Observation of domain morphology in twisted antimonene layers via moiré superlattice contrast with low energy electron microscopy.

Can low energy (1-20 eV) electron microscopy produce damage-free images of biological samples?

Low-Energy Electron Microscopy

This document is the final SAND Report for the LDRD Project 105877 - 'Novel Diagnostic for Advanced Measurements of Semiconductor Devices Exposed to Adverse Environments' - funded through the Nanoscience to Microsystems investment area. Along with the continuous decrease in the feature size of semiconductor device structures comes a growing need for inspection tools with high spatial resolution and high sample throughput. Ideally, such tools should be able to characterize both the surface morphology and local conductivity associated with the structures. The imaging capabilities and wide availability of scanning electron microscopes (SEMs) make them an obvious choice for imaging device structures. Dopant contrast from pn junctions using secondary electrons in the SEM was first reported in 1967 and more recently starting in the mid-1990s. However, the serial acquisition process associated with scanning techniques places limits on the sample throughput. Significantly improved throughput is possible with the use of a parallel imaging scheme such as that found in photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM). The application of PEEM and LEEM to device structures relies on contrast mechanisms that distinguish differences in dopant type and concentration. Interestingly, one of the first applications of PEEM was a study ofmore » the doping of semiconductors, which showed that the PEEM contrast was very sensitive to the doping level and that dopant concentrations as low as 10{sup 16} cm{sup -3} could be detected. More recent PEEM investigations of Schottky contacts were reported in the late 1990s by Giesen et al., followed by a series of papers in the early 2000s addressing doping contrast in PEEM by Ballarotto and co-workers and Frank and co-workers. In contrast to PEEM, comparatively little has been done to identify contrast mechanisms and assess the capabilities of LEEM for imaging semiconductor device strictures. The one exception is the work of Mankos et al., who evaluated the impact of high-throughput requirements on the LEEM designs and demonstrated new applications of imaging modes with a tilted electron beam. To assess its potential as a semiconductor device imaging tool and to identify contrast mechanisms, we used LEEM to investigate doped Si test structures. In section 2, Imaging Oxide-Covered Doped Si Structures Using LEEM, we show that the LEEM technique is able to provide reasonably high contrast images across lateral pn junctions. The observed contrast is attributed to a work function difference ({Delta}{phi}) between the p- and n-type regions. However, because the doped regions were buried under a thermal oxide ({approx}3.5 nm thick), e-beam charging during imaging prevented quantitative measurements of {Delta}{phi}. As part of this project, we also investigated a series of similar test structures in which the thermal oxide was removed by a chemical etch. With the oxide removed, we obtained intensity-versus-voltage (I-V) curves through the transition from mirror to LEEM mode and determined the relative positions of the vacuum cutoffs for the differently doped regions. Although the details are not discussed in this report, the relative position in voltage of the vacuum cutoffs are a direct measure of the work function difference ({Delta}{phi}) between the p- and n-doped regions.« less

