Dislocation Formation via an r-Plane Slip Initiated by Plastic Deformation during Nano-Indentation of a High Quality Bulk GaN Surface

Abstract

Bulk GaN substrates are of significant interest because they offer both high quality and low dislocation densities. Our group has previously reported the formation and movement of dislocations in high quality bulk GaN in response to nano-indentation. We have also proposed a mechanism involving an r-plane (-1012) slip initiated by plastic deformation during a pop-in event, a theory that was supported by molecular dynamics simulations. Herein, we present experimental evidence for this r-plane (-1012) slip mechanism in an indented GaN surface using nano-indentation with an indenter having a smaller radius (∼100 nm) and imparting a lower pop-in load (∼400 μN) compared to the values applied in our previous studies. In addition, this study included TEM observations immediately after the plastic deformation, such that cross-sectional TEM images of the indented surface of the c-plane bulk GaN were acquired just after the pop-in event. The pyramidal dislocation line of an r-plane slip was clearly observed and was inclined by 43° relative to the c-plane surface. Neither a basal c-plane slip nor a prism m-plane slip occurred as a result of dislocation multiplication as secondary or tertiary slip systems, even though these slips had been identified when employing a larger radius indenter and a higher pop-in load. From these experimental results, we were able to confirm that plastic deformation in bulk GaN is initiated via an r-plane slip.

Linear dependence of dislocation pattern size on the imprint width and scratch width on (0001) GaN

“…This large discrepancy of the first pop-in load appears to be caused by the crystal quality, type of indenter, or tip shape imperfection of the indenter. However, we could not find any consistency in [29][30][31][32][33][34][35]. In this experiment, we speculate that the first pop-in is related to an indentation mode change from spherical contact to Berkovich contact, because the load (0.2-0.3 mN = displacement of 12.4-15 nm) in which the first pop-in occurred is close to the X-axis intercept (0.32 mN = displacement of 16 nm) of the approximate line of the dependence of d i on the SqL.…”

“…The first pop-in at the low load range is not the threshold of elastic deformation to elastic-plastic deformation, according to the hysteresis of the load-displacement curve with maximum loads of 100 and 200 µN. The load range (200-300 µN) in which the first pop-in occurred is much lower than the reported first pop-in loads (0.4-40 mN) [29][30][31][32][33][34][35] on GaN (0001). This large discrepancy of the first pop-in load appears to be caused by the crystal quality, type of indenter, or tip shape imperfection of the indenter.…”

“…Therefore, the generation of dislocations with b = 1/3 < 11 23> on { 11 22} is the most probable, and this type of dislocation determines the penetration depth of dislocations around the imprint induced by indentation and confined under the imprint [23,24]. If we assume that dislocations which are induced around the imprint by Berkovich indentation are similar to those induced by Vickers indentation, the generated dislocations are b = 1/3 < 12 13> on {1 212} in a triangular area [23], b = 1/3 < 12 10> on (0001) in a flower pattern [23,24,36,37], and b = 1/3 < 12 10> on {10 10} in a rosette pattern [29,32,35,36,[38][39][40][41]. Although the generation of lateral dislocation loops with <11 20>/(0001) and <11 20>/{1 100} slip systems are unlikely because of the zero Schmid factor, the slip system <11 20>/(0001) becomes active according to the contact of the facets of the Berkovich indenter with GaN in the higher load range, because the load direction changes from [000 1] to the direction perpendicular to the facet of the Berkovich indenter.…”

Deformation and fracture in (0001) and (10-10) GaN single crystals

“…The majority of studies on mechanical properties concern GaN thin film, with limited number of investigations of c-plane GaN single crystals describing separately either crack formation at higher loads or dislocations and mechanical properties [14][15][16][17][18]. Nonpolar orientations for GaN thick films or single crystals have received less attention [19,20], while there has been a recent publication that studies the main dislocation slip system in bulk GaN [21]. The present study covers three deformation stages: elastic deformation at low loads, the pop-in and dislocation propagation and finally crack formation, both in c-plane and m-plane GaN single crystals, by utilising indentation methods at nanoscale and microscale and covering loads that range in four orders of magnitude (from 10 −4 up to 2 N).…”