Surface Oxide Removal Research Articles

Mechanistic Insights into the Reduction of CO2 on Tin Electrodes using in Situ ATR-IR Spectroscopy

The reduction of CO2 on tin cathodes was studied using in situ attenuated total reflectance infrared spectroscopy (ATR-IR). Thin films of a mixed Sn/SnOx species were deposited onto a single-crystal ZnSe ATR crystal. Peaks centered at about 1500, 1385, and 1100 cm–1, attributed to a surface-bound monodentate tin carbonate species, were consistently present under conditions at which CO2 reduction takes place. It was shown that these peaks are only present at potentials where CO2 reduction is observed. Moreover, these peaks disappear if the pH of the reaction is too low or if the tin surface is chemically etched to remove surface oxide. Sn6O4(OH)4 and SnO2 nanoparticles were shown to be catalytically active for CO2 reduction, and insights into the oxidation state of the catalytically active species are gained from a comparison of the catalytic behavior of the two nanoparticle species. From these experiments, a mechanism governing the reduction of CO2 on tin electrodes is proposed.

CH4/Ar/H2/SF6 Plasma Etching for Surface Oxide Removal of Indium Bumps

Plasma etching for surface indium oxide removal by methane/argon/hydrogen/sulfur hexafluoride (CH4/Ar/H2/SF6) mixture has been implemented. The morphology of the indium bumps was not deteriorated after the plasma etching. High-resolution O 1s x-ray photoelectron spectroscopy (XPS) proved that the In–O component decreased from 44.5% for the nonetched sample to 10.8% for the sample after plasma etching. The surface modification of the indium bumps might be in the form of doped fluorine according to the XPS results. The zero-bias resistance derived from current–voltage (I–V) measurements for plasma-etched infrared detectors was comparable to that for nonetched ones, indicating that such plasma treatment is suitable for processing sensitive materials such as mercury cadmium telluride.

Effects of different CdCl2 annealing methods on the performance of CdS/CdTe polycrystalline thin film solar cells

In this paper, the effects of different CdCl2 annealing methods, including vapor annealing and dip-coating annealing, on the performance of CdS/CdTe polycrystalline thin-film solar cells are studied. After annealing, the samples are lightly etched with 1% bromine in methanol to remove surface oxides. Both annealing methods give CdTe polycrystalline thin films with good crystallinity and complete structure. For solar cells containing the annealed CdTe films, cell efficiency first increases and then decreases as the concentration of CdCl2 solution used for dip-coating annealing increases, and the optimized CdCl2 concentration is 12%. The uniformity of the performance of all cells is analyzed by calculating the relative standard deviation for each parameter. The uniformity of cell performance can be improved dramatically by dip-coating annealing instead of vapor annealing. Most notably, an appropriate concentration of CdCl2 (12%) acts as a protective layer that is conducive to realizing uniform high-performance CdS/CdTe solar cells. According to the location of depletion regions, the CdTe films treated by dip-coating annealing show a relatively low doping concentration, except for the sample treated with a CdCl2 concentration of 6%, which is consistent with the changes of short-circuit current density of the cells. It is believed that these results can be applied to the large-scale production of CdTe polycrystalline thin-film solar cells.

Comparison of different chelating agents to enhance reductive Cr(VI) removal by pyrite treatment procedure

New technologies involving in-situ chemical hexavalent chromium [Cr(VI)] reduction to trivalent chromium [Cr(III)] with natural Fe(II)-containing minerals can offer viable solutions to the treatment of wastewater and subsurface systems contaminated with Cr(VI). Here, the effects of five different chelating agents including citrate, EDTA, oxalate, tartrate and salicylate on reductive Cr(VI) removal from aqueous systems by pyrite were investigated in batch reactors. The Cr(VI) removal was highly dependent on the type of ligand used and chemical conditions (e.g., ligand concentration). While salicylate and EDTA had no or little effect on Cr(VI) removal, the ligands including citrate, tartrate and oxalate significantly enhanced Cr(VI) removal at pH < 7 relative to non-ligand systems. In general, the efficiency of organic ligands on Cr(VI) removal decreased in the order: citrate ≥ oxalate ≈ tartrate > EDTA > salicylate ≈ non-ligand system. Organic ligands enhanced Cr(VI) removal by 1) removing surface oxide layer via the formation of soluble Fe–Cr-ligand complexes, and 2) enhancing the reductive iron redox cycling for the regeneration of new surface sites. While citrate, oxalate and tartrate eliminated the formation of surface Cr (III)–Fe(III)-oxides, the surface phase Cr (III) species was observed in the presence of EDTA and salicylate indicating that Cr(III) complexed with EDTA and salicylate sorbed or precipitated onto pyrite surface, thereby blocking the access of CrO42− to pyrite surface. The binding of Fe(III) with the disulfide reactive sites (≡Fe–S–S–Fe(III)) was essential for the regeneration of new surface sites through pyrite oxidation. Although Fe(III)–S species was detected at the pyrite surface in the presence of citrate, oxalate and tartrate, Fe(III) complexed with EDTA and salicylate did not strongly interact with the disulfide reactive sites due to the formation of non-sorbing Fe(III)-ligand complexes. The absence of surface Fe(III)–S species indicated that no new reactive sites were generated through Fe redox cycling in the presence of salicylate and EDTA.

