Showing papers on "Band offset published in 2007"

Band‐Offset Engineering for Enhanced Open‐Circuit Voltage in Polymer–Oxide Hybrid Solar Cells

Abstract: The power conversion efficiency of organic and hybrid solar cells is commonly reduced by a low open-circuit voltage (VOC). In these cases, the VOC is significantly less than the energy of the lowest energy absorbed photon, divided by the elementary charge q. The low photovoltage originates from characteristically large band offsets between the electron donor and acceptor species. Here a simple method is reported to systematically tune the band offset in a π-conjugated polymer–metal oxide hybrid donor–acceptor system in order to maximize the VOC. It is demonstrated that substitution of magnesium into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the VOC of poly(3-hexylthiophene) (P3HT)–ZnMgO planar devices. The VOC is seen to increase from 500 mV at x = 0 up to values in excess of 900 mV for x = 0.35. A concomitant increase in overall device efficiency is seen as x is increased from 0 to 0.25, with a maximum power-conversion efficiency of 0.5 % obtained at x = 0.25, beyond which the efficiency decreases because of increased series resistance in the device. This work provides a new tool for understanding the role of the donor–acceptor band offset in hybrid photovoltaics and for maximizing the photovoltage and power-conversion efficiency in such devices.

Valence band offset of InN∕AlN heterojunctions measured by x-ray photoelectron spectroscopy

Abstract: The valence band offset of wurtzite-InN∕AlN (0001) heterojunctions is determined by x-ray photoelectron spectroscopy to be 1.52±0.17eV. Together with the resulting conduction band offset of 4.0±0.2eV, a type-I heterojunction forms between InN and AlN in the straddling arrangement.

Recent Progress on 1.55- $\mu{\hbox {m}}$ Dilute-Nitride Lasers

Abstract: We review the recent developments in GaAs-based 1.55-mum lasers grown by molecular beam epitaxy (MBE). While materials growth is challenging, the growth window appears to be relatively broad and is described in detail. The key considerations for producing high-quality GalnNAsSb material emitting at 1.55-mum regime are examined, including the nitrogen plasma conditions, ion removal from the nitrogen flux, surfactant- mediated growth, the roles of various V-II ratios, the growth temperature, the active region thermal budget, proper annealing, and composition. We find that emission may be tuned throughout the 1.55-mum communications band without penalty to the optical quality varying only one parameter - the total growth rate. This powerful result is validated by the demonstration of low-threshold edge-emitting lasers throughout the 1.55-mum regime, including threshold current densities as low as 318 A/cm2 at 1.54 mum. Additional characterization by Z-parameter techniques, cavity length studies, and band offset measurements were performed to better understand the temperature stability of device performance. Lasing was extended as far as 1.63 mum under nonoptimized growth conditions. The GaAs-based dilute-nitrides are emerging as a very promising alternative to InP-based materials at 1.55-mum due to their high gain, greater range of achievable band offsets, as well as the availability of lattice-matched AlAs-GaAs materials and native oxide layers for vertical-cavity surface-emitting lasers (VCSELs). Indeed, this effort has enabled the first electrically injected C-band VCSEL on GaAs.

PbSe Nanocrystal/TiOx Heterostructured Films: A Simple Route to Nanoscale Heterointerfaces and Photocatalysis

Abstract: A simple approach to heterostructured thin film catalysts consisting of PbSe nanocrystals and layer-by-layer deposited TiOx is presented. Strong quantum confinement raises the conduction band of PbSe nanocrystals above that of TiOx, leading to a type II band offset. Photogenerated electrons in PbSe nanocrystals are transferred to catalytic TiOx which in turn initiate catalysis. Photocatalytic activity of the heterostructured films is found to be dependent on the size of the PbSe nanocrystals with the onset of catalysis coinciding at a photon energy of ∼2.5 times the band gap of the nanocrystals. Photocatalysis in the visible spectral region out to 650 nm is demonstrated.

