GaN Nanostructures Research Articles

Morphology-Controlled Reststrahlen Band and Infrared Plasmon Polariton in GaN Nanostructures.

Due to ultrabright and stable blue light emission, GaN has emerged as one of the most famous semiconductors of the modern era, useful for light-emitting diodes, power electronics, and optoelectronic applications. Extending GaN's optical resonance from visible to mid- and-far-infrared spectral ranges will enable novel applications in many emerging technologies. Here we show hexagonal honeycomb-shaped GaN nanowall networks and vertically standing nanorods exhibiting morphology-dependent Reststrahlen band and plasmon polaritons that could be harnessed for infrared nanophotonics. Surface-induced dipoles at the edges and asperities in molecular beam epitaxy-deposited nanostructures lead to phonon absorption inside the Reststrahlen band, altering its shape from rectangular to right-trapezoidal. Excitation of such surface polariton modes provides a novel pathway to achieve far-infrared optical resonance in GaN. Additionally, surface defects in nanostructures lead to high carrier concentrations, resulting in tunable mid-infrared plasmon polaritons with high-quality factors. Demonstration of morphology-controlled Reststrahlen band and plasmon polaritons make GaN nanostructures attractive for infrared nanophotonics.

Crystal side facet-tuning of GaN nanowires and nanofins grown by molecular beam epitaxy

GaN nanostructures are promising for a broad range of applications due to their 3D structure, thereby exposing non-polar crystal surfaces. The nature of the exposed crystal facets, i.e., whether they are a-, m-plane, or of mixed orientation, impacts the stability and performance of GaN nanostructure-based devices. In this context, it is of great interest to control the formation of well-defined side facets. Here, we show that we can control the crystal facet formation at the nanowire sidewalls by tuning the III–V ratio during selective area growth by molecular beam epitaxy. Especially, the N flux serves as a tool for controlling the growth kinetics. In addition, we demonstrate the growth of GaN nanofins with either a- or m-plane side facets. Based on our observations, we present the underlying nanostructure growth mechanisms. Low temperature photoluminescence measurements show a correlation of the formation of structural defects like stacking faults with the growth kinetics. This article demonstrates the controlled selective epitaxy of GaN nanostructures with defined crystal side facets on large-scale available AlN substrates.

Analysis and characterization of GaN nanostructure by electrochemical etching

Nanostructured GaN layers have been fabricated by electrochemical and laser-induced etching (LIE) processes based on n-type GaN thin films grown on the Si (111) substrate with AlN buffer layers. The effect of varying current and laser power density on the morphology of the GaN layers is investigated. The etched samples exhibited a dramatic increase in photoluminescence intensity as compared to the as grown samples. The average diameter of the GaN crystallites was about 7–10 nm, as determined from the PL data The Raman spectra also displayed stronger intensity peaks, which were shifted and broadened as a function of etching parameters. A strong band at 522 cm−1 is from the Si (111) substrate, and a small band at 301 cm−1, due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm−1 and 568 cm−1due to E2 (low) and E2 (high) respectively.

GaN on Si (111) nanostructure for solar cells application

The fabrications of nanostructured GaN by electrochemical etching and laser-induced etching have been attempted. Laser processing has the advantages of not causing damage or contamination, as well as of special selectivity with high resolution and high efficiency. The studies on the fundamental properties of these nanostructures are very important due to their unique structural and optical properties relative to the bulk form of the corresponding material. To the best of our knowledge, this has never been reported before.

Assessment of the Enhancement for the Excitation Emission in Porous Gan using Photoluminescence

ABSTRACT: This work aims to assess the enhancement of optical properties for porous GaN nanostructures, which fabricated by Photoelectrochemical etching under different current densities. The changing of optical properties for different samples were investigated by Photoluminescence (PL) spectroscopy. A strong near band-gap-edge emission (NBE) was detected with peak energy 3.40 eV for as-grown and sample etched at 5mA/cm2, while its 3.41 and 3.42 eV for samples etched with 10 and 20mA/cm2 respectively. Also, another peak emission from the sapphire substrate at peak 1.7 eV was observed. The PL peak intensity of the porous samples have increased with increasing the porosity, while the FWHM of the near-band-edge peak was decreased in 5 and 10mA/cm2 samples compared to as-grown non-etched sample, indicated that the pore size decreased with etching current density and porosity. Finally, the change of refractive with porosity was investigated in the porous GaN nanostructure.

