Epitaxy of semiconductor-superconductor nanowires.

Due to increasing demand of nanowires (NWs) in the areas of electrical and photonic devices applications, it is imperative to try to improve their properties that are found to degrade their device performances. This thesis provides an extensive investigation of InAs based NWs for use in the photodetection applications. To achieve this goal, the structural and optical properties of InAs NWs and InAs NW-based quantum materials (e.g., InAsSb NWs, InAs/AlSb NWs and InAs/GaSb core-shell NWs) have been investigated. The NW samples were grown by molecular beam epitaxy (MBE); self-catalysed droplet epitaxy was used as the growth mechanism for them. To improve the optical properties of InAs NWs, we further optimised the MBE growth for the NWs based on our previous growth understanding, which resulted in InAs NWs give strong room temperature photoluminescence (PL) at room temperature. We also notified that the substrate doping type gives significant effect on PL emission. In order to improve the axial growth rate for the epitaxy of NW devices, we developed a new 3-step growth technique: step 1 – droplet seeding, step – 2: NW growth initiation, step - 3 NW growth at high growth rate. The above new growth method yielded high quality InAs NWs with much higher axial growth rate compared to conventional growth methods. This method offers cost-effectiveness and reduction in time consumption. The NW samples grown by this method demonstrated denser, longer and enhanced optical properties. The thesis also studied the incorporation of Sb into the InAs NWs to synthesis InAsSb NWs which is very challenging but has many interesting device applications. Through comprehensive growth study, we demonstrated the realization of InAsSb NWs with increased Sb content through reducing growth rate of the NWs. For the first time, we produced optically active InAsSb NWs with 19% of Sb content which gives PL emission at a long wavelength of 5.1 훍m at 10 K. This achievement reveals that our InAsSb NWs could be used for infrared photodetectors and emitters operating in entire mid wavelength infrared spectral range (MWIR), e.g. 3-5 훍m. It is well known that NW structures suffer from severe surface states which degrade the resulting devices performance, due to the large surface to volume ratio. To overcome this problem, in this thesis, we developed several advanced NW-based quantum materials with a core-shell structure (heterojunction), e.g., the NW core is passivated with shell layer of different material. Two novel core-shell NW materials namely, InAs/AlSb and InAs/GaSb were grown. Transmission electron microscopy (TEM) and electron diffraction x-ray (EDX) confirmed the success of the core-shell structure. More importantly, the PL study indicates a massive enhancement in the PL emission, by 5 times in comparison with the bare InAs NWs. Temperature dependence PL measurements proved that surface states were significantly eliminated. Our core-shell NW structure is a great success in surface passivation. Mesa device of bare InAs NWs were fabricated and its current-voltage (I-V) were tested in room temperature. I-V measurements showed that the NW device is a working device with relatively high dark current of 0.0011A and negative photocurrent of (-8.9526×104A). Single NW field effect transistors (FET) were fabricated using bare InAs NWs and InAs/ AlSb core-shell NWs and operated as photodetectors. The dark current measurements reveal that the bare InAs NW device gives a high dark current 1.5×10-6 A, while the core-shell NW device has a much more supressed dark current of 2.8×10-8 A, which is 186 times less than bare InAs NW device. This is a further evidence of the surface passivation induced by the shell layer, which is important for fabricating photodetectors with high directivity. The photocurrent study show that the bare InAs device gives a photocurrent of 0.2 um leading to a signal-to-noise ratio of 13%, while core-shell NW devices exhibited anomalous photocurrent behaviour, e.g. The device gives a signal-to-noise ratio of 80%, which is 6 times higher than the bare InAs NW device. These striking device properties were attributed to the efficient surface passivation caused by the shell layer of the materials. The anomalous photocurrent behavior was attributed to the trap centres in the shell layer. Our study demonstrates the great potential of the core-shell structure in the use of highly efficient infrared photodetectors.

