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January 6, 1997

Volume 70, Issue 1,  pp. 1-140

Large third-order optical nonlinearity in Au:SiO2 composite films near the percolation threshold

H. B. Liao, R. F. Xiao, J. S. Fu, P. Yu, G. K. L. Wong, and Ping Sheng

Appl. Phys. Lett. 70, 1 (1997) (3 pages)

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Very large third-order optical nonlinearity, chi(3) ~ 2.5 × 10–6 esu, measured by a degenerate four wave mixing method using a short pulse (70 picosecond) laser, has been found in the rapid-thermal annealed Au:SiO2 composite films at concentrations below the Au percolation threshold. The dependence of the chi(3) on Au concentration, p, follows a cubic power law. The maximum figure of merit, chi(3)/alpha (with alpha being the absorption coefficient) is about 10–11 esu cm. We explain this result as due to local field enhancement arising from the Mie resonance of the Au nanoclusters, with strong interaction between the nanoclusters further promoting the effect. ©1997 American Institute of Physics.
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42.65.Ky, 42.70.Nq, 78.66.Sq

Enhanced absorption by stimulated emission in three excited states of Er3 + in yttrium aluminum garnet

Yoshinobu Maeda and Masatoshi Migitaka

Appl. Phys. Lett. 70, 4 (1997) (3 pages)

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Using a laser diode modulated at 250 and 500 MHz, dependence of enhanced absorption on both sample length and temperature was investigated in an erbium doped yttrium aluminum garnet crystal. The transmitted laser intensity decreased with a decay time of 300 ps for sample lengths greater than 3.0 mm and temperatures higher than 50 K when the incident laser intensity increased. In addition, an emission peak was observed near 835 nm corresponding to the transition from the 4S3/2 level to the 4I13/2 in Er3+. It was suggested that the enhanced absorption was caused by stimulated emission in three excited states. ©1997 American Institute of Physics.
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42.70.Hj, 42.50.-p, 78.45.+h

Using microcontact printing to generate amplitude photomasks on the surfaces of optical fibers: A method for producing in-fiber gratings

John A. Rogers, Rebecca J. Jackman, George M. Whitesides, Jefferson L. Wagener, and Ashish M. Vengsarkar

Appl. Phys. Lett. 70, 7 (1997) (3 pages)

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This letter describes a method for producing in-fiber gratings that reduces the effects of mechanical and optical instabilities limiting other methods. In this technique, opaque lines formed on the outside of the fiber using a procedure known as microcontact printing, serve as an amplitude photomask for exposure to ultraviolet light. Long-period fiber optic attenuators formed by ths technique demonstrate its advantages. ©1997 American Institute of Physics.
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42.81.Bm, 42.79.Dj

Integrated intracavity quasi-phase-matched second harmonic generation based on periodically poled Nd:LiTaO3

Kazi Sarwar Abedin, Takehiro Tsuritani, Manabu Sato, and Hiromasa Ito

Appl. Phys. Lett. 70, 10 (1997) (3 pages)

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Integrated intracavity quasi-phase-matched second harmonic generation (QPM-SHG) has been demonstrated in a periodically domain inverted Nd:LiTaO3 crystal which functions simultaneously as a gain and nonlinear medium. Lasing at 1.082 µm and simultaneous QPM-SHG at 541 nm has been established by pumping near 806 nm. The effect of doping on the domain reversal voltage of LiTaO3 and the photorefractive effect have also been investigated. It is found that optical damage can be avoided by heating above 80 °C. ©1997 American Institute of Physics.
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42.65.Ky, 77.80.Dj, 42.70.Nq

Spontaneous emission factor in oxide confined vertical-cavity lasers

D. V. Kuksenkov, H. Temkin, K. L. Lear, and H. Q. Hou

Appl. Phys. Lett. 70, 13 (1997) (3 pages)

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We report on measurements of the spontaneous emission factor for oxide-confined InGaAs vertical cavity surface emitting lasers. The spontaneous emission factor is determined as a function of the active layer volume from the measurement of small-signal harmonic distortion at threshold. For a 3 × 3 µm oxide aperture device we obtain spontaneous emission factor of 4.2 · 10–2 at room temperature. ©1997 American Institute of Physics.
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42.55.Px, 42.55.Sa

Soft x-ray pulse generation from femtosecond laser-produced plasma with reduced debris using a metal-doped glass target

H. Nakano, T. Nishikawa, and N. Uesugi

Appl. Phys. Lett. 70, 16 (1997) (3 pages)

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We have demonstrated soft x-ray short pulse generation from femtosecond laser-produced plasma with much lower debris using metal-doped glass as a target. Soft x-ray emission (50–200 Å) from a gold-doped glass target was about 40% of that from a solid gold target while the density of gold in the doped-glass was less than 0.001 vol % and the target was transparent at the wavelength of the laser light. Due to the low metal density, the particulate deposition, which is one of the serious obstacles to putting laser-produced plasma x-ray into practical use, was greatly reduced by the use of metal-doped glass as a target. ©1997 American Institute of Physics.
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52.25.Nr, 52.40.Nk, 52.50.Jm

Fiber Sagnac interferometer temperature sensor

A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De La Rosa

Appl. Phys. Lett. 70, 19 (1997) (3 pages)

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A modified Sagnac interferometer-based fiber temperature sensor is proposed. Polarization independent operation and high temperature sensitivity of this class of sensors make them cost effective instruments for temperature measurements. A comparison of the proposed sensor with Bragg grating and long-period grating fiber sensors is derived. A temperature-induced spectral displacement of 0.99 nm/K is demonstrated for an internal stress birefringent fiber-based Sagnac interferometer. ©1997 American Institute of Physics.
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07.20.Dt, 42.81.Pa, 07.60.Vg, 07.60.Ly

