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Twinning superlattices (TSLs) are a growing class of semiconductor structures proposed as a means of phonon and optical engineering in nanowires (NWs). In this work, we examine TSL formation in Te-doped GaAs NWs grown by a self-assisted vapor-liquid-solid mechanism (with a Ga droplet as the seed particle), using selective-area molecular beam epitaxy. In these NWs, the TSL structure is comprised of alternating zincblende twins, whose formation is promoted by the introduction of Te dopants. Using transmission electron microscopy, we investigated the crystal structure of NWs across various growth conditions (V/III flux ratio, temperature), finding periodic TSLs only at the low V/III flux ratio of 0.5 and intermediate growth temperatures of 492 to 537 °C. These results are explained by a kinetic growth model based on the diffusion flux feeding the Ga droplet.

GaAs-InGaAs-GaAs core-shell-shell nanowire (NW) structures were grown by gas source molecular beam epitaxy using the selective-area, self-assisted, vapor-liquid-solid method. The structural, morphological, and optical properties of the NWs were examined for different growth conditions of the InGaAs shell. With increasing In concentration of the InGaAs shell, the growth transitioned from preferential deposition at the NW base to the Stranski-Krastanov growth mode where InGaAs islands formed along the NW length. This trend is explained within a nucleation model where there is a critical In flux below which the conformal growth is suppressed and the shell forms only at the NW base. Low growth temperature produced a more uniform In distribution along the NW length but resulted in quenching of the photoluminescence (PL) emission. Alternatively, reducing the shell thickness and increasing the V/III flux ratio resulted in conformal InGaAs shell growth and quantum dot-like PL emission. Our results indicate a pathway toward the conditions for conformal InGaAs shell growth required for satisfactory optoelectronic performance.

Recent investigations of III-V semiconductor nanowires have revealed periodic zinc-blende twins, known as twinning superlattices, that are often induced by a high-impurity dopant concentration. In the present study, the relationship between the nanowire morphology, crystal structure, and impurity dopant concentration (Te and Be) of twinning superlattices has been studied in GaAs nanowires grown by molecular beam epitaxy using the self-assisted (with a Ga droplet) vapor-liquid-solid process. The contact angle between the Ga droplet and the nanowire top facet decreased linearly with the dopant concentration, whereas the period of the twinning superlattices increased with the doping concentration and was proportional to the nanowire radius. Our model, which is based entirely on surface energetics, is able to explain a unified formation mechanism of twinning superlattices in doped semiconductor nanowires.

III-V compound semiconductor nanowires (NWs) are being developed for the next generation of optoelectronic devices such as photodetectors, photovoltaics, betavoltaics and thermoelectrics. The self-assisted vapor-liquid-solid method is now a well-established technique for the growth of III-V NWs on silicon substrates. In this method, an array of holes in a SiO2 film is used for metal droplet formation, which seeds the growth of vertically oriented NWs within a periodic array. The free lateral surfaces of NWs allow elastic relaxation of lattice misfit strain without the generation of dislocations, permitting unique heterostructures and the direct integration of III-V materials on silicon substrates. Furthermore, NWs permit high optical absorption due to an optical antenna effect. The optical absorption in NW arrays can exceed that due to a thin film of equivalent thickness, enabling high efficiency NW-based photonic devices. Furthermore, optical resonances that depend on the NW diameter allow multispectral absorption. Some of the challenges associated with NW materials and devices, including quantum dot formation, will be illustrated.

