Showing papers on "Band offset published in 2003"

Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures

Abstract: Type-II band engineered quantum dots (CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures) are described. The optical properties of these type-II quantum dots are studied in parallel with their type-I counterparts. We demonstrate that the spatial distribution of carriers can be controlled within the type-II quantum dots, which makes their properties strongly governed by the band offset of the comprising materials. This allows access to optical transition energies that are not restricted to band gap energies. The type-II quantum dots reported here can emit at lower energies than the band gaps of comprising materials. The type-II emission can be tailored by the shell thickness as well as the core size. The enhanced control over carrier distribution afforded by these type-II materials may prove useful for many applications, such as photovoltaics and photoconduction devices.

Diluted II-VI Oxide Semiconductors with Multiple Band Gaps

Abstract: We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.

Atomic and electronic structure of the Si/SrTiO3 interface

Abstract: We predict theoretically the wetting conditions for the layer-by-layer growth of thin ${\mathrm{SrTiO}}_{3}$ (STO) films on Si. The result is in agreement with our recent experiments. The state of the art band offset calculations identify two different possibilities for the band alignment at the Si-STO interface. A very small conduction band offset is predicted for the structure without the chemically induced localization at the interface, and a 0.9 eV conduction band offset is predicted for the structure with chemically induced localized interface states.

Band offset measurements of the Si3N4/GaN (0001) interface

Abstract: X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were used to measure electronic states as Si3N4 was deposited on clean GaN (0001) surfaces. The n-type (2×1018) and p-type (1×1017) GaN surfaces were atomically cleaned in NH3 at 860 °C, and the n-and p-type surfaces showed upward band bending of ∼0.2±0.1 eV and downward band bending of 1.1±0.1 eV, respectively, both with an electron affinity of 3.1±0.1 eV. Layers of Si (∼0.2 nm) were deposited on the clean GaN and nitrided using an electron cyclotron resonance N2 plasma at 300 °C and subsequently annealed at 650 °C for densification into a Si3N4 film. Surface analysis was performed after each step in the process, and yielded a valence band offset of 0.5±0.1 eV. Both interfaces exhibited type II band alignment where the valence band maximum of GaN lies below that of the Si3N4 valence band. The conduction band offset was deduced to be 2.4±0.1 eV, and a change of the interface dipole of 1.1±0.1 eV was observed for Si3N4/GaN interfac...

Band offset measurements of the GaN (0001)/HfO2 interface

Abstract: Photoemission spectroscopy has been used to observe the interface electronic states as HfO2 was deposited on clean n-type Ga-face GaN (0001) surfaces. The HfO2 was formed by repeated deposition of several monolayers of Hf followed by remote plasma oxidation at 300 °C, and a 650 °C densification anneal. The 650 °C anneal resulted in a 0.6 and 0.4 eV change in band bending and valence band offset, respectively. The final annealed GaN/HfO2 interface exhibited a valence band offset of 0.3 eV and a conduction band offset of 2.1 eV. A 2.0 eV deviation was found from the electron affinity band offset model.

Band offset induced threshold variation in strained-Si nMOSFETs

Abstract: Due to the offset in the valence band, strained-Si nMOSFETs exhibit a -100 mV threshold shift and 4% degradation of the subthreshold slope per each 10% increase of Ge content in the relaxed SiGe layer. The correlation between the threshold shift and strained layer thickness is investigated based on device simulations. In a certain range of the strained-Si layer thickness, the threshold and subthreshold slope change gradually, posing a concern of larger device parameter variation. A larger threshold distribution is observed in devices fabricated with a strained layer thickness comparable to the depletion depth.

Band offset determination of Zn0.53Cd0.47Se/Zn0.29Cd0.24Mg0.47Se

Abstract: The interband transitions of a single quantum well structure of Zn0.53Cd0.47Se/Zn0.27Cd0.23Mg0.50Se (lattice matched to InP) were evaluated using contactless electroreflectance at room temperature. From a comparison of the measured optical transitions with those calculated using the envelope function approximation we determined that the conduction band offset for this system is given by the parameter Qc=ΔEc/ΔE0=0.82±0.02, which yields ΔEc of 590 meV. Such a large conduction band offset may be useful for the design of quantum cascade lasers and other devices based on intersubband transitions.

