Silicon is such an important substance that people named a valley after it-Silicon Valley. But this is not surprising for silicon, because today, the highly developed technology of human beings is based on silicon.
Silicon is a semiconductor material with conductivity between metal and insulator. In a computer chip, applying a small voltage can make silicon switch between conductive state and insulating state, generate binary digital information of "0" and "1", and then complete the logical operation. This control of current, coupled with high stability and reliability, has made silicon materials occupy a dominant position in the electronics industry for more than 60 years. Whether it's a smart phone or a pacemaker, the chips inside are made of silicon, and the emerging solar energy industry also uses silicon as the main material because it can convert light into electricity.
The demand for silicon is so great that you may have an illusion that the above characteristics of silicon are unique, but this is not the case. For example, germanium is also a common semiconductor material. In addition, there are gallium arsenide, gallium phosphide, cadmium sulfide, zinc sulfide and other semiconductor materials. Silicon's status today is mainly due to its quantity, not its performance. As the second most abundant element on earth, the low cost of silicon makes human beings very "useful".
However, the atomic structure of silicon limits its ability to conduct electrons, and the utilization of its electrical properties has reached its peak. Now a top-level high-end chip can integrate 5 billion transistors, which are the basic units for controlling current switches. This is close to the upper limit of integral. If you try to add more transistors, the heat generated by defects in silicon materials will reduce the efficiency of the chip, which is the main reason for the stagnation of processor speed in the past decade. If electronic devices want to be faster, cheaper and more compact, silicon as we know it today is likely to be rejected.
In terms of solar cells, the prospect of silicon is actually not very good. Silicon is less likely to absorb light because it is an indirect band-gap semiconductor material (semiconductors can be divided into direct band-gap semiconductors and indirect band-gap semiconductors, the former can directly perform photoelectric conversion, and the latter needs phonons, that is, the energy of lattice vibration, for photoelectric conversion). Therefore, the efficiency of traditional silicon solar cells is very low.
Where are the substitutes?
In order to make a substantial breakthrough, many elements and compounds that can replace silicon have been proposed. As for the material of computer chips, it has been proposed to replace silicon with graphene. This material is tougher than steel, and the transmission speed of electrons on its surface is much faster than that of silicon. However, graphene is difficult to produce on a large scale and lacks one of the most critical characteristics-band gap. The band gap allows the semiconductor device to turn off and perform a "logic" operation. So as far as logical application is concerned, graphene can be said to be hopeless. As for the new materials of solar cells, people have tried some compounds with direct band gaps, such as cadmium telluride and gallium arsenide, but these materials either contain rare and expensive elements or highly toxic heavy metals, which will cause great damage to the environment.
After trying again and again, scientists couldn't find a perfect substitute, so they came back with a new idea: maybe, to solve the problem, we need to make a fuss about silicon. After all, silicon is a mature material, which is non-toxic. People have equipped it with many industrial facilities, as long as we can turn it into "new silicon" with the best characteristics of other materials. Facts have proved that this transformation is scientifically possible. An element can be very different according to the arrangement of its atoms, such as graphene, which is a two-dimensional lattice of carbon. The arrangement of atoms has changed, and the same atoms can also become dazzling diamonds.
Si24 with direct band gap
Some scientists are already implementing this idea, such as Timothy Strobel of the Carnegie Institution in Washington. In 20 14, he and his colleagues announced the manufacture of a new type of silicon-si24, which can become a direct band gap only by atomic contraction.
In fact, the discovery of Si24 was just an accident. Strobel and his colleagues compressed silicon and sodium together to make a shiny blue crystal Na4Si24, and then wanted to measure the resistance of this composite crystal. To measure the resistance, it is necessary to glue the electrode to the crystal and heat it. Strobel found that when heated to 40℃, sodium ions in the crystal began to escape, and the electrical characteristics of the crystal also changed. This is an unexpected result. Usually, silicon compounds will form a cage lattice, and smaller sodium ions will be trapped in the lattice of Si and cannot escape at very high temperature. Na4Si24 does not form a cage lattice, but a corridor lattice. When the temperature rises, sodium ions can easily escape. When heated to 100℃, the content of sodium per 1000 atoms should not exceed one. If the temperature rises a little, a truly new type of silicon, Si24, will be made.
