High frequency physics and broadband instrumentation in CMOS silicon quantum devices

The increasing interest in Si-based nanostructures for quantum information purposes is motivated by the advantages offered by the physical properties of the material and by the maturity of the industrial CMOS technology which ensures a real chance of scalability. In such wide framework this thesis has investigated coherence-related effects of single electrons confined in silicon impurity or quantum dot in presence of oscillating electric fields. High frequency excitations in the MHz – GHz range on the one hand are demonstrated to be detrimental for coherent electron transfers; on the other hand they represent a tool for non-invasive and scalable charge detection through reflectometry. The research activity has also concerned the development and assembly of a new setup for broadband manipulation and current sensing of nanoscaled MOSFETs at cryogenic temperatures. Quantum transport measurements at 4 K in a single-gate FET evidence a hitherto unobserved selection rule on valley quantum numbers of the electrons. Here the 6-fold valley degeneracy typical of bulk Si is lifted by the confinement and electric field: the source-drain conduction is mediated by the energy levels of a single P atom that selects the valley state of the electron under tunneling. Analogously to Coulomb blockade for charges and Pauli blockade for spins, this valley blockade determines the transport suppression by the orthogonality of valley-orbital degrees in the reservoirs and at the impurity site. The conservation of the electron valley index is further confirmed by the observation of spin-valley Kondo transitions at the neutral charge state of the atom. The quantum transport is then driven out of equilibrium by an external field at several GHz frequencies and powers. The spin coherent fluctuations sustaining the Kondo effect are quenched by strong ac fields because of the spin-flips induced by electron-photon couplings. By contrast, the electron valley parity is not altered and the valley blockade phenomenology is fully preserved at several powers. Interestingly, very small excitations of ~ 100 MHz are exploitable to measure physical mechanisms of transport at the nanoscale through phase sensitive detection by radio frequency reflectometry. By means of a new dual-port reflectometric apparatus several aspects of the ultra-low temperature transport of a nanodevice are investigated. The multiplexing scheme exploiting the double-gate geometry of the sample allows clearer and more complete measurement of the charge stability diagrams than standardly used one-port setups. The dispersive detection of spin-dependent transitions makes gate-based reflectometry a promising yet barely explored technique combining high sensitivity and large bandwidth. Transport data are critically compared with reflectometry. Finally, the development and characterization at 4 K of a cryogenic modular setup for electrical measurements in multigate devices is reported. Degradation of ns voltage signals for electrical manipulation is minimized and custom cryogenic electronics allows low-noise current sensing. The versatile approach adopted for such platform can be replicated in more complicate systems like cryostats.