Ultra‐thin,high performance tunnel junctions for III–V multijunction cells

Tunnel junctions (TJ) made of p‐Al_0.1 Ga_0.9As/n‐GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra‐thin TJs with excellent performance can be realized. Even for a 7.5/6‐nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600 A/cm². The series resistance of the TJs was found to be at a constant level of 0.6 ± 0.2 mΩ cm² for all total thicknesses of the TJ in the 13.5–40 nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6‐nm thick TJ, compared with a cell with a 20/20‐nm thick TJ, gained 0.53 ± 0.05 mA/cm² in short circuit current to a value of 14.8 mA/cm². Copyright © 2012 John Wiley & Sons, Ltd.     

Explanation for the dark I–V curve of III–V concentrator solar cells

The measurement of the dark I–V curve is one of the most straightforward methods for characterizing solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. In this paper, an explanation of the dark I–V curve for concentrator III–V solar cells is presented using a 3D (three‐dimensional) model in order to provide a proper data fit that provides meaningful physical parameters that are also compatible and coherent with a data fit from illumination curves. The influence on the dark I–V curve of the most significant series resistance components of concentrator solar cells is also analysed concluding that only the vertical component as well as the front contact‐specific resistance can be assessable by means of this characterization method while both emitter and metal sheet resistances cannot be detected. For comparison purposes, the same experimental data have been fitted by means of a traditional two‐diode model showing that, although an accurate dark I–V curve fitting can be achieved, the extracted parameters are unable to reproduce illumination data since lumped models assume the same ohmic losses distribution for both dark and illumination conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Orientation‐Controlled Selective‐Area Epitaxy of III–V Nanowires on (001) Silicon for Silicon Photonics

Monolithic integration of III–V nanowires on silicon platforms has been regarded as a promising building block for many on‐chip optoelectronic, nanophotonic, and electronic applications. Although great advances have been made from fundamental material engineering to realizing functional devices, one of the remaining challenges for on‐chip applications is that the growth direction of nanowires on Si(001) substrates is difficult to control. Here, catalyst‐free selective‐area epitaxy of nanowires on (001)‐oriented silicon‐on‐insulator (SOI) substrates with the nanowires aligned to desired directions is proposed and demonstrated. This is enabled by exposing {111} planes on (001) substrates using wet chemical etching, followed by growing nanowires on the exposed planes. The formation of nanowire array‐based bottom‐up photonic crystal cavities on SOI(001) and their coupling to silicon waveguides and grating couplers, which support the feasibility for on‐chip photonic applications are demonstrated. The proposed method of integrating position‐ and orientation‐controllable nanowires on Si(001) provides a new degree of freedom in combining functional and ultracompact III–V devices with mature silicon platforms.     

Long‐term energy production of III–V triple‐junction solar cells on the Martian surface

Based on theoretical considerations, optimum triple‐junction bandgap combinations are determined in order to achieve highest electrical energy production for a mission on the Martian surface. The solar cell structures analysed in this contribution are based on the Ga_yIn_1–yP, Ga_xIn_1–xAs and Ge material system. A comparison of theoretical and already realised triple‐junction solar cell structures is presented. For the evaluation of long‐term electrical energy production, different geographic and climatic Martian scenarios are considered. Copyright © 2010 John Wiley & Sons, Ltd.     

Wafer reuse for repeated growth of III–V solar cells

The epitaxial lift‐off (ELO) technique can be used to separate a III–V solar cell structure from its underlying GaAs or Ge substrate. ELO from 4‐inch Ge wafers is shown and 2‐inch GaAs wafer reuse after lift‐off is demonstrated without degradation in performance of the subsequent thin‐film GaAs solar cells that were retrieved from it. Since a basic wet chemical smoothing etch procedure appeared insufficient to remove all the surface contamination, wafer re‐preparation is done by a chemo‐mechanical polishing procedure. Copyright © 2010 John Wiley & Sons, Ltd.     

Designing III–V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells featuring a Si subcell also serving as substrate. Subject to this Si bandgap constraint, we design optimum cell structures that we show depart from the unconstrained ideal. In order to progress to manufacturable designs, the use of III–V materials is considered, using a novel growth method capable of yielding low defect density III–V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current–voltage characteristics using the same parameters. The method yields a single‐parameter fit in terms of the Shockley–Read–Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few per cent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III–V and Si materials. Copyright © 2014 John Wiley & Sons, Ltd.     

Two‐terminal metal‐inter‐connected multijunction III–V solar cells

A novel bonding approach with an interface consisting of a metal and dielectric is developed, and a “pillar‐array” metal topology is proposed for minimal optical and electrical loss at the interface. This enables a fully lattice‐matched two‐terminal, four‐junction device that consists of an inverted top two‐junction (2J) cell with 1.85 eV GaInP/1.42 eV GaAs, and an upright lower 2J cell with ~1 eV GaInAsP/0.74 eV GaInAs aimed for concentrator applications. The fabrication process and simulation of the metal topology are discussed along with the results of GaAs/GaInAs 2J and (GaInP + GaAs)/GaInAs three‐junction bonded cells. Bonding‐related issues are also addressed along with optical coupling across the bonding interface. Copyright © 2014 John Wiley & Sons, Ltd.     

Degradation study of III–V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (Al_xGa_1−x)_0.51In_0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Assessing material qualities and efficiency limits of III–V on silicon solar cells using external radiative efficiency

The paper presents a quantitative approach to the investigation and comparison of the material qualities of III–V on silicon (III–V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high‐efficiency III–V/Si solar cells. This result yields several implications for the design of high‐efficiency III–V/Si solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

We report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10⁻⁴ to mid 10⁻⁵ Ohm cm² range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.     

Photolithographic Route to the Fabrication of Micro/Nanowires of III–V Semiconductors

Nano/microwires of semiconducting materials (e.g., GaAs and InP) with triangular cross‐sections can be fabricated by “top–down” approaches that combine lithography of high‐quality bulk wafers (using either traditional photolithography or phase‐shift optical lithography) with anisotropic chemical etching. This method gives good control over the lateral dimensions, lengths, and morphologies of free‐standing wires. The behaviors of many different resist layers and etching chemistries are presented. It is shown how wire arrays with highly ordered alignments can be transfer printed onto plastic substrates. This “top–down” approach provides a simple, effective, and versatile way of generating high‐quality single‐crystalline wires of various compound semiconductors. The resultant wires and wire arrays have potential applications in electronics, optics, optoelectronics, and sensing.     

Dendrite‐Assisted Growth of Silicon Nanowires in Electroless Metal Deposition

This article concerns the detailed investigations on the silver dendrite‐assisted growth of single‐crystalline silicon nanowires, and their possible self‐assembling nanoelectrochemistry growth mechanism. The growth of silicon nanowires was carried out through an electroless metal deposition process in a conventional autoclave containing aqueous HF and AgNO₃ solution near room temperature. In order to explore the mechanism and prove the centrality of silver dendrites in the growth of silicon nanowires, other etching solution systems with different metal species were also investigated in this work. The morphology of etched silicon substrates strongly depends upon the composition of the etching solution, especially the metal species. Our experimental results prove that the simultaneous formation of silver dendrites is a guarantee of the preservation of free‐standing nanoscale electrolytic cells on the silicon substrate, and also assists in the final formation of silicon nanowire arrays on the substrate surface.     

One‐Step Preparation of Coaxial CdS–ZnS and Cd1–xZnxS–ZnS Nanowires

Preparation of coaxial (core–shell) CdS–ZnS and Cd_1–xZn_xS–ZnS nanowires has been achieved via a one‐step metal–organic chemical vapor deposition (MOCVD) process with co‐fed single‐source precursors of CdS and ZnS. Single‐source precursors of CdS and ZnS of sufficient reactivity difference were prepared and paired up to form coaxial nanostructures in a one‐step process. The sequential growth of ZnS on CdS nanowires was also conducted to demonstrate the necessity and advantages of the precursor co‐feeding practice for the formation of well‐defined coaxial nanostructures. The coaxial nanostructure was characterized and confirmed by high‐resolution transmission electron microscopy and corresponding energy dispersive X‐ray spectrometry analyses. The photoluminescence efficiencies of the resulting coaxial CdS–ZnS and Cd_1–xZn_xS–ZnS nanowires were significantly enhanced compared to those of the plain CdS and plain Cd_1–xZn_xS nanowires, respectively, owing to the effective passivation of the surface electronic states of the core materials by the ZnS shell.     

