Dangling Bond States Research Articles

The [formula omitted]-reconstructed cleavage surface of diamond: A challenging test for experiment and theory

We have recently measured for the first time–by using Reflectance Anisotropy spectroscopy- the optical gap between dangling-bond states on a single-domain diamond C(111)-2 × 1 surface. The experimental result we have obtained (1.47±0.02 eV ) represents the conclusion of the long-standing effort to determine if this surface is metallic or semiconducting. The large surface gap (even larger for possible excitonic effects) opens a new scientific question: at the moment neither state-of-the-art band calculations have succeeded in obtaining it, nor has its physical origin been explained.

Conductivity of theSi(111)7×7dangling-bond state

Conductivity of the metallic dangling-bond state of adatoms on Si(111)7x7 clean surface was determined through passivation of the only electrical channel by similar to 0.1 monolayer Na adsorption. Through systematic measurements of electron transport and photoemission spectroscopy during the Na deposition, Si(111)7x7 was found to exhibit a metal-to-insulator transition. The decrease in conductivity through the transition, which is attributed to the conductivity of the dangling-bond state, was 2-4 mu Omega(-1) square(-1). The value is smaller than the two-dimensional Ioffe-Regel limit and the mean-free path is too short to apply the Boltzmann picture.

Electron-carrier generation by edge dislocations in InN films: First-principles study

Electronic structures of edge dislocations in InN films are studied using the first-principles calculation. We found that dangling-bond states of In atoms localized in the dislocation core are located above the conduction-band bottom and thus supplies the electron carriers to the conduction band of bulk InN, in agreement with the experimental suggestion by Wang et al. [Appl. Phys. Lett. 90 (2007) 151901]. Moreover, it is shown that the Fermi energy in the conduction band has the tendency to be pinned at the energy positions of N-related dangling-bond states.

Structural and electronic properties of identical-sized Zn nanoclusters grown on Si(111)-(7×7) surfaces

Identical-sized Zn nanoclusters have been grown on Si(111)-(7x7) surfaces at room temperature. In situ scanning tunneling microscopy (STM) studies and first-principles total energy calculations show that room-temperature grown Zn nanoclusters tend to form the seven-Zn-atom structure with one excess Zn atom occupying characteristically the center of the cluster. The evolution of the surface electronic structures measured by scanning tunneling spectroscopy reveals that the formation of Zn nanoclusters is responsible for the saturation of the metallic Si adatom dangling bond states at about -0.3 and +0.5 V and causes the semiconducting characteristics of the nanoclusters. Furthermore, the Zn nanocluster in a faulted half unit cell empties the filled surface dangling bond state of the closest edge Si adatoms in the nearest neighboring uncovered unfaulted half unit cells at about -0.3 V, leading to the suppressed height of the closest edge Si adatoms in the filled-state STM images.

Limitations of a simplified dangling bond recombination model for a-Si:H

The validity of a widely used simple closed-form expression for the recombination associated with dangling bonds in hydrogenated amorphous silicon (a-Si:H) is linked to the relative position of the distribution of the dangling bond states with respect to the quasi-Fermi levels for trapped electrons and holes. However, these quasi-Fermi levels for traps have not been derived before. In this work, we derive the four relevant quasi-Fermi levels for traps associated with dangling bonds in a-Si:H and clarify the limitations of the simple model.

Tip-Induced Reduction of the Resonant Tunneling Current on Semiconductor Surfaces

We report scanning tunneling microscope measurements showing a substantial decrease of the current, almost to zero, on the Si(111)-(7x7) reconstruction in the near-to-contact region under low bias conditions. First principles simulations for the tip-sample interaction and transport calculations show that this effect is driven by the substantial local modification of the atomic and electronic structure of the surface. The chemical reactivity of the adatom dangling bond states that dominate the electronic density of states close to the Fermi level and their spatial localization result in a strong modification of the electronic current.

Valence Surface Electronic States on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.

Staebler–Wronski-like formation of defects at the amorphous-silicon–crystalline silicon interface during illumination

The effective surface recombination velocity of amorphous-silicon-coated crystalline silicon wafers is measured after illumination for various durations to investigate the stability of the surface passivation. We develop a defect model to determine the densities of dangling bond states at the a-Si:H/c-Si interface from fitting the experimental lifetime data. The surface recombination velocity of both p-type and n-type substrates is Seff=3±1 cm/s at τn=1015 cm−3 in the as-deposited state. Illumination induces an increase to Seff=16±5 cm/s due to an increase in the dangling bond density by one order of magnitude. This increase is reversible by annealing at 300 °C for 5 min.

