N2 Annealing Research Articles

Introduction of high tensile strain into Ge-on-Insulator structures by oxidation and annealing at high temperature

It is demonstrated in this work that a high-temperature thermal process including oxidation and N2 annealing at 850 °C can provide a tensile strain of ∼0.58% at maximum into Ge-on-Insulator (GOI) structures without any special patterning or external stressors. The different impacts of oxidation and annealing on tensile strain generation, surface roughness, and crystal qualities in the GOI structures fabricated by Ge condensation and wafer bonding are systematically examined. A tensile strain of 0.47% is achieved without severe thermal damages under the optimal thermal process condition, which indicates the high potential of the present method for improving the performance of GOI n-channel metal-oxide-semiconductor field-effect transistors. The influence of thermal expansion mismatch between Ge and SiO2 is suggested as a possible physical origin of the high amount of tensile strain into GOI structures.

Growth of tungsten bronze phase out of niobate perovskite phase for opto‐ferroelectric applications

AbstractEngineering the optical bandgaps of classic ferroelectrics from the typical ultraviolet range down to the visible range is an emerging methodology of developing the next‐generation optoelectric and opto‐ferroelectric devices including ferroelectric solar cells, light‐driven transistors and modulators, and multi‐sensors/energy harvesters. Recently, a material interface comprised of a pseudo‐morphotropic phase boundary between the tungsten bronze and perovskite phases of the KNBNNO [(K,Na,Ba)x(Ni,Nb)yOz] has been reported to be an effective approach for bandgap engineering while retaining excellent ferroelectricity and piezoelectricity of the perovskite‐phased KNBNNO. However, this approach requires the compositions of the materials to be determined at the synthesis stage, leaving little room for any further modification of the microstructure and functional properties at the post‐processing stage. This paper presents a post‐processing method, that is, atmospheric annealing in N2 and O2, to grow the necessary tungsten bronze phase out of the perovskite phase in the KNBNNO. This method is advantageous over the previously reported because it enables to grow the tungsten bronze–perovskite interface region independent of the initial composition. The distinctive electrical properties and the giant tunability of photoconductivity of the tungsten bronze phase, the perovskite phase, and the interface are characterized in detail in this paper, supporting the exploitation of fabricating opto‐ferroelectric devices using the reported method which is compatible and comparable with some of the post‐processing methods applied in the silicon industry.

Facile fabrication of TiO2-SiO2-C composite with anatase/rutile heterostructure via sol-gel process and its enhanced photocatalytic activity in the presence of H2O2

Constructing anatase/rutile heterostructure in TiO2 based materials is a quite powerful approach to enhance their photocatalytic activities. Herein, by simply annealing the sol-gel derived TiO2-SiO2 composite in N2 atmosphere at 850 °C, TiO2-SiO2-C composite (CTS-850) with anatase/rutile heterostructure has been successfully prepared, while the counterpart prepared in air contains only anatase phase. It was proven that the residual organic groups in the sol-gel process were converted into carbon species upon N2 annealing, during which TiO2 in the composite was partially reduced, not only leaving lots of oxygen vacancies on its surface but also promoting the phase transformation. By turning the annealing temperature and atmosphere, a series of control products were further synthesized. Among these samples, the CTS-850 showed the best photocatalytic performance toward Rhodamine B degradation in the presence of H2O2, which was mainly due to its lowest band gap and the enhanced sensitization of H2O2 by oxygen vacancies. Moreover, the photocatalytic activity of CTS-850 remained unchanged after five cycles and a proper mechanism was also proposed.

Diffusion of dopants and impurities in β-Ga2O3

The understanding and availability of quantitative measurements of the diffusion of dopants and impurities in Ga2O3 are currently at an early stage. In this work, we summarize what is known about the diffusivity of the common donor dopants, Sn, Ge, and Si, as well as some of the deep acceptors, N, Fe, and Mg, and donors, Ir. Two commonly encountered interstitial impurities are H and F, the former through growth and processing ambients and the latter through its use in plasmas used for stripping dielectrics from Ga2O3. Both are found to have high diffusion coefficients and an effect on electrical conductivity, and H shows anisotropy in its diffusion behavior. Si, Ge, and Sn implanted into bulk β-Ga2O3 at total doses from 2 × 1013 to 2 × 1015 cm−2 and annealed at 1100 °C for 10–120 s in either O2 or N2 ambients showed a significant effect of the annealing ambient on the donor's diffusivity. In the case of O2 annealing, there was extensive redistribution of the Si, Sn, and Ge across the entire dose range, while, in sharp contrast, the use of N2 annealing suppressed this diffusion. The N2 ambient also suppressed loss of dopants to the surface, with >90% of the initial dose retained after annealing at 1100 °C for 120 s, compared to 66%–77% with O2 anneals under the same conditions.

