Letter pubs.acs.org/NanoLett
Anelastic Behavior in GaAs Semiconductor Nanowires Bin Chen,† Qiang Gao,‡ Yanbo Wang,† Xiaozhou Liao,*,† Yiu-Wing Mai,† Hark Hoe Tan,‡ Jin Zou,§ Simon P. Ringer,†,∥ and Chennupati Jagadish‡ †
School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia § Materials Engineering and Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, QLD 4072, Australia ∥ Australian Centre of Microscopy and Microanalysis, The University of Sydney, Sydney, NSW 2006, Australia ‡
S Supporting Information *
ABSTRACT: The mechanical behavior of vertically aligned single-crystal GaAs nanowires grown on GaAs(111)B surface was investigated using in situ deformation transmission electron microscopy. Anelasticity was observed in nanowires with small diameters and the anelastic behavior was affected by the crystalline defects in the nanowires. The underlying mechanism for the observed anelasticity is discussed. The finding opens up the prospect of using nanowire materials for nanoscale damping applications. KEYWORDS: Anelasticity, GaAs nanowires, in situ deformation, transmission electron microscopy, crystalline defects
W
microdomains. Fundamental investigations of deformation mechanisms have been relatively well explored and understood in these materials. So far, however, no investigation of anelasticity in brittle single-crystal materials has been reported. In this study, we present in situ deformation transmission electron microscopy (TEM) investigations of single-crystal GaAs semiconductor nanowires (NWs), demonstrating concrete experimental evidence that anelasticity can occur in single-crystal nanomaterials. Previous studies showed that NWs tend to have larger elastic limit and were stronger than their coarse-grained counterparts.13 Brittle materials in the form of one-dimensional NWs can even demonstrate exceptional plasticity.14,15 Nevertheless, little attention has been given to the details of the elastic/anelastic deformation process. Thus, a basic understanding is necessary to explore the deformation mechanisms in these materials, since the reliability and even the functionality of NW-based devices depend significantly on their mechanical properties.16−19 Vertically aligned single-crystal GaAs NWs were epitaxially grown on a GaAs(111)B substrate using Au nanoparticles as catalysts in a metal−organic chemical vapor deposition system. Trimethylgallium and AsH3 were used as the precursors together with ultrahigh-purity hydrogen as the carrier gas. The diameter of the NWs was controlled by the size of Au nanoparticles. Majority of the as-grown NWs are defect free. Detailed description of the GaAs NW growth can be found in
hen materials are subjected to an external stress, they will experience elastic deformation followed by irreversible plastic deformation (for ductile materials) when the external stress is higher than the yield stress of the materials and finally fracture. The elastic deformation is reversible, that is, the materials will resume their original shapes once the applied stress is retracted. In most materials, the recovery is immediate, but in some materials there is a delay in shape recovery after the applied stress has been removed. This time-dependent behavior is called anelasticity.1 Materials with anelasticity have been widely used in damping systems to reduce or even eliminate noise for a broad range of industrial, electronic, and structural applications.2,3 Anelasticity has been observed in metallic alloys, ceramics, bulk metallic glasses, and polymers. In general, anelasticity is due to time-dependent structural evolutions at the microscopic and atomic scales that are attendant to the deformation. However, the specific mechanisms responsible for the anelasticity in different materials are different. For example, phase transformations, reversible motion of twins, and cooperative motion of many atoms at grain boundaries are the dominant mechanisms for the anelasticity in Cu−Al−Ni alloys,4 In−Ti alloys,5,6 and nanocrystalline Au,7 respectively. In ceramic materials, such as tetragonal zirconia polycrystalline ceramics (TZP),8 yttria-stabilized zirconia (YSZ),9 and plasmasprayed YSZ,10 phase transformations, grain reorientation, and microcrack opening/closure, respectively, are suggested responsible for the anelastic behavior. The anelasticity in Zrbased metallic glasses11 and polymers (e.g., poly(methyl methacrylate) abbreviated to PMMA)12 is due to structural relaxations through the flow of free volume zones or © XXXX American Chemical Society
Received: April 2, 2013 Revised: June 5, 2013
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refs 20 and 21. The morphological characteristic of grown GaAs NWs was investigated using scanning electron microscopy (SEM, Hitachi S4500), and their structural characteristic was determined using TEM (JEOL JEM-3000F). In situ TEM compression tests were performed using a Hysitron PI 95 TEM PicoIndenter with a flat diamond punch in a JEM-2100 TEM.22−26 Specimens for in situ TEM tests were mounted in such a way that a compressive force was applied along the axial direction of the GaAs NWs. The entire deformation processes were recorded in TEM by real-time videoing at the speed of 30 frames per second. Figure 1a shows an SEM image of the as-grown GaAs NWs. The NWs were deliberately grown with a low density to
Figure 2. A series of TEM images extracted from Movie 1 in the online Supporting Information showing the anelastic behavior of a GaAs NW with a diameter of ∼25 nm. (a) Before deformation and (b) during deformation, bending contours are marked with arrows. (c) Immediately after the external stress was completely released. Contrast variation is marked by arrows and the gap between the current and original positions of the NW tip is indicated by two dotted arrows. (d,e) The NW gradually reverted to its original shape. Dotted ellipses mark the region for contrast comparison. (f) At the instant when the NW completely returned to its original shape. The inset in (f) shows the whole length of the NW (the scale bar is 200 nm). Figure 1. (a) An SEM image of vertically aligned GaAs NWs. (b) A typical bright-field TEM image of a GaAs NW. (c) A typical highresolution TEM image of a NW.
