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.     

SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layer formation during plasma sputtering of Si and SiO<Subscript>2</Subscript> targets

Deposition of SiO _x layers of variable composition onto silicon wafers was performed by co-sputtering of spaced Si and SiO₂ targets in argon plasma. Coordinate dependences of the thickness and refractive index of separately deposited Si and SiO₂ layers and the SiO _x layer grown during co-sputtering of targets were determined using optical techniques. It was shown that the SiO _x layer composition is not equal to a simple sum of thicknesses of separately deposited Si and SiO₂ layers. The coordinate dependences of the Si and SiO₂ layer thicknesses were calculated. To fit the calculated and experimental data, it is necessary to assume that no less than 10% of silicon is converted to dioxide during co-sputtering. A comparison of the coordinate dependences of the IR absorbance in SiO₂ and SiO _x layers with experimental ellipsometric data confirmed the presence of excess oxygen in the SiO _x layer. Taking into account such partial oxidation of sputtered silicon, composition isolines in the substrate plane were calculated. After annealing of the SiO _x layer at 1200°C, photoluminescence was observed in a wafer area predicted by calculations, which was caused by the formation of quantum-size Si nanocrystallites. The photoluminescence intensity was maximum at x = 1.78 ± 0.3, which is close to the composition optimum for ion-beam synthesis of nanocrystals.     

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.     

Variation in the Optical Properties of the SiC-SiO<Subscript>2</Subscript> Composite Antireflection Layer in Crystalline Silicon Solar Cells by Annealing

This study showed the effects of annealing on a sol–gel-derived SiC-SiO₂ composite antireflection (AR) layer and investigated the optical and photovoltaic properties of crystalline silicon (Si) solar cells. The SiC-SiO₂ composite AR coating showed a considerable decrease in reflectance from 7.18% to 3.23% at varying annealing temperatures of 450–800°C. The refractive indices of the SiC-SiO₂ composite AR layer were tuned from 2.06 to 2.45 with the increase in annealing temperature. The analysis of the current density–voltage characteristics indicated that the energy conversion efficiencies of the fabricated Si solar cells gradually increased from 16.99% to 17.73% with increasing annealing temperatures of 450–800°C. The annealing of the SiC-SiO₂ composite AR layer in Si solar cells was crucial to improving the optical, morphological, and photovoltaic properties.     

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO _x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO _x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO₂ glasses and films slightly deficient in oxygen.     

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.     

Thermal evolution of the morphology, structure, and optical properties of multilayer nanoperiodic systems produced by the vacuum evaporation of SiO and SiO2

The alternate vacuum evaporation of SiO and SiO₂ from separate sources is used to produce amorphous a-SiO _x /SiO₂ multilayer nanoperiodic structures with periods of 5–10 nm and a number of layers of up to 64. The effect of annealing at temperatures T _a = 500–1100°C on the structural and optical properties of the nanostructures is studied. The results of transmission electron microscopy of the samples annealed at 1100°C indicate the annealing-induced formation of vertically ordered quasiperiodic arrays of Si nanocrystals, whose dimensions are comparable to the a-SiO _x -layer thickness in the initial nanostructures. The nanostructures annealed at 1100°C exhibit size-dependent photoluminescence in the wavelength range 750–830 nm corresponding to Si nanocrystals. The data on infrared absorption and Raman scattering show that the thermal evolution of structural and phase state of the SiO _x layers with increasing annealing temperature proceeds through the formation of amorphous Si nanoinclusions with the subsequent formation and growth of Si nanocrystals.     

Degradation pattern of thin HfO2 films on Si(100) under ultrahigh-vacuum annealing: An investigation by x-ray photoelectron spectroscopy and low-energy ion scattering

The evolution of microstructure and phase structure of ultrathin HfO₂ films on Si(100) under ultrahigh-vacuum annealing is investigated in situ by x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS). The onset temperature of degradation is found to depend on film thickness. It is established that, for HfO₂ (4 nm)/SiO₂ (1 nm)/Si(100) specimens, 5-min annealing at about 900°C causes silicon (LEIS evidence) to appear on the surface, the silicon being uncombined with oxygen or the metal (XPS evidence). A longer annealing at the same temperature produces HfSi_x; annealing at 950°C converts the entire HfO₂ film into polycrystalline silicide whose grains are partly oriented as the Si substrate. With respect to annealing in a low-oxygen environment, the experimental results support a model whereby the degradation of an ultrathin HfO₂ film starts with the formation of nanopores by clustering of oxygen vacancies, whose density increases sharply due to partial desorption of oxygen; HfO_x with x < 2 then forms in the vicinity of vacancy clusters. It is concluded that the formation of hafnium silicide, the end product of HfO₂ degradation, starts in Si surface areas at the bottom of nanopores.     

