Dopant Segregation Effects Research Articles

Dopant segregation effects on ohmic contact formation in nanoscale silicon

The impact of boron dopant segregation on the carrier density in silicon nanowires and ohmic contact formation is studied. Based on electrical measurements of individual nanowires and a secondary ion mass spectroscopy study, we show that an oxidation at high temperatures reduces the active carrier density of the silicon nanowires due to the onset of dopant segregation. On the other hand, improved ohmic contact resistivities as low as 6E-7 Ω⋅cm2 is achieved by dopant surface segregation during the thermal formation of platinum silicided contacts. We show, that the contact resistivity decreases with increasing contact pad area for silicided, as well as non-silicide nanowires. Furthermore, it is also demonstrated that the achieved ohmic contacts can be used to realize junctionless nanowire transistors.

Screening and manipulation by segregation of dopants in grain boundary of Silicon carbide: First-principles calculations

The effect of segregation behavior of non-metallic dopants (H/He/O) and metallic dopants (Be/Al/Mg/Y) on the performance of grain boundary (GB) in SiC has been systematically investigated by first-principles calculations. Firstly, the GB energy and excess volume of different GBs have been studied to evaluate the stability of GB and the capacity to accommodate dopant atoms. The solution energies of dopant atoms greatly reduce in the GB region compared with those in the bulk, which makes the dopant atoms inside the grain tend to segregate and aggregate near the GB. The driving force of GB on dopant segregation generally decreases with the increase of distance from GB plane, and the preferential site of dopant is closely correlated with the atomic size of dopant. In addition, H and Y atom possesses the lowest segregation energy at the interstitial and substitutional site near the GB, respectively. Next, the segregation of single dopant induced the changes in the strength and stability of GB have been explored. It is found that non-metallic dopants have the significant embrittlement effects on GB strength. However, the segregation of most metallic dopants could strengthen the GB and Mg atom has the most significant strengthening effect on the GB. The stability of GB can be greatly improved by segregation of Al and Y dopants. Besides, the aggregation of H atoms has the obvious embrittlement effect on the GB. Furthermore, the co-segregation behavior of different dopants has also been explored. Be and Mg dopants have the most significant inhibition effect on the segregation of detrimental impurities H/He/O due to the repulsive interaction between dopant atoms. The present results provide a new insight into the effect of dopant segregation on GB properties and are expected to be a useful guidance for screening the chemical composition and manipulating the performance of SiC-based ceramics.

Selective dopant segregation modulates mesoscale reaction kinetics in layered transition metal oxide

Incorporation of foreign elements into the cathode material is broadly adopted by both academia and industry to improve the battery performance. The lack of an in-depth understanding for the underlying mechanism, however, makes it a largely try-and-error process with unsatisfactory efficiency and effectiveness. This is particularly true for the electrochemical reaction kinetics that is heterogeneous over a broad range of length scales and is determined collectively by the cathode’s electronic structure, lattice configuration, and micro-morphology. Here we unveiled a facet-dependent dopant segregation effect in Zr-modified single-crystal LiNi0.6Co0.2Mn0.2O2 cathode. By forming kinetically favored corners on the cathode particles, the presence of a trace amount of Zr critically modulates the mesoscale reaction kinetics. Our findings suggest that a delicately controlled dopant distribution is a viable strategy for designing the next-generation battery cathode with superior structural and chemical robustness.

Research Update: Ab initio study on resistive memory device optimization trends: Dopant segregation effects and data retention in HfO2−x

Segregation energy trends and their charge state dependence were established for Group II to Group VI substitutional metal dopants in HfO2 using density functional theory. Corroborating the segregation energy with dopant-oxygen bond lengths and charge state stability, strong effects are predicted for Group II and Group III p-type dopants, which can easily reverse their segregation trend due to −2 charge state preference. Transitions between segregation and isolation may occur around 1.5 eV above the valence band maximum, with the exception of Al, which remains barely stable in its segregated form. In Al-doped HfOx, the switching characteristics of filaments formed near Al dopants show subtle changes and the OFF state data retention is degraded. A comprehensive assessment on configurational Al-VO interaction, charge state dependence, and migration energy changes points to the fact that to achieve OFF state data retention improvements, it will be necessary to engineer the filament interaction with Al to stabilize configurations that favor vacancy filament dissolution. Another mode of failure can result from subsequent charge trapping during the operation of the device, which ultimately prohibits the filament dissociation.

