Based on the formation of large islands, the coalescence of these islands easily occurs during subsequent growth, which leads to the quasi-two-dimensional growth of GaN films 2D as depicted in Fig. The GaN graphite heterostructure is clearly visible in Fig. FFT diffraction patterns for the graphite layer region 1 clearly show the plane of graphite Fig. Further, the FFT diffraction patterns of the interface region 3 indicate that GaN films grew along the graphite plane Fig. It has been reported that the AlN nucleation layers is predominantly the wurtzite hexagonal phase [ 23 ].
Based on our experiment results, it can be concluded that the nucleation layers tend to form the hexagonal structure when Al is added to it, which allows the subsequent growth of the GaN films with the hexagonal structure.
The introduction of defects by O-plasma treatment reduces the activation energy needed for atomic bonding, increasing the nucleation density of the graphite surface. In addition, adding Al can effectively improve the nucleation rate due to its high adsorption energy and low migration barrier with graphite, thus forming dense nucleation layers and promoting the subsequent growth of GaN films.
This study accelerates the fabrication of optoelectronic devices using high purity graphite as a substrate. Nano Lett 16 6 — Nanoscale Res Lett 11 1 Ryu HY Investigation into the anomalous temperature characteristics of InGaN double quantum well blue laser diodes using numerical simulation.
Nanoscale Res Lett 12 1 Jpn J Appl Phys 55 8 Kang S, Mandal A, Chu JH et al Ultraviolet photoconductive devices with an n-GaN nanorod-graphene hybrid structure synthesized by metal-organic chemical vapor deposition. Sci Rep Surf Sci 8 — Proc IEEE 10 — RSC Adv 5 2 — Appl Phys Lett 70 4 — Appl Phys Lett 91 23 Nat Commun 5 5 Jpn J Appl Phys 56 8 Appl Phys Express 6 6 Npg Asia Mater 4:e Adv Mater 24 4 — Walton D Nucleation of vapor deposits.
J Chem Phys 37 10 — Usher BF, Robins JL The role of preferred surface sites in the nucleation and growth of gold on sodium chloride I: the influence of low energy electrons. Thin Solid Films 3 — J Vac Sci Technol A 33 6 Phys Rev Lett 97 18 Zhu J, Deng H, Xue W et al Effect of low temperature oxygen plasma treatment on microstructure and adhesion force of graphene. Appl Surf Sci — J Cryst Growth 1 — Finsy R On the critical radius in Ostwald ripening.
Langmuir 20 7 — Appl Phys Lett 77 21 — Schmelzer JWP On the determination of the kinetic pre-factor in classical nucleation theory. J Non-Cryst Solids 52—54 — Number density of active sites and nucleation rates per site. J Electroanal Chem 1 — Adv Funct Mater 24 42 — Download references. TL and CL conceived the study. TL and CL analyzed the data. All authors read and approved the final manuscript. Correspondence to Tianbao Li. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Reprints and Permissions.
Search all SpringerOpen articles Search. Abstract The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. Results and Discussion In the general film deposition process, nucleation sites often appear in particular locations on the substrate, such as defects, atomic layer steps, and impurity atoms [ 16 , 17 ]. Full size image. References 1. Nano Lett 16 6 — Article Google Scholar 2. Sci Rep Article Google Scholar 6. Surf Sci 8 —88 Article Google Scholar 7. Nat Commun 5 5 Article Google Scholar Adv Mater 24 4 — Article Google Scholar Langmuir 20 7 — Article Google Scholar The horizontal dashed line denotes the boundary between the layers grown before and after oxygen annealing, and the upper layer is Al-polar.
This means that the polarity of the N-polar matrix is inverted due to the oxygen annealing, while the polarity of Al-polar columns persists. In the magnified view of the IDB Fig. The triangular contrast pattern, where the polarity inversion occurs, corresponds well to the already proposed IDB model based on the polytypoid phase of Al x O y N z 26 , 27 , The IDB can be described by two interpenetrating N-polar and Al-polar wurtzite lattices sharing a common anion sub-lattice. Cations occupy both the upper Al-polar lattice and lower N-polar lattice symmetric tetrahedral sites.
