Self-catalyzed Growth Of GaAs Nanowires Research Articles

Growth of high-crystallinity uniform GaAs nanowire arrays by molecular beam epitaxy* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61674021, 11674038, 61704011, 61904017, 11804335, and 12074045), the Developing Project of Science and Technology of Jilin Province, China (Grant No. 20200301052RQ), the Project of Education Department of Jilin Province, China (Grant No. JJKH20200763KJ), and the Youth Foundation

The self-catalyzed growth of GaAs nanowires (NWs) on silicon (Si) is an effective way to achieve integration between group III–V elements and Si. High-crystallinity uniform GaAs NW arrays were grown by solid-source molecular beam epitaxy (MBE). In this paper, we describe systematic experiments which indicate that the substrate treatment is crucial to the highly crystalline and uniform growth of one-dimensional nanomaterials. The influence of natural oxidation time on the crystallinity and uniformity of GaAs NW arrays was investigated and is discussed in detail. The GaAs NW crystallinity and uniformity are maximized after 20 days of natural oxidation time. This work provides a new solution for producing high-crystallinity uniform III–V nanowire arrays on wafer-scale Si substrates. The highly crystalline uniform NW arrays are expected to be useful for NW-based optical interconnects and Si platform optoelectronic devices.

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.

Radial Growth of Self-Catalyzed GaAs Nanowires and the Evolution of the Liquid Ga-Droplet Studied by Time-Resolved in Situ X-ray Diffraction.

We report on a growth study of self-catalyzed GaAs nanowires based on time-resolved in situ X-ray structure characterization during molecular-beam-epitaxy in combination with ex situ scanning-electron-microscopy. We reveal the evolution of nanowire radius and polytypism and distinguish radial growth processes responsible for tapering and side-wall growth. We interpret our results using a model for diameter self-stabilization processes during growth of self-catalyzed GaAs nanowires including the shape of the liquid Ga-droplet and its evolution during growth.

Decoupling the Two Roles of Ga Droplets in the Self-Catalyzed Growth of GaAs Nanowires on SiOx/Si(111) Substrates

Liquid Ga droplets play a double role in the self-catalyzed growth of GaAs nanowires on Si(111) substrates covered with a native SiOx layer: they induce the formation of nanosized holes in SiOx and then drive the uniaxial nanowire growth directly onto the underlying Si. The independent control of the two mechanisms is a prerequisite for mastering the growth of nanowires, but it is challenging in a conventional growth procedure where they both take place under the same droplets. To that end, we have developed an in situ procedure where the Ga droplets used for the formation of SiOx holes are removed before new Ga droplets drive the growth of GaAs nanowires. In that way, it was possible to study the interaction between Ga droplets and SiOx, to create holes in SiOx with controlled number density and size, and, finally, to grow GaAs nanowires only within those holes. Our results show unprecedented control of the nanowire nucleation with unique possibilities: (1) deliberate control of the number density of nan...

Droplet-Confined Alternate Pulsed Epitaxy of GaAs Nanowires on Si Substrates down to CMOS-Compatible Temperatures.

We introduce droplet-confined alternate pulsed epitaxy for the self-catalyzed growth of GaAs nanowires on Si(111) substrates in the temperature range from 550 °C down to 450 °C. This unconventional growth mode is a modification of the migration-enhanced epitaxy, where alternating pulses of Ga and As4 are employed instead of a continuous supply. The enhancement of the diffusion length of Ga adatoms on the {11̅0} nanowire sidewalls allows for their targeted delivery to the Ga droplets at the top of the nanowires and, thus, for a highly directional growth along the nanowire axis even at temperatures as low as 450 °C. We demonstrate that the axial growth can be simply and abruptly interrupted at any time without the formation of any defects, whereas the growth rate can be controlled with high accuracy down to the monolayer scale, being limited only by the stochastic nature of nucleation. Taking advantage of these unique possibilities, we were able to probe and describe quantitatively the population dynamics of As inside the Ga droplets in specially designed experiments. After all, our growth method combines all necessary elements for precise growth control, in-depth investigation of the growth mechanisms and compatibility with fully processed Si-CMOS substrates.

Growth optimization for self-catalyzed GaAs-based nanowires on metal-induced crystallized amorphous substrate

The growth of monocrystalline semiconductor nanowires on arbitrary substrates via the metal-induced crystallization (MIC) process extends the possible combinations of substrates and epitaxial active materials. However, it is still difficult to accomplish high-density vertical nanowire growth on the MIC polycrystalline Si(111) substrate. Here, the authors report on the growth of self-catalyzed GaAs nanowires by molecular beam epitaxy on MIC polycrystalline Si(111) substrates with different surface oxide conditions. Forming the surface oxide by annealing the freshly hydrofluoric acid-etched MIC polycrystalline Si(111) substrate in an ambient atmosphere is found to be a key step to grow high-density GaAs nanowires. Moreover, the addition of Sb during nanowire growth improves the density of vertical nanowires. Photoluminescence measurements reveal a high optical quality of the GaAs nanowires, indicating that the nanowires grown on MIC polycrystalline Si(111) substrate may be used as building blocks for semiconductor nanowire optoelectronic devices on arbitrary substrates.

Lithography-free oxide patterns as templates for self-catalyzed growth of highly uniform GaAs nanowires on Si(111)

We report self-catalyzed growth of GaAs nanowires (NWs) on Si/SiOx patterns fabricated by a lithography-free method. The patterns are defined using droplet epitaxy of GaAs nanocrystals, spontaneous oxidation, and thermal annealing. We investigate the influence of the size and density of the nucleation sites on the NW growth process and show that this approach enables the fabrication of highly uniform GaAs NWs with controllable density. The pattern fabrication and NW growth process are studied and discussed in relation to the surface morphology and chemical properties of the Si/SiOx patterns. Furthermore, the optical quality of the NWs is investigated by photoluminescence experiments performed for GaAs–AlGaAs core–shell NWs.

