Query: Carrier type control of WSe 2 field- effect transistors by thickness modulation

Candidate papers:

abstract: Control of the carrier type in 2D materials is critical for realizing complementary logic computation. Carrier type control in WSe2 field effect transistors (FETs) is presented via thickness engineering and solid state oxide doping, which are compatible with state of the art integrated circuit (IC) processing. It is found that the carrier type of WSe2 FETs evolves with its thickness, namely, p type 15 nm) This layer dependent carrier type can be understood as a result of drastic change of the band edge of WSe2 as a function of the thickness and their band offsets to the metal contacts. The strong carrier type tuning by solid state oxide doping is also demonstrated, in which ambipolar characteristics of WSe2 FETs are converted into pure p type, and the field effect hole mobility is enhanced by two orders of magnitude. The studies not only provide IC compatible processing method to control the carrier type in 2D semiconductor, but also enable to build functional devices, such as, a tunable diode formed with an asymmetrical thick WSe2 flake for fast photodetectors.
author_names: Changjian Zhou

Yuda Zhao

Salahuddin Raju

Yi Wang

Ziyuan Lin

Mansun Chan

Yang Chai
title: Carrier type control of WSe2 field effect transistors by thickness modulation and MoO3 layer doping
year: 2016

abstract: A new sensitive gas sensor based on a self assembled monolayer field effect transistor (SAMFET) was used to detect the biomarker nitric oxide. A SAMFET based sensor is highly sensitive because the analyte and the active channel are separated by only one monolayer. SAMFETs were functionalised for direct NO detection using iron porphyrin as a specific receptor. Upon exposure to NO a threshold voltage shift towards positive gate biases was observed. The sensor response was examined as a function of NO concentration. High sensitivity has been demonstrated by detection of ppb concentrations of NO. Preliminary measurements have been performed to determine the selectivity. 2010 Elsevier B.V. All rights reserved. Human noses can perceive hundreds of thousands of different odor molecules [1] The olfactory system consists of an array of receptors, each of which detects a limited number of substances. This complex system warns us about dangers such as fire, air pollutants or spoiled food. In the past decades electronic noses have been developed that mimic the human olfactory system [2] An electronic nose comprises a gas sampling unit and an array of chemical sensors. Various transducers can be used like carbon black or conducting polymer based chemiresistors, metal oxide semiconductor field effect transistors, and surface or bulk acoustic wave resonators [3] The sensors themselves are not selective; a fingerprint of the smell is ob. All rights reserved. ronics, Zernike Insti, Nijenborgh 4, 9747 s.com (A. Andringa) tained and a neural network is incorporated for pattern recognition. An emerging application is the detection of the biomarker nitric oxide (NO) NO plays an important role in biological functions by acting as a neurotransmitter and by regulating the relaxation of blood vessels [4] Furthermore NO is a marker for airway inflammations such as asthma [5] Measurement of the NO concentration in exhaled breath is applied to diagnose and monitor the inflammation and the obtained information is used as a tool to manage the asthma treatment. NO detection is based on electrochemical, optical or electrical techniques [6,7] Typically, NO is detected indirectly as NO is first converted into NO2 by e.g. CrO3 or ozone [8] Although NO sensors are commercially available [9] there is a demand for small, sensitive NO transducers for point of care use [10,11] Here we introduce a new sensitive gas sensor based on self assembled monolayer field effect transistors (SAMFET) The proof of principle is demonstrated by direct Fig. 1. Schematic representation of the operation principle of a SAMFET sensor. The transistor consists of an organic semiconductor, a source and drain electrode, a gate dielectric and a gate. A conducting channel is formed at the semiconductor gate dielectric interface. On the left side, analyte molecules are absorbed on top of a thick semiconductor layer. The current modulation is limited by the thickness of the semiconductor. The electrostatic interactions are stronger when the distance between the analyte and the channel is decreased. Hence a monolayer thick semiconductor (right) yields the ultimate sensing performance. Fig. 2. Transfer characteristics of a typical SAMFET in vacuum with a 1 lm SiO2 gate dielectric in the linear and saturated regime. The device exhibited p type behavior with a pinch off voltage around 0 V. The inset shows a schematic cross section of the SAMFET sensor. The chemical structures of the SAM molecule (left) and the NO receptor Fe(TPP)Cl (above) are shown. 