Nanocrystalline Ga Research Articles

Effect of Thermal Oxidation on the Structure, Surface Texturing, and Microstructure Evolution in Nanocrystalline Ga─O─N Films

Effect of Thermal Oxidation on the Structure, Surface Texturing, and Microstructure Evolution in Nanocrystalline Ga─O─N Films

Realization and optimization of enhanced and spectral selective photoluminescence in size and phase controlled nanocrystalline Ga2O3 films made by pulsed laser deposition

• Nanocolumnar β-Ga 2 O 3 thin films were made by pulsed-laser deposition. • Performed detailed structural, interfacial and optical characterization of Ga 2 O 3 films. • Controlled processing conditions allow to tune the surface/interface microstructure. • Spectral selective photoluminescence demonstrated from nanotextured β-Ga 2 O 3 films. Realization and optimization of the optical properties for tunable and/or enhanced emission characteristics is critical to further advance the field of optoelectronics, photonics, and electronics for extreme environment applications. While recent advancements made to realize high-quality single crystals of Ga 2 O 3 , an interesting material for electronics and optoelectronics, strategies to obtain enhanced functionality and desired optical properties in nanocrystalline Ga 2 O 3 without needing to expensive epitaxial systems or quite cumbersome equipment or rare-earth dopants remains a challenging problem for further development. In this context we have demonstrated high yield spectral selective photoemission, from pulsed laser deposited (PLD) un-doped Ga 2 O 3 thin films with extensive control over nanocrystalline growth. Structural, interfacial and morphological investigation reveals the formation of perfect nanocrystalline seed layer promoting further growth of compact nano-columns with nano-textured atop. Optical spectroscopy demonstrates the desired control over band-edge absorption and realization of ideal photo-transition in perfectly oriented nano-columnar PLD β-Ga 2 O 3 films. Ellipsometry shows the growth parameters dependent evolution of refractive index over broad spectral range. The findings as presented and described here provide evidence for controlled growth and realization of nanocrystalline β-Ga 2 O 3 for functional device applications in optoelectronics in addition to provide a platform to further explore their futuristic technological applications.

Nanocrystalline Ga–Zn Oxynitride Materials: Minimized Defect Density for Improved Photocatalytic Activity?

Abstract We present an alternative synthesis strategy for developing nanocrystalline (Ga1−xZnx)(N1−xOx) semiconductors known to be very efficient photoabsorbers. In a first step we produce mixtures of highly crystalline β-Ga2O3 and wurtzite-type ZnO nanoparticles by chemical vapor synthesis. (Ga1−xZnx)(N1−xOx) nanoparticles of wurtzite structure are then formed by reaction of these precursor materials with ammonia. Microstructure as well as composition (zinc loss) changes with nitridation time: band gap energy, crystallite size and crystallinity increase, while defect density decreases with increasing nitridation time. Crystallite growth results in a corresponding decrease in specific surface area. In the UV regime photocatalytic activity for overall water splitting can be monitored for samples both before and after nitridation. We find a significantly lower photocatalytic activity in the nitrided samples, even though the crystallinity is significantly higher and the defect density is significantly lower after nitridation. Both properties should have led to a lower probability for charge carrier recombination, and, consequently, to a higher photocatalytic activity.

Nanocrystalline Ga2O3 films deposited by spray pyrolysis from water-based solutions on glass and TCO substrates

High-quality Ga2O3 films are obtained by spray pyrolysis from aqueous solutions through optimization of the solution composition and the spraying process parameters.

