An important issue in the technology of QCLs is the capability to extract heat out of the laser active area in order to reduce the increase of the temperature. High temperature not only reduces most performance metrics but also decreases device lifetime and impairs its reliability, leading to degradation of laser mirrors and destruction of the device.
In this paper, we report on the investigation of the temperature of QCLs based on different designs. QCLs are complex, multilayer structures, which require high current and voltage to polarize the structure in order to obtain level alignment. This results in high heat generation. Thermal limitations in case of QCL are the most critical factor decreasing the performance of a device. High electrical power combined with relatively low wall-plug efficiency results in high-temperature increase in the active core. Efficient heat dissipation is difficult due to hundreds of layers impeding thermal conductivity of the structure. Moreover, the materials composing the gain region are ternaries with a composition of roughly 50%, what results in thermal conductivity lower by a factor of 10 than in case of bulk InP. Knowledge of the temperature is gained through unique temperature measurement technique – CCD thermoreflectance (CCD TR). This method allows for rapid thermal characterization of QCLs, as the registration of high-resolution map of the whole facet of the device lasts only several seconds. CCD-TR allows accurate evaluation of the thermal characteristics of quantum cascade lasers.
Here, we report on the influence of design on thermal properties of QCLs. The design of waveguides and optical confinement in QCLs is essential. The increase of the optical confinement was frequently achieved by placing the active core between two InGaAs layers. However, low thermal conductivity of InGaAs layers results in inefficient dissipation of heat from the active core. By removing or significant reduction of the layers’ thickness, observed temperatures of the active core are significantly lower. The modifications include Experimental investigation proves that performance improvements can be gained by introducing modifications into the design of the structure. Based on experimental data, methods to further improve the performance of QCLs are discussed.