896 A. Andringa et al. Organic Electronics 11 (2010) 895 898 detection of NO. The operation principle is elucidated in Fig. 1. The left side shows a standard organic field effect transistor consisting from top to bottom of an organic semiconductor, a source and drain electrode, a gate dielectric and a gate. Upon applying a bias to the gate, charge carriers are accumulated at the gate dielectric semiconductor interface. A conducting channel is formed with a thickness of approximately 1 2 nm [12] Analyte molecules absorbed on top of the semiconductor can modulate the charge transport in the channel by electrostatic interactions. However, it has been demonstrated by Huang et al. [13] that the sensitivity of such a sensor is dependent on the thickness of the active layer. The response of organic transistors to nerve agent simulants increased dramatically with decreasing layer thickness, due to the strong distance dependence of the electrostatic interactions. Torsi et al. [14] have reported chiral sensors and argue that the sensing is restricted to the conducting channel. The sensitivity did not increase with decreasing layer thickness but this could be due to the granular nature of the semiconducting films. Finally, sensitivity enhancement of up to an order of magnitude has been reported in transistors by using ultra thin semiconducting films [15] Hence a semiconductor of only one monolayer thickness should yield the most sensitive gas sensor, as shown in Fig. 1 on the right. A SAMFET has recently been reported by Smits et al. [16] The monolayer consists of molecules with a semiconducting quinquethiophene core and an aliphatic spacer that is attached to the gate dielectric with a monofunctional anchoring group. The chemical structure of chloro1/211 d5 ethyl 2;2 5;2 5;2 5;2 0000 quinque thien 5 ylThundecyl dimethylsilane is shown in the inset of Fig 2. The SAMFETs were fabricated on heavily doped n type Si wafers, acting as common gate electrode, with a 1000 nm thermally oxidized SiO2 layer as gate dielectric. Gold source and drain electrodes were defined by conventional photolithographic methods, resulting in ring transistors with a channel length of 10 lm and a width of 2500 lm. Semiconducting monolayers were self assembled from a toluene solution on a HF activated SiO2 dielectric, as described previously [16] The SAMFETs were annealed in vacuum at 110 C for one hour to remove residual water and solvents. Electrical measurements were performed under vacuum using an Agilent 4155B semiconductor parameter analyzer. Possible gate bias stress effects in the electrical measurements were prevented by using a short integration time of less than 1 ms per step. A typical transfer curve is presented in Fig. 2. The mobility is about 0.01 cm/Vs and the current modulation six decades, in good agreement with previous reports. The response of the SAMFETs was measured by admitting small amounts of NO, diluted in nitrogen carrier gas, to the closed chamber. The gas pressure in this static system was used to calculate the partial NO concentration at 1 bar. The response of the SAMFET itself to NO is indistinguishable from random drift of the threshold voltage. To make the SAMFET specific for NO, a porphyrin receptor was used. Porphyrins are known to bind NO in e.g. biological systems [17] Here we used iron tetraphenylporphyrin chloride (Fe(TPP)Cl, Sigma Aldrich) that was previously used to detect NO in solution with a molecular controlled semiconductor resistor [18] In the ideal case this receptor is grafted on, or incorporated into, the monolayer. Here, to demonstrate the concept, we used a thin spin coated film. The porphyrin receptor was dissolved in toluene, 1.6 mg/ ml, and thin layers were spin coated at 800 rpm. The films Fig. 4. Linear plot of the transfer characteristics of the SAMFET sensor. The measurements performed in vacuum and nitrogen were identical. The transfer curve measured 30 min after exposure is shifted towards positive values for increasing NO concentrations. The inset shows the threshold voltage shift as a function of NO concentration. The detection limit is as low as sub 100 ppb NO. A. Andringa et al. Organic Electronics 11 (2010) 895 898 897 are only 10 nm thick and they contain a lot of pinholes. Therefore the diffusion of the nitric oxide is not limiting the detection rate. The addition of porphyrin on the SAMFET had no significant influence on the performance of the SAMFET. The chemical structure and the device layout are schematically depicted in the inset of Fig. 2. The transfer curve of the SAMFET with the porphyrin shifts upon exposure to NO. We observe that the field effect mobility remains unaffected; the only effect is a change in the threshold voltage towards positive gate bias. This clearly points to an increase of fixed negative interface charges upon exposure to NO. However, the reaction is not instantaneous, the threshold voltage shifts with time. A typical example is presented in Fig. 3. A possible reason might be the slow supply of negatively charged minority carriers needed to convert NO into NO x an effect that is presently under study. The threshold voltage shift takes about half an hour to saturate. Hence, in order to arrive at a dose response curve, the transfer curves were measured 30 min after NO exposure. To exclude any influence of competing charging effects, the threshold voltage of the SAMFET was monitored in the absence of NO. This measurement did not reveal a change of threshold voltage in time. The response to NO of the SAMFET without porphyrin was also investigated. These reference measurements are shown in Fig. 3. Only the combination of the SAMFET and the receptor in NO atmosphere yields a shift in threshold voltage. We made many sensors and investigated the response to NO. Fig. 3 discussed above showed a typical measurement for 2.7 ppm NO. Concentrations in the ppm regime can reproducibly be detected. In exceptional cases however a much higher sensitivity has been measured. An example is presented in Fig. 4 where the transfer curves measured in the linear regime are presented on a linear scale as a function of the NO concentration. The transfer Fig. 3. The response of the SAMFET sensor as a function of time for a fixed NO concentration (2.7 ppm) The threshold voltage shifts towards more positive voltages and saturates after approximately 30 min. Reference measureme
author_names: Anne-Marije Andringa

