Showing papers on "Heterojunction published in 1996"

Relationship between electroluminescence and current transport in organic heterojunction light‐emitting devices

Abstract: We measure the current–voltage and electroluminescencecharacteristics of single‐heterojunction, vacuum‐deposited organic light‐emitting devices(OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate‐based devices,electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices.

Exciton diffusion and dissociation in a poly(p-phenylenevinylene)/c60 heterojunction photovoltaic cell

Abstract: We report measurements of the photovoltaic response of two‐layer photocells formed with layers of the conjugated polymer poly(phenylenevinylene), PPV and fullerene, C60, formed between indium‐tin oxide and aluminum electrodes. Peak quantum efficiencies of up to ∼9% (electrons collected per incident photon) were measured under short‐circuit conditions. We model the photovoltaic response as arising from excitons photogenerated in the PPV layer which are able to diffuse to the interface with the C60 layer where they are ionized. We obtain a value for the exciton diffusion range of 7±1 nm, both from the spectral response and from the absolute efficiency. We demonstrate that the branching ratio for the creation of singlet excitons from absorbed photons is close to unity.

Determination of defect distributions from admittance measurements and application to Cu(In,Ga)Se2 based heterojunctions

Abstract: A method to deduce energy distributions of defects in the band gap of a semiconductor by measuring the complex admittance of a junction is proposed. It consists of calculating the derivative of the junction capacitance with respect to the angular frequency of the ac signal corrected by a factor taking into account the band bending and the drop of the ac signal over the space charge region of the junction. Numerical modeling demonstrates that defect distributions in energy can be reconstructed by this method with high accuracy. Defect distributions of polycrystalline Cu(In,Ga)Se2 thin films are determined by this method from temperature dependent admittance measurements on heterojunctions of Cu(In,Ga)Se2 with ZnO that are used as efficient thin film solar cells.

Shortest wavelength semiconductor laser diode

Abstract: A group III nitride based separate confinement heterostructure (SCH) single quantum well (SQW) structure, with an active layer thickness as small as 1.5 nm, was fabricated. It shows the shortest lasing from semiconductor lasers by current injection at room temperature to date. Line width is as little as 0.15 nm.

Quantum conductance of carbon nanotubes with defects

Abstract: We study the conductance of metallic carbon nanotubes with vacancies and pentagon-heptagon pair defects within the Landauer formalism. Using a tight-binding model and a Green's function technique to calculate the scattering matrix, we examine the one-dimensional to two-dimensional crossover in these systems and show the existence of metallic tube junctions in which the conductance is suppressed for symmetry reasons. \textcopyright{} 1996 The American Physical Society.

Red-Emitting Semiconductor Quantum Dot Lasers

Abstract: Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at ∼707 nanometers was observed at 77 kelvin with a threshold current of 175 milliamperes for a 60-micrometer by 400-micrometer broad area laser. An external efficiency of ∼8.5 percent at low temperature and a peak power greater than 200 milliwatts demonstrate the good size distribution and high gain in these high-quality QDs.

Measurement of AlN/GaN (0001) heterojunction band offsets by x‐ray photoemission spectroscopy

Abstract: X‐ray photoemission spectroscopy has been used to measure the valence band offset ΔEv for the AlN/GaN (0001) heterojunction interface. The heterojunction samples were grown by reactive molecular beam epitaxy on 6H–SiC (0001) substrates. A nested interface band alignment with ΔEv=1.36±0.07 eV is obtained (ΔEc/ΔEv=52/48).

The Coulomb Blockade in Coupled Quantum Dots

Abstract: Individual quantum dots are often referred to as “artificial atoms.” Two tunnel-coupled quantum dots can be considered an “artificial molecule.” Low-temperature measurements were made on a series double quantum dot with adjustable interdot tunnel conductance that was fabricated in a gallium arsenide-aluminum gallium arsenide heterostructure. The Coulomb blockade was used to determine the ground-state charge configuration within the “molecule” as a function of the total charge on the double dot and the interdot polarization induced by electrostatic gates. As the tunnel conductance between the two dots is increased from near zero to 2 e 2 / h (where e is the electron charge and h is Planck9s constant), the measured conductance peaks of the double dot exhibit pronounced changes in agreement with many-body theory.

