Applied Surface Science (v.252, #11)

Contents (iv-v).

Preface by Natarajan Chandrasekhar; Chris Boothroyd; Venkatesh Narayanamurti; Stuart S.P. Parkin; J. Campbell Scott (3921).

We report on the interface characterization of InGaN/GaN multiple quantum wells with indium aggregation grown by metalorganic chemical vapor deposition. The interface related microstructure was analyzed by high-resolution transmission electron microscopy, high-resolution X-ray diffraction and high angle annular dark field. Luminescence measurements were carried out by micro-photoluminescence measurement. In addition, quantitative determination of the indium concentration inside the ultra-small dots was attempted. We demonstrate that the quantum dots are coherent and the interfaces remain sharp. The In content inside ∼2 nm InGaN dots is about 65% determined by spectrum imaging in energy-filtered transmission electron microscopy combined with multiple linear least squares fitting, which is slighter higher than the value obtained either from HRTEM or theoretical calculations. This discrepancy is briefly discussed but demands further studies for complete understanding.
Keywords: InGaN/GaN;

Interfacial phenomena control the performance of devices employing various oxides and designed to realize phenomena such as spin injection and magnetic coupling. Here we review our work on spin injection into oxide superconductors and briefly mention work on magnetic heterostructures. The focus will be on geometries prepared using ozone-assisted, oxide-molecular beam epitaxy. The implications for our results of recent applications of probes of buried interfaces will be discussed.
Keywords: Spin injection; Cuprates; Manganites; Heterostructures;

Prediction of barrier inhomogeneities and carrier transport in Ni-silicided Schottky diode by A.R. Saha; C.B.- Dimitriu; A.B. Horsfall; S. Chattopadhyay; N.G. Wright; A.G. O’Neill; C.K. Maiti (3933-3937).
Based on Quantum Mechanical (QM) carrier transport and the effects of interface states, a theoretical model has been developed to predict the anomalous current–voltage (IV) characteristics of a non-ideal Ni-silicided Schottky diode at low temperatures. Physical parameters such as barrier height, ideality factor, series resistance and effective Richardson constant of a silicided Schottky diode were extracted from forward IV characteristics and are subsequently used for the simulation of both forward and reverse IV characteristics using a QM transport model in which the effects of interface state and bias dependent barrier reduction are incorporated. The present analysis indicates that the effects of barrier inhomogeneity caused by incomplete silicide formation at the junction and the interface states may change the conventional current transport process, leading to anomalous forward and reverse IV characteristics for the Ni-silicided Schottky diode.
Keywords: Carrier transport; Barrier inhomogeneity; Schottky diode; Quantum Mechanical;

Effect of Ru crystal orientation on the adhesion characteristics of Cu for ultra-large scale integration interconnects by Hoon Kim; Toshihiko Koseki; Takayuki Ohba; Tomohiro Ohta; Yasuhiko Kojima; Hiroshi Sato; Shigetoshi Hosaka; Yukihiro Shimogaki (3938-3942).
The adhesion of Cu on Ru substrates with different crystal orientations was evaluated. The crystal orientation of sputter deposited Ru could be changed from (1 0 0) to (0 0 1) by annealing at 650 °C for 20 min. The adhesion of Cu was evaluated by the degree of Cu agglomeration on Ru. Cu films on annealed Ru films with the (0 0 1) crystal orientation showed 28% lower RMS values and 50% lower Ru surface coverage than Cu as-deposited on Ru having the (1 0 0) crystal orientation after annealing at 550 °C for 30 min, which suggest that Cu wettability on the Ru(0 0 1) was better than that on the Ru(1 0 0) plane. The low lattice misfit of 4% between Cu(1 1 1) and Ru(0 0 1) may be the reason for this good adhesion property.
Keywords: Cu wettability; Ru glue layer; Ru crystal orientation; Lattice misfit;

We have investigated Cs and Na doping in copper phthalocyanine (CuPc) and tris(8-hydroxyquinoline) aluminum (Alq) using photoemission spectroscopy. We observed valence and core level spectra changes at different doping levels, and found that the doping induces an energy level shift that can be seen in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by a significant modification of organic energy levels, such as the introduction of a new gap state, new core level components and a change of binding energies. Furthermore, we observed that the energy level shift in the first stage depends in a semi-logarithmic fashion on the doping concentration, whose slope cannot be explained by the conventional model used in inorganic semiconductors. These results indicate that the molecular nature and strong correlation must be considered for doping in organic semiconductors.
Keywords: Organic semiconductors, Doping, Interface electronic structure, Energy level shift;

