Atomic Layer Deposition Al2O3 Layer Research Articles

Tuning the charge carrier polarity of roll-to-roll gravure printed carbon nanotube-based thin film transistors by an atomic layer deposited alumina nanolayer.

Charge carrier polarity tuning in printed thin film transistors (TFTs) is a crucial step in order to obtain complementary printed devices. In this work, we studied the effect of an Al2O3 passivation layer on printed single-walled carbon nanotube (SWCNT) based TFTs to tune the charge carrier polarity. By varying the atomic layer deposition (ALD) temperature and Al2O3 layer thickness, we can tune the doping degree of Al2O3 to tailor the polarity of printed SWCNT-based TFTs (SWCNT-TFTs). The precise control of threshold voltage (Vth) and polarity from p-type to well-balanced ambipolar, and n-type SWCNT-TFTs is successfully demonstrated with high repeatability by optimizing the ALD temperature and Al2O3 layer thickness based on 20 printed samples per test. As a proof-of-concept, inverter logic circuits using the SWCNT-TFT with different polarity types are demonstrated. The ambipolar device-based inverter exhibits a voltage gain of 3.9 and the CMOS-based inverter exhibits a gain of approximately 4.3, which is comparable to the current roll-to-roll (R2R) printed inverter circuits. Different thicknesses of Al2O3 layer, coated by the ALD at different temperatures and thicknesses, provide a deep understanding of the device fabrication and control process to implement the tailored doping method to efficiently realize R2R printed SWCNT-TFT-based complementary electronic devices.

Growth behavior of Ir metal formed by atomic layer deposition in the nanopores of anodic aluminum oxide.

The conformal coating or surface modification in high aspect ratio nanostructures is a tough challenge using traditional physical/chemical vapor deposition, especially for metal deposition. In this work, the growth behavior of iridium (Ir) metal formed by atomic layer deposition (ALD) in anodic aluminum oxide (AAO) templates was explored deeply. It is found that the surface hydrophilicity is crucial for the nucleation of ALD Ir. An in situ ALD Al2O3 layer with an ultra-hydrophilic surface can greatly promote the nucleation of ALD Ir in AAO nanopores. The effect of the Ir precursor pulse time, diameter, and length of AAO nanopores on the infiltration depth of ALD Ir was investigated systematically. The results show that the infiltration depth of ALD Ir in AAO nanopores is in proportion to the pore diameter and the square root of the Ir precursor pulse time, which follows a diffusion-limited model. Furthermore, the Ir precursor pulse time to obtain conformal Ir coating throughout all the AAO channels is in proportion to the square of the aspect ratio of AAO templates. In addition, the conformal Ir deposition in AAO nanopores is also related to the Ir precursor purge time and the O2 partial pressure. Insufficient Ir purge time could cause a CVD-like reaction, leading to the reduction of the infiltration depth in AAO. Higher O2 partial pressure can facilitate Ir nucleation with more Ir precursor consumption at the entrance of nanopores, decreasing the infiltration depth in AAO nanopores, so appropriate O2 partial pressure should be chosen for ALD Ir in high aspect ratio materials. Above all, our research is valuable for surface modification or coating of metal by ALD in high aspect ratio nanostructures for 3D microelectronics, nano-fabrication, catalysis and energy fields.

The effects of doping and coating on degradation kinetics in perovskites

In the last few years, perovskite-based solar cells have seen rapid improvements of their power conversion efficiencies (PCE) that have already reached silicon cell values. The commercial use of perovskite cell technology remains, however, a challenge due to durability issues. In fact, perovskite films when exposed to moisture and oxygen undergo degradation within a couple of hours or days. In order to address these problems, several strategies have been developed in recent years, including surface passivation and coating. Herein, we analyze and compare two perovskite stability improvement approaches, each aimed at the reduction of the oxygen diffusion, one by adding chlorine to perovskite precursors in order to block iodine vacancy sites, and the other by coating the perovskite layers with protective Al2O3 films using Atomic Layer Deposition (ALD). The study provides an experimental verification of the degradation mechanism linking it to oxygen diffusion into the perovskite films. As shown by X-ray photoelectron spectroscopy (XPS) analysis, the addition of chlorine into the iodine-based perovskite film slows down the oxygen diffusion, while X-ray diffraction (XRD) measurements show improved stability of such iodide perovskite with chlorine addition. Characteristic times for oxygen diffusion into subsurface region as well as for degradation process of film volume are provided and discussed. XRD measurements are also used to show that an additional significant stability improvement can also be achieved when the films are coated with a protective ALD Al2O3 layer.

