Elevated Sample Temperatures Research Articles

Single entangled photon pair emission from an InGaAs/GaAs quantum dot up to temperatures of 30 K

AbstractPhoton pairs emitted from the biexciton‐exciton cascade of a single self‐assembled InGaAs/GaAs quantum dot can be polarization‐entangled. The degree of entanglement is reduced by the fine structure splitting of the involved exciton level. We measure this splitting in high resolution using a scanning Fabry‐Pérot interferometer. By quantum state tomographywe can determine the twophoton polarization state.We are able to verify the conditions of entangled or classically correlated photon pairs in full consistence with the observed fine structure splittings. The entanglement remains relatively unaffected for elevated sample temperatures where we observe reliable entanglement up to 30 K. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The formation of the thin-film solar cell absorber CuInS 2 by annealing of Cu–In–S stacked elemental layer precursors — A comparison of selenisation and sulfurisation

We present results of in-situ X-ray diffraction experiments on the formation of CuInS 2 thin film solar cell absorbers. The experiments have been performed while annealing Cu–In–S stacked elemental layer precursors produced by sputtering and thermal evaporation to investigate the crystallisation process of the chalcopyrite CuInS 2. Rietveld refinement has been performed to obtain the quantitative phase evolution of crystalline phases while annealing. The annealing process is characterised by a rapid sulfurisation of the initially present intermetallic alloy Cu 11In 9 forming the sulfide phases CuS, Dg–Cu 2 − x S, InS and/or CuIn 5S 8. The chalcopyrite CuInS 2 crystallises at elevated sample temperatures by the consumption of these sulfide phases as educts. Three different chalcopyrite formation reactions have been identified by an analysis of the quantitative phase evolution. A comparison to earlier investigations on the formation of CuInSe 2 from Cu–In–Se precursors is presented to show similarities and differences of sulfurisation and selenisation processes. The chalcopyrite forms from chalcogenide educts in both cases. However, distinct differences concerning the chalcogenisation kinetics of sulfur and selenium containing Cu–In precursors have been revealed. The chalcogenisation of the intermetallic alloy phase Cu 11In 9 proceeds extremely fast for Cu–In–S precursors as compared to Cu–In–Se samples.

Dynamic behaviour of a nanoscale-patterned phospholipid thin film on mica and silicon studied by atomic force microscopy

The temporal evolution of the morphology of nanoscale-patterned phospholipid thin films on mica and silicon surfaces has been investigated with an atomic force microscope (AFM). The AFM images reveal that nanoscale scratch lines on thin films prepared on mica contract with time and eventually form roundish holes. An elevated sample temperature accelerates this morphological evolution. We model such an evolution based on the interplay of the thin-film surface line tension and the friction between the thin film and the substrate. The results show that the temperature-dependent contraction is governed by the ratio of the friction coefficient and the surface line tension. The friction at the lipid/mica interface decreases to a seventh as the sample temperature rises from 18 to 60 °C. This model is supported by experiments on silicon surfaces, on which contraction of the scratch patterns is limited because of an expected greater interfacial friction.

Annealing Effect on Characteristics of p<sup>+</sup>n 4H-SiC Diode Formed by Al Ion Implantation

The I-V characteristics of p+n 4H-SiC diode formed by Al ion implantation have been investigated as a function of annealing temperature. Al ions are implanted at the elevated sample temperature of 500 oC in order to fabricate p-type doped layer on the n-type epitaxial layer, grown on n+ 4H-SiC substrate. The implanted sample is annealed using electron bombardment annealing system in the annealing temperature ranging from 1700 to 1900 oC. The Al implanted sample, annealed below 1800 oC shows the deteriorated I-V characteristics in which the forward current includes the resistive current components and the reverse current is in the order of 10-4 A/cm2. The p+n diode formed by annealing at 1900 oC reveals the forward current without extra-current components and the reverse current as low as 10-6 A/cm2. It is suggested that the annealing above 1900 oC is effective in reducing the implantation-induced defect at the interface between Al implanted p+ layer and the underlying n-type epitaxial layer.

Oxidation of Pd(553): From ultrahigh vacuum to atmospheric pressure

The oxidation of a vicinal Pd(553) surface has been studied from ultrahigh vacuum (UHV) to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below ${10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$ and that the (553) surface is preserved. In the pressure range between ${10}^{\ensuremath{-}6}$ and $1\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$ and at a sample temperature of $300\char21{}400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, a surface oxide forms and rearranges the (553) surface facets and forming (332) facets. Most of the surface oxide can be described as a PdO(101) plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the $(\sqrt{6}\ifmmode\times\else\texttimes\fi{}\sqrt{6})$ ${\mathrm{Pd}}_{5}{\mathrm{O}}_{4}$ surface oxide can be observed on (111) terraces larger than those of the (332) terraces. Increasing the O pressure above $1\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$ results in the disappearance of the (332) facets and the formation of PdO bulk oxide.