Annealing-induced evolution of optical properties of the multilayered nanoperiodic SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/ZrO<Subscript>2</Subscript> system containing Si nanoclusters

The photoluminescence, infrared absorption, and Raman spectra of amorphous multilayered nanoperiodic a-SiO _x /ZrO₂ structures produced by vacuum evaporation and then annealed at different temperatures (500–1100°C) are studied. It is established that the evolution of the optical properties with increasing annealing temperature is controlled by sequential transformation of Si clusters formed in the SiO _x layers from nonphase inclusions to amorphous clusters and then to nanocrystals. The finally formed nanocrystals are limited in sizes by the thickness of the initial SiO _x layers and by chemical reactions with ZrO₂.     

Crystallization of Amorphous Si<Subscript>0.6</Subscript>Ge<Subscript>0.4</Subscript> Nanoparticles Embedded in SiO<Subscript>2</Subscript>: Crystallinity Versus Compositional Stability

Si_0.6Ge_0.4 nanocrystals, of diameter <5 nm, embedded in SiO₂ in the form of single layers (2.1 × 10¹² nanoparticles cm^–2) and five-period multilayers (above 10¹³ nanoparticles cm^–2) have been fabricated using a low-thermal-budget process consisting of deposition by low-pressure chemical vapor deposition and crystallization by rapid thermal annealing at several temperatures and for different times. The crystallization process was monitored by Raman spectroscopy and transmission electron microscopy. The loss of integrity and compositional changes of the nanoparticles during the annealing process were characterized by Rutherford backscattering spectrometry. During the annealing process, crystallization and Ge out-diffusion have been observed to compete with each other. Annealing of samples with nanoparticles of 4.6 nm diameter at low temperature (750°C) yields poor crystallization of the nanoparticles and causes the Ge to leave them by a pure diffusive mechanism, thus destroying their integrity. At higher temperatures (≥800°C), crystallization takes place in a short period of time (<30 s) and diffusion from the crystallized material is initially hindered. For samples with nanoparticles of 3.3 nm diameter, partial crystallization is detected at 800°C and 900°C and the crystalline quality is improved in both cases as the annealing time increases. Also, the detection capabilities of the Raman spectroscopy system for the detection of a certain density of SiGe nanocrystals of given diameter and composition have been explored and the lower limit estimated.     

Thermal Stability Study of Cu(MoN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) Seed Layer on Barrierless Si

This study reports the good thermal stability of a sputtered Cu(MoN _x ) seed layer on a barrierless Si substrate. A Cu film with a small amount of MoN _x was deposited by reactive co-sputtering of Cu and Mo in an Ar/N₂ gas mixture. After annealing at 560°C for 1 h, no copper silicide formation was observed at the interface of Cu and Si. Leakage current and resistivity evaluations reveal the good thermal reliability of Cu with a dilute amount of MoN _x at temperatures up to 560°C, suggesting its potential application in advanced barrierless metallization. The thermal performance of Cu(MoN _x ) as a seed layer was evaluated when pure Cu is deposited on top. X-ray diffraction, focused ion beam microscopy, and transmission electron microscopy results confirm the presence of an ∼10-nm-thick reaction layer formed at the seed layer/Si interface after annealing at 630°C for 1 h. Although the exact composition and structure of this reaction layer could not be unambiguously identified due to trace amounts of Mo and N, this reaction layer protects Cu from a detrimental reaction with Si. The Cu(MoN _x ) seed layer is thus considered to act as a diffusion buffer with stability up to 630°C for the barrierless Si scheme. An electrical resistivity of 2.5 μΩ cm was obtained for the Cu/Cu(MoN _x ) scheme after annealing at 630°C.     

The effect of O<Subscript>2</Subscript> ambient annealing on the microstructure of Cu(Mg) in the form of a Cu(Mg)/SiO<Subscript>2</Subscript>/Si multilayer

The effect of annealing in an O₂ ambient on Cu(Mg)/SiO₂/Si multilayer films was investigated. As-deposited Cu(Mg)/SiO₂/Si multilayer samples with film thicknesses in the 1,000–3,000 ? range were annealed for 30 min in oxygen ambients at pressures ranging from vacuum to 100 mtorr. The results showed that annealing in an 8-mtorr O₂ ambient significantly decreased the electrical resistivity of a 1,000 ? sample from 10.5 μΩ-cm to 3.7 μΩ-cm. Annealing in the O₂ ambient enhanced Mg diffusion to the surface in comparison to vacuum annealing. Furthermore, O₂ ambient annealing leads to excessive grain growth. However, the effect of O₂ ambient annealing on resistivity is less when the thickness of the film increases.     