(Invited) Epitaxial Growth of Ge/III-V Films and Hetero-Layer Lift-off for Ultra-Thin GeOI Fabrication

Monolithic 3D (M3D) integration has been thought of as an attractive idea to realize multi-functional heterogeneous integration devices. M3D integration enables us to combine logic devices with different functional devices such as memory, sensor, or optics to create extremely compact and efficient system-on-chips. Ge and III-V compound semiconductors are expected to be implemented as high mobility channel materials and optical components which cannot be realized with Si. The layer transfer technique with low processing temperature has great potential for stacking non-lattice-matched crystalline materials for M3D ICs. Here, we discuss the layer transfer technology utilizing high quality epitaxial growth, low-temperature direct bonding and advanced epitaxial lift-off (ELO) process, which reveals the applicability of transferred layers for heterogeneous integration.The low temperature ELO process is essential for realizing reliable layer transfer with different materials because the process is almost free from the issues of thermal expansion mismatch. Process flow of GaAs or Ge layer transfer with ELO process is described in Fig. 1. First of all, a lattice-matched single crystal GaAs or Ge layer is epitaxially grown on GaAs substrate with an AlAs release layer by MOCVD. Subsequently, Al2O3 layer is deposited on the epitaxial layer by atomic layer deposition (ALD). ALD Al2O3 layer serves as the buried oxide (BOX) and the passivation layer for Ge or GaAs surface. Then, Ge or GaAs layer is patterned using photolithography. The patterned GaAs or Ge/AlAs/GaAs donor wafer is then bonded on host substrate by direct bonding technique with wet activation process. Finally, the epitaxial GaAs or Ge layer is transferred on host substrates by splitting GaAs donor wafer from AlAs layer with HCl side etching thanks to extremely high etching selectivity. Note that the fabrication of patterned transfer layer is effective in shortening the required time of ELO process. In this ELO process, the thickness of a transferred layer can be control by epitaxial layer thickness. After the layer transfer process, the released donor GaAs wafer can be reclaimed and reused for the subsequent re-epitaxial growth to reduce the process cost.Figs. 2 (a) and (b) show cross-sectional SEM and TEM images of fabricated GeOI substrate with different BOX structures. Ge pattern edge shows no peeling from SiO2(100nm)/Si substrate, indicating strong bonding strength at the pattern edge. In Fig. 2 (b), the bonding interface is clearly observed at Al2O3(5nm)/SiO2(50nm) interface formed by the wet activation process. The bonding interface exhibits very smooth and no defect. There is no observable defect in transferred Ge layer, suggesting dislocation density is below TEM detection limit. Compared to Smart-CutTM technique, ELO process has significant advantages in terms of crystal quality, thickness control and the surface flatness thanks to totally low temperature processing.To reduce wafer-scale thickness variation, etching stopper (ES) technique with high etching selectivity is quite useful. As a donor wafer, we prepare Ge active layer (20nm)/SiGe ES layer (5nm)/Ge buffer layer/AlAs/GaAs structures. Here, the SiGe layer serves as ES against the etching of Ge buffer layer to control the final GeOI thickness and attain the wafer-scale thickness uniformity. In addition, Si passivation on Ge surface was performed during initial epitaxial growth of donor wafer since Si passivation was proved to be effective in reducing interfacial trap density (D it) between Ge and oxide. After ELO process, Ge buffer layer /SiGe ES layer/Ge active layer / Al2O3 /SiO2/Si heterostructure was obtained. Then, the top Ge buffer layer was removed in HCl/H2O2 solution. We found the etching selectivity was more than 30 between Ge and SiGe layer, which is high enough to yield very smooth SiGe surface. Finally, SiGe layer was removed by TMAH preferentially.Further conformal thinning of Ge active layer was performed by the cyclic dry oxidation/wet digital etching (DE) at room temperature. In addition to thickness control with the etching rate of about 0.8 nm/cycle, it is found that dozens digital etching (DDE) process is effective for reducing Ge surface roughness. Finally, UTB GeOI substrates with high crystal quality down to 3 nm have been successfully fabricated through SiGe ES and DDE processes. Ultra-smooth GeOI surface with reduced thickness fluctuation was confirmed after DDE by TEM cross-sectional images, resulting in significant improvement of device performances. Figure 1

Atomic-Layer-Deposition Assisted Formation of Wafer-Scale Double-Layer Metal Nanoparticles with Tunable Nanogap for Surface-Enhanced Raman Scattering

