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26 Oct 1992

Volume 61, Issue 17, pp. 2001-2118

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X‐ray laser frequency near‐doubling and generation of tunable coherent x rays in plasma

P. L. Shkolnikov, A. E. Kaplan, M. H. Muendel, and P. L. Hagelstein

Appl. Phys. Lett. 61, 2001 (1992); http://dx.doi.org/10.1063/1.108339 (3 pages) | Cited 5 times

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We identify and evaluate plasmas in which efficient x‐ray laser frequency near‐doubling is expected for a number of available x‐ray lasers by means of four‐wave mixing with Nd or KrF lasers. In some of these plasmas, four‐wave mixing of coherent x rays and tunable optical radiation may result in tunable coherent x‐ray radiation powerful enough for x‐ray laser spectroscopy.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.55.Vc X- and γ-ray lasers
52.38.-r Laser-plasma interactions

Haze‐free polymer dispersed liquid crystals utilizing linear polarizers

John L. West, David S. Fredley, and J. Craig Carrell

Appl. Phys. Lett. 61, 2004 (1992); http://dx.doi.org/10.1063/1.108340 (2 pages) | Cited 4 times

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Polymer dispersed liquid crystal (PDLC) light shutters are scattering in the OFF state and transparent in the ON state for normally incident light. PDLC shutters appear hazy in the ON state when viewed at oblique angles as a result of light scattering. The haze is caused by the effective increase in the refractive index of the liquid crystal. However, only one polarization of the incident radiation is scattered. Therefore, PDLC shutters with a linearly polarized sheet on the incident surface are haze‐free for an entire viewing plane. While addition of a polarizer reduces transmission through the PDLC to less than 50%, it also offers a simple means of producing haze‐free PDLC shutters for applications where viewing is predominantly in one plane.
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42.79.Kr Display devices, liquid-crystal devices
42.25.Fx Diffraction and scattering

Tunable wavelength conversion using a liquid crystal filter and a bistable laser diode

Hiroyuki Tsuda, Katsuhiko Hirabayashi, Hidetoshi Iwamura, and Takashi Kurokawa

Appl. Phys. Lett. 61, 2006 (1992); http://dx.doi.org/10.1063/1.108341 (3 pages) | Cited 3 times

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A tunable wavelength converter has been constructed using a liquid crystal Fabry–Perot interferometer and a multiple quantum well bistable laser diode. Input light pulses with a wavelength of 1495–1515 nm are converted to light pulses with a wavelength of 1485–1525 nm.
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42.79.Nv Optical frequency converters

THz pulses from the creation of polarized electron‐hole pairs in biased quantum wells

Paul C. M. Planken, Martin C. Nuss, W. H. Knox, D. A. B. Miller, and K. W. Goossen

Appl. Phys. Lett. 61, 2009 (1992); http://dx.doi.org/10.1063/1.108342 (3 pages) | Cited 37 times

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We report generation of coherent terahertz electromagnetic transients from GaAs/Al0.3Ga0.7As quantum wells in perpendicular fields. Although, at low temperature, the quantum well barriers suppress the transport current perpendicular to the layers by at least two orders of magnitude compared to bulk, we observe terahertz signals that are comparable in strength to those generated from bulk GaAs surfaces. This directly proves that the field‐induced polarization of photoexcited electron‐hole pairs is an important mechanism for the generation of terahertz radiation at semiconductor surfaces.
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79.20.Ds Laser-beam impact phenomena
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Pz Photoconduction and photovoltaic effects

Analysis of distributed‐feedback lasers: A recursive Green’s function approach

K. B. Kahen

Appl. Phys. Lett. 61, 2012 (1992); http://dx.doi.org/10.1063/1.108343 (3 pages) | Cited 3 times

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A recursive Green’s function method is introduced for obtaining the exact modal characteristics of aperiodic distributed‐feedback lasers. Additionally, based on scaling arguments, a new expression for the coupled‐mode theory coupling coefficient, κ, is derived, resulting in increased accuracy for coupled‐mode calculations.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.By Design of specific laser systems

