Electric-field effects on the infra-red absorption of H in a SiO2 film

Summary A simple model of a localized, anharmonic oscillator is developed to simulate the dynamics of hydrogen in amorphous SiO₂. The absorption coefficient in the presence of a strong electric field is evaluated in order to make a comparison with experimental multiple internal reflectance infra-red spectrum.

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Riassunto Per descrivere la dinamica di uno ione idrogeno impiantato in una matrice amorfa di SiO₂, è stato sviluppato un semplice modello di oscillatore anarmonico localizzato. I dati sperimentali relativi allo spettro infrarosso elettromodulato (in configurazione di riflettanza interna multipla) sono stati riprodotti attraverso il calcolo del coefficiente di assorbimentoK in presenza di un forte campo elettrico statico.

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Резюме Развивается простая модель локализированниго ангармонического осциллятора для моделирования динамики водорода в аморфном SiO₂. Вычисляются козффициенты поглощения в присутствии сильного электрического поля с целью сравнения с экспериментальным инфракрасным спектром при многократном внутреннем отражении.

     

Absorption lineshape ofF A centres in alkali halides

Summary The lineshapes of the absorption bands ofF _A centres in alkali halides have been studied for the first time. The new method used for this investigation is based on the determination of the overlap between theF _A1 andF _A2 bands from luminescence measurements. The experimental results have been compared with calculated values deduced from the theoreticalF _A bands of different shapes. For bothF _A (I) centres in KCl: Na⁺ andF _A (II) centres in KCl: Li⁺ and RbCl: Li⁺ the absorption lineshape at low temperature is much closer to a sum of two Lorentzian curves than that of two Gaussian or Poissonian bands. This result shows an unexpected difference with theF centres, whose absorption lineshape is known to be Poissonian at the same temperatures.

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Riassunto Sono state studiate per la prima volta le forme di riga delle bande di assorbimento dei centriF _A in alogenuri alcalini. Il nuovo metodo usato per questa indagine è basato sulla determinazione attraverso misure di luminescenza della sovrapposizione fra le bandeF _A1 eF _A2 . I risultati sperimentali sono stati confrontati con i valori dedotti da bandeF _A teoriche di diverse forme. La forma di riga dell'assorbimento a bassa temperatura, sia per i centriF _A (I) in KCl: Na⁺ sia per i centriF _A (II) in KCl: Li⁺ e RbCl: Li⁺, è approssimata molto meglio da una somma di due curve Lorentziane che da quella di due bande Gaussiane o Poissoniane. Questo risultato rivela una differenza imprevista rispetto ai centriF, il cui assorbimento ha una forma di riga Poissoniana alle stesse temperature.

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Резюме Впервые исследуются формы линий зон поглощения дляF _A центров в щелочно-галоидных соединениях. Предлагается новый метод исследования. Этот метод основан на определении перекрытия междуF _A иF _A2 зонами из измерений люминесценции. Экспериментальные результаты сравниваются с вычисленными величинами, полученными из теоретическихF _A зон различной формы. ДляF _A (I) центров в KCl: Na⁺ иF _A (II) центров в KCl: Li⁺ и RbCl: Li⁺ форма линии поглощения при низких температурах оказывается ближе к сумме двух Лоренцевых кривых, а не двух Гауссовых или Пуассоновых зон. Этот результат обнаруживает неожиданное отличие отF центров, форма линии поглощения которых, как известно, представляет Пуассонову кривую при тех же температурах.

     

Revisited vibronic model for Li+ ion and F A (Li) centre in alkali halides

Summary The controversial far-infrared optical absorption in Li-doped alkali halides is now attributed to transitions between the low-lying energy levels of an anharmonic oscillator composed of three halogen-ion pairs centred at the cation site which vibrate in theT _1u ungerade mode. This oscillator is electronically coupled to the impurity dragging it into rotation over 〈111〉 sites. Several experimental features derive simply from our model such as the absence of any clear-cut lithium-isotope effect and the low-frequency range. The capital effect of an F centre at a 〈001〉 nearest-neighbour (nn) site is to reduce the dimensionality of vibration forcing the impurity to rotate in the perpendicular (001)-plane. This upgrades the optical frequency in proportion to (3/2)^1/2 and shrinks the off-centre displacement. We finally show that recent computer-simulation data on Li-doped alkali halides provide a solid ground for regarding the Li off-centredness as a pseudo-Jahn-Teller phenomenon. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

