Showing papers on "Heterojunction published in 1999"

Quantum dot heterostructures

Abstract: Fabrication Techniques for Quantum Dots. Self-Organization Concepts on Crystal Surfaces. Growth and Structural Characterization of Self-Organized Quantum Dots. Modeling of Ideal and Real Quantum Dots. Electronic and Optical Properties. Electrical Properties. Photonic Devices. References. Index.

Modeling photocurrent action spectra of photovoltaic devices based on organic thin films

Abstract: We have modeled experimental short-circuit photocurrent action spectra of poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophene) (PEOPT)/fullerene (C60) thin film heterojunction photovoltaic devices. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the PEOPT/C60 interface. The internal optical electric field distribution inside the devices was calculated with the use of complex indices of refraction and layer thickness of the materials as determined by spectroscopic ellipsometry. Contributions to the photocurrent from optical absorption in polymer and fullerene layers were both necessary to model the experimental photocurrent action spectra. We obtained values for the exciton diffusion range of 4.7 and 7.7 nm for PEOPT and C60, respectively. The calculated internal optical electric field distribution and resulting photocurrent action spectra were used in or...

Electronic and optical properties of strained quantum dots modeled by 8-band k⋅p theory

Abstract: We present a systematic investigation of the elastic, electronic, and linear optical properties of quantum dot double heterostructures in the frame of eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory. Numerical results for the model system of capped pyramid shaped InAs quantum dots in GaAs (001) with ${101}$ facets are presented. Electron and hole levels, dipole transition energies, oscillator strengths, and polarizations for both electron-hole and electron-electron transitions, as well as the exciton ground-state binding energy and the electron ground-state Coulomb charging energy are calculated. The dependence of all these properties on the dot size is investigated for base widths between 10 and $20$ nm. Results for two different approaches to model strain, continuum elasticity theory, and the Keatings valence force field model in the linearized version of Kane, are compared to each other.

