The results of photoelectric properties and electroluminescent studies of structures ZnO/porous Si/p-type c-Si/Al and ZnO/porous Si/p-n c-Si junction/Al are presented. Porous Si is prepared by stain etching of c-Si covered with thin Al film. The transparent ZnO film allows light emission through the top surface of the device under forward electrical bias. Photocurrent is observed under reverse bias and a photovoltaic effect is measured on the p-n junction PS device. The model based on injection of minority carriers through a narrow energy barrier into the porous Si and the presence of the barrier at the interface porous Si/c-Si is suggested for describing the electrical, photoelectric and luminescent properties of the structures.

We propose a method for studies in deph profile of porous silicon (PS) using atomic force microscopy (AFM) to remove PS layers from the surface. The abrasion were made aplying forces between 10-11 to 10-13 N with the AFM liquid cell measuring method. These range of forces is enough to remove very thin PS layers of the surface at each scan the equipment makes. With this method we can make studies and see the PS diferences in depth profile made by different process or different manufacturing conditions.

Large photo-induced absorption signals were observed in free standing porous silicon films in the spectral range from 1.1 to 2.5 eV using a femtosecond pump and probe technique. The measured nonlinear signal has a very fast component with relaxation constants from 800 fs to tens of picoseconds superimposed on the slow-relaxing thermal background. The spectral structure and relaxation dynamics of the short-lived component of transient absorption show the presence of molecular-like Si complexes with well defined energy levels and spectrally uniform picosecond relaxation dynamics.

Properties of Porous Silicon (PS) subjected to gamma irradiation in an air ambient and in vacuum have been studied. Gamma-irradiation was carried out in usual room atmosphere, and in evacuated tubes. The samples of porous silicon were investigated by means of photoluminescence (PL) and infrared (IR) absorption. For the samples irradiated under atmospheric conditions, the PL intensity increases by several times as the dose increases up to 1019photons/cm2 and after that point it starts to go down. For a dose of ˜9.1019photons/cm2 the PL intensity falls by a factor of ˜50. The position of the spectrum maximum does not change, but the half-width of the spectrum increases by 2 or 3 times. In the case of gamma-irradiation in vacuum the PL intensity gradually decreases with increasing dose. IR absorption measurements showed variations of the intensity of Si-H and Si-O bands with the irradiation dose. Enhancement of PL at lower doses has been explained by irradiation assisted oxidation of PS at atmospheric conditions and its degradation at higher doses by suppresion of the light-emitting centers.

Photoluminescence (PL) spectra of porous silicon (PS) samples are studied. Effect of mechanical stresses in substrate on PL intensity is shown. The quantum yield (QY) of PS luminescence is estimated by comparing PL spectra of PS and rhodamine 6G.

Influence of different adsorbates on recovery of porous Si luminescence was in situ investigated for samples preheated under ultra-high vacuum conditions. Exposure to simple adsorbates (O2, H2, N2 up to a pressure of 10-3 to 10-1 Torr), long exposure to air at the atmosphere pressure, or immersion into distilled water fail to recover the red luminescence. On the other hand, we found that the red luminescence can be recovered by adsorption of water vapour onto the sample surface.

The red luminescence of as-prepared porous Si can be caused by water molecules (together with possible impurities) adsorbed in the pores. The immense surface of the pores provides a large number of emitting sites. The porous Si layer can filter for the emitted light, cutting off the wave region shorter than yellow. The combination of these two factors (emission and filtering) can result in the visible red-orange luminescence of porous Si.

We demonstrate experimentally that linear polarization of porous Si photoluminescence depends significantly on the excitation geometry and describe this effect within the framework of a dielectric model in which porous Si is considered as an aggregate of slightly deformed, elongated and flattened, dielectric elliptical Si nanocrystals with preferred orientation in the [100] direction. The theoretical best-fit analysis of the experimental data allows us to get certain information concerning the shapes and orientation of the ellipsoids.