Enhanced pressure-free bonding using mixture of Cu and NiO nanoparticles

Pressure-free bonding using mixed Cu–NiO nanoparticles was examined. The effect of mixing NiO nanoparticles with Cu nanoparticles on bonding strength was identified for a NiO content between 0.005 and 2wt%. The bonding layer, which presented a high bonding strength, was thin and dense. The Ni in the bonding layer was segregated at the surface and at grain boundaries of crystalline Cu grains as a metallic Cu–Ni alloy. These results suggest that the effect of the NiO addition is caused by the enhancement of sintering by surface diffusion in the bonding layer, resulting from sufficient removal of surface oxides on the Cu nanoparticles through their reaction with the NiO nanoparticles.

Wet-Chemical Approaches for Atomic Layer Etching of Semiconductors: Surface Chemistry, Oxide Removal and Reoxidation of InAs (100)

An approach for wet-chemical atomic layer etching (WALE) of semiconductors is described. The surface chemistry of InAs was investigated for HCl/H2O2 solutions suitable for controlled etching in the low etch rate range (<0.1–10 nm min−1). Kinetic studies were performed using inductively coupled plasma – mass spectrometry (ICP-MS). As for GaAs and InGaAs, the importance of the Cl− ion for the etching kinetics is demonstrated and a chemical reaction scheme is presented to help understand the surface chemistry. A detailed study of an alternative two-step etching process was performed. A quantitative ICP-MS analysis of the oxide formed in O3/H2O solution and the dissolution in HCl was performed suggesting that the removal of oxidized In products is the slow step in the dissolution reaction. The reoxidation of oxide-free InAs (100) surfaces in air is discussed. The etch rate range and the surface morphology control after etching show that the investigated wet-chemical approach for atomic-layer etching is a valid candidate for advanced CMOS processing.

ICP-RIE etching of polar, semi-polar and non-polar AlN: comparison of Cl2/Ar and Cl2/BCl3/Ar plasma chemistry and surface pretreatment

We report a comprehensive investigation of inductively-coupled plasma reactive ion etching (ICP-RIE) of polar (0001) c-plane, semi-polar (11–22) and non-polar (11–20) a-plane AlN epilayers and show that under optimized conditions a combination of BCl3-based surface oxide removal pretreatment and Cl2/Ar ICP etching allows fast etch rates (750 nm min−1) with a smooth surface morphology. We compare samples of different orientation etched in Cl2/Ar and Cl2/BCl3/Ar plasmas, with and without BCl3/Ar ICP pretreatment, and show that the effective removal of surface oxide is a crucial step for reliable ICP-RIE etching of AlN layers. For such pretreated samples, optimization of etch parameters such as RF power, ICP power, and chamber pressure then permit very high etch rates to be obtained with a smooth surface morphology. We also study the effect of varying the BCl3 fraction in BCl3/Cl2/Ar plasmas on the etch rate and surface morphology and find that increasing the BCl3 fraction reduces the etch rate for AlN. However, above 20% BCl3 content, samples with and without pre-treatment show similar etch rates.

Ionic liquid based approach for single-molecule electronics with cobalt contacts.

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane(di)thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions.

Fabrication of Polycrystalline Si Films by Silicide-Enhanced Rapid Thermal Annealing and Their Application to Thin Film Transistors

