High-temperature annealing of macroporous silicon in an inert-gas flow

Interest in the sintering of macroporous silicon is due to the possibility of purposefully modifying its structure. The annealing of macroporous structures in an atmosphere of Ar, instead of H₂, simplifies the requirements to equipment and safety engineering. The sintering of macroporous silicon as result of annealing at T = 1000–1280°C in a horizontal tube purged with high-purity gases: Ar or Ar + 3%H₂ is examined. Experiments were conducted with layers having deep cylindrical macropores produced by the electrochemical etching of samples with seed pits on their surface (ordered pores) and without seeds (random pores). The morphology of the porous structure and the changes in this structure upon annealing are studied with electron and optical microscopes. It is shown that, depending on the pore diameter and treatment temperature, the following transformation occurs: the pore surface is smoothed, pores are closed and a surface crust is formed, cylindrical pores are spheroidized and decompose into isolated hollow spheres, and a fine structure and faceting are formed. It is shown that the (111) planes have the minimal surface energy. It is found that the annealing of macroporous silicon in an inert gas leads to strong thermal etching, which is manifested in the fact that the porosity increases or even the porous layer at the sample edge fully disappears. Moreover, an oxide layer appears as a film, beads, or long filaments forming a glass wool upon annealing, especially at low temperatures. These features can be attributed to the presence of trace amounts of an oxidizing agent in the inert gas, which causes the formation of highly volatile SiO and products formed in the reaction involving this compound.     

Photoluminescence study of the structural evolution of amorphous and crystalline silicon nanoclusters during the thermal annealing of silicon suboxide films with different stoichiometry

The effect of the stoichiometry of thin silicon suboxide films on the processes of the formation and evolution of silicon nanoclusters during thermal annealing is studied by photoluminescence measurements. The samples are produced by the thermal sputtering of a SiO powder in an oxygen atmosphere, with the subsequent deposition of a 500 nm-thick SiO _x layer onto a Si substrate. The morphological properties and size of Si nanoclusters are explored by analyzing the photoluminescence spectra and kinetics. A comparative study of the luminescence properties of thin SiO _x layers with different stoichiometric parameters, x = 1.10, 1.29, 1.56, and 1.68, is accomplished for samples annealed at different temperatures in the range 850 to 1200°C. The dependences of the photoluminescence decay time on the annealing temperature, the stoichiometric parameter of the initial silicon suboxide film, and the nanocluster size are studied.     

Monte Carlo simulation of the effect of silicon monoxide on silicon-nanocluster formation

Silicon-nanocluster formation upon the annealing of SiO _x (1 ≤ x < 2) layers is studied with the use of the lattice Monte Carlo model. The simulation is performed taking into account an additional mechanism of silicon transport due to the diffusion of silicon-monoxide (SiO) particles. It is demonstrated that the presence of SiO in the system leads to the growth of a critical silicon-nanocluster nucleus and can increase the nanocluster growth rate. Silicon-nanocluster formation upon the annealing of SiO _x layers occurs only for the composition with x < 1.8. Upon the annealing of SiO _x layers on a silicon substrate, a region depleted of silicon nanoclusters is observed in the layer adjacent to the substrate, which allows the formation of silicon nanoclusters in the SiO₂ matrix at a certain distance from the Si/SiO₂ interface.     

Photoluminescence of amorphous and crystalline silicon nanoclusters in silicon nitride and oxide superlattices

The photoluminescence properties of silicon nitride and oxide superlattices fabricated by plasmaenhanced chemical vapor deposition are studied. In the structures annealed at a temperature of 1150°C, photoluminescence peaks at about 1.45 eV are recorded. The peaks are defined by exciton recombination in silicon nanocrystals formed upon annealing. Along with the 1.45-eV peaks, a number of peaks defined by recombination at defects at the interface between the nanocrystals and silicon-nitride matrix are detected. The structures annealed at 900°C exhibit a number of photoluminescence peaks in the range 1.3–2.0 eV. These peaks are defined by both the recombination at defects and exciton recombination in amorphous silicon nanoclusters formed at an annealing temperature of 900°C. The observed features of all of the photoluminescence spectra are confirmed by the nature of the photoluminescence kinetics.     