Producción Científica, Ridge microstructures were prepared by reactive ion etching (RIE) of a series of stacked InAsxP$_{1-x}$ quantum wells (QWs) with step graded compositions grown on InP by molecular beam epitaxy. These microstructures were characterized by low temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each of the QWs was clearly identified and a model for their line shape was implemented. PL line-scans were measured across etched ridge stripes of various widths in an optical cryostat, with a spatial resolution of 1 µm. The model for the PL spectra allowed accurate extraction of the local PL integrated intensities, spectral positions and line widths. Two different RIE processes, using CH4/H2 and CH4/Cl2, were investigated. The PL line-scans showed strong variations of the integrated PL intensities across the etched stripes. The PL intensities for all QWs increased gradually from the edge to the center of the ridge microstructures, over a length scale of 10–20 µm. On the other hand, the spectral peak position of the PL lines remained constant (within an accuracy of 0.2–0.4 meV, depending on which QW was considered) across the microstructures. These observations are discussed in terms of the mechanical stress induced by the RIE processes, the relaxation of the biaxial built-in compressive stress in the InAsP QWs (induced by the free surfaces at the vertical etched sidewalls), and also by the non-radiative recombination at these sidewalls. 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 RIE processing., Natural Sciences and Engineering Research Council of Canada (grants RGPIN-2018-04015 and RGPAS-2018-522624), Junta de Castilla y León (project VA283P18), Ministerio de Economía, Industria y Competitividad (project RTI2018-101020-B-I00)

International audience; We report on the selective area growth of InAs nanowires (NWs) by the catalyst-free vapor−solid method. Well-ordered InAs NWs were grown on GaAs(111)B and Si(111) substrates patterned with a dielectric mask using hydride vapor phase epitaxy (HVPE). Vertical and high aspect ratio InAs NWs with a hexagonal shape were grown on both GaAs and Si substrates. The impact of the growth conditions on the InAs morphology was investigated. The final shape of the InAs crystal was tuned from a NW to a nanoplatelet by controlling growth conditions such as growth temperature, vapor phase composition, and mask pattern. The influence of the aperture size on the nucleation density and then on the morphology of InAs is discussed. Small openings resulted in the formation of a single nucleus per hole, which was then converted to a NW. For larger apertures, the number of nuclei increased, leading to both three-dimensional crystals and NWs. The effect of growth temperature and the III/V ratio on the kinetics and thermodynamics of InAs growth is also discussed. The growth was first optimized on a GaAs(111)B substrate and then performed on Si, which is more suitable to develop devices. Finally, the absorbance and photoluminescence measurements were carried out on the InAs NW arrays, demonstrating the high potential of HVPE-grown InAs NWs for future multispectral photo-detection devices.

GaAs nanowire (NW) arrays were grown on Si substrates by selective-area molecular beam epitaxy (MBE) using the self-assisted vapor-liquid-solid (VLS) method. In the self-assisted VLS method, the GaAs NW diameter is determined by the size of a Ga droplet at the top of the NW. Absorption of the solar spectrum for photovoltaic devices is optimum with a GaAs NW diameter of ∼170 nm, requiring a Ga droplet during the growth of a similar size. Control of the Ga droplet size, and therefore the NW diameter, is demonstrated by controlling the V/III flux ratio during the MBE growth. A V/III flux ratio near unity rapidly increased the Ga droplet volume during the NW growth, thereby creating an inverse-tapered (funnel-shaped) NW morphology. Numerical simulations indicated that the inverse-tapered NW showed strong broadband absorption of the solar spectrum due to the absorption of consecutively longer wavelengths toward the base of the NW.

Hybrid organic–inorganic halide perovskites have emerged as a disruptive technology in a number of fields, and recently, there has been increased interest in developing nanostructured perovskite materials, due to their extremely high photoluminescence quantum yields, optical absorption, and tolerance for defects. In this study, we report on the development of a facile room temperature synthesis method for high density monodispersed metal–organic halide perovskite nanoparticles using a diblock copolymer reverse micelle deposition (RMD) method. Compared to traditional ligated methods, we show that diblock copolymer micelle templating allows greater control over the size distribution due to controlled nucleation and crystal growth. By separating the precursor solvation and reaction steps through micelle templating, we show that micelle templating is a universal, atmospheric approach to producing a variety of perovskite nanoparticles, including methylammonium lead iodide (MAPbI3), methylammonium lead bromide ...

