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

Low temperature silicon epitaxy on hydrogen terminated Si(100) surfaces

Si deposition on H terminated Si(100)-2x1 and 3x1 surfaces at temperatures 300-530 K is studied by scanning tunneling microscopy. Hydrogen apparently hinders Si adatom diffusion and enhances surface roughening. The post-growth annealing effect is analyzed. Hydrogen is shown to remain on the growth front up to at least 10 ML. The dihydride units on the 3x1 surfaces further suppress the Si adatom diffusion and increase surface roughness.Comment: 26 pages, 9 figure

STM characterization of the Si-P heterodimer

We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to study the behavior of adsorbed phosphine (PH$_{3}$) on Si(001), as a function of annealing temperature, paying particular attention to the formation of the Si-P heterodimer. Dosing the Si(001) surface with ${\sim}$0.002 Langmuirs of PH$_{3}$ results in the adsorption of PH$_{x}$ (x=2,3) onto the surface and some etching of Si to form individual Si ad-dimers. Annealing to 350$^{\circ}$C results in the incorporation of P into the surface layer to form Si-P heterodimers and the formation of short 1-dimensional Si dimer chains and monohydrides. In filled state STM images, isolated Si-P heterodimers appear as zig-zag features on the surface due to the static dimer buckling induced by the heterodimer. In the presence of a moderate coverage of monohydrides this static buckling is lifted, rending the Si-P heterodimers invisible in filled state images. However, we find that we can image the heterodimer at all H coverages using empty state imaging. The ability to identify single P atoms incorporated into Si(001) will be invaluable in the development of nanoscale electronic devices based on controlled atomic-scale doping of Si.Comment: 6 pages, 4 figures (only 72dpi

Encapsulation of phosphorus dopants in silicon for the fabrication of a quantum computer

The incorporation of phosphorus in silicon is studied by analyzing phosphorus delta-doped layers using a combination of scanning tunneling microscopy, secondary ion mass spectrometry and Hall effect measurements. The samples are prepared by phosphine saturation dosing of a Si(100) surface at room temperature, a critical annealing step to incorporate phosphorus atoms, and subsequent epitaxial silicon overgrowth. We observe minimal dopant segregation (5 nm), complete electrical activation at a silicon growth temperature of 250 degrees C and a high two-dimensional electron mobility of 100 cm2/Vs at a temperature of 4.2 K. These results, along with preliminary studies aimed at further minimizing dopant diffusion, bode well for the fabrication of atomically precise dopant arrays in silicon such as those found in recent solid-state quantum computer architectures.Comment: 3 pages, 4 figure

The micrometeoroid complex and evolution of the lunar regolith

The interaction of the micrometeoroid complex with the lunar surface is evidenced by numerous glass-lined microcraters on virtually every lunar surface exposed to space. Such craters range in size from less than .1 micron to approximately 2 sq cm diameter. Using small scale laboratory cratering experiments for calibration, the observed crater-sized frequency distributions may be converted into micrometeoroid mass distributions. These lunar mass distributions are in essential agreement with satellite data. Some physical properties of micrometeoroids may be deduced by comparing lunar crater geometries with those obtained in laboratory experiments. The proponderance of circular outlines of lunar microcraters necessitates equidimensional, if not spherical, micrometeoroids

Atomically precise placement of single dopants in Si

We demonstrate the controlled incorporation of P dopant atoms in Si(001), presenting a new path toward the creation of atomic-scale electronic devices. We present a detailed study of the interaction of PH3 with Si(001) and show that it is possible to thermally incorporate P atoms into Si(001) below the H-desorption temperature. Control over the precise spatial location at which P atoms are incorporated was achieved using STM H lithography. We demonstrate the positioning of single P atoms in Si with similar to1 nm accuracy and the creation of nanometer wide lines of incorporated P atoms

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