XPS studies of differences in contamination behaviour of amorphous and crystalline Ge

X-ray photoelectron spectroscopy studies of amorphous Ge films reveal small peaks associated with the L₃M₄₅M₄₅ Auger peak, which grow upon standing in (10^?5 torr) vacuum. The peaks disappear after annealing to crystallisation and do not reappear upon standing in similar vacuum. They are ascribed to oxygen aggregating near the mouths of pores in a-Ge.     

Dependence of the steepness of the exponential absorption tail on the chemical composition of materials

The quantity characterizing the steepness of the exponential tail of the fundamental absorption edge is studied. It is shown, that the magnitude of of binary compounds can be roughly estimated from their chemical composition in a way described in this paper.Boní II, Praha 4, Czechoslovakia.The author thanks Ing. D. Leal, CSc for his valuable discussions concerning this work.     

Invar behaviour in crystalline and amorphous alloys

A review is given of the wide range of ferromagnetic materials which exhibit Invar characteristics. It is proposed to illustrate the thesis that Invar behaviour is very general among such materials and does not necessarily demand the following conditions to be satisfied: (1) presence of iron; (2) incipient antiferromagnetism; (3) heterogeneous alloying; (4) closeness to a martensite transformation.

For most but not all of the materials discussed weak itinerant ferromagnetism accompanies and causes Invar characteristics. An example counter to this behaviour is Fe₃Pt. Here, Pettifor and Roy proposed as the origin of Invar behaviour a more general itinerant electron instability than is characteristic of weak itinerancy.     

Temperature dependance of the fundamental absorption edge in CdTe

Optical measurements made on CdTe put in light an extrinsic transition previously used to determine the interband edge of this compound. The donor level involved has a depth of 30 meV inside the bandgap at 77°K and of 60meV at 300°K. The value obtained for its temperature coefficient, suggests an association of this level with the L minimum of the conduction band.Also, determined is the true optical bandgap of CdTe between many conflicting results. One obtains: E_g = 1.529 meV with a temperature coefficient of ?3.10^?4eV/°K.     

The fundamental absorption band of amorphous germanium

The index of attenuationk() of evaporated films of amorphous Ge was measured in the spectral range of 1.95 to 5.3 eV by measuring the transmissivity of samples with thicknesses varying from 560 to 1510 Å. It is shown that in comparison with crystalline Ge the absorption is significantly shifted towards lower energies and the peak at 4·5 eV is absent. Using these data and the data onk() determined in other spectral ranges up to 25 eV by various authors, the index of refractionn() was calculated by Kramers-Kronig analysis. The absorption of amorphous Ge is compared with that of crystalline Ge and the differences are discussed.The authors thank Mr. Z. Balej for the preparation of the thin films used in our measurements, Dr. J. Nadrchal for help with the calculations, Dr. T. M. Donovan for the information on his measurements of photoemission before its publication and Prof. R. Grigprovici for helpful discussions.     

Transverse electroreflectance of GaSe at the fundamental absorption edge

The transverse electroreflectance of GaSe has been measured near the fundamental edge at 4.2 K. The observed structures have been interpreted in terms of the Franz-Keldysh oscillations, the exciton ground state, the first excited state of exciton, and the phonon replica of the exciton ground state.     

Anisotropy in the absorption spectrum of GaTe at the fundamental edge

The wavelength-modulated absorption coefficient of GaTe is measured at the fundamental edge at room temperature. The light is directed perpendicular to the cleavage plane, and its polarization is rotated in the plane itself. The optical constants are seen to vary appreciably for different polarizations, both in intensity and in the energy of the main peak at the absorption edge. A model is proposed to account for the experimental data.     

Determination of optical constants near the fundamental absorption edge in semiconductors

Abstract

The photoconductivity of semiconductors was considered under the following general conditions: the surface recombination velocity was not equal to zero, the space charge near the surface is taken into account, the value of absorption coefficient was not limited. Solving both the continuity and Poisson equations for an illuminated thick specimen, we obtained a solution which suggests a new combined measurement in order to determine various optical constants. The absorption coefficient and the diffusion length of electrons in CZ grown n-type Si (N _a = 10¹²-10¹³ cm^?3) were measured at room temperature, using the new technique.     

