Internal Photoemission Of Electrons Research Articles

Variability of band alignment between WS2 and SiO2: Intrinsic versus extrinsic contributions

Internal photoemission of electrons was used to determine the energy position of the top valence band of mono- and few-layer WS2 on an SiO2/Si substrate. It was found, contrary to density functional theory calculations, that the valence band top in WS2 shifts up in energy with respect to the conduction band minimum of SiO2 with decreasing number of monolayers. At the same time, the band alignment of WS2 with SiO2 appears to be less sensitive to the WS2 synthesis route than in the previously studied case of the MoS2/SiO2 interface, indicating less extrinsic WS2 variability.

A mechanism of effect of optical excitation on resistive switching in ZrO2(Y) films with Au nanoparticles

A mechanism of effect of optical excitation on resistive switching of dielectric films with embedded metal nanoparticles (MNPs) has been proposed. The mechanism is related to a charging of the MNPs due to internal photoemission of electrons from the MNPs, which results in an increasing of the electric field strength at the MNP surfaces that, in turn, promotes the rupture and restoring of the conductive filaments in the dielectric film. The increasing of the electric filed strength due to the MNP charging as a function of the MNP sizes and positions inside the dielectric films was evaluated using a simple model taking into account the mirror charges on the conductive electrodes of a memristor stack. The single electron charging was found to be essential at small MNP radii (∼1 nm). The proposed mechanism was confirmed experimentally by studying the photoexcitation-induced charging of Au MNPs (with average radius 1.6–1.8 nm) embedded into a 10 nm thick ZrO2(Y) film by Kelvin probe force microscopy.

Processing Stability of Monolayer WS2 on SiO2

Using internal photoemission of electrons, the energy position of the valence band top edge in 1 monolayer WS2 films on top of SiO2 thermally-grown on Si was monitored to evaluate the stability of the WS2 layer with respect to two critically important technological factors: exposure to air and the transfer of WS2 from the growth substrate (sapphire) onto SiO2. Contrary to previous results obtained for WS2 and MoS2 layers synthesized by metal film thermal sulfurization in H2S, the valence band top of metal-organic chemical vapor deposition grown WS2 is found to remain at 3.7 ± 0.1 eV below the conduction band bottom edge of SiO2 through different growth runs, transfer processing, and storage in air for several months. This exceptional stability indicates WS2 as a viable candidate for the wafer-scale technology implementation.

Internal photoemission of electrons from 2D semiconductor/3D metal barrier structures

Understanding the energy alignment of electronic bands, which originate from ultrathin MoS2 layers and metal electrodes attached to them, is crucial for the design of MoS2-based electronic devices. We have applied internal photoemission spectroscopy (IPE) to analyze this alignment. We demonstrate that IPE can yield the barrier heights in the metal/ two-dimensional semiconductor/insulator stacks when the top metal electrode is sufficiently thin for allowing both the photoexcitation of electrons and their transport towards the insulator. The electron barrier at the interface between Al and monolayer (1ML) of MoS2 is estimated at 0.7 eV, and this value explains the experimentally observed attenuated quantum yield contribution from the aluminum. Based on the relative energies of the low-energy threshold position and the Fermi level of aluminum at the interface with the SiO2 insulator, we provide a simple explanation for the observed current photoinjection at the interface between aluminum and 1ML MoS2.

The influence of metallic Bi in BiVO4 semiconductor for artificial photosynthesis

BiVO4 is a non-titania inorganic photocatalyst recognized as an effective visible-light-driven semiconductor that has been shown to be effective for CO2 reduction. However, some characteristics, such as a low separation rate of photogenerated electron-hole pairs and low mobility of electron-hole carriers, are still challenges to the widespread use of this semiconductor. In this paper, the influence of metallic Bi on the CO2 photoreduction activity was evaluated for the BiVO4 semiconductor. Bi–BiVO4 catalysts were prepared by microwave heating at 240 °C, employing different reaction times and magnetic stirring regimes. Metallic Bi improved the catalytic activity of BiVO4 for CO2 reduction, enhancing the absorption of visible light and promoting internal photoemission of electrons in the metal-semiconductor interface, which improves the electron density in the surface of the catalyst. This resulted in an astonishing concentration of methanol; Bi–BiVO4 prepared at 240 °C, for 5 min, and without magnetic stirring, produces around 5.0 mmol L−1 g−1catalyst of methanol and 40 μmol L−1 g−1catalyst of acetone after 240 min of reaction. The mechanism of charge transfer between the BiVO4 and the metallic Bi is influenced by the size of the microsphere crystallites, moreover, the production of methanol increased as the Bi grain size decreased.

