Forming Gas Environment Research Articles

Thermal annealing ambiance effect on phosphorus passivation and reactivation mechanisms in silicon-based Schottky diodes hydrogenated by MW-ECR plasma

The main objective of this work is to investigate the effect of thermal annealing in forming gas atmosphere on the mechanism of deactivation and reactivation of phosphorus in silicon-based Schottky diodes. Firstly, the microwave plasma power, initial phosphorus concentration in the samples and hydrogen flux were fixed as 650 W, 1015 cm–3, and 30 sccm, respectively, to investigate the behavior of different working parameters of diodes, specifically the duration and temperature of hydrogenation. Secondly, few samples hydrogenated at 400 °C for 1 h were annealed under the forming gas (10% H2 + 90% N2) within the temperature range from 100 to 700 °C for 1 h. The profiles of active phosphorus concentration were monitored by evaluating the change in concentration of phosphorus after hydrogenation or thermal annealing in a forming gas environment through capacitance-voltage measurements. The obtained results depict the temperature and duration of hydrogenation, which ultimately reveals the complex behavior of phosphorous and hydrogen in silicon. However, the phosphorus passivation rate is homogeneous over all the depths measured at 400 °C. The thermal annealing in a forming gas indicates the increase in passivation rate of phosphorus as a function of annealing temperature, till the passivation rate attains saturation in the sample annealed at 400 °C. At higher temperatures, a decrease in the concentration of phosphorous-hydrogen complexes is observed due to the dissociation of these complexes and reactivation of phosphorus under thermal effect.

Substoichiometric Tuning of the Electronic Properties of Titania

Reduced titania phases such as Ti2O3, Ti3O5, and TixO2x-1 (the magńeli phase, 4 ≤ x ≤10) are collectively a promising class of materials due to the stoichiometric tunability of their electronic structure. This flexibility, combined with their relatively high electrical conductivity, high corrosion resistance, and photostability makes reduced titania a candidate for many applications. In this work, stable reduced titania films were synthesized on silicon substrates via pulsed laser deposition (PLD) under a forming gas environment at elevated temperatures. The atomic/electronic structure and the electrical properties of the PLD reduced titania films were investigated using four-point probe, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy (XAS), and ultraviolet photoelectron spectroscopy (UPS). The PLD films demonstrated a notable decrease in resistivity compared to those of stoichiometric TiO2 films reported in the literature. Raman spectroscopy and XAS revealed that the observed resistivities in these films are correlated to substoichiometric titania phases (Ti2O3 between 400-600 °C and the Ti3O5-like at 700 °C) present in them. UPS measurements on the sub-stoichiometric titania indicated a decrease in the work functions with a decrease in the titanium oxidation state. The ability to tune such electrical properties with careful control of preparation is key to using reduced titania as a flexible platform for material design.

Tunable Electrocatalytic Activity of Atomically Thin Film Platinum Catalyst Electrodeposited on Sub-Stoichiometric Titania

Platinum (Pt) is an essential catalyst for many reactions including, but not limited to, oxidation/reduction, hydrogenation, and water gas shift. Despite its common use, the major disadvantages of platinum lie within its high cost due to its low abundance, and propensity for deactivation through poisoning. One way to address this setback is to reduce its loading content in catalyst design. Recently, our group has successfully synthesized a wetted atomic layer- by- layer Pt on carbon-based and noble metal supports via the surface limited redox replacement mechanism. This methodological approach produces atomically thin platinum films with the unprecedented electronic tuning of the catalyst activity induced by the supporting material. Implementing transition metal oxides as the support with tuned electronic properties (such as work function) can dictate the activity of the surface catalyst atoms through vicinity effects. This research project focuses on the electrochemical deposition and structural characterization of atomically thin-filmed platinum supported on oxygen deficient titanium oxide (TixOy) using imaging and spectroscopic techniques. TixOy supports are used to alleviate corrosion issues caused by the Vulcan XC-72, a known carbon support that is commonly used in fuel cells. TixOy films of varying oxygen vacancies are synthesized via pulsed laser deposition at elevated substrate temperatures in a forming gas environment. Four-point probe measurements, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and ultraviolet photoelectron spectroscopy (UPS) are used to examine the chemical and electrical properties of the TixOy supports. It has been revealed that the films deposited at 400-600oC display a Ti2O3-like phase while the films deposited at 700oC revealed a Ti3O5-like phase. The resistivity of stoichiometric titania films was immeasurable; however, by increasing the substrate deposition temperatures the films became more conductive. UPS demonstrated deviations within the work function of the TiO2, Ti2O3, and Ti3O5-like films. Dissimilarities of the work function values demonstrate transition metal oxide tunability in the energy required for moving an electron from the surface of the solid to an overlying material. To implement the growth of the atomically thin-filmed Pt monolayers, a single layer of graphene is transferred onto TixOy supports prior to the electrochemical deposition of Pt. Pt monolayers were synthesized using surface limited redox replacement mechanism, a spontaneous technique which initially relies on the electrochemical deposition of a sacrificial metal. XPS confirmed the growth of Pt overlayers and the cyclic voltammogram confirmed the low loading of platinum. Future experimental studies are geared towards probing the catalytic activity and durability of the Pt catalyst as a function of varying oxygen concentrations within the TixOy support. The novel electrocatalysts alleviate some of the major problems within fuel cell technology by simultaneously reducing the amount of platinum catalyst needed and modifying its performance through support induced catalytic activity tunability.

