Showing papers in "Applied Physics Letters in 1994"

Candela‐class high‐brightness InGaN/AlGaN double‐heterostructure blue‐light‐emitting diodes

Abstract: Candela‐class high‐brightness InGaN/AlGaN double‐heterostructure (DH) blue‐light‐emitting diodes(LEDs) with the luminous intensity over 1 cd were fabricated As an active layer, a Zn‐doped InGaN layer was used for the DH LEDs The typical output power was 1500 μW and the external quantum efficiency was as high as 27% at a forward current of 20 mA at room temperature The peak wavelength and the full width at half‐maximum of the electroluminescence were 450 and 70 nm, respectively This value of luminous intensity was the highest ever reported for blue LEDs

Magneto‐impedance effect in amorphous wires

Abstract: Recent experiments have discovered a giant magneto‐impedance (MI) effect in FeCoSiB amorphous wires. This effect includes a sensitive change (as much as 60%) in a high frequency wire voltage by an applied dc magnetic field and is thus a high frequency analog of giant magnetoresistance. We consider this phenomenon in terms of ac complex resistance or impedance. The giant MI effect is demonstrated to arise from a combination of a skin effect and a strong field dependence of the circumferential magnetic permeability associated with circular domain wall movements. The theoretical results agree satisfactorily with the existing experimental data.

Tapping mode atomic force microscopy in liquids

Abstract: Tapping mode atomic force microscopy in liquids gives a substantial improvement in imaging quality and stability over standard contact mode. In tapping mode the probe‐sample separation is modulated as the probe scans over the sample. This modulation causes the probe to tap on the surface only at the extreme of each modulation cycle and therefore minimizes frictional forces that are present when the probe is constantly in contact with the surface. This imaging mode increases resolution and reduces sample damage on soft samples. For our initial experiments we used a tapping frequency of 17 kHz to image deoxyribonucleic acid plasmids on mica in water. When we imaged the same sample region with the same cantilever, the plasmids appeared 18 nm wide in contact mode and 5 nm in tapping mode.

Laser‐induced breakdown by impact ionization in SiO2 with pulse widths from 7 ns to 150 fs

Abstract: Results of laser‐induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse‐width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.

Reliability and degradation of organic light emitting devices

Abstract: We present a simple encapsulation technique for organic light emitting devices (OLEDs). By studying the degradation of a population of OLEDs, we show that the lifetime of encapsulated devices is increased by more than two orders of magnitude over that of unencapsulated devices. In both cases, degradation is primarily due to the formation of nonemissive regions, or dark spot defects. By studying the structure and evolution of the dark spots, we infer that the growth of electrode defects limits device lifetime. Hermetic packaging of OLEDs is essential if they are to be used in commercially viable flat panel displays.

White light‐emitting organic electroluminescent devices using the poly(N‐vinylcarbazole) emitter layer doped with three fluorescent dyes

Abstract: White light‐emitting electroluminescent devices were fabricated using poly(N‐vinylcarbazole) (PVK) as a hole‐transporting emitter layer and a double layer of 1,2,4‐triazole derivative (TAZ) and tris(8‐quinolinolato)aluminum(III) complex (Alq) as an electron transport layer. The PVK layer was doped with fluorescent dyes such as blue‐emitting 1,1,4,4‐tetraphenyl‐1,3‐butadiene, green‐emitting coumarin 6, and orange‐emitting DCM 1. A cell structure of glass substrate/indium‐tin‐oxide/doped PVK/TAZ/Alq/Mg:Ag was employed. White emission covering a wide range of the visible region and a high luminance of 3400 cd/m2 were obtained at a drive voltage of 14 V.

