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Journal of Applied Physics

Issue 7 • Date Oct 2011

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Displaying Results 1 - 25 of 143
  • Adaptive oxide electronics: A review

    Page(s): 071101 - 071101-20
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    Novel information processing techniques are being actively explored to overcome fundamental limitations associated with CMOS scaling. A new paradigm of adaptive electronic devices is emerging that may reshape the frontiers of electronics and enable new modalities. Creating systems that can learn and adapt to various inputs has generally been a complex algorithm problem in information science, albeit with wide-ranging and powerful applications from medical diagnosis to control systems. Recent work in oxide electronics suggests that it may be plausible to implement such systems at the device level, thereby drastically increasing computational density and power efficiency and expanding the potential for electronics beyond Boolean computation. Intriguing possibilities of adaptive electronics include fabrication of devices that mimic human brain functionality: the strengthening and weakening of synapses emulated by electrically, magnetically, thermally, or optically tunable properties of materials.In this review, we detail materials and device physics studies on functional metal oxides that may be utilized for adaptive electronics. It has been shown that properties, such as resistivity, polarization, and magnetization, of many oxides can be modified electrically in a non-volatile manner, suggesting that these materials respond to electrical stimulus similarly as a neural synapse. We discuss what device characteristics will likely be relevant for integration into adaptive platforms and then survey a variety of oxides with respect to these properties, such as, but not limited to, TaOx, SrTiO3, and Bi4-xLaxTi3O12. The physical mechanisms in each case are detailed and analyzed within the framework of adaptive electronics. We then review theoretically formulated and current experimentally realized adaptive devices with functional oxides, such as self-programmable logic and neuromorphic circuits. Fina- - lly, we speculate on what advances in materials physics and engineering may be needed to realize the full potential of adaptive oxide electronics. View full abstract»

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  • Transmission of terahertz wave through one-dimensional photonic crystals containing single and multiple metallic defects

    Page(s): 073101 - 073101-8
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    We investigated numerically and experimentally the transmission of terahertz (THz) waves through single and multiple metallic defects created in a one-dimensional (1D) photonic crystal (PC) by inserting single metallic wires or arrays of parallel metallic wires into the air-gap defect of the 1D PC. The transmission properties of the metallic defect modes generated in the photonic bandgap (PBG) were characterized by using THz time-domain spectroscopy. For single metallic defects, it was found that the appearance the defect mode depends not only on the diameter of the metallic wires but also on the polarization of the THz wave. For transverse magnetic (TM) polarized waves whose electric fields are parallel to the metallic wires, the incident THz wave is generally split into two identical parts. In sharp contrast, the excitation of surface plasmon polaritons (SPPs) with enhanced field intensity is observed for transverse electric (TE) polarized waves whose electric fields are perpendicular to the metallic wires. In both cases, two resonant modes with reduced transmittance are observed in the PBG. While the resonant mode related to SPPs is found at the long-wavelength side of the original defect mode, the resonant mode without the excitation of SPPs appears at the short-wavelength side. Numerical simulation based on the finite-difference time-domain (FDTD) technique revealed that the electric field of SPPs is more tightly confined at the surface of the metallic wire when it is placed in the PC, implying that the confinement of a THz wave in the propagation direction will facilitate the localization of SPPs in the transverse direction. For two parallel metallic wires, the defect mode was found to depend on the separation between them. If they are widely separated, then the excitation of SPPs is similar to that observed in single metallic wires. However, the excitation of dipole-like SPPs does not occur for two closely packed metallic wires because of their large lateral - - size. It was also revealed that two parallel metallic wires with a small diameter and a narrow separation could be employed to achieve a significant enhancement, as large as 21.6, for the electric field in between them. More interestingly, the enhancement factor becomes larger when the confinement of the electric field in the propagation direction is increased. For an array of four widely separated wires whose lateral dimension is wider than the diameter of the THz beam, only one resonant mode is observed at the long-wavelength side of the original defect mode. The experimental observations are in good agreement with the simulation results based on the FDTD technique. The enhanced concentration of the electric field of SPPs at the surfaces of metallic defects may be useful for focusing and sensing of THz waves. View full abstract»

