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Showing papers in "Applied Physics Letters in 1990"


Journal ArticleDOI
TL;DR: In this paper, free standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography using electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers.
Abstract: Indirect evidence is presented that free‐standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography. The novel approach uses electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers. Mesoporous Si layers of high porosity exhibit visible (red) photoluminescence at room temperature, observable with the naked eye under <1 mW unfocused (<0.1 W cm−2) green or blue laser line excitation. This is attributed to dramatic two‐dimensional quantum size effects which can produce emission far above the band gap of bulk crystalline Si.

7,393 citations


Journal ArticleDOI
TL;DR: In this article, an electron wave analog of the electro-optic light modulator is proposed, where magnetized contacts are used to preferentially inject and detect specific spin orientations.
Abstract: We propose an electron wave analog of the electro‐optic light modulator. The current modulation in the proposed structure arises from spin precession due to the spin‐orbit coupling in narrow‐gap semiconductors, while magnetized contacts are used to preferentially inject and detect specific spin orientations. This structure may exhibit significant current modulation despite multiple modes, elevated temperatures, or a large applied bias.

4,268 citations


Journal ArticleDOI
TL;DR: In this article, the effect of varying the solution pH on the surface structure was studied by measuring the SiH stretch vibrations with infrared absorption spectroscopy, and the surface was found to be very homogeneous with low defect density (<0.5%) and narrow vibrational linewidth.
Abstract: Aqueous HF etching of silicon surfaces results in the removal of the surface oxide and leaves behind silicon surfaces terminated by atomic hydrogen. The effect of varying the solution pH on the surface structure is studied by measuring the SiH stretch vibrations with infrared absorption spectroscopy. Basic solutions ( pH=9–10) produce ideally terminated Si(111) surfaces with silicon monohydride ( 3/4 SiH) oriented normal to the surface. The surface is found to be very homogeneous with low defect density (<0.5%) and narrow vibrational linewidth (0.95 cm−1 ).

1,250 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that Si microcrystallites were embedded in a Si oxide matrix for the sample which emitted the light, and the inverse relation between emission energy and the square of the crystallite size indicates that carrier confinement in the Si micro-crystallite causes this photoluminescence phenomenon.
Abstract: Visible photoluminescence was observed in ultrafine Si particles at room temperature. Transmission electron microscopy revealed that Si microcrystallites were embedded in a Si oxide matrix for the sample which emitted the light. The emission energy depended on crystallite size in the range from 2.8 to 5 nm. The inverse relation between emission energy and the square of the crystallite size indicates that carrier confinement in the Si microcrystallites causes this photoluminescence phenomenon.

793 citations


Journal ArticleDOI
TL;DR: In this article, a simple reflection technique for measuring the electrooptic coefficient of nonlinear optical polymeric films is described, based on the polarization rotation of a laser beam due to the electro-optic effect.
Abstract: A simple reflection technique for measuring the electro‐optic coefficient of nonlinear optical polymeric films is described. The technique is based on the polarization rotation of a laser beam due to the electro‐optic effect. It requires no waveguiding and can be used to measure polymeric films during, as well as after, the electrical poling process. The results are in excellent agreement with those obtained from the more conventional but much more time consuming waveguiding method.

760 citations


Journal ArticleDOI
TL;DR: In this paper, the chemical modification of hydrogen-passivated n-Si surfaces by a scanning tunneling microscope (STM) operating in air is reported, and the modified surface regions have been characterized by STM spectroscopy, scanning electron microscopy (SEM), time-of-flight secondary ion mass spectrometry (TOF SIMS), and chemical etch/Nomarski microscopy.
Abstract: The chemical modification of hydrogen‐passivated n‐Si (111) surfaces by a scanning tunneling microscope (STM) operating in air is reported. The modified surface regions have been characterized by STM spectroscopy, scanning electron microscopy (SEM), time‐of‐flight secondary‐ion mass spectrometry (TOF SIMS), and chemical etch/Nomarski microscopy. Comparison of STM images with SEM, TOF SIMS, and optical information indicates that the STM contrast mechanism of these features arises entirely from electronic structure effects rather than from topographical differences between the modified and unmodified substrate. No surface modification was observed in a nitrogen ambient. Direct writing of features with 100 nm resolution was demonstrated. The permanence of these features was verified by SEM imaging after three months storage in air. The results suggest that field‐enhanced oxidation/diffusion occurs at the tip‐substrate interface in the presence of oxygen.

723 citations


Journal ArticleDOI
TL;DR: In this article, an organic electroluminescenters with multilayered thin-film structures which emitted bright blue light were constructed and two empirical guides for the selection of blue-emitting materials were established.
Abstract: Organic electroluminescent (EL) devices with multilayered thin‐film structures which emitted bright blue light were constructed. Two empirical guides for the selection of blue‐emitting materials were established. The keys to obtain the EL cells with high EL efficiency were excellent film‐forming capability of an emitter layer and the appropriate combinations of emitter and carrier transport materials for avoiding the formation of exciplexes. In one of our organic electroluminescent devices, blue emission with a luminance of 700 cd/m2 was achieved at a current density of 100 mA/cm2 and a dc drive voltage of 10 V.

691 citations


Journal ArticleDOI
TL;DR: In this article, a real-time optical microscope was developed that operates on the same principle as a liquid immersion microscope, with the liquid replaced by a solid lens of high refractive index material.
Abstract: A new type of real‐time optical microscope has been developed that operates on the same principle as a liquid immersion microscope, with the liquid replaced by a solid lens of high refractive index material. Using a lens with an index n=2 and 436 nm illumination, this microscope has resolved 100 nm lines and spaces and has demonstrated a factor of two improvement in the edge response over a confocal microscope.

683 citations


Journal ArticleDOI
TL;DR: In this paper, the Raman spectrum of nanophase TiO2 was used to determine the oxygen-titanium ratio, and the thermogravimetric analysis studies used to calculate the ratio were discussed.
Abstract: Calibration curves of the Raman spectrum of nanophase TiO2 to the material’s oxygen stoichiometry are presented. The thermogravimetric analysis studies used to determine the oxygen‐titanium ratio are also discussed.

594 citations


PatentDOI
TL;DR: In this paper, a dither motion is applied to the tip at a first frequency in a direction substantially normal (22) to the plane of the sample surface (18), and then at a second frequency the motion is simultaneously applied to a sample (20) at a parallel direction (24) parallel to the surface plane (18) to reduce the detected background signal.
Abstract: A near field optical microscopy method and apparatus eliminates the necessity of an aperture for scanning a sample surface (18) and greatly reduces the detected background signal. A small dimension tip (14), on the order of atomic dimension, is disposed in close promity to the sample surface (18). A dither motion is applied to the tip (14) at a first frequency in a direction substantially normal (22) to the plane of the sample surface (18). Dither motion is simultaneously applied to the sample (20) at a second frequency in a direction substantially parallel (24) to the plane of the sample surface (18). The amplitude of the motions are chosen to be comparable to the desired measurement resolution. The end (12) of the tip (14) is illuminated by optical energy. The scattered light from the tip (14) and surface (18) is detected at the difference frequency for imaging the sample surface (18) at sub-wavelength resolution without the use of an aperture. Alternatively, the tip (14) is maintained stationary and the sample (20) undergoes motion in the two directions.

566 citations


Journal ArticleDOI
TL;DR: In this article, a selective conversion of high composition (AlAs)x(GaAs)1−x layers into dense transparent native oxide by reaction with H2O vapor (N2 carrier gas) at elevated temperatures (400 °C) is presented.
Abstract: Data are presented on the conversion (selective conversion) of high‐composition (AlAs)x(GaAs)1−x layers, e.g., in AlxGa1−xAs‐AlAs‐GaAs quantum well heterostructures and superlattices (SLs), into dense transparent native oxide by reaction with H2O vapor (N2 carrier gas) at elevated temperatures (400 °C). Hydrolyzation oxidation of a fine‐scale AlAs(LB)‐GaAs(Lz) SL (LB +Lz≲100 A), or random alloy AlxGa1−xAs (x≳0.7), is observed to proceed more slowly and uniformly than a coarse‐scale ‘‘alloy’’ such as an AlAs‐GaAs superlattice with LB + Lz≳200 A.

Journal ArticleDOI
TL;DR: In this article, an atomic force microscope capable of measuring, simultaneously yet separately, lateral (frictional) and normal forces is described, and a direction-dependent feature, absent in topological images, is found when scanning stepped surfaces of NaCl(001) in ultrahigh vacuum.
Abstract: An atomic force microscope capable of measuring, simultaneously yet separately, lateral (‘‘frictional’’) and normal forces is described. A direction‐dependent feature, absent in topological images, is found when scanning stepped surfaces of NaCl (001) in ultrahigh vacuum. A simple model is presented to account for this observation.

