The Enhanced Performance of Oxide Thin-Film Transistors Fabricated by a Two-Step Deposition Pressure Process.

It is usually difficult to realize high mobility together with a low threshold voltage and good stability for amorphous oxide thin-film transistors (TFTs). In addition, a low fabrication temperature is preferred in terms of enhancing compatibility with the back end of line of the device. In this study, α-IGZO TFTs were prepared by high-power impulse magnetron sputtering (HiPIMS) at room temperature. The channel was prepared under a two-step deposition pressure process to modulate its electrical properties. X-ray photoelectron spectra revealed that the front-channel has a lower Ga content and a higher oxygen vacancy concentration than the back-channel. This process has the advantage of balancing high mobility and a low threshold voltage of the TFT when compared with a conventional homogeneous channel. It also has a simpler fabrication process than that of a dual active layer comprising heterogeneous materials. The HiPIMS process has the advantage of being a low temperature process for oxide TFTs.

Internal moisture barrier layer for improving high-humidity reliability of miniature light emitting diode die without encapsulation.

Atomic layer deposited Al2O3 films are incorporated into miniature light emitting diodes (mini-LEDs) as an internal moisture barrier layer. The experimental results show that the water vapor transmission rate reaches ≤10-4 g/m2/day when the Al2O3 thickness is ≥40 nm. The mini-LED with a 40 nm-thick Al2O3 layer shows negligible degradation after 1000 h of 85°C/85% relative humidity testing, whereas the device without an Al2O3 layer fails after only 500 h due to delamination occurring at the GaN surface. Current-voltage characteristics of the device without an Al2O3 moisture barrier layer indicate an increase in series resistance and ideality factor. This study provides a simple, light-weighting method to have a satisfactory encapsulation function for miniature LEDs.

Growth of GaN Thin Films Using Plasma Enhanced Atomic Layer Deposition: Effect of Ammonia-Containing Plasma Power on Residual Oxygen Capture.

In recent years, the application of (In, Al, Ga)N materials in photovoltaic devices has attracted much attention. Like InGaN, it is a direct band gap material with high absorption at the band edge, suitable for high efficiency photovoltaic devices. Nonetheless, it is important to deposit high-quality GaN material as a foundation. Plasma-enhanced atomic layer deposition (PEALD) combines the advantages of the ALD process with the use of plasma and is often used to deposit thin films with different needs. However, residual oxygen during growth has always been an unavoidable issue affecting the quality of the resulting film, especially in growing gallium nitride (GaN) films. In this study, the NH3-containing plasma was used to capture the oxygen absorbed on the growing surface to improve the quality of GaN films. By diagnosing the plasma, NH2, NH, and H radicals controlled by the plasma power has a strong influence not only on the oxygen content in growing GaN films but also on the growth rate, crystallinity, and surface roughness. The NH and NH2 radicals contribute to the growth of GaN films while the H radicals selectively dissociate Ga-OH bonds on the film surface and etch the grown films. At high plasma power, the GaN film with the lowest Ga-O bond ratio has a saturated growth rate, a better crystallinity, a rougher surface, and a lower bandgap. In addition, the deposition mechanism of GaN thin films prepared with a trimethylgallium metal source and NH3/Ar plasma PEALD involving oxygen participation or not is also discussed in the study.

Deposition Mechanism and Properties of Plasma-Enhanced Atomic Layer Deposited Gallium Nitride Films with Different Substrate Temperatures.

Gallium nitride (GaN) is a wide bandgap semiconductor with remarkable chemical and thermal stability, making it a competitive candidate for a variety of optoelectronic applications. In this study, GaN films are grown using a plasma-enhanced atomic layer deposition (PEALD) with trimethylgallium (TMG) and NH3 plasma. The effect of substrate temperature on growth mechanism and properties of the PEALD GaN films is systematically studied. The experimental results show that the self-limiting surface chemical reactions occur in the substrate temperature range of 250-350 °C. The substrate temperature strongly affects the crystalline structure, which is nearly amorphous at below 250 °C, with (100) as the major phase at below 400 °C, and (002) dominated at higher temperatures. The X-ray photoelectron spectroscopy spectra reveals the unintentional oxygen incorporation into the films in the forms of Ga2O3 and Ga-OH. The amount of Ga-O component decreases, whereas the Ga-Ga component rapidly increases at 400 and 450 °C, due to the decomposition of TMG. The substrate temperature of 350 °C with the highest amount of Ga-N bonds is, therefore, considered the optimum substrate temperature. This study is helpful for improving the quality of PEALD GaN films.

