Subsequent High-temperature Annealing Research Articles

Thermal recrystallization of short-range ordered WS2 films

The integration of van der Waals materials in nanoelectronic devices requires the deposition of few-layered MX2 films with excellent quality crystals covering a large area. In recent years, astonishing progress in the monolayer growth of WS2 and MoS2 was demonstrated, but multilayer growth resulted often in separated triangular or hexagonal islands. These polycrystalline films cannot fully employ the specific MX2 properties since they are not connected in-plane to the other domains. To coalesce separated islands, ultrahigh-temperature postdeposition anneals in H2S are applied, which are not compatible with bare silicon substrates. Starting from the deposition of stoichiometric short-ordered films, the present work studies different options for subsequent high-temperature annealing in an inert atmosphere to form crystalline films with large grains from stoichiometric films with small grains. The rapid thermal annealing, performed over a few seconds, is compared to excimer laser annealing in the nanosecond range, which are both able to crystallize the thin WS2. The WS2 recrystallization temperature can be lowered using metallic crystallization promoters (Co and Ni). The best result is obtained using a Co cap, due to the circumvention of Co and S binary phase formation below the eutectic temperature. The recrystallization above a critical temperature is accompanied by sulfur loss and 3D regrowth. These undesired effects can be suppressed by the application of a dielectric capping layer prior to annealing. A SiO2 cap can suppress the sulfur loss successfully during annealing and reveals improved material quality in comparison to noncapped films.

Down-shifting emission by charge transfer band in porous silicon infiltrated with Eu3+ and Gd3+ ions

Down-shifting emission properties of europium (Eu3+) and gadolinium (Gd3+) ions incorporated in porous silicon (PS) layers are presented. Different samples were prepared by electrochemical etching of p+-type crystalline silicon wafers followed by simple impregnation with rare earth ions from nitrate solutions of europium and gadolinium and subsequent high temperature annealing in air. The samples exhibited room temperature photoluminescence (PL) and the excitation spectra present a wide excitation band around 233 nm which can be associated to the charge transfer band transitions. This band is attributed to O2−(2p)→Eu3+(4f) ligand-to-metal charge transfer. The down-shifting emission for Gd+3:Eu3+ was analyzed by a simple rate equation model to study the interaction between the O2− ions and Eu3+ ions. The exact solution of this model agrees very well with the experimental results.

Rearrangement of Surface Structure of 4<sup>o</sup> Off-Axis 4H-SiC (0001) Epitaxial Wafer by High Temperature Annealing in Si/Ar Ambient

Mechanism of surface roughening caused by the polishing induced subsurface damage on 4o off-cut 4H-SiC (0001) substrate during thermal etching, CVD epitaxial growth, and the subsequent high temperature annealing was investigated in the wide temperature range of 1000-1800°C. Different from the previous study based on a macroscopic characterization by optical microscopy, microscopic characterization based on a scanning electron microscopy (SEM) was employed in this study. By utilizing the SEM operated under various conditions, disordered step arrangements as well as stacking faults and dislocations were imaged. The obtained results revealed that the SFs cause the fluctuation in the step kinetics, resulting in the step bunching formation during the thermal process.

Effect of coupled annular electromagnetic stirring and intercooling on the microstructures, macrosegregation and properties of large-sized 2219 aluminum alloy billets

AbstractCoarse microstructures and severe macrosegregation always occur in large-sized billets prepared by normal direct chill casting, and cannot be eliminated during subsequent deformation or high temperature annealing. This study proposes a new melt treatment method coupled to annular electromagnetic stirring and intercooling, and applies it to direct chill casting of large-sized 2219 aluminum alloy billets. Compared to the normal direct chill casting, the microstructure and second phase were refined and homogenized, and the macrosegregation of Cu was alleviated. Accordingly, the average tensile and yield strengths, elongation and hardness were significantly improved and remained relatively stable at different positions on the billet. Tensile fracture tests showed that annular electromagnetic stirring coupled with intercooling decreases the amount of porosity in the structure.

Polarized luminescence of nc-Si–SiOx nanostructures on silicon substrates with patterned surface

Polarization characteristics and spectra of photoluminescence (PL) of nc-Si–SiOx structures formed on the patterned and plane c-Si substrates are studied. The interference lithography with vacuum chalcogenide photoresist and anisotropic wet etching are used to form a periodic relief (diffraction grating) on the surface of the substrates. The studied nc-Si–SiOx structures were produced by oblique-angle deposition of Si monoxide in vacuum and the subsequent high-temperature annealing. The linear polarization memory (PM) effect in PL of studied structure on plane substrate is manifested only after the treatment of the structures in HF and is explained by the presence of elongated Si nanoparticles in the SiOx nanocolumns. But the PL output from the nc-Si–SiOx structure on the patterned substrate depends on how this radiation is polarized with respect to the grating grooves and is much less dependent on the polarization of the exciting light. The measured reflection spectra of nc-Si–SiOx structure on the patterned c-Si substrate confirmed the influence of pattern on the extraction of polarized PL.