Nitride Semiconductors and Devices

Abstract: 1. Introduction.- 2. General Properties of Nitrides.- 2.1 Crystal Structure of Nitrides.- 2.2 Gallium Nitride.- 2.2.1 Chemical Properties of GaN.- 2.2.2 Thermal and Mechanical Properties of GaN.- 2.3 Aluminum Nitride.- 2.3.1 Thermal and Chemical Properties of AlN.- 2.3.2 Mechanical Properties of AlN..- 2.3.3 Electrical Properties of AlN.- 2.3.4 Optical Properties of AlN.- 2.4 Indium Nitride.- 2.4.1 Crystal Structure of InN.- 2.4.2 Mechanical and Thermal Properties of InN.- 2.4.3 Electrical Properties of InN.- 2.4.4 Optical Properties of InN.- 2.5 Ternary and Quaternary Alloys.- 2.5.1 AlGaN Alloy.- 2.5.2 InGaN Alloy.- 2.5.3 InAIN Alloy.- 2.6 Substrates for Nitride Epitaxy.- 2A Appendix: Fundamental Data for Nitride Systems.- 3. Electronic Band Structure of Bulk and QW Nitrides.- 3.1 Band-Structure Calculations.- 3.2 Effect of Strain on the Band Structure of GaN.- 3.3 k*p Theory and the Quasi-Cubic Model.- 3.4 Quasi-Cubic Approximation.- 3.5 Confined States.- 3.6 Conduction Band.- 3.7 Valence Band.- 3.8 Exciton Binding Energy in Quantum Wells.- 3.9 Polarization Effects.- 3A Appendix.- 4. Growth of Nitride Semiconductors.- 4.1 Bulk Growth.- 4.2 Substrates Used.- 4.2.1 Conventional Substrates.- 4.2.2 Compliant Substrates.- 4.2.3 Van der Waals Substrates.- 4.3 Substrate Preparation.- 4.4 Substrate Temperature.- 4.5 Epitaxial Relationship to Sapphire.- 4.6 Growth by Hydride Vapor Phase Epitaxy (HVPE).- 4.7 Growth by OMVPE (MOCVD).- 4.7.1 Sources.- 4.7.2 Buffer Layers.- 4.7.3 Lateral Growth.- 4.7.4 Growth on Spinel (MgAl2O4).- 4.8 Molecular Beam Epitaxy.- 4.8.1 MBE Growth Systems.- 4.8.2 Plasma-Enhanced MBE.- 4.8.3 Reactive-Ion MBE.- 4.8.4 Reactive MBE.- 4.8.5 Modeling of the MBE-Like Growth.- 4.9 Growth on 6H-SiC (0001).- 4.10 Growth on ZnO.- 4.11 Growth on GaN.- 4.12 Growth of p-Type GaN.- 4.13 Growth of n-Type InN.- 4.14 Growth of n-Type Ternary and Quaternary Alloys.- 4.15 Growth of p-Type Ternary and Quaternary Alloys.- 4.16 Critical Thickness.- 5. Defects and Doping.- 5.1 Dislocations.- 5.2 Stacking-Fault Defects.- 5.3 Point Defects and Autodoping.- 5.3.1 Vacancies, Antisites and Interstitials.- 5.3.2 Role of Impurities and Hydrogen.- 5.3.3 Optical Signature of Defects in GaN.- 5.4 Intentional Doping.- 5.4.1 n-Type Doping with Silicon, Germanium and Selenium.- 5.4.2 p-Type Doping.- a) Doping with Mg.- 5.4.3 Optical Manifestation of Group-II Dopant-Induced Defects in GaN.- a) Doping with Beryllium.- b) Doping with Mercury.- c) Doping with Carbon.- d) Doping with Zinc.- e) Doping with Calcium.- f) Doping with Rare Earths.- 5.4.4 Ion Implantation and Diffusion.- 5.5 Defect Analysis by Deep-Level Transient Spectroscopy.- 5.6 Summary.- 6. Metal Contacts to GaN.- 6.1 A Primer for Semiconductor-Metal Contacts.- 6.2 Current Flow in Metal-Semiconductor Junctions.- 6.2.1 The Regime Dominated by Thermionic Emission.- 6.2.2 Thermionic Field-Emission Regime.- 6.2.3 Direct Tunneling Regime.- 6.2.4 Leakage Current.- 6.2.5 The Case of a Forward-Biased p-n Junction.- 6.3 Resistance of an Ohmic Contact.- 6.3.1 Specific Contact Resistivity.- 6.3.2 Semiconductor Resistance.- 6.4 Determination of the Contact Resistivity.- 6.5 Ohmic Contacts to GaN.- 6.5.1 Non-Alloyed Ohmic Contacts.- 6.5.2 Alloyed Ohmic Contacts.- 6.5.3 Multi-Layer Ohmic Contacts.- 6.6 Structural Analysis.- 6.7 Observations.- 7. Determination of Impurity and Carrier Concentrations.- 7.1 Impurity Binding Energy.- 7.2 Conductivity Type: Hot Probe and Hall Measurements.- 7.3 Density of States and Carrier Concentration.- 7.4 Electron and Hole Concentrations.- 7.5 Temperature Dependence of the Hole Concentration.