In recent decades, extensive research has been engaged into III-V semiconductor nanowires. It has been widely recognized that they have promising applications for electronic and optoelectronic devices due to their intrinsic material and unique geometry. Currently, the controllable growth of binary III-V nanowires can be achieved via Au-catalysed vapor-liquid-solid method in both metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) system. Conventionally, the key parameters for tuning III-V nanowires’ growths include the growth temperatures, precursor flow rates and the catalyst sizes. However, it has been found that the component addition, i.e. dopants and III/V incorporation, also influenced the controllable growth of III-V nanowires. Therefore, it is meaningful to understand this effect and its underlying influencing mechanism.Doping is a conventional way to introduce impurity atoms into host semiconductor to improve material properties by inducing free electrons or vacancies. The incorporation of impurity atoms can induce electrical property improvement, meanwhile may make the nanowire growth process more complex. However, the effect of doping on nanowire growth is still unclear. In this project, Sn-doped Au-catalysed GaAs nanowires were synthesized in MOCVD with a range of growth parameter including varied growth temperatures and tetraethyl-tin flow rates. The systematic characterizations on as-grown nanowires were carried out by electron microscopy. It was found that the Sn addition influenced the nanowires growth rate and structural quality in different ways at different growth temperatures.Suitable and tuneable bandgaps of ternary III-V nanowires make them suitable for electronic and optoelectronic devices, such as light emission diodes and tandem solar cells. However, elemental segregations have been a long-standing issue in the ternary nanowires, which could induce the core-shell heterostructures in the nanowires. The formation mechanism of core and shell need to be further clarified in order to control the compositional configuration in nanowires, which is meaningful for optimizing their performances in devices. In this project, InGaP nanowires were synthesized in MOCVD and their morphological, structural and compositional characteristics were investigated systematically by electron microscopy. The formation mechanism of the core-shell structure and the axial compositional gradient were discussed. Furthermore, the effect of catalyst size on the growth of hierarchical structured InGaP nanowires was demonstrated.

InAs is an important III-V material for mid-infrared applications due to its unique properties such as direct narrow bandgap (Eg ~ 0.36 eV at 300 K), high electron mobility (~33,000 cm2/Vs at 300 K), and low ohmic contact resistivity. In nanowire (NW) geometry, it can be monolithically integrated onto the mature, commercially dominant Si CMOS platform, showing great promise as building blocks for next generation mid-infrared electronic and optoelectronic devices. This thesis mainly investigates the molecular beam epitaxial (MBE) growth and optical properties of InAs-based nanowires. Two types of catalyst-free NWs were grown by MBE: selective area InAs NWs on Si, utilizing an electron-beam lithographically patterned silicon nitride mask; and randomly-positioned InAs NWs on un-patterned Si substrates. Details of both growth techniques were presented. A diameter series of selective-area InAs NWs, and a diameter series, a length series of randomly-nucleated InAs/InAlAs core shell NWs were fabricated for subsequent optical measurements. The development of functional NW devices requires a thorough knowledge in the NW carrier recombination dynamics. However, there is no publication which thoroughly resolved the three recombination processes − Shockley-Read-Hall (SRH), radiative and Auger − in InAs-based core-shell NWs. To fill this gap, ultrafast pump probe spectroscopy and external quantum efficiency measurement were performed on selective-area InAs/InAlAs core-shell NWs grown on Si, from which their SRH, radiative and Auger coefficients were extracted. It is discovered that the InAs/InAlAs NWs have a very low Auger rate (ten-fold smaller than planar zincblende InAs) and a high radiative rate, which results in a high estimated 77 K peak internal quantum efficiency of 22%. This suggests that InAs-based NWs show promise as high efficiency mid-infrared emitters. The Shockley-Read-Hall defect assisted recombination rates from different parts of the InAs-based NWs have never been completely separated out. To address this problem, ultrafast measurements were performed on two sets of randomly-nucleated InAs/InAlAs NWs grown on Si, independently varying the NW length and diameter. The carrier recombination rates at various NW regions: end facets, sidewall, and interior were disentangled. It is discovered that the carrier recombination in the NW interior is non-trivial compared to the surface recombination, especially at 293 K. Surface recombination is dominated by carrier recombination on the InAs/InAlAs NW sidewall, while contributions from the impure, highly-strained base are negligible. The surface and interior recombination rates in the selective-area InAs NWs grown on lithographically patterned Si substrates were also resolved. We found that even without an InAlAs shell, the 77 K surface recombination velocity was slightly smaller than that of the randomly-nucleated InAs/InAlAs core-shell NWs. In addition, an exceptionally long InAs NW interior minority carrier lifetime of 8.7\ ns was measured. Transmission electron microscopy showed a high density of stacking fault defects within the NWs, suggesting that interior recombination lifetime can be further prolonged by improving NW interior crystal quality. Finally, to explore the material properties of the GaInAsSb quaternary alloy, two GaInAsSb detectors with 2.6 µm cutoff wavelength, and p-doped to different levels of 3×1016 cm-3 and 3×1017 cm-3 were grown by MBE, fabricated and characterized. In the limit of infinite mesa area, R0A was extrapolated to be 21.1Ω-cm2 for p-3×1016 cm-3 detector, and 95.4Ω-cm2 for p-3×1017 cm-3 detector. Measurements showed that detector p-doped to 3×1017 cm-3 have overall better performance than detector p-doped to 3×1016 cm-3, which makes it a more preferable absorber doping level choice for the 2.6 µm GaInAsSb detector.

