Dangling Bond States Research Articles

Unoccupied band structure of wurtzite GaN(0001)

We report an inverse photoemission study of the unoccupied states of thin-film n-type wurtzite GaN. For incident electron energies below 30 eV, free-electron bands do not provide a good description of the initial state. However, using a calculated quasiparticle band structure for the initial state, we can obtain good agreement between our measurements and the calculated low-lying conduction bands. No evidence of unoccupied surface states is observed in the probed part of the Brillouin zone, confirming earlier angle resolved photoemission studies, which identified the surface states on GaN(0001) as occupied dangling bond states, resonant with the valence band.

Electronic excitations on the alkali-metal (Na,Cs)-adsorbed Si(111)7×7 surfaces

Electronic excitation spectra of the alkali-metal (AM) (Na,Cs)-adsorbed Si(111)7 × 7 surfaces have been investigated by using high-resolution electron energy loss spectroscopy. We find that AM induces metal-semiconductor transitions at the early stage of adsorption by completely filling half-filled Si adatom dangling-bond states. A loss peak of energy less than 0.31 eV appears as a precursor of the semiconducting phases, which disappears with increasing coverage, resulting in the formation of energy gaps of 0.43eV and 1.08 eV for Cs and Na, respectively. Both Cs- and Na-adsorbed surfaces become metallic at saturation, in sharp contrast with the semiconducting K-saturated surface, We discuss the physical origin of the AM-induced loss peaks and its implications for the electrical phase transitions.

Strongly nonlinear luminescence in oxidized porous silicon films

Partially oxidized free-standing porous silicon films show a strongly superlinear increase in photoluminescence (PL) intensity above a threshold cw excitation intensity of ∼10 W/cm 2. The PL-intensity increase can be expressed by a power law with n∼9 as a function of the excitation intensity. The PL-peak wavelength of this emission is slightly redshifted from that at low-excitation levels. These changes are fully reversible and reproducible, but not observed in samples on substrate. We attribute this behavior to thermal reexcitation of carriers trapped at the dangling bond states in initially nonluminescent Si nanocrystallites.

Ultrafast Processes of Highly Excited Carriers in CdSxSe1-x-Doped Glass

Femtosecond pump-probe experiments were conducted, and transient absorption spectra showing ultrafast dynamic processes of highly excited carriers were observed in a glass doped with microcrystalline CdSxSe1-x (average particle diameter of 2 nm). A relaxation process with a time constant of about 2 ps was obtained. This was the first observation of a 2-ps dynamic process occurring over a very wide spectral range, and the process can be understood as the relaxation of the excited carriers in the dangling-bond surface states. A residual increment in absorption was evident after the relaxation.

Interactions ofSrF2andPrF3with TiC(111) and Si(111) surfaces studied by low-energyD+scattering spectroscopy

On the basis of neutralization of scattered low-energy ${\mathrm{D}}^{+}$ ions, the nature of the bonding of ionic molecules adsorbed on metal and semiconductor surfaces has been investigated. It is found that ${\mathrm{SrF}}_{2}$ reacts with the active dangling-bond states of the TiC(111) surface and the ionic bonding between ${\mathrm{Sr}}^{2+}$ and ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions is strongly weakened. However, the ionicity of the adsorbates recovers after oxygenation or hydrogenation of the TiC(111) surface since H or O passivates the dangling-bond states at the interface. On the other hand, the dangling bond of Si(111) has relatively little effect on the ionic Sr-F bond formation and rather dissociation of ${\mathrm{SrF}}_{2}$ is promoted at an elevated temperature due to preferential reaction of F with Si. In terms of ${\mathrm{PrF}}_{3},$ ionicity is strongly reduced on both Si(111) and TiC(111) surfaces and oxygenation of the surface has very little effect, suggesting that ${\mathrm{PrF}}_{3}$ is dissociatively adsorbed and Pr forms covalent or metallic bonds with the substrate.

Theoretical and experimental study of the quasistatic capacitance of metal-insulator–hydrogenated amorphous silicon structures: Strong evidence for the defect-pool model

The density of localized states in hydrogenated amorphous silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ is studied by means of the quasistatic capacitance technique applied to metal-insulator $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ structures. Calculations in the framework of the defect-pool model show that the changes in the quasistatic capacitance versus gate bias curves (qs-CV curves) after bias annealing reveal the changes in the density of dangling-bond states predicted by the model, and are sensitive to the defect-pool parameters. The comparison of theoretical qs-CV curves with experimental curves obtained in a wide range of bias-anneal voltages ${V}_{\mathrm{ba}}$ on several kinds of structures (top gate oxide, top gate nitride, and the most commonly used bottom gate nitride structures) strongly support the defect-pool model, and values for the model parameters are deduced. It is shown that for all structures the dominant phenomenon for bias annealing at positive ${V}_{\mathrm{ba}}$ (i.e., under electron accumulation) is the creation of defects in the lower part of the gap in the $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ Bias annealing under hole accumulation reveals the creation of defects in the upper part of the gap of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H},$ but the precise dependence of the qs-CV curves upon ${V}_{\mathrm{ba}}$ depends on the nature of the insulator\char21{}$a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ interface. In particular, it is affected by a higher density of interface trap levels in the top gate nitride structures, and by hole injection and trapping from the $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ into the nitride layer in the bottom gate nitride structures.

