| Summary: | 碩士 === 國立成功大學 === 微電子工程研究所碩博士班 === 96 === Vertical structure GaN-based light-emitting diodes (VM-LEDs) have attracted much attention. This is because its n-side up structure design with metallic substrates enables much better current spreading, light extraction efficiency, and thermal dissipation capability as compared with conventional lateral conducting sapphire substrate GaN-based LEDs.
Essentially, current spreading length provided by n-GaN is around 120 µm. This is sufficient for those devices with chip sizes under 300x300 µm2. However, as the chip size goes up for the pursuit of increment in luminous flux per chip, one chip could not be effectively fully covered. The light extraction efficiency per unit area lowered, power conversion efficiency decreases, and device performance corrupts under high injection current eventually. Use of electrode geometry design, transparent conductive layers (TCL), and surface engineering have been sequentially proposed and reported with stimulating results.
In this thesis, we managed to release the situation by implementing micro-array structures on standard VM-LEDs (broad-area LEDs). 2x2 and 8x8 micro-array VM-LEDs(µ-LEDs) were fabricated. Based on the experimental results, 102% and 140% improvements over broad-area LEDs in the normalized light output power (Lop) at 100 mA were observed on 2x2 and 8x8 µ-LEDs, respectively. The improvement increased with the array number, because the proposed µ-LED enable better and effective current spreading per unit area and reflected in the normalized Lop. Together with the comparable current-voltage (I-V) characteristics, the power conversion efficiency per area of 2x2 and 8x8 µ-LEDs was 91.6% and 116% superior.
We further boost the 8x8 µ-LEDs and broad-area LEDs up to 700 mA for Lop-I measurement (not normalized) to examine stability of both device performance under high injection current. One saw that the Lop broad-area LEDs dropped at 390 mA. In the contrast, the Lop of µ-LEDs remained steady response throughout the testing region, and led that of broad-area LEDs at 410 mA.
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