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15 Jun 2002

Volume 91, Issue 12, pp. 9461-10231

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Structure, microstructure, and magneto-optical properties of laser deposited Bi3Fe5O12/Gd3Ga5O12(111) films

S. Kahl, S. I. Khartsev, A. M. Grishin, K. Kawano, G. Kong, R. A. Chakalov, and J. S. Abell

J. Appl. Phys. 91, 9556 (2002); http://dx.doi.org/10.1063/1.1477605 (5 pages) | Cited 13 times

Online Publication Date: 30 May 2002

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Bi3Fe5O12 films were deposited onto single crystal Gd3Ga5O12(111) substrates by pulsed laser deposition. X-ray diffraction patterns and cross sectional transmission electron micrographs show epitaxial film growth and a flat interface between the film and substrate. However, there are small inclusions at the interface and a network of cracks is observed by optical microscope. The refractive index is 3.4 at 800 nm wavelength. The optical transmission is low below 540 nm but increases to 90% for a 1.8 μm thick film at 850 nm. The magneto-optical Faraday rotation is high, with a peak value of −24 deg/μm at 560 nm. The angle of Faraday ellipticity reaches a maximum value of 15 deg/μm at 545 nm. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
78.66.Nk Insulators
75.70.Ak Magnetic properties of monolayers and thin films
78.20.Ls Magneto-optical effects
81.15.Fg Pulsed laser ablation deposition
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Transformation kinetics for the shock wave induced phase transition in cadmium sulfide crystals

M. D. Knudson and Y. M. Gupta

J. Appl. Phys. 91, 9561 (2002); http://dx.doi.org/10.1063/1.1478790 (11 pages) | Cited 11 times

Online Publication Date: 30 May 2002

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Initial stage kinetics of the cadmium sulfide (CdS) phase transition was investigated using picosecond time-resolved electronic spectroscopy in plate-impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in CdS single crystals shocked along a and c axes to stresses ranging between 35 and 90 kbar, which is above the phase-transition threshold stress of approximately 30 kbar. Significant difference in the transformation kinetics was observed for the two crystal orientations. At sufficiently high instantaneous stress, above approximately 60 to 70 kbar for a axis and 50 kbar for c axis, transformation to a metastable state appears to reach a constant state within the 100 ps time resolution. At lower instantaneous stresses, an incubation period on the order of several nanoseconds is observed prior to the onset of electronic changes that mark the onset of the structural change. The subsequent increase in absorbance was quite rapid, with a constant state being reached within the first few nanoseconds after the onset of the structural changes. These results suggest that the nucleation process determines the transformation rate. This insight into transformation kinetics, along with the transformation mechanism obtained from the high-stress experiments, was used to develop a phenomenological model, incorporating ideas of nucleation and growth in martensitic transformations, to simulate the time-dependent extinction of light observed in our experiments. The calculational results incorporating both extinction due to light absorption by the daughter phase volumes and scattering of light by small volumes of the daughter phase were in good agreement with experimental observations. Finally, the orientational differences observed in the transformation kinetics were interpreted in terms of the differences in the elastic-plastic response for the two orientations. © 2002 American Institute of Physics.
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62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
64.60.Q- Nucleation
81.30.Kf Martensitic transformations
81.05.Dz II-VI semiconductors
78.47.-p Spectroscopy of solid state dynamics
71.20.Nr Semiconductor compounds

Electron irradiation induced expansion in amorphous chalcogenide films

J. S. Romero, A. G. Fitzgerald, and K. Mietzsch

J. Appl. Phys. 91, 9572 (2002); http://dx.doi.org/10.1063/1.1478135 (3 pages) | Cited 3 times

Online Publication Date: 30 May 2002

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An expansion of the film surface is observed in amorphous germanium based chalcogenide films when an electron beam is focused in pulsed or fixed mode on the film surface. For thin films of 0.1–1 μm thick the expansion extends laterally over a range of 0.5–1.5 μm and the vertical expansion extends from 0.1–0.6 μm. Increasing the current density of the probe results in an increase in the thickness and a decrease in the width of these features. This phenomenon can be explained in terms of theoretical electrostatics. © 2002 American Institute of Physics.
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61.80.Fe Electron and positron radiation effects
68.55.-a Thin film structure and morphology
61.82.Fk Semiconductors

Metastable and equilibrium phase formation in sputter-deposited Ti/Al multilayer thin films

