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15 Feb 2012

Volume 111, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 043501 (2012); http://dx.doi.org/10.1063/1.3680881 (8 pages)

Gregory J. McGraw and Stephen R. Forrest
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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Effect of interface alloying and band-alignment on the Auger recombination of heteronanocrystals

J. I. Climente, J. L. Movilla, and J. Planelles

J. Appl. Phys. 111, 043509 (2012); http://dx.doi.org/10.1063/1.3684981 (5 pages) | Cited 1 time

Online Publication Date: 21 February 2012

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We report a numerical study of the effect of interface alloying and band alignment on the Auger recombination processes of core/shell nanocrystals. Smooth interfaces are found to suppress Auger recombination, the strength of the suppression being very sensitive to the core size. The use of type-II structures constitutes an additional source of suppression, especially when the shell confines electrons rather than holes. We show that “magic” sizes leading to negligible Auger recombination [Cragg and Efros, Nano Letters 10, 313 (2010)] should be easier to realize experimentally in nanocrystals with extended interface alloying and wide bandgap.
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79.20.Fv Electron impact: Auger emission

Evolution of SiHx hydrides during the phase transition from amorphous to nanocrystalline silicon films

C. Garozzo, R. A. Puglisi, C. Bongiorno, C. Spinella, S. Mirabella, R. Reitano, S. Di Marco, M. Foti, and S. Lombardo

J. Appl. Phys. 111, 043510 (2012); http://dx.doi.org/10.1063/1.3686136 (8 pages)

Online Publication Date: 21 February 2012

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This paper investigates the morphological evolution of hydrogenated amorphous silicon layers obtained by plasma enhanced chemical vapor deposition at different H dilutions in the regime close to the formation of the nanocrystalline phase. The role of hydrogen in the transition from the amorphous to the crystalline phase is investigated by accurate structural and chemical characterization, from the early stages of nucleation, where the nuclei present size slightly larger than the critical nucleus, i.e., about 0.8 nm in radius, up to the formation of crystalline grains larger than 30 nm in radius. A correlation between the structural characteristics of such crystalline phase and the bonding mechanism of Si with H through multiple hydrides, such as Si-H2 and Si-H3 is found, particularly the tri-hydrides are found to be directly correlated to the shape and the size of the nanocrystallites present in the films. The multiple hydrides are found to play a role also in the electrical characteristics of p-i-n a-Si:H solar cells whose intrinsic layer is realized in the above H dilution conditions. An explanation of the experimental data in terms of the different bonding mechanism of H in the Si matrix is provided.
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81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.kg Semiconductors
73.61.Cw Elemental semiconductors
64.70.Nd Structural transitions in nanoscale materials
64.60.qj Studies of nucleation in specific substances
88.40.jj Silicon solar cells

In situ study of the growth properties of Ni-rare earth silicides for interlayer and alloy systems on Si(100)

J. Demeulemeester, W. Knaepen, D. Smeets, A. Schrauwen, C. M. Comrie, N. P. Barradas, A. Vieira, C. Detavernier, K. Temst, and A. Vantomme

J. Appl. Phys. 111, 043511 (2012); http://dx.doi.org/10.1063/1.3681331 (13 pages) | Cited 1 time

Online Publication Date: 21 February 2012

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We report on the solid-phase reaction of thin Ni-rare earth films on a Si(100) substrate, for a variety of rare earth (RE) elements (Y, Gd, Dy, and Er). Both interlayer (Ni/RE/〈Si〉) and alloy (Ni-RE/〈Si〉) configurations were studied. The phase sequence during reaction was revealed using real-time x-ray diffraction whereas the elemental diffusion and growth kinetics were examined by real-time Rutherford backscattering spectrometry. All RE elements studied exert a similar influence on the solid phase reaction. Independent of the RE element or its initial distribution a ternary Ni2Si2RE phase forms, which ends up at the surface after NiSi growth. With respect to growth kinetics, the RE metal addition hampers the Ni diffusion process even for low concentrations of 2.5 at. %, resulting in the simultaneous growth of Ni-rich silicide and NiSi. Moreover, the formation of Ni2Si2RE during NiSi growth alters the Ni diffusion mechanism in the interlayer causing a sudden acceleration of the Ni silicide growth. Besides a significant effect on the silicide growth, we have found that adding 5 at. % Er (relative to Ni) lowers the NiSi Schottky barrier height on n-type Si(100) by approximately 0.1 eV for the interlayer and alloy configuration.
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68.55.A- Nucleation and growth
68.35.Fx Diffusion; interface formation
73.30.+y Surface double layers, Schottky barriers, and work functions

