Kan-Hao Xue

Prediction of Semimetallic TetragonalHf2O3andZr2O3from First Principles

Tetragonal semimetallic phases are predicted for Hf(2)O(3) and Zr(2)O(3) using density functional theory. The structures belong to space group P4[over ¯]m2 and are more stable than their corundum counterparts. Many body corrections at first order confirm their semimetallic character. The carrier concentrations are very similar for both materials, and are estimated as 1.8×10(21) cm(-3) for both electrons and holes, allowing for electric conduction. This could serve as a basic explanation for the low resistance state of hafnia-based resistive random access memory.

Prediction of semimetallic tetragonal Hf2O3 and Zr2O3 from first principles.

Tetragonal semimetallic phases are predicted for Hf(2)O(3) and Zr(2)O(3) using density functional theory. The structures belong to space group P4[over ¯]m2 and are more stable than their corundum counterparts. Many body corrections at first order confirm their semimetallic character. The carrier concentrations are very similar for both materials, and are estimated as 1.8×10(21) cm(-3) for both electrons and holes, allowing for electric conduction. This could serve as a basic explanation for the low resistance state of hafnia-based resistive random access memory.

Grain boundary composition and conduction in HfO2: An ab initio study

We investigate the electronic properties of HfO2 grain boundaries employing a simple Σ5 (310)/[001] grain boundary model based on the cubic phase. Our calculations show the emergence of unoccupied defect states 0.4 eV below the conduction band due to the under-coordination of certain Hf ions in the grain boundary. They also show that migration of metal interstitials such as Hf and Ti to the grain boundary is energetically favorable, turning the grain boundary region metallic. This scenario may create leakage paths in poly-crystalline HfO2 or serve as the conduction mechanism in resistive random access memories.

A Combined Ab Initio and Experimental Study on the Nature of Conductive Filaments in

Through ab initio calculations, we propose that the conductive filaments in Pt/HfO2/Pt resistive random access memories are due to HfOx suboxides, possibly tetragonal, where x ≤ 1.5. The electroforming process is initiated by a continuous supply of oxygen Frenkel defect pairs through an electrochemical process. The accumulation of oxygen vacancies leads to metallic suboxide phases, which remain conductive even as ultranarrow 1-nm2 filaments embedded in an insulating HfO2 matrix. Our experiments further show that the filaments remain as major leakage paths even in the OFF-state. Moreover, thermal heating may increase the OFF-state resistance, implying that there are oxygen interstitials left in the oxide layer, which may recombine with the oxygen vacancies in the filaments at high temperature.

{\rm Pt}/{\rm Hf}{\rm O}_{2}/{\rm Pt}

A model for resistance random access memory (RRAM) is proposed. The RRAM under research utilizes certain transition metal oxide (TMO) such as NiO which shows unipolar switching behavior. The existence of metal/insulator states is not explained by filaments but attributed to different Hubbard U values, which stems from an electron correlation effect. Current-voltage formulae are given both on the metal and insulator sides by putting the appropriate solutions of Hubbard model into the mesoscopic Meir-Wingreen transport equation. The RESET phenomenon is explained by a sufficient separation of Fermi levels in the electrodes and hence a Mott transition can be triggered in the anodic region due to a lack of electrons. The SET behavior originates from a tunneling current which removes the insulating region near the anode. Several experimental evidences are also presented to support this model. The model also serves as the theoretical prototype of Correlated Electron Random Access Memory (CeRAM) which is defined to be a TMO RRAM whose working mechanism is based on the strong electron correlation effects.

