Materials and interfaces issues in pentacene/PTCDI-C8 ambipolar organic field-effect transistors with solution-based gelatin dielectric

Gelatin is a natural protein, which works well as the gate dielectric for pentacene/N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C₈) ambipolar organic field-effect transistors (OFETs) in air ambient and in vacuum. An aqueous solution process was used to form the gelatin gate dielectric film on poly(ethylene terephthalate) (PET) by spin-coating and subsequent casting. Pentacene morphology and interface roughness are two major factors affecting the electron and hole field-effect mobility (μ_FE) values of pentacene/PTCDI-C₈ ambipolar OFETs in vacuum and in air ambient. In contrast, water absorption in gelatin has higher contribution to the electron and hole μ_FE values in air ambient. The ambipolar performance of pentacene/PTCDI-C₈ ambipolar OFETs depends on their layer sequence. For example, when PTCDI-C₈ is deposited onto pentacene, i.e. in the structure of PTCDI-C₈/pentacene, unbalanced ambipolar characteristics appear. In contrast, better ambipolar performance occurs in the structure of pentacene/PTCDI-C₈. The optimum ambipolar characteristics with electron μ_FE of 0.85 cm² V⁻¹ s⁻¹ and hole μ_FE of 0.95 cm² V⁻¹ s⁻¹ occurs at the condition of pentacene (40 nm)/PTCDI-C₈ (40 nm). Surprisingly, water absorption plays a crucial role in ambipolar performance. The device performance changes tremendously in pentacene/PTCDI-C₈ ambipolar OFETs due to the removal of water out of gelatin in vacuum. The optimum ambipolar characteristics with electron μ_FE of 0.008 cm² V⁻¹ s⁻¹ and hole μ_FE of 0.007 cm² V⁻¹ s⁻¹ occurs at the condition of pentacene (65 nm)/PTCDI-C₈ (40 nm). The roles of layer sequence, relative layer thickness, and water absorption are proposed to explain the ambipolar performance.     

The metal-induced crystallization of poly-Si and the mobility enhancement of thin film transistors fabricated on a glass substrate

In this study, we report on the fabrication of poly-crystalline silicon (poly-Si) using the metal-induced crystallization (MIC) method and its application to thin film transistors (TFTs). The top gate of the p-type TFTs, whose active layer used MIC poly-Si annealed for 1 h at 650 °C, showed a field effect mobility (μ_FE) of 7.5 cm²/V s. By increasing the crystallization time to 5 h, the quality of the MIC poly-Si was improved. The μ_FE increased from 7.5 to 15 cm²/V s. In order to enhance the channel mobility, the Si dangling bonds, which were produced during the transformation from the amorphous phase to the poly-crystalline phase of silicon (Si), were reduced by using plasma hydrogenation. Measurements show that the μ_FE reached 45 cm²/V s after passivation by an inductively coupled plasma chemical vapor deposition (ICPCVD) system.     

Natural polyelectrolyte: Major ampullate spider silk for electrolyte organic field-effect transistors

The major ampullate (MA) silk collected from giant wood spiders Nephila pilipes consists of 12% glutamic acid (Glu) and 4% tyrosine (Tyr) acidic amino residues. The MA silk may act as a natural polyelectrolyte for organic field-effect transistors (OFETs). Pentacene and F₁₆CuPc OFETs were fabricated with the MA silk thin film as the gate dielectric. The MA silk thin film with surface roughness of 4 nm and surface energy of 36.1 mJ/m² was formed on glass using a hexafluoroisopropanol (HFIP) organic process. The MA silk gate dielectric in pentacene OFETs may improve the field-effect mobility (μ_FE,sat) value in the saturation regime from 0.11 in vacuum to 4.3 cm² V⁻¹ s⁻¹ in air ambient at ca. 70% RH. The corresponding threshold voltage (V_TH) value reduced from −6 V in vacuum to −0.5 V in air ambient. Similar to other polyelectrolytes, the changes of μ_FE,sat and V_TH may be explained by the generation of electric double layers (EDLs) in the MA silk thin film in air ambient due to water absorption.     

