Lilin Liu

Efficiency degradation behaviors of current/thermal co-stressed GaN-based blue light emitting diodes with vertical-structure

With this work, we demonstrate a three-stage degradation behavior of GaN based LED chips under current/thermal co-stressing. The three stages in sequence are the initial improvement stage, the platform stage, and the rapid degradation stage, indicating that current/thermal co-stressing activates positive effects and negative ones simultaneously, and the dominant degradation mechanisms evolve with aging time. Degradation mechanisms are discussed. Electric current stress has dual characters: damaging the active layers by generating defects and at the same time improving the p-type conductivity by activating the Mg-dopant. High temperature stresses will promote the effects from electric current stresses. The activation of the Mg-dopant will saturate, whereas the generation of defects is carried on in a progressive way. Other mechanisms, such as deterioration of ohmic contacts, also operate. These mechanisms compete/cooperate with each other and evolve with aging time, resulting in the observed three-stage de...

Super multi-view three-dimensional display through spatial-spectrum time-multiplexing of planar aligned OLED microdisplays

Existing super multi-view (SMV) technologies depend on ultra-high resolution two-dimensional (2D) display panel or large number of 2D display panels to obtain dense sub-viewing-zones for constructing more natural three-dimensional (3D) display by pure spatial-multiplexing. Through gating the spatial-spectrum of each OLED microdisplay, the present work proposes a new SMV technology combining time- and spatial-multiplexing based on planar-aligned OLED microdisplays. The inherent light emission characteristics of OLED, i.e. large divergence angle, guarantees a homogeneous light intensity distribution on the spectrum plane, which is a necessary condition for successful time multiplexing. The developed system bears with low requirements on the number of 2D display panels. The factors influencing the lateral display resolution limit are discussed and the optimum value is deduced. Experimentally, a prototype system with 60 sub-viewing-zones is demonstrated by 12 OLED microdisplays. The horizontal interval between adjacent sub-viewing-zones is 1.6mm.

The investigation of LED's reliability through highly accelerated stress testing methods

The white light emitting diode technology is developing rapidly in recent years. Evaluating the lifetime and failure rates of white LED devices in short time periods is still a big problem. We propose to use highly accelerated stress testing (HAST) methods to investigate the reliability of LED devices in 168 hours. Biased highly accelerated temperature and humidity tests are conducted. The present work shows that under HAST conditions, the degradation of optical flux obeys exponential law also. Based on the Arrhenius equation, an accelerating factor of 81.4 is obtained for HAST methods. Normal accelerated life tests are carried out for comparison usage. The lifetimes of 70% lumen maintenance estimated by HAST experiments, normal ALT experiments and the manufacturer are at the same order, implying that highly accelerated stress tests can be an effective way to estimate LEDs lifetime quickly. Finally, a statistical analysis of the lifetime distribution through Nelson estimation and TTT curve indicates that GaN-based wLED fails in an increasing trend. Efforts must be conducted to improve the reliability of GaN-based LEDs.

Multiview three-dimensional display with continuous motion parallax through planar aligned OLED microdisplays.

Existing multiview three-dimensional (3D) display technologies encounter discontinuous motion parallax problem, due to a limited number of stereo-images which are presented to corresponding sub-viewing zones (SVZs). This paper proposes a novel multiview 3D display system to obtain continuous motion parallax by using a group of planar aligned OLED microdisplays. Through blocking partial light-rays by baffles inserted between adjacent OLED microdisplays, transitional stereo-image assembled by two spatially complementary segments from adjacent stereo-images is presented to a complementary fusing zone (CFZ) which locates between two adjacent SVZs. For a moving observation point, the spatial ratio of the two complementary segments evolves gradually, resulting in continuously changing transitional stereo-images and thus overcoming the problem of discontinuous motion parallax. The proposed display system employs projection-type architecture, taking the merit of full display resolution, but at the same time having a thin optical structure, offering great potentials for portable or mobile 3D display applications. Experimentally, a prototype display system is demonstrated by 9 OLED microdisplays.

Super multi-view three-dimensional display technique for portable devices

Portable display devices, such as intelligent telephones and panel PCs, have become parts of modern peoples daily life. Their mainstream display interfaces are based on two-dimensional (2D) images. Although some three-dimensional (3D) technologies have been proposed for portable devices, comfortable visual effects are untouched until now. A super multi-view (SMV) system with comfortable 3D effects, constructed by a group of OLED microdisplay/projecting lens pairs, is proposed in this paper. Through gating different segments of each projecting lens sequentially and refreshing the virtual image of the corresponding microdisplay synchronously, the proposed SMV system greatly decreases the demand on the number of employed microdisplays and at the same time takes a thin optical structure, endowing great potential for portable devices.

