1456 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 7, JULY 2005 Fabrication of Highly Reflecting Epitaxy-Ready Si–SiO Bragg Reflectors S. Akiyama, F. J. Grawert, J. Liu, Student Member, IEEE, K. Wada, G. K. Celler, Member, IEEE, L. C. Kimerling, Member, IEEE, and F. X. Kaertner, Senior Member, IEEE Abstract—A complementary metal–oxide–semiconductor-com- patible process for fabrication of highly reflecting Si–SiO Bragg mirrors with a crystalline top layer was developed. It comprises only one step of wafer-bonding and allows for subsequent epitaxial growth on the mirror. A six-pair reflector centered at 1400 nm with a 99% bandwidth of 700 nm and a surface roughness of 0.136 nm is demonstrated. Index Terms—High index contrast (HIC) reflector, optical sub- strate, photonic bandgap material, silicon Bragg reflector. S ILICON is the material of choice in large-scale integrated electronics. The maturity of existing fabrication technology combined with the high index contrast (HIC) of silicon and its thermal oxide renders it attractive for photonic applications and for fulfilling the long-held vision of co-integrated electronic and photonic circuits on a single chip [1], [2]. One of the key compo- nents for integrated silicon optoelectronics are highly reflecting mirrors, terminated with crystalline silicon layers that allow for subsequent crystalline epitaxial growth. Highly reflecting mir- rors are the fundamental building block for a number of de- vices, such as optical microcavities [3] that, for example, en- able tailoring of spontaneous emission from gain media such as Er-doped silicon, embedded in the cavity [4] and other ap- plications based on cavity quantum electrodynamics. In addi- tion, highly reflecting Si–SiO mirrors increase the quantum ef- ficiency in resonant cavity-enhanced photodetectors, while pre- serving high-speed operation [5]. Finally, the HIC ( , ) not only furnishes Si–SiO Bragg mirrors with high reflectance and broad optical bandwidth, but also ren- ders them omnidirectional, leading to a continuous stopband for all incident angles and polarizations [6]. These properties make Si–SiO mirrors attractive for solar cells, semiconductor saturable absorbers [7] and as reflective substrates in integrated optics. Common to all these applications is the challenge to fabricate Si–SiO Bragg mirrors with a crystalline silicon top layer for subsequent epitaxial growth and with low interfacial roughness Manuscript received January 27, 2005; revised March 24, 2005. This work was supported by the National Science Foundation (NSF) under Grant ECS- 0322740, Grant ONR-N00014-02-1-0717, and Air Force Office of Scientific Research (AFOSR) Grant FA9550-04-1-0011. S. Akiyama, J. Liu, K. Wada, and L. C. Kimerling are with the Department of Materials Science and Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA 02139 USA. F. J. Grawert and F. X. Kaertner are with the Department of Electrical Engineering and Computer Science, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: felixg@mit.edu). G. K. Celler is with Soitec USA, Peabody, MA 01960 USA. Digital Object Identifier 10.1109/LPT.2005.850005 Fig. 1. Fabrication process of the Si–SiO mirror. preventing scattering losses. A number of approaches based on different manufacturing techniques have been demonstrated. First of all, Si–SiO Bragg mirrors can be fabricated by chem- ical vapor deposition (CVD) of the individual layers or by repeated deposition of silicon layers and subsequent partial thermal oxidation, leading to a Si–SiO bilayer. While inex- pensive and straightforward, this method results in amorphous or polycrystalline material rather than in crystalline silicon films. We found that the surface roughness increases with each deposited bilayer, making it impossible to increase reflectance beyond a certain value by adding more layers. An alternative deposition process relies on electrochemical etching for the fabrication of porous silicon layers with indexes between 1.5 and 2.7 [4], [8]. However, the resulting material is porous, preventing subsequent crystalline epitaxial growth, and grain formation limits the highest achievable reflectance. In contrast, crystalline silicon top layers have been obtained by repeated application of the separation-by-implanted-oxygen process resulting in Si–SiO mirrors with 90% reflectance [9]. The dis- advantage of this process lies in the large defect density in the silicon layer caused by ion implantation and the dependence of interface morphology on the thickness ratio of Si–SiO layers. Finally, repeated application of the Smart Cut process has resulted in Si–SiO mirrors comprising up to three bilayers that are terminated with high-quality crystalline silicon layers [5], [10]. While providing films of highest quality, low yield and high cost limit the practical applicability of this process. A pos- sible solution is the recent approach to deposit Si–SiO mirrors from the backside in holes etched through handle and buried oxide (Fig. 1) of a silicon-on-insulator wafer (SOI-wafer) by means of CVD [11]. This process does not compromise the crystalline film quality of the SOI-layer serving as the starting material, and a reflectance in excess of 99% has been demonstrated. Here we extend this approach to the fabrication of Si–SiO Bragg reflectors with high-quality crystalline top layers that cover the entire surface area of a wafer and exhibit a very high reflectance. 1041-1135/$20.00 © 2005 IEEE