GaInN Light-emitting Diodes Research Articles

Comparative Study on Temperature‐Dependent Internal Quantum Efficiency and Light–Extraction Efficiency in III‐Nitride–, III‐Phosphide–, and III‐Arsenide–based Light‐Emitting Diodes

This study attempts to understand and elucidate the factors limiting/determining the external quantum efficiency (EQE) of light‐emitting diodes (LEDs) depending on material systems, i.e., III‐arsenide (GaAs), III‐phosphide (AlGaInP), and III‐nitride (GaInN), via the temperature measurements (30–500 K). The behaviors of EQEs are investigated carefully in terms of the thermal droop and efficiency droop, revealing that the thermal droop in the AlGaInP and GaAs LEDs, while the efficiency droop in the GaInN LEDs, is a critical factor limiting the EQE. To deepen the insight, the EQE is separated into internal quantum efficiency (IQE) and light‐extraction efficiency (LEE). Further, the IQE is separated into radiative efficiency (RE) and injection efficiency (IE). The analysis shows that the LEE plays a significant role in the thermal droop for the AlGaInP and GaAs LEDs. Meanwhile, the IE and RE play a significant role in the EQE reduction of the blue and red LEDs at high temperatures and high current injection.

Analysis of GaInN light-emitting diodes with rods and lenses fabricated by direct optical wiring technology

Analysis of GaInN light-emitting diodes with rods and lenses fabricated by direct optical wiring technology

CMOS-Based Optoelectronic On-Chip Neural Interface Device

On-chip neural interface devices based on CMOS image sensor technology are proposed and demonstrated. The devices were designed with target applications to optogenetics in bioscience. Multifunctional CMOS image sensors equipped with an addressable on-chip electrode array were integrated with a functional interface chip that contained embedded GaInN light emitting diodes (LEDs) and electrodes to create a neural interface. Detailed design information regarding the CMOS sensor chip and the functional interface chip including the packaging structure and fabrication processes are presented in this paper. The on-chip optical stimulation functionality was demonstrated in an in vitro experiment using neuron-like cells cultured on the proposed device. key words: CMOS image sensor, optogenetics, on-chip bioimaging, optical stimulation, GaInN LED, implantable electronics

Effect of a p-type ZnO insertion layer on the external quantum efficiency of GaInN light-emitting diodes

The external quantum efficiency (EQE) of a GaInN green light-emitting diode (LED) is improved by inserting a p-type ZnO layer between the indium tin oxide electrode and the p-type GaN layer. Several hypotheses are discussed to explain the EQE improvement in the LED with the ZnO layer. It is concluded that higher hole injection efficiency and better electron confinement explain the EQE improvement, which is supported by the results of device simulations showing that the EQE is sensitive to the polarization sheet charge density at the interface between the last quantum barrier and electron-blocking layer.

Polarization-Engineered High-Efficiency GaInN Light-Emitting Diodes Optimized by Genetic Algorithm

A genetic algorithm is employed to find an optimum epitaxial structure of multiple quantum wells (MQWs) and electron-blocking layer (EBL) for a GaInN-based light-emitting diode (LED). The optimized LED is composed of locally Si-doped quantum barriers (QBs) in the MQWs and a quaternary heterostructured AlGaInN EBL having a polarization-induced electric field directed oppositely to that of a conventional AlGaN EBL. The optimized LED shows 15.6% higher internal quantum efficiency, 24.6% smaller efficiency droop, and 0.21 V lower forward voltage at 200 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> comparing to the reference LED, which has fully Si-doped QB and 20-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> N EBL. We find that local Si doping near the QB/QW interface compensates the negative polarization-induced sheet charge at the interface and reduces electric field in the QWs, thereby enhancing electron-hole wave function overlap. In addition, the inverted polarization field in the quaternary EBL provides a high barrier for electrons but a low barrier for holes, resulting in enhanced electron-blocking and hole-injection characteristics.

