In 2007, Prof. Zhonglin Wang pointed out the so-called piezotronic effect based on the observation of the local Schottky-barrier height (SBH) modulated by the strain along the piezoelectric direction, i.e. the c-axis of ZnO. In 2015, Zhenfu Zhao of Wang’s group reported an interesting result by applying compressive forces on GaN nanowires along the polarc-direction and the nonpolar m-direction respectively. Two distinguished effects of piezotronic and piezoresistive had been clarified in well-designed experiments. In order to gain the high signal-noise ratio in the nano-scaled measurement, Nanowin’s ultra-high quality HVPE-GaN films of c- and m-plane with dislocation density less than 5x106 cm-2 were adopted to produce the nanowires in top-down etching process.
Their experiment started with 300μm c-plane and m-plane GaN as substrates. A 400nm-thick SiO2 film was firstly deposited on the GaN substrate by plasma-enhanced chemical vapor deposition.Then a thin Ni metal film added on top of SiO2 by electron-beam evaporation. Subsequently, the Ni/SiO2-coated GaN substrate was annealed to form Ni nano-spheres on the surface. Reactive ion etching (RIE) was used to etch the SiO2 layer and the Ni mask was removed by diluted hydrochloric acid. The residue of SiO2 served as a mask for the growth of c-plane and m-plane GaN nanowires(NWs), which were formed by inductively coupled plasma reactive ion etching (ICP-RIE) with the mixture of BCl3/Cl2/Ar. Finally, the SiO2 mask was removed with hydrofluoric acid wet-etching.
The Pt-coated AFM probe was positioned at the surface of the GaN NWs, and the silver paste was adhered to the bottom GaN substrate in forming the source and drain electrodes. The experimental results with theoretical analysis revealed that the normal compressive force effectively modulates the SBH of Pt on c-plane GaN NWs, which is referred to as the piezotronic effect. However, the normal compressive force cannot change the electron transport characteristics in m-plane GaN NWs, where the crystal polarization axis is perpendicular to the force applied. Therefore, a piezoresistive effect occurs.
Further, an m-plane GaN piezotronic transistor exhibits a high sensitivity to the transverse force. The piezoelectric circuit integrated by c-plane GaN and m-plane GaN piezotronic transistors will effectively respond to any external force, which has potential applications in energy collection, biomedical sciences, strain sensors, or human machine interfaces. We believe Nanowin’s high-quality GaN substrates will contribute to the fabrications of new optoelectronic devices.