Effect of dopant segregation and negative differential mobility on multi-quantum well activation energy

It is shown that the inclusion of dopant segregation, negative differential mobility and thermal assisted tunnelling to the calculation of dark current activation energy provides a much better fitting to the experimental results than geometrical or Poole–Frenkel models and handles well its voltage asymmetry and nonlinearity. Besides, the transport model obtained is able to fit the current–voltage curves for both bias polarities with almost the same transport parameters. A practical method to estimate the segregation length is discussed. A consequence of this method is a more accurate set of values for the transport parameters of the structure, considering direct and reverse bias.

3D Unsteady Simulation of the Transport Characteristics in the LEC Melt of In-Doped GaAs

A three-dimensional and time-dependent turbulent mathematical model is established for the mass, heat, momentum and dopant transport in the LEC melt of In-doped GaAs. The Solution scheme for the dopant segregation effect at the melt/crystal interface is put forward. Grid testing calculations have been performed for the choice of the grid. The turbulent mathematical model and numerical methodology are used to simulate the melt convections and dopant transports in the previously published experiments, and the numerical accuracy is validated by comparing the results with the experimental data in different model setups. Some transport characteristics in the LEC melt of In-doped GaAs have been concluded. Owing to the interacting forces associated with different length scales in the LEC melt, the fluid flow is non-axisymmetric. Because of the competition between buoyancy, Marangoni force, centrifugal force and the Coriolis force, the temperature fluctuates in the melt. Due to the segregation, the InAs concentration increase in the axial direction.

Thermally-Induced Dopant Segregation Effects on the Space Charge Layer and Ionic Conductivity of Nanocrystalline Gadolinia-Doped Ceria

Grain boundaries are known to block ionic conduction across grain boundaries in oxide ion conductors due to adjacent space charge layers. Since the positively charged grain boundary core is intensified with a high local concentration of defects such as oxygen vacancies, uniform distribution of a dopant may mitigate the formation of space charge layers and enhance the ionic conductivity. To investigate the dopant segregation effect on the space charge layer and ionic conductivity, we provided thermal energy to nanocrystalline gadolinia-doped ceria (GDC) thin film by post-annealing at different temperatures of 700°C, 900°C, and 1100°C. STEM-EELS analysis demonstrates strong dopant segregation and a higher Ce3+ content near the grain boundary than in the bulk after post-annealing. The concurrent segregation of dopants and Ce3+ ions implies that once thermal treatment is applied to nanocrystalline GDC thin films, complete space charge layers are formed while the non-thermally treated GDC film with a uniform distribution of dopants has less of a space charge effect and exhibits superior ionic conductivity.

A study on NiGe-contacted Ge n+/p Ge shallow junction prepared by dopant segregation technique

In this work, the effect of dopant segregation on the NiGe/n-Ge contact is studied by experiments and first-principles calculations. Both Al-contacted and NiGe-contacted n+/p junctions were fabricated. Phosphorus and arsenic ions were Implanted Before Germanide (IBG) formation or Implanted After Germanide (IAG) formation. The NiGe-contacted junction always exhibit higher forward current than the Al-contacted junction due to dopant segregation. First principles calculations predict that phosphorus atoms tend to segregate on both NiGe side and Ge side while arsenic atoms tend to segregate at Ge side. Since phosphorus has higher activation level and lower diffusion coefficient than arsenic, we propose a phosphorus IBG+arsenic IAG process. Shallow n+/p junction with junction depth 90nm below the NiGe/Ge interface is achieved. The lowest and average contact resistivity is 2×10−6Ωcm2 and 6.7×10−6Ωcm2, respectively. Methods which can further reduce the junction depth and contact resistivity are suggested.

