Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes

Two states tunneling current for MIS (Al/SiO2/p-Si) coupling tunneling diodes, i.e., read and gate diodes, were observed with ultra-thin oxide. For the gate MIS tunnel diode, after the electrical treatments of positive/negative voltage stressing for sufficient time, high and low current states can be achieved with electron trapping/de-trapping in the device edge [1]. With the aid of separated read diode, the two states could be enhanced by coupling effect. It is known that the tunnel current of MIS(p) structure is strongly dependent on the effective Schottky barrier height of majority carriers, a slight change of the effective Schottky barrier height by the surrounding signal would induce a considerable change of the tunneling current. It was proposed that the increase of lateral minority carrier diffusion current results in the decrease of the effective Schottky barrier height at the device edge, which leads to the increase of the current of MIS tunnel diode [2]. In this work, two states phenomenon of read tunneling current behavior induced by constant voltage stress on nearby MIS gate capacitor was demonstrated. Fig. 1 shows the device structures of top view and cross section. The radius of the inner circle of read diode, Rin is 125 µm. The space width between inner read diode circle and outer gate diode ring, S, is 30µm. The outside radius of ring, Rout is 576 µm. The thickness of oxide is 2.4 nm. Fig. 2 shows the currents of the inner circle MIS read tunnel diode ITD versus VTD , after applying two different electrical treatments of constant voltage stress on outer the ring MIS gate tunnel diode, VG . The set state is defined as the I-V curve after the stressing voltage VG of -2 V for 300 s, whereas the reset state is 2 V for 200 s. The fresh I-V curve is without applying positive/negative voltage stressing on the gate device. It is clear that the I-V curves of read diode are diverged out at VG > -0.5 V. The current decreases after applying VG of -2 V for 300 s. The mechanism is due to the decreasing of lateral diffusion current. While applying negative voltage stress VG of -2 V for 300 s on gate diode, the electrons trap in oxide layer under the outer ring region. Hence, the minority carrier density under the ring region decreases after negative voltage stress VG , and the gradient of carrier concentration also decreases. It leads to lower lateral diffusion current and change the Schottky barrier at the edge of the circle read device. Due to Schottky barrier height increasing, the tunneling current is lower than fresh I-V curve. Fig. 3 shows retention characteristics with various read voltage. It is noted that the negative current is caused by negative read voltage. Fig. 3(a) shows current-time (I-t) curves after VG of -2 V for 300 s and Fig. 3(b) shows I-t characteristic after VG of 2 V for 200 s. Since the electrons are de-trapped after the positive voltage stressing, the reset currents in Fig. 3(b) remain at constant values. It is obvious that the restored reset state currents in Fig. 3(a) are consistent the currents in Fig. 3(b). Fig. 4(a) shows normalized curves of Fig. 3(a). Fig. 4(b) is for MIS diode without ring structure, the retention degrades with the increasing of read voltage. Nevertheless, for the structure in this work with the electrical operators of stressing on outer ring and read on circle, the performances of retention are not declined with the change of read voltage from -0.2 V to 1.8 V. It might exist an optimal read voltage which gives rise to the best retention performance. This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. NSC 102-2221-E-002-183-MY3 and MOST 103-2622-E-002-031.

A new current equation for graphene/semiconductor or graphene/metal junctions in graphene-based electronic devices is proposed based on the thermionic emission theory. Temperature-dependent current density predicted by the proposed model agrees well with those experimental data reported in the literature. It can also explain the electric field and temperature-dependent effective Schottky barrier height observed in experiments. This is because a high drift velocity in graphene and its dependence on temperature can lead to a change in the effective Schottky barrier height. Due to the nonlinearity between current and temperature, the Richardson’s law will be broken down. The proposed model will benefit to better understand the current transport mechanism in graphene-like materials and graphene-based electronic devices.

