Light-emitting Porous Silicon Research Articles

Formation Mechanism and Pore Size Control of Light-Emitting Porous Silicon

The mechanisms of silicon dissolution and pore formation during the formation of porous silicon layers (PSLs) are investigated in the HF-water electrolyte solution. As HF concentration increases in the electrolyte, the depth of pores increases while the pore diameter decreases. It is found that hydroxide ion ( OH-) plays an apparently significant role in the reaction with silicon. The formation of silicon oxide is observed on the silicon surface during PSL formation. The dissolution mechanism of silicon is presented, based on spectroscopic analyses for the porous silicon surface. The area of initially formed silicon oxide on the silicon wafer surface determines the pore diameter. With increasing amount of OH- ions, or equivalently decreasing HF concentration, the thickness of the silicon walls between pores is reduced, revealing the blue shift of photoluminescence energy due to the quantum size effect.

Origin of multiple-peak photoluminescence spectra of light-emitting porous silicon.

Origin of multiple-peak photoluminescence spectra of light-emitting porous silicon.

Energy band lineup at the porous-silicon/silicon heterointerface measured by electron spectroscopy

The energy band gap of light-emitting porous silicon is determined by high-resolution electron energy loss spectroscopy, and the valence band edge of porous silicon with respect to its Fermi level is measured by ultraviolet photoelectron spectroscopy. By combining the results with that measured from clean Si, a picture of band lineup at the porous-silicon/p-Si heterointerface is proposed, in which 70% of the total band gap discontinuity occurs at the valence band edge.

Writing electronic nanometer structures into porous Si films by scanning tunneling microscopy

Under conditions of increased tunnel current and voltage, with the electron flow directed towards the sample, nanometer-scale structures have been written into ultrathin (∼20 nm) light-emitting porous silicon (PS) films using scanning tunneling microscopy in a high-vacuum environment. For the writing process, a threshold voltage of ∼4.5 V is observed and the resulting dimensions range between 20 and 50 nm. Depending on the writing parameters, the modified regions relax or remain stable during the observation time of several days at room temperature. These results can be assigned, in the first case, to a disruption of a small number of bonds, followed by reconfiguration, and a charging of dangling bond sites, followed by carrier release, in near surface regions. In the latter case, the creation of time-stable defect states within the PS layer is proposed.

Correlation between photoluminescence and surface species in porous silicon: Low-temperature annealing

Photoluminescence (PL) and Fourier-transform infrared (FTIR) measurements have been performed on light-emitting porous silicon (LEPSi) after annealing at temperatures below 600 °C. Two different kinds of samples with different surface morphologies and different initial concentrations of chemically bonded hydrogen were studied. In hydrogen-rich samples we have observed an increase of PL intensity at temperatures up to 250 °C, which correlated with an increase of Si—H bond concentration. A correlation between PL peak wavelength and the ratio of Si—O bonds over Si—H bonds has been demonstrated.

Instability of the structure of light-emitting porous silicon

Instability of the structure of light-emitting porous silicon

Blue emission in porous silicon: Oxygen-related photoluminescence.

A blue-photoluminescence (PL) band, centered at 2.6 eV, was observed in thermally and chemically oxidized light-emitting porous silicon layers with an efficiency that can exceed 0.1%. The PL decay is found to be independent of excitation intensity and detection wavelength, and to be nonexponential with a characteristic time of \ensuremath{\sim}1 nsec. A correlation between the intensity of the blue PL and the intensity of the infrared absorption related to bonded oxygen has been established. These results are examined in relation to the possible mechanisms for the blue-PL band.

Raman investigation of light-emitting porous silicon layers: Estimate of characteristic crystallite dimensions

Light-emitting porous silicon layers prepared on two types of boron-doped p and p+-Si substrates were studied by Raman spectroscopy in a broad frequency range (0–1100 cm−1). Standard phonon confinement model supported by new results from low-frequency scattering permit us to estimate consistently the characteristic dimensions of nanometric crystallites in porous silicon, without invoking complex size distribution. A sizable fraction of a highly disordered (a-Si) phase in the p-type samples is detected, and there is no clear spectral evidence of other Si-based compounds.

Ion implantation of porous silicon

We have investigated the properties of light-emitting porous silicon after ion implantation and successive annealing through continuous-wave photoluminescence (CWPL) and time dependent photoluminescence (TDPL) spectroscopies. Implantation was performed with phosphorus, boron and silicon ions of different doses and energies. Low dose dopant implantation keeps or even increases the CWPL intensity and increases the TDPL decay time. High dose dopant implantation and silicon self-implantation reduce the CWPL intensity and slightly decrease the TDPL decay time.

