bluuebunny/crossref_metadata_2025 huggingface dataset:
Native Oxide and the Residual Stress of Thin Mo and Ta Films
Authors: Parfitt L. J. , Rek Z. U. , Yalisove S. M. , Bilello J. C.
Published: April 2011

Residual stress changed substantially between 2.5 and 80 nm film thickness in very thin Mo and Ta coatings sputtered onto Si substrates with native oxide. For both Mo and Ta, the thinnest films had a high compressive stress on the order of 2 to 3 GPa, and the stress relaxed, or became slightly tensile, with increasing film thickness. The coatings were examined using a variety of advanced characterization techniques including Auger Electron Spectroscopy (AES), Grazing Incidence X-ray Scattering (GIXS), and High Resolution Transmission Electron Microscopy (HRTEM). AES showed the presence of 0 and C contamination at the interface between the substrate and the film; these impurities originated from the adsorbed species and the native oxide on the surface of the wafers. GIXS analysis of the Mo films showed that the lattice of the thinnest layers was considerably expanded compared to the interplanar spacing of bulk, pure Mo. This expansion was caused by incorporation of impurities from the substrate into interstitial sites, which caused the very high compressive stress. HRTEM showed that the sputter deposition process did not alter the thickness of the native oxide layer, suggesting that the Mo and Ta films interacted with only the adsorbed impurities and the top one or two layers of the SiO2. Mo films deposited onto clean W layers were tensile, which supported the hypothesis that impurities caused the high compressive stresses in the films deposited onto SiO2. Grain growth and phase transformations contributed to the relaxation in stress observed in the thicker films.
Real-time Characterization of the Optical Properties of an ultra-thin Surface Reaction Layer during Growth
Authors: Dietz N. , Sukidib N. , Harrisb C. , Bachmann K. J.
Published: April 2011

The average optical properties of an ultra-thin surface reaction layer (SRL) during growth by pulsed chemical beam epitaxy (PCBE) can be quantitatively accessed by p-polarized reflectance spectroscopy (PRS), as demonstrated on the example of heteroepitaxial GaP growth. Under PCBE growth conditions, the surface of the substrate is exposed to pulsed ballistic beams of tertiarybutyl phosphine [TBP, (C4H9)PH2] and triethylgallium [TEG, Ga(C2H5)3]. The pulsed precursor supply causes a period in composition and thickness modulated SRL, monitored as a fine structure that is superimposed on interference oscillations, resulting from back reflection at the substratelayer interface with increasing layer thickness. The amplitude of this fine structure undergoes a period amplitude modulation and exhibits turning points at which the response to the first precursor pulse changes sign. The turning points can be characterized by the expression R4(Φmax)=R4(Φmin), which describes the maximal and minimal values of the temporally modulated phase factor in the SRL, using a four layer stack description. The positions of these turning points are not affected by the thickness of the SRL, which allows the computation of the average complex dielectric function of the SRL independent of its thickness. In the next step, the average thickness of the SRL can be extracted from the amplitude of the observed fine structure.
Stress-Driven Wrinkling oF α-Alumina Films Grown oN Fe-Cr-Al by Thermal Oxidation
Authors: Tolpygo Vladimir K. , Clarke David R.
Published: April 2011

A thin α-Al2O3 film forms on Fe-20w/oCr-5w/oAl alloys when they are heated in air to about 1000°C. This thermally grown oxide exhibits a wrinkled morphology within minutes and with an amplitude and characteristic wavelength that increases with oxidation time. Measurements of the residual strain in the oxide, using Cr3+ photoluminescence piezospectroscopy, indicate that the wrinkling is associated with stress relaxation during the course of the thickening of the oxide. AFM and piezospectroscopic measurements reveal a strong time dependence of the growth stress and oxide wrinkling, as well as a marked dependence on the crystallographic orientation of the underlying Fe-Cr-Al substrate. Small additions (about 0.3 w/o) of yttrium to the metal completely suppress the wrinkling phenomenon although it is found not to lessen the growth stresses in the oxide film.
Stress and its Effect on Surface Morphology in Multi-Layer Ti-Cu Films
Authors: Zhou Ping , Buchman Saps , Gray Chris , Turneaure John , Turlo James E.
Published: April 2011

We have characterized the stress and its effects on surface morphology for multi-layer thin films of about 3μm total thickness, consisting of three or seven layers of Ti and Cu. These films constitute the electrostatic suspension electrodes for the gyroscope housings of the Relativity Mission Gravity Probe B. Full understanding of surface morphology is critical for meeting the complex requirements of this application.

The residual stresses have been measured using a laser curvature technique, while the surface morphology was studied by scanning electron microscopy (SEM). We find that the surface morphology depends strongly on the stress, which evolves with the Ti-Cu multi-layer period. Average stress and the resulting surface roughness decrease for thinner Cu layers (increased total number of layers). Seven layer Ti-Cu films with low stress and very smooth surface have been successfully used for the electrodes of the gyroscope housings.
Slow Decay of Reflection High Energy Electron Diffraction Oscillations in Cal1-xMgxF2
Authors: Kushibe Mitsuhiro , Shusterman Yuriy V. , Yakovlev Nikolai L. , Schowalter Leo J.
Published: April 2011

