Mobility of charge carriers in porous silicon layers

The (conduction) mobility of majority charge carriers in porous silicon layers of the n and p types is estimated by joint measurements of electrical conductivity and free charge carrier concentration, which is determined from IR absorption spectra. Adsorption of donor and acceptor molecules leading to a change in local electric fields in the structure is used to identify the processes controlling the mobility in porous silicon. It is found that adsorption of acceptor and donor molecules at porous silicon of the p and n types, respectively, leads to a strong increase in electrical conductivity, which is associated with an increase in the concentration of free carrier as well as in their mobility. The increase in the mobility of charge carriers as a result of adsorption indicates the key role of potential barriers at the boundaries of silicon nanocrystals and may be due to a decrease in the barrier height as a result of adsorption.     

Photoluminescence and EPR of porous silicon formed on <Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript> and <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript> single crystals implanted with boron and phosphorus ions

The effect of combined doping by shallow donor and acceptor impurities on boosting the quantum yield of porous-silicon photoluminescence (PL) in the visible and near IR range was studied using phosphorus and boron ion implantation. Nonuniform doping of samples and subsequent oxidizing annealing were performed before and after porous silicon was formed on silicon single crystals strongly doped by arsenic or boron up to ≈10¹⁹ cm^?3. The concentration of known P_b centers of nonradiative recombination was controlled by electron paramagnetic resonance. It is shown that there is an optimal joined content of shallow donors and acceptors that provides a maximum PL intensity in the vicinity of the red part of the visible spectrum. According to estimates, the PL quantum yield in the transitional n ⁺⁺-p ⁺ or p ⁺⁺-n ⁺ layer of porous silicon increases by two orders of magnitude as compared to that in porous silicon formed on silicon not subjected to ion irradiation.     

Shallow thermal donors in nitrogen-doped silicon single crystals

Czochralski-grown nitrogen-doped silicon crystals contain shallow thermal donors (STD) which are not present in reference crystals. In the course of annealing at 600 or 650°C, the STD concentration reaches saturation and this concentration scales with nitrogen content N as N^1/2. This implies that an STD includes only one nitrogen atom and that the most likely model of the STD defect is the NO_m complex of an interstitial nitrogen atom with m oxygen atoms. The number m is estimated as, on the average, m=3 from data on the temperature dependence of the equilibrium constant for the complex formation reaction     

Effect of the Lattice and Spin-Phonon Contributions on the Temperature Behavior of the Ground State Splitting of Gd<Superscript>3+</Superscript> in SrMoO<Subscript>4</Subscript>

The temperature behavior of the EPR spectra of the Gd³⁺ impurity center in single crystals of SrMoO₄ in the temperature range T = 99–375 K is studied. The analysis of the temperature dependences of the spin Hamiltonian b ₂ ⁰ (T) = b₂(F) + b₂(L) and P ₂ ⁰ (T) = P₂(F) + P₂(L) (for Gd¹⁵⁷) describing the EPR spectrum and contributing to the Gd³⁺ ground state splitting ΔE is carried out. In terms of the Newman model, the values of b₂(L) and P₂(L) depending on the thermal expansion of the static lattice are estimated; the b₂(F) and P₂(F) spin-phonon contributions determined by the lattice ion oscillations are separated. The analysis of b ₂ ⁰ (T) and P ₂ ⁰ (T) is evidence of the positive contribution of the spin-phonon interaction; the model of the local oscillations of the impurity cluster with close frequencies ω describes well the temperature behavior of b₂(F) and P₂(F).     

Mechanism of primary self-organization in porous silicon with a regular structure

A mechanism for self-organization of a regular system of pores in porous silicon is proposed. According to this mechanism, the self-organization obeys the general kinetic laws for a system of charge carriers. The mean interpore spacing in porous silicon prepared from p-Si and the anodizing current density required for synthesizing porous silicon through anodic etching are evaluated in terms of the proposed mechanism. The results obtained are in good agreement with the available experimental data. The dependence of the mean interpore spacing on the carrier concentration in the initial silicon is predicted to be similar to the function L(n) ～ n^?1/2. The validity of the proposed mechanism is confirmed by computer simulation.     

