Random Dopant Distribution Research Articles

Enhancing the Thermoelectric Properties of Conjugated Polymers by Suppressing Dopant-Induced Disorder.

Doping is a crucial strategy to enhance the performance of various organic electronic devices. However, in many cases, the random distribution of dopants in conjugated polymers leads to the disruption of the polymer microstructure, severely constraining the achievable performance of electronic devices. Here, it is shown that by ion-exchange doping polythiophene-based P[(3HT)1-x-stat-(T)x] (x=0 (P1), 0.12 (P2), 0.24 (P3), and 0.36 (P4)), remarkably high electrical conductivity of >400S cm-1 and power factor of >16µWm-1K-2 are achieved for the random copolymer P3, ranking it among highest ever reported for unaligned P3HT-based films, significantly higher than that of P1 (<40Scm-1, <4µWm-1K-2). Although both polymers exhibit comparable field-effect transistor hole mobilities of ≈0.1cm2V-1s-1 in the pristine state, after doping, Hall effect measurements indicate that P3 exhibits a large Hall mobility up to 1.2cm2V-1s-1, significantly outperforming that of P1 (0.06cm2V-1s-1). GIWAXS measurement determines that the in-plane π-π stacking distance of doped P3 is 3.44Å, distinctly shorter than that of doped P1 (3.68Å). These findings contribute to resolving the long-standing dopant-induced-disorder issues in P3HT and serve as an example for achieving fast charge transport in highly doped polymers for efficient electronics.

Interface engineering of phase separation in SrRuO3/SrTiO3 hybrid superlattices

Most observed phase separation phenomena in complex oxides occur in systems with chemical dopants or structural defects, and theories have established the strong connection between phase separation and the random distribution of chemical dopants. Recent experiments on fabricated high-quality oxide superlattices also confirmed that the phase separation is suppressed in the clean systems without chemical disorders. Thus far, phase separation in strongly correlated oxides without the need of chemical dopants or structural defects has not been fully demonstrated. Here, we have built chemically ordered hybrid superlattices using prototypical ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ perovskite oxides. Contrary to previous understandings, we observe phase separation of two magnetic phases with different spin easy axes. We elucidate this phenomenon through first-principles calculations that the hybrid superlattices have a spontaneous structural instability, leading to a coexistence of ferromagnetic and antiferromagnetic phases. Our findings provide an alternative pathway other than chemical doping to introduce phase separation in correlated oxides and imply that phase separation can exist in clean systems without the need of chemical disorders.

Structural and spectroscopic properties of phosphate–tungsten glasses doped with Nd3+ and Tb3+

Glasses of composition 90NaPO3-10WO3-xNd2O3, (0 ≤ x ≤ 1.7 mol%) and 90NaPO3-10WO3-yTbF3, (0 ≤ y ≤ 5.0 mol%) have been prepared and characterized. 31P and 23Na magic-angle spinning (MAS) NMR and 23Na{31P} rotational echo double resonance results are consistent with previously published data, indicating the presence of Q(2) and Q(1)1 W units. In addition, linear dependences of 31P MAS-NMR linewidths and spin-lattice relaxation rates and the electron spin-spin relaxation behavior (in the case of Nd2O3 doping) are consistent with a random dopant distribution, For Nd2O3- doped glass samples, absorption spectra in the visible and infrared regions were collected and analysed, and photoluminescence excitation and emission spectra were obtained under 575 nm excitation. Using the Judd-Ofelt formalism, the Ω2, Ω4 and Ω6 parameters were determined and by using them, the radiative lifetime (τr) were calculated for the 4F3/2 excited state. The fluorescence quantum efficiency value for this level (QE = τf/τr) of 13% was determined for glass with x = 1.0. The photophysical properties of Tb- doped glasses were also studied and upon UV excitation (372 nm) the glasses show emissions in the green (540 nm, 5D4 → 7F5 transition) and blue (434 nm, 5D3 → 7F4). The relative luminescence intensity ratio 5D3 → 7F5/5D4 → 7F5 is higher for lower Tb3+ content (0.5 Tb glass sample), because for higher Tb3+ contents, a cross-relaxation energy transfer process takes place favoring the green emission.