Here, a new promising perovskite structure of KMnF3 has been fabricated and characterized, which yields bandgap of 1.6 eV with fascinating moisture-resistance and phase stability. Investigation of structural, optical, stability and transport properties have done by XRD, SEM, UV–vis–NIR spectroscopy, photoluminescence and electrical conductivity test. Such examination indicated the high carrier mobility (18 cm2/V s) and density (1014/cm3) even after a long interval between each excitation. These transport properties are comparable to that of the organic perovskite, indicating the importance of KMnF3 for solar device applications.

The commonly employed methyl-ammonium containing perovskites are unstable at high temperature or under moisture attack, thus limiting their commercial applications. To overcome this barrier, a new perovskite possessing both thermal stability and water resistance has been constructed, involving pyrrolidinium rings (CH2)4NH2PbI3 (PyPbI3), through a simple solution-processing method. It presents not only prolonged moisture resistance and a favorable bandgap of 1.80 eV, as determined by P-XRD and UV absorption, but also excellent thermal stability, as verified via the in situ XRD technique. The new structure may be applicable to perovskite solar cells with improved thermal stability and water resistance.

International audience; We report for the first time on the hydride vapor phase epitaxy (HVPE) growth of long (26 μm) InAs nanowires on Si(111) substrate grown at a standard rate of 50 μm h−1. The nanowires grow vertically along the (111)B direction and exhibit a well faceted hexagonal shape with a constant diameter. The effect of the experimental parameters, growth temperature and III/V ratio, is investigated. The thermodynamic and kinetic mechanisms involved during the growth of such long nanowires are identified. It is demonstrated that growth occurs through direct condensation of InCl and As4/As2 gaseous species. Dechlorination of adsorbed InCl molecules is the limiting step at low temperature. Structural analysis through high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field (HAADF) imaging was performed. The high As4 partial pressure of the HVPE environment induces the presence of both wurtzite and zinc-blende phases. The results emphasize the potential of the low cost HVPE technique for the monolithic integration of arrays of long InAs nanowires on silicon.

A piezoelectrically driven metallic nanoprobe is installed inside a scanning electron microscope to perform local characterization of the field emission properties of InSb nanopillars. The tip-shaped anode can be precisely positioned at sub-micron distances from the emitters to collect electrons from areas as small as 1μm2 under the application of an external bias up to 100 V. Current-voltage characteristics are measured for cathode-anode separation down to 500 nm and are analyzed in the framework of the Fowler-Nordheim theory. We give estimation of performance parameters such as the field enhancement factor and the turn-on field and their dependence on the cathode-anode separation distance. We demonstrate the time stability of the emitted current for several minutes. Finally, we perform a finite element electrostatic simulation to calculate the electric field in proximity of the nanopillars and we evaluate the effective emitting area as well as the screening effect due to presence of other pillars in close vicinity. We show that InSb nanopillars are very stable emitters that allow current density as high as 104 A/cm2 and excellent time stability, crucial characteristics to envisage device exploitation.

Nuclear magnetic resonance (NMR) uses a large ensemble of molecules in a test tube (a liquid). The spin states of nuclei within the atoms of the molecules act as the qubits. Each molecule is a quantum computer.

Solid-state spin qubits are based on the electron spin in solid materials. We want to manufacture a spin system by confining/trapping a single electron in a quantum two-level system containing spin up or spin down, and then use electron spin resonance to control the spin. This chapter will discuss methods of trapping single electrons and implementing quantum computing in solid materials.

Quipu is an ancient Incan method of storing information in knots (Fig. 26.1). Topological quantum computing is a method of quantum information storage and processing using “topological” knots. In this chapter, we will examine topological quantum computing.

DiVincenzo criterion #5 from Chap. 18 states that the qubit lifetimes should be long compared to the duration of the algorithm. However, quantum systems are fragile. Unwanted external perturbations from the environment (e.g., electromagnetic waves, thermal excitations, charge, etc.) can interact with the quantum system, such that the system becomes entangled with the environment. Quantum computing presents two conflicting requirements: qubits should be isolated from the environment for long qubit lifetimes (DiVincenzo criterion #5), while also possessing good addressability (DiVincenzo criterion #2 and #4). We need to be able to address qubits without letting the noisy environment leak into the system. Due to the difficulty of this challenge, present day quantum algorithms have been demonstrated with only a few 10s of qubits. We need a way of correcting errors, known as quantum error correction (QEC).