Evolutions on surface chemistry, microstructure, morphology and electrical characteristics of SnO2/p-Si heterojuction under various annealing parameters

The influences of annealing temperature and ambient on surface chemistry, microstructure, morphology and electrical characteristics of Tin Oxide (SnO2)/Si heterojuctions are systematically identified. Metallic and oxygen deficient SnO2-x bonds have been consistently treated to stoichiometric SnO2 with annealing temperatures up to 600 °C in nitrogen environment. It has been observed that further increase in the annealing temperature causes the formation of the silicate layer. Presence of hydrogen in forming gas environment causes formation of the oxygen deficient SnO2-x, while the presence of the oxygen contributes to the formation of the stoichiometric SnO2 layer in oxygen annealing ambient. Parasitic phases significantly affect the crystalline structure of the films. Surface roughness of the samples decreases up to 1.02 nm which is strongly correlated with stress and strain on the film. On the other hand, oxidations of the dangling bonds and/or hydrogen diffusion cause variations in the total dipole moments which significantly change the capacitance. Depending on the annealing parameters, effective charge densities and interface trap densities decrease to 1011 cm−2, while the ideality factor and barrier potential are improved to 1.43 and 0.59 eV, respectively. Owing to the crystallization of the SnO2 layer, leakage current increase at low temperature annealing. However, passivation of the dangling bonds becomes more effective than phase crystallization at higher temperatures on the leakage characteristics. The current study shows that any changes in the surface chemistry directly affect the device performance.

Enhanced Visible Light Absorption of MoS(2-X)/TiO2/Ti Multilayered Heterojunction Photoelectrode

Design of efficient heterojunction photoelectrode for solar driven photoelectrochemical water splitting is one of the foremost challenges in the development of a solar hydrogen production system [1]. Representatively, n-type titanium dioxide (TiO2) has been extensively investigated as a promising photoelectrode material for photoexcited hydrogen from water. However, most attempts on modifying this material for improving the utilization of light in the visible region of the solar spectrum have met with limited performance [2]. As Molybdenum Disulfide (MoS2) has been examined potentially as efficient hydrogen evolution catalyst containing no precious metals [3]. Herein, we fabricated a novel multilayered heterojunction photoelectrode MoS(2-x)/TiO2/Ti by effortlessly spraying liquid nanoflake MoS2 on the surface of TiO2substrate after thermal oxidation of Ti foil, following thermally textualizing process, and then investigated the visible light characteristics. The whole fabrication process of the samples, as noted respectively a, b, c and d in the fig.1(1), consists of three steps, thermal oxidation of clean Ti foils in high temperature tube furnace at 500 ℃ for 3 hours creating TiO2 film layer on the surface, then spraying liquid nanoflake MoS2 (commercially available) on the surface of TiO2 (top and back side) developing layers of MoS2 film as top covering the TiO2 after 24 hours drying in a drying oven, and subsequently thermal annealing in the tube at 500 ℃ for 1 hour in forming gas environment (N2/H2 = 95%/5%). The elemental and quantity analysis of elemental dispersive X-ray spectrum (EDS, fig.1(2)) on surficial area shows expected TiO2 and MoS2-x existing. The scanning electron microscope (SEM) images for as-annealed sample d are shown in the fig.1 (3) and (4) with different resolutions. After annealing in the forming gas atmosphere, the surface shows signs of dissociation thereby increasing the defects density, including edge sites. This is because during the thermal annealing in hydrogen environment, partly sulfur atoms in MoS2 would be removed as forming H2S gas resulting into the creation of vacancies and active edges, while the remained excess Mo atoms form metal clusters on the film [3]. Finally, for comparison, the absorption spectrum of four samples is measured as displayed in the fig.2 in detail. The processed samples all show obvious absorption enhancement in the wavelength from 400 to 800 nm. Specially, the annealed sample d maintains good feature of visible light adsorption feature, indicating the formation of multilayered heterojunction without destroying the composite function of TiO2 as absorbing light energy material and MoS2as catalyst excessively. In conclusion, the MoS(2-x)/TiO2/Ti multilayered heterojunction photoelectrode with enhanced visible light absorption are fabricated through a facile method, which the multilayered heterojunction present stable structure and composite function. What’s more, this novel low-cost heterojunction photoelectrode posses potential application in the photoelectrochemical water splitting system. Acknowledgments: The author gratefully acknowledges the financial support from Norwegian Research Council FRINATEK programme (231416/F20), and China Scholarship Council (CSC, Grant No.201506930002). The Research Council of Norway is acknowledged for the support to the NorFab, project number 245963/F50.