Machine Learning Approach for Predicting the Effect of Statistical Variability in Si Junctionless Nanowire Transistors

This letter investigates the possibility to replace numerical TCAD device simulations with a multi-layer neural network (NN). We explore if it is possible to train the NN with the required accuracy in order to predict device characteristics of thousands of transistors without executing TCAD simulations. Here, in order to answer this question, we present a hierarchical multi-scale simulation study of a silicon junctionless nanowire field-effect transistor (JL-NWT) with a gate length of 150 nm and diameter of an Si channel of 8 nm. All device simulations are based on the drift-diffusion (DD) formalism with activated density gradient (DG) quantum corrections. For the purpose of this letter, we perform statistical numerical experiments of a set of 1380 automictically different JL-NWTs. Each device has a unique random distribution of discrete dopants (RDD) within the silicon body. From those statistical simulations, we extract important figures of merit (FoM) such as OFF-current (IOFF) and ON-current (ION), subthreshold slope (SS), and voltage threshold (VTH). Based on those statistical simulations, we train a multi-layer NN and we compare the obtained results with a general linear model (GLM). This shows the potential of using NN in the field of device modeling and simulation with a potential application to significantly reduce the computational cost.

Single-charge band-to-band tunneling via multiple-dopant clusters in nanoscale Si Esaki diodes

The electrostatic potential of p+-n+ junctions, as in Esaki (tunnel) diodes, originates from the Coulomb potentials of ionized dopants in the depletion-layer, but it has been modeled so far based on uniform space-charge regions, ignoring the discrete and random dopant distribution. This model can explain well the band-to-band tunneling (BTBT) between the opposite bands of the quasineutral regions (conduction band in the n+-region and valence band in the p+-region). In this letter, we show that a BTBT transport model should contain the mechanism of tunneling via “inherent” localized bandgap-states, created by dopant-induced potential fluctuation, which becomes detectable as a parallel transport mechanism in nanoscale Esaki diodes. This is manifested by the observation of single-charge (SC) BTBT at 5.5 K in nanoscale Si Esaki diodes. Numerical analysis of nanoscale p+-n+ junctions with random dopant-atom distributions suggests that SC-BTBT is mediated by a potential dip created by a number of dopants “clustered” near each other, i.e., by a multiple-dopant cluster.

Impact of the Gate and Insulator Geometrical Model on the Static Performance and Variability of Ultrascaled Silicon Nanowire FETs

We investigate the effect of the geometrical model adopted for the gate electrode and for the insulator enveloping the access regions on the full-quantum simulation of ultrascaled nanowire FETs (NW-FETs). We compare the results obtained in the “minimal” geometry commonly used in simulations with those obtained in a more realistic one, able to fully account for the gate fringing effects. We evaluate the impact of the model geometry on the static performance of NW-FETs and discuss the interplay with the surface roughness and the random distribution of dopants. We find that the ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio evaluated in the minimal geometry can be remarkably underestimated in short devices, notably in the case of small length-to-width ratio. The roughness-induced current degradation and the sensitivity to the surface roughness variability can also suffer from nonnegligible underestimations when evaluated in this geometry. Finally, we point out that an inaccurate description of the device electrostatics is expected to result in an overestimation of the sensitivity to the doping-induced variability.

Ferromagnetic clusters induced by a nonmagnetic random disorder in diluted magnetic semiconductors

In this work, we analyze the nonmagnetic random disorder leading to a formation of ferromagnetic clusters in diluted magnetic semiconductors. The nonmagnetic random disorder arises from randomness in the host lattice. Including the disorder to the Kondo lattice model with random distribution of magnetic dopants, the ferromagnetic–paramagnetic transition in the system is investigated in the framework of dynamical mean-field theory. At a certain low temperature one finds a fraction of ferromagnetic sites transiting to the paramagnetic state. Enlarging the nonmagnetic random disorder strength, the paramagnetic regimes expand resulting in the formation of the ferromagnetic clusters.

Spin dynamics in paramagnetic diluted magnetic semiconductors

Microscopic properties of low-energy spin dynamics in diluted magnetic semiconductor are addressed in a framework of the Kondo lattice model including random distribution of magnetic dopants. Based on the fluctuation-dissipation theorem, we derive an explicit dependence of the spin diffusion coefficient on the single-particle Green function which is directly evaluated by dynamical mean-field theory. In the paramagnetic state, the magnetic scattering has been manifested to suppress spin diffusion. In agreement with other ferromagnet systems, we also point out that the spin diffusion in diluted magnetic semiconductors at small carrier concentration displays a monotonic $1/T$-like temperature dependence. By investigating the spin diffusion coefficient on a wide range of the model parameters, the obtained results have provided a significant scenario to understand the spin dynamics in the paramagnetic diluted magnetic semiconductors.

