Showing papers by "Hoi Sing Kwok published in 1997"
TL;DR: In this article, the switching bistability of twisted nematic liquid crystal cells is investigated based on the interaction between dynamic flow and director rotation, and it is shown that there exists a general type of bistable twisted director configuration.
Abstract: We investigate the switching bistability based on the interaction between dynamic flow and director rotation in twisted nematic liquid crystal cells. Numerical calculation shows that there exists a general type of bistable twisted director configuration. Two specific cases are verified experimentally.
59 citations
TL;DR: In this article, the authors examined the pressure-distance scaling law for pulsed laser deposition for several thin film systems and found that the scaling law is due to the plasma dynamics occurring within the laser plasma plume near the location of the substrate.
45 citations
TL;DR: Reflective twisted nematic displays where the output polarizer is eliminated are analyzed and demonstrated in this article, where it is shown that excellent black and white displays can be obtained using film compensation and over 90% reflectance from a compensated reflective display has been obtained with low color dispersion.
Abstract: Reflective twisted nematic displays where the output polarizer is eliminated are analyzed and demonstrated. The absence of the rear output polarizer has many advantages, including much higher brightness and higher resolution. In this article, we examine the solution space for the design of such reflective displays. Both the cases of a simple reflective display and the use of a retardation film within the display to compensate for color dispersion are explored. It is shown that excellent black and white displays can be obtained using film compensation. Experimentally, over 90% reflectance from a compensated reflective display has been obtained with low color dispersion. The optical response of this reflective display was also found to be quite fast and suitable for video rate applications.
42 citations
TL;DR: Reflective twisted nematic liquid crystal displays consisting of just one input polarizer, a liquid crystal cell, and a rear reflector are discussed in this paper, where all such display modes reported in the literature can be depicted systematically in a series of parameter space diagrams.
Abstract: Reflective twisted nematic liquid crystal displays consisting of just one input polarizer, a liquid crystal cell, and a rear reflector are discussed. We first show that all such display modes reported in the literature can be depicted systematically in a series of parameter space diagrams. Then by making use of such diagrams as a guide, we show that other new high quality reflective display modes can be obtained. The three parameters that are varied are the input polarizer angle α, the liquid crystal twist angle φ, and the cell gap–liquid crystal birefringence product dΔn. A full numerical simulation which takes into account of the liquid crystal director deformation under the application of an electric field was used to optimize such displays. Experimental results agree well with the theoretical predictions.
28 citations
01 Jan 1997
TL;DR: In this paper, the authors investigated a scheme for driving reflective bistable cholesteric displays, which relies on fast switching from the planar to the focal conic state and showed that 1 ms per line scanning speed is possible using conventional passive matrix drivers.
Abstract: We investigate a new scheme for driving reflective bistable cholesteric displays. It relies on fast switching from the planar to the focal conic state. We show that 1 ms per line scanning speed is possible using conventional passive matrix drivers. In addition, there is no optical delay in the switching process.
27 citations
TL;DR: This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions and can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve.
Abstract: In the waveguiding limit, a twisted nematic liquid crystal cell behaves as an achromatic polarization rotator. We propose and demonstrate the application of such a polarization rotator to convert unpolarized light into linearly polarized light with almost 100% efficiency. This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions. It can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve. Both transmittive and reflective light valves can be used. The temperature dependence of the achromatic polarization rotator is also studied.
18 citations
TL;DR: In this article, it was shown that c-axis oriented GaN films can be formed if a thin zinc oxide (ZnO) buffer layer was first grown on the virgin substrate.
Abstract: Gallium nitride (GaN) thin films have been grown on several kinds of substrates (fused silica, single crystal silicon, and sapphire) by liquid-target pulsed laser deposition at substrate temperature of 600 °C and in the presence of ammonia (NH3) gas. X-ray diffraction, scanning electron microscopy, tunneling electron microscopy, Rutherford backscattering (RBS), and UV/VIS spectrometry were used to characterize the as-grown GaN films. It is shown that c-axis oriented GaN films can be formed if a thin zinc oxide (ZnO) buffer layer was first grown on the virgin substrate. Without the buffer layer the GaN films were found to be polycrystalline with randomly oriented grains. Once the ZnO buffer layer was used, the GaN films immediately grew in a columnar structure with its c-axis normal to the surface of the film. RBS data have shown that the obtained films are stoichiometric GaN. It was also found that the surface morphology and optical transparency of the GaN films were greatly improved by the ZnO buffer layer.
