Characterization of ITO/porous silicon LED structures

The electrical behavior and the electroluminescence (EL) obtained from n- and p-type ITO/porous silicon LEDs have been characterized simultaneously at different temperatures. Stability and aging in air were investigated, and means for avoiding their detrimental effects in the experiments are suggested. The dominating current carrying mechanism responsible for visible light emission in both substrate types has been identified to be Fowler–Nordheim tunneling. This emphasizes the contribution of embedded nanoparticles (quantum dots) rather than the role of nanowires in efficient EL.     

Singular Characteristics of Optical Thue–Morse Multilayers Composed of PT‐Symmetric Elements

Aperiodic 1D Thue–Morse (TM) multilayer optical structures composed of two parity‐time‐symmetric (PT‐symmetric) elements are constructed. The transfer matrix and scattering matrix are utilized for singular propagation characteristic analysis of the structures. The structures display interesting and singular properties, including an unusual eigenvalue spectra, transparency, and unidirectional reflectionless and unidirectional invisibility. Additionally, even‐generation and odd‐generation structures show a significant difference in the aforementioned four properties. The main reason for this is the symmetry difference between the two structures: for even‐generation structures, each individual element as well as the entire system with respect to the PT‐symmetry; while for the odd‐generation structures, each individual element has PT‐symmetry; however, the system as a whole does not have PT‐symmetry. The propagation characteristics of the entire structure, which is not a PT‐symmetric system being composed of PT‐symmetric elements, have not yet been reported. This work can contribute to the understanding of the TM sequence, as well as contribute to understanding the influence of the degree of PT‐symmetry on the singular optical propagation characteristics of aperiodic optical structures. Additionally, this may open new possibilities for important applications, such as the design of a diverse family of all‐optical devices with intriguing behaviors.     

Optical properties of multilayered Period-Doubling and Rudin-Shapiro porous silicon dielectric heterostructures

To investigate the optical properties in quasi-regular porous-silicon-based dielectric Period-Doubling and Rudin-Shapiro multilayer systems, we study here the reflection of light from these structures. The Period-Doubling and Rudin-Shapiro structures are fabricated in such a way that the optical thickness of each layer is one quarter of 600 and 640 nm respectively. We find that porous silicon Period-Doubling dielectric multilayers could demonstrate the optical properties similar to the classical periodic Febry–Perot interference filters with one or multiple resonant peaks, but with an advantage of having total optical thickness much lesser than the periodic structures. Additionally, light propagation in porous silicon Rudin-Shapiro structures is investigated for the first time, both theoretically and experimentally. The reflectance spectra of the structures exhibit photonic band gaps centered at predetermined wavelengths. In both cases, numerical simulation of light transmission is performed using transfer matrix method.     

Bistability and stationary gap solitons in quasiperiodic photonic crystals based on Thue–Morse sequence

The nonlinear properties of quasiperiodic photonic crystals based on the Thue–Morse sequence are investigated. The intrinsic asymmetry of these one-dimensional structures for odd generation numbers results in bistability thresholds which are sensitive to propagation direction. Along with resonances of perfect transmission, this feature allows to obtain strongly nonreciprocal propagation and to create an all-optical diode. The efficiency of two schemes is compared: passive and active when an additional short pump signal is applied to the system. The existence of stationary gap solitons in quasiperiodic photonic crystals is shown numerically, and their difference from the Bragg case is emphasized.     

Surface wave photonic device based on porous silicon multilayers

Porous silicon is widely studied in the field of photonics due to its interesting optical properties. In this work, we present theoretical and first experimental studies of a new kind of porous silicon photonic device based on optical surface wave. A theoretical analysis of the device is presented using plane-wave approximation. The porous silicon multilayered structures are realized using electrochemical etching of p⁺-type silicon. Morphological and optical characterizations of the realized structures are reported.     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.     

Current–voltage and low-frequency noise characteristics of structures with porous silicon layers exposed to different gases

Current–voltage and noise characteristics of porous silicon (PS)/single crystalline silicon (SCS) samples were measured under exposure to dry air, air +0.4% CO, dry air +1.7% CO, and dry air+ethyl alcohol vapor. The samples have a sandwich structure comprising Al/PS/SCS/Al. For the dry air +CO mixtures, the noise level was sensitive not only to the presence of CO but also to its percentage, and an increase of the CO concentration led to a change in the spectral density function of the low-frequency noise.     

