Dislocation luminescence in GaN single crystals under nanoindentation
© Huang et al.; licensee Springer. 2014
Received: 24 August 2014
Accepted: 4 November 2014
Published: 1 December 2014
This work presents an experimental study on the dislocation luminescence in GaN by nanoindentation, cathodoluminescence, and Raman. The dislocation luminescence peaking at 3.12 eV exhibits a series of special properties in the cathodoluminescence measurements, and it completely disappears after annealing at 500°C. Raman spectroscopy shows evidence for existence of vacancies in the indented region. A comprehensive investigation encompassing cathodoluminescence, Raman, and annealing experiments allow the assignment of dislocation luminescence to conduction-band-acceptor transition involving Ga vacancies. The nanoscale plasticity of GaN can be better understood by considering the dislocation luminescence mechanism.
KeywordsGaN Nanoindentation Dislocation Luminescence
GaN-related III-nitride materials have gained an unprecedented attention due to their wide-ranging applications such as short-wavelength optoelectronic devices , high-electron-mobility transistor , and semiconductor lasers . However, due to the lack of large-sized bulk materials, the majority of GaN-related alloys or structures are grown heteroepitaxially on foreign substrates such as sapphire or SiC. Consequently, those alloys or structures usually contain a high density of dislocations which can have detrimental effects on the performance of devices. In spite of the considerable progress made in the last decade in GaN, an in-depth understanding of the properties of dislocation is needed due to their paramount importance in the growth of most conventional semiconductor materials and in the manufacture of semiconductor devices. However, the optical and electronic properties of as-grown dislocations may be greatly affected by the unintentionally introduced impurities and defects during the growth process. Thus, it is interesting to clarify intrinsic optical properties of dislocations both in basic research and technological applications.
Nanoindentation is an ideal technique for studying the fundamental behaviors and properties of dislocations in a crystal by introducing dislocations into a small volume that is initially defect-free. Consequently, nanoindentation experiments and simulations can be used to demonstrate mechanisms governing dislocation nucleation in a broad range of fields and applications [4, 5]. Especially, there has also been a considerable effort to determine the properties of plastic deformation in GaN epilayers and GaN bulk crystals using indentation techniques [6–14]. Local strain fields of the indentation have been studied by a micro-Raman spectroscopy [11, 13], and the formation of contact-induced dislocations has been investigated via cathodoluminescence (CL) spectroscopy [6–9, 11] and transmission electron microscopy (TEM) [6–8, 10, 12]. However, most of these earlier studies mainly focused on the microstructure of the indentation-induced dislocations in GaN; the fundamental dislocation luminescence mechanism of GaN is not understood fully. This work presents a comprehensive study encompassing nanoindentation, CL, and Raman techniques aimed at revealing the origin of the dislocation luminescence in GaN.
A 1.5-mm-thick freestanding GaN layer with an area size of about 20 mm × 20 mm was selected for the indentation tests. The thick GaN layer grown by hydride vapor phase epitaxy on the c-plane of sapphire substrate was self-separated during cooling down from the growth temperature. The dislocation density of the GaN freestanding layer was about 5 × 105 cm−2 as estimated by the etch pit density. The background carrier concentration was about 1 × 1016 cm−3 from the analysis of the Hall data.
Nanoindentation tests were performed on the GaN (0001) surface using a nanoindentation system (Nano Indenter G200, Agilent Technologies, Inc., Santa Clara, CA, USA). A Berkovich indenter tip with a radius of curvature of 50 nm was employed for indentation experiments. The strain rate was set at 0.05 s−1 during nanoindentation tests. Scanning electron microscopy (Quanta 400 FEG, FEI, Hillsboro, OR, USA) - cathodoluminescence (MonoCL3+, Gatan, Inc., Pleasanton, CA, USA) system was used to characterize the indentation. The Raman spectra measured by a LabRAM HR 800 spectrometer (LabRAM HR 800 spectrometer, HORIBA Scientific, Edison, NJ, USA) were excited with the 633.28-nm He-Ne laser allowing for a lateral resolution of better than 1 μm.
Results and discussion
In fact, the dislocations are widely thought to be a non-radiative center in wurtzite GaN; they are not likely to manifest themselves by luminescence, unless point defects are trapped at them . Therefore, the most probable origin of VL is due to point defects.
From the above discussion, one can glean the obvious that the VL is related to a native point defect introduced by the indentation. Among all the native point defects in GaN, VGa appears to be the best candidate, since the transition energy from the conduction band to the 0/− transition level of VGa is estimated at about 3.15 eV , which is very close to the photon energy of VL. In addition, VGa was found to anneal out in long-range migration processes at 500 to 600 K [28, 29], which is consistent with the vanishing of VL in indented GaN after annealing at 500°C. The assignment of the VL peak to VGa is also supported by the Raman spectra, since the Raman spectra have found evidence for the existence of Ga vacancies in the indented region. Therefore, the most plausible cause for the VL is the VGa.
In conclusion, the VL band peaking at about 3.12 eV from the region near the dislocations is characterized and identified. A comprehensive study encompassing CL measurements, annealing experiments, and Raman analysis allow the assignment of VL band to e-A transitions involving VGa. A formation mechanism of vacancies by the motion of jogged dislocations is proposed to explain the dislocation luminescence in GaN single crystals under nanoindentation. The nanoscale plasticity of GaN can be better understood by considering that not only the dislocation mechanisms but also the nucleation of point defects are involved in the deformation.
Huang is currently a Postdoctoral Associate in the Center of Characterization and Analysis, SINANO, CAS. K. Xu, J. F. Wang, J. C. Zhang, and G. Q. Ren are professors in the Center of Characterization and Analysis, SINANO, CAS. Y. M. Fan is a PhD student in the Center of Characterization and Analysis, SINANO, CAS.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61306001, 61274127, 11327804, 61325022), the National Basic Research Program of China (973 Program No. 2012CB619305), the National High Technology Research and Development Program of China (863 Program) (Grant No. 2014AA032605), STS-Network Plan, CAS (KFJ-EW-STS-043), the Natural Science Foundation of Jiang Su (Grant Nos. BK2012630), and the Su Zhou International Technology Cooperation Program (Grant Nos. SH201225).
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