Superlattices and Microstructure& Vol. 12, No. I, 1992 47 COHERENT MAGNETOTUNNELING IN COUPLED QUANTUM WELLS FOR DIFFERENT LANDAU LEVELS A. Hernandez-Cabrera, P. Aceituno, and H. Crua Dpto. de Fisica Fundamental y Experimental. Universidad de La Laguna 38204-La Laguna. Tenerife. Canary Islands. Spain (Received 28 February 1992) In this work, we have studied the coherent tunneling oscillations of electrons in a Gal_xAlxAs-GaAs asymmetric double quantum well heterostructure under external applied electric and magnetic fields along the growth direction of the structure. In this coupled quantum well system, it is found that magnetic quantiaation collapses the resonant condition for coherent oscillations between the coupled quantum well states into a discrete set ‘of coherent tunneling channels. These tunneling channels depend on the magnetic field value and on the Landau level number. We show the possibility of a direct observation of these tunneling processes by means of an oscillating luminescence signal. l.- Introduction In the last few years, resonant tunneling through quantum well heterostructures has attracted considerable attention due to its possible application ultra-high-speed electronic devices [l,:P. Mainly static features of the tunneling phenomena in semiconductor heterostructures have been investigated, while dynamical aspects have not been studied thoroughly because the characteristic time scale of the process is of the order of picoseconds or less, which is shorter than the measurable time by commonly available methods [3]. Nevertheless, dynamics should be investigated since it not only determines the ultimate speed of tunneling devices, but it elucidates fundamental aspects of electronic waves in solids, particularly their behavior in the time domain [4-61. There is considerable current interest in the process of coherent tunneling in semiconductor heterostructures. Luryi [7] has discussed the possibility of observing the coherent oscillations in asymmetric coupled quantum wells (ACQW) at resonance. The escape time of electrons photoexcited into the quantum well of a double-barrier resonant tunneling structure has been recently measured by time-resolved luminescence [3]. In this paper, we report a theoretical study of the longitudinal magnetic field effect on the electron charge oscillations in a Ga Al As-GaAs The method of l-x x ACQW system. calculation used in this work is based on the transfer matrix approach and the Airy functions as analytical solutions of the electron effective-mass equation. One possible application of the coupled quantum wells is the reduction of the recovery time constant in one quantum well when we coupled it to another one through a thin barrier [4.5]. In this work, we show how an external magnetic field perpendicular to the semiconductor layers collapses the resonant condition for coherent tunneling in an ACQW system into a discrete set of tunneling channels obtaining a different value for the electronic oscillation period for each different Landau level index. A direct observation of the variation of this period by means of an experiment is proposed. 2.- Method of Calculation Consider the heterostructure illustrated in Fig.1. This structure consists on two quantum wells separated by a Ga,_xAlxAs barrier of height VO, and thickness L. Both wells represent pure GaAs layers of thickness d, and da for the left and right quantum well, respectively. This heterostructure (in the inset of fig.1) is our ACQW system. We no" consider the influence of an electric field applied perpendicular to the layers. In Fig.1 we plot the resonant tunneling condition: at a fixed applied electric field (Fr) we align the electron energy levels of both quantum wells in the conduction band 07494036/92/050047 +06 SOa.OO/O 0 1992 Academic Press Limited