Paramagnetic Defect Density Research Articles

Structural and electrical properties of metastable defects in hydrogenated amorphous silicon

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed dependence of the paramagnetic defect density on the Doppler S parameter indicates that porous, nanosized void-rich materials exhibit higher spin densities, while dense, divacancy-dominated materials show smaller spin densities. However, after light soaking more similar spin densities are observed, indicating a long-term defect creation process in the Staebler-Wronski effect that does not depend on the a-Si:H nanostructure. From ESE decays it appears that there are fast and slowly relaxing defect types, which are linked to various defect configurations in small and large open volume deficiencies. A nanoscopic model for the creation of light-induced defects in the a-Si:H nanostructure is proposed.

Spin counting in electrically detected magnetic resonance via low-field defect state mixing

The work herein describes a method that allows one to measure paramagnetic defect densities in semiconductor and insulator based devices with electrically detected magnetic resonance (EDMR). The method is based upon the mixing of defect states which results from the dipolar coupling of paramagnetic sites at low magnetic fields. We demonstrate the measurement method with spin dependent tunneling in thin film dielectrics; however, the method should be equally applicable to paramagnetic defect density measurements in semiconductors via the more commonly utilized EDMR technique called spin dependent recombination.

Disorder and defect formation mechanisms in molecular-beam-epitaxy grown silicon epilayers

We investigate the role of disorder, stress and crystallite size in determining the density of defects in disordered and partially ordered silicon thin films deposited at low or moderate temperatures by molecular beam epitaxy. We find that the paramagnetic defect density measured by electron spin resonance (ESR) is strongly dependent on the growth temperature of the films, decreasing from ~2·1019cm−3 at 98°C to ~1·1018cm−3 at 572°C. The physical nature of the defects is strongly dependent on the range of order in the films: ESR spectra consistent with dangling bonds in an amorphous phase are observed at the lowest temperatures, while the ESR signal gradually becomes more anisotropic as medium-range order improves and the stress level (measured both by X-ray diffraction and Raman spectroscopy) is released in more crystalline films. Anisotropic ESR spectra consistent with paramagnetic defects embedded in an epitaxial phase are observed at the highest growth temperature (572°C).

Magnetic properties of carbon nanotube poly(ether-ester) nanocomposites

The magnetic and electronic properties of composites consisting of oxidized multiwall carbon nantubes (MWNTs) dispersed in elastomeric poly(ether-ester) segmented block copolymer have been studied by means of electron spin resonance (ESR) and dc magnetization measurements. A marked reduction in the MWNT diamagnetic response is identified, indicative of substantial hole doping related to the oxygen functional groups on the oxidized carbon nanotube’s surface. Both ESR and the static magnetization reveal considerable enhancement of the spin susceptibility due to an excessive increase in the density of paramagnetic defects, which are sensitive to the dynamical polymer relaxation and thus to the MWNT-polymer interfacial coupling.

Photoinduced Leakage Currents in Silicon Carbon Nitride Dielectrics for Copper Diffusion Barriers

The current conduction in silicon carbon nitride (SiCN) dielectric films subjected to ultraviolet (UV) illumination at room temperature has been investigated. After exposure of SiCN single-layer and SiCN–SiO2 double-layer films to 4.9-eV UV illumination, leakage currents through both films substantially increased under positive and negative gate biases. The current increased as a function of the change in paramagnetic defect density, which was obtained from electron spin resonance signals of the SiCN and SiCN–SiO2 films subjected to 4.9-eV UV illumination. We suggest that the paramagnetic defects, which have been identified as neutrally charged silicon dangling bonds, act as generation centers and that their formation is responsible for the current increase. The conduction mechanisms of the photoinduced leakage current are discussed in detail.

Electron Spin Resonance and Photoluminescence Study of Charge Trap Centers in Silicon Nitride Films and Fabrication of Proposed Oxide–Nitride–Oxide Sidewall 2-bit/Cell Nonvolatile Memories

We have proposed a novel oxide–nitride–oxide (ONO)-sidewall 2-bit/cell nonvolatile memory and fabricated 70-nm-node nonvolatile memory devices. For low-pressure chemical-vapor-deposition (LPCVD)-SiN films, with increasing SiH4/NH3 mixture gas ratio, we have found from ESR and PL evaluation that the paramagnetic defect density increases and some PL emission energy levels become deeper. We consider that the energy-level shift is due to the effects of trap potential overlapping, where the trap centers are generated at the excess silicon atoms in the SiN films. In this study, a SiH4/NH3 mixture gas ratio of less than 1:100 was used to suppress the potential overlapping. As a result, we have also shown that the proposed memory device has high performance and excellent scalability.

