Effect of adsorption of the donor and acceptor molecules at the surface of porous silicon on the recombination properties of silicon nanocrystals

The effect of adsorption of the donor and acceptor molecules on the spectra of photoluminescence and electron spin resonance (ESR) of microporous silicon is studied. It is found that photoluminescence of microporous silicon is quenched, the photoluminescence peak shifts to shorter wavelengths, and the intensity of the ESR signal increases after adsorption of molecules of nitrogen dioxide and pyridine. The results obtained are interpreted using a model of radiative excitonic recombination in porous silicon that takes into account the formation of both the charged (NO₂)^? and (C₅H₅N)⁺ complexes and defects (e.g., dangling bonds at the silicon surface) at the surface of silicon nanocrystals.     

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

Porous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (10¹⁶/cm³) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p⁺ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p⁺ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p⁺ substrate interface (S ≅ 10³ cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.     

The effect of cathode materials on reactive ion etching of silicon and silicon dioxide in a CF4 plasma

Silicon and silicon dioxide have been Reactive Ion Etched in a CF₄ plasma using a diode sputtering configuration to achieve etching. Pressures ranged from 20 to 100 millitorr and power densities to the RF cathode were between 0.1 and 1.0 W/cm². The effect of cathode material on the quality of etched surfaces and on etch rates has been investigated. It has been observed that the etch rate of silicon decreases as the area of silicon exposed to the plasma is increased and that this silicon loading effect is strongly influenced by the material covering the balance of the cathode. For instance, the silicon loading effect is much more pronounced when silicon dioxide rather than aluminum is used to cover the balance of the cathode. This silicon loading effect was investigated further by varying RF power. It was found that loading a silicon dioxide covered cathode with silicon wafers decreases the dependence of silicon etch rate on power. The silicon dioxide etch rate and its dependence on RF power are the same whether silicon, silicon dioxide or aluminum is used to cover the balance of the cathode. Possible explanations for these experimental results will be discussed.     

Multilayer photosensitive structures based on porous silicon and rare-earth-element compounds: Study of spectral characteristics

The spectral characteristics of the specular reflectance, photosensitivity, and photoluminescence (PL) of multilayer structures based on porous silicon with rare-earth-element (REE) ions are investigated. It is shown that the photosensitivity of these structures in the wavelength range of 0.4–1.0 μm is higher than in structures free of REEs. The structures with Er³⁺ ions exhibit a luminescence response at room temperature in the spectral range from 1.1 to 1.7 μm. The PL spectrum of the erbium impurity is characterized by a fine line structure, which is determined by the splitting of the ⁴ I _15/2 multiplet of the Er³⁺ ion. It is shown that the structures with a porous layer on the working surface have a much lower reflectance in the entire spectral range under study (0.2–1.0 μm).     

Electroluminescence from porous silicon in the cathodic reduction of persulfate ions: Degree of reversibility of the tuning effect

The temporal evolution of the spectra of cathodic electroluminescence from porous silicon in an electrolyte containing persulfate ions S₂O ₈ ^2? was studied in the galvanostatic mode. It was shown that irreversible changes in luminescence properties of porous silicon occur under cathodic polarization. These changes are manifested in a decrease in the signal intensity and a long-wavelength shift of the electroluminescence (EL) spectrum when the substrate potential remains virtually unchanged (pseudo-tuning). The irreversibility of the change in luminescence parameters is related to a concurrent electrochemical oxidation of the surface of porous silicon, which hinders the bipolar injection of carriers into luminescence-active crystallites. The results obtained suggest that the degradation phenomena observed under cathodic polarization are due to those same processes which are responsible for EL excitation, which casts doubt on the interpretation of the tuning effect, known in the literature, as a consequence of a purely electronic process in porous silicon.     