Effect of annealing environment on the photoelectrochemical water oxidation and electrochemical supercapacitor performance of SnO2 quantum dots

SnO2 quantum dots (SQD) were prepared by utilizing the soft-chemical approach. The formed SQD's were annealed in two kinds of environments: air and nitrogen (N2). Each annealing environment resulted in significant improvement in the performance of water oxidation and electrochemical supercapacitor performance. The specific capacitance of the SQD's under the N2 annealing process (SQD-N2) shows significantly better electrochemical performance. A specific capacitance of 79.13 F/g was achieved for SQD-N2 sample by applying a current of 1 mA, which was approximately 1.5 times greater than that of the pristine SQD's. A cycle stability of 99.4% over 5000 cycles was achieved by SQD-N2. The process of nitrogen annealing environment brings down the bandgap from 3.37 to 1.9 eV. The SQD-N2 sample shows the highest photocurrent over SQD and SQD-Air samples. From the LSV study, SQD-N2 shows the photocurrent density of 4.82 mA/cm2, which is 1.43 times greater than pristine SQD sample. The nitrogen-annealing environment provides the optimal environment to tune the average crystallite size and crystallinity nature of SQD's to improve the optical properties like bandgap to enhance the water oxidation and also electrochemical performance.

Microstructure and optical properties of sputter-deposited Ga2O3 films

We report on the properties of gallium oxide (Ga2O3) thin films deposited on c-plane sapphire substrates using radio frequency magnetron sputtering under various conditions. The parameters varied included the composition of the deposition gas, the substrate temperature, and postdeposition annealing temperature. The optical characteristics obtained by UV-VIS spectroscopy showed excellent transparency of 90%–95% for all the films obtained. The structural and compositional properties of the films were determined using x-ray diffraction and energy dispersive spectrometry measurements. The films deposited in Ar at 400 °C showed diffraction peaks at 18.6°, 37.2°, and 58.2°, which are attributed to diffraction peaks from (2¯01), (4¯02), and (6¯03) planes of β-Ga2O3. Postdeposition annealing in N2 at 400–900 °C did not make any improvement in the crystalline quality of the films. The addition of tin in the films produced transparent films whose optical bandgaps decreased with increasing tin concentration in the films.

Chemical Stability of IrO2 Top Electrodes in Ferroelectric Hf0.5Zr0.5O2‐Based Metal–Insulator–Metal Structures: The Impact of Annealing Gas

The IrO2 top electrode chemistry of ferroelectric IrO2/Hf0.5Zr0.5O2 (HZO)/IrO2 metal–insulator–metal (MIM) structures dependent on rapid thermal annealing in different gas atmospheres (nitrogen, oxygen, and forming gas [FG]) is investigated. Using hard X‐ray photoelectron spectroscopy (HAXPES), the strongly modified chemical states of the IrO2 layer dependent on the choice of annealing gas atmosphere are observed. For O2 and N2 anneals, the IrO2 electrode remains either unaffected or is just slightly chemically attacked at the top. In contrast, FG annealing causes a complete reduction of the IrO2 top electrode into metallic Ir. Surprisingly, oxygen is detected—unbound to Ir—which is incorporated in the metallic Ir layer. This mobile oxygen is thought to affect the electrical behavior of the IrO2/HZO/IrO2 device. In addition, it may serve as a test sample for future studies of the root causes of the role of oxygen‐vacancy interactions at the interface, which can influence the performance instabilities in HZO‐based MIM structures, such as wake‐up, imprint, and fatigue.

Impact of Iridium Oxide Electrodes on the Ferroelectric Phase of Thin Hf0.5Zr0.5O2 Films

Thin film metal–ferroelectric–metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O2, and N2 annealing atmospheres significantly change the ferroelectric performance. The use of oxygen‐deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake‐up effect. It is found that oxygen‐rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties.

