Yuh-Renn Wu

Electronic and optical properties of InGaN quantum dot based light emitters for solid state lighting

In this paper, we have made a systematic study of the electronic and optical properties of InGaN based quantum dot light emitters. The valence force field model and 6×6k⋅p method have been applied to study the band structures in InGaN or InN quantum dot devices. Piezoelectric and spontaneous polarization effects are included. A comparison with InGaN quantum wells shows that InGaN quantum dots can provide better electron-hole overlap and reduce radiative lifetime. We also find that variation in dot sizes can lead to emission spectrum that can cover the whole visible light range. For high carrier density injection conditions, a self-consistent method for solving quantum dot devices is applied for better estimation of device performance. Consequences of variations in dot sizes, shapes, and composition have been studied in this paper. The results suggest that InGaN quantum dots would have superior performance in white light emitters.

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.

Analyzing the physical properties of InGaN multiple quantum well light emitting diodes from nano scale structure

We report on the influence of nanoscale indium fluctuations on physical properties for multiple quantum well (QW) light emitting diodes (LEDs). A commercial grade c-plane LED was analyzed by atom probe tomography, and the indium composition distribution was extracted. The influence of the degree of fluctuation and number of quantum wells were analyzed by a two-dimensional Poisson and drift-diffusion solver with very fine mesh and compared to the experimental result and a simple normal quantum well model. The studies show that the indium fluctuation will significantly impact the device’s internal quantum efficiency, droop behavior, and current-voltage curves. Including the influence of indium-fluctuation gives a better prediction of the device performance.

Strain-enhanced photoluminescence from Ge direct transition

Strong enhancement of Ge direct transition by biaxial-tensile strain was observed. The reduction in band gap difference between the direct and indirect valleys by biaxial tensile strain increases the electron population in the direct valley, and enhances the direct transition. The band gap reduction in the direct and indirect valleys can be extracted from the photoluminescence spectra and is consistent with the calculations using k⋅p and deformation potential methods for conduction bands and valence bands, respectively.

The influence of random indium alloy fluctuations in indium gallium nitride quantum wells on the device behavior

In this paper, we describe the influence of the intrinsic indium fluctuation in the InGaN quantum wells on the carrier transport, efficiency droop, and emission spectrum in GaN-based light emitting diodes (LEDs). Both real and randomly generated indium fluctuations were used in 3D simulations and compared to quantum wells with a uniform indium distribution. We found that without further hypothesis the simulations of electrical and optical properties in LEDs such as carrier transport, radiative and Auger recombination, and efficiency droop are greatly improved by considering natural nanoscale indium fluctuations.

Size-Dependent Strain Relaxation and Optical Characteristics of InGaN/GaN Nanorod LEDs

In this paper, InGaN/GaN nanorod LEDs with various sizes are fabricated using self-assembled Ni nanomasks and inductively coupled plasma-reactive ion etching. Photoluminescence (PL) characteristics exhibit size-dependent, wavelength blue shifts of the emission spectra from the nanorod LEDs. Numerical analyses using a valence force field model and a self-consistent Poisson, Schrodinger, and drift-diffusion solver quantitatively describe the correlation between the wavelength blue shifts and the strain relaxation of multiple quantum wells embedded in nanorods with different averaged sizes. Time-resolved PL studies confirm that the array with a smaller size exhibits a shorter carrier lifetime at low temperature, giving rise to a stronger PL intensity. However, the PL intensity deteriorates at room temperature, compared to that of a larger size, possibly due to an increased number of surface states, which decreases the nonradiative lifetime, and hence reduces the internal quantum efficiency.

Influence of polarity on carrier transport in semipolar (2021¯) and (202¯1) multiple-quantum-well light-emitting diodes

We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the semipolar (202¯1) and (2021¯) orientations of free-standing GaN. For semipolar MQW LEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar (2021¯) MQW LEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than (202¯1) MQW LEDs.

Performance and polarization effects in (112¯2) long wavelength light emitting diodes grown on stress relaxed InGaN buffer layers

Long wavelength (525–575 nm) (112¯2) light emitting diodes were grown pseudomorphically on stress relaxed InGaN buffer layers. Basal plane dislocation glide led to the formation of misfit dislocations confined to the bottom of the InGaN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the device active region. The change of the stress state of the quantum well due to one-dimensional plastic relaxation altered the valence band structure, which produced a significant shift in polarization of emitted light. Devices grown on relaxed buffers demonstrated equivalent output power compared to those for control samples without relaxation.

Two dimensional electron gases in polycrystalline MgZnO/ZnO heterostructures grown by rf-sputtering process

This paper reports the formation of two-dimensional electron gas (2DEG) in rf-sputtered defective polycrystalline MgZnO/ZnO heterostructure via the screening of grain boundary potential by polarization-induced charges. As the MgZnO thickness increases, the sheet resistance reduces rapidly and then saturates. The enhancement of the interfacial polarization effect becomes stronger, corresponding to a larger amount of resistance reduction, when the Mg content in the cap layer increases. Monte Carlo method by including grain boundary scattering effect as well as 2D finite-element-method Poisson and drift-diffusion solver is applied to analyze the polycrystalline heterostructure. The experimental and Monte Carlo simulation results show good agreement. From low temperature Hall measurement, the carrier density and mobility are both independent of temperature, indicating the formation of 2DEG with roughness scattering at the MgZnO/ZnO interface.

Gate leakage suppression and contact engineering in nitride heterostructures

We present a self-consistent approach to examine current flow in a general metal–polar heterostructure junction. The approach is applied to examine properties of three classes of junctions that are important in devices: (i) GaN/AlGaN structures that are used in nitride heterojunction field effect transistors; (ii) GaN/AlGaN/high-κ insulator structures for potential application in very small gate devices to suppress gate tunneling current; and (iii) GaN/AlGaN/polar insulator junctions with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if a dielectric thicknesses of ∼20 A can be achieved.