Modeling Si nanoprecipitate formation in SiO2 layers with excess Si atoms

Computer simulations based on the Monte Carlo method are used to analyze processes leading to the formation of luminescence centers in SiO₂ implanted with Si ions. The simulations, which take place in a two-dimensional space, mimic the growth of silicon nanoprecipitates in layers containing several at.% of excess silicon. It is assumed that percolation clusters made up of neighboring Si atoms form first. As the annealing temperature increases, these clusters grow and compactify into nano-sized inclusions of a well-defined phase. It is shown that a dose dependence arises from an abrupt enhancement of the probability of forming direct Si-Si bonds when the concentration of silicon exceeds ∼1 at. %. Under these conditions, percolation chains and clusters form even before annealing begins. The effect of the temperature of subsequent anneals up to 900 °C is modeled via the well-known temperature dependence of Si diffusion in SiO₂. It is assumed that annealing at moderate temperatures increases the mobility of Si atoms, thereby facilitating percolation and development of clusters due to an increase in the interaction radius. Intrinsic diffusion processes that occur at high temperatures transform branching clusters into nanoprecipitates with well-defined phase boundaries. The dose and temperature intervals for the formation of precipitates obtained from these simulations are in agreement with the experimental intervals of dose and temperatures corresponding to the appearance of and changes in luminescence. Fiz. Tekh. Poluprovodn. 33, 389–394 (April 1999)     

Annealing-induced evolution of the structural and morphological properties of a multilayer nanoperiodic SiO x /ZrO2 system containing Si nanoclusters

The structural and morphological properties of nanoperiodic structures produced by the alternate vacuum evaporation of SiO and ZrO₂ followed by annealing at temperatures of 500–1100°C are studied by the transmission electron microscopy of a transverse cross section. Upon annealing at temperatures below 700°C, the layers are amorphous. Upon annealing at 900°C and 1000°C, nanocrystals separated by twinned boundaries or amorphous regions are formed in the ZrO₂ layers. The formation of Si nanocrystals in the SiO _x layers occurs upon annealing at 1000°C and 1100°C. At 1100°C, because of the reaction between SiO _x and ZrO₂, spherical Si _x Zr _y O _z -type nanocrystals are formed in place of the ZrO₂ layers; the nanocrystal diameters exceed the initial layer thickness. The annealing-induced structural evolution is consistent with the previously considered behavior of the optical and luminescence properties of the system.     

Enhanced thermal stability of a sputtered titanium-nitride film as a diffusion barrier for capacitor-bottom electrodes

The effect of a thin RuO_x layer formed on the Ru/TiN/doped poly-Si/Si stack structure was compared with that on the RuO_x/TiN/doped poly-Si/Si stack structure over the post-deposition annealing temperature ranges of 450–600°C. The Ru/TiN/poly-Si/Si contact system exhibited linear behavior at forward bias with a small increase in the total resistance up to 600°C. The RuO_x/TiN/poly-Si/Si contact system exhibited nonlinear characteristics under forward bias at 450°C, which is attributed to no formation of a thin RuO_x layer at the RuO_x surface and porous-amorphous microstructure. In the former case, the addition of oxygen at the surface layer of the Ru film by pre-annealing leads to the formation of a thin RuO_x layer and chemically strong Ru-O bonds. This results from the retardation of oxygen diffusion caused by the discontinuity of diffusion paths. In particular, the RuO_x layer in a nonstoichiometric state is changed to the RuO₂-crystalline phase in a stoichiometric state after post-deposition annealing; this phase can act as an oxygen-capture layer. Therefore, it appears that the electrical properties of the Ru/TiN/poly-Si/Si contact system are better than those of the RuO_x/TiN/poly-Si/Si contact system.     

