Doping Silicon Quantum Dots With Boron And Phosphorus

Among all kinds of semiconductor in nanoscale region, silicon quantum dots (Si QDs) hold great promise for widespread applications in various fields such as optoelectronics, photovoltaics, electronics, energy storage and bioimaging. This is largely due to the nontoxicity and abundance of Si and the compatibility of Si-QD technology with well-established bulk-Si technology. Analogous to bulk Si, significant efforts have been devoted to doping Si QDs to realize the full potential of Si QDs. Up to now, doping has been experimentally realized for gas-phase-synthesized freestanding Si QDs that are passivated by hydrogen, and solid-phase-synthesized Si QDs that are embedded in SiO2. However, the doping behavior and underneath mechanism of Si QDs embedded in SiO2 have not been fully understood given the fact that few theoretical work has been carried out on Si QDs embedded in SiO2. Moreover, although size effect on the properties of undoped Si QDs has been largely studied, the dependence of properties of doped Si QDs on QD size has not been systematically investigated.In this thesis, we study the doping behaviors of B and P in Si QDs embedded in SiO2 based on density functional theory. Also the influence of structural relaxation on the properties of P-doped Si QDs is investigated. By using a nonthermal plasma system, we synthesize B-hypderdoped Si QDs with different sizes and work out the size-dependent properties for B-hypderdoped Si QDs. Following states the key results of this work:(1) We simulate the doping of Si QDs embedded in SiO2 by constructing Si@SiO2 and Si@dbSiO2 models in which Si QDs are completely covered by a thin layer of SiCte. P is the most likely incorporated into the sub-interface of Si@SiO2. However, for Si@dbSiO2 with a dangling bond at the Si/SiO2 interface, a P atom prefers passivating the dangling bond. P-induced deep energy levels are radiative, leading to light emission with energies smaller than the bandgaps.(2) The doping of B in Si QDs embedded in SiO2 (Si@SiO2 and Si@dbSiO2) is investigated. B is the most likely incorporated into the sub-interface or surface oxidation layre of Si@SiO2. For Si@dbSiO2, instead of passivating the dangling bond, a B atom prefers substituting a Si atom in the Si/SiO2 region. A serious structural disorder induced by B doping may lead to the damping of photoluminescence of Si QDs.(3) B and P codoped Si QDs embedded in SiO2 (Si@SiO2 and Si@dbSiO2) are simulated. The results show that they are more likely to form a B-P dipole in the sub-interface of Si@SiO2 in term of thermodynamic, otherwise they will introduce acceptor and donor energy levels in the bandgap. This may lead to different emission energy of the photoluminescence of Si QDs.(4) We study the influence of structural relaxation on the structure, electronic and optical properties of P-doped Si QDs by means of density functional theory. It is found that structural relaxation makes differences in the energy-level schemes, binding energy and optical absorption of P-doped Si QDs. With the increase of the concentration of P these changes induced by structural relaxation become more serious.(5) Si QDs hyperdoped with different B doping levels are considered. The dependence of the physical properties including localized surface plasmon resonance (LSPR) of hyperdoped Si QDs on the QD size is examined. The largest Si QDs are less affected by disorder and charge carrier surface scattering. As a result, there exists a range of doping concentrations where the LSPR energy can be blueshifted as the QD size decreases.