Crystallinity Effect on Electrical Properties of PEALD-HfO2 Thin Films Prepared by Different Substrate Temperatures.

Hafnium oxide (HfO2) thin film has remarkable physical and chemical properties, which makes it useful for a variety of applications. In this work, HfO2 films were prepared on silicon through plasma enhanced atomic layer deposition (PEALD) at various substrate temperatures. The growth per cycle, structural, morphology and crystalline properties of HfO2 films were measured by spectroscopic ellipsometer, grazing-incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), field-emission scanning electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy. The substrate temperature dependent electrical properties of PEALD-HfO2 films were obtained by capacitance-voltage and current-voltage measurements. GIXRD patterns and XRR investigations show that increasing the substrate temperature improved the crystallinity and density of HfO2 films. The crystallinity of HfO2 films has a major effect on electrical properties of the films. HfO2 thin film deposited at 300 °C possesses the highest dielectric constant and breakdown electric field.

Deposition Mechanism and Characterization of Plasma-Enhanced Atomic Layer-Deposited SnOx Films at Different Substrate Temperatures.

The promising functional tin oxide (SnOx) has attracted tremendous attention due to its transparent and conductive properties. The stoichiometric composition of SnOx can be described as common n-type SnO2 and p-type Sn3O4. In this study, the functional SnOx films were prepared successfully by plasma-enhanced atomic layer deposition (PEALD) at different substrate temperatures from 100 to 400 °C. The experimental results involving optical, structural, chemical, and electrical properties and morphologies are discussed. The SnO2 and oxygen-deficient Sn3O4 phases coexisting in PEALD SnOx films were found. The PEALD SnOx films are composed of intrinsic oxygen vacancies with O-Sn4+ bonds and then transformed into a crystalline SnO2 phase with increased substrate temperature, revealing a direct 3.5−4.0 eV band gap and 1.9−2.1 refractive index. Lower (<150 °C) and higher (>300 °C) substrate temperatures can cause precursor condensation and desorption, respectively, resulting in reduced film qualities. The proper composition ratio of O to Sn in PEALD SnOx films near an estimated 1.74 suggests the highest mobility of 12.89 cm2 V−1 s−1 at 300 °C.

Role of Ambient Hydrogen in HiPIMS-ITO Film during Annealing Process in a Large Temperature Range.

Indium tin oxide (ITO) thin films were prepared by high power impulse magnetron sputtering (HiPIMS) and annealed in hydrogen-containing forming gas to reduce the film resistivity. The film resistivity reduces by nearly an order of magnitude from 5.6 × 10-3 Ω·cm for the as-deposited film to the lowest value of 6.7 × 10-4 Ω·cm after annealed at 700 °C for 40 min. The role of hydrogen (H) in changing the film properties was explored and discussed in a large temperature range (300-800 °C). When annealed at a low temperature of 300-500 °C, the incorporated H atoms occupied the oxygen sites (Ho), acting as shallow donors that contribute to the increase of carrier concentration, leading to the decrease of film resistivity. When annealed at an intermediate temperature of 500-700 °C, the Ho defects are thermally unstable and decay upon annealing, leading to the reduction of carrier concentration. However, the film resistivity keeps decreasing due to the increase in carrier mobility. Meanwhile, some locally distributed metallic clusters formed due to the reduction effect of H2. When annealed at a high temperature of 700-800 °C, the metal oxide film is severely reduced and transforms to gaseous metal hydride, leading to the dramatic reduction of film thickness and carrier mobility at 750 °C and vanish of the film at 800 °C.

Compact Ga2O3 Thin Films Deposited by Plasma Enhanced Atomic Layer Deposition at Low Temperature.

