Comparative studies of lunar, Martian, and Mercurian craters and plains

The spatial distribution of lunar smooth plains is not consistent with experimental simulations of melt rock emplacement during cratering in layered materials. Nor is it consistent with the location of melt rocks (suevite) near the Ries basin. Lunar smooth plains surrounding Imbrium are most extensive in areas where pre-existing craters are most degraded. This observation suggests that plains form by impact of basin and local primary crater ejecta, together with deposition of debris excavated by the resultant secondary cratering events. Craters within the belt of smooth plains surrounding the Caloris basin on Mercury are most degraded nearest the basin; this suggests that Mercurian smooth plains must, at least in part, be emplaced in a manner similar to plains surrounding the Imbrium basin. Mercurian uplands have a primary crater population deficient in small crater diameters (less than approximately 30 km). Lunar uplands far from major basins also have a crater population deficient in small crater sizes. Martian cratered terrain exhibits a similar crater deficiency, which was previously interpreted as due to obliteration of small craters (less than approximately 30 km) by some surface process. A crater size distribution deficient in small sizes (less than approximately 30 km) on the Mercurian, lunar, and Martian uplands has implications for the origin of debris bombarding the inner solar system during the period recorded by these surfaces. It is proposed that during late heavy bombardment, the inner solar system was inundated with bodies that broke up under tidal fission as they approached the planets. Such a mechanism would lend to production of a crater population deficient in small crater sizes, and it would also explain the large degree of spatial clustering of primary craters on Mercury, the moon, and Mars

Simulation study of GaAsP/Si tandem solar cells

A model, adapted from the Shockley-Queisser detailed balance model to tandem solar cells with a monolithically grown GaAsxP1-x top junction on a Si bottom junction, has been developed. Updated data have been used for the absorption spectrums. Two surface geometries, flat and ideally textured, have been investigated. As an important improvement over existing models, the effects of threading-dislocations related Shockley-Read-Hall recombinations in the GaAsxP1-x cell, due to the lattice mismatch between the GaAsxP1-x epilayers and the Si substrate, have been taken into consideration. Auger recombinations in the Si bottom cell and luminescent coupling between the cells have also been considered. For a dislocation free 2-μm thick top cell, maximal theoretical efficiencies of 41.6% and 39.1% have been calculated for a textured and a flat surface, respectively. For threading dislocation (TD) densities below 10^4 cm^-2, the impact of TDs in the GaAsxP1-x layers on the solar cell performances is very limited. With TD densities over 10^5 cm^-2, the top cell open circuit voltage is reduced, hence the overall efficiency. For TD densities over 4×10^6 cm^-2, as the diffusion length of minority carriers in the base gets smaller than the base thickness, the short circuit current in the top GaAsxP1-x cell is also reduced, resulting in a decrease in the optimal top cell bandgap. Using non ideal EQEs and surface recombination rates from published experimental data, the long-term efficiency potential of the investigated technology has been estimated to be ~35.1% for an ideally textured GaAsxP1 x/Si tandem cell with a TD density of 10^5 cm^-2 (~33.0% with a flat surface)

External Quantum Efficiency modeling of GaAs solar cells grown on Si: a method to assess the Threading Dislocation Density

A method is reported in order to determine an upper bound for the Threading Dislocation (TD) density in experimental GaAs solar cells grown lattice-mismatched on Si. The method is based on the modeling of the devices’ External Quantum Efficiency (EQE), using the classic drift-diffusion model, or Hovel model. The model is fitted to experimental EQE measurements, using the diffusion length of minority carriers as the sole fitting parameter. Assuming low surface recombination velocities at both interfaces, a lower bound for the diffusion length of minority carriers is determined. Considering non-radiative recombinations on TDs as the dominant recombination pathway, this lower bound for the diffusion length of minority carriers can be converted to an upper bound for the TD density, using the NTT model. This method is then used to assess the TD density in GaAs solar cells grown on Si by Molecular Beam Epitaxy, using Strained Layer Superlattice (SLS) Dislocation Filter Layers (DFLs) coupled with Thermal Cycle Annealing (TCA) steps in order to reduce the TD density in the active region of the devices. Upper bounds for the TD densities in the low 10^{7}cm^{-2} are thus extracted from the devices’ experimental EQE measurements