Strain effects on the surface optical transitions of GaAs

The surface optical properties of GaAs on GaP-strained layers have been analyzed in situ by chemical modulation spectroscopy. A clear surface-related optical transition has been identified, and the effect of strain on this transition has been studied, showing a blueshift of 30 meV even for epitaxies as thin as 1 ML of GaAs deposited on top of GaP. This small shift is consistent with a transition between dimer states and dangling-bond states of the Ga-Ga dimers.

A new carbon rich model of the [formula omitted] surface reconstruction

This paper presents an analysis of the energetics of a few possible candidates for the structural model of the α-SiC(0001) 3 × 3 surface reconstruction. The analysis is based on quantum chemical calculations of MNDO type (AM1 parametrization). The results indicate that out of the models tested, the energetically most stable structure is made of C 6 rings bonded to the stoichiometric substrate via a 2/3 monolayer of silicon. The structure has one Si dangling bond per 3 × 3 surface unit cell (SUC) and as such should normally represent a conducting surface. However, instead of being exposed, the dangling bonds here are localized on the Si substrate atoms below the centers of the C 6 rings. The photoemission signal from the dangling bond states is then likely to be suppressed, thus making the surface appear semiconducting in photoemission experiments [L.I. Johansson, F. Owman, P. Mårtensson, Surf. Sci. 360 (1996) L478]. Moreover, the pattern of the core level shifts of the SiC(0001) 3 × 3 surface [L.I. Johansson, F. Owman, P. Mårtensson, Phys. Rev. B 53 (1996) 13793] agrees with the number of charge inequivalent Si and C atoms around the surface in the proposed model. Finally, the measured Si(LVV)/C(KLL) Auger intensity ratio of the surface at different stages of annealing [T. Tsukamoto et al., Surf. Sci. 371 (1997) 316] agrees with the proposed model of the reconstruction.

Low-temperature photoemission study of the surface electronic structure ofSi(111)7×7

The electronic structure of the $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface has been studied in detail at a sample temperature of 55 K with high-energy-resolution angle-resolved photoemission. The photoemission spectra show a previously undetected surface-state structure at \ensuremath{\approx}0.5 eV below the Fermi level that we assign to dangling-bond states located mainly to the adatoms near the corner holes of the $7\ifmmode\times\else\texttimes\fi{}7$ reconstruction. Detailed dispersion curves obtained at 55 K are presented for the three uppermost, dangling-bond derived, surface states. Surface sensitive Si $2p$ spectra obtained in this low-temperature study still show a comparatively broad line shape. Possible reasons for this are discussed.

Temperature dependence of DC conductivity of as-deposited and annealed selenium films

The linear DC dark- and photo-current-voltage characteristics observed for as deposited (Ts=25°C) and annealed (Ta=70°C) thermally-evaporated selenium s a films in the temperature range 150-300K were used to deduce the respective temperature dependencies of the dark-conductivity σdark and photo-conductivity σph of these samples. Both XRD and SEM studies confirmed the amorphous state ph of the as-deposited films and the hexagonal-like polycrystalline structure of the annealed films. The ln σ versus 1/T plots of the highly resistive amorphous selenium (a-Se) samples revealed only one thermally-activated process in the region 250-300K for both σdark and σph with an activation energy Eσ= 0.73eV, dark ph sigma and 0.49eV, respectively. This has been interpreted considering the conduction models based on the existence of the inherent charged defect centres (dangling bonds D−, D+) in the band tails. The conductivity of the annealed specimens was at least three orders of magnitude larger than that of the as-deposited ones and the temperature-dependence of their σdark and σph exhibited two different dark ph conduction mechanisms; one being operative in the high-temperature region (230-300K) and the other being dominant at low temperatures (220-150K). As these annealed samples were polycrystalline, the former activated process has been analysed on the basis of grain-boundary-limited conduction with Eσ= 0.17eV (σdark) and 0.15eV (σph). The low-temperature conductivity of these sample has dark ph been described in terms of variable range hopping conduction, of Mott's type, which appears to be applicable in polycrystalline solids at relatively low temperatures. The observed reduction in the grain-boundary potential barriers and in the degree of disorder on illuminating the annealed selenium films has been understood by the phenomenon of neutralization of the native dangling bond states present at the interface of the grain boundaries of these samples by the photo-generated charge carriers.

