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.     

The formation of silicon nanocrystals in SiO<Subscript>2</Subscript> layers by the implantation of Si ions with intermediate heat treatments

The effect of heat treatments at 1100°C on an ion-beam synthesis of Si nanocrystals in SiO₂ layers is studied. The ion-implanted samples are subjected either to a single heat treatment after the total ion dose (10¹⁷ cm^?2 has been implanted, two heat treatments (a heat treatment after the ion implantation of each half of the total dose), or three heat treatments (a heat treatment after each third of the dose). The total duration of the heat treatments is maintained at 2 h. It is found that the intermediate heat treatments lead to a shift of the Raman spectrum of the nanocrystals to longer wavelengths and to a shift of the photoluminescence spectrum to shorter wavelengths. Study using electron microscopy shows that the size of the nanoprecipitates decreases, which is accompanied by the disappearance of the characteristic features of crystallinity; however, the features of photoluminescence remain characteristic of the nanocrystals. The experimental data obtained are accounted for by a preferential drain of Si atoms to newly formed clusters, which is consistent with the results of a corresponding numerical simulation. It is believed that small nanocrystals make the main contribution to photoluminescence, whereas the Raman scattering and electron microscopy are more sensitive to larger nanocrystals.     

Effect of ion dose and annealing mode on photoluminescence from SiO2 implanted with Si ions

This paper discusses the photoluminescence spectra of 500-nm-thick layers of SiO₂ implanted with Si ions at doses of 1.6×10¹⁶, 4×10¹⁶, and 1.6×10¹⁷ cm⁻² and then annealed in the steady-state region (30 min) and pulsed regime (1 s and 20 ms). Structural changes were monitored by high-resolution electron microscopy and Raman scattering. It was found that when the ion dose was decreased from 4×10¹⁶ cm⁻² to 1.6×10¹⁶ cm⁻², generation of centers that luminesce weakly in the visible ceased. Moreover, subsequent anneals no longer led to the formation of silicon nanocrystallites or centers that luminesce strongly in the infrared. Annealing after heavy ion doses affected the photoluminescence spectrum in the following ways, depending on the anneal temperature: growth (up to ∼700 °C), quenching (at 800–900 °C), and the appearance of a very intense photoluminescence band near 820 nm (at >900 °C). The last stage corresponds to the appearance of Si nanocrystallites. The dose dependence is explained by a loss of stability brought on by segregation of Si from SiO₂ and interactions between the excess Si atoms, which form percolation clusters. At low heating levels, the distinctive features of the anneals originate predominantly with the percolation Si clusters; above ∼700 °C these clusters are converted into amorphous Si-phase nanoprecipitates, which emit no photoluminescence. At temperatures above 900 °C the Si nanocrystallites that form emit in a strong luminescence band because of the quantum-well effect. The difference between the rates of percolation and conversion of the clusters into nanoprecipitates allows the precipitation of Si to be controlled by combinations of these annealings. Fiz. Tekh. Poluprovodn. 32, 1371–1377 (November 1998)     

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.     

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.     

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.     

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.     

Raman and infrared spectroscopy of GaN nanocrystals grown by chloride-hydride vapor-phase epitaxy on oxidized silicon

Raman and infrared spectroscopy were applied to study nanocrystalline GaN films grown by chloride-hydride vapor-phase epitaxy on SiO₂/Si(111) substrates at T=520°C. It was ascertained that GaN nanocrystals are formed on the oxidized silicon surface at a rate of 10^?2 nm/s. It was shown that the peaks in the Raman spectra E₂(high)=566 cm^?1 and A₁(LO)=730 cm^?1 correspond to the elastically strained GaN wurtzite structure. It was detected that a peak related to E₁(TO)=558 cm^?1 arises in the infrared spectra, which shows that elastic stresses in the nanocrystals are insignificant.     

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.     

Effect of boron ion implantation and subsequent anneals on the properties of Si nanocrystals

To study the effect of implantation of 10¹³–10¹⁶ cm^?2 of boron ions and subsequent steady-state thermal or pulsed (20 ns) laser anneals on the properties of Si nanocrystals in SiO₂, methods of photoluminescence and Raman scattering are used. Implantation of B ions quenched the photoluminescence caused by dimensional quantization. A comparison with the effect of other ions shows that an increase in the mass of incident particles leads to an increase in the contribution of elastic losses to the photoluminescence quenching. This circumstance is accounted for by the binding of the generated defects into complexes that are not the center of nonradiative recombination. Our studies confirmed the promotion of crystallization of nanoprecipitates as a result of the introduction of an impurity and also revealed special features related to the small size of boron atoms. It is shown that the postimplantation laser-induced anneals are efficient methods for recovering photoluminescence; this efficiency is caused by the possible short-term melting of nanocrystals. Notwithstanding the evidence indicating that boron enters the nanocrystals, there is no indication that free holes appear. It is believed that this phenomenon is caused by the fact that the depth of impurity levels is larger in nanocrystals.     

Characteristic photoluminescence band in Si-implanted SiO2 grown on Si wafer

A possible mechanism for the photoemission from Si nanocrystals in an amorphous SiO₂ matrix fabricated by ion implantation is reported. We have measured the implantation dose dependence as well as the oxidation effect of the photoluminescence behavior of Si nanocrystals in SiO₂ layers fabricated by ion implantation and a subsequent annealing step. After annealing, a photoluminescence band, peaking just below the 1.7 eV was observed. The peak energy of the photoluminescence was found to be affected by the dose of implanted Si ions, but to be independent of annealing time and excitation photon energy. We also present experimental results of an oxidation induced continuous peak energy shift of the photoluminescence peak, up to around 1.8 eV. This peak energy, however, was found to return to its previous position with re-annealing. These results indicate that, whilst the excitation photons are absorbed by Si nanocrystals, the emission is not simply due to electron–hole recombination inside the Si nanocrystals, but is related to the presence of defects, most likely located at the interface between the Si nanocrystals and the SiO₂, for which the characteristic energy levels are affected by cluster–cluster interactions or the roughness of the interface.     