Journal ArticleDOI
TL;DR: In this paper, direct evidence for interplanar relaxation in islands at the initial stages of strained epitaxy was presented using molecular beam epitaxically deposited In 0.5Ga0.5As on GaAs(100), and the existence of atomic displacements in the substrates to unexpectedly large depths of ∼150 A was found.
Abstract: Direct evidence for interplanar relaxation in islands at the initial stages of strained epitaxy is presented using molecular beam epitaxically deposited In0.5Ga0.5As on GaAs(100). Concomitant existence of atomic displacements in the substrates to unexpectedly large depths of ∼150 A is found. In incoherent islands, defects are found to be introduced symmetrically near the island edges.

Journal ArticleDOI
TL;DR: In this paper, a novel approach to produce p-type ZnSe epitaxial layers is reported which involves nitrogen atom beam doping during molecular beam epitaxy growth, which achieves acceptor concentrations as large as 3.4×1017 cm−3.
Abstract: A novel approach to producing p‐type ZnSe epitaxial layers is reported which involves nitrogen atom beam doping during molecular beam epitaxial growth. Net acceptor concentrations as large as 3.4×1017 cm−3 have been measured in nitrogen atom beam doped ZnSe/GaAs heteroepitaxial layers which represents the highest acceptor concentration reported to date for ZnSe:N epitaxial material grown by molecular beam epitaxy. In addition, light‐emitting diodes based on ZnSe:N/ZnSe:Cl, p‐n homojunctions have been found to exhibit dominant electroluminescence in the blue region of the visible spectrum at room temperature.

Journal ArticleDOI
TL;DR: In this article, the heat equation for an axially heated cylinder with a thermally conductive boundary at the periphery has been solved using both a full numerical solution and an analytic approximation which assumes only radial heat flow.
Abstract: In order to estimate deleterious effects caused by heating in continuous‐wave end‐pumped solid‐state lasers, the heat equation has been solved for an axially heated cylinder with a thermally conductive boundary at the periphery Steady‐state thermal profiles are developed using both a full numerical solution and an analytic approximation which assumes only radial heat flow The analytic solution, which is in good agreement with the numerical solution, is utilized to obtain an expression for the thermal focusing due to temperature‐induced refractive index changes For Nd:YAG, 1 W of pump power deposited as heat is predicted to cause a thermal focusing length comparable to the cavity length of a typical diode end‐pumped laser

Journal ArticleDOI
TL;DR: In this paper, a bare semiconductor wafer was illuminated by femtosecond optical pulses, and electromagnetic waves radiated from the surface and formed collinear diffraction-limited electromagnetic beams in the inward and outward directions.
Abstract: We have generated electromagnetic beams from a variety of semiconductors. When a bare semiconductor wafer was illuminated by femtosecond optical pulses, electromagnetic waves radiate from the surface and form collinear diffraction‐limited electromagnetic beams in the inward and outward directions. The amplitude and phase of the radiated field depend on carrier mobility, the strength and polarity of the static internal field at the semiconductor surface.

Journal ArticleDOI
TL;DR: In this paper, a multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x ≥ 0.4).
Abstract: Epitaxial liftoff is an alternative to lattice‐mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin‐film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self‐bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin‐film atomic lattices can be simultaneously imaged, showing only a thin (20–100 A) amorphous layer in between.

Journal ArticleDOI
TL;DR: Arsenic precipitates have been observed in GaAs low-temperature buffer layers (LTBLs) used as "substrates" for normal molecular beam epitaxy growth as mentioned in this paper.
Abstract: Arsenic precipitates have been observed in GaAs low‐temperature buffer layers (LTBLs) used as ‘‘substrates’’ for normal molecular beam epitaxy growth. Transmission electron microscopy has shown the arsenic precipitates to be hexagonal phase single crystals. The precipitates are about 6±4 nm in diameter with a density on the order of 1017 precipitates per cm3. The semi‐insulating properties of the LTBL can be explained in terms of these arsenic precipitates acting as ‘‘buried’’ Schottky barriers with overlapping spherical depletion regions. The implications of these results on LTBL resistivity stability with respect to doping and anneal temperature will be discussed as will the possible role of arsenic precipitates in semi‐insulating liquid‐encapsulated Czochralski‐grown bulk GaAs.