G. Lucadamo, K. Barmak, C. Lavoie, C. Cabral, and C. Michaelsen

J. Appl. Phys. 91, 9575 (2002); http://dx.doi.org/10.1063/1.1477257 (9 pages) | Cited 9 times

Online Publication Date: 30 May 2002

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The sequence and kinetics of metastable and equilibrium phase formation in sputter deposited multilayer thin films was investigated by combining in situ synchrotron x-ray diffraction (XRD) with ex situ electron diffraction and differential scanning calorimetry (DSC). The sequence included both cubic and tetragonal modifications of the equilibrium TiAl3 crystal structure. Values for the formation activation energies of the various phases in the sequence were determined using the XRD and DSC data obtained here, as well as activation energy data reported in the literature. © 2002 American Institute of Physics.
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68.65.Ac Multilayers
81.05.Bx Metals, semimetals, and alloys
81.30.Bx Phase diagrams of metals, alloys, and oxides
68.55.-a Thin film structure and morphology
82.60.Cx Enthalpies of combustion, reaction, and formation
68.35.Fx Diffusion; interface formation
66.30.Ny Chemical interdiffusion; diffusion barriers
61.66.Dk Alloys

Effect of chemical stress on diffusion in a hollow cylinder

W. L. Wang, Sanboh Lee, and J. R. Chen

J. Appl. Phys. 91, 9584 (2002); http://dx.doi.org/10.1063/1.1477624 (7 pages) | Cited 17 times

Online Publication Date: 30 May 2002

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The effect of chemical stress on diffusion in a hollow cylinder for plane strain and zero axial force has been investigated. Two diffusion processes of constant surface stress and constant surface potential are studied. No matter what the plane strain or zero axial force is the influence of chemical stress on the diffusion process of constant surface potential is similar to that of constant surface stress. Chemical stress enhances both the diffusion coefficient and the concentration. For a given time, the level of concentration becomes lower with a greater ratio of outer radius to inner radius for constant surface potential with given FXf and for constant surface stress with given FXi. F=2Emath/[9(1−v)RT] where E, math, R, and T are the Young’s modulus, partial molal volume, gas constant, and absolute temperature, respectively; Xi and Xf are the initial and final mole fractions at boundary surfaces. The results are also compared with those in thin plates and solid cylinders reported in the literature. © 2002 American Institute of Physics.
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66.30.Dn Theory of diffusion and ionic conduction in solids
05.60.Cd Classical transport
02.70.Bf Finite-difference methods

Influence of deposition conditions on Ir/IrO2 oxygen barrier effectiveness

C. U. Pinnow, I. Kasko, N. Nagel, S. Poppa, T. Mikolajick, C. Dehm, W. Hösler, F. Bleyl, F. Jahnel, M. Seibt, U. Geyer, and K. Samwer

J. Appl. Phys. 91, 9591 (2002); http://dx.doi.org/10.1063/1.1471574 (7 pages) | Cited 24 times

Online Publication Date: 30 May 2002

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The influence of the deposition temperature during the reactive sputtering process on the microstructure of thin Ir and IrO2 films deposited on oxidized Si substrates was investigated and related to the oxygen barrier effectiveness. For this purpose differential thermal analysis combined with residual gas analysis by mass spectrometry was used for the investigation of the microstructural and chemical behavior of the as-sputtered IrO2 films upon heating. Moreover, in situ stress relaxation analyses up to 900 °C, in and ex situ x-ray diffraction measurements were done for various annealing conditions. The investigated polycrystalline IrO2 films exhibited a large compressive stress and a distorted lattice due to the sputter deposition process. It is demonstrated that a high deposition temperature involves a delayed relaxation of the IrO2 grains which is causing an extrinsic, enhanced defect controlled oxygen mobility for the annealing temperatures below the recrystallization. The well-known low intrinsic oxygen diffusivity was only found in those samples which show—in addition to the recovery process—a recrystallization at low temperatures and thus a formation and growth of a new generation of grains with a lattice spacing as in bulk IrO2. Moreover, the oxygen diffusion in Ir films was investigated and the oxygen was found to penetrate the Ir films very quickly at elevated temperatures. The microstructure of the films was investigated by cross sectional transmission electron microscopy and it is shown that the cold-sputtered columnar IrO2 films protect the underlying layers from oxidation during a 700 °C high temperature oxygen anneal with an optimized Ir/IrO2 oxygen barrier stack. © 2002 American Institute of Physics.
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68.35.Fx Diffusion; interface formation
66.30.J- Diffusion of impurities
81.15.Cd Deposition by sputtering
68.55.-a Thin film structure and morphology
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.Cc Kinetics of defect formation and annealing
68.60.Bs Mechanical and acoustical properties
68.37.Lp Transmission electron microscopy (TEM)
85.50.Gk Non-volatile ferroelectric memories

