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Here we present results highlighting the differences that soiling and cleaning have on concentrated photovoltaic arrays (CPV) specifically focusing on ultra high concentration, >1000 Suns, photovoltaic (UHCPV) arrays in a desert environment. Soiling losses in UHCPV arrays differ from those of conventional photovoltaic (PV) arrays, to illustrate this difference we present a direct comparison between a UHCPV array and a conventional PV array showing that soiling has an approximately five times larger effect on the UHCPV array compared to a PV array. The soiling effect has been measured by taking current–voltage curves of the arrays and monitoring the power output of a system’s photovoltaic inverter before and after cleaning. The results indicate that in a desert environment there will be some extreme weather events that require cleaning of all types of PV modules. They also indicate that even if it is not economically feasible to clean a PV system, that a CPV system is different enough in its response to soiling and cleaning that further studies and modeling are warranted. We also discuss our current cleaning process, dust prevention methods, and cleaning schedules in an attempt to stimulate further study in these areas.

A method for growing heteroepitaxial Si films on sapphire was developed using a 6 nm thin Al layer at substrate temperature of 600 °C. Subsequently, the growth of Si nanowires was demonstrated on these films at 490 °C without breaking vacuum. We characterized the properties of the Si films by Raman scattering, X-ray diffraction and transmission electron microscopy and show that the crystal quality and dopant control are promising for photovoltaic applications.

We discuss the benefits of using metals other than Au to catalyze the growth of Si and Ge nanowires, emphasizing the opportunities that these non-conventional materials provide for tailoring electronic and structural nanowire properties. However, since these metals are more reactive than Au, their use creates constraints on wire growth conditions as well as difficulties in post-growth characterization. These issues are illustrated for Si and Si/Ge nanowires grown from Al, Cu and AuAl starting materials. The vacuum requirements for the deposition of the reactive metals are discussed as well as the effect of atmospheric exposure on the structure of wires observed post-growth with electron microscopy.

A review and expansion of the fundamental processes of the vapor-liquid-solid (VLS) growth mechanism for nanowires is presented. Although the focus is on nanowires, most of the concepts may be applicable to whiskers, nanotubes, and other unidirectional growth. Important concepts in the VLS mechanism such as preferred deposition, supersaturation, and nucleation are examined. Nanowire growth is feasible using a wide range of apparatuses, material systems, and growth conditions. For nanowire growth the unidirectional growth rate must be much higher than growth rates of other surfaces and interfaces. It is concluded that a general, system independent mechanism should describe why nanowires grow faster than the surrounding surfaces. This mechanism is based on preferential nucleation at the interface between a mediating material called the collector and a crystalline solid. The growth conditions used mean the probability of nucleation is low on most of the surfaces and interfaces. Nucleation at the collector-crystal interface is however different and of special significance is the edge of the collector-crystal interface where all three phases meet. Differences in nucleation due to different crystallographic interfaces can occur even in two phase systems. We briefly describe how these differences in nucleation may account for nanowire growth without a collector. Identifying the mechanism of nanowire growth by naming the three phases involved began with the naming of the VLS mechanism. Unfortunately this trend does not emphasize the important concepts of the mechanism and is only relevant to one three phase system. We therefore suggest the generally applicable term preferential interface nucleation as a replacement for these different names focusing on a unifying mechanism in nanowire growth. (Less)

we present results that provide fundamental insights on how to experimentally tailor the planar defect density and even the crystal structure in III-V metal-particle-seeded nanowires, where zinc blende is the stable bulk crystal structure. We have grown GaP nanowires with metal-organic vapor-phase epitaxy under different conditions: pulsing of the Ga source, and Continuous growth with and without In background. The dominant crystal structure of the nanowires is zinc blende, which when grown under continuous conditions has a high density of twin planes perpendicular to the growth direction. Using pulsed growth we observed that the twin plane separations were much longer than those observed for continuous growth with an In background. On the other hand, during continuous growth, under In-free conditions, a considerable amount of the wurtzite-phase forms. Our results suggest that it might be possible to predict the conditions necessary for the growth of wires with perfect crystal structure. We interpret our results in terms of the supersaturation during growth. (C) 2008 Elsevier B.V. All rights reserved. (Less)

