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Beta-phase gallium oxide ([Formula: see text]-Ga2O3) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the [Formula: see text]-phase, numerous other polymorphs exist and understanding the competition between phases is critical to control practical devices. The phase formation sequence of Ga2O3, starting from amorphous thin films, was determined using lateral-gradient laser spike annealing at peak temperatures of 500–1400 °C on 400 μs to 10 ms timescales, with transformations characterized by optical microscopy, x-ray diffraction, and transmission electron microscopy (TEM). The resulting phase processing map showed the [Formula: see text]-phase, a defect-spinel structure, first nucleating under all annealing times for temperatures from 650 to 800 °C. The cross-sectional TEM at the onset of the [Formula: see text]-phase formation showed nucleation near the film center with no evidence of heterogeneous nucleation at the interfaces. For temperatures above 850 °C, the thermodynamically stable [Formula: see text]-phase was observed. For anneals of 1–4 ms and temperatures below 1200 °C, small randomly oriented grains were observed. Large grains were observed for anneals below 1 ms and above 1200 °C, with anneals above 4 ms and 1200 °C resulting in textured films. The formation of the [Formula: see text]-phase prior to [Formula: see text]-phase, coupled with the observed grain structure, suggests that the [Formula: see text]-phase is kinetically preferred during thermal annealing of amorphous films, with [Formula: see text]-phase subsequently forming by nucleation at higher temperatures. The low surface energy of the [Formula: see text]-phase implied by these results suggests an explanation for the widely observed [Formula: see text]-phase inclusions in [Formula: see text]-phase Ga2O3 films grown by a variety of synthesis methods.

Here, we have explored the thermal stability of α-(Al,Ga)2O3 grown by the molecular-beam epitaxy on m-plane sapphire under high-temperature annealing conditions for various Al compositions (i.e., 0%, 46%, and 100%). Though uncapped α-Ga2O3 undergoes a structural phase transition to the thermodynamically stable β-phase at high temperatures, we find that an aluminum oxide cap grown by atomic layer deposition preserves the α-phase. Unlike uncapped α-Ga2O3, uncapped α-(Al,Ga)2O3 at 46% and 100% Al content remain stable at high temperatures. We quantify the evolution of the structural properties of α-Ga2O3, α-(Al,Ga)2O3, and α-Al2O3 and the energy bandgap of α-Ga2O3 up to 900 °C. Throughout the anneals, the α-Ga2O3 capped with aluminum oxide retains its high crystal quality, with no substantial roughening.

ScxAl1−xN (x = 0.18–0.40) thin films of ∼28 nm thickness grown on metal polar GaN substrates by molecular beam epitaxy are found to exhibit smooth morphology with less than 0.5 nm roughness and predominantly single-phase wurtzite crystal structure throughout the composition range. Measurement of the piezoelectric d33 coefficient shows a 150% increase for lattice-matched Sc0.18Al0.82N relative to pure aluminum nitride, whereas higher Sc contents exhibit lower piezoelectric coefficients. The electromechanical response of the epitaxial films correlates with the crystal quality and the presence of zinc blende inclusions, as observed by high-resolution electron microscopy. It is further found that the polarity of the epitaxial ScxAl1−xN layers is locked to the underlying substrate. The measured electromechanical properties of epitaxial ScxAl1−xN, their relation to the atomic crystal structure and defects, and its crystal polarity provide useful guidance toward the applications of this material.

Interband Zener tunneling of electrons has been recently used in III-nitride semiconductor based light emitters to efficiently inject holes into p-cladding layers. Zener tunneling probabilities can be significantly enhanced if crystal symmetry-induced internal polarization fields assist the dopant-induced built-in electric fields of tunnel junctions because of the large reduction of the tunneling distance. In a metal-polar buried tunnel junction geometry, such electric field alignment needs an AlN interlayer at the tunnel junction. Because AlN is a larger bandgap semiconductor than GaN, it is not clear a priori if the net tunneling probability is reduced or enhanced compared to a homojunction. By combining theoretical modeling with experimental blue light emitting diodes, we find that the large tunneling enhancement due to the polarization field and band realignment overcome the reduction in tunneling due to the larger bandgap of AlN. Compared to a homojunction tunnel-junction, the inclusion of AlN in the tunnel junction is found to lower the turn-on and operating voltages and increase the wall-plug efficiency. This proves that polarization-induced AlN tunnel junctions are superior to homojunctions at low injection currents, resulting in higher optical emission intensity and superior uniformity.

