ESR studies of thermally oxidized silicon wafers

We have performed ESR measurements on oxidized (111) silicon wafers and found two ESR centers. One, the previously known P_b center,was found to be a two level interface trap by corona bias studies. The other one, previously known as “damaged silicon” center was found to be a silicon center, probably related to the oxide growth process.     

Structure of multiple-vacancy (oxygen) centers in irradiated silicon

Analysis of a new paramagnetic spin 1 spectrum, labeled Si-S1, indicates that it corresponds to the neutral charge state of the Si-B1 center in an excited spin-triplet state. An extension of this analysis to the Si-P1, -P2, -P3, -P4, and -P5 centers suggest that these centers comprise a family of defects which consist of a string of neighboring vacancies in a single (110) plane. In particular, it appears that the Si-P1 center is an odd ( 3)-vacancy defect; the Si-P2 center is a 2-vacancy, oxygen defect; the Si-P3 center is a 4-vacancy defect; the Si-P4 center is a 3-vacancy, oxygen defect of symmetry lower than C_2v ; and the Si-P5 center is a 3-vacancy, oxygen defect having C_2v symmetry.     

Capturing properties of a threefold coordinated silicon atom in silicon nitride: Positive correlation energy model

The electronic structure and capturing properties of threefold coordinated silicon atoms (≡Si·) and the Si-Si bond in silicon nitride (Si₃N₄) were studied using the ab initio density functional theory. The results show that the previously proposed negative correlation energy (NCE) model is not applicable to Si₃N₄. The NCE model was proposed for interpreting the absence of the ESR signal for threefold coordinated silicon defects and suggested that an electron can transfer between two silicon defects. We proposed that the absence of this ESR signal is due to the creation of neutral diamagnetic Si-Si defects in Si₃N₄. This model offers the most fundamental theory for explaining the hole localization (memory) effect in silicon nitride.     

LPCVD法制备的高纯半绝缘4H-SiC晶体ESR谱特性

利用电子自旋共振波谱（ESR）仪,分析由低压化学气相沉积（LPCVD）法制备的高纯半绝缘4H-SiC材料本征缺陷.结果发现,在暗场条件下获得的缺陷信息具有碳空位（V_C）及其络合物的特征;谱线具有半高宽较大、峰谷明显不对称的特点.分析认为造成ESR谱线半高宽较大及峰谷不对称现象的主要原因是测试温度较高.同时,吸收谱中峰谷不对称现象及较大半高宽现象的出现还与不对称的晶格结构及缺陷浓度的不均匀分布有关.在110 K测试温度下,能级上的电子分布对ESR谱特性影响很小. 关键词： 低压化学气相沉积 高纯半绝缘4H-SiC 电子自旋共振 本征缺陷     

Luminescence and structure of nanosized inclusions formed in SiO2 layers under double implantation of silicon and carbon ions

Luminescent and structural characteristics of SiO₂ layers exposed to double implantation by Si⁺ and C⁺ ions in order to synthesize nanosized silicon carbide inclusions have been investigated by the photoluminescence, electron spin resonance, transmission electron microscopy, and electron spectroscopy methods. It is shown that the irradiation of SiO₂ layers containing preliminary synthesized silicon nanocrystals by carbon ions is accompanied by quenching the nanocrystal-related photoluminescence at 700–750 nm and by the enhancement of light emission from oxygen-deficient centers in oxide in the range of 350–700 nm. Subsequent annealing at 1000 or 1100°C results in the healing of defects and, correspondingly, in the weakening of the related photoluminescence peaks and also recovers in part the photoluminescence of silicon nanocrystals if the carbon dose is less than the silicon dose and results in the intensive white luminescence if the carbon and silicon doses are equal. This luminescence is characterized by three bands at ～400, ～500, and ～625 nm, which are related to the SiC, C, and Si phase inclusions, respectively. The presence of these phases has been confirmed by electron spectroscopy, the carbon precipitates have the sp ³ bond hybridization. The nanosized amorphous inclusions in the Si⁺ + C⁺ implanted and annealed SiO₂ layer have been revealed by high-resolution transmission electron microscopy.     

