Top-gate amorphous IGZO thin-film transistors with a SiO buffer layer inserted between active channel layer and gate insulator

In this paper, top-gate thin-film transistors (TFTs) using amorphous In-Ga-Zn-O as the n-channel active layer and SiO₂ as gate insulator were fabricated by radio frequency magnetron sputtering at room temperature. In this device, a SiO layer was used to be a buffer layer between active layer and gate insulator for preventing the damage of the InGaZnO surface by the process of sputtering SiO₂ with relatively high sputtering power. The thickness of buffer layers was studied and optimized for enhancing the TFTs performances. Contrasting to the TFTs without buffer layer, the optimized thickness of 10 nm SiO buffer layer improved the top-gate TFTs performances greatly: mobility increases 30%, reached 1.29 cm²/V s, the I_on/I_off ratio increases 3 orders, and the trap density at the interface of channel/insulator decreases about 1 order, indicated that the improvement of semiconductor/dielectric interface by buffering the sputtering power.     

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.     

Extraction of density-of-states in amorphous InGaZnO thin-film transistors from temperature stress studies

The instability of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) with different active layer thicknesses under temperature stress has been investigated through using the density-of-states (DOS). Interestingly, the a-IGZO TFT with 22 nm active layer thickness showed a better stability than the others, which was observed from the decrease of interfacial and semiconductor bulk trap densities. The DOS was calculated based on the experimentally-obtained activation energy (E_A), which can explain the experimental observations. We developed the high-performance Al₂O₃ TFT with 22 nm IGZO channel layer (a high mobility of 7.4 cm²/V, a small threshold voltage of 2.8 V, a high I_on/I_off 1.8 × 10⁷, and a small SS of 0.16 V/dec), which can be used as driving devices in the next-generation flat panel displays.     

Enhanced photosensitivity of InGaZnO-TFT with a CuPc light absorption layer

A copper phthalocyanine (CuPc) organic semiconductor is capped onto an amorphous indium–gallium–zinc-oxide (InGaZnO) thin film transistor (TFT) to enhance the photosensitivity of InGaZnO-TFT. The CuPc organic semiconductor is served as a light absorption layer and forms a p–n junction with the InGaZnO film. After 60 s white light illumination, light responsivity (R) of InGaZnO-TFT with a CuPc light absorption layer reaches a value of 148.5 A/W at a gate-source voltage (V_GS) of 20 V, which is much larger than that (31.2 A/W) of the conventional InGaZnO-TFT. The results are attributed to the following mechanism. First, a CuPc layer is employed as the light absorption layer. Second, CuPc/InGaZnO p–n junction enables the injection of electron into InGaZnO film. Our results indicate that using CuPc as light absorption layer is an effective approach to improve the photosensitivity of InGaZnO-TFT.     

Effect of MgO buffer layer thickness on the electrical properties of MgZnO thin film transistors fabricated by plasma assisted molecular beam epitaxy

We report the fabrication and electrical characteristics of thin film transistors based on MgZnO thin films with different thicknesses of MgO buffer layer. The MgZnO thin films with MgO buffer layers were grown on SiO₂/p-Si substrates by plasma assisted molecular beam epitaxy. The effects of the buffer layer thickness on the structural properties of MgZnO films are investigated by X-ray diffraction, and the results show that the crystal quality of the MgZnO film is enhanced with 4 nm MgO buffer layer. The MgZnO TFT with 4 nm MgO buffer layer exhibits an n-type enhancement mode characteristics with a field effect mobility of 1.85 cm²/V s, a threshold voltage of 27.6 V and an on/off ratio of above 10⁶.     

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.     