- 7.6 Temperature Dependence of the Electron Concentration.- 7.7 Multiple Occupancy of the Valence Bands.- 7A Appendix: Fermi Integral.- 8. Carrier Transport.- 8.1 Ionized Impurity Scattering.- 8.2 Polar-Optical Phonon Scattering.- 8.3 Piezoelectric Scattering.- 8.4 Acoustic Phonon Scattering.- 8.5 Alloy Scattering.- 8.6 The Hall Factor.- 8.7 Other Methods Used for Calculating the Mobility in n-GaN.- 8.8 Measured vis. a vis. Calculated Mobilities in GaN.- 8.9 Transport in 2D n-Type GaN.- 8.10 Transport in p-Type GaN and AlGaN.- 8.11 Carrier Transport in InN.- 8.12 Carrier Transport in AlN.- 8.12.1 Transport in Unintensionally-Doped and High-Resistivity GaN.- 8.13 Observation.- 9. The p-n Junction.- 9.1 Heterojunctions.- 9.2 Band Discontinuities.- 9.2.1 GaN/AIN Heterostructures.- 9.2.2 GaN/InN and AIN/InN.- 9.3 Electrostatic Characteristics of p-n Heterojunctions.- 9.4 Current-Voltage Characteristics on p-n Junctions.- 9.4.1 Generation-Recombination Current.- 9.4.2 Surf ace Effects.- 9.4.3 Diode Current Under Reverse Bias.- 9.4.4 Effect of the Electric Field on the Generation Current.- 9.4.5 Diffusion Current.- 9.4.6 Diode Current Under Forward Bias.- 9.5 Calculation and Experimental I-V Characteristics of GaN Based p-n Juctions.- 9.6 Concluding Remarks.- 10. Optical Processes in Nitride Semiconductors.- 10.1 Absorption and Emission.- 10.2 Band-to-Band Transitions.- 10.2.1 Excitonuc Transitions.- 10.3 Optical Transitions in GaN.- 10.3.1 Excitonic Transitions in GaN.- a) Free Excitons.- b) Bound Excitons.- c) Exciton Recombination Dynamics.- d) High Injection Levels.- 10.3.2 Free-to-Bound Transitions.- 10.3.3 Donor-Acceptor Transitions.- 10.3.4 Defect-Related Transitions.- a) Yellow Luminescence.- b) Group-II Element Related Transitions.- 10.4 Optical Properties of Nitride Heterostructures.- 10.4.1 GaN/AlGaN Heterostructures.- 10.4.2 InGaN/GaN and InGaN/InGaN Heterostructures.- 11. Light-Emitting Diodes.- 11.1 Current-Conduction Mechanism in LED-Like Structures.- 11.2 Optical Output Power.- 11.3 Losses and Efficiency.- 11.4 Visible-Light Emitting Diodes.- 11.5 Nitride LED Performance.- 11.6 On the Nature of Light Emission in Nitride-Based LEDs.- 11.6.1 Pressure Dependence of Spectra.- 11.6.2 Current and Temperature Dependence of Spectra.- 11.6.3 I-V Characteristics of Nitride LEDs.- 11.7 LED Degradation.- 11.8 Luminescence Conversion and White- Light Generation With Nitride LEDs.- 11.9 Organic LEDs.- 12. Semiconductor Lasers.- 12.1 A Primer to the Principles of Lasers.- 12.2 Fundamentals of Semiconductor Lasers.- 12.3 Waveguiding.- 12.3.1 Analytical Solution to the Waveguide Problem.- 12.3.2 Numerical Solution of the Waveguide Problem.- 12.3.3 Far-Field Pattern.- 12.4 Loss and Threshold.- 12.5 Optical Gain.- 12.5.1 Gain in Bulk Layers.- 12.5.2 Gain in Quantum Wells.- 12.6 Coulombic Effects.- 12.7 Gain Calculations for GaN.- 12.7.1 Optical Gain in Bulk GaN.- 12.7.2 Gain in GaN Quantum Wells.- 12.7.3 Gain Calculations in Wz GaN QW Without Strain.- 12.7.4 Gain Calculations in WZ QW With Strain.- 12.7.5 Gain in ZB QW Structures Without Strain.- 12.7.6 Gain in ZB QW Structures with Strain.- a) Pathways Through Excitons and Localized States.- 12.7.7 Measurement of Gain in Nitrides.- a) Gain Measurement via Optical Pumping.- b) Gain Measurement via Electrical Injection (Pump) and an Optical Probe.- 12.8 Threshold Current.- 12.9 Analysis of Injection Lasers with Simplifying Assumptions.- 12.10 Recombination Lifetime.- 12.11 Quantum Efficiency.- 12.12 Gain Spectra of InGaN Injection Lasers.- 12.13 Observations.- 12.14 A Succinct Review of the Laser Evolution in Nitrides.- References.

