Self-catalyzed GaAs Nanowires Research Articles

Crystal phase control in self-catalyzed GaAs nanowires grown on pre-etched Si substrates

The size and density of the etched pinholes on the Si substrate influence the equivalent V/III flux ratio, which enables the growth of high-quality GaAs nanowires.

Self-consistent modeling of MBE self-catalyzed GaAs nanowire growth

Self-catalyzed GaAs nanowires are synthesized by molecular beam epitaxy at various arsenic fluxes and growth temperatures. The growth of GaAs nanowires is simulated considering the kinetics of material transport inside the catalyst droplet. The re-evaporation coefficient of arsenic is estimated for the given growth conditions. Calculated nanowire growth rate is in satisfactory agreement with the experimental data.

In Situ Monitoring of MBE Growth of a Single Self-Catalyzed GaAs Nanowire by X-ray Diffraction

In Situ Monitoring of MBE Growth of a Single Self-Catalyzed GaAs Nanowire by X-ray Diffraction

Structural and optical properties of Be-doped high-quality self-catalyzed GaAs nanowires

Crystal-phase control and crystalline quality improvement of GaAs nanowires (NWs) have been realized by dopant (Be) incorporation in GaAs NWs. We demonstrate the improvement of crystalline quality by X-ray diffraction (XRD) spectra combined with high-resolution transmission electron microscopy (HRTEM). The crystal-phase control from the wurtzite (WZ)/zinc blende (ZB) mixed phase to the pure ZB phase under the effect of Be doping is clearly revealed by Raman spectra combined with HRTEM. The photoluminescence (PL) revealed the free exciton and WZ/ZB type-II emission peaks of undoped GaAs NWs transform into Be impurity-related emission peak of Be-doped GaAs NWs.

Simultaneous Selective Area Growth of Wurtzite and Zincblende Self-Catalyzed GaAs Nanowires on Silicon.

Selective area epitaxy constitutes a mainstream method to obtain reproducible nanomaterials. As a counterpart, self-assembly allows their growth without costly substrate preparation, with the drawback of uncontrolled positioning. We propose a mixed approach in which self-assembly is limited to reduced regions on a patterned silicon substrate. While nanowires grow with a wide distribution of diameters, we note a mostly binary occurrence of crystal phases. Self-catalyzed GaAs nanowires form in either a wurtzite or zincblende phase in the same growth run. Quite surprisingly, thicker nanowires are wurtzite and thinner nanowires are zincblende, while the common view predicts the reverse trend. We relate this phenomenon to the influx of Ga adatoms by surface diffusion, which results in different contact angles of Ga droplets. We demonstrate the wurtzite phase of thick GaAs NWs up to 200 nm in diameter in the Au-free approach, which has not been achieved so far to our knowledge.

Microstructural evolution in self-catalyzed GaAs nanowires during in-situ TEM study

The microstructural evolutions in self-catalyzed GaAs nanowires (NWs) were investigated by using in situ heating transmission electron microscopy (TEM). The morphological changes of the self-catalyst metal gallium (Ga) droplet, the GaAs NWs, and the atomic behavior at the interface between the self-catalyst metal gallium and GaAs NWs were carefully studied by analysis of high-resolution TEM images. The microstructural change of the Ga-droplet/GaAs-NWs started at a low temperature of ∼200 °C. Formation and destruction of atomic layers were observed at the Ga/GaAs interface and slow depletion of the Ga droplet was detected in the temperature range investigated. Above 300 °C, the evolution process dramatically changed with time: The Ga droplet depleted rapidly and fast growth of zinc-blende (ZB) GaAs structures were observed in the droplet. The Ga droplet was completely removed with time and temperature. When the temperature reached ∼600 °C, the decomposition of GaAs was detected. This process began in the wurtzite (WZ) structure and propagated to the ZB structure. The morphological and atomistic behaviors in self-catalyzed GaAs NWs were demonstrated based on thermodynamic considerations, in addition to the effect of the incident electron beam in TEM. Finally, GaAs decomposition was demonstrated in terms of congruent vaporization.