17 citations
Patent•
27 Feb 1997
TL;DR: In this article, an active matrix liquid crystal display device based on voltage control of the birefringence (negative or positive) of the liquid crystal cell was proposed. But this device was based on a ground plate consisting of a piece of glass with a transparent conductive coating, such as indium tin oxide.
Abstract: The invention relates to an active matrix liquid crystal display device based on voltage control (non-zero driving voltage) of the birefringence (negative or positive) of the liquid crystal cell. In the device a ground (earth) plate comprises a piece of glass with a transparent conductive coating, such as indium tin oxide. A silicon wafer with integrated circuits on it has a coating of polymer for aligning liquid crystal molecules. The coated glass also has a coating for aligning the liquid crystal molecules.
15 citations
TL;DR: In this paper, a polycrystalline germanium thin film was formed by pyrolysis of germane gas in a low-pressure reactor, and high Hall effect mobility on the order of 300 cm 2 /V s was obtained, even at a relatively low deposition temperature of 300°.
Abstract: Thin films of polycrystalline germanium were formed by the pyrolysis of germane gas in a low-pressure reactor. Process parameters investigated were deposition temperature in the range 250 to 350°C and pressure in the range 300 to 600 mTorr. The properties of the film have been characterized using transmission electron microscopy and x-ray diffraction for structural analysis, atomic force microscopy for surface morphology analysis, secondary ion mass spectroscopy for compositional analysis, and Hall effect measurement for electrical parameter extraction, etc. High Hall effect mobility on the order of 300 cm 2 /V s was obtained, even at a relatively low deposition temperature of 300°. This makes the germanium thin films potentially very promising for low-temperature device processing.
15 citations
TL;DR: In this article, the Hall effect measurement was employed to study the isothermal annealing of boron or phosphorus implanted polycrystalline Si/sub 1-x/Ge/sub x/ thin films, with x varying from 0.3-0.55.
Abstract: Hall effect measurement was employed to study the isothermal annealing of boron or phosphorus implanted polycrystalline Si/sub 1-x/Ge/sub x/ thin films, with x varying from 0.3-0.55. X-ray diffraction and cross-sectional transmission electron microscopy were used to study the crystal structure, whereas X-ray photoelectron spectroscopy was used to determine the film composition and the chemical bonding states of the elements. In low-temperature (/spl les/600/spl deg/C) annealing, the conductivity, the dopant activation, and the Hall effect mobility decreased during extended annealing. The effective activation of phosphorus was less than 20% and decreased with increasing Ge content. Boron activation could reach above 70%. It was also found that Si/sub 1-x/Ge/sub x/ could be oxidized at 600/spl deg/C in a conventional furnace even with N/sub 2/ protection, especially for phosphorus doped films with high Ge content. Consequently, a low-temperature SiO/sub 2/ capping layer is necessary during extended annealing.
13 citations
TL;DR: In this paper, a study on the reaction of Ni and amorphous Si0.68Ge0.32 film on SiO2 was performed at 520°C in a conventional furnace.
Abstract: A study on the reaction of Ni and amorphous Si0.68Ge0.32 film on SiO2 is reported. The reaction was performed at 520° C in a conventional furnace. The resulting film was characterized using X-ray photoelectron spectroscopy (XPS) and Raman scattering spectroscopy. Ni induced crystallization of SiGe was confirmed by the Raman spectra. XPS results indicate Ni piled up at or near the interface of the crystallized SiGe and the SiO2 substrate. The small amount of Ni inside the SiGe layer exists in more of a silicide- or germanide-like form. Ni enhanced oxidation of SiGe was found during the reaction and the oxidized layer was found to be a mixture of oxides of Si and Ge, with Ge piling up at the surface.
TL;DR: In this article, a two-step heat treatment process was employed to crystallize low pressure deposited thin films of amorphous germanium and high Hall effect electron mobility was obtained.
