Nanocolumn Diameter Research Articles

The effects of copper doping on morphology and room-temperature photoluminescence of ZnO nanocolumns.

In this study, a versatile vapor phase transport method for the synthesis and copper-doping of ZnO nanocolumns is demonstrated. Doping percentage (up to 5%) showed no effect on the wurtzite structural phase of ZnO nanocolumns. However, a decrease in nanocolumn diameter (cross-sectional length of longest side or diagonal) due to doping was observed by scanning electron microscopy. Reduced rate of electron-hole recombination was inferred from a decrease in the intensity of the near-band edge emission peak shown in room-temperature photoluminescence spectra. Expression of structural defects in both doped and undoped nanocolumns suggest p-type conductivity. Observed copper-doping effects show promise for utilizing such structures as electrode components in dye-sensitized solar cells.

Self-organized formation of a-few-nanometer sized nanocolumns in chalcogenide-oxide nanocomposite film

Nanocomposite films are thin films with the addition of nano-sized second phases, and they are very promising for improving properties of chalcogenide films, such as superconducting vortex pinning. However, there are very few reports on the chalcogenide nanocomposite films, although many researchers are studying the oxide nanocomposite films. Since the process conditions for chalcogenide films are different from those for oxide films, the chalcogenide nanocomposite films are expected to have a characteristic structure. Here, we show fabrication of chalcogenide nanocomposite films containing self-organized oxide nanocolumns. FeSe+SrTiO3(STO) nanocomposite films were prepared using pulsed laser deposition, where the FeSe target with the STO sector was laser-ablated. The FeSe matrix kept the epitaxial relationship with LaAlO3 substrates even in the nanocomposite films, but the crystallinity and orientation of FeSe were degraded. Transmission electron microscopy observation clarified that Sr-Ti-O nanocolumns with the diameter of 2-5 nm and the spacing of <10 nm were formed at the deposition temperature of 450-550 ºC and the STO content of <25areal%. The formation of the FeSe+STO nanocomposite was affected by the low deposition temperature and interface instability. As a result, the nanocolumn diameter in the present chalcogenide-oxide nanocomposite was smaller than that in the oxide-oxide nanocomposites. This nanocolumn morphology (shape, diameter, and spacing) is expected to effectively influence the properties of chalcogenides.

Effect of oxygen flow rate on physical properties of Al-doped ZnO transparent conducting films prepared by reactive dc magnetron sputtering using metallic Zn:Al target

In this work, Al-doped ZnO transparent conducting thin films were prepared by a reactive dc magnetron sputtering method on slide glass substrate using single metallic Zn:Al (2 wt.% Al) target with the oxygen flow rate 1, 3, 5, 7 and 9 sccm, respectively. From XRD patterns, the films prepared at R(O2) of 1 sccm showed amorphous characteristics. In contrast, the films prepared at R(O2) of 3, 5, 7 and 9 sccm showed the preferred orientation (002) plane of hexagonal structure. Surface morphology and diameter of nanocolumns of the films were observed by AFM and FESEM. From transmittance spectra, energy gap value was found to vary between 3.24 and 3.32 eV. Electrical resistivity and Hall effect measurements were performed on the films with van der Pauw configuration. The temperature-dependent conductivity was performed in the range 20-300 K. Three types of conduction mechanisms were expected. Thermally activated band conduction at the high temperature range (250-300 K), Mott variable-range hopping (Mott-VRH) at the low temperature range (100-210 K) and Efros-Shklovskii variable-range hopping (ES-VRH) at the very low temperature range (<100 K) can be found in the films prepared with different oxygen flow rate conditions. Except for the oxygen flow rate at 5 sccm, the behavior of weakly localized electrons was observed. Then, the sets of parameters explaining the properties of localized electrons in each conduction regime were determined. The results indicated that the reactive dc magnetron sputtering using a single metallic Zn:Al (2 wt.% Al) target is suitable for TCO films since it can be easily obtained at low temperatures with good physical properties.

