Electron–hole liquid in low-dimensional silicon–germanium heterostructures

A brief review is given of the studies in which quasi-two-dimensional spatially-direct and dipolar electron–hole liquids in Si/SiGe/Si type-II heterostructures with a low Ge content in the SiGe layer were discovered and investigated.     

Si–Si bond as a deep trap for electrons and holes in silicon nitride

A two-stage model of the capture of electrons and holes in traps in amorphous silicon nitride Si₃N₄ has been proposed. The electronic structure of a “Si–Si bond” intrinsic defect in Si₃N₄ has been calculated in the tight-binding approximation without fitting parameters. The properties of the Si–Si bond such as a giant cross section for capture of electrons and holes and a giant lifetime of trapped carriers have been explained. It has been shown that the Si–Si bond in the neutral state gives shallow levels near the bottom of the conduction band and the top of the valence band, which have a large cross section for capture. The capture of an electron or a hole on this bond is accompanied by the shift of shallow levels by 1.4–1.5 eV to the band gap owing to the polaron effect and a change in the localization region of valence electrons of atoms of the Si–Si bond. The calculations have been proposed with a new method for parameterizing the matrix elements of the tightbinding Hamiltonian taking into account a change in the localization region of valence electrons of an isolated atom incorporated into a solid.     

Aromaticity of planar Si6 rings in silicon–lithium clusters

Ipsocentric calculations of current density at the B3LYP/6-311++G(2d,2p) level show that the planar Si₆ ring supports a diatropic π ring current of about half the strength of the equivalent π current in benzene, both in the presumed global optimum geometry of Si₆Li₆ and in geometries occupying higher-energy local minima, corroborating the attribution of aromaticity to this silicon analogue of benzene.     

Characterization of nanostructured CuO–porous silicon matrix formed on copper-coated silicon substrate via electrochemical etching

A pulsed anodic etching method has been utilized for nanostructuring of a copper-coated p-type (100) silicon substrate, using HF-based solution as electrolyte. Scanning electron microscopy reveals the formation of a nanostructured matrix that consists of island-like textures with nanosize grains grown onto fiber-like columnar structures separated with etch pits of grooved porous structures. Spatial micro-Raman scattering analysis indicates that the island-like texture is composed of single-phase cupric oxide (CuO) nanocrystals, while the grooved porous structure is barely related to formation of porous silicon (PS). X-ray diffraction shows that both the grown CuO nanostructures and the etched silicon layer have the same preferred (220) orientation. Chemical composition obtained by means of X-ray photoelectron spectroscopic (XPS) analysis confirms the presence of the single-phase CuO on the surface of the patterned CuO–PS matrix. As compared to PS formed on the bare silicon substrate, the room-temperature photoluminescence (PL) from the CuO–PS matrix exhibits an additional weak ‘blue’ PL band as well as a blue shift in the PL band of PS (S-band). This has been revealed from XPS analysis to be associated with the enhancement in the SiO₂ content as well as formation of the carbonyl group on the surface in the case of the CuO–PS matrix.     

Formation of SiH bonds on the surface of microcrystalline silicon covered with SiOx by HF treatment

Infrared absorption due to the surface chemical bonds on microcrystalline silicon (μc-Si) was investigated with recycling procedures of thermal oxygenation accompanied with dehydrogenation and HF etching. SiH bonds were reformed on the surface of μc-Si by HF etching of oxygenated μc-Si. The mechanism of the SiH bond formation was proposed. It was suggested that formation of =SiH₂,

SiH₂

_x or -SiH₃ group depended on the kind of a crystalline plane.     