Conduction band offset at the InN∕GaN heterojunction

Abstract: The conduction-band offset between GaN and InN is experimentally determined. InN∕n-type GaN isotype heterojunctions grown by molecular beam epitaxy are observed to exhibit Schottky-junction like behavior based on rectifying vertical current flow. From capacitance-voltage measurements on the heterojunction, the Schottky barrier height is found to be ∼0.94eV. The photocurrent spectroscopy measurement by backside illumination reveals an energy barrier height of 0.95eV across the heterojunction, consistent with the capacitance measurement. By combining electrical transport, capacitance-voltage, and photocurrent spectroscopy measurement results, the conduction band offset between InN and GaN is estimated to be ΔEC=1.68±0.1eV.

The electronic structure change with Gd doping of HfO2 on silicon

Abstract: Gd-doped HfO2 films deposited on silicon substrates undergo a crystallographic change from monoclinic to fluorite (cubic) phase with increasing Gd concentrations. The crystallographic phase change is accompanied by a small increase in the valence bandwidth and in the apparent band offset in the surface region. Electrical measurements show pronounced rectification properties for lightly doped Gd:HfO2 films on p-Si and for heavily-doped Gd:HfO2 films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level.

Improved High-Temperature Leakage in High-Density MIM Capacitors by Using a TiLaO Dielectric and an Ir Electrode

Abstract: We have fabricated high-kappa TaN/Ir/TiLaO/TaN metal-insulator-metal capacitors. A low leakage current of 6.6 times 10-7 A/cm2 was obtained at 125degC for 24-fF/mum2 density capacitors. The excellent device performance is due to the combined effects of the high-kappa TiLaO dielectric, a high work-function Ir electrode, and large conduction band offset.

Electronic structure and band alignment at the HfO2∕4H-SiC interface

Abstract: To evaluate the potential of HfO2 as a gate dielectric in SiC power metal-oxide-semiconductor field effect transistors (MOSFETs), the band alignment at the HfO2∕4H-SiC interface was determined by x-ray photoelectron spectroscopy (XPS) measurements and first-principles calculations using density functional theory (DFT) For XPS studies, HfO2 films were grown on 4H-SiC (0001) by a thermal atomic layer deposition process A valence band offset of 174eV and a conduction band offset of 070eV were determined based on the valence band and core-level spectra DFT simulations of the Si-terminated 4H-SiC (0001) surface found a 1×1 relaxed structure whereas simulations of the C-terminated surface observed a 2×1 reconstruction to form C–C dimers We studied two m-HfO2∕4H-SiC (0001) supercells based on these surfaces and valence band offset values of 209 and 147eV, and conduction band offset values of 035 and 097eV, respectively, were predicted The consistently low conduction band offset may not provide an adeq

Determination of the valence band offset of wurtzite InN /ZnO heterojunction by x-ray photoelectron spectroscopy

Abstract: The valence band offset (VBO) of the wurtzite InN/ZnO heterojunction is directly determined by x-ray photoelectron spectroscopy to be 0.82 +/- 0.23 eV. The conduction band offset is deduced from the known VBO value to be 1.85 -/+ 0.23 eV, which indicates a type-I band alignment for InN/ZnO heterojunction. (C) 2007 American Institute of Physics.

Temperature-dependent interfacial chemical bonding states and band alignment of HfOxNy∕SiO2∕Si gate stacks

Abstract: Temperature-dependent interfacial chemical bonding states and band alignment of HfOxNy∕SiO2∕Si gate stacks have been investigated by X-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry (SE). By means of the chemical shifts of Hf 4f, Si 2p, O 1s, and N 1s core-level spectra, it has been found that the chemical stability of the HfOxNy∕SiO2∕Si stacks strongly depends on the annealing temperature. Analysis of temperature-dependent band alignment of HfOxNy∕SiO2∕Si stacks suggests that the valence band offset ΔEv increases slowly from 1.82eV for as-grown film to 2.55eV for annealed film at 700°C; however, the values of conduction band offset ΔEc only demonstrates a slight change in the vicinity of 1.50eV. From the band offset viewpoint, HfOxNy∕SiO2∕Si gate stack could be a promising candidate for high-k gate dielectrics.