Evaluation of the effective work-function of monolayer graphene on silicon dioxide by internal photoemission spectroscopy

Internal photoemission of electrons from uncapped monolayer graphene to insulating SiO2 has been observed in samples prepared by water-intercalation based graphene transfer. The barrier height between the graphene Fermi level and the oxide conduction band bottom was reproducibly found to be 4.1–4.2 eV. Moreover, this value was weakly sensitive to the contacting metal work function (Al, Cu, Au). This barrier height corresponds to an effective work function of graphene close to 5.0 eV, which is nearly 0.5 eV higher than the usually reported vacuum value.

Impact of MoS2 layer transfer on electrostatics of MoS2/SiO2 interface

Using internal photoemission of electrons from few-monolayer thin MoS2 films into SiO2 we found that the MoS2 layer transfer processing perturbs electroneutrality of the interface, leading to an increase of the electron barrier height by ≈0.5–1 eV as compared to the case of the same films synthesized directly on SiO2. This effect is associated with the formation of an interface dipole, tentatively ascribed to interaction of H2O molecules with the SiO2 surface resulting in the incorporation of silanol (SiOH) groups. This violation of the interface electroneutrality may account for additional electron scattering in ultrathin transferred films and threshold voltage instabilities. Post-transfer annealing in H2S is shown to reduce the transfer-induced interface degradation.

Dark Conductivity and Photoconductivity of Nonaqueous Liposomes: a New Method for Measuring the Phase-Transition Temperatures of Lipid Membranes

A new method is developed to measure the phase-transition temperatures in artificial phospholipid membranes. This method is based on studying the temperature dependence of dark conductivity and photoconductivity in a symmetric cell with current-conducting indium–tin oxide (ITO) electrodes. Internal electron photoemission into a thin liposome layer is induced by visible and near IR light from the ITO electrodes. This method is applied to study the lyotropic phases in 1,2-dipalmitoyl-rac-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) with ethylene glycol (EG) and glycerol (G). The results of time-of-flight measurements are used to calculate the carrier mobilities in liposome vesicles. The measurement results are compared with the results obtained by dc conductometry. We are the first to detect the effect of a positive temperature coefficient of resistivity in a liquid-crystal phase. The proposed method makes it possible to detect the phase transitions in lyotropic liquid-crystal systems and, hence, can be used to create biocompatible drug carriers based on thermosensitive liposomes.

Conductive atomic force microscopy study of the photoexcitation effect on resistive switching in ZrO2(Y) films with Au nanoparticles

We report on the experimental observation of the effect of optical excitation on resistive switching in ultrathin ZrO2(Y) films with single-layered arrays of Au nanoparticles. The samples were prepared by depositing nanometer-thick Au films sandwiched between two ZrO2(Y) layers by magnetron sputtering followed by annealing. Resistive switching was studied by conductive atomic force microscopy by measuring cyclic current-voltage curves of a probe-to-sample contact. The contact area was illuminated by radiation of a semiconductor laser diode with the wavelength corresponding to the plasmon resonance in an Au nanoparticle array. The enhancement of the hysteresis in cyclic current-voltage curves due to bipolar resistive switching under illumination was observed. The effect was attributed to heating of Au nanoparticles due to plasmonic optical absorption and a plasmon resonance, which enhances internal photoemission of electrons from the Fermi level in Au nanoparticles into the conduction band of ZrO2(Y). Both factors promote resistive switching in a ZrO2(Y) matrix.

Band alignment at interfaces of synthetic few-monolayer MoS2 with SiO2 from internal photoemission

Electron band alignment at interfaces of SiO2 with directly synthesized few-monolayer (ML) thin semiconducting MoS2 films is characterized by using field-dependent internal photoemission of electrons from the valence band of MoS2 into the oxide conduction band. We found that reducing the grown MoS2 film thickness from 3 ML to 1 ML leads to ≈400 meV downshift of the valence band top edge as referenced to the common energy level of the SiO2 conduction band bottom. Furthermore, comparison of the MoS2 layers grown by a H-free process (sputtering of Mo in sulfur vapor) to films synthesized by sulfurization of metallic Mo in H2S indicates a significant (≈500 meV) electron barrier increase in the last case. This effect is tentatively ascribed to the formation of an interface dipole due to the interaction of hydrogen with the oxide surface.

