Basic Research on Silicon Quantum Computation Based on Isotope Control and Neutron Irradiation

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In Si-based quantum computer, ^<31>P should be distributed in an array at a distance of about 20 nm in spin-free silicon (spin zero ^<28>Si or ^<30>Si) as a qubit. In the present work, in order to fabricate its basic device structure, we aim to perform the experiments on the epitaxial growth of isotopically controlled silicon layers and the transformation of ^<30>Si to ^<31>P by neutron irradiation. This method for distributing ^<31>P in Si is superior to the ion-implantation technique, which is inherently random to the depth. For this purpose, first, epitaxial growth technique of almost 100% ^<30>Si was developed by the gas source MBE method using enriched ^<30>SiH_4, which was purchased from Kurchatov laboratory, Russia. Thus this spin-free Si epitaxial layer can be available to the base for Si-based quantum computer. Using this technique, we succeeded to fabricate the natural Si/^<30>Si/natural Si (^<28>Si:^<29>Si:^<30>Si=92.2:4.7:3.1) isotope double hetero-structures by supplying alternately normal SiH_4 and enriched ^<30>SiH_4. Then, neutron irradiation was carried out to transform ^<30>Si to ^<31>P at Japan Atomic Energy Research Institute with thermal neutron flux of 1×10^<14> cm^<-2> s^<-1> for 16 h. By the precise secondary ion mass spectroscopy (CAMECA-SIMS) measurement, the formation of ^<31>P from ^<30>Si was confirmed. Its concentration was to be about 5×10^<16> cm^<-3>w, which was almost equal to the theoretically predicted value. If the neutron irradiation is performed to the ^<29>Si/^<30>Si/^<29>Si double hetero-structures, nuclear spin ^<31>P will be distributed in a spin-free silicon (^<31>Si ), which will be used as a basic structure of Si-based quantum computer.