Optical properties of a laser dye in a solid-state polymeric host

Dhiraj K. Sardar and Felipe S. Salinas

J. Appl. Phys. 91, 9598 (2002); http://dx.doi.org/10.1063/1.1476967 (5 pages) | Cited 1 time

Online Publication Date: 30 May 2002

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The optical properties of C28H20N2O2B2F4H2O, an organic laser dye embedded in a solid plastic host, have been characterized for a number of laser wavelengths in the visible region. The index of refraction of the dye in a plastic host is measured by the conventional method of minimum deviation at these wavelengths. The one-dimensional two flux Kubelka–Munk method based on the diffusion approximation has been employed to determine the absorption, scattering, and scattering anisotropy coefficients of the dye in a plastic host from the measurements of total diffuse transmittance, total diffuse reflectance, and collimated transmittance using double-integrating spheres. The total attenuation coefficients obtained by this method have been compared with those determined by the collimated transmission and from the total attenuation measurement on a Cary-14 spectrophotmeter. © 2002 American Institute of Physics.
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42.55.Mv Dye lasers
78.55.Kz Solid organic materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Model for the director and electric field in liquid crystal cells having twist walls or disclination lines

G. Panasyuk and D. W. Allender

J. Appl. Phys. 91, 9603 (2002); http://dx.doi.org/10.1063/1.1477613 (10 pages) | Cited 5 times

Online Publication Date: 30 May 2002

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Two examples of the director structure and electric field in patterned electrode liquid crystal cells are studied using a recently developed calculational model. First, a display cell that exhibits a homeotropic to multidomainlike transition with twist wall structures has been considered for a liquid crystal with positive dielectric anisotropy. The model elucidates the behavior of the electric field. Calculations show good agreement between the model and direct computer solution of the Euler–Lagrange equations, but the model is at least 30 times faster. Second, the possibility that a cell has ±1/2 disclination lines instead of a wall defect is probed. A temperature dependent estimate for the size of the defect core is given, and the total free energy of the cell with disclination lines was calculated and compared with the corresponding value for the same cell with wall defect structure. © 2002 American Institute of Physics.
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61.30.Jf Defects in liquid crystals
61.30.Cz Molecular and microscopic models and theories of liquid crystal structure
64.70.M- Transitions in liquid crystals
65.20.-w Thermal properties of liquids
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Experimental investigation and modeling of diffusion in the InP/(In,Ga)As heterostructures

Jiri Bursik, Dmitri V. Malakhov, Yaoji Wang, George C. Weatherly, and Gary R. Purdy

J. Appl. Phys. 91, 9613 (2002); http://dx.doi.org/10.1063/1.1477264 (9 pages) | Cited 3 times

Online Publication Date: 30 May 2002

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The interdiffusion of lattice-matched InP/(In,Ga)As superlattice structures (nominally undoped, p-doped and n-doped) has been studied by analytical electron microscopy (x-ray analysis) using a field emission gun, scanning transmission electron microscope. The point-spread function of the electron beam was used to correct the experimental data (obtained as x-ray maps, 50×50 nm in area) in order to derive diffusion profiles of the group V elements (As, P) after annealing. The results, showing a marked asymmetry in the As profiles after annealing, are interpreted using a model based on the coherent diffusion of the group III and V elements on their own sublattices, each of which is treated as a regular solution. A mathematical procedure, based on the minimization of the difference between the measured and predicted concentration profiles, is employed to compute the two lattice-specific composition-dependent velocities from the experimental diffusion profiles. A good agreement is found between the experimental measurements and the predictions of the model. The role of coherency strains in the interdiffusion process is discussed. © 2002 American Institute of Physics.
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66.30.Ny Chemical interdiffusion; diffusion barriers
68.65.Cd Superlattices
68.35.Fx Diffusion; interface formation
68.37.Lp Transmission electron microscopy (TEM)
68.35.Ct Interface structure and roughness
81.40.Gh Other heat and thermomechanical treatments
61.72.Cc Kinetics of defect formation and annealing

Study of light emission from GaN/AlGaN quantum wells under power-dependent excitation

