Showing papers on "Heterojunction published in 1995"

Charge separation and photovoltaic conversion in polymer composites with internal donor/acceptor heterojunctions

Abstract: The photosensitivity of semiconducting polymers can be enhanced by blending donor and acceptor polymers to optimize photoinduced charge separation. We describe a novel phase‐separated polymer blend (composite) made with poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene], MEH‐PPV, as donor and cyano‐PPV, CN‐PPV, as acceptor. The photoluminescence and electroluminescence of both component polymers are quenched in the blend, indicative of rapid and efficient separation of photogenerated electron‐hole pairs with electrons on the acceptor and holes on the donor. Diodes made with such a composite semiconducting polymer as the photosensitive medium show promising photovoltaic characteristics with carrier collection efficiency of 5% electrons/photon and energy conversion efficiency of 0.9%, ∼20 times larger than in diodes made with pure MEH‐PPV and ∼100 times larger than in diodes made with CN‐PPV. The photosensitivity and the quantum yield increase with reverse bias voltage, to 0.3 A/W and 80% electrons/photon respectively at −10 V, comparable to results obtained from photodiodes made with inorganic semiconductors.

Properties of Metal Silicides

Abstract: Crystal structure mechanical and thermal properties growth and phase transformations thermochemical properties band structure Schottky barrier heights heterojunctions and interfaces electrical transport optical properties and functions impurity diffusion self-diffusion hyperfine interactions.

Radiative recombination in type-ii gasb/gaas quantum dots

Abstract: Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Franz–Keldysh oscillations in modulation spectroscopy

Abstract: In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s‐i‐n+/s‐i‐p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The cas...

Electronic structure of the semimetals Bi and Sb

Abstract: We have developed a third-neighbor tight-binding model, with spin-orbit coupling included, to treat the electronic properties of Bi and Sb. This model successfully reproduces the features near the Fermi surface that will be most important in semimetal-semiconductor device structures, including (a) the small overlap of valence and conduction bands, (b) the electron and hole effective masses, and (c) the shapes of the electron and hole Fermi surfaces. The present tight-binding model treats these semimetallic properties quantitatively, and it should, therefore, be useful for calculations of the electronic properties of proposed semimetal-semiconductor systems, including superlattices and resonant-tunneling devices.

Radiative states in type-II GaSb/GaAs quantum wells.

Abstract: We have studied optical properties of staggered band line-up (type-II) heterostructures based on strained GaSb sheets in a GaAs matrix. The giant valence-band offset characteristic to this heterojunction leads to an effective localization of holes in ultrathin GaSb layers. An intense photoluminescence (PL) line caused by radiative recombination of localized holes with electrons located in the nearby GaAs regions is observed. The separation of nonequilibrium electrons and holes in real space results in a dipole layer and, thus, in the formation of quantum wells for electrons in the vicinity of the GaSb layer. The luminescence maximum shifts towards higher photon energies with rising excitation density reflecting the increase in the electron quantization energy. A bimolecular recombination mechanism is revealed in PL and in time-resolved PL studies. In the case of pseudomorphic monolayer-thick GaSb layers, the radiative exciton ground state does not exist. Accordingly, small absorption coefficients and a featureless behavior of the band-to-band calorimetric absoprtion spectrum are found in the vicinity of ${\mathit{k}}_{\mathit{x},}$y=0. Remarkable enhancement of the absorption coefficient with a characteristic onset is observed for heavy holes with ${\mathit{k}}_{\mathit{x},}$yg0. Radiative states in the continuum of heavy-hole subbands are revealed also in temperature-dependent PL studies. The experimentally measured onset energies point out the importance of GaSb heavy- and light-hole mixing effects. We demonstrate intense luminescence from staggered band line-up GaSb-GaAs heterostructures up to room temperature.

Thin film II–VI photovoltaics

Abstract: With the exception of HgSe and HgTe, II–VI compounds are direct gap semiconductors with sharp optical absorption edge and large absorption coefficients at above bandgap wavelengths. Device quality polycrystalline films of II–VI compounds can be prepared from inexpensive raw materials by a number of low-cost methods. They are well-suited for thin film solar cells and provide an economically viable approach to the terrestrial utilization of solar energy. Thin film II–VI solar cells are usually of the heterojunction type consisting of a high bandgap window (or collector) and a lower bandgap absorber. The grain boundary effects in polycrystalline II–VI films are considerably less pronounced than those in III–V films and can be passivated, at least partially, by chemical treatment. The use of CdS, ZnO, ZnSe and Cd1 − xZnxS as the window and the use of CdTe and Cd1 − xZnxTe as the absorber are reviewed in this paper. The fabrication and characteristics of a number of the thin film solar cell structures are discussed with emphasis on the thin film CdS CdTe solar cell.