Amorphous (a-Si) films were epitaxially crystallized on a very thin large-grained poly-Si seed layer by a silicide- enhanced rapid thermal annealing (SERTA) process. The poly-Si seed layer contained a small amount of nickel silicide which can enhance crystallization of the upper layer of the a-Si film at lower temperature. A 5-nm thick poly-Si seed layer was then prepared by the crystallization of an a-Si film using the vapor-induced crystallization process in a NiCl2 environment. After removing surface oxide on the seed layer, a 45-nm thick a-Si film was deposited on the poly-Si seed layer by hot-wire chemical vapor deposition at 200 o C. The epitaxial crystallization of the top a-Si layer was performed by the rapid thermal annealing (RTA) process at 730 o C for 5 min in Ar as an ambient atmosphere. Considering the needle-like grains as well as the crystallization temperature of the top layer as produced by the SERTA process, it was thought that the top a-Si layer was epitaxially crystallized with the help of NiSi2 precipitates that originated from the poly-Si seed layer. The crystallinity of the SERTA processed poly-Si thin films was better than the other crystallization process, due to the high-temperature RTA process. The Ni concentration in the poly-Si film fabricated by the SERTA process was reduced to 1 × 10 18 cm �3 . The maximum field- effect mobility and substrate swing of the p-channel poly-Si thin-film transistors (TFTs) using the poly-Si film prepared by the SERTA process were 85 cm 2 /V·s and 1.23 V/decade at Vds = �3 V, respectively. The off current was little increased under reverse bias from 1.0 × 10 �11 A. Our results showed that the SERTA process is a promising technology for high quality poly- Si film, which enables the fabrication of high mobility TFTs. In addition, it is expected that poly-Si TFTs with low leakage current can be fabricated with more precise experiments.

Low-Temperature Solid-State Bonding Using Hydrogen Radical Treated Solder for Optoelectronic and MEMS Packaging

Many optoelectronic and MEMS devices require hermetic sealing for protection of the optoelectronic elements and movable fragile parts to ensure stable and reliable performance characteristics. Most commonly used bonding techniques for hermetic sealing are glass-frit bonding, anodic bonding, solder bonding, and eutectic bonding. For many applications, low temperature bonding is desirable because the temperature sensitive materials or components degrade and thermally induced stresses can be created during bonding. Furthermore, since the hermetic sealing is typically the final processing step in device fabrication, bonding method that is tolerant to surface roughness is desirable.In this study, a low-temperature solid-state bonding process at bonding temperature lower than 200 °C using hydrogen radical treated Sn-3.0Ag-0.5Cu (wt%) solder without flux is investigated for potential application as hermetic packaging. The interfacial reaction between solder and Au is studied by means of scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The bonding quality is evaluated by means of die-shear test and leak rate test.The re-oxidation of hydrogen radical treated Sn was studied by X-ray photoelectron spectroscopy (XPS). Fig. 1 shows the typical XPS spectra of Sn plate (purity: 5N) treated with hydrogen radicals (a) and argon fast atom beam (FAB) (b). The XPS Sn (3d2/5) spectra have been used to follow the surface oxide growth on Sn surfaces as a function of exposure time to air after removal of surface oxide. Two peaks are due to the presence of Sn metal and SnOx. The surface of Sn plate treated with Ar FAB contained a large amount of SnOxthan that with hydrogen radicals after exposure time to air of over 20 h. These results indicate differences between the removal of surface oxide by chemical reaction of hydrogen radicals and that by physical bombardment using fast atom beams. The inhibitory effect of re-oxidation of hydrogen radical treated solder [1] was confirmed by XPS measurement.Fig. 2 shows the process flow of the low-temperature solid-state bonding. Sn-Ag-Cu solder paste (melting point: 217 °C) is deposited by screen printing technique. Then, the surface oxides of Sn-Ag-Cu particles in the paste are reduced by hydrogen radical treatment at 180 ºC. After that, the solder is reflowed at 230 °C for 90 s with additional hydrogen radical treatment for 3 sec. During the reflow process, the Sn-Ag-Cu particles completely coalesce and solder pattern is formed on the wettable metal parts of the substrate [2]. Then, the Sn-Ag-Cu patterns are exposed to air and bonded with Au films without flux at temperatures below the melting point.Successful bonding was obtained at a bonding temperature above 150 °C for 10 min. Fig. 3 shows the SEM image and EDX map of the cross section of the bonded interface under the application of a bonding pressure of 150 MPa and held at 170 °C for 30 min. The Sn-Ag-Cu solder pattern was deformed to achieve enough contact with Au thin film (thickness: 500 nm) and Au-Sn intermetallic compounds were formed during bonding process as shown in Fig. 3(b). No clear interface corresponding to the original Au surfaces is visible. To evaluate the bonding strength, die-shear test was performed and the die shear strength was over 30 MPa. Fracture mainly occurred in the inside of solder layer, which indicates that solder layer was well bonded with Au.The Sn-Ag-Cu sealing ring pattern (width: 100 μm, size: 2.4 mm × 2. 4 mm, thickness: 5-10 μm) was formed on a glass substrate (2.8 × 2.8 mm2) by hydrogen radical reflow process for hermetic leak rate testing. The cavity structure (cavity volume: 1.9 × 10-3 cm3) with Ti (thickness: 50 nm)/Au (thickness: 500 nm) sealing ring was also fabricated on a Si substrate. Hermeticity was evaluated by measuring the resonance characteristics of photothermally excited microcantilevers inside the cavity [3]. The samples bonded at the low temperature of 170 °C under the application of a bonding pressure of 150 MPa showed leakage rates of less than 1.0 × 10−9 Pa·m3 s−1, which is the rejection limit defined by the MIL-STD-883G specification.In conclusion, hydrogen radical treated Sn-Ag-Cu solder patterns were bonded with Au thin films at a lower temperature than the melting point of Sn-Ag-Cu without flux. This process is expected to be applicable for chip-size packaging of optoelectronic and MEMS devices.

Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications

The cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.

Barrier height enhancement of Ni/GaN Schottky diode using Ru based passivation scheme

Wet chemical passivation of n-GaN surface using Ru based solution has been reported. X-ray photoelectron spectroscopy characterization of the GaN surface revealed removal of surface oxides by the introduction of Ru complex species. Ni/n-GaN Schottky barrier diodes were fabricated on passivated GaN and a remarkable improvement in Schottky barrier height from 0.76 eV to 0.92 eV was observed.

Heat Induced Passivation of CuInSe2 Surfaces: A Strategy to Optimize the Efficiency of Chalcopyrite Thin Film Solar Cells?

Despite the success of chalcopyrite thin film solar cells, many open questions are related to the complex defect physics at the interface between the n‐type window layer and the p‐type chalcopyrite absorber, which largely determines the device efficiency. Therefore, this study aims to clarify the defect physics of chalcopyrite thin film surfaces, which is investigated by scanning tunneling spectroscopy, photoelectron, and inverse photoelectron spectroscopy. After removing surface oxides by a wet chemical KCN treatment and subsequent annealing at 280 °C in ultrahigh vacuum, a complete passivation of defect levels is observed, which goes along with a type inversion and an enlarged band gap at the surface. Therefore, this sample state consolidates three exclusively beneficial properties, which potentially minimize interface recombination losses and increase the open circuit voltage in completed devices. In contrast, oxidation of the surface by annealing in air reduces the surface band bending and creates a high density of charge compensated defect levels, implying exclusively detrimental effects on the device performance due to the presence of oxygen at the window/absorber heterojunction. These results are discussed in view of previous models suggesting a passivation of defect levels upon oxygenation of the interface.

Galvanic Displacement of Gallium Arsenide Surface: A Simple and Low-Cost Method to Deposit Metal Nanoparticles and Films

Herein, we report galvanic displacement of metal nanoparticles and films onto single-crystalline GaAs (100) substrates, a simple and cost-effective method to fabricate highly controlled metal/semiconductor interface. A time-resolved surface analysis of Au/GaAs system was conducted and microscopic mechanism of galvanic displacement was elucidated in detail. Quantitative temporal XPS measurements of the Au/GaAs interface showed that, initially, fast Au growth was slowed down as the deposition process proceeded. This was attributed to growing oxide layer blocking hole conduction and causing quenching of the deposition process. Addition of various inorganic acids, which function as oxide etchants, was found to enhance deposition rates by effectively removing surface oxide, with HF the most effective. Various precious metals, such as Pt and Ag, could be deposited onto GaAs through galvanic displacement, which demonstrates the versatility of the method.

Identification of photo-induced spin-triplet recombination centers situated at Si surfaces and Si/SiO2 interfaces

A recombination center at Si surfaces and Si/SiO2 interfaces was identified using highly sensitive spin-dependent recombination (SDR) detection of electron paramagnetic resonance (EPR). The defect (Si-KU1) has a spin-triplet (S = 1) state owing to excitation by band-edge light employed in measurement. The Si-KU1 SDR-EPR spectrum exhibits orthorhombic symmetry along two equivalent ⟨110⟩ axes running parallel to the interface, which disappears upon mechanical lapping of the surface oxide or heat treatment above 400 °C. However, removal of surface oxide with diluted HF solution sharpens the Si-KU1 SDR-EPR lines. A plausible structural model of the center responsible for Si-KU1 is presented.

