Influence of process steps on the performance of microcrystalline silicon thin film transistors

Bottom gate microcrystalline silicon thin film transistors (μc-Si TFT) have been realized with two types of films: μc-Si(1) and μc-Si(2) with crystalline fraction of 80% and close to 100% respectively. On these TFTs we applied two types of passivation (SiN_x and resist). μc-Si TFTs with resist as a passivation layer present a low leakage current of about 2.10^− 12 A for V_G = − 10 and V_D = 0.1V an ON to OFF current ratio of 10⁶, a threshold voltage of 7 V, a linear mobility of 0.1 cm²/V s, and a sub-threshold voltage of 0.9 V/dec. Microcrystalline silicon TFTs with SiN_x as a passivation present a new phenomenon: a parasitic current for negative gate voltage (− 15 V) causes a bump and changes the shape of the sub-threshold region. This excess current can be explained by and oxygen contamination at the back interface.     

Copper phthalocyanine thin-film field-effect transistor with SiO2/Ta2O5/SiO2 multilayer insulator

Top-contact Copper phthalocyanine (CuPc) thin-film field-effect transistor (TFT) with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer as the dielectric was fabricated and investigated. With the multi-layer dielectric, drive voltage was remarkably reduced. A relatively large on-current of 1.1 × 10⁻⁷ A at a V_GS of −15 V was obtained due to the strong coupling capability provided by the STS multilayer gate insulator. The device shows a moderate performance: saturation mobility of μ_sat = 6.12 × 10⁻⁴ cm²/V s, on-current to off-current ratio of I_on/I_off = 1.1 × 10³, threshold voltage of V_TH = −3.2 V and sub-threshold swing SS = 1.6 V/dec. Atomic force microscope images show that the STS multilayer has a relative smooth surface. Experiment results indicate that STS multilayer is a promising insulator for the low drive voltage CuPc-based TFTs.     

Characteristics of transparent ZnO based thin film transistors with amorphous HfO2 gate insulators and Ga doped ZnO electrodes

Coplanar type transparent thin film transistors (TFTs) have been fabricated on the glass substrates. The devices consist of intrinsic ZnO, Ga doped ZnO (GZO), and amorphous HfO₂ for the semiconductor active channel layer, electrode, and gate insulator, respectively. GZO and HfO₂ layers were prepared by using a pulsed laser deposition (PLD) and intrinsic ZnO layers were fabricated by using an rf-magnetron sputtering. The transparent TFT exhibits n-channel, enhancement mode behavior. The field effect mobility, threshold voltage, and a drain current on-to-off ratio were measured to be 14.7 cm²/Vs, 2 V, and 10⁵, respectively. High optical transmittance (> 85%) in visible region makes ZnO TFTs attractive for transparent electronics.     

High mobility bottom gate nanocrystalline-Si thin-film transistors

We developed high mobility bottom gate nanocrystalline (nc)-Si thin-film transistors (TFTs). nc-Si film was deposited using inductively coupled plasma chemical vapor deposition method on SiN_x gate insulator. Because of good film crystallinity and low ion damage, we could get high performance TFT characteristics. Our TFT showed field effect mobility of 9.4 cm² V^− 1 s^− 1 for electrons. These results showed that bottom gate nc-Si TFT could be used in applications such as next generation high definition television and organic light-emitting diode display.     

Improvement in the negative bias temperature stability of ZnO based thin film transistors by Hf and Sn doping

We assessed the performance of ZnO TFTs using Si₃N₄ gate dielectrics after various treatments. A remarkable improvement in the transfer characteristics was obtained for the O₂ plasma treated ZnO TFT and SiO₂ interlayer deposited ZnO TFT. Also, we developed amorphous hafnium-zinc-tin oxide (HZTO) thin film transistors (TFTs) and investigated the influence of hafnium (Hf) doping on the electrical characteristics of the hafnium-zinc oxide (HZO) thin film transistors. Doping with Hf can decrease the carrier concentration, which may result from a decrease of the field effect mobility, and reduce oxygen vacancy related defects in the interfacial layer. Adding tin (Sn) can suppress the growth of a crystalline phase in the HZTO films. The HZTO TFTs exhibited good electrical properties with a field effect mobility of 14.33 cm²/Vs, a subthreshold swing of 0.97 V/decade, and a high I_ON/OFF ratio of over 10⁹.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