Leakage current and paramagnetic defects in SiCN dielectrics for copper diffusion barriers

We report the properties of paramagnetic defects in SiCN films which are used as copper diffusion barriers. Electron spin resonance (ESR) signals with a g value of 2.003 and the ΔH_PP of 1.1-1.2 mT were observed for as-grown and UV-illuminated SiCN films. These characteristics of the observed ESR signals are very similar to those of the K⁰ center in N-rich silicon nitrides. We also show that a substantial increase in the leakage current occurs by exposing the SiCN films to UV illumination. We suggest that the paramagnetic defects generated by the UV illumination are responsible for this current increase.     

Nonequilibrium polarization of triplet centers in silicon

The Liouville equation is considered for the density operator of a triplet center with phenomenological population and decay constants (K₊ and K_-). In the case of anisotropy of K- with respect to the internal magnetic axis of the triplet, the spin polarization and intensities of the EPR lines of the triplet center in a strong magnetic field are calculated. On this basis, an interpretation of experiments on Si-Si centers in irradiated silicon is given. Quantum-chemical calculations of the model of the center in the form of a cluster OSi₄H₁₂ are performed, taking account of the spin-orbital interaction, the anisotropy of which agrees with the EPR spectra of Si-Si centers in silicon.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 97–101, June, 1989.     

Annealing Induced Degradation of Thermal SiO2 On (100)Si: Point Defect Generation

The structural degradation of thermal SiO₂ on (100)Si under isochronal post oxidation vacuum annealing (POVA) has been probed by electron spin resonance (ESR). The degradation process, studied in the range T_an = 950–1250 °C, is established as intense point defect generation including E′_γ, E′_δ, EX and the elusive predominant degradation center S. Thermally activated generation is revealed over broad T_an ranges for the two most populous defects, S and E′_γ, with a common activation energy ～1.6eV. Depth profiling after heating at 1200°C shows that the S centers predominantly reside near the oxide borders, generally in anticorrelation with the E′_γ distribution. The S center susceptibility has been inferred from conventional ESR signal intensity monitoring as well as from revealed anisotropic demagnetisation effects. It is found Curie-Weiss type with critical temperature of ～ 1.3 K. Newly observed weak hyperfine structure may comply with the S center being an E′-like defect.     

Electron paramagnetic resonance (EPR) and photo-EPR studies of aggregate centers with two iron atoms in silicon

We have investigated iron by EPR (electron paramagnetic resonance) and photo-EPR in initially boron doped silicon samples in which the iron concentration exceeded the boron content. A new EPR spectrum, showing orthorhombic-I symmetry was observed and could be fitted by an effective spin Hamiltonian with the parameters S=3/2, g=2.07, and E/D=0.68. We identify this spectrum as a new modification of a Fe₂B center which has the same symmetry but different configuration of the constituent atoms.Furthermore, we were able to determine the donor level of the old Fe₂B center to E=E_V+0.57±0.02 eV above the valence band.We have also investigated the Fe₂ donor. According to our straightforward interpretation the energy levels of the transitions from Fe₂⁺ to Fe₂⁰ and from Fe₂⁰ to Fe₂⁺ were determined as E=E_V+0.61±0.02 eV and E=E_C-0.67±0.02 eV, respectively, suggesting a lattice relaxation on electron capture, which is unusual for transition metal centers in silicon. PACS 71.55.Cn; 76.30.Fc     

Studies of the electronic and vibrational signatures of the unusual bonding geometries in melt-quenched amorphous silicon

Tight-binding molecular dynamics simulations have been performed to investigate the effect of quenching rate of the Si melt on the resulting local structure of amorphous silicon. Different quenching rates were used to cool liquid silicon in the simulations to demonstrate that the choice of quenching rates significantly influences the resulting local structure. The calculated pair correlation functions show that the local structure is sensitive to the thermal processing of the liquid silicon melt. The use of cooling rates higher than 10^?13 K s^?1 appears to prevent the activation of the required structural re-arrangements necessary to stabilise the networks, causing unexpected bonding geometries to develop. The electronic signatures of the defects show that only the triangular defect structure contributes resonance states to the conduction band tail. Also, the vibrational signature of the triangular structure shows a high energy transverse optical mode at 95 meV, indicating that the defect is likely to be unstable at 300 K, although both defects contribute minimal states to the mid-gap level.     