The lattice anharmonicity-induced broadenings of defect spectral lines

Summary The effect of the anharmonicity of the host lattice to the defect optical absorption and emission is studied. Anharmonicity contributes to the defect optical linewidths via phonon lifetime effect and defect-phonon scattering mediated by a second phonon. Numerical estimates of these contributions indicate that the anharmonic broadenings may not be negligible in some insulating crystalline hosts.     

a-Si:H films: Influence of P- and B-doping on infra-red absorption bands

Summary The infra-red spectra of P- and B-doped hydrogenated amorphous silicon films grown by means of an evaporation method have been measured. The SiH _n stretching and wagging (or rocking) absorption bands have been examined by varying the dopant content measured by Auger analysis. On increasing the doping, a reduction of hydrogen content has been noticed with an enhancing of zeroth-order moment of Si-H stretching at the expense of the Si-H₂ one. Analogous results for B-doped films have been observed. Since no frequency shift towards higher frequencies of the stretching bands has been measured, the observed changes of the intensity of the bands have been attributed to structure modifications rather than to variations in the electronegativity in the neighbours of the silicon atom. In particular, the monohidride configuration with an overall decrease in the number of defects seems to be favoured by the deposition technique adopted. This work was supported in part by GNSM-CNR, M.P.I. and CNR ?Progetto Finalizzato Energetica?.     

On the effective field and optical constants of alkali halides containingF colour centres

Summary The Onsager and Lorentz local fields ofF colour centres are compared and their change as a function of angular frequency and temperature is also considered. The change of these fields is proportional to the change of the refractive index produced byF centres. Some approximations about the explicit temperature dependence of the refractive-index change are also presented.

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Riassunto I campi locali di Onsager e Lorentz diF centri di colore sono confrontati e si considera il loro cambiamento in funzione della frequenza angolare e della temperatura. Il cambiamento di questi campi è proporzionale al cambiamento dell'indice di rifrazione prodotto daF centri. Si presentano anche alcune approssimazioni riguardo alla esplicita dipendenza dalla temperatura del cambiamento dell'indice di rifrazione.

     

Disorder and optical characterization of evaporation deposited a-Si: H films

Summary We have found, from optical transmission and photoconductivity measurements, that the width of the Urbach's tail and the optical-band gap value depend on the amount of disorder present in the network ofa-Si: H films. The nature of the disorder, being it thermal or structural, affects in the same way the behaviour of the absorption coefficient and supports the hypothesis that the value of the optical-band gap depends only indirectly on the overall hydrogen content. Work partially supported by CNR ?Progetto Finalizzato Energetica? and by the Energy Committee of Sicilian Region.     

Optical investigations of Mn2+ ion in Cs2CdCl4 and CsCaCl3 single crystals

Summary Polarized absorption, emission and excitation spectra of Cs₂CdCl₄∶Mn²⁺ and emission as well as excitation spectra of CsCaCl₃∶Mn²⁺ single crystals at temperatures down to 77 K are reported. The general features of the spectra are fruitfully analysed within the framework of the strong field scheme of Tanabe and Sugano including Trees' correction. Some finer structure of the crystal field independent levels are observed which are rationalized in terms of vibrational mechanism. A detailed analysis of the spectra indicates octahedral site symmetry around the central metal ion. The authors of this paper have agreed to not receive the proofs for correction.     

Polarized absorption spectra of Co2+ in Na2Cd3Cl8 single crystals

Summary The polarized optical absorption spectra of Na₂Cd₃Cl₈: Co²⁺ in the range of 15 000 to 40 000 cm⁻¹ down to 15 K are reported. The Co²⁺ ion is found to occupy the Cd²⁺ sites in octahedral geometry and the spectra are interpreted satisfactorily in terms of a cubic ligand field model including spin-orbit coupling. The observed crystal field spectra are well reproduced withB=745 cm⁻¹,C=3410 cm⁻¹,Dq=700 cm⁻¹ and ζ (spin-orbit interaction) =520 cm⁻¹. No spectral evidence for tetragonal distortion is observed.     

Optical absorption of the F center as function of the temperature

Abstract

The temperature shift of the F absorption band for KBr and KCl single crystals was experimentally and theoretically determined. The theoretical calculations are based on the pseudopotential method using different wave functions and assumed a linear thermal expansion coefficient for the crystalline lattice. The change of F-center absorption energy due to collisions with the thermally vibrating lattice is calculated. This contribution is small. The measurements agree fairly well with the results obtained using the present model.     