Electrical mechanism analysis of Al2O3 doped zinc oxide thin films deposited by rotating cylindrical DC magnetron sputtering

Cost efficient and large area deposition of superior quality Al₂O₃ doped zinc oxide (AZO) films is instrumental in many of its applications, including solar cell fabrication due to its numerous advantages over indium tin oxide (ITO) films. In this study, AZO films were prepared by a highly efficient rotating cylindrical direct current (DC) magnetron sputtering system using an AZO target, which has a target material utilization above 80%, on glass substrates in argon (Ar) ambient. A detailed analysis on the electrical, optical, and structural characteristics of AZO thin films was performed for the solar cell, as well as display applications. The properties of films were found to critically depend on deposition parameters, such as sputtering power, substrate temperature, working pressure, and film thickness. A low resistivity of ~ 5.5 × 10^− 4 Ω cm was obtained for films deposited at 2 kW, keeping the pressure, substrate temperature and thickness constant at 3 mTorr, 230 °C and ~ 1000 nm respectively. This was due to an increase in carrier mobility and large grain size. Mobility is found to be controlled by ionized impurity scattering within the grains, since the mean free path of carriers is much smaller than the grain size of the films. The AZO films showed a high transparency of ~ 90% in the long wavelength region. Our results offer a cost-efficient AZO film deposition method that can fabricate films with significant low resistivity and high transmittance that can be applied in thin-film solar cells, as well as thin film transistor (TFT) and non-volatile memory (NVM).     

High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric

A high-performance ZnO thin film transistor （ZnO-TFT） with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer gate insulator is fabricated by sputtering at room temperature. Compared to ZnO-TFTs with sputtering SiO₂ gate insulator, its electrical characteristics are significantly improved, such as the field effect mobility enhanced from 11.2 to 52.4 cm²/V s, threshold voltage decreased from 4.2 to 2 V, and sub-threshold swing improved from 0.61 to 0.28 V/dec. The improvements are attributed to the high gate capacitance (from 50 to 150 nF/cm²) as well as nice surface morphology by using dielectric with high～k Ta₂O₅ sandwiched by SiO₂ layers. The capacitance-voltage characteristic of a metal-insulator-semiconductor capacitor with the structure of Indium Tin Oxide/STS/ZnO/Al was investigated and the trap charges at the interface or bulk is evaluated to be 2.24 × 10¹² cm⁻². From the slope of C⁻² versus gate voltage, the doping density N_D of ZnO is estimated to be 1.49 × 10¹⁶ cm⁻³.     

Deterioration and recovery in the resistivity of Al-doped ZnO films prepared by the plasma sputtering

A hot-cathode plasma sputtering technique was used for fabricating the highly transparent and conducting aluminum-doped zinc oxide (AZO) films on glass substrates from a disk-shaped AZO (Al₂O₃: 2 wt.%) target. Under particular conditions where the target voltage was V_T = −200 V and the plasma excitation pressure was P_S = 1.5 × 10⁻³ Torr, the lowest resistivity of 4.2 × 10⁻⁴ Ω cm was obtained at 400 nm, and this was associated with a carrier density of 8.7 × 10²⁰ cm⁻³ and a Hall mobility of 17 cm²/V s. From the annealing experiment of the AZO films in the oxygen and nitrogen gases of the atmospheric pressure it was revealed that both the oxygen vacancies and the grain boundaries in the polycrystalline AZO film played an important role in the electrical properties of the film.     