Hydrated bovine serum albumin as the gate dielectric material for organic field-effect transistors

Bovine serum albumin (BSA) is a natural protein with good hydration ability which contains acidic and basic amino acid residues of ca. 34% in total. In vacuum, pentacene organic field-effect transistors (OFETs) with BSA as the gate dielectric exhibits a field-effect mobility value (μ_FE,sat) of 0.3 cm² V⁻¹ s⁻¹ in the saturation regime and a threshold voltage (V_TH) of ca. −16 V. BSA is easy to be hydrated in air ambient. Electrical properties of BSA in vacuum and hydrated BSA in air ambient are characterized. Similar to polyelectrolyte, hydrated BSA may act the gate dielectric with the formation of electric double-layer capacitors (EDLCs) to improve the device performance. In a relative humidity of 47%, the μ_FE,sat value increases to 4.7 cm² V⁻¹ s⁻¹ and the V_TH reduces to −0.7 V. Generation of mobile ions in hydrated BSA and the formation of EDLCs are discussed.     

Synthesis,size-dependent optoelectronic and charge transport properties of thieno(bis)imide end-substituted molecular semiconductors

The synthesis of two new thieno(bis)imide (TBI, N) end functionalized oligothiophene semiconductors is reported. In particular, trimer (NT3N) and pentamer (NT5N) have been synthesized and characterized. Two different synthetic approaches for their preparation were tested and compared namely conventional Stille cross coupling and direct arylation reaction via C–H activation. Theoretical calculations, optical and electrochemical characterization allowed us to assess the role of the π-conjugation extent, i.e., of the oligomer size on the optoelectronic properties of these materials. In both TBI ended compounds, due to the strong localization of the LUMO orbital on the TBI unit, the LUMO energy is almost insensitive to the oligomer size, this being crucial for the fine-tailoring of the energy and the distribution of the frontier orbitals. Surprisingly, despite its short size and contrarily to comparable TBI-free analogues, NT3N shows electron charge transport with mobility up to μ_N = 10⁻⁴ cm² V⁻¹ s⁻¹, while increasing the oligomer size to NT5N promotes ambipolar behavior and electroluminescence properties with mobility up to μ_N = 0.14 cm² V⁻¹ s⁻¹ and to μ_P = 10⁻⁵ cm² V⁻¹ s⁻¹.     

Simulation and fabrication of high voltage AlGaN/GaN based Schottky diodes with field plate edge termination

In this paper, we report the breakdown voltage (BV) of AlGaN/GaN based Schottky diodes with field plate edge termination. Simulation and fabrication of AlGaN/GaN Schottky diodes were carried out. The simulations were performed using the commercial 2-D device simulator DESSIS. From the simulations, it is found that for a given gate-Ohmic distance (L_gd) of 10 μm, 2DEG of 1 × 10¹³ cm⁻² and field plate length (L_FP) of 2.5 μm, highest BV can be obtained for a silicon nitride thickness of 8000 Å and this BV value is more than 5 times that for a Schottky diode without field plate. The breakdown voltages were also simulated for different field plate lengths. The BV values obtained on the fabricated Schottky diodes are compared with the simulation data and the experimental results follow the trend obtained from the simulation. Simulations were also carried out on a Schottky diode with field plate placed over a stepped insulator with L_gd = 10 μm, L_FP = 5 μm and 2DEG = 1 × 10¹³ cm⁻² and the obtained BV values are about 7 times that without field plate.     

A simple droplet pinning method for polymer film deposition for measuring charge transport in a thin film transistor

Thin-film field-effect transistors (FETs) are widely used to evaluate charge transport properties of semiconducting polymers. Discovery of high performance materials require design and synthesis of new polymers. However, most polymers require multi-step synthesis and are difficult to be obtained in a large scale for comprehensive device evaluations. Here, we report a simple method to cast semiconducting polymer films from solutions with polymer concentration as low as 0.5 mg/mL, which is substantially less than typical values (∼10 mg/mL) used in conventional spin coating method. Here, we demonstrate that using this method, our cast films of a previously-reported polymer (PDPP-TT2T) exhibited field-effect mobility (μ_hole = 0.89 ± 0.13 cm² V⁻¹ s⁻¹, μ_e = 0.025 ± 0.005 cm² V⁻¹ s⁻¹), which is comparable to the reported values using the same device geometry. Furthermore, we extend this method to examine cast films of a pair of polymers (PDPP-3T-Ref, PDPP-3T-Si) to study the effect of siloxane substitution in the side chains on the molecular packing and their subsequent FET performance. We observed that shorter π-stacking distance (3.61 Å) for the siloxane-terminated polymer, when compared to that for the reference polymer (3.73 Å), resulted in improved FET performance (e.g., μ_hole = 0.63 ± 0.046 cm² V⁻¹ s⁻¹ for PDPP-3T-Si vs μ_hole = 0.17 ± 0.062 cm² V⁻¹ s⁻¹ for PDPP-3T-Ref). Taken together, this work presents an efficient alternative film-casting approach to produce polymer FETs that consumes much less material for their fabrication, lending viability for evaluation of various polymeric materials.     

