Semiconductor nanocrystals offer great promise for developing novel devices because quantum confinement in nanocrystals leads to a variety of interesting optical and electronic properties, including tunable discrete electronic energy levels, enhanced light emission efficiency, exciton multiplication, single-photon emission, and single-electron/hole charging. In particular, silicon nanocrystals have been of great interest because silicon is the most important material for current integrated circuit industry. Unlike bulk silicon, silicon nanocrystals, which have dimensions of just a few nanometers, have tunable electronic properties by varying the size, and give rise to novel properties that will be useful for future applications, such as silicon-based light sources, solar cells, nonvolatile memories, and single-electron devices, However, in order to realize such applications, the fundamental properties of the silicon nanocrystal quantum dot needs to be understood clearly.
In this thesis, the electronic properties such as long-term charge storage, Coulomb blockade effect, size-dependent carrier injection dynamics, and absolute values of electronic energy levels are studied on the silicon nanocrystal quantum dots embedded in silicon nitride films grown by plasma-enhanced chemical vapor deposition.
The effect of in-situ hydrogen passivation on the charge storage characteristics was investigated for applications to nonvolatile silicon nanocrystal memories. To study the effect of in-situ hydrogen passivation, two types of silicon quantum dots in silicon nitride films were grown by SiH₄+ N₂and SiH₄+ NH₃plasma and metal-insulator-semiconductor(MIS) devices containing the silicon quantum dots were fabricated. The MIS devices with a silicon nitride grown by SiH₄and NH₃plasma exhibited a very low charge loss rate due to the reduction in charge loss paths through the hydrogen passivation of defects in the silicon nitride matrix and the interface between the silicon quantum dot layer and the substrate, compared to those with a silicon nitride grown using SiH₄and N₂plasma.
A room-temperature Coulomb blockade effect was observed in silicon quantum dots(Si QDs) spontaneously grown in a silicon nitride film. The metal-insulator-metal device containing the Si QDs showed a clear Coulomb staircase and differential conductance peaks at room temperature. The size distribution of the Si QDs determined by high-resolution transmission electron microscopy suggests that the measured single electron addition energy of 67 meV can be attributed to the charging energy of 63 meV of the Si QDs with the largest diameter of 4.7 nm among the various-sized Si QDs.
We report the strong size dependence of carrier injection dynamics in quantum-confined silicon nanocrystals that is investigated by using tunneling capacitor devices containing size-controlled silicon nanocrystals. As the diameter of silicon nanocrystals increases, the threshold voltage for carrier injection decreases and the number of injected carriers increases, resulting from the quantum size effect. The tunneling time for the carrier injection is shortened by about two orders of magnitude with increasing the diameter of silicon nanocrystals from 3.43 ㎚ to 4.97 ㎚, and this can be attributed to the enhanced resonant tunneling in larger silicon nanocrystals.
Finally, we report that the electronic structure of silicon nanocrystals embedded in an insulating matrix can be identified by using a capacitance spectroscopy. The absolute position of the lowest conduction and the highest valence levels of silicon nanocrystals is revealed and the significantly enhanced quantum confinement over the theoretical prediction is observed in the conduction levels of the tensile-strained silicon nanocrystals in silicon nitride films. The size-dependent band-gap energies estimated by capacitance spectroscopy agree well with those measured by photoluminescence spectroscopy.