Electrical characterization of n/p-type nickel silicide/silicon junctions by Sb segregation

In this paper, n/p-type nickel-silicided Schottky diodes were fabricated by incorporating antimony atoms near the nickel silicide/Si junction interface and the electrical characteristics were studied through measurements and simulations. The effective Schottky barrier height (SBH) for electron, extracted from the thermionic emission model, drastically decreased from 0.68 to less than 0.1 eV while that for hole slightly increased from 0.43 to 0.53 eV. In order to identify the current conduction mechanisms, the experimental current-temperature-voltage characteristics for the n-type diode were fitted based on various models for transport of charge carrier in Schottky diodes. As the result, the large change in effective SBH for electron is ascribed to trap-assisted tunneling rather than barrier height inhomogeneity.     

Charge transfer induced polarity switching in carbon nanotube transistors

We probed the charge transfer interaction between the amine-containing molecules hydrazine, polyaniline, and aminobutyl phosphonic acid and carbon nanotube field effect transistors (CNTFETs). We successfully converted p-type CNTFETs to n-type and drastically improved the device performance in both the ON- and OFF-transistor states, utilizing hydrazine as dopant. We effectively switched the transistor polarity between p- and n- type by accessing different oxidation states of polyaniline. We also demonstrated the flexibility of modulating the threshold voltage (Vth) of a CNTFET by engineering various charge-accepting and -donating groups in the same molecule.     

Study of gaseous interactions in carbon nanotube field-effect transistors through selective Si(3)N(4) passivation

Carbon nanotube field-effect transistors with Si(3)N(4) passivated source and drain contacts and exposed carbon nanotube channel show n-type characteristics in air. In contrast, by passivating only the source contact, a diode-like behavior with a maximum current rectification ratio of 4.6 × 10(3) is observed. The rectifying characteristic vanishes in a vacuum but recovers once the devices are exposed to air. From our experiments, key parameters, such as critical gas pressure, adsorption energy of oxygen molecules and the contact barrier height modulation, can be obtained for studying the gaseous interaction in the carbon nanotube devices.     

Effects of the local environment on CNT-based nanoelectronics: development of a microenvironment probe station

In this study, we report the development of a microenvironment probe station capable of detecting the effect of small changes to the local environment around a carbon nanotube conduction channel. The microenvironment probe station is highly versatile and is used to characterize alterations in carbon nanotube field effect transistor electrical behavior in response to changes in temperature, gas species, infrared and ultraviolet light. All devices were electrically characterized in atmospheric, ultrahigh vacuum and oxygen-rich environments. The results suggest that devices could be changed from n-type at 1 x 10(-8) torr through an intermediate ambipolar state at 1 x 10(-4) torr to p-type at atmosphere solely by increasing the oxygen concentration. The average resistance of these carbon nanotube field effect transistors after annealing was observed to decrease by approximately 54% from their initial value under ultrahigh vacuum to their final value in the presence of pure oxygen while corresponding threshold voltages shifts were also observed. Illumination with infrared light resulted in a approximately 10% increase in drain current with an estimated response time <1 fs due to photon-induced electron-hole pair generation. Illumination with ultraviolet light resulted in approximately 5-15% reduction in drain current due to photon-induced desorption of oxygen adsorbate.     

Observation of the urbach tail in the effective density of States in carbon nanotubes

While a number of studies have reported evidence of localized states in carbon nanotube devices, the density distribution of these states has not been reported until now. By measuring trap emission current in carbon nanotube field-effect transistors, we observe a prominent exponential tail in the density of states near the band edge. Since continuous distributions of localized states are typically associated with highly disordered systems, this observation was quite unexpected in carbon nanotubes, which are nearly ideal crystals. This continuum of localized states may explain a variety of phenomena in carbon nanotube systems, including the nearly universal lack of n-type conduction in strongly gated field-effect transistors. While our focus is on carbon nanotubes, this phenomenon may be ubiquitous to low-dimensional semiconductors in nonvacated environments.     

Transmission through the band-gap states in Schottky-barrier carbon nanotube transistors

We have studied the minimum off-state leakage current of ultrascaled Schottky-barrier carbon nanotube transistors (SBCNTs) with midgap Schottky-barrier source/drain contacts. The off-state leakage current is separated into two parts: thermal emission around the top of the Schottky barrier and tunneling through the evanescent band-gap states. Because the transmission through deep band-gap states makes a dominant contribution for ultrascaled SBCNTs, the off-state minimum leakage current increases exponentially with decreasing scaling length of SBCNTs.     