Structural,compositional and electrical characterization of Si-rich SiOx layers suitable for application in light sensors

Metal-Oxide-Silicon (MOS) structures containing silicon nanoparticles (SiNPs) in three different gate dielectrics, single SiO_x layer (c-Si/SiNPs-SiO_x), two-region (c-Si/thermal SiO_x/SiNPs-SiO_x) or three-region (c-Si/thermal SiO₂/SiNPs-SiO_x/SiO₂) oxides, were prepared on n-type (100) c-Si wafers. The silicon nanoparticles were grown by a high temperature furnace annealing of sub-stoichiometric SiO_x films (x=1.15) prepared by thermal vacuum evaporation technique. Annealing in N₂ at 700 or 1000 °C leads to formation of amorphous or crystalline SiNPs in a SiO_x amorphous matrix with x=1.8 or 2.0, respectively. The three-region gate dielectric (thermal SiO₂/SiNPs-SiO₂/SiO₂) was prepared by a two-step annealing of c-Si/thermal SiO₂/SiO_x structures at 1000 °C . The first annealing step was carried out in an oxidizing atmosphere while the second one was performed in N₂. Cross-sectional Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy have proven both the nanoparticle growth and the formation of a three region gate dielectric. Annealed MOS structures with semitransparent aluminum top electrodes were characterized electrically by current/capacitance–voltage measurements in dark and under light illumination. A strong variation of the current at negative gate voltages on the light intensity has been observed in the control and annealed at 700 °C c-Si/SiNPs-SiO_x/Al structures. The obtained results indicate that MOS structures with SiO_1.15 gate dielectric have potential for application in light sensors in the NIR–Visible Light–UV range.     

Effect of chemical treatment on photoluminescence spectra of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layers with built-in Si nanocrystals

The effect of chemical treatment in saturated vapors of ammonia and acetone on the spectral composition and intensity of photoluminescence in porous SiO _x films containing Si nanocrystals (nc-Si) is studied. The porosity of the SiO _x films is provided by oblique vacuum deposition of thermally evaporated silicon or silicon monoxide on polished silicon substrates. The kinetics of adsorption of the vapors is monitored by variations in the frequency of a quartz oscillator on which the films to be studied are deposited. As a result of chemical treatment followed by high-temperature annealing of the SiO _x films at the temperature 950°C, a new band, absent from the as-prepared films, appears in the photoluminescence spectrum at shorter wavelengths. The peak position and intensity of the band depend, correspondingly, on the composition of the film and on the time duration of the treatment. It is found that the new photoluminescence band is quenched upon exposure to laser radiation at the wavelength 488 nm. The quenching is more pronounced at the band peak. The possibility of controlling the characteristics of photoluminescence of the porous structures by chemical treatment is shown.     

Deposition and characterization of Pb TiO3 thin films on silicon wafers using metalorganic sources

Ferroelectric PbTiO₃ thin films were deposited on bare silicon and Pt/SiO₂/Si substrates by metalorganic chemical vapor deposition in a temperature range from 270 to 550°C. The deposition of a single phase PbTiO₃ thin film did not occur on bare silicon substrates. Instead a double layer of lead-silicate and PbTiO₃ was formed owing to a serious diffusion of lead and oxygen ions into silicon substrates. But on Pt/SiO₂/Si substrates, a single phase PbTiO₃ oriented parallel to a-and c-axis was grown at a substrate temperature as low as 350°C even without a high temperature post-annealing. To get an optimal film, a precise control of input gas composition and also a deposition in a low temperature range from 350 to 400°C are necessary.     

Physical model and results of numerical simulation of the degradation of a Si/SiO2 structure as a result of annealing in vacuum

A theoretical model is developed for the evolution of P _b-centers at a Si/SiO₂ boundary during annealing in vacuum. The model takes into account diffusion of atomic and molecular hydrogen and the reactions between the hydrogen and these centers at the boundary. The reaction constants are calculated in the diffusion approximation. The results of these calculations are found to agree with experiment in the temperature range 480°–800 °C and oxide thickness range 200–1024 Å for the (111) and (100) facets of silicon.     

Raman studies of silicon nanocrystals embedded in silicon suboxide layers

Raman spectroscopy is used for the study of SiO _x (x ≈ 1) layers subjected to thermal annealing at the temperatures from 950 to 1200°C to form Si nanocrystals inside the layers. From comparison of the experimental data with the model of spatial confinement of phonons, the volume fractions of the crystalline and amorphous Si phases in the layers are determined. It is established that, as the annealing temperature is increased, the average dimensions of Si nanocrystals increase from 4 to 6.5 nm. This is attributed to the coarsening of nanocrystals due to crystallization of the amorphous Si phase and to the processes of coalescence of neighboring nanocrystals at the highest temperatures of annealing.