Article pubs.acs.org/JACS
Electrochemical Liquid Phase Epitaxy (ec-LPE): A New Methodology for the Synthesis of Crystalline Group IV Semiconductor Epifilms Joshua Demuth,†,∥ Eli Fahrenkrug,†,∥ Luyao Ma,† Titilayo Shodiya,† Julia I. Deitz,‡ Tyler J. Grassman,‡ and Stephen Maldonado*,†,§ †
Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109, United States Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, United States § Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109, United States ‡
S Supporting Information *
ABSTRACT: Deposition of epitaxial germanium (Ge) thin films on silicon (Si) wafers has been achieved over large areas with aqueous feedstock solutions using electrochemical liquid phase epitaxy (ec-LPE) at low temperatures (T ≤ 90 °C). The ec-LPE method uniquely blends the simplicity and control of traditional electrodeposition with the material quality of melt growth. A new electrochemical cell design based on the compression of a liquid metal electrode into a thin cavity that enables ec-LPE is described. The epitaxial nature, low strain character, and crystallographic defect content of the resultant solid Ge films were analyzed by electron backscatter diffraction, scanning transmission electron microscopy, high resolution X-ray diffraction, and electron channeling contrast imaging. The results here show the first step toward a manufacturing infrastructure for traditional crystalline inorganic semiconductor epifilms that does not require high temperature, gaseous precursors, or complex apparatus.
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INTRODUCTION Optoelectronic technologies hinge on the methods to synthesize crystalline inorganic materials and to fabricate their heterojunctions with extreme quality and fidelity.1 The ability to deposit unstrained films of crystalline Ge on crystalline Si is particularly challenging due to the large lattice mismatch (4.2%).2 Although some strategies exist for high quality Ge/Si heterojunctions based on multistep epitaxy and/or annealing,2−4 aspect ratio trapping,5 or compositionally graded Si1−xGex buffers exist,6,7 they are complex, involve high temperatures, and are time-consuming. The reliance on toxic/ corrosive gaseous precursors and elevated temperatures further complicate integrating Ge epilayers on preexisting Si device platforms.8,9 We previously demonstrated the concept that bulk liquid metals can simultaneously facilitate the electroreduction of oxidized semiconductor precursors and the formation of zerovalent semiconductor nano- and microcrystals,10−12 affording a pathway to simplify preparation of crystalline semiconductors. However, to date, an analogous process for macroscopic, crystalline semiconductor films has never been conceived. In this work, we describe the invention of a new approach specifically for the preparation of single-crystalline semiconductor films across macroscopic areas that can be intrinsically simple without sacrificing crystallographic quality. Herein, we show a hybrid strategy that avoids excessive temperatures and can be run under benign (benchtop) © 2017 American Chemical Society
conditions with intrinsically inexpensive/simple equipment, nonflammable precursors, and no toxic waste products for the synthesis/deposition of epitaxial Ge thin films. We present for the first time the growth of Ge “epifilms” on Si wafers using large-area, thin liquid eutectic gallium indium (e-GaIn) films that act as dual electrodes/solvents in an electrochemical liquid phase epitaxial (ec-LPE) process (Figure 1). We first present a compression cell design that stabilizes liquid metal films for this purpose. We then show data that illustrate ec-LPE with this cell performed at ambient pressure, with an aqueous feedstock solution, and only at temperatures below the boiling point of water. Although the conditions for ec-LPE are less extreme than, require less ancillary equipment than, and are at least as tunable as vapor/vacuum based deposition strategies, the results argue that the crystallographic properties of the asprepared Ge films and their heterojunctions with Si are of high quality. The prospects for ec-LPE as a realizable methodology for semiconductor film deposition that maintains the crystallographic quality of liquid-phase epitaxy growths13 with the simplicity and control of low temperature electrodeposition14 are discussed. Received: February 28, 2017 Published: May 9, 2017 6960
DOI: 10.1021/jacs.7b01968 J. Am. Chem. Soc. 2017, 139, 6960−6968
Article
Journal of the American Chemical Society
Figure 1. (a) Summary view of an electrochemical liquid phase epitaxy (ec-LPE) film growth concept with a thin liquid metal electrode with thickness h. (b) Schematic depiction of the relevant steps in ec-LPE, where an oxidized species Xn+ dissolved in the electrolyte is reduced to X0 by heterogeneous electron transfer, partitions into the liquid metal film, and then diffuses some distance away from the liquid electrolyte/liquid metal interface before precipitating out. When h is sufficiently thin, preferential heterogeneous nucleation and crystal growth is possible at the bottom interface between the liquid metal and a seed substrate.
Figure 2. (a) Exploded view of the ec-LPE cell developed for this work. The cell has a compression stack design to facilitate the formation of a stable liquid metal film over a large two-dimensional area. Upon assembly, liquid metal is flowed into a thin cavity with the side walls and bottom defined by photoresist patterned on a seed substrate, respectively, and the top comprised of a porous membrane that is permeable to electrolyte flow but resists infusion by the liquid metal. (b) Optical photograph of an ec-LPE cell for film areas ∼ 4.8 cm2.
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membrane was to permit flow of the liquid electrolyte to allow the formation of a liquid electrolyte/liquid metal interface that facilitated heterogeneous reduction of GeO2 dissolved in the electrolyte. The flux of species through the membrane was determined by the thickness and ionic strength of the electrolyte, but current densities of 10−3 A cm−2 were routinely supported without approaching the compliance voltage limits of the employed potentiostat. The uncompensated resistance across the membrane measured before each ec-LPE experiment and was
