In a recent research paper, Graphenea demonstrated the use of graphene as an intermediary layer to grow GaN-on-silicon devices.
Semiconductors fabricated from third-group periodic table elements and nitrogen (III-nitride semiconductors) such as indium nitride (InN), aluminum nitride (AlN), and gallium nitride (GaN), exhibit exotic properties, attracting interest for a number of applications such as high-frequency and high-power transistors, laser diodes, and LEDs.
Applied Physics Express, a publication of IOP Publishing and the Japan Society of Applied Physics, featured the results of the study, which was carried out in partnership with researchers from Ritsumeikan University (Japan), MIT (USA), Dongguk University and Seoul National University (Korea).
Silicon as Substrate
Various materials have been explored for use as substrates for epitaxially growing III–nitride semiconductors. However, silicon (Si) is an abundantly available material with attractive capabilities such as high quality at a low price, conductivity control, and ease of fabricating large substrates.
The C-plane (0001) crystallographic orientation of GaN has been the most successful one to date. Various GaN-based devices such as field-effect transistors (FETs) and LEDs on Si substrates have been made and commercialized using this crystal structure. In these devices, the Si substrate possesses a (111) crystal orientation rather than Si’s more technologically mature (100) structure used in most electronic devices.
It is essential to grow GaN films on Si(100) substrates to commercialize high-tech GaN-based electronic and optoelectronic devices. However, growing high-quality epitaxial GaN films on Si(100) is difficult due to the different symmetries in cubic (100) Si and hexagonal (0001) GaN.
Fabrication of GaN-on-Silicon Devices
In the research work described in Graphenea’s paper, GaN transistors on silicon were successfully fabricated when the two structures were mechanically pressed against each other. The researchers demonstrated the use of graphene as an intermediary layer to epitaxially grow GaN(0001) on Si(100).
Since graphene’s hexagonal lattice has the same symmetry as that of GaN, the GaN molecules naturally follow the graphene structure. Transfer of graphene onto silicon wafers take place at the same time. Using different lab studies, the researchers showed that their method yielded the best GaN(0001) layers on Si(100) illustrated so far.
The method involves growth of CVD graphene on copper, followed by transfer of graphene onto a silicon substrate when the copper is etched away, using Graphenea’s well-known transfer technique. A radio-frequency molecular beam epitaxy (RF-MBE) method is then used to directly grow GaN on graphene.
The method uses a high power RF wave to induce a reaction of gas molecules in an epitaxy chamber, leading to the fine deposition of a molecular thin film on a substrate in a well-controlled manner. Here, GaN films are deposited on the graphene substrate.
Reflection high-energy electron diffraction analysis was used for in situ monitoring of the GaN structure during growth. High-resolution X-ray diffraction analysis and scanning electron microscopy investigated the films subsequent to the growth, revealing a hexagonal symmetry for GaN and c-axis growth (Figure 1). In addition, the films had larger grain size than GaN grown on Si(100) in the absence of graphene.
Figure 1. SEM images of GaN grown on graphene/Si(100)
Although the Graphenea’s method is inferior to GaN grown on sapphire in terms of uniformity of the orientation, it yielded the best quality of GaN films of Si(100) demonstrated to date.
From the transmission electron microscopy (TEM) studies, the researchers found that the resulting GaN crystal structure has defects, which already exist in the first 10nm contacting the graphene. Research is underway in order to achieve GaN with improved quality and reduced defects.