The resistive switching phenomena of HfO2 films grown by metalorganic chemical vapor deposition was studied for the application of ReRAM devices. In the fabricated Pt/HfO2/TiN memory cells, the bipolar resistive switching characteristics were observed, and the set and reset states were measured to be as low as 7 uA and 4 uA, respectively, at VREAD = 1 V. Regarding the resistive switching performance, the stable RS performance was observed under 40 repetitive dc cycling test with the small variations of set/reset voltages and currents, and good retention characteristics over 105 s in both LRS and HRS. These results show the possibility of MOCVD grown HfO2 films as a promising resistive switching materials for ReRAM applications.
The surface and electronic structure of MOCVD-grown layers of Ga(0.92)In(0.08)N have been investigated by means of photoemission. An additional feature at the valence band edge, which can be ascribed to the presence of In in the layer, has been revealed. A clean (0001)-(1x1) surface was prepared by argon ion sputtering and annealing. Stability of chemical composition of the investigated surface subjected to similar ion etching was proven by means of X-ray photoemission spectroscopy.
We carefully investigate three important effects including postgrowth activation annealing, delta (δ) dose and p+-GaN layer thickness and experimentally demonstrate their influence on the electrical properties of GaN p-n homojunction diodes with a tunnel junction (TJ)-based p-contact. The p-n diodes and TJ structures were monolithically grown by metalorganic chemical vapor deposition (MOCVD) in a single growth step. By optimizing the annealing time and temperature for magnesium (Mg) activation and introducing δ-doses for both donors and acceptors at TJ interfaces, a significant improvement in …
We developed a thermodynamic supersaturation model for plasma-assisted metalorganic chemical vapor deposition of InN. The model is based on the chemical combination of indium with plasma-generated atomic nitrogen ions. Indium supersaturation was analyzed for InN films grown by PA-MOCVD with varying input flow of indium precursor. Raman spectroscopy, X-ray diffraction, and atomic force microscopy provided feedback on structural properties and surface morphology of grown films. Growth parameter variation effect on In supersaturation was analyzed. InN films grown at varying growth parameters resu…
In this letter, we design and demonstrate an improved MOCVD grown reverse Al-composition graded contact layer to achieve low resistance contact to MOCVD grown ultra-wide bandgap (UWBG) Al0.70Ga0.30N channel metal semiconductor field-effect transistors (MESFETs). Increasing the thickness of the reverse graded layer was found to improve contact layer resistance significantly, leading to contact resistance of 3.3x10^-5 Ohm.cm2. Devices with gate length, LG, of 0.6 microns and source-drain spacing, LSD, of 1.5 microns displayed a maximum current density, IDSMAX, of 635 mA/mm with an applied gate v…
A new record-high room temperature electron Hall mobility ($μ_{RT} = 194\space cm^{2}/V\space s$ at $n\sim 8\times 10^{15}\space cm^{-3}$) for $β$-Ga2O3 is demonstrated in the unintentionally doped thin film grown on (010) semi-insulating substrate via metalorganic chemical vapor deposition (MOCVD). A peak electron mobility of $\sim 9500\space cm^{2}/V\space s$ is achieved at 45 K. Further investigation on the transport properties indicate the existence of sheet charges near the epi-layer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epi…
We have selectively grown thin epitaxial GaAs films on Ge substrates with the aid of a 200 nm thin SiO2 mask layer. The selectively grown structures have lateral sizes ranging from 1 um width up to large areas of 1 by 1 mm2. The growth with the standard growth procedure for GaAs growth on Ge substrates reveals a limited amount of GaAs nucleation on the mask area and strong loading effects caused by diffusion of group III precursors over the mask area and in the gas phase. Reduction of the growth pressure inhibits GaAs nucleation on the mask area and reduces the loading effects strongly, but fa…
Device applications involving topological insulators (TIs) will require the development of scalable methods for fabricating TI samples with sub-micron dimensions, high quality surfaces, and controlled compositions. Here we use Bi-, Se-, and Te-bearing metalorganic precursors to synthesize TIs in the form of nanowires. Single crystal nanowires can be grown with compositions ranging from Bi2Se3 to Bi2Te3, including the ternary compound Bi2Te2Se. These high quality nanostructured TI compounds are suitable platforms for on-going searches for Majorana Fermions.
Ge-on-Si structures in-situ doped with phosphorus or arsenic via metal organic chemical vapor deposition (MOCVD) were investigated. Surface roughness, strain, threading dislocation desnity, Si-Ge interdiffusion, dopant diffusion, and photoluminescence were characterized to study the impacts of defect annealing and Si substrate offcut effects on the Ge film quality and most importantly, the light emission properties. All samples have a smooth surface (roughness < 1.5 nm), and the Ge films have a small tensile strain of 0.2%. As-grown P and As-doped Ge films have threading dislocaiton densities …
In this Letter, four-point magnetotransport of high mobility InGaAs/InP heterointerfaces is measured from 1.6 K to 300 K and from 0 to 15 T, and an analysis is shown whereby the mobility and density of the two-dimensional (2D) accumulation layer can be separately characterized from that of the parallel conducting dopant layer over all but a small intermediate temperature range. Standard magnetotransport regimes are defined as the temperature increases from 1.6 K to 300 K, namely quantum Hall (QH), Shubnikov de Haas (SdH), and Drude regimes (D), and in the QH and D regimes different analyses ar…
Domain structures of 320 nm thin epitaxial films of ferroelectric PbTiO3 grown by MOCVD technique in identical conditions on SmScO3 and TbScO3 perovskite sub- strates have been investigated by Raman spectroscopy and piezoresponse force microscopy techniques. Phonon frequency shifts and typical domain structure motifs are discussed. The results reveal strikingly different domain structure architecture: domain structures of the PbTiO3 film grown on SmScO3 have dominantly a-domain orientation while strongly preferential c-domain orientation was found in the PbTiO3 film grown on the TbScO3 substra…
Ammonia (NH3) is commonly used as group V precursor in gallium nitride (GaN) metalorganic chemical vapor deposition (MOCVD). The high background carbon (C) impurity in MOCVD GaN is related to the low pyrolysis efficiency of NH3, which represents one of the fundamental challenges hindering the development of high purity thick GaN for vertical high power device applications. This work uses a laser-assisted MOCVD (LA-MOCVD) growth technique to address the high-C issue in MOCVD GaN. Carbon dioxide (CO2) laser with wavelength of 9.219 um was utilized to facilitate NH3 decomposition via resonant vib…
BiFeO3 thin films have been grown on Pt/Ti/SiO2/Si substrates with pulsed laser deposition using Au as the top electrode. The resistive switching property of the Au/BiFeO3/Pt stack has been significantly improved by carefully tuning the oxygen pressure during the growth, and a large switching ratio of ~4500 has been achieved. The deposition pressure modifies the concentration of oxygen vacancies and the rectifying behavior of the Au/BiFeO3 junction, and consequently influences the resistive switching behavior of the whole stack. The switching takes place homogeneously over the entire electrode…
Substitutionally-doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional Rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 parts-per-million (ppm) by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10x. Ab initio models indicate the free-energy of sulfur-vacancy formation is increased along the…
The effect of film morphology on its surface chemistry and band structure has been analyzed for gallium nitride epitaxial films grown by molecular beam epitaxy. The film morphology has been studied using scanning electron microscopy and atomic force microscopy, and the bandstructure, defect and emission properties have been studied by X ray photoelectron spectroscopy and cathodoluminescence spectroscopy. It was found that the highly porous GaN nanowall network shows the highest relative conductivity and does not have defect related luminescence. The flatter films were more resistive and showed…
Using metal-ferroelectric junctions as switchable diodes was proposed several decades ago. This was shown to actually work in PbZr(1-x)TixO3 (PZT) by Blom et al. [P.W. M. Blom et al., Phys. Rev. Lett. 73, 2107 (1994)], who reported switching in the rectification direction and changes of the current of about 2 orders of magnitude upon switching the polarization direction of the ferroelectric layer. This form of resistive switching enables the read out of a ferroelectric memory state at higher speed compared to the capacitive design, without destroying the information in each reading cycle. Rece…
This work demonstrates the first nonpolar vertical GaN on GaN pn power diodes grown on m-plane free standing substrates by MOCVD. The SEM and HRXRD results showed the good crystal quality of the homoepitaxial nonpolar structure with low defect densities. The CL result confirmed the nonpolar p GaN was of high quality with considerably reduced deep level states. At forward bias, the device showed good rectifying behaviors with a turn-on voltage of 4.0 V, an on-resistance of 2.3 mohmcm2, and a high on off ratio of 1e10. At reverse bias, the current leakage and breakdown were described by the trap…
Current-voltage characteristics and resistive switching in epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_3$/SrMnO$_3$/La$_{0.67}$Sr$_{0.33}$MnO$_3$ multilayer
School of Physical Sciences, Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学物理科学学院）；Special Centre for Nanoscience, Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学纳米科学特别中心）
