Bond-centered Hydrogen Research Articles

Identification of the donor and acceptor states of the bond-centered hydrogen–carbon pair in Si and diluted SiGe alloys

The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si1 − xGex alloys (0 ≤ x ≤ 0.070) are investigated by high-resolution Laplace deep level transient spectroscopy measurements and first-principles calculations. By exploiting the presence of Ge atoms close to a substitutional C atom, we show that E90 and H180 belong to the same C–H pair (labeled CH1BC) with H in a bond-centered configuration (C—HBC—Si). The relative energies of the various configurations of the CH pair are calculated, and the complete vibrational spectra in the lowest-energy structures for each charge state are predicted.

Light-induced hydrogen evolution from hydrogenated amorphous silicon: Hydrogen diffusion by formation of bond centered hydrogen

The light-induced hydrogen evolution (LIHE) from amorphous (a-) Si:H by the order of at. % is observed during white light soaking (WLS) of 100–400 mW/cm2 at 350–500 K or ultra violet light soaking (UVLS) of 30–120 mW/cm2 at 305–320 K in a vacuum. The thermal desorption spectroscopy indicates that LIHE originated from bonded hydrogen takes place through the diffusion of light-induced mobile hydrogen (LIMH) with the activation energy of 0.5 eV. LIMH is assigned to bond centered hydrogen and the hydrogen diffusion process becomes prominent when LIMH can leave from a-Si:H such under light soaking in a vacuum above room temperature. For H2 in microvoids, the hydrogen evolution rate is governed by the surface barrier and its activation energy of 1.0 eV in dark decreases to 0.4 eV under WLS or UVLS.

Can Hydrogen be Incorporated inside Silicon Nanocrystals?

Hydrogen is omnipresent during the synthesis and processing of silicon nanocrystals (Si NCs). It is generally assumed that the incorporation of hydrogen leads to the passivation of Si dangling bonds at the NC surface. However, it is also speculated that hydrogen may be incorporated inside Si NCs. In this work the formation energy and probability of hydrogen in its three configurations, i.e., hydrogen molecules, bond-centered atomic hydrogen, and antibonding atomic hydrogen, are calculated to rigorously evaluate the incorporation of hydrogen inside Si NCs. We find that hydrogen cannot be incorporated inside Si NCs with a diameter of a few nanometers at temperatures up to 1500 K.

Hydrogen shallow donors in ZnO and rutile TiO2

A combined study of IR absorption, photoconductivity, photoluminescence and Raman measurements in ZnO samples supports the theoretical suggestions of a shallow bond-centered hydrogen donor and a shallow hydrogen donor within the oxygen vacancy. In rutile TiO2 we also identify a shallow hydrogen donor in contrast to recent theoretical predictions. A possible solution to this obvious discrepancy is proposed.

Reconfiguration and dissociation of bonded hydrogen in silicon by energetic ions

We report in situ infrared measurements of ion-induced reconfiguration and dissociation of bonded hydrogen associated with various defects in silicon at low temperatures. Defect-associated Si-H complexes were prepared by low-temperature proton implantation in silicon followed by room-temperature annealing. As a result of subsequent low-temperature $^{3}\mathrm{He}$ ion irradiation, we observed (1) ion-induced dissociation of Si-H complexes, (2) a notable difference in the dissociation rate of interstitial- and vacancy-type defects, and, unexpectedly, (3) the growth of bond-centered hydrogen, which is generally observed in association with low-temperature proton implantation. These findings provide insight into the mechanisms responsible for the dissociation of hydrogen bonds in silicon and thus have important implications for bond-selective nanoscale engineering and the long-term reliability of state-of-the-art silicon semiconductor and photovoltaic devices.

Temperature Induced Evolution of Bond-Centered Hydrogen (BCH) Defects in Crystalline Silicon: Dynamical, Electronic, Vibrational and Optical Signatures

ABSTRACTThe thermal stability, evolution and structure of the bond-centered-hydrogen (BCH) defect in crystalline silicon, its temperature induced dissociation, and the new H complexes formed have been studied in the temperature range from 50 K to 650 K by first-principles molecular dynamics (MD). We demonstrate that BCH is stable at 60 K, but decays at and above 310 K in agreement with experimental results. The dissolved BCH forms new complexes: transitional interstitials, stable monohydride-like and monohydride/dihydride-like complexes. The calculated asymmetric vibrational frequency of H in the BCH complex is 2000 cm-1, very close to the experimental values. Calculated vibrational frequencies, electron charge densities, electron densities of states (DOS), and optical spectra demonstrate noticeable differences for the different geometries with the BCH, interstitial and monohydride-like complexes, especially in the vicinity of the energy gap. The BCH complex is found to induce characteristic donor states below the conduction band, and raises the Fermi level to above the donor state energies.

