Piezotronics : A new field of strain - engineered functional semiconductor devices

P iezoelectric materials are the key functional component in many devices, such as sensors, actuators, ultrasonic transducers, sonar systems, and energy scavengers. These applications take advantage of the direct or reverse piezoelectric effects caused by simultaneous shifts in positive and negative charge centers within the primitive unit cell in response to mechanical deformation. Ideal piezoelectric materials also are perfectly dielectric. However, most piezoelectric materials are wide bandgap semiconductors that have a finite amount of free charges. The polar field resulting from the direct piezoelectric effect naturally interacts with charged species present in the solid in a Coulombic manner, and, thus, influences charge carrier distribution throughout the solid. This interaction also exists in many widely used semiconductors that are piezoelectric, such as ZnO, GaN, and CdS. Nevertheless, this polar field–charge interaction effect long has been overlooked by piezoelectric and semiconductor researchers until the recent emergence of the field known as piezotronics.1 Piezotronics is a new field that deals with the coupling of piezoelectric polarization (Ppz) with semiconductor properties to design new devices with novel functionalities and enhanced capabilities (Fig. 1). The general principle of piezotronics lies on the Ppz-induced internal and external free charge redistribution that can tune the local interfacial band structure and, thus, provide a mechanism to engineer the charge transport properties without altering the interface structure or chemistry.2-4 In a heterojunction, the effect of the energy state discontinuity is profound, with electronic transport properties that are exquisitely sensitive to the magnitude of the discontinuity. It then follows axiomatically that the electronic properties of the heterojunction system can be tailored by precise modification of the interfacial energetics. To that end, Ppz could have a significant influence on the heterostructure’s electronic properties. In 2006, the piezotronic phenomenon was first demonstrated in a Ppz-gated ZnO nanowire (NW) transistor. 5 The great promise of the piezotronic principle has been explored since then in a variety of semiconductor systems as a means for gating transistors, switching diodes, augmenting the quantum efficiency of light-emitting diodes (LEDs), improving photovoltaic (PV) performance, and optimizing catalytic ability.6 This emerging field has quickly attracted researchers worldwide from a wide range of disciplines, including materials science, physics, chemistry, electrical engineering, and mechanical engineering. This article outlines the basic principles, current research progress, and promising future of the new, interdisciplinary research field of piezotronics. Inspired by the human eye, this curved photodetector made of flexible germanium could eliminate the distortion that occurs in conventional photolenses.