Dangling Bond Defects Research Articles

Top-down fabrication of Si nanotube arrays using nanoimprint lithography and spacer patterning for electronic and optoelectronic applications

Silicon (Si) nanotube arrays are expected to be a promising material for application in electronics and optoelectronics due to their large specific surface area with axially hollow spaces, whereas realizing smooth surfaces, high aspect ratios, and controlled dimensions remains a challenge. Moreover, it is also necessary to estimate various aspects of Si nanotubes such as their surface damage and anti-reflective properties. Here, we demonstrate a top-down fabrication of Si nanotube arrays with wall thicknesses of ∼40 to ∼10 nm using nanoimprint lithography (NIL) and spacer patterning. The Bosch process yields the nanotubes with smooth surfaces, long lengths (∼1000 nm), and no noticeable distortion or deformation. Raman scattering and electron spin resonance (ESR) measurements show their high crystal quality with a low density of surface dangling-bond defects. Furthermore, a significant enhancement of the anti-reflection effect of nanotubes and its dimension dependence is demonstrated and investigated by UV–Vis–NIR spectrophotometry as well as numerical simulations.

Enhancing the Performance of Perovskite Light-Emitting Diodes via Synergistic Effect of Defect Passivation and Dielectric Screening

HighlightsSynergistic passivation mechanism of the dual functional compound of potassium bromide for metal halide perovskite films, that is through chemical bonding and physical screening towards the various types of defects.Metal-ion additives can simultaneously passivate the defects of halide vacancies with bromine anions and dangling bond defects with metal cations.Achieve the maximum external quantum efficiency of ~21% and maximum luminance of ~60,000 cd m−2.

Comparison of three titanium-precursors for atomic-layer-deposited TiO2 for passivating contacts on silicon

Atomic layer-deposited (ALD) TiO2 thin films on silicon were deposited using titanium tetrachloride (TiCl4), titanium tetraisopropoxide (TTIP), and tetrakis(dimethylamino)titanium (TDMAT) together with water vapor as the oxidant at temperatures ranging between 75 and 250 °C. The Si surface passivation quality of as-deposited and isothermally annealed samples was compared using photoconductance lifetime measurements in order to calculate their effective surface recombination velocities Seff. A low Seff of 3.9 cm/s (J0s=24fA/cm2) is achieved for as-deposited TiCl4-TiO2 at 75 °C when a chemically grown (i.e., from RCA cleaning) SiOx interface layer is present. Depositing TTIP-TiO2 at 200 °C on a chemically grown SiOx interface layer yields equivalent Seff values; however, in this case, TTIP-TiO2 requires a 5–15 min postdeposition forming gas anneal at 250 °C. In contrast, TDMAT-TiO2 was not found to provide a similar level of passivation with/without a chemically grown SiOx interface layer and postdeposition anneal. Modeling of the effective lifetime curves was used to determine the magnitude of the effective charge densities Qf in the TiO2 films. In all cases, Qf was found to be of the order of ∼1011 q cm−2, meaning field-effect passivation arising from ALD TiO2 is relatively weak. By comparing the material properties of the various TiO2 films using ellipsometry, photothermal deflection spectroscopy, Raman spectroscopy, elastic recoil detection analysis, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we find experimental support for the role of Cl (in conjunction with hydrogen) playing a beneficial role in passivating dangling bond defects at the Si surface. It is concluded that low deposition temperature TiCl4 processes are advantageous, by providing the lowest Seff without any postanneal and a comparatively high growth per cycle (GPC).

Carrier injection induced degradation of nitrogen passivated SiC–SiO2 interface simulated by time-dependent density functional theory

The performance of SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs) degrades seriously after a period of continuous operation. To directly understand this issue, we conduct real-time time-dependent density functional theory (TDDFT) simulations on a series of nitrogen passivated SiC–SiO2 interfaces to monitor the interaction between carriers and interface atoms. We find that the nitrogen passivation always leaves behind two local states near the VBM, which gives a chance to the strong interaction between channel carriers and C–N bonds, and finally results in the generation of C dangling bond defects. These processes are vividly presented and confirmed by the TDDFT simulation. Additionally, the results show that the new defects are more easily formed by the passivated C cluster than the passivated Si vacancy. These studies provide physical insights into the degradation mechanisms of working SiC MOSFETs, while simultaneously demonstrating the advantage of TDDFT as a crucial tool for investigating defect generation dynamics in semiconductor devices.

