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In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials., Science Advances, 9 (16), ISSN:2375-2548

We exploit the full capabilities of electrical scanning probe microscopy (e-SPM) to inspect the charge carrier transport of an electrically contacted, buried phosphorus, 2D nanowire (P-wire) in operando.

Germanium has emerged as an exceptionally promising material for spintronics and quantum information applications, with significant fundamental advantages over silicon. However, efforts to create atomic-scale devices using donor atoms as qubits have largely focussed on phosphorus in silicon. Positioning phosphorus in silicon with atomic-scale precision requires a thermal incorporation anneal, but the low success rate for this step has been shown to be a fundamental limitation prohibiting the scale-up to large-scale devices. Here, we present a comprehensive study of arsine (AsH3 ) on the germanium (001) surface. We show that, unlike any previously studied dopant precursor on silicon or germanium, arsenic atoms fully incorporate into substitutional surface lattice sites at room temperature. Our results pave the way for the next generation of atomic-scale donor devices combining the superior electronic properties of germanium with the enhanced properties of arsine/germanium chemistry that promises scale-up to large numbers of deterministically-placed qubits.

Dopant impurity species can be incorporated into the silicon (001) surface via the adsorption and dissociation of simple precursor molecules. Examples include phosphine (PH3), arsine (AsH3), and diborane (B2H6) for the incorporation of phosphorus, arsenic, and boron, respectively. Through exploitation of precursor surface chemistry, the spatial locations of these incorporated dopants can be controlled at the atomic scale via the patterning of a hydrogen lithographic resist layer using scanning tunneling microscopy (STM). There is strong interest in the spatial control of bismuth atoms incorporated into silicon for quantum technological applications; however, there is currently no known precursor for the incorporation of bismuth that is compatible with this STM-based lithographic method. Here, we explore the precursor chemistry (adsorption, diffusion, and dissociation) of bismuth trichloride (BiCl3) on Si(001). We show atomic-resolution STM images of BiCl3 exposed Si(001) surfaces at low coverage and combine this with density functional theory calculations to produce a model of the surface processes and the observed features. Our results show that, at room temperature, BiCl3 completely dissociates to produce bismuth ad-atoms, ad-dimers, and surface-bound chlorine, and we explain how BiCl3 is a strong candidate for a bismuth precursor compound compatible with lithographic patterning at the sub-nanometer scale.

Nominally-pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunnelling microscopy (STM) images of black phosphorus reveal the presence of long-ranged double-lobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP bandgap. The localised negative charge of the tin impurities induces hydrogenic states in the bandgap and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images., Comment: 19 pages, 4 figures

In this work, we present a novel resistless patterning technique on hydrogen-terminated Si (100) using EUV-induced surface reaction. We has successfully demonstrated a direct Si pattern transfer of half-pitch 75 nm Si gratings using the EUV resistless patterning technique. The EUV-induced surface reactions could achieve direct patterning on functionalized Si surface and potentially overcome the resolution limitations of conventional photoresists in the semiconductor industry. Moreover, this approach can be used for better understanding of EUV exposure mechanisms by preparing simple molecular systems and studying the EUV-induced reactions.

Motivated by recent advances in fabricating artificial lattices in semiconductors and their promise for quantum simulation of topological materials, we study the one-dimensional dimerized Fermi-Hubbard model. We show how the topological phases at half-filling can be characterized by a reduced Zak phase defined based on the reduced density matrix of each spin subsystem. Signatures of bulk-boundary correspondence are observed in the triplon excitation of the bulk and the edge states of uncoupled spins at the boundaries. At quarter-filling we show that owing to the presence of the Hubbard interaction the system can undergo a transition to the topological ground state of the non-interacting Su-Schrieffer-Heeger model with the application of a moderate-strength external magnetic field. We propose a robust experimental realization with a chain of dopant atoms in silicon or gate-defined quantum dots in GaAs where the transition can be probed by measuring the tunneling current through the many-body state of the chain., 11 pages, 7 figures

Defects play an important role for surface reconstructions and therefore also influence the substrate growth. In this work we present a first principle calculation for the Ge(001) surface without and with tin impurities incorporated into the top surface layer. By mapping the system onto an Ising-type model, with interaction constants taken from density functional theory, the stability of the surface reconstructions under the influence of different concentrations of tin impurities is explored. This approach allows us to simulate the possible phase transitions for the different surface reconstructions including the local structure around the tin impurity atoms. In addition, we compare our theoretical results with experimental STM images on clean and Sn-doped Ge(100) surfaces.

