A folded leg Ka-band RF MEMS shunt switch with amorphous silicon (a-Si) sacrificial layer

By employing a thin silicon sacrificial cap layer for silicide formation, we have fabricated both Pd2Si/Si1−xGex and PtSi/Si1−xGex Schottky-barrier infrared detectors with extended cutoff wavelengths and near-ideal leakage characteristics. Substantial deviation in the spectral response from Fowler’s theory was observed in some silicide/Si1−xGex detectors fabricated using this technique. This deviation was attributed to a parasitic barrier at the interface due to the excess silicon sacrificial layer left unconsumed by the silicide formation process. A simple model which modifies Fowler’s theory to take into account the reduced injection efficiencies of the photoexcited holes through the Si barrier was proposed. Good agreement between this model and the experimental data has been achieved.

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2020Multi-Stimuli-Responsive Field-Effect Transistor with Conjugated Polymer Entailing Spiropyran in the Side Chains Jing Ma†, Jianwu Tian†, Zitong Liu, Dandan Shi, Xisha Zhang, Guanxin Zhang and Deqing Zhang Jing Ma† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Jianwu Tian† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Zitong Liu *Corresponding author: E-mail Address: [email protected]; E-mail Address: [email protected]; Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) , Dandan Shi Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Xisha Zhang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Guanxin Zhang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) and Deqing Zhang *Corresponding author: E-mail Address: [email protected]; E-mail Address: [email protected]; Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) https://doi.org/10.31635/ccschem.019.201900056 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Multi-stimuli-responsive field-effect transistors (FETs) with organic/polymeric semiconductors have received increasing attention. Herein, we report a novel strategy for fabricating multi-stimuli-responsive polymeric semiconductors through the incorporation of spiropyran (SP) groups in the polymer side chains. The semiconducting performances of resultant FETs with a diketopyrrolopyrroles (DPP)-based conjugated donor–acceptor (D–A) polymer, that entails SP units in the side chains ( pDSP-1), could be modulated reversibly through UV and visible light irradiations, or UV light irradiation and heating, or acid treatment and heating. Our studies reveal that during the reversible transformations of closed, open, and the protonated forms of spiropyran, achieved by light irradiations, heating, and under acidic conditions, a large dipole moment changes occur, which triggers the reversible variation of semiconducting performance of the FETs. Download figure Download PowerPoint Introduction In the last few decades, the rapid developments of organic and polymeric semiconductors with high charge mobilities have been witnessed.1–5 Newly conjugated molecules and polymers have been designed, synthesized, and new device fabrication methods have been proposed in order to improve the semiconducting performance of organic semiconductors.6–20 Thin-film field-effect transistors (FETs) based on organic/polymeric semiconductors have shown potential applications in memory devices, flexible displays, and wearable electronics.21–24 In recent years, FETs with external-field tunable electrical properties other than electrical voltage have been of considerable interest.25–40 Since the output current could be controlled simultaneously and independently by voltage, as well as other external fields, it is possible to integrate complex functionalities, such as memory devices, switches, and complex logic operations, in a single FET device.41–43 Often, light is used as the external stimuli for optically tunable FETs.22–40 In fact, their fabrications have been achieved by utilizing photochromic molecules, able to undergo reversible light-induced interconversions between two isomers.41 For instance, Samorì and co-workers25 blended diarylethene derivative with P3HT [poly(3-hexylthiophene)] to generate a semiconducting film, which showed optically reversible FET with characteristic bi-stable states upon alternating UV and visible light irradiations. Additionally, Nuckolls and co-workers26 built optically reversible FETs using spiropyrans (SP) in the dielectric layer. Further, recent work by our lab and others27 incorporated azobenzene groups into the flexible alkyl side chains of a conjugated polymer, which was used successfully to fabricate optically tunable FETs with rapid photoresponsiveness. In addition to light, other external inputs, such as heating and introducing chemical species are also of high interest in the generation of multi-stimuli-responsive FETs.44–46 Herein, we report the fabrication of multi-stimuli-responsive FETs with the semiconducting D–A polymer (diketopyrrolopyrrole (DPP)-quaterthiophene conjugated polymer) with the substituted photochromic SP groups in the side chains ( pDSP-1), shown in Scheme 1. The incorporation of SP into the D–A polymer was based on the following considerations: (1) The closed-form, SP, should transform reversibly into the open-form, merocyanine (MC), and the protonated merocyanine (MCH+) forms, after UV or visible light irradiations, heating, addition of acid, and even the application of mechanical force.47,48 (2) By comparison, the open-form, MC, must possess a larger dipole moment (∼17.7 D) than the untransformed, closed-SP-form (∼4.3 D).49 Such a change in the dipole moment, presumably, generates carrier scattering sites within the semiconducting thin film and; as a result, the charge transporting behavior should be reduced. Thus, this became the basis of our fabrication of multi-stimuli-responsive FETs coupled with pDSP-1. Beside, pDSP-1 is a typical diketopyrrolopyrroles (DPP)-based conjugated donor–acceptor (D–A) polymer, which is known for its high charge mobility and good photostability.50–54 In pDSP-1, the SP moieties were incorporated into the side chains through covalent bonding; accordingly, the SP units were distributed uniformly within the semiconducting layer, and thus, the stability of the resulting device was enhanced. In comparison, SP was used in constructing responsive FETs via physical blending with the semiconducting polymer or by interfacial/dielectric modifications.26,30–35 The results revealed that the current and performance of the thin FETs coupled with pDSP-1 device could be reversibly tuned in a relatively faster manner by alternating UV and visible light irradiations, or by UV light irradiation, followed by heating, or by addition of acid, followed by heating. Scheme 1 | Chemical structure of pDSP-1 and the schematic representation of the multi-stimuli-responsive FET architecture. For comparative mechanistic studies, pDSP-5, containing different contents of SP groups in the side chains, was also prepared. Download figure Download PowerPoint Results and Discussion Synthesis and characterization of pDSP-1 The SP side chains conjugated DPP-based D–A polymer, pDSP-1, was prepared by Stille co-coupling of compounds 3 (with SP-containing linear alkyl chains) and 4 (with branching alkyl chains) with the bis(stannyl) compound 5 (see ). The molar ratio of compounds 3 and 4 was set at 1∶1 to enable the formation of the polymer pDSP-1, in which the molar ratio of the linear chains with SP units versus the branching alkyl chains is 1∶1. For comparison (see below), the polymer, pDSP-5, for which the molar ratio of the linear chains with SP units versus the branching alkyl chains was decreased to 1∶5, was prepared under similar conditions. After purification, pDSP-1 and pDSP-5 were obtained in 80% and 87% yields, respectively. pDSP-1 and pDSP-5 were characterized by 1H NMR, solid 13C NMR, and by elemental analysis. The molar ratios of SP-containing side chains versus branching side chains in pDSP-1 and pDSP-5 were calculated to be ∼1.09∶1 and ∼1∶4.67, being close to the respective feed ratios, based on the elemental analysis data. The number averaged molecular weights (Mns) and polydispersities (PDIs) of pDSP-1 and pDSP-5, determined by high-temperature gel permeation chromatography in o-dichlorobenzene at 140 °C, were 16.8 kg mol−1 and 2.9, and 26.0 kg mol−1 and 1.4, respectively. pDSP-1 and pDSP-5 could be dissolved in 1,1,2,2-tetrachloroethane (TeCA) to reach a concentration of ∼5 mg mL−1 at 100 °C. They could not dissolve in chloroform and nonhalogenated solvents, such as toluene and tetrahydrofuran (THF). Both pDSP-1 and pDSP-5 were thermally stable with decomposition temperatures of 309 °C and 371 °C (at a weight loss of 5%) based on thermogravimetric analysis (TGA) data (). Apparently, the incorporation of SP units in the