Martin Kirkengen

Direct generation of charge carriers in c-Si solar cells due to embedded nanoparticles

It is known that silicon is an indirect band gap material, reducing its efficiency in photovoltaic applications. Using surface plasmons in metallic nanoparticles embedded in a solar cell has recently been proposed as a way to increase the efficiency of thin-film silicon solar cells. The dipole mode that dominates the plasmons in small particles produces an electric field having Fourier components with all wave numbers. In this work, we show that such a field creates electron-hole-pairs without phonon assistance, and discuss the importance of this effect compared to radiation from the particle and losses due to heating.

Generalized two-tier relevance filtering of computer game update events

In this work-in-progress paper we present a relevance filtering scheme for a two-tier server architecture optimized for massive multiplayer online games. We distinguish between interest management of server tier game state and bandwidth adaptation of concentrator tier client link thresholds, making the concentrator tier totally application independent. An initial prototype has been implemented, demonstrating significant reductions in update event rate without loss of playability.

The role of excess minority carriers in light induced degradation examined by photoluminescence imaging

A new approach to investigate light induced degradation (LID) effects in boron-doped silicon has been developed. By studying spatial variations in LID resulting from localized carrier excitation (spot-LID), it is verified that the generation of the boron-oxygen complexes responsible for the degradation is directly related to the presence of excess minority carriers. Through the examination of the diffused minority carrier density distribution (during light exposure), from an exposed into an unexposed wafer area compared to the observed defect generation, we are able to monitor the generation of excess carrier induced defects over a range of carrier concentrations. The results show that very low concentrations of minority excess carrier densities are sufficient to generate the defects. For the investigated material carrier concentrations down to 1.7 ± 0.2 × 109 cm−3 are observed to cause lifetime degradation.

The Coulomb gap and low energy statistics for Coulomb glasses

We study the statistics of local energy minima in the configuration space of two-dimensional lattice Coulomb glasses with site disorder and the behavior of the Coulomb gap depending on the strength of random site energies. At intermediate disorder, i.e., when the typical strength of the disorder is of the same order as the nearest-neighbor Coulomb energy, the high energy tail of the distribution of the local minima is exponential. We furthermore analyze the structure of the local minima and show that most sites of the system have the same occupation numbers in all of these states. The density of states (DOS) shows a transition from the crystalline state at zero disorder (with a hard gap) to an intermediate, probably glassy state with a Coulomb gap. We analyze this Coulomb gap in some detail and show that the DOS deviates slightly from the traditional linear behavior in 2D. For finite systems these intermediate Coulomb gap states disappear for large disorder strengths and only a random localized state in which all electrons are in the minima of the random potential exists.Dedication: This paper is dedicated to Thomas Nattermann, our dearest friend, brilliant colleague, and outstanding teacher.

Nonlinear acoustic and microwave absorption in disordered semiconductors

Nonlinear hopping absorption of ultrasound and electromagnetic waves in amorphous and doped semiconductors is considered. It is shown that even at low amplitudes of the electric ~or acoustic! field the nonlinear corrections to the relaxational absorption appear anomalously large. The physical reason for such behavior is that the nonlinear contribution is dominated by a small group of close impurity pairs having one electron per pair. Since the group is small, it is strongly influenced by the field. An external magnetic field strongly influences the absorption by changing the overlap between the pair components’ wave functions. It is important that the influence is substantially different for the linear and nonlinear contributions. This property provides an additional tool to extract nonlinear effects. The subject of this article is nonlinear microwave and acoustic properties of amorphous semiconductors and lightly doped crystalline semiconductors in the regime of hopping conductance. We are interested in absorption to electron transitions between localized states associated with defects or impurity atoms. We consider the case where the absorption is due to electron hopping within pairs of neighboring defects containing one electron per pair. The distance between the centers within the pair must be small enough to allow tunneling, while the distance to other impurities should be large enough to prevent tunneling to impurities outside the pair. In a weakly doped semiconductor we can expect these pairs to be relatively rare, and triplets of the same kind will be even less likely. Thus a natural basis to treat the problem is the so-called two-level approximation according to which only the lowest energy level of each of two neighboring impurities are taken in consideration. This approach, as well as its range of applicability, was first discussed in detail by Pollack and Geballe. 1 For brevity, in the following we shall discuss the case of acoustic attenuation and then specify what changes in the formulas should be introduced to allow for electromagnetic absorption. An external ac electric or acoustic field causes transitions between the electron states. Direct interlevel transitions leading to absorption of quanta give rise to the so-called resonant absorption. For low intensities, the resonant contribution to the absorption coefficient of an acoustic wave can be expressed as 2 G (res) 5~a 1v/s!tanh~\v/2kBT!, ~1!

