We report resonantly excited photoluminescence (PL) spectroscopy of highly porous silicon. In the PL spectra we observe satellite structure due to the participation of momentum-conserving phonons in the optical transitions. The momentum-conserving role of these phonons, together with their energies and relative coupling strengths, demonstrate beyond doubt that crystalline silicon, which has already been shown to be the dominant constituent of unoxidised porous silicon, also forms the luminescent material. We show that the theory of quantum confinement in crystalline silicon wires can explain our results and those of other experiments, if the electron-hole interaction, andthe localisation of carriers by fluctuations in wire width, are taken into account.

Picosecond decay dynamics of luminescent porous silicon has been studied using the second harmonics (SH) of a cw modelocked YLF laser and a synchroscan streak camera. Picosecond luminescence decay shows nonexponential behavior that becomes large with decreasing emission energy. When increasing hydrogen termination on the surface of a Si microcrystal occurs, this picosecond luminescence decay becomes faster. Our experimental results indicate that there are two luminescent states in porous Si : a weak luminescent quantum confinement state and a strong luminescent surface localized state.

A detailed ESR study has been performed on porous silicon on both <100> and <111> p-type silicon substrates prepared using anodization in HF under a range of conditions and the results are correlated with the light emission properties. It is found that the ESR spectra are dependent upon the orientation of the samples. The ESR defect centers are identified to be the Pb centers or Pbo centers of the Si-SiO2 system from the g-value anisotropy maps. The variation of the spin density Ns with annealing conditions has also been studied for samples annealed either in nitrogen or oxygen ambient at 200°C for various time intervals. It is concluded that the increase or decrease of Ns are due to the generation or elimination of the Pb or Pbo centers in conjunction with the oxidation process during annealing. From PL study of these samples, it is found that there is no simple correlation between the spin density and the PL intensity. However, a blue shift in the PL peak position was observed both in samples after a post-annealing etch in HF solution, and in samples annealed in oxygen without a post-annealing etch. This blue shift supports the quantum confinement model of light emission from porous silicon.

Photoluminescence in porous silicon on sapphire (SOS) prepared through chemical staining and photo-initiated chemical etch is demonstrated. SOS consists of single crystal quality material on a transparent substrate through which photoluminescence can be excited and observed. Porous silicon films as thin as 270 nm are shown to photoluminesce. This study explores the effect of varying film thickness and crystal quality on the photoluminescent emission from porous silicon. Introduction of crystal defects by ion implantation is used to create controlled damaged layers prior to fabricating porous silicon. No photoluminescence is observed from chemically stained or electrochemically etched amorphized layers. Corroborative results are obtained using LPCVD deposited amorphous silicon layers.

We have detected two dominant paramagnetic centers in porous silicon by electron paramagnetic resonance (EPR). One of them is isotropic, assigned to a defect in amorphous silicon oxide in the porous silicon layer. The other is anisotropic, and is very much like a Pb center at a planar Si/SiO2 interface. This EPR center is unambiguously identified as an •Si≡Si3 moiety, a silicon with dangling orbital, back-bonded to three silicon atoms, by 29 Si hyperfine structure (HFS) associated with the dangling orbital, and 29 Si superHFS from three neighboring silicon atoms, as similarly observed in the usual planar surface Pb structure. The dangling orbitals are highly localized and heavily p character. The disposition of dangling orbitals is evidence that the skeletal structure of luminescent porous silicon is crystalline and has a lattice which is aligned and continuous with the wafer substrate. The possibility that these centers are the major photoluminescent killers or quenchers is not supported by our hydrogen annealing experiments.

Porous silicon (PSI) samples with high photoluminescence (PL) efficiency were examined using microscopic mapping techniques including scanning election microscopy (SEM) and spatially resolved photoluminescence (SRPL). Studies of the growth conditions indicate that the homogeneity of the PL and the surface roughness of the sample depend on the preparation procedure of the PSI layer. SEM and spatially-resolved reflectance scans of an n-type sample with a PSI layer on the order of 100 μm reveal a highly fractured surface. SRPL of the same sample shows non uniform PL on a scale of 5 μm. Cross-sectional analysis of the samples with SRPL and SEM reveals the intricate multilayer structure of the PSI film. The top portion of the PSI film is largely responsible for the PL and is composed of isolated column-like structures. We have also observed cross-shaped structures reminiscent of stress fractures on the surface of PSI films using SEM or optical microscopy. Furthermore, atomic force microscopy (AFM) and SEM measurements of the surface of PSI films of intermediate thickness reveal that samples which appear smooth on ̃ lμm scale are actually covered with clumped structures on the order of 100 to 150 nm.

