all-electronic-nanosecond-resolved-scanning-tunneling-microscopy

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new ...

University of Alberta

National Research Council of Canada, Edmonton

Electrical transport through molecules has been much studied since it was proposed that individual molecules might behave like basic electronic devices, and intriguing single-molecule electronic effects have been demonstrated. But because transport properties are sensitive to structural variations on the atomic scale, further progress calls for detailed knowledge of how the functional properties of molecules depend on structural features. The characterization of two-terminal structures has become increasingly robust and reproducible, and for some systems detailed structural characterization of molecules on electrodes or insulators is available. Here we present scanning tunnelling microscopy observations and classical electrostatic and quantum mechanical modelling results that show that the electrostatic field emanating from a fixed point charge regulates the conductivity of nearby substrate-bound molecules. We find that the onset of molecular conduction is shifted by changing the charge state of a silicon surface atom, or by varying the spatial relationship between the molecule and that charged centre. Because the shifting results in conductivity changes of substantial magnitude, these effects are easily observed at room temperature.

Field regulation of single-molecule conductivity by a charged surface atom.

It is now a well-known fact that the phase of electron waves is altered by external magnetic fields via the Aharonov-Bohm effect. This implies that any electron interference effects will be to some degree affected by the presence of such fields. In this study we examine the distortion effects of external (constant and variable) magnetic fields on electron interference and holography. For digital holography, the reconstruction of the object is done via numerical calculations and this leaves the door open for correcting phase distortions in the hologram reconstruction. We design and quantitatively assess such correction schemes, which decidedly depend on our knowledge of the magnetic field values in the holographic recording process. For constant fields of known value we are able to correct for magnetic distortions to a great extent. We find that variable fields are more destructive to the holographic process than constant fields. We define two criteria, related respectively to global and local contrast of the hologram to establish the maximum allowed external field which does not significantly hinder the accuracy of in-line holographic microscopy with electrons.

In-line holographic electron microscopy in the presence of external magnetic fields.

Future nanoscale integrated circuits will require the realization of interconnections using molecular-scale nanostructures; a practical fabrication scheme would need to be largely self-assembling and operate on a large number of like structures in parallel. The self-directed growth of organic molecules on hydrogen-terminated silicon(100) [H-Si(100)] offers a simple method of realizing one-dimensional molecular lines. In this work, we introduce the ability to change the growth direction and form more complex, contiguous shapes. Numerous styrene and trimethylene sulfide L shapes were grown on a H-Si(100)-3x1 surface in parallel with no intermediate surface lithography steps, and similar shapes were also grown using allyl mercaptan and benzaldehyde on H-Si(100)-2x1. Registered scanning tunneling microscopy (STM) images and high-resolution electron energy loss spectroscopy (HREELS) were used to investigate the growth process.

Self-directed growth of contiguous perpendicular molecular lines on H-Si(100) surfaces.

It has previously been shown that multimolecular organic nanostructures form on H-Si(100)-2×1 via a radical mediated growth process. In this mechanism, growth begins through the addition of a molecule to a silicon surface dangling bond, followed by the abstraction of a neighboring H atom and generation of a new dangling bond on the neighboring site. Nanostructures formed by this mechanism grow along one edge of a dimer row. Here, we explored the possibility of using lithographically prepared, biased metal contacts on the silicon surface to generate an electric field that orients molecules during the growth process to achieve growth in the perpendicular-to-row direction. The formation of some nanostructures in a direction that was nearly perpendicular to the dimer rows was achieved, whereas such features were not formed in the absence of the field. Analysis of the scanning tunneling microscopy images suggests that the formation of these nanostructures may involve self-templating effects in addition to dangling bond diffusion rather than a straightforward addition∕abstraction mechanism. These initial results offer some indication that a molecular pattern writer can be achieved.

Indications of field-directing and self-templating effects on the formation of organic lines on silicon.

