GaN Nanostructures Research Articles

Laser wavelength effect on GaN nanostructure films morphological properties deposited by PLD technique

Laser wavelength effect on GaN nanostructure films morphological properties deposited by PLD technique

The Characterization of Nanostructured GaN Prepared via Low Temperature Photoelectrochemical Etching at Different Etching Period

The Characterization of Nanostructured GaN Prepared via Low Temperature Photoelectrochemical Etching at Different Etching Period

Electromechanical properties and piezoelectric potentials of one-dimensional GaN nanostructures: A bond relaxation investigation

From the viewpoint of atomic bond relaxation, an analytical approach was put forward to elucidate the physical origins of crystal size and cross-sectional shape dependency of piezoelectric potentials in GaN nanowires and nanotubes. It is demonstrated that (i) size-induced increase in piezoelectric potential is attributed to the coupling effect of the rising piezoelectric coefficient and both the reducing dielectric constant and elastic constant caused by the surface atomic coordination number loss, bond energy perturbation, and surface-to-volume ratio rising; (ii) as the number of sides for polygonal nanowires or nanotubes with the same equivalent radius decreases, the surface-to-volume ratio rises, and the piezoelectric potential increases; and (iii) the nanotubes can generate a piezoelectric potential higher than their nanowire counterparts due to their larger surface-to-volume ratios. The proposed formulation offers a scientific basis for the fabrication, optimization, and modulation of one-dimensional GaN-based piezoelectric nanometer devices.

Porous GaN Nanopyramids: Advancing Beyond Conventional Nanostructures for High‐Brightness InGaN/GaN Quantum Wells Emission

AbstractGaN nanostructures hold significant promise in advancing nanoscale light‐emitting devices. However, significant progress remains elusive, possibly due to the absence of innovative approaches. Beyond the smaller size of GaN nanostructures, the main interest lies in growing InGaN active layers on semi‐ and non‐polar orientations. Nevertheless, it is evident that while leveraging such a method holds initial promise, this approach may eventually reach its limits, prompting the need for fresh perspectives. In this manuscript, electrochemical etching is utilized to fabricate porous GaN nanopyramids. Given the absence of prior studies on electrochemical etching of GaN nanostructures, its etching behavior is investigated, revealing its selectivity concerning doping concentration and geometry. Furthermore, how the choice of electrolyte can differently impact the optical response of the etched template is explored. Subsequently, an InGaN/GaN quantum wells shell is grown as the active region. The significance of the approach is evidenced by a tenfold enhancement in photoluminescence emission. Porous nanopyramids demonstrate a higher internal quantum efficiency of 44.7% and faster radiative recombination time. Additionally, finite difference time domain simulation reveals a twofold increase in light extraction efficiency. This research will lead the way toward the development of a new class of nanostructures offering potential advancements in optoelectronic devices.

Growth of β-Ga2O3 nanostructures by thermal oxidation of GaN-on-sapphire for optoelectronic devices applications

Heterostructures of wide-bandgap semiconductors, like β-Ga2O3/ GaN, are being used in many high-power, high-frequency electronic and optoelectronic devices. This paper presents the growth of β-Ga2O3 thin film and nanostructures by thermally oxidizing (at 1000 °C) the planar and nano-structured GaN surfaces respectively. The nano-structures of GaN (having 300–500 nm long vertical nano-kinks) are fabricated by metal-assisted chemical etching of GaN surface for 1–2 hours. The thermally grown β-Ga2O3 samples are polycrystalline. The chemical analysis of the samples using X-ray photoelectron spectroscopy (XPS) showed enhancement of Ga-O peak intensity (at 20.21–20.3 eV) at the expense of the Ga-N peak intensity (at 19.2–19.7 eV) in the thermal oxidation process. The O1s XPS spectra of the oxidized samples exhibited prominent signature of O2- ions (530.7–530.9 eV) representing the formation of Ga2O3 with presence of adsorbed hydroxyl group at the surface (532.2 – 532.4 eV). The valance band XPS spectra disclosed the formation of an additional Ga(4 s)–O(2p) hybridization energy state, and the valence band maxima increased from 2.3 eV to 3.25 eV. The Raman spectra of the oxidized samples exhibited multiple Ag and Bg vibrational modes of β-Ga2O3 along with intense E2(high), A1(LO) modes of GaN. The time-resolved photoluminescence spectra of the samples demonstrated fast transition of charge carriers from excited state to ground state with 460–820 pico-seconds average career lifetime.

