Showing papers on "Band offset published in 2014"

Band alignment and electrical properties of Al2O3/β-Ga2O3 heterojunctions

Abstract: The band alignment of Al2O3/n-Ga2O3 was investigated by x-ray photoelectron spectroscopy (XPS). With a band gap of 6.8 ± 0.2 eV measured for Al2O3, the conduction and valence band offsets at the interface were estimated to be 1.5 ± 0.2 eV and 0.7 ± 0.2 eV, respectively. The conduction band offset was also obtained from tunneling current in Al2O3/n-Ga2O3 (2¯01) metal-oxide-semiconductor (MOS) diodes using the Fowler-Nordheim model. The electrically extracted value was in good agreement with the XPS data. Furthermore, the MOS diodes exhibited small capacitance-voltage hysteresis loops, indicating the successful engineering of a high-quality Al2O3/Ga2O3 interface.

Interface control and modification of band alignment and electrical properties of HfTiO/GaAs gate stacks by nitrogen incorporation

Abstract: Effects of nitrogen incorporation on the interface chemical bonding states, optical dielectric function, band alignment, and electrical properties of sputtering-derived HfTiO high-k gate dielectrics on GaAs substrates have been studied by angle resolved X-ray photoemission spectroscopy (ARXPS), spectroscopy ellipsometry (SE), and electrical measurements. XPS analysis has confirmed that the interfacial layer of a HfTiO/GaAs gate stack is suppressed effectively after nitrogen incorporation. Analysis by SE has confirmed that reduction in band gap and increase in refractive index are observed with the incorporation of nitrogen. Reduction in valence band offset and increase in conduction band offset have been observed for a HfTiON/GaAs gate stack. Electrical measurements based on metal-oxide-semiconductor (MOS) capacitors have shown that the MOS capacitor with a HfTiON/GaAs stacked gate dielectric annealed at 600 °C exhibits low interface-state density (2.8 × 1012 cm−2 eV−1), small gate leakage current (2.67 × 10−5 A cm−2 at Vg = Vfb + V), and large dielectric constant (25.8). The involved mechanisms may originate from the decrease in the interface state density and the increase in the conduction band offset. The appropriate band offset relative to GaAs and excellent interface properties render HfTiON/GaAs as promising gate stacks in future III–V-based devices.

Atomically thin heterostructures based on single-layer tungsten diselenide and graphene.

Abstract: Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local ...

Band offset and negative compressibility in graphene-MoS2 heterostructures.

Abstract: We use electron transport to characterize monolayer graphene-multilayer MoS2 heterostructures. Our samples show ambipolar characteristics and conductivity saturation on the electron branch that signals the onset of MoS2 conduction band population. Surprisingly, the carrier density in graphene decreases with gate bias once MoS2 is populated, demonstrating negative compressibility in MoS2. We are able to interpret our measurements quantitatively by accounting for disorder and using the random phase approximation (RPA) for the exchange and correlation energies of both Dirac and parabolic-band two-dimensional electron gases. This interpretation allows us to extract the energetic offset between the conduction band edge of MoS2 and the Dirac point of graphene.

Impact of the Band Offset for n-Zn(O,S)/p-Cu(In,Ga)Se $_{2}$ Solar Cells

Abstract: The conduction-band offset (CBO) of the Zn(O, S)/Cu(In,Ga)Se2 heterojunction can play a significant role in the performance of solar cells. The individual electron affinities and bandgaps are controlled by the oxygen-to-sulfur and gallium-to-indium ratios, and the resulting offsets can range from +1.3 eV in the “spike” direction to -0.7 eV in the “cliff” direction if the full range of the two ratios is considered. The optimal CBO of near +0.3 eV can be achieved with various combinations of the two ratios. The traditional CdS emitter is near optimal for the commonly used 1.15-eV Cu(In,Ga)Se 2 (CIGS) but less optimal for higher Ga. The flexibility with Zn(O,S) emitters ranging from above 90% oxygen for CIS down to 50% oxygen for CGS allows an optimal CBO over the full gallium range. Assuming that other factors remain constant, the optimal offset should also be able to reduce the loss in cell efficiency between room temperature and temperatures more typical of field conditions by about 1% absolute.

