Rapid liftoff of epitaxial thin films
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Electronic devices made from single crystal thin films attached to inexpensive support substrates offer reduced material costs compared to wafer-based devices; however, scalable and inexpensive processes for producing these single crystal film structures have remained elusive. In this work, we describe a new approach for fabricating these structures. In our approach, an epitaxial film is grown on a single crystal template and is then separated from its growth surface via fracture along a weak heteroepitaxial interface between the single crystal film and its growth substrate. We show that epitaxial films of Si, Ge, and GaAs, with thicknesses ranging from 100 nm to 1 lm, grown on epitaxial CaF2 overlayers on Si ,111. substrates, can be transferred to glass substrates by inducing fracture along the heteroepitaxial interface between the semiconductor film and CaF2, or between CaF2 and the Si wafer, assisted by the presence of water as in moisture-assisted cracking. I. INTRODUCTION
The active region of a wafer-based semiconductor device typically occupies only a small fraction of the wafer thickness, resulting in unnecessarily high materials costs for a wide range of devices such as solar cells, LEDs, lasers, and transistors. Single crystal thin films attached to inexpensive support substrates can replace wafers, and be used to make equivalent, if not better, devices, but with lower materials costs and the possibility of mechanical flexibility. In principle, there are two ways to accomplish this. One can either separate a thin layer from a single crystal wafer or ingot, or, one can use a single crystal as a substrate for epitaxial growth of a single crystal thin film, which is then removed from the substrate. In both cases, the goal is to reuse the wafer or ingot as many times as possible to keep material costs to a minimum. Based on these principles, several approaches have been explored over the last 30 years or so, including (i) epitaxial Si separation via porous Si perforation layers,1 (ii) layer spalling via stressed overlayers,2–4 (iii) layer separation via ion-implantationinduced microcracks,5 (iv) and liftoff of epitaxial GaAs via lateral chemical (HF) etching of GaxAl1 xAs release layers.6 Although these efforts represent substantial progress toward these goals, each technique suffers from at least one major limitation. In the first three techniques, separation of the thin film from its single crystal substrate relies on some form of cleavage, which is efficient from a throughput and materials utilization standpoint; however, the location of fracture is not precisely controlled, resulting in films with high surface roughness (techniques i, iii), a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.226 2564
J. Mater. Res., Vol. 28, No. 18, Sep 28, 2013
http://journals.cambridge.org
Downloaded: 13 Mar 2015
thickness variation (ii, iii), and poor thickness accuracy (ii, iii). In process iv, however, the film’s separation plane is controlled by the location of the thin GaxAl1 xAs bu
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