Probing the Carrier Capture Rate of a Single Quantum Level

Dual-Level Enhanced Nonradiative Carrier Recombination in Wide-Gap Semiconductors: The Case of Oxygen Vacancy in SiO2

The conventional single-defect-mediated Shockley-Read-Hall model suggests that the nonradiative carrier recombination rate in wide-band gap (WBG) semiconductors would be negligible because the single-defect level is expected to be either far from valence-band-maximum (VBM) or conduction-band-minimum (CBM), or both. However, this model falls short of elucidating the substantial nonradiative recombination phenomena often observed experimentally across various WBG semiconductors. Owing to more localized nature of defect states inherent to WBG semiconductors, when the defect charge state changes, there is a pronounced structural relaxation around the local defect site. This suggests that a defect at each charge state may exhibit a few distinct local configurations, namely, a stable configuration and a few metastable/transit state configurations. Consequently, a dual-level nonradiative recombination model should more realistically exist in WBG semiconductors. In this model, through the dual-level mechanism, electron and hole trap levels are different from each other and could be closer to the CBM for the electron trap and closer to the VBM for the hole trap, respectively; therefore, this significantly increases the corresponding electron and hole capture rates, enhancing the overall process of nonradiative recombination, and explains the experimental observations. In this work, taking technically important SiO2 as an illustrative example, we introduce the dual-level mechanism to elucidate the mechanism of nonradiative carrier recombination in WBG semiconductors. Our findings demonstrated strong alignment with available experimental data, reinforcing the robustness of our proposed dual-level model. Our fundamental understanding, therefore, provides a clear physical picture of the issue and can also be applied to predict the defect-related nonradiative carrier recombination characteristics in other WBG materials.

Emergence of metallic surface states and negative differential conductance in thinβ-FeSi2films on Si(001)

The electronic properties of the surface ofβ-FeSi₂have been debated for a long. We studied the surface states ofβ-FeSi₂films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory calculations. STM simulations using the surface model proposed by Romanyuket al(2014Phys. Rev.B90155305) reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned by a surface state that appeared in the bulk band gap of theβ-FeSi₂film, irrespective of the polarity of the substrate. We also observed negative differential conductance at ∼0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states ofβ-FeSi₂.

Quantum Dot Acceptors in Two-Dimensional Epitaxially Fused PbSe Quantum Dot Superlattices

Oriented attachment of colloidal quantum dots allows the growth of two-dimensional crystals by design, which could have striking electronic properties upon progress on manipulating their conductivity. Here, we explore the origin of doping in square and epitaxially fused PbSe quantum dot superlattices with low-temperature scanning tunneling microscopy and spectroscopy. Probing the density of states of numerous individual quantum dots reveals an electronic coupling between the hole ground states of the quantum dots. Moreover, a small amount of quantum dots shows a reproducible deep level in the band gap, which is not caused by structural defects in the connections but arises from unpassivated sites at the {111} facets. Based on semiconductor statistics, these distinct defective quantum dots, randomly distributed in the superlattice, trap electrons, releasing a concentration of free holes, which is intimately related to the interdot electronic coupling. They act as acceptor quantum dots in the host quantum dot lattice, mimicking the role of dopant atoms in a semiconductor crystal.

Recent Progress on the Scanning Tunneling Microscopy and Spectroscopy Study of Semiconductor Heterojunctions

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

Single-Electron Tunneling PbS/InP Heterostructure Nanoplatelets for Synaptic Operations

Power consumption, thermal management, and wiring challenge of the binary serial architecture drive the search for alternative paradigms to computing. Of special interest is neuromorphic computing, in which materials and device structures are designed to mimic neuronal functionalities with energy-efficient non-linear responses and both short- and long-term plasticities. In this work, we explore and report on the enabling potential of single-electron tunneling (SET) in PbS nanoplatelets epitaxially grown in the liquid phase on InP, which present these key features. By extrapolating the experimental data in the SET regime, we predict and model synaptic operations. The low-energy (<fJ), high-speed (MHz) operation and scalable fabrication process of the PbS/InP nanoplatelets make such a nanoscale system attractive as neuromorphic computing building blocks.

Gigahertz Frame Rate Imaging of Charge-Injection Dynamics in a Molecular Light Source

Light sources on the scale of single molecules can be addressed and characterized at their proper sub-nanometer scale by scanning tunneling microscopy-induced luminescence (STML). Such a source can be driven by defined short charge pulses while the luminescence is detected with sub-nanosecond resolution. We introduce an approach to concurrently image the molecular emitter, which is based on an individual defect, with its local environment along with its luminescence dynamics at a resolution of a billion frames per second. The observed dynamics can be assigned to the single electron capture occurring in the low-nanosecond regime. While the emitter's location on the surface remains fixed, the scanning of the tip modifies the energy landscape for charge injection into the defect. The principle of measurement is extendable to fundamental processes beyond charge transfer, like exciton diffusion.

