Dopant Surface Segregation Research Articles

Exploring the Surface Segregation of Rh Dopants in PtNi Nanoparticles through Atom Probe Tomography Analysis

Exploring the Surface Segregation of Rh Dopants in PtNi Nanoparticles through Atom Probe Tomography Analysis

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.

Dopant Redistribution and Activation in Ga Ion-Implanted High Ge Content SiGe by Explosive Crystallization during UV Nanosecond Pulsed Laser Annealing

Explosive crystallization (EC) is often observed when using nanosecond-pulsed melt laser annealing (MLA) in amorphous silicon (Si) and germanium (Ge). The solidification velocity in EC is so fast that a diffusion-less crystallization can be expected. In the contacts of advanced transistors, the active level at the metal/semiconductor Schottky interface must be very high to achieve a sub-10−9 ohm·cm2 contact resistivity, which has been already demonstrated by using the dopant surface segregation induced by MLA. However, the beneficial layer of a few nanometers at the surface may be easily consumed during subsequent contact cleaning and metallization. EC helps to address such kind of process integration issues, enabling the optimal positioning of the peak of the dopant chemical profile. However, there is a lack of experimental studies of EC in heavily-doped semiconductor materials. Furthermore, to the best of our knowledge, dopant activation by EC has never been experimentally reported. In this paper, we present dopant redistribution and activation by an EC process induced by UV nanosecond-pulsed MLA in heavily gallium (Ga) ion-implanted high Ge content SiGe. Based on the obtained results, we also highlight potential issues of integrating EC into real device fabrication processes and discuss how to manage them.

3D Simulation for Melt Laser Anneal Integration in FinFET’s Contact

Process integration feasibility of UV nanosecond melt laser annealing (MLA) in 14 nm node generation FinFET's contact for dopant surface segregation and activation is assessed by using a 3D TCAD simulation tool. In a n-type source/drain (S/D) in-situ phosphorous doped epilayer, Sb ion implantation is performed, considering the advantage of its surface segregation in lowering of the contact resistivity. The simulation results show that the heat sources created by the laser irradiation are confined mainly in the replacement metal gate (RMG) part, suggesting a potential interest of controlling the polarization of laser light to enlarge the process window by reducing the laser absorption in the RMG part. Also, the estimated solidification front velocity (V) in the MLA-induced epilayer regrowth (~4 m/s) satisfies the requirements (~1 m/s <; V <; ~15 m/s) to enable the surface segregation and metastable activation of the dopants. The surface segregation is also experimentally confirmed in the FinFET contact module.

Size effect of electronic properties in highly arsenic-doped silicon nanowires

The unique electrostatic properties of semiconductor nanowires enable the realization of novel transistor types by the possibility to use surround gate architectures resembling ideal gate electrostatic control. Nevertheless one fundamental issue of semiconducting nanowire channels is the reliable control of doping to adjust the charge carrier concentration. Indeed, as dimensions scale down the surrounding media and the interfaces become more important. In this study we experimentally investigate the role of surface depletion and dielectric mismatch on the electronic charge transport of highly arsenic doped and bottom-up grown silicon nanowires. Electrical characterization of silicon nanowires (SiNWs) synthesized by Au catalyzed vapour-liquid-solid (VLS) growth and in-situ arsine (AsH3) doping is reported for the first time. We demonstrate that high n-type doping is possible by adjusting the dopant precursor flow ratio during growth. Based on electrical measurements of individual nanowires, reproducible donor concentrations of up to 5.2 × 1019 cm−3 could be revealed. By measuring the electrical characteristics for individual nanowires in dependence of their radius, we show that the electrically active carrier density drastically reduces for small nanowires at radii much larger than those at which quantization or dopant surface segregation effects are expected to occur. Furthermore, enhancement of the contact transparency for small radii nanowires is demonstrated through dopant segregation upon metal silicidation. Size dependent measurement of electrical characteristics revealed improved contact resistivities as low as 1.4 × 10−11 Ωm2.

Dopant segregation and CO adsorption on doped Fe3O4 (1 1 1) surfaces: A first-principle study

The water–gas shift reactions (WGS) used to produce hydrogen in industry are usually catalyzed by iron-based catalysts, in which the Fe3O4 was confirmed to be the catalytically active phase of iron oxide. In this work, based on the first-principle simulations, we theoretically study the relation between the dopant segregation and activities of the doped Fe3O4 for adsorbing CO. The surface electronic structure is proved to be a fundamental criterion for CO adsorption on doped Fe3O4 surfaces, which can be obtained by theoretical calculations. The surface segregation of dopants generally determine the effects of dopants on the surface properties, and the CO adsorption aims at the special reaction of WGS. It is worth emphasizing that, for a series of experiments on doped Fe3O4 catalyzed WGS, a unified explanation can be given based on the present results. Our theoretical calculations are well consistent with the recent experimental results at around 400 °C. Therefore, segregation could have decisive roles on the nature of WGS catalysis by iron oxides. This work is instructive for exactly depicting the catalytic activities of materials based on first-principle calculations, and is also helpful to search highly active catalysts.