Improvement of the photorefractive efficiency of BaTiO3 by γ irradiation

Thomas R. Moore and Robert W. Boyd

Appl. Phys. Lett. 61, 2015 (1992); http://dx.doi.org/10.1063/1.108344 (3 pages) | Cited 2 times

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We report that the photorefractive response of as‐grown BaTiO3 crystals can be improved by γ irradiation from a Co60 source. An increase in photorefractive response was observed with irradiation doses exceeding 106 rad, with a maximum observed increase in two‐beam coupling gain of ∼15%. We interpret our results within the context of a well‐known model of photorefractivity in BaTiO3 and conclude that γ irradiation increases the effective trap density and possibly also increases the normalized conductivity in samples where both holes and electrons contribute to photorefractivity.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
61.80.Ed γ-ray effects

Meander‐type wavelength demultiplexer with weighted coupling

H. Venghaus, C. Bornholdt, F. Kappe, H.‐P. Nolting, and C. M. Weinert

Appl. Phys. Lett. 61, 2018 (1992); http://dx.doi.org/10.1063/1.108345 (3 pages) | Cited 6 times

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An experimental and theoretical investigation of meander‐type wavelength demultiplexers with weighted coupling is reported. Numerical simulations have been performed combining a two‐dimensional finite difference analysis and a suitably developed coupling mode method. Devices have been realized based on rib waveguides in GaInAsP/InP with filter center wavelengths around 1.3 μm. With weighted coupling the sinc2(λ)‐type transmission characteristics of standard meander couplers change and the sidelobes of the latter devices are efficiently suppressed. We further show the determination of optimum weighted coupling for wavelength filters meeting given requirements with respect to channel separation, maximum allowable crosstalk, and total device length.
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42.79.Sz Optical communication systems, multiplexers, and demultiplexers
42.82.-m Integrated optics

Epitaxial growth of vanadyl‐phthalocyanine ultrathin films on hydrogen‐terminated Si(111) surfaces

Hirokazu Tada, Takafumi Kawaguchi, and Atsushi Koma

Appl. Phys. Lett. 61, 2021 (1992); http://dx.doi.org/10.1063/1.108346 (3 pages) | Cited 31 times

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Ultrathin films of vanadyl phthalocyanine (VOPc) have been grown on hydrogen‐terminated Si(111) surfaces by molecular beam epitaxy. Epitaxial growth was examined on the two types of substrates. Reflection high energy electron diffraction studies have revealed that VOPc molecules form commensurate lattices [−3[3 1]4], [−1[4 3]3], and [1[4 4]1] on the surface terminated homogeneously with monohydride. On the other hand, epitaxial growth did not occur on the surface terminated with the mixture of polyhydride. Homogeneity and microscopic flatness of the substrate surface seem to be important factors for the epitaxial growth of VOPc on the hydrogen‐terminated Si substrate.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.-a Thin film structure and morphology

Young’s modulus of a thin film determined from the peak shifts of its membrane modes

A. Fartash

Appl. Phys. Lett. 61, 2024 (1992); http://dx.doi.org/10.1063/1.108347 (3 pages)

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The perturbations of membrane modes of a circular film due to its plate rigidity is studied. A single mathematical expression valid for all orders of resonant modes is derived, and a new method for measuring the Young’s modulus is described that is based on finding the frequency shifts of thin‐film membrane modes. The Young’s moduli of textured aluminum and nickel films are measured and found in good agreement with their calculated literature values [D. Baral, J. B. Ketterson, and J. E. Hilliard, J. Appl. Phys. 57, 1076 (1985)].  
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68.60.Bs Mechanical and acoustical properties
07.10.-h Mechanical instruments and equipment
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Liquid precursor two‐step aluminum thin‐film deposition on KrF‐laser patterned palladium

