Irradiation damage in carbon-doped silicon irradiated at low temperatures by 2 MeV electrons

Abstract

Irradiation of silicon by 2 MeV electrons at 130°K leads to the removal of carbon from substitutional sites and the formation of centres with axial symmetry having vibrational modes at 921 and 930 cm^?1 for ¹²C; large isotope shifts are found in crystals containing ¹³C and ¹⁴C. This centre is considered to involve interstitial carbon atoms but not oxygen impurities. On annealing such irradiated crystals to room temperature the concentration of these centres is reduced and a new transient centre involving carbon has been detected. Further annealing leads to the formation of the well known 11.56μm absorption band in pulled crystals and it is shown that this may be correlated with another band at 1115.5 cm^?1. Again large isotope effects are found in crystals containing ¹³C and ¹⁴C and this centre is ascribed to a [O-C] complex involving an interstitial carbon atom.     

Irradiation and heating effects in amethyst crystals from brazil

Abstract

In this work we report optical absorption spectroscopy study of thermal and irradiation effects on samples of amethyst from Minas Gerais and Rio Grande do Sul, Brazil. Three bands were studied: 10500 cm^?1 (k), 18300 cm^?1 (θ) and 28000 cm^?1 (ζ). Thermal and irradiation effects shows that the θ and ζ bands belongs to a same center and the k band to another center. The isothermal decay and irradiation growth of these band reveal a complex kinetics. The optical absorption bands of amethyst from Minas Gerais do not recover the prmitive absorbance after being bleached at 470°C and irradiated. This sample heated at 470°C in highly reducing atmosphere gets a yellow-brown color. The amethyst from Rio Grande do Sul treated at 400°C gets, also, a yellow-brown color. We suggest this color is probalbly due to the formation of Fe₂O₃ submicroscopc segregate crystals due to the diffusion of Fe ions and oxygen vacancies.     

Creation and optical properties of stable and metastable hole-trapped centres in beryllium oxide

Abstract

The paper is devoted to study of formation mechanisms and optical absorption of the hole-trapped centers in neutron, electron-impulse and X-irradiated BeO crystals. V⁰ and V^? centers are found out to be formed as a result of neutron irradiation creating cation Frenkel pairs. Within the transient absorption decay kinetics, we registered a component whose thermal-time properties coincide with those of the luminescence of triplet self-trapped excitons. A number of absorption bands from the V_B center and exciton hole nucleus are interpreted as transitions between the O^? ion p-levels splitted by the crystal field, as well as polaron transitions and transitions into the valence band.     

Optical properties and radiation effects in layered crystals of (CnH2n+nNH3)2MCl4: n = 1, 2, 3; M = Cd,Cu, Mn

Abstract

A new absorption band has been found at 5.10 eV in (C _n H_{2n + 1}NH₃)₂CdCl₄: n = 1, 2, 3 in addition to the absorption bands of CdCl₂ whose electronic structure resembles the former crystals. The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10eV at room temperature (RT) to 5.48 eV at liquid nitrogen temperature. This large peak shift is attributed to a structural phase transition between these two temperatures. A new type of electron center has been found in these crystals (M = Cd, Mn; n = 1, 2, 3) irradiated with X-rays at 15 K in addition to the Cl₂ ^?. This shows optical absorption bands (IR bands) in the infrared region of 10 ～ 20 kcm ^?1. The IR bands are assigned to an electron center where an electron is trapped at an ammonium site in the neighborhood of a Cl^? vacancy.     

Infrared studies of single crystals of polycaprolactam

Infrared studies of the α-crystalline form of polycaprolactam (nylon 6) have confirmed the assignment of the 1288 and 1210-cm^?1 bands to a unique fold conformation. The γ-crystalline phase has a band at 1212 cm^?1 which overlaps the 1210-cm^?1 fold band in samples containing both α and γ phases. The 1288-cm^?1 band was, therefore, used to monitor regular fold content. No unique fold bands were detected in the γ form. Irregular folds present in solution-grown crystals regularize to the unique fold conformation upon annealing. Crystals which originally exhibit slight differences in regular folding as a result of different crystallization history have similar amounts of regular folds upon annealing at 220°C. Crystals grown from solutions of very dilute concentrations of polymer have different amounts of regular folding. The annealing behavior of crystals prepared from two different solution concentrations reflect the processes of regularization and long period increase.     

