Light-emitting Porous Silicon Research Articles

Quantitative Determination of Color Coordinates for Lumninescent Materials

On the basis of the photoluminescence spectra, a spectrophotometric methodology is developed to determine the color coordinates of luminescent materials such as the red light-emitting porous silicon film, the green light-emitting aluminate-based phosphor and the blue light-emitting liquid crystal.

Preparation of tunable silicon q-dots through ultrasound

Silicon quantum dots (QDs) have been prepared through ultrasound treatments of light-emitting porous silicon layers (PSL) electrochemically etched from a p + type crystalline silicon (c-Si). The sonication treatments allowed separating the porous fraction from the bulk of c-Si as well as to mechanically reduce their dimensions. The ultrasounds processes have been carried out in two different organic solvents (toluene and tetrahydrofuran), and allowed obtaining silicon QDs emitting light in the blue–green part of the visible spectrum (estimated QDs diameter around 5 nm). Moreover, by adding the proper chemicals in the solvents, such as alkenes, or simply paraffin oil, we have stabilized the QDs achieving surface modification and observed an effect on size reduction. Photoluminescence spectra of the QDs, TEM images and a preliminary micro-FTIR investigation of functionalised QDs will be presented in this paper.

Properties of light-emitting porous silicon formed by stain etching in HF∕KIO3 solution under light illumination

The structural and optical properties of stain-etched porous silicon are investigated using optical microscopy, scanning electron microscopy, ex situ atomic force microscopy (AFM), photoluminescence (PL), and Fourier transform infrared (FTIR) spectroscopy. The stained layers were formed in a 50% HF solution with the addition of an oxidizing agent of KIO3 on n-type silicon substrates with and without light illumination of λ=532nm. Strong yellowish emission is observed only when porous silicon is formed under light illumination and in the limited KIO3 concentrations. Surprisingly enough, no PL emission is observed on stained surfaces formed without light illumination. The surface morphology as characterized by AFM reveals that light illumination can produce microscopic roughnesses on the stained surfaces. No specific chemical species on the stained surfaces can be identified by FTIR spectroscopy. The origin of the yellowish luminescence is hypothesized to be due to the quantum-size effect that enables the nondirect optical transitions via a momentum conservation relaxation.

Formation of micrometer-sized electrical contacts on light-emitting porous silicon films

In the aqueous electrolyte of copper (II) chloride, metallic conducting thin films have been electrochemically deposited onto the top surfaces of porous silicon (PS) films. Employing the scanning electron microscopy (SEM), we have investigated the surface morphology for an original PS film, a lightly deposited PS film and a heavily deposited PS film, respectively. On the basis of the SEM micrographs, the surface roughness of each PS film is quantitatively analyzed. Our results have demonstrated that the originally rough surfaces of PS films can be smoothed by the electrochemical deposition so that electrical contacts can be easily formed on PS films. On the smooth bed of copper microcrystals, both center-hollowed and center-solid equilateral triangles are observed in the sizes of several micrometers.

Spectroscopic investigation of light-emitting porous silicon photoetched in aqueous HF∕I2 solution

The optical properties of porous silicon (PSi) photoetched in aqueous HF∕I2 solution are investigated using spectroellipsomety (SE), electroreflectance (ER), photovoltage (PV), photoconductivity (PC), photoluminescence (PL), and Fourier transform infrared (FTIR) spectroscopy. The PSi layers were formed in a HF∕I2 solution on n-Si substrates under Xe lamp illumination. The SE ε(E) and related data show an interference oscillation in the region below E∼3 eV, where the PSi material is nearly transparent. The PV and PC spectra reveal three individual peaks A, B, and C at ∼1.2, ∼1.7, and ∼2.5 eV, respectively, arising from the PSi layer itself. Peak C is also observed in the ER spectrum, together with a broadened E1 peak at ∼3.4 eV. Change in the fundamental-absorption-edge nature (EgX) from the indirect gap in crystalline silicon to the quasidirect gap in PSi is found in the PV and PC spectra. The PL spectrum shows a broad peak at ∼2.0 eV(B). Peaks A, B, and C observed in the PSi layer may originate from the nondirect optical transitions at and above the lowest absorption edges EgX (A and B) and EgL(C). The quantum-mechanical size effect, i.e., a relaxation of the momentum conservation, makes possible the nondirect or quasidirect transitions at and above EgX and EgL in porous materials. The FTIR data support that the PL emission is due to the surface-sensitive quantum confinement effect.

