Remote Epitaxy of III-N Membranes on Amorphous Boron Nitride

Gallium Nitride (GaN) is a wide bandgap binary semiconductor material, which can be fabricated by epitaxy crystal growth such as MBE. GaN owns may physical properties such as wide bandgap and high breakdown electric field. These merits make GaN a promising material for devices such as Light-Emitting diode (LED) and High-Electron-Mobility Transistor (HEMT). Knowing the merits of GaN compared with other semiconductor materials, remote epitaxy appears as a new way to manufacture free-standing GaN films recently. Compared with other methods of fabricating free-standing film such as Laser Lift Off (LLO), Controlled Spalling and Epitaxial Lift Off (ELO), remote epitaxy has smoother lift-off interface, higher substrate reusability, thinner membranes and faster process.<br/>So far, both graphene and Hexagonal Boron Nitride (hBN) have been shown in previous papers as promising 2D materials for remote epitaxy growth. Graphene can be transferred to the substrate from Copper (Cu) foil or SiC substrate or directly grown on the substrate by Chemical Vapor Deposition (CVD). Graphene transferred by PMMA (wet transfer) always results in polymer residuals on the graphene surface, which leads to bad epitaxial film quality. Although less residuals appear during dry transfer process (transferred by Nickle), the process results in the holes on the graphene, which can finally lead to the spall of the substrate during exfoliation. CVD grown graphene shows better graphene quality compared with transferred graphene. However, contaminations generated during sample transferring process between chambers, and graphene leads to high Carbon doping concentration. Besides graphene, hBN is also reported as the platform for remote epitaxy. However, unlike graphene, hBN is a polar material, which means epilayer is guided by the ionic field of the substrate as well as the hBN interlayer. Depending on the thickness of the transferred hBN, the epitaxial layer can be dominated by either of the two phases.<br/>Aiming to solve the problems mentioned above, in situ MBE grown amorphous Boron Nitride (aBN) is developed as the 2D material for remote epitaxy. aBN is grown in MBE chamber with the help of high temperature effusion Boron cell and plasma assisted nitrogen source. aBN is a more transparent 2D material with no ionic field compared with hBN. Epilayer is purely seeded by the substrate in the remote epitaxy process. This MBE in situ grown aBN fully eliminates the contaminations generated during 2D material transferring process and sample reloading process. This technique enables the growth of ultrathin free-standing single crystalline GaN films. This high-quality free-standing film enables the high sensitivity Surface Acoustic Wave (SAW) sensor which can make a conformal contact on human skin. By transferring the GaN on a substrate with higher heat conductivity, GaN based device heat dissipation can be improved. Multiple GaN/aBN layers can be stacked together with single MBE growth to enhance the fabrication rate of the free-standing films. Furthermore, unlike normal epitaxy methods, the epilayer of remote epitaxy spontaneously relaxes due to the slippery surface of the 2D material. The Van der Waal force on the 2D material doesn’t limit the relaxation like the covalent bond. Therefore, high quality relaxed InGaN can be obtained with this method.<br/>The synthesis temperature of MBE grown aBN can go as low as 300 °C, which broadens the application field of aBN based remote epitaxy. In the future, this technique has the potential to be applied to more semiconductor materials.