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1 Nov 1994

Volume 76, Issue 9, pp. 4935-5603

Page 1 of 4 Pages Next Page | Jump to Page

Characterization of defects in Si and SiO2−Si using positrons

P. Asoka‐Kumar, K. G. Lynn, and D. O. Welch

J. Appl. Phys. 76, 4935 (1994); http://dx.doi.org/10.1063/1.357207 (48 pages) | Cited 172 times

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In the past few years, there has been rapid growth in the positron annihilation spectroscopy (PAS) of overlayers, interfaces, and buried regions of semiconductors. There are few other techniques that are as sensitive as PAS to low concentrations of open‐volume‐type defects. The characteristics of the annihilation gamma rays depend strongly on the local environment of the annihilation sites and are used to probe defect concentrations in a range inaccessible to conventional defect probes, yet which are important in the electrical performance of device structures. We show how PAS can be used as a nondestructive probe to examine defects in technologically important Si‐based structures. The discussion will focus on the quality of overlayers, formation and annealing of defects after ion implantation, identification of defect complexes, and evaluation of the distribution of internal electric fields. We describe investigations of the activation energy for the detrapping of hydrogen from SiO2−Si interface trap centers, variations of interface trap density, hole trapping at SiO2−Si interfaces, and radiation damage in SiO2−Si systems. We also briefly summarize the use of PAS in compound semiconductor systems and suggest some future directions.
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61.72.-y Defects and impurities in crystals; microstructure
71.55.-i Impurity and defect levels
78.70.Bj Positron annihilation

Stark effect and excitonic tunneling escape process in semiconductor quantum wells

A. Hernández‐Cabrera, P. Aceituno, and H. Cruz

J. Appl. Phys. 76, 4983 (1994); http://dx.doi.org/10.1063/1.357208 (6 pages) | Cited 3 times

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In this work, we have numerically integrated in space and time the effective mass Schrödinger equation for an exciton in a semiconductor quantum‐well structure. Considering a Coulomb interaction between the electron‐hole pair and an external electric field, we have studied the excitonic tunneling escape process from semiconductor quantum wells. Our method of calculation has been applied to types‐I, ‐II, and ‐III quantum‐well superlattices. In addition, we present the calculated excitonic lifetimes for the GaAs/GaAlAs, InAs/GaSb, and HgTe/HgCdTe systems under an external electric field. In the HgTe/CdTe system, the possibility of having similar electron and hole lifetime values is also found if the applied electric field is large enough.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
71.35.-y Excitons and related phenomena
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect

Optimization of two dimensional gratings for very long wavelength quantum well infrared photodetectors

G. Sarusi, B. F. Levine, S. J. Pearton, K. M. S. Bandara, R. E. Leibenguth, and J. Y. Andersson

J. Appl. Phys. 76, 4989 (1994); http://dx.doi.org/10.1063/1.357209 (6 pages) | Cited 17 times

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We have performed a detailed study of two‐dimensional grating coupling for quantum well infrared photodetectors in the very long wavelength spectral region λ∼16–17 μm. Using calculations based on the modal expansion method we quantitatively explain the double peaked responsivity spectrum. By optimizing the grating parameters we achieve a normal incidence responsivity and detectivity which are three times larger than the 45° angle of incidence geometry.
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42.79.Dj Gratings
85.60.Gz Photodetectors (including infrared and CCD detectors)
78.30.Fs III-V and II-VI semiconductors

Enhanced photorefractive performance in a photorefractive polymeric composite

Maciek E. Orczyk, Bogdan Swedek, Jaroslaw Zieba, and Paras N. Prasad

J. Appl. Phys. 76, 4995 (1994); http://dx.doi.org/10.1063/1.357210 (4 pages) | Cited 17 times

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We characterize the photorefractive behavior of a photorefractive multicomponent polymer composite of PVK‐TCP:C60:DEANST. Efficient plasticization of the host polymeric matrix and utilization of a nonlinear chromophore with a large dipole moment provide a large poling‐induced electro‐optic coefficient. Diffraction efficiencies as high as 40% and asymmetric net two‐beam coupling gain coefficients in excess of 130 cm−1, surpassing those of known inorganic single‐crystalline photorefractive media, are reported.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Jk Polymers and organics