We present results on the effect of seed particle reconfiguration on the growth of short InAs and InP nanowire segments. The reconfiguration originates in two different steady state alloy compositions of the Au/In seed particle during growth of InAs and InP. From compositional analysis of the seed particle, the In content in the seed particle is determined to be 34 and 44% during InAs and InP growth, respectively. When switching between growing InAs and InP, transient effects dominate during the time period of seed particle reconfiguration. We developed a model that quantitatively explains the effect and with the added understanding we are now able to grow short period (

Nanowires of zinc blende crystal structure, grown in the B direction usually have a large number of twin plane defects. In order to investigate this phenomenon, we grow GaP nanowires with metal-organic vapor phase epitaxy. By rotating the nanowires in a high resolution transmission electron microscope, we show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets. Due to the alternating orientations of these twin octahedra, the sidewalls of the nanowires can be described as microfaceted surfaces with an overall orientation of 11 1 2}, but composed of alternating 11 1 I}A and {1 I 1}B facets. Moreover, the segment thicknesses follow exponential distributions, which show that there is a certain probability of twin plane formation, which is independent of segment thickness. (c) 2006 Elsevier B.V. All rights reserved. (Less)

This review focuses on material challenges associated with III-V co-integration with Si for future CMOS. There is a huge volume of literature on this topic as implementation of III-V monolithic integration with Si has been the holy grail for last four decades; targeting a wide range of applications including RF devices, LEDs, lasers, photo-detectors and the like. The key drivers have been the cost reduction, scalability with Si wafer diameter, and accessibility to highly scaled integrated circuits next to III-V devices. With the current focus on CMOS the pace of progress on monolithic integration has accelerated by leaps and bounds partly because of its vast impact on CMOS scaling, and partly due to the aggressive CMOS roadmap requirements. The discussion below concentrates on In 0.53 Ga 0.47 As channel which is the dominant III-V material being pursued for future technology. Despite the narrow focus, fundamental and engineering challenges posed by this material encompass a broad range of material topics including epitaxial growth, crystallographic defects and their dynamics during growth and subsequent processing, clever device architecture to alleviate adverse impact of defects on device leakage, and innovative engineering for material improvement.

We describe a process for increasing power efficiency of concentrator photovoltaic systems by optimizing the lens-to-cell spacing. We find that there is an optimum defocus position with improved power output and reduced sensitivity to pointing errors, which in combination can result in a more than 10% enhancement. The improvement can be realized by minor changes to module cases which should not require changes to other manufacturing, installation, or component costs. In fact optimizing the defocus position allows for lower costs per unit power due to increased power and relaxed system tolerances. The paper focuses on detailed data illuminating the behavior of ultra high concentration photovoltaic modules. While one can look forward to optimizing defocus through sufficiently detailed simulation, at present, we find that an empirical determination of optimum defocus is necessary. The data reveals that even without design parameters changing, supply chain changes can have a significant impact on the optimum defocus - data from five different module configurations with components from different manufacturing lots are presented. These different configurations serve to illustrate the consequences of component changes and the importance of verifying the optimum defocus. A discussion of the effects that are important to determining the optimum defocus and which underlie these differences is included.

We use metal–organic vapour phase epitaxy for growth investigations of epitaxial nanowires in III–V materials, such as GaAs, GaP, InAs, and InP. In this paper we focus on gold assisted growth of nanowires. The nature of the metal particle—whether it is in the solid or liquid state—is discussed. For InAs and InP we have demonstrated that gold assisted wires can only grow at temperatures where the particle is solid. We continue with a discussion concerning the kinetic aspects of nanowire growth. Under common growth conditions one observes that thinner wires grow faster than thicker wires, contrary to what was described in the early days of whisker growth. We address and resolve this discrepancy by discussing a simple transport model and comparing the supersaturations of different systems. Finally, we describe the morphology of epitaxial III–V nanowires with emphasis on the crystal structure.