side chains did not affect the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of pDSP-1 and pDSP-5. On the basis of cyclic voltammograms obtained from cyclic voltammetry (CV) analysis (), the HOMO/LUMO energy levels of pDSP-1 and pDSP-5 were estimated to be −5.18/−3.69 and −5.22/−3.67 eV, respectively (), being close to those of DPP-based D–A polymers reported previously.55–57 Absorption spectral variations for pDSP-1 upon light irradiations, heating, and protonation The SP groups in pDSP-1 were expected to switch reversibly between the closed-form (SP) and the opened form (MC) upon UV and visible light irradiations. As shown in , new absorption peaks in the range of 500–600 nm appeared in the UV-vis absorption spectrum after UV light irradiation of a solution of pDSP-1, and these peaks disappeared after further application of visible light irradiation or heating the solution. Similarly, new absorptions peaks ∼420 nm were observed for the solution of pDSP-1 after the addition of trifluoroacetic acid (TFA or CF3COOH) (see ), and these absorptions disappeared after heating the solution further. We attributed the absorption at ∼420 nm to the formation of the MCH+ form, based on previous reports.47,48 Additionally, such absorption spectral changes could be reversibly repeated for 5 cycles. Moreover, the correlation of the absorption spectral variations with the transformations of SP into MC or MCH+ forms also matched with the data obtained for thin films of pDSP-1. Figure 1a shows the absorption spectra of thin film of pDSP-1 before and after UV light irradiation at 365 nm for 30 s, followed by exposure to visible light irradiation at 470 nm for 10 min. A new absorption at ∼590 nm emerged after UV light irradiation. According to previous studies,47,48 the absorption from 550 to 600 nm is due to the formation of the MC form. Furthermore, visible light irradiation resulted in the gradual decrease of the absorption at 590 nm (see Figure 1b). Visible light irradiation for 10 min was required for the complete disappearance of the absorption due to the MC form. In contrast, this 590 nm peak due to the MC form disappeared quickly after heating the thin film of pDSP-1 at 80 °C for 2 min, as shown in Figure 1c. Indeed, the reversible variation of the absorption at 590 nm for thin film of pDSP-1 could be repeated for five cycles after alternating UV light irradiation and heating at 80 °C, as depicted in Figure 1d. These results indicate that the reversible interconversion between the SP form in pDSP-1 and MC form occurs at fast rates upon UV light irradiation and heating at 80 °C. Moreover, new absorption around 420 nm appeared after the thin film of pDSP-1 was treated with the vapor of TFA (500 ppm for 1 min), and this new peak vanished after further heating at 80 °C for 5 min (Figure 1e,f). We used monomer 3 (see ) as a model for pDSP-1 and pDSP-5 to estimate the conversion ratio by 1H NMR. The 1H NMR spectra of monomer 3 and those after UV light irradiation and treatment with CF3COOD are shown in , respectively. The chemical shifts around 8.65–8.60 ppm, 7.79–7.58, and 6.67–6.64 ppm were due to the MC form (see ), whereas the chemical shifts around 8.50 ppm, 8.29–8.16 ppm, and 7.64 ppm were attributable to the MCH+ form. Based on these 1H NMR data, ∼39.5% of SP units were converted to the MC form after UV irradiation at 365 nm for 30 s. Similarly, ∼82.5% SP units were transformed into the MCH+ moieties after addition of 0.03 M deuterium TFA (d-TFA or CF3COOD). However, the estimated conversion ratios might not reflect the conversion efficiencies of SP into MC (after UV light irradiation) and SP into MCH+ (after treatment with CF3COOH) in thin films of pDSP-1 and pDSP-5. This is because the structures of monomer 3 and pDSP-1/ pDSP-5 are different; thus, their conversion efficiencies are expected to be different both in solution and as thin films. Figure 1 | UV-vis absorption spectra of thin film of pDSP-1: (a) After successive UV light (365 nm for 30 s) and visible light (470 nm for 10 min) irradiations for five cycles. (b) The reversible variations of absorption intensity at 590 nm after alternating UV and visible irradiations. (c) After successive UV light irradiation (365 nm for 30 s) and heating (80 °C for 2 min) for five cycles. (d) The reversible variations of absorption intensity at 590 nm after alternating UV and heating treatments. (e) After successive acid [(CF3COOH) vapor (500 ppm) for 1 min] treatment and heating (80 °C for 5 min) for five cycles. (d) The reversible variations of absorption intensity at 420 nm after alternating acid and heating treatments. Download figure Download PowerPoint Multi-stimuli-responsive FETs We first explored the semiconducting performances of pDSP-1 by fabrication of bottom-gate, bottom-contact (BGBC) FETs before UV light irradiation. Predictably, a thin film of pDSP-1 exhibited a typical p-type semiconducting behavior under ambient conditions based on the transfer and output curves (). As shown in , hysteresis was minimal for the transfer and output characteristics. The maximum/average saturated and linear hole mobilities of thin films of pDSP-1 were measured to be 0.20/0.16 and 0.09/0.07 cm2 V−1 s−1, respectively, after thermal annealing at 160 °C, with on/off ratio of 106–107 (). We further studied the tunable semiconducting properties of FETs with thin films of pDSP-1 after UV light irradiation and heating at 80 °C. Figure 2a and show the transfer and output curves after UV light irradiation at 365 nm for 30 s, followed by heating at 80 °C for 2 min. Undoubtedly, the transfer curve was altered upon UV light irradiation, due to the transformation of the SP moieties of pDSP-1 to MC. Upon heating, the transfer curve was restored, attributable to the conversion of MC to SP. The output curve shows the same tendency (), suggesting that the reversible transformation between SP and MC forms. Such reversible tuning could be repeated for 5 cycles upon the successive UV light irradiation and heating treatment. As shown in Figure 2b and Table 1, the pristine average IDS was measured to be 6.5 × 10−5 A (VGS = VDS = −60 V), and it decreased to 2.1 × 10−5 A after UV light irradiation for 30 s. The drain current increased almost to the initial value after heating the device for 2 min. Figure 2b depicts the reversible variation of the device current for five cycles after alternating UV light irradiation and heating. In addition, the average hole mobility of pDSP-1 thin film could also be reversibly tuned between 0.16 and 0.05 cm2 V−1 s−1. Interestingly, the variation of IDS after UV light irradiation and heating matched well with the variation of the absorption spectra change of pDSP-1 in association with the transformation of the SP and MC forms of SP units in pDSP-1 (Figure 2b). Figure 2 | UV-vis absorption spectra showing: (a) variations in the transfer curves based on different pDSP-1 treatments after alternating UV light irradiation (365 nm for 30 s) and heating (80 °C for 2 min) and (b) the reversible modulation of IDS (VDS = VGS = −60 V) and absorption intensity at 590 nm for five UV/heating cycles. W = 1440 μm, L = 5 μm. I is the drain current upon UV or thermal treatment, while I0 is the initial drain current. A is the absorbance upon UV or thermal treatment, while A0 is the initial absorbance. Download figure Download PowerPoint Table 1 | Average Mobilities (μh), Drain Currents, and Current Variation Ratios for pDSP-1- and pDSP-5-Based FET Devices After UV Light Irradiation or Acid Treatment Polymer μh/cm2 V−1 s−1 I0 Current (10−5 A) Stimulus μh/cm2 V−1 s−1 I Current (10−5 A) Current Variation Ratio (%) pDSP-1 0.16a 6.5a UV 0.05b 2.1b 67d Acid 0.06c 2.3c 65d pDSP-5 0.60a 17.5a UV 0.30b 10.0b 43d Acid 0.28c 9.5c 46d aInitial average mobilities and drain currents (VDS = VGS = −60 V) of pDSP-1 and pDSP-5; baverage mobilities and drain currents after UV irradiations; caverage mobilities and drain currents after acid treatment; dcurrent variation ratio = (I0–I)/I0. We also explored the reversible variations of the semiconducting performances of FETs coupled with pDSP-1 upon either alternating UV and visible light irradiations or alternating acid treatment and heating at 80 °C. As discussed above, the device current and the hole mobility of the thin film of pDSP-1 decreased after UV light irradiation. As shown in Figure 3a,b and , the transfer and output curves for FETs that were treated with UV light, were almost restored to their respective initial ones after further irradiation with visible light at 470 nm for 10 min. Accordingly, the device current (IDS at VGS = VDS = −60 V) was restored to the initial value after further visible light irradiation. Figure 3 | UV-vis absorption spectra showing: (a) variation of transfer curves based on pDSP-1 after alternating UV light (365 nm for 30 