Accurate SIMS Doping Profiling of Aluminum-Doped Solid-Phase Epitaxy Silicon Islands Yann Civale, a,z Lis K. Nanver, a Stefano G. Alberici, b Andrew Gammon, b and Ian Kelly b a Laboratory of Electronic Components, Technology, and Materials, Delft Institute of Microsystems and Nanoelectronics-DIMES, Delft University of Technology, 2628 CT Delft, The Netherlands b Evans Analytical Group, Brunel University, Uxbridge UB8 3PH, United Kingdom A procedure has been implemented for a quantitative aluminum-doping profiling of m-scale aluminum-induced solid-phase- epitaxy SPE Si islands formed at 400°C. The aluminum concentration was measured to be 1–2  10 19 cm -3 , which is about 10 times higher than previously reported electrical activation levels. The elemental concentration was measured by secondary-ion- mass-spectroscopy SIMS on arrays of SPE Si islands grown by a recently developed process that allows control of the island geometry. © 2008 The Electrochemical Society. DOI: 10.1149/1.2836739 All rights reserved. Manuscript submitted November 8, 2007; revised manuscript received December 26, 2007. Available electronically January 28, 2008. Low-temperature Si crystallization techniques 1,2 are attracting wide attention due to their many potential applications, such as semiconducting nanowires, 3-5 thin-film transistors, 6 interconnects, 7 and shallow-junction formation. 8 One method that has proven itself in actual device fabrication is the use of metals like nickel and aluminum to mediate the transport of Si and thus lower the crystal- lization temperature of Si. 9 The doping of the resulting monocrys- talline Si  c-Si by the metal is an important factor for understanding and predicting the device performance, but up until now little quan- titative information has been available on this subject. In the case of Al, substitutionally incorporated dopants will act as acceptors. This means that the Al-mediated Si crystallization also offers a low- temperature means of creating p-doped regions. In the literature, the incorporated Al-concentration is often assumed to be above the equilibrium Al solid solubility in Si that is extremely low at tem- peratures below 500°C, slightly above 10 18 cm -3 . 10 In principle, the concentration of Al in Si can be profiled by secondary-ion-mass- spectroscopy SIMS analysis. However, in the Al-mediated c-Si material the size of the crystals is typically smaller than the SIMS analysis area and the dimensional control is poor. 11,12 Moreover, in systems where large c-Si areas are grown, the surface is usually not uniform enough to allow an accurate SIMS analysis. In this article, the Al-doping concentration in a recently devel- oped sub-500°C aluminum-/amorphous-Si  -Si solid-phase epi- taxy SPE process is extracted. This SPE process has been demon- strated to provide high-quality, ultra-abrupt, Al-doped p + -n elevated junctions down to nanoscale dimensions. 13 Moreover, both the height and lateral dimensions of the crystallized regions can be well controlled by adjusting the layer-stack geometry and the thermal processing parameters. Information on the Al-doping level was ac- quired in our past work through the fabrication and electrical char- acterization of simple SPE-Si-based devices such as p + contacts, p + -n diodes, or p-n-p bipolar junction transistors. From these stud- ies, the electrically active part of the Al doping in the bulk-Si of the islands was found to be about 2  10 18 cm -3 . In the present work, the high degree of controllability of the growth mechanism has been used to create areas of SPE-Si islands that are suitable for a quanti- tatively correct SIMS profiling of the elemental Al concentration in the islands. A detailed description of the fabrication of the SPE-Si islands and the SIMS measurement procedure is given. The results show that the elemental concentration of Al in SPE-Si grown at 400°C is about 1–2  10 19 cm -3 , which suggests that not all the incorporated Al dopants are electrically active. Experimental Fabrication of SIMS analysis area.— The process flow used for fabricating an SPE Si-island array with a total area of about 60  60 m 2 and containing 90 similar islands, each approximately 1.4  1.4 m 2 in size, is presented in Fig. 1. First, 1.4 m wide contact windows were opened by conventional lithography through a 30 nm thick thermal silicon dioxide  SiO 2  to the  100 Si sub- strate by using buffered hydrofluoric acid HF 1:7. Then, the native SiO 2 was removed from the Si surface by dip-etching in HF 0.55% immediately followed by the physical vapor deposition PVD of first 200 nm Al containing 1% Si and then 20 nm -Si. Both layers were deposited by using an argon flow of 100 sccm at room tem- perature and without vacuum break in order to prevent the formation of an aluminum oxide  Al 2 O 3  interface. The Al /-Si layer stack was then patterned using reactive-ion etching to form 5  5 m 2 large islands around the contact windows, in order to provide the optimal supply of Si for exactly filling the whole contact window with SPE-Si. In this manner, a practically ideal SPE-Si growth se- lectivity can be achieved with respect to the competing process of Si nucleation on the SiO 2 in which the windows are etched. 14 The epitaxy itself is induced by a thermal anneal at 400°C for 40 min in aN 2 /H 2 10:1 mixture at atmospheric pressure. After the growth, the aluminum transport layer was removed in a solution of diluted z E-mail: y.civale@dimes.tudelft.nl Figure 1. SPE process flow: a thermal oxidation, b contact window defi- nition, c Al /-Si PVD deposition, d Al /-Si layer-stack patterning, e SPE-Si island growth and transport layer removal, and f oxide removal. Electrochemical and Solid-State Letters, 11 4 H74-H76 2008 1099-0062/2008/114/H74/3/$23.00 © The Electrochemical Society H74 Downloaded 23 Feb 2012 to 131.180.130.114. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp