Showing papers on "Heterojunction published in 1997"

High-mobility Si and Ge structures

Abstract: Silicon-based heterostructures have come a long way from the discovery of strain as a new and essential parameter for band structure engineering to the present state of electron and hole mobilities, which surpass those achieved in the traditional material combination by more than an order of magnitude and are rapidly approaching the best III - V heteromaterials. It is the purpose of this article to report on the most recent developments, and the performance level achieved to date in this material system, in a concise and critical manner. The first part of this review is concerned with the structural and electronic properties of the lattice-mismatched Si/SiGe heterostructure. Emphases are put on the effects of strain both on the band structure and on the band offsets, as well as on means to actually control the strain in a stack of heteroepitaxial layers. The second part is dedicated to the transport properties of low-dimensional carrier systems in Si/SiGe and Ge/SiGe heterostructures. The prospects and limitations of the different layer concepts are discussed in terms of scattering mechanisms and experimental results. This part also reviews the most recent magneto-transport experiments on quantum wires and quantum point contacts, which became possible by the enhanced mean free paths in these materials. The third part covers the device aspects of these high-mobility materials, which is of special interest, because silicon-based heterostructures can significantly enhance the performance level of contemporary Si devices without sacrificing the essential compatibility with standard Si technologies. The recent achievements in this application-driven research field, but also the foreseeable problems and limitations, are discussed, and an assessment of the possible role of such heterodevices in future microelectronic circuits is given.

Ferromagnetic order induced by photogenerated carriers in magnetic iii-v semiconductor heterostructures of (in,mn)as/gasb

Abstract: We report the inducement of a ferromagnetic order by photogenerated carriers in a novel III-V--based magnetic semiconductor heterostructure $p$-(In,Mn)As/GaSb grown by molecular beam epitaxy. At low temperatures $(l35\mathrm{K})$, samples preserve ferromagnetic order even after the light is switched off, whereas they recover their original paramagnetic condition above 35 K. The results are explained in terms of hole transfer from GaSb to InMnAs in the heterostructure, which enhances a ferromagnetic spin exchange among Mn ions in the InMnAs layer.

Nanoscale metal/self-assembled monolayer/metal heterostructures

Abstract: We present the investigation of novel metal/organic monolayer/metal heterostructure diodes. Our technique provides well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal that thermal emission of electrons over a barrier of 0.22 eV dominates for electron injection from Ti into the organic layer while the transport for electron injection from Au into the organic layer satisfies the formula for hopping conduction.

Heterostructure integrated thermionic coolers

Abstract: Thermionic emission in heterostructures is proposed for integrated cooling of high power electronic and optoelectronic devices. This evaporative cooling is achieved by selective emission of hot electrons over a barrier layer from the cathode to the anode. It is shown that with available high electron mobility and low thermal conductivity materials, and with optimized conduction band offsets in heterostructures, single-stage room temperature cooling of up to 20°–40° over thicknesses of the order of microns is possible.

Measurement of piezoelectrically induced charge in GaN/AlGaN heterostructure field-effect transistors

Abstract: Electron concentration profiles have been obtained for AlxGa1−xN/GaN heterostructure field-effect transistor structures Analysis of the measured electron distributions demonstrates the influence of piezoelectric effects in coherently strained layers on III-V nitride heterostructure device characteristics Characterization of a nominally undoped Al015Ga085N/GaN transistor structure reveals the presence of a high sheet carrier density in the GaN channel which may be explained as a consequence of piezoelectrically induced charges present at the Al015Ga085N/GaN interface Measurements performed on an Al015Ga085N/GaN transistor structure with a buried Al015Ga085N isolation layer indicate a reduction in electron sheet concentration in the transistor channel and accumulation of carriers below the Al015Ga085N isolation layer, both of which are attributable to piezoelectric effects