XPS study of the chemical bonding in hydrogenated amorphous germanium–carbon alloys

A systematic study of the chemical bonding in hydrogenated amorphous germanium–carbon (a-Ge_1-xC_x:H)alloys using X-ray photoelectron spectroscopy (XPS) is presented. The films, with carbon content ranging from 0 at. % to 100 at. %, were prepared by the rf co-sputtering technique. Raman spectroscopy was used to investigate the carbon hybridization. Rutherford backscattering spectroscopy (RBS) and XPS were used to determine the film stoichiometry. The Ge 3d and C 1s core levels were used for investigating the bonding properties of germanium and carbon atoms, respectively. The relative concentrations of C–Ge, C–C, and C–H bonds were calculated using the intensities of the chemically shifted C 1s components. It was observed that the carbon atoms enter the germanium network with different hybridization, which depends on the carbon concentration. For concentrations lower than 20 at. %, the carbon atoms are preferentially sp³ hybridized, and approximately randomly distributed. As the carbon content increases the concentration of sp² sites also increases and the films are more graphitic-like. Received: 4 May 1999 / Accepted: 24 November 1999 / Published online: 24 March 2000     

Observations of the stress developed in Si inclusions following plastic flow in the matrix of an Al–Si–Mg alloy

Abstract

Measurements are made of the stress developed in near-spherical elastic inclusions in an elastic plastic matrix under both tension and compression loading. Two experimental conditions are reported. The first case is where no thermal mismatch exists between the inclusions and the matrix, so that the stress in the inclusion is purely a result of the misfit in the elastic moduli and of the distortion of the plastic slip-line field around the inclusion. The observations are believed to be the first such and are in qualitative agreement with finite-element modelling for idealised inclusion distributions. The second case is the more usual one where a thermal misfit stress exists and observations are reported of the stress relief effects caused by the interaction of the plasticity-induced stress with the thermal and elastic misfit stresses.     

Method of calculating the phase composition of SiC–Si–C materials obtained by silicon infiltration of carbon matrices

The synthesis of SiC–Si–C materials by siliconizing porous carbon matrices has been considered, and a method of determining their phase composition has been devised. Preforms of two types have been siliconized, i.e., biomorphic carbon matrices prepared by wood pyrolysis and artificial porous graphites prepared by mixing and compacting carbon powders with an organic binder. The calculated phase compositions are in good agreement with microstructure metallographic examination data.     

Investigation of muon states in silicon and germanium by field-quenching and RFμSR

The temperature dependence of the three states of positive muons in the semiconductors with diamond structure ( ⁺ in diamagnetic states ^d and paramagnetic muonium Mu and Mu^*) have been investigated on six Si (pure, B and P doped) and four Ge (ultrapure, CZ-grown undoped, Ga and Sb doped) single crystals by longitudinal field-quenching and radio-frequency ⁺SR. Clear evidence for the transition Mu^{* d} is found. The influence of light-induced charge-carriers is shown to be quite different in p- and n-type material.The work has been supported by the Bundesministerium für Forschung und Technologie in Bonn, Germany, under contract no. 03-SE3STU.     

Study of solid–liquid interface in water dispersions of microcrystalline celluloses

Interaction on the solid–liquid surface in dispersions of microcrystalline cellulose (MCC) with various particle sizes has been studied by means of rheological methods. It was shown that the MCC dispersions possess shear-thinning rheological properties. An inversely proportional relationship between the average particle size of the MCC particles and the viscosity of the dispersions was discovered. This phenomenon is explained by the decrease of water mobility with increase in the specific surface of the MCC particles. Irreversible closing of the MCC pores reduces the viscosity of water dispersions. Addition of some water-soluble polymers leads to a considerable increase in viscosity due to formation of macromolecular net composed of solid particles.     

A review of visible-range Fabry–Perot microspectrometers in silicon for the industry

This review presents microspectrometers in silicon for the industry for measuring light in the visible range, using the Fabry–Perot interferometric technique. The microspectrometers are devices able to do the analysis of the individual spectral components in a given signal and are extensively used on spectroscopy. The analysis of the interaction between the matter and the radiated energy can found huge applications in the industrial sector. The microspectrometers can be divided on three types, determined by the dispersion element or the used approach and can be found microspectrometers based on prisms, gratings interferometers. Both types of microspectrometers can be used to analyze the spectral content ranging from the ultraviolet (UV, below 390 nm), passing into the visible region of the electromagnetic spectrum (VIS, 390–760 nm) up to the infrared (IR, above 760 nm). The microspectrometers in silicon are versatile microinstruments because silicon-compatible techniques can be used to assembly both the optical components with the readout and control electronics, thus resulting high-volume with high-reproducibility and low-cost batch fabrications. A compensation technique for minimizing the scattered light effects on interferometers was implemented and is also a contribution of this paper. Fabry–Perot microspectrometers for the visible range are discussed in depth for use in industrial applications.     

