Yuh-Renn Wu - Academia.edu (original) (raw)

Papers by Yuh-Renn Wu

Processes, 2022

The efficiency of micro-light-emitting diodes (μ-LEDs) depends enormously on the chip size, and t... more The efficiency of micro-light-emitting diodes (μ-LEDs) depends enormously on the chip size, and this is connected to sidewall-trap-assisted nonradiative recombination. It is known that the internal quantum efficiency (IQE) of aluminum gallium indium phosphide (AlGaInP)-based red μ-LEDs is much lower than that of nitride-based μ-LEDs. To establish the major reasons giving rise to this huge IQE discrepancy, we examined the limiting factors in the two structures. For the nitride-based InGaN quantum wells, the influences of random alloy fluctuations were examined. A two-dimensional Poisson and drift-diffusion solver was applied to analyze these issues.

Physical Review Applied, 2022

For nitride-based blue and green light-emitting diodes (LEDs), the forward voltage V for is large... more For nitride-based blue and green light-emitting diodes (LEDs), the forward voltage V for is larger than expected, especially for green LEDs. This is mainly due to the large barriers to vertical carrier transport caused by the total polarization discontinuity at multiple quantum well and quantum barrier interfaces. The natural random alloy fluctuation in QWs has proven to be an important factor reducing V for. However, this does not suffice in the case of green LEDs because of their larger polarization-induced barrier. V-defects have been proposed as another key factor in reducing V for to allow laterally injection into multiple quantum wells (MQWs), thus bypassing the multiple energy barriers incurred by vertical transport. In this paper, to model carrier transport in the whole LED, we consider both random-alloy and V-defect effects. A fully two-dimensional drift-diffusion charge-control solver is used to model both effects. The results indicate that the turn-on voltages for blue and green LEDs are both affected by random alloy fluctuations and V-defect density. For green LEDs, V for decreases more due to V-defects, where the smaller polarization barrier at the V-defect sidewall is the major path for lateral carrier injection. Finally, we discuss how V-defect density and size affects the results. Given that V-defects play an important role in carrier injection, it is important to understand the role of random alloy fluctuations and of V-defects in carrier injection. The V-defect and random alloy effects may be simulated separately in three-dimensional simulations. 8,15 On the one hand, V-defects range from a few hundred nanometers in size to the µm scale, allowing for large mesh sizes. On the other hand, random alloy fluctuations occur on a scale of a few nanometers, requiring mesh elements to be as small as possible to account for such small-scale fluctuations. Including both effects in a three-dimensional simulation requires unreasonable computer memory (> a few TB) and computing times. However, to understand the influence of random alloy fluctuation

Nanophotonics, 2020

Semiconductor structures used for fundamental or device applications most often incorporate alloy... more Semiconductor structures used for fundamental or device applications most often incorporate alloy materials. In “usual” or “common” III–V alloys, based on the InGaAsP or InGaAlAs material systems, the effects of compositional disorder on the electronic properties can be treated in a perturbative approach. This is not the case in the more recent nitride-based GaInAlN alloys, where the potential changes associated with the various atoms induce strong localization effects, which cannot be described perturbatively. Since the early studies of these materials and devices, disorder effects have indeed been identified to play a major role in their properties. Although many studies have been performed on the structural characterization of materials, on intrinsic electronic localization properties, and on the impact of disorder on device operation, there are still many open questions on all these topics. Taking disorder into account also leads to unmanageable problems in simulations. As a pre...

Physical Review Materials, 2020

The presence of alloy disorder in III-nitride materials has been demonstrated to play a significa... more The presence of alloy disorder in III-nitride materials has been demonstrated to play a significant role in device performance through effects such as carrier localization and carrier transport. Relative to blue light emitting diodes (LEDs), these effects become more severe at green wavelengths. Because of the potential fluctuations that arise due to alloy disorder, full three-dimensional (3D) simulations are necessary to accurately relate materials properties to device performance. We demonstrate experimentally and through simulation that increased quantum well (QW) number in c-plane green LEDs contributes to excess driving voltage, and therefore reduced electrical efficiency. Experimentally, we grew an LED series with the number of QWs varying from one to seven and observed a systematic increase in voltage with the addition of each QW. Trends in LED electrical properties obtained from 3D simulations, which account for the effects of random alloy fluctuations, are in agreement with experimental data. Agreement is achieved without the need for adjusting polarization parameters from their known values. From these results, we propose that the polarization induced barriers at the GaN/InGaN (lower barrier/QW) interfaces and the sequential filling of QWs both contribute significantly to the excess forward voltage in multiple QW c-plane green LEDs.