A simple high-throughput approach is presented in this work to fabricate the Au nanoparticles (NPs)/nanogap/Au NPs structure for surface enhanced Raman scattering (SERS). This plasmonic nanostructure can be prepared feasibly by the combination of rapid thermal annealing (RTA), atomic layer deposition (ALD) and chemical etching process. The nanogap size between Au NPs can be easily and precisely tuned to nanometer scale by adjusting the thickness of sacrificial ALD Al2O3 layer. Finite-difference time-domain (FDTD) simulation data indicate that most of enhanced field locates at Au NPs nanogap area. Moreover, Au NPs/nanogap/Au NPs structure with smaller gap exhibits the larger electromagnetic field. Experimental results agree well with FDTD simulation data, the plasmonic structure with smaller nanogap size has a stronger Raman intensity. There is highly strong plasmonic coupling in the Au nanogap, so that a great SERS effect is obtained when detecting methylene blue (MB) molecules with an enhancement factor (EF) over 107. Furthermore, this plasmonic nanostructure can be designed on large area with high density and high intensity hot spots. This strategy of producing nanoscale metal gap on large area has significant implications for ultrasensitive Raman detection and practical SERS application.

Organic/inorganic multilayer thin film encapsulation via initiated chemical vapor deposition and atomic layer deposition for its application to organic solar cells

A thin film encapsulation (TFE) was fabricated by alternating deposition of organic and inorganic layers via initiated chemical vapor deposition (iCVD) and atomic layer deposition (ALD). A highly crosslinked organosilicone polymer, poly(1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane) (pV3D3) was successfully adopted as the organic layer for the fabrication of TFE. The film formed a stable and smooth interface with ALD Al2O3 layer. The multilayer TFE consisting of pV3D3 and Al2O3 alternating layers exhibited outstanding optical transparency as well as excellent barrier property. Finally, the multilayer TFE was applied monolithically to organic solar cells (OSCs), and the shelf life of the cells was increased immensely, suggesting that the developed organic/inorganic multilayer TFE is an efficient barrier film for elongating the lifetime of organic electronics.

DC characteristics of ALD-grown Al2O3/AlGaN/GaN MIS-HEMTs and HEMTs at 600 °C in air

To the best of our knowledge, the 600 °C device characteristics detailed here reflect the highest operation temperature reported for AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) in air which supports the realization of electronics for high-temperature applications (e.g., space exploration, combustion and downhole). The high-temperature response of Al2O3/AlGaN/GaN MIS-HEMTs with Al2O3 deposited by plasma-enhanced atomic layer deposition (ALD) as the gate dielectric and passivation layers was examined here. More specifically, the DC current–voltage response and the threshold voltage characteristics of the MIS-HEMTs were evaluated to temperatures up to 600 °C in air. For comparison, the response of AlGaN/GaN HEMTs without the ALD Al2O3 layer was also measured. It was observed that the HEMTs failed above 300 °C accompanied by a ∼500 times increase in leakage current and observation of bubbles formed in active region of gate. On the contrary, the MIS-HEMTs continued to operate normally up to 600 °C. However, within the 30 min period exposed to 600 °C the MIS-HEMT degraded permanently. This was observed at 20 °C after return from operation at 600 °C as a change in threshold voltage and saturation drain current. The failure of the HEMTs is suggested to be due to the diffusion of gate metals (Ni and Au) into the active regions of the AlGaN/GaN heterostructure, which creates additional leakage current pathways. The impact of strain relaxation and interfacial trapped charges on threshold voltage as a function of temperature was studied using an energy band-gap model. The ALD Al2O3 gate dielectric layer acts as a diffusion barrier to the Ni and Au gate metals, thus enabling short-term operation of MIS-HEMTs to 600 °C, the highest operation temperature reported for this device architecture to date.

Properties of slow traps of ALD Al2O3/GeOx/Ge nMOSFETs with plasma post oxidation

The realization of Ge gate stacks with a small amount of slow trap density as well as thin equivalent oxide thickness and low interface state density (Dit) is a crucial issue for Ge CMOS. In this study, we examine the properties of slow traps, particularly the location of slow traps, of Al2O3/GeOx/n-Ge and HfO2/Al2O3/GeOx/n-Ge MOS interfaces with changing the process and structural parameters, formed by atomic layer deposition (ALD) of Al2O3 and HfO2/Al2O3 combined with plasma post oxidation. It is found that the slow traps can locate in the GeOx interfacial layer, not in the ALD Al2O3 layer. Furthermore, we study the time dependence of channel currents in the Ge n-MOSFETs with 5-nm-thick Al2O3/GeOx/Ge gate stacks, with changing the thickness of GeOx, in order to further clarify the position of slow traps. The time dependence of the current drift and the effective time constant of slow traps do not change among the MOSFETs with the different thickness GeOx, demonstrating that the slow traps mainly exist near the interfaces between Ge and GeOx.