Effect of Si and C concentration on the microstructure, and the mechanical, tribological and electrochemical properties of nanocomposite TiC/a-SiC:H/a-C:H coatings prepared by plasma enhanced chemical vapor deposition

Duanjie Li, Salim Hassani, Suzie Poulin, Jerzy A. Szpunar, Ludvik Martinu, and Jolanta E. Klemberg-Sapieha

J. Appl. Phys. 111, 043512 (2012); http://dx.doi.org/10.1063/1.3684602 (9 pages) | Cited 1 time

Online Publication Date: 22 February 2012

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The nanocomposite TiC/a-SiC:H/a-C:H (presented as Ti-Si-C) coatings attract considerable interest due to their possible applications such as wear protective coatings, diffusion barriers, and materials for solar cells and electrical contacts. In order to explore new film properties and open new opportunities, in the present work, we prepare a series of C-rich Ti-Si-C coatings with different Si and C concentrations using plasma enhanced chemical vapor deposition, and we systematically investigate the effect of elemental composition on the microstructure, and on the mechanical, tribological and electrochemical properties. XRD and XPS analyses demonstrate that the Ti-Si-C coatings mainly consist of nanocrystalline (nc-) TiC embedded in amorphous (a-) SiC:H and a-C:H matrices. Ti-Si-C coatings with a high Si concentration possess enhanced mechanical properties (high hardness), while those with additional C exhibit superior tribological behaviors. The increase of Si and/or C concentrations leads to a grain size refinement of the TiC nanocrystals and to an expansion of the amorphous phase. This in turn substantially enhances their corrosion resistance. Ti-Si-C coatings with the highest Si or C contents exhibit the best corrosion performance among the tested samples by improving the corrosion resistance of the SS410 substrate by a factor of ∼400.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.65.Kn Corrosion protection
81.40.Pq Friction, lubrication, and wear
81.40.Rs Electrical and magnetic properties related to treatment conditions
68.60.Bs Mechanical and acoustical properties
62.20.Qp Friction, tribology, and hardness

Formation of fullerene superlattices by interlayer bonding in twisted bilayer graphene

Andre R. Muniz and Dimitrios Maroudas

J. Appl. Phys. 111, 043513 (2012); http://dx.doi.org/10.1063/1.3682475 (6 pages) | Cited 1 time

Online Publication Date: 22 February 2012

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Based on first-principles density functional theory calculations, we report a novel class of carbon nanostructures consisting of superlattice arrangements of caged fullerene configurations of various sizes embedded within planes of twisted bilayer graphene. Formation of these structures is the outcome of interlayer C-C bonding between pairs of graphene planes chemically modified with certain patterns of chemisorbed hydrogen and rotated with respect to each other by angles around 30°. A specific subclass of these nanostructures preserves the main features of the electronic structure of pristine single-layer graphene. Our study proposes possible functionalization strategies to systematically tailor the electronic properties of bilayer graphene.
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71.20.Tx Fullerenes and related materials; intercalation compounds
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.21.Cd Superlattices
81.16.Rf Micro- and nanoscale pattern formation
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Study on the effect of pressure on the properties of intrinsic point defects in monoclinic zirconia: Ab initio calculations

Shijun Zhao, Jianming Xue, Yugang Wang, and Sha Yan

J. Appl. Phys. 111, 043514 (2012); http://dx.doi.org/10.1063/1.3682766 (10 pages)

Online Publication Date: 22 February 2012

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First-principles calculations were performed to investigate the effects of external pressure on the properties of intrinsic point defects in monoclinic zirconia. Our results show that when the applied external pressure increases from atmospheric pressure to 14.9 GPa, the formation energies of oxygen vacancies decrease with increasing pressure, while the formation energies of the cation and anion interstitials increase all over the pressure regime investigated. Among them the most remarkable change occurs in neutral zirconium interstitial with an increase of 2.21 eV. In particular, the formation energy of zirconium vacancy depends strongly on its charge state. For all the chemical potential and Fermi level considered, the cation and anion vacancies are the most stable defects in all cases. The present calculations also reveal that the formation energies of Frenkel defects could be raised by external pressures, while the energetics of Schottky defects are not much affected. Moreover, the charge state transition of defects is found to be greatly influenced by the external pressures, and thus the stability diagram of defects in ZrO2 is strongly dependent on external pressures. Our findings suggest that external pressures should raise the concentration of oxygen vacancies significantly.
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61.72.jj Interstitials
61.72.jd Vacancies

