The reliably of GaN-based VCSEL with different ITO

In order to realize the device reliably of operation , we also measure the damage current density of four kinds of VCSEL devices by mean of EL measurement system.

Fig5.16 are the voltage and output power versus current density diagram (LIV curve) , and we choose the current density when the LIV curve change unusually as the damage current density of the devices. Because of the same epitaxial quality of

Fig 5.16(a) to (d) , the damage current is higher for ITO layer deposited by sputter than by E-gun. Besides, the devices with 210nm ITO has the lowest damage current density(~17kA/cm²) . It maybe attributed to the blocking of heat dissipation for the thicker ITO layer.

Fig 5.16 Voltage and output power versus current density diagram of VCSEL with different ITO structures

5.4 Summary

Table 5.1 is the overall electrical and optical characteristics of GaN-based VCSEL with four different ITO structures. From this table , the VCSEL devices with 30nm ITO layer have better electrically characteristics , such as series resistance and roll over current density , than the devices with 210nm ITO layer . Owing to lower optical absorption for 30nm ITO , the quality factor is also higher . This result has reached the goal we set before the experiment. Next, we should find the best ITO structure for our VCSEL devices. Compared with different deposition methods for the same thickness , the electrically characteristics of VCSELs with ITO layer deposited by sputter are better than deposited by E-gun. The 30nm crystalline and 10nm amorphous + 20nm crystalline ITO thin film deposited by sputter have similar electrically and optical performance of VCSEL devices , but the former suffers from inhomogeneous current spreading and large amount of leaky light , so the 10nm amorphous + 20nm crystalline ITO thin film is the best choice for our VCSEL devices.

Table 5.1 The overall electrical and optical characteristics of GaN-based VCSEL with four different ITO structures

Chapter 6

Conclusions and future work

6.1 Conclusions

In this report, we have designed and fabricated the electrical pumping GaN-based VCSELs with hybrid mirrors and four different ITO structures. The VCSEL resonant cavity is including the high-reflectivity AlN/GaN bottom DBRs (99.3%), about 7λ GaN-cavity, transparent conduct layer (ITO), and SiO

2/Ta

2O

5 dielectric DBRs (99%).

The series resistance of VCSEL devices with four different ITO structures are between 150Ω and 202Ω, and this value is better than the devices we reported previously (530Ω). However, the quality factor of VCSEL devices with 30nm ITO layer is about 1600, slightly lower than the devices without ITO layer (1900), but higher than the devices with 210nm ITO layer of about 700 due to the optical absorption in ITO layer. Compared the results with the same thickness ITO film deposited by E-gun and sputter, the devices with 30nm ITO layer by sputter has better electrically characteristics, such as series resistance(~165Ω) , roll over current density(~13kA/cm²), and damage current density (~43kA/cm²). Among VCSEL devices with four different ITO structures, the devices with 10nm amorphous plus 20nm crystalline ITO film have the similar electrically and the best optical

performance than 30nm crystalline ITO caused less emitted light to leak from the cavity. In summary, the 10nm amorphous plus 20nm crystalline ITO deposited by sputter is the most suitable material as the transparent contact layer to improve our VCSEL performance and characteristic.

6.2 Future work

According to the study in this thesis , there are several work for our group to continue and improve. One is the heat dissipation problem due to worse thermal conductivity of sapphire substrate. We can package our devices into TO cans or use pulsed current source for current injection to avoid large thermal effect in our devices.

The other is to get better current and optical confinement. To solve this problem , we have design the new VCSELs structure shown in Fig6.1 (a) and (b). These isolation layers of lower refractive index can efficiently upgrade both the current and optical confinement. We expect these structures can finally reduce the threshold condition for the VCSEL devices.

Fig7.1 (a) the devices with AlN current blocking layer (b) the devices with ions

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