of the resistivity, the donor activation energy was estimated to vary from 23to180meV as x was increased from 0.7 to 1.0.

Enhanced p-type conduction in GaN and AlGaN by Mg-δ-doping

Mg-δ-doping in GaN and AlGaN epilayers has been investigated by metalorganic chemical vapor deposition. It was demonstrated through electrical, optical, and structural studies that Mg-δ-doping improves not only p-type conduction, but also the overall quality of p-type GaN and AlGaN epilayers. A twofold (fivefold) enhancement in lateral (vertical) p-type conduction have been achieved for GaN and AlGaN epilayers. It is argued that the observed dislocation density reduction (of about one order of magnitude) is due to the growth interruption in the Mg-δ-doping duration that partially terminates the dislocation propagation in the growth direction. Furthermore, Mg-δ-doping also reduces Mg impurity self-compensation and enhances hole concentrations in Mg-δ-doped GaN or AlGaN.

AlGaN-based ultraviolet light-emitting diodes grown on AlN epilayers

AlGaN-based deep-ultraviolet light-emitting diode (LED) structures, which radiate light at 305 and 290nm, have been grown on sapphire substrates using an AlN epilayer template. The fabricated devices have a circular geometry to enhance current spreading and light extraction. Circular UV LEDs of different sizes have been characterized. It was found that smaller disk LEDs had higher saturation optical power densities but lower optical powers than the larger devices. This trade-off between power and power density is a result of a compromise between electrical and thermal resistance, as well as the current crowding effect (which is due to the low electrical conductivity of high aluminum composition n- and p‐AlGaN layers). Disk UV LEDs should thus have a moderate size to best utilize both total optical power and power density. For 0.85mm×0.85mm interdigitated LEDs, a saturation optical power of 2.9mW (1.8mW) at 305nm (290nm) was also obtained under dc operation.

Growth and optical properties of InxAlyGa1−x−yN quaternary alloys

InxAlyGa1−xN quaternary alloys with different In and Al compositions were grown by metalorganic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. It was observed that the dominant optical transition at low temperatures in InxAlyGa1−xN quaternary alloys was due to localized exciton recombination, while the localization effects in InxAlyGa1−xN quaternary alloys were combined from those of InGaN and AlGaN ternary alloys with comparable In and Al compositions. Our studies have revealed that InxAlyGa1−xN quaternary alloys with lattice matched with GaN epilayers (y≈4.8x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensity (or quantum efficiency) in InxAlyGa1−x−yN quaternary alloys than that of GaN. The quantum efficiency of InxAlyGa1−xN quaternary alloys was also enhanced significantly over AlGaN alloys with a comparable Al content. These results ...In{sub x}Al{sub y}Ga{sub 1-x}N quaternary alloys with different In and Al compositions were grown by metalorganic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. It was observed that the dominant optical transition at low temperatures in In{sub x}Al{sub y}Ga{sub 1-x}N quaternary alloys was due to localized exciton recombination, while the localization effects in In{sub x}Al{sub y}Ga{sub 1-x}N quaternary alloys were combined from those of InGaN and AlGaN ternary alloys with comparable In and Al compositions. Our studies have revealed that In{sub x}Al{sub y}Ga{sub 1-x}N quaternary alloys with lattice matched with GaN epilayers (y{approx}4.8x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensity (or quantum efficiency) in In{sub x}Al{sub y}Ga{sub 1-x-y}N quaternary alloys than that of GaN. The quantum efficiency of In{sub x}Al{sub y}Ga{sub 1-x}N quaternary alloys was also enhanced significantly over AlGaN alloys with a comparable Al content. These results strongly suggested that In{sub x}Al{sub y}Ga{sub 1-x-y}N quaternary alloys open an avenue for the fabrication of many optoelectronic devices such as high efficient light emitters and detectors, particularly in the ultraviolet region.