Visible light emission from silicon dioxide with silicon nanocrystals

An electroluminescent MOS structure was developed using silicon wafers covered by thermal silicon dioxide containing silicon nanocrystals. Efficiency of the structure was sufficient for observation to be possible with the naked eye in daylight conditions under DC polarization. Silicon nanocrystals were produced using silicon ion implantation followed by subsequent annealing at 1100 °C in a nitrogen atmosphere. Three separate bands of emitted light at wavelengths of ∼400-500 nm (blue), ∼500-600 nm (green), and ∼650-850 nm (red) were observed and found to be related to specific regions of the implanted silicon concentration profile. For a single energy implant, each of the emitted light bands originated from a separate depth region of the silicon dioxide layer containing silicon nanocrystals. The spectrum of the emitted light was found to depend on the excess silicon concentration profile. For practical applications, the color of the emitted light can be controlled by adjustment of the implantation parameters and MOS structuring process.     

Diffusion of promethium in silicon

The first investigations have been made on the diffusion of promethium in silicon. In the temperature range from 1100 to 1250 °C the diffusion constant of promethium increases from ∼1×10⁻¹³ cm²/s to ∼1.5×10⁻¹² cm²/s. The temperature dependence of the diffusion coefficient can be described by D = 5 x 10⁻¹ exp[-(3.3 eV/kT]cm²/s. Pis’ma Zh. Tekh. Fiz. 23, 46–50 (January 26, 1997)     

Diffusion of erbium in silicon

Diffusion of erbium in silicon has been investigated by the electric method. The erbium diffusion coefficient in the temperature range 1150–1250 °C increases from 1.4×10⁻¹³ to 6.2×10^{− 13} cm²·s⁻¹. The values obtained for the diffusion coefficient of erbium in silicon are in good agreement with data obtained by the method of tagged atoms. Pis’ma Zh. Tekh. Fiz. 24, 68–71 (January 26, 1998)     

Doping of silicon layers from a sublimating erbium source in molecular beam epitaxy

The preparation of light-emitting rare-earth-doped (Er) silicon structures has been considered. A new technique of erbium doping of silicon layer during its growth by the molecular beam epitaxy method is suggested.     

Deposition kinetics of silicon dioxide from tetraethylorthosilicate by PECVD

Tetraethylorthosilicate (TEOS)-based SiO₂ films were prepared in an RF reactor at various discharge powers and two substrate temperatures, and infra-red absorption spectra were taken for each deposited film using an FTIR spectrometer. Based on the FTIR observations, a model for the plasma and surface kinetics was developed. Using a proposed three-step reaction mechanism at the surface of a two-dimensional lattice, the growth rate of the films and the incorporation rate of Si---O---Si bridges into the films were derived as functions of the densities of the film precursors, oxygen atoms and molecular oxygen ions. The densities are further given by the uniform discharge model of our previous work as a function of the discharge power. According to the model, in the regime of high ratio of oxygen to TEOS flow the growth rate is governed by the film precursors generated from TEOS, and the incorporation rate is additionally determined by oxygen radicals and molecular oxygen ions and by the dehydration process. The growth rate was measured in μm/min and the relative incorporation rate was obtained from the absorption intensity of Si---O---Si rocking mode in FTIR spectra. The formulation of the model was compared with the experimental results, and on the validity of the model was discussed.     

Cobalt ferrite nanoparticles in a mesoporous silicon dioxide matrix

We have studied magnetic nanoparticles of cobalt ferrite obtained by the extraction-pyrolysis method in a mesoporous silicon dioxide (MSM-41) molecular sieve matrix. The X-ray diffraction data show evidence for the formation of CoFe₂O₄ particles with a coherent scattering domain size of ∼40 nm. Measurements of the magnetization curves showed that powders consisting of these nanoparticles are magnetically hard materials with a coercive field of H _c(4.2 K) = 9.0 kOe and H _c(300 K) = 1.8 kOe and a reduced remanent magnetization of M _r/M _s(4.2 K) = 0.83 and M _r/M _s(300 K) = 0.49. The shape of the low-temperature (4.2 K) magnetization curves is adequately described in terms of the Stoner-Wohlfarth model for randomly oriented single-domain particles with a cubic magnetic anisotropy.     

Quasi-monolayer deposition of silicon dioxide

SiO₂ films were deposited layer by layer from a new silicon source gas, i.e. tetra-iso-cyanate-silane (Si(NCO)₄). An average growth rate of about 0.17 nm per cycle was achieved by a cyclic process of alternating reaction of the substrate surface with Si(NCO)₄ and H₂O respectively. The detailed deposition characteristics together with chemical and physical properties of the deposited film were evaluated with ellipsometry, Fourier transform IR spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy.     