Orientation of ring hexavacancies under bending of silicon crystals

The orientation of ring hexavacancies, namely, the V ₆ vacancy clusters (B ₈₀ ⁴ centers), in single-crystal silicon is investigated under tension. It is demonstrated that hexavacancies under tension of the crystal along the [111] crystallographic direction are oriented in the [?111], [?1–11], and [1–11] directions. This orientation is explained by the reversal of the sign of the relative change in the length of the 〈110〉 bonds in V ₆ hexavacancies under tension of the crystal along the [111] direction with respect to the case of compression along the same direction. The pressure dependences of the intensity ratio I ₁/I ₂ of the lines X ₈₀ ⁴ (J ₁) and X ₇₂ ⁴ (J ₂) in the spectrum of the recombination radiation of excitons bound to the V ₆ hexavacancies are obtained. These dependences characterize the degree of orientation of the V ₆ hexavacancies as a function of the pressure acting on the V ₆ hexavacancies under compression and tension of the silicon crystal. A method is proposed for the orientation of V ₆ hexavacancies in the case of simultaneous compression and tension of the crystal under bending.     

Estimates of the spontaneous polarization in silicon carbide

Various approaches to calculating the spontaneous polarization P _sp for different polytypes of silicon carbide are discussed. Our estimates combined with data reported by other authors reveal a considerable scatter in the values of P _sp for the 2H-SiC polytype (from ?1.11 to ?4.32 × 10^?2 C/m²). The need for further studies is stressed.     

Ionization mechanisms of aluminum acceptor impurity in silicon

Processes of ionization of shallow acceptor centers (ACs) in silicon are studied. In crystalline silicon samples with phosphorus (1.6×10¹³, 2.7×10¹³, and 2.3×10¹⁵cm^?3) and boron (1.3×10¹⁵cm^?3) impurities, _μAl impurity atoms were produced by implantation of negative muons. It is found that thermal ionization is the main mechanism for ionizing the Al acceptor impurity in both p-type and n-type silicon with an impurity concentration of ?10¹⁵cm^?3 at T>45 K. The thermal ionization rate of Al ACs in Si varies from ～10⁵ to ～10⁶s^?1 in the temperature range 45–55 K.     

X-ray reflectivity and photoelectron spectroscopy of superlattices with silicon nanocrystals

The structural properties and features of the chemical composition of SiO _x N _y /SiO₂, SiO _x N _y /Si₃N₄, and SiN _x /Si₃N₄ multilayer thin films with ultrathin (1–1.5 nm) barrier SiO₂ or Si₃N₄ layers are studied. The films have been prepared by plasma chemical vapor deposition and have been annealed at a temperature of 1150°С for the formation of silicon nanocrystals in the SiO _x N _y or SiN _x silicon-rich layers with a nominal thickness of 5 nm. The period of superlattices in the studied samples has been estimated by X-ray reflectivity. The phase composition of superlattices has been studied by X-ray electron spectroscopy using the decomposition of photoelectron spectra of the Si 2p, N 1s, and O 1s levels into components corresponding to different charge states of atoms.     

Nonmonotonic temperature dependence of the Hall resistance of a 2D electron system in silicon

For a 2D electron system in silicon, the temperature dependence of the Hall resistance ρ_xy(T) is measured in a weak magnetic field in the range of temperatures (1–35 K) and carrier concentrations n where the diagonal resistance component exhibits a metallic-type behavior. The temperature dependences ρ_xy(T) obtained for different n values are nonmonotonic and have a maximum at T_max ～ 0.16T_F. At lower temperatures T < T_max, the change δρ_xy(T) in the Hall resistance noticeably exceeds the interaction quantum correction and qualitatively agrees with the semiclassical model, where only the broadening of the Fermi distribution is taken into account. At higher temperatures T > T_max, the dependence ρ_xy(T) can be qualitatively explained by both the temperature dependence of the scattering time and the thermal activation of carriers from the band of localized states.     

