Institute of Semiconductor Physics

The ever increasing requirements for electrical performance of on-chip wiring has driven three major technological advances in recent years. First, copper has replaced Aluminum as the new interconnect metal of choice, forcing also the introduction of damascene processing. Second, alternatives for SiO2 with a lower dielectric constant are being developed and introduced in main stream processing. The many new resulting materials needs to be classified in terms of their materials characteristics, evaluated in terms of their properties, and tested for process compatibility. Third, in an attempt to lower the dielectric constant even more, porosity is being introduced into these new materials. The study of processes such as plasma interactions and swelling in liquid media now becomes critical. Furthermore, pore sealing and the deposition of a thin continuous copper diffusion barrier on a porous dielectric are of prime importance. This review is an attempt to give an overview of the classification, the characteristics and properties of low-k dielectrics. In addition it addresses some of the needs for improved metrology for determining pore sizes, size distributions, structure, and mechanical properties.

We show that ellipsometric porosimetry can be used for the measurement of the pore size distribution in thin porous films deposited on top of any smooth solid substrate. In this method, in situ ellipsometry is used to determine the amount of adsorptive, which is adsorbed/condensed in the film. Changes in refractive index and film thickness are used to calculate the quantity of adsorptive present in the film. Room temperature porosimetry based on adsorption of vapor of organic solvents has been developed. In this article, a method of calculation of pore size distribution and results of measurements on mesoporous and microporous xerogel films is discussed. Examination of the validity of the Gurvitsch rule for various organic adsorptives (toluene, heptane, and carbon tetrachloride) is carried out to assess the reliability of measurements of pore size distributions by ellipsometric porosimetry.

The structural properties of clinopyroxene NaScSi 2 O 6 have been investigated using the X-ray powder diffraction refinement, and the luminescence properties of Eu 2+ and Eu 2+ /Mn 2+ -activated NaScSi 2 O 6 have been studied to explore the new materials for phosphor-converted white light ultraviolet light-emitting diodes (UV-LEDs). Eu 2+ was introduced into the NaScSi 2 O 6 host in the reducing atmosphere, and the preferred crystallographic positions of the Eu 2+ ions were determined based on the different structural models of the NaScSi 2 O 6 host. The as-obtained NaScSi 2 O 6:Eu 2+ phosphor shows greenish yellow emission with the broad-band peak at 533 nm, and efficient energy transfer (ET) takes place between Eu 2+ and Mn 2+ in NaScSi 2 O 6, leading to a series of color-tunable phosphors emitting at 533 and 654 nm for the designed NaScSi 2 O 6:Eu 2+,Mn 2+ phosphors under excitation at 365 nm. The ET mechanism of Eu 2+ and Mn 2+ has also been evaluated. We have demonstrated that NaScSi 2 O 6:Eu 2+ and NaScSi 2 O 6:Eu 2+,Mn 2+ materials exhibit great potential to act as the effective phosphors for UV-LEDs.

Rates of depopulation by blackbody radiation (BBR) and effective lifetimes of alkali-metal $nS$, $nP$, and $nD$ Rydberg states have been calculated in a wide range of principal quantum numbers $n\ensuremath{\le}80$ at the ambient temperatures of 77, 300, and 600 K. Quasiclassical formulas were used to calculate the radial matrix elements of the dipole transitions from Rydberg states. Good agreement of our numerical results with the available theoretical and experimental data has been found. We have also obtained simple analytical formulas for estimates of effective lifetimes and BBR-induced depopulation rates, which well agree with the numerical data.

Atomic hydrogen, produced by thermal dissociation of H2 molecules inside a hot tungsten capillary, is shown to be an efficient tool for multiple recleaning of degraded surfaces of high quantum efficiency transmission-mode GaAs photocathodes within an ultrahigh vacuum (UHV) multichamber photoelectron gun. Ultraviolet quantum yield photoemission spectroscopy has been used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure. For photocathodes grown by the liquid-phase epitaxy technique, the quantum efficiency is found to be stable at about 20% over a large number of atomic hydrogen cleaning cycles. A slow degradation of the quantum efficiency is observed for photocathodes grown by metal-organic chemical vapor deposition, although they reached a higher initial quantum efficiency of about 30%–35%. Study of the spatial distributions of photoluminescence intensity on these photocathodes proved that this overall degradation is likely due to insertion of a dislocation network into the mechanically strained photocathode heterostructures during multiple heating cycles and is not due to the atomic hydrogen treatment itself.