We report the optical, electrical, and structural properties of Si doped $β$-Ga$_2$O$_3$ films grown on (010)-oriented $β$-Ga$_2$O$_3$ substrate via HVPE. Our results show that, despite growth rates that are more than one order of magnitude faster than MOCVD, films with mobility values of up to 95 cm$^2$V$^{-1}$s$^{-1}$ at a carrier concentration of 1.3$\times$10$^{17}$ cm$^{-3}$ can be achieved using this technique, with all Si-doped samples showing n-type behavior with carrier concentrations in the range of 10$^{17}$ to 10$^{19}$ cm$^{-3}$. All samples showed similar room temperature photolu…
We report a study of the relaxation time of the restoration of the resistive superconducting state in single crystalline boron-doped diamond using amplitude-modulated absorption of (sub-)THz radiation (AMAR). The films grown on an insulating diamond substrate have a low carrier density of about 2.5x10^{21} cm^{-3} and a critical temperature of about 2 K. By changing the modulation frequency we find a high-frequency rolloff which we associate with the characterstic time of energy relaxation between the electron and the phonon systems or the relaxation time for nonequilibrium superconductivity. …
A multilayer perovskite thin film resistive memory device has been developed comprised of: a Pr0.7Ca0.3MnO3 (PCMO) perovskite oxide epitaxial layer on a YBCO bottom thin film electrode; a thin yttria stabilized zirconia (YSZ) buffer layer grown on the PCMO layer, and a gold thin film top electrode. The multi-layer thin film lattice structure has been characterized by XRD and TEM analyses showing a high quality heterostructure. I-AFM analysis indicated nano granular conductivity distributed uniformly throughout the PCMO film surface. With the addition of the YSZ buffer layer, the pulse voltage …
We investigate ferroelectric- and resistive switching behavior in 18-nm-thick epitaxial BaTiO$_3$ (BTO) films in a model electrolyte-ferroelectric-semiconductor (EFS) configuration. The system is explored for its potential as a ferroelectric microelectrode in bioelectronics. The BTO films are grown by pulsed laser deposition (PLD) on semiconducting Nb-doped (0.5 wt\%) SrTiO$_{3}$ (Nb:STO) single crystal substrates. The ferroelectric properties of the bare BTO films are demonstrated by piezoresponse force microscopy (PFM) measurements. Cyclic voltammetry (CV) measurements in EFS configuration, …
The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resis…
Transition metal oxide memristors, or resistive random-access memory (RRAM) switches, are under intense development for storage-class memory because of their favorable operating power, endurance, speed, and density. Their commercial deployment critically depends on predictive compact models based on understanding nanoscale physico-chemical forces, which remains elusive and controversial owing to the difficulties in directly observing atomic motions during resistive switching, Here, using scanning transmission synchrotron x-ray spectromicroscopy to study in-situ switching of hafnium oxide memri…
The nanoscale resistive switching characteristics of gallium phosphide (GaP) thin films directly grown on Si are investigated as a function of incident light. Firstly, as-grown GaP films show a high RON/ROFF (~10^4), shown to arise from the formation of conductive channels along the grain boundaries. It is proposed that point defects (most likely Ga interstitials) and structural disorder at the grain boundaries provide the ideal environment to enable the filamentary switching process. Next, we explored if such defects can give rise to mid-gap states, and if so could they be activated by photon…
The evidence for proximity-induced superconductivity in heterostructures of topological insulators and high-Tc cuprates has been intensely debated. We use molecular beam epitaxy to grow thin films of topological insulator Bi2Te3 on a cuprate Bi2Sr2CaCu2O8+x, and study the surface of Bi2Te3 using low-temperature scanning tunneling microscopy and spectroscopy. In few unit-cell thick Bi2Te3 films, we find a V-shaped gap-like feature at the Fermi energy in dI/dV spectra. By reducing the coverage of Bi2Te3 films to create nanoscale islands, we discover that this spectral feature dramatically evolve…
We report here gate-tunable memristors based on monolayer MoS2 grown by chemical vapor deposition (CVD). These memristors are fabricated in a field-effect geometry with the channel consisting of polycrystalline MoS2 films with grain sizes of 3-5 um. The device characteristics show switching ratios up to 500, with the resistance in individual states being continuously gate-tunable by over three orders of magnitude. The resistive switching results from dynamically varying threshold voltage and Schottky barrier heights, whose underlying physical mechanism appears to be vacancy migration and/or ch…
The resistivity of different films and structures containing fluorinated graphene (FG) flakes and chemical vapor deposition (CVD) grown graphene of various fluorination degrees under tensile and compressive strains due to bending deformations was studied. Graphene and multilayer graphene films grown by means of the CVD method were transferred onto the flexible substrate by laminating and were subjected to fluorination. They demonstrated a weak fluorination degree (F/C lower 20%). Compressive strains led to a strong (one-two orders of magnitude) decrease in the resistivity in both cases, which …
We report the fabrication and performance of all-metallic three-terminal devices with tantalum diselenide thin-film conducting channels. For this proof-of-concept demonstration, the layers of 2H-TaSe2 were exfoliated mechanically from single crystals grown by the chemical vapor transport method. Devices with nanometer-scale thicknesses exhibit strongly non-linear current-voltage characteristics, unusual optical response, and electrical gating at room temperature. We have found that the drain-source current in thin-film 2H-TaSe2-Ti/Au devices reproducibly shows an abrupt transition from a highl…
