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A wide variety of materials and processes for high k dielectrics and metal gate electrodes have been studied as replacements for poly-Si/SiO2 or SiON in advanced CMOS devices. Care must be taken with the interfacial layer to control not only the nitrogen content but its spatial location. Nanocrystallization of the high k dielectric and the corresponding formation of charge and trapping levels associated with defects in the dielectric present one of the current challenges. Control of the workfunction of the gate electrode is shown to depend on many variables, including oxygen content and the material used for the capping layer on the metal gate. The hafnium oxide family of materials, along with metal alloy gates, is seen to provide the best solution for equivalent oxide thicknesses (EOT's) < 0.7 nm, but higher k dielectrics and thinner interfacial layers are needed below 0.7 nm.

This paper reports on a study of the contributions to the image contrast of high-angle annular dark field (HAADF) images acquired in scanning transmission electron microscopy. Experimental HAADF images were obtained from a model system consisting of an epitaxial perovskite PbTiO 3 film grown on a SrTiO 3 single crystal. This sample allowed for the study of the intensities of a wide range of atomic numbers. The main objective of the paper was to quantify the influence of TEM foil thickness on the image contrast, but the effects of the annular detector inner angle and the probe forming lens focus were also studied. Sample thicknesses ranging from ∼10 nm to more than 400 nm were investigated. The image contrast was relatively insensitive to changes in inner angle. The main impact of sample thickness was a rapid increase in a background intensity that contributed equally to the intensities of the atomic columns and the channels between them. The background intensity and its increase with thickness reflected the average atomic number of the crystal. Subtraction of the background intensity allowed for a quantitative interpretation of image contrast in terms of atomic numbers and comparison with multislice image simulations. The consequences for the analysis of interfaces in terms of atom column occupancies are discussed.

Bismuth zinc niobate (BZN), a high dielectric constant material that combines low dielectric losses with an electric-field dependent permittivity, was implemented as the dielectric in metal-insulator-metal (MIM) capacitors. Dielectric properties up to 20 GHz were evaluated by measuring reflection coefficients with a vector network analyzer. The dielectric constant was around 180 and the quality factor remained greater than 100 up to 5 GHz for 64 μ m2 devices. No onset of dielectric relaxation, as seen in bulk ceramic data, could be detected in the measured frequency range. The results show that BZN thin films have great potential for low loss, tunable microwave devices.

High frequency loss measurements are complicated by parasitic electrode contributions to total device properties. In order to assess film properties and create accurate models for simulation, parasitic electrode contributions must be separated from film properties. Analysis of capacitance relaxation data reveals that the increase in losses at high frequencies in our devices is due to the electrode-film interface, not the film. This frequency-dependent resistance cannot be isolated by shorted structure measurement alone. Using a combination of measured Q data and calculated Q data from the relaxation data, the electrode-film interface contribution to loss can be determined.

The microstructure and orientation relationships of epitaxial (111)-oriented SrTiO3 thin films grown by radio frequency magnetron sputtering on epitaxial (111)-oriented Pt/Ti electrodes on sapphire were investigated using x-ray diffraction, conventional and scanning transmission electron microscopy. We show that the epitaxial growth of (111)-oriented SrTiO3 films was promoted by thin Ti adhesion layers underneath the Pt electrode. The SrTiO3 films nucleated with two twin-related orientation variants, rotated by 180° about the 〈111〉 surface normal. The twin boundaries were oriented approximately normal to the film plane, but no strong preference for a specific boundary plane was observed. Growth mechanisms and the relationships to the dielectric properties are discussed.

Novel metal oxide films and new metal gates are currently being developed for future generations of Si based field-effect transistors as the SiO2 gate dielectric and polycrystalline Si gate electrode are reaching scaling limits. These gate stacks are often comprised of sub-nanometer layers. Device properties are increasingly controlled by the complex structure and chemistry of interfaces between the layers. Electron energy-loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) is capable of providing insights into interfacial chemistry and local atomic structure with a spatial resolution unmatched by any other technique. Using gate stacks with Hf-silicate dielectrics as examples, we demonstrate the capabilities of STEM/EELS for analyzing the interfacial chemistry of novel gate stacks. We show that a priori unknown reaction layers of a few A thickness can be detected and identified even in the presence of substantial interfacial roughness that may obscure such layers in a high-resolution image. We discuss some experimental aspects of STEM/EELS chemical profiling applied to gate stacks and the factors affecting the interpretation. In particular, the effects of interfacial roughness, beam spreading, elemental analysis in a heavily scattering matrix, and the interpretation of the EELS core-loss fine-structures from ultrathin layers are discussed. (c) 2004 Elsevier B.V. All rights reserved.