Substrate Doping and Defect Influence on P-Rich InP(001):H Surface Properties

Density-functional theory calculations on P-rich InP(001):H surfaces are presented. Depending on temperature, pressure and substrate doping, hydrogen desorption or adsorption will occur and influence the surface electronic properties. For p-doped samples, the charge transition levels of the P dangling bond defects resulting from H desorption will lead to Fermi level pinning in the lower half of the band gap. This explains recent experimental data. For n-doped substrates, H-deficient surfaces are the ground-state structure. This will lead to Fermi level pinning below the bulk conduction band minimum. Surface defects resulting from the adsorption of additional hydrogen can be expected as well, but affect the surface electronic properties less than H desorption.

Interfacial Passivation Engineering for Highly Efficient Quantum Dot Light-Emitting Diodes via Aromatic Amine-Functionalized Dipole Molecules.

Blue quantum dot (QD) light-emitting diodes (QLEDs) exhibit unsatisfactory operational stability and electroluminescence (EL) properties due to severe nonradiative recombination induced by large numbers of dangling bond defects and charge imbalance in QD. Herein, dipolar aromatic amine-functionalized molecules with different molecular polarities are employed to regulate charge transport and passivate interfacial defects between QD and the electron transfer layer (ETL). The results show that the stronger the molecular polarity, especially with the -CF3 groups possessing a strong electron-withdrawing capacity, the more effective the defect passivation of S and Zn dangling bonds at the QD surface. Moreover, the dipole interlayer can effectively reduce electron injection into QD at high current density, enhancing charge balance and mitigating Joule heat. Finally, blue QLEDs exhibit a peak external quantum efficiency (EQE) of 21.02% with an operational lifetime (T50 at 100 cd m-2) exceeding 4000 h.

Oxide Glasses for Removal of Ammonia and Nitrogen Derivatives from Industrial Wastewater

Adsorption is a rapid and known method to treat wastewater and remove contaminants such as ammonia (NH3+) and nitrogen derivatives. Herein, novel adsorption materials are demonstrated to eliminate the most hazard industrial and municipal contaminants including ammonia and nitrogen derivatives. Oxide glasses as new adsorbing media are beneficial for wastewater treatment due to dangling bond defects and non-bridging oxygen, which act as adsorption centers. Oxide glasses are characterized by their low cost and simple preparation method. Different types of oxide glasses including borate, phosphate, silicate, and germinate glasses are used as adsorbents to estimate the glass type of the optimal removal efficiency. It is found that the higher removal efficiency is exhibited for both borate and silicate glasses. Therefore, by preparing hybrid borate-silicate (borosilicate) glass, the best efficiency is achieved. The influences of boundary conditions including contact time, adsorption temperature, and adsorbent dosage on the efficiency of adsorption process are demonstrated. The optimal removal efficiency is achieved when using borosilicate glass sample (as adsorbent) with contact time of 90 min, adsorption temperature of 70 °C, and adsorbent dosage of 1.5 mg/100 ml. Finally, by comparing our results with the previous adsorption treatment works, it is found that adsorption capacity of ammonia reached to 9.12 mg/g, which is a valid and acceptable value.

Dangling Bond Defects on Si Surfaces and Their Consequences on Energy Band Diagrams: From a Photoelectrochemical Perspective

Using silicon in multijunction photocells leads to promising device structures for direct photoelectrochemical water splitting. In this regard, photoelectron spectra of silicon surfaces are used to investigate the energetic condition of contact formation. It is shown that the Fermi‐level position at the surface differs from the values expected from their bulk doping concentrations, indicating significant surface band bending which may limit the overall device efficiency. In this study, the influence of different surface preparation procedures for p‐ and n‐doped Si wafers on surface band bending is investigated. With the help of photoemission and X‐ray absorption spectroscopy, Si dangling bonds are identified as dominating defect centers at Si surfaces. These defects lead to an occupied defect band in the lower half and an unoccupied defect band in the upper half of the Si bandgap. However, partial oxidation of the defect centers causes a shift of defect bands, with only donor states remaining in the Si bandgap. Source‐induced photovoltages at cryogenic temperatures indicate that partial surface oxidation also decreases the recombination activity of these defect centers. It is shown that defect distribution, defect concentration, and source‐induced photovoltages need to be considered when analyzing Fermi‐level pinning at Si surfaces.

Photoelectron Properties and Organic Molecules Photodegradation Activity of Titania Nanotubes with CuxO Nanoparticles Heat Treated in Air and Argon.

