Formation mechanism of SiGe nanorod arrays by combining nanosphere lithography and Au-assisted chemical etching

Chih-Chung Lai 1 Yun-Ju Lee 0 Ping-Hung Yeh 0 Sheng-Wei Lee 1 0 Department of Physics, Tamkang University , Danshui District, New Tapei, 25137, Taiwan 1 Institute of Materials Science and Engineering, National Central University , Jhongli, 32001, Taiwan The formation mechanism of SiGe nanorod (NR) arrays fabricated by combining nanosphere lithography and Auassisted chemical etching has been investigated. By precisely controlling the etching rate and time, the lengths of SiGe NRs can be tuned from 300 nm to 1m. The morphologies of SiGe NRs were found to change dramatically by varying the etching temperatures. We propose a mechanism involving a locally temperature-sensitive redox reaction to explain this strong temperature dependence of the morphologies of SiGe NRs. At a lower etching temperature, both corrosion reaction and Au-assisted etching process were kinetically impeded, whereas at a higher temperature, Au-assisted anisotropic etching dominated the formation of SiGe NRs. With transmission electron microscopy and scanning electron microscopy analyses, this study provides a beneficial scheme to design and fabricate low-dimensional SiGe-based nanostructures for possible applications. - Introduction Over the past few decades, intensive research efforts have been devoted to the fabrication and characterization of Si-based nanostructures due to their intrinsic physical properties, high packing density, and compatibility with current Si technology [1]. Self-assembled Si-based nanostructures are of particular interest because self-assembly provides a possible way to realize nanostructures without process-induced damages, which are frequently observed in the samples defined by electron (e)-beam lithography or reactive ion etching (RIE) [2,3]. Ge/Si has become a model system for the fabrication and investigation of nanometerscale heteroepitaxy due to their moderate lattice mismatch (4.2%) [4,5]. The fabrication of SiGe nanowire arrays is one of the most interesting topics [6,7]. Recently, the use of Si-based nanowires as high-performance devices or sensors has been extensively reported [8-12]. There are several methods to fabricate nanowire structures, such as e-beam lithography [13] and vapor-liquid-solid growth [14-16], and metal-assisted chemical etching [17-20]. Previous works have demonstrated that nanosphere lithography (NSL) provides an efficient way to fabricate selforganized, ordered, and close-packed sphere arrays [21,22]. However, there have been few studies paying attention on the formation mechanism of SiGe NRs. In this work, we fabricated SiGe NR arrays by combing NSL and Au-assisted chemical etching. The influences of chemical etching conditions on the morphologies of as-etched SiGe NRs were investigated to clarify their formation mechanism. Experimental details The schematic depiction of experimental procedures is shown in Figure 1. p-Type (001)-oriented Si wafers 10 to 25 cm in size and 100 mm in diameter were used in the present study. All SiGe heterostructures used in this study were grown at 550C in a multi-wafer ultra-high vacuum chemical vapor deposition (UHV/CVD) system. Before epitaxial growth, the Si wafers are dipped in a 10% HF solution to achieve the hydrogen-passivated surface and then transferred into an UHV/CVD system. A 50-nm-thick Si buffer layer was first grown and followed by growth of a 2-m-thick SiGe buffer layer and a Figure 1 Schematic diagram of fabrication process of SiGe NRs. (a) TEM image of 1-m-thick Si0.8Ge0.2 epilayer with the SiGe buffer layer grown on Si wafer. (b) PS nanospheres in diluted colloidal form were drop-placed onto the freshly prepared hydrophilic SiGe substrate. (c) The sizes of the PS spheres were reduced by RIE treatment. (d) A 20-nm-thick Au film was deposited by e-beam evaporation. (e) SiGe NRs were formed by Au-assisted wet chemical etching. (f) Finally, Au film is removed by an Au etchant. 1-m-thick Si0.8Ge0.2 uniform epilayer, as shown in Figure 1a. The wafer with the SiGe epilayer was sliced into 1 1 cm2 as the templates. Next, polystyrene (PS) nanospheres in diluted colloidal form were then dropplaced onto the freshly prepared hydrophilic substrate, as illustrated in Figure 1b. An area of a monolayer of polystyrene nanospheres then forms upon complete water evaporation under ambient condition. Subsequently, RIE process using O2 plasma with a power of 30 W was employed to reduce the sizes of the PS nanospheres, as illustrated in Figure 1c. In this step, the PS nanospheres with a reduced size formed non-closely packed arrays on the surface. Next, a 20-nm-thick gold film was then deposited onto the substrate by e-beam evaporation, as illustrated in Figure 1d. Due to the PS monolayer mask, an Au film with patterned nanohole arrays was formed. The patterned Au thin film acts as a catalyst in the following Au-assisted etching process. The samples were then dipped into the freshly prepared etching solution (2:1:2:5 (v/v) HF/H2O2/C2H5OH/DI water mixture) to form SiGe NR arrays under various etching conditions, as shown in Figure 1e. The diameter, spacing, and density of SiGe NRs should be defined by the starting reduced size of PS nanospheres. Once SiGe NRs were formed, the Au metal was washed away using an Au etchant (3:1 (v/v) HCl/HNO3 mixture), leaving PS nanospheres on the surface as labels for observing the etched SiGe NRs. Finally, the morphologies and microstructures of the resulting SiGe NRs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in conjunction with an energy dispersion spectrometer (EDS). Results and discussion Top-view SEM images of self-assembled PS nanosphere arrays before and after RIE treatment are shown in Figure 2a, b, respectively. Prior to the RIE treatment, the PS nanospheres were closely packed on the SiGe substrate with a uniform diameter of 600 nm. After the RIE treatment, the PS nanospheres were then trimmed to an average size of 420 nm. These reduced sizes of PS nanospheres were also found to be deformed to a truncated shape, possibly due to the thermal heating effect during the O2 plasma RIE process. Figure 3 shows the temperature dependence of the morphologies of SiGe NR arrays etched at temperatures from 5C to 25C. The etching time was fixed at 20 min for all SiGe NRs. By the fact that PS nanospheres are on top of the etched structure, we can verify that the etched SiGe nanostructures are composed of the as-grown SiGe epilayer. At lower etching temperatures (5C to 15C) as shown in Figure 3a, b, the etched SiGe nanostructures show a necklike body with a thin diameter underneath the truncated PS nanospheres. The maximal height of the etched SiGe nanostructures was limited to be about 300 nm. However, by increasing the etching temperature to 20C and 25C, the etched SiGe nanostructures became apparently longer (about 1 m at 25C), i.e., the formation of SiGe NRs. These results demonstrated that the morphologies of etc (...truncated)