Mark-Jan Spijkman

Edsger C P Smits

Simon Gijsbert Josephus Mathijssen

Paul A van Hal

Sepas Setayesh

Nicolaas Petrus Willard

Oleg V Borshchev

Sergei A Ponomarenko

Paul W M Blom

Dago M de Leeuw
title: University of Groningen Gas sensing with self assembled monolayer field effect transistors
year: 2018

abstract: We propose to investigate the two dimensional heterojunction interlayer tunneling field effect transistor (Thin TFET) The objectives of this research are; 1) to understand the vertical tunneling process between two 2 D semiconductor layers, and 2) to demonstrate Thin TFETs, otherwise stated, vertical 2D crystal TFETs with tunneling aligned with the gate field. Along with common metals (e.g. Au, Ti, Pt) and chemicals found in the cleanroom, we will be using 2D layered materials Mx2, where M tungsten (w) tin (Sn) molybdenum (Mo) and x sulfide (S) selenide (Se) telluride (Te) Summary of Research: Tunneling FETS promise sub60 mV/dec of sub threshold swing (SS) which cannot be realized in traditional MOSFETs. With this type of device, we can greatly reduce the amount of supply voltage (VDD) that is required to turn on devices, keeping offstate current sufficiently low. To realize TFETs with sub60 mV/dec of SS, strong gate control should be required to modulate the entire portion of conducting channel. The atomic scale thickness of layered twodimensional (2D) materials makes it possible to achieve strong gate modulation across the channel, and their surface with no dangling bond provides excellent condition for steep SS [1,2] Among various layered materials, we focus on WSe2 and SnSe2 to form nearly broken bandgap alignment, which is a desirable condition for tunneling devices. SnSe2 was exfoliated from bulk materials using polydimethylsiloxane (PDMS) and examined under microscope to find a flake desirable to be stacked together. Then the target flake was transferred onto SiO2/Si substrate and the same process was done for WSe2 flakes to locate it on top of SnSe2 flakes. Veeco atomic force microscope (AFM) was used to characterize the thickness of exfoliated flakes. Since the band gap of layered materials depends on the number of layers, it is important to know the flake thickness for appropriate band alignment. The typical size of exfoliated flakes is micronscale, which means the pattern for electrodes cannot be easily formed with traditional photolithography facilities. JEOL 6300 electron beam lithography system was used, which allows for the formation of submicron size patterns. After the electron beam exposure using resist poly methyl methacrylate (PMMA) followed by development, 5 nm of Cr and 50 nm of Pd were deposited as contact metals using the SC4500 evaporator. Figure 1 shows the stacked SnSe2 (bottom) and WSe2 (top) flakes with deposited metal electrodes. Instead of SiO2 as a backgate oxide, solid polymer, polyethylene oxide cesium perchlorate (PEO:CsClO4) was adopted as a topgate dielectric layer [3] With high capacitance and no electric conductivity, this polymer is a good candidate for highefficiency gate dielectrics required for steep SS. The devices were topgated with side metal pad near the flakes as shown in Figure 1. As it takes time for Cs+ and ClO4 ions to respond to changing gate voltage, 10 seconds of sweep delay was applied per 10mVchange in gate voltage. For TFETs operation, the band alignment of two flakes should be modulated to change the size of tunneling window for carriers. When gate voltage is applied, the relative position of Fermi level in band diagram is shifted to the upper or lower side accordingly.
author_names: Suresh Vishwanath

Rusen Yan

Mingda Li

Xiang Li

Hyunjea Lee
title: Two Dimensional Heterojunction Interlayer Tunneling Field Effect Transistor Thin TFET CNF Project Number 2324 15
year: 2017