Thermal Stability of Deuterated C60 Films

The thermal stability and electronic structure of deuterated C60 films were studied by temperature-resolved mass spectroscopy and ultraviolet photoionization/photoelectron spectroscopy, UPS. The films were prepared by exposing C60 layers predeposited onto HOPG to a flux of thermal energy atomic deuterium. The initial stage of on-top deuteration at room temperature is associated with the formation of a capping layer that exhibits a higher thermal stability than pristine C60 films. The sublimation heat of the capping layer increases with the deuterium dose applied and levels off at a value of 2.15 ± 0.05 eV/cage at deuterium exposures in the range of 1016 D/cm-2. Capping layers appear to kinetically hinder further deuteration of subsurface C60. This hindrance can be reduced by performing the initial (and subsequent) deuteration at elevated sample temperatures. Further deuterium exposure leads to the formation of higher deuterofullerenes (e.g., C60D36). These exhibit sublimation enthalpies of 1.39 ± 0.1 eV/cage, considerably lower than the capping layers but comparable to pristine C60 films. Capping layers are air-resistant and amenable to the fabrication of stable multilayered [C60/C60Dx]m structures.

Effect of an Electric Field on the Carrier Collection Efficiency of InAs Quantum Dots

Individual and multiquantum dots of InAs are studied by means of microphotoluminescence in the case where, in addition to the principal laser exciting photoluminescence, second infrared laser is used. It is demonstrated that the absorption of the infrared photons effectively creates free holes in the sample, which leads to both a change in the charge state of a quantum dot and to a considerable reduction of their photoluminescence signal. The latter effect is explained in terms of effective screening of the internal electric field, facilitating carrier transport along the plane of a wetting layer, by the surplus holes from the infrared laser. It is shown that the effect of quenching of quantum dot photoluminescence gradually disappears at increased sample temperature (T) and/or dot density. This fact is due to the essentially increased value of quantum dot collection efficiency, which could be achieved at elevated sample temperatures for individual quantum dots or even at low T for the case of multiquantum dots. It is suggested that the observed phenomena can be widely used in practice to effectively manipulate the collection efficiency and the charge state of quantum-dot-based optical devices. © 2005 Pleiades Publishing, Inc.

Vanadium surface oxides on Pd(111): A structural analysis

Scanning tunneling microscopy studies of vanadium oxides grown on Pd(111) show interesting structures especially in the low-coverage region. Evaporation of V in an oxygen background at elevated sample temperature $(250\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ results in the formation of a nonperiodic honeycomb-like structure growing from the steps, which starts to transform into an ordered phase at a vanadium coverage of $\ensuremath{\approx}0.2\mathrm{ML}$ (monolayer). At 0.31 ML the entire surface is covered by this well-ordered open $(4\ifmmode\times\else\texttimes\fi{}4)$ structure. Annealing this structure in ${\mathrm{H}}_{2}$ atmosphere transforms the phase into a ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ surface oxide with $(2\ifmmode\times\else\texttimes\fi{}2)$ periodicity, whose optimal coverage is reached at 0.5 ML vanadium. Models for both ordered structures have been suggested before on the basis of ab initio density-functional theory (DFT) calculations and molecular-dynamics simulations and these models are now unambiguously confirmed by quantitative low-energy electron-diffraction (LEED) analyses. In the $(4\ifmmode\times\else\texttimes\fi{}4)$ phase, the V atoms are surrounded by four oxygen atoms in an unusual tetrahedral coordination leading to a ${\mathrm{V}}_{5}{\mathrm{O}}_{14}$ stoichiometry. This tetrahedral coordination allows the oxide to adopt open loosely packed two-dimensional (2D) and 1D structures, which are stabilized by the surface-oxide interface energy. Furthermore, it is shown that state of the art DFT calculations can indeed predict complex structures exactly as well as that modern quantitative LEED is capable of dealing with very large unit cells.

Thermal oxidation of (0001) 4H-SiC at high temperatures in ozone-admixed oxygen gas ambient

The method of oxidation by atomic oxygen has been developed for gate oxide formation in SiC metal–oxide–semiconductor (MOS) devices. Ozone (O3)–admixed oxygen (O2) gas is introduced into the cold-wall oxidation furnace, where atomic oxygen in a ground state is formed by thermal decomposition of O3 molecules at elevated sample temperatures. The growth rate of oxide in the O3-admixed gas shows a maximum at around 666.4 Pa and 950–1200 °C, whereas the rate in pure O2 gas is negligible below 6664.5 Pa. Interface trap density (Dit) of the MOS capacitors fabricated using atomic oxygen strongly depends on the oxidization temperature; oxidation at 1200 °C results in significant reduction of Dit in comparison with that at 950 °C.

