Showing papers in "Applied Physics Letters in 1986"

Two‐layer organic photovoltaic cell

Abstract: A thin‐film, two‐layer organic photovoltaiccell has been fabricated from copper phthalocyanine and a perylene tetracarboxylic derivative. A power conversion efficiency of about 1% has been achieved under simulated AM2 illumination. A novel feature of the device is that the charge‐generation efficiency is relatively independent of the bias voltage, resulting in cells with fill factor values as high as 0.65. The interface between the two organic materials, rather than the electrode/organic contacts, is crucial in determining the photovoltaicproperties of the cell.

Extraction of Schottky diode parameters from forward current-voltage characteristics

Abstract: It is shown that by using the forward current density‐voltage (J‐V) characteristics of a Schottky diode, a plot of d(V)/d(ln J) vs J and a plot of the function H(J) vs J, where H(J)≡V−n(kT/q)ln(J/A**T2), will each give a straight line. The ideality factor n, the barrier height φB, and the series resistance R of the Schottky diode can be determined with one single I‐V measurement. This procedure has been used successfully to study thermal annealing effects of W/GaAs Schottky contacts.

Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer

Abstract: Atmospheric pressure metalorganic vapor phase epitaxial growth and characterization of high quality GaN on sapphire (0001) substrates are reported. Using AlN buffer layers, GaN thin films with optically flat surfaces free from cracks are successfully grown. The narrowest x‐ray rocking curve from the (0006) plane is 2.70’ and from the (2024) plane is 1.86’. Photoluminescence spectra show strong near band edge emission. The growth condition dependence of crystalline quality is also studied.

Macromolecular electronic device: Field-effect transistor with a polythiophene thin film

Abstract: The first solid‐state field‐effect transistor has been fabricated utilizing a film of an organic macromolecule, polythiophene, as a semiconductor. The device characteristics have been optimized by controlling the doping levels of the polymer. The device is a normally off type and the source (drain) current can be modulated by a factor of 102–103 by varying the gate voltage. The carrier mobility and the transconductance have also been determined to be ∼10−5 cm2/V s and 3 nS, respectively, by means of electrical measurements.

Field controlled light scattering from nematic microdroplets

Abstract: The light scattering and electro‐optic response of new material with display potential are investigated. The materials consist of microdroplets of nematic liquid crystals which are spontaneously formed in a solid polymer at the time of its polymerization. Droplet size, spacing, and distribution are readily controlled in these materials to allow optimization of displays based upon electrically controlled light scattering from the liquid crystal droplets. Preliminary experimental and theoretical studies of the light scattering properties show these materials to offer new features suitable for many display applications.

Simple technique for separating the effects of interface traps and trapped‐oxide charge in metal‐oxide‐semiconductor transistors

Abstract: A new technique is presented for separating the threshold‐voltage shift of a metal‐oxide‐semiconductor transistor into shifts due to interface traps and trapped‐oxide charge. This technique is applied to threshold‐voltage shifts on an n‐channel transistor that result from ionizing radiation.

Antiresonant reflecting optical waveguides in SiO2‐Si multilayer structures

Abstract: A new type of optical waveguide utilizing an antiresonant reflector is described. Implementation in the SiO2‐Si system gave losses as low as 0.4 dB/cm for the TE mode. The TM mode loss is >60 dB/cm, making the device an excellent planar technology integrated optic polarizer.

Wafer bonding for silicon‐on‐insulator technologies

Abstract: A silicon wafer bonding process is described in which only thermally grown oxide is present between wafer pairs. Bonding occurs after insertion into an oxidizing ambient. It is proposed the wafers are drawn into intimate contact as a result of the gaseous oxygen between them being consumed by oxidation, thus producing a partial vacuum. The proposed bonding mechanism is polymerization of silanol bonds between wafer pairs. Silicon on insulator (SOI) is produced by etching away all but a few microns of one of the bonded pair. Capacitor measurements show a 27 μs minority‐carrier lifetime and no degradation of the SOI‐insulator interface. In addition, there is negligible charge at the bonding interface making the technique attractive for three‐dimensional as well as planar SOI applications.

Second harmonic generation in poled polymer films

Abstract: We report the observation of second harmonic generation in a new class of organic polymeric materials, namely, electric field poled polymer glasses. Both the observed second harmonic coefficient (d33=6.0±1.3×10−9 esu, at 1.58 μm wavelength) and the poling process are described by a thermodynamic model that we have developed. The ability to form thin films from these materials may qualify them for integrated optics applications.

