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The density, crystallinity, and microstructure of reactively sputtered amorphous Ge2Sb2Te5 thin films have been assessed as a function of deposition temperature. The continuous density increase was observed with increasing substrate temperature between room temperature and 200 °C. The films deposited at room temperature are amorphous and exhibit a columnar structure with a lateral size of cells in the 10–15 nm range. Cells consist of high-density interior with boundaries with the density lower by ∼9% due to incorporation of pores. The pores and the columnar microstructure can be eliminated by deposition at 80 °C while still preserving the amorphous phase. The density of pore- and stress-free amorphous Ge2Sb2Te5 is 6.16 g/cm3 and is only 1.5% lower than the crystalline Ge2Sb2Te5 with NaCl structure. [ABSTRACT FROM AUTHOR]

The morphology of basal plane dislocations (BPDs) in 4H-SiC homoepitaxial layers was investigated using plane-view transmission x-ray topography and molten KOH etching. Based on the morphologies three types of BPDs are distinguished which include interfacial dislocations, curved dislocations and circular loop dislocations around micropipes.

The Si-Si bond along the dislocation line induces a deep level in the SiC band gap. This state serves as both a radiative and nonradiative recombination center and converts the electron-hole recombination energy into activation energy for the dislocation motion.

Surfaces of homoepitaxially grown TiO2-terminated SrTiO3(001) were studied in situ with scanning tunneling microscopy and spectroscopy. By controlling the Ti/Sr ratio, two-dimensional domains of highly ordered linear nanostructures, so-called “nanolines,” are found to form on the surface. To further study how the surface structure affects the band structure, spectroscopic studies of these surfaces were performed. Our results reveal significantly more band bending for surfaces with the nanolines, indicative of an acceptor state associated with these features. Additionally, an in-gap state is observed on nanoline surfaces grown under high oxygen deficient conditions. This state appears to be the same as that observed previously, arising from the (++/+) transition level of surface oxygen vacancies. [ABSTRACT FROM AUTHOR]

The generation and evolution of defects in 4H-SiC p-n junctions due to carrier injection under forward bias are investigated by synchrotron white beam x-ray topography, electroluminescence imaging, and KOH etching. Low angle grain boundaries are observed to act as one of a number of possible nucleation sites of stacking faults.

A study was conducted to analyze deep traps in undoped n-GaN layers prepared by organometallic vapor phase epitaxy on sapphire substrates. A number of electrical properties of GaN samples were considered in the study. Cathodoluminescence spectra were determined at 100 K while energy traps were computed from the dependence of the peak positions on the heating rate. Results indicated the presence of high concentrations of traps supporting energy levels between 0.1 and 0.2 eV in undoped GaN samples.

Research was conducted to examine the impact of the intermediate buffer layer on the structural and optical characteristics of gallium nitride (GaN) films grown on the oxygen and zinc surfaces of zinc oxide substrates. These properties were analyzed using techniques such as photoluminescence, atomic force microscopy, x-ray double diffraction, transmission electron microscopy and reflectivity. Differences in thermal expansion coefficients indicate that the GaN epilayers were grown with compressive strain.

Shallow and deep centers were examined by temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undoped n-AlGaN samples grown by organometallic vapor phase epitaxy. The shallow-deep transition occurred at about x=0.2 in the low conductivity Al(sub x)Ga(sub 1-x)N series and at about x=0.5 for the high conductivity series. Several PL bands were found in AlGaN and the band at 3.05 eV was caused by a radiative transition between deep donors in the upper part of the band gap and holes in the valence band or on shallow acceptors.

Analysis of the influence of dimethylaluminum methoxide (DMAlMO) doping on the compensation of shallow tellurium donors in GaAs and the characteristics of deep-level defects by using Hall-effect and deep-level transient spectroscopy measurements reveals that DMAlMO doping induced oxygen-related deep level defects compensate shallow tellurium donors for concentration as high as 10 to the 19th power per centimeter cubed. The major electron traps with activation energies of 0.93 and 0.74 eV, attributed to complexes of aluminum and oxygen involving two and one Al atoms, respectively, are responsible for the compensation.

Photoluminescence (PL) studies of GaAs layers doped with dimethylaluminum methoxide during vapor phase epitaxy reveals a near-band-edge PL increase at low growth temperatures corresponding to oxygen incorporation. Near-band-edge PL quenching probably occurs due to oxygen-related nonradiative recombination centers. The appearance of several deep PL peaks correspond to defect formation.

Ultraviolet photoluminescence, transmission electron microscopy and KOH etching were used to characterize extended defects in 4H-SiC epilayers grown at high growth rates (18 μm/h). Layers exhibited high densities of in-grown stacking faults and dislocation half-loops. The stacking faults were 8H Shockley-type faults. The Burgers vector of the dislocation half-loops was in the (0001) basal plane. Both defects nucleate within the epilayer at early stages of growth. Defect nucleation is directly correlated with high initial growth rate and is not related to any defects/heterogeneities in the substrate or epilayer. Epilayer growth by nucleation of two-dimensional islands is proposed as a possible mechanism for the formation of both defects, through nucleation of faulted Si-C bilayers. [ABSTRACT FROM AUTHOR]

Filamentary conduction via dislocations has been considered to be a mechanism driving resistive switching in SrTiO3 single-crystals. This possible mechanism is further investigated by fabricating Pt-SrTiO3-Pt lateral devices on high dislocation density areas as well as dislocation-free areas of single-crystal SrTiO3, and using electron channeling contrast imaging and dislocation-selective wet chemical etching to track pre-existing dislocations and dislocations nucleated during electrical biasing. Device size, compliance levels, and vacancy concentration were observed to impact dislocation formation. The susceptibility of SrTiO3 to dislocation formation and strategies to avoid it by reducing power dissipation are discussed. The presence of dislocations is found to have a negligible effect on the device resistive switching behavior. Dislocation-free resistive switching devices are demonstrated for reduced single-crystalline SrTiO3. [ABSTRACT FROM AUTHOR]

Resistive switching is frequently associated with local heating of the switching structure. The mechanical effect of such heating on Pt/SrTiO3 (001) Schottky barriers and on Pt/SrZrO3/SrRuO3/SrTiO3 switching devices was examined. The extent and magnitude of Joule heating was assessed using IR microscopy at power dissipation levels similar to what others have reported during electroforming. Lines aligned along the [100] and [010] directions were observed spreading laterally around the locally heated area imaged by IR. Atomic force microscopy, transmission electron microscopy. and electron channeling contrast imaging suggest these lines are slip lines due to the plastic deformation induced by the local compressive stresses created by Joule heating. The deformation pattern is identical to that produced by nanoindentation. The implications of deformation for resistive switching systems are discussed. [ABSTRACT FROM AUTHOR]

Capacitance-voltage characteristics of high quality Pt Schottky diodes fabricated on oxygen-vacancy-doped SrTiO3 single crystals were used to obtain the oxygen vacancy profiles within one microns of the Pt interface. Computer simulations based on solving the drift-diffusion equations for electrons and ionized vacancies were performed to understand the experimentally observed oxygen vacancy profile's time-evolution at room temperature and 0 V applied bias. Building upon this understanding, the diode's room temperature profile evolution under -35 V applied bias was analyzed to yield a vacancy mobility value of 1.5 × 10-13 cm2/V·s at an electric field of 500 kV/cm. This mobility is 8 orders of magnitude too low to produce nanosecond resistance switching in thin film devices. The applicability of the results to oxygen-migration-based resistance switching is discussed relative to recent observations and modeling. [ABSTRACT FROM AUTHOR]