LUMINESCENCE BEHAVIOR AND MECHANISM OF LIGHT-EMITTING POROUS SILICON

In this review article, we give a new insight into the luminescence mechanism of porous silicon. First, we observed a “pinning” characteristic of photoluminescent peaks for as-etched porous silicon samples. It was explained as resulting from the discontinuous variation of the size of Si nanostructures, i.e. the size quantization. A tight-binding calculation of the energy band gap widening versus the dimension of nanoscale Si based on the closed-shell Si cluster model agrees well with the experimental observations. Second, the blue-light emission from porous silicon was achieved by using boiling water treatment. By investigating the luminescence micrographic images and the decaying behaviors of PL spectra, it has been shown that the blue-light emission is believed to be originated from the porous silicon skeleton rather than the surface contaminations. The conditions for achieving blue light need proper size of Si nanostructures, low-surface recombination velocity, and mechanically strong skeleton. The fulfillment of these conditions simultaneously is possible but rather critical. Third, the exciton dynamics in light-emitting porous silicon is studied by using the temperature-dependent and picosecond time-resolved luminescence spectroscopy. A direct evidence of the existence of confined excitons induced by the quantum size effect has been revealed. Two excitation states are found to be responsible for the visible light emission, i.e. a higher lying energy state corresponding to the confined excitons in Si nanostructures and a lower lying state related with surfaces of Si wires or dots. A picture of the carrier transfer between the quantum confined state and the surface localized state has been proposed. Finally, we investigated the transient electroluminescence behaviors of Au/porous silicon/Si/Al structure and found it is very similar to that of an ordinary p-n junction light-emitting diode. The mechanism of electroluminescence is explained as the carrier injection through the Au/porous silicon Schotky barrier and the porous silicon/p-Si heterojunction into the corrugated Si wires, where the radiative recombination of carriers occurs.

Spectroscopic investigation of electroluminescent porous silicon

Light-emitting porous silicon films have been obtained by anodic etching p-type Si samples in a HF-ethanol solution. Porous Si samples efficiently luminesce at room temperature in the visible region. A degradation of the luminescence intensity with time is observed. Micro-Raman spectroscopy of free-standing porous silicon layers indicates phonon confinements as well as a strong laser heating effects. The surface chemical composition and the effect of electron-beam irradiation has been investigated through Auger spectroscopy. The Si LVV Auger transition dominates the spectrum, even in aged samples. The Si line shape gives evidence of a covalent bond between the porous Si surface atoms and some adsorbed species. A prolonged electron irradiation results in a strong variation of the surface chemical composition, with an anomalous carbon accumulation. Gold thin films have been deposited on the porous Si surface to form metal-semiconductor junctions. Schottky diodes with large rectifying ratio, ideality factor, and series resistance are obtained. When the junction is forward biased, electroluminescence is observed. Electroluminescence degrades with time while the current does not. When the junction is reverse biased a significant photocurrent is obtained. The results are discussed in the framework of the surface state emission model for the luminescence.

Carrier Transport in Porous Silicon Light-Emitting Diodes

ABSTRACTThis work presents a systematic investigation of carrier injection and transport mechanisms in light emitting porous silicon (LEPSI). In addition to optical characterization of the LEPSI film, the electrical characteristics of LEPSI light emitting diodes (LEDs) and the device parameter effects on electroluminescence (EL) efficiency are explored including junction structure, LEPSI layer thickness, and chemical treatment. Au/LEPSI device structures exhibit a power-law current-voltage relationship (I∼Vm) and LEPSI pn junction device structures exhibit an exponential current-voltage relationship (I∼eeV/nkT). The carrier transport mechanisms are discussed, and theoretical simulations match well with the experimental data. An increase in EL efficiency has been achieved by adjusting the LEPSI layer thickness, implementing chemical treatment, and improving the quality of the contacts. The EL response to the frequency modulation of LEPSI LEDs was studied, resulting in a cutoff frequency in excess of 100 kHz. LEPSI LEDs have demonstrated stable EL at applied voltages as low as 5 volts with EL efficiencies of 0.01%.

