Interfacial Band Offsets Research Articles

Saturation Thickness of Stacked SiO2 in Atomic-layer-deposited Al2O3 Gate on 4H-SiC

Abstract High-k materials as an alternative dielectric layer for SiC power devices have the potential to reduce interfacial state defects and improve MOS channel conduction capability. Besides, under identical conditions of gate oxide thickness and gate voltage, the high-k dielectric enables a greater charge accumulation in the channel region, resulting in a larger number of free electrons available for conduction. However, the lower energy band gap of high-k materials leads to significant leakage currents at the interface with SiC, which greatly affects device reliability. By inserting a layer of SiO2 between the high-k material and SiC, the interfacial barrier can be effectively widened and hence the leakage current will be reduced. In this study, the optimal thickness of the intercalated SiO2 was determined by investigating and analyzing the gate dielectric breakdown voltage and interfacial defects of a dielectric stack composed by atomic-layer-deposited Al2O3 layer and thermally nitride SiO2. Current-voltage and high-frequency capacitance-voltage measurements were performed on metal-oxide-semiconductor test structures with 35 nm thick Al2O3 stacked on 1-, 2-, 3-, 6-, or 9 nm thick nitride SiO2. Measurement results indicate that the current conducted through the oxides was affected by the thickness of the nitride oxide and the applied electric field. Finally, a saturation thickness of stacked SiO2 that contributed to dielectric breakdown and interfacial band offsets was identified. The findings in this paper provides guideline for the SiC gate dielectric stack design with the breakdown strength and the interfacial state defects considered.

Tailoring the Interfacial Band Offset by the Molecular Dipole Orientation for a Molecular Heterojunction Selector (Adv. Sci. 21/2021)

Molecular Heterojunction In article number 2101390 by Gunuk Wang and co-workers, a tailored molecular heterojunction selector is implemented by controlling its interfacial band offset. Based on diverse interfacial band offsets achieved by changing the heterojunction constituents, such as the molecular dipole moment direction, types of two-dimensional transition metal dichalcogenide, and work function of the electrodes, the nonlinearity is widely tuned. The suggested molecular heterojunction selector is depicted in array and binary numbers.

Optical spectra and interfacial band offsets of pulse-laser-deposited metal-oxides: SnO2, TiO2, and ZnO

Transparent conducting oxides are electrically conductive materials with high optical transmittance in the visible region of the spectrum and are useful in a wide range of applications. In this study, the optical spectra of a set of single-phase transparent conducting oxides TiO2, ZnO, and SnO2 grown by pulse laser deposition are measured by vacuum ultraviolet spectroscopic ellipsometry and the optical bandgaps are determined to be 3.30 ± 0.05 eV, 3.13 ± 0.05 eV, and 3.95 ± 0.05 eV, respectively. Differences between these values and previous measurements are discussed. SnO2 and ZnO optical responses at the bandgap reveal that they are a direct bandgap, while TiO2 appears to show an indirect type. For the interfacial electronic characteristics, the internal photoemission measurement shows that the electronic barriers of these naturally n-type-doped metal oxides adjacent to an Al2O3 layer originate from the Fermi level in their conduction bands. The band offset determination shows that the barrier heights are similar and have a small internal field dependence. The work functions are then estimated from the measured barrier heights.

Band structure and electronic transport across Ta2O5/Nb:SrTiO3 interfaces

Resistive switching devices promise significant progress in memory and logic technologies. One of the hurdles toward their practical realization is the high forming voltages required for their initial activation, which may be incompatible with standard microelectronic architectures. This work studies the conduction mechanisms of Ta2O5 layers, one of the most studied materials for memristive devices, in their initial, as-fabricated state (“pre-forming”). By separating this aspect and resolving the current mechanisms, we provide the input that may guide future design of resistive switching devices. For this purpose, Ta2O5 layers were sputtered on conductive Nb:SrTiO3 substrates. Ta2O5/Nb:SrTiO3 structures exhibit diode behavior with an ideality factor of n ≈ 1.3 over four current decades. X-ray photoelectron spectroscopy analysis of the interfacial band offsets reveals a barrier of 1.3 ± 0.3 eV for electrons injected from the semiconductor into Ta2O5. Temperature-dependent current–voltage analysis exhibits rectifying behavior. While several conduction mechanisms produce good fits to the data, comparing the physical parameters of these models to the expected physical parameters led us to conclude that trap-assisted tunneling (TAT) is the most likely conduction mechanism. Fitting the data using a recent TAT model and with the barrier that was measured by spectroscopy fully captures the temperature dependence, further validating this conduction mechanism.