We compare the effectiveness of in situ thermal cleaning with that of Al-assisted cleaning of native surface oxides of bulk AlN for homoepitaxial growth by molecular beam epitaxy. Thermal deoxidation performed at 1450 ° C in vacuum results in voids in the AlN substrate. On the other hand, Al-assisted deoxidation at ≈ 900 ° C results in high-quality AlN homoepitaxy, evidenced by clean and wide atomic terraces on the surface and no extended defects at the growth interface. This study shows that Al-assisted in situ deoxidation is effective in removing native oxides on AlN, providing a clean surface to enable homoepitaxial growth of AlN and its heterostructures; furthermore, it is more attractive over thermal deoxidation, which needs to be conducted at much higher temperatures due to the strong bonding strength of native oxides on AlN.

Atomic-scale electron spectroscopy is used to determine the local electronic structure of atomic-layer-deposited HfO2 gate dielectrics as a function of annealing conditions. Oxygen core-loss spectra from monoclinic crystallites exhibit a more strongly pronounced crystal-field splitting with increasing anneal temperature up to 1000 °C, consistent with a decrease in point defects. Concomitantly, electrical measurements of the same structures show a correlated reduction of fixed charge. An unintentional ∼5 A SiO2 layer is observed at the top interface, between the HfO2 and poly-Si electrode. No Hf–silicate intermixing is detected at either interface on a scale down to 2 A.

We have examined the interfaces in Zr-silicate gate dielectrics grown on Si substrates using electron energy loss spectroscopy. The Zr-silicate interface is found to be stable with the Si substrate and the polycrystalline silicon (poly-Si) electrode under annealing to 1050 °C. At this interface, a 0.35 nm wide Zr-free interface region is observed in the as-deposited film, and does not change on annealing. The Zr-free region is too thin to take on the bulk SiO2 electronic structure, and thus is unlikely to compromise the dielectric properties of the device. For films with an Al electrode, a 2 nm reaction layer forms at the Zr-silicate interface.

In this letter, we report on electrical and microstructural properties of ultrathin zirconia dielectrics grown on SiO2 by ultraviolet (UV) ozone oxidation and natural oxidation (no UV light). Capacitance–voltage (C–V) measurements were performed at multiple frequencies on capacitors fabricated from a ZrO2–SiO2 stack. It was found that the C–V curves from samples grown by natural oxidation were distorted and showed severe frequency dependence while samples grown with UV light exposure under otherwise identical conditions had superior electrical behavior. Loss tangent measurements and detailed electron energy loss spectroscopy studies performed on the two samples revealed that the sample grown by natural oxidation was highly oxygen deficient, and this led to its poor electrical properties.

In this letter, we report on the growth of ultrathin films of zirconia on silicon oxide and silicon oxynitride passivated (001) Si by ultraviolet ozone oxidation of Zr metal precursor layers. The oxidation kinetics has been measured using an accelerator-based nuclear reaction analysis. It was found that oxide films up to 55 A could be grown at room temperature by oxidation at 600 Torr while oxidation at 80 mTorr is self-limiting at 20 A. The interfaces between the dielectric and the substrate have been characterized by scanning transmission electron microscopy. The ZrO2 films were found to be crystalline as grown. Electrical measurements on capacitor structures with 30-A-thick ZrO2 films grown on native oxide on silicon show a capacitance–voltage hysteresis of 15 mV and a capacitance-based equivalent oxide thickness of 17 A at 100 kHz.

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproduciblemetal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinnerfilms with sharp MITs are discussed, including the Volmer-Weber type growth mode due to thehigh energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2substrate, which we quantify using low angle annular dark field scanning transmission electronmicroscopy in conjunction with electron energy loss spectroscopy. We find that controlling islandcoalescence on the (001) surface and minimization of cation interdiffusion by using a low growthtemperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2films with abrupt MIT behavior. VC 2015 AIP Publishing LLC.

