Introduction

Electron diffraction is a common tool for analyzing the surface structures of crystalline materials. The first diffraction patterns using electrons were aquired in 1927, only three years after de Broglie's discovery of the wave nature of matter.^[1]Electron diffraction is like X-ray diffraction in principle, but unlike X-rays, electrons have strong electrostatic interactions with matter. This strong interaction limits the electrons ability to penetrate further than a few atomic layers into the sample, resulting in good surface sensitivity.

Modern electron diffraction has split into a large variety of distinct techniques. They can be classified based on the energy or the application. Common uses of electron diffraction are low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), electron diffraction in transmission electron microscopy (TEM, involving several different diffraction modes), and electron backscatter diffraction (EBSD, often used with Scanning Electron Microscopy). ^[1][2][3]

Low-Energy Electron Diffraction (LEED)

LEED is typically done using accelerating voltages of 20 - 200 V. ^[1]This low energy causes very limited electron penetration into the sample, allowing us to investigate the surface. LEED is therefore a method for studying the crystallography of surfaces or thin films using backscattered electrons.^[2][4]

In LEED, the position and intensity of diffraction peaks is monitored against electron energy. The electron source in LEED usually produces an electron beam of around 1 mm, which hits the sample perpendicularly. Between the electron source and the sample are grids and a phosphor screen. The purpose of the grids is to allow only elastically scattered electrons through. The phosphor screen is kept at a high positive potential to show clear fluorescence when it is struck by electrons. These spots form a periodic diffraction pattern, which can then be analyzed.^[4]

Reflection High-Energy Electron Diffraction (RHEED)

RHEED is used for determining the surface crystal structure, but it can also be used to monitor the growth mode and deposition rate of films. A key difference to LEED is that the electrons strike the sample at grazing angles (0.1 – 5 °). The energy of the electron beam in RHEED is typically 10 – 50 keV. ^[4][5]

The high energy of the beam is enough to penetrate hundreds of nanometers into most materials, but the grazing angle of incidence limits the interaction to the topmost layer of atoms. The scattered electrons are collected on a phosphor screen and form a diffraction pattern.  When monitoring the growth of thin-films, the relative intensity of the diffracted beam can be plotted against time or angle. ^[4][5] 

The ability to monitor the growth of films is based on the low angle of the electron beam. Films that grow layer by layer with a smooth surface will result in a RHEED pattern of lines and streaks ^[4]. Structured surfaces with 3D topography will instead create diffraction spots. This difference can be used to evaluate the growth of a film in situ. RHEED is suited to both low and high pressures^[5].