Carrier relaxation and thermal activation of localized excitons in self-organized InAs multilayers grown on GaAs substrates

Abstract: We have investigated the temperature dependence of photoluminescence (PL) properties of a number of self-organized InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 to 3 ML. The temperature dependence of InAs exciton emission and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML. The fast redshift of PL energy and an anomalous decrease of linewidth with increasing temperature were observed and attributed to the efficient relaxation process of carriers in multilayer samples, resulting from the spread and penetration of the carrier wave functions in coupled InAs quantum dots. The measured thermal activation energies of different samples demonstrated that the InAs wetting layer may act as a barrier for the thermionic emission of carriers in high-quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to escape thermally from the localized states. \textcopyright{} 1996 The American Physical Society.

Dominance of tunneling current and band filling in InGaN/AlGaN double heterostructure blue light‐emitting diodes

Abstract: Measurement of the room temperature forward bias current‐voltage behavior of InGaN/AlGaN double heterostructure blue light‐emitting diodes demonstrates a significant departure from the usual Is exp(qV/ nkT) behavior where n is the ideality factor which varies between 1 and 2. The observed current‐voltage behavior at room temperature may be represented as I=2.7×10−11 exp(5.7V) which suggests a tunneling mechanism. Measurement of the electroluminescence for currents from 0.5 to 100 mA demonstrates that the emission peak shifts to higher energy while increasing in intensity. The shifting peak spectra is due to band filling, a process which results from the injection of holes via tunneling into an empty acceptor impurity band and vacant valence band tails. At currents near 100 mA, a non‐shifting band‐to‐band emission approaches the intensity of the shifting peak spectra. The active layer of these diodes is codoped with both the donor Si and the acceptor Zn.

Enhancement and depletion mode GaN/AlGaN heterostructure field effect transistors

Abstract: We report on the fabrication and characterization of Al0.1Ga0.9N/GaN heterojunction field effect transistors, both an enhancement mode and a depletion mode with a low pinchoff voltage, suitable for digital integrated circuit applications. For an enhancement mode device with a 1 μm gate length and 5 μm drain‐to‐source separation, the dc transconductance is around 23 mS/mm. Connecting the enhancement mode device as a switching transistor and a depletion mode device as a load, we demonstrate an AlGaN/GaN inverter.

Room temperature lasing from InGaAs quantum dots

Abstract: Alternating molecular beam epitaxy is used to form InGaAs quantum dots by utilising the two-dimensional to three-dimensional Stranski-Krastanow growth transition. The quantum dots are embedded in a separate confinement heterostructure to form laser diodes. Lasing is observed from excited states in the quantum dots from room temperature down to 80 K. Pronounced state-filling is observed in the quantum dot lasers at room temperature. As the temperature is decreased, the state-filling becomes less pronounced, which compensates for the bandgap increase and leads to lasers whose lasing wavelength is very weakly dependent on temperature.

Stresses and strains in epilayers, stripes and quantum structures of III - V compound semiconductors

Abstract: Stresses and strains in heterostructures have dominated semiconductor research during the last ten years. We review the theory and experimental work on stresses in III - V semiconductor heterostructures in this paper. First large-area lattice mismatched layers (InGaAs and InGaP layers grown on GaAs or InP substrates) and thermally strained layers (GaAs, GaP and InP layers grown directly on Si) are considered. The stresses in large-area epilayer - substrate structures are easy to model because substrate distortion can be neglected and strain in the epilayer is a simple biaxial tetragonal distortion. Calculated splitting of band edges, modification of bandgaps and shifts of Raman modes show good agreement with experiments. Edges of a stripe relax stress in the stripes and induce stress in the substrate. Since stresses in the stripe and the substrate are coupled, calculation of stresses in stripes and substrates is more involved. Recent finite element (FE) calculations of these stresses are discussed in detail and compared with the approximate analytical models and with luminescence and Raman measurements. FE calculations of stresses in buried quantum wires, stressor-induced quantum wires and quantum dots are also discussed. The results of these calculations are used to determine stability and luminescence of the quantum wires and dots and compared with the experimental results. Finally self-organized quantum dots consisting of islands formed during the 3D growth of InAs layers on GaAs are discussed. A possible explanation of the recent observation that they are formed in vertical columns embedded in GaAs is suggested. .