Chemical optimization of self-assembled carbon nanotube transistors

We present the improvement of carbon nanotube field effects transistors (CNTFETs) performances by chemical tuning of the nanotube/substrate and nanotube/electrode interfaces. Our work is based on a method of selective placement of individual single walled carbon nanotubes (SWNTs) by patterned aminosilane monolayer and its use for the fabrication of self-assembled nanotube transistors. This method brings a relevant solution to the problem of systematic connection of self-organized nanotubes. The aminosilane monolayer reactivity can be used to improve carrier injection and doping level of the SWNT. We show that the Schottky barrier height at the nanotube/metal interface can be diminished in a continuous fashion down to an almost ohmic contact through these chemical treatments. Moreover, sensitivity to 20 ppb of triethylamine is demonstrated for self-assembled CNTFETs, thus opening new prospects for gas sensors taking advantages of the chemical functionality of the aminosilane used for assembling the CNTFETs.     

Modulating the performance of carbon nanotube field-effect transistors via Rose Bengal molecular doping

A simple, reliable, and large scale ambient environment doping method for carbon nanotubes is a highly desirable approach for modulating the performance of nanotube based electronics. One of the major challenges is doping carbon nanotubes to simultaneously offer a large shift in threshold voltage and an improved subthreshold swing. In this paper, we report on modulating the performance of carbon nanotube field-effect transistors (CNTFETs) by rationally selecting doping molecules. We demonstrated that Rose Bengal sodium salt (RB-Na) molecular doping can effectively shift the threshold voltage (ΔVth) of CNTFETs up to ～6 V, decrease the subthreshold swing down to 130 mV/decade, and increase the effective field-effect mobility to 5 cm2 V(-1) s(-1). It is also shown that CNTFETs doped with Rose Bengal lactone (RBL) show a smaller variation in ΔVth (～2 V) and subthreshold swing than those doped by RB-Na, which can be attributed to the difference in their molecular structures. The observed right-shift of the threshold voltage is attributed to the positive charge doping of the nanotube conduction channel from Rose Bengal molecules. The resultant lowering of the subthreshold swing is due to the reduced Schottky barrier at the CNT/metal/molecule interface. This room temperature chemical doping approach provides an efficient, simple, and cost-effective method to fabricate highly reliable and high-performance nanotube transistors for future nanotube based electronics.     

Effect of diameter variation in a large set of carbon nanotube transistors

A study involving a large number of carbon nanotube transistors reveals that the nanotube diameter and the metal contact material play key roles in determining the on- and off-state currents of these devices. The results are discussed in terms of the Schottky barrier at the metal-semiconductor junction and the variation of this barrier relative to the alignment of energy levels between the carbon nanotube and the metal.     

