Electrical and optical characteristics of GaN-based LEDs with various doping profiles multi-quantum wells

In this thesis, the InGaN/GaN MQW LEDs with various barrier doping structures are investigated. As-grown samples are characterized with low temperature and room temperature photoluminescence measurement for the comparison of the relative internal quantum efficiency of different doping structures. The result reveals that higher internal quantum efficiency is achieved for more doped barriers LED. From the result of the electroluminescence measurement, LED samples with more doped barriers shows little blue-shift due to the injected carriers. We explain this phenomenon by Coulomb screening of the piezoelectric-field-induced quantum-confined Stark effect (QCSE). With an increasing number of doped-barriers, the internal field is more strongly compensated so that the magnitude of blue-shift due to the injected current is reduced.
From the total radiation flux measurement, the sample with six doped barriers and highest internal quantum efficiency doesn’t give highest radiation flux as expected. Sample with 3 doped and 3 undoped barriers gives the highest output radiation flux for all injection currents instead. It is also revealed that sample with all barriers doped displays an earlier saturation of the radiation flux. The saturation of the radiation flux is a strong indication that carriers overflow the active region as current densities increase.
The junction temperature measurement is also performed to investigate the influence of the junction temperature to the electrical and optical properties. We measure the junction temperature for all samples using forward voltage method. The result reveals that the sample with all doped barriers gives lowest junction temperature under the same operation condition.　From the junction temperature-varied optical properties measurement, we find that the luminous efficiency will decay more quickly at low current operation. However, when the current density increases to the saturation capacity of the quantum well, the decay ratio will reach a constant.