Atomic Layer Deposited Al2O3 Films Research Articles

Reducing contact resistance of MoS2-based field effect transistors through uniform interlayer insertion via atomic layer deposition.

Molybdenum disulfide (MoS2), a semiconducting two-dimensional layered transition metal dichalcogenide (2D TMDC), with attractive properties enables the opening of a new electronics era beyond Si. However, the notoriously high contact resistance (RC) regardless of the electrode metal has been a major challenge in the practical applications of MoS2-based electronics. Moreover, it is difficult to lower RC because the conventional doping technique is unsuitable for MoS2 due to its ultrathin nature. Therefore, the metal-insulator-semiconductor (MIS) architecture has been proposed as a method to fabricate a reliable and stable contact with low RC. Herein, we introduce a strategy to fabricate MIS contact based on atomic layer deposition (ALD) to dramatically reduce the RC of single-layer MoS2 field effect transistors (FETs). We utilize ALD Al2O3 as an interlayer for the MIS contact of bottom-gated MoS2 FETs. Based on the Langmuir isotherm, the uniformity of ALD Al2O3 films on MoS2 can be increased by modulating the precursor injection pressures even at low temperatures of 150 °C. We discovered, for the first time, that film uniformity critically affects RC without altering the film thickness. Additionally, we can add functionality to the uniform interlayer by adopting isopropyl alcohol (IPA) as an oxidant. Tunneling resistance across the MIS contact is lowered by n-type doping of MoS2 induced by IPA as the oxidant in the ALD process. Through a highly uniform interlayer combined with strong doping, the contact resistance is improved by more than two orders of magnitude compared to that of other MoS2 FETs fabricated in this study.

Rear Surface Passivation for Ink‐Based, Submicron CuIn(S, Se)2 Solar Cells

AbstractA N, N‐dimethylformamide and thiourea‐based route is developed to fabricate submicron (0.55 and 0.75 µm) thick CuIn(S,Se)2 (CISSe) thin films for photovoltaic applications, addressing challenges of material usage, throughput, and manufacturing costs. However, reducing the absorber film thickness below 1 µm in a regular CISSe solar cell decreases the device efficiency due to losses at the highly‐recombinative, and mediocre‐reflective Mo/CISSe rear interface. For the first time, to mitigate the rear recombination losses, a novel rear contacting structure involving a surface passivation layer and point contact openings is developed for solution processed CISSe films and demonstrated in tangible devices. An atomic layer deposited Al2O3 film is employed to passivate the Mo/CISSe rear surface while precipitates formed via chemical bath deposition of CdS are used to generate nanosized point openings. Consequently, Al2O3 passivated CISSe solar cells show an increase in the open‐circuit voltage (VOC) and short‐circuit current density when compared to reference cells with equivalent absorber thicknesses. Notably, a VOC increase of 59 mV contributes to active area efficiencies of 14.2% for rear passivated devices with 0.75 µm thick absorber layers, the highest reported value for submicron‐based solution processed, low bandgap CISSe solar cells.

Analyzing the Li-Al-O Interphase of Atomic Layer-Deposited Al2O3 Films on Layered Oxide Cathodes Using Atomistic Simulations.

