Metal-semiconductor Schottky Contact Research Articles

Tribological-behaviour-controlled direct-current triboelectric nanogenerator based on the tribovoltaic effect under high contact pressure

Dynamic metal–semiconductor Schottky contact interfaces suffer from wear even in low-friction direct-current triboelectric nanogenerators (DC-TENGs), which may affect their working stability and limit their practical applications. In this study, the mechanism and relationship between triboelectrification and tribological characteristics of the metal–semiconductor heterojunction interface with a ball-on-flat configuration under a high contact pressure were systematically studied to simultaneously obtain the high triboelectric output and low wear rate of the tribological-behaviour-controlled DC TENG (TCDC-TENG). The working mechanism of the TCDC-TENG could be attributed to the tribovoltaic effect. An increased normal load and sliding frequency enhanced the triboelectric output while increasing the wear loss of the TCDC-TENG. Furthermore, both the triboelectric output and wear loss per unit time increased with the increase in the applied friction power, which was a product of the coefficient of friction (CoF), normal load (FN), and sliding speed (v). By adding polyalphaolefin SpectraSyn 4 as a lubricant, the CoF was lowered from 0.76 to 0.16, and the wear loss considerably decreased by 99.5% after 20,000 cycles of reciprocating sliding, while maintaining almost constant DC output voltage. This study not only presents the strong correlation between triboelectrification and tribology characteristics based on the tribovoltaic effect but also provides a new strategy for the semiconductor-based DC-TENGs for achieving a stable DC triboelectric output and wear resistance.

Effect of charge coupling on breakdown voltage of high voltage trench-gate-type super barrier rectifier

With the rapidly increasing demands pertaining to high voltage applications in modern power electronic systems, power devices have become widely used in today’s power applications. As a major carrier device without unreliable metal-semiconductor Schottky contact, super barrier rectifier (SBR) has been one of promising substitutions for traditional diodes since it was first introduced, owing to its excellent performance and reliability. The main principle behind SBR approach is to create an adjustable potential barrier in the MOS channel. The height of this barrier can be easily adjusted by the doping concentration in the channel and by the oxide thickness. Trench-gate-type SBR (TSBR) with a trench gate is so designed that the junction-type field-effect transistor effect of planar gate structure enables TSBR to be eliminated to have ultralow forward voltages and a good tradeoff between the forward voltages and reverse leakage currents. However, the charge coupling effect under reverse bias, which is usually neglected and not intensively studied, plays an important role in determining the breakdown voltage of TSBR for high voltage applications (above 200 V). In this paper, the two-dimensional electric field distribution influenced by the charge coupling effect is explained and verified by the analytical model and device simulations with TCAD software Sentaurus. Adjusting the key parameters including the trench depth, oxide thickness and mesa width can improve the tradeoff between the forward voltage drops and breakdown voltages. The optimization of key parameters can provide the significant guidance for designing the device structure. Furthermore, some considerations for designing the TSBRs are discussed in this paper. In addition, a novel TSBR with the stepped oxide structure (SO-TSBR) is proposed and demonstrated. Similar to, yet different from, the stepped oxide structure for dual trench MOSFET, the stepped oxide design equipped with this new rectifier possesses the ability to enhance the forward conduction. As indicated by the simulation results, the SO-TSBR reduces the forward voltage drop by 51.49% at a forward current density of 2.5 A/cm<sup>2</sup> compared with the normal structure of TSBR, with almost the same breakdown voltage of 270 V. The optimized TSBRs and SO-TSBRs are very promising rectifiers that can be used in high power electronic systems, because their breakdown voltages are both greater than 250 V.

A useful model to interpret the experimental I-V-T and C-V-T data of spatially inhomogeneous metal-semiconductor rectifying contacts

ABSTRACT The temperature-dependent capacitance-voltage (C-V) and current-voltage (I–V) characteristics of the Ti/p-InP/ZnAu Schottky contacts were investigated in this study. The current across a metal-semiconductor Schottky contact can vary sensitively with the change of the Schottky barrier heights (SBHs) and sample temperature. Thereby, the value of the C-V barrier height increased and the value of the I–V barrier height decreased with a decrease in temperature. Such a change was ascribed to a Gaussian distribution of the spatial inhomogeneity of the SBHs over whole range of the measurement temperatures. Thus, the temperature dependences of the I–V and C-V characteristics were interpreted by a quantitative analysis of spatial distribution (Gaussian distribution) of the SBHs presented as a useful model. Thus, this quantitative analysis was determined from the change of values with (2kT −1).