Morphology of GaN(0001) and GaN(0001̄) surfaces: Persistence of surface clusters

We report a scanning tunneling microscopy and low energy electron diffraction study of GaN(0001) and GaN(0001̄) surfaces. Surface preparation methods included anneal, N2+ sputter/anneal, and chemical etching with Br2. The onset of surface facet formation by thermal anneal is 1050 K, determined by electron diffraction, whereas the onset of facet formation by Br2 etching at elevated sample temperatures is 765 K. For all surfaces studied, facetted and unfacetted, nanosize clusters are observed in scanning tunneling microscopy images regardless of treatment. The presence and persistence of these clusters is discussed, as well as their potential impact on studies of GaN surface chemistry and surface electronic structure.

Self-organized nanodot formation on MgO(100) by ion bombardment at high temperatures

The production of self-organized, spatially dense nanodots on a MgO(100) surface by Ar+ bombardment at elevated sample temperatures (700 and 1000 °C) and ion energies in the range 0.5–5 keV is reported. The results show that at elevated temperatures, the size of the nanodots can be modified by varying the ion energy in a well-defined manner. The nanodot diameter is found to increase with increasing temperature and decrease with increasing ion flux and increasing ion energy. The dot diameter is several tens of nanometers and the height is several nanometers. These results can be interpreted in terms of a recently developed theoretical model.

Influence of boron concentration on the enhanced diffusion observed after irradiation of boron delta-doped silicon at 570°C

A sequence of three boron spikes, grown in silicon by molecular beam epitaxy (MBE) and characterized by secondary-ion mass spectrometry, has been employed to investigate the mechanisms of boron diffusion during irradiation with 2.5 MeV protons at an elevated sample temperature of 570°C. The energy of the proton beam has been chosen so that the generation rate of point defects can be considered as uniform throughout the boron delta-doped silicon. The influence of the B concentration (ranging from 5×10 15 to 3×10 18 cm −3 in different samples) on the enhanced diffusion of boron have been studied in detail. For each concentration the B diffusion coefficient is increased by a factor greater than 5×10 4 under irradiation as compared to the B diffusion coefficient at 570°C in unirradiated areas. In the first approximation this enhancement is independent on the depth position of the boron spike but depends on the boron concentration. The diffusion of boron under irradiation exhibits an exponential behavior. This kind of diffusion has already been reported in the literature for boron diffusion in silicon during oxidation experiments at 600°C. The direct comparison with oxidation experiments where Si self-interstitials are injected in a non-uniform way provides further information on the basics of the boron diffusion mechanisms in silicon.

In Situ X-ray Absorption Spectroscopy in the Soft Energy Range: Novel Prospects for the Chemical Characterization of Solid State Surfaces at High Pressure And High Temperature

AbstractAn instrument equipped with total electron yield detectors was designed and constructed for in situ X-ray absorption spectroscopy (XAS) investigations in the soft X-ray range (100 eV ≤ hν ≤ 1000 eV) at elevated pressures (mbar range) and sample temperatures (T ≤ 1000 K) [1]. This allows, for the first time, XAS studies in a surface-sensitive mode of the light elements (Z = 3-15). Furthermore, the gas phase XAS and the surface-related XAS of the solid state phase can be collected simultaneously in order to correlate the gas/solid reaction rate with the surface electronic structure under working conditions in a flow-through mode.The novel experimental tool represents a contribution to the experimental overcoming of the “pressure gap” in material science. In this work examples are presented belonging to the field of heterogeneous catalysis [2-4] and to the reactivity of diamond surfaces [5]. Additionally, prospects for in situ studies in material science will be given.

The interface formation of PTCDA on Se-modified GaAs(100) surfaces

The interface formation between the molecular beam deposited molecular semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), and Se-treated GaAs(100)-(2×1) surface was investigated by soft X-ray photoemission spectroscopy (SXPS) using synchrotron radiation. For room temperature growth, no chemical bond is observed. At elevated sample temperature up to 350°C after the growth, all PTCDA except one monolayer desorbs from the surface. A loss of Se dimer atoms on top of the surface, which is in accordance with the number of anhydride oxygen atoms of PTCDA in a unit cell, indicates the reaction with PTCDA molecules upon annealing. The growth mode and the band banding upon PTCDA deposition on Se-treated GaAs(100) are also discussed.