Manganese centers with various valency in alkali halides

Abstract

Optical absorption bands and epr spectra of NaCl: Mn⁺⁺ X-irradiated at liquid nitrogen and room temperature are studied. Manganese epr spectra are divided into three classes, based on the value of hyperfme splitting constants. It is shown that the epr spectra with the hyperhe splitting constants of around 250 × 10-⁴, 80 × 10^?4 and 20 × 10^?4 cm^?1 are caused by Mn⁺, Mn⁺⁺ and Mn⁹, respectively. Formation of several manganese centers by X-irradiation is discussed. It is suggested that optical absorption bands at 410 and 440 nm are caused by Mn^O at the lattice site and that the Mn^O center which gives an epr spectrum with A = 15. 5 × 10^?4 cm^?1 is associated with a Mn⁰ center which is not at a substitutional site.     

Formation and nature of swirl defects in silicon

Point defect agglomerates in dislocation-free silicon crystals, usually called “swirls”, have been investigated by means of high-voltage electron microscopy. It was found that a single swirl defect consists of a dislocation loop or a cluster of dislocation loops. By contrast experiments it could be shown that these loops are formed by agglomeration of self-interstitial atoms. Generally the loops have a/2〈110〉 Burgers vectors, but in specimens with high concentrations of carbon (～10¹⁷ cm^?3) and oxygen (～10¹⁶ cm^?3) also dislocation loops including a stacking fault were observed. In crystals grown at growth rates higher thanv=4 mm/min no swirls are observed; lower growth rates do not markedly affect the size and shape of the dislocation loops. With decreasing impurity content (particulary of oxygen and carbon) the swirl density decreases, whereas the dislocation loop clusters become larger and more complex. A model is presented which describes the formation of swirls in terms of agglomeration of silicon self-interstitials and impurity atoms.     

Silicon negative ion implantation induced vacancy defects in thermally grown SiO2 thin films

ABSTRACT

Thermally grown SiO₂ thin films on a silicon substrate implanted with 100?keV silicon negative ions with fluences varying from 1?×?10¹⁵ to 2?×?10¹⁷ ions cm^?2 have been investigated using Electron spin resonance, Fourier transforms infrared and Photoluminescence techniques. ESR studies revealed the presence of non-bridging oxygen hole centers, E′-centers and P_b-centers at g-values 2.0087, 2.0052 and 2.0010, respectively. These vacancy defects were found to increase with respect to ion fluence. FTIR spectra showed rocking vibration mode, stretching mode, bending vibration mode, and asymmetrical stretching absorption bands at 460, 614, 800 and 1080?cm^?1, respectively. The concentrations of Si–O and Si–Si bonds estimated from the absorption spectra were found to vary between 11.95?×?10²¹ cm^?3 and 5.20?×?10²¹ cm^?3 and between 5.90?×?10²¹ cm^?3 and 3.90?×?10²¹ cm^?3, respectively with an increase in the ion fluence. PL studies revealed the presence of vacancies related to non-bridging oxygen hole centers, which caused the light emission at a wavelength of 720?nm.     

Optical properties of group-V atom-vacancy pairs in silicon

Abstract

Two optical absorption bands are identified as arising from the negative charge state of each of the P-, As-, and Sb-vacancy pairs in silicon. The more prominent band for each is at 6150, 6000, and 5500 cm^?1, respectively. A second broader partially overlapping band at ～8500 cm^?1 is also present for each. Dichroism produced in the bands by uniaxial stress reveals a static Jahn-Teller distortion of opposite sign to that for the neutral state previously studied by EPR. The absorption bands can be identified as electron transitions from filled to empty orbitals of a simple one-electron molecular orbital model for the defect.     

First-principles study of LiBaF3 crystals containing interstitial fluoride

The positions of the interstitial fluoride atoms in LiBaF₃ crystal are simulated by GULP. It is found that the formation energy is low when interstitial fluoride atoms at the center of the edge of the cube formed by Ba ions. The electronic structures and absorption bands for the perfect LiBaF₃ crystal and the LiBaF₃ crystal containing interstitial fluoride have been calculated using density functional theory code CASTEP. It is predicted that the interstitial fluoride atom combine with a formal lattice fluoride ion forming H center causes the 320 nm absorption band.     

