Porous silicon layers have been formed which exhibit photoluminescence (PL) peaks that do not blueshift with increasing porosity. Hydrogen desorption experiments have been performed in vacuum under anneals between 230°C–390°C. Simultaneous, in-situ PL measurements show that the PL intensity decreases and disappears with decreasing hydrogen content. Infrared spectroscopy (IR) measurements also show loss of hydride species in the same temperature range. These results indicate that hydrogen is important in the PL process. We suggest that silicon hydrides are the source of PL in porous SI.

Photoluminescence spectra have been measured in porous silicon following electrochemical etching in dilute hydrofluoric acid (HF). The effects of HF concentration during etching on the efficiency and peak wavelength of photoluminescence have been investigated. The effects of temperature between 25°C and 200°C on PL spectra have been recorded. Photoluminescence lifetimes as a function of wavelength have been studied following ultrashort UV photoexcitation. A number of lifetime components in the decay are observed the longest in good agreement over the wavelength range of 500 to 600 nm with a silicon quantum wire model. At longer wavelengths a departure from lifetimes of the wire model is observed and two hypotheses for the discrepancy are presented.

A method for synthesizing crystalline and/or amorphous silicon nanoparticles (3 – 30 nm diameter) is discussed. The photoluminescence (PL) behaviour of these particles is studied as crystallinity, size, surface stoichiometry, and excitation frequency are varied. Orange-red photoluminescence (PL), similar to that reported for “porous silicon,” is observed. The wavelength dependence of this PL is not a function of size, but rather particle crystallinity. A separate PL feature, near 430 nm, is reported. No size dependent effects are observed for this feature.

A novel fabrication technique involving the use of focused ion beam (FIB) selective implantation to fabricate nanostructures on crystalline Si substrates in conjunction with anisotropic etching is described. Using this maskless & resistless approach, Si nanostructures were fabricated by FIB implantation of Ga+ at doses from 1015 to 1016/cm 2. Wet etching in KOH/IPA does not attack the implanted region, while it removes the underlying Si anisotropically, with a very low etch rate on the {111} planes. The result is a cantilever-like structure whose thickness is dependent on the implantation energy and dose. Pre-etching rapid thermal annealing at 600°C for 30 sec does not prevent structure fabrication and post-etching RTA does not affect the shape of the structures.

Microcrystalline silicon embedded in silicon oxide has been prepared by means of partial oxidation of porous silicon produced anodically from degenerate p-Si wafers. Their optical properties such as absorption coefficients and luminescence have been characterized. Results show blue shifts in absorption and photoluminescence spectra in a visible wavelength region with decreasing size of the microcrystalline Si in the Si oxide matrix. The quantum size effect is discussed as well as possible origins of the observed visible luminescence, including light emission from as-anodized (or H-chemisorbed) porous silicon.

A procedure for generating colloidal suspensions of Si exhibiting luminescence, attributed to quantum confinement effects, is described. Samples of n- or p-type Si, that have been electrochemically etched to form porous Si, can be ultrasonically dispersed into methylene chloride, acetonitrile, methanol, toluene, or water solvents, forming a suspension of fine Si particles that luminesce. Transmission electron microscopy analyses show the Si particles to have irregular shapes, with diameters ranging from many microns to nanometers. Luminescent, composite polystyrene/Si films can be made by the addition of polystyrene to a toluene suspension of the Si nanoparticles and casting of the resulting solution.

Interaction of the solvents tetrahydrofuran, diethyl ether, methylene chloride, toluene, o-xylene, benzene, and methanol with luminescent porous n- Si (PS) results in reversible quenching of the luminescence associated with this material. The degree of quenching ranges from 99% – 50%, and scales with solvent dipole moment. Reaction with gaseous Cl2, Br2, or I2 results in irreversible quenching, associated with a surface reaction that removes Si-H bonds. Total luminescence quenching is observed on treatment of a PS wafer with a solution of the electron donor ferrocene in toluene, suggesting that charge transfer quenching may also be operative in this material. Luminescence is partially recovered by rinsing the PS in pure toluene. The data show that photoluminescence of PS is highly sensitive to surface adsorbates, suggesting that carrier trapping is easily induced in this material.

