Silicon-germanium self-assembled quantum dot growth and applications in nanodevices
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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.