Nanowires are rod or whisker-like structures with length on the order of microns and diameter from tens to hundreds of nanometers. They represent a new class of three-dimensional materials, and the next step in the evolution of conventional two-dimensional thin films, quantum wells, or heterostructures. Nanowires are typically fabricated by the assistance of foreign metal catalysts, such as Au, that collect deposited material, resulting in localized growth of nanowires. However, the use of foreign metal particles can result in contamination of the nanowires and reduction in the carrier lifetime, which degrades device performance. In the present work, we present the self-assisted growth of GaP nanowires by molecular beam epitaxy, using Ga droplets as a seed particle without the use of any foreign metal catalysts. Growth of the nanowire occurs by selective-area epitaxy using a patterned array of holes in an SiOx mask. The holes collect Ga adatoms forming a Ga droplet that seeds the nanowire growth. The size of the Ga droplet can be controlled by a novel evaporation process, resulting in ultra-thin nanowire structures. GaAs heterostructures were introduced into the GaP nanowires during growth resulting in quantum dots. The quantum dots are encapsulated in GaP, resulting in passivation of the QD surfaces. Photoluminescence emission was observed from the QDs in the visible range. The emission wavelength is tunable by the size or composition of the QDs. This process results in controlled luminescence emission with application in single photon sources, light emitting diodes, or laser diodes.

Abstractmagnified imageSignificant substrate‐dependent variations in germanium nanowire (NW) growth by molecular beam epitaxy (MBE) were found, although nominally identical substrates were used. To investigate this phenomenon, the surfaces of Ge(111) wafers polished with different polishing cloths and different pressures during planarization were studied. Characterization by means of atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed differences in surface roughness, which has an influence on the respective catalyst droplets' areal densities. These differences, however, manifest on larger scales and can only be determined by statistical evaluation of sufficiently large areas on the sample. This led to the conclusion, that even minor surface irregularities from chemical‐mechanical polishing (CMP) on otherwise atomically flat substrates can have a paramount influence on NW growth, if the scale on which these irregularities occur matches the scale or distance of the used catalyst droplets. It is assumed that small protrusions on the surface serve as pinning points to the droplets. This behavior promotes the vertical growth of NWs and avoids lateral movement of droplets or classical island formation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Group-III Nitrides Catalyzed Transformations of Organic Molecules

