Seminarium Fizyki Ciała Stałego

Group-III nitride nanowires (NWs) offer a promising alternative to planar heterostructures. Due to their one-dimensional geometry, generation and propagation of threading dislocations is effectively suppressed, enabling the growth of high-quality epitaxial structures even on structurally dissimilar or amorphous substrates. In addition, complicated heterostructures can be ideally grown in the form of NW with a crystallographic quality not achievable in the case of comparable planar structures. Therefore, application of group III-nitride NWs is considered as a promising pathway toward new generation of opto- and microelectronic devices. I will start the talk with an overview of processes active during catalyst-free self-assembled growth of GaN NWs by plasma-assisted MBE on amorphous substrates (e.g. nitridated Si) and an impact of these processes on arrangement [1-3], optical properties [4,5] and polarity [6] of structures formed. Although controlling the density of self-assembled NWs remains difficult and their spatial distribution is inherently random, spontaneous formation of group III-nitride NWs continues to be the mainstream approach. Next I will discuss the case of self-assembled growth of GaN NWs on polycrystalline metal substrates. Metal buffer layers show high electrical and thermal conductivities and optical reflectivity, and provide both a nucleation layer for growth and potentially offer a backside electrical contact for bottom-up optoelectronic structures which is crucial for electrically-driven devices. However, an epitaxial link of GaN to randomly oriented metallic grains creates a problem with controlling tilt of the NWs relative to the substrate surface. I will show that the arrays of device-relevant well-oriented vertical NWs can be obtained by MBE despite a random orientation of metallic grains on which epitaxially linked NWs nucleate. The respective model of geometrical selection of vertical NWs will be presented [7]. In addition, I will show that low resistive ohmic electrical contact is formed if metallic ZrN buffer layer on Si or sapphire substrate is used for nucleation of GaN NWs [8].Finally, selective area growth (SAG) will be presented as an effective strategy to suppress coalescence-induced defects and to achieve spatially ordered nanowire arrays. By confining nucleation to lithographically defined places, SAG enables precise control over nanowire position and morphology [9]. In its homoepitaxial variant, when GaN NWs are grown on patterned GaN substrate, both N-polar and Ga-polar NWs can be obtained by selecting substrate with the appropriate polarity. This gives an additional degree of freedom in designing devices, in contrast to self-assembled NWs which are typically N-polar. The growth mechanism, shape evolution and ways of controlling alignment of GaN NWs grown by SAG on GaN/sapphire templates will be discussed.[1] M. Sobanska et al., Nanotechnology 27 (2016) 325601[2] M. Sobanska et al., Nanotechnology 30 (2019) 154002[3] A. Wierzbicka et al., Appl. Surf. Sci. 425 (2017) 1014[4] K.P. Korona et al., J. Luminescence 155 (2014) 293[5] M. Sobanska et al., J. Appl. Phys. 118 (2015) 184303[6] M. Sobanska et al., Electronics 9 (2020) 1904[7] K. Olszewski et al., Nanomaterials 13 (2023) 2587[8] S. Tiagulsky et al., Nanoscale 17 (2025) 8111[9] M. Sobanska et al. Nanotechnology 31 (2020) 184001