The Numerical Simulation of the Nanosecond Switching of a <Emphasis Type="Italic">p</Emphasis>-SOS Diode

Abrupt high-density reverse current interruption has been numerically simulated for switching from forward to reverse bias in a silicon p⁺P₀n⁺ structure (p-SOS diode). It has been shown that the current interruption in this structure occurs as a result of the formation of two dynamic domains of a strong electric field in regions in which the free carrier concentration substantially exceeds the concentration of the doping impurity. The first domain is formed in the n⁺ region at the n⁺P⁰ junction, while the second domain is formed in the P⁰ region at the interface with the p⁺ layer. The second domain expands much faster, and this domain mainly determines the current interruption rate. Good agreement is achieved between the simulation results and the experimental data when the actual electric circuit determining the electron–hole plasma pumping in and out is accurately taken into account.     

Quantum-Chemical Simulation of the Solvent Effect on Spontaneous Emission of Singlet Oxygen

A molecular simulation of the solvent effect on radiative rate constant k _r of singlet oxygen is carried out. This study included a search for the most probable conformations of the complexes of molecules of singlet oxygen and ten solvents and calculation of dipole moments M of transitions a¹Δ _g –b¹Σ _g ⁺ (M _a–b ) and a¹Δ _g –X³Σ _g ^- (M _a–X ) of the oxygen molecule for them. Averaging of M _a–b by conformations, taking into account the probability of their formation for complexes without atoms with a large atomic number (Cl, S), yields values that, as a rule, correlate well with the behavior of k _r in the experiment. Taking into account the possibility of decreasing the distance (compared to equilibrium) between molecules in a collision complex at room temperature made it possible to achieve satisfactory agreement of the calculated and experimental data also for complexes with CCl₄, C₂Cl₄, and CS₂. The obtained data indicate that a number of factors affect k _r . The correlation of k _r with molecular polarizability in a number of cases is due, on the one hand, to its effect on the strength of dispersion interactions in the complex and, on the other hand, to the fact that it to some extent reflects the position of the upper filled orbitals of the solvent molecule. Both factors affect the degree of mixing of the π orbitals of the singlet oxygen molecule with the orbitals of the solvent molecule, which, as was found earlier, facilitates the activation of the a¹Δ _g –b¹Σ _g ⁺ transition and the borrowing of its intensity by the a¹Δ _g –X³Σ _g ^- transition.     

Scattering the Geometry of Weighted Graphs

Given two weighted graphs (X, b_k, m_k), k =?1,2 with b₁ ～ b₂ and m₁ ～ m₂, we prove a weighted L¹-criterion for the existence and completeness of the wave operators W_±(H₂, H₁, I_1,2), where H_k denotes the natural Laplacian in ?²(X, m_k) w.r.t. (X, b_k, m_k) and I_1,2 the trivial identification of ?²(X, m₁) with ?²(X, m₂). In particular, this entails a general criterion for the absolutely continuous spectra of H₁ and H₂ to be equal.     

Specific features of the three-electron <Emphasis Type="Italic">KL</Emphasis><Stack><Subscript>2, 3</Subscript><Superscript>2</Superscript></Stack> ionization of silicon atoms upon near-threshold photoabsorption

The shape and relative intensity of the group of the Kα_5–8 satellites (radiative transitions KL _{2, 3} ² )-L _{2, 3} ³ of Si atoms are experimentally studied upon photoabsorption near and far from the KL _{2, 3} ² ionization threshold. The satellites were excited near the ionization threshold by lines of the characteristic L spectrum and bremsstrahlung radiation from Nb and Mo anodes and far from the threshold by the L spectrum and bremsstrahlung radiation from an Ag anode and by monochromatized Kα_{1, 2} radiation from a Ti anode. It is established that the probability P(L _{2, 3} ² ) of formation of two additional 2p vacancies during KL _{2, 3} ² photoabsorption of Si atoms near the energy threshold is by a factor of 1.5 lower than that during photoionization in a more distant energy region beyond the threshold. At the same time, the P(L _{2, 3} ² )/P(L _{2, 3}) ratio remains invariable for the absorbed photons throughout the energy range studied. It is demonstrated that, as the KL _{2, 3} ² ionization threshold is approached, an intensity redistribution occurs among the components of the group of the Kα_5–8 lines, which reflects a decrease in the excitation cross section ratio σ(⁴ P)/σ(² P) of the ⁴ P and ² P terms of the KL _{2, 3} ² configuration. A conclusion is drawn that the effects of suppression of the generation of P terms of higher multiplicity during the KL _{2, 3} and KL _{2, 3} ² near-threshold photoionizations are of a common nature.     