Droplet epitaxy is a recently developed variant of molecular beam epitaxy (MBE) which is used to form compound semiconductor quantum structures. This usually involves the deposition of group III material liquid droplets on a substrate followed by crystallisation of the droplets under As flux. The electronic properties of quantum structures depend sensitively on the size and shape of nanoscale rings formed in the crystalisation process and so it is important to understand how to control the formation of these features for potential device applications. A significant difficulty in studying quantum structure formation is the presence of the large As flux which limits real-time studies using conventional techniques such as scanning tunnelling microscopy (STM). The first aim of this thesis is therefore to develop a III-V low energy electron microscope (LEEM) system for in situ, real time imaging of droplet epitaxy. The thesis begins with the development of the III-V LEEM system and its application to study Langmuir evaporation of GaAs (Chapters 2 and 3). We find the III-V LEEM can achieve in situ, real time observation of droplet nucleation, evolution, motion and coalescence. This establishes the feasibility of studying droplet epitaxy using III-V LEEM. As a prelude to studying droplet epitaxy, in Chapter 4 we consider the thermodynamics of Ga droplet formation during Langmuir evaporation of GaAs (001). The congruent evaporation temperature Tc plays a critical role in this process. Below Tc, Ga and As evaporate from the surface at equal rates, preserving substrate stoichiometry. However, above Tc, As evaporates more rapidly than Ga leaving behind Ga-rich droplets on the surface. At Tc, the droplets are stable and neither shrink nor grow, which provides an accurate measure of Tc. In Chapter 5 we apply this condition for droplet stability to experimentally measure Tc in the presence of As flux using III-V LEEM. This dependence is explained by modifying the thermodynamic model for evaporation to incorporate As flux. This work provides a method of controlling congruent evaporation which is important for MBE growth, droplet epitaxy, surface preparation and modifying droplet motion. The creation of droplets above Tc during Langmuir evaporation provides a potential means of self-assembling droplet arrays for subsequent crystallisation under As flux. It is therefore important to understand how the droplet size distribution evolves with time during this process. This is considered in Chapters 6 and 7 where we apply real-time surface electron microscopy to make movies of how droplet arrays evolve. Surprisingly, new Ga droplets are seen to form in regions cleared by the coalescence of larger droplets. A simple Monte Carlo model incorporating daughter droplet generation by coalescence is used to reproduce and explain the major features of our experimental droplet size distributions. In Chapter 8 we study droplet epitaxy of GaAs in real-time using surface electron microscopy which provides new insights into the dynamics of Ga droplet crystallisation under As flux. The resulting movies are used as the basis of a theoretical model for quantum ring formation which can qualitatively explain the origin of quantum features observed under a variety of experimental conditions. The model predicts that Ga adatom diffusion, under differing conditions of temperature and As flux, chiefly controls the quantum structure morphology. Local droplet etching (LDE) has received significant attention over recent years as a means of fabricating nanoscale holes in semiconductor surfaces. The technique offers the significant advantage that it avoids the need for lithographic processes and can be applied to a wide range of materials. In Chapter 9 we utilise III-V LEEM to study the time-evolution of Ga droplet etching of GaAs under As flux. A theoretical model of the etching process is developed from the movies which requires a minimum number of assumptions and is simply based on the liquid droplet maintaining a composition close to its equilibrium liquidus value. Conclusions and further work is considered in Chapter 10.

Low Energy Electron Microscopy and Normal Incidence Vleed

GaAs based communication and optoelectronic devices are widely used in our daily lives. Applications range from mobile phones and satellite communications to laser pointers, printers, barcode readers and DVD players. The components making up these devices are composed of thin layers of III-V semiconductor material, the thicknesses of which must be finely controlled. This is achieved by growing such layers via molecular beam epitaxy (MBE) with atomic layer precision. In addition to current technologies, the long-term research objective in III-V materials is to utilise variants of MBE to fabricate new quantum structures of nanoscale dimensions for new device applications. However, despite this current and future technological importance, the real-space imaging of III-V MBE surface growth dynamics is restricted by the presence of large incident As flux which restricts the use of conventional imaging techniques. To address this issue, the main goal of this thesis is therefore to develop a unique surface electron microscope to study the surface dynamics of III-V materials in real-time during MBE growth. The thesis begins with the design and development of a III-V low energy electron microscope (LEEM). The incorporation of III-V MBE and a high As flux, in particular, required numerous modifications to a commercial LEEM instrument (Elmitec LEEM III). These are described in detail in Chapter 2. Following the development of the LEEM it is important to understand the contrast associated with quantum structure formation. With this in mind, theories of mirror electron microscopy (MEM) were developed and experimentally verified using Ga droplets and their associated surface trails as convenient test objects. This work resulted in the development of the Laplacian and caustic theories of MEM imaging which is fully described in Chapters 3, 4 and 5. Finally, based on the advances in instrumental development and imaging, proof-of-principle applications were undertaken to confirm that III-V and other materials could be investigated under high As flux. These included the control of the GaAs (001) congruent evaporation temperature by As flux (Chapter 6), the asymmetric coalescence of Ga droplets during Langmuir evaporation (Chapter 7), and the dynamic behaviour of As on Si(111) at high temperatures (Chapter 8).

