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Geometrical selection of GaN nanowires grown by plas-ma-assisted MBE on polycrystalline ZrN layers

Version 1.0

Sobańska, Marta, 2024, "Geometrical selection of GaN nanowires grown by plas-ma-assisted MBE on polycrystalline ZrN layers", https://doi.org/10.18150/2TK6RB, RepOD, V1

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Description

The results contained in the repository show that unidirectional supply of material fluxes in molecular beam epitaxy (MBE) together with the shadow effect favor growth of well-oriented NWs even despite random orientation of grains to which the NWs are epitaxially linked [1]. We propose a geometrical model describing an impact of MBE chamber geometry on the orientation of the NWs. Specifically, we calculated effective area of the nanowire’s top facet S_eff that is in the line of sight of an effusion cell as a function of nanowire tilt relative to substrate normal (α) and substrate rotation angle (ω). The S_eff value is equivalent to the number of atoms incorporated on the NW top facet, which controls axial growth rate of the NW if adatom surface diffusion from the NW sidewalls is negligible, as assumed in the model. For example, this is the case of metal-limited MBE growth of nitride nanowires. As expected, the maximum of S_eff corresponds to the NW facing directly in the direction of the Ga effusion cell (α = 40º, ω = 0º). However, during rotation of the substrate the fastest growing nanowires are those perpendicular to the substrate. To verify the model we grew GaN NWs on ZrN polycrystalline buffers. Without substrate rotation the NWs grew uniformly arranged at α = 40º, ω = 0º, exactly as predicted by our model. However, standard growth with rotation of the substrate led to densely packed NWs with significantly limited tilt dispersion. We attribute this behavior to geometrical selection of NWs in densely packed NW array via the shadowing of strongly tilted NWs by preferentially grown vertical ones.

A description of the results can be found in the file "readme.txt".

Subject

Physics

Keyword

GaN nanowires, ZrN buffer layers, molecular beam epitaxy, nanowire orientation, geometrical selection

Related Publication

Olszewski, K.; Sobanska, M.; Dubrovskii, V.G.; Leshchenko, E.D.;Wierzbicka, A.; Zytkiewicz, Z.R. Geometrical Selection of GaN Nanowires Grown by Plasma-Assisted MBE on Polycrystalline ZrN Layers. Nanomaterials 2023, 13, 2587. https://doi.org/10.3390/nano13182587 https://doi.org/10.3390/nano13182587 doi: 10.3390/nano13182587

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CC0 Creative Commons Zero 1.0

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		readme.txt Plain Text - 4.8 KB - Aug 1, 2024 - 0 Downloads MD5: 0fcc754753e243cc71370f95506b71f0 License: CC0 Creative Commons Zero 1.0 Data description.	Preview Download
		Fig.1.png PNG Image - 45.2 KB - Aug 1, 2024 - 0 Downloads MD5: bcf7702c739739348f6b786fd317b5ec License: CC0 Creative Commons Zero 1.0 (a) θ/2θ X-ray diffraction scan and (b) GIXRD scan of ZrN polycrystalline film on Si(111) substrate.	Preview Download
		Fig.2.png PNG Image - 540.5 KB - Aug 1, 2024 - 0 Downloads MD5: e5c2e324d87b4eaedf90872f6f43c4b6 License: CC0 Creative Commons Zero 1.0 (a) SEM image of GaN NWs grown on ZrN buffer. The NWs are mostly oriented along the substrate normal. (b) Small GaN crystallites between the NWs.	Preview Download
		Fig.3.png PNG Image - 85.0 KB - Aug 1, 2024 - 0 Downloads MD5: 59296b94c421306368164ccfb1ad1b33 License: CC0 Creative Commons Zero 1.0 SEM image of GaN NWs grown on ZrN buffer without substrate rotation. The sample was cleaved in the plane defined by the normal to the substrate and the line-of-sight of the Ga source. The dashed line shows the Ga beam direction in the MBE system.	Preview Download
		Fig.4.png PNG Image - 797.6 KB - Aug 1, 2024 - 0 Downloads MD5: a7318d8f4abb244b618f8b00aa53e925 License: CC0 Creative Commons Zero 1.0 (a – d) SEM images of different parts of GaN NW sample with increasing NW density, (e) shows the FWHM values of XRD ω-scan representing the tilt dispersion of the NW ensemble at the respective parts of the sample.	Preview Download
		Fig.5.png PNG Image - 476.6 KB - Aug 1, 2024 - 0 Downloads MD5: 034457639650fc87b28b9496d43bd0e9 License: CC0 Creative Commons Zero 1.0 Illustration of MBE growth of an inclined NW with the tilt angle α, from a Ga beam with the angle φ= 40o; ω denotes the angle related to substrate rotation. Effective area S_eff of (b) the NW top facet and (c) NW sidewalls as a function of α and ω at a fixed φ=40o. Horizontal dashed lines in (b) and (c) mark the Ga supply direction. (d) Normalized contributions of the top and sidewall (SW) facets into the total axial growth rate versus the NW tilt angle at φ=40o. Numerical solutions are obtained by summing up the elements of the contour plots of (b) and (c) over ω at a fixed α and dividing the result by the number of elements. Analytical solutions are obtained from Equations (2) to (4). The approximate solution is obtained using Equation (5).	Preview Download
		Fig.6.png PNG Image - 122.6 KB - Aug 1, 2024 - 0 Downloads MD5: 2ff1e6208fad54c2d70ebf7ac074f4a6 License: CC0 Creative Commons Zero 1.0 Total axial NW growth rates in the units of v, deconvoluted into the contributions of the top and sidewall (SW) facets versus the NW tilt angle. The curves are obtained from Eq. (1) at a fixed Ga diffusion length λ_SW=40 nm and φ=40o for different NW radii: (a) R=50 nm, (b) R=35 nm, and (c) R=20 nm. In all cases, the total axial growth rate decreases with α. The decrease is steeper for larger NW radii.	Preview Download
		Fig.7.png PNG Image - 126.7 KB - Aug 1, 2024 - 0 Downloads MD5: 99016ae1504ecdfd7f0d7e1f84cc1a54 License: CC0 Creative Commons Zero 1.0 . (a) Illustration of the geometrical shadowing effect on a NW with the tilt angle α, originating from the circle of neighboring vertical NWs. Rotating substrate does not influence the height of vertical NWs L_0, but reduces the effective height of inclined NW to L_α cosα because, for each rotation angle ω, the full length L_α is partly shadowed by one vertical NW in a circle. The average distance between the NWs equals P. From geometrical consideration, the full shadowing of a given NW occurs when its height plus Pcotanφ equals the length of vertical NWs. (b) Critical deposition thickness of GaN 〖vt〗_c as a function of the NW tilt angle at a fixed φ=40o and three different NW densities given in the legend. The critical time t_c at which all the NWs having the tilt angle α stop growing strongly decreases with α and N. Therefore, more inclined NWs stop growing faster, and finally only vertically aligned NWs survive in the whole NW ensemble.	Preview Download
		equations.PNG PNG Image - 23.2 KB - Aug 1, 2024 - 0 Downloads MD5: a72c3ce10961faad6bafb1a5d0ce075a License: CC0 Creative Commons Zero 1.0 Equations udes for calculations.	Preview Download

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