Hydrogen in ZnO

Abstract The results of a combined study of Raman scattering, IR absorption, photoluminescence, and photoconductivity on ZnO are presented. Two shallow donors—hydrogen at the bond-centered lattice site, H BC , and hydrogen bound in an oxygen vacancy, H O —were identified. Donor H BC has an ionization energy of 53 meV. The recombination of an exciton bound to H BC gives rise to the 3360.1 ± 0.2 meV photoluminescence line. The 1 s → 2 p donor transition at 330 cm - 1 is detected in the Raman scattering and photoconductivity spectra. The stretch mode of the associated O–H bond is detected in IR absorption at 3611 cm - 1 . The H O donor in ZnO has an ionization energy of 47 meV. The excitonic recombination at H O leads to the previously labeled I 4 line at 3362.8 meV. Photoconductivity and Raman spectra reveal the 1 s → 2 p donor transition at 265 cm - 1 . It is shown that H BC migrating through the ZnO lattice forms electrically inactive interstitial H 2 . Vibrational modes of H 2 , HD, and D 2 were identified at 4145, 3628, and 2985 cm - 1 , respectively. These results suggest that interstitial H 2 is responsible for the “hidden” hydrogen in ZnO.

Effect of hydrogen doping in ZnO thin films by pulsed DC magnetron sputtering

This study examined the role of hydrogen impurities in highly oriented ZnO thin films. Hydrogen intentionally incorporated was found to play an important role as a donor in n-type conduction, improving the free carrier concentration. The increase in the conductivity of ZnO thin films was attributed to the two centers assigned to isolated hydrogen atoms in the anti-bonding sites as well as bond-centered interstitial hydrogen located between the Zn–O bonds and Zn vacancy passivated by one or two hydrogen atoms. Micro Raman spectroscopy showed two additional modes at approximately 501 and 573 cm −1. These two peaks were attributed to damage to the crystal lattice, which could be explained by the optical-phonon branch at the zone boundary and host lattice defects, such as vacancy clusters, respectively.

Infrared absorption spectroscopy on OH–Ni complex in hydrothermally grown ZnO

The microstructure and combination behavior of transitional metal Ni and hydrogen (H) in hydrothermally grown ZnO were investigated by infrared absorption spectroscopy. An infrared absorption peak at 2782.9 cm−1 was observed in ZnO crystal hydrogenated in H2 gas at 725 °C for 30 min. Isotope substitution experiments and polarized infrared absorption spectra revealed that this peak results from the stretch local vibrational mode of a single interstitial hydrogen bound to oxygen, with OH bond oriented at an angle of 108° to the c axis of ZnO. Different from the mode at 3577.3 cm−1 induced by the OH–Li complex in the same sample, the mode at 2782.9 cm−1 is ascribed to the OH–Ni complex related to a bond-centered hydrogen, which is sitting in the basal plane of the hexagonal lattice between the substitutional Ni at a Zn site and O. Moreover, this OH–Ni complex is thermally stable up to 500 °C.

Copper dihydrogen complex in ZnO

Local vibrational modes of a copper-dihydrogen complex ${\text{CuH}}_{2}$ in ZnO have been identified by using IR absorption spectroscopy. Two O-H modes at 3347 and $3374\text{ }{\text{cm}}^{\ensuremath{-}1}$ are observed after Cu in-diffusion at $1200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and subsequent hydrogenation at $725\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ in sealed ampoules filled with an ${\text{H}}_{2}$ gas. Isotope substitution experiments revealed that the defect consists of two equivalent hydrogen atoms. From uniaxial stress measurements, the defect was found to have monoclinic symmetry. Based on these results, the observed local modes are ascribed to a defect consisting of a substitutional Cu atom and two adjacent bond-centered hydrogen atoms, each of which is bound to the oxygen atoms in the basal plane of the defect. The hydrogen motion around the Cu atom was found to be thermally activated with an activation energy of $0.29\ifmmode\pm\else\textpm\fi{}0.04\text{ }\text{eV}$. The apparent thermal stability of ${\text{CuH}}_{2}$ depends on the Cu doping profile and the interaction with a hydrogen species that is invisible in IR absorption.