Observations of crack propagation along a Zr-doped alumina grain boundary.

Structural ceramics are typically used in polycrystalline form. It is well known that polycrystalline ceramics often show the intergranular fracture. To improve their mechanical properties, transition metals can be used as dopants into a bulk material, which tend to segregate into the grain boundaries[1]. However, the effect of dopant segregation on grain boundary fracture is still uncertain. In order to investigate the fracture behavior of a dopant-segregated grain boundary, we observed the crack propagation of a Zr-doped alumina grain boundary by in situ nanoindentation in a transmission electron microscope (TEM), and characterized the fracture surface by scanning TEM (STEM).An alumina bicrystal with a Zr-doped Σ13 grain boundary was fabricated by diffusion bonding at 1500(o)C for 10 hours in air, where a face of one crystal was coated by Zr metal in advance to the bonding (Fig.1a). A TEM sample was prepared from the bicrystal by mechanical grinding and Ar ion milling. For in situ indentation, the sample had a free edge perpendicular to the grain boundary (Fig.1b). The indentation experiment was performed by using a double-tilt indentation holder (Nanofactory) and JEM-2010 (200kV, JEOL). The fracture surface was further observed by high angle annular dark field (HAADF) STEM (ARM-200F, 200kV, JEOL).jmicro;63/suppl_1/i20-a/DFU064F1F1DFU064F1Fig. 1.(a) Schematic illustrations of bicrystal fabrication by diffusion bonding and (b) Bright field TEM image showing the geometric arrangement of the in situ nanoindentation experiment In the in situ TEM nanoindentation experiment, at first a crack was introduced in bulk close to the grain boundary and propagated with the amount of indentation. After the crack reached the grain boundary, it preferentially propagated along the grain boundary. To identify the crack pass at the atomic level, the STEM analysis was performed. We found that three-atomic-layer Zr was formed in the unbroken region of the grain boundary, whereas one to three Zr layers remained on the fracture surface. This indicates that the crack propagated within the segregation region of Zr in the grain boundary. In the presentation, we will discuss the crack propagation behavior and the atomic structure of the fracture surfaces in detail.

Growth of Semi-Insulating GaAs Crystal with Uniform Si Doping

An improved double crucible method of melt replenishing to counteract the dopant segregation effect in order to grow homogeneous doped GaAs crystals is brought forward, and its validity and feasibility are demonstrated by analytical and numerical study. The numerical results show that the new method can suppress the increase of dopant concentration near the growth interface and can maintain the homogeneity of dopant distribution along the radial direction. The positions of replenishing melt exit almost have no effects on the axial and radial distribution homogeneity of dopant Si in the crystal. So the new method has many distinct advantages: it can improve the crystallization rate and size, the operation is flexible, etc.

Reduction of thermal conductivity in dually doped ZnO by design of three-dimensional stacking faults

A design paradigm for fabrication of high quality thermoelectric material was demonstrated by using wurtzite ZnO based materials. To prepare the three-dimensional stacking faults in the microstructure of ZnO, Ga2O3 and In2O3 which were guided by atomistic simulation were selected as doping oxides in wurtzite ZnO. TEM microanalysis experimentally confirmed two kinds of stacking faults which were along the basal-plane (i.e. {0001}) and pyramidal-plane (i.e. {10–14}) in the microstructure of ZnO dually doped with small amount of Ga2O3 and In2O3. Also, EDS analysis results indicated the dopant segregation effect at stacking faults which was predicted by our atomistic simulation results. By varying doping levels, it was found that the (Ga0.004, In0.004)Zn0.992O with dense three-dimensional stacking faults revealed low lattice thermal conductivity (1.7 W m−1 K−1 at 773 K) and high thermo-electric performance (ZT: 0.19 at 773 K) in the present work. Based on all experimental results, it is expected that the combined method of atomistic simulation, microanalysis and processing route design will provide us great opportunity for design of three-dimensional stacking faults in the microstructure of ZnO with high performance.