Band offset and negative compressibility in graphene-MoS2 heterostructures

Abstract: We use electron transport to characterize monolayer graphene - multilayer MoS2 heterostructures. Our samples show ambipolar characteristics and conductivity saturation on the electron branch which signals the onset of MoS2 conduction band population. Surprisingly, the carrier density in graphene decreases with gate bias once MoS2 is populated, demonstrating negative compressibility in MoS2. We are able to interpret our measurements quantitatively by accounting for disorder and using the random phase approximation (RPA) for the exchange and correlation energies of both Dirac and parabolic-band two-dimensional electron gases. This interpretation allows us to extract the energetic offset between the conduction band edge of MoS2 and the Dirac point of graphene.

Interface stoichiometry control to improve device voltage and modify band alignment in ZnO/Cu2O heterojunction solar cells

Abstract: The interface stoichiometry of cuprous oxide (Cu2O) was controlled by adjusting the O2 and Zn partial pressures during ZnO sputter deposition and measured by high-resolution X-ray photoelectron spectroscopy of ultrathin (<3 nm) ZnO films on Cu2O. Open-circuit voltage measurements for ZnO/Cu2O heterojunctions under AM1.5 illumination were measured and it was found that a stoichiometric interface can achieve the voltage entitlement dictated by the band alignment, whereas the non-stoichiometric interface showed large open-circuit voltage deficits. These results highlight not only the need for stoichiometric interfaces in Cu2O devices, but also a reproducible experimental method for achieving stoichiometric interfaces that could be applied to any potential heterojunction partner. Additionally, valence-band offset measurements indicated changing the interface stoichiometry shifted the band alignment between Cu2O and ZnO, which accounts for the variation in previously reported band offset values.

Double-heterojunction nanorods

Abstract: As semiconductor heterostructures play critical roles in today's electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will enable the use of solution-processable colloidal quantum dots in future devices. Such heterojunctions incorporated into colloidal nanorods may be especially promising, since the inherent shape anisotropy can provide additional benefits of directionality and accessibility in band structure engineering and assembly. Here we develop double-heterojunction nanorods where two distinct semiconductor materials with type II staggered band offset are both in contact with one smaller band gap material. The double heterojunction can provide independent control over the electron and hole injection/extraction processes while maintaining high photoluminescence yields. Light-emitting diodes utilizing double-heterojunction nanorods as the electroluminescent layer are demonstrated with low threshold voltage, narrow bandwidth and high efficiencies.

Exploration of Zn resonance levels and thermoelectric properties in I-doped PbTe with ZnTe nanostructures.

Abstract: Motivated by the theoretically predicted Zn resonant states in the conduction band of PbTe, in the present work, we investigated the effect of Zn substitution on the thermoelectric properties in I-doped n-type PbTe. The room temperature thermopower values show good agreement with the theoretical Pisarenko plot of PbTe up to a carrier concentration of 4.17 × 1019 cm–3; thus, the presence of Zn resonance levels is not observed. Because of the low solubility of Zn in PbTe, a second phase of coherent ZnTe nanostructures is observed within the PbTe host matrix, which is found to reduce the lattice thermal conductivity. The reduced lattice thermal conductivity in PbTe by ZnTe nanostructures leads to notable enhancement in the figure of merit with a maximum value of 1.35 at 650 K. In contrast to the recent literature, the carrier mobility is not found to be affected by the band offset between ZnTe nanostructures and PbTe. This is explained by the quantum tunneling of the charge carrier through the narrow offset ...

Surface band bending and band alignment of plasma enhanced atomic layer deposited dielectrics on Ga- and N-face gallium nitride

Abstract: The effects of surface pretreatment, dielectric growth, and post deposition annealing on interface electronic structure and polarization charge compensation of Ga- and N-face bulk GaN were investigated. The cleaning process consisted of an ex-situ wet chemical NH4OH treatment and an in-situ elevated temperature NH3 plasma process to remove carbon contamination, reduce oxygen coverage, and potentially passivate N-vacancy related defects. After the cleaning process, carbon contamination decreased below the x-ray photoemission spectroscopy detection limit, and the oxygen coverage stabilized at ∼1 monolayer on both Ga- and N-face GaN. In addition, Ga- and N-face GaN had an upward band bending of 0.8 ± 0.1 eV and 0.6 ± 0.1 eV, respectively, which suggested the net charge of the surface states and polarization bound charge was similar on Ga- and N-face GaN. Furthermore, three dielectrics (HfO2, Al2O3, and SiO2) were prepared by plasma-enhanced atomic layer deposition on Ga- or N-face GaN and annealed in N2 ambient to investigate the effect of the polarization charge on the interface electronic structure and band offsets. The respective valence band offsets of HfO2, Al2O3, and SiO2 with respect to Ga- and N-face GaN were 1.4 ± 0.1, 2.0 ± 0.1, and 3.2 ± 0.1 eV, regardless of dielectric thickness. The corresponding conduction band offsets were 1.0 ± 0.1, 1.3 ± 0.1, and 2.3 ± 0.1 eV, respectively. Experimental band offset results were consistent with theoretical calculations based on the charge neutrality level model. The trend of band offsets for dielectric/GaN interfaces was related to the band gap and/or the electronic part of the dielectric constant. The effect of polarization charge on band offset was apparently screened by the dielectric-GaN interface states.