(Invited) Internal Photoemission of Electrons from 2-Dimensional Semiconductors

Few monolayer (ML) thin transition metal dichalcogenide (TMD) semiconductors attract considerable interest because of their tunable properties relevant to a broad range of device applications. In particular, few-layer MoS2 has already been shown to deliver significant advantages in terms of transistor channel downscaling. The performance of the MoS2-based devices critically depends on the band alignment with other materials since the alignment determines the electrostatics of the stack, height of the tunneling barriers, contact resistance, etc. However, barrier characterization at interfaces of few-layer two-dimensional (2D) materials represents an experimental challenge: The electronic transport across these interfaces is strongly affected by hydrocarbon residues of the flake transfer process, non-homogeneities (e.g., domain boundaries), and defects in the material itself. Therefore, it is important to find a technique allowing one to reliably determine the 2D-semiconductor bandgap edge energies with respect to the bands of other materials. In the present work we will show that this goal can be achieved using the spectroscopy of Internal PhotoEmission (IPE) of electrons from MoS2 and WS2 into an insulating layer. By using 2- and 4-ML thick films of these TMDs grown on top of a SiO2 layer as the prototype interfaces, we observe electron IPE from the 2D photo-emitters and determine the corresponding energy barriers. The studied samples were prepared by thermal evaporation of a thin Mo or W film on SiO2/Si or HfO2(2 nm)/SiO2/Si substrates. Next, the samples were transferred to a furnace and sulfurized in pure H2S(10 or 100 mbar) at 800 °C resulting in the formation of a large-area (in the range of cm2) poly-crystalline MoS2 or WS2 films with crystallites of 50 nm size [1]. Cross-sectional TEM images reveal the characteristic layered structure. The Raman spectrum exhibits two distinct peaks corresponding to the in-plane (E2g) and the out-of-plane (A1g) vibrations in TMD layer, respectively. This observation reveals conversion of the metal film into 2H-MoS2 or 2H-WS2 hexagonal polytypes. By varying the thickness of the initially deposited metal, it appears possible to produce samples with 4 or 2 ML thick MoS2 and WS2 films without contamination of the sulfide/oxide interfaces. Electrical contacts were fabricated by evaporation of 100-nm thick Au or Al pads on top of the TMD films [2]. Fabrication of large area laterally uniform MoS2 and WS2 few-ML thin films enabled us to perform IPE experiments which address one of the fundamental issues concerning energy band alignment between different TMD materials. By observing electron photoemission from the valence band (VB) of MoS2 or WS2 into the conduction band (CB) of SiO2 underlayer, one can find the energy barriers. These correspond to the VB top energies of the two photo-emitting TMD materials referenced to the same energy level of the oxide CB bottom edge [3]. The IPE quantum yield spectra reveal the same field-dependent energy onset of the IPE current in the energy range 3.5-4.0 eV for both MoS2 and WS2 as well as in their stacked heterojenctions. Since the same photocurrent spectra were also obtained when using Al as the contact pad material instead of Au, the TMD layer can be identified as the sole source of photoelectrons [2]. This conclusion is independently affirmed by the observed significant decrease of the quantum yield if reducing the thickness of MoS2 electrode from 4 ML to 2 ML. The observed field dependence of the IPE spectra agrees well with the image-force barrier lowering at the TMD/SiO2 interface. The energy barrier of Fe =4.2±0.1 eV between the top of the MoS2 VB and the bottom of the SiO2 CB can be found by extrapolating this dependence to zero field. No significant difference between spectral thresholds from MoS2 and WS2 is found suggesting that the VBs of of these TMDs are nearly aligned. The same conclusion can be reached if analyzing the IPE yield spectra from MoS2/WS2 and WS2/MoS2 heterostructures. This result would suggest the validity of the “common anion rule” for sulfides of metals similarly to the earlier studied case of insulating metal oxides [3]. Recent IPE experiments addressing the effects of TMD layer transfer on top of insulating substrate will also be discussed. D. Chiappe et al., Adv. Mater. Interfaces 1500635 (2015).V. V. Afanasev et al. Microelectron. Eng. 147 294 (2015).V. V. Afanas’ev, Adv. Condens. Matter Phys. 301302 (2014).