Cathodoluminescence measurements of suboxide band-tail and Si dangling bond states at ultrathin Si–SiO2 interfaces

We have directly observed cathodoluminescence (CL) in ultrahigh vacuum over a broad spectral range (0.7–4.0 eV) from ultrathin 5 nm layers of remote plasma enhanced chemical vapor deposition grown a-SiO2:H deposited on silicon substrates. In the infrared regime, luminescence is observed at 0.8 eV, consistent with the presence in the as-deposited film of Si dangling bond localized states, as well as at 1.1 eV due to band edge emission. In the optical regime, three peaks (1.9, 2.7, and 3.4 eV) are observed showing evidence for band tail state emission from an amorphous silicon–oxygen bonded suboxide region in the film, with smaller contributions from either substrate related c-Si or defect containing, stoichiometric SiO2. CL spectra with a range of beam voltages indicate that the stoichiometric SiO2 is very close to the free surface of the film, possibly due to oxidation of the air-exposed wafer or due to a nonuniformity in the film. When the films are annealed in situ in stages up to 500 °C, we observe no change in the shape of the a-SiOx:H peak at 1.9 eV, showing the stability of this suboxide to such temperatures. These observations are consistent with CL measurements of thicker films of a-SiO2:H and a-SiOx, and demonstrate the utility of CL spectroscopy for the study of ultrathin dielectric studies.

Ab initiocalculation of the stoichiometry and structure of the (0001) surfaces of GaN and AlN

We have investigated the stoichiometry and the atomic and electronic structure of the anion- and cation-terminated (0001) surfaces of wurtzite-phase GaN and AlN, using ab initio local-orbital calculations based on the local-density approximation and the pseudopotential method. All stable surface configurations studied differ in atomic composition and periodicity from the ideal bulklike termination. We compare the total energy computed for various $p(2\ifmmode\times\else\texttimes\fi{}2)$ geometries of GaN and AlN(0001). Vacancy structures are found to be the most stable configurations for the anion- and cation-terminated surfaces. For metal-rich growth conditions, our calculations favor the adsorption of metal atoms on the cation-terminated surface. Anion- and cation-derived dangling-bond states appear in the bulk band gap as a result of the formation of vacancies or the adsorption of group-III atoms. Flat surfaces of both types are found to be stabilized by a $\frac{3}{4}$ ML adsorption of hydrogen.

Changes in the Poole–Frenkel coefficient with current induced defect band conductivity of hydrogenated amorphous silicon nitride

Current induced conductivity in metal–semiconductor–metal thin film diodes containing hydrogenated amorphous silicon-rich alloys is determined by the concentration of silicon dangling bond states generated by the energy released during hole-electron recombination. This mechanism enables a range of defect concentrations to be introduced and results in a change in conductivity as carriers move between charged states in a defect band. Using a-SiNx:H with band gaps in the ranges 2.3 to 2.9 eV, we have measured the field dependence of the conductivity after stressing for increasing times and defect densities. By using the relationship between defect concentration and conductivity we have been able to establish that the apparent Poole–Frenkel coefficient (βPF) changes from βPF to 1/2βPF in a gradual way as the number of defects increase in a given material. Furthermore, in the transition region where βPF changes from the classical to the anomalous value, βPF is linearly dependent on total number of defects.

Doping and temperature dependence of paramagnetic states in microcrystalline silicon

Continuous wave and pulsed electron spin resonance (ESR) in the dark and under light illumination are used to study the doping and temperature dependence of defect, conduction electron and dopant states in microcrystalline silicon. Two resonances are found, which are attributed to silicon dangling bond states and located in different structural environments of the material. Light-induced ESR suggests that these two states have different energies, while in dark ESR, both show a wide and flat distribution in energy. The intensity of a third resonance from conduction electrons shows an almost 1:1 correlation with conductivity over three orders of magnitude. All resonances exhibit a Curie-like temperature dependence and the linewidth of the conduction electron resonance increases with temperature due to a decrease in the spin lattice relaxation time. Hyperfine interaction with phosphorus in n-type material and indications of hole states in p-type material are reported.

Adsorption of acetylene and ethylene on the Si(001)2×1 surface studied by NEXAFS and UPS

The adsorption of acetylene (C 2H 2) and ethylene (C 2H 4) on the Si(001)2×1 surface has been investigated by near edge X-ray adsorption fine structure (NEXAFS) and ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation. Upon molecular adsorption, a drastic decrease of the π* resonance (C 2H 4) and an energy shift of the σ* resonance (both C 2H 2 and C 2H 4) were observed in the NEXAFS spectra. The intensity of dangling bond states of Si in the UPS spectra diminished significantly by adsorption of both molecules. These results provide firm evidence for the structure model with Si dimers preserved and the di-σ bonding between the adsorbed molecules and Si dimers.

