Hydrogen nanosensor working near room temperature made of porous silicon covered by the TiO<sub>2-x</sub> or
ZnO<Al> thin film was realized. Porous silicon layer was formed by electrochemical anodization on a <i>p-</i> and <i>n</i>-type
silicon surface. Thereafter, <i>n</i>-type TiO<sub>2-x</sub> and ZnO<Al> thin films were deposited onto the porous silicon surface by
electron-beam evaporation and magnetron sputtering, respectively. Platinum catalytic layer and gold electric contacts
were for further measurements deposited onto obtained structures by ion-beam sputtering. The sensitivity of
manufactured structures to 1000-5000 ppm of hydrogen was studied. Results of measurements showed that it is possible
to realize a hydrogen nanosensor which has relatively high sensitivity and selectivity to hydrogen, durability, and short
recovery and response times. Such a sensor can also be a part of silicon integral circuit and work near room
We study current-voltages and low-frequency noise characteristics of the metal--porous silicon--silicon single crystal--metal structure with 50% and 73% porosity of porous silicon. The study is performed in dry air and in a mix of dry air
with carbon monoxide of different concentrations. The Hooge noise parameter α<i><sub>H</sub></i>
and the parameter γ in the
frequency dependence of the noise voltage spectral density
S<i><sub>U</sub></i>(ƒ) were determined from experimental data. High
sensitivity of spectral dependence of noise to gas concentration allows offering powerful method for determination of gas
concentration in the air or environment.
We present the results of investigations of double-layer thin-film hydrogen sensors that show high
sensitivity at low operating temperatures and improved reliability. These hydrogen sensors are manufactured
using the both ion-plasma assisted sputtering and sol-gel technique. It was established that the highest
sensitivity of the sensors occurred at 100-130°C. The hydrogen sensitivity depends on hydrogen
concentration linearly starting at 50 ppm, and reaches 10<sup>4</sup> at 5000 ppm. The response time was 1-2 s and the
recovery times were less than 10 s.
We show that compared to constant power supply, pulse heating of the sensor improves the stability of
the sensor, reduces the sensitivity to humidity, and reduces performance drift. Various possibilities of
reducing CO gas cross sensitivity are also presented.
Ceramic semiconductor photoelectrodes made of the Fe<sub>2</sub>O<sub>3</sub>-Nb<sub>2</sub>O<sub>5</sub> solid solutions were synthesized. The spectral and
capacitance-voltage characteristics of the photoelectrodes were determined, and the dynamic polarization with chopped
light was investigated. The anodic photocurrent onset potential, the flat band potential and the shallow and deep donor
density of these materials were determined. The threshold photon energies corresponding to the inter-band optical
transitions near the edge of the fundamental absorption of the semiconductor photoelectrode were calculated. Analysis
of the frequency dispersion of the real and imaginary parts of the complex impedance of photoelectrochemical cell was
carried out. On the basis of this analysis, equivalent circuits describing the structure of the double electrical layer of the
semiconductor - electrolyte interface were proposed and their parameters were calculated. The main limiting steps of the
electrode processes, which determine the electrode polarization and current, are determined.
Reflectance spectrum calculations of double- and triple-layer antireflection coatings based on porous silicon layer are performed using the optical matrix approach method. Obtained results are compared with the reflectance spectrum of the SiO<sub>2</sub>/TiO<sub>2</sub> double-layer antireflection coating. A low reflectance value of both double- and triple-layer antireflection coatings made of porous silicon is observed in comparison to that SiO<sub>2</sub>/TiO<sub>2</sub> antireflection coating. These results are of importance for solar cells application.
