Large Capture Cross Section Research Articles

Defect Distribution And Metastability in Chalcopyrite Semiconductors

AbstractDeep levels in chalcopyrite based heterojunctions are investigated by capacitance techniques. A method is presented which allows the determination of defect distributions from admittance measurements. For CuInSe2 a defect level at 280 meV above the valence band with an attempt to escape frequency of about 1011 s-1 is detected. A characteristic defect structure consisting of a shallow defect level and a broad deep structure is observed for Cu(In, Ga)Se2. The depth of the shallow defect is affected by annealing treatments which are necessary to achieve the optimum performance of the devices. CulnS2grown under an excess of CuS exhibits a high density of shallow defects whereas for Cu-poor CuInS2 defects close to midgap position with a large capture cross section are observed. These defects are metastable with respect to current injection and illumination relaxing to the equilibrium state around room temperature. The increase of the defect density after illumination results in a reduced bucking current. CV measurements can be interpreted in terms of a high density of deep states.

Determination of the Isotopic Compositions of Samarium and Gadolinium by Thermal Ionization Mass Spectrometry

Because of the large neutron capture cross sections for 149 Sm, 155 Gd, and 157 Gd, accurate and sensitive isotopic analyses of samarium and gadolinium can be used to investigate natural neutron irradiation exposure. In this paper, chemical separation procedures and mass spectrometric methods for the precise determination of samarium and gadolinium isotopic compositions are described. Isotopic ratios other than 144 Sm/ 152 Sm, 152 Gd/ 160 Gd, and 154 Gd/ 160 Gd for both samarium and gadolinium in commercial reagents, artificially neutron-irradiated reagents, and geological reference rocks are quantified within 0.001% precision at the 95% confidence level. The present method can detect effects caused by time-integrated neutron flux of 1.3×10 14 neutrons/cm 2 in the environmental samples

Feasibility study on transmutation of long-lived actinides and fission products in BWR core

Recycling of long-lived nuclides into a nuclear reactor is one of the measures for the reduction of radioactive hazard potential on the high level wastes (HLW) after a long storage period. We have considered such a fuel cycle that Np, Am, I and Tc are recycled to the light water reactors (LWRs) with U and Pu. Since LWR neutron spectrum will provide a large neutron capture cross section for most of nuclides, capture (and successive decay/fission) reaction in LWR is one of possibility to transmute longlived nuclide into a short-lived or stable one. Proper amount of long-lived nuclide recycles in LWR and a removal of short-lived products at reprocessing will avoid the cumulative increase of long-lived nuclides. Taking a BWR for a LWR sample, conditions for equilibrium material balance for long-lived nuclides per reactor, reactivity penalty and reactor core performances are presented.

Electron-spin-resonance evidence for an impurity-related E′-like hole trapping defect in thermally grown SiO2 on Si

Using electron spin resonance (ESR), a new electrically active point defect in thermally grown SiO2 films on Si has been detected. The defect has a large capture cross section for electrons when it is paramagnetic and holes when it is diamagnetic (ESR inactive). The g-tensor values, symmetry, and microwave power saturation characteristics are all similar to those of the well-known E′ family of amorphous SiO2 defect centers.

Observation and electronic characterization of ‘‘new’’ E′ center defects in technologically relevant thermal SiO2 on Si: An additional complexity in oxide charge trapping