Inkjet Printing of Single‐Crystalline Bi2Te3 Thermoelectric Nanowire Networks

Large‐scale and low‐cost fabrication techniques are needed to commercialize highly efficient, nanomaterial‐based thermoelectric generators (TEGs) for use in small‐scale, flexible applications (e.g., wearable energy harvesters). This study presents the first demonstration of inkjet‐printed networks of phase‐pure, single‐crystalline Bi2Te3 thermoelectric nanowires (BTNWs) that are amenable to large‐scale production. The BTNWs are synthesized via chemical batch processing and formulated into a jettable ink that is printed onto glass substrates and subsequently annealed in nitrogen and forming gas environments. The inkjet‐printed BTNWs annealed in forming gas provide the most favorable results with comparable thermoelectric performances to bulk Bi2Te3 materials (Seebeck coefficient up to 140 µV K−1) while approximately utilizing only 1% to 3% of the telluride materials found in their bulk counterparts. Thus, these printed BTNWs help pave the way for the development of low‐cost and scalable TEGs.

Improved Si/SiOx interface passivation by ultra-thin tunneling oxide layers prepared by rapid thermal oxidation

We analyze the influence of different oxidation methods on the chemical passivation quality of silicon oxide-nanolayers on crystalline silicon wafers with surface photo voltage and quasi-steady-state photo conductance measurements. We present a simple method by means of rapid thermal oxidation (RTO) and subsequent annealing in forming gas, which requires no complex surface pre-treatment or surface pre-conditioning after cleaning. This technique allows a reproducible preparation of high-quality ultra-thin oxide-nanolayers (1.3–1.6nm) with a nearly intrinsic energetic distribution of interface states and a defect density of states of only 1×1012cm−2eV−1 at the minimum of the distribution. These results are compared with silicon oxide-nanolayers prepared by wet chemical oxidation and plasma oxidation where only a slight reduction of the interface defect density is achieved by subsequent anneal in forming gas environment. Furthermore, it is shown that applying the RTO oxide-nanolayer as an intermediate layer between Si and an a-SiNx:H layer, leads to a significant improvement of the surface passivation quality.

Chemical Nature and Control of High-k Dielectric/III-V Interfaces

III-V semiconductor surfaces are notoriously difficult to control as their native oxides are not stable. For the growth of high-k dielectric, interface passivation layers have recently been incorporated into InP/high-k dielectric stacks using different approaches, including atomic layer deposition (ALD) to grow thin aluminum silicate layers and deposition of a buffer silicon layer prior to aluminum oxide growth. While beneficial effects have been observed on the interface electrical properties, little is known about the chemical composition and formation mechanisms of these interfacial layers. Futhermore, there is growing evidence that elemental diffusion from the substrate or its oxide may impair electrical properties. There is therefore a need for chemical characterization of surfaces and interfaces under typical processing conditions. Combined in-situ IR measurements and ab initio calculations of InP(100) surfaces reveal that a mild annealing (~300oC) typically needed for atomic layer deposition leads to marked chemical changes at the surface, with the formation of surface InPO4-species (P=O and P-OH species). The initial interaction of trimethylaluminum with this hydrophosphonated surface is characterized by the formation of P-O-Al and dative bonding between Al and the oxygen atom of neighboring P=O moiety, requiring several cycles to complete this interfacial layer. This chemical interface transformation and subsequent impact on the quality of an ALD-grown Al2O3 layer can be substantially altered by silicon deposition using trisilane exposure. Comparison of several silicon precursors [SiCl4, Si(OCH3)4 and Si3H6] by in-situ spectroscopy reveals that Si3H6 best reacts with the hydrophosphonate-terminated surface and reduces the native oxide by forming a SiOx layer that prevents interaction of TMA with the native oxide, leading to a denser Al2O3 layer. Deposition of a thin aluminum silicate barrier layer by ALD is also investigated, and Al from TMA found to play a key role in activating silicon precursors. However, TMA reaction with Si-modified surfaces leads to efficient methyl transfer to Si, a source of interfacial carbon contamination. Finally, mass transport is examined in HfO2/In0.53Ga0.47As stacks. Diffusion of indium through HfO2 is observed after post deposition annealing in N2 or forming gas environments by low energy ion scattering and X-ray photo electron spectroscopy, and found to be consistent with changes in interface layer thickness observed by transmission electron microscopy. Prior to post processing, elemental arsenic is detected in the form of arsenic oxide at the surface of ALD-grown HfO2 and is desorbed upon annealing at 350oC. Indium diffusion occurring upon annealing, resulting in a reduction of the interfacial layer thickness and a potential densification of HfO2, is confirmed by an increase in capacitance.