O. Lehmann and M. Stuke

Appl. Phys. Lett. 61, 2027 (1992); http://dx.doi.org/10.1063/1.108348 (3 pages) | Cited 9 times

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High quality aluminum films are deposited using a two‐step process. KrF‐laser mask patterned palladium catalyst areas are selectively covered with aluminum using triethylamine alane as precursor. The precursor is supplied directly in the liquid phase, so this technique can also work under atmospheric pressure conditions in an inert gas atmosphere. One micrometer thick aluminum metal films can be generated spatially selective with high deposition rate (t≲10 s).
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68.55.-a Thin film structure and morphology
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Exact results in linear thermomechanics of fluid‐saturated porous media

James G. Berryman and Graeme W. Milton

Appl. Phys. Lett. 61, 2030 (1992); http://dx.doi.org/10.1063/1.108349 (3 pages) | Cited 8 times

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Exact results are derived relating the various bulk moduli (frame, unjacketed, and pore) and heat capacity of inhomogeneous fluid‐saturated porous media to the pertinent thermal expansion coefficients and component moduli when only two porous solid constituents are present. The porous solids need not be space filling or in perfect welded contact, so these results apply to materials containing some voids or cracks interspersed between the two constituents in addition to the interior voids associated with their pores. The key ratio of fluid pressure to temperature change producing uniform expansion or contraction is shown to be independent of porosity.
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62.20.-x Mechanical properties of solids
46.25.Cc Theoretical studies
83.10.Ff Continuum mechanics
91.60.Ba Elasticity, fracture, and flow

Stabilization of C‐49 Ti(AlxSi1−x)2 observed from the reaction between TiSi2 and Al

I. Chung, H. G. Nam, and R. W. Bene’

Appl. Phys. Lett. 61, 2033 (1992); http://dx.doi.org/10.1063/1.108350 (3 pages)

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The solid state reaction between aluminum and titanium silicide was studied over the temperature range of 450–800 °C. The phase formed from the reaction was different from Ti7Al5Si12, which has been frequently reported as the reaction product. Also, two other phases were reported nearly identical to those observed in terms of the similarity in the structure type and lattice parameters, namely, ternary C‐49 Ti2AlSi3 and binary C‐49 Ti(AlxSi1−x)2 which is the metastable C‐49 TiSi2 with Al solid solution. Based on the detailed microscopic and spectroscopic analysis, it was suggested that the observed phase is likely the metastable C‐49 Ti(AlxSi1−x)2 stabilized by substitution of Si with Al. Even though we did not find the maximum value of x, approximately 10% of Si was found to be substituted with Al for the sample annealed at 800 °C for 30 min.
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81.30.-t Phase diagrams and microstructures developed by solidification and solid-solid phase transformations
68.35.Md Surface thermodynamics, surface energies
64.70.K- Solid-solid transitions

Apparent super resolution in high‐contrast photoresists

Martin Peckerar, Milton Rebbert, and Ganesh Gopalakrishnan

Appl. Phys. Lett. 61, 2036 (1992); http://dx.doi.org/10.1063/1.108351 (3 pages)

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Optoelectronic modulators and couplers frequently require high‐aspect ratio feature definition in thick (≥10 μm) gold. The common process used involves up plating with a photoresist guide. Theory argues that feature boundaries would be degraded due to both diffractive spreading and scattering of the incident light in the resist. This is not observed and near‐90° sidewalls appear obtainable. This letter demonstrates this effect and proposes an explanation for it.
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42.82.Cr Fabrication techniques; lithography, pattern transfer
42.25.Fx Diffraction and scattering
85.40.Hp Lithography, masks and pattern transfer

Partially exposed polymer dispersed liquid crystals for boundary layer investigations

Devendra S. Parmar and Jag J. Singh

Appl. Phys. Lett. 61, 2039 (1992); http://dx.doi.org/10.1063/1.108352 (3 pages) | Cited 7 times

