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Quantum engineering at the silicon surface using dangling bonds.

Schofield SR, Studer P, Hirjibehedin CF, Curson NJ, Aeppli G, Bowler DR - Nat Commun (2013)

Bottom Line: Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface.We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image.Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

View Article: PubMed Central - PubMed

Affiliation: London Centre for Nanotechnology, University College London, London WC1H 0AH, UK. s.schofield@ucl.ac.uk

ABSTRACT
Individual atoms and ions are now routinely manipulated using scanning tunnelling microscopes or electromagnetic traps for the creation and control of artificial quantum states. For applications such as quantum information processing, the ability to introduce multiple atomic-scale defects deterministically in a semiconductor is highly desirable. Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface. We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image. Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

No MeSH data available.


Related in: MedlinePlus

Energy-band diagrams for STM imaging of DBs.(a,b) Filled- and empty-state conditions for a single DB. The DB is negatively charged under filled-state imaging conditions, but can be either negative or positive under empty-state conditions, depending on whether it is in equilibrium with the substrate or not (see text). (c,d) Band diagrams for empty-state imaging of the double DB; its three bound states are indicated as calculated in Fig. 3j. In panel c, two alignments of the tip Fermi level with respect to the surface bands are shown (solid and dashed line), corresponding to two different bias conditions (note: the expected small increase in the band bending in the substrate is not shown in this diagram for clarity). Changing the tunnelling current magnitude and, hence, the tip-sample separation also changes the alignment of the tip and substrate bands; this is illustrated in panel d where two different tip-sample separations are shown by solid and dashed lines. The equivalence of decreasing the tunnelling current and increasing the sample bias is further highlighted in the STM data shown in Supplementary Fig. S3 and discussed in Supplementary Note S3.
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f2: Energy-band diagrams for STM imaging of DBs.(a,b) Filled- and empty-state conditions for a single DB. The DB is negatively charged under filled-state imaging conditions, but can be either negative or positive under empty-state conditions, depending on whether it is in equilibrium with the substrate or not (see text). (c,d) Band diagrams for empty-state imaging of the double DB; its three bound states are indicated as calculated in Fig. 3j. In panel c, two alignments of the tip Fermi level with respect to the surface bands are shown (solid and dashed line), corresponding to two different bias conditions (note: the expected small increase in the band bending in the substrate is not shown in this diagram for clarity). Changing the tunnelling current magnitude and, hence, the tip-sample separation also changes the alignment of the tip and substrate bands; this is illustrated in panel d where two different tip-sample separations are shown by solid and dashed lines. The equivalence of decreasing the tunnelling current and increasing the sample bias is further highlighted in the STM data shown in Supplementary Fig. S3 and discussed in Supplementary Note S3.

Mentions: As expected, we observe negatively charged DBs in filled-state images2223, evident from an extended enhancement that decays with radial distance from the DB (Fig. 1c). However, the charge state of the DB is strongly influenced by the STM tip. It is known that the electric field emanating from the STM tip induces large shifts in the surface energy bands when imaging H-terminated Si(001), such that the Fermi level (EF) of the substrate lies near the conduction/valence band edge under typical filled-/empty-state imaging conditions2425 as shown in Fig. 2a. However, it is much less well recognized that the consequence for is that empty-state bias the DB ground state will lie above the bulk Fermi level (Fig. 2b), and that under equilibrium conditions this will result in a positively charged DB. We confirm this experimentally by observing the characteristic signature of DBs becoming positively charged under empty-state tunnelling conditions—a sharp-edged disc of increased height surrounding the DB, which has an average diameter of 12 nm in Fig. 1b. The edge of this disc indicates the tip-DB radial separation distance at which the electric field of the STM tip causes the DB to become positively ionized by raising its energy above the bulk EF (Fig. 2b), analogous to recent observations of tip-induced ionization of Si donor atoms in GaAs26. However, DBs on n-type Si(001):H do not remain positively charged as the tip moves closer to them; instead, they again become negatively charged when the tip comes within a certain minimum distance from the DB due to non-equilibrium charging. This is evident by a depression region that surrounds each DB (~5 nm across in Fig. 1b). The STM tip is similar to the gate in a field-effect transistor; a negative tip bias results in an electron depletion region at the surface that decouples the DB from the n-type bulk (Fig. 2b). The DB is then analogous to the island of a single electron transistor, with the tip and substrate forming the source and drain, respectively. Exactly as for a single electron transistor, the electron occupation of the DB is then governed by the electron transfer rates on and off the DB, rather than by equilibrium with the bulk substrate17272829. Further discussion of this phenomenon, together with data illustrating the bias dependence of the depression and enhancement regions are given in Supplementary Note S1 and Supplementary Fig. S1, respectively. Thus, the DB is negatively charged in both bias polarities when directly imaged with the STM under the measurement conditions shown in this article, and the band alignment and tunnelling processes during DB imaging are well described by Fig. 2a.