Alumina surface coatings are commonly applied to layered oxide cathode particles for lithium-ion battery applications. Atomic layer deposition (ALD) is one such surface coating technique, and ultrathin alumina ALD films (<2 nm) are shown to improve the electrochemical performance of LiNixMnyCo1-x-yO2 materials, with groups hypothesizing that a beneficial Li-Al-O product is being formed during the alumina ALD process. However, the atomic structure of these films is still not well understood, and quantifying the interface of ultrathin (∼1 nm) ALD films is an arduous experimental task. Here, we perform molecular dynamics simulations of amorphous alumina films of varying thickness in contact with the (0001) LiCoO2 (LCO) surface to quantify the film nanostructure. We calculate elemental mass density profiles through the films and observe that the Li-Al-O interphase extends ∼2 nm from the LCO surface. Additionally, we observe layering of Al and O atoms at the LCO-film interface that extends for ∼1.5 nm. To access the short-range order of the amorphous film, we calculated the Al coordination numbers through the film. We find that while [4]Al is the prevailing coordination environment, significant amounts of [6]Al exist at the interface between the LiCoO2 surface and the film. Taken together, these principal findings point to a pseudomorphic Li-Al-O overlayer that approximates the underlying layered LiCoO2 lattice but does not exactly replicate it. Additionally, with sufficient thickness, the Li-Al-O film transitions to an amorphous alumina structure. We anticipate that our findings on the ALD-like, Li-Al-O film nanostructure can be applied to other layered LiNixMnyCo1-x-yO2 materials because of their shared crystal structure with LiCoO2. This work provides insight into the nanostructure of amorphous ALD alumina films to help inform their use as protective coatings for Li-ion battery cathode active materials.

&lt;i&gt;In Situ&lt;/i&gt; ALD-TEM System to Study Atomic Nucleation Phenomena - Enabled by Ultrathin Large Aspect Ratio Free-Standing Shell Structures

Today, Atomic Layer Deposition (ALD) sets the limits for achieving nanometer precision of thin, yet almost dense films. However, the initial growth process, determining possible film thinness, is poorly understood. A better understanding can be obtained with the help of in-situ characterization during film growth with high spatial and chemical resolution. Transmission Electron microscopy (TEM) would be a suitable and widely available technique to accomplish this objective. However, standard instruments have differing vacuum requirements than those necessary for ALD. During ALD, TEM detectors could be damaged as they are being exposed to corrosive volatile chemical compounds. Here we present a dedicated TEM holder design, where ALD deposition occurs inside a microchip containing a large area cavity surrounded by thin film Al2O3 membranes. These membranes act as windows for TEM characterization while decoupling the ALD process from the TEM environment. The microchip consists of longitudinal large overhang shell structures, themselves made by ALD and etched in an HF vapor etch processes. The set-up, which includes controlled heating, was tailored to ALD requirements, and passed a vacuum-pressure test. Post-mortem inspection of film growth on a silicon sample chip demonstrates the successful formation of an ALD Al2O3 film with 40 nm thickness inside the cavity. These results demonstrate the potential of the system to enable a range of experiments on growth phenomena that may lead to even thinner films and better control of interfaces than previously possible.

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.

High Field-Effect Mobility and On/Off Current Ratio of p-Type ALD SnO Thin-Film Transistor

High-performance p-type thin-film transistors (TFTs) with high field-effect mobility and high on/off current ratios (Ion/Ioff) were fabricated by engineering the microstructure and surface morphology of the atomic layer-deposited (ALD) SnO channel layer. ALD SnO films grown at a deposition temperature of 225 °C showed excellent crystallinity and dense, smooth surfaces, which resulted in superior field-effect mobility of 6.13–7.24 cm2/V·s without using a high-temperature postannealing process. Optimization of the SnO channel thickness suppressed the off-state leakage current, which consequently yielded excellent TFT switching performance, with an Ion/Ioff value of 104–105. Additionally, backchannel passivation by the ALD Al2O3 film improved the subthreshold swing characteristics by reducing surface defect states. These results suggest that synergistic control of the microstructure, surface morphology, and thickness of ALD SnO channels is crucial for achieving high-performance p-type SnO TFTs.

Influence of deposition temperature on microstructure and gas-barrier properties of Al2O3 prepared by plasma-enhanced atomic layer deposition on a polycarbonate substrate.