S. P. Łepkowski, T. Suski, P. Perlin, V. Yu. Ivanov, M. Godlewski, N. Grandjean, and J. Massies

J. Appl. Phys. 91, 9622 (2002); http://dx.doi.org/10.1063/1.1476082 (7 pages) | Cited 14 times

Online Publication Date: 30 May 2002

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We have performed a study of excitation power-dependent spectra of GaN/AlGaN single quantum wells (QWs). First, the experimental “blueshift” of the emission energy, due to screening of internal piezoelectric fields, was compared with the model calculations based on self-consistent solution of Schroedinger and Poisson equations. We found that, even for the highest applied levels of excitation power (2.5 MW/cm2), only 0.5×1012 cm−2 carriers were present in the QW layers. Second, we analyzed the evolution of power-dependent spectra of two single QW having different widths. For the thinner QW (2.1 nm), the peak corresponding to a QW photoluminescence (PL) emission dominates the entire spectrum in the whole range of the used excitation power. In the case of the wider QW (4.4 nm), for sufficiently high excitation power, we observe the effect of PL quenching. Using the rate equation model we show that the observed effect of the PL quenching can be associated with the reduction of exciton binding energy due to the many body interactions in the QW. © 2002 American Institute of Physics.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.21.Fg Quantum wells
71.35.Lk Collective effects (Bose effects, phase space filling, and excitonic phase transitions)

Temperature effects on the 1.0 and 1.3 μm emission lines of Nd3+ in a LaSc3(BO3)4 crystal host

Dhiraj K. Sardar, Joe A. French, Francisco Castano, Anthony Sayka, and Felipe S. Salinas

J. Appl. Phys. 91, 9629 (2002); http://dx.doi.org/10.1063/1.1476963 (6 pages) | Cited 2 times

Online Publication Date: 30 May 2002

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A detailed investigation of the temperature effect on widths and shifts of the spectral lines of Nd3+ in LaSc3(BO3)4 has been performed. The spectral lines investigated correspond to the inter-Stark transitions R1→Y3 (1062 nm) and R1→Z4 (1344 nm) within the 4F3/2→4I11/2 and 4F3/2→4I13/2 intermanifold transitions, respectively. The widths of these lines and their shifts have been measured as a function of temperature. The linewidths are found to increase with increasing temperature for both transitions; the 1062 nm line shifts to the longer wavelengths, whereas the 1344 nm line shifts to the shorter wavelengths. The theory of phonon-ion interactions has been employed to explain the temperature dependencies of widths and shifts. These interactions give rise to the phonon relaxation processes that are responsible for both the broadening and shifts of the spectral lines. © 2002 American Institute of Physics.
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78.55.Hx Other solid inorganic materials
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
63.20.-e Phonons in crystal lattices
42.70.Hj Laser materials

Polymer absorbers containing magnetic particles: Effect of polymer permittivity on wave absorption in the quasi-microwave band

Morihiko Matsumoto and Yoshimori Miyata

J. Appl. Phys. 91, 9635 (2002); http://dx.doi.org/10.1063/1.1475372 (3 pages) | Cited 16 times

Online Publication Date: 30 May 2002

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The effect of polymer permittivity on wave absorption in the quasi-microwave band has been elucidated for polymer composites containing magnetic metal particles. Absorbers composed of these composites can be thin when their real permittivities are high and their imaginary parts are low. However, in general, for polymer composites the real part actually increases as the imaginary part increases in this band. This implies that in designing absorbers we must choose a polymer species on the crossing area of the real and imaginary parts of the permittivity. Furthermore, to obtain thin absorbers, we should choose a polymer species with high real permittivities and low imaginary parts. © 2002 American Institute of Physics.
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78.70.Gq Microwave and radio-frequency interactions
77.22.Ch Permittivity (dielectric function)
75.50.Tt Fine-particle systems; nanocrystalline materials

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

C. Adelmann, J. Brault, D. Jalabert, P. Gentile, H. Mariette, Guido Mula, and B. Daudin

J. Appl. Phys. 91, 9638 (2002); http://dx.doi.org/10.1063/1.1471923 (8 pages) | Cited 67 times

Online Publication Date: 30 May 2002

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The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed. © 2002 American Institute of Physics.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.Dv Composition, segregation; defects and impurities
68.55.A- Nucleation and growth
81.05.Ea III-V semiconductors
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Ja Surface and interface dynamics and vibrations
68.43.Mn Adsorption kinetics
52.77.Dq Plasma-based ion implantation and deposition

