Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction.

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

The Consequences of Random Sequential Adsorption for the Precursor Packing and Growth-Per-Cycle of Atomic Layer Deposition Processes.

Atomic layer deposition (ALD) processes are known to deposit submonolayers of material per cycle, primarily attributed to steric hindrance and a limited number of surface sites. However, an often-overlooked factor is the random sequential adsorption (RSA) mechanism, where precursor molecules arrive one-by-one and adsorb at random surface sites. Consequently, the saturation coverage of precursors significantly deviates from ideal closed packing. In this study, we investigated the influence of RSA on precursor adsorption saturation and, consequently, on the growth per cycle (GPC) of the ALD processes. Our simulations revealed that the RSA model leads to a 22% to 40% lower surface density compared to the reference case of ordered packing. Furthermore, based on the precursor shape and size, we estimated GPC values with an average accuracy of 0.05 Å relative to experimental literature data. This work shows the critical role of RSA in ALD, emphasizing the need to consider this mechanism for a more accurate process design and optimization.

Computational Investigation of Precursor Blocking during Area-Selective Atomic Layer Deposition Using Aniline as a Small-Molecule Inhibitor.

Area-selective atomic layer deposition using small-molecule inhibitors (SMIs) involves vapor-phase dosing of inhibitor molecules, resulting in an industry-compatible approach. However, the identification of suitable SMIs that yield a high selectivity remains a challenging task. Recently, aniline (C6H5NH2) was shown to be an effective SMI during the area-selective deposition (ASD) of TiN, giving 6 nm of selective growth on SiO2 in the presence of Ru and Co non-growth areas. In this work, using density functional theory (DFT) and random sequential adsorption (RSA) simulations, we investigated how aniline can effectively block precursor adsorption on specific areas. Our DFT calculations confirmed that aniline selectively adsorbs on Ru and Co non-growth areas, whereas its adsorption on the SiO2 growth area is limited to physisorption. DFT reveals two stable adsorption configurations of aniline on the metal surfaces. Further calculations on the aniline-functionalized surfaces show that the aniline inhibitor significantly reduces the interaction of Ti precursor, tetrakis(dimethylamino)titanium, with the non-growth area. In addition, RSA simulations showed that the co-presence of two stable adsorption configurations allows for a high surface inhibitor coverage on both Co and Ru surfaces. As the surface saturates, there is a transition from the thermodynamically most favorable adsorption configuration to the sterically most favorable adsorption configuration, which results in a sufficiently dense inhibition layer, such that an incoming precursor molecule cannot fit in between the adsorbed precursor molecules. We also found that, as a result of the catalytic activity of the metallic non-growth area, further reactions of inhibitor molecules, such as hydrogenolysis, can play a role in precursor blocking.

Surface Chemistry during Atomic Layer Deposition of Pt Studied with Vibrational Sum-Frequency Generation.

A detailed understanding of the growth of noble metals by atomic layer deposition (ALD) is key for various applications of these materials in catalysis and nanoelectronics. The Pt ALD process using MeCpPtMe3 and O2 gas as reactants serves as a model system for the ALD processes of noble metals in general. The surface chemistry of this process was studied by in situ vibrational broadband sum-frequency generation (BB-SFG) spectroscopy, and the results are placed in the context of a literature overview of the reaction mechanism. The BB-SFG experiments provided direct evidence for the presence of CH3 groups on the Pt surface after precursor chemisorption at 250 °C. Strong evidence was found for the presence of a C=C containing complex (e.g., the form of Cp species) and for partial dehydrogenation of the surface species during the precursor half-cycle. The reaction kinetics of the precursor half-cycle were followed at 250 °C, showing that the C=C coverage saturated before the saturation of CH3. This complex behavior points to the competition of multiple surface reactions, also reflected in the temperature dependence of the reaction mechanism. The CH3 saturation coverage decreased significantly with temperature, while the C=C coverage remained constant after precursor chemisorption on the Pt surface for temperatures from 80 to 300 °C. These SFG results have resulted in a better understanding of the Pt ALD process and also highlight the surface chemistry during thin-film growth as a promising field of study for the BB-SFG community.

Atmospheric-Pressure Plasma-Enhanced Spatial ALD of SiO2 Studied by Gas-Phase Infrared and Optical Emission Spectroscopy.

