Chemical bath deposition of CdS channel layer for fabrication of low temperature-processed thin-film-transistors

～66 nm thick CdS film with a hexagonal structure was uniformly generated via a low temperature-processed chemical bath deposition at 80 °C using a complexing agent of ethylenediaminetetraacetic acid and its crystal structure, surface morphology, optical transmittance, and Raman scattering property were measured. Grown CdS film was used as a channel layer for the fabrication of bottom-gate, top-contact thin-film-transistor (TFT). The TFT device with 60 °C-dried channel layer exhibited a poor electrical performance of on-to-off drain current ratio (I_on/I_off) of 5.1 × 10³ and saturated channel mobility (μ_sat) of 0.10 cm²/Vs. However, upon annealing at 350 °C, substantially improved electrical characteristics resulted, showing I_on/I_off of 5.9 × 10⁷ and μ_sat of 5.07 cm²/Vs. Furthermore, CdS channel layer was chemically deposited in an identical way on a transparent substrate of SiN_x/ITO/glass as part of transparent TFT fabrication, resulting in I_on/I_off of 5.8 × 10⁷ and μ_sat of 2.50 cm²/Vs.     

The effect of annealing in forming gas on the a-IGZO thin film transistor performance and valence band cut-off of IGZO on SiNx

In this investigation, the carrier concentration gradient between channel and contact region is achieved to improve the Thin film Transistors (TFT) performance by employing annealing at 350 °C in forming gas (N₂ + 5% H₂). The contact region is covered with Mo metal and the channel region is only exposed to forming gas to facilitate the diffusion controlled reaction. The TFT using a-IGZO active layer is fabricated in ambient of Ar:O₂ in ratio 60:40 and the conductivity of the order of 10⁻³ S/cm is measured for as-deposited sample. The electrical conductivity of an annealed sample is of the order of 10² S/cm. The device performance is determined by measuring merit factors of TFT. The saturation mobility of magnitude 18.5 cm²V⁻¹ s⁻¹ has been determined for W/L (20/10) device at 15 V drain bias. The extrapolated field effect mobility for a device with channel width (W) 10 μm is 19.3 cm²V⁻¹ s⁻¹. The on/off current ratio is 10⁹ and threshold voltage is in the range between 2 and 3 V. The role of annealing on the electronic property of a-IGZO is carried out using X-ray photoelectron spectroscopy (XPS). The valance band cut-off has been approximately shifted to higher binding energy by 1 eV relative to as-deposited sample.     

A study of electrical enhancement of polycrystalline MgZnO/ZnO bi-layer thin film transistors dependence on the thickness of ZnO layer

A polycrystalline MgZnO/ZnO bi-layer was deposited by using a RF co-magnetron sputtering method and the MgZnO/ZnO bi-layer TFTs were fabricated on the thermally oxidized silicon substrate. The performances with varying the thickness of ZnO layer were investigated. In this result, the MgZnO/ZnO bi-layer TFTs which the content of Mg is about 2.5 at % have shown the enhancement characteristics of high mobility (6.77–7.56 cm² V⁻¹ s⁻¹) and low sub-threshold swing (0.57–0.69 V decade⁻¹) compare of the ZnO single layer TFT (μ_FE = 5.38 cm² V⁻¹ s⁻¹; S.S. = 0.86 V decade⁻¹). Moreover, in the results of the positive bias stress, the ΔV_on shift (4.8 V) of MgZnO/ZnO bi-layer is the 2 V lower than ZnO single layer TFT (ΔV_on = 6.1 V). It reveals that the stability of the MgZnO/ZnO bi-layer TFT enhanced compared to that of the ZnO single layer TFT.     

