Dopant-defect Interactions Research Articles

Giant electrostriction in bulk RE (III) substituted CeO2: Effect of RE-[formula omitted] interaction and RE concentration

Recent discovery of giant electrostriction in bulk polycrystalline ceramics and thin films of rare earth (RE (III)) substituted ceria (CeO2) driven by electroactive defect complexes and their coordinated elastic response, expands the material spectrum for electrostrain applications beyond the conventional piezoelectric materials. Bulk ceramics provide an ideal platform to systematically study electrostriction as a function of parameters such as RE3+-defect (VO••) interaction and RE concentration. In this work, we perform structure-property correlation studies in bulk ceramics of RE3+ substituted ceria (RECO) with RE=Y, La and Gd, all of them having weak dopant-defect interaction, with Y- VO•• being attractive, La-VO•• being repulsive, and Gd-VO•• being neutral. We employ large signal measurements to measure the electrostrictive coefficients of all the RECO ceramics, with varying concentration of RE3+, and show that an attractive defect-dopant interaction with atleast 20% of substituent ion concentration yields both giant M and Q electrostrictive response.

Dopant-defect interactions in Mg-doped GaN via atom probe tomography

In this work, an in-depth analysis of atomic level dopant-defect interactions in N-polar GaN:Mg was performed using atom probe tomography (APT). The 3D visualization of ion distribution revealed the formation of spherical Mg-rich clusters and the segregation of Mg dopant toward dislocations in MOCVD-grown GaN:Mg. Impurities, such as oxygen and hydrogen, were identified and detected adjacent to Mg-rich dislocations. Crystal stoichiometry around the defect regions was investigated to understand how the defects can influence dopant diffusion. Non-stoichiometric regions of N:Ga were found adjacent to Mg-rich dislocations and overlapping with some Mg-rich clusters, indicating dopant-defect interplay. Variations in N:Ga were not proportional to the Mg content, suggesting that the micro-features (clusters and dislocations) interact differently with local chemistry. Techniques for defining the quality of an APT experiment through invalidation of artifacts are also demonstrated. Mg-rich defects and variations in N:Ga were found to be independent of artifacts related to the evaporation field in APT.

Impact of oxygen on gallium doped germanium

Germanium (Ge) has advantageous materials properties and is considered as a mainstream material for nanoelectronic applications. Understanding dopant–defect interactions is important to form well-defined doped regions for devices. Gallium (Ga) is a key p-type dopant in Ge. In the present density functional theory study, we concentrate on the structures and electronic structures of Ga doped Ge in the presence of Ge vacancies and oxygen. We provide information on the defect structures and charge transfer between the doped Ga atom and the nearest neighbor Ge atom. The calculations show that the presence of Ga on the Ge site facilitates the formation of nearest neighbor Ge vacancies at 0.75 eV. The formation of interstitial oxygen is endoergic with the formation of −2 charge in both bulk Ge and Ga substituted Ge although the substitution of Ga has slightly less impact on the oxygen interstitial formation.

Electromechanical dopant–defect interaction in acceptor-doped ceria

Oxygen defective cerium oxide CeO2−δ exhibits a non-classical giant electromechanical response that is superior to that of lead-based electrostrictors.

Dopant-defect interactions in highly doped epitaxial Si:P thin films

Clustering configurations and reactions within in-situ doped epitaixal Si:P films were investigated. In-situ highly doped epitaxial Si:P films (HDSiP) were grown by reduced-pressure chemical vapor deposition with phosphorus concentrations up to 4.4 × 1021 cm−3. Additional P activation in Si:P films into metastable states can be achieved by a 1200 °C millisecond laser anneal with negligible dopant diffusion. Dopant deactivation takes place readily during subsequent low temperature annealing for HDSiP despite negligible diffusion during the same time. Diffusion analysis of buried marker layers suggest a high concentration of interstitials being released from HDSiP and Si:P layers doped with sub 1 × 1021 cm−3 P concentrations. Upon thermal annealing. In addition, extrinsic dislocation loop formation was observed in sufficiently high P concentration HDSiP films. It is proposed that a high concentration of grown-in P-interstitial clusters exist concurrently with vacancy clusters theorized in HDSiP thin films.