Adiabatic quantum computing (AQC) is an approach to quantum computing based on the adiabatic theorem. The adiabatic theorem states that if a Hamiltonian, \( \widehat{H} \), changes slowly in time, then the state remains in the ground state. By starting with some simple Hamiltonian and evolving it to some \( \widehat{H} \) representing the problem to be solved, the lowest energy configuration becomes the solution to the problem.

The analysis of the preceding Chapter was for a constant magnetic field. From Exercise 14.9, we see that the magnetic field is very large (> 1 T) for the Zeeman splitting to exceed the thermal energy. This is desired to prevent unwanted thermal excitation between the Zeeman split energy levels from affecting our qubit states. However, it is difficult to change such large static B-fields rapidly enough for quantum gates. Also, the resulting Larmor frequency ωo is very fast (≫GHz). We would like to have a slower rotation to get better control. To use smaller fields and slow down the precession, we use a technique called “electron spin resonance” (ESR).

In trapped ion quantum computing (TIQC), ions are created and trapped to form a qubit register. Ions can be suspended and trapped in free space using electromagnetic fields. The qubits, defined by the electronic states of the ions, are manipulated and read out using lasers.

Quantum computers can use a superposition to run many states simultaneously through an algorithm, rather than sequentially as in a classical computer. This principle is called quantum parallelism. This chapter refines our notion of quantum parallelism and introduces computational complexity to quantify the power of quantum computers.

Optical (also known as photonic or bosonic) quantum computing uses photons, so-called “flying qubits”, to encode \( \left| 0 \right\rangle \) or \( \left| 1 \right\rangle \). Several different methods exist for encoding \( \left| 0 \right\rangle \) and \( \left| 1 \right\rangle \), including the photon polarization or the spin angular momentum state (polarization encoding), different optical paths in an optical system (path encoding), the spatial distribution of light such as different optical modes in an optical waveguide (orbital angular momentum encoding), and time or frequency encoding. We have already examined the case of polarization encoding for quantum communication in Chap. 6. Let us consider the key optical elements used for optical quantum computing, using path encoding as an example.

In the previous chapter, we examined electron spin resonance (ESR) used to implement single-qubit gates. In this chapter, the dynamics of a two-level quantum system and ESR will be examined in more detail. We consider how the electron cycles back and forth between two energy levels.

Quantum key distribution (QKD) provides a means of secure communication between two parties. QKD exploits the principle that you cannot eavesdrop on a quantum communication channel without producing a detectable disturbance.

We present a model for the growth kinetics of III-V nanowires which describes the time evolution of the monolayer coverage and group V concentration in the droplet. Special emphasis is put on the stopping effect at very low group V contents, where 2D island rapidly fills a fraction of monolayer and then grows much slower at the rate of the droplet refill from vapor. Such a regime has far-reaching implications in the periodically changing morphology of the growth interface, nucleation statistics, and size distributions within the ensembles of III-V nanowires.

A review of models for determining the thermoelectric transport coefficients S (Seebeck coefficient), σ (electrical conductivity), and κ e (electronic thermal conductivity) is presented, for the cases of bulk and nanowire structures, along with derivations and a discussion of calculation methods. Results for the transport coefficients in GaAs, InAs, InP and InSb are used to determine the thermoelectric figure of merit, where an enhancement by two orders of magnitude is found for the nanowire case as compared with the bulk. The optimal electron concentration is determined as a function of nanowire diameter for both background and modulation doped nanowires.