Size dependent melting behaviors of nanocrystalline in particles embedded in amorphous matrix

J. Mu, Z. W. Zhu, H. F. Zhang, H. M. Fu, A. M. Wang, H. Li, and Z. Q. Hu

J. Appl. Phys. 111, 043515 (2012); http://dx.doi.org/10.1063/1.3686624 (4 pages)

Online Publication Date: 23 February 2012

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The composites of In nanoparticles embedded in Al-based amorphous matrix were synthesized. As the content of In increases, the average size of In nanoparticles increases. The melting behaviors of embedded In nanoparticles were investigated, indicating that the melting temperature is suppressed, and the smaller the size is, the lower the melting temperature is. It is confirmed by comparing the differential scanning calorimetry curves with those of the size distribution. The size dependent melting behaviors of nanoparticles were discussed with the thermodynamic model.
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64.70.D- Solid-liquid transitions
81.07.Wx Nanopowders

Ab initio many-body study of the electronic and optical properties of MgAl2O4 spinel

Shengli Jiang, Tiecheng Lu, Yao Long, and Jun Chen

J. Appl. Phys. 111, 043516 (2012); http://dx.doi.org/10.1063/1.3686727 (7 pages) | Cited 4 times

Online Publication Date: 23 February 2012

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The electronic structure and optical properties of MgAl2O4 spinel with and without oxygen vacancies have been studied in the framework of many-body perturbation theory. By considering the self-energy of electrons, we reasonably describe the bandgap of perfect MgAl2O4 and the defect energy levels of MgAl2O4 containing oxygen vacancies. With the inclusion of electron-hole interaction by solving Bethe-Salpeter equation, the calculated dielectric functions and reflectivity spectrum all are in agreement well with the experimental results for perfect MgAl2O4. Our results show that the sharp peak near 7.8 eV in the experimental absorption spectrum is attributed to the excitonic states. The oxygen vacancies produce some new defect energy levels in the forbidden gap. The optical absorption peaks at 5.3 eV, 4.75 eV and 3.2 eV are induced by the VO0 and VO1+ vacancies.
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71.20.Ps Other inorganic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.72.jd Vacancies
71.15.-m Methods of electronic structure calculations

Dissolution of Sn in a SnPb solder bump under current stressing

Ying-Ta Chiu, Kwang-Lung Lin, and Yi-Shao Lai

J. Appl. Phys. 111, 043517 (2012); http://dx.doi.org/10.1063/1.3682480 (8 pages)

Online Publication Date: 24 February 2012

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An in situ SEM/energy dispersive spectroscopy (EDS) analysis revealed that the dispersed Sn-rich phase of the 95Pb-5Sn solder in a 95Pb-5Sn/63Sn-37Pb composite solder bump dissolved under current stressing. The dissolution was not prominent until the current density reached a threshold value of between 3.3 × 104 and 4.2 × 104 A/cm2. Supersaturation over thermal solubility of Sn-in-Pb was detected with current stressing. A polarity dissolution model was proposed for the dissolution behavior of the Sn-rich phase under current stressing. The dissolution mechanism under current stressing was discussed in relation to electromigration and thermomigration behavior of SnPb solders.
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64.75.Bc Solubility
66.30.Qa Electromigration
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods

Structural study of GaP layers on misoriented silicon (001) substrates by transverse scan analysis

H. Jussila, S. Nagarajan, T. Huhtio, H. Lipsanen, T. O. Tuomi, and M. Sopanen

J. Appl. Phys. 111, 043518 (2012); http://dx.doi.org/10.1063/1.3686711 (6 pages) | Cited 4 times

Online Publication Date: 24 February 2012

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This paper examines the structural properties of gallium phosphide layers by high resolution x-ray diffraction and atomic force microscopy measurements. GaP layers are grown on misoriented and nominally exactly oriented silicon (001) substrates by metalorganic vapor phase epitaxy. Structural characterization is performed by reciprocal lattice map and transverse scan measurements of (00l)-reflections (l = 2, 4, 6). Transverse scan line profiles of GaP layers on exactly oriented and misoriented substrates are compared thoroughly and antiphase disorder related satellite peaks are observed on exactly oriented substrates. In addition, results imply that antiphase disorder is self-annihilated on misoriented substrates. The dependence of crystallographic tilt on growth temperature indicates structural coherence. Williamson-Hall-like plot of transverse scans reveals the lateral correlation length of crystalline defects of 79 nm which gives the average size of the mosaic crystallites. In addition, the mosaicity of the GaP layer is 0.042°.
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68.47.Fg Semiconductor surfaces
61.72.Mm Grain and twin boundaries
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.ag Semiconductors

