Cracking thin-solid GaN films into nanobelts
IMRE researchers reveal a novel technique for fabricating gallium nitride based nanobelts with enhanced functions and applications
Researchers from Institute of Materials Research and Engineering (IMRE), Singapore, claim to have fabricated InGaN/GaN hetero-structured nanobelts that set a new benchmark for performance.
The team can control the structure and doping of these belts, which makes them more attractive candidates for piezotronic and optoelectronic devices.
It is possible to make the nanobelts as long as desired, and they can be arranged in parallel arrays for integration into devices.
Fabrication involves MOCVD growth of an InGaN/GaN bilayer film that has an asymmetric elastic stress. When the bilayer film is detached from its substrate, the strain relaxation of the bilayer film drives itself to curve and crack along a certain direction, guided by the asymmetrically stored stress.
To asymmetrically store the elastic stress, r-plane sapphire was selected as the substrate, so that the c-axis of the GaN-related layers lies in the growing plane (i.e., parallel to the surface plane). For easy detachment of the bilayer film, ZnO sacrificial template was introduced before the MOCVD growth of InGaN/GaN heterostructure.
MOCVD deposition enables the control of the doping and thickness of the InGaN/GaN bilayer. The ZnO template was first epitaxially grown on an r-plane sapphire substrate by rf-magnetron sputtering. A 50 nm GaN film was then epitaxially grown by MOCVD on the ZnO sacrificial template.
To improve the crystal quality, magnesium-doping was employed as the surfactant to enhance the mobility of the adatoms on the growing surface. The structure was ended by a 150 nm InGaN thin film grown on the GaN layer, forming the InGaN/GaN bilayer. The in-plane asymmetrical stress induced by ZnO and InGaN was stored in the InGaN/GaN bilayer. When the ZnO buffer layer was laterally etched off, the bilayer was detached and cracked.
Guiding by the asymmetrical stress, the cracks propagate within the bilayer along the c-axis (lying parallel to the surface plane), forming the parallel nanobelts array. The length of the nanobelts can be varied, determined by the sample size while parallel nanobelts array can be transferred to any desired substrates with the help of wax.
The crack density and the width of the InGaN/GaN nanobelts are simply controlled by the amount of elastic stress stored in the bilayer film, which is dependent on the indium composition within the InGaN layer and the InGaN-to-GaN thickness ratio.
Further details of this work will be described in a paper accepted for publication by H. F. Liu et al in Nano Energy.