Engineering carrier transport across organic heterojunctions by interface doping

Abstract: Two common hole transporting materials, 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (MTDATA) and N,N′-diphenyl-N,N′-bis(1-naphthyl) (1,1′-biphenyl)-4,4′diamine (NPB), and a p-type dopant tetrafluorotetracyanoquinodimethane (F4-TCNQ) were used to build various hole-only heterojunction devices. Both experimental results and theoretical modeling show that the current flow in such devices is limited by the heterojunction potential barrier or band offset at the MTDATA/NPB interface. It was found that the device current flow can be modulated to increase or decrease dramatically by introducing a 2nm p-doped NPB:F4-TCNQ or MTDATA:F4-TCNQ interlayer, respectively. The observed phenomena were discussed using quasi-Fermi energy level realignment at the doped/undoped organic-organic interface.

Novel SONOS-Type Nonvolatile Memory Device with Suitable Band Offset in HfAlO Charge-Trapping Layer

Abstract: Satisfactory operation and reliability characteristics of SONOS-type flash devices are achieved by an optimal Hf/Al content in HfAlO charge-trapping layer. Results indicate that operation performance can be improved by a suitable band offset of HfAlO charge-trapping layer. High-speed operation can be realized by adopting CHEI programming and F-N erasing for NOR flash applications.

HfTiAlO dielectric as an alternative high-k gate dielectric for the next generation of complementary metal-oxide-semiconductor devices

Abstract: In this letter, the authors report on the material and electrical characterizations of high dielectric constant (k) oxide HfTiAlO for the next generation of complementary metal-oxide semiconductors. Crystallization temperature has been improved to 800–900°C versus that of HfO2. The substitution of Ti and Al in the HfO2 cubic structure results in an increased dielectric constant and an acceptable barrier height. The extracted dielectric constant is 36, and the band offset relative to the Si conduction band is 1.3eV. An equivalent oxide thickness of 11A and low leakage have been achieved with good interfacial properties.

Optical investigation methods for SiC device development: application to stacking faults diagnostic in active epitaxial layers

Abstract: We present a review of the different optical techniques that can be used to investigate the presence of as- grown and/ or process- induced stacking faults ( SFs) in 4H - SiC epitaxial layers A SF is always a finite admixture of different polytypes, and we begin with a brief review of the systematic of SiC polytype structure and electronic properties Next, we discuss the optical signature and compare with the results of several model calculations, taking successively into account the effect of valence band offset, internal polarization and non-homogeneity of the potential well Finally, we consider cathodo-luminescence and micro- photoluminescence techniques and show that, in both cases, some screening of the built- in electric field can be achieved

X-ray photoemission spectroscopy determination of the InN/yttria stabilized cubic-zirconia valence band offset

Abstract: The valence band offset of wurtzite InN(0001)/yttria stabilized cubic-zirconia (YSZ)(111) heterojunctions is determined by x-ray photoemission spectroscopy to be 1.19±0.17eV giving a conduction band offset of 3.06±0.20eV. Consequently, a type-I heterojunction forms between InN and YSZ in the straddling arrangement. The low lattice mismatch and high band offsets suggest potential for use of YSZ as a gate dielectric in high-frequency InN-based electronic devices.

Discrete monolayer light emission from GaSb wetting layer in GaAs

Abstract: Distinct light emission peaks from monolayers of GaSb quantum wells in GaAs were observed. Discrete atomic layers of GaSb for the wetting layer prior to quantum dot formation give rise to transition peaks corresponding to quantum wells with 1, 2, and 3 ML. From the transition energies the authors were able to deduce the band offset parameter between GaSb and GaAs. By fitting the experimental data with the theoretical calculated result using an 8×8 k∙p Burt’s Hamiltonian along with the Bir-Picus deformation potentials, the strain-free (fully strained) valence band discontinuity for this type-II heterojunction was determined to be 0.45eV (0.66eV).

Band edge discontinuities and carrier transport in c-Si/porous silicon heterojunctions

Abstract: We have prepared light emitting nanocrystallline porous silicon (PS) layers by electrochemical anodization of crystalline silicon (c-Si) wafer and characterized the c-Si/PS heterojunctions using temperature dependence of dark current–voltage (I–V) characteristics. The reverse bias I–V characteristics of c-Si/PS heterojunctions are found to behave like the Schottky junctions where carrier transport is mainly governed by the carrier generation-recombination in the depletion region formed on the PS side. Fermi level of c-Si gets pinned to the defect levels at the interface resulting in ln(I) ∝ V1/2. The barrier height in the reverse bias condition is shown to be equal to the band offset at the conduction band edges. An energy band diagram for the c-Si/PS heterojunction is proposed.

