| dc.description.abstract | In this thesis, we demonstrate thin-GaN LEDs bonded on AlN substrate. The electric, optical and thermal properties of thin-GaN LEDs bonding on AlN and Si substrates were analyzed and compared.
One side published AlN substrate was used in this study. Root-mean-square (RMS) roughness of published surface was about 40 nm. 3-ω method was used to determine the thermal conductivity of AlN and Si substrates. The thermal conductivities of AlN and Si substrates were 179 W/m-k and 130 W/m-k respectively.
LED wafers were bonded on 380-μm-thick AlN, 630-μm-thick AlN, and 525-μm-thick Si substrates by Au-Ag bonding method. After wafer bonding, laser lift-off method was employed to take sapphire substrate off. Then thin-GaN LEDs with a chip size of 1 × 1 mm2 were fabricated. Forward voltages of these three kinds of LEDs with an input current of 350 mA were all about 4 V. The output power of LEDs bonding on 380-μm-thick AlN, 630-μm-thick AlN, and 525-μm-thick Si substrates which was measured in an input current of 350 mA is about 32 ± 4 mW, 34 ± 5 mW, and 33 ± 7 mW, respectively. This indicated that the electric and optical performance of thin-GaN LED bonded on AlN is as good as LEDs bonded on Si substrate. The thermal resistances and junction temperatures of LEDs were measured by diode forward voltage method. The junction temperatures of LEDs bonded on 380-μm-thick AlN, 630-μm-thick AlN, and 525-μm-thick Si substrates were about 112 ± 5 oC、113 ± 7 oC and 117 ± 5oC , respectively. The thermal resists of LEDs bonding on 380-μm-thick AlN, 630-μm-thick AlN, and 525-μm-thick Si substrates were about 18 ± 4 oC/W、18 ± 1.4 oC/W and 、17.6 ± 4.3 oC/W, respectively. Since thermal conductivity of AlN is higher than that of Si, thin-GaN LEDs bonded on AlN substrate should have better heat dissipation than LEDs bonded on Si substrate. However, the difference of thermal resistances between LEDs bonded on 380-μm-thick AlN, 630-μm-thick AlN, and 525-μm-thick Si substrates is not obvious. This should be due to that surface roughness of AlN was larger than that of Si therefore more voids were formed between Au-Ag interface when bonded on AlN. The voids between Au-Ag interface would result in the increase of thermal resistance. Hence the difference of thermal resistances between LEDs bonded on AlN and Si substrates is not obvious.
| en_US |