S. G. Cherkova

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.

Effect of the ion-energy loss rate on defect formation during implantation in silicon nanocrystals

The effect of irradiation with He+, F+, and P+ ions with various energies on photoluminescence and structure of Si nanocrystals is studied. It is established that, at low intensities of ion losses, quenching of photoluminescence is provided by individual atomic displacement. However, as this intensity is increased, quenching is accompanied by an increase in nuclear losses. It is believed that, in low-density displacement cascades, mobile defects predominantly drain to the surface, where they form the centers of nonradiative recombination. In contrast, mobile defects partially form stable structural defects within the nanocrystals in dense cascades. It is sufficient to accumulate ∼0.06 dpa for amorphization of Si nanocrystals at 20°C; dependence of this effect on the intensity of the ion energy loss was not observed. It was also noted that there is a low probability of annihilation of vacancies and interstitials within Si nanocrystals; this effect is attributed to the presence of an energy barrier.

Coulomb blockade of the conductivity of SiOx films due to one-electron charging of a silicon quantum dot in a chain of electronic states

The electrical characteristics of metal-oxide-semiconductor (MOS) structures with silicon nanoparticles embedded in silicon oxide have been studied. The nanocrystals are formed by decomposition of an oversaturated solid solution of implanted silicon during thermal annealing at a temperature of ∼1000°C. At liquid-nitrogen temperature, a stepped current-voltage characteristic is observed in a MOS structure consisting of Si nanocrystals in a SiO2 film. The stepped current-voltage characteristic is, for the first time, quantitatively described using a model in which charge transport occurs via a chain of local states containing a silicon nanocrystal. The presence of steps is found to be associated with one-electron charging of the silicon nanocrystal and Coulomb blockade of the probability of a hop from the nearest local state to the conducting chain. The local states in silicon dioxide are assumed to be related to an excess of silicon atoms. The presence of such states is confirmed by measurements of the differential conductance and capacitance. For MOS structures implanted with silicon, the differential capacitance and conductance are found to be higher, compared to the reference structures, in the range of biases exceeding 0.2 V. In the same bias range, the conductance is observed to decrease under ultraviolet irradiation due to a change in the population of the states in the conductivity chains.

Light-emitting Si nanostructures formed in SiO2 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.

Formation of light-emitting nanostructures in layers of stoichiometric SiO2 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.

The effect of composition on the formation of light-emitting Si nanostructures in SiOx layers on irradiation with swift heavy ions

The SiOx layers different in composition (0 < x < 2) are irradiated with Xe ions with the energy 167 MeV and the dose 1014 cm−2 to stimulate the formation of light-emitting Si nanostructures. The irradiation gives rise to a photoluminescence band with the parameters dependent on x. As the Si content is increased, the photoluminescence is first enhanced, with the peak remaining arranged near the wavelength λ ≈ 600 nm, and then the peak shifts to λ ≈ 800 nm. It is concluded that the emission sources are quantum-confined nanoprecipitates formed by disproportionation of SiOx in ion tracks due to profound ionization losses. Changes in the photoluminescence spectrum with increasing x are attributed firstly to the increase in the probability of formation of nanoprecipitates and then to the increase in their dimensions; the latter effect is accompanied with a shift of the emission band to longer wavelengths. The subsequent quenching of photoluminescence is interpreted as a result of the removal of quantum confinement in nanoprecipitates and their coagulation.

Effect of high-power nanosecond and femtosecond laser pulses on silicon nanostructures

The effect of high-power nanosecond (20 ns) and femtosecond (120 fs) laser pulses on silicon nanostructures produced by ion-beam-assisted synthesis in SiO2 layers or by deposition onto glassy substrates is studied. Nanosecond annealing brings about a photoluminescence band at about 500 mn, with the intensity increasing with the energy and number of laser pulses. The source of the emission is thought to be the clusters of Si atoms segregated from the oxide. In addition, the nanosecond pulses allow crystallization of amorphous silicon nanoprecipitates in SiO2. Heavy doping promotes crystallization. The duration of femtosecond pulses is too short for excess Si to be segregated from SiO2. At the same time, such short pulses induce crystallization of Thin a-Si films on glassy substrates. The energy region in which crystallization is observed for both types of pulses allows short-term melting of the surface layer.

Formation of silicon nanocrystals in Si—SiO2—α-Si—SiO2 heterostructures during high-temperature annealing: Experiment and simulation

Experiments and simulations are performed to study the formation of silicon nanocrystals (Si-NCs) in multilayer structures with alternating ultrathin layers of SiO2 and amorphous hydrogenized silicon (α-Si:H) during high-temperature annealing. The effect of annealing on the transformation of the structure of the α-Si:H layers is studied by methods of high-resolution transmission electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. The conditions and kinetics of Si-NC formation are analyzed by the Monte Carlo technique. The type of the resultant crystalline silicon clusters is found to depend on the thickness and porosity of the original amorphous silicon layer located between SiO2 layers. It is shown that an increase in the thickness of the α-Si layer in the case of low porosity leads to the formation of a percolation silicon cluster instead of individual Si nanocrystals.

Phase separation as a basis for the formation of light-emitting silicon nanoclusters in SiOx films irradiated with swift heavy ions

This paper presents a study of the effect of swift heavy Xe ions of energy 130–167 MeV at doses of 1012–1014 cm−2 and Bi ions of 700 MeV at doses of 3·1012–3·1013 cm−2 on films of stoichiometric thermal silicon dioxide, silicon dioxide films with ion-implanted excess silicon, and SiOx films with the stoichiometric parameter x varying from 0 to 2. According to electron microscopy and Raman spectroscopy data, irradiation with the swift heavy ions resulted in the formation of silicon nanoclusters. The luminescence spectra depended on the size, number, and structure of the Si nanoclusters formed. Their size can be controlled by varying both the effect parameters (primarily, the ion energy loss per unit length of the track) and the stoichiometric composition of the films.

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.