Flexible metal-insulator-metal capacitor using plasma enhanced binary hafnium-zirconium-oxide as gate dielectric layer

We have used a sol-gel spin-coating process to fabricate a new metal-insulator-metal capacitor comprising 10-nm thick binary hafnium-zirconium-oxide (Hf_xZr_1−xO₂) film on a flexible polyimide (PI) substrate. The surface morphology of this Hf_xZr_1−xO₂ film was investigated using atomic force microscopy and scanning electron microscopy, which confirmed that continuous and crack-free film growth had occurred on the PI. After oxygen plasma pre-treatment and subsequent annealing at 250 °C, the film on the PI substrate exhibited a low leakage current density of 3.22 × 10⁻⁸ A/cm² at −10 V and maximum capacitance densities of 10.36 fF/μm² at 10 kHz and 9.42 fF/μm² at 1 MHz. The as-deposited sol-gel film was oxidized when employing oxygen plasma at a relatively low temperature (∼250 °C), thereby enhancing the electrical performance.     

Charge trapping and interface characteristics in normally-off Al2O3/GaN-MOSFETs

Normally-off GaN-MOSFETs with Al₂O₃ gate dielectric have been fabricated and characterized. The Al₂O₃ layer is deposited by ALD and annealed under various temperatures. The saturation drain current of 330 mA/mm and the maximum transconductance of 32 mS/mm in the saturation region are not significantly modified after annealing. The subthreshold slope and the low-field mobility value are improved from 642 to 347 mV/dec and from 50 to 55 cm² V⁻¹ s⁻¹, respectively. The I_D-V_G curve shows hysteresis due to oxide trapped charge in the Al₂O₃ before annealing. The amount of hysteresis reduces with the increase of annealing temperature up to 750 °C. The Al₂O₃ layer starts to crystallize at a temperature of 850 °C and its insulating property deteriorates.     

Transport and interface states in high-κ LaSiOx dielectric

The paper presents the results of capacitance-voltage, conductance-frequency and current-voltage characterization in the wide temperature range (140-300 K) as well as results of low temperature (5-20 K) thermally stimulated currents (TSC) measurements of metal-oxide-semiconductor (MOS) structures with a high-κ LaSiO_x dielectric deposited on p- and n-type Si(1 0 0) substrate. Interface states (D_it) distribution determined by several techniques show consistent result and demonstrates the adequacy of techniques used. Typical maxima of interface states density were found as 4.6 × 10¹¹ eV⁻¹cm⁻² at 0.2 eV and 7.9 × 10¹¹ eV⁻¹cm⁻² at 0.77 eV from the silicon valence band. The result of admittance spectroscopy showed the presence of local states in bandgap with activation energy E_a = 0.38 eV from silicon conductance band, which is in accord with interface states profile acquired by conductance method. Low-temperature TSC spectra show the presence of shallow traps at the interface with activation energies ranging from 15 to 32 meV. The charge carrier transport through the dielectric film was found to occur via Poole-Frenkel mechanism at forward bias.     

Deposition and structuring of Ag/AgCl electrodes inside a closed polymeric microfluidic system for electroosmotic pumping

The design, fabrication and analysis of a low voltage electroosmotic (eo) pump with integrated Ag/AgCl electrodes are shown. The fabrication was based on casting the hydrophilic polymer NOA63, capping the NOA63 trenches with a glass-slide and subsequently depositing Ag/AgCl electrodes by a flow of electroless solution and structuring the electrodes by microfluidic stopvalves. The herewith obtained eo pump embedded into the microfluidic system had a capillary cross-section of 65 μm × 55 μm and its flow rate was determined to be 0.12 nl s⁻¹ V⁻¹ U_eh within the range of applied voltages U_eh from −1.5 up to 1.5 V.     

