Performances of 650 nm AlGaInP-based RCLEDs and MBRCLEDs…

In this investigation, we designed three different light emission aperture sizes of 84, 60, and 40 µm which were shown in figure 2.7. The large aperture was designed for higher output power devices requirement and small aperture devices were applied for higher speed applications. These two fundamental devices properties are trade-off and the epi-structure should be optimized for different requirements. Figure 2.8 described the typical light-current-voltage (L-I-V) characteristics of RCLEDs with 84, 60, and 40 µm aperture sizes.

The output power of 84, 60 and 40 µm device without epoxy encapsulated under 20 mA forward current operation were 3, 2 and 1.6 mW, respectively. From I-V curves, the devices series resistance were 8.4, 14.7, 27 ohm, respectively. Although the 40 µm device presents the lowest output power and the power saturate at lower driving current due to the high current density, the maximum power was 1.6 mW and still suitable to transmitting data through 50 m POF. Figure 2.9 shows the wall-plug efficiency and power characteristics of 84 µm devices.

The wall-plug efficiency was as high as 12% and the output power could achieve over 3.5 mW at 20 mA with epoxy encapsulated. R. Wirth et al., who had shown the output power and maximum wall-plug efficiency of 2.9 mW and 9.5% from the same 84 µm emission aperture size and 20 mA driving current and with epoxy [18]. The output power could be compared with to other groups which have even larger aperture size under the equal measurement conditions [14], [17]. In order to approach device temperature insensitive, properly detuning the wavelength between resonant cavity and quantum well is necessary. The output power variation versus detuning wavelength was shown in figure 2.10. The optimized wavelength detuning is 15 nm and the emission power droop from 20°C to 85°C is less than -2dB. P.

Sipilä et al. who had shown the output power drop versus temperature is -3.8dB from 20°C to

The detuning wavelength will also affect the far field pattern and coupling efficiency in POF applications, although POF has larger fiber core (1 mm) than glass optical fiber (GOF).

This investigation was presented that the device with 15 nm detuning wavelength exhibited the far field pattern at 50% power angle of 134°. Figure 2.11 shows that the coupling efficiency into 0.5 NA POF of transmitter remained a high value (>50%) in wide driving current range (10 ~ 90 mA) due to the higher half power angle and output power. This coupling efficiency could be compared with M. Dumitrescu et al. [13] report under the same conditions of 0.5 NA POF and 84 µm devices, where their coupling efficiency was less than 35% from 0 mA to 40 mA operation current. Figure 2.12 shows the cut-off frequency -3dB standard. In addition, the devices rise and fall-time is 2.86 ns and 1.08 ns respectively using Hamamatsu Optical Oscilloscope C8188 test equipment. An open eye-diagram with error free was obtained at Ibias=30 mA. Since the f-3dB of the 40 µm RCLEDs was 313MHz, the 40 µm devices could achieve IEEE 1394b s400 standard. The 40 µm RCLEDs eye diagram achieved 622 Mbit/s and shown in figure 2.13. In order to achieve high speed RCLEDs, RCLEDs

coefficient, J_inject is the injection current density, and d is the thickness of active layer. From this equation, there are several methods to modulation RCLEDs devices speed. Firstly, the carrier lifetime τ could be reduced by inserting some non-radiate recombine centers in the active region. But this method also decreased the internal efficiency of this structure, which is not desirable. The second way is that increases the current density of injection or decreases the size of emission window. However, these two ways will cause the power droop during higher forward current injection. The last method is reducing the active layer thickness, it could be increase the devices speed without sacrificing other performance. The cut-off frequency and output power of 84 µm RCLEDs devices with single QW were obtained to be 235 MHz and 2 mW at 20 mA, respectively. This performance was suitable for IEEE 1394b s400 standard applications. M.Guina et al. had also shown the data of bandwidth versus current, and its cut-off frequency of 84 µm devices is 125 MHz under 20 mA current operating [20]. In addition, an error free open eye diagram for the transceiver using single QW RCLEDs devices at Ibias = 30 mA was obtained to be 500 Mbit/s data rate through a graded-index POF over 50 m as shown in figure 2.14.

