The effect of oxygen content on the performance of low-voltage organic phototransistor memory

Optical writing and electrical erasing organic phototransistor memory (OPTM) is a promising photoelectric device for its novel integration of photosensitive and memory properties. The performance of OPTM can be influenced by the trap density of the gate dielectric layer. Here, we occupy tantalum pentoxide (Ta₂O₅), which is a prospective material in microelectronics field, as the gate dielectric. By increasing the oxygen content from 10% to 50% during the fabrication process of Ta₂O₅, it is found that the mobility and the photoresponsivity of OPTMs are significantly enhanced about 10 times and the retention time is greatly increased to 8.4 × 10⁴ s as well. As far as we know, this is the first example that the modulation of oxygen content can improve the OPTM performance. Furthermore, the change of the oxygen content gives rise to the alteration of the threshold voltage and memory window, of which the absolute values of all the threshold voltage are below 5 V which is low enough to reduce the power consumption. It is found that the oxygen content can influence the surface roughness and surface energy of Ta₂O₅ films, which alter the nucleation and orientation of semiconductor layers, change the contact resistance and modulate the electron trap density in the Ta₂O₅ films.     

Defect modification in ZnInSnO transistor with solution-processed Al2O3 dielectric by annealing

The properties of solution-processed Al₂O₃ thin films annealed at different temperatures were thoroughly studied through thermogravimetry–differential thermal analysis, UV–vis-NIR spectrophotometer measurements, scanning electron microscopy, X-ray diffraction, atomic force microscopy and a series of electrical measurements. The solution-processed ZnInSnO thin films transistors (TFTs) with the prepared Al₂O₃ dielectric were annealed at different temperatures. The TFTs annealed at 600 °C have displayed excellent electrical performance such as the field-effect mobility of 116.9 cm² V⁻¹ s⁻¹ and a subthreshold slope of 93.3 mV/dec. The performance of TFT device could be controlled by adjusting the annealing temperature. The results of two-dimensional device simulations demonstrate that the improvement of device performance are closely related with the reduction of interface defects between channel and dielectric and subgap density of stats (DOS) in the channel layer.     

Low-Pressure chemical vapour deposition of tantalum pentoxide films for ulsi devices using tantalum pentaethoxide as precursor

A laminar flow low-pressure chemical vapour deposition (LPCVD) system (LAM Integrity^TM) has been used to deposit tantalum pentoxide (Ta₂O₅) from Ta(OEt)₅ films in the presence of oxygen (O₂) at 470 °C at a typical deposition rate of 4 nm min^?1. Uniformities of <1.5% (SD 1σ) over a 150 mm silicon substrate were obtained. The layers were annealed under different conditions. It was discovered that the films did not change their stoichiometry as determined by Rutherford backscattering (RBS). The as-deposited films were amorphous but became crystalline (β-Ta₂O₅) at temperatures > 700 °C. The transmission electron microscopy (TEM) results on crystallisation behaviour were supported by X-ray diffraction data. The electrical properties of the Ta₂O₅ films have been characterised using MIS (metal/insulator/silicon) capacitor structures. Leakage values of <10^?6 A cm^?2 at 6 MV cm^?1 equivalent applied electric field and breakdown strengths of >7 MV cm^?1 at 1.6 μA were obtained for annealed layers. Compound dielectric constants (native silicon oxide thickness of about 2.5 nm plus Ta₂O₅ of various thicknesses) between 14 and >30 have been measured. The electrical properties reveal the potential use of Ta₂O₅ as a storage capacitor dielectric in 64 and 256 Mbit DRAM (dynamic random access memory) devices.     

Constant current stress of lightly Al-doped Ta2O5

The response of lightly Al-doped Ta₂O₅ stacked films (6 nm) to constant current stress (CCS) under gate injection (current stress in the range of 1 to 30 mA/cm² and stressing time of 50–400 s) has been investigated. The stress creates positive oxide charge, which is assigned to oxygen vacancies but it does not affect the dielectric constant of the films. The most sensitive parameter to the stress is the leakage current. Different degradation mechanisms control the stress-induced leakage current (SILC) in dependence on both the stress conditions and the applied measurement voltage. The origin of SILC is not the same as that in pure and Ti- or Hf-containing Ta₂O₅. The well known charge trapping in pre-existing traps operates only at low level stress resulting in small SILC at accumulation. The new trap generation plays a key role in the SILC degradation and is the dominant mechanism controlling the SILC in lightly Al-doped Ta₂O₅ layers.     