The quantum efficiency of the photo-electric effect in germanium for the 0.3–2 μ wavelength region

The absolute value of the quantum efficiency of the internal photoelectric effect in germanium in the 0.3–2 wavelength region was measured by means of the photovoltaic effect on a p-n junction. Two different regions were found. In the region from the infra-red absorption edge of germanium to a wavelength of 5750Å a practically constant number of electron-hole pairs, approximately equal to one, is produced per photon absorbed. In the second region from a wavelength of 5750Å and lower the number of generated pairs of current carriers is proportional to the total energy absorbed. In this region an energy of 2.5 eVis necessary to form an electron-hole pair.     

Influence of “ytterbium nanofilm–silicon Si(111)” interfaces on the valence of ytterbium

The work function of ytterbium films of nanometer thickness (from 1 to 16 monolayers) has been measured. The films have been prepared by sputtering of ytterbium in an ultrahigh vacuum on n- and p-type Si(111)7 × 7 silicon substrates with an electrical resistivity from 1 to 20 Ω cm. It has been shown that, in the films with a thickness of less than 8 monolayers, the work function depends nonmonotonically on the amount of ytterbium deposited on the surface (Friedel oscillations), whereas in the films with a thickness of more than 8 monolayers, the work function takes on a constant value (3.3 eV) that exceeds the work function for macroscopic samples (2.6 eV). This difference is associated with the fact that, during the formation of an Yb–Si interface, the large difference in the work functions of ytterbium and silicon (4.63 eV) leads to the transfer of a significant fraction of electrons from the metal to the semiconductor. This transfer of electrons from the film to silicon is accompanied by the lowering of the Yb 5d level below the Fermi level. As a result, the valence of the metal and, accordingly, the work function increase.     

Specific features of thermal resistance of silicon in the temperature range 105–130 K

Careful experimental investigations into the behavior of the thermal resistance of single-crystal silicon are carried out in the immediate vicinity of the temperature of an anharmonicity sign inversion (T _i =121.1 K), where phonon thermal resistance approaches zero. An anomalous positive deviation of the total thermal resistance (W) from the linear part of the temperature dependence with a maximum at 121.1 K is found in the temperature range 105–130 K. The temperature behavior of W in this range indicates that the mean free path of phonons is limited by a characteristic size of structural defects and that its temperature dependence exhibits specific features in the vicinity of T _i . It is established that the character of the temperature dependence of W above and below T _i is different. A linear functional relation between the total thermal resistance and the isobaric thermal strain is revealed at positive and negative anharmonicities of atomic vibrations.     

Absolute efficiency calibration of high-purity germanium detector in the range 120–8500 keV

This article reports the efficiency response curve of the high-purity germanium detector over the wide energy range, covering from 120 to 8500 keV. The efficiencies were measured for different counting geometries by using point radionuclide standards (mono-energetic as well as multi-gamma emitters) supplied by IAEA and the capture gamma-ray facility installed at PINSTECH nuclear reactor PARR-1. The measured efficiencies were required to fit with a suitable fitting function for interpolation within the energy range of interest. Several fitting functions were proposed in the literature covering different energy ranges. The functions giving the best fit to experimental data are presented. The work has successfully extended the response curve beyond 1500–8500 keV, which is the region where the standard calibration radionuclides are not available. The thermal neutron capture gamma-ray facility provided the collimated neutron beam, extracted from the core of the reactor and made to react with ammonium chloride target to produce the capture gamma rays for determining the efficiencies in the extended region. It was found that the capture gamma-ray provides a satisfactory solution to extend the absolute efficiency calibration in the MeV range. It was also found that the fitting function that is linear in its parameter was highly satisfactory up to 1500 keV but proved insufficient upto 8500 keV. The exponential function giving the good fit over the range has been presented. Good agreement has been found between the experimentally measured absolute efficiencies and the predicted result.     

