Thermodynamic stability of high phosphorus concentration in silicon nanostructures

Abstract

Doping of Si nanocrystals (NCs) has been the subject of a strong experimental and theoretical debate for more than a decade. A major difficulty in the understanding of dopant incorporation at the nanoscale is related to the fact that theoretical calculations usually refer to thermodynamic equilibrium conditions, whereas, from the experimental point of view, impurity incorporation is commonly performed during NC formation. This latter circumstance makes impossible to experimentally decouple equilibrium properties from kinetic effects. In this report, we approach the problem by introducing the dopants into the Si NCs, from a spatially separated dopant source. We induce a P diffusion flux to interact with the already-formed and stable Si NCs embedded in SiO 2 , maintaining the system very close to the thermodynamic equilibrium. Combining advanced material synthesis, multi-technique experimental quantification and simulations of diffusion profiles with a rate-equation model, we demonstrate that a high P concentration (above the P solid solubility in bulk Si) within Si NCs embedded in a SiO 2 matrix corresponds to an equilibrium property of the system. Trapping within the Si NCs embedded in a SiO 2 matrix is essentially diffusion limited with no additional energy barrier, whereas de-trapping is prevented by a binding energy of 0.9 eV, in excellent agreement with recent theoretical findings that highlighted the impact of different surface terminations (H-or O-terminated NCs) on the stability of the incorporated P atoms.

Ab initio studies of the optoelectronic structure of undoped and doped silicon nanocrystals and nanowires: the role of size, passivation, symmetry and phase

“…105 This is in agreement with the observation that for Si-NCs embedded in a SiO 2 matrix, the matrix provides a strong barrier to P diffusion, inducing P segregation in the Si rich region. 106,107 By contrast, still consistent with experiments, 108 the diffusion of B toward the matrix is signicantly easier. Once more the structural relaxation around the impurity plays a signicant role.…”

Diffusion doping route to plasmonic Si/SiOxnanoparticles

“…The problem may be partially related to a kinetically controlled doping regime, as substitutional doping of silicon cores by phosphorus has been shown to be thermodynamically preferable in case of hydroxyl or surface oxide passivation. [25][26][27] The studies of post-synthetic doping of Si SNPs are rather scarce [28][29][30][31] and concern embedded Si nanocrystals only, those cannot be considered for industrial application. DLs up to 6% at were reported for phosphorus.…”

Absence of free carriers in silicon nanocrystals grown from phosphorus- and boron-doped silicon-rich oxide and oxynitride

“…Si NCs of a few nanometres in diameter (i.e., quantum dots) represent a good model system to study doping at the nanoscale. They can be fabricated by various methods [ 7 – 9 ] and doped either during growth [ 7 ] or post-growth [ 10 ]. A recent review provides a broad overview of all available techniques and approaches [ 11 ].…”