The dielectric function of porous silicon layers depends strongly on the electronic properties of the nanostructure of the silicon skeleton. In this paper we discuss the main effects, as determined from spectroscopic ellipsometric measurements of the dielectric function of porous layers formed on p-doped material. Finite-size effects and the high inner surface area of the nanostructure lead to relaxation of k-momentum conservation as defined for infinite crystals and therefore to a broadening of the features in <ε> arising from interband critical points. In addition a small threshold energy shift is observed when the percolation of the structure is reduced. However, this shift is too small to explain red photoluminescence as a consequence of a pseudo-direct gap whose energy is blue-shifted by confinement.

We present a photoluminescence study of erbium implanted into porous silicon (Er:PSi) with two different Si porosities, a) Er:PSi with a purple appearance and b) with a green-yellow appearance. Er was implanted with a dose of 1×1015 Er/cm2 at 380 keV and annealed at 650°C for 30 minutes. Room-temperature 1.54μm Er3+ emission was observed from both samples. The emission from purple Er:PSi was four times stronger than that from green-yellow Er:PSi. In contrast, visible luminescence from green-yellow Er:PSi was found to be stronger than that from purple Er:PSi. Temperature quenching and power dependence was investigated to elucidate the excitation mechanisms of Er3+ in porous silicon. The results support a correlation between nanostructures of porous Si and 1.54 μm Er3+ luminescence.

Germanium (Ge) films were deposited on substrates whose temperature was kept at room (Ge-RT) or liquid nitrogen temperature (Ge-LNT) by the cluster-beam evaporation technique. The Raman spectra of both films with a double peak suggest that the crystal structure is not the ordinary diamond but the tetragonal one. The critical temperature for the phase transformation from the tetragonal into the diamond structure is found much higher than that for the Ge nanostructures deposited by the gas-evaporation technique. The Ge-LNT sample exhibits photooxidation and photoluminescence (PL) when it is exposed to the UV light. Their PL and optical absorption characteristics are strongly influenced by a combination of the photo-oxidation and thermal annealing treatments.

We discuss the mechanism of red luminescence from surface-oxidized Si nanostructures. The interface state between crystalline Si and SiO2 surface layer plays an essential role in efficient visible luminescence. The crystalline Si nanostructures with a disorder potential of interface states show complicated luminescence properties.

We present semi-empirical tight-binding and ab-initio local density calculations demonstrating the (meta)stability of self-trapped excitons in silicon nanocrystallites. These are obtained not only for surface dimer bonds passivated for instance by hydrogen atoms or by silicon oxide but also for “normal” nearest-neighbors bonds. Light emission from these trapped excitons is predicted in the infrared or in the visible. We are thus led to the interpretation that part of the luminescence is due to such localized states while optical absorption is characteristic of quantum confinement effects. These conclusions should extend to other semiconductor crystallites.

We have investigated on the effect of different substrate surfaces in changing the microstructure of μc-SixGe1-x:H films prepared by reactive magnetron sputtering. Films were deposited on hydrogen terminated Si(111), Si(100) surfaces, and surfaces chemical and plasma oxides. The thin film microstructure was characterized by Fourier transform infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman scattering.

Synthesis of Ge nanocrystals in SiO2 is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The SiO2films containing Ge nanocrystals show intense visible photoluminescence at room temperature that is very similar in spectral features to that of SiO2 containing Ge nanocrystals synthesized by other methods, such as co-sputtering and hydrothermal reduction. The dependence of the measured peak luminescence energy on the nanocrystal diameter shows a poor correlation compared to the calculated sizedependent exciton energy for Ge ‘quantum dot’ states. The measured luminescence lifetimes are much shorter than those predicted by calculated radiative decay rates for the observed size range. The photoluminescence spectra show only a weak temperature dependence. In addition, very similar photoluminescence spectra have also been observed from Xe+-implanted SiO2with damage profiles similar to Ge+-implanted SiO2. Furthermore, the luminescence has been shown to be reversibly quenched by deuterium. These results indicate that the process responsible for visible photoluminescence is not the radiative recombination of excitons in Ge ‘quantum dots’ but is instead related to luminescent radiative defect centers in the matrix or at the nanocrystal/matrix interface.

