Low Doping Region Research Articles

CHARGE-SPIN FREEZING AND FINITE SIZE SCALING IN YBa2Cu3O6+x

The charge and spin freezing phenomena in high T c YBa 2 Cu 3 O 6+x superconductor is investigated in the low doping region (0<x<0.4) by the zero-field muon spin relaxation (ZF-μSR) technique. For x>0.2, the observed spin freezing is correlated to the charge freezing of the holes in the Cu(2)O2 planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa 2 Cu 3 O 6+x.

Stripes and superconductivity in high- Tc superconductors

The ground state of the three-band Hubbard model with d and p orbitals is investigated by using a quantum variational Monte Carlo method (VMC) with focus on the underdoped region of high- T c superconductors. The antiferromagnetic (AF) state is considerably stable compared to the d-wave pairing state in the underdoped region, and is further stabilized by introducing spin modulations and stripes. A possibility of pure d-wave pairing state in the underdoped region is small since this state is overwhelmed by the AF ordering for small doping. The VMC evaluations show that the superconducting state is realized as a coexistence state of d-wave and AF states in the low-doping region. In particular, the non-uniform pairing state along with stripes is possible with the oscillating superconducting order parameter in space.

Electrically Induced Junction MOSFET for High Performance Sub-50nm CMOS Technology

AbstractDegrading of short-channel effects (SCE) e.g. Drain-Induced-Barrier-Lowering (DIBL), charge-sharing etc., as CMOS devices are scaled into the sub-50nm regime, is a major roadblock for ULSI technologies. This problem can be circumvented to some extent by a proper scaling of MOSFET vertical dimensions (junction depths, oxide thickness etc.). In this work we propose a novel implementation of an electrically induced junction (EJ) MOSFET. An EJ-MOSFET is different from conventional CMOS device in that the gate voltage electrically induces the shallow source-drain extensions (SDEs). In such a device the SDEs are underneath the gate and contain low-doped regions of opposite conductivity as that of deep source-drain (S/D). In order to turn ON the device, a voltage is applied at the gate of EJ-MOSFET device, such that these low doped regions below poly-Si gate get inverted and serve as SDEs. Consequently, the effective channel length in this condition is the distance between these low-doped regions. On the contrary, at any gate voltage less than that required for inverting these regions, no SDEs are induced, and the effective channel length is equal to the physical separation between the deep S/D junctions.

Properties of the transient of avalanche transistor switching at extreme current densities

Avalanche transistor switching at extreme currents is studied under conditions in which the charge of the excess carriers drastically rebuilds the collector field domain, causing fast switching and a low residual voltage across the switched-on device. The dynamic numerical model includes carrier diffusion and considers different dependencies of the velocities and ionization rates for the electrons and holes in the electric field. These dependences determine the principal difference in the switching process between n/sup +/-p-n/sub 0/-n/sup +/ and p/sup +/-n-p/sub 0/-p/sup +/ structures. Reasonably good agreement is found between the simulated and measured temporal dependences of the collector current and voltage drop across the device for a particular type of avalanche transistor. Certain differences in the switching delay can partly be attributed to limitations in the one-dimensional (1-D) approach. It is now certain that collector domain reconstruction defines the transient in a n/sup +/-p-n/sub 0/-n/sup +/ transistor at high currents, but is not very pronounced in a p/sup +/-n-p/sub 0/-p/sup +/ transistor. Some nontrivial features of the device operation are found, depending on the semiconductor structure. In particular, it is shown that the thickness of the low-doped collector region affects mainly the switching delay, and does not significantly effect the current rise time.

Local density of states near the surface of high- Tc superconductors under magnetic field based on the t– J model

Quasiparticle states near the surface of d-wave superconductors in a magnetic field are studied based on the t– J model on the 2D square lattice within the Gutzwiller approximation. We perform a calculation on the local density of states near the (1 1 0) surface, by changing the hole concentration in the presence of applied magnetic field. In the low-doping region, the zero-energy peak (ZEP) is not sensitive to the applied magnetic field. With the increase of the hole concentration, the ZEP splits into two due to the broken time reversal symmetry state with induced subdominant s-wave like component. Even in this case, the position of split peak is insensitive to the applied magnetic field.