Hydrogen-induced splitting in silicon over a buried layer heavily doped with boron

Formation of interior hydrogen-passivated surfaces in hydrogen-implanted single-crystal Si containing a buried layer heavily doped with boron is investigated. With the use of the infrared absorption spectroscopy, it is shown that, upon annealing, the composition of hydrogen-containing defects in Si samples containing a buried heavily doped layer is the same as in Si samples that do not have such a layer. However, the presence of a heavily doped layer enhances the blistering and exfoliation of a thin silicon film from the Si sample, and the activation energies of the relevant processes change. Thus, the process of development of cavities in such layers changes upon thermal annealing. The depth at which hydrogen-passivated surfaces are formed corresponds to the projected range of H ions in Si, which also corresponds to the depth at which the B-doped layer is located. When a thin exfoliated film is transferred onto an insulator to form a silicon-on-insulator structure, the surface roughness of the film decreases by a factor of 2–5.     

Surface texture of single-crystal silicon oxidized under a thin V<Subscript>2</Subscript>O<Subscript>5</Subscript> layer

The process of surface texturing of single-crystal silicon oxidized under a V₂O₅ layer is studied. Intense silicon oxidation at the Si–V₂O₅ interface begins at a temperature of 903 K which is 200 K below than upon silicon thermal oxidation in an oxygen atmosphere. A silicon dioxide layer 30–50 nm thick with SiO₂ inclusions in silicon depth up to 400 nm is formed at the V₂O₅–Si interface. The diffusion coefficient of atomic oxygen through the silicon-dioxide layer at 903 K is determined (D ≥ 2 × 10^–15 cm² s^–1). A model of low-temperature silicon oxidation, based on atomic oxygen diffusion from V₂O₅ through the SiO₂ layer to silicon, and SiO _x precipitate formation in silicon is proposed. After removing the V₂O₅ and silicon-dioxide layers, texture is formed on the silicon surface, which intensely scatters light in the wavelength range of 300–550 nm and is important in the texturing of the front and rear surfaces of solar cells.     

Thin Amorphous Si/Si3N4-Based Light-Emitting Device Prepared With Low Thermal Budget

This letter reports for the first time on an electrically pumped silicon light-emitting device with a thin multilayer stacked amorphous silicon (alpha-Si, in thickness of 3-7 nm)/silicon nitride (~10 nm) structure. The observed photoluminescence (PL) is tunable from ~700 to ~670 nm, and intensity increases by decreasing the alpha-Si thickness. The PL intensity can be enhanced through postdeposition annealing at relatively low temperatures and a short annealing time (e.g., as optimized at 700degC/10min). Electroluminescence from devices that are built upon the proposed structure originates from electron-hole pair recombination, and the carrier injection mechanism is through Frenkel-Poole tunneling. Our proposed structure, being highly complimentary metal-oxide-semiconductor compatible, benefits from a low thermal budget process coupled with an accurate layer thickness control.     

Infrared spectroscopy of bonded silicon wafers

Infrared spectra of multiple frustrated total internal reflection and transmission for silicon wafers obtained by direct bonding in a wide temperature range (200–1100°C) are studied. Properties of the silicon oxide layer buried at the interface are investigated in relation to the annealing temperature. It is shown that the thickness of the SiO₂ layer increases from 4.5 to 6.0 nm as the annealing temperature is increased. An analysis of the optical-phonon frequencies showed that stresses in the SiO₂ relax as the annealing temperature is increased. A variation in the character of chemical bonds at the interface between silicon wafers bonded at a relatively low temperature (20–400°C) is studied in relation to the chemical treatment of the wafers’ surface prior to bonding. Models of the process of low-temperature bonding after various treatments for chemical activation of the surface are suggested.     

Ultra-Shallow junctions in silicon using amorphous and polycrystalline silicon solid diffusion sources

The inter-dependence of diffusion behavior and grain microstructure in amorphous silicon/polysilicon-on-single crystal silicon systems has been studied for rapid thermal and furnace annealing for P and BF₂ implants. It is found that the changes of microstructure during annealing play a major role in determining the diffusion profiles in the substrate as well as in the polysilicon layer. For P doping, a drive-in diffusion results in a much larger grain microstructure for as-deposited amorphous silicon than for as-deposited polysilicon, which leads to the formation of shallower junctions in the substrate for the first case. For B doping, there is little difference in the final microstructure and junction depth between the two cases. The P and B junctions formed in the substrate are found to be laterally very uniform in spite of expected doping inhomogeneities due to polysilicon grain boundaries both for as-deposited amorphous silicon diffusion sources and for as-deposited polysilicon diffusion sources.     

