No abstract

This Communication presents the important finding that certain chalcogenide materials, well-known from rewritable optical recording, allow resistive memory states that are a combination of two electrically-induced (reversible) switching processes, i.e., an actual amorphous-crystalline phase transformation and a (electrolytic) polarity-dependent resistance change. Nanometer-sized crystalline marks were written electrically in amorphous Ge 2 Sb 2+x Te 5 films using atomic force microscopy (AFM), and their resistance was found to depend on the polarity of the applied voltage with a resistance difference of three orders of magnitude. However, no contrast in current has been detected between the crystalline higherresistance state and the surrounding amorphous phase. This resistance switching is reversible for bias voltages well below the threshold voltage required to induce the phase transformation. The switching mechanism is attributed to the solidstate electrolytic behavior due to the presence of excess Sb in the films. Our results render exciting technological opportunities for data storage and encryption by combining both switching concepts.Following his seminal work in 1968, [1] Ovshinsky demonstrated in chalcogenide alloys a fast reversible transformation between amorphous and crystalline phases induced by electrical or optical (laser) pulses. [2][3][4] The two phases exhibited clear contrast in electrical and optical properties and, hence, these materials were suggested for binary data-storage applications. However, it took considerable time before rewritable optical compact discs (CD) and digital versatile discs (DVD) based on these findings came to the market. In recent years, the main focus of phase-change data-storage research returned to resistance switching. So-called chalcogenide or phase-change random access memory (CRAM/PRAM) and ovonic unified memory (OUM) based on the phase-dependent resistance switching are currently under intense investigations, [5][6][7][8][9][10][11][12][13] because they show great promise as next-generation nonvolatile solid-state memory replacing flash memory. In certain chalcogenides a special phenomenon of polaritydependent resistance switching (induced by an electric field) has been identified. [14][15][16][17][18][19] This is related to the solid-state electrolytic character and high ionic conductivities of chalcogenides, and hence is called ionic/electrolytic switching. For one polarity, the chalcogenide medium is electrically conductive by forming conducting filamentary pathways between electrodes, whereas for the reverse polarity it becomes relatively insulative or at least less conductive because of rupture of the previously formed electrical pathways. Memory elements (or structures) based on this switching have been demonstrated in some Ag-saturated chalcogenides including Ag-S, [14,15] AgGe-Se, [16,17] Ag-Ge-Te, [18] and Ag-In-Sb-Te. [19] This switching seems more attractive for applications than phase-dependent switching, because i) it can be performed at lower volta...

Encapsulated conducting probes that can sustain high currents are used to study the nanoscale properties of thin-film stacks comprising of a phase-change chalcogenide, Ge(2)Sb(2)Te(5). Scaling studies on this promising candidate for random-access memory devices had thus far required extensive lithography and nanoscale growth. This seriously hampers rapid materials characterization. This article describes the use of two key techniques, an encapsulated conductive probe and its use in retraction mode, whereby the attractive force between tip and sample is used to maintain electrical contact. The effective transformation of nanoscale dots of amorphous Ge(2)Sb(2)Te(5) into the crystalline state is achieved and the electrical conductivity of the transformed structures is probed. The use of retraction force microscopy in a robust manner is demonstrated by reading the conductivity of the crystalline dots. Both these techniques could enable rapid electrical characterization of nanoscale materials, without extensive nanopatterning, thus reducing material development cycles.

In this paper, we demonstrate reversible resistance switching in a capacitorlike cell using a Ge-Sb-Te film that does not rely on amorphous-crystalline phase change. The polarity of the applied electric field switches the cell resistance between lower-and higher-resistance states, as was observed in current-voltage characteristics. Moreover, voltage pulses less than 1.25 V showed this switching within time scales of microseconds with more than 40% contrast between the resistance states. The latter are found to be nonvolatile for months. The switching could also be achieved at nanoscales with atomic force microscopy with a better resistance contrast of three orders of magnitude.

No abstract