Nanorod Light-emitting Diodes Research Articles

Significant performance enhancement of InGaN/GaN nanorod LEDs with multi-layer graphene transparent electrodes by alumina surface passivation

Nanotextured surfaces provide an ideal platform for efficiently capturing and emitting light. However, the increased surface area in combination with surface defects induced by nanostructuring e.g. using reactive ion etching (RIE) negatively affects the device’s active region and, thus, drastically decreases device performance. In this work, the influence of structural defects and surface states on the optical and electrical performance of InGaN/GaN nanorod (NR) light emitting diodes (LEDs) fabricated by top-down RIE of c-plane GaN with InGaN quantum wells was investigated. After proper surface treatment a significantly improved device performance could be shown. Therefore, wet chemical removal of damaged material in KOH solution followed by atomic layer deposition of only 10 alumina as wide bandgap oxide for passivation were successfully applied. Raman spectroscopy revealed that the initially compressively strained InGaN/GaN LED layer stack turned into a virtually completely relaxed GaN and partially relaxed InGaN combination after RIE etching of NRs. Time-correlated single photon counting provides evidence that both treatments—chemical etching and alumina deposition—reduce the number of pathways for non-radiative recombination. Steady-state photoluminescence revealed that the luminescent performance of the NR LEDs is increased by about 50% after KOH and 80% after additional alumina passivation. Finally, complete NR LED devices with a suspended graphene contact were fabricated, for which the effectiveness of the alumina passivation was successfully demonstrated by electroluminescence measurements.

Double-Heterojunction Nanorod Light-Emitting Diodes with High Efficiencies at High Brightness Using Self-Assembled Monolayers

Size-tunable, narrow-bandwidth emission, low threshold voltage, and high external quantum efficiency (EQE) make quantum dot light-emitting diodes (QD-LEDs) promising in next-generation display and lighting technologies. However, the maximum efficiencies are often observed in a relatively low-current, low-brightness regime, and the efficiency droop leads to less-than-ideal performance at high-luminance conditions useful for many applications. Here, we examine solution-processed, double-heterojunction nanorod (DHNR)-LEDs with self-assembled monolayer (SAM) modified indium tin oxide (ITO) electrodes. The SAMs can modify the surface and the work function of ITO, facilitate hole transport into the device, and therefore improve charge balance in DHNR-LEDs. Extremely bright DHNR-LEDs with maximum luminance over 100 000 cd/m2 are demonstrated. Furthermore, maximum efficiencies appear at high luminance conditions that can be achieved at very low bias and current density (e.g., 3.1 V and 53 mA/cm2 at ∼10 000 cd/m2,...

The effects of nanocavity and photonic crystal in InGaN/GaN nanorod LED arrays.

InGaN/GaN nanorod light-emitting diode (LED) arrays were fabricated using nanoimprint and reactive ion etching. The diameters of the nanorods range from 120 to 300 nm. The integral photoluminescence (PL) intensity for 120 nm nanorod LED array is enhanced as 13 times compared to that of the planar one. In angular-resolved PL (ARPL) measurements, there are some strong lobes as resonant regime appeared in the far-field radiation patterns of small size nanorod array, in which the PL spectra are sharp and intense. The PL lifetime for resonant regime is 0.088 ns, which is 40 % lower than that of non-resonant regime for 120 nm nanorod LED array. At last, three dimension finite difference time domain (FDTD) simulation is performed. The effects of guided modes coupling in nanocavity and extraction by photonic crystals are explored.

Polarized Emission From InGaN/GaN Single Nanorod Light-Emitting Diode

InGaN/GaN single nanorod (NR) light-emitting diodes (LEDs) have successfully been fabricated by nanoimprint lithography and focused ion beam-induced deposition. The current-voltage characteristics of single NR LEDs show low leakage current of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> A at a reverse bias of -5 V and turn-ON voltage of ~3.8 V. Linear polarization-oriented parallel to the c-axis with a degree of ~50% was discovered from the electroluminescence emission of single NR LEDs.

Regularly patterned non-polar InGaN/GaN quantum-well nanorod light-emitting diode array.

The growth and process of a regularly patterned nanorod (NR)- light-emitting diode (LED) array with its emission from sidewall non-polar quantum wells (QWs) are demonstrated. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR LED device is similar to that of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence spectral shift under reversed bias confirms that the emission of the fabricated NR LED comes from non-polar QWs.

Strong modification of spontaneous emission rate in nanorod light-emitting diode structures.

