Formation Of Native Oxide Research Articles

Fluorocarbon-Capped Aluminum Nanoparticles: Synthesis, Characterization, Combustion, and Comparison to Hydrocarbon-Capped Aluminum Nanoparticles

ABSTRACT Aluminum nanoparticles (NPs) are attractive as fuel additives in propulsion or pyrotechnic applications, but their combustion is inhibited by native oxide layer formation, which also reduces the energy content. Capping Al NPs with fluorocarbon ligands may inhibit native oxide formation and create an intimate mixture of fuel with an oxidizer capable of reacting exothermically with both Al and the passivating oxide layer. We present an approach to producing Al NPs capped by perfluorotetradecanoic acid (PFTDA), based on rapid mechanical attrition of NPs followed by PFTDA capping under mild conditions. The materials were characterized by dynamic light scattering, electron microscopy with energy dispersive spectroscopy, elemental analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and infrared spectroscopy. Ignition and combustion properties were studied using bomb calorimetry, thermogravimetric analysis with differential scanning calorimetry, and photographic analysis of both flame and laser ignition of particle samples. For comparison, Al NPs were produced identically but capped with a hydrocarbon (oleic acid). PFTDA-capped NPs were found to ignite at lower laser power and burn far more vigorously than oleic acid-capped NPs, despite the observation that oleic acid capping resulted in a thinner native oxide layer on the air-exposed NPs. PFTDA-capped NPs were also found to have shorter ignition delay and lower ignition energy compared to commercially available Al NPs. XPS and XRD were used to probe the combustion products for the PFTDA-capped NPs, confirming formation of AlF3 and other products. Density functional theory was used to calculate the heat of formation of PFTDA, which was then used to calculate reaction enthalpies for reactions in both inert and oxidizing environments.

Native Oxidation of HgCdTe Compound Semiconductors and Its Impact on Infrared Detectors Performances

HgCdTe surface processing and exposition to the atmospheric environment promote the formation of surface oxides, which have been reported to influence the electrical behavior of devices. This work investigates the formation of HgCdTe of air native oxides on epitaxial Hg(1-x)Cd(x)Te layers after a standard wet etching procedure (x ≃ 0.2 to 0.7). Real-Time Spectroscopic Ellipsometry (RTSE) studies were performed to monitor initial native oxide formation in a clean-room environment and revealed a two-stage kinetic of oxide formation. X-ray Photoelectron Spectroscopy (XPS) was used to assess the evolution of the surface chemistry. After etching, the surfaces exhibited an excess of tellurium that oxidized during early oxide formation (< 45min). Longer air exposures resulted in oxide thicknesses correlated to the alloy composition, indicating a cadmium contribution to further oxidize the material. The combination of techniques allowed the investigation of the formation mechanisms surrounding the native oxide of wet-etched HgCdTe surfaces.

In-situ deposited anti-aging TiN capping layer for Nb superconducting quantum circuits

Abstract The performance of Nb superconducting quantum devices is predominantly limited by dielectric loss at the metal–air interface, where Nb2O5 is considered the main loss source. Here, we suppress the formation of native oxides by in-situ deposition of a TiN capping layer on the Nb film. With TiN capping layers, no Nb2O5 forms on the surface of the Nb film. The quality factor Q i of the Nb resonator increases from 5.6 × 105 to 7.9 × 105 at low input power and from 6.8 × 106 to 1.1 × 107 at high input power. Furthermore, the TiN capping layer also shows good aging resistance in Nb resonator devices, with no significant performance fluctuations after one month of aging. These findings highlight the effectiveness of TiN capping layers in enhancing the performance and longevity of Nb superconducting quantum devices.

