Deposition of Germanium Nanowires from Hexamethyldigermane: Influence of the Substrate Pretreatment

Abstract

Germanium nanowires ͑GeNWs͒ were synthesized by low pressure chemical vapor deposition of hexamethyldigermane ͑GeMe 3 ͒ 2 at 490°C and a pressure of 90-100 Pa. GeNWs of several nanometers in diameter and a few micrometers in length were deposited onto substrates made of stainless steel, Fe, Mo, Ta, W, Si, and SiO 2 . The influence of surface pretreatment of the substrates ͑roughening of surface, grooves made by a diamond tip or Ge thermal evaporation͒ is discussed. GeNW deposits were studied using scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, and energy-dispersive X-ray analyses.Nowadays, the progress in microelectronics has been driven by nanoscale technology for two-and one-dimensional structures. Therefore, nano-objects ͓nanowires ͑NWs͒, nanotubes, nanorods, nanopillars, nanoribbons, etc.͔ made of novel materials and alloys with unique properties have been studied and tested in detail in microelectronic systems. Gradually, such nano-objects are implemented into silicon technology to update and enhance the power and efficiency of microelectronic circuits, solar cell arrays, catalyst systems, etc.Germanium is considered to replace silicon in some applications as it has a higher electron and hole mobility ͑3900 and 1900 cm 2 /V s, respectively͒ than silicon ͑1500 and 450 cm 2 /V s, respectively͒. Moreover, the Bohr radius of germanium is larger ͑24.3 vs 4.3 nm for silicon͒ and, therefore, quantum phenomena are more readily established. 1 Field emission transistors using germanium nanowires ͑GeNWs͒ have already been reported. [2][3][4] The most commonly used methods for the preparation of GeNWs are the vapor-liquid-solid ͑VLS͒ technique and the seedassisted deposition processes using catalyst nanoparticles ͑mostly Au͒. 5-7 However, the seeds act as traps for free charge carriers and therefore shorten their lifetime. The seed atoms could also migrate through the circuit structure and degrade its electronic properties. Although some attempts were made to manufacture NW microelectronic structures using Au seed nanocolloids, 8,9 the whole manufacturing process is complicated by the steps of introducing and removing Au nanoparticles from the fabricated NW structure.A solution to this problem is based on a noncatalyst process that avoids using heteroatom seeds. Some of them seem not to be convenient for standard microelectronic arrays because they anticipate other chemicals as in electrochemical etching applying a HClcontaining aqueous electrolyte. 10 The best results of the noncatalyst growth of GeNWs have been obtained by chemical vapor deposition ͑CVD͒ procedures.The first approach relates to the idea of intermetallic sites serving as initial centers of GeNW growth. Mathur et al. 11 proposed an intermetallic concept of the self-catalyst initial stage of GeNWs grown onto iron substrates. They assumed the formation of Fe-Ge intermetallic nuclei on an iron substrate durin...

A Review of Self-Seeded Germanium Nanowires: Synthesis, Growth Mechanisms and Potential Applications

“…Mathur et al [225] also explored the growth mechanism of Ge nanowires grown on Fe substrates. No evidence of spherical seeds was found at the tips of the nanowires, unlike those reported by Dřínek et al [141] for Ge nanowire growth with same Fe substrates. Recently, Dřínek et al [142] also reported the formation of core-shell Ge-Si/C nanowires in the absence of external catalytic seeds.…”

“…Substrates only accommodate catalytic droplets (Ge nanoparticles in this case) for further nanowire growth. Dřínek et al [141] screened a series of substrates such as stainless steel, Fe, Mo, Ta, W and SiO 2 at a reaction temperature of 490 • C, with (GeMe 3 ) 2 as the Ge precursor, to evaluate the role of the substrates in self-seeded Ge nanowire growth. Even though Fe, Mo and Ta form intermetallic phases with Ge, only Fe does it at the reaction temperature of 490 • C, while the others form at temperatures of~700 and 600 • C, respectively [224].…”

“…W and SiO 2 do not present any intermetallic alloy with Ge [224]. Considering the required necessity of adhesion and limited mobility of the precursor atoms until they anchor at a specific surface site to form a nucleus nanocrystal, Dřínek et al [141] proposed the physicochemical properties of the substrate surface (such as defects, imperfections and present impurities) as the decisive feature for nanowire formation. However, the presence of the methyl groups during precursor decomposition was not considered.…”

“…Broad diffraction peaks determined by X-ray Diffraction (XRD) pointed towards low crystallinity in the nanowire samples, further confirmed by the TEM observation of a small yield of amorphous Ge(a-Ge) phase. Dřínek et al [141] utilised alternative commercial precursors such as (GeMe 3 ) 2 for the Ge nanowire growth without the presence of any additional catalysts. A growth temperature of 490 • C lead to the formation of high-quality Ge nanowires on substrates such as Ta, SiO x and W. While kinking was absent in the nanowires (see Figure 2c) bending of the nanowires was observed (see Figure 2d) which may inhibit their application in electronic/optoelectronic devices.…”

Tribology of Ge thin films on stainless steel

“…Subsequently, the Ge thin films were deposited onto the steel substrates. The deposition procedure has been described elsewhere (Drinek et al, 2010;Krabac et al, 2012); nevertheless, the discussed deposition process was carried out at 650°C for 90 minutes and under the pressure of 40-80 Pa during the deposition process. Moreover, hexamethyldigermane (technical grade, Sigma-Aldrich Corporation, USA) was used as a volatile precursor.…”