We prepared polymer-based encapsulation films by plasma-enhanced atomic layer deposition (PEALD) of Al2O3 film on a polycarbonate (PC) substrate at 80-160 °C to fabricate Al2O3/PC barrier films. The thermal and dynamic mechanical properties of the PC substrate, the structural evolution of PEALD Al2O3 films, the optical transmission, surface morphology, and gas-barrier properties of Al2O3/PC film are all studied in this work as a function of temperature. The glass transition temperature T g of the PC substrate is about 140 °C, and the coefficient of thermal expansion increases significantly when the temperature exceeds T g. Increasing the deposition temperature from 80 to 160 °C for Al2O3 film deposited over 300 cycles increases the density from 3.24 to 3.45 g cm-3, decreases the thickness from 44 to 40 nm, and decreases the O/Al content ratio from 1.525 to 1.406. Al2O3/PC films deposited at 80-120 °C have no surface cracks, whereas surface cracks appear in samples deposited near or above 140 °C. Upon increasing the deposition temperature, the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) of Al2O3/PC films decrease significantly at temperatures below T g, and then increase at temperatures near to or above T g due to cracks in the films. The optimal deposition temperature is 120 °C, and the minimum WVTR and OTR of Al2O3/PC film are 0.00132 g per (m2 24 h) and 0.11 cm3 per (m2 24 h 0.1 MPa), respectively. The gas-barrier properties of the Al2O3/PC films are attributed to both the densification of the Al2O3 film and the cracks, which are caused by the shrinkage of the PC substrate.

Study of mechanical degradation of freestanding ALD Al2O3 by a hygrothermal environment and a facile protective method for environmentally stable Al2O3: toward highly reliable wearable OLEDs.

Al2O3 deposited via atomic layer deposition (ALD) has been used as an insulating and barrier film for thin-film transistors, organic electronics, and microelectromechanical systems. However, ALD Al2O3 films are easily degraded by hydrolysis under harsh hygrothermal conditions, owing to their poor environmental stability. In this study, the mechanical properties and water-vapor transmission rate (WVTR) of environmentally degraded Al2O3 films were investigated by varying the temperature and relative humidity (RH). The hygrothermal environment led to surface and pinhole-concentrated degradation based on aluminum hydroxide, which caused an increased WVTR and reduced elongation of the films in harsher environments. In particular, the elongation of the degraded Al2O3 films was reduced to 0.3%, which is one-third of that of as-deposited Al2O3, and their WVTR increased on the order of 10-1 g m-2 day-1, which is more than 1000 times that of as-deposited Al2O3. Therefore, we introduced a functional silane-based inorganic-organic hybrid layer (silamer) onto the Al2O3 films to improve their environmental stability. The silamer helped preserve the characteristics of Al2O3 films by forming a strong and continuous aluminate phase of Al-O-Si at their interface in hygrothermal environments. Furthermore, the silamer-capped Al2O3 was shown to be an environmentally stable encapsulation for application in wearable organic devices.

Release Rate Studies of 5-Aminosalacylic Acid Coated with Atomic Layer-Deposited Al2O3 and ZnO in an Acidic Environment.

5-Aminosalicylic acid (5-ASA) is a first-line defense drug used to treat mild cases of inflammatory bowel disease. When administered orally, the active pharmaceutical ingredient is released throughout the gastrointestinal tract relieving chronic inflammation. However, delayed and targeted released systems for 5-ASA to achieve optimal dose volumes in acidic environments remain a challenge. Here, we demonstrate the application of atomic layer deposition (ALD) as a technique to synthesize nanoscale coatings on 5-ASA to control its release in acidic media. ALD Al2O3 (38.0 nm) and ZnO (24.7 nm) films were deposited on 1 g batch powders of 5-ASA in a rotatory thermal ALD system. Fourier transform infrared spectroscopy, scanning electron microscopy, and scanning/transmission electron microscopy establish the interfacial chemistry and conformal nature of ALD coating over the 5-ASA particles. While Al2O3 forms a sharp interface with 5-ASA, ZnO appears to diffuse inside 5-ASA. The release of 5-ASA is studied in a pH 4 solution via UV-vis spectroscopy. Dynamic stirring, mimicking gut peristalsis, causes mechanical attrition of the Al2O3-coated particles, thereby releasing 5-ASA. However, under static conditions lasting 5000 s, the Al2O3-coated particles release only 17.5% 5-ASA compared to 100% release with the ZnO coating. Quartz crystal microbalance-based etch studies confirm the stability of Al2O3 in pH 4 media, where the ZnO films etch 41× faster than Al2O3. Such results are significant in achieving a nanoscale coating-based drug delivery system for 5-ASA with controlled release in acidic environments.