Using electron-spin resonance (ESR), we demonstrate that several E′ variant precursors exist in a variety of technologically significant thermally grown thin SiO2 films on Si. The E′ variants include two varieties with the ubiquitous Eγ′ line shape (zero-crossing g=2.0005, O3≡Si⋅) and a second very narrow line shape (zero-crossing g=2.0019, structure unknown). We tentatively label the g=2.0019 defect EP for provisional E′ and distinguish the Eγ′ variants Eγn′ (neutral) and Eγp′ (positive). We combine ESR, capacitance versus voltage electrical measurements, and charge injection sequences to compare the electronic properties of the defects. We find that paramagnetic EP defects are positively charged while paramagnetic Eγ′ centers can be either positively charged or, under some circumstances, neutral. We find that EP precursors have a very large capture cross section for holes (σ=10−13 cm2) and that paramagnetic EP defects have an even larger capture cross section for electrons (σ=10−12 cm2). Both EP capture cross sections are an order of magnitude greater than those of the Eγp′ defects. We find that EP centers are distributed much more broadly throughout the oxide than either the Eγp′ or Eγn′ defects. We also find a two order of magnitude variation in EP density dependent upon processing variations. In addition, EP centers, unlike the Eγ′ variations, are not stable at room temperature. With their large capture cross section for holes and even larger capture cross section for electrons, EP defects may be relevant to device reliability and charge trapping under conditions of a low, relatively pure hole fluence such as in hot hole injection in short n-channel metal-oxide-semiconductor field-effect transistors.

Electrical characterization of neutron irradiation induced defects in undoped epitaxially grown n-GaAs

Undoped n-GaAs, grown by organometallic vapor phase epitaxy, was irradiated with neutrons from a clinical p(66)/Be(40) source for a range of fluences. Deep level transient spectroscopy (DLTS), employing Pd Schottky barrier diodes, indicated that four electron traps, En1, En2, En4, and En5, with energy levels at 0.04, 0.14, 0.36, and 0.66 eV, respectively, below the conduction band were created during neutron radiation. Their introduction rates varied from 1 cm−1 for the En1 to 11 cm−1 for the En5. It was found that the En1, En2, and En4 defects have DLTS ‘‘signatures’’ similar to the E1, E2, and E3 point defects introduced during high energy electron irradiation, indicating their point defect nature. The En5 has a very large capture cross section, its emission rate exhibits a strong electric field dependence, and there are indications that it has a band-like energy distribution, that results in a broad DLTS peak. We speculate that this trap is related to the presence of extended defects in the neutron irradiated GaAs.

Electron trapping properties of Ge-implanted SiO 2

The electron trapping characteristics of Ge-implanted SiO 2 have been studied. The results indicate that every Ge-atom constitutes an electron trap with large capture cross section. This trap is only slightly affected by an anneal at 400°C in forming gas whereas it can be removed by an extended anneal in N 2 at 1000°C.

Classical trajectory simulation of the cluster–atom association reaction I–Arn+I→I2+nAr. I. Capture of iodine by the I(Ar)12 cluster

The atom–cluster association reaction I(Ar)n+I→I2+nAr (n=12) is studied theoretically as a prototypical model of the effect of microscopic solvation on reaction dynamics. Classical trajectory methods are employed to model the dynamics. This paper focuses on the initial capture of I by the I(Ar)12 cluster. Two distinct minimum energy configurations for I(Ar)12 are considered: Ar6(I)Ar6, an icosahedron with I located at the center of the cluster; and IAr12, an icosahedron with I replacing one of the vertex Ar atoms. Both the structure and the temperature dependence of the capture cross section are investigated. Capture rate constants at temperatures of 10 and 30 K are computed. Capture cross sections for Ar6(I)Ar6+I predicted by a Langevin model agree well with those computed by classical trajectory simulation, revealing that the capture process under investigation is determined by the long range interaction potential. In comparison with its gas phase counterpart I+I, Ar6(I)Ar6+I has a much larger capture cross section. One of the most important roles played by the microscopic solvation of chemical reactants in clusters is this enhancement of the cross section for the initial capture process.

DEEP LEVEL STUDIES OF P-HEMT STRUCTURE GROWN BY MBE

The deep levels of P-HEMT structure grown by MBE have been studied using DLTS technique. DLTS Spectra of samples show that high temperature electron traps, having larger capture cross sections and concentrations, are measured in n-AlGaAs layer of P-HEMT structure. These traps may correlate strongly with oxygen content of n-AIGaAs layer and be responsible for electrical parameters of P-HEMT structure. The experimental results also show that DLTS technique may be a tool of optimisation design of practical devices.