Tailoring of Interfacial Band Offsets by an Atomically Thin Polar Insulating Layer To Enhance the Water-Splitting Performance of Oxide Heterojunction Photoanodes.

An important factor in the performance of photoelectrochemical water splitting is the band edge alignment of the photoelectrodes for efficient transport and transfer of photogenerated carriers. Many studies for improving charge transfer ability between the electrode and the electrolyte have been reported, while research to improve charge transfer at the interface of the photoactive semiconductor and the conducting substrate is largely lacking. Here, we demonstrate that the water-splitting performance of an oxide heterostructured photoelectrode can be increased 6-fold by inserting an atomically thin polar LaAlO3 interlayer compared with that of an oxide heterostructure without an insertion to modify interfacial band offsets. The electrically lowered Schottky barrier is driven by the atomically thin layer, and the charge transfer resistance between the oxides is reduced by up to 2 orders of magnitude upon insertion of LaAlO3, a wide-gap (5.6 eV) insulator. We show that the critical thickness of the polar layer for enhancing the charge transfer is 3 unit cells. The dipole moment from the polar sheets of LaAlO3 introduces an internal electric field, which modifies the effective band offsets in the device. This work serves as a proof of concept that photoelectrochemical performance can be improved by manipulating the band offsets of the heterostructure interface, suggesting a new design strategy for heterostructured water-splitting photoelectrodes.

The Middle Road Less Taken: Electronic-Structure-Inspired Design of Hybrid Photocatalytic Platforms for Solar Fuel Generation

The development of efficient solar energy conversion to augment other renewable energy approaches is one of the grand challenges of our time. Water splitting, or the disproportionation of H2O into energy-dense fuels, H2 and O2, is undoubtedly a promising strategy. Solar water splitting involves the concerted transfer of four electrons and four protons, which requires the synergistic operation of solar light harvesting, charge separation, mass and charge transport, and redox catalysis processes. It is unlikely that individual materials can mediate the entire sequence of charge and mass transport as well as energy conversion processes necessary for photocatalytic water splitting. An alternative approach, emulating the functioning of photosynthetic systems, involves the utilization of hybrid systems wherein different components perform the various functions required for solar water splitting. The design of such hybrid systems requires the multiple components to operate in lockstep with optimal thermodynamic driving forces and interfacial charge transfer kinetics. This Account describes a new class of nanoscale heterostructures comprising M xV2O5 nanowires, where M is a p-block cation with a ( n - 1) d10 ns2 np0 electronic configuration characterized by a stereoactive lone pair of electrons and x is its stoichiometry, interfaced with II-VI semiconductor quantum dots (QDs). Photocatalytic water splitting involves the transfer of excited-state holes from QDs to mid-gap states (derived from the stereoactive lone pairs of p-block cations) of nanowires, hole transport through nanowires, the reduction of protons at a QD-immobilized catalyst, and water oxidation at an anode. The M xV2O5/QD architectures provide a vast design space for evolutionary optimization of function with considerable tunability of composition and structure of the individual components as well as of the interfacial structure, thereby facilitating programmability of absorption spectra, energetic offsets, and charge-transfer reactivity. The available design space spans choice of the p-block cation M, its stoichiometry x, the composition and size of various QDs, and the nature of the nanowire/QD interface. This multivariate parameter space has been navigated by integrating first-principles modeling, diversified synthesis, spectroscopic measurements, and catalytic evaluation to facilitate the rational design of several generations of heterostructures and the systematic improvement of their photocatalytic performance. The electronic structures of the target heterostructures are predicted by DFT calculations in light of the revised lone pair model of stereoactive structural distortions and evaluated by hard X-ray photoelectron spectroscopy such as to systematically tune the interfacial band offsets. Central to this approach is the development of a topochemical "etch-a-sketch" intercalation approach that allows for facile installation of p-block cations in metastable polymorphs of V2O5. The interfacial charge transfer kinetics of M xV2O5/QD heterostructures is further evaluated by transient absorption spectroscopy to measure excited-state charge-transfer dynamics and is found to depend sensitively on interfacial structure and the thermodynamic driving forces in accordance with Marcus theory. The integration of theory and experiment has allowed for the design of viable photocatalytic architectures exemplified by the exceptional catalytic performance of β-Pb xV2O5/CdX (X= S, Se) architectures, which has subsequently been elaborated to other lone-pair M xV2O5 compounds, demonstrating the effective exploitation of the opportunities for programmability available in the design space.