Thermomechanical properties of aluminum oxide thin films made by atomic layer deposition

In microelectromechanical system devices, thin films experience thermal processing at temperatures some cases exceeding the growth or deposition temperature of the film. In the case of the thin film grown by atomic layer deposition (ALD) at relatively low temperatures, post-ALD thermal processing or high device operation temperature might cause performance issues at device level or even device failure. In this work, residual stress and the role of intrinsic stress in ALD Al2O3 films grown from Me3Al and H2O, O3, or O2 (plasma ALD) were studied via post-ALD thermal processing. Thermal expansion coefficient was determined using thermal cycling and the double substrate method. For some samples, post-ALD thermal annealing was done in nitrogen at 300, 450, 700, or 900 °C. Selected samples were also studied for crystallinity, composition, and optical properties. Samples that were thermally annealed at 900 °C had increased residual stress value (1400–1600 MPa) upon formation of denser Al2O3 phase. The thermal expansion coefficient varied somewhat between Al2O3 made using different oxygen precursors. For thermal-Al2O3, intrinsic stress decreased with increasing growth temperature. ALD Al2O3 grown with plasma process had the lowest intrinsic stress. The results show that ALD Al2O3 grown at 200 and 300 °C is suitable for applications, where films are exposed to post-ALD thermal processing even at temperature of 700 °C without a major change in optical properties or residual stress.

Characterization of ALD Ta2O5, Al2O3, and Ta2O5/Al2O3 Nanolaminate as Metal-Insulator-Metal Capacitor Dielectric for GaAs HBT Technology

Tantalum pentoxide (Ta2O5), aluminum oxide (Al2O3), and Ta2O5/Al2O3 nanolaminate deposited using atomic layer deposition (ALD) have been characterized and evaluated as metal-insulator-metal (MIM) capacitor dielectric in GaAs hetero-junction bipolar transistor (HBT) technology. The results show that the capacitor with 60 nm of ALD Ta2O5 and Al2O3 capacitor dielectric films resulted in a capacitance density of 3.84x10-15 F/μm2 and 1.37x10-15 F/μm2 and a dielectric constant of 26.1 and 9.3, respectively, while the capacitor with 60 nm of ALD Ta2O5/Al2O3 nanolaminate capacitor dielectric film resulted in capacitance density of 2.40x10-15 F/μm2 and a dielectric constant of 16.3. The capacitance density of capacitor using these three films increased marginally by 2.2% to 4.3%, when the temperature was increased from 25oC to 125oC. No significant change in capacitance density of these ALD films was observed, when the applied voltage was varied from -5 V to +5 V. The leakage current density of the ALD Al2O3, Ta2O5, and Ta2O5/Al2O3 nanolaminate films at an electric field of 3 MV/cm is 2.0x10-16 A/μm2, 6.8x10-8 A/μm2, and 9.2x10-13 A/μm2, respectively. The 60 nm ALD Al2O3 film has higher breakdown voltage of 46 V, compared to the 60 nm ALD Ta2O5 film and the 60 nm Ta2O5/Al2O3 nanolaminate film, which has a breakdown voltage of 22.5 V and 32.7 V, respectively. As the temperature was increased from 25oC to 125oC, the leakage current increased and breakdown voltage decreased for all three ALD films.