Electrical and optical properties of infrared photodiodes using the InAs/Ga1−xInxSb superlattice in heterojunctions with GaSb

Abstract: The InAs/Ga1−xInxSb strained‐layer superlattice (SLS) holds promise as an alternative III–V semiconductor system for long wavelength infrared detectors. In this article, we present the first investigation, to the best of our knowledge, of heterojunction photodiodes using this new material. The devices were grown by molecular beam epitaxy on GaSb substrates, and are comprised of a 38 A InAs/16 A Ga0.64In0.36Sb SLS used in double heterojunctions with GaSb contact layers. The structures were designed to optimize the quantum efficiency while minimizing transport barriers at the heterointerfaces. The photodiodes are assessed through the correlation of their performance with the SLS material quality and the detector design. X‐ray diffraction, absorption, and Hall measurements are used to determine the SLS material properties. The electrical and optical properties of the photodiodes are determined using current–voltage and spectral responsivity measurements. At 78 K, these devices exhibit rectifying electrical behavior and photoresponse out to a wavelength of 10.6 μm corresponding to the SLS energy gap. The responsivity and resistance in these thin‐layered (0.75 μm), unpassivated photodiodes result in a detectivity of 1×1010 cm √Hz/W at 8.8 μm and 78 K. Based upon the performance of these devices, we conclude that high‐sensitivity operation of long‐wavelength photovoltaic detectors at temperatures well in excess of conventional III–V band gap‐engineered systems, and potentially in excess of HgCdTe, is feasible using this material system.

Low interface state density oxide‐GaAs structures fabricated by in situ molecular beam epitaxy

Abstract: Several oxide‐GaAs heterostructures were fabricated using in situ multiple‐chamber molecular beam epitaxy. The oxides include SiO2, MgO, and Ga2O3(Gd2O3), all evaporated by an electron beam method. The SiO2 and Ga2O3(Gd2O3) films are amorphous while the MgO films are crystalline and part of the films are epitaxially grown on GaAs(100). Among these heterostructures, the Ga2O3(Gd2O3)–GaAs shows a photoluminescence intensity comparable to that of Al0.45Ga0.55As–GaAs, and forms accumulation and inversion layers as measured from capacitance voltage measurement in quasistatic and high frequency modes.

Finite Size Effects on Electroluminescence of Nanoscale Semiconducting Polymer Heterojunctions.

Abstract: Among the novel finite size effects observed in a series of nanoscale (25−100 nm) semiconducting polymer heterojunctions, such as polyquinoline(PPQ)/poly(p-phenylene vinylene) (PPV), is the reversible switching of electroluminescence (EL) colors (orange/red ↔ green) by the applied voltage. The finite size effects are related to the charge carrier ranges in semiconducting polymers and are expected to be important in electronic and optoelectronic devices from organic semiconductors.

Accurate small-signal modeling of HFET's for millimeter-wave applications

Abstract: In this paper we discuss the small-signal modeling of HFET's at millimeter-wave frequencies. A new and iterative method is used to extract the parasitic components. This method allows calculation of a /spl pi/-network to model the heterojunction field-effect transistor (HFET) pads, thus extending the validity of the model to higher frequencies. Formulas are derived to translate this /spl pi/-network into a transmission line. A new and general cold field-effect transistor (FET) equivalent circuit, including a Schottky series resistance, is used to extract the parasitic resistances and inductances. Finally, a new and compact set of analytical equations for calculation of the intrinsic parameters is presented. The real part of Y/sub 12/ is accounted for in these equations and its modeling is discussed. The accounting of Re(Y/sub 12/) improves the S-parameter modeling. Model parameters are extracted for an InAlAs/InGaAs/InP HFET from measured S-parameters up to 50 GHz, and the validity of the model is evaluated by comparison with measured data at 75-110 GHz.