Circumventing the ammonia-related growth suppression for obtaining regular GaN nanowires by HVPE

Selective area growth by hydride vapor phase epitaxy of GaN nanostructures with different shapes was investigated versus the deposition conditions including temperature and ammonia flux. Growth experiments were carried out on templates of GaN on sapphire masked with SiN x . We discuss two occurrences related to axial and radial growth of GaN nanowires. A growth suppression phenomenon was observed under certain conditions, which was circumvented by applying the cyclic growth mode. A theoretical model involving inhibiting species was developed to understand the growth suppression phenomenon on the masked substrates. Various morphologies of GaN nanocrystals were obtained by controlling the competition between the growth and blocking mechanisms as a function of the temperature and vapor phase composition. The optimal growth conditions were revealed for obtaining regular arrays of ∼5 μm long GaN nanowires.

Concentrated Solar Light Photoelectrochemical Water Splitting for Stable and High-Yield Hydrogen Production.

Photoelectrochemical water splitting is a promising technique for converting solar energy into low-cost and eco-friendly H2 fuel. However, the production rate of H2 is limited by the insufficient number of photogenerated charge carriers in the conventional photoelectrodes under 1 sun (100mWcm-2) light. Concentrated solar light irradiation can overcome the issue of low yield, but it leads to a new challenge of stability because the accelerated reaction alters the surface chemical composition of photoelectrodes. Here, it is demonstrated that loading Pt nanoparticles (NPs) on single crystalline GaN nanowires (NWs) grown on n+-p Si photoelectrode operates efficiently and stably under concentrated solar light. Although a large number of Pt NPs detach during the initial reaction due to H2 gas bubbling, some Pt NPs which have an epitaxial relation with GaN NWs remain stably anchored. In addition, the stability of the photoelectrode further improves by redepositing Pt NPs on the reacted Pt/GaN surface, which results in maintaining onset potential >0.5V versus reversible hydrogen electrode and photocurrent density >60mAcm-2 for over 1500 h. The heterointerface between Pt cocatalysts and single crystalline GaN nanostructures shows great potential in designing an efficient and stable photoelectrode for high-yield solar to H2 conversion.

Structural and optical characterization of dilute Bi-doped GaN nanostructures grown by molecular beam epitaxy

We have carried out detailed studies on the epitaxy and characterization of dilute Bi-doped GaN nanostructures. A comprehensive investigation of Bi-doped GaN nanowires and quasi-film epitaxial growth conditions has been performed. Scanning electron microscopy studies show that lowering the GaBiN growth temperature causes gradual changes in top c-plane nanowire morphology due to the incremental incorporation of foreign Bi atoms. This trend is further substantiated by the secondary ion mass spectroscopy analysis of a multi-layer Bi-doped GaN quasi-film. However, it is also found that the amount of Bi incorporation into the GaN lattice is relatively independent of the N2 flow rate variation under the growth conditions investigated. Furthermore, room-temperature micro-Raman spectra show that there are additional peaks near 530, 650, and 729 cm−1 wave numbers in the Bi-doped GaN samples, which can primarily be attributed to Bi local vibrational modes, indicative of a small amount of Bi incorporation in the GaN lattice. Moreover, phonon calculations with density functional theory indicate that Bi replacing the N sites is the likely origin of the experimentally measured Raman modes. X-ray photoelectron spectroscopy measurements have also been obtained to deduce the electronic interaction between the Bi dopant atom and the GaN nanostructure. Such one-dimensional nanowires permit the synthesis of dislocation-free highly mismatched alloys due to strain relaxation, allowing efficient light absorption and charge carrier extraction that is relevant for solar energy harvesting and artificial photosynthesis.

Confinement of Excitons within GaN 1D Nanoarchitectures Formed on AlN Molecular Steps

AbstractThe fabrication and the optical properties of ultrathin GaN quantum wells on AlN (0001) vicinal surfaces with macrosteps are studied. The employed growth condition spontaneously restricts the GaN thickness to one monolayer (ML) on a planar plane. However, because of a higher density of dangling bonds at macrosteps, 2 ML GaN can be stabilized only at step edges, which creates quantum‐wire‐like 1D nanostructures with a dimension of a few nanometers for the lateral confinement. Exciton (carrier) confinement within the 1D GaN nanostructure is predicted theoretically and is confirmed experimentally by strongly polarized photoluminescence with in‐plane twofold symmetry at a wavelength of ≈250 nm at room temperature. This study demonstrates one path to realize quantum confinement within a fine nanoarchitecture composed of nitride semiconductors and might provide clues to the future creation of novel low‐dimensional nanostructures for the UV–vis spectral range.

