| Summary: | 博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 92 === In this dissertation, six approaches are presented to improve future solid-state lighting devices: AlGaInP and InGaN LEDs. Therein, four approaches are related to how to promote the brightness and improve the reliability for AlGaInP LEDs. Another two are concerning the promotion of brightness and ESD withstand voltage for InGaN LEDs. Finally, we also proposed and fabricated an alternative white LED.
In respect of research on AlGaInP LEDs, we have proposed a modulation-doped GaP/InGaP superlattice to improve the current-spreading ability of conventional structure and tried to explain the functioning of superlattice by means of the dynamic resistance versus applied voltage. With the benefit of superlattice, the luminous intensity was increased to 1.16 times higher than that of conventional structure. In addition, less heating effect was observed in terms of the EL spectra with different operating current. Then we investigated the effect of blocking depth on the optoelectronic characteristics of AlGaInP LEDs with different chip size. It was found that deeper blocking layer led to higher luminous intensity but sacrifice the electric property, i.e., the forward voltage was slightly increased. Under the operation of 20 mA, the brightness of AlGaInP LEDs with small size increased to 1.19 times and that with large size increased to 1.16 times. The third approach is about the concept of meshed contact layer applied to ITO-assisted AlGaInP LEDs. Via the modification of contact layer, not only the brightness was significantly increased but the reliability was better than conventional structures. In this study, we performed numerical calculation to verify the uniform distribution of injected carriers for the proposed structure with a meshed contact layer. Utilizing the calculation results can roughly estimate the increased ratio contributed by the application of meshed contact layer. Besides, we also developed a measurement system for junction temperature of devices. The fourth research reported the effect of sulfide passivation on AlGaInP LEDs. The reverse leakage current of AlGaInP LEDs with passivation was dramatically reduced to be one thousandth of that without passivation. Meanwhile, the brightness was thus increased. We also found that the activation energy, calculated from logarithmic reverse current versus inverse temperature, was close to half of energy band gap of constituent material. This indicates the sulfide passivation can perfectly mend the leaky junction along the perimeter of AlGaInP LEDs, so the generation current within the depletion region can strongly dominate the reverse leakage current. From the surface morphology measured by scanning probe microscopy (SPM), we found that the GaP surface got smoother with the longer time, taken for immersing LEDs into sulfide solution. It suggested that the brightness increase have nothing to do with the critical angle loss. Based on the simulation of transmittance, on the other hand, we also exclude the fresnel loss from the possible mechanisms responsible for brightness increase. It was finally concluded that the effective carrier injection is the primary cause for the brightness increase.
With regard to the research on InGaN LEDs, we first proposed a lateral current-blocking structure to promote the brightness. The operating voltage was also increased due to the decrease of contact area. After comparing the performance of InGaN LEDs with different configuration of lateral current-blocking holes, we concluded that the brightness increase mainly resulted from the improvement of lateral current-spreading instead of the enhancement of light extraction. Next, we paid attention to the investigation into the ESD issue of InGaN LEDs. It was found that the p-type InGaN/GaN superlattice enabled the InGaN LED to withstand higher voltage (from 490 V to 600 V) under Human-Body-Model test condition. We first proposed an equivalent capacitor model, by which it was shown that the ESD robustness has something to do with the capacitance and effective area of devices. The activation energy of p-GaN was 157 meV and that of p-superlattice was 91 meV, which were measured by temperature-dependent Hall Effect Measurement. Due to the piezoelectric field of superlattice, the hole concentration was substantially increased, resulting in a lower resistivity of 0.59 ��-cm. Judging from where the breakdown happens and material melts, we think low resistivity of p-superlattice assisting in uniformity of electric field all over the device is one of the reasons for enhancing ESD robustness. On the other hand, the InGaN LED of larger reverse capacitance can also reasonably account for the enhancement of ESD robustness in view of the circuit analysis.
At last, a tandem-type white LED was proposed and successfully fabricated. Its features lie in simple fabrication process free of fluoresced material and requirement of single driving circuit. So far the luminous efficiency of the original tandem-type white LED is 14.56 lm/W at an operating current of 20 mA.
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