Au nanoparticles are commonly used as seeds for epitaxial growth of III-V semiconductor nanowires. However, the interaction between Au and In-containing III-V materials makes it difficult to control the growth of more complex nanowire structures in materials such as InAs. Here we report the growth of InAs nanowires and branched nanotrees using Au and Au-In nanoparticles. We show that the initial composition of the particle does not affect the morphology of the first-generation nanowires, nor does it affect the final composition of the particle after growth. However, when the Au-In particles were used to seed a second generation of nanowires, producing nanotrees, the branches exhibited a 2-3 times higher growth rate and more regular shape than those seeded by pure Au particles. This result is attributed to the decreased interaction between the seed particle and the trunk nanowires when Au-In particles are used. Thus the incorporation of In into the seed particle during particle production allows for modification of the particle-wire interaction.

Most III-V compound semiconductor nanowires seeded by metal particles grow preferentially in a (over bar 1 over bar 1 over bar 1)B direction (B wires) and most commonly with many stacking faults perpendicular to the growth direction. If growth proceeds in an alternate direction, defect-free growth has been observed. We present experimental results for the growth of GaAs nanowires in a previously uninvestigated growth direction, a A direction (A wires). One novelty is that a {111} A growth plane, like a {over bar 1 over bar 1 over bar 1} B, is a close packed plane where the stacking sequence can be interrupted forming stacking faults, but unlike the B wires the A wires lack stacking faults. It is also observed that, when grown under equivalent conditions, the growth rate of the A wires is approximately twice that of the B wires. Additionally, B wires have a hexagonal cross section with three {11 over bar 2} and three {11 over bar 2} side facets. A wires, on the other hand, have only three major side facets which are of the {11 over bar 2} type, giving them a triangular cross section. (c) 2005 Elsevier B.V. All rights reserved. (Less)

The use of metal organic vapor-phase epitaxy (MOVPE) for growth of one-dimensional nanostructures in the material systems GaAs, GaP, InAs and InP is investigated. Some kinetic effects are discussed, especially the general finding that in MOVPE thinner whiskers grow faster than thicker whiskers. Effects of growth temperature on growth rate and shape of the whiskers, the effects of different growth directions on the perfection of the materials and the possibilities to grow heterostructures in axial and lateral directions are reported. Ways to overcome the randomness in whisker growth by controlled seeding of the Au particles and by using lithography for site control are demonstrated.

A gas-phase method has been developed for producing size-controlled nanoparticles by thermal cracking of iron pentacarbonyl. The method includes the formation of iron particles from vapor emanating from the cracking of the volatile compound and subsequent compacting of the selected particles. Different temperature steps were involved in the process, and their effects on the particle production were investigated. On-line differential mobility measurements and off-line transmission electron microscopy (TEM) were used to study the morphology, composition and structure of the generated particles. The aim of the study was to produce nanoparticles of pure iron. After transportation in air, the composition of the particles was analyzed by TEM and found to be magnetite, Fe3O4. The oxidation may be attributed to the exposure to air and humidity during the transport. The phase of the uncontaminated particles could not be determined. Tests for growth of silicon nanowhiskers using nanoparticles produced with this method were successful.

We present results of ideal epitaxial nucleation and growth of III−V semiconductor nanowires on silicon substrates. This addresses the long-time challenge of integrating high performance III−V semiconductors with mainstream Si technology. Efficient room-temperature generation of light on silicon is demonstrated by the incorporation of double heterostructure segments in such nanowires. We expect that advanced heterostructure devices, such as resonant tunneling diodes, superlattice device structures, and heterostructure photonic devices for on-chip communication, could now become available as complementary device technologies for integration with silicon.

We recently reported that monolayers on silicon are formed, and silicon surfaces concomitantly patterned, when native oxide-coated silicon is scribed with a diamond-tipped instrument in the presence of reactive liquids. Notably, monolayers were prepared (and are prepared in this work) in an open laboratory with reagents that are not degassed. However, while this method is facile, the features originally produced using 2-3 N of force on a diamond tip are irregular, broad (∼100 μm), and deep (∼5 μm). Reducing the force to 0.08 N using an improved tip holder yields narrower features (∼10 μm), but the best features made with a diamond tip using the lighter force still remain quite deep (∼0.1 μm) and rough. Here we show that substantially sharper and shallower features are produced by (a) wetting hydrogen-terminated silicon with a reactive compound and (b) scribing it with a 1 / 3 2 in. tungsten carbide ball with a low force (∼0.08 N). It is remarkable that (i) the depth of these features is only 10-20 A and (ii) their edge widths are sharp (submicron resolution). The resulting features are invisible to the naked eye but are observable by atomic force microscopy, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. Both Si(100) and Si(111) were successfully modified. Miniature hydrophobic corrals made with this technique were loaded with solutes, for example, colloidal carbon, semiconductor nanocrystals, and DNA, from aqueous solutions with a simple dip. Under appropriate conditions colloidal carbon selectively deposits onto functionalized lines but not in between them.