Influences of p-type layer structure and doping profile on the temperature dependence of the foward voltage characteristic of GaInN light-emitting diode

Many GaInN light-emitting diodes (LEDs) are subjected to a great temperature variation during their serving. In these applications, it is advantageous that GaInN LEDs have a weak temperature dependence of forward voltage. However, the factors determining the exact temperature dependence of the forward voltage characteristics are not fully understood. In this paper, two series of GaInN LEDs are prepared for investigating the correlation between the epitaxial structural and the temperature dependence of the forward voltage characteristics. The forward voltage characteristics of samples are studied in a temperature range from 100 K to 350 K. The curves of forward voltage versus temperature (dV/dT) are compared and analyzed. For the three samples in series I, according to the barrier thickness and emitting wavelength, they are designated as blue multiquantum well (MQW) with thin barrier (sample A), blue MQW with thick barrier (sample B), and green barrier with thick barrier (sample C) respectively. Their structures of active region including the insertion layer between n-GaN and MQW, the MQW, and the emitting wavelength are different from each other. However, the same slopes of dV/dT at room temperature (300 K± 50 K) are observed in the samples. Moreover, samples B and C with the same p-type layer design also have the same slopes of dV/dT at cryogenic temperatures. Sample A with a much thinner p-type layer shows a lower slope than samples B and C. Based on the these experimental data, it is deduced that the intrinsic physic properties of active region such as structure and emission wavelength have a little influence on the variation of the slope of dV/dT either at room temperature or at cryogenic temperatures. Moreover, the Mg concentration of the p-GaN main region determines the slope of dV/dT at cryogenic temperatures. Low doping concentration leads to a high slope of dV/dT.#br#In order to find the decisive factor determining the slope of dV/dT at room temperature, three samples in series II are grown. For sample E, at the MQW-EBL (electron blocking layer) interface, the Mg concentration increases very slowly while an abruptly varying doping profile is observed for samples D and F. The slopes of samples D and F are both -1.3 mV·K-1. This is very close to the calculation value of the lower bond for the change in forward voltage (-1.2 mV·K-1). Meanwhile, the slope of sample E is -2.5 mV·K-1, which is much higher than those of samples D and F. Thus, it is suggested that the major factor influencing the slope of dV/dT at room temperature is the Mg doping profile of the initial growth stage of the p-AlGaN electron blocking layer. These phenomena are mainly attributed to the changes of the activation energy of p-AlGaN and p-GaN, since it relies on the doping concentration and temperature. Our findings clarify the roles of active region, p-AlGaN and p-GaN in the temperature dependence of the forward voltage characteristic. More importantly, the results obtained in this study are helpful for optimizing the growth parameters to achieve LED devices with forward voltage that has a low sensitivity to temperature.

Enhanced Performance of GaInN LEDs by Abrupt Mg Doped p-AlGaN Electron Blocking Layer

Performance improvements in GaInN blue light-emitting-diodes (LEDs) including decreased wavelength shift, enhanced quantum efficiency, and reduced operating voltage are obtained by an abruptly and heavily Mg-doped p-AlGaN electron blocking layer. Moreover, the commonly observed efficiency collapse at cryogenic temperatures for conventional LEDs is not found in abrupt doped LEDs. Under high level injection conditions, the conventional LEDs have much higher electric field in the p-type neutral region than the abruptly doped LEDs at cryogenic temperatures. The higher electric field leads to the greater electron leakage, as well as the cause of efficiency collapse.

GaInN-based light emitting diodes embedded with wire grid polarizers

The use of liquid crystal displays (LCDs) has become prevalent in our modern, technology driven society. We demonstrate a linearly polarized GaInN light-emitting diode (LED) embedded with a wire-grid polarizer (WGP). A derivation of rigorous coupled-wave analysis is given; starting from Maxwell’s equations and finishing by matching the boundary conditions in the grating and other regions of interest. Simulated results are shown for various grating parameters, including different metals used for the grating and the metal-line dimensions. An LED fabrication process is developed for demonstrating WGP-LEDs. A clear polarization preference for the light coming out of the WGP-LED is experimentally demonstrated with a polarization ratio over 0.90, which is in good agreement with simulation results.

