Ordered High‐Density Si [100] Nanowire Arrays Epitaxially Grown by Bottom Imprint Method
Ordered High‐Density Si [100] Nanowire Arrays Epitaxially Grown by Bottom Imprint Method
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DOI：
10.1002/adma.200802156
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发表时间：
2009-07
影响因子：
29.4
通讯作者：
Zhang Zhang-Zhang;Tomohiro Shimizu;S. Senz;U. Gösele
中科院分区：
文献类型：
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作者：
Zhang Zhang-Zhang;Tomohiro Shimizu;S. Senz;U. Gösele
在所有一维纳米材料中，硅纳米线（SiNWs）因其在纳米电子、光电子和热电子器件中的潜在应用而特别受到关注。因此，人们一直在努力开发各种技术来控制SiNWs的合成。蒸汽-液-固（VLS）是一种在金属催化剂的帮助下在单晶衬底上生长外延纳米线的广泛应用的方法。然而，使用VLS化学气相沉积（CVD）工艺，在裸硅衬底上主要获得了三个外延生长方向，其中不包括h100i生长方向。考虑到传统的基于Si（100）晶片的硅微纳电子学，在Si（100）晶片上实现垂直生长的高密度外延Si[100]纳米线阵列具有重要意义。特别是对于使用垂直纳米线阵列的器件，排序和尺寸分布也是重要的参数。阳极氧化铝（AAO）膜被广泛用作制备各种纳米结构的模板。对于孔径和厚度可调的AAO薄模板，人们一直致力于改善其有序性。Masuda等人发现的方法依赖于多孔氧化铝通道底部经过长时间的首次阳极氧化步骤后气孔的自排序，这可以产生由均匀大小的平行通道组成的自组装蜂窝阵列。在选择性蚀刻第一次阳极氧化后，可在第二次阳极氧化中得到有序的AAO模板。光刻方法允许制造纳米线阵列与控制尺寸和良好定义的生长位置通过图案模板。通过聚焦离子束（FIB）对沉积在硅衬底上的Al薄膜进行图案化，已经实现了远程有序AAO模板，尽管该工艺受到高成本和顺序写入所需的长时间的限制。为了在两步阳极氧化后使用定义良好的AAO模板来指导Si纳米线阵列的外延生长，最重要的问题是生长的纳米线与衬底之间的良好界面。没有与衬底的界面，sinw已经在AAO模板的通道内合成。在CVD工艺之前，将Au催化剂种子插入AAO孔的中间，在孔内形成SiNWs，两端被Au包裹。这种方法在没有外延生长的情况下，对晶体生长方向的控制较差，并且从模板中释放的sinw难以集成到垂直阵列器件中。Lombardi等人将AAO膜转移到Si（111）衬底上，作为Au催化剂种子的掩膜。随后，他们成功制备了具有高密度和明确尺寸分布的垂直生长的外延Si[111]纳米线阵列。然而，这种方法对于Si[100]纳米线来说是不可能的。为了实现[100]的生长方向，我们之前的工作成功地利用直接阳极氧化的AAO模板在Si（100）衬底上垂直生长外延Si[100]纳米线。在高频辅助化学沉积的帮助下，我们成功地解决了金和硅在孔隙底部直接接触的问题。然而，对于在硅片上直接沉积Al层来说，很难达到足够高的厚度来实现长时间的首次阳极氧化，这对有序化很重要。接下来的阳极氧化加上化学蚀刻会导致硅表面形貌发生不必要的变化。因此，到目前为止，垂直高密度Si[100]纳米线阵列在孔隙有序、尺寸分布窄的AAO模板中外延生长尚未实现。在本文中，我们报告了一种新的方法，我们称之为底部压印（BI）方法，用于在Si（100）衬底上生长高密度外延Si[100]纳米线阵列，具有明确的有序和狭窄的尺寸分布。与在硅衬底上直接阳极氧化的AAO模板相比，采用长时间首次阳极氧化的AAO薄膜更方便我们的目的。采用成熟的两步阳极氧化法制备了高纯度铝膜。此外，随着多孔薄膜结构转移到制备的硅衬底上，我们可以避免任何化学污染和表面形貌修饰。我们方法的中心思想是将薄的透孔AAO膜直接粘合到预沉积Au层覆盖的Si（100）衬底上，形成保形接触。与氧化铝相比，金是一种软金属，杨氏模量和硬度随温度和压痕载荷的增加而降低。在适当的温度和负载下，可以将AAO模板较硬的底部结构压印到Au层中，形成均匀分离的纳米颗粒阵列。最后，在UHV-CVD过程中，由VLS机制催化的SiNWs在与AAO模板相同的顺序和大小的孔内外延生长。薄（<500 nm）的AAO膜由脆性陶瓷膜组成，在去除阻挡层下的铝后难以处理。在这里，我们采用了一种改进的方法，该方法是为印迹而开发的。二次阳极氧化后，在AAO表面自旋涂覆稀释的聚苯乙烯（PS）以填充气孔，形成填充孔道的聚合物柱。PS层在处理脆弱的aaa膜方面起着重要的作用，并且在选择性蚀刻屏障层时也可以防止孔扩大。为了压印的目的，超薄的aa膜
2009 WILEY-VCH Verlag Gmb Among all 1D nanomaterials, silicon nanowires (SiNWs) in particular attracted attention because of their potential usage in nanoelectronic, optoelectronic, and thermolelectronic devices. Therefore, considerable effort has been devoted to developing the controlled synthesis of SiNWs by various techniques. Vapor–liquid–solid (VLS) is one widely used method that can grow epitaxial nanowires on single-crystal substrates with the help of metal catalysts. However, using a VLS chemical vapor deposition (CVD) process, mainly three epitaxial growth directions on bare silicon substrates were obtained, which do not include the h100i growth direction. Considering conventional Si micro/nanoelectronics based on Si (100) wafers, it is meaningful to realize vertically grown high-density epitaxial Si [100] nanowire arrays on Si (100) wafers. Especially for devices using vertical-nanowire arrays, ordering and size distribution are also important parameters. Anodic aluminum oxide (AAO) membranes are widely used as templates to prepare various nanostructures. Much effort has been devoted to improving the ordering of thin AAO templates with their adjustable pore size and thickness. The method discovered by Masuda et al. relies on self-ordering of pores at the bottom of porous alumina channels after a long first anodization step, which can give rise to a self-assembled honeycomb array of uniformly sized parallel channels. An ordered AAO template can be obtained in a second anodization after selective etching off the first anodized one. Lithographic methods allow the fabrication of nanowire arrays with controlled size and welldefined growth positions through patterned templates. Longrange ordered AAO templates have already been achieved