Theoretical study of GaAs-based quantum dot lasers and microcavity light emitting diodes
Quantum dots are semiconductor nanostructures that act as artificial atoms by confining electrons and holes in three-dimensions. Self-organized QDs sit “on top of” a wetting layer, so that the QDs’ zero-dimensional levels are electronically coupled to the wetting layer’s 2-D density of levels. A theoretical model is presented that is capable of describing both nonequilibrium carrier distributions in the QD zero-dimensional levels at low temperature, as well as quasiequilibrium distribution for higher temperature operation. Due to the larger thermalization rate out of higher energy QDs of the ensemble, rearrangement of the carrier distribution could occur, which produces asymmetric gain spectrum, and leads to the novel behavior that the threshold current density can in fact decrease with increasing temperature for a QD laser, and thus show a negative characteristic temperature coefficient. The narrowing of the full-wdith half-maximum (FWHM) of the spontaneous emission spectrum could occur due to this gain spectrum narrowing. At room temperature the carrier transport is so fast that the most important characteristics of QD lasers can be analyzed using quasi-equilibrium solutions to the rate equations. The closely spaced hole levels result in a thermal smearing of the hole population among many hole states, and cause a large fraction of injected holes that do not occupy the QDs’ ground states, so that injected carriers can be wasted by not optically coupling to the lasing mode. P-doping is proposed to build in excess holes and overcome the thermal effect of the close hole spacings. High characteristic temperature coefficient is predicted for these p-doped QD lasers, large modulation bandwidth with zero or negative chirp is also predicted. According to Fermi’s golden rule, the spontaneous emission probability of an active emitter is given by the electronic transition probability from the excited state to the ground state times the photon density of states. Therefore, by tailoring the surrounding environment of an active emitter, both the spontaneous emission rate and the direction of emission will be altered. We study a QD VCSEL with very small mode volume, and show that the modulation bandwidth has a strong dependence on the mode volume. The pulse response is also strongly mode volume sensitive, and at small mode volume the relaxation oscillation is greatly suppressed, even allowing multi-gigabit data transmission. In planar microcavity LEDs enhanced efficiency and narrower spectral linewidth are achieved through the Purcell effect. Quantum dots are useful to obtain the necessary electronic confinement to very small apertures and can provide a short spontaneous lifetime. 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. We study the performance of an apertured QD MCLED, and find that very high efficiency apertured QD-MCLED can be achieved with small mode volume and narrow QD inhomogeneous linewidth, while only a moderate cavity quality factor Q is needed.