Electron resonance study of hydrogen-containing WO3 films

We have searched at 4.2K for the electron spin resonance of localized electrons in dehydrated amorphous and recrystallized WO₃ films immediately after coloring by hydrogen injection from an electrolyte and after room temperature bleaching in air. No resonance could be observed before or after bleaching. Undried, colored samples show a variety of strong resonances spread over 7kG which vary from sample to sample and which change in nature and intensity as the samples bleach. We interpret these results to mean OH ions are not involved in the coloring process except for an effect on the diffusion rate and are not necessary for bleaching. If OH ions are present the protons will form defect centers with them as bleaching occurs. These results show that previous 77K EPR studies were incorrectly interpreted as evidence for the presence of W⁵⁺ in colored samples. Our negative results argue against the intervalence transfer and small polaron models used to explain the optical data. We suggest that if coloration is due to W⁵⁺ , the electrons are localized over larger regions containing a number of tungsten sites and there is not a unique localized configuration to give a well defined electron resonance. Spin pairing is also a possibility. However, the EPR is also consistent with other coloration models.     

Temperature influence on performance degradation of hydrogenated amorphous silicon solar cells irradiated with protons

This paper reports temperature influence on radiation degradation of hydrogenated amorphous silicon (a‐Si : H) solar cells. Degradation behaviors of a‐Si : H solar cells irradiated with protons at 331 K are compared with that at 298 K (room temperature). Variations with time in the post‐irradiation electrical properties are also investigated. It is found that the radiation degradation of the electrical properties at 331 K is significantly smaller than that at room temperature. Also, all the electrical properties (short‐circuit current, open‐circuit voltage, output maximum, and fill factor) recover with time after irradiation even at room temperature. The characteristic time of thermal annealing of short‐circuit current is larger as the temperature is higher. These results indicate that temperature during irradiation and elapsed time from irradiation to measurement is an important parameter for radiation degradation of a‐Si : H solar cells. Therefore, these parameters should be controlled in conducting the ground radiation tests. Copyright © 2012 John Wiley & Sons, Ltd.     

Electron spin resonance study of ion-implanted polymers

The effect of ion implantation on the polymers PAN (polyacrylonitrile), PPO (poly 2,6-dimethylphenylene oxide) and PPS (p-polyphenylene sulfide) is studied using electron spin resonance. ESR measurements on these polymers were performed as a function of ion species and fluence in the temperature range 10 <T< 300 K. The unpaired carrier concentration increases with increasing fluence and is independent of the ion species used for implantation in this study (⁸⁴Kr,⁸⁰Br,⁷⁵As,⁴⁰Ar and¹⁴N). It is therefore concluded that the carrier concentration is related to the structural damage and not to chemical doping effects. From the shape of the ESR line, the ratio of the relaxation times for one-dimensional to three-dimensional spin diffusion is determined to be larger than 1000. The temperature dependence of the unpaired carrier concentration shows a strong deviation from a Curie law behavior, which can be explained by assuming that a defect band is formed with a bandgap due to strong Coulomb interaction between electrons on the defect sites.     

Detection of nitrogen incorporation in nm-thin HfO2 layers on (1 0 0)Si by electron spin resonance

We report on a low-temperature electron spin resonance (ESR) study of (1 0 0)Si/HfO₂ entities with ultrathin layers of amorphous (a)- HfO₂ deposited by distinct chemical vapor deposition (CVD) techniques using chemically different precursors. The incorporation of N is revealed in (1 0 0)Si/HfO₂ structures with ultrathin a-HfO₂ films deposited by CVD using Hf(NO₃)₄ as precursor: Upon ⁶⁰Co γ-irradiation, a prominent ESR powder pattern is observed, which via ESR measurements at two observational frequencies has been incontrovertibly identified as originating from NO₂ radicals (density 55 at ppm). The molecules are found to be stabilized and likely homogeneously distributed in the a-HfO₂ network. Based on symmetry considerations, it is suggested that during deposition, N is incorporated in the HfO₂ network as neutral N≡O₃ precursors, which are transformed into ESR-active NO₂ radicals upon γ-irradiation. The N incorporation appears inherent to the particular nitrado CVD process, an aspect that may bear on the electrical properties of the insulator, such as, e.g., introducing charge traps.     

