Nanoscratch Characterization of GaN Epilayers on c- and a-Axis Sapphire Substrates
© The Author(s) 2010
Received: 13 May 2010
Accepted: 26 July 2010
Published: 7 August 2010
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© The Author(s) 2010
Received: 13 May 2010
Accepted: 26 July 2010
Published: 7 August 2010
In this study, we used metal organic chemical vapor deposition to form gallium nitride (GaN) epilayers on c- and a-axis sapphire substrates and then used the nanoscratch technique and atomic force microscopy (AFM) to determine the nanotribological behavior and deformation characteristics of the GaN epilayers, respectively. The AFM morphological studies revealed that pile-up phenomena occurred on both sides of the scratches formed on the GaN epilayers. It is suggested that cracking dominates in the case of GaN epilayers while ploughing during the process of scratching; the appearances of the scratched surfaces were significantly different for the GaN epilayers on the c- and a-axis sapphire substrates. In addition, compared to the c-axis substrate, we obtained higher values of the coefficient of friction (μ) and deeper penetration of the scratches on the GaN a-axis sapphire sample when we set the ramped force at 4,000 μN. This discrepancy suggests that GaN epilayers grown on c-axis sapphire have higher shear resistances than those formed on a-axis sapphire. The occurrence of pile-up events indicates that the generation and motion of individual dislocation, which we measured under the sites of critical brittle transitions of the scratch track, resulted in ductile and/or brittle properties as a result of the deformed and strain-hardened lattice structure.
GaN-related III–nitride materials are highly attractive semiconductor materials because of their great potential for the development of optoelectronic devices in blue/green light emitting diodes, semiconductor lasers, and optical detectors [1–4]. The most common orientation of sapphire used as a substrate for GaN is c-axis sapphire. Although GaN epilayers on sapphire substrates generally exhibit a large lattice mismatch (ca. 13.9%), causing in-plane tensile strain of the sample, the lattice mismatch of the GaN films on a-axis (11 0) sapphire is less (2%) than that on c-axis (0001) sapphire (13.9%), suggesting that excellent quality GaN can be grown with improved surface morphology . Furthermore, compared with bulk single crystals, the deformation properties of thin films are more strongly correlated with their geometrical dimensions and defect structure of the material. Indeed, misfit dislocations at the interface play an important role in determining such properties as carrier mobility and luminescence efficiency. Unfortunately, mechanical damages to GaN epilayers, such as film cracking and interface delamination caused by thermal stress or chemical–mechanical polishing, usually decrease the processing yield and the reliability of their applications in microelectronic devices [6–8]. Surface measurements have been made possible through the development of instruments that continuously measure force and displacement during the process of making an indentation [9–12]. Slip band movement [13, 14] and dislocation nucleation mechanisms  have been proposed to explain “pop-in” events. Most of these studies have been performed using c-axis GaN epilayers and bulk single crystals . This nanoscratch technology, which directly processes the surfaces of materials using a diamond particle or tip of nano size, is attractive for several reasons: the free selection of materials, the simple alternation of the design principles, and the convenient initial facilities [17, 18]. The values of H (Hardness) and residual stress are two of the most significant parameters for characterizing tribological film [19, 20]. Comparisons of the nanotribological behavior of GaN epilayers grown on c- and a-axis sapphire substrates have not been reported previously in detail.
In this article, we describe our investigation into the nanotribological characterization of GaN epilayers. We investigated the pile-up-induced impairment of GaN epilayers on c- and a-axis sapphire substrates using a nanoscratch system and atomic force microscopy (AFM).
The GaN epilayers used in this study were grown using metal-organic chemical vapor deposition (MOCVD) onto both c-plane (0001) and a-plane (11 0) sapphire substrates. To fabricate the GaN epilayers, a 10-nm-thick AlN buffer layer was grown on the sapphire substrate, and then both GaN epilayers (thickness: ca. 2 μm) were grown on top of the buffer layer through MOCVD at 1,100°C, using triethylgallium (TEGa), trimethylaluminum (TMAl), and ammonia (NH3) as the gallium, aluminum, and nitrogen sources, respectively. The GaN epilayers grown on c-plane (0001) and a-plane (11 0) sapphire had a  orientation and a [1 1 0] orientation.