A Review on Ge Nanocrystals Embedded in SiO2 and High‐k Dielectrics

In this article, the work on Ge nanocrystals embedded in dielectric films formed by phase separation from supersaturated solid solutions is reviewed. Different methods to synthesize supersaturated solid solutions are covered, e.g., magnetron sputtering, ion implantation, and chemical vapor deposition. The phase separation is activated by subsequent high temperature annealing. Important parameters that influence the formation and properties of the Ge nanocrystals are discussed. Various matrix materials like SiO2, Al2O3, HfO2, HfAlOx, Lu2O3, ZrO2, TaZrOx, and Si3N4 are reported in the literature. The influence of the matrix on the formation and properties of the Ge nanocrystals is addressed in this review. Ge nanocrystals are investigated for applications such as charge storage nodes in nonvolatile memory devices or as silicon technology compatible light emitters. A key to establish these applications seems to be embedding the Ge nanocrystals in still amorphous matrices. This could help to avoid grain boundaries that act as leakage paths for electrical charges (unfavorable for nonvolatile memories) and facilitate defects, which could act as recombination centers (unfavorable for light emitters). A further important point is the synthesis of size and position controlled Ge nanocrystals. Both aspects are reviewed for Ge nanocrystals embedded in the mentioned matrix materials.

Lightweight, flexible SiCN ceramic nanowires applied as effective microwave absorbers in high frequency

For the first time, the lightweight, flexible SiCN nanowires with highly efficient electromagnetic wave absorbing performance were prepared by electrospinning with subsequent high temperature annealing in nitrogen atmosphere. The nanowires are composed of cubic-SiC, free carbon, and silicon nitride nanocrystals. By means of controlling annealing time, the microstructure and chemical composition of as-prepared nanowires can be well regulated. The nanowires exhibit excellent flexibility with no fracture whatever conditions they are under for instance bending, twisting, folding, or twining. Carbon backbone was inserted into ceramic matrix to toughen the nanowire. The dielectric properties and electromagnetic (EM) wave absorption performance of nanowires were investigated between 2 and 18 gigahertz (GHz). The nanowires display great EM absorbing behaviour with an optimal reflection loss (RL) of −53.1 dB and the effective absorption bandwidth (EAB, RL below −10 dB) covers whole Ku band (12.4–18.0 GHz) at a small thickness of 1.95 mm. The distinctive microstructure of the nanowires give rise to their outstanding mechanical and EM performance that endow the nanowires potential to be utilized as lightweight, ultrastrong radar wave absorbers in harsh environments.

Synthesis of finely dispersed NaBaPO_4:Eu^2+ phosphors and structural investigation of their centers of luminescence

A technique for the synthesis of NaBaPO4:Eu2+ phosphors with increased dispersity via sol-gel precipitation and subsequent high-temperature annealing in molten NaCl is developed. The synthesized NaBaPO4:Eu2+ phosphor has a nanocrystalline structure and its annealing in molten NaCl is shown to reduce the average particle size by a factor of 3.8 while increasing the content of the finely dispersed (i.e., particle sizes <1 μm) fraction of the phosphor by a factor of 11 by comparison with the phosphor synthesized by the traditional sol-gel method. We establish that the synthesized NaBaPO4:Eu2+ phosphors contain four types of luminescence centers and construct their resulting energy diagram.

Metal doped tubular carbon nitride (tC3N4) based hematite photoanode for enhanced photoelectrochemical performance

Metal doped tubular carbon nitride (M-tC3N4) photocatalyst is synthesized and successfully integrated with the hematite thin films photoanodes for enhanced photoelectrochemical water splitting performance. SEM and TEM analysis revealed that the tubular structures of carbon nitride are formed due to cobalt doping and subsequent high temperature (700 °C) annealing under nitrogen atmosphere. It has been demonstrated that cobalt based bimetal doping with Fe or Ni is useful in enhancing the photoelectrochemical performance in terms of high photocurrent density, low photocurrent onset potential and improved photo-stability under the visible light illumination. The prepared CoNi-tC3N4/α-Fe2O3 photoanode exhibits swift separation of photo-induced change carriers and achieved a photocurrent density of ∼2.73 mA/cm2 at 1.23 V vs. RHE, which is significantly high as compared to g-C3N4/α-Fe2O3, Co-tC3N4/α-Fe2O3 and CoFe-tC3N4/α-Fe2O3 photoanodes. The remarkably high photoelectrochemical performance is probably due to the tailored structural morphology which increases the surface area of the catalyst, high optical absorption in visible region and improved electronic properties which enhances the carrier concentration and finally efficient separation and transfer of photogenerated charge carriers at the semiconductor-electrolyte junction.

