New study reveals a deeper insight into common imperfections found in transition-metal dichalcogenides (TMDs), which are being considered as a potential substitute for silicon in computer chips. The research also sets the groundwork for creating smaller features through etching.
Silicon computer chips have been a staple for over 50 years. The current smallest features on chips are about 3 nanometers, which is incredibly tiny compared to a human hair’s width of roughly 80,000 nanometers. Shrinking the features on chips is crucial to keep up with the increasing demand for more memory and processing power in handheld devices. However, we are nearing the limits of what can be achieved with traditional materials and methods.
Researchers at the Princeton Plasma Physics Laboratory (PPPL), under the U.S. Department of Energy, are leveraging their skills in physics, chemistry, and computer simulations to drive the development of the next wave of computer chips. Their focus is on processes and materials that will enable the production of chips with smaller features.
“Most of our current electronic devices rely on silicon-based chips, which are three-dimensional. Now, there is a growing interest in two-dimensional chips made from 2D materials,” explained Shoaib Khalid, an associate research physicist at PPPL. Although these materials technically exist in three dimensions, their extreme thinness – often just a few atomic layers thick – has led to their classification as 2D materials.
Khalid, in collaboration with PPPL’s Bharat Medasani and Anderson Janotti from the University of Delaware, delved into a potential alternative to silicon: transition-metal dichalcogenides (TMD). Their latest study, published in the journal 2D Materials, explores the structural variations that can occur in TMDs, the reasons behind these variations, and their impact on the material. This knowledge is crucial for refining the manufacturing processes required to produce next-gen computer chips. The ultimate aim is to develop plasma-based manufacturing systems capable of fabricating TMD-based semiconductors with precise specifications for various applications.
TMD: A minuscule metal layer cake
A TMD can be as thin as three atoms high, resembling a tiny metal layered cake. The “bread” comprises a chalcogen element (oxygen, sulfur, selenium, or tellurium), while the filling consists of a layer of transition metal drawn from groups 3 to 12 in the periodic table.
In its bulk form, a TMD spans five or more atomic layers arranged in a crystal lattice. Ideally, the atoms should form a consistent and precise pattern throughout the lattice. However, minor deviations, termed defects, can be found, like a missing atom in the pattern or an atom in an unexpected position. These defects, though seemingly errors, can actually influence the material positively.
Some defects in TMDs, for instance, can enhance the material’s electrical conductivity. Whether beneficial or detrimental, it is essential for scientists to comprehend why defects arise and how they impact the material to either integrate or eliminate them as necessary. Understanding these common defects also aids researchers in interpreting outcomes from prior TMD experiments.
“When bulk TMDs are produced, they exhibit an excess of electrons,” noted Khalid, highlighting the mystery around the presence of these surplus negatively charged particles. “Through this study, we elucidate that hydrogen could be the cause of these excess electrons.”
The team reached this insight by calculating the energy needed to create various TMD defect types. They explored defects related to chalcogen vacancies, already observed in TMDs, as well as defects associated with hydrogen due to its common presence during chip manufacturing. Researchers are particularly interested in identifying defects with low formation energy as these are more likely to occur, requiring minimal energy input!
The researchers then probed the influence of each low-formation-energy defect on the material’s electrical charge status, particularly focusing on how each defect arrangement could alter the material’s charge. They discovered that one defect configuration involving hydrogen produces excess electrons, resulting in negatively charged semiconductor material, known as n-type material. Computer chips are typically crafted using combinations of n-type and positively charged (p-type) semiconductor materials.
Illuminating the Absence of Chalcogens
The paper also delves into another defect type known as a chalcogen vacancy, where an oxygen, sulfur, selenium, or tellurium atom is missing, depending on the TMD type. The researchers aimed to elucidate the outcomes of previous experiments conducted on bulk TMD molybdenum disulfide flakes. These experiments, involving light exposure on the TMD, unveiled unexpected light frequencies emitted by the material. The researchers linked these unexpected frequencies to electron movements associated with the chalcogen vacancy.
“This defect is quite common and can often be observed through scanning tunneling microscope images when growing TMD films,” Khalid explained. “Our research outlines a method to investigate the presence of these vacancies in bulk TMDs. We clarified past experimental findings in molybdenum disulfide and extended the same predictions to other TMDs.”
The procedure recommended by the researchers involves defect analysis in TMDs using photoluminescence measurement techniques to identify the emitted light frequencies. The peak frequency of light emissions can be used to determine the electron configurations within the TMD atoms and ascertain the presence of chalcogen defects. The study provides insights into the light frequencies emitted by five TMD types with chalcogen vacancies, including molybdenum disulfide, thereby serving as a guideline for future investigations into chalcogen vacancies.
