Bond-centered Site Research Articles

Hydrogen donor formation in a solution-processed ZnO film by UV light irradiation

A transparent ZnO film was deposited by the spin-spray method at a low temperature of 90 °C. Ultraviolet (UV) light irradiation to the film for 60 min decreased the film resistivity drastically by three orders of magnitude from 55.3 to 4.43 × 10−2 Ω cm. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile and 1H solid-state nuclear magnetic resonance (NMR) spectroscopy analysis revealed that the as-deposited ZnO film before UV treatment contained a large amount of water in the ZnO crystal lattice, and hydrogen donors, including interstitial hydrogen in bond-centered sites (Hi), substitutional hydrogen on the oxygen lattice site (HO), and three O–H bonds in a zinc vacancy (VZn−H3), were generated in the ZnO film after UV treatment. From these results, the mechanism of the hydrogen donor formation was proposed in which UV irradiation to photocatalytic ZnO decomposed the water in the ZnO crystal lattice to form H+ and OH− ions, which associated with the oxygen and zinc vacancies, resulting in the formation of Hi, HO, and VZn−H3.

Subthreshold photodissociation of H2 in silicon

A photodissociation of interstitial ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ in single-crystalline Si by a laser field at 532 nm is reported. It is shown that in the course of Raman scattering measurements with a power density of around $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ W/${\mathrm{cm}}^{2}$ the ${\mathrm{H}}_{2}({\mathrm{D}}_{2}$) signal decreases with a characteristic time of a few minutes. At temperatures below 100 K the molecule transforms into a metastable state known as ${\mathrm{H}}_{2}^{*}$, which includes one hydrogen atom located at the bond-centered site and the second one at the antibonding position along the same $\ensuremath{\langle}111\ensuremath{\rangle}$ direction. At temperatures above 100 K both dimers dissociate, whereby the products of fragmentation, inaccessible for the Raman scattering, are proposed to be atomic interstitials.

Engineering nitrogen- and hydrogen-related defects in ZnO nanowires using thermal annealing

The chemical bath deposition (CBD) of ZnO nanowires (NWs) is of high interest, but their formation occurs in a growth medium containing many impurities including carbon, nitrogen, and hydrogen, rendering the accurate determination of predominant crystal defects as highly debated. In addition to the typical interstitial hydrogen in bond-centered sites $({\mathrm{H}}_{\mathrm{BC}})$ and zinc vacancy-hydrogen $({V}_{\mathrm{Zn}}\text{\ensuremath{-}}n\mathrm{H})$ complexes, we reveal that nitrogen-related defects play a significant role on the physical properties of unintentionally doped ZnO NWs. We show by density functional theory that the ${V}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{N}}_{\mathrm{O}}\text{\ensuremath{-}}\mathrm{H}$ defect complex acts as a deep acceptor with a relatively low formation energy and exhibits a prominent Raman line at $3078\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1}$ along with a red-orange emission energy of \ensuremath{\sim}1.82 eV in cathodoluminescence spectroscopy. The nature and concentration of the nitrogen- and hydrogen-related defects are found to be tunable using thermal annealing under oxygen atmosphere, but a rather complex, fine evolution including successive formation and dissociation processes is highlighted as a function of annealing temperature. ZnO NWs annealed at the moderate temperature of 300 \ifmmode^\circ\else\textdegree\fi{}C specifically exhibit one of the smallest free charge carrier densities of $5.6\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$ along with a high mobility of $\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\mathrm{s}$ following the analysis of longitudinal optical phonon-plasmon coupling. These findings report a comprehensive diagram showing the complex interplay of each nitrogen- and hydrogen-related defect during thermal annealing and its dependence on annealing temperature. They further reveal that the engineering of the nitrogen- and hydrogen-related defects as the major source of crystal defects in ZnO NWs grown by CBD is capital to precisely control their electronic structure properties governing their electrical and optical properties in any devices.

