On the effective cross-section for recombination of free carriers with dipole centres

An approximate calculation of the effective cross-section for recombination δ_eff of free charge carriers with dipole centres for small mean path lenght l of free carriers and not very small gas-kinetical cross-section of recombination σ_k is conducted. A formula for the σ_eff suitable also for larger l and small σ_k is suggested.     

Extremely low surface recombination in 1 Ω cm n‐type monocrystalline silicon

A key requirement in the recent development of highly efficient silicon solar cells is the outstanding passivation of their surfaces. In this work, plasma enhanced chemical vapour deposition of a triple layer dielectric consisting of amorphous silicon, silicon oxide and silicon nitride, charged extrinsically using corona, has been used to demonstrate extremely low surface recombination. Assuming Richter's parametrisation for bulk lifetime, an effective surface recombination velocity S_eff = 0.1 cm/s at Δn = 10¹⁵ cm^–3 has been obtained for planar, float zone, n ‐type, 1 Ω cm silicon. This equates to a saturation current density J_0s = 0.3 fA/cm², and a 1‐sun implied open‐circuit voltage of 738 mV. These surface recombination parameters are among the lowest reported for 1 Ω cm c‐Si. A combination of impedance spectroscopy and corona‐lifetime measurements shows that the outstanding chemical passivation is due to the small hole capture cross section for states at the interface between the Si and a‐Si layer which are hydrogenated during nitride deposition. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A nonperturbative stability theory of quantum field theory models

A nonperturbative method of analysis of the stability problem of quantum field theory models is proposed. The method consists in the systematic analysis of the functional dependence on boson field B of the effective boson Lagrangian S^eff(B) consisting of the fermion term S_l^F(B), constraint term S_l^FP(B) and the boson self-interaction term S_l(B). A new heat kernel representation for S_l^F(B) is derived in which counterterms are calculated in the explicit functional form by means of the analytic renormalization method. Using these results the instabillity of Yukawa₄, four-Fermi₄, and the massive Gürsey models is demonstrated.     

Quasiparticle recombination and 2Δ-phonon-trapping in superconducting tunneling junctions

The experimental recombination lifetime τ_eff of quasiparticles in superconducting films in general exceeds the intrinsic recombination lifetime τ _R by phonon trapping. On the basis of geometric acoustic propagation and reabsorption of phonons emitted in quasiparticle recombination, τ_eff is calculated as a function of film thicknessd taking into account longitudinal and transverse phonon reabsorption, bulk loss processes and acoustical phonon transmission into the substrate. With increasing thicknessd three characteristic ranges are found: range 1 with film thicknessd small compared to the phonon reabsorption mean free path Λ _w range 2 withd larger than Λ _w and dominating boundary losses, and range 3, also withd larger than Λ _w but with dominating bulk losses. For very smalld the relation between τ_eff and τ _R , the intrinsic recombination lifetime, contains only the limiting angle of total reflection of phonons within the superconducting film. Therefore, τ _R can be directly obtained by τ _eff measurements and from the sound velocities of the film-substrate system. Range 2 is characterized by a linear dependence of τ _eff ond. In this range it is not possible to obtain τ _R from τ _eff measurements, however, τ _eff allows a determination of the phonon boundary transmission. Range 3 shows no thickness dependence of τ _eff ond in the limit of larged values. In this range a further method for obtaining τ _R from τ _eff values is suggested.     

Metal carbide-induced negative flatband voltage shift in TaCx and HfCx/HfO2 gate stacks

We systematically investigated the role of the top interface for TaC_x and HfC_x/HfO₂ gate stacks on the effective work function (Φ_m,eff) shift by inserting a SiN layer at the gate/HfO₂ top interface or HfO₂/SiO₂ bottom interface. We found that Φ_m,eff of the TaN gate electrode on HfO₂ was larger than that on SiO₂ because of the HfO₂/SiO₂-bottom-interface dipole. On the other hand, we found that Φ_m,eff values of the TaC_x and HfC_x gate electrodes on HfO₂ agree with Φ_m,eff on SiO₂. This is because the potential offset of the opposite direction with respect to the bottom interface dipole appears at the metal carbide/HfO₂ interface. It is thus concluded that the top interface in the metal carbide/HfO₂ gate stacks causes the negative Φ_m,eff shift.     

Level-density parameters in the back-shifted Fermi gas model

The parameters a and δ_eff appearing in the back-shifted Fermi gas model are determined for about 3000 nuclei on the basis of modern estimated experimental data and the proposed systematics. For 272 of these nuclei, the parameters are deduced from experimental data on the cumulative numbers of low-lying levels and on mean spacings between S-wave neutron resonances at the neutron binding energy in the nuclei. For 952 nuclei, the parameter δ_eff is calculated by using the cumulative numbers of low-lying levels and values of the parameter a that were obtained via an interpolation from the points corresponding to the aforementioned 272 nuclei. For the remaining nuclei, the parameters a and δ_eff are obtained on the basis of the proposed systematics. An expression is constructed for taking into account the damping of shell effects with increasing excitation energy of nuclei. The results are compared with those from other studies.     