Chemistry, electronic structure and electrical behavior at the interfaces between copper phthalocyanine (CuPc) and Mg with a reverse formation sequence were investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and current–voltage (IV) measurements. A chemical reaction occurs between CuPc and Mg irrespective of the deposition sequence. Despite having different reaction zone thicknesses, both the CuPc-on-Mg and the Mg-on-CuPc interfaces exhibit chemistry-induced gap states and identical carrier injection barriers, which are confirmed by the symmetric electrical behavior obtained from IV characteristics of devices with a structure of Mg/CuPc/Mg. These findings contrast with those expected from physisorptive noble metal–CuPc interfaces and suggest that strong local chemical bonding is a primary factor determining molecular level alignment at reactive metal–CuPc interfaces.
Keywords: Metal–organic interface; CuPc; Chemical bonding; Barrier formation;

Improved electrical and optical properties of MEH-PPV light emitting diodes using Ba buffer layer and porphyrin by Amit Kumar; P.K. Bhatnagar; P.C. Mathur; K. Tada; M. Onoda (3953-3955).
It has been found that insertion of a thin Ba buffer layer between the Al electrode and the MEH-PPV layer results in a significantly higher current density in ITO/MEH-PPV/Al polymer light-emitting diodes due to a reduction of the potential barrier at the cathode–polymer interface. The photoluminescence is found to increase with the addition of porphyrin-containing platinum as the central atom, showing that some of the triplet excitons decay radiatively as a result of mixing porphyrin.
Keywords: PLED; MEH-PPV; Buffer layer; Porphyrin;

Tuning the charge transport through a metal–molecule–metal junction by changing the interface properties is widely studied and is of paramount importance for applications in molecular electronic devices. We used current sensing atomic force microscopy (CSAFM) as a tool to study the contact resistance of metal–molecule–metal (MmM) junctions formed by sandwiching self-assembled monolayers (SAMs) of alkanethiols with various end groups (–CH3, –OH and –NH2) between Au(1 1 1) substrates and Au coated AFM tips. The effect of interface chemistry on charge transport through such SAMs with varying end groups was studied in an inert, non-polar liquid (hexadecane) environment. We find that the contact resistances of these MmM junctions vary significantly based on the end group chemistry of the molecules.
Keywords: Metal–molecule–metal junctions; Self-assembled monolayer; Current sensing atomic force microscopy;

An admittance spectroscopy technique has been developed for the interfaces between organic monolayers and silicon. The present work involves the development of an effective equivalent circuit to represent the silicon/organic-monolayer system, and the development of a parameter extraction procedure, which yields the monolayer capacitance and the monolayer thickness, the flat-band voltage, the silicon doping density, the silicon surface potential, the interface trap density, the interface trap capture cross-section and the interface trap energy. This technique was applied to three types of silicon/organic-monolayer system.
Keywords: Organic monolayers; Silicon–organic interface; Admittance spectroscopy; Self-assembly;

Self-assembly of doped semiconductor nanocrystals leading to the formation of highly luminescent nanorods by K. Manzoor; V. Aditya; S.R. Vadera; N. Kumar; T.R.N. Kutty (3968-3971).
Meso-scale self-assembly of doped semiconductor nanocrystals leading to the formation of monocrystalline nanorods showing enhanced photo- and electro-luminescence properties are reported. Polycrystalline ZnS: Cu+–Al3+ nanoparticles of zinc-blended (cubic) structure with an average size of ∼4 nm were aggregated in aqueous solution and grown into nanorods of length ∼400 nm and aspect ratio ∼12. Transmission electron microscope (TEM) images indicate crystal growth mechanisms involving particle-to-particle oriented-attachment assisted by sulphur–sulphur catenation leading to covalent-linkage. The nanorods exhibit self-assembly dependant luminescence properties such as quenching of the lattice defect-related emissions accompanied by enhancement of dopant-related emission, efficient low-voltage electroluminescence (EL) and super-linear voltage-brightness EL characteristics. This study demonstrates the technological importance of aggregation based self-assembly in doped semiconductor nanosystems.
Keywords: Nanostructures; Semiconductors; Luminescence;

Metallic thin film depth measurements by X-ray microanalysis by F.L. Ng; J. Wei; F.K. Lai; K.L. Goh (3972-3976).
In this study, a low-cost technique, energy dispersive spectroscopy (EDS), was used to explore the application of X-ray microanalysis in depth determination of metallic films. Al, Ni and Au films with varied thicknesses from 50 to 400 nm were deposited on silicon (Si) substrates by magnetron sputtering. Electron beam energies ranging from 4 to 30 keV were applied while other parameters were kept constant to determine the electron beam energy required to penetrate the films. The effect of the atomic number of the metallic films on the penetration capability of the electron beam was investigated. Based on the experimental results, mathematical models for Al, Ni and Au films were established and the interaction volume was simulated using a Monte Carlo program. The simulations are in good agreement with the experimental results. Al/Ni/Au multilayers were also studied.
Keywords: Penetration depth; Monte Carlo modeling; Thin films;