Reduction of defect states of tantalum oxide thin films with additive elements

Electrical properties of tantalum oxide (Ta₂O₅) thin films formed by metalorganic decomposition have been studied with respect to their defect states. A shallow band of defect states, which are the origin of hole traps, is detected by thermally stimulated current. The defect states in the Ta₂O₅ films are fully suppressed with additive elements such as TiO₂ and WO₃. It is considered that oxygen vacancies are fully compensated by Ti⁴⁺ ions or W⁶⁺ ions that are substitutionally incorporated into Ta⁵⁺ sites.     

Low-voltage,high-performance n-channel organic thin-film transistors based on tantalum pentoxide insulator modified by polar polymers

Polar polymers (polyfluorene copolymers, PFN–PBT) with different polarities are utilized to modify the surface of tantalum pentoxide (Ta₂O₅) insulator in n-channel organic thin-film transistors (OTFTs). A high mobility of 0.55 cm²/Vs, high on/off current ratio of 1.7 × 10⁵, and low threshold voltage of 2.8 V are attained for the OTFT with the modification polymers, the performances of which are much better than those of OTFT with only Ta₂O₅ insulator. The performances of the OTFT with only Ta₂O₅ insulator are only 0.006 cm²/Vs in mobility, 5 × 10³ in on/off ratio, and 12.5 V in threshold voltage. Furthermore, it is found that the threshold voltage of the OTFTs with PFN–PBT modification layer is easily tuned by polarities of the polymers. Further studies show that self-assembly dipole moments in the polymers play an important role in the improvement of the OTFT performances.     

Antimony sulfide (Sb2S3) thin films by pulse electrodeposition: Effect of thermal treatment on structural,optical and electrical properties

Antimony sulfide films have been deposited by pulse electrodeposition on Fluorine doped SnO₂ coated glass substrates from aqueous solutions containing SbCl₃ and Na₂S₂O₃. The crystalline structure of the films was characterized by X-ray diffraction, Raman spectroscopy and TEM analysis. The deposited films were amorphous and upon annealing in nitrogen/sulfur atmosphere at 250 °C for 30 min, the films started to become crystalline with X-ray diffraction pattern matching that of stibnite, Sb₂S₃, (JCPDS 6-0474). AFM images revealed that Sb₂S₃ films have uniformly distributed grains on the surface and the grain agglomeration occurs with annealing. The optical band gap calculated from the transmittance and the reflectance studies were 2.2 and 1.65 eV for as deposited and 300 °C annealed films, respectively. The annealed films were photosensitive and exhibited photo-to-dark current ratio of two orders of magnitude at 1 kW/m² tungsten halogen radiation.     

Improved reliability of NO treated NH3-nitrided oxide with regard to O2 annealing

NH₃-nitrided oxide has been annealed in NO or O₂ gas at 1000°C for 1 min and the electrical characteristics, charge trapping and time-dependent dielectric breakdown (TDDB) have been studied. It was observed that in the F–N region before stress the conduction current is the same for all the samples but intrinsic breakdown is earlier for O₂ annealed NH₃-nitrided oxide. After stress, the leakage current increases abruptly for the O₂ annealed NH₃-nitrided oxide. The TDDB characteristics have been measured for wet, NO and O₂ annealed NH₃-nitrided oxide. It was observed that the endurance of wet and NO annealed NH₃-nitrided oxide is the same, but the O₂ annealed NH₃-nitrided oxide has a lot of initial failure at the same stress of −11 MV cm⁻¹. From the experimental results, it can be said that NO annealing not only removes H which comes from the NH₃ nitridation but also improves the oxide reliability by replacing strained Si–O bonds for a stable Si–N bond.     

