Full Length ArticleInGaN nanocolumn growth self-induced by in-situ annealing and ion irradiation during growth process with molecular beam epitaxy method
Introduction
Annealing is a traditional technique in material processing, especially for alloys forging [1], [2], which can eliminate the defects and residual stress, reduce the hardness, generate phase transition in steel, and so forth. These applications of annealing have the identical effects on III-nitrides semiconductors. In addition, many research groups, worldwide, made great efforts to investigate the micro-structural changes of GaN [3], [4], [5], [6], [7], InGaN [3], [4], [5], [8], [9], [10], [11], [12], [13], [14] and InAlN [3], [4], [15] induced by break of chemical bonds in annealing process. This makes it more attractive to remove the phase separation in rich indium content alloys according to the method of annealing [14], [16], [17]. Besides that, for GaN related semiconductors, annealing is usually applied to activate dopants [18] and prepare ohmic contact [19].
Comparing with the huge number of works about the annealing, however, there are only a limited number of investigations about the GaN surface treatments using nitrogen ions at elevated temperature [21], [22], [23], [24], [25]. The most famous effect caused by ions on GaN surface was interpreted into the Bradley-Harper (BH) theory [26]: if the incident ion beam, at low glancing angle, did not react chemically with the solid, the ripple would form on surface and it can be considered in general to be results of competition between a curvature-driven roughness, introduced by ion sputtering, and smoothing induced by surface diffusion. Whereafter, the nano-sculpting technique, on this basis of BH theory, was developed to assist the growth by molecular beam epitaxy (MEB), on the aim of eliminating the dislocations [20], [21], [27], [28].
At the same time, GaN nanocolumns (NCs) by MBE gain much attraction for their high aspect ratio about lightening [29] and photodetecting [30]. GaN NCs have advantages to be grown catalyst-free or without substrate pre-patterning. In other words, the self-assembled GaN NCs do not require any noble metal catalyst or expensive semiconductor microfabrication. Furthermore, the self-assembly method is a novel growth mode by MBE and it is valuable for fundamental researches on theory and application. Although the self-assembled NCs were first achieved in 1997 [31], the formation mechanisms of GaN NCs haven’t been elucidated clearly until now.
In fact, the self-induced NCs must go through three steps: Forming the NC nuclei, elongating the nuclei and ending up with the NC coalescence [32]. It is easy to understand that nucleating is a surface migration behavior which is governed by surface morphology. Meanwhile, the NCs elongation, also easily affected by the surface morphology, can be achieved by Volmer-Weber growth, which protects the NCs from coalescing laterally. As described above, annealing and ion irradiation can alter microstructures of alloys, which may put forming and elongating the NCs nuclei into practice to grow self-induced NCs. In this letter, self-assembled InGaN NC nuclei could be simply obtained by in-situ annealing and ion irradiation (thermal and ion etching, TIE) during growth, which can be considered as origins of growing self-assembled NCs. Because of the decomposition of In-N bonds during TIE process and strain relaxation at the subsequent growing stage, multiphase domains generated and lattices misorientation took place, respectively, which impelled the growth of NCs.
Section snippets
Experimental
The InGaN-based NC structures, were grown by plasma assisted MBE on c-plane sapphire. The group III fluxes including high-purity elemental In and Ga were supplied by effusion cells. Active nitrogen was produced by an RF plasma source, using a nitrogen flow rate of about 3 sccm. That resulted in an N-limited growth rate of about 0.8 μm/h. The onset of sample growth began of with a thin AlN nucleation layer at about 800 °C. The growth temperature was lowered to about 700 °C for GaN layer with
Results and discussions
Since the axial growth rate of NCs is much faster than the lateral, the NCs form with one-dimensional geometrical shape, which determines more compositional variation along the axial direction∥[0001]. Hence, an overview on compositional change, as shown in Fig. 2, was studied by EDS line scan from the substrate to the top of NCs. As expected, because of the effect of TIE treatment, the Ga and In composition abruptly vary on the interface of InGaN template and InGaN NCs, where the Ga content
Conclusion
In the present paper, we have provided a new approach for synthesizing self-induced InGaN NCs, which was initiated with forming NC nuclei on annealed and ion bombarded InGaN template. Because of TIE treatments, the NC nuclei were isolated by corroded V-pits and took shape. After TIE, the amorphous alloys were filled in the corroded V-pits and the z-GaN were discovered on the sidewalls of V-pits. Both of them could be considered as the results from TIE. The amorphous alloys played a key role to
Acknowledgments
This work was supported by Natural Science Foundation of Jiangsu Province (BK20160883), the NSFC (61604080, 61574079, 61634002 and 61474060), University Science Research Project of Jiangsu Province (16KJB140011 and 14KJB510020) and NUPTSF (NY214154).
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