0 Quantum Injection Dots Eliade Stefanescu Center of Advanced Studies in Physics of the Romanian Academy Romania 1. Introduction In optoelectronics, quantum dots are essential elements for coupling a device to an electromagnetic field in the infrared domain of the frequency spectrum. Such a dot is a small semiconductor region, with a forbidden band specific to a given application, embedded in the active i-region of the p-i-n junction of a laser structure (1), or in the sensitive i-region of the p-i-n junction of a photovoltaic structure (2). These quantum dots have the advantage of U c U 3 i n E 1 E 0 U 1 U 2 U 0 U 00 U 4 U v p I e I h U 0 y z G − √ 1 −TG √ TG x 3 x 1 x 0 x 2 x 4 x 5 x T 0 = 0 T ✲ GaAs GaAs Al x Ga 1−x As GaAs GaAs Al x Ga 1−x As Al x Ga 1−x As Fig. 1. Quantum injection dot with the energy levels E 1 and E 0 , in a Fabry-Perot resonator with transmission coefficients of the mirrors T 0 = 0 and T > 0. By quantum transitions between these levels, a super radiant electromagnetic field with two counter-propagating waves of amplitudes G and √ 1 −TG is generated. being feasible for a large class of applications (transition frequencies). However, they have the disadvantage that, being embedded in a bulk region, any injection or photovoltaic process includes transport processes through the active (sensitive) region, which are very dissipative. In such a system of quantum dots, an injected electron in the bulk active region of a laser structure has a large probability to recombine with an electron hole before reaching a quantum dot, where this electron becomes an active one, contributing to the laser field generation. Similarly, an optically excited electron in a quantum dot of a photo-voltaic bulk active region, has a large probability to meet another quantum dot, and to decay in the ground state of this 13 www.intechopen.com