Transfer by direct photo etching of poly(vinylidene flouride) using X-rays

A direct pattern transfer method has been developed by photo etching poly(vinylidene fluoride) (PVDF) using X rays (1-16 keV) from a synchrotron storage ring. The ability to pattern thin film of PVDF, a piezoelectric, pyroelectric and ferroelectric polymer, has potential applications in the areas of MEMS, nonlinear optics, and nonvolatile ferroelectric random access memory technology. Without the use of any reactive chemical gas, a maximum etched depth in excess of 9 /spl mu/m is achieved. The etched depth for a given photon energy approaches saturation with respect to exposure time. An in situ mass spectrometry revealed the evolution of hydrogen, fluorine, and hydrogen fluoride species. The etched regions turned dark in color indicating a possible increase in the fraction of carbon atoms. The X-ray transmittance of photo etched PVDF approached that of a pure carbon as the exposure time is increased. Upon etching the root mean-square surface roughness of the etched portion increased by more than a factor of two. The rate of etching increased at elevated sample temperatures.

High Density Plasma Etching of Ta2O5-Selectivity to Si and Effect of UV Light Enhancement

AbstractEtch rates up to 1200 Åmin−1 for Ta2O5 were achieved in both SF6/Ar and Cl2/Ar discharges under Inductively Coupled Plasma conditions. The etch rates with N2/Ar or CH4/H2/Ar chemistries were an order of magnitude lower. There was no effect of post deposition annealing on the Ta2O5 etch rates, at least up to 800 °C. Selectivities to Si of ∼1 were achieved at low source powers, but at higher powers the Si typically etched 4-7 times faster than Ta 20 5. UV illumination during ICP etching in both SF6/Ar and Cl2/Ar produced significant enhancements (up to a factor of 2) in etch rates due to photo-assisted desorption of the TaFx products. The UV illumination is an alternative to employing elevated sample temperatures during etching to increase the volatility of the etch products and may find application where the thermal budget should be minimized during processing.

Thermal annealing of deep ultraviolet (193 nm) induced compaction in fused silica

Different 1995–1996 ultraviolet (UV) grade experimental fused silica samples were evaluated for their resistance to UV-induced compaction at 193 nm under elevated sample temperature conditions. We found that the compaction rate decreases with increasing sample temperature. Compaction recoveries were observed at temperatures as low as 120 °C, this suggests that the recovery of UV-induced compaction in fused silica is a thermally excited process with a low activation energy. Isothermal annealing experiments were performed on two different sets of precompacted fused silica samples. An activation energy of 0.13 eV was found for both cases.

New experimental technique: X-ray absorption spectroscopy detector for in situ studies in the soft X-ray range (250 eV ⩽ hv ⩽ 1000 eV) under reaction conditions

A novel tank reactor equipped with t otal electron yield (TEY) detectors was designed and constructed for in situ X-ray absorption spectroscopy (XAS) investigations in the soft X-ray range (250 eV ⩽ by ⩽ 1000 eV) under elevated pressures (up to 20 mbar) and sample temperatures (up to 1000 K). This allows, for the first time, XAS studies in the TEY mode at the K-absorption edges of the low atomic number elements like carbon, nitrogen and oxygen with high surface sensitivity. Furthermore, the gas-phase XAS and the surface-related XAS of the catalyst can be collected simultaneously in order to correlate the gas/solid reaction rate with the electronic structure of the sample surface and near-surface region. This work presents the experimental technique and the oxygen K- and copper L 2,3 X-ray absorption near-edge structures (XANES) of Cu(poly) obtained under various gas environments and at reaction temperature.

SIMS analysis of epitaxial layers for power- and micro-electronics

This paper gives an overview of rencent secondary ion mass spectrometry (SIMS) studies of impurities and dopants in epitaxial layers of silicon and 6H silicon carbide (SiC). Detection limits in the 10 12 cm −3 range are demonstrated for transition metal impurities like Ti in SiC. Hydrogen is found to be mobile in SiC at temperatures in excess of 600°C despite strong trapping by defects and dopant atoms, and the effective diffusion coefficient exhibits an activation energy of ∼ 3.5 eV. In epitaxially grown Si layers, containing Ge delta distributions, profile broadening and shift during sputtering by Ar + ions are accurately described by recoil mixing. For O 2 + ions oxide formation and surface swelling must also be considered. Further, at elevated sample temperature Ge is found to segregate out of the SiO 2 surface layer formed during oxygen bombardment, consistent with a larger heat of oxide formation for Ge than Si and a high enough mobility in SiO 2.

Formation of dry etched gratings in GaN and InGaN

Sub-micron periodic gratings with pitch ~3,000A were formed in GaN and InGaN using holographic lithography and room temperature electron cyclotron resonance (ECR) BClg3/N2 dry etching at moderate microwave (500 W) and radio frequency (100 W) powers. The process produces uniform gratings without the need for elevated sample temperatures during the etch step.