Titania is very famous photocatalyst for decomposition of organic pollutants. Its photocatalytic properties significantly depend on the morphology and chemical composition of the samples. Herein, the TiO2 nanotubes/CuxO nanoheterostructures have been synthesized and the effect of heat treatment performed in molecular atmospheres of air and argon on their photoelectrochemical and photocatalytic properties has been studied. The prepared samples have a higher reaction rate constant compared to TiO2 nanotubes in the decomposition reaction of methylene blue molecules. It is established that in argon treated nanoheterostructures, the copper oxide is present in two phases, CuO and Cu2O, while in air treated ones there is only CuO. In the TiO2 nanotubes/CuxO samples, Cu2+ ions and molecular O2- radicals were detected while in TiO2 nanotubes only carbon dangling bond defects are present. The dynamics of O2- radicals under illumination are discussed. It was shown that the TiO2 nanotubes do not exhibit photocatalytic activity under visible light. The mechanism of the photocatalytic reaction on the surface of the TiO2 nanotubes/CuxO samples was proposed. It is assumed that a photocatalytic decomposition of organic molecules under visible light at the surface of the nanoheterostructures under investigation is realized mainly by the reaction of these molecules with photogenerated O2- radicals. The results obtained are completely original and indicate the high promise of the prepared photocatalysts.

Analysis of effects of dangling-bond defects in doped a-Si:H layers in heterojunction silicon solar cells with different electron affinities of ITO contacts

The effects of dangling-bond defects in doped hydrogenated amorphous-silicon layers (p-a-Si:H and n-a-Si:H) in heterojunction silicon (SHJ) solar cells are studied in relation to applied Indium-Tin-Oxide (ITO) contacts with different electron affinities. A state-of-the-art numerical model of the SHJ solar cell was employed, including ITO contacts as full, volumetric semiconductor layers, applying the trap-assisted, band-to-band and direct tunnelling mechanisms at heterointerfaces in the device. The levels of dangling bond defect concentrations were varied in both p-a-Si:H and n-a-Si:H layers and ITOs with two different electron affinities were considered at both sides of the device. We show that the effects of the defects on the short-circuit current density, open-circuit voltage, fill factor and conversion efficiency of the device become more pronounced if ITOs with non-optimal electron affinities are used. Possibility to reach higher doping levels of the doped a-Si:H layers would mitigate the effects of its dangling bond states, which becomes more important if ITO electron affinity is not optimized to the doped a-Si:H layers. We demonstrate that the reduced efficiency due to the increase in dangling-bond density originates from the decrease of the fill-factor and open-circuit voltage, whereas the short-circuit current density has a small effect on efficiency for the chosen variation span. The reduction of the fill-factor is further explained by a drop in maximum-power-point voltage, which is more pronounced if optimization of ITO electron affinity is not taken into account.

Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films

Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si1-xGex:H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors.

Electron paramagnetic resonance study of silicon-28 single crystal for realization of the kilogram

To realize the kilogram with high accuracy using the x-ray crystal density method, mass deficit correction to account for the number of vacancy defects in silicon-28 crystals is necessary. In this paper, we present the results of electron paramagnetic resonance (EPR) spectroscopy experiments performed on a silicon-28 crystal. The crystal was cut from near the radial center of the Si28–23Pr11 crystal boule where a relatively high vacancy defect concentration was expected within the boule, which was grown by the floating zone method. We obtained both the in-phase and out-of-phase EPR spectra, which focused on the fast- and slow-spin relaxation centers, respectively. Based on EPR measurements both in the dark and under illumination, the concentrations of phosphorus impurities with unpaired electrons were found to be between 1.6 × 1012 cm−3 and 3.4 × 1012 cm−3. In contrast, EPR-active vacancy defects in the silicon-28 crystal were not detected at the detection sensitivity of the present measurements (around 1 × 1012 cm−3), although at the Si/SiO2 interface, dangling-bond defects (P b0 centers) were detected with an areal density between 0.4 × 1012 cm−2 and 1.7 × 1012 cm−2. The experimental detection of a larger amount of phosphorus donors under illumination indicates the presence of compensation centers (electronic levels acting as acceptors) with a concentration of at most 1.6 × 1012 cm−3. We determined that such a concentration is mainly compensated by the boron acceptors, and that there is little contribution of electrically active vacancy defects, 0.0(6) × 1012 cm−3. This value is much smaller than the previously reported concentrations of 1 × 1014 cm−3 to 4 × 1014 cm−3 for vacancy defects determined by positron annihilation spectroscopy.