The nanotribological properties of GaN epilayers were determined by combining AFM (Digital Instruments Nanoscope III) together with a nanoindentation measurement system (Hysitron), operated at a constant scan speed of 2 μm s−1. For the GaN epilayer/sapphire systems, constant forces of 2,000 and 4,000 μN were applied. The maximum load was then maintained while forming 10-μm-long scratches. Surface profiles before and after scratching were obtained by scanning the tip at a 0.02-mN normal load (i.e., a load sufficiently small that it produced no measurable displacement). After scratching, the wear tracks were imaged using AFM.
Critical lateral forces and values of μ determined from nanoscratch trace depths within GaN films on c- and a-axis sapphire substrates
Normal load (µN)
Coefficient of friction
Lateral force (µN)
The fluctuation profile from a nanoscratch tests does not depend exclusively on the plasticity or the value of H; it is also related to the adhesive strength between the film and the substrate. While the contact area between the tool and the GaN surface increases, the pressure under the tool becomes insufficient to drive the transformation to a denser crystal structure. Thus, the deformation theory cannot be accommodated in a ductile manner. From studies of nanomachining processing, both tribological and chemical effects, rather than physical deformation and fracture, are believed to become dominant. In this scenario, the average measured residual stress in the cracking zone is much lower than that in the crack-free zone, because the elastic strain is released by cracking. From the nanotribological point of view, curvature and/or distribution in the values of μ signal the onset of adhesive failure, such as cracking or delamination resulting from the interaction between the sliding stylus and the debris formed on the nanoscratch track [21, 22]. Several factors can affect the value of H of a film, including the packing factor, stoichiometry, residual stress, preferred orientation, and grain size. In our experiments, the orientation of the GaN sample not only affected its nanotribological performance but also its scratching resistance; thus, the volume of the removed material from nanoscratch tests can be measured to determine the role of the orientation of the GaN sample. This approach can be used to explain the nanotribological behavior of the GaN sample; for example, the profile of GaN/a-axis sapphire sample reveals more serious wear of the components (Fig. 1) and more unwanted self-excited oscillations (Fig. 2) than that of the GaN/c-axis sapphire sample. Thus, the GaN/c-axis sapphire sample revealed relatively small oscillations with respect to the ramped load. Taken together, our findings reveal that the nanoscratch deformation of the GaN samples was influenced primarily by the orientation of the sapphire substrate. The mechanisms for the dislocation recovery from elastic and/or plastic deformation appear to be associated with the activation of dislocation sources brought about by the nanoscratching of the GaN sample. The plastic deformation prior to nanoscratching was associated with the individual movement of a small number of new nucleation sites; large shear stress was quickly accumulated underneath the indenter tip. When the local stress underneath the tip reached high-level cycles, a burst of collective dislocation movement on the slip system was activated, resulting in a release of local stress. The extensive interactions between the dislocations slipping along the surface of the GaN/a-axis sapphire sample, therefore, confined the brittle transition part of the scratch track, resulting in ductile and/or brittle properties because of the deformed and strain-hardened lattice structure.
We employed a combination of nanoindentation and AFM techniques to investigate the contact-induced deformation behavior of GaN films on c- and a-axis sapphire substrates. We observed three separate scratch processes in the ductile, brittle transition (elastic–plastic deformation), and brittle regions. AFM morphological studies of the bulge edge scenarios provided evidence for significant reductions in the average scratch depth for the GaN/c-axis sapphire. It suggested that the substrate orientation dominated the extent of ploughing in the GaN epilayers during the scratching process. In addition, this discrepancy suggested that c-axis sapphire–grown GaN epilayers have higher shear resistance than those grown on a-axis sapphire. Pile-up events indicated the generation and motion of individual dislocations measured under the critical brittle transition part of the scratch track, result in ductile and/or brittle properties.
This research was supported by the National Science Council of the Republic of China (NSC-98-2221-E-009-069) and by the National Nano Device Laboratories in Taiwan (NDL97-C04SG-088, NDL97-C05SG-087).
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