Comparing Thermal Durability and Effects of Annealing Temperature on Characteristics of Hydrogen-Doped ZnO, AZO, and GZO Thin Films

In this work, undoped, aluminum-, and gallium-doped ZnO thin films (ZnO-H, AZO-H, and GZO-H, respectively) deposited on soda-lime glass substrates by magnetron sputtering method in a gas mixture of hydrogen and argon are annealed at various temperatures in the range of 200–500°C in air to evaluate the durability of those films under annealing temperature. From photoluminescence spectra, formation of point defects, especially oxygen vacancies, when hydrogen diffuses out of the films at high annealing temperature is exhibited via a significant increase of visible emissions. We find out that carrier concentration and Hall mobility of AZO-H and ZnO-H films dramatically decrease, while those of GZO-H film are still stable as the annealing temperature increased from 200°C to 300°C. We proposed a model for interpreting the thermal durability of GZO-H film that, at an annealing temperature of 300°C, Ga3+ ions located at adjacent Zn sites can push hydrogen atoms, which are broken out of the antibonding sites which are perpendicular to the c -axis (AB┴), into bond center sites paralleled to the c -axis (BC//). The movement of hydrogen from AB┴ to BC// site also gives rise to the durability of electrical properties of GZO-H films at the high annealing temperature.

Insight into segregation sites for oxygen impurities at grain boundaries in silicon

The three-dimensional distribution of oxygen atoms segregated at Σ9{114} grain boundaries (GBs) in Czochralski-grown silicon ingots is analyzed within a high spatial resolution of less than 0.5 nm by atom probe tomography combined with a focused ion beam (FIB) operated at −150 °C. The analysis reveals a segregation of oxygen atoms within a range of 2.5 nm across the GB plane, which is much narrower in comparison with the previous reports obtained using a conventional FIB. The oxygen concentration profile accurately reflects the distribution of the segregation sites, which exist at bond-centered sites under tensile stresses above 2 GPa, as calculated by ab initio local stress calculations.

Experimental characterization of hydrogen adsorption sites for H/W(111) using low energy ion scattering

Low energy ion scattering (LEIS) and direct recoil spectroscopy (DRS) are among the few experimental techniques that allow for the direct detection of hydrogen on a surface. The interpretation of LEIS and DRS measurements, however, is often made difficult by complexities that can arise from complicated scattering processes. Previously, these complexities were successfully navigated to identify the exact binding configurations of hydrogen on a few surfaces using a simple channeling model for the projectile ion along the surface. For the W(111) surface structure, this simple channeling model breaks down due to the large lateral atomic spacing on the surface and small interlayer spacing. Instead, our observed hydrogen recoil signal can only be explained by considering not just channeling along the surface but also scattering from subsurface atoms. Using this more complete model, together with molecular dynamics (MD) simulations, we determine that hydrogen adsorbs to the bond-centered site for the W(111)$+\text{H}$(ads) system. Additional MD simulations were performed to further constrain the adsorption site to a height $h=1.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ and a position ${d}_{\mathrm{BC}}=1.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ along the bond between neighbors in first and second layers. Our determination of the hydrogen adsorption site is consistent with density functional theory simulation results in the literature.

