(Invited) Hydrogen Sensing Characteristics of Gallium Nitrides with Various Crystal Planes

Hydrogen has driven great attention as one of the most promising alternative energies. It does not produce any harmful byproducts such as carbon dioxide and nitrogen oxide during the conversion. However, energy conversion using hydrogen is still at research stage due to the possibility of explosion in the air even though it is almost matured in the respect of scientific technology. It is very important to detect low concentration of hydrogen promptly without false alarm in that it can promote the use of hydrogen energy to daily life applications.

     Recently, we demonstrated ultra-sensitive hydrogen sensors based on GaN using platinum nano-networks grown by facile solution method. Extremely large surface to volume ratio of nano-networks dramatically improved the sensitivities of the hydrogen sensors. Size, density and distribution of nano-networks can be controlled and optimized by the synthesis conditions and coating methods, and the selective area deposition of platinum nano-networks on the GaN surface was achieved for the fabrication of sensors.

     Nonpolar and semipolar GaN is of great current interest due to its promise for eliminating internal electric fields, which exist in conventional c-plane III-nitrides. In the polar crystal orientations, both spontaneous and piezoelectric polarization cause the quantum-confined Stark Effect (QCSE), resulting in the reduction of radiative recombination rate in quantum wells used as active layers in LEDs due to the separation of wavefunctions of electron and hole. There have been recent many reports on growth and fabrication of LEDs with nonpolar and semipolar GaN on SiC, LiAlO₂, Al₂O₃ and bulk GaN substrates.

     In this study, we investigated dependence of hydrogen sensing characteristics on various crystal planes of GaN including conventional gallium polar c-plane (0001), nitrogen polar c-plane (000-1), a-plane (11-20), m-plane (1-100), and semipolar plane (11-22). Density functional theory indicates much higher affinity of nitrogen in GaN to hydrogen and stronger bonding. Each plane has its own surface configuration and shows different hydrogen responsivity. In case of (11-22) plane GaN, its current response to hydrogen is very large even though its surface is terminated with gallium atoms, since nitrogens position right beneath the gallium atoms.