Influence of Hydrogen on Alloy Catalyst and Growth Mechanism from VLS-Hydride-Vapor-Phase-Epitaxy of GaN Nanowires

Nanowires, with their unique one-dimensional structure that enhances their electronic performance, are primarily grown via the vapor-liquid-solid (VLS) method. The growth of gallium nitride (GaN) nanowires requires high temperatures; hence, sustainable alloy catalysts must be applied instead of pure metal catalysts in conventional VLS growth. This bottom-up approach, in collaboration with the hydride vapor phase epitaxy (HVPE), is known for its cost-effectiveness and potential for large-scale production. The typical carrier gas in HVPE is nitrogen, while our research found that the addition of hydrogen in the transport of Ga precursor influenced the composition of alloy catalysts. This observation necessitates further investigation to understand the impact on the growth and properties of GaN nanowires.<br/>In this study, we explore GaN nanowires grown from Ni/Au alloy catalysts utilizing either pure nitrogen or nitrogen with hydrogen as carrier gases on substrates of MoS₂, c-sapphire, and Si (111). The morphology of GaN nanowires was significantly altered from wire-like to rod-like after adding hydrogen to the nitrogen carrier gas. The optical quality of GaN nanowires was enhanced, demonstrated by a pronounced PL peak at the near-band edge of GaN and a marked reduction in defect-related PL yellow emissions. These effects were especially notable in the nanowires grown on MoS₂ substrates. Additionally, the microstructural and elemental analyses using scanning transmission electron microscope (STEM) and energy dispersive X-ray spectroscopy (EDXS) reveal that introducing hydrogen into nitrogen carrier gas during GaN nanowire growth substantially influences the catalyst composition and nanowire growth orientation. At the growth temperature, the Ni/Au reacts with Ga precursor gas to form NiGa nanocrystals embedded in the AuGa liquid droplets. In a pure nitrogen growth environment, the overall catalyst droplets are larger in size, containing a floating NiGa crystal embedded in liquid AuGa with a lower Ga atomic concentration. This condition results in the formation of m-oriented GaN nanowires. In comparison, with the hydrogen introduction into the nitrogen carrier gas, the overall catalyst droplets became smaller, where a faceted NiGa nanocrystal was attached to the nanowire growth front, and the liquid AuGa contained the higher Ga atomic concentration. This leads to the predominance of c-oriented GaN nanowires. In other words, the NiGa nanocrystals were observed floating within the catalyst droplets of m-oriented GaN nanowires, whereas, in c-oriented GaN nanowires, they were attached to the growth front, exhibiting the epitaxial relationship with GaN nanowires. It is speculated that the condition of the NiGa nanocrystals within the liquid AuGa enables the control of vapor-liquid-solid (VLS) or vapor-solid-solid (VSS) growth, thus making the growth orientation of nanowires adjustable. These fundamental insights reveal the critical role of the growth environment, whether hydrogen or hydrogen with nitrogen, in enabling precise control of GaN nanowires and expanding their potential applications.