Multilevel Resistive Memory Switching in Amorphous Ternary High K-Dielectric Oxide LaGdO3 Thin Films Grown By Pulsed Laser Deposition

The conventional memory technologies are facing the scaling issues as the semiconductor devices are rapidly approaching the miniaturization limits. Recently the electric-field controlled multilevel resistive memory switching phenomenon in metal oxides has attracted considerable attention to develop next generation low power, high speed, rugged and high density nonvolatile resistive random access memory (RRAM) devices. Multilevel switching of the resistance in RRAM devices promises high capacity memory with capability of storing multiple bits in one device. Generally the materials employed for this application are in polycrystalline form which limits the continuous downscaling when memory cell size becomes comparable to the grain diameter. Amorphous materials free from grain boundaries are capable of offering homogeneous structure to avoid such issue. It is hence well reasoned that amorphous high-k gate dielectrics, which have already been demonstrated to be compatible with semiconductor transistor technologies, may be good choices for RRAM applications. Amongst other high-k gate dielectrics materials currently being explored for the development of RRAM, lanthanum based amorphous high-k oxides have emerged as potential candidates.  In this paper we report, for the first time (to the best of our knowledge), multilevel resistive switching (RS) in RRAM devices based on amorphous thin films of LaGdO₃(LGO), their resistive switching characteristics, and associated conduction mechanisms.

About 50 nm thick amorphous LGO films were grown on commercial Pt/TiO₂/SiO₂/Si substrates using pulsed laser deposition at a substrate temperature of ~ 300 ºC and oxygen partial pressure of ~ 2×10^-3 Torr. Top electrodes of ~ 70 nm thick Pt film with a typical diameter of ~ 80 µm were used to construct MIM capacitors. The RS characteristics and conduction mechanisms of these devices were studied through current-voltage (I-V) measurements in the top-bottom configuration. The as grown devices were found to be initially in high resistance state (HRS) ~ 40 MΩ and did not show any resistance switching behavior until the applied bias voltage was increased to ~7 V (initial forming voltage) with a current compliance (I_CC) of 10 mA at which device switched to low resistance stae (LRS). After forming process the device showed reliable and repeatable switching between HRS to LRS with nearly constant resistance ratio ~ 10⁶ and well defined and non-overlapping switching voltages. To achieve multilevel resistance switching, the compliance currents was varied in the range of 1-10 mA during the set process i.e. during switching from HRS to LRS. It was observed that the higher the I_CC imposed on the device, the lower resistance value was obtained in LRS. Under different I_CC values of 1, 5 and 10mA, three different resistances of LRS having values ~ 2400, 170 and 10Ω were obtained with a nearly constant value of resistance of HRS. All the four states showed good endurance up to 50 cycles provided the desired I_CC is maintained along with retention characteristics over 10³ seconds. The temperature dependent measurement of resistances of the device in different resistance states revealed that all the three low resistance states were metallic with increasing resistance temperature coefficient (α) on increasing resistance of LRS while HRS showed semiconducting behavior. Based on this and XPS studies we propose that the observed resistive  switching can be explained by the formation/rupture of conductive filaments formed out of oxygen vacancies and metallic Gd atom while multilevel conduction in the device can be explained by the variation in the diameter of conducting filament which was further supported by variation in α. The observed reproducible and nonvolatile multilevel resistive switching, good contrast in  HRS to LRS resistance ratio, high endurance, and amorphous structure make LGO as a promising material for the future nonvolatile multi-bit RRAM devices.