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Ridge microstructures were prepared by etching through samples consisting of a series of stacked InAs x P 1 − x quantum wells (QWs) with step graded composition grown on InP by molecular beam epitaxy. Different etching techniques were used: wet etching with HCl/H 2 O and reactive ion etching with CH 4 /H 2. These microstructures were characterized by low-temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each QW was clearly identified. The PL was measured in detail across etched ridge stripes of various widths. Variations of the integrated PL intensities across the etched stripes were observed. The PL intensities for all QWs increase gradually from the edge to the center of the ridge microstructures. The PL intensity measured at the ridge center is systematically reduced for ridges which are 10 or 20 μ m wide as compared to ridges which are 30 μ m wide or larger. On the other hand, the spectral peak position of the PL lines remained constant with high accuracy (0.2–0.4 meV) across the microstructures. These observations are discussed in terms of the different possible mechanisms which determine the PL intensity variations, namely, non-radiative recombination at the etched walls and effects of stray electric fields which result from the etching process. Based on this discussion, we compare quantitatively the different etching processes which we have used. Altogether, this study illustrates the contribution that specially designed test structures, coupled with advanced spectroscopic characterization, can provide to the development of semiconductor photonic devices (e.g., lasers or waveguides) involving etching processes. [ABSTRACT FROM AUTHOR]

A model used to simulate and optimize the performance of nanowire-based betavoltaic generators is developed. The optimum nanowire array geometry is established for devices made of silicon, gallium arsenide, and gallium phosphide for both nickel-63 and tritium sources by computing the energy capture efficiency for each case. The captured power for nanowire devices is shown to be drastically greater compared to planar betavoltaic generators with maximum improvement factors of approximately 7, 3, 5, and 9 for devices utilizing radioisotope sources of nickel-63, nickel citrate, tritium, and tritiated butyl, respectively. In addition, nanowire devices do not suffer from self-shielding effects, a large limitation in conventional, planar betavoltaics. By coupling the spatial distribution of electron–hole pair generation rate calculated from Monte Carlo simulations and a semiconductor charge-transport model, the diode design is optimized for the maximum power output. The top performing devices, utilizing a tritium source, exhibited an output power of approximately 4, 6, and 2 μW cm−2 for silicon, gallium arsenide, and gallium phosphide, respectively. Overall device efficiencies were found to range from 4% to 10%, surpassing several betavoltaic devices reported in the literature. It was also found that, contrary to the traditional betavoltaic design, semiconductors with higher bandgaps do not necessarily result in the best device performance due to additional material parameters, such as surface recombination velocity. Potential improvements for nanowire-based betavoltaic generators are suggested for additional investigation. [ABSTRACT FROM AUTHOR]

The geometry of a gallium phosphide nanowire (NW) array has been optimized for maximum current generation in a betavoltaic (BV) device. The energy capture efficiency for various device geometries with different radioisotope source compounds was calculated in GEANT4. A validation of GEANT4 for BV device simulation was performed by comparing a model output with the available bulk semiconductor BV performance data, followed by predictions of the performance of NW-based devices. The pitch and the diameter of the NWs were found to have the most significant impact on the β-generated current density, with the optimum diameter-to-pitch ratios ranging from 0.55 to 0.8, depending on the source. The energy capture efficiency improved when low energy beta (β) emitters were used. For devices utilizing 63Ni source compounds, the β-generated current densities approached 0.95 μA cm−2, representing an improvement by a factor as high as 5.8 compared to planar devices. In the case of 3H source compounds, the generated current density was 3.05 μA cm−2, a factor of 15.5 larger than comparable planar devices. However, NW devices utilizing sources with a higher decay energy, such as 147Pm, did not demonstrate any improvements over planar geometries. Using the results for optimum NW geometries, NW-based or other nanostructured devices could be made to surpass the present commercial BV batteries. [ABSTRACT FROM AUTHOR]

Nonradiative processes are studied in InGaAs/InP quantum wells (QWs) as a function of the pump power and the temperature, using time-resolved photoluminescence. Intravalley disorder induced direct and intervalley phonon-assisted indirect Auger processes are found to be responsible for nonradiative recombination in low mobility and high mobility quantum wells, respectively. Both Auger processes are spatially separated: the phonon-assisted and direct Auger recombinations take place in the central part of the QW and near the interfaces, respectively. The recombination rate corresponding to the phonon-assisted Auger process is shown to increase with the temperature, while no influence of the temperature was detected for the rate of the disorder induced direct Auger process. The presented data point to the X and/or L valleys as final states for the Auger electron in the intervalley Auger process. Moreover, Auger recombination associated with different confined levels is studied. We show that the conditions for the intervalley phonon-assisted Auger processes are fulfilled for the ground states of the confined electrons and holes, while the nonradiative recombination due to the excited states is dominated by the direct Auger process. [ABSTRACT FROM AUTHOR]

