We report that crystalline phases with ferroelectric behavior can be formed in thin films of SiO2 doped hafnium oxide. Films with a thickness of 10 nm and with less than 4 mol%. of SiO2 crystallize in a monoclinic/tetragonal phase mixture. We observed that the formation of the monoclinic phase is inhibited if crystallization occurs under mechanical encapsulation and an orthorhombic phase is obtained. This phase shows a distinct piezoelectric response, while polarization measurements exhibit a remanent polarization above 10 C/cm2 at a coercive field of 1 MV/cm, suggesting that this phase is ferroelectric. Ferroelectric hafnium oxide is ideally suited for ferroelectric field effect transistors and capacitors due to its excellent compatibility to silicon technology

The transition metal oxides ZrO(2) and HfO(2) as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO(2) thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO(2)-ZrO(2) mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc2(1), whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.

Structural incommensurate modulation rule in hexagonal Ba(Ti1-xMx)O3-δ (M = Mn, Fe) multiferroics AIP Advances 2, 042129 (2012) Water assisted gate induced temporal surface charge distribution probed by electrostatic force microscopy J. Appl. Phys. 112, 084329 (2012) Influence of target composition and deposition temperature on the domain structure of BiFeO3 thin films AIP Advances 2, 042104 (2012) Nanodomain structures formation during polarization reversal in uniform electric field in strontium barium niobate single crystals J. Appl. Phys. 112, 064117 (2012) The effect of the top electrode interface on the hysteretic behavior of epitaxial ferroelectric Pb(Zr,Ti)O3 thin films with bottom SrRuO3 electrode J. Appl. Phys. 112, 064116 (2012) Additional information on J. Appl. Phys. Structural and electrical evidence for a ferroelectric phase in yttrium doped hafnium oxide thin films is presented. A doping series ranging from 2.3 to 12.3 mol% YO 1.5 in HfO 2 was deposited by a thermal atomic layer deposition process. Grazing incidence X-ray diffraction of the 10 nm thick films revealed an orthorhombic phase close to the stability region of the cubic phase. The potential ferroelectricity of this orthorhombic phase was confirmed by polarization hysteresis measurements on titanium nitride based metal-insulator-metal capacitors. For 5.2 mol% YO 1.5 admixture the remanent polarization peaked at 24 lC=cm 2 with a coercive field of about 1.2 MV=cm. Considering the availability of conformal deposition processes and CMOS-compatibility, ferroelectric Y:HfO 2 implies high scaling potential for future, ferroelectric memories.

for the VCM effect. [ 15 , 18 , 19 ] The VCM effect may take place at dislocations that act as conducting fi laments, [ 18 ] or homogenously over a somewhat larger interface region, [ 20 ] or both within the same sample. [ 21 ] To fulfi ll the requirements of a suitable nonvolatile memory, a cell should be scalable down to a few nanometers. As a further important requirement a WRITE voltage of a few volts must be suffi cient to switch a cell within less than 100 ns and a READ voltage of a few tenths of a volt should leave the resistance state unchanged for ten years. This requirement corresponds to a nonlinear voltage acceleration of the kinetics of many orders of magnitude, also known as the voltage-time dilemma. The origin of this strong voltage acceleration has been attributed to electric-fi eld-enhanced ion-hopping mobility, although possible contributions of temperature-activated ion mobility have also been mentioned. [22][23][24][25][26][27][28] Another explanation for this effect in the literature is the electric-fi eld-enhanced recombination/generation of oxygen vacancies. [ 29 , 30 ] Herein, we report experimental results of the switching kinetics of interface-type VCM cells based on epitaxial SrTiO 3 thin fi lms as a model system, which reveal a highly nonlinear voltage-time dependency. Based on a 2D axisymmetric fi nite element simulation model, which allows a quantitative discrimination of fi eld and temperature contributions, we are able to identify the temperature acceleration as the origin of this nonlinearity. Using this model we discuss the scaling properties of the VCM cell concept and prove its feasibility on the nanoscale. Electrical CharacterizationThin-fi lm devices consisting of a SrTiO 3 (STO) thin fi lm grown epitaxially on a conducting Nb-doped SrTiO 3 (STO:Nb) and a Ti top electrode were characterized by performing quasistatic current-voltage ( I-V ) studies as well as by defi ned voltagepulse measurements. The voltage was always applied to the top electrode of the devices. The quasistatic I-V sweep was also employed to execute the initial electroforming process and to achieve a defi ned OFF state prior to the pulse studies. An overview of the setup and the measurement procedure is shown in Figure 1 . Before and after a SET voltage pulse was applied to a memory cell, its resistance was measured quasistatically to determine the resistance change. The quasistatic I-V sweep Origin of the Ultra-nonlinear Switching Kinetics in Oxide-Based Resistive SwitchesExperimental pulse length-pulse voltage studies of SrTiO 3 memristive cells are reported, which reveal nonlinearities in the switching kinetics of more than nine orders of magnitude. The results are interpreted using an electrothermal 2D fi nite element model. The nonlinearity arises from a temperature increase in a few-nanometer-thick disc-shaped region at the Ti electrode and a corresponding exponential increase in oxygen-vacancy mobility. The model fully reproduces the experimental data and it provides essential design rules for opti...