The growth of multi-layer germanium-tin (GeSn) quantum wells offers an intriguing pathway towards the integration of lasers in a CMOS platform. An important step in growing high quality quantum well interfaces is the formation of an initial wetting layer. However, key atomic-scale details of this process have not previously been discussed. We use scanning tunneling microscopy combined with density functional theory to study the deposition of Sn on Ge(1 0 0) at room temperature over a coverage range of 0.01 to 1.24 monolayers. We demonstrate the formation of a sub-2% Ge content GeSn wetting layer from three atomic-scale characteristic ad-dimer structural components, and show that small quantities of Sn incorporate into the Ge surface forming two atomic configurations. The ratio of the ad-dimer structures changes with increasing Sn coverage, indicating a change in growth kinetics. At sub-monolayer coverage, the least densely packing ad-dimer structure is most abundant. As the layer closes, forming a two-dimensional wetting layer, the more densely packing ad-dimer structure become dominant. These results demonstrate the capability to form an atomically smooth wetting layer at room temperature, and provide critical atomic-scale insights for the optimization of growth processes of GeSn multi-quantum-wells to meet the quality requirements of optical GeSn-based devices.

Over the last two decades, prototype devices for future classical and quantum computing technologies have been fabricated, by using scanning tunneling microscopy and hydrogen resist lithography to position phosphorus atoms in silicon with atomic-scale precision. Despite these successes, phosphine remains the only donor precursor molecule to have been demonstrated as compatible with the hydrogen resist lithography technique. The potential benefits of atomic-scale placement of alternative dopant species have, until now, remained unexplored. In this work, we demonstrate successful fabrication of atomic-scale structures of arsenic-in-silicon. Using a scanning tunneling microscope tip, we pattern a monolayer hydrogen mask to selectively place arsenic atoms on the Si(001) surface using arsine as the precursor molecule. We fully elucidate the surface chemistry and reaction pathways of arsine on Si(001), revealing significant differences to phosphine. We explain how these differences result in enhanced surface immobilization and in-plane confinement of arsenic compared to phosphorus, and a dose-rate independent arsenic saturation density of $0.24{\pm}0.04$ ML. We demonstrate the successful encapsulation of arsenic delta-layers using silicon molecular beam epitaxy, and find electrical characteristics that are competitive with equivalent structures fabricated with phosphorus. Arsenic delta-layers are also found to offer improvement in out-of-plane confinement compared to similarly prepared phosphorus layers, while still retaining >80% carrier activation and sheet resistances of $

Control of dopants in silicon remains the most important approach to tailoring the properties of electronic materials for integrated circuits, with Group V impurities the most important n-type dopants. At the same time, silicon is finding new applications in coherent quantum devices, thanks to the magnetically quiet environment it provides for the impurity orbitals. The ionization energies and the shape of the dopant orbitals depend on the surfaces and interfaces with which they interact. The location of the dopant and local environment effects will therefore determine the functionality of both future quantum information processors and next-generation semiconductor devices. Here we match observed dopant wavefunctions from low-temperature scanning tunnelling microscopy (STM) to images simulated from first-principles density functional theory (DFT) calculations. By this combination of experiment and theory we precisely determine the substitutional sites of neutral As dopants between 5 and 15A below the Si(001):H surface. In the process we gain a full understanding of the interaction of the donor-electron state with the surface, and hence of the transition between the bulk dopant (with its delocalised hydrogenic orbital) and the previously studied dopants in the surface layer., 12 pages; accepted for publication in Phys. Rev. B