Magnesium is incorporated into the growth of Ca1-xMgxF2 to reduce the lattice constant of fluorite (CaF2) which is 0.6% larger than that of Si at room temperature. When grown epitaxially on Si(111) substrates at 300°C, the lattice constant of the alloy became smaller than that of Si by 1.5% when the Mg concentration was around 20%. At higher Mg concentrations, the lattice constant did not decrease any further. This invariability of the lattice constant was caused by a phase separation of the Ca1-xMgxF2 layer into a Mg-rich region and a Mg-deficient region. When the growth temperature was increased, the critical Mg concentration for the phase separation became smaller. When Ca1-xMgxF2 was grown on vicinal Si(111) substrates, the reflection high energy electron diffraction (RHEED) intensity oscillations reflected no change in the composition, suggesting segregation of a Mg-rich phase along the steps. Nevertheless, the oscillations in the intensity of the specular spot for Ca1-xMgxF2 lasted longer than those observed for pure CaF2, suggesting a flatter surface for the alloy. Scanning tunneling microscopy (STM) observations support this model.
Structural Studies Of Epitaxial CdF2 Layers on Si(111)
Authors: Khilko A. Yu. , Kyutt R. N. , Mosina G. N. , Sokolov N. S. , Shusterman Yu. V. , Schowalter L. J.
Published: April 2011

Epitaxial CdF2 layers, which may be used in light-emitting devices integrated with silicon, were grown by Molecular Beam Epitaxy (MBE). Characterization of the layers by Rutherford Backscattering Spectroscopy (RBS), X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) showed that optimal growth temperature lies in the range 60–80°C. The sticking coefficient of CdF2 molecules was found to decrease at temperatures above 100°C. Different modes of misfit strain relaxation were observed above and below that temperature.
Atomic Force Microscopy Study of Hard Coating Films Prepared by Pulsed Laser Deposition Method
Authors: Chan Hsieh-Li , Kumar Ashok , Sanderson L. , Weimer J. J.
Published: April 2011

Aluminum nitride (AIN), silicon nitride (Si3N4), and silicon carbide (SIC) thin films have been snthesized using pulsed laser deposition (PLD) techniques. AIN and Si3N4 films were deposited on Si (100) substrates at different temperatures (room temperature to 600°C) and partial pressures (15 mTorr N2 to 200 mTorr N2). SiC films were deposited on Si (100) substrates at different temperatures (room temperature to 400°C) in high vacuum. The atomic force microscopy (AFM) is a useful tool for studying the surface topography of these technologically interesting thin film surfaces. This paper discusses in detail AFM analysis of thin film coatings.
Observation of Rheed Intensity Oscillations During PbSe/CaF2/Si(111) MBE
Authors: Liu W. K. , Fang X. M. , McCann P. J. , Santos M. B.
Published: April 2011

RHEED intensity oscillations observed during MBE growth of CaF2 on Si(111) and PbSe on CaF2/Si(111) are presented. The effects of substrate temperature and initial nucleation procedure are investigated. Strong temporal oscillations of the specular beam intensity are found to be most readily observed at temperatures below 200°C for both CaF2 and PbSe. Growth rates measured as a function of cell temperatures exhibit Arrhenius behavior with activation energies of 5.0 eV and 1.93 eV for CaF2 and PbSe, respectively. The relatively high activation energy obtained for CaF2 is consistent with the high melting point and sublimation energy of ionic fluorides.
Pulsed Laser Deposition of Barium Zirconate Thin Films for Neutral Imaging Applications
Authors: Leuchtner R. , Yanochko R. , Krzanowski J. , Brock W. , Quinn J.
Published: April 2011

Low work function surfaces offer great promise as thermionic converters and neutral conversion surfaces due to the relative ease with which electrons may be removed. BaZrO3 surfaces were prepared using pulsed laser deposition (PLD) and their materials properties evaluated for potential use as a neutral conversion surface for space applications. Single layer films were fabricated at temperatures ranging from 110°C to 600°C, and as a function of background ambient pressure. The degree of crystallinity of the resulting films was measured using x-ray diffraction (XRD) and the surface morphology evaluated with atomic force microscopy (AFM). The film quality was found to be directly related to substrate temperature: an increase in grain size from ˜ 10 nm to >50 nm and improved crystal orientation with respect to the substrate were observed as the deposition temperature was increased from 110°C to 600°C.
In-Situ Optical Diagnostics During Pulsed-Laser Deposition of Magnetoresistive La-Ca-Mn-O Films on Silicon Substrates
Authors: Kung P.-J. , Cosgrove J. E. , Kinsella K. , Hamblen D. G.
Published: April 2011

During pulsed-laser deposition of La0.67Ca0.33MnO3 films on silicon substrates, a system that consists of visible optical-emission spectroscopy (OES) and Fourier transform infrared (FT-IR) spectroscopy is employed to perform in-situ diagnosis of the laser-induced plume and to monitor the substrate temperature and the film thickness. The effects of oxygen pressure, laser fluence, and distance from the target surface on emission spectra were studied. In FT-IR measurements, the slopes of the reflectance versus wavenumber curves were observed to increase with film thickness and hence with time, which provides end-point detection during the film growth. La0.67Ca0.33MnO3 films with (100), (110), and mixed orientations, depending on the substrate temperature, were deposited on yttria-stabilized zirconia (YSZ) buffered Si(100) and Si(111) substrates. In a magnetic field of 5 T, the maximum magnetoresistance (MR) values of 250% at 195 K and 164% at 140 K were observed in the as-deposited (110) and (100) films, respectively.
Estimation of Minimum Liquidus Free Energy Concentration for Silicide and Germanide Systems
Authors: Nam H. G. , Cho N.-I.
Published: April 2011