Low Energy Electron Microscopy (LEEM) is a child of surface science and was motivated by the desire to image surfaces with Low Energy Electron Diffraction (LEED) beams in a manner similar to that used in transmission electron microscopy of crystalline specimens which produce transmission diffraction beams. In surface science it is usual to monitor surface changes with surface-sensitive probes and, therefore, LEEM instruments were designed from the very beginning to allow a large variety of in-situ experiments [1]. This chapter will not describe the basics, possibilities and limitations of LEEM and its extensions which are amply discussed in recent reviews [2–4], It will rather focus on the possibilities of LEEM for in-situ studies as illustrated by work of the author and his collaborators.

Surface studies by low-energy electron microscopy (LEEM) and conventional UV photoemission electron microscopy (PEEM)

For surface science, the 1980s were the decade in which the microscopes arrived. The scanning tunneling microscope (STM) was invented in 1982. Ultrahigh vacuum transmission electron microscopy (UHVTEM) played a key role in resolving the structure of the elusive Si(111)-7 × 7 surface. Scanning electron microscopy (SEM) as well as reflection electron microscopy (REM) were applied to the study of growth and islanding. And low-energy electron microscopy (LEEM), invented some 20 years earlier, made its appearance with the work of Telieps and Bauer.LEEM and TEM have many things in common. Unlike STM and SEM, they are direct imaging techniques, using magnifying lenses. Both use an aperture to select a particular diffracted beam, which determines the nature of the contrast. If the direct beam is selected (no parallel momentum transfer), a bright field image is formed, and contrast arises primarily from differences in the scattering factor. A dark field image is formed with any other beam in the diffraction pattern, allowing contrast due to differences in symmetry. In LEEM, phase contrast is the third important mechanism by which surface and interface features such as atomic steps and dislocations may be imaged. One major difference between TEM and LEEM is the electron energy: 100 keV and above in TEM, 100 eV and below in LEEM. In LEEM, the imaging electrons are reflected from the sample surface, unlike TEM where the electrons zip right through the sample, encountering top surface, bulk, and bottom surface. STM and TEM are capable of ~2 Å resolution, while LEEM and SEM can observe surface features (including atomic steps) with -100 Å resolution.

Palladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2–6 monolayers (ML) thick on Ru(0001), and ∼20 ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750 K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880 K, have a relaxed in-plane lattice spacing. All the films present an fcc stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fcc twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps.

The growth of Ge on Ag:Si(111)-√3×√3-R30° has been studied by low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED) and x-ray photoemission electron microscopy (XPEEM). For submonolayer adsorption of Ag at 550°C, the Ag terminated √3×√3-R30° domains decorate the step edges of the substrate. The wetting layer growth and Ge island nucleation on such a step-edge decorated surface is quite similar to Ge growth on bare Si(111)-7×7. During Ge deposition, the √3×√3-R30° domains dissolve and small Ag terminated 3×1 domains are formed that are distributed over the whole surface. Larger 3×1 domains are found only at the circumference of the three-dimensional (3D) Ge islands. From the Ge 3D island morphology, size distribution and density it is concluded that in this submonolayer Ag pre-adsorption scenario there is only little influence of the Ag on the growth kinetics and island geometry. This is completely different for Ge growth on an entirely covered Ag:Si(111)√3×√3-R30° surface. As compared to growth on bare Si(111)-7×7, a strong increase of the diffusion length is observed that leads to a drastic reduction of the island density. Also the island morphology is strongly affected by Ag pre-adsorption in this regime. Instead of triangular islands, we observe huge, irregularly shaped islands that rather resemble a discontinuous Ge film. [DOI: 10.1380/ejssnt.2010.221]