We investigate the relationship between microstructure and dielectric properties of textured SrTiO3 thin films deposited by radio-frequency magnetron sputtering on epitaxial Pt electrodes on sapphire substrates. The microstructures of Pt electrodes and SrTiO3 films are studied by transmission electron microscopy, atomic force microscopy, and X-ray diffraction. SrTrO3 films grown on as-deposited and annealed Pt electrodes, respectively, consist of a mixture of (111)- and (110)-oriented grains. Temperature-dependent dielectric measurements show that differences in texture and microstructure are reflected in the Curie-Weiss behavior of the SrTiO3 films. Phenomenological models that account for the effects of thermal mismatch strain on the dielectric behavior are developed for different film textures. The models predict that at a given temperature, paraelectric (111)-oriented films of SrTiO3 on tensile substrates will have a higher Curie-Weiss temperature and a greater dielectric constant than (110)-oriented films or bulk SrTiO3. The experimental dielectric behavior is compared with the predictions from theory, and different contributions, such as interfacial layers, film stress, and microstructure, to the Curie-Weiss behavior are discussed.

The sensing ability of individual SnO(2) nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.

High-k metal oxide gate dielectrics may be required to extend Moore’s law of semiconductor device density scaling into the future. However, growth of a thin SiO2-containing interface layer is almost unavoidable during the deposition of metal oxide films onto Si substrates. This limits the scaling benefits of incorporating high-k dielectrics in future transistors. A promising approach, in which oxygen-gettering metal overlayers are used to engineer the thickness of the SiO2-based interface layer between metal oxide and Si substrate after deposition of the metal oxide layer, is reported. Using a Ti overlayer with high solubility for oxygen on ZrO2 or HfO2 dielectrics, the effective removal of the low-k interface layer at 300K has been confirmed by electron microscopy and spectroscopy techniques. Significant enhancement of the gate capacitance density, while retaining low leakage current densities, has also been demonstrated for these interface-engineered high-k gate stacks.

Fluorothiol-capped fcc gold nanoparticles, with a mean diameter of approximately 4 nm have been studied using total scattering powder neutron diffraction at 15 and 300 K on the NPDF diffractometer at the Manuel Lujan Jr. Neutron Scattering Center. The results are compared with bulk gold powder. A real space analysis of the structure of the nanoparticles has been performed using the pair distribution function (PDF). This provides valuable information and reveals several interesting features. The PDF pattern for the nanoparticles is significantly attenuated compared to bulk gold due to the finite size of the particles; the attenuation is consistent with the 4 nm size of the particles. The nanoparticles have a slightly smaller lattice parameter at both temperatures but the nearest neighbor Au–Au vectors are entirely symmetrical and there is no evidence for structural relaxation associated with atoms near the surface. The vector between surface Au atoms and sulfur of the thiol cap layer can be observed at ∼2.4 A. The study points to the great power of total neutron scattering in ensemble-averaged analysis of structure in the nano regime.

Crystallization of hafnia and zirconia and their alloys with silica and lanthana was studied in bulk and thin film samples by thermal analysis, X-ray diffraction and electron microscopy. Crystallization temperatures of hafnia and zirconia increase by more than 300 °C with increase of surface/interface area of the amorphous phase. Crystallization temperatures of zirconia and hafnia alloys with silica and lanthana increase with dopant content and exceed 900 °C for 50 mol% SiO2 and LaO1.5. Energies for tetragonal HfO2 and ZrO2 interfaces with amorphous silica were derived from their crystallization enthalpies from silicates as 0.25 ± 0.08 and 0.13 ± 0.07 J/m2, respectively. The crystallization pathways in bulk powders and films of zirconia and hafnia can be interpreted as resulting from thermodynamic stabilization by the surface energy term of tetragonal and amorphous phases over monoclinic. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission electron microscopy and x-ray diffraction were used to study SrTiO3 films grown on platinized (0001) Al2O3 substrates. The Pt films were epitaxial with an orientation relationship described by (111)Pt‖(0001)Al2O3 and [110]Pt‖[10¯10]Al2O3. SrTiO3 films with two different Sr to Ti ratios, 1.02 and 1.4, were deposited by radio-frequency magnetron sputtering. In the film with a large amount of Sr excess, the grain sizes were smaller and a high density of planar defects was observed. The films were predominantly (111) textured, but a weaker (110) texture component was also found, independent of stoichiometry. While the (111) texture could be explained with the excellent lattice match with (111) Pt, the (110) textured grains had a large mismatch with the Pt electrode. We propose that the presence of the (110) oriented grains is due to nucleation at Pt surface defects. Planar defects in the films with a large amount of Sr excess served to accommodate the nonstoichiometry. Comparison with homoepitaxial SrTiO3 films showed that the density of planar defects in the SrTiO3 films on (111)Pt/Al2O3 is insufficient to accommodate all the excess Sr. The influence of the film microstructure on the dielectric properties is also discussed.