On the cds/cuinse2 conduction-band discontinuity.

Abstract: Recent calculations of the electron affinity difference between CdS and CuInSe2 indicate that the conduction band (CB) minimum of CuInSe2 is below the CB minimum of CdS. As a consequence, a spike occurs in the CB at the CdS/CuInSe2 interface. Such a spike is commonly considered as in conflict with good photovoltaic performance of heterojunction solar cells. It is outlined here that the simple assumption of thermionic emission across the junction can explain an unimpeded electron transport in the case of an n+p structure (n‐type window, p‐type absorber), even when a spike in the CB occurs.

Multiphonon-assisted tunneling through deep levels: A rapid energy-relaxation mechanism in nonideal quantum-dot heterostructures.

Abstract: The presence of a deep-level trap coupled to a quantum-dot heterostructure is shown to provide a rapid energy-relaxation pathway through which electrons may thermalize. A capture process is considered whereby a free conduction-band electron is captured into the ground conduction-band state of a quantum dot by multiphonon-assisted tunneling through the trap. As an example calculation, transition rates for a 5 nm radius ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dot coupled to the defect M1 are calculated as a function of separation between the quantum dot and the deep level. For separations less than \ensuremath{\approxeq}10 nm these rates are found to be in excess of ${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 4.2 K. The result suggests that the presence of point defects may serve to enhance the luminescence efficiency of quantum-dot material. The physical situation described in this paper could only arise if the spatial distribution of defects were strongly correlated with that of the quantum-dot structures, e.g., through formation of interface states or point defects as a consequence of the growth process. With this caveat, the proposed mechanism may possibly explain the failure to observe a significant phonon bottleneck effect in recent work on ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-dot structures [e.g., Appl. Phys. Lett. 64, 2815 (1994)].

Properties of strained and relaxed silicon germanium

Abstract: Strained layers in general crystal structure phase diagrams ordering interfaces thermal properties mechanical properties lattice vibrations surface properties segregation during growth optical functions band structure and offsets effective masses carrier mobilities magnetotransport injection across a heterostructure device-related structures on Si substrates.

Enhanced SiGe heterojunction bipolar transistors with 160 GHz-f/sub max/

Abstract: Double-mesa type SiGe heterojunction bipolar transistors (HBTs) have been improved by increasing the base Gummel number and by using a thin, highly doped launcher layer between the base and the collector. In addition, the contact resistance of the base contact has been reduced. Hence, it was possible to obtain a record maximum frequency of oscillation up to 160 GHz for a 2-emitter finger HBT in common emitter configuration.

Influence of platinum interlayers on the electrical properties of RuO2/Pb(Zr0.53Ti0.47)O3/RuO2 capacitor heterostructures

Abstract: Lead zirconate titanate, Pb(ZrxTi1−x)O3 or PZT, thin films grown on RuO2 electrodes by the sol‐gel process have excellent resistance to polarization fatigue, but they generally have two drawbacks. The films have high leakage currents and large property variation. In this letter we show that the use of a thin Pt layer (100 A) deposited on the bottom RuO2 electrode to fabricate RuO2/PZT/Pt/RuO2/(100)MgO capacitors has two important effects. It reduces capacitor leakage by two to four orders of magnitude and it significantly reduces the large property variation. In addition, these capacitors retain their excellent resistance to polarization fatigue which is characteristic of the RuO2/PZT/RuO2 heterostructure.

Visible luminescence from semiconductor quantum dots in large ensembles

Abstract: Visible luminescence has been obtained from ensembles of defect‐free, InxAl(1−x)As islands of ultrasmall dimensions embedded in AlyGa(1−y)As cladding layers. These structures were grown by molecular beam epitaxy and studied with low‐temperature photoluminescence (PL) and transmission electron microscopy. Visible luminescence is produced using various compositions of InxAl(1−x)As/AlyGa(1−y)As. Quantum dot size distributions, planar densities, dot heights, and wetting layer thicknesses are presented and correlated with the PL spectra.

Optical device with low electrical and thermal resistance bragg reflectors

Abstract: A compound-semiconductor optical device and method. The optical device is provided with one or more asymmetrically-graded heterojunctions between compound semiconductor layers for forming a distributed Bragg reflector mirror having an improved electrical and thermal resistance. Efficient light-emitting devices such as light-emitting diodes, resonant-cavity light-emitting diodes, and vertical-cavity surface-emitting lasers may be formed according to the present invention, which may be applied to the formation of resonant-cavity photodetectors.

