Density-Functional Studies Of The Surface Chemistry And Doping Of Silicon Nanocrystals

The remarkable optical properties of silicon nanocrystals (Si NCs) have enabled Si NCs to be one of the most promising materials for the realization of silicon-based optoelectronics. Recently, a number of experiments have shown that the optical properties of Si NCs are strongly affected by impurities. In the nanometer-sized regime, however, it is tremendously challenging to directly pinpoint impurities that are responsible for the optical behavior. This leads to the imperative need to use quantum-mechanical methods to study the effect of impurities on the optical properties of Si NCs. On the basis of density functional theory, we have investigated the formation of oxygen (O), nitrogen (N), phosphorus (P), boron (B) and manganese (Mn) at the surface of Si NCs and in Si NCs. The optical properties of impurity-incorporated Si NCs are studied. The main findings of our work are as folows.(1) Ab initio methods based on density functional theory are employed to investigate the bonding of O at the oxide/nanocrystal (NC) interface after hydrogen (H)-passivated silicon (Si) NCs are oxidized. Besides the well-known quantum confinement effect, the type of O bonding and the oxidation state of Si at the oxide/NC interface are found to significantly affect the optical properties of oxidized Si NCs. After oxidation, the excitation energies of Si35 and Si66 increase, while those of Si87 and Si123 decrease. We show that oxidation-induced redshifts in the light emission from Si NCs do not always result from defective O such as doubly bonded O at the oxide/NC interface. When Si atoms at the oxide/NC interface are mainly in low oxidation states, backbond O at the interface per se results in the redshifts in the light emission from Si NCs. When Si atoms at the oxide/NC interface are mainly in high oxidation states, Si3+=O at the interface leads to the redshifts in the light emission from Si NCs. It is found that for Si NCs with perfect oxide/NC interface (i.e. O at the interface is all backbond O) the seriously weakened next-nearest-neighboring SisSi of Si3+ readily breaks after excitation. At the oxide/NC interface, Si2+=O induced strong electronic localization and Si2+=O and Si3+=O induced reduction of interface polarization stabilize the geometry of oxidized Si NCs at the excited state. The electronic localization of severely stressed bridge O at the oxide/NC interface is relatively weak. This facilitates the breaking of nearest-neighboring Si-Si at the oxide/NC interface as oxidized Si NCs are excited.(2) By means of first-principle energy calculations, we determine that N is most likely incorporated at the surface of Si NCs to form a double interstitial-N pair (DINP). The time-dependent density functional theory with the hybrid B3LYP exchange-interaction kernel is used for studying the effect of DINP on the electronic structures and optical properties. It is found that DINP introduces localized electronic states near the bandedge of Si NCs, leading to the formation of bound exciton. The DINP-induced bound exciton results in a strong emission with radiative lifetime of nanoseconds, consistent with experiments. For Si NCs smaller than 1.5 nm in diameter, the geometry of a Si NC at excited state can be stabilized by DINP. This significantly shrinks the stoke shift, blueshifting the emission energy to the blue and ultraviolet range.(3) The doping of P provides an additional means to control the optical properties of silicon nanocrystals (Si NCs). The P-doping-induced changes in the optical properties of Si NCs, however, have not been consistently understood. On the basis of first-principles calculations, we explain the P-doping-induced infrared absorption of Si NCs and the effect of P doping on the light emission from Si NCs. The explanations are enabled by the investigation of the locations of P in Si NCs, including a variety of locations at the surface of Si NCs. We show that the light emission from Si NCs critically depends on the location of P. Transitions involving P-doping-induced defect energy levels lead to the infrared absorption of Si NCs.(4) By means of first-principles study in the framework of density functional theory we have found that B prefers residing at the surface of silicon nanocrystals (Si NCs), similar to P. Different from P, B induces surface restructuring when B is one- or two-coordinated at the NC surface. B doping does not significantly change the bandgap of Si NCs. But in most cases B introduces deep energy levels in the bandgap of Si NCs. This explains the B-doping induced quenching of band-edge light emission usually observed in experiments. The negligible infrared absorption of B-doped Si NCs may result from that only three-coordinated B is formed at the NC surface. The electronic transitions involving the energy levels induced by these three-coordinated B are not in the infrared range.(5) The hybrid B3LYP function within the framework of density functional theory is employed to study the energetic stability of Mn-doped Si NCs with Mn at substitutional or interstitial site inside a Si NC. In most cases, Mn prefers to substitute the subsurface Si atom. Only in extremely Si-rich condition, Mn can be incorporated at the interstitial site beneath the (100) facet. After Mn doping, the spin multiplicity of a Si NC become quartet, regardless of the location of Mn. The electronic states with majority spin are strongly affected by quantum confinement, while those with minority spin are quantum-confinement-independent. The optical properties of Si NCs are studied by the use of time-dependent density-functional theory with hybrid B3LYP exchange-interaction kernel. When the NC size changes, spin flipping of the electronic transition related to the optical gap of Si NCs with Mn at subsurface substitutional site may occur, significantly modifying the optical gap of Si NCs. While the electronic transition related to the optical gap of Si NCs with Mn at the interstitial site beneath the (100) facet does not spin-flip with size.