Thermodynamic functions of Co-Si and Au-Si systems were studied. The validity of these functions were confirmed by successfully calculating phase diagrams. It was revealed that the composition of the first nucleated compound is close to the concentration of the minimum free energy of the liquid alloy with respect to the two solid components (ΔG) of the binary systems. In addition, the minimum ΔG concentration was found to be located by interpolating the portion of the liquidus, where the liquid alloy is in equilibrium with the two solid constituents, into the central region of the diagram where compounds exist. The minimum ΔG concentration of other silicide and germanide systems were estimated by the suggested interpolation method. A new model for predicting the first nucleated compound in silicide as well as germanide systems was proposed based on the findings.
Van Der Waals Epitaxy of Transition Metal Dichalcogenides Using Metal Organic Precursors (Movdwe)
Authors: Tiefenbacher S. , Pettenkofer C. , Jaegermann W.
Published: April 2011

Epitaxial WS2 films have been grown on MoTe2 (0001) van der Waals faces using W(CO)6 and SnS2 as precursors. Despite a lattice mismatch of 10.3 % the film shows high crystalline quality. The interface is atomically abrupt and shows no interface dangling bonds as deduced from photoemission results (XPS, UPS). The growing overlayer nucleates on the flat (0001) terraces and not on steps as suggested from STM-measurements. For thin epilayers Moiré superstructures in LEED have been measured for the first time in semiconductor heteroepitaxy. From the intensity variation of the LEED spots with energy a corrugation of the overlayer film can be deduced. Based on these results a mechanism of van der Waals epitaxy is suggested.
New Silicon-Carbon Materials Incorporating Si4C Building Blocks
Authors: Chandrasekhar D. , Kouvetakis J. , Murran J. Mc , Todd M. , Smith David J.
Published: April 2011

Novel precursor chemistry and ultrahigh-vacuum chemical vapor deposition have been used to deposit Si1-yCyth in films on (001) Si substrates. Films with carbon compositions ranging up to 20 at. % were deposited at substrate temperatures of 600–750°C using interactions of C(SiH3)4 or C(SiH2Cl)4 (C-H free precursors incorporating Si4C tetrahedra) and SiH4 gas mixtures. The composition of the resulting materials was obtained by Rutherford backscattering spectrometry including carbon resonance analysis. Cross-sectional transmission electron microscopy and infrared spectroscopy were used to provide microstructural and bonding information respectively. The effect of precursor chemistry on the composition and structure of the materials is discussed.
Cross-Sectional Transmission Electron Microscopy of Si-Based Nanostructures
Authors: Sidorov Maxim V. , Smith David J.
Published: April 2011

This work demonstrates the successful application of the precision cross-sectioning technique to the characterization of two types of Si-based nanostructures. Careful wedge-polishing of an array of metal-coated poly-Si microlines gave electron transparency over areas as broad as 1.5 mm across. A single, specific, SET (Single Electron Transistor), having dimensions of 4 × 4 μm2, was cross-sectioned for examination using conventional and high-resolution TEM imaging.
GaxIn1-xP/GaP Heterostructures on Si(001) Substrate
Authors: Sukidi N. , Dietz N. , Rossow U. , Bachmann K. J.
Published: April 2011

In this contribution we report on the real-time monitoring of low temperature growth of epitaxial GaxIn1-xP/GaP heterostructures on Si(100) by pulse chemical beam epitaxy, using tertiary butylphosphine (TBP), triethylgallium (TEG), and trimethylindium (TMI) as source materials. Both step-graded and continuously graded heterostructures have been investigated. The composition of the GaxIn1-xP epilayers has been analyzed by various techniques including X-ray diffraction, Rutherford backscattering, Auger, and Raman spectroscopy. Good correlation has been found between X-ray diffraction, RBS, and Vegard's law compositional analysis. We used Ppolarized Reflectance Spectroscopy (PRS) and Laser Light Scattering (LLS) to monitor the growth rate and surface morphology during growth. The information gained by these techniques has been utilized in the improvement of the surface preconditioning as well as to optimize the initial heteroepitaxial nucleation and overgrowth process. We studied the optical response to the compositional changes in the surface reaction layer (SRL) during the exposure of the surface to either sequential or synchronous pulses of TEG and TMI. The cross sectional TEM analysis indicates that the synchronous exposure results in an abrupt GaxIn1-xP/GaP interface while the sequential exposure does not which may suggest a compositionally graded interlayer formation. For heteroepitaxial GaxIn1-xP films on Si, a buffer layer of GaP is found to be necessary for optimum uniformity of the GaxIn1-xP layer. The selective growth of GaxIn1-xP on Si(001) is accessed.
Composite Materials with Adjustable Thermal Expansion for Electronic Applications
Authors: Shi J. D. , Pu Z. J. , Wu K. ‐H. , Larkins G.
Published: April 2011

In this paper, we discuss a newly characterized compound, ZrW2Og, that has been introduced into the composite materials with an adjustable and low thermal expansion for electronic applications. Offering a negative coefficient of thermal expansion (CTE) of approximate ‐9xlO–6/°C in a large temperature range, ZrW2Og was used as a particle filler in polymer‐matrix composites. The paper presents two kinds of composites, that is, polyester and epoxy with various volume fractions of ZrW20g. The CTEs of the polyester/ZrW2Og and epoxy/ZrW2Og composites have been proven adjustable in the ranges of 94 to 56x10–6 /°C and 54 to 18х 10–6 /°C, respectively, with ZrW2Og filler from 0 to 30 vol%. In addition, the analysis about the interfaces between the matrices and filler indicated that the interfaces may be beneficial to reduce the overall thermal expansion of the composites. The methods to further decrease composite CTEs are also discussed.
Technique to Measure Thermodynamic Adhesion of Copper-Polyimide Interfaces
Authors: Menezes Marlon E. , Roberton I.M. , Birnbaum H.K.
Published: April 2011