STUDIES ON HIGH TEMPERATURE TRAP OF AlGaAs/GaAs GRIN SCH SQW LASER FABRICATED BY MBE

The high temperature traps of AlGaAs/GaAs GRIN-SCH SQW lasers fabricated by MBE has been studied using DLTS technique.The majority and minority carrier DLTS spectra show that high temperature trap, having larger capture cross section and concentration, exist in n-AlGaAs layer of lasers. This trap may correlate strongly with oxygen content of AlGaAs layer grown by MBE. It may be responsible for the degradation of laser performance. High temperature hole trap may spatially localize between the interface of xAl= 0.2→0.43 and xAl= 0.43 n-AlGaAs layers. And high temperature electron trap may spatially localize in the interface regions of discontinous variation Al mole fraction of AlGaAs layer with xAl= 0.43.

One- and two-electron capture in collisions of slow B4+ and Be4+ ions with helium.

One-electron and two-electron capture in slow ${\mathrm{B}}^{4+}$-He and ${\mathrm{Be}}^{4+}$-He collisions are studied within the semiclassical close-coupling description with two-electron atomic basis sets. For ${\mathrm{B}}^{4+}$-He collisions, we find large two-electron capture cross sections to 2l2l' doubly excited projectile states. Double-capture cross sections to 2${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}^{\mathit{e}}$, 2s2p $^{1}$${\mathit{P}}^{\mathit{o}}$, and 2${\mathit{p}}^{2}$ $^{1}$${\mathit{D}}^{\mathit{e}}$ states and the relative M distributions are found to be in good agreement with recent experiments. Earlier data on ${\mathrm{B}}^{3+}$ production are found to contain large portions of double transfer followed by autoionization. For the ${\mathrm{Be}}^{4+}$-He system, the summed cross sections from this work agree with an earlier theoretical study within the adiabatic model. Discrepancies remain, however, for this system in the distribution of double transfer over final states.

A new measurement method for trap properties in insulators and semiconductors: Using electric field stimulated trap-to-band tunneling transitions in SiO2

A new experimental technique for characterization of traps in insulators and semiconductors and its speed-up derivative are presented which use trap-to-band tunneling emission of electrons. Equations are given which allow the energy levels to be calculated from experimental emission decay data. The expressions are valid for electrically active defects with a discrete energy level or a distribution of levels. The numerical error in calculating the energy level from experimental data is 1%–2% for a given set of energy-band parameters such as the effective masses and the parabolicity of the dispersion relation in the energy gap. The techniques are applied to electron traps in thermally grown integrated-circuit grade SiO2 to illustrate their accurate and utility. An energy distribution of shallow charged oxide traps is found throughout about 0–3 eV below the oxide conduction band edge with a peak density of states at about 0.9 to 1.0 eV. The centroid of the charged oxide traps is about 82 Å from the SiO2/Si interface. The same density–energy curve is observed for oxide traps in many thermally grown dry oxide films thicker than 150 Å. A decrease in the density of the charged oxide traps is found for oxides less than 100 Å. A distribution of capture cross sections versus trap energy depth, ranging from 10−15 to 10−17 cm2, is measured at low oxide electric field (1.1 MV/cm). The shallower traps have a larger capture cross section than the deeper traps. The measured energy level at the peak density is consistent with the recent theoretical anticipation of Robertson and Rudra and Fowler for an oxygen vacancy which relaxes to form a silicon–silicon bond in the oxide [(Si—O)3≡Si—Si≡(O—Si)3].

