Silicon-germanium self-assembled quantum dot growth and applications in nanodevices
Abstract
Device feature sizes have been reduced over the years and are now
approaching 100nm at mass production levels. To achieve further scaling down
of MOSFETs, single-electron devices utilizing the Coulomb blockade effect will
likely be the basic element of future solid-state electronics. To develop these
devices into commercial products, room temperature operation capability and
nano-scale structures are necessary. Silicon or SiGe quantum dots embedded in
an insulator have potential application for room temperature operation of singleelectron
transistor and nonvolatile memory devices.
In first study, direct growth of Ge dots on insulators, such as nitrided
oxides, which are suitable for tunneling oxides (3nm) in memory devices were
achieved. Germanium dots were grown at different temperatures on various
dielectric substrates, including Si3N4, SiO2, oxynitride, NH3-annealed oxynitride,
NH3-annealed nitride, and N2O-annealed nitrided oxide. The characteristics of
the Ge dots were investigated using atomic force microscopy (AFM), Auger
electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) in
order to find the mechanism of the Ge dot formation. On N2O annealed nitrided
oxide films, we obtained Ge dots with heights and diameters of 3.2nm and 11nm,
respectively. No Ge dots were formed on surfaces of the other dielectric
substrates studied at 550ºC. Our experimental results suggest that the surface of
N2O annealed nitrided oxide contains a large amount of defects such as dangling
bonds, which act as Ge nucleation sites.
In second study, SiGe quantum dot growth between 500ºC and 525ºC
using a Si2H6/GeH4/H2 based chemistry was studied. The nucleation and growth
characteristics of the SiGe dots were quantified by measuring the nuclei density
and the concentration of Ge on the oxide using scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and AFM. For dielectrics, both
SiO2 and hafnium oxide (HfO2), high-K gate dielectric which uses thicker layer
for reduced leakage, improved resistance to boron diffusion, and better reliability
characteristics were used. The effect of GeH4 and Si2H6 pretreatment on the SiO2
surface was also investigated. It was found that Si atoms dominate the formation
of the critical nuclei and Ge atoms impinge on these Si atoms to grow the SiGe
dots. The number of Si atoms that terminate defect sites on SiO2 and the Si2H6
partial pressure determine the densities of SiGe dots. The growth of SiGe dots is
limited by the GeH4 partial pressure, which determines the activation energy of
disilane decomposition in the surface-reaction–limited regime and the number of
hydrogen desorption sites on the substrate.
In another study, the effects of charging voltage on charge retention
characteristics of SiGe dots with ZrO2 tunneling oxide were demonstrated. Using
this ZrO2 high-K dielectric tunneling oxide we achieved a lower write voltage
and an improved retention time compared to SiGe dots with a SiO2 tunneling
oxide. The discharge behavior in terms of a logarithmic charge decay of the ZrO2
device was similar to that of Si dots embedded in SiO2
Finally, a SiGe dot floating-gate flash memory with HfO2 tunneling oxide
was developed. Using SiGe dots and HfO2 tunneling oxide, which allows for a
thicker physical oxide thickness than an equivalent SiO2 tunneling oxide without
sacrificing non-volatility, a low program/erase voltage as well as good endurance
and charge retention characteristics can be achieved. This demonstrates that the
SiGe dots with HfO2 tunneling oxide can replace Si/Ge dots with SiO2 tunneling
oxide as a floating gate and have a high potential for further scaling of floating
gate memory devices.
Department
Description
text