## 3 Results  
Figure 4a shows the SEM image of the Au/GaN nanopillars, which displays a regular periodic array. A continuous metal film is formed on the surface and sidewall of the nanopillar, as presented in Figure 4b, which is beneficial to the following electrical testing procedure. The Au films in the top and sidewall of the GaN nanopillar had a thickness of 50 nm and 25 nm, respectively. The reflection spectra and transmission spectra were measured using a Fourier transform infrared spectrometer. The reflection spectra and transmission spectra were normalized to a gold mirror and air, respectively. Figure 5a presents the measured reflection (R) and transmission (T) spectra of the planar-type device and the Au/GaN-nanopillar device. The absorption coefficient (A) can be calculated from the reflection coefficient (R) and transmission coefficient (T) as A = \(1-\mathrm{T}-\mathrm{R}\). The optical absorption of the Au/GaN planar-type device and the Au/GaN-nanopillar device is shown in Figure 5b. The absorption efficiency of the Au/GaN-nanopillar structure in the SWIR range is enhanced to over 60%, which is five times higher than the average for the planar structure. In addition, compared with the simulated absorption spectrum in Figure 2b, the measured absorption peak is basically located in the similar wavelength, but widened, which may result from the size fluctuation of the Au/GaN nanopillars. Figure 3: Schematic diagram of the preparation process for the Au/GaN-nanopillar array-based hot-electron photodetectors; the Au/GaN-nanopillar arrays have a period of T and a height of H. (**a**) SiN\({}_{\mathrm{x}}\) film was deposited on the surface of n-GaN. (**b**) Assmled monolayers of polystyrene particles. (**c**) A non-close-packed PS sphere monolayer was formed by reactive ion etching. (**d**) The PS sphere pattern transferred to the SiN\({}_{\mathrm{x}}\) layer by RIE. (**e**) GaN-nanopillar arrays were obtained by dry etching. (**f**) The preparation of the Schottky contact and Ohmic contact. The dry etching process has a significant influence on the n-GaN surface properties. The ICP etching of the n-GaN surface creates high-density N vacancies acting as donors [26; 27], which results in the formation of Ohmic-like contacts, as shown in Figure 6a. The current of the Au/GaN-nanopillar device in the voltage range of \(-1\) V\(\sim\)0 V in the absence of incident light is large (blue line in Figure 6a), which is harmful to the photoresponse performance. It is necessary to improve the Schottky contact to reduce the dark current. Therefore, before the metal deposition, the etched GaN nanopillars were annealed in nitrogen ambience to recover the etching damage. As illustrated in Figure 6a, an appropriate annealing temperature, for example 700 \({}^{\circ}\)C, is beneficial to improve the rectifying characteristics of the Schottky contact. Figure 6b shows the I-V characteristics for the Au/GaN planar-type device (black line), Au/GaN-nanopillar device without annealing (blue line), and Au/GaN-nanopillar device annealed at 700 \({}^{\circ}\)C for 30 s (red line). The rectifying characteristics of Au/GaN became worse after ICP etching, and were partially recovered by rapid-thermal annealing, which can effectively eliminate the etch-induced lattice disorder and point defects [28; 29]. Compared with the planar structure (black line in Figure 6b), the rectification effect of the Au/GaN-nanopillar structure is not fully recovered. This is because some dry-etch-induced damage (e.g., point defects) is irreversible. Figure 4: (**a**) Typical SEM image of the Au/GaN-nanopillar arrays (scale bar, 2 \(\upmu\)m). (**b**) Cross-section of the Au/GaN nanopillar (scale bar, 200 nm). Figure 5: The measured reflection, transmission (**a**), and absorption (**b**) spectra of the Au/GaN planar-type device and the Au/GaN-nanopillar device.