Interdigitated back-contacted crystalline silicon solar cells fully manufactured with atomic layer deposited selective contacts

The interdigitated back-contacted (IBC) solar cell concept has been extensively studied for single-junction cells and more recently as a good choice for three-terminal tandem devices. In this work, carrier-selective contacts based on transition metal oxides deposited by atomic layer deposition (ALD) technique are applied to IBC c-Si(n) devices. In the first part of the study, we develop a hole-selective contact based on thin ALD vanadium oxide (V2O5) layers without using an amorphous silicon interlayer. The ALD process has been optimised, i.e. number of ALD cycles and deposition temperature, as a trade-off between surface passivation and contact resistivity. Noticeable surface passivation with recombination current densities around 100 fA/cm2, as well as reasonable contact resistivity values below 250 mΩcm2 are reached using 200 ALD V2O5 cycles deposited at a deposition temperature of 125 °C (∼10 nm layer thickness). The optimised ALD V2O5-based contact is combined with both an ALD TiO2-based electron-selective contact and an excellent surface passivation in non-contacted regions provided by ALD Al2O3 films, to form a fully ALD IBC c-Si(n) solar cell scheme. Fabricated devices yield photovoltaic efficiencies and pseudo efficiencies, i.e. calculated without series resistance losses, of 18.6% and 21.1% respectively (3 cm × 3 cm device area). These results reveal the potential of the ALD technique to deposit transition metal oxide (TMO) films as selective contacts on high efficiency devices, paving the way of using low thermal-budget, low cost and highly scalable processes for a highly demanding IBC solar cell architecture in the photovoltaic industry.

Effects of carbon impurity in ALD-Al2O3 film on HAXPES spectrum and electrical properties of Al2O3/AlGaN/GaN MIS structure

In this study, the effects of carbon impurity in the atomic layer deposited-Al2O3 film on the hard X-ray photoelectron spectroscopy (HAXPES) spectra and the electrical properties of metal–insulator–semiconductor (MIS) structure were measured. The carbon concentration in the Al2O3 film was adjusted by varying the deposition conditions (precursor: trimethylaluminum, Al(CH3)3, or dimethylaluminum hydride, Al(CH3)2H, oxidant, and deposition temperature) in the atomic layer deposition process. The HAXPES measurements revealed the correlation between full width at half maximum (FWHM) of Al 1 s and O 1 s spectra and the carbon concentration in the Al2O3 film. Furthermore, the negative charges in the Al2O3 film could change the FWHM, attributed to the carbon impurity. The correlation between the carbon concentration and the electrical characteristics of the Al2O3/AlGaN/GaN MIS structure was analyzed. The interface state density and the effective charge density were dependent on the carbon concentration in the Al2O3 film.

Influence of growth temperature on dielectric strength of Al2O3 thin films prepared via atomic layer deposition at low temperature

Thin films grown via atomic layer deposition (ALD) suffer from insufficient growth rate and unreliability for temperature-sensitive electronic substrates. This study aimed to examine the growth characteristics and dielectric strength of ALD Al2O3 films grown at low temperatures (≤ 150 °C) for potential application in flexible electronic devices. The growth rate of the Al2O3 films increased from 0.9 to 1.1 Å/cycle with increasing temperature and saturated at growth temperatures ≥ 150 °C, which is the critical temperature at which a complete oxidation reaction occurred. The dielectric strength was also improved with increasing growth temperature, and the films grown at 150 °C showed a high breakdown field strength (~ 8.3 MV/cm), attributable to the decrease in the carbon impurities and oxygen defects, as confirmed by X-ray photoelectron spectroscopy. Even at low growth temperatures (≤ 150 °C), ALD Al2O3 films showed an overall amorphous structure and extremely smooth surfaces regardless of the growth temperature.

Complete stress release in monolayer ALD-Al2O3 films based on mechanical equilibrium homeostasis to realize a bending radius of 1 mm.