Semi-transparent pentacene thin film transistors with NiOx electrode operating at low voltages

We demonstrate the fabrication of semi-transparent pentacene-based thin-film transistors (TFTs) with thin poly-4-vinylphenol (PVP)/high-k yttrium oxide (YO_x) double gate dielectric layers and also with thermally-evaporated NiO_x source/drain (S/D) electrodes which show a transmittance of ∼ 30-40% and sheet resistance range of 100-200 Ω/□ (controlled by deposition rate). Our pentacene TFTs with PVP (45 nm)/YO_x (100 nm) layers operated at less than − 5 V, exhibiting a decent saturation mobility (maximum 0.83 cm²/Vs) and on/off current ratios of 10⁴. When the sheet resistance of our semi-transparent NiO_x electrode increased from 100 Ω/□ to 200 Ω/□, the field mobility of our TFT decreased but was found to be still effective as 0.32 cm²/Vs.     

Effects of substrate on the structural, electrical and optical properties of Al-doped ZnO films prepared by radio frequency magnetron sputtering

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on substrates including alkali-free glass, quartz glass, Si, and SiO₂ buffer layer on alkali-free glass by using radio frequency magnetron sputtering. The effects of different substrates on the structural, electrical and optical properties of the AZO films were investigated. It was found that the crystal structures were remarkably influenced by the type of the substrates due to their different thermal expansion coefficients, lattice mismatch and flatness. The AZO film (100 nm in thickness) deposited on the quartz glass exhibited the best crystallinity, followed sequentially by those deposited on the Si, the SiO₂ buffer layer, and the alkali-free glass. The film deposited on the quartz glass showed the lowest resistivity of 5.14 × 10^− 4 Ω cm among all the films, a carrier concentration of 1.97 × 10²¹ cm^− 3 and a Hall mobility of 6.14 cm²/v·s. The average transmittance of this film was above 90% in the visible light spectrum range. Investigation into the thickness-dependence of the AZO films revealed that the crystallinity was improved with increasing thickness and decreasing surface roughness, accompanied with a decrease in the film resistivity.     

Structural and functional properties of Al:ZnO thin films grown by Pulsed Laser Deposition at room temperature

Current research on transparent conductive oxides (TCOs) is focusing on indium-free TCOs, such as Al-doped ZnO (AZO), as an alternative to indium-tin oxide. In this work, AZO thin films were grown by Pulsed Laser Deposition at room temperature in oxygen atmosphere. The O₂ pressure was varied from 0.01 Pa to 10 Pa, highlighting the effects of defect formation and oxygen vacancies on the film properties. Structural properties were characterized by X-ray diffraction and Scanning Electron Microscopy, while functional properties were characterized by measurement of electrical conductivity, Hall mobility, carrier density and optical transmission. At an optimal deposition pressure of 2 Pa, optical transparency in the visible range and minimum resistivity (4.5 ? 10^− 4 Ω cm) were found, comparable to state-of-the-art TCOs. Mean value of visible transparency was shown to increase with increasing pressure, up to 88% at a deposition pressure of 10 Pa.     

The influence of fabrication process on top-gate thin-film transistors

Amorphous indium zinc oxide (a-IZO) thin-film transistors (TFTs) with bottom- and top-gate structures were fabricated at room temperature by direct current (DC) magnetron sputter in this research. High dielectric constant (κ) hafnium oxide (HfO₂) films and a-IZO were deposited for the gate insulator and the semiconducting channel under a mixture of ambient argon and oxygen gas, respectively. The bottom-gate TFTs showed good TFT characteristics, but the top-gate TFTs did not display the same characteristics as the bottom-gate TFTs despite undergoing the same process of sputtering with identical conditions. The electrical characteristics of the top-gate a-IZO TFTs exhibited strong relationships with sputtering power as gate dielectric layer deposition in this study. The ion bombardment and incorporation of sputtering ions damaged the interface between the active layer and the gate insulator in top-gate TFTs. Hence, the sputtering power was reduced to decrease damage while depositing HfO₂ films. When using 50 W DC magnetron sputtering, the top-gate a-IZO TFTs showed the following results: a saturation mobility of 5.62 cm²/V-s; an on/off current ratio of 1 × 10⁵; a sub-threshold swing (SS) of 0.64 V/decade; and a threshold voltage (V_th) of 2.86 V.     

