Physica B 279 (2000) 220}223 Self-sustained oscillations caused by magnetic "eld in a weakly-coupled GaAs/AlAs superlattice B.Q. Sun*, J.N. Wang, D.S. Jiang, J.Q. Wu, Y.Q. Wang, W.K. Ge NLSM, Institute of Semiconductors, P.O. Box 912, CAS, Beijing 100083, People's Republic of China Physics Department, Hong Kong University of Science and Technology, Hong Kong, People's Republic of China Abstract We have investigated the in#uence of transverse magnetic "eld B up to 14 T at 1.6 K on the tunneling processes of electric "eld domains in doped weakly coupled GaAs/AlAs superlattices. Three regimes, i.e. stable "eld domains, current self-sustained oscillations and averaged "eld distribution, are successively observed with increasing B. The mechanisms of switching-over among these regimes are due to B-induced modi"cation of the dependence of the e!ective electron drift velocity on electric "eld. The simulated calculation gives a good agreement with the observed experimental results.  2000 Published by Elsevier Science B.V. All rights reserved. PACS: 73.40.GK; 73.20.DX; 73.50.Dn Keywords: Transverse magnetic "eld; Field domains; Self-sustained oscillations The inhomogeneous electric "eld distribution and the high- and low-"eld domains in weakly coupled superla- ttices (SL) originate from the sequential resonant tunnel- ing between the subbands. The occurrence of electric "eld domains is attributed to the existence of negative di!er- ential velocity (NDV) in doped SL with increasing bias voltage. Recently, damped and undamped temporal self- sustained oscillations of the current were observed in the MHz and GHz ranges for SL with a certain range of doping concentration biased by a "xed DC voltage [1}3]. The origin of current self-oscillations is an oscillat- ing charge-accumulated domain boundary. Above and below this doping concentration, the self-oscillations dis- appear completely, resulting in the build-up of stable electric "eld domains at higher doping concentrations or in a uniform electric "eld distribution at lower doping concentrations. The variation of carrier concentrations can be realized experimentally by photo-excitation of the undoped SLs. We have previously reported that * Corresponding author. Fax: #86-10-62325372. E-mail address: bqsun@red.semi.ac.cn (B.Q. Sun) the switching-over between stable "eld domains and the current oscillations can also be observed by limiting injected current [3]. Under the transverse B, the NDV will be modi"ed due to B-induced redistribution of the tunneling electron momentum and energy. Therefore, it is possible to observe the switching-over between di!er- ent regimes of stable, dynamic (current self-oscillation) "eld domains and uniform "eld distribution by increas- ing B. In this paper, the tunneling processes of electric "eld domains are investigated under a transverse B up to 14 T at 1.6 K. The sawtooth-like current branch amplitudes in I}V characteristics decrease and then disappear with increasing B. On further increasing B, the dynamic elec- tric "eld domain (temporal current self-oscillation) oc- curs. The observed results can be interpreted based on B-induced modi"cation of NDV. A discrete drift model is used to simulate the experimental results. The typical sample investigated in this work was doped GaAs/AlAs SL diode grown by MBE in a VGMKII system. The SL consists of 40 periods of 14 nm GaAs well and 4 nm AlAs barrier. The center 10 nm of the well was doped with Si (n"210 cm). The area of the diode is 0.01 mm. The I}< characteristics of the 0921-4526/00/$ - see front matter  2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 7 5 0 - 4