Semiconductor nanowire (NW) based heterostructures are a promising material system for next generation optoelectronic devices, such as flexible solar cells and light emitting diodes [1]. Their reduced contact area and surface strain relaxation allow for epitaxial growth on lattice-mismatched substrates, a key advantage for integration of different III-V semiconductors with existing silicon-based technology.

Position-controlled NWs can be grown in ordered arrays on Si to improve uniformity and device integration. This is commonly performed by using a SiO₂ thin film as a mask. Patterning of circular holes in the mask (Fig. 1(a)) allows for site-specific NW growth in predefined patterns and positions. To date, this is performed using lithography techniques such as electron beam lithography or nanoimprint lithography [2]. Important processing parameters include oxide thickness, hole diameter and pattern pitch, requiring several steps to be optimized in order to achieve a high yield of uniform NWs [3]. Additionally, the catalytic particle is rarely centered in the hole, leading to undesirable asymmetry in the NW cross-sections [4].

In this work, the parameter space for direct patterning of NW growth substrates by focused ion beam (FIB) is explored (Fig. 1). Self-catalyzed GaAsSb NWs were grown using molecular beam epitaxy (MBE) on a FIB patterned Si(111) substrate with 40 nm thermal oxide, where hole size, dose and Ga-beam overlap were systematically varied (Fig. 1(a-c)). It is expected that a higher degree of flexibility and control can be attained using FIB compared to the conventionally used resist-based patterning techniques. In addition, patterning by FIB leads to Ga implantation in both Si and SiO₂, which could positively affect the self-catalyzed NW growth and the properties of the NW-substrate system in a unique way.

After MBE growth, three distinct growth regimes can be recognized, present in all arrays (Fig. 1(d-e)): The smallest (10 nm pattern) diameter row features a high yield (≤ 80%) of straight NWs. As the hole diameter increases there is initially a transition to more parasitic crystal growth and finally multiple (2-5) NWs grow within each hole. As the dose increases between arrays in each column, the patterned diameter for these transitions decreases proportionally. The results demonstrate that using FIB the parameter space can be mapped out efficiently within a single growth session and that growth can be tuned between aligned single NWs, 2D parasitic crystals and multiple NWs per hole. Transmission electron microscopy and electrical testing of single NWs directly on the growth substrate [5] will be used to refine the structural analysis and study the electrical properties of these NWs. It is expected that in addition to the flexibility of FIB patterning, III-V NWs grown on FIB-patterned Si will exhibit novel properties due to the implantation of Ga and the altered NW-substrate interface.

References:

[1] Joyce, H. J. et al., Prog. Quant. Electron., 35, 23–75 (2011), DOI: 10.1016/j.pquantelec.2011.03.002

[2] Munshi, A. M. et al., Nano Lett., 14, 960–966 (2014), DOI: 10.1021/nl404376m

[3] Plissard, S. et al., Nanotechnology, 22, 275602 (2011), DOI: 10.1088/0957-4484/22/27/275602

[4] Nilsen, J. S. et al., J. Phys. Conf. Ser., 644, 012007 (2015), DOI: 10.1088/1742-6596/644/1/012007

[5] Fauske, V. T. et al., J. Microsc., (2015, in press), DOI: 10.1111/jmi.12328

Acknowledgements: This work is supported by the Research Council of Norway through funding for the NorFab (197411) facility and the FRINATEK (214235) program.

Figures:

Figure 1: SEM characterization of a NW dose-diameter growth series. (a) FIB-milled hole array with linearly increasing hole diameter. (b) 30 arrays of 25x25 holes after NW growth. The three columns each contain ten arrays, using linearly increasing dose. Columns differ in beam overlap. (c) Cross-section of a test array. The penetration in the mask and substrate can be seen to increase with increasing hole diameter. (d, e) Arrays where holes were milled using the lowest (d) and median (e) dose, both illustrating the transition between ordered single NW growth, 2D growth and multiple NW growth with transitions at different hole sizes depending on dose.

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