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.     

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.     

Effect of tin on the processes of silicon-nanocrystal formation in amorphous SiO x thin-film matrices

The effect of tin on the processes of silicon-nanocrystal formation in amorphous silicon oxide (a-SiO _x , x ≈ 1.15) thin-film matrices is studied. It is established that the tin impurity accelerates the processes of crystallization of amorphous silicon. After heat treatment in an argon atmosphere, silicon crystallites embedded in the tin-containing silicon oxide (a-SiO _x Sn) matrix are smaller in size (6–9 nm) compared to crystallites in a-SiO _x (≥10 nm). It is shown that, upon annealing of a-SiO _x Sn at temperatures increased from 800 to 1100°C, the volume fraction of the crystalline phase increases from 20 to 80%. At the same time, in the samples free from tin, the silicon crystalline phase appears only upon annealing at 1000°C and 1100°C, and the volume fraction of the crystalline phase is 45 and 65%, respectively.     

Formation of silicon nanocrystals in multilayer nanoperiodic <Emphasis Type="Italic">a</Emphasis>-SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/insulator structures from the results of synchrotron investigations

The problem of the efficiency of the controllable formation of arrays of silicon nanoparticles is studied on the basis of detailed investigations of the electronic structure of multilayer nanoperiodic a-SiO _x /SiO₂, a-SiO _x /Аl₂О₃, and a-SiO _x /ZrO₂ compounds. Using synchrotron radiation and the X-ray absorption near edge structure (XANES) spectroscopy technique, a modification is revealed for the investigated structures under the effect of high-temperature annealing at the highest temperature of 1100°C; this modification is attributed to the formation of silicon nanocrystals in the layers of photoluminescent multilayer structures.     

Influence of irradiation with swift heavy ions on multilayer Si/SiO2 heterostructures

The influence of Xe ions with an energy of 167 MeV and a dose in the range 10¹²-3 × 10¹³ cm^?2 on heterostructures consisting of six pairs of Si/SiO₂ layers with the thicknesses ～8 and ～10 nm, correspondingly, is studied. As follows from electron microscopy data, the irradiation breaks down the integrity of the layers. At the same time, Raman studies give evidence for the enhancement of scattering in amorphous silicon. In addition, a yellow-orange band inherent to small-size Si clusters released from SiO₂ appears in the photoluminescence spectra. Annealing at 800°C recovers the SiO₂ network, whereas annealing at 1100°C brings about the appearance of a more intense photoluminescence peak at ～780 nm typical of Si nanocrystals. The 780-nm-peak intensity increases, as the irradiation dose is increased. It is thought that irradiation produces nuclei, which promote Si-nanocrystal formation upon subsequent annealing. The processes occur within the tracks due to strong heating because of ionization losses of the ions.     

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.     

Structural transformations and silicon nanocrystallite formation in SiOx films

The results of a comprehensive study by the methods of IR absorption, Raman scattering, photoluminescence (PL), and electron spin resonance (ESR) of SiO_x films prepared by thermal evaporation of SiO in a vacuum are presented. The nature of structural transformations occurring on annealing the films is determined. Annealing in the temperature range 300–600°C gives rise to a PL band at 650 nm, presumably related to structural defects in SiO_x film. Raising the annealing temperature further leads to healing of such defects and quenching of the PL band. Silicon precipitates pass from the amorphous to the crystalline state on being annealed at T _ann=1100°C, which gives rise to a new PL band at 730 nm. ESR spectra of P _b centers were recorded at the interface between randomly oriented silicon nanocrystallites and SiO₂.     