Geometry and diameter dependence of the electronic and physical properties of GaN nanowires from first principles

We present a comprehensive first-principles investigation of the atomic and electronic structures of gallium nitride nanowires, and examine the dependence on nanowire diameter and shape. We consider nanowires in the [0001] growth direction, with diameters ranging from $8\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and investigate the influence of saturating the dangling bonds at the edges of nanowires. We find that unsaturated nanowires are semiconducting and contain dangling bond states in the region of the band gap, the positions of which remain rather constant with varying diameter. Saturating the nanowires with hydrogen removes these states, and the band gap decreases with increasing nanowire diameter. For the unsaturated wires there is a considerable contraction of the Ga-N bond lengths at the edge of the wires of 6.0%--7.4%, while for saturated wires it is 1.5%. We also calculate the heat of formation of the nanowires and find that as the diameter of the nanowire increases, the average relative stability of the nanowire increases, as intuitively expected.

First-principles investigation of an epitaxial silicon oxynitride layer on a6H‐SiC(0001)surface

In a recent publication, Shirasawa et al. [Phys. Rev. Lett. 98, 136105 (2007)] have experimentally shown that incorporation of nitrogen at the interface of a silicate adlayer on $6H\text{\penalty1000-\hskip0pt}\mathrm{Si}\mathrm{C}(0001)$ leads to the formation of a well ordered, highly stable epitaxial silicon oxynitride (SiON) layer without dangling bond states. We investigate the structural and electronic properties of this intriguing system by employing density functional theory with self-interaction-corrected pseudopotentials. Our results corroborate the structural model inferred from low-energy electron diffraction. In addition, our calculated filled- and empty-state scanning tunneling microscopy images are in excellent agreement with the experimental data, clearly revealing that O and Si surface states of the silicate double layer on top of the system, respectively, give rise to the observed images. The calculated surface band structure exhibits a surface band gap of $9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in very good agreement with the scanning tunneling spectroscopy data. The physical origin of this amazingly large gap is clarified.

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ∼10nm). Using this technique, the position of the surface Fermi level EFs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ∼1015states/cm3 can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100nm) layers, and the minimum achievable distance of EFs from the conduction band is ∼360mV for doping with 104ppm PH3. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.

Modelling the light induced metastable effects in amorphous silicon

AbstractWe present results of computer simulations of the light induced degradation of amorphous silicon solar cells. It is now well established that when amorphous silicon is illuminated the density of dangling bond states increases. Dangling bond states produce amphoteric electronic mid‐gap states which act as efficient charge trapping and recombination centres. The increase in dangling bond states causes a decrease in the performance of amorphous silicon solar cells. To show this effect, a modelling approach has been developed which uses the density of localised states with exponentially increasing band‐tails and dangling bond defect states distribution chosen according to the defect pool model. The calculation of the evolution of dangling bond state density during illumination has been achieved through a dynamic scaling relation derived from a defect creation model. The approach considers the amphoteric nature of the dangling bond state and thus accounts for the contributions of the different charge states of the dangling bond during the degradation process. This paper attempts to describe the simulation approach which calculates the defect density as a function of energy, position in the solar cell and illumination time. In excellent agreement with other workers, our simulation results show that the increase in the density of neutral dangling bond states during illumination is higher than of the charged states. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

シリコンおよびゲルマニュームナノアイランドの微細構造と局所電子状態

To study the relation between the strain and the electronic structure in two-dimensional (2D) nanoislands of Si and Ge on the Si(111)-7×7 surface, we made the nanoislands the same size and measured with the angle resolved ultraviolet photoelectron spectroscopy (ARUPS), the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We also calculated the relation based on the density-functional theory. In the calculation, the dangling-bond state at the strained adatom on the nanoisland (SR state) shifts to lower energy, which has a liner dependence with the height of the adatoms. The ARUPS spectrum and the STS show characteristic peaks corresponding to the SR state, whose energy depends on the deformation of the adatom. The height of the adatom on the nanoisland estimated from the energy difference is consistent with a result of the STM measurement. The strain of adatoms can be estimated from the electronic structure.

Probing the Carrier Capture Rate of a Single Quantum Level

The performance of many semiconductor quantum-based structures is governed by the dynamics of charge carriers between a localized state and a band of electronic states. Using scanning tunneling spectroscopy, we studied the transport of inelastic tunneling electrons through a prototypical localized state: an isolated dangling-bond state on a Si(111) surface. From the saturation of the current at an energy resonant with this state, the hole capture rate by the dangling bond was determined. By further mapping the spatial extension of its wave function, the localized nature of the level was found to be consistent with the small magnitude of its cross section. This approach illustrates how the microscopic environment of a single defect critically affects its carrier dynamics.