Explanation of Light/Dark Superposition Failure in CIGS Solar Cells

Abstract: CIGS solar cells in many cases show a failure of light/dark superposition of their current-voltage (J-V) curves. Such failure generally becomes more pronounced at lower temperatures. J-V measurements under red light may also show an additional distortion, known historically as the “red kink”. The proposed explanation is that a secondary barrier results from the conduction band offset between CIGS and the commonly employed CdS window layer. This barrier produces a second diode with the same polarity and in series with the primary photodiode. The secondary-diode barrier height is modified by photoinduced changes of trap occupancy in the CdS layer, hence creating a voltage shift between dark and light conditions. Numerical modeling of the proposed explanation, including a band offset consistent with experimental and theoretical values, gives a very good fit to measured light and dark J-V curves over a wide temperature range. It also predicts the observed difference between illuminated J-V curves with photon energy above the CdS band gap, and those with sub-band-gap illumination.

C–V characterization of strained Si/SiGe multiple heterojunction capacitors as a tool for heterojunction MOSFET channel design

Abstract: Capacitance–voltage (C–V) characteristics are used to investigate double heterojunction strained Si/SiGe MOS capacitors. Structures of this type potentially form the channels of CMOS devices based on the strained Si/SiGe material system. The technique represents a fast and non-destructive method to determine important characteristics such as layer thicknesses, threshold voltages and band offsets. Moreover, it contributes to the design of optimum heterostructures for CMOS. Experimental C–V data are compared with simulation and complementary results including SIMS and TEM to confirm the accuracy of the technique.

Bandgap states in transition-metal (Sc, Y, Zr, and Nb)-doped Al2O3

Abstract: We have investigated the electronic structure of transition-metal (TM=Sc, Y, Zr, and Nb)-doped Al2O3 by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) In valence bands of these TM-doped Al2O3 measured by XPS, the highest occupied levels and the shapes of valence bands are almost unchanged from the pure alumina On the other hand, XAS spectra obtained at the oxygen K-edge show that Nb- and Zr-doped Al2O3 show localized d states in the bandgap below the conduction band minimum, while Y- and Sc-doped Al2O3 have d states inside the conduction band of the original Al2O3 This implies that Y- and Sc-doped Al2O3 will show little bandgap degradation and maintain the same band offset as silica at the Si interface, and can serve as promising candidates for an alternative gate oxide

Valence band alignment with a small spike at the CuI/CuInS2 interface

Abstract: From the point of view of the “doping pinning rule,” application of p-type buffer layer materials for CuInS2 solar cells may lead to record levels of cell efficiency due to an optimal band offset at the interface. Under simplified simulation conditions, an increase in efficiency of up to about 18% was predicted. Evaluation of the valence band offset between CuI and single crystalline CuInS2 by photoelectron spectroscopy displays a spike height of (0.1±0.2) eV, optimal for solar cell applications. The positions of the valence band edge were determined by assuming a parabolic distribution of the density of states.

Bonding and band offset in N2O-grown oxynitride

Abstract: Using high-resolution angle-resolved x-ray photoelectron spectroscopy (ARXPS) measurements, the chemical bonding, and valance-band offset of ultrathin (16 and 24 A) N2O-grown oxide were studied. We confirmed that the composition of N2O-grown oxide is mainly silicon oxide with both the concentration and band offset values measured using ARXPS. The surface density of nitrogen is about (3±1)×1014 cm−2 near the Si/dielectric interface. The valence- and conduction-band offsets for N2O-grown oxide are the same as those for the Si/SiO2 interface because the nitrogen content is too low to have any pronounced effects. In addition, we found that most of the nitrogen atoms at the interface appeared in the form of Si–N bonding instead of N–O bonding.