Diffusion barrier properties of ionized metal plasma deposited tantalum nitride thin films between copper and silicon dioxide

Tantalum nitride thin films with different thickness are sputtered deposited on silicon dioxide in the Cu/TaN/SiO₂/Si multiplayer structure. Using resistivity analyses, X-ray diffraction, scanning electron microscopy and Rutherford backscattering spectroscopy, this work examines the impact of varying layer thickness on the crystal structure, resistivity, intermixing and reactions at the interfaces before and after annealing. The thinner the film thickness of TaN, the severe the reactions at the interface of Cu/TaN and consumed more conductive Cu. All the structures shown similar degradation process, and were found to be stable up to 500°C for 35 min. Accelerated grain growth and agglomeration were also observed after annealing temperature higher than 550°C at all Cu surfaces of the samples.     

Hydrogen plasma etching of silicon dioxide in a hollow cathode system

Chemical etching of various materials has been observed when hydrogen plasmas are used in material processing. In the case of the deposition of diamond films the preferential etching of sp² bonded carbon is considered to be of fundamental importance. A few papers have been published which have indicated that etching by hydrogen ions is different to that by hydrogen atoms. In this paper we describe the etching of silicon dioxide by hydrogen which was plasma-activated in a molybdenum-lined RF hollow cathode. The etch rate was seen to be thermally activated but decreased with increasing plasma power. The addition of a few percentage of helium increased the etch rate. The application of a − 50 V bias to the sample holder almost doubled the etch rate indicating the importance of ion bombardment for the chemical reaction. At high plasma powers and substrate temperatures in excess of 450 °C a thin molybdenum deposit was formed on the quartz samples.     

Strong visible electroluminescence from Ge- and Sn-implanted silicon dioxide layers

Silicon-based light emission is a key feature to make a real step into the world of integrated optical systems, laboratory-on-chip applications and high performance optical communication. One of the most promising approaches is ion implantation into thin SiO₂ films. In this paper, the electroluminescence (EL) properties of Sn-implanted SiO₂ layers are investigated and compared with those of Ge-implanted SiO₂ layers. Strong EL in the blue-violet spectral region with a power efficiency of 0.025% is extracted from Sn-implanted oxide layers. Similar to the case of Ge, the main emission at 3.2 eV is attributed to a radiative T₁→S₀ transition of an Sn-related oxygen deficiency center, the EL intensity increases linearly over several orders of magnitude and the stability reaches comparable values. In contrast to the case of Ge, a low-energy shoulder appears in the EL spectrum of Sn-implanted oxides. Finally, the suitability of Sn-implanted oxides for optoelectronic applications is discussed.     

Diffusion and solid solubility of chromium in silicon

The diffusion and the solid solubility of Cr in Si were investigated by radiochemical and electrical methods. At a diffusion temperature of 1250 °C the doping profile could be approximated by an error function. Assuming the error function holds also at lower temperatures the diffusion coefficient (D) was determined in the temperature range between 1100 and 1250 °C by the pn-junction method. An expression D=0.01 exp (?1.0 eV/kT) was found. The solid solubility increases exponentially from 2.2 · 10¹³/cm³ 900 °C to 2.5 · 10¹⁵/cm³ at 1280 °C.     

Diffusion of hydrogen in silicon: Diffusion-reaction model

A diffusion-reaction model is proposed as the mechanism for the diffusion of hydrogen in amorphous and crystalline silicon. In this model molecular hydrogen dissolves and diffuses interstitially in the open silicon structure. Dissolved molecular hydrogen reacts with silicon to form SiH groups. Equations derived for this model give profiles that fit well with experimental hydrogen profiles in amorphous silicon. Other experimental features, such as steps in hydrogen and deuterium concentrations at interfaces, exponential profiles at short times, and a decrease of the effective diffusion coefficient with increasing time, arise naturally in the diffusion-reaction model. Received: 20 February 2000 / Reviewed and accepted: 17 May 2000     

Investigation of silicon etching and silicon dioxide bubble formation during silicon oxidation in HCl-oxygen atmospheres

Experiments performed to characterize the silicon etching occurring at the silicon-silicon dioxide interface during thermal oxidation of silicon in dry oxygen-HCl atmospheres at 1100 °C are described. The etching was accompanied by bubble formation or bowing of the silicon dioxide film and ultimately by complete loss of adhesion of the thermally grown film. Pre-oxidation experiments with dry oxygen suggest that silicon etching is primarily dependent upon the chlorine concentration at the silicon-silicon dioxide interface, with oxide growth prior to HCl addition having a small effect.     

Diffusion of aluminum into silicon nitride films

Aluminum diffusion into silicon nitride films at temperatures in the range 450–530°C was studied by Auger electron spectroscopy in conjunction with depth profiling. The activation energy for the diffusion of aluminum and the diffusion coefficient were found to be 2.0±0.3 eV and (7.3±3.5) x 10^-3 cm² s^-1, respectively. The chemical effects in the KLL aluminum Auger spectra together with the compositional depth profiles suggest that the migration of aluminum is dominated by volume diffusion which involves the reaction of aluminum with oxygen.     