Red‐Emitting InGaN‐Based Nanocolumn Light‐Emitting Diodes with Highly Directional Beam Profiles

Various InGaN/GaN pn‐junction nanocolumn arrays arranged in a triangle lattice are grown on the same substrate while changing the nanocolumn diameters (Dn‐GaN) of the underlying n‐side GaN region under the lattice constant (L) of 340 nm. The nanocolumn diameter increases during the growth of the active InGaN and p‐side GaN regions. The periodic arrangement of nanocolumns leads to a photonic crystal effect. Redshift of the band edge wavelength, from 573 to 629 nm, is observed as Dn‐GaN increases from 159 to 282 nm. This phenomenon can be explained by the fact that the larger filling factor increases the effective refractive index of the nanocolumn system, resulting in a redshift of the band edge. The light diffraction at the photonic band edge induces the directional radiation beam from the surface of the nanocolumn system. Using the nanocolumn array with Dn‐GaN = 260 nm, the red‐emitting (λ = 637 nm) nanocolumn light‐emitting diodes with the radiation angle of ±30° are demonstrated.

ZnO nanocolumns synthesized by chemical bath process and spray pyrolysis: Ultrasonic and mechanical dispersion of ZnO seeds and their effect on optical and morphological properties

Hexagonal ZnO nanocolumns were synthesized by chemical bath deposition on previously deposited ZnO nanoseeds on Corning glass substrates, at deposition temperatures from 300 to 550 °C in steps of 50 °C, by ultrasonic spray pyrolysis technique. The ZnO nanoseeds were previously suspended in the spraying solution, with and without ultrasonic dispersion. Scanning Electron Microscopy study of surface morphology was carried out revealing the formation of hexagonal ZnO nanocolumns; the smaller diameters were obtained for ZnO seed layers deposited at substrate temperature between 400 and 450 °C. The best vertical alignment was observed on nanocolumns grown from ultrasonic-dispersed ZnO nanoseeds deposited at 400 °C. Raman spectroscopy was used to evaluate the crystal quality of the ZnO nanocolumns, a relationship was determined between nanocolumns stress with a shift to lower wavenumbers of E2H mode, the ZnO seeds deposition temperature and the nanocolumn diameters. Intrinsic defects of ZnO nanocolumns were studied with photo and cathodoluminescence spectroscopy. Also, photoluminescence life time measurements were carried out. Samples grown from ultrasonically dispersed ZnO seeds generated nanocolumns with greater stoichiometry and purity.

Study on ultra-high sensitivity piezoelectric effect of GaN micro/nano columns

High-quality GaN micro/nano columns were prepared with self-organized catalytic-free method. Young’s modulus of GaN nanocolumns were measured under both compressive stress and tensile stress. It was found that the Young’s modulus decreases with the increasing of nanocolumn diameter due to the increase of face defect density. Furthermore, we measured the piezoelectric properties and found that there was a 1000-fold current increase under a strain of 1% with a fixed bias voltage of 10 mV. Based on the Schottky Barrier Diode model, we modified it with the effect of polarization charge, image charge and interface state to analyze the experiment results which reveals that the strong piezopolarization effect plays an important role in this phenomenon. Therefore, the GaN nanocolumns has a great prospect to be applied in high-efficiency nanogenerators and high-sensitivity nanosensors.

Spectrally-broadened multimode lasing based on structurally graded InGaN nanocolumn photonic crystals suitable for reduction of speckle contrast

Bottom-up grown structurally graded InGaN-based nanocolumn photonic crystals, in which nanocolumns were arranged in triangular lattice and the nanocolumn diameter changed one-dimensionally from 93 to 213 nm with a fixed lattice constant of 250 nm, were fabricated. The spatial distribution of the diameter resulted in random-laser-like operation under optical excitation. A broad multi-wavelength lasing spectrum with more than 10 peaks was obtained with a full width at half maximum of 27 nm at 505 nm wavelength as well as lowering of the polarization degree, which is expected to be suitable for speckle contrast reduction in laser projection display applications.

