Excitons in Si nanocrystals

The states of electron-hole pairs in spherical silicon nanocrystals are theoretically studied using the “multiband” effective-mass approximation in the limit of an infinitely high potential barrier at the boundary. The degeneracy of the states at the top of the valence band is taken into account in the spherical approximation, and the ellipsoidal character of the electronic spectrum in the conduction band is allowed for. Coulomb interaction-induced corrections to the energy of an electron-hole pair are found.     

Vacancies in a Si(111) thin film

We investigated the electronic structure of an ideal vacancy in the Si(111) thin film by using empirical tight binding method. The supercell model used in our calculations predicted vacancy related states in general agreement with previous works. For the vacancy near the surface, it is found that the bound state energies shift to higher energies as the vacancy moves toward the surface. At the surface, however, it was seen that the vacancy bound state mix with the dangling bond surface states. Considering energy locations in the bond gap, we propose that vacancies created in the surfaceregion may account for the peak (at about ～0.5 eV above the valence band edge) in the density of interface states observed at the interface of the Si-SiO₂ junction.     

Excitonic and quasiparticle gaps in Si nanocrystals

We present calculations of the one- and two-particle excitations in silicon nanocrystals. The one-particle properties are handled in the GW approximation, and the excitonic gap is obtained from the Bethe-Salpeter equation. We develop a tight binding version of these methods to treat clusters up to 275 atoms. The self-energy and Coulomb corrections almost exactly cancel each other for crystallites with radius larger than 0.6 nm. The result of this cancellation is that one-particle calculations give quite accurate values for the excitonic gap of crystallites in the most studied range of sizes.     

Photoluminescence studies of Si nanocrystals

Summary We report room temperature time-resolved photoluminescence (PL) and temperature dependence of continuous wave (cw) PL studies of high fluence (from 3·10¹⁶ to 3·10¹⁷ cm⁻²) Si⁺-implanted thermal SiO₂ layers after annealing at high temperature (T=1000°C). Such measurements were related to TEM analysis of samples. Nancocrystals were observed at TEM only a samples implanted at higher fluence. In these samples a near infrared PL signal peaked at approximately 1.5 eV with decay time of about 100 μs is present. Besides, in all samples a light emission is present in the green region of the spectrum. The intensity of the emission shows large variations with ion fluence, and is characterized by 0.4, 2 and 7 ns decay times. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce) Torino, 12–13 October 1995.     

Direct bandgap optical transitions in Si nanocrystals

JETP Letters - The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the...     

Direct bandgap optical transitions in Si nanocrystals

The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the Γ-point decreases with size reduction: quantum confinement enhances radiative recombination across the direct bandgap and introduces its “red“ shift for smaller grains. We postulate to identify the frequently reported efficient blue emission (F-band) from Si nanocrystals with this zero-phonon recombination. In a dedicated experiment, we confirm the “red“ shift of the F-band, supporting the proposed identification.     

Thermodynamic analysis of Si doping in GaN

Equilibrium calculations of Si-doping in GaN are investigated using the Gemini code. The method of the calculation is based on the minimisation of the Gibbs free energy. Experimental growth conditions are used for the calculation. The variables are the amount of the dopant and the temperature. The results show the formation of a solid Si₃N₄ compound with a certain quantity of the input SiH₄, that is the silicon precursor in our MOVPE system. Si₃N₄ formation can explain the limitation of Si incorporation and the surface roughening as revealed by MOVPE Si doped layers.     

Optical gap of P-doped Si nanocrystals

The optical gap of Si nanocrystals strongly doped with phosphorus has been calculated. The energy levels of the electron and hole ground states in the Si nanocrystal were found as functions of the nanocrystal size and the P concentration in the framework of the envelope function approximation and under the assumption of uniform impurity distribution over the nanocrystal volume. It is shown that introducing phosphorus into the nanocrystal leads to a decrease in its optical gap. This decrease is related to a strong shift of the ground electron level produced by the short-range part of the Coulomb electron-P-ion interaction.     

Temperature-dependent photoluminescence of arsenic-doped Si nanocrystals

The characteristics of temperature-dependent photoluminescence (PL) from Si nanocrystals and effects of arsenic-doping (As-doping) were investigated. The Si nanocrystals on a p-type Si substrate were prepared by low pressure chemical vapor deposition and post-deposition thermal oxidation. The As-doping process was carried out using the gas-phase-doping technique. Temperature-dependent PL from Si nanocrystals exhibited considerable differences between samples with/without As-doping. Phase transition between electron-hole liquid and free exciton was observed in the undoped Si nanocrystals, leading to the increase in PL intensity with temperature less than 50 K. Electron emission from As-doped Si nanocrystals to the p-Si substrate was responsible for the significant increase in PL intensity with temperature greater than 50 K. Characteristics of light emission from Si nanocrystals will facilitate the development of silicon-based nanoscaled light-emitting devices.     

Photoluminescence of Si nanocrystals under selective excitation

Photoluminescence of Si nanocrystals passivated by different alkanes (hexane, octane, and tridecane) was studied at room temperature. It is shown that the emission band shape is not affected by the length of the carbon chain in the alkanes used for passivation. A pronounced fine structure of the photoluminescence band consisting of peaks separated by 150-160 meV was observed under resonant excitation. The structure is interpreted by predominant contribution from Si nanocrystal groups with particular stable size/shape existing in addition to the previously reported nanocrystals with “magic” numbers of Si atoms. The contribution of these stable nanocrystals is revealed using selective resonant photoexcitation to the higher energy states in the discrete energy spectrum of such nanocrystals.     

