Steady State Photocapacitance Study of Semiconductor/Electrolyte Junctions II. Surface State Distribution and Charge Transfer Mechanisms

2018

J. Phys. Chem. C

Bismuth thin films constitute a promising nanostructure for the fabrication of spin-based devices. To achieve this goal, it is necessary to obtain high-quality Bi layers with controlled and reproducible properties. Therefore, studies focused on the understanding of the nucleation process and the correlation between the growth conditions and the film properties are of great interest. In this work, we have studied the electrodeposition of Bi thin films onto GaAs(111)B substrates at different overpotentials. In Part I, we have analyzed the nucleation of the films by means of potentiostatic curves. The current density transients have been deconvoluted into individual processes taking into account the energy band diagram of the semiconductor-electrolyte interface. The deconvolution of the current density transients indicates that Bi electrodeposition follows a 3D nucleation controlled by diffusion, accompanied by concurrent processes such as both proton adsorption and reduction. The competition of these processes is controlled by the energy distribution of the surface states at the semiconductor-electrolyte interface and determines the nucleation process. The correlation between the properties of the Bi films and the Bi/GaAs interface with the nucleation process, i.e., with overpotential, is discussed in detail in Part II of this work.

supporting

confidence: 87%

mentioning

confidence: 99%

Electrodeposition of Bi Thin Films on n-GaAs(111)B. I. Correlation between the Overpotential and the Nucleation Process

2018

J. Phys. Chem. C

“…In our case, OCP ≈ 0.07 V vs Ag/AgCl, which is equivalent to -4.77 eV with respect to the vacuum level [22]. The position of the EF approximately coincides with the energy of the highest occupied level of SSint, measured by photocapacitance spectroscopy (E1 = EC,s − 0.98 eV) [47]. This indicates that surface dangling bonds are filled, i.e., saturated by adsorbed species, which is the reason for the absence of surface reconstructions in liquid media [46].…”

Section: Energy Band Diagram Of the Seisupporting

confidence: 69%

Electrodeposition of Bi films on H covered n-GaAs(111)B substrates

2019

Electrochimica Acta

We have investigated how the presence of an adsorbed hydrogen layer affects the nucleation and properties of Bi layers grown by dc electrodeposition at different overpotentials on n-GaAs(111)B substrates with a carrier concentration of 1.3 •10 17 cm -3 in darkness and at 300 K. The kinetics of Bi(III) ions reduction is controlled by the overpotential but also negatively affected by the adsorbed hydrogen layer, as deduced from the deconvolution of the current density transients recorded during the nucleation of the films.The surface morphology and the structural properties of the Bi films are correlated with the nucleation process and therefore, influenced by both the overpotential and the adsorbed hydrogen layer. At low overpotentials, porous and rough Bi films with a low crystal quality are obtained due to the low rate of proton and Bi(III) ion reduction. As the overpotentials raises, the rate of these reactions increase leading to flatter and more compact Bi films with a higher crystal quality. The electrical properties of the Bi/n-GaAs interface depend on the interfacial states whose origin is again the combined effect of the adsorbed hydrogen layer and growth overpotential.

“…[39] This position approximately coincides with that obtained by photocapacitance spectroscopy for the highest occupied level of surface states associated with surface unsaturated bonds (E 1 = E C,s − 0.98 eV). [40] This indicates that surface dangling bonds (SS int ) are filled, i.e., saturated by adsorbed species, being the reason for the absence of surface reconstructions in liquid media [41]. Depending on the type of ion adsorbed at the surface, the OCP can slightly vary from 70 mV (adsorbed protons) to 100 mV (adsorbed anions) due to the different effect that each type of ion produces on the surface dipole.…”

Section: Continuous Illumination Electroless Deposition Of Bi Flakesmentioning

confidence: 99%

Use of light for the electrochemical deposition of Bi on n-GaAs substrates

2019

Electrochimica Acta

In this work, we have explored the possibility of using light to remove the adsorbed hydrogen layer that blocks the GaAs substrate surface when electrodepositing Bi thin films on lower-doped n-GaAs(111)B substrates. A light pulse of a few seconds applied under opencircuit (zero-current) conditions before starting the Bi electrodeposition in darkness has a small effect on the structural, morphological and interfacial electrical properties of the Bi film in comparison to layers deposited without the use of light. The potentiostatic curves recorded during the Bi nucleation show that the light pulse does not remove the adsorbed hydrogen layer but modifies the n-GaAs surface, inhibiting the reduction of Bi(III) ions. The atomic force microscopy analysis of the n-GaAs surface corroborates that the light degrades the surface by inducing photo-oxidation reactions, phenomenon that is correlated to the photocorrosion of the substrate. To maintain the electrical neutrality during photocorrosion, proton reduction and electroless deposition of Bi occur in parallel to the photo-oxidations.The simultaneity of these processes and the inhibition of Bi(III) ions to get reduced on those areas of the n-GaAs surface chemically altered enables the electroless deposition of unconnected Bi flakes with morphological, structural and interfacial electrical properties close to the state of the art of Bi thin films. Only the out of plane crystal quality of the Bi flakes show a small detriment whereas the Schottky barrier height slightly increases.