Efficient dye‐sensitized charge separation in a wide‐band‐gap p‐n heterojunction

Abstract: We report the fabrication and performance of a dye‐sensitized p‐n heterojunction formed from a planar interface between two wide‐band‐gap semiconductors, n‐TiO2 and p‐CuSCN, which contains an intervening monolayer of a sulforhodamine B dye. When exposed to visible light, the photoexcited dye molecules transfer electrons to the n‐TiO2 and holes to the p‐CuSCN. The absorbed‐photon‐to‐current efficiency (APCE) is ≳70% and the open circuit voltage is ≊500 mV. This heterojunction is the solid‐state analog of the dye‐sensitized photoelectrochemical interfaces used in photography and photovoltaics. The high quantum efficiency and voltage show that it is possible to simultaneously optimize both the dye/n‐type and dye/p‐type interface for efficient forward charge injection and slow charge combination in a solid‐state device.

Empirical tight‐binding calculation of the branch‐point energy of the continuum of interface‐induced gap states

Abstract: The band lineup at metal–semiconductor contacts as well as at semiconductor heterostructures may be described by one and the same physical concept, the continuum of interface‐induced gap states. These intrinsic interface states derive from the virtual gap states (ViGS) of the complex semiconductor band structure and their character varies from predominantly donorlike closer to the valence band to mostly acceptorlike nearer to the conduction band. Calculations are presented of the respective branch points for elemental and binary as well as ternary compound semiconductors which make use of Baldereschi’s concept of mean‐value points in the Brillouin zone [Phys. Rev. B 7, 5212 (1973)], Penn’s idea of dielectric band gaps [Phys. Rev. 128, 2093 (1962)], and the empirical tight‐binding approximation (ETB). The results are as follows. First, at the mean‐value point the band gaps calculated in the GW approximation have the same widths as the dielectric band gaps. Second, the ETB approximation reproduces the GW va...

Effective-mass theory for InAs/GaAs strained coupled quantum dots

Abstract: In the framework of effective-mass envelope-function theory, the optical transitions of InAs/GaAs strained coupled quantum dots grown on GaAs (100) oriented substrates are studied. At the Gamma point, the electron and hole energy levels, the distribution of electron and hole wave functions along the growth and parallel directions, the optical transition-matrix elements, the exciton states, and absorption spectra are calculated. In calculations, the effects due to the different effective masses of electrons and holes in different materials are included. Our theoretical results are in good agreement with the available experimental data.

Photoluminescence studies of self‐assembled InSb, GaSb, and AlSb quantum dot heterostructures

Abstract: Photoluminescence (PL) spectroscopy has been performed on a set of self‐assembled InSb, GaSb, and AlSb quantum dot (QD) heterostructures grown on GaAs. Strong emission bands with peak energies near 1.15 eV and linewidths of ∼80 meV are observed at 1.6 K from 3 monolayer (ML) InSb and GaSb QDs capped with GaAs. The PL from a capped 4 ML AlSb QD sample is weaker with peak energy at 1.26 eV. The PL bands from these Sb‐based QD samples shift to lower energy by 20–50 meV with decreasing excitation power density. This behavior suggests a type II band lineup. Support for this assignment, with electrons in the GaAs and holes in the (In,Ga,Al)Sb QDs, is found from the observed shift of GaSb QD emission to higher energies when the GaAs barrier layers are replaced by Al0.1Ga0.9As.

Low threshold PbEuSeTe/PbTe separate confinement buried heterostructure diode lasers

Abstract: Continuous wave (cw) operating temperature of 223 K was achieved with molecular beam epitaxy grown separate confinement buried heterostructure (SCBH) PbTe diode lasers with PbEuSeTe electrical and optical confinement layers. This is the highest cw operating temperature reported for midinfrared diode lasers. The active region of the SCBH diode lasers varies laterally to form a crescent‐shaped waveguide with a maximum thickness of 0.15 μm and a lateral width of 2 μm. Exceptionally low threshold currents of 102 mA at 200 K, 166 mA at 210 K, and 249 mA at 215 K were measured.

