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Formation And Device Applications Of Compound Semiconductor Quantum Nanostructures

The so-called nanotechnology has recently made a great progress. Thus, the possibilities of constructing novel quantum electronic devices consisting artificial quantum structures such as quantum wells, wires, dots and single and multiple tunneling barriers, have become realistic. In this talk, the present status and key issues of research on the formation and device applications of compound semiconductor quantum nanostructures are presented and discussed, introducing recent results obtained by the author's group at RCIQE. Use of a UHV-based growth and processing system with suitable non-destructive characterization capabilities is a promising approach for formation of high-density arrays of defect free quantum nanostructures. An MBE based system of such a nature schematically shown in Fig. 1, which is installed at RCIQE, is described and its features are discussed. In spite of the superiority of the Si -based technology in the present and near-future ULSIs, 111-V materials seem to be more promising for high-density integration of quantum nanodevices, because, unlike Si, only 111-V materials allow formation of uniform, high density arrays of position-controlled, defect-free quantum wires and dots by combination of the EB-lithography and the selective MBE or MOVPE epitaxy on patterned or masked substrates. At RCIQE, the authors's group is engaged in formation of high density quantum wires and dots of InGaAs by selective MBE growth on pattered InP substrates. As an example, the preparation sequence and SEM and CL images of a wire-dot coupled structure for fabrication of single electron transistors (SETS) are shown in Fig.2. Status and future prospects of such a technology are discussed. Surface passivation becomes also a critical issue for quantum nanostructures. A unique Si interface control layer based structure, schematically shown in Fig.3, is being investigated at RCIQE for formation of high quality insulator-semiconductor interfaces on 111-V materials. Its formation and characterization using the UHV-based system in Fig. 1 are discussed. As for device applications, one can think of two lunds of electronic devices in the quantum regime, i.e., "quantum wave devices" and "single electron devices", since electrons manifest predominantly either wave-nature or particle-nature depending on their environments. In Japan, a multi-university national project dedicated to single electron devices ("SED" Project) is currently going (Head Investigator: H. Hasegawa, RCIQE, Period: April 1996 March 2000). Latest results of this "SED" project are briefly mentioned in the talk. At RCIQE, we were interested in both of quantum wave devices and single electron devices. To provide stronger electron confinement than that in previous split gate devices, we have proposed and tested two kinds of new Schottky gate structures which provide stronger electron confinement. They are Schottky in-plane gate (IPG) and Schottky wrap gate (WPG) structures, shown in Fig.4(a). Using these gate structures, we have fabricated quantum wire (QWR) transistors, gated Aharonov-Bohm (A-B) ring devices, IPG-based GaAs wave coupler devices and single electron devices, as shown in Fig.4(b). A SEM image of a WPG SET and its conductance oscillation are shown in Fig.5 (a) and (b). Present status of such device efforts at RCIQE are also presented and discussed.