Strong correlation between capacitance and breakdown voltage of GaInN/GaN light-emitting diodes

Investigating the relationship between the breakdown voltage and the capacitance of GaInN light-emitting diodes (LEDs), we find that a lower capacitance due to weaker internal electric field in depletion region or wider depletion width at the pn junction results in lower reverse leakage current and thus larger breakdown voltage. The measured breakdown voltage and capacitance of LEDs show a strong correlation, opening a nondestructive and non-intrusive way to estimate the breakdown voltage of an LED based on the capacitance-voltage measurement.

Mesa-Free III-V Nitride Light-Emitting Diodes with Flat Surface

A mesa-free pn-junction GaInN light-emitting diode (LED) structure is proposed and demonstrated. Some of the p-type top surface area of the LED epitaxial wafer is converted from p-type to n-type by means of Si-ion implantation. The resulting pn-junction LED wafer has cathode and anode contact points at the same surface height so that no dry-etching process is needed to access the n-type region. The measured specific contact resistance on the type-converted n-type layer and the light-output power of the mesa-free pn-junction LED are very comparable to those obtained by a reference LED.

GaInN light-emitting diodes using separate epitaxial growth for the p-type region to attain polarization-inverted electron-blocking layer, reduced electron leakage, and improved hole injection

A GaInN light-emitting diode (LED) structure is analyzed that employs a separate epitaxial growth for the p-type region, i.e., the AlGaN electron-blocking layer (EBL) and p-type GaN cladding layer, followed by wafer or chip bonding. Such LED structure has a polarization-inverted EBL and allows for uncompromised epitaxial-growth optimization of the p-type region, i.e., without the need to consider degradation of the quantum-well active region during p-type region growth. Simulations show that such an LED structure reduces electron leakage, reduces the efficiency droop, improves hole injection, and has the potential to extend high efficiencies into the green spectral region.

Analysis of the temperature dependence of the forward voltage characteristics of GaInN light-emitting diodes

The forward voltage characteristics of GaInN light-emitting diodes are studied in the temperature range of 80 K to 450 K. The forward-voltage-vs.-temperature curve has a “two-slope” characteristic with a slope of dVf/dT = −1.7 mV/K and −8.0 mV/K at room temperature and cryogenic temperatures, respectively. To investigate the two-slope characteristic, we perform transmission-line-model measurements on p-type GaN and show that both p-type contact and sheet resistance decrease drastically with increasing temperature. We conclude that dVf/dT in the high-slope region is limited by p-type sheet and contact resistance, and in the low-slope region by the GaN pn junction properties.

Identifying the cause of the efficiency droop in GaInN light-emitting diodes by correlating the onset of high injection with the onset of the efficiency droop

An unequivocal correlation between the onset of high injection and the onset of the efficiency droop is demonstrated in GaInN light-emitting diodes over a wide range of temperatures. The diode voltage at the onset of high injection and the voltage at the onset of the efficiency droop are correlated by the equation VHigh-injection onset + ΔV ≈ VDroop onset. The excess voltage, ΔV, determined to be 0.3 V, drops partially over the p-type neutral region. The resulting electric field sweeps electrons out of the active region and results in substantial electron leakage despite high barriers that confine the carriers to the active region.

Efficiency droop in light‐emitting diodes: Challenges and countermeasures

AbstractEfficiency droop, i.e. the loss of efficiency at high operating current, afflicts nitride‐based light‐emitting diodes (LEDs). The droop phenomenon is currently the subject of intense research, as it retards the advancement of solid‐state lighting which is just starting to supplant fluorescent as well as incandescent lighting. Although the technical community does not yet have consented to a single cause of droop, this article provides a summary of the present state of droop research, reviews currently discussed droop mechanisms, and presents a recently developed theoretical model for the efficiency droop. In the theoretical model, carrier leakage out of the active region caused by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, is discussed in detail. The model is in agreement with the droop's key behaviors not only for GaInN LEDs but also for AlGaInP LEDs.

