The effect of implantation of P ions on the photoluminescence of Si nanocrystals in SiO<Subscript>2</Subscript> layers

The effects of implanting 10¹³–10¹⁶ cm^?2 P ions and subsequent annealing at 600–1100°C on the photoluminescence of Si nanocrystals formed preliminarily in SiO₂ layers were studied. Quenching of the 780-nm luminescence band related to nanocrystals was observed immediately after the implantation of 10¹³ cm^?2 P ions. The recovery of emission from partially damaged nanocrystals was noticeable even after annealing at 600°C; however, heat treatments at 1000–1100°C were needed when the amorphization-threshold dose was exceeded. Intensification of luminescence was observed as a result of heat treatments of SiO₂ layers implanted with low doses of P ions; if the P content was higher than ～0.1 at. %, the recovery of luminescence was promoted. The former effect is attributed to impact crystallization of nanoprecipitates. The latter effect is attributed to the promotion of crystallization by an impurity and (along with the dose dependence of postannealing luminescence) is considered an indication that P atoms penetrate into the Si nanocrystals. Contrary to a number of estimations, introducing P does not result in the quenching of luminescence due to Auger recombination. This discrepancy is attributed to the interaction of charge carriers with the nuclei of donors.     

Charge spectroscopy of SiO<Subscript>2</Subscript> layers with embedded silicon nanocrystals modified by irradiation with high-energy ions

The method of charge deep-level transient spectroscopy (Q-DLTS) is used to study and compare the ejection of charge carriers from silicon nanocrystals (NCs) located in an ordered or random way in the SiO₂ matrix. It is shown that, in all cases, this ejection is a thermally activated process. The parameters of energy barriers characterizing the processes of ejection of charge carriers from the levels of nanocrystals in the layers NCs:SiO₂ before (random distribution) and after their modification by irradiation with high-energy ions (ordered distribution of nanocrystals) are determined. It is found that the activation energies for ejection of charge carriers from nanocrystals and the size of nanocrystals estimated from the difference between energies of two levels observed by the Q-DLTS method decrease as the ion fluence is increased. The density of nanocrystals observed by the Q-DLTS method decreases by approximately an order of magnitude as a result of irradiation with fluence of 10¹²–10¹³ cm⁻² in comparison with an initial unirradiated structure; this decrease is due to formation of conducting chains of nanocrystals in tracks.     

Excitation of Er<Superscript>3+</Superscript> ions in SiO<Subscript>2</Subscript> with Si nanocrystals

Probabilities of excitation of erbium ions via Coulomb interaction with carriers localized in silicon nanocrystals embedded in SiO₂, in recombination and intraband relaxation of these carriers, have been calculated.     

Atomic-force-microscopy visualization of Si nanocrystals in SiO<Subscript>2</Subscript> thermal oxide using selective etching

The topography and local hardness of the etched surfaces of layers of SiO₂ thermal oxide that contained Si nanocrystals in its bulk were studied using atomic-force microscopy. The Si nanocrystals were obtained by implanting Si⁺ ions into the oxide with subsequent high-temperature annealing. It is shown that the use of selective etching that removes the oxide material makes it possible to reveal Si nanocrystals that appear in the profile of etched surfaces in the form of nanohillocks with a height of up to 2–3 nm. These values are in satisfactory agreement with the average radius of Si nanocrystals in the SiO₂ oxide layer. Independent confirmation of the Si-nanocrystal observation was obtained by comparing the topography of etched surfaces with the local-hardness maps obtained for the same surfaces; in these maps, the hillocks appear as sites at the surface with a reduced hardness. The phase precipitation of implanted Si is also observed in the form of extended flat clusters oriented in the oxide bulk parallel to the oxide surface. The suggested method for revealing the Si nanocrystals and clusters incorporated into the oxide provides a convenient way to study the specific features of nucleation growth and spinodal decomposition in the Si solid solution in the SiO₂ oxide.     

Light-emitting Si nanostructures formed in SiO<Subscript>2</Subscript> on irradiation with swift heavy ions

SiO₂ layers containing implanted excess Si are irradiated with Xe ions with an energy of 130 MeV and doses of 3 × 10¹²–10¹⁴ cm⁻². In the samples irradiated with a dose of 3 × 10¹² cm⁻², ∼10¹² cm⁻² segregated clusters 3–4 nm in dimension are detected by transmission electron microscopy. With increasing dose, the dimensions and number of these clusters increase. In the photoluminescence spectrum, a 660- to 680-nm band is observed, with the intensity dependent on the dose. After passivation of the sample with hydrogen at 500°C, the band disappears, but a new ∼780-nm band typical of Si nanocrystals becomes evident. On the basis of the entire set of data, it is concluded that the 660- to 680-nm band is associated with imperfect Si nanocrystals grown in the tracks of Xe ions due to high ionization losses. The nonmonotonic dependence of the photoluminescence intensity on the dose is attributed to the difference between the diameters of tracks and the diameters of the displacements’ cascades responsible for defect formation.     

