Germanium quantum dot is a promising candidate in view of applications in photonic components, and thermoelectronics, etc. Precisely, to apply germanium quantum dots for optoelectronics, the quantum dots have to be smaller as compared to the effective Bohr radius in a bid to confine the electrons as well as holes into the dots independently and to achieve a density in the order of 1011/cm2. This is for efficient light transmission as well as receiving.
A number of methods, through self-assembly and lithographical processes, have been incepted to manufacture quantum dots. The lithographical processes allows for the control of dot size and the their arrangement into desired locations. Unfortunately, to make the dots in the quantum size, fine process technologies through extreme UV and electron beam lithographical processes are needed. On the other hand, reducing the size of the dot and putting germanium quantum dots on the desired place remain demanding for self-assembly processes.
Carbon-mediated growth on silicon substrate have been researched extensively in the fabrication of germanium quantum dots. Germanium nucleates on the silicon surface therefore avoiding the regions where carbon-silicon bonds are formed. For this reason, the carbon-mediated surface reconstruction presents a useful approach for germanium quantum dots fabrication.
Researchers led by Professor Katsuyoshi Washio from Tohoku University in Japan, in a bid to scale-down dot size to quantum size and consequently increase dot density to the order of 1011/cm2, they investigated the effects of germanium growth conditions including temperature of germanium and its growth rate. As a consequence, nucleation probability of germanium adatoms was important in the carbon-mediated germanium dot formation. Their work is published in peer-reviewed journal, Thin Solid Films.
The authors prepared the samples by a solid-source molecular beam epitaxy system equipped with an electron gun for carbon sublimation as well as a Knudsen cell for germanium evaporation. Carbon coverage of 0.25 monolayer was then deposited at a substrate temperature of 200 °C, which was followed by a high temperature treatment in order to react carbon with silicon surface. The authors then deposited germanium equivalent to 3nm thick. They later investigated the diameter of the resulting germanium quantum dots and surface morphology through atomic force microscopy.
The dots were scaled down and the dot density was raised as germanium growth rate up to approximately 2nm/min. However, both the dot size and density were observed to be saturated beyond that. The authors found that the effect of germanium growth temperature was quite strong at slow growth rates; however, it was insensitive at higher growth rates.
The researchers found that the minimum mean dot diameter as well as maximum dot density were approximately 25nm and 1.1×1011/cm2, respectively, at a growth temperature of 450 °C. These outcomes indicate that germanium growth condition of fast growth rate or even low growth temperature functions efficiently to improve nucleation of germanium adatoms and subsequently enable the formation of germanium quantum dots by maximizing the impact of carbon-mediated surface reconstruction.
Yuhki Satoh, Yuhki Itoh, Tomoyuki Kawashima, and Katsuyoshi Washio. Effects of Ge growth rate and temperature on C-mediated Ge dot formation on Si (100) substrate. Thin Solid Films, volume 621 (2017), pages 42–46.Go To Thin Solid Films