Electronic properties of interfaces between perylene derivatives and GaAs(001) surfaces : Organic-inorganic semiconductor interfaces

Abstract: The adsorption of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMe-PTCDI) on differently treated n-doped GaAs(100) surfaces was investigated using high-resolution photoemission spectroscopy. The chemical interaction between the molecules and the semiconductor substrate is found to be weak; core level photoemission spectra show no additional chemically shifted peaks, indicating the absence of any covalent/ionic bond formation. Only a sharpening of the core level spectra is observed for a coverage lower than one monolayer and this is attributed to a reduction of inhomogeneous band bending at the surface. This is interpreted in terms of preferential sticking of the organic molecules to surface defects. The energy offset between the occupied states in the substrate and the organic film is directly derived from ultraviolet photoemission spectroscopy measurements. Interface dipoles are found to form according to the electron affinities of the substrates and PTCDA films at the interfaces and, consequently, the vacuum level alignment rule does not hold. For vanishing interface dipole the lowest unoccupied molecular orbital of PTCDA is found to align with the conduction band minimum of GaAs resulting in electron affinity of 4.12 eV for PTCDA. This provides an energy gap in the range of 2.44-2.55 eV, which is larger than the onset of optical absorption. The same procedure is applied to DiMe-PTCDI layers.

Copper phthalocyanine on InSb(111)A: interface bonding, growth mode and energy band alignment : Organic-inorganic semiconductor interfaces

Abstract: The growth of the organic semiconductor CuPc on the InSb(111)A surface at 300 K has been studied using photoelectron spectroscopy. Core level emission data obtained using low energy synchrotron radiation reveal that the interface is abrupt with very weak bonding between the InSb surface atoms and the adsorbed molecules. The coverage dependence of the substrate and overlayer core level peak intensities follows the prediction of a uniform growth mode at high growth rates, but the organic film follows a Stranski-Krastanov growth mode at lower growth rates. C 1s and N Is photoelectron emission data obtained with Mg Kα radiation confirm that the CuPc molecules are intact within the layer, and shake-up satellites associated with benzene and pyrrole C and N peaks provide an insight into the energy and spatial distribution of the highest occupied and lowest unoccupied molecular orbitals. Photoelectron emission from the occupied bonding states of the CuPc and the valence band states of InSb provides the band offset for the filled states and the overall energy band profile for this organic-inorganic heterojunction. The presence of an interface dipole at the interface disproves a simple band alignment based on the vacuum level; the energy bands have a nested arrangement where both band edges in the InSb lie within the HOMO-LUMO gap of the CuPc.

Effects of covalency, p-d coupling, and epitaxial strain on the band offsets of II-VI semiconductors

Abstract: Using the first-principles all-electrons method, we have systematically studied the natural band offsets among zinc blende BeX, MgX, and ZnX (X= S, Se, Te). We show that ZnX, which has large anion p-cation d repulsion, always has higher natural valence band maximum (VBM) than BeX and MgX, whereas BeX, which shows strong covalency, has higher natural VBM than MgX due to kinetic-energy-induced valence band broadening. However, epitaxial strain could reverse these trends. We found that for these isovalent semiconductors, the band offset is not sensitive to interface atomic compositions.

Optimization of type-II heterostructures for the tuning region in tunable laser diodes

Abstract: Type-II superlattices are very attractive as tuning regions in tunable laser diodes exploiting the free-carrier plasma effect. Due to the confining of the electrons and holes in different layers, the recombination of the electron–hole pairs is drastically suppressed. As a result, the carrier density, and hence the tuning efficiency for type-II superlattices, is enhanced by more than an order of magnitude in the small current regime as compared to bulk tuning regions. However, with increasing carrier density the recombination rate increases because of the stronger penetration of the electron and hole wavefunctions into the corresponding barriers. For high carrier densities, the capability of type-II superlattices depends critically on design parameters such as as layer thickness, band offset and strain. Our calculations show that, for an optimized type-II superlattice, the tuning range can be significantly enhanced by more than a factor of 3 for a given current.