Evolution of silicon bulk lifetime during III–V‐on‐Si multijunction solar cell epitaxial growth

The evolution of Si bulk minority carrier lifetime during the heteroepitaxial growth of III–V on Si multijunction solar cell structures via metal‐organic chemical vapor deposition (MOCVD) has been analyzed. In particular, the impact on Si lifetime resulting from the four distinct phases within the overall MOCVD‐based III–V/Si growth process were studied: (1) the Si homoepitaxial emitter/cap layer; (2) GaP heteroepitaxial nucleation; (3) bulk GaP film growth; and (4) thick GaAs_yP_1‐y compositionally graded metamorphic buffer growth. During Phase 1 (Si homoepitaxy), an approximately two order of magnitude reduction in the Si minority carrier lifetime was observed, from about 450 to ≤1 µs. However, following the GaP nucleation (Phase 2) and thicker film (Phase 3) growths, the lifetime was found to increase by about an order of magnitude. The thick GaAs_yP_1‐y graded buffer was then found to provide further recovery back to around the initial starting value. The most likely general mechanism behind the observed lifetime evolution is as follows: lifetime degradation during Si homoepitaxy because of the formation of thermally induced defects within the Si bulk, with subsequent lifetime recovery due to passivation by fast‐diffusing atomic hydrogen coming from precursor pyrolysis, especially the group‐V hydrides (PH₃, AsH₃), during the III–V growth. These results indicate that the MOCVD growth methodology used to create these target III–V/Si solar cell structures has a substantial and dynamic impact on the minority carrier lifetime within the Si substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

Quantitative Analysis of Current–Voltage Characteristics of Semiconducting Nanowires: Decoupling of Contact Effects

A metal‐semiconductor‐metal (M‐S‐M) model for quantitative analysis of current–voltage (I–V) characteristics of semiconducting nanowires is described and applied to fit experimental I–V curves of Bi₂S₃ nanowire transistors. The I–V characteristics of semiconducting nanowires are found to depend sensitively on the contacts, in particular on the Schottky barrier height and contact area, and the M‐S‐M model is shown to be able to reproduce all experimentally observed I–V characteristics using only few fitting variables. A procedure for decoupling contact effects from that of the intrinsic parameters of the semiconducting nanowires, such as conductivity, carrier mobility and doping concentration is proposed, demonstrated using experimental I–V curves obtained from Bi₂S₃ nanowires and compared with the field‐effect based method.     

Deformation of Top‐Down and Bottom‐Up Silver Nanowires

Atomistic simulations are employed to probe the deformation behavior of experimentally observed top‐down and bottom‐up face‐centered cubic silver nanowires. Stable, <110> oriented nanowires with a rhombic and truncated‐rhombic cross section are considered, representative of top‐down geometries, as well as the multiply twinned pentagonal nanowire that is commonly fabricated in a bottom‐up approach. The tensile deformation of a stable, experimentally observed structure is simulated to failure for each nanowire structure. A detailed, mechanistic explanation of the initial defect nucleation is provided for each nanowire. The three geometries are shown to exhibit different levels of strength and to deform by a range of mechanisms depending on the nanowire structure. In particular, the deformation behavior of top‐down and bottom‐up nanowires is shown to be fundamentally different. The yield strength of nanowires ranging from 1 to 25 nm in diameter is provided and reveals that in addition to cross‐sectional diameter, the strength of the nanowires is strongly tied to the structure. This study demonstrates that nanowire structure and size may be tailored for specific mechanical requirements in nanometer‐scale devices.     

