Effects of interface disorder on valley splitting in SiGe/Si/SiGe quantum wells

Jiang, Zhengping; Kharche, Neerav; Boykin, Timothy B.; Klimeck, Gerhard

doi:10.1063/1.3692174

Cited by 24 publications

(12 citation statements)

References 34 publications

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“…We can see that the VS of Si can vary significantly for different atomic configurations of barriers at the same composition X b . This is consistent with the recent calculation showing that specific atomic arrangements at the interface region can result in distinct VS (however the assumed Si 3 Ge luzonite structure is difficult for experimental realization)30. Also, the critical role of atomic resolution and symmetry is apparent by considering a system of short-period Si–Ge superlattices located directly on a substrate (that is, no active Si layer in Fig.…”

Section: Resultssupporting

confidence: 90%

Genetic design of enhanced valley splitting towards a spin qubit in silicon

et al. 2013

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The long spin coherence time and microelectronics compatibility of Si makes it an attractive material for realizing solid-state qubits. Unfortunately, the orbital (valley) degeneracy of the conduction band of bulk Si makes it difficult to isolate individual two-level spin-1/2 states, limiting their development. This degeneracy is lifted within Si quantum wells clad between Ge-Si alloy barrier layers, but the magnitude of the valley splittings achieved so far is small—of the order of 1 meV or less—degrading the fidelity of information stored within such a qubit. Here we combine an atomistic pseudopotential theory with a genetic search algorithm to optimize the structure of layered-Ge/Si-clad Si quantum wells to improve this splitting. We identify an optimal sequence of multiple Ge/Si barrier layers that more effectively isolates the electron ground state of a Si quantum well and increases the valley splitting by an order of magnitude, to ∼9 meV.

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Section: Resultssupporting

confidence: 90%

Genetic design of enhanced valley splitting towards a spin qubit in silicon

et al. 2013

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“…The appearance and persistence of the center peak with B > 0 is a key prediction of the theory of the valley Kondo effect and arises physically from an interference of valley conserving and valley non-conserving processes. This suggests that valley index is not always conserved during tunneling, a subject of some debate, 12,13,15,[22][23][24][25][26][27] and in accord with recent effective mass calculations of the effects of atomic step disorder at the quantum well interface. 17 Secondly, given a non-zero ∆, the QD electrons must occupy an excited state rather than the ground state viewed in terms of single-particle levels.…”

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confidence: 70%

Signatures of the valley Kondo effect in Si/SiGe quantum dots

et al. 2014

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We report measurements consistent with the valley Kondo effect in Si/SiGe quantum dots, evidenced by peaks in the conductance versus source-drain voltage that show strong temperature dependence. The Kondo peaks show unusual behavior in a magnetic field that we interpret as arising from the valley degree of freedom. The interplay of valley and Zeeman splittings is suggested by the presence of side peaks, revealing a zero-field valley splitting between 0.28 to 0.34 meV. A zero-bias conductance peak for non-zero magnetic field, a phenomenon consistent with valley nonconservation in tunneling, is observed in two samples. 1The valley degree of freedom of conduction band electrons is one of several intriguing properties distinguishing silicon from III-V materials. The six-fold valley degeneracy in bulk Si is reduced to two-fold in Si/SiGe heterostructures due to the confinement of electrons in a two-dimensional electron gas (2DEG). The resulting valley splitting ∆ in strained Si quantum dots (QD) and quantum point contacts is typically of order 0.2 meV. 1,2 A particularly interesting manifestation of valley physics would be the valley Kondo effect. The spin 1/2 Kondo effect comprehensively studied in GaAs quantum dots 3-9 is usually observed when there is an odd number of electrons in the QD, in which the spin of an unpaired electron is screened by spins in the leads to form a singlet, resulting in a conductance resonance at zero dc bias. When a magnetic field B is applied, the QD spin states are split by the Zeeman energy E z = gµ B B, g being the Landé factor and µ B the Bohr magneton. The spin-Kondo resonance then splits into two peaks at eV SD = ±E z , where V SD is the applied source-drain voltage. 3,10 On the other hand, Kondo effects in Si/SiGe QDs have only rarely been reported, 11 and there have been recent studies of dopants in a Si fin-type field effect transistor. 12 This is perhaps not surprising, since for the valley Kondo effect to occur, the energy associated with the Kondo temperature T K must be larger than the valley splitting ∆, i.e. k B T K > ∆ where k B is the Boltzmann constant, a rather stringent condition. Nonetheless, how the valley degeneracy in Si affects the Kondo effect in Si/SiGe QDs has been investigated theoretically 13,14 and found to share some resemblance with carbon nanotubes. 15 The addition of the valley degree of freedom allows for a new set of phenomena to emerge, since both spin and valley indices can be screened. Measurement of the valley Kondo effect could therefore help probe the nature of valley physics in Si/SiGe QDs, a topic of great importance considering their potential application in quantum computation. 16 In particular, the question of how the valley index of an electron changes 17 as it tunnels on and off a QD can be illuminated.Here we report measurement of unconventional Kondo effects in two different Si/SiGe QD samples, one in perpendicular magnetic field B ⊥ and both in in-plane magnetic field B . In both samples we observe conductance enhancement in tw...

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“…The splitting at a normal (front) Si/SiO 2 interface is at least one order of magnitude smaller and is undetectable for a negative V BG value due to the inhomogeneous broadening of the conduction onset voltage. We should mention here that the physical origin of the large valley splitting remains unclear, and is still under debate although there are several proposals regarding the origin such as interface states at Si/SiO 2 39, roughness27 or disorder40.…”

Section: Resultsmentioning

confidence: 98%

Electric tuning of direct-indirect optical transitions in silicon

Noborisaka

Nishiguchi

Fujiwara

2014

Sci Rep

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Electronic band structures in semiconductors are uniquely determined by the constituent elements of the lattice. For example, bulk silicon has an indirect bandgap and it prohibits efficient light emission. Here we report the electrical tuning of the direct/indirect band optical transition in an ultrathin silicon-on-insulator (SOI) gated metal-oxide-semiconductor (MOS) light-emitting diode. A special Si/SiO2 interface formed by high-temperature annealing that shows stronger valley coupling enables us to observe phononless direct optical transition. Furthermore, by controlling the gate field, its strength can be electrically tuned to 16 times that of the indirect transition, which is nearly 800 times larger than the weak direct transition in bulk silicon. These results will therefore assist the development of both complementary MOS (CMOS)-compatible silicon photonics and the emerging “valleytronics” based on the control of the valley degree of freedom.

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Effects of interface disorder on valley splitting in SiGe/Si/SiGe quantum wells

Cited by 24 publications

References 34 publications

Genetic design of enhanced valley splitting towards a spin qubit in silicon

Genetic design of enhanced valley splitting towards a spin qubit in silicon

Signatures of the valley Kondo effect in Si/SiGe quantum dots

Electric tuning of direct-indirect optical transitions in silicon

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