Characteristics of sputtered Al-doped ZnO films for transparent electrodes of organic thin-film transistor

Aluminum-doped ZnO (AZO) thin-films were deposited with various RF powers at room temperature by radio frequency (RF) magnetron sputtering method. The electrical properties of the AZO film were improved with the increasing RF power. These results can be explained by the improvement of the crystallinity in the AZO film. We fabricated the organic thin-film transistor (OTFT) of the bottom gate structure using pentacene active and poly-4-vinyl phenol gate dielectric layers on the indium tin oxide gate electrode, and estimated the device properties of the OTFTs including drain current-drain voltage (I_D-V_D), drain current-gate voltage (I_D-V_G), threshold voltage (V_T), on/off ratio and field effect mobility. The AZO film that grown at 160 W RF power exhibited low resistivity (1.54 × 10^− 3 Ω·cm), high crystallinity and uniform surface morphology. The pentacene thin-film transistor using the AZO film that's fabricated at 160 W RF power exhibited good device performance such as the mobility of 0.94 cm²/V s and the on/off ratio of ~ 10⁵. Consequently, the performance of the OTFT such as larger field-effect carrier mobility was determined the conductivity of the AZO source/drain (S/D) electrode. AZO films prepared at room temperature by the sputtering method are suitable for the S/D electrodes in the OTFTs.     

Zinc oxide thin film transistors using MgO-Bi1.5Zn1.0Nb1.5O7 composite gate insulator on glass substrate

We report on high mobility ZnO thin film transistors (TFTs) (< 5 V), utilizing a room temperature grown MgO-Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) composite gate insulator on a glass substrate. 30 mol% MgO added BZN composite gate insulators exhibited greatly enhanced leakage current characteristics (~< 2 × 10^− 8 A/cm² at 0.3 MV/cm) due to the high breakdown strength of MgO, while retaining an appropriate high-k dielectric constant of 32. The ZnO-TFTs with MgO-BZN composite gate insulators showed a high field-effect mobility of 37.2 cm²/Vs, a reasonable on-off ratio of 1.54 × 10⁵, a subthreshold swing of 460 mV/dec, and a low threshold voltage of 1.7 V.     

Improvement in the bias stability of zinc oxide thin-film transistors using Si3N4 insulator with SiO2 interlayer

The performance of ZnO thin film transistors (TFT) subjected to SiO₂ interlayer treatments on Si₃N₄ insulators was investigated. In the case of a SiO₂ interlayer of 10 nm on Si₃N₄ insulator, a drastic improvement in device performance was obtained. ZnO TFT with this interlayer showed reduced trap density between the Si₃N₄ and ZnO channel, bringing remarkable improvement in bias stability characteristics. These devices show good performance and exhibit a high field-effect mobility of 6.41 cm²/Vs, an on/off current ratio of 10⁸, and a subthreshold swing of 1.46 V/decade. Also, the turn-on voltage shifted from − 2 V to − 6 V with negligible changes in the subthreshold swing and field effect mobility after total stress time.     

Evaluation of Y2O3 gate insulators for a-IGZO thin film transistors

In this work, Y₂O₃ was evaluated as a gate insulator for thin film transistors fabricated using an amorphous InGaZnO₄ (a-IGZO) active layer. The properties of Y₂O₃ were examined as a function of various processing parameters including plasma power, chamber gas conditions, and working pressure. The leakage current density for the Y₂O₃ film prepared under the optimum conditions was observed to be ~ 3.5 × 10^− 9 A/cm² at an electric field of 1 MV/cm. The RMS roughness of the Y₂O₃ film was improved from 1.6 nm to 0.8 nm by employing an ALD (Atomic Layer Deposition) HfO₂ underlayer. Using the optimized Y₂O₃ deposition conditions, thin film transistors (TFTs) were fabricated on a glass substrate. The important TFT device parameters of the on/off current ratio, sub-threshold swing, threshold voltage, and electric field mobility were measured to be 7.0 × 10⁷, 0.18 V/dec, 1.1 V, and 3.3 cm²/Vs, respectively. The stacked insulator consisting of Y₂O₃/HfO₂ was highly effective in enhancing the device properties.     

