The porous silicon (PS) surface is derivatized with ethanol, triethylsilanol and formic acid as well as oxidized with water. The two reactions used to prepare these surfaces are discussed, and FTIR spectra of the products are presented. Surface-modified PS retains 10-40% of its original photoluminescence. PS-derivatives display reversible luminescence quenching by gas phase water, ethanol, acetonitrile and benzene. The extent of quenching varies with different PS-derivatives depending on the interaction of the chemical vapor with the modified PS surfaces.

We have investigated the sensitivity of blue (PLmax = 480 nm) and red (PLmax = 660 nm) emitting porous silicon samples to various chemical adsorbates. Steady-state and time-resolved photoluminescence measurements and FTIR spectroscopy were employed to characterize the photophysical and optical effects induced by chemical exposure. The red samples, which are hydrogen terminated, exhibit quenching and recovery of photoluminescence intensity and broadening of the Si-Hx stretch bands upon exposure to liquid methanol. This behavior is attributed to the ability of the Si-Hx specie on the surface of the PSI to interact with the solvent molecules which temporarily traps the electrons and causes PL loss and Si-Hx broadening. The blue samples, which are oxygen terminated, display similar sensitivity to methanol. This sensitivity is attributed to the solvent's ability to change the surface passivation and thereby introduce competitive radiative and nonradiative recombination channels. The origin of the blue PL is discussed.

Fourier transform infrared spectroscopy (FTIR) was used to investigate the reaction of methanol with porous silicon and hydrogen passivated porous silicon. At 300 K methanol adsorbs onto hydrogen free porous silicon by cleavage of the O-H bond. Both of the resulting surface species, Si-H and Si-OCH3, were determined to be stable up to ∼500 K. Above 500 K the Si-OCH3 moiety decomposes by breakage of the C-O and C-H bonds. The resulting carbon and oxygen were incorporated into the porous layer and additional Si-H surface species were detected. Further heating to 900 K removed the Si-H surface species. Adsorption of methanol onto hydrogen-passivated porous silicon did not occur until 600 K. At temperatures beyond 600 K, oxygen and carbon incorporation into the porous layer and Si-OCH3, Si-CH3, and Si-H surface species were seen. The previously unseen Si-CH3 surface species is believed to be stabilized by oxygen incorporation.

Near-edge and extended x-ray absorption fine structure measurements from a wide variety of H-passivated porous Si samples and oxidized Si nanocrystals, combined with electron microscopy, ir-absorption, α-recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For luminescent porous Si samples peaking in the visible region, i. e., ≤700nm, their mass-weighted-average structures are determined here to be particles–not wires, whose short-range character is crystalline – not amorphous, and whose dimensions – typically <15 Å – are significantly smaller than previously reported or proposed. These results depend only on sample luminescence behavior, not on sample preparation details, and thus have general implications in describing the mechanism responsible for visible luminescence in porous silicon. New results are also presented which demonstrate that the observed luminescence is unrelated to either the photo-oxidized Si species in porous Si or the interfacial suboxide species in the Si nanocrystals.

The microstructure of anodically prepared porous silicon films was determined using a novel x-ray diffraction technique. This technique uses double-crystal diffractometry combined with position-sensitive x-ray detection to efficiently and quantitatively image the reciprocal space structure of crystalline materials. Reciprocal space analysis of newly prepared, as well as aged, p+ porous silicon films showed that these films exhibit a very broad range of crystallinity. This material appears to range in structure from a strained, single-crystal, sponge-like material exhibiting long-range coherency to isolated, dilated nanocrystals embedded in an amorphous matrix. Reciprocal space analysis of n+ and p+ porous silicon showed these materials are strained single-crystals with a spatially-correlated array of vertical pores. The vertical pores in these crystals may be surrounded by nanoporous or nanocrystalline domains as small as a few nm in size which produce diffuse diffraction indicating their presence. The photoluminescence of these films was examined using 488 nm Ar laser excitation in order to search for possible correlations between photoluminescent intensity and crystalline microstructure.

Using electrochemically prepared porous Si with hydride covered internal surfaces, we determine the H-content by thermal effusion and measurement of the total gas evolution. The amount of Si is obtained by weighing. In this way, we derive a value of the ratio of H- to Si-atoms and from the latter a value of the linear dimension of the luminescing particles

We discuss the dependence of the dielectric function on the nanocrystalline size of porous silicon layers. The layers were grown by a standard electrochemical process and characterized by spectroscopic ellipsometry. By a lineshape analysis values of the critical point energies and broadening parameters of the Interband critical points were derived. In order to obtain further information about the nanocrystallites, the preparation conditions were varied (HF concentration, NH4F was added) or the layers were further treated by etching and arsenic deposition. The lineshape analysis values indicate that the layers consist mainly of nanocrystallites, or more accurately, that the electrons are confined to regions of a few nanometers in size. Furthermore, there is strong indication that some preparation conditions may leave these nanocrystallites heavily strained .

