Dopant depletion in the near surface region of thermally prepared silicon (100) in UHV

Pitters, Jason; Piva, P. G.; Wolkow, Robert A.

doi:10.1116/1.3694010

Cited by 41 publications

(57 citation statements)

References 38 publications

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“…The broadening of the lowest energy peak is always essentially independent of tip height, and the transport line shape matches what is expected for broadening due to coupling to a thermally broadened Fermi sea 32 at liquid Helium temperature. We attribute this to weak tunnel coupling of the donor to a buried Fermi sea, as expected for flash annealed Si substrates 33 .…”

Section: S1 Experimental Proceduresmentioning

confidence: 86%

Spatial metrology of dopants in silicon with exact lattice site precision

Usman¹,

Bocquel²,

Salfi³

et al. 2016

Nature Nanotech

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The aggressive scaling of silicon-based nanoelectronics has reached the regime where device function is affected not only by the presence of individual dopants, but more critically their position in the structure. The quantitative determination of the positions of subsurface dopant atoms is an important issue in a range of applications from channel doping in ultra-scaled transistors to quantum information processing, and hence poses a significant challenge. Here, we establish a metrology combining low-temperature scanning tunnelling microscopy (STM) imaging and a comprehensive quantum treatment of the dopant-STM system to pin-point the exact lattice-site location of sub-surface dopants in silicon. The technique is underpinned by the observation that STM images of sub-surface dopants typically contain many atomic-sized features in ordered patterns, which are highly sensitive to the details of the STM tip orbital and the absolute lattice-site position of the dopant atom itself. We demonstrate the technique on two types of dopant samples in silicon -the first where phosphorus dopants are placed with high precision, and a second containing randomly placed arsenic dopants. Based on the quantitative agreement between STM measurements and multi-million-atom calculations, the precise lattice site of these dopants is determined, demonstrating that the metrology works to depths of about 36 lattice planes. The ability to uniquely determine the exact positions of subsurface dopants down to depths of 5 nm will provide critical knowledge in the design and optimisation of nanoscale devices for both classical and quantum computing applications.As we approach the ultimate regime of Feynman's vision 1 of nanotechnology based on atom-by-atom fabrication 2-5 , there is a critical need to match advances in miniaturisation with atomically precise metrology. In conventional CMOS 6 and tunnelling field effect 7,8 transistors the key relationship between doping profile and performance is now dominated by the positions of just a few dopant atoms, and currently cannot be quantitatively determined. Beyond conventional nanoelectronic devices, in quantum processors based on phosphorus dopants in silicon 9 the precise locations of the individual dopants is critical to the design and operation of spin-based quantum logic gates. Previous studies on locating subsurface dopant positions in semiconductors either provide donor depths based on statistical evidence 10 , or only qualitatively locate dopants within a few nanometers region (of the order of 2.5 nm or more)11 . The key metrological challenge in all of the ultra-scaled applications is the ability to determine the position of dopant atoms in the silicon crystal substrate with lattice-site precision, which will drastically transform our understanding at the most fundamental scale leading to devices with optimised functionalities.In this work, we present an atomically precise metrology and demonstrate the pinpointing of the position of subsurface phosphorous (P) and arsenic (As) dopants in si...

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Section: S1 Experimental Proceduresmentioning

confidence: 86%

Spatial metrology of dopants in silicon with exact lattice site precision

Usman¹,

Bocquel²,

Salfi³

et al. 2016

Nature Nanotech

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“…Hydrogen passivation was carried out by dosing with 9 monolayers of atomic hydrogen. This flash anneal procedure is known from secondary ion mass spectroscopy to deplete the upper ∼ 10 nm of the wafer of arsenic dopants 34 , but is shallow enough to maintain sufficient coupling to the conduction impurity band to obtain a measurable single-electron tunneling current through donor-bound states. Donors found by subsurface dopant imaging, as described in the main text, were residual impurities statistically distributed throughout the depletion region 34,38 .…”

Section: Methodsmentioning

confidence: 99%

“…• C, which are expected to have too deep (∼ 100 nm) a surface depletion and too few residual arsenic dopants 34 . Measurements were performed using an Omicron low temperature scanning tunneling microscope (LT-STM) operating in ultra-high vacuum at a temperature of 4.2 K. Current I was measured as a function of sample voltage U using ultra-low noise electronics, and dI/dU was obtained by numerical differentiation.…”