MOS devices with tetragonal ZrO2 as gate dielectric formed by annealing ZrO2/Ge/ZrO2 laminate

A Ge-stabilized tetragonal ZrO₂ (t-ZrO₂) film with permittivity (κ) of 36.2 was formed by depositing a ZrO₂/Ge/ZrO₂ laminate and a subsequent annealing at 600 °C, which is a more reliable approach to control the incorporated amount of Ge in ZrO₂. On Si substrates, with thin SiON as an interfacial layer, the SiON/t-ZrO₂ gate stack with equivalent oxide thickness (EOT) of 1.75 nm shows tiny amount of hysteresis and negligible frequency dispersion in capacitance-voltage (C-V) characteristics. By passivating leaky channels derived from grain boundaries with NH₃ plasma, good leakage current of 4.8 × 10⁻⁸ A/cm² at V_g = V_fb − 1 V is achieved and desirable reliability confirmed by positive bias temperature instability (PBTI) test is also obtained.     

Comparison of short and long wavelength absorption electron donor materials in C60-based planar heterojunction organic photovoltaics

Buckminsterfullerene, C₆₀-based planar heterojunction (PHJ) organic photovoltaics (OPVs) have been created using a short wavelength absorption (λ_max = 490 nm) electron-donating bis(naphthylphenylaminophenyl)fumaronitrile (NPAFN). NPAFN exhibits a hole mobility greater than 0.07 cm² V⁻¹ s⁻¹ as determined by its field-effect transistor. It can be attributed to such hole mobility that enables a thin layer (<10 nm) NPAFN in PHJ OPV, ITO/NPAFN/C₆₀/bathocuproine/Al. Because of the low lying HOMO energy level (5.75 eV) of NPAFN and relatively high ionization potential ITO (∼5.58 eV), such OPVs exhibit a very high open circuit voltage of ∼1.0 V, relatively high fill factor of 0.60, and a relatively high shunt resistance of 1100 Ω cm⁻², which all compensate for a relatively low short circuit current of 3.15 mA cm⁻² due to the short absorption wavelength and inferred short exciton diffusion length of NPAFN. Altogether, NPAFN OPVs display a power conversion efficiency (η_PC) of 2.22%, which is better than other long wavelength absorption materials in similar PHJ OPVs, such as pentacene (λ_max 670 nm, HOMO 5.12 eV, η_PC 1.50%) and copper phthalocyanine (λ_max 624, 695 nm, HOMO 5.17 eV, η_PC 1.43%).     

Ionization potential dependent air exposure effect on the MoO3/organic interface energy level alignment

We reported an ionization potential (IP) dependent air exposure effect on the MoO₃/organic interface energy level alignment by carrying out in situ ultraviolet photoelectron spectroscopy and synchrotron light based X-ray photoelectron spectroscopy investigations. The electronic structures at MoO₃/organic interfaces comprising various π-conjugated small organic molecules with different IP on MoO₃ substrate have been systematically investigated. For the molecules with low IP, MoO₃/organic interface electronic structures remained almost unchanged after air exposure. In contrast, for the molecules with high IP, the highest occupied molecular orbital (HOMO) leading edge (or hole injection barrier) increases gradually with the increasing molecule IP after air exposure. For the MoO₃/copper-hexadecafluorophthalocyanine (F₁₆CuPc, IP: ∼6.58 eV) interface, air exposure can induce a significant downward shift of the HOMO level as large as ∼0.80 eV.     

Schottky barrier SOI-MOSFETs with high-k La(2)O(3)/ZrO(2) gate dielectrics

Schottky barrier SOI-MOSFETs incorporating a La₂O₃/ZrO₂ high-k dielectric stack deposited by atomic layer deposition are investigated. As the La precursor tris(N,N′-diisopropylformamidinato) lanthanum is used. As a mid-gap metal gate electrode TiN capped with W is applied. Processing parameters are optimized to issue a minimal overall thermal budget and an improved device performance. As a result, the overall thermal load was kept as low as 350, 400 or 500 °C. Excellent drive current properties, low interface trap densities of 1.9 × 10¹¹ eV⁻¹ cm⁻², a low subthreshold slope of 70-80 mV/decade, and an I_ON/I_OFF current ratio greater than 2 × 10⁶ are obtained.     