Visible light emission from porous silicon has recently been demonstrated by several groups. We have found that annealing of porous silicon samples in an inert atmosphere over 250C reduces the emission intensity, almost fully quenching it by 450C. We believe that this is due to the removal of passivating hydrogen, based on measurements of infrared absorption due to Si-H vibrational modes. We have also found that we can partially recover the emission intensity in these annealed porous silicon layers by passivation in a hydrogen plasma. We have therefore demonstrated that hydrogen passivation is necessary for the observation of visible emission from porous silicon. We have also shown that thermal oxidation of porous silicon over 200C reduces the PL emission. We propose that this reduction is also due to the loss of hydrogen passivation during the oxidation.

This paper attempts to address the question of the role played by dimensionality in the observed optical transitions of porus silicon and related material systems. The effect of the degree of porosity on the transition energies, line widths and thermal stability. In addition more subtle effects observed in the thermal behaviour of the spectra which may relate to carrier trapping in a random potential of an imperfect localised system are reported. Comparisons with the spectral behaviour which would be predicted by quantum scale confinement of electronic particles are made.

We report photoluminescence emission and photoluminescence excitation studies of porous silicon obtained from p-type Si(111) wafers over the range of 0.9–13.0 eV (∼400–1400 nm). Strong IR emission above and below the bulk silicon band gap at ∼1.09 eV (1135 nm) at 300 K was observed. This luminescence due to intrinsic band-to-band recombination, was found to be enhanced by two orders of magnitude or more over the IR spectrum from an unanodized wafer. The visible luminescence peak that has been attributed to quantum confinement effects in porous silicon was located at ∼1.8 eV (689 nm). We studied the high intensity IR and visible PL in detail as a function of temperature (4–700 K), laser excitation energy (1.96–3.80 eV), and laser power (0.1–1000 mW). The measured shift in the bulk silicon band gap was approximately linear from 300–700 K (∼−0.22 meV/K), while the visible peak red-shifted by ∼− 1.24 meV/K over the same range. Below 300 K they both had comparable temperature dependences. The visible PL was investigated at room temperature in magnetic fields up to 15 T; no discernible shift in the peak position or change in the peak intensity was observed.

The visible photoluminescence of electrochemically etched silicon wafers is studied by cw- and pulsed-laser excitation. Raman data and infrared transmission measurements on the same samples prove the presence of oxygen and hydrogen in different bonding configurations in the luminescent layers. Identical optical properties are found for chemically synthesized siloxene (Si6O3H6) and its derivates. We present evidence that the origin of the strong room temperature luminescence in “porous” silicon can be traced to siloxene derivates present in the samples.

Stain films on Si wafers produced in solutions of HF:HNO3:H2O have been studied for over 30 years [1], and have been suggested [1] to be similar in nature to the anodically-etched porous Si films first demonstrated by Uhlir [2]. More recently, it was shown that stain films produced by etching Si in solutions of NaNO2 in HF and CrO3 in HF were similar in structure to porous Si films produced by anodic etching [3]. In fact, in the etching of Si by HF:HNO3:H3O solutions, the oxidation reaction chemistry is recognized to be the same as that of anodic oxidation, with points on the Si surface behaving randomly as localized anodes and cathodes [4]

The photoluminescence (PL) and X-band ODMR of porous Si layers is described and discussed. The layers were prepared by anodizing the (100) face of a Si wafer at 20 mA/cm2 in 20% HF for 5 mai and passively soaking them in 36% HF for up to 10 hrs. The PL was broad and featureless, extending from ˜1.5 to ˜2.1 eV and peaking at 1.68 eV. Its intensity slightly increased upon cooling to 90 K, and then strongly decreased at lower temperatures. A ˜20 G wide asymmetric PL-enhancing ODMR was observed at g ˜2.0031 ±I 0.0009, which could be fit to a sum of two Gaussians. Their g-values were slightly temperature dependent. The ODMR intensity strongly decreased with increasing temperature, and was unobservable above ˜80 K. The results are compared to the optical properties of hydrogenated amorphous Si.

