Metal-induced Crystallization Research Articles

Evaluation of extremely thin polycrystalline germanium films and their TFT performance fabricated at 400 °C by Cu-induced crystallization on a glass substrate

Germanium (Ge) has high mobility and is more suitable for low-temperature device processes than silicon (Si); thus, it has attracted attention as an upper-layer transistor for monolithic three-dimensional (M3D) integration. We evaluated in detail the crystalline quality of extremely thin polycrystalline-Ge (poly-Ge) thin films thinner than 15 nm fabricated by metal-induced crystallization using copper (Cu-MIC) at 400 °C using micro-Raman scattering, in-plane X-ray diffraction, transmission electron microscopy (TEM), TEM energy dispersive X-ray spectroscopy, and TEM electron diffraction. Additionally, the films were applied to p-ch double-gate (DG) poly-Ge thin-film transistors (TFTs), and their performance was evaluated. As a result, a high I on/I off ratio of 2.8 × 103 was realized by crystallization at 400 °C with a ratio equal to that of crystallization at 500 °C. This study demonstrated the feasibility of Cu-MIC DG poly-Ge TFTs for M3D applications.

Low temperature metal induced crystallization of orientation-preferred polycrystalline germanium with n- and p-type doping for device applications

A metal-induced crystallization (MIC) doping method is proposed for the fabrication of n- and p-type doping polycrystalline germanium (poly-Ge). This method involves the use of amorphous- (α-) Ge/metal/α-Ge/crystal-Ge stacked structure to achieve orientation-preferred poly-Ge on Ge (100) substrate at a very low temperature of 240℃. Aluminum (Al) and bismuth (Bi) metal layers are selected for p- and n- type doping sources, respectively. The thickness ratio of metal to α-Ge is optimized to obtain dense poly-Ge. Raman spectrum, scanning electron microscopy and electron backscattered diffraction characterizations confirm that the crystallized α-Ge exhibits a preferred orientation along (100) with a smooth surface. The p- and n- type doping concentrations are evaluated to be 2.2×1021 cm−3 and 2.7×1019 cm−3, respectively, by Hall effect measurement. Furthermore, p-type poly-Ge/n-Ge and n-type poly-Ge/p-Ge junctions are formed with significant rectification ratios of 103 and 102@±2 V, respectively. These findings indicate that the low-temperature MIC approach holds great potential for the fabrication of n- and p-type poly-Ge, enabling the development of Ge-based active devices at low temperature.

Four-terminal polycrystalline-silicon vertical thin-film transistors on glass substrates

This study aimed to fabricate n-channel four-terminal (4 T) polycrystalline silicon (poly-Si) vertical thin-film transistors (VTFTs) with submicron gate lengths on a glass substrate. The 4 T VTFTs have a poly-Si active layer crystallized via metal-induced crystallization using nickel (Ni-MIC), a top gate (TG), and a bottom gate (BG). Here, the TG covers all the channel regions, and the BG affects only a part of the channel region. A double-gate drive (simultaneous operation of both gates) displayed a high I on/I off ratio and small s.s. compared with those of a single-gate drive. In addition, the threshold voltage (V th) of the TG drive varied depending on the BG voltages (V BG), with a γ-value (=ΔV th/ΔV BG) of 0.17. This value is smaller than the theoretically expected value because of the localized effects of the BG on the channel region and the inferior crystalline quality of the Ni-MIC poly-Si film.

Effects of crystallinity of silicon channels formed by two metal-induced lateral crystallization methods on the cell current distribution in NAND-type 3D flash memory

Two metal-induced lateral crystallization (MILC) methods are proposed as candidate techniques to enhance cell current in future ultra-high-density NAND-type 3D flash memory devices. The channel crystallinity differs depending on the MILC method. In a single MILC, the channel is composed of single-crystal Si, whereas in a regional MILC, the channel comprises multiple crystal grains that are larger than those of the conventional polycrystalline Si. Using transmission electron microscopy, the inhibiting factor of MILC was modeled to reveal that the two MILC approaches result in different cell current distributions that are related to their degree of crystallinity. A comparison of these two cell current distributions in a 3D flash memory with over 900 word-line stacks showed that the single MILC delivers a higher median cell current with outliers on the lower side. In contrast, the regional MILC delivers a lower median cell current without outliers on the lower side.