s) and visible light (470 nm for 10 min) irradiations; (b) the reversible modulation of IDS (VDS = VGS = −60 V) and absorption intensity at 590 nm for five UV/vis cycles. (c) Variation of transfer curves based on pDSP-1 after alternating acid (500 ppm for 1 min) and heating (80 °C for 5 min) treatment; (d) the reversible modulation of IDS (VDS = VGS = −60 V) and absorption intensity at 420 nm for five acid/heating cycles. W = 1440 μm, L = 5 μm. I is the drain current upon external stimuli treatment, while I0 is the initial drain current. Similarly, A is the absorbance upon external stimuli treatment, while A0 is the initial absorbance. Download figure Download PowerPoint As discussed earlier, and also, displayed in Scheme 1 and Figure 1e, the SP units in pDSP-1 could be transformed into the protonated MC (MCH+) units after exposure to CF3COOH vapor, and the MCH+ moieties, in turn, could switch back to SP upon heating. For this reason, we investigated the variation of semiconducting performances of FETs- pDSP-1 after exposure to the vapor of CF3COOH and heating at 80 °C. First, BGBC FETs were treated with CF3COOH vapor at a concentration of 500 ppm for 1 min, followed by thermal treatment of the devices at 80 °C for 5 min. The transfer and output curves were measured separately before the treatments and after exposure to CF3COOH vapor and heating at 80 °C. As shown in Figure 3c,d, and , both transfer and output curves were varied after exposure to CF3COOH vapor, but the curves were almost restored after further thermal treatment. As listed in Table 1, the average device current (IDS) decreased from 6.5 × 10−5 to 2.3 × 10−5 A with a resultant current variation ratio [(I0−I)/I0] of 65%. After heating at 80 °C, the device current reincreased to 6.5 × 10−5 A. Such current variation could be repeated reversibly by alternating exposures to CF3COOH vapor and thermal treatment. Beside, for drain current, the variation of threshold voltages (Vths) was also observed, as shown in . For instance, Vth for the BGBC FET with pDSP-1 was varied from −5 to −3 V to −10 to −8 V after UV light irradiation. The reversible variation of the semiconducting performance was also observed by treatments with other acids, such as acetic acid (CH3COOH) and hydrochloric acid (HCl), followed by heating, as shown in . These results demonstrated that the semiconducting performances of FETs with pDSP-1 entailing SP units in the side chains could tune reversibly not only by alternating UV light irradiation, visible light irradiation or heating but also by treatment with CF3COOH vapor and heating. Notably, the transfer curves of FETs with pDSP-1 in which the SP units transformed into MC forms could be restored gradually after the devices were left in air for 2 h (see ). This also applied to FETs with pDSP-1 in which the SP units could convert into the MCH+ forms, as depicted in . These results could be attributable to both MC and MCH+ forms able to switch back to the SP forms at room temperature. Thus, the incorporation of SP units in the side chains of pDSP-1 enabled the semiconducting polymer to be multi-stimuli-responsive; accordingly, multi-stimuli-responsive FETs could be fabricated via coupling with such stimuli-responsive semiconducting polymer. Mechanism of FETs/pDSP-1 reversible modulation We further investigated the mechanism of the reversible modulation of the semiconducting performance of FETs with pDSP-1, which is associated with the reversible transformation among SP, MC, and MCH+ upon light irradiations, heating, and acid treatment. We first examined whether the transformation of SP units in pDSP-1 into MC units could influence the interchain packing, thin-film crystallinity, and morphology. We performed geometry measurements by grazing incidence wide-angle X-ray scattering (GIWAXS) on thin film of pDSP-1 untreated and external stimuli-treated samples. The GIWAXS patterns collected for thin film of pDSP-1 before and after UV light irradiation for 30 s and subsequent heating at 80 °C for 2 min are shown in . Our results showed that the 2D-GIWAXS patterns and the corresponding line-cut profiles of thin film of pDSP-1 after UV light irradiation and subsequent heating were almost the same as that of the untreated thin-film (). Both lamellar, including (100), (200), (300), and (400) peaks, and π–π stacking (010) signals for the pristine thin film of pDSP-1 were almost identical to those of the respective thin film after sequential treatments with UV light and heating (). shows the atomic force microscopy (AFM) height images for the untreated spin-coated thin film of pDSP-1 and those after UV light irradiation, followed by heating. The results also revealed no distinct differences in thin-film morphology, consistent with the root-mean-square roughness (RRMS), which remained unchanged for the thin film of pDSP-1 after treatments with UV light and heating. These studies demonstrated that thin-film crystallinity and were not by the transformation of SP units in pDSP-1 into the MC We also measured the of the pristine thin film of pDSP-1 and those after exposure to CF3COOH vapor and sequential thermal treatment at 80 °C (see ) and obtained similar the thin-film of pDSP-1 was also not further that its geometry was not by the treatments. the transformation of SP units in pDSP-1 into the MC units is associated with variations of the dipole moment, that the dipole moment of the SP form is whereas the MC form a dipole moment of In fact, as shown in Figure the for thin film of pDSP-1 were measured to be and before and after UV light irradiations, respectively. Such variation is consistent with the MC form a larger dipole moment than the SP form. is that the MCH+ form, achieved by treatment of the SP form with as the also possess a large dipole According to previous species such as MC and with large dipole might as charge carrier scattering sites by the which to decrease of the FETs device We also observed threshold voltage changes (), which might be due to the variations of charge or semiconducting and we could the reversible tuning of the semiconducting performances of FETs coupled with pDSP-1 to the dipole moment change among SP, MC or MCH+ forms, as in Figure Figure 4 | UV-vis absorption spectra the reversible of based on thin-film of pDSP-1 under UV light irradiation (365 nm for 30 s) and heating treatment (80 °C for 2 Download figure Download PowerPoint Figure 5 | mechanism for multi-stimuli-responsive FETs based on SP-containing semiconducting polymer. Download figure Download PowerPoint further that SP moieties of pDSP-1 are for the charge the polymer pDSP-5 of pDSP-1, Scheme in which the molar ratio of the SP-containing alkyl chains versus the branching ones is 1∶5, was prepared. Thus, polymer pDSP-5 contents of SP units than pDSP-1. After to we that the semiconducting performances of FETs coupled with pDSP-5 could also be tuned reversibly by light irradiations, and heating in the same as FETs with pDSP-1 (see ). However, as shown in Table 1, after UV light irradiation for 30 s, the average device current (IDS at VGS = VDS = −60 V) of pDSP-5 decreased to of the initial current. This further upon exposure of FETs coupled with pDSP-5 to UV and light irradiation, during which the current variation ratio to than that for the FETs device coupled with pDSP-1 under the same conditions. On the other the device current variation ratio exhibited similar after treatment with CF3COOH for FETs coupled with pDSP-5 and pDSP-1. Additionally, as shown in , the for thin film of pDSP-5 were measured to be and before and after UV light irradiations, respectively, and to be than the respective pDSP-1 (see Figure Thus, we from this comparative analysis because pDSP-5 a of SP units than pDSP-1, this resulted in the formation of of MC or MCH+ units after UV light irradiation or treatment with CF3COOH the charge within the thin film of pDSP-5 was by the UV light irradiation or treatment with CF3COOH vapor, with pDSP-1. We report a novel strategy for the of multi-stimuli-responsive polymeric semiconductors through the of SP groups in the side chains of a DPP-based D–A polymer by the of the reversible transformation of SP into MC or MCH+ under light irradiations, and heating. The semiconducting performances of FETs coupled with pDSP-1 that SP units could tune reversibly after alternating UV and visible light irradiations, alternating UV light irradiation, and heating, or alternating protonation with CF3COOH and heating. it was to fabricate multi-stimuli-responsive FETs with such semiconducting SP units in the side chains. Our current results that the molecular transformations among SP, MC, and MCH+ in the side chains of conjugated polymers could affect the charge within the semiconducting thin films. Accordingly, our studies new for applications of semiconducting is of are no