It is a real pleasure and honor for me to write this preface to the festschrift in honor of Marc Ilegems on the occasion of his 65th birthday. As all of his collaborators, I have always had a great pleasure to work with Marc and appreciated a lot his competence, dedication and subtlety in all matters. It has been very difficult, among the amount of work carried out by Marc throughout his carrier, to choose a topic that would allow for a self‐contained book. It would have been possible for example to center the book on GaN based structures and devices. I however used my privilege, as a guest editor, to choose the field where my own collaboration with the group of Marc has been the most fruitful, the field of semiconductor microcavities. Marc Ilegems was born in Antwerp, Belgium, on April 19th 1940. He studied at the Ecole polytechnique of the Free University of Brussels (ULB) where he graduated in July 1963 with “ Great Distinction ” and got a degree in Electrical Engineering. He obtained further a Certificate in Telecommunications in July 1964. From October 1963 to September 1964, and, after an interruption for military service, from January 1965 to August 1965, he was employed as a research assistant in the Laboratory for Electronics of the University of Brussels, working on parametric frequency division using nonlinear capacitive elements. He was awarded a fellowship from the Fullbright Foundation in May 1966 and enrolled in the Master's degree program at Stanford University, California, from September 1966 to June 1967. In July 1967 he started his doctoral research under Professor Gerald L. Pearson in the Electrical Engineering Department, working on the growth by liquid phase epitaxy of AlGaAs mixed crystals and on the study of the infrared reflectance of Al x Ga 1– x As crystals. The results of this work 1 , 2 , published in 1969 and 1970, were amongst the very first publications on this new and very important semiconducting material. His results have been validated by numerous later studies and publications on the subject. In September 1969, he has been enrolled as a Member of Technical Staff at the Solid State Electronics Research Laboratory, Bell Laboratories, Murray Hill, in the group directed at the time by Dr. Morton B. Panish. Over the time period 1969–1972 he worked on the study of the liquid–solid phase diagrams of ternary and quaternary semiconducting compounds in collaboration with M. B. Panish and A. Jordan 3 , 4 . This was both experimental and modeling work. In parallel, he worked on the vapor phase growth of GaP, and, following the publication of the first paper by J. Pankove and coworkers at the RCA Laboratories on the growth of GaN, on the vapor phase epitaxy of gallium‐nitride compounds 5 . The work on the nitrides, in collaboration essentially with R. Dingle, D. Sell and H. Montgomery, led to the first detailed papers on the luminescent properties 6 and the bandstructure of GaN 7 paving the way to the present use of GaN for blue light emitters. The first temperature dependent electrical characterizations of GaN epitaxial layers published in 1972 8 , reported mobility values of 440 and 780 cm 2 /Vs at room temperature and 55 K, respectively, values which have stood as absolute records in the literature for over 20 years. Because of the inability to achieve p‐type doping in GaN at the time, the emphasis on the work in the 1972 to 1977 period shifted to the growth and characterization of GaAs and AlGaAs materials by the then new technique of molecular beam epitaxy (MBE) initiated by Al Cho of the same laboratory. Significant results from this work were the first demonstration of the use of beryllium as a p‐type dopant in GaAs 9 as it is still used worldwide today, the first demonstration of a load‐lock system for MBE growth 10 , allowing for the first time to obtain AlGaAs layers with high luminescent efficiencies, the first demonstration of selective area growth by MBE using shadow masks 11 , and the first demonstration, with J. P. Van der Ziel, of the growth of monolithic GaAs/AlAs Bragg mirrors 12 , which six years later have become the key elements in the development of vertical cavity surface emitting lasers (VCSELs) and opened up the field of semiconductor planar microcavities for quantum optics. In September 1977, Marc Ilegems joined the Ecole Polytechnique Fédérale de Lausanne (EPFL) as an associate Professor and Director of the newly created Interdepartmental Institute for Microelectronics, with the dual mission to establish a physics and technology oriented microelectronics curriculum for students in Lausanne and to develop a research program complementary to and/or in cooperation with the existing Microelectronics R&D programs developed in the Centre Electronique Horloger in Neuchâtel. In 1988, the Interdepartmental Institute for Microelectronics merged with the Optoelectronics Laboratory headed by Prof. F. K. Reinhart to form the new Institute for Micro and Opto‐electronics. Marc Ilegems has been the director of this institute for 10 years, from 1988 till 1998 before he became the head of the physics department. The direction of Marc Ilegems has always been smooth and very efficient. From 1998 to 2000, M. Ilegems served as the Head of the Department of Physics at EPFL, again with great competence and dedication. He then has taken the responsibility of the Swiss National Center of Competence in Research in Quantum Photonics, which groups together the competencies of the leading laboratories in Switzerland in the field of photonics. During his time at EPFL the research carried out by the group of Marc Ilegems has first concentrated on the field of silicon technology with a large program around the modeling and optimization of short channel length CMOS transistors, hot electron injection, trapping and breakdown in non‐volatile electrically programmable memories, and low frequency noise measurements as a tool to study degradation in MOSFETs. Marc has also launched a research in III–V materials and devices initially centered around the application of the molecular beam (MBE) and chemical beam epitaxy (CBE) techniques, with activities directed, on the one hand, towards materials growth and characterization, and on the other hand to the realization and study of a wide variety of electronic and optoelectronic devices. Examples of the work carried out, in general in the framework of doctoral thesis projects, include: – study of doping, oxygen incorporation, and deep level defects in GaAs and AlGaAs layers and high electron mobility transistors (HEMTs) using transient capacitance and low frequency noise measurements, – development of the CBE and selective area growth techniques for GaInAs and GaInAsP‐based quantum well structures and diode lasers 13 , – realization and study of high speed GaAs and GaInAs‐based photodetectors at 850 to 1300 nm wavelengths, 2D electron gas high electron mobility transistors (HEMTs) based on pseudomorphic GaInAs on GaAs or lattice matched GaInAs on InP substrates, and GaAs‐based quantum Hall resistance standards, – realization of high finesse semiconductor microcavities with distributed Bragg reflectors 14 and study of their luminescent properties in the cavity‐polariton regime, demonstration of room temperature cavity polaritons 15 , – development of high brightness microcavity light emitting diodes 16 , – study of coupled cavities and demonstration of dual‐wavelength lasing in coupled microcavities 17 . Starting in 1997, the research interest partially shifted to the growth and study of nitride‐based wide bandgap materials and devices. Research topics included the growth of thick GaN layers by hydride vapor phase epitaxy to be used as pseudo‐substrates, and the achievement of reduced defect densities by using epitaxial lateral overgrowth 18 , the growth by metal organic vapor phase epitaxy of GaN‐based light emit