High-Temperature Superconducting Multilayers and Heterostructures Grown by Atomic Layer-By-Layer Molecular Beam Epitaxy

Abstract: Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) ofhigh-tem­ perature superconductors (HTSC) and other complex oxides has been developed. Thin films with atomically flat surfaces and abrupt interfaces can be produced an atomic layer at a time. Samples are engineered by stacking molecular layers of diff erent compounds, by adding or omitting atomic monolayers, and by doping within specified monolayers. Novel artificial HTSC compounds such as Bi2Sr2 Ca7CugOx (2278), as well as various heterostructures , have been synthesized in this way. This unique synthetic capability has allowed several fundamental physics issues, such as the dimensionality of the HTSC state, long-range proximity effects, resonant tunneling with a specified number of hops, etc, to be addressed. Trilayer Josephson junctions with uniform and reproducible properties have been fabricated; phase-locked operation of two junctions has also been demonstrated. Finally, titanate slabs only 4-A thick have been grown without pin-holes and shown to provide tunneling barriers for c-axis transport.

Structure and properties of epitaxial ba0.5sr0.5tio3/srruo3/zro2 heterostructure on si grown by off-axis sputtering

Abstract: We report the growth and characterization of epitaxial Ba0.5Sr0.5TiO3/SrRuO3/ZrO2 on Si for potential charge storage applications. Both Ba0.5Sr0.5TiO3 (BST) and SrRuO3 (SRO) are grown (110)‐oriented on yttrium‐stabilized ZrO2 (YSZ) (100)‐buffered Si. These films show a high degree of crystallinity with minimal interdiffusion at the interfaces as evidenced from x‐ray diffraction, Rutherford backscattering, and transmission electron microscopy. Studies on the in‐plane crystallographic relations between the layers revealed an interesting rectangle‐on‐cube epitaxy between BST/SRO and YSZ. The dielectric constant and loss tangent of the BST dielectric layer are 360 and 0.01 at 10 kHz, respectively. The leakage current density is lower than 4×10−7 A/cm2 at 1 V. A strong frequency dependence on both dielectric constant and loss tangent is observed in 1–10 MHz frequency range. This is attributed to the effect of a series resistance in the measurement loop, which is likely related to the bottom SrRuO3 electrode.

Band alignment and carrier injection at the porous‐silicon–crystalline‐silicon interface

Abstract: The current‐voltage characteristics and the photoresponse of metal‐porous Si–p‐type Si heterostructures have been studied. It is shown that the common interpretation of the current‐voltage characteristics, which assumes that the current is limited by the Schottky barrier at the metal‐porous Si interface, is wrong. An alternative explanation based on the electric‐field dependence of the porous Si conductivity is suggested. It is shown that the rectifying behavior originates from a depletion inside the c‐Si substrate at its interface to the porous Si.

Conduction-band spin splitting and negative magnetoresistance in A3B5 heterostructures

Abstract: The quantum interference corrections to the conductivity are calculated for an electron gas in asymmetric quantum wells in a magnetic field. The theory takes into account two different types of the spin splitting of the conduction band: the Dresselhaus terms, both linear and cubic in the wave vector, and the Rashba term, linear in wave vector. It is shown that the contributions of these terms into magnetoconductivity are not additive, as it was traditionally assumed. While the contributions of all terms of the conduction-band splitting into the D'yakonov-Perel' spin relaxation rate are additive, in the conductivity the two linear terms cancel each other, and, when they are equal, in the absence of the cubic terms the conduction-band spin splitting does not show up in the magnetoconductivity at all. The theory agrees well with experimental results and enables one to determine experimentally parameters of the spin-orbit splitting of the conduction band.

Theoretical study of electron transport in gallium nitride

Abstract: This paper describes characteristic electron transport properties for GaN in bulk and quantum well structures. First, ensemble Monte Carlo calculations of steady‐state electron drift velocity in bulk GaN are presented as a function of applied electric field for different lattice temperatures. At 300 K, the calculated peak steady‐state drift velocity is 2.8×107 cm/s and the threshold field is 160 kV/cm. It is found that the peak steady‐state electron drift velocity decreases only slightly by about 20% as the temperature increases from 300 to 600 K while the threshold field increases slightly by about 20%. Therefore, in addition to its high temperature stability, GaN has a low temperature coefficient making it ideal for high temperature applications. For electron transport in heterostructures, quantum mechanical calculations of the electron capture rate in GaN‐based quantum wells as a function of well thickness are also presented. An oscillatory behavior of the electron capture rate as a function of quantum...

Growth of germanium‐carbon alloys on silicon substrates by molecular beam epitaxy

Abstract: Metastable Ge1−yCy alloys were grown by molecular beam epitaxy as homogeneous solid solutions having a diamond lattice structure. The substrates were (100) oriented Si wafers and the growth temperature was 600 °C. We report on measurements of the composition, structure, lattice constant, and optical absorption of the alloy layers. In thick relaxed layers, C atomic fractions up to 0.03 were obtained with a corresponding band gap of 0.875 eV. These alloys offer new opportunities for fundamental studies, and for the development of silicon‐based heterostructure devices.