Amorphous Gallium oxide (Ga2O3) thin films were grown by plasma-enhanced atomic layer deposition using O2 plasma as reactant and trimethylgallium as a gallium source. The growth rate of the Ga2O3 films was about 0.6 Å/cycle and was acquired at a temperature ranging from 80 to 250 °C. The investigation of transmittance and the adsorption edge of Ga2O3 films prepared on sapphire substrates showed that the band gap energy gradually decreases from 5.04 to 4.76 eV with the increasing temperature. X-ray photoelectron spectroscopy (XPS) analysis indicated that all the Ga2O3 thin films showed a good stoichiometric ratio, and the atomic ratio of Ga/O was close to 0.7. According to XPS analysis, the proportion of Ga3+ and lattice oxygen increases with the increase in temperature resulting in denser films. By analyzing the film density from X-ray reflectivity and by a refractive index curve, it was found that the higher temperature, the denser the film. Atomic force microscopic analysis showed that the surface roughness values increased from 0.091 to 0.187 nm with the increasing substrate temperature. X-ray diffraction and transmission electron microscopy investigation showed that Ga2O3 films grown at temperatures from 80 to 200 °C were amorphous, and the Ga2O3 film grown at 250 °C was slightly crystalline with some nanocrystalline structures.

Drugstore Beetle Damage Pattern of Glue Paste-Lined Oil Paintings.

A glue paste lining is a traditional conservation treatment used to reinforce the aged canvas of oil paintings. Several insect infestation cases concerning glue paste-lined oil paintings have been reported around the world, particularly in European countries. In 2008, Chimei Museum, a private museum located in Tainan, Taiwan, was affected by a severe beetle infestation of oil paintings. To confirm the infesting insects and to quantify the damage pattern for further development of control and monitoring methods, three severely damaged paintings were examined along with the restoration procedure. A total of four insect species were collected. The drugstore beetle, Stegobium paniceum (L.) (Coleopter: Ptinidae), was the primary pest found in all three paintings investigated and was identified based on morphological and genetic data. Thrips and booklice were considered secondary pests, and a Pteromalid wasp was speculated to have a parasitic relationship with the drugstore beetle. Drugstore beetle larvae mainly bored into the glue paste layer and original canvas and required only 5.94 ± 1.38 mm3 of feed to grow from egg to pupa. Their bores were not evenly distributed, and most of them were found in the shaded area covered by the stretcher and outer frame. The body length of drugstore beetles varied, ranging from 1.67 to 2.75 mm, which may explain the various sizes of exit holes on gummed tape surrounding the frame. Detailed and quantified information on drugstore beetle's pattern of damage provided in this study will be beneficial for further developing conservation practices and inspection methods.

Role of Interfacial Oxide in the Preferred Orientation of Ga2O3 on Si for Deep Ultraviolet Photodetectors.

It is generally known that a layer of amorphous silicon oxide (SiO2) naturally exists on the surface of silicon, resulting in the growth of gallium oxide (Ga2O3) that is no longer affected by substrate crystallinity during sputtering. This work highlights the formation energy between the native amorphous nano-oxide film formed on the Si substrate and monoclinic ß-Ga2O3 dominating the preferred orientation prepared for deep ultraviolet photodetectors. The latter were deposited on p-type silicon (p-Si) with (111) orientation using radio frequency sputtering at 600 °C and post rapid thermal annealing (RTA). The X-ray diffraction (XRD) results indicate both as-deposited and postannealing films with the (400) preferred orientation for a layer thickness of 100 nm. However, slight random orientation with the amorphous structure is mixed in the preferred one for the as-deposited film with a thickness of 200 nm and reduced after being annealed at 800 °C, which is observed by XRD and transmission electron microscopy. Meanwhile, thermal-induced massive twin boundaries (TBs) and stacking faults (SFs) were generated when annealed at 1000 °C, owing to the relaxation of lattice strain by the coherent interface. The interfacial bonding energy per unit area (E i) between ß-Ga2O3 films with various facets ((001), (010), (100), and (2̅01)) and amorphous SiO2 was calculated using density functional theory. The E i of ß-Ga2O3 (100)/SiO2 reveals the highest value (0.289 eV/Å2), which is consistent with the (100) preferred orientation of deposited films. The (100) preferred orientation is the driving force for TBs and SFs. The discrimination of responsivities and the photo/dark current contrast ratio (I ph/I dark) are inversely proportional to the amorphous structure, grain boundaries, TBs, and SFs. Therefore, optimum metal-semiconductor-metal photodetector performance is achieved for RTA-treated samples at 800 °C with an I ph/I dark of 3.91 × 102 and a responsivity of 0.702 A/W (λpeak = 230 nm) at 5 V bias for a 200 nm thin film.

