The I-V and impedance characteristics of p and n-type silicon electrodes in HF solutions have been determined. Three different I-V regimes are observed, one of which is associated with the on-set of localized dissolution. The formation of porous silicon takes place via a surface state mediated charge transfer mechanism. The position of the main recombination-generation center is estimated at 400 mV above the valence band edge. Localized dissolution is initiated at or close to active adsorption sites. It is then favoured because of geometrical field enhancement effects. Porous silicon has a surface chemistry that can be significant in luminescence.

Lasing of nanocrystalline CuCl embedded in a NaCl single crystal wasobserved for the first time. Lasing takes place at 77 K in nanocrystalline CuCl sandwiched between dielectric mirrors under the pulsed ultraviolet laser excitation. The lasing transition is that from bi-exciton to exciton. The lasing is observed up to 108 K. The optical gain of nanocrystalline CuCl is almost the same as that of bulk crystals in spite of the low concentration of CuCl in the NaCl matrix.

The origin of visible photoluminescence of nanocrystalline Ge in SiO2 glassy matrix has been studied. Spectroscopic analyses of nanocrystalline Ge indicate that the room-temperature photoluminescence comes from nanocrystalline Ge of diameter of 4 nm or less. High-resolution electron microscopic studies imply that the structure of nanocrystalline Ge of diameter ≤ 4 nm differs from the diamond structure. These data suggest that new nanostructure crystalline Ge having a character of direct optical transition exhibits the visible photoluminescence.

Recently we presented a formation mechanism for micro-porous silicon which is based on a depletion of holes in the porous region due to quantum confinement. This theory allows predictions concerning the dependence of the porous morphology on the formation conditions. It is the purpose of this work to check whether these predictions are in accordance with experimental observations.

We have studied the visible photoluminescence (PL) and microstructure of porous Si layers (PSLs) fabricated by the chemical etching of annealed amorphous Si (a-Si). The a-Si layers were partially crystallized by annealing between 550°C-1150°C in N2 and the PSL formed by etching in a HF-HNO3-based solution. No visible PL was observed after etching of unannealed a-Si. Visible PL was detected after etching a-Si layers first annealed at temperatures ≥725°C, coinciding with the observation of Si microcrystallites in the annealed layer prior to etching. The results suggest that an initial crystalline structure is important for fabricating luminescent PSLs.

Heavily Boron-doped micrometer-thick amorphous (a-Si:B:H) or nanocrystalline (nc-Si) silicon layers have been deposited on a variety of conductive substrates by the 50 KHz PECVD of SiH4 / B2H6 / H2 mixtures at 320°C. These films have been partially electro-oxidized in a HF solution and then anodically oxidized in water in a manner similar to that yielding luminescent porous monocrystalline silicon layers (PcSL). Although the anodized films are x-ray amorphous, they yield intense luminescence properties at room temperature very similar to those of anodically oxidized PcSL with a similar vibrational spectrum. In both anodically oxidized materials, aging is shown to improve the external quantum efficiency. In amorphous anodized layers, optical microscopy under UV excitation showed strongly luminescent strain-related heterogeneities (20 μ,m in diameter) connected by non luminescent channels. The incidence of our results on the current debate about the origin of visible room-temperature luminescence in porous silicon and Si:O:H systems is discussed.

Porous silicon which has been prepared by a “dry” technique, that is, by spark erosion, yields similar photoluminescence spectra as anodically etched porous silicon which has been prepared in aqueous solutions. Fourier transform infrared spectra reveal that the dominant features in spark eroded silicon are the Si-O-Si stretching mode and the Si-O-Si bending mode. No infrared vibrational modes characteristic for siloxene have been found in spark eroded Si. Results from X-ray and electron diffraction studies suggest that spark eroded Si involves minute polycrystalline silicon particles which are imbedded in an amorphous matrix.

Nanocrystalline Ge suitable for photoluminescence studies was synthesized from hydrothermally oxidized alloys of Si60Ge40 using steam at 25 MPa and 475°C. Annealing in either an inert atmosphere (UHP N2) or in flowing forming gas (80:20 N2:H2) results in Ge precipitation. In the first case the Ge precipitates are large and concentrated in the bottom portion of the oxide and result from Si reduction of GeO2. In the case of flowing forming gas the Ge precipitates are distributed diroughout die oxide and are formed by reaction between H2 and GeO2. The precipitated samples were analyzed using Raman spectroscopy and cross sectional TEM. Preliminary photoluminescence data reveal that oxide samples excited by the 325 nm line of a HeCd laser after annealing in H2 at both 750 and 800°C for 30 min show strong visible luminescence while unannealed samples do not.