The electron transport and recombination processes of photoexcited electron-hole pairs were studied in InGaAs/InP single quantum wells. Comprehensive transport data analysis reveals a asymmetric shape of the quantum well potential where the electron mobility was found to be dominated by interface-roughness scattering. The low-temperature time-resolved photoluminescence was employed to investigate recombination kinetics of photogenerated electrons. Remarkable modification of Auger recombination was observed with variation of the electron mobility. In high mobility quantum wells, the increasing pump power resulted in a new and unexpected phenomenon: a considerably enhanced Auger non-radiative recombination time. We propose that the distribution of the photoexcited electrons over different conduction band valleys might account for this effect. In low mobility quantum wells, disorder-induced relaxation of the momentum conservation rule causes inter-valley transitions to be insignificant; as a consequence, the non-radiative recombination time is reduced with the increase in pump power. Thus, interface-roughness scattering was found responsible for both transport properties and dynamic optical response in InGaAs/InP quantum wells. [ABSTRACT FROM AUTHOR]

A method is presented to improve the quantitative determination of dopant concentration in semiconductor nanowire (NW) arrays using secondary ion mass spectrometry (SIMS). SIMS measurements were used to determine Be dopant concentrations in a Be-doped GaAs thin film and NW arrays of various pitches that were dry-etched from the same film. A comparison of these measurements revealed a factor of 3 to 12 difference, depending on the NW array pitch, between the secondary Be ion yields of the film and the NW arrays, despite being identically doped. This was due to matrix effects and ion beam mixing of Be from the NWs into the surrounding benzocyclobutene that was used to fill the space between the NWs. This indicates the need for etched NWs to be used as doping standards instead of 2D films when evaluating NWs of unknown doping by SIMS. Using the etched NWs as doping standards, NW arrays of various pitches grown by the vapour-liquid-solid mechanism were characterized by SIMS to yield valuable insights into doping mechanisms. [ABSTRACT FROM AUTHOR]

GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor−liquid−solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume. [ABSTRACT FROM AUTHOR]

We report on the surface passivation of Au-assisted Te-doped GaAs nanowires (NWs) grown by metalorganic vapor phase epitaxy. The electrical properties of individual free standing NWs were assessed using a tungsten nano-probe inside a scanning electron microscope. The diameter independent apparent resistivity of both strained and relaxed passivated NWs suggests the unpinning of the Fermi level and reduction of sidewalls surface states density. Similar currentvoltage properties were observed for partially axially relaxed GaAs/GaP NWs. This indicates a negligible contribution of misfit dislocations in the charge transport properties of the NWs. Low temperature micro-photoluminescence (μ-PL) measurements were also carried out for both uncapped and passivated GaAs NWs. The improvement of the integrated (μ-PL) intensity for GaAs/GaP NWs further confirms the effect of passivation. [ABSTRACT FROM AUTHOR]

The control of the doping in nanowires (NWs) is of fundamental importance for the implementation of NW-based devices. A method is presented to obtain local information about doping by monitoring the Fermi-energy position within the bandgap at the surface along single NWs through spatially resolved x-ray photoemission spectroscopy. The experimental results are complemented by theoretical simulations of the carrier profile, taking into account the presence of electronic surface states and quantifying the impact of carrier depletion at the NW surface. This combined approach allows to determine the effect of the incorporation of Si dopants in GaAs NWs following different growth protocols, such as vapor-liquid-solid axial growth or vapor-solid radial growth, and in the resulting core-shell structures and axial junctions. The method also revelaed the strong dependence of the resulting doping on the morphology of the single NW (orientation, shell thickness). This approach can be easily applied to other nanoscale objects, allowing the direct observation of how doping (or junctions, or adsorbates,...) may locally affect the position of the Fermi level at the surface, which is a crucial factor in several application fields, such as photovoltaic and photocatalysis. [ABSTRACT FROM AUTHOR]