Interface Engineering of Next Generation Lithium Metal Anodes

Protection of lithium metal is of significant interest to the battery community: as the lightest and most electropositive alkali metal, lithium has a low density, resulting in an extremely high specific capacity value of 3.86Ah/g and energy density of 11,701Wh/kg, making it the holy grail of battery anodes. However, due to the extreme reactivity of lithium metal surfaces, controlled protection and characterization are extremely difficult. We have developed a bespoke integrated deposition, transfer, and characterization system capable of coating lithium metal anodes using Atomic layer deposition (ALD), characterizing the surface chemistry using XPS/SEM/UPS/Auger spectroscopy, and transfer directly from UHV into an Ar glovebox for battery assembly, disassembly, and electrochemical testing. After testing, the metal anodes can be transferred directly back to the XPS/SEM/UPS/Auger system for characterization. Protection coatings for lithium metal require the coatings be conformal, easily deposited at low temperature, and stable against organic electrolytes. Preventing direct contact of the lithium metal surface with the electrolyte and any dissolved species that would ordinarily react will ensure that the lithium atoms are oxidized in a reversible manner, rather than losing ions/electrons to highly stable, low ionic conductivity surface species such as Li2CO3. Lithium metal protection may allow the relaxation of purity requirements for both electrolytes and, in the case of Li-O2 batteries, cathode gas purity. To date, Li metal protection has been demonstrated with coatings of chlorosilanes[1] and other polymeric[2], organic[3], and inorganic coatings, but ALD has not been applied directly to lithium metal anodes for the purposes of protection. Known for conformality, low temperature deposition, excellent thickness control, ALD serves as an excellent permeation barrier, suggesting that it can be pinhole-free a promising technique for this application. We have developed ALD processes suitable for the protection of Li metal, and we assess the behavior of these functional ALD coatings electrochemically using over-potential studies and electrochemical impedance spectroscopy measurements. Surface characterization by semi in-situ XPS of protected and pristine lithium metal subjected to both gaseous environments and electrolytes with controlled/measured H2O content will be discussed, where the surface characterization is accomplished without ambient air exposure. We show that protection of lithium metal is not only possible with ALD, but that it is effective at preventing the tarnishing of the lithium metal surface due to reaction with atmospheric H2O and CO2. Using our unique capabilities, we are able to probe the chemistry of both protected and unprotected lithium metal, while using in-situ gas dosing we can decouple and characterize the interface chemistry and reactions. We demonstrate as a proof of concept that a 14 nm thick, ALD Al2O3 layer can protect the Li surface from corrosion due to atmosphere, sulfur, and electrolyte exposure. Furthermore, using Li-S battery cells as a test system, we demonstrate an improved capacity retention using ALD protected anodes over cells assembled with bare Li metal anodes for up to 100 cycles. This work has implications and impact on the metallic lithium anode battery systems Li-O2, Li-S and Li-NMC. Additionally, it is promising to pursue corresponding protection layers for Mg ad Na, where similar deleterious surface reactions may apply. Fundamental understanding of the controlled surface protection of lithium by ALD coatings could be extended to these other next-generation metal anode battery systems, and indeed could have a beneficial impact on the cyclability and storage lifetimes of lithium metal anode batteries.

Room Temperate Bonding of Al2O3 Layers by Atomic Layer Deposition on Polyimide Substrates