Tantalum-Doped TiO2 Prepared by Atomic Layer Deposition and Its Application in Perovskite Solar Cells.

Tantalum (Ta)-doped titanium oxide (TiO2) thin films are grown by plasma enhanced atomic layer deposition (PEALD), and used as both an electron transport layer and hole blocking compact layer of perovskite solar cells. The metal precursors of tantalum ethoxide and titanium isopropoxide are simultaneously injected into the deposition chamber. The Ta content is controlled by the temperature of the metal precursors. The experimental results show that the Ta incorporation introduces oxygen vacancies defects, accompanied by the reduced crystallinity and optical band gap. The PEALD Ta-doped films show a resistivity three orders of magnitude lower than undoped TiO2, even at a low Ta content (0.8-0.95 at.%). The ultraviolet photoelectron spectroscopy spectra reveal that Ta incorporation leads to a down shift of valance band and conduction positions, and this is helpful for the applications involving band alignment engineering. Finally, the perovskite solar cell with Ta-doped TiO2 electron transport layer demonstrates significantly improved fill factor and conversion efficiency as compared to that with the undoped TiO2 layer.

Deposition and Characterization of RP-ALD SiO2 Thin Films with Different Oxygen Plasma Powers.

In this study, silicon oxide (SiO2) films were deposited by remote plasma atomic layer deposition with Bis(diethylamino)silane (BDEAS) and an oxygen/argon mixture as the precursors. Oxygen plasma powers play a key role in the quality of SiO2 films. Post-annealing was performed in the air at different temperatures for 1 h. The effects of oxygen plasma powers from 1000 W to 3000 W on the properties of the SiO2 thin films were investigated. The experimental results demonstrated that the SiO2 thin film growth per cycle was greatly affected by the O2 plasma power. Atomic force microscope (AFM) and conductive AFM tests show that the surface of the SiO2 thin films, with different O2 plasma powers, is relatively smooth and the films all present favorable insulation properties. The water contact angle (WCA) of the SiO2 thin film deposited at the power of 1500 W is higher than that of other WCAs of SiO2 films deposited at other plasma powers, indicating that it is less hydrophilic. This phenomenon is more likely to be associated with a smaller bonding energy, which is consistent with the result obtained by Fourier transformation infrared spectroscopy. In addition, the influence of post-annealing temperature on the quality of the SiO2 thin films was also investigated. As the annealing temperature increases, the SiO2 thin film becomes denser, leading to a higher refractive index and a lower etch rate.

Properties and Mechanism of PEALD-In2O3 Thin Films Prepared by Different Precursor Reaction Energy.

Indium oxide (In2O3) film has excellent optical and electrical properties, which makes it useful for a multitude of applications. The preparation of In2O3 film via atomic layer deposition (ALD) method remains an issue as most of the available In-precursors are inactive and thermally unstable. In this work, In2O3 film was prepared by ALD using a remote O2 plasma as oxidant, which provides highly reactive oxygen radicals, and hence significantly enhancing the film growth. The substrate temperature that determines the adsorption state on the substrate and reaction energy of the precursor was investigated. At low substrate temperature (100-150 °C), the ratio of chemically adsorbed precursors is low, leading to a low growth rate and amorphous structure of the films. An amorphous-to-crystalline transition was observed at 150-200 °C. An ALD window with self-limiting reaction and a reasonable film growth rate was observed in the intermediate temperature range of 225-275 °C. At high substrate temperature (300-350 °C), the film growth rate further increases due to the decomposition of the precursors. The resulting film exhibits a rough surface which consists of coarse grains and obvious grain boundaries. The growth mode and properties of the In2O3 films prepared by plasma-enhanced ALD can be efficiently tuned by varying the substrate temperature.