Quantum engineering at the silicon surface using dangling bonds.

Schofield SR, Studer P, Hirjibehedin CF, Curson NJ, Aeppli G, Bowler DR - Nat Commun (2013)

Energy-band diagrams for STM imaging of DBs.(a,b) Filled- and empty-state conditions for a single DB. The DB is negatively charged under filled-state imaging conditions, but can be either negative or positive under empty-state conditions, depending on whether it is in equilibrium with the substrate or not (see text). (c,d) Band diagrams for empty-state imaging of the double DB; its three bound states are indicated as calculated in Fig. 3j. In panel c, two alignments of the tip Fermi level with respect to the surface bands are shown (solid and dashed line), corresponding to two different bias conditions (note: the expected small increase in the band bending in the substrate is not shown in this diagram for clarity). Changing the tunnelling current magnitude and, hence, the tip-sample separation also changes the alignment of the tip and substrate bands; this is illustrated in panel d where two different tip-sample separations are shown by solid and dashed lines. The equivalence of decreasing the tunnelling current and increasing the sample bias is further highlighted in the STM data shown in Supplementary Fig. S3 and discussed in Supplementary Note S3.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC3644071&req=5

f2: Energy-band diagrams for STM imaging of DBs.(a,b) Filled- and empty-state conditions for a single DB. The DB is negatively charged under filled-state imaging conditions, but can be either negative or positive under empty-state conditions, depending on whether it is in equilibrium with the substrate or not (see text). (c,d) Band diagrams for empty-state imaging of the double DB; its three bound states are indicated as calculated in Fig. 3j. In panel c, two alignments of the tip Fermi level with respect to the surface bands are shown (solid and dashed line), corresponding to two different bias conditions (note: the expected small increase in the band bending in the substrate is not shown in this diagram for clarity). Changing the tunnelling current magnitude and, hence, the tip-sample separation also changes the alignment of the tip and substrate bands; this is illustrated in panel d where two different tip-sample separations are shown by solid and dashed lines. The equivalence of decreasing the tunnelling current and increasing the sample bias is further highlighted in the STM data shown in Supplementary Fig. S3 and discussed in Supplementary Note S3.
Mentions: As expected, we observe negatively charged DBs in filled-state images2223, evident from an extended enhancement that decays with radial distance from the DB (Fig. 1c). However, the charge state of the DB is strongly influenced by the STM tip. It is known that the electric field emanating from the STM tip induces large shifts in the surface energy bands when imaging H-terminated Si(001), such that the Fermi level (EF) of the substrate lies near the conduction/valence band edge under typical filled-/empty-state imaging conditions2425 as shown in Fig. 2a. However, it is much less well recognized that the consequence for is that empty-state bias the DB ground state will lie above the bulk Fermi level (Fig. 2b), and that under equilibrium conditions this will result in a positively charged DB. We confirm this experimentally by observing the characteristic signature of DBs becoming positively charged under empty-state tunnelling conditions—a sharp-edged disc of increased height surrounding the DB, which has an average diameter of 12 nm in Fig. 1b. The edge of this disc indicates the tip-DB radial separation distance at which the electric field of the STM tip causes the DB to become positively ionized by raising its energy above the bulk EF (Fig. 2b), analogous to recent observations of tip-induced ionization of Si donor atoms in GaAs26. However, DBs on n-type Si(001):H do not remain positively charged as the tip moves closer to them; instead, they again become negatively charged when the tip comes within a certain minimum distance from the DB due to non-equilibrium charging. This is evident by a depression region that surrounds each DB (~5 nm across in Fig. 1b). The STM tip is similar to the gate in a field-effect transistor; a negative tip bias results in an electron depletion region at the surface that decouples the DB from the n-type bulk (Fig. 2b). The DB is then analogous to the island of a single electron transistor, with the tip and substrate forming the source and drain, respectively. Exactly as for a single electron transistor, the electron occupation of the DB is then governed by the electron transfer rates on and off the DB, rather than by equilibrium with the bulk substrate17272829. Further discussion of this phenomenon, together with data illustrating the bias dependence of the depression and enhancement regions are given in Supplementary Note S1 and Supplementary Fig. S1, respectively. Thus, the DB is negatively charged in both bias polarities when directly imaged with the STM under the measurement conditions shown in this article, and the band alignment and tunnelling processes during DB imaging are well described by Fig. 2a.

Bottom Line: Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface.We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image.Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

View Article: PubMed Central - PubMed

Affiliation: London Centre for Nanotechnology, University College London, London WC1H 0AH, UK. s.schofield@ucl.ac.uk

ABSTRACT
Individual atoms and ions are now routinely manipulated using scanning tunnelling microscopes or electromagnetic traps for the creation and control of artificial quantum states. For applications such as quantum information processing, the ability to introduce multiple atomic-scale defects deterministically in a semiconductor is highly desirable. Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface. We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image. Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

No MeSH data available.


Related in: MedlinePlus