A. G. Cherkov

Effect of quantum confinement on optical properties of Ge nanocrystals in GeO2 films

Germanium dioxide films containing Ge nanocrystals are studied. The films have been prepared by two methods: (i) deposition from supersaturated GeO vapors with subsequent decomposition of metastable germanium monoxide into a heterophase Ge:GeO2 system, and (ii) formation of anomalously thick native germanium oxides with a GeO2(H2O) chemical composition by a catalyst-accelerated oxidation of germanium. The films, which have been prepared on various substrates, are studied using the photoluminescence technique, Raman spectroscopy, spectral ellipsometry, and high-resolution electron microscopy. In the GeO2 films with built-in Ge nanoclusters, intense photoluminescence is detected at room temperature. The nanocluster sizes are estimated from the position of the Raman peak related to localized optical phonons. The correlation between a decrease in the nanocluster size and the shift of the photoluminescence peaks to the blue spectral region as the relative Ge content decreases is revealed. The presence of nanoclusters is confirmed by the data obtained from high-resolution electron microscopy. The correlation of the optical gap calculated taking into account the quantum confinement of electrons and holes in the nanoclusters with the experimentally observed luminescence peak is established. It can be concluded from the data obtained that the Ge nanoclusters constructed in the GeO2 matrix represent type I quantum dots.

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.

Wavelength-selective enhancement of the intensity of visible photoluminescence in hydrogen-ion-implanted silicon-on-insulator structures annealed under high pressure

Characteristic features of the visible photoluminescence (PL) spectra were studied in silicon-on-insulator (SOI) wafers following high-dose (3×1017cm−2) ion implantation of hydrogen and annealing at high hydrostatic pressures. The PL behavior of the SOI material was compared with that of hydrogen-implanted bulk Si. Annealing at a pressure above 6kbars produced a wavelength-selective increase (∼37 times) in the intensity of the visible PL from the implanted SOI structures. The results are explained in terms of the effect of an optical resonant cavity formed between the air/SOI and the Si∕SiO2 interfaces as a result of the high-pressure annealing.

Behavior of germanium ion-implanted into SiO2 near the bonding interface of a silicon-on-insulator structure

The properties of germanium implanted into the SiO2 layers in the vicinity of the bonding interface of silicon-on-insulator structures are studied. It is shown that, under conditions of high-temperature (1100°C) annealing, germanium nanocrystals are not formed, while the implanted Ge atoms segregate at the Si/SiO2 bonding interface. It is established that, in this case, Ge atoms are found at sites that are coherent with the lattice of the top silicon layer. In this situation, the main type of traps is the positive-charge traps; their effect is interpreted in the context of an increase in the surface-state density due to the formation of weaker Ge-O bonds. It is found that the slope of the drain-gate characteristics of the back MIS transistors increases; this increase is attributed to an increased mobility of holes due to the contribution of an intermediate germanium layer formed at the Si/SiO2 interface.

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.

Femtosecond pulse crystallization of thin amorphous hydrogenated films on glass substrates using near ultraviolet laser radiation

Femtosecond laser treatments (second harmonic of Ti-sapphire laser, λ ≈ 400 nm wavelength, <30 fs pulse duration) were applied for crystallization of thin hydrogenated amorphous silicon films on glass substrates. The concentration of atomic hydrogen in the films was varied from 10 to ≈35%. The energy densities (laser fluences) for crystallization of the films with thicknesses from 20 to 130 nm were found. Assumedly, non-thermal processes (plasma annealing) take place in phase transition caused ultra-fast pulses. The developed approach can be used for creation of polycrystalline silicon films on non-refractory substrates.

Precipitation of boron in silicon on high-dose implantation

AbstractPrecipitation of boron implanted in silicon with a dose of 1 × 1016 cm−2 is studied in relation to the concentration of substitutional boron

Light-emitting Si nanostructures formed in SiO2 on irradiation with swift heavy ions

C_{B_0 }