We report the observation of ferroelectricity in capacitors based on hafnium-zirconium-oxide. Hf0.5Zr0.5O2 thin films of 7.5 to 9.5 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remnant polarization of 16 μC/cm2 and a high coercive field of 1 MV/cm were observed. Further proof for the ferroelectric nature was collected by quasi-static polarization-voltage hysteresis, small signal capacitance-voltage, and piezoelectric measurements. Data retention characteristics were evaluated by a Positive Up Negative Down pulse technique. No significant decay of the initial polarization state was observed within a measurement range of up to two days.

The realization of logic operations within passive crossbar memory arrays is a promising approach to expand the fields of application of such architectures. Material implication was recently suggested as the basic function of memristive crossbar junctions, and single bipolar resistive switches (BRS) as well as complementary resistive switches (CRS) were shown to be capable of realizing this logical functionality. Based on a systematic analysis of the Boolean functions, we demonstrate here that 14 of 16 Boolean functions can be realized with a single BRS or CRS cell in at most three sequential cycles. Since the read-out step is independent of the logic operation steps, the result of the logic operation is directly stored to memory, making logic-in-memory applications feasible.

Memristive cells based on different physical effects, that is, phase change, valence change, and electrochemical processes, are discussed with respect to their potential to overcome the voltage–time dilemma that is crucial for an application in storage devices. Strongly non‐linear switching kinetics are required, spanning more than 15 orders of magnitude in time. Temperature‐driven and field‐driven crystallization, threshold switching, ion migration, as well as redox reactions at interfaces are identified as relevant mechanisms. In phase change materials the combination of a reversible threshold switching and extremely large crystal growth velocities at high voltages enables ultra‐fast resistive switching whereas lower voltages will not be sufficient to overcome the energy barrier for crystallization. In electrochemical cells it depends on the voltage regime, which mechanism is the rate‐determining one for switching. While electro‐crystallization dominates at low voltages, electron transfer in the medium voltage range and a mixture of electron transfer and ion migration at high voltages. In valence change materials, ion migration is found to be accelerated by a combined effect of electric field and local temperature increase due to Joule heating. All discussed types of resistive switches can provide sufficient non‐linearity of switching kinetics for overcoming the voltage time dilemma.

We investigated phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) by temperature dependent polarization and x-ray diffraction measurements. If heated under mechanical confinement, the orthorhombic ferroelectric phase reversibly transforms into a phase with antiferroelectric behavior. Without confinement, a transformation into a monoclinic/tetragonal phase mixture is observed during cooling. These results suggest the existence of a common higher symmetry parent phase to the orthorhombic and monoclinic phases, while transformation between these phases appears to be inhibited by an energy barrier