Generation and annealing characteristics of paramagnetic centers in oxidized 3C-SiC and 6H-SiC

Electron paramagnetic resonance studies show that defects are activated in oxidized 3C-SiC and 6H-SiC by heat-treatments at temperatures greater than 800°C in dry (<1 ppm H₂O) N₂ or O₂ ambients. Annealing in forming gas (7% H₂, 93% N₂) at 700°C completely passivates the centers induced by the dry heat-treatment. By contrasting the generation and annealing kinetics for these centers with the well studied Si dangling bond, we suggest that the centers in the oxidized SiC are C dangling bonds created by hydrogen effusion during the dry heat-treatment.     

Hole trapping in E-beam irradiated SiO2 films

Low energy (25 kV) electron beam irradiation of MOS capacitors is shown to produce neutral hole traps in thin ‘radiation hardened’ SiO₂ films. These traps are found in an uncharged state after irradiation and are populated by passing a small hole current, generated by avalanche breakdown of then-type silicon substrate, through the oxide. From the time dependence of the observed trapping, a capture cross-section between 1 × 10∼⁻¹³ and 1 × 10⁻¹⁴ cm² is deduced. The trap density is found to depend on the annealing conditions and incident electron beam dosage. The density of traps increases with incident electron beam exposure. Once introduced into the oxide by the radiation the traps can be removed by thermal anneals at temperatures above 500° C. Parallels between electron and hole trapping on these neutral centers are strong evidence for an amphoteric uncharged trap generated by ionizing radiation.     

Identification of a Three-Site Defect in Semi-Insulating 4H-SiC

Using electron paramagnetic resonance (EPR), we observe a defect that increases significantly with annealing temperature. This spin S = 1 defect is characterized by g _x ≈ g _y ≈ g _z = 2.0056 and a fine structure splitting D ∼ 100 G whose principal axis lies in a plane perpendicular to the c-axis. Also resolved are several hyperfine interactions with the low abundance ²⁹Si and ¹³C neighboring nuclei. A careful analysis of the intensity of these hyperfine lines allows a precise determination of the identity and the quantity of interacting nuclei. This center is diamagnetic in the ground state but can be excited into a paramagnetic triplet state by sub-bandgap light. We identify this new defect as a three-site vacancy involving V _C-V _Si-V _C. The angular dependence of the ¹³C hyperfine interaction supports the proposed model. These may be the simplest of a family of more complex and extended defects that play a role in the semi-insulating (SI) character of SiC.     

MgAl2O4透明陶瓷电子辐照及退火效应

用能量 1 .7Me V,束流强度 0 .7μA· cm-2 ,总注量 6 .3× 1 0 16 cm-2的电子辐照镁铝尖晶石透明陶瓷 ,并对辐照后的样品进行等时退火。利用 PAT、UV- VIS和 FI- IR测试 ,表明辐照前3 3 6 0 cm-1处有吸收峰 ,通过电子辐照样品在 2 3 7nm和 3 70 nm处产生吸收带 ,3 3 6 0 cm-1处吸收峰消失 ,主要是由 VOH-1心及辐照产生的 F心和 V型色心引起的。通过退火 ,可消除 F心及 V型色心吸收峰。     

单晶硅中氢与辐照缺陷的相互作用

对NTD氢区熔单晶硅进行了不同温度下等时退火,采用Hall电学方法测量了电阻率、迁移率随退火温度的变化规律.利用红外吸收技术测量了单晶硅氢区熔退火前后及NTD氢区熔单晶硅不同退火温度下与氢、辐照缺陷有关的红外振动吸收峰变化,对辐照缺陷的退火行为进行了探讨.实验证实NTD氢区熔单晶硅在150～650℃范围内等时退火具有显著特点:在500℃下退火,出现电阻率极小值,即出现浓度很高的过量浅施主;P型向N型转变温度为400℃,迁移率恢复温度为500℃,载流子恢复温度为600℃,均明显低于NTD氩区熔单晶硅转型温度及迁移率和载流子恢复温度,这与氢积极参与辐照缺陷相互作用直接相关.     

Electron microscopic study on residual defects of Al+ or B+ implanted 4H-SiC

The residual defects of Al⁺- or B⁺-implanted 4H-SiC were studied in combination with annealing temperature and implantation temperature using cross-sectional transmission electron microscopy technique. Noticeable defects structure is not observed before post-implantation annealing. But after annealing, a lot of black spots appear in the implanted layer. These black spots are composed of a dislocation loop, parallel to {0001} of 4H-SiC, and strained area at the upper and lower sides of the dislocation loop. This defect structure and its size do not depend on implantation temperature and implanted ion species. The size of defect area depends only on post-implantation annealing temperature. The size grows, when post-annealing temperature is raised.     