The effect of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface

We have studied the effects of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface. The surface was exposed to various doses of Ar + ions accelerated towards the surface at 500 eV. X-ray photoelectron spectroscopy (XPS) spectra of the irradiated H-terminated Si(0 0 1) surface reveal the appearance of peaks that are associated with the presence of cleaved Si bonds. Ultraviolet photoelectron spectroscopy (UPS) spectra of the irradiated Si(0 0 1)2 × 1 surface show that the dimer dangling-bond surface state decays monotonically with increasing dose. These results, coupled with previous scanning tunneling microscopy (STM) studies, indicate that the breaking of dimers, and possibly the creation of adatom-like defects, during ion irradiation are responsible for the changes in the electronic structure of the valence band for this surface.

Electronic structure and defect states of transition films from amorphous to microcrystalline silicon studied by surface photovoltage spectroscopy

In this paper, surface photovoltage spectroscopy (SPS) is used to determine the electronic structure of the hydrogenated transition Si films. All samples are prepared by using helicon wave plasma-enhanced chemical vapour deposition technique, the films exhibit a transition from the amorphous phase to the microcrystalline phase with increasing temperature. The film deposited at lower substrate temperature has the amorphous-like electronic structure with two types of dominant defect states corresponding to the occupied Si dangling bond states (D0/D−) and the empty Si dangling states (D+). At higher substrate temperature, the crystallinity of the deposited films increases, while their band gap energy decreases. Meanwhile, two types of additional defect states is incorporate into the films as compared with the amorphous counterpart, which is attributed to the interface defect states between the microcrystalline Si grains and the amorphous matrix. The relative SPS intensity of these two kinds of defect states in samples deposited above 300°C increases first and decreases afterwards, which may be interpreted as a result of the competition between hydrogen release and crystalline grain size increment with increasing substrate temperature.

Atomic and electronic structures of [formula omitted] surfaces

The atomic and electronic structures of 6H-SiC ( 0 0 0 1 ¯ ) - 3 × 3 reconstructions were analyzed by high-resolution medium energy ion scattering (MEIS) combined with photoelectron spectroscopy. We prepared three types of (3 × 3) surfaces by (a) annealing the Si-rich (2 × 2) surface at 1030 °C, (b) annealing the 3 × 3 silicate surface at 1050 °C, and (c) annealing the RCA-treated surface at 1050 °C in ultra-high vacuum. The present MEIS analysis reveals the fact that the (3 × 3) surfaces consist of a Si-adlayer (1.1 ML), C-adlayer (0.5 ML) and C-adatoms (1/3 ML)/C-adlayer (2/3 ML) on the 1st C–Si bilayer for samples (a), (b) and (c), respectively. Observation of the valence band spectra shows that all the surfaces are semiconducting and have dangling bond states in the band gap (1.4 eV below the Fermi level for sample (a)). The Si-rich surface (a) has one surface-related component in Si 2p and in contrast, the C-rich surfaces (b and c) have two surface-related components in C 1s spectra. It is shown that the (3 × 3) reconstructions of the SiC ( 0 0 0 1 ¯ ) are categorized into the above three types and take different atomic configurations. For the Si-rich (3 × 3) surface, the present analysis supports the structure model proposed by Hoster et al. [H.E. Hoster, M.A. Klakov, B. Bullemer, Surf. Sci. 382 (1997) L658]. The probable structures of the C-rich (3 × 3) surfaces are also discussed in detail.

Surface Dangling-Bond States and Band Lineups in Hydrogen-Terminated Si, Ge, andGe/SiNanowires

We report an ab initio study of the electronic properties of surface dangling-bond (SDB) states in hydrogen-terminated Si and Ge nanowires with diameters between 1 and 2 nm, Ge/Si nanowire heterostructures, and Si and Ge (111) surfaces. We find that the charge transition levels epsilon(+/-) of SDB states behave as a common energy reference among Si and Ge wires and Si/Ge heterostructures, at 4.3+/-0.1 eV below the vacuum level. Calculations of epsilon(+/-) for isolated atoms indicate that this nearly constant value is a periodic-table atomic property.

Electron Transport via Local Polarons at Interface Atoms

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

Band-gap opening in metallic carbon nanotubes adsorbed on H∕Si(001)

A recent scanning tunneling microscope nanolithography technique can fabricate one-dimensional “dangling-bond (DB) wire” by the selective removal of H atoms from a H-passivated Si(001) surface along the Si dimer row. We here theoretically investigate the bonding geometry, band structure, and binding mechanism of an armchair (3,3) single-walled carbon nanotube (CNT) adsorbed on the DB wire. We find that the formation of C–Si bonds between the CNT and the DB wire gives rise to hybridization between the carbon π-bond states and the Si dangling-bond states. This hybridization breaks the rotational symmetry of the (3,3) CNT whose π-bonding and π-antibonding bands cross at the Fermi level. As a result, the adsorbed CNT opens an energy gap of ∼0.1eV, yielding a metal-to-semiconductor transition.