Luminescence from porous silicon doped with erbium–ytterbium complexes

The study of photoluminescence (PL) from porous silicon (PS) containing complexes of gadolinium oxychloride with Er³⁺- and Er³⁺–Yb³⁺ is reported. The concentration dependencies of PL intensity of PS with Er³⁺ containing complex have been studied. The dependencies have retained the main features that are characteristic of the pure complex for both IR and visible regions of the PL spectra. This allows interpretation of PL processes in complex-containing PS lthrough the concept of multiplication of low-energy electron excitations and cross-relaxation degradation of higher excited states. It has been shown that introducing Yb³⁺ ions into the complex significantly increases the PL intensity. Mechanisms associated with defect formation, the intrinsic conversion of excitation energy within Yb³⁺, and the conversion within Er³⁺ ions followed by transferring of excitation energy to the Yb³⁺ ions has been considered. The PL polarization with excitation in the visible is reported as well.     

Light emission from porous silicon single and multiple cavities

Experimental and theoretical techniques are used to examine the effects of microstructuring on the optical properties of multilayer, single and multiple microcavity structures fabricated from porous silicon. Measurements of the reflectivity and photoluminescence spectra of three multilayer samples are presented. The results are modelled using a transfer matrix technique including a negative absorption term to represent the effect of spontaneous emission which gives luminescence. The emitted light is strongly controlled by the optical modes of the structures and very good agreement is observed between theory and experiment.     

On the origin of 1.5 μm luminescence in porous silicon coated with sol–gel derived erbium-doped Fe2O3 films

Sol–gel derived Fe₂O₃ films containing about 10 wt% of Er₂O₃ were deposited on porous silicon by dipping or by a spin-on technique followed by thermal processing at 1073 K for 15 min. The samples were characterized by means of PL, SEM and X-ray diffraction analyses. They exhibit strong room-temperature luminescence at 1.5 μm related to erbium in the sol–gel derived host. The luminescence intensity increases by a factor of 1000 when the samples are cooled from 300 to 4.2 K. After complete removal of the erbium-doped film by etching and partial etching the porous silicon, the erbium-related luminescence disappears. Following this, luminescence at 1.5 μm originating from optically active dislocations (“D-lines”) in porous silicon was detected. The influence of the conditions of synthesis on luminescence at 1.5 μm is discussed.     

Optical properties of erbium-doped porous silicon waveguides

Planar and buried channel porous silicon waveguides (WG) were prepared from p⁺-type silicon substrate by a two-step anodization process. Erbium ions were incorporated into pores of the porous silicon layers by an electrochemical method using ErCl₃-saturated solution. Erbium concentration of around 10²⁰ at/cm³ was determined by energy-dispersive X-ray analysis performed on SEM cross-section. The luminescence properties of erbium ions in the IR range were determined and a luminescence time decay of 420 μs was measured. Optical losses were studied on these WG. The increased losses after doping were discussed.     

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.     

Polarization phenomena in the optical properties of porous silicon

We examine the polarization memory effect for porous Si excited by linearly polarized light. The various observations for the red-luminescing, slow band are discussed in the general framework of particle shape asymmetry. We show that because of the intrinsically nonlinear luminescence response, measurement parameters influence the polarization response. The preparation of porous Si with photoassisted etching is found to control the polarization retention parameter ρ. Using linearly polarized light during etching produces in-plane asymmetries. We find a substantial ρ-anisotropy linked to crystal symmetry planes and axes as a consequence of anisotropic etching. The effects are discussed with reference to current models of the light emission mechanism.     

Ellipsometric study of refractive index anisotropy in porous silicon

Porous Si layers of different thicknesses were prepared by anodising p⁺-type Si substrates with a resistivity of 0.01 Ω cm. The porosity of the samples ranged from 23% to 62%. The refractive index values for the ordinary and extraordinary rays were determined by multiple angle of incidence ellipsometry, from which an optical anisotropy parameter varying from 13% to 20% was obtained. The porous Si layers were modelled as uniaxially anisotropic films on an isotropic substrate, with an optical axis perpendicular to the sample surface. The morphological anisotropy which is typical for the p⁺-type porous Si with a predominating cylindrical geometry is responsible for these optical properties. All the porous Si layers studied were found to be optically negative.     

