Single GaN Nanowires Research Articles

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.

Microwave Schottky Diodes based on Single GaN Nanowires

Microwave Schottky Diodes based on Single GaN Nanowires

Light-stimulated low-power artificial synapse based on a single GaN nanowire for neuromorphic computing

The fast development of the brain-inspired neuromorphic computing system has ignited an urgent demand for artificial synapses with low power consumption. In this work, it is the first time a light-stimulated low-power synaptic device based on a single GaN nanowire has been demonstrated successfully. In such an artificial synaptic device, the incident light, the electrodes, and the light-generated carriers play the roles of action potential, presynaptic/postsynaptic membrane, and neurotransmitter in a biological synapse, respectively. Compared to those of other synaptic devices based on GaN materials, the energy consumption of the single-GaN-nanowire synaptic device can be reduced by more than 92%, reaching only 2.72×10−12 J. It is proposed that the oxygen element can contribute to the synaptic characteristics by taking the place of the nitrogen site. Moreover, it is found that the dynamic “learning−forgetting” performance of the artificial synapse can resemble the behavior of the human brain, where less time is required to relearn the missing information previously memorized and the memories can be strengthened after relearning. Based on the experimental conductance for long-term potentiation (LTP) and long-term depression (LTD), the simulated network can achieve a high recognition rate up to 90% after only three training epochs. Such few training times can reduce the energy consumption in the supervised learning processes substantially. Therefore, this work paves an effective way for developing single-nanowire-based synapses in the fields of artificial intelligence systems and neuromorphic computing technology requiring low-power consumption.

Density control of GaN nanowires at the wafer scale using self-assembled SiN x patches on sputtered TiN(111)

The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh (>10 μm−2) or ultralow (<1 μm−2) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN x patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by {100} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN x atoms prior to the GaN growth. By varying the amount of pre-deposited SiN x , the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN x patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III–V semiconductors nucleus grown on inert surfaces like 2D materials.

Numerical determination of field emission performance of GaN nanowire arrays

Determining the field emission turn-on field of the GaN nanowire array can better adapt to the application of field-assisted photocathode. A numerical simulation of the field emission behavior based on the finite difference method is proposed. Considering the influence of nanowire radius, height, cathode-anode spacing and interwire spacing on the field emission of GaN nanowires. The simulation proves that field enhancement factor of a single GaN nanowire is the most excellent, while emission electrons is relatively small. When GaN nanowires have a larger aspect ratio, it will have a larger enhancement factor and a small required turn-on electric field. Due to an enhanced field shielding, the enhancement factor of the GaN nanowire array reduces as interwire spacing reduces. Cathode-anode spacing hardly influences enhancement factor of GaN nanowire array, while potential distributions are affected by cathode-anode spacing to a certain extent.

Microwave Schottky diodes based on single GaN nanowires

A series of Schottky diodes based on single GaN nanowires has been fabricated. Based on the data of small-signal frequency analysis (parameter S21) of diode structures at various bias voltages, the parameters of the corresponding equivalent electrical circuit were determined. It is shown that the cutoff frequency of the fabricated diodes reaches 27.5 GHz. Keywords: GaN, nanowires, microwave band, Schottky diode.

Сверхвысокочастотные диоды Шоттки на основе одиночных нитевидных нанокристаллов GaN

A series of Schottky diodes based on single GaN nanowires has been fabricated. Based on the data of small-signal frequency analysis (parameter S21) of diode structures at various bias voltages, the parameters of the corresponding equivalent electrical circuit were determined. It is shown that the cutoff frequency of the fabricated diodes reaches 27.5 GHz.

Electronic performance of GaN nanowires cathode under electric field

The built-in electric field of exponential doping can promote the concentration of the photogenerated carrier center to the top surface of the nanowire. The external electric field also can bend the motion trajectory of the emitted electrons toward the collecting side. These field-assisted methods promote the quantum efficiency. In this paper, the emission theory of a single GaN nanowire photocathode is studied for the first time. The effects of height and width of the nanowire, wavelength, intensity of electric field on quantum efficiency of uniformly doped or exponentially doped GaN nanowire photocathodes were explored. It shows that the top of the exponentially doped cathode has a higher quantum efficiency than uniformly doped cathode. With the absence of the field, quantum efficiency of a uniformly doped cathode reaches a maximum value of 55.29% when the width is 150 nm and the wavelength is 220 nm. The form of exponentially doped cathode can generate an internal field. With the internal field, a maximum value rises 56.73% when the height is 900 nm and the wavelength is 230 nm. The theoretical results can direct the preparation of this kind of photocathode.