An atmospheric-pressure plasma-enhanced spatial atomic layer deposition (PE-s-ALD) process for SiO2 using bisdiethylaminosilane (BDEAS, SiH2[NEt2]2) and O2 plasma is reported along with an investigation of its underlying growth mechanism. Within the temperature range of 100-250 °C, the process demonstrates self-limiting growth with a growth per cycle (GPC) between 0.12 and 0.14 nm and SiO2 films exhibiting material properties on par with those reported for low-pressure PEALD. Gas-phase infrared spectroscopy on the reactant exhaust gases and optical emission spectroscopy (OES) on the plasma region are used to identify the species that are involved in the ALD process. Based on the identified species, we propose a reaction mechanism where BDEAS molecules adsorb on -OH surface sites through the exchange of one of the amine ligands upon desorption of diethylamine (DEA). The remaining amine ligand is removed through combustion reactions activated by the O2 plasma species leading to the release of H2O, CO2, and CO in addition to products such as N2O, NO2, and CH-containing species. These volatile species can undergo further gas-phase reactions in the plasma as indicated by the observation of OH*, CN*, and NH* excited fragments in OES. Furthermore, the infrared analysis of the precursor exhaust gas indicated the release of CO2 during precursor adsorption. Moreover, this analysis has allowed the quantification of the precursor depletion yielding values between 10 and 50% depending on the processing parameters. Besides providing insights into the chemistry of atmospheric-pressure PE-s-ALD of SiO2, our results demonstrate that infrared spectroscopy performed on exhaust gases is a valuable approach to quantify relevant process parameters, which can ultimately help evaluate and improve process performance.

Atomic layer deposition of LiF using LiN(SiMe3)2 and SF6 plasma.

Lithium fluoride films were prepared by atomic layer deposition (ALD) using a new route in which LiN(SiMe3)2 is used as a precursor and SF6 plasma as a coreactant. Conformal LiF films were deposited at 150 °C at a growth rate of ∼0.4 Å per cycle. All deposited films were polycrystalline and slightly lithium-rich with a composition of LiF0.8, independently of the plasma conditions (e.g. exposure time, pressure and power). The levels of H, C, N, O, Si, and S were all <1 at%. Spectroscopic ellipsometry measurements were carried out over the wavelength range of 140-2480 nm and showed a refractive index of 1.37 at 633 nm for films deposited using 1 s plasma exposure time. We conclude that short plasma exposures are preferred, since a prolonged exposure time leads to an increase in optical absorption and lower growth per cycle values. Furthermore, mass spectrometry measurements revealed the formation of SiMe3F species during both half-cycles, originating from the reaction between the precursor ligands and fluorine species present either at the surface or in the plasma. Moreover, the SF6 plasma step led to the formation of fluorocarbon species, suggesting that dissociation and recombination in the plasma takes place. Overall, this work demonstrates that SF6 plasma offers a promising alternative to other coreactants for ALD of high purity lithium fluoride.

Large area, patterned growth of 2D MoS2 and lateral MoS2-WS2 heterostructures for nano- and opto-electronic applications.

The patterned growth of transition metal dichalcogenides (TMDs) and their lateral heterostructures is paramount for the fabrication of application-oriented electronics and optoelectronics devices. However, the large scale patterned growth of TMDs remains challenging. Here, we demonstrate the synthesis of patterned polycrystalline 2D MoS2 thin films on device ready SiO2/Si substrates, eliminating any etching and transfer steps using a combination of plasma enhanced atomic layer deposition (PEALD) and thermal sulfurization. As an inherent advantage of ALD, precise thickness control ranging from a monolayer to few-layered MoS2 has been achieved. Furthermore, uniform films with exceptional conformality over 3D structures are obtained. Finally, the approach has been leveraged to obtain in-plane lateral heterostructures of 2D MoS2 and WS2 thin films over a large area which opens up an avenue for their direct integration in future nano- and opto-electronic device applications.

Effect of an electric field during the deposition of silicon dioxide thin films by plasma enhanced atomic layer deposition: an experimental and computational study.