Properties of light-emitting porous silicon photoetched in aqueous HF∕FeCl3 solution

The formation of yellow-light-emitting porous silicon (PSi) layers in a HF solution with adding an oxidizing agent FeCl3 is presented. The PSi layers are formed by photoetching under Xe lamp illumination. The photoluminescence (PL) intensity is strongly dependent on the FeCl3 concentration and shows a maximum at x∼25 wt % [50 wt % HF:(x wt % FeCl3 in H2O)=1:1]. The surface topography as characterized by atomic force microscopy reveals features on the order of 20−100 nm with a root-mean-squares roughness of ≤2 nm. The Fourier-transform infrared spectroscopy shows a new absorption peak at ∼1100 cm−1, which is assigned to the surface oxide stretching mode and grows larger with increasing etching time. The stain etched samples also show PL emission, but they are synthesized only at higher x concentrations (≥20 wt %). The PSi formation mechanism can be explained with the aid of a surface energy-band diagram of n-type silicon in the HF∕FeCl3 electrolyte.

Electroluminescence from porous silicon formed by photoetching in an HF/I2 solution

We demonstrate the formation of a light-emitting porous silicon (PSi) layer by photoetching in an HF/I2 solution. The use of a Xe lamp makes possible a large and homogeneous PSi layer on an n-type Si(100) wafer. An insulating layer has been formed on the PSi layer by chemical oxidation in an acidic solution. These techniques are used to fabricate electroluminescent (EL) devices of metal/insulator/semiconductor (MIS) type. It is shown that the present device is superior with respect to emission efficiency to the conventional Schottky- or metal/semiconductor-type device.

Effects of selenium treatment on composition and photoluminescence properties of porous silicon

In this work, an ultrasonically enhanced anodic electrochemical etching is developed to fabricate light-emitting porous silicon material. Porous silicon layer is fabricated in n-type (1 0 0) oriented silicon using HF solution and treated in selenious acid to increase the photoluminescence intensity. It is found that the increase of photoluminescence intensity after selenious acid treatment is higher in the intact zones and lower in the detached zones of ultrasonic excitation. The photoluminescence appears as a non-monotonous function of time exposure of selenious acid treatment. Surface chemical composition analysis by X-ray photoelectron spectroscopy shows formation of Si–Se x and Si–Se x –O y on the surface of porous silicon treated with the selenious acid.

Step voltage with periodic hold-up etching: A novel porous silicon formation

A novel etching method for preparing light-emitting porous silicon (PS) is developed. A gradient steps (staircase) voltage is applied and hold-up for different periods of time between p-type silicon wafers and a graphite electrode in HF based solutions periodically. The single applied staircase voltage (0–30 V) is ramped in equal steps of 0.5 V for 6 s, and hold at 30 V for 30 s at a current of 6 mA. The current during hold-up time (0 V) was less than 10 μA. The room temperature photoluminescence (PL) behavior of the PS samples as a function of etching parameters has been investigated. The intensity of PL peak is initially increased and blue shifted on increasing etching time, but decreased after prolonged time. These are correlated with the study of changes in surface morphology using atomic force microscope (AFM), porosity and electrical conductance measurements. The time of holding-up the applied voltage during the formation process is found to highly affect the PS properties. On increasing the holding-up time, the intensity of PL peak is increased and blue shifted. The contribution of holding-up the applied steps during the formation process of PS is seen to be more or less similar to the post chemical etching process. It is demonstrated that this method can yield a porous silicon layer with stronger photoluminescence intensity and blue shifted than the porous silicon layer prepared by DC etching.