The goal of this thesis is to master the synthesis of GaAs nanowires ensembles on Si for their application in solar cells. Semiconductor nanowires present promising characteristics for photovoltaic applications: they benefit from their longitudinal high aspect ratio geometry to enhance light absorption, minimize material consumption and efficiently collect the carriers. To fully unleash their potential, the following properties have to be controlled: number density, diameter and orientation. The latter is of utmost importance to have uniform junctions and to avoid leakages/shortcuts, whereas number density and diameter allow to tune light absorption and minimize material utilization. Our nanowires have been grown by molecular beam epitaxy (MBE), a well-known technique for the high crystalline quality and atomically sharp interfaces in thin film applications. Moreover, to develop a scalable technique and to avoid any possible contamination we used a self-assembly and self-catalyzed approach, which involves only Ga and As, without any patterning of the surface. In a first place we studied the occurrence of GaAs nanowires growth for different types of silicon oxides, such as thermal oxide, native oxide and hydrogen silsesquioxane (HSQ). We determined the critical thicknesses to achieve nanowire growth and investigated the influence of surface roughness. This comparison study lead us to choose native oxide as oxide of choice for GaAs nanowires growth on Si. With this type of oxide, reproducibility and uniformity of results outpaced the others. Successively we developed a simple technique to control native oxide thickness and characterized the chemical composition and wetting. Once the behavior of the oxide properties as a function of oxide thickness was clarified we studied their influence over nanowire growth. We found that impacted the overall possibility of nanowires growth and to control their orientation with respect to the substrate. The root cause of the change in growth morphology was identified to be in the different thermal stability of the oxides with different compositions, and the wetting properties. The understanding of the influence of the surface properties over nanowires nucleation was of paramount importance to achieve reproducible, uniform and scalable growth of vertical nanowires. Once full control over the substrate was achieved, we investigated the tailoring of diameter and density by growth conditions using the self-assembly of Ga droplets. We demonstrated an approach to tailor diameter-density distribution that minimize nanowires-array reflectivity. These results give a clear pathway on how to obtain fully controlled nanowires growth in terms of diameter, density and orientation, paving the way to the development of GaAs nanowires based solar cells on Si.