Characterization of optical waveguides in KTiOPO4 by second harmonic spectroscopy

M. G. Roelofs, A. Suna, W. Bindloss, and J. D. Bierlein

J. Appl. Phys. 76, 4999 (1994); http://dx.doi.org/10.1063/1.357211 (8 pages) | Cited 21 times

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Segmented optical waveguides were fabricated in KTiOPO4 by Rb/Ba ion exchange. The waveguides were characterized by measuring, as a function of input optical wavelength, the Bragg‐reflected power, and the intensity of second harmonic (SH) radiation generated within the waveguides. Seven peaks in the SH spectra are assigned to different interactions involving the two lowest‐order spatial modes at the fundamental wavelength and the three lowest‐order spatial modes at the SH wavelength. ‘‘Combined mode’’ SH interactions are found which sum spectrally degenerate fundamental waves which are spatially nondegenerate. Analysis of the spectra is shown to be a convenient method of determining the effective indices of the propagating modes, and these then determine the refractive index depth profile of the waveguide. The relative intensities of the SH peaks are dependent on the depth of the region within the ion‐exchanged area which has a reversal of the ferroelectric polarization direction (reversed domain). In some waveguides, the spectra imply that the area of reversed domain is buried, and does not extend completely to the surface.
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42.79.Gn Optical waveguides and couplers
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Acoustic transient generation by holmium‐laser‐induced cavitation bubbles

T. Asshauer, K. Rink, and G. Delacrétaz

J. Appl. Phys. 76, 5007 (1994); http://dx.doi.org/10.1063/1.357212 (7 pages) | Cited 26 times

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The acoustic effects of free‐running 2.12 μm Cr:Tm:Ho:YAG laser pulses delivered in water are studied. Laser pulses of 10 to 1200 mJ energy and 230 μs duration (full width at half‐maximum) are used. Delivery fiber diameters of 200–600 μm are investigated. Combined fast flash video imaging and needle probe hydrophone pressure sensing are used. The experimental results show that the laser‐induced water vapor bubbles can generate strong acoustic transients at the bubble collapse several hundreds of μs after the start of the laser pulse. Pressures of up to 3600 bar are measured. Above a laser fluence threshold of 40 J/cm2 the pressure amplitude increases sharply, reaching a maximum value between 100 and 200 J/cm2. At higher fluences up to more than 1000 J/cm2, the pressure amplitude is found to decrease again. A two‐phase mechanism is proposed to describe the complex bubble dynamics generated by the free‐running pulses: The isotropic expansion of an initially superheated water volume is followed by a continuous ablation phase. The results suggest a mechanism of possible unwanted acoustic damage during Holmium laser medical applications in a liquid environment.
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43.25.Yw Nonlinear acoustics of bubbly liquids
42.62.Be Biological and medical applications
87.50.W- Optical/infrared radiation effects

Hyperbolic temperature profiles for laser surface interactions

Ali Vedavarz, Kunal Mitra, and Sunil Kumar

J. Appl. Phys. 76, 5014 (1994); http://dx.doi.org/10.1063/1.357213 (8 pages) | Cited 16 times

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This study theoretically analyzes the transient temperature distributions in laser irradiated materials by considering a hyperbolic heat conduction model. Exact and limiting mathematical solutions for the temperature distributions are developed and important parameters are identified. Traditional Fourier transient heat conduction models are parabolic in nature, which imply an infinite speed of propagation of the thermal signal in the material. Hyperbolic non‐Fourier models have been introduced to account for the finite speed of the thermal wave. The effects of finite speed are significant in short‐pulse applications where the time period of the laser input is comparable to the thermal characteristic time of the material, and the resultant temperature variations are significantly different from that of traditional infinite‐speed Fourier predictions. Two different types of materials, biological materials and inorganic solids, are considered for laser‐surface interactions in the study. The parameter of greatest significance is found to be the ratio of thermal characteristic length to the laser beam width. Values of this parameter in the range between 0.1 and 3, corresponding to different applications, are examined and local temperature maximas, or hot spots, are found to occur at initial time periods for values greater than ∼0.2.
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44.10.+i Heat conduction
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Atomic‐scale and microscale friction studies of graphite and diamond using friction force microscopy