Adsorption of Diospyrin on the surface of CC/AlN/AlP/GaN Nanotubes: A DFT investigation

• Distribution of charges all over the systems is evidence of chemical improvement. • Enhancement of the bioactivity with nanocage complexes. • Nanocages/nanotubes could be a good contestant for drug discovery. • Solvent effects of DO in nanocage systems greatly affect drugability. Density Functional Theory calculations were applied to explore the adsorption behaviors of Diospyrin (DO) on the CC/AlN/AlP/GaN nanostructures in gas and solvent phases. The drug molecule has a distinct binding characteristic on different nanotubes/Nano cages. The calculated adsorption energy for DO was found to be −86.97, for DO-AlN complex are −25.99 for DO-AlP, −2.77 for DO-CC, and −67.27 kcal/mol for DO-GaN complexes. Drug adsorption on AlN and GaN is more powerful than AlP and CC nanotubes. Furthermore; the drug adsorption in the studied systems is found to be more favorable in the aqueous phase than in the gas phase. We are anticipated that our findings on drug nanotubes’ adsorption and electronic properties will open a new way for drug delivery experts in developing new drug delivery vehicles.

An investigation on GaN/ porous-Si NO2 gas sensor fabricated by pulsed laser ablation in liquid

Pulsed-laser ablation in liquid was used to prepare GaN nanostructure. The P-type GaN nanostructure was deposited onto the porous-silicon substrate through the drop-casting method for NO2 gas-sensor fabrication. Ablation was performed in ethanol using two laser wavelengths, namely, 532 and 1064 nm. The XRD pattern showed a high and sharp peak at 2θ= 29.49°, indicating enhanced GaN formation using a 532 nm laser wavelength. AFM and FESEM analyses confirmed increased GaN grain growth at the same wavelength. The optical reflectance of the GaN sample showed higher reflectance at 532 nm than at 1064 nm. The optical-energy bandgap was more elevated at 532 nm than at 1064 nm. Photoluminescence analysis revealed that the 532 nm sample had a higher-intensity peak than the 1064 nm one. Device-performance studies showed the most enhanced sensor response (158.49%), highest sensitivity (2.109 ppm), and best response time (13.5 s) at 250 °C for the sample prepared using 532 nm laser wavelength.

High-Efficient Water Splitting Using Nanostructured Conical GaN

Nanostructured GaN has many advantages in water splitting, such as stronger light absorption, shorter migration distance and more reactive sites. Here, we have introduced a conical nanostructured GaN with the significant improvement of water splitting performance. The fabrication method is simple without any extra mask, including electrochemical etching and ICP etching only, which largely reduces the complexity and cost for fabricating nanostructured GaN. By making use of the large number of pores formed in electrochemical etching, a conical GaN nanostructure can be prepared using ICP etching afterwards. Compared with the control planar GaN, the reflectivity of the conical GaN decreases by ∼2.57 times, indicating the light absorption of the conical GaN increases significantly. The corresponding water splitting current density of the conical GaN has also been measured, which is 0.69 mA cm−2 at 1 V, ∼6.27 times higher than that of control planar GaN (0.11 mA cm−2). Our work provides a simple method to fabricate high-performance nanostructured GaN, and broadens the application of III-nitrides for efficient energy storage and conversion.