Optically functional surface structures for GaN-based light-emitting diodes

The unrestricted control of the surface structure and refractive index would allow for new and improved functionalities in optoelectronic devices. Specifically, micro-patterned graded-refractive-index (GRIN) coatings can enable control of emission pattern and promote light extraction in GaInN light-emitting diodes (LEDs). We design and demonstrate coatings that are patterned into arrays of GRIN micro-pillars, each composed of five dielectric layers of (TiO2)x(SiO2)1−x with the bottom layer (adjacent to semiconductor) having the highest refractive index and the top layer (adjacent to air) having the lowest one. The GRIN micro-pillars, including their planar geometric shape and size, are structured for emission pattern control and maximum light-extraction efficiency. It is shown that the peak emission intensity of the GRIN LEDs is controllable from ±20° to ±50° off the surface-normal. In addition, LEDs patterned with an array of four-pointed-star-shaped GRIN micro-pillars with a pillar-size of 4 μm and spacing between neighboring pillars of 4 μm show a 155% enhancement in light-output power over an uncoated planar reference LED.

Strong light-extraction enhancement in GaInN light-emitting diodes patterned with TiO2 micro-pillars with tapered sidewalls

An effective method to enhance the light extraction for GaInN light-emitting diodes (LEDs) is reported. The method employs TiO2 micro-pillars with tapered sidewalls, which are refractive-index-matched to the underlying GaN. The tapered micro-pillars are fabricated by using reflowed photoresist as mask during CHF3-based dry etch, with O2 added in order to precisely control the taper angle. LEDs patterned with TiO2 micro-pillars with tapered sidewalls show a 100% enhancement in light-output power over planar reference LEDs. The measured results are in good agreement with ray-tracing simulations, showing strong potential of optical surfaces that are controlled in terms of refractive index and lateral structure.

Polarized light emission from GaInN light-emitting diodes embedded with subwavelength aluminum wire-grid polarizers

We demonstrate a back-emitting (sapphire-substrate emitting) linearly polarized GaInN light-emitting diode (LED) embedded with a subwavelength-sized aluminum wire-grid polarizer (WGP). Rigorous coupled wave analysis is implemented to study the polarization characteristics of such a WGP LED. The aluminum nanowire grating with a period of 150 nm is located on the sapphire backside of a GaInN LED structure and is fabricated by electron-beam lithography and inductively coupled plasma reactive-ion etching. A polarization ratio of 0.96 is demonstrated for a WGP GaInN LED in good agreement with simulation results.

Emission pattern control and polarized light emission through patterned graded-refractive-index coatings on GaInN light-emitting diodes

Patterned graded-refractive-index (GRIN) coatings that offer the controllability of far-field emission pattern and polarization properties of GaInN light-emitting diodes (LEDs) are investigated. Compared with a planar reference LED, the light-output power of an LED with patterned GRIN coatings (GRIN LED) is enhanced by about 69%. Furthermore, the GRIN LED has bidirectional emission peaks at about 45° off-surface-normal and polarized light emission with the maximum polarization ratio occurring at the same angle, i.e. the intensity maximum and the polarization-ratio maximum coincide. The large off-surface-normal emission of the GRIN LED results from the strong light extraction through the sidewalls of the GRIN pillars, which is in good agreement with results predicted from ray-tracing simulations.

Efficiency droop in AlGaInP and GaInN light-emitting diodes

At room temperature, AlGaInP pn-junction light-emitting diodes (LEDs) emitting at 630 nm do not exhibit an efficiency droop. However, upon cooling the AlGaInP LEDs to cryogenic temperatures, they show a pronounced efficiency droop. We attribute the efficiency droop in AlGaInP LEDs to electron-drift-induced reduction in injection efficiency (i.e., carrier leakage out of the active region) mediated by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, with the concentration disparity exacerbated at low temperatures.

Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities

The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes (LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.