Self‐organized growth of regular nanometer‐scale InAs dots on GaAs

Abstract: The deposition of InAs on GaAs proceeds first by two‐dimensional (2D) growth and above a 1.75‐monolayer coverage by the formation of single‐crystal dots on a residual 2D wetting layer. By atomic force microscopy measurements, we show that the first dots formed are in the quantum size range (height 30 A, half‐base 120 A), that the dispersion on their sizes is remarkably low (±10%), and that they are located fairly regularly (interdot distance 600 A). Upon further growth, density and shapes do not change but sizes increase up to double values before coalescence occurs. Self‐organized growth in strained structures is then shown to be a simple and efficient way of building regular quantum dots.

Giant magnetoresistance in magnetic multilayered nanowires

Abstract: Giant magnetoresistance (GMR) is observed in a new type of nanostructured material consisting of magnetic multilayered nanowires formed by electrodeposition into nanometer-sized pores of a template polymer membrane. The composition of these nanowires is modulated over nanometer length scales with distinct magnetic and nonmagnetic metallic layers. Magnetoresistance measurements with the current perpendicular to the layers were performed on the array of parallel nanowires. GMR of about 15% was observed at room temperature on Co/Cu multilayered nanowires. (C) 1994 American Institute of Physics.

Melt processing for obtaining NdBa2Cu3Oy superconductors with high Tc and large Jc

Abstract: A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four‐probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M‐H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x<0.1) among the Nd1+xBa2−xCu3Oy solid solutions in a reduced oxygen atmosphere.

Giant magnetic field dependent impedance of amorphous fecosib wire

Abstract: The impedance of an amorphous Fe4.3Co68.2Si12.5B15 wire (100 μm diameter) exhibits an extraordinarily large frequency dependent resistance in addition to the previously reported frequency dependent wire reactance. The frequency response of both the resistance and reactance is almost entirely suppressed by an axial magnetic field HA<150 Oe, resulting in a typical magnetoresistance for frequencies f<1 MHz of the order of the dc wire resistance. The magnetoresistance at f=1 MHz is ΔR/Rsat=370%. As the bulk of the magnetic response occurs for HA<5 Oe, this system shows great technological promise. We give a quantitative analysis of the phenomenon, which is rooted in classical electrodynamics.

Complete microcrystalline p-i-n solar cell—Crystalline or amorphous cell behavior?

Abstract: Complete μc‐Si:H p‐i‐n solar cells have been prepared by the very high frequency glow discharge method. Up to now, intrinsic μc‐Si:H has never attracted much attention as a photovoltaic active material. However, an efficiency of 4.6% and remarkably high short circuit current densities of up to 21.9 mA/cm2 due to an enhanced absorption in the near‐infrared could be obtained. First light‐soaking experiments indicate no degradation for the entirely μc‐Si:H cells. Voltage‐dependent spectral response measurements suggest that the carrier transport in complete μc‐Si:H p‐i‐n cells may possibly be cosupported by diffusion (in addition to drift).

Native-Oxide Defined Ring Contact for Low Threshold Vertical-Cavity Lasers

Abstract: Data are presented characterizing a new process for fabrication of vertical‐cavity surface‐emitting lasers based on the selective conversion of high Al composition epitaxial AlGaAs to a stable native oxide using ‘‘wet oxidation.’’ The native oxide is used to form a ring contact to the laser active region. The resulting laser active regions have dimensions of 8, 4, and 2 μm. The lowest threshold laser is achieved with the 8‐μm active region, with a minimum threshold current of 225‐μA continuous wave at room temperature.

High-efficiency cuinxga1-xse2 solar cells made from (inx,ga1-x)2se3 precursor films

Abstract: In, Ga, and Se were coevaporated to form precursor films of (Inx,Ga1−x)2Se3. The precursors were then converted to CuInxGa1−xSe2 by exposure to a flux of Cu and Se. The final films were smooth, with tightly packed grains, and had a graded Ga content as a function of film depth. Photovoltaic devices made from these films showed good tolerance in device efficiency to variations in film composition. A device made from these films resulted in the highest total‐area efficiency measured for any non‐single‐crystal, thin‐film solar cell, at 15.9%.