Resistive Random Access Memory

Using first-principles evolutionary simulations, we have systematically investigated phase stability in the Hf-O system at pressure up to 120 GPa. New compounds Hf5O2, Hf3O2, HfO and HfO3 are discovered to be thermodynamically stable at certain pressure ranges and a new stable high-pressure phase is found for Hf2O with space group Pnnm and anti-CaCl2-type structure. Both P62m-HfO and P4m2-Hf2O3 show semimetallic character. Pnnm-HfO3 shows interesting structure, simultaneously containing oxide O2- and peroxide [O-O]2- anions. Remarkably, it is P62m-HfO rather than OII-HfO2 that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-Hf2O, Imm2-Hf5O2, P31m-Hf2O and P4m2-Hf2O3 phases also show superior mechanical properties, these phases can be quenched to ambient pressure and their properties can be exploited.

A non-filamentary model for unipolar switching transition metal oxide resistance random access memories

The low temperature metal organic decomposition techniques of ferroelectric bismuth titanate (BIT) thin films were investigated. BIT was found to be crystallized by rapid thermal processing at 450 °C. The stoichiometric Bi4Ti3O12 sample exhibited (117) orientation, while the Bi4.8Ti3O13.2 sample, with 20% excess bismuth, possessed a/b axes orientation with (117) component. Pt/Bi4.8Ti3O13.2/Pt ferroelectric capacitors were fabricated with temperature confined below 450 °C. The saturated 2Pr value was 31.1 μC/cm2. Such method is valuable for ferroelectric memories at 65 nm technology node and beyond because low temperature processes are required for the stability of interconnect material nickel silicide.

Pressure-induced novel compounds in the Hf-O system from first-principles calculations

A method of making transition metal oxide materials that result in resistive switching properties stable over time and temperature is described. We have developed an ultra low temperature (≤450°C) process for carbonyl ligand modified NiO thin films based on the chemical solution deposition (CSD) for correlated electron random access memory (CeRAM) applications. CeRAMs form the general class of devices that use the electron-electron interaction as the primary mode of operation. These devices are fabricated in the conductive state (born-ON), thus, they do not require electroforming to enter the variable resistance state. Several process parameters such as film stoichiometry, thickness, annealing temperature and ambient have been investigated to optimize CeRAMs properties. We present the coordination number ‘fine tuning’ in NiO ultra thin films via carbonyl ligand doping that regulate the number of oxygen vacancies and the surface excess of metal ions. CeRAMs contrary to just standard NiO based resistive mem...

Low temperature preparation of ferroelectric bismuth titanate thin films

The crystal orientations and electrical properties of Bi4Ti3O12 (BIT) and Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were studied and compared. Stoichiometric BIT and BLT samples were deposited on Pt(111) substrates and crystallized at 750 °C to get mixed orientations. The BIT sample exhibited a/b axes orientation with (117) component, while the BLT sample was more c-axis oriented. The 2Pr values of such BIT and BLT were 34.3 μC/cm2 and 25.7 μC/cm2, respectively. Nevertheless, BLT has much better leakage current and polarization saturation properties. In order to obtain c-axis oriented BIT thin films, excess bismuth was used. BIT with 15% excess bismuth processed at 750 °C was purely c-axis oriented and only possessed a 2Pr value of 4.9 μC/cm2, while that of 15% excess bismuth BLT sample was 18.3 μC/cm2. This can be attributed to the large polarization anisotropy in BIT, and this anisotropy is reduced by lanthanum doping.

Material and process optimization of correlated electron random access memories

The switching properties and characterization of correlated electron random Access Memories (CeRAMs) are described herein. High temperature retention, cycle dispersion and optimization, cycle Fatigue, and switching parameter optimization have been investigated. CeRAM’s display initially conductive or “born-ON” behavior without the need for the high electroforming voltages usually required for other transition metal oxide based resistive memories. Nonvolatile data retention at elevated temperatures up to 573 K (300 °C) in addition to a wide operating range from 4 to 423 K for CeRAM has been confirmed. CeRAMs also show exceptional read endurance with no evidence of fatigue out to 1012 cycles. Desirable scaling characteristics for high density memory application have also been shown for CeRAMs due to a widening of the read window and consistent write window as devices are scaled down.