Size-dependent donor and acceptor states in codoped Si nanocrystals studied by scanning tunneling spectroscopy

Abstract

The electrical and optical properties of semiconductor nanocrystals (NCs) can be controlled, in addition to size and shape, by doping. However, such a process is not trivial in NCs due to the high formation energy of dopants there. Nevertheless, it has been shown theoretically that in the case of B and P (acceptor/donor) codoped Si-NCs the formation energy is reduced relative to that of single type doping. Previous comprehensive measurements on ensembles of such codoped Si-NCs have pointed to the presence of donor and acceptor states within the energy gap. However, such a conjecture has not been directly verified previously. Following that, we investigate here the electronic properties of B and P codoped Si-NCs via Scanning Tunneling Spectroscopy. We monitored the quantum confinement effect in this system, for which the energy gap changed from ∼1.4 eV to ∼1.8 eV with the decrease of NC diameter from 8.5 to 3.5 nm. Importantly, all spectra showed two in-gap band-states, one close to the conduction band edge and the other to the valence band edge, which we attribute to the P and B dopant levels, respectively. The energy separation between these dopants states decrease monotonically with increasing NC diameter, in parallel to the decrease of the conduction-to-valence bands separation. A fundamental quantity that is derived directly for these Si-NCs is the intrinsic like position of the Fermi energy, a non-trivial result that is very relevant for understanding the system. Following the above results we suggest an explanation for the character and the origin of the dopants bands.

Electrical and chemical properties of vacancy-ordered lead free layered double perovskite nanoparticles

“…However, the tunneling did show any evidence for in-gap states, unlike some cases where in-gap states giving rise to red-shifted PL of Si nanocrystals were found. 28,29 It should be noted, however, that in the case of the Si NPs the PL signal was much larger compared to what we observed here, signifying a larger relative amount of in-gap states, and they were also located at the Si nanocrystals surface, two important factors that making them easier to detect. Moreover, the band gaps of the Si nanocrystals studied in ref.…”

“…The STM topographic images were typically measured with sample-bias and current set values of V = 2–2.2 V (to be well above the CBM) and I = 0.5 nA, whereas the tunneling I – V curves were acquired with set values (before disabling the feedback loop for spectrum acquisition) of V = 1.8 V and I = 0.5 nA (in attempt to be as close as possible to the CBM and observe also in-gap states, but at the same time avoid apparent gap enhancement due to voltage distribution between the two tunnel junctions involved in the double-barrier configuration. 13,28 Our d I /d V − V tunneling spectra, which are proportional to the local density of states, were numerically derived from curves resulting by averaging over 5 I – V characteristics taken on each NP, in each of which the current was recorded, and averaged over, 64 times for every bias value.…”

Solution Processing of Hydrogen-Terminated Silicon Nanocrystal for Flexible Electronic Device

“…This mechanism has been discussed in amorphous semiconductor 49 , organic semiconductor 50 , and nanoparticle films [51][52][53] . Our Si-NC thin film also has a dispersive charge transport because energy levels of NCs have a distribution due to size dispersion as discussed later 54,55 .…”

Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets

“…These results are compared with optical absorption and photoluminescence (PL) measurements as well as with theoretical modeling to study the NPL’s thickness-dependent electronic level structure. The STS and optical measurements provide complementary information, since the former measures the fundamental (quasiparticle, QP) energy gap of the particles rather than the excitonic band gap monitored by the latter. − In addition, STS can directly reveal the density of states (DOS) of supra-bandgap levels, information that can, in-turn, provide insight into optical results. , Indeed, such combination revealed the artificial atom character of colloidal QDs, ,, and the one-dimensional (1D) character of colloidal semiconductor nanorods . Notably, the level structure extracted from these measurements can be interpreted via theoretical modeling utilizing atomistic empirical pseudopotential calculations of the NPLs’ electronic DOS along with band-structure calculations employing multiband effective mass.…”