Electron paramagnetic resonance and optically-detected magnetic resonance of donors in AlxGa1-xAs

The Electron Paramagnetic Resonance (EPR) and Optically-Detected Magnetic Resonance (ODMR) work on Si-donors in Al _{x Ga1- x} As is reviewed in the context of the shallow-deep bistability (DX) problem. Three donor states are important. Little work has been published on donors tied to theT-minimum. However, there are many results forX-donors. In AlAs/GaAs heterostructures, well-resolved spectra reveal a donor state comprised of independentX _x andX _y valleys with theXz valley unpopulated due to the hetero-epitaxial strain. As Al mole fraction decreases, intervalley coupling is evident from the line positions and linewidths. The published attempts to observe and identify the deep (relaxed) state are inconclusive. Some suggestions for future work are presented.     

Valence band discontinuity of the (0001) 2H-GaN / (111) 3C-SiC interface

X-ray and UV photoelectron spectroscopies were used to measure the valence band discontinuity at the interface between (0001) 2H-GaN films and 3C-SiC (111) substrates. For GaN films grown by NH₃ gas source molecular beam epitaxy on (1×1) 3C-SiC on-axis surfaces, a type I band alignment was observed with a valence band discontinuity of 0.5±0.1 eV. A type I band alignment was also determined for GaN films grown on (3×3) 3C-SiC, but with a larger valence band discontinuity of 0.8±0.1 eV.     

The role of oxygen in electron cyclotron resonance etching of silicon carbide

The electron cyclotron resonance (ECR) etching of silicon carbide (SiC) was studied using SF₆ + O₂ based plasma. The role of O₂ was studied by varying the O₂ flow rate while keeping the total gas flow constant. It was found that oxygen enhances the etch rate at low O₂ fraction through releasing more fluorine atoms, while lowers the etch rate at high O₂ fraction by diluting fluorine atoms and forming an oxide-like layer. The etched surface roughness was found to be affected by the surface oxidation and oxygen ion related physical ion bombardment. The role of oxygen in chemical etching of carbon was found to be insignificant. In general, the etched surface is smooth and free of micromasking effect that can arise from Al contamination and C rich layer.     

Heterojunction diodes in 3C-SiC/Si system grown by reactive magnetron sputtering: Effects of growth temperature on diode rectification and breakdown

3C-SiC/Si heterojunction diodes were prepared by reactive magnetron sputtering of pure Si in CH₄-Ar discharge on Si(111) substrates kept at temperatures (T_s) ranging from 800 to 1000°C. A good diode rectification process started for films grown at T_s≤900°C. Heterojunction diodes grown at T_s = 850°C showed the best performance with a saturation current density of 2.4 × 10⁻⁴ A cm⁻². Diode reverse breakdown was obtained at a voltage of −110 V. The doping concentration (N_d) of the 3C-SiC films was calculated from 1/C² vs V plot to be 3 × 10¹⁵ cm⁻³. Band offset values obtained were −0.27 and 1.35 eV for the conduction and valence band, respectively. X-ray diffraction analysis revealed the film grown at T_s = 850°C to be single-phase 3C-SiC. The full width at half maximum of the 3C-SiC(111) peak was only 0.25 degree. Cross-sectional transmission electron microscopy showed the film to be highly (111)-oriented with an epitaxial columnar structure of double positioning domain boundaries.     

Improvement of the crystallinity of 3C-SiC films by lowering the electron temperatures in the afterglow plasma region using triode plasma CVD

Crystalline SiC films were grown at low temperatures by triode plasma chemical vapor deposition (CVD) using dimethylchlorosilane diluted with hydrogen as the source gas. Influences of the grid bias on the electron temperature in the discharge region and in the afterglow region, and on the properties of the SiC films such as crystallinity, chemical bonding structure, and composition were investigated. Under negative grid bias conditions, the electron temperature in the discharge region increased and that in the afterglow region became about one-tenth of that under positive bias conditions. The crystallinity of the SiC films grown under low electron temperatures in the afterglow plasma region was remarkably improved and the composition of the films became stoichiometric. Under the negative grid bias, a high density of active hydrogen radicals was generated in the discharge region, diffused toward the substrate surface, and extracted the weak bonds or excessive methyl groups from the growing film surface under low electron temperature. As a result of these processes, SiC films with good crystallinity were grown.     