Role of MW-ECR hydrogen plasma on dopant deactivation and open-circuit voltage in crystalline silicon solar cells

Plasma hydrogenation is an efficient method to passivate intergrain and intragrain defects of polycrystalline silicon (pc-Si) solar cells. The hydrogenation experiments were carried out in hydrogen plasma generated in an electron cyclotron resonance system controlling different operating parameters such as microwave power (P _MW), process time (t _H) and hydrogenation temperature (T _H) for a fixed hydrogen flux of 30 sccm. The hydrogenation of n⁺pp⁺ pc-Si solar cells resulted in an improvement in the open-circuit voltage. The improvement was correlated with the dopant deactivation due to the formation of boron–hydrogen bonding. This was demonstrated from the changes in the doping level after hydrogenation of n⁺p diode structures made using single crystalline silicon as a reference material. It was found that deactivation of boron was more pronounced at high microwave plasma power, in good agreement with the high open-circuit voltage values obtained on pc-Si mesa cells. On the other hand, the effect of longer hydrogenation time and higher temperature resulted in a decrease of boron deactivation, while an increase in V _oc with a tendency of saturation at high T _H was observed. Reasons for such behavior were thoroughly explained.     

Assessment of the relaxation of the autocorrelation function for the <Emphasis Type="Italic">Ã</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">B</Emphasis><Subscript>2</Subscript> excited state of the NO<Subscript>2</Subscript> molecule

Based on the Bixon-Jortner model of two coupled vibrational ensembles, an analytical expression for the autocorrelation function |P(t)|² of the à ² B ₂ of the term of the NO₂ molecule is derived. Proceeding from the known relaxation parameters of the autocorrelator, a fast method for estimating the nonadiabatic matrix element of the \(\tilde A^2 B_2 - \tilde X^2 A_1\) transition in NO₂ is proposed.     

Energy scan of cross sections for <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>–</Superscript> annihilation into quarkonia and light hadrons in the Belle experiment

The cross sections of the reactions e⁺e^– → ?(nS)π⁺π^? (n = 1, 2,3) and e⁺e^– → h _b (nP)π⁺π^? (n = 1, 2) are measured as a function of the cms collision energy from their thresholds up to 11.02 GeV using the data of the Belle experiment operating at the KEKB e⁺e^– collider. The peaks of the ?(10 860) and ?(11020) resonances are observed in the cross sections with an insignificant contribution of the continuum. The decay ?(11020) → h _b (nP)π⁺π^? is found to fully proceed through intermediate isovector states Z _b (10610) and Z _b (10650).     

Conductivity anisotropy ofn-type silicon in the range of warm and hot carriers

The electric conductivity ofn-type silicon was measured as a function of the field intensity in different crystallographic directions at temperatures between 78 and 275 °K. From the data at medium fields (range of warm carriers) the coefficientsβ ₀ andγ ₀ ofSchmidt-Tiedemann's anisotropy theory were determined. Especially at low temperatures these coefficients are different for lightly and heavily doped crystals. The differences can be explained by the influence of ionized impurity scattering in addition to lattice scattering. The repopulation of the energy valleys of the conduction band as a function of the field intensity was calculated from the ratioγ ₀/β ₀ and — in the range of hot carriers — from the conductivities measured in <001> and <111> directions. A maximum increase of population of a cool valley was found to be between 0.5 and 1.2 of the zero-field population, depending on the particular sample, the field intensity being about 0.5 kV/cm and the lattice temperature 89 °K.