Hafnium aluminate films with different compositions were deposited at room temperature by jet vapor deposition. The as-deposited films were amorphous. After annealing at 1100 °C, the microstructure of the films was analyzed by high-resolution transmission electron microscopy, electron diffraction and electron energy loss spectroscopy (EELS). The crystalline phase in pure HfO2 films was monoclinic. With an increase in the Al content in the films, the amount of metastable HfO2 with a tetragonal distorted fluorite structure increased. In addition, the grain sizes decreased, making the detection of crystallization by x-ray diffraction difficult. No crystalline Al2O3 phase could be unambiguously detected in electron diffraction patterns in films with up to 30 mol % Al2O3. However, an Al-rich intergranular phase was identified by EELS. Films with ∼64 mol % Al2O3 crystallized as tetragonal HfO2 and metastable cubic Al2O3 with the spinel structure. The complex microstructures of the films should be considered in ...

Radio-frequency sputtering was used to deposit near-stoichiometric Bi1.5Zn1.0Nb1.5O7 (BZN) films. Composition, crystallinity, and phase purity of the films were investigated by Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy (TEM). X-ray diffraction detected cubic pyrochlore in films that were annealed above 400 °C. Films were fully crystallized at 750 °C. TEM and electron diffraction confirmed the cubic pyrochlore structure of the grains. Dielectric constant and loss were measured using planar Si/SiO2/Pt/BZN/Pt and Al2O3/Pt/BZN/Pt capacitor structures, respectively. After annealing at 750 °C, films on Pt coated silicon wafers showed a permittivity of 170, low dielectric losses, and a large electric field tunability of the dielectric constant at a measurement frequency of 1 MHz. Dielectric loss tangents improved when substrates were moderately heated during deposition.

Using the concept of metastable phase diagrams, we discuss the microstructure evolution during annealing of amorphous ZrO2–SiO2 and HfO2–SiO2 thin films for gate dielectric applications. These systems are characterized by a low solid solubility, a liquid miscibility gap and a kinetic barrier to the formation of the complex, crystalline silicate. We show that phase partitioning is expected for most compositions. Compositions within the metastable extensions of the spinodal are unstable and will spontaneously unmix in the amorphous phase upon heating. Hafnia- or zirconia-rich phases will subsequently crystallize to form HfO2 or ZrO2. Most compositions outside the metastable extensions of the liquid phase miscibility gap must phase separate above the crystallization temperature by nucleation of crystalline HfO2 or ZrO2 out of an amorphous silica-rich matrix. We present calculations of the metastable extensions of the miscibility gap and spinodal. The calculations predict that SiO2-rich compositions, investigated for gate dielectric applications, will show spinodal decomposition if they contain less than ~90 mol% SiO2 at the typical device processing temperature of 1000°C. Experimental studies of Hf-silicate films with three different compositions, between ~40 and 80 mol% HfO2 that lie inside and outside the miscibility gap, respectively, are presented. All three compositions show phase separation. Despite the different pathways of microstructure evolution, the final phase separated microstructures are similar. Experimental verification of the pathways that lead to these microstructures requires further studies.

For this paper, we used radio-frequency (rf) sputter deposition to synthesize epitaxial La0.5Sr0.5CoO3−∂ (LSCO) films. We investigated the influence of sputter deposition parameters, in particular, oxygen partial pressure, plasma power, total sputter pressure, and post-deposition cooling atmosphere on film composition, microstructure, and electrical resistivity. We show that rf sputtering from a single target can produce LSCO films with La/Sr and (La + Sr)/Co ratios of unity and with low electrical resistivities of about 1 mΩ cm. Film microstructures were characterized by high-resolution transmission electron microscopy and x-ray diffraction. Formation of an ordered film superlattice, most likely due to oxygen vacancy ordering, was observed. In this paper, we discuss the relationship between the film microstructure and the electrical resistivity.

A single-crystal barium oxotitanate(III, IV) of approximate composition Ba5:93(Ti 3þ 11:86Ti 4þ 28:14ÞO80, containing mixed-valence Ti, was grown from a borate flux. The crystal structure was identified as hollandite type by single-crystal X-ray diffractometry. Electron-energy-loss spectroscopy of Ti L2;3 and O K edges was used to determine chemical shifts related to the presence of mixed-valence Ti in the crystal. Comparison of Ti L2;3 and O K energy-loss near-edge structure

Low-temperature magnetotransport studies are reported for (112)Cd3As2 films grown on (111)CdTe by molecular beam epitaxy as a function of the Cd3As2 film thickness. All films show Shubnikov-de Haas oscillations. An even-integer quantum Hall effect is observed for films thinner than 70 nm. For the thinnest films, the bulk is gapped and transport at low temperatures occurs only via the gapless, two-dimensional states. The lowest Landau level is reached at ∼10 T, and the longitudinal resistance nearly vanishes at the plateaus in the Hall resistance. The results are discussed in the context of the current theoretical understanding of topological surface states in three-dimensional Dirac semimetals.