Silicon nanocrystal formation upon annealing of SiO2 layers implanted with Si ions

The formation of silicon nanocrystals in SiO₂ layers implanted with Si ions was investigated by Raman scattering, X-ray photoelectron spectroscopy, and photoluminescence. The excess Si concentration was varied between 3 and 14 at. %. It was found that Si clusters are formed immediately after implantation. As the temperature of the subsequent annealing was raised, the segregation of Si accompanied by the formation of Si-Si₄ bonds was enhanced but the scattering by clusters was reduced. This effect is attributed to the transformation of loosely packed clusters into compact, separate-phase nanoscale Si precipitates, with the Raman peak observed at 490 cm^?1 being related to surface scattering. The process of Si segregation was completed at 1000°C. Nevertheless, characteristic nanocrystal photoluminescence was observed only after annealing at 1100°C. Simultaneously, scattering in the range 495–520 cm^?1, typical of nanocrystals, appeared; however, the “surface-related” peak at 490 cm^?1 persisted. It is argued that nanocrystals are composed of an inside region and a surface layer, which is responsible for their increased formation temperature.     

Ion-beam synthesis of InSb nanocrystals in the buried SiO2 layer of a silicon-on-insulator structure

The ion-beam synthesis of InSb nanocrystals in the buried SiO₂ layer of a silicon-on-insulator structure is investigated. The distributions of In and Sb atoms after annealing at a temperature of T _a = 500–1100°C are studied. It is established that the redistribution of implanted atoms is unsteadily dependent on the annealing temperature. The formation of InSb nanocrystals occurs at Ta ≥ 800°C near the Si/SiO₂ interface and at a depth corresponding to the mean paths R _p . Analysis of the profiles of implanted atoms and of the structure and depth distribution of nanocrystals formed allows an inference regarding the two-stage character of formation of the InSb phase. In the initial stage, antimony precipitates are formed; further the precipitates serve as nuclei for indium and antimony to flow to them.     

Structural and Luminescent Properties of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>(Si) Nanocomposite Films

With a view to creating Si LEDs, the structural and luminescent properties of SiO _x N _y films containing Si nanocrystals in the SiO _x N _y matrix are studied experimentally. It is found that the film structure (nanocrystal size and concentration, the presence of an amorphous phase, etc.) and the spectrum and intensity of photoluminescence (PL) and electroluminescence (EL) are strongly dependent on the Si stoichiometric excess δ and annealing conditions. At δ≈ 10%, unannealed films are amorphous and contain Si clusters of size < 2 nm, as deduced from the TEM and microdiffraction data obtained. Annealing at 800–1000°C for 10–60 min produces Si crystals 3–5 nm in size with a concentration of ≈10¹⁸ cm^?3. The annealed films exhibit room-temperature PL and EL over the wavelength range 400–850 nm with intensity peaks located at 50–60 and 60–70 nm, respectively. The PL and EL spectra are found to be qualitatively similar. This suggests that both the PL and the EL should be associated with the formation of luminescent centers at nanocrystal–matrix interfaces and in boundary regions. However, the two phenomena should differ in the mechanism by which the centers are excited. With the EL, excitation should occur by impact processes due to carrier heating in high electric fields. It is found that as δ increases, so does the proportion of large amorphous Si clusters with a high density of dangling bonds. This enhances nonradiative recombination and suppresses luminescence.     

The influence of irradiation and subsequent annealing on Si nanocrystals formed in SiO2 layers

Luminescent Si nanocrystals formed in SiO₂ layers were irradiated with electrons and He⁺ ions with energies of 400 and 25–130 keV, respectively. The effects of irradiation and subsequent annealing at 600–1000°C were studied by the methods of photoluminescence and electron microscopy. After irradiation with low doses (～1 displacement per nanocrystal), it was found that photoluminescence of nanocrystals was quenched but the number of them increased simultaneously. After irradiation with high doses (～10³ displacements per nanocrystal), amorphization was observed, which is not characteristic of bulk Si. The observed phenomena are explained in terms of the generation of point defects and their trapping by Si-SiO₂ interfaces. Photoluminescence of nanocrystals is recovered at annealing temperatures below 800°C; however, an annealing temperature of about 1000°C is required to crystallize the precipitates. An enhancement of photoluminescence observed after annealing is explained by the fact that the intensities of photoluminescence originated from initial nanocrystals and from nanocrystals formed as a result irradiation are summed.     

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.     

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.     

