Dependence Of Generation Rate Research Articles

Theoretical model for spatial separation of dominating recombination region in a-Si:H/c-Si structures

In this paper we describe a theoretical model for spatial quantitative separation and determination of dominating recombination region in structures with amorphous/crystalline silicon heterojunction. This model is based on analysis of ideality factor, experimentally obtained from open circuit voltage dependence of generation rate; characteristic energy, corresponding to the maximum possible quasi Fermi level separation, experimentally obtained from temperature dependence of open circuit voltage. Compared to the existing approaches in this work we consider recombination in the bulk of amorphous silicon.

Analysis of lifetime distribution of defect luminescence in hydrogenated amorphous silicon

The lifetime distribution of defect photo-luminescence (PL) in a-Si:H has been analyzed quantitatively by obtaining a characteristic lifetime for the distribution. The generation rate dependence and the temperature variation of the characteristic lifetime have been obtained for the defect PL. Decrease of the lifetime with increasing generation rate, i.e., the nature of non-geminate recombination, has been observed for the defect PL of 0.83 and 0.95eV. Temperature variation of the characteristic lifetime of the PL has also been studied. The radiative recombination rate weakly depends on temperature in the case of 0.83eV while it increases with increasing temperature in the case of 0.95–1.46eV. Changes of the radiative recombination processes with increasing temperature are discussed.

Phonon-assisted tunnelling as a process determining current dependence on field andtemperature in MEH-PPV diodes

In this paper we would like to show the applicability of phonon-assistedtunnelling theories for explanation of temperature-dependent current–voltage(I–V)characteristics of diodes based on organic thin films of conjugated polymers, such as poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). For this purpose theI–V characteristics measured for MEH-PPV by Campbell et al (1997 J. Appl. Phys. 82 6326),Lupton and Samuel (1999 J. Phys. D: Appl. Phys. 32 2973), and van Woudenbergh et al (2001 Appl. Phys. Lett. 79 1697) are compared with the free carrier generation ratedependence on field strength using the phonon-assisted tunnelling theories. A strongdependence of charge carrier mobility on temperature is also explained by this model.The presented model allows us not only to explain the temperature variation ofI–V and mobility data, but also to estimate the density of traps taking part in the current flow.

Decay from steady‐state photocurrent in amorphous semiconductors

This paper presents analysis, computer modeling and experimental measurements of the photoconductive decay which occurs on cessation of illumination, in amorphous semiconductors. We explore the processes of relaxation of the excess carrier distributions using a capture-rate analysis, and examine the relative roles of re-trapping and recombination in exemplar cases of exponential trapping state profiles, with monomolecular and bimolecular recombination. We classify decay under strong or weak retrapping conditions which can be selected for by the prior steady generation rate, and this results in a rich variety of possible decay behavior. We examine several plausible intuitive explanations of the decay process, and show in detail that these explanations fail to predict accurately the speed and generation rate dependence of the decay. Results of experimental measurements of the decay from steady state and the impulse response in films of a-Si1−xCx:H are presented. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transient decay from the steady state in the photoconductivity of amorphous semiconductors

We introduce a modification to the theory developed by G. J. Adriaenssens, S. D. Baranovskii, W. Fuhs, J. Jansen, and \O. \Okt\u [Phys. Rev. B 51, 9661 (1995)] for the initial decay of the photoconductivity in amorphous semiconductors. With the same physical model and application of the concepts of multiple trapping we derive a theory based upon an emission-rate analysis for the decaying free electron density. We obtain an easy-to-use analytical expression for exponential band tails that links the decay characteristics to the band-tail parameter. A comparison with results from full numerical solutions confirms the validity of our analytical approach and the improvement with respect to the results by Adriaenssens et al. We also present solutions, supported by numerical modeling, that relate the power-law exponents for the generation rate dependence of the steady-state photoconductivity with that of the response time for the decay.

Generation of thermal donors in silicon: oxygen aggregation controlled by self-interstitials

The analysis of kinetic data on thermal donors (TDS) leads to a general model of oxygen aggregation: the aggregates which arise differ not only in size but also in the number of emitted self-interstitials, m. Each line of aggregates-of a particular m-may contain a TD family. The well known family of double donors observed in in absorption spectra is most likely to reside at m=2. This family is often a minor one in comparison with another family of double TDS which resides at m=1 and is characterized by somewhat shallower energy levels. Self-interstitials (SiIs) produced by aggregates have a strong effect on the TD generation as (i) the equilibrium concentration ratio between the adjacent lines depends on the SiI concentration, CI, and (ii) oxygen transport is sometimes enhanced by SiIs though oxygen attachment to the aggregates is often controlled not by monomer migration (enhanced or not) but by migration of the aggregates themselves. The dependence of generation rate on CI includes both rising and falling parts as well as plateaux (the classical fourth-power law is an example of such a plateau). The actual value of CI depends on the density of SiI sinks (self-interstitial clusters) which appears to be a most important parameter in the TD kinetics.

