Piezotronics and Piezo-Phototronics

Starting from the road map for microelectronics, the focus of future electronics will be on functionalities toward personal, portable, polymer, sensor, and self-powering applications. The integration of these characteristics with the fast speed and high density as defined by Moore’s law will lead to the development of smart systems and self-powered systems. This chapter first introduces the basic physics of piezotronics and piezo-phototronics from band structure theory. Then blue-prints for future impacts and applications of piezotronics and piezo-phototronics are presented. The role anticipated to be played by piezotronics in the era of “Beyond Moore” is similar to the mechanosensation in physiology that provides a direct human “interfacing” with CMOS technology. It presents a paradigm shift for developing revolutionary technologies for force/pressure triggered/controlled electronic devices, sensors, MEMS, human–computer interfacing, nanorobotics, touch-pad, solar cell, photon detector and light-emitting diodes.

The most fundamental physics for piezotronics and piezo-phototronics is in the presence of a piezoelectric potential (piezopotential) in semiconductor structured materials, such as the wurtzite structure. This chapter introduces the fundamental theory for calculating the piezopotential distribution in nanostructures with and without considering the presence of doping. The finite conductivity possessed by the material can partially screen the regional piezopotential having an opposite polarity to the type of doping, but cannot completely cancel the polarization charge due to the dielectric property of the material and the moderate doping level. The effect of piezopotential on the local contact in electrical measurements is also discussed, and a through-end model is proposed for understanding the transport properties of nanowire-based devices. This model will be adopted in future chapters for understanding the I–V characteristics of the devices.

Using the basic transport equations, this chapter gives the theory of charge transport in piezotronic devices. Besides presenting the formal theoretical frame work, analytical solutions are presented for cases like the metal–semiconductor contact and p–n junction under simplified conditions. Numerical calculations are given for predicting the current–voltage characteristics of a general piezotronic transistor: metal–ZnO nanowire–metal device. This study is important for understanding the working principle and characteristics of piezotronic devices, but also for providing guidance for device design.

By using the piezopotential as the “gating voltage” for tuning/controlling interface charge transport, this chapter presents the fundamental principle and fabrication of piezotronic transistors using horizontal and vertical oriented nanowires. The piezotronic transistor is a two-terminal transistor without a gate electrode. The replacement of an external voltage gating by an inner crystal potential gating makes it possible to fabricate arrays of devices using vertical nanowires that can be individually addressed/controlled. This is advantageous for fabricating a high density device matrix for electro-mechanical transduction, such as sensors and touch pad technology.

In this chapter, by utilizing the gating effect produced by the piezopotential in a nanowire under externally applied deformation, piezotronic transistors have been fabricated; one can use them for the universal logic operations such as NAND, NOR and XOR gates as has been demonstrated for performing piezotronic logic operations. The mechanical–electronic logic units are an important step toward the basic design of complex systems in human–CMOS interfacing, touch pad technology and active flexible, nanorobotics, active flexible electronics, microfluidics and MEMS.

In this chapter, we treat the piezoelectrically modulated resistive switching device based on a piezotronic nanowire, through which the write/read access of the memory cell is programmed via electromechanical modulation. Adjusted by the strain-induced polarization charges created at the semiconductor/metal interface under externally applied deformation by the piezoelectric effect, the resistive switching characteristics of the cell can be modulated in a controlled manner, and the logic levels of the strain stored in the cell can be recorded and read out, which has the potential for integrating with NEMS technology to achieve micro/nanosystems capable for intelligent and self-sufficient multidimensional operations.

Effective electron–hole separation at a p–n junction is important for the efficiency of a solar cell. Band structure modification at the junction can be achieved by the piezo-phototronic effect, which is demonstrated in this chapter for tuning the solar cell output made using poly(3-hexylthiophene) (P3HT)-ZnO micro/nanowire and a n-CdS/p-Cu₂S coaxial nanowire. This effect offers a new concept for improving solar energy conversation efficiency by designing the orientation of the nanowires and the strain to be purposely introduced in the packaging of the solar cells.

An effective electron–hole separation at a Schottky contact or p–n junction is important for the efficiency of a photon detector. In this chapter, we demonstrate how the piezo-phototronic effect can be used to largely improve the responsivity of a photon detector in a whole range from visible to UV. After a systematic study on the Schottky barrier height change with tuning the strain and the excitation light intensity, an in-depth understanding is provided about the physical mechanism of the coupling of piezoelectric, optical and semiconducting properties. Our results show that the piezo-phototronic effect can enhance the detection sensitivity more than fivefold for pW levels light detection.

As a classical device, the performance of an LED is dictated by the structure of the p–n junction and the characteristics of the semiconductor materials. Once an LED is made, its efficiency is determined largely by the local charge carrier densities and the time at which the charges can remain at the vicinity of the junction. The latter is traditionally controlled by growing a quantum well or using a built-in electronic polarization for “trapping” electrons and holes in the conduction and valence bands, respectively. This is a rather complex and expensive process involving MBE and MOCVD, and more importantly, the width and potential well depth of the quantum well are fixed once the growth is complete. In this chapter, we describe the piezo-phototronic effect on the light emitting of a n-ZnO–p-GaN structure for illustrating its general impact to LED. The emission intensity and injection current at a fixed applied voltage have been enhanced by a factor of 17 and 4 after applying a 0.093 % compressive strain, respectively, and the corresponding conversion efficiency has been improved by a factor of 4.25 in reference to that without applying strain! The absolute external efficiency has reached 7.82 %. This hugely improved performance is suggested to arise from an effective increase in the local “biased voltage” as a result of the band shift caused by piezopotential and the trapping of holes/electrons at the interface region in a channel created by the piezopotential near the interface. The study shows that the piezo-phototronic effect can be very effectively used for enhancing the efficiency of energy conversion in today’s green and renewable energy technology without using the sophisticated nanofabrication procedures that are of high cost and complex. The physical model presented can be expanded to many other materials.

Photoelectrochemical (PEC) processes are fundamental for photon water splitting and energy storages. The key to the PEC efficiency is dictated by the charge generation and separation processes. In this chapter, we present the piezoelectric on PEC, in which a consistent enhancement or reduction of photocurrent was observed when tensile or compressive strains were applied to the ZnO anode, respectively. The photocurrent variation is attributed to a change in barrier height at the ZnO/electrolyte interface. We also introduce a fundamental mechanism that directly hybridizes the two processes into one, using which the mechanical energy is directly converted and simultaneously stored as chemical energy without going through the intermediate step of first converting into electricity. By replacing the polyethylene (PE) separator as for conventional Li battery with a piezoelectric poly(vinylidene fluoride) (PVDF) film, the piezoelectric potential from the PVDF film as created by mechanical straining acts as a charge pump to drive Li ions to migrate from cathode to the anode accompanying with charging reactions at electrodes. This new approach can be applied to fabricating a self-charging power cell (SCPC) for sustainable driving micro/nano-systems and personal electronics.