A preview of some of the most interesting papers appearing in theApril 2008 issue of the Journal of the SID. To obtain access to these articles on-line, please go to www.sid.org
Edited by Aris Silzars
H. S. Oh
K. H. Kim
J. H. Lee
J. H. Baek
Y. M. Yu
J. S. Kwak
Korea PhotonicTechnology Institute
To improve the DBR growth conditions, we enumerated as many factors as possible: (i) differential pressure between run and vent line, (ii) growth temperature, (iii) V/III ratio, (iv) growth pressure, and (v) growth rate. For a differential pressure between run and vent line, the vent flow was regulated to minimize the pressure difference between run and vent line. In the preliminary experiment, two LED structures were grown under different DBR growth rates of 1 and 2 μm/hour, respectively. We found that the device performance was not different when changing the growth rate of the DBR structure from 1 to 2 μm/hour. Therefore, the growth temperature, AsH3 flow, and growth pressure were selected as experimental parameters.
TABLE 1 — Experimental growth conditions of a DBR set by DOE (Design of Experiment).
National Central University
Abstract — A method of calculating the luminance and luminance uniformity of a bottom LED backlight is proposed and demonstrated. Both the power consumption and brightness uniformity as a function of screen brightness, screen size, backlight thickness, transmittance of the LCD panel, reflective cavity efficiency, gain, cone angle of the enhancement films, LED array configuration, average luminous flux, radiation pattern, and input power of individual LEDs. Moreover, a 42-in. LCD TV using this backlight design approach was fabricated. The bottom backlight incorporates an array of RGGB 4-in-1 multi-chip LEDs within a highly reflective box behind a diffuser and a dual brightness-enhancement film. The brightness uniformity can be predicted within an accuracy of 94% and the luminance level within an accuracy of 96%.
A direct or bottom RGB LED backlight is a key concept in large-area LCDs because it does not use a light guide, is flat, and is easy to assemble. Simply, an array of LEDs is placed in a cavity directly behind the LCD panel. Moreover, a bottom backlight enables local dimming and scanning of LEDs in time with the video displayed on the screen. This feature can increase the color contrast and reduce both the movement blurring and power consumption.
FIGURE 6 — (Left) Constructed LED backlight. (Inset) a RGGB 4-in-1 multi-chip LED. (Right) The LED-based 42-in. LCD TV displaying images.
Abstract — Light extraction from InGaN-based light-emitting diodes (LEDs) on which microlens arrays were integrated using ray-tracing methods was simulated. Enhancement of the total light extraction and beam shaping in the forward direction of the output of microlens-integrated LEDs compared to conventional LEDs were observed. The diameter, curvature radius, and density of the microlens arrays on the LEDs were varied and the optimal conditions for external efficiency was determined.
We considered the case in which several lenses were integrated onto one LED, as shown in Fig. 4. In this case, we calculated the light-extraction efficiency according to the variation in the radius of curvature of the lenses and the variation in the pitch of the lens array. The choice of these parameters in our simulations was reasonable because the variation in the curvature radius of the lenses or number of lenses on the LED can illustrate various aspects of their effects on light extraction from LEDs.
FIGURE 4 — Schematic diagram of a LED with an integrated microlens array. We calculated the light-extraction efficiency for differences in the radius of the curvature of the lenses and the numbers of lenses.
Jong Hyeob Baek
Korea Photonics Technology Institute
Abstract — The structural, electrical, and optical properties of GaN epilayers grown on various ion-implanted sapphire(0001) substrates by MOCVD were investigated. GaN or AlN buffer layers and pre-treatment were indispensably introduced before GaN-epilayer growth. The ion-implanted substrate's surface had decreased internal free energies during the growth of the ion-implanted sapphire(0001) substrates. The crystal and optical properties of the GaN epilayers grown in ion-implanted sapphire(0001) substrates were improved. Also, an excessively roughened and modified surface caused by ions degraded the GaN epilayers. Not only the ionic radius but also the chemical species of implanted sapphire(0001) substrates improved the properties of the GaN epilayers grown by MOCVD. It is obvious that the ion-implanted pre-treatment of sapphire(0001) substrates can be an alternative pre-treatment procedure for GaN deposition and has the potential to improve the properties of the GaN epilayers on sapphire(0001) substrates.
GaN-based materials and related nitride compounds have many attractive chemical and physical properties for use in high-temperature and high-power electronics, as well as blue and ultraviolet light-emitting diodes (LEDs) and laser diodes (LDs). Also, the wurtzite GaN structure has several advantages, including a direct bandgap of 3.4 eV at room temperature for high efficiency in optoelectronic devices, high radiation hardness, and complete solubility among the binary compounds and forms a continuous solid solution with InN and AlN, which have the same wurtzite structure and direct band gap of 1.9 and 6.2 eV, respectively.
