Kinetic study of H-terminated silicon nanowires oxidation in very first stages
© Bashouti; licensee Springer. 2013
Received: 9 October 2012
Accepted: 27 November 2012
Published: 21 January 2013
Oxidation of silicon nanowires (Si NWs) is an undesirable phenomenon that has a detrimental effect on their electronic properties. To prevent oxidation of Si NWs, a deeper understanding of the oxidation reaction kinetics is necessary. In the current work, we study the oxidation kinetics of hydrogen-terminated Si NWs (H-Si NWs) as the starting surfaces for molecular functionalization of Si surfaces. H-Si NWs of 85-nm average diameter were annealed at various temperatures from 50°C to 400°C, in short-time spans ranging from 5 to 60 min. At high temperatures (T ≥ 200°C), oxidation was found to be dominated by the oxide growth site formation (made up of silicon suboxides) and subsequent silicon oxide self-limitation. Si-Si backbond oxidation and Si-H surface bond propagation dominated the process at lower temperatures (T < 200°C).
KeywordsSilicon nanowires Oxidation Kinetics Activation energy
During the last decade, silicon nanowires (Si NWs) have been studied extensively to be employed in the modern electronic industry in the direction of the size reduction and efficiency boost of the devices . Because of the high surface to volume ratio, Si NWs’ properties depend firmly on their surface conditions and surface terminations, in particular. The oxidation of Si NWs, when exposed to ambient air, is believed to have a detrimental effect on their electrical properties due to the low quality of the oxide, giving rise to the uncontrolled interface states and enhanced carrier recombination rates . This necessitates protection of Si NWs’ surfaces against oxidation via termination by various chemical moieties (i.e., alkyls and alkenyls) [3, 4]. However, to better prevent oxide formation, a deeper understanding of the Si NW’s oxidation mechanisms and kinetics is essential. For planar Si, the widely known Deal-Grove (DG) model considers the interfacial oxidation reaction and oxidant diffusion as the major rate-determining reaction steps for short and long oxidation times, respectively . DG model has undergone a number of modifications due to imprecise prediction of the oxidation behavior at low temperatures (T ≤ 700°C) in convex/concave surfaces and for very thin oxide layers [6–8]. Specifically, in sufficiently small Si NWs (d ≤ 44 nm), oxidation can be completely retarded by the compressive stress normal to the oxide/NW interface [9, 10]. Nevertheless, the studies on the oxidation mechanisms of Si NWs have been focused mostly on the formation of thick oxide layers at relatively high temperatures and long times, overlooking the early stages of oxidation which involve removal of surface functionalities and suboxides formation.
In this article, thermal stability of hydrogen-terminated Si NWs of 85-nm average diameter was investigated by means of the surface-sensitive X-ray photoelectron spectroscopy (XPS) for a variety of temperatures and times. H-terminated surfaces are of importance since they are considered as the starting surfaces for further functionalization of Si NWs [11–15]. The different kinetic behavior for the three transient silicon suboxides and SiO2 has been shown. Growth regimes were mainly addressed by four different phenomena including Si-Si backbond oxidation, surface bond propagation, suboxide growth site formation, and self-limited oxidant diffusion. A preliminary oxidation mechanism, elucidating the influence of time and temperature on the role of latter factors, is outlined.
Synthesis of initial Si NWs
To produce Si NWs, the vapor–liquid-solid (VLS) technique for silane (SiH4) gas, assisted by gold (Au) as silane decomposition catalyst, was employed. Prior to the VLS process, the native oxides on substrates of Si(111) have to be removed through etching in diluted HF. A thin gold layer of 2 nm in thickness was then sputtered on the etched substrates. After being transferred to the VLS operation chamber, the substrates were subjected to temperature and pressure of ≈580°C and ≈ 5 × 10−7 mbar for 10 min, as to be annealed. Subsequently, to grow nanowires on the surface, temperature was reduced to ≈520°C and a gas mixture of 5 to 10 ccm (standard cm3 min−1) Ar and 5 ccm SiH4 was introduced for 20 min at a pressure ranging from 0.5 to 2 mbar.