The droplet contact angle and morphology of the growth interface (vertical, tapered or truncated facets) are known to affect the zincblende (ZB) or wurtzite (WZ) crystal phase of III–V nanowires (NWs) grown by the vapor-liquid-solid method. Here, we present a model which describes the dynamics of the morphological evolution in self-catalyzed III–V NWs in terms of the time-dependent (or length-dependent) contact angle or top nanowire radius under varying material fluxes. The model fits quite well the contact angle dynamics obtained by in situ growth monitoring of self-catalyzed GaAs NWs in a transmission electron microscope. These results can be used for modeling the interface dynamics and the related crystal phase switching and for obtaining ZB-WZ heterostructures in III–V. [ABSTRACT FROM AUTHOR]

Numerical optimization has been used to determine the optimum junction design for core–shell nanowires used in betavoltaic generators. A genetic algorithm has been used to calculate the relative thickness, height, and doping of each segment within silicon, gallium arsenide, and gallium phosphide nanowires. Using the simulated spectra and energy deposition of nickel-63, nickel citrate, tritium, and tritiated butyl, devices with power output and overall efficiency up to 8 µW.cm−2 and 12%, respectively, have been predicted. Compared to previously investigated axial nanowires, the core–shell structures simulated here have realized drastic improvements by reducing surface recombination for longer nanowires. In addition, core–shell nanowires are shown to be capable of nearly matching the ideal performance predicted for this device structure. A new approach for calculating the practical upper limit of betavoltaic performance is presented and additional methods for improvement are discussed. [ABSTRACT FROM AUTHOR]

InAs nanowires with AlxIn1-xP or AlxIn1-xAs shells were grown on GaAs substrates by the Au-assisted vapour-liquid-solid method in a gas source molecular beam epitaxy system. Core diameters and shell thicknesses were measured by transmission electron microscopy (TEM). These measurements were then related to selected area diffraction patterns to verify either interface coherency or relaxation through misfit dislocations. A theoretical strain model is presented to determine the critical shell thickness for given core diameters. Zincblende stiffness parameters are transformed to their wurtzite counterparts via a well known tensor transformation. An energy criterion is then given to determine the shell thickness, at which coherency is lost and dislocations become favourable. Our model only considers axial strain relieved by edge dislocations since they were the only type of dislocation observed directly by TEM. [ABSTRACT FROM AUTHOR]

The reflectance, transmittance, and absorptance of GaAs nanowire (NW) arrays are calculated by solving Maxwell's equations using the finite element method. The model is compared with measurement results from well-ordered periodic GaAs NW arrays fabricated by dry etching. The model results are also compared with the reflectance measured from NWs grown by the Au-assisted vapor-liquid-solid method. The optimum NW diameter, periodicity (spacing between NWs), and length are determined to maximize absorptance of the AM1.5G solar spectrum and short circuit current density in a NW array solar cell. The optimum NW diameter, period, and length were 180 nm, 350 nm, and 5 μm, respectively, giving a photocurrent density from the NW of 27.3 mA/cm2 and corresponding to 91.3% absorption of the AM1.5 G solar spectrum. The photocurrent density saturated for NW lengths greater than 5 μm. A gold nanoparticle at the top of the NWs (used in the vapor-liquid-solid NW growth process) substantially reduced the optimum photocurrent density, while a polymer filling the space between NWs and a planar indium tin oxide contact had a relatively minor influence. [ABSTRACT FROM AUTHOR]

The combination of photoluminescence and magneto-transport measurements is used to study the single-particle relaxation time and the transport scattering time in short-period InGaAs/InP superlattices. Both the single-particle relaxation times of the electrons and of the holes were obtained in the same samples and were shown to be determined by the remote-impurity scattering. The transport scattering time for electrons was found to be dominated by the interface-roughness scattering with lateral length Λ=10 nm and height Δ = 0.13 nm. We also discuss the importance of multiple-scattering effects for small well widths and of alloy scattering for large well widths. [ABSTRACT FROM AUTHOR]

The continuity and Poisson equations are solved numerically to obtain J-V characteristics and photoconversion efficiency of a two-junction solar cell. The cell consists of a top junction comprised of nanowires with bandgap of 1.7 eV grown on a bottom junction comprised of a Si substrate. The lattice relaxation possible in nanowires permits lattice-mismatched III-V material growth on Si, thereby achieving the optimum bandgaps in a two-junction cell. The model indicates a limiting efficiency of 42.3% under a concentration of 500 Suns (AM1.5 D spectrum). This limiting efficiency is similar to that calculated for the planar lattice-matched triple-junction Ge/InGaAs/InGaP cell. Methods of fabricating the nanowire/Si cell are discussed including requirements for nanowire sidewall surface passivation. The model indicated that passivation of the nanowire sidewall surfaces that produces a surface recombination velocity of 3000 cm·s-1 and surface trap density of 1012 cm-2 should be sufficient to yield high efficiency solar cells. [ABSTRACT FROM AUTHOR]