Reflectance spectroscopy has been used to obtain the dielectric function of the solid phase of porous silicon. The method is based on a fit of a parameterized dielectric function model to measured spectra. A crucial step in the procedure is the 'dielectric averaging' of the microscopic dielectric function of the pore wall material to the macroscopic effective dielectric function which governs the optical properties.

Results are given for heavily and moderately p-doped samples of various porosities. For the latter large differences to bulk silicon have been found. The obtained dielectric functions are compared to the results of band structure calculations taken from literature.

The peak energy of the room temperature photoluminescence of porous silicon is compared with the bandgap determined from photoelectron spectroscopy measurements for a series of porous silicon samples prepared under different conditions. The photoluminescence bandgap is found to be smaller than the photoelectron spectroscopy bandgap, but exhibits the same trend with preparation conditions. The width of both the photoluminescence spectrum and the L-absorption edge increases when the current density during the preparation is increased or the sample is allowed to soak in HF after preparation.

We combine photoluminescence excitation spectroscopy and photoconductivity to extract information about the bandgap and particle size distribution of porous silicon. This allows us to specify the influence of size dispersion and to show that different methods to determine absorption probe different parts of the size distribution.

The defect properties of as-etched, annealed and thermally oxidized nano-porous Si as well as self supporting macro-porous silicon layers are studied by electron paramagnetic resonance (EPR), photoluminescence (PL), optically detected magnetic resonance (ODMR), and electron nuclear double resonance (ENDOR). The paramagnetic defects observed are dangling bonds closely related to the Pb-center, the Si/SiO2 interfacial defect. In EPR a minimum defect density of 1016 cm−3 is observed for as-etched material, it reaches a maximum of 8 x 1018 cm−3 for samples annealed around 400°C. We quantitatively correlate the defect density with hydrogen desorption data and luminescence efficiency. In the ODMR experiments the same dangling bond centers are observed on the 1.7 eV luminescence band, but with increased sensitivity in the infra-red emission band at 1.15 eV. Electron nuclear double resonance experiments show that the dangling bonds are not solely bonded to Si neighbors but involve hydrogen and fluorine.

The spectroscopic evidence that the main visible photoluminescence (PL) band of porous silicon (the 'S' band) originates from quantum confined crystalline silicon is presented, and arguments that claim to invalidate this evidence are analysed in detail. We find that a careful study of all the spectroscopic data provides strong support for the quantum confinement model. Additionally we consider the interesting issue of the luminescence spectrum of a single silicon quantum dot.

Photoluminescence (PL) properties from anodically oxidized porous silicon (PS) have been investigated. Large temperature coefficients (-0.5 meV/K) of PL peak energy are observed for relatively strongly oxidized PS specimens which show saturation of PL peak blue shift upon further anodic oxidation. Time-resolved PL data show that the PL decay is characteristic of thermal activation process with an energy of 9-27 meV at low temperatures between 80 and 180 K. Those results can be explained using a luminescence model which assumes several PL centers in the energy gap and considers thermally activated and tunneling processes.

The visible and the infrared photoluminescence bands in porous Si have been studied at low temperature for two series of samples: one in which the size of the crystallites has been varied and another in which the degree of surface degradation has been changed. It is shown that the relation of the two bands can be explored for characterization of the porous Si bandstructure. The size- and the surface dependence of the valence band and of the conduction band related states is discussed. A model is proposed for explanation.

The photoluminescence (PL) infrared (IR)-band of p-doped porous Si (PS) films is studied by steady-state and time-resolved PL and by photoluminescence excitation (PLE) in detail. In analogy to the S-band in the visible the IR-band shifts to higher energies with reduced average nanocrystal size. The IR- and S-bands are very different in their decay behavior and in their recombination lifetimes. The temperature-dependent PL intensity shows non-exponential decay with lifetime distributions in the nsec-µsec range in contrast to the stretched exponential decay shape of the S-band corresponding to lifetime distributions in the μsec -msec range. The origin of the IR-band is likely related to radiative recombination at deep defects in Si nanocrystals with quantum-upshifted band gaps.