Electronic structure of a single oxygen vacancy in rutile TiO2

Summary I report calculations of the electronic structure of an ideal (undistorted) oxygen vacancy in rutile TiO₂ using the equation-of-motion method. I use a full phenomenological tight-binding Hamiltonian due to Vos to describe the electronic structure. The bulk band structure is also discussed in detail using the traditional Green’s-function method. I illustrate an efficient way to calculate the perfect-crystal Green’s-function matrix elements for any structure, including those that belong to non-symmorphic space groups. The results show that the oxygen vacancy in rutile TiO₂ results in deeply localized states in its vicinty.     

On the lattice dynamics of AgGaS2

Summary The vibrational spectrum and one-phonon density of states of a chalcopyrite crystal AgGaS₂ are calculated in an extended Keating’s model with two-bond-stretching and one-bond-bending force constants. Three charges of ions and three force constants are determined by a least-square fitting to experimental frequencies of long-wave phonons taken from Raman-scattering experiments. The calculated constant-volume specific heat, Debye temperature and elastic constants, of AgGaS₂ are in agreement with the experimental data of other authors.     

Computation of impurity Green’s functions in the tight-binding scheme

Summary A general method to calculate electronic Green’s functions in the tight-binding scheme is presented. The integrations are systematically reduced to the irreducible volume of the first Brillouin zone, so that a very short time is necessary for the computer’s work. The importance of symmetry planes, lines and points is stressed.

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Riassunto Si presenta un metodo generale per il calcolo delle funzioni di Green elettroniche nell’approssimazione degli elettroni fortemente legati. Le integrazioni sono sistematicamente ridotte al volume irriducibile della prima zona di Brillouin, in modo da ridurre notevolmente il tempo di calcolo. Si mette in risalto l’importanza di piani, linee e punti di simmetria.

     

Resonant raman in impurity molecules: an exact solution in the presence of harmonic and anharmonic vibrational potential

Summary Resonant Raman response of impurity molecules in nonmetallic solid matrix is solved exactly. The model embodies the nonradiative interaction between impurity molecule and the nearest-neighbour molecules in the solid. The interatomic vibration of the excited electronic level of the molecule is modelled by harmonic and anharmonic potential. Frank-Condon and anharmonic effects are discussed by selected numerical examples.

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Riassunto Il tensore Raman risonante di molecole di impurezza in solidi molecolari è calcolato esattamente. Il modello di hamiltoniana proposto contiene la diseccitazione non radiativa della molecola di impurezza mediante l'interazione con la matrice solida ed inoltre le vibrazioni interatomiche della molecola sono rappresentate da un potenziale armonico o anarmonico. L'effetto Frank-Condon ed il potenziale anarmonico delle vibrazioni della molecola sono risolti esattamente per la prima volta e discussi mediante esempi numerici.

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Резюме Определятся реазонансный Раманоский отклик от молекул примеси в неметаллической твердой матрице. Предложенная модель включает нерадиационное взаимодействие между молекулой примеси и ближайшими молекулами твердого тела. Межатомное колебание возбужденного злектронного уровня молекулы моделируется с помощью гармонического и ангармонического потенциала. На численных примерах обсуждвются зффект Франка-Кондона и ангармонические зффекты.

     

Thermal quenching of extrinsic photoluminescence of GaAs-Ga1-x Al x As single quantum well

Summary The temperature dependence of the emission intensity due to conduction band-to-neutral acceptor recombination (e-A⁰) is investigated in a GaAs/Ga_0.7Al_0.3As single quantum well. It is shown that the thermal quenching of the (e-A⁰) emission peak is not monotonous with temperature. The increase of the (e-A⁰) emission intensity up to about 30 K is interpreted as a consequence of the ionization of shallow donors, while the decrease in emission intensity at higher temperature is due to ionization of neutral acceptors.     

Deep-level spectroscopy by analysis of isothermal capacitance transients

Summary Deep-level spectrometry of semiconductor has been carried out by measuring isothermal capacitance transients. From the recorded transients, DLTS signals have been reconstructed by software to give Arrhenius plots. Transients have been acquired in two time ranges to increase the emissivity interval on which to observe the trap decay. The method has been tested on Mo/GaAs Schottky barriers and a comparison has been carried out with results obtained on the same diodes using a commercial DLTS system. The authors of this paper have agreed to not receive the proofs for correction.     