The growth, chemical, structural, mechanical, and optical properties of oxide thin films deposited by plasma enhanced atomic layer deposition (PEALD) are strongly influenced by the average-bias voltage applied during the reaction step of surface functional groups with oxygen plasma species. Here, this effect is investigated thoroughly for SiO2 deposited in two different PEALD tools at average-bias voltages up to -300 V. Already at a very low average-bias voltage (< -10 V), the SiO2 films have significantly lower water content than films grown without biasing together with the formation of denser films having a higher refractive index and nearly stoichiometric composition. Substrate biasing during PEALD also enables control of mechanical stress. The experimental findings are supported by density functional theory and atomistic simulations. They demonstrate that the application of an electric field during the plasma step results in an increased energy transfer between energetic ions and the surface, directly influencing relevant surface reactions. Applying an electric field during the PEALD process leads to SiO2 thin films with significantly improved properties comparable to films grown by ion beam sputtering.

Radiomics approach to distinguish between well differentiated liposarcomas and lipomas on MRI.

BACKGROUND: Well differentiated liposarcoma (WDLPS) can be difficult to distinguish from lipoma. Currently, this distinction is made by testing for MDM2 amplification, which requires a biopsy. The aim of this study was to develop a noninvasive method to predict MDM2 amplification status using radiomics features derived from MRI. METHODS: Patients with an MDM2-negative lipoma or MDM2-positive WDLPS and a pretreatment T1-weighted MRI scan who were referred to Erasmus MC between 2009 and 2018 were included. When available, other MRI sequences were included in the radiomics analysis. Features describing intensity, shape and texture were extracted from the tumour region. Classification was performed using various machine learning approaches. Evaluation was performed through a 100 times random-split cross-validation. The performance of the models was compared with the performance of three expert radiologists. RESULTS: The data set included 116 tumours (58 patients with lipoma, 58 with WDLPS) and originated from 41 different MRI scanners, resulting in wide heterogeneity in imaging hardware and acquisition protocols. The radiomics model based on T1 imaging features alone resulted in a mean area under the curve (AUC) of 0·83, sensitivity of 0·68 and specificity of 0·84. Adding the T2-weighted imaging features in an explorative analysis improved the model to a mean AUC of 0·89, sensitivity of 0·74 and specificity of 0·88. The three radiologists scored an AUC of 0·74 and 0·72 and 0·61 respectively; a sensitivity of 0·74, 0·91 and 0·64; and a specificity of 0·55, 0·36 and 0·59. CONCLUSION: Radiomics is a promising, non-invasive method for differentiating between WDLPS and lipoma, outperforming the scores of the radiologists. Further optimization and validation is needed before introduction into clinical practice.

ANTECEDENTES: Es difícil distinguir los liposarcomas bien diferenciados (well-differentiated liposarcomas, WDLPS) de los lipomas. En la actualidad, esta distinción se realiza mediante la prueba de amplificación del gen MDM2 por biopsia. El objetivo de este estudio fue predecir de forma no invasiva el estado de amplificación del gen MDM2 para diferenciar los lipomas de los WDLPS utilizando características radiómicas a partir de la resonancia magnética. MÉTODOS: Se incluyeron los pacientes remitidos al instituto Erasmus MC entre 2009-2018 por un lipoma MDM2 negativo o WDLPS MDM2 positivo y las resonancias magnéticas potenciadas en T1 correspondientes antes del tratamiento. Cuando estaban disponibles, se incluyeron otras secuencias de MRI en el análisis radiómico. Se describieron la intensidad, forma y textura de la región tumoral. Para la clasificación se utilizaron varios modelos de aprendizaje automático (machine learning). La evaluación se realizó mediante una validación cruzada aleatoria 100x. Se comparó el rendimiento de los modelos con la clasificación realizada por tres radiólogos expertos. RESULTADOS: Se incluyeron 116 pacientes (58 lipomas, 58 WDLPS) y 41 aparatos de MRI, con una gran heterogeneidad en las técnicas y protocolos para la adquisición de imágenes. El modelo radiómico basado únicamente en las características de las imagen en T1 dio como resultado una AUC media de 0,83, con una sensibilidad de 0,68 y una especificidad de 0,84. Un análisis adicional incorporando las imágenes ponderadas en T2 mejoró el modelo con una AUC media de 0,89, una sensibilidad de 0,74 y una especificidad de 0,88. Los tres radiólogos obtuvieron una AUC de 0,74/0,72/0,61, una sensibilidad de 0,74/0,91/0,64 y una especificidad de 0,55/0,36/0,59, respectivamente. CONCLUSIÓN: La radiómica es un método prometedor y no invasivo para diferenciar entre WDLPS y lipomas, superando la valoración de los radiólogos. Sin embargo, se necesita la optimización y validación de esta técnica antes de su introducción en la práctica clínica diaria.