Enhancement of electrical stability of a-IGZO TFTs by improving the surface morphology and packing density of active channel

a-IGZO films were deposited on Si substrates by d.c sputtering technique with various working power densities (p_d) in the range of 0.74–2.22 W/cm². The correlation between material properties and their effects on electrical stability of a-IGZO thin-film transistor (TFTs) was studied as a function of p_d. At a p_d of 1.72 W/cm² a-IGZO film had smoothest surface roughness (0.309 nm) with In-rich and Ga-poor cation compositions as a channel. This structurally ordered TFTs exhibited a high field effect mobility of 9.14 cm²/Vs, a sub-threshold swing (S.S.) of 0.566 V/dec, and an on–off ratio of 10⁷. Additionally, the V_th shift in hysteresis loop is almost eliminated. It was shown that the densification of the a-IGZO film resulted in the reduction of its interface trap density (1.83 × 10¹² cm^?2), which contributes for the improvement in the electrical and thermal stability.     

Room temperature fabrication Oxide TFT with Y2O3 as a gate oxide and Mo contact

In this investigation, an operating voltage as low as 5 V has been achieved for Oxide TFT with Y₂O₃ as a gate oxide and a-IGZO as an active layer. The OTFT has been fabricated at room temperature using RF sputter. The mobility and threshold voltages are 11.3 cm²/V s and 3.4 V for the device with W/L = 0.8, respectively. The annealing at 400 °C in N₂ containing 5% H₂ ambient has been utilized to improve the electrical performance of TFT. The on-off current which is determined by gate dielectric has been observed to be 10⁴. It has also been observed that the dielectric properties of gate oxide deteriorate on annealing. The dielectric constant of Y₂O₃ is observed in the range between 5.1 and 5.4 measured on various devices.     

Investigation of the electrical properties and stability of HfInZnO thin-film transistors

In this study, amorphous HfInZnO (a-HIZO) thin films and related thin-film transistors (TFTs) were fabricated using the RF-sputtering method. The effects of the sputtering power (50–200 W) on the structural, surface, electrical, and optical properties of the a-HIZO films and the performance and NBIS stability of the a-HIZO TFTs were investigated. The films’ N_e increased and resistivity decreased as the sputtering power increased. The 100 W deposited a-HIZO film exhibited good optical and electrical properties compared with other sputtering powers. Optimization of the 100 W deposited a-HIZO TFT demonstrated good device performance, including a desirable μ_FE of 19.5 cm²/Vs, low SS of 0.32 V/decade, low V_th of 0.8 V, and high I_on/I_off of 10⁷, respectively. The 100 W deposited a-HIZO TFT with Al₂O₃ PVL also exhibited the best stability, with small V_th shifts of -2.2 V during NBIS testing. These high-performance a-HIZO thin films and TFTs with Al₂O₃ PVL have practical applications in thin-film electronics.     

Effect of Ge doping on the electrical properties of amorphous Zn–Sn–O thin films

We report the effect of germanium doping on the active layer of amorphous Zinc–Tin-Oxide (a-ZTO) thin film transistor (TFT). Amorphous thin film samples were prepared by RF magnetron sputtering using single targets composed of Zn₂Ge_0.05Sn_0.95O₄ and Zn₂SnO₄ with variable oxygen contents in the sputtering gases. In comparison with undoped, Ge-doped a-ZTO films exhibited five order of magnitude lower carrier density with a significantly higher Hall-mobility, which might be due to suppressed oxygen vacancies in the a-ZTO lattice since the Ge substituent for the Sn site has relatively higher oxygen affinity. Thus, the bulk and interface trap densities of Ge-doped a-ZTO film were decreased one order of magnitude to 7.047 × 10¹⁸ eV⁻¹cm⁻³ and 3.52 × 10¹¹ eV⁻¹cm⁻², respectively. A bottom-gate TFT with the Ge-doped a-ZTO active layer showed considerably improved performance with a reduced SS, positively shifted V_th, and two orders of magnitude increased I_on/I_off ratio, attributable to the doped Ge ions.     

