G. A. Kachurin

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.

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.

SiOx layer formation during plasma sputtering of Si and SiO2 targets

Deposition of SiOx 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 SiOx layer grown during co-sputtering of targets were determined using optical techniques. It was shown that the SiOx 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 SiOx layers with experimental ellipsometric data confirmed the presence of excess oxygen in the SiOx layer. Taking into account such partial oxidation of sputtered silicon, composition isolines in the substrate plane were calculated. After annealing of the SiOx 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.

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.

The mechanisms of impurity redistribution on laser-annealing of ion-implanted semiconductors

Abstract Ion-implanted impurity profiles were studied after laser annealing in different regimes. It was found that the redistribution of impurities in silicon occurs after annealing with both millisecond and nanosecond laser pulses. The character of redistribution depends on the power density of the light beam. In the case of relatively low power-density the most probable mechanism of impurity migration seems to be interstitial diffusion. For high power-densities the redistribution is caused by the flux of excess vacancies or by the recrystallization of the melted surface layer.

The formation of silicon nanocrystals in SiO2 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.

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.

The effect of implantation of P ions on the photoluminescence of Si nanocrystals in SiO2 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.

Phase transitions in a-Si:H films on a glass irradiated by high-power femtosecond pulses: Manifestation of nonlinear and nonthermal effects

The crystallization of amorphous silicon films 90-nm thick irradiated by laser pulses (a wavelength of 800 nm and a pulse duration of 120 fs) is investigated using Raman scattering spectroscopy and electron microscopy. The absorption coefficient for 800-nm low-power probe radiation by an a-Si:H film is small but can increase owing to nonlinear effects for high-power pulses. According to the estimates, the energy absorbed in the film is insufficient for its heating and complete melting but is sufficient for the generation of free charge carriers with a density of about 1022 cm−3. The electron and phonon temperatures in this case are strongly different and silicon becomes unstable. Thus, the action of such a short laser pulse cannot be reduced only to the heating effects and subsequent phase transitions through the liquid or solid phase.

Short-wavelength photoluminescence of SiO2 layers implanted with high doses of Si+, Ge+, and Ar+ ions

The short-wavelength (400–700 nm) photoluminescence (PL) spectra of SiO2 layers implanted with Si+, Ge+, and Ar+ ions in the dose range 3.2×1016–1.2×1017 cm−2 are compared. After Ar+ implantation an extremely weak luminescence, which vanishes completely after annealing for 30 min at 400 °C or 20 ms at 1050 °C, was observed. After implantation of group-IV elements the luminescence intensities were 1 to 2 orders of magnitude higher, and the luminescence remained not only with annealings but it could also increase. The dose and heating dependences of the luminescence show that it is due to the formation of impurity clusters and this process is more likely to be of a percolation than a diffusion character. For both group-IV impurities an intense blue band and a weaker band in the orange part of the spectrum were observed immediately after implantation. The ratio of the excitation and emission energies of the blue luminescence is characteristic of oxygen vacancies in SiO2; its properties are determined by the direct interaction of group-IV atoms. On this basis it is believed that the centers of blue PL are chains of Si (or Ge) atoms embedded in SiO2. The orange luminescence remained after annealings only in the case of Si+ implantation. This is attributed directly to the nonphase precipitates of Si in the form of strongly developed nanometer-size clusters.