Time-resolved studies of the visible photoluminescence in porous silicon with three different coverages of the internal surface are reported. We use aged, naturally oxidized porous Si (oxihydride), rapid thermal processed material (oxide) and samples stored in HF (pure hydride). A new, fast luminescence band in the blue-green spectral range and with response time less than 100 ns is observed at room temperature in each of the samples, although with different intensities. The observations prove that this is not an oxide-defect luminescence. We speculate on mechanisms for the origin of the fast luminescence in nanometer-size crystallites of Si.

Porous Si formed by anodization is studied by means of high-resolution scanning electron microscopy, optical microscopy, micro-photoluminescence spectroscopy, high-resolution transmisson electron microscopy, and micro-Raman spectroscopy. Surface morphologies of porous Si films are varied by changing the anodization conditions. High-resolution scanning electron microscopy measurements show that in all these films, porous Si fine structures of visible photoluminescent regions, which are confirmed with optical microscopy and micro-photoluminescence spectroscopy, are composed of particlelike structure of various sizes from several nm to several tens of nm. Transmission electron microscopy shows that these particles are composed of single-crystal Si spheres covered with an oxide layer. The peak of the Raman spectrum from the particlelike structure is shifted to lower energy and is wider than that for bulk Si, possibly due to lattice expansion around large surface fractions. These results suggest that the effective photoluminescence of porous Si is due to three-dimensional carrier confinement in quantum boxes.

The photoluminescence and infrared absorbance of electrochemically anodized porous silicon samples were examined as a function of hydrofluoric acid (HF) etching time. Transmission FTIR spectroscopy measurements revealed that the infrared absorbance from silicon hydrogen surface species was largest for the initial porous silicon samples and immediately decreased with HF etching. In contrast, the photoluminescence did not appear until after HF etching times of 20–80 minutes, depending on initial sample porosity. Subsequently, the photoluminescence intensity increased, reached a maximum, and then progressively decreased versus HF etching time. These HF etching results demonstrate that there is no direct correlation between the photoluminescence and the silicon hydrogen surface species.

We elaborate on the explanation of radiative emission involving surface-confined states on the nanocrystallites of porous Si. After reviewing the evidence for the existence of such states, we give a model description of the origin of visible luminescence in porous Si. The model accounts for different spectral luminescence bands with distinct energies and relaxation times.

The optical properties of porous Si (p-Si) are compared to those of siloxene and its derivatives in order to gain more insight into the mechanism of the luminescence observed in p-Si. We report new results of photoluminescence (PL), photoluminescence excitation (PLE), time-dependent and pressure-dependent photoluminescence, and optically detected magnetic resonance (ODMR). Important information about the structural, electronic, and microscopic nature of the two classes of materials are deduced from these experiments. Annealed siloxene and p-Si show very similar properties, suggesting that siloxene-related structures, e.g. electrically isolated Si6-rings, might be responsible for the luminescence in p-Si. The Si-planes in as-prepared siloxene, with their green luminescence, are metastable and are readily oxidized into red-luminescent siloxene configurations.

Small-angle neutron scattering (SANS) measurements of four electrochemically etched, porous silicon (PS) samples have been performed over a wide wavevector transfer (Q) range. The intermediate to high Q results can be modeled with a non-particulate, random phase model. Correlation length scales on the order of 1 to 2 nm thought to characterize the PS skeleton have been deduced from the SANS data. The microstructural anisotropy was studied tilting two of the samples with respect to the neutron beam. These samples exhibited an asymmetric scattering pattern at intermediate Q (0.1 ≤ Q ≥ 0.6 nm-1) in this condition. Photoluminescence spectra from all four samples have been recorded as well. A correlation appears to exist between the SANS and photoluminescence measurements. An x-ray diffraction measurement of one sample demonstrates that the PS layer retains the silicon lattice structure. Significant peak broadening is observed that we interpreted as a quasi-particle size effect The PS particle size calculated from the x-ray diffraction measurement is equal to the correlation length obtained in the SANS measurement.