Theoretical and experimental studies of electronic band structure for GaSb1−xBix in the dilute Bi regime

Abstract: Photoreflectance (PR) spectroscopy was applied to study the band gap in GaSb1−xBix alloys with Bi < 5%. Obtained results have been interpreted in the context of ab initio electronic band structure calculations in which the supercell (SC) based calculations are joined with the alchemical mixing (AM) approximation applied to a single atom in the cell. This approach, which we call SC-AM, allows on the one hand to study alloys with a very small Bi content, and on the other hand to avoid limitations characteristic of a pure AM approximation. It has been shown that the pure AM does not reproduce the GaSb1−xBix band gap determined from PR while the agreement between experimental data and the ab initio calculations of the band gap obtained within the SC-AM approach is excellent. These calculations show that the incorporation of Bi atoms into the GaSb host modifies both the conduction and the valence band. The shift rates found in this work are respectively −26.0 meV per % Bi for the conduction band and 9.6 meV per % Bi for the valence band that consequently leads to a reduction in the band gap by 35.6 meV per % Bi. The shifts found for the conduction and valence band give a ~27% (73%) valence (conduction) band offset between GaSb1−xBix and GaSb. The rate of the Bi-related shift for the split-off band is −7.0 meV per % Bi and the respective increase in the spin–orbit split-off is 16.6 meV per % Bi.

X-ray photoelectron spectroscopy analysis and band offset determination of CeO2 deposited on epitaxial (100), (110), and (111)Ge

Abstract: The oxidation states, interface, and band alignment properties of physical vapor deposited CeO2 films on epitaxial (100), (110), and (111)Ge were investigated by x-ray photoelectron spectroscopy (XPS). The cross-sectional transmission electron microscopy demonstrated the polycrystalline nature of the CeO2 film. XPS analysis showed multiple Ce3d and Ce4d oxidation states with a mixture of Ce3+ and Ce4+ components existing in CeO2. Angular resolved XPS investigations indicate that the CeO2 films mostly consist of Ce4+ oxidation states while the Ce3+ oxidation states are preferentially present near the surface. The CeO2/(100)Ge, CeO2/(110)Ge, and CeO2/(111)Ge structures showed almost identical valence band offset (VBO) values of 1.6, 1.5, and 1.6 eV, respectively, using XPS measurements from Ce3d core level (CL) peaks. These (VBO) values were also supported by XPS measurements from shallow Ce4d CL binding energy peaks. The conduction band offset values between CeO2/Ge were ∼1.3 eV using the measured optical bandgap of CeO2. The XPS spectral analysis of cerium oxidation states and the measured band offset parameters for carrier confinement would offer an important path for the future design of Ge-based metal-oxide semiconductor devices.

Band-gap engineering at a semiconductor - crystalline oxide interface

Abstract: The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to electrically coupling crystalline oxides with semiconductors to realize functional behavior is controlling the manner in which their bands align at interfaces. Here we apply principles of band gap engineering traditionally used at heterojunctions between conventional semiconductors to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZr$_{x}$Ti$_{1-x}$O$_3$ and Ge, in which the band-gap of the former is enhanced with Zr content $x$. We present structural and electrical characterization of SrZr$_{x}$Ti$_{1-x}$O$_3$-Ge heterojunctions for $x$ = 0.2 to 0.75 and demonstrate the band offset can be tuned from type-II to type-I, with the latter being verified using photoemission measurements. The type-I band offset provides a platform to integrate the dielectric, ferroelectric and ferromagnetic functionalities of oxides with semiconducting devices.