Japanese Journal of Applied Physics, 2020

Physical Review B, 2020

Carrier transport across polar n-type InGaN/GaN multiple quantum wells (MQWs) has been studied by... more Carrier transport across polar n-type InGaN/GaN multiple quantum wells (MQWs) has been studied by time-resolved photoluminescence (PL) using an optical marker technique. Efficiency of the hole transfer into the marker well experienced a nonmonotonous temperature dependence. First, as the temperature was lowered below room temperature, the number of transferred holes decreased because of the decreased efficiency of the thermionic emission. However, when the temperature was lowered below ∼80 K, the number of transferred holes experienced a significant rise. In addition, the low-temperature hole transport across the MQW structure was very fast, <3 ps. These features indicate that the low-temperature hole transport across the MQWs is ballistic or quasiballistic. Comparison of PL data for structures with different MQW parameters suggests that at low temperatures the hole mean-free path is about 10 nm. Probably, hole transport via light hole and split-off valence bands contributes to this high value.

Extended Abstracts of the 2010 International Conference on Solid State Devices and Materials, 2010

Introduction Recently, Ⅲ-nitride-based optoelectronic devices, such as InGaN/GaN laser diodes hav... more Introduction Recently, Ⅲ-nitride-based optoelectronic devices, such as InGaN/GaN laser diodes have been used for various applications, including BD players, overhead projectors, and laser printers. However, for a c-plane InGaN/GaN quantum well (QW), there exists a strong build-in electric field due to the accumulation of spontaneous and piezoelectric polarization charges at the interface, leading to the quantum-confine Stark effect (QCSE). To avoid the internal field, the growth of non-polar InGaN/GaN quantum wells along (112 0) direction (a-plane), or along (11 00) direction (m-plane) 1 has garnered intensive research interests. As a result of eliminating QCSE, the non-polar light emitting sources exhibit not only high internal quantum efficiency, but also strong polarization 2 , which must be taken into account for device design. The development of threshold-less lasers have been proposed by raising the spontaneous emission rate through a photonic crystal (PC) microcavity with a high quality factor (Q-factor) and a small modal volume (V m) 3,4. However the design of PC microcavities mainly focuses on the value of Q/V m , which was proposed by Purcell 5 first in 1968. In the ideal case, the polarization of cavity modes is assumed to be aligned with the polarization of electric charges, which is not true for strongly y-polarized emission from a-plane InGaN/GaN QWs. In this work, we demonstrate the enhanced spontaneous emission (SpE) from an a-plane InGAN/GaN QW by optimizing Q, V m , and polarization properties of a PC microcavity slab using a finite difference time domain (FDTD) approach. The enhancement factors where the spontaneous emission coupled to the dominant cavity modes are calculated and investigated for different quantum well thicknesses and Indium compositions.

Physical Review B, 2017

Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urb... more Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urbach tail observed in InGaN alloy quantum wells of solar cells and LEDs by biased photocurrent spectroscopy is shown to be characteristic of the ternary alloy disorder. The broadening of the absorption edge observed for quantum wells emitting from violet to green (indium content ranging from 0 to 28%) corresponds to a typical Urbach energy of 20 meV. A 3D absorption model is developed based on a recent theory of disorder-induced localization which provides the effective potential seen by the localized carriers without having to resort to the solution of the Schrödinger equation in a disordered potential. This model incorporating compositional disorder accounts well for the experimental broadening of the Urbach tail of the absorption edge. For energies below the Urbach tail of the InGaN quantum wells, type-II well-to-barrier transitions are observed and modeled. This contribution to the below bandgap absorption is particularly efficient in near-UV emitting quantum wells. When reverse biasing the device, the well-to-barrier below bandgap absorption exhibits a red shift, while the Urbach tail corresponding to the absorption within the quantum wells is blue shifted, due to the partial compensation of the internal piezoelectric fields by the external bias. The good agreement between the measured Urbach tail and its modeling by the new localization theory demonstrates the applicability of the latter to compositional disorder effects in nitride semiconductors.