Atomic layer deposition (ALD) gas barrier layer coatings on polymer substrates have been studied to address the problem of short-lifetime of polymer devices. ALD Al2O3 layers deposited with trimethylaluminium (TMA) and H2O vapor precursors are relatively dense and conformal, so ALD Al2O3 layers are good candidates for effective gas barriers against water vapor and oxygen permeation. In order to seal such ALD Al2O3 coated polymer substrates without organic adhesives, the bonding technique of choice has to satisfy low temperature bonding requirements. Utilizing a high temperature bonding process would damage thermally sensitive polymer substrates during thermal compression bonding. Furthermore the gas barrier property of organic adhesives would be too low for effective sealing. The polymer substrates used in this study are polyimide (PI) sheets from XENOMAX® from Toyobo Co., Ltd that are widely used as substrates for flexible electronics. These polyimide sheets have extremely smooth surfaces (RMS ~1 nm), which are conducive for bonding, because good large contact areas can be obtained. These polyimide substrate sheets of 35 μm thickness were coated with Al2O3 films of 250 ALD cycles using different process temperatures ranging from 80 ºC, 150 ºC and 250 ºC, while 100 ALD cycles were used at 250 ºC. The warpage of the polyimide sheets, the surface morphology, and the results of bonding for these ALD Al2O3 coated polyimide samples were investigated as a function of different ALD deposition temperatures. The polyimide sample processed at the higher ALD Al2O3 temperature of 250 ºC exhibited much larger warpage, which is detrimental for bonding. We prepared PI sheets whose size was 10 cm x 10 cm. The ALD Al2O3 on polyimide samples deposited at 80, 150, and 250 ºC display warpage, and these edges lifted up to 1 mm, 3 mm, and 5 mm from a horizontal plane, respectively. This warpage is attributed to the difference of thermal expansion coefficients between the polyimide sheet and the deposited ALD Al2O3 layer. The surface morphology of the ALD layer was studied using atomic force microscopy (AFM) surface profiling. The sample deposited at a lower ALD temperature has smoother surface morphologies. Our ALD Al2O3 films synthesized at 80 ºC exhibited a very smooth surface morphology indicative of amorphous material, while the Al2O3 film synthesized at 250 ºC was observed to possess a granular structure demonstrating recrystallization into a polycrystalline film. The rough grain structure of ALD Al2O3 films deposited at 250 ºC exceeds the critical surface roughness and as a consequence prevents surfaces from bonding. For the bonding experiment, all PI samples coated by ALD Al2O3 were bonded using a modified method of Surface Activated Bonding (SAB) employing a nano-adhesion layer. First, ALD Al2O3 layers of about 25 nm are deposited on the surface of the polyimide substrates using trimethylaluminium (TMA) and water as precursors. Next, the Al2O3-coated surfaces are moved inside a high vacuum bonding machine and cleaned using Ar-ion beam bombardment. After the surface cleaning process step, Si nano-adhesive layers are deposited via ion-beam sputtering. Subsequently, the surface of the nano-adhesion layers are activated using the Ar-ion beam SAB method. Finally, surfaces were brought into close contact and bonded under compression in ultra-high vacuum. The structure of the bond interface was PI/Al2O3/Si/Si/Al2O3/PI. Here the thickness of each ALD Al2O3 layer and Si layer are 25 nm and 10 nm, respectively. We compared the results of the bonding of PI films coated at different ALD process temperatures. All bonding processes were performed at room temperature in ultra high vacuum. While the ALD Al2O3 samples deposited at 150 ºC suffered various non-bonded areas due to warpage and surface roughness, the ALD films deposited at 80 ºC revealed near perfect bonding without any voids, which is desirable for tight sealing of gas barrier layers. Room temperature bonding of ALD Al2O3 films on Si and polyimide substrates was successfully performed. Due to high temperature gradient induced warpage of the polymer substrate and elevated surface roughness of the ALD layer, a lower process temperature of 80 ºC for ALD synthesis of Al2O3 provided optimum gas barrier conditions. The results of our bonding experiments demonstrate that larger bonded areas were achieved when the ALD synthesis of Al2O3 gas barrier layers was performed at 80 ºC. This bonding method is expected to be used for future microelectronics packaging and gas barrier sealing for polymer substrate.

Interface Engineering of Next Generation Lithium Metal Anodes Using Atomic Layer Deposition