(Invited) Internal Photoemission of Electrons from 2-Dimensional Semiconductors

Experimental analysis of photocurrent spectra in Si/SiO2/transition metal dichalcogenide (TMD) structures indicates the dominant contribution of internal photoemission (IPE) of electrons from the TMD valence band. Spectral distributions of this signal can be used to determine energy position of the band edges in the TMD film with respect to the common reference level of the oxide conduction band as demonstrated for the cases of sulfurization-grown MoS2, WS2, and their heteostructures. Furthermore, IPE allows one to trace the impact of sample processing on the electrostatic potential distribution at the TMD/oxide interface.

Re-distribution of oxygen at the interface between \u03b3-Al2O3 and TiN

Interface of TiN electrode with γ-Al2O3 layers was studied using near edge X-ray absorption fine structure, conventional X-ray photoelectron spectroscopy and photoelectron spectroscopy with high energies. Despite the atomic-layer deposited Al2O3 being converted into thermodynamically-stable polycrystalline cubic γ-phase by high-temperature (1000 or 1100 °C) anneal, our results reveal formation of a thin TiNxOy (≈1-nm thick) interlayer at the interface between γ-Al2O3 film and TiN electrode due to oxygen scavenging from γ-Al2O3 film. Formation of the TiO2 was not observed at this interface. As environmental effect, a strong oxidation resulting in formation of a TiO2(1.4 nm)/TiNxOy(0.9 nm) overlayers on the top of the TiN electrode is traced. Development of O-deficiency of γ-Al2O3 is observed and related to the polarization anisotropy due to the preferential orientation of spin states involved in the X-ray absorption in the plane parallel to the surface. Investigation of the TiN electrode reveals the predominantly “stretched” octahedra in its structure with the preferential orientation relative the interface with γ-Al2O3. This anisotropy can be correlated with ≈200 meV electron barrier height increase at the O-deficient TiN/γ-Al2O3 interface as compared to the TiN/γ-Al2O3 barrier formed under abundant oxidant supply condition as revealed by internal photoemission of electrons from TiN into the oxide.

Oxidation-induced electron barrier enhancement at interfaces of Ge-based semiconductors (Ge, Ge1 − xSnx, SiyGe1 − x − ySnx) with Al2O3

Experiments on internal photoemission of electrons at interfaces of SiyGe1−x−ySnx binary and ternary alloys (0≤x≤0.11; 0≤y≤0.19) with amorphous insulating Al2O3 reveal that the application of an additional oxidation step (5min in dry O3 at 300°C) after atomic layer deposition of first ≈1nm of alumina results in a significant increase of the electron barrier height (by ≈0.4–0.5eV) as compared to the conventionally grown Al2O3 layers. Furthermore, this supplemental oxidation step facilitates the removal of Ge sub-oxides from the interface. The observed electron barrier enhancement is suggested to be caused by formation of a wide gap germanate interlayer between the Ge-based semiconductors and alumina.

Effect of La doping on interface barrier between Si‐passivated Ge and insulating HfO2

AbstractBy analyzing internal photoemission of electrons from Si/SiOx‐passivated Ge into insulating HfO2 we found that insertion of additional La interlayer between SiOx and HfO2 leads to dramatic increase (more than by factor of 20) of the barrier transparency. However, no measurable variation of the interface barrier height is observed suggesting that La induces intermixing of near‐interface oxide stack resulting in development of additional density of states corresponding to conduction band of LaOx and HfOx sub‐networks. At the same time, photoemission results indicate the presence of discrete positive charges in the near‐interface oxide layer which may explain the observed ∼1 V shift of capacitance‐voltage curves. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Physico-Chemical Characterization of Anodic Oxides on Hf as a Function of the Anodizing Conditions

Anodic films were grown to 5 V (Ag/AgCl) on mechanically polished Hf in 0.1 M ammonium biborate and 0.1 M NaOH. Independent of the anodizing conditions, the photoelectrochemical characterization allowed the observation of optical transitions at 3.25 eV, i.e. at photon energy lower than the bandgap of HfO2. They are attributed to localized states inside the gap of the oxide induced by the presence of oxygen vacancies. From the cathodic photocurrent spectra, it was possible to estimate an energy threshold of ∼2.15 eV for internal electron photoemission phenomena. The impedance measurements proved the formation of insulating oxides with ɛ = 19. The anodizing occurs under a high field regime with an activation energy of 1.1 eV and an activation distance of 3.8 Å.