In-situ monitoring and control of hydrogenated amorphous silicon–germanium band-gap profiling during plasma deposition process

In-situ germanium content monitoring and its characteristics in SiH₄/GeH₄/H₂ plasmas was studied during hydrogenated amorphous silicon–germanium (a-SiGe:H) film depositions. Since an appropriate band-gap profiling in a-SiGe:H deposition is very important to achieve high efficiency solar cell, the accurate monitoring and control of Ge contents are required. In this work, we found the spectral intensity ratio of silicon atom (288.2 nm) and germanium atom (303.9 nm) emission has strong relation with Ge content in plasmas. In typical, band-gap energy of films was decreased with the increasing of gas flow ratio GeH₄/SiH₄. However, at different total flow rate of GeH₄, the band-gap was different for same gas flow ratio cases because the Ge content in plasmas was changed due to the changes of electron temperature by hydrogen dilution. On the other hand, the emission intensity ratio Ge/Si detected the band-gap variation. Using this method, therefore, we measured and control Ge/Si to make a U-shape band-gap profile which was proved by an ellipsometer and Auger electron spectroscopy depth profile analysis.     

Generation of far-infrared radiation by hot holes in germanium and silicon inE ⊥H fields

The experimental results obtained by the investigation of stimulated FIR emission from dopedp-type germanium andp-type silicon by hot holes in crossedE andH fields at = 10 and 80 K are reported. The analysis of the emission intensity fromp-type germanium as a function ofE andH fields permits us to draw a conclusion about the important role of quantization of the energy spectrum of light holes and the contribution of light hole transitions with n = 2 to the amplification of FIR radiation. A new region of generation is demonstrated inp-type germanium under uniaxial stress. The first experimental results on stimulated FIR emission fromp-type silicon are reported.     

Skin dominance of the dielectric–electronic–phononic–photonic attribute of nanoscaled silicon

Nanoscaled or porous silicon (p-Si) with and without surface passivation exhibits unusually tunable properties that its parent bulk does never show. Such property tunability amplifies the applicability of Si in the concurrent and upcoming technologies. However, consistent understanding of the fundamental nature of nanoscaled Si remains a high challenge. This article aims to address the recent progress in this regard with focus on reconciling the tunable dielectric, electronic, phononic, and photonic properties of p-Si in terms of skin dominance. We show that the skin-depth bond contraction, local quantum entrapment, and electron localization is responsible for the size-induced property tunability. The shorter and stronger bonds between undercoordinated skin atoms result in the local densification and quantum entrapment of the binding energy and the bonding electrons, which in turn polarizes the dangling bond electrons. Such local entrapment modifies the Hamiltonian and associated properties such as the band gap, core level shift, Stokes shift (electron–phonon interaction), phonon and dielectric relaxation. Therefore, given the known trend of one property change, one is expected to be able to predict the variation of the rest based on the notations of the bond order–length–strength correlation and local bond average approach (BOLS-LBA). Furthermore, skin bond reformation due to Al, Cu, and Ti metallization and O and F passivation adds another freedom to enhance or attenuate the size effect. The developed formulations, spectral analytical methods, and importantly, the established database and knowledge could be of use in engineering p-Si and beyond for desired functions.     

Blue–violet photoluminescence from colloidal suspension of nanocrystalline silicon in silicon oxide matrix

We report room temperature visible photoluminescence (PL), detectable by the unaided eye, from colloidal suspension of silicon nanocrystals (nc-Si) prepared by mechanical milling followed by chemical oxidation. The PL bands for samples prepared from Si wafer and Si powder peak at 3.11 and 2.93 eV respectively, under UV excitation, and exhibit a very fast (～ns) PL decay. Invasive oxidation during chemical treatment reduces the size of the nc-Si domains distributed within the amorphous SiO₂ matrix. It is proposed that defects at the interface between nc-Si and amorphous SiO₂ act as the potential emission centers. The origin of blue–violet PL is discussed in relation to the oxide related surface states, non-stoichiometric suboxides, surface species and other defect related states.