Reversible Control of the Mn Oxidation State in SrTiO3 Bulk Powders.

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO3) bulk powder. We employ NaBH4 as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn2+ dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti3+ in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy. In addition to Mn2+, Mn4+, and the Mn2+ complex with an oxygen vacancy, we also observe a sharp resonance in the EPR spectrum of heavily reduced Mn-doped SrTiO3. This sharp signal is tentatively assigned to surface superoxide ion that is formed by the surface electron transfer reaction between Ti3+ and O2. The ability to control the relative amounts of various paramagnetic defects in SrTiO3 provides many possibilities to study in a model system the impact of tunable dopant-defect interactions for spin-based electronic applications or visible-light photocatalysis.

Evolution of phosphorus-vacancy clusters in epitaxial germanium

The E centers (dopant-vacancy pairs) play a significant role in dopant deactivation in semiconductors. In order to gain insight into dopant-defect interactions during epitaxial growth of in situ phosphorus doped Ge, positron annihilation spectroscopy, which is sensitive to open-volume defects, was performed on Ge layers grown by chemical vapor deposition with different concentrations of phosphorus (∼1×1018–1×1020cm−3). Experimental results supported by first-principles calculations based on the two component density-functional theory gave evidence for the existence of mono-vacancies decorated by several phosphorus atoms as the dominant defect type in the epitaxial Ge. The concentration of vacancies increases with the amount of P-doping. The number of P atoms around the vacancy also increases, depending on the P concentration. The evolution of Pn–V clusters in Ge contributes significantly to the dopant deactivation.

Defect chemistry and thermodynamic properties of proton dissolution into BaZr0.9Y0.1O3 − δ

Proton concentration in BaZr0.9Y0.1O3−δ was measured by highly-accurate thermogravimetry as functions of temperature and p(H2O), and thermodynamic properties of the proton dissolution were discussed. Apparent Gibbs free energy change of the proton dissolution reaction changes with proton concentration even at a constant temperature. This means the proton dissolution in BaZr0.9Y0.1O3−δ is not ideal-solution-like. The deviation from the ideal solution became more significant with increasing temperature i.e. decreasing the proton concentration. This indicated the interaction between oxygen vacancies or electron hole, dopant-defect interaction, or lattice deformation may cause the proton dissolution in BaZr0.9Y0.1O3−δ especially at higher temperature.

Diffusion ofn-type dopants in germanium

Germanium is being actively considered by the semiconductor community as a mainstream material for nanoelectronic applications. Germanium has advantageous materials properties; however, its dopant-defect interactions are less understood as compared to the mainstream material, silicon. The understanding of self- and dopant diffusion is essential to form well defined doped regions. Although p-type dopants such as boron exhibit limited diffusion, n-type dopants such as phosphorous, arsenic, and antimony diffuse quickly via vacancy-mediated diffusion mechanisms. In the present review, we mainly focus on the impact of intrinsic defects on the diffusion mechanisms of donor atoms and point defect engineering strategies to restrain donor atom diffusion and to enhance their electrical activation.

First principles study of pyrophosphate defects and dopant–defect interactions in stronium-doped lanthanum orthophosphate

LaPO4 has been actively studied for proton conductor applications, due to its stability and proton uptake in humid atmospheres over intermediate temperature ranges. An important process underlying the application of this and related materials for proton-conductor applications is the hydrolysis of pyrophosphate defects. In this work we undertake density-functional-theory (DFT) calculations of the relative energetics of pyrophosphate defects and protons in LaPO4, including their binding with divalent dopant cations. Due to the low symmetry of the monazite crystal structure for LaPO4, there exists four symmetry-distinct pyrophosphate defect configurations; DFT calculations are used to identify the most stable configuration, which is 0.24 eV lower in energy than all others. Further, from supercell calculations with 1.85 mol% Sr doping, we investigate the dopant-binding energies for pyrophosphate defects to be 0.37 eV, which is comparable to the value of 0.34 eV calculated for proton–dopant binding energies in the same system. These results establish that dopant–defect interactions further stabilize proton incorporation, with the hydration enthalpies when the dopants are nearest and furthest from the protons and pyrophosphate defects being −1.66 eV and −1.37 eV, respectively. Even though our calculations show that dopant binding enhances the enthalpic favorability of proton incorporation, they also suggest that such binding is likely to substantially lower the kinetic rate of hydrolysis of pyrophosphate defects.