We report results of photoluminescence (PL), FTIR and ESR investigations on nanoporous silicon (PS) where a reversible PL intensity relaxation effect in the chemically oxidised material is observed. To be activated the effect needs, however, additional preparation steps including light irradiation and ageing in ambient atmosphere. After illumination with visible light, the PL intensity is remarkably diminished. However it recovers in the dark within the time scale of minutes to hours under ambient atmosphere at room temperature. This cycle can be repeated several times. We show that the variation of the PL intensity is anticorrelated to an ESR signal attributed to silicon dangling bonds. From the IR spectrum, however, no significant change of the pore surface chemical structure can be observed during a cycle. Therefore we conclude that the variation of the PL intensity is rather controlled by a metastable change in the number of dangling bond centers than by modification of the surface chemistry in the porous silicon system.

A pulsed, high-power TEA CO2 laser with lines in the region from 9.2 to 10.6 μm has been used to irradiate luminescent porous Si samples. The visible luminescence quenches and then recovers to its initial value on a time scale of one hour. It is found that the quenching is efficient when the IR wavelength is within the Si-O absorption band. We suggest that the resonant excitation of the Si-O bonds results in a metastable reconfiguration of the oxygen together with the creation of dangling bonds. These non-radiative centers are responsible for the PL quenching.

Photoluminescence from porous silicon can be quenched reversibly by a variety of molecular species. Quenching pathways for chemically incorporated surface species, physisorbed species (that undergo no net chemical transformation), and for electron donating reagents have all been identified. For systems involving charge transfer quenching, the concentration dependence typically follows a Stern-Volmer type of relationship, with the more easily oxidized molecules producing the largest Stern-Volmer slopes (most efficient quenching). The slope of the Stern-Volmer plot is dependent upon the particular wavelength that the Stern-Volmer data are obtained from. Data interpretation is complicated by the fact that porous Si shows a wavelength dependent emission lifetime, although when this is taken into account the data can be qualitatively described within the context of a driving-force dependent quenching model. A simple model for understanding the various photoluminescence quenching phenomena observed with porous Si is presented, involving an ensemble of emissive states with energy dependent lifetimes and Stern-Volmer quenching behavior. The model adequately accounts for both red and blue spectral shifts that have been observed upon photoluminescence quenching.

Electronic states of photocarriers in porous silicon have been investigated by photomodulated infrared and photoluminescence spectroscopies. Wet porous silicon is only weakly yellow-green luminescent, and contains photocarriers which exhibit a Drude-like absorption characteristic of free carriers. On the contrary, once dried or anodically oxidized, porous silicon becomes strongly red luminescent and photocarriers exhibit a broad Gaussian-like absorption, characteristic of localized carriers, which correlates in frequency, response time and intensity with the photoluminescence. This behavior does not appear to be related to surface chemistry. Instead, non-wetting of the surface seems to be mandatory in order to obtain localization of the photocarriers and strong red luminescence. This suggests that the driving effect for switching from green to red luminescence could be the dielectric screening of the Coulombic interactions by the embedding medium in which porous silicon is maintained. The electronic states bound to the potential generated by a charge in a cylindrical silicon wire have then been computed for various values of the dielectric constant of the embedding medium. It is shown that, when the dielectric constant is low, one may account for the red luminescence in terms of recombination through such shallow states, and that the finite time needed for the dielectric relaxation in electrolytes may also account for an inefficient trapping in such states in a wet environment. Preliminary luminescence results are consistent with the predictions of the model.

The luminescence in red-emitting porous silicon exhibits a distribution of lifetimes in the μsec time domain at room temperature and in the msec time domain at cryogenic temperatures. However, the luminescence and carrier dynamics in porous silicon display transients that vary from much less than 1 psec to ∼ 1 sec, depending on the measurement conditions and sample preparation. We have investigated the carrier dynamics in porous silicon by two time-resolved techniques. The blue photoluminescence of oxidized porous silicon has been measured with 100 ps time resolution as a function of the oxidation method, emission wavelength, excitation intensity and measurement temperature. The blue luminescence has a distinct origin from the well-studied red luminescence and we attribute it to defects in the oxide. Femtosecond photoinduced absorption measurements have been performed on thin red-emitting porous silicon films. The wavelength and intensity dependence of the recovery are interpreted in terms of trapping and of Auger recombination at high excitation intensity. Our results also show conclusively that red-emitting porous silicon is not a direct gap semiconductor.