No abstract

Van der Waals epitaxy provides a fertile playground for the monolithic integration of various materials for advanced electronics and optoelectronics. Here, a previously unidentified nanorod-assisted van der Waals epitaxy is developed and nearly single-crystalline GaN films are first grown on amorphous silica glass substrates using a graphene interfacial layer. The epitaxial GaN-based light-emitting diode structures, with a record internal quantum efficiency, can be readily lifted off, becoming large-size flexible devices. Without the effects of the potential field from a single-crystalline substrate, we expect this approach to be equally applicable for high-quality growth of nitrides on arbitrary substrates. Our work provides a revolutionary technology for the growth of high-quality semiconductors, thus enabling the hetero-integration of highly mismatched material systems.

In the absence of inversion symmetry along the c-direction, group III-nitride materials on sapphire substrates (AlN, GaN, InN) and their alloys can exhibit either an N-polar or a metal-(Al, Ga, In) polar surface. [1] Here, N-polar or metalpolar refers to the situation where the epilayer grows along the c or −c direction. Although having identical bulk properties (e.g., refractive index, bandgap, etc.), the two polarities exhibit noticeable differences in surface energy, [2] growth mode, [3] nonlinear optical property, [4] and susceptibility to chemicals. [5] Manipulating the polarity, therefore, is a fertile playground for discovering novel device concepts in form of heteropolar junctions, such as nonlinear optical devices, [3,4,6] polar discontinuity devices, [7] and high-performance GaN high-electron-mobility transistors (HEMT). [8] Particularly, a number of The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metalorganic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.