Initial Growth Study of Atomic-Layer Deposition of Al2O3 by Vibrational Sum-Frequency Generation.

The initial growth during the atomic-layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) and water was studied on two starting surfaces: SiO2 and -H-terminated Si(111) [H/Si(111)]. In situ spectroscopy ellipsometry (SE) showed virtually immediate growth of Al2O3 on both surfaces, although for H/Si(111) a reduced growth-per-cycle was observed in the initial 20 cycles. The underlying surface chemistry during the initial cycles of ALD was monitored with in situ broadband sum-frequency generation (BB-SFG) spectroscopy. For the SiO2 surface, the -CH3 surface groups were followed revealing that only the first TMA half-cycle deviates from the steady-growth regime. The reaction cross section of the initial TMA half-cycle (σTMA = 2.0 ± 0.2 × 10-18 cm2) was a factor of 3 lower than the cross section of the TMA half-cycle during the steady-growth regime of ALD (σTMA = 6.5 ± 0.6 × 10-18 cm2). All H2O half-cycles, including the first, showed steady-growth behavior with a corresponding reaction cross section (σH2O = 4.0 ± 0.4 × 10-20 cm2). Therefore, only the first ALD cycle was affected by initial growth effects on the SiO2 starting surface, in line with the SE data. For the H/Si(111) surface, the Si-H groups were monitored with BB-SFG spectroscopy, revealing a reaction cross section of σTMA = 3.1 ± 0.3 × 10-18 cm2 for the first TMA half-cycle on H/Si(111); a factor two lower than that during the steady regime of Al2O3. These results demonstrate that the chemistry during the initial growth regime of Al2O3 ALD on SiO2 and H/Si(111) shows subtle but measurable differences compared to the steady-growth regime.

Data on dopant characteristics and band alignment of CdTe cells with and without a ZnO highly-resistive-transparent buffer layer.

Photovoltaic enhancement of cadmium telluride (CdTe) thin film solar cells using a 50 nm thick, atomic-layer-deposited zinc oxide (ZnO) buffer film was reported in "Enhancement of the photocurrent and efficiency of CdTe solar cells suppressing the front contact reflection using a highly-resistive ZnO buffer layer" (Kartopu et al., 2019) [1]. Data presented here are the dopant profiles of two solar cells prepared side-by-side, one with and one without the ZnO highly resistive transparent (HRT) buffer, which displayed an open-circuit potential (Voc) difference of 25 mV (in favor of the no-buffer device), as well as their simulated device data. The concentration of absorber dopant atoms (arsenic) was measured using the secondary ion mass spectroscopy (SIMS) method, while the density of active dopants was calculated from the capacitance-voltage (CV) measurements. The solar cell simulation data was obtained using the SCAPS software, a one-dimensional solar cell simulation programme. The presented data indicates a small loss (around 20 mV) of Voc for the HRT buffered cells.

Isotropic Atomic Layer Etching of ZnO Using Acetylacetone and O2 Plasma.

Atomic layer etching (ALE) provides Ångström-level control over material removal and holds potential for addressing the challenges in nanomanufacturing faced by conventional etching techniques. Recent research has led to the development of two main classes of ALE: ion-driven plasma processes yielding anisotropic (or directional) etch profiles and thermally driven processes for isotropic material removal. In this work, we extend the possibilities to obtain isotropic etching by introducing a plasma-based ALE process for ZnO which is radical-driven and utilizes acetylacetone (Hacac) and O2 plasma as reactants. In situ spectroscopic ellipsometry measurements indicate self-limiting half-reactions with etch rates ranging from 0.5 to 1.3 Å/cycle at temperatures between 100 and 250 °C. The ALE process was demonstrated on planar and three-dimensional substrates consisting of a regular array of semiconductor nanowires (NWs) conformally covered using atomic layer deposition of ZnO. Transmission electron microscopy studies conducted on the ZnO-covered NWs before and after ALE proved the isotropic nature and the damage-free characteristics of the process. In situ infrared spectroscopy measurements were used to elucidate the self-limiting nature of the ALE half-reactions and the reaction mechanism. During the Hacac etching reaction that is assumed to produce Zn(acac)2, carbonaceous species adsorbed on the ZnO surface are suggested as the cause of the self-limiting behavior. The subsequent O2 plasma step resets the surface for the next ALE cycle. High etch selectivities (∼80:1) over SiO2 and HfO2 were demonstrated. Preliminary results indicate that the etching process can be extended to other oxides such as Al2O3.

