Showing papers on "Schottky barrier published in 2004"

ZnO nanowire field-effect transistor and oxygen sensing property

Abstract: Single-crystal ZnO nanowires are synthesized using a vapor trapping chemical vapor deposition method and configured as field-effect transistors. Electrical transport studies show n-type semiconducting behavior with a carrier concentration of ∼107cm−1 and an electron mobility of ∼17cm2∕Vs. The contact Schottky barrier between the Au/Ni electrode and nanowire is determined from the temperature dependence of the conductance. Thermionic emission is found to dominate the transport mechanism. The effect of oxygen adsorption on electron transport through the nanowires is investigated. The sensitivity to oxygen is demonstrated to be higher with smaller radii nanowires. Moreover, the oxygen detection sensitivity can be modulated by the gate voltage. These results indicate that ZnO holds high potential for nanoscale sensing applications.

Carbon Nanotube Field-Effect Transistors with Integrated Ohmic Contacts and High-κ Gate Dielectrics

Abstract: High-performance enhancement-mode semiconducting carbon nanotube field-effect transistors (CNTFETs) are obtained by combining ohmic metal-tube contacts, high-dielectric-constant HfO2 films as gate insulators, and electrostatically doped nanotube segments as source/drain electrodes. The combination of these elements affords high ON currents and subthreshold swings of 70-80 mV/decade and allows for low OFF currents and suppressed ambipolar conduction. The doped source and drain approach resembles that of MOSFETs and can impart excellent OFF states to nanotube FETs under aggressive vertical scaling. This presents an important advantage over devices with a metal source/drain, or devices commonly referred to as Schottky barrier FETs.

Tunneling versus thermionic emission in one-dimensional semiconductors.

Abstract: This Letter focuses on the role of contacts and the influence of Schottky barriers on the switching in nanotransistors. Specifically, we discuss (i) the mechanism for injection from a three-dimensional metal into a low-dimensional semiconductor, i.e., the competition between thermionic emission and thermally assisted tunneling, (ii) the factors that affect tunneling probability with emphasis on the importance of the effective mass for transistor applications, and (iii) a novel approach that enables determination of barrier presence and its actual height.

A numerical study of scaling issues for Schottky-barrier carbon nanotube transistors

Abstract: We performed a comprehensive scaling study of Schottky-barrier (SB) carbon nanotube transistors using self-consistent, atomistic scale simulations. We restrict our attention to SB carbon nanotube field-effect transistors (FETs) whose metal source-drain is attached to an intrinsic carbon nanotube channel. Ambipolar conduction is found to be an important factor that must be carefully considered in device design, especially when the gate oxide is thin. The channel length scaling limit imposed by source-drain tunneling is found to be between 5 nm and 10 nm, depending on the off-current specification. Using a large diameter tube increases the on-current, but it also increases the leakage current. Our study of gate dielectric scaling shows that the charge on the nanotube can play an important role above threshold.

Hysteretic current-voltage characteristics and resistance switching at an epitaxial oxide Schottky junction SrRuO$_{3}$/SrTi$_{0.99}$Nb$_{0.01}$O$_{3}$

Abstract: Transport properties have been studied for a perovskite heterojunction consisting of SrRuO$_{3}$ (SRO) film epitaxially grown on SrTi$_{0.99}$Nb$_{0.01}$O$_{3}$ (Nb:STO) substrate. The SRO/Nb:STO interface exhibits rectifying current-voltage ($I$-$V$) characteristics agreeing with those of a Schottky junction composed of a deep work-function metal (SRO) and an $n$-type semiconductor (Nb:STO). A hysteresis appears in the $I$-$V$ characteristics, where high resistance and low resistance states are induced by reverse and forward bias stresses, respectively. The resistance switching is also triggered by applying short voltage pulses of 1 $\mu$s - 10 ms duration.

N-type Schottky barrier source/drain MOSFET using ytterbium silicide

Abstract: Ytterbium silicide, for the first time, was used to form the Schottky barrier source/drain (S/D) of N-channel MOSFETs. The device fabrication was performed at low temperature, which is highly preferred in the establishment of Schottky barrier S/D transistor (SSDT) technology, including the HfO/sub 2/ gate dielectric, and HaN/TaN metal gate. The YbSi/sub 2 - x/ silicided N-SSDT has demonstrated a very promising characteristic with a recorded high I/sub on//l/sub off/ ratio of /spl sim/10/sup 7/ and a steep subthreshold slope of 75 mV/dec, which is attributed to the lower electron barrier height and better film morphology of the YbSi/sub 2 - x//Si contact compared with other self-aligned rare earth metal-(Erbium, Terbium, Dysprosium) silicided Schottky junctions.