Single GaN Nanowires for Extremely High Current Commutation

An array of GaN nanowires (NWs) on Si substrate is synthesized by molecular beam epitaxy. Measurements of electrical properties at room temperature of single NWs transferred to an auxiliary substrate demonstrate that the current density reaches an extremely high level of 2 MA cm−2 without NW damage. Taking into account the limited conductivity of the NW periphery due to electron surface states, that is, the conduction channel narrowing, the effective current density can reach 3 MA cm−2.

Numerical study of Schottky diode based on single GaN NW on Si

Numerical modelling of Schottky diode formed by single GaN nanowire on Si substrate was performed. Two metals, namely, gold and platinum forming the Schottky barrier were considered. The potential barrier height was calculated taking into account occurrence of image force as well as Fermi level pinning at metal/semiconductor interface. Volt-ampere characteristics were obtained for different NW doping levels considering Wentzel–Kramers–Brillouin tunneling model. Cutoff frequencies are evaluated from current-time relaxation curves for different NW lengths and doping levels. It is shown that such diode structure demonstrates high-speed performance with cutoff frequency in the range from 0.1 to 0.9 THz for both studied metals.

Investigation of the effect of the doping order in GaN nanowire p–n junctions grown by molecular-beam epitaxy

We analyse the electrical and optical properties of single GaN nanowire p–n junctions grown by plasma‐assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p–n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p–n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 1018 cm−3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology.

Nano-spectroscopic and nanoscopic imaging of single GaN nanowires in the sub-diffraction limit

The optical spatial resolution limit prohibits a precise spectroscopic characterization of objects at the nanoscale. However, plasmonic assisted microscopy at the nanoscale can overcome the diffraction limit owing to the confinement of the localized electric field. We report the spectroscopic imaging of a single GaN nanowire (NW) of diameter ∼200 nm with the help of Au nanoparticle assisted plasmonic tip-enhanced Raman spectroscopy (TERS), well below the diffraction limit of ∼750 nm. The challenge in the achievement was that the insignificant value of change of polarizability in the partially ionic III–V nitrides made the Raman scattering intensity too low for spectroscopic studies. Consequently, the enhancement factors for different observed Raman modes for the wurtzite GaN NW were also found to be significantly low. Spectroscopic imaging of such low scattering efficiency group III-nitride single GaN NW is achieved in the sub-diffraction limit by careful selection of the TERS substrate. Similarly, the nanoscopic optical imaging of a single GaN NW of diameter ∼10 nm using the confined electromagnetic wave is also achieved with the help of the plasmonic assisted near-field scanning optical microscopic technique in the sub-diffraction limit.

Polarized Raman studies of single GaN nanowire and GaN/AlN hetero-nanowire structures

We report the Raman results obtained from single GaN, GaN/AlN core–shell, and GaN/AlN branched nanowires (NWs). Polarized Raman spectra from a single GaN NW showed strong anisotropic behavior, in agreement with the Raman polarization selection rules for a wurtzite crystal, indicating the high crystalline quality of the NW. A Raman spectrum from a single GaN NW revealed several optical phonons, including A1(TO), A1(LO), and E2H phonons at 530.5, 724.2, and 567.0 cm−1, respectively. Fabricating an AlN shell layer on the side wall of the GaN core NW shifted the GaN A1(LO) phonon energy upward by 5.7 cm−1, indicating that compressive strain occurred in the GaN core. The formation of AlN nanorod branches on the GaN/AlN core–shell surface shifted the A1(LO) phonon energy downward toward the value of the GaN NW, indicating a relaxation of the compressive strain in the GaN core. In particular, a broad phonon response was observed at 691.8 cm−1 on the low-energy shoulder of the GaN A1(LO) phonon peak. A careful analysis of this mode identified that the 691.8 cm−1 mode corresponded to a defect-related phonon, not to a surface optical phonon.

Effect of the Conductive Channel Cut-Off on Operation of n+–n–n+ GaN NW-Based Gunn Diode

Nowadays modern science and technology require development of efficient electromagnetic emitters covering far GHz and THz ranges. Semiconductor elements with negative differential resistance (NDR) such as Gunn diodes based on GaAs and InP are well-established powerful microwave emitters. In this work the Gunn diode structure based on single GaN nanowire with the potential of THz device development was proposed and investigated both theoretically and experimentally. According to the results of the numerical modeling, the Gunn generation can be obtained in the proposed geometry. Dependence of the doping concentration on the NW length necessary for the electrons domain formation was obtained. The nanostructures were grown via molecular beam epitaxy technique. Contacts to single nanowires were fabricated with the combination of e-beam lithography and thermal evaporation. It was shown experimentally that the volt-ampere characteristics of the GaN nanowire structure possess current saturation region. Experimental results correspond well to proposed numerical model.

