Growth and characterization of germanium nanowires on a flexible aluminium substrate by electron beam evaporation
- 1.3k Downloads
For the first time, Germanium (Ge) nanowires have been grown on a gold (Au) coated flexible aluminum (Al) foil substrate in high vacuum (1 × 10−5 mbar) by electron-beam evaporation of germanium using the vapor–liquid–solid mechanism at a substrate temperature of 380°C. The grown nanowires have been analyzed for their structural, morphological and chemical properties by employing standard techniques X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy, respectively. X-ray diffraction measurements revealed the formation of cubic Ge phase highly oriented in (111) reflection in plane with the Al foil substrate. The morphological observations by SEM have shown the randomly grown nanowires with an average length and diameter of 600 ± 50 and 100 ± 10 nm, respectively, for a deposition time of 30 min. TEM investigation revealed single crystalline nanowires with free of defects. The wettability studies by contact angle measurement have confirmed the hydrophobic nature of the Ge NWs film surface with contact angle for water 110° ± 1°. The growth mechanism of Ge nanowires on Al foil substrate has also been discussed.
KeywordsGe nanowires VLS growth mechanism Scanning electron microscopy Flexible substrate Hydrophobicity Thin films e-beam evaporation
Crystalline semiconductor nanowires (NWs) have attracted enormous research interest in recent years, owing to their potential use as building blocks for future nanoscale electronic devices (Cui and Lieber 2001). One dimensional (1D) nanostructures of semiconductors such as Si, Ge, GaAs, GaN and InAs have been grown by different physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques for possible applications in field-effect transistors (FET) (Liang et al. 2007), light emitting diodes (LED) (Qian et al. 2005), Photo Detectors (Ahn and Park 2007) and Solar cells (Tsakalakos et al. 2007). Among these, 1D semiconductor nanostructures, Germanium NWs are of particular interest for high performance nano electronic devices due to their high carrier mobility and low effective mass (Law et al. 2004; Xiang et al. 2006). Ge NWs have been extensively grown by various methods such as pulsed laser ablation (PLD), chemical vapour deposition (CVD) and vapour transport techniques (Wang 2007). Among the techniques, electron beam evaporation (EBE) technique is quite simple in operation to obtain uniform nanowires compared to CVD technique. It is much inexpensive than molecular beam epitaxy (MBE), and being a non-UHV technique, has much higher throughput which makes it interesting for industrial applications. In the literature, there are only few reports on Ge nanowires growth by e-beam evaporation technique (Kumar et al. 2010; Pecorea et al. 2010; Sivakov et al. 2007).
Flexible substrates such as poly (dimethyl siloxane) (PDMS), polymer substrates, poly (ethylene terephthalate) (PET), thin foils of Al, Cu, have gained increasing attention in the semiconductor industry, due to their light weight and cost in the fabrication process. Ge NWs find applications in fabricating electronic devices (Duan et al. 2003; Lee et al. 2009; Forrest 2004), chemical sensors (Mcalpine et al. 2007). The advantages of Al substrates over Si substrates are flexible, low cost and easy handling. Large area Al substrates are also available commercially. To the best of our knowledge, so far there have been no reports on the growth of Ge NWs on flexible Al substrates by e-beam evaporation. The purpose of depositing on flexible substrate such as Al foil is, to use the Ge NWs as anode for high density Li ion batteries (Chan et al. 2008; Kim et al. 2010). Experiments are under progress to make device fabrication using Ge NWs grown on Al foil substrate.
Water repellence (hydrophobic property) is desirable and important for many industrial and biological processes, such as the prevention of the adhesion of snow to antennae (Nakajima et al. 2001), the prevention of contamination (Coulson et al. 2000), stain-resistant textiles and cell mobility (Barady 1994; Blossey 2003). Hydrophobic surfaces are generally prepared by combining the surface roughness at both the micro and nanoscales with low surface-energy materials. This effect occurs in natural systems quite frequently in the case of a lotus leaf, the self-cleaning process is done by water droplets rolling on the surface and removing dirt and debris. Nature accomplishes this phenomenon through the generation of surface topography that displays structures at micro and nanometre scales. Artificial hydrophobic surfaces have been prepared using various strategies (Coffinier et al. 2007; Furstner et al. 2005; Shiu et al. 2004; Oner and McCarthy 2000).
