Silicon nanowires prepared by electron beam evaporation in ultrahigh vacuum
© Xu et al.; licensee Springer. 2012
Received: 10 February 2012
Accepted: 6 May 2012
Published: 6 May 2012
One-dimensional silicon nanowires (SiNWs) were prepared by electron beam evaporation in ultrahigh vacuum (UHV). The SiNWs can be grown through either vapor–liquid-solid (VLS) or oxide-assisted growth (OAG) mechanism. In VLS growth, SiNWs can be formed on Si surface, not on SiO2 surfaces. Moreover, low deposition rate is helpful for producing lateral SiNWs by VLS. But in OAG process, SiNWs can be grown on SiO2 surfaces, not on Si surfaces. This work reveals the methods of producing large-scale SiNWs in UHV.
KeywordsSilicon nanowires Preparation Vapor–liquid-solid Oxide-assisted growth Ultrahigh vacuum
One-dimensional (1D) nanomaterials have stimulated great interest due to their importance in basic academic research and potential technology applications [1, 2]. It is widely accepted that 1D nanomaterials not only play vital roles as interconnects and functional units for nanodevices, but also provide opportunities to investigate the dependence of electrical, thermal, and mechanical properties on the dimensionality and size reduction [2–4]. Among all 1D nanomaterials, silicon nanowires (SiNWs) are particularly attractive [5–9] because of the center role of silicon (Si) in semiconductor industry. Moreover, a wealth of traditional knowledge about Si material is helpful for understanding the relationships between its properties and nanostructures. In addition, the mature Si-based technology can be exploited to fabricate future nanodevices.
Many methods have been developed to prepare 1D SiNWs [5–16]. However, most SiNWs were produced in air atmosphere or low vacuum conditions [5–12]. Synthesis of SiNWs in ultrahigh vacuum (UHV) is helpful for obtaining highly pure products, as well as better evaluating their properties and understanding the related mechanisms. Therefore, the growth of SiNWs in UHV has attracted considerable attention recently [13–16]. Schubert et al. reported the preparation of vertical Si nanowhiskers in UHV by molecular beam epitaxy through vapor–liquid-solid (VLS) growth . Similarly, Irrera et al. prepared vertical SiNWs in UHV by electron beam evaporation (EBE) through VLS [14, 15]. Differently, Xu et al. introduced a new method to synthesize SiNWs in UHV by EBE through oxide-assisted growth (OAG) mechanism .
In this work, we demonstrated the feasibility of producing lateral SiNWs in UHV by EBE through both VLS and OAG mechanisms. The critical factors for 1D nanowire formations were also discussed.
Si(111) wafers sized 1 × 1 cm2 were chosen as the deposition substrates. Before being used, the Si substrates were ultrasonically cleaned in methanol for 15min followed by a dip in a diluted hydrofluoric acid (HF) solution for removing organic contaminations and surface oxides. The SiO2/Si(111) substrates were prepared by deposition of SiO2 films on Si(111) wafers using a plasma-enhanced chemical vapor deposition system (Orion II, Trion Technology, Clearwater, FL, USA) under the conditions of 13.56MHz, 600W, 0.6Torr, 300°C, and a N2O/SiH4 flux ratio of 100/150sccm. These SiO2/Si(111) substrates were ultrasonically cleaned with successive rinses of acetone and methanol. After having been dried by N2 gas, the cleaned substrates were transferred immediately to a UHV EBE system (Balzers ULS 400, Balzers Ltd., Liechtenstein, Switzerland) for depositions at constant rate of 0.02nm/s. The base pressure was 2 × 10−10mbar, and the process pressure was maintained at 1 × 10−7mbar or below during the depositions.
The as-deposited materials were characterized exsitu by tapping mode atomic force microscopy (AFM, Nanoscope III, Veeco Instruments, Inc., Plainview, NY, USA) and scanning electron microscopy (SEM, FEI XL30S-FEG, FEI Company, NE Dawson Creek Drive, Hillsboro, OR, USA), respectively.
Results and discussion
If a 100-nm-thick SiO2/Si(111) was used as the substrate, the case is different after the same Au and Si depositions. Figure 1c reveals that only 0D nanoparticles, but no any 1D nanowires, were formed on the SiO2 surface. The large particles in Figure 1c are Au, while the small ones are Si. Figure 1a and c suggest that molten Au-Si alloys are formed on the Si surface, and thus 1D SiNWs growth by VLS is induced (Figure 1a) [5–7,10-13]. But on the SiO2 surface, the interactions of Au-O and Si-O between the substrate and arriving species prevent the coalescence of Au and Si. Therefore, no Au-Si alloys can be created on the SiO2 surface, and thus no wires are yielded through VLS (Figure 1c). Although Pecora et al. similarly deduced the negative effects of SiO2 layer on the growth of SiNWs , they ignored the interactions between the substrate and arriving species. We believe that such interactions are critical in physical vapor deposition processes.
Large-scale SiNWs were successfully prepared by EBE in UHV through both VLS and OAG mechanisms. In VLS growth, Au-Si alloys created on the HF-treated Si substrates catalytically induce the growth of 1D SiNWs at 700°C. However, the Au-O and Si-O interactions inhibit the formation of Au-Si alloys, and thus no SiNWs can be grown through VLS on the SiO2 surfaces. In OAG process, the strong interactions of Si-Si between the Si substrates and deposited Si species prevent the formation of 1D SiNWs. In contrast, SiO2 surfaces provide suitable interactions so that SiNWs can be grown at 700°C through OAG on such substrates. This work reveals the methods to produce 1D SiNWs by EBE in UHV.
Atomic force microscopy
Electron beam evaporation
Scanning electron microscopy
We would like to thank Professor Zhongfan Liu and Professor Huizhong Huang in Peking University, Proessor Zhanpin Li in Beijing Electron Spectroscopy Center of Tsinghua University for their assistance. This work is supported by the National Natural Science Foundation of China (NSFC 61071032).
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