Papers by Keyword: Lattice Strain

Abstract: Multi-step high energy ion implantation enables uniform doping to depths up to 12 µm in 4H-SiC epiwafers for superjunction devices but extent of lattice damage is of significant concern for device fabrication. 4H-SiC wafers with 12 µm thick epilayers implanted with Al ions at a concentration of 5 x 10¹⁶ cm^-3 using the Tandem Van de Graaff accelerator at Brookhaven National Laboratory across an energy range of 13.8 to 65.7 MeV and at different temperatures: room temperature, 300 °C, and 600 °C were analyzed by reciprocal space mapping measurements. Implanted layers exhibited tensile strains that decreased with increasing implantation temperature indicating dynamic annealing effect reduces lattice damage. On annealing at 1700 °C, for recovery of the lattice strain, RSM measurements show that the highest implantation temperatures have the lowest residual strains. In addition, the use of a Silicon Energy-Filter for Ion Implantation (EFII) for Al implantation in a single stage to the depth of 15µm is found to further reduce lattice strain compared to wafers implanted without EFII to the same doping concentration. This preliminary study will assist in optimizing the implantation and annealing conditions for the development of superjunction devices.

Abstract: 4H-SiC wafers with 12 µm epilayers were blanket implanted to a depth of 12 µm with 5 x 10¹⁶ cm^-3 Al ions via Tandem Van de Graaff accelerator located at Brookhaven National Laboratory with energy range of 13.8 to 65.7 MeV at room temperature, 300 °C and 600 °C. High resolution X-ray diffraction measurements reveal the implanted layers are characterized by tensile strains. However, the dynamic annealing process reduces the level of tensile strains as the temperature of implantation is increased. Analysis indicates that the implant temperature of 600 °C is not sufficient to minimize lattice damage due to implantation and a higher implantation temperature will be required. This preliminary experiment will guide the optimization of implantation conditions for fabricating superjunction devices.

Abstract: A novel high energy implantation system has been successfully developed to fabricate 4H-SiC superjunction devices for medium and high voltages via implantation of dopant atoms with multi-energy ranging from 13 to 66 MeV to depths up to 12um. Since the level of energies used is significantly higher than those employed for conventional implantation, lattice damage caused by such implantation must be characterized in detail to enhance the understanding of the nature of the damage. In regard to this, by employing the novel high energy system, 4H-SiC wafers with 12μm thick epilayers were blanket implanted by Al atoms at energies ranging from 13.8MeV to 65.7MeV and N atoms at energies ranging up to 42.99MeV. The lattice damages induced by the implantation were primarily characterized by Synchrotron X-ray Plane Wave Topography (SXPWT). 0008 topographs recorded from the samples are characterized by an intensity profile consisting of multiple asymmetric diffraction peaks with an angular separation of only 2” (arcseconds). Using Rocking-curve Analysis by Dynamical Simulation (RADS) program, diffracted intensity profile was used to extract the corresponding strain profile indicating an inhomogeneous strain distribution across the depth of the implanted layer.

Abstract: 4H-SiC wafers with 12 um epilayer were implanted at the Tandem Van de Graaff facility at Brookhaven National Laboratory with tunable energy from 13 MeV up to 66 MeV. Lattice strains introduced by the implantation process were characterized in detail by synchrotron rocking curve X-ray topography (SXRCT) and reciprocal space maps (RSMs). It is observed that the strain levels correlate with the atomic mass and energy of acceleration of the dopant atoms.