POLYCRYSTALLINE THIN FILM SOLAR CELLS:Present Status and Future Potential

Abstract: ▪ Abstract Polycrystalline thin film solar cells on copper indium diselenide (CulnSe2) and its alloys and cadmium telluride (CdTe) appear to be the most promising candidates for large-scale application of photovoltaic energy conversion because they have shown laboratory-efficiences in excess of 15%. Heterojunction devices with n-type cadmium sulfide (CdS) films show very low minority carrier recombination at the absorber grain boundaries and at the metallurgical interface, which results in high quantum efficiences. Open circuit voltages of these devices are relatively low owing to the recombination in the space charge region in the absorber. Further improvements in efficiency can be achieved by reducing the recombination current, especially in devices based on CulnSe2 and its alloys. Low-cost manufacturing of modules requires better resolution of a number of other technical issues. For modules based on CulnSe2 and its alloys, the role of Na and higher deposition rates on device performance need to be bett...

External radiative quantum efficiency of 96% from a GaAs/GaInP heterostructure

Abstract: GaAs/GaInP double heterostructures are index matched with ZnSe hemispheres to increase the coupling of photoluminescence out of the device. We measure external quantum efficiencies as large as 96% at room temperature using a bolometric calibration technique. When the carriers are optically injected near the bandgap energy, the luminescence is blueshifted by up to 1.4 kT. In this case, external efficiencies exceeding 97.5% would yield optical refrigeration in the solid state.

Evidence of a Hybridization Gap in ``Semimetallic'' InAs/GaSb Systems

Abstract: InAs/GaSb composite quantum wells sandwiched by AlSb are studied by using capacitance-voltage, quantum Hall, and three-terminal transfer measurements. Our data reveal a positive energy gap resulting from the hybridization of in-plane dispersions of electrons in InAs and holes in GaSb for conventionally recognized ``semimetallic'' InAs/GaSb heterostructures.

FET having a Si/SiGeC heterojunction channel

Abstract: Si and SiGeC layers are formed in an NMOS transistor on a Si substrate. A carrier accumulation layer is formed with the use of a discontinuous portion of a conduction band present at the heterointerface between the SiGeC and Si layers. Electrons travel in this carrier accumulation layer serving as a channel. In the SiGeC layer, the electron mobility is greater than in silicon, thus increasing the NMOS transistor in operational speed. In a PMOS transistor, a channel in which positive holes travel, is formed with the use of a discontinuous portion of a valence band at the interface between the SiGe and Si layers. In the SiGe layer, too, the positive hole mobility is greater than in the Si layer, thus increasing the PMOS transistor in operational speed. There can be provided a semiconductor device having field-effect transistors having channels lessened in crystal defect.

Small valence-band offsets at GaN/InGaN heterojunctions

Abstract: The band discontinuities between GaN and InN, as well as InGaN alloys, are key parameters for the design of nitride-based light emitters Values reported to date are subject to large uncertainties due to strain effects at this highly mismatched interface We have investigated the band lineups using first-principles calculations with explicit inclusion of strains and atomic relaxations at the interface We find that the “natural” valence-band offset between unstrained InN and GaN is 03 eV Prescriptions are given, including the band shifts, due to strains at a pseudomorphic interface

Laser diodes based on beryllium-chalcogenides

Abstract: Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2.

Persistent photoconductivity in a two-dimensional electron gas system formed by an AlGaN/GaN heterostructure

Abstract: Persistent photoconductivity (PPC) effect associated with a two-dimensional electron gas (2DEG) in an AlGaN/GaN heterojunction device has been observed. As a consequence, the device was observed to be sensitive to light and the sensitivity was associated with a permanent photoinduced increase in the 2DEG carrier mobility and density. By formulating the PPC buildup and decay kinetics, we attributed the observed increase in the 2DEG carrier density and mobility to the photoionization of deep level impurities (DX like centers) in the AlGaN barrier material. In the PPC state, we were able to continuously vary the 2DEG carrier density in a single sample through photoexcitation and it has been found that the 2DEG carrier mobility increases almost linearly with the carrier density in the 2DEG channel. At 10 K, an electron mobility of 5800 cm2/V⋅s has been obtained in a PPC state. Implications of these observations on the device applications based on AlGaN/GaN heterojunctions have also been discussed.

