# Output Properties of Transparent Submount Packaged FlipChip Light-Emitting Diode Modules
## 3 Results and Discussions  
The optical output intensity of the FCLEDs mounted on different submounts was measured as a function of the driving current, i.e., L-I curves as shown in Figure 3. It was observed that FCLEDs mounted on the DSB sapphire submount had the highest lumens at 150 mA and the LEDs mounted on the silicon exhibited the lowest output values. As to the glass-mounted LEDs, their performance was in between. There are some special tendencies that need to be emphasized: at a low bias current region, due to the transparent property of the glass, its performance was close to that of the DSB samples; at a high current operation region, the heat accumulation-induced droop phenomenon is clearly observed. Thus, these results are thought to be due to the combination effects of both the transparency and thermal conductivity of each submount, respectively. Although the heat-conducting Si submount shown here exhibits a linear IV curve without any droop phenomenon, it still showed a disadvantage for realizing a high-brightness LEDs FC mounting when compared to those samples mounted on transparent sapphire or glass. The FCLEDs mounted on the DSB sapphire substrate showed a light output power of 0.12 W at a current of 140 mA, which is comparable to the results from other groups [26; 27]. FCLEDs mounted on DSB sapphire showed an 80% increase in the light output power when compared to its non-transparent counterpart Si with good thermal conductivity. Glass and sapphire both are transparent substrates, and at a small bias current, the transparent submount dominates the light extraction efficiency. Therefore, the output from LEDs mounted on glass is almost consistent with the LEDs mounted on DSB sapphire. However, due to the thermal conductivity difference, the glass-mounted LEDs' output power intensity at a high current operation region starts to droop, which implies that at a high current operation region the thermal conductivity dominates. In general, the LED thermal accumulation behaviors can be deduced from the shift of the corresponding light emission wavelength spectra. Thus, Figure 4 is further focused on the respective light spectra for FCLEDs on different submounts at varying operation currents. From Figure 4, a clear shift in the peak wavelength for different samples with the varying currents can be seen. In the GaN-based blue LED, the common blue shift in the low-injected-current region and good thermal dissipation samples are obtained for both Si and DSB samples. However, at a high-injected-current region the glass sample showed a serious red shift. To summarize the results of Figure 4, blue LEDs are used in this experiment, and their peak wavelength mostly revolves between 459 nm to 460 nm. As shown in Figure 4, it is observed that the peak wavelength goes on decreasing (blue shift) for FCLEDs mounted on sapphire and silicon which both have good thermal conductivity. However, the peak wavelength variation for FCLEDs mounted  
Figure 3: L–I curves for the different FCLED samples. The inset figure shows the transparent submount bidirectional LED module. on glass, at a region of higher than 100 mA, showed a red shift from 460.4 nm to 462.4 nm which is believed to be due to the thermal accumulation. For a driving current less than 100 mA, a small blue shift was observed for FCLED on Si and sapphire submounts due to the band-filling effect of the localization energy states caused by indium composition fluctuation in the InGaN multiple quantum wells, which is also known as the quantum confined Stark effect (QCSE) [28]. The Si and sapphire have a thermal conductivity much higher than that of glass, and thus provide a good heat sink. Thermal conductivities used in the above ANSYS simulation for glass, Si and sapphire are 1.05 W/m. \({}^{\circ}\)C, 124 W/m. \({}^{\circ}\)C and 35 W/m. \({}^{\circ}\)C, respectively, as mentioned in the ANSYS engineering toolbox. Due to the excessive heat accumulation in LEDs on glass substrates, the light output intensity drops and a red shift in the peak wavelength was observed.