Nanometer-thick molecular beam epitaxy Al films capped with in situ deposited Al2O3—High-crystallinity, morphology, and superconductivity

Achieving high material perfection in aluminum (Al) films and their associated Al/AlOx heterostructures is essential for enhancing the coherence time in superconducting quantum circuits. We grew Al films with thicknesses ranging from 3 to 30 nanometers (nm) epitaxially on sapphire substrates using molecular beam epitaxy (MBE). An integral aspect of our work involved electron-beam (e-beam) evaporation to directly deposit aluminum oxide (Al2O3) films on the freshly grown ultrathin epitaxial Al films in an ultra-high-vacuum (UHV) environment. This in situ oxide deposition is critical for preventing the oxidation of parts of the Al films, avoiding the formation of undesired native oxides, and thereby preserving the nm-thick Al films in their pristine conditions. The thicknesses of our Al films in the study were accurately determined; for example, coherence lengths of 3.0 and 20.2 nm were measured in the nominal 3.0 and 20 nm thick Al films, respectively. These Al films were epitaxially grown on sapphire substrates, showing an orientational relationship, denoted as Al(111)⟨21¯1¯⟩∥sapphire(0001)[21¯1¯0]. The Al/sapphire interface was atomically ordered without any interfacial layers, as confirmed by in situ reflection high-energy electron diffraction (RHEED) and cross-sectional scanning transmission electron microscopy (STEM). All sample surfaces exhibited smoothness with a roughness in the range of 0.1–0.2 nm. The Al films are superconducting with critical temperatures ranging from 1.23 to around 2 K, depending on the film thickness.

Material Changes of Bimetallic Ag–Cr, Fe, Co, Ni, Cu, Sn Electrocatalysts during Alkaline Oxygen Reduction in Fundamental Versus Alkaline Membrane Exchange Fuel Cell Conditions

The more apparent challenges of climate change drastically increase the global demand for sustainable fuel production and usage.1 In this context, hydrogen fuel cells (FCs) powered by renewable electricity are among the promising technologies.2 Addressing the sluggish cathode reaction in FCs, the oxygen reduction reaction (ORR), these devices hold immense potential for sustainable energy conversion and storage. An alkaline environment allows for the utilization of more abundant and cost-effective metals in contrast to current platinum-based catalysts. Silver (Ag), for instance, exhibits high selectivity for the ORR, i.e., forming hydroxide instead of peroxide.3 When combined with secondary metals, bimetallic Ag catalysts show enhanced activity, making them viable alternatives to platinum-group materials for ORR.In our highly collaborative research, we investigated the material transformations of bimetallic Ag thin films with chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and tin (Sn) as secondary metals for ORR electrocatalysis in fundamental rotating disk electrode (RDE) setups and the application device, anion-exchange membrane FC (AEMFC). Grazing-incidence X-ray diffraction (GI-XRD) conducted prior to electrochemical testing reveals no evidence of alloying except for the Ag-Sn bimetallic thin film. However, X-ray photoelectron spectroscopy (XPS) indicates a shift of the Ag 3d peaks to higher binding energies for all catalysts, suggesting a hybridization of Ag and the secondary metals. The composition of the metal surface, native oxide formation, ORR performance, and material changes post-RDE testing vary distinctly among the Ag bimetallic catalysts depending on the secondary metal. Intriguingly, performance is either enhanced (Ag-Co, Ag-Sn), maintained with reduced Ag content (Ag-Fe, Ag-Cr), or falls in between the two monometallic materials (Ag-Ni) compared to monometallic Ag thin films. Density functional theory (DFT) calculations offer insights into the catalyst performances and changes in metal oxidation states and surface composition prior and post electrocatalysis. AEMFC tests reveal promising performances under device conditions contradicting our fundamental RDE measurements. The integration of fundamental and application device tests, along with material characterization and DFT calculations, enables a comprehensive exploration of material changes under varying electrochemical conditions. This underscores the complexity of bimetallic catalysts and the importance of testing under device conditions, positioning Ag-bimetallic catalysts as highly promising alternatives to platinum-group metal catalysts for alkaline ORR.