Co-sputtered Zn1-xMgxO films and interfacial band offsets at heterojunctions with SnS-CUB

Zinc magnesium oxide, Zn1−xMgxO, is a desirable candidate for a buffer/window layer in thin film solar cells. We deposited Zn1−xMgxO film with hexagonal structure by co-sputtering of ZnO and Mg targets. Band gap of the Zn1−xMgxO films was found to increase from 3.48 to 3.83 eV with the increase in Mg content in the film. We used X-ray photo-electron spectroscopy (XPS) to examine the interfaces formed between such Zn1−xMgxO films and the recently reported cubic structured SnS (SnS-CUB) thin films of bandgap 1.72 eV. The valence band maxima of the materials determined from XPS show that the conduction band offset (CBO) for Zn0.88Mg0.12O/SnS-CUB interface is 0.39 eV. The CBO, 0.52 eV of Zn0.88Mg0.12O/SnS-CUB hetero junction was also calculated from electron affinities of Zn0.88Mg0.12O and SnS-CUB thin films which is in agreement with the CBO calculated from the XPS measurements.

Band alignment of semiconductors and insulators using dielectric-dependent hybrid functionals: Toward high-throughput evaluation

The band alignment of prototypical semiconductors and insulators is investigated using first-principles calculations. A dielectric-dependent hybrid functional, where the nonlocal Fock exchange mixing is set at the reciprocal of the static electronic dielectric constant and the exchange correlation is otherwise treated as in the Perdew-Burke-Ernzerhof (PBE0) hybrid functional, is used as well as the Heyd-Scuseria-Ernzerhof (HSE06) hybrid and PBE semilocal functionals. In addition, these hybrid functionals are applied non-self-consistently to accelerate calculations. The systems considered include C and Si in the diamond structure, BN, AlP, AlAs, AlSb, GaP, GaAs, InP, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe in the zinc-blende structure, MgO in the rocksalt structure, and GaN and ZnO in the wurtzite structure. Surface band positions with respect to the vacuum level, i.e., ionization potentials and electron affinities, and band offsets at selected zinc-blende heterointerfaces are evaluated as well as band gaps. The non-self-consistent approach speeds up hybrid functional calculations by an order of magnitude, while it is shown using HSE06 that the resultant band gaps and surface band positions are similar to the self-consistent results. The dielectric-dependent hybrid functional improves the band gaps and surface band positions of wide-gap systems over HSE06. The interfacial band offsets are predicted with a similar degree of precision. Overall, the performance of the dielectric-dependent hybrid functional is comparable to the $G{W}_{0}$ approximation based on many-body perturbation theory in the prediction of band gaps and alignments for most systems. The present results demonstrate that the dielectric-dependent hybrid functional, particularly when applied non-self-consistently, is promising for applications to systematic calculations or high-throughput screening that demand both computational efficiency and sufficient accuracy.

Role of Carrier Mobility and Band Alignment Engineering on the Efficiency of Colloidal Quantum Dot Solar Cells

We investigate the physics-based design of colloidal quantum dot (CQD) solar cells using self-consistent computational modeling. The significance of band alignment engineering and optimized carrier mobility is quantitatively explored as a function of subbandgap defect density ( Nt ) in the bulk CQD. For Nt $ \leq $ 10 15 cm-3, band alignment engineering near the interface of CQD and the metal contact could significantly improve open-circuit voltage by suppressing the forward-bias dark current. This effect could enhance cell efficiency up to ∼37% for thinner ( $ 1 μm) CQD layers. For the thicker ( $ > $ 1 μm) CQD layer, the effect of band engineering is diminished, as the forward-bias dark current becomes diffusion limited and less dependent on the interfacial band offsets. An optimal carrier mobility in CQD lies in the range ∼10-2–100 cm2/V⋅s and shows variation as a function of CQD layer thickness and the interfacial band offset. For Nt $ \approx {10^{14}}$ cm-3, an optimally designed cell could provide $\sim$ 20% efficiency under AM1.5G solar spectrum without employing advanced structural optimizations such as the nanostructured electrodes. These physical insights contribute to a better understanding of the quantum dot solar cell design that is needed for improving the efficiency for this emerging low-cost solar cell technology.