3.3 /spl mu/m 'W' quantum well light emitting diode

Abstract: Recent studies suggest that the radiative conversion efficiency of mid-infrared semiconductor devices is limited by non-radiative Auger mechanisms Band structure engineering techniques, such as the introduction of strain or the use of type-II band offset materials, have been shown to reduce the effect of Auger recombination Results from light emitting diodes (LEDs) with an active region consisting of ten InAs/GaInSb/InAs/AlGaAsSb type-II ‘W’ quantum wells grown by molecular beam epitaxy (MBE) on GaSb substrates are described At room temperature, the device was characterised by a slope efficiency of 98 µW/A at low currents, which dropped at higher currents due to heating This corresponded to an internal efficiency of approximately 26%

Strain effects in the common-cation II–VI heterostructures: case of ZnS/ZnSe superlattices

Abstract: The electronic band-structures of the strained-layer ZnS/ZnSe (001) superlattices (SLs) have been investigated using the sp3s* tight-binding method, which includes the strain and spin–orbit effects. The SL band-structures are studied versus the biaxial strain, layer thickness, and band offsets. The results suggest that the common-cation II–VI heterojunction exhibit a vanishingly small conduction-band offset (CBO). It is shown that the SL valence-band top state is always a heavy-hole localized within ZnSe slabs; whereas the conduction-band edge state (electron) is sensitive to the biaxial strain (or VBO). To assess the strain effects, we considered three differently strained SLs corresponding to the three substrates: (i) ZnSe; (ii) ZnS0.5Se0.5; and (iii) ZnS. The results show that all the studied SLs are of type-I except those strained to ZnS (case iii), that may exhibit type-I to type-II transition. One striking result obtained here is the existence of a critical VBO (Vc0.76 eV) that predicts such transition, and particularly the fact that this value is independent of the strain state (substrate) (i.e. all SLs whose VBO is smaller than Vc are of type-I, else are of type-II). The comparison of our theoretical results to the photoluminescence experiments yields valuable information about the strain morphology as well as the structural and optical qualities of the experimental samples.

Heterojunction semiconductor device having an intermediate layer for providing an improved junction

Abstract: A heterojunction for a semiconductor device. The heterojunction has a first region (116) formed from a first semiconductor material having a first conductivity type, a second region (108) formed from a second semiconductor material having a second conductivity type, and an intermediate layer (110) between the first region (116) and the second region (108). The band line-up of the first region (116), the intermediate layer (110), and the second region (108) has no bound states in its conduction band (152) and no bound states in its valence band (154). The intermediate layer (110) has a thickness small enough to allow electrons to tunnel from the first region (116) to the second region (108) with negligible attenuation. The semiconductor device may be a heterojunction bipolar transistor. The conduction band (152) of the intermediate layer (110) has a higher energy level than the conduction bands (152) of the first and second regions (116, 108).

Models of GaSb/InAs type-II infrared detectors at very long wavelengths: band offsets and interface bonds

Abstract: We report a series of studies on GaSb/InAs superlattices, pseudomorphically strained to GaSb buffer layers. These heterostructures have recently been grown using molecular beam epitaxy for very long wavelength infrared photodetectors. We calculated the valence band alignment with the widely used model solid theory and evaluated the electronic band structure by employing an empirical pseudopotential (EP) scheme. The absorption coefficient was subsequently calculated at the far-infrared range of the spectrum, using density matrix theory. This approach predicted optical cut-off wavelengths, showing poor agreement with values obtained from absolute spectral responsivity measurements. Variation of the bond types at the interfaces (InSb- or GaAs-like, or a combination) led to significant changes in the cut-offs and absorption magnitude. However, no combination of interface bonds gave rise to results which were consistent with the experimental cut-offs. Addressing the band offset, we employed a recently published, interface-specific model as an alternative to the model solid theory-derived value. Including this model in our EP scheme, we obtained good agreement with experiment for a superlattice containing an InSb-like bond at each heterojunction, the configuration which had been fabricated in the photodetectors.