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.

Abstractmagnified imageThe growth of self‐catalyzed InAs nanowires on Si(111) substrates via vapour–solid (VS) and vapour–liquid–solid (VLS) growth mechanisms is investigated using molecular beam epitaxy. For both mechanisms, the substrate preparation plays a crucial role. In this context, the required thin oxide layer for the VS growth of the nanowires is obtained by treating the HF‐cleaned Si substrate with hydrogen peroxide. For the VLS growth, Ga is predeposited on the unprocessed Si substrate. The Ga forms droplets, which etch the native oxide and create the necessary pinholes.magnified image(© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications.

For the Ga-assisted growth of GaAs nanowires on Si(111) substrates by molecular beam epitaxy, growth temperature, As flux, and Ga flux have been systematically varied across the entire window of growth conditions that result in the formation of nanowires. A range of GaAs structures was observed, progressing from pure Ga droplets under negligible As flux through horizontal nanowires, tilted nanowires, vertical nanowires, and nanowires without droplets to crystallites as the As flux was increased. Quantitative analysis of the resulting sample morphology was performed in terms of nanowire number and volume density, number yield and volume yield of vertical nanowires, diameter, length, as well as the number and volume density of parasitic growth. The result is a growth map that comprehensively describes all nanowire and parasitic growth morphologies and hence enables growth of nanowire samples in a predictive manner. Further analysis indicates the combination of global Ga flux and growth temperature determines the total density of all objects, whereas the global As/Ga flux ratio independently determines the resultant sample morphology. Several dependencies observed here imply that all objects present on the substrate surface, i.e. both nanowires and parasitic structures, originate from Ga droplets.

On the growth of InAs nanowires by molecular beam epitaxy

The control of optical and transport properties of semiconductor heterostructures is crucial for engineering new nanoscale photonic and electrical devices with diverse functions. Core–shell nanowires are evident examples of how tailoring the structure, i.e., the shell layer, plays a key role in the device performance. However, III–V semiconductors bandgap tuning has not yet been fully explored in nanowires. Here, a novel InAs/AlSb core–shell nanowire heterostructure is reported grown by molecular beam epitaxy and its application for room temperature infrared photodetection. The core–shell nanowires are dislocation‐free with small chemical intermixing at the interfaces. They also exhibit remarkable radiative emission efficiency, which is attributed to efficient surface passivation and quantum confinement induced by the shell. A high‐performance core–shell nanowire phototransistor is also demonstrated with negative photoresponse. In comparison with simple InAs nanowire phototransistor, the core–shell nanowire phototransistor has a dark current two orders of magnitude smaller and a sixfold improvement in photocurrent signal‐to‐noise ratio. The main factors for the improved photodetector performance are the surface passivation, the oxide in the AlSb shell and the type‐II bandgap alignment. The study demonstrates the potential of type‐II core–shell nanowires for the next generation of photodetectors on silicon.

Self‐catalyzed GaAs nanowire (NW) growth via the vapor–liquid–solid mechanism is investigated by a theoretical model including the kinetics of material transport inside the catalyst droplet. The proposed model allows the description of nucleation and growth of 2D islands on the top facet of GaAs NWs. Analytical expressions for the growth rate of the disk‐shaped GaAs island due to the volume diffusion of species in the droplet and for the attachment rate of GaAs pairs to the critical island are derived. As a result, the duration of the droplet refilling stage and the island growth stage at typical growth conditions of self‐catalyzed GaAs NWs are obtained by a self‐consistent calculation. Also, the time evolution of the droplet composition and the island radius are found. The derived equations for the island growth rate can be applied for modeling of catalyst‐assisted growth of other III–V compounds. The results of the modeling are in good agreement with the experimental data on self‐catalyzed GaAs NW growth via molecular beam epitaxy and can be used for the optimization of the NW growth conditions.