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A new configuration termed partially exposed polymer dispersed liquid crystal in which the liquid crystal microdroplets dispersed in a rigid polymer matrix are partially entrapped on the free surface of the thin film deposited on a glass substrate is reported. Optical transmission characteristics of the partially exposed polymer dispersed liquid crystal thin film in response to an air flow induced shear stress field reveal its potential as a sensor for gas flow and boundary layer investigations.
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61.30.-v Liquid crystals
42.79.Kr Display devices, liquid-crystal devices

Enhanced breakdown voltages in strained InGaAs/GaAs structures

J. P. R. David, M. J. Morley, A. R. Wolstenholme, R. Grey, M. A. Pate, G. Hill, G. J. Rees, and P. N. Robson

Appl. Phys. Lett. 61, 2042 (1992); http://dx.doi.org/10.1063/1.108354 (3 pages) | Cited 7 times

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The breakdown voltage (VBD) in a semiconductor is usually proportional to its band‐gap (Eg) through the dependence of the impact ionization process on the threshold voltage (Eth). It has recently been suggested that strain can cause Eth to increase even when Eg decreases, raising the possibility of narrow band‐gap materials with large VBD. By growing a range of strained InGaAs/GaAs multiple quantum well (MQW) pin diode structures and measuring VBD, we show that the presence of strained InGaAs increases VBD confirming that it has a larger Eth than GaAs.
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73.50.Fq High-field and nonlinear effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Etch masks for microfabrication produced by electron beam sublimation of graphitic carbon deposited as semimetallic amorphous carbon thin films

G. A. Porkolab and E. D. Wolf

Appl. Phys. Lett. 61, 2045 (1992); http://dx.doi.org/10.1063/1.108355 (3 pages) | Cited 2 times

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High quality etch masks for nanometer plasma processing can be formed from thin films of semimetallic amorphous carbon that are deposited by electron beam sublimation of graphitic carbon. These films are amorphous, hard, semimetallic, and mirror‐reflective. These electron beam sublimation deposited (EBSD) semimetallic amorphous carbon (semimetallic a‐C) thin films can be routinely deposited up to at least 400 nm thick and patterned by SF6 plasma reactive ion etching (RIE) via standard photoresist masks. They are demonstrated to be excellent etch masks on gallium arsenide, silicon, and germanium substrates using chemically assisted ion beam etching (CAIBE), also known as ion beam assisted etching (IBAE), reactive ion beam etching (RIBE), and RIE. The carbon etch masks have fine grain, low chemical reactivity, low sputter rates, and high thermal stability. Finally, the EBSD semimetallic a‐C can be readily stripped by SF6 or O2 or H2 plasmas.
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85.40.Hp Lithography, masks and pattern transfer
81.65.-b Surface treatments
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Lattice relaxed impurity and persistent photoconductivity in nitrogen doped 6H‐SiC

A. S. Dissanayake and H. X. Jiang

Appl. Phys. Lett. 61, 2048 (1992); http://dx.doi.org/10.1063/1.108328 (3 pages) | Cited 10 times

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The impurity properties in n‐type N doped single crystal 6H‐SiC semiconductors have been studied by conductivity measurements. A slow decay of the dark current and persistent photoconductivity (PPC) have been observed at low temperatures. Different energy barriers have been obtained at different conditions. Measured donor binding energy, the electron capture barrier, and the emission barrier are 34.6±1.0, 14.9±0.3, and 45.6±1.2 meV, respectively. Our results suggest that PPC in SiC has a similar origin as that in AlxGa1−xAs. However the amount of lattice relaxation in SiC is much smaller compared with that in AlxGa1−xAs. Experimental results reported in this letter will help to understand the properties of lattice relaxed impurities. It also explains why different energy levels have been obtained previously by different methods.
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72.40.+w Photoconduction and photovoltaic effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.55.Ht Other nonmetals

Effect of one monolayer of surface gold atoms on the epitaxial growth of InAs nanowhiskers

