Katrien Strubbe

Electrochemistry and Photoetching of n‐GaN

The (photo)electrochemical behavior of n-GaN[0001] in various aqueous solutions was studied using rotating-disk voltammetry, cyclic voltammetry, and electrical impedance measurements. It was found that the bandedges of the semiconductor shift over 60 mV/pH unit, indicating acid-base equilibria at the interface. In H 2 SO 4 and KOH solutions, the photocurrent under anodic bias is associated with the oxidation of the semiconductor according to a three-equivalent reaction, leading to dissolution and roughening of the surface. In 1.2 M HCl solutions, n-GaN is stabilized for anodic decomposition due to the competing oxidation of Cl - ions. In the presence of oxalic acid and citric acid, anodic photocurrent multiplication was observed. Under cathodic polarization in the dark, Fe 3+ , Ce 4+ , HIO 3 , in acid medium and Fe(CN) 3- 6 in alkaline medium are electrochemically reduced at a diffusion-limited rate. In I M KOH a high reactivity for O 2 /H 2 O reduction is observed, explaining why n-GaN can be photoetched under open-circuit conditions in this solution.

A comparative study of electrochemically formed and vacuum-deposited n-GaAs/Au Schottky barriers using ballistic electron emission microscopy (BEEM)

A comparative study between n-GaAs/Au contacts, formed by electrochemical deposition or by vacuum evaporation, is presented. The main parameter, the barrier height ?B, was determined using three methods, i.e.?classical current-voltage and capacitance-voltage measurements as well as STM-based ballistic electron emission microscopy (BEEM). The latter method allowed us to determine the distribution of ?B over the contact area on a nanometre scale and showed that the electrochemically made contacts are inhomogeneous. The main result, confirmed by the three methods, was that ?B was higher for the electrochemically deposited contacts than for the evaporated ones. This higher value is attributed to O- groups, present at the interface during the electrochemical metallization, and forming interfacial dipoles with the Au, leading to an increase of the barrier.

Bromine‐Methanol as an Etchant for Semiconductors: A Fundamental Study on GaP I . Etching Behavior of n‐ and p‐Type

Electrochemical and etching experiments were performed at n- and p-type GaP single crystals in the commonly used etchant bromine-methanol to investigate the fundamental aspects of the etching reaction. The etching properties of these methanolic bromine solutions were similar to those of bromine solutions in which water is used as the solvent; thus, e.g., as in water, the etching kinetics and morphologies at the (111) and (111) faces are markedly different. In many cases of practical etching, methanol may be substituted by water as the solvent for bromine. The results allow us to propose an overall reaction equation for the etch process as well as a detailed mechanism involving radical decomposition intermediates of the semiconductor

Photoelectrochemical reactions at the n-GaN electrode in 1 M H2SO4 and in acidic solutions containing Cl− ions

The photoelectrochemical behaviour of n-GaN in contact with 1 M H2SO4 and with acidic solutions containing Cl− ions was studied using rotating-ring-disk voltammetry, electrochemical impedance spectroscopy and etching experiments. It was found that n-GaN is stabilized against photoanodic decomposition in the presence of Cl− ions due to the competing oxidation of Cl− to Cl2. The competition kinetics were interpreted on the basis of a mechanism, in which intrinsic surface states take part in the photoanodic oxidation of Cl−. The participation of such intrinsic surface states may explain the discrepancies found in the literature concerning the photoelectrochemical behaviour of n-GaN.

On the etching of GaP single crystals in aqueous bromine solutions

Abstract The etch rate of n- and p-GaP single crystals was measured in aqueous Br 2 solutions, using a flow-cell set-up. The etching process was found to occur by a chemical mechanism, involving the formation of radical decomposition intermediates. The difference in etching kinetics between the (111)- and the (111)-face resulted in a difference in the morphology of the surface after etching. Incorporating the GaP crystals in an electrochemical cell gave a strong interrelationship was found between the cathodic reduction of Br 2 , the chemical and the anodic etching of GaP.

