Ga-doped ZnO Nanowires Research Articles

Enhanced self-powered ultraviolet photoresponse of ZnO nanowires/p-Si heterojunction by selective in-situ Ga doping

In the current work, Ga-doped ZnO nanowires are grown by employing double step chemical bath deposition (CBD) method on Si substrate for fabricating the n-Ga:ZnO nanowires/p-Si heterojunction photodiodes. The doping concentrations have been systematically varied (1%–5%) for obtaining superior photo-responsive properties. The morphology, chemical composition and crystalline nature of the grown nanowires are studied by using FESEM, EDS and XRD, respectively. The vacancies/defect states and energy bandgap are studied from CL and UV–vis spectra. The photo-electric measurements indicate that optimized Ga-doped ZnO nanowires/p-Si heterojunction provides enhanced performance in terms of photo-to-dark current ratio (>2 × of undoped), response time, responsivity (0.192 A/W) and EQE (62.7%) in self-powered mode. At zero bias, 16.6% enhancement in responsivity has been observed for optimized Ga-doped ZnO nanowires/p-Si heterojunction as compared to undoped one with a highly selective UV-A (320 nm–400 nm) spectrum.

Growth of n-Ga doped ZnO nanowires interconnected with disks over p-Si substrate and their heterojunction diode application

In this paper, the heterojunction diode based on n-Ga doped ZnO nanowires interconnected with disks/p-Si assembly was fabricated and their low-temperature electrical properties were examined. The Ga-doped ZnO nanowires interconnected with disks were grown over p-Si substrate and studied by numerous techniques to understand the structural, compositional and morphological characteristics. Electrical properties, at lowtemperatures ranging from 77 K–295 K, were examined for the fabricated heterojunction diode assembly both in reverse and forward biased conditions which exhibited an excellent stability over all the temperature range. The detailed electrical characterizations revealed that the current decreases gradually from 1.9 μA, to 0.87 μA to 0.84 μA when temperature increases from 77 K, 100 K to 150 K and then increases gradually from 1.86 μA–3.36 μA and to 9.95 μA when temperature increases from 200 K–250 K and to 295 K, respectively. Both the highest rectifying ratio at 100 K and the lowest one at 295 K occur in the voltage range of 2–5 V.

Polarity Control in Growing Highly Ga-Doped ZnO Nanowires with the Vapor-Liquid-Solid Process.

Surface behavior modification by forming surface-transparent conductive nanowires (NWs) is an important technique for many applications, particularly when the polarities of the NWs can be controlled. The polarities of Ga-doped ZnO (GaZnO) NWs grown on templates of different polarities under different growth conditions are studied for exploring a polarity control growth technique. The NWs are formed on Ga- and N-face GaN through the vapor-liquid-solid (VLS) process using Ag nanoparticles as growth catalyst. The NWs grown on templates of different polarities under the Zn- (O-) rich conditions are always Zn (O) polar. During the early stage of NW growth, because the lattice sizes among different nucleation islands formed at the triple-phase line are quite different, high-density planar defects are produced when lateral growths from multiple nucleation islands form a GaZnO double bilayer. In this situation, frequent domain inversions occur, and GaZnO polarity is unstable. Under the Zn- (O-) rich conditions, because the lateral growth rate of GaZnO in the Zn- (O-) polar structure is higher due to more available dangling bonds, the growth of the Zn- (O-) polar structure dominates NW formation such that the NW eventually becomes Zn (O) polar irrespective of the polarity of the growth template. Therefore, the polarity of a doped-ZnO NW can be controlled simply by the relative supply rates of Zn and O during VLS growth.

Crystal Facet Engineering in Ga-Doped ZnO Nanowires for Mid-Infrared Plasmonics

The metal–organic chemical vapor deposition growth of various Ga-doped ZnO nanostructures for plasmonics is investigated, with a particular focus on the nanowire facet transformations induced by the addition of trimethylgallium in the gas phase. For nonintentionally doped spontaneous ZnO nanowires, the aspect ratio is strongly decreased due to residual Ga in the reactor, and the shape evolves rapidly toward Christmas-tree-like and hierarchical structures upon intentional Ga doping. Regarding ZnO/ZnO:Ga core–shell structures, a change of the smooth initial M-oriented facets occurs, with the development of {2021} surfaces, and further {1011} and {0001} surfaces. Interestingly, a similar evolution of the lateral roughness is observed in Au-catalyzed doped nanowires. High concentrations of Ga in the grown nanostructures are revealed by photoluminescence and confirmed by Rutherford backscattering spectrometry. First photoacoustic measurements show an optical absorption at 6 μm, evidencing that the degenerate...