Highly stable hafnium–tin–zinc oxide thin film transistors with stacked bilayer active layers

Hf–Sn–Zn–O (HTZO) thin films were prepared on SiO₂/SiN_x substrates at room temperature by the direct current (DC) magnetron sputtering of Hf-doped Sn–Zn–O targets. The characteristics of films with different amounts of Hf were analyzed. Amorphous HTZO films were obtained by increasing the Hf content, while polycrystalline films have not shown with Hf doping. With the proper Hf concentration in the HTZO films (∼2.0 atomic % Hf/(Hf + Sn + Zn + O)), HTZO films demonstrated good performance as an oxide semiconductor channel material in thin film transistors (TFTs) with a field effect mobility (μ_FE) of 10.9 cm²V⁻¹ s⁻¹, an on/off current ratio of 10⁹, and a subthreshold voltage swing of 0.71 V/decade.     

低栅极电压控制下带有phenyltrimethoxysilane单分子自组装层的有机薄膜晶体管场效应特性研究

制作了底栅极顶接触有机薄膜晶体管器件,60 nm的pentacene被用作有源层,120 nm热生长的SiO₂作为栅极绝缘层.通过采用不同自组装修饰材料对器件的有源层与栅极绝缘层之间的界面进行修饰,如octadecyltrichlorosilane （OTS）,phenyltrimethoxysilane （PhTMS）,来比较界面修饰层对器件性能的影响.同时对带有PhTMS修饰层的OTFTs器件低栅极电压调制下的场效应行为及其载流子的传输机理进行研究.结果得到,当|V 关键词： 有机薄膜晶体管 自组装单分子层 场效应迁移率 低栅极调制电压     

Room-temperature and solution-processed vanadium oxide buffer layer for efficient charge injection in bottom-contact organic field-effect transistors

We introduce a room temperature and solution-processible vanadium oxide (VO_x) buffer layer beneath Au source/drain electrodes for bottom-contact (BC) organic field-effect transistors (OFETs). The OFETs with the VO_x buffer layer exhibited higher mobility and lower threshold voltages than the devices without a buffer layer. The hole mobility with VO_x was over 0.11 cm²/V with the BC geometry with a short channel length (10 μm), even without a surface treatment on SiO₂. The channel width normalized contact resistance was decreased from 98 kΩ cm to 23 kΩ cm with VO_x. The improved mobility and the reduced contact resistance were attributed to the enhanced continuity of pentacene grains, and the increased work function and adhesion of the Au electrodes using the VO_x buffer layer.     

Effect of buffer layers on the property of Pb(Zr,Ti)O3-Pb(Mn,W,Sb,Nb)O3 thin films grown by pulsed laser deposition

New ferroelectric Pb(Zr,Ti)O₃-Pb(Mn,W,Sb,Nb)O₃ (PZT-PMWSN) thin film has been deposited on a Pt/Ti/SiO₂/Si substrate by pulsed laser deposition. Buffer layer was adopted between film and substrate to improve the ferroelectric properties of PZT-PMWSN films. Effect of a Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Pb_0.72La_0.28)Ti_0.93O₃ (PLT) buffer layers on the stabilization of perovskite phase and the suppression of pyrochlore phase has been examined. Role of buffer layers was investigated depending on different types of buffer layer and thickness. The PZT-PMWSN thin films with buffer layer have higher remnant polarization and switching polarization values by suppressing pyrochlore phase formation. The remnant polarization, saturation polarization, coercive field and relative dielectric constant of 10-nm-thick PLT buffered PZT-PMWSN thin film with no pyrochlore phase were observed to be about 18.523 μC/cm², 47.538 μC/cm², 63.901 kV/cm and 854, respectively.     

The role of a ZnO buffer layer in the growth of ZnO thin film on Al2O3 substrate

A ZnO buffer layer and ZnO thin film have been deposited by the pulsed laser deposition technique at the temperatures of 200 C and 400 C, respectively. Structural, electrical and optical properties of ZnO thin films grown on sapphire (Al₂O₃) substrate with 1, 5, and 9 nm thick ZnO buffer layers were investigated. A minute shift of the (101) peak was observed which indicates that the lattice parameter was changed by varying the thickness of the buffer layer. High resolution transmission electron microscopy (TEM) was used to investigate the thickness of the ZnO buffer layer and the interface involving a thin ZnO buffer between the film and substrate. Selected area electron diffraction (SAED) patterns show high quality hexagonal ZnO thin film with 30 in-plane rotation with respect to the sapphire substrate. The use of the buffer can reduce the lattice mismatch between the ZnO thin film and sapphire substrate; therefore, the lattice constant of ZnO thin film grown on sapphire substrate became similar to that of bulk ZnO with increasing thickness of the buffer layer.     