Organic‐inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4

Abstract: Using the combination of a layered perovskite compound (C6H5C2H4NH3)2PbI4 (PAPI), which forms a stable exciton with a large binding energy owing to its low‐dimensional semiconductor nature and exhibits sharp and strong photoluminescence from the exciton band, and an electron‐transporting oxadiazole derivative, we fabricated an organic–inorganic heterostructure electroluminescent (EL) device. The EL spectrum of the device corresponded well to the photoluminescence spectrum of the PAPI film; the emission was peaking at 520 nm and half‐width of the emission was about 10 nm at liquid‐nitrogen temperature. Further, highly intense EL of more than 10 000 cd m−2 was performed at 2 A cm−2 at liquid‐nitrogen temperature in the device.

Tapping mode atomic force microscopy in liquid

Abstract: We show that standard silicon nitride cantilevers can be used for tapping mode atomic force microscopy (AFM) in air, provided that the energy of the oscillating cantilever is sufficiently high to overcome the adhesion of the water layer. The same cantilevers are successfully used for tapping mode AFM in liquid. Acoustic modes in the liquid excite the cantilever. o­n soft samples, e.g., biological material, this tapping mode AFM is much more gentle than the regular contact mode AFM. Not o­nly is the destructive influence of the lateral forces minimized, but more important, the intrinsic viscoelastic properties of the sample itself are effectively used to ''harden'' the soft sample.

Bright red light‐emitting organic electroluminescent devices having a europium complex as an emitter

Abstract: Organic electroluminescent (EL) devices with a trivalent europium (Eu) complex as an emitter were fabricated. Triple‐layer‐type cells with a structure of glass substrate/indium‐tin oxide/ triphenyldiamine derivative (TPD)/Eu complex: 1,3,4‐oxadiazole derivative (PBD)/aluminum complex (Alq)/Mg:Ag exhibit bright red luminescence upon applying dc voltage. The EL spectrum consists of extremely sharp emission bands, which is a typical luminescence spectrum of the Eu complex. Luminance of 460 cd/m2 with an emission peak at 614 nm is achieved at a drive voltage of 16 V. This is the highest luminance so far obtained for the EL cells having a Eu complex as an emitter.

Multilayered organic electroluminescent device using a novel starburst molecule, 4,4’,4‘‐tris(3‐methylphenylphenylamino)triphenylamine, as a hole transport material

Abstract: A novel π‐conjugated starburst molecule, 4,4’,4‘‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA), which forms a stable amorphous glass, functions as an excellent hole transport material for organic electroluminescent devices. An electroluminescent device consisting of double hole transport layers of m‐MTDATA and 4,4’‐bis(3‐methylphenylphenylamino)biphenyl and an emitting layer of tris(8‐quinolinolato)aluminum exhibits a high luminance efficiency and significant durability.

Implantation and transient B diffusion in Si: The source of the interstitials

Abstract: Implanted B and P dopants in Si exhibit transient enhanced diffusion (TED) during initial annealing, due to Si interstitials being emitted from the region of the implant damage. The structural source of these interstitials has not previously been identified. Quantitative transmission electron microscopy measurements of extended defects are used to demonstrate that TED is caused by the emission of interstitials from specific defects. The defects are rodlike defects running along 〈110〉 directions, which consist of interstitials precipitating on {311} planes as a single monolayer of hexagonal Si. We correlate the evaporation of {311} defects during annealing at 670 and 815 °C with the length of the diffusion transient, and demonstrate a link between the number of interstitials emitted by the defects, and the flux of interstitials driving TED. Thus not only are {311} defects contributing to the interstitial flux, but the contribution attributable to {311} defect evaporation is sufficient to explain the whole ...