Reigniting innovation in the transistor

Today the transistor is integral to the electronic circuitry that wires our lives. When Bardeen and Brattain first observed an amplified signal by connecting electrodes to a germanium crystal they saw that their 'semiconductor triode' could prove a useful alternative to the more cumbersome vacuum tubes used at the time [1]. But it was perhaps William Schottky who recognized the extent of the transistor's potential. A basic transistor has three or more terminals and current across one pair of terminals can switch or amplify current through another pair. Bardeen, Brattain and Schottky were jointly awarded a Nobel Prize in 1956 'for their researches on semiconductors and their discovery of the transistor effect' [2]. Since then many new forms of the transistor have been developed and understanding of the underlying properties is constantly advancing. In this issue Chen and Shih and colleagues at Taiwan National University and Drexel University report a pyroelectrics transistor. They show how a novel optothermal gating mechanism can modulate the current, allowing a range of developments in nanoscale optoelectronics and wireless devices [3]. The explosion of interest in nanoscale devices in the 1990s inspired electronics researchers to look for new systems that can act as transistors, such as carbon nanotube [4] and silicon nanowire [5] transistors. Generally these transistors function by raising and lowering an energy barrier of k(B)T( -1), but researchers in the US and Canada have demonstrated that the quantum interference between two electronic pathways through aromatic molecules can also modulate the current flow [6]. The device has advantages for further miniaturization where energy dissipation in conventional systems may eventually cause complications. Interest in transistor technology has also led to advances in fabrication techniques for achieving high production quantities, such as printing [7]. Researchers in Florida in the US demonstrated field effect transistor behaviour in devices fabricated from chemically reduced graphene oxide. The work provided an important step forward for graphene electronics, which has been hampered by difficulties in scaling up the mechanical exfoliation techniques required to produce the high-quality graphene often needed for functioning devices [8]. In Sweden, researchers have developed a transistor design that they fabricate using standard III-V parallel processing, which also has great promise for scaling up production. Their transistor is based on a vertical array of InAs nanowires, which provide high electron mobility and the possibility of high-speed and low-power operation [9]. Different fabrication techniques and design parameters can influence the properties of transistors. Researchers in Belgium used a new method based on high-vacuum scanning spreading resistance microscopy to study the effect of diameter on carrier profile in nanowire transistors [10]. They then used experimental data and simulations to gain a better understanding of how this influenced the transistor performance. In Japan, Y Ohno and colleagues at Nagoya University have reported how atomic layer deposition of an insulating layer of HfO(2) on carbon nanotube field effect transistors can change the carrier from p-type to n-type [11]. Carrier type switching-'ambipolar behaviour'-and hysteresis of carbon nanotube network transistors can make achieving reliable device performance challenging. However studies have also suggested that the hysteretic properties may be exploited in non-volatile memory applications. A collaboration of researchers in Italy and the US demonstrated transistor and memory cell behaviour in a system based on a carbon nanotube network [13]. Their device had relatively fast programming, good endurance and the charge retention was successfully enhanced by limiting exposure to air. Progress in understanding transistor behaviour has inspired other innovations in device applications. Nanowires are notoriously sensitive to gases such as CO, opening opportunities for applications in sensing using one-dimensional nanostructure transistors [12]. The pyroelectric transistor reported in this issue represents an intriguing development for device applications of this versatile and ubiquitous electronics component [3]. As the researchers point out, 'By combining the photocurrent feature and optothermal gating effect, the wide range of response to light covering ultraviolet and infrared radiation can lead to new nanoscale optoelectronic devices that are suitable for remote or wireless applications.' In nanotechnology research and development, often the race is on to achieve reliable device behaviour in the smallest possible systems. But sometimes it is the innovations in the approach used that revolutionize technology in industry. The pyroelectric transistor reported in this issue is a neat example of the ingenious innovations in this field of research. While in research the race is never really over, as this work demonstrates the journey itself remains an inspiration. References [1] Bardeen J and Brattain W H 1948 The transistor, a semi-conductor triode Phys. Rev 74 230-1 [2] Shockley W B, Bardeen J and Brattain W H 1956 The nobel prize in physics www.nobelprize.org/nobel_prizes/physics/laureates/1956/# [3] Hsieh C-Y, Lu M-L, Chen J-Y, Chen Y-T, Chen Y-F, Shih W Y and Shih W-H 2012 Single ZnO nanowire-PZT optothermal field effect transistors Nanotechnology 23 355201 [4] Tans S J, Verschueren A R M and Dekker C 1998 Room-temperature transistor based on a single carbon nanotube Nature 393 49-52 [5] Cui Y, Zhong Z, Wang D, Wang W U and Lieber C M 2003 High performance silicon nanowire field effect transistors Nano Lett. 3 149-52 [6]Stafford C A, Cardamone D M and Mazumdar S 2007 The quantum interference effect transistor Nanotechnology 18 424014 [7] Garnier F, Hajlaoui R, Yassar A and Srivastava P 1994 All-polymer field-effect transistor realized by printing techniques Science 265 1684-6 [8] Joung D, Chunder A, Zhai L and Khondaker S I 2010 High yield fabrication of chemically reduced graphene oxide field effect transistors by dielectrophoresis Nanotechnology 21 165202 [9] Bryllert T, Wernersson L-E, L¨owgren T and Samuelson L 2006 Vertical wrap-gated nanowire transistors Nanotechnology 17 S227-30 [10] Schulze A et al 2011 Observation of diameter dependent carrier distribution in nanowire-based transistors Nanotechnology 22 185701 [11] Moriyama N, Ohno Y, Kitamura T, Kishimoto S and Mizutani T 2010 Change in carrier type in high-k gate carbon nanotube field-effect transistors by interface fixed charges Nanotechnology 21 165201 [12] Bartolomeo A D, Rinzan M, Boyd A K, Yang Y, Guadagno L, Giubileo F and Barbara P 2010 Electrical properties and memory effects of field-effect transistors from networks of single-and double-walled carbon nanotubes Nanotechnology 21 115204 [13] Liao L et al 2009 Multifunctional CuO nanowire devices: P-type field effect transistors and CO gas sensors Nanotechnology 20 085203.     