Properties of Ge nanocrystals formed by implantation of Ge<Superscript>+</Superscript> ions into SiO<Subscript>2</Subscript> films with subsequent annealing under hydrostatic pressure

The influence of hydrostatic compression on the implantation-induced synthesis of Ge nanocrystals in SiO₂ host was studied. It is found that high-temperature annealing under pressure leads to retardation of Ge diffusion in SiO₂. It is shown that unstressed Ge nanocrystals are formed as a result of conventional annealing (under atmospheric pressure). Annealing under pressure is accompanied by formation of hydrostatically stressed Ge nanocrystals. The stress in Ge nanocrystals was determined from optical-phonon frequencies in the Raman spectra. The shift of Raman resonance energy (E₁, E₁ + Δ₁) corresponds to the quantization of the ground-state energy for a two-dimensional exciton at the critical point M₁ of germanium. It is ascertained that a photoluminescence band peaked at 520 nm is observed only in the spectra of the films which contain stressed Ge nanocrystals.     

Formation of light-emitting nanostructures in layers of stoichiometric SiO<Subscript>2</Subscript> irradiated with swift heavy ions

Thermally grown SiO₂ layers have been irradiated with 700-MeV Bi ions with doses of (3–10) × 10¹² cm⁻². It is found that, even after a dose of 3 × 10¹² cm⁻², a photoluminescence band in the region of 600 nm appears. Its intensity levels off at a dose of ∼5 × 10¹² cm⁻². The nature of the emission centers is studied by the methods of infrared transmission, Raman scattering, X-ray photoelectron spectroscopy, ellipsometry, and the reaction to passivating low-temperature anneals. It is established that irradiation brings about a decrease in the number of Si-O bonds with a relevant increase in the Si-Si bonds. It is assumed that the photoluminescence is caused by nanostructures containing an excess Si and/or having a deficit of O. The reaction of reduction of SiO₂ proceeds in ion tracks due to high levels of ionization and heating within these tracks. The dose dependence is used to estimate the diameter of a track at 8–9 nm.     

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.     

Ordered arrays of Si nanocrystals in SiO<Subscript>2</Subscript>: Structural,optical, and electronic properties

It is found that irradiation of SiO₂ layers containing Si nanocrystals with high-energy heavy ions induces profound structural modifications—specifically, the formation of vertically ordered arrays of nanocrystals along the ion tracks. This effect results in substantial changes in the electrical properties (the conductivity and capacitance—voltage characteristics) and optical (photoluminescence) properties of the layers containing nanocrystals.     

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.     

Reduction in absorption in quartz/Si,quartz/Si/SiO<Subscript>2</Subscript>, and SiC/Si/SiO<Subscript>2</Subscript> structures on laser treatment

The effect of laser radiation on the optical absorption spectra of the quartz/Si, quartz/Si/SiO₂, and SiC/Si/SiO₂ systems is studied. The effects of modification of the transparency of the structures are clarified.     

Gas-phase etching of SiO<Subscript>2</Subscript> layers in an HF/C<Subscript>2</Subscript>H<Subscript>5</Subscript>OH mixture

This paper describes a technique for dry etching SiO₂ layers in MEMS technologies without the moving elements sticking. Etching the sacrificial SiO₂ in anhydrous HF (hydrofluoric acid in the gas phase) allows avoiding the subsequent complex operations of cleaning and drying, which are mandatory in the case of liquid etching. Using the HF/C₂H₅OH anhydrous mixture under low pressures makes it possible to prevent water condensation, which is due to etching in HF vapor, and allows one to employ gas-phase etching in surface MEMS technologies. The mechanisms and physicochemical processes taking place when etching thermal SiO₂ are discussed. The rate of etching thermal SiO₂ films is investigated at temperatures ranging from 30 to 50°С and under chamber pressures ranging from 10 to 20 kPa.     

Traps with near-midgap energies at the bonded Si/SiO<Subscript>2</Subscript> interface in silicon-on-insulator structures

Capture centers (traps) are studied in silicon-on-insulator (SOI) structures obtained by bonding and hydrogen-induced stratification. These centers are located at the Si/SiO₂ interface and in the bulk of the split-off Si layer. The parameters of the centers were determined using charge deep-level transient spectroscopy (Q-DLTS) with scanning over the rate window at fixed temperatures. Such a method allows one to study the traps near the Si midgap at temperatures near 295 K. It is shown that the density of traps with a continuous energy spectrum, which are located at the bonded Si/SiO₂ interface, decreases by more than four orders of magnitude at the mid-gap compared with the peak density observed at the activation energy E _a ≈0.2–0.3 eV. The capture centers are also found in the split-off Si layer of the fabricated SOI structures. Their activation energy at room temperature is E _a =0.53 eV, the capture cross section is 10^?19 cm², and the concentration is (0.7–1.7)×10¹³ cm^?3. It is assumed that these capture centers are related to deep bulk levels induced by electrically active impurities (defects) in the split-off Si layer close to the Si/SiO₂ interface.     