Growth and Properties of Ultrathin Epitaxial Layers, Volume 8
Since the HAADF-intensity of the cation columns is associated with the occupation of the respective sites, thus, within the IDB, the gradual change of the cation sites occupation, from N-polar to Al-polar sites establishes Al-polarity. Theoretical stick-and-ball model of Al 9 O 3 N 7 is shown for comparison.
The anion positions small balls are shared between N red and O yellow atoms. The cation positions big blue balls are occupied by Al and Al vacancies, thus displayed with different filling ratios.
Intentional polarity conversion of AlN epitaxial layers by oxygen | Scientific Reports
The stick-and-ball model of Al 9 O 3 N 7 is shown along with the experimental image on Fig. The transition interface between N-polar and Al-polar wurtzite on the experimental image consists of 13—14 monolayers instead of 4 monolayers in the Al 9 O 3 N 7 model by Asaka et al. We ascribe this to the three-dimensional irregular shape of the IDB resulting in a projection of the inclined facets of the IDB onto the image plane that apparently increases the width of the IDB in the image.
To prove that the inversion domain boundary consists of Al x O y N z and to quantify the oxygen content, we analyze it by spatially-resolved electron energy-loss spectroscopy and map the oxygen integrated intensity Fig. In particular, we studied a region, in which the N-polar matrix and an Al-polar domain lie aside from each other. As described above, the N-polar domain switches its polarity, while the Al-polar domain does not see schematic representation in the inset. The integrated line profile shows that oxygen is predominantly present within the lateral inversion boundary where the N polar column switches to Al polarity but is not measurable in the Al-polar columnar domain i.
EELS analysis of the inversion domain boundary. The rise of oxygen signal within the IDB is correlated with a drop of the nitrogen signal within the same area — an indication that oxygen atoms partially substitute nitrogen atoms within the AlN wurtzite lattice, likewise in the Al 9 O 3 N 7 structure mentioned above, where the oxygen and nitrogen atoms share the anion sites. To estimate the oxygen content within the IDB, we utilize the equation relating the EELS integrated intensities and concentrations of two elements 29 :.
Our investigation shows that the polarity of N-polar AlN films can be intentionally switched by an oxygen annealing as a mediating step in MOVPE growth. The structure qualitatively agrees with the Al 9 O 3 N 7 model that forms during nitridation of the sapphire substrate and also promotes polarity inversion as proposed by Mohn et al.
Al-polar surfaces, however, do not switch their polarity after oxygen treatment, and Al x O y N z is not present. Thus, the AlN film, deposited after oxygen annealing, is predominantly Al-polar as opposed to the mainly N-polar AlN formed during the first growth step before annealing. The fact that we do not observe a conversion from Al polarity to N-polarity might be related to the differences in the oxygen incorporation into the different surfaces.
Previous experimental and theoretical studies show that oxygen atoms more favorably adsorb on N-polar GaN and AlN surfaces than on metal polar ones since the adsorption energy at high oxygen coverage is significantly lower for the —1 -polar AlN and GaN surface compared to the -polar surface 1 , 30 , 31 , 32 , It is interesting to note, that the total amount of oxygen incorporated into the interface between N-polar AlN and overgrown Al-polar is limited to an equivalent of 1.
This is precisely the amount found by Westwood et al. These authors explained this limitation by charge compensation necessary to fulfill the Pauling rule. A straightforward consideration may explain this similarity between our results: Above we describe the IDB by two interpenetrating wurtzite lattices that share a common anion sub-lattice.
Assuming that substitutional oxygen in the AlN lattice acts as a shallow donor and provides one free electron per incorporated oxygen atom, an amount of 1.
