Isotopically Engineered Si as a Promising Material for Spintronics and Semiconductor-Based Nuclear Spin Quantum Computers

Abstract

Silicon is the most exploitable material in the modern electronics and semiconductor industry. More than 90% of semiconductor devices on the market are made from silicon. Natural silicon consists of three stable isotopes with atomic mass 28 (92.21%), 29 (4.70%) and 30 (3.09%). At present, isotopic enrichment of Si is used in electronics for two goals: (i) fabrication of substrates with a high level of doping and homogeneous distribution of impurities and (ii) fabrication of substrates with enhanced heat conduction, which allows further chip miniaturization. For the first purpose, enrichment of Si with Si³⁰ is used, because after irradiation of a Si ingot by the thermal neutron flux in a nuclear reactor, this isotope transmutes into a phosphorus atom which is a donor impurity in Si. This neutron transmutation doping is based on the nuclear reaction: ₁₄Si³⁰ + n ⁰ → ₁₄Si³¹ → ₁₅P³¹ + e ⁻. Enrichment of Si with Si³⁰ allows one to significantly increase the level of doping (up to a factor of 30) with a high homogeneity of the impurity distribution. The second purpose is achieved in Si highly enriched with isotope Si²⁸ up to 99.85%, because mono-isotopic Si is characterized by enhanced thermal conductivity. A new potential of isotopically engineered Si comes to light because of novel areas of fundamental and applied scientific activity connected with spintronics and a semiconductor-based nuclear spin quantum computer where electron and/or nuclear spins are the object of manipulation. In this case, control of the abundance of nuclear spins is extremely important, because even if only the electron spin is considered as a carrier of information, interaction with the nuclear spin system determines the electron spin decoherence time. Fortunately, Si allows such a control, because only isotope Si²⁹ has a non-zero nuclear spin. Therefore, enrichment or depletion of Si with isotope Si²⁹ will lead to the creation of a material with a controlled concentration of nuclear spins, and even without nuclear spins. One might deliberately vary the isotopic composition to produce layers, wires and dots that could serve as nuclear spin qubits with a controlled number of nuclear spins. A method is suggested to prepare a Si-nanowire from Si enriched with Si³⁰ with array of P atoms, introduced by NTD technique, which could serve as nuclear-spin qubits in the Kane’ model of NSQC.