M. Yazawa, M. Koguchi, A. Muto, M. Ozawa, and K. Hiruma

Appl. Phys. Lett. 61, 2051 (1992); http://dx.doi.org/10.1063/1.108329 (3 pages) | Cited 90 times

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This letter shows that selective heteroepitaxy of nanometer‐scale InAs whiskers on SiO2‐patterned GaAs substrates [Yazawa, Koguchi, and Hiruma, Appl. Phys. Lett. 58, 1080 (1991)] is induced by surface contamination with Au resulting from the fluorocarbon plasma etching process used to etch the SiO2 mask. We demonstrate that high densities (≂1010/cm2) of InAs nanowhiskers 20–30 nm in diameter can be epitaxially grown on InAs(111)B substrates onto which 1 monolayer of Au atoms had been deposited. This wirelike growth appears to be induced by ultrafine alloy droplets generated by the reactions between Au‐clusters and InAs substrates.
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68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Arsine flow requirement for the flow modulation growth of high purity GaAs using adduct‐grade triethylgallium

B. L. Pitts, D. T. Emerson, and J. R. Shealy

Appl. Phys. Lett. 61, 2054 (1992); http://dx.doi.org/10.1063/1.108330 (3 pages) | Cited 1 time

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Using arsine and triethylgallium with flow modulation, organometallic vapor phase epitaxy can produce high purity GaAs layers with V/III molar ratios near unity. We have estimated that under appropriate growth conditions the arsine incorporation efficiency into epitaxial GaAs can exceed 30%. The arsine flow requirement for obtaining good morphology has been identified over a range of substrate temperatures using adduct‐grade triethylgallium. The process described reduces the environmental impact and life safety risk of the hydride based organometallic vapor phase epitaxial method.
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81.15.Kk Vapor phase epitaxy; growth from vapor phase

Pulsed excimer laser ablated barium titanate thin films

D. Roy and S. B. Krupanidhi

Appl. Phys. Lett. 61, 2057 (1992); http://dx.doi.org/10.1063/1.108305 (3 pages) | Cited 48 times

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Thin films of BaTiO3 were deposited on platinum coated silicon substrates by excimer laser (248 nm) ablation at 600 °C or ex situ crystallized at about the same annealing temperature. Films deposited at 600 °C showed good crystallinity and were characterized for ferroelectricity, dielectric constant, dielectric loss, leakage current, and C‐V characteristics. The films showed a dielectric constant of 220, a dissipation factor of 0.02, a leakage current of 1.8×10−6 A/cm2 at a bias of 5 V, and a charge storage density of about 40 fC/μm2 at a field of 0.15 MV/cm.
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81.15.Jj Ion and electron beam-assisted deposition; ion plating
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.55.-g Dielectric thin films

Growth morphology of epitaxial ErAs/GaAs by x‐ray extended range specular reflectivity

P. F. Miceli, C. J. Palmstrøm, and K. W. Moyers

Appl. Phys. Lett. 61, 2060 (1992); http://dx.doi.org/10.1063/1.108306 (3 pages) | Cited 12 times

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The growth morphology of ErAs on [001] GaAs with thicknesses ranging from 2 atomic layers to 400 Å is investigated using x‐ray specular reflectivity. This epitaxial system displays rich morphological behavior and we observe the evolution from discontinuous to continuous layers at just a few monolayers and qualitative changes occur in layer thickness fluctuations for thicker films having high dislocation densities. We also demonstrate that the reflectivity of a heteroepitaxial system can be measured and modeled over an extended angular range, starting from grazing angles and continuing through the regions of Bragg scattering at higher angles. Disorder in epitaxial layers typically gives a mosaic line broadening transverse to the reflectivity and we show that transverse line shape considerations are crucial to these measurements.
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61.05.cf X-ray scattering (including small-angle scattering)
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Consideration of the thermal energy in hydrodynamic transport models

Matthias Curow

Appl. Phys. Lett. 61, 2063 (1992); http://dx.doi.org/10.1063/1.108307 (3 pages) | Cited 5 times

