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.     

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.     

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.     

Ground-state structure of beryllium-related acceptor impurity centers in silicon

The infrared absorption spectra of some beryllium-related acceptor impurity centers in silicon have been measured. Their Fourier self-deconvoluted spectra have been obtained clearly demonstrating that the ground states of some impurity centers have a splitting feature while some consist of only a single component.     

Analytic evaluation of diffused impurity layers in silicon

An analytic method is outlined for the calculation of average conductivity of diffused layers in silicon for a complementary error function and Gaussian impurity profiles and the results are found to be in good agreement with those obtained by the numerical integration method. It is shown that the results for Gaussian distribution are obtained directly from the derivations for an error-function distribution. The method is also useful for the estimation of the temperature coefficient of the diffused layers.     

Special features of recombination of nonequilibrium charge carriers in porous silicon with different nanostructure morphologies

Photoluminescence of porous silicon with different nanostructure morphologies and silicon monocrystalline wafers used as substrates was studied comparatively. The photoluminescence intensity of mesoporous and nanoporous silicon was established to be related to the excitation intensity by the quadratic and linear dependences, respectively. A model of recombination processes in the semiconductor nanocrystal systems is suggested. The experimental results are in good agreement with the model predictions.     

Drift mobility of carriers in porous silicon

The drift mobility of carriers in porous silicon has been studied in a wide temperature range (190–360 K) at electric field strengths of 2×10³–3×10⁴ V/cm. An exponential temperature dependence of the hole drift mobility with an activation energy of d ～ 0.14 eV was established. The density of localized states controlling the transport is evaluated.     

Average mobilities of carriers in subdoped silicon layers

The average mobilities of carriers in subdoped silicon layers as a function of surface as well as background concentrations are calculated using the universal formula for the average mobilities of carriers in nonhomogeneous doped layers along with Irvin's average conductivity curves. The results are shown graphically.     

Depletion of charge carriers in electronic polycrystalline silicon

Silicon Si is the material of choice for reliable and low-cost different applications in nano- and optoelectronics, from integrated circuits (IC) to TFTs and flat-panel displays. Some processes taking place in polycrystalline Si samples of finite length; for example, carrier depletion, is of particular interest. Until now processes of carrier depletion were studied only for monocrystalline Si and Ge. Here, we report for the first time the results of the studies carried out in polycrystalline Si specimens of n-type conductivity at room temperature in order to clarify the effect of crystal structure on electric field-induced properties. Samples with columnar and grain type of crystal structure were chosen for simplest non-destructive electric investigations. It was shown that depletion of carriers was observed in samples of columnar crystal structure; at the same time, specimens of grain type structure showed a synthetic mechanism: exclusion+tunneling of carriers through intergranular boundaries. The effect of heat treatment of the samples under 900 and 1100 °C on the described properties was also studied. The experimental results are numerically modeled, and a summary of investigations seems to be useful for future applications of polycrystalline Si.     

Velocity of surface carriers in inversion layers on silicon

Velocity-field curves for surface free-carriers in silicon are determined from measurements on resistivegate IGFETs. The measurements were performed on n-channel devices fabricated on both (100) and (111) substrates and on p-channel devices fabricated on (100) substrates. The channel length of the devices is ～8 μm and the impurity concentration of the substrates is ～ 10¹⁵ cm^?3. The dependence of velocity on the field strength along the channel is found to be well approximated by an empirical relationship involving three parameters: low-field mobility μ₀, a critical field $\text{E}{}_{\mathrm{cy}}$ signalling the onset of velocity saturation, and a parameter α that determines the curvature between the constant-mobility and constant-velocity branches of the curve. The curve-fitting parameters are given in tabular form for the two n-channel and one p-channel systems studied. The dependence of the velocity-field curves on temperatures in the range 100–350K is also reported.     

Effects of local photoexcitation of high-concentration charge carriers in silicon

The phenomena occurring at the local pulsed photoexcitation of intrinsic nonequilibrium highconcentration charge carriers in silicon are experimentally investigated. The effect of a substantial increase in the lifetime of photoexcited charge carriers is found. It is shown that the effect of a substantial increase in the lifetime of charge carriers is caused by a change in the degree of degeneracy and displacement of the impurityrecombination level towards the Fermi level due to local thermoelastic deformation of the crystal and the corresponding distribution of the concentration of nonequilibrium charge carriers.     

Interaction of copper impurity with radiation defects in silicon doped with boron

The spectrum of deep levels formed in boron-doped Czochralski-grown silicon single crystals as a result of interaction of radiation defects with copper impurity is studied. It is shown that, irrespective of the order of introduction of defects (both in the case of low-temperature copper diffusion into crystals preliminarily irradiated with electrons and in the case of irradiation of the samples contaminated with copper), the same set of deep levels appears. In addition to conventional radiation defects, three types of levels have been detected in the band gap of copper-containing crystals. These levels include the level E _v + 0.49 eV (already mentioned in available publications), the level E _v + 0.51 eV (previously not related to copper), and a level close to the donor level of a vacancy. Based on the analysis of concentration profiles, the interstitial carbonoxygen pair is excluded from possible precursors of the copper-containing center with level E _v + 0.49 eV.     