Optical force microscopy with silicone rubber waveguides

S. Herminghaus, M. Riedel, P. Leiderer, M. Bastmeyer, and C. Stürmer

Appl. Phys. Lett. 70, 22 (1997) (3 pages)

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A simple technique is described for optically imaging the lateral distribution of normal forces exerted onto a flat surface. It is based on the detuning of a silicone rubber planar waveguide by the forces to be investigated. The method is demonstrated by imaging the contact line force of a sessile water droplet on the surface, with a force resolution better than µN. It is shown that the lateral resolution may be much better than the decay length of the waveguide modes used. ©1997 American Institute of Physics.
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07.60.-j, 68.10.Et, 87.80.+s

Laser emission of a flash-lamp pumped Rhodamine 6 G solid copolymer solution

Oscar G. Calderón, J. M. Guerra, A. Costela, I. García-Moreno, and R. Sastre

Appl. Phys. Lett. 70, 25 (1997) (3 pages)

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A Rhodamine 6 G solid solution in a copolymer of 2 hydroxyethyl methacrylate and methyl methacrylate have been flash-lamp pumped in an untuned laser cavity. The pumping energy threshold to obtain laser emission is 20 percent higher than in equivalent methanol solutions. With a flash-lamp electrical excitation average energy of 150 joules per pulse, no photodegradation was observed in the laser emission after more than 30 pumping pulses. There was an absence of local irregular fluctuations found typically in high Fresnel number dye lasers. An interpretation based on the assumption of stereospecificity in the lodging cavity of dye molecules, is consistent with the high photostability and the absence of local fluctuations obtained with this polymeric solid-state laser. ©1997 American Institute of Physics.
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42.55.Mv, 42.60.Pk

Noise gain in single quantum well infrared photodetectors

A. Carbone and P. Mazzetti

Appl. Phys. Lett. 70, 28 (1997) (3 pages)

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A new approach to the calculation of current noise in single quantum well infrared photodetectors is proposed. The modulation noise due to the fluctuation of the emitter barrier potential is taken into account by considering the correlation between the elementary pulses constituting the excess current injected from the emitter when the quantum well is depleted by electrons. A simple relationship between the noise gain and the photoconductive gain of the device is obtained. A comparison with experiments is also reported. ©1997 American Institute of Physics.
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85.60.Gz, 73.50.Td, 07.57.Kp

Model near field calculations for optical data storage readout

A. Madrazo and M. Nieto-Vesperinas

Appl. Phys. Lett. 70, 31 (1997) (3 pages)

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We investigate the near field distribution of light diffracted from deep grooves of sub-wavelength lateral dimensions, carved on a flat metallic surface. It is shown that, in spite of multiple interaction, the reflected field presents, for s polarization, peaks very localized on the groove positions, and whose amplitude increases with the groove depth. For p polarization, however, the near field does not follow the surface profile. This permits us to establish for s polarization a threshold in order to distinguish signals produced on reflection from grooves having different profiles. This is of guidance in diffractive modeling of readout systems of high density optical disks. ©1997 American Institute of Physics.
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42.79.Vb, 42.25.Fx

Absorption spectra of Se and HgI2 chains in channels of AlPO4-5 single crystal

Z. K. Tang, Michael M. T. Loy, Jiesheng Chen, and Ruren Xu

Appl. Phys. Lett. 70, 34 (1997) (3 pages)

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The absorption spectra are reported for isolated Se and HgI2 semiconductor chains accommodated in AlPO4-5 (AFI) channels of diameter 7.3 Å. The lowest electronic excitation states of the isolated chains are shifted to higher energy from the band edge transitions of their bulk crystals. The blue shifts are qualitatively explained by the quantum confinement effects of carriers in a one-dimensional wire. The experimental result of the Se/AFI is in good agreement with the theoretical expectation based on the effective-mass-approximation. The result of the HgI2/AFI, however, does not agree with the calculation. The different behaviors of the Se/AFI and the HgI2/AFI are expected to result from their different electronic structures. ©1997 American Institute of Physics.
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78.40.Fy, 73.20.Dx, 78.20.Ci

Improvement of ultrathin gate oxide and oxynitride integrity using fluorine implantation technique

Prasenjit Chowdhury, Anthony I. Chou, Kiran Kumar, Chuan Lin, and Jack C. Lee

Appl. Phys. Lett. 70, 37 (1997) (3 pages)

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The effects of fluorine on ultrathin gate oxide and oxynitride (~ 40 Å) have been studied. The incorporation of fluorine was done by fluorine ion implantation into polycrystalline silicon (polysilicon) gate followed by a high-temperature drive-in step. It has been found that the integrity of oxide has been improved with the incorporation of fluorine as demonstrated by the reduction of stress-induced leakage current and interface trap generation. Furthermore, unlike thicker dielectrics (> 100 Å) for which the charge-to-breakdown (QBD) values decrease with increasing fluorine concentration, QBD's remain the same as those of the control samples for the ultrathin thickness regime. The mechanism for oxide quality improvement by F will also be discussed in the letter. ©1997 American Institute of Physics.
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85.40.Ry, 85.30.De, 61.72.Tt

InAsSbP/InAsSb/InAs laser diodes (lambda =3.2 µm) grown by low-pressure metal–organic chemical-vapor deposition