Physically based compact device models for circuit modelling applications

Physically based compact device models of the MOSFET, as well as PiN diode and insulated gate bipolar transistor (IGBT) are presented in this paper. For the correct description of static and dynamic behaviour of power bipolar devices (IGBT and PiN diode) a 1D module for the drift zone (low doped n-base region) is presented that incorporates conductivity modulation and non-quasistatic charge storage effect. Finally, it is possible to transform, relatively easily, these electric models into the electrothermal models by adding an extra node (thermal node) to the electrical compact models. This thermal node will store information about junction temperature of the active device and it represents a connection between the device and rest of the circuit thermal network.

Scanning capacitance microscopy investigations of SiC structures

We have applied scanning capacitance microscopy (SCM) to investigate SiC structures grown by vapour-phase epitaxy. The SCM technique is evaluated using n- and p-type doping staircase structures with doping concentrations ranging from 1016 to 1020cm−3. The n- and p-type doping was obtained by doping SiC with nitrogen and aluminium, respectively. The sample cross-sections for SCM were obtained by simple cleaving. For doping levels above 1017cm−3 the SCM data are consistent with doping data obtained independently from secondary ion mass spectroscopy (SIMS). Treating the samples with diluted hydrofluoric acid significantly improves the SCM signal for the low-doped regions. The SCM technique has been used to investigate doping redistribution in patterned regrowth of n- and p-type SiC around dry-etched mesas. In both cases, contrast variations were seen close to the mesa walls, indicative of doping variations; lower and higher incorporation for p- and n-type, respectively. The observations are shown to be consistent with the expected trends in dopant incorporation in the SiC material.

Superconducting and SDW condensation energies in the 2D Hubbard model and the meaning to the stripe phase

The variational Monte Carlo calculations give a finite bulk-limit value of superconducting (SC) condensation energy E cond in the 2D Hubbard model with next-nearest-site transfer energy t′ when electron density ρ≥0.84 with −0.25≤ t′≤−0.10; our energy unit is the transfer energy between the nearest-neighbor (n.n.) sites. However, the SDW E cond computed by the same method is larger than the SC one in such a region. This suggests that the SC region is very restricted, if it exists. When we improved the trial wave function taking into account the n.n.-site correlation, the SC E cond increased moderately while the SDW E cond decreased considerably. With this modified variational Monte Carlo method we have found a set of parameter values for which the bulk-limit SC E cond is finite whereas that for the SDW vanishes. The SC E cond obtained in both ways around the optimal doping is in fair agreement with the experimental value ∼0.26 meV/site for YBCO estimated from the specific heat and the critical magnetic field, in contrast to the case of the t– J model which gives a value larger by almost two orders of magnitude. In the low-doping region the SDW E cond is overwhelmingly larger than the SC E cond. Further, the striped SDW state seems more stable here. This is considered to be the energetic origin of the stripe features and presumably of the consequent pseudogap in the underdoped region.

Incommensurate modulation of spin magnetic susceptibility in single band Hubbard model

By means of a cumulant expansion method, we investigate the incommensurate phase in the low doping region of the two dimensional single-band Hubbard model. We calculate the spin magnetic susceptibility as a function of doping and analyze the effect of a Van Hove singularity on a commensurate-incommensurate transition. A comparison with the experimental results of LSCO is discussed.

POSSIBLE EVIDENCE OF CHARGE-STRIPE ORDERING IN THE ab-PLANE RESISTIVITY OF STRONGLY UNDERDOPED La2-xSrxCuO4 SINGLE CRYSTALS

We measured the ab-plane resistivity of La 2-x Sr x CuO 4 underdoped single crystals (x = 0.052 and x=0.06) between 4.2 and 300 K by using an AC version of the Van der Pauw technique. We suggest that a possible scenario of charge-stripe ordering at the lowering of the temperature can be delineated by starting from the main features of the ρ ab (T) curves of these samples. In particular, the deviation from the linearity of the ab-plane resistivity, occurring at a temperature T ch , could be related to the beginning of the charge localization, and the upturn of ρ ab to the progressive pinning of the resulting charge stripes. By starting from an analysis of our ab-plane resistivity curves, we determined the temperature of charge ordering T ch for our samples, thus extending in consistent way the T ch vs. x curve reported in literature to the very low-doping region of the phase diagram.