Modeling of defects, dopant diffusion and clustering in silicon

Ion implantation is a very well established technique to introduce dopants in semiconductors. This technique has been traditionally used for junction formation in integrated circuit processing, and recently also in solar cells fabrication. In any case, ion implantation causes damage in the silicon lattice that has adverse effects on the performance of devices and the efficiency of solar cells. Alternatively, damage may also have beneficial applications as some studies suggest that small defects may be optically active. Therefore it is important an accurate characterization of defect structures formed upon irradiation. Furthermore, the technological evolution of electronic devices towards the nanometer scale has driven the need for the formation of ultra-shallow and low-resistive junctions. Ion implantation and thermal anneal models are required to predict dopants placement and electrical activation. In this article, we review the main models involved in process simulation, including ion implantation, evolution of point and extended defects and dopant-defect interactions. We identify different regimes at which each type of defect is more relevant and its inclusion in the models becomes crucial. We illustrate in some examples the use of atomistic modeling techniques to gain insight into the physics involved in the processes as well as the relevance of the accuracy of models.

Defect engineering strategies for germanium

After years of neglect germanium is emerging as a mainstream material for microelectronics applications. This is partially due to the introduction of high-k dielectrics that have allowed the substitution of native oxides. Nevertheless, there are still difficulties concerning the fast diffusion and deactivation of n-type dopants. The aim of this review is to show how density functional theory (DFT) calculations can aid to the understanding of dopant-defect interactions and the design of defect engineering strategies to optimize the properties of doped germanium.

Electronegativity and doping in semiconductors

Charge transfer predicted by standard models is at odds with Pauling’s electronegativities but can be reconciled by the introduction of a cluster formation model [Schwingenschlögl et al., Appl. Phys. Lett. 96, 242107 (2010)]. Using electronic structure calculations, we investigate p- and n-type doping in silicon and diamond in order to facilitate comparison as C has a higher electronegativity compared to Si. All doping conditions considered can be explained in the framework of the cluster formation model. The implications for codoping strategies and dopant-defect interactions are discussed.

Effect of Microstructure of Nitrogen-Doped Graphene on Oxygen Reduction Activity in Fuel Cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.

Physical understanding of cryogenic implant benefits for electrical junction stability

We investigate the effect of cryogenic temperature implants on electrical junction stability for ultra shallow junction applications for sub-32 nm technology nodes and beyond. A comprehensive study was conducted to gain physical understanding of the impact of cryogenic temperature implants on dopant-defect interactions. Carborane (C2B10H12) molecule, a potential alternative to monomer boron was implanted in carbon preamorphized silicon substrates at cryogenic implant temperatures. Results indicate implants at cryogenic temperatures increase dopant activation with reduced diffusion, resulting in lower sheet resistance for a lower junction depth. Further, this study emphasizes the benefits of co-implants performed at cryogenic temperatures as alternative to traditional preamorphizing implants.

Front-end process modeling in silicon

Front-end processing mostly deals with technologies associated to junction formation in semiconductor devices. Ion implantation and thermal anneal models are key to predict active dopant placement and activation. We review the main models involved in process simulation, including ion implantation, evolution of point and extended defects, amorphization and regrowth mechanisms, and dopant-defect interactions. Hierarchical simulation schemes, going from fundamental calculations to simplified models, are emphasized in this Colloquium. Although continuum modeling is the mainstream in the semiconductor industry, atomistic techniques are starting to play an important role in process simulation for devices with nanometer size features. We illustrate in some examples the use of atomistic modeling techniques to gain insight and provide clues for process optimization.

