Higher Doping Values Research Articles

Z_{2} Parton Phases in the Mixed-Dimensional t-J_{z} Model.

We study the interplay of spin and charge degrees of freedom in a doped Ising antiferromagnet, where the motion of charges is restricted to one dimension. The phase diagram of this mixed-dimensional t-J_{z} model can be understood in terms of spinless chargons coupled to a Z_{2} lattice gauge field. The antiferromagnetic couplings give rise to interactions between Z_{2} electric field lines which, in turn, lead to a robust stripe phase at low temperatures. At higher temperatures, a confined meson-gas phase is found for low doping whereas at higher doping values, a robust deconfined chargon-gas phase is seen, which features hidden antiferromagnetic order. We confirm these phases in quantum MonteCarlo simulations. Our model can be implemented and its phases detected with existing technology in ultracold atom experiments. The critical temperature for stripe formation with a sufficiently high hole concentration is around the spin-exchange energy J_{z}, i.e., well within reach of current experiments.

Low‐Temperature Selective Growth of Heavily Boron‐Doped Germanium Source/Drain Layers for Advanced pMOS Devices

The peculiarities of heavily boron‐doped germanium, selectively grown at low temperature by means of a cyclic deposition and etch chemical vapor deposition process, are investigated through the analysis of the structural and electrical material properties. The incorporation of B in Ge can exceed 6 × 1020 cm−3, close to a factor 100 above the solubility limit, without any significant degradation of the Ge:B crystalline quality, although high B‐doping induces an unwanted contraction of the Ge lattice. Micro‐Hall effect measurements and the multiring circular transmission line method are used to evaluate the active carrier concentrations and resistivities of Ti/Ge:B contacts. Even though the resistivity of as‐grown layers saturates for chemical B concentrations approaching 1 × 1021 cm−3 and increases beyond that level, a contact resistivity below 3 × 10−9 Ω cm2 is obtained for the highest active doping concentration, showing that a compromise must be found to decrease the total contact resistance. Finally, first principles simulations are used to understand dopant deactivation mechanisms in the Ge:B system. In conclusion, the formation of boron‐interstitial clusters is most likely the cause for electrical performance degradation at high doping values.

Improvement of methane storage in nitrogen, boron and lithium doped pillared graphene: A hybrid molecular simulation

The aim of this study is to investigate the storage capacity of methane on the doped pillared graphene using molecular simulation. To this end, a pillared graphene containing two parallel graphene sheets with two vertical carbon nanotubes as holders was selected. This carbon structure was doped using nitrogen, boron and lithium atoms to enhance the gas storage capacity. A hybrid molecular simulation - a combination of molecular dynamics and grand canonical Monte Carlo simulation - was applied to simulate the storage capacity of adsorbent. The results showed that in all systems, doping could enhance the storage capacity of pillared graphene in comparison to pure structure. This improvement was more significant for lithium doped structures while the enhancement of methane storage capacity was estimated 28% higher than that of pristine pillared graphene. In all systems, the storage capacity improvement could be reinforced by increasing the doping percentage of dopant atoms, but this difference was more noticeable for the lithium doped structure. Furthermore, lithium doped pillared graphenes at all levels of dopant and nitrogen doped structures with high doping values (equal or greater than18.5%) were shown to meet the recent target set by U.S. Department of Energy for methane storage capacity.

Numerical simulation of the impact of design parameters on the performance of back-contact back-junction solar cell

This work presents a study based on electro-optical numerical simulations of the impact of geometrical and doping parameters on main figures of merit of crystalline silicon back-contact back-junction solar cells. State-of-the-art physical models in combination with two-dimensional simulations performed by a TCAD tool have been adopted to carry out an extensive and detailed analysis of the influence of many fabrication parameters on performance. The studied design parameters are the doping level in front surface field (FSF), back surface field (BSF) and emitter, and the main geometrical parameters. A doping level value that allows the maximization of the efficiency for the three regions can be clearly identified. In particular, for BSF and emitter, an efficiency degradation is observed for relatively lower doping values and is ascribed to the higher contact recombination while for higher doping values the Auger recombination plays a significant role in reducing the ultimate efficiency. In FSF region the recombination due to defects at the front interface is the main limiting mechanisms for efficiency. On the basis of our analysis, a marked sensitivity of the efficiency to the gap and pitch size is caused by the series resistance increase. The efficiency exhibits a maximum value for an emitter coverage fraction (R) of 85 %. However, in the case of lower emitter coverage, Auger, Shockley---Read---Hall (SRH): in bulk and at interfaces are detrimental for the cell conversion efficiency.