Band edge discontinuities and carrier transport in c-Si/porous silicon heterojunctions

Abstract: We have prepared light emitting nanocrystalline porous silicon (PS) layers by electrochemical anodization of crystalline silicon (c-Si) wafer and characterized the c-Si/PS heterojunctions using temperature dependence of dark current-voltage (I-V) characteristics. The reverse bias I-V characteristics of c-Si/PS heterojunctions are found to behave like Schottky junctions where carrier transport is mainly governed by the carrier generation-recombination in the depletion region formed on the PS side. Fermi level of c-Si gets pinned to the defect levels at the interface resulting in ln(I) proportional to V^0.5. The barrier height in the reverse bias condition is shown to be equal to the band offset at the conduction band edges. An energy band diagram for the c-Si/PS heterojunction is proposed.

Device Design of High–Speed Source–Heterojunction–MOS Transistors (SHOTs): Optimization of Source Band Offset and Graded Heterojunction

Abstract: We have studied the device design of 15-nm highspeed n-channel source-heterojunction-MOS transistors (SHOTs) utilizing high-velocity electron injection from the source into the channel region and using the conduction-band-offset energy between the source and the channel regions. The band-offset energy near the source region and the length of the graded heterojunction are key parameters for realizing high-speed operation of SHOTs. A 2D device simulator indicates that the enhancement in transconductance Gm in SHOTs on an SOI substrate over conventional SOI-MOSFETs without source-heterojunction structures strongly depends on the source conduction-band-offset value DeltaEc and the length of graded-heterojunction structures LH in SHOTs. Moreover, the Gm enhancement of SHOTs is affected by the drain and the gate biases. We have shown that, with fully optimized DeltaEc and LH values, the Gm enhancement of SHOT due to high-velocity electron injection can be achieved in a whole range of drain bias. The optimized SHOT is quite promising for high-speed CMOS devices in the 10-nm regime.

A Unified Model for Gate Capacitance–Voltage Characteristics and Extraction of Parameters of Si/SiGe Heterostructure pMOSFETs

Abstract: A unified model for gate capacitance-voltage characteristics of Si/SiGe heterostructure pMOSFETs is presented. This model is applicable to buried-channel, surface-channel, and dual-channel Si/SiGe heterostructure pMOSFETs. The results from the model are compared with the experimental results and are found to be in excellent agreement. A simple and accurate method for the extraction of parameters such as the valence band offset, Si cap layer thickness, threshold voltages, and substrate doping is also presented in this paper.

Electron emission properties of relaxation-induced traps in InAs/GaAs quantum dots and the effect of electronic band structure

Abstract: The electron-emission properties of relaxation-induced traps in InAs/GaAs quantum dots (QDs) are studied in detail using capacitance-voltage (C-V) profiling and bias-dependent deep-level transient spectroscopy. Strain relaxation is shown to induce a threading-dislocation-related trap in the top GaAs layer and a misfit-dislocation-related trap near the QD. The threading trap decreases its electron-emission energy from 0.63 to 0.36 eV from sample surface toward the QD, whereas the misfit trap gradually increases its electron-emission energy from 0.28 to 0.42 eV from near the QD toward the GaAs bottom layer, indicating that both traps near the QD have lower electron-emission energies. Hence, the emission-energy change is attributed to the related traps across the QD interface where a band offset exists. The C-V profiling at 300 K shows extended carrier depletion near the QD. As temperature is increased, an electron-emission peak emerges at the QD followed by a prominent peak, suggesting that the trap respons...

Band discontinuity measurements of the wafer bonded InGaAs/Si heterojunction

Abstract: p-type InGaAs∕Si heterojunctions were fabricated through a wafer fusion bonding process. The relative band alignment between the two materials at the heterointerface was determined using current-voltage (I-V) measurements and applying thermionic emission-diffusion theory. The valence and conduction band discontinuities for the InGaAs∕Si interface were determined to be 0.48 and −0.1eV, respectively, indicating a type-II band alignment.