Ultradense and planarized antireflective vertical silicon nanowire array using a bottom-up technique
© Dupré et al.; licensee Springer. 2013
Received: 11 January 2013
Accepted: 27 February 2013
Published: 9 March 2013
The production and characterization of ultradense, planarized, and organized silicon nanowire arrays with good crystalline and optical properties are reported. First, alumina templates are used to grow silicon nanowires whose height, diameter, and density are easily controlled by adjusting the structural parameters of the template. Then, post-processing using standard microelectronic techniques enables the production of high-density silicon nanowire matrices featuring a remarkably flat overall surface. Different geometries are then possible for various applications. Structural analysis using synchrotron X-ray diffraction reveals the good crystallinity of the nanowires and their long-range periodicity resulting from their high-density organization. Transmission electron microscopy also shows that the nanowires can grow on nonpreferential substrate, enabling the use of this technique with universal substrates. The good geometry control of the array also results in a strong optical absorption which is interesting for their use in nanowire-based optical sensors or similar devices.
Semiconductor nanowires are now widely implemented as active elements in devices for various applications such as energy harvesting [1, 2], microelectronics , or sensors [4, 5]. In order to achieve high performances, high densities of nanowires are required to increase efficiency or sensitivity of devices [6, 7]. In this purpose, top-down etching of a semiconductor wafer is the most commonly used technique [7–9]. However, the requirement of a bulk wafer prevents the realization of cost-effective devices. Some groups therefore choose to use bottom-up techniques and produce nanowires using catalytic processes such as chemical vapor deposition (CVD) [10–12], allowing the growth of nanowires on noncrystalline substrates [13, 14]. However, the production of high-density arrays of aligned nanowires is challenging with this technique because it requires a control of the density and localization of the metallic catalyst seeds. Furthermore, if the substrate is not oriented in the preferential growth direction, it is impossible to achieve arrays with aligned nanowires because of their random orientations on the substrate. Various solutions are investigated to create high-density networks of nanowires using a bottom-up approach. For instance, dense networks of gold droplets can be realized by dewetting a thin layer of gold deposited on the surface of a substrate , but the density is not as high as with top-down techniques, and the size of the catalyst particles is hardly controlled. Another interesting solution is to lithographically pattern a substrate with catalyst particles [16, 17], which is time and money consuming in the case of e-beam lithography to achieve nanoscale dimensions.
We describe a new bottom-up method to produce silicon nanowire arrays which present a very high density and height homogeneity. Nanowires are grown by gold-catalyzed CVD in the vapor–liquid-solid (VLS) mode using an anodic aluminum oxide (AAO) membrane with cylindrical nanopores as growth template. This guided nanowire growth is used to create arrays of vertically aligned nanowires with densities up to 1010 cm−2 on substrates oriented in another direction than the preferential one [18, 19]. It is usually admitted that in this case, growth has to be stopped before the nanowires reach the surface of the AAO template. It indeed prevents any structural anomalies such as kinks and increases of the nanowires’ diameter due to the catalyst getting out of the template. This leads to a difficult control and inhomogeneities in the length of the nanowires depending on the size of the initial gold catalyst. However, a planarized silicon nanowire matrix is of great interest to achieve reproducible and homogeneous top contacts or structural processing . In this paper, we show that a combination of ultrasonic agitation, gold-chemical etching, and silicon plasma etching enables the achievement of high-density arrays of silicon nanowires with a very good length control and homogeneity on a silicon substrate. The nanowires have a good crystalline quality, and the array features good antireflective properties that could be useful for their implementations in devices such as detectors.
Structural characterizations were carried out using a Zeiss Ultra 55 SEM (Carl Zeiss, Inc., Oberkochen, Germany) and a Jeol 3010 transmission electron microscope (TEM, JEOL Ltd., Akishima-shi, Japan). Grazing incidence X-ray diffraction (GIXD) was performed at the BM2-D2AM beamline of the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Reflectivity measurements were carried out with a homemade optical setup.
Results and discussion
SEM pictures of Figure 2 clearly show the very high density of individualized nanowires. Based on the number of nanowires counted on SEM images, we estimate the density to around 8×109 nanowires cm−2 for a sample in which growth template was made at 40 V. It is also clear that nanowires were guided in the nanopores during their growth as revealed by the roughness of their surface: the morphology of the nanopores’ sidewalls was transferred to the growing nanowires which were thoroughly filling them (Figure 2e,f). The combination of standard microelectronics processes with the confined VLS growth of silicon nanowires therefore enabled the production of arrays of nanowires presenting similar features than with top-down techniques: their density is very high and every single nanowire is well individualized.
Since the nanowires’ diffraction peak appears at a lower scattering vector than the substrate one, the silicon lattice parameter is slightly dilated in the nanowires compared to bulk silicon. The calculated strain using Equation 3 is Δa/a = 1.9 × 10− 3 which is one order of magnitude greater than for gold-catalyzed silicon nanowires which grew freely . This increased strain could be explained by the forced growth in the nonpreferential  crystallographic direction or by the effects at the interface between the growing nanowires and their Al2O3 growth template, but this still needs further investigation.
Two types of nanowires therefore grew in the AAO template, one in epitaxy with the (100) substrate and another one with no crystalline relation with it, each type being clearly detected with a separate technique. Using SEM pictures such as the one of Figure 2e, it is not possible to visually differentiate between the two types of wires since they are all well individualized and fully guided in the nanopores. The most likely cause for the nonepitaxial nanowire growth is a partial deoxidation of the silicon substrate during the vapor HF step before catalyst electrodeposition. If the silicon surface at the bottom of a pore is only partially deoxidized, the remaining native oxide would disturb the initial growth steps by screening the substrate and therefore preventing a good epitaxy. This effect is known and described in the case of copper electrodeposition in nanoporous alumina . In the opposite case of a thorough deoxidation of the nanopore, the resulting nanowires would grow in epitaxy with the silicon substrate.
Silicon nanowire arrays were produced presenting top-down features but using a bottom-up CVD process. A very high density was reached with a planarized overall surface and long-range periodicity leading to interesting optical behavior such as an increased light absorption. Silicon nanowires are monocrystalline and grew on a nonpreferential (100) silicon substrate, opening the way to the use of this technique on noncrystalline universal substrates such as glass or metals.
The authors would like to thank Marc Zelsmann for his help in the deposition of thick aluminum. Special thanks go to the BM2-D2AM beamline staff of ESRF for their technical support. This work was financially supported by the French Ministère de la Défense-Direction Générale de l’Armement and by the Region Rhône-Alpes Scientific Research Department via Clusters de Micro et Nanotechnologies.
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