RF-MEMS switches may be divided into capacitive and metal-to-metal types in terms of type of connection. In capacitive switches in the OFF-state the beam does not attach to the transmission line due to the presence of a thin dielectric layer and it gets released easily. Capacitive switches are better compared with the metal-to-metal types due to their ability to transmit signals of higher frequency and power. In this paper, a novel RF MEMS shunt capacitive switch with a unique spring design is proposed. The proposed design benefits from high isolation, low loss, low actuation voltage, small size and weight compared to similar switches and is designed for application in the Ka-band. Step structure is used in the mechanical design of the switch to reduce the air gap between the bridge and the transmission line that has resulted in reduction of actuation voltage to 2.2 V. The proposed switch has been placed upon a coplanar waveguide (CPW) line with an impedance of 50 Ohms. $${\text{SiO}}_{2}$$ with a thickness of 1000 A has been used as dielectric to increase isolation and down-state capacitance. The unique design of the spring results in the reduction of stress to a minimum at all points of the switch especially the springs at the time of applying the actuation voltage. This advantage would reduce switch failures as time passes and results in increased life of the switch. Up-state capacitance is 93.3 fF and down-state capacitance is 2.64 pF. Therefore, a capacitive ratio of 28 is obtained. The dynamic behavior of the switch is studied by using the COMSOL Multiphysics software package and the RF characteristics have been obtained by using the HFSS software and show good RF performance. The results show that in the up-state, the designed switch including S11 is less than − 11.5 dB and S21 is more than − 0.85 dB at 1–40 GHz. In the down-state, the switch has excellent isolation in the Ka-band. Maximum isolation is − 71 dB that occurs at the resonance frequency of 30.5 GHz.