We describe a new approach for achieving controlled spatial placement of VLS-grown nanowires that uses an oxygen-reactive seed material and an oxygen-containing mask. Oxygen-reactive seed materials are of great interest for electronic applications, yet they cannot be patterned using the approaches developed for noble metal seed materials such as Au. This new process, nanoscale chemical templating, takes advantage of the reactivity of the blanket seed layer by depositing it over a patterned oxide that reacts with the seed material to prevent nanowire growth in undesired locations. Here we demonstrate this technique using Al as the seed material and SiO2 as the mask, and we propose that this methodology will be applicable to other reactive metals that are of interest for nanowire growth. The method has other advantages over conventional patterning approaches for certain applications including reducing patterning steps, flexibility in lithographic techniques, and high growth yields. We demonstrate its application with standard and microsphere lithography. We show a high growth yield and fidelity, with no NWs between openings and a majority of openings occupied by a single vertical nanowire, and discuss the dependence of yield on parameters.

This paper discusses the development and deployment of an ultra-high concentrating PV module that utilizes concentration above 1400X on multijunction solar cells. The development process included the selection of cell assemblies, primary and secondary optics, and focal distance. The systems were deployed in Saudi Arabia inside the Solar Village near Riyadh and in Khafji near the border of Saudi and Kuwait, following the deployment of first prototype in Yorktown, NY. Data from operation in those areas are shown here, and next steps of optimizing the module performance are discussed.

Surface modification and patterning at the nanoscale is a frontier in science with significant possible applications in biomedical technology and nanoelectronics. Here we show that an atomic force microscope (AFM) can be employed to simultaneously pattern and functionalize hydrogen-terminated silicon (111) surfaces. The AFM probe was used to break Si–H and Si–Si bonds in the presence of reactive molecules, which covalently bonded to the scribed Si surface. Functionalized patches and patterned lines of molecules were produced. Linewidths down to 30 nm were made by varying the force at the tip.

Carrier lifetimes of Si micro/nanowires grown by the vapor-liquid-solid method are measured using an extension of the classic contactless photoconductivity decay method. The samples measured consist of a thin aggregated film of oxide passivated wires on a fused silica carrier. Au catalyzed wires in the 392-730 nm diameter range are studied. Recombination in these wires is controlled by the surface or near surface effects, not bulk Au impurities. The lifetimes of Au- and Al-catalyzed wires of comparable diameter are measured. The Al wires are found to have slightly longer lifetimes than those grown with Au at a comparable diameter. Across all samples, the lifetimes measured range was from 0.2 to 1.0 ns. The surface controlled nature of the recombination measured implies larger diameter wires will offer better performance in devices that rely on minority carrier transport.

Practical multi receiver ultra high (1000+ Suns) concentration photovoltaic (UHCPV) systems experience large radiation, thermal and electrical loads in addition to large power density transients under routine operation. This report is a summary of the issues involved in determining the practical limits to concentration. How high is too high? Explorations into UHCPV have both theoretical and experimental aspects. Understanding the theoretical device physics and circuit limitations is often essential to determining which experiments to do and in interpreting results. On the experimental side the work can be divided into two fields depending on the type of light source. The first is artificial or simulated sources and the second is working in the field with direct solar irradiation. Both fields have advantages and disadvantages. Direct solar radiation was selected for the current experiments due to the low cost and ability to produce ultra high concentrations (4000+) over relatively large areas (25+ mm2). Several experimental examples from these direct solar measurements shed light on some of the basic theories of how concentrated light affects the performance of multi junction photovoltaic cells. Out of these examples and theoretical foundations we conclude that for practical devices the first order constraint to optimum efficiency at ultra high concentrations is the series resistance. We also present a simple model based on published data and our results that can be used to predict the total system series resistance needed to optimize a system for a particular concentration.