Protection of lithium metal is of significant interest to the battery community: as the lightest and most electropositive alkali metal, lithium has a low density, resulting in an extremely high specific capacity value of 3.86Ah/g and energy density of 11,701Wh/kg, making it the holy grail of battery anodes. However, due to the extreme reactivity of lithium metal surfaces, controlled protection and characterization are extremely difficult. We have developed a bespoke integrated deposition, transfer, and characterization system capable of coating lithium metal anodes using Atomic layer deposition (ALD), characterizing the surface chemistry using XPS/SEM/UPS/Auger spectroscopy, and transfer directly from UHV into an Ar glovebox for battery assembly, disassembly, and electrochemical testing. After testing, the metal anodes can be transferred directly back to the XPS/SEM/UPS/Auger system for characterization. Protection coatings for lithium metal require the coatings be conformal, easily deposited at low temperature, and stable against organic electrolytes. Preventing direct contact of the lithium metal surface with the electrolyte and any dissolved species that would ordinarily react will ensure that the lithium atoms are oxidized in a reversible manner, rather than losing ions/electrons to highly stable, low ionic conductivity surface species such as Li2CO3. Lithium metal protection may allow the relaxation of purity requirements for both electrolytes and, in the case of Li-O2 batteries, cathode gas purity. To date, Li metal protection has been demonstrated with coatings of chlorosilanes[1] and other polymeric[2], organic[3], and inorganic coatings, but ALD has not been applied directly to lithium metal anodes for the purposes of protection. Known for conformality, low temperature deposition, excellent thickness control, ALD serves as an excellent permeation barrier, suggesting that it can be pinhole-free a promising technique for this application. We have developed ALD processes suitable for the protection of Li metal, and we assess the behavior of these functional ALD coatings electrochemically using over-potential studies and electrochemical impedance spectroscopy measurements. Surface characterization by semi in-situ XPS of protected and pristine lithium metal subjected to both gaseous environments and electrolytes with controlled/measured H2O content will be discussed, where the surface characterization is accomplished without ambient air exposure. We show that protection of lithium metal is not only possible with ALD, but that it is effective at preventing the tarnishing of the lithium metal surface due to reaction with atmospheric H2O and CO2. Using our unique capabilities, we are able to probe the chemistry of both protected and unprotected lithium metal, while using in-situ gas dosing we can decouple and characterize the interface chemistry and reactions. We demonstrate as a proof of concept that a 14 nm thick, ALD Al2O3 layer can protect the Li surface from corrosion due to atmosphere, sulfur, and electrolyte exposure. Furthermore, using Li-S battery cells as a test system, we demonstrate an improved capacity retention using ALD protected anodes over cells assembled with bare Li metal anodes for up to 100 cycles. This work has implications and impact on the metallic lithium anode battery systems Li-O2, Li-S and Li-NMC. Additionally, it is promising to pursue corresponding protection layers for Mg ad Na, where similar deleterious surface reactions may apply. Fundamental understanding of the controlled surface protection of lithium by ALD coatings could be extended to these other next-generation metal anode battery systems, and indeed could have a beneficial impact on the cyclability and storage lifetimes of lithium metal anode batteries.

Next-Generation Lithium Metal Anode Engineering via Atomic Layer Deposition.

Lithium metal is considered to be the most promising anode for next-generation batteries due to its high energy density of 3840 mAh g(-1). However, the extreme reactivity of the Li surface can induce parasitic reactions with solvents, contamination, and shuttled active species in the electrolyte, reducing the performance of batteries employing Li metal anodes. One promising solution to this issue is application of thin chemical protection layers to the Li metal surface. Using a custom-made ultrahigh vacuum integrated deposition and characterization system, we demonstrate atomic layer deposition (ALD) of protection layers directly on Li metal with exquisite thickness control. We demonstrate as a proof-of-concept that a 14 nm thick ALD Al2O3 layer can protect the Li surface from corrosion due to atmosphere, sulfur, and electrolyte exposure. Using Li-S battery cells as a test system, we demonstrate an improved capacity retention using ALD-protected anodes over cells assembled with bare Li metal anodes for up to 100 cycles.

Enhanced electrochemical stability of high-voltage LiNi0.5Mn1.5O4 cathode by surface modification using atomic layer deposition

We report improved cycling performance of high-voltage LiNi0.5Mn1.5O4 cathode for lithium-ion batteries by modifying its surface with ultrathin layer coating of Al2O3 (<1 nm) using atomic layer deposition (ALD) technique. Four and six layers of ALD Al2O3 were coated directly on the porous LiNi0.5Mn1.5O4 nanoparticle (~200 nm) slurry composite. Electrochemical characterization results show that LiNi0.5Mn1.5O4 electrode with four and six layers of ALD Al2O3 maintained 92 and 98 % of their initial capacity after 200 cycles, respectively, in comparison to bare LiNi0.5Mn1.5O4 electrode which showed 84 % capacity retention. We show that the ALD Al2O3 coated cathode surface is protected from undesired side reactions occurring at the electrode/electrolyte interface. For the first time, we used XPS studies to investigate the surface chemistry of ultrathin ALD Al2O3 coated LiNi0.5Mn1.5O4 and confirmed that the ultrathin ALD Al2O3 layer reduces side reactions involving organic components of the electrolyte decomposition product in high-voltage cathodes.