Effect of hydrogen plasma treatment on transparent conducting oxides

Abstract: The effect of hydrogen plasma treatment on indium tin oxide (ITO), fluorine‐doped tin oxide (FTO), and indium‐doped zinc oxide (IZO) films has been studied. X‐ray photoelectron spectroscopy analysis shows that ITO and FTO surfaces get reduced to yield elemental indium and tin, respectively. Annealing of the plasma treated films in air leads to re‐oxidation of the reduced surface and the electro‐optical properties are recovered. In contrast, IZO films are not reduced by plasma treatment and show no changes in the electrical and optical properties. The surface of plasma treated IZO films shows a higher binding energy O(1s) peak probably due to OH or OH...O species which appear to form a protective layer against plasma degradation.

Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics

Abstract: Amorphous powders of Ni32Ti68 and of Ni45Nb55 were synthesized by mechanical alloying (MA) starting from either a mixture of pure metal powders (in the appropriate molar ratio) or from powders of the crystalline intermetallics NiTi2 and Ni45Nb55, respectively. For both alloys, the peak temperature increase (above the average processing temperature) in the powder particles trapped between colliding balls is estimated at 38 K. Thus, the amorphization is attributed to a process other than the formation of local melts followed by the rapid solidification of these melts into the amorphous phase. The amorphization by MA starting from a mixture of pure crystalline powders is attributed to a solid state interdiffusion reaction, the kinetics of which is controlled by the excess point and lattice defects generated by plastic deformation. The amorphization by MA starting from powders of crystalline intermetallics is attributed to the accumulation of point and lattice defects which raise the free energy of the faulted intermetallic above that of the amorphous alloy.

Low‐temperature silicon epitaxy by ultrahigh vacuum/chemical vapor deposition

Abstract: We have successfully demonstrated the use of a novel chemical vapor deposition technique, ultrahigh vacuum/chemical vapor deposition, to deposit homoepitaxial silicon layers of high crystalline perfection at low temperatures (T≥750 °C). Rutherford backscattering and transmission electron microscopy showed the transition to epitaxial silicon growth took place in the range 700–750 °C, and secondary ion mass spectrometry showed typical oxygen and carbon levels to be near the detection limits of the technique 1016–1017 cm−3. In addition, abrupt dopant transitions have been demonstrated, with B levels dropping four orders of magnitude, 1019–1015 B/cm3, in the first 1000 angstroms of an intrinsic epilayer.

Scanning thermal profiler

Abstract: A new high‐resolution profilometer has been demonstrated based upon a noncontacting near‐field thermal probe. The thermal probe consists of a thermocouple sensor with dimensions approaching 100 nm. Profiling is achieved by scanning the heated sensor above but close to the surface of a solid. The conduction of heat between tip and sample via the air provides a means for maintaining the sample spacing constant during the lateral scan. The large difference in thermal properties between air and solids makes the profiling technique essentially independent of the material properties of the solid. Noncontact profiling of resist and metal films has shown a lateral resolution of 100 nm and a depth solution of 3 nm. The basic theory of the new probe is described and the results presented.

Optically detected carrier confinement to one and zero dimension in GaAs quantum well wires and boxes

Abstract: Carrier confinement to one and zero degrees of freedom has been achieved in artificial quantum well wires and boxes fabricated in the GaAs‐GaAlAs system. Low‐temperature cathodoluminescence measurements show new luminescence lines attributed to transitions arising from ground and excited levels of electrons within these low dimensional structures.

New approach to the high quality epitaxial growth of lattice‐mismatched materials

Abstract: We have reconsidered the problem of the critical layer thickness hc for growth of strained heterolayers on lattice‐mismatched substrates, using a new approach which allows us to determine the spatial distribution of stresses in a bi‐material assembly and include the effects of a finite size of the sample. The possibility of dislocation‐free growth of lattice‐mismatched materials on porous silicon substrates is discussed as an example of a more general problem of heteroepitaxial growth on small seed pads of lateral dimension l, having a uniform crystal orientation over the entire substrate wafer. It turns out that for a given mismatch f, the critical film thickness hlc strongly depends on l, rising sharply when the latter is sufficiently small, l≲lmin. The characteristic size lmin( f ) below which, effectively, hlc( f )→∞, is determined in terms of the experimentally known (or calculated for growth on a monolithic substrate) function h∞c( f )≡hc( f ). When l≲lmin, then the entire elastic stress in the epit...

