GaN Nanocrystals Research Articles

Size Shrinkage of GaN Nanocrystalline Grains Induced by Eu Doping

Undoped nanocrystalline GaN grains with the size changing from ∼ 11 to ∼31 nm have been obtained by the combustion method, where grain size was controlled by the nitridation temperature changing from 850 to 1050°C. The same procedure has been applied in order to obtain Eu-doped GaN powders. It has been observed that the introduction of Eu ions reduces the size of GaN grains by 30 ± 10%. In photoluminescence spectra, in addition to the Eu-related emission, strong UV bands (associated with the GaN bandgap-related emission) and VIS bands (associated with the GaN defect/surface-related emission) have been clearly observed. After the Eu-ion incorporation into GaN nanocrystals, the relative intensity of UV and VIS bands changes in favor of VIS emission because of the Eu-induced size shrinkage, i.e., the surface-to-volume ratio increase.

Synthesis and optical properties of In-doped GaN nanocrystals

Indium-doped GaN nanocrystals with 5% and 10% In have been prepared by a low temperature solvothermal method using hexamethyldisilazane as the nitriding reagent. The nanocrystals show Raman bands at lower frequencies compared to GaN. Photoluminescence spectra of the In-doped GaN nanocrystals exhibit an increase in the FWHM with the decrease in the PL band energy, the band energy itself decreasing with increase in the In content.

Synthesis of nanocrystalline GaN by the sol–gel method

Single-phase wurtzite GaN nanocrystals with an average diameter of 11 ± 3 nm were synthesized by the sol–gel technique from readily available Ga(NO 3) 3. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) confirmed they had a hexagonal structure and a narrow size distribution of the nanocrystals. X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurement showed that the GaN powder was of single-phase wurtzite structure with a considerable fraction of structural defects such as twin and stacking faults. The IR spectrum showed that only the Ga–N stretch is present at 600 cm −1. The EDX pattern of as-prepared product showed their ratio approximate to 1:1. Room temperature photoluminescence (PL) measurement exhibited the band-edge emission of GaN at about 390 nm and defect emission peak at 610 nm.

Ferromagnetism in Mn-doped GaN nanocrystals prepared solvothermally at low temperatures

3% and 5% Mn-doped GaN nanocrystals of different sizes, with the average diameters in the range of 4–18nm, have been prepared by two independent routes under solvothermal conditions starting with two different precursors. The reaction temperature was around 350°C in all the preparations. The nanocrystals so prepared exhibit ferromagnetism with magnetization (M) and Curie temperature (TC) values increasing with percent of Mn and particle size. The observation of ferromagnetism in Mn-doped GaN nanocrystals prepared at relatively low temperatures is of significance in understanding this potential in spintronics materials.

TEM study of indium and gallium nitride nanocrystals in silica gasses obtained by the sol‐gel method

GaN and InN nanocrystals in silica glasses prepared by the sol-gel method were studied by transmission electron microscopy techniques. Morphology, structure and phase composition of silica gel containing Ga or In as function of the calcination and nitridation temperature were investigated.

Optical properties of GaN nanocrystals embedded into silica matrices

Abstract The purpose of this work was to obtain GaN nanocrystals (GaN-nc) embedded into silica-based matrix and to investigate their optical properties by photoluminescence (PL) spectroscopy. GaN-nc have been obtained both by the sol–gel chemistry and by the combustion method (CM). The GaN-nc obtained by CM have been introduced into the silica-based matrix during the formation of the film in the spin-on technique. Strong emission at ∼3.4 eV has been observed for the films doped by GaN-nc whereas no emission in UV has been observed for GaN-nc obtained by sol–gel chemistry.

Three-dimensional GaN nano-structure fabrication by focused ion beam chemical vapor deposition

We have demonstrated a position controlled GaN nano-structure fabrication by ion beam mediated growth methods. For this fabrication, we used two different methods. One is the “droplet epitaxy” which is a micro-crystal growth from Ga droplet followed by nitridation. By this method, the GaN micro-crystals in about 800nm were found to be grown and cathode luminescence in broad wavelength (400–600nm) were observed. Another one is the focused ion beam assisted chemical vapor deposition. Ga precursor gas and atomic N radicals impinged onto the surface simultaneously during the irradiation. By this method, the GaN nano-crystals of 200nm×100nm block with 50nm height was fabricated and strong near-band edge emission at 3.37eV from GaN were observed. From the obtained results, ion beam assisted chemical vapor deposition proved to be a promising method to make GaN nano-crystal arrays.