To our knowledge, there are no reports on wettability studies on germanium nanowires film surfaces. Here, we report for the first time, the growth of pure Ge NWs on Au coated Al foil substrate by e-beam evaporation in high vacuum (1 × 10−5 mbar) with hydrophobic property. The morphology, composition, micro structural and wettability properties of Ge NWs have been studied by SEM, XPS, TEM, XRD, Raman and contact angle measurement, respectively.
In the present investigation, to grow Ge NWs by the e-beam evaporation method, pure Ge ingots (purity 99.99% by Blazers) loaded into water-cooled graphite crucible has been used as a source for e-beam evaporation. The rate and thickness of the deposited Ge has been monitored by a water-cooled quartz crystal monitor. An Al substrate of approximate 160 μm thick has been cleaned in an ultrasonic bath with acetone, and de-ionized water and subsequently, loaded into a sputter coating system and deposited with an Au film of 3 nm thickness.
We have employed BALTEC SCD 500 sputter coating system to deposit gold droplets. For this, after placing the pre-cleaned Al foil substrate in the chamber, the system has been pumped down to high vacuum (1 × 10−5 mbar) and flushed with Ar (99.999%) several times and switch on high voltage for sputtering the gold target. The distance between the substrate and gold target is fixed at 10 cm to obtain uniform gold film.
The Al substrate was heated to the desired temperature by a in-house built substrate heater and the substrate temperature was calibrated prior to deposition by a thermocouple. Au coated Al substrates have been transferred into the e-beam evaporation chamber and was evacuated using a diffusion pump and rotary pump combination. Au coated Al substrates have been annealed in situ at 450°C under a 1 × 10−5 mbar vacuum for 25 min prior to the deposition of Ge to form Au droplets. After annealing, the substrate was cooled to 380°C and Ge has been evaporated by the e-beam at the rate of ~0.2 nm/s for 30 min. The applied (emission) current to evaporate germanium is 25 mA and the distance between the source and substrate is 20 cm.
The obtained Ge nanowires on the Al foil substrate has been subjected to adhesion test by using tape. No peeling of the film containing nanowires is observed indicating the good adhesion of the Ge NWs on the Al foil substrate.
The morphological investigation on the Ge NWs has been examined by a scanning electron microscope (Model: SEM FEI Quanta 200). The micro structural properties of the Ge NWs have been evaluated by a transmission electron microscope (Model: TEM, Technai F-30). The structural properties of the films were characterized by X-ray diffraction (XRD, Bruker D8 Advance) and compositional properties were carried out by X-ray Photoelectron spectroscopy (Model: Multilab 2000 Thermo Scientific system) spectroscopy equipped with an Al X-ray source (hν = 1,253.6 eV). A Phonon confinement study was carried out by Raman spectroscopy (Model: Lab RAM HR Raman system) equipped with a charge coupled device detector (CCD) at working temperature of −70°C. The sample is illuminated by the 514 nm line of an argon ion laser focused at 100× objective. All the data have been recorded using 2 mW of laser power and for 20 s of acquisition. The hydrophobic nature of Ge NWs film has been investigated at ambient atmosphere, by contact angle measurement using deionized water.
Results and discussion
Figure 4b shows the XPS spectra of Ge 3d core level, obtained from the GeNWs. The core level Ge 3d spectrum indicates the binding energy of Geo at 29.05 eV and that of Ge+4 at 32.25 eV, respectively. This observation is in good agreement with the XPS database (Castain 1992). The presence of a Geo peak implies that the main body of the Ge NWs is composed of pure Ge. It is evident that, there is a slight formation of the GeO2 (32.25 eV) on the surface of the formed Ge NWs, and this may be due to exposure of samples to atmosphere (Molle et al. 2006).