Visible luminescence of nanocrystalline AlN:Er thin film by co-deposition of AlN,Er, and SiO2

We report a visible luminescence of Er³⁺ ions in an amorphous-nanocrystalline AlN:Er thin film prepared by co-deposition using AlN, Er, and SiO₂ targets. A PL emission spectrum of Er³⁺ in the AlN:Er film annealed at 750 °C showed a strong bluish green emission of Er³⁺ in the amorphous-nanocrystalline AlN:Er thin film, which is attributed to the intra-4fEr³⁺ transitions of ²H_11/2 → ⁴I_15/2 and ⁴F_7/2 → ⁴I_15/2. It was found that crystallite diameters were between 3 and 5 nm by high-resolution transmission electron microscopy. The occurrence of the strong Er³⁺ emission in the annealed AlN:Er thin film with a mixture of amorphous and nanocrystalline phases may be contributed to an increase in the number of excitation Er³⁺ centers and a presence of oxygen related to Er³⁺ excitation and recombination process in the AlN:Er thin film.     

Measurement of optical and surface properties of PLZT thin films

Lanthanum-modified lead zirconate titanate (Pb_0.93La_0.07(Zr_0.3Ti_0.7)_0.93O₃, PLZT7/30/70) thin films with and without a seeding layer of PbTiO₃ (PT) were successfully deposited on indium-doped tin oxide (ITO) coated glass substrate via spin coating in conjunction with a sol–gel process, and a top transparent conducting thin film of SnO₂ was also prepared in the same way. The thicknesses of PLZT and PT layers are 0.5 μm and 24 nm, respectively. The retardance of PLZT film was measured by a new heterodyne interferometer and enhanced by application of a seeding layer of PT. The Pockels linear electro-optical coefficient of PLZT film with a PT layer was determined to be 3.17 × 10^?9 m/V when the refractive index is considered as 2.505, which is one order larger than 1.4 × 10^?10 m/V for PLZT12/40/60 doped with Dy reported in the literature. The root-mean-square (rms) roughness of PLZT thin film with a PT layer (R_rms = 6.867 nm) was larger than that of PLZT film (R_rms = 0.799 nm). From the comparisons, the average transmittance of PLZT film with a PT seeding layer was 77.01%, which was a little smaller than that of PLZT film (around 80.75%). Experimental results imply that the PT seeding layer plays a key role in the increase of retardance value, leading to a higher Pockels coefficient.     

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.     

Electro-optic properties of epitaxial Sr<Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films grown on MgO substrates using Li<Subscript>x</Subscript>Ni<Subscript>2-x</Subscript>O buffer layer

Textured Li_xNi_2-xO (LNO) thin films have been fabricated on (001)MgO substrates by pulsed laser deposition technique. The as-deposited LNO films shows a conductivity of 2.5×10^-3 Ω m and possess a transmittance of about 35% in the visible region. Subsequent deposition of Sr_0.6Ba_0.4Nb₂O₆ (SBN60) thin film on these LNO-coated MgO substrates resulted in a textured SBN layer with a 〈001〉 orientation perpendicular to the substrate plane. Phi scans on the (221) plane of the SBN layer indicated that the films have two in-plane orientations with respect to the substrate. The SBN unit cells were rotated in the plane of the film by ± 8.2° as well as ± 45° with respect to the LNO/MgO substrate. Besides the highly (00l)-orientation, the SBN films also exhibited a dense microstructure as shown by scanning electron microscopy. The electro-optic coefficient (r₃₃) of the SBN film was measured to be 186 pm/V. On the basis of our results, we have demonstrated that the LNO film can be used as a buffer layer as well as a transparent bottom electrode for waveguide applications. The SBN/LNO heterostructure is also a suitable candidate for integrated electro-optics devices. PACS  42.79.Gn; 42.82.Et; 78.20.Ci     

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.     