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  • Efficiency droop behaviors of the blue LEDs on patterned sapphire substrate

    Page(s): 073102 - 073102-4
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    In this paper, different efficiency behaviors of blue LEDs fabricated on the conventional sapphire substrate (C-LEDs) and patterned sapphire substrate (PSSLEDs) by metal organic chemical vapor deposition are investigated. Maximum external quantum efficiency (EQE) of PSSLEDs shows about 23.5 % improvement compared with that of C-LEDs. However, PSSLEDs have lower peak-efficiency-current at which the EQE reaches maximum, and suffer more serious efficiency droop. Besides, slight changes of efficiency droop for these two type LEDs in aging experiments indicate that structure defects do not play major roles for efficiency droop, and faster broadenings of the full width at half maximum of the electro-luminescence (EL) spectrum when injection current increases suggest stronger band filling effects in PSSLEDs. From the rate equation analyses, the value of coefficient C' is found to be two orders larger than Auger coefficient, and PSSLEDs have a greater coefficient C', which implies more carriers overflow from QWs in PSSLEDs. Therefore, it is suggested that, compared to Auger recombination, carrier overflow from QWs would play a more important part in the efficiency droop of blue LEDs. View full abstract»

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  • Self-heating and athermal effects on the electroluminescence spectral modulation of an AlGaInP light-emitting diode

    Page(s): 073103 - 073103-7
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    We report on the investigation of electroluminescence (EL) modulation depth as a function of photon energy in a high-power double-heterojunction AlGaInP light-emitting diode (LED). At low frequencies, the modulation spectrum exhibits the features as follows: (i) a dip at the photon energy close to the EL spectrum peak; (ii) an almost wavelength-independent modulation enhancement in the long-wavelength wing of the EL spectrum; and (iii) a linear increase of the modulation depth with photon energy in the short-wavelength wing of the EL spectrum. With increasing modulation frequency, these features diminish but do not disappear. A model, which quantitatively accounts for the experimental data has been introduced. At low frequencies, the spectral modulation features are due mainly to the modulation of EL efficiency, bandgap energy and the carrier distribution function caused by the oscillation of junction temperature. In the high-frequency limit when the oscillation of junction temperature fades, the spectral modulation features can be understood in terms of asymmetrical EL band broadening due to athermal many-body effects. The investigation of spectrally resolved EL modulation was shown to be applicable for the estimation of small (∼0.1 K) junction temperature oscillation and thermal relaxation time constants of LEDs emitting due to band-to-band recombination of free carriers. View full abstract»

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  • Broadband near ultra violet sensitization of 1 μm luminescence in Yb3+-doped CeO2 crystal

    Page(s): 073104 - 073104-7
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    Broadband spectral modification of near ultra violet (UV) light to infra-red (IR) light is investigated in Yb3+-doped CeO2 polycrystalline ceramics sintered in different atmospheres (air, oxygen, and 95%N2-5%H2). The intense Yb3+ photoluminescence (PL) peaked at 970 nm was observed by the UV excitation at around 390 nm in the samples except those sintered under N2-H2. A broad photoluminescence excitation (PLE) band of Yb3+ luminescence peaked at 390 nm corresponds to the absorption band and the photocurrent excitation band in the non-doped CeO2 crystal, which are also in accordance with the PLE band of Eu3+ luminescence in the Eu3+-doped CeO2. Judging from these results, the PLE band is attributed to the charge transfer (CT) band from O2- to Ce4+, but not to the CT from O2- to Yb3+. From the sintering atmosphere dependence of the PL and PLE, we found that the oxygen vacancies and Ce3+ impurities are not responsible for the 390 nm-absorption band but they work as a quenching center for the Yb3+ luminescence. View full abstract»

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  • Influence of wetting layers and quantum dot size distribution on intermediate band formation in InAs/GaAs superlattices

    Page(s): 073105 - 073105-5
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    We examine the influence of the wetting layers (WLs) and the quantum dot (QD) size distribution on the sub-bandgap external quantum efficiency (EQE) of QD solar cells. We use a finite-element Schrödinger-Poisson model that considers QD and wetting layer shapes, sizes, and spacings from cross-sectional scanning tunneling and atomic force micrographs. A comparison between experiments and computations reveals an insignificant contribution of the WL to the sub-bandgap EQE and a broadening of sub-bandgap EQE associated with a variation in QD sizes in the growth direction. View full abstract»