Electrically switchable volume gratings in polymer‐dispersed liquid crystals

Abstract: We report electrical switching of the diffraction efficiency in volume Bragg gratings written holographically in polymer‐dispersed liquid crystals (PDLCs). Scanning electron microscopy confirms the volume nature of the gratings and shows that they consist of periodic PDLC planes. The diffraction efficiency can be switched from a high value (∼50%) to a value near zero at fields ∼11 V/μm.

Low resistance ohmic contacts on wide band‐gap GaN

Abstract: We report a new metallization process for achieving low resistance ohmic contacts to molecular beam epitaxy grown n‐GaN (∼1017 cm−3) using an Al/Ti bilayer metallization scheme. Four different thin‐film contact metallizations were compared during the investigation, including Au, Al, Ti/Au, and Ti/Al layers. The metals were first deposited via conventional electron‐beam evaporation onto the GaN substrate, and then thermally annealed in a temperature range from 500 to 900 °C in a N2 ambient using rapid thermal annealing techniques. The lowest value for the specific contact resistivity of 8×10−6 Ω cm2, was obtained using Ti/Al metallization with anneals of 900 °C for 30 s. X‐ray diffraction and Auger electron spectroscopy depth profile were employed to investigate the metallurgy of contact formation.

Thermal and ambient-induced deflections of scanning force microscope cantilevers

Abstract: The deflection of scanning force microscope cantilevers, metal coated on one side, is significantly influenced by both thermal heating and variations in relative humidity. For constant relative humidity, the deflection of the cantilever drifts due to laser heating and eventually reaches a steady‐state value. For a thermally stabilized cantilever, the deflection varies linearly with relative humidity. Exposure to other vapors, such as mercury, changes the inherent deflection of the cantilever. Relative amounts of adsorbates on the cantilever can be estimated from shifts in the cantilever resonance frequency with picogram mass resolution. The cantilever deflection as well as changes in resonance frequency due to vapor adsorption can be used as basis for novel chemical sensors.

Nanoscale patterning and oxidation of H‐passivated Si(100)‐2×1 surfaces with an ultrahigh vacuum scanning tunneling microscope

Abstract: Nanoscale patterning of the hydrogen terminated Si(100)‐2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope. Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H‐passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.

Polyaniline as a transparent electrode for polymer light-emitting diodes: Lower operating voltage and higher efficiency

Abstract: We demonstrate that by using polyaniline (PANI) or a combination of (PANI) and indium/tin oxide (ITO) as the transparent anode of a polymer light‐emitting diode with poly[2‐methoxy‐5‐(2’‐ethyl‐ hexyloxy)‐1,4‐phenylene vinylene] (MEH‐PPV), as the active layer, device performance can be significantly improved. The operating voltage can be reduced by ∼30%–50% and the quantum efficiency can be increased by ∼30%–40% with respect to the devices using ITO alone as the hole‐injecting anode. The barrier height at the PANI/MEH‐PPV interface is estimated to be ∼0.08–0.12 eV, approximately half of that at the ITO/MEH‐PPV interface.

Terahertz optical rectification from 〈110〉 zinc‐blende crystals

Abstract: We report the study of optically induced terahertz (THz) electromagnetic radiation from 〈110〉 oriented zinc‐blende crystals. This work extends our previous studies of 〈100〉 and 〈111〉 GaAs. Excellent agreement between calculated results and experimental data indicates that, under conditions of moderate optical fluence and normal incidence on the unbiased sample, second‐order optical rectification is the major nonlinear process that generates THz radiation.

Electromagnetic radiation shielding by intrinsically conducting polymers

Abstract: The shielding efficiency of various intrinsically conducting polymers (ICPs) as a function of their intrinsic properties (conductivity and dielectric constant), thickness, and temperature is determined. Two types of shielding, reflection and absorption, by ICPs are discussed. The high shielding efficiencies of highly conducting doped polyaniline, polypyrrole, and polyacetylene are reported and compared to that of copper. The easy tuning of intrinsic properties by chemical processing suggests the wide applications of ICPs, especially polyaniline for shielding.