2420 アルミナ結晶中のらせん転位近傍への不純物偏析 : ハイブリッド量子/古典分子動力学解析

2420 アルミナ結晶中のらせん転位近傍への不純物偏析 : ハイブリッド量子/古典分子動力学解析

Impact of stochastic accelerations on dopant segregation in microgravity semiconductor crystal growth

The residual accelerations that are typically present in microgravity environments (g-jitters) contain a broad spectrum of frequencies and may be modeled as stochastic processes. Their effects on the quality of the semiconductor crystals are analyzed here quantitatively with direct numerical simulation. In particular we focus on the dopant segregation effects due to thermosolutal convection as a function of the parameters characterizing the statistics of the stochastic force. The numerical simulation is specified for material parameters of two doped semiconductors (Ge:Ga and GaAs:Se) in realistic conditions of actual microgravity environments. As a general result, we show that the segregation response is strongly dominated by the low-frequency part of the g-jitter spectrum. In addition, we develop a simplified model of the problem based on linear response theory that projects the dynamics into very few effective modes. The model captures remarkably well the segregation effects for an arbitrary time-dependent acceleration of small amplitude, while it implies an enormous reduction of computer demands. This model could be helpful to analyze results from real accelerometric signals and also as a predictive tool for experimental design.

Dopant Segregated Pt and Ni Germanosilicide Schottky S/D p-MOSFETs with Strained Si-SiGe Channel

Dopant segregated Pt and Ni germanosilicide (PtSiGe and NiSiGe) Schottky source/drain (S/D) transistors were demonstrated on Si1-xGex substrates. The Schottky barrier height of Pt-Si0.7Ge0.3 was decreased by 0.2 eV compared to Pt- Si due to the smaller band-gap of SiGe and the dopant segregation effect. Schottky p-MOSFETs with Pt germanosilicided S/D junctions and strained Si/SiGe channels have also been demonstrated with 65 % higher drive current and lower leakage current compared with their Ni counterparts.

A novel split-gate MOSFET design realized by a fully silicided gate process for the improvement of transconductance and output resistance

Work function tuning of nickel silicide (NiSi) gates was utilized to fabricate a novel split-gate MOSFET with improved device performance. The MOSFET with a NiSi split gate has been achieved by implanting antimony into the polysilicon gate from the drain side with a tilt angle, followed by a full nickel-silicidation process. The laterally nonuniform antimony implantation causes the NiSi gate work function to vary from the source side to the drain side due to the dopant segregation effect. Improved current drive and output resistance are observed in the MOSFET with such a NiSi split gate. Metal gate advantages and NiSi process simplicity were also realized in the split-gate process, and gate oxide quality did not degrade due to the low temperature process. This split-gate design is expected to be applicable in the nanoscale regime by optimizing process conditions.

Quantum Computer Development with Single Ion Implantation

Spins of single donor atoms are attractive candidates for large scale quantum information processing in silicon. Formation of devices with a few qubits is crucial for validation of basic ideas and development of a scalable architecture. We describe our development of a single ion implantation technique for placement of single atoms into device structures. Collimated highly charged ion beams are aligned with a scanning probe microscope. Enhanced secondary electron emission due tohigh ion charge states (e.g., 31P13+, or 126Te33+)allows efficient detection of single ion impacts. Studies of electrical activation of low dose, low energy implants of 31P in silicon show a drastic effect of dopant segregation to the SiO2/Si interface,while Si3N4/Si retards 31P segregation. We discuss resolution limiting factors in ion placement, and process challenges forintegration of single atom arrays with control gates and single electron transistors.