We pursue dynamic charge and occupancy modulation of silicon dangling bond sites on H-Si(100)-2 × 1 with a biased scanning tunneling microscope tip and demonstrate that the reactivity and mechanism of product formation of cyclobutylmethylketone (CBMK) on the surface at the active sites may be thus spatially regulated. Reactivity is observed to be dependent on the polarity between tip and surface while the area over which reactivity modulation is established scales according to the dopant concentration in the sample. We account for these observations with examination of the competition kinetics applicable to the CBMK/H-Si reaction and determine how said kinetics are affected by the charge state of DB sites associated with reaction initiation and propagation. Our experiments demonstrate a new paradigm in lithographic control of a self-assembly process on H-Si and reveal a variant to the well-known radical mediated chain reaction chemistry applicable to the H-Si surface where self-assembly is initiated with dative bond formation between the molecule and a DB site.

Organic nanostructures on hydrogen-terminated silicon report on electric field modulation of dangling bond charge state.

Field ion microscopy (FIM) using neon imaging gas was used to evaluate a W(111) nanotip shape during a nitrogen assisted etching and evaporation process. Using appropriate etching parameters a narrow ring of atoms centered about the tip axis appears in a helium generated image. Etching of tungsten atoms continues exclusively on the outside of this well-defined ring. By replacing helium imaging gas with neon, normally inaccessible crystal structure of a tip apex is revealed. Comparison of the original W(111) tip (before etching) and partly etched tip shows no atomic changes at the tip apex revealing extraordinarily spatially selective etching properties of the etching process. This observation is an important step towards a detailed understanding of the nitrogen assisted etching and evaporation process and will lead to better control over atomically defined tip shapes.

Field ion microscope evaluation of tungsten nanotip shape using He and Ne imaging gases.

While it is known that the Si-(7×7) is a conducting surface, measured conductivity values differ by 7 orders of magnitude. Here we report a combined STM and transport method capable of surface conductivity measurement of step-free or single-step containing surface regions and having minimal interaction with the sample, and by which we quantitatively determine the intrinsic conductivity of the Si-(7×7) surface. We found that a single step has a conductivity per unit length about 50 times smaller than the flat surface. Our first principles quantum transport calculations confirm and lend insight into the experimental observation.

Conductivity of Si(111)-(7×7): the role of a single atomic step.

Here we report the direct observation of single electron charging of a single atomic dangling bond (DB) on the H-Si(100)-2×1 surface. The tip of a scanning tunneling microscope is placed adjacent to the DB to serve as a single-electron sensitive charge detector. Three distinct charge states of the dangling bond--positive, neutral, and negative--are discerned. Charge state probabilities are extracted from the data, and analysis of current traces reveals the characteristic single-electron charging dynamics. Filling rates are found to decay exponentially with increasing tip-DB separation, but are not a function of sample bias, while emptying rates show a very weak dependence on tip position, but a strong dependence on sample bias, consistent with the notion of an atomic quantum dot tunnel coupled to the tip on one side and the bulk silicon on the other.

Single-electron dynamics of an atomic silicon quantum dot on the H-Si(100)-(2×1) surface.

A new technique for the fabrication of highly sensitive qPlus sensor for atomic force microscopy (AFM) is described. The focused ion beam was used to cut then weld onto a bare quartz tuning fork a sharp micro-tip from an electrochemically etched tungsten wire. The resulting qPlus sensor exhibits high resonance frequency and quality factor allowing increased force gradient sensitivity. Its spring constant can be determined precisely which allows accurate quantitative AFM measurements. The sensor is shown to be very stable and could undergo usual UHV tip cleaning including e-beam and field evaporation as well as in situ STM tip treatment. Preliminary results with STM and AFM atomic resolution imaging at 4.5 K of the silicon Si(111)-7×7 surface are presented.

New fabrication technique for highly sensitive qPlus sensor with well-defined spring constant.

