Photoelectron microscopy and applications in surface and materials science

High-temperature annealing of gallium arsenide in vacuum causes excess evaporation of arsenic, with accumulation of gallium as liquid droplets on the surface. Using real-time in situ surface electron microscopy, we found that these droplets spontaneously run across the crystal surface. Running droplets have been seen in many systems, but they typically require special surface preparation or gradient forces. In contrast, we show that noncongruent evaporation automatically provides a driving force for running droplets. The motion is predicted and observed to slow and stop near a characteristic temperature, with the speed increasing both below and above this temperature. The same behavior is expected to occur during the evaporation of similar III-V semiconductors such as indium arsenide.

https://science.sciencemag.org/content/sci/324/5924/236.full.pdf

Running droplets of gallium from evaporation of gallium arsenide

The imaging of surfaces by means of photoexcitation electron emission microscopy (PEEM) has recently received considerable interest. This is mainly due to the extended use and availability of brilliant synchrotron radiation in the soft x-ray regime which generally facilitates studies with surface specificity and chemical selectivity. The most popular application of the x-ray PEEM (XPEEM) technique concerns studies of magnetic systems and phenomena. By exploiting the high degree of circular or linear polarization of the synchrotron light, the magnetic microstructure in both ferromagnets and antiferromagnets can be visualized. In this contribution we demonstrate the unique potential and the versatility of the PEEM approach, and review the current status with a certain emphasis on experiments with soft x-ray excitation. In some cases, the high-energy excitation studies can be complemented by laboratory experiments employing threshold photoemission with ultraviolet light (UV-PEEM). Current limitations and future developments and perspectives of the PEEM technique applied to magnetic systems are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/65/12/202/pdf

Investigating surface magnetism by means of photoexcitation electron emission microscopy

Space-, time- and spin-resolved photoemission

An ovel instrument for imaging ESCA is described. It is based on a tandem arrangement of two hemispherical energy analysers used as an imaging energy filter. The main spherical aberration (α 2 -term) of the analyser is corrected by the antisymmetry of the tandem configuration. The kinetic energy range useable for imaging extends up to 1.6 keV; this is compatible with Mg and Al Kα laboratory x-ray sources. First experiments on the chemical surface composition of a Cu0.98Bi0.02 polycrystal, a GaAs/AlGaAs heterostructure and Ag crystallites on Si(111) have been performed using synchrotron radiation. The results reveal an energy resolutio no f190 meV and a lateral resolution (edge resolution) of 120 nm. Besides elimination of the analyser’s spherical aberration, the tandem arrangement largely retains the time structure of the electron signal, unlike a singl eh emispherical analyser.

http://iopscience.iop.org/article/10.1088/0953-8984/17/16/004/pdf

Nanoelectron spectroscopy for chemical analysis: a novel energy filter for imaging x-ray photoemission spectroscopy

Measurements performed in an electron microscope with the mirror operation mode are most sensitive to local electric fields and geometrical roughness of any kind of the object being studied. The object with a geometrical relief is equivalent to a smooth surface with an effective distribution of microfields. Electrons forming the image interact with the local microfields for an extended time: during approach to the object, deceleration and acceleration away from the object. As a result, the electron trajectories can be strongly distorted, and the contrast changes essentially, leading to image deformation of details of the object under investigation and to lowering of the resolution. These effects are theoretically described and are illustrated by experiments. An analysis of these effects enables the real size and the shape of the object involved to be reconstructed.

Peculiarities of imaging one‐ and two‐dimensional structures using an electron microscope in the mirror operation mode

In the present paper a general analytic expression has been obtained and confirmed by a computer simulation which links the surface roughness of an object under study in an emission electron microscope and it's resolution A quantitative derivation was made for the model case when there is a step on the object surface It was shown that the resolution is deteriorated asymmetrically relative to the step The effect sets a practical limit to the ultimate lateral resolution obtainable in an emission electron microscope

Resolution deterioration in emission electron microscopy due to object roughness

Under investigation by emission electron microscopy, the shape and size of three-dimensional objects are distorted because of the appearance of a characteristic potential relief and a possible contact potential difference between the particles and the substrate. An estimation of these effects for spherical particles is made. It is shown that the apparent size of particles observed in an emission electron microscope (EEM) could be increased as well as decreased depending on the relation between the work functions of the particle and the substrate. The corresponding formulae are given and several possibilities are shown which permit us to determine from the EEM image the real size of particles and their work function relative to the substrate.

Imaging of three-dimensional objects in emission electron microscopy.

An emission electron microscope without restriction of the electron beams was used to visualize and measure the distribution of electric fields and potentials on the surface under study. Investigations of this kind can be performed in an emission electron microscope without any aperture diaphragm. The potentialities of this method have been demonstrated using measurements with a silicon p-n junction to which a voltage has been applied in the reverse direction. The quantitative analysis becomes more complicated if the specimen is characterized by a heterogeneous intensity distribution of the electron emission from different areas of its surface. In the latter case two images obtained at different accelerating voltages (i.e. different voltages of the microscope extractor) provide the information necessary for an analysis of electric field and potential distributions.

Measurement of the electric field distribution and potentials on the object surface in an emission electron microscope without restriction of the electron beams

An emission electron microscope was used for visualization and measurement of the distribution of electric fields and potentials on the surface under study. The contrast of microfields is caused by the fact that slow-moving electrons emitted from the object surface are deflected by these fields. The measurements were performed on a p-n junction to which a voltage was applied. It is shown that the type of contrast from the p-n junction can be reversed depending on the position of the contrast aperture restricting the electron beam. The same result was obtained by means of a computer simulation.

N.N. Sedov

Papers

Peculiarities of imaging one‐ and two‐dimensional structures using an electron microscope in the mirror operation mode

Resolution deterioration in emission electron microscopy due to object roughness

Imaging of three-dimensional objects in emission electron microscopy.

Measurement of the electric field distribution and potentials on the object surface in an emission electron microscope without restriction of the electron beams

Use of emission electron microscope for potential mapping in semiconductor microelectronics.