Ju‐Ai Ruan and Bharat Bhushan

J. Appl. Phys. 76, 5022 (1994); http://dx.doi.org/10.1063/1.357214 (14 pages) | Cited 46 times

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Friction between graphite and diamond surfaces against a sharp silicon nitride tip was measured using a friction force microscope (FFM). Atomic‐scale friction images of a freshly cleaved highly oriented pyrolytic graphite exhibited the same periodicity as that of the graphite surface; however, the peaks in friction profiles and those in corresponding topography profiles were displaced relative to each other. Using the Fourier expansion of the interaction potential, the conservative interatomic forces between the FFM tip and the graphite surface have been calculated. It is shown that the variations in atomic‐scale friction and the observed displacement between the peaks in the frictional (or lateral) force and those in the corresponding topography can be explained by the variations in interatomic forces in the normal and lateral directions. Thus, the observed variation in friction force may not necessarily occur as a result of the commonly believed atomic‐scale stick‐slip process, but can be due to variation in the intrinsic lateral force between the sample and the FFM tip. At large scan sizes (50×50 nm2 or larger), the variation of friction for graphite and a single‐crystal (IIa) diamond was found to correlate with the variations in the local slope of the sample surface, suggesting that a ratchet mechanism is responsible for variations in microscale friction.
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46.55.+d Tribology and mechanical contacts
81.40.Pq Friction, lubrication, and wear
68.35.Gy Mechanical properties; surface strains

Two‐dimensional model of the ignition phase of high‐pressure glow discharges

G. Simon and W. Bötticher

J. Appl. Phys. 76, 5036 (1994); http://dx.doi.org/10.1063/1.357215 (11 pages) | Cited 10 times

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A self‐consistent two‐dimensional (2D) model of large volume inhomogeneously preionized transient high‐pressure glows as used in XeCl lasers is described. The basic concept is to use a curvilinear orthogonal coordinate system generated by conformal mapping of a cartesian system. The model uses the local field approximation and is based on a cartesian 2D model of J.‐P. Boeuf and L. C. Pitchford [IEEE Trans. Plasma Sci. 19, 286 (1991)]. As only direct ionization is taken into account, applications are limited to the ignition phase. Application to a discharge between cylindrical rods predicts a high degree of E‐field homogenization due to space charges and demonstrates the limits of parallel resistor models. Inside the cathode sheath an explosive formation of a very thin highly ionized layer is predicted. This sheath ignition is also seen in experiments.
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52.65.-y Plasma simulation
52.80.Hc Glow; corona

Dependence of keV x‐ray generation on the temporal and spatial separation of two KrF laser pulses

R. Bobkowski, J. N. Broughton, R. Fedosejevs, R. J. Willis, and M. R. Cervenan

J. Appl. Phys. 76, 5047 (1994); http://dx.doi.org/10.1063/1.357216 (7 pages) | Cited 1 time

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The generation of keV x‐ray emission from plasma produced by a 15 mJ 90 ps KrF laser pulse focused to an 8 μm spot on a copper target preceded by a similar 8 mJ pulse is studied experimentally. The change in efficiency of generation of keV x rays by the second pulse due to the presence of plasma produced by the preceding laser pulse on target is investigated as a function of the spatial and temporal separation of the laser pulses. When closely overlapped in space and time the pair of pulses behave like a single higher intensity pulse. When separated in space by more than 100 μm the two pulses behave independently generating x rays at the same rate as they would as two separate pulses. However, when overlapped in space and separated in time the x‐ray generation by the second pulse is enhanced for small temporal separations and then suppressed for larger temporal separations due to the plasma created by the first pulse. These results can be explained by the initial production of hot plasma which enhances x‐ray generation followed by the production of an extended colder plasma from the unloading of the shock heated target surface which suppresses keV x‐ray production by the second pulse. The observed time of transition from enhancement to reduction of x‐ray generation agrees with the time for the hot initial plasma due to the first laser pulse to expand and cool significantly.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.70.La X-ray and γ-ray measurements

Thermo‐optic investigation of heating during electrical breakdown in avalanche photodiodes