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  • Luminescence of black silicon fabricated by high-repetition rate femtosecond laser pulses

    Page(s): 073106 - 073106-4
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    We studied the photoluminescence (PL) from black silicon that was fabricated using an 800 nm, 250 kHz femtosecond laser in air. By changing the scan velocity and the fluence of the femtosecond laser, the formation of the PL band between the orange (600 nm) and red bands (near 680 nm) could be controlled. The red band PL from the photoinduced microstructures on the black silicon was observed even without annealing due to the thermal accumulation of high-repetition rate femtosecond laser pulses. The orange band PL was easily quenched under 532 nm cw laser irradiation, whereas the red band PL was more stable; this can be attributed to “defect luminescence” and “quantum confinement”, respectively. View full abstract»

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  • Triangular resonator with surface plasmon resonance around the critical angle

    Page(s): 073107 - 073107-5
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    A novel triangular resonator with total internal reflection and attenuated total reflection (ATR) mirrors is proposed. The resonance characteristics are analyzed for the first time. The optimum gold thickness in the ATR mirror is 33 nm and the resonance angle is 17.777°. Extinction ratio greater than 66 dB could be achieved with the appropriate choice of resonance angle and coupling ratios. The triangular ring resonator with an incident angle of 18° inside the resonator was also fabricated and measured. The resulting free spectral range of the resonator is approximately 0.85 nm near 1558 nm, and the on-off ratio was about 3.5 dB. These resonance characteristics of the triangular resonator are found to be useful in realizing bio/chemical sensors when the ATR mirror carries a special electromagnetic wave called a surface plasmon wave. View full abstract»

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  • Reduced auger recombination in mid-infrared semiconductor lasers

    Page(s): 073108 - 073108-6
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    A quantum-design approach to reduce the Auger losses in λ = 2 μm InGaSb type-I quantum well edge-emitting lasers is reported. Experimentally realized structures show a ∼3 × reduction in the threshold, which results in 4.6 × lower Auger current loss at room temperature. This is equivalent to a carrier lifetime improvement of 5.7 × and represents about a 19-fold reduction in the equivalent “Auger coefficient.” View full abstract»

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  • Photoluminescence origins of the porous silicon nanowire arrays

    Page(s): 073109 - 073109-7
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    We investigate the photoluminescence (PL) behavior of the porous silicon nanowire (PSiNW) arrays synthesized via metal-assisted electroless etching method on the n-Si (100) substrate. Two PL bands with different origins dependent on the post-chemical treatments were detected. The red emission band, the peak position of which is insensitive to temperature and excitation source, is considered to originate from the excitons localized at the interface between the Si nanostructure and the oxide layer. An anomalous blue shift of the near-infrared PL band was observed when the temperature increased from 80 to 290 K. The maximum intensity appears around 160 K and the emission energy is strongly dependent on the excitation energy and power. The triplet-singlet state transition is introduced to explain the PL behavior for this emission band. The absorption spectra are also induced to confirm the PL origins. View full abstract»

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  • The direct measurement of ablation pressure driven by 351-nm laser radiation

    Page(s): 073110 - 073110-4
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    The instantaneous scaling of ablation pressure to laser intensity is directly inferred for ramp compression of diamond targets irradiated by 351-nm light. Continuously increasing pressure profiles from 100 to 970 GPa are produced by direct-drive laser ablation at intensities up to 7 × 1013 W/cm2. The free-surface velocity on the rear of the target is used to directly infer the instantaneous ablation-pressure profile at the front of the target. The laser intensity on target is determined by laser power measurements and fully characterized laser spots. The ablation pressure is found to depend on the laser intensity as P(GPa)=42(±3)[I(TW/cm2)]0.71(±0.01). View full abstract»

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  • Tunable terahertz-mirror and multi-channel terahertz-filter based on one-dimensional photonic crystals containing semiconductors