Low temperature chemical vapor deposition growth of β-SiC on (100) Si using methylsilane and device characteristics

Abstract: The growth properties of β-SiC on (100) Si grown by rapid thermal chemical vapor deposition, using a single precursor (methylsilane) without an initial surface carbonization step, were investigated. An optimun growth temperature at 800 °C was found to grow single crystalline materials. A simple Al Schottky barrier fabricated on n-type SiC grown on Si substrates exhibited a “hard” reverse breakdown of 13 V with a positive temperature coefficient of 2×10−4 °C−1 up to 120 °C, indicating an avalanche mechanism. A Pt Schottky barrier fabricated on n-type SiC grown on tilted Si substrates to improve the surface morphology exhibited a breakdown voltage of 59 V, with a negative temperature coefficient. From the analysis of the electrical field distribution, the breakdown probably occurred at interface defects between SiC and Si, as suggested by Raman spectroscopy. To investigate minority transport behavior, SiC/Si heterojunction bipolar transistors (HBTs) were fabricated and compared to Si bipolar junction transistors. The collector currents of the SiC/Si HBTs were similar to those of Si control transistors, because both devices had the same base structures. Compared to Si control transistors, the base currents of SiC/Si HBTs increased. It seems that the interface defects between Si and SiC act as recombination centers to deplete back-injected holes, instead of being the barrier to stop hole currents, and thus to increase the base currents of SiC/Si HBTs.

Carrier transport in thin films of silicon nanoparticles

Abstract: The electrical and electroluminescence characteristics of heterostructure systems containing thin films of visibly emitting silicon nanoparticles are shown to be controlled by carrier transport through the nanoparticulate films. A conduction mechanism encompassing both geometric and electronic effects most effectively relates the high resistivity with structural properties of the films. Heterostructure devices are constructed with silicon nanoparticle active layers produced by pulsed laser ablation supersonic expansion. The observed temperature-dependent photoluminescence, electroluminescence, and $I\ensuremath{-}V$ characteristics of the devices are consistent with a model in which carrier transport is controlled by space-charge-limited currents or tunneling through potential barriers on a percolating lattice.

Quantum shift of band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes

Abstract: We report on the band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells. In these devices, we observe that the stimulated emission peak wavelength shifts to shorter values with decreasing well thickness. From the comparison of the results of the quantum mechanical calculations of the subbands energies with the measured data, we estimate the effective conduction- and valence-band discontinuities at the GaN–In0.13Ga0.87N heterointerface to be approximately 130–155 and 245–220 meV, respectively. We also discuss the effect of stress on the estimated values of band discontinuities.

Novel photovoltaic devices based on donor-acceptor molecular and conducting polymer systems

Abstract: Interest in organic photocells was significantly renewed a few years ago due to discovery of extremely fast photo-induced charge transfer (CT) from conducting polymer (CP) to fullerene C/sub 60/ molecules in CP(C/sub 60/) composites. This allowed creation of interfacial junction photocells with relatively high efficiencies. It has been recognized that those are not conventional p/n type semiconducting junction devices, but are so called donor-acceptor (D A) photocells, reminiscent of reaction centers of natural photosynthetic systems. Even more efficient photovoltaic devices have recently been fabricated with interpenetrating D-A networks which allow electron-hole separation to take place throughout the bulk of the device. In the present paper we demonstrate how these photocells can be further improved; we describe new experimental results, give general discussion of the efficiency of such devices, and analyze strategies for design of better D-A type photocells. Our recent experimental results on sensitization by inserting an excitonic layer between donor and acceptor layers multilayered structures, and by doping the D-layer by A-molecules in D(A)/A cells are briefly discussed. The organic photocell, which consists of the multilayer structure: ITO/poly(2,5-dioctyloxy-phenylenvinylene) (OOPPV)/octaethylporphine (OEP)/C/sub 60//Al, has been fabricated. Photocurrent yield spectra are interpreted by light absorption at OEP layer, exciton migration, and charge generation at both organic heterojunctions. That is, double heterojunctions of OOPPV/OEP and OEP/C/sub 60/ contribute to the charge generation by excitonic dissociation. We also report new type of "fractal network" photovoltaic devices made of two conducting polymers, particularly D and A derivatives of poly(phenylene vinylene). Conceptual theoretical work and modeling have been carried out to understand the ways for improvement of the device performance. Strategies to improve each step involving selective doping have been suggested to improve each step. Selective doping can provide low serial resistance and create internal electric fields for the collection of charges, while in the undoped parts excitons are effectively photogenerated. Ways to increase the effective use of absorbed photon energy and filling factor are also suggested.