CdS nanofilms: Synthesis and the role of annealing on structural and optical properties

Suresh Kumar, Pankaj Sharma, and Vineet Sharma

J. Appl. Phys. 111, 043519 (2012); http://dx.doi.org/10.1063/1.3688042 (6 pages) | Cited 4 times

Online Publication Date: 27 February 2012

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Cadmium sulfide (CdS) nanofilms have been deposited on the glass substrate using the chemical bath technique. Cadmium chloride and thiourea have been used as Cd2+ and S2− ion sources with ammonia as a complexing agent in the presence of a nonionic surfactant. The deposited films have been annealed in air at 373 K, 473 K, 573 K, and 673 K ± 5 K temperature. The effect of the annealing on the structure, morphology, and optical properties of CdS nanofilms has been studied. CdS films have been characterized by X-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, and UV-Vis-NIR spectrophotometer. The CdS films have been found to be nanocrystalline in nature with the zinc blende structure. The direct bandgap has been determined. Various parameters, viz. the grain size, inter-planer spacing, lattice constant, dislocation density, microstrain, surface morphology, absorption coefficient, and optical bandgap has been calculated and found to vary with annealing. The results have been explained on the basis of structural, surface, and optical changes.
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68.55.ag Semiconductors
78.30.Fs III-V and II-VI semiconductors
78.66.Hf II-VI semiconductors
78.40.Fy Semiconductors
68.55.J- Morphology of films
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Effects of P implantation and post-implantation annealing on defect formation in ZnO

X. J. Wang, W. M. Chen, F. Ren, S. Pearton, and I. A. Buyanova

J. Appl. Phys. 111, 043520 (2012); http://dx.doi.org/10.1063/1.3687919 (7 pages) | Cited 3 times

Online Publication Date: 27 February 2012

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Photoluminescence (PL) and optically detected magnetic resonance (ODMR) techniques are utilized to examine the effects of P implantation and post-implantation annealing on defect formation in ZnO single crystals. From ODMR, the main defects created by ion implantation include oxygen and zinc vacancies as a well as a deep donor labeled as PD. The formation of the PD defect is likely promoted by the presence of P as it could only be detected in the P-containing ZnO. The VO and PD centers are found to exhibit low thermal stability and can be annealed out at 800 °C. On the other hand, a new set of defects, such as Z, T, and D* centers, is detected after annealing. Based on measured spectral dependences of the ODMR signals, the VO, VZn, and PD centers are shown to participate in spin-dependent recombination processes related to red emissions, whereas the Z, T, and D* centers are involved in radiative recombination over a wide spectral range of 1.55–2.5 eV. From the PL measurements, combined effects of implantation and annealing also lead to appearance of a new PL band peaking at ∼3.156 eV, likely due to donor-acceptor-pair recombination. The formation of the involved deep acceptor is concluded to be facilitated by the presence of P.
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61.72.uj III-V and II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing
61.72.jd Vacancies
78.55.Et II-VI semiconductors

Refractive index of sodium iodide

G. E. Jellison, Jr., L. A. Boatner, J. O. Ramey, J. A. Kolopus, L. A. Ramey, and D. J. Singh

J. Appl. Phys. 111, 043521 (2012); http://dx.doi.org/10.1063/1.3689746 (4 pages) | Cited 1 time

Online Publication Date: 28 February 2012

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The refractive index of sodium iodide, an important scintillator material that is widely used for radiation detection, is based on a single measurement made by Spangenberg at one wavelength using the index-matching liquid immersion method (Z. Kristallogr. 57, 494 (1923)). In the present paper, we present new results for the refractive index of sodium iodide as measured by the minimum deviation technique at six wavelengths between 436 nm (n = 1.839 ± 0.002) and 633 nm (n = 1.786 ± 0.002). These six measurements can be fit to a Sellmeier model, resulting in a χ2 of 1.02, indicating a good fit to the data. In addition, we report on ellipsometry measurements, which suggest that the near-surface region of the air sensitive NaI crystal seriously degrades, even in a moisture-free environment, resulting in a significantly lower value of the refractive index near the surface. First-principles theoretical calculations of the NaI refractive index that agree with the measured values within 0.025-0.045 are also presented and discussed.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.-q Absorption and reflection spectra: visible and ultraviolet
78.70.Ps Scintillation
back to top Electronic Structure and Transport

Impact of incomplete ionization of dopants on the electrical properties of compensated p-type silicon