Effect of alteration, formation absorption, and standoff on the response of the thermal neutron porosity log in gabbros and basalts: Examples from Deep Sea Drilling Project‐Ocean Drilling Program Sites

This study focuses on the effects of hydrous alteration minerals, formation absorption, and standoff on the response of the thermal neutron porosity log in the basaltic and gabbroic rocks logged at sites 395, 418, 504, 642, and 735 during the Ocean Drilling Program. The concentration of hydrogen present in the rocks in the form of free water (pore space) and bound water (hydrous minerals) is the primary factor controlling the neutron elastic scattering process, while the presence of other elements, such as chlorine, gadolinium, boron, lithium, and samarium in the fluids and in the rock matrix can largely affect the thermal diffusion phase. These neutron absorbers cause an increase of the capture cross section, and in turn of the apparent thermal porosity. Further perturbations occur when the recording conditions depart from those under which the tool has been calibrated; a large and irregular hole diameter and a lack of eccentralization both produce erroneous porosity readings. The effect of hydrous alteration minerals on the thermal neutron porosity log has been estimated from 922 core oxide measurements using an analysis program that calculates the slowing‐down length and converts it into apparent porosity. The results show that the computed apparent porosity ranges from less than 1% in fresh basalts and gabbros to about 30% in highly altered units. Depending on the alteration mineral assemblage, natural gamma ray, capture cross section, or hydrogen logs have been used to continuously predict the effect of bound hydrogen at each site. Corrected porosities generally show excellent agreement with core data for massive units, whereas they are higher for pillow basalts and fractured zones. The discrepancy is interpreted as the result of (1) difference in the volume of rock investigated (core specimens do not sample large vugs and fractures) and (2) frequent variations in the hole size and lack of tool contact with the borehole wall (standoff), not completely accounted for in the corrections. High capture cross section values measured on selected basalt samples seem to be associated with more altered basalts, suggesting a larger concentration of strong neutron absorbers in secondary minerals. The concentrations of elements with large capture cross section (Σ) measured at three of the five sites studied yield a maximum increase of 1% in the apparent neutron porosity. This is too low to explain the porosity values, some greater than 7%, observed in some of the fresh and unfractured massive units. A 1‐ to 2‐inch standoff due to the lack of eccentralization of the tool string used in the Ocean Drilling Program is considered the most probable cause of such discrepancy. This interpretation is confirmed by the neutron porosity values recorded at site 735, where the nuclear string was for the first time fully eccentralized, and only a correction for borehole size and the presence of hydrous minerals has been applied.

Polysiliodn Annealing and its Inflience on Floating Gate Eeprom Degation

AbstractExperimental results on the influence of prooessing on degradation of thin silicon dioxide films are reported. The high temperature polysilicon oxidation degrades the integrity and robustness of thermally grown dry oxide as shown by constant current charge to breakdown measuremeits.Furthermore the influence of high temperature processing on the charge trapping properties of the tunnel oxide (silicon dioxide) are ascertained. The details of electron trapping its capture cross section, areal concentration and trapping efficiency as studied by high field dark current-voltage method is presented. Generally, the dominant −15 electron trap with a large capture cross section of 10 cm2 is observed and is believed to be due to phosphorus near polysilicon/thin oxide interface. In addition the QBD reduction is correlated to outdiffusion of water from the thin oxide as seen by reduction in the hysteretic instability of I-V curves.

Computer analysis of the negative differential resistance switching phenomenon of double-injection devices

By directly solving the semiconductor differential equations for the double-injection (DI) devices involving two interacting deep levels, the authors studied the negative differential resistance switching characteristic and its relationship with the device dimension, doping level, and dependence on the deep impurity profile. Computer simulation showed that although one can increase the threshold voltage by increasing the device length, the excessive holding voltage that would follow would put this device in a very limited application such as pulse power source. The excessive leakage current in the low conductance state also jeopardizes the attempt to use the device for any practical purpose. Unless there are new materials and deep impurities found that have a large differential hole and electron capture cross sections and a reasonable energy bandgap for low intrinsic carrier concentration, no significant improvement in the fate of DI devices is expected in the near future. >

Defect Levels in the Near-Surface Region of 2.0 MeV 16O+ Ion Implanted n-GaAs.