Although in essence, Si24 is still considered as an indirect bandgap semiconductor, electrons can jump directly and perform photoelectric conversion as long as a small stress is applied, such as squeezing it by 2%. At present, the best silicon solar cells are only 25% efficient (25% of light energy is converted into electricity). Scientists generally believe that the upper limit of solar cell efficiency is 33%, and Si24 can make the efficiency of solar cells closer to this upper limit or even higher.
BC8 releases electrons.
The upper limit of 33% is based on the assumption that every incident photon will excite a free electron that can conduct electricity. In fact, if the microscopic quantum effect is considered, some materials may excite multiple free electrons at a time, and a silicon nanoparticle named BC8 can generate multiple free electrons based on a single photon.
The researchers simulated the behavior of BC8 with the help of a supercomputer at Lawrence Berkeley National Laboratory. This silicon structure is formed under high pressure, but it is also stable under normal pressure. The simulation results show that silicon BC8 nanoparticles do generate multiple electron-hole pairs based on a single photon. Using BC8 can improve the efficiency of solar cells to 42%, exceeding the upper limit of 33%, which is of great significance. In addition, if the parabolic reflector is used to collect sunlight for new solar cells, the efficiency of BC8 can even reach 70%.
Unfortunately, BC8 can only work under ultraviolet radiation, but it can't work normally under visible light radiation when combined with traditional silicon nanoparticles. However, scientists from Harvard University published a paper pointing out that when ordinary silicon solar cells are irradiated by laser, the energy emitted by laser is enough to generate local high voltage and form silicon BC8 nanocrystals. Therefore, if the existing solar cells are put under pressure or irradiated with laser, the efficiency of solar cells may be improved-Strobel has been testing this idea.
Almost perfect silylene
In the field of computer chips, another new type of silylene has been making breakthroughs. Silicone resin is a kind of graphene-like material, which is composed of silicon with single atomic thickness. Silene, like graphene, has extraordinary conductivity, but it has a band gap, which can theoretically realize logical operation.
2065438+In February 2005, Deki Akinwand of the University of Texas announced that they had made the first silicon transistor. This caused an uproar, because silylene was extremely difficult to prepare. Unlike graphene, this material can be peeled off from large graphite layer by layer with adhesive tape. In order to obtain silylene, scientists need to heat silicon in vacuum and then deposit steam on silver blocks, which is a very complicated process. In addition, a single siloxane is extremely unstable in air. Even up to 20 14, some scientists are still questioning whether silene really exists.
Although the technology of the silylene transistor made by Akinwand is ingenious, it cannot be applied to practice in a short time, because the exposed silylene is destroyed in two minutes. If you want to prolong the life of the silicon transistor, you must add some protective layers to it. However, this breakthrough encourages scientists to try the technology of manufacturing silicon-based transistors in various ways, which will lead to a revolution in the electronics industry. After all, the transmission speed of electrons on the surface of silene is 654.38+0 million times faster than that of ordinary silicon, and the collision between high-speed electrons is greatly reduced, thus reducing the heat generated by dense chips and making electronic devices smaller. In addition, transistors made of silene will be very thin. Scientists are also considering adding BC8 and Si24 to future electronic products, and integrating optical and electronic devices into a single chip. This hybrid chip can use light and electrons to transmit signals, which greatly improves the speed and the amount of data that can be carried. Therefore, in the future, silicon will have a fantastic prospect and continue to occupy the center of the electronic industry stage.
(This article is from the third issue of big science and technology * scientific mystery 20 16)