J. H. Hunt and R. B. Holmes

J. Appl. Phys. 76, 5054 (1994); http://dx.doi.org/10.1063/1.357217 (9 pages) | Cited 1 time

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We have experimentally determined the internal temperature rise caused by electrical breakdown in a silicon avalanche photodiode by observing changes in the diode’s optical properties. The spatial profile and temporal dependence of the temperature rise are both measured as a function of photodiode voltage. The results are consistent with a thermo‐optic theoretical model, which assumes a scaling of the width of the breakdown filament that increases with the magnitude and duration of the steady‐state breakdown current. The radius of the optical modulation was experimentally observed to range from about 1.5 to 3 μm. A single free parameter is used to obtain consistency between theory and experiment: the breakdown filament’s full width at half maximum is assumed to equal 1.28 μm for an overvoltage of 10 V and a delay of 96 ns, for the diode studied. Modulation of the read beam was observed at a level as low as 10±6 absorbed photons per write pulse, demonstrating near‐photon‐counting response.
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85.60.Dw Photodiodes; phototransistors; photoresistors
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
72.20.Ht High-field and nonlinear effects

Electron‐energy‐loss spectroscopy of mass‐selected ion‐beam‐deposited diamondlike carbon

J. Kulik, Y. Lifshitz, G. D. Lempert, J. W. Rabalais, and D. Marton

J. Appl. Phys. 76, 5063 (1994); http://dx.doi.org/10.1063/1.357218 (7 pages) | Cited 59 times

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Amorphous diamondlike carbon (DLC) films grown by low‐energy mass‐selected ion‐beam deposition have been examined by electron‐energy‐loss spectroscopy (EELS). Films grown using deposition energies of 50, 120, and 300 eV have been studied. For these deposition energies, all films exhibit similar EELS characteristics indicating a very high degree of sp3 bonding. The bulk plasmon resonance is intermediate between that of graphite and that of diamond; however, the properties of the low‐energy‐loss spectra of the DLC films are more similar to those of diamond. The near‐K‐edge carbon EELS data from the films exhibit a π∗ feature which is much smaller than that of graphite or evaporated carbon. The use of previously proposed computational methods on the near‐K‐edge EELS data indicates that over 80% of the carbon atoms are sp3 bonded. The size of the π∗ feature is larger for smaller plasmon energies, as expected. The present data are in accord with other analyses of similar films that indicate a broad (∼30–300 eV) energy window for diamondlike film formation.
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73.20.-r Electron states at surfaces and interfaces
81.15.Jj Ion and electron beam-assisted deposition; ion plating

Reliability test of popular fractal techniques applied to small two‐dimensional self‐affine data sets

S. Talibuddin and J. P. Runt

J. Appl. Phys. 76, 5070 (1994); http://dx.doi.org/10.1063/1.358490 (9 pages) | Cited 12 times

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The fractal dimensions of five fractional Brownian motion (fBm) surfaces of 257×257 pixel size, with Hurst exponent H ranging from 0.1 to 0.9, were computed by profile, contour, and surface area analyses. A technique was deemed reliable if it demonstrated accuracy, consistency and sensitivity. Of all the techniques examined, surface area analysis methods, namely, two‐dimensional pyramid and Peleg methods, were found to be most reliable and efficient for the data size studied. Hence, these were employed in a preliminary fractal analysis of poly(methyl methacrylate) and poly(styrene) fracture surfaces. The surfaces were imaged at scan sizes ranging from 1–6.5 μm by atomic force microscopy (AFM). The images indicated the existence of fractal structure and a high degree of roughness at microstructural scales for both the surfaces. These observations were supported by the results of the two surface area analysis techniques. A more conclusive study was prevented by severe scoring of the surfaces by the AFM tip at smaller scan sizes and the availability of only a narrow range of scan sizes.
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61.43.Bn Structural modeling: serial-addition models, computer simulation
61.43.Hv Fractals; macroscopic aggregates (including diffusion-limited aggregates)
68.35.B- Structure of clean surfaces (and surface reconstruction)
02.70.-c Computational techniques; simulations

Carbon doping and growth rate reduction by CCl4 during metalorganic chemical‐vapor deposition of GaAs

