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

Abstract

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.

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

“…In darkness, the OCP dark ≈ 70 mV indicates that the GaAs surface is covered by a H ads layer. [10,12] When the light is switched on, the OCP shifts cathodically, reaching a value of OCP light ≈ -280 mV, in agreement with previous work [18]. After 7 s, which is denoted as LP as abovementioned, the illumination is switched off and the OCP acquires a value of OCP LP ≈ 40 mV.…”

“…This is a consequence of the doping level of the n-GaAs substrate, which is low enough to produce a wide space charge region that avoids the necessary electrons tunneling from the electrolyte to the substrate for the anodic reaction. [12] The onset potential of the Bi(III) ion reduction (defined as the intersection of the rising current of the cathodic peak with its baseline) is E onset (Bi) ≈ -230 mV in both cases, which is approximately the onset potential of H + reduction. This is a consequence of the presence of the H ads layer on the n-GaAs surface which blocks surface sites that are required for the reduction of Bi(III) ions.…”

“…c show that the nucleation model accurately describes the experimental transients, similarly to our previous work. [8,12] In both cases the charging of the ScEI (j ScEI , red dashed line) is undetectable for the growth potential and time step used. [8,12] The anion desorption (j des , blue dashed line) has a similar time constant ( des ) in both cases, i.e., the kinetics of this process is not altered by the LP.…”

“…This is a consequence of the presence of the H ads layer on the n-GaAs surface which blocks surface sites that are required for the reduction of Bi(III) ions. [13,10,12] When H + reduction is activated, some surface sites become free , and Bi (III) ions can get reduced on the n-GaAs surface. Consequently, in H ads -covered n-GaAs substrates, Bi(III) ion reduction takes place simultaneously to H + reduction.…”

“…[10] Finally, the electrodeposition conditions have a strong impact not only on the morphological and crystal quality of the film but also on the electrical properties of the interface. [11,12] However, one of the main drawbacks of electrodepositing Bi onto n-GaAs is the presence of an adsorbed hydrogen (H ads ) layer at the substrate surface that results from the interaction between the n-GaAs surface states and the protons of the electrolyte. [13] Although it might protect the n-GaAs surface form oxidation, it also hinders the nucleation of Bi(III) ions avoiding the growth of compact and high quality Bi thin films.…”