| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 101 === Wide and direct band gap compound semiconductor material GaN has been studied extensively for the applications of solid state lighting. However, the low light-extraction efficiency is the bottleneck for high-power light-emitting diodes (LEDs). Moreover, the InGaN/GaN quantum well usually suffer from strong built-in electric field because of lattice mismatch induced strain. The built-in electric field can cause low internal quantum efficiency due to a reduced overlapping between the electron and hole wave functions.
The nanorod structure can not only release the strain caused by the lattice mismatch, but also improve the extraction efficiency by the higher surface to volume ration. Thus, we begin to study GaN based nanorod structured devices. The GaN based nanord structure also have been studied widely but most of the research are restricted at material characterization. The process to fabricate nanorod structure LEDs is difficult especially on the large reversed current issue.
We developed a stable process to fabricate InGaN/GaN based nanorod light emitting diode arrays. The combination method of passivation layer and polishing process was applied to fabricate p-type metal contact on the top of nanorod structure. The nanorod LED demonstrates a reverse current only nano ampere level under -5V bias voltage.
We applied Raman spectroscopy to observe the strain relaxation of nanorod structure. The equation of the correlation between Raman wave-number and strain of InGaN is applied to estimate the strain within InGaN quantum wells. To study the process created side wall defect, the low-temperature electroluminescent (EL) spectra of InGaN/GaN nanorod arrays were also explored and compared with those of planar LEDs. The extraction efficiency of nanorod structure was also calculated by the low temperature PL and EL measurement.
We further discovered that the choice of nanorod passivation materials results in the variation of strain in the InGaN/GaN quantum wells, and thus the corresponding change of light emission properties. The results were further investigated by performing Raman measurement to understand the strain of nanorods with different passivation materials and by calculating the optical transition energy of the devices under the influence of strain-induced deformation potential and the piezoelectric polarization field.
At last but not least, the efficiency droop effect is also discussed in our nanorod structure LEDs. The simulation and measurement consider not only the quantum efficiency but also the EL spectra, which will make the simulation more close to the real situation. Our results show that Auger recombination dominates at low-level currents. However, the increase number of leakage carriers out of quantum wells is responsible for the efficiency droop at high current injection levels.
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