We report enhancement-mode composite-channel (In0.53Ga0.47As/InAs/In0.53Ga0.47As) MOSFETs fabricated using a substitutional-gate process, with n+ relaxed InAs source-drain regions formed by regrowth by molecular beam epitaxy. A device with 70-nm gate length and 2-nm In0.53Ga0.47As/3.5-nm InAs/3-nm In0.53Ga0.47As channel showed a peak transconductance of greater than 0.76 mS/μm at Vds = 0.4 V and showed Id = 0.5 mA/μm at Vds = 0.4 V and Vgs - Vth = 0.7 V. The subthreshold swing at Vds = 0.1 V was 130 mV/dec.

Electron energy-loss spectroscopy and high-resolution transmission electron microscopy were used to investigate ZrO2 layers grown by electron-beam evaporation in a molecular-beam epitaxy system. ZrO2/Si layers were investigated before and after uncapped annealing at 1000 °C under different oxygen partial pressures. The thickness of a SiO2-like, low-dielectric constant layer at the silicon interface was found to depend on the oxygen partial pressure during annealing. At oxygen partial pressures of about 10−4 torr the interfacial silicon oxide thickness increased through oxygen diffusion through the ZrO2 layer and silicon consumption at the interface. At oxygen partial pressures in the range of approximately 10−5 torr, only a thin (1 nm) interfacial silicon oxide layer was present, as required for low-equivalent oxide thicknesses of gate stacks incorporating alternative oxides. Further reduction of the oxygen partial pressures (about 10−7 torr) during annealing resulted in zirconium silicide formation at the interface. ZrO2 films annealed at the optimal partial pressure for a thin interfacial oxide were found to crystallize and contain no silicon. High-resolution analytical capabilities afforded by scanning transmission electron microscopy techniques proved essential in analyzing the stability of these ultrathin layers.

We report on the growth of epitaxial Sr2RuO4 films using a hybrid molecular beam epitaxy approach in which a volatile precursor containing RuO4 is used to supply ruthenium and oxygen. The use of the precursor overcomes a number of issues encountered in traditional MBE that uses elemental metal sources. Phase-pure, epitaxial thin films of Sr2RuO4 are obtained. At high substrate temperatures, growth proceeds in a layer-by-layer mode with intensity oscillations observed in reflection high-energy electron diffraction. Films are of high structural quality, as documented by x-ray diffraction, atomic force microscopy, and transmission electron microscopy. The method should be suitable for the growth of other complex oxides containing ruthenium, opening up opportunities to investigate thin films that host rich exotic ground states., In press, APL Mater

Three-dimension (3D) Dirac semimetals are a newly discovered class of topological materials that have attracted significant interest. Their electronic structure exhibits a linear dispersion relation around the Fermi energy, making them a three-dimensional analogue of graphene. Epitaxial heterostructures of these materials open a path for the manipulation of topological electronic states by strain engineering, electron confinement, or electrostatic gating, all of which may enable new electronic devices. Here, we discuss the growth of epitaxial thin films of Cd3As2, a 3D Dirac semimetal, by molecular beam epitaxy. We report on their structure, magnetotansport properties, and field gating experiments using high-k dielectrics. The films are shown to grow in the low-temperature, vacancy ordered, tetragonal Dirac semimetal phase. Extremely high room temperature mobilities (~20,000 cm2/Vs) are achieved in epitaxial layers on III-V substrates. Unusual transport characteristics are observed, such as a negative longitudinal magnetoresistance, i.e., when the magnetic field is applied parallel to the direction of electric current. We vary the film parameters and study their influence on the magnetotransport to distinguish between ‘classical’ phenomena and effects originating from the unique electronic structure.

Integration of ultrathin ferroelectric thin films with semiconductors is of interest for negative capacitance transistors that exhibit internal voltage gain, which may allow for scaling the supply voltage of low power circuits. In this study, BaTiO3 thin films were grown on doped SrTiO3 channels using molecular beam epitaxy. The BaTiO3 films are ferroelectric despite their low thickness (∼10 nm). Parallel plate capacitor devices exhibit anti-clockwise hysteresis, and a comparison with reference structures without BaTiO3 shows that the polarization in the BaTiO3 thin films is switchable and controls the charge density in the channel. Field effect transistors were fabricated to study the effect of ferroelectricity on the transistor characteristics. Anti-clockwise hysteresis and a shift in threshold-voltage are observed in the output characteristics of the transistors. These properties make these heterostructures a suitable system for studying negative capacitance effects.