J. Diaz, H. Yi, A. Rybaltowski, B. Lane, G. Lukas, D. Wu, S. Kim, M. Erdtmann, E. Kaas, and M. Razeghi

Appl. Phys. Lett. 70, 40 (1997) (3 pages)

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We report metal–organic chemical-vapor deposition-grown double heterostructure InAsSbP/InAsSb/InAs diode lasers emitting at 3.2 µm operating at temperatures up to 220 K with threshold current density of 40 A/cm2 at 77 K and characteristic temperature up to 42 K. Output powers as high as 260 mW in pulse mode and 60 mW in continuous wave operation have been obtained from an uncoated 100 µm stripe-width broad-area laser at 77 K. Comparison with theory shows that there is no significant nonradiative recombination mechanism for these lasers at 77 K. ©1997 American Institute of Physics.
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42.55.Px, 81.15.Gh, 78.66.Fd

Heat capacity measurements of Sn nanostructures using a thin-film differential scanning calorimeter with 0.2 nJ sensitivity

S. L. Lai, G. Ramanath, L. H. Allen, and P. Infante

Appl. Phys. Lett. 70, 43 (1997) (3 pages)

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We have developed a new thin-film differential scanning calorimetry technique that has extremely high sensitivity of 0.2 nJ. By combining two calorimeters in a differential measurement configuration, we have measured the heat capacity and melting process of Sn nanostructures formed via thermal evaporation with deposition thickness down to 1 Å. The equivalent resolution of the calorimeter is 1 nanogram in mass or 0.4 Å in thickness. We have observed a decrease of up to 120°C in the melting point of Sn nanostructures. ©1997 American Institute of Physics.
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65.40.+g, 07.20.Fw, 68.60.Dv

Ferroelectric properties of (Pb0.97La0.03)(Zr0.66Ti0.34)0.9875O3 films deposited on Si3N4-coated Si substrates by pulsed laser deposition process

Tzu-Feng Tseng, Rong-Pyng Yang, Kuo-Shung Liu, and I-Nan Lin

Appl. Phys. Lett. 70, 46 (1997) (3 pages)

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(Pb0.97La0.03)(Zr0.66Ti0.34)0.9875O3, PLZT, thin films deposited on either LaNiO3 (LNO) or LNO/Pt coated Si3N4/Si substrates, possessing good ferroelectric properties, were successfully obtained by the pulsed laser deposition process. Using LNO/Pt as double layer electrodes not only resulted in PLZT films with superior electric properties, but also of better handling endurance. The former is attributed to the low surface resistivity of electrode materials (i.e., rhoLNO/Pt = 0.5 m Omega cm) due to the bypassing effect of the Pt layer, whereas the latter is ascribed to the induction of compressive stress on PLZT and LNO layers due to a relatively larger thermal expansion coefficient (CTE) of the Pt layer. The ferroelectric properties of (PLZT )LNO/pt films were Pr = 16.5 µC/cm2 and Ec = 63.5 kV/cm, while the dielectric constant and leakage current were K = 1.028 and Je <= 8 × 10–6 A/cm2 (under 150 kV/cm), respectively. Their fatigue life was longer than 2 × 109cycles under action of 300 kV/cm pulse. ©1997 American Institute of Physics.
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77.80.-e, 77.84.Dy, 81.15.Fg

Heterogeneous nucleation of coherently strained islands during epitaxial growth of Ge on Si(110)

M. Krishnamurthy, Bi-Ke Yang, J. D. Weil, and C. G. Slough

Appl. Phys. Lett. 70, 49 (1997) (3 pages)

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We report on the molecular beam epitaxial growth of Ge on Si(110) surfaces. High temperature cleaning (oxide desorption) results in the formation of shallow faceted pits distributed randomly on the Si(110) surface. Deposition of Ge at temperatures between 600 and 725 °C leads to preferential nucleation along the pit edges forming elongated islands, which subsequently grow to compact three-dimensional coherent islands. The observation of strained islands in close proximity to each other offers insights into their nucleation behavior and strain relaxation. Our observations suggest heterogeneous nucleation as a possible method for fabricating assemblies of quantum dots. ©1997 American Institute of Physics.
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68.55.Jk, 81.15.Hi, 81.05.Cy

The growth of AlGaAs–InGaAs quantum-well structures by molecular beam epitaxy: Observation of critical interdependent effects

Robert N. Bicknell-Tassius, Kyeong Lee, April S. Brown, Georgianna Dagnall, and Gary May

Appl. Phys. Lett. 70, 52 (1997) (3 pages)

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Critical interdependent effects have been observed in the growth of AlGaAs/InGaAs quantum-well structures by molecular beam epitaxy. It is shown that statistical experimental design is an effective method for quickly optimizing complex device structures. This technique is very useful for the optimization of processes with a large number of interdependent parameters, and allows for the clear visualization and separation of complex interwoven effects. In the present work, we show the importance of the oxide desorption process for the optimal growth of AlGaAs-containing structures. ©1997 American Institute of Physics.
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81.15.Hi, 68.65.+g, 81.05.Ea

Inductive-coupling-nitrogen-plasma process for suppression of boron penetration in BF<sub>2</sub><sup>+</sup>-implanted polycrystalline silicon gate

T. S. Chao and C. H. Chu

Appl. Phys. Lett. 70, 55 (1997) (2 pages)

