Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599-3290, United States.
ACS Appl Mater Interfaces. 2017 Oct 25;9(42):37105-37111. doi: 10.1021/acsami.7b08162. Epub 2017 Oct 11.
Although silicon (Si) nanowires (NWs) grown by a vapor-liquid-solid (VLS) mechanism have been demonstrated for a range of photonic, electronic, and solar-energy applications, continued progress with these NW-based technologies requires increasingly precise compositional and morphological control of the growth process. However, VLS growth typically encounters problems such as nonselective deposition on sidewalls, inadvertent kinking, unintentional or inhomogeneous doping, and catalyst-induced compositional gradients. Here, we overcome several of these difficulties and report the synthesis of uniform, linear, and degenerately doped Si NW superlattices with abrupt transitions between p-type, intrinsic, and n-type segments. The synthesis of these structures is enabled by in situ chlorination of the NW surface with hydrochloric acid (HCl) at temperatures ranging from 500 to 700 °C, yielding uniform NWs with minimal nonselective growth. Surprisingly, we find the boron (B) doping level in p-type segments to be at least 1 order of magnitude above the solid solubility limit, an effect that we attribute to a high incorporation of B in the liquid catalyst and kinetic trapping of B during crystallization at the liquid-solid interface to yield a highly nonequilibrium concentration. For growth at 510 °C, four-point-probe measurements yield active doping levels of at least 4.5 × 10 cm, which is comparable to the phosphorus (P) doping level of n-type segments. Because the B and P dopants are in sufficiently high concentrations for the Si to be degenerately doped, both segments inhibit the etching of Si in aqueous potassium hydroxide (KOH) solution. Moreover, we find that the dopant transitions are abrupt, facilitating nanoscale morphological control in both B- and P-doped segments through selective KOH etching of the NW with a spatial resolution of ∼10 nm. The results presented herein enable the growth of complex, degenerately doped p-n junction nanostructures that can be explored for a variety of advanced applications, such as Esaki diodes, multijunction solar cells, and tunneling field-effect transistors.
尽管通过气相-液相-固相（VLS）机制生长的硅（Si）纳米线（NWs）已经在一系列光子、电子和太阳能应用中得到了证明，但这些基于 NW 的技术的持续发展需要对生长过程的组成和形态进行越来越精确的控制。然而，VLS 生长通常会遇到一些问题，例如在侧壁上的非选择性沉积、意外的扭结、无意或不均匀的掺杂以及催化剂诱导的组成梯度。在这里，我们克服了其中的一些困难，并报告了具有 p 型、本征和 n 型段之间的突然转变的均匀、线性和简并掺杂 Si NW 超晶格的合成。这些结构的合成是通过在 500 至 700°C 的温度范围内用盐酸（HCl）对 NW 表面进行原位氯化来实现的，这导致了具有最小非选择性生长的均匀 NW。令人惊讶的是，我们发现 p 型段中的硼（B）掺杂水平至少比固溶度极限高 1 个数量级，我们将这种效应归因于液体催化剂中 B 的高掺入以及在液体-固体界面处结晶过程中 B 的动力学捕获，从而产生了非常非平衡的浓度。对于在 510°C 下的生长，四点探针测量得出的活性掺杂水平至少为 4.5×10 cm，这与 n 型段中的磷（P）掺杂水平相当。由于 B 和 P 掺杂剂的浓度足够高，使得 Si 可以简并掺杂，因此这两个段都抑制了 Si 在水性氢氧化钾（KOH）溶液中的蚀刻。此外，我们发现掺杂剂的转变是突然的，这使得通过选择性地用 KOH 蚀刻 NW 可以在 B 和 P 掺杂段中实现纳米级形态控制，空间分辨率约为 10nm。本文所介绍的结果能够实现复杂的、简并掺杂的 p-n 结纳米结构的生长，这些结构可以探索用于各种先进应用，如 Esaki 二极管、多结太阳能电池和隧道场效应晶体管。
