Individual GaN Nanowires Research Articles

Role of the surface density of states in understanding size-dependent surface band bending in GaN nanowires

In semiconductor nanostructures, surface states play a very important role in determining the surface potential or contact potential difference (CPD). We measure the CPD value of individual GaN nanowires (NWs) using Kelvin probe force microscopy (KPFM). The corresponding surface band bending (SBB) is calculated from the measured CPD value and is found to increase with a decrease in the NW size. In order to investigate the size dependence of the SBB, the electronic density of states of the surface atoms of the nanowires at different values of the diameter are calculated using the first-principle density functional theory (DFT). The calculated density of states of the nanowires revealed size-dependent occupancy and shift of conduction band edge. The relative position of the conduction band edge corroborates with size-dependent SBB values obtained from KPFM measurements.

Conductive AFM study of the electronic properties of individual epitaxial GaN nanowires

In this work, we use conductive atomic force microscopy (CAFM) to study the impact of substrate surface preparation and buffer layer composition on the electrical transport properties of GaN nanowires (NWs). I-V curves of single NWs from seven differently prepared samples were obtained. The tip of atomic force microscope (AFM) was used as a top conductive electrode to create stable electric contact to NW free upper grain, while the bottom contact was established between the highly doped Si substrate and a grounded sample holder of the AFM device. Single NW I-V curves were compared to those of NW arrays. The difference between them was discussed.

GaN nanowires as probes for high resolution atomic force and scanning tunneling microscopy

GaN nanowires are potential candidates for use in scanning probe microscopy due to their well-defined, reproducible, geometric shapes, their hardness, and their light guiding properties. We have developed and investigated probes for high resolution atomic force microscopy and scanning tunneling microscopy utilizing GaN nanowires as probes. The nanowires are n-doped and the morphology of the nanowires has been tailored for scanning probe microscopy by growing them with a sharp tip for measurements and high thickness for robustness. The individual GaN nanowires were removed from their growth substrate and attached onto commercial atomic force microscopy cantilevers or etched tungsten wires for scanning tunneling microscopy. A standard scanning electron microscope equipped with a nanoprobe, a focused ion beam column and a gas injection system was used to locate, transfer, and attach the nanowires. We evaluated the properties of the GaN probes on different substrates including HOPG, Au, SiO2, InAs, and GaAs. We demonstrate both atomic force microscopy and scanning tunneling microscopy measurements with single atomic layer resolution and evaluate the robustness of the tips by monitoring them before and after scanning. Finally, we explore the use of the tips for scanning tunneling spectroscopy demonstrating that reliable results, which can reveal information on the electronic properties of the surface-tip system, are obtainable. The fundamental properties of these probes, which are demonstrated in this work, show promise for future use of the probes in exploring semiconductor-semiconductor tunneling junctions at the nanoscale as well as for other scanning probe techniques where high resolution is required.

Controlling the persistence of photoconductivity through additional sub-bandgap photoexcitation in individual m-axial GaN nanowires

The persistence of photoconductivity after switching off the photoexcitation is investigated in individual m-axial n-GaN nanowires as a function of temperature. At room temperature, photoconductivity is found to decay with a time scale of several hours. The capture barrier height is estimated to be ∼450 meV from the stretched exponential fitting of the decay characteristics recorded at different temperatures. This energy value is found to be much less than the surface band-bending energy of ∼770 meV, which is believed to act as the capture barrier in this system. This finding indicates the tunneling of electrons through the top part of the band-bending barrier. Interestingly, the decay rate of photoconductivity is observed to reduce significantly when the photoconductivity in these wires is quenched by an additional sub-bandgap illumination prior to the switching off the photoexcitation. A rate equation model is proposed to explain the upward band bending at the surface as well as the persistent photoconductivity effect in terms of the transfer of holes between the valence band and acceptor-type surface states of the nanowires. Photoconductivity decay profiles simulated from the model are found to match very well with the experimental data recorded at different temperatures in both quenched and unquenched cases.

Photocurrent modulation under dual excitation in individual GaN nanowires.