abstract: Dear editor, Two dimensional (2D) semiconductors have emerged as one of the most promising material candidates for next generation electronic devices [1] Owing to the broad range of bandgap diversity and the pristine interface, 2D semiconductors have triggered tremendous research interest for various device applications, especially in tunnel field effect transistors (TFETs) [2 4] To date, most of the experimental demonstrations of 2D based TFETs mainly focus on either n type or p type device, and are realized in different material systems, which is not preferred for highly integrated circuits. In [5] both n type and p type TFETs based on the reconfigured structure and black phosphorus (BP) material have been realized through electrical modulation of carrier types of the source and the drain. However, due to the inherent small band gap of BP, the leakage current and on/off current ratio of devices are unsatisfactory. Complementary TFETs based on the same 2D material system with superior performance are still in urgent need. Moreover, most of the 2D TFET reports focus on the reduction of effective tunnel barrier height for drive current enhancement, while few studies have been reported for the improvement of output characteristics. Some typical features in the output curves of TFETs, such as super linear onset phenomenon, could degrade the dynamic property and static noise margin of digital circuits [6] Therefore, both transfer and output characteristics should be considered for the complementary TFET design. Taking into consideration of transfer and output characteristics, both n type and p type TFETs are designed and experimentally demonstrated based on the WSe2/SnS2 van der Waals heterostructure. The type II band alignment of WSe2/SnS2 heterostructure with reduced effective tunnel barrier height is beneficial for high tunnel current, and the thicknesses of WSe2 and SnS2 in n TFET and pTFET are optimized separately to suppress the super linear onset in output curves. The fabricated complementary TFETs show on/off current ratio of 10, which is much higher than that of the previously reported BP TFETs [5] The tunneling mechanism of fabricated devices is also confirmed by the temperature dependence characteristics. Device structure and design principle. Figures 1(a) and (b) show the schematic structures of the bottom gated complementary TFETs based on van der Waals heterostructure in this study. The SiO2 and highly doped Si are used as the gate dielectric and the bottom gate, respectively. The channel layer is designed to be under the source layer so that the channel potential could be controlled by the bottom gate with the better gate controllability. Besides, taking into consideration of both transfer and output characteristics, the 2D based complementary TFETs are designed according to the following principles. Take the pTFET for example.
author_names: Rundong Jia

Liang Che Chen

Qianqian Huang

Ru Huang
title: Complementary tunneling transistors based on WSe2/SnS2 van der Waals heterostructure
venue: Science China Information Sciences
year: 2019

abstract: 2010 WILEY VCH Verlag Gm The creation of functional interfaces by combining different classes of materials is one of the emerging interests in condensed matter science. This trend will become increasingly important for developing novel processing techniques or device concepts to realize new applications having vastly superior performance and much lower production costs than achievable with current devices. A big challenge lies beyond well established semiconductor devices exemplified by silicon metal oxide semiconductor field effect transistors (MOSFETs) and high electron mobility transistors based on modulation doped III V heterostructures. In this regard, recently there have been remarkable advances in the growth of abrupt interfaces between metal oxides, for example, demonstrating high mobility 2D electron gas (2DEG) at LaAlO3/SrTiO3 and MgxZn1 xO/ ZnO. In both interfaces, a strong built in potential arises from the polarization mismatch, resulting in the formation of the 2DEG. A sophisticated field effect control method has recently been developed for the former by using a scanning atomic force microscope (AFM) tip as a nanometer scale gate, whose principle relies on the retention of charge trapping at the surface states. Metal semiconductor FET (MESFET) and conventional MOSFETstructures can in principle be used for these oxides, yet a number of nontrivial issues remain for realizing practical applications. The notion of a Schottky barrier is a purely electrostatic effect arising from surface space charges in semiconductors, but its energy height tends to remain intact in covalent bonding semiconductors due to the presence of a surface pinning state (i.e. the Bardeen limit) Oxide semiconductors, on the other hand, inherently accord with the Schottky model (i.e. Schottky limit) In reality, however, the quality of the Schottky contacts on oxides is highly sensitive to the processes used for chemical cleaning of the surface and metallization with noble metals such as gold, silver, and platinum. Instead of addressing the subtle issues of conventional processes, we chose a totally different approach. The experiments reported here are based on a simple spin coating process to make a MESFET structure with unconventional materials, in which a 2DEG channel is tunable from a normally on state to a dilute off state. The MESFET devices, which are composed of wide bandgap MgxZn1 xO/ZnO heterostructures and a conducting polymer, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) exhibited excellent performance including a 2D electron mobility in excess of 20 000 cmV 1 s 1 at 2 K. The 2D quantum transport inMgxZn1 xO/ZnO heterostructures was successfully modulated by an electrostatic field effect via the polymer oxide interface. The chemical structure of PEDOT:PSS is shown in the inset of Figure 1a. A polythiophene backbone of PEDOT molecules (broken line) forms a hole conduction pathway and PSS molecules act as acceptors. The PEDOT:PSS thin film has a resistivity as low as several milliohm centimeters, a large work function of 5.0 eV, suitable for making a Schottky contact with n type semiconductors, and high optical transparency in a wavelength range from 250 to 800 nm (Fig. 1a) By combining this transparent polymer Schottky contact with ZnO based materials, high performance visible blind photodiodes have been demonstrated. The schematic device structure used in this study is shown in Figure 1b. Three heterostructures having different Mg contents (x) and thicknesses (d) of the MgxZn1 xO layer were prepared, as listed in the inset table in Figure 1b. We fabricated two individual devices located close to one another for magnetotransport and current/capacitance voltage measurements, respectively (Fig. 1c) The devices showed remarkable stability in their electrical properties during measurements at temperatures from 300 to 2K, repeated intermittently over the course of months. Figure 1d shows typical current voltage (I V) curves for sample A at 300 and 2K. A similarly large rectification ratio of 10 and ideality factor (n) near unity at 300 K were obtained for all devices, indicating a homogeneous Schottky contact over a wide area. Figure 1e shows the gate voltage (VG) dependencies of capacitance (C, black curve) and longitudinal resistivity (rxx, orange curve) measured for sample B at 2 K, indicating consistent properties along the carrier depletion direction. As VG was swept
author_names: Masaki Nakano