Initial applications of the molecular model to compute defect vibrations of oxygen in silicon

Abstract

We have employed the molecular model introduced first by Jaswal to compute the vibrational spectra of oxygen bearing defects in a silicon crystal. This was done in the context of a silicon molecular cluster with outer valencies terminated by hydrogen. We employ the MINDO/3 semi-empirical electronic structure method to compute the total energy of the molecular cluster. We examine the conditions in applications of the molecular model required for accurate predictions of oxygen local-mode vibrational frequencies. We find that the oxygen atom and its nearest neighbor silicon atoms must be allowed to vibrate. The nearest-neighbor and next nearest-neighbor shells of silicon atoms must be allowed to relax from their lattice positions. The outermost relaxed shell of silicon atoms should be bonded to silicon atoms in their lattice positions. We apply the molecular model to three defects of crystalline silicon; interstitial oxygen, oxygen in a vacancy (the A-center), and two oxygen atoms in a vacancy. Comparison of our computed local-mode oxygen vibration frequencies with experiment shows the computed oxygen local-mode frequencies to be almost uniformly 10% greater than those observed. Isotope shifts fit experiment equally well. We conclude that the molecular model represents an accurate and efficient approach for the computation of defect local mode vibrational frequencies for oxygen and other defects in crystalline silicon.     

Characteristics of the formation of radiation defects in silicon structures

The method of deep-level transient spectroscopy is used to investigate aspects of the formation of radiation defects in silicon p ⁺-n diffusion structures when bombarded by accelerated electrons. It is shown that for base thicknesses of the p ⁺-n structures in the range 0.2–0.6mm a substantial change in the concentration of the radiation defects formed in this way is observed, having a maximum at 0.25 mm. Below 0.2 mm and above 0.6 mm the concentration of radiation defects exhibits a weak dependence on base thickness. The observed effect is explained by variation of the relative concentrations of vacancies and interstitial silicon atoms in the base during formation of p ⁺-n pairs. Zh. Tekh. Fiz. 69, 121–123 (January 1999)     

17O12C17O and 18O12C17O spectroscopy in the 1.6 μm region

Near infrared spectra of a carbon dioxide sample enriched with oxygen-17 have been recorded using a high-resolution continuous scan Fourier transform interferometer fitted with a femto OPO/Idler laser source. Cavity enhanced absorption has been achieved in a static gas cell allowing an optimal rms noise equivalent absorption of 1.2?×?10^?10?cm^?1?Hz^?1/2 per spectral element to be reached, corresponding to α_min?=?10^?8?cm^?1. Spectra were calibrated against acetylene reference line positions. Three bands in the 3ν₁?+?ν₃ tetrad in both ¹²C¹⁷O₂ and ¹²C¹⁷O¹⁸O have been identified and rotationally analyzed, as well as some related hot bands, eight of which are newly reported and three with their analysis updated compared with a preliminary report (X. de Ghellinck d’Elseghem Vaernewijck et al., Chem. Phys. Lett. 514, 29 (2011)).     

Interstitial carbon showing up in the absorption spectra of natural diamonds

Natural diamonds are studied by Fourier-transform IR (FTIR) spectroscopy, and it is shown that B2 centers in them form through intermediate stages, which are accompanied by the appearance of absorption bands with maxima near 1550 and 1526 cm⁻¹. The concentration of interstitial carbon atoms in the centers responsible for these bands may be several times higher than the concentration of the interstitials in B ² defects.     

Fluorescence of Crowned Butadienyl Dye and Its Metal Complexes

The absorption and fluorescence spectra of complexes of butadienyl dye (1) with lithium, sodium, magnesium, and calcium cations in MeCN were investigated. The addition of Li, Na, Mg, and Ca perchlorates to the solution of dye 1 in acetonitrile results in a significant (up to 5900-cm^–1) short-wavelength shift of the absorption spectrum and a small (about 200-cm^–1) short-wavelength shift of the fluorescence spectrum. The recoordination reaction in metal complexes of 1 takes place by intramolecular mechanism. The fluorescence quantum yield of 1Li⁺, 1Na⁺, and 1Ca²⁺ is approximately two times higher than that for 1. It was supposed that Li⁺ cation coordinates predominantly with oxygen atoms of macrocycle and, hence, influences weakly macrocycle nitrogen atoms conjugated with a molecule -system.     

