Highly n-type doped InGaN films for efficient direct solar hydrogen generation

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Abstract

We report on the metal organic vapour phase epitaxy (MOVPE) and characterization of n-type InGaN films for application in direct solar water splitting. The 40 nm thick films exhibit an In fraction of 15% and show good structural quality and a surface root mean square (RMS) roughness of only 0.7 nm. An electron concentration of 9 × 1018 cm-3 at a mobility of 185 cm2/Vs is assessed. The emission peak in photoluminescence (PL) spectra taken at room temperature (RT) is at 2.8 eV, which is consistent to transmission data and the expected bandgap value for this composition. Illumination of the InGaN electrode with a Xe lamp in a 1 M NaOH electrolyte solution at zero external voltage induces a photocurrent (PC) of 0.13 mA/cm2 with a pronounced overshoot in the first few seconds, which we explain by the discharging of a Helmholtz bilayer capacitor. The PC linearly increases with applied voltage while the estimated total conversion efficiency peaks at 0.6 V. Although hydrogen bubbles visibly evolve from the Pt counter electrode, total conversion efficiencies are limited to 0.05%, which is ascribed to incomplete utilization of the optical excitation spectrum and low thickness of both the InGaN film and the space charge region at the semiconductor surface. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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