Fabricating well controlled nanostructures and obtaining precise structural, electrical, and optical information from them are essential for understanding die intrinsic properties of silicon (Si) nanostructures, which in turn is important for exploring the potential of quantum confinement induced light emission from crystalline Si. A combination of high resolution electron beam lithography, anisotropic reactive ion etching (RIE), and thermal oxidation has been successfully applied to obtain sub-5 nm Si columnar structures [1]. A transmission electron microscopy (TEM) technique has also been used to characterize die precise structural dimensions of these columns [1]. To obtain the electrical and optical information, a process based on polyimide planarization was developed to establish electrical contacts to these nanostructures. The same process is also applicable for fabricating device structures to study electrically pumped optical response. Preliminary transport studies have confirmed current conduction through die Si nano-pillars and yielded an estimate of die conductivity.

Intense photoluminescence has been obtained from nanocrystal-line silicon prepared in completely dry processing by the optimizing the crystallite size and the chemical passivation of the grain boundaries due to a controlled postoxidation of the plasma deposited films.

Thin films of CaF2 containing layers of Si nanocrystals were grown epitaxially on Si(111) substrates by MBE. When Si is deposited epitaxially on CaF2 it forms islands and, by choosing appropriate growth conditions, a composite of Si nanocrystals embedded in a CaF2 host crystal is obtained which we have observed with TEM. We varied the effective Si concentration in the composite region from 1% to 25%. The CaF2 could be grown with high crystal quality as demonstrated by ion channeling results which gave Ca xmin's as low as 4.7%. Raman spectra of the CaF2 containing Si nanocrystallites showed significant shifts toward lower energies. EUipsometry measurements were performed on a number of samples over a range of effective Si concentrations and Maxwell-Garnett effective medium theory was used to fit the data. A key technological advantage to this approach is that the epitaxial Si/CaF2 composite material is readily integrable into VLSI processing.

We report on a technique for generating Si-clusters in the nanometersize regime. The cluster source is capable of generating micron-thick cluster films in a few minutes. The cluster films luminesce in thevisible and infrared. Source construction is discussed. Electron micrographs of the clusters are shown. Luminescence and optical absorption spectra for the clusters are presented.

We have begun a systematic study of the chemistry of light emitting porous Si (LEPOS) using X-ray photoemission spectroscopy and time-of-flight elastic recoil nuclear scattering. The relation of chemical composition to preparation parameters and its evolution with rapid thermal annealing in N2:H2 and in vacuum were studied. The relation between chemical and photoluminescence properties and stability was investigated. The composition of LEPOS is extremely sensitive to preparation conditions and to subsequent treatment. The concentration of C and F in addition to Si, H and O vary with thermal treatment conditions and with time after preparation. However, preparation under the cleanest possible conditions increase both reproducibility and stability.

Through a post treatment of light emitting porous silicon in boilingwater, a large blue shift of its photoluminescence (PL) spectrum hasbeen observed and a stable blue-green light emission at the peak wavelength down to 500 nm is achieved. The effect of boiling water treatment is suggested to be a kind of oxidation, which could reduce thesize of the Si column, fill up some micropores and strengthen the Siskeleton. The photoluminescence microscopic observation shows that the surface of blue light emitting porous silicon is composed of manysmall uniformly light-emitting domains at the size of several tens of μm. Fourier transform infrared reflection (FTIR) measurements show that the formation of Si-H bonds is not responsible for the visible luminescence in the very thin Si wires.

The aim of this paper is to provide a better understanding of photoluminescent porous silicon (PS) microstructure in relation to their electronic properties: absorption band edge shift1 and quantum confinement hypothesis1,2, dielectric constant evolution and electroluminescence characteristics.

Results concerning the p type PS microstructure characterization by X ray diffraction and electron microscopy are presented showing a noticeable decrease in crystallite size and surface area with decreasing substrate doping and increasing porosity.