Transfer of physically-based models from process to device simulations: Application to advanced SOI MOSFETs

Dopant implantation, followed by spike annealing is one of the main focus areas in the simulation of silicon processing due to its ability to form highly-activated ultra-shallow junctions. Coupled with the growing interest in the use of silicon-on-insulator (SOI) wafers, modelling and simulation of the influence of SOI structure on damage evolution and ultra-shallow junction formation on one hand, and on electrical MOSFET device characteristics on the other hand, are required. In this work, physically-based models of dopant implantation and diffusion, including amorphization, defect interactions and evolution, as well as dopant–defect interactions in both bulk silicon and SOI are integrated within a unique simulation tool to model the different physical mechanisms involved in the process of ultra-shallow junction formation. The application to 65 nm SOI MOSFET devices demonstrated the strong impact of the process simulation models on the simulated electrical device characteristics, in particular for both defect evolution and defect dopant interaction with the additional silicon/buried oxide (Si/BOX) interface. Simulation results of the threshold voltage ( V th) and the variation of the on- and off-state currents of the explored structures are in good agreement with experimental data and can provide important insight for optimizing the process in both bulk silicon and SOI technologies.

Understanding of Carbon/Fluorine Co-implant Effect on Boron-Doped Junction Formed during Soak Annealing

The formation of highly activated ultrashallow junctions is one of the main challenges for the forthcoming generation of complementary metal oxide semiconductor (CMOS) devices. Co-implantation of impurities such as carbon (C) or fluorine (F) is an attractive technique. However, junction optimization can only be achieved with a complete understanding of the underlying physical mechanisms. In this paper, the effect of co-implant on boron (B)-doped preamorphized silicon during the soak annealing is extensively studied. atoms are located in the middle range between the concentration profiles and the end-of-range (EOR) defect band, with the aim of reducing the interactions of dopants with the interstitials released from EOR region. Isochronal annealing study is performed to investigate the impact of codoping on the dopant de/reactivation behavior. It is shown that transient enhanced diffusion can be reduced by both co-implant schemes. The B-doped junction formed with the C co-implant is relatively stable and dopant deactivation is inhibited, while it is presumed that F atoms form complexes, which reduces the B activation level. A physical insight on the dopant-defect interactions associated with co-implant is established through the combination of diffusion and activation studies during soak annealing.

Physical insight into ultra-shallow junction formation through atomistic modeling

We use atomistic simulations to gain physical understanding of relevant dopant–defect interactions involved in junction formation. The analysis of the energetics of B–Si interstitial clusters (BICs) indicates that a high Si interstitial supersaturation is necessary to nucleate BICs in crystalline Si, but only a Si interstitial supersaturation slightly larger than that set by BICs is enough to cause the stabilization and growth of preexisting BICs. We have analyzed the mechanisms associated to B uphill diffusion and deactivation in preamorphized Si upon subsequent annealing. Both phenomena occur simultaneously and they are the result of the trapping of B atoms by preexisting B clusters in the high concentration region.

Modeling and Simulation of the Influence of SOI Structure on Damage Evolution and Ultra-shallow Junction Formed by Ge Preamorphization Implants and Solid Phase Epitaxial Regrowth

AbstractPreamorphization implant (PAI) prior to dopant implantation, followed by solid phase epitaxial regrowth (SPER) is of great interest due to its ability to form highly-activated ultra-shallow junctions. Coupled with growing interest in the use of silicon-on-insulator (SOI) wafers, modeling and simulating the influence of SOI structure on damage evolution and ultra-shallow junction formation is required. In this work, we use a kinetic Monte Carlo (kMC) simulator to model the different mechanisms involved in the process of ultra-shallow junction formation, including amorphization, recrystallization, defect interaction and evolution, as well as dopant-defect interaction in both bulk silicon and SOI. Simulation results of dopant concentration profiles and dopant activation are in good agreement with experimental data and can provide important insight for optimizing the process in bulk silicon and SOI.