Charge transfer and mobility enhancement at CdO/SnTe heterointerfaces

Abstract: We report a study of the effects of charge transfer on electrical properties of CdO/SnTe heterostructures. A series of structures with variable SnTe thicknesses were deposited by RF magnetron sputtering. Because of an extreme type III band offset with the valence band edge of SnTe located at 1.5 eV above the conduction band edge of CdO, a large charge transfer is expected at the interface of the CdO/SnTe heterostructure. The electrical properties of the heterostructures are analyzed using a multilayer charge transport model. The analysis indicates a large 4-fold enhancement of the CdO electron mobility at the interface with SnTe. The mobility enhancement is attributed to reduction of the charge center scattering through neutralization of the donor-like defects responsible for the Fermi level pinning at the CdO/SnTe interface.

Type I and type II band alignments in ZnO/MgZnO bilayer films

Abstract: We report the change in the type of band alignments due to an increase in the dopant (Mg) concentration in pulsed laser deposited ZnO/MgZnO bilayer film. The band offset measurements were carried out from the core level shifts as well as valence band maxima in the single as well as the bilayer films. The change in the type of band alignment is attributed to the surface enrichment of Mg at the heterojunction.

Band alignment of n-SnO2/p-GaN hetero-junction studied by x-ray photoelectron spectroscopy

Abstract: X-ray photoelectron spectroscopy has been used to study band offsets at the n-SnO2/p-GaN hetero-junction. The valence-band offset (�E V) and the conduction-band offset (�E c) are determined to be 0.97 ± 0.2 eV and 0.77 ± 0.2 eV respectively, indicating that the n-SnO2/p-GaN hetero-junction has a type II band alignment.

Intrinsic origin of interface states and band offset profiling of nanostructured LaAlO3/SrTiO3 heterojunctions probed by element-specific resonant spectroscopies

Abstract: The origin of electronic states at the basis of the 2DEG found in conducting LaAlO 3 /SrTiO 3 interfaces (5 u.c. LaAlO 3 ) is investigated by resonant photoemission experiments at the Ti L 2,3 and La M 4,5 edges. As shown by the resonant enhancement at the Ti L 2,3 edge, electronic states at E F receive a dominant contribution from Ti 3d states. Both Ti and La resonance effects in the valence-band region are used to estimate the valence-band maxima at the two sides of the junction. Through a comparison with the valence-band states of the LaAlO 3 and SrTiO 3 parent compounds, we reconstruct the band diagram of the heterojunction, which is revealed to be type I (straddling gap), with a large notch of the band profile at the interface as compared with the reference insulating (3 u.c. LaAlO 3 ) interface.

SiGe composition and thickness effects on NBTI in replacement metal gate / high-κ technologies

Abstract: The dependence of NBTI on SiGe thickness and composition for epitaxially grown layers on (100) and (110) Si substrates is studied in detail. It is found that SiGe thickness has no significant impact on NBTI at lower Ge%. However, lower NBTI degradation was observed with increasing Ge%, even though the interface state densities (N it ) increase with respect to Si. This improved NBTI is due to band offset limited V T , indicating that the improvement is substrate related rather than interface related. The physical mechanism is then discussed in terms of Ge%-induced variation in the band alignment.

Designing band offset of a-SiO:H solar cells for very high open-circuit voltage (1.06 V) by adjusting band gap of p–i–n junction

Abstract: We have matched the world’s highest open-circuit voltage (Voc: 1.06 V) achieved to date for a layered structure comprised of a glass/tin oxide (SnO2)/hydrogenated amorphous silicon oxide (a-SiO:H) (p–i–n)/back electrode. For the purposes of this study, we adjusted the band gaps of each layer (p–i–n) to improve overall film quality. Fine-tuning of band profiles with reference to activation energy and optical band gap allowed us to offset the conduction band and the valence band of each layer (p–i–n) and thus improve the built-in potential rather than the electron conductivity, Fourier transform infrared spectroscopy, transmittance or reflectance ratio, resulting in a high Voc. To fully exploit the characteristics of wide-band-gap materials and prevent problems with absorbance, we employed commercially available SnO2 in the front transparent conductive oxide instead of zinc oxide. Using our deposition and evaluation technologies to build a wide-band-gap single solar cell, we succeeded in matching the world’s highest Voc of 1.06 V (Eff: 5.38%, Jsc: 8.15 mA/cm2, FF: 0.624).

