Xiren Zhang

Electronic transport characterization of silicon wafers by laterally resolved free-carrier absorption and multiparameter fitting

Laterally resolved modulated free-carrier absorption (MFCA) is applied to the simultaneous determination of the electronic transport properties of semiconductor wafers. A rigorous three-dimensional carrier diffusion model is used to fit the observed dependences of the MFCA signal amplitude and phase on the separation between the pump and probe laser spots, measured at several modulation frequencies covering an appropriate range. This leads to a simultaneous and unambiguous determination of the values of three transport parameters, namely, the minority-carrier lifetime tau, the carrier diffusivity D, and the front surface recombination velocity s(1). The extracted values for a n-type Si wafer with a resistivity of 7-10 Omega cm are 53 mu s (tau), 16.6 cm(2)/s (D), and < 200 cm/s (s(1)), respectively. (c) 2006 American Institute of Physics.

Fabrication and Magnetization Reversal Processes for Co/Cu Multilayer Nanowires

Co/Cu multilayer nanowires fabricated in an array using anodized aluminium oxide (AAO) template has been investigated. Experimental conditions are optimized to fabricate Co/Cu multilayer systems with fixed Cu and variable Co layer thicknesses. Magnetization reversal mode is found to depend on the Co layer thickness. A transition occurs from coherent rotation to a combination of coherent and curling rotation at around t(Co) = 60 nm with increasing t(Co). The reversal modes have been investigated using the magnetic hysteresis loops measured at room temperature for Co/Cu nanowires placed at various angles between the directions of the nanowire axis and external fields using a vibrating sample magnetometer. The magnetic easy axis changes from the direction perpendicular to nanowires to that parallel to the nanowires at around t(Co) = 60 nm, indicating a change in the magnetization reversal mode. The reversal mode for the nanowires with thin disk-shaped Co layers is of a coherent rotation type, while that for long rod-shaped Co layers can be explained by a combination of coherent rotation and a curling mode.

Determination of the critical interspacing for the noninteracting magnetic nanoparticle system

The dipole–dipole interactions of monodisperse Fe3O4 nanoparticles (NPs) can be directly controlled by a uniform SiO2 shell with different thickness, i.e., different interspacings. Thus, the interacting strength of a serial of Fe3O4–SiO2 NPs system can be revealed by fitting the blocking temperature TB measured at ac fields to the Vogel–Fulcher law. The interspacing over five times of diameter for less than 8.0 nm Fe3O4 NPs is the critical value to achieve a noninteracting system. Furthermore, a general equation to evaluate critical interspacing for noninteracting magnetic NPs systems with different sizes and saturation magnetizations was calculated by Monte Carlo method.

Accuracy analysis for the determination of electronic transport properties of Si wafers using modulated free carrier absorption

Computer simulations are carried out to investigate the sensitivity of simultaneous determination of three electronic transport properties (carrier lifetime, carrier diffusivity, and front surface recombination velocity) of silicon wafers by modulated free carrier absorption (MFCA) via a multiparameter fitting procedure. The relative accuracy of the transport parameter determination by laterally resolved MFCA (LR-MFCA), in which the amplitude and phase are measured as functions of the pump-probe-beam separation at several modulation frequencies covering an appropriate range, and by conventional frequency-scan MFCA (FS-MFCA), in which only the modulation frequency dependences of the amplitude and phase are recorded, is theoretically analyzed and experimentally estimated by calculating the dependence of the mean square variance on individual transport parameter via a multiparameter estimation process. Simulated and experimental results show that the determination of the transport properties of silicon wafers by LR-MFCA are more accurate, compared with that by FS-MFCA. Comparative experiments are performed with a silicon wafer and the estimated uncertainties of the carrier diffusivity; lifetime and front surface recombination velocity are approximately +/- 3.7%, +/- 25%, and +/- 35% for LR-MFCA and +/- 7.5%, +/- 31%, and +/- 24% for FS-MFCA, respectively.