Section: Methodsmentioning

confidence: 99%

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Spatially resolving valley quantum interference of a donor in silicon

Salfi¹,

Mol²,

Rahman³

et al. 2014

Nature Mater

105

187

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Electron and nuclear spins of donor ensembles in isotopically pure silicon experience a vacuum-like environment, giving them extraordinary coherence. However, in contrast to a real vacuum, electrons in silicon occupy quantum superpositions of valleys in momentum space. Addressable single-qubit and two-qubit operations in silicon require that qubits are placed near interfaces, modifying the valley degrees of freedom associated with these quantum superpositions and strongly influencing qubit relaxation and exchange processes. Yet to date, spectroscopic measurements only indirectly probe wavefunctions, preventing direct experimental access to valley population, donor position, and environment. Here we directly probe the probability density of single quantum states of individual subsurface donors, in real space and reciprocal space, using scanning tunneling spectroscopy. We directly observe quantum mechanical valley interference patterns associated with linear superpositions of valleys in the donor ground state. The valley population is found to be within 5% of a bulk donor when 2.85 ± 0.45 nm from the interface, indicating that valley perturbation-induced enhancement of spin relaxation will be negligible for depths > 3 nm. The observed valley interference will render two-qubit exchange gates sensitive to atomic-scale variations in positions of subsurface donors. Moreover, these results will also be of interest to emerging schemes proposing to encode information directly in valley polarization. Fabrication of devices1,2 , spin readout 1 , and quantum control of spins 3,4 in silicon has been accomplished at the single-donor level. However, addressable control and coupling within qubit arrays requires local gates and control interfaces, whose atomic-scale potentials strongly influence electronic valley degrees of freedom [5][6][7][8][9][10][11][12][13][14][15][16] . While these unconventional orbital degrees of freedom play no role in conventional silicon microelectronics, they invariably arise in quantized states in indirect gap materials, and are pervasive in quantum electronics. In silicon, valley physics determines qubit relaxation rates 13,17,18 and are predicted to strongly influence two qubit gates Here, individual states of subsurface donors were measured using cryogenic scanning tunneling spectroscopy. Quantum mechanical valley interference patterns were observed in real space, associated with linear quantum superpositions of wavevectors in the six conduction band valleys of silicon. Enabled by high-accuracy empirical determination of donor depth and electric field, we perform a parameter-free comparison with atomistic theory establishing that the z-valley population is ∼ 38 ± 2% for a 2.85 ± 0.45 nm deep donor, perturbed by only ∼ 5% compared with ∼ 33.3% for a donor in bulk silicon. Consequently, donors more than 3 nm deep should not experience a significant valley-repopulation induced increase in spin-lattice relaxation. Moreover, the nearly bulklike valley interference observed will render two-qubit excha...

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“…19 and Ref. 20 -together. The first one reports ARPES experiments at higher temperatures on germanium and silicon samples.…”

Section: E Postulated Scenariomentioning

confidence: 99%

Fermi level pinning at the Ge(001) surface—A case for non-standard explanation

Wojtaszek

Zuzak

Godlewski

et al. 2015

Journal of Applied Physics

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To explore the origin of the Fermi level pinning in germanium we investigate the Ge(001) and Ge( 001):H surfaces. The absence of relevant surface states in the case of Ge(001):H should unpin the surface Fermi level. This is not observed. For samples with donors as majority dopants the surface Fermi level appears close to the top of the valence band regardless of the surface structure. Surprisingly, for the passivated surface it is located below the top of the valence band allowing scanning tunneling microscopy imaging within the band gap. We argue that the well known electronic mechanism behind band bending does not apply and a more complicated scenario involving ionic degrees of freedom is therefore necessary. Experimental techniques involve four point probe electric current measurements, scanning tunneling microscopy and spectroscopy.

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Dopant depletion in the near surface region of thermally prepared silicon (100) in UHV

Cited by 41 publications

References 38 publications

Spatial metrology of dopants in silicon with exact lattice site precision

Spatial metrology of dopants in silicon with exact lattice site precision

Spatially resolving valley quantum interference of a donor in silicon

Fermi level pinning at the Ge(001) surface—A case for non-standard explanation

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