Mobility improvement of 4H-SiC (0001) MOSFETs by a three-step process of H2 etching, SiO2 deposition, and interface nitridation

4H-SiC(0001) metal-oxide-semiconductor field-effect transistors (MOSFETs) and MOS capacitors were fabricated by the following procedures: H2 etching, SiO2 deposition, and nitridation, and their electrical characteristics were evaluated. Substantially low interface state densities (4–6 × 1010 cm−2 eV−1) and high channel mobilities (80–85 cm2 V−1 s−1) were achieved by N2 annealing or NO annealing after H2 etching and SiO2 deposition. The threshold voltage of the MOSFETs fabricated with N2 annealing was shifted negatively when the oxide was formed by deposition. On the other hand, normally-off operation and high channel mobility were compatible for the MOSFETs fabricated with NO annealing.

Tuning the hierarchical pore structure of graphene oxide through dual thermal activation for high-performance supercapacitor

Herein, we introduce a simple method to prepare hierarchical graphene with a tunable pore structure by activating graphene oxide (GO) with a two-step thermal annealing process. First, GO was treated at 600 °C by rapid thermal annealing in air, followed by subsequent thermal annealing in N2. The prepared graphene powder comprised abundant slit nanopores and micropores, showing a large specific surface area of 653.2 m2/g with a microporous surface area of 367.2 m2/g under optimized conditions. The pore structure was easily tunable by controlling the oxidation degree of GO and by the second annealing process. When the graphene powder was used as the supercapacitor electrode, a specific capacitance of 372.1 F/g was achieved at 0.5 A/g in 1 M H2SO4 electrolyte, which is a significantly enhanced value compared to that obtained using activated carbon and commercial reduced GO. The performance of the supercapacitor was highly stable, showing 103.8% retention of specific capacitance after 10,000 cycles at 10 A/g. The influence of pore structure on the supercapacitor performance was systematically investigated by varying the ratio of micro- and external surface areas of graphene.

InAlN/GaN HEMT on Si With fmax = 270 GHz

Device surface properties are critical for its performance such as channel electron density, leakage current, subthreshold swing (SS), and noise in gallium nitride high-electron-mobility transistors (HEMTs). In this article, the improved surface property of InAlN/GaN HEMTs with forming gas (FG, 5% H2, and 95% N2) annealing is demonstrated. X-ray photoelectron spectra (XPS) show that the number of Ga–O bonds decreases while that of the Ga–N bonds increases, an indication of the surface native oxide removal after FG annealing. Compared with N2 annealing, an increase of two-dimensional electron gas (2DEG) electron density with FG annealing is determined by both energy band simulation and capacitance–voltage measurement. Transmission line measurement (TLM) shows that N2 annealing offers a lower ohmic contact resistance ( ${R}_{\text {C}}$ ) while FG annealing features a lower sheet resistance ( ${R}_{\text {sheet}}$ ). Herein, a FG/N2 two-step ohmic contact annealing is developed to achieve a SS of 110 mV/dec, a transconductance ( ${g}_{\text {m}}$ ) peak of 415 mS/mm, a record low drain-induced barrier lowing (DIBL) of 65 mV/V, and a record high power gain cutoff frequency ( ${f}_{\text {max}}$ ) of 270 GHz on a 50-nm InAlN/GaN HEMT on Si.

Preparation of Fe0.8Mn1.54Ni0.66O4 Nano-Ceramics by Sol-gel auto combustion and N2 Annealing for High Reliable NTC Thermistor

Fe0.8Mn1.54Ni0.66O4 nano-ceramics have been successfully prepared by sol-gel auto combustion. The microstructure and phase of these samples was observed by using Scanning Electron Microscopy and X-Ray Diffraction. The diameters of Fe0.8Mn1.54Ni0.66O4 ceramic particles pre-fired at 800℃ range from 52 to 83nm. The powder sintered at above 1050℃ has the compact and uniform spinel structure. We have investigated the electrical characteristics of these thermistors at different sintering temperatures and concluded that the sample sintered at 1200℃ is sufficient to form the appropriate spinel phase. Moreover, the thermistor annealed for 72h at 450~550℃ in N2 atmosphere has the drift value of <0.7%.

