GaAs-based quantum dot vertical-cavity surface-emitting lasers and microcavity light emitting diodes
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Due to quantum dots’ atom-like density of states, quantum dot lasers have been expected to exhibit ultralow threshold current, temperature-insensitive operation, high modulation speed due to narrow spectral linewidth, high material gain and high differential gain. Quantum dot optoelectronic devices have advanced rapidly since the advent of self-organized QD growth technique. To obtain temperature-insensitive low threshold QD lasers, ground state lasing is highly desirable, the devices should operate close to transparency and far below the saturation, and thermal population of the upper energy levels needs to be suppressed. Three stack InAs/GaAs QD edge-emitting lasers are fabricated and characterized with either 500 Å or 300 Å GaAs spacer thickness. For 500 Å spaced QD lasers, a combination of wide energy separation of 95 meV and relatively high internal efficiency of 74% leads to low threshold operation and high characteristic temperature To of 126 K beyond room temperature. In semiconductor vertical-cavity surface-emitting lasers (VCSELs), extremely high reflectivities are required for both top and bottom mirrors. Critical VCSEL design issues include alignment between the cavity resonance and the optical gain peak, lateral optical confinement, high contrast-ratio DBR mirrors. Ground state lasing of a 1.07 mm oxide-confined InGaAs/GaAs QD VCSEL is demonstrated using intracavity contacts and low loss cavity design, with the room temperature lasing threshold of ~ 700 mA and ~ 270 mA for 10 mm and 2 mm devices, respectively. The much lower experimental cavity Q is shown due to the excessive distributed loss in the upper dielectric mirror. In planar microcavity light-emitting diodes (MCLEDs) enhanced efficiency and narrower spectral linewidth are achieved through modifying the optical mode structure. To fully take advantage of the enhanced mode coupling provided by the microcavity, it is important to electronically confine the carriers to small optical mode volume, which can be achieved in apertured QD-MCLEDs. Apertured QD-MCLEDs are first demonstrated showing the efficiency enhancement with reduced mode size. The highest efficiency of 16 % for apertured QD-MCLED is achieved by designing resonance tuning at low temperature of 160 K.