M. Forster, A. Cuevas, E. Fourmond, F. E. Rougieux, and M. Lemiti

J. Appl. Phys. 111, 043701 (2012); http://dx.doi.org/10.1063/1.3686151 (7 pages) | Cited 2 times

Online Publication Date: 17 February 2012

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This paper investigates the importance of incomplete ionization of dopants in compensated p-type Si and its impact on the majority-carrier density and mobility and thus on the resistivity. Both theoretical calculations and temperature-dependent Hall-effect measurements demonstrate that the carrier density is more strongly affected by incomplete ionization in compensated Si than in uncompensated Si with the same net doping. The previously suggested existence of a compensation-specific scattering mechanism to explain the reduction of mobility in compensated Si is shown not to be consistent with the T-dependence of the measured carrier mobility. The experiment also shows that, in the vicinity of 300 K, the resistivity of compensated Si has a much weaker dependence on temperature than that of uncompensated silicon.
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61.72.uf Ge and Si
81.05.Cy Elemental semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
72.20.Pa Thermoelectric and thermomagnetic effects

First-principles study on electronic structures and magnetic properties of AlN nanosheets and nanoribbons

Chang-wen Zhang

J. Appl. Phys. 111, 043702 (2012); http://dx.doi.org/10.1063/1.3686144 (6 pages) | Cited 1 time

Online Publication Date: 21 February 2012

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Based on first-principles calculations, the effects of the intrinsic defects and edge states on electronic structures and magnetic properties of AlN nanosheets (NSs) and nanoribbons (NRs) are investigated. In comparison to Al-defective AlN NS, N-defective systems can be easily achieved in experiments, and show a ferromagnetic (FM) property with Curie temperatures above room temperature. For quasi one-dimensional (1D) single-layer zigzag (ZZ) and armchair (AC) AlNNRs with and without edge atoms passivated by hydrogen, the bare and H-passivated AC and ZZNRs are found to be nonmagnetic (NM) semiconductors, whereas a FM character occurs in bare ZZNRs. We also find that the bandgap in H-passivated NRs decreases with the increase of the width of NRs, while bandgap in bare ACNRs increases with increasing the ribbon width. More interesting, in the case of multilayer ZZNRs, when the number of Al-N layers are even, they show NM semiconducting characters, while the odd-layer ZZNRs exhibit magnetic behaviors. Our predicted diverse and tunable electronic and magnetic properties endow AlN nanostructures potential applications in electronics and spintronics.
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71.20.Nr Semiconductor compounds
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.Lf Electronic structure of magnetic nanoparticles
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Pp Magnetic semiconductors

Modeling metastabilities in chalcopyrite-based thin film solar cells

Koen Decock, Paweł Zabierowski, and Marc Burgelman

J. Appl. Phys. 111, 043703 (2012); http://dx.doi.org/10.1063/1.3686651 (7 pages) | Cited 3 times

Online Publication Date: 21 February 2012

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Cu(In,Ga)Se2-based thin film solar cell devices exhibit metastable electrical behavior. This behavior is often ascribed to intrinsic defects that can change configuration accompanied by large lattice relaxations. We extended the thin film solar cell simulation software scaps to enable the simulation of the metastable behavior of this kind of defects. The statistics that are needed to describe metastable defects are discussed. The procedure that has been implemented is introduced, and special attention is paid to the convergence of the method for high defect densities. The model is demonstrated by simulating the effect of voltage induced metastabilities on the capacitance-voltage characteristics. Some of the features present in the measured apparent doping density profiles can be directly related to presence of metastable defects.
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88.40.jn Thin film Cu-based I-III-VI2 solar cells
73.61.Le Other inorganic semiconductors
61.72.up Other materials

Thermoelectric properties of Zn-doped GaSb

Chang-eun Kim, Ken Kurosaki, Hiroaki Muta, Yuji Ohishi, and Shinsuke Yamanaka

J. Appl. Phys. 111, 043704 (2012); http://dx.doi.org/10.1063/1.3678012 (3 pages)

Online Publication Date: 22 February 2012

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III-V compounds with the zinc blende structure have good power factors due to their high carrier mobility. In the present study, Zn-doped GaSb (Ga1−xZnxSb, x = 0.001, 0.002, 0.005, and 0.01) samples were fabricated and the thermoelectric (TE) properties of the samples were measured. Hall measurements were conducted to investigate the carrier transport properties and the results revealed a highest power factor of 2.02 W/m K2 for the sample with x = 0.002 at 573 K, of which the largest weighted mobility was also obtained. However, Zn-doped GaSb also exhibits high thermal conductivity (24.3 W/m K for the sample with x = 0.002 at room temperature). The maximum TE efficiency of 0.23 was obtained at 873 K for the sample with x = 0.01.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.80.Ey III-V and II-VI semiconductors
66.70.Df Metals, alloys, and semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.My Galvanomagnetic and other magnetotransport effects