Near‐field optical data storage using a solid immersion lens

Abstract: A near‐field optical technique, using a new type of solid immersion lens (SIL), has been developed and applied to the writing and reading of domains in magneto‐optic material. The SIL is a truncated glass sphere which serves to increase the numerical aperture of the optical system by n2, where n is the index of refraction of the lens material. Using a SIL made from n=1.83 glass and illuminating with 780 nm light, we have achieved a 317 nm spot size. We have resolved a 500 nm period grating, and written and read 350 nm diameter magnetic domains. The technique should be capable of a 125 nm focused spot size using blue light.

Formation of artificial batio3/srtio3 superlattices using pulsed laser deposition and their dielectric properties

Abstract: We have formed strained dielectric superlattices of BaTiO3 (BTO) and SrTiO3 (STO) by a pulsed laser deposition technique. A large strain of 400–500 MPa is introduced at the interface between the BTO and STO layers. A large dielectric constant of 900 was observed with a stacking periodicity of 2 unit cells/2 unit cells. The superlattices show drastically different electrical behavior from that of the solid solution (Sr,Ba)TiO3 films. Broad maxima of the dielectric constants occur around 40–50 °C and the values remain large even for a temperature above 200 °C.

Giant magnetoresistance in La1-xSrxMnOz films near room temperature

Abstract: We report the observation of giant magnetoresistance near room temperature in ferromagnetic films of La1−xSrxMnOz for 0.16≤x≤0.33. For B=5 T, the maximum magnetoresistance ratio [R(0)−R(B)]/R(0) of an annealed film is 60% at 260 K for x=0.2, and 35% at 330 K for x=0.33. Annealed films have higher Curie temperature (Tc), a larger saturation moment and a larger magnetoresistance effect near Tc than do as‐grown films. The temperature dependence of resistivity for all the samples investigated is unusual, activated above Tc and metallic below Tc. This and the giant magnetoresistance are possibly explained by scattering from magnetic polarons which dominate the transport near Tc.

Giant magnetoresistance of nanowires of multilayers

Abstract: A new technique is required which enables tailoring of the morphology of a metallic nanostructured material down to the 10 nm length scale. Using nanoporous nuclear track etched membranes as templates for electrodeposition, an assembly of wires with diameters as low as 30 nm could be obtained. Alternating the electrodeposition of two metals resulted in multilayers grown perpendicular to the wire axis. Layer thicknesses as low as 2 nm could be reached. Application is demonstrated by making wires 6 μm long, 80 nm in diameter, having a succession of either Co and Cu layers or of (Ni,Fe) and Cu layers. Wires containing layers of 5–10 nm in thickness exhibited a giant magnetoresistance. The current was naturally perpendicular to the layers. At ambient temperature, a magnetoresistance of 14% for Co/Cu and of 10% for (Fe,Ni)/Cu was observed.

Very large magnetoresistance in perovskite‐like La‐Ca‐Mn‐O thin films

Abstract: Colossal magnetoresistance with more than a thousandfold change in resistivity (ΔR/RH=127 000% at 77 K, H=6 T) has been obtained in epitaxially grown La‐Ca‐Mn‐O thin films. This magnetoresistance value is about three orders of magnitude higher than is typically seen in the giant‐magnetoresistance‐type metallic, superlattice films. The temperature of peak magnetoresistance is located in the region of metallic resistivity behavior. As the magnetoresistance peak occurs not at the temperature of magnetic transition but at a temperature where the magnetization is still substantial, the spin‐disorder scattering is not likely to be the main mechanism in these highly magnetoresistive films. The peak can be shifted to near room temperature by adjusting processing parameters. Near‐room‐temperature ΔR/RH values of ∼1300% at 260 K and ∼400% at 280 K have been observed. The presence of grain boundaries appears to be very detrimental to achieving large magnetoresistance in the lanthanum manganite compounds. The fact th...