Systematic Study of the Growth of Aligned Arrays of α‐Fe2O3 and Fe3O4 Nanowires by a Vapor–Solid Process

Growth of aligned and uniform α‐Fe₂O₃ nanowire (NW) arrays has been achieved by a vapor–solid process. The experimental conditions, such as type of substrate, local growth and geometrical environment, gas‐flow rate, and growth temperature, under which the high density α‐Fe₂O₃ NW arrays can be grown by a vapor–solid route via the tip‐growth mechanism have been systematically investigated. The density of the α‐Fe₂O₃ NWs can be enhanced by increasing the concentration of Ni atoms inside the alloy substrate. The synthesized temperature can be as low as 400 °C. Fe₃O₄ NWs can be produced by converting α‐Fe₂O₃ NWs in a reducing atmosphere at 450 °C. The transformation of phase and structure have been observed by in situ transmission electron microscopy. The magnetic and field‐emission properties of the NWs indicate their potential applications in nanodevices.     

Cover Picture: Photolithographic Route to the Fabrication of Micro/Nanowires of III–V Semiconductors (Adv. Funct. Mater. 1/2005)

The cover shows a patterned assembly of GaAs nanowires with their ends tethered to a bulk single‐crystal wafer as described on p. 30 by Rogers and co‐workers. These wires, which have triangular cross‐sections, were fabricated via a top–down process that combines photolithography and anisotropic chemical etching. Nano/microwires of semiconducting materials (e.g., GaAs and InP) with triangular cross‐sections can be fabricated by “top–down” approaches that combine lithography of high‐quality bulk wafers (using either traditional photolithography or phase‐shift optical lithography) with anisotropic chemical etching. This method gives good control over the lateral dimensions, lengths, and morphologies of free‐standing wires. The behaviors of many different resist layers and etching chemistries are presented. It is shown how wire arrays with highly ordered alignments can be transfer printed onto plastic substrates. This “top–down” approach provides a simple, effective, and versatile way of generating high‐quality single‐crystalline wires of various compound semiconductors. The resultant wires and wire arrays have potential applications in electronics, optics, optoelectronics, and sensing.     

High‐Yield Fabrication and Electrochemical Characterization of Tetrapodal CdSe,CdTe, and CdSexTe1–x Nanocrystals

The high‐yield fabrication of tetrapodal CdSe, CdTe, and CdSe_xTe_1–x nanocrystals is systematically studied. CdSe nanocrystals are prepared by first controlling the synthesis of high‐quality wurtzite CdSe and zinc blende CdSe nanocrystals at a relatively high temperature (260 °C) by selecting different ligands. Then, based on the phase control of the CdSe nanocrystals, two nanoparticle‐tailoring routes (i.e., a seed‐epitaxial route and ligand‐dependent multi‐injecting route) are used, and a high yield of CdSe tetrapods is obtained. CdTe nanocrystals are prepared by adjusting the ligand composition and the ratio of Cd to Te; CdTe tetrapods are synthesized in high yield using a mixed ligand that does not contain alkylphosphonic acids. Moreover, the nanoscale Te powder (Te nanowires/nanorods), which is highly soluble in the ligand solvent, is first used as a Te source to synthesize CdTe nanocrystals, which remarkably enhanced the output of the CdTe nanocrystals in one reaction. Furthermore, composition‐tunable ternary CdSe_xTe_1–x alloyed tetrapods are synthesized on a large scale, for the first time, by thermolyzing the mixture of the organometallic Cd precursor and the mixed (Se + Te) source in a mixed‐ligand solution. The CdSe, CdTe, and CdSe_xTe_1–x nanocrystals are characterized by transmission electron microscopy (TEM), high‐resolution TEM, selected‐area electron diffraction, X‐ray diffraction, and UV‐vis and photoluminescence (PL) spectroscopy. Interesting nonlinear, composition‐dependent absorption and PL spectra are observed for the ternary CdSe_xTe_1–x alloyed nanocrystals. The band‐edge positions of the nanocrystals of CdSe, CdSe_xTe_1–x, and CdTe are systematically studied by cyclic voltammetry.