Arsenic Pathways in Self-Catalyzed Growth of GaAs Nanowires

Self-catalyzed growth of GaAs nanowires by molecular beam epitaxy on (111)Si substrates is investigated by introducing AlxGa1–xAs time markers. The nanowire elongation rate is found to be radius-independent, constant at substrate temperatures below 650 °C and linearly increasing with the incoming arsenic flux. The basic question of which pathways are followed by the arsenic species contributing to nanowire growth is clarified. The flow rate of As atoms directly impinging on the Ga catalyst drop is significantly smaller than the As consumption by nanowire growth. Thus, supplementary As atoms are necessary to explain the actual elongation rate. We show that surface diffusion of adsorbed Asx species toward the catalyst cannot account for the missing atoms. On the other hand, the reevaporation of Asx species from the substrate and from nanowire sidewall surfaces can act as an efficient secondary arsenic source. We argue that a sufficient amount of these species can be intercepted by the Ga drop and add up with the direct As impingement to explain the actual elongation rate.

Evidence for Structural Phase Transitions Induced by the Triple Phase Line Shift in Self-Catalyzed GaAs Nanowires

Self-catalyzed growth of GaAs nanowires are widely ascribed to the vapor-liquid-solid (VLS) mechanism due to the presence of Ga particles at the nanowire tips. Here we report synthesis of self-catalyzed GaAs nanowires by molecular-beam epitaxy covering a large growth parameter space. By carefully controlling the Ga flux and its ratio with the As flux, GaAs nanowires without Ga particles and exhibiting a flat growth front are produced. Using scanning electron microscopy and high-resolution transmission electron microscopy, we compare the growth rate and structure, especially near the growth front, of the nanowires with and without Ga droplets. We find that regardless of whether Ga droplets are present on top, the nanowires have a short wurtzite section following the zinc-blende bulk structure. The nanowires without Ga droplets are terminated by a thin zinc-blende cap, while the nanowires with Ga droplets do not have such a cap. The bulk zinc-blende phase is attributed to the Ga droplet wetting the sidewall during growth, pinning the triple phase line on the sidewall. The zinc-blend/wurtzite/(zinc-blende) phase transitions at the end of growth are fully consistent with the triple phase line shifting up to the growth front due to the progressive consumption of the Ga in the droplet by crystallization with As. The results imply an identical VLS growth mechanism for both types of GaAs NWs, and their intricate structures provide detailed comparison with and specific experimental verification of the recently proposed growth mechanism for self-catalyzed III-V semiconductor nanowires ( Phy. Rev. Lett. 2011 , 106 , 125505 ). Using this mechanism as a guideline, we successfully demonstrated controllable fabrication of two distinct types of axial superlattice GaAs NWs consisting of zinc-blende/defect-section and wurtzite/defect-section units.

Vapor-liquid-solid and vapor-solid growth of self-catalyzed GaAs nanowires

We report on the morphological and structural properties of GaAs nanowires nucleated by self-catalyzed vapor-liquid-solid processes by molecular beam epitaxy on Si-treated GaAs substrates. We found that GaAs nanowires display zincblende and/or wurtzite phase depending on the As/Ga abundance ratio at the growth front, that determines the size and supersaturation of the Ga nanoparticles at the nanowire tip. We also found that even when growth conditions lead to the disappearance of such Ga nanoparticles, preferential one-dimensional growth continues through a vapor-solid mechanism. The nanowire portions grown by vapor solid mechanism display zincblend structure.

Structural Phase Control in Self-Catalyzed Growth of GaAs Nanowires on Silicon (111)

Structural Phase Control in Self-Catalyzed Growth of GaAs Nanowires on Silicon (111)

Structural Phase Control in Self-Catalyzed Growth of GaAs Nanowires on Silicon (111)

Au free GaAs nanowires with zinc blende structure, free of twin planes and with remarkable aspect ratios, have been grown on (111) Si substrates by molecular beam epitaxy. Nanowires with diameters down to 20 nm are obtained using a thin native oxide layer on the Si substrates. We discuss how the structural phase distribution along the wire length is controlled by the effective V/III ratio and temperature at the growth interface and explain how to obtain a pure twin plane free zinc blende structure.

Position controlled self-catalyzed growth of GaAs nanowires by molecular beamepitaxy

GaAs nanowires are grown by molecular beam epitaxy using a self-catalyzed,Ga-assisted growth technique. Position control is achieved by nano-patterning aSiO2 layer with arrays of holes with a hole diameter of 85 nm and a hole pitch varying between 200 nm and 2 µm.Gallium droplets form preferentially at the etched holes acting as catalyst for the nanowire growth.The nanowires have hexagonal cross-sections with {110} side facets and crystallize predominantlyin zincblende. The interdistance dependence of the nanowire growth rate indicates a change of theIII/V ratio towards As-rich conditions for large hole distances inhibiting NW growth.

Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy

Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy withoutthe use of any outside metal catalyst. The growth occurs on Si facets obtained by thecleavage of Si(100) substrates. The growth has been obtained with or without Gapre-deposition. In both cases two kinds of nanowires have been obtained. The wires of thefirst type clearly present a Ga droplet at their free end and have a lattice structurethat is wurtzite for wide regions beneath the Ga droplet. The second type, incontrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainlyzincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowiresare longer than the others and thinner on average. The experimental findingssuggest that the two types of nanowires grow after different growth processes.