Influence of Cu column under-bump-metallizations on current crowding and Joule heating effects of electromigration in flip-chip solder joints

Y. C. Liang, W. A. Tsao, Chih Chen, Da-Jeng Yao, Annie T. Huang, and Yi-Shao Lai

J. Appl. Phys. 111, 043705 (2012); http://dx.doi.org/10.1063/1.3682484 (7 pages) | Cited 2 times

Online Publication Date: 22 February 2012

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The electromigration behavior of SnAg solder bumps with and without Cu column under-bump-metallizations (UBMs) has been investigated under a current density of 2.16 × 104 A/cm2 at 150 °C. Different failure modes were observed for the two types of samples. In those without Cu column UBMs, when SnAg solder bumps that had implemented 2 μm Ni UBMs were current stressed at 2.16 × 104 A/cm2, open failure occurred in the bump that had an electron flow direction from the chip side to the substrate side. However, in those with Cu column UBMs, cracks formed along the interface of Cu6Sn5 intermetallic compounds and the solder on the substrate side in the Sn-3.0Ag–0.5Cu solder bump that had an electron flow direction from the substrate side to the chip side. A three-dimensional simulation of the current density distribution was performed in order to obtain a better understanding of the current crowding behavior in solder bumps. The current crowding effect was found to account for the void formation on both the chip and the substrate side for the two kinds of solder bumps. One more important finding, as confirmed by infrared microscopy, is that the alleviation of current crowding by Cu column UBMs also helped decrease the Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2 μm Ni UBMs.
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66.30.Qa Electromigration
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mt Cracks
61.72.Qq Microscopic defects (voids, inclusions, etc.)

High temperature thermoelectric properties of the type-I clathrate Ba8AuxSi46−x

C. Candolfi, U. Aydemir, M. Baitinger, N. Oeschler, F. Steglich, and Yu . Grin

J. Appl. Phys. 111, 043706 (2012); http://dx.doi.org/10.1063/1.3682585 (7 pages)

Online Publication Date: 22 February 2012

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The thermoelectric properties of the type-I clathrate Ba8AuxSi46−x (4.10 ≤ x ≤ 6.10) were characterized from 300 to 700 K. Increasing the Au concentration leads to a transition from an n-type (x < 5.43) to a p-type (x ≥ 5.43) electrical conduction. The experimental data are well described by a single-parabolic-band model assuming a single scattering mechanism of the charge carriers in this temperature range. The lattice thermal conductivity, inferred from degeneracy-adjusted Lorenz numbers, is low regardless of the composition. However, the measured values are significantly lower in the p-type samples possibly due to a combination of a higher degree of disorder in the crystal structure at high Au contents and an enhanced phonon-charge carrier coupling. Even though high thermopower values are achieved, the high electrical resistivity remains the main obstacle to push the dimensionless figure of merit ZT (∼0.2 at around 600 K for x = 5.59) beyond the level of the best Si-based clathrate compounds.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.60.+g Mixed conductivity and conductivity transitions
72.80.Sk Insulators
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
61.66.Fn Inorganic compounds

The impact of Ge codoping on the enhancement of photovoltaic characteristics of B-doped Czochralski grown Si crystal

Mukannan Arivanandhan, Raira Gotoh, Tatsuro Watahiki, Kozo Fujiwara, Yasuhiro Hayakawa, Satoshi Uda, and Makoto Konagai

