Recently, the spectrum group and the device process group of Nano-X have made important achievements in the tuning of AlGaN/GaN heterojunction photodetectors by Al nanoparticle arrays. The simulation software, scanning electron microscopy (SEM), atomic force microscopy (AFM), focused ion beam etching (FIB) and photocurrent testing of Nano-X support all the simulation and test data.

In this work, by fabricating Al nanoparticle arrays in AlGaN surface using the AAO template transferring method, significant broadband ultraviolet (UV) photoresponse enhancement was demonstrated on AlGaN/GaN heterojunction photodetectors. By deliberately designing the close-packed Al NP arrays, the broadband UV plasmonic resonance with large optical field absorption and strong interface field enhancement are enabled, hence, the highest responsivity exceeding 8.1 A/ W and maximum external quantum efficiency of 3500% was obtained at the resonance wavelength 292 nm, revealing more than 80 times the excellent enhancement in responsivity. Specifically, owing to coupling among NPs at the Al/AlGaN interface, the smaller size Al NP array exhibits an excellent photoresponse enhancement encompassing the entire UV band compared to the relatively larger size Al NP array. The device structure diagram is shown in Figure 1. The work was published in Physical Chemistry Chemical Physics with the title of "Broadband ultraviolet plasmonic enhanced AlGaN/GaN heterojunction photodetectors with close-packed Al nanoparticle arrays ".

Figure 1: (a) Schematic diagram of AlGaN/GaN UV detection based on plasmonic nanoparticle arrays. (b) Atomic Force microscope (AFM) images. (c) Dark field scanning TEM images of cross-sectional AlGaN/GaN heterojunctions.

As is well-known, the plasmonic resonance near field enhancement, the spectral shape, and plasmon resonance peaks position of the extinction in metallic nanoparticle arrays located on high-index substrates are highly sensitive to their sizes, internal gaps, and substrate optical index. The team systematically simulated the electric field enhancement and extinction spectra of Al NP arrays on the GaN substrate. Then, the coupling or the interactions of the metal NPs among NP array could not only tune the resonance peak position of the plasmonic extinction spectrum but also influence the field distribution between nanoparticles. The absorption spectra of Al NParrays with particle sizes of 40 nm and 100 nm were obtained and compared with the scattering spectrum of the corresponding single Al particle on the GaN substrate.The simulation result in figure 2(a) suggests that, for the close-packed Al NP array on GaN with a particles gap of 20 nm and particle size of 40 nm, the main plasmonic resonance peak is originated from the dipole_substrate mode resonance of individual NP, which presents an obvious blue-shift with the decrease in the particle space. The near field distribution of NP array resonances in the particle gap of 20 nm presents a slight air-side shift, both localized at the air and GaN side of the particle, as shown in the inset of Fig. 2(a). Meanwhile, the close-packed nanoparticle array exhibits a strong and broad resonance peak, covering the entire UV band (UV-A to UVC), which is consistent with the response wavelength in our AlGaN/GaN heterojunction detection.In contrast, for the close-packed Al NP array on the GaN substrate with an Al particle size of 100 nm and period of 120 nm, it can be found that the main plasmonic resonance peak is derived from the coupling of dipole-air mode resonances of individual NP, as shown in figure 2(b). This dipole-air resonance mode of the Al NP array on GaN shows a similar field distribution as that of a single Al particle on GaN substrate, localized at the Al/air interface.

Figure. 2: Simulated absorption spectra of the Al NPs array on AlGaN/GaN substrate as a function of array period for the particle size of (a) d = 40 nm and (b) d = 100 nm. The corresponding scattering spectra of a single Al NP (d = 40 and 100 nm) on AlGaN/GaN are also shown. The insets show the electric field intensity distribution maps at the XZ plane for the Al NPs array on the AlGaN/GaN substrate at the peak wavelength of the coupled dipole-substrateplasmonic mode (d = 40 nm and p = 60 nm) and the coupled dipole-air plasmonic mode (d = 100 nm and p = 120 nm).

In order to further verify the high absorption efficiency of Al nano-array with an interval of 20 nm, The team experimentally prepared Al nanoparticle array with 20 nm narrow spacing and a large area by AAO template transfer method. The fabrication process of a close-packed Al NP array over a large area using AAO masks on an AlGaN/GaN substrate is presented in Fig. 3(a).

Figure 3: Preparation process and SEM morphology of Al nanoparticle array

Figure. 4:(a) Spectral response and (b) EQE of AlGaN/GaN-based detector with Al NP arrays of different particle sizes and periods and without Al nanoparticles under 10 V applied bias.

Figure. 5:  Responsivity versus applied bias at a fixed wavelength of (a) 290 nm and (b) 360 nm for the AlGaN/GaN-based detector with Al NP arrays and without Al nanoparticles. (c) and (d) Schematics of the energy band diagrams of the Al NP array coupled AlGaN/GaN photodetector and the charge transfer process under UV illumination and an applied bias V.

Finally, the team tested photocurrent of the detector device. As shown in Figure 4, broadband enhancement of AlGaN/GaN heterojunction UV detector was realized based on Al nanoparticle array, and the highest response rate of more than 8.1 A/ W and the maximum external quantum efficiency of 3500% were obtained at the resonant wavelength of 292 nm. Moreover, it is found that the coupling between NPS at the Al/AlGaN interface shows an excellent optical response enhancement covering the entire UV band compared with the relatively large Al NP arrays. According to the simulation analysis and combined with the model of the charge transfer through a Schottky junction between Al and GaN for harvesting photocharges, as presented in Fig. 5(c) and (d), the dramatic broadband UV enhancement of responsivities for close-packed Al NP array-assisted AlGaN/GaN PDs might be attributed to three possible mechanisms: the enhancement of small-size array depends on light absorption and hot electron emission; The large-size array enhancement depends on the far-field scattering effect.

This work is important for obtaining an effective method to implement metal plasmonic enhanced heterojunction wideband ultraviolet detectors. Among them, the Nano-X plays a very good supporting role in the exploration of the mechanism of test analysis.