Si-SiO 2 interface passivation by plasma NH 3 and atomic H

Plasma ammonia treatment at 400 °C leads to de-passivation of a fully hydrogenated Si-SiO 2 interface, and to passivation of a fully de-hydrogenated Si-SiO 2 interface. Plasma NH 3 exposure causes irreversible Si surface damage and degradation of thermal stability. Atomic hydrogen exposure, although it results in similar effects on the Si-SiO 2 interface, does not introduce additional defects or a decrease of the Si surface thermal stability. The difference between plasma NH 3 exposure and atomic H exposure is speculated to be due to either the nitridation of Si-SiO 2 interface or radiation damage resulting from plasma NH 3 exposure. EPR measurements indicate changes of the paramagnetic defect properties and an increase in the paramagnetic defect density generated by plasma NH 3 exposure.

Crystallization Kinetics of Cubic Boron Nitride in the B-N-Mg-H-F System

The kinetics of the h-BN → c-BN transformation in a mixture of composition 78% h-BN + 5% NH4F + 2% B + 15% Mg are studied at 5 GPa and temperatures from 1670 to 1910 K. The activation energy of c-BN formation, temperature-dependent reaction rate constant, strength of the forming c-BN particles, and density of paramagnetic defects in c-BN are evaluated.

Defects and structure of hydrogenated microcrystalline silicon films deposited by different techniques

The development of structure and paramagnetic defects of microcrystalline silicon films prepared by plasma enhanced chemical vapour deposition at very high frequency (VHFPECVD), at radio frequency and in the high pressure, high power regime (RFPECVD) and by the hot wire CVD (HWCVD) are studied. The relationship between paramagnetic defects and structural configuration of μc-Si:H films obtained under deposition conditions near the amorphous-microcrystalline structural transition is investigated using results from electron spin resonance (ESR) such as spin density, g-value, ESR peak to peak line width Δ H pp, and crystalline volume fraction as calculated from Raman spectroscopy. Similar relationships between the structure transition from crystalline to amorphous and the density and spectral shape of paramagnetic defects are observed for all deposition methods. Low substrate temperatures and deposition conditions close to the transition to amorphous growth are found beneficial for low spin densities in accordance with the optimum growth conditions for high efficiency solar cells.

Paramagnetic structural defects and conductivity in hydrogenated nanocrystalline carbon-doped silicon films

The behavior of paramagnetic defects and dark conductivity in heterogeneous samples of hydrogenated nanocrystalline carbon-containing silicon (nc-SiC:H) prepared by the photo-CVD method is studied. It is shown that, with increasing carbon content in nc-SiC:H, a phase transition from a nanocrystalline to an amorphous structure occurs, producing a reduction in paramagnetic defect density and a significant decrease in the dark conductivity.

Microstructural modifications induced by rapid thermal annealing in plasma deposited SiOxNyHz films

The effect of rapid thermal annealing (RTA) processes on the structural properties of SiOxNyHz films was investigated. The samples were deposited by the electron cyclotron resonance plasma method, using SiH4, O2 and N2 as precursor gases. For SiOxNyHz films with composition close to that of SiO2, which have a very low H content, RTA induces thermal relaxation of the lattice and improvement of the structural order. For films of intermediate composition and of compositions close to SiNyHz, the main effect of RTA is the release of H at high temperatures (T>700 °C). This H release is more significant in films containing both Si–H and N–H bonds, due to cooperative reactions between both kinds of bonds. In these films the degradation of structural order associated to H release prevails over thermal relaxation, while in those films with only N–H bonds, thermal relaxation predominates. For annealing temperatures in the 500–700 °C range, the passivation of dangling bonds by the nonbonded H in the films and the transition from the paramagnetic state to the diamagnetic state of the K center result in a decrease of the density of paramagnetic defects. The H release observed at high annealing temperatures is accompanied by an increase of density of paramagnetic defects.