Band alignment of semiconductors from density-functional theory and many-body perturbation theory

The band lineup, or alignment, of semiconductors is investigated via first-principles calculations based on density functional theory (DFT) and many-body perturbation theory (MBPT). Twenty-one semiconductors including C, Si, and Ge in the diamond structure, BN, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe in the zinc-blende structure, and GaN and ZnO in the wurtzite structure are considered in view of their fundamental and technological importance. Band alignments are determined using the valence and conduction band offsets from heterointerface calculations, the ionization potential (IP) and electron affinity (EA) from surface calculations, and the valence band maximum and conduction band minimum relative to the branch point energy, or charge neutrality level, from bulk calculations. The performance of various approximations to DFT and MBPT, namely the Perdew-Burke-Ernzerhof (PBE) semilocal functional, the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, and the $GW$ approximation with and without vertex corrections in the screened Coulomb interaction, is assessed using the $GW{\ensuremath{\Gamma}}^{1}$ approximation as a reference, where first-order vertex corrections are included in the self-energy. The experimental IPs, EAs, and band offsets are well reproduced by $GW{\ensuremath{\Gamma}}^{1}$ for most of the semiconductor surfaces and heterointerfaces considered in this study. The PBE and HSE functionals show sizable errors in the IPs and EAs, in particular for group II-VI semiconductors with wide band gaps, but are much better in the prediction of relative band positions or band offsets due to error cancellation. The performance of the $GW$ approximation is almost on par with $GW{\ensuremath{\Gamma}}^{1}$ as far as relative band positions are concerned. The band alignments based on average interfacial band offsets for all pairs of 17 semiconductors and branch point energies agree with explicitly calculated interfacial band offsets with small mean absolute errors of both $\ensuremath{\sim}0.1\text{eV}$, indicating a good overall transitivity of the band offsets. The alignment based on IPs from selected nonpolar surfaces performs comparably well in the prediction of band offsets at most of the considered interfaces. The maximum errors are, however, as large as 0.3, 0.4, and 0.7 eV for the alignments based on the average band offsets, branch point energies, and IPs, respectively. This margin of error should be taken into account when performing materials screening using these alignments.

Electrical performance of polymer ferroelectric capacitors fabricated on plastic substrate using transparent electrodes

Polymer-based flexible ferroelectric capacitors have been fabricated using a transparent conducting oxide (ITO) and a transparent conducting polymer (PEDOT:PSS). It is found that the polarization fatigue performance with transparent oxide electrodes exhibits a significant improvement over the polymer electrodes (20% vs 70% drop in polarization after 106 cycles). This result can be explained based on a charge injection model that is controlled by interfacial band-offsets, and subsequent pinning of ferroelectric domain walls by the injected carriers. Furthermore, the coercive field (Ec) of devices with our polymer electrodes is nearly 40% lower than reported values with similar polymer electrodes. Surprisingly, this difference was found to be related to the dry etching process used to define the top electrodes, which is reported for the first time by this group. The temperature dependence of relative permittivity of both devices shows a typical first order ferroelectric-to-paraelectric phase transition, but with a reduced Curie temperature compared to reference devices fabricated on Pt.

Hybrid functional study on structural and electronic properties of oxides

The hybrid functional method within the HSE06 scheme is tested on various oxides such as TiO 2, SrTiO 3, ZnO, SnO 2, MgO, SiO 2, and Al 2O 3. Since the canonical mixing parameter still underestimates the energy gap, we optimize it by fitting the energy gap to the experimental value. It is found that optimized values lie between 0.2 and 0.4 depending on the material. The structural properties are examined and the lattice parameters calculated with the HSE06 functional are in better agreement with experiment compared to (semi)local functional results. The relative shifts in the valence and conduction edges are provided, which can serve as first-order corrections to the semilocal functional results on defect levels or interfacial band offsets.

Organic/Inorganic Hybrids for Solar Energy Generation

AbstractOrganic and hybrid (organic/inorganic) solar cells are an attractive alternative to traditional silicon-based photovoltaics due to low-temperature, solution-based processing and the potential for rapid, easily scalable manufacturing. Using oxide semiconductors, instead of fullerenes, as the electron acceptor and transporter in hybrid solar cells has the added advantages of better environmental stability, higher electron mobility, and the ability to engineer interfacial band offsets and hence the photovoltage. Further improvements to this structure can be made by using metal oxide nanostructures to increase heterojunction areas, similar to bulk heterojunction organic photovoltaics. However, compared to all-organic solar cells, these hybrid devices produce far lower photocurrent, making improvement of the photocurrent the highest priority. This points to a less than optimized polymer/metal oxide interface for carrier separation. In this article, we summarize recent work on examining the polymer structure, electron transfer, and recombination at the polythiophene-ZnO interface in hybrid solar cells. Additionally, the impact of chemical modification at the donor-acceptor interface on the device characteristics is reviewed.