Scanning tunneling spectroscopy at very low temperatures on homogeneously disordered superconducting titanium nitride thin films reveals strong spatial inhomogeneities of the superconducting gap Delta in the density of states. Upon increasing disorder, we observe suppression of the superconducting critical temperature Tc towards zero, enhancement of spatial fluctuations in Delta, and growth of the Delta/Tc ratio. These findings suggest that local superconductivity survives across the disorder-driven superconductor-insulator transition.

This paper reports the development of new phosphors using the chemical unit cosubstituting solid solution design strategy.

Tuning and optimizing luminescent properties of oxonitridosilicates phosphors are important for white light-emitting diode (WLED) applications. To improve the color rendering index, correlated color temperature and thermal stability of layer-structured M Si 2 O 2 N 2:Eu (M = Sr, Ba) phosphors, cation substitutions have been used to adjust their luminescent properties. However, the underlying mechanisms are still unclear. In this research, a series of (Sr 1– x Ba x )Si 2 O 2 N 2:Eu (0 ≤ x ≤ 1) compounds were prepared by solid-state reaction, after which systematic emission variations were investigated. The crystal structures of (Sr 1– x Ba x )Si 2 O 2 N 2:Eu (0 ≤ x ≤ 1) are nominally divided into three sections, namely, Phase 1 (0 ≤ x ≤ 0.65), Phase 2 (0.65 < x < 0.80), and Phase 3 (0.80 ≤ x ≤ 1) based on the X-ray diffraction measurements. These experimental results are further confirmed by optimizing the crystal structure data with first-principle calculations. Continuous luminescence adjustments from green to yellow are observed in Phase 1 with gradual replacement of Sr 2+ with Ba 2+, and the abnormal redshift is clarified through extended X-ray absorption fine structure analysis. Sr(Eu)–O/N bond length shrinkage in local structure causes the redshift emission, and the corresponding luminescence mechanism is proposed. Controllable luminescence in Phase 2 (from blue to white) and Phase 3 (from cyan to yellowish green) are observed. Based on the high-resolution transmission electron microscopy and selected area electron diffraction analysis, the two kinds of luminescence tuning are attributed to phase segregation. This study serves as a guide in developing oxonitride luminescent materials with controllable optical properties based on variations in local coordination environments through cation substitutions.

New compound discovery is of interest in the field of inorganic solid-state chemistry. In this work, a whitlockite-type structure Sr1.75Ca1.25(PO4)2 newly found by composition design in the Sr3(PO4)2-Ca3(PO4)2 join was reported. Crystal structure and luminescence properties of Sr1.75Ca1.25(PO4)2:Eu(2+) were investigated, and the yellow-emitting phosphor was further employed in fabricating near-ultraviolet-pumped white light-emitting diodes (w-LEDs). The structure and crystallographic site occupancy of Eu(2+) in the host were identified via X-ray powder diffraction refinement using Rietveld method. The Sr1.75Ca1.25(PO4)2:Eu(2+) phosphors absorb in the UV-vis spectral region of 250-430 nm and exhibit an intense asymmetric broadband emission peaking at 518 nm under λex = 365 nm which is ascribed to the 5d-4f allowed transition of Eu(2+). The luminescence properties and mechanism are also investigated as a function of Eu(2+) concentration. A white LED device which is obtained by combining a 370 nm UV chip with commercial blue phosphor and the present yellow phosphor has been fabricated and exhibit good application properties.