Germanium NWs were grown both perpendicular and inclined with respect to the substrate as shown in the SEM images (Fig. 1a, b). The growth mechanism of Ge NWs grown by e-beam evaporation is similar to the mechanism proposed for Ge, Si NWs grown by e-beam evaporation (Pecorea et al. 2010; Irrera et al. 2009). The growth of NWs can be explained in terms of two competing processes: germanium atoms directly impinging on the gold droplet and germanium atoms adsorbing on the substrate. Germanium atoms impinging on the gold droplet form an alloy with the gold droplet, and causing super saturation of alloy results in a growth of germanium nanowire occurred just at the liquid/solid interface of the alloy and the substrate. This process results in the growth of NWs with a gold particle at the end of the each wire. This mechanism clearly shows the VLS mechanism. Atoms impinging on the substrate also have a fundamental role in the axial growth; in fact, growth conditions allow them to easily diffuse over the sample in such a way that some of the atoms can diffuse along the growing Ge sidewalls. These atoms can be adsorbed at the solid germanium/liquid eutectic interface under the gold droplet. Therefore, they may be having an important contribution to the NWs growth. To our knowledge, as on today in the literature, the maximum length grown with molecular beam epitaxy, electron beam evaporation methods is about 2 micrometer (Pecorea et al. 2010). The main reason for not getting lengthy nanowires is adatom diffusion length of growth species. It plays a key role in nanowires growth by EBE (Artoni et al. 2011; Xu et al. 2011). Initial stage of nanowire growth depends on growth species which deposit on the substrate contribute to nanowire growth. When length becomes larger than the diffusion length, then the length of the nanowire depends mainly on the diffusion of adatoms fall that adsorb directly onto the NW side walls or on the catalyst particle surface. Therefore, after few nm nanowire growth, growth become slow and at the same time 2D layer deposit increases on the substrate, and nanowire grows by collecting growth species from the sidewalls and catalyst. If we increase time of deposition gold diffuse away from the original seed particle, as a result, the nanowire cannot grow in lengthy any more, but over growth on the side walls occurs.
For the first time, Ge nanowires have been successfully grown on a flexible Al foil substrate by e-beam evaporation technique under high vacuum (1 × 10−5 mbar) at 380°C substrate temperature using Au as a catalyst. SEM measurements indicated that the grown Ge NWs have been oriented in a random direction with respect to the substrate. XRD and Raman studies revealed the crystalline nature of the NWs and also the phonon confinement effect in Ge NWs. XPS studies revealed that the grown NWs are bit oxidised due to exposure of the samples to the atmosphere. From the TEM studies, the growth of the NWs follows the VLS mechanism. The as grown nanowires were single crystalline in nature and free of defects. Contact angle measurement revealed the hydrophobic nature of the Ge NWs film surface. Experiments are under progress to grow the Ge NWs in vertical direction perpendicular to the substrate with minimum length of 1 micron for practical applications in high speed electronics, high density Li ion batteries, low k dielectric materials.
Authors would like to acknowledge Nano Center—Indian Institute of Science, Bangalore for providing the HRSEM and TEM-EDX facility.
- Castain J (1992) Hand book of X-ray photoelectron spectroscopy edited by (Perkin-Elmer Corp, EenPraire, Minnisota 55344, USA)Google Scholar
- Duan X, Niu C, Sahi V, Chen J, Parce JW, Empedocles S, Goldman JL (2003) High-performance thin-film transistors using semicondcutor nanowires and nanoribbons. Nature 425:274–278Google Scholar
- Jin G, Tang YS, Liu JL, Wang KL (1999) Growth and study of self-organized Ge quantum wires on Si(111) substrates. Appl Phys Lett 74:2741–2743Google Scholar
- Molle A, Md. Bhuiyan NK, Tallarida G, Fanciulli M (2006) In situ chemical and structural investigations of the oxidation of Ge (001) substrates by atomic oxygen. Appl Phys Lett 89:83504–83506Google Scholar
- Pecorea EF, Irrera A, Artoni P, Boninelli S, Bongiorno C, Spinella C, Priolo F (2010) Hetroepitaxial growth and facetting of Ge nanowires on Si (111) by electron–beam evaporation. Electro Solid State Lett 13:K53–K55Google Scholar
- Qian F, Gradecak S, Yat L, Yen C, Lieber CM (2005) Core/multishell nanowire hetrostructures as multicolour high efficiency light emitting diodes. Nano Lett 5:2287–2291Google Scholar
- Wang D (2007) Synthesis and properties of germanium nanowires. Pure Appl Chem 79:55–65Google Scholar
This article is published under license to BioMed Central Ltd. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.