New semiconductor materials in microelectronics

Properties of non-silicon systems are discussed. In addition to Si, the use of materials possessing a higher mobility and a shorter lifetime (Ge, GaAs and other), permits to increase working frequencies and switching speed, to expand the temperature range of the microelectronic devices operation. The problem can be solved by obtaining thin and homogenous epitaxial semiconductor films and insulating layers.     

Electron microscopic investigations of the recombination activity of dislocations in silicon

By combined application of scanning electron microscopy (EBIC technique) and transmission electron microscopy conclusions on the electrical activity (recombination efficiency) of individual dislocations in silicon are drawn.     

Effect of oxygen impurities on recombination luminiscence in some alkali halide crystals

The low-energy emission spectra of nominally pure NaCl, KCl, KBr, and KI crystals are examined at different electron beam densities and at temperatures between 80 and 300 K. The results obtained for the crystals of the same chemical formula but with different OH⁻ contents, or subjected to the action of the air atmosphere, show that the emission bands considered are related with the presence of oxygen ions formed during radiolysis of such impurities like OH⁻ or H₂O. The decay of pertinent emission observed at higher radiation doses is explained by the transformation of simple oxygen ions into the neutral, inactive O₂-molecules.     

Interpretation of the switching effect in amorphous semiconductors as a recombination instability

For an explanation of the switching effect appearing on thin layers of amorphous semiconductors, a double-injection model is developed where the presence of diffusion currents in front of the electrodes is assumed in the high-resistance state. In the diffusion regions recombination takes place. The cause of the switching effect is seen in a recombination instability. Because of a saturation of the recombination current the life time of the free carriers becomes long enough for the diffusion length to extend over the full length and the high-resistance state to collapse.     

Voids in hydrogenated amorphous silicon materials

Effusion measurements of hydrogen and of implanted helium are used to characterize the presence of voids in hydrogenated amorphous silicon (a-Si:H) materials as a function of substrate temperature, hydrogen content, etc. For undoped plasma-grown a-Si:H, interconnected voids are found to prevail at hydrogen concentrations exceeding 15–20 at.%, while isolated voids which act as helium traps appear at hydrogen concentrations ≤ 15 at.%. The concentration of such isolated voids is estimated to some 10¹⁸/cm³ for device-grade undoped a-Si:H deposited at a substrate temperature near 200 °C. Higher values are found for, e.g., doped material, hot wire grown a-Si:H and hydrogen-implanted crystalline Si. The results do not support recent suggestions of predominant incorporation of hydrogen in a-Si:H in (crystalline silicon type) divacancies, since such models predict a concentration of voids (which act as helium traps) in the range of 10²¹/cm³ and a correlation between void and hydrogen concentrations which is not observed.     

Positron lifetime in inhomogeneous metallic materials

A positron is considered in an inhomogeneous jellium model according to a formalism following from the theory of liquids. This leads to a nonlocal treatment of the screening of this particle. Annihilation rates have been found inferior to the ones for a homogeneous electron gas, in agreement with positron lifetime measurements.     

New photosensitive materials

The emergence and growth of some far reaching applications have triggered quite a few new developments in the field of photosensitive materials.Among those applications of growing technical and economical interest, we concentrate on both the “solar photovoltaic conversion” and the “xerographic” type applications (e.g. laser printing and recording).     

Bulk non-radiative recombination reduction in nGaAs due to an annealing under Ga-Al melt

It is shown by microcathodoluminescence and electron beam induced current investigations that efficiency of radiative recombination and the values of hole diffusion lengths in nGaAs substrate increase (up to L_p ∽ 5 μm) due to an annealing under Ga Al melt. The values of the annealing critical temperature exceeding of which leeds to bulk non-radiative recombination drop as well as optimal values of annealing temperature and duration are determined. In pAlGaAs–(p-n)GaAs solar cells based on annealed substrates the values of open circuit voltage U_oc = 1.16 – 1.18 V under 500-fold concentrated solar radiation as well as values of photocurrent comparable with the best results of solar cells with epitaxial nGaAs are achieved.     