Ab initio treatment of silicon-hydrogen bond rupture at Si/SiO2 interfaces

Even after more than 50 years of development, a major issue in silicon-based technology is the understanding of the $\mathrm{Si}/{\mathrm{SiO}}_{2}$ interface and its defects, particularly the unsaturated silicon dangling bonds which have to be passivated by hydrogen during fabrication. Although it is well known that hydrogen dissociation from an initially passivated interfacial Si dangling bond results in an electrically active defect, there is still no consensus on the actual microscopic Si--H bond-breaking mechanism, despite a significant research effort. The most thorough theoretical study in the field was published by Tuttle and Van de Walle 20 years ago. Although it was then suggested that the hydrogen dissociates most likely into a bond-center site, no clean argument for bond rupture could be given at that time. In order to take a fresh look at this highly important problem, we employ the latest $ab$ initio methods available, including the method of well-tempered metadynamics and nudged-elastic-band calculations based on density functional theory (DFT). This allows us to study the interactions of a Si--H bond with its realistic environment in a three-dimensional Si/$a--{\mathrm{SiO}}_{2}$ interface in considerable detail. Using classical force fields and well-tempered metadynamics in conjunction with DFT, we provide new insights into the dissociation kinetics. We find that one of the previously suggested dissociation paths only leads into a neutral, metastable state which would not facilitate bond dissociation. By sampling the configuration space in greater detail than ever before, we propose a trajectory whereby the H first moves towards an adjacent Si and in a second step relaxes into a configuration between the next-nearest Si--Si bond. The final statistical analysis on a large number of defects on this amorphous interface yields potential energy surfaces and barriers which are in excellent agreement with experimental values.

Electronic structure of interstitial hydrogen in In-Ga-Zn-O semiconductor simulated by muon

We report on the local electronic structure of an interstitial muon (Mu) as pseudohydrogen in In-Ga-Zn oxide (IGZO) semiconductors studied by muon spin rotation/relaxation (μSR) experiment. In polycrystalline (c-) IGZO, it is inferred that Mu is in a diamagnetic state, where the μSR time spectra under zero external field are perfectly described by the Gaussian Kubo-Toyabe relaxation function with the linewidth Δ serving as a sensitive measure for the random local fields from In/Ga nuclear magnetic moments. The magnitude of Δ combined with the density functional theory calculations for H (to mimic Mu) suggests that Mu occupies Zn-O bond-center site (MuBC) similar to the case in crystalline ZnO. This implies that the diamagnetic state in c-IGZO corresponds to MuBC+, thus serving as an electron donor. In amorphous (a-) IGZO, the local Mu structure in the as-deposited films is nearly identical to that in c-IGZO, suggesting MuBC+ for the electronic state. In contrast, the diamagnetic signal in heavily hydrogenated a-IGZO films exhibits the Lorentzian Kubo-Toyabe relaxation, implying that Mu accompanies more inhomogeneous distribution of the neighboring nuclear spins that may involve a Mu− H−-complex state in an oxygen vacancy.

Development of the ReaxFF Reactive Force Field for Inherent Point Defects in the Si/Silica System.

We redeveloped the ReaxFF force field parameters for Si/O/H interactions that enable molecular dynamics (MD) simulations of Si/SiO2 interfaces and O diffusion in bulk Si at high temperatures, in particular with respect to point defect stability and migration. Our calculations show that the new force field framework (ReaxFFpresent), which was guided by the extensive quantum mechanical-based training set, describes correctly the underlying mechanism of the O-migration in Si network, namely, the diffusion of O in bulk Si occurs by jumping between the neighboring bond-centered sites along a path in the (110) plane, and during the jumping, O goes through the asymmetric transition state at a saddle point. Additionally, the ReaxFFpresent predicts the diffusion barrier of O-interstitial in the bulk Si of 64.8 kcal/mol, showing a good agreement with the experimental and density functional theory values in the literature. The new force field description was further applied to MD simulations addressing O diffusion in bulk Si at different target temperatures ranging between 800 and 2400 K. According to our results, O diffusion initiates at the temperatures over 1400 K, and the atom diffuses only between the bond-centered sites even at high temperatures. In addition, the diffusion coefficient of O in Si matrix as a function of temperature is in overall good agreement with experimental results. As a further step of the force field validation, we also prepared amorphous SiO2 (a-SiO2) with a mass density of 2.21 gr/cm3, which excellently agrees with the experimental value of 2.20 gr/cm3, to model a-SiO2/Si system. After annealing the a-SiO2/Si system at high temperatures until below the computed melting point of bulk Si, the results show that ReaxFFpresent successfully reproduces the experimentally and theoretically defined diffusion mechanism in the system and succeeded in overcoming the diffusion problem observed with ReaxFFSiOH(2010), which results in O diffusion in the Si substrate even at the low temperature such as 300 K.