Photovoltaics is a renewable energy that can reduce pollution and climate change effects. A serious alternative to fossil energy, the solar industry is showing exponential growth in production. Currently about 90% of fabricated solar modules are made of crystalline silicon. Materials costs represent about 45% of the total module price. One way to make the photovoltaic industry more competitive is to reduce the consumption of silicon and increase the efficiency of solar cells. Approaches to trimming materials costs include replacing crystalline silicon with silicon thin films deposited on cheap substrates at low temperatures. However, the efficiency of solar cells made using this technique is limited by the low quality of the material. Cutting thinner wafers of crystalline silicon from the ingot can also decrease costs, but mechanical problems limit this method. We are developing silicon thin films that are epitaxially grown on a sacrificial layer and transferred to a cheap foreign substrate.1 To maximize the potential of this material, we combine it with rear contact solar cell technology. Figure 1 illustrates the process we use to create these solar cells. First, we create a sacrificial porous silicon layer on top of a monocrystalline silicon substrate. A high-quality single-crystal silicon film is then grown by epitaxy on this surface. The rear contact solar cell is created on this film. All metallic contacts are deposited on the rear side of the cell to avoid shadowing effects and to allow high efficiencies. A low-cost substrate (glass or ceramic) is then stuck on the back side of the cell. Using the sacrificial layer, the cell is separated from the initial wafer, which can be recycled several times.2 Figure 1. Process to create solar cells with lower cost and higher efficiency.