Polarons in wide-band-gap molecular materials: Polysilanes

Illumination of polysilanes leads to the formation of free charge carriers. Due to the charge–phonon coupling and molecule deformation, polarons are formed. Their occurrence can be detected by thermostimulated luminescence (TSL). Polaron binding energy E_p can be determined by fitting experimental dependences of charge carrier mobility vs. applied electric field measured at different temperatures. E_p for poly[methyl(phenyl)silylene] films was found to be 0.29 eV.     

Trap-controlled transient current injection in amorphous materials

A theory for trap-controlled transient current injection is presented. The basic equations governing a carrier transport in amorphous materials are solved analytically for an arbitrary energy distribution of traps in the gap. Two special models, opposite with respect to each other, are treated: (1) the model of monoenergetic trapping level; and (2) the model of trapping levels distributed uniformly over a wide energy range. The theoretical results obtained are in agreement with existing transient current injection data on some amorphous materials.     

Manifestation of optical activity in different materials

Various manifestations of optical activity (OA) in crystals and organic materials are considered. Examples of optically active enantiomorphic and nonenantiomorphic crystals of 18 symmetry classes are presented. The OA of enantiomorphic organic materials as components of living nature (amino acids, sugars, and proteins) is analyzed. Questions related to the origin of life on earth are considered. Examples of differences in the enantiomers of drugs are shown. The consequences of replacing conventional left-handed amino acids with additionally right-handed amino acids for living organisms are indicated.     

Influence of non-stoichiometry on the recombination activity of dislocations in undoped semi-insulating GaAs crystals

The effect of non-stoichiometry of gallium arsenide melt composition on the recombination activity of dislocations in undoped semi-insulating GaAs crystals is studied and analyzed. It is shown that the deviation from the stoichiometric melt composition is one of the major factors affecting the recombination activity of dislocations in undoped semi-insulating GaAs crystals. namely: the recombination activity of dislocations is increased when the arsenic-rich melt is used, and, on the contrary, is decreased when the gallium-rich melt is used. The regularities observed are explained by the complex processes of interaction of dislocations with the non-stoichiometry-induced intrinsic point defects.     

Sol-gel-derived organic-inorganic hybrid materials

Optically transparent organic-inorganic hybrid coating materials have been prepared by a sol-gel process. Four different types of the coating material produced by TWI in Cambridge, UK using the patented Vitresyn^® method, all identical in terms of the starting materials, but differing in terms of their relative proportions, have been examined. Tetraethoxysilane was used as the primary inorganic precursor and urethane acrylate was used as the source of the organic component. 3-(Trimethoxysilyl)propyl methacrylate was used as both a secondary inorganic source and a silane coupling agent to improve the compatibility of the organic and inorganic phases. The degree of chemical interaction of the organic and inorganic phases after processing was determined by ²⁹Si and ¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopy. The effect of the relative amount of inorganic starting component in these hybrid materials on their thermal properties was investigated through differential scanning calorimetry and thermogravimetric analysis. Similar degrees of chemical interaction between the organic and inorganic phases were found in all four samples. T³, Q³ and Q⁴ are the main cross-linking network structures in these hybrid systems, the relative proportions of which are determined by the relative proportions of the starting materials.     

Minerals as advanced materials

A resume is given on relationships between minerals and materials. It is shown that Nature has many advantages over technology which can be profitably exploited for finding new materials with useful properties. However, Nature has also inherent limitations. Therefore, it seems advisable to use the knowledge of minerals as a treasure which can be tapped for getting inspired on the way to finding new materials. Several examples are given, including those of zorite/ETS‐4/ETS‐10, boracites, zircon, kesterite, a natural quasicrystal, opals, ultramarine and mayenite. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)