Charng-Gan Tu

Efficiency improvement of a vertical light-emitting diode through surface plasmon coupling and grating scattering

The enhancement of output intensity, the generation of polarized output, and the reduction of the efficiency droop effect in a surface plasmon (SP) coupled vertical light-emitting diode (LED) with an Ag nano-grating structure located between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN quantum wells (QWs) are demonstrated. In fabricating the vertical LED, the patterned sapphire substrate is removed with a photoelectrochemical liftoff technique. Based on the reflection measurement from the metal grating structure and the numerical simulation result, it is found that the localized surface plasmon (LSP) resonance induced around the metal grating crest plays the major role in the SP-QW coupling process although a hybrid mode of LSP and surface plasmon polariton can be generated in the coupling process. By adding a surface grating structure to the SP-coupled vertical LED on the n-GaN side, the output intensity is further enhanced, the output polarization ratio is further increased, and the efficiency droop effect is further suppressed.

Cross-sectional sizes and emission wavelengths of regularly patterned GaN and core-shell InGaN/GaN quantum-well nanorod arrays

The cross-sectional sizes of the regularly patterned GaN nanorods (NRs) and InGaN/GaN quantum-well (QW) NRs of different heights and different hexagon orientations, which are grown on the patterned templates of different hole diameters, pitches, and crystal orientations, are compared. It is found that the cross-sectional size of the GaN NR, which is formed with the pulsed growth mode, is mainly controlled by the patterned hole diameter, and the thickness of the sidewall QW structure is mainly determined by the NR height. The cross-sectional size variation of GaN NR is interpreted by the quasi-three-dimensional nature of atom supply amount for precipitating a two-dimensional disk-shaped NR segment. The variation of the sidewall QW structure is explained by the condition of constituent atom supply in the gap volume between the neighboring NRs. Also, we compare the cathodoluminescence emission wavelengths among those samples of different growth conditions. Generally speaking, the QW NR with a smaller height,...

Light-emitting device with regularly patterned growth of an InGaN/GaN quantum-well nanorod light-emitting diode array

A light-emitting device consisting of a two-dimensional regularly patterned InGaN/GaN quantum well (QW) nanorod (NR) light-emitting diode (LED) array is implemented and characterized. The NR p-i-n structure includes n-GaN NR core and essentially conformal p-GaN shell. The active regions include nonpolar sidewall QWs and polar top-face QWs. A conformal layer of transparent GaZnO of low resistivity is deposited onto the NR LED structure for spreading the injection current over the sidewalls. It is found that the blue-shift range of the output spectral peak in increasing injection current is smaller than that of a planar LED of about the same operation wavelength in a similar variation range of injection current density although it is nonzero. The small blue-shift range is attributed to the mixed emission contributions from the nonpolar sidewall QWs and polar top-face QWs.

Modulation behaviors of surface plasmon coupled light-emitting diode.

The modulation bandwidths of the light-emitting diodes (LEDs) of different mesa sizes with and without surface plasmon (SP) coupling effect are compared. Due to the significant increase of carrier decay rate, within the size range of LED square-mesa from 60 through 300 micron and the injected current-density range from 139 through 1667 A/cm², the SP coupling can lead to the enhancement of modulation bandwidth by 44-48%, independent of the variations of LED mesa size or injected current level. The enhancement ratios of modulation bandwidth of the samples with SP coupling with respect to those of the samples without SP coupling are lower than the corresponding ratios of the square-root of photoluminescence decay rate due to the increases of their RC time constants (the product of device resistance and capacitance). The increases of the RC time constants in the samples with SP coupling are attributed to the increases of their device resistance levels when the Ag nanoparticles and GaZnO dielectric interlayer are added to the LED surface for effectively inducing SP coupling.

Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction.

For enhancing the light extraction of a light-emitting diode, surface grating fabrication based on a simple method of combining photoelectrochemical (PEC) etching with phase mask interferometry has been demonstrated. To understand the optimum grating period in forming a surface grating on a vertical light-emitting diode (VLED), we construct a Llyods interferometer within PEC electrolyte (KOH) to fabricate surface gratings of various periods on VLEDs for comparing their light extraction efficiencies. Also, to compare the effectiveness of light extraction enhancement between surface grating and rough surface, VLEDs with the rough surfaces fabricated with two different KOH wet etching methods are fabricated. The comparisons of VLED characterizations show that among those grating VLEDs, the light extraction is more effective in a VLED of a smaller grating period. Also, compared with VLEDs of rough surfaces, the grating VLEDs of short grating periods (<2 μm) have the higher light extraction efficiencies, even though the root-mean-square roughness of the rough surface is significantly larger than the grating groove depth.