Equivalent states of muonium and implanted hydrogen in silicon (experiment)

Main experimental data on the hydrogen-like states with an anisotropic hyperfine structure forming in silicon single crystals in the implantation of high energy muons and protons are presented. The characteristics of the “anomalous” muonium (Mu^*) and hydrogen-containing silicon AA9 states studied by the muon spin rotation (μSR) and ESR techniques in silicon with a due inclusion of the isotope effect are shown to be similar, thus suggesting the existence of two equivalent structures in silicon, Mu^* and AA9, differing only in the mass of the paramagnetic center.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Application of crystalline silicon surface oxidation to silicon heterojunction solar cells

We study the effect of ultra-thin oxide (SiO_x) layers inserted at the interfaces of silicon heterojunction (SHJ) solar cells on their open-circuit voltage (V_OC). The SiO_x layers can be easily formed by dipping c-Si into oxidant such as hydrogen peroxide (H₂O₂) and nitric acid (HNO₃). We confirm the prevention of the undesirable epitaxial growth of Si layers during the deposition of a-Si films by the insertion of the ultra-thin SiO_x layers. The formation of the SiO_x layers by H₂O₂ leads to better effective minority carrier lifetime (τ_eff) and V_OC than the case of using HNO₃. c-Si with the ultra-thin SiO_x layers formed by H₂O₂ dipping, prior to deposition of a-Si passivation layers, can have high implied V_OC of up to ∼0.714 V.     

Electric field influence on spin polarization vector behaviour of the negative muon in silicon

When a negatively charged muon is captured by a silicon atom, the atom is transformed into a solitary acceptor center similar to an Al atom. An external electric field influences the formation process of the neutral acceptor center (A.C.). It is shown in this article that the behaviour of the muon polarization vector changes appreciably in electric fields with intensities E\,\gtrsim\,10 V/cm. We estimate the muon spin relaxation rate \varLambda_μ in the so‐called “dirty limit”; for example, interaction between an internal electric field from charged impurities and a nonzero dipole moment of the A.C. is taken into account. A phonon mechanism is proposed to explain the temperature dependence of \varLambda_μ. We also estimate the value of the paramagnetic shift of the muon spin precession frequency \delta\omega/\omega₀ which is also temperature dependent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Bulk model of rapid thermal oxidation of silicon

A model of rapid thermal oxidation of silicon in dry oxygen based on the reaction of volume oxidation is constructed. It is assumed that the coefficient of oxygen diffusion for silicon dioxide decreases because of internal compressive stress, which is at a maximum near the SiO₂-Si interface; as the distance from the interface increases, this stress decreases according to the time-dependent exponential law because of viscoelastic relaxation from the value of the diffusion coefficient for strained oxide to that for fused quartz. The characteristic relaxation time of the coefficient of oxygen diffusion in silicon dioxide correlates with the relaxation time of internal stress in silicon dioxide films on silicon and with the relaxation time of the refraction index. Because the refraction index is related to the density of silicon dioxide, we arrive at the conclusion that the relaxation of the diffusion coefficient is related not only to the relaxation of internal mechanical stress, but also to the relaxation of the density of silicon dioxide.     