Ultrathin Epitaxial Properties of Growth Layers And
Schematic representation of charge compensation within inversion domain boundary between N-polar light blue balls as Al cations and Al-polar dark blue balls as Al cations interpenetrating lattices. Small red and yellow balls denote N and O atoms, respectively. The black arrow schematically indicates a transfer of electrons from an oxygen donor to N dangling bonds in the IDB. Summarizing, we have demonstrated a way to intentionally invert N-polar AlN layers to Al-polar by introducing annealing in an oxygen-rich environment into the MOVPE growth process.
We showed that the oxygen annealing induces an inversion domain boundary consisting of a self-limited aluminum-oxynitride structure, which forms due to oxygen incorporation into N-polar surfaces and establishes Al-polarity of the film. Incorporation of an equivalent of 1. This effect offers a method of controlled polarity inversion of N-polar films along the growth direction during MOVPE growth and realization of N-Al hetero-polar interfaces.
Besides, the selective effect of oxygen incorporation on N-polar surfaces offers a way to suppress unwanted N-polarity in the growth of metal polar III-Nitride nanowires. It might also be used to control the size of N- and Al-polar domains in mixed-polar buffers, thus offering a method to reduce the dislocation density in AlN buffer layers for deep-UV emitters by a lateral overgrowth of domains with different polarities.
A mixture of H 2 and N 2 acts as a carrier gas. Trimethylaluminum and NH 3 gas are used as Al and nitrogen sources, respectively. The total pressure in the reactor during the AlN deposition is mbar. After the first step of AlN deposition, the process is interrupted, and the sample is transferred into a plasma-assisted molecular beam epitaxy MBE chamber.
The samples are then thinned to electron transparency by argon ion milling at accelerating voltages decreasing stepwise from 4 to 0. The values of convergence and EELS collection semi-angles remained constant for each experiment and are 28 mrad and 24 mrad, respectively. EELS measurement are performed in a fast acquisition mode This mode allows suppressing the effect of strong radiation damage of the specimen while maintaining high spatial resolution, at the expense of a low signal-to-noise ratio.
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request. Zywietz, T. The adsorption of oxygen at GaN surfaces. Fichtenbaum, N. Impurity incorporation in heteroepitaxial N-face and Ga-face GaN films grown by metalorganic chemical vapor deposition.
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Growth , — Neugebauer, J. Ambacher, O. Amano, H. Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer. Nakamura, S. Rouviere, J. MRS Internet J. Nitride Semicond. Fuke, S. Influences of initial nitridation and buffer layer deposition on the morphology of a GaN layer grown on sapphire substrates. Paduano, Q. Effect of initial process conditions on the structural properties of AlN films. Matsuoka, T. Funato, M. Heteroepitaxy mechanisms of AlN on nitridated c- and a-plane sapphire substrates.
Alloing, B. On the polarity of GaN micro- and nanowires epitaxially grown on sapphire and Si substrates by metal organic vapor phase epitaxy and ammonia-molecular beam epitaxy. Coulon, P. Dual-polarity GaN micropillars grown by metalorganic vapour phase epitaxy: Cross-correlation between structural and optical properties. Duff, A. B - Condens. Matter Mater. Armstrong, J. Interactions between Light Waves in a Nonlinear Dielectric. Fejer, M. Quasi phase-matching second harmonic generation: tuning and tolerances.
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Quantum Electron. Chowdhury, A. Second-harmonic generation in periodically poled GaN. Romano, L. Faceted inversion domain boundary in GaN films doped with Mg. Grandjean, N.
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Control of the polarity of GaN epilayers using a Mg adsorption layer. Atomic structure of pyramidal defects in Mg-doped GaN. B 68 Pezzagna, S. Polarity inversion of GaN 0 0 0 1 by a high Mg doping. Mohn, S. Stolyarchuk, N. Impact of sapphire nitridation on formation of Al-polar inversion domains in N-polar AlN epitaxial layers.
Definition of Epitaxial Growth
Serneels, R. Status Solidi 58 , — Inversion domains in GaN grown on sapphire. Asaka, T.