Low-temperature-deposited, transparent, high-barrier-performance atomic layer deposition (ALD) Al2O3 films are widely utilized to protect organic optoelectronic devices. However, because the internal compression residual stresses result in poor mechanical properties, these films are unable to realize independent encapsulation. In this work, we propose a pre-bending and substrate thermal expansion process to fabricate low-residual-stress ALD-Al2O3 films by multistep adjustment of mechanical equilibrium homeostasis, compare and analyze the key properties of the films to determine the optimal processing conditions, and adjust the surface area differences between the film and the substrate to acquire a flat-bottom encapsulation substrate. Therefore, fewer cracks form in ALD-Al2O3 deposited at 40 °C, and no significant increase in the water-vapor-transmission rate (WVTR) occurred after multiple bending processes at a 3 mm radius. Furthermore, both the bending radius for the first crack generation and crack saturation density were optimal, resulting in excellent mechanical stability, and a bending radius of 1 mm was achieved. Finally, we confirm the feasibility of the monolayer ALD-Al2O3 prepared by our process for applications in OLEDs. The encapsulated OLED maintained 94% of its initial efficiency after 10 days of bending in a harsh environment at 30 °C and 90% relative humidity.

Deposit and etchback approach for ultrathin Al2O3 films with low pinhole density using atomic layer deposition and atomic layer etching

­Ultrathin Al2O3 atomic layer deposition (ALD) films with low pinhole density were fabricated using a deposit and etchback approach. This strategy was able to avoid the pinholes that originated during nonuniform nucleation of Al2O3 ALD films. In this method, an Al2O3 ALD film was deposited to a thickness greater than the desired thickness to reduce the number of pinholes and form a more continuous Al2O3 ALD film. Subsequently, the Al2O3 ALD film was etched back to a smaller thickness using thermal Al2O3 atomic layer etching (ALE). The number of pinholes in the resulting Al2O3 ALD film was measured by the percentage yield of metal-insulator-metal (MIM) capacitors based on an Ag/Al2O3/Al structure that did not have an electrical short. The device yield was improved using the deposition and etchback approach. For example, using device areas of 0.01 mm2, Al2O3 ALD films that were grown to 5 nm in the MIM capacitor gave a yield of 30%–40%. In contrast, Al2O3 ALD films that were grown to 24 nm and then etched back to 5 nm to form the MIM capacitor provided a yield of 65%–75%. This increase in yield of approximately 100% indicates that the deposit and etchback approach can improve the yield of MIM devices based on ultrathin Al2O3 ALD films. Although this method has been previously suggested to improve the quality of ultrathin films, this report is believed to be the first demonstrated application of using the deposit and etchback approach for device fabrication. Additional experiments revealed that a portion of the yield improvement can be attributed to the fluorination of the Al2O3 ALD films that produced a volume expansion when forming AlF3. This expansion may produce a compressive stress that helps to close the pinholes. The dielectric constant was also measured for Al2O3 ALD films versus Al2O3 film thickness. The dielectric constant was the same for as-deposited Al2O3 ALD films and Al2O3 ALD films that were first grown to 24 nm and then etched back to smaller thicknesses. This agreement indicates that the dielectric constant can be understood in terms of a series capacitor model and that Al2O3 ALE does not affect the electrical properties of the Al2O3 films.

Smoothing surface roughness using Al2O3 atomic layer deposition

Al2O3 atomic layer deposition (ALD) was used to smooth the roughness on silicon wafers obtained prior to chemical mechanical polishing (CMP). The initial silicon wafers had an average RMS surface roughness of 3.3 nm as determined by atomic force microscopy (AFM) measurements. AFM line scans also measured an average lateral spacing of ≈490 nm between the surface asperities. The RMS roughness decreased and the average lateral spacing increased progressively with number of Al2O3 ALD cycles. After 3000 Al2O3 ALD cycles that deposit an Al2O3 film thickness of 370 nm, the RMS roughness reduced to 1.5 nm and the average lateral spacing between the surface asperities increased to ≈890 nm. Additional Al2O3 ALD cycles produced little change in the RMS roughness or average lateral spacing. The efficiency of the smoothing decreased when the lateral distance between the surface asperities was much larger than the Al2O3 ALD film thickness. Power spectral density (PSD) analysis revealed that the ALD smoothing was most effective for surface topographical features with lateral spacings in the range of 10s to 100s of nanometers. Reflectivity studies of silver films deposited on the silicon wafers also demonstrated that Al2O3 ALD smoothing improved the optical performance of reflective mirrors.