The influence of the surface and interfaces of semiconductor-metal Al/n<sup>+</sup>Si-nSi/Al and Ag/n<sup>+</sup>Si-nSi/Ag structures with different aluminum and silver contact coating layers on the level and behavior of the low frequency current noise spectra was experimentally investigated. It is shown that the level of low frequency noise strongly depends on the material and form of the contact coating layers. At the room temperature, at the frequency 10 Hz, the noise level for Al/n<sup>+</sup>Si-nSi/Al structures is equal to ~ 10<sup>-15</sup> A<sup>2</sup>/Hz, for the Ag/n<sup>+</sup>Si-nSi/Ag structures is equal to ~ 10<sup>-17</sup>-10<sup>-18</sup> A<sup>2</sup>/Hz. It seems possible that the interface conditions modification, which in its turn mirrors at the processes of surface reflection and refraction of electrons and phonons, affects on the relaxation processes of longer-wavelength electron distribution function fluctuation and thus on mobility fluctuation and 1/f noise level as well. On the base of noise spectral characteristics of the mentioned structures the peculiarities of the acoustic phonons refraction on the semiconductor-metal flat interface are experimentally investigated. Several practical aspects related with so-called "refraction points" are discussed. It is proposed that, by manipulation of those phonons "refraction points" at hetero-interface, it will be possible to suppress the part of 1/f noise level, which arises in the volume of semiconductor. It is supposed that the hetero-interface by itself is not the source ("generator") of the 1/f noise, but probably is a factor of the volume 1/f -noise "reduction".
Process of the origin and relaxation of the fluctuation of distribution function of conduction electrons in space-homogenous and non-degenerate equilibrium semiconductors is discussed. The fluctuations of electron distribution function, formed as result of the internal fluctuations of the phonon system, are studied. The physical mechanisms of the origin and following slow (diffusion) damping of the equilibrium fluctuations of the electron and phonon distribution functions are represented. It is shown that in low frequency region the Fourier-component of distribution function fluctuations of predominantly long wavelength electrons and phonons are depends on frequency by law ω<sup>-1/2</sup>.
The giant optical non-linearity of liquid crystals was used to develop a device for measuring intensity of laser beams. The sensitivity of the device was substantially increased by substitution of one of glass substrates of the liquid crystal cell by a photosensitive semiconductor. It is shown that this is due to decreased anchoring energy of LC molecules on the semiconductor surface. The peculiarities of giant optical non-linearity in a liquid crystal cell with one semiconductor substrate are investigated. The discovered regularities allow supposing that the observed decrease in anchoring energy is caused by the semiconductor surface states recharging at light generation.
The two main causes of origin of the mobility fluctuation of the electrons in homogeneous, unlimited, and non-degenerated semiconductors are discussed. It is shown that the mobility fluctuation is conditioned by the symmetric component of the fluctuation of the distribution function, i.e. by the fluctuations of the conduction electrons energy. On the base of the developed quasi-classical model the spectrum of electrons lattice mobility fluctuations is calculated. In the frequency wide variation range it has 1/<i>f</i> form.
The influence of the built-in electric field (e.g. several potential barriers, impurity gradients, etc) at presence of external crossed electric and magnetic fields on the level of low-frequency noise is theoretically studied by the use of interrelated Langevin type Boltzmann transport equations for the systems of electrons and phonons for non-degenerate n-type semiconductors. At the first time it is shown that within the context of the proposed problem the built-in field causes origin of separate 1/f-noise component, which, besides the main parameters of semiconductor, depends also on the value of this field. The spectral density of this component by the form is similar to the Hooge's empirical formula. For the parameter analogous to the Hooge parameter <b>α</b> the comparison with experimental data for n-type Si is carried out. It is shown that for range of values of the built-in electric field from 50 to 600 V/m at low temperatures from 77K to 150K, the longitudinal component of the analog of the Hooge parameter varies from 10<sup>-6</sup> up to 10<sup>-4</sup>, which in some cases may exceed corresponding values of the generally observed 1/f-noise. The transversal component of the Hooge parameter analog has square dependence on external magnetic field intensity; even for low temperature region, at values of magnetic field from 50 to 400 A/m it has very low values and varies from 10<sup>-15</sup> up to 10<sup>-11</sup>. Basing on the calculations a physical model describing the origin of built-in component of 1/f-noise is proposed. It explains the mechanism of origin of the additional component and helps to shed light on the origin of the general 1/f-noise.
The results presented in this paper demonstrate the possibility of using tin dioxide films as sensitive elements for alcohol sensors. The tin dioxide films were deposited by the spray pyrolysis method on alumina substrates as well as on n<SUP>+</SUP> porous silicon upper layer of silicon diode structures. Room temperature, under atmospheric pressure measurements of the parameters of the obtained structures revealed their high sensitivity to the mixture of ethyl alcohol vapors and air. The optimum concentration of precursor solution for precipitation of tin dioxide films and technological regimes for the deposition of tin dioxide films and formation of a porous silicon layer with appropriate thickness and porosity by electrochemical anodization were found.