Optical properties of GaN synthesized by implantation of nitrogen ions into GaAs

Implantation of N ions with doses of 3 × 1 0 16 and 3 × 1 0 17 cm - 2 and energies of 100 keV into semiinsulating (1 0 0) GaAs substrates has been performed in order to synthesize the GaN compound. The samples have been annealed in a furnace at 750 °C for 30 min in N gas flow or under GaAs powder and then characterized by 300 and 77 K cathodoluminescence, infrared reflection, Raman spectroscopy and secondary ion mass spectroscopy measurements. All optical techniques have detected the presence of GaN phase in the samples. No noticeable dependence of CL spectra on annealing conditions leading to different As and N losses from the surface has been observed. SIMS measurements have been performed to show N atom redistribution after annealing. The results of the study are consistent with the presence of buried layers with differently oriented GaN nanocrystals.

Radiative Recombination in GaN Nanocrystals at High Intensities of Optical Excitation

The spectra of spontaneous UV luminescence of GaN nanocrystals excited by optical pumping at power densities ranging from 50 W/cm2 to 50 MW/cm2 are studied. At room temperature, radiation peaks related to the emission of free excitons and recombination of electron-hole plasma are revealed. The spectral characteristics of the emission of the electron-hole plasma in GaN is studied in the wide temperature range of 77–550 K.

Fast High-Density Low-Pressure Plasma Synthesis of GaN Nanocrystals

AbstractGallium Nitride is of interest due to its direct bandgap, which allows for efficient emission in the near-UV range. Bulk GaN is already in use in solid-state devices that exploit its emissive properties, however, the promise of GaN nanocrystals as tunable emitters for use in light-emitting devices and lasers has led to the recent exploration of nanocrystalline GaN synthesis routes. Here we discuss the use of nonthermal plasmas for the synthesis of nanocrystalline pow-ders of GaN. The particles were examined using transmission electron microscopy and x-ray photoelectron spectroscopy.

Size- and shape-controlled GaN nanocrystals grown on Si(111) substrate by reactive epitaxy

We report the investigation of coherent GaN nanocrystals spontaneously formed by nitrogen-plasma-assisted reactive epitaxy with Ga droplets supported on the single crystalline Si3N4(0001)∕Si(111) surface. The distribution of grown GaN nanocrystals, as revealed by scanning electron microscopy and cross-sectional transmission electron microscopy, is very uniform in size (∼16nm) and shape and the distribution width is significantly narrower than that of Ga droplets deposited in the Volmer–Weber mode. By using high-resolution electron microscopy, the shape and crystalline structure of the self-assembled GaN nanocrystals can be determined to be truncated triangular pyramids formed by the facets of the GaN wurtzite lattice.

Raman scattering from LO–phonon–plasmon coupled modes in Ag-coated GaN nanocrystals

Relationships are studied governing the Raman scattering spectra in GaN nanocrystals grown by HVPE on substrates of oxidized silicon with Ag atoms deposited in a UHV system. It is found that the intensity of an LO–phonon–plasmon coupled mode in GaN nanocrystals is increasing initially as metal atoms are deposited on their surface to a thickness of 3 nm and then decreasing. It is suggested that the intensity increase of this mode is due to increasing near-surface barrier height and the space-charge layer width.

Hollow boron nitride (BN) nanocages and BN-nanocage-encapsulated nanocrystals.

Hollow boron nitride (BN) nanocages (nanospheres, image on the left) and BN-nanocage-encapsulated GaN nanocrystals (right) have been synthesized by using a homemade B-N-O precursors. The as-prepared BN hollow nanocages have typically spherical morphologies with diameters ranging from 30 to 200 nm. The nanocages have crystalline structures. Peanutlike nanocages with double walls have also been observed; their internal space is divided into seperated compartments by the internal walls. The method is extended to sheathe nanocrystals with BN nanocages; BN-shell/GaN-core nanostructures have been successfully fabriacted. The method may be generally applicable to the fabrication BN-sheathed nanocrystals.

New Solvothermal Routes for GaN Nanocrystals

GaN nanocrystals (see Figure) of hexagonal structure and varying diameters have been prepared by the reaction of hexamethyldisilazane (HMDS) with gallium cupferron or GaCl3 under solvothermal conditions. The nanocrystals show a size‐dependent photoluminescence band in the 260–320 nm region, characteristic of quantum confinement.