First‐Principles Computational Screening of Dopants to Improve the Deacon Process over RuO2

AbstractDoping of metal oxides is a promising route to further optimize their catalytic performance. To guide corresponding experimental endeavors, we performed a DFT‐based computational screening study for a wide range of metal dopant atoms in rutile RuO2. With a focus on the Deacon process, that is, the catalytic oxidation of HCl to chlorine and water, we used the rate‐controlling Cl desorption energy as a reactivity descriptor. As stability descriptors, we employed the dopant surface segregation energy and as the dopant thermodynamic stability against precipitation into the metal oxide or bulk oxide grains. In the oxygen‐rich conditions of the Deacon process, particularly the instability against oxide precipitation, represents a strong limitation. In this respect, doping with Cu appears as an optimum compromise between stability and catalytic activity enhancement.

Fermi energy tuning with light to control doping profiles during epitaxy

The influence of light stimulation and photogenerated carriers on the process of dopant surface segregation during growth is studied in molecular beam epitaxially grown Si-doped GaAs structures. The magnitude of surface segregation decreases under illumination by above-bandgap photons, wherein splitting of the quasi Fermi levels reduces the band bending at the growth surface and raises the formation energy of compensating defects that can enhance atomic diffusion. We further show that light-stimulated epitaxy can be used as a practical approach to diminish dopant carry-forward in device structures and improve the performance of inverted modulation-doped quantum wells.

Surface Control of Epitaxial Manganite Films via Oxygen Pressure.

The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. This is of special importance in the systems with multiple competing interactions. We have used in situ scanning tunneling microscopy to image surfaces of La5/8Ca3/8MnO3 films grown by pulsed laser deposition. The atomically resolved imaging was combined with in situ angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O2 leads to mixed-terminated film surfaces, with B-site (MnO2) termination being structurally imperfect at the atomic scale. A relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La,Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction.

KINETICS OF DOPANT ACCUMULATION IN THE ADSORPTION LAYER DURING MOLECULAR-BEAM EPITAXY

A kinetic model of the doping processes in molecular beam epitaxy is developed. The dopant incorporation into the growing crystal is assumed to occur via both blocking dopant atoms at the kink positions by the host atoms and atomic exchange between dopant adatoms and atoms of the topmost crystalline layer. The dopant surface segregation is treated as accumulation of dopant atoms in energetically favorable adsorption positions on the surface. Two segregation pathways are considered: climbing of dopant adatoms over moving steps and jumping of the dopants out of the topmost crystalline layer. It is shown that increasing supersaturation in the adlayer, e.g. with decreasing temperature or increasing growth rate, leads to more efficient blocking of the dopant atoms and, as a consequence, to the decreasing surface segregation. The supersaturation is partially reduced with appearance of 2D islands on the terraces and creation of kinks at the 2D island edges. This results in weaker growth rate and temperature dependences of the surface segregation ratio. Very strong (superexponential) dependence of the surface segregation ratio on the growth temperature is possible under the condition of the intensive jumping of the dopants out of the topmost crystalline layer. The reason is that the probability of «immurement» of the dopant atoms by the moving step depends exponentially on the jumping-out rate constant. The model reproduces experimentally observed segregation behavior of Sb on Si(100).

Surface Segregation Behavior of B, Ga, Sb, and As Dopant Atoms on Ge(100) and Ge(111) Examined with a First-principles Method

To understand surface segregation behaviour of B, Ga, As, and Sb dopant atoms on Ge (100) and Ge (111) surfaces, the potential energies of these dopant atoms in the five top layers of the surfaces were evaluated by first-principles molecular orbital calculations of model clusters. The obtained potential energy curves of these dopant atoms for Ge (100) closely resemble those for Si (100) surface. On the other hand, Ge (111) surface exhibits different potential profiles as compared with Ge (100) surface. The thermal activation energy between the adsorbed state and sub-surface state was also evaluated for each dopant atom. These energy data determining the driving force and dynamical property of dopant segregation help us to understand the difference in dopant surface segregation phenomena on Ge(100) and Ge(111) surfaces from an atomistic point of view.

