## 3 Results and Discussion  
Electroluminescence (EL) and light output power-current-voltage (L-I-V) characteristics of the light signal transmitter are measured by a probe station connected to a semiconductor device parameter analyzer (Agilent, B1500A, Santa Clara, CA, USA), a spectrometer in the ultraviolet range and an optical power meter. The dominant peak of the EL spectrum is about 380 nm in the ultraviolet range as shown in Figure 5a. The transmitter exhibits typical current-voltage characteristics (black curve in Figure 5b) of a LED, with a relatively low turn-on voltage (about 3 V). Light output power versus the current measured by microscopy system and fibred-coupled optical power meter is a linear growth curve (blue curve in Figure 5b). With the increase of current, the dominant peak of the transmitter remains stable and the light output power is lineally modulated. It demonstrates that the light signal transmitter has a suitable optoelectronics performance for optical communication in the ultraviolet range. As can be seen in Figure 6, C-V (current-voltage) characteristics of the light signal transmitter presents the negative capacitance behavior under AC signal with different frequency (10 KHz-1 MHz) in positive bias voltage range. The junction capacitance is positive under negative bias voltage. As the positive bias voltage increases, the junction capacitance decreases and drops down to negative value after reaching the turn-on voltage in positive bias voltage range. The modulation bandwidth and transmission speed could be improved by reducing RC time constant, and a lower RC time constant is always accompanied with lower absolute value of negative junction capacitance under positive bias voltage [20; 21]. The absolute value of negative junction capacitance increases with lower AC signal frequency and higher positive bias voltage. Under 4 V bias voltage, the absolute value of negative junction capacitance with 10 KHz, 100 KHz and 1 MHz AC signal are 105.3 pF, 58.3 pF and 22.5 pF, respectively. Because there are few studies about the capacitance characterization of a light emitter in the ultraviolet range, we compared the absolute value of negative junction capacitance in our study with the capacitance characterization of a III-Nitrides light emitter in the visible range [22; 23; 24]. Figure 4: (**a**) Three dimensional AFM (atomic force microscope) images of the active area (ridge structure) of the light signal transmitter; (**b**) cross-section size of the active area. It demonstrates that the absolute value of the negative junction capacitance in our study is smaller than the III-Nitrides light emitter in the visible range in the above references. A free-space data transmission test as shown in Figure 7a was conducted to study the transmission speed of the light signal transmitter. The transmit signal was loaded on light signal transmitter by arbitrary waveform generator (Keysight, 81160A, Santa Rosa, PH, USA) with 250 Mbps random binary sequence. A 40\(\times\) objective lens with a numerical aperture of 0.75 was set 80 cm from the transmitter to capture optical signal. The optical signal was sent to avalanche photodiode (Hamamatsu, C12702-12, Hamamatsu, Japan) to amplify the received light signal to electrical signal. The electrical signal was divided into two channels with low-pass and high-pass filters, and is finally characterized by an Agilent DSO9254A digital storage oscilloscope. The transmit signal and received signal are shown in Figure 7b, and the signal waveform is well preserved during transmission. The clear open eye diagrams of free-space data transmission test at 250 Mbps are clearly observed in Figure 7c. Three-dB modulation bandwidth of transmitter at a 40 mA current is shown in Figure 7d, and it achieves about 260 MHz (tested by vector network analyzer, Keysight, E5080A). It demonstrates that the light signal transmitter is capable of achieving high-speed transmission in optical communication in the ultraviolet range.