Electronic properties of zinc‐blende GaN, AlN, and their alloys Ga1−xAlxN

Abstract: The electronic properties of wide‐energy gap zinc‐blende structure GaN, AlN, and their alloys Ga1−xAlxN are investigated using the empirical pseudopotential method Electron and hole effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Γ and those of the conduction band at Γ and X are obtained for GaN and AlN, respectively The energies of Γ, X, L conduction valleys of Ga1−xAlxN alloy versus Al fraction x are also calculated The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices based on these materials in the blue light range application

Scaling laws for the reduction of threading dislocation densities in homogeneous buffer layers

Abstract: In the heteroepitaxial growth of films with large misfit with the underlying substrate (linear mismatch strains in excess of 1%–2%) the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggest that the TD density in relaxed homogeneous buffer layers can be divided into three regimes: (i) an entanglement region near the film/substrate interface corresponding to TD densities of ∼1010–1012 cm−2; (ii) a falloff in TD density that is inversely proportional to the film thickness h, applicable to densities in the range ∼107–109 cm−2; and (iii) saturation or weak decay of the TD density with further increase in film thickness. Typical saturation densities are on the order of ∼106–107 cm−2. In this article, we show that the TD reduction may be described in terms of effective lateral motion of TDs with increasing film thickness. An analytic model is developed that successfully predicts both the 1/...

Design of Si/SiGe heterojunction complementary metal-oxide-semiconductor transistors

Abstract: An optimized Si/SiGe heterostructure for complementary metal-oxide semiconductor (CMOS) transistor operation is presented. Unlike previous proposals, the design is planar and avoids inversion of the Si layer at the oxide interface. The design consists of a relaxed Si/sub 0.7/Ge/sub 0.3/ buffer, a strained Si quantum well (the electron channel), and a strained S/sub 1-x/Ge/sub x/ (0.7>x>0.5) quantum well (the hole channel). The channel charge distribution is predicted using a 1-D analytical model and quantum mechanical solutions. Transport is modeled using 2-D drift-diffusion and hydrodynamic numerical simulations. An almost symmetric performance of p- and n-transistors with good short-channel behavior is predicted. Simulated ring oscillators show a 4- to 6-fold reduction in power-delay product compared to bulk Si CMOS at the 0.2-/spl mu/m channel length generation.

Stresses and strains in lattice‐mismatched stripes, quantum wires, quantum dots, and substrates in Si technology

Abstract: We discuss recent advances made in the theory and measurements of stresses and strains in Si‐based heterostructures containing submicron‐ and micron‐size features Several reports on theoretical as well as experimental studies of stresses in the substrates with local oxidation of silicon structures on the surface have been published recently With the advent of GeXSi1−X strained layers and stripes extensive studies of both the stripe and the substrate stresses have also been made Unlike the previous calculations and analytical models, recent finite element (FE) calculations take into account the coupling between the film–substrate stresses without making the approximation that the interface is rigid or that there is no variation of stresses in the stripes in a direction perpendicular to the interface The results of these calculations have been compared with the analytical models and limitations of the analytical models have been pointed out Micro‐Raman measurements of the stresses in the stripes, quant

High quality GaN–InGaN heterostructures grown on (111) silicon substrates

Abstract: We report on the low pressure metal organic chemical vapor deposition of single crystal, wurtzitic layers of GaN and GaN/InGaN heterostructures on (111) GaAs/Si composite substrates. The structural, optical, and electrical properties of the epitaxial layers are evaluated using x‐ray diffraction, transmission electron microscopy, photoluminescence, and measurements of minority carrier diffusion length. These measurements demonstrate high quality of GaN grown on the composite substrate.

Growth and characterization of AlInGaN quaternary alloys

Abstract: We report on the deposition of AlyInxGa1−x−yN in the (0