The correlation of γ-irradiation,particle size and their effects on physical properties of AgInSe2 nanostructure thin films

In this work the effect of γ-irradiation on the optical and electrical properties of near stoichiometric AgInSe₂ nanostructure thin films have been characterized. The XRD pattern of ingot AgInSe₂ powder prepared by solid state reaction showed tetragonal polycrystalline single-phase structure. The thin films of thickness 75 nm were prepared by inert gas condensation (IGC) technique at using constant Ar flow and substrate temperature of 300 K.Thin films were exposed to annealing process at 473 K for 2 h in vacuum of 10⁻² Torr. The amorphous and tetragonal nanocrystalline structures were detected for as-deposited and annealed films respectively by grazing incident in-plane X-ray diffraction (GIIXD) technique. The structure and average particle size of annealed irradiated films by different γ-doses from 0 to 4 Mrad were determined using high resolution transmission electron microscope (HRTEM). Optical transmission, reflection and absorption spectra were studied for both annealed unirradiated and irradiated films. Two optical transitions for each annealed unirradiated and film exposed to γ-irradiation doses from 0 to 4 Mrad were observed. The evaluated E_g1 due to 1^st transition have decreased from 1.52 to 1.44 eV and E_g2 due to 2^nd transition have decreased from 2.83 to 2.30 eV as the particle size increased from 7.3 to 9.5 nm by raising the irradiation dose up to 1 Mrad. The behavior of d.c. electrical conductivity with temperature that measured under vacuum was examined for all films under investigation. The evaluated activation energies due to irradiation doses are ranging from 0.58 to 0.68 eV.     

Effects of oxygen concentration on the properties of Al-doped ZnO transparent conductive films deposited by pulsed DC magnetron sputtering

The effect of oxygen concentration on the properties of Al-doped ZnO (AZO) transparent conductive films has been investigated on the films deposited by pulsed DC magnetron sputtering using a cylindrical ZnO target containing 2 wt% Al. AZO films were deposited at 230 °C to the thickness of about 1000 nm and the oxygen concentration was controlled by varying the O₂/Ar supply ratio from 0 to 0.167. With the increasing O₂/Ar ratio, crystallinity of the AZO films deteriorated while the film surface became smooth. Accompanying this, electrical properties also deteriorated significantly. When the O₂/Ar ratios were 0 and 0.033, the AZO films showed metallic conduction behavior with the electrical resistivity in the mid 10^?4 Ω cm range. However, when the ratios were 0.100 and 0.167, the films showed poor electrical conduction behavior similar to semiconductors as deduced from the transmittance behavior. Spectroscopic analysis showed that such deteriorating properties are due to the formation of condensed oxide group through the reaction between excess oxygen and dopant aluminum.     

Study of sensitivity and selectivity of α-Fe2O3 thin films for different toxic gases and alcohols

This paper presents a detailed study on the sensitivity and selectivity of α-Fe₂O₃ thin films produced by deposition of Fe and post-deposition annealed at two temperatures of 600 °C and 800 °C with flow of oxygen for application as a sensor for toxic gases including CO, H₂S, NH₃ and NO₂ and alcohols such as C₃H₇OH, CH₃OH, and C₂H₅OH. The crystallographic structure of the samples was studied by X-ray diffraction (XRD) method while an atomic force microscope (AFM) was employed for surface morphology investigation. The electrical response of the films was measured while they were exposed to various toxic gases and alcohols in the temperature range of 50–300 °C. The sample annealed at higher temperature showed higher response for different gases and alcohols tested in this work which can be due to the higher resistance of this sample. Results also indicated that the α-Fe₂O₃ thin films show higher selectivity to NO₂ gas relative to the other gases and alcohols while the best sensitivity is obtained at 200 °C. The α-Fe₂O₃ thin film post-deposition annealed at 800 °C also showed a good stability and reproducibility and a detection limit of 10 ppm for NO₂ gas at the operating temperature of 200 °C.     

