Enhancing GaN Nanowires Performance Through Partial Coverage with Oxide Shells

Results posted here concern studies on GaN nanowires crystallised by molecular beam epitaxy coated with the wide bandgap oxides: AlOx and HfOx shells of nominal thickness varying from 1 to 20 nm, obtained via atomic layer deposition (ALD) . The scanning electron microscopy images show that the shells of different nominal thicknesses exhibit varying morphology. Specifically, as the number of ALD deposition cycles decreases, the shells take the form of islands. This leads to partial coverage of the nanowires. To explore the changes in the GaN core’s crystal lattice, strain was calculated based on X-ray diffraction, photoluminescence, and Raman spectroscopy. A great agreement between all techniques was found, indicating the presence of in-plane compressive strain.Moreover, in both photo- (PL) and cathodoluminescence (CL) spectra, luminescence enhancement was observed for the nanowires with partial coatings. The observation was confirmed through temperature-dependent PL and CL measurements . It was expected that the shells would enhance luminescence by passivating surface states and inducing a flat-band effect. However, we found that thick shells may reduce signal intensity by compromising structural quality, increasing strain gradient, and scattering light. Hence, the optimum shell thickness occurs when a balance between all these factors is achieved. What is more, the partial coating was confirmed to successfully protect nanowires against photoadsorption, despite their non-homogenity. We propose that the shells grow in the Stranski-Krastanov mode, which assumes that the additional monolayer is deposited prior to the formation of islands, providing sufficient protection for the cores.

本文发布的结果涉及通过分子束外延（molecular beam epitaxy）制备的氮化镓（GaN）纳米线相关研究，该纳米线经宽带隙氧化物包覆：其壳层为氧化铝（AlOx）与氧化铪（HfOx），标称厚度范围为1至20 nm，通过原子层沉积（ALD）制备得到。扫描电子显微镜图像显示，不同标称厚度的壳层呈现出差异化的形貌特征。具体而言，随着原子层沉积循环次数减少，壳层呈现岛状结构，导致纳米线的包覆不完全。为探究氮化镓核的晶格演变，研究基于X射线衍射、光致发光（PL）与拉曼光谱计算了体系的应变。三种表征技术所得结果高度吻合，表明体系存在面内压应变。此外，在光致发光与阴极射线发光（CL）光谱中，包覆不完全的纳米线均表现出发光增强现象。该结论通过变温光致发光与阴极射线发光测试得到了验证。此前人们认为，壳层可通过钝化表面态并诱导平带效应来提升发光性能。但本研究发现，过厚的壳层会损害结构质量、增大应变梯度并散射光线，从而降低信号强度。因此，最优壳层厚度需在上述各项因素间取得平衡。更值得关注的是，尽管包覆层存在不均匀性，不完全包覆仍可有效保护纳米线免受光吸附作用。研究提出，壳层以斯特兰斯基-克拉斯塔诺夫生长模式（Stranski-Krastanov mode）生长：即先沉积额外单原子层，再形成岛状结构，从而为纳米线核提供充分保护。