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO3 films depositedby reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cationstoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity(k33), with the greatest decrease in films of the same composition observed for films containingplanar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivitycan be modified by as much as 80%—from 11.5W m1K1 for stoichiometric homoepitaxialSrTiO3 to 2W m1K1 for strontium-rich homoepitaxial Sr1þdTiOx films—by incorporating (SrO)2Ruddlesden-Popper planar defects.VC 2015 AIP Publishing LLC.

Crystallinity and texturing of RF sputtered c-axis aligned crystal InGaZnO4 (CAAC IGZO) thin films were quantified using X-ray diffraction techniques. Above 190 °C, nanocrystalline films with an X-ray peak at 2θ = 30° (009 planes) developed with increasing c-axis normal texturing up to 310 °C. Under optimal conditions (310 °C, 10% O2), films exhibited a c-axis texture full-width half-maximum of 20°. Cross-sectional high-resolution transmission electron microscopy confirmed these results, showing alignment variation of ±9° over a 15 × 15 nm field of view and indicating formation of much larger aligned domains than previously reported. At higher deposition temperatures, c-axis alignment was gradually lost as polycrystalline films developed.

The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1–5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases.

We demonstrate the strain release of LaAlO3 epitaxial films on SrTiO3 (001) by inserting ultrathin “SrAlOx” buffer layers. Although SrAlOx is not a perovskite, nor stable as a single phase in bulk, epitaxy stabilizes the perovskite structure up to a thickness of 2 unit cells (uc). At a critical thickness of 3 uc of SrAlOx, the interlayer acts as a sliding buffer layer, and abruptly relieves the lattice mismatch between the LaAlO3 film and the SrTiO3 substrate, while maintaining crystallinity. This technique may provide a general approach for strain relaxation of perovskite films far below the thermodynamic critical thickness.

We have fabricated MgO-barrier magnetic tunnel junctions with a Co/Ni switching layer to reduce the demagnetizing field via interface anisotropy. With a fcc-(111) oriented Co/Ni multilayer combined with an FeCoB insertion layer, the demagnetizing field is 2 kOe and the tunnel magnetoresistance can be as high as 106%. Room-temperature measurements of spin-torque switching are in good agreement with predictions for a reduced critical current associated with the small demagnetization for antiparallel-to-parallel switching. For parallel-to-antiparallel switching the small demagnetization field causes spatially nonuniform reversal nucleated at the sample ends, with a low energy barrier but a higher switching current.

The three-dimensional reconstruction of a porous low-dielectric constant film (κ=2.5), resolving pores as small as 1 nm, was achieved using annular dark-field scanning transmission electron tomography, enabling quantitative measurements of the pore morphologies and size distribution. Most large pores were elliptical. Together with log-normal pore-size distribution, this suggests pore coalescence during the material’s growth. Ellipsometric porosimetry indicates a high degree of interconnectivity between pores. Tomography shows the material exhibits little large-scale pore connectivity, thus placing an upper limit on the size of the interconnections at below 1 nm. Systematic errors in the tomographic and ellipsometric size distributions appear to be largely complementary.

Precipitates formed during the growth of manganite thin films by pulsed laser deposition have been an obstacle for fabricating high quality devices incorporating these ferromagnetic metals. In order to analyze the nature of these precipitates, we have investigated their spectroscopic and structural properties by scanning transmission electron microscopy. For LaMnO3 films, crystallites of Mn3O4 are found to segregate out from stoichiometric films to accommodate a net cation off-stoichiometry during growth. By tuning the laser spot conditions, these precipitates can be eliminated.

Atomic-scale electron spectroscopic imaging on sputtered magnetic tunnel junctions (MTJs) with a thin

We report the structural properties of homoepitaxial (100) SrTiO3 films grown by reactive molecular-beam epitaxy (MBE). The lattice spacing and x-ray diffraction (XRD) rocking curves of stoichiometric MBE-grown SrTiO3 films are indistinguishable from the underlying SrTiO3 substrates. Off-stoichiometry for both strontium-rich and strontium-poor compositions (i.e., Sr1+xTiO3+δ films with −0.2

The use of boron-alloyed electrodes with the radio frequency (rf) sputter deposition of MgO yields magnetic tunnel junctions (MTJs) with Mg–B–O tunnel barriers. After annealing, such MTJs can exhibit very high tunneling magnetoresistance (TMR) in the thin (∼1.0 nm) barrier regime. Scanning tunneling spectroscopy of Mg–B–O layers reveals a better defined, but smaller band gap in comparison to that of thin MgO. We produced Fe60Co20B20/Mg–B–O/Ni65Fe15B20 MTJs where after a 350 °C annealing the Ni–Fe–B free electrode crystallizes into a highly textured (001)-normal body centered cubic (bcc) crystal structure and the MTJs achieve 155% TMR.