A contact angle measurement technique has been used to determine the thermodynamic adhesive strength between nano-sized copper clusters and polyimide. The copper clusters are formed under UHV conditions by using ∼ mono-layer per minute deposition rates to assist equilibrium cluster formation. Temperatures of the polyimide substrate during the deposition process are maintained below Tg, which ensures no diffusion of the copper atoms into the polyimide matrix. Values of the strength of adhesion from these contact angle measurements are in reasonable agreement with predictions made using Fowkes’ theory of non-polar interfacial adhesion, but are at least one order of magnitude less than the value obtained by mechanical tests. The limitations of this technique due to deformation at the polyimide surface, as well as kinetic factors constraining equilibrium cluster formation will be discussed.
Spalling of Cu-Sn Compound Spheroids in Solder Joints on Si Wafers
Authors: Kim H.K. , Tu K.N.
Published: April 2011

In reacting eutectic SnPb solder with Ti/Cu and Cr/Cu/Au thin film metallization on Si wafers, we have observed spalling of Cu6Sn5 spheroids when the solder consumes the Cu. The formation of the spheroids is assisted by the ripening reaction among the compound grains. In addition we have observed an asymmetric spalling phenomenon using a sandwich structure, in which two wafers were soldered face-to-face. The spalling occurs predominantly at the interface below the solder joint. It suggests that gravity plays a role.
Structural Reliability of Direct-Chip-Attaches Bonded With Anisotropic Conductive Film
Authors: Lam D.C.C. , Shen C. , Karim JF Xie, Z. , Tong P.
Published: April 2011

Structural reliability is a major concern in using anisotropic conductive films (ACF) in the electrical connection of chips or chip carriers to substrates. The relationship between electrical and structural failure is experimentally examined using a custom designed test chip. In-situ electrical measurements along with post failure microstructural examination revealed that electrical failure (open circuit) is caused by interfacial cracking of ACF/bump interface. An engineering analysis of the influence of assembly geometry revealed that tall relative bump height and large underbump adhesive thickness decrease the driving force for interfacial cracking. However, when relative bump height is increased and underbump adhesive thickness is reduced, increased compression from increased relative bump height dominate over increased shear resulting in a net decrease in crack propagation driving force. In general, tall relative bump height can reduce the potential for electrical failure in ACF interconnect when the ACF/bump interfacial fracture is the dominant failure mode.
Comparison of ICl and IBr for Dry Etching of III-Nitrides
Authors: Vartuli C. B. , Lee J. W. , MacKenzie J. D. , Pearton S. J. , Abernathy C. R. , Shul R. J.
Published: April 2011

ICl/Ar ECR discharges provide the fastest dry etch rates reported for GaN, 1.3 µm/min. These rates are much higher than with Cl2/Ar, CH4/H2/Ar or other plasma chemistries. InN etch rates up to 1.15 µm/min and 0.7 µm/min for In0.5Ga0.5N are obtained, with selectivities up to 5 with no preferential loss of N at low rf powers and no significant residues remaining. The rates are much lower with IBr/Ar, ranging from 0.15 µm/min for GaN to 0.3 µm/min for InN. There is little dependence on microwave power for either chemistry because of the weakly bound nature of IC1 and IBr. In all cases the etch rates are limited by the initial bond breaking that must precede etch product formation and there is a good correlation between materials bond energy and etch rate. The fact that low microwave power can be employed is beneficial from the viewpoint that photoresist masks are stable under these conditions, and there is no need for use of silicon nitride or silicon dioxide. Selectivities for GaN over A1N with IC1 and IBr are still lower than with Cl2- only.
Wide Bandgap Semiconductors for Cold Cathodes: A Theoretical Analysis
Authors: Lerner Peter , Cutler P. H. , Miskovsky N. M.
Published: April 2011

In this paper we describe the field emission from wide band-gap semiconductor thin film electron sources as a three-step process. Internal field emission is the mechanism for electron injection at the metal-semiconductor cathode interface. Under an internal field, electrons injected into the conduction band can propagate quasi-ballistically through the thin semiconductor film. At the vacuum interface, they are field emitted across a PEA or NEA surface. Consistent with the electron injection mechanism we have done molecular dynamics simulations for GaN films with an initial energy distribution corresponding to a Fowler-Nordheim (FN) spectrum. Results demonstrate quasi-ballistic propagation and approximate preservation of the FN energy distribution. Furthermore, high levels of n-doping in GaN (∼ 1017cm−3) do not inhibit transport in thin films (<0.1 μm).
Degradation of Single-Quantum Well InGaN Green Light Emitting Diodes Under High Electrical Stress
Authors: Osiński Marek , Perlin Piotr , Eliseev Ptr G , Liu Gungtan , Barton Daniel L
Published: April 2011

We performed a degradation study of high-brightness Nichia single-quantum well AlGaN/InGaN/GaN green light-emitting diodes (LEDs). The devices were subjected to high current electrical stress with current pulse amplitudes between 1 A and 7 A and voltages between 10 V and 70 V with a pulse length of 100 ns and a repetition rate of 1 kHz. The study showed that when the current amplitude was increased to the 6 A - 7.5 A range, a fast (about 1 s) degradation occurred, with a visible discharge between the p and n-type electrodes. Subsequent failure analysis revealed severe damage to metal contacts which lead to the formation of shorts in the surface plane of diode. For currents smaller than 6 A, a slow degradation was observed as a decrease in optical power and an increase in the reverse current leakage. After between 24 and 100 hours however, a rapid degradation occurred which was similar to the rapid degradation observed at higher currents. Failure analysis results suggest that carbonization of the plastic encapsulation material on the diode surface leads to the discharge which destroys the diode.
Degradation of InGaN/AlGaN LED on Sapphire Substrate Grown by MOCVD
Authors: Egawa T. , Ishikawa H. , Jimbo T. , Umeno M.
Published: April 2011