Low‐Temperature Formation of Well‐Aligned Nanocrystalline Si/SiOx Composite Nanowires

Well‐aligned nanocrystalline (nc)‐Si/SiO_x composite nanowires have been deposited on various substrates at 120 °C using SiCl₄/H₂ in a hot‐filament chemical vapor deposition reactor. Structural and compositional analyses reveal that silicon nanocrystals are embedded in the amorphous SiO_x nanowires. The nc‐Si/SiO_x composite nanowires are transparent in the range 500–900 nm. Photoluminescence spectra of the nc‐Si/SiO_x composite nanowires have a broad emission band, ranging from 420 to 525 nm. Water vapor from the chamber wall plays a crucial role in the formation of the well‐aligned nanowires. A possible mechanism for the formation of the composite nanowires is suggested.     

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.     

Radiative recombination in Ge<Superscript>+</Superscript>-implanted SiO<Subscript>2</Subscript> films annealed under hydrostatic pressure

Photoluminescence (PL) spectra and PL excitation spectra were recorded at room temperature from SiO₂ films implanted with Ge⁺ ions and annealed at temperature T _a =450–1100°C under hydrostatic pressure P=12 kbar. The emergence of features in the violet and green bands of the PL and PL excitation spectra correlates with the formation of hydrostatically strained Ge nanocrystals. The shift of the PL bands to higher energies, which occurs as the annealing temperature is raised to T _a ≥800°C, can be attributed to a shift of the energy levels related to the radiative recombination centers, which is caused by the increasing deformation potential. The observed PL is accounted for by the enhanced probability of direct radiative transitions in Ge nanocrystals with an X-like conduction band.     

Kinetics of structural and phase transformations in thin SiO<Subscript>x</Subscript> films in the course of a rapid thermal annealing

Infrared spectroscopy and analysis of photoluminescence spectra have been used to study variations in the composition of the oxide phase in a SiO_x film and the precipitation of the Si phase in the course of a rapid thermal annealing for 1–40 s at temperatures of 500–1000°C. Kinetics of phase segregation has been observed for the first time at temperatures of 600–700°C: an increase in the amount of precipitated silicon as the annealing duration increases followed by an eventual leveling off. The phase separation is brought to completion in a time as short as 1 s at temperatures higher than 900°C. The diffusion coefficient is estimated in the context of a model of the diffusion-controlled formation of Si nanoparticles. The obtained values of the diffusion coefficient exceed, by five to ten orders of magnitude, those of the silicon diffusion coefficients in SiO₂ and Si and are comparable to the diffusion coefficients of the oxygen contained in these structures. It is assumed that oxygen mobility forms the basis for the mechanism of structural and phase transformations in the SiO_x layers and for the formation of Si nanoparticles in the course of annealing.     

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.     

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.     

Formation of photoluminescence centers during annealing of SiO2 layers implanted with Ge ions

Photoluminescence (PL), Raman scattering, and the Rutherford backscattering of α-particles were used to study the formation of the centers of radiative-recombination emission in the visible region of the spectrum on annealing of the SiO₂ layers implanted with Ge ions. It was found that the Ge-containing centers were formed in the as-implanted layers, whereas the stages of increase and decrease in the intensities of PL bands were observed following an increase in the annealing temperature to 800°C. The diffusion-related redistribution of Ge atoms was observed only when the annealing temperatures were as high as 1000°C and was accompanied by formation of Ge nanocrystals. However, this did not give rise to intense PL as distinct from the case of Si-enriched SiO₂ layers subjected to the same treatment. It is assumed that, prior to the onset of Ge diffusion, the formation of PL centers occurs via completion of direct bonds between the neighboring excess atoms, which gives rise to the dominant violet PL band (similar to the PL of O vacancies in SiO₂) and a low-intensity long-wavelength emission from various Ge-containing complexes. The subsequent formation of centers of PL with λ_m～570 nm as a result of anneals at temperatures below 800°C is explained by agglomeration of bonded Ge atoms with formation of compact nanocrystalline precipitates. The absence of intense PL following the high-temperature anneals is believed to be caused by irregularities in the interfaces between the formed Ge nanoc-rystals and the SiO₂ matrix.