Self-catalyzed Molecular Beam Epitaxy Research Articles

Wafer-scale integration of GaAs/AlGaAs core-shell nanowires on silicon by the single process of self-catalyzed molecular beam epitaxy.

GaAs/AlGaAs core-shell nanowires, typically having 250 nm diameter and 6 μm length, were grown on 2-inch Si wafers by the single process of molecular beam epitaxy using constituent Ga-induced self-catalysed vapor-liquid-solid growth. The growth was carried out without specific pre-treatment such as film deposition, patterning, and etching. The outermost Al-rich AlGaAs shells form a native oxide surface protection layer, which provides efficient passivation with elongated carrier lifetime. The 2-inch Si substrate sample exhibits a dark-colored feature due to the light absorption of the nanowires where the reflectance in the visible wavelengths is less than 2%. Homogeneous and optically luminescent and adsorptive GaAs-related core-shell nanowires were prepared over the wafer, showing the prospect for large-volume III-V heterostructure devices available with this approach as complementary device technologies for integration with silicon.

A Study on the Effects of Gallium Droplet Consumption and Post Growth Annealing on Te-Doped GaAs Nanowire Properties Grown by Self-Catalyzed Molecular Beam Epitaxy

In this work, the effects of arsenic (As) flux used during gallium (Ga) seed droplet consumption and the post-growth annealing on the optical, electrical, and microstructural properties of self-catalyzed molecular beam epitaxially grown tellurium (Te)-doped GaAs nanowires (NWs) have been investigated using a variety of characterization techniques. NWs using the same amount of As flux for growth of the seed droplet consumption demonstrated reduced density of stacking faults at the NW tip, with four-fold enhancement in the 4K photoluminescence (PL) intensity and increased single nanowire photocurrent over their higher As flux droplet consumption counterparts. Post-growth annealed NWs exhibited an additional low-energy PL peak at 1.31 eV that significantly reduced the overall PL intensity. The origin of this lower energy peak is assigned to a photocarrier transition from the conduction band to the annealing assisted Te-induced complex acceptor state (TeAsVGa−). In addition, post-growth annealing demonstrated a detrimental impact on the electrical properties of the Te-doped GaAs NWs, as revealed by suppressed single nanowire (SNW) and ensemble NW photocurrent, with a consequent enhanced low-frequency noise level compared to as-grown doped NWs. This work demonstrates that each parameter in the growth space must be carefully examined to successfully grow self-catalyzed Te-doped NWs of high quality and is not a simple extension of the growth of corresponding intrinsic NWs.

Self-catalyst growth and characterization of wurtzite GaAs/InAs core/shell nanowires

Research on nanowire (NW) growth of III-V semiconductors including GaAs and InAs demonstrated the ability to grow in both wurtzite (WZ) and zincblende (ZB) polymorphs. However, the control of crystal phase in self-catalyzed NW growth is still a remaining challenge. In this study we report a controlled growth of GaAs/InAs core/shell nanowires in WZ phase using self-catalyzed molecular beam epitaxy (MBE). The GaAs NWs having pure WZ crystal were achieved and attributed to the effect of small wetting angle, which is realized by supplying high V/III ratio. Furthermore, the WZ formation is shown to be uninfluenced by NW diameter. For the wetting angle larger than 90°, mixed phase starts to be observed. The InAs shell is planarly grown on six m-plane facets of the GaAs core NWs leading to the same strain states along a and c axes of growth plane, in which tensile and compressive strains are observed for GaAs core and InAs shell, respectively. High-resolution transmission electron microscopy (HR-TEM) reveals a few misfit dislocations (~0.03 nm−1) at GaAs/InAs interface indicating insignificant strain relief via creating misfit dislocation. Electrical characterization of the hetero-wires shows that the InAs shell exhibits n-type conduction. A room-temperature sheet carrier concentration at zero gate-bias of 2.5 × 1012 cm−2 and the corresponding carrier mobility of ~ 900 cm2 V−1 s−1 are demonstrated.