Scalable nano-integration strategy: Controllable three-dimensional monocrystalline GaN nanostructures from nanobelts to nanonetwork

The effective and scalable assembly of 1D nano-units into ordered 3D monocrystalline networks remains a challenge. Based on the crystal structure and growth characteristics of GaN, we have designed a specific epitaxy strategy and successfully achieved large area self-networking of 3D monocrystalline nanostructure on Au-coated β-LiGaO2 (100) substrate formed by the interconnection of GaN nanobelts grown along three equivalent directions of the m-axis [011̅0], [101̅0] and [11̅00]. This well-aligned 3D GaN nanostructure grown by simple CVD method has good reproducibility and the yield of cross-linked junctions are quite high. The mono-crystallinity of this structure were proved by X-ray diffraction (XRD) and transmission electron microscope (TEM) results, especially at the crosslinked junctions. The overall epitaxial relationships between GaN and LiGaO2 was analyzed and the defect-mediated anisotropic VS growth mechanism was deduced from thorough characterization of individual nanobelts. Besides, the morphology of 3D GaN nanostructure including length, width and height was found to be controllable. The E2 (high) phonon peak at 567.4 cm−1 in the Raman spectrum and the strong ultraviolet emission peak centered at 363.3 nm (3.41 eV) in the photoluminescence (PL) spectrum indicate that the 3D GaN nanostructures are nearly strain-less and have high crystalline quality. This large-scale 3D monocrystalline network not only retains the advantage of 1D nanomaterials, but also become easier to operate compared with individual nanowire. The epitaxy assembly strategy is expected to ensure the property homogeneity of chip-scale devices for their real applications and provide a reference for further growth of tunable 3D structures of other materials.

Formaldehyde chemical sensor based on the Pt- and Pd-decorated GaN nanotubes

The current piece of research aimed at investigating how the decoration of Pt and Pd atoms impacts on the formaldehyde detection ability of a gallium nitride nanotube (GaNNT) through density functional theory calculations. Based on the results, the interaction between GaNNT and formaldehyde was a physical adhesion with a sensing response of around 6.7. After decorating Pt and Pd atoms into the GaNNT surface, the adsorption energies changed from −6.6 to −20.6 and –22.5 kcal/mol, respectively. In addition, there was a noticeable increase in the sensing response to 51.3 and 98.5, which revealed an increase in the sensitivity GaNNTs. The findings revealed that the decoration of Pd atom increases the sensitivity of GaNNT to formaldehyde more than the Pt atom. The theoretically obtained results also pinpointed to the fact that decorating GaN nanostructures with metals can render them suitable for practical uses.

Nanoscale Schottky contacts to GaN: Theoretical study and a brief review

Nanostructured GaN materials, including nanowires and nanorods, are advantageous for nanoscale devices, owing to their higher surface-to-volume ratio than thin films. Despite the technological progress, there exist many issues to be solved for commercial applications. To realize nanostructured GaN devices, it is essential to figure out thoroughly the current transport mechanisms with regard to the nanoscale contact size. Experimental and theoretical studies have shown that the transport properties in nanoscale Schottky contacts are quite different from those in thin-film based contacts. In this article, theoretical models reported by other researchers are considered to calculate the potential profiles near the metal/GaN interfaces and the current transport mechanisms in nanoscale contacts are investigated. In addition, the results on the nanoscale Schottky contacts to GaN are reviewed to throw light on important issues in GaN nanoscale devices.

Phonon modes and topological phonon properties in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x superlattices