This paper presents a new monolithic microwave integrated circuit (MMIC) based on coplanar waveguide (CPW) design for a tunable resonator based on RF MEMS. This RF structure, which can be used for system on chip (SOC), is constituted with MEMS Bridge placed between two meander inductors and the tenability is controlled by a variable applied DC voltage. Moreover, this device presents a compactness characteristic and the possibility to operate at high frequencies. The resonant frequency and the bandwidth can be changed easily by changing the bridge gap of the RF MEMS. The numerical simulations of this novel structure of a tunable RF MEMS resonator were performed with the electromagnetic solvers CST MWS (Computer simulation Technology Microwave Studio) and validated by the more accurate electromagnetic solver HFSS (High Frequency Structural Simulator). The simulation results, for three different spacing of the bridge gap, show that the tunable frequency band are between 10 and 40 GHz with the two electromagnetic solvers and exhibiting three resonant frequencies (21, 23.1 and 24.6 GHz).The simulation results of the return loss using CST achieves 29 dB with an insertion loss less than 1 dB; However, the HFSS simulation shows similar performance in the resonant frequencies and in the bandwidth giving better results in terms of the return loss (about 35dB instead of 29 dB) and showing a good adaptation.

We have demonstrated a fabrication process for fabricating MEMS cantilever and bridge structures using a novel high-temperature piezoelectric polyimide as the structural material. The process consists of conventional lithography and metallization processes and uses a sacrificial layer of photoresist. Electrodes are fabricated on the upper and lower surfaces of the suspended structures and a self-test electrode is fabricated underlying each suspended component for testing. Particular attention is paid to the baking and curing of the polymer films used for structural, sacrificial and lithography layers in order to maintain the structural integrity of the devices throughout the fabrication process. A key aspect to accomplishing this is the use of a chemically imidized polyimide solution to eliminate the high curing temperature normally required to form polyimide. Using this approach, the entire process sequence, through the removal of the sacrificial layer, is kept below 125/spl deg/C. This process has application to building a variety of microelectromechanical sensors and actuators.