A theoretical analysis is presented for the combination of stoichiometry changes within a metal contact to the effective Schottky barrier height of a metal/semiconductor interface. Results are shown for a metal/n-CdTe interface, and the stoichiometry changes within the metal contact due to Cd-richness and Te-richness are considered. Current-transport, X-ray photoelectron spectroscopy and photoluminescence studies have shown that Te-rich surfaces exhibit barrier heights of 0.72 +/- 0.02 eV and Cd-rich surfaces exhibit barrier heights of 0.93 +/- 0.02 eV. Here, it is shown that for Te-rich coverage of above 40% area of the contact the effective barrier height lies within the experimental error of the measured value 0.72 +/- 0.02 eV. For Te-rich coverage of less than 40% area and greater than 1% of the contact, the effective barrier height may take a range of values between 0.74 eV to 0.83 eV. However, when the Te-rich coverage becomes less than 1% of the contact, the effective barrier height changes drastically from 0.83 eV to 0.93 eV. This aggregate area effect on the barrier height gives strong support to previous experimental observations on Sb and Au contacts on n-CdTe surfaces. A majority of the diodes fabricated on bromine methanol etched surfaces yielded the lower barriers of similar to 0.72 eV, and a few diodes yielded higher barriers of similar to 0.93 eV, although the preparation procedures were identical for all diodes.

Ferroelectric semiconducting field-effect transistors (FeS-FETs) based on two-dimensional materials exhibit nonvolatile resistive switching, making them promising candidates for next-generation memory and neuromorphic computing. However, the mechanisms governing resistive switching in α-In2Se3 lateral devices remain unresolved, particularly regarding the relative contributions of channel and contact resistance. In this study, Kelvin probe force microscopy (KPFM) was employed to spatially resolve the gate-poling-dependent contact and channel resistances in α-In2Se3 FeS-FETs, while scanning photocurrent microscopy (SPCM) was used to quantify changes in effective Schottky barrier height at the metal contacts. Both contact and channel resistances were found to increase (decrease) with positive (negative) poling, with the contact resistance modulation correlating with changes in Schottky barrier height. Control experiments on as-exfoliated multidomain flakes confirmed that spontaneous polarization influences both channel and contact resistances. However, typical clockwise resistive switching characteristics can be observed even in the absence of detectable ferroelectric polarization switching. Furthermore, typical gate-poling conditions lead to the formation of stacking defects observed by ex situ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The observed defects can impede domain wall motion, providing a rationale for the lack of an abrupt switching threshold and a possible mechanism of coupling in-plane fields to out-of-plane polarization. We conclude that resistive switching in α-In2Se3 lateral channel devices is often influenced by both reversible polarization switching and irreversible defect formation, highlighting the need for improved domain wall control and defect mitigation strategies to enhance FeS-FET performance for reliable memory applications.

We fabricated Pt‐functionalised hydrogen gas sensors on AlGaN/GaN heterojunction platform and investigated the influence of GaN‐cap layer on the sensing characteristics. Pt‐Schottky diodes with GaN‐cap layer exhibited a larger change of Schottky barrier height than ones with no GaN‐cap layer when hydrogen gas was detected. Technology computer‐aided design simulation indicated that the increase of electron concentration at heterojunction can be magnified by a larger change of barrier height. The AlGaN/GaN FET‐type sensors with Pt catalyst on the gate area demonstrated significant enhancement of hydrogen gas sensitivity from 16 to 35% at 200°C when GaN cap layer was employed.

Rapid thermal annealing of nickel-dysprosium (Ni-Dy) film stacks on silicon (Si) was investigated, and formation of the nickel disilicide (NiSi2) phase was observed. The formation mechanism for the NiSi2 phase was elucidated. The nucleation, growth, and distribution of the inverted NiSi2 pyramids can be explained from both the thermodynamic and kinetic aspects of the solid-state reaction. In addition, lowering of the effective electron Schottky barrier height (ΦBn,eff) of NiSi2 on Si was observed. The high electric field at the tips of the inverted NiSi2 pyramids increases the tunneling probability of electrons, and results in thermionic field emission being the dominant carrier transport mechanism at the NiSi2/Si interface. This contributes significantly to an increase in reverse bias current and gives a reduced ΦBn,eff. An analytical expression for the localized electric field is derived and it is found to be as high as ∼1.9 × 106 V/cm based on our experimental result.

With assistance from a nano-manipulator system inside a scanning electron microscope chamber, mechanical and electrical properties of ZnO nanorings were investigated. The change of a fractured nanoring to nearly straight nanobelts was strong evidence to support the previously proposed electrostatic-force-induced self-coiling model, and our computational simulation results indicated the fracture force was 25–30 μN. The contact between a tungsten tip of the manipulator and a ZnO nanoring was confirmed as the Schottky type; therefore, the change of I–V curves of the nanoring under compression was attributed to the Schottky barrier height changes.