Microwave microscopy enables three-dimensional characterization of atomically thin semiconductor structures with nanometer precision., It is now possible to create atomically thin regions of dopant atoms in silicon patterned with lateral dimensions ranging from the atomic scale (angstroms) to micrometers. These structures are building blocks of quantum devices for physics research and they are likely also to serve as key components of devices for next-generation classical and quantum information processing. Until now, the characteristics of buried dopant nanostructures could only be inferred from destructive techniques and/or the performance of the final electronic device; this severely limits engineering and manufacture of real-world devices based on atomic-scale lithography. Here, we use scanning microwave microscopy (SMM) to image and electronically characterize three-dimensional phosphorus nanostructures fabricated via scanning tunneling microscope–based lithography. The SMM measurements, which are completely nondestructive and sensitive to as few as 1900 to 4200 densely packed P atoms 4 to 15 nm below a silicon surface, yield electrical and geometric properties in agreement with those obtained from electrical transport and secondary ion mass spectroscopy for unpatterned phosphorus δ layers containing ~1013 P atoms. The imaging resolution was 37 ± 1 nm in lateral and 4 ± 1 nm in vertical directions, both values depending on SMM tip size and depth of dopant layers. In addition, finite element modeling indicates that resolution can be substantially improved using further optimized tips and microwave gradient detection. Our results on three-dimensional dopant structures reveal reduced carrier mobility for shallow dopant layers and suggest that SMM could aid the development of fabrication processes for surface code quantum computers.

We characterize the incorporation of Ba adatoms into the Ge(001) surface, resulting in the formation of one-dimensional structures with an internal 2×3 periodicity, after the deposition of Ba atoms at 970 K or at room temperature followed by a 770 K anneal. Scanning tunneling microscopy (STM) data were compared with theoretically simulated STM images generated by density functional theory electronic structure calculations. Excellent agreement between experiment and simulation was found when using an adopted structural model that assumes partial removal of the surface Ge dimers in the [1–10] surface direction and subsequent addition of a single Ba atom to the substrate second layer. Structural assignments for a number of defects observed within regions of the 2×3 reconstruction were also obtained.

Electronic devices made from organic materials have the potential to support a more ecologically friendly and affordable future. However, the ability to fabricate devices with well-defined and reproducible electrical and optical properties is hindered by the sensitivity to the presence of chemical impurities. Oxygen in particular is an impurity that can trap electrons and modify conductive properties of some organic materials. Until now the 3-dimensional profiling of oxygen species in organic semiconductors has been elusive and the effect of oxygen remains disputed. In this study we map out high-spatial resolution 3-dimensional distributions of oxygen inclusions near the surface of single crystal rubrene, using Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). Channels of diffused oxygen, 'oxygen pillars', are found extending from uniform oxygen inclusion layers at the surface. These pillars extend to depths in excess of 1.8 {\mu}m and act as an entry point for oxygen to diffuse along the ab-plane of the crystal with at least some of the diffused oxygen molecularly binding to rubrene. Our investigation of surfaces at different stages of evolution reveals the extent of oxygen inclusion, which affects rubrene's optical and transport properties, and is consequently of importance for the reliability and longevity of devices., Comment: Submitted to Physical Chemistry Chemical Physics

Recently, phosphorous structures in silicon have been of interest theoretically and experimentally due to their relevance in the field of quantum computing. Coherent control of the orbital states of shallow donors in silicon has been demonstrated in bulk doped samples. Here we discuss the fabrication techniques required to 1) obtain patterned two dimensional dilute sheets of impurities in silicon of controlled doping densities 2) get them to act as targets for a terahertz laser. Scanning tunnelling microscope hydrogen lithography enables patterning of impurity features in silicon with a resolution from 1nm to tens of nm. Molecular beam epitaxy is used for a protective thin-film crystalline silicon growth over the impurity sheet. Electron beam lithography coupled with reactive ion etching allows features from tens to hundreds of microns to be etched into the silicon with 10 to 20nm resolution. The experimental readout is achieved via illumination of the silicon target by terahertz light and subsequent electrical detection. The electrical signal comes from coherent and non-linear excitations of the impurity electrons. This detection technique enables the precision condensed matter samples to remain intact after exposure to the free electron laser pulse.

We have studied single Si dangling bonds on the Si(0 0 1) surface using scanning tunnelling microscopy (STM) and density functional theory (DFT) calculations. The Si dangling bonds are created by the chemisorption of single hydrogen atoms forming a Si–Si–H hemihydride. At room temperature, the hemihydride induces static buckling on adjacent Si–Si dimers. In the STM measurements, we observe that the orientation of the static buckling pattern can be reversed with tip-sample bias and influenced by the substrate doping. Our DFT calculations yield a correlation between the electron occupancy of the hemihydride Si dangling bond and the buckling orientation around it.

Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that the structure of isolated Si dangling bonds on the Si(001) surface is critically influenced by the doping of the silicon substrate. Substitutional phosphorus atoms in the surface form Si−P heterodimers that contain a single, isolated Si dangling bonda common feature of many adsorbate species and surface defects on Si(001). In our STM measurements, the Si−P heterodimer is observed to adopt a different appearance on n- and p-type substrates. DFT calculations show that this difference arises due to the creation and occupation of a localized defect state associated with the Si dangling bond. Depending on the substrate doping, charge is transferred into the Si dangling bond leading to a relaxation of the geometric and electronic structure.

An ordered alkaline-earth submonolayer on a clean $\mathrm{Si}(001)$ surface provides a template for growth of the atomically sharp, crystalline Si-oxide interface that is ubiquitous in the semiconductor device industry. It has been suggested that submonolayers of Sr or Ba on $\mathrm{Ge}(001)$ could play a similar role as on structurally identical $\mathrm{Si}(001)$, overcoming known limitations of the $\mathrm{Ge}(001)$ substrate such as amorphization of its oxidation layers. In this paper the initial stage of the Ba oxidation process, i.e., adsorption and organization of Ba atoms on the $\mathrm{Ge}(001)$ surface as a function of temperature $(270\ensuremath{-}770\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ for coverage 1.0 monolayer (ML) and $0.15\ensuremath{-}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered $\mathrm{Ge}(001)$ surface. They originate from isolated Ba adatoms, isolated Ba ad-dimers, and the Ba ad-dimers assembled into short-range, randomly distributed chains that run across the Ge dimer rows. We find from both STM measurements and DFT calculations that the latter is the dominant structure on $\mathrm{Ge}(001)$ with increasing coverage.

We present a comparative scanning tunnelling microscopy (STM) study of two features on the Si(0 0 1) surface with a single dangling bond. One feature is the Si–P heterodimer—a single surface phosphorus atom substituted for one Si atom of a Si–Si dimer. The other feature is the Si–Si–H hemihydride—a single hydrogen atom adsorbed to one Si atom of a Si–Si dimer. Previous STM studies of both surface species have reported a nearly identical appearance in STM which has hampered an experimental distinction between them to date. Using voltage-dependent STM we are able to distinguish and identify both heterodimer and hemihydride on the Si(0 0 1) surface. This work is particularly relevant for the fabrication of atomic-scale Si:P devices by STM lithography on the hydrogen terminated Si(0 0 1):H surface, where it is important to monitor the distribution of single P dopants in the surface. Based on the experimental identification, we study the lateral P diffusion out of nanoscale reservoirs prepared by STM lithography.

The ability to identify and control the pathway by which chemical reactions occur at the level of individual atoms will be of enormous benefit in the coming age of nanotechnology. Here, we present a detailed atomic-resolution scanning tunneling microscopy (STM) study of one such pathway: the adsorption, surface diffusion and dissociation of phosphine (PH3) on silicon (001). We show that PH3 dissociates on Si(001) to produce PH2 + H, PH + 2H and P + 3H moieties. In addition, we show that PH2 molecules are readily able to diffuse on Si(001) at room temperature, resulting in the formation of hemihydride dimers on low dosed Si(001) surfaces. In addition we show that control over the processes of diffusion and surface incorporation can be achieved using STM-based hydrogen lithography; a technique that is now used in many STM laboratories. [DOI: 10.1380/ejssnt.2006.609]

We present a review of a detailed fabrication strategy for the realisation of nano and atomic-scale devices in silicon using phosphorus as a dopant and a combination of ultra-high vacuum scanning probe microscopy and silicon molecular beam epitaxy (MBE). In this work we have been able to overcome some of the key fabrication challenges to the realisation of atomic-scale devices including the identification of single P dopants in silicon, the controlled incorporation of P atoms in silicon with atomic precision and the minimisation of P segregation and diffusion during Si encapsulation. Recently, we have combined these results with a novel registration technique to fabricate robust electrical devices in silicon that can be contacted and measured outside the ultra-high vacuum environment. We discuss the importance of our results for the future fabrication of atomic-scale devices in silicon.