Published information on polymer composite solar cells was screened to pin down the most critical parameters for a further improvement of these photovoltaic systems towards higher efficiency and stability. Largely irreversible separation of photocharge between the polymer (e.g. PPV) and the fullerene with subsequent percolation of electrons (e.g. to the Al contact) and holes (to the ITO contact), under the symmetry breaking influence of an electric field imposed by the work function difference of the contacts (Al and ITO respectively) accounts for photocurrent generation. The energetics is determined by n-p hetero junction, which is formed between ITO and the polymer/fullerene composite. Positive polarization of the ITO contact leads to the generation of an interfacial potential drop and an increase of the effective ITO work function. This explains the generation of a photovoltage reaching more than 0.8 Volt. Parameters limiting solar cell performance are discussed.
Impedance characteristics of semiconductor barrier-injection transit-time diodes (BARITT) structures made of Silicon Carbide containing quantum wells in the drift region are theoretically examined. It is shown that the magnitude of the negative dynamic resistance can be increased due to trapping and escape effects of injected charge carriers in quantum wells. It is shown that the negative resistance of the BARITT structure made of different polytypes of SiC is one order of magnitude higher in absolute value in comparison with the Si structure, all other factors being equal. In the proposed structure significantly higher operation frequencies can be realized in comparison with usual BARITT'S.
Processes in a high temperature superconducting thin film circuit with nonlinear active resistance and inductances are theoretically examined. A possibility to control values of equivalent parameters of the circuit by optical radiation modulated on the intensity is shown.
In this reference processes in a high temperature superconducting (HTSC) thin film circuit with nonlinear parametric resistance and inductance are theoretically examined. The fact that the active and reactive components of the impedance of HTSC film are modulated by the law of optical signal is accepted as a basic precondition for this discussion. By the analogy with the Josephson phenomenon we give certain phases for superconducting and normal states and accept that the alternating components of the photocurrent I<SUB>Phi</SUB> and photovoltage U created by the radiation can be presented as a phase difference between superconducting and normal states respectively. Based on this, the equation for the equivalent circuit of the film presents a complicated differential heterogeneous equation of second rank. As a particular state of the homogeneous equation has been investigated and the Mathieu equation has been obtained. In a result the expression for the gain in power is obtained, which depends on the parameters of the optical signal and thin film. The possibility to control the gain in power by the selection of various values of the depth of modulation of the active and reactive components of the surface impedance, the intensity of radiation and other parameters of the HTSC film is shown.
A method was developed for the theoretical calculation of the reflectance for the antireflection coatings made of porous silicon. Although porous silicon has been the focus of interest for the past years, still a satisfactory concept for calculating the reflectance features is lacking. Towards this end, we suggest a new concept that is a combination of the optical matrix method and a graded-bandgap model of porous silicon, which takes into account the gradient in porosity. The calculations based on the optical matrix method were carried out for common antireflection coatings, as well, for MgF<SUB>2</SUB>, ZnS and Ta<SUB>2</SUB>O<SUB>5</SUB> in order to prove feasibility of the method. A rather good agreement with experimental data was found for all types of antireflection coatings. It is shown that the model of porous silicon as a graded-band semiconductor is valid and is a handy method for the reflectance calculation for porous silicon.
Bifacial sunlit solar cells made of silicon p<SUP>+</SUP>nn<SUP>+</SUP> structures are investigated theoretically. It is shown that the short circuit current, open circuit voltage, fill-factor and efficiency strongly depend on both p<SUP>+</SUP>n junction parameters and n<SUP>+</SUP>n isojunction. Possibilities of manufacturing high-efficiency bifacial solar cells using silicon p<SUP>+</SUP>nn<SUP>+</SUP> structures are discussed.
An effect of the increase in the efficiency of solar cells (in particular, common silicon solar cells) is described when these solar cells are immersed in an isotropic liquid dielectric. The presence of a dielectric thin film results in an increase in the efficiency of solar cells by 40-60% from the reference value. The current-voltage characteristic, fill factor and other main parameters of these solar cells are analyzed. The mechanisms responsible for the increase in the efficiency of solar cells are discussed.