Microscopic analysis of voltage noise operation mode in SiGe/Si bipolar heterojunctions: Influence of the SiGe strained layer

A Monte Carlo analysis of bipolar transport and voltage fluctuations in a p+-Si/n-Si0.7Ge0.3 heterojunction and in a p+n Si homojunction under different operation regimes is presented. Comparison of the spectral density of voltage fluctuations at low frequency, SV(x,0), between both structures reveals the strong effect of the SiGe layer on the noise behavior in the heterojunction. Alloy scattering hinders the electron mobility enhancement expected from the removal of valley degeneracy in the SiGe layer. Despite this mobility reduction, the greater accumulation of carriers in the low-doped region supported by the valence band discontinuity reduce SV(x,0) in the heterojunction for a given average voltage. This study also reveals the impact of hot carriers on noise performance in the quasisaturation regime of both diodes.

Comparative analysis of minority carrier transport in npn bipolar transistors with Si, Si1−xGex, and Si1−yCy base layers

Here we present a comparative analysis of vertical minority carrier transport in Si, Si0.925Ge0.075, Si0.998C0.002, and Si0.99C0.01 base layers of bipolar transistors. We show that a conventional transit time analysis for extracting the minority carrier mobilities fails for doping profiles containing a low doped emitter region. The contribution of locally compensated charge storage, called neutral charge storage, in the emitter-base depletion region must not be neglected. To overcome drawbacks of the simple transit time analysis, we use 2D device simulations to obtain an improved understanding of the measured high-frequency parameters. Taking into account the real doping profiles and device structures, and using a calibrated parameter set for strained SiGe, the simulation results for the Si, Si1−xGex, and Si1−yCy (y≤0.2%) base layer transistors reproduce very well the measured transit times (assuming the Si data for the electron mobility μn) in the heteroepiaxial base layers. In the case of higher carbon concentration (y=1%), the electron mobility is reduced by a factor of two.

Ground-state properties of the Hubbard model by Lanczos diagonalizations

We investigate the Hubbard model on a two-dimensional cluster of 18 sites using Lanczos algorithm for different strengths of Coulomb repulsion in the low doping region. Energies and correlation functions are given for half-filling and two holes. In the strong coupling regime we compare our results with the $t\ensuremath{-}J$ model. We confirm the tendency of holes to repel, although a significant value of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductive order parameter is measured. These data represent a benchmark for future quantum simulations.

Electronic States and Mixing of Different Symmetry or Antiferromagnetism in Vortex Cores in dx2-y2-wave Superconductivity

The microscopic structure of vortices in high-Tc superconductors is studied using the two-dimensional t-J model. The spatial dependences of order parameters are determined self-consistently using the Gutzwiller approximation, in which the effect of correlation is into account. In the high-doping region, the pair potential has d x2-y2-wave nature and the local density of states in the vicinity of the core shows a zero-energy peak. However, in the low-doping region, a spatially oscillating (extended) s-wave-type order parameter is locally induced around the vortex core. As a result, the local density of states near the core shows a splitting of the zero-energy peak. This gives a possible explanation for the experimental data of scanning tunneling spectroscopy for YBCO. The possibility of the antiferromagnetic vortex core is also studied by modifying the Gutzwiller approximation. It is found that, close to the boundary to the antiferromagnetically ordered state, the antiferromagnetic correlation develops inside the vortex core, even if the bulk state is the pure d-wave state. In this case the local density of states near the core does not have a zero-energy peak but instead shows a gap-like feature, which can be observed experimentally. Finally the sign change of the charge of the vortex core as a function of the doping rate is found.