Optical and structural properties of SiOxNyHz films deposited by electron cyclotron resonance and their correlation with composition

SiO x N y H z films were deposited from O2, N2, and SiH4 gas mixtures at room temperature using the electron cyclotron resonance plasma method. The absolute concentrations of all the species present in the films (Si, O, N, and H) were measured with high precision by heavy-ion elastic recoil detection analysis. The composition of the films was controlled over the whole composition range by adjusting the precursor gases flow ratio during deposition. The relative incorporation of O and N is determined by the ratio Q=φ(O2)/φ(SiH4) and the relative content of Si is determined by R=[φ(O2)+φ(N2)]/φ(SiH4) where φ(SiH4), φ(O2), and φ(N2) are the SiH4, O2, and N2 gas flows, respectively. The optical properties (infrared absorption and refractive index) and the density of paramagnetic defects were analyzed in dependence on the film composition. Single-phase homogeneous films were obtained at low SiH4 partial pressure during deposition; while those samples deposited at high SiH4 partial pressure show evidence of separation of two phases. The refractive index was controlled over the whole range between silicon nitride and silicon oxide, with values slightly lower than in stoichiometric films due to the incorporation of H, which results in a lower density of the films. The most important paramagnetic defects detected in the films were the K center and the E′ center. Defects related to N were also detected in some samples. The total density of defects in SiOxNyHz films was higher than in SiO2 and lower than in silicon nitride films.

Correlation between photoluminescence efficiency and density of paramagnetic defects in Er-doped hydrogenated amorphous silicon

Photoluminescence and electron paramagnetic resonance in layers of hydrogenated amorphous silicon doped with erbium and oxygen have been investigated. The intensity of Er-related photoluminescence (1.54 μm) depends non-monotonically on the ratio between erbium and oxygen content varied from 0.1 to 3. The photoluminescence efficiency correlates with the density of spin centers (most likely Si dangling bonds (DBs)) that gives evidence for the role of Si DBs in the electronic excitation of Er-ions.

Hydrogen and disorder in diamond-like carbon

Diamond-like carbon is a system of rather high disorder as it has a wide optical absorption tail and a high density of paramagnetic defects. The defect density remains high even in DLCs containing 30-60% hydrogen, so hydrogen does not appear to passivate defects well unlike in a-Si:H. To investigate the role of hydrogen on the disorder in DLCs we have investigated the effect of low concentrations of hydrogen on the disorder in ta-C, by introducing 10(-6)-10(-3) mbar hydrogen into the deposition of ta-C by filtered cathodic vacuum are (FCVA), which corresponds to 0.1-15 at.% hydrogen in the films. Higher pressures of hydrogen reduces the ionisation leading to sp(2) bonding, and ultimately the thermalisation of the plasma leads to nanotubes and fullerenes. The deposited ta-C:H films were investigated by electron energy loss spectroscopy (EELS), Raman spectroscopy, optical measurements, electronic transport and N-15 resonant nuclear reaction analysis. Plasma characterisation with a retarding field analyser showed that the ion current density remains nearly unchanged in the pressure range used to deposit the films. Raman measurements indicate the onset of clustering of sp(2) sites when the hydrogen pressure exceeds 2 x 10(-4) mbar. We find that small amounts of hydrogen increase the optical gap up to 2X10(-6) mbar hydrogen pressure, and then the band gap decreases continuously. The absorption tail sharpens by the addition of hydrogen, as measured by photothermal deflection spectroscopy (PDS) and thus confirms the Raman measurements that suggest that the order in the material increases with increasing hydrogen content.

Relations between the deposition conditions, the microstructure and the defects in PECVD hydrogenated amorphous carbon films; influence on the electronic density of states

The relationships between the deposition conditions, the growth mechanisms, the microstructure and the electronic density of states of hydrogenated amorphous carbon (a-C:H) films prepared by PECVD from hydrocarbons are not yet fully understood. We therefore performed a systematic study using several complementary techniques to determine the changes in the microstructure and in the optical properties of a-C:H samples deposited from a dual micro-wave/radio-frequency discharge in methane as a function of the negative r.f. bias voltage applied to the substrate −Vb. The results reveal the existence of two successive film growth regimes when varying −Vb from −30 to −600 V, i.e. when increasing the ion bombardment during deposition, which lead to different types of sp2 C atoms clustering and H incorporation, but to the same density of paramagnetic defects. Models of the a-C:H microstructure are proposed in each case, and their influence on the electronic states density is analysed in detail. It is shown that the H content and the proportion of sp2 C atoms play a minor role on the electronic properties, as compared to the nature, the size, the number and probably the distortions of the π-bonded clusters.