The possibility of using porous silicon layers as antireflection coating instead of the antireflection coatings in common silicon solar cells was investigated. A technology for the manufacture of porous silicon antireflection layers on the emitter of n<SUP>+</SUP> - p junctions with the already deposited contacts grid was developed. The careful investigation of the photovoltaic and optical characteristics of solar cells with porous silicon antireflection coating compared with the performances of solar cells using a well known ZnS antireflection coating is presented. It is shown that the formation of the porous layer under optimum technological regimes leads to decrease of reflection and improvement of the main photovoltaic parameters - short-circuit current and open-circuit voltage. A means of determining the porous silicon layer thickness through capacitance measurements is suggested.
The idea of the quantum dot solar cell, which provides a novel direction to the high-efficiency solar cell problem, is developed. A theoretical model is presented for a practical p-i-n quantum dot solar cell built on the base of the self-organized InAs/GaAs system. Study the advantages of the use of quantum dots in active region for photon absorption in the long-wavelength part of spectrum and increase in the efficiency of the conversion of solar energy in electrical power is carried out. Problems of theory and experiment of low-dimensional solar cells (quantum well and quantum dot solar cells) are discussed.
In the present paper it is shown that one can present the porous semiconductor as a set of clusters of silicon atoms surrounded by SiO<SUB>x</SUB>, as well as the single crystalline silicon substrate can be considered as an infinite cluster also. The formulae for the estimation of variable porosity of the material (including the value of critical porosity-- the percolation threshold, after which the characteristic phenomena are expected in porous silicon) and the forbidden bandgap value of clusters are suggested as functions of the sizes of nanocrystallites. A new fractal model of pore creation on the surface of a material is proposed also. The cases of semi-spherical, conical (V-groove dielectric isolation technology) and cylindrical (U-groove dielectric isolation technology) are considered. Formulae for the formed surface area S, material porosity p as a function of the depth and fractal dimension are obtained.
In this paper we present an IR-radiometer circuit that allows to carry out measurements of week photoelectric signals on considerably lower level of the inherent noise than known IR- detector systems for (delta) equals 4.5 - 5; 8 - 13 micrometers ((mu) ) atmospheric windows. Contrary to similar developments, here additional circuit solutions suggested to decrease preamplifier noise, for the amplitude stabilization of photosensitive bridge feeding pulses to correct basic and useful signals phases as well as to improve characteristics of the synchronous detector.
The impedance and noise characteristics of a semiconductor punch-through structure are theoretically examined for an operation under a condition when quantum wells (QWs) are present in the transit-time region of the structure. It is shown that the magnitude of the negative dynamic resistance can be increased under the influence of the trapping and escape effects of injected carriers by quantum wells. It is expected that the structure proposed have significantly higher operation frequencies in comparison with usual barrier-injection transit-time diode. It is shown also that the noise measure decreased under an influence injected charge carriers captured by QWs with the increase of the ration of the emission time of electrons emitted out of QWs due to the thermal excitation to the capture time of free charge carriers. The frequency band where this phenomenon takes place is narrowed and displaced to a lower frequency range.
Processes in a modulation amplifier on a high temperature superconducting film with nonlinear parametric inductance and active resistance are theoretically examined. The expression is obtained in this paper for the average power on the inductance. A possibility of a parametric regeneration on the modulation frequency of the surface impedance of a HTSC film offers at the irradiation of the latter by an optical radiation modulated on the intensity.
The intensities of electro- (EL) and photoluminescence (PL) in graded-bandgap semiconductor structures operating in the double injection mode are calculated. The cases of the band- to-band radiative recombination and radiative recombination via centers are considered. The dependencies of the intensity of luminescence on current and incident radiation intensity for different lengths of the base and energy bands gradients are analyzed. It is shown that linear, quadratic or cubic dependencies for the EL intensity on current with smooth transitions between them are possible for the band- to-band radiative recombination case. In the dependence of the PL intensity on incident radiation intensity, besides the linear term, there is a quadratic one getting sharper with an increase in the current.