The characterization of porous silicon layers by optical reflectance spectroscopy in the infrared, visible and UV is presented. A fit of simulated to measured spectra is used to interprete the experimental results. We stress that the microgeometry of the porous system determines the optical properties to a large extent and must be taken into account in a correct way in order to achieve reliable results.

We have studied effects of hydrogen-termination on optical properties of porous Si. The hydrogen concentration in the samples was controlled by photochemical etching after the electrochemical anodization. Transmission electron microscopy examination shows that the size of Si crystallites in porous silicon ranges from 3 to 10 nm and no significant size changes occur by the photochemical etching process. The PL spectra, the PL excitation spectra, and the picosecond PL decay rate were sensitive to the photochemical etching process. Spectroscopic analysis suggests that the radiative recombination occurs at the near-surface region of the Si crystallites.

The structure of aged porous silicon (PS) has been investigated using reflectivity, photoluminescence (PL), optical absorption and photoconductivity decay. An optical anisotropy in a perpendicular to surface direction is observed for PS samples below a critical porosity of 70%. The photostability of the PL spectra imply three different structures of PS, these should be attributed to quantum size structures, polymeric type structures and fluor-related structures. The absorption edge is deconvoluted into a band-to-band absorption with energy ranging between Eg = 1.6–2 eV and an absorption shoulder at hu ≤ Eg. The high energy PL band around 2.3 eV resembles that of fluoroindate glass. The observed rapid lifetime at high injection level is attributed to excitonic Auger-type recombination.

Rapid photoluminescence intensity degradation are observed in porous silicon layers formed at different preparation conditions. The decay rate, ranging from 0.5 to tens of seconds, is found to be a function of illumination intensity, sample temperature and emission wavelength. We attributed the degradation effect to photochemical reactions on the surface.

Highly porous silicon, well passivated via an anodic oxidation process, is a stable and efficient visible light emitter showing a 3% photoluminescence efficiency at room temperature. Luminescence decay times are on the order of 100 μs at room temperature and 10 ms at low temperature. Above room temperature the de-excitation is dominated by non-radiative processes well describe by a tunnelling escape of carriers from confined regions. The “anomalous” luminescence behaviour showing a dramatic increase of the lifetimes upon cooling associated with a decrease of the intensity is explained by the temperature dependence of the effective radiative recombination rates due to a population redistribution among two excited states with very different radiative relaxation rates.

Rapid thermal oxidation of porous silicon leads to desorbtion of hydrogen from the inner surface and formation of a thin oxide layer. Despite this dramatic change in the chemical composition oxidized microporous silicon (miPS) shows photoluminescence (PL) in the visible region. This is contradictory to the idea that the observed PL originates from chemical compounds like siloxene or polysilane, which would require a certain stoichiometry. If silicon microcrystallites are still present in the oxidized miPS, the observed luminescence can be explained in terms of a quantum confinement effect. It is the purpose of this work to prove the existence of microcrystallites in oxidized miPS using electron microscopy and X-ray diffraction.

We studied the defect properties present in rapid thermal oxidized porous silicon (RTOPS) by Electron Spin Resonance (ESR). Two different types of defects are distinguished, one similar to the ones observed in damaged c-Si, and in a-Si. The second one is probably related to the Pbo center at the Si/SiO2 interface. The minimum density of 1016 cm-3 is observed for the as etched and for the 900°C oxidized samples, but reaches a maximum of 8×1018 cm-3for the 600°C samples. The PL intensity anticorrelates with the defect densities, which shows that nonradiative recombination via defects is a very powerful channel in quenching the PL efficiency.

Luminescent oxidised porous Si is produced by rapid thermal annealing of the anodised material in a dry oxygen ambient. Its light-emitting properties are studied by both photoluminescence and cathodoluminescence methods. The structure of the oxidised material is examined by transmission electron microscopy, while its oxygen content is determined by X-ray microanalysis. These investigations show that crystalline Si nanostructures remain in the oxidised porous material and account for its luminescence properties. The work demonstrates that the speculated importance of either Si-based amorphous phases or the interesting material, siloxene, in this regard is unrealistic.