Structure and energetics of Si nanocrystals embedded in a-SiO2

We develop realistic models of Si nanocrystals embedded in a-SiO2 using a Monte Carlo approach. The interface structure and its energetics are studied as a function of the nanocrystal size. We find that the low-energy geometries at the interface are Si-O-Si bridge bonds. Remarkably, their fraction strongly declines as the size becomes smaller. Concurrently, the embedding causes substantial deformation in such small nanocrystals. Based on these findings, an alternative explanation is given for the reduced optical gaps in this size regime.     

Erbium-doped Si nanocrystals: optical properties and electroluminescent devices

In the last decade, a strong effort has been devoted towards the achievement of efficient light emission from silicon. Among the different approaches, rare-earth doping and quantum confinement in Si nanostructures have shown great potentialities. In the present work, the synthesis and properties of low-dimensional silicon structures in SiO₂ will be analyzed. All of these structures present a strong room temperature optical emission, tunable in the visible by changing the crystal size. Moreover, Si nanocrystals (nc) embedded in SiO₂ together with Er ions show a strong coupling with the rare earth. Indeed each Si nc absorbs energy which is then preferentially transferred to the nearby Er ions. The signature of this interaction is the strong increase of the excitation cross section for an Er ion in the presence of Si nc with respect to a pure oxide host. We will show the properties of Er-doped Si nc embedded within Si/SiO₂ Fabry–Pérot microcavities. Very narrow, intense and highly directional luminescence peaks can be obtained. Moreover, the electroluminescence (EL) properties of Si nc and Er-doped Si nc in MOS devices are investigated. It is shown that an efficient carrier injection at low voltages and quite intense room temperature EL signals can be achieved, due to the sensitizing action of Si nc for the rare earth. These data will be presented and the impact on future applications discussed.     

Fine structure effects and phase transition of Xe nanocrystals in Si

We report on an X-ray diffraction study performed on Xe agglomerates obtained by ion implantation in a Si matrix. At low temperature, Xe nano-crystals were formed in Si with different average sizes according to the preparation procedure. High resolution diffraction spectra were detected as a function of the temperature, in the range 15–300 K, showing evidence of fine structure effects in the growth mode of the Xe nanocrystals. We report the first experimental observation of fcc crystalline agglomerates with a lattice parameter expanded by the epitaxial condensation on the Si cavities, whereas for small agglomerates randomly oriented evidence of a contracted lattice was found. For these nanocrystals, a solid-to-liquid transition temperature, size dependent, was detected; above the transition temperature, a fluid phase was observed. Neither overpressurized clusters were detected at any temperature, nor preferential binary size distribution as reported for a metal matrix.     

Microscopic phonon theory of Si/Ge nanocrystals

Microscopic phonon theory of semiconductor nanocrystals (NCs) is reviewed in this paper. Phonon modes of Si and Ge NCs with various sizes of up to 7 nm are investigated by valence force field theory. Phonon modes in spherical SiGe alloy NCs approximately 3.6 nm (containing 1147 atoms) in size have been investigated as a function of the Si concentration. Phonon density-of-states, quantum confinement effects, as well as Raman intensities are discussed.      

Ag纳米颗粒对富Ag二氧化硅薄膜电致发光谱的影响

利用磁控溅射和热退火在硅衬底上制备了Ag纳米颗粒镶嵌的氧化硅薄膜(SiO2∶ Ag),制作了电致发光结构ITO/SiO2∶Ag/p-Si,观测到了可见区的电致发光.发现薄膜中的Ag纳米颗粒不仅成倍地提高器件的发光强度,还明显地移动电致发光的峰位.Ag含量越高,颗粒越大,发光峰位越红移.氧化硅中的发光中心与纳米Ag间的电...     

Room-temperature below bulk-Si band gap photoluminescence from P and B co-doped and compensated Si nanocrystals with narrow size distributions

Thin films consisting of the layers of phosphorus (P) and boron (B) co-doped Si nanocrystals (Si-ncs) and glass spacer layers were prepared and their photoluminescence properties were studied. Cross-sectional TEM observations revealed the growth of Si-ncs with narrow size distributions. The samples exhibited PL below the band gap energy of bulk Si crystal at room temperature. The low-energy PL is considered to arise from the transitions between donor and acceptor states in compensated Si-ncs. The successful formation of narrow size distribution co-doped Si nanocrystals promotes the study of the optical properties of compensated Si nanocrystals.     

Soft X-ray fluorescence and photoluminescence of Si nanocrystals embedded in SiO2

We have used ion-beam mixing to form Si nano-crystals in SiO₂ and SiO₂/Si multilayers, and applied photoluminescence and soft-X-ray emission spectroscopy to study the nanoparticles. Ion-beam mixing followed by heat treatment at 1100 °C for 2 h forms the Si nanocrystals. The ion-beam-mixed sample shows higher PL intensity than that of a Si-implanted SiO₂ film. Photon and electron-excited Si L_2,3 X-ray emission measurements were carried out to confirm the formation of Si nanocrystal in SiO₂ matrix after ion-beam mixing and heat treatment. It is found that Si L_2,3 X-ray emission spectra of ion-beam-mixed Si monolayers in heat-treated SiO₂ films lead to noticeable changes in the spectroscopic fine structure. Received: 20 November 1999 / Accepted: 17 April 2000 / Published online: 5 October 2000     

Investigation of doping in Si crystals by means of electroreflectance

Summary In this work the importance of electroreflectance is evidenced in two specific applications concerning studies of semiconductor surfaces: i) determination of junction depths and carrier profiles, ii) mapping of inhomogeneity curves of the carrier distribution on Si surfaces. New information in both cases is obtained and briefly discussed. Work partially supported by Progetto Finalizzato Energetica (Sottoprogetto Energia Solare) of consiglio Nazionale delle Ricerche.