Metal-induced lateral crystallization of germanium thin films

Recent advances in polycrystalline Ge layers formed through solid-phase crystallization have demonstrated carrier mobilities superior to those of single-crystal Si. In this study, we thoroughly examined the phenomenon of metal-induced lateral crystallization in Ge, a topic that had garnered significant attention in Si research and had shown promise in improving the performance of thin-film transistors. Of the 24 metals tested (Au, In, Bi, Pb, Ga, Ag, Al, Sn, Zn, Sb, Fe, Nb, Mg, Mn, Co, Cr, Mo, Zr, Cu, Ni, Pd, Ta, Ti, and W), 20 were found to promote the low-temperature (<400 °C) lateral growth of Ge layers on glass. Based on the reaction type (eutectic or compound), reaction temperatures, diffusion coefficients, and domain matching analyzed using a materials informatics method, the growth morphology was categorized into three types: alloy, dendrite, and uniform. The knowledge of metal-induced growth obtained in this study has broad implications for a variety of materials and holds potential for developing the next generation thin-film devices with common and inexpensive substrates.

The role of tin atoms on the crystallization of amorphous germanium films

The thorough investigation of the GeSn system was able to present some new (or intermediary) properties that, if properly applied, can provide interesting outcomes. One of these refer to the crystallization of amorphous Ge as induced by the presence of Sn, also known as metal-induced crystallization. According to it, depending on the relative composition of the GeSn system Ge crystallites can be produced at rather low temperatures in a gradual and very controllable way. This forms the basis of this work that investigated the Sn-induced crystallization of sputter-deposited amorphous Ge films − as a function of the Sn concentration (∼0−30 at.% range) and temperature of thermal annealing (from 25 to 600 °C). In all cases, the atomic structure of the films was investigated by Raman spectroscopy that corroborates the influence of Sn atoms in reducing the crystallization temperature and improving the quality of the Ge crystallites. Accordingly, this work suggests the suitability of the GeSn system in producing cost-effective poly-crystalline Ge films.

Selective area multilayer graphene synthesis using resistive nanoheater probe

Graphene has been a material of interest due to its versatile properties and wide variety of applications. However, production has been one of the most challenging aspects of graphene and multilayer graphene (MLG). Most synthesis techniques require elevated temperatures and additional steps to transfer graphene or MLG to a substrate, which compromises the integrity of the film. In this paper, metal-induced crystallization is explored to locally synthesize MLG directly on metal films, creating an MLG-metal composite and directly on insulating substrates with a moving resistive nanoheater probe at much lower temperature conditions (~ 250 °C). Raman spectroscopy shows that the resultant carbon structure has properties of MLG. The presented tip-based approach offers a much simpler MLG fabrication solution by eliminating the photolithographic and transfer steps of MLG.

Fabrication of an atomically smooth Ge(111) surface by Au-induced crystallization at 170 °C

Crystalline Ge layer fabricated via layer-exchange metal-induced crystallization is a promising candidate as a seed layer for the epitaxial growth of III–V semiconductor thin films for multijunction solar cells. However, small crystalline islands that grow on top of the crystalline Ge layer are a problem, which roughens the surface and hinders subsequent epitaxial growth. Considering the effect of heating rate on the Au-induced crystallization behavior of Ge, it is found that the temperature required for the island growth in the top Ge layer was higher than that for the bottom layer. By carefully choosing the annealing conditions, the growth of the top Ge layer can be avoided resulting in an atomically smooth Ge(111) surface.

High Mobility IZTO Thin-Film Transistors Based on Spinel Phase Formation at Low Temperature through a Catalytic Chemical Reaction.

In this paper, In0.22 Znδ Sn0.78- δ O1.89- δ (δ = 0.55) films with a single spinel phase are successfully grown at the low temperature of 300 °C through careful cation composition design and a catalytic chemical reaction. Thin-film transistors (TFTs) with amorphous In0 .22 Znδ Sn0.78- δ O1.89- δ (δ = 0.55) channel layers have a reasonable mobility of 41.0 cm2 V-1 s-1 due to the synergic intercalation of In and Sn ions. In contrast, TFTs with polycrystalline spinel In0 .22 Znδ Sn0.78- δ O1.89- δ (δ = 0.55) channel layers, achieved through a metal-induced crystallization at 300 °C, exhibit a remarkably high field-effect mobility of ≈83.2 cm2 V-1 s-1 and excellent stability against external gate bias stress, which is attributed to the uniform formation of the highly ordered spinel phase. The relationships between cation composition, microstructure, and performance for the In2 O3 -ZnO-SnO2 ternary component system are investigated rigorously to attain in-depth understanding of the roles of various crystalline phases, including spinel Zn2- y Sn1- y In2 y O4 (y = 0.45), bixbyite In2-2 x Znx Inx O4 (x = 0.4), rutile SnO2 , and a homologous compound of compound (ZnO)k (In2 O3 ) (k = 5). This work concludes that the cubic spinel phase of Zn2- y Sn1- y In2 y O4 (y = 0.45) film is a strong contender as a substitute for semiconducting polysilicon as a backplane channel ingredient for mobile active-matrix organic light-emitting diode displays.