We report an optical degradation of an InGaN/AIGaN double-heterostructure light-emitting diode (LED) on a sapphire substrate grown by metalorganic chemical vapor deposition. The InGaN/AIGaN LED exhibited an optical output power of 0.17 mW, external quantum efficiency of 0.2 %, and the peak emitting spectrum at 437 nm with full width at half-maximum of 63 nm under 30 mA dc operation at 300 K. The InGaN/AIGaN LED showed the optical degradation under high injected current density. Electroluminescence, electron-beam induced current and cathodoluminescence observations show that the degraded InGaN/AIGaN LED exhibits formation and propagation of dark spots and a crescent-shaped dark patch, which act as nonradiative recombination centers. The values of degradation rate under injected current density of 0.1 kA/cm2 were determined to be 1.1 × 10-3, 1.9 × 10-3 and 3.9 × 10-3 h-1 at ambient temperatures of 30, 50 and 80°C, respectively. The activation energy of degradation was also determined to be 0.23 eV.
Spontaneous and Stimulated Recombination in the Nitrides
Authors: Hangleiter A. , Scholz F. , Härle V. , Im J. S. , Frankowsky G.
Published: April 2011

Both spontaneous and stimulated emission processes are essential ingredients for constructing a laser from the nitrides. Based on our picosecond time-resolved photoluminescence studies we show that spontaneous radiative recombination is strongly influenced by excitonic effects, both in bulk GaN and in quantum wells. Particularly in quantum wells, localization of excitons plays an important role. We have studied the optical gain spectra in GaInN/GaN and GaN/AlGaN double heterostructures and quantum wells, grown by LP-MOVPE, using the stripe excitation method. Both room temperature and low temperature measurements were performed. Based on our results, we discuss the physical mechanism of optical gain in the nitrides as well as consequences for laser operation. We show that localization or, equivalently, the formation of quantum dot like structures, governs the optical gain mechanism in the nitrides.
Gain Spectra and Stimulated Emission in Epitaxial (In,Al) GaN Thin Films
Authors: Wiesmann D. , Brener I. , Pfeiffer L. N. , Khan M. A. , Sun C. J. , Chang C. S. , Fang W. , Chuang S. L.
Published: April 2011

We measured the emission of (In,Al) GaN films under high intensity optical pumping both in the direction parallel and perpendicular to the film growth. In the edge emission geometry we determine the gain magnitude from the variable stripe length (VSL) method. We use the spontaneous emission collected perpendicular to the layer plane to calculate the spectral dependence of the gain. Theoretical calculations are in good agreement with those experimentally determined gain spectra. We also show that the observalion of a stimulated emission peak perpendicular to the film is predominantly due to scattering of the in-plane stimulated emission but without ruling out contributions from microstructures in the films.
Optical Quenching of Photoconductivity in GaN Photo-conductors
Authors: Huang Z.C. , Mott D.B. , Shu P.K. , Zhang R. , Chen J.C. , Wickenden D.K.
Published: April 2011

We report the first observation of optical quenching of photoconductivity in GaN photoconductors at room temperature. Three prominent quenching bands were found at Ev+1.44, 1.58 and 2.20 eV, respectively. These levels are related to the three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He-Ne laser shining on the detector.
An Optical Waveguide Formed by Aluminum Nitride thin film on Sapphire
Authors: Tang Xiao , Yuan Yifang , Wongchotigul K. , Spencer Michael G.
Published: April 2011

We have investigated an optical waveguide formed by aluminum nitride (AlN) thin film on sapphire. A good quality AlN thin film on sapphire substrate was prepared by metal organic chemical vapor deposition (MOCVD) in this laboratory. A rutile prism coupler was employed to display the waveguide modes (N-lines) with wavelengths of 632.8, 532.1, 514.5 and 488.0 nm. The refractive index and thickness of the waveguide material is obtained by prism-coupler measurement. The dispersion curve of the AlN film is given and the dispersion equation is derived. The attenuation in the waveguide is evaluated by scattering loss measurements using a fiber probe. The attenuation coefficient alpha (α) is 1.5- 2.1 cm−1 depending on the sample and the different modes of waveguide. The accuracy of the measurement is discussed.
In-Plane Optical Anisotropies of AlxGa1-xN films in their Regions of Transparency
Authors: Rossow U. , Edwards N. V. , Bremser M. D. , Kern R. S. , Liu H. , Davis R. F. , Aspnes D. E.
Published: April 2011

GaN, AlxGa1–xN, and AlN layers exhibit interference oscillations and bandgap-related features in their reflectance-difference (-anisotropy) (RD/RA) spectra. We concentrate on the interpretation of interference-related data, providing a general expression for these optical anisotropics and discussing mechanisms that originate in the layers themselves. These include anisotropic strain in the plane of the layer, a tilt of the c axis with respect to the surface normal, and non-normal-incidence illumination. We estimate the magnitudes of these contributions, and show that they are consistent with those observed. In principle these contributions can be separated by their different azimuthal dependences. The complex pattern of the data for AlxGa1–xN and AlN indicate that contributions from several layers are present.
In-situ Real Time Spectroscopic Ellipsometry Applied to the Surface Monitoring of Semiconductors
Authors: Boher P. , Stehle J. L.
Published: April 2011