Optoelectronic properties of the Sb/III-Sb interface induced by laser photooxidation

Laser photooxidation is used to form a surface antimony layer on p-type GaSb and GaAs0.06Sb0.94. The emerged Sb/III-Sb interface is studied by a combination of the Raman and photoluminescence spectroscopy supported by Kelvin probe microscopy and atomic force microscopy. The laser power density controls the thickness and structure of the Sb layer. Laser photooxidation thickens the amorphous Sb layer that emerged after native oxide formation. The thickening decreases the bandgap photoluminescence intensity and increases surface band bending. Further increase in the laser power density forms a multilayered antimonene phase with a rhombohedral structure. The crystalline Sb layer decreases the surface band bending due to changes in the work function. For the crystalline layer, the PL intensity further decreases. The effective work function model explains the experimental results. Finally, the study shows that it is possible to modify surface optoelectronic properties with a submicrometer lateral resolution.

Electron Microscopy Investigation of Porous Transport Layer Coatings

The flow of reactants and biproducts to and from the anode in polymer electrolyte membrane (PEM) water electrolyzers is dictated by the properties of the porous transport layer (PTL). The PTL is made of titanium (Ti) fibers or sintered particles to provide stable thermal and electronic conduction pathways at the high overpotentials and acid conditions experienced at the anode. To prevent the formation of a thick native oxide on the Ti PTL surface, precious metal coatings are required. These coatings enable and maintain a reduced interfacial resistance between the PTL and catalyst layer. Understanding the impact of surface treatments and coatings on the morphology and composition of this interface is an important step to minimizing interfacial resistance, degradation and production costs. In this work, ion and electron beam techniques, including focused ion beam (FIB), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and scanning transmission electron microscopy (STEM), are used to investigate PTL coatings at the nanoscale.A series of fiber-type PTLs with a range of platinum (Pt) coating thicknesses were synthesized using physical vapor deposition. The uniformity of the coatings was analyzed using SEM-EDS in both plan-view and cross-section. Sections of individual fibers were then removed using FIB lift-out technique and thinned to less than 50 nanometers for further analysis by STEM. The thickness and uniformity of the Pt coating was quantified from STEM images for each Pt loading and correlated with interfacial resistance measurements. Changes to the thickness and composition of the coatings and oxide layers after single cell tests will also be reported alongside additional insights from correlative characterization approaches like atomic force microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. Overall, these findings will help steer the development of PTL treatments and coatings for highly durable and efficient PEM water electrolyzers towards meeting the U.S. Department of Energy’s Hydrogen Shot to reduce the cost of clean hydrogen production to $1 per kg by 2031.Funding for this work was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the H2NEW Consortium. Electron microscopy research conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.

Role of 5 wt.% Mg Alloying in Al on Corrosion Characteristics of Al-Mg Coating Deposited by Plasma Arc Thermal Spray Process.

The corrosion of steel structures in coastal areas is a major issue. Therefore, in the present study, the protection against the corrosion of structural steel is carried out by depositing 100 μm thick Al and Al-5 Mg coatings using a plasma arc thermal spray process, immersing them in 3.5 wt.% NaCl solution for 41 days (d). To deposit such metals, one of the best known processes, arc thermal spray, is frequently used, but this process has severe defects and porosity. Thus, to minimize the porosity and defects of arc thermal spray, a plasma arc thermal spray process is developed. In this process, we used normal gas to create plasma instead of argon (Ar) and nitrogen (N2) with hydrogen (H) and helium (He). Al-5 Mg alloy coating exhibited uniform and dense morphology, where it reduced more than four times the porosity compared to Al, where Mg fills the voids of the coating, resulting in greater bond adhesion and hydrophobicity. The open circuit potential (OCP) of both coatings exhibited electropositive values due to the formation of native oxide in Al, while in the case of Al-5 Mg, the coating is dense and uniform. However, after 1 d of immersion, both coatings showed activation in OCP, owing to the dissolution of splat particles from the corner where the sharp edges are present in the Al coating, while Mg preferentially dissolved in the Al-5 Mg coating and made galvanic cells. Mg is galvanically more active than Al in the Al-5 Mg coating. Due to the capacity of the corrosion products to cover the pores and defects, both coatings stabilized the OCP after 13 d of immersion. The total impedance of the Al-5 Mg coating is gradually increased and is higher than the Al, which can be attributed to the uniform and dense coating morphology where Mg dissolves and agglomerates to form globular corrosion products and deposit over the surface, thereby causing barrier protection. The defect bearing corrosion products on Al coating led to the cause having a higher corrosion rate than the Al-5 Mg coating. A total of 5 wt.% mg in the Al coating improved the corrosion rate by a rate of 1.6 times compared to the pure Al in the 3.5 wt.% NaCl solution after 41 d of immersion.