Numerical simulation of current-voltage (J-V) characteristics of III-V nanowire core-shell p-n junction diodes under illuminated conditions is presented with an emphasis on optimizing the nanowire design for photoconversion efficiency. Surface recombination and depletion effects are found to play a dominant role in the J-V characteristics. The impact of surface charge density, surface recombination velocity, doping concentration, and nanowire geometry are investigated. Investigation of contacting methodology indicated that solar cell efficiency is degraded with electrical contacts on the sidewalls of the nanowire due to Fermi level pinning at the metal/semiconductor interface. On the other hand, contacts on the top of nanowires with sidewall passivation provide solar cell performance close to the detailed balance efficiency limit of ∼30%. Elimination of the thin film between nanowires produces a smaller dark current and improved cell performance. [ABSTRACT FROM AUTHOR]

The present work describes the development of a hybrid GaAs-aptamers biosensor for the label-free detection of adenosine 5′-triphosphate (ATP). The implemented sensing strategy relies on the sensitivity of the GaAs photoluminescence (PL) emission to the local environment at its surface. Specifically, GaAs substrates were chemically modified with thiol-derivatized oligonucleotide aptamers following conventional condensed-phase deposition techniques and exposed to the target ATP molecules. The resulting modification in the PL intensity is attributed to a specific biorecognition interaction between the aptamer receptors and the ATP target and, more importantly, the accompanying ligand-induced structural change in the aptamer conformation. Since the negatively charged aptamer probes are covalently anchored to the substrate surface, the sensing mechanism can be understood in terms of a change in the surface charge distribution and thereby, the width of the nonemissive GaAs surface depletion layer. Biosensors fabricated with aptamer probes of various lengths indicate a strand length-dependent nature of the luminescence response to the biorecognition events, with longer aptamers yielding a greater PL enhancement. Results provided by several control experiments demonstrate the sensitivity, specificity, and selectivity of the proposed biosensor in accurately identifying ATP. Modeling the performance data by means of Poisson–Boltzmann statistics in combination with the GaAs depletion layer model shows a good correlation between the structural conformation of the aptamers and the PL yield of the underlying substrate. Collectively, the results described within indicate the promise of the prospective luminescence-based GaAs-aptamer biosensor for use in real-time sensing assays requiring a straightforward and efficient means of label-free analytical detection. [ABSTRACT FROM AUTHOR]

The structural and optical properties of GaAsP/GaP core-shell nanowires grown by gas source molecular beam epitaxy were investigated by transmission electron microscopy, Raman spectroscopy, photoluminescence (PL), and magneto-PL. The effects of surface depletion and compositional variations in the ternary alloy manifested as a redshift in GaAsP PL upon surface passivation, and a decrease in redshift in PL in the presence of a magnetic field due to spatial confinement of carriers. [ABSTRACT FROM AUTHOR]

This study provides a deeper insight into the chemistry and physics of the common engineering practice of using a proximity cap, while annealing compound semiconductors such as GaAs. We have studied the cases of a GaAs proximity cap, a Si proximity cap, and no proximity cap. Using x-ray photoelectron spectroscopy, it has been found that annealing increases the gallium to arsenic ratio in the oxide layer in all cases. During the annealing of UV-ozone oxidized GaAs, it has been observed that GaAs proximity caps also serve as a sacrificial layer to accelerate the desorption of oxide species. In all cases surface deterioration due to pit formation has been observed, and the depth of pits is found to depend on the effective role played by the capping material. Energy dispersive x-ray analysis provides additional evidence that pits mainly consist of elemental As and gallium oxide, with most of the elemental As situated at the pit-substrate interface. Deposition of a thin layer of gold and subsequent annealing to 500 °C for 300 s under different capping conditions shows the use of a proximate cap to be practically insignificant in annealing Au deposited films. [ABSTRACT FROM AUTHOR]

A comprehensive study of the oxide formed on GaAs by exposure to ultraviolet light and ozone (UV-ozone), and the evolution of Au nanoparticles on UV-ozone treated GaAs(100) substrates, has been carried out using x-ray photoelectron spectroscopy. The UV-ozone oxide has been found to consist of oxides of gallium and arsenic in addition to having a significant contribution from elemental arsenic concentrated mostly at the substrate-oxide interface. An intermediate suboxide species -AsxGa2O is found to make up a substantial part of the oxide layer, mostly near the surface region, whereas the interface to bulk GaAs is dominated by Ga2O3. As2O3 is found to be the dominant oxide in the initial UV-ozone oxide layer, while the oxide layer becomes rich in Ga2O3 as the sample is annealed at 500 °C for 300 s in a dry nitrogen environment. It is also found that gold does not alloy with Ga during the formation of gold nanoparticles by dewetting from the UV-ozone oxidized GaAs substrate. [ABSTRACT FROM AUTHOR]