Effect of Ag doping on the performance of ZnO thin film transistor

Ag-doped zinc oxide (SZO) thin film transistors (TFTs) have been fabricated using a back-gate structure on thermally oxidized and heavily doped p-Si (100) substrate. The SZO thin films were deposited via pulsed laser deposition (PLD) from a 1, 3, and 5 wt.% Ag-doped ZnO (1SZO, 3SZO, and 5SZO, respectively) target using a KrF excimer laser (λ, 248 nm) at oxygen pressure of 350 mTorr. The deposition carried out at both room-temperature (RT) and 200 °C. The SZO thin films had polycrystalline phase with preferred growth direction of (002) as well as a wurtzite hexagonal structure. Compare with ZnO thin films, the SZO thin films were characterized by confirming the shift of (002) peak to investigate the substitution of Ag dopants for Zn sites. The as-grown SZO TFTs deposited at RT and 200 °C showed insulator characteristics. However the SZO TFTs annealed at 500 °C showed good n-type TFT performance because Ag was diffused from Zn lattice site and bound themselves at the high temperature, and it caused generation of electron carriers. The post-annealed 5SZO TFT deposited at 500 °C exhibited a threshold voltage (V_th) of 11.5 V, a subthreshold swing (SS) of 2.59 V/decade, an acceptable mobility (μ_SAT) of 0.874 cm²/V s, and on-to-off current ratios (I_on/off) of 1.44 × 10⁸.     

Deposition, characterization and biological application of epitaxial Li:ZnO/Al:ZnO double-layers

Double-layers of lithium doped ZnO (LZO) and aluminum doped ZnO (AZO) are grown on r-cut sapphire (r-Al₂O₃) crystal substrates by pulsed-laser deposition. The epitaxial double-layers are a-axis lattice oriented to the substrate. The LZO/AZO/r-Al₂O₃ samples have high optical transmission in the visible range and a bandgap energy of E_g = 3.28 eV according to the absorption edge of ZnO. The AZO bottom layers are electrically conductive (resistivity at room temperature ρ ~ 10^− 3 Ω cm) and LZO top layers are highly resistive (ρ ≥ 10⁵ Ω cm). Acoustic shear mode resonances in r-Al₂O₃ are excited by employing electric fields to the piezoelectric LZO layer (frequency interval 1.5-3 GHz). For biological applications, Madin-Darby canine kidney cells are cultivated on Platinum coated LZO/AZO/r-Al₂O₃ samples. Osmotic pressure applied to the cells increases or reduces the cell volume depending on the osmolarity of the medium.     

Effect of N2O/SiH4 flow ratios on properties of amorphous silicon oxide thin films deposited by inductively-coupled plasma chemical vapor deposition with application to silicon surface passivation

Silicon oxide (SiO_x) thin films have been deposited at a substrate temperature of 300 °C by inductively-coupled plasma chemical vapor deposition (ICP-CVD) using N₂O/SiH₄ plasma. The effect of N₂O/SiH₄ flow ratios on SiO_x film properties and silicon surface passivation were investigated. Initially, the deposition rate increased up to the N₂O/SiH₄ flow ratio of 2/1, and then decreased with the further increase in N₂O/SiH₄ flow ratio. Silicon oxide films with refractive indices of 1.47-2.64 and high optical band-gap values (>3.3 eV) were obtained by varying the nitrous oxide to silane gas ratios. The measured density of the interface states for films was found to have minimum value of 4.3 × 10¹¹ eV⁻¹ cm⁻². The simultaneous highest τ_eff and lowest density of interface states indicated that the formation of hydrogen bonds at the SiO_x/c-Si interface played an important role in surface passivation of p-type silicon.