The photo-response properties of vapor-liquid-solid (VLS) grown [101[combining macron]0] oriented individual GaN nanowires of the diameter ranging from 30 to 100 nm are investigated under the joint illumination of above and sub-bandgap lights. When illuminated with above-bandgap light, these wires show persistent photoconductivity (PPC) effects with long build-up and decay times. The study reveals the quenching of photoconductivity (PC) upon illumination with an additional sub-bandgap light. PC recovers when the sub-bandgap illumination is withdrawn. A rate equation model attributing the PPC effect to the entrapment of photo-generated holes in the surface states and the PC quenching effect on the sub-bandgap light driven release of the holes from the trapped states has been proposed. The average height of the capture barrier has been found to be about 400 meV. The study also suggests that the capture barrier has a broad distribution with an upper cut-off energy of ∼2 eV.

Ultrasensitive and Highly Selective Photodetections of UV-A Rays Based on Individual Bicrystalline GaN Nanowire.

The detection of UV-A rays (wavelength of 320-400 nm) using functional semiconductor nanostructures is of great importance in either fundamental research or technological applications. In this work, we report the catalytic synthesis of peculiar bicrystalline GaN nanowires and their utilization for building high-performance optoelectronic nanodevices. The as-prepared UV-A photodetector based on individual bicrystalline GaN nanowire demonstrates a fast photoresponse time (144 ms), a high wavelength selectivity (UV-A light response only), an ultrahigh photoresponsivity of 1.74 × 107 A/W and EQE of 6.08 × 109%, a sensitivity of 2 × 104%, and a very large on/off ratio of more than two orders, as well as robust photocurrent stability (photocurrent fluctuation of less than 7% among 4000 s), showing predominant advantages in comparison with other peer semiconductor photodetectors. The outstanding optoelectronic performance of the bicrystalline GaN nanowire UV-A photodetector is further analyzed based on a detailed high-resolution transmission electron microscope (HRTEM) study, and the two separated crystal domains within the GaN nanowires are believed to provide separated and rapid carrier transfer channels. This work paves a solid way toward the integration of high-performance optoelectronic nanodevices based on bicrystalline or horizontally aligned one-dimensional semiconductor nanostructures.

Investigation of electrical properties of individual GaN nanowire-based ferroelectric field effect transistor

Ferroelectric field effect transistors (FeFETs) using individual GaN nanowire as the conducting channel and Pb(Zr0.52Ti0.48)O3 (PZT) thin film as the gate dielectric were fabricated, and their electrical properties were investigated. The curves of the transfer characteristics for the individual GaN nanowire-based FeFET are of counterclockwise hysteresis loops, as the gate voltage was swept from negative to positive and then back. The memory window is about 5 V, and an on/off current ratio is up to 103 at zero gate voltage, indicating the feasibility of one-bit ferroelectric memory. The physical mechanism for the memory effect could be attributed to the reversible carrier concentration in individual GaN nanowire, which is modulated by the switchable remnant polarization of PZT thin film. The results may be helpful for the potential application of GaN nanowire in nonvolatile memory.

Cathodoluminescence of stacking fault bound excitons for local probing of the exciton diffusion length in single GaN nanowires

We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42 eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location.

Time-resolved photoluminescence spectroscopy of individual GaN nanowires

We investigate individual and ensembles of free-standing GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy using time-resolved photoluminescence (TRPL) spectroscopy. The PL transients of individual NWs always show a single exponential decay, independent of the excitation density. However, different NWs exhibit different decay times ranging from about 50 to 400 ps. In contrast, the PL transients of the NW ensemble always exhibit a nonexponential decay even at low excitation densities. We conclude that this nonexponential PL decay of the NW ensemble is a superposition of the different decay times of the different individual NWs in the ensemble. The origin of these different decay times is attributed to the impact of surface recombination.

Controlled dielectrophoretic nanowire self-assembly using atomic layer deposition and suspended microfabricated electrodes

Effects of design and materials on the dielectrophoretic self-assembly of individual gallium nitride nanowires (GaN NWs) onto microfabricated electrodes have been experimentally investigated. The use of TiO2 surface coating generated by atomic layer deposition (ALD) improves dielectrophoretic assembly yield of individual GaN nanowires on microfabricated structures by as much as 67%. With a titanium dioxide coating, individual nanowires were placed across suspended electrode pairs in 46% of tests (147 out of 320 total), versus 28% of tests (88 out of 320 total tests) that used uncoated GaN NWs. An additional result from these tests was that suspending the electrodes 2.75 μm above the substrate corresponded with up to 15.8% improvement in overall assembly yield over that of electrodes fabricated directly on the substrate.