Chemical Reaction and Ion Bombardment Effects of Plasma Radicals on Optoelectrical Properties of SnO2 Thin Films via Atomic Layer Deposition.

In this study, the effect of radical intensity on the deposition mechanism, optical, and electrical properties of tin oxide (SnO2) thin films is investigated. The SnO2 thin films are prepared by plasma-enhanced atomic layer deposition with different plasma power from 1000 to 3000 W. The experimental results show that plasma contains different amount of argon radicals (Ar*) and oxygen radicals (O*) with the increased power. The three deposition mechanisms are indicated by the variation of Ar* and O* intensities evidenced by optical emission spectroscopy. The adequate intensities of Ar* and O* are obtained by the power of 1500 W, inducing the highest oxygen vacancies (OV) ratio, the narrowest band gap, and the densest film structure. The refractive index and optical loss increase with the plasma power, possibly owing to the increased film density. According to the Hall effect measurement results, the improved plasma power from 1000 to 1500 W enhances the carrier concentration due to the enlargement of OV ratio, while the plasma powers higher than 1500 W further cause the removal of OV and the significant bombardment from Ar*, leading to the increase of resistivity.

Air Annealing Effect on Oxygen Vacancy Defects in Al-doped ZnO Films Grown by High-Speed Atmospheric Atomic Layer Deposition.

In this study, aluminum-doped zinc oxide (Al:ZnO) thin films were grown by high-speed atmospheric atomic layer deposition (AALD), and the effects of air annealing on film properties are investigated. The experimental results show that the thermal annealing can significantly reduce the amount of oxygen vacancies defects as evidenced by X-ray photoelectron spectroscopy spectra due to the in-diffusion of oxygen from air to the films. As shown by X-ray diffraction, the annealing repairs the crystalline structure and releases the stress. The absorption coefficient of the films increases with the annealing temperature due to the increased density. The annealing temperature reaching 600 °C leads to relatively significant changes in grain size and band gap. From the results of band gap and Hall-effect measurements, the annealing temperature lower than 600 °C reduces the oxygen vacancies defects acting as shallow donors, while it is suspected that the annealing temperature higher than 600 °C can further remove the oxygen defects introduced mid-gap states.

Suppression of Oxygen Vacancy Defects in sALD-ZnO Films Annealed in Different Conditions.

Zinc oxide (ZnO) has drawn much attention due to its excellent optical and electrical properties. In this study, ZnO film was prepared by a high-deposition-rate spatial atomic layer deposition (ALD) and subjected to a post-annealing process to suppress the intrinsic defects and improve the crystallinity and film properties. The results show that the film thickness increases with annealing temperature owing to the increment of oxide layer caused by the suppression of oxygen vacancy defects as indicated by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectra. The film transmittance is seldom influenced by annealing. The refractive index increases with annealing temperature at 300-700 °C, possibly due to higher density and crystallinity of the film. The band gap decreases after annealing, which should be ascribed to the decrease in carrier concentration according to Burstein-Moss model. The carrier concentration decreases with increasing annealing temperature at 300-700 °C since the oxygen vacancy defects are suppressed, then it increases at 800 °C possibly due to the out-diffusion of oxygen atoms from the film. Meanwhile, the carrier mobility increases with temperature due to higher crystallinity and larger crystallite size. The film resistivity increases at 300-700 °C then decreases at 800 °C, which should be ascribed primarily to the variation of carrier concentration.

Effect of Annealing Temperature on Spatial Atomic Layer Deposited Titanium Oxide and Its Application in Perovskite Solar Cells.

