Piezotronics for sensors and energy technology
Strain-induced polarization charges in piezoelectric semiconductors modulate the electronic and optoelectronic processes of charge carriers at the interface, opening up novel applications.
New technologies for developing electronics/optoelectronics with tunable functionalities and performance are critical to emerging applications in wearable electronics, communications, robotics, prosthetics, and biomedical treatments. For these applications, the active and adaptive interactions between devices and stimuli (e.g., from the human body) are essential. Although mechanical stimuli are ubiquitous and abundant in the environment for potential interactions with these electronics/optoelectronics,1–3 they are difficult to implement with conventional silicon devices.
The piezoelectric effect has been widely used for electromechanical sensing, actuating, and energy harvesting. Charge polarization occurs in response to mechanical deformation in materials lacking inversion symmetry. Conventional piezoelectric materials such as Pb(ZrxTi1−x)O3 (PZT) and polyvinylidene fluoride (PVDF) are insulating and hence not suitable for constructing functional electronics or optoelectronics. Because of this, the effect of mechanically induced polarization on charge carriers in piezoelectric materials has long been overlooked. Semiconductor materials such as zinc oxide (ZnO), gallium nitride, and cadmium sulfide with wurtzite or zinc blende structure also possess piezoelectric properties. However, they have relatively small piezoelectric coefficients and so have not been used as extensively in piezoelectric sensors and actuators.4
Despite this, their combination of piezoelectric polarization with semiconductor properties gives rise to novel fundamental phenomena and has novel device applications, leading to increasing interest in the emerging field of ‘piezotronics’ since it was first identified in 2006.5 The piezotronic effect modulates charge carrier transport across a metal-semiconductor (M-S) barrier or p-n junction by mechanical deformation. Strain-induced charge polarization results from redistributed free carriers and band structure changes near the interface (see Figure 1).1,2,4 Generally, the negative piezoelectric polarization charges induced near the barrier/junction interface repel electrons, further depleting the interface. Positive piezoelectric polarization charges attract electrons toward the interface, reducing depletion. Strain-induced polarization charges hence effectively modulate the local interfacial band structure and charge carrier transport. ‘Piezotronics’ describes electronic devices using these interfacial piezoelectric polarization charges as gate-controlling signals. Such devices are fundamentally different in operation from, for example, the electrically gated field effect transistor.
By replacing the external gating voltage with strain-induced polarization charges for controlling charge transport, we have developed two-terminal strain-gated piezotronic transistors.6, 7 More complex functionalities have also been developed following these preliminary demonstrations, such as strain-gated piezotronic logic nanodevices for performing mechanically modulated electronic logic operation. Another example is a piezotronic strain memory device with write/read access of the memory cell programmed via electromechanical modulation for electrically recording and reading out the logical levels of applied strain.6, 8
Eliminating the gate electrode in a piezotronic transistor enables novel 3D structuring. Using the piezoelectric polarization charges created at the M-S interface to modulate transport process of local charge carriers, we could apply the piezotronic effect to design independently addressable two-terminal transistor arrays. In this way, we converted mechanical stimuli applied to the devices into local electronic controlling signals. Based on this concept, we designed and developed a way to integrate circuitry of vertical ZnO nanowire (NW) transistors in 3D on a large scale (92×92 tactile pixels in 1cm2) as a flexible force/pressure-sensor matrix for artificial skin (see Figure 2).5,7
Our artificial skin exhibits performance comparable to human skins. Its integration complexity, spatial resolution, and response are superior to any previous tactile sensing nanodevices. Remarkably, the strain-gated vertical piezotronic transistors have several unique capabilities not available in existing technologies. First, they can detect changes to the device shape in situ in real time and feed back the sensed changes to calibrate other system functions, for instance, for an artificial tissue or prosthetic device. Second, self-powered active tactile sensing emulates the physiological operations of biological mechanoreceptors, such as human hair follicles in the skin and hair cells in the cochlea and can stimulate neurons, for instance, for regenerative therapy in prosthetic skins. The mechanism is completely different from the piezoresistive effect (see Table 1). The change from a three-terminal configuration to a two-terminal configuration significantly simplifies layout design and circuitry fabrication while maintaining effective control over individual devices.
|Piezoresistive effect||Piezotronic effect|
|Linear current-voltage curve||Non-linear ‘rectifying’ current-voltage curve|
|Symmetric effect on end-contacts||Asymmetric effect on end-contacts|
|No polarity||Strong polarity|
|‘Volume’ effect||‘Interface’ effect|
|No switch function||Switch function|
The piezoelectric polarization charges can also control the charge separation, transport, and recombination in optoelectronic processes,2, 9,10 which is the piezo-phototronic effect. This effect enhances the performance of photocells,11–13 the sensitivity of photodetectors, and the external efficiency of an LED.14, 15 Furthermore, strain-controlled LED emission can be used to directly ‘image’ the force/pressure distribution on a device with micrometer resolution,16 by using strain-induced polarization charges to tune local light-emitting intensity from individual NW-LEDs. The 2D distribution of light-emission intensity then becomes a map of the pressure distribution on the surface (see Figure 3).5 This approach is novel in relying on the piezoelectric polarization charges for a stable, fast response and parallel-detection strain-sensor arrays. The output signal is electroluminescence, which is easy to integrate with photonic technologies for fast data transmission, processing, and recording, and may enable the development of highly intelligent human-machine interfaces. This may represent a major step toward on-chip recording of mechanical signals by optical means.
In summary, the essence of the emerging research and applications in piezotronics and piezo-phototronics relies on the coupling between strain-induced polarization and semiconductor properties in piezoelectric semiconductor materials. Piezotronics provides a novel approach for modulating device characteristics by tuning the junction/contact properties. Until now, it was not possible to do this without modifying the interface structure or chemistry. We expect piezotronics and piezo-phototronics to enable technological advances in sensing, human-electronics interfacing, robotics, biomedical therapy, prosthetics, bioimaging, and optical microelectromechanical systems. Future work will include producing the tactile pixel arrays from single NWs instead of bundles to improve the sensitivity of the arrays by at least three orders of magnitude and integrating the arrays onto CMOS silicon devices.