Photoelectrochemical properties of porous silicon based novel photoelectrodes

Porous silicon interfaces have been modified with nitrided TiO₂ (TiON) nanoparticles to develop highly efficient photoelectrodes. Photoelectrodes were prepared by impregnating the electrochemically prepared porous silicon microchannels with titanium oxynitride. Photocatalytic measurements were carried out on titanium oxynitride particles in water‐methanol mixture and the results showed a dependence on the nitrogen concentration. Among the photoelectrodes used for photocurrent measurements, porous silicon impregnated with TiO₂ nitrided at 600 °C showed maximum photocurrent increase after exposure to sunlight‐type radiation. The enhancement in photocurrent was one order more for the porous silicon/titanium oxynitride hetero‐structure than that of polished silicon/titanium oxynitride hetero‐structure. Photoelectrodes thus prepared were found to have stable performance for a period of six months. This observation promises the possibility of using porous silicon/titanium oxynitride hetero‐structures as efficient electrodes for photovoltaic cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I₂ iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I₂ molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ～10¹⁸?10¹⁹ cm^?3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P _b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I₂ molecules.     

Space charge limited current in porous silicon and anatase (TiO<Subscript>2</Subscript>)

A comparative study of the transient space-charge-limited current in porous silicon and porous anatase (TiO₂) has been carried out. The room-temperature drift mobilities of electrons in porous silicon and porous anatase, determined from the transit times, are, respectively, 10^?2 and 5×10^?6 cm² V^?1 s^?1. The specific features of space-charge-limited currents in porous anatase are discussed.     

Influence of pyridine molecule adsorption on concentrations of free carriers and paramagnetic centers in porous silicon layers

The influence of adsorption of donor pyridine (C₅H₅N) molecules on free-hole and defect concentrations in porous silicon layers differing in the morphology of composing nanocrystals and pores, as well as in the boron doping concentration, was studied using infrared and electron spin resonance spectroscopy. It was shown that the dependence of the hole concentration on the pyridine vapor pressure is controlled by the initial boron doping level of porous silicon, while the number of defects, i.e., dangling silicon bonds, is almost unchanged during adsorption for all sample types. For samples on substrates with a boron concentration of ～10²⁰ cm^?3, a decrease in the number of holes at low pyridine pressures is observed and is attributed to hole capture by surface states of adsorbed C₅H₅N molecules. At pyridine pressures close to the saturated vapor pressure, the hole concentration in porous silicon layers increases, which is associated with hole “trap” depopulation due to an increase in the permittivity of the silicon nanocrystal neighborhood under conditions of C₅H₅N vapor condensations in sample pores.     

Study on the Thermoelectric Properties of CVD SiC Deposited with Inert Gases

We deposited silicon carbide (SiC) by the chemical vapor deposition (CVD) method using the inert gases Ar and He. It was confirmed that SiC deposited with inert gases had a porous microstructure and high carbon content. We also studied the thermoelectric properties. SiC deposited with He gas had lower electrical and thermal conductivity compared with SiC deposited with Ar gas. Both samples using Ar and He exhibited a negative Seebeck coefficient, indicating n-type semiconductor behavior. The calculated figure of merit (Z) of SiC deposited with inert gases was improved compared with SiC deposited with H₂ or N₂ gas. The value for SiC deposited with He was higher than that for SiC deposited with Ar. The thermoelectric properties of porous silicon carbide deposited with inert gases were also compared with those of silicon carbide deposited with hydrogen or nitrogen gas.     

Effect of deposition methods on dielectric breakdown strength of PECVD low-k carbon doped silicon dioxide dielectric thin films

The effect of deposition methods on dielectric breakdown strength of PECVD low-k dielectric carbon doped silicon dioxide films is investigated. I-V measurements were performed using metal-insulator semiconductor structures for carbon doped silicon dioxide thin films with various thicknesses by single deposition station and six sequential deposition systems. I-t measurements are also performed for films with the thickness of 32 nm prepared using both deposition methods. Comparison studies have been carried out for the thickness dependence, temperature dependence, conduction mechanism and time dependence of dielectric breakdown for carbon doped silicon dioxide with single layer and six sub-layers. Results demonstrated that both films follow the newly obtained relationship between dielectric strength E_B and thickness d, i.e. E_B∝(d−d_c)⁻ⁿ, but with a lower exponential factor n and a larger thickness limit d_c for films with six sub-layers. It is also demonstrated that films with six sub-layers have a higher dielectric strength in all the thickness and temperature ranges, a thickness independent thermal behavior and a longer lifetime under constant voltage stressing. This indicates that by tuning the deposition methods smaller thickness with desired dielectric properties can be achieved.     