Pressure-Controlled Layer-by-Layer to Continuous Oxidation of ZrS2(001) Surface

Understanding oxidation mechanisms of layered semiconducting transition-metal dichalcogenides (TMDC) is important not only for controlling native oxide formation but also for synthesis of oxide and oxysulfide products. Here, reactive molecular dynamics simulations show that oxygen partial pressure controls not only the ZrS2 oxidation rate but also the oxide morphology and quality. We find a transition from layer-by-layer oxidation to amorphous-oxide-mediated continuous oxidation as the oxidation progresses, where different pressures selectively expose different oxidation stages within a given time window. While the kinetics of the fast continuous oxidation stage is well described by the conventional Deal-Grove model, the layer-by-layer oxidation stage is dictated by reactive bond-switching mechanisms. This work provides atomistic details and a potential foundation for rational pressure-controlled oxidation of TMDC materials.

Sequential Use of Orthogonal Self‐Assembled Monolayers for Area‐Selective Atomic Layer Deposition of Dielectric on Metal

AbstractAlthough there have been several demonstrations of area‐selective atomic layer deposition (AS‐ALD) of dielectric on dielectric in metal/dielectric patterns, the reverse process of selective dielectric on metal (DoM) is not as well developed due to the challenge of inhibiting only the dielectrics. Unavoidable native oxide formation on metals tends to lead to similar surface chemical properties between metal and dielectric substrates, decreasing the selectivity in inhibitor adsorption. Hence, to achieve DoM, preventing unwanted inhibitor adsorption on metals is critical. This study demonstrates a two‐step strategy of first applying a dodecanethiol (DDT) self‐assembled monolayer (SAM) on a Cu/SiO2 pattern to protect the Cu surfaces from subsequent deposition of an octadecyltrimethoxysilane (OTMS) inhibitor, which then selectively forms an OTMS SAM on SiO2. It is further shown that by removing the DDT protector with thermal treatment before AS‐ALD, subsequent ALD growth on Cu is not affected while ALD remains blocked on the OTMS‐covered SiO2 regions. Using this strategy, DoM is demonstrated with selectivity above 0.9 after 5.6 nm of ZnO and 1.5 nm of Al2O3 ALD. This work presents a new approach to expand the material systems available to AS‐ALD which may help enable more applications in microelectronics, optoelectronics, and energy.

Electronic work function modulation of phosphorene by thermal oxidation

In this study, we evaluate the variation of the work function of phosphorene during thermal oxidation at different temperatures. The ultraviolet photoelectron spectroscopy results show an N-shaped behaviour that is explained by the oxidation process and the dangling-to-interstitial conversion at elevated temperatures. The exfoliation degree and x-ray photoelectron spectroscopy confirm the formation of native oxides in the top-most layer that passivates the material. Ex-situ XPS reveals the full oxidation of monolayers at temperatures higher than 140 °C, but few-layer phosphorene withstands the thermal oxidation even up to 200 °C with slight modifications of the A 2 g/A 1 g and A 2 g/B 2g vibrational mode ratios and a weak fluorescence in the Raman spectra of the heat-treated samples.

