Correlation between Structural and Photoelectrochemical Properties of GaN Porous Nanostructures Formed by Photo-Assisted Electrochemical Etching

Abstract

We investigated the correlation between structural and photoelectrochemical properties of GaN porous nanostructures formed by photo-assisted electrochemical etching. The porous nanostructures were formed during light irradiation of the top-surface of homo-epitaxial layers grown on freestanding GaN substrates. The pore depth, wall thickness, and surface morphology of porous nanostructures were strongly influenced by the way holes generated by the light irradiation were supplied. Such structural features influenced the optical properties of GaN porous nanostructures. The photoluminescence peaks measured on GaN porous nanostructures were shifted to higher energies because of the quantum confinement in the thin GaN walls between pores. Formation of porous nanostructure decreased the photoreflectance of the GaN surface, and the smallest reflectance was obtained from the porous sample having large pores on its surface after the ultrathin layer with small pores had been removed by surface-etching. The photoelectrochemical response measured on GaN porous nanostructures in a NaCl electrolyte were drastically enhanced by the unique features of those structures, such as low photoreflectance and large surface area. The largest photocurrents were obtained from the sample from which H 3 PO 4 treatment had removed the ultrathin layer without thinning the pore walls. Photoelectrochemical systems based on semiconductor photoelectrodes have recently attracted much attention due to their potential use in the next generation of green technologies such as water splitting for fuel cells, artificial photosynthesis, and so on.1-5 Among the photoelectrode materials, GaN is one of the most attractive because of its chemical stability and its potential to achieve direct photoelectrolysis by solar power without the consumption of electric power. [6][7][8] In addition, the bandgap energy of GaN-based materials can be varied from about 0.65 to 6.0 eV by alloying them with InN and AlN, which enables us to design various functional photoelectrodes not only for spectral matching of solar light but also for the electrochemical reduction of CO 2 to carbohydrate. 9 One of the common approaches to improving conversion efficiency is to form nanostructures on the photoelectrode surface in order to increase its surface area. Most reported GaN nanostructures have been made using selective-area growth 10,11 or a dry etching process such as reactive ion etching. 12,13 There are, however, severe limitations on increasing the density of nanostructures because most approaches use lithography for defining the size and position of the nanostructures. And when a dry etching process is involved, the etching damage induced by ion bombardment is not negligible [14][15][16] and could significantly degrade the photoelectrochemical efficiency.One alternative approach is an electrochemical-fabrication process, which can form various semiconductor nanostructures in a self-assembled fashion. The most well-known application of an electrochemical process is the formation...

Large photocurrents in GaN porous structures with a redshift of the photoabsorption edge

“…As shown in Fig. 3(b), as for the porous electrode, larger photocurrents were observed, reflecting the large surface area of the electrode, and its low photo-reflectance properties due to the high-density array of pores [16]. The photocurrents obtained at 0.5 V are re-plotted as a function of light wavelength in Fig.…”

“…From a detailed investigation of photocurrent-voltage characteristics, they arrived at their conclusion that the enhancement of photoresponse was attributed to GaN porous structures with a low-density defect surface, on which the surface recombination was suppressed as compared with that on the unetched surface. We have recently shown [16] that the photoresponse of GaN porous structures is also enhanced by the reduction of the surface photo-reflectance in addition to the increase of pore depth or surface area. The larger photocurrents obtained in this study can be similarly explained by the specific features of porous structures as described above.…”

Precise thickness control in recess etching of AlGaN/GaN hetero-structure using photocarrier-regulated electrochemical process

“…Generally, inhomogeneous etching of a semiconductor results from localized carrier transfer by high-electric field applied at the electrolyte/semiconductor interface. [19][20][21] Similar to the case with the carrier transfer in a Schottky barrier diode, [22][23][24][25] localized carrier transfer may be caused by inhomogeneous potential distribution due to the concentration of the electric field on crystallographic disorders such as dislocations and vacancies. Since inhomogeneous etching observed under this condition will be the origin of rough surfaces, the use of photo-holes generated in the GaN layer is unfavorable for the fabrication of recessed-gate structures.…”

Precise Structural Control of GaN Porous Nanostructures Utilizing Anisotropic Electrochemical and Chemical Etching for the Optical and Photoelectrochemical Applications

“…15 However, the pore depth could not be controlled linearly by etching time, resulting in difficulty with forming pores deeper than a micro-meter. This was because charge carriers, most of which were generated near the top surface, were expended preferentially in the lateral etching of pore walls that appeared on the top surface.…”

“…[11][12][13][14] Our group has also succeeded in forming GaN porous nanostructures by photo-assisted EC etching and found the energy-conversion efficiency to be enhanced compared with the planar substrate. 15 However, the pore depth could not be controlled linearly by etching time, resulting in difficulty with forming pores deeper than a micro-meter. This was because charge carriers, most of which were generated near the top surface, were expended preferentially in the lateral etching of pore walls that appeared on the top surface.…”