Single Crystal Thin Films of Silicon on Graphene Enabled by Solid Phase Epitaxy

One of the challenges in thin film nanotechnology is to create scalable large area high quality thin single crystal semiconductor layers that can be transferred to arbitrary substrates, including amorphous substrates such as glass. Solid phase epitaxy of an amorphous layer using a crystalline seeding substrate followed by lift-off and transfer of the thin film to another “handling” substrate and reuse of the seeding substrate may be one possible approach provided it can be accomplished in a cost-effective manner (for large scale, low-cost applications). The challenge here lies in being able to control the strength of the interfacial bonding between the seeding substrate and the amorphous thin film that is to be crystallized and transferred. The bonding should be strong enough for high quality solid phase epitaxy, at the same time the subsequent debonding should be a facile process.<br/>We approach this problem by using a perforated graphene layer as an interlayer between a silicon substrate and an amorphous silicon film deposited upon it. Perforations within the graphene lead to intimate epitaxial silicon-amorphous silicon contact with a goal of enabling solid phase epitaxy upon annealing of the samples. At the same time the presence of the graphene sheet and its weak bonding with the silicon on either side may enable a lower interfacial energy for delamination of the crystallized film. These expectations have been supported via molecular dynamics simulations. Based upon the expectations we have carried experiments that demonstrate that amorphous silicon films can indeed be epitaxially crystallized and delaminated via a peel-off process.<br/>In our experiments, the solid phase epitaxy of silicon process starts with patterning monolayer graphene on a silicon (100) wafer using an electron-beam lithography tool. Following this, the graphene is carefully etched, and the amorphous silicon is deposited using a plasma enhanced chemical vapor deposition tool. The amorphous silicon deposition is preceded with native oxide removal, which is the key step in this process. The amorphous silicon is then annealed in a vacuum chamber equipped with a reflection high-energy electron diffraction (RHEED) setup in order to monitor the film crystallization. Our results indicate that annealing a 15-75 nm amorphous silicon layer at pyrometer measured temperatures of 600-700 °C for a duration of 60-360 minutes leads to the RHEED pattern showing a single crystal symmetry on the surface, implying epitaxial crystallization in some areas of the film. Currently, we are working on improving the process conditions to achieve a complete epitaxial crystallization of the film. We then then use a spalling technique [1] to successfully delaminate the amorphous silicon from the substrate. We will discuss results from our crystallization and delamination experiments. We will also discuss approaches and results for replacing the electron beam patterning technique (selected for the experiments because of controllability for these exploratory experiments) with a scalable and significantly cheaper self-assembly based process. We will also discuss the molecular dynamics results that examines the feasibility of this method.<br/>[1] J. Kim, C. Bayram, H. Park, C.-W. Cheng, C. Dimitrakopoulos, J. a. Ott, K.B. Reuter, S.W. Bedell, and D.K. Sadana, Nat. Commun. 5, 4836 (2014)<br/>This work was partially supported by a Vannevar Bush Fellowship under the program sponsored by the Office of the Undersecretary of Defense for Research and Engineering (OUSD (R&E)) and The Office of Naval Research as the executive manager for the grant. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.