Bench Talk

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Piezoelectric devices represent a large class of devices that produce an electrical charge under an applied stress or mechanical load. Any physical deformation generates an electrical charge. Piezoelectric devices have been used in many different applications and electronic systems.

Like many large device classes, piezoelectric devices have numerous device subclasses. For example, one subclass within piezoelectric devices is piezotronics. Even within the piezotronics subclass is a further subclass called piezo-phototronics.

Piezo-phototronic devices use the principles of piezotronics but are geared towards more optoelectronic and photonic applications—and in turn use general piezoelectric principles that show new effects under certain circumstances.

Piezo-phototronic devices exhibit the piezotronics effect, which is when a piezopotential is used as a gate voltage to tune and control the charge carrier properties of the device. However, the key distinguishing feature between the materials used in piezotronic and piezo-phototronic systems is that piezo-phototronic materials are also responsive to light.

Piezo-phototronic systems revolve around the use of 1D semiconducting nanomaterials, as the piezoelectric effect produces a voltage potential that can be harnessed using these nanomaterials to tune, control, and modulate different electronic systems. In this article we look at what piezo-phototronic materials are and how they can be used in different optoelectronic devices.

Piezo-phototronics work using the piezo-phototronic effect. However, there is typically a focus on optoelectronic applications compared to the sensing and computing applications of piezotronic devices. Piezo-phototronic systems operate by using a combination of piezoelectric, semiconductor, and photonic properties. To understand the fundamental principles of piezo-phototronics, we need to look at the overarching piezoelectric principles that are responsible for their operation.

All piezoelectric devices—including both piezotronic and piezo-phototronic devices—exhibit the piezoelectric effect. The piezoelectric effect is the generation of an electrical voltage in a material when it is subject to a mechanical stress, strain, or deformation. The ions in the lattice rearrange to generate a positive charge on one side of the material and a negative charge on the other side.

This rearrangement of charges also generates a large dipole moment in materials that are non-centrosymmetric. Facilitating a dipole moment across the material generates an electrical potential known as a piezoelectric potential.

This potential is the fundamental principle and feature of both piezotronic and piezo-phototronic materials, with the main difference between the two systems being that when their principles are applied to photo-devices and optoelectronic devices, they are known as piezo-phototronic materials. This means that the materials used in piezo-phototronics need to exhibit some level of photoexcitation properties, whereas piezotronic devices do not.

Moreover, piezo-phototronic devices tend to be used with photo-induced currents as well, and these materials are ultimately used to improve the performance of different optoelectronic devices by modulating how much current flows through a device at a given time. Due to the photoexcitation requirements and ability to be used in optoelectronic devices, the nanowires investigated in piezo-phototronic applications so far have typically been ZnO, GaAs, and InN nanowires.

One of the biggest potential applications for piezo-phototronic materials is in solar cells. Solar cells using piezotronic principles are based around either metal-semiconductor interfaces or p-n junctions and a depletion zone is formed at the junction of the two materials. This depletion zone is full of neutrally charged species, with the photon-generated electron-hole pairs separated on either side of the junction.

Solar cells use a large electric field in the depletion zone to assist with the separation of the positive hole and negative electron charge carriers. When a photon hits the junction, the charge carriers get energy and move across the junction, breaking down the depletion zone until enough charge carriers have recombined to create a barrier once again between the two charge carrier species. At this point, the photons are removed since the energy source for the charge carrier movement is removed, leading to the recombination of charge carriers.

When piezo-phototronic materials are used in the solar cell junction, the polarization charges generated under strain can modify the electronic band structure at the interface. The electronic bands are deformed in these junctions, leading to discontinuity known as a Schottky barrier, which can have different ‘heights’ depending on how close it is to the interface. The Schottky barrier height positioning has a key influence on how many charge carriers pass through the junction, as it is a function of the different electronic bands near the interface.

By using the polarization charges under strain, it enables the band structure and better control of charge carrier generation, separation, and transport across the junction. This also means that the conduction and valence bands can be decreased, consequently lowering the Schottky barrier height of the junction that accelerates the separation of electrons and holes and reduces the possibility of recombination. So, by modulating and tuning the charge carriers across the junction, piezo-phototronic materials offer a way to control and enhance the performance of solar cells and improve their power conversion efficiencies (PCEs).

Photodetector operations are based on the separation of photon-generated electron-hole pairs across either a p-n junction or a Schottky barrier. This means that the Schottky barrier height is an important factor for determining the sensitivity of the device. Like with solar cells, inducing a strain in the piezo-phototronic material can help to reduce the Schottky barrier height, making it easier for charge carriers to pass through the junction. This has enabled photodetectors (especially UV photodetectors) to be created with a higher level of sensitivity and even be used when the illumination intensity is weak.

As a result, piezo-phototronic materials can significantly improve the performance of photodetectors that have weak light illumination, but the induced strain has little effect on the sensitivity to stronger light illumination. In fact, with elevated light illumination levels, the photodetectors can perform worse because newly generated charge carriers accumulate, screening the piezopotential and reducing it.

So, the enhancement of piezo-phototronic materials in photodetectors is geared more towards weak-illumination photodetectors. The piezopotential can be used to modulate and control the Schottky barrier height, which tunes the performance and responsivity of the detector—especially at low light levels. They are also suited to low-light illumination detectors because the piezo-phototronic strain-induced charges have no effect on the dark current—the current flowing through the detector when there is no incident photon flux—which is beneficial for low-intensity light detection as it preserves the low dark current characteristics of the device.

The final area is light emitting diodes (LEDs). Like many of the technologies listed here, piezo-phototronic LEDs rely on an efficient carrier injection, recombination, and extraction to function effectively. Like the other areas, the piezo-phototronic effect can enhance the efficiency of an LED. This is because the depletion zone width (formed under recombination) and the internal field is reduced during the forward-biased voltage. Additionally, the injection current and light-emitting intensity under the same forward voltage are increased when the device is strained and the piezo-phototronic material is producing a piezopotential.

Piezo-phototronic materials share many characteristics with piezotronic materials, and while they share the same piezopotential properties that help to modulate charge carriers and electric currents at different semiconducting interfaces, the photoexcitation and photoresponsivity properties of the materials make them suitable for a range of optoelectronic devices. The generation of piezoelectric charges through 1D semiconducting nanowires is offering new ways to improve and control the properties of solar cells, LEDs, and photodetectors.