Effect of N2* and N on GaN nanocolumns grown on Si(111) by molecular beam epitaxy

The self-induced growth of GaN nanocolumns (NCs) on SixN1−x/Si (111) is investigated as a function of the ratio of molecular to atomic nitrogen species generated via plasma assisted molecular beam epitaxy. Relative concentrations of the molecular and atomic species are calculated using optical emission spectroscopy. The growth rate (GR), diameter, and density of NCs are found to vary with the molecular to atomic nitrogen species relative abundance ratio within the plasma cavity. With increasing ratio, the GR and diameter of NCs increase while the density of NCs seems to be decreasing. The morphologies and the coalescence of GaN NCs exhibit a trend for molecular/atomic ratios up to 11, beyond which they still change but at a lower rate. The detrimental effect of taperedness of the NCs decreases with increasing molecular/atomic ratios. This is possibly because of reduction in radial growth in NCs due to increase in diffusivity of nitrogen species with increasing ratios.

Effect of oxygen flow rate on the ultraviolet sensing properties of zinc oxide nanocolumn arrays grown by radio frequency magnetron sputtering

Highly transparent metal–semiconductor–metal ultraviolet (UV) photoconductive sensors were fabricated using thin (less than 100nm in thickness), dense, small-diameter ZnO nanocolumn arrays prepared via low-power, catalyst-free radio frequency (RF) magnetron sputtering at different oxygen flow rates ranging from 0 to 25sccm. The FESEM images revealed the average nanocolumn diameter decreased with increasing oxygen flow rate. The transmittance spectra show that with the introduction of oxygen, the transmittance of the nanocolumn arrays in the visible region improves relative to that of a film prepared in the absence of oxygen with values greater than 95%. The UV responsivity and sensitivity were significantly improved for sputtered ZnO nanocolumn arrays prepared at oxygen flow rates up to 10sccm, with the highest values of 9.70mA/W and 2.20×104. Furthermore, the responsivity and sensitivity decreased at oxygen flow rates greater than 10sccm, which can be attributed to the increased electrical resistance of the nanocolumn arrays. Our findings indicate that a high-performance UV photoconductive sensor can be realised using very thin sputtered ZnO nanocolumn arrays and that such a sensor would exhibit high sensitivity.

Selective-area growth of GaN nanocolumns on Si(111) substrates for application to nanocolumn emitters with systematic analysis of dislocation filtering effect of nanocolumns

The growth of highly uniform arrays of GaN nanocolumns with diameters from 122 to 430 nm on Si (111) substrates was demonstrated. The employment of GaN film templates with flat surfaces (root mean square surface roughness of 0.84 nm), which were obtained using an AlN/GaN superlattice (SL) buffer on Si, contributed to the high-quality selective-area growth of nanocolumns using a thin Ti mask of 5 nm thickness by rf-plasma-assisted molecular beam epitaxy. Although the GaN template included a large number of dislocations (dislocation density ∼1011 cm−2), the dislocation filtering effect of nanocolumns was enhanced with decreasing nanocolumn diameters (D). Systematic transmission electron microscopy (TEM) observation enabled us to explain the dependence of the dislocation propagation behavior in nanocolumns on the nanocolumn diameter for the first time. Plan-view TEM analysis was performed for nanocolumns with D = 120–324 nm by slicing the nanocolumns horizontally at a height of ∼300 nm above their bottoms and dislocation propagation through the nanocolumns was analyzed by the cross-sectional TEM observation of nanocolumns with D ∼ 200 nm. It was clarified that dislocations were effectively filtered in the bottom 300 nm region of the nanocolumns, the dislocation density of the nanocolumns decreased with decreasing D, and for narrow nanocolumns with D < 200 nm, dislocation-free crystals were obtained in the upper part of the nanocolumns. The dramatic improvement in the emission properties of GaN nanocolumns observed with decreasing diameter is discussed in relation to the decreased dislocation density. The laser action of InGaN/GaN-based nanocolumn arrays with a nanocolumn diameter of 170 nm and a period of 200 nm on Si under optical excitation was obtained with an emission wavelength of 407 nm. We also fabricated red-emitting InGaN-based nanocolumn light-emitting diodes on Si that operated at a wavelength of 652 nm, demonstrating vertical conduction through the AlN/GaN SL buffer to the Si substrate.