Low Actuation Wideband RF MEMS Shunt Capacitive Switch

This letter presents a D-Band (110-170 GHz) RF-MEMS based Single-Pole Double-Throw (SPDT) switch fabricated in a 0.13 μm SiGe BiCMOS technology. The on-wafer S-parameter measurements of the RF-MEMS based SPDT switch show beyond state of the art RF performances, 1.42 dB insertion loss and 54.5 dB isolation at 140 GHz. The SPDT switch consists of a tee junction connected to two Single-Pole Single-Throw (SPST) RF-MEMS switches. The RF-MEMS switch is actuated using 60 V actuation voltage and provides less than 10 μs switch-on and switch-off times. To the best of the authors' knowledge, the results achieved in this study are the lowest insertion loss and the highest isolation of a SPDT reported in D-band.

Epoxy based photo polymer - SU8 is evaluated as microwave packaging material for packaging of Radio Frequency Micro Electro Mechanical Systems (RFMEMS). Standard and Slotted Coplanar Waveguide transmission lines are chosen as test vehicles. Transmission lines of lengths up to 10mm are formed on ST-Cut Quartz substrates. The fabricated transmission lines are subjected to wafer level packaging. The measured S-parameter data shows a significant difference prior to and after packaging. Further, the effects due to the packaging are modeled using lumped components and the measured data are validated with simulations in Advanced Design System. The measured and simulated data agree favorably with each other, for both before and after packaging.