Зависимость скорости роста и структуры III-V нитевидных нанокристаллов от площади сбора адатомов на поверхности подложки

A theoretical analysis is presented for the growth rate and structure of III-V nanowires depending on the collection area of group III adatoms on the substrate surface. An expression for the maximum possible nanowire vertical growth rate is obtained and different reasons are analyzed for its suppression in different technologies. It is shown that the maximum growth rate is proportional to the collection area and inversely proportional to the squared nanowire radius. It is demonstrated that self-catalyzed GaAs nanowires grow or shrink radially at large or small adatom collection areas, respectively, having the zincblende crystal phase in both cases. The wurtzite phase forms in GaAs nanowires growing only axially at the intermediate adatom collection areas.

Reasons of Crystallite Formation during the Self-Catalyzed GaAs Nanowire Growth

During the self-catalyzed GaAs nanowire growth formation of parasitic GaAs crystallites is observed. The reasons for crystallite formation are explained on the base of Monte Carlo simulation results. During simultaneous deposition of gallium and arsenic on the GaAs(111)B substrate coated by a silicon oxide film, liquid gallium droplets nucleate on the oxide surface. After nucleation, droplets enlarge in size with time and etch the oxide layer. Formation of GaAs crystal structures becomes possible only after the Ga droplet contacts underlying crystal substrate. It is shown that excessively high arsenic and gallium deposition rates lead to the crystallite formation at the initial growth stage. The GaAs crystallites collect part of the deposited gallium and arsenic decreasing their surface concentration, thereby, adjusting the growth conditions for the nanowire growth. Therefore, during the self-catalyzed GaAs nanowire growth the self-regulation of growth conditions takes place.

Modeling the dynamics of interface morphology and crystal phase change in self-catalyzed GaAs nanowires

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.

Low-Temperature In-Induced Holes Formation in Native-SiOx/Si(111) Substrates for Self-Catalyzed MBE Growth of GaAs Nanowires.

The reduction of substrate temperature is important in view of the integration of III–V materials with a Si platform. Here, we show the way to significantly decrease substrate temperature by introducing a procedure to create nanoscale holes in the native-SiOx layer on Si(111) substrate via In-induced drilling. Using the fabricated template, we successfully grew self-catalyzed GaAs nanowires by molecular-beam epitaxy. Energy-dispersive X-ray analysis reveals no indium atoms inside the nanowires. This unambiguously manifests that the procedure proposed can be used for the growth of ultra-pure GaAs nanowires.

Interatomic potential for atomistic simulation of self-catalyzed GaAs nanowires growth

Semiconductor nanowires are long time candidates for numerous applications. Complete understanding of their most common growth process (vapour-liquid-solid), however, is essential in order to achieve the crystalline quality that applications demand. To date, most of the information gathered to understand vapour-liquid-solid growth comes from ex situ and in situ electron microscopy images. Still, those approaches might miss some important features due to time and length scales involved. In this sense, computational simulations can provide additional insights into the process. Yet, so far, no computational simulation was able to consistently reproduce the growth of more than one monolayer of a semiconductor nanowire through the vapour-liquid-solid method. By using a force matching software (PotFit), we developed a new GaAs interatomic potential for molecular dynamics simulations. Our potential can reproduce several features of the growth of self-catalyzed GaAs nanowires, including: formation of zinc blende and wurtzite layers, layer-by-layer formation of the solid phase, b-polar surface growth preference and the dependency between growth rate and precursor flow.

Phase Selection in Self-catalyzed GaAs Nanowires.

Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.

Crystal phase engineering of self-catalyzed GaAs nanowires using a RHEED diagram.

It is well known that the crystalline structure of the III–V nanowires (NWs) is mainly controlled by the wetting contact angle of the catalyst droplet which can be tuned by the III and V flux. In this work we present a method to control the wurtzite (WZ) or zinc-blende (ZB) structure in self-catalyzed GaAs NWs grown by molecular beam epitaxy, using in situ reflection high energy electron diffraction (RHEED) diagram analysis. Since the diffraction patterns of the ZB and WZ structures differ according to the azimuth [11̄0], it is possible to follow the evolution of the intensity of specific ZB and WZ diffraction spots during NW growth as a function of the growth parameters such as the Ga flux. By analyzing the evolution of the WZ and ZB spot intensities during NW growth with specific changes of the Ga flux, it is then possible to control the crystal structure of the NWs. ZB GaAs NWs with a controlled WZ segment have thus been realized. Using a semi-empirical model for the NW growth and our in situ RHEED measurements, the critical wetting angle of the Ga catalyst droplet for the structural transition is deduced.