Kinetics and mechanisms of the thermal degradation of amorphous carbon films

Li Hong Zhang, Hao Gong, and Jian Ping Wang

J. Appl. Phys. 91, 9646 (2002); http://dx.doi.org/10.1063/1.1478133 (6 pages) | Cited 2 times

Online Publication Date: 30 May 2002

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The thermal stability of amorphous carbon thin films is significantly improved by increasing the carbon-plasma energy in dc magnetron sputtering. The on-set decomposition temperature in air increased from 335 °C to 525 °C, while the apparent activation energy ΔE in the early decomposition stage increased from approx. 90 to 180 kJ/mol when the pressure in the sputtering chamber changed from 16×10−3 Torr to 1×10−3 Torr. Thermogravimetry (TG) analysis clearly showed that with the increase of carbon-plasma energy, the amount of unbounded and loosely bounded molecules in the structure reduced significantly. A study of decomposition mechanisms based on isothermal TG analysis showed that the decomposition of the films could best be expressed in an apparent nth order reaction, or −[ln(1−α)]n∼t. The apparent reaction order n reduced with the increase of temperature, and also with the carbon-plasma energy during film deposition. The temperature effect on n is explained from the diffusion resistance of decomposed products. The film structure deposited at higher energy carbon plasma showed higher bonding ratio and structure density, which yielded the film of higher thermal stability, ΔE, and lower n values. © 2002 American Institute of Physics.
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68.60.Dv Thermal stability; thermal effects
81.05.U- Carbon/carbon-based materials
68.55.-a Thin film structure and morphology
81.15.Cd Deposition by sputtering
68.55.A- Nucleation and growth
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
61.43.Er Other amorphous solids

Modeling of elastic deformation of multilayers due to residual stresses and external bending

Chun-Hway Hsueh

J. Appl. Phys. 91, 9652 (2002); http://dx.doi.org/10.1063/1.1478137 (5 pages) | Cited 81 times

Online Publication Date: 30 May 2002

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A general closed-form solution for elastic deformation of multilayers due to residual stresses and external bending is derived. Based on the general solution, simplified solutions for residual stress distributions in multiple layers of thin films on a thick substrate are obtained. These simplified solutions can be expressed as functions of either mismatch strains between film layers and substrate or the curvature of the system. The simplified solution for the special case of one film layer on a substrate is also presented, and its accuracy is discussed. © 2002 American Institute of Physics.
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68.35.Gy Mechanical properties; surface strains
68.65.Ac Multilayers
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep

Reduction of intensity from coatings on cathodoluminescent phosphors: MgO or Al2O3 on Y2O3:Eu or Y2SiO5:Tb

W. J. Thomes, C. H. Seager, and P. H. Holloway

J. Appl. Phys. 91, 9657 (2002); http://dx.doi.org/10.1063/1.1478798 (6 pages) | Cited 3 times

Online Publication Date: 30 May 2002

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Coatings of MgO and Al2O3 were deposited on Y2O3:Eu and Y2SiO5:Tb field emission display phosphors by pulsed laser deposition. Steady-state and pulsed cathodoluminescence (CL) data were collected over a range of electron beam energies up to 4 keV. The reduced CL intensities from coated phosphors are consistent with a reduction in the energy of the incoming primary electrons. This energy loss can be attributed to inelastic collisions between the incident electron and the electrons of atoms in the coating layer. Monte Carlo simulations were used to verify the modeled results. © 2002 American Institute of Physics.
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78.60.Hk Cathodoluminescence, ionoluminescence
78.66.Nk Insulators
81.15.Fg Pulsed laser ablation deposition

Atomic level characterization of ultrathin flat cobalt disilicide film with three crystalline domains

Takumi Ohtomo, Tadahiro Kawasaki, and Yoshizo Takai

J. Appl. Phys. 91, 9663 (2002); http://dx.doi.org/10.1063/1.1478797 (4 pages) | Cited 2 times

Online Publication Date: 30 May 2002

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The crystal structure and interface structure of ultrathin cobalt disilicide films, prepared on a Si(001) substrate at 400 °C, have been examined using transmission electron microscopy (TEM). Analysis of the electron diffraction data reveals that there are three general types of epitaxy between the silicon substrates and the cobalt disilicide films: (1) CoSi2(110)//Si(001), CoSi2[001]//Si[110]; (2) CoSi2(110)//Si(001), CoSi2[math10]//Si[110]; and (3) CoSi2(001)//Si(001), CoSi2[110]//Si[110]. Using cross-sectional high-resolution TEM for all three types of epitaxy showed that (1) the cobalt disilicide films form an ultrathin flat layer and (2) the interface between the substrate and the film is abrupt with an atomic level for all three types of epitaxy. Dark-field observations also revealed unique localized distributions of three different types of crystalline domains. The domains of CoSi2(110) had elliptic and those of CoSi2(001) had round shapes that were several hundred nanometers in diameter. Each domain consisted of small crystalline epitaxial particles approximately 5–30 nm in diameter as well as an amorphous phase consisting of a cobalt disilicide crystal structure characterized by short-range order. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
68.35.Ct Interface structure and roughness
68.37.Lp Transmission electron microscopy (TEM)