GaN based heterostructure for high power devices

Abstract: We discuss the potential of GaN-based field effect transistor for high-power, high-temperature operation. At room temperature, the GaN/AlGaN doped channel HFETs (DC-HFETs) demonstrated highest frequency operation among all wide band gap semiconductor devices because of excellent transport properties of two dimensional electron gas at the AlGaN/GaN heterointerface and a large sheet carrier concentration in the device channel. CW operation at 10 and 15 GHz was also recently reported. Monte Carlo simulations indicate that short-channel GaN devices should have transported superior even to GaAs. However, improved thermal and microwave designs are required in order to take advantage of these material properties for applications in high power devices.

SiC/111-V-nitride heterostructures on SiC/SiO2 /Si for optoelectronic devices

Abstract: A low-cost Si-based construction for optical and electronic bulk-heterostructure devices and multiple-quantum-well devices in which the active layers of the device are SiC or AlGaN or InGaN or InAlN. Material quality is high, and the MQW devices such as blue light lasers or LEDs have stable pseudomorphic layers with low defect densities. The low-cost large-area 3C SiC substrate is created by converting 100% of a 100-500 angstrom (Å) layer of Si in a silicon-on-insulator wafer to 3C SiC with propane at 1300 degrees C. The SiO2 layer provides strain-free support for the "perfect" 3C SiC crystal layer. Direct-gap wurtzite nitride heterostructures, bulk or pseudomorphic MQW, are grown upon an (0001) 6H SiC epilayer on the (111) 3C SIC substrate, or directly upon the (111) 3C SiC substrate. For zincblende heterostructures, a (100) 3C SiC substrate is used.

Quantum dot memory cell

Abstract: A channel layer and a spacer layer form a heterojunction therebetween. A V-shaped groove is formed in the spacer layer. The sharp bottom of the V-shaped is located above the heterojunction interface. On the bottom of the V-shaped groove a plurality of quantum dots are formed in a line and discretely. A gate electrode is formed above the quantum dots. A source electrode is connected to the heterojunction interface to form an ohmic contact therebetween. A drain electrode is connected to the heterojunction interface to form an ohmic contact therebetween. The quantum dots are arranged between the source and drain electrodes.

Fine Structure of Excitonic Levels in Semiconductor Nanostructures

Abstract: A brief survey is given on recent work on the fine structure of zero-dimensional excitons in semiconductors. The paper focuses on microscopic mechanisms of anisotropic exchange splitting of the localized-exciton radiative doublet in type-I and type-II heterostructures and on polarized photoluminescence of excitons confined in self-assembled quantum dots.

Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface

Abstract: We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN–AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented. We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230±0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect.

Low pressure organic vapor phase deposition of small molecular weight organic light emitting device structures

Abstract: A new technique for the deposition of amorphous organic thin films, low pressure organic vapor phase deposition (LP-OVPD), was used to fabricate organic light emitting devices (OLEDs) consisting of a film of aluminum tris-(8 hydroxyquinoline) (Alq3) grown on the surface of a film of N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′biphenyl-4-4′diamine. The resulting heterojunction OLED was found to have a performance similar to conventional, small molecular weight OLEDs grown using thermal evaporation in vacuum. The LP-OVPD grown device has an external quantum efficiency of 0.40±0.05% and a turn-on voltage of approximately 6 V. The rapid throughput demonstrated with LP-OVPD has the potential to facilitate low cost mass production of conventional small molecule based OLEDs, and its use of low vacuum in a horizontal reactor lends itself to roll-to-roll deposition of organic films for many photonic device applications.