Solar cell fabrication using edge-defined film-fed growth (EFG) silicon wafers

The insufficient supply of polysilicon is limiting the growth of the segment of the photovoltaic industry using silicon materials. Because it is grown directly in the form of ribbon from a silicon melt, edge-defined film-fed growth (EFG) silicon ribbon is a promising alternative for cutting down wafer costs by reducing the polysilicon consumption and eliminating kerf loss. In this paper, we will discuss the various properties that can be achieved with for low cost and high-efficiency EFG silicon ribbon solar cell fabrication. Boron-doped p-type EFG ribbon silicon wafers with resistivities of 2-4 Ω cm and a size of 125 mm × 125 mm were used. The major fabrication steps we studied were mixed acid (HF, HNO₃, DI water) texturing, phosphorus diffusion with POCl₃, thermal oxide growth for surface passivation, laser process for edge isolation, and PECVD of SiN_x:H for surface passivation and antireflection coating. By optimizing the processing steps, we achieved a conversion efficiency, open circuit voltage, short circuit current, and fill factor as high as 14.5%, 584 mV, 32.1 mA/cm², and 77%, respectively.     

Synthesis of silicon nanocrystals in aluminum-doped SiO2 film by laser ablation method

The effects of aluminum (Al-) doping in SiO₂ film containing silicon-nanocrystal (nc-Si) dots were investigated by photoluminescence (PL) and electron spin resonance (ESR) measurements. The observed PL peak center showed a blueshift due to reduction of size of nc-Si dots as a result of the Al doping followed by annealing within a range of 600–800 °C. For the samples annealed at 1000 °C, the PL intensity showed increases with increasing concentration of Al atoms in the SiO₂. The ESR results obtained from all the samples, however, revealed that the density of defects causing the PL quenching did not show decrease by the Al doping. Therefore, the enhancement of the PL intensity by the Al doping seemed to be caused probably by the increase in the density of nc-Si dots.     

Development of Hybrid-Type Pressure Cell for High-Pressure and High-Field ESR Measurement

A hybrid-type piston-cylinder pressure cell for the electron spin resonance (ESR) measurement has been developed. The cylinder of this pressure cell consists of a NiCrAl inner cylinder and a CuBe outer sleeve, and all inner parts are made of zirconium oxide which has good transmittance to the millimeter and submillimeter waves. We confirmed that the pressure reaches 2.1 GPa. We have also developed a transmission-type high-field ESR system having two different modulation methods for this pressure cell. A test measurement without pressure cell for the two-dimensional orthogonal-dimer spin system of SrCu₂(BO₃)₂ has been done successfully in the wide frequency region. The combination of this electromagnetic wave transmission-type pressure cell and this high-field ESR system is a promising tool for the study of the pressure-induced phase transition of SrCu₂(BO₃)₂.     

Effect of ultrasonic treatment on the defect structure of the Si–SiO2 system

The effect of ultrasonic treatment (UST) on the defect structure of the Si–SiO₂ system by means of electron spin resonance (ESR), metallography, MOS capacitance technique and secondary ions mass-spectroscopy (SIMS) is presented. The non-monotonous dependence of the defect densities on the US wave intensity has been observed. The influence of the UST frequency on the ESR signal intensity of the defect centres has been studied. It is shown that observed changes in the ESR intensity are caused by vibrational dissipative mechanisms which are a function of defect centre type and crystallographic orientation. The influence of the UST on the Si–SiO₂ interface properties depends on the time of oxidation. The density of point defects at the Si–SiO₂ interface can be reduced and its electrical parameters improved by an appropriate choice of the UST and oxidation conditions.     

ESR spectral studies of a Cu(II) salicylidene tyrosine complex

A novel Schiff base derived from salicylidene and tyrosine and its copper(II) complex have been synthesized and characterized. The composition of the complex is K[CuL(Ac)] · H₂O, where L = H₁₃C₁₆NO₄. Electron spin resonance (ESR) spectra of the copper(II) complex were investigated at different temperatures and in various solvents. The second-order effect and the relaxation effect were observed in the solution spectrum at room temperature and satisfactorily explained by the spin Hamiltonian. The bonding parameters of the Cu(II) complex were calculated with spectral parameters from ESR spectra at low temperature. Its bonding characterization and stability were discussed. The result shows that both the in-plane σ-bond and the in-plane π-bond in the complex play an important role.