A group of researchers from IBM and KAIST has reported a solution to a 140-year-old mystery in physics. The study published in Nature last month unravels the physical properties of semiconductors in considerably greater detail. The study also helps to develop new and enhanced semiconductor materials.
Researchers led by Professor Byungha Shin from the Department of Material Sciences and Engineering and Dr Oki Gunawan at IBM developed a new formula and method that allows both minority and majority carrier information such as their mobility and density to be simultaneously extracted. The method also enables gaining more understanding about diffusion lengths, carrier lifetimes, and the recombination process.
This new discovery and technology will enable semiconductor progress in both current and evolving technologies. Being the fundamental building blocks of the present-day digital electronics age, semiconductors offer us a multitude of devices that are advantageous to modern life.
If the physics of semiconductors must be truly appreciated, it is highly vital to gain insights into the basic properties of the charge carriers within the materials, whether the particles are positive or negative, how densely they are packed within the material, and their speed under the influence of an applied electric field.
In 1879, Edwin Hall, a physicist, discovered a technique to identify those properties, upon finding that a magnetic field will deflect the movement of electronic charges within a conductor, and that it is possible to quantify the amount of deflection as a voltage perpendicular to the flow of the charge.
Several years after Hall’s discovery, scientists also identified that the Hall effect can be measured with light through “photo-Hall experiments.” While performing such experiments, the light produces multiple carriers or electron–hole pairs in the semiconductors.
However, the basic Hall effect only offered an understanding of the dominant charge carrier, or the majority carrier. Scientists were not able to identify the properties of both the carriers (the minority and majority carriers) at the same time. The information related to the property of both the carriers is vital for several applications that make use of light, for example, solar cells and other optoelectronic devices.
In the photo-Hall experiment performed by the KAIST-IBM researchers, both carriers contribute to variations in the Hall coefficient and conductivity. The main understanding comes from quantifying the Hall coefficient and conductivity as a function of light intensity.
The Hall coefficient curve, which is hidden in the trajectory of the conductivity, unravels vital new information: the mobility of both the carriers differs. This relationship can be neatly expressed as Δµ = d (σ2H)/dσ.
The researchers solved this equation for the mobility and density of both the minority and majority carriers as a function of light intensity. They named the new method Carrier-Resolved Photo Hall (CRPH) measurement. If the light illumination intensity is known, the carrier lifetime can be determined in a similar manner.
Apart from achieving progress in theoretical understanding, advances in experimental methods were also crucial to achieve this breakthrough. The method necessitates a clean Hall signal measurement, which could be difficult for materials for which the Hall signal is weak when additional undesirable signals exist (for example, under powerful light illumination) or due to low mobility.
The newly devised photo-Hall method enables a remarkable amount of information to be extracted from semiconductors. When compared to the classic Hall measurements from which only three parameters can be obtained, this new method yields up to seven parameters at each tested light intensity level.
These include the mobility of both the hole and the electron, their carrier density under light, the recombination lifetime, and the diffusion lengths for holes, electrons, and ambipolar types. It is possible to repeat all of these N times (or the number of light intensity settings used in the experiment).
This novel technology sheds new light on understanding the physical characteristics of semiconductor materials in great detail.
Byungha Shin, Professor, Department of Material Sciences and Engineering, KAIST
According to Dr Gunawan, “This will help accelerate the development of next-generation semiconductor technology such as better solar cells, better optoelectronics devices, and new materials and devices for artificial intelligence technology.”