First-principles theoretical analysis of transition-metal doping of ZnSe quantum dots

We present a systematic analysis of the underlying mechanism of transition-metal doping in ZnSe nanocrystals, using first-principles density functional theory calculations. Our analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets. We find that the chemical potentials of the growth precursor species determine the surface structure and morphology of the nanocrystals. We report binding energies for Mn adsorption onto ZnSe surfaces and find that all the anion-rich surfaces contribute toward dopant adsorption onto ZnSe nanocrystal surface facets. Beyond a critical value of dopant surface coverage, these adsorbed dopants may induce structural transitions in low-Miller-index surface facets, resulting in morphological transitions of the ZnSe nanocrystals. In addition, the dopant binding-energy dependence on the dopant surface concentration explains the doping difficulties during nanocrystal growth. Finally, we report surface segregation energy profiles for Mn dopant segregation on low-Miller-index ZnSe nanocrystal surface facets. We find that, under conditions that render ZnSe(001)-(2 × 1) as the dominant dopable surface of ZnSe nanocrystals, Mn dopants do not have a tendency to segregate on this surface; this guarantees that the dopants remain incorporated into the core regions of the nanocrystal instead of escaping to the surface.

Understanding the role of Si doping on surface charge and optical properties: Photoluminescence study of intrinsic and Si-doped InN nanowires

In the present work, the photoluminescence (PL) characteristics of intrinsic and Si-doped InN nanowires are studied in detail. For intrinsic InN nanowires, the emission is due to band-to-band carrier recombination with the peak energy at $\ensuremath{\sim}$0.64 eV (at 300 K) and may involve free-exciton emission at low temperatures. The PL spectra exhibit a strong dependence on optical excitation power and temperature, which can be well characterized by the presence of very low residual electron density and the absence or a negligible level of surface electron accumulation. In comparison, the emission of Si-doped InN nanowires is characterized by the presence of two distinct peaks located at $\ensuremath{\sim}$0.65 and $\ensuremath{\sim}$0.73--0.75 eV (at 300 K). Detailed studies further suggest that these low-energy and high-energy peaks can be ascribed to band-to-band carrier recombination in the relatively low-doped nanowire bulk region and Mahan exciton emission in the high-doped nanowire near-surface region, respectively; this is a natural consequence of dopant surface segregation. The resulting surface electron accumulation and Fermi-level pinning, due to the enhanced surface doping, are confirmed by angle-resolved x-ray photoelectron spectroscopy measurements on Si-doped InN nanowires, which is in direct contrast to the absence or a negligible level of surface electron accumulation in intrinsic InN nanowires. This work elucidates the role of charge-carrier concentration and distribution on the optical properties of InN nanowires.

Donor deactivation in silicon nanostructures

The operation of electronic devices relies on the density of free charge carriers available in the semiconductor; in most semiconductor devices this density is controlled by the addition of doping atoms. As dimensions are scaled down to achieve economic and performance benefits, the presence of interfaces and materials adjacent to the semiconductor will become more important and will eventually completely determine the electronic properties of the device. To sustain further improvements in performance, novel field-effect transistor architectures, such as FinFETs and nanowire field-effect transistors, have been proposed as replacements for the planar devices used today, and also for applications in biosensing and power generation. The successful operation of such devices will depend on our ability to precisely control the location and number of active impurity atoms in the host semiconductor during the fabrication process. Here, we demonstrate that the free carrier density in semiconductor nanowires is dependent on the size of the nanowires. By measuring the electrical conduction of doped silicon nanowires as a function of nanowire radius, temperature and dielectric surrounding, we show that the donor ionization energy increases with decreasing nanowire radius, and that it profoundly modifies the attainable free carrier density at values of the radius much larger than those at which quantum and dopant surface segregation effects set in. At a nanowire radius of 15 nm the carrier density is already 50% lower than in bulk silicon due to the dielectric mismatch between the conducting channel and its surroundings.

Si ∕ Si Ge n-type resonant tunneling diodes fabricated using in situ hydrogen cleaning

In situ hydrogen cleaning to reduce the surface segregation of n-type dopants in SiGe epitaxy has been used to fabricate Si∕SiGe resonant tunneling diodes in a joint gas source chemical vapor deposition and molecular beam epitaxial system. Diodes fabricated without the in situ clean demonstrate linear current-voltage characteristics, while a 15min hydrogen clean produces negative differential resistance with peak-to-valley current ratios up to 2.2 and peak current densities of 5.0A∕cm2 at 30K. Analysis of the valley current and the band structure of the devices suggest methods for increasing the operating temperature of Si∕SiGe resonant tunneling diodes as required for applications.

