## 2 Materials and Methods  
III-Nitride epitaxial layer of GaN-on-silicon platform (Lattice Power Corporation, Suzhou, China) consists of \(\sim\)25 nm thick p-contact GaN layer, \(\sim\)500 nm thick p-cladding AlGaN layer, \(\sim\)52 nm InGaN/GaN MQWs (4 pairs of quantum well, 3 nm InGaN/10 nm Al\({}_{0.1}\)Ga\({}_{0.9}\)N per pair), \(\sim\)750 nm thick n-cladding AlGaN layer, \(\sim\)2.45-\(\mu\)m-thick n-contact AlGaN layer, \(\sim\)1030 nm thick undoped GaN layer and \(\sim\)750 nm thick AlGaN buffer layer. Metal films with Pd 30 nm/Pt 30 nm/Au 50 nm are deposited on III-Nitride epitaxial layer by electron beam (EB) evaporation (step a). The structures of the active area are patterned on metal films and transferred to III-Nitride epitaxial layer by inductively coupled plasma (ICP) etching for III-V materials with a 320 nm etching depth (step b). Then, a silicon oxide  
Figure 1: Schematic of high-speed light signal transmitter for optical communication based on ultraviolet radiation.  
layer for electrical isolation is deposited on III-Nitride epitaxial layer by plasma enhanced chemical vapor deposition (PECVD), and it is patterned by photolithography and wet etching with a BHF (buffer hydrofluoric acid) etching solution (step c). The region for exposing n-type III-nitride is patterned and etched by ICP etching with 2 \(\upmu\)m depth (step d). Finally, p/n electrodes (Ti 50 nm/Pt 100 nm/Au 500 nm) are formed by EB evaporation and lift-off process (step e). Figure 2 shows the fabrication process of the light signal transmitter based on ultraviolet radiation.  
Figure 3 shows the optical microscope image of the light signal transmitter based on ultraviolet radiation. The inset image in Figure 3 is an enlarged SEM (scanning electron microscope) image of the active area of the light signal transmitter. As mentioned above, the modulation bandwidth and transmission speed of the ultraviolet optical communication systems are significantly influenced by the absolute value of transmitter capacitance, which depends on the area of the active area. The light signal transmitter with an ultra-small active area is fabricated by a double-etching process.  
Figure 4a presents three-dimensional AFM (atomic force microscope) images of the active area (ridge structure) of the light signal transmitter. The roughness of the top surface of the ridge structure  
Figure 3: Optical microscope image of the light signal transmitter based on ultraviolet radiation. Inset: Enlarged SEM (scanning electron microscope) image of the active area (ridge structure) of the light signal transmitter.  
Figure 2: Fabrication process of the light signal transmitter based on ultraviolet radiation.  
with electrode Pd/Pt/Au is less than 50 nm. As shown in Figure 4b, the width and height of the active area is about 2.74 \(\upmu\)m and 365 nm. The sidewall of the active area has high steepness and small roughness. The thickness of Pd/Pt/Au electrode is 110 nm, and the etching depth of the active area is about 255 nm. P-type GaN layer and part of p-cladding AlGaN layer are removed by the first etching step. The ultra-small active area has a width of 2.74 \(\upmu\)m and an area of 850 \(\upmu\)m\({}^{2}\). The absolute value of transmitter capacitance could be greatly reduced. The etching depth of the second etching step for exposing n-type III-Nitride is 2.3 \(\upmu\)m, etching to n-contact AlGaN layer. The deep etching for n-type III-Nitride could reduce parasitic resistance and capacitance, thus the second etching step is also beneficial to improve the modulation bandwidth and transmission speed of the transmitter [19; 20].