Influence of high-pressure deformation and annealing on the structure and properties of a bulk MgB2 superconductor

A synthesized MgB2 superconductor has been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and by the measurements of the superconducting characteristics and microhardness after cold high-pressure deformation in a Toroid chamber and in Bridgman anvils and subsequent high-temperature annealing. A nanocrystalline structure is formed in the superconductor after high-pressure treatment, but internal cracks appear, and the critical current density decreases strongly. The annealing leads to a coarsening of the structure and to an increase in the critical current density up to 5.8–6.7 × 104 А/сm2, which is more than three times greater than that in the initial state.

Ultra-High Aspect Ratio InP Junctionless FinFETs by a Novel Wet Etching Method

Junctionless FinFETs with an array of ultra-high aspect ratio (HAR) fins, enabled by inverse metal-assisted chemical etching, are developed to achieve high on-current per fin. The novel device fabrication process eliminates dry etching-induced plasma damage, high energy ion implantation damage, and subsequent high-temperature annealing thermal budget, ensuring interface quality between the high- $k$ gate dielectric and the HAR fin channel. Indium phosphide junctionless FinFETs, of record HAR (as high as 50:1) fins, are demonstrated for the first time with excellent subthreshold slope (63 mV/dec) and ON/OFF ratio ( $3\times 10^{5}$ ).

Evolution of properties in composite carbon films under thermal conditions and in the presence of a catalyst

The characteristics of composite carbon films are experimentally studied. The films are synthesized under concurrent conditions of the magnetron and plasmatron sputtering of molybdenum disilicide and polyphenyl methylsiloxane (PPMS 2/5). Automodulation of the elemental and phase composition is found and studied in the direction of film deposition onto the substrate. The stability of the automodulation parameters to subsequent high-temperature annealing is examined.

Anomalies of dielectric properties and conductivity in single domain LiNbO3:Zn crystals

ABSTRACTA study of the temperature dependence of dielectric constant, conductivity, and piezoelectric modulus in the single-domain state of LiNbO3 crystals modified by Zn admixture at threshold concentration is reported. Unipolarity of the LiNbO3:Zn crystals is observed to increase after treatment of brand-new samples by high-temperature electro-diffusion annealing and by subsequent high-temperature annealing of short-circuited samples. The observed effects are explained as a result of meta-stable residual domains collapsing at high temperature the collapse being assisted by disintegration of charged clusters stabilizing domain walls. The rise of unipolarity is accompanied by anomalies on the σ(T) and ϵ(T) dependences and by a significant increase of the value of the piezo-modulus.

Atomic Layer Deposition of Lithium Titanate on 3D-Structured 300 Mm Silicon Substrates

Thin film battery technology delivers high added value for autonomous microsystems in e.g. wireless sensor networks, Internet of Things (IoT) applications and medical implants. The transition to solid state electrolytes means significant improvement of the battery safety and even opens possibilities for integration into silicon technology. The 3D micro battery concept, including deposition of functional layers on structured surfaces with high aspect ratio, has been shown to improve both, capacity and rate performance of all-solid-state batteries [1]. Our aim is the development and characterization of functional stacks allowing direct integration into microsystems. For CMOS compatible design and production, we manufacture functional electrode and electrolyte layers on 300 mm wafer using standard industrial thin film deposition equipment. Atomic layer deposition (ALD) is a vapor-phase technique based on sequential, self-limiting surface reactions enabling conformal deposition of thin films. In this work, alternate atomic layer deposition of titanium oxide and lithium carbonate is demonstrated. The formation of the desired spinel lithium titanate (LTO) is investigated by subsequent high temperature anneals. Although its comparatively low capacity LTO is a promising electrode material for 3D integration. It stands out due to its zero-strain characteristics and the three dimensionality of ion transport [2]. [1] Y. Wang, B. Liu, Q. Li, S. Cartmell. S. Ferrara, Z. Deng, and J. Xiao, Journal of Power Sources 286 (2015) 330-345. [2] V. Miikkulainen, O. Nilsen, M. Laitinen, and T. Sajavaara, RSC Advances 3 (2013) 7537.

Bipolar Degradation of 6.5 kV SiC pn-Diodes: Result Prediction by Photoluminescence

The stability of 6.5 kV pn-diodes is dependent on the absence of critical crystal defects, such as basal plane dislocations. In this paper, we present a method to detect these defects on wafer level by utilizing photoluminescence (PL). The PL scan is performed immediately after epitaxy and also after the implantation process steps with subsequent high temperature annealing. The analysis of both scans enables the prediction of devices that will drift due to bipolar degradation, and devices that will exhibit non-drifting behaviour. To validate this PL scanning technique, forward bias electrical stress tests have been performed on the fabricated 6.5 kV pn-diodes.