Functional perovskite oxides may enable entirely new electronic device concepts, ranging from negative capacitance to charge amplification in phase change devices. A major challenge is the intrinsically poor charge carrier mobility of most perovskite oxides, typically no better than 1 - 10 cm2V-1s-1 at room temperature. Recent reports of room temperature mobilities of ~300 cm2 V-1s-1 in single crystals of La-doped BaSnO3 have therefore generated significant excitement. In addition, BaSnO3has a wide band gap (~ 3 eV), which makes it of interest as a new transparent conducting oxide and as a wide band gap semiconductor for power electronics. Furthermore, it would allow for integration of functional perovskite oxides on a lattice- and symmetry-matched, high-mobility, semiconducting channel. Electronic devices require the growth of high-quality films with low defect densities. High-mobility BaSnO3 thin films have been challenging to grow. Molecular beam epitaxy (MBE) is a low energetic deposition technique and routinely produces the highest mobility semiconductor and functional oxide films. However, stoichiometry control in MBE of perovskite oxides can be challenging. We show that in the MBE of perovskite stannates using a Sn metal source, volatile SnO consumes all active oxygen in the growth environment, leaving behind only Sn droplets at low growth temperatures. No film growth occurs at high temperatures. We discuss a modified MBE approach, which supplies pre-oxidized SnO x . This technique addresses issues in the stoichiometry controls in MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO3. A second challenge is the lack of lattice-matched substrates. We discuss the effect of lattice mismatch on the growth and transport properties of BaSnO3 by comparing the properties of films grown on SrTiO3 and PrScO3 substrates. We show that the carrier mobility doubles as the lattice mismatch is reduced by a factor of two, indicating that high misfit dislocation densities are the main limiting factor in further improving carrier mobilities. We demonstrate room temperature electron mobilities of 150 cm2 V-1s-1 at carrier densities of 7×1019 cm-3 in BaSnO3 films grown on PrScO3 using the modified MBE technique. We will discuss the prospects for further improving the carrier mobilities and reducing the carrier densities.

We report InGaAs-channel MOSFETs using recessed InP spacer layers in the regrown source and drain. By replacing narrow band-gap InGaAs with wide band-gap InP within the high-field region near the drain end of the channel, band-to-band tunneling (BTBT) leakage is significantly reduced. A 30 nm gate length device using InP spacers shows a minimum I off ∼60 pA/µm, approximately 100∶1 smaller than a similar device using InGaAs source/drain spacers. A FET using InP spacers, with 45 nm gate length, and with a 3 nm ZrO 2 gate oxide shows I on =150 µA/µm at I off =1 nA/µm and V DS =0.5 V. The low off-state leakage current observed with InP source/drain spacers makes InGaAs MOS technology viable for low-power logic.

We report the fabrication of low-leakage rectifying Pt and Au Schottky diodes and Au-gated metal-semiconductor field effect transistors (MESFETs) on n-type SrTiO3 thin films grown by hybrid molecular beam epitaxy. In agreement with previous studies, we find that compared to Pt, Au provides a higher Schottky barrier height with SrTiO3. As a result of the large dielectric constant of SrTiO3 and the large Schottky barrier height of Au, the Au-gated MESFETs are able to modulate ~1.6 x 1014 cm-2 electron density, the highest modulation yet achieved using metal gates in any material system. These MESFETs modulate current densities up to ~68 mA/mm, ~20x times larger than the best demonstrated SrTiO3 MESFETs. We also discuss the roles of the interfacial layer, and the field-dependent dielectric constant of SrTiO3 in increasing the pinch off voltage of the MESFET., Accepted for publication in Applied Physics Letters

Because InGaAs has a small bandgap, InGaAs MOSFETs are vulnerable to large band-to-band tunneling currents, arising near the drain end of the channel. This sets a minimum off-state leakage current, and can render the device unsuitable for VLSI applications at sub-10-nm gate lengths. Here we report two techniques to reduce band-to-band tunneling leakage in InGaAs MOSFETs. By using either a recessed InP wide bandgap spacer in the regrown source/drain region or, alternatively, an InP channel cap on top of InGaAs channel, off-state leakage can be reduced to 1 nA/μm for 22 nm gate length devices. These two techniques pave the way for application of III-V InGaAs MOSFETs in low power logic.

We report MOSFETs with 25-nm gate length (L g ), extremely thin 2.5 nm InAs channels and 0.7/3.0 nm (physical) Al 2 O x N y /ZrO 2 gate dielectrics, and 12 nm In 0.53 Ga 0.47 As vertical spacers in the raised epitaxial source/drain. The FETs establish key new DC performance records, at VLSI-relevant gate lengths (25 nm), including 0.50 mA/μm on-current (at 100 nA/μm I off and 0.5 V V DD ) and 77 mV/dec. subthreshold swing (SS) at V DS =0.5 V. At 1 μm L g and V DS =0.1 V, the minimum subthreshold swing is 61 mV/dec., a record low for InAs/InGaAs, indicating high interface quality.

