Memory Effect Involving Fib-Induced Ga-Nanocrystals In GaAs/AIGaAs Heterojunction FETs

Abstract

Nanofabrication using focused-ion-beam (FIB) technique has made it possible to fabricate various nano-structures where electrons exhibit low dimensionality and/or the Coulomb blockade effect [l]. However, single-electron effects are normally observed in these structures only at very low temperature, since the room temperatureoperation is possible only in nm-size devices. Here, we investigate effects of implanted impurities on the transport of AlGaAdGaAs FET by measuring its voltage-current characteristics after FIB implantation with high dose (>10'5cm-2) Ga ions. As the dose increases, the Ga ions are expected to form gallium nanocrystals in the vicinity of projected raqge (Rp) which trap electrons between the channel and the control gate. This process is interesting as .a new phenomenon in nano-structures and may be important as a method to form new single electronmemory devices at room temperature. In this work, we show that a novel memory function can be achieved by using the charge-transfer process between FIB-induced Ga-nanocrystals and a nearby channel. Samples used for transport properties are prepared on a modulation-doped GaAs/AlGaAs single heterostructure grown by molecular beam epitaxy (MBE) on a (001) oriented GaAs substrate. The implantation is performed at room temperature using the lOOkV focused Ga ion beam and 2 ~ 1 0 ' ~ c m ~ dose density. Figure l(a) shows a schematic view of the device structure used in the experiment. It has a channel whose nominal width (W) and length (L) are both 1 ,U m. lOOkV Ga+ ions focused to a spot size of 0.2 , U m are implanted (black lines in Fig. l(b)). The effective channel confined by FIB induced depletion layers is less than 0.2 ,U m. We investigate the drain current Id and the device capacitance C as a function of gate voltage Vg. The reproducible hysteresis is measured on the current (Id)-voltage (Vg) experiments when the control gate voltage is swept back and forth with respect to the source (Fig.2). No hysteresis has been observed in other Ga beam induced FETs fabricated in the same process with a low dose density. There is a possibility that heavy dose of Ga ions has led to the formationof Ga-nanocrystals near the channel (Fig. 1 (c)).Therefore, it is likely that the hysteresis is caused by the charge transfer between Ga-nanocrystals and the effective channel. Note that Ga nanocrystals are formed on the channel due to the unfocused ions and/or lateral spreading of scattered ions. Ga nanocrystals produced as an three-dimensionally confined quantum dot embedded in a capacitor, separated from effectivechannel by a shottky barrier and from the other by a non-doped AlGaAs spacer layer. In this memory device the potentialchange elc associated with the transfer of a single electron is a non-negligible quantity This transistor, in which channel region is locally constricted by highdose Ga-FIB implantation, may, therefore, operate as a kind of single-electron memory device at room temperature with controllable injection, storage, and removal of an electron from Gananocrystals. The injection of an electron occurs from the laterally confined channel via direct tunneling only when the control gate is forward biased with respect to the source and drain. Then the stored charge screens the control gate charge and decreases the conduction in the channel as the threshold voltage of the transistor gets more positive. In summary, a memory effect caused by the direct tunneling into FIB induced nanocrystals is found to yield a hysteresis in the conduction of GaAdAlGaAs FETs. [ l ] Y Hirayama, K. Ploog : J. Appl. Phys. 72, p3022 (1992).