A new kind of Real Time Spectroscopie Ellipsometer (RTSE) system is presented in detail. A multichannel analyser with photointensifier allows one to get spectroscopie measurements over the 0.25–0.85μm wavelength range with resolution λ/Δλ better than 500 and a good signal to noise ratio even for samples with poor reflectance. Precision and reproducibility of the system are around 0.3 and 0.1% for the two ellipsometric parameters. The measurement speed can be increased up to 15 spectra/s but must be adapted accurately to the process under control. An example of the heating process of a GaAs substrate measured in-situ by RTSE is presented and the temperature and native oxide thickness are deduced.
Excitation Intensity and Temperature Dependent Photoluminescence Behavior of Silicon Nanoparticles
Authors: Werwa E. , Seraphin A. A. , Kolenbrander K. D.
Published: April 2011

The luminescence properties of silicon nanoparticles have been studied as a function of the excitation light intensity, the temporal Nature of the excitation source, and of sample temperature. The excitation intensity dependence of the luminescence was found to depend strongly on the temporal Nature of the excitation source. Under high intensity excitation from a pulsed 355 nm source, the photoluminescence (PL) intensity saturates and the peak PL wavelength shifts to the blue at room temperature. This behavior persists at reduced temperature. In contrast, under high intensity excitation using a cw 488 nm source at room temperature, the PL intensity saturates but does not shift in wavelength. At reduced temperatures, there is no saturation of luminescence intensity with high intensity cw excitation. These differences indicate that photogenerated carrier recombination occurs via different pathways depending on the temporal profile of the excitation, with cw excited samples following the expected Auger pathway while pulsed samples exhibit a state filling mechanism. Auger models for the pulsed behavior are found to be inconsistent with the experimental data. The temperature dependence of the PL from a pulsed excited sample for a constant excitation intensity was also monitored. The variation of the peak emission wavelength was found to be similar in magnitude to that observed for amorphous silicon, suggesting that structural disorder may play a role in the luminescence. The change in emission intensity was fairly weak, indicating enhanced carrier confinement, as would be expected in a quantum confined system.
Raman Scattering and Photoluminescence of Spontaneously Ordered Ga0.5In0.5P Alloy
Authors: Li G. H. , Liu Z. X. , Han H. X. , Wang Z. P. , Dong J. R. , Wang Z. G.
Published: April 2011

Samples of the spontaneously ordered Ga0.5In0.5P alloys were grown by the MOCVD method on [001]-oriented GaAs substrates. The thickness of the epitaxal layer is about 2 μm. Raman scattering and photoluminescence spectra have been measured at room temperature. The result from photoluminescence measurement indicates that the direct-band gaps of the spontaneously ordered samples are lower than that of the disordered sample. Three scattering peaks have been observed in the Raman spectra, corresponding to the GaP-like LO, InP-like LO and InP-like TO modes in the alloys, respectively. The frequencies of the GaP- and InP-like LO modes increase with the decrease of the band-gap of the ordered alloys. It is related to the formation of the (GaP)1/(InP)1 monolayer superlattice along [111] direction in the ordered alloys. The polarization properties of the ordered alloys are similar to those of the bulk III-V semiconductors with the zinc-blende structure.
Growth and Characterization of Zn1-xMnxSe1-ySy Epilayers and Related Heterostructures
Authors: Flack F. S. , Samarth N. , Semendy F.
Published: April 2011

We describe the growth of (Zn,Mn)(S,Se) epitaxial layers of high structural quality on (100) GaAs substrates. Double crystal x-ray diffraction (DCXRD) measurements indicate that quaternary epilayers nearly lattice-matched with GaAs are characterized by DCXRD curves with a foil width at half maximum in the range 30 – 60 arc seconds. Photoluminescence (PL) spectroscopy is employed to map the variation of the energy gap of the quartenary alloys over a wide range of alloy compositions. Finally, temperature dependent PL is used to examine the viability of (Zn,Mn)(S,Se) alloys as confining layers for ZnSe and (Zn,Cd)Se quantum wells. While efficient exciton confinement is demonstrated through the observation of robust PL from such quantum wells up to high temperatures, the renormalization of the quartenary band gap by spin fluctuations leads to a rapid decrease in confinement with increase in temperature.
Raman Scattering and Photoluminescence Measurements on II-VI Semiconductor Nanocrystals as a Function of Pressure and Particle Size
Authors: Schroeder John , Lee Mierie , Silvestri Markus R. , Hwang Lih-Wen , Persans Peter D.
Published: April 2011

Pressure tuned photoluminescence and Raman scattering were employed to study the structural phase transition from wurtzite to rocksalt structure of CdSxSei1-x nanocrystals embedded in a glass matrix as a composite material and in the colloidal form. In both the composite and the colloidal form the increased phase stability of the wurtzite to rocksalt structure is reversible and size dependent. In contrast the phase transition in bulk semiconductors of cadmium-sulfide is sharp but for the nanocrystals it is broad and occurs at higher pressures. A size dependent study of the phase stability was conducted and a correlation between size and increase in phase stability was definitely established. For the nanocrystals in the composite form the difference in surface tension in both phases allowed for a model that predicted the tendencies observed in optical spectra. For the colloidal nanocrystals a model based on surface tension and volume effects (defects) gave good agreement in explaining the observations. The above results are discussed with respect to surface tension and deep level defects and their pressure dependence.
In-Situ Photoluminescence Investigation of the Initial Porous Silicon Formation in 0.2M NH4F (pH 3.2)
Authors: Dittrich T. , Timoshenko V. Y. , Rappich J.
Published: April 2011

