Nanomechanical behavior of single taper-free GaAs nanowires unravelled by in-situ TEM mechanical testing and molecular dynamics simulation

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Abstract

Nanowire-based devices have been widely applied in optoelectronics, sensors, generators and spectroscopy. These nanowires are typically subjected to mechanical conditions during manufacture or operation. Thus, a compressive understanding of nanowire mechanical properties is increasingly required. However, only limited results have been reported owing to the challenges inherent in nanomechanical testing, particularly for quantitative tensile deformation. Herein, taper-free zinc blende GaAs nanowires with a 120 nm diameter are grown along the [111]B direction using metalorganic vapor phase epitaxy. The mechanical properties and fracture mechanisms of these single-phase GaAs nanowires are explored by in-situ uniaxial tensile deformation inside a transmission electron microscope, followed by molecular dynamics simulations. Under tensile stress, GaAs nanowires deform overally elastically until sudden brittle fracture at 3.79% strain. The fracture strength and elastic modulus are experimentally determined as 4.0 and 109.5 GPa, respectively, which are much smaller than other reported results based on compression. The tensile deformation and fracture mechanisms are further explored using molecular dynamics simulations, and the effects of different crystal structures on the GaAs nanowire mechanical behavior are discussed. These results assess the mechanical behavior of single GaAs nanowires and present critical insights into the reliable design of engineering nanodevices.

Introduction

Semiconductor nanowires exhibit important applications for optoelectronic devices, medical sensors and spectroscopy [[1], [2], [3]]. Among the III-V compounds, GaAs nanowires have been extensively used for near-infrared lasing [4], efficient solar cells [1,5] and photodetector applications due to their superior optical qualities such as a direct bandgap [2,6], efficient light absorption and high electron mobility [7]. The optical and electronic quality of GaAs nanowires typically depends on the well-recognized factors such as surface recombination [4] and crystal structure/defects [5,[8], [9], [10], [11]]. Meanwhile, it has also been reported that a mechanical response (i.e., strain, stress, and perturbation) significantly affects the carrier mobility, optical and magnetic transition of homojunction GaAs light emission diodes (LEDs) [12], ZnO-based nanogenerators [13], Si n- and p-MOSFETs [14], Au nanowire interconnects [15], and Si/Ge thermoelectric nanomaterials [16] in their associated nanodevices.

For instance, it has been reported that mechanical stress causes degradation in homojunction GaAs LEDs [12], while a self-healing phenomenon was observed and mechanically facilitated in ultrathin Au nanowires [15]. A high output voltage (50 mV) with 6.8% energy conversion efficiency was obtained by mechanically stretching and releasing a single ZnO wire between 0.05 and 0.1% change in strains [13]. Mechanical deformation can induce positive and negative voltages in ZnO micro/nanowires owing to imposed tensile and compressive strains, respectively [17,18]. Mechanical strain applied at the Si/Ge interfaces enabled tuning of the band structure of superlattice nanowires [15]. Transition between indirect–direct bandgaps could form in Si nanowires, which was generated by an externally-applied strain [19]. We note that nanowires are subjected to mechanical loading/unloading conditions during synthesis, device fabrication processes and operation, which can induce mechanical failure or fracture. In particular, the mechanical behavior of materials under loading varies dramatically with dimensions, from bulk to the micro- or nanoscale [10,[20], [21], [22], [23]]. Thus, the mechanical behavior and corresponding failure physics require clarification to achieve reliable integration of nanowires into devices.

However, such information is limited because of the technical challenges inherent in the experimental mechanical testing of nanowires, in particular for in-situ quantitative tensile measurements. Over the past two decades, the development of nanomanipulation techniques has made mechanical characterization of nanowires partially workable [22,24,25]. Precise mechanical testing of nanowires can now be realized via in-situ tensile tests within a transmission electron microscope (TEM) coupled with a PicoIndenter transducer [[26], [27], [28]]. This methodology depends on a suitable nanowire size, delicate manipulation, a high-speed charge-coupled device (CCD) camera, and high resolution of the displacement/load [28]. Owing to these challenges, most reports concerning the mechanical deformation of nanowires/nanoneedles have been obtained using nanocompression in a TEM [[9], [10], [15], [29], [30], [31]] and scanning electron microscope (SEM) [32]; nanotension in a TEM [[33], [34], [35], [36]] and SEM [37,38]; nanolooping in an optical microscope [22]; and other nanobending tests [24]. However, for most of these experiments, the stress applied at both nanowire ends may not be uniaxially fixed on an identical plane. Misalignment will spontaneously occur during tensile deformation, leading to spurious effects and experimental errors. There are limited reports of uniaxial tensile tests on a single nanowire [26,28,35,38]. Additionally, the tested GaAs nanowires have obvious taper or defects [10,31], which can cause inaccuracies in mechanical tests. To our knowledge, no quantitative tensile tests of GaAs nanowires were currently available.

