Epitaxial metal - GaAs contacts via electrodeposition

The fabrication of epitaxial metal-GaAs contacts via electrodeposition and the electrical properties of the Schottky diodes are reported in this dissertation. Epitaxial electrodeposition of copper, iron, cobalt, iron-nickel, and bismuth on GaAs was discovered to rely on three major factors: the preparation of the GaAs surface by (NH$_{4}$)OH etching, the addition of (NH$_{4}$)$_{2}$SO$_{4}$ to the electrolytes, and the control of deposition current density. The surface preparation by (NH$_{4}$)OH provides a hydrophilic surface likely due to passivation via a layer of hydroxide bonds. Ammonium sulphate inhibits oxidation of the metal cations and likely acts as a sur factant both on the metal and GaAs surfaces. Control of adatom flux, similar to vacuum deposition techniques such as molecular beam epitaxy, determined the crystallinity, varying from polycrystalline to epitaxial to dendritic for current densities from 0.01 to 1 mA/mm$^{2}$. The effects of other electrodeposition conditions including pH and electrolyte temperature were also investigated. Neutral electrolytes are needed for copper, iron, nickel-iron alloy, and cobalt epitaxial deposition, while acidic solution works for bismuth deposition indicating that the control of hydrogen evolution may be important. Cobalt nanodisc formation was obtained for lower temperatures (2 - 22 $^{\circ}$C) while optimal copper, iron, and bismuth epitaxy required higher deposition temperatures, 53, 56, and 70 $^{\circ}$C, respectively. The growth is via island nucleation and coalescence. The iron films develop a small decrease in lattice constant with residual compressive stress, which is a function of temperature. This is likely due to an impurity such as oxygen. Copper and cobalt formed nanometer scale reacted interfaces with GaAs, while iron and bismuth/GaAs formed abrupt interfaces. The electrical properties of these Schottky diodes were, nevertheless, found to be close to ideal and comparable to vacuum deposited diodes. Fe and Co/GaAs diodes showed identical electrical properties for GaAs (100), (110) and (111)B substrate orientations, consistent with uniform and defect-free interfaces. Cu and Bi/GaAs showed a much greater orientational dependence perhaps due to the larger lattice mismatch. Iron and nickel-iron alloy films showed anisotropic magnetic properties consistent with single crystalline material. These metal/GaAs contacts are potentially interesting for spintronics applications.