Polarization-induced charge and electron mobility in algan/gan heterostructures grown by plasma-assisted molecular-beam epitaxy

Abstract: The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.

Bound-to-continuum intersubband photoconductivity of self-assembled InAs quantum dots in modulation-doped heterostructures

Abstract: We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications

Abstract: This review covers a spectrum of optoelectronic properties of and uses for semi-insulating semiconductor heterostructures and thin films, including epilayers and quantum wells. Compensation by doping, implantation, and nonstoichiometric growth are described in terms of the properties of point defects and Fermi level stabilization and pinning. The principal optical and optoelectronic properties of semi-insulating epilayers and heterostructures, such as excitonic electroabsorption of quantum-confined excitons, are described, in addition to optical absorption by metallic or semimetallic precipitates in these layers. Low-temperature grown quantum wells that have an arsenic-rich nonstoichiometry and a supersaturated concentration of grown-in vacancies are discussed. These heterostructures experience transient enhanced diffusion and superlattice disordering. The review discusses the performance of optoelectronic heterostructures and microcavities that contain semi-insulating layers, such as buried heterostructure stripe lasers, vertical cavity surface emitting lasers, and optical electroabsorption modulators. Short time-scale applications arise from the ultrashort carrier lifetimes in semi-insulating materials, such as in photoconductors for terahertz generation, and in saturable absorbers for mode-locking solid state lasers. This review also comprehensively describes the properties and applications of photorefractive heterostructures. The low dark-carrier concentrations of semi-insulating heterostructures make these materials highly sensitive as dynamic holographic thin films that are useful for adaptive optics applications. The high mobilities of free carriers in photorefractive heterostructures produce fast dielectric relaxation rates that allow light-induced space-charge gratings to adapt to rapidly varying optical fringe patterns, canceling out environmental noise during interferometric detection in laser-based ultrasound, and in optical coherence tomography. They are also the functional layers in high-sensitivity dynamic holographic materials that replace static holograms in Fourier imaging systems and in experimental Tbit/s optical systems. Semi-insulating heterostructures and their applications have attained a degree of maturity, but many critical materials science issues remain unexplored.

Spontaneous and piezoelectric polarization effects in III-V nitride heterostructures

Abstract: The role of spontaneous and piezoelectric polarization in III–V nitride heterostructures is investigated Polarization effects and crystal polarity are reviewed in the context of nitride heterostructure materials and device design, and a detailed analysis of their influence in nitride heterostructure field-effect transistors is presented The combined effects of spontaneous and piezoelectric polarization are found to account well for carrier concentrations observed in AlGaN/GaN transistor structures with low to moderate Al concentrations, while the data for higher Al concentrations are consistent with defect formation in the AlGaN barrier Theoretical analysis suggests that incorporation of In into the barrier and/or channel layers can substantially increase polarization charge at the heterojunction interface The use of polarization effects to engineer Schottky barrier structures with large enhancements in barrier height is also discussed, and electrical characteristics of transistors with conventional a

Band lineup of layered semiconductor heterointerfaces prepared by van der Waals epitaxy: Charge transfer correction term for the electron affinity rule

Abstract: The occurrence of quantum dipoles at layered materials semiconductor heterointerfaces was investigated by photoemission spectroscopy (PES). Due to the unique properties of layered compounds the prepared interfaces are essentially free of the structural problems known from the usually investigated heterosystems composed of III–V, IV or II–VI materials allowing the detailed investigation of electronic phenomena at the interfaces. We investigated heterostructures composed of epitaxial layers of SnS2 and SnSe2 on different single crystalline layered chalcogenide substrates (WSe2, MoS2, MoTe2, and GaSe). The epilayers were grown by van der Waals epitaxy (vdWe) on the (0001) plane of the substrate crystals. For every system the valence band offset was determined by careful evaluation of the PES data as a function of the film thickness. Using published values for the band gaps and the experimentally determined work functions and surface potentials the band lineup for each system was determined. The band offsets ...