Characterization of the magnetic properties of a GdBa2Cu3O7/La0.75Sr0.25MnO3 superlattice using off-axis electron holography by T. Kasama; M.S. Moreno; R.E. Dunin-Borkowski; S.B. Newcomb; N. Haberkorn; J. Guimpel; P.A. Midgley (3977-3983).
Off-axis electron holography is used to characterize the magnetic properties of a GdBa2Cu3O7/La0.75Sr0.25MnO3 superlattice below the Curie temperature of the manganite layers, in both cross-sectional and plan-view geometry. The samples were prepared for electron microscopy using focused ion beam milling. Differences between the magnetic properties of successive manganite layers are observed in the cross-sectional sample. Magnetic ripple contrast and weakly magnetic regions are observed in plan-view geometry. Although the results may be affected by sample preparation for electron microscopy, the observed differences between the magnetic properties of the manganite layers are consistent between the different samples examined.
Keywords: Manganite; High-T C superconductor; Superlattice; Cross-sectional sample geometry; Off-axis electron holography; Focused ion beam milling;

Quantitative measurement of image intensity in transmission electron microscope images by Wenbang Qu; Chris Boothroyd; Alfred Huan (3984-3988).
We have made a thorough comparison of the ability of image simulations to predict the contrast in high-resolution electron microscope lattice images of GaAs. Simulations of the diffracted beam intensities from thickness fringes generally agreed with observations to within ∼20% over a range of GaAs thicknesses up to 150 nm. Likewise, simulations of lattice images agreed qualitatively with experimental lattice images over a range of defocus and sample thicknesses up to 20 nm. However, using the same parameters as for the diffracted beam intensities, lattice fringe amplitudes were calculated to be typically two to three times higher than observed experimentally.
Keywords: High-resolution transmission electron microscopy; Lattice images; Energy-filtered images; Stobbs factor;

Non-equilibrium grain boundary cosegregation of Mo and P by Li Li; Er-Bao Liu; Qing-Fen Li; Zhen Li (3989-3992).
The segregation of P at grain boundaries is believed to be an important cause of temper embrittlement in steels. As an alloy element, Mo may reduce the embrittlement. However, the concentration measured by Auger electron spectroscopy at the grain boundary in 2.25Cr1MoV and 12Cr1MoV showed that the concentration of P increased with that of Mo, which indicates that Mo and P cosegregated to the grain boundary in Cr–Mo steels.
Keywords: Grain boundary; Cosegregation; Auger electron spectroscopy;

With the aim of understanding the thermal stability of binary immiscible metallic multilayers, we propose a generally size-dependent thermodynamic criterion for determining the interface alloying in multilayers, with respect to the size-dependent interface energy of binary metal systems. Taking the copper/tungsten bilayer as an example, we obtain the interfacial alloying phase diagram based on the proposed thermodynamic model. Our theoretical predictions are consistent with experiments, implying that the size-dependent thermodynamic criterion of the thermal stability could be expected to be applicable to many multilayers.
Keywords: Interface; Multilayered films; Thermodynamics; Copper/tungsten;

Monte Carlo simulation of grain growth in polycrystalline materials by C. Ming Huang; C.L. Joanne; B.S.V. Patnaik; R Jayaganthan (3997-4002).
Understanding the kinetics of grain growth, under the influence of second phase (such as impurities, voids and bubbles) is fundamental to advances in the control of microstructural evolution. As a precursor to this objective, we have investigated the grain growth kinetics in a polycrystalline material using a standard Q-state Potts’ model under Monte Carlo settings. Based on physical reasoning, new modifications are suggested to circumvent some of the disadvantages in the basic Potts model. The efficacy of these modifications vis-à-vis the basic model is verified. The influence of second phase particles on the impurity loaded grain boundaries is investigated for the study of grain growth kinetics.
Keywords: Grain growth; Monte Carlo simulation; Potts model; Micro structural evolution;

We present a self-consistent model of spin transport in a ferromagnetic (FM)–semiconductor (SC)–FM trilayer structure with interfacial barriers at the FM–SC boundaries. The SC layer consists of a highly doped n2+ AlGaAs–GaAs 2DEG while the interfacial resistance is modeled as delta potential (δ) barriers. The self-consistent scheme combines a ballistic model of spin-dependent transmission across the δ-barriers, and a drift-diffusion model within the bulk of the trilayer. The interfacial resistance (R I) values of the two junctions were found to be asymmetric despite the symmetry of the trilayer structure. Transport characteristics such as the asymmetry in R I, spin-injection efficiency and magnetoresistance (MR) are calculated as a function of bulk conductivity σ s and spin-diffusion length (SDL) within the SC layer. In general a large σ s tends to improve all three characteristics, while a long SDL improves the MR ratio but reduces the spin-injection efficiency. These trends may be explained in terms of conductivity mismatch and spin accumulation either at the interfacial zones or within the bulk of the SC layer.
Keywords: Spin-injection efficiency; Spin-diffusion length; Magnetoresistance;