Hydrogen interactions with low-index surface orientations of tungsten

We report on density functional theory calculations that have been performed to systematically investigate the hydrogen-surface interaction as a function of surface orientation. The interactions that were analyzed include stable atomic adsorption sites, molecular hydrogen dissociation and absorption energies, migration pathways and barriers on tungsten surfaces, and the saturation coverage limits on the (1 1 1) surface. Stable hydrogen adsorption sites were found for all surfaces. For the reconstructed W(1 0 0), there are two primary adsorption sites: namely, the long-bridge and short-bridge sites. The threefold hollow site (3F) was found to be the most stable for W(1 1 0), while the bond-centered site between the first and second layer was found to be most stable for the W(1 1 1) surface. No bound adsorption sites for H2 molecules were found for the W surfaces. Hydrogen (H) migration on both the (1 0 0) and (1 1 0) surfaces is found to have preferred pathways for 1D motion, whereas the smallest migration barrier for net migration of H on the W(1 1 1) surface leads to 2D migration. Although weaker H interactions are predicted for the W(1 1 1) surface compared to the (1 0 0) or (1 1 0) surfaces, we observe higher H surface concentrations of Θ = 4.0 at zero K, possibly due to the corrugated surface structure. These results provide insight into H adsorption, surface saturation coverage and migration mechanisms necessary to describe the evolution from the dilute limit to concentrated coverages of H.

Heat and charge transport in bulk semiconductors with interstitial defects

Interstitial defects are inevitably present in doped semiconductors that enable modern-day electronic, optoelectronic or thermoelectric technologies. Understanding of stability of interstitials and their bonding mechanisms in the silicon lattice was accomplished only recently with the advent of first-principles modeling techniques, supported by powerful experimental methods. However, much less attention has been paid to the effect of different naturally occurring interstitials on the thermal and electrical properties of silicon. In this work, we present a systematic study of the variability of heat and charge transport properties of bulk silicon, in the presence of randomly distributed interstitial defects (Si, Ge, C and Li). We find through atomistic lattice dynamics and molecular dynamics modeling studies that, interstitial defects scatter heat-carrying phonons to suppress thermal transport-1.56% of randomly distributed Ge and Li interstitials reduce the thermal conductivity of silicon by $\sim$ 30 and 34 times, respectively. Using first principles density functional theory and semi-classical Boltzmann transport theory, we compute electronic transport coefficients of bulk Si with 1.56% Ge, C, Si and Li interstitials, in hexagonal, tetrahedral, split-interstitial and bond-centered sites. We demonstrate that hexagonal-Si and hexagonal-Ge interstitials minimally impact charge transport. To complete the study, we predict the thermoelectric property of an experimentally realizable bulk Si sample that contains Ge interstitials in different symmetry sites. Our research establishes a direct relationship between the variability of structures dictated by fabrication processes and heat and charge transport properties of silicon. The relationship provides guidance to accurately estimate performance of Si-based materials for various technological applications.

Polarity-Dependent High Electrical Conductivity of ZnO Nanorods and Its Relation to Hydrogen

A statistical analysis of the electrical properties of selective area grown O- and Zn-polar ZnO nanorods by chemical bath deposition is performed by four-point probe resistivity measurements in patterned metal contact and multiprobe scanning tunneling microscopy configurations. We show that ZnO nanorods with either polarity exhibit a bulklike electrical conduction in their core and are highly conductive. O-polar ZnO nanorods with a smaller mean electrical conductivity have a nonmetallic or metallic electrical conduction, depending on the nano-object considered, while the vast majority of Zn-polar ZnO nanorods with a larger mean electrical conductivity present a metallic electrical conduction. We reveal, from Raman scattering and spatially resolved 5 K cathodoluminescence measurements, that the resulting high carrier density of ZnO nanorods with O or Zn polarity is due to the massive incorporation of hydrogen in the form of interstitial hydrogen in bond-centered sites (HBC), substitutional hydrogen on the ...