The interfacial properties of lattice-matched InGaAs/InP superlattice (SL) structures grown by gas source molecular beam epitaxy were investigated by high resolution x-ray diffraction (HRXRD). SLs with various periods were grown to determine the contributions of the interface layers to the structural properties of the SLs. The HRXRD curves exhibited a number of features indicative of interfacial layers, including weak even-order satellite peaks, and a zero-order diffraction peak that shifted toward lower diffraction angles with decreasing SL period. A detailed structural model is proposed to explain these observations, consisting of strained InAsP and InGaAsP monolayers due to the group-V gas switching and atomic exchange at the SL interfaces. [ABSTRACT FROM AUTHOR]

Presents information on a study which analyzed the incorporation of the group V components in indium (In) gallium arsenic phosphorus (P) grown lattice matched to InP by gas source molecular beam epitaxy. Experimental procedure; Background on the thermodynamic model used in the study; Conclusions.

Semiconductor nanowires (NWs) represent a new class of materials and a shift from conventional two-dimensional bulk thin films to three-dimensional devices. Unlike thin film technology, lattice mismatch strain in NWs can be relaxed elastically at the NW free surface without dislocations. This capability can be used to grow unique heterostructures and to grow III-V NWs directly on inexpensive substrates, such as Si, rather than lattice-matched but more expensive III-V substrates. This capability, along with other unique properties (quantum confinement and light trapping), makes NWs of great interest for next generation optoelectronic devices with improved performance, new functionalities, and reduced cost. One of the many applications of NWs includes energy conversion. This review will outline applications of NWs in photovoltaics, thermoelectrics, and betavoltaics (direct conversion of solar, thermal, and nuclear energy, respectively, into electrical energy) with an emphasis on III-V materials. By transitioning away from bulk semiconductor thin films or wafers, high efficiency photovoltaic cells comprised of III-V NWs grown on Si would improve performance and take advantage of cheaper materials, larger wafer sizes, and improved economies of scale associated with the mature Si industry. The thermoelectric effect enables a conversion of heat into electrical power via the Seebeck effect. NWs present an opportunity to increase the figure of merit (ZT) of thermoelectric devices by decreasing the thermal conductivity (κ) due to surface phonon backscattering from the NW surface boundaries. Quantum confinement in sufficiently thin NWs can also increase the Seebeck coefficient by modification of the electronic density of states. Prospects for III-V NWs in thermoelectric devices, including solar thermoelectric generators, are discussed. Finally, betavoltaics refers to the direct generation of electrical power in a semiconductor from a radioactive source. This betavoltaic process is similar to photovoltaics in which photon energy is converted to electrical energy. In betavoltaics, however, energetic electrons (beta particles) are used instead of photons to create electron-hole pairs in the semiconductor by impact ionization. NWs offer the opportunity for improved beta capture efficiency by almost completely surrounding the radioisotope with semiconductor material. Improving the efficiency is important in betavoltaic design because of the high cost of materials and manufacturing, regulatory restrictions on the amount of radioactive material used, and the enabling of new applications with higher power requirements. [ABSTRACT FROM AUTHOR]

A method is presented for maximizing the yield and crystal phase purity of vertically aligned Auassisted GaAs nanowires grown with an SiOx selective area epitaxy mask on GaAs (111)B substrates. The nanowires were grown by the vapor-liquid-solid (VLS) method in a gas source molecular beam epitaxy system. During annealing, Au VLS seeds will alloy with the underlying GaAs substrate and collect beneath the SiOx mask layer. This behavior is detrimental to obtaining vertically aligned, epitaxial nanowire growth. To circumvent this issue, Au droplets were pre-filled with Ga assuring vertical yields in excess of 99%. [ABSTRACT FROM AUTHOR]

Indium tin oxide (ITO) was deposited by RF sputtering on n-GaAs nanowires grown by the Au-assisted vapor–liquid–solid process in a molecular beam epitaxy (MBE) system. The ITO formed an Ohmic contact with n-doped (n = 8 × 1018 cm−3) GaAs nanowires with a specific contact resistance of <1.41 Ω cm2. Insertion of a 25 nm thick indium layer between 500 nm thick ITO and the GaAs nanowires resulted in a reduction of specific contact resistance to <0.13 Ω cm2 after annealing at 400 °C for 30 s. The In/ITO film had an average transmittance of 89% from 400 to 900 nm and a sheet resistance of 13 Ω/□, which is well suited for nanowire-based optoelectronic applications. [ABSTRACT FROM AUTHOR]