AIP Advances, 2016

Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum wel... more Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well light-emitting diodes

MRS Proceedings, 2009

ZnO has shown great promise for the application in optoelectronic devices. Since the modulation o... more ZnO has shown great promise for the application in optoelectronic devices. Since the modulation of conductivity is one of the key issues in device performances, we have applied the Monte Carlo method to analyze the mobility of poly-crystalline MgZnO/ZnO heterostructure thin film layer in this paper. The effects of the grain boundary scattering, ionized impurity scattering, as well as phonon scattering are considered. Our study shows that with a design of modulation doping by including the effects of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed to improve the mobility of the ZnO layer by order(s) of magnitude. Simulation results are also confirmed by our experimental works that polarization effects play an important role to attract carriers and to increase the mobility.

Nitride Semiconductor Light-Emitting Diodes (LEDs), 2014

Abstract: A polarization-induced electric field fundamentally limits nitride-based LEDs grown on ... more Abstract: A polarization-induced electric field fundamentally limits nitride-based LEDs grown on the c-plane. Nonpolar and semipolar LEDs have potential for superior performance through high internal quantum efficiency over a wide spectral region and low efficiency droop due to improved carrier transport and high compositional homogeneity. Nonpolar and semipolar LEDs possess unique electroluminescent characteristics such as polarized light emission and reduced wavelength shift due to the lift of degeneracy in the conduction band and mitigated quantum-confined Stark effect. In epitaxial growth, the surface morphology and defect generation mechanism on nonpolar and semipolar planes differ from those on the c-plane due to the anisotropic surface geometry and tilted slip systems. LED chips are designed to enhance the light extraction efficiency of nonpolar and semipolar LEDs grown on free-standing GaN substrates.

IEEE Journal of Quantum Electronics, 2014

The unique properties of nonpolar GaN lightemitting diodes (LEDs) have the advantages of generati... more The unique properties of nonpolar GaN lightemitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.

Journal of Applied Physics, 2014

In this paper, we describe the influence of the intrinsic indium fluctuation in the InGaN quantum... more 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. V

Journal of Applied Physics, 2014

Articles you may be interested in Polarized light extraction in m-plane GaN light-emitting diodes... more Articles you may be interested in Polarized light extraction in m-plane GaN light-emitting diodes by embedded photonic-crystals Appl.

Journal of Applied Physics, 2010

This paper reports the formation of two-dimensional electron gas ͑2DEG͒ in rf-sputtered defective... more This paper reports the formation of two-dimensional electron gas ͑2DEG͒ 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.

Journal of Applied Physics, 2010

Journal of Applied Physics, 2013

We report calculations of the strain dependence of the piezoelectric field within InGaN multi-qua... more We report calculations of the strain dependence of the piezoelectric field within InGaN multi-quantum wells light emitting diodes. Such fields are well known to be a strong limiting factor of the device performance. By taking into account the nonlinear piezoelectric coefficients, which in particular cases predict opposite trends compared to the commonly used linear coefficients, a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. We propose that such reduction of the field can be obtained by including a metamorphic InGaN layer below the multiple quantum well active region. V

Journal of Applied Physics, 2013

This paper presents the findings of investigating core-shell multiple quantum well nanowire light... more This paper presents the findings of investigating core-shell multiple quantum well nanowire lightemitting diodes (LEDs). A fully self-consistent three dimensional model that solves Poisson and driftdiffusion 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 coreshell 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 paper presents a discussion on the influences of the spacing between each nanowire on corresponding nanowire diameters. V

Japanese Journal of Applied Physics, 2013

We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) gro... more We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) grown on the free-standing GaN (202̄1) substrate with a forward voltage as low as 2.8 V at 20 mA. A low p-GaN growth temperature is required to prevent the structure deterioration during the p-GaN growth. The reduction of the forward voltage was observed as the emission wavelength increased in the (202̄1) SQW LEDs, which is attributed to its reversed polarization-related electric field compared to the conventional c-plane LEDs.

IEEE Transactions on Electron Devices, 2012

This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral... more This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral and vertical lightemitting 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.