Photoconductivity in hydrogenated amorphous silicon. I. Thermal emission and hopping of trapped charges

Abstract The photoconductivity of hydrogenated amorphous silicon is found to be the result of three different processes that have been identified using the dual-beam-modulated photoconductivity technique: the release of electrons trapped either in D− states or in the conduction-band tail, and a quenching process related to recombination. Each process follows its own generation rate dependence. A model has been established in which the above three processes add up to give the observed steady-state photoconductivity. The relative contributions of these processes have been determined over large temperature and light flux ranges. This allowed us to show firstly that the thermal release of electrons from gap states, either from D− or from the band tail, is still an important process at 90 K, secondly that the relative contributions of D− and tail states change drastically at about 165 K, that is when the electron quasi-Fermi level is located at about 0.3 eV below the mobility edge and thirdly that at low tempe...

Defect Relaxation in a-Si:H Studied by Defect Absorption and Luminescence

The experimental results about defect relaxation in a-Si:H have been reported recently on several transient measurements, such as forward-bias-transient-current,1 optical bias effect on electron drift,2 and photo-capacitance transient.3 In this paper we report the steady state results of the bias effect on sub-band-gap absorption and photon-emission spectra measurements in undoped samples.The optical bias effects the sub-band absorption. In additional to an enhancement of the sub-band gap absorption, the absorption threshold moved from 0.8 eV to 0.6 eV for undoped samples under optical bias at room temperature. The effects are larger in the light-soaked-state than in the annealed-state.We also report on the generation rate dependence on photoluminescence (PL) spectra line shape. We found that the energy position of the defect PL band shifted from 1.1 eV to 0.9 eV when the excitation level increased 4,000 times.While motion of the quasi-Fermi level as a explanation cannot be eliminated, defect relaxation seems more plausible.

Light intensity dependence of the photoconductivity of hydrogenated amorphous silicon

Abstract The physical meaning of the exponent γ characterizing the light intensity dependence of the photoconductiyity σ p in hydrogenated amorphous silicon is investigated on the basis of a dual-beam modulated photoconductiyity (DBMP) analysis. The frequency-dependent spectrum can be resolyed into three components which at high temperatures are due to thermal re-emission of electrons from D− centres, thermal re-emission of electrons from tail states and electron-hole recombination. It is found that each component follows its own generation rate dependence with an exponent γ i ≠ γ. On this basis a model is established which allows firstly σ p to be related to the components separated by DBMP, secondly each component to be characterized by its relative weight a i such that Σa i γ i,= 1, thirdly the dependence of γ on the position of the quasi-Fermi level in the gap to be related to the variations in Σa i and fourthly the weights a i and the subgap density of states to be quantitatively correlated. Experime...

Trapping and recombination in amorphous hydrogenated silicon studied by dual-beam-modulated photoconductivity

Abstract The frequency-dependent spectrum of dual-beam-modulated photoconductivity reveals that various processes contribute to steady-state photoconductivity in undoped a-Si: H. These are 1. electron thermal emission from doubly-occupied dangling bonds 2. electron thermal emission from localized states of the conduction-band tail, and 3. quenching of photoconductivity which proceeds by reducing the density of electrons trapped on dangling bonds. These results are obtained on the basis of a model which in particular allows the coefficient γ characterizing the generation rate dependence of photoconductivity (σpαfy p) to be related to the coefficients γi(i = 1—3) characterizing processes 1—3: γ = a 1γ1 + a 2γ2 — a 3γ3, where a i is a dimensionless parameter describing the relative contribution of process i. The temperature and the photogeneration rate dependences of these processes studied in the annealed and in the light-soaked states are used to identify the nature of the quenching process.

Field enhanced carrier generation in MOS-capacitors containing defects

Minority carrier generation lifetime was measured as a function of depletion width on wafers with different densities of defects. The results were compared with theoretical predictions for field enhanced carrier emission from coulomb centers. We found that capacitors with a very low lifetime due to stacking faults or metallic precipitations show a drastically enhanced field dependence of generation rate. Agreement with theory can be obtained if we assume a locally enhanced electric field at the defects which can be one order of magnitude higher than the average bulk field.

Effect of Novobiocin and Its Combination with Tetracycline, Chloramphenicol, Erythromycin, and Lincomycin on the Microbial Generation of Escherichia coli

Inhibition of the steady-state generation of Escherichia coli by the bacteriostatic antibiotic novobiocin is linearly related to drug concentration in the range of 0 to 30 mug/ml. Increased cell sizes result because the drug inhibits cell division. The generation rate dependence on drug concentration depends on the nonionized fraction of novobiocin and is invariant with inoculum size or medium composition. However, the antibacterial activity of novobiocin decreases as the concentration of nutrients and Mg(2+) increases, although the inhibitory action of novobiocin on generation rate remains unchanged for concentrations of Mg(2+) above 8.1 x 10(-4) M. Novobiocin is synergistic in combinations with tetracycline in broth, but not when the Mg(2+) was maintained at 4.05 x 10(-3) M. Combinations of novobiocin with the 50S ribosomal subunit inhibitors chloramphenicol, erythromycin, or lincomycin are antagonistic, and the degree of growth inhibition is determined only by that component of the binary combination that would have the greater potency if it were acting alone.