No abstract

In this study, Chemical Mechanical Planarization (CMP) of (0001) GaN surface, using two different slurries containing colloidal silica and alumina as abrasive nanoparticles, has been reported. Effect of processing parameters, such as concentration of the oxidizer, downward pressure, head rpm, base rpm, pH of the slurries, and type and concentration of abrasive particles, on the material removal rate (MRR) and surface quality (roughness) have been studied in details. The maximum MRR has been found to be ∼39 nm/hr and ∼85 nm/hr for slurries containing SiO 2 and Al 2 O 3 abrasives, respectively, under 38 kPa pressure, 90 rpm base speed (100 rpm for Al 2 O 3 containing slurry), 30 rpm carrier speed, slurry pH 1 (2 for Al 2 O 3 containing slurry), 0.3 M Oxidizer concentration, and 3.75 wt% abrasive particle concentration. RMS surface roughness of 1.3 Å and 0.7 Å, over scanning area of 10 μm × 10 μm and 5 μm × 5 μm, respectively, has been achieved on polished Ga-faced GaN surface for SiO 2 containing slurry using optimized slurry chemistry and processing parameters.Gallium nitride and related iii-v nitride semiconductors are becoming increasingly popular as the candidate material for use in optoand high power and high frequency electronic devices because of their wide bandgaps, high carrier mobility, high saturated electron drift velocity and high breakdown field. 1,2 GaN, in particular, is suitable for variety of electronic and optoelectronic applications because of its unique capabilities of amplifying high-frequency RF signals without distortion, can withstand high temperatures, and emits blue and green lights. However, an important issue in development and commercialization of GaN based power or optoelectronics is the unavailability of suitable device grade (epi-ready) wafer. The ideal choice would be to use homoepitaxial substrate. But, due to difficulties in growth of bulk GaN wafer these substrates are not readily available. The most important challenge for further development of GaN based technology is the need to improve the quality of GaN films by reducing defects caused by the heteroepitaxial growth. GaN, most commonly, is grown heteroepitaxially on sapphire substrate. 1,3 Due to large difference in thermal expansion and lattice mismatch between sapphire and GaN, a large number of crystal defects and rough surface is generated during the growth process, which limits further application of the grown film. 4,5 It is essential to produce an atomically flat, smooth and defect free GaN surface to realize the full potential of this material. Conventional planarization techniques cannot be effectively applied to GaN surfaces because of its extreme mechanical hardness and chemical inertness toward many chemicals. Mechanical polishing using diamond abrasive produces scratches on the surface. Chemical mechanical planarization (CMP) using colloidal silica has been proven successful for preparing defect free GaN surfaces. Numerous reports are available on CMP of several other materials including Al 2 O 3 , 6 SiO 2 , 7 a...

Magnetic and magnetoelastic properties of Zn-doped cobalt-Magnetic and magnetoelastic properties of Zn-doped cobaltferrites-CoFe ferrites-CoFe 2−x 2−x Zn Zn x x O O 4 4 (x =

Nanocrystalline particulates of Er doped cobalt-ferrites CoFe(2−x)ErxO4 (0 ≤ x ≤ 0.04), were synthesized, using sol-gel assisted autocombustion method. Co-, Fe-, and Er- nitrates were the oxidizers, and malic acid served as a fuel and chelating agent. Calcination (400–600 °C for 4 h) of the precursor powders was followed by sintering (1000 °C for 4 h) and structural and magnetic characterization. X-ray diffraction confirmed the formation of single phase of spinel for the compositions x = 0, 0.01, and 0.02; and for higher compositions an additional orthoferrite phase formed along with the spinel phase. Lattice parameter of the doped cobalt-ferrites was higher than that of pure cobalt-ferrite. The observed red shift in the doped cobalt-ferrites indicates the presence of induced strain in the cobalt-ferrite matrix due to large size of the Er+3 compared to Fe+3. Greater than two-fold increase in coercivity (∼66 kA/m for x = 0.02) was observed in doped cobalt-ferrites compared to CoFe2O4 (∼29 kA/m).

Er-substituted cobalt-ferrites CoFe2−xErxO4 (0 ≤ x ≤ 0.04) were synthesized by sol-gel assisted auto-combustion method. The precursor powders were calcined at 673–873 K for 4 h, subsequently pressed into pellets and sintered at 1273 K for 4 h. X-ray diffraction (XRD) confirmed the presence of the spinel phase for all the compositions and, additional orthoferrite phase for higher compositions (x = 0.03 and 0.04). The XRD spectra and the Transmission Electron Microscopy micrographs indicate that the nanocrystalline particulates of the Er-substituted cobalt ferrites have crystallite size of ∼120–200 nm. The magnetization curves show an increase in saturation magnetization (MS) and coercivity (HC) for Er-substituted cobalt-ferrites at sub-ambient temperatures. MS for CoFe2O4, CoFe0.99Er0.01O4, CoFe0.98Er0.02O4, and CoFe0.97Er0.03O4 peak at 89.7 Am2/kg, 89.3 Am2/kg, 88.8 Am2/kg, and 87.1 Am2/kg, respectively, at a sub-ambient temperature of ∼150 K. HC substantially increases with decrease in temperature for all the compositions, while it peaks at x = 0.01−0.02 at all temperatures. The combination of Er content—x ∼ 0.02 and the temperature—∼5 K provides the maximum HC ∼ 984 kA/m. Er-substituted cobalt-ferrites have higher cubic anisotropy constant, K1, compared to pure cobalt-ferrite at ambient/sub-ambient temperatures. K1 gradually increases for all compositions in the temperature decreasing from 300 to 100 K. While K1 peaks at ∼150 K for pure cobalt-ferrite, it peaks at ∼50 K for CoFe0.99Er0.01O4, CoFe0.98Er0.02O4, and CoFe0.96Er0.04O4. The MS (∼88.7 Am2/kg), at 5 K, for Er substituted cobalt-ferrite is close to the highest values reported for Sm and Gd substituted cobalt-ferrites. The MS (∼83.5 Am2/kg) at 300 K for Er-substituted cobalt-ferrite is the highest among the lanthanide series element substituted cobalt-ferrites. The HC (at 5 K) for Er substituted cobalt-ferrite is close to the highest values observed for La, Ce, Nd, Sm, and Gd substituted cobalt-ferrites.