Post-Anodization Implantation and CVD Techniques for Passivation of Porous Silicon

ABSTRACTProper surface passivation is critical for achieving stable, efficient PL from light-emitting porous silicon (LEPSi). As-anodized LEPSi is passivated by hydrogen which desorbs at a temperature as low as 400 °C. For device purposes, it is necessary that porous Si can tolerate at least 450 °C for post anodization annealing/metallization steps. We have established that, if the hydrogen at the surface is substituted by oxygen, the resulting Si-Ox passivation is significantly more stable. One way of achieving this is to implant low energy/low dose oxygen to form a thin coating of SiO2 on the surface. Post implantation FTIR data report the absence of Si-H peaks. XPS data indicate the formation of nearly stoichiometric SiO2 at the surface. Similar results were achieved by implanting with nitrogen to form Si3N4. As an alternative to implantation, we have deposited thin capping layers of SiO2, Si3N4 and SiC by plasma-enhanced chemical vapor deposition (PECVD) which resulted in a similar degree of passivation. Wafers were pre-treated at 400 °C to remove hydrogen from the surface. Finally, we carried out a low-pressure CVD (LPCVD) oxide deposition on LEPSi. Post implantation/CVD annealing was done at temperatures up to 600 °C. In most cases, little or no change was observed in the resultant PL intensities.

Study of the Raman peak shift and the linewidth of light-emitting porous silicon

The correlation between the Raman peak shift and the linewidth of porous silicon is studied. The experimental result does not fit with the relationship predicted by the phonon confinement model. By taking into account both the phonon confinement and the effect of strain, the calculated Raman line shape coincides fairly well with the measured spectrum. The built-in strain of porous silicon varies with the porosity of the sample and is on the order of 10−3.

EFFECT OF PHONON CONFINEMENT AND STRAIN ON RAMAN SPECTRA FROM LIGHT-EMITTING POROUS SILICON

The typical Raman spectrum from light-emitting porous silicon shows a sharp peak near 520cm －１ . The peak wavenumber decreases with the increase of porosity. The lineshape analysis shows that both phonon confinment and strain effects in nanocrystal silicon must be taken into account for the measured Raman spectra. The strain in porous silicon film is estimated and the result is consistent with that given by X-ray diffraction studies. No light scattering from amorphous phase is observed.

Photolithographic patterning of porous silicon

Regions of light-emitting porous silicon fabricated by anodic electrochemical etching have been patterned by photolithography. Samples were spin-coated with photoresist, and contact photolithography was used to define the desired mask pattern. Exposure of unmasked sample regions to an isotropic oxygen plasma in a barrel reactor results in a dramatic reduction of the visible photoluminescence. The resulting patterns exhibit high contrast, with a minimum feature size of 4 μm. The patterns are remarkably stable, withstanding both chemical treatment and atmospheric contamination.

Nanocrystallites in luminescent porous silicon characterized by Raman scattering

Light-emitting porous silicon (PS) layers prepared on p- and p + -Si substrates were studied by Raman scattering in a broad frequency range. Dimensions of nanometric crystallites in PS were consistently estimated using new results from low-frequency scattering (LOFIRS) besides the implications from standard phonon confinement models. Whereas some fraction of a-Si phase in the p-type PS is detected, there is no clear spectral evidence of complex size distributions or of the presence of Si compounds.

Improvement of electroluminescence properties of light-emitting porous silicon

By applying some post-pore-forming treatments, we have achieved solid state electroluminescence of an Au/PS/Si structure. Stable visible light emission is observed under forward bias with a threshold voltage and current density down to 6 V and 30 mA cm-2.

Magnetic properties of light-emitting porous silicon

We have investigated magnetic properties of porous silicon prepared by anodical etching of n- and p-type Si wafers. Measurements with a SQUID magnetometer have revealed in our samples ferromagnetism characterized by hysteretic behaviour, saturation in fields above 1.3 T and a remanent moment persisting up to T c ⋍ 570 K. Electron paramagnetic resonance investigations have shown the presence of paramagnetic centers related with silicon dangling bonds.

Porous silicon light-emitting p-n junction

The fabrication and properties of a light-emitting porous silicon device incorporating a p-n junction are presented. By means of a selective anodization of a sandwich of p + on n-substrate, a nanoporous recombination region within the p-n junction is formed. The device shows rectifying characteristics with a considerable series resistance. When driven under forward bias, the diode emits light with linear dependence on the current. The measured external quantum efficiency is of the order of 10 -2%. The internal quantum efficiency should be at least one order of magnitude higher.