Inter-band and intra-band reflections in graphene–insulator–superconductor junctions with zigzag or armchair edge

We analyze electron–electron and Andreev reflections (AR) for a graphene–insulator–superconductor junction for zigzag and armchair edges, where the insulator is modeled as a potential barrier characterized by a strength. We calculate the reflection probabilities and differential conductance using the Bogoliubov–de Gennes–Dirac (BdGD) equations. For low doping values and zigzag edge the reflection coefficients have the same behavior that in a graphene–superconductor junction. However for high doping values the reflection probabilities have a periodicity of πwith the strength barrier values. For high doping values and armchair edge the electron–electron reflections associated to K′ valley increase and AR associated to K valley decrease. We compare our results with the differential conductance obtained by the Green formalism. We show that the effect of barrier strength for high doping resembles the behavior when a hopping between graphene and superconductor interfaces is considered.

Analysis of the Impact of Doping Levels on Performance of back Contact-Back Junction Solar Cells

In this work, by exploiting two-dimensional (2-D) TCAD numerical simulations, we performed a study of the impact of the doping levels on the main figures of merit in the different regions of a crystalline silicon Back-Contact Back-Junction (BC-BJ) solar cell: the emitter, the Back Surface Field (BSF) and the Front Surface Field (FSF). The study is supported by a dark loss analysis which can highlight the contribution of several recombination mechanisms to the total diode saturation current. The efficiency curve as a function of doping level exhibits a bell-shape with a clearly identifiable optimum value for the three regions. The decrease in efficiency observed at lower doping values is explained in terms of higher contact recombination for BSF and emitter, and in terms of higher surface recombination for FSF. The efficiency decrease observed at higher doping values is ascribed to the higher surface recombination for FSF and Auger recombination for all cases.

Evidence for a Lifshitz transition in electron-doped iron arsenic superconductors at the onset of superconductivity

The phase diagram of a correlated material is the result of a complex interplay between several degrees of freedom, providing a map of the material's behavior. One can understand (and ultimately control) the material's ground state by associating features and regions of the phase diagram, with specific physical events or underlying quantum mechanical properties. The phase diagram of the newly discovered iron arsenic high temperature superconductors is particularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca, Sr, Ba, T being transition metals), the simultaneous structural/magnetic phase transition that occurs at elevated temperature in the undoped material, splits and is suppressed by carrier doping, the suppression being complete around optimal doping. A dome of superconductivity exists with apparent equal ease in the orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonal state with no long range magnetic order. The question then is what determines the critical doping at which superconductivity emerges, if the AFM order is fully suppressed only at higher doping values. Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that critical changes in the Fermi surface (FS) occur at the doping level that marks the onset of superconductivity. The presence of the AFM order leads to a reconstruction of the electronic structure, most significantly the appearance of the small hole pockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitz transition, at the onset of superconductivity. Superconductivity and magnetism are competing states in the iron arsenic superconductors. In the presence of the hole pockets superconductivity is fully suppressed, while in their absence the two states can coexist.

MAGNETIC, ELECTRIC AND EQUILIBRIUM PROPERTIES OF YBa2Cu3-x(PO4)xO7-δ HIGH TEMPERATURE SUPERCONDUCTING SYSTEM

The new YBa 2 Cu 3-x( PO 4)x O 7-δ (x = 0; 0.03; 0.06; 0.09; 0.12) superconducting material was synthesized by the standard solid state reaction recipe. X-ray diffraction results show the effective substitution of copper for phosphate and also show no evidence of any impurity related with phosphate content. Resistivity, DC magnetization and heat capacity measurements reveal a superconducting transition for all samples with a maximum critical temperature up to 97 K, corresponding to the x = 0.03 sample. For higher doping values, the critical temperature decreases in an approximately linear response. In agreement with the charge transfer model, the critical temperature decreases with the carrier density. For x = 0.03, the critical temperature shows a rise up due to an approximation to the optimum density which separates over-doped and under-doped regimes. The maximum critical temperature achieved is one of the highest ever seen for the 123-type superconductors.