Study of silicon doped with zinc ions and annealed in oxygen

The results of studies of the surface layer of silicon and the formation of precipitates in Czochralski n-Si (100) samples implanted with ⁶⁴Zn⁺ ions with an energy of 50 keV and a dose of 5 × 10¹⁶ cm^–2 at room temperature and then oxidized at temperatures from 400 to 900°C are reported. The surface is visualized using an electron microscope, while visualization of the surface layer is conducted via profiling in depth by elemental mapping using Auger electron spectroscopy. The distribution of impurity ions in silicon is analyzed using a time-of-flight secondary-ion mass spectrometer. Using X-ray photoelectron spectroscopy, the chemical state of atoms of the silicon matrix and zinc and oxygen impurity atoms is studied, and the phase composition of the implanted and annealed samples is refined. After the implantation of zinc, two maxima of the zinc concentration, one at the wafer surface and the other at a depth of 70 nm, are observed. In this case, nanoparticles of the Zn metal phase and ZnO phase, about 10 nm in dimensions, are formed at the surface and in the surface layer. After annealing in oxygen, the ZnO · Zn₂SiO₄ and Zn · ZnO phases are detected near the surface and at a depth of 50 nm, respectively.     

A model of formation of fixed charge in thermal silicon dioxide

A quantitative model of formation of fixed charge (Q _f ) in silicon dioxide during thermal oxidation of silicon is developed. The value of Q _f is governed by the number of interstitial silicon atoms in the vicinity of the Si-SiO₂ interface; these atoms are formed as a result of the processes of their generation and recombination at the interface and also due to their diffusion to the depth of dioxide. The model makes it possible to describe a decrease in the fixed charge as the oxidation temperature is increased and in the case of annealing in neutral media for silicon dioxide on silicon with orientations (100) and (111) in a wide range of temperatures.     

A deep level transient spectroscopy study of rapid thermal annealed arsenic implanted silicon

Deep level traps are observed in silicon that has been implanted with high doses of arsenic and subsequently annealed by rapid thermal annealing. The doses studied create enough damage to form a surface amorphous layer. Annealing temperatures, implant fluence, and the presence of a surface amorphous layer contribute to the type of trap observed. These results show evidence for a clustering/declustering mechanism of arsenic in silicon during rapid thermal annealing.     

Electronic states on silicon surface after deposition and annealing of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The spectrum of the photoconductivity induced by the polarization field of charges at surface states and traps in the film bulk has been analyzed to determine the energy band diagram at the c-Si-SiO _x interface and the changes in the electronic states after the film annealing. It is found that the energy bands are bent at the Si-SiO _x interface and the Si surface is enriched in electrons. In equilibrium the photocurrent peak at 1.1 eV is due to the band-to-band transitions in the silicon part of the interface. Annealing shifts the peak to higher energies; this shift increases with an increase in the annealing temperature from 650 to 1000°C. This effect is accompanied by a decrease in the photocurrent at ≤1.1 eV and weakening of the band-edge photoluminescence near the Si surface. The changes revealed are explained by the formation of an oxide layer with Si nanoclusters at the Si-SiO _x interface upon annealing. This process is caused by oxygen diffusion from the SiO _x film, which occurs mainly via defects on the Si wafer surface. The photoconductivity spectrum of the samples charged by short-term application of a negative potential to silicon exhibits electronic transitions in the SiO _x film, both from the matrix electronic states and from the states of the defects and Si nanoclusters in the film.     

Monolithic 3-D silicon photonics

A monolithic CMOS compatible process has been developed to realize vertically integrated devices in silicon. The method involves the implantation of an oxygen into a patterned silicon substrate to form buried guiding structures. These buried devices are separated from a surface silicon layer by an intervening layer of silicon dioxide formed through the implantation process. Photolithography and etching is used to define devices on the surface silicon layer. The method has been utilized to realize the vertically coupled microdisk resonators and a variety of microresonator-based integrated optical elements. A new method for extraction of the unloaded Q of a cavity from its measured spectrum is also described.     