Long-term Cyclability of Substoichiometric Silicon Nitride Thin Film Anodes for Li-ion Batteries

Silicon has been the subject of an extensive research effort aimed at developing new anode materials for lithium ion batteries due to its large specific and volumetric capacity. However, commercial use is limited by a number of degradation problems, many of which are related to the large volume change the material undergoes during cycling in combination with limited lithium-diffusivity. Silicon rich silicon oxides (SiOx), which converts into active silicon and inactive lithium oxide during the initial lithiation, have attracted some attention as a possible solution to these issues. In this work we present an investigation of silicon rich amorphous silicon nitride (a-SiNx) as an alternative convertible anode material. Amorphous SiN0.89 thin films deposited by plasma enhanced chemical vapour deposition show reversible reactions with lithium when cycled between 0.05 and 1.0 V vs. Li+/Li. This material delivers a reversible capacity of approximately 1,200 mAh/g and exhibits excellent cycling stability, with 41 nm a-SiN0.89 thin film electrodes showing negligible capacity degradation over more than 2,400 cycles.

Coherent charge transport through an interface between a superconductor and hopping insulator : Role of barrier properties

At low temperatures and voltages tunneling transport through an interface between a superconductor and hopping insulator is dominated by coherent two-electron tunneling between the Cooper-pair condensate and pairs of localized states Kozub et al., Phys. Rev. Lett. 96, 107004 2006. By detailed analysis of such transport we show that the interface resistance is extremely sensitive to the properties of the tunneling barriers, as well as to the asymptotic behavior of the localized states. In particular, a dramatic cancellation takes place for hydrogenlike impurities and ideal barriers. However, some disorder can lift the cancellations, restoring the interface transport. We also study the non-Ohmic behavior of the interface resistor and show that it is sensitive to the Coulomb correlation of the occupation probabilities of the involved localized states. It is expected that the non-Ohmic contribution to the I-V curve will experience pronounced mesoscopic fingerprint fluctuations.

Nonlinear acoustic and microwave absorption in glasses

A theory of weakly-nonlinear low-temperature relaxational absorption of acoustic and electromagnetic waves in dielectric and metallic glasses is developed. Basing upon the model of two-level tunneling systems we show that the nonlinear contribution to the absorption can be anomalously large. This is the case at low enough frequencies, where freqeuency times the minimal relaxation time for the two-level system are much less than one. In dielectric glasses, the lowest-order nonlinear contribution is proportional to the waves intensity. It is negative and exhibits anomalous frequency and temperature dependencies. In metallic glasses, the nonlinear contribution is also negative, and it is proportional to the square root of the waves intensity and to the frequency. Numerical estimates show that the predicted nonlinear contribution can be measured experimentally.

Substoichiometric Silicon Nitride – An Anode Material for Li-ion Batteries Promising High Stability and High Capacity

Silicon is often regarded as a likely candidate to replace graphite as the main active anode material in next-generation lithium ion batteries; however, a number of problems impacting its cycle stability have limited its commercial relevance. One approach to solving these issues involves the use of convertible silicon sub-oxides. In this work we have investigated amorphous silicon sub-nitride as an alternative convertible silicon compound by comparing the electrochemical performance of a-SiNx thin films with compositions ranging from pure Si to SiN0.89. We have found that increasing the nitrogen content gradually reduces the reversible capacity of the material, but also drastically increases its cycling stability, e.g. 40 nm a-SiN0.79 thin films exhibited a stable capacity of more than 1,500 mAh/g for 2,000 cycles. Consequently, by controlling the nitrogen content, this material has the exceptional ability to be tuned to satisfy a large range of different requirements for capacity and stability.

Slow relaxation and equilibrium dynamics in a two-dimensional Coulomb glass: Demonstration of stretched exponential energy correlations

We have simulated energy relaxation and equilibrium dynamics in Coulomb glasses using the random energy lattice model. We show that in a temperature range where the Coulomb gap is already well developed T = 0.03– 0.1, the system still relaxes to an equilibrium behavior within the simulation time scale. For all temperatures T, the relaxation is slower than exponential. Analyzing the energy correlations of the system at equilibrium C, we find a stretched exponential behavior, C =e � /0 . We study the temperature dependence of 0 and . 0 is shown to increase faster than exponentially with decreasing T. is proportional to T at low temperature and approaches unity for high temperature. We define a time from these stretched exponential correlations and show that this time corresponds well with the time required to reach equilibrium. From our data it is not possible to determine whether diverges at any finite temperature, indicating a glass transition, or whether this divergence happens at zero temperature. While the time dependence of the system energy can be well fitted by a random walker in a harmonic potential for high temperatures T =1 0, this simple model fails to describe the long time scales observed at lower temperatures. Instead we present an interpretation of the configuration space as a structure with fractal properties and the time evolution as a random walk on this fractal-like structure.