Fabrication of silicon quantum dots by pulsed-gas plasma processes and their properties

Shunri Oda
Research Center for Quantum Effect Electronics
Tokyo Institute of Technology
O-Okayama, Meguro-ku, Tokyo 152, Japan
 

We have proposed a novel method for the fabrication of nanocrystalline silicon (nc-Si) quantum dot structures with size less than 10nm and a very small spread of size (1nm).[1-4] Nanocrystalline silicon is formed by very-high-frequency plasma decomposition of silane [5] and coalescence of radicals. The basic idea for the formation of uniform structure is separation of the nucleation and the crystal growth processes. We have introduced hydrogen gas pulse into a silane plasma, that enhances nucleation of nc-Si particles. The nuclei grow in size in the silane plasma during the off state of hydrogen gas supply. The next hydrogen gas pulse forces nc-Si particles grown in the previous cycle out of the plasma cell into the deposition chamber and the next nucleation of nc-Si is enhanced simultaneously. Nanocrystalline silicon particles can be deposited onto any kind of substrate at room temperature. Typically, as substrates we used carbon microgrids for TEM measurements, quartz for photoluminescence measurements [6] and thermal oxidized silicon with patterned polycrystalline silicon electrodes for electrical measurements. Nanocrystalline silicon particles are not stick firmly onto the substrates but adsorbed by the Van der Waals force. Hence the position of nc-Si particles can be manipulated by AFM tip while observing the image. We have also found that nc-Si particles are deposited preferentially at the steps of substrate surfaces. Natural oxide which covers the surface of nc-Si plays very important role in the characteristics of nc-Si. First, the oxide serves as a potential barrier which controls charge and energy quantization, and tunneling current. Second, the oxide passivates the surface dangling bonds resulting in reduction in electron traps and enhancement of luminescence efficiency. Third, the oxide serves as glue for nc-Si to fix to the substrates. Electrical properties of nc-Si were measured using nanostructured electrodes with a very small gap of 25nm prepared by EB lithography and ECR reactive ion etching. [7] Current-voltage characteristics from a single dot and an array of multiple dots were measured at various temperature between 77-300K. Some of the structures in I-V curves can be interpreted by a model of single electron tunneling.

1. S. Oda and M. Otobe, Materials Research Society Symp. Proc. 358, 721 (1995).
2. M. Otobe and S. Oda, Materials Research Society Symp. Proc. 377, 51 (1995).
3. A. Itoh, T. Ifuku, M. Otobe and S. Oda, Materials Research Society Symp. Proc. 452, in press (1997).
4. T. Ifuku, M. Otobe, A. Itoh and S. Oda, Jpn. J. Appl. Phys. 36, in press (1997).
5. S. Oda, Plasma Sources Sci. Technol., 2, 26 (1993).
6. Y. Kanemitsu, S. Okamoto, M. Otobe and S. Oda, Phys. Rev. B55, R7375 (1997).
7. A. Dutta, M. Kimura, Y. Honda, M. Otobe, A. Itoh and S. Oda, Jpn. J. Appl. Phys. 36, in press (1997).