NW to fully recover to its original shape (Figure 2f). Since the original length of the NW was ∼1000 nm (inset of Figure 2f), the total anelastic strain of the NW was estimated to be 0.5− 1.0% (calculated based on two assumptions, that is, strains of ∼0.5% for elastic bending and ∼1.0% for compressive deformation; see details in the Supporting Information), which is larger than that in brittle bulk materials (∼0.2%).27 NW diameter plays an important role in the anelastic behavior of the NWs. Figure 3 shows three snapshot images of
facilitate in situ deformation testing. All the NWs have a uniform morphology and are aligned vertically to the substrate. Figure 1b shows a typical bright-field TEM image of a GaAs NW in which a Au nanoparticle is located at the tip of the NW, confirming that the NWs were indeed catalyzed by Au nanoparticles. Figure 1c is a high-resolution TEM image of a section of GaAs NW in Figure 1b and shows that an amorphous layer with a thickness of about 2 nm covers the GaAs NW. It is believed that this amorphous layer is an oxide layer formed when the NW was exposed to air after unloading from the growth chamber. It should be pointed out that most of the semiconductor NWs have a very thin nonremovable amorphous surface layer due to the inevitable oxidation. Figure 2 presents a series of in situ deformation TEM images extracted from Movie 1 in the Supporting Information. Figure 2a shows a straight single NW with a diameter of ∼25 nm before an external stress was applied. During loading, the NW gradually bent/buckled and bending contours can be witnessed along the NW (marked with arrows in Figure 2b). When the diamond punch was just fully retracted, that is, no external force was applied to the NW (Figure 2c), some bending contours still remained on the NW. Because the NW base position was fixed and the NW length was a constant when no force was applied to the NW, anelasticity was easily confirmed by using the original NW tip position as the reference and by checking the time needed for the tip to reach the original position after the compression force was released. The NW tip was ∼10 nm away from its original position before loading, as indicated by two dotted arrows in Figure 2c. With the elapse of time, the contrast of bending contours disappeared gradually along the NW (refer to elliptical regions in Figure 2d,e). According to our real time recording, it took about 116 s for the
Figure 3. TEM images of a GaAs NW with a diameter of ∼55 nm extracted from Movie 2 in the Supporting Information. (a) Before deformation, (b) during loading, and (c) at the moment the external stress was completely removed. Bending contours in (b) are marked with arrows. The NW almost immediately returned to its original state when the stress was released.
a NW with a diameter of ∼55 nm before, during, and immediately after loading. The images were extracted from Movie 2 in the Supporting Information. Similar to the situation found in Figure 2, bending contours (marked with arrows in Figure 3b) were seen on the NW during the loading process. However, the NW instantaneously reverted to its original shape upon unloading (Figure 3c), indicating no obvious anelasticity in NWs with large diameters. This result suggests that only NWs with small diameters exhibit anelastic behavior. It has been well documented that lattice defects in a crystalline material such as stacking faults can significantly affect B
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its mechanical behavior. For this reason, thin GaAs NWs with high densities of stacking faults were deformed to understand their anelastic behavior, and an example is shown in Figure 4.
Figure 5. (a) A typical atomic resolution TEM image of the interface in a NW without stacking faults. (b) A typical atomic resolution TEM image of the interface in a NW with stacking faults. SF is the abbreviation for stacking faults.
