Liangchen Wang

Color filter-less technology of LED back light for LCD-TV

Back Light Unit (BLU) and Color Filter are the two key components for the perfect color display of Liquid Crystal Display (LCD) device. LCD can not light actively itself, so a form of illumination, Back Light Unit is needed for its display. The color filter which consists of RGB primary colors, is used to generate three basic colors for LCD display. Traditional CCFL back light source has several disadvantages, while LED back light technology makes LCD obtain quite higher display quality than the CCFL back light. LCD device based on LED back light owns promoted efficiency of display. Moreover it can generate color gamut above 100% of the NTSC specification. Especially, we put forward an idea of Color Filter-Less technology that we design a film which is patterned of red and green emitting phosphors, then make it be excited by a blue light LED panel we fabricate, for its special emitting mechanism, this film can emit RGB basic color, therefore replace the color filter of LCD device. This frame typically benefits for lighting uniformity and provide pretty high light utilization ratio. Also simplifies back light structure thus cut down the expenses.

Plasma-induced damage in GaN-based light emitting diodes

The effects of plasma induced damage in different conditions of ICP and PECVD processes on LEDs were presented. For ICP mesa etch, in an effort to confirm the effects of dry etch damage on the optical properties of p-type GaN, a photoluminescence (PL) measurement was investigated with different rf chuck power. It was founded the PL intensity of the peak decreased with increasing DC bias and the intensity of sample etched at a higher DC bias of -400V is less by two orders of magnitude than that of the as-grown sample. Meanwhile, In the I-V curve for the etched samples with different DC biases, the reverse leakage current of higher DC bias sample was obviously degraded than the lower one. In addition, plasma induced damage was also inevitable during the deposition of etch masks and surface passivation films by PECVD. The PL intensity of samples deposited with different powers sharply decreased when the power was excessive. The PL spectra of samples deposited under the fixed condition with the different processing time were measured, indicating the intensity of sample deposited with a lower power did not obviously vary after a long time deposition. A two-layer film was made in order to improve the compactness of sparse dielectric film deposited with a lower power.

Design and optimization of dielectric optical coatings for GaN based high bright LEDs

Different types of dielectric optical coatings for GaN based high bright LEDs were designed and discussed. The optical coatings included the anti-reflection (AR) coating, high-reflection (HR) coating, and omni-directional high reflection coating. Main materials for the optical coatings were dielectric materials such as SiO2, Ta2O5 and Al2O3, which were different from the metallic reflector such as Ag usually used now. For the application of anti-reflection coating in GaN LEDs, it was introduced into the design of transparent electrodes with transparent materials such as ITO to form combined transparent electrodes. With the design of P, N transparent electrodes using the AR coating and ITO for GaN LEDs, the extraction efficiency was improved by about 15% experimentally. For the dielectric high-reflection coating, it has higher reflectivity and lower absorption than the metal reflector, and it was supposed to improve the extraction efficiency obviously. While the dielectric omni-directional reflection coating using dielectric materials was also designed and discussed in this article, since which was anticipated to improve the extraction efficiency furthermore. Using SiO2 and Ta2O5, the average reflectivity of a design of all dielectric omni-directional high reflection coating on the sapphire surface was over 94%.

The effects of sapphire substrates processes to the LED efficiency

We investigate the relation between the thickness of sapphire substrates and the extraction efficiency of LED. The increasing about 5% was observed in the simulations and experiments when the sapphire thickness changed from 100μm to 200μm. But the output power increasing is inconspicuous when the thickness is more than 200μm. The structure on bottom face of sapphire substrates can enhance the extraction efficiency of GaN-based LED, too. The difference of output power between the flip-chip LED with smooth bottom surface and the LED with roughness bottom surface is about 50%, where only a common sapphire grinding process is used. But for those LEDs grown on patterned sapphire substrate the difference is only about 10%. Another kind of periodic pattern on the bottom of sapphire is fabricated by the dry etch method, and the output of the back-etched LEDs is improved about 50% than a common case.