Tuning Photoresponse of Ultrathin ZnO Films Using Few Angstrom Al2O3 Overlayers

Ultra-thin ZnO films can be deposited with precise thickness control, good conformality and uniformity by atomic layer deposition (ALD). This strategy has been widely used in optical devices and solar cells.[i] However, ZnO with a thickness less than 10 nm – where surface effects dominate electronic transport properties, has been found to possess a high degree of electronic defects.[ii] These defects originate on the surface as unterminated, dangling bonds and degrade electronic and optoelectronic transport properties.[iii]To understand the phenomena further, we report here temperature-based (80K-320K) conductance and photoresponse (Ion/Ioff) properties of 6.1 nm ZnO. Further, we track these parameters as a function of ALD Al2O3 ‘overlayers’ deposited on the same ZnO film. The temperature based Ion/Ioff is shown in Figure 1, left and the maximum Ion/Ioff is shown as a function of ALD Al2O3 overlayers in Figure 1, right. Based on these data, the Ion/Ioff ratio initially increases with the number of ALD Al2O3 layers and maximizes at 7 cycles of Al2O3 overlayer. It then decreases for additional ALD Al2O3 cycles. A maximum Ion/Ioff of 11.4x is obtained with 7 cycles of ALD Al2O3.Activation energy (Ea) for conduction will be estimated using conductance vs. temperature curves.[iv] Accompanying data using XPS and photoluminescence will structurally probe the surface modification of the ZnO due to addition of the ALD Al2O3monolayers. These results lead to insights into the passivation mechanisms of ultrathin ZnO films with applications in a wide variety of electronic and optoelectronic devices. Figure 1. Left - The Ion/Ioff vs. temperature (left) and, Right - The maximum Ion/Ioff plotted with number of ALD Al2O3 cycles. (*Ion/Ioffis defined by the light conductance over the dark conductance.)[i] a) Lee, D.-J., Kim, H.-M., Kwon, J.-Y., Choi, H., Kim, S.-H., Kim, K.-B. Adv. Funct. Mater. 2011, 21, 448-455. b) Wang, J.-C., Weng, W.-T., Tsai, M.-Y., Lee, M.-K., Lee, M.-k., Horng, S.-F. Perng, T.-P., Kei, C.-C., Yu, C.-C. Meng, H.-F. J. Mater. Chem. 2010, 20, 862-866.[ii] a) Cheun, H., Fuentes-Hernandez, C., Zhou, Y., Potscavage, W. J., Kim, S.-J., Shim, J., Dindar, A., Kippelen, B. J. Phys. Chem. C 2010, 114, 20713-20718. b) Na, J.-S., Scarel, G., Parsons, G. N. J. Phys. Chem. C 2010, 114, 383-388.[iii] Cheun, H., Fuentes-Hernandez, C., Zhou, Y., Potscavage, W. J., Kim, S.-J., Shim, J., Dindar, A., Kippelen, B. J. Phys. Chem. C 2010, 114, 20713-20718.[iv] Wu, F., Tian, L., Kanjolia, R., Singamaneni, S., Banerjee, P. ACS Appl. Mater. Interfaces, 2013, 5, 7693-7697.

Performance improvement of inverted organic solar cells by adding ultrathin Al2O3 as an electron selective layer and a plasma enhanced chemical vapor deposition of SiOx encapsulating layer

In this paper, we report the performance improvement of inverted organic solar cells by adding an ultrathin electron selective layer of Al2O3 prepared between the indium tin oxide (ITO) electrode and the active transport layer through atomic layer deposition (ALD). We evaluated the cell shelf-life after encapsulating with SiOx-coated polyethylene terephthalate, where the SiOx layer was made by plasma enhanced chemical vapor deposition (PECVD). It was found that the devices with ALD Al2O3 have a higher open circuit voltage than those without the ALD Al2O3 layer. Al2O3 deposited on an ITO electrode decreased the work function of ITO. Furthermore, based on the current density–voltage curves of the initial devices showing a pronounced S-shape, we soaked the cells with the ultraviolet (UV) light process. Then we obtained a higher efficiency in these ALD Al2O3 treated devices. With a careful analysis by atomic force microscopic and X-ray photoelectron spectroscopy, we believe that the UV light soaking process affected both ITO and Al2O3. Further, after the encapsulation by PECVD SiOx, our devices achieved a shelf-life of over 500h for 50% retained cell efficiency.

Electrical Measurement Under Atmospheric Conditions of PbSe Nanocrystal Thin Films Passivated by Remote Plasma Atomic Layer Deposition of Al$_{\bf 2}$O $_{\bf 3}$

PbSe nanocrystal thin-film transistors (TFTs) were passivated using remote plasma atomic layer deposition (ALD) of a ∼10 nm thick Al <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> O <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex></formula> film at 150 °C. By using a highly reactive remote oxygen plasma source, the time for one complete ALD cycle was about 15 s with growth rates of ∼1.1 Å/cycle. The effective mobilities measured under atmospheric condition from Al <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $_{2}$</tex></formula> O <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex></formula> -passivated PbSe nanocrystal TFTs were comparable to the values reported previously for air-free PbSe nanocrystal TFTs, demonstrating that ALD Al <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> O <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex></formula> layers prevent oxidation and degradation of nanocrystal films from air exposure. The variation in the effective mobility of passivated devices was also found to be negligible under ambient conditions over a period of 30 days. The results show that remote plasma ALD processing of Al <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> O <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex></formula> is capable of producing an effective passivation layer on air-sensitive nanocrystals with high deposition rates at reduced temperature.

