Synthetic diamond wafers for device applications

As a material with excellent mechanical and functional properties, diamond is greatly needed in a wide range of fields. The high cost of natural diamond pushes forward the research of synthetic diamond. Diamond can be divided into single crystal diamond (SCD) and polycrystalline diamond (PCD). PCD has higher concentration of defects comparing to SCD due to the presence of grain boundary and the electrical properties of devices made by PCD are significantly limited as a result [Citation15]. Therefore, SCD is a better choice than PCD if you want to develop the potential of diamond as an electronic and optical device ( UV / radiation detector, field effect transistor, diode, etc. ).

Diamond can be synthesized by two main methods: high pressure and high temperature (HPHT) and chemical vapor deposition (CVD) . The HPHT method mimics the formation of natural diamond, which transforms graphite into diamond under HPHT. HPHT method is of low cost and high yield. However, this method also has some problems. For example, impurities from air or the catalysts can be easily introduced into diamond during the synthesizing process . Also, the synthetic diamond size is limited by the HPHT reaction chamber size, making HPHT diamond not an ideal choice for the semiconductor applications.

Another synthesizing method is CVD , which allows a lower temperature and pressure condition to synthesize diamond. This technique, especially the micro plasma CVD (MPCVD), has been widely used for growing high-quality and large-size diamond. As shown in Figure 1(a–d), the research of MPCVD SCD growth has been conducted from several different aspects, including the preparation with high growth rate, low dislocation density, large size, and flat surface.

It is necessary to increase diamond growth rate as there is a strong demand in both industrial production and academic research . The diamond growth mechanism has been investigated extensively . One efficient strategy is by increasing plasma density during the growth process , which can be realized by increasing growth pressure or/and growth power. The addition of nitrogen can improve the diamond growth rate as well . C. Yan et al.  achieved a diamond growth rate of 150 μm/h under a growth pressure of 200 Torr with nitrogen addition. The optical image of the synthesized diamond is shown in Figure 1(a). The sample is yellow due to the presence of single substitutional nitrogen. Q. Liang et al. increased growth pressure to 300 Torr and achieved a growth rate of 165 μm/h.

Low dislocation density is important for the electronic applications of diamond . A small number of defects would significantly weaken the electrical properties of diamond semiconductor devices. Some serious defects may even lead to the direct failure of the device. The intrinsic defect within diamond seed is the main defect source for the synthesized diamond. Several strategies can be used to minimize the influence from the seed defects. Takeuchi et al. proposed a two-step growth method for growing diamond with low dislocation density, in which case a buffer layer of low dislocation density was synthesized at low growth rate first, followed by the high-speed growth of low-dislocation diamond on the buffer layer. The use of high-quality seed crystal is also important. Mokuno et al.  synthesized low-dislocation diamond using high-quality diamond substrate. The dislocation density is <100 cm−2 for the diamond substrate seed and ∼400 cm−2 for the synthetic diamond after growth (Figure 1(b)).

Large size is another important parameter . Many microelectronics chips have a special demand of the size for integration of large numbers of transistors. Moreover, more chips can be fabricated from one single large-size wafer. Therefore, obtaining large (single crystal) diamond wafers with relatively low cost is another key point of the commercial application of diamond semiconductor devices. Mosaic growth is a popular strategy to realize large size diamond . In this method, several small square diamond substrates are pieced together for growing one large diamond . Figure 1(c) shows a 20 mm × 40 mm size diamond wafer prepared through this method. However, defects such as stacking faults, twins, and dislocations can easily form in the junction region between the adjacent diamond seeds during the mosaic growth. Heteroepitaxial growth (i.e. growing diamond on silicon or sapphire) can also achieve the large-size diamond growth, but with a high dislocation density .

The flat surface of diamond is important as well , which is not only essential for diamond optics, but also ideal for the heterogeneous integration of diamond with other materials (e.g. GaN). Okushi et al.  successfully synthesized diamond with an atomically flat (001) surface, of which the roughness was reduced to 0.04 nm. However, the low growth rate of only tens of nanometers per hour cannot meet the requirement of industrial production. Bogdan et al. improved diamond growth rate to 5 μm/h and guaranteed a low surface roughness of about 0.6 nm at the same time (Figure 1(d)).

Despite all these progresses, it is hard to achieve all the above requirements (high growth rate, low dislocation density, flat surface, etc.) in one sample, and tradeoffs are often made. For example, nitrogen addition is a common method to increase diamond growth rate , but this will introduce nitrogen impurity into diamond, which is not ideal for electronic-grade SCD. Another example is by using low growth temperature and methane content to grow diamond with low dislocation density or high-quality surface , but these growth parameters will result in low plasma density and reduce diamond growth rate significantly. Some strategies can be used to handle these conflicts. For example, it is feasible to achieve both high speed and high quality growth by using specially designed, high-power MPCVD device . High device power has high requirement for MPCVD device. And it is more difficult to modulate plasma status under high power condition. With the development of MPCVD device, the improvement of seed crystal quality and diamond synthesizing technique, one can expect to see mass production of large-size and high-quality SCD in near future.

CSMH focuses on the research and production of diamond wafers. At present, it has diamond wafers, diamond heat sinks, GaN on diamond, AlN on diamond and other products. Among them, high-power semiconductor lasers packaged by diamond heat sinks have been used in optical communications. In the fields of laser diodes, power transistors, and electronic packaging materials, it can provide customers with diamond thermal management solutions.