Atsushi Tsukazaki

Akira Ohtomo

Kazunori Ueno

Shunsuke Akasaka

Hiroyuki Yuji

Ken Nakahara

Tomoteru Fukumura

Masashi Kawasaki
title: Electric field [corrected] control of two dimensional electrons in polymer gated oxide semiconductor heterostructures.
venue: Advanced materials
year: 2010

abstract: Functional Oxides with strongly correlated electrons show characteristic phenomena such as colossal magnetoresistance, metal insulator transition, caused by strong coupling among spin, charge and orbital degrees of freedom. Among many correlated oxides, vanadium dioxide (VO2) is the prototypical material, possessing a dramatic resistance and magnetism changes between a metallic state with paramagnetism and an insulating state with non magnetism induced by dimerized singlet spins of vanadium atoms around 340 K. This metal insulator transition (MIT) can be induced by a variety of external stimuli, such as thermal variation, lattice strain and electric field [1] Controlling of MIT by an electric field is especially expected toward the realization of Mott transistor. In this research, we fabricated VO2 nano wire channel based field effect transistors [2] with a side gate type, bilayer insulating gates, air gap side gate type so as to discuss their modulation mechanism. Fig. 1(a) shows planer type FET device geometry, where VO2 thin films were deposited on TiO2 (001) substrate [3] and Al2O3 (0001) substrate [4] by using a pulsed laser deposition technique and formed as nanowire channel and side gate patters with a variety of width through nanoimprint lithography method. The electrical transport measurement was conducted in dried N2 to neglect electrochemical reaction. The change rate in resistance (defined as (Roff Ron)/Roff, where Roff and Ron is the resistance of off bias and on bias states, respectively) on Al2O3 (0001) substrate is 0.4% at gate bias VG 30 V, while the rate on TiO2 (001) substrate is 4.5% (Fig. 1(b) which is 10 times higher than that using Al2O3 (0001) substrate. It is considered that the huge difference was caused by obtaining single crystalline epitaxial VO2 on TiO2 (001) substrate. Furthermore, the change rate in resistance depend on wire width of VO2 channels, that is, when reducing wire width of VO2 channels rom 3000 nm to 300 nm on TiO2 (001) substrate, resistance modulation ratio enhanced from 0.7 to 4.5 This indicated the advantage of nanowire and the modulation rate will be drastically enhanced in narrower width. Next, epitaxial VO2 nanowire based FETs with high k inorganics Ta2O5/organic polymer parylene C hybrid solid gate insulator [5 7] were prepared to enhance induced carrier density. In the typical device configuration, the width of VO2 nanowire is 100 nm; the thickness of parylene C is 80 nm; and the overlay Y doped Ta2O5 is of 250 nm thick. Their change rate in resistance gradually increased at each given gate bias from 280 K to 295 K, which is close to the phase transition temperature, and suddenly became almost zero at 300 K in metallic state. The maximum value of resistance modulation is up to 8.58% for VG= 30 V, near the phase transition temperature at 295 K. Fig. 2 plotted the Fig. 1(a) AFM image of double side gate FET, (b) Time dependence resistance switch by applying gate bias for VO2 nanowire channel on TiO2 (001) substrate
author_names: H Tanaka

T Kanki
title: Nanoscale electrostatic and electrochemical transistors in correlated oxides
year: 2017