Meta-lens light-sheet fluorescence microscopy for in vivo imaging

Abstract Light-sheet fluorescent microscopy has become the leading technique for in vivo imaging in the fields of disease, medicine, and cell biology research. However, designing proper illumination for high image resolution and optical sectioning is challenging. Another issue is geometric constraints arising from the multiple bulky components for illumination and detection. Here, we demonstrate that those issues can be well addressed by integrating nanophotonic meta-lens as the illumination component for LSFM. The meta-lens is composed of 800-nm-thick GaN nanostructures and is designed for a light-sheet well-adapted to biological specimens such as the nematode Caenorhabditis elegans (C. elegans). With the meta-lens, the complexity of the LSFM system is significantly reduced, and it is capable of performing multicolor fluorescent imaging of live C. elegans with cellular resolution. Considering the miniature size and plane geometry of the meta-lens, our system enables a new design for LSFM to acquire in vivo images of biological specimens with high resolution.

Synthesized of GaN Nanostructure Using 1064 nm Laser Wavelength by Pulsed Laser Ablation in Liquid

GaN nanostructure was Synthesized using Pulsed laser ablation in liquid ethanol with Nd:YAG laser at pulsed laser ablation energy of 1600 mj and laser wavelength of 1064 nm. The nanoparticle was deposited using the drop cast method on the prepared porous silicon substrate. The structural and optical properties of the prepared GaN were studied. XRD pattern shows a high and sharp peak of pSi peak at 2θ =28.74 reflected from (111) plane and exhibits h-GaN rise at 2θ =34.54, 2θ =37.49, 2θ= 48.19 and 2θ=57.99 which are reflected from (002), (100), (102), (110) planes respectively where (002) plane has the highest peak than others. AFM and FESM proved an increase in the grain size of GaN. The reflectance of GaN (81.79%) at the wavelength (306nm) and has an energy band gap of (3.9eV).

Carbon Coating of Gan Nanostructures for Enhanced Sensitivity and Selectivity of Chemical Vapours

Carbon Coating of Gan Nanostructures for Enhanced Sensitivity and Selectivity of Chemical Vapours

Foam-like GaN: Study on the controlled tuning of pore size by R group change in amino acid etchant and its ultra-high photocurrent response

Relative to ionic liquids, in addition to mild physical and chemical properties, and are environmentally friendly, amino acids possess a smaller molecular weight as well. Herein, we developed a foamed GaN (F-GaN) nanostructures with uniform appearance and smooth pore wall, which prepared by the photoelectrochemical etching (PECE) in amino acid etchant. The controlled adjustment of the average pore size from 27.33 nm to 56.41 nm was achieved by the change of R group, verifying the amino acids with different molecular weight can effectively adjust the pore size of the F-GaN. In addition, the highest pore density was estimated to 1.34 × 1011 cm−1, which reached a peak value in the field of GaN wet etching ever before. Importantly, the highest photocurrent of the as-obtained F-GaN was 21 times than the planar GaN, which demonstrated F-GaN nanostructures had great potential in the photoelectric and optical devices.

DFT analysis of crystal polarity on graphene surface

We report an ab-initio study of the preferred polarity for wurtzite GaN nanostructures on virtual graphene substrates. By means of the density functional theory analysis we show that N-polar nanostructures on graphene are energetically favorable in comparison to Ga-polar. These finding are in agreement with experimentally observed N-polarity of wurtzite GaN nanowires grown on graphene substrate. We believe that the revealed polarity preference is of importance for piezoelectric and optoelectronic device design.

High performance conical nanostructured GaN-based photodetectors

Here, we report on a new type of conical nanostructured GaN (CNG), and the corresponding high performance GaN-based metal–semiconductor–metal (MSM) photodetectors (PDs). Compared with the control planar GaN-based MSM PDs, the photocurrent increases by ∼600 times at 0.4 V. The responsivity of the CNG-based PDs can reach ∼2 × 104 A W−1 at 4 V, increased by ∼2000 times compared to the planar GaN-based PDs. The specific detectivity of the CNG-based PDs reaches the maximum at 1 V, ∼1014 Jones, which is more than 800 times to that of the planar GaN-based PDs. Our work paves the way to develop high-performance GaN-based PDs using a new nanostructure for detecting weak optical signals.

Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins

Due to their intrinsically large surface-to-volume ratio, nanowires and nanofins interact strongly with their environment. We investigate the role of the main air constituents nitrogen, oxygen and water on the efficiency of radiative recombination in GaN nanostructures as a function of different surface treatments and at temperatures up to 200 °C. Oxygen and water exposures exhibit a complex behavior as they can both act quenching and enhancing on the photoluminescence intensity dependent on the temperature. For oxygen, these characteristics are already observed for low concentrations of below 0.5% in nitrogen. While the photoluminescence intensity changes induced by oxygen occur independently of illumination, the influence of water is light-induced: it evolves within tens of seconds under ultraviolet light exposure and is heavily influenced by the nanostructure pre-treatment. In contrast to observations in dry atmospheres, water prevents a recovery of the photoluminescence intensity in the dark. Combined measurements of the electrical current through GaN nanofins and their photoluminescence intensity reveal the environmental influence on the interaction of non-radiative recombination processes and changes in the surface band bending of the nanostructures. Several investigated solvents show an enhancing effect on the PL intensity increase, peaking in c-hexane with a 26-fold increase after 6 min of light exposure. Stabilization of the PL intensity was achieved by a passivation of the GaN surface with Ga x O y , and ZnO shells. Surprisingly, Al2O3 coatings resulted in a highly instable PL intensity during the first minutes of illumination. Our findings reveal the high importance of controlled environmental conditions for the investigation of nanostructures, especially when aimed at their applications in the fields of environmental sensing, photo-catalysis and light-emitting diodes.

Tuning the Charge Transfer Dynamics of the Nanostructured GaN Photoelectrodes for Efficient Photoelectrochemical Detection in the Ultraviolet Band

AbstractThe intriguing surface sensitivity of the single‐crystalline semiconductor nanowires offers tremendous opportunity in tuning the physical properties of nanophotonic and nanoelectronic devices for versatile applications. Particularly, in the pursuit of emerging photoelectrochemical (PEC)‐type devices, significant efforts have been devoted to understanding the charge transfer dynamics between the nanowires and the electrolyte. Here, a PEC‐type ultraviolet photodetector consisting of GaN p‐n junction nanowires as photoelectrodes is constructed. It is found that two competing charge transport processes at the nanowires’ surface as well as in the p‐n junction co‐determine the photoresponsive behavior of the device. Furthermore, the surface platinum (Pt) decoration has successfully tuned the charge transfer dynamics by enhancing the charge transport efficiency at the surface, resulting in a twenty‐fold increase of the photocurrent compared to the pristine GaN nanowires. Theoretical calculations reveal that the newly formed electronic states at the Pt/GaN interface account for the improved charge transfer at the surface, and the optimal hydrogen adsorption energy contributes to the boosted PEC reaction rate. The synergy of these two effects uncover the underlying mechanism of the high photoresponse of the constructed Pt/GaN‐nanowires‐based PEC photodetectors.

The growth behaviors and high controllability of GaN nanostructures on stripe-patterned sapphire substrates

The growth behaviors of multi-dimensional GaN nanostructures including simultaneous epitaxial growth of films and horizontal nanowires on the sapphire substrates were observed by making substrates per-patterned, leading to some interesting phenomena, thereby helping us understand the characteristics of GaN growth more comprehensively and clearly. The nonpolar a-plane GaN films grown on the Au-coated areas were formed by many triangular pyramids. And there are two kinds of GaN horizontal nanowires in the Au-free areas containing straight-growing nanowires and U-shaped nanowires. The detailed mechanism of those growth phenomena is proposed: when Au catalysts and Ga are sufficient, the growth of GaN is mainly controlled by vapor-solid (VS) mechanism, gradually growing into films. While when Au catalysts are insufficient, the growth of GaN mainly follows vapor-liquid-solid (VLS) mechanism. The difference between the growth of straight and U-shaped nanowires reflects that ±polarity has an important impact on the growth direction of nanowires, making the nanowires take an U-turn during the growth. In addition, the GaN nanostructures show a high degree of controllability. The width and length of nanowires, the continuity of films, even the presence of Au particles at the end of nanowires can be controlled, which provides a simple and convenient synthesis strategy.

