Retardation in the Oxidation Rate of Nanocrystalline Silicon Quantum Dots

  • PDF / 412,625 Bytes
  • 6 Pages / 612 x 792 pts (letter) Page_size
  • 113 Downloads / 207 Views

DOWNLOAD

REPORT


RETARDATION IN THE OXIDATION RATE OF NANOCRYSTALLINE SILICON QUANTUM DOTS

J. OMACHI, R. NAKAMURA, K. NISHIGUCHI and S. ODA Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan   ABSTRACT    Using very-high-frequency (VHF) plasma decomposition of SiH4 and pulsed gas technique, we have successfully prepared nanocrystalline silicon (nc-Si) quantum dots having average diameter of 8 nm and dispersion of 1 nm. The role of natural oxide is very important. It controls the size of nc-Si dots. Of particular interest is that the oxidation of these dots can be self limited, due to the stress induced near Si/oxide interface, which would allow further reduction of size and improvement in dispersion. This paper deals with the systematic study of oxidation process of nc-Si dots. Nc-Si dots formed in an Ar plasma with SiH4 gas pulses are deposited onto a Pt mesh The dots are then oxidized at 750, 800 and 850°C from 20 minutes to 15 hours. The dimensions of the residual nc-Si and the grown oxide are measured directly from the TEM micrographs and analyzed. For comparison, field oxide is investigated using ellipsometry. Retardation in the oxidation rate of nc-Si is observed. The mechanism of the reduction of oxidation rate in nc-Si is discussed taking into account the effect of stress. INTRODUCTION In future quantum effect devices, e.g., single electron transistors and single electron memories, small conducting islands will be major components. When these devices are to be operated at room temperature, nc-Si dots are required to have a diameter of less than 5 nm. Using very-high-frequency (VHF) plasma decomposition of SiH4 and pulsed gas technique, we have successfully prepared nanocrystalline silicon (nc-Si) quantum dots with an average diameter of 8 nm and dispersion of 1 nm[1],[2]. Recently, it is reported that the oxidation rate of lithographically patterned Si columns (Si nanocolumns) is retarded by the stress induced near Si/oxide interface[3],[4]. Moreover, in Si nanocolumns, the self-limiting effect of oxidation is observed. In principle these same effects should be observed for nc-Si dots, which would allow the control of nc-Si dot size with a small dispersion. EXPERIMENT Nc-Si dots are deposited onto a Pt mesh for simple TEM sample preparation. Before deposition, the Pt mesh is coated by a CVD derived SiO2 film to prevent nc-Si from reacting Pt mesh. After coating, Pt mesh is annealed in O2 atmosphere at 900°C to remove C, OH Ø and other impurities. A schematic diagram of nc-Si deposition system is shown in Figure.1. The electrodes of plasma cell are capacitively coupled. The stainless steel plate with an orifice of 6 mm diameter and 2 mm length, separating the UHV chamber from the plasma cell, is used as the grounded electrode. The deposition rate is monitored by a quartz crystal sensor during deposition. The F5.3.1

TITANIUM GETTERING PUMP

EXCHANGE CHAMBER

SUBSTRATE SHUTTER

TURBO MOLECULAR PUMP

UHV CHAMBER QUARTZ CRYSTAL THICKNESS MONIT