How to make boron nitride conduct electricity?

Cubic boron nitride is a promising semiconductor material. By adding different impurities in raw materials, doping in high-pressure synthetic raw materials or through vacuum diffusion after synthesis, the conductivity of boron nitride can be effectively changed.

1. Classification and properties of boron nitride

Boron nitride is a crystal composed of nitrogen atoms and boron atoms. In addition to the common hexagonal boron nitride (white graphite), there are cubic boron nitride (CBN), rhombohedral boron nitride (RBN), wurtzite type Boron nitride (WBN) and other variants, scientists have even discovered two-dimensional boron nitride crystals with similar properties to graphene. The boron nitride that is often said generally refers to cubic boron nitride or hexagonal boron nitride.

1.1 Hexagonal boron nitride

It has a layered crystal structure similar to graphite, and its physical and chemical properties are also similar to graphite. It is white powder in normal state, and it is loose, lubricated, easy to absorb moisture, and light in weight. In addition, it is similar to graphite in terms of thermal conductivity, high temperature resistance, and chemical stability. Of course, its properties are not the same as graphite. For example, boron nitride is an excellent insulating material, while graphite is a good conductor of electricity.

1.2 Cubic boron nitride

Cubic boron nitride has excellent physical and chemical properties, its hardness is second only to diamond, and it also has high strength, so it has application prospects in many fields.

In addition, cubic boron nitride is currently the semiconductor material with the highest temperature, which has high thermal conductivity and good semiconductor characteristics. However, the existing preparation methods all have inherent shortcomings that are difficult to overcome, so that it is not easy to make them widely used.

2. Why boron nitride is not conductive?

Boron nitride is an important superconducting material, and its excellent superconducting properties lead the research and development of a new round of superconducting materials. But at the same time, boron nitride is a non-conductive material, so how do we explain this paradox?

Currently, there are several explanations for why boron nitride does not conduct electricity. First of all, the atomic composition of boron nitride is extremely simple, consisting of boron atoms and nitrogen atoms, so the orientation of boron nitride is lower than that of other materials, which results in the limitation of electrons in boron nitride, which cannot moves, making the boron nitride non-conductive. Secondly, the nitrogen atom in boron nitride itself has strong electric absorption, which makes the external electric field have a great influence on the electron level of boron nitride, so that boron nitride does not work under a certain electric field strength, that is, nitriding The electrical properties of boron do not change significantly with the increase of the applied electric field, making it non-conductive.

In addition, boron nitride adopts a quasi-one-dimensional structure due to its chemical structure, which also affects the conductive characteristics of boron nitride. In crystals, ions form clusters that can destroy charged particles. These ions allow only one charged particle to pass through in a cluster, which means that the passage of electrons is greatly restricted, so that the solid does not conduct electricity. Boron nitride is a typical example of ion clusters hindering the passage of electrons and not conducting electricity.

Although boron helium is a non-conductive material, its special electrical properties make it superconducting. When boron nitride is at low temperature, its resistivity will be greatly reduced, resulting in superconducting properties. Under high temperature conditions, boron nitride is still in a non-conductive state, which shows that low temperature has an important impact on the energy level of boron fluoride electrons, so that it has superconducting properties.

In short, the non-conductivity of boron nitride is related to its simple atomic structure, the electric absorption of nitrogen atoms, the quasi-one-dimensional chain structure and the low temperature to its electronic energy level and other important reasons. Although boron nitride is a non-conductive material, it still has good superconducting properties, and its important role is undoubtedly a great revelation to material science.

3. How to make boron nitride conductive?

Cubic boron nitride is a synthetic material. As early as 1961, Wentorf pointed out that cubic boron nitride is a semiconductor material, and impurities such as Si and Be can be added to the synthetic raw materials to obtain N-type or P-type semiconductors. These discussions and experiments have laid the foundation for the in-depth study of the semiconductor properties of cubic boron nitride and its development and application.

The production of conductive boron nitride is usually carried out at high temperature and pressure. Wentorf added 0.01% to 1% Be (in the form of metal or salt) to the synthetic raw materials to synthesize P-type materials under high pressure. The crystal is blue, similar to the case of adding boron to synthetic diamond. The resistivity of the crystal at room temperature is 10³Ω•cm. The resistance of the crystal was measured at a temperature of 25-44°C, and the activation energy was estimated to be 0.19-0.23eV. Due to the small size and uneven growth of the crystal, the Hall effect and absorption spectrum could not be measured.

At the same time, Wentorf carried out experiments on doping impurities such as Si, KCN, methane or sulfur in the Li-N-hBN system, and obtained cubic boron nitride semiconductors with N-type characteristics. The typical value is 105~107Ω•cm, and the crystal color is yellow, brown or brownish red. Wentorf also pointed out that cBN p-n junctions can be made by improving experimental methods. However, due to limited experimental conditions, it is extremely difficult to synthesize large and uniform crystals.

In the late 1980s, due to the development of thin film technology, especially the diamond-cubic boron nitride coating technology, scientists used the combination of solid-film technology to study the cubic boron nitride (bulk material)-diamond (film) heterojunction , and showed excellent semiconducting properties, and prepared cubic boron nitride materials with semiconducting properties. In 1986, O. Mishima and others from the Japan Inorganic Research Institute synthesized large cubic boron nitride single crystals (3mm) using the temperature gradient method used by Wentorf to grow large-grain single crystal diamonds, thus providing a basis for the research of cubic boron nitride semiconductors. conditions. They added Be with a mass fraction of 1% to the synthetic raw material, under the conditions of a pressure of 5.5GPa and a temperature of 1800°C, using Li•CaBN as a catalyst, and deposited in a high-pressure synthesis chamber for 20 hours, a black Be-doped material was grown. Bulk cubic boron nitride single crystal. The single crystal has P-type semiconductor characteristics, and its resistivity at room temperature is 102~10³Ω•cm. On this basis, the same conditions are used to deposit Si on the p-type cubic boron nitride semiconductor material, thereby forming a p-n junction. The p-n junction still maintains good rectification characteristics at high temperature (530°C). However, due to its low purity and limited integrity during crystal growth, it has a certain impact on the characteristics of the p-n junction. Therefore, they believe that in order to improve the application characteristics of cubic boron nitride semiconductors, two factors must also be considered: one is to be able to achieve ohmic contact, and the other is to improve related synthesis technologies to produce high-temperature and wide-bandgap semiconductor devices with special uses.

In 1992, Taniguchi et al. mixed Be powder into the Li-based catalyst and hBN mixture to synthesize Be-containing cubic boron nitride powder, and then sintered the cubic boron nitride powder at a certain temperature and pressure to obtain cubic ammonia. Boron oxide polycrystals exhibit good semiconducting properties.

Pure cubic boron nitride crystals are non-conductive. According to the above research, conductivity can be obtained by magazine doping. Doping in high-pressure synthesis—adding different amounts of impurities such as Si and Be to the raw materials for synthesis; vacuum and high temperature doping—putting the synthesized sheet-like cubic boron nitride crystals into crucibles and covering them with pure Si and Be powders , and then put it into a high-temperature vacuum furnace to diffuse doping at different temperatures and time combinations. Whether it is doped in high-pressure synthetic raw materials or vacuum diffusion after synthesis, the conductive properties of crystals can be effectively changed.

Cubic boron nitride is a promising semiconductor material. By adding different impurities to the raw materials, P-type and N-type semiconductors can be obtained by high-pressure synthesis, and the cubic boron nitride single crystal obtained by high-pressure synthesis can also be passed through high temperature Vacuum diffusion doping method to obtain P, N type semiconductor materials.