abstract: Organic thin film transistors (OTFTs) with the viewpoints of low cost/low temperature processing, mechanical flexibility, and large area coverage, have spurred tremendous interest within the development of consumable electronics and optoelectronics such as flat panel display drivers, radio frequency identification tags, smart cards, and sensors. (1 5) The typical architecture of OTFTs has adopted inorganic metal oxide semiconductor field effect transistors in which the source and drain lie on the same plane above or below the semiconductor layer. Therefore, the performance of conventional TFTs is determined by two design parameters, the channel length (L) and field effect mobility (mFE) in accordance with tSD 1/4 L 2 (mFEVDD) where tSD is the source drain response time and VDD the supply voltage. (6) Although, substantial progress has been achieved in developing new organic materials with higher mFE, typically confined to 1cm 2 V 1 s 1 (7) for conventional OTFTs, they still meet the lower operation frequencies and the higher operation voltages for some applications, which are mainly due to the low carrier mobility of organic semiconductors, and their frequency responses were also determined by the capacitance of the gate dielectric used. The design of TFTs with high performance relies mainly on the channel dimensions, for example the channel length. Hence, varying the channel length is a critical parameter in enhancing the carrier transport properties across the drain/source in planar type TFTs. However, shortening the channel length suffers from the incapability of integrating organic devices into nanolithographic techniques. Moreover, the contact resistance of semiconductors/metals limits the performance of devices while shortening the channel length down to sub micrometer scales. (8) An alternative approach to shorten the channel length is to adopt a vertical structure for transistors. (9 12) Since the conductive channel is in a perpendicular direction to the substrate, the channel length can be precisely controlled by organic film thickness down to the micrometer or sub micrometer regimes. Owingtothisadvantageoftheshortchannel length,theoperation frequency and the operation voltage can be dramatically improved, even when we use the existing organic semiconductors as channel materials. Accordingly, extensive progress has been achieved in the production of high performance organic vertical type transistors (OVTs) such as polymer grid triodes, (13) hot carrier transistors, (14,15) gate source drain transistors, (16,17) and metal based organic transistors. (18) Although these OVTs present high device performances as compared to the conven tional OTFTs, they still lack apparent saturation regions, which limit their use in low power and high frequency electronic applications. In this communication, we demonstrate a promising vertical type transistor, organic base modulation triode (OBMT) which is fabricated employing two back to back diodes on a flexible plastic substrate. The device exhibits current modulations with apparent saturation whileapplying various supply voltages to the base electrode. The operation mechanism could be attributed to the part of the induced current that flows through the base electrode rather than recombining at the base electrode. The effective mobility of the device is investigated by fitting the current voltage characteristics at high voltages to the space charge limited current (SCLC) model. Since the OBMT exhibits an operation behavior similar to that of conventional OTFTs, an inverter operated at low supply voltages can also be achieved by integrating a p channel OBMT with a load resistor; such an inverter circuit and their voltage transfer curves are also demonstrated.
author_names: Shiau-Shin Cheng

Y C Chuang

Dhananjaya Kekuda

Chun-Wei Ou

Meng-Chyi Wu

Chih-Wei Chu
title: Organic Base Modulation Triodes and Their Inverters on Flexible Substrates
year: 2009

abstract: n type SiGe modulation doped hetero field effect transistors (MODFET's) with a 0.25 mm Schottky gate on a Si/sub 0.55/Ge/sub 0.45/ buffer are presented. The layer structure was designed to enable elevated sheet carrier densities of n/sub s/=7.0x10/sup 12/ cm/sup 2/ at moderate electron mobilities of 1050 cm2/Vs. Reducing the thickness of the cap layers enhances the control of the gate on the 2DEG and leads to a high transconductance of 320 mS/mm. Targeting analog applications, we focused on large current densities around 400 mA/mm. Due to advanced RF characteristics the 100 GHz hurdle of fmax was passed for the first time with fmax(U)=120 GHz and fT was determined at 42 GHz.
author_names: M Zeuner

Thomas Hackbarth

G Hock

D Behammer

Ulf Konig
title: High frequency SiGe n MODFET for microwave applications
venue: IEEE Microwave and Guided Wave Letters
year: 1999

abstract: Transition metal dichalcogenide (TMD) with layered structure, has recently been considered as promising candidate for next generation flexible electronic and optoelectronic devices because of its superior electrical, optical, and mechanical properties.[1] Scalability of thickness down to a monolayer and van der Waals expitaxial structure without surface dangling bonds (consequently, native oxides) make TMD based thin film transistors (TFTs) that are immune to the short channel effect (SCE) and provide very high field effect mobility ~200 cm/V sec that is comparable to the universal mobility of Si) respectively.[2] In addition, an excellent photo detector with a wide spectral range from ultraviolet (UV) to close infrared (IR) is achievable with using WSe2, since its energy bandgap varies between 1.2 eV (bulk) and 1.8 eV (monolayer) depending on layer thickness.[3] However, one of the critical issues that hinders the successful integration of WSe2 electronic and optoelectronic devices is the lack of a reliable and controllable doping method. Such a component is essential for inducing a shift in the Fermi level, which subsequently enables wide modulations of its electrical and optical properties. In this work, we demonstrate n doping method for WSe2 on poly 4 vinylphenol and poly (melamine coformaldehyde) (PVP/PMF) insulating layer and adjust the doping level of WSe2 by controlling concentration of PMF in the PVP/PMF layer. We investigated the doping of WSe2 by PVP/PMF layer in terms of electronic and optoelectronic devices using Raman spectroscopy, electrical measurements, and optical measurements. REFERENCES [1] B. Radisavljevic, et. al. "Single Layer MoS2 Transistors." Nat. Nanotechnol, Vol.6, pp.147 150, 2011. [2] D. Liu, et. al. "Role of Metal Contacts in Designing High Performance Monolayer n type WSe2 Field Effect Transistors." Nano. Lett. Vol.13, pp.1983 1990, 2013. [3] W. Choi, et. al. "High Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared." Adv. Mater. Vol.24, pp.5832 5836, 2012.
author_names: Hyohyun Nam

