Nanopillar Light-emitting Diodes Research Articles

Characterization of GaN‐Based Nanopillar Light‐Emitting Diodes on Multicrystalline Si Substrates: Insights into Emitting‐Color Distribution Characteristics

In a previous study, GaN‐based nanopillars are grown vertically on a multicrystalline Si substrate by inserting (In)GaN steering crystals. In addition, blue‐green and white light‐emitting diodes (LEDs) are prepared on this substrate for the first time using a double heterotype p–n junction. Herein, the emission‐color distribution characteristics of this type of LED are analyzed in depth, and a related luminescence principle is proposed. Each nanopillar plus an electrode is considered a nanopillar LED, and the emission color of each nanopillar LED is inferred. The indium distribution in the InGaN active region is predicted based on the luminous color and corresponding spectra. Simultaneously, the morphology of related materials, electrical properties of LEDs, and chromaticity coordinates of luminous colors are analyzed and discussed.

Optical-Emission Properties of GaN-Based Nanopillar Light-Emitting Diodes Prepared on Non-Polished and Non-Single-Crystalline Substrates

GaN-based nanopillar crystals are directly grown on multicrystalline Si and amorphous-carbon-coated graphite substrates whose surfaces are not mirror-polished. Light-emitting diodes (LEDs) of a double-hetero structure are prepared from the nanopillar crystals, and their optical–emission properties are investigated. Despite the substrate type and surface conditions, moderate light emissions are obtained from nanopillar LEDs though the light emissions are not always homogeneous, especially in the LEDs prepared on the graphite-based substrate. Nevertheless, these results will lead to realizations of novel large-area light-emitting devices.

Ultrathin Tactile Sensors with Directional Sensitivity and a High Spatial Resolution.

An ultrathin tactile sensor with directional sensitivity and capable of mapping at a high spatial resolution is proposed and demonstrated. Each sensor node consists of two gallium nitride (GaN) nanopillar light-emitting diodes. Shear stress applied on the nanopillars causes the electrons and holes to separate in the radial direction and reduces the light intensity emitted from the nanopillars. A sensor array comprising 64 sensor nodes was designed and fabricated. Two-dimensional directional sensitivity was experimentally confirmed with a dynamic range of 1-30 mN and an accuracy of ±1.3 mN. Tracking and mapping of an external force moving across the sensor array were also demonstrated. Finally, the proposed tactile sensor's sensitivity was tested with a fingertip gently moving across the sensor array. The sensor successfully registered the finger movement's direction and fingerprint pattern.

Nonpolar GaN-based nanopillar green light-emitting diode (LED) fabricated by using self-aligned In3Sn nanodots

We fabricate nonpolar InGaN/GaN single quantum-well based a-plane nanopillar green light-emitting diode (LED). The top-down fabrication method was used for fabricating nanopillar LED from the planar LED, where self-aligned In3Sn nanodots were used as an etching mask. Fabricated nanopillars have high yield with large height-to-diameter aspect ratio found by scanning-electron-microscopy characteristics. The size of nanopillars depends on the size of In3Sn nanodots which are fabricated from the ITO (indium tin oxide) thin film by 3% HCl solution. It is systematically investigated that the size of nanodots depends on the ITO thickness rather than the etching time. In order to achieve uniform current injection into nanopillars, a p-type transparent ITO contact is selectively deposited on the top of each nanopillar. The ITO contact layer consists of a slanted oblique-angle (85○/−85○) layer followed by a thick blanket layer for uniform current distribution. The macroscopic optoelectronic characteristics show the uniform current distribution over the chip area, which is also interactively explained.

Enhanced localisation effect and reduced quantum-confined Stark effect of carriers in InGaN/GaN multiple quantum wells embedded in nanopillars

A conventional planar InGaN/GaN multiple quantum well (MQW)-based light emitting diode (LED) producing a blue-green dual-wavelength spectrum, due to phase separation in the InGaN well layers, is grown, and fabricated into a nanopillar (NP) LED array structure with an etching depth penetrating through the MQW region using a top-down dry etching process. Excitation power and temperature dependences of the photoluminescence (PL) spectra of the planar and NP samples have shown that the fabrication of the nanotexture not only decreases the quantum-confined Stark effect (QCSE), but also enhances the carrier localisation effect, both in the InGaN matrix region and in the In-rich quasi-quantum dot (QD) region, due to the reduction of the strain relaxation-induced piezoelectric field in the MQWs embedded in the nanopillars. Moreover, the fabrication of the nanotexture also improves the light extraction efficiency (LEE), due to the increased light-extracting surface area and light-guiding effect. These results are also consistent with the measured quantum efficiencies of the two samples.