Electrical properties of lightly Ga-doped ZnO nanowires

We investigated the growth, crystal structure, elemental composition and electrical transport characteristics of ZnO nanowires, a promising candidate for optoelectronic applications in the UV-range. Nominally-undoped and Ga-doped ZnO nanowires were grown by metal-organic chemical vapor deposition. Photoluminescence measurements confirmed the incorporation of Ga via donor-bound exciton emission. With atom-probe tomography we estimated an upper limit of the Ga impurity concentration (). We studied the electrical transport characteristics of these nanowires with a W-nanoprobe technique inside a scanning electron microscope and with lithographically-defined contacts allowing back-gated measurements. An increase in apparent resistivity by two orders of magnitude with decreasing radius was measured with both techniques with a much larger distribution width for the nanoprobe method. A drop in the effective carrier concentration and mobility was found with decreasing radius which can be attributed to carrier depletion and enhanced scattering due to surface states. Little evidence of a change in resistivity was observed with Ga doping, which indicates that the concentration of native or background dopants is higher than the Ga doping concentration.

Comparison of Ga-doped and Ag-doped ZnO Nanowire Gas-sensor Sensitivity and Selectivity

Pure ZnO, ZnO nanowires doped with 3 wt.% Ga (3GZO) and doped with 3 wt.% Ag (3SZO) were grown by a hot-walled pulse laser deposition (HW-PLD) technique. The optical and chemical properties of Ga and Ag doped nanowires was analyzed. Nanowires were determined to be under 200 nm in diameter and several μm in length. Change of significant resistance was observed and the gas detection sensitivities of ZnO, 3GZO and 3SZO nanawires were compared. The sensitivities of ZnO, 3GZO, and 3SZO nanowire sensors were measured at 300℃ for 1 ppm of ethanol gas at 97%, 48%, and 203%, respectively.

Ga-doped ZnO nanowire nanogenerator as self-powered/active humidity sensor with high sensitivity and fast response

High sensitivity and fast response piezo-humidity sensing have been realized from Ga-doped ZnO nanowire (NW) nanogenerator (NG) as self-powered/active gas sensor. The piezoelectric output generated by Ga-doped ZnO NW NG can not only act as a power source for driving the device, but also as a sensing signal for detecting humidity. Upon exposure to 80% relative humidity (RH) at room temperature, the piezoelectric output of Ga-doped ZnO NWs decreases from 0.56 V (in 45% RH) to 0.12 V, and the sensitivity is up to 358, higher than that of undoped ZnO NWs. In addition, the response time of Ga-doped ZnO NWs is 5 s, much shorter than that of undoped ZnO NWs. Such high performance can be ascribed to that Ga3+ ions can provide many donor defects and produce high local charge density/strong electrostatic field. The present results demonstrate a feasible approach for realizing high performance self-powered/active humidity sensor.

Sensing Properties of Ga-doped ZnO Nanowire Gas Sensor

Pure ZnO and ZnO nanowires doped with 3 wt.% Ga (&#x2018;3GZO&#x2019;) were grown by pulsed laser deposition in a furnace system. The doping of Ga in ZnO nanowires was analyzed by observing the optical and chemical properties of the doped nanowires. The diameter and length of nanowires were under 200 nm and several <TEX>${\mu}m$</TEX>, respectively. Changes of significant resistance were observed and the sensitivities of ZnO and 3GZO nanowires were compared. The sensitivities of ZnO and 3GZO nanowire sensors measured at 300&#x2103; for 1 ppm of ethanol gas were 97% and 48%, respectively.

Comparison of Optical Properties of Ga-doped and Ag-doped ZnO Nanowire Measured at Low Temperature

Pristine ZnO, 3 wt.% Ga-doped (3GZO) and 3 wt.% Ag-doped (3SZO) ZnO nanowires (NWs) were grown using the hot- walled pulse laser deposition (HW-PLD) technique. The doping of Ga and Ag in ZnO NWs was observed by analyzing the optical and chemical properties. We optimized the synthesis conditions, including processing temperature, time, gas flow, and distance between target and substrate for the growth of pristine and doped ZnO NWs. The diameter and length of pristine and doped ZnO NWs were controlled under 200 nm and several μm, respectively. Low temperature photoluminescence (PL) was performed to observe the optical property of doped NWs. We clearly observed the shift of the near band edge (NBE) emission by using low temperature PL. In the case of 3GZO and 3SZO NWs, the center photon energy of the NBE emissions shifted to low energy direction using the Burstein Moss effect. A strong donor- bound exciton peak was found in 3 GZO NWs, while an acceptor-bound exciton peak was found in 3SZO NWs. X-ray photoelectron spectroscopy (XPS) also indicated that the shift of binding energy was mainly attributed to the interaction between the metal ion and ZnO NWs.