J. Appl. Phys. 111, 043707 (2012); http://dx.doi.org/10.1063/1.3687935 (6 pages) | Cited 1 time

Online Publication Date: 23 February 2012

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The effect of Ge codoping on minority carrier lifetime in boron (B)-doped Czochralski-silicon (CZ-Si) crystals was investigated. The minority carrier lifetime increased from 110 to 176 µs as Ge concentration was increased from zero to 1 × 1020 cm−3 in B/Ge codoped CZ-Si crystals. Light-induced degradation (LID) experiments showed that B-doped CZ-Si degrades rapidly, while B/Ge codoped CZ-Si degrades more slowly. Moreover, the flow pattern defect (FPD) density of grown-in micro-defects (GMD) in as-grown B/Ge codoped CZ-Si decreased with increasing Ge concentration. From the infrared (IR) absorption studies, it was observed that the interstitial oxygen (Oi) concentration decreased as Ge concentration increased in the crystal. The suppressed LID effect in the B/Ge codoped CZ-Si appears to be related to the low concentration of B-O associated defects, possibly because Ge doping retards the Oi diffusion in addition to the low Oi concentration present (evidenced from IR studies). The mechanism by which the Ge concentration influences the reduction of FPDs and Oi concentration is discussed in terms of Ge-vacancy defect formation during post-growth cooling of the ingots.
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61.72.uf Ge and Si
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.jj Interstitials
61.72.jd Vacancies
81.10.Fq Growth from melts; zone melting and refining
72.40.+w Photoconduction and photovoltaic effects

Epitaxial Ag(001) grown on MgO(001) and TiN(001): Twinning, surface morphology, and electron surface scattering

J. S. Chawla and D. Gall

J. Appl. Phys. 111, 043708 (2012); http://dx.doi.org/10.1063/1.3684976 (10 pages) | Cited 1 time

Online Publication Date: 23 February 2012

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Epitaxial Ag(001) layers were deposited on MgO(001) in order to study electron surface scattering. X-ray reflection indicates 3D layer nucleation with a high rms surface roughness of 1.0 nm for a layer thickness d = 3.5 nm. X-ray diffraction shows that {111} twins form at d < 11 nm, followed by 2nd generation twinning for 11 nm < d < 120 nm. Increasing the growth temperature from 25 to 150 °C suppresses 2nd generation twinning and reduces the twin density by 2 orders of magnitude. In situ deposition of epitaxial 2.5-nm-thick TiN(001) underlayers prior to Ag deposition results in twin-free single-crystal Ag(001) with 10 × smoother surfaces for d = 3.5 nm. This is attributed to a better wetting on the higher energy TiN(001) than MgO(001) surface, resulting in the absence of 3D nuclei with exposed {111} facets, which facilitate twin nucleation. The twinned Ag/MgO layers have a higher resistivity ρ than the single crystal Ag/TiN layers at both 298 and 77 K, due to electron scattering at grain and twin boundaries. The ρ for single-crystal Ag layers increases with decreasing d, which is well explained with known surface scattering models and provides specularity parameters for the Ag-vacuum and the Ag-air interfaces of p = 0.8 ± 0.1 and 0.4 ± 0.1, respectively. A comparison with corresponding epitaxial Cu(001) layers shows that ρAg < ρCu for d > 50 nm, consistent with known bulk values. However, ρAg > ρCu for d < 40 nm. This is attributed to the larger electron mean free path for electron-phonon scattering and a correspondingly higher resistivity contribution from surface scattering in Ag than Cu. In contrast, air exposure causes ρAg < ρCu for all d, due to diffuse scattering at the oxidized Cu surface and the correspondingly higher Cu resistivity.
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73.61.At Metal and metallic alloys
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.at Other materials
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)
61.72.Mm Grain and twin boundaries
81.05.Bx Metals, semimetals, and alloys
68.55.-a Thin film structure and morphology

Thermoelectric prospects of nanomaterials with spin-orbit surface bands

T. E. Huber, K. Owusu, S. Johnson, A. Nikolaeva, L. Konopko, R. C. Johnson, and M. J. Graf

J. Appl. Phys. 111, 043709 (2012); http://dx.doi.org/10.1063/1.3686206 (5 pages)

Online Publication Date: 23 February 2012

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Nanostructured composites and nanowire arrays of traditional thermoelectrics, like Bi, Bi1-xSbx, and Bi2Te3, have metallic Rashba surface spin-orbit bands featuring high mobilities rivaling that of the bulk for which topological insulator behavior has been proposed. Nearly pure surface electronic transport has been observed at low temperatures in Bi nanowires, with diameter around the critical diameter, 50 nm, for the semimetal-to-semiconductor transition. The surface contributes strongly to the thermopower, actually dominating for temperatures T < 100 K in these nanowires. The surface thermopower was found to be –1 T μV/K2, a value that is consistent with theory. We show that surface electronic transport together with boundary phonon scattering leads to enhanced thermoelectric performance at low temperatures of Bi nanowire arrays. We compare with bulk n-BiSb alloys, optimized CsBi4Te6, and optimized Bi2Te3. Surface dominated electronic transport can be expected in nanomaterials of the other traditional thermoelectrics.
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72.15.Jf Thermoelectric and thermomagnetic effects
73.25.+i Surface conductivity and carrier phenomena
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Charge transport in amorphous low bandgap conjugated polymer/fullerene films

