Core-Shell Tunnel Junction Nanowire White-Light-Emitting Diode

Shape and Composition Evolution in an Alloy Core-Shell Nanowire Heterostructure Induced by Adatom Diffusion

A core-shell nanowire heterostructure is an important building block for nanowire-based optoelectronic devices. In this paper, the shape and composition evolution induced by adatom diffusion is investigated by constructing a growth model for alloy core-shell nanowire heterostructures, taking diffusion, adsorption, desorption and incorporation of adatoms into consideration. With moving boundaries accounting for sidewall growth, the transient diffusion equations are numerically solved by the finite element method. The adatom diffusions introduce the position-dependent and time-dependent adatom concentrations of components A and B. The newly grown alloy nanowire shell depends on the incorporation rates, resulting in both shape and composition evolution during growth. The results show that the morphology of nanowire shell strongly depends on the flux impingement angle. With the increase in this impingement angle, the position of the largest shell thickness on sidewall moves down to the bottom of nanowire and meanwhile, the contact angle between shell and substrate increases to an obtuse angle. Coupled with the shell shapes, the composition profiles are shown as non-uniform along both the nanowire and the shell growth directions, which can be attributed to the adatom diffusion of components A and B. The impacts of parameters on the shape and composition evolution are systematically investigated, including diffusion length, adatom lifetime and corresponding ratios between components. This kinetic model is expected to interpret the contribution of adatom diffusion in growing alloy group-IV and group III-V core-shell nanowire heterostructures.

Manipulation of Photoelectrochemical Water Splitting by Controlling Direction of Carrier Movement Using InGaN/GaN Hetero-Structure Nanowires

We report the improvement in photoelectrochemical water splitting (PEC-WS) by controlling migration kinetics of photo-generated carriers using InGaN/GaN hetero-structure nanowires (HSNWs) as a photocathode (PC) material. The InGaN/GaN HSNWs were formed by first growing GaN nanowires (NWs) on an Si substrate and then forming InGaN NWs thereon. The InGaN/GaN HSNWs can cause the accumulation of photo-generated carriers in InGaN due to the potential barrier formed at the hetero-interface between InGaN and GaN, to increase directional migration towards electrolyte rather than the Si substrate, and consequently to contribute more to the PEC-WS reaction with electrolyte. The PEC-WS using the InGaN/GaN-HSNW PC shows the current density of 12.6 mA/cm² at -1 V versus reversible hydrogen electrode (RHE) and applied-bias photon-to-current conversion efficiency of 3.3% at -0.9 V versus RHE. The high-performance PEC-WS using the InGaN/GaN HSNWs can be explained by the increase in the reaction probability of carriers at the interface between InGaN NWs and electrolyte, which was analyzed by electrical resistance and capacitance values defined therein.

Observation of polarity-switchable photoconductivity in III-nitride/MoSx core-shell nanowires

III-V semiconductor nanowires are indispensable building blocks for nanoscale electronic and optoelectronic devices. However, solely relying on their intrinsic physical and material properties sometimes limits device functionalities to meet the increasing demands in versatile and complex electronic world. By leveraging the distinctive nature of the one-dimensional geometry and large surface-to-volume ratio of the nanowires, new properties can be attained through monolithic integration of conventional nanowires with other easy-synthesized functional materials. Herein, we combine high-crystal-quality III-nitride nanowires with amorphous molybdenum sulfides (a-MoS_x) to construct III-nitride/a-MoS_x core-shell nanostructures. Upon light illumination, such nanostructures exhibit striking spectrally distinctive photodetection characteristic in photoelectrochemical environment, demonstrating a negative photoresponsivity of -100.42 mA W^-1 under 254 nm illumination, and a positive photoresponsivity of 29.5 mA W^-1 under 365 nm illumination. Density functional theory calculations reveal that the successful surface modification of the nanowires via a-MoS_x decoration accelerates the reaction process at the electrolyte/nanowire interface, leading to the generation of opposite photocurrent signals under different photon illumination. Most importantly, such polarity-switchable photoconductivity can be further tuned for multiple wavelength bands photodetection by simply adjusting the surrounding environment and/or tailoring the nanowire composition, showing great promise to build light-wavelength controllable sensing devices in the future.

Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs

Multi-color emission from coaxial GaInN/GaN multiple-quantum-shell (MQS) nanowire-based light-emitting diodes (LEDs) was identified. In this study, MQS nanowire samples for LED processes were selectively grown on patterned commercial GaN/sapphire substrates using metal-organic chemical vapor deposition. Three electroluminescence (EL) emission peaks (440, 540, and 630 nm) were observed, which were primarily attributed to the nonpolar m-planes, semipolar r-planes, and the polar c-plane tips of nanowire arrays. A modified epitaxial growth sequence with improved crystalline quality for MQSs was used to effectively narrow the EL emission peaks. Specifically, nanowire-based LEDs manifested a clear redshift from 430 nm to 520 nm upon insertion of AlGaN spacers after the growth of each GaInN quantum well. This demonstrates the feasibility of lengthening the EL emission wavelength since an AlGaN spacer can suppress In decomposition of the GaInN quantum wells during ramping up the growth temperature for GaN barriers. EL spectra showed stable emission peaks as a function of the injection current, verifying the critical feature of the non-polarization of GaN/GaInN MQSs on nanowires. In addition, by comparing EL and photoluminescence spectra, the yellow-red emission linked to the In-fluctuation and point defects in the c-plane MQS was verified by varying the activation annealing time and lowering the growth temperature of the GaInN quantum wells. Therefore, optimization of MQS nanowire growth with a high quality of c-planes is considered critical for improving the luminous efficiency of nanowire-based micro-LEDs/white LEDs.

