Nanodevices produced with focussed ion beams

In directly writing the 30 nm focus of a focussed Ga-ion beam (FIB) with an energy of 100 keV we define insulating lines in two-dimensional electronic layers in semiconductors. Ga ions act in GaAs and silicon as deep impurities or p-type doping, respectively. In this way the insulation by such written lines is due to lateral depletion within npn-like interfaces. In writing two FIB lines with a close spacing we define conducting channels between them. In applying a voltage of several Volts to the adjacent areas of the channel relative to it we can tune the effective width of the channel in the range of a few 100 nm to zero and obtain thus a one-dimensional field-effect-transistor-type structure. This transistor exhibits a pure lateral field effect and is thus topologically very different to current transistor concepts. Due to its particular geometry it is called in-plane-gate (IPG) transistor? since the gate and the channel are in the same plane. The fabrication of this type of transistor is thus completely maskless and does not require any alignment procedures since gate, source and drain are all written in the same writing process. Due to the computer-control of the beam deflection even more complex structures are just a question of software and do not need a set of specific masks or photoresist like in the classical lithography. The required line ion dose is of the order of 10(6) cm(-1) which means that there are about 100 ions per mu m implanted. For devices with maximum micron dimensions only a few hundred ions need thus to be implanted. With typical beam currents of up to 30 pA there are 2 x 10(8) ions available in a second and it is thus realistic to assume that millions of devices can be written per second with the focused ion beam. In this way the sequential character of the FIB implantation does not limit the fabrication speed severely due to the low required ion doses. Room temperature as well as low temperature operation of this device are presented and discussed in the framework of applications in nanoelectronics. The very low capacitance between the IPG and the channel is discussed. Especially the specularity of ballistic electrons at such an IPG is investigated. In changing the gate voltage and its sign, this specularity can be varied by 20%. It is related to electron transport in undisturbed or ion-implanted regions which leads to velocity-modulated systems. (C) 1998 Elsevier Science B.V.