Jeong‐Seok Lee, In Kim, Byung‐Doo Choe, and Weon Guk Jeong

J. Appl. Phys. 76, 5079 (1994); http://dx.doi.org/10.1063/1.357219 (6 pages) | Cited 10 times

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The electrical, structural, and optical properties of GaAs grown by metalorganic chemical‐vapor deposition using CCl4 have been studied and the growth rate reduction by CCl4 under various growth conditions has been investigated. Hole concentrations ranging from 2×1016 to 1.8×1020 cm−3 have been obtained by varying V/III ratio and growth temperature. From Hall, x‐ray, and low‐temperature photoluminescence measurements, a low compensation is ensured. A growth rate reduction up to 50% has been observed. The dependence of the growth rate reduction on the growth temperature, the V/III ratio, and the CCl4 mole fraction was investigated. It is believed that the growth rate reduction is caused not by etching of solid GaAs but by reduction of Ga species through the formation of GaCl in gas phase.
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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.uj III-V and II-VI semiconductors
73.61.Ey III-V semiconductors

Production of semi‐insulating layers in n‐doped InP by Fe implantation

A. Carnera, A. Gasparotto, M. Tromby, M. Caldironi, S. Pellegrino, F. Vidimari, C. Bocchi, and C. Frigeri

J. Appl. Phys. 76, 5085 (1994); http://dx.doi.org/10.1063/1.357220 (10 pages) | Cited 9 times

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A detailed study of Fe implantation and damage annealing in indium phosphide is presented. The technological goal was to obtain thermally stable semi‐insulating layers in n‐type InP. Different characterization techniques were employed, including structural (x‐ray diffraction, Rutherford backscattering spectrometry, and transmission electron microscopy), chemical (secondary ions mass spectrometry), and electrical (current‐voltage) measurements. Both undoped and n‐type (Sn) doped substrates were implanted with Fe doses ranging from 5×1011 to 2.2×1014 cm−2 and annealed at a temperature of 650 °C. The high doses used to compensate n+ doping caused amorphization of the material. The reordering process of the amorphous layers and its influence on the Fe redistribution properties were studied in detail. The activation of the implanted Fe atoms after annealing was derived. Although the recovery process of the amorphized layer appears to be rather complex, our results show that good crystal quality and full compensation can be reached also for n+ doped substrates, leading to resistivity values above 2×107 Ω cm, even starting from an initial doping level as high as 1.4×1018 cm−3.
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61.72.uj III-V and II-VI semiconductors
61.72.S- Impurities in crystals
72.80.Ey III-V and II-VI semiconductors

Dynamic force microscopy in liquids

M. Dreier, D. Anselmetti, T. Richmond, U. Dammer, and H.‐J. Güntherodt

J. Appl. Phys. 76, 5095 (1994); http://dx.doi.org/10.1063/1.357221 (4 pages) | Cited 17 times

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We applied dynamic force microscopy in a liquid environment to silanized and derivatized glass surfaces, InGaAs, as well as to biological materials such as hexagonally packed intermediate layers of deinococcus radiodurans. The vertical and lateral resolution were estimated to be <1 Å and 7–10 nm, respectively. Upon immersing the cantilever into water, the resonance frequency was found to be reduced by a factor of two and the Q factor was lowered to 20–30. The experimental working distance between sensor and sample was determined with approach curves indicating that the range of interaction in water is much shorter compared to air.  
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
68.35.B- Structure of clean surfaces (and surface reconstruction)
87.64.Dz Scanning tunneling and atomic force microscopy

Influence of metal/n‐InAs/interlayer/n‐GaAs structure on nonalloyed ohmic contact resistance

