Dangling Bond Defects Research Articles

A low thermal impact annealing process for SiO2-embedded Si nanocrystals with optimized interface quality

Silicon nanocrystals (Si NCs) for 3rd generation photovoltaics or optoelectronic applications can be produced by several industrially compatible physical or chemical vapor deposition technologies. A major obstacle for the integration into a fabrication process is the typical annealing to form and crystallize these Si quantum dots (QDs) which involves temperatures ≥1100 °C for 1 h. This standard annealing procedure allows for interface qualities that correspond to more than 95% dangling bond defect free Si NCs. We study the possibilities to use rapid thermal annealing (RTA) and flash lamp annealing to crystallize the Si QDs within seconds or milliseconds at high temperatures. The Si NC interface of such samples exhibits huge dangling bond defect densities which makes them inapplicable for photovoltaics or optoelectronics. However, if the RTA high temperature annealing is combined with a medium temperature inert gas post-annealing and a H2 passivation, luminescent Si NC fractions of up to 90% can be achieved with a significantly reduced thermal load. A new figure or merit, the relative dopant diffusion length, is introduced as a measure for the impact of a Si NC annealing procedure on doping profiles of device structures.

Multinuclear NMR and EPR Studies on Si-Doped Diamond-like Carbon

Recently, there is high interest in the development of diamond-like carbon (DLC) for a variety of applications in high strength, low-wear and low-friction film coatings. For instance, tool surfaces comprised of DLC are useful for machining electronic and optical components. Another example involved durable coatings for magnetic storage devices. Performance enhanced materials can be prepared by incorporating a few percent Si into the DLC. This has been done using plasma immersion ion-implantation and deposition (PIIID) by Professor Robert Carpick and his group at the University of Pennsylvania, who have provided materials for our investigation. During sample fabrication, the physical/chemical properties can be tailored, and correspondingly a large and variable combination of bulk and surface structures including dangling bonds will be present. In order to better understand these materials, we have carried out a study of the atomic level structure of Si-doped DLC using solid-state 1H, 13C and 29Si nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). NMR is used to quantify the protons and investigate the carbon and silicon local structural environments, while EPR is employed to measure the unpaired electron spin concentration which is attributed to dangling bond defects. The ratio of carbon hybridizations (sp2:sp3) and the hydrogen content are two significant factors that gauge the mechanical properties of DLCs. We have found a sp2:sp3 ratio of 58:42, as measured by 13C MAS NMR for a Si-DLC sample, which contrasts with earlier studies of pure DLC materials.(1,2,3) Qualitatively, further evidence of the differences between sp2 and sp3 carbons is borne out through bonding scenarios with hydrogen, as this can be observed with the enhanced sp2 signal intensity via 13C-1H cross-polarization (CP). The normalized 1H NMR signal intensity gives an unexpectedly large hydrogen content as much as 5% by weight, though some of this is attributed to adsorbed water. In addition to these findings, 29Si-1H CP experiments provide direct evidence of silicon-hydrogen bonds. Unpaired electron spins in dangling bond defects are quantified from normalized EPR spectra. We measure roughly 2 × 1020unpaired electron spins/gram for Si-DLC. These preliminary measurements provide an interesting glimpse into the structure of PIIID prepared Si-doped DLCs. Many questions remain concerning the distributions of sp2 and sp3 carbons, Si-tetrahedra, etc., as well as how the structure can incorporate so many protons and defects. Similar NMR/EPR measurements for annealed samples (50o to 350oC) are in progress, as these are expected to provide valuable information concerning structural changes resulting from treatment at elevated temperature, which is important for device applications.Reference:(1) P.K. Chu, L.Li, Mater. Chem. Phys., 96(2006) 253-277(2) A.I.Shames, A.M.Panich,et al., J Phys. Chem. Solids, 63(2002) 1993-2001(3) N. Zumbulyadis, J.Chem.Phys., 86(1987) 1162-1166Acknowledgement:This work was supported by the National Science Foundation through a subcontract to Hunter College from the University of Pennsylvania.