Radio Frequency Micro Electro Mechanical Systems (RF MEMS) are devices showing exceptional potential to satisfy the demands of emerging RF electronic technologies, including those considered for high-power applications, such as for long distance communication systems. Operation in this regime requires an alternative way of thinking for these devices and, for example, a more accurate control of the pull-in voltage is of major importance due to the self-actuation effect. Therefore, the studies focusing on the features of the moving bridges are of great importance. This work presents the fabrication of a full family of RF MEMS switches suitable for high-power implementations having bridges deposited by Au electroplating and released using purely standard wet processes, as well as a carefully designed experimental study of their pull-in voltage. Depositing the bridge of the high-power RF MEMS by using only a single electroplating step makes the device fabrication easier, whilst the utilization of a purely wet release process is an asset. This method relies on low temperature processes, applicable simultaneously in bridges with various geometrical and perforation details without the need of any specialised infrastructure. The experimentally obtained results suggest that for this technology the bridge thickness is a critical factor for controlling the pull-in characteristics between devices fabricated in the same run. Moreover, it is revealed that for thicker bridges, geometry and hole perforation effects are more pronounced. This technology is therefore suitable for developing RF MEMS where the bridge thickness could be potentially utilized for enabling optimization engineering between devices that should be fabricated in the same run but need to satisfy diverse specifications during their operation.

This work presents additional arguments about the enhancement of optical and electrical properties of SiNWs obtained by chemical etching and the beneficial effect of the formation of ultrathin film of SiO2 by a heat treatment at 900 and 1000 °C reported recently by Karyaoui et al. in [Eur. Phys. J. Appl. Phys. 58 , 20103 (2012)]. In the actual study, we report the effect of this thermal oxidation treatment on the morphological and optoelectronic properties of silicon nanowires (SiNWs). The SEM and AFM observations exhibit indeed a remarkable change of the surface shape after this oxidation. On the other hand, the LBIC measurements of the SiNWs reveal that the thermal treatment improves the optoelectronic response at 900 °C and the effective diffusion length increases from 150 μ m for untreated SiNWS to 235 μ m after thermal treatment.

Heterostructures of low-dimensional semiconducting materials, such as transition metal dichalcogenides (MX2), are promising building blocks for future electronic and optoelectronic devices. The patterning of one MX2 material on top of another one is challenging due to their structural similarity. This prevents an intrinsic etch stop when conventional anisotropic dry etching processes are used. An alternative approach consist in a two-step process, where a sacrificial silicon layer is pre-patterned with a low damage plasma process, stopping on the underlying MoS2 film. The pre-patterned layer is used as sacrificial template for the formation of the top WS2 film. This study describes the optimization of a cyclic Ar/Cl2 atomic layer etch process applied to etch silicon on top of MoS2, with minimal damage, followed by a selective conversion of the patterned Si into WS2. The impact of the Si atomic layer etch towards the MoS2 is evaluated: in the ion energy range used for this study, MoS2 removal occurs in the over-etch step over 1–2 layers, leading to the appearance of MoOx but without significant lattice distortions to the remaining layers. The combination of Si atomic layer etch, on top of MoS2, and subsequent Si-to-WS2 selective conversion, allows to create a WS2/MoS2 heterostructure, with clear Raman signals and horizontal lattice alignment. These results demonstrate a scalable, transfer free method to achieve horizontally individually patterned heterostacks and open the route towards wafer-level processing of 2D materials.

This paper presents the design a capacitive shunt type RF-MEMS switch with high isolation, high switching speed and low actuation voltage for Ka-band applications. The proposed RF-MEMS switch has chosen different structures to reduce the actuation voltage. This paper investigates various analysis such as Electromechanical and RF characteristics of the proposed switch. The beam material taken as the gold and the dielectric is taken as Si3N4 with ℰr as 7.5 and the pull-in voltage of proposed switch obtained as 3.37 V. The proposed different RF-MEMS switches have been analyzed over the range of 26.5–40 GHz frequency. The clamped-clamped structure type RF MEMS Switch is having high isolation as − 46.37 dB at 40 GHz. The perfection of RF losses makes these switches as a good choice for a high frequency of K-band.

Electromechanical characterization of low actuation voltage RF MEMS capacitive switches based on DC CV measurements

We report a radio frequency micro electromechanical system (RFMEMS) device integrated with phononic crystals (PnC) that provide a Lamb-wave resonator with frequency-selective heat management, power handling capability, and more efficient electromechanical coupling at ultra high frequency (UHF) and low microwave bands. The integrated device is fabricated in a silicon-on-insulator (SOI) aluminum nitride (AlN) platform and boosts thermal performance by 40%, power handling by 3 dB, and coupling coefficient by three times. Design approach is scalable to higher frequencies.