Crystal Growth and Characterization of n-GaN in a Multiple Quantum Shell Nanowire-Based Light Emitter with a Tunnel Junction

Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n⁺⁺/ p⁺⁺-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n⁺⁺-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p⁺⁺-GaN and 3-nm-thick n⁺⁺-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p⁺⁺-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 10¹⁷ cm^-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p⁺⁺-GaN region was calculated to be 1.1 × 10²⁰ cm^-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

Gain and bandwidth of InP nanowire array photodetectors with embedded photogated InAsP quantum discs

Here we report on the experimental results and advanced self-consistent real device simulations revealing a fundamental insight into the non-linear optical response of n⁺-i-n⁺ InP nanowire array photoconductors to selective 980 nm excitation of 20 axially embedded InAsP quantum discs in each nanowire. The optical characteristics are interpreted in terms of a photogating mechanism that results from an electrostatic feedback from trapped charge on the electronic band structure of the nanowires, similar to the gate action in a field-effect transistor. From detailed analyses of the complex charge carrier dynamics in dark and under illumination was concluded that electrons are trapped in two acceptor states, located at 140 and 190 meV below the conduction band edge, at the interface between the nanowires and a radial insulating SiO_x cap layer. The non-linear optical response was investigated at length by photocurrent measurements recorded over a wide power range. From these measurements were extracted responsivities of 250 A W^-1 (gain 320)@20 nW and 0.20 A W^-1 (gain 0.2)@20 mW with a detector bias of 3.5 V, in excellent agreement with the proposed two-trap model. Finally, a small signal optical AC analysis was made both experimentally and theoretically to investigate the influence of the interface traps on the detector bandwidth. While the traps limit the cut-off frequency to around 10 kHz, the maximum operating frequency of the detectors stretches into the MHz region.

Vertically stacked RGB LEDs with optimized distributed Bragg reflectors

The design and fabrication of a vertically stacked red-green-blue (RGB) light-emitting diode (LED) with novel, to the best of our knowledge, wavelength-selective distributed Bragg reflectors (DBRs) are demonstrated. The two DBRs are optimized to achieve selective reflectance in the RGB spectral region through theoretical calculations and simulation modeling. The insertion of optimal DBRs into the stack structure can effectively reflect downward emission from the upper chip without filtering the emission from the lower chips, thereby increasing the luminous efficiency for white emission with a color temperature range of 3000-8000 K by 1.6-7.4%. The optical performances of stacked devices with and without DBRs are thoroughly studied, verifying the effectiveness of the proposed wavelength-selective DBR structure.

Heat Dissipation in Flexible Nitride Nanowire Light-Emitting Diodes

We analyze the thermal behavior of a flexible nanowire (NW) light-emitting diode (LED) operated under different injection conditions. The LED is based on metal-organic vapor-phase deposition (MOCVD)-grown self-assembled InGaN/GaN NWs in a polydimethylsiloxane (PDMS) matrix. Despite the poor thermal conductivity of the polymer, active nitride NWs effectively dissipate heat to the substrate. Therefore, the flexible LED mounted on a copper heat sink can operate under high injection without significant overheating, while the device mounted on a plastic holder showed a 25% higher temperature for the same injected current. The efficiency of the heat dissipation by nitride NWs was further confirmed with finite-element modeling of the temperature distribution in a NW/polymer composite membrane.

Development of Monolithically Grown Coaxial GaInN/GaN Multiple Quantum Shell Nanowires by MOCVD

Broadened emission was demonstrated in coaxial GaInN/GaN multiple quantum shell (MQS) nanowires that were monolithically grown by metalorganic chemical vapor deposition. The non-polar GaInN/GaN structures were coaxially grown on n-core nanowires with combinations of three different diameters and pitches. To broaden the emission band in these three nanowire patterns, we varied the triethylgallium (TEG) flow rate and the growth temperature of the quantum barriers and wells, and investigated their effects on the In incorporation rate during MQS growth. At higher TEG flow rates, the growth rate of MQS and the In incorporation rate were promoted, resulting in slightly higher cathodoluminescence (CL) intensity. An enhancement up to 2-3 times of CL intensity was observed by escalating the growth temperature of the quantum barriers to 800 °C. Furthermore, decreasing the growth temperature of the quantum wells redshifted the peak wavelength without reducing the MQS quality. Under the modified growth sequence, monolithically grown nanowires with a broaden emission was achieved. Moreover, it verified that reducing the filling factor (pitch) can further promote the In incorporation probability on the nanowires. Compared with the conventional film-based quantum well LEDs, the demonstrated monolithic coaxial GaInN/GaN nanowires are promising candidates for phosphor-free white and micro light-emitting diodes (LEDs).