Hydrogen sensing performance of a GaN-based Schottky diode with an H2O2 treatment and electroless plating approach

A new electroless plating (EP)-Pd/GaOx/GaN Schottky diode-type hydrogen sensor is fabricated and comprehensively studied herein. The GaOx dielectric is formed as a result of a proper H2O2 treatment on the GaN surface. In addition, a 20-period sensitization/activation process and an appropriate EP approach of the Pd catalytic layer are employed to facilitate a high-performance metal-semiconductor (M-S) Schottky contact. Experimentally, an extremely high hydrogen sensing response of 5.5 × 106 (under 1% H2/air gas) and a relatively low detection limit of 5 ppm H2/air are obtained at 300 K. The corresponding response and recovery times are 22 s and 21 s at 300 K, respectively. A new data transmission algorithm based on the Kalman algorithm is also proposed in this work. According to the simulation results, the data transmission volume can be reduced by 90%. The average recovery error rate is less than 0.38%. Furthermore, our developed reduced redundancy method can pre-warn the user of sensor failure. Based on the advantages mentioned above, therefore, the studied device is promising for high-performance hydrogen sensing and Internet of Things (IoT) applications.

Single GaAs Nanowire/Graphene Hybrid Devices Fabricated by a Position-Controlled Microtransfer and an Imprinting Technique for an Embedded Structure.

We developed a new technique to fabricate single nanowire devices with reliable graphene/nanowire contacts using a position-controlled microtransfer and an embedded nanowire structure in a planar junction configuration. A thorough study of electrical properties and fabrication challenges of single p-GaAs nanowire/graphene devices was carried out in two different device configurations: (1) a graphene bottom-contact device where the nanowire-graphene contact junction is formed by transferring a nanowire on top of graphene and (2) a graphene top-contact device where the nanowire-graphene contact junction is formed by transferring graphene on top of an embedded nanowire. For the graphene top-contact devices, graphene-nanowire-metal devices, where graphene is used as one electrode and metal is the other electrode to a nanowire, and graphene-nanowire-graphene devices, where both electrodes to a nanowire are graphene, were investigated and compared with conventional metal/p-GaAs nanowire devices. Conventional metal/p-GaAs nanowire contact devices were further investigated in embedded and nonembedded nanowire device configurations. A significantly improved current in the embedded device configuration is explained with a "parallel resistors model" where the high-resistance parts with the metal-semiconductor Schottky contact and the low-resistance parts with noncontacted facets of the hexagonal nanowires are taken into consideration. Consistently, the nonembedded nanowire structure is found to be depleted much easier than the embedded nanowires from which an estimation for a fully depleted condition has also been established.

Photoinduced triboelectric polarity reversal and enhancement of a new metal/semiconductor triboelectric nanogenerator

We reported a novel metal/semiconductor triboelectric nanogenerator (TENG) based on Au and TiO2 as the friction layers. Upon illumination, the current polarity rapidly reverses compared to the dark state. The negative and positive short-circuit current approximately increase by 12 and 2 times, respectively. Particularly, a photocurrent plateau appears due to the formation of metal-semiconductor Schottky contact in the device. By carefully analyzing the charge transfer in dark and under illumination, it is found that the accumulation of a large number of photoelectrons on the surface of TiO2 and the increase of conductivity are the reasons for the reversal current polarity and significant current increase under illumination. Our work provides a new approach for improving TENG output and a new understanding about the effect of surface states on the triboelectric performance especially the polarity of TENG.

Simultaneously Solving the Photovoltage and Photocurrent at Semiconductor–Liquid Interfaces

In this work, we present a general theoretical and numerical approach for simultaneously solving the photovoltage and photocurrent at semiconductor–liquid interfaces. Our methodology extends drift-diffusion methods developed for metal–semiconductor Schottky contacts in the device physics community into the domain of semiconductor–liquid “pseudo-Schottky” contacts. This model is applied to the study of photoelectrochemical anodes, utilized in the oxidative splitting of water. To capture both the photovoltage and photocurrent at semiconductor–liquid interfaces, we show that it is necessary to solve both the electron and hole current continuity equations simultaneously. The electron continuity equation is needed to primarily capture the photovoltage formation at photoanodes, whereas the hole continuity equation must be solved to obtain the photocurrent. Both continuity equations are solved through coupled recombination and generation terms. Moreover, to capture charge transfer at the semiconductor–liquid int...