Diffusion of In0.53Ga0.47As elements through hafnium oxide during post deposition annealing

Diffusion of indium through HfO2 after post deposition annealing in N2 or forming gas environments is observed in HfO2/In0.53Ga0.47As stacks by low energy ion scattering and X-ray photo electron spectroscopy and found to be consistent with changes in interface layer thickness observed by transmission electron microscopy. Prior to post processing, arsenic oxide is detected at the surface of atomic layer deposition-grown HfO2 and is desorbed upon annealing at 350 °C. Reduction of the interfacial layer thickness and potential densification of HfO2, resulting from indium diffusion upon annealing, is confirmed by an increase in capacitance.

Oxygen incorporation and dipole variation in tantalum nitride film used as metal-gate electrode

Tantalum nitride (TaN) films were used as gate electrodes in MOS capacitors fabricated with 8-nm-thick SiO2 as gate dielectric, and also used in Schottky diodes on n-type Si (100) substrates. TaN films with 20- and 100-nm-thick layers presented electrical resistivity of 439 and 472 μΩ cm, respectively. XPS measurements on these TaN film surfaces show oxygen incorporation, which can be related to air exposure. MOS capacitors with TaN/SiO2/Si/Al and Al/TaN/SiO2/Si/Al structures, and Schottky diodes with TaN/Si/Al and Al/TaN/Si/Al structures, were fabricated on the same substrates. These devices were electrically characterized by capacitance–voltage (C–V) and current–voltage (I–V) measurements after sintering in a conventional furnace in a forming gas environment at 450 °C, for different times between 0 and 30 min. From C–V measurements of the MOS capacitors, the extracted TaN work function, effective charge densities, and flatband voltage values were found to be between 4.23 and 4.42 eV, −1011 and −1012 cm−2, and −0.12 and 0.25 V, respectively. From I–V measurements of the Schottky diodes, work function and ideality factor values between 4.40 and 4.53 eV, and 1.0 and 1.9, respectively, were extracted. The variation of the TaN work function (extracted from C–V and I–V curves), flatband voltage, and ideality factor values were related to dipole variations (at the TaN/SiO2 interface), Ta and N vacancies in the TaN film, and oxygen incorporation on the TaN film surface. These results can contribute to work function engineering area for MOS technology.

Die attach properties of Zn–Al–Mg–Ga based high-temperature lead-free solder on Cu lead-frame

Several candidate alloys have been suggested as high-temperature lead-free solder for Si die attachment by different researchers. Among them, Zn-Al based alloys have proper melting range and excellent thermal/electrical properties. In this study, Zn-Al-Mg-Ga solder wire was used to attach Ti/Ni/Ag metallized Si die on Cu lead-frame in an automatic die attach machine. Die attachment was performed in a forming gas environment at temperature ranging from 370 to 400 C. At the interface with Cu lead- frame, CuZn4 ,C u 5Zn8 and CuZn intermetallic compound (IMC) layers were formed. At the interface with Si, Al3Ni2 IMC formed when 200 nm Ag layer was used at the die back and AgZn and AgZn3 IMC layers when the Ag layer was 2,000 nm thick. Microstructure of the bulk solder consists of mainly two phases: one with a brighter contrast (about 80.9 wt% Zn) and the other one is a mixture of light (about 73.7 wt% Zn) and dark phases (about 45 wt% Al). Zn-Al-Mg-Ga solder wetted well on Cu lead-frame, covered entire die area and flowed in all directions under the Si die. Less than 10% voids were found in the die attach samples at die attach temperatures of 380 and 390 C. Die shear strength was found within the acceptable limit (21.8-29.4 MPa) for all the die attach temperatures. Die shear strength of standard Pb-Sn solder was also measured for comparison and was found to be 29.3 MPa. In electrical test, maximum deviation of output voltage after 1,000 thermal cycles was found 12.1%.