On mobility of electrons in a shallow Fermi sea over a rough seafloor

Several doped semiconductors, in contrast to heavily-doped silicon and germanium, host extremely mobile carriers, which give rise to quantum oscillations detectable in relatively low magnetic fields. The small Fermi energy in these dilute metals quantifies the depth of the Fermi sea. When the carrier density exceeds a threshold, accessible thanks to the long Bohr radius of the parent insulator, the local seafloor is carved by distant dopants. In such conditions, with a random distribution of dopants, the probability of finding an island or a trench depends on on the effective Bohr radius, and the carrier density, n. This picture yields an expression for electron mobility with a random distribution of dopants: , in reasonable agreement with the magnitude and concentration dependence of the low-temperature mobility in three dilute metals whose insulating parents are a wide-gap (SrTiO3), a narrow-gap (PbTe) and a ‘zero’-gap (TlBiSSe) semiconductor.

Numerical study of inhomogeneity effects on Hall measurements of graphene films

This paper presents a two-dimensional model calculation of inhomogeneous graphene films which incorporates a random distribution of dopants (leading to electron and hole puddles) for analysis of Hall measurements. The model predicts significant effects of inhomogeneity on the Hall coefficient, which can lead to an underestimate of carrier mobility. We investigate the effect of parameters including size of puddles, local charge density deviation, and device sizes. The inhomogeneity of epitaxial graphene generated by steps and terraces of SiC substrates is also discussed. The simulation results quantify possible statistical errors in Hall mobility measurements, Dirac point estimation and non-uniformity of scaled devices over wafers.

Site preference of Mg acceptors and improvement of p-type doping efficiency in nitride alloys

We perform first-principles density functional calculations to investigate the effect of Al and In on the formation energy and acceptor level of Mg in group-III nitride alloys. Our calculations reveal a tendency for the Mg dopants to prefer to occupy the lattice sites surrounded with Al atoms, whereas hole carriers are generated in In- or Ga-rich sites. The separation of the Mg dopants and hole carriers is energetically more favourable than a random distribution of dopants, being attributed to the local bonding effect of weak In and strong Al potentials in alloys. As a consequence, the Mg acceptor level, which represents the activation energy of Mg, tends to decrease with increasing numbers of Al next-nearest neighbours, whereas it increases as the number of In next-nearest neighbours increases. Based on the results, we suggest that the incorporation of higher Al and lower In compositions will improve the p-type doping efficiency in quaternary alloys, in comparison with GaN or AlGaN ternary alloys with similar band gaps.

Phonon transport in nanowire with contacts: Size and doping

The phonon transport in Lennard-Jones silicon wire with contacts is investigated using non-equilibrium Green’s function. With the size decreasing, the significant reduction in the number of phonon modes leads to a smaller thermal conductance density. The dopant (Ge) atoms are used to substitute the atom in the wire to study the doping effect. For thin wire, its thermal conductance is very sensitive to the location of dopants. It is also found that the interior atom substitution has more impact on the thermal conductance over surface atom; substitution near contact surface reduces thermal conductance significantly; thermal conductance is suffering a 10%–20% variation due the random distribution of dopants; 17% of Ge content is sufficient to reduce thermal conductance by 80%.

Spin Glass State and Other Magnetic Structures with Their Symmetries in Terms of the Fibre Bundle Approach

The fibre bundle approach has been applied to derive the explicit formulas presenting all the eight fundamental magnetic structures and their symmetry groups including the spin glass state. The explanation of the uniqueness of the spin glass state has its roots in the appearance of the probability function p(i, j) in the second term of the assumed Hamiltonian. This term actually describes the random distribution of either dopants or defects in the ferromagnetic matrix under the percolation threshold. On this basis the Gaussian type randomness was derived for both the general global magnetic coupling constant and the magnetization vector, the latter effect bringing to the statistical features of the magnetic structure and the magnetic symmetry group of the spin glass state.

Fabrication of hybrid metal island/silicon single electron transistor

A novel process for silicon single electron transistors (Si-SET) is presented, combining chemical mechanical polishing (CMP) with the advantages of silicon processing. To eliminate effects caused by random distribution of dopants in the island, The Si-SET’s doped-Si island is replaced with an aluminum or tantalum island by integrating a metal CMP technique with our nano-encapsulation process. The fabricated hybrid Al-island Si-SET has ∼10 nm silicon on insulator lines as the source and drain leads. It shows characteristic Coulomb blockade oscillations with a charging energy of ∼4 meV. The fabrication results demonstrate the scalability of the SET island and the tunnel junction size in a manufacturable top-down approach.

