TRANSPORT AND TUNNELING IN ATOMIC-SCALE, ULTRADOPED ACCEPTOR-BASED QUANTUM DEVICES IN SILICON

Abstract

Atomically-precise acceptor-based qubits are promising candidates for spin-based quantum computing due to their inherently strong spin-orbit interaction and the feasibility of fast electrical control using gates. Despite their theoretical promise, experimental studies of acceptor qubits have been limited due to challenges in fabrication and process development. Throughout my Ph.D. research, I addressed these challenges by developing comprehensive fabrication processes combined with quantum tunneling and transport measurements to investigate atomically-precise acceptor doping and devices, culminating in the realization of the first atomically-precise single-hole transistor (SHT), which is a critical component for single-acceptor-atom qubit readout.

Initial investigations focused on Al dopants in Si, where I conducted detailed investigations into AlCl3 adsorption on Si(100) surfaces and subsequent annealing effects, utilizing scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Magneto-transport measurements conducted on AlCl3 exposed Si samples after dopant incorporation and Si capping failed to produce a noticeable electrical activation of Al dopants, despite Al concentrations exceeding 10^19 cm^-3 in the δ-doped region, as verified by secondary ion mass spectrometry (SIMS). This result, combined with the observation that AlCl3 readily adsorbed and formed extended chlorinated aluminum chains on the surface post-annealing, points towards a dopant deactivation process. Ultimately, this limits the suitability of AlCl3 for practical atomically-precise doping applications.

Studies with BCl3 demonstrated barrierless dissociative adsorption onto Si surfaces. Experimental validation through STM, SIMS, and Hall measurements confirmed ultrahigh doping densities, exceeding 10^21 cm^-3, could be achieved with minimal thermal processing. Atomically-precise, area selective deposition was demonstrated utilizing STM-patternable, monatomic resists of either hydrogen or chlorine, enabling the fabrication of atomic-scale wires that exhibit ohmic conduction down to mK temperatures.

I studied the mechanism of hole tunneling in an atomically-precise SHT, in which the dimensions were defined with sub-nanometer precision utilizing STM-based lithography. Coulomb peaks and Coulomb diamonds were readily observed and indicative of single-hole resonance tunneling associated with 310 ± 20 boron atoms confined within a 13 ± 0.5 nm x 14 ± 0.5nm region in Si. Analysis using the Wentzel–Kramers–Brillouin (WKB) approximation and capacitance modeling via a three-dimensional Poisson solver highlighted that hole tunneling was dominated by carriers with the lowest effective mass, i.e. light holes. DFT calculations further supported these observations. These results not only provide fundamental insight into the tunneling mechanism of holes in Si, but they pave the way for future explorations and measurements of single acceptor atom qubits in Si.