Perspectives on UVC LED: Its Progress and Application

High-quality epitaxial layers are directly related to internal quantum efficiency. The methods used to design such epitaxial layers are reviewed in this article. The ultraviolet C (UVC) light-emitting diode (LED) epitaxial layer structure exhibits electron leakage; therefore, many research groups have proposed the design of blocking layers and carrier transportation to generate high electron–hole recombination rates. This also aids in increasing the internal quantum efficiency. The cap layer, p-GaN, exhibits high absorption in deep UV radiation; thus, a small thickness is usually chosen. Flip chip design is more popular for such devices in the UV band, and the main factors for consideration are light extraction and heat transportation. However, the choice of encapsulation materials is important, because unsuitable encapsulation materials will be degraded by ultraviolet light irradiation. A suitable package design can account for light extraction and heat transportation. Finally, an atomic layer deposition Al2O3 film has been proposed as a mesa passivation layer. It can provide a low reverse current leakage. Moreover, it can help increase the quantum efficiency, enhance the moisture resistance, and improve reliability. UVC LED applications can be used in sterilization, water purification, air purification, and medical and military fields.

Band alignment modulation of atomic layer deposition-prepared Al2O3/β-Ga2O3 heterojunction interface by deposition temperature

The band alignment between oxygen plasma-assisted atomic layer deposition Al2O3 films and β-Ga2O3 (−201) substrates under different deposition temperatures was characterized by x-ray photoelectron spectroscopy. As the deposition temperature increased from 30 to 200 °C, all the heterojunctions exhibited a type-I alignment. The bandgap of Al2O3 enlarged from 6.26 ± 0.1 to 6.81 ± 0.1 eV, leading to the conduction band offset varying linearly from 1.39 ± 0.1 to 1.95 ± 0.1 eV, while the valence band offset was insensitive. This difference was attributed to Al ion deficiency and hydroxyl groups induced by an inadequate reaction of trimethylaluminum under low deposition temperatures, which was proved by secondary ion mass spectrometry (SIMs) and Fourier-transform infrared spectroscopy (FTIR). These findings could facilitate the design of a CBO-controllable Al2O3/β-Ga2O3 heterojunction through deposition temperature.

Voltage-Controlled Anodic Oxidation of Porous Fluorescent SiC for Effective Surface Passivation.

This study investigated the fabrication of porous fluorescent SiC using a constant voltage-controlled anodic oxidation process. The application of a high, constant voltage resulted in a spatial distinction between the porous structures formed inside the fluorescent SiC substrates, due to the different etching rates at the terrace and the large step bunches. Large, dendritic porous structures were formed as the etching process continued and the porous layer thickened. Under the conditions of low hydrofluoric acid (HF) concentration, the uniformity of the dendritic porous structures through the entire porous layer was considerably improved compared with the conditions of high HF concentration. The resulting large uniform structure offered a sizable surface area, and promoted the penetration of atomic layer-deposited (ALD) Al2O3 films (ALD–Al2O3). The emission intensity in the porous fluorescent SiC was confirmed via photoluminescence (PL) measurements to be significantly improved by a factor of 128 after ALD passivation. With surface passivation, there was a clear blueshift in the emission wavelength, owing to the effective suppression of the non-radiative recombination rate in the porous structures. Furthermore, the spatial uniformity of emitted light was examined via PL mapping using three different excitation lasers, which resulted in the observation of uniform and distinctive emissions in the fluorescent SiC bulk and porous areas.