Fabrication and characterization of semiconductor nanowires for thermoelectric application 5 6

Catalytic Etching Mechanism 93 Chapter 5 Investigation on the Catalytic Etching Mechanism of Silicon 5.1 Introduction Metal assisted catalytic etching was used in the fabrication of silicon nanowires (SiNw) for testing the thermal conductivity measurement setup and potential thermoelectric application(for potential thermoelectric application?). The catalytic etching process attracted increasing attention recently because there is a need for nanostructures with specific orientation as explained in section 2.4.1 in Chapter 2. Catalytic etching has many advantages such as being a simple and inexpensive process. It is able to control parameters such as diameter, length and orientation of the nanostructures. The etching process can also produce nanowires with high crystalline quality. [75,76] [Quote some relevant refs.] Some aspects of the actual reaction mechanism in catalytic etching are still unclear because of the difficulty in accessing and characterizing the etching interface which is covered by the metal catalyst. Currently, there are two possible models proposed to explain the catalytic etching mechanism [80]. [Quote refs.] One model states that the etching takes place at the interface between the metal catalyst and the silicon substrate. In the other model, silicon atoms diffuse up through the metal layer and react at the interface between the metal catalyst and the hydrofluoric acid/hydrogen peroxide (HF/H2O2) solution. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used in this work to reveal more information on the actual mechanism that takes place during catalytic etching. For example, whether there is any Catalytic Etching Mechanism 94 Si diffusion through the metal catalyst during catalytic etching and at which interface (i.e., Si-metal or metal-solution interface) did the etching action takes place. Recently, Huang et al. [79] made use of an anodic aluminium oxide (AAO) template mask to produce Si nanowires by catalytic etching. For the reduction-oxidation (redox) reaction in the catalytic etching process to occur, the catalyst needs to have a higher electronegativity than Si so that electrons can be pulled away from Si atoms and the oxidation of Si can take place [97]. In the work of Huang et al., a non-catalyst metal that has a lower electronegativity than Si, such as chromium (Cr), was deposited onto the AAO and used as a blocking material for the catalytic etching process. After removing the AAO, a blanket layer of Au catalyst was deposited to produce Cr/Au dots (at regions which are originally the pores of the AAO) and Si regions covered by Au. Those areas of Si protected by the Cr/Au dots will remain after etching in the HF/H2O2 solution, leaving behind regular array of Si nanowires with diameters that can be adjusted depending on the pore diameter in the AAO mask. In this work, experiments were carried out to investigate the effect of a bi-layer of two different metals on catalytic etching of Si so as to understand better the actual mechanism involved. 5.2 Effect of the metal film thickness on the etching process HF of 4.6M and H2O2 of 0.44M were used as the etching solution in this experiment. The samples were cleaned as discussed in the sample preparation section in Chapter 3. Since Cr/Au was verified to be an effective protective metal layer that can block etching [79], Cr/Au (10/30 nm) markers were prefabricated to make comparison with Catalytic Etching Mechanism 95 the surrounding etched Si areas not covered by the markers. Using a standard optical lithography process, micron-sized marker patterns, formed by 10 nm Cr and 30 nm Au through evaporation, were formed on a Si (100) surface. The Si (100) substrate with the markers were then used as the starting substrate for deposition of the bi-layer metals before subjecting the samples to chemical etching in the HF/H2O2 etching solution. The marker regions were not expected to be etched as the underlying Si in these regions are covered by the Cr/Au (10/30 nm) layer and another bi-layer metal, and Cr/Au (10/30 nm) had been demonstrated to block the chemical etching [79]. As for the remaining non-marker regions where the underlying Si was just covered by the bi-layer metal, whether chemical etching takes place or not depended on the bi-layer metal materials selected and the thickness of the layers. Figure 43 shows the SEM images of two etched Si samples with Ti/Au bi-layer of different thickness deposited on top of the Si marker sample. Both samples were etched in a fresh solution with the same HF/H2O2 composition for 5 minutes. Figure 43(a) shows the sample that has a bi-layer of Ti/Au (5/10 nm) where 5 nm of Ti was first deposited on the Si substrate with markers, followed by 10 nm of Au. It can be seen clearly that chemical etching has taken place in the non-marker regions. The etched depth was about 6 µm. In Figure 43(b), a bi-layer of Ti/Au (5/15 nm) was deposited on the Si substrate with markers and the sample was etched for 5 minutes; however, there was only very limited (negligible) etching observed in the non-marker regions. Although Ti itself has lower electronegativity than Si and can act as a blocking layer in etching, Ti will react with HF and get dissolved. The only difference in the two samples is the thickness of the protective Au layer above Ti. From the results, it shows that at least 15 nm of Au is required to protect the Ti underneath. Catalytic Etching Mechanism 96 Therefore only a bi-layer of Ti/Au with 15 nm of Au on 5 nm of Ti will be able to be used as an effective blocking layer for catalytic etching. Figure! "#! $%&! '()*+! ,-! ./+! +.0/+1! $'! ()23+2! 4)(56+! 7'./! 8)9! :';;?@! A(9! )A1! 8B9! :';;?>!A(9!B'C6)D+2!1+5,4'.+1!,A!./+!$'!()23+2!4)(56+E! A similar experiment was repeated with Cr/Au as the blocking bi-layer metal with two different thicknesses of the Au layer (10 nm and 15 nm) and 5 nm of Cr investigated. Both Figures 44(a) and 44(b) show some chemical etching in the non-marker regions, although this is somewhat limited, after the samples were immersed in the HF/H2O2 etching solution for 5 minutes. This shows that 5 nm of Cr is still sufficient as a blocking layer. This is due to the fact that Cr does not react with HF or H2O2. Although 10 nm of Au is not enough to block HF and H2O2, the Cr/Au blocking layer still remained intact after the reaction. Therefore, summarizing the results from Figures 43 and 44, the reactivity of the blocking material with the etching solution has to be taken into account when choosing an appropriate blocking material in addition to the thickess of the bi-layer. Catalytic Etching Mechanism 97 !"#$%&'((')*+'",-#&'./'01&'&021&3')"'4-,56&'7"01'8-9':%;=' @,9'A"B6-C&%'3&5.4"0&3'.@'01&')"',-%D&%'4-,56&E' 5.3 XPS results on the catalytic etching mechanism To check if Si has diffused through the catalyst metal layer during the catalytic etching process, XPS technique is used. Figure 45 shows the sample where XPS analysis was carried out. 21nm of Au was deposited on a Si substrate with a shadow mask. !"#$%&' (=' )-,56&' 7"01' 2"%2$6-%' [...]... !"#$%&'()'*+,'-.&/0%1'2%34 '53 /10" 36- '7' 168 '9'120&%'" 36' &0/:;'* %3 ... !"#$%&'()'*+,'-.&/0%1'2%34 '53 /10" 36- '7' 168 '9:';