Dielectric nanospheres are important components for photonic applications, where interactions between whispering gallery modes (WGMs) can be used to construct photonic band structures. Using the electromagnetic fields generated by relativistic electrons in a monochromated 200 keV scanning transmission electron microscope, we record electron energy loss spectral features reflecting the density of states (DOS) of the electric-type WGMs in SiO2 nanospheres over an energy range extending into the far-ultraviolet regime. These results demonstrate possibilities for mapping the local DOS of photonic systems in an electron microscope with a spatial resolution of a few nanometers.

The ability to detect individual impurity atoms has been greatly enhanced by the development of aberration-corrected electron microscopes. The reduced depth of focus potentially enables three-dimensional reconstructions of impurity atoms from through-focal series. We test the robustness of this depth-sectioning method for detecting impurity atoms in gate oxides using multislice simulations. For amorphous materials, dopants can be reliably imaged, and are accurately described by a simpler three-dimensional linear imaging model. For crystalline materials, however, channeling artifacts can render the signal uninterpretable. These artifacts can be eliminated by orienting the crystal slightly off the zone axis, which still preserves atomic resolution.

Surface segregation effects play an important role in the growth of traditional III-V semiconductor heterointerfaces. Here we show that segregation processes can also set an upper limit to the obtainable interface sharpness in perovskite oxide heterostructures. In particular, the structure of LaVO3∕SrTiO3 superlattices was studied on the atomic scale by electron microscopy and spectroscopy. The vanadate layers exhibit a growth asymmetry, with diffuse lower and atomically abrupt upper interfaces, caused by preferential Sr surface segregation. Switching the SrTiO3 termination layer does not change the interface abruptness, which excludes the interfacial polar discontinuity as the driving force for the observed growth asymmetry.

Electron energy-loss spectroscopy at subnanometer resolution is used to investigate CoFeB∕MgO∕CoFeB magnetic tunnel junctions grown by electron-beam evaporation and radio frequency (rf) sputtering before and after annealing. Gap states were observed in the MgO layer for both growth methods although the rf-sputtered MgO layer showed significantly more gap states. Asymmetry in oxygen bonding between the top and bottom CoFeB∕MgO interfaces was also observed. Moreover, significant amounts of diffused B as BOx were observed in the rf-sputtered MgO layer. A Mg underlayer between the MgO layer and the bottom electrode greatly reduced BOx formation in the barrier upon annealing.

Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies.

As integrated circuits have shrunk, conventional electron microscopies have proven inadequate for imaging complicated interconnect structures due to the overlap of features in projection. These techniques produce transmission functions with a nonmonotonic dependence of intensity on thickness for common microelectronic materials, making them unsuitable for tomography. We report the use of an incoherent bright field imaging technique in a scanning transmission electron microscope optimized for the three-dimensional reconstruction of thick copper microelectronic structures. Predictable behavior of the signal in samples up to ∼1μm thick allows us to reconstruct and quantify the shape and volume of stress voids within Ta-lined interconnects.

High-performance metal-oxide-semiconductor field effect transistors (MOSFETs) on III–V semiconductors have long proven elusive. High-permittivity (high-κ) gate dielectrics may enable their fabrication. We have studied hafnium oxide and aluminum oxide grown on gallium arsenide by atomic layer deposition. As-deposited films are continuous and predominantly amorphous. A native oxide remains intact underneath HfO2 during growth, while thinning occurs during Al2O3 deposition. Hydrofluoric acid etching prior to growth minimizes the final interlayer thickness. Thermal treatments at ∼600°C decompose arsenic oxides and remove interfacial oxygen. These observations explain the improved electrical quality and increased gate stack capacitance after thermal treatments.