Y. Shiraishi, N. Furuhata, and A. Okamoto

J. Appl. Phys. 76, 5099 (1994); http://dx.doi.org/10.1063/1.357222 (12 pages) | Cited 9 times

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We have investigated in detail the influence of interlayer structures on nonalloyed ohmic contact resistance (ρc), in terms of the crystalline defects and the potential barrier at the interlayer/GaAs interface. The interlayer structures are a graded‐band‐gap InAs/GaAs strained‐layer superlattice (graded SLS), a graded‐band‐gap InGaAs, and conventional SLSs without graded band gaps. A two‐layer transmission line model indicates that the barrier resistance in the interlayer highly depends on the interlayer structure: ≤5×10−8 Ω cm2 for the graded SLS and graded InGaAs interlayers and 10−5–10−6 Ω cm2 for the conventional SLS interlayers. To explain the large dependence of the interlayer structure, first, the density and distribution of the misfit dislocations and stacking faults caused by the large lattice mismatch between InAs and GaAs have been investigated in detail by high‐resolution transmission electron microscopy. In the graded SLS and conventional SLS interlayers, the influence of the high‐density depletion regions spread near the crystalline defects is found to be negligible because of the high doping concentrations (∼1019 cm−3) in the interlayers. Second, the potential barrier at the interlayer/GaAs interface has been investigated by simulating the barrier resistance. The potential barrier profile is calculated self‐consistently with Poisson’s equation and the Schrödinger equation. Tunneling current through the barrier is analyzed using the Wentzel–Kramers–Brillouin approximation or the numerical wave solution to the Schrödinger equation. The graded SLS interlayer has the effectively smooth conduction band profile without the barriers, which is similar to that of the graded InGaAs interlayer, because of its short period SLS. In the conventional SLS interlayers, the reasonable barrier heights of 0.14–0.26 eV obtained by this simulation indicates that these barriers are the dominant factor which increases the contact resistances. For the low‐resistance nonalloyed ohmic contact, therefore, a smooth conduction band profile without band discontinuity is more predominant than the reduction in the crystalline defect density.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.40.Cg Contact resistance, contact potential

X‐ray‐diffraction analysis of complex structures in ZnCdSe/ZnSe strained‐layer superlattices

Kiichi Nakashima and Yoshihiro Kawaguchi

J. Appl. Phys. 76, 5111 (1994); http://dx.doi.org/10.1063/1.357223 (7 pages) | Cited 1 time

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The procedure for analyzing the discrimination of fundamental peaks in x‐ray‐diffraction measurements of strained‐layer superlattice (SLS) structures is generalized and refined for application to more complex structures, including both lattice‐relaxed and coherently strained layers. Two criteria are proposed for investigating lattice relaxation and coherent deformation of relevant layers. A ZnCdSe/ZnSe SLS grown on a GaAs substrate with a ZnSe buffer layer is structurally analyzed by the generalized procedure with the two criteria, demonstrating its effectiveness in analyzing such complex structures. It is revealed from this analysis that the ZnSe buffer layer is almost totally lattice relaxed relative to the GaAs substrate and that the ZnCdSe/ZnSe SLS layer is coherently strained relative to the relaxed ZnSe buffer layer. Quantitative analysis with a kinematical step model also confirms the above results in terms of peak position and peak intensity profile. The full width at half‐maximum of the SLS peaks in the experimental profiles, however, is much broader than that of the calculated profiles. Based on this difference, some consideration of the peak broadening mechanism is offered from the viewpoint of broadening symmetry, revealing that this analysis works as a simple checking method on the peak broadening mechanism.
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61.05.C- X-ray diffraction and scattering
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Observation of degradation processes of Al electrodes in organic electroluminescence devices by electroluminescence microscopy, atomic force microscopy, scanning electron microscopy, and Auger electron spectroscopy

L. M. Do, E. M. Han, Y. Niidome, M. Fujihira, T. Kanno, S. Yoshida, A. Maeda, and A. J. Ikushima

J. Appl. Phys. 76, 5118 (1994); http://dx.doi.org/10.1063/1.357224 (4 pages) | Cited 75 times

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Degradation of top electrodes is one of the most important factors to determine the lifetimes of organic electroluminescence (EL) devices. An organic EL device [indium thin oxide (ITO)/N,N′‐diphenyl‐N,N′‐bis(3‐methylphenyl)‐(1,1′‐biphenyl)‐4, 4′‐diamine (TPD)/tris(8‐hydroxy‐ quinoline)aluminum (Alq3)/Al] was prepared and a morphological change of the Al top electrode was observed during and/or after applying voltage by atomic force microscopy and scanning electron microscopy (SEM). The change in the electrode surface, i.e., the increase in surface roughness was observed during the current flow. The degradation process started from faint dark core parts and propagated into disks with different rates depending on the magnitude of applied voltage. Degraded sites of the Al electrode, which were analyzed as aluminum oxide by Auger electron spectroscopy, protruded into the air on the organic layers. In SEM images of a life‐end electrode, discontinuities due to crevasse formation in the organic layers sandwiched by the ITO base and the metal top electrodes were observed in many places. These results confirm that one of the most crucial factors of the degradation process was deformation of metal and organic layers due to heat, gas evolution, and oxidation caused by applied voltage.
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
85.60.Pg Display systems

