By Avraham David Sherwood
The Editor in Chief
A new study published in Nature magazine describes research and breakthrough collaboration led by IBM. The solution enables the development of new and improved semiconductor materials.
Nowadays, semiconductors are the cornerstones of the digital and electronic era. They provide us with huge quantities of useful devices in our modern lives, including computers, smartphones and other mobile devices. Improvements in semiconductor functionality and performance enable their next-generation applications for computation, sensing and energy conservation. However, researchers have for many years been faced with the inability to understand how the electronic charges in semiconductor devices and advanced semiconductor materials limit our ability to make further progress.
A new study published in Nature magazine describes IBM-led research and breakthrough collaboration on a problem that has plagued the physics world for some 140 years. The solution enables the development of new and improved semiconductor materials.
In order to truly understand the semiconductor physics we must first know the basic properties of charge carriers of the various materials and to know whether these particles are positive or negative, what their velocity is in a particular electric field and what their density is in a given material. In 1987, physicist Edwin Hall discovered a way by which these properties can be determined and is depicted as a magnetic field diverting the motion of the electronic charge with a conductive material, so the degree of deviation can be measured as a perpendicular to the charge current, as shown in Figure 1a. The Voltage, known as Hall Voltage, removes the mystery from essential information about semiconductor charge carriers, including are they negative or positive electrons or semi-particles called “holes”? What is the velocity of their movement in an electric field or their mobility? (Μ) and what is their density (n) in a semiconductor?
A 140-year-old secret
For decades after Hall’s discovery, other researchers have realized that they can also measure Hall’s effect with light. The experiments they conducted were called light experiments in light and are shown in Figure 1b. In these experiments, the illumination of the semiconductor generates a large number of charge carriers or pairs of “holes”. Understanding the basic Hall effect provided insights only on the positive charge carriers (which constitute most carriers). The researchers were unable to understand the features of the two carriers at the same time. This information is essential for many applications involving light such as solar cells and other optoelectronic devices.
The IBM Research study published in Nature explains one of Hall’s biggest puzzles. Researchers from the Korea Institute for Advanced Science and Technology (KAIST), the Korea Chemical Technology Research Institute (KRICT), Duke University, and IBM have discovered an innovative formula and technique that can simultaneously extract information on the majority and minority carriers, such as density and mobility characteristics, and generate additional insights On the lifetime of the carriers, the length of the diffusion and the process of reunification (recombination)
In the Hall light experiment, both carriers contribute to changes in the conduction (σ) and the coefficient of Hall (H) and are therefore proportional to the Hall voltage relation to the magnetic field. The main insight comes from the conduction measurement and Hall coefficient as a function of light intensity. The transmission curve – Hall coefficient (σ-H), hidden in the transmission path, reveals new information: the difference in the mobility of the two carriers. The study summarizes these relationships elegantly as follows: Δµ = d (σ²H) / dσ.
When the density of the majority carriers is known from traditional Hall measurement in the dark, we can solve both the mobility and the density of the majority and minority carriers as a function of light intensity. The team called the new Carrier-Resolved Photo Hall CRPH measurement technique. When the intensity of the illumination is known, the duration of the subject’s life can similarly be calculated. These relationships and their associated solutions have been hidden for nearly 150 years since the discovery of the Hall effect.
Although extensive theoretical knowledge has been obtained, it is of critical importance to advance the experimental techniques to enable the new technique. The new technique requires clean measurement of Hall signal, which is a challenging task in materials whose weak signal is weak (e.g. due to low mobility) or in the presence of other undesirable signals such as under strong light conditions. To do this, there is a shape to perform the Hall measurement with an oscillating magnetic field (AC). In the same way as listening to the radio, the frequency of the desired station must be selected while rejecting all other frequencies that are noise. The CRPH technique takes us a step further and selects not only the desired frequency but also the phase of the oscillating magnetic field in a technique called “lock in detection”. The concept of AC Hall measurement has long been known, but the traditional technique using an electromagnetic coil system to generate an AC magnetic field has been ineffective.
An impressive discovery that foreshadows the future
What happens in the science world is that discoveries in one field are making progress in another. In 2015, IBM Research reported a physical phenomenon that was unknown until then and called a confinement effect of a magnetic field; This phenomenon was called the camel honey effect. It occurs between two bipolar crossing lines when they exceed critical lengths, as shown in Figure 2a. This effect is a key feature that allows a new type of natural magnetic trap called a parallel bipolar (PDL) line as shown in Figure 2b. The PDL magnetic trap can be used as a new platform for various sensor applications such as tilt meter and seismograph (earthquake sensor).
These new and other sensing systems are being explored by the IBM Research team developing a Big Data analytics platform called the IBM Physical Analytics Integrated Repository Service PAIRS. The same PDL component has another unique application and when rotated it is used as the ideal Hall Experiment light system to achieve strong, unidirectional and harmonically pure oscillation in the magnetic field (Figure c2). In addition, the system provides plenty of space to illuminate a large area on top of the example, which is critical to the Hall light experiment.
The latest development of Hall’s light technique allows us to extract a surprising amount of semiconductor information. Compared to the three parameters produced in classical Hall measurement, the new technique produces up to seven parameters at each light intensity tested. These parameters include the mobility of the electron and the hole, the density of their carriers under light, the lifetime of the recombination, the diffusion lengths of the electrons, holes and the type of ambipolar. All of these can be repeated N times (that is, as a number of degrees of light intensity used in the experiment).
Discovering the new discovery and developing the new technologies will help us advance the semiconductor innovations based on existing and emerging technologies. We now have the knowledge and tools needed to understand the physical properties of semiconductor materials at a more detailed level. To illustrate, we can now accelerate the development of next-generation semiconductor technology such as: improved solar cells, higher quality optoelectronic devices, materials and other devices for artificial intelligence technologies.
 O. Gunawan et al., Carrier-resolved photo-Hall effect, Nature (2019).
 E. H. Hall, On a new action of the magnet on electric currents, Am. J. Math. 2, 287 (1879).
 “A new effect in electromagnetism discovered – 150 years later”, IBM Research Blog, https://www.ibm.com/blogs/research/2017/10/new-effect-electromagnetism, (2017); and references therein.