Stimulated emission from optically pumped GaN quantum dots

Abstract: Stimulated emission was observed from optically pumped GaN quantum dots in an AlxGa1−xN separate confinement heterostructure fabricated on 6H-SiC(0001) substrate by metal organic chemical vapor deposition. Nanostructural GaN quantum dots, with an average size of ∼10 nm width, ∼1–2 nm height, and density of ∼1011 cm−2, were self-assembled on the AlxGa1−xN cladding layer surface. The stimulated emission peak was observed at ∼3.48 eV, which is ∼50 meV lower than that of spontaneous emission. The excitation power dependence on the emission intensity clearly indicates threshold pump power density of 0.75 MW/cm2 for the onset of stimulated emission.

Evidence of stress dependence in SiO2/Si3N4 encapsulation-based layer disordering of GaAs/AlGaAs quantum well heterostructures

Abstract: Spatial selectivity of layer disordering induced in GaAs/AlGaAs quantum well heterostructures using SiO2 and Si3N4 capping and annealing was investigated using low temperature photoluminescence in conjunction with cross-sectional transmission electron microscopy. Comparative study reveals opposite behaviors for patterned Si3N4 covered with SiO2 and patterned SiO2 covered with Si3N4. In the former, layer disordering occurs in the regions located under the SiO2 strips and in the latter, layer disordering surprisingly occurs under the Si3N4 strips while it is inhibited in the SiO2-capped areas. These results are in agreement with a proposed interdiffusion model based on the effect on Ga vacancy diffusion of the stress distribution generated in the heterostructure during annealing by the capping layers. This work clearly demonstrates that the diffusion of point defects, such as the Ga vacancies, which are responsible for the layer disordering, can be piloted by the stress field imposed to the semiconductor an...

Fabrication of n-type nickel doped B 5 C 1 + δ homojunction and heterojunction diodes

Abstract: We have successfully nickel doped a boron carbide (B5C) alloy film. The nickel doped boron-carbide (Ni-B5C1+δ) thin films were fabricated from a single source carborane cage molecule and nickelocene [Ni(C5H5)2] using plasma enhanced chemical vapor deposition. Nickel doping transforms the highly resistive undoped film from a p-type material to an n-type material. This has been verified from the characteristics of diodes constructed of Ni-B5C1+δ on both n-type silicon and p-type B5C. The homojunction diodes exhibit excellent rectifying properties over a wide range of temperatures.

A Scanning Tunneling Microscopy-Reflection High Energy Electron Diffraction-Rate Equation Study of the Molecular Beam Epitaxial Growth of InAs on GaAs(001), (110) and (111)A–Quantum Dots and Two-Dimensional Modes

Abstract: The growth modes of InAs on the three low index orientations of GaAs during molecular beam epitaxy (MBE) are very different, despite a constant lattice mismatch of ≈7%. Coherent three-dimensional (3D) growth occurs only on (001) surfaces; on the other two orientations strain relaxation involves misfit dislocation formation and a continuous two dimensional growth mode. Strain is therefore not a sufficient condition to induce 3D growth. Reflection high-energy electron-diffraction and scanning tunnelling microscopy observations confirm that an intermediate `wetting layer' is formed on (001)-oriented substrates prior to the formation of quantum dots. The thickness and composition of this layer is dependent on both growth temperature and the amount of InAs deposited, but it is always an (In, Ga)As alloy. We have also confirmed that substantial mass transport occurs during quantum dot formation and that the dots themselves have an alloy composition. A model to account for at least some of these effects, based on rate equations, is introduced.