As the ultimate miniaturization of semiconductor devices approaches, it is imperative that the effects of single dopants be clarified. Beyond providing insight into functions and limitations of conventional devices, such information enables identification of new device concepts. Investigating single dopants requires sub-nanometre spatial resolution, making scanning tunnelling microscopy an ideal tool. However, dopant dynamics involve processes occurring at nanosecond timescales, posing a significant challenge to experiment. Here we use time-resolved scanning tunnelling microscopy and spectroscopy to probe and study transport through a dangling bond on silicon before the system relaxes or adjusts to accommodate an applied electric field. Atomically resolved, electronic pump-probe scanning tunnelling microscopy permits unprecedented, quantitative measurement of time-resolved single dopant ionization dynamics. Tunnelling through the surface dangling bond makes measurement of a signal that would otherwise be too weak to detect feasible. Distinct ionization and neutralization rates of a single dopant are measured and the physical process controlling those are identified.

Time-resolved single dopant charge dynamics in silicon.

A single line of dangling bonds (DBs) on Si(100)-2 × 1:H surface forms a perfect metallic atomic-wire. In this work, we investigate quantum transport properties of such dangling bond wires (DBWs) by a state-of-the-art first-principles technique. It is found that the conductance of the DBW can be gated by electrostatic potential and orbital overlap due to only a single DB center (DBC) within a distance of ∼16 Å from the DBW. The gating effect is more pronounced for two DBCs and especially, when these two DB "gates" are within ∼3.9 Å from each other. These effective length scales are in excellent agreement with those measured in scanning tunnelling microscope experiments. By analyzing transmission spectrum and density of states of DBC-DBW systems, with or without subsurface doping, for different length of the DBW, distance between DBCs and the DBW, and distance between DB gates, we conclude that charge transport in a DBW can be regulated to have both an on-state and an off-state using only one or two DBs.

Quantum Transport in Gated Dangling-Bond Atomic Wires.

Negative differential resistance remains an attractive but elusive functionality, so far only finding niche applications. Atom scale entities have shown promising properties, but the viability of device fabrication requires a fuller understanding of electron dynamics than has been possible to date. Using an all-electronic time-resolved scanning tunneling microscopy technique and a Green's function transport model, we study an isolated dangling bond on a hydrogen terminated silicon surface. A robust negative differential resistance feature is identified as a many body phenomenon related to occupation dependent electron capture by a single atomic level. We measure all the time constants involved in this process and present atomically resolved, nanosecond time scale images to simultaneously capture the spatial and temporal variation of the observed feature.

Time-Resolved Imaging of Negative Differential Resistance on the Atomic Scale.

The origin of bond-resolved atomic force microscope images remains controversial. Moreover, most work to date has involved planar, conjugated hydrocarbon molecules on a metal substrate thereby limiting knowledge of the generality of findings made about the imaging mechanism. Here we report the study of a very different sample; a hydrogen-terminated silicon surface. A procedure to obtain a passivated hydrogen-functionalized tip is defined and evolution of atomic force microscopy images at different tip elevations are shown. At relatively large tip-sample distances, the topmost atoms appear as distinct protrusions. However, on decreasing the tip-sample distance, features consistent with the silicon covalent bonds of the surface emerge. Using a density functional tight-binding-based method to simulate atomic force microscopy images, we reproduce the experimental results. The role of the tip flexibility and the nature of bonds and false bond-like features are discussed.

Indications of chemical bond contrast in AFM images of a hydrogen-terminated silicon surface.