Controlling barrier height and spectral responsivity of p-i-n based GeSn photodetectors via arsenic incorporation.

GeSn compounds have made many interesting contributions in photodetectors (PDs) over the last ten years, as they have a detection limit in the NIR and mid-IR region. Sn incorporation in Ge alters the cut off wavelength. In the present article, p-i-n structures based on GeSn junctions were fabricated to serve as PDs. Arsine (As) is incorporated to develop n-GeSn compounds via a metal induced crystallization (MIC) process followed by i-GeSn on p-Si wafers. The impact of As and Sn doping on the strain characteristics of GeSn has been studied with high resolution transmission electron microscopy (HRTEM), X-ray diffraction and Raman spectroscopy analyses. The direct transitions and tuning of their band energies have been investigated using diffuse reflectance UV-vis spectroscopy and photoluminescence (PL). The barrier height and spectral responsivity have been controlled with incorporation of As. Variation of As incorporation into GeSn Compounds shifted the Raman peak and hence affected the strain in the Ge network. UV-vis spectroscopy showed that the direct transition energies are lowered as the Ge-As bonding increases as illustrated in Raman spectroscopy investigations. PL and UV-vis spectroscopy of annealed heterostructures at 500 °C showed that there are many transition peaks from the UV to the NIR region as result of oxygen vacancies in the Ge network. The calculated diode parameters showed that As incorporation leads to an increase of the height barrier and thus dark current. Spectral response measurements show that the prepared heterojunctions have spectral responses in near UV and NIR regions that gives them opportunities in UV and NIR photodetection-applications.

Toward White Light Random Lasing Emission Based on Strained Nano Polygermanium Doped with Tin via Metal-Induced Crystallization (MIC)

Solubility of Sn in Ge network gives it a preference for photonic applications, because of the direct transition in GeSn alloy. Here, we employed the metal-induced crystallization (MIC) process of amorphous Ge and Si via Sn as a novel mechanism to incorporate Sn inside Ge and Si networks. (Al/Si/Sn/Ge/Sn) and (Al/Ge/Sn/Ge/Sn) multilayers are deposited by thermal vacuum evaporation on different substrates. The devices are annealed under low vacuum at 500 °C to incorporate the oxygen for band-gap tuning. The structure of Ge-doped nanocrystals is investigated. The direct transition and band-gap values have been estimated using diffuse reflectance spectroscopy and photoluminescence (PL) measurements. PL indicated that the junctions have emissions from visible to NIR regions that make them promising as optically pumped white-light sources as well as waveguide applications, and the impact of the base substrate on enhancing the emission has been investigated via PL measurements. Electroluminescence measurements show that the prepared heterostructures on fluorine-doped tin oxide (FTO) substrate have sharp random lasing spikes over the range of PL samples spectra as the sample can lase randomly by light scattering through the Ge-doped nanocrystalline materials. The charge carrier lifetime measurements show high lifetime for the prepared sample. These give them the chance to be a candidate for white-light random laser diode applications.

A Novel Ni-Al Alloy Metal Induced Lateral Crystallization Process for Improved Channel Conduction in 3-D NAND Flash

In this letter, a Ni-Al alloy based metal induced lateral crystallization (MILC) process in a vertical Si channel, from 3-D NAND flash cell, is reported. Alloying Ni with Al improves NiSi2 nucleation, resulting in a favorable channel morphology after the silicide transport. We show that it is possible to achieve better ON-current and gate control over the existing Ni-only MILC process with no observable impact on the memory performance.

Metal-induced crystallization of amorphous semiconductor films: Nucleation phenomena in Ag-Ge films

The initial stages of interfacial interactions between Ag nanoparticles and amorphous Ge (a-Ge) films, in particular the mechanism and kinetics of metal-induced crystallization (MIC) of a-Ge films are investigated herein. Unsupported Ag-Ge films formed by PVD were used as a model. In situ high-resolution (S)TEM and low-loss monochromated STEM-EELS techniques were used for real-time observation of the nucleation and growth of crystalline Ge and intermediate phases, as well as mapping of chemical elements at the reaction zone. We show that crystallization is activated at the metal–semiconductor interface around ≈200 °C, followed by lateral crystallization of a-Ge film at temperatures above ≈400 °C. It was revealed that the MIC process occurs through intermediate phases. Metastable Ag0.8Ge0.2 hcp phase was identified at the reaction front at elevated temperatures, thus revealing the non-equilibrium melting of a eutectic alloy as the most probable mechanism of Ag-induced crystallization of a-Ge film. We show that the MIC process is activated only if the size of Ag nanoparticles exceeds a critical value, which is ≈8 nm in our case and equal to the thickness of the a-Ge film.