Vibrational lifetimes of light impurities in silicon

Light impurities in covalent materials, such as H or O in Si, are characterized by high-frequency local vibrational modes (LVMs) far above the phonon frequencies of the host crystal. Upon excitation, such LVMs are expected to have long lifetimes since their decay involve multi-phonon processes. Yet, time-resolved infrared absorption spectroscopy reveals that the vibrational lifetimes of almost identical LVMs sometimes differ by up to two orders of magnitude. Indeed, the low-temperature lifetimes of the 2062cm−1 mode of the H2∗ complex, the 1998cm−1 mode of bond-centered hydrogen HBC+ and the 2072cm−1 mode of the di-vacancy di-hydrogen complex VH·HV are 4, 8 and 295ps, respectively. More surprising still, the measured lifetime of the asymmetric stretch of interstitial O in Si changes by almost an order of magnitude with the isotope of O or one of its Si neighbors. In this paper, ab-initio molecular dynamic simulations in periodic supercells are used to calculate vibrational lifetimes. The calculations predict the correct lifetimes of the various defects in the range 50<T<200K and provide critical insight into the decay processes.

First-principles theory of the temperature dependence of vibrational lifetimes for light impurities in Si

First-principles theory in Si periodic supercells is used to calculate the vibrational lifetime of the asymmetric stretch of bond-centered hydrogen (H + BC) as a function of temperature. We discuss the method used to calculate lifetimes and introduce a general method to simulate the background temperature of a supercell in equilibrium without thermalization or thermostat.

On the nature of hydrogen-related shallow donors in ZnO

Vapor-phase grown ZnO samples treated with hydrogen plasma were studied by means of photoconductivity and IR absorption spectroscopy. Three bands at 180 , 240 , and 310 cm - 1 were observed in the photoconductivity spectra of hydrogenated ZnO. These are identified as electronic transitions of three independent hydrogen-related shallow donors. Two electronic transitions from the H–I defect previously associated with the bond-centered hydrogen were found in IR absorption spectra at 1430 and 1480 cm - 1 . Based on the energies of these transitions H–I was ruled out as a candidate for the hydrogen-related shallow donor in ZnO.

Theory of anharmonicity on bond-centered hydrogen oscillators in silicon

An anomalous positive shift when the mass of the Si host isotope is increased has been observed recently [Pereira et al., Physica B 340-342 (2003) 697] for the asymmetric stretch frequency of the bond-centered proton in Si ( H BC + ) . On the other hand, the usual downward isotope shift was observed for the analogous bond-centered deuteron ( D BC + ) . Based on phenomenological and ab initio modeling studies, we explain the observed puzzling effect. We introduce a Si–H–Si linear model that accounts well for the observations when anharmonicity, volumetric effects due to the host-isotope mass change, and the coupling of the Si–H–Si unit to the lattice are taken into account. The positive isotope shift for H BC + results from anharmonic A 2 u + A 1 g mode mixing and volumetric effects.

Anharmonicity and lattice coupling of bond-centered hydrogen and interstitial oxygen defects in monoisotopic silicon crystals

We discuss the vibrational dynamics of bond-centered protons $({\mathrm{H}}_{\mathrm{BC}}^{+})$ and deuterons $({\mathrm{D}}_{\mathrm{BC}}^{+})$ in monoisotopic ($^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$) silicon crystals, based on joint infrared absorption measurements and ab initio modeling studies. Protons and deuterons have been implanted at temperatures below $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and in situ--type infrared absorption measurements have subsequently been performed at $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A major absorption line is observed at $1998\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ after proton implantation, which has previously been ascribed to a local mode of ${\mathrm{H}}_{\mathrm{BC}}^{+}$. We find that the ${\mathrm{H}}_{\mathrm{BC}}^{+}$ mode at $1998\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ displays an anomalous (positive) frequency shift when the Si isotope mass is increased, unlike the analogous ${\mathrm{D}}_{\mathrm{BC}}^{+}$ mode at $1448\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which shows a negative shift. This effect cannot be described with a purely harmonic model. We show that the mode frequencies are accurately accounted for with a simple model based on a linear $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}$ structure when anharmonicity, volumetric effects due to the host-isotope mass, and the coupling of the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}$ unit to the lattice are taken into account. Interstitial oxygen $({\mathrm{O}}_{i})$ atoms in silicon, also located at the bond-center site, are as well investigated in a parallel way. The relative contributions of the different terms of the vibrational model to the mode frequency of ${\mathrm{H}}_{\mathrm{BC}}^{+}$ and ${\mathrm{O}}_{i}$ are compared. The anomalous (positive) isotope shift of ${\mathrm{H}}_{\mathrm{BC}}^{+}$ results from mixing via anharmonicity of ${A}_{2u}$ and ${A}_{1g}$ modes of the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}$ unit, which shows that a reliable vibrational model has to take into consideration the local structure of the defect. The mode frequency of the ${\mathrm{O}}_{i}$ defect exhibits the normal (negative) isotope shift, because the relatively modest contribution of anharmonicity diminishes the importance of the mode mixing. The effect of the defect-lattice coupling on the stretch-mode frequencies of ${\mathrm{H}}_{\mathrm{BC}}^{+}$ and ${\mathrm{O}}_{i}$ is also discussed.