PECVD-AlOx/SiNx Passivation Stacks on Silicon: Effective Charge Dynamics and Interface Defect State Spectroscopy

The charge dynamics and the interface defect state density of AlOx/SiNx passivation stacks deposited by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) wafers are investigated. High frequency (1MHz) capacitance voltage (C-V) measurements were performed on stacks in the as deposited state and after an annealing step. C-V sweeps reveal an initially high negative charge density for the as deposited sample, activated by the thermal budget during SiNx deposition. However, this charge state is unstable and reduced owning to electron detrapping and emission into the c-Si upon applying moderate voltages. In the annealed sample, the AlOx/SiNx stack has a stable negative fixed charge. Both for as deposited and for annealed samples, applying a positive or negative constant gate voltage stress (Vstress) enhances or reduces the negative effective charge density (Qox,eff), respectively. Injection of charges from the c-Si into traps in the AlOx/SiNx stack is identified as the mechanism responsible for this behavior. We conclude that in addition to fixed negative charges trapping of negative charges near the interface is a crucial mechanism contributing to the total effective negative charge of the stack. Their contribution depends on the temperature and duration of the thermal treatment. Additionally, a large Vstress leads to generation of additional Si dangling bond defects over the entire c-Si bang gap at the c-Si/AlOx interface.

Metal Removal and Defect Passivation Performed on Si Wafers for Solar Cell Use by HCN Treatments

Cleaning of Si wafers for solar cell use with HCN solutions can remove metal contaminants almost completely. Fe and Ni atoms with ∼2 × 1011 and ∼3 × 1010 atoms/cm2 concentrations reside on Si even after formation of pyramidal textured Si surfaces produced by anisotropic alkaline etching. The Fe and Ni concentrations decrease to less than ∼6 × 109 atoms/cm2 (i.e., detection limit of a total-reflection X-ray fluorescence (TXRF) spectrometer for Fe) and ∼1 × 1010 atoms/cm2, respectively, after cleaning with pH 11.5 HCN solutions containing 1.5 M isopropanol (IPA). Minority carrier lifetime for the pyramidal textured p-type single crystalline Si specimens increases from 58 to 220 μs by the HCN treatment with IPA. The micro-roughness on Si(111) surfaces treated with HCN solutions containing IPA is much less (i.e., 0.27 nm) than that without IPA (i.e., 0.52 nm). Adsorption of IPA on Si retards the etching rate from 32.7 to 8.6 nm/min, leading to enhancement of anisotropic etching, thus resulting in the formation of more complete pyramidal structure. Cleaning with pH 11.5 HCN solutions without IPA increases reflectivity due to isotropic etching, while that with IPA slightly decreases it due to the formation of more complete pyramidal textured surfaces. The increase in the minority carrier lifetime caused by HCN cleaning with IPA is attributable to i) removal of metal contaminants, ii) passivation of Si dangling bond defect states by CN− ions, and iii) a decrease in the defect density due to a decrease in the micro-roughness.

Time constants of spin-dependent recombination processes

We present experiments to systematically study the time constants of spin-dependent recombination processes in semiconductors using pulsed electrically detected magnetic resonance (EDMR). The combination of time-programmed optical excitation and pulsed spin manipulation allows us to directly measure the recombination time constants of electrons via localized spin pairs and the time constant of spin pair formation as a function of the optical excitation intensity. Using electron nuclear double resonance, we show that the time constant of spin pair formation is determined by an electron capture process. Based on these time constants we devise a set of rate equations to calculate the current transient after a resonant microwave pulse and compare the results with experimental data. Finally, we critically discuss the effects of different boxcar integration time intervals typically used to analyze pulsed EDMR experiments on the determination of the time constants. The experiments are performed on phosphorus-doped silicon, where EDMR via spin pairs formed by phosphorus donors and Si/SiO2 interface dangling bond defects is detected.