Comparative study on luminescence extraction strategies of LED by large-scale fabrication of nanopillar and nanohole structures

Light extraction and current injection are two important considerations in the development of high efficiency light-emitting-diodes (LEDs), but usually cannot be satisfied simultaneously in nanostructure patterned devices. In this work, we investigated near-UV LEDs with nanopillar and nanohole patterns to improve light extraction efficiency. Photoluminescence (PL) intensities were enhanced by 8.0 and 4.1 times for nanopillar and nanohole LEDs compared to that of planar LED. Nanopillar LED exhibits higher PL emission than that of the nanohole LED, attributing to a convex shape sidewall for more effective outward light scattering, and reduction of quantum-confined-stark-effect owing to strain relaxation. However, nanopillar LED exhibits lower electroluminescence intensity than the nanohole sample, which calls for further optimization in carrier distributions. Experimental results were further supported by near-field electric field simulations. This work demonstrates the difference in optical and electrical behaviors between the nanopillar and nanohole LEDs, paving the way for detailed understanding on luminescence extraction mechanisms of nanostructure patterned UV emitters.

Monolithic integration of individually addressable light-emitting diode color pixels

Monolithic integration of individually addressable light-emitting diode (LED) color pixels is reported. The integration is enabled by local strain engineering. The use of a nanostructured active region comprising one or more nanopillars allows color tuning across the visible spectrum. In the current work, integration of amber, green, and blue pixels is demonstrated. The nanopillar LEDs exhibit an electrical performance comparable to that of a conventional thin-film LED fabricated on the same wafer. The proposed platform uses only standard epitaxy and a similar process flow as a conventional LED. It is also shown that the emission intensity can be linearly tuned without shifting the color coordinate of individual pixels.

Diameter-dependent photoluminescence properties of strong phase-separated dual-wavelength InGaN/GaN nanopillar LEDs

In this paper, strong phase-separated blue/green dual-wavelength InGaN/GaN nanopillar (NP) light emitting diodes (LEDs) with the same NP density and various NP diameters were fabricated using focused ion beam etching. Micro-Raman spectroscopy was used to show the effect of NP diameter on the strain relaxation in the multi-quantum-wells (MQWs). The effect of NP diameter on optical behaviors of the strong phase-separated dual-wavelength InGaN/GaN NP LEDs was investigated for the first time by using micro-photoluminescence (PL) spectroscopy. The blue shifts of PL peak energies of the NP LEDs showed that the NP LED with a smaller diameter exhibited a larger strain relaxation in the MQWs, as confirmed by micro-Raman results. And the blue shift of green emission was smaller than that of blue emission. The total integrated PL intensities from the NP arrays were enhanced compared to the as-grown sample due to the increased recombination rate and light extraction efficiency. The enhancement factor decreased with decreasing the NP diameter in our experiments, which indicated that the loss of active volume was gradually dominant for the luminous efficiency of NP LEDs compared to the increased recombination rate and light extraction efficiency.

Ultracompact Position-Controlled InP Nanopillar LEDs on Silicon with Bright Electroluminescence at Telecommunication Wavelengths

Highly compact III–V compound semiconductor active nanophotonic devices integrated with silicon are important for future low power optical interconnects. One approach toward realizing heterogeneous integration and miniaturization of photonic devices is through nanowires/nanopillars grown directly on silicon substrates. However, to realize their full potential, the integration of nanowires/nanopillars with silicon-based electronics must be made scalable via precise control of nanopillar site and dimensions. Here we demonstrate the first electrical-pumped InGaAs/InP multiquantum-well (MQW) light emitting diodes (LED) using nanopillar array directly grown on a Si substrate with site control, with current conduction directly through the silicon. The growth is via catalyst-free, low-temperature metal organic chemical vapor deposition, which is CMOS compatible. We report excellent optical properties including long minority carrier lifetimes and room-temperature lasing under optical pumping. InGaAs/InP quantum w...

Fabrication and photoluminescence of strong phase-separated InGaN based nanopillar LEDs

A strong phase-separated InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) producing a blue-green dual-wavelength spectrum has been grown, and fabricated into two nanopillar array structures with different etching depths: one fabricated only on the p-GaN layer and the other penetrating through the active layers. Micro-photoluminescence (μ-PL) spectra have shown that compared with the as-grown (planar) LED, light extraction efficiency (LEE) is improved for both the shorter and the longer nanopillar array structures due to the increased light-extracting surface area and light-guiding effect, in contrast, internal quantum efficiency (IQE) is improved for only the longer one due to the relaxation of strain in the MQWs embedded in the nanopillars. Furthermore, the strain relaxation-induced peak blue-shift value is smaller for the green emission from In-rich quantum dots (QDs) than the blue emission from the InGaN matrix, and decreases for both the emissions with increasing temperature. The former is attributed to the quantum-confined Stark effect (QCSE) in the strong localized QDs of small size being smaller than in the InGaN matrix, the latter is due to the decrease of the thermal-mismatch strain in the MQWs with increasing temperature.

