Pergnmon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA &did-State Elecrraaics zyxwvutsrqponmlkjihgfedcbaZYXWV Vol. 40. zyxwvutsrqponmlkjihgfedcbaZY Nos 1-8, zyxwvutsrqponmlkjihgfedcb pp. zyxwvutsrqponmlkjihgfed 4 4 1-+51. 19% Copyright (i3 1996 Elsevier Science Ltd 003%1101(%)00308-8 Printed in Great Britain. All rights reserved 0038-I lOI/ 515.00 + 0.00 RESONANT MAGNETOTUNNELLING SPECTROSCOPY OF A QUANTUM LOOP A. NOGARET’, M. J. GOMPERTZ’, P. C. MAIN’, L. EAVES’, M. HENINI’, T. J. FOSTER’, S. P. BEAUMONT’ and H. McLELLAND’ ‘Department of Physics, University of Nottingham, Nottingham NG7 2RD. U.K. %epartment of Electronics and Electrical Engineering, University of Glasgow, Glasgow Gl2 SQQ, U.K. Abstract-We report on tunnelling transport through a novel quantum device consisting of a double barrier structure fabricated as a submicron square loop in the plane of growth. We find two kinds of quantum confinement coexist in such a device: near the current onset, tunelling starts through zero-dimensional states located in the four corners of the loop, whereas at higher bias the resonant current exhibits kinks due to one-dimensional confinement in the arms of the loop. The use of a high magnetic field permits us to measure the binding energy of the dot ground state to the first one-dimensional subband in the connecting wires. This value is then compared to existing theory. Moreover, we show that the different kinds of lateral confinement are not only evidenced in the resonant structure, but also in the scattering assisted replica peaks. Low dimensional semiconductor quantum devices usually consist of a quantum well to which a lateral confining potential is added by means of either sidewall depletion[ I] or electrostatic gating[2]. The use of ionized donors placed inside the quantum well[3] or the exploitation of an inhomogeneous strain[4] have recently been introduced as alternative methods. Theoretical models predict the formation of bound states of reduced dimensionality in bulged[5,6] or twisted (one- or two-dimensional) channels with constant width[7]. The existence of these states has, so far, been revealed by extra resonances in the propagation of microwaves through metallic cavities(81. Similar experiments are difficult to perform in electron waveguides because their finite electrical conductivity drastically reduces the lifetime of the bound states. Optical spectroscopy has produced evidence for low energy transition lines ascribed to a one-dimensional channel formed at the intersection between two quantum wells[9]. Our experiment provides the first evidence for geometrically induced bound states in quantum transport. For this, we investigate tunnelling trans- port through a quantum wire bent at right angles and closed onto itself to form a square loop. We demonstrate the existence of zero-dimensional and one-dimensional states located in the corners and the arms of the loop, respectively. Near the current onset, the dot incrementally charges with electrons, and tunnelling is controlled by Coulomb blockade. Tun- nelling through one-dimensional subbands in the well results in an additional structure being superposed on the main resonant tunnel current peak, and is identically replicated on the LO phonon satellite in the valley. Dots fabricated using this novel method can be controlled by the geometry, as well as the size of the patterned wire, they are also natural candidates for the realization of coupled dot arrays operating in the single electron regime. Our double barrier structure comprises a 10.2 nm wide GaAs quantum well between two asymmetric (AlGa)As barriers, 5.6 and 6.6nm wide, respect- zyxwvutsrqp ively. Two 20 nm spacer layers, one on each side, followed by 50 nm n = 2 x lOI cm -’ doped elec- trodes appear effective in preventing the segregation of impurities into the active region since no sub- threshold current structure was observed in any sample studied[3]. The submicron square loop shown in Fig. I was patterned by electron beam lithography and reactive ion etching. The top contact is composed of Pd/Ge/Au/NiCr to perform the functions of both ohmic contact and dry etch mask. PdGe has several advantages over the more traditional Au/Ge/Ni. Firstly, the low annealing temperature (325°C) limits the diffusion of excess Ge to within 20 nm of the surface and prevents the contamination of the active region. Secondly, the compound has an excellent pattern stability after alloying which is crucial if one requires good control of lateral confinement. Finally, the metal may be deposited in layers as thin as a few 10s of nanometres allowing no limitation in down- sizing the pattern to be transferred by lift-off. Such ohmic contacts appear to be very robust against time and thermal cycling. The devices are then insu- lated using low power SIC& reactive ion etching to avoid sidewall electrical damage. Current-voltage 447