Journal ArticleDOI
TL;DR: In this article, a double-heterostructure indium-tin-oxide substrate/hole transport layer/emitter layer/electron transport layer was fabricated by vacuum vapor deposition.
Abstract: Organic electroluminescent devices with a double‐heterostructure indium‐tin‐oxide substrate/hole transport layer/emitter layer/electron transport layer/MgAg have been fabricated by vacuum vapor deposition. The organic carrier transport and emitter layers were composed of amorphous films. In the double‐heterostructure devices, the luminance continued to lie in high level, even when the emitter thickness was 50 A. The confinement of charge carriers and molecular excitons within a narrow emitter layer was achieved.

Journal ArticleDOI
TL;DR: In this paper, the authors used a 9-carbon model compound to describe a proposed mechanism for homoepitaxial growth of diamond from methyl radicals on a hydrogenated, electrically neutral (100) surface.
Abstract: We use a 9‐carbon model compound to describe a proposed mechanism for homoepitaxial growth of diamond from methyl radicals on a hydrogenated, electrically neutral (100) surface. We estimate enthalpy and entropy changes for each step in the mechanism using group additivity methods, taking into account the types of bonding and steric repulsions found on the (100) surface. Rate constants are estimated based on analogous reactions for hydrocarbon molecules, while gas phase species concentrations are taken from our previous measurements. The rate equations are then integrated. The method, which contains no adjustable parameters or phenomenological constants, predicts a growth rate of between 0.06 and 0.6 μm/h, depending on the local details of the surface. Uncertainties related to the use of a model compound rather than diamond are discussed. The analysis demonstrates that the proposed mechanism is feasible.

Journal ArticleDOI
TL;DR: In this article, a GaAs quantum well infrared detector having a cutoff wavelength of λc=10.7 μm was used to achieve a blackbody detectivity of 1.0×1010 cm (Hz) 1/2/W at T=68 K, a temperature which is readily achievable with a cryogenic cooler.
Abstract: By increasing the quantum well barrier width, we have dramatically reduced the tunneling dark current by an order of magnitude and thereby significantly increased the blackbody detectivity D*BB. For a GaAs quantum well infrared detector having a cutoff wavelength of λc=10.7 μm, we have achieved D*BB =1.0×1010 cm (Hz)1/2/W at T=68 K, a temperature which is readily achievable with a cryogenic cooler.

Journal ArticleDOI
TL;DR: In this article, the authors used a scanning tunneling microscope to directly imaged n−alkane layers adsorbed at the liquid/graphite interface using a high degree of two-dimensional ordering.
Abstract: We have directly imaged n‐alkane layers adsorbed at the liquid/graphite interface using a scanning tunneling microscope. The layers possessed a high degree of two‐dimensional ordering. The adsorbate was observed to enhance the tunneling current, and the atomic structure of the images was dominated by features associated with the substrate. These systems are excellent vehicles for studies concerning the imaging mechanism of adsorbed organic layers because of their stability and simplicity.

Journal ArticleDOI
TL;DR: In this article, the voltage-current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model were described in a manner consistent with the resistive transition behavior.
Abstract: We describe the voltage‐current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model in a manner consistent with the resistive transition behavior. The magnitude of the activation energy, and its temperature and magnetic field dependences, are readily derived from the experimentally observed power law characteristics and show a (1−T/Tc)3/2 type of behavior near Tc. The activation energy is a nonlinear function of the current density and it enables the determination of the shape of the flux line potential well.

Journal ArticleDOI
TL;DR: In this article, a simple and accurate extended Jones matrix representation for the twisted nematic liquid crystal display at the oblique incidence was obtained for the first time, which is quite close to those obtained by the faster 4×4 matrix method with spectrum averaging to account for the nonzero bandwidth of the incident light.
Abstract: A simple and accurate extended Jones matrix representation for the twisted nematic liquid‐crystal display at the oblique incidence was obtained for the first time. The results obtained by this extended Jones matrix representation are quite close to those obtained by the faster 4×4 matrix method with spectrum averaging to account for the nonzero bandwidth of the incident light. However, the computation time for the extended Jones matrix method is less than half that of the faster 4×4 matrix method without spectrum averaging. Furthermore, at normal incidence, this extended Jones matrix representation reduces to the ordinary Jones matrix representation.