(Invited) Interface Treatment Related Defects During Ge Gate Stack Formation

Germanium as a high mobility channel material is currently of considerable interest since Ge has both higher electron and hole mobility as compared to silicon (1). The deposition of high-k dielectrics on Ge has been a significant challenge because of high interface defects density, Dit (2). By considering the interface reaction kinetics, thermal budget various atomic layer deposition (ALD) schemes and interface treatments can be identified to get a stable dielectric with a robust interface. However, understanding of the interface kinetics and defect distribution into gate stack and/or Ge is still limited.In this work, we report the impact of different interface treatments on the p type Ge prior to any high-k layer deposition on defect generation. Ge surface was oxidized by a controlled chemical oxidation, slot-plane-antenna plasma oxidation and oxidation through vapor O3 treatment before ALD deposition of 1nm Al2O3/3.5nm ZrO2 for gate stack formation. These devices were characterized to evaluate the interface and possible bulk defects by deep level transient spectroscopy (DLTS), capacitance spectroscopy, conductance method in addition to standard I-V and C-V measurements. DLTS has the advantage to have the spectrum comparison among the interface treatments. Signature of these spectra can reveal the difference, which is more accurate than the defects estimated by Conductance method. DLTS also provides the signature of slow traps (~1ms) (Fig. 1) originating from the bulk or interface depending on the type of GeOx formation.Furthermore, we evaluate the defect distribution when the gate stack was subjected to a slot-plane antenna plasma oxidation prior to metal deposition. A TiN/Hf1-xZrxO2/Al2O3/Ge gate stack with six different Zr content was investigated (Fig. 2). Al2O3 was used to impede the Ge diffusion into the gate stack. Interface state density (Dit) and C-V hysteresis show that the mid-gap Dit tend to increase with decrease in EOT, possibly due to the undesirable native oxide formation on Ge.The author would like to thank Drs. Y.M. Ding (Western Digital Corporation, Shanghai, China), M.N. Bhuyian (Globalfoundries, Malta, NY, USA), K.L. Ganapathi (Indian Institute of Technology Madras, India), N. Bhat, (Indian Institute of Science, Bangalore, India) and K. Tapily, R. D. Clark, S. Consiglio, C. S. Wajda, G. Nakamura, and G. J. Leusink of TEL Technology Center, America, Albany, NY, for their help in this work. References Arimura et al, IEDM Technical Digest, (2020)N. Bhuyian et al, ECS Journal of Solid-St. Sci. and Tech., vol. 7(2), N1-N6, (2018)G. Kolla et al, J. Vac. Sci. Technol. B, 36(2), 021201 (2018)Ding et al, IEEE Trans. on Dev. and Materials Reliability, 17(2), 349-354, (2017)Ding, et al, J. of Vac. Sci. and Technol. B, 34(2), 021203, 2016. Figure 1

Unveiling oxidation mechanism of bulk ZrS2

Transition metal dichalcogenides have shown great potential for next-generation electronic and optoelectronic devices. However, native oxidation remains a major issue in achieving their long-term stability, especially for Zr-containing materials such as ZrS2. Here, we develop a first principles-informed reactive forcefield for Zr/O/S to study oxidation dynamics of ZrS2. Simulation results reveal anisotropic oxidation rates between (210) and (001) surfaces. The oxidation rate is highly dependent on the initial adsorption of oxygen molecules on the surface. Simulation results also provide reaction mechanism for native oxide formation with atomistic details.Graphic

Synthesis of model Al-Al2O3 multilayer systems with monolayer oxide thickness control by circumventing native oxidation

A hybrid thin film deposition system was built by combining physical vapor deposition (PVD) with atomic layer deposition (ALD) without breaking high vacuum, which was achieved by a chamber-in-chamber design. The vacuum is better than 1 × 10−5 Pa during the entire sample transfer process between the ALD and PVD chambers. The Al-Al2O3 multilayers synthesized by the PVD-ALD integrated system consist of ALD Al2O3 sublayers between pure PVD Al sublayers. The Al2O3 sublayers, with incremental thickness of 1 nm from 1 to 10 nm, effectively interrupted the grain growth of the ~250 nm thick Al sublayers. Native surface oxide formation on the PVD deposited pure Al sublayers was circumvented by keeping the substrate in high vacuum between processing steps. The pure Al sublayers are constituted of equiaxed grains with no epitaxial or texture relationship with the neighboring layers. The Al grain boundaries are parallel to the film growth direction and extend the height of each Al sublayer, to form a brick-and-mortar type microstructure. In this system, ALD allows precise control of the thickness of each ultrathin Al2O3 layer, to thicknesses below the native surface oxide thickness of pure Al, and avoids further natural oxidation through the low-oxygen environment of the PVD-ALD system.