Enhancement of multiferroic and magnetocapacitive properties in nanocrystalline Mg-doped GaFeO3

We have study the effect of Mg doping on structural, magnetic, electric and magnetocapacitive properties of nanocrystalline Ga1−xMgxFeO3 (GMFO, 0 ≤ x ≤ 0.10) prepared by a modified Pechini method. The presence of a single crystalline phase identified as the orthorhombic structure with Pc21n space in all the GMFO samples. Magnetization measurements reveal that with increasing Mg-content up to 0.10, the ferrimagnetic transition temperature (TC) increases from 302 to 325 K. It is also found that the Mg-doped GaFeO3 (GFO) samples exhibit the characteristics of ferroelectricity at room temperature. Furthermore, Mg substitution in GFO brings in the balance of structural distortion, cation distribution, and magnetic exchange interaction, which affects both ferrimagnetism and ferroelectricity. The simultaneous enhancement of magnetization, electric polarization and magnetocapacitance of Mg-doped GFO samples are prominent candidates for the applications of multiferroic devices.

Electrical transport mechanisms and structure of hydrogenated and non-hydrogenated nanocrystalline Ga1−xMnxAs films

The mechanisms of electrical conductivity in hydrogenated and non-hydrogenated nanocrystalline Ga1−xMnxAs (0.000⩽x⩽0.081) films were analyzed, first from a macroscopic perspective, followed by microscopic analysis to investigate the energy levels for trapping electric charges. The analysis of the current–voltage and resistivity–temperature characteristics allowed the development of a model based on the morphology and structure of the films. This model takes into account the main aspects of the transport above 300K. Space charge limited current (SCLC) mechanism was observed in Mn-free films and is associated with deep trap states located at 0.10 and 0.22eV below the conduction band. In samples containing Mn, the dark conductivity is highly dependent on the presence of hydrogen. This effect was related to the grain boundaries and interstitial regions of the films, in which the density of gap states is expected to be reduced by the presence of hydrogen.

Synthesis of nanocrystalline Ga–TiO2 powders by mild hydrothermal method and their visible light photoactivity

In this work, nano-sized gallium doped titanium dioxide (Ga–TiO2) powders with different dopant concentrations (0.1, 0.6 and 1.2mol%) were synthesized by a mild hydrothermal method dwelling at 150°C for 10h without any post heat treatment. The synthesized samples were characterized by X-ray diffraction (XRD), Brunauer–Emment–Teller (BET) specific surface area, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–visible light diffuse-reflection spectra (DRS). The photocatalytic activities were evaluated via photocatalytic decolorization of methyl orange under 420–780nm visible light irradiation. Due to the small particle size, large specific surface area, improved visible light response and enhanced separation of photo-excited carriers, Ga–TiO2 photocatalysts exhibit better photocatalytic activity than that of Degussa P25 or un-doped TiO2, which prepared under the same processing conditions. Among all the Ga doped TiO2 photocatalysts, 0.6mol% Ga–TiO2 shows the best photo-decolorization efficiency reaching up to 82%.

Structure and Magnetic Properties of Ga Substituted Ni-Ferrites

Nanocrystalline Ga doped nickel ferrite [(NiFe2-xGaxO4 (x=0.0, 0.1, 0.3, 0.5 and 0.7)] powders have been synthesized by sol-gel auto-ignition method and the effect of non-magnetic gadillum content on the nanosize particles and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with spinel structure. The calculated grain size from XRD data have been verified using transmission electron microscopy (TEM). TEM photograph shows that the powders consist of nanometer sized grain. The size of nanoparticles decreases as the non magnetic Ga content increases. Magnetic hysteresis loops were measured at room temperature with maximum applied magnetic field of 20 KOe. As Ga content increases, the measured magnetic hysteresis curves became border and saturation magnetization (MS) increased up to x= 0.3 and further increase of x leads the magnetization to decrease. The results are explained according to the assumed cation distribution.