High yield of self-catalyzed GaAs nanowire growth on silicon (111) substrate templated by focused ion beam patterning

We report and detail a lithography-free method to pattern Si substrates. In particular, a focused Ga ion beam is used to create regular patterns of holes which serve as a template for the growth of vertically aligned GaAs nanowires (NW)s on Si(111) substrates using self-catalyzed molecular beam epitaxy. We show that the hole diameter plays a crucial role in the growth of the NWs at the drilled holes. The critical parameters defining the width of the holes are: ion dose quantities, wet etching procedures, and high-temperature steps at the process of growth. As a result, we obtained a yield of more than 80% for vertically aligned NW. Compared to other methods of patterning our approach provides the following advantages: (i) it is a lithography-free procedure, (ii) allows for quick patterning process and hole diameter optimization within a small window of trial and error, (iii) and provides potential applicability for different material systems.

Correlated optical and structural analyses of individual GaAsP/GaP core–shell nanowires

We report on the structural and optical properties of GaAs0.7P0.3/GaP core–shell nanowires (NWs) for future photovoltaic applications. The NWs are grown by self-catalyzed molecular beam epitaxy. Scanning transmission electron microscopy (STEM) analyses demonstrate that the GaAsP NW core develops an inverse-tapered shape with a formation of an unintentional GaAsP shell having a lower P content. Without surface passivation, this unintentional shell produces no luminescence because of strong surface recombination. However, passivation of the surface with a GaP shell leads to the appearance of a secondary peak in the luminescence spectrum arising from this unintentional shell. The attribution of the luminescence peaks is confirmed by correlated cathodoluminescence and STEM analyses of the same NW.

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...

Suppression of axial growth by boron incorporation in GaAs nanowires grown by self-catalyzed molecular beam epitaxy

The addition of boron to GaAs nanowires grown by self-catalyzed molecular beam epitaxy was found to have a strong effect on the nanowire morphology, with axial growth greatly reduced as the nominal boron concentration was increased. Transmission electron microscopy measurements show that the Ga catalyst droplet was unintentionally consumed during growth. Concurrent radial growth, a rough surface morphology and tapering of nanowires grown under boron flux suggest that this droplet consumption is due to reduced Ga adatom diffusion on the nanowire sidewalls in the presence of boron. Modelling of the nanowire growth puts the diffusion length of Ga adatoms under boron flux at around 700–1000 nm. Analyses of the nanowire surfaces show regions of high boron concentration, indicating the surfactant nature of boron in GaAs.

Light-Emitting GaAs Nanowires on a Flexible Substrate.

Semiconductor nanowire-based devices are among the most promising structures used to meet the current challenges of electronics, optics and photonics. Due to their high surface-to-volume ratio and excellent optical and electrical properties, devices with low power, high efficiency and high density can be created. This is of major importance for environmental issues and economic impact. Semiconductor nanowires have been used to fabricate highperformance devices, including detectors, solar cellsand transistors. Here, we demonstrate a technique for transferring large-area nanowire arrays to flexible substrates while retaining their excellent quantum efficiency in emission. Starting with a defect-free self-catalyzed molecular beam epitaxy (MBE) sample grown on a Si substrate, GaAs core-shell nanowires are embedded in a dielectric, removed by reactive ion etching and transferred to a plastic substrate. The original structural and optical properties, including the vertical orientation, of the nanowires are retained in the final plastic substrate structure. Nanowire emission is observed for all stages of the fabrication process, with a higher emission intensity observed for the final transferred structure, consistent with a reduction in nonradiative recombination via the modification of surface states. This transfer process could form the first critical step in the development of flexible nanowire-based light-emitting devices.

