Multilayer Heterojunction Research Articles

Acoustic-phonon transmission in multilayer heterojunctions

Using a transfer matrix method, we investigate the transmission property of the acoustic-phonon modes in symmetrical multilayer heterojunctions consisting of many different heterojunctions (GaAs/AlxiGa1-xiAs) within elastic continuum approximation. Our results show that the oscillation amplitude and frequency are different between transmission curve in such structure and transmission curve in the superlattice (GaAs/AlAs) with the same thicknesses of constituent layers. The features of transmission spectrum are influenced by the distribution of the concentration xi(i denotes the i th AlxiGa1-xiAs slab on both sides of the symmetrical axis) . When the concentration xi in AlxiGa1-xiAs slab decreases with the increase of i, the curve in transmission spectrum is flat except in the vicinity of the main valley. When the concentration xi increases with the increase of i, the oscillation amplitude of the curve increases and the main valley is split. Our results also show that the transmission coefficient is sensitive to the thicknesses of constituent layers, especially the thickness of AlxiGa1-xiAs. In addition, it is found that the number of heterojunctions influences the transmission of acoustic phonon.

Photovoltaic cells from a soluble pentacene derivative

Multilayer heterojunction photovoltaic cells were fabricated using a solution processable pentacene derivative as the donor layer. Upon annealing of these devices, the open circuit voltage was found to gradually increase with temperature up to 265 °C. Further improvement in the photovoltaic performance was achieved with the incorporation of mobile ions into the donor material for concentrations of lithium triflate up to 1.4 mg/ml. Finally, after introducing an exciton blocking layer, an optimized white-light (100 mW/cm2) power conversion efficiency of 0.52% was achieved.

Synthesis of 2-phenylquinoline-based ambipolar molecules containing multiple 1,3,4-oxadiazole spacer groups

A series of donor–acceptor type ambipolar electroluminescence dyes with the general structure PQ(OXD) n T (where n = 1, 2 and 3) were prepared, in which PQ is 2-phenylquinoline, T is diphenylamine which constituted the hole transporting triphenylamine moiety with an adjacent phenyl ring, and OXD is an electron transporting 2-phenyl-1,3,4-oxadiazole repeating unit. The compounds fluoresced bluish green to green hue in solid-state, exhibited a positive solvatochromism in solution and their quantum efficiency decreased rapidly with increase in n. The materials are thermally stable with glass transition temperature ( T g) ranging from 83 °C ( n = 1) to 130 °C ( n = 3). Cyclic voltammetry studies indicated the HOMO remained relatively unchanged with n while the LUMO decreased (away from the vacuum level) with an increase in the number of OXD. For single layer homojunction OLEDs, highest efficiency was obtained when n = 1 (max luminous 3300 cd/m 2 and current efficiency 0.9 cd/A), whereas for multilayer heterojunction OLEDs, best results was achieved for compounds with n = 1 or 2 assuming the role of the HT layer (over 8200 cd/m 2 max and 2.0 cd/A). Formation of exciplexes led to significant red-shift and lower emission efficiency for the compound with n = 3.

Molecular light emitting heterojunctions based on fully conjugated heterocyclic aromatic rigid-rod polymer poly- p-phenylenebenzobisthiazole

Poly- p-phenylenebenzobisthiazole, PBT, is a heterocyclic aromatic polymer having a fully conjugated rigid-rod backbone, excellent stabilities and opto-electronic properties. Multilayer heterojunctions of PBT and conjugated dyes of 3-(2-benzothiazoly)-7-(diethylamino), Coumarin 6 (or C 6), and tris-(8-hydroxyquinoline) aluminum, Alq 3, were processed using PBT as a hole injection and transport layer. A PBT with an intrinsic viscosity of 18.0 dL/g was dissolved in acids and spun onto an indium-tin-oxide (ITO) substrate followed by coagulation to remove the acid. The low molar dyes were evaporated directly to, or onto the PBT film adhered to the ITO substrate. UV–Vis absorption spectra showed direct band gaps for PBT, C 6 and Alq 3 at correspondingly 2.6, 2.4, and 2.9 eV. Scanning electron microscope revealed that the heterojunction layer for PBT and the dyes were, respectively, about 23 and 50 nm in thickness. Aluminum was evaporated onto the heterojunction as electron injector for light-emitting diode (LED). By varying the layer ordering and composition, it was evidenced that PBT was a p-type material of high hole mobility; both C 6 and Alq 3 were n-type materials of a low and a high electron mobility, respectively. Significantly reduced LED threshold voltage and enhanced electroluminescence intensity were resulted from the dyes by introducing a PBT layer showing that the holes were the minority carrier for the heterojunction LEDs.