A thin-film transistor with a very thin a-Si:H layer and its application for an LC panel

One of the disadvantages of applying an a-Si:H thin-film transistor (TFT) to an active matrix-addressed liquid crystal (LC) panel is that a TFT with an a-Si:H has a very large photo-leakage current because of the high photo-conductivity of an a-Si:H itself.We have tried decreasing the photo-leakage current by varying the thickness of an a-Si:H layer (L) in TFTs and investigated the characteristics of TFTs, mainly drain voltage versus drain current containing photo-leakage current (I _ph).As a result, it is shown that lnI _ph is proportional to InL, and its gradient is 1.5–2.0. We assume that the thinner an a-Si:H layer is, the more effective the recombination of carriers at the interface states is forI _ph.We have applied TFT with a very thin a-Si:H layer (30nm) to a full-color active matrix-addressed LC panel for a moving picture display and realized a display of good quality under illuminated condition of 5×10⁴lx without a shading layer in it.     

铜-钼源漏电极对非晶氧化铟镓锌薄膜晶体管性能的改善

在铜(Cu)和非晶铟镓锌氧化物(a-IGZO)之间插入30 nm厚的钼(Mo)接触层, 制备了具有Cu-Mo源漏电极的a-IGZO薄膜晶体管(TFT). Mo接触层不仅能够抑制Cu与a-IGZO有源层之间的扩散, 而且提高了Cu电极与玻璃基底以及栅极绝缘层的结合强度. 制备的Cu-Mo结构TFT与纯Cu 结构TFT相比, 具有较高的迁移率(～9.26 cm²·V^-1·s^-1)、更短的电流传输长度(～0.2 μm)、更低的接触电阻(～1072 Ω)和有效接触电阻率(～1×10^-4Ω·cm²), 能够满足TFT 阵列高导互联的要求.     

Characterization of magnesium oxide gate insulators grown using RF sputtering for ZnO thin-film transistors

A ZnO thin-film transistor (TFT) with an MgO insulator was fabricated on a silicon (100) substrate using a radiofrequency magnetron sputtering system. The MgO insulator was deposited using the same deposition system; the total pressure during the deposition process was maintained at 5 mTorr, and the oxygen percentage of O₂/(Ar + O₂) was set at 30%, 50%, or 70%. The process temperature was maintained at below 300 °C. The dielectric constant of the MgO thin layer was approximately 11.35 with an oxygen percentage of 70%. This ZnO TFT displayed enhanced transistor properties, with a field-effect mobility of 0.0235 cm² V⁻¹ s⁻¹, an I_ON/I_OFF ratio of ∼10⁵, and an SS value of 1.18 V decade⁻¹; these properties were superior to those measured for the MgO insulators synthesized using oxygen percentages of 30% and 50%.     

The effect of post-metal annealing on the electrical performance and stability of two-step-annealed solution-processed In2O3 thin film transistors

In this work, solution-processed indium oxide (In₂O₃) thin film transistors (TFTs) were fabricated by a two-step annealing method. The influence of post-metal annealing (PMA) temperatures on the electrical performance and stability is studied. With the increase of PMA temperatures, the on-state current and off-state current (I_on/I_off) ratio is improved and the sub-threshold swing (SS) decreased. Moreover, the stability of In₂O₃ TFTs is also improved. In all, In₂O₃ TFT with post-metal annealing temperature of 350°С exhibits the best performance (a threshold voltage of 4.75 V, a mobility of 13.8 cm²/V, an I_on/I_off ratio of 1.8 × 10⁶, and a SS of 0.76 V/decade). Meanwhile, the stability under temperature stress (TBS) and positive bias stress (PBS) also show a good improvement. It shows that the PMA treatment can effectively suppress the interface trap and bulk trap and result in an obviously improvement of the In₂O₃ TFTs performance.     

