Single-electron transistor in strained Si/SiGe heterostructures

Abstract

A split gate technique is used to form a lateral quantum dot in a two-dimensional electron gas of a modulation-doped silicon/silicon-germanium heterostructure. e-beam lithography was employed to produce split gates. By applying negative voltages to these gates the underlying electron gas is depleted and a lateral quantum dot is formed, the size of which can be adjusted by the gate voltage. We observe single-electron operation with Coulomb blockade behavior below 1K. Gate leakage currents are well controlled, indicating that the recently encountered problems with Schottky gates for this type of application are not an inherent limitation of modulation-doped Si/SiGe heterostructures, as had been speculated. With the introduction of the Si/SiGe heterobipolar transistor into large scale production, Si-based heterostructures have become an important material system for electronic high-performance device with full compatibility with standard Si technologies 1 . Meanwhile, the first digital CMOS circuits with selectively grown SiGe epilayers are commercially available, 2 and intense research and development is dedicated to the fabrication of SiGe pseudosubstrates and SiGe-on-insulator substrates for further device applications. In terms of basic research, great effort is directed toward spintronic and quantum computing as potential techniques for future computation and encrypting facilities 3,4 . Si-based heterostructures have distinct advantages in these fields because of the extremely long spin coherence times 5,6 , which are attributed to the small spin-orbit interaction, and the low natural abundance of isotopes with nuclear spin. Moreover, in this material system the interaction between the spins of the conduction electrons and the nuclei can be further reduced or even completely suppressed by employing enriched 28 Si, and Ge with a depleted 73 Ge isotope. This way, matrix materials free of nuclear spins are conceivable in addition to the unrivalled ultra-large scale integration capabilities of Si technologies.On a device level, single electron transistors (SET) with carrier confinement in all three directions of space are considered a key-component for spin manipulation and programmable entanglement 7 . While SET development for laboratory applications have reached a mature state in heterosystems based on III-V compound semiconductors, only few Si/SiGe SETs have been reported 8,9,10 . Moreover, none of these was achieved by the usual split-gate technique that is considered a precondition for efficient coupling of quantum dots and for high integration. It was pointed out in some of these papers that a lateral quantum dot cannot be achieved by Schottky split gates due to detrimental leakage currents through threading dislocations in the crystal and Fermi level pinning. Here we demonstrate that excessive leakage currents are not an intrinsic limitation of modulation-doped Si/SiGe heterostructures: Our split-gate SETs show well-behaved Coulomb blockade and very low leakage currents of the Pd Schottky gates.A high-mobi...

Aluminum oxide for an effective gate in Si/SiGe two-dimensional electron gas systems

“…For the Pd gate the onset of the noticeable leakage current, i.e. I G > 0.1 nA, is not visible down to V G = −5.8 V. This result is consistent with a previous report for Pd [16]. The leakage current I G through the mesa is smallest for the Al 2 O 3 /Al gate (see figure 1(c)).…”

“…Furthermore, a long electron spin lifetime [9] has been confirmed for Si/SiGe QDs. However, gate leakage in Si/SiGe QDs can still constitute a bottleneck especially when multiple qubits are considered where many gate electrodes are required [13][14][15][16][17][18].…”

Pauli spin blockade and lifetime-enhanced transport in a Si/SiGe double quantum dot

“…[37][38][39] The quantum well was nominally 12 nm thick, and it contained a two-dimensional electron gas of density n = 4 × 10 11 cm −2 and mobility 40, 000 cm 2 /Vs. The gate design for this device, reproduced from Ref.…”

“…The data we discuss here were acquired from a double quantum dot formed in a top-gated Si/Si 0.7 Ge 0.3 heterostructure. [37][38][39] The quantum well was nominally 12 nm thick, and it contained a two-dimensional electron gas of density n = 4 × 10 11 cm −2 and mobility 40, 000 cm 2 /Vs. The gate design for this device, reproduced from Ref.…”