FIGURE 1 — Depth distribution of (a) He+–, (b) Ar+–, and (c) Xe+-ion implantation in sapphire(0001) substrates.
S. L. Hwang, K. H. Kim,
H. S. Jeon, C. H. Lee,
S. H. Hong, I. H. Heo,
M. Yang, H. S. Ahn,
S. W. Kim, S. C. Lee,
I. S. Cho, W. T. Lim,
J. H. Lee, S. K. Shee
Korea Maritime University
Abstract — The selective area growth (SAG) of a InGaN/AlGaN light-emitting diode (LED) is performed by using mixed-source hydride vapor-phase epitaxy (HVPE) with a multi-sliding boat system. The SAG-InGaN/AlGaN LED consists of a Si-doped AlGaN cladding layer, an InGaN active layer, a Mg-doped AlGaN cladding layer, and a Mg-doped GaN capping layer. The carrier concentration of the n-type AlxGa1– xN (x ~ 16%) cladding layer depends on the amount of poly-Si placed in the Al–Ga source. The carrier concentration is varied from 2.0 x 1016 to 1.1 x 1017 cm–3. Electroluminescence (EL) characteristics show an emission peak wavelength at 426 nm with a full width at half-maximum (FWHM) of approximately 0.47 eV at 20 mA. It was found that the mixed-source HVPE method with a multi-sliding boat system is a candidate growth method for III-nitride LEDs.
Wide-bandgap III-nitride semiconductors are one of the most promising materials for the application of short-wavelength light emitters.III-nitride-based heterostructures have also received much attention by optical devices, such as light-emitting diodes (LEDs) and laser diodes (LDs) operating in the blue and green regions. These performances are based on multilayer epitaxial structures grown by metal-organic chemical vapor deposition (MOCVD) or molecular-beam epitaxy (MBE) methods. HVPE allows the growth of low-defect-density material that incorporates a high proportion of aluminum (Al) in the AlGaN layers without severely degrading the crystal quality.
FIGURE 5 — Top view of the SAG-InGaN/AlGaN LED grown by mixed-source HVPE with a multi-sliding boat system.
National Kaohsiung First University of Science and Technology
Abstract — This paper describes the development of a design method for a prism pattern for an LCD light-guide plate to improve the uniformity of its exiting light. First, the prism surface of the light-guide plate is divided into several equal regions. With the aid of ASAP simulation, this method uses the mean light flux of all regions as a reference value to adjust the distribution density of the prism pattern for each region. Curve fitting is then performed to provide a smoothly changing distribution density for further improvement of the exiting light uniformity. ASAP results demonstrate that the illuminance uniformity for a 2.5-in. light-guide plate is substantially improved from 45% to 90.9% by using this design method.
To reduce the number of prism sheets [(i.e., to reduce the cost of the backlight module (BLM)], a light-guide plate with prisms (i.e.,V-shaped grooves) on its surface(s) was invented, such that the prism surface possesses the function of condensing light, as shown in Fig. 1(b). Therefore, the prism LGP provides higher brightness than the scattering-dot LGP for the LCD BLM, and thus this type of the LGP is considered in this paper.
FIGURE 1 — LCD backlight module and light-guide plate: (a) The schematic diagram for an LCD backlight module with a side-type LGP; (b) a wedge-shaped light-guide plate with a prism pattern.
The University of Electro-Communications
Abstract — A thick-film ceramic-sheet PDP provides a long sustain discharge gap of 0.45 mm, enabling the use of positive column discharges. The discharges are established in the middle of the discharge space and are completely free from touching the surface of substrates. This allows for the reduction in diffusion losses of the charged particles. To further improve the efficacy, delayed D pulses are applied to the address electrodes during the sustain period. Although the pulses only draw a little current, they perturb the electric field, reducing the peak discharge current and hence resulting in higher efficacy and luminance. The efficacy and luminance increase by 35% and 38%, respectively, with the delayed D pulses. These pulses are incorporated into the contiguous-subfield erase-addressing drive scheme for TV application. A short gap of 70 μm between the sustain and data electrodes generates a fast-rising discharge and allows a high-speed addressing of 0.25 μsec. This provides 18 contiguous subfields for the full-HD single-scan mode, with 70% light emission duty. A luminous efficacy of 6.0 lm/W can been attained using Ne + 30% Xe 47 kPa, a sustain voltage of 320 V, and a sustain frequency of 3.3 kHz, when the luminance is 157 cd/m2. Alternatively, the panel can achieve 4.2 lm/W and 1260 cd/m2 by increasing the sustain frequency to 33 kHz.