Processes, 2022

The efficiency of micro-light-emitting diodes (μ-LEDs) depends enormously on the chip size, and t... more The efficiency of micro-light-emitting diodes (μ-LEDs) depends enormously on the chip size, and this is connected to sidewall-trap-assisted nonradiative recombination. It is known that the internal quantum efficiency (IQE) of aluminum gallium indium phosphide (AlGaInP)-based red μ-LEDs is much lower than that of nitride-based μ-LEDs. To establish the major reasons giving rise to this huge IQE discrepancy, we examined the limiting factors in the two structures. For the nitride-based InGaN quantum wells, the influences of random alloy fluctuations were examined. A two-dimensional Poisson and drift-diffusion solver was applied to analyze these issues.

Physical Review Applied, 2022

For nitride-based blue and green light-emitting diodes (LEDs), the forward voltage V for is large... more For nitride-based blue and green light-emitting diodes (LEDs), the forward voltage V for is larger than expected, especially for green LEDs. This is mainly due to the large barriers to vertical carrier transport caused by the total polarization discontinuity at multiple quantum well and quantum barrier interfaces. The natural random alloy fluctuation in QWs has proven to be an important factor reducing V for. However, this does not suffice in the case of green LEDs because of their larger polarization-induced barrier. V-defects have been proposed as another key factor in reducing V for to allow laterally injection into multiple quantum wells (MQWs), thus bypassing the multiple energy barriers incurred by vertical transport. In this paper, to model carrier transport in the whole LED, we consider both random-alloy and V-defect effects. A fully two-dimensional drift-diffusion charge-control solver is used to model both effects. The results indicate that the turn-on voltages for blue and green LEDs are both affected by random alloy fluctuations and V-defect density. For green LEDs, V for decreases more due to V-defects, where the smaller polarization barrier at the V-defect sidewall is the major path for lateral carrier injection. Finally, we discuss how V-defect density and size affects the results. Given that V-defects play an important role in carrier injection, it is important to understand the role of random alloy fluctuations and of V-defects in carrier injection. The V-defect and random alloy effects may be simulated separately in three-dimensional simulations. 8,15 On the one hand, V-defects range from a few hundred nanometers in size to the µm scale, allowing for large mesh sizes. On the other hand, random alloy fluctuations occur on a scale of a few nanometers, requiring mesh elements to be as small as possible to account for such small-scale fluctuations. Including both effects in a three-dimensional simulation requires unreasonable computer memory (> a few TB) and computing times. However, to understand the influence of random alloy fluctuation

Nanophotonics, 2020

Semiconductor structures used for fundamental or device applications most often incorporate alloy... more Semiconductor structures used for fundamental or device applications most often incorporate alloy materials. In “usual” or “common” III–V alloys, based on the InGaAsP or InGaAlAs material systems, the effects of compositional disorder on the electronic properties can be treated in a perturbative approach. This is not the case in the more recent nitride-based GaInAlN alloys, where the potential changes associated with the various atoms induce strong localization effects, which cannot be described perturbatively. Since the early studies of these materials and devices, disorder effects have indeed been identified to play a major role in their properties. Although many studies have been performed on the structural characterization of materials, on intrinsic electronic localization properties, and on the impact of disorder on device operation, there are still many open questions on all these topics. Taking disorder into account also leads to unmanageable problems in simulations. As a pre...

Physical Review Materials, 2020

The presence of alloy disorder in III-nitride materials has been demonstrated to play a significa... more The presence of alloy disorder in III-nitride materials has been demonstrated to play a significant role in device performance through effects such as carrier localization and carrier transport. Relative to blue light emitting diodes (LEDs), these effects become more severe at green wavelengths. Because of the potential fluctuations that arise due to alloy disorder, full three-dimensional (3D) simulations are necessary to accurately relate materials properties to device performance. We demonstrate experimentally and through simulation that increased quantum well (QW) number in c-plane green LEDs contributes to excess driving voltage, and therefore reduced electrical efficiency. Experimentally, we grew an LED series with the number of QWs varying from one to seven and observed a systematic increase in voltage with the addition of each QW. Trends in LED electrical properties obtained from 3D simulations, which account for the effects of random alloy fluctuations, are in agreement with experimental data. Agreement is achieved without the need for adjusting polarization parameters from their known values. From these results, we propose that the polarization induced barriers at the GaN/InGaN (lower barrier/QW) interfaces and the sequential filling of QWs both contribute significantly to the excess forward voltage in multiple QW c-plane green LEDs.