Field-emission properties of individual GaN nanowires grown by chemical vapor deposition

Single crystalline GaN nanowires were synthesized using chemical vapor deposition. Devices containing individual GaN nanowires were fabricated using contact printing. The local turn-on electric field at the tip of the GaN nanowires was compared to that of other nanomaterials. The quality of contact between GaN nanowires and metal electrodes was found to affect the field-emission behavior significantly. It was also observed that the field-emission behavior of individual GaN nanowires follows the conventional Fowler-Nordheim model in the range of applied electric fields.

Individual GaN Nanowires Exhibit Strong Piezoelectricity in 3D

Semiconductor GaN NWs are promising components in next generation nano- and optoelectronic systems. In addition to their direct band gap, they exhibit piezoelectricity, which renders them particularly attractive in energy harvesting applications for self-powered devices. Nanowires are often considered as one-dimensional nanostructures; however, the electromechanical coupling leads to a third rank tensor that for wurtzite crystals (GaN NWs) possesses three independent coefficients, d(33), d(13), and d(15). Therefore, the full piezoelectric characterization of individual GaN NWs requires application of electric fields in different directions and measurements of associated displacements on the order of several picometers. In this Letter, we present an experimental approach based on scanning probe microscopy to directly quantify the three-dimensional piezoelectric response of individual GaN NWs. Experimental results reveal that GaN NWs exhibit strong piezoelectricity in three dimensions, with up to six times the effect in bulk. Based on finite element modeling, this finding has major implication on the design of energy harvesting systems exhibiting unprecedented levels of power density production. The presented method is applicable to other piezoelectric NW materials as well as wires manufactured along different crystallographic orientations.

Strong piezoelectricity in individual GaN nanowires

Abstract

Characterizing Atomic Composition and Dopant Distribution in Wide Band Gap Semiconductor Nanowires Using Laser-Assisted Atom Probe Tomography

Characterization of atomic composition and spatially resolved dopant distribution in wide band gap semiconducting nanowires is critical for their applications in next-generation nanoelectronic and optoelectronic devices. We have applied laser-assisted atom probe tomography to measure the spatially resolved composition of wide band gap semiconducting undoped GaN nanowires and Mg-doped GaN nanowires. Stoichiometric evaporation of individual GaN nanowires was achieved, and optimal experimental conditions to characterize the concentration and spatial distribution of the dopant in the Mg:GaN nanowire samples were established. Extremely mild operating conditions, with laser pulse energy as low as 3 pJ, are required to avoid preferential loss of nitrogen and achieve stoichiometric evaporation. The role of nanowire morphology in the selection of optimal experimental conditions is discussed in the context of thermal transport within the nanowire under a heat load imposed by the pulsing laser. The results of this work are expected to help guide similar atom probe tomography studies of related wide band gap III–V semiconductor alloys, which will facilitate a better understanding of material response and will help develop structure–property relationships.

Highly selective GaN-nanowire/TiO2-nanocluster hybrid sensors for detection of benzene and related environmentpollutants

Nanowire–nanocluster hybrid chemical sensors were realized by functionalizinggallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO2) nanoclusters for selectively sensing benzene and other related aromaticcompounds. Hybrid sensor devices were developed by fabricating two-terminaldevices using individual GaN NWs followed by the deposition ofTiO2 nanoclusters using RF magnetron sputtering. The sensor fabrication process employedstandard microfabrication techniques. X-ray diffraction and high-resolutionanalytical transmission electron microscopy using energy-dispersive x-ray andelectron energy-loss spectroscopies confirmed the presence of the anatase phase inTiO2 clusters afterpost-deposition anneal at 700 °C. A change of current was observed for these hybrid sensors when exposed tothe vapors of aromatic compounds (benzene, toluene, ethylbenzene, xylene andchlorobenzene mixed with air) under UV excitation, while they had no response tonon-aromatic organic compounds such as methanol, ethanol, isopropanol,chloroform, acetone and 1,3-hexadiene. The sensitivity range for the noted aromaticcompounds except chlorobenzene were from 1% down to 50 parts per billion (ppb)at room temperature. By combining the enhanced catalytic properties of theTiO2 nanoclusters with the sensitive transduction capability of the nanowires, an ultra-sensitiveand selective chemical sensing architecture is demonstrated. We have proposed amechanism that could qualitatively explain the observed sensing behavior.

