Influence of the Chemical Properties of the Substrate on Silicon Quantum Dot Nucleation

Abstract

We have studied the influence of SiO 2 surface properties on the nucleation and growth of silicon quantum dots ͑Si-QDs͒ deposited by SiH 4 low-pressure chemical vapor deposition ͑LPCVD͒. First, the effect of siloxane groups ͑Si-O-Si͒ strain at the SiO 2 surface layer, characterized by Fourier transform infrared ͑FTIR͒ spectroscopy, is studied. We evidenced an increase of Si-QD nucleation with the strain of siloxane groups in the SiO 2 substrate layer. Second, the Si-QD nucleation strongly depends on the surface silanol group ͑Si-OH͒ density. This density, controlled by chemical and thermal treatments, is measured by multiple internal reflexion ͑MIR͒ FTIR. Very high Si-QD densities larger than 10 12 /cm 2 are obtained on highly hydroxylated SiO 2 .During the last few years, silicon quantum dots ͑Si-QDs͒ have been studied for nanoelectronics applications. Their unique physical properties, size confinement effect, and coulomb blockade phenomena make Si-QDs suitable for use in new silicon-based devices like single electron transistors 1 or quantum dot floating gate memories. 2 For room-temperature operation of such devices, nanometric size silicon dots ͑Ͻ10 nm͒ are required.Low-pressure chemical vapor deposition ͑LPCVD͒ is a good way to obtain Si-QDs for industrial applications because of its metal-oxide field effect transistor ͑MOSFET͒ technology compatibility. By controlling the early stages of the Si film growth, silicon crystallites of nanometer size ͑5 nm͒ are obtained. 3 It has been shown that Si-QDs elaborated by SiH 4 LPCVD could exhibit coulomb blockade at room temperature 4 but to successfully integrate Si-QDs in devices their alignment must be controlled. Results were recently obtained by Baron et al. 5 who deposited an ordered array of Si-QDs by SiH 4 LPCVD on a substrate realized by wafer bonding with a periodic strain field at the surface.In order to obtain operating devices, size, size uniformity, and Si-QDs density must also be controlled with great precision and reproducibility. For floating gate memory applications, densities between 10 11 and 10 12 /cm 2 are required. To fabricate a single electron transistor with a lateral current transport, the spacing between dots should be lower than 2 nm. Typically, for a dot size of 5 nm, these conditions lead to a density of 3 ϫ 10 12 Si-QDs/cm 2 . 6 Si-QD size and density are piloted by pressure and temperature conditions. The chemical nature of the substrates 3 and its physical properties such as stress, roughness, or defects can play an important role in silicon nucleation. Voutsas and Hatalis 7 showed that the chemical properties of the SiO 2 surface also strongly affect the first stages of silicon deposition.In this paper, we separately investigated the influence of ͑i͒ the strain of siloxane ͑Si-O-Si͒ groups and ͑ii͒ the surface silanol ͑Si-OH͒ density on thermally grown SiO 2 layers on Si-QD nucleation by SiH 4 LPCVD. These characteristics are controlled by the oxidation process and postoxidation chemical and thermal treatments, respectively. Experim...

Development of an original model for the synthesis of silicon nanodots by Low Pressure Chemical Vapor Deposition

“…These physical properties are strongly dependent on the ND size distribution, area density and shape [4,[6][7]. Area densities of 10 12 NDs/cm 2 corresponding to NDs of 5 nm in diameter spaced 5 nm apart are sought for optimal device performance [7,8].…”

“…Area densities of 10 12 NDs/cm 2 corresponding to NDs of 5 nm in diameter spaced 5 nm apart are sought for optimal device performance [7,8]. As a consequence, it is mandatory to have a thorough understanding of the phenomena involved in the nucleation, growth and coalescence of NDs.…”

“…Siloxane bonds are known to be more thermally stable and less reactive than silanol bonds [11]. Miyazaki et al [12], Mazen [13] and Mazen et al [7] have shown that pre-treatment of SiO 2 surfaces with hydrofluoric acid (HF) increases the area density of silanol bonds and also, proportionally, that of silicon NDs synthesized by LPCVD while the ND size decreases. The use of SiO 2 substrates with high Si-OH bond densities is then a key to reach high NDs densities useful for quantum devices [7,11].…”

“…Miyazaki et al [12], Mazen [13] and Mazen et al [7] have shown that pre-treatment of SiO 2 surfaces with hydrofluoric acid (HF) increases the area density of silanol bonds and also, proportionally, that of silicon NDs synthesized by LPCVD while the ND size decreases. The use of SiO 2 substrates with high Si-OH bond densities is then a key to reach high NDs densities useful for quantum devices [7,11]. Mazen et al [7] have shown, using Fourier Transform Infra-Red (FTIR) spectroscopy that the silanol bond area density of a HF treated SiO 2 substrate is close to 10 14 cm −2 .…”

“…The use of SiO 2 substrates with high Si-OH bond densities is then a key to reach high NDs densities useful for quantum devices [7,11]. Mazen et al [7] have shown, using Fourier Transform Infra-Red (FTIR) spectroscopy that the silanol bond area density of a HF treated SiO 2 substrate is close to 10 14 cm −2 .…”