Fabrication of transparent p-n heterojunction thin film diodes based entirely on oxide semiconductors

Abstract: All oxide-based, transparent polycrystalline p–n heterojunctions on a glass substrate were fabricated. The structure of the diode was n+-ZnO electrode/n-ZnO/p-SrCu2O2/In2−xSnxO3 electrode on the substrate. The contact between the n- and p-type semiconducting oxides was found to be rectifying. The ratio of forward current to the reverse current exceeded 80 within the range of applied voltages of −1.5 to +1.5 V and the estimated diode factor (n value) was 1.62. The diode structure was fabricated on a glass plate with the total thickness of 1.3 μm and possessed an optical transmission of 70%–80% in the visible region.

Mesoporous oxide junctions and nanostructured solar cells

Abstract: Mesoporous films of wide-band gap semiconductor oxides are an important new class of electronic materials. They are constituted by a network of nanocrystalline particles of oxides, such as titania, niobia or zinc oxide, sintered together to allow for charge carrier transport to take place. The pores between the nanoparticles are filled with an electrolyte or a solid state organic hole conductor forming an interpenetrating heterojunction of very large contact area. These junctions exhibit extraordinary opto-electronic properties due to their large surface area to volume ratio leading to applications in different domains, such as photovoltaics, intercalation batteries, electrochromic and electroluminescent displays, photocatalysis and chemical sensors. Of particular interest are dye-sensitized heterojunctions, where photo-induced charge separation occurs at the interface between the mesoporous oxide and the hole conductor or the electrolyte. Photovoltaic cells based on this concept form a viable alternative to conventional silicon cells. Solar to electric power conversion efficiencies exceeding 10% have been reached with mesoporous titania films derivatized with molecular charge transfer sensitizers and used in conjunction with organic iodide/triiodide-based redox electrolytes. Long-term accelerated light-soaking tests have shown the system to be intrinsically stable. This article summarized recent developments in this field including a discussion of solid state dye-sensitized heterojunctions employing spirobifluorene-connected arylamines as hole transport materials.

Charge Separation in Solid-State Dye-Sensitized Heterojunction Solar Cells

Abstract: Dye-sensitized nanocrystalline solar cells are presently under intensive investigation, as they offer an attractive alternative to conventional p--n junction devices. Solid-state versions have been described where the electrolyte present in the pores of the malodorous oxide film is replaced by a large band gap p-type semiconductor. In this way, a solid-state heterojunction of very large contact area is formed. Light is absorbed by the dye that is located at the interface. Upon excitation, the dye injects electrons into the conduction band of the oxide and is regenerated by hole injection into the p-type conductor. High incident photon-to-electric current conversion efficiencies have been achieved recently with a cell consisting of a dye-derivatized mesoporous TiO{sub 2} film contacted by a new organic hole conductor. The great advantage of such systems with regard to conventional p--n junctions is that only majority carriers are involved in the photoelectric conversion process. Moreover, these are generated by the dye precisely at the site of the junction where the electric field is maximal, enhancing charge separation. Photoelectric conversion by conventional solar cells involves minority carriers whose lifetime is restricted due to recombination. As they are generated throughout the semiconductor and away from the junction, expensive high-purity materialsmore » are required in order to maintain the minority carrier diffusion length at a level where current losses are avoided. While the dynamics of photoinduced redo processes in photoelectrochemical systems have been studied in great detail, little is known about the electron-transfer dynamics in solid-state sensitized junctions. Here the authors report for the first time on the direct observation of photoinduced, interfacial charge separation across a dye-sensitized solid-state heterojunction by means of picosecond transient absorption laser spectroscopy.« less

Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga)Se2 films and devices

Abstract: Post-deposition air-annealing effects of Cu(In,Ga)Se2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 °C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells shows that the Cu(In,Ga)Se2 surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 °C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent wi...