Raman spectra from electrochemically etched porous silicon are correlated with photoluminescence (PL) data from the same spots of the sample. This correlation is consistent with optical properties of quantum confinement. The dielectric constant determined from angle resolved ellipsometry gives values far below that of bulk silicon. This reduction is due to the combined effects of voids as well as quantum confinement. The PL spectrum shows a weak high energy peak around 2.8eV in addition to the strong broad peak at 1.5 to 1.9eV. The temperature dependence of PL resembles that of bound excitons such as Si:S, having a thermal dissociation energy of 100 meV near room temperature. The radiation life time changes from tens of microseconds near room temperature to a few milliseconds at liquid helium temperatures. The rapid increase in lifetime and decrease in PL intensity at low temperatures indicates that phonons are probably involved.

A microcrystalline model for the light emitting portion of porous silicon is outlined. Confinement to a short length scale induces an effective direct dipole matrix element for radiative recombination. The radiative recombination time is strongly size (hence confinement induced energy shift) dependent, and in the microsecond regime for blue shifts of ˜1 eV. Trends and comparison to experiment are discussed.

The structure of porous silicon exhibiting efficient visible photoluminescence has been characterized by using Fourier transformed infrared absorption, Raman scattering and x-ray diffraction. It is shown that the lattice spacing in the porous Si layer expands by about 0.3% in the direction perpendicular to the surface and also a partially disordered structure is existing. Electron beam irradiation causes desorption of hydrogen and fluorine bonds which terminate the surface, resulting in the quenching of the visible luminescence. The chemical etching of such layer has led to complete recovery of the luminescence intensity as well as the hydrogen and fluorine bonds termination.

The luminescent characteristics of porous silicon are affected by the preparation conditions as well as subsequent treatment. Oxidation, which easily occurs in ambient conditions under illumination, greatly reduces the luminescence efficiency. Vertical inhomogeneity, which is inherent from the preparation procedure, can complicate interpretation of results, especially following treatments which non-uniformly affect the porous material. These results point out the need to pay careful attention to the sample's structure and history, as well as the necessity of controlling this degradation in order to realize practical luminescent devices.

For light-emitting porous Si there has been a severe problem with instability and degradation of the light emission. We report that a stabilization of the emission intensity and the peak energy can be achieved in air by a proper laser irradiation, In-situ photoluminescence measurements were performed to monitor the degradation and stabilization process under different conditions and parameters, such as laser power, laser wavelength and environment (ambient atmosphere of certain gas or ultra high vacuum). We found oxygen is the major cause for the emission degradation in this laser enhanced adsorption process, and the laser heating effect can be excluded. For a comparison we study the reversible thermal heating and quenching process. We also discuss microwave and ECR plasma passivation results.

Thermal annealing studies of the photoluminescence (PL) intensity and Fourier-transform infrared (FTIR) spectroscopy have been performed concurrently on porous Si. A sharp reduction in the PL intensity is observed for annealing temperatures > 300 °C and this coincides with desorption of hydrogen from the SiH2 surface species. The role of silicon hydride species on the photoluminescence intensity has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H-plasma. The as-passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH2 termination on the surface. Photoluminescence measurements and transmission Fourier-transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH2) coincides with an increase in the photoluminescence intensity. To achieve electroluminescence it will be beneficial to generate carriers with sufficient energy to populate the states of the quantum-confined Si structures. A viable method to accomplish this is to utilize a wide-bandgap heterojunction injector such as GaP. Toward that end we report the successful formation of porous Si buried underneath GaP islands and we demonstrate that the buried porous Si layer exhibits strong photoluminescence.

The photoluminescence and electroluminescence of light emitting porous silicon (LEPOS) is described. The porous silicon is made by anodic dissolution of silicon in HF with an applied electrical current and illumination with visible light. Photoluminescence is observed using ultraviolet light, visible electroluminescence is achieved by applying a voltage to a solid state contact on top of the porous layer. The luminescence, the structure and the composition of the LEPOS are studied.