Compound materials for reversible, phase‐change optical data storage

Abstract: Results of rapid (laser induced) and slow (heating stage induced) crystallization studies on Te1−xGex, 0≤x≤0.6, are reported. The time it takes to laser crystallize varies with x by more than four orders of magnitude. Films with stoichiometric compositions, Te and GeTe, can be crystallized using laser pulses of less than 100 ns duration. Unlike Te, which spontaneously crystallizes at room temperature, GeTe has a crystallization temperature of >150 °C. From these results we argue that, in general, compound materials allow realization of fast‐switching, reversible, phase‐change optical recording media. Furthermore, this fast‐switching capability, from the amorphous to the crystalline state and back, can be attained simultaneously with long term data (amorphous phase) stability, provided the melting temperature of the compound is sufficiently high.

Interface traps and Pb centers in oxidized (100) silicon wafers

Abstract: The band‐gap energy distribution of Pb centers on oxidized (100) Si wafers has been determined and compared with interface electrical trap density Dit. Two different Pb centers are observed on (100) Si: Pb0, which has the structure ⋅Si≡Si3, and is essentially identical to the sole Pb center observed on (111) Si; and Pb1, of presently uncertain identity, but clearly different in nature from Pb0. By electric field‐controlled electron paramagnetic resonance (EPR) and capacitance‐voltage (C‐V) measurements, it is found that Pb0 has its (0↔1) electron transition at Ev+0.3 eV and its (1↔2) transition at Ev+0.85 eV. Similarly, Pb1 has its (0↔1) transition at Ev+0.45 eV and its (1↔2) transition at Ev+0.8 eV. The Pb band‐gap density correlates qualitatively and quantitatively with the electrical trap density Dit from C‐V analysis; nonbonded Pb orbitals are found to be the source of about 50% of the characteristic traps in dry‐oxidized, unannealed (100) Si wafers.

Band alignments of coherently strained GexSi1−x/Si heterostructures on 〈001〉 GeySi1−y substrates

Abstract: The self‐consistent ab initio pseudopotential results of C. G. Van de Walle and R. M. Martin [J. Vac. Sci. Technol. B 3, 1256 (1985)] have been combined with a phenomenological deformation potential theory to estimate the band gap and band offsets for coherently strained multilayers of GexSi1−x/Si for growth on 〈001〉 GeySi1−y substrates. It is found that ΔEc is negligible and the narrower GexSi1−x gap falls within the wider Si gap (type I band alignment) if the Si in the multilayers is cubic, whereas ΔEc can be appreciable and the GexSi1−x conduction‐band edge tends to be higher in energy than the Si conduction‐band edge(type II band alignment) if both the Si and the GexSi1−x are strained. In particular, the present results resolve the seeming paradox which arose from interpretations of modulation doping experiments using heterojunctions grown either on Si〈001〉 substrates or on an unstrained alloy buffer layer.

Super-resolution fluorescence near-field scanning optical microscopy

Abstract: A one‐dimensional near‐field scanning optical microscope (NSOM) operating in the fluorescence mode has been demonstrated. NSOM line scans of both metallic edges and fluorescent gratings have been obtained and quantitatively compared to both scanning electron micrographs and conventional optical micrographs of the same structures. The sharpness of the near‐field scans indicates resolution of <100 nm.

Electro-optic and dielectric properties of KTiOPO4

Abstract: The electro‐optic and dielectric properties of potassium titanyl phosphate (KTP) have been measured from dc to 1 GHz and are compared with other electro‐optic materials. KTP is shown to possess a combination of properties that make it unique for a variety of electro‐optic modulator applications.

Effect of nonlinear gain reduction on semiconductor laser wavelength chirping

Abstract: A simple wavelength chirping formula is presented which includes small nonlinearities in the net gain such as spectral hole burning. The two dominant terms are a laser‐structure‐independent derivative or ‘‘transient’’ chirp and a structure‐dependent ‘‘adiabatic’’ chirp, each with distinctly different lightwave system consequences. The two chirp contributions are indirectly related through their mutual association with relaxation oscillations. Time‐resolved spectral measurements on a number of different laser structures support the results.

Optical spectroscopy of ultrasmall structures etched from quantum wells

Abstract: We have studied the photoluminescence and photoexcitation spectra of ultrasmall structures, of approximately 500 A in dimension, which we refer to as quantum ribbons and quantum disks. These are fabricated from GaAs‐AlGaAs quantum wells grown by molecular beam epitaxy and patterned by electron beam lithography. Contrary to our expectation, photoluminescence from these structures is extremely efficient. The excitation spectra of the two types of small structures differ greatly from each other and from that of the as‐grown quantum wells. These differences may be a result of the confinement of the carriers in these small structures.

