Chi-Kang Li

Study on the Current Spreading Effect and Light Extraction Enhancement of Vertical GaN/InGaN LEDs

This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral and vertical light-emitting diodes (LEDs). Specifically, this study uses a fully 2-D model that solves drift-diffusion and Poisson equations to investigate current flow paths and radiative recombination regions. The ray-tracing technique was used to calculate the LEE of the top surface. First, this study discusses the current spreading effect of the lateral and conventional vertical LED and determines the efficiency droop even with a transparent conducting layer. Different electrode configurations in the vertical LED were tested to optimize the current spreading effect, which, in turn, suppresses the carrier leakage and mitigates the efficiency droop under high injection conditions. This study also discusses the wall-plug efficiency in overall cases to identify the design rules for higher power conversion efficiency.

Three dimensional numerical study on the efficiency of a core-shell InGaN/GaN multiple quantum well nanowire light-emitting diodes

This paper presents the findings of investigating core-shell multiple quantum well nanowire light-emitting diodes (LEDs). A fully self-consistent three dimensional model that solves Poisson and drift-diffusion equations was employed to investigate the current flow and quantum-confined stark effect. The core-shell nanowire LED showed a weaker droop effect than that of conventional planar LEDs because of a larger active area and stronger recombination in nonpolar quantum wells (QWs). The current spreading effect was examined to determine the carrier distribution at the sidewall of core-shell nanowire LEDs. The results revealed that a larger aspect ratio by increasing the nanowire height could increase the nonpolar-active area volume and reduce the droop effect at the same current density. Making the current spreading length exceed a greater nanowire height is critical for using the enhancement of nonpolar QWs effectively, when an appropriate transparent conducting layer might be necessary. In addition, this...

Computer simulation and analysis of fountain flow in filling process of injection molding

The viscous flow in the filling stage of injection molding can be described in terms of an one-dimensional fully developed main flow and a complex two-dimensional flow near the advancing front, which is often termed the ”fountain flow“. The transport characteristics in the front region of the mold flow are of increasing importance in injection process of composite materials such as resin injection molding (RIM). By using of finite element method, the simulation of non-isothermal viscous flow between two isothermal parallel plates with the generalized newtonian fluid is presented in detail. The ”un-folding” of the fluid particles towards the mold wall directly affects transport characteristics such as the distribution of temperature, the orientation and the concentration of molecule near the front in filling stage.

Optical Properties of the Partially Strain Relaxed InGaN/GaN Light-Emitting Diodes Induced by p-Type GaN Surface Texturing

Partial strain relaxation from the light-emitting diode (LED) with surface-textured p-GaN was observed. The textured device possesses less efficiency droop and a higher current level at the efficiency maximum, as compared with the planar one. The results suggest that surface roughening affects not only the external light extraction but also the internal quantum efficiency. Furthermore, the photoluminescent (PL) measurement at low temperature reveals that the percentage increment of the optical power of the textured LED over that of the planar LED becomes lower. In addition to the effect of frozen nonradiative defect states, the PL difference is related to the strain-correlated quantum-confined Stark effect.

Thin 3D Multiplication Regions in Plasmonically Enhanced Nanopillar Avalanche Detectors

We demonstrate a nanopillar (NP) device structure for implementing plasmonically enhanced avalanche photodetector arrays with thin avalanche volumes (∼ 310 nm × 150 nm × 150 nm). A localized 3D electric field due to a core-shell PN junction in a NP acts as a multiplication region, while efficient light absorption takes place via surface plasmon polariton Bloch wave (SPP-BW) modes due to a self-aligned metal nanohole lattice. Avalanche gains of ∼216 at 730 nm at -12 V are obtained. We show through capacitance-voltage characterization, temperature-dependent breakdown measurements, and detailed device modeling that the avalanche region is on the order of the ionization path length, such that dead-space effects become significant. This work presents a clear path toward engineering dead space effects in thin 3D-confined multiplication regions for high performance avalanche detectors for applications in telecommunications, sensing and single photon detection.

Optimum design of runner system balancing in injection molding

Abstract The runner-system design is of great importance to achieve a successful injection molding process of a family mold with multiple cavities. Using a Galerkin finite element method to simulate the flow of a one-dimensional tube runner-system including phase change in detail we present a power law model. The difference between the pressure and temperature at the gate entrance of all cavities can be taken as the objective function to be minimized because if all cavities are filled simultaneously, these pressure and temperature variations should be a minimum. The purpose is to fill all cavities at the same instant under the same conditions in order to prevent flashing of the mold and to produce parts of improved and uniform quality. In this paper we present a feasible means to optimize the runner design automatically by integrating optimization theory with a flow/thermo-simulation program.

Study on the Optimization for Current Spreading Effect of Lateral GaN/InGaN LEDs

This paper exhibits systematic results for lateral light emitting diodes (LEDs) with various conditions. The simulation results and circuit model are both included to describe the current spreading effect at the same time. A fully 2-D model that solves Poisson and drift-diffusion equations to investigate the current flow and radiative recombination distribution specifies the uniformity of the carrier distribution, which is combined with the Monte Carlo ray tracing technique to calculate the light extraction efficiency (LEE). This paper focuses on the modulation of the transparent conducting layer. In addition, this paper will discuss bottom emission LEDs addressing the current spreading effect and LEE compared with top emission LEDs. We also examine the droop effect to verify our discussion. A thorough analysis provides deep insights for achieving high efficiency lateral LEDs.

Scaling performance of Ga2O3/GaN nanowire field effect transistor

A three-dimensional finite element solver is applied to investigate the performance of Ga2O3/GaN nanowire transistors. Experimental nanowire results of 50 nm gate length are provided to compare with the simulation, and they show good agreement. The performance of a shorter gate length (<50 nm) is studied and scaling issues of the short-channel effect are analyzed. With a better surrounding gate design and a recessed gate approach, the optimal conditions for a 20 nm gate length are explored in this paper.

On the Efficiency Decrease of the GaN Light-Emitting Nanorod Arrays

Nanostructure light emitting arrays, with the mitigated quantum confined stark effect, provide a different angle to investigate the efficiency decrease in the GaN based LEDs. In this paper, the external quantum efficiency and the electroluminescent spectra of GaN based nanorod LEDs are characterized through experiments and simulations. The strains in the InGaN/GaNnanorods are varied with the choice of nanorod sidewall passivation materials. Our results indicate that Auger recombination dominates at low-level currents. However, even though the effect of Auger accounts fora higher percentage weighting, the increase number of leakage carriers out of quantum wells is responsible for the efficiency drop at high current levels.

3D numerical modeling of the carrier transport and radiative efficiency for InGaN/GaN light emitting diodes with V-shaped pits

In this paper, influence of a V-pit embedded inside the multiple quantum wells (MQWs) LED was studied. A fully three-dimensional stress-strain solver and Poisson-drift-diffusion solver are employed to study the current path, where the quantum efficiency and turn-on voltage will be discussed. Our results show that the hole current is not only from top into lateral quantum wells (QWs) but flowing through shallow sidewall QWs and then injecting into the deeper lateral QWs in V-pit structures, where the V-pit geometry provides more percolation length for holes to make the distribution uniform along lateral MQWs. The IQE behavior with different V-pit sizes, threading dislocation densities, and current densities were analyzed. Substantially, the variation of the quantum efficiency for different V-pit sizes is due to the trap-assisted nonradiative recombination, effective QW ratio, and ability of hole injections.