Formation of Cubic GaN Nanocrystals: A Study of the Thermal Stability of GaP Nanocrystals under Nitrogen

AbstractCubic GaN nanoparticles and nanorods have been successfully synthesized from the nitridation of nanocrystalline GaP at 500 °C for 30 min, by manipulating different heating rates in nitrogen. The results of the thermal stability studies of nanocrystalline GaP under N2 at different heating rates support the idea that cubic GaN can form from GaP via a N−P metathesis mechanism. The morphology of the cubic GaN can be controlled by variation of the external synthetic parameters such as the rate of heating. This work shows that it is very simple to synthesize cubic GaN nanoparticles or nanorods and opens a wide route towards detailed studies of the fundamental properties and potential applications of semiconductor nanocrystals. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Raman and infrared spectroscopy of GaN nanocrystals grown by chloride-hydride vapor-phase epitaxy on oxidized silicon

Raman and infrared spectroscopy were applied to study nanocrystalline GaN films grown by chloride-hydride vapor-phase epitaxy on SiO2/Si(111) substrates at T=520°C. It was ascertained that GaN nanocrystals are formed on the oxidized silicon surface at a rate of 10−2 nm/s. It was shown that the peaks in the Raman spectra E2(high)=566 cm−1 and A1(LO)=730 cm−1 correspond to the elastically strained GaN wurtzite structure. It was detected that a peak related to E1(TO)=558 cm−1 arises in the infrared spectra, which shows that elastic stresses in the nanocrystals are insignificant.

Towards controllable optical response of GaN quantum dots in alumina

GaN nanocrystals (in the wurtzite phase) have been synthesized by thermal annealing in reducing atmosphere of Ga++N+ sequentially implanted alumina. We show that the reduction of Ga local concentration (by lowering the implantation dose) yields to an overall increase in the intensity of the GaN photoluminescence signal, to a decrease of the PL bandwidth and to the shift of its onset towards higher energies, due to quantum confinement effects in GaN nanocrystals with a narrow size distribution. The interpretation of the optical results is supported by structural analyses. Moreover, by investigating different forming treatments for GaN synthesis, the key role of hydrogen in the annealing atmosphere is evidenced.

Red shifted-Photoluminescence of Ensembles of GaN Nano-Crystallites

AbstractWe present optical analysis concerning the redshift of the photoluminescence (PL) of ensembles of GaN nano-crystals. We found that the extent of the redshift depends on the laser power as well as on the ensemble size. For ensemble of size ∼ 30 μm, the laser power in our experimental specification impacted the PL energy and caused a redshift of up to 120 meV. This phenomena was not observed for a small ensemble of ∼ 1 νm or less. For the small ensemble the PL redshift was negligible and depended weakly on the laser power; similar behavior was found in GaN thin film. The above findings were observed in the PL of GaN nano-crystalline of Wurtzite as well as cubic structure. Our results point to a laser heating event occurring in the large ensemble; the emitted and scattered light is confined among the nano-crystallites thus causing the heating. For a small ensemble the light has a higher probability of diffusing outside the enclosure, and thus no laser heating occurs.

Growth of self-assembled nanosize InGaN/GaN multiple quantum wells embedded in amorphous SiN x by metalorganic vapor phase epitaxy

We report on the growth and photoluminescence of InGaN/GaN multiple quantum wells (MQWs) grown on GaN nanocrystals embedded in amorphous SiN x , grown on a p-type Si(1 0 0) substrate. p-Type Si(1 0 0) was thermally nitridated using NH 3 to form an amorphous SiN x layer in a metalorganic vapor phase epitaxy system. GaN nanocrystals were then grown on the SiN x layer, followed by the growth of InGaN/GaN MQWs on the GaN nanocrystals. These nanosize MQWs were capped with amorphous SiN x in a plasma enhanced chemical vapor deposition system. The findings show that nanosize InGaN/GaN MQWs can be embedded in amorphous SiN x grown on p-type Si(1 0 0) and that these self-assembled nanostructures may be used as new nanosize light-emitting sources as evidenced by photoluminescence from nanosize MQWs.

GaN nanocrystals from oxygen and nitrogen-based precursors

Wurtzite GaN nanocrystals were produced by the pyrolysis and crystallization of two different precursors, gallium dimethyl amide (GDA) and gallium isopropoxide (GIP), in an ammonia atmosphere between 600°C and 1200°C. Ammonia reacted with GDA at room temperature by transamination, followed by deamination of the amorphous precursor at higher temperatures. After crystallization to wurtzite GaN, grain growth occurred and was dominated by evaporation–condensation. Low-temperature pyrolysis of GIP in an ammonia atmosphere yielded gallium oxynitride nanocrystals. Higher processing temperatures transformed the nanocrystalline powders to wurtzite GaN. GaN powders exhibited the room temperature photoluminescence characteristic of wurtzite GaN. Nitrogen vacancies present in the powder appear to be potential candidates for the yellow luminescence.