(Invited) Interface Engineering of High-k Dielectrics and Metal Contacts for High Performance Top-Gated MoS2 FETs

The 2D materials, in particular Transition Metals Di-chalcogenides (TMD), are potential candidates to scale CMOS technology beyond Silicon. The ability to deposit ultrathin TMD films, down to a mono-layer, offers excellent electrostatic control of the channel by the gate electrode. This enables aggressive scaling of the channel length of transistor, due to superior short channel performance. In particular, few layer Molybdinum Disulphide (MoS2), with a bandgap comparable to Silicon, has emerged as a promising candidate to to enable continued transistors scaling. However, high performance and reliable gate dielectric and contact formation continues to be an open problem. Since the impurity doping strategies for MoS2 are still not clear, the ohmic contact formation is limited by the nature of metal-semiconductor schottky contact. Despite the Fermi level pinning close to conduction band of MoS2, a low source/drain contact resistance value for NMOS transistor is limited by the surface state effects. The problem is exasperated due to large variations in contact resistance values within the wafer, possibly due to surface defects and stoichiometric variations. We demonstrate that appropriate surface preparation can drastically enhance the performance of ohmic contacts on MoS2. In particular, we present a surface-state engineering technique using sulfur treatment of MoS2 before the contact formation. The transistors (L=W= ~1μm) are formed on exfoliated few layer MoS2 flakes (5-7 nm thick). The flakes are treated with ammonium sulphide solution (NH4)2S (Sigma Aldrich, 40% solution in H2O) for 5 min at a temperature of 50 °C, followed by De-ionized water rinse and N2 blow-dry. Nickel (Ni) and Palladium (Pd) contacts are formed using lift-off process. Through extensive material and electrical characterization, we demonstrate that the sulfur treatment reduces to schottky barrier height by 81 meV (Ni) and 135 meV (Pd) due to stronger pinning of Fermi level. This in turn lowers the contact resistance by as much as 10 times with concomitant increase in mobility by a factor of 2. We demonstrate record drain current of 170μA/μm, with complete mitigation of contact resistance and hence drain current variability. Due to the inert basal plane of 2D materials, it is difficult nucleate the growth of dielectrics using CVD and/or ALD techniques, which are routinely used for high-k dielectric integration on Silicon. Although some techniques have been proposed to nucleate the dielectric growth on MoS2, the device performance is inferior due to interface defects. To circumvent this issue, we develop a very high performance HfO2 thin film using electron beam evaporation. The O2 flow rate is optimized during e-beam evaporation to achieve 30nm HfO2 films with k=19 and EOT=6.1 nm. The interface trap density is estimated to be 8.7x1011 /cm2, resulting in a very high field effect mobility value of 63 cm2/V-s. The HfO2 gate oxide results in a very low leakage current of 7x10-7 A/cm2. The top gated transistors fabricated with this dielectric yield one of the highest reported drain current value of 180 μA/μm, with near ideal subthreshold behaviour (60 to 75 mV/decade). We also utilize 30nm HfO2 dielectric as a substrate to deposit MOS2 flakes, instead of conventional 300nm SiO2 substrate. Finally we demonstrate an NMOS inverter circuit using HfO2 top-gated devices using a depletion mode NMOS transistor as a resistive load. The Inverter exhibits an output to input gain of about 9. Figure 1

Boosting photoresponse in silicon metal-semiconductor-metal photodetector using semiconducting quantum dots.

Silicon based metal-semiconductor-metal (MSM) photodetectors have faster photogeneration and carrier collection across the metal-semiconductor Schottky contacts, and CMOS integratibility compared to conventional p-n junction photodetectors. However, its operations are limited by low photogeneration, inefficient carrier-separation, and low mobility. Here, we show a simple and highly effective approach for boosting Si MSM photodetector efficiency by uniformly decorating semiconducting CdSe quantum dots on Si channel (Si-QD). Significantly higher photocurrent on/off ratio was achieved up to over 500 compared to conventional Si MSM photodetector (on/off ratio ~5) by increasing photogeneration and improving carrier separation. Furthermore, a substrate-biasing technique invoked wide range of tunable photocurrent on/off ratio in Si-QD photodetector (ranging from 2.7 to 562) by applying suitable combinations of source-drain and substrate biasing conditions. Strong photogeneration and carrier separation were achieved by employing Stark effect into the Si-QD hybrid system. These results highlight a promising method for enhancing Si MSM photodetector efficiency more than 100 times and simultaneously compatible with current silicon technologies.