Surface segregation of Si and Mg dopants in MOVPE grown GaN films revealed by X‐ray photoemission spectro‐microscopy

AbstractThe surface segregation of Mg and Si dopant species in GaN films has been investigated by means of combined X‐ray photoemission spectroscopy and microscopy. For both species, a segregation tendency was detected. Comparing Mg doped samples before and after Ar ion sputtering, a Mg surface enrichment is concluded on that extends at least over several atomic layers. For Si doped films, the formation of facetted cracks with threefold symmetry was observed. The Si surface concentration was found to be enhanced at the facets of these cracks by a factor of 2.5 with respect to the c ‐plane surface. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth of Fe-doped GaN by RF plasma-assisted molecular beam epitaxy

Iron-doped GaN films were grown using plasma-assisted molecular beam epitaxy (PA-MBE) for various III/V ratios. Iron concentrations were determined by calibrated secondary-ion mass spectroscopy (SIMS), and films were characterized by high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The iron incorporation strongly depended on the growth conditions, with increasing incorporation with decreasing III/V ratio. The behavior was explained by promotion of surface segregation of iron by the gallium adlayer and droplets. AFM indicated no effect of iron doping on surface morphology. Precipitation of intermetallic FeGa 3 was found for gallium droplet regime growth and high iron fluxes. Nitrogen-rich growth resulted in significantly enhanced iron incorporation and reduced segregation, and nitrogen-rich iron-doped films were grown for iron concentrations of up to 1×10 19 cm −3 with no evidence of iron-related phase precipitation or crystalline degradation. Highly-doped nitrogen-rich films were overgrown with gallium-rich GaN, resulting in specular surface morphologies and sharp turn-on and turn-off of the iron profile in SIMS. Similarities to the reported behavior of manganese doping of GaN suggests that surface segregation of transition metal dopants may be important for PA-MBE growth.

Suppression of surface segregation of silicon dopants during molecular beam epitaxy of (411)A In[sub 0.75]Ga[sub 0.25]As∕In[sub 0.52]Al[sub 0.48]As pseudomorphic high electron mobility transistor structures

The authors achieved considerable suppression of surface segregation of Si dopants in In0.75Ga0.25As∕In0.52Al0.48As high electron mobility transistor (HEMT) structures grown on (411)A InP substrates by molecular beam epitaxy (MBE). The (411)A HEMT structures were conventionally grown at a high substrate temperature (Ts) of 540°C in order to form the extremely flat (411)A heterointerfaces. This results in considerable surface segregation of Si dopants. Surface segregation of Si dopants was suppressed by lowering growth temperature of the top InAlAs barrier layer (TB) down to 450°C with keeping Ts of 540°C for other parts. Sheet carrier concentration (Ns) of two-dimensional electron gas (2DEG) was measured as a function of thickness (Lb) of the top InAlAs barrier for TB=450 and 540°C samples. Observed Ns of the TB=540°C sample vanished when Lb approached to 7nm, while 2DEG with of Ns∼2×1012cm−2 remained for the TB=450°C sample. Surface segregation of Si-sheet-doped (411)A InAlAs layers grown at TB was also ...

Kinetic Modeling Of Dopant And Impurity Surface Segregation During Vapor Phase Growth: Multiple Mechanism Approach

ABSTRACTWe propose a new kinetic model for surface segregation during vapor phase growth that accounts for multiple segregation mechanisms, including mechanisms for terrace mediated exchange and step edge mediated exchange. The major result of the model is an analytic expression for the experimentally measured segregation length and profile broadening that can be readily calculated without the need for numerical simulations. We compare the model to experimental measurements for the temperature dependence of segregation of Sb in Si(001). The model is able to accurately describe both the anomalous segregation at low temperature and the transition between equilibrium and kinetically limited segregation at high temperature. An excellent agreement is obtained using realistic energies and pre-exponential factors for the kinetic rate constants. The model can be applied to other segregating systems in planar geometries, including metallic and III-V semiconducting thin films.

Deposition and Characterization of Transparent Thin Films of Zinc Oxide Doped with Bi and Sb

Undoped and Bi/Sb doped highly transparent ZnO films were grown adopting metal organic chemical vapor deposition with suitable combinations of Zn(C5F6HO2)2·2H2O·(CH3OCH2CH2)2O, Bi(C6H5)3, and Sb(C6H5)3 precursors. Hexagonal ZnO phases were always found. Doping levels ranged from 1.4 to 7.4 and from 0.2 to1.8 atomic % for Bi and Sb, respectively. UV−vis measurements indicated that the film transmittance was usually as high as 90% in the visible and near-infrared region. Dopant surface segregation was found in X-ray photoelectron spectroscopy experiments. Preliminary resistivity measurements indicated that lightly doped ZnO films are semiconducting while greater doping levels could result in potential varistor properties.