Electrodeposited semiconductor nanostructures & epitaxial thin films for flexible electronics

Single-crystal Si is the bedrock of semiconductor devices due to the high crystalline perfection which minimizes electron-hole recombination, and the dense native silicon oxide which minimizes surface states. To expand the palette of electronic materials beyond planar Si, an inexpensive source of highly ordered material is needed that can serve as an inert substrate for the epitaxial growth of grain boundary-free semiconductors, photonic materials, and superconductors. There is also a need for a simple, inexpensive, and scalable fabrication technique for the growth of semiconductor nanostructures and thin films. This dissertation focuses on the fabrication of semiconducting nanowires (polycrystalline Ge & epitaxial ZnO) and epitaxial thin films (Au & Cu₂O) using electrodeposition from an aqueous solution at ambient conditions as a simple benchtop process. Paper I describes a simple one-step electrodeposition of Ge nanowires on an indium-tin oxide substrate decorated with In nanoparticles. An In metal acts both as a catalyst for electrodeposition and as a solvent for recrystallization of the nanowires at ambient conditions. Ge nanowires are an attractive anode material for Li-ion batteries, due to their larger theoretical capacity compared to graphite. Paper II presents a scheme for epitaxial electrodeposition of ultrathin Au films on Si as an inexpensive proxy for single crystal Au for the electrodeposition of epitaxial Cu₂O thin films. A detailed study of the epitaxial growth, morphology, junction characteristics, and crystallinity is performed for both the Au and Cu₂O thin films. Paper III describes a technique for epitaxial lift-off of wafer-scale Au foils as transparent, single-crystal and flexible substrates for flexible electronics. The Au foils offer the order of traditional single-crystal semiconductors without the constraint of a rigid substrate. An organic light emitting diode is presented to evaluate the flexibility and transparency of Au foils. To study the single crystal nature of Au foil an epitaxial Cu₂O thin film inorganic diode with an improved diode quality factor is demonstrated --Abstract, page iv

Electrodeposition of Protocrystalline Germanium from Supercritical Difluoromethane

We report results for the electrochemistry of the germanium(II) tri-halide anions, [GeCl3]?, [GeBr3]? and [GeI3]?, in supercritical difluoromethane containing 60?mm [NnBu4][BF4] at 19.1?MPa and 358?K. The voltammetry shows mass-transport-limited currents for reduction to germanium at gold on the first scan. There is no evidence of a germanium stripping peak and, on subsequent scans, the electrode slowly passivates with the deposition of approximately 0.4??m of material. The redox potentials for the reduction of the three tri-halides are in the order [GeCl3]? <[GeBr3]? <[GeI3]?, with the iodide being the most easily reduced complex. Electrodeposition of germanium onto TiN electrodes from supercritical difluoromethane at 19.1?MPa and 358?K, using either 16?mm [EMIM][GeI3] with 60?mm [EMIM][BF4] or 16?mm [NnBu4][GeI3] with 60?mm [NnBu4][BF4], gave deposition rates of 2–3??m?h?1. Raman spectroscopy and transmission electron microscopy showed that the resulting germanium films were protocrystalline, containing nanocrystals of germanium embedded in an amorphous germanium matri

Electrodeposited Germanium Nanowires

Germanium (Ge) is a group IV semiconductor with superior electronic properties compared with silicon, such as larger carrier mobilities and smaller effective masses. It is also a candidate anode material for lithium-ion batteries. Here, a simple, one-step method is introduced to electrodeposit dense arrays of Ge nanowires onto indium tin oxide (ITO) substrates from aqueous solution. the electrochemical reduction of ITO produces in nanoparticles that act as a reduction site for aqueous Ge(IV) species, and as a solvent for the crystallization of Ge nanowires. Nanowires deposited at 95 °C have an average diameter of 100 nm, whereas those deposited at room temperature have an average diameter of 35 nm. Both optical absorption and Raman spectroscopy suggest that the electrodeposited Ge is degenerate. the material has an indirect bandgap of 0.90-0.92 eV, compared with a value of 0.67 eV for bulk, intrinsic Ge. the blue shift is attributed to the Moss-Burstein effect, because the material is a p-type degenerate semiconductor. on the basis of the magnitude of the blue shift, the hole concentration is estimated to be 8 × 1019 cm-3. This corresponds to an in impurity concentration of about 0.2 atom %. the resistivity of the wires is estimated to be 4 × 10-5 ·cm. the high conductivity of the wires should make them ideal for lithium-ion battery applications