Photoconductivity and infrared absorption study of hydrogen-related shallow donors in ZnO

Vapor phase grown ZnO samples treated with hydrogen and∕or deuterium plasma were studied by means of photoconductivity and infrared (IR) absorption spectroscopy. Three bands at 180, 240, and $310\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ were observed in the photoconductivity spectra of hydrogenated ZnO. These are identified as electronic transitions of three independent hydrogen-related shallow donors. Two electronic transitions from the $\mathrm{H}\text{\penalty1000-\hskip0pt}\mathrm{I}$ defect previously associated with the bond-centered hydrogen [E. V. Lavrov et al., Phys. Rev. B 66, 165205 (2002)] were found in IR absorption spectra at 1430 and $1480\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Based on the energies of these transitions $\mathrm{H}\text{\penalty1000-\hskip0pt}\mathrm{I}$ was ruled out as a candidate for the hydrogen-related shallow donor in ZnO.

Local modes of bond-centered hydrogen in Si:Ge and Ge:Si

Local vibrational modes of bond-centered hydrogen have been identified in Ge-doped Si (Si:Ge) and Si-doped Ge (Ge:Si) with in-situ-type infrared absorption spectroscopy. The infrared absorbance spectra recorded at $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ immediately after implantation of the very dilute Si:Ge and Ge:Si alloys with protons reveal in each case four lines located at $\ensuremath{\sim}2000$ and $\ensuremath{\sim}1800\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, respectively. The dominating band in all spectra arises from quasi-isolated positively charged bond-centered hydrogen $({\mathrm{H}}_{\mathrm{BC}}^{+})$ centers, whereas the remaining features originate from different ${\mathrm{H}}_{\mathrm{BC}}^{+}$ defects perturbed locally by a nearby Ge in Si:Ge or Si in Ge:Si located in the first-, second- and third-nearest neighbor sites. These assignments are based on the analysis of the local mode frequencies and their absorption intensities, together with the isotopic shifts observed when hydrogen is replaced by deuterium, and the thermal stability of the modes. Our results are compared with previous experimental and ab-initio theory investigations.

Local vibrational modes of bond-centered H in [formula omitted], and [formula omitted] crystals

The local vibrational mode of positively charged bond-centered hydrogen (HBC+) has been investigated in Si crystals enriched with 28Si,29Si, or 30Si isotopes. Protons and deuterons were implanted into each sample at temperatures below 20K, and in-situ-type infrared absorption measurements were subsequently performed at 8K. The isotope shifts observed for the hydrogen and deuterium modes display opposite behavior with increasing mass of the silicon isotope. This behavior cannot be accounted for with a purely harmonic potential. We apply a simple model based on the linear molecule Si–H–Si, where the potential associated with the stretching of each Si–H bond is approximated by the Morse potential. The local mode frequencies and the isotope shifts obtained with our model are in excellent agreement with those observed.

Forms of Hydrogen and Hydrogen Diffusion in Realistic Models of a-Si:H

ABSTRACTUsing ab initio density functional calculations we investigate the energetics of hydrogen in a-Si:H starting from a basic supercell that contains 142 atoms and is completely free of defects. The study includes isolated H atoms bonded to dangling bonds, H atoms bonded to dangling bonds in regions of clustered H, bond centered hydrogen, and molecular hydrogen. In particular, the difference between clustered and isolated hydrogen has been largely ignored in the past. Energetics of doped as well as undoped cells are considered. The results are discussed with particular emphasis on H diffusion and the replacement of atoms in molecular hydrogen with those bonded.

Weakly bound carbon-hydrogen complex in silicon

Local vibrational modes of a weakly bound carbon-hydrogen complex in silicon have been identified with infrared-absorption spectroscopy. After implantation of protons at \ensuremath{\sim}20 K and subsequent annealing at 180 K, two carbon modes at 596 and 661 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and one hydrogen mode at 1885 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are observed. The three modes originate from the same complex, which is identified as bond-centered hydrogen in the vicinity of a nearby substitutional carbon atom. Ab initio theory has been applied to calculate the structure and local modes of carbon-hydrogen complexes with hydrogen located at the first, second, and third nearest bond-center site to substitutional carbon. The results support our assignment.