Silicon-based Coulomb blockade thermometer with Schottky barriers

A hybrid Coulomb blockade thermometer (CBT) in form of an array of intermittent aluminum and silicon islands connected in series via tunnel junctions was fabricated on a thin silicon-on-insulator (SOI) film. Tunnel barriers in the micrometer size junctions were formed by metal-semiconductor Schottky contacts between aluminium electrodes and heavily doped silicon. Differential conductance through the array vs. bias voltage was found to exhibit characteristic features of competing thermal and charging effects enabling absolute temperature measurements over the range of ∼65 to ∼500 mK. The CBT performance implying the primary nature of the thermometer demonstrated for rather trivial architecture attempted in this work paves a route for introduction of Coulomb blockade thermometry into well-developed contemporary SOI technology.

Analytical Expression for Thermionic Transport Through Isotype Heterojunction Interfaces of Arbitrary Doping Ratio

We present an analytical expression for the current density across an isotype heterojunction valid for arbitrary doping concentration ratios. This result generalizes the standard expression found in the literature, which is limited by the assumption that the doping concentration ratio is equal to one. This result relies on the solution of a transcendental equation associated with the heterojunction boundary conditions by means of the Lambert W function. As done in the derivation of the standard expression, the generalization only considers thermionic emission, but the method can equally be applied for other transport mechanisms. The general result mathematically contains the expression for the current density across a metal-semiconductor Schottky contact as a limiting case, unifying the treatment of these two heterointerfaces into a single general analytical description.

Characterization of Photoinduced Current in Poly-Si Solar Cell by Employing Photoconductive Atomic Force Microscopy (PC-AFM)

In this study, we have attempted to characterize the photovoltaic effect in real-time measurement of photoinduced current in a poly-Si-based solar cell using photoconductive atomic force microscopy (PC-AFM). However, the high contact resistance that originates from the metal-semiconductor Schottky contact disturbs the current flow and makes it difficult to measure the photoinduced current. To solve this problem, a thin metallic film has been coated on the surface of the device, which successfully decreases the contact resistance. In the PC-AFM analysis, we used a metal-coated conducting cantilever tip as the top electrode of the solar cell and light from a halogen lamp was irradiated on the PC-AFM scanning region. As the light intensity becomes stronger, the current value increases up to <TEX>$200{\mu}A$</TEX> at 80 W, as more electrons and hole carriers are generated because of the photovoltaic effect. The ratio of the conducting area at different conditions was calculated, and it showed a behavior similar to that generated by a photoinduced current. On analyzing the PC-AFM measurement results, we have verified the correlation between the light intensity and photoinduced current of the poly-Si-based solar cell in nanometer scale.

Electronic structure of the indium tin oxide/nanocrystalline anatase (TiO2)/ruthenium-dye interfaces in dye-sensitized solar cells

The electronic structure of two interfaces commonly found in dye-sensitized photovoltaic cells based on nanocrystalline anatase TiO2 (“Grätzel cells”) was investigated using photoemission spectroscopy (PES). X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements were carried out on the indium tin oxide (ITO)/TiO2 and the TiO2/cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium dye (“N719” or “Ruthenium 535-bisTBA”) interfaces. Both contacts were investigated using a multistep deposition procedure where the entire structure was prepared in vacuum using electrospray deposition. In between deposition steps the surface was characterized with XPS and UPS resulting in a series of spectra, allowing the determination of the orbital and band lineup at the interfaces. The results of these efforts confirm previous PES measurements on TiO2/dye contacts prepared under ambient conditions, suggesting that ambient contamination might not have significant influence on the electronic structure at the dye/TiO2 interface. The results also demonstrate that there may be a significant barrier for electron injection at the sputtered ITO/TiO2 interface and that this interface should be viewed as a semiconductor heterojunction rather than as metal-semiconductor (Schottky) contact.

