Graphenea's latest research paper demonstrates the growth of commercially important GaN-on-silicon devices using graphene as an intermediary layer.
Semiconductors made of third-group periodic table elements and nitrogen (III-nitride semiconductors), including GaN, AlN, InN, and their alloys, have attracted considerable attention because of their outstanding material properties leading to many applications for light-emitting diodes (LEDs), laser diodes (LDs), and high-frequency and high-power transistors.
Although many types of substrate have been tried for the epitaxial growth of III–nitride semiconductors, the abundant silicon (Si) is considered one of the most attractive substrates owing to its high quality at a low cost, ease of obtaining large substrates, and conductivity control. Thus far, the most successful crystallographic orientation of GaN has been so-called “c-plane (0001)”. Using this crystal structure, researchers and engineers have fabricated and commercialized GaN-based devices such as LEDs and field-effect transistors (FETs) on Si substrates. The Si substrate in those devices has a (111) crystal orientation, as opposed to the more technologically mature (100) structure of Si, employed in many electronic devices.
For the commercialization of advanced GaN-based electronic and optoelectronic devices, growth on Si(100) substrates is essential. However, owing to the different symmetries in hexagonal (0001) GaN and cubic (100) Si, it is still difficult to grow high-quality epitaxial GaN films on Si(100). The most advanced research successfully made GaN transistors on silicon ones by mechanically pressing the two structures against each other.
Image: SEM images of GaN grown on graphene/Si(100).
In our research, we show that graphene can be used as an intermediary layer enabling the epitaxial growth of GaN(0001) on Si(100). The hexagonal lattice of graphene has the same symmetry as that of GaN, such that the GaN molecules naturally follow the structure of graphene. At the same time, graphene is commonly transferred on top of silicon wafers. Using a variety of lab analyses, we show that our method results in the best GaN(0001) layers on Si(100) demonstrated to date.
The procedure starts with regular CVD-grown graphene on copper. We use our well-known transfer technique to move the graphene to a silicon substrate, as the copper is etched away. GaN is then grown directly on graphene using radio-frequency molecular beam epitaxy (RF-MBE). In RF-MBE, a technique mastered by our Japanese co-authors, a high power RF wave triggers a reaction of gas molecules in an epitaxy chamber, resulting in a fine, well-controlled deposition of a molecular thin film on a substrate. In this case, the substrate is graphene, while the film is GaN.
The structure of GaN was monitored in situ (during growth) by reflection high-energy electron diffraction analysis. After the growth, the films were investigated by scanning electron microscopy and high-resolution X-ray diffraction analysis. The analyses revealed that the GaN assumed a hexagonal symmetry, indicating c-axis growth. The grain size was larger than that of GaN grown on Si(100) without graphene. The results also showed that in terms of uniformity of the orientation, our method did not yield results as good as those for GaN grown on sapphire, but did show the best quality of GaN films of Si(100) seen to date.
Further transmission electron microscopy (TEM) studies showed that the resultant GaN crystal structure suffers from defects which are already present in the first 10nm which touch the graphene. Research is ongoing to improve the quality of the GaN and reduce the defects.
The research was published in Applied Physics Express (a publication of IOP Publishing and the Japan Society of Applied Physics) in collaboration with researchers from MIT (USA), Ritsumeikan University (Japan), Seoul National University and Dongguk University (Korea).