Enhanced optical properties of nanopillar light-emitting diodes by coupling localized surface plasmon of Ag/SiO2 nanoparticles

We fabricated nanopillar light-emitting diodes (LEDs) embedded with Ag/SiO2 nanoparticles, and investigated the energy coupling processes between the localized surface plasmons of nanoparticles and the active quantum well regions of nanopillar LEDs. These nanoparticle-embedded nanopillar LEDs showed considerable increases in photoluminescence and electroluminescence intensities, compared with reference nanopillar LEDs. The observed optical enhancement was explained by the increased spontaneous emission rate caused by energy coupling from excitons in the InGaN/GaN quantum well active region of the LEDs to the localized surface plasmon modes of the Ag/SiO2 nanoparticles. A strongly enhanced characteristic photoluminescence decay also confirmed such an explanation.

Fabrication and improvement of nanopillar InGaN / GaN light-emitting diodes using nanosphere lithography

Surface-patterning technologies have enabled the improvement of currently existing light-emitting diodes (LEDs) and can be used to overcome the issue of low quantum efficiency of green GaN-based LEDs. We have applied nanosphere lithography to fabricate nanopillars on InGaN/GaN quantum-well LEDs. By etching through the active region, it is possible to improve both the light extraction efficiency and, in addition, the internal quantum efficiency through the effects of lattice strain relaxation. Nanopillars of different sizes are fabricated and analyzed using Raman spectroscopy. We have shown that nanopillar LEDs can be significantly improved by applying a combination of ion-damage curing techniques, including thermal and acidic treatment, and have analyzed their effects using x-ray photoelectron spectroscopy.

Axial Diffusion Barriers in Near-Infrared Nanopillar LEDs

The growth of GaAs/GaAsP axial heterostructures is demonstrated and implemented as diffusion current barriers in nanopillar light-emitting diodes at near-infrared wavelengths. The nanopillar light-emitting diodes utilize an n-GaAs/i-InGaAs/p-GaAs axial heterostructure for current injection. Axial GaAsP segments are inserted into the n- and p-GaAs portions of the nanopillars surrounding the InGaAs emitter region, acting as diffusion barriers to provide enhanced carrier confinement. Detailed characterization of growth of the GaAsP inserts and electronic band-offset measurements are used to effectively implement the GaAsP inserts as diffusion barriers. The implementation of these barriers in nanopillar light-emitting diodes provides a 5-fold increase in output intensity, making this a promising approach to high-efficiency pillar-based emitters in the near-infrared wavelength range.

Sidewall passivation for InGaN/GaN nanopillar light emitting diodes

We studied the effect of sidewall passivation on InGaN/GaN multiquantum well-based nanopillar light emitting diode (LED) performance. In this research, the effects of varying etch rate, KOH treatment, and sulfur passivation were studied for reducing nanopillar sidewall damage and improving device efficiency. Nanopillars prepared under optimal etching conditions showed higher photoluminescence intensity compared with starting planar epilayers. Furthermore, nanopillar LEDs with and without sulfur passivation were compared through electrical and optical characterization. Suppressed leakage current under reverse bias and four times higher electroluminescence (EL) intensity were observed for passivated nanopillar LEDs compared with unpassivated nanopillar LEDs. The suppressed leakage current and EL intensity enhancement reflect the reduction of non-radiative recombination at the nanopillar sidewalls. In addition, the effect of sulfur passivation was found to be very stable, and further insight into its mechanism was gained through transmission electron microscopy.

Optical properties of nanopillar AlGaN/GaN MQWs for ultraviolet light-emitting diodes.

Nanopillar AlGaN/GaN multiple quantum wells ultraviolet light-emitting diodes (LEDs) were fabricated by nanosphere lithography and dry-etching. The optical properties of the nanopillar LEDs were characterized by both temperature-dependent and time-resolved photoluminescence measurements. Compared to an as-grown sample, the nanopillar sample has a PL emission peak blue-shift of 7 meV, a 42% enhanced internal quantum efficiency at room temperature and a reduced radiative recombination lifetime from 870 picosecond to 621 picosecond at 7K. These results are directly from the suppressed quantum confined stark effect that is due to the strain relaxation in the nanopillar MQWs, further revealed by micro-Raman measurement. Additionally, finite-difference time domain simulation also proves better light extraction efficiency in the nanopillar LEDs.

