Doping Dependence of Thermal Oxidation on n-type 4H-SiC

The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was investigated. The oxidation was performed in the temperature range 1000C to 1200C for samples with nitrogen doping in the range of 6.5e15/cm3 to 9.3e18/cm3, showing a clear doping dependence. Samples with higher doping concentrations displayed higher oxidation rates. The results were interpreted using a modified Deal-Grove model. Linear and parabolic rate constants and activation energies were extracted. Increasing nitrogen led to an increase in linear rate constant pre-exponential factor from 10-6m/s to 10-2m/s and the parabolic rate constant pre-exponential factor from 10e9m2/s to 10e6m2/s. The increase in linear rate constant was attributed to defects from doping-induced lattice mismatch, which tend to be more reactive than bulk crystal regions. The increase in the diffusion-limited parabolic rate constant was attributed to degradation in oxide quality originating from the doping-induced lattice mismatch. This degradation was confirmed by the observation of a decrease in optical density of the grown oxide films from 1.4 to 1.24. The linear activation energy varied from 1.6eV to 2.8eV, while the parabolic activation energy varied from 2.7eV to 3.3eV, increasing with doping concentration. These increased activation energies were attributed to higher nitrogen content, leading to an increase in effective bond energy stemming from the difference in C-Si (2.82eV) and Si-N (4.26eV) binding energies. This work provides crucial information in the engineering of SiO2 dielectrics for SiC MOS structures, which typically involve regions of very different doping concentrations, and suggests that thermal oxidation at high doping concentrations in SiC may be defect mediated.Comment: 13 pages. 9 figures, accepted as a transiction in IEEE electron device. TED MS#8035

Magnetotransport properties of a magnetically modulated two-dimensional electron gas with the spin-orbit interaction

We study the electrical transport properties of a two-dimensional electron gas with the Rashba spin-orbit interaction in presence of a constant perpendicular magnetic field $(B_0 \hat z)$ which is weakly modulated by ${\bf B_1} = B_1 \cos (q x) \hat z$, where $B_1 \ll B_0$ and $q = 2 \pi/a$ with $a$ is the modulation period. We obtain the analytical expressions of the diffusive conductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and produce beating patterns in the amplitude of the Weiss and Shubnikov-de Haas oscillations. We show that the Rashba strength can be determined by analyzing the beating pattern in the Weiss oscillation. We find a simple equation which determines the Rashba spin-orbit interaction strength if the number of Weiss oscillations between any two successive nodes is known from the experiment. We compare our results with the electrically modulated 2DEG with the Rashba interaction. For completeness, we also study the beating pattern formation in the collisional and the Hall conductivities.Comment: 11 pages, 5 figures, re-written with new result

Comparative Study on Priority Based QOS Aware Mac Protocols for WSN

n Wireless Sensor Network (WSN), QoS (Quality of S\ud ervice) in sensor application plays a very importan\ud t\ud role. QoS based routing is required to ensure the b\ud est use of nodes in WSN. In this paper, a comparati\ud ve\ud study of QoS based routing in Media Access Control\ud (MAC) protocols are presented based on the traits t\ud o\ud solve problems like prioritization, timeliness, rel\ud iability etc. The study mainly focuses on some prio\ud rity\ud based QoS protocols used in WSN and a comparison am\ud ong them. The study reveals that among the five\ud mentioned protocols; QMAC, PRIMA, DB-MAC, RAP, GTS;\ud PRIMA shows the best performance in the\ud category of Packet Prioritization, Scheduling Schem\ud e, Queue Type, Energy Awareness and Qo

Structural, elastic, and electronic properties of newly discovered Li2PtSi3 superconductor: Effect of transition metals

First-principles calculations within the density functional theory (DFT) with GGA-PBE exchange-correlation scheme have been employed to predict the structural, the elastic and the electronic properties of newly discovered lithium silicide superconductor, Li2PtSi3, for the first time. All the theoretical results are compared with those calculated recently for isostructural Li2IrSi3. The present study sheds light on the effect of replacement of transition metal element Ir with Pt on different mechanical, electronic, and superconducting properties. The effect of spin-orbit coupling on electronic band structure was found to be insignificant for Li2PtSi3. The difference in superconducting transition temperatures of Li2PtSi3 and Li2IrSi3 arises primarily due to the difference in electronic energy density of states at the Fermi level. Somewhat reduced Debye temperature in Li2PtSi3 plays a minor role. We have discussed the implications of the theoretical results in details in this study.Comment: Submitted for publicatio

Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi$_2$O$_6$ in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below $\sim$100 K. This lattice modulation is characterized by a resolution limited wave vector {\bf q}$_{IC}$=(0,$\sim$0.13,0) and its 2$^{nd}$ and 3$^{rd}$ harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.Comment: 4 pages, 4 figure

Finite element simulation of mechanical properties of graphene sheets

© Published under licence by IOP Publishing Ltd. Graphene is the material for the twenty first century applications. In this paper, the elastic properties of monolayer and double layer Graphene sheets, typically less than 10nm in size are investigated through linear finite element simulations. The effect of aspect ratio, sizes and chirality of the Graphene sheet on the Young's modulus, Shear modulus and Poisson's ratio are studied. By using structural mechanics approach combining atomistic and equivalent continuum techniques, the Young's modulus, shear modulus and the Poisson ratio were found and they slightly increase with the aspect ratio but decrease with the size of the Graphene sheet. These simulated properties compliment the mechanical properties of Graphene found in literature