by focused ion beam (FIB) patterning of Al films deposited on silicon substrates, although this process is limited by high costs and the long time required for sequential writing. In order to use the well-defined AAO templates after two-step anodization to direct the epitaxial growth of Si nanowire arrays, the most important issue is a good interface between the grown nanowires and the substrate. Without an interface to the substrate, SiNWs have already been synthesized inside the channels of AAO templates. Au catalyst seeds were inserted into the middle of pores of AAO before the CVD process, which formed SiNWs inside the pores, with both ends capped by Au. This method allows poor control over the crystallographic growth direction without epitaxial growth, and the SiNWs released from the template have been difficult to integrate into vertical-array devices. Lombardi et al. transferred an AAO membrane onto a Si (111) substrate as mask of Au catalyst seeds. Subsequently, they successfully prepared vertically grown epitaxial Si [111] nanowire arrays with high density and well-defined size distribution. However, this approach is not possible for Si [100] nanowires. To realize the [100] growth direction, our previous work successfully utilized directly anodized AAO templates to grow epitaxial Si [100] nanowires vertically on Si (100) substrates. We successfully resolved the task of direct contacting between Au and silicon at the bottom of the pores with the help of HF-assisted electroless deposition. However, for the direct deposition of an Al layer on a silicon wafer, a high-enough thickness for the long-time first anodization is not easy to achieve, and is important for the ordering. The following anodization plus chemical etching result in unwanted alteration of the Si-surface morphology. Therefore, up to now, epitaxial growth of vertical high-density Si [100] nanowire arrays in AAO templates with well-ordered pores and a narrow size distribution has not been achieved. In this paper, we report a novel approach, which we call bottom imprint (BI) method, for growing high-density epitaxial Si [100] nanowire arrays on Si (100) substrates with well-defined ordering as well as a narrow size-distribution. In comparison with directly anodized AAO templates on silicon substrates, thin AAO membranes using long-time first anodization are more convenient for our purpose. Highly ordered AAOmembranes can be produced by the well-developed two-step anodization of highpurity Al foils. Furthermore, with the pore-opened thin film structure transferred onto the prepared silicon substrate, we can avoid any chemical contamination and surface-morphology modification. The central idea of our approach is to bond thin pore-through AAO membranes directly onto Si (100) substrates covered by pre-deposited Au layers, forming conformal contact. Au is a soft metal compared with alumina, both Young’s modulus and hardness decrease as the temperature and indentation load increase. Under proper temperature and load, it is possible to imprint the harder bottom structure of the AAO template into the Au layer to form homogeneously separated nanoparticle arrays. Finally, in a UHV-CVD process, SiNWs catalyzed by the VLS mechanism grow epitaxially inside the pores with the same ordering and size as the AAO template. The thin (<500 nm) AAO membranes consist of a brittle ceramic film, and are difficult to handle after removal of the aluminum beneath the barrier layer. Herein, we adopt a modification of a method that was developed for imprints. After the second anodization, a diluted polystyrene (PS) is spin-coated on the surface of AAO to fill the pores, which forms polymer pillars filling the channels. The PS layer plays an important role in the handling of the fragile AAOmembrane, and it also prevents pore widening when selectively etching off the barrier layer. For imprint purposes, the ultrathin AAOmembrane