FIGURE 1 — Structure of thick-film ceramic sheet (TFCS) PDP.
FIGURE 2 — Cross-sectional view of TFCS PDP along the data electrode.
Abstract — Coatings of indium tin oxide (ITO) nanoparticles on different flexible polymer substrates were investigated with respect to the achievable sheet resistance and their electrical behavior under oscillatory bending. As substrate materials, polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polyetheretherketone (PEEK), and polyimide (PI) were chosen, the surface resistances on the different polymer substrates were compared as a function of annealing temperature and surface topography. The surface topography, which has a strong influence on the surface resistance, was characterized by means of a white-light confocal (WL-CF) microscope. On the PET substrate, which exhibits the smoothest surface, the coating of ITO nanoparticles shows the lowest sheet resistance of 2 kΩ/4 for a layer thickness of 3 μm and an annealing temperature of 200°C. Furthermore, the electrical behavior of coatings of ITO nanoparticles under oscillatory bending was investigated using a special device. These coatings show a cyclic change of the conductivity which can be explained by an alternating compression and extension of crack flanks under the applied stress. Due to the growing number of cracks with increasing number of cycles, a decrease of the conductivity is observed in the bent state as well as in the balanced state.
The transmittance of visible light was investigated using UV/VIS spectroscopy to characterize the influence of the layer thickness on the transmittance of ITO-nanoparticle coatings. In Fig. 6, the transmittance of a 100-μm-thick PET film coated with 3- and 4-μm-thick layers of annealed ITO nanoparticles is plotted as a function of wavelength and compared to uncoated PET films and commercially available ITO-sputtered PET films.
FIGURE 6 — Transmittance of visible light through uncoated PET films and PET films coated with annealed ITO nanoparticles.
James S. Speck
Steven P. DenBaars
University of California
Abstract — This article addresses spontaneously polarized light emission from GaN-based light-emitting diodes (LEDs) fabricated on electrically non-polar crystallographic orientations and application of spontaneously polarized emission for backlighting of liquid-crystal displays (LCDs). The first half of the article describes polarized light emission from GaN-based LEDs and its role in solid-state lighting technology. The second half reports on our experimental work to explore the potential of non-polar LEDs for LCD backlighting applications. Optical transmission of non-polar LED emission was characterized through a liquid-crystal layer. Extinction ratios of 0.21 were measured between zero and an applied bias voltage to the liquid-crystal cells. These extinction ratios are not particularly high yet; nevertheless, the experiment has demonstrated the potential of such non-polar LEDs for LCD backlighting.
LED samples used in the present experiments were fabricated by using the metal organic chemical-vapor deposition method on anm-plane-oriented GaN substrate. LED structures consisted of a multiple QW stack of InGaN/GaN that was sandwiched by n-type GaN and p-type GaN/AlGaN layers. A conventional mesa structure (active area ~300 x 300 μm2) was fabricated on an LED wafer to make electrical contacts. Indium tin oxide was used as a p-type contacting layer. A schematic of the LED is drawn in Fig. 4.
FIGURE 4 — An LED die with a common mesa structure. A stack of the multiple quantum-wells (MQW) is grown on an n-type GaN layer and is followed by p-type AlGaN and p-type GaN layers. A dry-etching technique is used to make the mesa structure on the grown LED wafer (~350 μm thick). Metal contacts and bonding pads are deposited via vacuum techniques. The LED wafer with metal contacts is cut into discrete LED dies, and gold wires are attached to the pads.
Abstract — The temperature dependence of residual DC voltage was studied based on the adsorption and desorption of ions in the liquid-crystal (LC) layer to and from the interface between the LC and alignment layers during the application of an external DC offset voltage. The relaxation process of the adsorbed ions during the open-circuit state was also studied after applying the DC offset voltage. Those processes were found to follow the Arrhenius rule, and a new evaluation parameter related to the temperature is proposed for the design of the LC and alignment-layer materials.
There are several problems with the reliability of the latest LCDs. One of the serious problems is the generation of residual DC voltage (VrDC), which leads to the generation of image sticking or a residual image. VrDC is the DC offset voltage generated inside a LC cell after applying an external DC offset voltage, and relates to the existence of ions inthe LC layer as an impurity. It is generally known that the degree of ionization changes with a shift in temperature. In most cases, the concentration of the ion increases with increasing temperature. The fact indicates that VrDC also changes with shifting temperature.
FIGURE 1 — VrDC as a function of time for external DC offset voltage at various temperatures.