Thermal stability of Ir-Mn∕Co-Fe-B∕Al-O∕Co-Fe-B tunnel junctions

The thermal stability of magnetic tunnel junctions with structures of Ta(5)∕Cu(30)∕Ta(5)∕Ni79Fe21(5)∕Ir22Mn78(12)∕Co62Fe20B18(4)∕Al(0.8)-oxide∕Co62Fe20B18(4)∕Cu(30)∕Ta(5) (thicknesses unit in nanometers) has been investigated. The tunnel magnetoresistance (TMR) shows a large increase up to 54.4% after annealing at 265 °C due to the improved characteristic properties of the barrier and the interface between the barrier and the ferromagnetic electrodes. The TMR was observed to decrease drastically above the annealing temperature of 310 °C accompanied by a notable increase of junction resistance and coercivity of the free layer. The amorphous Co62Fe20B18 layers seem to behave as a barrier of diffusion, preventing the migration of Mn or Cu atoms into the interface between the barrier and the ferromagnetic layers. This may cause the drastic decrease of TMR due to the deterioration of the barrier and its interface with Co62Fe20B18 layers. The observed crystallization in the amorphous Co62Fe20B18 layers is consi...

Sensitivity analysis of laterally resolved free carrier absorption determination of electronic transport properties of silicon wafers

Simulations are performed to investigate the uniqueness of simultaneous determination of electronic transport properties (the carrier lifetime, the carrier diffusivity, and the front surface recombination velocity) of silicon wafers by laterally resolved modulated free carrier absorption (MFCA) and multiparameter fitting. The dependences of MFCA amplitude and phase on these transport properties at different pump-probe-beam separations and modulation frequencies are analyzed. The uncertainties of the fitted parameter values are analyzed by investigating the dependences of a mean square variance including both the amplitude error and phase error on corresponding electronic transport parameters. Simulation results show that the electronic transport parameters can be determined accurately through fitting experimental MFCA data carrying both frequency- and space-domain information of carrier diffusion to a rigorous MFCA model. Among the three transport parameters, the carrier diffusivity can be determined most precisely, with an uncertainty of less than +/- 5%, due to the highest sensitivity of the laterally resolved MFCA signal to the diffusivity. The highly accurate determination of the diffusivity further improves the precision of the carrier lifetime and the front surface recombination velocity values simultaneously determined via multiparameter fitting. Experiments were performed with a silicon wafer and the results were in good agreement with the theoretical simulations

Effect of Magnetic Field Annealing Upon Co-Rich Nanowires

The magnetic field (MF) annealing of dc electrodeposited ferromagnetic nanowire arrays of compositions Co94Fe06 , Co91Fe07B2 was done in the direction of their easy axis of magnetization (in plane) at 2650 C under 800 Oe applied magnetic field for 2 h. An induced magnetic anisotropy (MA) and a large increase in saturation magnetization (Ms) were manifested by the changes in hysteresis loops of the nanowire samples before and after the annealing process. The structural changes were investigated using X-ray diffraction and transmission electron microscopy (TEM). The shape of nanowires, the predicted natural texture with c-axis in the plane of membrane (because of high concentration of cobalt) and diffusional pair ordering of unlike atoms along the direction of external applied field were thought to be the causes of induced MA. The reannealing of samples in absence of MF at 400degC and 600degC produced no significant changes in hysteresis loops instead of removing the MA but with evolution of crystalline phase in both the samples. Therefore, the shape of nanowire was concluded to play the major role in persistence of induced MA in nanowire samples after high temperature annealing

Accurate determination of electronic transport properties of semiconductor wafers with spatially resolved photo-carrier techniques

Spatially resolved photo-carrier techniques, in which a modulated and tightly focused laser beam with photon energy larger than the band gap of the semiconductor material under investigation is employed to generate free carriers, are applied to the simultaneous determination of the electronic transport properties of semiconductor wafers, that is, the carrier lifetime, the carrier diffusivity, and the front surface recombination velocity. The simultaneous determination is fulfilled by measuring and multi-parameter fitting the modulation-frequency-dependent three-dimensional distribution of photo-generated free carriers in a semiconductor wafer to a rigorous three-dimensional theoretical model. The uncertainties of the fitted parameter values are analyzed by investigating the dependence of a mean square variance including both amplitude and phase errors on the corresponding transport parameters and compared to that obtained by the conventional frequency-scan approach, in which only the frequency dependence of the amplitude and phase of the photo-carrier signals are recorded at the excitation spot. Both theoretical simulations and experimental results show that the accuracy of the simultaneous multi-parameter determination is greatly improved by the spatially resolved information of the photo-generated free-carrier diffusion in the semiconductor wafers.