Damage accumulation and dopant migration during shallow As and Sb implantation into Si

The damage evolution and concomitant dopant redistribution as a function of ion fluence during ultra shallow, heavy ion implants into Si have been investigated using medium energy ion scattering (MEIS) and secondary ion mass spectrometry (SIMS). These studies involved As and Sb ions implanted at room temperature, at energies of 2.5 and 2 keV to doses from 3 × 10 13 to 5 × 10 15 cm −2. MEIS is capable of detecting both the displaced atom and implant profiles with sub-nanometre depth resolution. These studies show that for doses up to 1 × 10 14 cm −2 (at which an amorphous layer is formed) the damage build up does not follow the energy deposition function. Instead it proceeds through the initial formation of a ∼4 nm wide amorphous layer immediately under the oxide, that grows inwards into the bulk with increasing dose. This behaviour is explained in terms of the migration of some of the interstitials produced along the length of the collision cascade to the oxide or amorphous/crystal Si interface, where their trapping nucleates the growth of a shallow amorphous layer and the subsequent planar growth inwards of the damage layer. Although for doses ⩾4 × 10 14 cm −2 the As depth profiles agreed well with TRIM calculations, for lower doses As was observed to have a shallower profile, ∼2 nm nearer to the surface. This behaviour is related the growth of the amorphous layer and ascribed to the movement of As into the near-surface amorphous layer (probably mediated by point defect migration) in which the larger dopant is accommodated more easily. SIMS studies have confirmed this dopant segregation effect. Shallow Sb implants also exhibit this novel dopant movement effect for low doses in combination with a damage evolution similar to As.

Epitaxial Si-based tunnel diodes

Tunneling devices in combination with transistors offer a way to extend the performance of existing technologies by increasing circuit speed and decreasing static power dissipation. We have investigated Si-based tunnel diodes grown using molecular beam epitaxy (MBE). The basic structure is a p + layer formed by B delta doping, an undoped spacer layer, and an n + layer formed by Sb delta doping. In the n-on-p configuration, low temperature epitaxy (300–370°C) was used to minimize the effect of dopant segregation and diffusion. In the p-on-n configuration, a combination of growth temperatures from 320 to 550°C was used to exploit the Sb segregation to obtain a low Sb concentration in the B-doped layer. Post-growth rapid thermal anneals for 1 min in the temperature interval between 600 and 825°C were required to optimize the device characteristics. J p, the peak current density, and the peak-to-valley current ratio (PVCR), were measured at room temperature. An n-on-p diode having a spacer layer composed of 4 nm Si 0.6Ge 0.4, bounded on either side by 1 nm Si, had a J p=2.3 kA/cm 2 and PVCR=2.05. A p-on-n tunnel diode with an 8 nm Si spacer (5 nm grown at 320°C, 3 nm grown at 550°C) had a J p=2.6 kA/cm 2 and PVCR=1.7.

Surface segregation of transition metals in sol-gel silicafilms

Silica sol-gel films (SGFs) doped with various transition metalsderived by spin-coating on glass substrates and silicon wafers have beenstudied. Precursor solutions were prepared by mixing tetraethylorthosilicate,ethanol, water and nitrates of the transition metals (Cr, Mn, Fe, Co, Ni andCu) using HCl as a catalyst. Remarkable segregation of metals at the surfacewas observed for films doped with Cu and Co. For Mn- and Ni-doped films thiseffect was much less, and it vanished for the films doped with Cr or Fe. Theinfluence of boron and phosphorus upon the chemical state and the distributionof metal atoms was also analysed. The structure, optical properties andcomposition of the SGFs were studied and some pathways to control the dopantsegregation effect were realized.

Analysis of the dopant segregation effects at the floating zone growth of large silicon crystals

A computer simulation is carried out to study the dopant concentration fields in the molten zone and in the growing crystal for the floating zone (FZ) growth of large (> 100mm) Si crystals with the needle-eye technique and with feed/crystal rotation. The mathematical model developed in the previous work is used to calculate the shape of the molten zone and the velocity field in the melt. The influence of melt convection on the dopant concentration field is considered. The significance of the rotation scheme of the feed rod and crystal on the dopant distribution is investigated. The calculated dopant concentration directly at the growth interface is used to determine the normalized lateral resistivity distribution in the single crystal. The calculated resistivity distributions are compared with lateral spreading resistivity measurements in the single crystal.