Nanoscale anisotropic plastic deformation in single crystal GaN
© Huang et al; licensee Springer. 2012
Received: 8 October 2011
Accepted: 22 February 2012
Published: 22 February 2012
Elasto-plastic mechanical deformation behaviors of c-plane (0001) and nonpolar GaN single crystals are studied using nanoindentation, cathodoluminescence, and transmission electron microscopy. Nanoindentation tests show that c-plane GaN is less susceptible to plastic deformation and has higher hardness and Young's modulus than the nonpolar GaN. Cathodoluminescence and transmission electron microscopy characterizations of indent-induced plastic deformation reveal that there are two primary slip systems for the c-plane GaN, while there is only one most favorable slip system for the nonplane GaN. We suggest that the anisotropic elasto-plastic mechanical properties of GaN are relative to its anisotropic plastic deformation behavior.
PACS: 62.20.fq; 81.05.Ea; 61.72.Lk.
GaN-related III-nitride materials are attracting increasing attention for a wide range of device applications, such as light-emitting diodes , blue laser devices , and high electron mobility transistor . Besides, GaN also exhibits attractive mechanical properties for potential device manufacture such as making microelectromechanical systems and making surface acoustic wave device [4, 5]. Therefore, there is an increasingly growing interest in the elasto-plastic mechanical behaviors of GaN material [6–16].
The previous investigations in the elasto-plastic mechanical behaviors of GaN were mainly carried out on a c-plane GaN. The only few works performed on nonpolar ((11-20) and (10-10) planes) GaN epitaxial layers, however, were largely affected by the substrate or the growth induced stacking faults [6–16]. Moreover, despite it is well known that GaN crystallizes in the wurtzite structure is characterized by a high anisotropy, a study on anisotropic elasto-plastic mechanical behaviors of GaN is still lacking.
This work attempts to present nanoindentation experiments performed on the three principal surfaces, (0001), (11-20), and (10-10), of GaN single crystals. Furthermore, the underlying activated slip systems which govern the plastic deformation of GaN are identified by cathodoluminescence (CL) and transmission electron microscopy. Finally, a comparison between the c-plane and nonpolar GaN in mechanical properties and plastic deformation is presented.
The samples under investigations were grown by hydride vapor phase epitaxy in homemade reactor. The c-plane GaN samples were obtained from a 1.5-mm thick free standing GaN wafer with dislocation density lower than 106 cm-2. Also, the a-plane and m-plane samples were obtained from a 1.5-mm thick free standing GaN wafer by cleaving along the (11-20) and (10-10) planes, respectively. The crystal orientations were identified by X-ray diffraction. The mechanical properties of GaN crystals were conducted with a nanoindentation system (Nano indenter G200, Agilent Technologies Inc., Santa Clara, CA, USA). The analytical method developed by Oliver and Pharr  was adopted to determine the hardness (H) and Young's modulus (E) of the GaN crystals from indentations with a maximum penetration depth of 500 nm. Scanning electron microscopy (Quanta 400 FEG, FEI Company, Hillsboro, OR, USA) and its attached cathodoluminescence (MonoCL3+) (SEM-CL) were combined to characterize the indentation-induced plastic deformation at room temperature. The cross-sectional transmission electron microscopy (XTEM) samples were prepared from the indents by means of a dual-beam focused ion beam (Nova 220) station with Ga ions. A FEI Tecnai G2 F20 S-Twin TEM operated at an accelerating voltage of 200 kV was used to study the microstructures of obtained XTEM samples.
Results and discussion
Indenter tip, contact plane, Young's modulus (E), and hardness (H) of GaN single crystal
333.61 ± 2.70
19.04 ± 0.23
330.56 ± 2.72
15.31 ± 0.20
329.74 ± 3.26
15.24 ± 0.21
317.34 ± 4.12
20.16 ± 0.48
308.26 ± 3.82
15.87 ± 0.41
306.96 ± 3.98
16.23 ± 0.44
Values were obtained from nanoindentation tests on GaN single crystal. The data from the indentation tests represent average data from at least 20 valid initial values. Also, the mean square error of each average data is given in the table. Values of E and H were calculated by Oliver and Pharr method  using load-depth curve of Figure 1.
In order to fully understand the deformation mechanism of c-plane GaN and nonpolar GaN under nanoindentation, the damages induced by conical tip were investigated by CL and XTEM (the damages induced by Berkovich tip is similar to but have less symmetry than those induced by conical tip).
To further elucidate the slip band of nanoindentation-induced deformation, an indent with maximum depth of 500 nm in c-plane GaN is investigated by XTEM. Figure 2b displayed a dark-field XTEM picture in the vicinity of the indented region. The picture displays a microstructure of the slip bands, characterized by features of aligned bright lines in the photograph. It can be seen clearly that the slip bands aligned parallel to the (0001) basal plane intersect the slip bands propagating along the (10-11) pyramidal plane. The two sets of slip bands suggest that during the indentation, the rapidly increasing dislocations can glide collectively along those planes. Moreover, selected area diffraction patterns of the highly damaged regions directly beneath the indenter showed no evidence of structural transformations to other phases of GaN. Therefore, the dislocations gliding along both the (0001) basal planes and the (10-11) pyramidal planes should be the primary mechanism of plastic deformation on the c-plane GaN during loading. The result is consistent with the previous reports [6, 8, 10–12].
To summarize, the anisotropic mechanical properties and deformation behavior based on high-quality GaN bulk material were presented in this paper. Nanoindentation tests reveal that nonpolar GaN has lower values than the c-plane GaN in both Young's modulus and hardness. Also, CL and XTEM measurements show that there are two primary dislocation slip planes ((0001) and (10-11) planes) for c-plane GaN, while there is only one most favorable dislocation slip plane ((0001) plane) for nonplane GaN during plastic deformation. We suggest that the anisotropic elasto-plastic mechanical properties of GaN are relative to its anisotropic plastic deformation behavior.
We thank the help from Suzhou Nanowin Science and Technology Co., Ltd. (NANOWIN) for the growth of samples. This work was partly supported by the National Natural Science Foundation of China (grant no. 51002179).
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