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A novel and simple method to suppress the boron penetration in the BF<sub>2</sub><sup>+</sup>-implanted polycrystalline silicon gate is presented by using the inductive-coupling-nitrogen-plasma (ICNP) process. A nitrogen layer was found at the SiO2/Si interface by using this method. The result shows that the sample with the ICNP treatment exhibits a good suppression of boron penetration and improved electrical characteristics. ©1997 American Institute of Physics.
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85.30.Tv, 52.75.Rx, 85.40.Ry

Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n-type GaN

B. P. Luther, S. E. Mohney, T. N. Jackson, M. Asif Khan, Q. Chen, and J. W. Yang

Appl. Phys. Lett. 70, 57 (1997) (3 pages)

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We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7 × 1017 cm–3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8 × 10–6 Omega cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5 × 1017 cm–3) had resistivities of 7 × 10–6 and 5 × 10–6 Omega cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN. ©1997 American Institute of Physics.
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73.40.Cg, 73.40.Ns, 68.60.Dv

The suppression of misfit dislocation introduction in heavily carbon doped GaAs

S. P. Westwater, T. J. Bullough, T. B. Joyce, B. R. Davidson, and L. Hart

Appl. Phys. Lett. 70, 60 (1997) (3 pages)

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The relaxation of heavily carbon doped GaAs, grown on (001) GaAs substrates was investigated using transmission electron microscopy and x-ray diffraction. Misfit dislocation introduction occurred only significantly above Matthews and Blakeslee's critical thickness, with the onset of relaxation being delayed as the C concentration was increased. The resultant strain relaxation was highly anisotropic with the introduction of As-cored misfit dislocations being totally suppressed at the highest C levels, resulting in cracking of the epilayer. Locking of As-cored partial dislocations and a reduction in misfit dislocation velocity is proposed as the cause of the anisotropy and lack of relaxation. ©1997 American Institute of Physics.
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61.72.Ff, 61.72.Lk

Oxidation of Si(100) in nitric oxide at low pressures: An x-ray photoelectron spectroscopy study

A. Kamath, D. L. Kwong, Y. M. Sun, P. M. Blass, S. Whaley, and J. M. White

Appl. Phys. Lett. 70, 63 (1997) (3 pages)

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Ultrathin (8–23 Å) silicon oxynitrides have been studied in the temperature range of 560–1000 °C in 4 Torr of NO using a sequential growth and analysis approach. X-ray photoelectron spectroscopy indicates that with increasing growth temperature and time, a bonding structure with predominantly Si–O rather than Si–N formation is favored. Simultaneously, the average volume fraction of N (N/N+O) in the dielectric decreases, as a consequence of which the N1s binding energy increases by 0.2–0.8 eV from its initial value of 397.8 eV at a thickness of 8 Å. A correlation of the electrical characteristics of NO grown oxynitrides with nitrogen content and location has been made. A film growth mechanism that takes into account the removal of previously incorporated N by NO is also proposed. ©1997 American Institute of Physics.
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81.65.Mq, 81.05.Cy, 77.55.+f, 79.60.Dp

Electrical properties of oxides grown on strained Si using microwave N2O plasma

L. K. Bera, S. K. Ray, D. K. Nayak, N. Usami, Y. Shiraki, and C. K. Maiti

Appl. Phys. Lett. 70, 66 (1997) (3 pages)

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Microwave plasma oxidation (below 200 °C) of strained Si on relaxed Si1 – xGex buffer layers in N2O ambient is reported. The electrical properties of grown oxide have been characterized using a metal-oxide-semiconductor structure. Fixed oxide charge density and mid-gap interface state density are found to be 6 × 1010 cm–2 and 1.2 × 1011 cm – 2 eV – 1, respectively. The oxide on strained-Si samples has exhibited hole trapping behavior and moderately low interface state generation on constant current stressing. ©1997 American Institute of Physics.
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73.40.Qv, 73.61.Ng, 81.65.Mq

Micro-Raman characterization of the electronic properties of Si-doped GaAs layers grown on patterned substrates

J. Gerster, J. M. Schneider, C. Ehret, W. Limmer, R. Sauer, and H. Heinecke

Appl. Phys. Lett. 70, 69 (1997) (3 pages)

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The coupled modes of plasmons and LO phonons appearing in the Raman spectra of highly doped GaAs layers have been investigated in order to study the local incorporation character of Si on different crystal facets. The Si-doped GaAs layers were grown by molecular beam epitaxy on patterned GaAs (100) substrates with etched ridges forming (111)A and (111)B facets with a lateral extension of a few micrometers. The local type and concentration of free charge carriers have been determined from an analysis of the coupled-mode Raman spectra. It is shown that Si acts as a donor in the material grown on the (111)B facets and as an acceptor in the material grown on the (111)A facets. ©1997 American Institute of Physics.
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78.30.Fs, 73.20.Mf, 68.55.Ln, 81.05.Ea

Femtosecond differential transmission measurements on low temperature GaAs metal–semiconductor–metal structures

Ulrich D. Keil, Jørn M. Hvam, Sönke Tautz, Stefan U. Dankowski, Peter Kiesel, and Gottfried H. Döhler

Appl. Phys. Lett. 70, 72 (1997) (3 pages)

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We report on differential transmission measurements on low temperature grown (LT)-GaAs with and without applied electrical fields at different wavelengths. Electrical fields up to 100 kV/cm can be applied via an interdigitated contact structure to our LT GaAs samples which have been removed from the substrate by epitaxial lift off. In the presence of an electric field, both, the absorption bleaching due to phase space filling and field induced absorption changes due to the Franz–Keldysh effect contribute to the transmission changes. We observe an extended carrier lifetime with applied field. The response time of a biased metal–semiconductor–metal detector, therefore, exceeds the carrier life time of the substrate material. ©1997 American Institute of Physics.
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73.40.Sx, 72.20.Ht, 78.47.+p