This study investigates the characteristics of modifications in spontaneous emission (SE) from GaN-based nanorod light-emitting diode (LED) structures using the three-dimensional finite-difference time-domain method. The simulated nanorod LED structure is assumed to be enclosed by perfect conductors and includes InGaN multiple-quantum-well active layers emitting at 500 nm. In the simulation, the modification of the SE rate is calculated as the structural parameters of nanorods are varied. The SE rate is found to depend strongly on the radius of nanorods. Large enhancement of the SE rate is observed when the resonant modes are formed inside the nanorod cavity. For both the transverse-electric and transverse-magnetic modes, the SE rate of nanorod LED structures can be enhanced by more than six times when the radius or height of the nanorod is optimally chosen. This large increase in the SE rate can lead to considerable increase in the internal quantum efficiency of LEDs, making nanorod structures prime candidates for future high-efficiency LEDs than can overcome the efficiency limit of current LED structures.

Strong photonic crystal behavior in regular arrays of core-shell and quantum disc InGaN/GaN nanorod light-emitting diodes

We show that arrays of emissive nanorod structures can exhibit strong photonic crystal behavior, via observations of the far-field luminescence from core-shell and quantum disc InGaN/GaN nanorods. The conditions needed for the formation of directional Bloch modes characteristic of strong photonic behavior are found to depend critically upon the vertical shape of the nanorod sidewalls. Index guiding by a region of lower volume-averaged refractive index near the base of the nanorods creates a quasi-suspended photonic crystal slab at the top of the nanorods which supports Bloch modes. Only diffractive behavior could be observed without this region. Slab waveguide modelling of the vertical structure shows that the behavioral regime of the emissive nanorod arrays depends strongly upon the optical coupling between the nanorod region and the planar layers below. The controlled crossover between the two regimes of photonic crystal operation enables the design of photonic nanorod structures formed on planar substrates that exploit either behavior depending on device requirements.

Dependencies of the emission behavior and quantum well structure of a regularly-patterned, InGaN/GaN quantum-well nanorod array on growth condition.

To achieve green emission from the sidewall non-polar quantum wells (QWs) of a GaN nanorod (NR) light-emitting diode, regularly patterned InGaN/GaN QW NR arrays are grown under various growth conditions of indium supply rate, QW growth temperature, and QW growth time for comparing their emission wavelength variations of the top-face c-plane and sidewall m-plane QWs based on photoluminescence and cathodoluminescence (CL) measurements. Although the variation trends of QW emission wavelength by changing those growth conditions in the NR structure are similar to those in the planar structure, the emission wavelength range of the QWs on an NR is significantly shorter than that in a planar structure under the same growth conditions. Under the growth conditions for a longer NR QW emission wavelength, the difference of emission wavelength between the top-face and sidewall QWs is smaller. Also, the variation range of the emission wavelength from the sidewall QWs over different heights on the sidewall becomes larger. On the other hand, strain state analysis based on transmission electron microscopy is undertaken to calibrate the average QW widths and average indium contents in the two groups of QW of an NR. The variation trends of the calibrated QW widths and indium contents are consistent with those of the CL emission wavelengths from various portions of NR QWs.

Large enhancement of light extraction efficiency in AlGaN-based nanorod ultraviolet light-emitting diode structures

Light extraction efficiency (LEE) of AlGaN-based nanorod deep ultraviolet (UV) light-emitting diodes (LEDs) is numerically investigated using three-dimensional finite-difference time-domain simulations. LEE of deep UV LEDs is limited by strong light absorption in the p-GaN contact layer and total internal reflection. The nanorod structure is found to be quite effective in increasing LEE of deep UV LEDs especially for the transverse magnetic (TM) mode. In the nanorod LED, strong dependence of LEE on structural parameters such as the diameter of a nanorod and the p-GaN thickness is observed, which can be attributed to the formation of resonant modes inside the nanorod structure. Simulation results show that, when the structural parameters of the nanorod LED are optimized, LEE can be higher than 50% and 60% for the transverse electric (TE) and TM modes, respectively. The nanorod structure is expected to be a good candidate for the application to future high-efficiency deep UV LEDs.PACS41.20.Jb; 42.72.Bj; 85.60.Jb

Fabrication of Nanorod Light Emitting Diode by Ni Nano-cluster and Enhanced Extraction Efficiency