Impact of strain on the passivation efficiency of Ge dangling bond interface defects in condensation grown SiO2/GexSi1−x/SiO2/(1 0 0)Si structures with nm-thin GexSi1−x layers

Hydrogen passivation of germanium dangling bond defects observed as paramagnetic GePb1 centers at the GexSi1−x/SiO2 interfaces is studied as a function of Ge concentration and thickness of the GexSi1−x layer. By correlating the results obtained by three independent defect-sensitive methods – electron spin resonance spectroscopy, ac conductance of the GexSi1−x layer, and the positron annihilation spectroscopy – with the results of strain measurements by high-resolution X-ray diffractometry, we found that the density of the Ge dangling bonds reflects residual strain in the GexSi1−x layer. Furthermore, in the layers with high strain the hydrogen passivation efficiency of dangling bonds is found to decrease, suggesting a considerable spread in the activation energies of the passivation/depassivation reactions.

Electrically detected double electron–electron resonance: exchange interaction of P donors and P defects at the Si/SiO interface

We study the coupling of P dangling bond defects at the Si/SiO2 interface and 31P donors in an epitaxial layer directly underneath using electrically detected double electron–electron resonance (EDDEER). An exponential decay of the EDDEER signal is observed, which is attributed to a broad distribution of exchange coupling strengths J/2π from 25 kHz to 3 MHz. Comparison of the experimental data with a numerical simulation of the exchange coupling shows that this range of coupling strengths corresponds to 31P–P distances ranging from 14 nm to 20 nm.

Remote plasma-processing (RPP), medium range order, and precursor sites for dangling bond defects in “amorphous-Si(H)” alloys: Photovoltaic and thin film transistor devices

Remote plasma processing (RPP) provides pathways to the formation of photovoltaic (PV) and thin-film-transistor (TFT) devices that include buried interfaces. This is made possible by separate and independent control of (i) plasma excited O- and N-atom deposition precursors in a up-stream plasma chamber, combined with (ii) down-stream injection of Si- and Ge-atoms with control gas flow rates providing control of buried interface bonding at monolayer levels. Devices with intrinsic, B p-type and P n-type “a-Si(H)” & “a-Si,Ge(H)” layers require 10% bonded H in monolayer (SiH arrangements) and deposition and/or annealing at temperatures between 240 and 275°C. Deposition from SiH4 with either PH3 or B2H6 dopant gasses provides spectrally reflecting films which can be annealed yielding fine-grain films for gate, or source and drain regions for TFTs or FETs.

Quantum engineering at the silicon surface using dangling bonds

Individual atoms and ions are now routinely manipulated using scanning tunnelling microscopes or electromagnetic traps for the creation and control of artificial quantum states. For applications such as quantum information processing, the ability to introduce multiple atomic-scale defects deterministically in a semiconductor is highly desirable. Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface. We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image. Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

Dangling-bond defect in a-Si:H: Characterization of network and strain effects by first-principles calculation of the EPR parameters

The performance of hydrogenated amorphous silicon (a-Si:H) solar cells is severely affected by the light-induced formation of metastable defects in the material (Staebler-Wronski effect). The common notion is that the dangling-bond (db) defect, a threefold coordinated silicon atom, plays a key role in the underlying mechanisms. To support the characterization of this defect by electron paramagnetic resonance (EPR), we present in this work a first-principles study of the EPR parameters for a structural ensemble of the db defect. We show that the a-Si:H dangling bond is a network defect for which charge and spin localization substantially depend on the actual coordination of the db atom and the local geometric and electronic structure of the immediate surrounding. It consequently differs by its very nature from its crystalline counterpart, which is typically related to the presence of a vacancy. The application of hydrostatic strain to our models yields further insights into the dependence of the hyperfine interaction on the structural characteristics of the defect. The observed trends are shown to result from the interplay between delocalization and sp hybridization.