The nanopatterning of semiconductors and other surfaces in a controlled manor is of a great interest for industrial application. The current technique is a new method of controlling the spatial placement of the growth of nanowires (NWs) seeded with oxygen reactive materials such as aluminum, which is a standard metal in silicon process line. The technique is based about patterning a semiconductor substrate or other like substrate which is capable of forming a semiconductor alloy with an oxygen reactive element during a subsequent annealing step. Moreover, it does not require removal of the patterned compound oxide layer.

A promising field for future low cost, medium efficiency solar cell devices is the use of vapor-liquid-solid (VLS) grown nanowires or micropillars (NWs referring to both) as the active region of large scale (greater than 1 mm2 area) photovoltaic devices. There are several advantages of using NWs. The NWs can be doped as grown, helping with formation of a PV structure. NW-based PV structures require shorter carrier diffusion distances than are needed for a similarly thick planar absorber layer. At the same time, due to scattering and other optical phenomena the NW structure is able to trap more light and improve the overall light absorption. This, combined with the ability to grow nanowires on cheap substrates or reuse the growth substrate multiple times, makes NWs promising for future generation PV devices. In order for NWs to perform to their full potential several technical challenges need to be overcome. In this paper we will discuss these technical challenges in conjunction with the advantages of using NWs in large scale PV devices. We will also outline the progress that we and others have made in overcoming these challenges on the way to making nanowires a viable PV technology.

We have examined the formation of silicon nanowires grown by self-assembly from Si substrates with thin aluminum films. Postgrowth and in situ investigations using various Al deposition and annealing conditions suggest that nanowire growth takes place with a supercooled liquid droplet (i.e., the vapor-liquid-solid system), even though the growth temperatures are below the bulk Al/Si eutectic temperature. Wire morphology as a function of processing conditions is also described. It is shown that when Al environmental exposure is prevented before wire growth a wide process window for wire formation can be achieved. Under optimum growth conditions, it is possible to produce excellent crystal quality nanowires with rapid growth rates, high surface densities, low diameter dispersion, and controlled tapering. Photoelectron spectroscopy measurements indicate that the use of Al leads to active doping levels that depend on the growth temperature in as-grown nanowires and increase when annealed. We suggest that these structural and electronic properties will be relevant to photovoltaic and other applications, where the more common use of Au is believed to be detrimental to performance.

GaP B nanowires are dominated by (111) twins orthogonal to the growth direction and show well-developed {111} side-facets. Based on this, a 3D-model has been constructed with a cross-section of an octahedron used as a building block. The twins can be of ortho- or para type i.e. by 60° about the growth axis or 180° in the twin plane. The segment thickness variation follows an exponential distribution with a clear dependence on growth temperature. Multislice simulations show different features of the twin types that are useful for further characterisation.

Free-standing epitaxially grown nanowires provide a controlled growth system and an optimal interface to the underlying substrate for advanced optical, electrical, and mechanical nanowire device connections. Nanowires can be grown by vapor-phase epitaxy (VPE) methods such as chemical vapor deposition (CVD) or metal organic VPE (MOVPE). However, VPE of semiconducting nanowires is not compatible with several microfabrication processes due to the high synthesis temperatures and issues such as cross-contamination interfering with the intended microsystem or the VPE process. By selectively heating a small microfabricated heater, growth of nanowires can be achieved locally without heating the entire microsystem, thereby reducing the compatibility problems. The first demonstration of epitaxial growth of silicon nanowires by this method is presented and shows that the microsystem can be used for rapid optimization of VPE conditions. The important issue of the cross-contamination of other parts of the microsystem caused by the local growth of nanowires is also investigated by growth of GaN near previously grown silicon nanowires. The design of the cantilever heaters makes it possible to study the grown nanowires with a transmission electron microscope without sample preparation.