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.     

Photoluminescence of Si3N4 films implanted with Ge+ and Ar+ ions

The room-temperature photoluminescence emission and excitation spectra of Si₃N₄ films implanted with Ge⁺ and Ar⁺ ions were investigated as a function of the ion dose and temperature of subsequent annealing. It was established that the implantation of bond-forming Ge atoms during annealing right up to temperature T _a=1000 °C stimulates the formation of centers emitting in the green and violet regions of the spectrum. Implantation of inert Ar⁺ ions introduces predominantly nonradiative defect centers. Comparative analysis of the photoluminescence spectra, Rutherford backscattering data, and Raman scattering spectra shows that the radiative recombination is due not to quantum-well effects in Ge nanocrystals but rather recombination at the defects ≡Si-Si≡, ≡Si-Ge≡, and ≡Ge-Ge≡. Fiz. Tekh. Poluprovodn. 33, 559–566 (May 1999)     

Effects of nitrogen annealing on electron scatterings in SiSiO2 interface

The effective mobility of electrons at Si (100) surfaces was measured as a function of electron density N_s = 5 × 10¹¹?1 × 10¹³ cm^?2 at 4.2K for samples with and without annealing (10 min–2 hr) in nitrogen gas at 1000°C after wet thermal oxidation. A great part of the scattering by Coulomb and short-range potentials was reduced by a short (～10 min) anneal time, although the subsequent annealing resulted in a slight increase in the number of the scatterers. On the other hand, scattering by a surface roughness potential was reduced with increase in the anneal time. These scattering effects associated with N₂ annealing are discussed.     

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.     

Photoluminescence of nanocrystalline silicon films formed by pulsed laser-assisted deposition with the introduction of carbon

The effect of carbon on the photoluminescent properties of films consisting of quantum-dimensional Si nanocrystals in the SiO_x (x → 2) matrix is studied. The spectra of time-resolved photoluminescence in the photon-energy range of 1.4–3.2 eV and the infrared-absorption spectra in the wave-number range of 650–1500 cm^?1 were measured. It is established that the introduction of carbon in the presence of oxygen in the course of the pulsed laser-assisted deposition of the films brings about the white-blue emission spectrum and also an increase in the intensity and stability of photoluminescence. The effect of carbon on the photoluminescent properties of the films is related to the formation of the SiO₂ barrier phase instead of SiO_x (1 < x < 2), saturation of silicon dangling bonds at the surface of Si nanocrystals with larger sizes, and mechanical strengthening of Si nanocrystals with smaller sizes.     

Interaction of fullerene with single-crystal silicon

The optical properties of layers of C₆₀ fullerene on a silicon substrate are studied before and after a reducing annealing at 900–1050°;C in a hydrogen atmosphere in order to detect the formation of silicon-carbide clusters. It is shown, on the basis of Raman scattering, infrared absorption, time-resolved photoluminescence spectra, and ellipsometric measurements, that the SiC clusters are not detected at the accuracy of the methods used. After annealing, the layer is in the form of a porous hydrogen-rich film of disordered graphite, possibly with a small amount of fullerene molecules.     

Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N₂ or H₂ as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N₂ carrier gas show two additional broad peaks, which are ascribed to carbon sp² clusters related modes, spreading in the frequency range 1300–1600 cm^?1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ～520 cm^?1, considered as some defects related local vibrations, appears in the samples grown with N₂ as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp² clusters, ranges from 16.5 to 19.4 Å. However, in all the samples grown with H₂ as carrier gas, the bands related to carbon sp² clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200 cm^?1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900 °C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N₂ carrier gas, were only weakened.     

Low-temperature relaxation of elastic stresses in SiGe/Si heterostructures irradiated with Ge<Superscript>+</Superscript> ions

Pseudomorphic Si_0.76Ge_0.24/Si heterostructures grown by molecular-beam epitaxy were irradiated with 350-keV Ge⁺ ions at a temperature of 400°C so that the peak of the ions’ energy losses was located within the silicon substrate (deeper than the SiGe-Si interface). The effect of ion implantation on the relaxation of elastic stresses and the defect structure formed as a result of postimplantation annealing is studied. It is found that annealing at a temperature even as low as 600°C makes it possible to ensure a very high degree of relaxation of elastic stresses in the heterostructure and a comparatively low density of threading dislocations in the SiGe layer (<10⁵ cm^?2). The results obtained make it possible to suggest a method for the formation of thin SiGe/Si layers that feature a high degree of relaxation, low density of threading dislocations, and a good surface morphology.     

Formation of silicon nanocrystals by thermal annealing of low-pressure chemical-vapor deposited amorphous SiNx (x=0.16) thin films

Silicon nanocrystals have been produced by thermal annealing of SiN_x thin film obtained by low pressure chemical vapor deposition using a mixture between disilane and ammonia. Morphological, structural, and photoluminescence properties of the thin film were investigated using X-ray diffraction, scanning electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. The results revealed a high crystallinity of film with a crystalline volume fraction exceeded 70%, and a dominance of silicon nanocrystallites having the sizes within the range 2.5–5 nm and density ~1.98.10¹²/cm². The PL peaks consist of nanocrystalline silicon and amorphous silicon. The luminescence from the silicon nanocrystals was dominant.