Stable ultraviolet photoluminescence emission in n-type porous silicon

This very paper is focusing on the investigation of porous silicon preparation with n-type silicon wafer by means of electrochemical anodization in the dark and, particularly, on its stable ultraviolet photoluminescence emission. A lateral electrical potential was applied, for this purpose, on silicon wafers, driving the electrons away and letting holes appear on the surface of the silicon wafer to enhance the electrochemical etching process. Characterizations have been made with scanning electronic microscope, fluorescence spectrophotometer and Fourier transform infrared spectroscope. An ultraviolet photoluminescence emission of 370 nm is found in the as-prepared n-type porous silicon, which seems to be well associated with the formation of oxygen-related species (twofold coordinated silicon defect) during the anodic oxidation. The result characterized by photo-bleaching performance indicates that the ultraviolet photoluminescence emission is so stable—only 7% reduction within 3600 s. Meanwhile the morphology of as-prepared n-type porous silicon is investigated.     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

The room temperature oxidation of porous silicon

The room temperature oxidation of porous silicon was studied using isothermal methods. The oxidation was found to depend on the type of the porous silicon. The microcalorimetric signals from the oxidation of the p⁺- and n-type porous silicon in dry air were different. In humid air the signals from the oxidation could not be distinguished from the strong signal due to adsorption of water vapour, but when the samples were placed in water similar differences were observed. The reason for differences in reactions is discussed. The oxidation in different liquids was also studied. The signal from reactions in methanol and ethanol were found to be 100 times higher than in water. In FTIR studies the reaction gas produced by reactions between alcohols and the porous silicon, silane (SiH₄) was found in the gas. Traces of SiOCH₃ and SiOC₂H₅ groups were also found in FTIR spectra indicating Si---O---C_xH_y passivation of the surface.     

Photoluminescence of porous silicon coated by SILD method with LaF3 nanolayers

The method of lanthanum fluoride passivating layer synthesis in the matrix of porous silicon by successive ionic layer deposition was elaborated and optimized. Luminescence and FTIR of obtained structures demonstrate the crucial role of the chemical composition of silicon nanocrystallite surface in the formation of radiative recombination channels and in the stability of porous silicon photoluminescence. The combination of high optical transparency of LaF₃ layers and low recombination losses in silicon covered with such layers allows to recommend the lanthanum fluoride film as an effective passivating coating for silicon optoelectronics devices.     

Surface analysis of thermally annealed porous silicon

Quasi-monocrystalline porous silicon (QMPS) has high potential for photovoltaic application for its enhanced optical absorption compared to bulk silicon in the visible range of solar spectrum. In this study, QMPS was formed from low porosity (∼20-30%) porous silicon (PS) produced by electrochemical anodization, and thermal annealing in the temperature range 1050-1100 °C under pure hydrogen ambient for a duration of 30 min. We analyzed the material surface by grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (SIMS) study. The crystallinity was confirmed by GIXRD while FESEM studies revealed that the surface layer is pore free with voids embedded inside the body. AFM studies indicated relatively smooth and uniform surface and the dynamic SIMS study showed the depth profiles of impurities present in the material.     

Tuning the cathodoluminescence of porous silicon films

We have obtained intense cathodoluminescence (CL) emission from electron beam modified porous silicon films by excitation with electrons with kinetic energies below 2 keV. Two types of CL emissions were observed, a stable one and a non-stable one. The first type is obtained in well-oxidized samples and is characterized by a spectral peak that is red shifted with respect to the photoluminescence (PL) peak. The physically interesting and technologically promising CL is however the CL that correlates closely with the PL. Tuning of this CL emission was achieved by controlling the average size of the nanostructure thus showing that the origin of this CL emission is associated with the quantum confinement and the surface chemistry effects that are known to exist in the porous silicon system. We also found that the electron bombardment causes microscale morphological modifications of the films, but the nanoscale features appear to be unchanged. The structural changes are manifested by the increase in the density of the nanoparticles which explains the significant enhancement of the PL that follows the electron irradiation.