We report the mechanically induced formation of a silicon-hydrogen covalent bond and its application in engineering nanoelectronic devices. We show that using the tip of a noncontact atomic force microscope (NC-AFM), a single hydrogen atom could be vertically manipulated. When applying a localized electronic excitation, a single hydrogen atom is desorbed from the hydrogen-passivated surface and can be transferred to the tip apex, as evidenced from a unique signature in frequency shift curves. In the absence of tunnel electrons and electric field in the scanning probe microscope junction at 0 V, the hydrogen atom at the tip apex is brought very close to a silicon dangling bond, inducing the mechanical formation of a silicon-hydrogen covalent bond and the passivation of the dangling bond. The functionalized tip was used to characterize silicon dangling bonds on the hydrogen-silicon surface, which was shown to enhance the scanning tunneling microscope contrast, and allowed NC-AFM imaging with atomic and chemical bond contrasts. Through examples, we show the importance of this atomic-scale mechanical manipulation technique in the engineering of the emerging technology of on-surface dangling bond based nanoelectronic devices.

Atomic White-Out: Enabling Atomic Circuitry through Mechanically Induced Bonding of Single Hydrogen Atoms to a Silicon Surface.

We report on tuning the carrier capture events at a single dangling bond (DB) midgap state by varying the substrate temperature, doping type, and doping concentration. All-electronic time-resolved scanning tunneling microscopy (TR-STM) is employed to directly measure the carrier capture rates on the nanosecond time scale. A characteristic negative differential resistance (NDR) feature is evident in the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements of DBs on both n- and p-type doped samples. We find that a common model accounts for both observations. Atom-specific Kelvin probe force microscopy (KPFM) measurements confirm the energetic position of the DB's charge transition levels, corroborating STS studies. We show that under different tip-induced fields the DB can be supplied with electrons from two distinct reservoirs: the bulk conduction band and/or the valence band. We measure the filling and emptying rates of the DBs in the energy regime where electrons are supplied by the bulk valence band. We show that adding point charges in the vicinity of a DB shifts observed STS and NDR features due to Coulombic interactions.

Article	Year
Field regulation of single-molecule conductivity by a charged surface atom. Nature\| PubMed ID: 15931218	2005
In-line holographic electron microscopy in the presence of external magnetic fields. Ultramicroscopy\| PubMed ID: 17822853	2008
Self-directed growth of contiguous perpendicular molecular lines on H-Si(100) surfaces. The journal of physical chemistry. A\| PubMed ID: 17867660	2007
Indications of field-directing and self-templating effects on the formation of organic lines on silicon. The Journal of chemical physics\| PubMed ID: 21428656	2011
Organic nanostructures on hydrogen-terminated silicon report on electric field modulation of dangling bond charge state. Journal of the American Chemical Society\| PubMed ID: 22708929	2012
Field ion microscope evaluation of tungsten nanotip shape using He and Ne imaging gases. Ultramicroscopy\| PubMed ID: 23000917	2012
Conductivity of Si(111)-(7×7): the role of a single atomic step. Physical review letters\| PubMed ID: 24996100	2014
Single-electron dynamics of an atomic silicon quantum dot on the H-Si(100)-(2×1) surface. Physical review letters\| PubMed ID: 25014824	2014
New fabrication technique for highly sensitive qPlus sensor with well-defined spring constant. Ultramicroscopy\| PubMed ID: 26117434	2015
Time-resolved single dopant charge dynamics in silicon. Nature communications\| PubMed ID: 27782125	2016
Quantum Transport in Gated Dangling-Bond Atomic Wires. Nano letters\| PubMed ID: 28073256	2017
Time-Resolved Imaging of Negative Differential Resistance on the Atomic Scale. Physical review letters\| PubMed ID: 28084769	2016
Indications of chemical bond contrast in AFM images of a hydrogen-terminated silicon surface. Nature communications\| PubMed ID: 28194036	2017
Atomic White-Out: Enabling Atomic Circuitry through Mechanically Induced Bonding of Single Hydrogen Atoms to a Silicon Surface. ACS nano\| PubMed ID: 28719182	2017
Resolving and Tuning Carrier Capture Rates at a Single Silicon Atom Gap State. ACS nano\| PubMed ID: 29091424	2017

Robert A. Wolkow

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