Monolithic integration of four‐colour InGaN‐based nanocolumn LEDs

The monolithic integration of four-colour indium gallium nitride (InGaN)-based nanocolumn light-emitting diodes (LEDs) is demonstrated. In the integrated nanocolumn LED unit, blue-, sky-blue-, green- and yellow-emitting micro-LEDs (LEDs 1–4) with a 65 μm diameter circular indium tin oxide emission window were arrayed in a 2 × 2 square lattice with a lattice constant of 190 μm. LEDs 1–4 consisted of nanocolumn arrays arranged in a triangular lattice with a lattice constant of 300 nm and their nanocolumn diameters at the position of the InGaN/gallium nitride (GaN) multiple quantum wells (MQWs) were 119, 145, 188 and 231 nm, respectively. The increase in nanocolumn diameter from LED 1 to LED 4 resulted in increasing emission peak wavelengths, which were 465, 489, 510 and 570 nm for LEDs 1–4, respectively. On the same substrate, a red-emitting micro-LED was prepared, in which the nanocolumn diameter was increased to 260 nm by using a 350 nm-lattice-constant nanocolumn array. A combination of different lattice constants in an integrated LED unit is expected to contribute to the achievement of red–green–blue–yellow (RGBY)-colour-integrated nanocolumn LEDs.

Flip‐chip bonding and fabrication of well‐ordered nanocolumn arrays on sputter‐deposited AlN/Si (111) substrate

Employing a thin AlN film sputter‐deposited on Si substrates, well‐ordered GaN nanocolumn arrays were demonstrated by RF‐plasma‐assisted molecular‐beam‐epitaxy selective‐area growth. Dislocation‐free crystals were grown even on this cost‐effective framework, which included high‐density threading dislocations (&gt;3 × 1011 cm−2), by applying nanocolumn growth techniques. Single‐peak PL spectra from integrated InGaN/GaN MQW were obtained in red region and the peak wavelength changed from 640 to 690 nm with increasing nanocolumn diameter. This framework also contributes to a fabrication of vertical injection type flip‐chip structure, even if a nanocolumn device. Damage‐free wet‐etching removal of Si substrates resulted in exposure of flip‐chip well‐ordered nanocolumns array.(a) TEM image of nanocolumns array on sputter‐deposited thin AlN film/Si (111) substrate. (b) SEM image of flip‐chip well‐ordered nanocolumns array.

The early stage of formation of self-organized nanocolumns in thin films: Monte Carlo simulations versus atomic-scale observations in Ge-Mn

Formation kinetics of self-organized nanocolumns during epitaxial growth of a thin film composed of immiscible elements (A,B) has been investigated using Kinetic Monte Carlo simulations. Simulated nanostructures show a good agreement with those observed in Ge-Mn using Atom Probe Tomography and Transmission Electron Microscopy. Self organisation is observed although the rigid lattice simulations used do not account for misfit elastic strain. Simulations reveal that the final nanostructure, in term of number density and diameter of nanocolumns, is controlled by the early stages of growth of the film. The influence of both growth temperature and solute concentration on the nanostructure features is discussed in details.

Effects of Substrate Temperature on Indium Gallium Nitride Nanocolumn Crystal Growth

Indium gallium nitride films with nanocolumnar microstructure were deposited with varying indium content and substrate temperatures using plasma-enhanced evaporation on amorphous SiO2 substrates. FESEM and XRD results are presented, showing that more crystalline nanocolumnar microstructures can be engineered at lower indium compositions. Nanocolumn diameter and packing factor (void fraction) was found to be highly dependent on substrate temperature, with thinner and more closely packed nanocolumns observed at lower substrate temperatures.