International audience; Au droplets are used as a catalyst for the growth of nanowires on Si(111) substrate via the vapor-liquid-solid (VLS) mechanism. The dewetting of a Au thin film is the most common method to obtain these droplets. The control of this step is crucial to adjust the density and the diameter of the nanowires during VLS growth. When the Si(111) substrate is covered with a silicon dioxide layer, the kinetics of Au droplet formation is strongly modified. The dependence of the diameter and spatial distribution of the droplets on the surface have been studied by scanning electron microscopy with respect to the thickness of the silicon dioxide layer, the thickness of the Au film and the temperature of the substrate during deposition and post-deposition annealing. In-situ low energy electron microscopy and low energy electron diffraction revealed the dynamics of the Au droplet formation after annealing. The Au droplets are shown to catalyze the decomposition of silicon dioxide at high temperature (> 650-700 °C) and form a wetting layer of Au-(33)-Si(111). Consequently, the droplets absorb silicon atoms from the substrate, migrate perpendicular to the atomic steps and grow by the Smoluchowski ripening process.

InSb nanowire arrays with different geometrical parameters, diameter and pitch, are fabricated by top-down etching process on Si(100) substrates. Field emission properties of InSb nanowires are investigated by using a nano-manipulated tungsten probe-tip as anode inside the vacuum chamber of a scanning electron microscope. Stable field emission current is reported, with a maximum intensity extracted from a single nanowire of about 1$\mu A$, corresponding to a current density as high as 10$^4$ A/cm$^2$. Stability and robustness of nanowire is probed by monitoring field emission current for about three hours. By tuning the cathode-anode separation distance in the range 500nm - 1300nm, the field enhancement factor and the turn-on field exhibit a non-monotonic dependence, with a maximum enhancement $\beta \simeq $ 78 and a minimum turn-on field $E_{ON} \simeq$ 0.033 V/nm for a separation d =900nm. The reduction of spatial separation between nanowires and the increase of diameter cause the reduction of the field emission performance, with reduced field enhancement ($\beta, Comment: 20 pages, 6 figures

Producción Científica, Photoluminescence and cathodoluminescence spectral imaging were performed across rectangular stripes etched in samples with InAsxP1-x quantum wells of constant thickness and variable composition grown on InP. In particular, the effects of different etching chemistries (CH4/H2/Ar and Cl2/CH4/Ar) were investigated. The results discussed deal with modifications of the luminescence line shapes (which differ with etching process) and with the intensity variation of the emissions associated with the quantum wells across the stripes. The possible origins of these effects are investigated in terms of carrier recombination on the vertical sidewalls of the stripes and lateral diffusion of species from the plasma during etching. Cathodoluminescence measurements on samples under DC-bias also show the quantum confined Stark effect which is correlated to the material modifications induced by the etching.

Polarization-resolved photoluminescence was used to study spin relaxation of photoexcited holes in In 0.53Ga0.47As/InP quantum wells in a quantizing magnetic field as a function of temperature. At a temperature below 10 K, the circular polarization of the photoluminescence due to the spin-split valence band Landau levels was found temperature-independent. In this temperature range fast hole spin relaxation as compared to their lifetime leads to the photoluminescence circular polarization determined by the ratio of these times. Increasing temperature resulted in efficient hole spin thermalization in the Zeeman split valence band Landau levels and as a consequence, in vanishing photoluminescence polarization. Fits of the experimental data by the theory allowed a determination of the hole spin relaxation times related to different Landau levels and the corresponding hole effective g-factor. Direct measurements of the hole spin relaxation times prove the obtained results.

The incorporation of Si into vapor-liquid-solid GaAs nanowires often leads to p-type doping, whereas it is routinely used as an n-dopant of planar layers. This property limits the applications of GaAs nanowires in electronic and optoelectronic devices. The strong amphoteric behavior of Si in nanowires is not yet fully understood. Here, we present the first attempt to quantify this behavior as a function of the droplet composition and temperature. It is shown that the doping type critically depends on the As/Ga ratio in the droplet. In sharp contrast to vapor-solid growth, the droplet contains very few As atoms, which enhance their reverse transfer from solid to liquid. As a result, Si atoms preferentially replace As in GaAs, leading to p-type doping in nanowires. Hydride vapor phase epitaxy provides the highest As concentrations in the catalyst droplets during their vapor-liquid-solid growth, resulting in n-type dopant behavior of Si. We present experimental data on n-doped Si-doped GaAs nanowires grown by this method and explain the doping within our model. These results give a clear route for obtaining n-type or p-type Si doping in GaAs nanowires and may be extended to other III-V nanowires.

