Low energy electron microscopy at cryogenic temperatures.

We describe a cryogenic sample chamber for low energy electron microscopy (LEEM), and present first experimental results. Modifications to our IBM/SPECS aberration-corrected LEEM instrument are presented first. These include incorporation of mechanisms for cooling the sample and its surroundings, and reduction of various sources of heat load. Using both liquid nitrogen and liquid helium, we have reached sample temperatures down to about 15 K. We also present first results for low-temperature LEEM, obtained on a three-monolayer pentacene film. Specifically, we observe a reduction of the electron beam irradiation damage cross-section at 15 eV by more than a factor of five upon cooling from 300 K down to 52 K. We also observe changes in the LEEM-IV spectra of the sample upon cooling, and discuss possible causes.

Key Role of Very Low Energy Electrons in Tin-Based Molecular Resists for Extreme Ultraviolet Nanolithography.

Extreme ultraviolet (EUV) lithography (13.5 nm) is the newest technology that allows high-throughput fabrication of electronic circuitry in the sub-20 nm scale. It is commonly assumed that low-energy electrons (LEEs) generated in the resist materials by EUV photons are mostly responsible for the solubility switch that leads to nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine LEE microscopy (LEEM), electron energy loss spectroscopy (EELS), and atomic force microscopy (AFM) to study changes induced by electrons in the 0-40 eV range in thin films of a state-of-the-art molecular organometallic EUV resist known as tin-oxo cage. LEEM-EELS uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM postexposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer because of carbon loss. Remarkably, electrons with energies as low as 1.2 eV can induce chemical reactions in the Sn-based resist. Electrons with higher energies are expected to cause electronic excitation or ionization, opening up more pathways to enhanced conversion. However, we do not observe a substantial increase of chemical conversion (densification) with the electron energy increase in the 2-40 eV range. Based on the dose-dependent thickness profiles, a simplified reaction model is proposed where the resist undergoes sequential chemical reactions, first yielding a sparsely cross-linked network and then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm3 per incident electron in the energy range studied, which means that about 10 LEEs per molecule on average are needed to turn the material insoluble and thus render a pattern. Our observations are consistent with the observed EUV sensitivity of tin-oxo cages.

Growing a LaAlO3/SrTiO3 heterostructure on Ca2Nb3O10 nanosheets.

The two-dimensional electron liquid which forms between the band insulators LaAlO3 (LAO) and SrTiO3 (STO) is a promising component for oxide electronics, but the requirement of using single crystal SrTiO3 substrates for the growth limits its applications in terms of device fabrication. It is therefore important to find ways to deposit these materials on other substrates, preferably Si, or Si-based, in order to facilitate integration with existing technology. Interesting candidates are micron-sized nanosheets of Ca2Nb3O10 which can be used as seed layers for perovskite materials on any substrate. We have used low-energy electron microscopy (LEEM) with in-situ pulsed laser deposition to study the subsequent growth of STO and LAO on such flakes which were deposited on Si. We can follow the morphology and crystallinity of the layers during growth, as well as fingerprint their electronic properties with angle resolved reflected electron spectroscopy. We find that STO layers, deposited on the nanosheets, can be made crystalline and flat; that LAO can be grown in a layer-by-layer fashion; and that the full heterostructure shows the signature of the formation of a conducting interface.

Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate.

For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.

Reprint of Low-energy electron potentiometry.

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

Low-energy electron potentiometry.

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy.

High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the 'chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of.

Nanoscale measurements of unoccupied band dispersion in few-layer graphene.

The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only.

Robust graphene membranes in a silicon carbide frame.

We present a fabrication process for freely suspended membranes consisting of bi- and trilayer graphene grown on silicon carbide. The procedure, involving photoelectrochemical etching, enables the simultaneous fabrication of hundreds of arbitrarily shaped membranes with an area up to 500 µm(2) and a yield of around 90%. Micro-Raman and atomic force microscopy measurements confirm that the graphene layer withstands the electrochemical etching and show that the membranes are virtually unstrained. The process delivers membranes with a cleanliness suited for high-resolution transmission electron microscopy (HRTEM) at atomic scale. The membrane, and its frame, is very robust with respect to thermal cycling above 1000 °C as well as harsh acidic or alkaline treatment.

Electron-electron interaction in the magnetoresistance of graphene.

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.

Bottom-gated epitaxial graphene.

High-quality epitaxial graphene on silicon carbide (SiC) is today available in wafer size. Similar to exfoliated graphene, its charge carriers are governed by the Dirac-Weyl Hamiltonian and it shows excellent mobilities. For many experiments with graphene, in particular for surface science, a bottom gate is desirable. Commonly, exfoliated graphene flakes are placed on an oxidized silicon wafer that readily provides a bottom gate. However, this cannot be applied to epitaxial graphene as the SiC provides the source material out of which graphene grows. Here, we present a reliable scheme for the fabrication of bottom-gated epitaxial graphene devices, which is based on nitrogen (N) implantation into a SiC wafer and subsequent graphene growth. We demonstrate working devices in a broad temperature range from 6 to 300 K. Two gating regimes can be addressed, which opens a wide engineering space for tailored devices by controlling the doping of the gate structure.

Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide.

Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room temperature. It has the potential for technological applications as a successor of silicon in the post Moore's law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices. However, vacuum decomposition of SiC yields graphene layers with small grains (30-200 nm; refs 14-16). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach mu=2,000 cm (2) V(-1) s(-1) at T=27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.