Donor ionization in size controlled silicon nanocrystals: The transition from defect passivation to free electron generation

Abstract

We studied the photoluminescence spectra of silicon and phosphorus co-implanted silica thin films on (100) silicon substrates as a function of isothermal annealing time. The rapid phase segregation, formation, and growth dynamics of intrinsic silicon nanocrystals are observed, in the first 600 s of rapid thermal processing, using dark field mode X-TEM. For short annealing times, when the nanocrystal size distribution exhibits a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts further to the red than the intrinsic nanocrystals. These results indicate the existence of competing pathways for the donor electron, which depends strongly on the nanocrystal size. In samples containing a large density of relatively small nanocrystals, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface means that ionization results in a passivation of dangling bond (P b -centre) type defects, through a charge compensation mechanism. As the size distribution evolves with isothermal annealing, the density of large nanocrystals increases at the expense of smaller nanocrystals, through an Ostwald ripening mechanism, and the majority of phosphorus atoms occupy substitutional lattice sites within the nanocrystals. As a consequence of the smaller band-gap, ionization of phosphorus donors at these sites increases the free carrier concentration and opens up an efficient, non-radiative de-excitation route for photo-generated electrons via Auger recombination. This effect is exacerbated by an enhanced diffusion in phosphorus doped glasses, which accelerates silicon nanocrystal growth.

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

“…3 , the dependence of the PL spectra on the P-concentration in SRO:P and SRON:P is demonstrated. Here, all samples are H 2 -passivated and hence only the PL-quenching effect of P-incorporation is visible, not the PL-enhancement often observed for low P-concentrations and associated to dangling bond passivation by P [ 42 ]. Up to the level of ≈0.6 atom % P the PL intensity drops by less than 40% without any significant peak shift.…”

“…The TT-results are presented for both H 2 -passivated and unpassivated states without distinctive differences, but one remark concerning the interaction of Si-doping and hydrogen shall be made: While P in the Si NC system is known to passivate dangling bonds (DBs) at the Si/SiO 2 interface [ 7 , 42 ] similar to a post-annealing in H 2 , hydrogen treatments have also been shown to deactivate P-donors and B-acceptors in heavily doped Si nanowires [ 47 ] and in the bulk [ 48 – 50 ]. However, this type of dopant passivation solely relies on very reactive atomic hydrogen (rather than molecular H 2 gas) and requires much lower temperatures of 100–150 °C to be efficient.…”

Charge transfer of single laser crystallized intrinsic and phosphorus-doped Si-nanocrystals visualized by Kelvin probe force microscopy

“…Crowe et al also reported a transition from defect passivation by P atoms to free carrier generation by P donors as the increase of rapid thermal annealing time during the Si-NCs growth when the other conditions were kept unchanged. 35 Charge number in Figs. 6(b) and 6(c) is also calculated, which is þ1.3 e and þ1.2 e, respectively.…”

An Outlook on Silicon Nanocrystals for Optoelectronics

“…Interaction of Si-NCs with Er ions has attracted a lot of attention due to the possibility of operating in the telecommunications band around 1.5 µm. Recent studies have shown that phosphorus doping can result in either a quenching or an enhancement of the Si-NC's PL, depending on the dopant concentration and NC sizes [28,35]. Crowe et al [35] suggested the existence of competing pathways for the donor electron, which depend strongly on the NC size.…”

“…Recent studies have shown that phosphorus doping can result in either a quenching or an enhancement of the Si-NC's PL, depending on the dopant concentration and NC sizes [28,35]. Crowe et al [35] suggested the existence of competing pathways for the donor electron, which depend strongly on the NC size. For relatively small NCs, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface results in a passivation of dangling bond type defects as evidenced by an enhancement of the integrated intensity and corresponding blue-shift of the emission peak.…”