Transport properties of two-dimensional electron gas in AlGaAs/GaAs selectively doped heterojunctions with embedded InAs quantum dots

Abstract: Transport properties of two‐dimensional electron gas (2DEG) are studied in selectively doped GaAs/n‐AlGaAs heterojunctions, in which nanometer‐scale InAs dots are embedded in the vicinity of the GaAs channel. When the distance Wd between the InAs dot layer and the channel is reduced from 80 to 15 nm, the mobility μ of electrons at 77 K decreases drastically from 1.1×105 to 1.1 ×103 cm2/V s, while the carrier concentration increases from 1.1×1011 to 5.3×1011 cm−2. Such a reduction of mobility is found only when the average thickness of InAs layer is above the onset level (∼1.5 monolayer) for the dot formation. Origins of these changes in μ and Ns are discussed in connection with dot‐induced modulations of the electronic potential V(r) in the channel.

Room‐temperature 2.78 μm AlGaAsSb/InGaAsSb quantum‐well lasers

Abstract: We describe room‐temperature 2.78 μm AlGaAsSb/InGaAsSb multiquantum well lasers. Pulsed laser operation was observed at 15 °C with a threshold current of 1.1 A (10 kA/cm2), and a maximum power output of 30 mW, and a maximum differential quantum efficiency of 9%. Lasers operated pulsed up to 60 °C with a characteristic temperature of 58 K over the range of 0–40 °C. To date, 2.78 μm is the longest emission wavelength for a room‐temperature III–V laser.

Realization and modeling of a pseudomorphic (GaAs1−xSbx–InyGa1−yAs)/GaAs bilayer‐quantum well

Abstract: We have realized a (GaAs1−xSbx‐InyGa 1−yAs)/GaAs bilayer‐quantum well (BQW), which consists of two adjacent pseudomorphic layers of GaAs1−xSbx and InyGa1−yAs sandwiched between GaAs barriers. Photoluminescence was observed at longer wavelengths than those found for corresponding InyGa1−yAs/GaAs and GaAs1−xSbx/GaAs single quantum wells (SQW), which indicates a type‐II band alignment in the BQW. The longest 300 K emission wavelength achieved so far was 1.332 μm. For an accurate determination of the band offset between GaAs1−xSbx and GaAs, required for a theoretical modeling of the interband transition energies of these BQWs, a large set of GaAs1−xSbx /GaAs SQWs was prepared from which a type‐II band alignment was deduced, with the valence band discontinuity ratio Qv found to depend on the Sb concentration x (Qv=1.76+1.34 x). With this parameter it was possible to calculate the expected interband transition energies in a BQW structure without any adjustable parameters. The calculations are in agreement with ...

Two‐dimensional electron gas in GaN–AlGaN heterostructures deposited using trimethylamine‐alane as the aluminum source in low pressure metalorganic chemical vapor deposition

Abstract: In this letter, we report the fabrication of high quality GaN–Al0.1Ga0.9N heterostructures using trimethylamine‐alane as the aluminum source. The two‐dimensional electron gas (2DEG) with mobility values as high as 5000 cm2/V s at 150 K is demonstrated. We also present the results of our calculations of the 2DEG mobility at the GaN–Al0.1Ga0.9N heterointerface. Our calculations show that the mobility enhancement in the 2DEG is related to a much larger volume carrier concentration (compared to bulk GaN) and, therefore, to a larger Fermi energy and to a more effective screening of impurity and piezoelectric scattering.

Quantum-well intermixing for optoelectronic integration using high energy ion implantation

Abstract: The technique of ion‐induced quantum‐well (QW) intermixing using broad area, high energy (2–8 MeV As4+) ion implantation has been studied in a graded‐index separate confinement heterostructure InGaAs/GaAs QW laser. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating optoelectronic devices. Parameters controlling the ion‐induced QW intermixing, such as ion doses, fluxes, and energies, post‐implantation annealing time, and temperature are investigated and optimized using optical characterization techniques such as photoluminescence, photoluminescence excitation, and absorption spectroscopy.

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

Abstract: A design is presented and computer-simulated for high-mobility Si/SiGe heterojunction CMOS transistors. Comparing 0.2 /spl mu/m Si/SiGe FETs to bulk Si FETs, an increase is predicted in current drive of 125% and 23% in the p-FET and n-FET, respectively. For given propagation delay (55 ns), simulated loaded ring oscillators at 1.5 V exhibit 4.6 times reduction in power-delay-product compared to bulk Si CMOS oscillators of the same design rules operating at 2.5 V.