for the defect-free NW (Figure 5a), while the amorphous/ crystalline interface of the NW with stacking faults is no longer smooth but wavy (Figure 5b). Because each GaAs NW has a core/shell structure with a single crystal core and a thin amorphous surface layer and because the anelastic behavior is affected by the planar defects in the NWs, we anticipate that the interface between the amorphous surface and the crystalline core is a key factor in determining the anelastic behavior in the GaAs NWs. The mechanisms for anelasticity proposed in the published literature, including reversible phase transformations, reversible motion of twins, cooperative motion of many atoms at grain boundaries, microcrack opening/closure, grain reorientation, and flow of free volume zones or microdomains, cannot be used to explain the observed anelasticity in our GaAs NWs for the following reasons: (i) neither phase transformations nor microcracks were detected in the NWs; (ii) no twins were seen before, during, and after deformation; (iii) no grain boundary nor grain reorientation in the single-crystal NWs; and (iv) free volume or microdomain model cannot explain the sizedependent anelasticity of the NWs. On the other hand, because the thickness of amorphous layers remains a constant the thin NWs increase its ratio of the amorphous/crystalline interfacial area to the volume. Because introducing stacking faults significantly changes the amorphous/crystalline interfacial structure, the observed effect of NW size and stacking faults on the anelasticity in GaAs NWs indicates that the amorphous/ crystalline interface of the NWs plays a critical role on the mechanical behavior of the NWs. Under an applied stress, the NW experiences a deformation event during the loading process. Since the NW surface is covered with an amorphous layer, the atomic bonding at the interfacial region may not be as strong as that inside the NW crystal. After the same displacement, it is reasonable to propose that the residual stress and relaxation time of the amorphous surface layer and the crystal core of the NW are different during the unloading process and the relaxation also varies from one place to another because of the variation in the strain distribution in different places caused by the bending/buckling effect. When the external stress is removed, instead of an instantaneous elastic recovery of the stretched or shrunk bonds in a crystal, the whole NW gradually returns to its original shape with a time delay since the amorphous layer holds back the crystal core resulting in a slow recovery. However, for a NW with a large diameter the volume fraction of the interface decreases significantly and the interfacial effect on the mechanical behavior of the NW is no longer dominant. As
Figure 4. TEM micrographs extracted from Movie 3 in the Supporting Information, showing the mechanical response of a GaAs NW with a high density of stacking faults (diameter of ∼25 nm). (a) TEM image of the NW showing a high density of stacking faults; some of them are marked with arrowheads. (b) Before deformation and (c) during deformation in which bending contours are marked with arrows. (d) At the instant when the external stress was completely released, and the gap between the current and original positions of the NW tip is shown by two dotted arrows. (e) The NW gradually reverted to its original shape, (dotted ellipses in (d) and (e) mark the regions for contrast comparison). (f) At the instant when the NW completely returned to its original shape.
As can be seen from Figure 4a, the NW has a diameter of ∼25 nm with a high density of stacking faults (e.g., some of them are marked with arrowheads). A video (Movie 3) showing the deformation process of this NW can be found in the Supporting Information. Before loading, no bending contour was seen throughout the NW (Figure 4b). During the loading process, bending contours appeared, as arrowed in Figure 4c. When the external stress was just released, a certain amount of residual strain remained, as evidenced by the nonuniform diffraction contrast in the NW (refer to the region marked a dotted ellipse in Figure 4d). A ∼6 nm gap was observed between the current and the original tip positions (indicated by two dotted arrows in Figure 4d). The anelastic strain was estimated to be 0.3−0.6% based on two assumptions (details outlined in the Supporting Information), which is smaller than that of the NWs without stacking faults (i.e., 0.5−1.0%). The contrast variation on the NW gradually became fainter (Figure 4e) and finally disappeared and the NW recovered its original shape ∼29 s after unloading (Figure 4f). These results indicate that NWs with stacking faults have smaller anelastic strain and shorter recovery time than NWs without stacking faults. It should be noted that the side-walls of defected NWs have different morphology with respect to defect-free NWs.28,29 Therefore, the nonremovable amorphous layers around NWs can then have different morphology, which may affect their anelastic behavior. As a consequence, the morphological nature of amorphous layers around the NWs with and without stacking faults should be investigated. Figure 5 shows the highresolution TEM images and the morphological nature of the amorphous/crystalline interfaces of typical NWs with and without stacking faults, respectively. As anticipated, the interface is smooth and aligns with the NW growth direction C
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such, no obvious anelastic behavior is observed in the NWs with large diameters. For a NW with a high density of stacking faults, this leads to the formation of a wavy amorphous/crystalline interfacial structure (Figure 5b). These stacking faults in the NW enable easy stress transfer. Hence, when the applied load is removed, the residual stress in the amorphous layer can be effectively transferred along the planar stacking faults to the crystal core, resulting in a faster response for full recovery of the NW. Therefore, the NW with a high density of stacking faults experiences a relatively weak anelasticity. It should be noted that the effect of electron beam irradiation on the anelasticity cannot be ruled out. However, it is expected that the effect should not be significant on the observed anelastic behavior because (i) the electron beam intensity used in this study was very weak (∼10−3 A/cm2), and thereby the irradiation-induced temperature rise in the samples should be small30,31 and (ii) the recovery times for NWs with and without stacking faults were very different although all NWs had the same diameter (∼25 nm) and were illuminated under the same electron beam intensity. In summary, an in situ deformation TEM technique was used to study the mechanical behavior of GaAs NWs. The NWs exhibited a remarkable anelastic behavior when the applied stress was removed. Anelastic strains of approximately 0.5−1.0 and 0.3−0.6% were recorded in NWs with a diameter of ∼25 nm without and with stacking faults, respectively. The observed anelasticity was sensitive to the NW diameters and was significantly affected by the amorphous/crystalline interface at the surface of the NWs. We propose that this new finding in the fundamental nanomechanics of NWs opens up the prospect of using NWs in nanoscale damping systems.
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ASSOCIATED CONTENT
S Supporting Information *
Movies 1−3 from which snapshot images were extracted and shown in Figures 2−4, respectively, were provided. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is financially supported by the Australian Research Council. The Australian Microscopy & Microanalysis Research Facility and Australian National Fabrication Facility, both established under Australian Government’s National Collaborative Research Infrastructure Strategy Program, are also acknowledged for providing growth and characterization facilities for this study.
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REFERENCES
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