Band alignment at the In 2 S 3 /Cu 2 ZnSnS 4 heterojunction interface investigated by X-ray photoemission spectroscopy

Abstract: The band alignment at the In2S3/Cu2ZnSnS4 heterojunction interface is investigated by X-ray photoemission spectroscopy. In2S3 is thermally evaporated onto the contamination-free polycrystalline Cu2ZnSnS4 surface prepared by magnetron sputtering. The valence band offset is measured to be 0.46 ± 0.1 eV, which matches well with the valance band offset value 0.49 eV calculated using “transitivity” method. The conduction band offset is determined to be 0.82 ± 0.1 eV, indicating a ‘type I’ band alignment at the heterojunction interface.

Band Alignment and Band Gap Characterization of La 2 O 3 Films on Si Substrates Grown by Radio Frequency Magnetron Sputtering

Abstract: La2O3 films are grown on Si (100) substrates by the radio-frequency magnetron sputtering technique. The band alignment of the La2O3/Si heterojunction is analyzed by the x-ray photoelectron spectroscopy. The valence-band and the conduction-band offsets of La2O3 films to Si substrates are found to be 2.40±0.1 and 1.66±0.3eV, respectively. Based on O 1s energy loss spectrum analysis, it can be noted that the energy gap of La2O3 films is 5.18±0.2 eV, which is confirmed by the ultra-violet visible spectrum. According to the suitable band offset and large band gap, it can be concluded that La2O3 could be a promising candidate to act as high-k gate dielectrics.

Valence band offsets at Cu(In,Ga)Se2/Zn(O,S) interfaces

Abstract: The energy band alignment at interfaces between Cu-chalcopyrites and Zn(O,S) buffer layers, which are important for thin-film solar cells, are considered. Valence band offsets derived from X-ray photoelectron spectroscopy for Cu(In,Ga)Se2 absorber layers with CdS and Zn(O,S) compounds are compared to theoretical predictions. It is shown that the valence band offsets at Cu(In,Ga)Se2/Zn(O,S) interfaces approximately follow the theoretical prediction and vary significantly from sample to sample. The integral sulfide content of chemical bath deposited Zn(O,S) is reproducibly found to be 50–70%, fortuitously resulting in a conduction band offset suitable for solar cell applications with Cu(In,Ga)Se2 absorber materials. The observed variation in offset can neither be explained by variation of the Cu content in the Cu(In,Ga)Se2 near the interface nor by local variation of the chemical composition. Fermi level pinning induced by high defect concentrations is a possible origin of the variation of band offset.

Epitaxial Synthesis, Band Offset, and Photoelectrochemical Properties of Cubic Ga2S3 Thin Films on GaAs (111) Substrates

Abstract: Uniform and crack-free cubic Ga2S3 thin films have been epitaxially synthesized on n-GaAs (111) substrates by sulfurization. Atomic-force microscopy revealed that the Ga2S3 surface is dominated by nanoparticles of smaller than 50 nm in diameter. The nanoparticles, clustered into regular triangle structures that hierarchically packaged on GaAs, significantly reduced the reflectance of GaAs. Low-temperature photoluminescence revealed typical acceptor-like defects while X-ray photoemission spectroscopy revealed type-Iheterojunctionwithavalence-bandoffsetof0.6eVfortheGa2S3/GaAsheterostructure.Photoelectrochemicalpropertiesofthe Ga2S3/n-GaAs (111) heterojunction are studied and compared with those of bare n-GaAs (111) substrate in a typical three-electrode

Effect of band offset on carrier transport and infrared detection in InP quantum dots/Si nano-heterojunction grown by metalorganic chemical vapor deposition technique

Abstract: Epitaxy of III-V semiconductors on Si gets recent interest for next generation system on heterogeneous chip on wafer. The understanding of band offset is thus necessary for describing the charge transport phenomenon in these heterojunctions. In this work, x-ray photoemission spectroscopy has been used to determine the band offsets in a heterojunction made of InP quantum dots on Si. The valence and conduction band offset was found to be 0.12 eV and 0.35 eV, respectively, with a type-II band lineup. Deviation from theoretical prediction and previously published reports on quasi similar systems have been found and analyzed on the basis of the effect of strain, surface energy, shift in the electrostatic dipole and charge transfer at the interface. The carrier transport mechanisms along with different device parameters in the heterojunction have been studied for a temperature range of 180–300 K. This heterojunction is found to behave as an efficient infrared photodetector with an ON/OFF ratio of 21 at a reverse bias of 2 V. The corresponding rise and decay time was found to be 132 ms and 147 ms, respectively.