Nanoscale Interface Modification of LiCoO2 by Al2O3 Atomic Layer Deposition for Solid-State Li Batteries

Cycle stability of solid-state lithium batteries (SSLBs) using a LiCoO2 cathode is improved by atomic layer deposition (ALD) on active material powder with Al2O3. SSLBs with LiCoO2/Li3.15Ge0.15P0.85S4/77.5Li2S-22.5P2S5/Li structure were constructed and tested by charge-discharge cycling at a current density of 45 μA cm−2 with a voltage window of 3.3 ∼ 4.3 V (vs. Li/Li+). Capacity degradation during cycling is suppressed dramatically by employing Al2O3 ALD-coated LiCoO2 in the composite cathode. Whereas only 70% of capacity retention is achieved for uncoated LiCoO2 after 25 cycles, 90% of capacity retention is observed for LiCoO2 with ALD Al2O3 layers. Electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM) studies show that the presence of ALD Al2O3 layers on the surface of LiCoO2 reduces interfacial resistance development between LiCoO2 and solid state electrolyte (SSE) during cycling.

Room temperature atomic layer deposition of Al2O3 and replication of butterfly wings for photovoltaic application

In this paper, we present two key process steps for potential applications in the fabrication of low-cost and high-efficiency ultrathin monocrystalline silicon (mono-Si) solar cells to effectively harvest solar energy. One is to grow an Al2O3 passivation layer on the rear side of an Si wafer by a successive atomic layer deposition (ALD) at room temperature (25 °C). The other is to produce a bio-inspired antireflection structure on the front side of the Si wafer by replicating butterfly wing patterns. The capacitance–voltage measurements reveal that the successive ALD procedure can yield a higher negative charge density at the Al2O3/Si interface relative to the conventional one. The measurement results of quasi-steady-state photoconductance indicate that after annealing, the 25 °C ALD Al2O3 layers reach a similar passivation level to the p-type Si wafers compared to that deposited at 250 °C. With the help of an ALD Al2O3 layer, butterfly wing patterns (Hypochrysops polycletus) are replicated on a PMMA layer which is on an SiO2/Si stack. This work demonstrates the ability for replicating the natural photonic features on Si wafers and other substrates by using nanoimprint.

Effect of Processing Conditions on the Electrical Characteristics of Atomic Layer Deposited Al2O3 and HfO2 Films

An extensive mercury-probe capacitance-voltage characterization is carried out on Al2O3 and HfO2 films deposited on silicon by atomic layer deposition (ALD) at different temperatures and subjected to various high-temperature post-deposition thermal treatments. For the case of the Al2O3 layers, higher flat-band voltages (Vfb) for higher deposition temperatures are obtained and positive Vfb shifts are always registered after the different thermal annealing conditions studied; however, the impact is found to be significantly different depending on the deposition temperature. On the contrary, smaller changes are observed for the case of the studied HfO2 layers. Interestingly for practical applications, the hystereses, as well as the Vfb shifts, clearly diminish for higher deposition temperatures or after a thermal annealing treatment. The results obtained from ellipsometry suggest that significant physical changes occur under the highest temperature treatments in either, the ALD Al2O3 or HfO2 layers and/or its interface with the silicon substrate.

Passivation of Silicon Surfaces Using Atomic Layer Deposited Metal Oxides

AbstractSurface passivation of silicon substrates using atomic layer deposited Al2O3 and HfO2 thin films are assessed. Al2O3 and HfO2 dielectric layers with various thicknesses were deposited on both sides of n-type (100) FZ-Si substrates (resistivity 4 – 6 Ω-cm) at 200°C by atomic layer deposition (ALD) system. The effective excess carrier lifetime of as-deposited oxide/Si/oxide structure was measured by microwave-photoconductivity-decay (MWPCD) measurement technique and it was observed that the thicker ALD dielectrics lead to higher effective excess carrier lifetime and better surface passivation. The measurements showed average excess carrier lifetime values of 302 μs and 347 μs for as-deposited Al2O3 and HfO2 passivated Si substrates with 150 ALD cycles, respectively. MWPCD and capacitance-voltage (C-V) measurements suggest that as-deposited ALD HfO2 layer leads to a better surface passivation compared to as-deposited ALD Al2O3 layer. Further, the results suggest that there exist fixed negative charges in the bulk of the ALD dielectrics and this contributes to the field effect passivation of the silicon surfaces.