Oxygen diffusion and thermal donor formation in silicon

The information available on the diffusion of oxygen and on the formation of thermal donors in silicon is critically reviewed. In this context the effects of intrinsic point defects on the diffusion-controlled growth of oxygen precipitates is investigated in some detail. Seemingly contradictory experimental results on the diffusivity of oxygen in silicon at temperatures around 400° C are explained in terms offast-diffusing gas-like molecular oxygen in silicon. The concept of molecular oxygen is also invoked in a newly suggested model of thermal donor formation in silicon. The diffusivity of molecular oxygen in silicon is estimated to be around 10^–9cm²s^–1 at 450° C, almost nine orders of magnitude higher than the diffusivity of atomic oxygen in interstitial position.     

DLTS and PTIS of new donors in oxygen-doped germanium

Two new DLTS bands I and II have been observed in n-type germanium containing 10¹⁶ cm^-3 interstitial oxygen. The signature of II is: E_c?E_II = 0.031 eV, K = 4 × 10⁷ cm^?2s^?1; level I is about half as deep. In addision, the excitation spectrum of I is recorded by PTIS. It is composed of three hydrogenic donor series Ia, b, c with activation energies 17.25, 17.6 and 18.1 meV. The donors I and II are thought to correspond with the well known thermal oxygen donors. Energies for the latter were determined at 0.017 and 0.04 eV from Hall effect by Fuller and Doleiden. The possibility of double donors is examined, in analogy with the case of oxygen donors in silicon.     

Effect of 50 MeV Li+3 and 80 MeV C+5 ions’ beam irradiation on the optical,structural, chemical and surface topographic properties of PMMA films*

The self-standing films of polymethyl methacrylate (PMMA) were irradiated under vacuum with 50?MeV lithium (Li³⁺) and 80?MeV carbon (C⁵⁺) ions to the fluences of 3?×?10¹⁴, 1?×?10¹⁵, 1?×?10¹⁶ and 1?×?10¹⁷ ions µm^?2. The pristine and irradiated samples of PMMA films were studied by using ultraviolet–visible (UV–Vis) spectrophotometry, Fourier transform infrared, X-ray diffractrometer and atomic force microscopy. With increasing ion fluence of swift heavy ion (SHI), PMMA suffers degradation, UV–Vis spectra show a shift in the absorption band from the UV towards visible, attributing the formation of the modified system of bonds. E_g and E_a decrease with increasing ion fluence. The size of crystallite and crystallinity percentage decreases with increasing ion fluence. With SHI irradiation, the intensity of IR bands and characteristic bands of different functional groups are found to shift drastically. The change in (E_g) and (N) in carbon cluster is calculated. Shifting of the absorption band from the UV towards visible along with optical activity and as a result of irradiation, some defects are created in the polymer causing the formation of conjugated bonds and carbon clusters in the polymer, which in turn lead to the modification in optical properties that could be useful in the fabrication of optoelectronic devices, gas sensing, electromagnetic shielding and drug delivery.     

Infrared studies of chain folding in polymers. X. polycaprolactam

The 1210-, 1233-, and 1288—cm^?1 infrared bands in the α form of nylon 6 are shown to result from a unique conformation in a tightly folded chain with adjacent reentry. The assignment of the 1210- and 1288-cm^?1 bands to motions of a gauche nitrogen-methylene group in the fold and the 1233-cm^?1 band to motions of a gauche carboxyl-methylene group in the fold is supported by the infrared studies of the α form of nylon 6, the γ form of nylon 6, poly-D(-)β-methyl- ? -caproamide (PBCA), and the high- and low-temperature forms of the cyclic dimer of nylon 6. The γ- form of nylon 6 and PBCA do not exhibit a unique conformation in the fold. The 1198-cm^?1 band can be assigned to the trans conformation of the amide group which exists in the normal planar zigzag conformation in the α-form crystal. A mechanism for the α to γ iodine-induced transition in nylon 6 is proposed showing the role of the fold in maintaining a minimum of molecular disordering during the transformation.