    Page(s): 073111 - 073111-6
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    We demonstrate that tunable terahertz-mirrors and multichannel terahertz-filters can be carried out by taking into account the transmission properties of the one-dimensional photonic crystals (1DPCs) containing semiconductor materials with a tunable dielectric constant in the THz frequency range. Using the semiconductor InSb as the components in the 1DPCs, we have designed such a mirror and a filter and identified their tunability. The THz-mirror is tunable by changing the external temperature or the thickness of the components of the 1DPC. The operation frequency of the multichannel-filter can be tuned by changing the temperature and thickness of the component materials in the 1DPC, the channel number of the filter can be controlled by varying the number of the defect layers in the 1DPC, and the transmittance bandwidth can be adjusted by changing the period number of the 1DPC. With thermal tunability, structure controllability and narrow bandwidth, 1DPCs containing semiconductors open a promising way to fabricate tunable terahertz devices for future terahertz communications. View full abstract»

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  • C/CrC nanocomposite coating deposited by magnetron sputtering at high ion irradiation conditions

    Page(s): 073301 - 073301-6
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    CrC with the fcc NaCl (B1) structure is a metastable phase that can be obtained under the non-equilibrium conditions of high ion irradiation. A nano-composite coating consisting of amorphous carbon embedded in a CrC matrix was prepared via the unbalanced magnetron sputtering of graphite and Cr metal targets in Ar gas with a high ionized flux (ion-to-neutral ratio Ji/Jn = 6). The nanoscale amorphous carbon clusters self-assembled into layers alternated by CrC, giving the composite a multilayer structure. The phase, microstructure, and composition of the coating were characterized using x-ray diffraction, transmission electron microscopy, and aberration corrected scanning transmission electron microscopy coupled with electron energy loss spectroscopy. The interpretation of the true coating structure, in particular the carbide type, is discussed. View full abstract»

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  • A simple profile-fitting method to determine the metastable and resonant densities in a cold atmospheric pressure argon plasma jet

    Page(s): 073302 - 073302-10
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    A simple profile-fitting method is applied to determine the concentrations of metastable and resonant species in a cold atmospheric pressure argon plasma jet. This method is based on the analysis of the effect of self-absorption broadening on the profiles of spectral lines emitted from plasma. The Argon lines which correspond to transitions to metastable and resonant lower states were measured with high resolution optical emission spectroscopy. Fitting the measured line contours yields line-integrated densities (Nl × L) of metastable Ar(1s5) and Ar(1s3) of 1.2 × 1015 m-2 and 5.0 × 1014 m-2, respectively. For resonant species, it is difficult to perform a good density-estimation by fitting measured line contours because of the broad intrinsic resonant line shape and the low resonant densities in the plasma. Top-limits of 4.8 × 1015 m-2 and 7.2 × 1014 m-2 are estimated for resonant states Ar(1s2) and Ar(1s4), respectively. By comparing the fittings of metastable and resonant lines, it is found that the applicability of the profile-fitting routine is sensitive to the line-center optical thickness of emission lines (mainly determined by the lower states densities) and the sharpness of intrinsic line profiles of corresponding transitions. View full abstract»

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  • Electron density measurement of inductively coupled plasmas by terahertz time-domain spectroscopy (THz-TDS)

    Page(s): 073303 - 073303-8
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    The electron densities of argon inductively coupled plasmas were measured by terahertz time-domain spectroscopy (THz-TDS). At a low pressure, the electron densities were also measured with a Langmuir-type double probe and the validity of THz-TDS electron-density measurement in a plasma has been corroborated. As the input radio-frequency (RF) power increases, the plasma density and gas temperature increase, which makes the probe measurement less reliable or even impossible, due to the large heat load to the probe surface. On the contrary, the THz-TDS measurement is unaffected by the gas temperature and becomes more reliable due to the higher electron density at higher input power for plasma generation. View full abstract»

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  • Density and temperature measurement of OH radicals in atmospheric-pressure pulsed corona discharge in humid air

