## 2 Device Design and Fabrication
Therefore, the thickness of the gold film was set to 50 nm, and the height of the nanopillars was set to 200 nm. The calculated absorption efficiency of the Au/GaN-nanopillar structure at the peak wavelength could reach 80%. This is because the Au/GaN-nanopillar arrays can generate surface plasmons that have a local enhancement effect, as illustrated in Figure 2e, and an electric field with a high intensity was observed around the bottom corner of the nanopillar. The simulated Au/GaN-nanopillar arrays had a period of 800 nm and a height of 200 nm, and a 50 nm thick Au is assumed to conformally cover the GaN nanopillar. Figure 2f shows the hot-electron generation rate (G), which is proportional to the electric field intensity in the metal [21]. Hot electrons are mainly generated near the metal/GaN interface, which is in favor of the collection of hot electrons by GaN. Figure 1: (**a**) Schematic of the metal/GaN-nanopillar structure with period T, height H and metal film thickness t. (**b**) Schematic of the energy band diagram and the detection mechanism of the hot-electron photodetector, the light blue area is the depletion region. Figure 3 shows the schematic of the preparation process for the hot-electron photodetectors based on the Au/GaN-nanopillar arrays. First, GaN film was grown on a c-plane sapphire substrate using an AlXTRON 2000 HT metal-organic chemical vapor deposition (MOCVD) system. The thickness of the n-GaN film was about 6 \(\upmu\)m. A 200 nm thick SiN\({}_{x}\) was then deposited on the surface of n-GaN by plasma-enhanced chemical vapor deposition (PECVD). A monolayer of polystyrene (PS) spheres was prepared on the SiN\({}_{x}\) using a self-assembly technique. A non-close-packed PS sphere monolayer was formed by reactive ion etching (RIE) in O\({}_{2}\) plasma for a certain time. Then, GaN-nanopillar arrays could be obtained by two-step dry etching, as illustrated in Figure 3d,e, where the PS sphere pattern transferred to the SiN\({}_{x}\) layer by RIE in SF\({}_{6}\), and then through dry-etching of GaN nanopillars using Cl\({}_{2}\)/BCl\({}_{3}\)/Ar inductively coupled plasma (ICP). Finally, the Schottky contact and Ohmic contact were prepared sequentially. Utilizing the PS sphere monolayer as an etching mask can achieve a low-cost and large-area fabrication. For comparison measurements, planar structure devices without the GaN nanopillars were also prepared using the same wafer. The morphology of the samples was characterized by a scanning electron microscope (SEM, ZEISS Merlin). The optical property measurements were carried out on a Fourier transform infrared spectrometer (Bruker Vertex 80). The measurements of the current-voltage characteristics were performed with an Agilent 4155C parameter analyzer. An 1173 K blackbody with a long-wavelength pass filter (>1.1 \(\upmu\)m, Thorlabs) was chosen as the light source for characterizing the photocurrent of the devices. The wavelength-dependent responsivity was measured by illuminating the devices with a tunable laser (NKT Photonics), performing at a wavelength from 1300 to 2100 nm with a step of 25 nm. The photocurrent signal in response to each wavelength was measured using a lock-in amplifier (Stanford Research System SR830) with a 200 Hz chopper. Figure 2: (**a**) The parameters of the Au/GaN-nanopillar structure. (**b–d**) Calculated absorption spectra of the Au/GaN-nanopillar array with a varying period, height, and thickness, respectively. (**e**) Simulated distributions of the electric field intensity under optical excitation at a wavelength of 1.8 \(\upmu\)m. (**f**) The distribution of the hot-electron generation rate (G) under optical excitation at a wavelength of 1.8 \(\upmu\)m.