Cheng-Hung Lin

Threading dislocation evolution in patterned GaN nanocolumn growth and coalescence overgrowth

Threading dislocation (TD) evolution during patterned GaN nanocolumn (NC) growth and coalescence overgrowth with metal-organic chemical vapor deposition is studied based on the comparisons of NC and coalescence overgrowth samples of different NC cross-section diameters and spacing sizes. From the measurement results of depth-dependent x-ray diffraction and cross-section transmission electron microscopy, it is found that the TD density in an NC depends on the patterned hole size for NC growth. Also, the TD formation at the beginning of coalescence overgrowth is related to the NC spacing size. Although the TD density at the bottom of the overgrown layer is weakly dependent on NC and spacing sizes, at its top surface, the TD density strongly relies on NC size. Among the overgrowth samples of different NC diameters and spacing sizes with a fixed NC diameter/spacing ratio, the one with the smallest size and spacing leads to the lowest TD density, the largest lateral domain size, and the highest photoluminescen...

Effects of the intermediate SiO2 layer on polarized output of a light-emitting diode with surface plasmon coupling

The variation behaviors of the output intensity and polarization ratio of InGaN/GaN quantum well (QW) light-emitting diodes (LEDs) with surface plasmon (SP) coupling by inserting SiO2 intermediate layers between the p-GaN layers and surface Ag grating structures are demonstrated. The insertion of the SiO2 layer is expected to reduce the metal dissipation of SP energy and extend the near-field distribution range of the induced SP for generating more favored SP-QW coupling effects. The Ag grating period for optimizing SP-QW coupling is increased when a SiO2 layer is added to the device, which is consistent with the simulation results of the momentum matching of SP polariton and the resonance behavior of localized SP. The almost unpolarized outputs from other LED samples fabricated with an epitaxial structure of thicker p-GaN layer, which leads to weak SP-QW coupling, indicate that the observed polarization ratios are due to near-field SP-QW coupling, instead of far-field diffraction.

Improving emission enhancement in surface plasmon coupling with an InGaN/GaN quantum well by inserting a dielectric layer of low refractive index between metal and semiconductor

The improved emission enhancement in surface plasmon polariton (SPP) coupling with an InGaN/GaN quantum well (QW) by inserting a SiO2 layer of lower refractive index between the deposited Ag and GaN layers is experimentally and numerically demonstrated. The inserted SiO2 layer leads to reduced SPP dissipation rate, increased evanescent field intensity beyond a certain depth in GaN, and decreased SPP density of state. The combination of these factors can result in further emission enhancement of QW through SPP coupling. For light-emitting diode application, the elongated evanescent field coverage can release the constraint of thin p-type GaN for effective SPP coupling. More importantly, the reduced SPP dissipation can result in more effective emission in such an SPP-QW coupling mechanism.

Nitride Nanocolumns for the Development of Light-Emitting Diode

The progress of light-emitting-diode (LED) development based on nitride nanocolumn (NC) growth, including InGaN/GaN quantum-well (QW) growth on NCs and regularly arranged GaN NC growth, is first reviewed. Then, the coalescence-overgrowth results based on patterned GaN NC growth are introduced. The overgrowth quality dependence on NC cross-sectional size and NC spacing size is discussed. Generally, a smaller NC dimension and spacing size lead to higher overgrowth quality, including lower threading dislocation (TD) density and larger lateral domain size. Next, the emission enhancement results of blue- and green-emitting InGaN/GaN QW and LED structures based on NC growth and coalescence overgrowth are presented. Significant enhancements (up to ~100% output intensity increase in a blue LED) are demonstrated. For LED applications, the TD density reduction in an overgrown GaN template can more effectively enhance the emission efficiency of a blue LED, when compared with a green LED.

A GaN photonic crystal membrane laser

The implementation of a series of optically pumped GaN photonic crystal (PhC) membrane lasers is demonstrated at room temperature. The photonic crystal is composed of a scalene-triangular arrangement of circular holes in GaN. Three defect structures are fabricated for comparing their lasing characteristics with those of perfect PhC. It is observed that all the lasing defect modes have lasing wavelengths very close to the band-edge modes in the perfect PhC structure. Although those lasing modes, including band-edge and defect modes, have different optical pump thresholds, different lasing spectral widths, different quality factors (Q factors), and different polarization ratios, all their polarization distributions show maxima in the directions around one of the hole arrangement axes. The similar lasing characteristics between the band-edge and defect modes are attributed to the existence of extremely narrow partial band gaps for forming the defect modes. Also, the oriented polarization properties are due to the scalene-triangle PhC structure. In one of the defect lasing modes, the lasing threshold is as low as 0.82 mJ cm(-2), the cavity Q factor is as large as 1743, and the polarization ratio is as large as 25.4. Such output parameters represent generally superior lasing behaviors when compared with previously reported implementations of similar laser structures.

Light Extraction Enhancement of a GaN-Based Light-Emitting Diode Through Grating-Patterned Photoelectrochemical Surface Etching With Phase Mask Interferometry

The enhancement of light extraction by fabricating a surface grating structure around the mesa of a light-emitting diode (LED) with an approach combining photoelectrochemical (PEC) wet etching and phase mask interferometry is demonstrated. The PEC etching rate is controlled by the intensity of illuminating UV light, which is spatially modulated by the fringe pattern of phase mask interferometry, for forming the grating structure. Without affecting the resistance characteristics of the device, the diffraction of such a grating structure leads to LED output enhancement by > 43% on either the top or bottom side.

Photoelectrochemical Liftoff of Patterned Sapphire Substrate for Fabricating Vertical Light-Emitting Diode

A low-cost large-area effective sapphire substrate liftoff method based on the photoelectrochemical (PEC) etching technique is demonstrated. By preparing patterned sapphire substrate (PSS) with 1-D periodic grooves and an epitaxial structure with the grooves preserved to form tunnels, PEC electrolyte can flow along the tunnels to etch the bottom of the GaN layer for separating the PSS from the wafer-bonded epitaxial layer. Assisted by the device isolation procedure, the PSS liftoff of a quarter-wafer sample can be completed in 8 min. After a smoothing process of the exposed N-face surface after liftoff, a vertical light-emitting diode (LED) is fabricated for comparing its characteristics with those of a conventional LED.

Sapphire Substrate Liftoff With Photoelectrochemical Etching for Vertical Light-Emitting Diode Fabrication

The method of sapphire substrate liftoff for the fabrication of vertical nitride light-emitting diode (LED), based on the combination of the photoelectrochemical (PEC) etching and epitaxial lateral overgrowth (ELOG) techniques, is demonstrated. This method relies on the formation of connected voids during the ELOG process on a GaN template such that PEC electrolyte can approach the GaN portions above the SiO2 masks. Also, the GaN template must be thin enough for the illuminating ultraviolet light to reach the GaN portions above the SiO2 masks. It is shown that PEC etching starts from a very thin layer of GaN right above a SiO2 mask. It then extends into the window regions of ELOG to completely separate GaN from sapphire. The performances of a vertical LED are illustrated.

Fabrication of photonic crystal light-emitting diode with photoelectrochemical wet etching and phase mask interference

We demonstrate the high light-extraction efficiency by using the photoelectrochemical etching technique for forming photonic crystal structures on an InGaN/GaN quantum-well light-emitting diode through phase-mask interference. More than 90% increase of output power is observed.

GaN defect photonic crystal membrane laser

We report the lasing behaviors of a GaN defect photonic crystal membrane structure. The high Q factor and low threshold condition show the effective photon confinement with the full photonic band gap structure.