In Situ Nanomechanics of GaN Nanowires

The deformation, fracture mechanisms, and the fracture strength of individual GaN nanowires were measured in real time using a transmission electron microscope-scanning probe microscope (TEM-SPM) platform. Surface mediated plasticity, such as dislocation nucleation from a free surface and plastic deformation between the SPM probe (the punch) and the nanowire contact surface were observed in situ. Although local plasticity was observed frequently, global plasticity was not observed, indicating the overall brittle nature of this material. Dislocation nucleation and propagation is a precursor before the fracture event, but the fracture surface shows brittle characteristic. The fracture surface is not straight but kinked at (10-10) or (10-11) planes. Dislocations are generated at a stress near the fracture strength of the nanowire, which ranges from 0.21 to 1.76 GPa. The results assess the mechanical properties of GaN nanowires and may provide important insight into the design of GaN nanowire devices for electronic and optoelectronic applications.

Injection-level-dependent internal quantum efficiency and lasing in low-defect GaN nanowires

Measurements of temperature-dependent and time-resolved photoluminescence (PL) on individual GaN nanowires revealed PL lifetimes and values of internal quantum efficiency (IQE) that increased with excitation fluence. With sufficient injection levels, radiative recombination dominated within the nanowire temperature range of 75 K to 175 K, as indicated by the T3/2 temperature dependence of the free-exciton PL lifetimes for this bulk material. The IQE was close to unity here. Free-carrier recombination became more significant as temperatures increased toward room temperature, but excitonic recombination remained important with ultrashort excitation pulse fluences as high as 190 μJ/cm2. The IQE at room temperature fell to a value between 3% and 30% depending on the nature of the recombination, and, considering both excitonic and free-carrier recombination, the effective IQE was roughly 15%. Temperature-dependent measurements of lasing thresholds in optically pumped nanowires showed lower thresholds at temperatures where excitonic radiative recombination was strong, indicating a possible persistence of excitoniclike behavior with high injected carrier densities at temperatures below T = 170 K.

Sub-Band-Gap Photocurrent of an Individual Defective GaN Nanowire Measured by Conductive Atomic Force Microscopy

Sub-Band-Gap Photocurrent of an Individual Defective GaN Nanowire Measured by Conductive Atomic Force Microscopy

Origin of energy dispersion inAlxGa1−xN/GaNnanowire quantum discs with low Al content

Individual GaN nanowires containing ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}/\text{GaN}$ quantum discs (QDiscs) with Al content $x\ensuremath{\le}16\mathrm{%}$ have been investigated by microphotoluminescence, transmission electron microscopy, and theoretical modeling. Single quantum discs show narrow emission lines with a linewidth as low as 3 meV at energies above the GaN band gap while the emission of nanowires containing multiple quantum discs shows multiple peaks with total spectral broadening that depends on the Al content in the barrier. As assessed by simulations of the quantum confinement based on a three-dimensional effective-mass model, the main factors influencing the spectral dispersion are: (i) strain relaxation in the QDiscs, strongly affected by the presence of a lateral AlGaN shell with a progressively changing thickness formed during the barrier growth; (ii) monolayer fluctuations in the QDisc thickness.

Electrical and Optical Performance of Sublimation-Grown Long GaN Nanowires

Through a facile sublimation method, high-quality GaN nanowires with a length of several hundred micrometers were densely grown on amorphous substrates. The morphology and single-crystalline hexagonal structure of the GaN nanowires were characterized. Photoluminescence of the nanowires was studied, and a near-band-gap emission of wurtzite GaN was observed, indicating good optical quality of the GaN nanowires. Individual GaN nanowire devices were fabricated, and their photoconductivity and electrical transport properties were investigated. The results reveal that the sublimation-grown GaN nanowires possess outstanding UV sensitivity and an n-type gating effect.