Charged dust particulates, forming a layered crystal in the electrode sheath of a rf discharge, are known to show vertical alignment and an onset of characteristic oscillations below a threshold of neutral gas density. Here forces on the particulates due to the formation of positive space-charge clouds below the dust particles are calculated from Monte Carlo calculations of the ion motion in the sheath. The forces are shown to be attractive and nonreciprocal for the different crystal layers. From the Monte Carlo results an analytical lattice model is derived that quantitatively explains the experimental findings. \textcopyright{} 1996 The American Physical Society.

Magnetic and electric properties of some single crystals of complex composition of perovskite-type structure were investigated and coexistence of ferroelectric and antiferromagnetic properties was shown in them. It was found that single crystals YMnO3 and YbMnO3 of the new class of ferroelectrics, discovered by Bertaut and others, are also antiferromagnetics. The results of the investigation of solid solutions based on BiFeO3, which has a perovskite-type structure, are discussed. The opinion is expressed that BiFeO3 is probably not a ferroelectric. The main results concerning the thermodynamic theory of substances which are ferroelectric and ferromagnetic simultaneously are given in this report.

Scheelite related alkali-metal rare-earth double molybdate compounds with a general formula of ALn(MoO4)2 can find wide application as red phosphors. The crystal chemistry and luminescence properties of red-emitting CsGd1-xEux(MoO4)2 solid-solution phosphors have been evaluated in the present paper. A detailed analysis of the structural properties indicates the formation of isostructural scheelite-type CsGd1-xEux(MoO4)2 solid-solutions over the composition range of 0 ≤ x ≤ 1. The photoluminescence emission (PL) and excitation (PLE) spectra, and the decay curves were measured for this series of compounds. The critical doping concentration of Eu(3+) is determined to be x = 0.6 in order to realize the maximum emission intensity. The emission spectra of the as-obtained CsGd(1-x)Eux(MoO4)2 phosphors show narrow high intensity red lines at 592 and 615 nm upon excitation at 394 or 465 nm, revealing great potential for applications in white light-emitting diode devices.

While silicon and gallium arsenide are dominant materials in modern micro- and nanoelectronics, devices fabricated from them still use Si and GaAs substrates only separately. Integrating these materials on the (highest effeciency) substrate of Si has been the subject of much research effort for more than twenty years. This review systematizes and generalizes the current understanding of the fundamental physical mechanisms governing the epitaxial growth of GaAs and its related III–V compounds on Si substrates. Basic techniques avilable for improving the quality of such heterostructures are described, and recent advances in fabricating device-quality AIIIBV/Si heterostructures and devices on their bases are also presented.

The microcrystals of monoclinic europium molybdate, α-Eu 2 (MoO 4 ) 3, have been fabricated by solid-state synthesis at T = 753–1273 K for 300 h. The crystal structure of α-Eu 2 (MoO 4 ) 3 has been refined by the Rietveld method and was found to belong to the space group C 2/ c with unit cell parameters a = 7.5576(1), b = 11.4709(2), c = 11.5158(2) Å, and β = 109.278(1)° ( R B = 3.39%). About 40 narrow Raman lines have been observed in the Raman spectrum of the α-Eu 2 (MoO 4 ) 3 powder sample. The luminescence spectra of α-Eu 2 (MoO 4 ) 3 under excitation at 355 and 457.9 nm reveal domination of induced electric dipole transition 5 D 0 → 7 F 2 and the presence of ultranarrow lines at 5 D 0 → 7 F 0 and 5 D 1 → 7 F 0 transitions.

The Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 phosphors were synthesized by solid state reaction at temperature 1623 K and pressure 1.5 × 10(7) Pa in (95% N2 + 5% H2) atmosphere. Under the conditions, the compounds crystallize in the form of isolated euhedral partly faceted microcrystals ∼19 μm in size. The crystal structures of the Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 garnets have been obtained by Rietveld analysis. The photoluminescence (PL) and X-ray excited luminescence (XL) spectra obtained at room temperature indicate broad asymmetric bands with maxima near 519 and 540 nm for Y2.99Ce0.01Al5O12 and Lu2.98Ce0.01Y0.01Al5O12, respectively. The light source was fabricated using the powder Lu2.98Ce0.01Y0.01Al5O12 phosphor and commercial blue-emitting n-UV LED chips (λ(ex) = 450 nm). It is found that the CIE chromaticity coordinates are (x = 0.388, y = 0.563) with the warm white light emission correlated color temperature (CCT) of 6400 K and good luminous efficiency of 110 lm/W.