Low-band-gap, sublimable rhenium(I) diimine complex for efficient bulk heterojunction photovoltaic devices

The use of chlorotricarbonyl rhenium(I) diimine complex as photosensitizing molecule in photovoltaic cells is demonstrated. Unlike other transition-metal-based sensitizers, the complex is sublimable and has low band gap. It also exhibits bipolar charge transport character with relatively high carrier mobilities on the order of 10−3 cm2 V−1 s−1. Multilayer heterojunction and bulk heterojunction devices with fullerene as the electron accepting molecule were prepared. For the bulk heterojunction devices, the fill factor and power conversion efficiency under simulated solar light illumination were 0.56 and 0.48%, respectively. Atomic force microscopic images showed that the complex dispersed evenly with fullerene molecules in solid state.

Efficient Bulk Heterojunction Photovoltaic Cells using Sublimable Transition Metal Complex as Photosensitizers

ABSTRACTMultilayer bulk heterojunction photovoltaic cells using chlorotricarbonyl rhenium (I) bis(phenylimino)acenaphthene (Re-DIAN) complex as photosensitizer were studied. The complex is sublimable, has lower band gap, good thermal stability and good processibility. It has broad absorption in UV-visible region. Therefore, Re-DIAN exhibits good photosensitising property for photovoltaic cells. Multilayer bulk heterojunction photovoltaic cells with simple structures were fabricated base on Re-DIAN complex. The active layer consists of a blend of Re-DIAN and fullerene that were co-deposited in the same layer by vacuum deposition. The photovoltaic properties of the devices were studied by irradiaton under AM1.5 simulated solar light. The effects of changing the co-deposition film thickness, amount of Re-DIAN photosensitizer, and hole transporting materials were studied.

Determining the thickness and composition of SiGe heterostructures using an optical microscope

We demonstrate a novel method of determining the thickness and Ge concentration in multilayer SiGe heterojunctions using an optical microscope. The sample preparation is a two-step process: first the layer structure is bevelled under a shallow angle to enlarge the width of the layers, followed by a wet thermal oxidation step at low temperatures. Differences in the oxide thickness, due to the dependence of oxidation rate on Ge composition, result in colour differences in the layers that are visible under an optical microscope. The Ge concentration is calibrated using XRD results, and the measured layer thickness is compared to TEM results.

Resonant cavity enhancement in heterojunction GaAs/AlGaAs terahertz detectors

The room-temperature absorption and reflection spectra in the range of 5–100 μm (3–60 THz) for multilayer heterojunction interfacial work function internal photoemission (HEIWIP) GaAs/AlGaAs far-infrared (FIR) detectors are presented. Calculated results based on the free carrier absorption and interaction with optical phonons are found to be in good agreement with the experimental results. Experimental responsivity spectra demonstrate the expected maxima from the absorption measurements due to resonant cavity effects. It is shown that the resonance cavity architecture enhances the performance of the FIR HEIWIP detectors and further improvement is proposed through the use of n++ and p++ bottom contact layers or doped substrates.

Simulation of hydrogenated amorphous and microcrystalline silicon optoelectronic devices

This paper is concerned with the modelling and simulation of amorphous and microcrystalline silicon optoelectronic devices. The physical model and its mathematical formulation are extensively described. Its numerical reduction is also discussed together with the presentation of a computer program dedicated to the simulation of the electrical behaviour of such devices. This computer program, called ASCA (Amorphous Silicon Solar Cells Analysis), is capable of simulating, on one- and two-dimensional domains, the internal electrical behaviour of multi-layer structures, homojunctions and heterojunctions under simple or complex spectra illumination and externally applied biases. The applications of the simulator presented in this work are the analysis of μc/a-Si:H p-i-n photovoltaic cell in thermal equilibrium and illuminated by monochromatic light and the AM1.5 solar spectrum, with and without polarisation. We also study the appearance within the device of lateral components of the electric field and current density vectors when the illumination is not uniform.

