In-situ chemical doping of silicon nanowires by supercritical-fluid synthesis




Villarreal, Julián Enrique

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Silicon nanowires show promise as components in electronic devices and integrated circuits. The ability to chemically dope nanowires is desirable in order to enable further technological development of this material. In this study, phosphorus-doped (P-doped) silicon nanowires were synthesized in situ by the supercritical fluid-liquid-solid (SFLS) growth mechanism through addition of diphenylphosphine (DPP) or tris(trimethylsilyl)phosphine (P(SiMe3)3) as phosphorus dopants. Conduction electrons from P donor states in crystalline Si have been observed in the in-situ-doped nanowires by detection of a resonance peak at g = 1.998 via electron paramagnetic resonance spectroscopy (EPR) at 4.2 K. Elemental analysis via inductively coupled plasma-atomic emission spectroscopy (ICP-AES) has shown that the amount of P in in-situ-doped nanowires is commensurate with the dose of P precursor, and by appropriate dosing, the doping level can be modulated between 10^17 and 10^19 P atoms cm^−3 without an intolerable degradation in nanowire quality or yield. Field-effect transistors (FETs) were fabricated from the undoped and doped nanowires. An n-channel, field-effect response was in general not clearly observed in FETs fabricated using the in-situ-doped nanowires, though in some cases unipolar p-type and ambipolar behavior were observed with strong hysteresis regardless of the direction of the gate-voltage sweep. The poor performance of the tested devices might be attributed to the use of Au seeds, which introduce mid-gap trap states in Si, and to the presence of surface defects that can scatter charge carriers. In addition, ion implantation of silicon nanowires was performed, as an alternative to in-situ doping of P. The nanowires were subjected to ion implantation at targeted dopant concentrations similar to those of in-situ doping. FET transfer characteristics of these implantated nanowires were determined for as-received nanowires as well as post-implantation annealed nanowires. FET characterization was inconclusive, and EPR studies did not reveal a clear sign of conduction electrons from P. However, compelling changes in the surface states of the nanowires were revealed. In particular, for annealed nanowires, paramagnetic states associated with oxide defects in silicon were observed. Nevertheless, high-resolution transmission electron microscopy did not reveal widespread amorphization of silicon nanowires after ion implantation, as might be expected in bulk silicon. These results suggest that ion implantation of silicon nanowires has a strong effect on surface states, while leaving the crystalline core of the nanowires relatively intact.


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