Evaluation of the temperature stability of AlGaN/GaN heterostructure FETs

Abstract: Temperature stress experiments up to 800/spl deg/C have been applied to AlGaN/GaN FET's grown by MOVPE on sapphire and their individual technological building blocks. It was found that the temperature limit is given by the irreversible degradation of the intrinsic active heterostructure material itself during operation above 600/spl deg/C. The irreversible degradation was observed for both unconnected and electrically operated devices during temperature stress.

Metalorganic chemical vapor deposition of high mobility AlGaN/GaN heterostructures

Abstract: In this article, we discuss parameters influencing (a) the properties of thin AlxGa1−xN layers grown by metalorganic chemical vapor deposition and (b) the electrical properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1−xN/GaN heterojunction. For xAl>0.3, the AlxGa1−xN layers showed a strong tendency towards defect formation and transition into an island growth mode. Atomically smooth, coherently strained AlxGa1−xN layers were obtained under conditions that ensured a high surface mobility of adsorbed metal species during growth. The electron mobility of the 2DEG formed at the AlxGa1−xN/GaN interface strongly decreased with increasing aluminum mole fraction in the AlxGa1−xN layer and increasing interface roughness, as evaluated by atomic force microscopy of the surfaces prior to AlxGa1−xN deposition. In the case of modulation doped structures (GaN/AlxGa1−xN/AlxGa1−xN:Si/AlxGa1−xN), the electron mobility decreased with decreasing thickness of the undoped spacer layer and increasing sil...

Dispersion of polar optical phonons in wurtzite quantum wells

Abstract: Dispersion relations for polar optical phonon modes in wurtzite quantum wells (QW's) are obtained in the framework of the dielectric continuum model. It is found that anisotropy of the dielectric medium causes a number of qualitative peculiarities in the phonon spectra. Among these are the absence of the proper confinement for the oscillatory waves located in the QW, inversion of the order of symmetric and antisymmetric quasiconfined optical modes, formation of the finite energy intervals where such confined modes---which are found to be dispersive---can exist, penetration of the half-space phonons into the QW, etc. Some additional peculiarities, such as appearance of propagating modes, strong dispersion of long-wavelength half-space modes, and reduction of the number of interface modes, arise as a result of overlapping characteristic phonon frequencies of the surrounding material and the material of QW. Predicted phonon behavior leads to the conclusion that dependence of dielectric properties of ternary-binary low-dimensional wurtzite heterostructures on composition can serve as a powerful tool for the purposes of phonon spectrum engineering. In order to illustrate these results, the optical phonon spectra are calculated for an ${\mathrm{Al}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}/\mathrm{GaN}/{\mathrm{Al}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N} \mathrm{QW},$ an $\mathrm{AlN}/\mathrm{GaN}/\mathrm{AlN} \mathrm{QW},$ and for a $\mathrm{GaN}$ dielectric slab.

Semi-insulating C-doped GaN and high-mobility AlGaN/GaN heterostructures grown by ammonia molecular beam epitaxy

Abstract: A method of growing semi-insulating GaN epilayers by ammonia molecular beam epitaxy through intentional doping with carbon is reported. Thick GaN layers of high resistivity are an important element in GaN-based heterostructure field-effect transistors. A methane ion source was used as the carbon dopant source. The cracking of the methane gas by the ion source was found to be the key to the effective incorporation of carbon. High-quality C-doped GaN layers with resistivities greater than 106 Ω cm have been grown with high reproducibility and reliability. AlGaN/GaN heterostructures grown on the C-doped semi-insulating GaN-based layers exhibited a high-mobility two-dimensional electron gas at the heterointerface, with room-temperature mobilities typically between 1000 and 1200 cm2/V s, and liquid-nitrogen-temperature mobilities up to 5660 cm2/V s. The carrier density was almost constant, with less than 3% change over the measured temperature range.