Regularly patterned non-polar InGaN/GaN quantum-well nanorod light-emitting diode array

The growth and process of a regularly patterned nanorod (NR)- light-emitting diode (LED) array with its emission from sidewall non-polar quantum wells (QWs) are demonstrated. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR LED device is similar to that of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence spectral shift under reversed bias confirms that the emission of the fabricated NR LED comes from non-polar QWs.

Dependencies of the emission behavior and quantum well structure of a regularly-patterned, InGaN/GaN quantum-well nanorod array on growth condition

To achieve green emission from the sidewall non-polar quantum wells (QWs) of a GaN nanorod (NR) light-emitting diode, regularly patterned InGaN/GaN QW NR arrays are grown under various growth conditions of indium supply rate, QW growth temperature, and QW growth time for comparing their emission wavelength variations of the top-face c-plane and sidewall m-plane QWs based on photoluminescence and cathodoluminescence (CL) measurements. Although the variation trends of QW emission wavelength by changing those growth conditions in the NR structure are similar to those in the planar structure, the emission wavelength range of the QWs on an NR is significantly shorter than that in a planar structure under the same growth conditions. Under the growth conditions for a longer NR QW emission wavelength, the difference of emission wavelength between the top-face and sidewall QWs is smaller. Also, the variation range of the emission wavelength from the sidewall QWs over different heights on the sidewall becomes larger. On the other hand, strain state analysis based on transmission electron microscopy is undertaken to calibrate the average QW widths and average indium contents in the two groups of QW of an NR. The variation trends of the calibrated QW widths and indium contents are consistent with those of the CL emission wavelengths from various portions of NR QWs.

Growth of Highly Conductive Ga-Doped ZnO Nanoneedles

The molecular beam epitaxy growth of highly degenerate Ga-doped ZnO (GaZnO) nanoneedles (NNs) based on the vapor-liquid-solid (VLS) growth mode using Ag nanoparticles (NPs) as the growth catalyst is demonstrated. It is shown that when the growth substrate temperature is sufficiently high, a portion of a Ag NP can be melted for serving as the catalyst to precipitate GaZnO on the residual Ag NP and form a GaZnO NN. Record-low turn-on and threshold electric fields in the field emission test of the grown GaZnO NNs are observed. Also, a record-high field enhancement factor in field emission is calibrated. Such superior field emission performances are attributed to a few factors, including (1) the low work function and high conductivity of the grown GaZnO NNs due to highly degenerate Ga doping, (2) the sharp-pointed geometry of the vertically aligned GaZnO NNs, (3) the Ag doping in VLS precipitation of GaZnO for further reducing NN resistivity, and (4) the residual small Ag NP at the NN tip for making the tip even sharper and tip conductivity even higher.

Surface plasmon coupled light-emitting diode: Experimental and numerical studies

First, the experimental implementations and theoretical/numerical investigations of surface plasmon (SP) coupled InGaN/GaN quantum-well light-emitting diodes (LEDs) are reviewed. If the p-GaN layer in an LED can be thin, surface metal nanoparticle (NP) is an inexpensive structure for inducing effective SP coupling. When the p-GaN layer is thick, a few metal structures, including metal protrusion, buried metal NP, and embedded metal NP, can be used for effective SP coupling. In the numerical study, an algorithm, including the feedback effect of the induced SP resonance on the radiating behavior of the source dipole, has been proposed for studying the SP coupling effects with an embedded metal NP, a surface metal NP, and a metal protrusion. Then, the theoretical formulations and numerical algorithms for evaluating the radiated power enhancement in the coupling process between two radiating dipoles and the localized surface plasmon (LSP) induced on a nearby Ag NP are built. Three mechanisms are considered in the coupling process for radiated power enhancement, including the interference of the two phase-retarded radiation contributions from the two dipoles, the interaction between the two dipoles, and the LSP resonant coupling.

Combining High Hole Concentration in p-GaN and High Mobility in u-GaN for High p-Type Conductivity in a p-GaN/u-GaN Alternating-Layer Nanostructure

p-GaN/u-GaN alternating-layer nanostructures are grown with molecular beam epitaxy to show a low p-type resistivity level of 0.038 Ω-cm. The obtained low resistivity is due to the high hole mobility in the u-GaN layers, which serve as effective transport channels of holes diffused from the neighboring p-GaN layers. The Mg doping in a thin p-GaN layer can lead to a high Mg-doping concentration for supplying holes to the neighboring u-GaN layers. Simulations based on a 1-D drift diffusion charge control model and the Brooks-Herring theory of ionized impurity scattering are undertaken to first obtain the depth-dependent distributions of hole concentration, mobility, and, hence, resistivity. Then, weighted averaging processes are used for evaluating the effective hole concentration, mobility, and resistivity of a p-GaN/u-GaN alternating-layer nanostructure to give consistent results with the measured data.