Solution for high-performance Schottky-source/drain MOSFETs: Schottky barrier height engineering with dopant segregation technique

Abstract: A novel approach for achieving high-performance Schottky-source/drain MOSFETs (SBTs: Schottky Barrier Transistors) is proposed. The dopant segregation (DS) technique is employed and significant modulation of Schottky barrier height is demonstrated. The DS-SBT fabricated with the current CoSi/sub 2/ process show competitive drive current and better short-channel-effect immunity compared to the conventional MOSFET. In conclusion the DS-Schottky junction is useful for the source/drain of advanced MOSFETs.

Leakage mechanism in GaN and AlGaN Schottky interfaces

Abstract: Based on detailed temperature-dependent current–voltage (I–V–T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I–V–T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I–V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.

Electrical Nanoprobing of Semiconducting Carbon Nanotubes Using an Atomic Force Microscope

Abstract: We use an atomic force microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p region, with no Schottky barrier. In the n region, large contact resistances were found which dominate the transport properties.

Thermal annealing effects on Ni/Au based Schottky contacts on n-GaN and AlGaN/GaN with insertion of high work function metal

Abstract: Recent progress in GaN based high electron mobility transistors (HEMTs) has revealed them to be strong candidates for future high power devices with high frequency operation. In order to extract and utilize the favorable GaN material properties, however, there is still a lot to be investigated. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. A high work function metal such as Pt, Ir, Pd or Mo was inserted to the conventional Ni/Au Schottky contact to n-GaN and AlGaN/GaN epilayers, and the Schottky diodes were studied in detail with respect to the thermal annealing in nitrogen ambient. The electrical characteristics were found to be changed by the thermal treatment in each device. A drastic improvement was attained in the Ni/Pt(Ir)/Au system whereas degradation occurred in Ni/Mo/Au by RTA at 500 °C for 5 min. These phenomena were confirmed to be dependent on the work function of each inserted metal. The role of Ni in the Ni/Pt/Au system was also investigated, and it was found to be essential in obtaining better electrical performance in comparison with the diodes without Ni, such as Pt/Au or Ir/Au Schottky electrodes. The AlGaN/GaN HEMTs were fabricated using Ni/Pt/Au gate contacts. Reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing without degrading the transconductance of the transistor, and it was concluded that this technique was promising for high power AlGaN/GaN HEMT electronics.

High performance of potassium n-doped carbon nanotube field-effect transistors

Abstract: We describe a robust technique for the fabrication of high performance vertically scaled n-doped field-effect transistors from large band gap carbon nanotubes. These devices have a tunable threshold voltage in the technologically relevant range (−1.3 V⩽Vth⩽0.5 V) and can carry up to 5–6 μA of current in the on-state. We achieve such performance by exposure to potassium (K) vapor and device annealing in high vacuum. The treatment has a twofold effect to: (i) controllably shift Vth toward negative gate biases via bulk doping of the nanotube (up to about 0.6e−/nm), and (ii) increase the on-current by 1–2 orders of magnitude. This current enhancement is achieved by lowering external device resistance due to more intimate contact between K metal and doped nanotube channel in addition to potential reduction of the Schottky barrier height at the contact.

A new route to zero-barrier metal source/drain MOSFETs

Abstract: A new method for dramatically lowering the Schottky barrier resistance at a metal/Si interface by interposing an ultrathin insulator is demonstrated for the first time, with thermionic barriers less than those reported to date with silicides. Results with Er and near-monolayer thermal SiN/sub x/ at the interface are consistent with simulations of effective metal Fermi level separations from the silicon conduction band of 0.15 V for n-type Si and 45 mV for p-type Si. Simulations of advanced metal source/drain (S/D) ultrathin-body CMOS devices in comparison with competitive doped S/D devices show a significant performance advantage with a barrier to the conduction band of up to 0.1 V.

Improving organic transistor performance with Schottky contacts

Abstract: Organic field-effect transistors (OFETs) with non-Ohmic contacts, e.g., pentacene with gold electrodes, exhibit a linearly growing threshold voltage with increased film thickness due to tunnel injection [R. Schroeder et al., Appl. Phys. Lett. 83, 3201 (2003)]. In this letter, we demonstrate gold/pentacene OFETs with a low threshold voltage independent of pentacene thickness. By doping the pentacene in the contact area with FeCl3 (iron-III-chloride), the metal-insulator-type tunneling barrier was changed to a metal-semiconductor Schottky barrier. Since the injection through a Schottky barrier depends on the potential and not on the electric field, the threshold voltage is no longer a function of the semiconductor thickness. Through selective doping of the area under the electrode, the channel remains undoped, and large on/off ratios are retained.