The optical transmission of homogeneous free-standing PS layers of different porosities and substrate dopings is studied, showing no evidence of a direct energy gap in PS. On the contrary, a large blue shift of the optical absorption edge, taking into account the total Si mass content in the PS film, is demonstrated. This shift is well correlated with the crystallite size variations with porosity and substrate doping and is attributed to a quantum confinement of electronic wavefunctions in the nanocrystallites.

On the other hand, ellipsometry measurements show the PS absorption to be little affected by the microcrystalline structure of the material in the 3.5–5 eV range, i.e. above the direct band gap of bulk Si. This indicates that, if confirmed, the quantum confinement strongly affects the PS joint density of states in the vicinity of the Si band edge and, as could be expected, to a much lesser degree near the edge of the confining potential.

Capacitance voltage measurements of thin PS layers allow the determination of the dielectric constant which is shown to decrease with increasing porosity. This behavior is in reasonable agreement with theε values deduced from the transmission experiments in the near infrared. Furthermore, it is shown that this dependence on porosity is well accounted for by the Bruggeman effective medium approximation.

Finally, recent results concerning visible light emission from solidstate porous silicon devices will be presented: I-V characteristics, electroluminescence intensity and dynamic, quantum efficiency and device ageing.

Steady state and time-resolved photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy have been performed in situ during etching, on “as prepared” porous Si in air under laser exposure and on chemically oxidized porous Si. We suppose that PLdegradation of “as prepared” porous Si is caused by creating non-radiative defect centers during photooxidation. Chemically oxidized porous Si shows increased PL intensity and longer recombination lifetimes as compared to non-oxidized samples. We conclude, that an oxide layer with low defect density on the inner surface of chemically oxidized porous Si reduces the non-radiative recombination rate.

We describe the acquisition of Raman and photoluminescence (PL) spectra on porous silicon (PS) samples that emit visible light. Spectra were acquired in both ex situ experiments (after exposure to air) and in situ experiments (with the PS covered either with the hydrofluoric acid electrolyte used in the formation process or water). Our results generally show a correlation of blue-shifted PL with increased oxidation. In one set of ex situ experiments, however, we observed an inconsistency in the shift of the wavelengthof maximum luminescence intensity for PS samples that exhibit oxygenated character in the Raman spectra. A higher anodization current density produced a red shift in the PL spectra in one experiment, while chemical dissolution of the PS by hydrofluoric acid produced the well-known blue shift in the other case. In two in situ experiments, we observed very weak and red-shifted PL for a PS sample immersed in HF (compared to the same sample measured later in air) while in another we immersed air-exposed PS in water and observed a 15-fold increase in PL intensity along with a blue shift in the luminescence maximum.

We report the results of a systematic study of the influence of the anodization and etching conditions, and chemical and thermal treatments on the properties of light-emitting porous silicon (PSI). PSI layers with stable, high efficiency photoluminescence (PL) tunable from the near infrared to the yellow have been obtained using p-type and n-type substrates. As the temperature drops below 100 K, the as-anodized PSI layers display striking changes in the PL spectra, which are absent in PSI layers that have received a post-growth treatment.

On the basis of Raman, photoluminescence, and absorption studies of porous and nanoparticle silicon we propose that the strong luminescence in porous silicon results from strained silicon quantum dots. A silicon nanoparticle is a special Jahn-Teller system induced by extended electron states rather than localized state. Thus Raman scattering and photoluminescence in porous silicon are multi-phonon assisted free electronic transition processes, all observed anomalous properties of porous silicon can be clearly explained by using this strained quantum dot model.

We studied the microstructure of two types of light emitting porous silicon (PS), as-etched and rapid thermal oxidized and of material prepared according to the siloxene recipe by Small-Angle X-Ray Scattering (SAXS). In all three types of samples we found particles with nanometer dimensions. The average particle size in as-etched PS is ingood agreement with results achieved by TEM and X-ray diffraction. Shape analysis shows, that the PS skeleton consists of cylindrical shaped particles with an average heigth of 20Å and a diameter of 40 Å.

Si2p core level absorption and photoemission spectra are taken for different porous Si layers using synchrotron radiation, toknow the electronic structures of porous Si. The core level absorption spectra show the high energy shift of the conduction band which correlates with the photo-luminescence blue shift. The oxidation states of porous Si are clarified from the photoemission spectra.