Dependence of the Curie temperature on the effective de Gennes factor in ferromagnets with exchange frustration

Considering the magneto-diluted gadolinium-containing Adamian alloys, it is shown that the dependence of the Curie temperature on the de Gennes factor ξ = c(g ? 1)2J(J + 1) can be expanded for ferromagnets with the exchange frustration. It is shown that in this case it is necessary to replace J by S _eff and that the linear dependence of T _C on ξ_eff remains up to the pure spin-glass state with S _eff = 0.     

Magnetizations of a nanoparticle described by the transverse Ising model

The characteristic influences of size S, exchange interaction and transverse field on the longitudinal and transverse magnetizations of a ferroelectric small particle described by the transverse Ising model are investigated by the use of the standard mean-field theory. In particular, the longitudinal magnetization of a nanoparticle is strongly affected by the surface situations. The effective exponent β_eff of the longitudinal magnetization is also studied. We find some characteristic phenomena of β_eff, depending on the values of S and the ratios of the physical parameters. In relation of recent investigations, the thermal variations of longitudinal and transverse magnetizations in the nanoparticle, consisting of a ferromagnetic core with size S=3 surrounded by a ferromagnetic surface shell with an antiferromagnetic inter-shell coupling, are examined and some typical ferrimagnetic behaviors are found in them. In relation to these phenomena, the effects of surface dilution on the magnetizations are investigated and some novel features are found in the system with size S=3 surrounded by such a ferromagnetic diluted surface shell.     

Dynamical entanglement as a signature of chaos in the semiclassical limit

The relationship between classically chaotic dynamics and the entanglement properties of the corresponding quantum system is examined in the semiclassical limit. Numerical results are computed for a modified kicked top, keeping the classical dynamics constant while investigating the entanglement for several versions of the corresponding quantum system characterized by a different value of the effective Planck constant ℏ _eff. Our findings indicate that as ℏ _eff → 0, the apparent signatures of classical chaos in the entanglement properties, such as characteristic oscillations in the time-dependence of the linear entropy, can also be obtained in the regular regime. These results suggest that entanglement is not a universal marker of chaotic dynamics of the corresponding classical system.     

Effective magnetic anisotropy of nanocrystalline Nd-Fe-Ti-N hard magnetic alloys

The intermetallic compound Nd-Fe-Ti-N has been successfully synthesized by a mechanical alloying process. The structure and magnetic properties of the sample have been studied using X-ray diffraction and magnetic measurements. It is found that alloy exhibits a nanocrystalline ThMn₁₂-type tetragonal structure with lattice parameters of a=0.8723 nm and c=0.4896 nm. The saturation magnetization M_S and effective magnetic anisotropy K_eff of the compound have been determined by investigating magnetization processes. The calculated results based on the law of approach to magnetic saturation have been successfully used to determine the constant K_eff. The difference between observed and calculated values in magnetization is lower than 3%. Of all terms in the law of approach to saturation, it is the 1/H ² term, which is attributed more to non-compensated anisotropy energy, that has the prevailing effect for the compound. The absorption of nitrogen is found to increase unit cell volume, M_S and K_eff. Received: 28 October 1996 / Revised: 14 March 1997 and 4 August 1997 / Accepted: 8 August 1997     

Buffer-less Cu(In,Ga)Se2 solar cells by band offset control using novel transparent electrode

Cu(In,Ga)Se₂ (CIGS) solar cells without buffer layers have been demonstrated. Currently, CdS, Zn(O,S,OH), ZnS, or InS buffer layers are used in high efficiency CIGS solar cells to suppress interface recombination. One of the important parameters to reduce the recombination is the conduction band offset (CBO) between the buffer and CIGS layers. In this study, we have proposed the use of a novel transparent conductive oxide (TCO) which can control the CBO to reduce interface recombination and eliminate the buffer layers. The device simulation was used to verify the effect of CBO control theoretically. Then, the novel TCO material of ZnO_1?xS_x:Al prepared by co-sputtering of ZnO:Al₂O₃ and ZnS targets was fabricated to verify the CBO effect experimentally. The efficiency of a CIGS solar cell with a ZnO:Al/CIGS/Mo/soda-lime glass structure, i.e. buffer-less structure using a conventional TCO, was significantly low because of severe shunting. In contrast, the use of ZnO_1-xS_x:Al instead of ZnO:Al increased the shunt resistance of the CIGS solar cell, resulting in higher open-circuit voltage and efficiency. The result is the first proof of the concept of the buffer-less CIGS solar cells.     