Effects of boron doping on the structural and optical properties of silicon nanocrystals in a silicon dioxide matrix

Doping of Si nanocrystals is an important topic in the emerging field of Si nanocrystals based all-Si tandem solar cells. Boron-doped Si nanocrystals embedded in a silicon dioxide matrix were realized by a co-sputtering process, followed by high temperature annealing. The x-ray photoelectron spectroscopy B 1s signal attributable to Si-B (187?eV) and/or B-B (188?eV) indicates that the boron may exist inside Si nanocrystals. A higher probability of effective boron doping was suggested for Si-rich oxide films with a low oxygen content, Then, structural and optical properties were characterized with a focus on the effects of the boron content on Si quantum dots. The results show that as the boron content increases, the nanocrystal size is slightly reduced and the Si crystallization is suppressed. The photoluminescence intensity of the films is decreased as the boron content increases. This is due to boron-induced defects and/or Auger processes induced by effective doping. These results can provide optimal conditions for future Si quantum dot based solar cells.     

Pinpoint and bulk electrochemical reduction of insulating silicon dioxide to silicon

Silicon dioxide (SiO(2)) is conventionally reduced to silicon by carbothermal reduction, in which the oxygen is removed by a heterogeneous-homogeneous reaction sequence at approximately 1,700 degrees C. Here we report pinpoint and bulk electrochemical methods for removing oxygen from solid SiO(2) in a molten CaCl(2) electrolyte at 850 degrees C. This approach involves a 'contacting electrode', in which a metal wire supplies electrons to a selected region of the insulating SiO(2). Bulk reduction of SiO(2) is possible by increasing the number of contacting points. The same method was also demonstrated with molten LiCl-KCl-CaCl(2) at 500 degrees C. The novelty and relative simplicity of this method might lead to new processes in silicon semiconductor technology, as well as in high-purity silicon production. The methodology may be applicable to electrochemical processing of a wide variety of insulating materials, provided that the electrolyte dissolves the appropriate constituent ion(s) of the material.     

Diffusion of nickel in silicon below 475 °C

Nickel films were deposited on (100) and (111) surfaces of single-crystal silicon and were then annealed. The conditions under which the nickel is deposited determine whether or not an NiSi compound forms on annealing. It is postulated that defects are necessary for the formation of an NiSi compound at annealing temperatures below at least 475 °C, although the presence of defects may not necessarily cause the formation of a silicide. For substrate temperatures below 70 °C, defects are created during the vapor deposition of nickel on silicon. These defects always result in the formation of nickel silicide when the sample is annealed at higher temperatures. When nickel is deposited on defect-free silicon at temperatures of about 250 °C no defects are generated and, although interdiffusion of nickel and silicon occurs, silicide formation does not take place upon subsequent annealing below 475 °C. The activation energies for the diffusion of nickel into (100) silicon and (111) silicon were determined.     

Diffusion characteristics of boron and phosphorus in polycrystalline silicon

Grain boundary diffusion of dopants (boron and phosphorous) in silicon is discussed. The appropriate models and equations are presented both for semi-infinite and for thin film boundary conditions. Data reported in the literature for both thick and thin samples have generally been analyzed using models appropriate to diffusion in a homogeneous semi-infinite substrate. These data were reanalyzed using the appropriate boundary conditions and a more realistic model of the inhomogeneous nature of diffusion in polycrystalline samples. It was shown that, even though the relation between diffusion depth and time may be the same from bulk and grain boundary models, the diffusion coefficients determined from assuming the homogeneous semi-infinite solid may be several orders of magnitude in error for the pre-exponential factor, and the activation energy will also be wrong. Grain boundary diffusion coefficients for arsenic, boron and phosphorus in silicon were determined.     

Diffusion of arsenic along dislocations in epitaxial silicon films

⁷⁶As isotope has been diffused along stair rod dislocations associated with stacking faults in Si epitaxial layers. Diffusions were performed in a two-temperature zone furnace using neutron-activated elemental As as the diffusion source. Uniform densities of dislocations (>10⁶/cm²), with axes intercepting the diffusion surface, were obtained following the growth of epitaxial layers on Si substrates whose surfaces were damaged by ion implantation to high dosages. Sections were taken by an anodizing and stripping technique and their activity was determined by liquid scintillation methods. The resulting profiles revealed a composite shape indicative of normal lattice diffusion near the surface together with a deeply penetrating portion which can be unambiguously associated with rapid diffusion along dislocations. For temperatures in the range 950°–1050°C, the intrinsic diffusivity of As in the lattice is given by D_i=(0.51^+0.19_?0.14cm²/sec) exp (3.53±0.03 eV/kT) and for the dislocations KδD_d=((9.4±1.6)x10^?8cm³/sec) exp (2.56±0.02 eV/kT)     

Stress-induced leakage currents in thin silicon dioxide films

High-field stress applied to thin oxide films leads to an increase of dielectric leakage in low-field conditions. In this paper, the dependencies of the high-field-induced leakage on stress polarity/time and stress/measurement sequences are analyzed. Generation of neutral electron traps during the stress is confirmed to be the main cause of the low-field leakage enhancement.