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.     

Development of sulphurized SnS thin film solar cells

Thin films of tin sulphide (SnS) have been grown by sulphurization of sputtered tin precursor layers in a closed chamber. The effect of sulphurization temperature (T_s) that varied in the range of 150–450 °C for a fixed sulphurization time of 120 min on SnS film was studied through various characterization techniques. X-ray photoelectron spectroscopy analysis demonstrated the transformation of metallic tin layers into SnS single phase for T_s between 300 °C and 350 °C. The X-ray diffraction measurements indicated that all the grown films had the (111) crystal plane as the preferred orientation and exhibited orthorhombic crystal structure. Raman analysis showed modes at 95 cm⁻¹, 189 cm⁻¹ and 218 cm⁻¹ are related to the A_g mode of SnS. AFM images revealed a granular change in the grain growth with the increase of T_s. The optical energy band gap values were estimated using the transmittance spectra and found to be varied from 1.2 eV to 1.6 eV with T_s. The Hall effect measurements showed that all the films were p-type conducting nature and the layers grown at 350 °C showed a low electrical resistivity of 64 Ω-cm, a net carrier concentration of 2 × 10¹⁶ cm⁻³ and mobility of 41 cm² V⁻¹ s⁻¹. With the use of sprayed Zn_0.76Mg_0.24O as a buffer layer and the sputtered ZnO:Al as window layer, the SnS based thin film solar cell was developed that showed a conversion efficiency of 2.02%.     

Structure and properties of GZO thin films grown on ZnO buffer layers

Thin gallium-doped zinc oxide (in GZO the Ga₂O₃ contents are approximately 3 wt%) films having different ZnO buffer layers were deposited using radio frequency (rf) magnetron sputtering. The use of a grey-based Taguchi method to determine the processing parameters of ZnO buffer layer deposition has been studied by considering multiple performance characteristics. A Taguchi method with an L₉ orthogonal array, signal-to-noise (S/N) ratio, and analysis of variance (ANOVA) is employed to investigate the performance characteristics in the deposition operations. The effect and optimization of ZnO buffer deposition parameters (rf power, sputtering pressure, thickness, and annealing) on the structure, morphology, electrical resistivity, and optical transmittance of the GZO films are studied. Annealing treatment and reduction in thickness resulted in a decrease in root-mean-square (RMS) surface roughness of the ZnO buffer layer. Using the optimal ZnO buffer layer obtained by the application of the grey-based Taguchi method, the electrical resistivity of GZO films was decreased from 2.94×10⁻³ to 9.44×10⁻⁴ Ω cm and the optical transmittance in the visible region was slightly increased from 84.81% to 85.82%.     

Synthesis,spectral characterization of CuPcF16 and its application in organic thin film transistors using p-6p as inducing layer

An air-stable n-channel semiconductor material, CuPcF₁₆, was synthesized in a slightly modified procedure and characterized by infrared (IR), X-ray diffraction (XRD), UV–vis and fluorescence spectra. CuPcF₁₆ showed a monomer characteristic in THF and pyridine while exhibited an aggregation property in DMF. The CuPcF₁₆/p-6p (CuPcF₁₆ on p-6p) organic thin film transistors (OTFTs) using CuPcF₁₆ as an active layer and p-6p as an inducing layer was fabricated by the physical vapor deposition technique. Charge carrier field-effect mobility (μ), I_on/I_off and threshold voltage (V_T) of the CuPcF₁₆/p-6p OTFTs were 0.07 cm²/V s, 10⁵ and 5.28 V, respectively. The charge mobility of the OTFTs was two or even three times higher than that of the conventional single layer CuPcF₁₆-based OTFTs. The improved performance was attributed to the introduction of p-6p to form a highly oriented and continuous film of CuPcF₁₆ with the molecular π–π conjugated direction parallel to the substrate.