Growth of defect-free GaAsSbN axial nanowires via self-catalyzed molecular beam epitaxy

Bandgap reduction of 10% by incorporation of a dilute amount of N is reported for the first time, in axial GaAsSb nanowires (NWs) grown on Si (111) via Ga-assisted molecular beam epitaxy. Impact of N incorporation on the surface morphology, NW growth kinetics, and their structural and optical properties were examined. Dilute nitride NWs with Sb composition of 7 at% did not exhibit any noticeable planar defects, as revealed by the absence of satellite twin peaks in the selected-area diffraction pattern and high-resolution transmission electron microscopy imaging. Point defects were also minimal in as-grown dilute nitride NWs, as ascertained from the comparison of low-temperature photoluminescence spectra as well as the shape and shift of Raman modes, with in situ annealed NWs in different ambients. Evidence of enhanced incorporation of N was found in the NWs in situ annealed in N ambient, but with deteriorated optical quality due to simultaneous creation of N-induced defects. The lack of any noticeable defects in the as-grown GaAsSbN NWs demonstrates the advantage of the vapor–liquid–solid mechanism responsible for growth of axial configuration over the vapor–solid growth mechanism for core–shell NWs as well as their thin film counterpart, which commonly exhibit N-induced point defects.

Self-catalyzed molecular beam epitaxy growth and their optoelectronic properties of vertical GaAs nanowires on Si(111)

Self-assembled GaAs nanowires were grown by molecular beam epitaxy (MBE) on un-pretreated Si(111) substrates under different As4/Ga flux ratios (V/III ratios). It has been found that the fraction of vertical wires would be nearly 100% when the As4/Ga ratio arrives 90. The transmission electron microscopy (TEM) and micro-photoluminescence (PL) spectra results have indicated that the GaAs nanowires grown under a larger V/III ratio (90) have a pure ZB structure. Field-effect transistors (FET) based on single nanowire were fabricated with GaAs nanowires grown under the larger V/III ratio (90). The characteristics of the FET reveal a hole concentration of 3.919×1017cm−3 and a hole mobility of 0.417cm2V−1s−1. Photodetectors based on single nanowire and multiple nanowires structure with a metal-semiconductor-metal (MSM) electrode configuration have been proposed and demonstrated. All the photodetectors operating at room temperature exhibit good photoconductive performance, excellent stability, reproducibility and superior peak responsivity (87.67A/W under 5V for single nanowire photodetector).

Effects of Annealing on GaAs/GaAsSbN/GaAs Core-Multi-shell Nanowires.

The effects of ex-situ annealing in a N2 ambient on the properties of GaAs/GaAsSbN/GaAs core-multi-shell nanowires on Si (111) substrate grown by self-catalyzed molecular beam epitaxy (MBE) are reported. As-grown nanowires exhibit band edge emission at ~0.99 eV with a shoulder peak at ~0.85 eV, identified to arise from band tail states. A large red shift of 7 cm−1 and broadened Raman spectra of as-grown nanowires compared to that of non-nitride nanowires confirmed phonon localization at N-induced localized defects. On annealing nanowires to 750 °C, there was no change in the planar defects in the nanowire with respect to the as-grown nanowire; however, vanishing of the photoluminescence (PL) peak corresponding to band tail states along with enhanced band edge PL intensity, recovery of the Raman shift and increase in the Schottky barrier height from 0.1 to 0.4 eV clearly point to the efficient annihilation of point defects in these GaAsSbN nanowires. A significant reduction in the temperature-induced energy shift in the annealed nanowires is attributed to annihilation of band tail states and weak temperature dependence of N-related localized states. The observation of room temperature PL signal in the 1.3 μm region shows that the strategy of adding small amounts of N to GaAsSb is a promising route to realization of efficient nanoscale light emitters with reduced temperature sensitivity in the telecommunication wavelength region.

Growth of Vertical GaAs Nanowires on an Amorphous Substrate via a Fiber-Textured Si Platform

We demonstrate the vertical self-catalyzed molecular beam epitaxy (MBE) growth of GaAs nanowires on an amorphous SiO2 substrate by using a smooth [111] fiber-textured silicon thin film with very large grains, fabricated by aluminum-induced crystallization. This generic platform paves the way to the use of inexpensive substrates for the fabrication of dense ensembles of vertically standing nanowires (NWs) with promising perspectives for the integration of NWs in devices.