InGaAsN/GaAs heterojunction for multi-junction solar cells

Abstract: An InGaAsN/GaAs semiconductor p-n heterojunction is disclosed for use in forming a 0.95-1.2 eV bandgap photodetector with application for use in high-efficiency multi-junction solar cells. The InGaAsN/GaAs p-n heterojunction is formed by epitaxially growing on a gallium arsenide (GaAs) or germanium (Ge) substrate an n-type indium gallium arsenide nitride (InGaAsN) layer having a semiconductor alloy composition In x Ga 1−x As 1−y N y with 0 70%.

Fullerene-structured nanowires of silicon

Abstract: Silicon vapor from a magnetron sputter source was deposited onto highly oriented pyrolytic graphite, resulting in the formation of nanoscale wires. The structures were analyzed by scanning tunneling microscopy. The wires are from 3 to 7 nm in diameter and at least 100 nm long. They tend to be assembled parallel in bundles. In order to understand the observed quasi-one-dimensional structures, diamondlike and fullerenelike wire models are constructed. Molecular-orbit calculations yield binding energies and band gaps of such structures, and lead us to propose a fullerene-type ${\mathrm{Si}}_{24}$-based atomic configuration for nanowires of silicon.

Role of Spontaneous and Piezoelectric Polarization Induced Effects in Group-III Nitride Based Heterostructures and Devices

Abstract: The wurzite group-III nitrides InN, GaN, and AlN are tetrahedrally coordinated direct band gap semiconductors having a hexagonal Bravais lattice with four atoms per unit cell. As a consequence of the noncentrosymmetry of the wurzite structure and the large ionicity factor of the covalent metal–nitrogen bond, a large spontaneous polarization oriented along the hexagonal c-axis is predicted. In addition, group-III nitrides are highly piezoelectric. The strain induced piezoelectric as well as the spontaneous polarizations are expected to be present and to govern the optical and electrical properties of GaN based heterostructures to a certain extent, due to the huge polarization constants which are one of the most fascinating aspects of the nitrides. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in AlGaN/GaN, InGaN/GaN and AlInN/GaN heterostructures lead to the formation of two-dimensional carrier gases suitable for the fabrication of high power microwave frequency transistors.

Thin Films: Heteroepitaxial Systems

Abstract: Atomic interactions and surface processes in heteroepitaxy ordering in III-V semiconductor alloys transition metal thin film epitaxy the growth and structure of epitaxial metal-oxide/metal interfaces heteroepitaxy of disparate materials - from chemisorption to epitaxy in CaF2/Si (111) recent progress in high Tc superconducting heteostructures strain accommodation and relief in GeSi/Si heteroepitaxy heteroepitaxial growth modes and morphologies on GaAs surfaces molecular beam epitaxy of Sb-based semiconductors P-based semiconductor multilayers MBE growth of wide-gap refractory nitride epitaxial films epitaxial growth, microstructural characterizations, and optical physics of wide bandgap II-VI heterostructures molecular beam epitaxy of narrowgap IV-VI semiconductors

Time-resolved photoluminescence as a probe of internal electric fields in GaN-(GaAl)N quantum wells

Abstract: Very strong coefficients for spontaneous and piezoelectric polarizations have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry. Such polarizations influence significantly the mechanisms of radiative emissions in quantum-confinement heterostructures based on these materials. The photoluminescence decay time of excitons is used as a probe of internal electric fields in GaN-(Ga, Al)N quantum wells in various configurations of strain, well widths, and barrier widths. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN wells but also on the nonradiative escape of carriers through Ga1 - xAlyN barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the held in the wells is not a simple function of the strain of these layers via the only piezoelectric effect, but rather the result of the interplay of spontaneous and piezoelectric polarizations in both well and barrier materials. [S0163-1829(99)02923-9].

