Published: March 18, 2011 r 2011 American Chemical Society 1618 dx.doi.org/10.1021/nl200002x | Nano Lett. 2011, 11, 1618���1622 LETTER pubs.acs.org/NanoLett In Situ Nanomechanics of GaN Nanowires Jian Yu Huang,*,��� He Zheng,���,�� S. X. Mao,��� Qiming Li,��� and George T. Wang��� ���Sandia National Laboratories, Albuquerque, New Mexico 87185, United States ��� Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States �� Department of Physics, Center for Electron Microscopy and Key Laboratory of Acoustic and Photonic Materials and Devices, Wuhan University, Wuhan 430072, China bSupporting S Information Gband allium nitride (GaN) is a technologically important wide gap (Eg = 3.39 eV) semiconductor used in optoelec- tronic and high frequency and high power electronic applications.1 12 GaN thin films heteroepitaxially grown on sapphire or other substrates exhibit a high density of threading dislocations, which degrades the e���ciency and the lifetime of the GaN-based devices.13 19 In this context, GaN nanowires have the advantage of low or even no dislocations, which makes them attractive candidate for GaN-based applications. The electrical, optoelectrical, and thermal properties of GaN nanowires have been studied extensively, but relatively little is known on their mechanical properties.20 Such studies are important because GaN nanowires may be subjected to mechanical or electrical stress during device processing or operation, and such stresses may impact the properties of the nanowires for example via dislocation generation. In this work, the deformation, fracture mechanisms, and the fracture strength of individual GaN nano- wires were measured in real time using a transmission electron microscope scanning probe microscope (TEM-SPM) platform. The GaN nanowires were grown by Ni-catalyzed metal organic chemical vapor deposition (MOCVD) method on a sapphire substrate wafer,10,21,22 and the as-grown nanowires were triangular in cross-section (Figure 1) with stacking faults in the basal planes similar to that reported in the literature.23,24 The nanowire diameter varied from 100 nm to about 500 nm, and its growth direction was generally [1-210] (Figure 1). A tiny piece was cut off the main wafer and glued to an Al rod with diameter of about 280 ��m (Supporting Information Figure S1). Individual nanowires were then manipulated to approach either a nanoin- dentor or a flat-ended scanning tunneling microscopy probe (STM) for in situ compression experiments. In the former, the force-displacement could be directly measured and recorded by a computer. The compression was displacement controlled with the speed varying from 0.1 to 6.8 nm/s. Figure 2a d and Supporting Information movie M1 show that dislocations were nucleated from a free surface (Figure 2a,b), and then propagated along a prismatic plane of the nanowire, leading to a displacement of the upper segment with respect to the lower segment of the nanowire (Figure 2c) eventually the nanowire broke from the location where slippage had occurred (Figure 2d). Dislocation nucleation from a free surface has been observed in metallic nanomaterials,25,26 but it has not been reported in semiconductor nanomaterials to our knowledge. The result suggests that surface dislocation nucleation may be a general phenomena to many nanostructured materials, re- gardless of their metallic, semiconducting, or ceramic nature. Figure 2e i (Supporting Information movie M2) is another set of data showing the slip event before the fracture of the nanowire. There was an abrupt diameter change near the base of the nanowire, causing a surface step as pointed out by an arrowhead in Figure 2e. Slip or shear initiated from the step (Figure 2e), propagated along a prismatic slip plane (Figure 2f), and Received: January 1, 2011 Revised: February 13, 2011 ABSTRACT: The deformation, fracture mechanisms, and the fracture strength of individual GaN nanowires were measured in real time using a transmission electron microscope scanning probe microscope (TEM-SPM) platform. Surface mediated plasticity, such as dislocation nucleation from a free surface and plastic deformation between the SPM probe (the punch) and the nanowire contact surface were observed in situ. Although local plasticity was observed frequently, global plas- ticity was not observed, indicating the overall brittle nature of this material. Dislocation nucleation and propagation is a precursor before the fracture event, but the fracture surface shows brittle characteristic. The fracture surface is not straight but kinked at (10-10) or (10-11) planes. Dislocations are generated at a stress near the fracture strength of the nanowire, which ranges from 0.21 to 1.76 GPa. The results assess the mechanical properties of GaN nanowires and may provide important insight into the design of GaN nanowire devices for electronic and optoelectronic applications. KEYWORDS: GaN nanowire, nanomechanics, dislocation, plasticity, fracture, in-situ electron microscopy

1619 dx.doi.org/10.1021/nl200002x |Nano Lett. 2011, 11, 1618���1622 Nano Letters LETTER eventually the nanowire fractured along the same slip plane (Figure 2g i). The fracture surface is smooth. This shows again that fracture was initiated from a slip or a shear which was nucleated from a free surface. Dislocation nucleation and propagation right before fracture were captured in situ, as shown in Figure 3a d and Supporting Information movies M3 and M4. The nanowire may have fractured from the middle in a previous compression experiment, but the broken segment was still attached to the nanowire, forming a kink. As the nanowire was compressed, dislocations were nucleated about 50 nm away from the punch and nanowire contact (pointed out by an arrow in Figure 3b), crossed the nanowire along a prismatic plane (pointed out by an arrow in Figure 3c), and then escaped to the other free surface. Similar dislocation emission accompanying the nanowire fracture was also observed in other nanowires (Supporting Information Figure S2). This indicated that dislocation activity was a precursor of fracture. In many cases, residual dislocations were observed near the fracture surface (Figure 3e), and the dislocations were mostly 1/3 [11- 20] type (Figure 3g) with the slip plane being (1-100) (Figure 3f,g). Significant local plastic deformation was observed right before fracture in some nanowires. Figure 3h,i and Supporting Informa- tion movies M5 to M9 show the fracture process of GaN nanowires under compression. Note the plastic deformation induced by piling up of dislocations occurred in the contact surfaces between the nanowires and the punches (Figure 3h and Supporting Information movies M5 to M7). Plastic deformation was not obvious in the nanowires shown in Supporting Informa- tion movies M8 and M9. Interestingly, the fracture location is about 140 nm away from the contact surface (Figure 3h,i), and the fracture surface is very sharp from low-magnification TEM images (Figure 3i) with an inclined angle of 64�� with respect to the vertical direction. Similar fracture surfaces were observed in Figure 1. The cross-section of the GaN nanowire is triangular. (a) An atomic structural model of the cross-sectional view of the nanowire. The nanowire with a growth direction of [1-210] is enclosed by the (000-2), (-1011), and (10-11) planes. (b) A TEM image showing the triangular cross-section of the nanowire. The enclosed angles between different planes are consistent with the model shown in (a). (c) A plan-view of the triangular nanowire. (d) A general electron diffraction pattern from a GaN nanowire. Streaks along the (0002) diffraction series indicated stacking faults in the nanowires. Figure 2. (see also Supporting Information movies M1 and M2) Two sets of sequential TEM images showing fracture initiated from a slip or a shear nucleated from the free surface of the nanowire (pointed out by arrows). (a,b) and (e,f) Dislocation nucleation from free surfaces. (c,f) Slip of the dislocations. (d,h,i) Fracture of the nanowire. The punch was advanced at 0.5 and 0.7 nm/s in movies M1 and M2, respectively.