In situ observations of graphene growth and strong graphene‑nickel interactions by grazing incidence X-ray diffraction

The growth mechanisms of graphene on polycrystalline Ni by chemical vapor deposition using polymethyl methacrylate (PMMA) as solid-state carbon sources were investigated by in-situ two-dimensional grazing incidence X-ray diffraction (GIXRD). The results show that the formation of graphene can be described by four stages: The graphitization of solid-state carbon, carbon dissolution in Ni, graphene nucleation, and isothermal growth of graphene, suggesting that the graphitization mechanism is metal-induced crystallization (MIC) mechanism. The so-called induction is mutual, that is, Ni induces the growth of graphene, and reciprocally, graphene induces the rotation and reorientation of Ni. The reorientation of Ni is the enhancement of Ni(111). The rotation and reorientation of Ni occur simultaneously with the nucleation of graphene and continue throughout the growth of graphene. The rotation of Ni ends when the graphene growth stops. Further, the forces causing the rotation and reorientation of the surface Ni lattice are analyzed. It was discovered for the first time that grain rotation was caused by directional carbon diffusion and graphene-Ni interaction. In addition, when the growth temperature is the same, the time when graphene starts to grow is mainly related to the nickel matrix but not the carbon source.

Metal-Induced Crystallization in Metal Oxides.

ConspectusThe properties of a material depend upon its physical characteristics, one of these being its crystalline state. Next generation solid-state technologies will integrate crystalline oxides into thermal sensitive processes and composite materials. Crystallization of amorphous phases of metal oxides in the solid state typically requires substantial energy input to induce the amorphous to crystalline phase transformation. In the case of silica, the transformation to α-quartz in a furnace occurs above 1300 °C and that of titania, above 400 °C. These calcination processes are costly in energy but also often degrade complex material architectures or compositions.Thus, low temperature crystallization techniques are required that preserve macro- and mesostructures and complex elemental composition (e.g., organic-, metal-, and semiconductor-metal oxide hybrids/composites). Some solution-based techniques exist to directly fabricate crystalline metal oxides. However, these are not always compatible with the specificities of the system or industrial constraints. A postsynthetic, solid-state approach that reduces crystallization temperature in metal oxides is metal-induced crystallization (MIC).MIC is the introduction of catalytic amounts of a cation, which can be an s-block, p-block, or d-block cation, that migrates through the solid metal oxide lattice. The cation is thought to temporarily break metal oxide bonds, allowing [MOx] polyhedra to rotate and reform bonds with neighboring [MOx] groups in a lower energy crystalline phase. Depending on the system, the cation can favor or defavor the formation of a particular crystalline phase, providing a means to tune the purity and crystalline phase ratios. An advantage of MIC is that, although the crystallization occurs in the solid state, the crystallization process can be accomplished for particle suspensions in liquid media. In this case, the energy required to induce the crystallization can come from, for example, a microwave or an ultrasound bath. The crystallization of particles in suspension avoids aggregation from particle–particle sintering. In the case of thin films, the energy for crystallization typically comes from a laser or calcination.MIC is only recently being used as a low temperature metal oxide crystallization technique, despite being widely used in the semiconductor industry. Here, the mechanism and previous studies in MIC are presented for titania, silica, and other oxides. The beauty of this technique is that it is extremely easy to employ: cations can be incorporated into the system postsynthetically and then are often expelled from the lattice upon phase conversion. We expect MIC to enrich materials for photochromic, optoelectronic, catalyst, biological, and other applications.

Perspectives on metal induced crystallization of a-Si and a-Ge thin films.

In recent times, the metal induced crystallization (MIC) process in amorphous semiconductors (a-Si and a-Ge) has been extensively investigated by many researchers due to potential applications of crystalline semiconductors in high-density data storage devices, flat panel displays, and high performance solar cells. In this context, we have presented a review on different schemes of MIC in metal/a-Si and metal/a-Ge bilayer films (with stacking change) on various substrates under different annealing conditions. The parameters, which limit crystallization of a-Si and a-Ge have been analyzed and discussed extensively keeping in mind their applications in solar cells and flat panel displays. The MIC of a-Si and a-Ge films under ion beam irradiation has also been discussed in detail. At the end, some suggestions to overcome the limitations of the MIC process in producing better crystalline semiconductors have been proposed. We believe that this review article will inspire readers to perform a thorough investigation on various aspects of MIC for further development of high efficiency solar cells and high quality flat panel displays.