Grafting Methyl Groups on the Si(111) Surface as a Buffer Layer for van der Waals Epitaxial Growth of ZnO Nanorods in Chemical Bath Deposition

The absence of an epitaxial relation of material growth with silicon substrates usually results from the formation of native oxide on the substrate surface, especially in the lengthy solution process at elevated temperature. An effective method to impede surface oxidation and the consequent inheritance of the lattice structure of the substrate are beneficial for the epitaxial growth. In this study, we use the two-step halogenation/methylation process to create a methylated Si(111) surface for the growth of ZnO nanorods in a chemical bath deposition (CBD) process. The grafted methyl groups effectively passivate the Si(111) surface, and the deposited ZnO nanorods exhibit an epitaxial relation with the silicon substrate—ZnO[0001]//Si[111]; ZnO{11̅00}//Si{22̅0}. Because no chemical bonding is formed between the ZnO nanorods and the Si(111) substrate, this solution process exhibits the characteristics of the van der Waals epitaxial (vdWE) growth. The approach developed in this study paves a way for vdWE growth of nanostructures on Si substrates by the CBD process.

Microcharacterization of Interface Oxide Layer on Laser-Structured Silicon Surfaces of Plated Ni–Cu Solar Cells

Light-induced plating on laser-structured passivation layer openings is an alternative method to silver screen printing for fabrication of the front-side metal grid on silicon solar cells. Fundamental contact properties, such as adhesion and contact resistance, are determined at the highly inhomogeneous interface of laser-structured silicon and plated nickel. Usage of laser structuring and plating holds the risk that an interfacial oxide layer is incorporated to the contact stack. It is shown within this article that both native and laser-induced oxide formation influence contact properties. Although, a native oxide layer does not impede metal deposition during plating, the dynamics of the layer formation is shown to be changed. It is known that interface oxides harm the contact resistance, and it is reported that postplating annealing decreases the contact resistance. In this article, the microstructural basis of the temperature-induced changes is examined. By transmission electron microscopy and energy dispersive X-ray spectroscopy, no temperature-induced modification of the interface microstructure, such as silicide formation or metal-induced crystallization of the amorphous surface, is observed. Results suggest that the decrease is attributed to a temperature-induced change of the electrical properties of the interface oxide. Considering reports that partially closed interface oxide layers are an inherent feature of laser-structured plated contacts, the results presented motivate reconsidering the objective of postplating annealing. The results indicate that the beneficial impact of silicide formation is overrated for the case of laser-structured plated contacts, whereas the temperature-induced modifications on residual interface oxide layers are neglected in the scientific discussions on the given contact stack.

Elucidating mechanisms of oxide growth and surface passivation on zinc thin film electrodes in alkaline solutions using the electrochemical quartz crystal microbalance

We utilized electrochemical quartz crystal microbalance to understand electrochemical passivation of the zinc electrode in alkaline media and subsequent oxide growth and stripping. The formation of native zinc oxide on a near pristine Zn surface upon contact with 1 M KOH electrolyte was observed. The surface was seen to roughen with time. During Zn dissolution, the measured mass-charge ratio corresponded to the removal of ZnO and not merely Zn. The mechanism of passivation was determined to be caused by electrochemical etching to form pores. The etching leads to a build-up of solid ZnO and Zn(OH)2 due to electrochemical reactions on the surfaces of the pores. These reactions functionally close the pores off from the bulk. The cycle of Zn deposition and dissolution develops a porous network, which may be the origin of mossy Zn deposits that plague reversible Zn metal batteries.