Comparison of Fe/Schottky and Fe/Al2O3 tunnel barrier contacts for electrical spin injection into GaAs

Abstract: We compare electrical spin injection from Fe films into identical GaAs-based light-emitting diodes (LEDs) using different tunnel barriers—a reverse-biased Fe/AlGaAs Schottky contact and an Fe/Al2O3 barrier. Both types of structures are formed in situ using a multichamber molecular-beam epitaxy system. A detailed analysis of the transport data confirms that tunneling occurs in each case. We find that the spin polarization achieved in the GaAs using the Al2O3 barrier is 40% (best case; 30% typical), but the electrical efficiency is significantly lower than that of the Fe Schottky contact.

Ab initio study of Schottky barriers at metal-nanotube contacts

Abstract: The type of barrier at a metal/CNT junction is one of the key issues in nanotube electronics. Despite the extensive experimental work done to clarify this issue, there is no consensus in the nano-electronics community. We present here the first ab initio calculation on the Schottky barrier and tunneling barrier height of an idealized metal (Au, Pd, Pt) semiconducting (8,0) nanotube junction. All three metal species form Schottky barriers when contacting small diameter nanotubes. Two most important atomic geometrical factors influencing the Schottky barrier height are identified as the metal species and its surface orientation. Pd is found to have the lowest Schottky barrier. Our simulation results give useful insight into the on going experiments.

Correlation between photoelectric and optical absorption spectra of thermally evaporated pentacene films

Abstract: We have measured the spectral photoresponse of Al/pentacene Schottky junction photodiodes and optical absorption spectra of pentacene films thermally evaporated on glass. The photoelectric response exhibited the genuine highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition at 1.97 eV and interband absorption peaks at 2.3 and 2.5 eV. These peaks are also identified in the optical absorption spectra, but they are dominated by additional strong exciton peaks at 1.82 and 2.1 eV. By comparing these complementary measurements, we determine the HOMO–LUMO gap energy of 1.97 eV and the fundamental exciton binding energy of 0.15 eV for thin solid pentacene.

Multimode transport in Schottky-barrier carbon-nanotube field-effect transistors.

Abstract: We present a detailed study on the impact of multimode transport in carbon nanotube field-effect transistors. Under certain field conditions electrical characteristics of tube devices are a result of the contributions of more than one one-dimensional subband. Through potassium doping of the nanotube the impact of the different bands is made visible. We discuss the importance of scattering for a stepwise change of current as a function of gate voltage and explain the implications of our observations for the performance of nanotube transistors.

High-voltage single-crystal diamond diodes

Abstract: Demonstration of a 2.5-kV diamond diode is provided by electrical measurements using a circular gold Schottky contact, with an area >1 mm/sup 2/, on large area freestanding single-crystal diamond consisting of a thin high purity layer ( 1/spl times/0/sup 19/ [B]/cm/sup 3/) substrate with an ohmic back contact. The diode structures were fabricated using a microwave-assisted chemical vapor deposition process. The forward properties of the diode show a space charge limited current, with a forward-voltage drop of 2 V and a hole mobility of 4100/spl plusmn/400 cm/sup 2//Vs at room temperature. For temperatures between 300 K

Measurement of low Schottky barrier heights applied to metallic source/drain metal–oxide–semiconductor field effect transistors

Abstract: This article investigates the extraction of low Schottky barrier heights in the perspective of integration of metal–oxide–semiconductor field effect transistors (MOSFET) with a metallic source/drain. A test structure composed of two back-to-back junctions is proposed to characterize materials with a low Schottky barrier. To complete the proposed measurement setup, particular attention is placed on a Schottky transport model that continuously combines thermionic emission, field emission, and barrier lowering due to image charge. In the case of platinum silicide (PtSi) contact, it is shown that Arrhenius plots can be accurately reproduced over a wide range of temperature and applied bias. A consolidation of the measurement strategy and of the associated transport model is also performed through measurements and simulations on a long channel p-type Schottky barrier silicon-on-insulator MOSFET with PtSi source/drain. A excellent agreement between simulated and experimental current-voltage characteristics is o...

Theory of spin injection into conjugated organic semiconductors

Abstract: We present a theoretical model to describe electrical spin injection from a ferromagnetic contact into a conjugated organic semiconductor. In thermal equilibrium the magnetic contact is spin polarized, whereas the organic semiconductor is unpolarized. The organic semiconductor must be driven far out of local thermal equilibrium by an electric current to achieve significant spin current injection. However, if the injecting contact has metallic conductivity, its electron distribution cannot be driven far from thermal equilibrium by practical current densities. Thus, quasi-equilibration between the conjugated organic semiconductor and the metallic contact must be suppressed to achieve effective spin injection. This requires a spin-dependent barrier to electrical injection that may be due either to tunneling through the depletion region of a large Schottky barrier or to tunneling through a thin, insulating, interface layer. Schottky barrier formation on conjugated organic semiconductors differs from that on inorganic semiconductors inasmuch as contacts made to organic semiconductors often follow near-ideal Schottky behavior, thus permitting the energy barrier to electrical injection to be varied over a wide range by using metals with different work functions. In addition, insulating tunnel barriers to organic semiconductors based on organic molecules can be conveniently fabricated using self-assembly techniques.