Anomalous long-term degradation of photoluminescence in porous silicon layers

The main systematic features of degradation of photoluminescence of porous silicon layers during long-term storage in air are studied. A profound increase in the photoluminescence intensity and a shift of the photoluminescence peak to shorter wavelengths in the optical spectra are observed for all samples. It is found that the degree of degradation depends on the initial crystallographic orientation of the silicon substrate. The decrease in the average diameters of silicon nanoclusters in porous silicon samples is attributed to chemical processes that occur at the developed porous surface with participation of atmospheric oxygen.     

Accumulation of majority charge carriers in GaAs layers containing arsenic nanoclusters

The accumulation of electrons and holes in GaAs layers that contained As clusters and were sandwiched between n-and p-type buffer GaAs layers was revealed by capacitance-voltage measurements. As a result of majority-carrier accumulation, expansive depletion regions are formed in the adjoining buffer layers. Simulation of the capacitance-voltage characteristics, based on a numerical solution of the Poisson equation, shows that the accumulated charge density is ～1×10¹² cm^?2, which is comparable with the concentration of As nanoclusters determined by transmission electron microscopy.     

A Mössbauer study of a two-electron acceptor impurity of zinc in silicon

Mössbauer emission spectroscopy of the ⁶⁷Ga(⁶⁷Zn) isotope was used to show that the two-electron acceptor impurity of Zn is present in silicon only in the form of neutral (Zn⁰) or doubly ionized (Zn^2?) centers, depending on the Fermi-level position. Broadening of the spectra corresponding to the above centers indicates that the local symmetry of these centers is not cubic. The absence of the line corresponding to the singly ionized state (Zn^2?) of zinc in the Mössbauer spectra of partially compensated samples is regarded as evidence that the correlation energy is negative.     

Migration of excited charge carriers in arrays of phosphorus-doped silicon nanocrystals

The rate of tunnel migration of excited charge carriers (electrons and holes) in the array of silicon nanocrystals doped with phosphorus is calculated. It is shown that, starting from certain phosphorus concentrations dependent on the relation between the dimensions of the emitting and accepting nanocrystals, the rate of tunneling of electrons sharply decreases (by several orders of magnitude) and becomes lower than the rate of interband radiative recombination     

Interaction of infrared radiation with free carriers in mesoporous silicon

Features of absorption and reflection of infrared radiation in the range 500–6000 cm^?1 are investigated; these features are associated with free carriers in the layers of mesoporous Si (porosity, 60–70%) formed in single-crystal p-Si(100) wafers with a hole concentration of N _p ≈10²⁰ cm^?3. It is found that the contribution of free holes to the optical parameters of the samples decreases as the porosity of the material increases and further falls when the samples are naturally oxidized in air. The experimental results are explained in the context of a model based on the Bruggeman effective medium approximation and the Drude classical theory with a correction for additional carrier scattering in silicon residues (nanocrystals). A comparison between the calculated and experimental dependences yields a hole concentration in nanocrystals of N _p ≈10¹⁹ cm^?3 for as-prepared layers and shows a reduction of N _p when they are naturally oxidized.     

Formation and control of boron buried layers in silicon using anexcimer laser

Buried layers have been shown to enhance performance of Si MOSFETs in the deep submicrometer regime. Epitaxial growth methods such as atomic layer doping or ion implantation are currently used for the formation of such doping profiles. In this work, we propose an alternative approach, using a XeCl (λ=308 mn) pulsed excimer laser, for the fabrication of pulse-shaped B profiles in Si. This process affords simplicity, versatility, and independent control over the depth, width, and the height of the B buried layer     

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.     

Excitation of luminescence in porous silicon with adsorbed ozone molecules

A new effect of the excitation of luminescence in porous silicon during adsorption of ozone from the gaseous phase was investigated. The signals of ozone-induced luminescence and photoluminescence decay with time of ozone exposure in a strictly correlated way; simultaneously, an oxide-phase growth is observed in porous silicon. A linear relationship was found between the luminescence intensity and the amount of oxide phase formed in the presence of ozone. Correlated shifts in the spectra of ozone-induced luminescence and photoluminescence are observed if the porosity of silicon varies. A mechanism for this effect is proposed. According to this mechanism, in the case of the dissociative adsorption of ozone, the exothermic reaction of oxidation of backbonds of a silicon atom takes place on the surface of nanocrystallites. Energy released is spent for the excitation of electron spectrum of silicon crystallites. The radiative relaxation in the case of ozone excitation proceeds similarly to that of the photon excitation of luminescence in porous silicon.