Structural Changes in a Single GaN Nanowire under Applied Voltage Bias.

GaN nanowires (NWs) are promising building blocks for future optoelectronic devices and nanoelectronics. They exhibit stronger piezoelectric properties than bulk GaN. This phenomena may be crucial for applications of NWs and makes their study highly important. We report on an investigation of the structure evolution of a single GaN NW under an applied voltage bias along polar [0001] crystallographic direction until its mechanical break. The structural changes were investigated using coherent X-ray Bragg diffraction. The three-dimensional (3D) intensity distributions of the NWs without metal contacts, with contacts, and under applied voltage bias in opposite polar directions were analyzed. Coherent X-ray Bragg diffraction revealed the presence of significant bending of the NWs already after metal contacts deposition, which was increased at applied voltage bias. Employing analytical simulations based on elasticity theory and a finite element method (FEM) approach, we developed a 3D model of the NW bending under applied voltage. From this model and our experimental data, we determined the piezoelectric constant of the GaN NW to be about 7.7 pm/V in [0001] crystallographic direction. The ultimate tensile strength of the GaN NW was obtained to be about 1.22 GPa. Our work demonstrates the power of in operando X-ray structural studies of single NWs for their effective design and implementation with desired functional properties.

Axial p–n junction and space charge limited current in single GaN nanowire

The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p–n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p–n junction NWs grown by plasma-assisted molecular beam epitaxy. I–V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p–n junction NW. Thanks to an improved contact process, both the electric field induced by the p–n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p–n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116–125 nm. Following our previous work, the acceptor Na doping level was estimated to be 2–3 × 1017 at cm−3 assuming a donor level Nd of 2–3 × 1018 at cm−3. The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

Modeling the semiconductor devices with negative differential resistance based on nitride nanowires

We proposed a numerical model of the Gunn diode and resonant diode based on a single nitride nanowire. Important model parameters needed for development of the considered devices, namely layers thickness, composition and doping level were evaluated. It has been shown theoretically that the single GaN nanowire based Gunn diode is capable to operate in over 1 THz frequency regime.

Controlling charges distribution at the surface of a single GaN nanowire by in-situ strain

Effect of the strain on the charge distribution at the surface of a GaN semiconductor nanowire (NW) has been investigated inside transmission electron microscope (TEM) by in-situ off-axis electron holography. The outer and inner surfaces of the NW bent axially under compression of two Au electrodes were differently strained, resulting in difference of their Fermi levels. Consequently, the free electrons flow from the high Fermi level to the low level until the two Fermi levels aligned in a line. The potential distributions induced by charge redistribution in the two vacuum sides of the bent NW were examined respectively, and the opposite nature of the bounded charges on the outer and inner surfaces of the bent NW was identified. The results provide experimental evidence that the charge distribution at the surfaces of a single GaN NW can be controlled by different strains created along the NW.

Study of GaN nanowires converted from β-Ga2O3 and photoconduction in a single nanowire

The formation of GaN nanowires from β-Ga2O3 nanowires and photoconduction in a fabricated single GaN nanowire device has been studied. Wurtzite phase GaN were formed from monoclinic β-Ga2O3 nanowires with or without catalyst particles at their tips. The formation of faceted nanostructures from catalyst droplets presented on a nanowire tip has been discussed. The nucleation of GaN phases in β-Ga2O3 nanowires and their subsequent growth due to interfacial strain energy has been examined using a high resolution transmission electron microscope. The high quality of the converted GaN nanowire is confirmed by fabricating single nanowire photoconducting devices which showed ultra high responsivity under ultra-violet illumination.

Formation of ultralong GaN nanowires up to millimeter length scale and photoconduction study in single nanowire

Long length GaN nanowire is emerging as desirable requirement for nanodevice applications including sensors, catalytic functionalities and large scale device integration. An alternate approach to obtain ultralong GaN nanowires with length up to millimeter is reported here. High quality ultralong GaN nanowires are obtained from β-Ga2O3 nanowires. As-formed ultralong GaN nanowires with bandgap of 3.4eV exhibit wurtzite crystal structure. The high quality of as-formed ultralong GaN nanowires has been confirmed by XRD, Raman, TEM and photoluminescence. The photoconduction in single ultralong GaN nanowire shows high responsivity in order of ~103 and gains in order of ~104.