Microcontact printing of indium metal using salt solution “ink”

Poly(dimethlysiloxane) stamps were made from Si masters fabricated using photolithography and anisotropic etching. GaCl 3 and In(NO 3) 3 were microcontact printed onto Si substrates, creating arrays of micron size metal salt deposits. The In(NO 3) 3 deposits were further processed by annealing in an N 2 :H 2 (9:1) forming gas environment at 600 °C which converted the deposits into In metal. The ability to inexpensively pattern metal arrays on semiconductor surfaces has implications for ohmic contacts and, with additional processing, arrays of semiconductor crystallites for optoelectronic applications.

Excitonic fine structure and recombination dynamics in single-crystallineZnO

The optical properties of a high quality bulk $\mathrm{ZnO}$, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at $3.3605\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ having a linewidth of $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and dominating the PL spectrum at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This bound exciton transition was visible only below $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whereas the A-free exciton transition at $3.3771\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ persisted up to room temperature. A-free exciton binding energy of $60\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of $\ensuremath{\sim}170$ and $\ensuremath{\sim}864\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

Characteristics of Silicon Implanted Trap Memory in Oxide-Nitride-Oxide Structure

AbstractIn this work, non-volatile memory based on silicon nano trap was implemented by implanting silicon into oxide-nitride-oxide layers(ONO) on silicon. The charge storage effects in these memory structures were measured using capacitance-voltage techniques in polysilicon-ONO- silicon structures in terms of silicon implantation dose. Without silicon implantation, the devices did not the threshold shift (memory window). The memory window was found to be dependent on silicon implantation dose and DC bias (programming) voltage. The memory window did show any degradation by annealing in nitrogen or forming gas environment at 450 °C for 30 minutes. Nano trap memory elements were fabricated with 0.35 micron technology showed encouraging results up to 1e5 program/erase cylces.

Rapid thermal annealing effect on near-surface stoichiometry of GaAs by heavy-ion Rutherford backscattering

We have studied the near-surface stoichiometry profiles of GaAs wafers subjected to rapid thermal annealing in a forming gas environment using the Rutherford backscattering (RBS) technique with a 33-MeV 16O beam. Our measured profiles quantify the extent of arsenic loss at uncapped wafer surfaces for high-temperature anneals. We have observed that a two-temperature step anneal results in a stoichiometry profile which is nearly RBS indistinguishable from that of an unannealed wafer.

Diffusion of Metals in Silicon Dioxide

The diffusion coefficient and solubilities of several metals (Ag, Cu, Au, Pd, and Ti) have been investigated by bias temperature stress (BTS) of (MOS) structures using the candidate metal as the electrode. Temperatures ranged from 250° to 600°C, and the electric fields ranged from +106 to −106 V/cm in ambients of vacuum (10−6 torr), nitrogen, and forming gas . In dry , the activation energy for silver diffusion is found to be 1.24 eV in the temperature range of 275°–365°C. The diffusion coefficient for silver at 300°C in silicon dioxide is . Copper diffusion has an activation energy of 1.82 eV in the temperature range of 350°–450°C in a forming gas environment and a diffusion coefficient in at 450°C of . A thermodynamic model to predict the activation energy of the solid solubility of these metals in and an interstitial diffusion model, that includes both strain and electrostatic energies, which predicts the diffusion activation energy, have been developed. Diffusion coefficients are estimated from a closed form solution of the diffusion equation and the observed behavior of the metal in .

Formation of titanium hydrides in (Al, Ga)As double-heterostructure laser contacts and their effect on degradation rate

A cellular pattern observed in the top surface electron-beam-induced current image of p-metallized (Al,Ga)As double-heterostructure laser material is investigated and attributed to hydride formation in the grain boundaries of the Ti layer of the TiPt p metallization. This hydride formation occurs when the TiPt p metallization is annealed in a forming gas environment and leads to high initial device degradation rates. When the hydride formation in the Ti layer and therefore the presence of the cellular pattern is eliminated by annealing in a N2 environment, low initial device degradation rates are obtained. The initial device degradation rates for material annealed in N2 are equal to or better than those for devices having a weak cellular pattern and 4–5× lower than for devices with a moderate or strong cellular pattern.