Controlled growth of nanowires is an important, emerging research field with many applications in, for example, electronics, photonics, and life sciences. Nanowires of zinc blende crystal structure, grown in the 〈111〉B direction, which is the favoured direction of growth, usually have a large number of twin-plane defects. Such defects limit the performance of optoelectronic nanowire-based devices. To investigate this defect formation, we examine GaP nanowires grown by metal-organic vapour-phase epitaxy. We show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets, resulting in a microfaceting of the nanowires. We discuss these findings in a nucleation context, where we present an idea on how the twin planes form. This investigation contributes to the understanding of defect formation in nanowires. One future prospect of such knowledge is to determine strategies on how to control the crystallinity of nanowires.

This paper describes three advances in lab on a chip technology. First, it is shown that chemomechanical surface patterning can be performed using a commercially available liquid handler that has undergone only minor modifications. These capabilities are demonstrated by making and then characterizing smaller hydrophobic corrals, made with a diamond tip, than have previously been reported. Hydrophobic corrals are small enclosures on a surface that are ringed by hydrophobic lines. They hold droplets of high surface tension solutions. They allow a surface to be subdivided into individually addressable elements, thus providing a platform for conducting many simultaneous surface experiments with small (down to ca. 1 microL) liquid volumes. An important consequence of this work is that it makes chemomechanical surface patterning, which is a valuable and straightforward method for surface modification, much more accessible to the technical community. Second, it is shown that an entire array of hydrophobic corrals can be simultaneously coated with polyelectrolyte multilayers, but that the hydrophobic corrals still retain the ability to hold liquids after this deposition. The robotic arm of the liquid handler is again employed to manufacture this ultrathin film. Finally, as a demonstration of the capability of this technology to create complex patterned arrays on surfaces from solution for biological or nanostructured materials applications, and again employing the liquid handler, polyelectrolyte-coated hydrophobic corrals are individually addressed and loaded with a solution containing gold nanoparticles for independently specified times. The density and morphology of deposited nanoparticle monolayers were studied by scanning electron microscopy. The deposition of gold nanoparticles onto a chip occurred at a constant rate (0.5% min(-1)) over the range of times studied.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

InP nanowires are fabricated by organo-metallic vapour phase epitaxy and studied via photoluminescence measurements performed on single nanowires, finding evidence of state filling with increasing excitation power density. To increase flexibility in fabrication technology we developed a wet chemical procedure to grow a CdS shell on these wires. In these InP-CdS wires the luminescence efficiency was decreased with respect to the bare wires. The CdS capping procedure needs further investigations to improve the emission properties of nanowires, in order to become technically useful. We suggest as possible improvements of this technique to increase the bath temperature and/or illuminate the sample with UV radiation during the capping procedure.

We have developed a novel technology, called nanoparticle conversion, for producing compound semiconductor quantum dots (QDs) in which the dot size, surface density, position, and the materials system are all independently controlled. Nanoparticle conversion also lends itself to spatially controlled positioning of QDs. To demonstrate this technology we report the formation of InP QDs using nanoparticle conversion. We have produced QDs on substrates of different types by converting randomly and lithographically positioned nanoparticles into compound semiconductors in a chemical vapour deposition system. Electron microscopy and atomic force microscopy measurements reveal that the morphology of these QDs is similar to that of QDs produced by other techniques. Photo- and cathodoluminescence measurements show that the converted nanoparticles exhibit properties and behaviours typical of semiconductor QDs. These include quantum confinement, free-to-bound recombination and blinking. Production of multi-component QDs like InP, GaN, and InAsP on various substrates like Si, SiO2, and sapphire show that this technology can produce a wide variety of different types of QD on different substrates with minimal need for process optimization.

Nanometre-sized particles (1-100 nm) have unique properties receiving growing attention in wide areas of research. Here, a convenient method to deposit size-selected nanoparticles on surfaces by means of electrospraying colloidal suspensions in the aerosol phase is presented. We demonstrate the deposition of individual nanoparticles and the feasibility of this method in seeding gold particles for nanostructure growth. An advantage of the present method is the easy set-up and operation, using only commercially available machinery and substances. Problems regarding low deposition rates and colloidal remnants are approached, e. g. the aerosol flow is examined in a differential mobility analyser. This method is not material dependent and could be extended to deposit any colloidal particle.