Super Barrier Rectifier—A New Generation of Power Diode

The main principle behind the new super barrier rectifier (SBR) approach is to create the ldquosuperrdquo barrier for majority carriers without unreliable metal-semiconductor Schottky contact. The SBR technology creates such barrier in the MOS channel. The height of this barrier can be easily adjusted by the doping concentration in the channel. This paper demonstrates that the new power diodes combine high performance and high reliability for low voltage applications (below 100 V). The underlying concepts and analysis of operation are presented as well as the laboratory test results that compare performance and reliability between Schottky and the new SBR diode.

ZnO devices: Photodiodes and p-type field-effect transistors

The potential use of ZnO-based photonic and electronic devices has been demonstrated by the fabrication of prototype ultraviolet (UV) photodetector and field-effect transistor (FET) devices that contain films of p-type ZnO with arsenic as the p-type dopant. These p-type films have high crystalline quality and show long-term stability. The ZnO UV photodetectors are based on p-n junctions. The FETs are made with metal-semiconductor Schottky contacts on p-type ZnO and are normally off (enhancement) devices. The spectral and electrical characteristics of these devices are presented and explained.

Ferroelectric-like hysteresis loop in nonferroelectric systems

A ferroelectric-like hysteresis loop is obtained for a nonferroelectric system consisting of two back-to-back metal-semiconductor Schottky contacts with a large concentration of traps distributed over a finite thickness near the electrodes. The presented results show that a simple hysteresis loop measured at a single frequency is not always solid evidence of ferroelectricity in low-dimensional structures such as thin films or nanoscale structures.

LEAD-BASED FERROELECTRIC COMPOUNDS: INSULATORS OR SEMICONDUCTORS?

ABSTRACT Lead zirconate-titanate (PZT) epitaxial thin films are considered wide-gap p-type semiconductors with standard Schottky contacts. Capacitance-voltage, current-voltage, as well as the dielectric hysteresis are analyzed using a model based on the conventional metal-semiconductor Schottky contact in which the effect of the ferroelectric polarization on band-bending was considered. In addition, presence of the deep traps and its effect on the measured quantities were considered and discussed. For a single-crystal like PZT films with a remnant polarization of 40 μC/cm2 the free carrier concentration is estimated to about 3× 1018 cm−3, whereas the effective density of the fixed charges in the depletion region is of about 1.8 × 1019 cm−3.

Preferential regrowth of indium–tin oxide (ITO) films deposited on GaN (0001) by rf-magnetron sputter

Effects of thermal treatments on the electrical properties and microstructures of indium–tin oxide (ITO)/GaN contacts have been investigated using a rf-magnetron sputter deposition followed by rapid thermal annealing. ITO films annealed at 800 °C revealed Schottky contact characteristics with a barrier height corresponding to ITO’s work function of 4.62 eV. The evolution of electrical properties of ITO/GaN contacts was attributed to the preferential regrowth of In2O3 (222)//GaN (0001) with an ideal metal–semiconductor Schottky contact. The feasible use of ITO/GaN as a transparent Schottky contact would be realized by the enhanced regrowth of In2O3 at high temperature.

Spin-on metal-n-GaAs contacts: an interface state density analysis

Metal-semiconductor Schottky contacts were fabricated using spin-on Pt, Pd and Ru metals and GaAs. An interface state density analysis of these Schottky contacts was carried out and compared with those formed using conventionally evaporated metallic sources. The results show that the metal-semiconductor interface formed using spin-on metals is comparable with that of conventionally evaporated metallic sources.

Negative transconductance in monocrystalline (Al,Ga)As/NiAl/(Al,Ga)As semiconductor/metal/semiconductor tunneling transistors

The authors present the three-terminal transport characteristics of a resonant-tunneling semiconductor-metal-semiconductor (SMS) structure. The buried metal quantum well consists of a 3-nm-thick NiAl layer, epitaxially integrated in (Al,Ga)As, and is contacted by selectively removing the semiconductor overgrowth. The undoped AlAs tunneling barriers are 2 nm thick and are set back by 5 nm of undoped GaAs from the doped GaAs electrodes. The GaAs doping densities were adjusted to allow for the fabrication of emitter-up, collector-up, and symmetric transistors. The metal-semiconductor Schottky contacts between the NiAl and the cladding (Al,Ga)As layers were studied in order to characterize the individual interfaces and also to confirm the independence of the ultrathin buried metal electrode. Transistor action has been observed at room temperature in emitter-up structures with a wide (70 nm) undoped GaAs collector spacer. Room-temperature negative transconductance values as high as 1.4 mS/mm/sup 2/ have been obtained for large-area (80- mu m diameter) devices.