Electron transport in two dimensional electron gas formed at the heterojunction of AlxGa(1−x)N/GaN at microwave frequencies

Transport properties of the two dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN heterostructure ranging from 20 K to 300 K has been investigated theoretically considering the various scattering mechanisms like the acoustic, the piezo, the surface roughness, the alloy, the polar optical phonon and the dislocation scattering. The dc mobility is found to remain constant up to 150 K and then it decrease sharply with further increase in temperature. The ac mobility is also found to decrease with increase in the temperature. The real part of ac mobility, i.e. μ_r decreases with the increase in the frequency very fast initially and then gradually attains a steady value. The imaginary part of the ac mobility μ_im initially increases with the increase in the frequency and then decreases after reaching the maximum value. The value of the ac mobility reduces quite reasonably as the 2D carrier concentration increases at lower range of the frequency. At the carrier concentration of 5 × 10¹⁷ m⁻², the ac mobility remains constant through a wide range of frequencies. With the increase in the dislocation densities, the values of the ac mobility are found to decrease at the lower range of frequencies. The thermo electric power is positive at the 2D carrier concentration of 5 × 10¹⁶ m⁻², the value of which increases with the increase in the temperature and gradually attains a steady value. But the thermoelectric power at n_2D of 10¹⁷ m⁻² is found to be negative in the value. The value of ZT is found to increase with the temperature and attains the maximum value of 0.007 at 150 K and the value of ZT then decreases with increase in the temperature.     

Power performance and scalability study of AlGaAs/GaAs double-recessed pseudomorphic high electron mobility transistors on unthinned substrate for coplanar waveguide circuit applications

We have investigated the power performance and scalability of AlGaAs/GaAs Double-Recessed Pseudomorphic High Electron Mobility Transistors (DR-PHEMTs) at 10 GHz on an unthinned GaAs substrate for CoPlanar Waveguide (CPW) circuit applications. It was found that the output power varied linearly with the logarithm of the device’s gate width ranging from 200 to 1000 μm. It increased at a rate of 0.01 dB/μm. That worked out to a doubling of output power (or 3 dB) for every 300 μm increase in the gate width. Gain decreased at a rate of about 0.005 dB/μm while PAE generally improved when the gate width was increased. As for DC measurement, the maximum transconductance of the device was about 375 mS/mm at V_G = −0.5 V and V_DS = 3 V. The gate-drain breakdown voltage (BV_GD) measured was −20 V, defined at I_G = −1 mA/mm. The microwave performance of the devices was measured on-wafer using a load-pull system at a bias of V_G = −0.5 V and V_DS = 8 V. For a device with a gate width of 1 mm, its saturated CW output power, gain and PAE value at 10 GHz was 27.5 dBm (0.55 W), 8 dB and 48%, respectively. At this same set of bias conditions, the value of f_t and f_max was 40 and 80 GHz, respectively.     

Interface optimization using diindenoperylene for C60 thin film transistors with high electron mobility and stability

C₆₀-based organic thin film transistors (OTFTs) with high electron mobility and high operational stability are achieved with (1 1 1) oriented C₆₀ films grown by using template effects of diindenoperylene (DIP) under layer on the SiO₂ gate insulator. The electron mobility of the C₆₀ transistor is significantly increased from 0.21 cm² V⁻¹ s⁻¹ to 2.92 cm² V⁻¹ s⁻¹ by inserting the template-DIP layer. Moreover much higher operational stability is also observed for the DIP-template C₆₀ OTFTs. A grazing incidence X-ray diffraction and ultrahigh-sensitivity photoelectron spectroscopy measurements indicate that the improved electron mobility and stability arise from the decreased density of trap states in the C₆₀ film due to increased (1 1 1) orientation of C₆₀-grains and their crystallinity on the DIP template.     

Air-stability and bending properties of flexible organic field-effect transistors based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]

We have fabricated flexible field-effect transistors (FETs) using poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, as an active channel, poly(methyl methacrylate) (PMMA) as gate dielectric and biaxially oriented polyethyleneterephthalate (BOPET) as supporting substrate. The output and transfer characteristics of the devices were measured as a function of channel length. It has been observed that various OFET parameters viz. on–off ratio (∼10⁵), mobility (μ ∼ 10⁻⁴ cm² V⁻¹ s⁻¹), threshold voltage (V_th ∼ −14 V), switch-on voltage (V_so ∼ −6 V), subthreshold slope (S ∼ 7 V/decade) and trap density (N_it ∼ 10¹⁴ cm⁻² V⁻¹) are almost independent of the channel length, which suggested a very high uniformity of the PCDTBT active layer. These devices were highly stable under atmospheric conditions (temperature: 20–35 °C and relative humidity: 70–85%), as no change in mobility was observed on a continuous exposure for 70 days. The studies on the effect of strain on mobility revealed that devices are stable up to a compressive or tensile strain of 1.2%. These results indicate that PCDTBT is a very promising active layer for the air stable and flexible FETs.