The porous silicon (por-Si) formation in 0.2M NH4F (pH 3.2) is investigated in-situ by photoluminescence (PL). The p-type Si(100) samples are treated electrochemically in the galvanostatic regime starting from Ihe hydrogenated surface. Single pulses of a N2 laser are used to probe stroboscopically the radiative band-band recombination of the bulk c-Si and the PL of por-Si. The PL intensity of c-Si is correlated with the current density during the current-voltage scan and indicates changes of surface recombination by the onset of chemical reactions. The PL intensity of c-Si increases rapidly after switching off the anodic current while the PL intensity of por-Si is not influenced by the rapid current switch. This shows that the passivation of the surfaces of the Si nanostructures is not affected by the por-Si formation at the surface of the bulk c-Si.
Synchrotron Radiation Induced Optical Luminescence from Porous Silicon: Recent Observations
Authors: Coulthard I. , Sham T. K. , Jiang D.-T. , Tan K. H.
Published: April 2011

Photoluminescence from porous silicon was examined using synchrotron radiation as an excitation source. The tunability of the excitation source permitted a wide range of excitation energies ranging from VUV to X-rays. This permitted site selective excitation where specific core levels (i.e. Si-K, O-K, Si-2p) were probed. In high porosity samples, luminescence bands of both surface and bulk origins were observed. All experiments exhibited a common luminescence maximum typically in the orange-red region of the visible spectrum. At certain specific excitation energies particularly in the VUV region additional peaks related to sites with oxygen character were also observed. The VUV excitation luminescence spectra of the porous silicon remarkably resembled that of oxygen deficient amorphous silicon dioxide glasses.
Electron Microscopy of Mesoscale Arrays of Quantum-Confined CdS Nanoparticles Formed on DNA Templates
Authors: Pinizzotto Russell F. , Rho Young G. , Chen Yandong , Pirtle Robert M. , Pirtle Irma L. , Coffer Jeffery L. , Li Xin
Published: April 2011

This paper describes the fabrication method and initial characterization of self-assembled mesoscale arrays of quantum-confined CdS nanoparticles using DNA as a template for the overall shape. Three DNAs were used: the circular and linear forms of the plasmid pUCLeu4, and circular φ×174 RF II. In all three cases, the mesoscale lengths are consistent with the A-form of DNA. The structural signatures and crystallography were confirmed using conventional and high resolution transmission electron microscopy, and electron diffraction. Optical spectroscopy demonstrated that the particles display quantum-confinement effects. This research is a fundamental demonstration of the power of combining biochemical and solid-state processing techniques.
Ab-Initio Calculation of the Optical Properties of Silicon Quantum Wires
Authors: Ossicini Stefano , Biagini M. , Bertoni C. M. , Roma G. , Bisi O.
Published: April 2011

We studied the effect of H, O passivation and inter-wire interaction on the optical properties of nanoscale Si wires. We find that wires with diameters as small as 10–25 Å are active in the visible range. Inter-wire interaction leads to the presence of localized states which lower the band gap energy. The presence of dangling bonds generates broad features in the infrared region. O-Si bonds reduce the absorption threshold. These results are important for the discussions concerning absorption and luminescence in porous Si.
Excimer-Laser Crystallization of Silicon Films: Numerical Simulation of Lateral Solidification
Authors: Gupta Vikas V. , Jin Song H. , Im James S.
Published: April 2011

We have utilized a recently developed transient two-dimensional model for simulating localized beam-induced melting and solidification of thin silicon films on SiO2. Specifically, by tailoring the lateral beam profile, we simulate those situations that are encountered in the artificially-controlled superlateral growth (ACSLG) method, in which various techniques are utilized to irradiate the sample in preselected regions of a silicon film. The spatially and temporally localized character of heating is simulated by introducing a time-dependent two-dimensional heat-source function. The evolution of melt-creation and ensuing solidification is studied as a function of incident energy density and film thickness. The results show two distinct types of behavior as a function of incident energy density: at low energy densities, partial melting and predominantly vertical solidification occur; while at high energy densities, complete melting of the irradiated portion of the film is followed by rapid lateral solidification.
Purely Intrinsic Poly-Silicon Films by Hot Wire Chemical Vapor Deposition
Authors: Rath J. K. , Feenstra K. F. , Ruff D. , Meiling H. , Schropp R. E. I.
Published: April 2011

Poly-silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 °C). The optical gap of the poly-silicon films is 1.1 eV, though with a higher optical absorption than c-Si. The grains have a preferential orientation (220) perpendicular to the substrate with an average crystallite size of 70 nm. The crystalline volume fraction is 95% with complete coalescence of grains. Large structures up to 0.5 μm could be observed in the AFM micrograph. The activation energy (0.54 eV) and the low carrier concentration (1011 cm−3) indicate a fully intrinsic nature of the films. The μτ product of carriers is 7.1×10−7 cm2V−1 whereas the ambipolar diffusion length (LD) is 334 nm. The excellent photo-conductive properties are attributed to the low (∼1017 cm−3) defect density. The HWCVD poly-silicon films showed a very small temperature dependence of mobility, indicating negligible trapping of carriers at the grain boundaries. Preliminary n-i-p cells incorporating poly-silicon i-layer yielded 3.15 % efficiency.
Synthesis and Laser Spectroscopy of Monoclinic Eu3+: Y2O3 Nanocrystals
Authors: Bihari Bipin , Tissue Brian M.
Published: April 2011

