Critical Size for Carrier Delocalization in Doped Silicon Nanocrystals: A Study by Ultrafast Spectroscopy

Abstract

We present a comprehensive ultrafast spectroscopy-based study on the delocalization of doping-induced carriers in Si nanocrystals (NCs). To this end we prepare thin films of differently sized doped Si NCs and vary the doping configurations from singly P and B doping to simultaneously P and B co-doping. We show that the NC size orchestrates the level of delocalization of the doping-induced carriers. This can be understood in light of (1) the quantum confinement effect and (2) unscreened Coulomb interactions by moving further into the nanoscale. Both contributions affect the activation energy (ΔE) that is required to create free majority carriers. By varying the NC size in combination with the doping configuration we tune ΔE and control the delocalization of the doping-induced carriers. Most importantly, we show that there is a critical NC diameter of D critical ≈ 6 nm that describes the transition from a localized to a free carrier regime. In particular, our results show that optical bandgaps of ∼0.95 eV (optimal for carrier multiplication-facilitated solar cell power conversion) can be achieved in P−B co-doped Si NCs with D NC < D critical . These results indicate that the practical photovoltaic feasibility of co-doped Si NCs is not limited by the presence of some remaining free carriers in uncompensated NCs.

Donor–Acceptor Pair Recombination in Size-Purified Silicon Quantum Dots

“…In contrast to rapid exploration of applications in doped compound semiconductor QDs, impurity-doping in Si QDs is still at the stage of fundamental research despite its importance in microelectronics. Doped Si QDs have been studied by electron paramagnetic resonance (EPR) spectroscopy, and size dependence of the doping efficiency has been discussed. , The donor and acceptor energy levels were determined by the combination of photoemission yield spectroscopy and PL spectroscopy and by scanning tunneling spectroscopy. , Doping-induced carriers in Si QDs have been characterized by infrared absorption spectroscopy , and by ultrafast induced-absorption (IA) spectroscopy. , …”

“…12,13 Doping-induced carriers in Si QDs have been characterized by infrared absorption spectroscopy 14,15 and by ultrafast inducedabsorption (IA) spectroscopy. 16,17 In previous attempts, for the exploration of noble electrical and optical properties of Si QDs by doping the largest obstacle is the small solid solubility of impurities in Si crystal lattice. For example, even in the case of P, which has the largest solubility in Si crystal (∼1 × 10 21 cm −3 ), the calculated maximum dopant number in a 2 nm Si QD is 4.…”

Size-Dependent Asymmetric Auger Interactions in Plasma-Produced n- and p-Type-Doped Silicon Nanocrystals

“…We do note that Beard et al 26 did quantify the picosecond Auger lifetime of a charge-neutral biexciton in Si NCs; however, the fine details in terms of the individual trion components and the behavior of AR in (heavily) charged NC systems are yet to be understood. To attempt to address this knowledge gap, we have recently performed induced absorption (IA)-based investigations on plasma-produced P-doped 27 and sputter-produced P−Bcodoped 28,29 Si NC structures and roughly estimated the negative trion recombination lifetime. 27 Still, a complete photophysical model of AR in n-type Si NCs and, in particular, a quantification of the photophysical effects of p-type boron doping are barely understood.…”

“…27 Still, a complete photophysical model of AR in n-type Si NCs and, in particular, a quantification of the photophysical effects of p-type boron doping are barely understood. 7,29 In the present investigation, we monitor AR decay in both n-and p-type-doped Si NCs over a large size range (D NC ≈ 3−8 nm) that allows us to quantify the lifetimes of the negative and positive trionsthe simplest charged states in Si NCsfor this ubiquitous indirect semiconductor.…”

On the Location of Boron in SiO2‐Embedded Si Nanocrystals—An X‐ray Absorption Spectroscopy and Density Functional Theory Study

“…[ 1 ] Meanwhile, several reports support the concept that Si NCs ≤6 nm in diameter are not electronically p‐type‐doped (i.e., possess free carriers from B dopants) but that B in the NCs or the surrounding SiO 2 creates nonradiative defect states. [ 2–5 ]…”