Japanese Journal of Applied Physics, 2020

Physical Review B, 2020

Carrier transport across polar n-type InGaN/GaN multiple quantum wells (MQWs) has been studied by... more Carrier transport across polar n-type InGaN/GaN multiple quantum wells (MQWs) has been studied by time-resolved photoluminescence (PL) using an optical marker technique. Efficiency of the hole transfer into the marker well experienced a nonmonotonous temperature dependence. First, as the temperature was lowered below room temperature, the number of transferred holes decreased because of the decreased efficiency of the thermionic emission. However, when the temperature was lowered below ∼80 K, the number of transferred holes experienced a significant rise. In addition, the low-temperature hole transport across the MQW structure was very fast, <3 ps. These features indicate that the low-temperature hole transport across the MQWs is ballistic or quasiballistic. Comparison of PL data for structures with different MQW parameters suggests that at low temperatures the hole mean-free path is about 10 nm. Probably, hole transport via light hole and split-off valence bands contributes to this high value.

Extended Abstracts of the 2010 International Conference on Solid State Devices and Materials, 2010

Introduction Recently, Ⅲ-nitride-based optoelectronic devices, such as InGaN/GaN laser diodes hav... more Introduction Recently, Ⅲ-nitride-based optoelectronic devices, such as InGaN/GaN laser diodes have been used for various applications, including BD players, overhead projectors, and laser printers. However, for a c-plane InGaN/GaN quantum well (QW), there exists a strong build-in electric field due to the accumulation of spontaneous and piezoelectric polarization charges at the interface, leading to the quantum-confine Stark effect (QCSE). To avoid the internal field, the growth of non-polar InGaN/GaN quantum wells along (112 0) direction (a-plane), or along (11 00) direction (m-plane) 1 has garnered intensive research interests. As a result of eliminating QCSE, the non-polar light emitting sources exhibit not only high internal quantum efficiency, but also strong polarization 2 , which must be taken into account for device design. The development of threshold-less lasers have been proposed by raising the spontaneous emission rate through a photonic crystal (PC) microcavity with a high quality factor (Q-factor) and a small modal volume (V m) 3,4. However the design of PC microcavities mainly focuses on the value of Q/V m , which was proposed by Purcell 5 first in 1968. In the ideal case, the polarization of cavity modes is assumed to be aligned with the polarization of electric charges, which is not true for strongly y-polarized emission from a-plane InGaN/GaN QWs. In this work, we demonstrate the enhanced spontaneous emission (SpE) from an a-plane InGAN/GaN QW by optimizing Q, V m , and polarization properties of a PC microcavity slab using a finite difference time domain (FDTD) approach. The enhancement factors where the spontaneous emission coupled to the dominant cavity modes are calculated and investigated for different quantum well thicknesses and Indium compositions.

Physical Review B, 2017

Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urb... more Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urbach tail observed in InGaN alloy quantum wells of solar cells and LEDs by biased photocurrent spectroscopy is shown to be characteristic of the ternary alloy disorder. The broadening of the absorption edge observed for quantum wells emitting from violet to green (indium content ranging from 0 to 28%) corresponds to a typical Urbach energy of 20 meV. A 3D absorption model is developed based on a recent theory of disorder-induced localization which provides the effective potential seen by the localized carriers without having to resort to the solution of the Schrödinger equation in a disordered potential. This model incorporating compositional disorder accounts well for the experimental broadening of the Urbach tail of the absorption edge. For energies below the Urbach tail of the InGaN quantum wells, type-II well-to-barrier transitions are observed and modeled. This contribution to the below bandgap absorption is particularly efficient in near-UV emitting quantum wells. When reverse biasing the device, the well-to-barrier below bandgap absorption exhibits a red shift, while the Urbach tail corresponding to the absorption within the quantum wells is blue shifted, due to the partial compensation of the internal piezoelectric fields by the external bias. The good agreement between the measured Urbach tail and its modeling by the new localization theory demonstrates the applicability of the latter to compositional disorder effects in nitride semiconductors.