The fabrication of atomic-scale devices in silicon requires the encapsulation of dopant atoms which have been incorporated into the silicon surface at atomically precise positions using scanning tunnelling microscopy (STM) lithography. During silicon encapsulation, it is important to minimise segregation and diffusion of dopant atoms in order to retain the lithography defined device structure. Buried dopant imaging using STM is capable of imaging dopant atoms such as phosphorus after encapsulation in silicon several monolayers below the silicon surface, thus making it possible to check the integrity of the device structure. To fabricate buried phosphorus-doped samples, we use phosphine gas as a source of phosphorus atoms and incorporate the phosphorus atoms into a Si(001) surface during an annealing step. Molecular beam epitaxy is used to encapsulate the dopant atoms with several monolayers of silicon. After encapsulation, we hydrogen terminate the silicon surface in order to image the buried phosphorus dopants using STM. We show that a buried phosphorus atom appears as a bright glow superimposed on the silicon dimer structure in empty state STM images, whereas filled state images only show a very faint protrusion in the vicinity of the phosphorus atom. We highlight the importance of our results for the fabrication of atomic-scale devices.

We have used scanning tunneling microscopy (STM) to image charged defects on the clean (100)-(2×1) surface of p-type silicon. In the absence of “C”-type defects, band bending can occur during STM imaging, allowing near surface charge to influence the state density contributing to the tunnel current. As in the case of cleavage faces of III–V semiconductor crystals, the charge-induced band bending produces long range enhancements superimposed on the periodic surface lattice. The charged defects observed in this work are of the types commonly observed elsewhere in clean Si(100)-(2×1) STM studies, however, not all defects of a given type appear charged. This would indicate subtle differences in defect structure that are not obvious at higher sample bias. This work demonstrates the ability to observe charged features on the clean Si(100) surface, which will be important for current and future research focused on producing atomic scale electronic structures.

The construction of a scalable quantum computer in silicon, using single phosphorus atoms as qubits, presents a significant technological challenge. This paper describes recent results from a ‘bottom-up’ strategy to incorporate individual phosphorus atoms in silicon with atomic precision using a combination of advanced scanning tunnelling lithography techniques followed by low temperature silicon molecular beam epitaxial overgrowth. To date we have demonstrated (i) placement of individual phosphorus molecules at predetermined sites in the silicon surface using a hydrogen resist strategy, (ii) spatially controlled phosphorus incorporation into the silicon surface, (iii) minimisation of surface segregation by low temperature silicon encapsulation and (iv) complete electrical activation of the donors. Whilst these results bode well for the fabrication of silicon devices with atomically precise dopant profiles, we discuss the challenges that remain before a few qubit P in Si quantum computer prototype can be realised.

We present recent progress towards the fabrication of a silicon-based quantum computer, via a ‘bottom up’ strategy, which exploits a combination of scanning tunneling microscopy (STM) and molecular beam epitaxy. Following our recent success in the fabrication of an atomic array of P atoms in silicon we now address the critical issues leading to the encapsulation of this array in silicon. Exposing Si(0 0 1) to a low PH 3 dose at room temperature results in the formation of a number of species on the surface. We use bias dependent STM to characterise these species and identify PH x ( x =2,3) moieties and hemihydrides as the dominant surface adsorbates. We show that by controlled annealing we can incorporate single P atoms into the surface layer. We use STM lithography to depassivate regions of a H:Si(0 0 1) surface in order to confine the above adsorption/incorporation processes to nanometer sized areas.

We investigate the morphology of epitaxial Si layers grown on clean and on hydrogen terminated Si(0 0 1) to explore the growth strategy for the fabrication of a Si-based quantum computer. We use molecular beam epitaxy to deposit 5 monolayers of silicon at a temperature of 250 °C and scanning tunnelling microscopy to image the surface at room temperature after growth and after various rapid annealing steps in the temperature range of 350–600 °C. The epitaxial layer grown on the hydrogenated surface shows a significantly higher surface roughness due to a lower mobility of silicon surface atoms in the presence of hydrogen. Annealing at temperatures ≥550 °C reduces the roughness of both epitaxial layers to the value of a clean silicon surface. However, the missing dimer defect density of the epitaxial layer grown on the hydrogenated surface remains higher by a factor of two compared to the layer grown on clean Si(0 0 1). Our results suggest a quantum computer growth strategy in which the hydrogen resist layer is desorbed before the epitaxial silicon layer is grown at low temperature to encapsulate phosphorus quantum bits.

