Integration of Cobalt Ferromagnetic Control Gates for Electrical and Magnetic Manipulation of Semiconductor Quantum Dots

Abstract

The rise of electron spin qubit architectures for quantum computing processors has led to a strong interest in designing and integrating ferromagnets to induce stray magnetic fields for electron dipole spin resonance (EDSR). The integration of nanomagnets imposes however strict layout and processing constraints, challenging the arrangement of different gating layers and the control of neighboring qubit frequencies. This work reports a successful integration of nano-sized cobalt control gates into a multi-gate FD-SOI nanowire with nanometer-scale dot-to-magnet pitch, simultaneously exploiting electrical and ferromagnetic properties of the gate stack at nanoscale. The electrical characterization of the multi-gate nanowire exhibits full field effect functionality of all ferromagnetic gates from room temperature to 10 mK, proving quantum dot formation when ferromagnets are operated as barrier gates. The front-end-of-line (FEOL) compatible integration of cobalt is examined by energy dispersive X-ray spectroscopy and high/low frequency capacitance characterization, confirming the quality of interfaces and control over material diffusion. Insights into the magnetic properties of thin films and patterned control-gates are provided by vibrating sample magnetometry and electron holography measurements. Micromagnetic simulations anticipate that this structure fulfills the requirements for EDSR driving for magnetic fields higher than 1 T, where a homogeneous magnetization along the hard magnetic axis of the Co gates is expected. The FDSOI architecture showcased in this study provides a scalable alternative to micromagnets deposited in the back-end-of-line (BEOL) and middle-of-line (MOL) processes, while bringing technological insights for the FEOL-compatible integration of Co nanostructures in spin qubit devices.

Figures (7)

• Figure 1: a) Schematic of an FDSOI nanowire architecture featuring ferromagnetic gates for electrical and magnetic control of quantum dots with integrated MOL cobalt interconnects, and micromagnetic simulation results for the magnetic field gradient generated along the nanowire cross-section by the Co gates (in black) for an external magnetic field of 1 T oriented along the $x$-direction (the estimated position of the quantum dots is highlighted in green). b) SEM image of a fabricated double-dot device featuring cobalt barrier gates (B$_1$, B$_2$, and B$_3$) and palladium plunger gates (P$_1$ and P$_2$), with EDX-SEM analysis of the materials.
• Figure 2: Material analysis. a) EDX-TEM analysis of Cr-Co and Ti-Pd gates on FDSOI with a cutline highlighting the surface oxidation of cobalt (sample annealed for 10 minutes in forming gas at 300 °C). b) XPS results for e-beam evaporated Cr/Co thin films as-deposited and annealed at 300 °C in forming gas. $\theta$ is relative to the surface normal. c) VSM measurements of the same films annealed at different temperatures. d) Bright-field TEM micrographs and SAED patterns for the as-deposited and annealed Co thin films, with azimuthal integration of the Bragg peak intensities vs scattering vector to highlight the crystal phase and overlap with GIXRD measurement. The scale for the bright-field images is $200$ nm.
• Figure 3: Characterization of Co-FETs. a) High and low frequency C-V measurements of a Cr-Co/SiO$_2$ MOS-capacitor, with a zoom-in to highlight the minimal hysteresis induced by the gate stack. b) Extracted density of interface traps within the silicon band-gap for a Cr-Co/ALD-Al$_2$O$_3$ MOS-capacitor (the same gate stack of the fabricated multi-gate FDSOI nanowire). The inset showcases a comparison of $D_{it}$ between different gate/oxide layers around the Si flat-band potential. c) Transconductance value extracted for 60 Co-FETs measured before and after thermal treatment, the shaded colors indicate the standard deviation around the mean value.
• Figure 4: Electrical characterization of the fabricated FDSOI nanowire. Room temperature measurements (the gates activated in the voltage sweeps are colored in the TEM images): transcharacteristic curves for a) all gates with mean value and standard deviation of the leakage current from gates to channel (I$_{\mathrm{Gates-Drain}}$), b) Ti-Pd plunger gates at different barrier gates voltage, c) Cr-Co barrier gates at different plunger gates voltage, and d) bias curves for all gates above threshold. Cryogenic characterization of gates defining a quantum dot: transcharacteristic curves for e) all gates, f) one plunger gate, g-h) barrier gates at different plunger gate voltages.
• Figure 5: Quantum confinement induced by Co barrier gates at 10 mK. a) Coulomb blockade peaks for a quantum dot electrostatically induced by two ferromagnetic Co barrier gates. b) Coulomb blockade diamonds for the same dot at lower barriers voltage.