Soci Cesare, Bao Xin-Yu, Aplin David P R, Wang Deli
Department of Electrical and Computer Engineering, Jacobs School of Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0407, USA.
Nano Lett. 2008 Dec;8(12):4275-82. doi: 10.1021/nl801986r.
The epitaxial growth of GaAs nanowires (NWs) on GaAs(111)B substrates by metal-organic chemical vapor deposition has been systematically investigated as a function of relevant growth parameters, namely, temperature, arsine (AsH3) and trimethyl-gallium (TMGa) flow rates, growth time, and gold nanoparticle catalyst size. When growing in excess As conditions (V/III molar ratios greater than four), the NW growth rate is independent of AsH3 concentration, while it is linearly dependent on TMGa concentration, and it is thermally activated. The NW morphology is primarily affected by the growth temperature, with very uniform NWs growing at around 400 degrees C and severely tapered NWs growing above 500 degrees C. A simple phenomenological expression that allows prediction of the NW growth rate over a wide range of growth parameters has been derived. The growth rate dependence on the seed nanoparticle size has also been investigated, which reveals valuable information on the role of catalyst supersaturation and Ga surface diffusion in the growth mechanism. The NW growth rate is found to be almost independent of Au nanoparticle size down to diameters of approximately 20 nm over a wide range of temperatures and TMGa and AsH3 molar flows. For smaller NW radii, the growth rate becomes size-dependent and is strongly affected by the V/III molar ratio; at relatively low V/III ratios, smaller NWs grow more slowly due to the Gibbs-Thompson effect, while at higher V/III ratios (V/III >50), Ga adatom diffusion becomes the dominant mass-transport mechanism, and smaller NWs grow faster than larger ones. The growth-limiting mechanisms in the above growth regimes are finally discussed, and important quantities such as pyrolysis efficiency of the precursors, supersaturation, and surface diffusion length are deduced by comparing the experimental results with the NW growth rates predicted from first principles.
通过金属有机化学气相沉积法在GaAs(111)B衬底上外延生长GaAs纳米线（NWs），已作为相关生长参数的函数进行了系统研究，这些参数包括温度、砷化氢（AsH3）和三甲基镓（TMGa）的流量、生长时间以及金纳米颗粒催化剂的尺寸。在过量砷条件下生长（V/III摩尔比大于4）时，NW的生长速率与AsH3浓度无关，而与TMGa浓度呈线性相关，并且是热激活的。NW的形态主要受生长温度影响，在约400℃时生长出非常均匀的NW，而在500℃以上生长出严重锥形的NW。已经推导出一个简单的唯象表达式，可用于预测在广泛的生长参数范围内NW的生长速率。还研究了生长速率对种子纳米颗粒尺寸的依赖性，这揭示了有关催化剂过饱和度和Ga表面扩散在生长机制中的作用的有价值信息。发现在广泛的温度以及TMGa和AsH3摩尔流量范围内，直至直径约为20nm，NW的生长速率几乎与金纳米颗粒尺寸无关。对于较小的NW半径，生长速率变得与尺寸有关，并受到V/III摩尔比的强烈影响；在相对较低的V/III比下，由于吉布斯-汤姆逊效应，较小的NW生长较慢，而在较高的V/III比（V/III>50）下，Ga吸附原子扩散成为主要的质量传输机制，较小的NW比较大的NW生长得更快。最后讨论了上述生长模式中的生长限制机制，并通过将实验结果与从第一原理预测得到的NW生长速率进行比较，推导出了前驱体的热解效率、过饱和度和表面扩散长度等重要量。