Anisotropy and growth-sequence dependence of atomic-scale interface structure in InAs/Ga1 – xInxSb superlattices

A. Y. Lew, S. L. Zuo, E. T. Yu, and R. H. Miles

Appl. Phys. Lett. 70, 75 (1997) (3 pages)

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We have used cross-sectional scanning tunneling microscopy to study the atomic-scale interface structure of InAs/Ga1 – xInxSb superlattices grown by molecular beam epitaxy. Detailed, quantitative analysis of interface profiles obtained from constant-current images of both (110) and (11-bar 0) cross-sectional planes of the superlattice indicate that interfaces in the (11-bar 0) plane exhibit a higher degree of interface roughness than those in the (110) plane, and that the Ga1 – xInxSb-on-InAs interfaces are rougher than the InAs-on-Ga1 – xInxSb interfaces. The roughness data are consistent with anisotropy in interface structure arising from anisotropic island formation during growth, and in addition a growth-sequence-dependent interface asymmetry resulting from differences in interfacial bond structure between the superlattice layers. ©1997 American Institute of Physics.
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68.35.Ct, 68.65.+g, 61.16.Ch

Nonequilibrium hydrogen temperatures under diamond chemical vapor deposition conditions

Robert S. Sinkovits

Appl. Phys. Lett. 70, 78 (1997) (3 pages)

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Direct simulation Monte Carlo calculations indicate that significant differences can exist between the translational, rotational, and vibrational temperatures of molecular hydrogen under diffusion-dominated diamond chemical vapor deposition conditions. For hydrogen confined in a 1 cm gap between an activating source and a diamond substrate, the largest discrepancies between the translational and rotational temperatures of H2 occur at a pressure of approximately 2 Torr, but measurable differences persist for pressures exceeding 20 Torr. The observed trends are due to the inefficiency of intermolecular collisions at exciting the internal modes of H2. ©1997 American Institute of Physics.
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81.15.Gh, 33.20.Vq

Efficient doping of nitrogen with high activation ratio into ZnSe using a high-power plasma source

K. Kimura, S. Miwa, T. Yasuda, L. H. Kuo, C. G. Jin, K. Tanaka, and T. Yao

Appl. Phys. Lett. 70, 81 (1997) (3 pages)

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We have developed a high-power (5 kW) rf plasma source for nitrogen doping in ZnSe molecular beam epitaxy. Optical emission spectroscopy shows dominant atomiclike emissions around 800 nm due to excited neutral nitrogen atoms in the high power region and their intensities rapidly increase with increasing the rf power from 1 to 3 kW. The high net acceptor concentration (NA–ND) of 1.2 × 1018 cm – 3 was achieved at the growth temperature of 220 °C and the activation ratio [(NA–ND)/N] as high as 60%, which is the highest value so far obtained for NA–ND ~ 1018 cm – 3. Consequently, the PL spectrum showed well-resolved deep donor-acceptor pair emissions even with high NA–ND. ©1997 American Institute of Physics.
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61.72.Vv, 52.75.Rx, 81.05.Dz, 81.15.Hi

Observation of trap states in Er-doped InP by photoreflectance

Jiti Nukeaw, Jun Yanagisawa, Naoteru Matsubara, Yasufumi Fujiwara, and Yoshikazu Takeda

Appl. Phys. Lett. 70, 84 (1997) (3 pages)

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We have investigated room-temperature photoreflectance (PR) spectra in Er-doped InP grown by low-pressure organometallic vapor phase epitaxy. A new feature is clearly observed at 1.31 eV, accompanied with a feature due to the bandgap transition at 1.35 eV. The new feature is attributed to a transition involving an Er trap. The transition energy is independent of both doping processing techniques and concentrations of Er in the layers. The PR spectral width of the trap transition energy described by the broadening parameter, increases linearly with the logarithm of Er concentrations. ©1997 American Institute of Physics.
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71.55.Eq, 72.20.Jv, 78.40.Fy

Pure strain effect on reducing the chirp parameter in InGaAsP/InP quantum well electroabsorption modulators

Takayuki Yamanaka, Koichi Wakita, and Kiyoyuki Yokoyama

Appl. Phys. Lett. 70, 87 (1997) (3 pages)

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The pure strain dependence of the chirp parameter (linewidth enhancement factor) of electroabsorption in the 1.55 µm window for strain ranging from compression to tension is studied theoretically in InGaAsP strained quantum well (QW) structures. The small-signal chirp parameter for TE polarization is evaluated from calculated electroabsorption (EA) spectra based on k · p theory and their Kramers–Krönig transformed refractive index changes. It is found that both compressive and tensile strain in the well layer reduce the chirp parameter. In a tensile-strained QW, almost continuously negative values irrespective of applied electric fields occur at an optimized amount of strain. The compressive-strained QW lowers the chirp parameter to nearly zero or negative values, the amount of the reduction being proportional to strain amount. ©1997 American Institute of Physics.
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85.60.Bt, 78.20.Bh, 78.20.Jq, 42.79.Hp

Effect of illumination on the subband electronic structure of Si delta-doped GaAs

G. Li and C. Jagadish

Appl. Phys. Lett. 70, 90 (1997) (3 pages)