Hyung-Youl Park

Jin-Hong Park
title: Poly 4 vinylphenol and Poly (melamine co formaldehyde) based Tungsten Diselenide (WSe2) Doping Method
year: 2015

abstract: The discovery of high Tc superconductors has revealed promising applications in optoelectronics. However a basic requirement for the development of successful novel optronic devices is the ability to fabricate high quality Josephson, tunnel or three terminal devices in a reproducible, controllable and reliable way. Large area deposition of YBCO multilayers, whole wafer processing techniques are essential and in great progress. Nevertheless, the degradation of the YBa2Cu3O7 x surface layer close to the junction contacts is observed and induces severe limitations in the devices performances. Experimental evidence of such problems and possible solutions will be presented. 1. INTEREST OF HTC JUNCTIONS FOR PHOTON DETECTION Optoelectronic applications using high Tc superconducting oxides have recently attracted much attention [1 5J For instance the feasibility of sensitive high bandwidth infrared detectors with a wide optical band should provide important applications in the field of space, communications and military industries. In the past highly sensitive detectors have been made in Grain Boundary Josephson junctions (GBJ) formed in BaPbl xBixO3 (BPBO) 16] and more recently in Bal xKxBiO3 (BKBO) granular films 171. Similar results have been observed in low temperature NbAlOx Nb superconductive tunnel junction array (STJ) detectors which can be read using an integrated superconductive readout circuit [8] Up to now, YBa2Cu3O7 x and the higher Tc oxide phases deposited as granular films present much lower responsitivity to infrared radiation [9] Only transition edge microbolometers have been shown to be sensitive enough [5] but it should be difficult to make large array of them due to their narrow operating temperature range and frequency limitation. Superconductive junctions (i.e. Josephson, tunnel or Super Shottky) or three terminal devices (for instance field effect devices. look more promising as photodetectors. In such kind of devices the junctions act as a sensitive thermometer working in a large temperature range below Tc detecting a change of superconductive properties i.e. energy gap or Josephson current (J) decrease in the electrode (or channel) mainly due to thermal heating. When the superconducting electrodes are very thin and when the junction is well sinked to the cold substrate one might expect to observe also some fast non equilibrium effect detection due to flux flow, phase slip, kinetic inductance effect, etc. depending on the devices design [4][1OJ[111. The present difficulty to achieve junctions in YBCO has not yet permitted to make arrays of good junctions. In the present paper we present reliable whole wafer processing techniques for making various kinds of superconductive diodes on two inches R plane sapphire and on MgO wafers. Whe also try to analyze some of the characteristic behaviors and defects of experimental junctions made on YBCO based multilayers. 0 8194 0728 3/92/$4.O0 SPIE Vol. 1597 Progress in High Temperature Superconducting Transistors and Other Devices (1991)1135 2. YBa2Cu3O7 x MULTILAYER DEPOSITION AND WHOLE WAFER JUNCTIONS PROCESSING This study is limited to junctions fabrication on sputtered c axis epitaxial YBa2Cu3O7 films and multilayers deposited on R plane sapphire (q 2 inches) and on 2x2 cm2 MgO (100) wafers. Thin superconductive (10 nm to 300 nm) YBa2Cu3O7 x films are deposited from a single sintered YBa2Cu3O7 x target on heated substrates by the "direct" process, meaning that the oxidation stage of the films occured during the cooling phase after the deposition. Three deposition techniques have been tested in three separate vacuum multitarget systems a laser ablation system using an KrF excimer laser (X 248 nm) an Inverted Cylindrical Magnetron (1CM) sputtering system, a planar magnetron sputtering equipment. Each vacuum system contains other targets in order to deposit buffer layers or tunnel barriers in situ. YBCO films are grown on R plane sapphire covered by a YSZ buffer layer deposited at the same substrate temperature than for the YBCO deposition. In such a case, a thin buffer layer is necessary both to avoid the Al diffusion from the substrate and to fit the physical parameters of the film to those of the sapphire. The YSZ buffer and YBCO layers are grown at 750degC either by RF sputtering or by laser ablation depending on the system with a thickness ranging from 30 to 100 nm. The YSZ layer is (001) oriented on the sapphire with in plane orientation observed by pole figures. Deposition conditions have been given previously F12] 13] and the main properties of YBCO films are listed in table I. Sputtered films deposited on MgO are found to have in plane orientation, a low dechanneling amplitude Xmin 8% at 1.5 MeV and a low surface resistance at 77K following the w2 behavior in the 1 20 GHz range with Rs 1 m( at 20 GHz [131. Various kinds of in situ deposited trilayers have been processed as listed in table 2 using