Abstract: A two-step heat treatment process has been employed to crystallize low pressure deposited thin films of amorphous germanium. Large grain p -type polycrystalline germanium with a Hall effect hole mobility of greater than 300 cm 2 /Vs has been obtained. Films with near intrinsic conductivity, necessary for the construction of practical enhancement-mode insulated-gate thin film transistors, were obtained by introducing phosphorus as a compensating dopant. High Hall effect electron mobility of 245 cm 2 /Vs has been measured on the resulting n -type polycrystalline germanium thin films.
Proceedings Article•
01 Jan 1997
TL;DR: In this article, the brightness and contrast ratio of bistable twisted nematic displays are studied using the parameter space approach, and regions where the Bistable WN display shows the best contrast ratio are clearly shown.
Abstract: The brightness and contrast ratio of bistable twisted nematic displays are studied using the parameter space approach. By plotting the contrast ratio as a function of the twist angle and birefringence of the display, regions where the bistable twisted nematic display shows the best contrast ratio are clearly shown. We also studied the switching mechanisms of such displays.
Patent•
25 Mar 1997TL;DR: In this article, a reflective liquid crystal display which consists of just an input polarizer, the liquid crystal cell and a rear reflector which can be placed inside or outside of the cell is presented.
Abstract: The invention provides a reflective liquid crystal display which consists of just an input polarizer, the liquid crystal cell and a rear reflector which can be placed inside or outside of the liquid crystal cell. There is no rear polarizer and no retardation film compensation. The invention is capable of producing a high contrast, low chromatic dispersion display. Depending on the twist angle chosen for the liquid crystal cell, the display can be used in direct view or in a projection type display. The display can also be driven in an active matrix mode or in a passive matrix multiplexed drive mode. A full numerical simulation which takes into account the liquid crystal director deformation under the application of an electric field was used to optimize such displays.
14 Feb 1997
TL;DR: RefReflective liquid crystal displays that consist of just one polarizer and a rear reflector are presented in this article, and an operating mode is found that possess excellent quasi blackhvhite contrast and low colour dispersion.
Abstract: Reflective liquid crystal displays that consist of just one polarizer and a rear reflector are presented. No retardation films are used to compensate for colour dispersion. An operating mode is found that possess excellent quasi blackhvhite contrast and low colour dispersion. In terms of both performance and production cost, this new display mode is b&er than existing supertwisted nematic displays.
TL;DR: In this paper, the authors investigate the switching bistability based on the interaction between dynamic flow and director rota-tion in twisted nematic liquid crystal cells and show that there exists a general type of bistable twisted director configurations.
Abstract: We investigate the switching bistability based on the interaction between dynamic flow and director rota-tion in twisted nematic liquid crystal cells. Numerical calculation shows that there exists a general type of bistable twisted director configurations. Two specific cases are verified experimentally. Keywords: bistability, twisted nematic, liquid crystal displays, backflow, switching 1. INTRODUCTION Twisted nematic (TN) and super-twisted nematic (STN) liquid crystal displays (LCD) are used widelyin many information display systems. Recently, a bistable TN (BTN) LCD has been demonstrated 1,2• This BTN is based on a gross mismatch of the natural twist of the liquid crystal director and the rubbingboundary conditions of the two LC cell surfaces. If the LC cell alignment favors a -twist, then for somevalue of natural helical pitch P, the LC cell can exist either in the -twist state or the (4 + 2ir)-twiststate. Roughly speaking, in the absence of pretilt of the director, the natural twist of the LC cell shouldbe about 4 + ir in order for bistability to occur. Hence the d/P ratio, where d is the LC cell thickness,
01 Jan 1997
TL;DR: In this article, the authors search the parameter space of reflective nematic liquid crystal displays and discover two new operating modes that should have applications to both direct view and projection systems, the low twist RTN mode and the high twist RSTN mode.
Abstract: By searching the parameter space of reflective nematic liquid crystal displays, two new operating modes were discovered that should have applications to both direct view and projection systems. They are the low twist RTN mode and the high twist RSTN mode. We show that they are actually related to the MTN mode, the TNECB mode, the SCTN mode and the HFE mode.
