Inelastic light scattering and X-ray diffraction from thick free-standing porous silicon films

The results of Raman scattering and X-ray diffraction studies of thick, free-standing, porous Si layers with thickness up to 500 μm are presented. The Raman scattering spectra have a distinctive difference from previous data for porous Si films on Si substrate and for thin, free-standing, porous Si layers. The experimental data can be explained by a modified phonon confinement model that accounts for a comprehensive strained Si nanocrystal. The comprehensive strain is a tensile one, and the value of stress can be up to 3 GPa. This interpretation is supported by data of X-ray diffraction measurements.     

Self detachment of free-standing porous silicon membranes in moderately doped n-type silicon

In this article we describe a reliable etching method to fabricate porous silicon free-standing membranes (FSMs) based on a self detachment of the porous layer in moderately doped n-type silicon substrates. We found that stable growth of smooth and straight pores is restricted to a narrow range of etching conditions and, unlike p-type substrates, the lift-off of the membrane is a self-limited process that does not require a large burst of current. The detachment of the porous membrane is independent of the structure of the already porosified layer, meaning that the average pore diameter can be tuned from nano to macro size within the same membrane. We also demonstrate that, despite their limited thickness, FSMs are quite robust and can sustained further processing. Thus, the etching receipt we are proposing here extends the range of sensors and filters that can be fabricated using porous silicon technology.     

Red- and blue-light emission from free-standing porous silicon

Summary We report the observation of strong red- and blue-light emission in free-standing porous-silicon samples prepared fromn ⁺ substrates at different anodization current densities. The surface morphology of the free-standing samples has been analyzed by means of atomic-force microscopy. Upon excitation with nanosecond pulses at room temperature, both blue and red luminescence bands appear, peaked around 3.2 and 2.0 eV, respectively. An extensive study of the time-resolved behavior of the photoluminescence signal reveals different dynamical features for the two spectral regions. The observed long decay time (several μs) of the red band reflects the predominant effect of non-radiative processes and is consistent with models based on excition diffusion through the interconnected silicon nanocrystals (quantum dots) skeleton. The relatively fast blue-band decay time (≈400 ns) is shown to be related to non-radiative recombination through trap states at the silicon nanocrystal surface. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

Study of birefringence in porous silicon layers by IR Fourier spectroscopy

Birefringence in porous silicon layers prepared with different etching currents on a (110) substrate is studied by IR Fourier spectroscopy. The spectra exhibit beats in the intensity of transmitted and reflected radiation due to the summation of the intensities of the ordinary and extraordinary waves interfering in the porous layer. An analysis of the spectra shows the layers to exhibit properties of a negative uniaxial crystal with the optical axis lying in the layer plane. The difference between the refractive indices of the ordinary and extraordinary waves for a layer with a porosity of 80% reaches 18%. The experimental data are in agreement with the calculations based on the effective-medium approximation, which takes into account the anisotropy of silicon nanocrystal arrangement in a porous layer.     

Anomalous polarization of Raman scattering spectra from porous silicon

A violation of the polarization selection rules for Raman scattering is observed in porous silicon. This effect is caused by a weak disorientation of the quasi-one-dimensional silicon wires, with the crystal structure of the wires themselves and the macroscopic homogeneity of the material in optical experiments remaining intact. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 95–100 (25 January 1998)     

Anomalous evolution of photoluminescence in porous silicon in an electric field

The change produced in the photoluminescence (PL) of n-type porous silicon (por-Si) by irradiation with ultraviolet laser radiation in the presence of an external electric field has been investigated. A field effect, consisting of a large change in the integrated PL intensity when the field is switched on or off, was observed. When the field intensity E exceeds a critical value, the change in the PL becomes anomalous — an alternating sawtooth signal. A kinetic model is proposed wherein the experimental results are explained by a change in the density of neutral acceptor and hole surface states formed by hydrogen, oxygen, and fluorine atoms adsorbed on the surfaces of pores. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 357–361 (10 September 1997)     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Giant birefringence in anisotropically nanostructured silicon

We performed a study of the in-plane birefringence of anisotropically nanostructured Si layers, which exhibit a greater difference in the main value of the anisotropic refractive index than that of natural birefringent crystals. The anisotropy parameters were found to be strongly dependent on the typical size of the Si nanowires used to assemble the layers. This finding opens the possibility of an application of birefringent Si retarders to a wide spectral range for control of the polarization state of light.     