Fully leaky guided mode study of an orthoconic antiferroelectric liquid crystal cell deviating from perfect horizontal surface stabilization

Giusy Scalia, Per Rudquist, David S. Hermann, Koen D’havé, Sven T. Lagerwall, and J. Roy Sambles

J. Appl. Phys. 91, 9667 (2002); http://dx.doi.org/10.1063/1.1473228 (9 pages) | Cited 12 times

Online Publication Date: 30 May 2002

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The optical properties of an antiferroelectric liquid crystal (AFLC) cell slightly deviating from the ideal orthoconic condition, i.e., 45° tilt angle and fully horizontal surface stabilization (the molecular tilt plane parallel to the glass plates), have been investigated using the fully leaky guided mode (FLGM) technique. In contrast to standard optical investigative methods, such as polarization microscopy, the FLGM data give information about the orientation of the index ellipsoid or the molecular tilt plane, not only in the plane of the cell but in three dimensions. The sensitivity of the FLGM technique thus allows a detailed optical characterization on a mesoscopic scale and, in particular, has allowed us to discriminate, in the investigated cell, a tilt out of the substrate plane in the anticlinic state, thus a deviation from the horizontal surface stabilization condition. Orthoconic surface-stabilized AFLCs are, in the ideal case, uniaxial negative with the optic axis perpendicular to the glass plates and therefore provide a perfect dark state between crossed polarizers even if the direction of the smectic layer normal is not homogeneous in the plane of the cell. Small deviations from the ideal case lead to the appearance of a very small birefringence in the cell plane which, however, should not compromise the attractive optical properties of this class of AFLCs. Using laser diffraction we have verified that, even with the deviation from the horizontal surface stabilization detected by the FLGM study, a horizontal chevron geometry, clearly visible in the synclinic state, is not optically detectable in the anticlinic state of orthoconic AFLCs. We conclude that the attractive optical properties are preserved in a reasonably large interval around the ideal orthoconic conditions allowing for the practical exploitation of these materials as key components in liquid crystal displays and other electro-optic devices. © 2002 American Institute of Physics.
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61.30.-v Liquid crystals
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
78.20.Fm Birefringence

Modeling of the induced charge in semiconductor gamma-ray imaging array

M. Ifraimov, A. Ludwig, and Y. Nemirovsky

J. Appl. Phys. 91, 9676 (2002); http://dx.doi.org/10.1063/1.1471922 (9 pages) | Cited 1 time

Online Publication Date: 30 May 2002

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Pixelated two-dimensional (2D) arrays of gamma-ray spectrometers based on CdZnTe are of great interest due to the high spatial and energy resolution they provide. In contrast to a single spectrometer, in a 2D array, the field is not uniform and the induced charge is not proportional to the carrier distance from the anodes. For complete performance modelling of such an array, there is a need to calculate the charge induced on the anodes by a single photogenerated charge carrier. In this study, we present a solution based on the Moments Method, which considers the gaps between the anodes, i.e. mixed boundary conditions. This solution is compared with an approximation presented in previous studies. Good agreement is found in the region under an anode, however, in between anodes, more accurate results are achieved using the Moments Method. © 2002 American Institute of Physics.
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29.40.Wk Solid-state detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)

Ammonia-molecular-beam epitaxial growth and optical properties of GaN/AlGaN quantum wells

H. Tang, J. B. Webb, P. Sikora, S. Raymond, and J. A. Bardwell

J. Appl. Phys. 91, 9685 (2002); http://dx.doi.org/10.1063/1.1479756 (4 pages) | Cited 2 times

Online Publication Date: 30 May 2002

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GaN/AlGaN quantum wells on GaN templates have been grown on (0001) sapphire substrates using the ammonia-molecular-beam epitaxy technique. The GaN template layers were of the type used previously for growing high-mobility, heterostructure field-effect transistor structures. The photoluminescence properties of the quantum wells showed strong quantum-confined Stark effect in good agreement with theoretical calculations, as well as evidence of carrier localization due to in-plane well width fluctuation. At low temperature, the quantum well emissions were dominated by two or more localized exciton levels. Carrier thermalization to the lower localized level was observed as the temperature was raised. Evidence of interwell transfer of photocarriers was observed, with the holes appearing to transfer in the direction of the internal field within the AlGaN barrier. © 2002 American Institute of Physics.
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81.07.St Quantum wells
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Ea III-V semiconductors
78.67.De Quantum wells
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
73.21.Fg Quantum wells
78.20.Jq Electro-optical effects
71.35.Cc Intrinsic properties of excitons; optical absorption spectra