Ultra-thin Al2O3 capped with SiNx enabling implied open-circuit voltage reaching 720 mV on industrial p-type Cz c-Si wafers for passivated emitter and rear solar cells

In this study, we report on the passivation quality of atomic layer deposition grown ultra-thin Al2O3 and Al2O3 capped with plasma-enhanced chemical vapor deposition deposited SiNx on Cz p-type wafers for the rear side of a passivated emitter and rear cell (PERC). Different activation recipes using N2, forming gas (FG), and two-step annealing for different durations are investigated before SiNx deposition. The effect of different Al2O3 thicknesses and corresponding activation processes on the Al2O3/SiNx passivation performance, after a high temperature firing step, is studied to reach a new optimization toward higher efficiency and lower cost. A record high iVoc of 720 mV is obtained after firing step from Al2O3/SiNx stacks with Al2O3 thickness as thin as ∼2 nm with FG annealing. Our results demonstrate that, under well-optimized process conditions, ultra-thin Al2O3 thicknesses provide superior passivation quality as compared to the larger thicknesses which are commonly applied in the PERC industrial line and the potential for further improvement of industrial PERC solar cells in terms of cost reduction and efficiency.

Role of Oxygen in Two-Step Thermal Annealing Processes for Enhancing the Performance of Colloidal Quantum Dot Solar Cells.

Colloidal quantum dots (CQDs) have large surface-to-volume ratios; thus, surface control is critical, especially when CQDs are utilized in optoelectronic devices. Layer-by-layer solid-state ligand exchange is a facile and applicable process for the formation of conductive CQD solids through various ligands; however, achieving complete ligand exchange on the CQD surface without dangling bonds is challenging. Herein, we demonstrate that CQDs can be further passivated through two-step annealing; air annealing forms sulfonate bonding at (111) Pb-rich surfaces, and subsequent N2 annealing removes insulating oxygen layers from the (100) surfaces of CQDs. By subsequently conducting annealing treatment in two different environments, traps on the surface of CQDs could be significantly reduced. We achieved a 40.8% enhancement of the power conversion efficiency by optimizing each two-step annealing process.

Improvement of Both n- and p-Channel Mobilities in 4H-SiC MOSFETs by High-Temperature N₂ Annealing

Effects of high-temperature (1400 °C–1600 °C) N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> annealing on the interface states of 4H- SiC/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the channel mobility of 4H-SiC metal–oxide–semiconductor field-effect transistors (MOSFETs) were investigated. It is demonstrated that high-temperature N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> annealing is effective not only for the reduction of the interface state density near the conduction band edge but also for that near the valence band edge. N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> annealing improves channel mobility of both n- and p-channel MOSFETs as in the case of nitric oxide (NO) annealing. In particular, the field-effect mobility of the p-channel MOSFETs annealed in an N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambient was improved to 17 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, which is 30% higher than that of NO-annealed MOSFETs. The interface state density estimated from the subthreshold slopes in the MOSFETs is much higher than that extracted by the high–low method but is almost comparable to that estimated by the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\psi _{\text {s}}$ </tex-math></inline-formula> method in both the cases of interface states near the conduction and the valence band edges. These results suggest that there exist fast states (>1 MHz) near the valence band edge, and these fast states affect the channel mobilities of p-channel SiC MOSFETs as in the case of n-channel MOSFETs.

Robustness of Ferroelectricity in Hafnium-Zirconium Dioxide Films Deposited By Sputtering and Chemical Solution Deposition for Ferroelectric Transistor Applications

Since the discovery of ferroelectricity in HfO2 in 2011, extensive work begin to burst forth on HfO2-based ferroelectric materials and devices. Orthorhombic phase is responsible for ferroelectricity in HfO2, which is usually stabilized by the dopant such as Al, Zr, Si, la and Y. In particular, Zr doped HfO2 (HZO) has large Pr around 20 µC/cm2 and ferroelectricity can be obtained in a wide range of Zr composition. The orthorhombic phase is a metastable phase and Migita et al. [1] reported that the orthorhombic phase of sputtered HZO film was transferred to monoclinic phase by further annealing. In this work, robustness of ferroelectric phase against the further annealing process has been investigated using sputtered HZO and solution-derived Y-doped HZO films [2].Sputtered HZO films were prepared by room temperature deposition followed by 600 oC, 10 min annealing in N2. Y-doped HZO films were prepared by chemical solution deposition (CSD) by annealing the samples at 800 oC for 3 min in O2.[2] Ferroelectric properties were confirmed by polarization-electric field (P-E) and capacitance-voltage (C-V) measurements for both samples before the second time annealing. Then, both samples were further annealed in 400 and 600 oC in N2 to examine the robustness of ferroelectricity. It was found that sputtered HZO films become paraelectric and that X-ray diffraction (XRD) pattern shows diffraction peak from orthorhombic phase disappeared. On the other hand, CSD HZO films shows ferroelectric nature even after second time annealing with negligible monoclinic phase. Such a significant difference in robustness is presumably due to residual carbon in the CSD films.In addition, In2O3/(Y-)HZO structures are also fabricated for oxide channel thin film transistor applications and characterized, which will be presented at the conference.[1] Shinji Migita et al, Jpn. J. Appl. Phys. 58 SBBA07, 2019.[2] Mohit et al, Jpn. J. Appl. Phys. 59 SMMB02, 2020.