Cation distribution and dielectric properties of nanocrystalline gallium substituted nickel ferrite

Nano crystalline NiFe2−xGaxO4, 0.0 ⩽ x ⩽ 1 ferrite was prepared by citrate method. The structural and micro structural evolutions of the nanophase have been studied using X-ray powder diffraction and the Rietveld method. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is partially an inverse spinel. Furthermore, the Mössbauer spectra, measured at 20 K of all the samples show complete magnetic behavior, whereas at room temperature, are combination of ferromagnetic and superparamagnetic components indicating the particle size dependency. The inversion of Ni ions in a sample is affected by the percentage of Ga ions present, consistent with the results of X-ray data analysis. The a.c conductivity was measured in the temperature range 77–493 K and frequency range 50–100 kHz. The dielectric constant increase with increase Ga amount. The a.c conductivity results showed that nanocrystalline Ga doped NiFe2O4 ferrite samples follow the correlated barrier hopping (CBH) model. The activation energy increases from1.2 to 1.8 eV as the amount of Ga increases.

The important role of Mn 3+ in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga 1− x Mn x N) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga 1− x Mn x N films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn 3+ due to the thermal treatment. Mn 3+, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn 3+ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn 3+ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn x Ga y formation.

Columnar microstructure of nanocrystalline Ga 1− xMn xN films deposited by reactive sputtering

Ga 1− x Mn x N (0≤ x≤0.18) films grown onto amorphous silica substrate by reactive sputtering are characterised by high resolution transmission electron microscopy, energy dispersive spectroscopy, and energy filtered transmission electron microscopy. The electron transmission images and the electron diffraction patterns evidence the presence, at the substrate–film interface, of a few tens of nm thick intermediate layer with a high density of non-oriented nanocrystals (NCs). This intermediate layer represents the nucleation site for the subsequent growth of a compact Ga 1− x Mn x N columnar nanostructure, whose thickness (600–900 nm) is only limited by the deposition time. The columnar region shows a fibre texture with the c axis of the wurtzite nanocrystals corresponding to the column axis, both disposed perpendicular to the film surface. The thickness of the initial NC-rich layer and the coalescence of the nanocolumns are found to have a systematic dependence on the Mn concentration. No evidence of Mn segregation or of Mn rich phases is observed even for the samples with the highest Mn concentration. The correlation between the observed film microstructure and the reactive sputtering deposition parameters is discussed.

Influence of temperature and illumination on the characteristics of nanocrystalline Ga 0.29 Al 0.71As based heterojunction prepared by MOCVD

The heterojunction between nanocrystalline n-Ga 0.29 Al 0.71As and p-GaAs was fabricated by using metal organic chemical vapor deposition, MOCVD. The elemental composition of the prepared film was confirmed by energy dispersive X-ray (EDX) spectroscopy. The morphology and crystal structure of the film were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. This work explores the electrical properties of nanocrystalline n-Ga 0.29 Al 0.71As/p-GaAs heterojunction (HJ) in the temperature range 300–400 K. The n-Ga 0.29 Al 0.71As/p-GaAs heterojunction exhibits high rectifying behavior with a low saturation current density. While increasing temperature, the saturation current of the junction is increased and however, its series resistance decreased. At all temperatures the junction exhibited three types of transport mechanisms namely recombination, diffusion-limited thermionic emission, and space-charge-limited current mechanism, respectively depending on the applied bias-voltage. While increasing temperature, the diffusion potential of the barrier decreased linearly and the temperature sensitivity coefficient of the junction is found to be 5.3 mV/K. The C– V characteristics were measured at different testing signal frequencies. Their frequency dependence is related to the influence of a HJ series resistance on these characteristics. The plot of 1/ C 2 vs. the applied bias voltage behavior is linear, indicating the presence of abrupt junction. The effect of temperature and illumination on capacitance–voltage ( C–V) characteristics of n-Ga 0.29 Al 0.71As/p-GaAs heterojunction was investigated. The J– V characteristics of n-Ga 0.29 Al 0.71As/p-GaAs heterojunction device were studied under illumination of 100 mW/cm 2 at different temperature in the range 300–400 K. It can be observed that the open circuit voltage, V oc decrease continuously with increase in temperature, while J sc increases with increase in temperature. The temperature coefficient dV oc/ dT and dJ sc/ dT were obtained. The photovoltaic parameters are also studied under effect of different illumination intensities.