To address the performance of GaAs nanowires under mechanical conditions, the requirements include (a) an in-depth physical understanding of mechanical failure, (b) accurate in-situ TEM mechanical testing data at the nanoscale, and (c) an efficient simulation to predict mechanical failure. Twin-free and taper-free ZB GaAs nanowires can be grown by vapor-liquid-solid growth mechanism using a two-temperature growth process. Consequently, the possibility of defect incorporation as a result of unintentional radial growth is avoided. The uniform diameter and single crystal nature of the nanowire make the mechanical analysis simpler. In the present work, the mechanical behavior of defect-free GaAs nanowires was explored in situ under uniaxial tensile deformation in a TEM. Based on sophisticated sample preparation and a newly-developed push-to-pull (PTP) apparatus, the accurate stress–strain curve and associated fracture mechanisms of a single GaAs nanowire under tensile stress were investigated for the first time. Subsequently, comparison between the experimental results and molecular dynamics (MD) simulations was made for a thorough investigation. This work aims to deliver two fundamental perspectives: an in-situ observation of the deformation failure of a single nanowire coupled with MD simulation validation; and the precise mechanical properties of a single nanowire, including the elastic modulus, fracture strength, and fracture strain under uniaxial tension.

Section snippets

GaAs nanowires synthesis

The GaAs nanowire growth was conducted in a metal organic vapor phase epitaxy (MOVPE) reactor (Aixtron 200/4) at 100 mbar pressure with a horizontal gas flow rate of 15 L min−1 [9,39]. Trimethylgallium (TMGa) and arsine (AsH3) were used as the precursors for the Ga and As elements, respectively. The Au nanoparticles were prepared by depositing 100 nm diameter Au colloids on GaAs(111)B substrates pre-immersed in a poly-l-lysine solution. After Au colloid deposition, the GaAs(111)B substrates

Nanowire microstructural features

The morphology and structure of the GaAs nanowires studied in this work are shown in Fig. 3. GaAs nanowires grew virtually with taper-free morphology along the [111]B direction (Fig. 3a). TEM images in Fig. 3b confirm a uniform diameter distribution in the nanowires (100–120 nm). Each GaAs nanowire has an Au nanoparticle catalyst at its tip, as indicated in Fig. 3c. An indexed selected area electron diffraction (SAED) pattern was taken from the [110] zone axis, which identifies the ZB

Discussion

The experimental elastic modulus is determined as 109.5 GPa, which is smaller than the value measured under nanocompression by Wang et al. (123–118 GPa for diameters between 120 and 150 nm [10]) but larger than the bulk GaAs material (86 GPa) [50]. The elastic modulus calculated from MD simulation was 103.19 GPa under tensile deformation, which agrees well with the experimental data (109.5 GPa). The present work mainly focuses on the tensile response and fracture behavior. However, the diameter

Conclusions

In summary, the uniaxial tensile response of defect-free ZB GaAs nanowires with 120 nm diameters was investigated by in-situ TEM mechanical tests. In our case under tensile deformation, the GaAs nanowire overally broke via brittle fracture without the dislocation-mediated plasticity observed in previously reported bending tests. Indeed, there was no evidence of plastic deformation prior to fracture during the in-situ TEM tensile testing. The fracture strength along the [111] close-packed

CRediT authorship contribution statement

Zhilin Liu: Conceptualization, Funding acquisition, Data curation, Investigation, Writing - original draft, Investigation, Project administration, Writing - review & editing. Xiaoming Yuan: Conceptualization, Funding acquisition, Data curation, Investigation, Project administration, Writing - review & editing. Shiliang Wang: Investigation, Writing - review & editing. Sha Liu: Simulation, Data curation, Investigation, Writing - review & editing. Hark Hoe Tan: Investigation, Writing - review &

Declaration of competing interest

All authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by National Natural Science Foundation of China (No. 51702368), Hunan Provincial Natural Science Foundation of China (No. 2018JJ3684), and Innovation-Driven Project of Central South University (No. 2018CX045). Z. Liu appreciates the Marie Sklodowska-Curie Individual Fellowship program through the project MINIMAL (No. 749192). All authors acknowledge the Australian National Fabrication Facility - ACT Node for access to the GaAs nanowires, IMDEA Materials Institute and

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