Sequential resonant tunneling through a multiquantum well superlattice

Abstract: We report the observation of two negative conductance regions in the low‐temperature photocurrent‐voltage characteristic of tight‐binding multiquantum well (35 periods) 1‐μm‐thick Al0.48 In0.52As/ Ga0.47 In0.53As superlattices grown by molecular beam epitaxy. The two peaks occur at voltages corresponding to a potential energy drop across the superlattice period equal to the energy differences between the first two excited states and the ground state of the quantum wells. This provides direct evidence of sequential resonant tunneling between the ground and excited states of adjacent wells alternated with intrawell energy relaxation.

Generation of subpicosecond solitonlike optical pulses at 0.3 THz repetition rate by induced modulational instability

Abstract: We report the generation of 0.5 ps full width at half‐maximum optical pulses at >0.3 THz (tunable) repetition rate via an induced modulational instability in a single‐mode fiber. A 1.319‐μm neodymium:yttrium aluminum garnet laser is chosen as the carrier wave and the initial modulation on this carrier is introduced by mixing it with an external‐grating‐cavity InGaAsP laser. The use of this all‐optical modulating scheme and the nonlinear optical propagation effect in fiber (i.e., modulational instability) allows one to reach into the terahertz regime, which is about two orders of magnitude beyond the fastest data limited by the finite response of electronics.

Determination of the interdiffusion of Al and Ga in undoped (Al,Ga)As/GaAs quantum wells

Abstract: We have employed photoluminescence spectroscopy to determine the temperature dependence of the interdiffusion coefficient of Al and Ga in GaAs/Al03Ga07As quantum wells The position of the photoluminescence peaks, due to the n=1 electron to heavy‐hole transition, was measured before and after annealing the samples A variational calculation was employed to determine the expected position of these photoluminescence peaks and from this a value of the interdiffusion coefficient was extracted The interdiffusion process is characterized by an activation energy of about 6 eV leading to an interdiffusion coefficient at 850 °C of 4×10−19 cm2/s This technique allows for the measurement of small diffusion coefficients in a wide variety of material systems

Amorphization of transition metal Zr alloys by mechanical alloying

Abstract: Amorphous alloy powders of the types NiZr, CoZr, FeZr, and CuZr are produced by mechanical alloying from crystalline elemental powders. The alloying and amorphization process is monitored by microstructural investigations, x‐ray diffraction, and, where applicable, by magnetization measurements. The crystallization temperatures, determined by differential scanning calorimetry, are comparable to those measured for rapidly quenched and solid state reacted amorphous metals of the same compositions.

Epitaxial growth of rare-earth silicides on (111) Si

Abstract: Rapid heating with an electron beam has been used to react overlayers of rare‐earth (RE) metals with (111) Si, forming epitaxial layers of silicides of Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Under conventional furnace annealing, forming such silicides on Si typically leads to rough, pitted surfaces. The use of fast beam heating not only results in a much smoother surface topology but also helps promote epitaxial growth on (111) Si in both solid and liquid phase reactions. These epitaxial silicides have a hexagonal RESi∼1.7 structure (defected AlB2 type). Their orientation with the Si substrate is (0001)∥(111), with predicted lattice mismatches ranging from +0.83 to −2.55%.

Generation of subpicosecond electrical pulses on coplanar transmission lines

Abstract: Electrical pulses shorter than 0.6 ps were generated by photoconductively shorting a charged coplanar transmission line with 80 fs laser pulses. After propagating 8 mm on the line the electrical pulses broadened to only 2.6 ps.

Solid neon moderator for producing slow positrons

Abstract: Slow positrons can be obtained by moderating the energetic β+ particles from a radioactive source. We find that solid Ne makes a more efficient moderator than any other material known to date. The efficiency e, defined as the number of slow positrons per β+ emitted by the source, is (0.30±0.02)% for a flat layer of Ne covering a 22Na deposit. In a cylindrical geometry, e is (0.70±0.02)%, more than twice the previous best efficiency obtained with single‐crystal tungsten. The energy spectrum for Ne has a full width at half‐maximum of 0.58 eV, somewhat broader than the spectrum of positrons from a single‐crystal metal. Moderators made from the other solid rare gases have a much lower efficiency and a larger energy spread.