Patent•
25 Mar 1997TL;DR: In this article, a liquid crystal display is described in which the need for a conventional output polarizer is eliminated and the display comprises only the liquid crystal cell, a single input polarizer, a reflector placed behind the cell, and a retardation film placed between the reflector and the cell.
Abstract: A liquid crystal display is described in which the need for a conventional output polarizer is eliminated and the display comprises only a liquid crystal cell, a single input polarizer, a reflector placed behind the cell, and a retardation film placed between the reflector and the cell. A Jones matrix simulation approach is used in order to obtain the parameters corresponding to solution spaces for the above design and several combinations are disclosed that give good results.
TL;DR: In this paper, it was shown that ZnO/glass is a good alternative to bare glass for producing commercial ITO films and that the resistivity is dominated by impurity scattering in the range of 10K-300K.
Abstract: Indium Tin Oxide (ITO) thin films with low resistivities of 0.1∼0.2 mΩ-cm were deposited on various substrates such as YSZ, glass, and ZnO buffered glass by pulsed laser deposition (PLD). The X-ray rocking curve of crystalline (200) ITO films grown on (100) YSZ had a FWHM as narrow as 0.08°. ITO films grown on ZnO (0001) buffered glass had an single (222) orientation and the X-ray rocking curve had a FWHM of 2. 1°. Ultrathin ITO films of 3.6nm were fabricated on YSZ and their electrical properties were measured from 10K-300K. ITO films fabricated on ZnO buffered glass and bare glass were characterized by Hall effect measurements as a function of temperature. The results indicate that the resistivity of ITO films grown by PLD does not depend on the orientation or the structure of the thin film. The resistivity is dominated by impurity scattering in the range of 10K-300K. We show that ZnO/glass is a good alternative to bare glass for producing commercial ITO films.
TL;DR: In this paper, the phase matching condition for second harmonic generation and the half-wave voltage for Q-switching are interrelated and a self-consistent solution is obtained.
Abstract: Simultaneous Q-switching and second harmonic generation in KTP is analysed numerically. In this mode of operation, the phase matching condition for second harmonic generation and the half-wave voltage for Q-switching are interrelated. In this paper, this coupled numerical problem is solved iteratively until a self-consistent solution is obtained. The effective refractive index and effective electro-optical coefficient along the phase matching direction are defined and calculated in order to estimate the half-wave voltage for Q-switching with KTP. Additionally, the application of temperature and wavelength dispersion tuning to compensate for the constant retardation in this biaxial crystal is also studied. It is found that the operating temperature of the KTP crystal should be stable to within 0·1°C for exact retardation compensation.
Proceedings Article•
01 Jan 1997
TL;DR: In this paper, a chromatic parameter space (CPS) representation is proposed to represent the operation of liquid crystal displays (LCDs) and the real appearance of the LCDs in the static state (voltage off or nonselect state) can be shown in the CPS diagram, both in terms of brightness and color.