Effect of deposition temperature on the optical transmission and paramagnetic centers in pulsed laser deposited amorphous silicon carbide thin films

The optical transmission and paramagnetic centers in pulsed laser deposited amorphous silicon carbide films were investigated as a function of the deposition temperature (Td). As Td is raised from 200 to 650 °C, the optical gap of the films increases from about 2.0 eV up to a value of 2.2 eV, while the density of paramagnetic defects, measured by electron paramagnetic resonance (EPR), diminishes from (5.6 to 3.6)×1020 spins/cm3. EPR line shape analyses and g-value measurements indicate that the EPR active centers are clustered sp2 carbon related defects. At a measurement temperature of −190 °C, the exchange narrowed EPR linewidth, evaluated at 5.0±0.3 G, is nearly independent of Td. However, at room temperature, the EPR linewidth was found to increase with deposition temperature, thus suggesting a decrease in the size of the sp2 bonded clusters. All the structural changes revealed by EPR, as Td was raised from 200 to 650 °C, correlate well with the optical transmission data and with the increase in heteronuclear SiC bonding revealed by Fourier transform infrared spectroscopy. Finally, analysis of the thermal dependence of the EPR linewidth together with the conductivity of the a-SiC films have revealed the existence of different conduction mechanisms in the layers, depending on the deposition temperature.

Photoluminescence from polymer-like hydrogenated and nitrogenated amorphous carbon films

The mechanism of photoluminescence (PL) in hydrogenated amorphous carbon (a-C:H), and nitrogenated and hydrogenated amorphous carbon (a-C:H:N) thin films grown by radio frequency driven plasma enhanced chemical vapor deposition is still a subject of much debate. In this work, we investigate the PL signal obtained from a-C:H and a-C:H:N films, paying particular attention to the effect of nitrogen flow rate during growth, and postgrowth, ex situ annealing on the PL properties of the films. We also correlate the PL spectra to the electronic structure of the films. The films had a low paramagnetic defect density (1017 cm−3). The PL spectra were obtained using the 488 nm (2.54 eV) line of an argon ion laser, as the excitation source. It was observed that the nitrogenation of the films leads to the creation of new bands in the PL signal, which were correlated to the bond fraction of CN triple bonds, as measured by infrared spectroscopy.

Decrease of electron paramagnetic defect density and enhancement of electron field emission in annealed carbon films

We have studied the effect of the paramagnetic defects in carbon films on the field-emission properties. The paramagnetic defects in carbon films originating from the carbon dangling bonds were measured using electron spin resonance (ESR). We found a reduction of the dangling bond density in the annealed carbon films in N2/H2 atmosphere from ESR spectroscopy. The annealed films with lower defect densities, i.e., dangling bonds, showed an improved characteristic of higher emission current density as well as a lower turn-on electric field in the Fowler–Nordheim type tunneling. Also in the annealed carbon films a correlation is observed between the decrease of band gap estimated from the electrical conductivity and the reduction of dangling bond density. The earlier results indicate that the enhancement of the emission current level in the annealed carbon films is related to the decrease of the ESR centers.

Photoconductivity and photoluminescence processes in amorphous carbon

The photoconductivities of various sp 3 bonded amorphous carbons were studied to understand their recombination and photoluminescence processes. At temperatures >200 K, the photoconductivity of each type of carbon does not vary inversely with the paramagnetic defect density, indicating that paramagnetic defects are not the only recombination centres. This effect occurs because carrier demarcation levels lie in the band tails, so both defects and tail states act as recombination centres. The temperature regime <200 K is due to energy loss hopping. The localisation radius is not constant, so the capture radius cannot be treated as constant when describing luminescence efficiency in hydrogenated amorphous carbon (a-C:H) of different band gaps.

Photoluminescence in low defect density a-C:H and a-C:H:N

Films with paramagnetic defect density ∼10 17 cm −3 of hydrogenated amorphous carbon (a-C:H) and nitrogenated and hydrogenated amorphous carbon (a-C:H:N) were grown on an earthed electrode of a radio-frequency (RF) driven plasma enhanced chemical vapour deposition system. The photoluminescence spectra had two bands at 562 nm ( 2.20±0.05 eV ), and 612 nm ( 2.05±0.05 eV ). The intensity of the 612 nm band increased relative to the intensity of the 562 nm band with increasing incorporation of nitrogen. Fourier transform infra-red spectroscopy results indicated there is also an increase in the CN triple bond fraction. The 612 nm band is attributed to π ∗− π transitions within a CN triple bond cluster.