The white electroluminescence (EL) demonstrated from Si nanocrystals in a wider bandgap amorphous oxide matrix based structure has exciting opportunities in electroptic applications as well as novel LEDs. In this report, we review the electroluminescent properties of the devices for rapid thermally annealed samples at anneal temperatures ranging from 875°C to 1025°C. Depending upon the anneal conditions the EL spectra has shown two distinct spectral features; a strong emission peak at 380nm with a width of 50nm, and a broader features centered above 800nm,. Further, the I-V characteristics and corresponding EL spectra have been measured for sample temperatures ranging from 317K to 240K. In addition, Raman scattering estimated the mean particle sizes of the Si nanocrystals of 6.5nm and 8nm as well as provide insight to the nature of the amorphous matrix. The novel light emission from our devices demonstrates an exciting opportunity for Si nanocrystal (and nanocrystals in general) technology in a wide variety of applications.

We report the synthesis of size-classified silicon nanocrystals by applying differential mobility analysis to an originally polydisperse aerosol of ultrafine silicon crystallites. Using the described technique, the average nanocrystal size can be tuned by simply adjusting an electric field. Samples of uncoagulated silicon nanocrystals have been obtained and size control has been achieved within 15–20% in the 2–10 nm size regime.

Strong room temperature photoluminescence emission from thin films of Er nanoparticles embedded in a matrix of silicon is reported. The Er nanoparticles were produced by a pulsed laser ablation supersonic expansion technique. After a heat treatment at 500°C in an Ar-atmosphere, intense Er-related luminescence appears at λ = 1.54 gim, characteristic of intra-4f emission from Er3÷. Only a 50% reduction in photoluminescence intensity is observed as the temperature increases from 4 K to 300 K. A photocarrier mediated process is responsible for the excitation of the optically active Er-centers.

The electrical and optical properties of light emitting devices employing silicon nanocrystallites as the active material have been studied. Thin films of silicon nanocrystallites were deposited using a pulsed laser ablation supersonic expansion technique. In order to explore their potential for optoelectronic applications, these films have been incorporated into multilayer light emitting devices, using polymer carrier transport layers to enhance their efficiency. Device performance has been found to be weakly temperature dependent, but strongly dependent on the electrical transport properties of the material and the carrier injection mechanisms at the nanocrystal/electrode interface. A systematic study of simple electrode/nanocrystallite/electrode heterostructures has demonstrated that the electrode material and polarity of the applied bias control whether the devices operate in the interface limited or carrier transport limited regimes.

The visible photoluminescence from thin films of silicon nanocrystallites produced by a pulsed laser ablation supersonic expansion source has been studied to determine the role of particle surface and size in the emission behavior. Variation of nanocrystallite size and surface through a variety of chemical treatments and processing steps has shown that the visible emission can be explained using only a simple quantum confinement model. Particle size has been reduced through a series of acid etch and reoxidation cycles and compared to sample emission wavelength. The role of surface species in determining emission behavior has been studied by chemically altering the surfaces and comparing their surface properties as determined by x-ray photoelectron spectroscopy to their photoluminescence behavior. Nanocrystallite size controls the emission energy, while surface passivation only determines emission intensity, through control of excited carrier recombination pathways.

Extended x-ray absorption fine structure (EXAFS) technique has been employed to investigate the local structures around luminescent centers in nanocrystals of Mn-doped ZnS and Tb-doped Y2O3. Size-dependent local structural changes around Mn luminescent centers have been found in Mn-doped nanocrystals of ZnS by using Mn K-edge EXAFS. Local structures around Tb investigated by Tb Li-edge EXAFS also show substantial differences between bulk and nanocrystal samples. This structural information is useful for understanding the novel optical properties of doped nanocrystals.