Opto-electrical properties and chemisorption reactivity of Ga-doped ZnO nanopagodas

ZnO nanostructures with Ga doping prepared by metal–organic chemical vapor deposition developed an interesting nanopagoda shape with exposed {111}, {112}, {201}, and {101} lateral planes instead of {100} and {110} in the usually observed ZnO nanorods or nanowires. The types of exposed planes strongly affect those properties relating to surface reactivity. In this work, we report the opto-electrical properties and chemical reactivity of the Ga-doped ZnO nanopagodas and ZnO nanowires. Ultraviolet responses of photodetectors demonstrated that Ga-doped ZnO nanopagodas not only possessed a faster electron–hole generation and recombination rate but also exhibited higher photocatalytic activities than the ZnO nanowires. The improved performance of Ga-doped ZnO nanopagodas is due to the enhanced O2 and H2O chemisorption reactivity of the {111}, {112}, {201}, and {101} planes relative to the {100} and {110} planes.

Characterizations of Ga-Doped ZnO Nanowires Depending on Growth Temperature and Target-Substrate Distance in Hot-Walled Pulsed Laser Deposition

Using a hot-walled pulsed laser deposition (HW-PLD), nanowires (NWs) comprising 3 weight% Ga-doped ZnO (3GZO) have been successfully grown on a sapphire substrate. The structural and optical properties of 3GZO nanostructures have also been systematically investigated with respect to the target-substrate (T-S) distance and the growth temperature. The morphology transformations of nanostructures such as nano-horns, NWs, and clusters are strongly affected by growth temperatures due to different thermal energy. Also, the morphologies of nanostructures--including length, diameter, and density--are strongly affected by the T-S distance, illustrating a close correlation between the growth kinetics and the position in the plume formed by the particles from the GZO target. Also, the exciton that is bound to the neutral donor (D(0)X) peak of the 3GZO nanostructures is found at the low temperature PL spectra, indicating successful Ga-doping into ZnO NWs.

Temperature-dependent photoconductance of heavily doped ZnO nanowires

Ga-doped ZnO nanowires have been synthesized by a pulsed laser chemical vapor deposition method. The crystal structure and photoluminescence spectra indicate that the dopant atoms are well integrated into the ZnO wurtzite lattice. The photocurrent properties at different temperatures have been systematically investigated for nanowires configured as a three-terminal device. Among the experimental highlights, a pronounced semiconductor-to-metal transition occurs upon UV band-to-band excitation. This is a consequence of the reduction in electron mobility arising from the drastically enhanced Coulomb interactions and surface scattering. Another feature is the reproducible presence of two resistance valleys at 220 and 320 K upon light irradiation. This phenomenon originates from the trapping and detrapping processes in the impurity band arising from the native defects as well as the extrinsic Ga dopants. This work demonstrates that due to the dimensional confinement in quasi-one-dimensional structures, enhanced Coulomb interaction, surface scattering, and impurity states can significantly influence charge transport. Open image in new window

온벽 펄스 레이저 증착법을 이용해 합성한 Ga 도핑된 산화아연계 나노선 에탄올 가스 센서의 특성

We have investigated the sensing properties of ethanol gas sensor with pure ZnO and Ga-doped ZnO nanowires on Au coated (0001) sapphire substrates grown by hot walled pulsed laser deposition. Randomly aligned ZnO nanowires arrays were grown on a Au-electrode patterned under ambient conditions. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The average of length and diameter of the ZnO nanowires were <TEX>$8\;{\mu}m$</TEX> and 100 nm respectively, and confirmed by field emission scanning electron microscopy. Sensitivity chanege characterization of the gas sensor was found that measured sensitivities of the ethanol gas sensors were 83.3% and 68.3% at <TEX>$300^{\circ}C$</TEX> respectively.