The epitaxial growth of Ag on Si(111)-(7 x 7) surface and its (√3x√3)-R30 surface phase transformation

Ag is adsorbed in ultra-high vacuum on to the (7x7) reconstructed Si(111) surface with submonolayer coverage control with a deposition rate of 3-3 x 1012 atoms/cm2/sec. The initial stages of growth and intermediate equilibrium phase formation are determined by using low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) for structural information, and auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) for composition and interaction analyses. Room temperature (RT) adsorption results in the nearly epitaxial (1 x 1) surface phase growth in the simultaneous multi-layer growth mode. The quenching of the dangling bond states during adsorption is observed by monitoring thep-character of the Si LVV auger peak. For depositions carried out at high temperatures (HT), several plateaus in the auger uptake curve with the (√3 x √3)-R30° LEED structures are formed. It is observed that a minimum coverage of 0–33 monolayer (ML) is required for the formation of the (√3 x √3) phase and this phase causes the reappearance of thep-electron-related states that were quenched by 1.0 ML adsorption at RT. However the (√3 x √3) is observed for higher coverages (0.66 and 1.0 ML) also. The polar angle anisotropy of Si(2p) emission in XPD indicates the rearrangement of substrate Si atoms for the formation of the (√3 x √3) phase. The EELS data also shows relevant changes due to adsorption of Ag at RT and upon annealing. The results suggest the importance of controlled deposition parameters, the lack of which may have kept the determination of the nature and coverage of the (√3 x √3) surface phase unresolved in literature.

First-principles calculations of theSi(111)−c(2×8)surface

We present results of a first-principles total-energy calculation of the $\mathrm{Si}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)$ reconstruction. This structure has been observed in the quenched Si(111) surface. Similar to the case of $\mathrm{Ge}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)$, the surface unit cell presents asymmetries, related to small inequivalences of the rest atoms, adatoms, and first bilayer atoms. These asymmetries are mainly in the surface plane, but a small buckling of the rest atoms is also found. The inequivalences in the unit cell produce a splitting of the rest-atom dangling bond state, which explains the apparent difference in the atomic size of the rest atoms as seen in recent STM experiments.

Gap-State Distributions in Hydrogenated Amorphous Silicon-Germanium Evaluated Using Capacitance–Voltage Method

The density of gap states in hydrogenated amorphous silicon-germanium (a-SiGe:H) alloys deposited at temperatures of 150–250°C has been evaluated by using a capacitance–voltage (C–V ) method. It was found that the gap-state distribution for a-SiGe:H deposited at 150°C has a broad bump around 0.6 eV below the conduction band edge Ec which is associated with Ge dangling bonds. In contrast, a 250°C-deposited film exhibits a peak density of 8×1018 cm-3·eV-1 around 0.75 eV below the conduction band edge Ec, which might originate from Ge and/or Si dangling bond states. After AM1 (200 mW/cm2) light soaking, the gap-state density above the midgap is increased for films deposited below 200°C, while at 250°C deposition, defect states near the midgap are increased.

The Role of Charged Defects In Current Transport Through Hydrogenated Amorphous Silicon Alloys

ABSTRACTCurrent transport in metal-semiconductor-metal structures based on amorphous silicon alloys has been studied in relation to the density of dangling bond state defects. The density of defects was changed by varying alloy composition or by current stressing. We show that the change of current-voltage characteristics and activation energy with defect density and the onset of Poole-Frenkel conduction with composition require charged defects. It is found that there are more charged defects in amorphous silicon nitride (a-Si1−xNx:H) than in amorphous silicon carbide (a-Si1−xCx:H). In addition, an excess of negatively charged dangling bond defects compared to positively charged dangling bond defects is observed in a-Si1−xNx:H films. This is attributed to the presence of N4+ act as the donor states in silicon nitride. We find that the density of charged dangling bond defects can be higher than 1019cm−3.

Programmable 3-Dimensional Memories Based on Current Induced Conductivity in Hydrogenated Amorphous Silicon Nitride

ABSTRACTChanges of several orders of magnitude in the low field conductivity of hydrogenated amorphous silicon alloy metal-semiconductor-metal MSM devices can be obtained by current stressing. This feature is suitable for memory applications, since the device can be programmed from an unstressed state to a stressed state with a ratio greater than 104. The change in conductivity is attributed to the creation of a large concentration of silicon dangling bond states during current stressing which form a defect band with a low activation energy for current transport. In this paper, we consider the use of current induced conductivity in hydrogenated amorphous silicon nitride (a-SiNx:H) multi-layer structures and show that, in principle, highly complex three dimensional circuits could be made. In particular, the potential of this approach is illustrated by using a simple MSMSM device with two silicon rich silicon nitride layers. An array with lxi0 elements has been fabricated and programmed to store 28 bits of binary information with three outputs per input.