Dielectric Properties of Porous Reaction-Bonded Silicon Nitride with Different Additives

The influence of additives on the dielectric properties of porous silicon nitride ceramics with 0.6-mm apertures and 56% porosity prepared by reaction sintering has been studied. The results show that high porosity and large apertures in porous silicon nitride ceramics lower the dielectric constant and dielectric loss compared with dense silicon nitride ceramic. α-Si₃N₄ additive is beneficial to decrease the dielectric constant and dielectric loss. Addition of Y₂O₃ and La₂O₃ powders may result in formation of Y-Si-O-N and La-Si-O-N phases, and enhance the volume fraction of β-Si₃N₄ phase. The transition of Y³⁺ and La³⁺ weakly associated ions and vacancies in the Y-Si-O-N and La-Si-O-N systems leads to higher dielectric constant and dielectric loss.

     

Electrical and photoelectric properties of nanostructures obtained by electroless etching of silicon

The electrical and photoelectric properties of nanostructures with porous silicon layers obtained by electroless etching of silicon have been investigated. It is found that the photoelectric and photovoltaic properties of these structures depend on their morphology and are determined by not only the properties of the modified layer, but also the presence of possible barriers in the layered porous silicon. The ratio of the photoconductivity to the dark conductivity reached 10²−5 × 10². An open-circuit voltage V _oc was detected that amounted to ∼250 mV at an incident light power close to AM-1 (∼100 mW/cm²). In this case, the density of short-circuit current I _sc was about 20 μA/cm².     

The role of nitrogen in the formation of luminescent silicon nanoprecipitates during heat treatment of SiO2 layers implanted with Si+ ions

Twenty-five kiloelectronvolt Si⁺ ions with doses of (1–4)×10¹⁶ cm^?2 and 13-keV N⁺ ions with doses of (0.2–2)×10¹⁶ cm^?2 were implanted into SiO₂ layers, which were then annealed at 900–1100°C to form luminescent silicon nanoprecipitates. The effect of nitrogen on this process was deduced from the behavior of the photoluminescence spectra. It was found, for a certain ratio between the concentrations of implanted silicon and nitrogen, that the photoluminescence intensity increases significantly, and its peak shifts to shorter wavelengths. It is concluded that the number of precipitation nuclei increases owing to the interaction of nitrogen with excess silicon. Eventually, this results in an increase in the number of nanocrystals and in a decrease in their average sizes. In spite of introducing additional precipitation nuclei, the minimal concentrations of excess Si on the order of 10²¹ cm^?3 and heat treatments at temperatures higher than 1000°C were still required for the formation of nanocrystals.     

Characterization of an Oxidized Porous Silicon Layer by Complex Process Using RTO and the Fabrication of CPW‐Type Stubs on an OPSL for RF Application

This paper proposes a 10‐µm thick oxide layer structure that can be used as a substrate for RF circuits. The structure has been fabricated using an anodic reaction and complex oxidation, which is a combined process of low‐temperature thermal oxidation (500 °C, for 1 hr at H₂O/O₂) and a rapid thermal oxidation (RTO) process (1050 °C, for 1 min). The electrical characteristics of the oxidized porous silicon layer (OPSL) were almost the same as those of standard thermal silicon dioxide. The leakage current density through the OPSL of 10 µm was about 10 to 50 nA/cm² in the range of 0 to 50 V. The average value of the breakdown field was about 3.9 MV/cm. From the X‐ray photo‐electron spectroscopy (XPS) analysis, surface and internal oxide films of OPSL prepared by a complex process were confirmed to be completely oxidized. The role of the RTO process was also important for the densification of the porous silicon layer (PSL) oxidized at a lower temperature. The measured working frequency of the coplanar waveguide (CPW) type short stub on an OPSL prepared by the complex oxidation process was 27.5 GHz, and the return loss was 4.2 dB, similar to that of the CPW‐type short stub on an OPSL prepared at a temperature of 1050 °C (1 hr at H2O/O2). Also, the measured working frequency of the CPW‐type open stub on an OPSL prepared by the complex oxidation process was 30.5 GHz, and the return was 15 dB at midband, similar to that of the CPW‐type open stub on an OPSL prepared at a temperature of 1050 °C (1 hr at H₂O/O₂).     

