## 2 Device Design and Fabrication  
The metal/GaN-nanopillar device consists of a metal nanofilm deposited on the surface of GaN-nanopillar arrays, as presented in Figure 1a, which has the advantages of simple, large-scale, and low-cost manufacture. The planar structure cannot generate surface plasmons because the momentum of the photon is smaller than that of the surface plasmons of the same frequency [20]. Metal/GaN-nanopillar arrays can excite the surface plasmons because it can overcome the momentum mismatch problem. Surface plasmons based on metal/GaN-nanopillar arrays can effectively improve the light absorption efficiency. In addition, the back-incident mode (incidence from the semiconductor side) is adopted, which is favorable for the transport and collection of photon-excited hot electrons in the metal. The hot-electron photodetection process can be divided into three steps: metal absorbs photons to generate hot electrons, the transport of hot electrons in metal, and their collection in the semiconductor [21; 22; 23], as illustrated in Figure 1b. Basically, hot electrons with energies higher than the SBH can be collected by GaN with a high probability. According to the Schottky-Mott relationship, SBH (\(\Phi_{B}\)) can be written as \(\Phi_{B}=|W-X|\), where \(W\) and \(X\) are the work function of metal and the electron affinity of semiconductor, respectively [24]. The electron affinity of GaN is 4.1 eV, and Au with a work function of 5.1 eV is used for the preparation of the Schottky contact. Apart from the metal materials, the doping level of GaN has an important impact on the width of the depletion region, which will affect the properties of the Schottky contact. The device based on metal/GaN nanopillars consists of an Ohmic contact and a Schottky contact. The unintentional-doped GaN with a doping density of about 1 \(\times\) 10\({}^{16}\) cm\({}^{-3}\) prefers to form a Schottky contact; however, it is difficult to form a good ohmic contact necessary for the collection of the photocurrent. Therefore, a lightly Si-doped GaN with a doping concentration of 6 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) was adopted for the metal/GaN hot-electron photodetector. The finite-difference time-domain (FDTD) method was carried out to investigate the optical properties of the Au/GaN nanopillars. The periodic boundary conditions were used along the x and y directions, and the perfectly matched layer (PML) conditions were used along the z direction for a single cell. A plane wave with the electric component polarized along the x axis was employed as a stimulation source and was projected normally on the device. Two monitors were used to record the reflection coefficient (R) and transmission coefficient (T) during the simulation, and the absorption coefficient (A) can be obtained as A = \(1-\mathrm{T}-\mathrm{R}\). A significant enhancement on the absorption efficiency in the SWIR range can be expected when utilizing the metal/GaN-nanopillar structures, as shown in Figure 2b. The period of nanopillar array has a great influence on the absorption wavelength, and the absorption peak shifts to longer wavelengths with the increase in the nanopillar period. The period of the Au/GaN-nanopillar arrays in the following experiments was set to 800 nm. The surface plasmons resonance wavelengths can be expressed as \(\lambda_{i,j}\sim np/\sqrt{i^{2}+j^{2}}\) for the back-illumination mode [25], where \(i\) and \(j\) are integers indicating the order of the coupling, and \(n\) is the GaN refractive index. The resonance wavelength of the Au/GaN-nanopillar arrays with \(p\) = 0.8 \(\upmu\)m was calculated to be 1.84 \(\upmu\)m, which agrees well with the simulation results in Figure 2b. The height of the nanopillar and the thickness of the gold film had little effect on the absorption efficiency, as presented in Figure 2c,d, respectively. However, if the gold film was too thin, it was not conducive to the formation of a continuous film on the sidewalls of the GaN nanopillars.