Ab initio calculations of interfacial magnetism in Fe/Mo superlattices by Yousong Gu; Xiaoyuan Zhan; Zhen Ji; Yue Zhang (4009-4012).
Ab initio calculations have been performed on Fe/Mo(1 0 0) superlattices in order to study the interfacial magnetic properties and layer thickness effect on the magnetic moments. In most cases, the magnetic moments of interfacial Fe monolayers are always smaller than those of the inner layers, and the induced magnetic moments of interfacial Mo monolayers oriented in the opposite direction. Calculation results show that the Fe layers are ferromagnetic when n  = 3. As the thickness of the Mo layers increases, the influence of the Mo layer increases and the magnetic state of the Fe layer gradually changes into an antiferromagnetic or non-magnetic state. The change of magnetic moments of Fe/Mo superlattices is in agreement with the experimentally observed oscillation periods.
Keywords: Interfacial magnetism; Density functional theory; Fe/Mo superlattice;

Observation of oxygen contamination at ZnSe/GaAs interfaces using SIMS by F.S. Gard; J.D. Riley; K. Prince (4013-4015).
ZnSe epilayers were grown on GaAs (1 0 0) substrates using MBE. The native contamination (oxide and carbon) was removed in situ from the substrate surfaces by conventional thermal cleaning and by exposure to atomic hydrogen. A maximum substrate temperature of 600 °C was required for the thermal cleaning process, while a substrate temperature of 450 °C was sufficient to clean the substrate using hydrogen. ZnSe epilayers were also grown on As capped GaAs epilayers, which were decapped at a maximum temperature of 350 °C. SIMS profiles showed the existence of oxygen at the interface for all of the substrate preparation methods. The oxygen surface coverage at the interface was found to be 0.03% for the atomic hydrogen cleaned substrate and 0.7% for the thermally cleaned substrate.
Keywords: GaAs; ZnSe; MBE; SIMS;

Phase formation by ion beam mixing in the Ti/Si multilayer system by Veenu Sisodia; D. Kabiraj; W. Bolse; I.P. Jain (4016-4019).
The irradiation effect of 350 MeV Au+ ions on Ti/Si multilayers has been studied using Rutherford backscattering spectroscopy, X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXRD). Intermixing effects have been studied as a function of fluences of 0.46 × 1014, 1.82 × 1014 and 4.62 × 1014  cm−2. Rutherford backscattering spectra (RBS) confirm mixing at the interface. X-ray reflectivity patterns show damage at the interfaces with the absence of a continuous fringe pattern at high fluence doses in comparison to the pristine interface. Mixing leads to titanium di-silicide (TiSi2) phase formation as a shown by grazing incidence X-ray diffraction patterns. The observed intermixing is attributed to energy deposited by the incident ions in the electronic system of the target. Swift heavy ion irradiation induced intermixing increases with fluence.
Keywords: Swift heavy ion; Ion beam mixing; Irradiation;

Spectroscopy and imaging of metal–organic interfaces using BEEM by Linda Kunardi; Cedric Troadec; N. Chandrasekhar (4020-4022).
Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electron emission microscopy (BEEM) enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrier injection is observed for both Ag–PPP (poly-paraphenylene) and Ag–MEHPPV (poly-2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) interfaces. BEEM current images are found to correlate only marginally with the surface topography of the Ag film.
Keywords: BEEM; Hole injection; Schottky barrier; Interface;

Possible transition from space-charge-limited to injection-limited conduction in poly(3-hexylthiophene) thin films by Yi Zheng; Linda Kunardi; Cedric Troadec; Andrew T.S. Wee; N. Chandrasekhar (4023-4025).
Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150  μ m) has been studied using current–voltage characteristics at different temperatures. Space-charge-limited conduction (SCLC) with high injection barriers (1.3 eV) has been observed at all temperatures in the low electric field regime. A possible transition from SCLC to injection-limited conduction (ILC) is reported. The experimental results have been compared with the disorder-controlled injection model proposed by Arkhipov et al. [V.I. Arkhipov, H. von Seggern, E.V. Emilianova, Appl. Phys. Lett. 83 (2003) 5074; V.I. Arkhipov, E.V. Emilianova, Y.-H. Tak, H. Bässler, J. Appl. Phys. 84 (1998) 848; V.I. Arkhipov, U. Wolf, H. Bässler, Phys. Rev. B 59 (1999) 7514].
Keywords: P3HT thin films; Metal–organic interface; Charge injection; SCLC to ILC transition;