Si/4H–SiC heterostructure formation using metal-induced crystallization

The possibility to realize crystalline silicon (c-Si) layers on 4H–SiC from sputter-deposited, amorphous Si within a low-temperature process is investigated by applying metal-induced crystallization (MIC). In a first experimental series, the basic performance of sputter-deposited Al, Au and Ag thin films as crystallization agents is characterized at temperatures below 275 °C. The crystallinity and orientation of the films after MIC are evaluated using X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. The use of Al revealed the best orientational crystallization of Si, with a dominant <111> direction perpendicular to the interface. A strong influence of the metallic thin film microstructure on the MIC process was found. To enhance the film quality very thin Al layers were used, giving highly crystalline, epitaxially connected c-Si/4H–SiC interfaces with only few remaining Al islands.

Raman scattering spectroscopy for solid-phase and metal-induced crystallization of extremely thin germanium films on glass

Raman scattering spectroscopy was applied to characterize polycrystalline germanium (Ge) on noncrystalline substrates fabricated by solid-phase crystallization and metal-induced crystallization (MIC) using copper (Cu) and tin (Sn) as the catalyzer. The subject was focused on the material for thin-film transistors in which the precursors, with the thickness of only 15 nm, were deposited by sputtering. The optical phonon mode exhibited considerable asymmetry and was separated into the following three components: crystal, low-dimensional nanosized crystallite (NC), and amorphous. The width and frequency shift of the crystal component were analyzed using the phonon confinement effect, thermal stress, and Sn composition. The transition from amorphous to the NC state was associated with the disorder in the precursor. The addition of a restrained amount of Cu led to the evident progress of crystallization and simultaneous suppression of the NC transition. The Sn composition of 3 at% was insufficient to exhibit a noticeable MIC effect.

Evolution of phases and their thermal stability in Ge–Sn nanofilms: a comprehensive in situ TEM investigation

The group IV Ge–Sn system is a promising candidate for a variety of electronic and photonic applications due to its tunable bandgap and compatibility with Si-based technology. However, the peculiarities of the components interaction, phases evolution and, especially, their thermal stability are still far from a brimming comprehension. In the course of in situ TEM heating experiments, we demonstrate the high potential of using nanosized, layered Sn/amorphous-Ge films for the synthesis of Ge–Sn solid solutions with an enhanced Sn content. Particularly, we observe the formation of highly uniform, diamond structured Ge1-xSnx solid solution with Sn content of 33 at% by metal-induced crystallization (MIC) at 90 °C. The Ge0.67Sn0.33 alloy is stable in the range of 20–150 °C, whereat additional heating leads to partial Sn segregation and subsequent melting of the Sn-rich phase. The formation of the metastable and stable Sn-rich liquid phases, which was observed at 90 and 150–190 °C, respectively, has a key role in the system thermal stability. The temperatures and the observed complex interphase interactions are discussed in the frame of the size effects in nanoscaled structures.

Fabrication of Nanocrystalline Silicon Thin Films Utilized for Optoelectronic Devices Prepared by Thermal Vacuum Evaporation.

Metal-induced crystallization of amorphous silicon is a promising technique for developing high-quality and cheap optoelectronic devices. Many attempts tried to enhance the crystal growth of polycrystalline silicon via aluminum-induced crystallization at different annealing times and temperatures. In this research, thin films of aluminum/silicon (Al/Si) and aluminum/silicon/tin (Al/Si/Sn) layers were fabricated using the thermal evaporation technique with a designed wire tungsten boat. MIC of a:Si was detected at annealing temperature of 500 °C using X-ray diffraction, Raman spectroscopy, and field emission scanning electron microscopy. The crystallinity of the films is enhanced by increasing the annealing time. In the three-layer thin films, MIC occurs because of the existence of both Al and Sn metals forming highly oriented (111) silicon. Nanocrystalline silicon with dimensions ranged from 5 to 300 nm is produced depending on the structure and time duration. Low surface reflection and the variation of the optical energy gap were detected using UV–vis spectroscopy. Higher conductivities of Al/Si/Sn films than Al/Si films were observed because of the presence of both metals. Highly rectifying ideal diode manufactured from Al/Si/Sn on the FTO layer annealed for 24 h indicates that this device has a great opportunity for the optoelectronic device applications.