Charge state of luminescence centers in the Si-SiO<Subscript>2</Subscript> structures subjected to sequential implantation with silicon and carbon ions

Electroluminescence of Si-SiO₂ structures subjected to sequential implantation of silicon and carbon ions and to postimplantation annealing is studied. It is shown that two bands appear in the electroluminescence spectrum with energies of 2.7 and 4.3 eV as a result of ion implantation. After annealing, the band peaked at the energy of 3.2 eV appears in the spectrum; this band can be related to the formation silicon-carbide clusters. Charge characteristics of the structures under study are obtained. It is shown that the luminescence centers responsible for all bands are not charged.     

Modeling SiO<Subscript>2</Subscript> leakage currents caused by electrical overloads

A two-component model for the formation of leakage currents in SiO₂ in a strong electric field is proposed. In the model, agreement is found between the theoretical and experimental values of leakage currents in a wide range of electric field strength and the size of charge transferred.     

Crystallization behavior and ferroelectric properties of PbTiO<Subscript>3</Subscript>/Ba<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> sandwich thin film on Pt/Ti/SiO<Subscript>2</Subscript>/Si substrates

A PbTiO₃/Ba_0.85Sr_0.15TiO₃/PbTiO₃ (PT/BST15/PT) sandwich thin film has been prepared on Pt/Ti/SiO₂/Si substrates by an improved sol-gel technique. It is found that such films under rapid thermal annealing at 700°C crystallize more favorably with the addition of a PbTiO₃ layer. They possess a pure, perovskite-phase structure with a random orientation. The polarization-electric field (P-E) hysteresis loop and current-voltage (I-V) characteristic curves reveal that a PT/BST15/PT film exhibits good ferroelectricity at room temperature. However, no sharp peak, only a weak maximum, is observed in the curves of the dielectric constant versus temperature. The dielectric constant, loss tangent, leakage current density at 20 kV/cm, remnant polarization, and coercive field of the PT/BST15/PT film are 438, 0.025, 1.3 × 10⁻⁶ Acm⁻², 2.46 μCcm⁻², and 41 kVcm⁻¹, respectively, at 25°C and 10 kHz. The PT/BST15/PT film is a candidate material for high sensitivity elements for uncooled, infrared, focal plane arrays (UFPAs) to be used at near ambient temperature.     

Improvement in Performance of Carbon Nanotube Field-Effect Transistors on Patterned SiO<Subscript>2</Subscript>/Si Substrates

Horizontally aligned single-walled carbon nanotubes (SWNTs) were fabricated on patterned SiO₂/Si substrates with groove-and-terrace structures, which were obtained using electron-beam lithography and reactive ion etching. Scanning electron microscopy observation revealed that SWNTs were aligned in the direction parallel to the groove-and-terrace structures and were preferentially grown along the edges of terraces. Using aligned SWNTs as multichannels, carbon nanotube field-effect transistors (CNTFETs) were fabricated on the patterned SiO₂/Si substrates. This method will be promising to control the direction of SWNTs on SiO₂/Si substrates for fabrication of high-performance CNTFETs with high current outputs.     

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.     

Diffusion-Controlled growth of Ge nanocrystals in SiO<Subscript>2</Subscript> films under conditions of ion synthesis at high pressure

The growth of Ge nanocrystals in SiO₂ films is studied in relation to the dose of implanted Ge⁺ ions and the annealing temperature at a pressure of 12 kbar. It is established that the dependences of the nanocrystal dimensions on the content of Ge atoms and the annealing time are described by the corresponding root functions. The nanocrystal radius squared is an exponential function of the inverse temperature. The dependences correspond to the model of the diffusion-controlled mechanism of nanocrystal growth. From the temperature dependence of the nanocrystal dimensions, the diffusion coefficient of Ge in SiO₂ at a pressure of 12 kbar is determined: D = 1.1 × 10^–10 exp(–1.43/kT). An increase in the diffusion coefficient of Ge under pressure is attributed to the change in the activation volume of the formation and migration of point defects. Evidence in favor of the interstitial mechanism of the diffusion of Ge atoms to nanocrystal nuclei in SiO₂ is reported.     

The role of nitrogen in the formation of luminescent silicon nanoprecipitates during heat treatment of SiO2 layers implanted with Si+ ions

Twenty-five kiloelectronvolt Si⁺ ions with doses of (1–4)×10¹⁶ cm^?2 and 13-keV N⁺ ions with doses of (0.2–2)×10¹⁶ cm^?2 were implanted into SiO₂ layers, which were then annealed at 900–1100°C to form luminescent silicon nanoprecipitates. The effect of nitrogen on this process was deduced from the behavior of the photoluminescence spectra. It was found, for a certain ratio between the concentrations of implanted silicon and nitrogen, that the photoluminescence intensity increases significantly, and its peak shifts to shorter wavelengths. It is concluded that the number of precipitation nuclei increases owing to the interaction of nitrogen with excess silicon. Eventually, this results in an increase in the number of nanocrystals and in a decrease in their average sizes. In spite of introducing additional precipitation nuclei, the minimal concentrations of excess Si on the order of 10²¹ cm^?3 and heat treatments at temperatures higher than 1000°C were still required for the formation of nanocrystals.