Inductively coupled plasma–reactive ion etching of c- and a-plane AlGaN over the entire Al composition range: Effect of BCl3 pretreatment in Cl2/Ar plasma chemistry

Inductively coupled plasma (ICP)–reactive ion etching (RIE) patterning is a standard processing step for UV and optical photonic devices based on III-nitride materials. There is little research on ICP-RIE of high Al-content AlGaN alloys and for nonpolar nitride orientations. The authors present a comprehensive study of the ICP-RIE of c- and a-plane AlGaN in Cl2/Ar plasma over the entire Al composition range. The authors find that the etch rate decreases in general with increasing Al content, with different behavior for c- and a-plane AlGaN. They also study the effect of BCl3 deoxidizing plasma pretreatment. An ICP deoxidizing BCl3 plasma with the addition of argon is more efficient in removal of surface oxides from AlxGa1−xN than RIE alone. These experiments show that AlxGa1−xN etching is affected by the higher binding energy of AlN and the higher affinity of oxygen to aluminum compared to gallium, with oxides on a-plane AlGaN more difficult to etch as compared to oxides on c-plane AlGaN, specifically for high Al composition materials. The authors achieve reasonably high etch rate (∼350 nm/min) for high Al-content materials with a smooth surface morphology at a low DC bias of ∼−45 VDC.

Formation of low resistance ohmic contacts in GaN-based high electron mobility transistors with BCl3 surface plasma treatment

A BCl3 surface plasma treatment technique to reduce the resistance and to increase the uniformity of ohmic contacts in AlGaN/GaN high electron mobility transistors with a GaN cap layer has been established. This BCl3 plasma treatment was performed by an inductively coupled plasma reactive ion etching system under conditions that prevented any recess etching. The average contact resistances without plasma treatment, with SiCl4, and with BCl3 plasma treatment were 0.34, 0.41, and 0.17 Ω mm, respectively. Also, the standard deviation of the ohmic contact resistance with BCl3 plasma treatment was decreased. This decrease in the standard deviation of contact resistance can be explained by analyzing the surface condition of GaN with x-ray photoelectron spectroscopy and positron annihilation spectroscopy. We found that the proposed BCl3 plasma treatment technique can not only remove surface oxide but also introduce surface donor states that contribute to lower the ohmic contact resistance.

The effect of (NH4)2Sx passivation on the (311)A GaAs surface and its use in AlGaAs/GaAs heterostructure devices

We have studied the efficacy of (NH4)2Sx surface passivation on the (311)A GaAs surface. We report XPS studies of simultaneously-grown (311)A and (100) heterostructures showing that the (NH4)2Sx solution removes surface oxide and sulfidizes both surfaces. Passivation is often characterized using photoluminescence measurements; we show that while (NH4)2Sx treatment gives a 40–60 × increase in photoluminescence intensity for the (100) surface, an increase of only 2–3 × is obtained for the (311)A surface. A corresponding lack of reproducible improvement in the gate hysteresis of (311)A heterostructure transistor devices made with the passivation treatment performed immediately prior to gate deposition is also found. We discuss possible reasons why sulfur passivation is ineffective for (311)A GaAs, and propose alternative strategies for passivation of this surface.

Effects of surface treatment on the bonding quality of wafer-level Cu-to-Cu thermo-compression bonding for 3D integration

Various surface treatments are applied for surface oxide removal prior to wafer-level Cu-to-Cu thermo-compression bonding and the bonding quality is systematically analyzed in this work. Three methods are investigated: self-assembled monolayer (SAM) passivation, forming gas annealing and acetic acid wet cleaning. The surface conditions are carefully examined including roughness, contact angle and x-ray photoelectron spectroscopy (XPS) scan. The wafer pairs are bonded at 250 °C under a bonding force of 5500 N for a duration of 1 h in a vacuum environment. The bonding medium consists of a Cu (300 nm) bonding layer and a Ti (50 nm) barrier layer. The bonding quality investigation consists of two parts: hermeticity based on helium leak test and mechanical strength using four-point bending method. Although all samples under test with different surface treatment methods present an excellent hermetic seal and a robust mechanical support, the measurement results show that samples bonded after SAM passivation exhibit the best hermeticity and bonding strength for 3D integration application.

XPS Analysis of AlGaN/GaN Surface after Chemical and N-Containing Plasma Treatments

The effects of chemical treatment and nitride based plasma on an AlGaN/GaN heterostructure surface are investigated by XPS analyses. Different chemical treatments were evaluated in order to reduce the surface oxide, while N-based plasma treatments such as in-situ N2 and NH3, as well as decoupled forming gas plasmas were tested for nitridation effect on the surface. A 10% NH4OH solution proved to be the most efficient in removing surface oxides, with residual Ga oxides as low as 3% and Al oxides below the XPS detection limit. In terms of nitridation, a 500W decoupled forming gas plasma at 150{degree sign}C and a 50W N2 in-situ plasma at 350{degree sign}C increased drastically the N-presence at the surface, up to three times its original level. The nitride plasma also proved to be efficient in removing overall carbon contamination, as well as diminishing the surface rugosity.