The polar discontinuity [1] and electronic reconstruction at the interface of SrTiO3 and GdTiO3 leads to a unique high two dimensional electron gas (2DEG) of 3×10 14 electron/cm 2[2] , that is exactly half of the number of unit cells at the interface. The ability to modulate this high charge density could enable a new class of oxide electronics and plasmonics devices that harness extreme charge density. In this work show how heterostructures field effect transistors can be designed to enable modulation of over 10 14 cm -2 electron/cm 2 , where represents the highest charge density modulated in any field effector transistor to date.

Recently, InAs or In-rich InGaAs (In>53%) has been widely studied as the channel material for III-V FETs due to its superior electron transport properties over In 0.53 Ga 0.47 As. These materials provide excellent on-state characteristics, e.g. >2.5 mS/μm peak transconductanc (g m ) at V DS =0.5 V [1-3]. The narrow bandgap in these materials, however, causes band-to-band tunneling (BTBT) in the high drain-field region even at relatively low supply voltage of 0.5 V, thus resulting in high leakage at the off-state [1][3]. In our previous work, in order to address this issue, we incorporated a vertical spacer between the channel and N+ source/drain (S/D) to accommodate the depletion region near the channel-drain junction. The spacer significantly improved off-state characteristics such as off-state leakage, drain-induced barrier lowering (DIBL), and subthreshold swing (SS) without increasing the device footprint [3], [4]. In this work, by adopting a ~4 nm-thick In 0.53 Ga 0.47 As channel instead of a thick InAs channel, we have further improved the off-state characteristics at high V DS and achieved 81 mV/dec. minimum subthreshold swing (SS min ) for a 35 nm-L g device at V DS =0.5 V and 385 μA/μm on-current (I on ) at 100 nA/μm off-current (I off ) and V DD =0.5 V, which are the best SS min and I on from all reported In 0.53 Ga 0.47 As channel FETs.

Because of the low effective mass, MOSFETs using In-rich (x>53%) In x Ga 1-x As channels [1-5] exhibit high on-state current I on at low drain bias (V DS =0.5 V). However, the small bandgap of high-indium In x Ga 1-x As channels can lead to high off-state leakage I off due to band-to-band tunneling (BTBT) and impact ionization (I.I.). Earlier we had reported [1] that adding an unintentionally-doped (U.I.D.) InGaAs vertical spacer within the raised source/drain (S/D) of an InAs/InGaAs channel MOSFET substantially reduced I off . Here we compare the characteristics of FETs using a wide-bandgap U.I.D. InP vertical spacer to earlier results [1] using an InGaAs spacer, and to control devices using only a very thin spacer. We find that FETs using InP spacers have I off comparable to FETs using narrower-bandgap InGaAs spacers of similar thickness, suggesting that with the spacer, the observed I off at high V DS arises from BTBT or I.I. within the channel, and not within the high-field gate-drain spacer layer. Further, the wide-gap U.I.D. InP source spacer does not increase the threshold voltage V th , suggesting that the gated potential barrier remains in the channel and not in the source spacer region. We also compare the on-state characteristics of FETs using InAs/InGaAs channels and an N+ InP S/D. Unlike the findings of [6], we do not observe improved I on with the use of a wider-bandgap N+ source.

While the growth of III-As and III-P semiconductors is well-established, and their transport properties well-understood, the performance of high-frequency and VLSI electron devices can still be substantially improved. Here we review design principles, experimental efforts, and intermediate results, in the development of nm and THz electron devices, including nm InAs/InGaAs planar MOSFETs and finFETs for VLSI, InGaAs/InP DHBTs for 0.1-1 THz wireless communications and imaging, and ~5nm InAs/InGaAs Schottky diodes for mid-IR mixing.

The temperature dependent Hall mobility data from La-doped SrTiO3 thin films has been analyzed and modeled considering various electron scattering mechanisms. We find that a ~6 meV transverse optical phonon (TO) deformation potential scattering mechanism is necessary to explain the dependence of transport on temperature between 10-200 K. Also, we find that the low temperature electron mobility in intrinsic (nominally undoped) SrTiO3 is limited by acoustic phonon scattering. Adding the above two scattering mechanisms to longitudinal optical phonon (LO) and ionized impurity scattering mechanisms, excellent quantitative agreement between mobility measurement and model is achieved in the whole temperature range (2-300K) and carrier concentrations ranging over a few orders of magnitude (8x1017 cm-3 - 2x1020 cm-3)., Accepted for publication in Physical Review Letters

Quantum phase transitions are driven by quantum fluctuations that alter the nature of the electronic quasiparticles, resulting in phenomena such as non-Fermi liquid behaviour. Oxide heterostructures offer fundamentally new ways of manipulating quantum criticality. Here, we report on non-Fermi liquid behaviour in thin SrTiO3 quantum wells that are embedded in insulating, antiferromagnetic SmTiO3, as a function of temperature, quantum well thickness and SmTiO3 layer thickness in superlattices. Such quantum wells contain very high sheet carrier densities on the order of one electron per pseudocubic planar unit cell. We show that the quantum well thickness is a tuning parameter for non-Fermi liquid behaviour. Increasing the thickness by a single atomic layer and coupling in superlattices recover the Fermi liquid behaviour. The critical exponents, the symmetry of the order parameter, the role of carrier densities and symmetry-lowering distortions are discussed, and the results are compared with those of quantum wells embedded in ferrimagnetic GdTiO3.