Field emission and optical properties of Ga-doped ZnO nanowires synthesized via thermal evaporation

Ga-doped ZnO (GZO) nanowires have been synthesized by thermal evaporation with gallium metal as the dopant source. The morphology, microstructure and chemical composition were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The investigation confirmed that the GZO nanowires were the wurtzite hexagonal structures. These doped nanowires have diameters in the range 30–70nm and lengths of several hundreds of nanometers with growth direction along the (100) crystal plane. The optical properties from the cathodoluminescence (CL) and photoluminescence (PL) spectrum show that GZO nanowires exhibit a relative weak ultraviolet emission (UV) and a strong green emission. The UV emission for ZnO and GZO nanowires is attributed to near band-edge emission from recombination of free excitons. Furthermore, the green emission is attributed to oxygen vacancy and gallium impurity energy levels. Field emission measurements demonstrate that the GZO possesses good performance with a turn-on field of 3.4V/μm at a current density of 10μA/cm2, a threshold field of 5.4V/μm at a current density of 1mA/cm2, and a field-enhancement factor β of 5945. These results are very helpful for the design, fabrication and optimization of integrated optoelectronic nanodevices using GZO nanowires.

Optical and Electrical Properties of Ga-Doped ZnO Nanowire Arrays on Conducting Substrates

Large area Ga-doped ZnO nanowire arrays have been vertically grown on transparent conducting substrate by a simple chemical vapor deposition method. Experimental results reveal the well-aligned array morphology and the uniform Ga concentration in these nanowires with individual nanowires of single crystallinity. In particular, direct I−V measurements performed on a single nanowire on indium−tin oxide (ITO) substrate disclose both the low resistivity of nanowire and its Ohmic contact with the conducting substrate, which characteristics make them promising candidates for various optoelectronic and energy applications including photovoltaic cells.

Fabrication and characterization of Ga-doped ZnO nanowire gas sensor for the detection of CO

We demonstrate an efficient CO sensor using Ga-doped ZnO (GZO) nanowires (NWs). Various GZO NWs are synthesized with Au catalysts on sapphire substrates by hot-walled pulse laser deposition. The deposition temperature of ZnO NWs was in the range of 800–900 °C. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) characterizations indicate that the obtained NWs have the well-crystallized hexagonal structure with customized Ga-doping concentration of 0–5 wt.%. The NWs have the diameter of about 50 nm and the length of about 8 µm. After depositing the Ag electrodes on both sides of the NW cluster, the resistance change is checked with the exposure to CO gas in the self-designed gas chamber that can facilitate the detection of the resistance change and the control of gas flow as well as temperature . The detected resistance modulations are 1.0 kΩ and 83.2 kΩ in the cases of 3 wt.% GZO and pure ZnO NW clusters, respectively, indication that we successfully customize the sensitivity of the gas sensors by controlled doping.

Modification of structure and optical property of ZnO nanowires by Ga ion beam

AbstractBased on the focused ion beam (FIB) technology, we have prepared ZnO nanowires containing periodic nano-sized structures by an ultra thin Ga ion beam. ZnO nanowires can keep a good crystal quality after Ga ion bombardment. The cathodoluminescence (CL) spectroscopy study of the Ga-doped ZnO nanowires at low temperatures shows that the Ga doping effect can largely suppress the green emission that may mainly originate from the defects on the surfaces of ZnO nanowires.

Controlled synthesis of zigzagged and straight Ga-doped ZnO nanowires in hot-walled pulsed laser deposition

We synthesize zigzagged and straight Ga-doped ZnO nanowires (NWs) carefully controlling the doping-induced stress in the stacked structures of the individual NWs. The analysis of the zigzag shaping is approached with tracking the local stress migration in the NW crystals. Self-designed hot-walled pulsed laser deposition is employed to guarantee the wide window for the condition of successful NW formation. The density and kinetic energy of the laser-ablated particles from a target are optimized by tuning the laser operation condition, and the activation energy for switching the growing crystal directions is adjusted by controlling the synthesis temperature and the catalyst dimension to achieve the zigzagged morphology. The doping into the NWs is verified with photoluminescence measurement at low and room temperatures. Resultant zigzagged ZnO NWs successfully prepared with diversified Ga-doping concentration are analyzed, and compared with each other in terms of the morphology and growth kinetics.

Synthesis and analysis of resistance-controlled Ga-doped ZnO nanowires

We have synthesized the resistance-controlled Ga-doped ZnO nanowires employing self-designed hot-walled pulsed laser deposition, and investigated the status of the Ga-doping highlighting the chemical structure change, the stack-structured morphology, and the optical property of the nanowires. Especially the chemical structure is quantitatively evaluated, verifying that the substitutional Ga atoms and oxygen vacancies are proportional to the Ga-doping concentration. The resultant data of the controlled resistance ranging from 1.6 to 70 MΩ are presented.

Tunable n‐Type Conductivity and Transport Properties of Ga‐doped ZnO Nanowire Arrays

Tunable n‐Type Conductivity and Transport Properties of Ga‐doped ZnO Nanowire Arrays