AIP Advances, 2016

Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum wel... more Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well light-emitting diodes

MRS Proceedings, 2009

ZnO has shown great promise for the application in optoelectronic devices. Since the modulation o... more ZnO has shown great promise for the application in optoelectronic devices. Since the modulation of conductivity is one of the key issues in device performances, we have applied the Monte Carlo method to analyze the mobility of poly-crystalline MgZnO/ZnO heterostructure thin film layer in this paper. The effects of the grain boundary scattering, ionized impurity scattering, as well as phonon scattering are considered. Our study shows that with a design of modulation doping by including the effects of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed to improve the mobility of the ZnO layer by order(s) of magnitude. Simulation results are also confirmed by our experimental works that polarization effects play an important role to attract carriers and to increase the mobility.

Nitride Semiconductor Light-Emitting Diodes (LEDs), 2014

Abstract: A polarization-induced electric field fundamentally limits nitride-based LEDs grown on ... more Abstract: A polarization-induced electric field fundamentally limits nitride-based LEDs grown on the c-plane. Nonpolar and semipolar LEDs have potential for superior performance through high internal quantum efficiency over a wide spectral region and low efficiency droop due to improved carrier transport and high compositional homogeneity. Nonpolar and semipolar LEDs possess unique electroluminescent characteristics such as polarized light emission and reduced wavelength shift due to the lift of degeneracy in the conduction band and mitigated quantum-confined Stark effect. In epitaxial growth, the surface morphology and defect generation mechanism on nonpolar and semipolar planes differ from those on the c-plane due to the anisotropic surface geometry and tilted slip systems. LED chips are designed to enhance the light extraction efficiency of nonpolar and semipolar LEDs grown on free-standing GaN substrates.

IEEE Journal of Quantum Electronics, 2014

The unique properties of nonpolar GaN lightemitting diodes (LEDs) have the advantages of generati... more The unique properties of nonpolar GaN lightemitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.

Journal of Applied Physics, 2014

In this paper, we describe the influence of the intrinsic indium fluctuation in the InGaN quantum... more 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. V

Journal of Applied Physics, 2014

Articles you may be interested in Polarized light extraction in m-plane GaN light-emitting diodes... more Articles you may be interested in Polarized light extraction in m-plane GaN light-emitting diodes by embedded photonic-crystals Appl.

Journal of Applied Physics, 2010

This paper reports the formation of two-dimensional electron gas ͑2DEG͒ in rf-sputtered defective... more This paper reports the formation of two-dimensional electron gas ͑2DEG͒ 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.

Journal of Applied Physics, 2010

Journal of Applied Physics, 2013

We report calculations of the strain dependence of the piezoelectric field within InGaN multi-qua... more We report calculations of the strain dependence of the piezoelectric field within InGaN multi-quantum wells light emitting diodes. Such fields are well known to be a strong limiting factor of the device performance. By taking into account the nonlinear piezoelectric coefficients, which in particular cases predict opposite trends compared to the commonly used linear coefficients, a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. We propose that such reduction of the field can be obtained by including a metamorphic InGaN layer below the multiple quantum well active region. V

Journal of Applied Physics, 2013

This paper presents the findings of investigating core-shell multiple quantum well nanowire light... more This paper presents the findings of investigating core-shell multiple quantum well nanowire lightemitting diodes (LEDs). A fully self-consistent three dimensional model that solves Poisson and driftdiffusion 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 coreshell 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 paper presents a discussion on the influences of the spacing between each nanowire on corresponding nanowire diameters. V

Japanese Journal of Applied Physics, 2013

We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) gro... more We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) grown on the free-standing GaN (202̄1) substrate with a forward voltage as low as 2.8 V at 20 mA. A low p-GaN growth temperature is required to prevent the structure deterioration during the p-GaN growth. The reduction of the forward voltage was observed as the emission wavelength increased in the (202̄1) SQW LEDs, which is attributed to its reversed polarization-related electric field compared to the conventional c-plane LEDs.

IEEE Transactions on Electron Devices, 2012

This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral... more This study analyzes the current spreading effect and light extraction efficiency (LEE) of lateral and vertical lightemitting 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.