High-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and electron diffraction were used to study the microstructure of epitaxial La0.5Sr0.5CoO3−∂ thin films grown on (001) oriented LaAlO3 substrates. Films were characterized before and after annealing treatments under different oxygen partial pressures. EELS shows that annealing reduces the valence state of cobalt due to loss of oxygen. HRTEM image simulations show that the superstructure contrast observed in HRTEM can be explained by shifts of cations in planes containing ordered oxygen vacancies. The as-deposited film showed weak, short-range ordering of oxygen vacancies within nanometer-sized domains. The annealed film showed long-range order and a strong anisotropy in ordering, with the oxygen-deficient planes aligned parallel to the film/substrate interface. We propose that the anisotropy in ordering is a mechanism of stress relief in these films. Implications of the observed microstructure on the oxygen...

Epitaxial Pb(Mg1/3Nb2/3)O3 (PMN) and (l-x)(Pb(Mg1/3Nb2/3)O3)-x(PbTiO3) (PMN-PT) thin films have been deposited by metalorganic chemical vapor deposition at 700 – 780°C on (100) SrTiO3 and SrRuO3/SrTiO3 substrates. Room temperature values of 900 and 1.5%, were measured for the zero-bias permittivity and loss respectively, at 10 kHz for 220 nm thick pure PMN films. For PMN-PT films, the small-signal permittivity ranged from 1000 to 1500 depending on deposition conditions and Ti content; correspondingly low values for the zero-bias dielectric loss between 1 and 5% were determined for all samples. For PMN-PT with x of approximately 0.30–0.35, polarization hysteresis with Prμ18μC/cm2 was obtained. Initial piezoresponse results are discussed.

Atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) were used to study the defect structure of oxygen deficient strontium cobaltite (SrCoO 3− δ ) after repeated oxidation/reduction cycles at elevated temperatures. Ordered microdomains were found in the reduced sample. Z-contrast imaging showed ordering in alternate CoO 2 planes, characteristic for a brownmillerite-type vacancy ordered structure. EELS showed that the average valence state of cobalt had decreased after testing. In the tested sample, cobalt was found in +3 and +2 valence states, respectively. In addition, cobalt oxide precipitates were found at grain boundaries and in triple pockets in this sample, indicating a cobalt deficiency of the grains. The mechanisms for the formation of the observed microstructure are discussed.

Epitaxial, perovskite Pb(Mg1/3Nb2/3)O3–PbTiO3 films were grown by metalorganic chemical vapor deposition under various deposition conditions and characterized by x-ray diffraction and transmission electron microscopy. Pyrochlore free films were obtained under all deposition conditions used in this study. Magnesium-rich growth conditions lead to the formation of a Mg-rich impurity phase in the films, embedded as coherent lamellae parallel to the growth direction. Depending on the growth conditions, a wide variation in the stoichiometry and volume fraction of this impurity phase was found between samples, whereas morphology and crystal structure were found to be very similar.

In this paper, we investigate the role of grain boundaries in polycrystalline (Ba x Sr1−x )Ti1+y O3+z films, grown by metal organic vapor deposition, in the accommodation of nonstoichiometry, as well as their role in the strong composition dependence of the electric and dielectric behavior observed in these films. High-spatial resolution electron energy-loss spectroscopy is used for the analysis of composition and structural changes at grain boundaries, as a function of film composition. The existence of amorphous, titanium rich, TiO2-like phases at the grain boundaries of films with large amounts of excess Ti (y ≥ 0.08) may explain the non-monotonic resistance degradation behavior of the films as a function of Ti content. However, we show that a grain boundary phase model fails to explain the strong composition dependence of the dielectric behavior. Electron energy-loss spectra indicate a distortion of the Ti–O octahedra in the grain interiors in samples with increasing Ti excess. The decrease of the dielectric constant with increasing amounts of excess Ti is therefore more likely due to Ti accommodation in the grain interiors.

Thermal conductivity of a crystalline solid at high temperature is dominated by the Umklapp process because the number of high frequency phonons increases with temperature. It is challenging to reduce the thermal conductivity of crystalline solids at high temperature although it is widely known that, by increasing the atomic defect concentration, thermal conductivity of crystalline solids can be reduced at low temperature. By increasing the concentration of ErAs nanoparticles in In 0.53Ga 0.47As up to 6 atom %, we demonstrate a thermal conductivity reduction by almost a factor of 3 below that of In 0.53Ga 0.47As at high temperature. A theoretical model suggests that the mean free path of the low frequency phonons is suppressed by increasing the ErAs nanoparticle concentration.