Disorder effects produced by the Mn and H incorporations on the optical absorption edge of Ga1−xMnxAs:H nanocrystalline films

Abstract The optical absorption edges of nanocrystalline Ga 1− x Mn x As:H films (0.000 ⩽ x ⩽ 0.081) prepared by sputtering were analyzed. The influence of Mn and hydrogen incorporations were both investigated. The energy dispersive X-ray spectroscopy and X-ray diffraction measurements show that the films are nanocrystalline and do not display any evidence of Mn segregation, or of any other secondary phase formation. The transmittance measurements in the ultraviolet–visible–near infrared range allow us to calculate the absorption coefficient, the optical gap, and the Urbach energy. The hydrogenated Ga 1− x Mn x As films presented wider gaps and smaller Urbach energies than its non-hydrogenated counterparts. In the hydrogenated films a linear correlation was observed between the decrease of the optical gap and the increase of the Urbach energy, which we have attributed to potential fluctuations and disorder induced by the Mn incorporation.

Structural and vibrational analysis of nanocrystalline Ga1−xMnxN films deposited by reactive magnetron sputtering

The structural and vibrational properties of nanocrystalline Ga1−xMnxN films deposited by reactive magnetron sputtering were analyzed in a wide composition range (0<x<0.18). The films were structurally characterized using x-ray diffraction with Rietveld refinement. The corresponding vibrational properties were investigated using micro-Raman and Fourier transform infrared spectroscopies. The films present a high crystallized fraction, crystallites having wurtzite structure, and high orientation texture with the c axis oriented perpendicular to the substrate surface. Rietveld analysis indicates that Mn atoms are incorporated substitutionally into Ga positions and show that the ionic character of cation-N bonds along the c axis is favored by the Mn incorporation. No evidence for Mn segregation or Mn rich phases was found in the composition range analyzed. Micro-Raman scattering spectra and infrared absorption experiments showed progressive changes with the increase of x and monotonic shifts of the GaN TO and LO peaks to lower frequencies. The structural and vibrational analyses are compared and the influence of Mn on the static and dynamic properties of the lattice is analyzed.

Nanocrystalline Ga 1−xMn xN films grown by reactive sputtering

The growth of nanocrystalline Ga 1− x Mn x N (0.00⩽ x⩽0.18) films grown by reactive RF-magnetron sputtering is focused here for the first time. The films were grown in a N 2 atmosphere by co-sputtering technique using a Ga target covered with small pieces of Mn onto c-GaAs (1 0 0), c-Si (1 0 0) and amorphous SiO 2 substrates maintained at 500 K. Scanning electron microscopy and X-ray diffraction (XRD) experiments did not show any evidence for Mn segregation within the studied composition range. EDX measurements show that the Mn concentration is increased monotonically with the fraction of the target area covered by Mn. The XRD characterization show that the films are nanocrystalline, the crystallites having mean grain sizes in the 15–19 nm range and wurtzite structure with preferential growth orientation along the c-axis direction. The lattice parameters of α-GaN ( a and c) increase practically linearly with the increase of Mn incorporation. The changes in the structural properties of our films due to the Mn incorporation are similar to those that occur in ferromagnetic GaMnN single-crystal films.

A green hydrothermal route to GaOOH nanorods

GaOOH nanorods were synthesized by a green hydrothermal method at 200 °C using nanocrystalline Ga 2O 3 powders and distilled water as the starting materials, and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and thermogravimetric and differential scanning calorimetry (TG-DSC) analysis.