Hydrogen induced dipole at the Pt/oxide interface in MOS devices

Thanks to its good thermal stability, including resistance to oxidation, platinum (Pt) is widely used in prototyping a wide spectrum of electron devices ranging from metal‐oxide‐semiconductor (MOS) transistors to resistive switching memory cells. In this work, the energy barriers for electrons between the Fermi level of Pt and the conduction band of several oxide insulators (SiO2, Al2O3, HfO2, Hf0.8Al0.2Ox, Sr0.53Ti0.47O3) were determined by using internal photoemission of electrons. By combining this barrier value with the electron affinity of the particular oxide, the effective work function (EWF) of Pt was determined for different interfaces. As studied over the reference Pt/oxide/Si stacks de‐gassed in high vacuum at 400 °C, the EWF of Pt is found to differ significantly from the accepted vacuum WF value of 5.6 eV. The EWF is equal to 5.2 eV at the Pt/Al2O3 interface, 5.1 eV at Pt/HfO2, 5.3 eV at Pt/Hf0.8Al0.2Ox, 4.8 eV at Pt/SiO2, and 5.8 eV at the Pt/Sr0.53Ti0.47O3 interface indicating the presence of a polarization layer of which the contribution to the EWF depends on the oxide composition. Furthermore, annealing in H2 at 400 °C reduces the Pt EWF by ∼0.5 eV at all interfaces except for the Pt/Sr0.53Ti0.47O3 one. This observation indicates the formation of an additional H‐related dipole at the Pt/oxide interfaces and suggests that the vacuum WF of Pt cannot be used as the value relevant for the MOS properties.

Band alignment at interfaces of few-monolayer MoS2 with SiO2 and HfO2

Internal photoemission of electrons from 4- and 2-monolayer thick MoS2 films prepared by sulphurization of metallic Mo on top of SiO2 or HfO2/SiO2 insulating stacks is detected. This enables determination of the energy position of the MoS2 valence band which is found to be at 4.1–4.2eV below the SiO2 conduction band. At the interface with HfO2, a barrier height of 3.7eV is found, corresponding to an increase of the electron affinity of MoS2 by ≈0.5eV as compared to the SiO2 case. This suggests the presence of interface charges (or dipoles) in the interfacial HfO2 layer.

Band alignment at interfaces of amorphous Al2O3 with Ge1−xSnx- and strained Ge-based channels

Spectroscopy of internal photoemission of electrons from Ge and Ge1−xSnx (x ≤ 0.08) alloys into amorphous Al2O3 is used to evaluate the energy of the semiconductor valence band top. It is found that in Ge and Ge1−xSnx the valence bands are aligned within the measurement accuracy (±0.05 eV) irrespective of the strain imposed on the semiconductor or by the kind of passivating inter-layer applied between the semiconductor and alumina. This indicates that the Ge1−xSnx-stressor approach may be useful for strain engineering in p-channel Ge metal-oxide-semiconductor transistors.

Influence of metal electrode stoichiometry on the electron barrier height at CuxTe1−x/Al2O3 interfaces for CBRAM applications

Spectroscopy of Internal photoemission of electrons was applied to reveal the impact of the stoichiometry of CuxTe1−x (0⩽x⩽1) electrodes on the barrier height between the electrode Fermi level and the bottom of the Al2O3 conduction band. The barriers observed in the structures with atomic layer deposited alumina films (20 or 10nm) on (100) Si substrates are compared to the metal/SiO2 barriers in capacitors with thermally grown Si oxide. We found that the CuxTe1−x stoichiometry has a marginal impact on the barrier height, yielding the same value for the Cu metal electrode even for high Te concentration (80%). In the case of pure Te electrode the photoemission spectral threshold is found to be shifted towards higher photon energy by 0.5eV.