Influence of the local environment on Zn acceptors in the GaAs(110) surface

Individual Zn acceptors in the GaAs (110) surface layer are studied with a scanning tunneling microscope. Tunneling spectroscopy reveals a peak associated with a Zn acceptor state, whose linewidth and response to local band bending depend on proximity to neighboring subsurface Zn acceptors. Though identical in topographic images, surface-layer Zn with nearby subsurface neighbors exhibits a broad peak that is insensitive to band bending, while more isolated surface-layer Zn exhibit a narrow peak which is sensitive to band bending. These results are suggestive of a shallow-to-deep acceptor transition, driven by the random dopant distribution.

An Approach Based on Sensitivity Analysis for the Evaluation of Process Variability in Nanoscale MOSFETs

We propose an approach to evaluate the effect on the threshold-voltage dispersion of nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) of line-edge roughness, surface roughness, and random dopant distribution. The methodology is fully based on parameter sensitivity analysis, performed by means of a limited number of technology computer-aided design simulations or analytical modeling. We apply it to different nanoscale transistor structures, i.e., bulk 45-nm n-channel, 32-nm ultrathin-body silicon-on-insulator, and 22-nm double-gate MOSFETs. In all cases, our approach is capable of reproducing with very good accuracy the results obtained through 3-D atomistic statistical simulations at a small computational cost. We believe that the proposed approach can be a powerful tool to understand the role of the main variability sources and to explore the device design parameter space.

Spatial inhomogeneity inRFeAsO1−xFx(R=Pr, Nd) determined from rare-earth crystal-field excitations

We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO${}_{0.87}$F${}_{0.13}$, and NdFeAsO${}_{0.85}$F${}_{0.15}$. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4$f$ electrons of the Pr${}^{3+}$ and Nd${}^{3+}$ ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of ${T}_{c}$ compared to LaFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ is not due to rare-earth magnetism.

Simulation of Statistical Aspects of Charge Trapping and Related Degradation in Bulk MOSFETs in the Presence of Random Discrete Dopants

The distribution of fractional current change and threshold voltage shift in an ensemble of realistic 35 nm bulk negative-channel metal-oxide-semiconductor field-effect transistors caused by charge trapping on stress-generated defect states at the Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface is studied using 3-D statistical ¿atomistic¿ simulations. The simulations take into account the underlying random discrete dopant distribution in the transistors, which in conjunction with strategically positioned traps could result in rare but dramatic changes in the transistor characteristics. The evolution of threshold voltage distribution as a result of accumulation of trapped charges in the devices due to progressive negative bias temperature instability, positive bias temperature instability, or hot electron degradation is simulated and compared with simple analytical model predictions and recently published experimental measurements to demonstrate the necessity to consider statistical variability in realistic reliability simulations. The magnitude of the degradation in devices of different geometries is also investigated where minimal correlation is found.

Asymmetric gate capacitance and dynamic characteristic fluctuations in 16 nm bulk MOSFETs due to random distribution of discrete dopants

Characteristic variability of a transistor is a crucial issue for nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs). In this study, we explore the asymmetric sketch of the random dopant distribution near the source end and the drain end in 16 nm MOSFETs. Discrete dopants near the source and drain ends of the channel region induce rather different fluctuations in gate capacitance and dynamic characteristics. Based upon the observed asymmetry properties, a lateral asymmetry channel doping profile engineering is then proposed to suppress the random-dopant-induced characteristic fluctuations in the examined devices and circuits. The results of this study indicate that the fluctuations in average gate capacitance, circuit gain, 3 db bandwidth and unity-gain bandwidth for the cases with dopants near the drain side could be simultaneously reduced by 62.6%, 22.2%, 63.1% and 41.4%, respectively. Consequently, such a lateral asymmetry channel doping profile could be considered to design intrinsic parameter fluctuation resistant transistors.

Structural andTcinhomogeneities inherent to doping inLa2−xSrxCuO4superconductors and their effects on the precursor diamagnetism

The inhomogeneities inherent to the random distribution of Sr dopants in La_{2-x}Sr_xCuO_4 superconductors are probed by measuring the x-ray diffraction linewidths and the Meissner transition widths, and then consistently explained on the grounds of a simple model in which the local Sr content is calculated by averaging over distances close to the in-plane electronic mean free path. By taking into account these intrinsic bulk inhomogeneities with long characteristic lengths (much larger than the superconducting coherence length amplitudes), the precursor diamagnetism measured above Tc, a fingerprint of the superconducting transition own nature, is then explained for all doping levels in terms of the conventional Gaussian-Ginzburg-Landau approach for layered superconductors. These results also suggest that the electronic inhomogeneities observed in the normal state by using surface probes overestimate the ones in the bulk.