GaN surface states investigated by electrochemical studies

Abstract

We present a systematic study of electrochemically active surface states on MOCVD-grown n-type GaN in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy over a wide range of potentials and frequencies. In order to alter the surface states, the GaN samples are either etched or oxidized, and the influence of the surface treatment on the defect-mediated charge transfer to the electrolyte is investigated. Etching in HCl removes substoichiometric GaO x , and leads to a pronounced density of electrochemically active surface states. Oxidation effectively removes these surface states. Published by AIP Publishing. Recently, group-III nitride materials have attracted considerable interest in fields such as photo-electrochemistry and photocatalysis due to the favorable energy position of their band edges with respect to the redox levels of relevant species in liquid electrolytes. [1][2][3][4][5][6] The flexibility in doping III-nitrides in a controlled way enables the deterministic generation of either holes (for n-type doping) or electrons (for p-type doping) at the surface under illumination to selectively drive oxidation or reduction reactions, respectively. The presence of electronic defect states directly at or close to the semiconductor surface is known to influence charge transfer at semiconductor electrodes. Such electronically active surface defects, e.g., are responsible for surface band bending determining the separation of photo-excited charge carriers and laterally inhomogeneous charging of the surface due to local charge trapping. For a quantitative analysis of the influence of surface states on electrochemical processes and Schottky diodes, it is important to understand the mechanisms governing charge transfer across the GaN surface. In a previous work, 7 we have combined the contact potential difference (CPD) and photoconductivity measurements to investigate electronic processes at or close to illuminated GaN surfaces. We could show that localized surface defect states play a crucial role in the kinetics of photo-generated charges. Sachsenhauser et al. have investigated the charge transfer across the n-type SiC/electrolyte interface. 8 They performed cyclic voltammetry and impedance spectroscopy measurements over a wide range of potentials and frequencies. Adding the ferricyanide/ferrocyanide redox couple to the electrolyte made it possible to reveal the contribution of surface states to the charge transfer and determine the energy distribution of the surface states. In this work, we have investigated whether or not electrically active states located in the GaN band gap are also involved in the charge transfer to an electrolyte. To this end, we have systematically studied the n-type GaN grown by metal-organic chemical vapor deposition (MOCVD) in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy. Different surface treatments are applied in order to alter the surface states and investigate the influence of these on the details of defectmediated charge transfer.(000...

Band Bending of n-GaN under Ambient H2O Vapor Studied by X-ray Photoelectron Spectroscopy

“…Si-doped n-type GaN is one candidate for well-defined photoanodic materials for water splitting due to its chemical stability, − relatively longer diffusion length of carriers, , and proper band edge positions. ,− Owing to these advantages, n-GaN has been used for clarifying the mechanism of carrier transport through electrochemical measurement. ,− Winnerl et al investigated the electrochemical properties of n-GaN etched in HCl and passivated by oxidation . Their results indicate the possibility that surface states due to Ga dangling bonds on the n-GaN(0001) caused Fermi level pinning.…”

“…15,19−21 Winnerl et al investigated the electrochemical properties of n-GaN etched in HCl and passivated by oxidation. 20 Their results indicate the possibility that surface states due to Ga dangling bonds on the n-GaN(0001) caused Fermi level pinning. However, Chen and Kuo 22 and Sato et al 23 have reported that the Ga dangling bonds are theoretically compensated with the dissociative adsorption of H 2 O onto the GaN surface.…”

Photoluminescence Detection of Surface Oxidation Processes on InGaN/GaN Nanowire Arrays

“…Turning to the first question of similarity, we note that in the past several years an increasing body of observations has emerged that points to the fact that III-nitride surfaces, like silicon and silicon carbide, − also tend to form thin layers of natural oxide after exposure to air ambient conditions. − Another indication for the similarity of oxide and oxidized III-nitride surfaces is the similarity of the chemically sensitive photoluminescence that has been observed on MOX nanowire assemblies − and on InGaN/GaN opto-chemical transducers.…”

“…In all these cases, the photoactivated interaction between adsorbed molecules and the semiconductor nanostructures takes place at the nonpolar lateral sidewalls of the NWAs without any intentional coatings. In the past several years, however, an increasing body of observations has revealed that III-nitride surfaces, like silicon and silicon carbide, − also tend to form thin layers of natural oxide. − As a consequence, the surface-chemical interactions occurring at the nonpolar InGaN/GaN sidewalls are likely to bear some resemblance to more conventional metal oxide (MOX) nanowire assemblies which have been reported to exhibit chemically sensitive photoluminescence as well. − …”

Unassisted Highly Selective Gas-Phase CO2 Reduction with a Plasmonic Au/p-GaN Photocatalyst Using H2O as an Electron Donor

“…Although the CO 2 reduction can be achieved in Au/p-GaN heterostructures, the activity is still relatively low, possibly because of surface defects that can act as charge-trapping and/or carrier-recombination centers on GaN surfaces. Because oxides are commonly applied to passivate semiconductor surfaces, , we used a surface engineering approach via atomic layer deposition (ALD) to passivate the surface of our heterostructures. We introduced ALD-deposited Al 2 O 3 interfacial layers with different thicknesses and evaluated their influence on photocatalytic CO 2 reduction activity (see Methods).…”