Boubacar Traore

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}

We investigate in detail the effects of metal electrodes on the switching performance and conductive filament (CF) stability of HfO2-based RRAM. The current- voltage characteristics of the devices exhibit different electrodedependent RESET profiles which we attempt to clarify. With the insight from the experimental data, we employ first-principles calculations to have a better microscopic understanding of the devices. We study the charge injection, formation of Frenkel pairs, and diffusion of oxygen defects (oxygen vacancies Vo and oxygen interstitials Oi) that are important in the CF creation and stability during the device operation. Since the presence of Ti in RRAM has been associated with the creation of substoichiometric TiOy region at the Ti/HfO2 interface, we also explore different Ti and Hf suboxides to understand the possible composition of that interface. Our calculations suggest that the composition of the interface would be Ti2O/Hf2O3 from thermodynamic perspective. By combining the experimental and calculations results, we show that the concentration of oxygen interstitial (Oi) ions in the oxide after CF formation is larger for RRAM devices with inert electrodes (like Pt) compared with O reactive electrodes (like Ti) which results in degraded device performance. The lower Oi concentration in HfO2 layer with Ti electrodes results in improved CF thermal stability and device variability.

Resistive Random Access Memory

We study in detail the impact of alloying HfO₂ with Al (Hf_{1_x}Al₂xO_2+x) on the oxide-based resistive random access memory (RRAM) (OxRRAM) thermal stability through material characterization, electrical measurements, and atomistic simulation. Indeed, migration of oxygen atoms inside the dielectric is at the heart of OxRRAM operations. Hence, we performed comprehensive diffusion barrier calculations in HfO₂, Hf_{1_x}Al₂xO_2+x, and Hf_{1_x}TixO₂ relative to the oxygen vacancy (Vo) movement involved in low-resistance state (RON) thermal stability. Calculations are performed at the best level using ab initio techniques. This paper provides an insight on the improved RON stability of our Hf_{1_x}Al₂xO_2+x-based RRAM devices and predicts the degraded retention of Hf_{1_x}TixO₂-based RRAM measured in the literature. Our theoretical calculations link the origin of RON retention failure to the lateral diffusion of oxygen vacancies at the constriction/tip of the conductive filament in HfO₂-based RRAM.

HfO 2 -Based RRAM: Electrode Effects, Ti/HfO 2 Interface, Charge Injection, and Oxygen (O) Defects Diffusion Through Experiment and Ab Initio Calculations

Hybrid perovskites are on a trajectory toward realizing the most efficient single-junction, solution-processed photovoltaic devices. However, a critical issue is the limited understanding of the correlation between the degree of crystallinity and the emergent perovskite/hole (or electron) transport layer on device performance and photostability. Here, the controlled growth of hybrid perovskites on nickel oxide (NiO) is shown, resulting in the formation of thin films with enhanced crystallinity with characteristic peak width and splitting reminiscent of the tetragonal phase in single crystals. Photophysical and interface sensitive measurements reveal a reduced trap density at the perovskite/NiO interface in comparison with perovskites grown on poly(3,4-ethylene dioxy thiophene) polystyrene sulfonate. Photovoltaic cells exhibit a high open circuit voltage (1.12 V), indicating a near-ideal energy band alignment. Moreover, photostability of photovoltaic devices up to 10-Suns is observed, which is a direct result of the superior crystallinity of perovskite thin films on NiO. These results elucidate the critical role of the quality of the perovskite/hole transport layer interface in rendering high-performance and photostable optoelectronic devices.

On the Origin of Low-Resistance State Retention Failure in HfO 2 -Based RRAM and Impact of Doping/Alloying

Two-dimensional (2D) hybrid halide perovskites come as a family (B)2(A)n-1PbnX3n+1 (B and A= cations; X= halide). These perovskites are promising semiconductors for solar cells and optoelectronic applications. Among the fascinating properties of these materials is white-light emission, which has been mostly observed in single-layered 2D lead bromide or chloride systems (n = 1), where the broad emission comes from the transient photoexcited states generated by self-trapped excitons (STEs) from structural distortion. Here we report a multilayered 2D perovskite (n = 3) exhibiting a tunable white-light emission. Ethylammonium (EA+) can stabilize the 2D perovskite structure in EA4Pb3Br10-xClx (x = 0, 2, 4, 6, 8, 9.5, and 10) with EA+ being both the A and B cations in this system. Because of the larger size of EA, these materials show a high distortion level in their inorganic structures, with EA4Pb3Cl10 having a much larger distortion than that of EA4Pb3Br10, which results in broadband white-light emission of EA4Pb3Cl10 in contrast to narrow blue emission of EA4Pb3Br10. The average lifetime of the series decreases gradually from the Cl end to the Br end, indicating that the larger distortion also prolongs the lifetime (more STE states). The band gap of EA4Pb3Br10-xClx ranges from 3.45 eV (x = 10) to 2.75 eV (x = 0), following Vegards law. First-principles density functional theory calculations (DFT) show that both EA4Pb3Cl10 and EA4Pb3Br10 are direct band gap semiconductors. The color rendering index (CRI) of the series improves from 66 (EA4Pb3Cl10) to 83 (EA4Pb3Br0.5Cl9.5), displaying high tunability and versatility of the title compounds.

