Beyond Nanowire Size Constraints: New Epitaxy Strategies Towards Metastable III-V Layers
Metastable crystal phases of conventional semiconductors comprise enormous potential for high-performance electro-optical devices, while at the same time benefitting from the chemical similarity to their stable counterparts which enables the reuse of established processing technology. AlAs, AlSb, AlP, GaP and Ge all possess indirect bandgaps in their thermodynamically stable cubic phase, whereas both theoretical calculations as well as first experimental reports suggest their band transition to be direct when their lattice periodicity is changed to the hexagonal wurtzite (WZ) or lonsdaleite (LD) symmetry [1, 2]. These findings recently triggered intense research interest for creating efficient light sources in the important amber-green wavelength regime of the visible spectrum where a lack of suitable emitting materials currently limits the performance of LEDs and semiconductor lasers . Beyond that, direct bandgap Ge and SiGe compounds could even pave the way towards active group-IV optoelectronic devices. However, synthesizing these novel polytypes remains challenging and so far, has mainly been achieved in the form of thin nanowires, mostly by using Au catalysts and by compromising other important parameters such as crystal morphology or doping, hampering scientific and commercial exploitation. In this work, we demonstrate new techniques to synthesize layers in their metastable WZ phase. InP is particularly suited for this task as both the stable cubic and the WZ phase exhibit a direct, but distinctive bandgap, which allows efficient optical analysis. We use MOCVD and selective area epitaxy to grow pure WZ nanowires and fins. We then show two extensions of this technique to obtain planar layers: The first one is based on confined epitaxy and enables growth on standard (001)- oriented substrates . In a second, newly introduced approach we explore epitaxial layer overgrowth (ELO) on (111)Aoriented wafers. This allows to grow pure WZ layers on insulator exceeding areas of 100 μm2 , constituting a promising substrate for device fabrication. The material quality of the structures is determined by micro-photoluminescence (μ-PL), high-resolution scanning transmission electron microscopy (HR-STEM), atomic force microscopy (AFM), and cathodoluminescence (CL). We compare the investigated techniques, show their limitations and develop a general model to explain polymorphism in planar layers. This work was supported by the EU H2020 program SiLAS (Grant Agreement No. 735008).  De, A. et al. Phys. Rev. B 81, 155210 (2010).  Cartoixà, X. et al. Nano Lett. 17, 4753–4758 (2017).  Gagliano, L. et al. Nano Lett. 18, 3543–3549 (2018).  Staudinger, P. et al. Nano Lett. 18, 7856–7862 (2018).