Impurity-induced disordering in vertical-cavity surface-emitting lasers (VCSELs) has demonstrated enhanced performance such as higher modulation speeds, reduced series resistance, and higher-order mode suppression for singlemode operation. Initiated by the diffusion of Zn, impurity-induced disordering intermixes discrete AlGaAs-based distributed Bragg reflectors (DBR) pairs which leads to lower mirror power reflectivity and increased optical loss. When formed into an aperture where the center is non-disordered, suppression of higher-order transverse modes for high-power single-mode operation can be achieved. For maximal mode suppression, deep disordering apertures are desirable. However, due to the isotropic nature of diffusion, these apertures are limited to the lateral diffusion encroaching onto the fundamental mode. By tailoring the film stress of the SiNx diffusion mask, the capability to modify the diffusion front of the disordering aperture is demonstrated. Defined by their lateral-to-vertical (L/V) diffusion ratios, an L/V ratio of 3.7 to 0.90 is measured for corresponding SiNx diffusion mask strains ranging from a compressive -797 MPa to a tensile +347 MPa. This demonstrates that tensile strained diffusion masks limit the amount of lateral diffusion. To further reduce the lateral encroachment, increasingly tensile diffusion masks are deposited by modifying the SiH4/NH3 flow ratios. This diffusion mask is employed to fabricate high-power single-mode VCSELs designed for 850 nm emission. Compared to VCSELs fabricated with non-optimized disordering apertures, enhanced transverse-mode control is achieved and singlemode output power in excess of 3.8 mW with a side mode suppression ratio greater than 30 dB is measured.
Progress on the modeling, fabrication, and characterization of the transistor-injected quantum-cascade laser (TI-QCL) is presented. As a novel variant of the quantum cascade laser, the TI-QCL has been projected to have advantages over conventional QCLs in certain applications because of its 3-terminal nature. The separation of field and current is expected to allow separate amplitude and frequency modulation, and the location of the cascade structure in a p-n junction depletion region is expected to reduce free carrier absorption and improve efficiency. At the same time, the added complexity of the structure creates challenges in the realization of working devices. An overview of the basic operating principles of the TI-QCL is first given, and projected advantages discussed. Next, work on modeling GaAsbased TI-QCLs is presented, and a design for devices in this system is presented. Finally, work on fabrication and characterization of devices is examined and ongoing challenges are discussed. The role of quantum state alignment in the QCL region on electron-hole recombination in the base is also examined, showing the capability of using basecollector voltage to modulate the optical output from the direct-bandgap transistor base.
Recombination of carriers in the direct-bandgap base of a transistor-injected quantum cascade laser (TI-QCL) is shown to be controllable through the field applied across the quantum cascade region located in the transistor’s base-collector junction. The influence of the electric field on the quantum states in the cascade region’s superlattice allows free flow of electrons out of the transistor base only for field values near the design field that provides optimal QCL gain. Quantum modulation of base recombination in the light-emitting transistor is therefore observed. In a GaAs-based light-emitting transistor, a periodic superlattice is grown between the p-type base and the n-type collector. Under different base-collector biasing conditions the distribution of quantum states, and as a consequence transition probabilities through the wells and barriers forming the cascade region, leads to strong field-dependent mobility for electrons in transit through the base-collector junction. The radiative base recombination, which is influenced by minority carrier transition lifetime, can be modulated through the quantum states alignment in the superlattice. A GaAs-based transistor-injected quantum cascade laser with AlGaAs/GaAs superlattice is designed and fabricated. Radiative base recombination is measured under both common-emitter and common-base configuration. In both configurations the optical output from the base is proportional to the emitter injection. When the quantum states in the superlattice are aligned the optical output in the base is reduced as electrons encounter less impedance entering the collector; when the quantum states are misaligned electrons have longer lifetime in the base and the radiative base recombination process is enhanced.
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