Effect of substrate temperature on indium tin oxide (ITO) thin films deposited by jet nebulizer spray pyrolysis and solar cell application

This study focused on the effect of substrate temperature (350 °C, 400 °C, and 450 °C) on morphological, optical, and electrical properties of indium tin oxide (ITO) films deposited onto porous silicon/sodalime glass substrates through jet nebulizer spray pyrolysis for use in heterojunction solar cells. X-ray diffraction analysis confirmed the formation of pure and single-phase In₂O₃ for all the deposited films whose crystallinity was enhanced with increasing substrate temperature, as shown by the increasing (222) peak intensity. Morphological observations were conducted using scanning electron microscopy to reveal the formation of continuous dense films composed of nanograins. The UV–vis spectra revealed that the transmittance increased with increasing substrate temperature, reaching a value of over 80% at 450 °C. The photoelectric performance of the solar cell was studied using the I–V curve by illuminating the cell at 100 mW/cm². A high efficiency (η) of 3.325% with I_sc and V_oc values of 14.8 mA/cm² and 0.60 V, respectively, was attained by the ITO solar cell annealed at 450 °C.     

Thin RF sputtered and thermal Ta2O5 on Si for high density DRAM application

Tantalum pentoxide thin films on Si prepared by two conventional for modern microelectronics methods (RF sputtering of Ta in Ar + O₂ mixture and thermal oxidation of tantalum layer on Si) have been investigated with respect to their dielectric, structural and electric properties. It has been found that the formation of ultra thin SiO₂ film at the interface with Si, during fabrication implementing the methods used, is unavoidable as both, X-ray photoelectron spectroscopy and electrical measurements, have indicated. The initial films (as-deposited and as-grown) are not perfect and contain suboxides of tantalum and silicon which act as electrical active centers in the form of oxide charges and interface states. Conditions which guarantee obtaining high quality tantalum oxide with dielectric constant of 32–37 and leakage current density less than 10⁻⁷ A/cm² at 1.5 V applied voltage (Ta₂O₅ thickness equivalent to about 3.5 nm of SiO₂) have been established. These specifications make the layers obtained suitable alternative to SiO₂ for high density DRAM application.     

Study of amorphous Ta2O5 thin films by DC magnetron reactive sputtering

The dc magnetron reactive sputtering deposition of tantalum pentoxide (Ta₂O₅) thin films was investigated. By combining Schiller's criterion and Reith’s “target preoxidation” procedure, high quality Ta₂O₅ thin films were prepared at a high deposition rate of about lOOÅ;/min. The deposited films were amorphous, with a refractive index around 2.07 and a dielectric constant of 20. An optical transmit-tance of 98.6% was obtained for a 4500Â thick film. The leakage current density is 5 × 10^?9 A/cm² at an electric field strength of 1 MV/cm and its breakdown field strength is above 2 MV/cm. The temperature coefficient of capacitance for capacitors fabricated using the deposited films is approximately +230 ppm/°C. X-ray photoelectron spectroscopy shows that the films are stoichiometric tantalum pentoxide, Ta₂O₅, and exhibit good stability.     

Effects of annealing in O2 and N2 on the electrical properties of tantalum oxide thin films prepared by electron cyclotron resonance plasma enhanced chemical vapor deposition

The tantalum oxide thin films with a thickness of 14 nm were deposited at 95°C by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECRPECVD), and annealed at various temperatures (700∼850°C) in O₂ and N₂ ambients. The microstructure and composition of the tantalum oxide thin films and the growth of interfacial silicon oxide layer were investigated and were related to the electrical characteristics of the film. Annealing in an O₂ ambient led to a high dielectric constant (ε_r(Ta₂O₅) = 24) as well as a small leakage current (E_bd = 2.3 MV/cm), which were due to the improved stoichiometry and the decreased impurity carbon content. Annealing in an N₂ ambient resulted in poor and nonuniform leakage current characteristics. The as-deposited tantalum oxide films were crystallized into δ-Ta₂O₅ after annealing at above 750°C regardless of the ambient. The leakage current of the film abruptly increased after annealing at 850°C probably because of the stress caused by thermal expansion or contraction.     