various dielectric or semi insulating barriers and sputtered YBCO or Silver as counter electrode Superconducting Insulator Normal metal (SIN) Superconducting Insulator Superconducting (515) Super Schottky diodes and capacitances with a superconducting electrode. Whole wafer trilayer patterning "SNOP" Sandwiches structures formed of a base electrode, an intermediate layer and a counter electrode have been fabricated using the "SNOP" process. Such a trilayer process has been developped first for NbN oxide NbN and Nb AlOx Nb junctions [141 and extended later for submicrometer areas NbN MgO NbN junctions [15] The main advantages of "SNOP" (Selective Niobium Overlap Process) (figure 1) on other "trilayer" processes are: it needs only two stencil masks, simple "cross type" junctions are accurately defined in area far below 1 jm2, there is no parasitic capacity because there is no overlapping extra insulating layer deposition. 136 SPIE VoL 1597 Progress in High Temperature Superconducting Transistors and Other Devices (1991) The trilayers are patterned using conventional photolithography (Shippley AZ 140031) and Ar ion milling. Then a YSZ self aligned dielectric layer 50 nm thick is RF sputtered at room temperature on a rotating substrate before the base electrode resist stripping in acetone (YSZ Lift off) The interconnecting Ag layer is then DC sputtered at room temperature, insuring a low resistance contact. The interconnecting and counterelectrodes layers are patterned using argon ion milling or wet etching. The etching process is stopped on the surface of YBCO base electrode. Such a process shown in figure 2 precisely defines junctions electrodes down to micrometer sizes and allows to make arrays of 100 junctions in series of 10 x 10 jm2 areas. Cross type junctions made with "SNOP" whole wafer process have been found to have uniform I V curves on two inches wafers without shorts due to lithography or processing (a typical HR TEM cross section of a c oriented YBCO MgO YBCO SIS trilayer is shown on figure 3) They are suitable for checking the specific resistance of the dielectric barrier R(Ocm2) and its dependance on temperature, barrier thickness, electrical field as well as the specific capacitance of the diodes 15] 3. EFFECT OF ELECTRICAL FIELD ON YBa2Cu3O7.X SiOX Au CAPACITANCES The charge carrier modulation of 1% of very thin YB2Ca3O7 x films has been recently observed by J. Mannhart et al 16J in a MISFET structure by the application of an electric field of 4 106V/cm accross SrTiO3 dielectric layer. The relatively low carrier density of high Tc oxide superconductors supports the research of three terminal superconducting field effect devices using a superconducting channel made with a single very thin HTS film 10 nm) 17] or with a Josephson coupled HTS multilayer 18] Such kind of devices may also be interesting as an optical detector 18] For studying the effect of high electric field on HTS thin channel we have made YBa2Cu3O7 x SiOx Au capacitances as trilayers. In order to observe the field effect at high frequency the device requires a dielectric material with a high breakdown field (BD) high dielectric constant (c) (C.EBD 108 V/cm low losses, low leakage currents and limited charge trapping at the YBa2Cu3O7 x dielectric interface. Our first choice, evaporated SiOx is generally a good dielectric used in microelectronics with an intermediate e value. A 100 nm thick SiOx layer has been thermally evporated from a boat in a iO mbar oxygen atmosphere onto a 60 nm YBa2CuO7 x film sputtered with a hollow target [12J. Gold contact areas of 1 mm2 were preparated through a mechanical mask. After capacitance completion the YBCO transition is about 80K, 7K lower than a 60 nm YBCO film sputtered without post processing. The leakage current accross a typical capacitance shown in figure 4a) is found to be slightly unsymmetrical for a positive or negative bias on Au electrode. This might be due to non ohmic behavior of the YBa2Cu3O7 x SiOx interface and charging effects. The resistivity decreased from 3 1015 tkm to 2 1011 0cm for positive bias voltage of 1 V to 10 V respectively at 70K. For an electric field strength of 106 V crrr1 a leakage current of 37 nA through the insulating layer is low enough for practical devices. The breakdown voltage is about 40V which corresponds to a breakdown field of 4 106 Vcm1. Measurement of the real part and the imaginary part of the dielectric constant of the capacity has been performed for different temperatures and frequencies from 120 Hz to 1 MHz (figures 4b and c) SPIE Vol. 1597 Progress in High Temperature Superconducting Transistors and Other Devices (1991)1 137 For both YBCO SiO Au and Au SiO Au capacitances with the same SiO, layer thickness 100 nm) YBCO SiO Au capacitances give higher losses and permittivity values than Au SiO Au up to 100 kHz showing a loss peak probably due to some induced impurity level inside SiO, or at the YBCO SiO interface. At higher freq
author_names: Jean-Claude Villegier
title: HTCS Films for novel Optronic devices
venue: Microelectronic Processing
year: 1992