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The subband electronic structure of Si delta-doped GaAs grown by metal organic vapor phase epitaxy was investigated using magnetotransport measurements. The work focused on the effect of illumination. We found that illumination leads to a slight increase of the quasi-two dimensional electron gas density in the well. This increase does not depend on the illumination intensity. The illumination-generated electron density weakly increases with an increase in the Si delta-doping concentration. We also experimentally confirmed that illumination only slightly alters the electron densities of the occupied subbands and the illumination-generated electrons populate one previously empty subband. Only a fraction of the illumination-generated electrons persist but the newly occupied subband under illumination remains populated in the dark after removal of the illumination. The experimental results suggest that the DX centers are unlikely to be populated in our Si delta-doped GaAs regardless of the doping concentration. The weak and partially persistent photoconductivity effect observed in Si delta-doped GaAs may arise from ionization of other Si localized states.©1997 American Institute of Physics.
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71.20.Nr, 73.50.Jt, 73.61.Ey

A 40-nm-pitch double-slit experiment of hot electrons in a semiconductor under a magnetic field

Hiroo Hongo, Yasuyuki Miyamoto, Kazuhito Furuya, and Michihiko Suhara

Appl. Phys. Lett. 70, 93 (1997) (3 pages)

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We report a double-slit experiment of hot electrons in a semiconductor under a magnetic field. The pitch of the double slit buried in the semiconductor is 40 nm and the electron energy is of the order of 100 meV. By applying a magnetic field, the change in current that passes through the slits is observed at the segmented collector. The measured current shows a clear minimum around B=0 T, with this behavior agreeing with a theoretical calculation based on double-slit interference. Quantitative estimation is consistent with this order of current variation. We think that these results show evidence of the observation of hot electron interference by a double slit in a semiconductor. ©1997 American Institute of Physics.
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73.23.Ad, 85.30.St

1.3 µm strain-compensated InAsP/InGaP electroabsorption modulator structure grown by atmospheric pressure metal–organic vapor epitaxy

A. Ougazzaden, F. Devaux, E. V. K. Rao, L. Silvestre, and G. Patriarche

Appl. Phys. Lett. 70, 96 (1997) (3 pages)

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High quality 15-period strain-compensated InAsP/InGaP electroabsorption (EA) modulator structures have been grown by atmospheric pressure metal–organic vapor epitaxy. The incorporation of large compressive strain (~ 1.7%) in the InAsP wells and tensile strain (~ –1.8%) in the InGaP barriers necessitated the growth of a few InP monolayers between the wells and barriers. The high structural quality of such samples has been demonstrated by (cross-sectional transmission electron microscopy analysis to be free of misfit dislocations and thickness undulations. The detection of a sharp and abrupt room-temperature exciton peak both in the photoconductivity and photoluminescence measurements further confirmed their excellent optical quality. 100 µm cavity length EA modulators fabricated in these structures exhibited excellent performances namely, an extinction ratio higher than 20 dB for 2.5 V drive voltage, a 3 dB bandwidth over 20 GHz, and low coupling losses to fiber (less than 2.5 dB per facet). ©1997 American Institute of Physics.
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42.79.Hp, 78.20.Jq, 81.15.Kk, 85.60.Bt

Low resistance Ohmic contact scheme (~ µOmega cm2) to p-InP

Moon-Ho Park, L. C. Wang, J. Y. Cheng, and C. J. Palmstrøm

Appl. Phys. Lett. 70, 99 (1997) (3 pages)

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A low resistance Pd/Sb/Zn/Pd Ohmic contact based on the solid phase regrowth principle has been investigated to p-InP. Contact resistivity as low as ~ 2 × 10–6 Omega cm2 has been obtained for samples annealed at 500 °C for 1 min. The Ohmic behavior can be rationalized by the formation of a heavily doped surface layer or a heterojunction. The InSb phase responsible for the observed low resistivity is identified using the x-ray diffraction technique. ©1997 American Institute of Physics.
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73.40.Cg, 72.80.Ey, 73.40.Ns, 68.55.Nq

Effect of indirect Gamma-L and Gamma-X transfer on the carrier dynamics of InGaP/InAlP multiple quantum wells

C. S. Menoni, O. Buccafusca, M. C. Marconi, D. Patel, J. J. Rocca, G. Y. Robinson, and S. M. Goodnick

Appl. Phys. Lett. 70, 102 (1997) (3 pages)

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Indirect Gamma-L scattering within the well, and real space carrier transfer to the barrier X1c states are shown to significantly affect the carrier dynamics in In0.48Ga0.52P/In0.5Al0.5P multiple quantum wells. When carriers transfer to the indirect states occurs, the carrier dynamics is modified by the slow return of the carriers from the low mobility states to the well. As a result, the absorption recovery time increases by almost an order of magnitude. Carrier transfer to the indirect states also increases the carrier lifetime to values characteristic of indirect recombination. ©1997 American Institute of Physics.
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73.40.Kp, 73.20.Dx, 73.50.Gr

Resonant tunneling of electrons via 20 nm scale InAs quantum dot and magnetotunneling spectroscopy of its electronic states

M. Narihiro, G. Yusa, Y. Nakamura, T. Noda, and H. Sakaki

Appl. Phys. Lett. 70, 105 (1997) (3 pages)

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The resonant tunneling of electrons through a 20 nm scale InAs quantum dot bound by a pair of very thin AlAs barriers is studied. A well-resolved composite peak resulting from the ground 1s states was observed at 4.2 K in current–voltage characteristics. By investigating the effects of inplane magnetic fields, the shape of the wave function and the spatial extent of the first two electronic states are clarified. ©1997 American Institute of Physics.
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73.20.Dx, 73.40.Gk, 73.61.Ey