Spin Coating Epitaxial Films

Spin-coated films, such as photoresists for lithography or perovskite films for solar cells, are either amorphous or polycrystalline. We show that epitaxial films of inorganic materials such as cesium lead bromide (CsPbBr3), lead(II) iodide (PbI2), zinc oxide (ZnO), and sodium chloride (NaCl) can be deposited onto a variety of single-crystal and single-crystal-like substrates by simply spin coating either solutions of the material or precursors to the material. The out-of-plane and in-plane orientations of the spin-coated films are determined by the substrate. The thin stagnant layer of supersaturated solution produced during spin coating promotes heterogeneous nucleation of the material onto the single-crystal substrate over homogeneous nucleation in the bulk solution, and ordered anion adlayers may lower the activation energy for nucleation on the surface. The method can be used to produce functional materials such as inorganic semiconductors or to deposit water-soluble materials such as NaCl that can serve as growth templates

Response to Comment on Spin Coating Epitaxial Films

Lu and Tang claim that the spin-coated films in our study are not epitaxial. They assume that all of the background intensity in the x-ray pole figures of the spin-coated materials is due to randomly oriented grains. There is no evidence for randomly oriented grains in the 2θ x-ray patterns. The background intensity in the pole figures is also comparable to the background from the single-crystal substrates, which is inconsistent with their assumption

Electrodeposited Germanium Nanowires

Germanium (Ge) is a group IV semiconductor with superior electronic properties compared with silicon, such as larger carrier mobilities and smaller effective masses. It is also a candidate anode material for lithium-ion batteries. Here, a simple, one-step method is introduced to electrodeposit dense arrays of Ge nanowires onto indium tin oxide (ITO) substrates from aqueous solution. The electrochemical reduction of ITO produces In nanoparticles that act as a reduction site for aqueous Ge(IV) species, and as a solvent for the crystallization of Ge nanowires. Nanowires deposited at 95 °C have an average diameter of 100 nm, whereas those deposited at room temperature have an average diameter of 35 nm. Both optical absorption and Raman spectroscopy suggest that the electrodeposited Ge is degenerate. The material has an indirect bandgap of 0.90–0.92 eV, compared with a value of 0.67 eV for bulk, intrinsic Ge. The blue shift is attributed to the Moss–Burstein effect, because the material is a p-type degenerate semiconductor. On the basis of the magnitude of the blue shift, the hole concentration is estimated to be 8 × 10¹⁹ cm^–3. This corresponds to an In impurity concentration of about 0.2 atom %. The resistivity of the wires is estimated to be 4 × 10^–5 Ω·cm. The high conductivity of the wires should make them ideal for lithium-ion battery applications

Nanometer-Thick Gold on Silicon as a Proxy for Single-Crystal Gold for the Electrodeposition of Epitaxial Cuprous Oxide Thin Films

Single-crystal Au is an excellent substrate for electrochemical epitaxial growth due to its chemical inertness, but the high cost of bulk Au single crystals prohibits their use in practical applications. Here, we show that ultrathin epitaxial films of Au electrodeposited onto Si(111), Si(100), and Si(110) wafers can serve as an inexpensive proxy for bulk single-crystal Au for the deposition of epitaxial films of cuprous oxide (Cu₂O). The Au films range in thickness from 7.7 nm for a film deposited for 5 min to 28.3 nm for a film deposited for 30 min. The film thicknesses are measured by low-angle X-ray reflectivity and X-ray Laue oscillations. High-resolution TEM shows that there is not an interfacial SiO_<i>x</i> layer between the Si and Au. The Au films deposited on the Si(111) substrates are smoother and have lower mosaic spread than those deposited onto Si(100) and Si(110). The mosaic spread of the Au(111) layer on Si(111) is only 0.15° for a 28.3 nm thick film. Au films deposited onto degenerate Si(111) exhibit ohmic behavior, whereas Au films deposited onto n-type Si(111) with a resistivity of 1.15 Ω·cm are rectifying with a barrier height of 0.85 eV. The Au and the Cu₂O follow the out-of-plane and in-plane orientations of the Si substrates, as determined by X-ray pole figures. The Au and Cu₂O films deposited on Si(100) and Si(110) are both twinned. The films grown on Si(100) have twins with a [221] orientation, and the films grown on Si(110) have twins with a [411] orientation. An interface model is proposed for all Si orientations, in which the −24.9% mismatch for the Au/Si system is reduced to only +0.13% by a coincident site lattice in which 4 unit meshes of Au coincide with 3 unit meshes of Si. Although this study only considers the deposition of epitaxial Cu₂O films on electrodeposited Au/Si, the thin Au films should serve as high-quality substrates for the deposition of a wide variety of epitaxial materials