To effectively regulate thermal transport for the near-junction thermal management of GaN electronics, it is imperative to gain an understanding of the phonon characteristics of GaN nanostructures, particularly the topological phonon properties connected to low-dissipation surface phonon states. In this work, a comprehensive study on phonon modes and topological phonon properties is performed from first principles in (GaN) x /(AlN) x and (AlGaN) x /(GaN) x (x = 1,2,3) superlattices. Phonon modes, including the dispersion relation, density of states, and participation ratio, were calculated for six GaN superlattices. The participation ratio results did not reveal the localized phonon mode. In topological phonon analyses, it is found that Weyl phonons with a Chern number of 1(−1) are present in all six GaN superlattices, consisting of trivial (GaN) and nontrivial (AlN and AlGaN) combinations. These phonons are located on either side of the k z = 0 plane symmetrically in the Brillouin zone. With the increase in the number of phonon branches in superlattices, the number of Weyl phonon points also increases from dozens to hundreds. One Weyl phonon with significant and clean surface states is selected and analyzed for each GaN superlattice. Among them, the Weyl phonon in (GaN)2/(AlN)2 superlattice mainly results from the lattice vibrations of Al and Ga atoms, while the Weyl phonons in other superlattices mainly result from the lattice vibrations of N atoms. The Weyl phonons at opposite k z planes form pairs in (GaN)2/(AlN)2, AlGaN/GaN, and (AlGaN)2/(GaN)2. Effects of strain including biaxial and uniaxial strain on Weyl phonons in GaN/AlN and AlGaN/GaN superlattices are investigated. Results indicate that Weyl phonons persist in large strain states, however, no monoclinic trend is observed due to the accidental degeneracy of these superlattices. The investigation in this work is promising to provide a deeper understanding of phonon properties and the topological effects of phonons in GaN nanostructures.

Dispersive response of polar optical phonon modes on the dielectric and size effects in a nitride nanostructure

Within the framework of the macroscopic dielectric continuum model, the dispersive spectra of polar optical phonon modes of III-nitride based nanostructures with different inner and outer dielectric media are investigated in this paper. The dielectric and quantum size effects on the dispersive frequencies of phonon modes are focused and emphasized. In the case of GaN quantum rings (QRs), we found that the dielectric constant of the inner/outer medium could significantly influence the dispersive spectra of the surface optical (SO) phonon mode as the high/low-frequency SO phonon modes are mainly localized at the inner/outer surface of QRs, respectively. Moreover, such effects associated with different crystallographic structures of GaN, i.e., the wurtzite (WZ) and zinc blende (ZB) crystal structures, are also studied in detail, showing the different degenerating behavior and electron–phonon coupling strength. The results obtained in this study provide a beneficial opportunity to independently adjust dispersive spectra of the two branches of SO modes in GaN nanostructures; thus, it could be used to design and develop phonon-based detectors and sensors.

Preferential crystal orientation etching of GaN nanopillars in Cl2 plasma

The ability to fabricate organized, dense arrays of GaN nanostructures with high aspect ratios is of great interest for improving light extraction and absorption in optoelectronic devices, such as light-emitting diodes (LEDs) and Lasers.However, achieving this requires a patterning method that allows for the fabrication of GaN nanostructures with a controlled final shape and specific crystallographic facets. Our study shows that a top-down approach combining lithography and Cl2-based plasma etching processes offers the possibility of anisotropically transferring a pattern while revealing smooth nonpolar facets.Although GaN etching in Cl2 is driven by ion-enhanced chemical etching mechanisms, the chemical component is very strong, and Cl2 plasma leads to preferential crystallographic orientation etching of GaN. The mechanisms that drive facet formation are very similar to those involved in wet KOH etching. The etching ability of a specific crystallographic plane by chlorine atoms depends on the planar density and the N dangling bonds present on the plane. Predictably, GaN crystallographic planes will be etched in Cl2 plasmas as follows: a-type semipolar > m-type semipolar planes > a nonpolar plane > m polar planes > c polar planes. Crystallographic etching usually reveals the slowest etching planes, meaning the m-planes. However, consistently with the wet mechanisms, our study shows that the final revealed plane in Cl2 plasma will strongly depend on the surface curvature: in concave surfaces, fast-etch facets persist, while in convex surfaces, slow-etch facets persist. Consequently, Cl2 plasma etching of pillars (convex surfaces) will result in a-type facets, while etching of holes (concave surfaces) result in m-type facets.

The Fabrication of GaN Nanostructures Using Cost-Effective Methods for Application in Water Splitting

The adjustable bandgap, single crystal structure, and strong chemical inertness of GaN materials make them excellent candidates for water splitting applications. The fabrication of GaN nanostructures can enhance their water splitting performance by increasing their surface area, improving photon absorption, and accelerating photocatalytic reactions. Developing cost-effective methods to fabricate GaN nanostructures is crucial to promote the development of GaN-based materials in water splitting applications. In this review, we introduce the main cost-effective techniques for the fabrication of GaN nanostructures and highlight future development directions.

Selective Area Epitaxy of GaN Nanostructures: MBE Growth and Morphological Analysis

This work presents the selective area epitaxy of GaN nanostructures grown on Ga-polar GaN/sapphire substrates by plasma-assisted molecular beam epitaxy. We demonstrate three types of nanostructures, including nanowires, nanofins, and nanorings on GaN-on-sapphire templates as well as investigate the ways of controlling their morphology, and orientation of sidewall plus top facets. A range of growth conditions including low to high Ga flux were employed during selective area epitaxy to develop these nanostructures with homogenous geometry and near vertical and smooth sidewalls. Using appropriate growth conditions and mask patterning orientations, nonpolar nanofin grids containing both/either m-/a-planes with as low as 260 nm fin width and up to six interconnects are demonstrated with smooth sidewalls. Based on these experimental results, we developed a growth model that takes different sidewall facets and orientations into account. The model generalizes the experimental results well and explains the growth conditions for the nanostructures. This study serves to advance the understanding of selective area epitaxy for defining complex III-nitride nanostructures that constitute an active area of research and development in the fields of nanotechnology and nanoscience.

Tailoring and functionalizing the graphitic-like GaN and GaP nanostructures as selective sensors for NO, NO2, and NH3 adsorbing: a DFT study.

Langmuir adsorption of gas molecules of NO, NO2, and NH3 on the graphitic GaN and GaP sheets has been accomplished using density functional theory. The changes of charge density have shown a more important charge transfer for GaN compared to GaP which acts both as the electron donor while gas molecules act as the stronger electron acceptors through adsorption on the graphitic-like GaN surface. The adsorption of NO and NO2 molecules introduced spin polarization in the PL-GaN sheet, indicating that it can be employed as a magnetic gas sensor for NO and NO2 sensing. The partial electron density states based on "PDOS" graphs have explained that the NO and NO2 states in both of GaN and GaP nanosheets, respectively, have more of the conduction band between - 5 and - 10eV, while expanded contribution of phosphorus states is close to gallium states, but nitrogen and oxygen states have minor contributions. GaN and GaP nanosheets represent having enough capability for adsorbing gases of NO, NO2, and NH3 through charge transfer from nitrogen atom and oxygen atom to the gallium element owing to intra-atomic and interatomic interactions. Ga sites in GaN and GaP nanosheets have higher interaction energy from Van der Waals' forces with gas molecules.

Reprogrammable Binary and Ternary Optoelectronic Logic Gates Composed of Nanostructured GaN Photoelectrodes with Bipolar Photoresponse Characteristics

AbstractThe elementary electronic‐logic‐gates, performing basic logic functions using electric signals as input, act as a building block of modern digital circuits. Intriguingly, the optoelectronic‐logic‐gates (OLGs), composed of optical devices, are emerging as a new logic platform which enables faster and large‐capacity data transmission and processing by using photons as input. However, the strict operation principle of classic optical devices, for example, the unidirectional photoresponse of the photodetector, restricts the functional enrichment of OLGs. Herein, reprogrammable OLGs in a photoelectrochemical (PEC)‐environment are reported by employing gallium‐nitride semiconductor p–n nanowires as photoelectrodes where bidirectional photocurrent is achieved, leading to the demonstration of various binary OLGs including “NOT”, “XOR”, and “OR”. Strikingly, thanks to the versatile tunability of PEC‐photoelectrodes, the logic function of these OLGs is switchable by simply adjusting programming inputs including light intensity, bias voltage, electrolyte condition, and the physicochemical properties of the nanowire surface. Most importantly, unique ternary OLGs, for example, ternary “OR” gates, can also be realized based on binary ones by just tuning their applied bias for higher logic complexity applications, without changing the device architecture. Such reprogrammable binary and ternary OLGs could provide a new avenue toward next‐generation logic circuits for fast computing and data‐processing in the future.

Review of Recent Advances of GaN Nanowires Based Sensors

This paper presents a review of the recent advances of GaN nanowire-based sensors. GaN nanostructures of various forms including nanowires, nanotubes, nanofibers, nanoparticles and nanonetworks have been reported for several sensing applications due to their unique electrical, optical, and structural properties. The nature of GaN materials provides advantages for nanodevices compared to thin films due to its higher surface-to-volume ratio. In addition, GaN materials can absorb ultraviolet radiation which is a key feature in several optical applications. These advantages led to much attention and further study in the applications of GaN. In this paper, the recent advances of GaN sensing applications such as gas sensors, biosensors, and pressure sensors are presented. The performance of these sensors is demonstrated along with the structural, electrical, and optical properties of GaN.