Ambient and vacuum scanning tunneling spectroscopy of sulfur‐ and oxygen‐terminated gallium arsenide

R. M. Silver, J. A. Dagata, and W. Tseng

J. Appl. Phys. 76, 5122 (1994); http://dx.doi.org/10.1063/1.357225 (10 pages) | Cited 7 times

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Tunneling spectroscopy of sulfur‐ and oxygen‐terminated n‐ and p‐type GaAs (110) surfaces is reported for air and ultrahigh‐vacuum conditions. Simulations of the complete I‐V characteristics with explicit inclusion of surface states within the planar junction theory are described and compared to experiment. These results provide a comprehensive understanding of the interplay between tip‐induced and surface‐state‐induced band‐bending effects observed in the tunneling spectra of passivated semiconductor surfaces.
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
73.40.Gk Tunneling

Migration of nitrogen implanted into iron induced by the presence of a buried carbon‐rich layer

J. Jagielski, N. Moncoffre, G. Marest, and S. Fayeulle

J. Appl. Phys. 76, 5132 (1994); http://dx.doi.org/10.1063/1.357226 (4 pages) | Cited 3 times

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A redistribution of nitrogen atoms towards a buried carbon‐rich layer is observed for thin iron films implanted at 250 °C. The effect is attributed to the formation of ϵ‐Fe3−x(C,N) iron carbonitride. Above 300 °C the ϵ‐carbonitride phase becomes unstable and consequently the redistribution effect vanishes leading to well‐separated profiles of nitrogen and carbon atoms.  
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.up Other materials
66.30.Ny Chemical interdiffusion; diffusion barriers

Fracture of metal‐polymer line structures. I. Semiflexible polyimide

S. L. Chiu, J. Leu, and P. S. Ho

J. Appl. Phys. 76, 5136 (1994); http://dx.doi.org/10.1063/1.357227 (7 pages) | Cited 33 times

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The fracture characteristics of metal/polymer line structures formed by depositing Au/Cr lines on a semiflexible polyimide, pyromellitic dianhydride‐oxydianiline (PMDA‐ODA), substrate have been investigated using a stretch deformation technique. The delamination behavior, fracture morphology, fracture energy, and energy dissipation rate have been determined as a function of line width and thickness. The metal dimension was found to influence the crack formation mode and morphology. The experimental studies were supplemented by finite‐element analysis to evaluate the stress distribution and deformation energetics of the line structure, which takes into account the plastic deformation of the metal and the polymer. Results from this analysis show that the observed fracture characteristics can be attributed to the edge and thickness effects induced by metal confinement. Essentially, the deformation behavior is determined by the mechanical environment induced by metal confinement at the interface. Plastic deformation of both metal and polymer plays an important role in controlling the stress distributions as well as the deformation energetics. The fracture energy of the metal‐polyimide interface determined by an overall energy balance method was consistent with that obtained from energy dissipation rate. The average value is 25 J/m2 for the Au/Cr/PMDA‐ODA line structure.
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62.20.M- Structural failure of materials
68.60.Bs Mechanical and acoustical properties
77.84.Jd Polymers; organic compounds

Fracture of metal‐polymer line structures. II. Rigid‐rodlike polyimide

J. Leu, P. S. Ho, and S. L. Chiu

J. Appl. Phys. 76, 5143 (1994); http://dx.doi.org/10.1063/1.357228 (6 pages) | Cited 3 times

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The fracture behavior of the Au/Cr line structures formed on a rigid‐rodlike polyimide, biphenylenetetracarboxylic dianhydride‐phenylenediamine (BPDA‐PDA), film substrate has been investigated using a stretch deformation technique and compared with that of the Au/Cr line structures formed on a semiflexible polyimide, pyromellitic dianhydride‐oxydianiline. In general, the effects of metal line dimensions on the deformation behavior can be attributed to the changes in the mechanical environment induced by metal confinement at the line interface. This gives rise to a fracture behavior and geometrical dependence of these two polyimide structures which are qualitatively similar in most aspects. However, the fracture energy and crack propagation rate of these two polyimide line structures are quantitatively different, with values about twice higher for the BPDA‐PDA line structures. This cannot be accounted for solely by the different chemical bonding; instead, the high fracture toughness of the BPDA‐PDA structure has to be attributed to its superior mechanical properties, particularly its plastic deformation characteristics, which are related to its molecular structure.  
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62.20.M- Structural failure of materials
68.60.Bs Mechanical and acoustical properties
77.84.Jd Polymers; organic compounds