Direct phonon-assisted transitions near the fundamental absorption edge in CuInSe2

Summary The absorption spectrum near the fundamental absorption edge of an-type CuInSe₂ single crystal grown by the Bridgman method has been studied at 100 and 300 K. An absorption edge corresponding to the direct-band-gap transition is observed at about 1 eV. An additional absorption is observed at lower energies ((0.93÷0.95) eV) and is attributed to direct transitions assisted by long-wave-length optical phonons.

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Riassunto Lo spettro di assorbimento vicino al bordo di assorbimento fondamentale di un cristallo singolo di tipon di CuInSe₂, cresciuto col metodo di Bridgman, è stato studiato a 100 e 300 K. Si osserva un bordo di assorbimento in corrispondenza della transizione del gap di banda diretta a circa 1 eV. Un ulteriore assorbimento è osservato a energie inferiori ((0.93÷0.95) eV) ed è attribuito a transizioni dirette alla presenza di fononi ottici a grande lunghezza d'onda.

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Резюме При 100 К и 300 К исследуется спектр поглощения вблизи основного края поглощения монокристалла CuInSe₂ n-типа, выращенного по методу Ъридгмана, Вблизи 1 эВ обнаружен край поглощения, соответствующий прямому переходу. Дополнительное поглощение наблюдается при меньших энергиях ((0.93÷0.95) эВ) и приписывается прямым переходом, связанным с длинноволновыми оптическими фононами.

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Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

The absorption edge of amorphous As2S3

The absorption constantK of amorphous As₂S₃ was determined at the absorption edge in the range 1 to 10⁵ cm^–1. It is shown that at low energy levels up to about 10³ cm^–1 K depends exponentially on the photon energy; at higher absorption levels the energy dependence ofK is quadratic. The significance of these results is discussed. When sulphur is added to As₂S₃ the absorption edge has similar properties but is shifted towards lower energies.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be ?2×10^?4 eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm^?3 when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Photo-induced ESR and optical absorption edge shift in amorphous Ge-S films

Reversible shift of the optical absorption edge by the cycle of band-gap illumination and annealing has been observed for Ge-S films for the first time. Photo-induced ESR has been observed, and it is explained by two processes: (1)Increase in randomness of the structure by illumination, and (2)Trapping of free carriers excited by illumination by charged dangling bonds. The ESR signal caused by the former process does not disappear either by warming the film up to room temperature or by illuminating it with the mid-gap light, while that by the latter process disappears.     

Temperature variation of the fundamental absorption edge in AgGaSe2

Optical absorption measurements were made in the temperature range 9–300 K on the chalcopyrite semiconductor compound AgGaSe₂ and the optical energy gap E_G determined as a function of temperature T. In order to obtain the values of E_G as a function of T, the Elliot-Toyozawa model [R.J. Elliot, J. Phys. Rev. 108 (1957) 1384; D.D. Sell, P. Lawaets, Phys. Rev. Lett. 26 (1971) 311] was employed to perform the analysis of the optical absorption spectra. The resulting E_G vs. T curve was fitted to a semi-empirical model that takes into account both the thermal expansion and the electron–phonon interaction contributions. The results have been used to estimate values of the deformation potentials of the valence and conduction bands of the compound.     

Temperature dependence of the fundamental absorption edge in CuGaSe2

The temperature dependence of the fundamental absorption edge in CuGaSe₂ single crystals was determined in the temperature range from 15 to 300 K. Above about 120 K the gap energy changes linearly with temperature with dE_g/dT = ? (2.1 ± 0.1) eV K^?1. The downshift in dE_g/dT of the I–III–VI₂ compounds compared to their II–VI analogs is discussed accounting for the p-d hybridization of the uppermost valence band.