Emission microscopy and related techniques: Resolution in photoelectron microscopy, low energy electron microscopy and mirror electron microscopy

Spherical and chromatic aberration coefficients Cs and C of various immersion lenses forlow voltage SEM and LEEM are calculated. The minimun values of magnetic immersion lens are C5Cl mm. For the combined electrostatic and magnetic lenses, those values are at most C5=1 mm and CO.7 mm, when the specimen is free from the electrostatic field. Whenthe specimen is immersed in the electrostatic field, those values reduce to C50.2 mm and Cc=o.1 mm at 1 kV. Key words: SEM, LEEM, Objective lens, Spherical aberration, Chromatic aberration, Immersion lens, Cathode lens, Computer aided design 1. INTRODUCTION Importance of low accelerating voltage in scanning electron microscopy (SEM) increasesrecently for semiconductor and biological applications. A main difficulty of low voltage SEM(LVSEM) is its low spacial resolution due to the increase of the wave length and the increaseof AVIV0 ratio where AV is the energy spread and V0 the incident energy. The immersionobjective lens as well as the field emission electron gun is very useful in realizing the highresolution LVSEM. Low energy electron microscopy (LEEM) instruments are very useful forsurface studies of metals and semiconductors. The electrostatic immersion objective lensis essentially necessary for LEEM to get a low landing voltage and low aberrations.There are three kind of immersion lenses: the magnetic immersion lens, the retardingelectrostatic immersion lens and the combination of immersion electrostatic and magneticlenses. The latter two lenses are reviewed by Mullerova and Lenc (1992)1. Here we have to

Low energy electron microscopy (LEEM) is a surface imaging technique in which the surface is illuminated by an approximately parallel electron beam at near nonnal incidence. The image is fonned with those electrons which are elas­ tically backscattered into a small angular region around the surface nonnal. The limitation to a small angular region is necessary because of the large aberra­ tions of the objective lens which produces the primary image. This lens is a so-called cathode lens which not only has imaging properties but at the same time decelerates the fast electrons of the illuminating beam to the desired low energy at the specimen and re-accelerates the backscattered electrons to high energies again. In order to achieve this, the specimen is at a high negative po­ tential which differs from the potential of the emitter of the electron gun of the illumination system by Va = Eo/e, where Eo is the energy of the electrons at the specimen. Typical energy values are E = eV = 15-20keV for the fast electrons and 0 < Eo < 50 e V at the specimen. There are three fundamental quantities which are important in LEEM: resolution, intensity and contrast. These will be discussed in Sect. 12.1. Section 12.1 also describes how LEEM can be com­ bined with other surface characterization techniques, such as low energy electron diffraction (LEED), photoemission electron microscopy (PEEM) and other emis­ sion microscopies. Section 12.2 illustrates the applications of LEEM and of the associated techniques to the study of clean surfaces, while Sect. 12.3 presents examples of the power of LEEM in the study of surface layers. Section 12.4 gives an outlook for possible future developments. A brief summary (Sect. 12.5) concludes this chapter.

The Photoelectron Emission Microscope (PEEM) and Low Energy Electron Microscope (LEEM) are parallel-imaging electron microscopes with highly surface-sensitive image contrast mechanisms. In PEEM, the electron yield at the illumination wavelength determines image contrast, in LEEM, the intensity of low energy (&lt; 100 eV) electrons back-diffracted from the surface, as well as interference effects, are responsible for image contrast. Mirror Electron Microscopy is also possible with the LEEM apparatus. In MEM, no electron penetration into the solid occurs, and an image of surface electronic potentials is obtained.While the emission microscope techniques named above are not high resolution methods, the unique contrast mechanisms, the ability to use thick single crystal samples, their compatibility with uhv surface science methods and new material-growth methods, coupled with real-time imaging capability, make them very useful.These microscopes do not depend on scanning probes, and some are compatible with pressures up to 10-3 Torr and specimen temperatures above 1300K.