Abstract
The growth of GaN nanowires having a polar, wurtzite structure on nonpolar Si substrates raises the issue of GaN nanowire polarity. Depending on the growth procedure, coexistence of nanowires with different polarities inside one ensemble has been reported. Since polarity affects the optical and electronic properties of nanowires, reliable methods for its control are needed. In this work, we use Kelvin probe force microscopy to assess the polarity of GaN nanowires grown by plasma-assisted Molecular Beam Epitaxy on Si(111) substrates. We show that uniformity of the polarity of GaN nanowires critically depends on substrate processing prior to the growth. Nearly 18% of nanowires with reversed polarity (i.e., Ga-polar) were found on the HF-etched substrates with hydrogen surface passivation. Alternative Si substrate treatment steps (RCA etching, Ga-triggered deoxidation) were tested. However, the best results, i.e., purely N-polar ensemble of nanowires, were obtained on Si wafers thermally deoxidized in the growth chamber at ~1000 °C. Interestingly, no mixed polarity was found for GaN nanowires grown under similar conditions on Si(111) substrates with a thin AlOy buffer layer. Our results show that reversal of nanowires’ polarity can be prevented by growing them on a chemically uniform substrate surface, in our case on clean, in situ formed SiNx or ex situ deposited AlOy buffers.
Highlights
GaN nanowires (NWs) are found as promising building blocks for a future generation of electronic and optoelectronics devices
The growth of wurtzite GaN nanowires on nonpolar Si substrates raises the issue of GaN NW polarity, which is known to have a critical impact on the structural properties of GaN, such as the incorporation of dopants [1,2,3,4], surface reactivity [5] and thermal stability [6], as well as on the nucleation and growth of GaN NWs [7,8]
The most common procedures used to process Si substrates prior to epitaxial growth are tested to find their impact on polarity uniformity inside the ensemble of self-assembled GaN NWs grown by plasma-assisted molecular beam epitaxial (PAMBE)
Summary
GaN nanowires (NWs) are found as promising building blocks for a future generation of electronic and optoelectronics devices. NWs are almost strain-free objects without lattice misfit defects propagating into the crystalline structure even if grown on highly lattice-mismatched substrates These nanostructures facilitate, for instance, the integration of GaN-based devices with Si electronics. It is widely reported that the polarity of self-assembled NWs is determined at the GaN nucleation stage and can be reversed from N- to Ga-polar by the high local surface concentration of impurities like Mg, Si, Ti or O [20,23,24,25,26,27,28,29,30] These findings clearly indicate that proper substrate preparation is decisive for the achievement of homogeneous polarity. This shows the crucial role the chemistry at the GaN/Si(111) interface plays for the determination of GaN NWs polarity
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