Electronic properties of atomically coherent square PbSe nanocrystal superlattice resolved by Scanning Tunneling Spectroscopy

Rock-salt lead selenide nanocrystals can be used as building blocks for large scale square superlattices via two-dimensional assembly of nanocrystals at a liquid-air interface followed by oriented attachment. Here we report Scanning Tunneling Spectroscopy measurements of the local density of states of an atomically coherent superlattice with square geometry made from PbSe nanocrystals. Controlled annealing of the sample permits the imaging of a clean structure and to reproducibly probe the band gap and the valence hole and conduction electron states. The measured band gap and peak positions are compared to the results of optical spectroscopy and atomistic tight-binding calculations of the square superlattice band structure. In spite of the crystalline connections between nanocrystals that induce significant electronic couplings, the electronic structure of the superlattices remains very strongly influenced by the effects of disorder and variability.

Spin-polarized quantum transport in Si dangling bond wires

We report theoretical modeling of spin-dependent quantum transport properties of dangling bond wires (DBWs) on the Si(100)-2 × 1:H surface. A single spin-polarized dangling bond center (DBC) near the DBW may strongly affect transport as characterized by anti-resonances or dips in the transmission spectra. Such spin-dependent gating can be effective up to a distance of 1.5 nanometer between the DBW and the DBC. At the energies where anti-resonances occur, the scattering states of the system are found to be "attracted" to the DBC - rather than moving forward to the existing electrode. The variety of gating effects can be well organized by a physical picture, i.e. a strong hybridization or interaction between the spin-polarized DBW and DBC occurs with the same spin polarization (at DBW and DBC) and at similar energy. The sharp spin-resolved anti-resonance in the DBW gives rise to a spin-filtering effect up to 100% efficiency.

Trap-Free Heterostructure of PbS Nanoplatelets on InP(001) by Chemical Epitaxy

Semiconductor nanocrystalline heterostructures can be produced by the immersion of semiconductor substrates into an aqueous precursor solution, but this approach usually leads to a high density of interfacial traps. In this work, we study the effect of a chemical passivation of the substrate prior to the nanocrystalline growth. PbS nanoplatelets grown on sulfur-treated InP (001) surfaces at temperatures as low as 95 °C exhibit abrupt crystalline interfaces that allow a direct and reproducible electron transfer to the InP substrate through the nanometer-thick nanoplatelets with scanning tunnelling spectroscopy. It is in sharp contrast with the less defined interface and the hysteresis of the current-voltage characteristics found without the passivation step. Based on a tunnelling effect occurring at energies below the bandgap of PbS, we show the formation of a type II, trap-free, epitaxial heterointerface, with a quality comparable to that grown on a nonreactive InP (110) substrate by molecular beam epitaxy. Our scheme offers an attractive alternative to the fabrication of semiconductor heterostructures in the gas phase.

In situ biasing and off-axis electron holography of a ZnO nanowire

Quantitative characterization of electrically active dopants and surface charges in nano-objects is challenging, since most characterization techniques using electrons [1-3], ions [4] or field ionization effects [5-7] study the chemical presence of dopants, which are not necessarily electrically active. We perform cathodoluminescence and voltage contrast experiments on a contacted and biased ZnO nanowire with a Schottky contact and measure the depletion length as a function of reverse bias. We compare these results with state-of-the-art off-axis electron holography in combination with electrical in situ biasing on the same nanowire. The extension of the depletion length under bias observed in scanning electron microscopy based techniques is unusual as it follows a linear rather than square root dependence, and is therefore difficult to model by bulk equations or finite element simulations. In contrast, the analysis of the axial depletion length observed by holography may be compared with three-dimensional simulations, which allows estimating an n-doping level of 1 × 10¹⁸ cm^-3 and negative sidewall surface charge of 2.5 × 10¹² cm^-2 of the nanowire, resulting in a radial surface depletion to a depth of 36 nm. We found excellent agreement between the simulated diameter of the undepleted core and the active thickness observed in the experimental data. By combining TEM holography experiments and finite element simulation of the NW electrostatics, the bulk-like character of the nanowire core is revealed.

Resolving and Tuning Carrier Capture Rates at a Single Silicon Atom Gap State

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.

Quantum Transport in Gated Dangling-Bond Atomic Wires

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.

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

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.

Switching the charge state of individual surface atoms at Si(111)-√3 × √3:B surfaces

We show that each surface atom of heavily boron-doped, (111)-oriented silicon with a √3 × √3 reconstruction has electrically switchable two charge states due to the strong electron-lattice coupling at this surface. The structural and electronic properties of the two charge states as well as their energetics are uncovered by employing scanning tunneling microscopy measurements and density functional theory calculations, which reveals that one of the two is a two-electron bound state or surface bipolaron. We also execute the single-atom bit operations on individual surface atoms by controlling their charge states while demonstrating implementation of the atomic scale memory at a silicon surface with an unprecedented recording density.