Tunneling-enhanced recombination in Cu(In, Ga)Se2 heterojunction solar cells

Abstract: This letter presents an analytical model for tunneling-enhanced recombination current in the space charge region of semiconductor junctions. We investigate current–voltage characteristics of different types of Cu(In, Ga)Se2-based heterojunction solar cells in a temperature range from 100 to 340 K. The temperature dependence of the saturation current and of the diode ideality factor of these devices are well described by the closed form expressions derived by the present approach.

Thermal stability of supersaturated MgxZn1−xO alloy films and MgxZn1−xO/ZnO heterointerfaces

Abstract: We have examined the thermal stability of wurtzite-phase MgxZn1−xO alloy films and ZnO/MgxZn1−xO bilayer films with x exceeding the reported solubility limit of 0.04. When a Mg0.23Zn0.78O film was annealed, the segregation of MgO started at 850 °C and the band gap was reduced to the value of that for an x=0.15 film after annealing at 1000 °C. Mg0.15Zn0.85O films showed no change of the band gap even after annealing at 1000 °C. Therefore, we conclude that the thermodynamic solubility limit of MgO in MgxZn1−xO epitaxial film is about x=0.15. The thermal diffusion of Mg across the MgxZn1−xO/ZnO interface was observed only after annealing above 700 °C. Unlike other II–VI semiconductors, ZnO-based alloy films and heterointerfaces are stable enough for the fabrication of high-crystallinity heterostructures.

Observation of intermixing at the buried CdS/Cu(In, Ga)Se2 thin film solar cell heterojunction

Abstract: A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.

Superlattices of Metal and Metal−Semiconductor Quantum Dots Obtained by Layer-by-Layer Deposition of Nanoparticle Arrays

Abstract: Superlattices formed by arrays of Pt or Au nanoparticles have been obtained by layer-by-layer deposition by using dithiols as cross-linkers. The superlattices have been characterized by X-ray diffraction, photoelectron spectroscopy, and scanning tunneling microscopy. The core-level intensities of the metal and of the dithiol in the X-ray photoelectron spectra show the expected increase with successive depositions. The formation of such structures has been confirmed by depositing Pt and Au layers alternatively. Layers of metal and CdS nanoparticles have been deposited alternatively to obtain heterostructures.

A GaN/4H-SiC heterojunction bipolar transistor with operation up to 300°C

Abstract: We report on the fabrication and characterization of GaN/4H-SiC n-p-n heterojunction bipolar transistors (HBTs). The device structure consists of an n-SiC collector, p-SiC base, and selectively grown n-GaN emitter. The HBTs were grown using metalorganic chemical vapor deposition on SiC substrates. Selective GaN growth through a SiO2 mask was used to avoid damage that would be caused by reactive ion etching. In this report, we demonstrate common base transistor operation with a modest dc current gain of 15 at room temperature and 3 at 300°C.

Structural and electronic properties of composite BxCyNz nanotubes and heterojunctions

Abstract: The structure, stability and electronic properties of composite BxCyNz nanotubes and related heterojunctions have been studied using both ab initio and semi-empirical approaches. Pure BN nanotubes present a very stable quasi particle band gap around 5.5-6.0 eV independent of the tube radius and helicity. The bottom of the conduction bands is controlled by a nearly-free-electronn state localized inside the nanotube, suggesting interesting properties under doping. In the case of nanotubes with BC2N stoichiometry, we show that in the thermodynamic limit the system is driven towards segregation of pure C and BN sections. This demixing significantly affects the electronic properties of such materials. The same process of segregation into BC3 islands is evidenced in the case of B-doped carbon nanotubes. These spontaneous segregation processes lead to the formation of quantum dots or nanotube heterojunctions. In particular, C/BN superlattices or isolated junctions have been investigated as specific examples of the wide variety of electronic devices that can be realized using such nanotubes.