Interfacial properties of self-assembled GaN nanowires on pre-processed Al2O3(0001) surfaces
Introduction
Nucleation and growth of GaN NWs on inert sapphire surfaces by plasma-assisted molecular beam epitaxy (PAMBE), is a critical issue, since NWs do not emerge without the presence on metal catalyst or other previous substrate surface treatments [1]. Although alternative substrates as Si are frequently used, sapphire offers the unique advantage of being transparent to visible light, and thus being the appropriate substrate for GaN-based opto-chemical sensor devices [2], [3]. The use of an external catalyst degrades the structural and optical properties of NWs and should be avoided. Sapphire nitridation seems to promote the growth of GaN NWs by PAMBE [4], while in-situ preparation of the sapphire surface is required for catalyst-free selective-area growth of GaN NWs by metal-organic vapor phase epitaxy (MOVPE) [5]. It is already established that GaN NWs are nucleated on sapphire by PAMBE either catalyst-assisted [4], [6], [7] or catalyst-free. In the latter case a thin AlN nucleation layer is used which is either formed unintentionally [8], [9] or it is deposited prior the growth of the GaN NWs [10], [11].
In this work we have investigated the nanostructure and interfacial properties of spontaneously grown GaN NWs on sapphire (a) after deposition of a silicon nitride (SixNy) layer on nitridated Al2O3 surface, and (b) after deposition of pure Si on bare Al2O3 surface without intentional nitridation, prior to NWs growth. To address this issue, we have utilized the knowledge on the nucleation of catalyst-free GaN NWs on Si(111) substrates by PAMBE [12], [13], [14], [15], [16], [17]. Moreover, the technological importance of SixNy or Si as intermediate layers, to improve the structural characteristics of the GaN NWs has been established in several studies [e.g. [18] and references therein]. High-resolution transmission electron microscopy (HRTEM), scanning TEM (STEM) and energy dispersive X-ray spectroscopy (EDXS) were employed. In addition, geometric phase analysis (GPA) [19] and the projection method [20] were used to quantitatively evaluate the nanostructured features of the GaN/Al2O3 interface.
Section snippets
Materials and methods
Self-assembled GaN NWs were grown by PAMBE in Nitrogen-rich growth conditions along the wurtzite (000-1) direction. The 2″ wafers of c-plane sapphire substrates were metalized on the backside with 200 nm Ti and 70 nm Pt, since heat absorption of sapphire is insufficient to achieve optimum surface temperatures for GaN growth. In the first sample, GaN NWs were grown at Tsub=695 °C on the nitridated sapphire substrate, following pre-deposition of Si (TSi=1240 °C) with the nitrogen plasma source active
Results and discussion
The overall GaN NWs morphology grown on c-plane Al2O3, (a) over the SixNy layer, and (b) when pure Si deposited for 40 min, is illustrated in the XTEM images of Fig. 1. The NWs areal density was ~5×1010 cm−2 and basal stacking faults (BSFs) were the foremost crystal defects observed in both samples. In the (a) case, the height of NWs varied in the range of 130–280 nm, while their diameter increased gradually from the bottom (15–40 nm) to the top (30–45 nm), as a result of the coalescence of adjacent
Conclusions
The interfacial structures of catalyst-free GaN NWs, grown on surface-treated c-plane sapphire substrates by PAMBE, were investigated by TEM, quantitative HRTEM/ STEM and EDXS. It was shown that when SixNy was pre-deposited on nitridated Al2O3 surface slightly tilted GaN NWs were grown, as a result of the SixNy surface roughness, leading to a high degree of NWs coalescence. In this case, the formation of a semi-coherent thin crystalline AlN epilayer directly above the Al2O3 surface and inside
Acknowledgment
Work supported by the FP7 Project DOTSENSE (Grant no. STREP 224212).
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