Abstract: We demonstrate InGaN/GaN nanorod light emitting diode (LED) with significantly enhanced emission efficiency. Our nanorod LED has been fabricated by top down approach using self-assembled Ni nano-clusters as an etching mask during ICP etching. Nanorod LED has nowadays become very interesting to the researchers because of it ’ s strain relaxation effect that induced in InGaN/GaN MQWs. Our nanorod LED structure showed 9 nm (54meV) blue shift in peak emission wavelength compared to conventional LED which confirms the strain relaxation in InGaN/GaN MQWs and 2.8 times PL intensity enhancement which reflects the enhancement of internal quantum efficiency (IQE). Furthermore, extraction efficiency has been calculated using FDTD simulation. Our simulation results showed extraction efficiency up to 80% for nanorod LED. Keywords: Light emitting diode, Nano-cluster, Nanorod, Strain relaxation, Internal quantum efficiency, Light extraction efficiency, Finite Difference Time Domain (FDTD).

Enhanced Light Output of Dipole Source in GaN‐Based Nanorod Light‐Emitting Diodes by Silver Localized Surface Plasmon

The light output of dipole source in three types of light‐emitting diodes (LEDs), including the conventional planar LED, the nanorod LED, and the localized surface plasmon (LSP) assisted LED by inserting silver nanoparticles in the gaps between nanorods, was studied by use of two‐dimensional finite difference time domain method. The height of nanorod and the size of silver nanoparticles were variables for discussion. Simulation results show that a large height of nanorod induces strong wavelength selectivity, which can be significantly enhanced by LSP. On condition that the height of nanorod is 400 nm, the diameter of silver nanoparticle is 100 nm, and the wavelength is 402.7 nm, the light‐output efficiency for LSP assisted LED is enhanced by 190% or 541% as compared to the nanorod counterpart or the planar counterpart, respectively. The space distribution of Poynting vector was present to demonstrate the significant enhancement of light output at the resonant wavelength of LSP.

Flexible paper-based ZnO nanorod light-emitting diodes induced multiplexed photoelectrochemical immunoassay

ZnO nanorods inorganic-organic heterostructured light-emitting diodes have been demonstrated on a cheap/disposable paper substrate and applied in multiplexed photoelectrochemical immunoassay.

Light-emitting device with regularly patterned growth of an InGaN/GaN quantum-well nanorod light-emitting diode array

A light-emitting device consisting of a two-dimensional regularly patterned InGaN/GaN quantum well (QW) nanorod (NR) light-emitting diode (LED) array is implemented and characterized. The NR p-i-n structure includes n-GaN NR core and essentially conformal p-GaN shell. The active regions include nonpolar sidewall QWs and polar top-face QWs. A conformal layer of transparent GaZnO of low resistivity is deposited onto the NR LED structure for spreading the injection current over the sidewalls. It is found that the blue-shift range of the output spectral peak in increasing injection current is smaller than that of a planar LED of about the same operation wavelength in a similar variation range of injection current density although it is nonzero. The small blue-shift range is attributed to the mixed emission contributions from the nonpolar sidewall QWs and polar top-face QWs.

The fabrication of GaN-based nanorod light-emitting diodes with multilayer graphene transparent electrodes

GaN-based nanorod light-emitting diodes (LEDs) with multilayer graphene (MLG) transparent electrodes have been fabricated. Two types of nano-LEDs with graphene on and under the metal pads are fabricated and their performances are investigated. And LEDs with graphene on the metal-pads exhibiting lower forward voltage and higher electroluminescence intensity are obtained. Using scanning electron microscope and Raman spectroscopy, we have demonstrated that graphene transferred after the metal deposition remains intact and has much less damages than graphene under the metal during the fabrication of LEDs with nanorods.

A facile method for highly uniform GaN-based nanorod light-emitting diodes with InGaN/GaN multi-quantum-wells.

We report on a simple and reproducible method for fabricating InGaN/GaN multi-quantum-well (MQW) nanorod light-emitting diodes (LEDs), prepared by combining a SiO2 nanosphere lithography and dry-etch process. Focused-ion-beam (FIB)-deposited Pt was contacted to both ends of the nanorod LEDs, producing bright electroluminescence from the LEDs under forward bias conditions. The turn-on voltage in these nanorod LEDs was higher (13 V) than in companion thin film devices (3 V) and this can be attributed to the high contact resistance between the FIB-deposited Pt and nanorod LEDs and the damage induced by inductively-coupled plasma and Ga + -ions. Our method to obtain uniform MQW nanorod LEDs shows promise for improving the reproducibility of nano-optoelectronics.

