III-nitride semiconductors are promising candidates for the photoelectrodes in solar water splitting because of their stability in electrolyte and the wide bandgap to straddle the redox potentials for the evolutions of hydrogen and oxygen. However, the wide bandgap is not suitable for the absorption of visible photons and a tandem structure consisting of narrower-gap InGaN is necessary for high energy efficiency. Conventional InGaN layers grown on group-III polar surfaces has a large drawback of polarization-induced electric field which hinders carrier extraction to the surface of the semiconductor structure. We have proposed a novel tandem structure in which a thin AlN sandwiched by InGaN (or GaN) serves as a tunnel junction and the polarization-induced field assists the transport of carriers to the designated surfaces. This novel structure employs group-III polar crystal layers and it can be grown with conventional metalorganic vapor-phase epitaxy process. Furthermore, the structure functions as a photocathode which is more tolerant against surface corrosion compared with a photoanode. So far, the performance of a photocarhode by nitride semiconductors has been very poor due to the inferior crystal quality of p-type nitride layers. Our polarization-engineered tandem structure with a thin AlN interlayer allows a low-doped n-type layer on the electrode surface to function as a photocathode. As a result, the novel photocathode performs in much more stable manner than a photoanode using the same low-doped n-type layer.
The development of the new structure started from the optimization of layer thicknesses, especially for the AlN interlayer. Thinner layer is better for the tunneling of carriers but the polarization-induced field to align the conduction and valence band edges at both sides of the AlN needs thicker AlN layer. The next bottleneck existed in the crystal quality of the AlN layer. A thin AlN layer grown in a coherent manner to the (In)GaN layers at the both sides, with the maximum tensile strain in the AlN, was necessary to maximize the polarization-induced electric field. However, the AlN/(In)GaN interface is very susceptive to atomic interdiffusion. Crystal growth method was improved substantially in order to minimize such diffusion. Last but not least, the growth of high-quality InGaN is mandatory, which has been realized by using GaN native substrates.
Such structural optimization and the improved growth technology allows us to fabricate a n-InGaN/AlN/n-GaN tandem photocathode, and the onset potential of photocurrent exceeded the redox potential for oxygen evolution.