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The superlattice discovered in 1925: How did this scientific wonder change our understanding?
Since its discovery in 1925, the concept of superlattice has become one of the most important research fields in condensed matter physics and materials science. A superlattice is a periodic structure composed of layers of two or more materials, typically with single layers only a few nanometers thick. The uniqueness of this structure lies in its ability to significantly change the mechanical and electronic properties of the material, giving the superlattice a wide range of potential applications in semiconductors and optical devices.
The discovery of superlattice
The discovery of superlattices can be traced back to 1925, when scientists Johansson and Linde first identified this new type of structure by studying special X-ray diffraction patterns of gold-copper and palladium-copper systems. Subsequently, many researchers such as Bradley and Jay, Gorsky, Borelius, Dehlinger and Graf, Bragg and Williams, and Bethe also conducted experimental observations and theoretical explorations of superlattice, and deeply analyzed the evolution of atoms from a disordered state in the crystal lattice. The process of transforming into an ordered state.
The discovery of superlattice is not only a breakthrough in materials science, but also an important advancement in our understanding of the nature of crystal structure.
Mechanical properties of superlattice
According to the theoretical predictions of J.S. Koehler, shear resistance can be increased up to 100 times by alternating materials with high and low elastic constants to build nanolayers, because in these nanolayers the dislocations of the Frank–Read source cannot operation. In 1978, Lehoczky first confirmed this phenomenon in Al-Cu and Al-Ag systems. Subsequently, Barnett and Sproul et al. also conducted further experimental verification, proving that the mechanical hardness of superlattice materials is significantly enhanced.
Semiconductor Properties
In a semiconductor superlattice, if the materials used have different energy band intervals, each quantum well will establish new selection rules, affecting the flow conditions of charge in the structure. Since Esaki and Tsu proposed synthetic superlattices in 1970, much research has been done on the physics of such ultrafine semiconductors, now known as quantum structures. The concept of quantum confinement allows us to observe quantum size effects in isolated quantum well heterostructures, and this phenomenon is closely related to superlattices through tunneling effects. Therefore, discussions of the two are often based on the same physical principles, but the applications they each address differ in electrical and optical devices.
Types of semiconductor superlattice
The mini-ribbon structure of a semiconductor superlattice depends on the type of heterostructure, including Type I, Type II, and Type III. In the type I structure, the bottom of the conductive band and the top of the valence band subband are located in the same semiconductor layer, while in type II, the arrangement of the conductive and valence band subbands in real space and reciprocal space is intricate, so that the electrons and apertures are Restricted to different layers. Type III superlattice involves semi-metallic materials similar to HgTe/CdTe, which can be continuously adjusted from semiconductor to zero-bandgap material, and even transformed into a semi-metal with a negative bandgap.
Manufacture of superlattice
Currently, there are various manufacturing technologies for superlattice, but the most common methods are molecular beam epitaxy (MBE) and sputtering technology. Using these methods, it is possible to produce layers with a thickness of only a few atomic distances. For example, [Fe20V30]20 describes an alternating layer containing 20Å of iron (Fe) and 30Å of vanadium (V), repeated 20 times, to a final thickness of 1000Å, or 100nm. MBE technology is crucial for the fabrication of semiconductor superlattices.
The uniqueness of superlattice lies in the alternating interaction of the properties and structures of its constituent materials, which can open up the possibility of new applications.
Superlattice in other dimensions
With the popularity of two-dimensional electron gas (2DEG), researchers are trying to create structures that can be called two-dimensional artificial crystals. This structure is achieved by applying an additional modulation potential V(x,y) to the semiconductor boundary. Unlike the case of the classical one-dimensional superlattice mentioned above, this usually needs to be accomplished by depositing a suitable pattern of metal gates on the surface of the heterostructure or performing etching. When the amplitude of V(x,y) is larger, a new superlattice structure can be formed.
These studies show that superlattice is not only a collection of materials, but also a stage for physical phenomena. Future development may change our fundamental understanding of materials and their applications. So, how will the exploration and application of superlattices continue to shape future technology and life?