Bromine‐Methanol as an Etchant for Semiconductors: A Fundamental Study on GaP II . Interaction Between Chemical and Anodic Etching of p‐Type

Rotating disk and rotating ring-disk electrode experiments were performed to investigate the anodic behavior of dark p-type GaP electrodes in bromine-methanol solutions. A strong interaction occurred between chemical etching of the semiconductor by bromine and anodic decomposition. A mechanism for the anodic dissolution reaction is proposed in which electrochemical as well as chemical reaction steps participate. Marked differences between the polar (111) and (111) faces are observed and discussed

Copper Plating on Resistive Substrates, Diffusion Barrier and Alternative Seed Layers

We have studied electrochemical deposition of copper on ruthenium-tantalum (Ru-Ta) alloy, tantalum (Ta), and cobalt (Co) substrates using cyclic voltammetry and galvanostatic methods. We show that a single-step direct-plating from acidic Cu bath approach is favorable on thin Ru-Ta films, while it presents a significant challenge for plating on resistive Ta and Co substrates.

Electrochemical formation and properties of n-GaAs/Au and n-GaAs/Ag Schottky barriers: Influence of surface composition upon the barrier height

The barrier height ΦB of electrochemically formed n-GaAs/Au Schottky barriers is investigated as a function of deposition potential VDu2006. Two potential regions can be distinguished: one in which ΦB is comparable to the value of the contacts made by evaporation and another one in which ΦB is higher. This is explained on the basis of a different surface composition of GaAs in the respective potential regions. At more positive VDu2006, gold will be deposited upon a surface at which the Ga atoms are –O− terminated. This leads to the formation of a Auδ+–Oδ− dipole layer at the interface and hence to an increase in ΦBu2006. At more negative VDu2006, metallic Ga will be formed during the cathodic decomposition of nn-GaAs. As a consequence, surface Ga–O− groups will no longer be present, no surface dipole layers will be formed and ΦB will be lower. This interpretation is supported by results, obtained on n-GaAs/Ag Schottky barriers formed in solutions with different pH.

Adsorption/Desorption of Suppressor Complex on Copper: Description of the Critical Potential

The acid cupric sulfate solutions, used for Damascene copper plating, contain several additives which provide the differential deposition rate inside cavities for void-free fill. Most of the models for this process, available in the literature, can successfully predict trends. However, these models miss fines as they are based on physical adsorption only. Since the copper surface has a complex chemistry, a physicochemical approach is needed to describe all the reaction mechanisms involved. In this work we provide some further insight into the thermodynamics of copper deposition inhibition by the suppressor polymer. The quantitative participation of each solution component is discussed and the nadsorption/desorption reaction for the suppression is balanced based on the achieved results. In addition, a value for the standard critical potential was obtained which made possible the solubility constant for the Cu(I)-Suppressor-Cl complex to be estimated.

Electroreduction of Co2 + and Ni2 + at III-V Semiconductors and Properties of the Semiconductor/Metal Interfaces Formed

The electrochemical deposition of Ni and Co on GaAs and GaP at pH 5 was investigated by means of cyclic voltammetry and rotating disk voltammetry. The current‐potential behavior of the two semiconductors in dilute metal ion solutions appeared to be different: at n‐GaP, a stepwise reduction of the metal ions was observed, whereas at n‐GaAs this was not the case. This observation can be explained on the basis of two alternative reaction mechanisms for the reduction of the monovalent intermediate. Under well‐chosen conditions of deposition potential and metal ion concentration, it appeared to be possible to form adherent n‐GaAs/Co, n‐GaAs/Ni, and n‐GaP/Co Schottky contacts with good rectifying properties. The barrier height of the n‐GaAs/Ni contacts appeared to increase as a function of time, leading to values of which were dependent on the deposition potential . At n‐GaAs/Co and n‐GaP/Co junctions, this aging effect was absent, leading to values of the barrier height which were independent of . It was further observed that the occurrence of side reactions during metal deposition may exert a great influence upon the properties of the metal/semiconductor junctions formed.