Atomic Layer Deposition of Cobalt Using H2-, N2-, and NH3-Based Plasmas: On the Role of the Co-reactant.

This work investigates the role of the co-reactant for the atomic layer deposition of cobalt (Co) films using cobaltocene (CoCp2) as the precursor. Three different processes were compared: an AB process using NH3 plasma, an AB process using H2/N2 plasma, and an ABC process using subsequent N2 and H2 plasmas. A connection was made between the plasma composition and film properties, thereby gaining an understanding of the role of the various plasma species. For NH3 plasma, H2 and N2 were identified as the main species apart from the expected NH3, whereas for the H2/N2 plasma, NH3 was detected. Moreover, HCp was observed as a reaction product in the precursor and co-reactant subcycles. Both AB processes showed self-limiting half-reactions and yielded similar material properties, that is, high purity and low resistivity. For the AB process with H2/N2, the resistivity and impurity content depended on the H2/N2 mixing ratio, which was linked to the production of NH3 molecules and related radicals. The ABC process resulted in high-resistivity and low-purity films, attributed to the lack of NH x,x≤3 species during the co-reactant exposures. The obtained insights are summarized in a reaction scheme where CoCp2 chemisorbs in the precursor subcycle and NH x species eliminate the remaining Cp in the consecutive subcycle.

Initial stage of atomic layer deposition of 2D-MoS2 on a SiO2 surface: a DFT study.

In this study, we investigate the reactions involving Atomic Layer Deposition (ALD) of 2D-MoS2 from the heteroleptic precursor Mo(NMe2)2(NtBu)2 and H2S as the co-reagent on a SiO2(0001) surface by means of density functional theory (DFT). All dominant reaction pathways from the early stage of adsorption of each ALD reagent to the formation of bulk-like Mo and S at the surface are identified. In the metal pulse, proton transfer from terminal OH groups on the SiO2 to the physisorbed metal precursor increases the Lewis acidity of Mo and Lewis basicity of O, which gives rise to the chemical adsorption of the metal precursor. Proton transfer from the surface to the dimethylamido ligands leads to the formation and desorption of dimethylamine. In contrast, the formation and desorption of tert-butylamine is not energetically favorable. The tert-butylimido ligand can only be partially protonated in the metal pulse. In the sulphur pulse, co-adsorption and dissociation of H2S molecules give rise to the formation and desorption of tert-butylamine. Through the calculated activation energies, the cooperation between H2S molecules ('cooperative' mechanism) is shown to have a profound influence on the formation and desorption of tert-butylamine, which are crucial steps in the initial ALD deposition of 2D-MoS2 on SiO2. The cyclic ALD reactions give rise to the formation of a buffer layer which might have important consequences for the electrical and optical properties on the 2D layer formed in the subsequent homodeposition.

Characterization of nano-porosity in molecular layer deposited films.

Molecular layer deposition (MLD) delivers (ultra-) thin organic and hybrid materials, with atomic-level thickness control. However, such layers are often reported to be unstable under ambient conditions, due to the interaction of water and oxygen with the hybrid structure, consequently limiting their applications. In this contribution, we investigate the impact of porosity in MLD layers on their degradation. Alucone layers were deposited by means of trimethylaluminium and ethylene glycol, adopting both temporal and spatial MLD and characterized by means of FT-IR spectroscopy, spectroscopic ellipsometry, and ellipsometric porosimetry. The highest growth per cycle (GPC) achieved by spatial MLD resulted in alucone layers with very low stability in ambient air, leading to their conversion to AlOx. Alucones deposited by means of temporal MLD, instead, showed a lower GPC and a higher ambient stability. Ellipsometric porosimetry showed the presence of open nano-porosity in pristine alucone layers. Pores with a diameter in the range of 0.42-2 nm were probed, with a relative content between 1.5% and 5%, respectively, which are attributed to the temporal and spatial MLD layers. We concluded that a correlation exists between the process GPC, the open-porosity relative content, and the degradation of alucone layers.

Low-temperature plasma-enhanced atomic layer deposition of 2-D MoS2: large area, thickness control and tuneable morphology.

Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS2 on SiO2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.

Anti-stiction coating for mechanically tunable photonic crystal devices.