Fractal microstructures of light-emitting porous silicon films

By means of scanning electron microscopy and computer simulation, we have investigated the microstructures of a 23-μm-thick porous silicon (PS) film and a 6-μm-thick PS film. The two films give off strong visible emissions when excited by the 254nm light. For the 23-μm-thick PS film, both of its cross-sectional and top-surface morphologies exhibit self-similarity whose small-scale and large-scale microstructures resemble one another. For the 6-μm-thick PS film, self-similar cracks are developed on its top surface. Our results have demonstrated that the microstructures of PS films exhibit the characteristics of fractals. With box counting method, the fractal dimensions of the PS films are calculated to be about 2.3–2.6. Based on the model of diffusion-limited aggregation, the fractal growth processes have been simulated for the PS films.

Light-emitting porous silicon formed by photoetching in aqueous HF/KIO3 solution

The formation of light-emitting porous silicon (PSi) in a HF solution by adding an oxidizing agent KIO3 is presented. The PSi layers are formed by photoetching under Xe lamp illumination. The use of a Xe lamp enables the formation of large PSi layers on n-Si substrates. Yellow photoluminescence peaking at ∼2.0 eV is observed for the sample prepared in a HF/KIO3 solution for 6 min, which is about 80 times stronger than that formed in a pure HF solution for 105 min. The surface topography as characterized by atomic force microscopy reveals features of the order of 20–100 nm. The PSi formation mechanism can be successfully explained with the aid of a surface energy-band diagram of n-Si in the HF/KIO3 electrolyte.

The effect of chemical treatment on porous silicon: the role of alcohol

ABSTRACTWe investigated the effect of the chemical etching on light-emitting porous silicon. It is well known that the treatment with the mixture of HF acid and ethanol noticeably enhances photoluminescent efficiency but the mechanism is not clarified yet. We found that ethanol plays a key role to oxidize silicon hydride species that dissolves with the aid of HF acid. In addition, the chemical treatment with methanol or propanol instead of ethanol gave the same result.

Porous Silicon Influence of Etching Temperature on Microstructure and Luminescence

Electrochemical etching in solutions based on hydrofluoric acid has been widely used to form light-emitting porous silicon. However, the effects of a number of the experimental parameters on the quality of the porous silicon produced have yet to be fully investigated. In the present paper the influence of temperature and viscosity of the etching solution is evaluated in terms of the morphology and porosity of the porous silicon produced as well as the wavelength of the photoluminescence or electroluminescence subsequently emitted. It was found that under stimulation from a UV light source the wavelength of the photoluminescence emitted from the porous silicon films blueshifted with decreasing etching temperature. SEM and AFM investigations revealed that this blueshifting of the photoluminescence resulted from the production of smaller nanocrystals at the lower etching temperatures.

POROUS SILICON: INFLUENCE OF ETCHING TEMPERATURE ON MICROSTRUCTURE AND LUMINESCENCE

Electrochemical etching in solutions based on hydrofluoric acid has been widely used to form light-emitting porous silicon. However, the effects of a number of the experimental parameters on the quality of the porous silicon produced have yet to be fully investigated. In the present paper the influence of temperature and viscosity of the etching solution is evaluated in terms of the morphology and porosity of the porous silicon produced as well as the wavelength of the photoluminescence or electroluminescence subsequently emitted. It was found that under stimulation from a UV light source the wavelength of the photoluminescence emitted from the porous silicon films blueshifted with decreasing etching temperature. SEM and AFM investigations revealed that this blueshifting of the photoluminescence resulted from the production of smaller nanocrystals at the lower etching temperatures.