Chemical kinetics of the hydrogen-GePb1 defect interaction at the (100)GexSi1−x/SiO2 interface

A study of the hydrogen passivation/dissociation kinetics of the GePb1 (Ge dangling bond) defect at the (100) GexSi1−x/SiO2 interface shows that the data can be well described by the same generalized simple thermal model as applied to the Si Pb dangling bond defect at the Si/SiO2 interface, enabling inference of the relevant kinetic parameters. It is found that even for optimized treatment, only ∼60% of the GePb1 system can be electrically inactivated through binding to H, which is well below device grade level. This is concluded to be a direct consequence of the existence of excessive spreads in activation energies for GePb1 passivation/reactivation dissociation.

Multi-frequency electron spin resonance analysis of interfacial Ge dangling bond defects in condensation-grown (1 0 0)Si/SiO2/Si1–xGex/SiO2

The multi-frequency electron spin resonance (ESR) study of the GePb1 Ge dangling bond (DB) interface defect, appearing identically at both SiO2/GexSi1–x interfaces in condensation-grown SiO2/GexSi1–x/SiO2/(1 0 0)Si heterostructures (0.28 ≤ x ≤ 0.93), shows that the ESR signal width is dominated by inhomogeneous broadening due to a strain-induced spread in g. For the x = 0.73 case, this results in a frequency (ν)-dependent peak-to-peak broadening of ΔBppSB/ν = 0.62 G/GHz for the applied magnetic field B along the g3 principal axis Ge DB direction. Compared to the familiar Si Pb-type interface defects in (1 0 0)Si/SiO2, the enhanced ν-dependent broadening scales with the spin–orbit coupling constant ratio λ(Ge)/λ(Si). The natural inhomogeneous broadening due to unresolved 73Ge hyperfine interaction is found to be below ∼1.56 G. While the results adduce quantitative support for the assignment of GePb1 as a Ge DB-type interface center, it is concluded that the GeSi/SiO2 interface uniformity is of good level.

Acoustic spectroscopy and electrical characterization of Si/NAOS-SiO2/HfO2 structures

The MOS structure prepared on n-type Si substrate with SiO2/HfO2 gate dielectric layers was formed by 5nm HfO2 oxide deposited by atomic layer deposition on 0.6nm SiO2 oxide film prepared with nitric acid oxidation of Si (NAOS) in ∼100% HNO3 vapor. The set of this MOS structure was annealed in N2 atmosphere at 200, 300 and 400°C for 10min to stabilize the structure, to decrease the interface states density and leakage current density. The both acoustic deep level transient spectroscopy (A-DLTS) and acoustoelectric response signal versus gate voltage dependence (Uac–Ug characteristics) were used to characterize the interface states and the role of annealing treatment, except ordinary electrical investigation represented by current–voltage and capacitance–voltage measurements. The main interface deep centers with activation energies ∼0.30eV typical for dangling-bond type defects were observed as well as a particular influence of annealing treatment on the interface states. The obtained results are analyzed and discussed.

Optimal Size Regime for Oxidation-Resistant Silicon Quantum Dots

First-principles computations have been carried out to predict that appropriately terminated silicon quantum dots with diameters in the range of 1.2-2 nm will offer a superb resistance to oxidation. This is because surface treatments can produce dangling bond defect densities sufficiently low that dots of this size are unlikely to have any defect at all. On the other hand, these dots are large enough that the severe angles between facets do not expose bonds that are vulnerable to subsequent oxygen attack. The absence of both surface defects and geometry-related vulnerabilities allows even very short passivating ligands to generate an effective barrier, an important consideration for charge and exciton transport within quantum dot assemblies. Our computations, which employ many-body perturbation theory using Green functions, also indicate that dots within this size regime have optical and electronic properties that are robust to small amounts of inadvertent oxidation, and that any such oxygen incorporation is essentially frozen in place.

