Gačević Ž, Grandal J, Guo Q, Kirste R, Varela M, Sitar Z, Sánchez García M A
ISOM, Universidad Politécnica de Madrid Avda. Complutense 30, E-28040 Madrid, Spain.
GFMC, Departamento de Física de los Materiales & Instituto Pluridisciplinar, Universidad Complutense de Madrid, E-28040, Spain.
Nanotechnology. 2021 May 7;32(19):195601. doi: 10.1088/1361-6528/abe2c7.
Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20-30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1-2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy.
通过等离子体辅助分子束外延（PAMBE）在SiO/Si（111）衬底上生长自组装氮化铝纳米线（NWs）。结合原位反射高能电子衍射和非原位X射线衍射（XRD），我们发现纳米线生长时几乎无应变，优先垂直于非晶SiO中间层生长，且与Si(111)衬底不存在外延关系，正如预期的那样。扫描电子显微镜研究显示纳米线有显著的合并现象，这导致其直径逐渐增大，并形成具有非六边形横截面的柱状结构。利用扫描透射电子显微镜（STEM），发现纳米线的初始直径在20 - 30纳米范围内。此外，在衬底表面观察到形成了一层薄的（≈30纳米）多晶AlN层。关于AlN纳米线的结构质量，STEM测量显示形成了延伸的柱状区域，这些区域以几乎完美的金属极性纤锌矿排列生长，仅偶尔观察到延伸缺陷。STEM与电子能量损失谱相结合揭示了在纳米线表面形成了连续的氧化铝（1 - 2纳米）。低温光致发光测量显示在6.03 eV（2 K时）处有一个单一的近带边（NBE）发射峰，该值与XRD证明的纳米线几乎零应变一致，并且与通过其他生长技术合成的AlN体层所获得的值相符。在≈20 meV（2 K时）处发现的NBE发射的显著半高宽表明自由激子和束缚激子在这个单一发射带内混合在一起。最后，将通过PAMBE生长的本文报道的AlN纳米线的光学性质与通过各种技术生长的体相、外延和/或柱状AlN的光学性质进行了全面比较，这些技术包括：物理气相传输、金属有机气相外延、金属有机化学气相沉积和分子束外延。