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.

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.

The optical absorptance from arrays of GaAs nanowires (NWs) was examined by the finite element method. Absorptance in cylindrical NWs, frustum nanocones (with base wider than the top) and inverted frustum nanocones (with top wider than the base) was compared. The introduction of higher order HE1n modes, the red-shift of the HE1n modes along the NW length due to NW tapering, and the red-shift of the modes due to increase of the overall NW diameter all contribute to a broadening of the absorption spectrum in conical NWs as compared to NWs with a constant diameter. The optical reflectance versus NW top diameter shows a minimum due to a balance between reflectance from the top of the NWs and reflectance from the substrate between NWs. The optimum geometry for photovoltaic energy conversion was determined from the total photocurrent. An optimum photocurrent of 26.5 mAcm-2 was obtained, corresponding to a conical NW morphology with base diameter of 200 nm, top diameter of 110 nm, and length of 2000 nm. An optimized inverse tapered conical morphology gave comparable performance.

The efficiency of GaAs nanowire solar cells can be significantly improved without any new processing steps or material requirements. We report coupled optoelectronic simulations of a GaAs nanowire (NW) solar cell with vertical p-i-n junction and high band gap AlInP passivating shell. Our frequency-dependent model facilitates calculation of quantum efficiency for the first time in NW solar cells. For passivated NWs, we find that short-wavelength photons can be most effectively harnessed by using a thin emitter while long-wavelength photons are best utilized by extending the intrinsic region to the nanowire/substrate interface, and using the substrate as a base. These two easily implemented changes, coupled with the increase of NW height to 3.5 um with realistic surface recombination in the presence of a passivation shell, result in a NW solar cell with greater than 19% efficiency., Comment: 6 pages + 3 pages appendices

InAs nanowires (NWs) were grown on silicon substrates by gas source molecular beam epitaxy using five different growth modes: (1) Au-assisted growth, (2) positioned (patterned) Au-assisted growth, (3) Au-free growth, (4) positioned Au-assisted growth using a patterned oxide mask, and (5) Au-free selective-area epitaxy (SAE) using a patterned oxide mask. Optimal growth conditions (temperature, V/III flux ratio) were identified for each growth mode for control of NW morphology and vertical NW yield. The highest yield (72%) was achieved with the SAE method at a growth temperature of 440 °C and a V/III flux ratio of 4. Growth mechanisms are discussed for each of the growth modes.

InAsSb pillars were investigated for multispectral photodetection in the long wavelength infrared (LWIR) region. An InAs0.19Sb0.81 thin film was successfully grown on Si (1 0 0) substrate, utilizing an AlSb buffer layer to alleviate the large lattice mismatch. X-ray diffraction studies showed a majority [1 0 0] orientation of the as-grown films, with minor orientations arising as a result of twinning. Arrays of InAsSb pillars with diameters ranging from 1700 nm to 4000 nm were fabricated by a top-down reactive ion etching process. The arrays showed resonant optical absorption peaks in the LWIR region from 8 to 16 μm wavelength, dependent on the pillar diameter. The peak absorptance wavelength increased by 0.46 μm for each 100 nm increase in pillar diameter, demonstrating the multispectral tunability of such arrays.

International audience; 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.

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.

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.