Overcoming the thickness paradox: Systematical optimization of inverted polymer solar cells

Inverted structure comes out to be a promising alternative for making polymer solar cells (PSC) with high efficiency and long-term stability. Vertically stacked functional layers with planar shapes often suffer contradictions in holding high optical absorption and excellent charge transfer/hindrance capability to construct well performed inverted PSC devices. Here, we give an example of rational control of the thickness of electron transport layer (ETL), hole transport layer (HTL) and organic active layer (OAL) to achieve a synergistic effect on promoting the overall photovoltaic behaviors. With in-depth exploration of the interaction between device performance and layer thickness, we obtain the optimized device ITO/ZnO Ncs (45 nm)/P3HT:PCBM (70 nm)/MoO₃ (1 nm)/Ag (70 nm) exhibiting an V_oc of 0.63 V, J_sc of 12.52 mA/cm², FF of 54% and PCE of 4.26%.     

Control of electrical properties and gate bias stress stability in solution-processed a-IZO TFTs by Zr doping

Zr-doped indium zinc oxide (IZO) thin film transistors (TFTs) are fabricated via a solution process with different Zr doping ratios. The addition of Zr suppressed the carrier concentration in the IZO films, which was confirmed by Hall Effect measurements. As the amount of Zr was increased in the oxide active layer of TFTs, the subthreshold swing (S.S) reduced, the ON/OFF ratio improved, and the threshold voltage (V_th) shifted positively. Moreover, the starting points of the ON state for TFTs near the point zero gate voltage could be controlled by the addition of Zr. The 0.3% Zr-doped IZO TFT exhibited a high saturation mobility of 7.0 cm² V⁻¹ s⁻¹, ON/OFF ratio of 2.6 × 10⁶ and S.S of 0.57 V/decade compared the IZO TFT with 10.1 cm² V⁻¹ s⁻¹, 1.7 × 10⁶ and 0.75 V/decade. The Zr effect of the gate bias stability was examined. Zr-doped IZO TFTs were relatively unstable under a positive bias stress (PBS), whereas they showed good stability at a negative bias stress (NBS). The gate bias stability of the oxide TFTs were compared with the extracted parameters through a stretched-exponential equation. The characteristic trapping time under NBS of 0.3% Zr-doped IZO TFTs was improved from 8.3 × 10⁴ s for the IZO TFT to 3.1 × 10⁵ s.     

A poly-Si thin film transistor fabricated by new excimer laser recrystallization employing floating active structure

We fabricated a high-performance polycrystalline silicon (poly-Si) thin film transistor (TFT) by new excimer laser annealing (ELA) method employing floating active structure. The new simple ELA method produces large lateral grains exceeding 4 μm. A novel poly-Si TFT that exhibits very high mobility (μ_FE=331 cm²/V s) and low leakage current has been successfully fabricated by single laser irradiation on selectively floating a-Si layer. Uniform lateral grains have been obtained with wide ELA process window.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

An organic electrophosphorescent device driven by all-organic thin-film transistor using photoacryl as a gate insulator

Organic electrophosphorescent devices have been intensively investigated for using in full-color flat-panel display. Since the quantum efficiency of electrophosphorescent device decreases rapidly as the luminance increases, it is desirable to operate the electrophosphorescent display with active matrix rather than passive matrix. Here we report the study of driving electrophosphorescent diode with all-organic TFT. We obtained the maximum power luminance that was obtained about 90 cd/m². Turn-on voltage is approximately 10 V. Field effect mobility, threshold voltage, and on–off current ratio in 0.5-μm thick gate dielectric layer were 0.13 cm²/V s, −7 V, and 10⁶ A/A. The structure of electrophosphorescent diode is ITO/TPD/BCP:Ir(ppy)₃/BCP/Alq₃/Li:Al/Al. In organic TFT, photoacryl is used as an insulator and pentacene as an active layer.     

Low-voltage-drive and high output current ZnO thin-film transistors with sputtering SiO2 as gate insulator

Low-voltage-drive ZnO thin-film transistors (TFTs) with room-temperature radio frequency magnetron sputtering SiO₂ as the gate insulator were fabricated successfully on the glass substrate. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 4.2 V, a field effect mobility of 11.2 cm²/V s, an on/off ratio of 3.1 × 10⁶ and a subthreshold swing of 0.61 V/dec. The drain current can reach to 1 mA while the gate voltage is only of 12 V and drain voltage of 8 V. The C–V characteristics of a MOS capacitor with the structure of ITO/SiO₂/ZnO/Al was investigated. The carrier concentration N_D in the ZnO active layer was determined, the calculated N_D is 1.81 × 10¹⁶ cm⁻³, which is the typical value of undoped ZnO film used as the channel layer for ZnO-TFT devices. The experiment results show that SiO₂ film is a promising insulator for the low voltage and high drive capability oxide TFTs.     