Control of carrier lifetime of thick n-type 4H-SiC epilayers by high-temperature Ar annealing

We investigated the carrier lifetime and Z1/2 center density of thick n-type 4H-SiC epilayers, which were oxidized and subsequently annealed in Ar at high temperatures. The Z1/2 center density decreased below the detection limit in the region to, at least, a 130 µm depth by thermal oxidation. After subsequent high-temperature annealing, the Z1/2 center density increased with increasing annealing temperature, while the distribution of the Z1/2 center density was nearly uniform to a 130 µm depth. The carrier lifetime could be controlled from 26 to 2.4 µs by changing the annealing temperature from 1600 to 1800 °C.

Improved electrical properties of silicon quantum dot layers for photovoltaic applications

The low density of silicon quantum dots (Si QDs) in a dielectric matrix is one of the plausible reasons for the low power conversion efficiency (PCE) of Si QD solar cells due to large spacing between QD and substrate. In this study, the electrical properties of the Si QD layers were improved by increasing the density of Si QDs and reducing the barrier width. In the previous studies, Si QD layers were fabricated by alternating deposition of SiO2/SiOx layers and subsequent high temperature (1100°C) annealing, where the diffusion of Si species in the SiOx layer into Si substrate was observed. In order to prevent the diffusion, we propose the substitution of the SiOx layer with an amorphous Si (a-Si) layer. In [SiO2 (2nm)/SiOx (2nm)]×34 multilayer structures, the first few SiOx layers were substituted with a-Si layers. As a result, Si QDs could be well formed in the a-Si layer with high density and survived near the interfaces between the first Si QD layer and Si substrate. The distance of the nearest Si QDs from the Si substrates was also reduced from 7.2nm to 3.6nm, which results in the effective carrier transfer by tunneling between the Si QDs. The PCE of heterojunction B-doped Si QDs/crystalline Si solar cells was enhanced by using the Si QD structures with the improved electrical properties. Since the variations of open-circuit voltage and short-circuit current are not so large, the enhancement of PCE is mainly related with the improvement of fill factor due to the decrease of series resistance.

Features of phase transformations in a Ni V –Pt–Si system during stationary stepped annealing

Electron diffraction methods in reflection geometry are used to investigate the distinctive features of phase transformations that occur in a NiV–Pt–Si system during stationary stepped annealing. A platinum film 0.015–0.02 µm thick and then a vanadium-alloyed nickel film 0.08 µm thick are magnetron-sputtered on the surface of a silicon wafer of KEF-0.5 grade with (111) orientation. Annealing is performed in a nitrogen atmosphere. It is shown that depending on the degree of homogenization of the mutual Ni–Pt solid solution and the interaction at the Pt–Si interface at the first step of annealing, which lasts for long periods of time at temperatures of 240 or 350°C, subsequent high-temperature annealing at 550°C for 30 min leads to the formation of Ni1–xPtxSi- or Pt1–xNixSi silicides with NiSi- or PtSi-type crystal lattices.

Incorporation of air-cavity into sapphire substrate and its effect on GaN growth and optical properties

We propose a new GaN growth scheme using a cavity engineered sapphire substrate (CES), in which a patterned cavity array was formed on the sapphire surface. Amorphous alumina film was deposited by atomic layer deposition on a photoresist patterned sapphire substrate, and subsequent high temperature annealing resulted in the formation of a cavity array surrounded by a crystallized sapphire shell. During the GaN growth on CES, the GaN film filled up the space between the cavities and then grew laterally over them, leading to a completely coalesced pit-free smooth surface. The incorporation of cavities was observed to reduce the stress in GaN film by ~30%. The output power of a light emitting diode (LED) on CES at an input current of 20mA was measured to be 2.2 times higher than that on a planar sapphire substrate. It was also found that the dominant peak wavelength of the LEDs on CES showed a red-shift of 12nm due to higher In incorporation associated with presumably lower surface temperature in the presence of air-cavity.

Carbon out-diffusion mechanism for direct graphene growth on a silicon surface

Direct growth of graphene on silicon (Si) through chemical vapor deposition has predominantly focused on surface-mediated processes due to the low carbon (C) solubility in Si. However, a considerable quantity of C atoms was incorporated in Si and formed Si1−xCx alloy with a reduced lattice dimension even in the initial stage of direct graphene growth. Subsequent high temperature annealing promoted active C out-diffusion, resulting in the formation of a graphitic layer on the Si surface. Furthermore, the significantly low thermal conductivity of the Si1−xCx alloy shows that the incorporated C atoms affect the properties of a semiconductor adjacent to the graphene. These findings provide a key guideline for controlling desirable properties of graphene and designing hybrid semiconductor/graphene architectures for various applications.