Abstract: Variations in nitrogen doping concentration across the PVT-grown 4H-SiC wafers can induce significant lattice strain, leading to degradation on the performance of SiC-based power devices. A qualitative study on the lattice strain variation in facet and off-facet regions was carried out for 4H-SiC substrates. The lattice strain maps for 4H-SiC wafers were derived from the 11-20, 1-100 (transmission geometry) and 0008 (reflection geometry) using synchrotron double crystal contour mapping method. Results show that the lattice strain within the basal plane is isotropic, while along [0001] direction, lattice strain is one order of magnitude lower, indicating elastic anisotropy of strain due to doping in 4H-SiC crystals. The distribution of lattice strain inside the facet region is more uniform than in the off-facet region. Measurement of nitrogen doping concentration by Hall effect measurements shows over 45% of difference in doping level between heavily doped facet region and off-facet region. Existence of significant lattice distortion in facet region of 4H-SiC substrates was further confirmed by X-ray rocking curve measurements.

Abstract: Polycrystalline La_0.67Sr_0.33MnO₃ (LSMO) powder prepared via conventional solid state reaction was pressed into pellet form. The pellets became target to growth thin films on corning glass (LSMO-C), fused silica (LSMO-FS) and MgO (100) (LSMO-M) substrate via pulsed laser deposition (PLD) method. XRD results showed that all samples were hexagonal structure with R-3C space group. Thin films showed relatively smaller crystallite size compared to bulk samples. From Rietveld Refinement analysis, all thin films experienced lattice strain when deposited on different substrate. LSMO compound deposited in different substrate induced structure distortion and lattice strain. Compression along c-axis occurred when the lattice strain increased thus shifted the metal-insulator transition temperature to lower temperature and increased its resistivity.

Abstract: Aluminum doped zinc oxide (ZnO:Al) thin films were deposited on corning glass substrates using DC magnetron sputtering at various growth temperatures (27°C-400°C). X-rays diffraction spectroscopy (XRD) analysis showed the crystal structure of ZnO:Al thin films is wurtzite with c-axis orientation. By enhancing the growth temperature, the crystal size and the crystal stress are increase, while the resistivity of films decreases. Crystal size increase from 35 nm to 52 nm, the stress increase from -7.689 GPa to -5.126 GPa, while the resistivity decreases from 6.29 x 10⁴ Ωcm to 4.05 x 10³ Ωcm. Generally, the quality of crystal enhanced as the raising of growth temperature.

Abstract: ZnO, with direct wide band gap of 3.37 eV and high excitonic binding energy of 60 meV has been attracting much attention due to its wide range of applications, for transparent electronics, solar cells, and other optoelectronics device. We present a simple, single step process to produce ZnO nanotrees using co-precipitation method. As a precursor, zinc nitrate dehydrate was stabilized by hexamethylene tetraamine (HMTA) and 3-9 mM polyethylene glycol (PEG) was added at 180°C for 3-6 hours followed by residual polymer removal. Scanning Electron Microscopy revealed the typical rod-like branched nanostructures were achieved. For longer annealing time the PEG-assisted growth process indeed exhibited a distinctive c-direction inhibition responsible for the lateral growth and subsequent branching of ZnO, in which the branch growth in sample with PEG amount of 0.05 g is the slowest. Some amounts of PEG up to 0.03 g are sensitive to affect several parameters, such as, lattice stress, unit cell volume, density of film and dislocation density.

Abstract: Silicon carbide (SiC) is an important material that has the potential to be used in the nuclear industry. The surface of 0001 silicon carbide, 4H polytype wafer has been irradiated by electron accelerator at the accelerator voltage of 3 MeV and 10 mA beam current. Samples were irradiated to dose received of 1000 kGy, 1500 kGy and 2000 kGy. Characterization of the samples have been done using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the electron irradiation effect on structure as well as current-voltage tests to study electrical property.

Abstract: Tensile properties of ferrite lamella in pearlite under lattice strain are examined by a strain gradient crystal plasticity analysis. Tensile direction is made to be parallel to the lamella. Obtained results of macroscopic stress-strain relation of the lamella show significant increase of yield stress and strain hardening rate with the reduction of the lamella thickness and further increase of the yield stress with positive normal lattice strain parallel to the tensile direction in the ferrite layer. Whereas normal lattice strain perpendicular to the tensile direction contributes little to the tensile properties.