We present three important results relating to the fabrication of a quantum computer in silicon: (i) the interaction of the dopant gas phosphine with Si(001), (ii) a comparison of the morphology of epitaxial Si layers grown on clean and on monohydride-terminated Si(001), and (iii) a direct measure of the segregation/diffusion of incorporated P atoms during Si epitaxial growth and annealing. After low phosphine (PH3) dosing of a Si(001) surface dual bias scanning tunneling microscopy was used to identify the PH x(x = 2, 3) species on the surface. Subsequent annealing to 350°C resulted in the P atom from the PH x molecule being incorporated into the surface to form Si–P heterodimers. The threefold coordination that results from incorporation is expected to be advantageous for phosphorus quantum bit fabrication since it will reduce P segregation and diffusion during Si epitaxial overgrowth. One question to be addressed in the encapsulation process for quantum bits is whether the H resist layer needs to be removed or whether we can grow through the hydrogen layer. We demonstrate that five-monolayer-thick epitaxial Si layers deposited at low temperature (250°C) using molecular beam epitaxy have a significantly lower roughness and defect density when grown on a clean Si(001) surface compared to a H-terminated surface. Attempts to encapsulate phosphorus quantum bits at 260°C and to recover the surface quality of the epitaxial layer resulted in P atoms segregating and diffusing to the surface. These results suggest that the hydrogen layer is desorbed first before the P atoms are encapsulated in epitaxial silicon grown at very low temperature (below 250°C) to minimise phosphorus segregation.

Recognition of the potentially massive computational power of a quantum computer has driven a considerable experimental effort to build such a device. Of the various possible physical implementations, silicon-based architectures are attractive for the long spin relaxation times involved, their scalability, and ease of integration with existing silicon technology. However, their fabrication requires construction at the atomic scale-an immense technological challenge. Here we outline a detailed strategy for the construction of a phosphorus in silicon quantum computer and demonstrate the first significant step towards this goal-the fabrication of atomically precise arrays of single phosphorus bearing molecules on a silicon surface. After using a monolayer hydrogen resist to passivate a silicon surface we apply pulsed voltages to a scanning tunnelling microscope tip to selectively desorb individual hydrogen atoms with atomic resolution. Exposure of this surface to the phosphorus precursor phosphine results in precise placement of single phosphorus atoms on the surface. We also describe preliminary studies into a process to incorporate these surface phosphorus atoms into the silicon crystal at the array sites.

Scanning tunneling microscopy (STM) has been used to image charged defects on the clean Si(100)-(2×1) surface. Previous studies have shown that, in the absence of “C”-type defects, the surface does not pin the Fermi level, allowing near surface charge to influence the state density contributing to the tunneling current. As in the case of cleavage faces of III–V semiconductor crystals, the charge-induced band bending produces long-range enhancements superimposed on the periodic surface lattice. This is observed in empty-state STM images taken on n-type material. No band bending signature is seen in the filled-state images. This can be understood by considering the band structure at the surface, which has surface states within the band gap. The charged defects observed in this work are of the types commonly observed in clean Si(100)-(2×1) STM studies, however, not all defects of a given type appear charged. This would indicate subtle differences in structure or the influence of impurities. Predictions for p...

We present atomic force microscopy (AFM) measurements from a passivated silicon crystal miscut by 0.1° and show the etching regime to be significantly different from surfaces with a larger miscut angle. A simple kinetic model is developed to explain the results and is used to derive the optimal etching conditions for nominally flat Si(1 1 1)–(1×1)H. We show that small changes in miscut angle can alter the kinetic steady state and promote the formation of deep etch pits, even on the least stable, 〈 1 1 2〉 miscut surface. Collisions of steps with these pits result in arrays of stable, self-aligned `etch hillocks' over micron dimensions. Following preparation, we use AFM to observe the initial growth of native oxide on the Si(1 1 1)–(1×1)H surface, and demonstrate that AFM is a sensitive probe to surface oxidation in the sub-monolayer regime.