Charge transport in SiCN/Si heterostructures

The steady-state space-charge limited (SCL) current was investigated in amorphous silicon carbonitride (a-SiCN) films deposited on single crystal silicon substrates of p-type conductivity by plasma-enhanced chemical vapor deposition (PECVD) technique. The influence of the deposition time and thermal annealing on the mechanism of the SCL current is studied. It is shown that the SCL current can be limited by shallow or deep traps, depending on the preparation condition. Experimental evidences are obtained that the traps in the films under study are associated with dangling bond defects, presumably due to the decomposition of Si–H bonds. Band and trap parameters (mobility of electrons, density and energy of trap states) are estimated from the SCL current measurements.

Atomic-scale defects generated in the early/intermediate stages of dielectric breakdown in Si/SiO2 transistors

Electrically detected magnetic resonance and near-zero-field magnetoresistance measurements were used to study atomic-scale traps generated during high-field gate stressing in Si/SiO2 MOSFETs. The defects observed are almost certainly important to time-dependent dielectric breakdown. The measurements were made with spin-dependent recombination current involving defects at and near the Si/SiO2 boundary. The interface traps observed are Pb0 and Pb1 centers, which are silicon dangling bond defects. The ratio of Pb0/Pb1 is dependent on the gate stressing polarity. Electrically detected magnetic resonance measurements also reveal generation of E′ oxide defects near the Si/SiO2 interface. Near-zero-field magnetoresistance measurements made throughout stressing reveal that the local hyperfine environment of the interface traps changes with stressing time; these changes are almost certainly due to the redistribution of hydrogen near the interface.

Effects of 29Si and 1H on the near-zero field magnetoresistance response of Si/SiO2 interface states: Implications for oxide tunneling currents

We report on a study that offers fundamental physical insight into an important phenomenon in solid state device physics, tunneling in Si/SiO2. We observe near-zero field magnetoresistance via spin-dependent trap-assisted-tunneling in both unpassivated and passivated Si/SiO2 and 28Si/28SiO2 metal–insulator–semiconductor (MIS) capacitors. A previous report, which utilized electrically detected magnetic resonance and NZFMR on these devices, indicates a surprising conclusion: the observed trap-assisted tunneling spectra are dominated by silicon dangling bonds back bonded to silicon at the Si/SiO2 interface, Pb0 and Pb1 centers. In this study, the four sets of samples are virtually identical, apart from the presence or absence of either 1H and 29Si. We observed a substantial narrowing of the NZFMR response with the removal of 29Si nuclei and a substantial broadening with the addition of 1H. Since superhyperfine interactions between 29Si nuclei Pb at the Si/SiO2 interface are a full order of magnitude stronger than such interactions involving silicon dangling bonds defects (E′ centers) within the oxide, the NZFMR results strongly suggest a response dominated by Si/SiO2 interface trap defects. With the introduction of 1H magnetic nuclei to the interface after a forming gas anneal, linewidths and lines shapes of Si/SiO2 and 28Si/28SiO2 MIS capacitors were nearly identical. However, the amplitude of the NZFMR response is greatly reduced by the introduction of hydrogen by a fraction about equal to the reduction in the interface trap density. Our results further indicate that the rate limiting step in trap-assisted tunneling is the interface to oxide trapping event.

(Invited) Impact of Boron Doping and H2 Annealing on Light Emission from Ge/Si Core-Shell Quantum Dots

We have fabricated Si-QDs with boron-doped Ge core with an areal density as high as ~1011 cm-2 on ultrathin SiO2 and characterized their room temperature photoluminescence (PL) properties. With B doping to the Ge core, stable PL observed in the energy range of 0.62 - 0.85 eV was increased by a factor of ~1.5. Note that a new relatively-narrow component peaked at ~0.64 eV emerges in addition to four components derived from the spectral deconvolution of the PL spectrum of undoped QDs and is attributable to radiative recombination between the first quantized state in the conduction band of the Si clad and the B-acceptor level in the Ge core. With H2 post-anneal at 100°C, the integrated PL intensity of the B-doped QDs was reduced by ~35% and the 0.64 eV component disappeared. With increasing H2 post-anneal temperature up to 350°C, the PL intensity was increased to ~1.4 times the initial intensity before post-annealing, and the 0.64 eV component was recovered. The observed PL changes with post-anneal are attributable to hydrogen-induced passivation of B acceptors at 100°C and thermal dissociation of B-H complex at higher temperatures up to 350°C accompanied with hydrogen passivation of residual dangling bond defects as seen in the undoped case.

