P. W. M. Blom

Carrier‐carrier scattering induced capture in quantum well lasers

We present calculations of the carrier capture efficiency into various types of quantum well lasers. The carrier capture into a quantum well can be due to either optical phonon emission or carrier‐carrier scattering. Both capture mechanisms have been calculated and show oscillations as a function of the quantum well thickness. By optimizing the carrier capture efficiency the carrier accumulation in the barrier layers can be reduced, resulting in an improved modulation response and threshold current.

MEASUREMENT OF THE AMBIPOLAR CARRIER CAPTURE TIME IN A GaAs/AlxGa1-xAs SEPARATE CONFINEMENT HETEROSTRUCTURE QUANTUM WELL

The carrier capture in a separate confinement heterostructure quantum well has been studied both experimentally and theoretically. Our calculations show that the electron and hole capture time vary strongly as a function of the excess energy. At an excess energy of 40 meV, both capture times are equal resulting in an ambipolar capture process which allows a direct comparison between theory and experiment. We carried out subpicosecond luminescence spectroscopy experiments and deduce an ambipolar overall capture time of 20 ps, a number which for the first time is in agreement with theoretical predictions. The quantum mechanical overall capture time of 20 ps gives rise to a classical local capture time of 3 ps which is determined from a diffusion model.

Experimental and theoretical study of the carrier capture time

We present an experimental and theoretical study of the carrier capture time into a semiconductor quantum well. We observe for the first time the predicted oscillations of the phonon emission induced capture time experimentally and found good agreement with theory. Calculations show that not only does the LO phonon emission induced capture time (ph capture) oscillate as a function of well width, but also the carrier-carrier scattering induced capture time (c-c capture) oscillates by more than an order of magnitude as a function of the active layer design. Recently, it has been shown that the carrier capture time is directly related to the modulation bandwidth in a quantum well laser. As a result, it might be possible to tailor the modulation bandwidth by optimizing the capture efficiency using a proper design of the active layer in a quantum well laser.

Blocking of Γ→X transfer in GaAs/AlAs short period superlattices due to X-state band filling

We report an experimental study of the optical properties of a GaAs/AlAs short period superlattice (SPSL) by photoluminescence and time‐resolved two‐pulse correlation experiments. Our SPSL is designed in such a way that the lowest confined Γ state in the GaAs is only slight (30–50 meV) above the X states in the AlAs. Therefore the Γ→X transfer due to LO‐phonon emission or interface scattering is prohibited by X‐band filling at high excitation densities and small excess energies, which allows us to measure a Γ→X transfer time induced by LO‐phonon absorption of 20 ps. By adjusting the laser energy, excitation density, and temperature we are able to transform the emission spectrum of the SPSL completely from a type‐I into a type‐II transition.

Reduction of the threshold current in quantum-well lasers by optimization of the carrier capture efficiency

We have investigated the carrier capture mechanism in quantum well lasers and its relevance for device characteristics. It is demonstrated that the dependence of the threshold current on the structure parameters of the layers in the active region is highly correlated with the electron capture efficiency. From our calculations it appears that not only the LO-phonon induced capture process but also the carrier-carrier scattering induced capture process oscillate as a function of quantum well width. The predicted structure parameters for an optimum capture efficiency are equivalent for these scattering processes, because in both capture mechanisms these oscillations arise from oscillations in the wave function overlap. The carrier-carrier scattering starts to dominate the capture process for carrier densities larger than 1.1011 cm-2 in the quantum well. As a result an efficient capture process enhances the cooling of the carriers after injection, giving rise to the reduction of the carrier temperature and thus to a low threshold current. We find that a large capture efficiency improves the modulation response of a quantum well laser due to a smaller amount of carrier accumulation in the barrier. By maximizing the carrier capture efficiency in laser structures we for the first time are able to predict the structure parameters of the layers in the active region for an optimum laser performance.

Carrier capture time: relevance to laser performance

We present an experimental and theoretical study of the carrier capture time in a semiconductor quantum well. We observed for the first time the predicted oscillations of the phonon emission induced capture time experimentally and found good agreement with theory. Calculations show that not only the LO-phonon emission induced capture time (ph-capture) oscillates as a function of well width, but also the carrier-carrier scattering induced capture time (c-c capture) oscillates by more than an order of magnitude as a function of the active layer design. Moreover, the calculated amount of excess carrier heating also oscillates as a function of quantum well thickness. Recently, it has been shown that the carrier capture time is directly related to the nonlinear gain in a quantum well laser. As a result, the nonlinear gain can be tailored by optimizing the capture efficiency using a proper design of the active layer in a quantum well laser.

Ultrafast carrier capture in quantum well structures

We present an experimental and theoretical study of the carrier capture time into a semiconductor quantum well. The carrier capture time was obtained by measuring both the rise of the quantum well population using time-resolved luminescence measurements and the decay of the barrier population using pump-probe correlation experiments. In the first technique we compare the QW rise times after direct (below the barrier band gap) and indirect (above the barrier band gap) excitation, in order to eliminate the effects of relaxation and exciton formation in the quantum well. We report the first experimental observation of oscillations in the carrier capture time between 3 and 20 ps as a function of quantum well thickness, obtained from both techniques. The observed capture times are for the first time in agreement with theoretical predictions from an ambipolar capture model.