Does desorption affect the length distributions of nanowires?

State-of-the art models for statistical properties within the nanowire ensembles consider influx of precursors, reflection and surface diffusion of adatoms. These models predict a delay in the nanowire growth start and the evolution toward an asymmetric length distribution. We demonstrate here the effect of desorption of the nanowire material, which has not been considered so far in studies of the nanowire length distributions. We show that at the very beginning of growth the length distribution should be asymmetric due to the slow nucleation of nanowires. At longer times, the length distribution acquires a symmetric Gaussian shape due to the increased weight of desorption. The width of this distribution is larger than Poissonian and increases for higher ratio of desorption over deposition rate. Our model is consistent with the length evolution of organized self-catalyzed GaAs nanowires. We outline that desorption of the nanowire material should be minimized to achieve arrays of highly identical nanowires. These results are relevant for a wide variety of material systems.

Te incorporation and activation as n -type dopant in self-catalyzed GaAs nanowires

Dopant atoms can be incorporated into nanowires either via the vapor-liquid-solid mechanism through the catalyst droplet or by the vapor-solid growth on the sidewalls. Si is a typical n-type dopant for GaAs, but in nanowires it often suffers from a strongly amphoteric nature in the vapor-liquid-solid process. This issue can be avoided by using Te, which is a promising but less common alternative for $n$-type doping of GaAs nanowires. Here, we present a detailed investigation of Te-doped self-catalyzed GaAs nanowires. We use several complementary experimental techniques, such as atom probe tomography, off-axis electron holography, micro-Raman spectroscopy, and single-nanowire transport characterization, to assess the Te concentration, the free-electron concentration, and the built-in potential in Te-doped GaAs nanowires. By combing the experimental results with a theoretical model, we show that Te atoms are mainly incorporated by the vapor-liquid-solid process through the Ga droplet, which leads to both axial and radial dopant gradients due to Te diffusion inside the nanowires and competition between axial elongation and radial growth of nanowires. Furthermore, by comparing the free-electron concentration from Raman spectroscopy and the Te-atom concentrations from atom probe tomography, we show that the activation of Te donor atoms is $100%$ at a doping level of $4\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, which is a significant result in terms of future device applications.

Axial GaAs/Ga(As, Bi) nanowire heterostructures

Bi-containing III-V semiconductors constitute an exciting class of metastable compounds with wide-ranging potential optoelectronic and electronic applications. However, the growth of III-V-Bi alloys requires group-III-rich growth conditions, which pose severe challenges for planar growth. In this work, we exploit the naturally-Ga-rich environment present inside the metallic droplet of a self-catalyzed GaAs nanowire (NW) to synthesize metastable GaAs/GaAs1−xBix axial NW heterostructures with high Bi contents. The axial GaAs1−xBix segments are realized with molecular beam epitaxy by first enriching only the vapor–liquid–solid (VLS) Ga droplets with Bi, followed by exposing the resulting Ga-Bi droplets to As2 at temperatures ranging from 270 °C to 380 °C to precipitate GaAs1−xBix only under the NW droplets. Microstructural and elemental characterization reveals the presence of single crystal zincblende GaAs1−xBix axial NW segments with Bi contents up to (10 ± 2)%. This work illustrates how the unique local growth environment present during the VLS NW growth can be exploited to synthesize heterostructures with metastable compounds.

Impact of the Shadowing Effect on the Crystal Structure of Patterned Self-Catalyzed GaAs Nanowires.