Micro-Raman Investigation of Defects in a 4H-SiC Homoepilayer

Three types of defects, namely defect I, defect II, defect III, in the 4H-SiC homoepilayer were investigated by micro-raman scattering measurement. These defects all originate from a certain core and are composed of (I) a wavy tail region, (II) two long tails, the so called comet and (III) three plaits. It was found that there are 3C-SiC inclusions in the cores of defect II and defect III and the shape of inclusion determines the type of defect II or defect III. If the core contains a triangle-shaped inclusion, the defect III would be formed; otherwise, the defect II was formed. No inclusion was observed in the core of the defect I. The mechanisms of these defects are discussed.

Optimized design on High-power GaN-based Micro-LEDs

The structure of micro-LEDs was optimized designed. Optical, electrical and thermal characteristics of micro-LEDs were improved. The optimized design make micro-LEDs suitable for high-power device. The light extraction efficiency of micro-LEDs was analyzed by the means of ray tracing. The results shows that increasing the inclination angle of sidewall and height of mesa, and reducing the absorption of p and n electrode can enhance the light extraction efficiency of micro-LEDs. Furthermore, the total light output power can be boosted by increasing the density of micro-structures on the device. The high-power flip-chip micro-LEDs were fabricated, which has higher quantum efficiency than conventional BALEDs. When the number of microstructure in micro-LEDs was increased by 57%, the light output power was enhanced 24%. Light output power is 82.88mW at the current of 350mA and saturation current is up to 800mA, all of these are better than BALED which was fabricated in the same epitaxial wafer. The I-V characteristics of micro-LEDs are almost identical to BALED.

Research and fabrication of high power LEDs with transparent electrodes

In this paper we studied the influence of N electrode on the extraction efficiency of high power light-emitting diodes (LEDs). Simulation and experimental results show that comparing with traditional metal N electrodes the extraction efficiency of LEDs with transparent N electrode is increased by 15%, and it is easier in process than the other techniques. So we proposed a new kind of strip LEDs with transparent electrodes on both P-GaN and N-GaN. The design of transparent electrodes was trade-off between transmittance and resistance. At the same time, the strip structure has some advantages over the traditional square LEDs, which can increase the extraction efficiency and reduce the thermal resistance. Antireflective and high reflective optical coatings were also used in this design. The fabrication of LEDs with transparent electrodes on both P-GaN and N-GaN has been demonstrated. The output power of blue LEDs is 240mW at 350mA, forward voltage is below 3.5V. The luminous flux of white LEDs reached 65lm at 350mA.

Combined transparent electrodes for high power GaN-based LEDs with long life time

In this paper fabrication of high power light emitting diodes (LEDs) with combined transparent electrodes on both P-GaN and N-GaN have been demonstrated. Simulation and experimental results show that comparing with traditional metal N electrodes the efficacy of LEDs with transparent N electrode is increased by more than 10% and it is easier in process than the other techniques. Further more, combining the transparent electrodes with dielectric anti-reflection film, the extraction efficiency can be improved by 5%. At the same time, the transparent electrodes were protected by the dielectric film and the reliability of LEDs can be improved.

Heteroepitaxial Growth of 3C-SiC on Si (111) Substrate Using AlN as a Buffer Layer

Using AlN as a buffer layer, 3C-SiC film has been grown on Si substrate by low pressure chemical vapor deposition (LPCVD). Firstly growth of AlN thin films on Si substrates under varied V/III ratios at 1100oC was investigated and the (002) preferred orientational growth with good crystallinity was obtained at the V/III ratio of 10000. Annealing at 1300oC indicated the surface morphology and crystallinity stability of AlN film. Secondly the 3C-SiC film was grown on Si substrate with AlN buffer layer. Compared to that without AlN buffer layer, the crystal quality of the 3C-SiC film was improved on the AlN/Si substrate, characterized by X-ray diffraction (XRD) and Raman measurements.

Effect of indium tin oxide (ITO) current spreading layer on the current uniformity of vertical structure GaN-based light-emitting diodes

Compared with conventional GaN based LEDs structure, vertical electrodes structure can improve the current uniformity to some extend. However, due to the presence of N-GaN series resistance, current crowing phenomenon still exist at the electrode edge of vertical structure GaN-based LEDs. The most common solutions to overcome these problems consist in using transparent current spreading contact layer, generally made of indium tin oxide (ITO). In this paper, the effect of indium tin oxide (ITO) layer on the current uniformity and the current distribution are analyzed by theoretical calculation. At last, vertical structure GaN-based LEDs was fabricated and the electrical character was measured. Experiment result exhibits good agreement with the theoretical calculation.