Nanowire Piezo-Phototronic Photodetector: Theory and Experimental Design

TLDR: A theoretical model for describing the characteristics of a metal-nanowire-metal structured piezo-phototronic photodetector is constructed and numerical simulations fit well to the experimental results of a CdS and ZnO nanowire based visible and UV detector, respectively.

Abstract: one-dimensional structures of these materials are ideal for fabricating strain-controlled piezo-phototronic devices. The strain applied to cause the deformation of the nanowires is mainly through shape change of the fl exible substrate that supports the device. Such devices can be the basis for active fl exible electronics, which uses the mechanical actuation from the substrate for inducing new electronic/optoelectronic effects. As the piezopotential is controlled by externally applied mechanical deformation with different orientation and magnitude, the piezo-phototronics effect can be combined with fl exible optoelectronics to promote new device functions. Previously, we have demonstrated the enhancement of the sensitivity of UV photodetector, [ 6 ] the response of photocells, [ 7 ] and the emission effi ciency of light emitting diodes. [ 8 ] In these reports, the coupling between piezoelectric effect and photoexcitation has been investigated experimentally. Theoretical calculation of the piezopotential along ZnO nanowires under different strain has been carried out, [ 9‐11 ] and a theoretical framework has been built for the two-way coupling between the piezoelectric effect and semiconductor transport properties. [ 12 ] Theoretical study for the three-way coupling in piezophototronics remains to be investigated. Constructing such a model will not only provide an in-depth understanding about the experimental results, but also explore the core phenomena and build high performance devices. Besides piezo-phototronic effect, other factors such as piezoresistance effect and change of contact area or contact condition can also affect the device performance. It is important to distinguish the contribution made by the piezo-phototronics effect from these other factors through theoretical analysis. In this paper, we have constructed a theoretical model and fabricated corresponding experimental devices to study the piezo-phototronic photodetectors based on single-Schottky and double-Schottky contacted metal‐semiconductor‐metal (MSM) structures. We have coupled the photoexcitation and piezoelectric terms into basic current equations to study their infl uence on the fi nal device performance. Theoretically predicted results have been quantitatively verifi ed by photodetectors based on CdS nanowires for visible light and ZnO nanowires for UV light. Our experimental results show that the piezo-phototronic effect dominates the performance of the photodetector rather than other experimental factors. It is shown that the piezophototronic effect is signifi cantly pronounced at low light intensities, which is important for extending the sensitivity and application range of the photodetector. The conclusions drawn on Schottky contacts present the core properties of the effect and can easily be extrapolated to other structures like p-n junctions. Finally, based on the theoretical model and experimental results, we have proposed three criteria for describing the contribution made by the piezo-phototronic effect to the performance of the photodetectors, which are useful for distinguishing this effect from other factors in governing the performance of the photodetector. The theoretical model for two-way coupling in piezotronics has been developed in a previous report. [ 12 ] Here we adopt the same assumptions and follow similar methods, as schematically shown in Figure 1 . The depletion approximation is assumed for the Schottky contact. Piezoelectric polarization is induced in a semiconductor nanowire when it is subjects to strain, it is reasonable to assume that the piezo charges are distributed in a layer in the depletion zone, which tune the Schottky barrier height. The formation of an inner potential will drive the free charge carriers to redistribute. If there is no external bias, the inner electric fi eld and net charges should only exist in the depletion zone at static or quasi-static state. The Schottky contact current equation will be used as the basic starting point. The infl uence of photoexcitation and piezo-charges on the material band structure will be discussed, and the fi nal coupled term will be integrated into the current transport equation. To give more intuitive perspective of the piezo-phototronic effect, we have also carried out numerical simulations. A one-dimensional model and other simplifi cations are adopted for easy understanding. The core equations and conclusions are shown in the anayltical model below. Current Density for a Forward Schottky Contact : For a piezophototronic photodetector, a measurement of the photoninduced current is an indication of photon intensity. The coupling effect of piezoelectricity and photon excitation is also