A conducting tip of an atomic force microscope (AFM) is used to induce ultra-small oxide patterns on metallic (Ti) thin film and semiconducting (GaAs) surfaces. The oxide is used to deplete a shallow two-dimensional electron gas (2DEG) formed at a GaAs/AlGaAs interface, 274 A beneath the surface. The depleted portion of the 2DEG is rendered highly resistive and remains insulating over a wide voltage range at low temperatures. Furthermore, in-plane side gates, defined by oxide lines, are used to constrict the 2DEG into a narrow one-dimensional (1D) electron channel which exhibits quantized conductance at low temperatures. A relatively high sub-band energy spacing (∼5.5 meV) is achieved between the first two sub-bands, demonstrating the steep lateral confining potential provided by side gates through oxide walls.

We study subsurface arsenic dopants in a hydrogen-terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant-related features that fall into two classes, which we call As1 and As2. When imaged in occupied states, the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionization when imaged under empty-state conditions, similar to previous observations of acceptors in GaAs. Density functional theory calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction-band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled-state images and an isotropic depression in empty-state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

We describe an experiment based around a scanning tunnelling microscope (STM) designed for an advanced undergraduate teaching laboratory. The STM is used to study the kinetics of graphite oxidation by monitoring the density of oxidation pits that form on the surface of a highly oriented pyrolytic graphite (HOPG) sample which has been heated to 750-800C. Our experiment takes the STM beyond its traditional 'look-see' teaching role by exploiting the technique for the acquisition of semi-quantitative data. The students are required to calibrate the instrument, determine the optimum oxidation conditions experimentally and calculate the pit nucleation activation energy for planar graphite. They then determine the distribution of pit diameters as a function of oxidation time and explain their findings. We show typical results and describe the experimental arrangement.

We investigate the effect of an external magnetic field on the physical properties of the acceptor hole states associated with single Mn acceptors placed near the (110) surface of GaAs. Crosssectional scanning tunneling microscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported by theoretical calculations based on a tightbinding model of Mn acceptors in GaAs. For Mn acceptors on the (110) surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantly the magnetic anisotropy landscape. However the acceptor hole wavefunction is strongly localized around the Mn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed on deeper layers below the surface, the acceptor hole wavefunction is more delocalized and the corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However the magnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis. We predict that substantially larger magnetic fields are required to observe a significant field-dependence of the tunneling current for impurities located several layers below the GaAs surface., Comment: None

In order to fabricate precise atomic-scale devices in silicon using a combination of scanning tunnelling microscopy (STM) and molecular beam epitaxy it is necessary to minimize the segregation/diffusion of dopant atoms during silicon encapsulation. We characterize the surface segregation/diffusion of phosphorus atoms from a $\delta$-doped layer in silicon after encapsulation at 250$^{\circ}$C and room temperature using secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES), and STM. We show that the surface phosphorus density can be reduced to a few percent of the initial $\delta$-doped density if the phosphorus atoms are encapsulated with 5 or 10 monolayers of epitaxial silicon at room temperature. We highlight the limitations of SIMS and AES to determine phosphorus segregation at the atomic-scale and the advantage of using STM directly.

The growth of K on Si(100), at 150 K, has been studied in situ by helium atom scattering. The transition from a corrugated, semi-conducting surface charge density to a smooth, metallic surface is clearly visible in the data. Below one monolayer we identify a single ordered overlayer at a coverage corresponding to 0.5 ML with respect to the Si atom density. A close packed metal film develops near 1 ML and reaches maximum order at 1.25 ML. The density of the metal film is consistent with a c(4 × 2) structure with a lattice mismatch of 3.5% in the [011] direction (overlayer expansion) and 2.3% in the [011] direction (overlayer compression) or vice versa. At higher coverages, the film appears to develop by island growth.

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments.