The growth of regular arrays of uniform III-V semiconductor nanowires is a crucial step on the route toward their application-relevant large-scale integration onto the Si platform. To this end, not only does optimal vertical yield, length, and diameter uniformity have to be engineered, but also, control over the nanowire crystal structure has to be achieved. Depending on the particular application, nanowire arrays with varying area density are required for optimal device efficiency. However, the nanowire area density substantially influences the nanowire growth and presents an additional challenge for nanowire device engineering. We report on the simultaneous in situ X-ray investigation of regular GaAs nanowire arrays with different area density during self-catalyzed vapor-liquid-solid growth on Si by molecular-beam epitaxy. Our results give novel insight into selective-area growth and demonstrate that shadowing of the Ga flux, occurring in dense nanowire arrays, has a crucial impact on the evolution of nanowire crystal structure. We observe that the onset of Ga flux shadowing, dependent on array pitch and nanowire length, is accompanied by an increase of the wurtzite formation rate. Our results moreover reveal the paramount role of the secondary reflected Ga flux for VLS NW growth (specifically, that flux that is reflected directly into the liquid Ga droplet).

Gradients of Be-dopant concentration in self-catalyzed GaAs nanowires

Effective and controllable doping is instrumental for enabling the use of III–V semiconductor nanowires (NWs) in practical electronics and optoelectronics applications. To this end, dopants are incorporated during self-catalyzed growth via vapor–liquid–solid mechanism through the catalyst droplet or by vapor–solid mechanism of the sidewall growth. The interplay of these mechanisms together with the competition between axial elongation and radial growth of NWs can result in dopant concentration gradients along the NW axis. Here, we report an investigation of Be-doped p-type GaAs NWs grown by the self-catalyzed method on lithography-free Si/SiOx templates. The influence of dopant incorporation on the structural properties of the NWs is analyzed by scanning and transmission electron microscopy. By combining spatially resolved Raman spectroscopy and transport characterization, we are able to estimate the carrier concentration, mobility and resistivity on single-NW level. We show that Be dopants are incorporated predominantly by vapor–solid mechanism for low Be flux, while the relative contribution of vapor–liquid–solid incorporation is increased for higher Be flux, resulting in axial dopant gradients that depend on the nominal doping level.

X-ray Diffraction Analysis of the Angular Stability of Self-Catalyzed GaAs Nanowires for Future Applications in Solar-Light-Harvesting and Light-Emitting Devices

Semiconductor nanowires are a class of materials that recently have gained increasing interest in solar-cell applications and light-emitting devices. Finding reproducible processing conditions is fundamental for their future mass production. In this work, the stability of individual epitaxial GaAs nanowires (NWs) under molecular beam epitaxy (MBE) processing conditions is studied by means of a time-resolved in situ micro X-ray diffraction (μXRD) method and scanning electron microscopy. Our proposed μXRD method is a nondestructive characterization technique where individual nanoobjects of different dimensions, crystal orientations, and structures are detectable under MBE processing conditions. NWs were grown by self-catalyzed MBE onto prepatterned Si(111) substrate. When exposed to MBE processing conditions at 610 °C without supply of a source material, or with only an arsenic (As) supply, we observe evaporation from the facets with no indication of gallium (Ga) droplet formation. Furthermore, the NWs, whi...

Deterministic Switching of the Growth Direction of Self-Catalyzed GaAs Nanowires.

The typical vapor-liquid-solid growth of nanowires is restricted to a vertical one-dimensional geometry, while there is a broad interest for more complex structures in the context of electronics and photonics applications. Controllable switching of the nanowire growth direction opens up new horizons in the bottom-up engineering of self-assembled nanostructures, for example, to fabricate interconnected nanowires used for quantum transport measurements. In this work, we demonstrate a robust and highly controllable method for deterministic switching of the growth direction of self-catalyzed GaAs nanowires. The method is based on the modification of the droplet-nanowire interface in the annealing stage without any fluxes and subsequent growth in the horizontal direction by a twin-mediated mechanism with indications of a novel type of interface oscillations. A 100% yield of switching the nanowire growth direction from vertical to horizontal is achieved by systematically optimizing the growth parameters. A kinetic model describing the competition of different interface structures is introduced to explain the switching mechanism and the related nanowire geometries. The model also predicts that the growth of similar structures is possible for all vapor-liquid-solid nanowires with commonly observed truncated facets at the growth interface.