Selective synthesis of SiO₂ NWs on Si substrate and their adjustable photoluminescence

Abstract

Amorphous SiO₂ nanowires with diameters of 20–200 nm and lengths of several micrometers were selectively grown，at 1100 °C under ambient pressure，on the ultra-thin Au film coated silicon wafers，which were characterized by scanning electron microscope, Fourier transform infrared spectroscope, and high resolution transmission electron microscope. Using X-ray energy dispersion spectroscope equipped on the high resolution transmission electron microscope, the investigation was carried out on the micro-region chemical composition of the as-grown amorphous SiO₂ nanowire. The growth mechanism has been discussed, and it has been believed to be extended vapor-liquid-solid process. The SiO₂ layer, as the result indicates, can prevent the formation of Si–Au alloy from Si–Au interface. This work also discusses the photoluminescence emission from SiO_x centers at room temperature, it is found that by changing the oxygen, the photoluminescence intensity can be adjusted.

Introduction

One dimensional semiconductor nanostructures (SCNS) have aroused intense research interests due to their excellent properties and potential applications in optical-electronic device manufactories. A large number of nanostructures, taking nanorodes (NRs) and nanowires (NWs) for an example, of ZnO, GaN, Si and silica, have been synthesized over the past decade year. The vapor-liquid-solid (VLS) mechanism has been developed to explain the growth mechanism of these nanostructures. Meanwhile, some novel nanostructures, taking ZnO for an example, are grown on carbon nanotubes by thermal evaporation [1]. Owing great deal to their unique properties, nowadays, semiconductor nanowires are extensively being investigated as components for future integrated circuits (IC). A variety of electronic device prototypes have also been realized using NWs, such as field-effect transistors [2] and photodiodes to biosensors. Although, the SCNS can drastically reduce the cost and size and probably improve the device efficiency, these devices cannot easily be compatible with modern VLSI processes.

Amorphous SiO₂ coating has been widely used in IC manufactory because of its excellent insulation property. Recently, it is accepted more and more that SiO₂ nanostructures, such as nanoparticles and NWs, are stable luminescent phosphors [3]. The potentials of developing silicon-based nanostructures in optoelectronics device manufactories have been predicted from the significant discoveries on the luminescent performance in amorphous SiO₂ nanostructures [4]. So far, the SiO₂ NWs have been prepared by several approaches including thermal evaporation [5], [6], laser ablation [7], [8], sol-gel [9] and chemical vapor deposition [10]. Meanwhile, the SiO₂ NWs exhibit complicated photoluminescence properties. For instance, SiO₂ NWs formed with the assistance of Mg [11] as catalyst have three peaks at 380 nm, 414 nm and 434 nm. Yet, SiO₂ nanowires prepared with the assistance of Ni [12] as catalyst have a UV band ranged from 350 nm to 420 nm, and those synthesized by thermal evaporation [6] without any catalyst show a UV band centered at 400 nm with two additional green emission bands at 510 nm and 560 nm, respectively. These properties present the brilliant to the future micro-optoelectronic device, but the mechanism of these peaks is still divergent, and needs further study. For instance, there is an argument that whether these photoluminescence emissions intensively depend on the SiO_x defects, such as neutral oxygen vacancies (O≡Si–Si≡O), intrinsic diamagnetic defect centers (Si–O–Si), nonbonding oxygen hole centers (≡ Si–O), impurity atoms and interface state [13], [14], [15]. In addition, it is reported that the hydrogen was used industrially to dangling silicon bonds or interface traps [16]. Although the photoluminescence property has been reported in the literatures, the photoluminescence efficiency is still not high enough for practical application.

Hereby, we report a simple yet efficient method to selectively grow amorphous SiO₂ NWs by annealing the ultra-thin Au film coated silicon wafers found by accident. A growth model based on an extended vapor-liquid-solid (VLS) mechanism is discussed. Meanwhile, it will be worth pointing out that the photoluminescence performance can be tunable by changing the growth conditions, and the mechanism of it is also further discussed according to the experiment conditions.