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The thermal energy is an important parameter in hydrodynamic transport models used for systematic studies of the behavior of hot electron devices. This contribution shows that a careful consideration is necessary in order to avoid some solution details which are not only unphysical, but which can easily lead to numerical instabilities, too. Fairly good results are obtained at least for GaAs if the thermal energy is expressed via steady‐state results of Monte Carlo simulations as it is done for other transport parameters.
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85.30.De Semiconductor-device characterization, design, and modeling
72.20.Dp General theory, scattering mechanisms

Photoreflectance characterization of an InP/InGaAs heterojunction bipolar transistor structure

D. Yan, Fred H. Pollak, V. T. Boccio, C. L. Lin, P. D. Kirchner, J. M. Woodall, Russell C. Gee, and Peter M. Asbeck

Appl. Phys. Lett. 61, 2066 (1992); http://dx.doi.org/10.1063/1.108308 (3 pages) | Cited 26 times

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We have measured the photoreflectance (PR) spectrum at 300 K from a lattice‐matched InP/InGaAs heterojunction bipolar transistor structure grown by gas‐source molecular beam epitaxy. From the observed Franz–Keldysh oscillations we have evaluated the built‐in dc electric fields and associated doping profiles in the n‐InGaAs collector and n‐InP emitter regions. These donor concentrations are in agreement with capacitance‐voltage and secondary ion mass spectroscopy determinations, but are considerably lower than the intended values from the growth parameters. We have thereby detected a failure of the Si doping source which occurred during material growth using this contactless, nondestructive, and rapid characterization method. The energy of the InGaAs band gap from the PR spectrum also verifies the lattice‐matched nature of the system, further demonstrating the diagnostic and process control value of the PR technique.
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85.30.Pq Bipolar transistors
85.30.De Semiconductor-device characterization, design, and modeling
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Visible photoluminescence in crystallized amorphous Si:H/SiNx:H multiquantum‐well structures

Kunji Chen, Xinfan Huang, Jun Xu, and Duan Feng

Appl. Phys. Lett. 61, 2069 (1992); http://dx.doi.org/10.1063/1.108309 (3 pages) | Cited 64 times

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Visible photoluminescence has been observed in crystallized a‐Si:H/a‐SiNx:H multiquantum‐well structures at room temperature. The MQW heterostructures consisting of 72 layers were formed by computer controlled plasma‐enhanced chemical‐vapor deposition method and then crystallized by Ar+ laser annealing technique. The crystallinity and average grain size of the silicon microcrystals were determined by means of Raman and x‐ray diffraction spectroscopy. The crystallized samples with well‐layer thickness Ls=40 Å showed an intense photoluminescence which is peaked at 2.1 eV with a full width at half‐maximum of 0.25 eV. This is consistent with calculations based on the quantum confinement model.
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78.55.Hx Other solid inorganic materials
78.66.-w Optical properties of specific thin films
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Triple quantum well electron transfer infrared modulator

V. Berger, E. Dupont, D. Delacourt, B. Vinter, N. Vodjdani, and M. Papuchon

Appl. Phys. Lett. 61, 2072 (1992); http://dx.doi.org/10.1063/1.108310 (3 pages) | Cited 11 times

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In this letter, we present a new bicolor infrared modulator using intersubband absorptions in quantum wells. The structure consists of two wells of different widths on either side of a reservoir of electrons. Depending on the sign of the electric field, the electrons transfer either into the wide well or into the narrow well, and then give rise to an intersubband absorption of the corresponding wavelength. With no applied bias, all the carriers are located in the reservoir and no absorption occurs in the midinfrared wavelength range of interest. Two samples are studied by transmission measurements in a Fourier transform infrared spectrometer. The great importance of the space charge effects in this type of device is evidenced and simulated by a self‐consistent model.  
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42.79.Hp Optical processors, correlators, and modulators
78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
42.65.Pc Optical bistability, multistability, and switching, including local field effects
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