Resistivity scaling and electron surface scattering in epitaxial Co(0001) layers

In situ and ex situ transport measurements on epitaxial Co(0001)/Al2O3(0001) layers with thickness d = 7–300 nm are used to quantify the resistivity ρ scaling due to electron surface scattering. Sputter deposition at 300 °C followed by in situ annealing at 500 °C leads to single-crystal layers with smooth surfaces (&amp;lt;1 nm roughness) and an epitaxial relationship: Co[0001]‖Al2O3[0001] and Co[101¯0]‖Al2O3[112¯0]. The measured ρ vs d data are well described by the classical expression by Fuchs and Sondheimer at both 295 and 77 K, yielding a temperature-independent product of the bulk resistivity times the mean free path ρo × λ and an effective room temperature λ = 19.5 ± 1.0 nm. The resistivity increases by 9%–24% upon air exposure for layers with d ≤ 21 nm, indicating a transition from partially specular (p = 0.55 ± 0.05) to completely diffuse (p = 0) surface scattering during native oxide formation. The overall results suggest that Co exhibits a resistivity scaling that is comparable to W and approximately 2× smaller than that of Cu and that the resistance of narrow Co lines can be reduced considerably by engineering the Co-liner interface to facilitate specular electron scattering.

Surface smoothing and native oxide suppression on Zn doped aerotaxy GaAs nanowires

Aerotaxy, a recently invented aerosol-based growth method for nanostructures, has been shown to hold great promise in making III-V nanowires more accessible for cheap mass-production. Aerotaxy nanowire surface structure and chemistry, however, remains unexplored, which is unfortunate since this can influence (opto)electronic properties. We investigate the surfaces of aerotaxy grown GaAs nanowires using synchrotron based high resolution X-ray photoelectron spectroscopy and high resolution atomic force microscopy. We observe that increasing the concentration of the p-type dopant diethylzinc to very high levels during nanowire growth significantly changes the surface morphology and leads to a strong suppression of native surface oxide formation. Our findings indicate that up to 1.8 monolayers of Zn are present on the nanowire surface after growth. Finally, we find that this also influences the Fermi level pinning of the surface. We suggest that Zn present on the surface after growth could play a role in the strongly hindered oxidation of the III-V compound when exposed to air. The aerotaxy nanowires generally exhibit a round cross section, while a significant smoothening of the surface morphology along the nanowire appears for very high nominal doping levels likely as a result of slight reshaping during growth in the presence of Zn. Given that surface oxide and a rough morphology can be detrimental to nanowire electrical and optical performance, the ability to reduce them as a side effect of dopant introduction will benefit future applications. Finally, the observed hindering of oxidation during air transport can allow for reliable post-growth processing in separate systems.

Structural and optical properties of sulfur passivated epitaxial step-graded GaAs1-ySby materials

The impact of bulk and surface defect states on the vibrational and optical properties of step-graded epitaxial GaAs1-ySby (0 ≤ y ≤ 1) materials with and without chemical surface treatment by (NH4)2S was investigated. Tunable antimony (Sb) composition GaAs1-ySby epitaxial layers, grown by solid source molecular beam epitaxy (MBE), were realized on GaAs and Si substrates by varying key growth parameters (e.g., Sb/Ga flux ratio, growth temperature). Raman and photoluminescence (PL) spectroscopic analysis of (NH4)2S-treated GaAs1-ySby epitaxial layers revealed composition-independent Raman spectral widths and enhanced PL intensity (1.3×) following (NH4)2S surface treatment, indicating bulk defect-minimal epitaxy and a reduction in the surface recombination velocity corresponding to reduced surface defect sites, respectively. Moreover, quantification of the luminescence recombination mechanisms across a range of measurement temperatures and excitation intensities (i.e., varying laser power) indicate the presence of free-electron to neutral acceptor pair or Sb-defect-related recombination pathways, with detectable bulk defect recombination discernible only in binary GaSb PL spectra. In addition, PL analysis of the short- and long-term thermodynamic stability of sulfur-treated GaAs1-ySby/Al2O3 heterointerfaces revealed an absence of quantifiable atomic interdiffusion or native oxide formation. Leveraging the combined Raman and PL analysis herein, the quality of the heteroepitaxial step-graded epitaxial GaAs1-ySby materials can be optimized for optical devices.