Abstract: A chromatic parameter space (CPS) representation is proposed to represent the operation of LCDs. Both chrominance and luminance are visualized in the CPS diagrams. All the usual display modes, both transmissive and reflective, such as TN, ECB, OMI, STN, and SBE are shown clearly on the diagrams. Two designs with the help of CPS are proposed and demonstrated: multicolor STN and a novel fully-compensated reflective STN with high brightness and almost no chromatic dispersion. Introduction Optical modeling is very important in designing and optimizing liquid crystal displays (LCDs) . In previous work , we proposed a new systematic technique called parameter space (PS) representation of all nematic LCDs. This PS approach is very helpful in understanding the physical operation of various LCD modes and their relationships. With the help of the PS, we also analyzed the various different reflective LCD designs and developed several new reflective LCD operating modes . However, in our previous work, the PS is calculated only for a single wavelength, normally 550nm. Thus it does not show the appearance of the LCD under white light illumination. Even though the monochromatic PS can provide an idea of the chromatic dispersion behavior of the various LCD modes, it is still desirable to devise a scheme where the chromaticity of the display can be shown more clearly. It is particularly so for LCDs with considerable chromatic dispersion, such as STN displays. In this paper, a chromatic parameter space (CPS) representation is proposed and demonstrated. Both chrominance and luminance are visualized in the same diagram with the help of color representation. The real appearance of the LCDs in the static state (voltage-off or nonselect state) can be shown in the CPS diagram, both in terms of brightness and color. Using this new CPS, we analyzed the chromatic dispersion of various display modes, both transmissive and reflective. Furthermore, in this paper, we shall show two examples of the application of this CPS in designing new LCDs with good performance: (1) Multicolor STN (2) Fullycompensated reflective STN with high brightness and almost no chromatic dispersion. It is interesting to note that in the first case, we make use of chromatic dispersion to achieve multicolor display. In the second case, we do our best to eliminate chromatic dispersion. These two cases provide a good demonstration of how CPS can be used for LCD design and novel device development. Chromatic parameter space The reflectance or transmittance of an LCD is determined by four variables: input polarizer angle α, output polarizer angle γ , twist angle φ , and retardation value d∆n. Any pair of these variables can be used to generate a contour map which is called the PS diagram. The calculation assumes a fixed wavelength λ. The idea of the CPS is to repeat the PS calculation for all λ in the visual range ( 380nm ~ 780nm), and obtain the luminance and chrominance of the output assuming a standard white light input source. In this paper, CIE D65 is used as the incident light. From the output spectrum, we can obtain the CIE 1931 XYZ values. The XYZ color coordinate is transformed to the standard color space: sRGB, which is proposed by Hewlett-Packard and Microsoft . The sRGB tristimulus values can be computed using the following relationship:
TL;DR: In this article, a parameter space description of RLCD as a function of polarizer angle, liquid crystal twist angle and birefringence is discussed, which can be obtained from searching the parameter space systematically.
Abstract: Reflective LCDs (RLCDs) have high brightness and are free from viewing parallax. They also cost less materials to construct and are compatible with existing manufacturing and driving practices. In this paper, a parameter space description of RLCD as a function of polarizer angle, liquid crystal twist angle and birefringence is discussed. It is shown that all published RLCD modes can be depicted in this parameter space, including the twisted-nematic- electrically controlled birefringence (TN-ECB) modes, the hybrid field effect mode, the mixed-mode TN, and the self-compensated TN (SCTN) mode. Additionally we show several new RLCD modes, including the reflective TN (RTN) and the reflective STN (RSTN) which can be obtained from searching the parameter space systematically. All RLCD modes are related by a variation of the 3 LCD parameters. The RTN and RSTN modes have applications to both direct view and projection display systems. Sample RTN and RSTN displays were fabricated. Experimental results show good agreement with theoretical predictions.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Proceedings Article•
01 Jan 1997
TL;DR: In this paper, a new TN-ECB mode liquid crystal display is developed for reflective liquid crystal displays (RLCDs), which has a larger cell gap and lower operating voltage than most recently developed RLCDs.
Abstract: A new TN-ECB mode liquid crystal display is developed for reflective liquid crystal displays (RLCDs). This new TN-ECB mode has a larger cell gap and lower operating voltage than most recently developed RLCDs. It also has a small wavelength dispersion and has a good dark state at the low operating voltage. It is particularly suitable for reflective crystalline Si backplane CMOS AMLCDs.
14 Feb 1997
TL;DR: In this paper, a polycrystalline germanium thin film was formed by pyrolysis of germane gas in a low-pressure reactor, and high Hall effect mobility on the order of 300 cm 2 /V s was obtained, even at a relatively low deposition temperature of 300°.
Abstract: Thin films of polycrystalline germanium were formed by the pyrolysis of germane gas in a low-pressure reactor. Process parameters investigated were deposition temperature in the range 250 to 350°C and pressure in the range 300 to 600 mTorr. The properties of the film have been characterized using transmission electron microscopy and x-ray diffraction for structural analysis, atomic force microscopy for surface morphology analysis, secondary ion mass spectroscopy for compositional analysis, and Hall effect measurement for electrical parameter extraction, etc. High Hall effect mobility on the order of 300 cm 2 /V s was obtained, even at a relatively low deposition temperature of 300°. This makes the germanium thin films potentially very promising for low-temperature device processing.