Metal-assisted chemical etching in HF/H2O2 produces porous silicon

A simple and effective method is presented for producing light-emitting porous silicon (PSi). A thin (d<10 nm) layer of Au, Pt, or Au/Pd is deposited on the (100) Si surface prior to immersion in a solution of HF and H2O2. Depending on the type of metal deposited and Si doping type and doping level, PSi with different morphologies and light-emitting properties is produced. PSi production occurs on the time scale of seconds, without electrical current, in the dark, on both p- and n-type Si. Thin metal coatings facilitate the etching in HF and H2O2, and of the metals investigated, Pt yields the fastest etch rates and produces PSi with the most intense luminescence. A reaction scheme involving local coupling of redox reactions with the metal is proposed to explain the metal-assisted etching process. The observation that some metal remains on the PSi surface after etching raises the possibility of fabricating in situ PSi contacts.

Possibility of fabricating light-emitting porous silicon from gas phase etchants

We present a new technique for porous semiconductor formation which is based on the exposure of semiconductor surfaces to gas phase etchants. The technique offers the possibility of fabricating light-emitting devices by selectively exposing a silicon surface to HF vapor. Photoluminescence measurements reveal an efficient emission at around 750 nm. FTIR analysis confirm the existence of strong hydrogen incorporation and oxidation as evidenced from the local bonding environment of hydrogen and oxygen atoms.

Photoluminescence from nano porous silicon prepared by photoelectrochemical etching of n-type single crystalline silicon

AbstractThe influence of operating parameters in producing light-emitting porous silicon materials was investigated in ethanolic solutions of hydrofluoric acid. Photoluminescence spectra depended on applied potential, the intensity and wavelength of illumination, and electrolyte concentration. When the applied potential and the illumination wavelength increased, the photoluminescence shifted to longer wavelength. Change in HF concentration resulted in different intensity in photoluminescence.

Physical structure of light-emitting porous polycrystalline silicon thin films

Abstract This paper reports the surface electronic structure of light-emitting porous polycrystalline silicon (PPS) using X-ray photoelectron spectroscopy (XPS). We find that the PPS films with strong photoluminescence (PL) effect can only be observed in thin film with trace amount of silicon nanoclusters and the luminescence can be enhanced remarkably with proper passivation of the PPS surface. Incomplete oxidation of silicon (Si 3+ or Si 2+ ) does not lead to visible PL. We further estimate that the average size of silicon nanoclusters is in the range of 20–30 A in the sample having PL emission.

Spatial versus quantum confinement in porous amorphous silicon nanostructures

Light-emitting porous amorphous silicon has been produced by anodization in HF of hydrogenated amorphous silicon films. The maximal thickness of the porous films is limited by the onset of an instability which results in the formation of large channels short-circuiting the amorphous layer. This is due to the high resistivity of the amorphous silicon films as compared to that of the electrolyte. Confinement effects on the electron wavefunction are analyzed in situ using photoluminescence measurements in hydrofluoric acid and compared to those observed in porous crystalline silicon. For crystalline silicon, a huge blue shift of the photoluminescence is observable upon reducing the size of the structures by photo-etch, showing clear evidence of quantum confinement effects in this material. No shift has been observed when carrying out the same experiment with amorphous silicon. This indicates that the extent of the wavefunction in the bandtail states involved in luminescence is too small to be sensitive to confinement down to the minimum sizes of our porous material (\(\) 3 nm). Measurements of the width and the temperature dependence of the photoluminescence demonstrate that the Urbach energy does not change upon increasing the porosity, i.e., upon decreasing the size of the a-Si:H nanostructures, in contradiction with what has been reported in ultrathin a-Si:H multilayers.

Short time-scale radiative transfer in light-emitting porous silicon

In this work, a five level system is proposed to model the short time-scale radiative transfer in light-emitting porous silicon. Equations are derived for the various energy levels. The model is applied to picosecond differential transmission measurements made on porous silicon. A picture of short time-scale carrier dynamics is drawn considering the unique properties of confinement, high surface-to-volume ratio and disorder on several length scales.