Composition,structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films

Thin films of alumina (Al₂O₃) were deposited over Si 〈1 0 0〉 substrates at room temperature at an oxygen gas pressure of 0.03 Pa and sputtering power of 60 W using DC reactive magnetron sputtering. The composition of the as-deposited film was analyzed by X-ray photoelectron spectroscopy and the O/Al atomic ratio was found to be 1.72. The films were then annealed in vacuum to 350, 550 and 750 °C and X-ray diffraction results revealed that both as-deposited and post deposition annealed films were amorphous. The surface morphology and topography of the films was studied using scanning electron microscopy and atomic force microscopy, respectively. A progressive decrease in the root mean square (RMS) roughness of the films from 1.53 nm to 0.7 nm was observed with increase in the annealing temperature. Al–Al₂O₃–Al thin film capacitors were then fabricated on p-type Si 〈1 0 0〉 substrate to study the effect of temperature and frequency on the dielectric property of the films and the results are discussed.     

Structural,morphological, electrical and optical studies of Cr doped SnO2 thin films deposited by the spray pyrolysis technique

Tin oxide (SnO₂) and chromium (Cr) doped tin oxide (Cr:SnO₂) thin films were deposited on the preheated glass substrates at 673 K by spray pyrolysis. Concentration of Cr was varied in the solution by adding chromium (III) chloride hexahydrate from 0 to 3 at%. The effect of Cr doping on the structural, electrical and optical properties of tin oxide films is reported. X-ray diffraction pattern confirms the tetragonal crystal structure for undoped and Cr doped tin oxide films. Scanning electron microscopic photographs show the modification of surface morphology of tin oxide film due to varying concentration of Cr. X-ray photoelectron spectra of Cr:SnO₂ (3 at%) thin film revealed the presence of carbon, tin, oxygen, and chromium. Carrier concentration and mobility of the SnO₂ films decrease with increasing concentration of Cr and 0.5 at% Cr doped tin oxide film acquires a mobility of 70 cm²/V s. Average optical transmittance in the 550–850 nm range varies from 38% to 47% with varying Cr concentration in the solution.     

Effect of dissolved oxygen on thermal oxidation in Ta2O5/Ta sandwiches

Chemical analysis, structural investigations, and electrical capacitance measurements have been performed on Ta₂O₅/Ta foils annealed at 753K in air for different initial concentration of oxygen in Ta. It was shown that the initial concentration of oxygen in Ta plays a crucial role in formation of thermal nonstoichiometric oxide layer between tantalum and anodic Ta₂O₅, namely only Ta with low initial oxygen content covered with thin Ta₂O₅ layer is resistant to thermal oxidation. The obtained results may explain the degradation of real capacitors made of fine Ta powders.     

Effect of high-temperature annealing on lanthanum aluminate thin films grown by ALD on Si(1 0 0)

Amorphous lanthanum aluminate thin films were deposited by atomic layer deposition on Si(1 0 0) using La(ⁱPrCp)₃, Al(CH₃)₃ and O₃ species. The effects of post-deposition rapid thermal annealing on the physical and electrical properties of the films were investigated. High-temperature annealing at 900 °C in N₂ atmosphere leads to the formation of amorphous La-aluminosilicate due to Si diffusion from the substrate. The annealed oxide exhibits a uniform composition through the film thickness, a large band gap of 7.0 ± 0.1 eV, and relatively high dielectric constant (κ) of 18 ± 1.     

Deposition of tantalum oxide films by dual spectral source assisted metalorganic chemical vapor deposition (MOCVD)

Dual spectral source assisted metalorganic chemical vapor deposition (MOCVD) is an ideal technique for the deposition of high dielectric constant materials. Tungsten halogen lamps and a deuterium lamp are used as the sources of optical and thermal energy. In this paper, we have reported the deposition and characterization of tantalum penta oxide films. Ta₂O₅ films were deposited at 660°C for 15 min and annealed at 400°C for 1 h. The leakage current densities of 10.6 nm thick films are as low as 10₋₁₀ A/cm² for gate voltage under 4V. To the best of our knowledge, these are the best results reported to date by any researcher. The high energy photons used in the in-situ cleaning and deposition process play an important role in obtaining high quality films of Ta₂O₅.