Clinopyroxenes along the Jervisite (NaScSi2O6)-Diopside (CaMgSi2O6) join have been studied, and a solid-solution of the type (Na(1-x)Ca(x))(Sc(1-x)Mg(x))Si2O6 has been identified in the full range of 0 ≤ x ≤ 1. The powder X-ray patterns of all the phases indicate a structural similarity to the end compounds and show smooth variation of structural parameters with composition. The linear structural evolution of iso-structural (Na(1-x)Ca(x))(Sc(1-x)Mg(x))Si2O6 solid-solutions obeying Vegard's rule has also been examined and verified by high resolution transmission electron microscopy (HRTEM). The continuous solid-solutions show the same structural type, therefore the photoluminescence spectra of Eu(2+) doped samples possess the superposition of spectral features from blue-emitting component (CaMgSi2O6:Eu(2+)) and yellow-emitting one (NaScSi2O6:Eu(2+)). This indicates that the spectroscopic properties of (Na(1-x)Ca(x))(Sc(1-x)Mg(x))Si2O6 clinopyroxene solid-solutions are in direct relations with structural parameters, and it is helpful for designing color-tunable photoluminescence with predetermined parameters.

Abstract Feasibility of many emergent phenomena that intrinsic magnetic topological insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi 2 Te 4 )(Bi 2 Te 3 ) m with m = 0, ⋯, 6. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi 2 Te 4 ( m = 0) to MnBi 4 Te 7 ( m = 1) and MnBi 6 Te 10 ( m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for ( m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, respectively, realized in the m = 0, 1, 2 and m = 3 cases. For large m numbers a hitherto-unknown topologically nontrivial phase can be created, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi 2 Te 4 building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi 2 Te 4 )(Bi 2 Te 3 ) m allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

The cation substitution-dependent phase transition was used as a strategy to discover new solid solution phosphors and to efficiently tune the luminescence property of divalent europium (Eu 2+ ) in the M 3 (PO 4 ) 2:Eu 2+ (M = Ca/Sr/Ba) quasi-binary sets. Several new phosphors including the greenish-white SrCa 2 (PO 4 ) 2:Eu 2+, the yellow Sr 2 Ca(PO 4 ) 2:Eu 2+, and the cyan Ba 2 Ca(PO 4 ) 2:Eu 2+ were reported, and the drastic red shift of the emission toward the phase transition point was discussed. Different behavior of luminescence evolution in response to structural variation was verified among the three M 3 (PO 4 ) 2:Eu 2+ joins. Sr 3 (PO 4 ) 2 and Ba 3 (PO 4 ) 2 form a continuous isostructural solid solution set in which Eu 2+ exhibits a similar symmetric narrow-band blue emission centered at 416 nm, whereas Sr 2+ substituting Ca 2+ in Ca 3 (PO 4 ) 2 induces a composition-dependent phase transition and the peaking emission gets red shifted to 527 nm approaching the phase transition point. In the Ca 3– x Ba x (PO 4 ) 2:Eu 2+ set, the validity of crystallochemical design of phosphor between the phase transition boundary was further verified. This cation substitution strategy may assist in developing new phosphors with controllably tuned optical properties based on the phase transition.

The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between ``bright'' exciton ground states and novel ``dark'' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.

We investigate low-temperature transport properties of thin TiN superconducting films in the vicinity of the disorder-driven superconductor-insulator transition. In a zero magnetic field, we find an extremely sharp separation between superconducting and insulating phases, evidencing a direct superconductor-insulator transition without an intermediate metallic phase. At moderate temperatures, in the insulating films we reveal thermally activated conductivity with the magnetic field-dependent activation energy. At very low temperatures, we observe a zero-conductivity state, which is destroyed at some depinning threshold voltage V{T}. These findings indicate the formation of a distinct collective state of the localized Cooper pairs in the critical region at both sides of the transition.