Improvement of electrical characteristics of InGaZnO thin film transistors by using HMDSO/O2 plasma deposited SiOCH buffer layer

In this work, we present the performance improved InGaZnO thin film transistors by inserting low temperature processed 10 nm thick SiOCH buffer layers between SiN_x insulator and InGaZnO channel layer. The influences of oxygen flow rate during the deposition of SiOCH buffer layer have been intensively investigated. Basing on the analysis of hall effect measurement and Fourier transform infrared spectrum, the SiOCH buffer layer can effectively increase the carrier concentration of the channel layer by the hydrogen doping due to re-sputtering and diffusion effect. The InGaZnO thin film transistor with buffer layer exhibits an enhanced performance with mobility of 13.09 cm²/vs, threshold voltage of −0.55 V and I_on/I_off over 10⁶.     

Synthesis of TiO2 nanoparticles and spectroscopic upconversion luminescence of Nd3+-doped TiO2–SiO2 composite glass

Nd³⁺-doped TiO₂–SiO₂ composites were prepared by sol–gel method. Optical properties such as radiative life-time (τ), stimulated emission cross-section (σ_p) and branching ratio (β) were calculated using Judd–Ofelt theory. Violet to blue upconversion emissions at 380 nm (⁴D_3/2→⁴I_11/2), 399 nm (²P_3/2→⁴I_11/2), 420 nm (²D_5/2→⁴I_9/2) and 452 nm (²P_3/2→⁴I_13/2) were obtained under 578 nm xenon-lamp excitation. The choice of 578 nm is justified by the absorption spectra of the same samples, which shows a strong absorption peak at 578 nm. This 578 nm excitation pump produces upconversion in Nd³⁺ by a sequential two-photon absorption process.     

Quasi-solid state polymer electrolytes for dye-sensitized solar cells: Effect of the electrolyte components variation on the triiodide ion diffusion properties and charge-transfer resistance at platinum electrode

Quasi-solid state polymer electrolytes have been prepared from poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) as gelator for 1-ethyl-3-methylimidazolium based ionic liquids (with anions like trifluoromethanesulfonate [EMIM][TfO], bis(trifluoromethanesulfonyl)imide [EMIM][Tf₂N]) and polyacrylonitrile (PAN) for gelation of 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA] as well as I⁻/I₃⁻ as the redox couple. All electrolytes exhibit high ionic conductivity in the range of 10^− 3 S/cm. The effect of gelation, redox couple concentration, I⁻/I₃⁻ ratio, choice of cations and additives on the triiodide diffusion and charge-transfer resistance of the platinum/electrolyte interface (R_ct) were studied. The apparent diffusion coefficient of triiodide ion (D(I₃⁻)) at various iodide/triiodide ratios in liquid and gelified electrolytes has been calculated from measurements of the diffusion limited current (I_lim) in electrochemical cell resembling the set-up of a dye-sensitized solar cell. The charge-transfer resistance of the platinum/electrolyte interface as well as the capacitance of the electrical double layer (C_dl) have been calculated from impedance measurements. Electrolytes with reduced content of polymer (2.5 wt.%) were doped with Al₂O₃ particles of different sizes (50 nm, 300 nm, 1 μm). The dispersion of the particles proceeds by speedy stirring of the hot electrolyte and the addition of PAN provides a homogeneous suspension. The addition of Al₂O₃ particles causes a slight increase of the triiodide diffusion constants. Furthermore the suggested enhancement of the charge transfer rate shows a dependence on the size of the particles.