Direct measurement of free‐energy barrier to nucleation of crystallites in amorphous silicon thin films

Frank G. Shi

J. Appl. Phys. 76, 5149 (1994); http://dx.doi.org/10.1063/1.357229 (5 pages) | Cited 8 times

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A method is introduced to a measure the free‐energy barrier W∗, the activation energy, and activation entropy to nucleation of crystallites in amorphous solids, independent of the energy barrier to growth. The method allows one to determine the temperature dependence of W∗, and the effect of the preparation conditions of the initial amorphous phase, the dopants, and the crystallization methods on W∗. The method is applied to determine the free‐energy barrier to nucleation of crystallites in amorphous silicon (a‐Si) thin films. For thermally induced nucleation in a‐Si thin films with annealing temperatures in the range of from 824 to 983 K, the free‐energy barrier W∗ to nucleation of silicon crystals is about 2.0–2.1 eV regardless of the preparation conditions of the films. The observation supports the idea that a‐Si transforms into an intermediate amorphous state through the structural relaxation prior to the onset of nucleation of crystallites in a‐Si. The observation also indicates that the activation entropy may be an insignificant part of the free‐energy barrier for the nucleation of crystallites in a‐Si. Compared with the free‐energy barrier to nucleation of crystallites in undoped a‐Si films, a significant reduction is observed in the free‐energy barrier to nucleation in Cu‐doped a‐Si films. For a‐Si under irradiation of Xe2+ at 105 eV, the free‐energy barrier to ion‐induced nucleation of crystallites is shown to be about half of the value associated with thermal‐induced nucleation of crystallites in a‐Si under the otherwise same conditions, which is much more significant than previously expected. The present method has a general kinetic basis; it thus should be equally applicable to nucleation of crystallites in any amorphous elemental semiconductors and semiconductor alloys, metallic and polymeric glasses, and to nucleation of crystallites in melts and solutions.
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64.60.Q- Nucleation
61.43.Dq Amorphous semiconductors, metals, and alloys
68.55.-a Thin film structure and morphology

Diffusive buoyancy force and concentration‐dependent diffusivities

Y. Zimmels

J. Appl. Phys. 76, 5154 (1994); http://dx.doi.org/10.1063/1.357230 (7 pages)

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The effect of pressure gradients that develop in diffusion systems consisting of particulates dispersed in a continuous fluid is considered. It is shown that the gradient of chemical potential which drives the diffusion flux induces a pressure gradient that opposes this flux. This effect, which exists in addition to the induced bulk flow, is expressed in terms of a diffusive buoyancy force (DBF). For dispersions consisting of monodisperse particulates in a single‐component fluid, the net driving force is the negative product of the volume fraction occupied by the fluid and the gradient of the chemical potential of the particulates. For polydisperse particulates, the DBF is the negative product of the total volume fraction occupied by the particulates and the expectation of gradient of their chemical potential. The joint effect of the DBF and the hydrodynamic hindrance is expressed in terms of a concentration‐dependent diffusion coefficient. It is shown that the effect of the DBF yields a fundamental diffusion coefficient Dϕ, which is the product of the volume fraction occupied by the fluid 1−ϕ, and the Stokes–Einstein diffusion coefficient D. The intrinsic diffusion coefficient, which is defined as the product of 1−ϕ and Dϕ, thus becomes the product of the square of 1−ϕ and D. At steady state the concentration profile cannot be analytically linear unless the buoyancy and hydrodynamic effects are offset by changes of size, energy per particulate and the activity coefficient. Finally, implications regarding the diffusion equation and effects of combined fields on the DBF are considered.
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66.10.C- Diffusion and thermal diffusion
82.70.Kj Emulsions and suspensions
83.10.Pp Particle dynamics
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