Study On GaN-Based Light-Emitting Diodes For Visible Light Communications

With the breakthrough in GaN-based light-emitting diodes(LEDs),visible light communication(VLC)develops rapidly and shows high potential for applications.In recent years,it has attracted much attention for further research.Because of large and direct band-gap of GaN materials,GaN-based LEDs with high efficiency have been widely used in illumination and display applications.On the other hand,GaN-based LEDs as blue or green sources in VLC system can not only lower the cost but also increase the practical value of VLC system.The modulation bandwidth of LED is a key factor that affects transmission of VLC system.However,the research on the modulation mechanism of GaN-based LED is still not detailed.To solve this issue,in this study we fabricated the hybrid array of LEDs with different sizes and investigated the influence of device size on the characteristics of LEDs,including the modulation bandwidth and electrical properties.Here are the main results listed as follows:1.We design the experimental details and fabricate the hybrid array of LEDs with different sizes.Each unit contains twelve different devices,including eight micro devices with the diameters from 30?m to l00?m increased by l0?m and four large devices with the diameters from 150?m to 300?m increased by 50?m.For each unit,p-pads are individual while n-pad is common to all pixels.2.We set up a system for LED modulation bandwidth measurement,which can measure frequency response curves of LED and obtain the-3dB bandwidth.Maximum bandwidth of the system is 1.2GHz,supporting light spectrum ranged from 200nm to 1l00nm.The frequency response is recorded by a network analyzer.3.We measure the characteristics of the devices with different sizes,including the current-voltage(I-V),output power-current(P-I)and electroluminescence(E-L)properties.With the device size increasing,series resistor and threshold voltage at 20mA gradually decreases,while the maximum light output power increases.It indicates that device size has an impact on the performance of LED.The emission peak of the device exhibits blue shift with the current increasing.However,the devices with different sizes but the same current density have almost the same emission peak,which indicates that shielding effect of the built-in electric field in the light-emitting area depends on the injected carrier density.4.We investigate the influence of device size and inject current on the characteristics of modulation bandwidth,based on the measurements in different conditions.Among them,the device with the diameter of 30?m exhibits a large-3dB bandwidth of?165MHz with the inject current of 110mA.Measurement results show that modulation bandwidth gradually improves along with the current increasing,due to the decrease of carrier recombination lifetime.On the other hand,for the micro-LEDs with different sizes but the same current density,the modulation bandwidths are similar.These results show that the current density(carrier recombination lifetime)plays a key role in the bandwidth modulation.