Mount St. Helens aerosol evolution

Stratospheric aerosol samples were collected using a wire impactor during the year following the eruption of Mount St. Helens. Analysis of samples shows that aerosol volume increased for 6 months due to gas-to-particle conversion and then decreased to background levels in the following 6 months

Simulation study of GaAsP/Si tandem cells including the impact of threading dislocations on the luminescent coupling between the cells

A model, derived from the detailed balance model from Shockley and Queisser, has been adapted to monolithically grown GaAsP/Si tandem dual junction solar cells. In this architecture, due to the difference of lattice parameters between the silicon bottom cell – acting as the substrate – and the GaAsP top cell, threading dislocations (TDs) arise at the III-V/Si interface and propagate in the top cell. These TDs act as non-radiative recombination centers, degrading the performances of the tandem cell. Our model takes into account the impact of TDs by integrating the NTT model developed by Yamaguchi et. al.. Two surface geometries have been investigated: flat and ideally textured. Finally the model considers the luminescent coupling (LC) between the cells due to reemitted photons from the top cell cascading to the bottom cell. Without dislocations, LC allows a greater flexibility in the cell design by rebalancing the currents between the two cells when the top cell presents a higher short-circuit current. However we show that, as the TD density (TDD) increases, non-radiative recombinations take over radiative recombinations in the top cell and the LC is quenched. As a result, non-optimized tandem cells with higher short-circuit current in the top cell experience a very fast degradation of efficiency for TDDs over 10^4cm^-2. On the other hand optimized cells with matching currents only experience a small efficiency drop for TDDs up to 10^5cm^-2. High TDD cells therefore need to be current-matched for optimal performances as the flexibility due to LC is lost

Non-Oberbeck-Boussinesq effects in turbulent thermal convection in ethane close to the critical point

As shown in earlier work (Ahlers et al., J. Fluid Mech. 569, p.409 (2006)), non-Oberbeck Boussinesq (NOB) corrections to the center temperature in turbulent Rayleigh-Benard convection in water and also in glycerol are governed by the temperature dependences of the kinematic viscosity and the thermal diffusion coefficient. If the working fluid is ethane close to the critical point the origin of non-Oberbeck-Boussinesq corrections is very different, as will be shown in the present paper. Namely, the main origin of NOB corrections then lies in the strong temperature dependence of the isobaric thermal expansion coefficient \beta(T). More precisely, it is the nonlinear T-dependence of the density \rho(T) in the buoyancy force which causes another type of NOB effect. We demonstrate that through a combination of experimental, numerical, and theoretical work, the latter in the framework of the extended Prandtl-Blasius boundary layer theory developed in Ahlers et al., J. Fluid Mech. 569, p.409 (2006). The latter comes to its limits, if the temperature dependence of the thermal expension coefficient \beta(T) is significant.Comment: 18 pages, 15 figures, 3 table

Impact of the growth temperature on the performance of 1.70-eV Al 0.22 Ga 0.78 As solar cells grown by MBE

Growth of high material quality Aluminum Gallium Arsenide (AlxGa1-xAs) is known to be challenging, in particular with an Al content x above 20%. As a result, the use of AlxGa1-xAs in devices requiring high minority carrier lifetimes, such as solar cells, has been limited. Nonetheless, it has long been established that the substrate temperature is a key parameter in improving AlxGa1-xAs material quality. In order to optimize the growth temperature of 1.70-eV Al0.22Ga0.78As solar cells, five samples have been grown by Solid-Source Molecular Beam Epitaxy (SSMBE) at 580 °C, 600 °C, 620 °C, 640 °C, and 660 °C, respectively. A strong improvement in performance is observed with increasing the growth temperature from 580 °C to 620 °C. An open-circuit voltage above 1.21 V has in particular been demonstrated on the sample grown at 620 °C, translating into a bandgap-voltage offset Woc below 0.5 V. Above 620 °C, performances – in particular the short-circuit current density – moderately decrease. This trend is confirmed by photoluminescence, current density versus voltage characterization under illumination, and external quantum efficiency measurements

1.7eV Al0.2Ga0.8As solar cells epitaxially grown on silicon by SSMBE using a superlattice and dislocation filters