    Page(s): 073304 - 073304-7
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    Plasma application for environmental improvement is desirable, and it is worthwhile to clarify the behavior of OH radicals in nonthermal plasma. Under atmospheric-pressure humid air, the time evolutions and spatial distribution of relative density and rotational temperature of OH radicals are measured in pulsed positive corona discharge using laser-induced fluorescence with a tunable optical parametric oscillator laser. The density of OH radicals generated by discharge when 28 kV is applied is estimated to be about 1×1015cm-3 at 3 μs after discharge. The OH density increases with humidity. The rotational temperature rises after discharge. The rate of temperature rise increases with humidity. This phenomenon arises from fast vibration-to-translation energy relaxation of H2O. The spatial distributions of OH rotational temperature indicate that the temperature rises in the secondary streamer channel. View full abstract»

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  • Plasma treatments to improve metal contacts in graphene field effect transistor

    Page(s): 073305 - 073305-6
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    Graphene formed via chemical vapor deposition was exposed to various plasmas (Ar, O2, N2, and H2) in order to examine its effects on the bonding properties of graphene to metal. After exposing patterned graphene to Ar plasma, the subsequently deposited metal electrodes remained intact, enabling the successful fabrication of field effect transistor arrays. The effects of the enhanced adhesion between graphene and metals were more evident from the O2 plasma than the Ar, N2, and H2 plasmas, suggesting that a chemical reaction of O radicals imparts hydrophilic properties to graphene more effectively than the chemical reaction of H and N radicals or the physical bombardment of Ar ions. The electrical measurements (drain current versus gate voltage) of the field effect transistors before and after Ar plasma exposure confirmed that the plasma treatment is quite effective in controlling the graphene to metal bonding accurately without the need for buffer layers. View full abstract»

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  • Influence of discharge conditions on energetic hydrogen atoms in a glow discharge

    Page(s): 073306 - 073306-10
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    Influence of discharge conditions on fast hydrogen atoms in glow discharge is investigated using Balmer alpha emission spectroscopy. Investigation was performed in two orthogonal directions of observation in pure hydrogen. The shapes of the profiles are examined together with the space intensity distribution of the excessively broadened Balmer alpha line. It was found that line profile, space intensity distribution, and energy distribution of exited atoms strongly depend on voltage, pressure, and the reduced electric field. This confirms that fast H atoms are generated in charge exchange processes and neutralization of ions at cathode surface and not in a non-field process. View full abstract»

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  • Emission spectroscopy of a microhollow cathode discharge plasma in helium-water gas mixtures

    Page(s): 073307 - 073307-5
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    A dc microhollow cathode discharge (MHCD) plasma was generated inflowing helium gas containing water vapor. The cathode hole diameters were 0.3, 0.7, 1.0, and 2.0 mm, each with a length of 2.0 mm. Emission spectroscopy was carried out to investigate the discharge mode and to determine the plasma parameters. For the 0.3-mm cathode, stable MHCDs in an abnormal glow mode existed at pressures up to 100 kPa, whereas for larger diameters, a plasma was not generated at atmospheric pressure. An analysis of the lineshapes relevant to He at 667.8 nm and to Hα at 656.3 nm implied an electron density and gas temperature of 2 × 1014 cm-3 and 1100 K, respectively, for a 100-kPa discharge in the negative glow region. The dependence of the OH band, and Hα intensities on the discharge current exhibited different behaviors. Specifically, the OH spectrum had a maximum intensity at a certain current, while the H atom intensity kept increasing with the discharge current. This observation implies that a high concentration of OH radicals results in quenching, leading to the production of H atoms via the reaction OH + e- → O + H + e-. View full abstract»

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  • Theoretical and experimental study of the microwave cut-off probe for electron density measurements in low-temperature plasmas

    Page(s): 073308 - 073308-7
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    The microwave cut-off probe for the electron density measurement in low-temperature plasmas is described in this article. It is based on the wave cutoff in an unmagnetized plasma. The measurement principle is analyzed theoretically using a model of plasma slab. Because of the high-pass characteristic of plasma, the waves above the cut-off frequency can penetrate the plasma slab, whereas the lower frequency waves are reflected from the cut-off layer. Therefore, an obvious critical point can be observed in the wave transmission spectrum. The abscissa of the critical point indicates the cut-off frequency, which is directly related to the maximum electron density between transmitting/receiving antennas of the cut-off probe. The measured electron densities are in agreement with the data obtained by the Langmuir probe. Experimental results show that the microwave cut-off probe can be used to diagnose the plasmas with a wide range of parameters. View full abstract»