Optimisation of implantation conditions for the formation of buried SiO2 layers in silicon

The substrate temperature dependence of the IR transmission spectra of buried silicon dioxide layers formed by high dose ion implantation (~1017) was investigated for an ion dose ranging from 1017 to 1018 ions/cm2 at 250°C of 16O+ at 100 keV energy. The IR spectra indicate that the silicon/silicon-dioxide/silicon can be obtained after thermal annealing treatment of the implanted sample for 10 mm in hydrogen ambient at 1100°C.     

Formation mechanism of concave by dielectric breakdown on silicon carbide metal-oxide-semiconductor capacitor

Adjacent concaves are formed commonly on silicon carbide (SiC) MOS capacitor after time-dependent dielectric breakdown (TDDB). This paper describes the formation mechanism of the concave on the SiC MOS capacitor with aluminum gate electrode on thermally grown silicon dioxide gate dielectric by the dielectric breakdown. At the bottom of an approximately 450 nm-deep concave, a stack structure of the concave surface was found to be surface oxide/C-rich layer/Si-rich layer/SiC substrate. Some C-rich debris adhered on the surface of the concave. The concave surface was speculated to be formed by a sequence of the C-rich surface on the Si-rich surface, the debris adhered on the surface, and the oxide layer containing nitrogen and aluminum. Formation of the concave and its surface is explained based on the physical properties of SiC; (i) a peritectic decomposition of SiC to the solid phase carbon and the liquid phase solution containing silicon and carbon, (ii) a normal freezing process of the liquid phase solution, and (iii) a thermal decomposition on the concave surface to form a graphite layer.     

A permeable-base switching device using stacked layers of silicon, silicon dioxide, and silicon-rich silicon dioxide

A novel permeable-base device that is a type of solid-state analog to the vacuum tube has been-constructed using stacked layers of silicon, silicon dioxide, and silicon-rich silicon dioxide. The base or grid, which is formed from thin patterned polycrystalline silicon with degenerate doping, is separated from a single-crystal silicon substrate that acts as the collector or anode by a layer of silicon dioxide. The emitter or cathode is formed on top of the base using a stack of silicon dioxide, silicon-rich silicon dioxide, and degenerately doped polcrystalline silicon (known as an electron injector structure). Current flow from the emitter to the base and collector is modulated by the electric fields created in the silicon dioxide layers by the voltages applied to the various terminals. This unipolar device, which has only electrons carrying the current, is shown to be capable of operation over a wide range of voltages and gains depending on design.     

Specific features of erbium ion photoluminescence in structures with amorphous and crystalline silicon nanoclusters in silica matrix

Photoluminescence properties of the structures of amorphous and crystalline silicon nanoclusters with average sizes no larger than 4 nm in an erbium-doped silicon dioxide matrix were studied. It was found that the photoluminescence lifetime of Er³⁺ ions at a wavelength of 1.5 μm decreases from 5.7 to 2.0 ms and from 3.5 to 1.5 ms in samples with amorphous nanoclusters and with nanocrystals, respectively, as the Er³⁺ concentration increases from 10¹⁹ to 10²¹ cm^?3. The decrease in the erbium photoluminescence lifetime with the ion concentration is attributed to the effects of concentration-related quenching and residual implantation-induced defects. The difference between lifetimes for samples with amorphous and crystalline nanoclusters is interpreted as the effect of different probabilities of energy back transfer from Er³⁺ ions to the solid-state matrix in the structures under consideration.     

Radiation-thermal activation of silicon implanted in gallium arsenide

The layer density, density profile, and mobility of electrons in ²⁸Si-ion-doped layers of semiinsulating GaAs after radiation annealing with electron energy above and below the defect formation threshold and after thermal annealing in the temperature range T _a =590–830 °C are investigated. It is shown that for radiation annealing energy above the defect formation threshold ion-doped layers are formed with much lower annealing temperatures, and the degree of electrical activation of silicon in these layers is high and the density of electron mobility limiting defects is low. Fiz. Tekh. Poluprovodn. 33, 687–690 (June 1999)     

Effect of fast annealing on the electrical properties of SiO2/Si structures with thin layers of anodic silicon oxide

Effects of fast annealing on the properties of SiO₂/Si structures with thin, ～10 nm, layers of anodic silicon oxide formed on single-crystal Si substrates are studied in relation to the semiconducting properties of the silicon support and to the duration, temperature, and environment of thermal treatment. The optimal duration of high-temperature annealing of structures in an inert atmosphere for their application in the technology of nanosize MOS integrated circuits is determined.