A Power-Type Single GaN-Based Blue LED With Improved Linearity for 3 Gb/s Free-Space VLC Without Pre-equalization

A new GaN light-emitting diode (LED) structure, which has a 300-nm aluminum-doped zinc oxide transparent current spreading layer epitaxial layer grown on a standard GaN LED epistack used in commercial GaN LED products, shows improved power vs. voltage (P-V) linearity and is suitable for high-data rate visible light communication. Experimentally, a single GaN-based blue LED with a mesa diameter of 150 μm and a maximum optical power of 42 mW demonstrates a 3-Gb/s free-space data transmission speed. The modulation bandwidth reaches 600 MHz under the present experimental setup. The present work proves the practicability of enhancing the LEDs free-space data transmission ability through a P-V linearity improvement at the chip level.

Improved spatiotemporal-multiplexing super-multiview display based on planar aligned OLED microdisplays

Through gating spectrum plane of multiple planar aligned OLED microdisplays by a timely sequential manner, a super-multiview (SMV) three-dimensional (3D) display based on spatiotemporal-multiplexing was developed in our previous paper. But an upper limit of the allowable sub-viewing-zones (SVZs) for an OLED microdisplay did exist in the previous system, even if microdisplays with very high frame rates could be commercially available. In this manuscript, an improved spatiotemporal-multiplexing SMV displays system is developed, which removes the above limitation through controllable fusing of light beams from adjacent OLED microdisplays. The employment of a liquid-crystal panel as the gating-aperture array allows the improved system to accommodate multiple rows of OLED microdisplays for denser SVZs. Experimentally, a prototype system is demonstrated by 24 OLED microdisplays, resulting in 120 SVZs with an interval small to 1.07mm.

Generation of 360° three-dimensional display using circular-aligned OLED microdisplays

A 360° all-around multiview three-dimensional (3D) display system is proposed by using coarse-pitch circular-aligned OLED microdisplays. The magnified virtual color images projected from microdisplays serve as stereo images, which can create separate eyeboxes for the viewer. Through inserting baffles, a transitional stereo image assembled by two spatially complementary segments from adjacent stereo images is presented to a complementary fusing zone (CFZ) which locates between adjacent eyeboxes. For a moving observation point, the spatial ratio of the two complementary segments evolves gradually, resulting in continuously changing transitional stereo images and thus overcoming the problem of discontinuous moving parallax. Such a controllable light-ray fusing technology, assured by the inherent large divergent angle of OLED pixels, decreases the required number of display panels for 360° multiview 3D display greatly. A prototype display system with only 67 full-color OLED microdisplays is set up to demonstrate the 360° 3D color display. The develop system is freed from the dependence on mechanical moving elements, high-speed components and diffusion screens.

Size- and current-density-controlled tunable wavelength in gan-based leds for potential dense wavelength-division multiplexing application

A micro-LED array device with hybrid pixel sizes from 30 μm~60 μm is designed and fabricated. A wavelength tunable range of 20 nm is obtained, which shows the devices potential as a tunable light source to enable DWDM technology for visible light communication applications. For example, superimposing the luminescent spectra of μLEDs by different combinations, a series of spectra and light intensity values can be obtained. In addition, the FWHM of the complex spectrum broadens from 16 nm to 45 nm. Considered from the aspect of general lighting, a broader FWHM can increase lighting quality. The tunable peak wavelength is controlled by micro-LED size and input current density. Underlying mechanisms are discussed. The structure parameters of the arrayed-μLED device can be further optimized to satisfy requirements on communications and general lighting.

An explanation for catastrophic failures of GaN-based vertical structure LEDs subjected to thermoelectric stressing

Both accelerated life tests and engineering practices present a common behavior: LED products may encounter catastrophic failure under thermo-electric co-stressing. The present work proposes that the applied thermoelectric stresses themselves are not necessary conditions but pre-conditions for LED breakdown. The deformation and sliding of the metal lines on n-GaN under thermoelectric co-stressing cause electrodes to become dilapidated. Such dilapidated electrodes with rough edges will help to generate localized high electrical fields. The catastrophic failure occurs during the de-trapping process of space charges in and around threading dislocations when the applied electric field is removed and short circuits form between the charged dislocation cores and the tips of the dilapidated electrodes. A phenomenological relationship is given to describe the interactions between temperatures and applied electric currents on the diffusion-flux-accommodated sliding and deformation of patterned metal electrodes on vertical LEDs. Such a law is coincident with the practical trend of LED failure rates observed in accelerated life tests and engineering applications.