Integrated multi-sensor control of II–VI MBE for growth of complex IR detector structures

Next-generation HgCdTe infrared detectors and detector arrays require the growth of multilayer heterojunction structures with precisely controlled alloy composition and doping levels and minimal defect densities. Molecular beam epitaxy (MBE) provides the ability to produce such structures. However, in the absence of a real-time, in situ control methodology the extreme sensitivity of HgCdTe layer quality and doping efficiency on fundamental MBE variable such a substrate temperature and effusion cell flux provide serious challenges to the uniform and reproducible growth of such structures. In this paper, we describe an integrated, multi-sensor approach for monitoring and controlling the variables that are most important for MBE growth of HgCdTe device structures used in advanced multi-color infrared detectors and high speed, low-noise avalanche photodiodes. Substrate temperature, effusion cell flux, and layer composition are monitored using absorption-edge spectroscopy (ABES), optical flux monitoring (OFM), an spectroscopic ellipsometry (SE), respectively. Flexible, custom software has been developed and implemented for analysis of sensor inputs and feedback control of the MBE system in response to those inputs. The sensors and their application to growth of HgCdTe will be described, and the use of a custom software framework for data analysis and system control will be discussed.

MBE-grown HgCdTe multi-layer heterojunction structures for high speed low-noise 1.3–1.6 µm avalanche photodetectors

HgCdTe is an attractive material for room-temperature avalanche photodetectors (APDs) operated at 1.3–1.6 µm wavelengths for fiber optical communication applications because of its bandgap tunability and the resonant enhancement of hole impact ionization for CdTe fractions near 0.73. The HgCdTe based separate absorption and multiplication avalanche photodetector is designed and fabricated for backside illumination through a CdZnTe substrate. The multi-layer device structure is comprised of seven layers including 1). n+ contact 2). n− diffusion buffer 3). n− absorber 4). n charge sheet 5). n− avalanche gain 6). p to form junction, and 7).p+ contact. Several wafers were processed into 45 µm × 45 µm and 100 (µm × 100 µm devices. The mean value of avalanche voltage is 63.7 V measured at room temperature. At 1 GHz, the device shows a gain of about 7 for a gain-bandwidth product of 7 GHz. This first demonstration of an all molecular beam epitaxially grown HgCdTe multi-layer heterojunction structure on CdZnTe substrates represents a significant advance toward the goal of producing reliable room temperature HgCdTe high speed, low noise avalanche photodetectors.

Electrical Contacts to p‐ZnSe Based on HgSe and ZnTe

AbstractLow resistance ohmic contacts to p‐ZnSe involving the use of HgSe are studied in detail. HgSe does not form an ohmic contact with p‐ZnSe because of a residual valence band offset at the heterojunction of 0.5 to 0.6 eV. Annealing of HgSe/ZnSe:N does not result in an ohmic contact because a large miscibility gap in Hg1−xZnxSe prevents a HgZn interdiffusion and therefore the formation of a graded heterojunction. The molecular beam epitaxial (MBE) growth of Hg1−xZnxSe does not circumvent this problem because these epitaxial layers suffer from a compositional phase separation due to spinodal demixing during MBE growth. A significant improvement of HgSe based contacts could be achieved by the use of ZnSe1−xTex:N transition layers between HgSe and ZnSe:N. Either by abrupt, graded, or multilayer heterojunctions, very low contact resistivities are obtained. The growth of ZnSe1−xTex:N layers on top of ZnSe:N is shown to reduce the carrier concentration in ZnSe:N by several orders of magnitude. A nitrogen diffusion process is believed to be responsible for this effect.