C-V characteristics of metal-titanium dioxide-silicon capacitors

Titanium dioxide capacitors were fabricated on silicon wafers using electron-beam evaporation. The TiO₂ films varied in thickness from 500 to 2000 Å. Post-deposition oxidation at 1000°C in dry O₂ was used to promote stoichiometric conversion of the films to the rutile phase. Capacitive densities of greater than 2 pf/sq. mil were obtained (dielectric constants ranged from 4 to 40). For long oxidation times, significant silicon dioxide grows under the TiO₂ as a result of oxygen diffusing through the TiO₂ film. Titanium was also shown to diffuse into the silicon during the oxidation cycle resulting in an n-type diffusion. Surface state densities ranging from 10¹¹ to 5 × 10¹¹ cm^?2 eV^?1 at midgap were obtained for good devices. Longer oxidation times result in lower capacitance, leakage current and surface state density.     

A model of formation of fixed charge in thermal silicon dioxide

A quantitative model of formation of fixed charge (Q _f ) in silicon dioxide during thermal oxidation of silicon is developed. The value of Q _f is governed by the number of interstitial silicon atoms in the vicinity of the Si-SiO₂ interface; these atoms are formed as a result of the processes of their generation and recombination at the interface and also due to their diffusion to the depth of dioxide. The model makes it possible to describe a decrease in the fixed charge as the oxidation temperature is increased and in the case of annealing in neutral media for silicon dioxide on silicon with orientations (100) and (111) in a wide range of temperatures.     

Terahertz absorption and emission upon the photoionization of acceptors in uniaxially stressed silicon

Experimental data on the spontaneous emission and absorption modulation in boron-doped silicon under CO₂ laser excitation depending on the uniaxial stress applied along the [001] and [011] crystallographic directions are presented. Room-temperature radiation is used as the probe radiation. Low stress (less than 0.5 kbar) is shown to reduce losses in the terahertz region by 20%. The main contribution to absorption modulation at zero and low stress is made by A⁺ centers. Intersubband free hole transitions additionally contribute to terahertz absorption at higher stress. These contributions can be minimized by compensation.     

Effect of the initial doping level on changes in the free-carrier concentration in porous silicon during ammonia adsorption

Infrared spectroscopy is used to investigate the effect of ammonia adsorption on the concentration of equilibrium charge carriers in porous-silicon layers with various initial types of dopants at different concentrations. It is found that ammonia adsorption results in an increase in the number of free electrons in n-type samples up to a level exceeding 10¹⁸ cm^?3. In p-type samples, a nonmonotonic dependence of the charge-carrier concentration on ammonia pressure is observed. The obtained results are accounted for by the appearance of adsorption-induced shallow donor states that, along with the initial-dopant and surface-defect states, specify the charge-carrier type and concentration in the silicon nanocrystals of the porous layer after ammonia adsorption.     

Radiation hardness of porous silicon

The effect of irradiation by 300-keV Ar⁺ ions on the properties of electrochemically produced porous silicon is studied at doses of 5×10¹⁴–1×10¹⁶ cm⁻². Raman scattering and photoluminescence data are used to show that the radiation hardness of porous silicon layers is substantially greater than that of single crystal silicon. Fiz. Tekh. Poluprovodn. 31, 1126–1129 (September 1997)