Piperidine induced polarity conversion in single-walled carbon nanotube field effect transistors

Piperidine is found to be an efficient electron doping agent that converts as-prepared p-type single-walled carbon nanotube (SWCNT) field effect transistors (FETs) into n-type SWCNT-FETs. Electron transfer from the amine group in piperidine to the SWCNTs is suggested to be the origin of the p- to n-type conversion. The effect of electron doping is further supported by the Raman tangential G(+) and G(-)-peak downshift up to 3 cm(-1) without the peak broadening. No detectable change in the Raman D-peak suggests non-covalent attachment of piperidine to the SWCNTs. A low temperature (110?°C) Si(3)N(4) passivation layer is used to maintain the long term air stability of the converted n-type devices. A complementary SWCNT inverter is demonstrated through integrating the n- and p-type SWCNT-FETs.     

Self-aligned ballistic n-type single-walled carbon nanotube field-effect transistors with adjustable threshold voltage

Near ballistic n-type single-walled carbon nanotube field-effect transistors (SWCNT FETs) have been fabricated with a novel self-aligned gate structure and a channel length of about 120 nm on a SWCNT with a diameter of 1.5 nm. The device shows excellent on- and off-state performance, including high transconductance of up to 25 microS, small subthreshold swing of 100 mV/dec, and gate delay time of 0.86 ps, suggesting that the device can potentially work at THz regime. Quantitative analysis on the electrical characteristics of a long channel device fabricated on the same SWCNT reveals that the SWCNT has a mean-free-path of 191 nm, and the electron mobility of the device reaches 4650 cm(2)/Vs. When benchmarked by the metric CV/ I vs Ion/Ioff, the n-type SWCNT FETs show significantly better off-state leakage than that of the Si-based n-type FETs with similar channel length. An important advantage of this self-aligned gate structure is that any suitable gate materials can be used, and in particular it is shown that the threshold voltage of the self-aligned n-type FETs can be adjusted by selecting gate metals with different work functions.     

Quantum mechanical simulation of hole transport in p-type Si Schottky barrier MOSFETs

A full quantum-mechanical simulation of p-type nanowire Schottky barrier metal oxide silicon field effect transistors (SB-MOSFETs) is performed by solving the three-dimensional Schr?dinger and Poisson's equations self-consistently. The non-equilibrium Green's function (NEGF) approach is adopted to treat hole transport, especially quantum tunneling through SB. In this work, p-type nanowire SB-MOSFETs are simulated based on the 3-band k.p method, using the k.p parameters that were tuned by benchmarking against the tight-binding method with sp3s* orbitals. The device shows a strong dependence on the transport direction, due to the orientation-sensitive tunneling effective mass and the confinement energy. With regard to the subthreshold slope, the [110] and [111] oriented devices with long channel show better performance, but they are more vulnerable to the short channel effects than the [100] oriented device. The threshold voltage also shows a greater variation in the [110] and [111] oriented devices with the decrease of the channel length.     

Gate voltage dependent characteristics of p-n diodes and bipolar transistors based on multiwall CN(x)/carbon nanotube intramolecular junctions

The electrical transport characteristics of multiwall CN(x)/carbon nanotube intramolecular junctions were studied. The junctions could be used as diodes. We found that the rectification resulted from p-n junctions, not from metal-semiconductor junctions. The gate effect was very weak when the diodes were reverse biased. At forward bias, however, some of the p-n diodes could be n-type transistors. Experimental results supported the opinion that the gate voltage dependent property is derived from the Schottky barrier between the CN(x) part and the electrode. Using p-n diodes, a bipolar transistor with nanoscale components was built, whose behavior was very similar to that of a conventional planar bipolar transistor.     

Floating electrode transistor based on purified semiconducting carbon nanotubes for high source-drain voltage operation

We report floating-electrode-based thin-film transistors (F-TFTs) based on a purified semiconducting single-walled carbon nanotube (swCNT) network for a high source-drain voltage operation. At a high source-drain voltage, a conventional swCNT-TFT exhibited poor transistor performance with a small on-off ratio, which was attributed to the reduced Schottky barrier modulation at a large bias. In the F-TFT device, an swCNT network channel was separated into a number of channels connected by floating electrodes. The F-TFTs exhibited a much higher on-off ratio than a conventional swCNT-TFT with a single channel. This work should provide an important guideline in designing swCNT-TFTs for high voltage applications such as displays.     