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  • Radio frequency emission from high-pressure xenon arcs: A systematic experimental analysis of the underlying near-anode plasma instability

    Page(s): 073309 - 073309-10
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    High-pressure Xe discharge lamps at DC operation can show unwanted strong RF (radio-frequency) emission to beyond 1 GHz, correlated to a sharp periodic lamp-voltage instability in the near-anode plasma with a pulse repetition rate ε of 1–10 MHz. The physical origin of the instability is unclear. Here, its existence and pulse rate have been measured as a function of arc current I = 0.2–1.2 A and anode temperature Ta = 1700–3400 K independently, in experimental lamps with pure-tungsten electrodes and a Xe operating pressure around p = 10 MPa. Surprisingly, the instability is not affected by I or current density j but exists if Ta is lower than a threshold value around 2800–2900 K. The pulse rate ε is simply a rising linear function of the inverse anode temperature 1/Ta, with only a small I-dependent correction. The average anode heat load is slightly lower in the unstable regime and possibly depends on ε. The results allow a consistent re-interpretation of earlier and present experimental observations and should be both a valuable help in practical lamp engineering and a tight constraint for future theories of this effect. View full abstract»

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  • Raman spectra and dielectric function of BiCrO3: Experimental and first-principles studies

    Page(s): 073501 - 073501-8
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    We present the complex dielectric function of BiCrO3 thin films in the energy range of 0.73–9.8 eV determined using spectroscopic ellipsometry. By analyzing the absorption onset region, it is shown that the optical bandgap of BiCrO3 is indirect, with a value of 2.27 eV. The imaginary part of the BiCrO3 dielectric function, ɛ2, calculated using density functional theory in the generalized gradient approximation with an Hubbard potential of 3 eV agrees well with the experimentally determined one. Raman spectra of both polycrystalline and epitaxial thin films of BiCrO3 are reported. The temperature dependent Raman measurements indicate a structural phase transition at ∼400 K which was confirmed also by x-ray diffraction investigations. View full abstract»

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  • Plastic flow in shock-loaded silver at strain rates from 104 s-1 to 107 s-1 and temperatures from 296 K to 1233 K

    Page(s): 073502 - 073502-7
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    The evolution of elastic-plastic shock waves in 99.9% purity silver samples of 0.127 to 2.0 mm thickness has been studied in a series of VISAR-instrumented planar impact experiments with initial sample temperature varied from 296 to 1233 K. The decay of elastic precursor wave at 933, 1173, and 1233 K temperatures is approximately inversely proportional to the square root of the propagation distance. The latter corresponds to the cubic dependence of initial plastic strain rate, ranged from 104 s-1 to 106 s-1, on the shear stress. At fixed strain rates, the flow stress grows linearly with the temperature but the dependence becomes stronger near the silver melting point, 1234 K. An analysis of the rise times of the plastic shock waves shows that for the same level of shear stress the plastic strain rate at the shock front is significantly higher than that at the top of the elastic precursor wave. View full abstract»

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  • Origin of predominantly a type dislocations in InGaN layers and wells grown on (0001) GaN

    Page(s): 073503 - 073503-6
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    Threading dislocations that are of a type were observed to form locally in InGaN layers and wells containing 7%–15% indium. Direct correlations between a type dislocations and stacking faults in InGaN layers and wells were observed. The formation of these dislocations is attributed to the dissociation of Shockley partials bounding the stacking faults. View full abstract»

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  • Stress migration model for Cu interconnect reliability analysis

    Page(s): 073504 - 073504-5
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    Stress migration (SM) reliability data have been treated qualitatively to define pass or fail criteria in the past. However, realistic quantitative SM analysis and lifetime estimates for products were not available due to lack of a suitable SM model. In this paper, we establish a comprehensive SM model for quantitative stress-induced-voiding (SIV) risk analysis for 32 nm technology and beyond. It was found that the SIV risk is dependent on both stress temperatures and geometric structural line widths as driving forces. Based on the new SM model, the SM lifetime can be estimated from measurable SM data and accelerated SM test methods can be designed to meet the qualification criteria. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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P. James Viccaro
Argonne National Laboratory