Search

We report on the optical characterization of a nitrogen plasma source based on radiofrequency (RF) used to grow III-nitride materials by molecular beam epitaxy (MBE). Optical emission spectroscopy (OES) was used to study the nitrogen plasma response as a function of the RF power applied and the flow rate of molecular nitrogen. Analysis of the intensities of spectral signals assigned to atomic and molecular species and the ratio of these intensities is performed in detail. The OES results show that the plasma source studied is sensitive to the RF power applied to produce an atomic nitrogen signal, while varying the incoming flow impacts the signal of metastable nitrogen molecules; this outcome allows for the determination of conditions under which certain types of nitrogen species are favored over others. InN films were grown on AlN-buffered Si(111) substrates by MBE under different plasma operational parameters, where, according to the OES studies, atomic nitrogen or excited molecular nitrogen is favored in the plasma. In situ reflection high-energy electron diffraction, scanning electron microscopy, and x-ray diffraction techniques were employed to characterize the InN samples. It is found that the surface morphology of the InN films is highly sensitive to the plasma conditions. A transition in the growth mode from smooth compact films to coalesced islands and columnar structures is observed when the dominant reactive species is atomic nitrogen or excited molecular nitrogen. The results of the characterization are discussed and correlated with the reactive nitrogen species present in the plasma. [ABSTRACT FROM AUTHOR]

Cubic GaN (c-GaN) samples on GaAs (0 0 1) substrates were grown by RF-plasma-assisted molecular beam epitaxy, in which an As4 overpressure was employed for the nucleating layer. Photoreflectance spectra were obtained in the temperature range from 14 to 300 K. Two independent phenomena were noticed. The first one consisted in optical interference features below the c-GaN bandgap, whose origin is a thermo-optical effect: the ultraviolet perturbation beam changes the refractive index of the c-GaN. The second one represents electro-optical phenomena in which two classical band-to-band transitions occur: the first transition for c-GaN layer in which the temperature dependence reveals defects in the film attributed to a hexagonal fraction estimated previously between 3% and 10%, and a second transition for the GaAs substrate that shows Franz–Keldysh oscillations. [ABSTRACT FROM AUTHOR]

We report the growth of In0.145Ga0.855As0.132Sb0.868 layers on GaSb(100) substrates by the liquid phase epitaxy (LPE) technique using the ramp-cooling method. We achieved a near-lattice-matched epitaxial growth with a lattice mismatch of Δa/a = -4.1 x 10-4 between the quaternary layer and GaSb(100) substrate due to optimal growth parameters. Aberration-corrected scanning transmission electron microscope (AC-STEM) confirmed the high crystalline quality of the quaternary layer and the low lattice mismatch in the heterostructure, without the presence of linear or planar defects. Also, the Secondary Ion Mass Spectrometry (SIMS) technique evidenced a uniform distribution of the atomic elements along the quaternary layer and an abrupt interface between the In0.145Ga0.855As0.132Sb0.868 layer and the GaSb substrate. Plasmonphonon interactions were observed by Raman spectroscopy indicating that the crystalline quality increases at greater depth in the sample with respect to the surface. The quaternary layer presented a uniform and flat morphology, and luminescence emission attributed to the recombination of bound exciton states at 641 meV. The structural, chemical, and optical properties of the In0.145Ga0.855As0.132Sb0.868 layer demonstrated that it could be auspicious material for infrared range optoelectronic applications. Likewise, the LPE technique successfully shows that it should be used to grow near-lattice-matched heterostructures. [ABSTRACT FROM AUTHOR]

InN epitaxial films with cubic phase were grown by rf-plasma-assisted molecular beam epitaxy (RF-MBE) on GaAs(001) substrates employing two methods: migration-enhanced epitaxy (MEE) and conventional MBE technique. The films were synthesized at different growth temperatures ranging from 490 to 550 °C, and different In beam fluxes (BEPIn) ranging from 5.9 × 10-7 to 9.7 × 10-7 Torr. We found the optimum conditions for the nucleation of the cubic phase of the InN using a buffer composed of several thin layers, according to reflection high-energy electron diffraction (RHEED) patterns. Crystallographic analysis by high resolution X-ray diffraction (HR-XRD) and RHEED confirmed the growth of c-InN by the two methods. We achieved with the MEE method a higher crystal quality and higher cubic phase purity. The ratio of cubic to hexagonal components in InN films was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) and hexagonal ([formula]) planes measured by X-ray reciprocal space mapping (RSM). For MEE samples, the cubic phase of InN increases employing higher In beam fluxes and higher growth temperatures. We have obtained a cubic purity phase of 96.4% for a film grown at 510 °C by MEE. [ABSTRACT FROM AUTHOR]

In this work we compared the optical properties of AlGaN(50 nm)/GaN(5 nm) quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy (PAMBE) technique, on Si(111) substrates and sapphire substrates. Optical properties were acquired by photoluminescence (PL) and reflectance (R) spectroscopies. Reflectance spectra shows interference oscillations of the reflected beam at Air/AlGaN and the beam reflected at bottom interface with Si substrate. Such oscillations vanish at the band gap of GaN buffer layer (3.4 eV). The PL spectra of samples grown on Silicon substrate shows a photoluminescence modulation effect, attributed to interference of light emitted from the QWs that is reflected at different heterostructure interfaces. On sapphire substrate no modulation effect is present. This could be explained due to interfacial roughness and the smaller refractive index of sapphire in comparison to silicon. PL shows a strong emission around 3 eV, which is in agreement with recombination energy determined by self-consistent calculations, which consider a 4 MV/cm built-in electric field and low carrier densities. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Nanoparticles of manganese antimonate (MnSb2O6) were prepared using the microwave-assisted colloidal method for its potential application as a gas sensor. For the synthesis of the oxide, manganese nitrate, antimony chloride, ethylenediamine and ethyl alcohol (as a solvent) were used. The precursor material was calcined at 800 °C in air and analyzed by X-ray diffraction. The oxide crystallized into a hexagonal structure with spatial group P321 and cell parameters a = b = 8.8054 Å and c = 4.7229 Å. The microstructure of the material was analyzed by scanning electron microscopy (SEM), finding the growth of microrods with a size of around ~10.27 μm and some other particles with an average size of ~1.3 μm. Photoacoustic spectroscopy (PAS) studies showed that the optical energy band (Eg) of the oxide was of ~1.79 eV. Transmission electron microscopy (TEM) analyses indicated that the size of the nanoparticles was of ~29.5 nm on average. The surface area of the powders was estimated at 14.6 m2/g by the Brunauer–Emmett–Teller (BET) method. Pellets prepared from the nanoparticles were tested in carbon monoxide (CO) and propane (C3H8) atmospheres at different concentrations (0–500 ppm) and operating temperatures (100, 200 and 300 °C). The pellets were very sensitive to changes in gas concentration and temperature: the response of the material rose as the concentration and temperature increased. The results showed that the MnSb2O6 nanoparticles can be a good candidate to be used as a novel gas sensor. [ABSTRACT FROM AUTHOR]