Gas-phase condensation of CO2 laser-heated EU3+:Y2O3 ceramics produces monoclinic-phase nanocrystalline material. Transmission electron microscopy shows particle diameters in the range 7–30 nm for particles quenched at 60 °C under 400 Torr of nitrogen atmosphere. The optical spectra of nanocrystals produced from 0.1% Eu3+:Y2O3 starting material have narrow lines, and the 5D0 lifetimes are 1.8, 1.2 and, 1.3 ms for the three Eu3+ cation sites. Nanocrystals obtained from 0.7 –5 % Eu3+:Y2O3 starting material show line broadening and the presence of Eu2O3 secondary phase.
Atomistic Study of Crack Propagation and Dislocation Emission in Cu-Ni Multilayers
Authors: Clinedinst Jeff , Farkas Diana
Published: April 2011

We present atomistic simulations of the crack tip configuration in multilayered Cu-Ni materials. The simulations were carried out using molecular statics and EAM potentials. The atomistic structure of the interface was studied first for a totally coherent structure. Cracks were simulated near a Griffith condition in different possible configurations of the crack plane and front with respect to the axis of the layers. Results show that interface effects predominately control the mechanical behavior of the system studied.
Synthesis of Oxide-Coated Metal Clusters
Authors: Crane Robert A. , Matthews Jonathan T. , Andres Ronald P.
Published: April 2011

“Fish-eye” particles consisting of metal clusters (Ag, Cu) a few nanometers in diameter encapsulated within a thin layer (-1 nm) of silica are produced using aerosol synthesis procedures. We present a method for predicting stable “fish-eye” nanostructures and describe synthesis techniques for producing significant quantities of silica-encapsulated metal nanoparticles.

For many metal/oxide pairs, gas phase formation of oxide encapsulated metal particles is thermodynamically favorable. Using known surface free energies and binary phase diagrams, it is possible to predict whether SiO2-encapsulated metal clusters will form in the gas phase. Two conditions which must be satisfied are: 1) that the surface free energy of the metal is higher than that of Si; and 2) that the metal composition in the particle is greater than the eutectic composition in the metal/Si phase diagram. Ag-SiO2 and Cu-SiO2 are two examples of systems which readily form “fish-eye” structures.

Two types of gas phase cluster sources are used at Purdue for producing encapsulated metal nanoparticles. The Multiple Expansion Cluster Source (MECS) is a well established apparatus which produces small quantities (- 50 mg/hr) of very uniform materials using resistive heating for evaporation. The new Arc Cluster Evaporation Source (ACES) offers much higher production rates (>1 g/hr) using DC arc evaporation. These two cluster sources make possible the study of a unique class of materials.
Transition Dynamics in Ferroelectrics with Ordered Nanoregions
Authors: Siny LG. , Katiyar R. S. , Lushnikov S. G.
Published: April 2011

Raman scattering was used to study two model relaxor ferroelectrics, PbMg1/3Nb2/3O3 (PMN) with the 1:2 stoichiometric composition of Mg2+ and Nb5+ ions in the oxygen octahedrons and PbSc1/2Ta1/2O3 (PST) with the 1:1 stoichiometric composition of Sc3+ and Ta5+ ions. In spite of a different stoichiometric ratio the Raman spectra of both materials are consistent with the Fm3m space symmetry which implies the existence of similar 1:1 ordered clusters at least in nanoscale regions. The spectra show some anomalous features in the temperature range preceding a ferroelectric state, namely a broad central peak appears in PMN and a complex structure develops from the initially singlet line in PST. Those phenomena are considered as the dynamic features in course of evolution of the relaxors to a ferroelectric state. The preceding phase is characterized by a breakdown in the selection rules for Raman scattering, so some points in the Brillouin zone can contribute to the light scattering spectra. Comparing all available data, one can assume the determinant role of heterophase fluctuations in that process. The fluctuations in a special preceding phase are caused by a competition between two phases, namely between the ferroelectric phase and an additional nonpolar phase.
Consolidation and Evolution of Physical-Mechanical Properties of Nanocomposite Materials Based on High-Melting Compounds
Authors: Andrievski R. A. , Kalinnikov G. V. , Urbanovich V. S.
Published: April 2011

Nanocomposite materials of TiN/TiB2 system are prepared both by high pressure sintering of ultrafine powders and the films deposition using magnetron sputtering. Properties and structure of obtained paniculate and film materials derived from hardness measurements as well as TEM and SEM analyses are discussed and compared in detail.
Heterogeneous Nanocomposite Materials Based on Liquid Crystals and Porous Media
Authors: Sinha G. P. , Aliev F. M.
Published: April 2011

An effective way of preparing a variety of liquid crystal based nanocomposite materials is to disperse LC in porous media with different porous matrix structure, pore size and shape. We present the results of investigations of quasiequilibrium and dynamical properties of nematic and smectic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes) by light scattering, photon correlation and dielectric spectroscopies. Confining LC to nanoscale level leads to quantitative changes in physical properties and appearance of new behavior which does not exist in either of the components. Relaxation of director fluctuations which is characterized by single relaxation time in the bulk LC are transformed to a process with a spectrum of relaxation times in pores, which includes extremely slow dynamics typical for glass formers. Existence of developed interface in these materials leads to new dielectric properties such as an appearance of a low frequency relaxation of the polarization and modification of dipole rotation.