Introduction to Electron Beam Induced Current (EBIC)
The principle operating of electron beam induced current (EBIC) is to locally create electron hole pairs in a material using the beam of an electron beam. These electron and hole have different lifetimes and mobilities in a material.
When measuring the EBIC signal we are measuring the contribution of each of hole and electron components that make up the current signal that flows through a devices by completing a circuit. As the electron beam is moved you form an image of the response at each point.
The signal difference can be the result of junctions with electric fields and different energy levels, doping levels and impurities in a device, from crystalline defects or dangling bonds on a surface or interface.
The decay of a signal away from a junction or interface can measure key semiconductor device parameters such as diffusion lengths of minority carriers, relaxation times, and image the extent of depletion regions. Electron ands holes are highly sensitive to local differences in E-fields in a material and are excellent at characterizing crystalline defects which can locally alter generation and recombination rates. Since the technique was popularized in the early 1980’s, EBIC has been used to characterize a wide range of semiconductor materials and devices.
Recent developments have pushed the detection levels and resolution of EBIC images to the point where this technique can be used to investigate nano-electronic devices and a wider range of materials beyond traditional semiconductors.
Signals from Primary E-beam
When the electron beam impinges on a semiconductor material it release a substantial amount of energy. This energy can release characteristic X-rays that reveal elemental composition. Backscatter electrons that scatter off of the nucleus of the atoms and are sensitive to atomic weight. Secondary electrons are electrons excited in the materials that have enough energy to become released and travel freely in vacuum and are the primary imaging mode in SEMs.
The incident electron beam generates a large number electron-hole pairs in semiconductors in a spatially limited generation volume dependent on the incident energy of the beam and the scattering inside of the material. These electron-hole pairs can either recombine and release energy, radiative and give off light (cathode-luminescence), non-radiatively (generating heat) , or they can result in current following through the material (EBIC). The information that can be obtained changes with the device configuration and the circuit you connect it to.
Probing of Semiconductors
Locally a high number of electron hole pairs are generated in a small volume. Typically the carrier concentration of delta p and delta n is determined by the generation rate, and the recombination rate. The interaction volume is usually thought of as a teardrop shape that is dependent on the Z contrast, atomic weight of the material, its density and the energy of the incident electron beam.
To get a finer detail of this interaction volume, Monte Carlo simulations (Casino 3.2) of electron flight path and scattering events. Estimating the number of carriers generated in a volume can be calculated by taking the energy dispersed and dividing it by 3Eg, which is the mean excitation energy.
Depletion regions are readily imaged by EBIC due to efficient carrier collection and separation of generated electron hole pairs form the built-in electric field. On either side of a P-N junction the minority carrier diffusion lengths can be directly measured.
The study of defects and their properties is one of the widest used applications of EBIC. At crystalline defects, there are several factors that can cause contrast in charge collection. These sites can have states that exist in the middle of the band gap and can be a site of both recombination and generation. If the defect is charged it can locally bend bands around it this can lead to either a greater recombination or generation rates.
To aid in the contrast and detection limit, samples are made with a thin Au coating to create a Schottky junction or use an existing P-N junction, that can increase the collection efficiency. Defects appear as a site of recombination with dark contrast when compared to the surrounding signal from the junction.
Electron Beam Absorbed Current
Most EBIC measurements are conducted with two contacts one going to ground and the other going to a transimpedance amplifier that acts as a virtual ground. In this configuration the electrons and holes can be efficiently separated. If there is only one contact to the transimpedance amplifier, electron beam absorbed current (EBAC),then you are at not measuring carrier separation but the local differences between generation and recombination rates. The transimpedance amplifier will source or sink current to maintain charge neutrality. This is of interest because defects can be imaged outside of depletion regions without modifying the sample.
EBAC has mainly been used by industry for fault analysis of conductive lines in semiconductor integrated circuits. We have greatly expand the capabilities of EBAC that goes well beyond current uses and can image crystal domains in a wide range of semiconductor materials. EBAC can be sensitive to the anisotropy in crystal structures and has similar contrast to bright field TEM measurements.