Semiconductor nanowires (NWs) are a good candidate for future optoelectronic devices. However, the control of the essential parameters that determine the electronic and optical quality of NWs, such as crystal structure and incorporation of impurity dopants, are still challenging problems. Most III-V NWs exhibit crystal defects, which are typically randomly distributed zincblende twinning segments and stacking faults that can affect the optical and electrical properties of NW devices. The incorporation of intentional impurity dopants in NWs is important for the fabrication of p-n junctions and control of the electrical conductivity of NWs. The effect of Te and Be impurity dopant concentration on the crystal structure, surface roughness and optical properties of GaAs NWs will be presented. Four identical GaAs NW arrays were grown: an undoped sample (as a reference) and 6 samples with different Te and Be doping concentration. High resolution transmission electron microscopy (HRTEM) revealed an unusual superlattice twinning, with periodicity that became wider and more regular as the doping level increased. Twin boundaries in GaP are shown to act as an atomically narrow plane of wurtzite phase with a type-I homostructure band alignment. Twin boundaries and stacking faults (wider regions of the wurtzite phase) lead to the introduction of shallow trap states observed in photoluminescence studies. Controlling the surface roughness, the periodicity, and the width of twinning planes with doping concentration might open new possibilities for high efficiency NW-based thermoelectric devices, but also has wide implications for all NW devices.

The dynamics of differently spin-polarized carriers photoexcited in a system of Landau levels is investigated in an InGaAs/InP quantum well. Shake-up emission from Landau levels above the Fermi level is observed, and it is shown to significantly affect the recombination dynamics of Landau levels. The shake-up effect is found to occur due to the inter-Landau-level scattering, which manifests itself in a rapid decay of the emission from Landau levels. In addition, the inter-Landau-level scattering is shown to determine the time delay of photoluminescence response due to the relaxation of photoexcited electrons among Landau levels. The sharp minimum of the recombination time is observed between the quantum Hall phases with the filling factors $\ensuremath{\nu}=1$ and 2, and it is attributed to the formation of the metallic intermediate phase, which causes increasing overlap between the electron and hole wave functions.

Semiconductor nanowires (NWs) are a candidate technology for future optoelectronic devices. One of the critical issues in NWs is the control of impurity doping for the formation of p-n junctions. In this study, beryllium (p-type dopant) and tellurium (n-type dopant) in self-assisted GaAs NWs was studied. The GaAs NWs were grown on (111) Si by molecular beam epitaxy using the self-assisted method. The dopant incorporation in the self-assisted GaAs NWs was investigated using Raman spectroscopy, photoluminescence, secondary ion mass spectrometry and electron holography. Be-doped NWs showed similar carrier concentration as compared to thin film (TF) standards. However, Te-doped NWs showed at least a one order of magnitude lower carrier concentration as compared to TF standards. Dopant incorporation mechanisms in NWs are discussed.

The morphologies of self-assisted GaP nanowires grown by gas source MBE in regular arrays on silicon substrates are modeled by a kinetic equation for the nanowire radius versus the position along the nanowire axis. The most important growth parameter that governs the nanowire morphology is the V/III flux ratio. Sharpened nanowires with a stable radius equal to only 12 nm at a V/III flux ratio of 6 are achieved, demonstrating their suitability for the insertion of quantum dots.

A new dopant incorporation mechanism in Ga-assisted GaAs nanowires grown by molecular beam epitaxy is reported. Off-axis electron holography revealed that p-type Be dopants introduced in situ during molecular beam epitaxy growth of the nanowires were distributed inhomogeneously in the nanowire cross-section, perpendicular to the growth direction. The active dopants showed a remarkable azimuthal distribution along the (111)B flat top of the nanowires, which is attributed to preferred incorporation along 3-fold symmetric truncated facets under the Ga droplet. A diffusion model is presented to explain the unique radial and azimuthal variation of the active dopants in the GaAs nanowires.

We utilize rigorous coupled wave analysis (RCWA) to optimize the optical design of of GaAs nanowire solar cells. RCWA is highly accurate while being less computationally expensive than competing techniques. Using a simplex optimization method, we determine the optimal device geometries that maximize photocurrent density for bare nanowires and contacted nanowires in a dielectric material with a top ITO contact, finding agreement with previous work. We extend these results to include passivating shells, which are required for efficient devices. These optimizations give important guidance for geometric design of nanowire solar cell devices.