What's the best semiconductor material?

According to the latest issue of the journal science, from the United States at the Massachusetts institute of technology, the university of Houston, and other agencies conducted by a team of the experimental results show that a material called cubic boron arsenide overcome the two limitations of silicon as semiconductor: to provide a high mobility for electrons and holes, and has a good thermal performance.  Researchers say it may be the best semiconductor material ever discovered.

Semiconductor materials three generations

In all existing three generations of semiconductor material, the first generation of semiconductor material with silicon (Si), germanium (Ge), silicon material is the basis of the current mainstream logic chip and power device, is based on modern science and technology represented by silicon above the results of this type of semiconductor material, is more than 90% of the products are made from silicon as the substrate of the semiconductor.  However, the application of silicon in optoelectronics and high frequency and high power devices has been hindered, and the performance of silicon at high frequency is poor, which is not suitable for high voltage application scenarios.  

The second-generation semiconductor materials mainly refer to compound semiconductor materials, represented by gallium arsenide (GaAs) and Indium phosphide (InP), which are mainly used in the manufacture of high-speed, high-frequency, high-power and light-emitting electronic devices, and are excellent materials for the manufacture of high-performance microwave, millimeter wave devices and light-emitting devices.  It is widely used in satellite communication, mobile communication, optical communication, GPS navigation and other fields.  However, GaAs and InP materials are scarce and expensive, and they are toxic and pollute the environment. InP is even considered as a suspected carcinogenic substance. These shortcomings make the application of second-generation semiconductor materials have certain limitations.  

The third generation semiconductor refers to the emerging semiconductor materials such as SiC, GaN, ZnO, diamond (C), AlN with wide band gap (Eg > 2.3eV).

The third generation of wide bandgap power semiconductor materials has the advantages of high thermal conductivity, high breakdown field strength, high saturation electron drift rate and high bond energy, which can meet the new requirements of modern electronic technology for high temperature, high power, high voltage, high frequency and radiation resistance, and is the most promising material in the field of semiconductor materials.  It has an important application prospect in defense, aerospace, energy, communication, electric transportation, industry and other fields.  On the whole, SiC is the most mature wide bandgap power semiconductor material, followed by GaN, diamond, AlN and Ga2O3 have also become the international frontier research hotspot.

What's the secret to "Best Semiconductor Material"?  

Among the known materials, diamond and graphene materials have a thermal conductivity of more than 2000W•m-1• k-1, much higher than silicon (150W•m-1• k-1), which seems to be ideal for cooling electronic devices.  However, although diamond has been used for heat dissipation in some cases, it is not practical to use diamond for heat dissipation of electronic devices due to the high cost of natural diamond and the structural defects of man-made diamond films.  Due to the anisotropy of thermal conductivity and the difficulty of preparation, graphene materials also limit its wide application in device heat dissipation.  Other heat dissipation material with GaN (230 w, m - 1, the thermal conductivity K - 1), Al (240 w. k. m - 1-1), AlN (285 w. k. m - 1-1), Cu (400 w. k. m - 1-1) and SiC (490 w. k. m - 1-1), etc.  But there is a huge gap between its thermal conductivity and the ultra-high thermal conductivity of diamond and other materials.  

Early experiments showed that the thermal conductivity of cubic boron arsenide was almost 10 times that of silicon.  This is very attractive for cooling.  It has also been demonstrated that the material has a very good bandgap, a property that gives it great potential as a semiconductor material.  

The new study shows that boron arsenide has all the main qualities required for an ideal semiconductor because of its high mobility of electrons and holes.  This is important, the researchers note, because in semiconductors, positive and negative charges are equal.  Therefore, if you want to build a device, you want to have a material with less resistance to the movement of electrons and holes.  Heat is currently a major bottleneck for many electronics, and silicon carbide is replacing silicon as power electronics in the major electric vehicle industry, including Tesla, because it has three times the thermal conductivity of silicon, despite its lower electron mobility.  Boron arsenide has 10 times the thermal conductivity and much higher mobility than silicon, which could be a game changer.

Crystal growth method  

The difficulty in preparing BAs crystals is mainly affected by the following factors:  

(1) The melting point of boron (2076℃) is much higher than the sublimation temperature of arsenic (614℃);  

(2) Arsenic and related reactants are toxic;  

(3) the chemical stability of boron is very strong, it is difficult to react;  

(4) BAs will decompose into a more stable subcrystalline phase (B12As2) when the growth temperature exceeds 920℃.  

Therefore, it is difficult to use the melt method for crystal growth like GaAs and other group ⅲ-ⅴ semiconductor materials.  CVT can achieve chemical reaction control by precise temperature control in multi-temperature zone in a closed environment, which effectively solves the above problems and is the main method for BAs single crystal preparation.

Schematic of BAs crystal growth by CVT method

CVT method is a method of crystal growth by means of reversible reaction between solid phase and gas phase and auxiliary gas applied. The principle of BAs growth is shown in the figure above. In a vacuum-sealed quartz tube, high-purity As and B with a certain ratio are placed at the high temperature end As the source, while the low temperature end serves As the crystallization zone.  BAs single crystal is grown by controlling the temperature distribution of the source region and the growing region to realize the gas phase transmission.  I2 halogen elements or compounds were used as transport agents in the growth process to enhance transport efficiency.  Through the chemical reaction between the raw material and the transport agent, the gas convenient for transport is formed and transported to the crystal growth surface, where the crystal is deposited through the corresponding reverse reaction.  

Conclusion  

So far, cubic boron arsenide has only been manufactured and tested on a laboratory scale, these products are not uniform, and more work is needed to determine if cubic boron arsenide can be made in a practical, economical form.  But researchers say that in the near future, people may find some useful uses for the material, and its unique properties could make a significant difference.