Excited states and quantum confinement in room temperature few nanometre scale silicon single electron transistors

Abstract

Single nanometre scale quantum dots (QDs) have significant potential for many 'beyond CMOS' nanoelectronics and quantum computation applications. The fabrication and measurement of few nanometre silicon point-contact QD single-electron transistors are reported, which both operate at room temperature (RT) and are fabricated using standard processes. By combining thin silicon-on-insulator wafers, specific device geometry, and controlled oxidation, <10 nm nanoscale point-contact channels are defined. In this limit of the point-contact approach, ultra-small, few nanometre scale QDs are formed, enabling RT measurement of the full QD characteristics, including excited states to be made. A remarkably large QD electron addition energy ∼0.8 eV, and a quantum confinement energy ∼0.3 eV, are observed, implying a QD only ∼1.6 nm in size. In measurements of 19 RT devices, the extracted QD radius lies within a narrow band, from 0.8 to 2.35 nm, emphasising the single-nanometre scale of the QDs. These results demonstrate that with careful control, 'beyond CMOS' RT QD transistors can be produced using current 'conventional' semiconductor device fabrication techniques.

Room-Temperature Measurement of Electrostatically Coupled, Dopant-Atom Double Quantum Dots in Point-Contact Transistors

“…Atoms along or near the percolation path can then behave as QDs or interact in pairs to form DQD complexes, leading to current lines and peaks that define hexagonal regions in the charge-stability plot of I ds versus V gs1 and V gs2 . In our earlier work [13,19], single QD potential wells in similar devices were shown to be anharmonic, with diameters similar to that of the dopant separation. Variation in the dopant separation implies the following possibilities: (i) If dopant atoms in the SiO 2 barrier are very close, <2 nm, electronic states on these are delocalized, isolated QDs are not formed, and resistive conduction paths may form between source and drain regions, suppressing the observation of QD behavior.…”

“…Measurements of the electrical characteristics are confined to RT. It has been shown previously [13] that, in these highly doped point-contact devices, with decreasing temperature, singleelectron effects are masked by an increasingly significant potential barrier in series with the QD. This changes the shape and ultimately turns the device "off" and suppresses charge-stability regions in the measurement.…”

“…1(d)]. A geometric oxidation process [7,13,19] then both separates these contact regions by forming a SiO 2 tunnel junction at the point contact and leaves an array of isolated dopant atoms within the SiO 2 tunnel junction [ Fig. 1(e)]; this is shown in greater detail in Fig.…”

“…Ideally, an atomic-scale nanoelectronic circuit using multiple single-atom transistor elements would require control over both tunnel coupling and the device energy states. Recently, we measured RT "Coulomb diamond" single-electron charging characteristics, as a function of a single gate voltage, in a P dopant-atom QD transistor, where the P atom was embedded within a Si/SiO 2 /Si tunnel junction [13,19]. In this structure, it is possible to observe both resonant tunneling and Coulomb diamond single-electron charging effects through an atomic-scale P QD placed between large source and drain regions and switching of the tunnel current by an electrostatically coupled gate.…”

“…Single-electron device simulation is used to investigate evolution of the charge-stability region with varying capacitance and resistance. In combination with extracted tunnel capacitances and resistances, this allows experimental trends to be reproduced and provides information on the dopant-atom arrangement.At present, most single-atom transistors use donor [3,4,[9][10][11][12][13][14][15][16] and acceptor [1,2] substitutional impurity atoms within the Si channel of a field-effect transistor or single atoms deposited on atomically flat surfaces by scanning probe methods [4]. In these devices, electronic states associated with individual dopant-atom QDs [1,3,9,10] and with electrostatically coupled double QDs within a dopantatom array [14][15][16] are measured and interactions between dopant-atom arrays formed by single-ion implantation methods [11] are characterized.…”