Critical Role of Interface and Crystallinity on the Performance and Photostability of Perovskite Solar Cell on Nickel Oxide

We present the new homologous series (C(NH2)3)(CH3NH3)nPbnI3n+1 (n = 1, 2, 3) of layered 2D perovskites. Structural characterization by single-crystal X-ray diffraction reveals that these compounds adopt an unprecedented structure type, which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). Compared to the more common Ruddlesden-Popper (RP) 2D perovskites, the ACI perovskites have a different stacking motif and adopt a higher crystal symmetry. The higher symmetry of the ACI perovskites is expressed in their physical properties, which show a characteristic decrease of the bandgap with respect to their RP perovskite counterparts with the same perovskite layer thickness (n). The compounds show a monotonic decrease in the optical gap as n increases: Eg = 2.27 eV for n = 1 to Eg = 1.99 eV for n = 2 and Eg = 1.73 eV for n = 3, which show slightly narrower gaps compared to the corresponding RP perovskites. First-principles theoretical electronic structure calculations confirm the experimental optical gap trends suggesting that the ACI perovskites are direct bandgap semiconductors with wide valence and conduction bandwidths. To assess the potential of the ACI perovskites toward solar cell applications, we studied the (C(NH2)3)(CH3NH3)3Pb3I10 (n = 3) compound. Compact thin films from the (C(NH2)3)(CH3NH3)3Pb3I10 compound with excellent surface coverage can be obtained from the antisolvent dripping method. Planar photovoltaic devices from optimized ACI perovskite films yield a power-conversion-efficiency of 7.26% with a high open-circuit voltage of ∼1 V and a striking fill factor of ∼80%.

Tunable White-Light Emission in Single-Cation-Templated Three-Layered 2D Perovskites (CH3CH2NH3)4Pb3Br10-xClx

Ruddlesden–Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n-value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free carriers) and the exciton reduced mass, and their scaling with quantum well thickness, which are critical for designing efficient optoelectronic devices, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modeling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with both exciton reduced masses and binding energies decreasing, respectively, from 0.221 m0 to 0.186 m0 and from 470 meV to 125 meV with increasing thickness from n equals 1 to 5. Based on this study we propose a general scaling law to determine the binding energy of excitons in perovskite quantum wells of any layer thickness.Hybrid 2D layered perovskites are solution-processed quantum wells whose optoelectronic properties are tunable by varying the thickness of the inorganic slab. Here Blancon et al. work out a general behavior for dependence of the excitonic properties in layered 2D perovskites.

New Type of 2D Perovskites with Alternating Cations in the Interlayer Space, (C(NH2)3)(CH3NH3)nPbnI3n+1: Structure, Properties, and Photovoltaic Performance

This letter studies the intrinsic variability in oxide-based resistive RAM technology, highlighting the presence of a short range (≈40) correlation of resistances among cycles (for both low resistance state and high resistance state). Experimental results demonstrate the existence of a resistance correlation, and an analytical model is proposed in order to explain the findings. The presence of the correlation seems to indicate that the conductive filament, which is believed to be the basis of resistive RAM behavior, keeps for a limited number of cycling operations a memory of its morphology. The extension of this correlation depends on the programming conditions.

Scaling law for excitons in 2D perovskite quantum wells

Surface states are ubiquitous to semiconductors and significantly impact the physical properties and, consequently, the performance of optoelectronic devices. Moreover, surface effects are strongly amplified in lower dimensional systems such as quantum wells and nanostructures. Layered halide perovskites (LHPs) are two-dimensional solution-processed natural quantum wells where optoelectronic properties can be tuned by varying the perovskite layer thickness n, i.e., the number of octahedra spanning the layer. They are efficient semiconductors with technologically relevant stability. Here, a generic elastic model and electronic structure modeling are applied to LHPs heterostructures with various layer thickness. We show that the relaxation of the interface strain is triggered by perovskite layers above a critical thickness. This leads to the release of the mechanical energy arising from the lattice mismatch, which nucleates the surface reorganization and may potentially induce the formation of previously observed lower energy edge states. These states, which are absent in three-dimensional perovskites are anticipated to play a crucial role in the design of LHPs for optoelectronic systems.

Investigation of Cycle-to-Cycle Variability in HfO 2 -Based OxRAM

In this paper we present the recent advances in the understanding of microscopic mechanisms driving the resistive switching in ReRAM devices using ab initio theoretical methods. We highlight the complex interplay between interface reactions and charge injection in the generation of oxygen Frenkel pairs during the forming step. Energy barrier calculations suggest that the formation/destruction of the conductive filament can be due to movements of oxygen vacancies composing the filament or interaction with oxygen atoms released from the metal electrode.