Journal ArticleDOI
TL;DR: Using a source of freely propagating subpicosecond pulses of THz radiation, this article measured the absorption and dispersion of both N and P-type, 1 Ω cm silicon from 0.1 to 2 THz.
Abstract: Using a source of freely propagating subpicosecond pulses of THz radiation, we have measured the absorption and dispersion of both N‐ and P‐type, 1 Ω cm silicon from 0.1 to 2 THz. These results give the corresponding frequency‐dependent complex conductance over the widest frequency range to date. The data provide a complete view on the dynamics of both electrons and holes and are well fit by the simple Drude relationship.

Journal ArticleDOI
TL;DR: In this article, a two-dimensional electron gas (2DEG) was fabricated on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE).
Abstract: We have succeeded in fabricating a two-dimensional electron gas (2DEG) on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE). A (100) wafer previously prepared by MBE growth is reinstalled in the MBE chamber so that an in situ cleave exposes a fresh (110) GaAs edge for further MBE overgrowth. A sequence of Si-doped AlGaAs layers completes the modulation-doped structure at the cleaved edge. Mobilities as high as 6.1×10^5 cm^2/V s are measured in the 2DEG at the cleaved interface.

Journal ArticleDOI
TL;DR: In this paper, an improved high-efficiency silicon solar cell structure was proposed, which overcomes deficiencies in an earlier structure by locally diffusing boron into contact areas at the rear of the cells.
Abstract: Significant improvements in silicon solar cell performance are reported using an improved high‐efficiency silicon solar cell structure. This structure overcomes deficiencies in an earlier structure by locally diffusing boron into contact areas at the rear of the cells. Terrestrial energy conversion efficiencies up to 24% are reported for silicon cells for the first time. Air Mass 0 efficiencies lie in the 20–21% range, the first silicon cells to exceed 20% efficiency under space illumination.

Journal ArticleDOI
TL;DR: In this paper, a method has been found for preparing uniform silicon tips with a radius of curvature less than 1 nm, formed by oxidation of 5μm-high silicon cones through exploitation of a known oxidation inhibition of silicon at regions of high curvature.
Abstract: Electron emitters in vacuum microelectronic devices need sharp tips in order to permit electron emission at moderate voltages A method has been found for preparing uniform silicon tips with a radius of curvature less than 1 nm These tips are formed by oxidation of 5‐μm‐high silicon cones through exploitation of a known oxidation inhibition of silicon at regions of high curvature

Journal ArticleDOI
TL;DR: In this paper, a two-terminal, monolithic cascade solar cell with an efficiency of 27.3% was reported, which consists of a Ga 0.5In0.5P homojunction and a GaAs tunnel diode interconnect.
Abstract: A two‐terminal, monolithic cascade solar cell with an efficiency of 27.3% is reported. The device structure consists of a Ga0.5In0.5P homojunction grown epitaxially upon a GaAs homojunction, with a GaAs tunnel diode interconnect. The tandem combination of these two materials is lattice matched, and has a theoretical efficiency of 34%. The device was grown by metalorganic chemical vapor deposition at 700 °C, using trimethylgallium, trimethylindium, arsine, and phosphine as sources. The minority‐carrier transport properties of the Ga0.5In0.5P are shown to be relatively insensitive to variations of the growth temperature and phosphine overpressure. Other factors that affect the efficiency of the device are presented and discussed.

Journal ArticleDOI
TL;DR: In this article, the authors used laser light scattering with the laser beam rapidly rastered in a plane parallel to the rf electrode, and observed that there is significant negative charge on the particles.
Abstract: Particles generated in an argon plasma and suspended at the plasma/sheath boundary are localized by lateral trapping fields. In the commercial rf etching reactor used in this work, the particles and their motion in real time are observed by laser light scattering with the laser beam rapidly rastered in a plane parallel to the rf electrode. Repulsion between individual, relatively large particles is observed, verifying that there is significant negative charge on the particles. Two types of trapping regions are commonly seen: rings of particles around the outside edge of silicon wafers, and domes of particles over the centers of the wafers. It is shown that these effects are influenced by the topography of the electrode. In addition, particle densities >107 cm−3 for particles of diameter 0.2 μm are inferred from transmission studies for certain plasma conditions.