Phonon-mediated electron transport through CaO thin films

Scanning tunneling microscopy has developed into a powerful tool for the characterization of conductive surfaces, for which the overlap of tip and sample wave functions determines the image contrast. On insulating layers, as the CaO thin film grown on Mo(001) investigated here, direct overlap between initial and final states is not enabled anymore and electrons are transported via hopping through the conduction-band states of the oxide. Carrier transport is accompanied by strong phonon excitations in this case, imprinting an oscillatory signature on the differential conductance spectra of the system. The phonons show a characteristic spatial dependence and become softer around lattice irregularities in the oxide film, such as dislocation lines.

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

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.

Quantum engineering at the silicon surface using dangling bonds

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.

The ability to add and move individual atoms on a surface with a scanning tunnelling microscope enables precise control over the electronic quantum states of the surface. Schofield et al. show that removing hydrogen atoms from a passivated silicon surface can be used to generate and control such states.

Topological dangling bonds with large spin splitting and enhanced spin polarization on the surfaces of Bi2Se3

We investigate the topological surface state properties at various surface cleaves in the topological insulator Bi2Se3, via first principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS). While the typical surface termination occurs between two quintuple layers, we report the existence of a surface termination within a single quintuple layer where dangling bonds form with giant spin splitting owing to strong spin-orbit coupling. Unlike Rashba split states in a 2D electron gas, these states are constrained by the band topology of the host insulator with topological properties similar to the typical topological surface state, and thereby offer an alternative candidate for spintronics usage. We name these new states "topological dangling-bond states". The degree of the spin polarization of these states is greatly enhanced. Since dangling bonds are more chemically reactive, the observed topological dangling-bond states provide a new avenue for manipulating band dispersions and spin-textures by adsorbed atoms or molecules.

Negative differential resistance in a hybrid silicon-molecular system: resonance between the intrinsic surface-states and the molecular orbital

It has been a long-term desire to fabricate hybrid silicon-molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-molecular system that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × √3-Ag (R3-Ag/Si) surface and the molecular orbital of cobalt(II)-phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co(2+) ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance between the intrinsic surface-state band S(1) of the R3-Ag/Si surface and the localized unoccupied Co(2+)d(z(2)) orbital of the adsorbed CoPc molecules. We expect that such a mechanism can be generally used in other silicon-molecular systems.

Observation of room-temperature negative differential resistance in Gd-doped Si nanowires on Si(110) surface

The massively parallel arrays of highly periodic Gd-doped Si nanowires (SiNWs) self-organized on Si(110)-16 × 2 surface were investigated by scanning tunneling microscopy and spectroscopy. These periodic Gd-doped SiNWs are atomically precise and show equal size, periodic positions, and high-integration densities. Surprisingly, the scanning tunneling spectroscopy results show that each metallic-like, Gd-doped SiNW exhibits room-temperature negative differential resistance (RT-NDR) behavior, which can be reproducible with various Gd dopings and is independent of the tips. Such massively parallel arrays of highly ordered and atomically identical Gd-doped SiNWs with one-dimensional laterally confined RT-NDR can be exploited in Si-based RT-NDR nanodevices.

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.

Coulomb energy determination of a single Si dangling bond

Determination of the Coulomb energy of single point defects is essential because changing their charge state critically affects the properties of materials. Based on a novel approach that allows us to simultaneously identify a point defect and to monitor the occupation probability of its electronic state, we unambiguously measure the charging energy of a single Si dangling bond with tunneling spectroscopy. Comparing the experimental result with tight-binding calculations highlights the importance of the particular surrounding of the localized state on the effective charging energy.

Mapping active dopants in single silicon nanowires using off-axis electron holography

We demonstrate that state-of-the-art off-axis electron holography can be used to map active dopants in silicon nanowires as thin as 60 nm with 10 nm spatial resolution. Experiment and simulation demonstrate that doping concentrations of 10(19) and 10(20) cm(-3) can be measured with a detection threshold of 10(18) cm(-3) with respect to intrinsic silicon. Comparison of experimental data and simulations allows an estimation of the charge density at the wire-oxide interface of -1 x 10(12) electron charges cm(-2). Off-axis electron holography thus offers unique capabilities for a detailed analysis of active dopant concentrations in nanostructures.

Orbital and charge-resolved polaron states in CdSe dots and rods probed by scanning tunneling spectroscopy

Conduction electrons interact with lattice vibrations, leading to coupled electron-phonon states. This effect is of fundamental importance in understanding electron transport and energy relaxation in nanoscale systems. We report quantitative results on the electron-phonon coupling strength in CdSe quantum dots (QDs) and rods, obtained by low-temperature scanning tunneling microscopy. We resolve the polaron eigenstates arising from coupling of the electrons to longitudinal optical phonons. The electron-phonon coupling strength is dependent on the electron orbital symmetry, the number of added electrons, and the size and dielectric environment of the QD.