Magnetic birefringence of light in hematite

Peculiarities in the propagation of IR radiation in the easy-plane antiferromagnet α-Fe₂O₃ are investigated. In the geometry of the Cotton-Mouton effect, magnetic birefringence is measured and magnetooptical constants are estimated.     

White light luminescence from annealed thin ZnO deposited porous silicon

In this study, photoluminescence (PL) properties of annealed ZnO thin films deposited onto a porous silicon (PS) surface by rf-sputtering were investigated. A huge blue shift of luminescence from the ZnO deposited onto the PS surface and a broadband luminescence (white luminescence) across most of the visible spectrum were obtained after the heat treatment at 950 °C in air. The results of Fourier Transform Infrared Spectroscopy (FTIR) analysis suggested that the porous silicon surface was oxidized after ZnO deposition and the broadband luminescence was due to the conversion of Si-H bonds to Si-O-Si bonds on the PS surface. The underlying mechanisms of the broadband PL were discussed by using oxygen-bonding model for the PS and native defects model for ZnO. The experimental results suggested that the heat treatment provides a relatively easy way to achieve white luminescence from thin ZnO deposited porous silicon.     

Application of stain-etched porous silicon in light emitting diodes and solar cells

Porous silicon/c-Si heterostructures have been formed by the method of stain etching.The properties of light emitting diodes (LED) and solar cells have been studied. The transport mechanism of the diode has been investigated from the current–voltage characteristics measured at different temperatures (296–380 K). A model based on multi-step tunneling of carriers at reverse and low forward bias (<1 V) and on field tunneling across a narrow barrier at higher forward bias (>1.5 V) is proposed for the LED. In the case of the solar cells the porous silicon is formed in between the fingers of the front grid contact. Application of porous silicon in solar cells results in an increase of the short-circuit current and efficiency of the cells by about 30%.     

Anomalous thermoelastic effect in silicon

The equations of visco and thermoelasticity are discussed and the dispersion relation for a cubic material presented. The thermoelastic contribution to the acoustic attenuation in silicon is computed from 1 – 300K. Strong attenuation anomalies associated with negative thermal expansion are found in the vicinity of 17K and 125K. Comparison with experimental results is discussed. It is suggested that anharmonic effects are responsible for the anomalies seen in low frequency measurements and that several materials should exhibit similar behavior.     

Local order in light emitting porous silicon studied by XEOL and TEY

The local structure of porous silicon has been studied exciting its optical luminescence by X-rays (XEOL). The photoluminescence yield and the total electron yield (TEY), recorded simultaneously as a function of the X-ray energy at the Si K edge, give rise to the extended X-ray absorption fine structures (EXAFS). Analysis of EXAFS data confirms that the optical luminescence of porous Si originates from the nanocrystalline cores and shows that XEOL–EXAFS and TEY–EXAFS are sensitive to different Si local environment. It can be assumed that XEOL–EXAFS is related only to the light emitting sites while TEY–EXAFS is sampling both luminescent and non-luminescent Si sites.     

Cleavage cracking across twin boundaries in free-standing silicon thin films

Cleavage cracking across twin boundaries in free-standing silicon thin films is investigated in a microtensile fracture experiment. If the twist misorientation is relatively small, the crack front transmission can be quite smooth; otherwise the fracture surface may be either planar or broken down into parallel terrains. In all the cases, the local fracture resistance tends to increase. PACS 62.20.-x; 68.35.bg; 68.35.Gy; 68.37.-d; 62.20.mm     

Anomalous scattered light correlator

The space-time correlation function of the scattered radiation intensity of atoms situated in an external field is investigated. This function includes contributions of the normal +E^–> and anomalous –E–> complex field amplitude correlator. These correlators have components due to scattering by density fluctuations and to collective, diffraction effects. The spatial structure of these correlators is studied and the conditions for the measurement of the anomalous correlator are derived. The temporal properties of the anomalous correlator and the effect of Doppler broadening on the correlation properties of scattered radiation are also considered. The investigation of the anomalous correlator is of interest in high-resolution spectroscopy and may also be useful in the study of nonresonant media.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 70–73, February, 1982.     

Anomalous surface photoconductivity in hydrogenated amorphous silicon

The intensity and time dependence of the source-drain photocurrent is measured on hydrogenated amorphous silicon (a-Si:H) field effect transistors as a function of gate bias. The photocurrent increases rapidly, becomes weakly dependent on excitation intensity, and exhibits long decay times. A model in which the relaxation of the space charge dominates the photocurrent quantitatively predicts the experimental data. The results show that anomalous surface photoconductivity is often the dominant photoresponse of a-Si:H.