Low-Voltage High-Performance UV Photodetectors: An Interplay between Grain Boundaries and Debye Length

Abstract

Accurate detection of UV light by wearable low-power devices has many important applications including environmental monitoring, space to space communication, and defense. Here, we report the structural engineering of ultraporous ZnO nanoparticle networks for fabrication of very low-voltage high-performance UV photodetectors. A record high photo- to dark-current ratio of 3.3 × 10 and detectivity of 3.2 × 10 Jones at an ultralow operation bias of 2 mV and low UV-light intensity of 86 μW·cm are achieved by controlling the interplay between grain boundaries and surface depletion depth of ZnO nanoscale semiconductors. An optimal window of structural properties is determined by varying the particle size of ultraporous nanoparticle networks from 10 to 42 nm. We find that small electron-depleted nanoparticles (≤40 nm) are necessary to minimize the dark-current; however, the rise in photocurrent is tampered with decreasing particle size due to the increasing density of grain boundaries. These findings reveal that nanoparticles with a size close to twice their Debye length are required for high photo- to dark-current ratio and detectivity, while further decreasing their size decreases the photodetector performance.

High‐Temperature One‐Step Synthesis of Efficient Nanostructured Bismuth Vanadate Photoanodes for Water Oxidation

“…For all materials examined, n was ½, which is indicative of indirect transition, consistent with the previous literature . The porosity of the BiVO 4 films ( ε ) was estimated from the absorbance and SEM visible thickness, as described in Equation and . where d bulk (cm) is the thickness of BiVO 4 estimated from the absorbance ( A ) at 400 nm using an absorption coefficient of 1 × 10 5 cm −1 , and is the total film thickness determined by a cross‐sectional SEM imaging.…”

Effects of Grain Size on the UV‐Photoresponse of Zinc Oxide Thin Films Grown by Spray‐Pyrolysis

“…Interestingly, film ZO40 is formed of larger grains which have started to grow in the c direction in a step‐wise manner, as represented in Scheme . This is in agreement with the corresponding XRD pattern, which shows a diffraction peak corresponding to the (0002) plane of lower intensity with respect to that in samples ZO20 and ZO30, in agreement with the observations by Bo et al for porous ZnO films . Hexagonal platelets happen to grow until a limiting diameter at which they become the seed crystallites for a secondary nucleation process that takes place on {0001} faces.…”

“…Small grains are characterized by relatively wider depletion regions with respect to large grains. In fact, small grains may be fully depleted presenting overlapped potential barriers . On the other hand, large grains are only surface‐depleted.…”

Direct Catalyst‐Free Chemical Vapor Deposition of ZnO Nanowire Array UV Photodetectors with Enhanced Photoresponse Speed

“…Figure 3a shows I-V curves of ZnO NW array-based device with and without light illumination. Photo-to-dark current ratio (I ph /I dark ) could be improved by decreasing the diameter of ZnO NWs, [33,34] and etching discontinuous patterns on the substrates for ZnO NW growth. The high dark current of the ZnO NW array-based device resulted from the formation of a ZnO layer on the roughened SiO 2 surface.…”

“…We estimated the time constants of rise stage are t 1 ¼ 6.27 s and t 2 ¼ 37.1 s, and their relative weight factors are 85 and 15%, respectively, which give an average rise time constant of t r ¼ 11.02 s; the estimated time constants of the decay stage are t 1 ¼ 0.28 s and t 2 ¼ 6.85 s, with relative weight of 67 and 33%, respectively, producing an average decay time constant t d of 2.43 s. It was found that the rise process of NW array PD (1.38 s) is much faster than that of single NW PD (11.02 s), while the resetting process of ZnO NW array PD (4.05 s) is also comparable with that of single NW device (2.43 s). [6,9,11,14,16,19,20,22,25,27,[33][34][35][36][37][38][39][40][41][42] Compared to low temperature synthesized ZnO nanorods reported in reference, [35,36] ZnO NW arrays grown by CVD at high temperature have better crystal quality, which contributes to the high performance of our devices. Table 1 compares the photoresponse of different ZnO NW-based UV PDs as reported previously.…”