Ohmic contacts to n-GaN using PtIn2

D. B. Ingerly, Y. A. Chang, N. R. Perkins, and T. F. Kuech

Appl. Phys. Lett. 70, 108 (1997) (3 pages)

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A new metallization scheme has been developed to form Ohmic contacts to n-GaN. Contacts were fabricated by sputtering the intermetallic compound, PtIn2 on metal–organic vapor phase epitaxy grown n-GaN (n ~ 5 × 1017 cm–3) with some of the contacts subjected to rapid thermal annealing. Contacts in the as-deposited state exhibited nearly Ohmic behavior with a specific contact resistance of 1.2 × 10–2 Omega cm2. Contacts subjected to rapid thermal annealing at 800 °C for 1 min exhibited linear current–voltage characteristics and had specific contact resistances less than 1 × 10–3 Omega cm2. Auger depth profiling and glancing angle x-ray diffraction were used to examine the interfacial reactions of the PtIn2/n-GaN contacts. Consistent with estimated phase diagram information, the results from Auger depth profiling and glancing angle x-ray diffraction indicated the formation of (InxGa1 – x)N at the contact interface, which could be responsible for the Ohmic behavior of PtIn2 contacts. ©1997 American Institute of Physics.
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73.40.Cg, 73.40.Ns, 85.40.Ls, 61.10.Kw

Confinement potential and surface state density in deep-mesa etched quantum wires

S. P. Riege, T. Kurth, F. Runkel, D. Heitmann, and K. Eberl

Appl. Phys. Lett. 70, 111 (1997) (3 pages)

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We present an electrostatic model to determine the surface charge density and confinement potential in etched AlGaAs/GaAs quantum wires. In far-infrared transmission experiments we have determined the resonance frequencies of the dynamic eigenmodes of the quantum wire, which gives an independent determination of the confinement potential. We find good agreement with the modeled potential, which shows that this model gives a very good description of the electronic system. ©1997 American Institute of Physics.
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73.20.At, 73.20.Dx, 78.30.Fs

NbN/AlN/NbN tunnel junctions with high current density up to 54 kA/cm2

Zhen Wang, Akira Kawakami, and Yoshinori Uzawa

Appl. Phys. Lett. 70, 114 (1997) (3 pages)

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We report on progress in the development of high current density NbN/AlN/NbN tunnel junctions for applications as submillimeter wave superconductor-insulator-superconductor mixers. A very high current density up to 54 kA/cm2, roughly an order of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions with about 1 nm thick AlN barriers. The magnetic field and temperature dependence of critical supercurrents were measured to investigate the Josephson tunneling behavior of critical supercurrents in the high-Jc junctions. The junctions showed high-quality junction characteristics with a large gap voltage of 5 mV and sharp quasiparticle current rise (Delta Vg = 0.1 mV). The Rsg/RN ratio was about 5 with a Vm value of 14 mV measured at 4.2 K. ©1997 American Institute of Physics.
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74.80.Fp, 74.50.+r, 74.76.Db, 74.25.Nf

High trapped fields in bulk YBa2Cu3O7–delta samples at temperatures around 50 K

G. Fuchs, G. Krabbes, P. Schätzle, S. Gruß, P. Stoye, T. Staiger, K.-H. Müller, J. Fink, and L. Schultz

Appl. Phys. Lett. 70, 117 (1997) (3 pages)

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Bulk melt textured YBa2Cu3O7–delta samples with single grains of about 24 mm diameter were obtained by use of SmBa2Cu3O7–x seed crystals. The maximum trapped field B0 in the gap between two samples was investigated as function of temperature. B0 increased from 1 T at 77 K to 8.5 T at 51.5 K, which is the highest trapped field achieved in nonirradiated samples. At low temperatures, cracking of the samples was observed under magnetic pressure. In this temperature range, the trapped field is limited by the mechanical strength of the samples, for which a value of 25 MPa was estimated. ©1997 American Institute of Physics.
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74.72.Bk, 74.80.Bj, 74.25.Ld

Scaling behavior of 1/f noise in high-temperature superconductor Josephson junctions

A. Marx and R. Gross

Appl. Phys. Lett. 70, 120 (1997) (3 pages)

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Extensive studies of the low frequency 1/f noise in high-temperature superconducting (HTS) Josephson junctions of various types and materials have been performed for a wide range of operating parameters. The origin of the measured voltage fluctuations can be traced back to the trapping and release of charge carriers in trapping centers in an insulating barrier, giving rise to correlated fluctuations of the junction critical current Ic and normal-state resistance Rn. We observed a linear dependence of the normalized critical current and resistance fluctuations on Rn, which suggests a constant density of trapping centers for the HTS Josephson junctions. The scaling of the normalized fluctuations is in good agreement with the previously found scaling relation IcRn [proportional] 1/Rn and supports a junction model assuming a leaky tunnel barrier. ©1997 American Institute of Physics.
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74.40.+k, 74.50.+r, 74.76.Bz

Fast and sensitive suspended YBaCuO microbolometers using silicon separated by implanted oxygen substrates

Laurence Méchin, Jean-Claude Villégier, and Daniel Bloyet

Appl. Phys. Lett. 70, 123 (1997) (3 pages)