This article reviews recent developments in nanowire‐based photovoltaics (PV) with an emphasis on III–V semiconductors including growth mechanisms, device fabrication and performance results. We first review the available nanowire growth methods followed by control of nanowire growth direction and crystal structure. Important device issues are reviewed, including optical absorption, carrier collection, strain accommodation, design for high efficiency, tunnel junctions, Ohmic contact formation, passivation and doping. Performance data of III–V nanowire cells and the primary challenges in nanowire PV are summarized. Many of the issues discussed here are also applicable to other nanowire devices such as photodetectors. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) Part of Focus Issue on “Semiconductor Nanowires” (Eds.: Chennupati Jagadish, Lutz Geelhaar, Silvija Gradecak) To overcome the limitations of current III–V multi‐junction solar photovoltaic (PV) cells, III–V nanowire‐based PV is being aggressively pursued by numerous groups. This article provides a comprehensive review of nanowire‐based PV covering all aspects of design for high power conversion efficiency including nanowire growth methods, growth direction, crystal structure, optical absorption, carrier collection, strain accommodation, tunnel junctions, Ohmic contact formation, passivation and doping. [ABSTRACT FROM AUTHOR]

Numerical simulation of the photocurrent density is performed for a two-junction nanowire (NW)-on-silicon solar cell under AM1.5G illumination. The photocurrent density is determined for NW diameters from 100-250 nm, period (spacing) from 250-1000 nm, and length of 5 μm. The dependence of photocurrent density on NW bandgap is also determined. For each NW bandgap, the optimum diameter and period are determined to obtain current matching between the top NW cell and the bottom Si cell. [ABSTRACT FROM AUTHOR]

Dynamic and time-of-flight (TOF) secondary ion mass spectrometry (SIMS) was performed on vertically standing III-V nanowire ensembles embedded in Cyclotene polymer. By embedding the NWs in Cyclotene, the top surface of the sample was made planar, while the space between the NWs was filled to protect the background substrate from the ion beam, thus allowing for the NWs to be sputtered and analyzed evenly as a function of depth. Using thin film standards, SIMS analysis was used to calculate the impurity dopant concentration as a function of height in the NW ensemble. This marked the first use of conventional SIMS to accurately determine the doping density with excellent depth resolution. Additionally, this is the first presentation of SIMS as the only reported tool for characterizing the segment height uniformity of any arbitrary axial heterostructure NW ensemble. [ABSTRACT FROM AUTHOR]

The patterning of metal seed particles for the vapor-liquid-solid (VLS) growth of semiconductor nanowires is examined. We report the challenges of obtaining GaP nanowires grown by molecular beam epitaxy (MBE) from a patterned array of Au seed particles created by electron beam lithography. Transmission electron microscopy studies revealed the presence of a thin silicon oxide layer over the patterned Au seed particles. Au acted as a catalyst for silicon oxide growth at even moderate temperatures. In particular, the annealing of Au-patterned Si (1 1 1) substrates at low temperatures required for indium mounting (200-300 °C) was responsible for the formation of an oxide layer that was detrimental to Au-assisted growth. Removal of the oxide layer by a buffered HF etch prior to MBE sample loading enabled patterned VLS growth. [ABSTRACT FROM AUTHOR]

Self-assembled monolayers (SAMs) of octadecanethiol (ODT), CH3(CH2)17SH, were deposited on GaAs (100) substrates from liquid and vapor phases. Liquid-phase-deposited SAMs were prepared by immersing the substrate in a dilute solution of ODT and ethanol, while vapor-phase-deposited monolayers were prepared by exposing the GaAs surface to a stream of ODT vapor in an ultrahigh vacuum environment. The structural and optical properties of the resulting SAMs were examined with contact angle (CA) analysis, photoluminescence (PL) spectroscopy, high-resolution x-ray photoelectron spectroscopy (HRXPS), and spectroscopic ellipsometry. Although well-ordered films were formed from both deposition techniques, PL, CA analysis, and ellipsometry measurements revealed that the overall quality, structure, and long-term durability of the resulting SAMs depended on the preparation method. Specifically, time-dependent PL and CA analysis indicated an enhanced stability for vapor-deposited films stored under ambient conditions. Through HRXPS measurements, the attachment of the thiolate molecules to the GaAs substrates was shown to proceed through the formation of chemical bonds at both Ga and As surface sites, with the percentage of each bonding configuration dictated by the surface termination produced via the cleaning process used prior to the SAM deposition. Collectively, the results suggested that more robust monolayers exhibiting greater surface coverage, and therefore increased passivation and stability characteristics, are assembled from vapor phase. [ABSTRACT FROM AUTHOR]