Jung Yong Kim, Hyunduck Cho, Seunguk Noh, Yoonkyoo Lee, Young Min Nam, Changhee Lee, and Won Ho Jo

J. Appl. Phys. 111, 043710 (2012); http://dx.doi.org/10.1063/1.3686633 (7 pages)

Online Publication Date: 23 February 2012

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The structural and charge transport properties of a low bandgap copolymer poly(3-hexylthiophene -alt-6,7-dimethyl-4,9-bis-(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (P(3HT-MeTDQ)) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are investigated. Thermal analysis, X-ray scattering diffraction (XRD), atomic force microscopy and transmission electron microscopy (TEM) of P(3HT-MeTDQ) reveal that the polymer is amorphous in solid state. As the hole mobility of P(3HT-MeTDQ) was measured by the time-of-flight photoconductivity method, the mobility was 3.35 × 10−4 cm2/V s, which is very comparable to that of semicrystalline poly(3-hexyl thiophene). When the mobility of amorphous P(3HT-MeTDQ) was analyzed according to the Gaussian disorder model, the polymer has the energetic and positional disorders with the values of σ = 62 meV and Σ = 1.7, respectively, indicating that the polymer has a relatively narrow Gaussian distribution of transport states. Interestingly, when P(3HT-MeTDQ) is blended with PCBM, the amorphous P(3HT-MeTDQ) becomes partially ordered, as evidenced by observation of two discernible XRD peaks at 2θ = 5° (d = 17.7 Å) and 25.5° (d = 3.5 Å) corresponding to the interchain distance and π-stacking distance, respectively. The bicontinuous network morphology was identified at the blend with 60 wt. % PCBM by TEM, at which the charge carrier transport changes from hole-only to ambipolar.
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73.40.-c Electronic transport in interface structures
73.61.Ng Insulators
61.41.+e Polymers, elastomers, and plastics
78.70.Ck X-ray scattering
73.50.Dn Low-field transport and mobility; piezoresistance
78.56.-a Photoconduction and photovoltaic effects

Single-particle tunneling in doped graphene-insulator-graphene junctions

R. M. Feenstra, Debdeep Jena, and Gong Gu

J. Appl. Phys. 111, 043711 (2012); http://dx.doi.org/10.1063/1.3686639 (10 pages) | Cited 5 times

Online Publication Date: 23 February 2012

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The characteristics of tunnel junctions formed between n- and p-doped graphene are investigated theoretically. The single-particle tunnel current that flows between the two-dimensional electronic states of the graphene (2D–2D tunneling) is evaluated. At a voltage bias such that the Dirac points of the two electrodes are aligned, a large resonant current peak is produced. The magnitude and width of this peak are computed, and its use for devices is discussed. The influences of both rotational alignment of the graphene electrodes and structural perfection of the graphene are also discussed.
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73.22.Pr Electronic structure of graphene
73.40.Gk Tunneling
73.23.Hk Coulomb blockade; single-electron tunneling
73.63.-b Electronic transport in nanoscale materials and structures

The reduction of effective doping with extra dopant: n-Type doping of tris(8-hydroxyquinoline) aluminum with K

Hyunbok Lee, Sang Wan Cho, Jeihyun Lee, Pyung Eun Jeon, Kwangho Jeong, Jin Woo Lee, and Yeonjin Yi

J. Appl. Phys. 111, 043712 (2012); http://dx.doi.org/10.1063/1.3686704 (5 pages) | Cited 2 times

Online Publication Date: 23 February 2012

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We studied the n-type doping effect of K deposited on tris(8-hydroxyquinoline) aluminum (Alq3), which has been used for efficient organic semiconducting devices for the last decade. The K doped or inserted at the interface region of the Alq3/cathode has shown highly enhanced device characteristics and yet, peculiarly, extra doping of K has always deteriorated the device properties. We study the interfacial electronic structures of the Alq3–K system using in situ photoemission spectroscopy and a theoretical model to understand the origin of such deterioration. As the K doping progresses, the lowest unoccupied molecular orbital (LUMO) of pristine Alq3 is gradually filled and it becomes an occupied gap state. This reduction of LUMO density of states makes the electron injection diminished, which is the origin of the device deterioration.
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61.72.U- Doping and impurity implantation
81.05.Fb Organic semiconductors
82.30.-b Specific chemical reactions; reaction mechanisms
82.45.Fk Electrodes
79.60.-i Photoemission and photoelectron spectra
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
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