Erbium silicided n-type Schottky barrier tunnel transistors for nanometer regime applications

The theoretical and experimental current-voltage characteristics of Erbium silicided n-type Schottky barrier tunneling transistors (SBTTs) are discussed. The theoretical drain current to drain voltage characteristics show good correspondence with the experimental results in 10-/spl mu/m-long channel n-type SBTTs. From these results, the extracted Schottky barrier height is 0.24 eV. The experimentally manufactured n-type SBTTs with 60-nm gate lengths show typical transistor behaviors in drain current to drain voltage characteristics. The drain current on/off ratio is about 10/sup 5/ at low-drain voltage regime in drain current to gate voltage characteristics. However, the on/off ratio tends to decrease as the drain voltage increases. From the numerical simulation results, the increase of off-current is mainly attributed to the thermionic current and the increase of drain current is mainly attributed to the tunneling current.     

Reliability tests and improvements for Sc-contacted n-type carbon nanotube transistors

Scandium (Sc) contacted n-type carbon nanotube (CNT) field-effected transistors (FETs) with back and top-gate structure have been fabricated, and their stability in air were investigated. It was shown that oxygen and water molecules may affect both the nanotube channel and Sc/nanotube contacts, leading to deteriorated contact quality and device performance. These negative effects associated with the instability of n-type carbon nanotube FETs can be eliminated through passivating the CNT devices by a thin layer of atomic-layer-deposition grown Al₂O₃ insulator. After passivation, the n-type carbon nanotube FETs are shown to exhibit excellent atmosphere stability even after being tested and exposed to air for over 146 days, and then much smoother output characteristics and reduced gate voltage hysteresis from 1 to 0.1 V were demonstrated when compared with devices without passivation. Lasting power-on tests were also performed on the passivated CNT FETs under large gate stress and high drain current in air for at least 10 h, revealing null device degradation and sometimes even improved performance. These results promise that passivated CNT devices are reliable in air and may be used in practical applications.      

The role of metal-nanotube contact in the performance of carbon nanotube field-effect transistors

Single-wall carbon nanotube field-effect transistors (CNFETs) have been shown to behave as Schottky barrier (SB) devices. It is not clear, however, what factors control the SB size. Here we present the first statistical analysis of this issue. We show that a large data set of more than 100 devices can be consistently accounted by a model that relates the on-current of a CNFET to a tunneling barrier whose height is determined by the nanotube diameter and the nature of the source/drain metal contacts. Our study permits identification of the desired combination of tube diameter and type of metal that provides the optimum performance of a CNFET.     

Post annealing effect on the electrical properties of top-gate SWNT network transistors

High performance top-gate single walled carbon nanotube network transistors are fabricated with aluminum oxide (Al2O3) layer as a gate dielectric by atomic layer deposition. It exhibits large on/off ratio (>10(4)) due to selective growth of semiconducting tubes by the plasma enhanced chemical vapor deposition. I-V characteristics show p-type or n-type depending on the deposition temperature. We investigate the type dependent characteristics for the carrier polarities with the post annealing effect on the top-gate SWNT network transistors. The dramatic change in the polarity of the top-gate SWNT network transistors, from n-type to p-type due to conversion of I-V characteristics is observed by post-annealing at 350 degrees C for 30 minutes under vacuum. Our observation suggests that competition between electron transfer from the Al2O3 layers to the SWNT surface and electron capture by oxygen molecules adsorbed on the tube walls seems to be the key point for the V(th) change as a function of Al2O3 deposition temperature.     

Length scaling of carbon nanotube transistors

Carbon nanotube field-effect transistors are strong candidates in replacing or supplementing silicon technology. Although theoretical studies have projected that nanotube transistors will perform well at nanoscale device dimensions, most experimental studies have been carried out on devices that are about ten times larger than current silicon transistors. Here, we show that nanotube transistors maintain their performance as their channel length is scaled from 3 μm to 15 nm, with an absence of so-called short-channel effects. The 15-nm device has the shortest channel length and highest room-temperature conductance (0.7G?) and transconductance (40 μS) of any nanotube transistor reported to date. We also show the first experimental evidence that nanotube device performance depends significantly on contact length, in contrast to some previous reports. Data for both channel and contact length scaling were gathered by constructing multiple devices on a single carbon nanotube. Finally, we demonstrate the performance of a nanotube transistor with channel and contact lengths of 20 nm, an on-current of 10 μA, an on/off current ratio of 1 x 10?, and peak transconductance of 20 μS. These results provide an experimental forecast for carbon nanotube device performance at dimensions suitable for future transistor technology nodes.