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A process to fabricate epitaxial YBaCuO suspended microbolometers using silicon separated by implanted oxygen (SIMOX) substrates is reported. Unlike the other micromachining techniques, no process step is needed after film deposition and the YBaCuO air-bridge retains excellent superconducting qualities. The support membrane is constituted of a 80-nm-thick CeO2/YSZ buffer layer on a 150-nm-thick silicon layer. The critical temperature Tc(R = 0) is 88 K and the transition width is 2.5 K. Current-voltage characteristics directly measured on a 10-µm-wide, 100-µm-long suspended bridge show a critical current density of 7.5 × 105 A/cm2 at 80 K. The thermal conductance G of this bridge is 2 × 10–5 W/K. An optical noise equivalent power (NEP) of 6.1 × 10–12 W/sqrt(Hz) at 1 kHz is deduced from optical sensitivity and noise measurements. The time constant is 6 µs, which is the lowest value reported on such suspended structures. Comparison with a similar suspended bridge fabricated without a silicon layer shows that the sensitivity–bandwidth product of the SIMOX bridge is improved by one order of magnitude, thus demonstrating the feasibility of sensitive and fast YBaCuO bolometers. ©1997 American Institute of Physics.
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07.57.Kp, 74.76.Bz, 85.25.Pb

Ti and Ca substitution in SrRuO3 thin films by sequential deposition process

L. Miéville, T. H. Geballe, L. Antognazza, and K. Char

Appl. Phys. Lett. 70, 126 (1997) (3 pages)

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We report on the growth and properties of (Sr1 – xCax)RuO3 and Sr (Ru1 – xTix)O3 thin films obtained by sequential deposition of submonolayers from end members of each compound. Magnetization measurements as well as transport properties exhibit a very different behavior for each type of substitution. A simple model, which assumes a random distribution of the substituted sites, allows us to account quantitatively for the reduction of the magnetization with increased doping and further confirms the homogeneity of our samples. A strong distortion due to the presence of the Ti on the Ru site could explain the different behavior observed between both types of doping. ©1997 American Institute of Physics.
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75.70.Ak, 75.50.Dd, 68.55.Ln, 81.15.Fg

Structure and magnetism of the Fe/GaAs interface

A. Filipe, A. Schuhl, and P. Galtier

Appl. Phys. Lett. 70, 129 (1997) (3 pages)

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We study the magnetic properties of Fe thin films epitaxially grown on GaAs (001) for a large range of substrate temperature. Magnetization deficiency has been observed and studied. Its dependence on both thickness and temperature clearly show the existence of a nearly half-magnetized phase at the interface, covered by "as-bulk" Fe. Furthermore, reflection high-energy electron diffraction studies show a transition between two bcc structures with different crystalline parameters. Transmission electron microscopy confirms the formation of this interfacial phase, for which the compound Fe3Ga2 – xAsx seems to be the best candidate. ©1997 American Institute of Physics.
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75.70.Cn, 68.35.Ct, 61.14.Hg

The effect of the microstructure on the magnetic interactions in CoFe–AgCu granular films: From demagnetizing to magnetizing interactions

X. Batlle, V. Franco, A. Labarta, M. L. Watson, and K. O'Grady

Appl. Phys. Lett. 70, 132 (1997) (3 pages)

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The sign and strength of the dominant magnetic interactions in Co34Fe8Ag54Cu4 granular films were modified by changing the microstructure through annealing. Magnetic force micrographs showed that in the as-cast sample the magnetic moments of neighboring grains tended to be arranged parallel along a direction out of the film plane, forming elongated magnetic clusters that were themselves aligned antiparallel, with dominant demagnetizing interactions. This is a direct evidence that an uncompensated antiferromagneticlike microstructure is stabilized below the volume percolation threshold. However, in the sample annealed at 750 °C the particle growth led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. Thus, in this letter we present direct correlation of interactions effects with magnetic measurements and show that Delta M plots correlate with changes in the magnetic microstructure in these systems. ©1997 American Institute of Physics.
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75.70.Ak, 75.50.Bb, 81.40.Rs

High resolution imaging of thin-film recording heads by superparamagnetic magnetic force microscopy tips

S. H. Liou, S. S. Malhotra, John Moreland, and P. F. Hopkins

Appl. Phys. Lett. 70, 135 (1997) (3 pages)

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We have used superparamagnetic magnetic force microscopy (MFM) tips to obtain high spatial resolution MFM images of recording heads. Profiles of the magnetic field gradient above a thin-film recording head under 3 mA bias current to the head and various tip-head distance conditions are presented. At a low tip-head distance, the gap width, gap location, and gap-field structure can be well resolved in these MFM images. Superparamagnetic tips show promise for the magnetic imaging of recording heads with gap widths below 200 nm. ©1997 American Institute of Physics.
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07.79.Pk, 85.70.Kh, 75.50.Ss

Fatigue resistance in lead zirconate titanate thin ferroelectric films: Effect of cerium doping and frequency dependence

S. B. Majumder, Y. N. Mohapatra, and D. C. Agrawal

Appl. Phys. Lett. 70, 138 (1997) (3 pages)

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We demonstrate improvement in fatigue resistance and other ferroelectric properties through cerium doping in sol–gel derived lead zirconate titanate thin films. We have studied frequency dependence of fatigue behavior and show that the loss of polarization due to fatigue follows a universal scaling behavior with N/f2, where N is the number of switching cycles and f the frequency. The origin of the scaling is attributed to the drift of oxygen vacancies, which is the rate limiting process in the growth of the interface layer responsible for fatigue. Empirical fits for both undoped and cerium-doped samples show that switchable polarization follows stretched exponential decay with time or N/f. Cerium doping is shown to improve fatigue resistance by impeding the motion of oxygen vacancies. ©1997 American Institute of Physics.
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77.80.Fm, 77.84.Dy, 85.50.+k

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