The effect of the Schottky barrier height changes on the metal/EDT-treated (1,2-ethanedithiol) PbS nanocrystals film interface is considered. Also, the influence of shunts on the J-V characteristic and the Schottky barrier height is demonstrated, as well, the effect of silver oxide layer on the charge accumulation and tunneling. It is shown that the gold electrodes provide more stable results even when the Schottky barrier is formed, while the silver electrode provides more current. Keywords: semiconductor nanocrystals, Schottky barrier, charge carriers transport, thin films.

We report here the first results of our study of metal/III–V semiconductor contacts :Ag–GaAs (001) and Ag–InP (001). Clean GaAs (001) and InP (001) surfaces, under various conditions of stoichiometry, are obtained by ion etching followed by annealing and arsenic or phosphorous adsorption and carefully characterized by AES, LEED, and ELS. Using these techniques some correlations between surface stoichiometry, surface structures, and electronic surface states have been found. On these surfaces the metallization has been carried out by an MBE-like technique. The results obtained show that the Schottky barrier height changes according to the conditions of surface preparation. To explain these results, different hypotheses are discussed.

C– V analysis of rapidly thermal annealed SF 6 plasma treated AlGaN/GaN heterostructures

Density and energetic distributions of interface states between metal-semiconductor rectifying contacts in sub-micron GaAs MESFET and AlGaAs/InGaAs pseudomorphic high electron mobility transistors (HEMT's) have been studied. Electrical properties of the interface states between gate metal and semiconductor in sub-micron devices depend on growth technique, associated processing parameters and surface states on III-V semiconductors. Correlation between nonideal current-voltage (I-V) characteristics and interface states has been established through the bias dependence of ideality factor. Ideality factor determined from I-V characteristics of MESFET and HEMT increases with bias and then decreases after reaching a maximum. A theoretical model based on nonequilibrium approach has been used to determine the density of interface states and their energetic distribution from ideality factor. Essentially, Fermi level shifts with applied bias and Schottky barrier height changes due to trapping and detrapping of electrons by the interface states, and from these changes, density of interface states and their energetic distributions have been determined.

Electrocatalyst (EC)-modified semiconductor (SC) photoelectrodes are key elements of solar water-splitting systems. The SC|EC interface affects the composite photoelectrode behavior but is poorly understood. We uncover the role of EC activity and SC|EC interface properties using a range of metal (Ni, Fe, Ni-Fe, Co, Ir) oxide or (oxy)hydroxide ECs deposited on model single-crystal n-TiO2 photoanodes. The impedance and photoelectrochemical response of the system was nearly independent of EC oxygen evolution activity if the catalyst was deposited electrochemically as an ion-permeable (oxy)hydroxide or hydrous oxide. When dense oxides (e.g., ion-impermeable) ECs were used, the response depended strongly on the EC. These data demonstrate that the EC and SC interface structures are more important than the EC activity in determining the composite photoanode response, confirming recent SC|EC interface simulations for ion-permeable ECs. These results thus inform the design of high-performance water-oxidizing photoanodes with direct SC|EC interfaces.

Pt Si ∕ Si contacts were studied as a function of thickness. A change of Schottky barrier height up to 100mV from its bulk value was detected when PtSi thickness decreased to 6nm. One explanation is the quantum size effect, generally confirming theoretical predictions. This effect also results in an increase of contact resistance in Ohmic PtSi∕p-Si contacts. Furthermore, the nanoscale PtSi thin layer presented a poor electron screen from the outside contact layer of Al. It was found that for PtSi>50nm, the contact properties of Al∕PtSi∕Si is unchanged by the presence of the Al layer.

An XPS study of thin Pt and Ir silicide overlayer formation on Si(100)2 X 1 surfaces

Electrical properties of Bi2S3nanowires grown using a single source precursor in anodic aluminum oxide templates are sensitive to the relative humidity in an inert gas environment. Dynamic sensing dependency is obtained and shows presence of spontaneous resistance switching effect between low and high relative humidity states. Employing the thermionic field emission theory, heights of Schottky barriers are estimated from the current-voltage characteristics and in relation to the humidity response. The change of Schottky barrier height is explained by local changes in physically adsorbed water molecules on the surface of the nanowire.