Compensation of Native Defect Acceptors in Microcrystalline Ge and Si1-xGex Thin Films by Oxygen Incorporation: Electrical Properties and Solar Cell Performance

Undoped hydrogenated microcrystalline Ge (µc-Ge:H) films grown by plasma-enhanced chemical vapor deposition reveal high concentration of free holes (>1018 cm-3) when the films exhibit a high crystalline volume fraction. ESR and Hall-effect experiments suggest that the acceptor states arise from the native dangling bond defects at Ge crystalline grain boundaries. It is demonstrated that an intentional oxygen incorporation during the µc-Ge:H deposition reduces the hole concentration by two orders of magnitude. Furthermore, µc-Si1-xGex:H (x=0.1–0.3) alloy p–i–n solar cells show marked improvements in photocarrier collection properties upon oxygen incorporation into the i-layer in the order of 5×1018–1020 cm-3. These findings are explained by the effect of the compensation of the negatively charged Ge dangling bonds by oxygen donors.

Free-Standing Si and Ge, and Ge/Si Core-Shell Semiconductor Nanowires

The properties of free-standing silicon and germanium nanowires oriented along the [110] direction are studied using di erent rst principles methods. We show the corrections due to quasi-particles to the band structures obtained using the local-density approximation. The formation energies of B and P doped nanowires are calculated, both in the absence and presence of dangling bond defects and we link these to experimental results. Furthermore, we report on the phonon properties of pure Si and Ge nanowires, as well as Ge/Si core-shell nanowires, and discuss the di erences between them.

Nanogratings in fused silica: Formation, control, and applications

The authors investigated the formation of periodic subwavelength structures, so-called nanogratings, in the volume of fused silica. These self-organized structures emerge upon irradiation with ultrashort laser pulses, undergoing three distinct stages of growth from randomly distributed nanostructures to extended domains with uniform periodicity. The experiments revealed that the cumulative action of subsequent laser pulses is mediated by dangling-bond type defects. On shorter time scales, transient self trapped excitons may significantly enhance the formation process. Nanogratings exhibit an extremely large temperature stability up to 1150 °C. In combination with the possibility to precisely tune their form birefringence, nanogratings provide a powerful tool to realize, thermally stable complex phase elements.

The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries

Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder.However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses.

Atomic and Electrical Characterisation of Amorphous Silicon Passivation Layers

In this work, an extensive characterisation of intrinsic amorphous silicon (a-Si) passivation layers deposited on n- and p-type silicon is reported. Low temperature capacitance-voltage measurements are utilised to enable parameter extraction from the c-Si/a-Si interface and a-Si bulk. Electron spin resonance enables atomic identification of defects present. Results reveal the presence of electrically active defects at the c-Si/intrinsic a-Si interface (∼1x1012cm-2), and throughout the amorphous silicon layer bulk (∼8x1016cm-3), which are atomically identified as Pb0 centres and D centres silicon dangling bond defects, respectively. The value of this work is the atomic identification of these defects in this stack, coupled with their electrical activity. That they can be detected by these techniques demonstrates the power of the methodology used to assess and quantify these defects. Therein lies the significance of this work: a methodology capable of fundamentally optimising amorphous silicon processing from an atomic perspective.

Bonded hydrogen in nanocrystalline silicon photovoltaic materials: Impact on structure and defect density

We have performed a detailed structural and optical investigation of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma-enhanced chemical vapor deposition. The microstructural properties of these thin films are characterized and interpreted physically based on the growth mechanism. Infrared spectroscopy reveals that the bonded hydrogen in a platelet-like configuration, which is believed to be located at grain boundaries, greatly affects oxygen incursions into nc-Si:H thin films, whereas electron spin resonance observations link these incursions to the introduction of dangling bond defects. Consequently, we propose that in nc-Si:H thin films, high bonded-hydrogen content in grain boundaries is of great importance in forming hydrogen-dense amorphous tissues around the small crystalline grains, i.e., compact grain boundary structures with good passivation. Such structures effectively prevent post-deposition oxidation of grain boundary surfaces, which might lead to the formation of dangling bond defects.