Octadecanethiol (ODT), CH3(CH2)17SH, self-assembled monolayers (SAMs) were deposited on GaAs (100) substrates from solution and from vapor phase. The monolayers assembled from solution were prepared by incubating the substrate in a dilute millimolar ODT/ethanol solution, while vapor-deposited monolayers were prepared from vapor phase transport of ODT in an ultrahigh vacuum environment. The structural and optical characteristics of the resulting SAMs were examined with contact angle (CA) analysis, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry. Static CA measurements of the SAMs indicated the successful formation of highly organized monolayers by both preparation routes. In contrast, PL spectroscopy, AFM, and ellipsometry measurements showed differences in the optical and structural properties of SAMs prepared from liquid and vapor phases. The data obtained indicate that more densely packed monolayers exhibiting greater surface coverage and therefore, increased passivation characteristics are assembled from vapor phase. [ABSTRACT FROM AUTHOR]

The surface passivation of n-type GaAs nanowires (NWs) by ammonium polysulfide solution, (NH4)2Sx, is described. The passivation resulted in a two order of magnitude increase in current density in an ensemble NW device. A depletion and recombination model is used to explain the results in terms of a reduction in surface trap density upon passivation. The results are comparable to a previous passivation method using AlInP shells. The S passivation was found to be unstable according to the degradation in the ensemble NW conductivity after a 7 months exposure to air. [ABSTRACT FROM AUTHOR]

Poisson's equation is solved for a radial pn junction nanowire (NW) with surface depletion. This resulted in a model capable of giving radial energy band and electric field profiles for any arbitrary core/shell doping density, core/shell dimensions, and surface state density. Specific cases were analyzed to extract pertinent underlying physics, while the relationship between NW specifications and the depletion of the NW were examined to optimize the built-in potential across the junction. Additionally, the model results were compared with experimental results in literature to good agreement. Finally, an optimum device design is proposed to satisfy material, optical, and electrostatic constraints in high efficiency NW solar cells. [ABSTRACT FROM AUTHOR]

Structural and optical properties of heterostructured GaAs/AlGaAs/GaAs core/inner shell/outer shell nanowires are studied. Transmission electron microscopy and Raman scattering measurements unambiguously identify the presence of segments crystallized in zincblende and wurtzite phases, which spread to the shells. Four observed photoluminescence lines are assigned to the radiative recombination of photoexcited electrons confined in the center of the GaAs core and at the heterointerface between the outer GaAs shell and the inner AlGaAs shell with the holes localized at the heterointerface between the core and the inner AlGaAs shell; both recombinations take place in zincblende and wurtzite phases. One additional photoluminescence line is attributed to the spatially indirect recombination between the electrons in zincblende and the holes in wurtzite phases. The bandgap of the wurtzite phase and the band offsets between the zincblende and wurtzite phases are determined. [ABSTRACT FROM AUTHOR]

A photoluminescence (PL) study of the individual electron states localized in a random potential is performed in artificially disordered superlattices embedded in a wide parabolic well. The valence band bowing of the parabolic potential provides a variation of the emission energies which splits the optical transitions corresponding to different wells within the random potential. The blueshift of the PL lines emitted by individual random wells, observed with increasing disorder strength, is demonstrated. The variation of temperature and magnetic field allowed for the behavior of the electrons localized in individual wells of the random potential to be distinguished. [ABSTRACT FROM AUTHOR]

Electron mobility was studied in lattice-matched short-period InGaAs/InP superlattices as a function of the width of the wells. The decreasing mobility with decreasing well width was shown to occur due to the interface roughness. The roughnesses of InGaAs/InP and GaAs/AlGaAs interfaces were compared. Much smoother InGaAs/InP interfaces resulted in higher electron mobility limited by interface roughness. [ABSTRACT FROM AUTHOR]

InP nanowires (NWs) were grown by gas source molecular beam epitaxy on InP (111)B substrates, using Au nanoparticles as a growth catalyst. The rod-shaped NWs exhibited hexagonal sidewall facets oriented along the {-211} family of crystal planes for all NW diameters, indicating minimal sidewall growth. Stacking faults, when present, were concentrated near the NW tips, while NWs with lengths less than 300 nm were completely free of stacking faults. [ABSTRACT FROM AUTHOR]