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Abstract: Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.

Electron Tunneling at the TiO2/Substrate Interface Can Determine Dye-Sensitized Solar Cell Performance

Abstract: The electric potential distribution in dye-sensitized solar cells plays a major role in the operation of such cells. Models based on a built-in electric field which sets the upper limit for the open circuit voltage (Voc) and/or the possibility of a Schottky barrier at the interface between the mesoporous wide band gap semiconductor and the transparent conducting substrate have been presented. We show that I−V characteristics in the dark and upon illumination are very well explained by electron tunneling, rather than transport over a Schottky barrier, at this interface. Our calculations, based on tunnel currents, show that a discontinuity of the conduction band at the TiO2/FTO interface, rather than a built-in electric field, suffices for efficient electron transfer through this interface, and, thus, for efficient operation of this type of solar cell. Clearly, this will hold only if the photoinduced electrostatic potential barrier between the transparent conducting substrate and the mesoporous wide band ga...

High performance n-doped carbon nanotube field-effect transistors

Abstract: We describe a robust technique for the fabrication of high performance vertically scaled n-doped field-effect transistors from large band gap carbon nanotubes. These devices have a tunable threshold voltage in the technologically relevant range (-1.3V < V_th < 0.5V) and can carry up to 5-6 muA of current in the on-state. We achieve such performance by exposure to potassium (K) vapor and device annealing in high vacuum. The treatment has a two-fold effect to: (i) controllably shift V_th toward negative gate biases via bulk doping of the nanotube (up to about 0.6e/nm), and (ii) increase the on-current by 1-2 orders of magnitude. This current enhancement is achieved by lowering external device resistance due to more intimate contact between K metal and doped nanotube channel in addition to potential reduction of the Schottky barrier height at the contact.

High-speed Schottky-barrier pMOSFET with f/sub T/=280 GHz

Abstract: High-speed results on sub-30-nm gate length pMOSFETs with platinum silicide Schottky-barrier source and drain are reported. With inherently low series resistance and high drive current, these deeply scaled transistors are promising for high-speed analog applications. The fabrication process simplicity is compelling with no implants required. A sub-30-nm gate length pMOSFET exhibited a cutoff frequency of 280 GHz, which is the highest reported to date for a silicon MOS transistor. Off-state leakage current can be easily controlled by augmenting the Schottky barrier height with an optional blanket As implant. Using this approach, good digital performance was also demonstrated.

A 50-nm-gate-length erbium-silicided n-type Schottky barrier metal-oxide-semiconductor field-effect transistor

Abstract: The theoretical and experimental current–voltage characteristics of 50-nm-gate-length erbium-silicided n-type Schottky barrier metal-oxide-semiconductor field-effect transistors (SB-MOSFETs) are discussed. The manufactured 50-nm-gate-length n-type SB-MOSFET shows large on/off current ratio with low leakage current less than 10−4 μA/μm. The saturation current is 120 μA/μm when drain and gate voltage is 1 and 3 V, respectively. The experimental current–voltage characteristics of 50-nm-gate-length n-type SB-MOSFET are fitted using newly developed theoretical model. From the theoretical analysis, the off- and on-current is mainly attributed to the thermionic and tunneling current, respectively. The decrease of tunneling distance at silicon/silicide Schottky junction with the increase of drain voltage gives the increase of tunneling current. This phenomenon is explained by using drain-induced Schottky barrier thickness thinning effect.

Temperature-dependent characteristics of Pt Schottky contacts on n-type ZnO

Abstract: The Schottky barrier height of Pt contacts on n-type (n∼1016 cm−3) thin film ZnO deposited by pulsed laser deposition was obtained from current–voltage measurements as a function of temperature. The resulting values ranged from 0.61±0.04 eV at 25 °C to 0.46±0.06 eV at 100 °C with saturation current densities of 1.5×10−4 A cm−2 (25 °C) to 6.0×10−2 A cm−2 (100 °C), respectively. The reverse current magnitude was larger than predicted by thermionic emission alone. The measured barrier height for Pt on ZnO is similar to the value reported for both Au and Ag rectifying contacts on this material.