An EPR study of defects induced in 6H-SiC by ion implantation

Crystalline (0001) plane wafers of n-type 6H-SiC have been implanted at room temperature with 200 keV Ge⁺ ions in the dose range 10¹² to 10¹⁵ cm⁻². Electron paramagnetic resonance (EPR) measurements have been made on these samples both before and after annealing them at temperatures in the range room temperature to 1500°C. The as-implanted samples have a single isotropic and asymmetric line EPR spectrum whose width, ΔB_pp, increases with ion dose before falling when a buried continuous amorphous layer is produced. This increase is interpreted in terms of the change in the relative intensity of a line with g = 2.0028 ± 0.0002, ΔB_pp = 0.4 mT associated primarily with carbon dangling bonds in a-SiC and a line with g in the range 2.0033 to 2.0039 of uncertain origin. The variation with anneal temperature of the populations of these defects is reported.     

Optically detected magnetic resonance study of defects in undoped, Be-doped, and Mg-doped GaN

Undoped, Be-doped, and Mg-doped GaN samples were investigated with photoluminescence-detected electron paramagnetic resonance (PL-EPR) and electron-nuclear double resonance (PL-ENDOR). Two types of shallow donors and a deep level Be-related complex were measured in Be-doped GaN. One type of shallow donors is probably due to Si contamination. In all investigated samples, distant gallium ENDOR lines were observed which were split by a quadrupole interaction. From this splitting the electrical field gradients (efg’s) at the Ga nuclei in all investigated samples could be determined very precisely. The efg’s were correlated with strain.     

Molecular Metals Based on BEDT‐TTF Radical Cation Salts with Magnetic Metal Oxalates as Counterions: β″‐(BEDT‐TTF)4A[M(C2O4)3]·DMF (A = NH4+, K+; M = CrIII,FeIII)

Four (BEDT‐TTF)₄A[M(C₂O₄)₃]·DMF (DMF = dimethylformamide) salts of the organic donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT‐TTF) with metal oxalate anions, where A = (NH₄, K), M = Cr ( 1 ); A = NH₄, M = Fe ( 2 ); A = K, M = Cr ( 3 ); and A = NH₄, M = Cr ( 3′ ) were prepared by electrocrystallization. These salts were characterized by single‐crystal X‐ray diffraction, electron spin resonance (ESR) spectroscopy, electrical resistance measurements, and electronic band structure calculations. The structures (with space group C2/c) consist of alternating β″‐type layers of BEDT‐TTF and an approximately hexagonal network formed by the A⁺ cation and the metal, with the solvent molecule, DMF, occupying hexagonal cavities in the anion layer. All of the salts are two‐dimensional organic metals down to 4.2 K and do not exhibit superconductivity. Their electronic band structure is similar to that of the known organic superconductor β″‐(BEDT‐TTF)₄H₃O[Fe(C₂O₄)₃]·BN. The ESR spectra of salts 1 and 3′ are characterized by two resonances, one of Gaussian shape arising from the 3d localized electrons of Cr³⁺ and the other of Lorentzian (and Dysonian) shape due to the conduction electrons in the organic layers. On the basis of the calculated Fermi surfaces it is suggested that these salts could exhibit an interesting magnetoresistance behavior if disorder does not prevent the observation of the Shubnikov‐de Haas oscillations.     

Al2O3-SiC复相微波衰减材料的性能研究

采用热压烧结工艺制备了Al2O3-SiC复相微波衰减材料。通过网络分析仪,研究了SiC含量对材料的微波衰减性能的影响。结果表明,当w(SiC)<3%,复相材料具有选频衰减的频谱曲线,谐振峰位置基本不变;当w(SiC)为3%~16%,材料的谐振损耗峰向低频方向移动;当w(SiC)>16%时,材料呈现出宽频衰减特性。对复相材料的微波衰减机理作了初步探讨,弛豫损耗和电导损耗是其主要的衰减机理。     

Evolution of Deep Defect Centers in Semi-Insulating 4H-SiC Substrates under High-Temperature Annealing

The influence of postgrowth high-temperature anneals between 1400°C and 2400°C on the behavior of the D₁ center in semi-insulating 4H-SiC was studied by photoluminescence. The optical signature of D₁ was observed up to 2400°C with intensity maxima at 1700°C and 2200°C. It was also found that changes in the postannealing cooling rate drastically influence the behavior of the D₁ center and the concentrations of the V_C, V_Si, V_C–V_Si, and V_C–C_Si lattice defects observed from electron paramagnetic resonance experiments. The change in intensity of the D₁ defect has some correlation with the intensity change of the V_C–V_Si pair defect at temperatures above 1900°C. In addition, infrared photoluminescence spectroscopy studies showed that changes in the intensity of the D₁ defect at 2100°C to 2400°C annealing temperatures and variable cool-down rates have close correlation with intensity changes of the UD2 defect.