In this study, spatial atomic layer deposition (sALD) is employed to prepare titanium dioxide (TiO2) thin films by using titanium tetraisopropoxide and water as metal and water precursors, respectively. The post-annealing temperature is varied to investigate its effect on the properties of the TiO2 films. The experimental results show that the sALD TiO2 has a similar deposition rate per cycle to other ALD processes using oxygen plasma or ozone oxidant, implying that the growth is limited by titanium tetraisopropoxide steric hindrance. The structure of the as-deposited sALD TiO2 films is amorphous and changes to polycrystalline anatase at the annealing temperature of 450 °C. All the sALD TiO2 films have a low absorption coefficient at the level of 10-3 cm-1 at wavelengths greater than 500 nm. The annealing temperatures of 550 °C are expected to have a high compactness, evaluated by the refractive index and x-ray photoelectron spectrometer measurements. Finally, the 550 °C-annealed sALD TiO2 film with a thickness of ~8 nm is applied to perovskite solar cells as a compact electron transport layer. The significantly enhanced open-circuit voltage and conversion efficiency demonstrate the great potential of the sALD TiO2 compact layer in perovskite solar cell applications.

Zinc Oxide Films with High Transparency and Crystallinity Prepared by a Low Temperature Spatial Atomic Layer Deposition Process.

Zinc oxide (ZnO) attracts much attention owing to its remarkable electrical and optical properties for applications in optoelectronics. In this study, ZnO thin films were prepared by spatial atomic layer deposition with diethylzinc and water as precursors. The substrate temperature was varied from 55 to 135 °C to investigate the effects on the optical, electrical, and structural properties of the films. All ZnO samples exhibit an average transmittance in visible and near-infrared light range exceeding 80% and a resistivity in the range of (3.2-9.0) × 10-3 Ω·cm when deposited on a borosilicate glass with a refractive index of ≈1.52. The transmittance, band gap, refractive index, and extinction coefficient are rarely affected, while the resistivity only slightly decreases with increasing temperature. This technique provides a wide process window for depositing ZnO thin films. The results revealed that the films deposited at a substrate of 55 °C were highly crystalline with a preferential (1 0 0) orientation. In addition, the grains grow larger as the substrate temperature increases. The electrical properties and reliability of ZnO/PET samples are also studied in this paper.

Deposition of Silicon-Based Stacked Layers for Flexible Encapsulation of Organic Light Emitting Diodes.

In this study, inorganic silicon oxide (SiOx)/organic silicon (SiCxHy) stacked layers were deposited by a radio frequency inductively coupled plasma chemical vapor deposition system as a gas diffusion barrier for organic light-emitting diodes (OLEDs). The effects of thicknesses of SiOx and SiCxHy layers on the water vapor transmission rate (WVTR) and residual stress were investigated to evaluate the encapsulation capability. The experimental results showed that the lowest WVTR and residual stress were obtained when the thicknesses of SiOx and SiCxHy were 300 and 30 nm, respectively. Finally, different numbers of stacked pairs of SiOx/SiCxHy were applied to OLED encapsulation. The OLED encapsulated with the six-pair SiOx/SiCxHy exhibited a low turn-on voltage and low series resistance, and device lifetime increased from 7 h to more than 2000 h.

Enhanced Si Passivation and PERC Solar Cell Efficiency by Atomic Layer Deposited Aluminum Oxide with Two-step Post Annealing.

In this study, aluminum oxide (Al2O3) films were prepared by a spatial atomic layer deposition using deionized water and trimethylaluminum, followed by oxygen (O2), forming gas (FG), or two-step annealing. Minority carrier lifetime of the samples was measured by Sinton WCT-120. Field-effect passivation and chemical passivation were evaluated by fixed oxide charge (Qf) and interface defect density (Dit), respectively, using capacitance-voltage measurement. The results show that O2 annealing gives a high Qf of - 3.9 × 1012 cm-2, whereas FG annealing leads to excellent Si interface hydrogenation with a low Dit of 3.7 × 1011 eV-1 cm-2. Based on the consideration of the best field-effect passivation brought by oxygen annealing and the best chemical passivation brought by forming gas, the two-step annealing process was optimized. It is verified that the Al2O3 film annealed sequentially in oxygen and then in forming gas exhibits a high Qf (2.4 × 1012 cm-2) and a low Dit (3.1 × 1011 eV-1 cm-2), yielding the best minority carrier lifetime of 1097 µs. The SiNx/Al2O3 passivation stack with two-step annealing has a lifetime of 2072 µs, close to the intrinsic lifetime limit. Finally, the passivated emitter and rear cell conversion efficiency was improved from 21.61% by using an industry annealing process to 21.97% by using the two-step annealing process.