Sharp tip geometry and its effect on hardness in nanoindentation experiments

Kazuhiro Ikezawa and Tadashi Maruyama

J. Appl. Phys. 91, 9689 (2002); http://dx.doi.org/10.1063/1.1478794 (7 pages) | Cited 9 times

Online Publication Date: 30 May 2002

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In this article, we used an indenter, sharper than a Berkovich indenter, consisting of a triangular pyramid with a face angle of 80°, and compared the difference between our own and the Berkovich indenters. First, the cross-sectional shapes of the indenters were analyzed by the Shih, Yang, and Li model [C. W. Shih, M. Yang, and J. C. M. Li, J. Mater. Res. 6, 2623 (1991)]. The result suggested that our sharp indenter has more deviation from the ideal triangular pyramid than the Berkovich indenter, resulting from the difference in sharpness of the indenters. Second, we determined an area function for our indenter using fused silica and compared it with those of an ideal triangular pyramid and a Berkovich indenter. There was a larger difference between the real and ideal indenters in our indenter than in the Berkovich case. Third, hardness measurements indicated that hardness increases with increases in depth and reaches a constant value in both cases of fused silica and single-crystal Si. This dependence of hardness on depth is explained herein in a qualitative manner. © 2002 American Institute of Physics.
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81.70.Bt Mechanical testing, impact tests, static and dynamic loads
62.20.M- Structural failure of materials
62.20.Qp Friction, tribology, and hardness
68.35.Gy Mechanical properties; surface strains
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
68.35.B- Structure of clean surfaces (and surface reconstruction)

Interdiffusion, stress, and microstructure evolution during annealing in Co/Cu/Co trilayers

W. Brückner, S. Baunack, J. Thomas, M. Hecker, and C. M. Schneider

J. Appl. Phys. 91, 9696 (2002); http://dx.doi.org/10.1063/1.1479750 (5 pages) | Cited 4 times

Online Publication Date: 30 May 2002

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In order to find interdiffusion and microstructural changes on sputtered 90 nm Co/180 nm Cu/90 nm Co trilayers during annealing, the evolution of the stress and electrical resistance was studied in situ during temperature ramps and isothermal annealing as well as concentration-depth profiles, the grain morphology, and the phase and texture formation were investigated after heat treatment at various temperatures. Up to 450 °C, no distinct lattice interdiffusion was observed. Grain-boundary diffusion of Cu through the Co top layer to the surface as well as a distinct growth of Cu grains into the Co top layer start to occur at about 450 °C. The conclusions corroborate findings on Cu/Co nanoscale multilayers, which show a deterioration of the giant magnetoresistance effect at high temperatures due to similar microscopic mechanisms. © 2002 American Institute of Physics.
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68.65.Ac Multilayers
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
68.35.Fx Diffusion; interface formation
81.15.Cd Deposition by sputtering
66.30.Ny Chemical interdiffusion; diffusion barriers
61.72.Cc Kinetics of defect formation and annealing
61.72.Mm Grain and twin boundaries
75.47.De Giant magnetoresistance
75.50.Cc Other ferromagnetic metals and alloys

Theoretical mass sensitivity of Love wave and layer guided acoustic plate mode sensors

G. McHale, M. I. Newton, and F. Martin

J. Appl. Phys. 91, 9701 (2002); http://dx.doi.org/10.1063/1.1477603 (10 pages) | Cited 22 times

Online Publication Date: 30 May 2002

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A model for the mass sensitivity of Love wave and layer guided shear horizontal acoustic plate mode (SH–APM) sensors is developed by considering the propagation of shear horizontally polarized acoustic waves in a three layer system. A dispersion equation is derived for this three layer system and this is shown to contain the dispersion equation for the two layer system of the substrate and the guiding layer plus a term involving the third layer, which is regarded as a perturbing mass layer. This equation is valid for an arbitrary thickness perturbing mass layer. The perturbation, Δν, of the wave speed for the two-layer system by a thin third layer of density, ρp and thickness Δh is shown to be equal to the mass per unit area multiplied by a function dependent only on the properties of the substrate and the guiding layer, and the operating frequency of the sensor. The independence of the function from the properties of the third layer means that the mass sensitivity of the bare, two-layer, sensor operated about any thickness of the guiding layer can be deduced from the slope of the numerically or experimentally determined dispersion curve. Formulas are also derived for a Love wave on an infinite thickness substrate describing the change in mass sensitivity due to a change in frequency. The consequences of the various formulas for mass sensing applications are illustrated using numerical calculations with parameters describing a (rigid) poly(methylmethacrylate) wave-guiding layer on a finite thickness quartz substrate. These calculations demonstrate that a layer-guided SH–APM can have a mass sensitivity comparable to, or higher, than that of Love waves propagating on the same substrate. The increase in mass sensitivity of the layer guided SH–APMs over previously studied SH–APM sensors is of significance, particularly for liquid sensing applications. The relevance of the dispersion curve to experiments using higher frequencies or frequency hopping and to experiments using thick guiding layers is discussed. © 2002 American Institute of Physics.
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07.64.+z Acoustic instruments and equipment
43.20.Mv Waveguides, wave propagation in tubes and ducts
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Two different behavior patterns of photoluminescence depending on Mg doping rate in p-type GaN epilayers