Modal quantum logic and its dialogic foundation

In this paper we investigate a modal calculus of quantum metalogic which is complete and sound with respect to a dialogic semantics. This calculus calledM _eff(Q _eff) has three parts: one covering the formal metalogic, one reflecting the calculus of the object language, and one which is a link between object language and metalanguage. This third part is invariant with respect to a variation of the object language     

Effectiveness of charge separation from the long-lived second excited state of donors

In molecular zinc-porphyrin-based donor–acceptor systems, the electron transfer from the second singlet excited state S₂ is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized charge-separated state (20%). It is demonstrated that the quantum yield of ultrafast charge separation in donor–acceptor triads D–A₁–A₂ can be close to 100% in molecular systems with lifetimes of the S₂ state longer than 150 ps. As prototypes of such systems, donor–acceptor diads D–A₁ and triads D–A₁–A₂ are considered, wherein the xanthione molecule plays the role of a donor. The ranges of the model parameters are determined in which the efficiency of charge separation is high. The twostage photoinduced charge transfer is studied within the framework of a multichannel stochastic model that takes into account the reorganization of a polar solvent and a high-frequency intramolecular vibrational mode.     

Quantum efficiency and fatiguing behavior of various luminescence processes in a−As2S3

Measurements of the luminescence excitation spectra of amorphous As₂S₃ show that the quantum effieiencies of the subbandgap luminescence processes decrease whereas that of the band-to-band luminescence increases with increasing excitation photon energy. Furthermore, the band-to-band luminescence shows no fatigue, in contrast to the subbandgap processes. These results allow us to conclude that the usual defects play no role in the band-to-band luminescence process and that surface recombination is not important in the energy range investigated.     

Monte Carlo simulation of SU(2) gauge theory with fermions on a four-dimensional lattice

After integration over the fermions in an SU(2) lattice gauge theory, the effective fermionic action may be expressed as a sum over all possible closed gauge field loops with corresponding weight factors. We approximate this sum and perform a Monte Carlo simulation of a coupled fermion-gauge system on a 4⁴ lattice. We compare our results for 〈S_eff〉 and $\text{〈}\text{ψ}\text{ψ〉}$ for different values of the gauge field coupling β and fermion coupling κ with the free fermion theory on a lattice. 〈S_eff〉 turns out to be quite small for $\text{κ?}\text{1}\text{8}$.     

Investigation of polymer/inorganic hybrid photovoltaic device using quantum dots as the interface modifier

CdS quantum dots (QDs) were introduced as an interface modifier in the poly(3-hexylthiophene) (P3HT)/TiO₂ nanorod arrays hybrid photovoltaic device. The presence of CdS QDs interlayer was found to provide enhanced light absorption, increased interfacial recombination resistance at the P3HT/TiO₂ interfaces, thus leading to a lower recombination rate of the electrons due to the stepwise structure of band edge in P3HT/CdS/TiO₂, which accounts for the observed enhanced photocurrent and photovoltage of the hybrid solar cells. The optimized performance was achieved in P3HT/CdS/TiO₂ hybrid solar cells after deposition of CdS QDs for 10 cycles, with a power conversion efficiency of 0.57 %, which is nearly ten times higher than that of P3HT/TiO₂. The findings indicate that inorganic semiconductor quantum dots provide effective means to improve the performance of polymer/TiO₂ hybrid solar cells.     

Increased open‐circuit voltage of ZnO nanowire/PbS quantum dot bulk heterojunction solar cells with solution‐deposited Mg(OH)2 interlayer

An ultrathin Mg(OH)₂ layer was solution‐deposited onto the ZnO nanowires to solve the problem of interfacial charge recombination, caused by the increase of interfacial area in bulk heterojunction (BHJ) PbS colloidal quantum dot solar cells (CQDSCs). This Mg(OH)₂ interlayer efficiently passivated the surface defects of ZnO nanowires and provided tunnel barrier at ZnO/PbS interface. As a result, the charge recombination at ZnO/PbS interface was largely suppressed, proved by the significantly elongated electron lifetime and the increased open‐circuit voltage of the Mg(OH)₂‐involved BHJ CQDSCs. Careful thickness optimization of Mg(OH)₂ interlayer finally brought a ～33% increase in V_oc and ～25% improvement in power conversion efficiency.     

Influence of Laser radiation on the collisional-radiative recombination and ionization coefficients

It is shown that irradiation of a plasma by a Laser beam lowers the collisional-radiative coefficient for recombination (α) and increases the one for ionization (S) compared to unirradiated plasmas. Numerical values ofα andS are given for a hydrogen plasma irradiated by CO₂, Ruby, and Ar-Ion Lasers. The calculations are based on a collisional-radiative model in which photoionization and stimulated recombination intervene. Stimulated recombination has an important effect and can lead to saturation of free-bound transitions at high power density of the Laser beam as long as multiphoton ionization effects and plasma heating by inverse bremsstrahlung remain negligibly small.     

The chemical zeno effect in exchange-coupled radical Pairs: 1. Triplet radical pairs

It was shown that spin-selective intracage recombination could influence the limiting triplet-singlet conversion stage and, therefore, the whole chemical dynamics. An increase in the rate constant for intracage recombination w decreases the S-T evolution frequency and changes its character by transforming the oscillating spin conversion mode into “kinetic.” As a result, an increase in w can decrease the yield of intracage recombination products and increase the yields of products formed in competing extracellular radical reaction channels. The chemical Zeno effect and its consequences are an analogue of the quantum Zeno effect (quantum evolution “deceleration” caused by successive measurements), and spin-selective recombination is similar to quantum state measurements.