The Study On Thermal Theory And Fabrication Process Of High-power Vertical-cavity Surface-emitting Lasers

The research on high-power vertical-cavity surface-emitting lasers (HP VCSEL) has become a hot topic in recent years in china and abroad, because the HP VCSEL has many advantages over the conventional high power edge-emitting lasers. But the progress of the research on HP VCSEL is somewhat slow because of its serious self-heating effect. The effect will make HP VCSEL devices increase temperature and a great amount of heat occurred can increase the threshold current and reduce output power of the device. Thermal problem has become a key difficult issue in fabrication of HP VCSEL. In order to solve this problem and reduce the self-heating in the device and to improve the device heat dissipation, we have designed the 980 nm HP VCSEL with radial bridge electrodes on AlxNy film passivation layer, the main research work is as follows:1. The thermal theory of the HP VCSEL is introduced and the influence of thermal effect on the HP VCSEL has been analyzed. The mechanism of heat generation inside the HP VCSEL has been pointed out. It is explained that the Joule's heat generated in the P-DBR is the important source in the HP VCSEL. According to the definition of the thermal resistance we have analyzed and derived the thermal resistance formula of the HP VCSEL. It is indicated that the thermal resistance is directly proportional to the area of active region and inversely proportional to the distance between the heat source and heat sink. Finally we established the thermal model to calculate heat distribution inside the HP VCSEL on the basis of heat conduction theory.2. In order to improve the thermal characteristics and to increase the output power of HP VCSEL, we designed the epitaxial wafer on the basis of related theories. The active region of HP VCSEL is made of InGaAs/GaAs strain-compensated quantum well. The devices showed their good temperature characteristics due to the better electron and hole confinement in the strain-compensated quantum well materials. In order to improve the device temperature adaptability the gain peak wavelength is designed to operate at 970nm in device resonant cavity by regulating In content in wells and P content in barriers. The linearly graded structure is adopted in the HP VCSEL DBR so as to reduce the series resistance and to decrease Joule's heat of P-DBR.3. In order to improve the device heat dissipation and performance, we designed the structure of HP VCSEL with an AlxNy film as the passivation layer. The conventional circular injection electrode on the P side in bottom emitting HP VCSEL is turned into a radial bridge electrode. The analysis shows that the HP VCSEL with the radial bridge electrodes on AlxNy film passivation layer has lower P-DBR electric and thermal resistance than the traditional ones. Also, the device with the new structure has better heat dissipation performance.4. The processing of the HP VCSEL with the radial bridge electrodes on AlxNy film passivation layer, especially the wet etching, wet oxidation and preparing AlxNy films have been studied with experiments. By choosing the etching liquid and improving the etching condition, we eliminated the "dovetail" structure that conventionally happened on the lateral side of the etched wall. Wet oxidation experiments have been carried out. The results showed that the oxidation rate increases with the time by an exponential law. We have obtained the best condition of wet oxidation through the experiments. Studying the film preparation, we got the technique for making the AlxNy films with high resistance and high thermal conductivity.5. The HP VCSEL with radial brigde electrodes on AlxNy film passivation layer and its counterpart with the conventional electrodes on SiO2 film passivation layer have been made with the same epitaxial wafer and the same processing for comparison. The testing results of devices for the two kinds of structure showed that the temperature and opto-electric characteristics of HP VCSEL with the new structure are much better than those of the traditional one. Under CW operation at room-temperature, the threshold current of new structure device is 390 mA, the differential efficiency is 0.4W/A, the power conversion efficiency is 13.2% and the maximal output power is 1.6 times higher than that of the traditional devices. Its thermal resistance is 0.095℃/mW whereas it is 0.125℃/mW for the traditional device. The thermal resistance of new structure device is much lower than that of the traditional structure device.6. We have made the experimental setup for the beam quality characterization and applied for the national patent. The near-field pattern, far-field distribution, and the M2 factors of the new structure HP VCSEL have been tested with the setup.The testing results showed that the beam quality characterization of new structure device is much better than that of the traditional structure device.