Electrochemical Vapor Deposition of Semiconductors from Gas Phase with a Solid Membrane Cellby Sung Ki Cho, Fu-Ren F. Fan, Allen J. Bard

J. Am. Chem. Soc.

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Year
2015
DOI
10.1021/jacs.5b02878
Subject
Chemistry (all) / Colloid and Surface Chemistry / Biochemistry / Catalysis

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Text

Electrochemical Vapor Deposition of Semiconductors from Gas

Phase with a Solid Membrane Cell

Sung Ki Cho,§ Fu-Ren F. Fan, and Allen J. Bard*

Center for Electrochemistry, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712,

United States *S Supporting Information

ABSTRACT: We demonstrate the feasibility of semiconductor deposition via the electrochemical reduction of gaseous precursors by the use of an anhydrous proton-conducting membrane, the solid acid CsHSO4, at 165 °C. This membrane electrode assembly was operated within the oxidation of hydrogen on a porous Pt anode and the deposition of Si or Ge under bias at the cathode from chloride-based gaseous precursors; SiCl4 and GeCl4 in an Ar flow with a reduction potential over −1.0 V (vs RHE) ■ INTRODUCTION

Electrochemical deposition is one of the oldest and most widely used techniques for generating metal films. Its deposition reaction is driven by applying a potential that is more intensive and energy efficient than thermal processes, resulting in more economic production of material with good quality. Contrary to the vacuum-based technologies such as physical vapor deposition and chemical vapor deposition, electrochemical deposition is carried out in the liquid electrolyte containing ions, which could be incorporated in the deposit during deposition and become an impurity of the deposit film. The electrolyte is a major impurity source, and it is practically impossible to remove the impurity completely,1 whereby it has a significant effect on semiconductor electrodeposition when it is applied to a photovoltaic or other device, since the impurity level in the semiconductor is critical to its physical and electrical properties.

In this study, we explore the possibility of electrochemical deposition of metals or semiconductor without using liquid electrolyte, i.e., electrochemical deposition of metal from gas phase with solid electrolyte (membrane). There are several requirements for the operation of this process: (i) Membrane itself should not be easily reduced or oxidized electrochemically; (ii) metal precursor must be in the gas phase at the operating temperature of the membrane and be reducible electrochemically; (iii) conductive ion of the membrane should be coupled with the reduction reaction of metal source to satisfy the mass balance of conductive ion in the membrane.

Figure 1 shows a diagram of the reaction scheme for the reduction of a metal chloride on solid membranes.

While there has been little research on chloride-conducting membranes,2,3 many kinds of proton-conducting membranes such as sulfonated fluoropolymers (e.g., Nafion)4 or polyetheretherketone,5 polybenzimidazole doped with phosphoric acid,6 and perovskite-type oxides (e.g., zirconates and cerates)7 have been developed. In many cases, water molecules play an important role in their proton conduction as they pass proton in the hydrated form of H+(H2O)n, 8 which cannot be used for the reduction of metal chlorides, because they react chemically with water. Therefore, it is necessary to use an anhydrous pure proton conductor at a temperature well above ambient.

Solid acid, which has a structure of MHnXO4 {(M = K, Na,

Cs), (X = S, P, Se)}, is known as a proton-conducting membrane with a high conductivity along with the structural

Received: March 19, 2015

Figure 1. Conceptual diagrams of electrochemical deposition of metal from gas phase with (a) a proton-conducting membrane or (b) a chloride-conducting membrane.

Article pubs.acs.org/JACS © XXXX American Chemical Society A DOI: 10.1021/jacs.5b02878

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX change, called a superprotonic transition, at a critical temperature.8−10 Cesium hydrogen sulfate (CsHSO4) is a representative of solid acid, which has a conductivity of about 10−2 S/cm at the temperature above its superprotonic transition point (140 °C). It is a pure proton conductor9,10 as it has a reasonably high conductivity and a moderate operation temperature; this material is a good candidate for our purpose.

In this study, we report the silicon and germanium electrodeposition from the gas phase of silicon tetrachloride and germanium chloride. To our knowledge, it is the first report of electrodeposition from a gas-phase precursor. ■ EXPERIMENTAL SECTION

Experimental Materials. CsHSO4 was synthesized with a stoichiometric mixing of Cs2CO3 (Sigma-Aldrich, St. Louis, MO) and H2SO4 (Sigma-Aldrich, St. Louis, MO) in aqueous solution. The addition of acetone in the solution led to fast precipitation of CsHSO4 particles in the solution.9 The precipitate was filtered and dried at 60 °C in order to remove the solvent and water residues. X-ray diffractometry confirmed that powder has a monoclinic structure of

CsHSO4 (Figure S1a in the Supporting Information). The conductivity of CsHSO4 measured by AC impedance spectroscopy (CH Instruments model 660D potentiostat, Austin, TX) showed the superprotonic transition of the conductivity near 150 °C and its conductivity was 0.01 S/cm at 160 °C (Figure S1b, see the details in the Supporting Information), which are similar to those reported in the literature.9,10 Platinum black (Alfa Aesar, Ward Hill, MA) was used as electrode material for the reduction and oxidation of hydrogen gas.

In case of the electrolysis of metal chlorides, porous metal electrodes such as gold mesh (2000, 12.7 μm spacing, SPI Supplies, Inc., West

Chester, PA) or porous silver film (5 μm sized pore, SPI Supplies, Inc.,

West Chester, PA) were used as electrode. Contrary to metal powdertype electrode, well-defined geometry of mesh-type or porous metal film made it easier to observe the morphologies of deposits. Silicon tetrachloride (SiCl4, Sigma-Aldrich, St. Louis, MO) and germanium tetrachloride (GeCl4, Sigma-Aldrich, St. Louis, MO) were used as metal precursors during the electrolysis.

Fabrication of Membrane Electrode Assembly (MEA).

Platinum electrodes were prepared by loading Pt black suspended in toluene (Sigma-Aldrich, St. Louis, MO) solution on carbon paper (ElectroChem, Inc., Woburn, MA) via brushing. The loading amount of Pt black was about 5 mg/cm2. Pressing of CsHSO4 powder (0.5 g) and electrodes together at 340 MPa with mechanical press resulted in a MEA pellet with thickness of 1 mm and area of about 1.32 cm2. A single MEA has three electrodes. One (denoted as a working electrode) is a gold mesh of 0.4 × 0.3 cm2 or a porous silver film, which is used for the reduction of metal chloride. Another (denoted as a counter electrode) is placed on the opposite side of the membrane pellet, which is a 0.6 × 0.3 cm2 platinum electrode used for the oxidation of hydrogen gas. The other is placed next to the counter electrode, and it is a 0.4 × 0.3 cm2 platinum electrode used as a reference electrode during the electrochemical analysis (Figure 2).