Lattice-mismatched 1.7eV Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on Si substrates using Solid Source Molecular Beam Epitaxy (SSMBE). As a consequence of the 4%-lattice-mismatch, threading dislocations (TDs) nucleate at the interface between the Si substrate and III-V epilayers and propagate to the active regions of the cell. There they act as recombination centers and degrade the performances of the cell. In our case, direct AlAs/GaAs superlattice growth coupled with InAlAs/AlAs strained layer superlattice (SLS) dislocation filter layers (DFLSs) have been used to reduce the TD density from 1×10^9cm^-2 to 1(±0.2)×10^7cm^-2. Lattice-matched Al0.2Ga0.8As cells have also been grown on GaAs as a reference. The best cell grown on silicon exhibits a Voc of 964mV, compared with a Voc of 1128mV on GaAs. Fill factors of respectively 77.6% and 80.2% have been calculated. Due to the lack of an anti-reflection coating and the non-optimized architecture of the devices, relatively low Jsc have been measured: 7.30mA.cm^-2 on Si and 6.74mA.cm^-2 on GaAs. The difference in short-circuit currents is believed to be caused by a difference of thickness between the samples due to discrepancies in the calibration of the MBE prior to each growth. The bandgap-voltage offset of the cells, defined as Eg/q-Voc, is relatively high on both substrates with 736mV measured on Si versus 572mV on GaAs. The non-negligible TD density partly explains this result on Si. On GaAs, non-ideal growth conditions are possibly responsible for these suboptimal performances

Rayleigh-B\'{e}nard convection in a homeotropically aligned nematic liquid crystal

We report experimental results for convection near onset in a thin layer of a homeotropically aligned nematic liquid crystal heated from below as a function of the temperature difference $\Delta T$ and the applied vertical magnetic field $H$ and compare them with theoretical calculations. The experiments cover the field range 8 \alt h \equiv H/ H_{F} \alt 80 ($H_F =$ is the Fr\'eedericksz field). For $h$ less than a codimension-two field $h_{ct} \simeq 46$ the bifurcation is subcritical and oscillatory, with travelling- and standing-wave transients. Beyond $h_{ct}$ the bifurcation is stationary and subcritical until a tricritical field $h_t= 57.2$ is reached, beyond which it is supercritical. The bifurcation sequence as a function of $h$ found in the experiment confirms the qualitative aspects of the theoretical predictions. However, the value of $h_{ct}$ is about 10% higher than the predicted value and the results for $k_c$ are systematically below the theory by about 2% at small $h$ and by as much as 7% near $h_{ct}$. At $h_{ct}$, $k_c$ is continuous within the experimental resolution whereas the theory indicates a 7% discontinuity. The theoretical tricritical field $h_t^{th} = 51$ is somewhat below the experimental one. The fully developed flow above $R_c$ for $h < h_{ct}$ is chaotic. For $h_{ct} < h < h_t$ the subcritical stationary bifurcation also leads to a chaotic state. The chaotic states persist upon reducing the Rayleigh number below $R_c$, i.e. the bifurcation is hysteretic. Above the tricritical field $h_t$, we find a bifurcation to a time independent pattern which within our resolution is non-hysteretic.Comment: 15 pages incl. 23 eps figure

Al0.2Ga0.8As solar cells monolithically grown on Si and GaAs by MBE for III-V/Si tandem dual-junction applications

Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on silicon substrates by Molecular Beam Epitaxy. Due to the 4% lattice mismatch between AlGaAs and Si, Threading Dislocations (TDs) nucleate at the III-V/Si interface and propagate to the active region of the cells where they act as recombination centers, reducing the performances of the devices. In order to reduce the Threading Dislocation Density (TDD) in the active layers of the cells, InAlAs Strained Layer Superlattice (SLS) Dislocation Filter Layers (DFLs) have been used. For one of the samples, in-situ Thermal Cycle Annealing (TCA) steps have additionally been performed during growth. For comparison purposes, reference Al0.2Ga0.8As solar cells have been grown lattice-matched on GaAs. For the sample grown on Si without TCA, the TDD has been reduced from over 7×109cm-2 at the III-V/Si interface to 3×107cm-2 in the base of the cells. With TCA, the TDD has been reduced throughout the sample from over 3×109cm-2 in the initial epilayers to 8(±2)×106cm-2 in the base of the cells. For the best devices, the Voc improves from 833mV on Si without TCA to 895mV using TCA, compared with 1070mV for the reference sample grown lattice-matched on GaAs. Similarly the fill factor improves from 73.7% on Si without TCA to 74.8% using TCA, compared with 78.4% on GaAs. The high bandgap-voltage offset obtained both on Si and GaAs indicates a non-optimal bulk AlGaAs material quality due to non-ideal growth conditions