AbstractMeV ion implantation in GaAs is known to cause amorphization of the region at the end of the ion range. The near-surface region, however, is still crystalline albeit heavily compensated. We have carried out deep level transient spectroscopy (DLTS) studies of the defect levels in the near—surface region of n—GaAs samples implanted with different doses of 2.0 MeV 16O+ ions.A comparison between the defect structures in the original and the implanted samples shows that implantation produced a broad range of defect levels ranging from 0.58 to 0.3 eV from the conduction band edge. This broad range of defects has an unusually large capture cross—section. The intensities of the DLTS peaks increase with the dose of 160+ ions. The presence of EL2, which was present in the original samples, was not observed in the implanted samples.Results from measurements made on samples that have been implanted at 200°C and on implanted samples subjected to rapid thermal annealing will also be discussed.

Dynamically polarized proton filter as a neutron-spin polarizer using a 3He− 4He heat exchanger

A dynamically polarized proton filter was constructed as a low-energy polarizer. Protons in filter material were polarized by a similar method as the one for the polarized proton target used in high energy physics experiments. Filter material was immersed in liquid 4Hem which was cooled by liquid 3He through a heat exchanger from outside. Liquid 3He was removed from the neutron-beam path, since 3He has a very large capture cross section for the neutron. Considerable improvements in intensity and polarization of the neutron beam were achieved.

Deep levels and a possible d-x-like center in molecular beam epitaxial inxal1−xas

The work reported here was performed in order to establish whether or not complex defects like the D-X center can be present in InAlAs. Dominant deep electron and hole traps in lattice-matched In0.52Al0.48As/InP have been identified and characterized. The traps have activation energies ranging from 0.25 to 0.95 eV. The electron traps have very large capture cross-sections,~10−12-10−11 cm2, similar to attractive centers. The lattice-matched samples do not show any persistent photoconductivity effects at low temperatures. In strained In1−xAlxAs with 0.48 <x ≤ 0.57, an electron trap with a thermal ionization energy of 0.35 eV, a strong dependence of its concentration on donor doping and very small thermal capture cross-section, 1018 cm2, is identified. These samples exhibit persistent photoconductivity. We believe this trap is similar to the D-X center commonly observed in AlxGa1−xAs forx ≥ 0.28.

Photoionization cross-section studies of the platinum-donor center in silicon

The relative photoionization cross section of the platinum donor center in silicon was measured over the wavelength range of 2.4 to 3.9 μm by electrical deep-level optical spectroscopy on an n+p junction at 80 K. The data were analyzed in terms of the lattice-coupling model proposed by Ridley and Amato which was modified for valence-band nonparabolicity. Good agreement was obtained between the experimental results and the model calculations of the cross section with the energy level of the donor at 0.320±0.005 eV above the valence-band edge and a Huang–Rhys factor S of approximately 1.4. This S value corresponds to a Franck–Condon energy shift of 70 meV with a phonon energy of 50 meV. Previously reported photoionization data of the gold donor were also fit by the same model, yielding S≂0.4, a surprisingly small value. Estimates were made of the majority-carrier capture cross section for these two levels and for the platinum acceptor center in silicon, which was measured previously. These estimates, based on Ridley’s quantum-defect model and our measured S values, are several orders of magnitude smaller than the corresponding measured values, indicative of the complex nature of these 5d-transition elements in silicon. More detailed models, perhaps including anharmonicity of the defect vibrations, are required to understand these large capture cross sections.

Analysis of light-induced degradation in amorphous silicon alloy p-i-n solar cells

We present results of computer simulations of the characteristics of amorphous silicon alloy p-i-n solar cells in both undegraded and degraded conditions. Changes in device performance upon degradation correlate well with realistic changes in the minority carrier diffusion length of the intrinsic layer. The results of our model are in good agreement with experimental data and lead us to conclude that the observed degradation in the electronic properties of amorphous silicon alloys are only consistent with the observed increases in the localized state density if the photo-induced metastable defects have larger capture cross-sections than the localized states in as-deposited material.