Wei-Kai Wang

Defect reduction and efficiency improvement of near-ultraviolet emitters via laterally overgrown GaN on a GaN/patterned sapphire template

An approach to improve the defect density and internal quantum efficiency of near-ultraviolet emitters was proposed using a combination of epitaxial lateral overgrowth (ELOG) and patterned sapphire substrate (PSS) techniques. Especially, a complementary dot array pattern corresponding to the underlying PSS was used for the ELOG-SiO2 mask design. Based on the transmission-electron-microscopy and etch-pit-density results, the ELOG∕SiO2∕GaN∕PSS structure can reduce the defect density to a level of 105cm−2. The internal quantum efficiency of the InGaN-based ELOG-PSS light-emitting diode (LED) sample showed three times in magnitude as compared with that of the conventional GaN/sapphire one. Under a 20mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5mW, respectively. The enhanced output power could be due to a combination of the reduction in dislocation density (by ELOG) and improved light extraction efficiency (by PSS). Unlike the previ...

Effect of resonant cavity in wafer-bonded Green InGaN LED with dielectric and silver mirrors

The green InGaN-based resonant cavity light-emitting diodes (RCLEDs) on Si substrates were fabricated using laser liftoff and wafer bonding techniques. Five-pair TiO2--SiO2 distributed Bragg reflectors (reflectivity: 85%) and an Ag metal layer (reflectivity: 99%) were employed as the top and bottom mirrors, respectively. The light output power of the RCLED at room temperature is 1.5 times the magnitude of a similar structure without a resonant cavity at an injecting current density of 600A/cm2. The mode spectrum exhibits a line width of approximately 5.5 nm at the dominant peak wavelength of 525 nm, which indicates a quality factor of 100. Under various injection current densities, a low thermally induced red shift in the 525-nm emission peak was observed. This indicates that the resonant microcavity effect contributes to the stability of electroluminescence emission wavelength

Fabrication and efficiency improvement of micropillar InGaN∕Cu light-emitting diodes with vertical electrodes

We present a micropillar surface structure based on the enhancement of the light extraction efficiency of the near-ultraviolet (409nm) vertical-conducting InGaN light-emitting diode (LED) with an electroplated Cu substrate. The micropillar InGaN∕Cu LED (chip size: 1×1mm2) was fabricated using a combination of patterned sapphire substrate (PSS), laser lift-off, and copper electroplating processes. The PSS and Cu substrate can offer the advantages of dislocation reduction and thermal heat sink, respectively. It was found that the light output power (at 350mA) of the micropillar InGaN∕Cu LED sample can be improved by 39% as compared with that of the conventional InGaN∕Cu LED one. This significant enhancement in output power could be attributed to the increase of the extraction efficiency which is a result of the increase in photon escaping probability caused by scattering the emission light at the micropillar surface. The light extraction efficiency can be further optimized by tuning the micropillar spacing,...

InGaN-Based Light-Emitting Diodes with a Cone-Shaped Sidewall Structure Fabricated Through a Crystallographic Wet Etching Process

The InGaN-based light-emitting diodes (LEDs) were fabricated through a crystallographic etching process to increase their light extraction efficiency. After the laser scribing and the selective lateral wet etching processes at the LED chip edge region, the stable crystallographic etching planes were formed as the GaN {1012} planes and had an including angle with the top GaN (0001) plane measured as 40.3°. The AlN buffer layer acted as the sacrificial layer for the lateral wet process with a 27.5 μm/h etching rate. The continuous cone-shaped sidewall (CSS) structure of the treated LED has a larger light-scattering area and higher light extraction cones around the LED chips. The LED with the CSS structure around the chip edge region has a higher light output power compared to a conventional LED when measured in LED chip form.

Characteristics of Flip-Chip InGaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates

We report on the characteristics of high-power near-ultraviolet (425 nm) flip-chip InGaN light-emitting diodes (LEDs) fabricated onto a patterned sapphire substrate (PSS). When the PSS flip-chip LED (chip size: 1 mm2) operated at a 20 mA forward current at room temperature, the forward voltage and the light output power were 3.15 V and 6.8 mW, respectively. It was found that the PSS flip-chip LED has similar current–voltage characteristics to those of a conventional flip-chip LED. The luminance intensity of the PSS flip-chip LED was approximately 43% lager than that of the conventional flip-chip LED (at 100 mA). Moreover, the light output power was greatly increased by 59% for the PSS sample at a forward injection current of 350 mA compared with that of the conventional flip-chip LED. This result was attributed to the increase in the probability of photons escaping from the LED samples, resulting in the enhancement of light extraction efficiency. The effect of the PSS on the flip-chip LED structure has been simulated and shows a good correlation with the measured results.