Thermal leakage characteristics of Pt/SrTiO 3 /Pt structures Junwoo Son and Susanne Stemmer a) Materials Department, University of California, Santa Barbara, CA 93106-5050, U.S.A. a) Corresponding author; electronic mail: stemmer@mrl.ucsb.edu MATERIAL NAMES: strontium titanate (SrTiO 3 ), platinium (Pt), barium strontium titanate (Ba,SrTiO 3 )

Transmission electron microscopy (TEM) is used to study the effect of high-temperature tests on the amorphous grain boundary films in a commercial, liquid phase sintered silicon nitride ceramic. The thickness of amorphous films between silicon nitride grains is characterized by high-resolution TEM. In addition, electron energy-loss spectroscopy is applied to analyze the composition of grain boundary films. A smaller equilibrium film thickness is measured in the specimens after testing. This decrease in film thickness is due to changes in the intergranular film chemistry. The composition changes are caused by phase transformations in the secondary crystalline phases, which were observed by TEM and X-ray diffraction, and by crystallization of residual amorphous pockets during high-temperature testing.

Viscous energy dissipation in a liquid phase sintered silicon nitride, and the resulting internal friction, were investigated at temperatures up to 1300°C with uniaxial tension–compression, torsion pendulum, and impulse excitation tests. The amount of internal friction, which is lower after heat treatment of the silicon nitride, reaches a steady-state value only after long-term tests at high temperature. A low and a high (>50 MPa) stress amplitude regime are distinguished, revealing the existence of different anelastic deformation mechanisms. The evolution of the microstructure is investigated by X-ray diffraction, analysis of scanning electron microscopy images, and high resolution and analytical transmission electron microscopy in order to relate specific internal friction features to the silicon nitride microstructure.

In this paper, we use high-resolution transmission electron microscopy and electron energy-loss spectroscopy to study the microstructure of grain boundaries and the segregation of yttrium, respectively, in yttria-stabilized tetragonal polycrystalline zirconia, sintered from different high-purity powders. No amorphous films were observed at the grain boundaries, and only the sample containing the highest amount of silicon impurity showed presence of an amorphous silicate phase in all triple grain junctions. A strong yttrium segregation to the grain boundaries is observed in all samples, despite different grain sizes and impurity levels.

The focus dependence of atom column intensities in high-angle annular dark-field (HAADF) images is investigated as a function of sample thickness for two different materials, InP and SrTiO3. We show that even for very thick samples, the column intensities in both materials are maximized if the probe is focused near the entrance sample surface. Atom column intensities decrease for focus settings away from the optimum focus. The results show that HAADF through-focus series with a non-aberration corrected probe do not allow for atomically resolved depth sectioning in zone-axis crystals.

A monolithic Ku-band phase shifter employing voltage tunable Bi/sub 1.5/Zn/sub 1.0/Nb/sub 1.5/O/sub 7/ (BZN) thin film parallel plate capacitors is reported. BZN films were deposited by radio frequency magnetron sputtering on single-crystal sapphire substrates. A nine-section distributed coplanar waveguide loaded-line phase-shifter structure was designed. A differential phase shift of 175/spl deg/ was achieved with a maximum insertion loss of 3.5 dB at 15 GHz, giving a figure of merit /spl sim/50/spl deg//dB. To the best of our knowledge, this is the first demonstration of a monolithic tunable microwave circuit using BZN thin films.

The electrical and structural characteristics of SmTiO3/SrTiO3/SmTiO3 and GdTiO3/SrTiO3/GdTiO3 heterostructures are compared. Both types of structures contain narrow SrTiO3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO3 quantum wells embedded in GdTiO3 show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO3 remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO3 or in GdTiO3. We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state., Accepted for publication as a Regular Article in Physical Review B

Oxide-based resistive switching devices are promising candidates for new memory and computing technologies. Poor understanding of the defect-based mechanisms that give rise to resistive switching is a major impediment for engineering reliable and reproducible devices. Here we identify an unintentional interface layer as the origin of resistive switching in Pt/Nb:SrTiO3 junctions. We clarify the microscopic mechanisms by which the interface layer controls the resistive switching. We show that appropriate interface processing can eliminate this contribution. These findings are an important step towards engineering more reliable resistive switching devices., Understanding the resistive switching mechanism in oxide-based memories is an ongoing challenge. Here, the authors isolate an unintentional interfacial layer as the origin of resistive switching in Pt/Nb:SrTiO3 junctions, and show that suitable processing can remove this unwanted contribution.