Novel nanocrystalline Ga–Al–Zn complex oxide: catalyst for simultaneous treatment of NPAC and lean NO x

Nanocrystalline Ga–Al–Zn complex-oxide (designated here as nano-GAZ) has been successfully synthesized using hydrothermal method. Using TEM, BET, FTIR, DTA–TGA, and XRD, synthesized nano-GAZ particles are shown as high-surface-area nanoparticles comprising typical spinel crystal structures. The nano-GAZ catalyst was tested as a new deNPAC catalyst, using pyridine as the model NPAC compound. The reaction results show that pyridine could be completely oxidized over nano-GAZ catalyst above 400 °C. Furthermore, NO formation was significantly decreased due to the excellent performance of the nano-GAZ catalyst for the selective catalytic reduction of NO x with hydrocarbon in the presence of excess oxygen. An optimal reaction temperature at around 440–500 °C was observed to achieve complete catalytic oxidation of pyridine with the lowest NO x formation. These results demonstrate a promising approach for dual-mitigation of NPAC and NO x pollutants using nano-GAZ catalyst.

Optical properties of nanocrystalline Ga 1− xIn xSb/SiO 2 films

Ga 1− x In x Sb ( x=0.19, 0.38, 0.63) nanoparticles embedded in a SiO 2 matrix were grown on the glass substrates by radio-frequency magnetron co-sputtering. X-ray diffraction patterns strongly support the existence of nanocrystalline Ga 1− x In x Sb in the SiO 2 matrix. The changes in binding energies with Ga 1− x In x Sb nanocrystals deposition have been directly observed by X-ray photoemission spectroscopy, and these show the existence of Ga 1− x In x Sb nanocrystals in the SiO 2 matrix. Room-temperature Raman spectra show that the Raman peaks of the Ga 1− x In x Sb–SiO 2 composite film have a larger red-shift of about 95.3 cm −1 (longitudinal-optical mode) and 120.1 cm −1 (transverse-optical mode) than that of bulk GaSb, suggesting the existence of phonon confinement and tensile stress effects. Additionally, the room-temperature optical transmission data exhibit a large blue-shift with respect to that of the bulk semiconductor due to the strong quantum confinement effect.

Growth and optical properties of nanocrystalline Ga 0.81In 0.19Sb embedded in silica film

Nanocrystalline Ga 0.81In 0.19Sb embedded in silica film was grown by radio frequency (RF) magnetron co-sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) strongly support the existence of nanocrystalline Ga 0.81In 0.19Sb in the silica film. X-ray photoelectron spectroscopy further shows that the strong affinity of Si with oxygen produces a silicon dioxide layer wrapping the nanoparticles of the Ga 0.62In 0.38Sb. The room temperature optical transmission spectra show that the absorption edge exhibits a large blue shift of about 3.38 eV compared with that of the bulk semiconductor, suggesting the existence a of quantum confinement effect.

Micromagnetic analysis of nanocrystalline Ga- and Nb/Mo-doped NdFeB permanent magnets

Nanocrystalline as well as microcrystalline melt-spun ribbons of NdFeB alloys with small additions of Ga and Nb or Mo were prepared using the single-roller technique. Average grain sizes of 25 nm up to 500 nm were obtained by varying the wheel speed and the parameters of the heat treatment after rapid quenching. Systematic investigations of the relationship between microstructural effects and magnetic properties were carried out according to micromagnetic concepts. The theory of nucleation in single-domain particles is able to explain the magnetization reversal process, if several deteriorating effects of the microstructure are taken into account. The additives Ga and Nb/Mo both improve the coercive field. Ga is known to form new intergranular phases leading to a better decoupling of the grains. High-melting Nb/Mo-containing precipitates act as nucleation centers. Ga- and Nb/Mo-doped-samples show therefore small magnetization reversal intervals and a high reversibility of the first part of the demagnetization curve. Furthermore the concept of microstructural parameters α K and N eff is applied to an extensive investigation of the temperature dependence of coercivity. For different microstructures it turns out that α K reaches saturation as required by theory whereas a wide spectrum of N eff is found. Nevertheless a strong correlation of both α K and N eff restricts the improvement of the coercive field. Best critical fields at room-temperature up to 2.5 T have been obtained for annealed Ga-and Nb-doped samples with a fine-grained nanocrystalline and uniform microstructure.