M.Sc Thesis

M.Sc StudentZaknoon Bashir
SubjectInvestigation of Silicon Nanostructures by Scanning Probe
Microscopy and Electron Spectroscopy
DepartmentDepartment of Electrical and Computer Engineering
Supervisors PROFESSOR EMERITUS Gad Bahir
PROF. Alon Hoffman


Quantum dots (QDs) are coherent inclusions with nanometric dimensions of semiconductor material, in a different dielectric material with a wider energy band gap. In such structures, the free carriers exhibit three-dimensional confinement, which causes the formation of discrete energy levels. As a result, the electronic properties of the QDs are different from those found in bulk materials. QDs are often described as “artificial man-made atoms” due to their physical property resemblance.

In this work we present our correlated scanning tunneling spectroscopy (STS) results as well as structural investigations of the electronic structure and single-electron charging effect in single Si QDs. Silicon QDs were fabricated using conventional CMOS flow. After thermal growth of 1.5 nm tunnel oxide, silicon QDs were deposited using a low-pressure chemical vapor deposition (CVD) process. Following the deposition, the nanocrystals were passivated by thin SiOXNY shell.

Atomic force microscopy (AFM), scanning tunneling microscopy (STM), and high-resolution transmission electron microscopy (TEM) were used to determine the QDs' shape, size, surface density, and crystallinity. The composition information concerning the QDs and dielectric barriers was obtained from X-ray photo electron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). It was found that QDs have a spherical shape with a crystalline core having a diameter ranging from 2.5 to 7 nm and 8×1011 cm-2 QD surface density.

The STS measurements were performed in the double-barrier tunnel junction (DBTJ) configuration where the QD is placed between two macroscopic electrodes. The proper design of the double-barrier junction parameters enabled us, for the first time, to measure the electronic level structure and the single-electron charging effect of a single Si QD at room temperature (RT). The dependence of the Si QD band gap and the single-electron charging energy (Coulomb Blockade) on the QDs' diameter is presented and compared to model calculation. The measurements show that the degeneracy of the first energy level is 12 for QDs larger than 4 nm, as expected from theoretical considerations.  

These results are very important for a physical understanding of single-electron-based devices such as single electron transistors or single dot non-volatile memory (NVM) devices.