Formation of InGaN quantum dots in regularly arranged GaN nanocolumns grown by rf‐plasma‐assisted molecular‐beam epitaxy

AbstractInGaN quantum dots (QDs) were successfully integrated at the apex of each pyramid‐topped GaN nanocolumn. Various nanocolumn arrays with different nanocolumn diameters arranged in a triangular lattice were prepared on GaN templates by Ti‐mask selective‐area growth (SAG) with rf‐plasma‐assisted molecular‐beam epitaxy (rf‐MBE). The photoluminescence (PL) emission wavelength from the InGaN QDs shifted from 477 to 516 nm with increasing nanocolumn diameter from 206 to 326 nm. From the Arrhenius plot of PL integrated intensity, the PL internal quantum efficiency (IQE) was evaluated to be 48.4% for the 516‐nm‐wavelength sample. Threading dislocations at the bottom region of the nanocolumns were bent toward the sidewall and did not propagate to the upper active layer, which contributed to the higher PL efficiency. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Emission color control from blue to red with nanocolumn diameter of InGaN/GaN nanocolumn arrays grown on same substrate

A novel technology for controlling the In composition of InGaN quantum wells on the same wafer was developed, which paved the way for the monolithic integration of three-primary-color nano-light-emitting diodes. In the experiment, InGaN/GaN multiple quantum well nanocolumn arrays with nanocolumn diameters from 137 to 270 nm were prepared on the same substrate with the Ti-mask selective area growth by rf-plasma-assisted molecular beam epitaxy. The emission color changed from blue to red (from 479 to 632 nm in wavelength) with increasing nanocolumn diameter. The emission color change mechanism was clearly explained by the beam shadow effect of the neighboring nanocolumns.

Threading dislocation evolution in patterned GaN nanocolumn growth and coalescence overgrowth

Threading dislocation (TD) evolution during patterned GaN nanocolumn (NC) growth and coalescence overgrowth with metal-organic chemical vapor deposition is studied based on the comparisons of NC and coalescence overgrowth samples of different NC cross-section diameters and spacing sizes. From the measurement results of depth-dependent x-ray diffraction and cross-section transmission electron microscopy, it is found that the TD density in an NC depends on the patterned hole size for NC growth. Also, the TD formation at the beginning of coalescence overgrowth is related to the NC spacing size. Although the TD density at the bottom of the overgrown layer is weakly dependent on NC and spacing sizes, at its top surface, the TD density strongly relies on NC size. Among the overgrowth samples of different NC diameters and spacing sizes with a fixed NC diameter/spacing ratio, the one with the smallest size and spacing leads to the lowest TD density, the largest lateral domain size, and the highest photoluminescence efficiency. Also, the optical and crystal qualities at the surfaces of all the overgrowth samples are superior to those of a GaN template.

Photoluminescence of exciton and biexciton in GaN nanocolumns

We measured photoluminescence (PL) from excitons and biexcitons in GaN nanocolumns at low temperature and found that the PL spectra of excitons depend on the nanocolumn diameter. Taking into account the polaritonic effect of the excitons, calculation of the PL spectra revealed that the dependence on diameters causes a difference of PL intensity from side surfaces of the nanocolumns. At high excitation intensities, we also observed biexciton emissions and found that the biexciton binding energies are higher than those in bulk samples. Although the mechanism for the increase in the binding energy is not clear at present, we suppose that it arises from a spatial confinement effect due to the nanocolumn morphology.

Growth of high-In-content InAlN nanocolumns on Si (1 1 1) by RF-plasma-assisted molecular-beam epitaxy

In x Al 1− x N nanocolumns ( x In=0.73–1.00) of c-axis orientation were successfully self-organized on Si (1 1 1) substrates by RF-plasma-assisted molecular-beam epitaxy (RF-MBE). The height, diameter and density of nanocolumns were 0.7–1.1 μm, 40–130 nm, and 0.7–3.0×10 10 cm −2, respectively. With increasing In composition from 0.73 to 1.00, the room-temperature photoluminescence (RT-PL) peak wavelengths shifted from 1.06 to 1.87 μm. AlN nanocolumns (when x In=0) were also fabricated, and RT-PL was observed with a peak energy of 5.85 eV.