TEM studies of fine low dimensional modulated fringes (FLDMF) in GaAs/Ga1-xAlx As multilayer heterojunction structure grown by MBE

The characteristics of GaAs/Ga1-x Alx As superstructure grown by MBE are always noticed. Recently, as well known, cross-sectional trasmission electron microscopy (XTEM) has been used and many workers have observed the superstructure(multilayer heterojunction structure) of GaAs/Ga1-x Alx, As hetero-multilayer. They usually observed a wide fringes with several nanometers or even more in GaAs/GaAlAs multilayer structure.In this work, we have studied a phenomenon which was named"Fine Low Dimensional Modulated Fringes" of GaAs/GaAlAs heteromutilayer structure grown by MBE, using XTEM technique. It is found that the FLDMFs are an equal width and very narrow, the order of Å’s.The GaAs/GaAls layers that are examined were grown on (001) GaAs substrate by MBE. Cross-section TEM specimens with a wafer cleaved along (110) plane were prepared, and thinned by argon ion.A typical structure is given in Fig.l, the image is of superlattice(SL) and Fine Low Dimensional Modulated Fringes area of MBE grown GaAs/GaAlAs heteromutilayer.

New step impact electroluminescent devices

The step impact electroluminescent devices (SIED) and the multistep photon amplifier converted (SPAC) are proposed. The devices are a multilayered heterojunction structures in which the accelaration and collision excitation processes are spatially separated and permits independent optimization of each function in different materials. In collision excitation region, ballistic electrons excite the rare earth ions by direct impact. By changing conduction band step Δ E c we can tune up the energy of excited electrons to the resonance condition of a particular excited state of RE 3+ ions. The emission from the rare earth ions with transition wavelength shorter than semiconductor's band edge emission can be generated. The emission will contain narrow lines with gradual change in wavelength with temperature. The advantages of the new devices and excitation mechanism as discussed in detail. Laser action should also be obtainable by properly choosing device geometry. The laser oscillation at the rare earth transition assure the same wavelength from devices to devices with weak temperature dependence. The SIED structure is very promising structure for studying the direct impact excitation mechanism for a localized impurity in semiconductors. It can be used as a general tool for studying the excitation function of RE 3+ ions and other impurities with internal transitions like some transition metal impurities in III–V compounds.

Multi-layered heterojunction structure for millimeter-wave impatt devices

Abstract Multilayer GaAs-AlxGa1−xAs avalanche and transit time devices have been studied theoretically using a particle time domain simulation of their operation. For structures derived from quantum well superlattices rather good performances were obtained at 100 GHz. It is shown that these results are in agreement with the interpretation of a bandedge-discontinuity-assisted impact ionization and a better defined avalanche region.

Ion Implantation and Annealing in III-V Multilayer Heterojunctions

ABSTRACTThis paper reviews the status of implantation and annealing in III-V multilayer heterojunctions. The limitations on diffusion doping of multilayers are noted. Current heterojunction device applications for implantation are summarized. Issues relevant to the annealing of multilayers are discussed, including Si migration in modulation doped structures, the effectiveness of rapid annealing in reducing SI diffusion, and impurity enhanced diffusion in superlattices. The formation of damage in multilayers and the unique properties of strained multilayers are also discussed. Finally, implant activation studies are summarized.

New device applications of bandedge discontinuities in multilayer heterojunction structures

New device applications of bandedge discontinuities in multilayer heterojunction structures

New device applications of bandedge discontinuities in multilayer heterojunction structures

Recent work on new device applications of band edge discontinuities in molecular beam epitaxial multilayer heterostructures is discussed. Band discontinuties can be used to control the “real space” transfer of electrons between sets of layers (negative resistance, switching and storage devices). Another application deals with the possibility of enhancing the ionization rates ratio via the asymmetry between conduction and valence band discontinuities (superlattice photodetectors). Finally a multistage graded gap structure, the solid-state analog of a photomultiplier (staircase APD), has been disclosed.

Resonance impact ionization in superlattices

We propose an enhancement of the electron or hole impact ionization coefficients (α or β) by introducing resonant impact ionization states into the (conduction or valence) band by using suitable lattice matched multilayer heterojunctions (superlattices). Model calculations for the AlAs:GaAs superlattice indicate resonance enhancements can occur over a wide range of energy gaps (1.54–1.9 eV). The gap can be varied by choosing the appropriate ratio of the alternating layer thickness. This effect should be useful for improving the signal/noise ratio of avalanche photodiodes significantly.

Impact ionisation in multilayered heterojunction structures

Calculations are reported showing that in multilayered heterojunction structures the effective impact ionisation rates for electrons and holes can be very different, even if they are the same in the basic bulk materials. The reason for this is the difference in the band-edge discontinuities for electrons and holes and the lower phonon mean free path for holes in quantum well structures.