Site preference and diffusion behaviors of H influenced by the implanted-He in 3C-β SiC

SiC materials are potential plasma facing materials in fusion reactors. In this study, site preference and diffusion behaviors of H in the pure 3C-β SiC and in the He-implanted 3C-β SiC are investigated, on the basis of the first-principles calculations. We find that the most stable sites for H in the pure 3C-β SiC is the anti-bond site of SiC at the side of C (ABc), while it becomes the bond-center (BC) site of SiC in the He-implanted 3C-β SiC. Analysis on the electronic structures reveals that such change is attributed to the reduction of hybridization of CSi bonds induced by the implanted He. Moreover, the presence of He strongly affects the vibration features of the system in the high frequency region, causing a blue shift of 3.10 meV for CH stretch mode with H at ABc site and a red shift of 20.46 meV for that at BC site, with respect to that in the pure system. In the pure 3C-β SiC, H is diffusive with an energy cost of about 0.50 eV, preferring to rotate around the C atom in a SiC tetrahedron with an energy barrier of just about 0.10 eV. In contrast, in the He-implanted 3C-β SiC, the energy barrier for H migration goes up to be about 0.95 eV, indicating that the implanted-He blocks the diffusive H to some extent. Our calculations also show that the influence of He on H diffusion works in a short range, just covering the nearest neighbors.

Infrared absorption of hydrogen-related defects in ammonothermal GaN

Polarization controlled Fourier transform infrared (FTIR) absorption measurements were performed on a high quality m-plane ammonothermal GaN crystal grown using basic chemistry. The polarization dependence of characteristic absorption peaks of hydrogen-related defects at 3000–3500 cm−1 was used to identify and determine the bond orientation of hydrogenated defect complexes in the GaN lattice. Majority of hydrogen was found to be bonded in gallium vacancy complexes decorated with one to three hydrogen atoms (VGa-H1,2,3) but also hydrogenated oxygen defect complexes, hydrogen in bond-center sites, and lattice direction independent absorption were observed. Absorption peak intensity was used to determine a total hydrogenated VGa density of approximately 4 × 1018 cm−3, with main contribution from VGa-H1,2. Also, a significant concentration of electrically passive VGa-H3 was detected. The high density of hydrogenated defects is expected to have a strong effect on the structural, optical, and electrical properties of ammonothermal GaN crystals.

Heterogeneous nucleation and high orientation of ZnO nanorods on graphene

We report the realization of ZnO nanostructures on a graphene/SiO2/Si substrate by using a facile precursor-template synthesis technique through catalyst-free, seed-layer-free chemical vapor deposition. Vertical growth on the graphene buffer layers resulted in highly-oriented ZnO nanorods due to heterogeneous nucleation of the bond-centered sites on graphene. This demonstrates that the morphology of the structures is sensitive to the graphene substrate owing to the strong ZnOgraphene interaction. The photoluminescence exhibited distinctive band-edge emission and defect emission. The defect emission could be largely eradicated by covering the graphene and decreasing the ZnO-graphene spacing.

First-principles study of the lattice integration of palladium defects in doped germanium and silicon

Time-differential perturbed angular correlations spectroscopy of palladium in doped germanium has identified palladium-vacancy pairing in n-type antimony-doped, p-type gallium-doped and undoped germanium. In contrast, an equivalent study of palladium defects in doped silicon suggests a different scenario for the silicon host. Palladium-vacancy pairing has been proposed in n-type silicon irrespective of the dopant type (phosphorous, arsenic or antimony) but palladium–boron pairing has been speculated to occur in p-type boron-doped silicon. This thus raises the question: why does palladium pair with a dopant atom in p-type silicon, but with a vacancy in p-type germanium? Based on the density functional theory calculations carried out in this work, it is suggested that the size of the dopant and the host material both play a crucial role in determining the type of palladium-defect complex that is formed. The calculations predict a configuration with the palladium atom on a bond-centered interstitial site pairing with a semi-vacancy on either side in gallium-doped and antimony-doped silicon and germanium, respectively. Whereas, a configuration with the palladium atom on a bond-centered interstitial site pairing with the dopant was proposed in boron-doped silicon and germanium. In further support of the argument, in n-type phosphorous-doped materials the calculations predict a configuration with the palladium atom on a bond-centered interstitial site pairing with a semi-vacancy on either side in silicon, but a configuration with the palladium atom on a bond-centered interstitial site pairing with the phosphorous dopant in germanium.