Antiferromagnetism and phase separation in thet−Jmodel at low doping: A variational study

Using Gutzwiller-projected wave functions, I estimate the ground-state energy of the t-J model for several variational states relevant for high-temperature cuprate superconductors. The results indicate antiferromagnetism and phase separation at low doping both in the superconducting state and in the staggered-flux normal state proposed for the vortex cores. While phase separation in the underdoped superconducting state may be relevant for the stripe formation mechanism, the results for the normal state suggest that similar charge inhomogeneities may also appear in vortex cores up to relatively high doping values.

Effect of the doping level on the conductance of polymer–salts complexes in the presence of humidity

The conductance of complexes between polyacetylene-based polymers and inorganic salts (dopants) was investigated in the presence of humidity as a function of the salt concentration. The conductance reaches a maximum value by increasing the doping level. The decrease of conductance observed for high-doping values is due to enhanced ion interactions. In some cases, the conductance/doping plots show two maxima likely due to ion association, i.e. to the formation of double and triple ions. The trends of conductance and capacitance with the doping are very similar at constant relative humidity. This supports a previously suggested model, based on the assumption that films of polymer electrolytes behave, in the presence of humidity, as solid polymer structures surrounded by a liquid ion solution.

In-plane Mg doping in YBa2Cu3O7: influence on thesuperconducting anisotropy

Melt textured single domains of Mg-doped YBa2Cu3O7 superconductors with homogeneous cation distribution and narrowsuperconducting transitions have been prepared by top seeding growth. Thestrong reduction in the transition temperature, Tc, found in Mg substitutedYBa2(Cu1-xMgx)3O7 samples, suggests that Mgions occupy the CuO2 planes. For low doping concentrations, thedecrease in Tc is estimated as dTc/dx≈-12 K/at%Mg. Instead, for high doping values, a saturation of Mg substitution occurswithin the matrix, leading to MgO segregation. The maximum amount of Mg atomicsubstitution has been estimated as x≈4 at%, shifting Tc down toTc≈45 K. Studies of anisotropy of the superconducting statefrom magnetoresistance measurements reveal that, concomitant to the decreasein Tc, the mass anisotropy decreases with Mg doping. These results areinterpreted in the framework of non-magnetic impurity scattering effects ind-wave superconductors, where the main distinguishing feature has been thedevelopment of new quasi-particle states within the superconducting gap.

Drude weight and total optical weight in a t - t' - J model

We study the Drude weight D and the total optical weight K for a t - t' - J model on a square lattice that exhibits a metallic phase-modulated antiferromagnetic ground state close to half-filling. Within a suitable 1/N expansion that includes leading quantum-fluctuation effects, D and K are found to increase linearly with small hole doping away from the Mott metal - insulator transition point at half-filling. The slow zero-sound velocity near the latter transition identifies with the velocity of the lower-energy branch of the twofold excitation spectrum. At higher doping values, D and K eventually saturate and then start to decrease. These features are in qualitative agreement with optical conductivity measurements in doped antiferromagnets.

Drude weight and total optical weight in strongly correlated electron systems

We study the doping dependence of the Drude weightD and the total optical weightK in strongly correlated electron systems described by at-t′-J model on a square lattice. Within a suitable 1/N expansion that includes leading quantum-fluctuation effects,D andK are found to increase linearly with small hole doping away from the Mott metal-insulator transition point at half-filling. The slow zero-sound velocity near the latter transition identifies with the velocity of the lower-energy branch of the twofold excitation spectrum. At higher doping values,D andK eventually saturate and then start to decreases. These features are in qualitative agreement with optical conductivity measurements in the high-T c cuprate superconductors.

Er-Doped GaAs for High Speed Photoconductor Applications

ABSTRACTThe observation of 1.54 μm luminescence in erbium-doped-GaAs (GaAs:Er) and AlxGal-xAs, has stimulated research efforts because of their potential application to light sources in optical communications. In this paper, we study a different aspect of the photoresponse behavior of this material. The dependence of the carrier lifetime on the doping concentration of Erbium is investigated in GaAs:Er grown by MBE. A reduction in the carrier lifetime down to ∼1 ps is observed for the highest doping (∼5×1019 cm−3) investigated. Together with the high-resistivity observed for the higher doping values, this material serves as a novel photoconductor material for high-speed optoelectronics.