GaAs nanowires were passivated by AlInP shells grown by the Au-assisted vapor-liquid-solid method in a gas source molecular beam epitaxy system. Transmission electron microscopy confirmed a core-shell GaAs-AlInP structure. Current-voltage measurements on ensemble nanowires indicated improved carrier transport properties in the passivated nanowires as compared to their unpassivated counterpart. Similarly, individual nanowires showed improved photoluminescence intensity upon passivation. A detailed model is presented to quantify the observed improvements in nanowire conduction and luminescence in terms of a reduction in surface charge trap density and surface recombination velocity upon passivation. The model includes the effects of high-level injection, bulk recombination, and surface recombination. The model can be used as a tool for assessing various passivation methods. [ABSTRACT FROM AUTHOR]

High efficiency III–V nanowire solar cell: III–V nanowire‐based photovoltaics (PV) are a promising replacement for thin‐film solar cells, offering the potential for less material utilization, excellent anti‐reflection properties, lattice‐mismatched growth on silicon, and high power conversion efficiency. In the Review@RRL article by LaPierre et al. (pp. 815–830), the status of nanowire PV is summarized including issues related to nanowire growth, control of nanowire growth direction, crystal structure, optical absorption, carrier collection, strain accommodation, tunnel junctions, Ohmic contact formation, passivation and doping. Performance data of III–V nanowire cells and the primary challenges in nanowire PV are summarized. [ABSTRACT FROM AUTHOR]

Be-doped InP nanowires were grown by the gold-assisted vapour-liquid-solid mechanism in a gas source molecular beam epitaxy system. The InP nanowire length versus diameter [L(D)] dependence revealed an unexpected transition with increasing Be dopant concentration. At Be dopant concentration below ∼1018 cm-3, nanowires exhibited the usual inverse L(D) relationship, indicating a diffusion-limited growth regime. However, as dopant concentration increased, the nanowire growth rate was suppressed for small diameters, resulting in an unusual L(D) dependence that increased before saturating in height at about 400 nm. The cause of this may be a change in the droplet chemical potential, introducing a barrier to island nucleation. We propose a model accounting for the limitations of diffusion length and monolayer nucleation to explain this behaviour. [ABSTRACT FROM AUTHOR]

The effect of ammonium polysulfide solution, (NH4)2Sx, on the surface passivation of p-doped InP nanowires (NWs) was investigated by micro-photoluminescence. An improvement in photoluminescence (PL) intensity from individual NWs upon passivation was used to optimize the passivation procedure using different solvents, sulfur concentrations and durations of passivation. The optimized passivation procedure gave an average of 24 times improvement in peak PL intensity. A numerical model is presented to explain the PL improvement upon passivation in terms of a reduction in surface trap density by two orders of magnitude from 1012 to 1010 cm−2, corresponding to a change in surface recombination velocity from 106 to 104 cm s−1. The diameter dependence of the PL intensity is investigated and explained by the model. The PL intensity from passivated nanowires decreased to its initial (pre-passivation) value over a period of seven days in ambient air, indicating that the S passivation was unstable. [ABSTRACT FROM AUTHOR]

Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO2 films by the advective transport and deposition of Zn vapours obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW volume and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO+C) source–SiO2 substrate distance. We observe C flakes on the ZnO NWs/SiO2 substrates which exhibit short NWs that developed on both sides. The SiO2 and C substrates/NW interfaces were studied in detail to determine growth mechanisms. NWs on Au-seeded SiO2 were promoted by a rough ZnO seed layer whose formation was catalysed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW volume is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concentration and growth rate. [ABSTRACT FROM AUTHOR]

A detailed continuum model is presented for predicting the growth characteristics of GaAs nanowires during chemical beam epitaxy. The model describes the transport processes of Ga and As adatoms on the substrate and nanowire sidewalls, and through the nanoparticle and the nanowire-catalyst interface (NCI). The growth mechanisms of nanowires within the NCI are described using an extended step-flow kinetic model. The vapor-liquid-solid and vapor-solid-solid growth mechanisms are both described in the kinetic model. The growth rate of the nanowires, the surface and bulk concentrations of adatoms, and the role of transport processes of Ga and As adatoms during chemical beam epitaxy were investigated. The growth mechanisms of the nanowires were found to vary with increasing length of the nanowire. [ABSTRACT FROM AUTHOR]