S. M. Jeong, H. W. Shim, H. S. Yoon, M. G. Cheong, R. J. Choi, E.-K. Suh, and H. J. Lee

J. Appl. Phys. 91, 9711 (2002); http://dx.doi.org/10.1063/1.1479478 (5 pages) | Cited 10 times

Online Publication Date: 30 May 2002

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Mg-doped p-type GaN epilayers grown by metalorganic chemical vapor deposition exhibit two different types of photoluminescence (PL) characteristics depending on Mg doping rate; the existence of critical Mg concentration is found where both electrical and optical characteristics show an abrupt change in their behavior. In samples with relatively low Mg concentration, a band edge emission, a peak associated with shallow donor-acceptor pair recombination, and a 2.8 eV blue band emission appear in the PL spectrum. Intensities of all three PL peaks decrease with increasing annealing temperature; the blue band quenches most rapidly. In contrast, in samples grown with high ratio of Mg/Ga flow rate, only a strong blue emission band is observed and the intensity of the blue band increases with the annealing temperature suggesting a different origin from the blue band of lightly doped samples. From the annealing temperature dependence of the blue band intensity, we speculate that the emission mechanism of the blue band in relatively lightly Mg-doped GaN epilayers involves Mg-related deep level complexes such as substitutional/interstitial Mg-related complexes. On the other hand, a model involving a formation of pyramidal defects and the presence of Mg-rich inclusions which modifies the dominant optical transition can be a possible explanation for the origin of the blue band in heavily doped ones. © 2002 American Institute of Physics.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Cc Kinetics of defect formation and annealing
71.55.Eq III-V semiconductors
61.72.J- Point defects and defect clusters
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Strain relaxation of SiGe islands on compliant oxide

H. Yin, R. Huang, K. D. Hobart, Z. Suo, T. S. Kuan, C. K. Inoki, S. R. Shieh, T. S. Duffy, F. J. Kub, and J. C. Sturm

J. Appl. Phys. 91, 9716 (2002); http://dx.doi.org/10.1063/1.1479757 (7 pages) | Cited 41 times

Online Publication Date: 30 May 2002

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The relaxation of patterned, compressively strained, epitaxial Si0.7Ge0.3 films transferred to borophosphorosilicate (BPSG) glass by a wafer-bonding and etch-back technique was studied as an approach for fabricating defect-free Si1−xGex relaxed films. Both the desired in-plane expansion and undesired buckling of the films concurrently contribute to the relaxation. Their relative role in the relaxation process was examined experimentally and by modeling. Using x-ray diffraction, Raman scattering and atomic force microscopy, the dynamics of in-plane expansion and buckling of Si0.7Ge0.3 islands for island sizes ranging from 10 μm×10 μm to 200 μm×200 μm for anneal temperatures between 750 and 800 °C was investigated. Lateral relaxation is favored in small and thick islands, and buckling is initially dominant in large and thin islands. Raising the temperature to lower viscosity of the oxide enhances the rate of both processes equally. For very long annealing times, however, the buckling disappeared, allowing larger, flat, and relaxed islands to be achieved. Cross-sectional transmission electron microscopy observation on a relaxed Si0.70Ge0.30 island revealed no dislocations, confirming that SiGe relaxation on BPSG is a good approach to achieve high quality relaxed SiGe. © 2002 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
81.05.Hd Other semiconductors
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
62.20.F- Deformation and plasticity
78.66.Li Other semiconductors
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.43.Fg Adsorbate structure (binding sites, geometry)
68.55.-a Thin film structure and morphology
68.37.Lp Transmission electron microscopy (TEM)
61.72.Cc Kinetics of defect formation and annealing
68.37.Ps Atomic force microscopy (AFM)
78.30.-j Infrared and Raman spectra
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