We report that solitary bismuth and antimony atoms, incorporated at Si(111) surfaces, induce either positive or negative charge states depending on the site of the surface reconstruction in which they are located. This is in stark contrast to the hydrogenic donors formed by group V atoms in silicon bulk crystal and therefore has strong implications for the design and fabrication of future highly scaled electronic devices. Using scanning tunnelling microscopy (STM) and density functional theory (DFT) we determine the reconstructions formed by different group V atoms in the Si(111)2 × 1 surface. Based on these reconstructions a model is presented that explains the polarity as well as the location of the observed charges in the surface. Using locally resolved scanning tunnelling spectroscopy we are furthermore able to map out the spatial extent over which a donor atom influences the unoccupied surface and bulk electronic states near the Fermi-level. The results presented here therefore not only show that a dopant atom can induce both positive and negative charges but also reveal the nature of the local electronic structure in the region of the silicon surface where an individual donor atom is present.

The growth and structure of potassium on Ge(100) has been studied at 300 K using scanning tunnelling microscopy. In the early stages of growth (approximately 0.1 ML), constant current topographs show small growth nuclei in two distinct surface sites: between and atop the Ge(100) dimers, respectively. Growth of both sites proceeds anisotropically with a preference for the 〈011〉 direction. Topographs obtained at higher coverages suggest the growth is locally more isotropic. Images showing regions near to surface steps display a higher local coverage on the upper terrace than on the lower, suggesting a preferential nucleation of growth at steps on the upper terrace. Near saturation coverage, the K atoms form clear one-dimensional chains along the 〈011〉 direction, similar to those on the K/Si(100) surface. The tunnelling characteristics are observed to change dramatically close to saturation coverage, at which point the surface develops a metallic character.

We present a design for a simple, reliable, and robust storage container suitable for the transportation of silicon crystals between clean room and experiment after hydrogen passivation by a “wet-chemical” process. The container stores the crystal in an inert atmosphere that is depleted of the water and oxygen responsible for surface oxidation. An atomic-force microscopy study of the surfaces of stored crystals confirmed that the storage method was successful and that surface oxidation can be impeded for at least 24 h. Our design is also suitable for the storage of other systems that degrade under atmospheric conditions.

Cleaved rubrene crystals offer reproducible and unoxidized surfaces ideal for investigation of the environmental dependence of the charge transport properties. Using atomic force microscopy (AFM), we found that the ambient environment induces molecular reorganization on a cleaved rubrene surface. Nanoscale beads and molecular fingers are formed on new surfaces exposed to the atmospheric environment. The bead formation is found to be suppressed in the argon environment. Studied by conductive AFM, the beads show insulating behavior. The observations suggest that the bead formation process is related to oxidation and charge transport modification of the rubrene surface.

Strain induced by antiphase boundaries (APBs) in the Si(111) 2 x 1 surface is investigated using scanning tunneling microscopy (STM), laterally resolved scanning tunneling spectroscopy (STS), and density functional theory (DFT). We determine the structure of all identified APB reconstructions and show that a band shift of states close to the Fermi energy leads to the previously observed electronic contrast. The orientation of the band shift and the observed movement of APBs within the surface are explained by surface strain resulting from the excess free energy of the boundary. We demonstrate that the location of APBs and their associated strain can be precisely manipulated, making them an ideal model system to study and control strain at the atomic scale.

We fabricate a one-dimensional constriction in a shallow, δ-doped GaAs/AlxGa1−xAs two-dimensional electron gas, by locally oxidizing the surface using an atomic force microscope. The channel exhibits ballistic conduction with up to seven conductance plateaus, quantized in units of 2e2/h. The dependence of the device conductance on dc bias voltage reveals the energy separation of the first two subbands to be ΔE1,2=5.5(±0.3) meV, which allows ballistic conduction to be observed up to a temperature of 20 K. A lateral electric field, combined with the hard-walled confinement due to the insulating lines, allows manipulation of the electron wave function in a way which is not possible with surface-gated devices.

The adsorption of isolated H atoms on the Ge(001) surface is studied using density functional theory (DFT) and scanning tunneling microscopy (STM). Two stable adsorption positions that are found in DFT correspond to H atom attachment to an up-or down-buckled Ge dimer atom, respectively. Surprisingly, in the case where H bonds to the down-buckled Ge atom, we find that there is a redistribution of a unit of charge which leaves the net charge of the doubly occupied dangling bond of the unreacted Ge atom intact. This configuration is found to be the more stable of the two structures. Comparison to filled- and empty-state STM images confirms that this lowest energy structure is observed at room temperature. These results represent a fundamentally different picture of the physics and chemistry of H adsorption to Ge(001) compared with previous work.