Complex-time analysis of tunnelling processes

Summary The interpretation of the tunnelling time is apparently still an open question. In the attempt to clarify this problem we have performed a complex-time analysis in order to evaluate the semi-classical propagator, as a function of the energy, via the path integral method, for an anharmonic potential. This analysis allows us to do an evaluation of the tunnelling time and bounce duration. The connection with some solid-state physical problem is also discussed. To speed up publication, the authors have agreed not to receive proofs which have been supervised by the Scientific Committee.     

Mode coupling analysis of coherent quasi-elastic neutron scattering from fluorite-type materials approaching the superionic transition

Summary Neutron scattering experiments on SrCl₂, CaF₂ and PbF₂ have shown that intensity and width of the coherent diffuse quasi-elastic spectrum increase rapidly with temperature into the fast-ion conducting phase, the main feature in the integrated quasi-elastic intensity being a peak just beyond the (200) point along the [100] direction in scattering wave vector space. The Zwanzig-Mori memory function formalism is used in this work to analyse the quasi-elastic scattering cross-section from charge density fluctuations in terms of anharmonic couplings between the vibrational modes of the crystal. The two- and three-mode channels are examined for compatibility with the quasi-elastic neutron scattering evidence, on the basis of i) energy and momentum conservation and van Hove singularity arguments and ii) measured phonon dispersion curves along the main symmetry directions in SrCl₂, CaF₂, SrF₂ and BaF₂. The analysis identifies a specific microscopic role for the Raman-active optic branches. The eigenvectors of the relevant Raman-active and partner modes in the three-mode channel describe relative displacements of the two halogens in the unit cell superposed on relative displacements of the halogen and alkaline-earth components. This microscopic picture is thus consistent with the superionic transition being associated with the onset of dynamic disorder in the anionic component of the crystal.

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Riassunto Questo lavoro esamina la sezione d’urto per diffusione quasi elastica di neutroni da fluttuazioni di carica in cristalli di tipo fluorite in prossimità della transizione ad uno stato superionico, in relazione alle caratteristiche sperimentalmente osservate per lo spettro di diffusione quasi elastica da SrCl₂, CaF₂ e PbF₂. Si usa il formalismo della funzione di memoria di Zwanzig e Mori nell’ambito di un’approssimazione di accoppiamento anarmonico tra modi vibrazionali del cristallo. I processi di decadimento a due modi e tre modi sono analizzati mediante argomenti basati su leggi di conservazione e singolarità di van Hove, usando curve di dispersione per vibrazioni reticolari misurate in SrCl₂, CaF₂, SrF₂ e BaF₂. L’analisi attribuisce un ruolo specifico ai modi Raman nella dinamica ionica precorrente la transizione. Gli spostamenti atomici associati ai modi dominanti sono correlati all’attivazione di disordine dinamico nel sottoreticolo anionico che porta alla transizione.

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Резюме Эксперименты по рассеянию нейтронов на SrCl₂, CaF₂ и PbF₂ показывают, что интенсивность и ширина когерентного квази-упругого спектра быстро ывеличиваются с температурой причем основная особенность в проинтегрированной квази-упругой интенсивности представляет пик выше точки (200) вдоль направления [100] в пространстве волновых векторов рассеяния. В этой работе используется формализм функции памяти Цванцига-Мори для анализа поперечного сечения квази-упругого рассеяния на флуктуациях плотности заряда в терминах ангармонических связей между колебательными модами кристалла. Исследуются каналы двух- и трех-мод для подтверждения квази-упругого рассеяния неитронов, основываясь на 1) сохранении энергии и импульса и аргументах сингулярности ван Хова и на 2) измеренных фононных диспеионных кривых вдоль главных направлений симметрии в SrCl₂, CaF₂, SrF₂ и BaF₂. Проведенный анализ определяет микроскопическую роль Раман-активных оптических ветвей. Собственные векторы соответствующих мод в трех-модовых каналах описывают относительные смещения двух галогенов в элементарной ячейке, наложенные на относительные смещения галогенной и щелочно-земельной компонент. Эта микроскопическая картина согласется с суперионным переходом, который связан с началом динамического беспорядка в анионной компоненте кристалла.

     

Luminescence centers in Sc2O3

Sc₂O₃ luminescence spectra are studied. The spectra are separated into elementary bands by the Alentsev–Fock method. It is established that the luminescence spectra consist of a number of overlapping bands with maxima at 3.5; 3.05; 2.65; 2.35, and 2.05 eV. The band at 3.5 eV is interpreted as emission of self-localized excitons, and the other bands, as defect-center recombination. L’vov State University, 50, Dragomanov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 776–778, November–December, 1997.