GaN-based green resonant cavity light-emitting diodes

GaN-based resonant cavity light-emitting diodes (RCLEDs) have been successfully fabricated on Si substrate by laser lift-off and wafer bonding techniques. A five-pair TiO2/SiO2 dielectric distributed Bragg reflector (DBR) (with 85% reflectivity) and an Ag layer (with 99% reflectivity) were employed as top and bottom mirrors, respectively, for front emission RCLEDs. The room temperature light output power of the RCLED was 1.5 times that of similar LED structures without a top DBR mirror under 20 mA injection current. The cavity modes exhibit a linewidth of 5.5 nm at 525 nm wavelength, which corresponds to a quality factor about 100. Moreover, the full width at half maximum of the emission can be reduced to 35 nm, as a result of the effect of the resonant cavity.

Effects of Transparent Conductive Layers on Characteristics of InGaN-Based Green Resonant-Cavity Light-Emitting Diodes

InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) with indium–tin oxide (ITO) and Ni/Au transparent conductive layers (TCLs) have been fabricated on Si substrates by laser lift-off and wafer bonding techniques. The RCLED structure consisted of an InGaN/GaN multiple-quantum-well active layer between the top (5 pairs) and bottom (7.5 pairs) dielectric TiO2/SiO2 distributed Bragg reflectors. It was found that the cavity mode of the RCLED with an ITO TCL shows a linewidth of 4 nm at the main emission peak at 494 nm. The electroluminescence intensity of the ITO-RCLED sample is 1.73 times higher in magnitude than that of the Ni/Au-RCLED one. It was found that the quality factor of the InGaN RCLED structure increased from 84 to 120 when the Ni/Au TCL was replaced by ITO. The improvements in both the optical output and the quality factor could be attributed to the higher optical transmittance of the ITO TCL enhancing the spontaneous emission at its resonant wavelength.

1.48-kV enhancement-mode AlGaN/GaN high-electron-mobility transistors fabricated on 6-in. silicon by fluoride-based plasma treatment

Enhancement-mode AlGaN/GaN high-electron-mobility transistors (HEMTs) were fabricated on a Si substrate by fluorine plasma treatment without pre-etching. The threshold voltage shifted from −8 to 2.3 V, effectively converting the depletion-mode HEMT to the enhancement-mode HEMT. The leakage current was reduced by two orders of magnitude by CF4 plasma treatment. The device exhibited a superior performance with a maximum drain saturation current of 210 mA/mm at V GS = 10 V and a peak gain of 44.1 mS/mm. A low off-state gate leakage current of 10−6 mA/mm, I ON/I OFF ratio of approximately 108, and high breakdown voltage of 1480 V were obtained.

Influence of hydrogen implantation concentration on the characteristics of GaN-based resonant-cavity LEDs

GaN-based resonant-cavity light-emitting diodes (RCLEDs) have been fabricated on Si substrates with a current confinement by hydrogen (H) implantation. In order to ascertain the optimum implantation concentration for the current confinement layer of RCLEDs, the effects of implantation concentration (as-grown, 1013, 1014 and 1015 ions cm−2) on the characteristics of a p-GaN cladding layer were investigated in terms of Hall measurements, photoluminescence (PL) spectra and x-ray (XRD) diffraction. The properties of PL, XRD and contact resistance of the epi-LED wafers with different implantation concentrations were also analyzed. An optimum H-implantation concentration of 1014 ions cm−2 has been determined based on the current confinement performance. Under this condition, the 1014 ions cm−2 implanted RCLED sample shows the higher electroluminescence intensity than that of the SiO2-insulated RCLED one. Furthermore, the light emission pattern of the 1014 ions cm−2 implanted RCLED also shows a superior directionality. The improved results could be attributed to the better current and photon confinements laterally in the light aperture.

Characteristics of yttrium fluoride and yttrium oxide coatings for plasma process equipment prepared by atmospheric plasma spraying

In this study, yttrium fluoride (YF3) and yttrium oxide (Y2O3) coatings were prepared by an atmospheric plasma spraying technique and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). YF3 powders were sprayed at various plasma spraying powers of 9, 15, and 21 kW. The XRD result indicates that the YF3 coating shows preferred orientations and was well crystallized. The XPS results revealed a strong Y–F bond on the YF3 coating surface. A porosity value analysis showed that the porosity of the YF3 coating was lower than that of the Y2O3 coating. Moreover, the dielectric strength of the YF3 coating (22.65 kV/mm) was higher than that of the Y2O3 coating (14.42 kV/mm). This confirms that the YF3 coating exhibits a breakdown voltage of 4.97 kV, which is more than 1.5 times higher than that observed for the Y2O3 coating (3.29 kV). These results indicate that the YF3 coating has better mechanical and dielectric properties than the Y2O3 coating, indicating that the YF3 coating is a very attractive novel antiplasma and corrosion-resistant material.