A method to avoid the stiction failure in nano-electro-opto-mechanical systems has been demonstrated by coating the system with an anti-stiction layer of Al2O3 grown by atomic layer deposition techniques. The device based on a double-membrane photonic crystal cavity can be reversibly operated from the pull-in back to its release status. This enables to electrically switch the wavelength of a mode over ~50 nm with a potential modulation frequency above 2 MHz. These results pave the way to reliable nano-mechanical sensors and optical switches.

Effective Surface Passivation of InP Nanowires by Atomic-Layer-Deposited Al2O3 with POx Interlayer.

III/V semiconductor nanostructures have significant potential in device applications, but effective surface passivation is critical due to their large surface-to-volume ratio. For InP such passivation has proven particularly difficult, with substantial depassivation generally observed following dielectric deposition on InP surfaces. We present a novel approach based on passivation with a phosphorus-rich interfacial oxide deposited using a low-temperature process, which is critical to avoid P-desorption. For this purpose we have chosen a POx layer deposited in a plasma-assisted atomic layer deposition (ALD) system at room temperature. Since POx is known to be hygroscopic and therefore unstable in atmosphere, we encapsulate this layer with a thin ALD Al2O3 capping layer to form a POx/Al2O3 stack. This passivation scheme is capable of improving the photoluminescence (PL) efficiency of our state-of-the-art wurtzite (WZ) InP nanowires by a factor of ∼20 at low excitation. If we apply the rate equation analysis advocated by some authors, we derive a PL internal quantum efficiency (IQE) of 75% for our passivated wires at high excitation. Our results indicate that it is more reliable to calculate the IQE as the ratio of the integrated PL intensity at room temperature to that at 10 K. By this means we derive an IQE of 27% for the passivated wires at high excitation (>10 kW cm-2), which constitutes an unprecedented level of performance for undoped InP nanowires. This conclusion is supported by time-resolved PL decay lifetimes, which are also shown to be significantly higher than previously reported for similar wires. The passivation scheme displays excellent long-term stability (>7 months) and is additionally shown to substantially improve the thermal stability of InP surfaces (>300 °C), significantly expanding the temperature window for device processing. Such effective surface passivation is a key enabling technology for InP nanowire devices such as nanolasers and solar cells.

Ultralow Surface Recombination Velocity in Passivated InGaAs/InP Nanopillars.

The III-V semiconductor InGaAs is a key material for photonics because it provides optical emission and absorption in the 1.55 µm telecommunication wavelength window. However, InGaAs suffers from pronounced nonradiative effects associated with its surface states, which affect the performance of nanophotonic devices for optical interconnects, namely nanolasers and nanodetectors. This work reports the strong suppression of surface recombination of undoped InGaAs/InP nanostructured semiconductor pillars using a combination of ammonium sulfide, (NH4)2S, chemical treatment and silicon oxide, SiOx, coating. An 80-fold enhancement in the photoluminescence (PL) intensity of submicrometer pillars at a wavelength of 1550 nm is observed as compared with the unpassivated nanopillars. The PL decay time of ∼0.3 µm wide square nanopillars is dramatically increased from ∼100 ps to ∼25 ns after sulfur treatment and SiOx coating. The extremely long lifetimes reported here, to our knowledge the highest reported to date for undoped InGaAs nanostructures, are associated with a record-low surface recombination velocity of ∼260 cm/s. We also conclusively show that the SiOx capping layer plays an active role in the passivation. These results are crucial for the future development of high-performance nanoscale optoelectronic devices for applications in energy-efficient data optical links, single-photon sensing, and photovoltaics.

Plasma-assisted atomic layer deposition of conformal Pt films in high aspect ratio trenches.

To date, conventional thermal atomic layer deposition (ALD) has been the method of choice to deposit high-quality Pt thin films grown typically from (MeCp)PtMe3 vapor and O2 gas at 300 °C. Plasma-assisted ALD of Pt using O2 plasma can offer several advantages over thermal ALD, such as faster nucleation and deposition at lower temperatures. In this work, it is demonstrated that plasma-assisted ALD at 300 °C also allows for the deposition of highly conformal Pt films in trenches with high aspect ratio ranging from 3 to 34. Scanning electron microscopy inspection revealed that the conformality of the deposited Pt films was 100% in trenches with aspect ratio (AR) up to 34. These results were corroborated by high-precision layer thickness measurements by transmission electron microscopy for trenches with an aspect ratio of 22. The role of the surface recombination of O-radicals and the contribution of thermal ALD reactions is discussed.