Neutron Scattering Study of Temperature-Concentration Phase Diagram of (Cu1-xMgx)GeO3

In doped CuGeO3 systems, such as (Cu1-xZnx)GeO3 and Cu(Ge1-xSix)O3, the spin-Peierls (SP) ordering (T<Tsp) coexists with the antiferromagnetic (AF) phase (T<TN<Tsp). Tsp decreases while TN increases with increasing x in low doping region. For higher x, however, the SP state disappears and only the AF state remains. These features are common for all the doped CuGeO3 systems so far studied, indicating the existence of universal T-x phase diagram. Recently, Masuda et al. carried out comprehensive magnetic susceptibility (chi) measurements of (Cu1-xMgx)GeO3, in which doping concentration can be controlled significantly better than the Zn doped systems. They found that TN suddenly jumps from 3.43 to 3.98K at the critical concentration xc sim 0.023 and that a drop in chi corresponding to the SP ordering also disappears at x>xc. They thus concluded that there is a compositional phase boundary between two distinct magnetic phases. To clarify the nature of two phases, we performed neutron-scattering measurements on (Cu1-xMgx)GeO3 single crystals with various x. Analysis of the data at fixed temperature points as a function of doping concentration has revealed sudden changes of order parameters at the critical concentration xc=0.027 +- 0.001. At finite temperatures below TN, the drastic increase of the AF moment takes place at xc. The spin-Peierls order parameter delta associated with lattice dimerization shows a precipitous decrease at all temperature below Tsp. However, it goes to zero above xc only at the low temperature limit.

Chemically controlled properties of layered perovskites

The magnetic and magnetotransport properties of have been investigated. In the low doping region x < 0.5, the samples show similar properties to (x = 0), but magnetic transition temperature , insulator-metal transition temperature and the temperature of magnetoresistance (MR) maximum all decrease with x increasing. When the samples resemble (x = 1.2), without spontaneous magnetization and being insulating. The MR maximum of increases with Nd doping, and the highest MR ratio, , is observed in the transition region x = 0.5 and 0.6. It is shown that the properties of change gradually with Nd doping and can be chemically controlled by tuning the La/Nd ratio.

Theory of the quasiparticle spectra around a vortex in the two-dimensional t– t′– J model

The local density of states around a vortex core, which can be observed in scanning tunneling spectroscopy, is studied in the two-dimensional t– J model and t– t′– J model in order to clarify the effect of correlation. The spatial dependence of the order parameter is numerically determined self-consistently under a uniform magnetic field using the Gutzwiller approximation. It is found that an s-wave component is locally induced around the core in the low-doping region, which leads to a splitting of the zero-energy peak in the local density of states near the core center. This may explain the scanning tunneling spectroscopy experiment in YBa 2Cu 3O 7−δ.

Numerical evidence for SO(5) symmetry and superspin multiplets in the t– J model

In this work, we present numerical results which support SO(5) symmetry as a concept unifying superconductivity and antiferromagnetism in high-temperature superconductors. Using exact cluster diagonalization, we verify that the low-energy states of the t– J model, a widely used microscopic model for high T c cuperates, form SO(5) symmetry multiplets. Our results show that the d-wave superconducting ground states away from half-filling are obtained from the higher spin states at half-filling through SO(5) rotations. The dominant low-energy resonance in the dynamic spin correlation function corresponds to a 'π resonance' in the entire low doping region. The SO(5) symmetry is robust against the inclusion of longer- range Coulomb repulsion and next-nearest-neighbor hopping.

Incommensurate magnetism in cuprate materials

In the low-doping region, an incommensurate magnetic phase is observed in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$. By means of the composite-operator method we show that the single-band two-dimensional Hubbard model reproduces with good accuracy the experimental situation. In the higher-doping region, where experiments are not available, the incommensurability is depressed owing to the van Hove singularity near the Fermi level. By changing doping, the calculated incommensurability amplitude and the experimental critical temperature evolve in a similar way, suggesting a close relation between superconductivity and incommensurate magnetism.

EBIC analysis of SiC mesa diodes

Mesa etched p +n 6H-SiC diodes were investigated using EBIC. The EBIC signal was analysed using TMA Medici and Monte Carlo simulations. Space charge characteristics obtained from EBIC and static device simulations were compared. Surface charge density and minority carrier lifetime of the low doped n-type region were determined. The charge generation volume is described by an analytical approximation based on calculations using single scattering Monte Carlo simulation code. Results show large positive surface charge density of the order of 1×10 13 q/cm 2 present at the n-type SiC/SiO 2 interface. A common origin of the surface charge on n- and p-type SiC is suggested. The EBIC method, combined with simulation tools, was confirmed to be a valuable diagnostic tool for study of the junction termination and passivation in SiC devices.