The Correlation between the Preferred Orientation and Al Distribution of Al-Doped HfO2 Films By Plasma-Enhanced Atomic Layer Deposition

A high dielectric constant (k = 22.5) Al-doped HfO2 film was prepared on Si substrate by plasma-enhanced atomic layer deposition (PEALD) at a reduced thickness of ~ 30 nm by realizing a (200) preferentially oriented tetragonal structure. The realization of a (200) preferred orientation was able to be obtained by the interface control in PEALD. When using a conventional supercycle that consisted of the Al2O3 and HfO2 subcycles, the dielectric constants of tetragonal Al-doped HfO2 films rapidly decreased from 37 to 8.5 as the film thickness decreased from ~ 92 to ~ 9 nm due to the change of the preferred orientation from (200) to (111). To obtain (200) preferentially oriented tetragonal Al-doped HfO2 thin films at highly thin thickness, a modified supercycle that consisted of the Hf-Al-O subcycle and the HfO2 subcycle was designed to increase Al dopant distribution in HfO2 films. As a result, a (200) preferentially oriented tetragonal Al-doped HfO2 films with a high dielectric constant of 22.5 was obtained at a reduced thickness of ~ 30 nm. Also, the leakage current densities of the Pt/~ 30 nm-thick-tetragonal Al-doped HfO2 films/Si capacitor were about 10-5 A/cm2 for a field strength of -1 MV/cm due to the crystallization after N2 annealing process.

Enhanced visible-light photocatalytic performance of cadmium sulfide film via annealing treatment

In this study, cadmium sulfide (CdS) film prepared through a simple homogeneous precipitation method is used for methylene blue degradation. An enhanced visible-light photocatalytic performance of CdS film can be observed after the N2 annealing treatment. This enhancement in photocatalytic activity can be attributed to the higher crystallinity, better optical absorption, and efficient photogenerated carrier transfer performance of samples. The N2 annealed CdS films also display the optimum reproducibility in recycling reactions, which is beneficial for its practical application. This study provides a reference for preparing other catalysts with high photocatalytic activity and stability for degradation of dye under visible light.

Formation of high-quality SiC(0001)/SiO2 structures by excluding oxidation process with H2 etching before SiO2 deposition and high-temperature N2 annealing

We formed SiC/SiO2 structures by various procedures that excluded an oxidation process. We found that a SiC/SiO2 interface with a low interface state density near the conduction band edge of SiC (D it ∼ 4 × 1010 cm−2 eV−1 at E c −0.2 eV) is obtained for a fabrication process consisting of H2 etching of the SiC surface, SiO2 deposition, and high-temperature N2 annealing. D it is rather high without H2 etching, indicating that etching before SiO2 deposition plays a significant role in reducing D it. The key to obtaining low D it may be the removal of oxidation-induced defects near the SiC surface.

Improving the stability of Fe0.8Mn1.54Ni0.66O4 NTC thermistor with nano-powders and N2 annealing

Fe0.8Mn1.54Ni0.66O4 nano-ceramics have been successfully prepared by sol-gel auto combustion. The microstructure and phase of these samples were observed by using SEM and XRD. The diameters of Fe0.8Mn1.54Ni0.66O4 ceramic particles pre-fired at 800 °C range from 52 to 83 nm. The powder sintered at above 1050 °C has the compact and uniform spinel structure. We have investigated the electrical characteristics of these thermistors at different sintering temperatures and concluded that the sample sintered at 1200 °C is sufficient to form the appropriate spinel phase. Moreover, the thermistor annealed for 72 h at 450 ∼ 550 °C in N2 atmosphere has the drift rate of <0.7%.