GaAs pillar array-based light emitting diodes fabricated by metal-assisted chemical etching

We demonstrate GaAs pillar array-based light emitting diodes (LEDs) with axial p-i-n junctions fabricated using a room-temperature metal-assisted chemical etching (MacEtch) method. Variations in vertical etch rates for all three doping types of GaAs are investigated as a function of etching temperature, oxidant/acid concentration ratio, and dilution of the etching solution. Control over nanopillar morphologies is demonstrated, simply through modification of the etching conditions. Optical emission enhancement from the MacEtched p-i-n GaAs nanopillar LED is observed, relative to the non-etched planar counterpart, through room-temperature photoluminescence and electroluminescence characterization.

Improved Photoluminescence Efficiency in UV Nanopillar Light Emitting Diode Structures by Recovery of Dry Etching Damage

In this study, we have fabricated 375-nm-wavelength InGaN/AlInGaN nanopillar light emitting diodes (LED) structures on c-plane sapphire. A uniform and highly vertical nanopillar structure was fabricated using self-organized Ni/SiO2 nano-size mask by dry etching method. To minimize the dry etching damage, the samples were subjected to high temperature annealing with subsequent chemical passivation in KOH solution. Prior to annealing and passivation the UV nanopillar LEDs showed the photoluminescence (PL) efficiency about 2.5 times higher than conventional UV LED structures which is attributed to better light extraction efficiency and possibly some improvement of internal quantum efficiency due to partially relieved strain. Annealing alone further increased the PL efficiency by about 4.5 times compared to the conventional UV LEDs, while KOH passivation led to the overall PL efficiency improvement by more than 7 times. Combined results of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) suggest that annealing decreases the number of lattice defects and relieves the strain in the surface region of the nanopillars whereas KOH treatment removes the surface oxide from nanopillar surface.

Fabrication and characterization of high efficiency green nanopillar LED

InGaN/GaN green multiple quantum well (MQW) nanopillar light emitting diodes (LEDs), ∼150nm in diameter and 700nm in height, were fabricated by inductively coupled plasma etching using a Ni self-assembled nano-size mask. 2.5- and 7-fold improvement in the photoluminescence intensity was recorded for the 515 and 543nm green LED sample, respectively. Since the size and the shape of both samples were similar, it is believed that the higher In content LEDs would produce more strain relaxation through the fabrication of a nanopillar.

InGaN-Based Nano-Pillar Light Emitting Diodes Fabricated by Self-Assembled ITO Nano-Dots

InGaN/GaN based nano-pillar light emitting diodes (LEDs) with a diameter of 200-300 nm and a height of 500 nm are fabricated by inductively coupled plasma etching using self-assembled ITO nano-dots as etching mask, which were produced by wet etching of the as-deposited ITO films. The peak PL intensity of the nano-pillar LEDs was significantly higher than that of the as-grown planar LEDs, which can be attributed to the improvement of external quantum efficiency of the nano-pillar LEDs due to the large sidewall of the nano-pillars. We have also demonstrated electrical pumping of the InGaN/GaN based nano-pillar LEDs with a self-aligned TiO2 layer as a passivation of sidewall of the nano-pillars.

Nonradiative energy transfer between colloidal quantum dot-phosphors and nanopillar nitride LEDs

We present in this communication our study of the nonradiative energy transfer between colloidal quantum dot (QD) phosphors and nitride nanopillar light emitting diodes (LEDs). An epitaxial p-i-n InGaN/GaN multiple quantum-well (QW) heterostructure was patterned and dry-etched to form dense arrays of nanopillars using a novel etch mask consisting of self-assembled In3Sn clusters. Colloidal QD phosphors have been deposited into the gaps between the nanopillars, leading to sidewall coupling between the QDs and InGaN QW emitters. In this approach, close QW-QD contact and a low-resistance design of the LED contact layer were achieved simultaneously. Strong non-radiative energy transfer was observed from the InGaN QW to the colloidal QD phosphors, which led to a 263% enhancement in effective internal quantum efficiency for the QDs incorporated in the nanopillar LEDs, as compared to those deposited over planar LED structures. Time-resolved photoluminescence was used to characterize the energy transfer process between the QW and QDs. The measured rate of non-radiative QD-QW energy-transfer agrees well with the value calculated from the quantum efficiency data for the QDs in the nanopillar LED.