Electrically detected magnetic resonance and near-zero field magnetoresistance in 28Si/28SiO2

We report on electrically detected magnetic resonance (EDMR) and near-zero-field magnetoresistance (NZFMR) measurements observed through spin-dependent trap-assisted-tunneling on unpassivated Si/SiO2 metal–insulator–semiconductor capacitors comparing those containing silicon of natural isotopic abundance and silicon depleted of 29Si. Although our measurements involve monitoring the spin-dependence of the trap-assisted-tunneling current responsible for leakage across the oxide, the EDMR spectra resemble that of a combination of Pb0 and Pb1 silicon dangling bonds sites at the Si/SiO2 interface. Additionally, we observe a substantial narrowing of the NZFMR response with the removal of 29Si nuclei. The breadth of the NZFMR response is strongly influenced by hyperfine interactions. Since superhyerfine interactions between 29Si nuclei and silicon dangling bonds at the Si/SiO2 interface are a full order of magnitude stronger than such interactions involving silicon dangling bonds defects (E′ centers) within the oxide, the NZFMR results also strongly suggest a response dominated by Si/SiO2 interface trap defects. These results collectively suggest very strongly that the leakage currents that we observe involve tunneling from Si/SiO2 Pb dangling bonds to defects within the oxide. Our results thus offer fundamental insight into technologically important phenomena involving oxide leakage currents in metal–oxide–semiconductor devices such as stress induced leakage currents and time dependent dielectric breakdown.

On the characteristics of N-polar GaN Schottky barrier contacts with LPCVD SiN interlayers

We study the behavior of N-polar GaN Schottky diodes with low-pressure chemical vapor deposited (LPCVD) SiN interlayers and unveil the important role of an amphoteric miniband formed in this interlayer due to a previously identified and dominating Si dangling bond defect. Through analysis of temperature-dependent current–voltage (I–V–T), capacitance–voltage (C–V), and x-ray photoelectron spectroscopy measurements, we observe that when nickel is deposited on LPCVD SiN pretreated with hydrofluoric acid, the SiN/GaN interface is responsible for determining the overall system's barrier height. By contrast, contact formation on oxidized LPCVD SiN leads to a metal/SiN-dominant barrier. We, consequently, propose band diagrams that account for an amphoteric miniband in LPCVD SiN, leading to a new understanding of LPCVD SiN as a lossy dielectric with surface barrier-dependent behavior.

Imaging of Bandtail States in Silicon Heterojunction Solar Cells: Nanoscopic Current Effects on Photovoltaics

Silicon heterojunction (SHJ) solar cells represent a promising technological approach towards higher photovoltaics efficiencies and lower fabrication cost. While the device physics of SHJ solar cells have been studied extensively in the past, the ways in which nanoscopic electronic processes such as charge-carrier generation, recombination, trapping, and percolation affect SHJ device properties macroscopically have yet to be fully understood. We report the study of atomic scale current percolation at state-of-the-art a-Si:H/c-Si heterojunction solar cells under ambient operating conditions, revealing the profound complexity of electronic SHJ interface processes. Using conduction atomic force microscopy (cAFM), it is shown that the macroscopic current-voltage characteristics of SHJ solar cells is governed by the average of local nanometer-sized percolation pathways associated with bandtail states of the doped a-Si:H selective contact leading to above bandgap open circuit voltages ($V_{\mbox{OC}}$) as high as 1.2 V ($V_{\mbox{OC}}>e E_{\mbox{gap}}^{\mbox{Si}}$). This is not in violation of photovoltaic device physics but a consequence of the nature of nanometer-scale charge percolation pathways which originate from trap-assisted tunneling causing dark leakage current. We show that the broad distribution of local photovoltage is a direct consequence of randomly trapped charges at a-Si:H dangling bond defects which lead to strong local potential fluctuations and induce random telegraph noise of the dark current.

Observation of electrically detected electron nuclear double resonance in amorphous hydrogenated silicon films

We report on the electrical detection of electron nuclear double resonance (EDENDOR) through spin-dependent tunneling transport in an amorphous hydrogenated silicon thin film. EDENDOR offers a many orders of magnitude improvement over classical ENDOR and is exclusively sensitive to paramagnetic defects involved in electronic transport. We observe hyperfine interactions with 1H nuclei very close to silicon dangling bond defects. These observations substantially extend recent EDENDOR observations involving silicon vacancy defects and 14N hyperfine interactions with fairly distant nitrogen atoms in 4H-SiC bipolar junction transistors. We have improved the detection scheme utilized in the earlier study by combining magnetic field modulation with RF amplitude modulation; this combination significantly improves the operation of the automatic power leveling scheme and the overall sensitivity.