Si NWs hydrogen termination
The grown Si NWs has to be treated on their surface. Si NW were first cleaned by N2(g) flow for several seconds and then exposed in a sequence to buffered HF solution (pH = 5) and NH4F (40% in water) for 30 to 50 s and 30 to −180 s, respectively. H-terminated Si NWs were rinsed by water for less than 10 s per side to prevent the oxidation and dried in N2(g) for 10 s.
Oxide growth in Si NWs
To evaluate the thermal stability of hydrogen atoms bonded to NWs’ surfaces and find dominant oxidation mechanisms, H-Si NWs were annealed at atmospheric condition in six distinct temperatures of 50°C, 75°C, 150°C, 200°C, 300°C, and 400°C, each for five different time-spans: 5, 10, 20, 30, and 60 min. The annealing and hydrogen-termination processes were gentle in the sense that they did not melt the Si NWs or change their diameters.
Characterization of Si NWs
Pristine Si NWs were examined by scanning electron microscopy (SEM, Toshiba S-4800, Toshiba International (Europe) Ltd., Uxbridge, UK) with 5.0 kV voltage and 10.0 μA current, on top and side views. After each heating stage, the specimens were scanned by home-made XPS. Core level and valance band photoelectron spectra were excited by monochromatic Al K radiation (1,487 eV) and collected, at take-off angle of 35°, by a hemispherical analyzer with adjustable overall resolution between 0.8 and 1.2 eV. The surveys were conducted in various ranges of electron energies including the overall binding energy survey (0 to 1,000 eV) besides individual spectra for Si 2p (95.0 to 110.0 eV), C 1 s (282.0 to 287.0 eV) and O 1 s (520 to 550 eV) which were monitored more accurately in a discrete number of scans. All spectra were taken at room temperature in a UHV chamber of about 10−10 Torr pressure. The resulting XPS spectra were analyzed by spectral decomposition using the XPS peak software and their oxide levels were determined.
Results and discussion
Intensity of the silicon suboxides for the samples annealed at 150°C and 400°C
T = 150°C
T = 400°C
Intensity/oxidation time (min)
The growth of oxide in planar silicon in thick layers and at high temperatures has been successfully expressed by the Deal-Grove model. However, it breaks down in very thin oxide layers and has been modified considering the suboxides as nucleation sites (or oxide growth sites) that are necessary for oxide build-up . Through high-temperature oxidation, silicon suboxides exhibit relatively constant values after a sharp increase in their intensities. Therefore, in the early stages of Si NWs oxidation, formation of the growth sites composed of suboxides can be taken into account as the major mechanism.
In conclusion, the growth kinetics of the suboxides and silicon dioxide is highly dependent to temperature and time. At lower temperatures, oxidation is first controlled by backbond oxidation. After full oxidation of the backbonds, Si-H bond rupture dominates the process kinetics. At higher temperatures, suboxide nucleation sites (known as oxide growth sites) control the early stages of oxidation. After complete formation of the very first oxide monolayers, further oxidation is self-limited as the oxidant’s diffusion through the oxide layers is impaired. These findings suggest a perspective on more efficient methods to stabilize Si NWs against oxidation over the long term.
Scanning electron microscopy
- Si NWs:
X-ray photoelectron spectroscopy.
KS wishes to thank University of Erlangen-Nuremberg and the Elite Advanced Materials and Processes (MAP) graduate program for the MS thesis scholarship. MYB gratefully acknowledges the Max-Planck Society for the Post-Doctoral fellowship. SHC acknowledges the financial support by the FP7264 EU project LCAOS (nr. 258868, HEALTH priority) and the BMBF project (MNI priority) NAWION.
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