Van der Waals Epitaxy of c-Oriented Wurtzite AlGaN on Polycrystalline Mo Substrates for Enhanced Heat Dissipation

Abstract

The epitaxy of III-nitrides on metallic substrates is competitive due to the advantages of vertical carrier injection, enhanced heat dissipation, and flexible application in various IIInitride-based devices. However, the serious lattice mismatch, atom diffusion, and interface reaction under the rigorous growth conditions have caused enormous obstacles. Based on the thermal and chemical stability of the graphene layer, we propose the van der Waals epitaxy of c-oriented wurtzite AlGaN on the polycrystalline Mo substrate by high-temperature metal−organic chemical vapor deposition. The insertion of a graphene layer interrupts the chaotic epitaxial relationship between the polycrystalline metal and epilayers, resulting in the single-crystalline orientation along the wurtzite (0002) plane and residual stress release in AlGaN because of the weak van der Waals interaction. We also demonstrate that the epitaxy of AlGaN on Mo metal possesses enhanced heat dissipation ability, in which the epilayer temperature is controlled at only 28.7 °C by the heating of a ∼54 °C hot plate. The heat dissipation enhancement for the present epitaxial structures provides a desirable strategy for the fabrication of efficient ultraviolet devices with excellent stability and lifetime.

AlGaN UV Detector with Largely Enhanced Heat Dissipation on Mo Substrate Enabled by van der Waals Epitaxy

“…10−12 However, the construction method by growth of AlGaN on metallic substrates suffers from serious interfacial atoms diffusion and chemical reaction due to the high reactivity of metal in the rigorous epitaxial conditions, such as the high growth temperature (over 1000 °C). 13 Otherwise, the general polycrystalline nature of metallic substrates offers inconsistent epitaxial orientation, 13 which causes chaotic crystal growth and poor crystalline quality in AlGaN epilayers. A currently applied alternative strategy, including the laser-assisted epilayer lift-off, transfer, and bonding to metallic substrate, has avoided these difficulties mentioned above.…”

“…Up to now, the vdWs epitaxy of AlGaN-based materials has been widely investigated on the single crystalline sapphire substrate, particularly, the AlN nucleation regulation and polarity control through the insertion of 2D materials have been reported in our previous works. , Inspiring by the advantages of vdWs epitaxy, we consider that it is a promising strategy for the growth of AlGaN on polycrystalline metallic substrates, aiming at the easy fabrication of AlGaN-based optoelectronic devices. On the one hand, the impermeability for metal atoms through 2D materials would ensure the clear epitaxial interface by avoiding from interfacial atoms diffusion and chemical reaction; on the other hand, the remote epitaxial space at the interface screens the lattice field interaction of polycrystalline metals, potentially resulting in the single crystalline epitaxy of AlGaN …”

“…Meanwhile, the natural ductility makes metallic substrates easily being extruded into thin foils with high flexibility. These advantages reveal that metals are more competitive as the AlGaN epitaxy substrates for the fabrication of flexible/wearable devices with vertical carrier injection and enhanced heat dissipation. − However, the construction method by growth of AlGaN on metallic substrates suffers from serious interfacial atoms diffusion and chemical reaction due to the high reactivity of metal in the rigorous epitaxial conditions, such as the high growth temperature (over 1000 °C) . Otherwise, the general polycrystalline nature of metallic substrates offers inconsistent epitaxial orientation, which causes chaotic crystal growth and poor crystalline quality in AlGaN epilayers.…”

“…These advantages reveal that metals are more competitive as the AlGaN epitaxy substrates for the fabrication of flexible/wearable devices with vertical carrier injection and enhanced heat dissipation. − However, the construction method by growth of AlGaN on metallic substrates suffers from serious interfacial atoms diffusion and chemical reaction due to the high reactivity of metal in the rigorous epitaxial conditions, such as the high growth temperature (over 1000 °C) . Otherwise, the general polycrystalline nature of metallic substrates offers inconsistent epitaxial orientation, which causes chaotic crystal growth and poor crystalline quality in AlGaN epilayers. A currently applied alternative strategy, including the laser-assisted epilayer lift-off, transfer, and bonding to metallic substrate, has avoided these difficulties mentioned above. − Wang et al recently presented the transfer of GaN epilayer from the sapphire onto Mo substrate, a 355 nm laser was used for GaN lift-off and the multilayer Ti–Pd–Ti composite was served as a bonding layer to the Mo substrate .…”

“…On the one hand, the impermeability for metal atoms through 2D materials would ensure the clear epitaxial interface by avoiding from interfacial atoms diffusion and chemical reaction; on the other hand, the remote epitaxial space at the interface screens the lattice field interaction of polycrystalline metals, potentially resulting in the single crystalline epitaxy of AlGaN. 13 In present work, the growth of highly c-oriented AlGaN crystal and fabrication of UV detector with lateral metal-AlGaN-metal (M-AG-M) junctions on the polycrystalline Mo substrate are achieved by the vdWs epitaxy with a graphene insertion layer, and high-temperature annealed physical vapor deposition (PVD) AlN layer is first deposited on graphene for improving the AlGaN crystalline quality. The following AlGaN grown by metal organic chemical vapor deposition (MOCVD) has a high Al content of 92%, and the full width at halfmaximum (fwhm) of rocking curve along (0002) orientation is 1.12°, which is much better than that grown without (w/o) graphene insertion layer.…”