A Parametric Study of InGaN/GaN Nanorod Core-Shell LEDs

In this paper, we present simulation results on the optical and transport properties of InGaN/GaN core-shell nanorod light-emitting diodes. The influence of contact position, surface recombination, and doping configuration on internal quantum efficiency is examined. The qualitative behavior when adding an electron blocking layer and the dependence on In content have been studied.

Electroluminescence from InGaN/GaN multi-quantum-wells nanorods light-emitting diodes positioned by non-uniform electric fields

We report that the nanorod light-emitting diodes (LEDs) with InGaN/GaN multi-quantum-wells (MQWs) emitted bright electroluminescence (EL) after they were positioned and aligned by non-uniform electric fields. Firstly, thin film LED structures with MQWs on sapphire substrate were coated with SiO(2) nanospheres, followed by inductively-coupled plasma etch to create nanorod-shapes with MQWs, which were transferred to the pre-patterned SiO(2)/Si wafer. This method allowed us to obtain nanorod LEDs with uniform length, diameter and qualities. Dielectrophoretic force created by non-uniform electric field was very effective at positioning the processed nanorods on the pre-patterned contacts. After aligned by non-uniform electric field, we observed bright EL from many nanorods, which had both cases (p-GaN/MQWs/n-GaN or n-GaN/MQWs/p-GaN). Therefore, bright ELs at different locations were observed under the various bias conditions.

All-InGaN Phosphorless White Light Emitting Diodes: An Efficiency Estimation

In this theoretical study we investigate the efficiency potential of monolithic white light emitting diodes (LEDs) that are free of wavelength-converting phosphors and are based solely on the InGaN material system. For that purpose we develop a numerical model that handles multiple active layers of different emission wavelength and takes photon reabsorption and -emission as well as internal non-radiative and optical losses into account. It is applied both to thin film structures as well as novel nanorod LEDs featuring disc-like active layers. In both cases, the active layers may either consist of multiple thin quantum wells or a single thick, bulk-like InGaN layer.

Core–shell InGaN nanorod light emitting diodes: Electronic and optical device properties

AbstractIn this theoretical study, we investigate both electronic and optical properties of core–shell InGaN nanorods (NRs) for light emitting diodes (LEDs). These structures feature active layers wrapped around high aspect ratio NR cores, thus allowing for an enormous increase in active area. Hence, efficiency may be increased by operating at lower carrier densities, mitigating efficiency droop known to limit conventional InGaN LEDs. Due to poor conductivity of the outer pGaN shell, current spreading along the entire NR length is challenging and requires an additional cover of transparent conductive oxide in order to fully exploit the active area. At the same time, quantum wells (QWs) on core–shell NRs are grown on m‐plane rather than conventional c‐plane GaN facets. The calculations show that the absence of piezoelectric fields on these non‐polar facets results in strong reabsorption of the emitted photons. A comprehensive numerical model is applied, linking internal quantum efficiency (IQE) and light extraction via the process of photon recycling. In summary, increasing optical absorption and photon recycling losses limit extraction efficiency. While this partially compensates the gain in IQE with increasing active area, a net improvement of the external quantum efficiency (EQE) of +10% is achievable by the proposed design of a novel core–shell NR LED.

Characterization of InGaN-based nanorod light emitting diodes with different indium compositions

Blue and green InGaN/GaN-based nanorod array light emitting diodes (LEDs) with superior performance have been realized using self-assembled Ni nano-mask and dry etching techniques. Temperature-dependent photoluminescence measurement shows that the internal quantum efficiencies (IQEs) of the LED nanorods are significantly improved in comparison to the as-grown epiwafer, with enhancement factors of 2.8 and 1.5 for the green LED nanorods and blue LED nanorods, respectively. It is in good agreement with a theoretical calculation based on the reduction in the internal electrical field due to strain relaxation in strained InGaN/GaN QWs. As compared to the planar LEDs fabricated using the same wafer, the emission of both nanorod LEDs is greatly improved. More significant enhancement in the light output power is observed for the green nanorod LED, manifesting that the emission enhancement is mainly attributed to a significant enhancement in the IQE. Furthermore, the current-voltage characteristics of nanorod LEDs exhibit two distinct regions at moderate forward bias, in which diffusion-recombination process is involved to a large extent in later period in spite that tunnelling transport dominates over a wide range of bias. The reverse leakage current of nanorod LEDs is about one order of magnitude higher as compared to the planar ones.