The high performances MBRCLEDs, having a high thermal conductivity Si substrate, were fabricated via twice bonding technique of glue bonding and metal bonding processes. In figure 2.15.1 shows the scanning electron microscope (SEM) photograph of the MBRCLEDs devices with 300 x 300 µm² dimensions. The figure 2.15.2 shows the MBRCLEDs electron-luminescence (EL) state and light intensity profile. The emission area diameter is 84 µm and the dark area of the emission window is metal mesh line for current spreading. In the RCLEDs devices applied requirements, the temperature insensitively properties is a key point.

In order to approach the temperature insensitivity, proper detuning the wavelength between cavity and quantum well is necessary. In this study, both of the main epi-structure between

Figure 2.16 shows the light-current (L-I) curves of the MBRCLEDs and the RCLEDs were operated at room temperature (RT), 60°C, and 100°C ambiance. It is observed that the output power of MBRCLEDs decay were less than the RCLEDs. From RT to 100°C ambiance, the output power drop of MBRCLEDs was -0.31, -1.78, and -2.5 dB under 20, 50 and 70 mA current injection respectively. In addition, the output power droop of the RCLEDs were -1.75, -3.03, -5.47 dB during 20, 50, and 70mA current injection respectively. The wall-plug efficiency of the RCLEDs and the MBRCLEDs under room temperature operations were shown in figure 2.17. The maximum wall-plug efficiency of MBRCLEDs was 13.7% under 2.5 mA current injection. In addition, the wall-plug efficiency difference (∆ efficiency) between the MBRCLEDs and the RCLEDs under 20 and 70mA current operating were 1.2% and 1.9%, respectively. Since the thermal conductivity of GaAs is poor than Si substrate, the self-heating effect is relatively obvious in the RCLEDs especially. The phenomenon indicates that although the structure had been optimized detuning wavelength but the self-heating still needs a thermal conductivity path to release especially for high current injection. In junction temperature (Tj) measurement, a pulsed generator (KEITHLEY Model 2520) was used to obtain V_f variations in different current injection conditions with a short pulses time of 0.8 µsec. The junction temperature variations (∆^Tj) versus forward voltage of the MBRCLEDs and the RCLEDs under pulsed current operation at room temperature was shown in figure 2.18. It is significantly observed that the junction temperature of the RCLEDs is higher than the MBRCLEDs, the caused of this result is higher thermal resistance of the GaAs material than Si.

The RCLEDs performances will be degenerated with increasing temperature form internal device, and lower internal quantum efficiency was induced due to the more leakage current and non-radiative recombination. This serious leakage current in AlGaInP/ GaInP material is mainly attributed to the relatively low conduction band offset value. In the

MBRCLEDs fabrication processes, the epi-structure would undergo twice bonding processes, which could be damage the thin epi-layers. The above results were demonstrated clearly that the twice wafer bonding processes have not affected the epi-structure. Following, the MBRCLEDs far-field patterns versus different temperature ambiance operating were shown in figure 2.19. The MBRCLEDs were measured under 20 mA and without encapsulating epoxy. The 50% power angle (half-center brightness or 50% of the full luminosity) of the MBRCLEDs under RT and 80ºC is 134º and 126º, respectively. The cause of this result is wavelength detuning between quantum-well and DBR resonance. This special wavelength detuning (15nm) will affect the radiation pattern pass into rabbit ear and reduce the devices temperature sensitively. From figure 2.19 results, the 50% power angle offset is only 8º under RT to 80ºC ambiance operating due to the MBRCLEDs with a better thermal conductivity substrate. Figure 2.20 shows the life-time of the MBRCLEDs and the RCLEDs under a condition of 85°C and 20 mA driving current. Both of the power reliability decay ratio were less than 30% but the MBRCLEDs had a better reliability result due to an excellent thermal conductivity substrate. In figure 2.21 shows the eye diagram measurements were performed to estimate the ability of the MBRCLEDs for high-speed data communication under a condition of devices were operated in 200MHz and 20mA driving current. Eye diagrams at RT and 80ºC have a clear pattern and similar open eye diagram. As above-mentioned performances, the MBRCLEDs are more suitable for working in high temperature ambiance than the RCLEDs such as mobile components, industrial sensor and high-speed data rate transmission applications.