Effects of hydrogen atoms on surface conductivity of diamond film

To investigate the effects of surface chemisorbed hydrogen atoms and hydrogen atoms in the subsurface region of diamond on surface conductivity, models of hydrogen atoms chemisorbed on diamond with (100) orientation and various concentrations of hydrogen atoms in the subsurface layer of the diamond were built. By using the first-principles method based on density functional theory, the equilibrium geometries and densities of states of the models were studied. The results showed that the surface chemisorbed hydrogen alone could not induce high surface conductivity. In addition, isolated hydrogen atoms in the subsurface layer of the diamond prefer to exist at the bond centre site of the C-C bond. However, such a structure would induce deep localized states, which could not improve the surface conductivity. When the hydrogen concentration increases, the C-H-C-H structure and C-3Hbc-C structure in the subsurface region are more stable than other configurations. The former is not beneficial to the increase of the surface conductivity. However, the latter would induce strong surface states near the Fermi level, which would give rise to high surface conductivity. Thus, a high concentration of subsurface hydrogen atoms in diamond would make significant contributions to surface conductivity.

A first-principles study on hydrogen in ZnS: Structure, stability and diffusion

Abstract Based on first-principles calculations, the local structures and their energetic stability for impurity hydrogen (H) in semiconductor ZnS are investigated. H is most favorable to dwell in the bond center (BC) site in ZnS. The antibonding site of Zn (ABZn) has close energy with BC. The antibonding site of S (ABS) and interstitial (IH) site have 0.19 eV and 0.44 eV energy cost, separately. The bond strength with S and Zn determines the stability of impurity H in ZnS. Meanwhile, H is highly moveable in ZnS. At the room temperature, H can overcome the barrier to diffuse through the neighboring BC site.

Palladium-defect complexes in diamond and silicon carbide

Time Differential Perturbed Angular Correlations (TDPAC) studies, supported by Density Functional Theory (DFT) modelling, have shown that palladium atoms in silicon and germanium pair with vacancies. Building on these results, here we present DFT predictions and some tentative TDPAC results on palladium-defect complexes and site locations of palladium impurities in diamond and silicon carbide. For both diamond and silicon carbide, the DFT calculations predict that a split-vacancy V-PdBI-V complex is favoured, with the palladium atom on a bond-centred interstitial site having a nearest-neighbour semi-vacancy on either side. Consistent with experimental results, this configuration is also assigned to palladium complexes in silicon and germanium. For silicon carbide, the DFT modelling predicts furthermore that a palladium atom in replacing a carbon atom moves to a bond-centred interstitial site and pairs with a silicon vacancy to form a complex that is more stable than that of a palladium atom which replaces a silicon atom and then moves to a bond-centred interstitial site pairings with a carbon vacancy. These two competing alternatives differ by 8.94 eV. The favourable pairing with a silicon vacancy is also supported independently by TRIM Monte Carlo calculations, which predict that more silicon vacancies than carbon vacancies are created during heavy ion. implantation.

Origin of the lattice sites occupied by implanted Co in Si

We have investigated the lattice location of implanted in silicon. By means of emission channeling, three different lattice sites have been identified: ideal substitutional sites, displaced bond-centered sites and displaced tetrahedral interstitial sites. To assess the origin of the observed lattice sites we have compared our results to emission channeling studies on and and to Mössbauer spectroscopy experiments on , present in literature. The possible interpretation of several Mössbauer lines is discussed in the light of our new results on the lattice location. The conclusions are relevant for the microscopic understanding of some gettering techniques.