Evolution of hyperbranched polyglycerols as single-dopant carriers

Abstract

The self-assembly of macromolecules with each carrying one phosphorus atom may allow us to manipulate individual dopants at large scale. In this work, we applied the ring-opening multibranching polymerization process to synthesize hyperbranched polyglycerols (hbPGs). Nuclear magnetic resonance (NMR), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and size exclusion chromatography with multi-angle light scattering (SEC/MALLS) are employed to characterize the hbPGs molecules. The results indicate that each hbPGs molecule carries only one phosphorus atom. To drive the dopants into silicon, a monolayer of these hbPGs molecular carriers was immobilized on silicon surfaces, followed by rapid thermal annealing. Second ion mass spectroscopy (SIMS) indicates that every two hbPGs molecules contribute one phosphorus dopant to the Si substrate by thermal diffusion. Strategies were suggested to improve the dopant incorporation efficiency and the electrical ionization rate of the P dopants in Si.

Introduction

Traditional semiconductor devices such as metal-oxide-semiconductor field effect transistors (MOSFET) are created by introducing dopants into semiconductors [[1], [2], [3]]. Following the Moore’s law, modern MOSFET transistors have been scaled down to have a few discrete dopants in the device channel [4,5]. The random distribution of dopants results in a significant fluctuation in threshold voltages among MOSFET transistors [6,7], which in turn degrades the reliability of the integrated circuitry [8]. Deterministic doping at atomic level will help relieve this issue [9]. In 2006, Japanese scientists first demonstrated the deterministic doping arrays at atomic level by single ion implantation [5]. Later on a group led by M. Simmons in Australia demonstrated the control of single phosphorus dopant on silicon surfaces by scanning tunneling microscopy [10,11]. Transistors based on single phosphorus atoms were also demonstrated. In 1998, Kane first proposed a quantum computer architecture based on single phosphorus atom in silicon, which requires a precise control of single P atoms [12]. With the recent progress in single dopant manipulation, it looks quite promising to realize the Kane architecture for quantum computing [[13], [14], [15]]. Moreover, the dopant regulation will be useful for improving electronic properties of silicon-based biosensors, for example NO detection [16,17], bacteria detection [18,19], and plant cellular signal detection [20,21].

However, from the engineering point of view, the existing ion implantation and STM manipulation are time-consuming serial processes. It requires a parallel process to mass produce single dopants at arbitrary locations for commercialization. To data, linear polymer grafting with thermal decomposition can achieve the discrete atomic doping [22,23]. But, the self-limitation of linear polymer cannot occupied the specific doping region, owing to the cross-linking of linear polymers onto planar surface [24,25]. By employing the self-assembled monolayer doping technique (so-called molecular doping) [26,27], we previously proposed to use macromolecules as dopant carriers to control the dopant atom locations via self-assembly of the macromolecule carriers onto chemically pre-patterned silicon surfaces, as shown in Fig. 1 [28,29]. When the macromolecules and the patterns are comparable in size, a space-limiting process may occur, i.e. only one macromolecule will be immobilized on each pattern. Before the wide application of dopant polymer carriers in molecular monolayer doping and single electron transistors preparation, the effect of dopant carriers on electronic properties and grafting method should be well investigated. A large array of single dopants will be created if each macromolecule carries only one dopant atom and this dopant atom can be incorporated into silicon with a high success rate. In this work, we successfully synthesized hyperbranched polyglycerols (hbPGs) molecules which was carefully characterized by several techniques. The results indicate that each hbPGs molecule carries only one phosphorus atom. The phosphorus atoms carried by the molecules were then driven into the silicon substrate by rapid thermal annealing with a success rate of every two hbPGs molecules contributing one phosphorus dopant to the Si substrate. To improve the doping efficiency of hbPGs, different pre-dealing process, only Ar annealing and Ar annealing with peroxidation under oxygen ambient, were performed to remove the carbon contamination. Low Hall measurement and relative characterization demonstrated that the pre-oxidation can significantly improve the activation rate of doped phosphorus element.