Piezocatalytic Effect Induced Hydrogen Production from Water over Non-noble Metal Ni Deposited Ultralong GaN Nanowires.

Piezoelectric material-based catalysis that relies on an external stress-induced piezopotential has been demonstrated to be an effective strategy toward various chemical reactions. In this work, non-noble metal Ni-decorated ultralong monocrystal GaN nanowires (NWs) were prepared through a chemical vapor deposition (CVD) technique, followed by a photodeposition method. The piezocatalytic activity of the GaN NWs was enhanced by ∼9 times after depositing the Ni cocatalyst, generating hydrogen gas of ∼88.3 μmol·g-1·h-1 under ultrasonic vibration (110 W and 40 kHz), which is comparable to that of Pt-loaded GaN NWs. Moreover, Ni/GaN NWs with smaller diameters (∼100 nm) demonstrated superior piezocatalytic efficiency, which can be attributed to the large piezoelectric potential evidenced by both finite-element analysis and piezoresponse force microscopy measurements. These results demonstrate the promising application potential of non-noble metal loaded GaN nanostructures in hydrogen generation driven by weak mechanical energy from the surrounding environment.

Field emission of GaN nanofilms on Si substrates enhanced by hydrogen plasma treatment

The (002) oriented GaN nanostructured films prepared by pulsed laser deposition (PLD) on n-type Si (100) substrates were treated by H- and O-plasma, finding both treatments do not make the sizable changes in crystalline structure and morphology of GaN nanofilms. However, the field emission (FE) performance of GaN nanofilms is considerably improved by H-plasma treatment (H-GaN) but deteriorated by O-plasma treatment (O-GaN). The turn-on fields, Eon, for H- and O-plasma treated GaN nanofilms are determined to be 0.52 and 1.79 V μm−1, respectively, in contrast with that of pristine GaN nanofilms, 0.95 V μm−1. The improvement of FE performance by the H-plasma treatments could be attributed to the reasons of the reduced surface potential barrier, the increased electron concentration, the increased surface conductivity, and the increased effective emission area. The first-principles calculations represents that the experimental and theoretical work function results have the same trend with the descending sequence of O-plasma treated GaN > pristine GaN > the H-plasma treated GaN.

Controlled epitaxial growth of GaN nanostructures on sapphire (11–20) using laser molecular beam epitaxy for photodetector applications

We have grown various epitaxial GaN nanostructures on sapphire (11–20) substrates by tuning the buffer layer growth conditions in laser molecular beam epitaxy (LMBE) process. The pre-nitridation and buffer layer GaN growth at low temperature (LT) on sapphire (11–20) critically affect the surface morphology and structural properties. Granular GaN thin film (~ 160 nm) was grown on pre-nitridated sapphire whereas nano-column (NC)-GaN was obtained on LT-GaN buffer layer on bare sapphire having a height of ~ 370 nm at the growth temperature of 700 °C. Nano-porous (NP)-GaN was obtained with pore sizes in the range of 70 ~ 110 nm having vertical height of ~ 560 nm under similar growth conditions on LT-GaN buffered pre-nitridated sapphire. In-situ reflection high energy electron diffraction, high-resolution x-ray diffraction and Raman spectroscopy measurements indicated the epitaxial growth of c-axis oriented, wurtzite crystalline GaN nanostructures on sapphire (11–20) substrate with nearly negligible biaxial stress (0.03–0.23 GPa). Further, metal-semiconductor-metal (MSM) ultra-violet (UV) photodetectors were fabricated on epitaxial GaN nanostructures. The photo responsivity studies revealed that the NP-GaN MSM device has a photoresponse of ~ 358 mA/W at an applied bias of 1V. The photo-responsivity of NP-GaN MSM device is higher than that of GaN film (~ 36 mA/W) and NC-GaN (~7 mA/W) which revealed the importance of shape and size of GaN nanostructures on the responsivity of UV-photodetector devices. These results demonstrate the capability of LMBE technique to grow different GaN nanostructures on sapphire (11–20) substrate by tuning buffer layer conditions for their application as UV-photodetectors.