Germanium Films Research Articles

Selective ultrashort laser annealing of amorphous Ge/Si multilayer stacks

We report on single-short laser crystallization of Ge/Si multilayer stacks consisting of alternating amorphous nanosized films of silicon and germanium using near- and mid-infrared femtosecond and picosecond laser pulses. The phase composition of the irradiated stacks was investigated by the Raman scattering technique. Several non-ablative regimes of crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of one- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are analysed based on theoretical estimations and comparison with experimental data. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point. The possibility of ultrafast non-thermal phase transition in germanium in the studied regimes is also discussed.

Infrared analysis of catalytic CO2 reduction in hydrogenated germanium.

The oxidation and carbisation kinetics of porous amorphous and nano-crystalline hydrogenated germanium (a-Ge:H and nc-Ge:H) films exposed to ambient air and deionized water have been studied using vibration modes observed by infrared spectroscopy. Based on infrared analysis, a two-step process of first oxidation by water and secondly carbisation by carbon dioxide (CO2) is proposed that partly mimics the (photo-)catalytic processes in artificial (photo)synthesis. It is shown that water acts like the precursor for oxidation of porous a-Ge:H and nc-Ge:H in the first step. The incorporation of oxygen in a-Ge:H and nc-Ge:H alloys occurs preferentially at Ge-dangling bonds and not at the Ge–Ge back bonds like in hydrogenated silicon alloys (next of kin IV-valence element). The formation of germanium oxide (GeO) tissue at void surfaces locally creates Ge alloys with significantly lower energy levels for the valence band that can align with the half reaction for water reduction. The heterogeneous nature of a-Ge:H and nc-Ge:H oxidation will result in local catalytic generation of electrons and protons. It is proposed that these charge carriers and ions act as precursors for the second-step reaction based on carbisation that includes both the adsorption of CO2 and formation of CO and formaldehyde.

Sub-bandgap light absorption enhancement in germanium films through Berreman mode weak coupling to a microcavity mode

light absorption is a substantial problem that profoundly influences a wide range of disciplines. Whereas it is fundamentally restricted by the bandgap energy of the involved materials. Herein, we study the sub-bandgap light absorption in germanium films via Berreman mode (BE) and its enhancement through weak coupling to Fabry-Perot cavity mode. This enhancement is performed by integrating the semiconductor film into a microcavity structure and tune its resonance frequency to match the epsilon-near-zero (ENZ) wavelength of the film material in a planar multilayer structure. We ascertained that our approach of electric field confinement in the semiconductor layer could perform significant light absorption at large incidence angles. That provides a novel, general, and simple method to enhance the optical and optoelectronic responses of any ENZ material, especially semiconductors below their bandgap energies.

Optically Controlled Ultrafast Terahertz Metadevices with Ultralow Pump Threshold (Small 44/2021)

Ultrafast Terahertz Metadevices In article number 2104275, Jiangang Liang, Chunhai Fan, Chao Chang, and co-workers present a germanium hybrid metallic metasurface under dual-stimulus control. Photoexcitation of germanium film enables 100% modulation depth of Fano resonance and ultrafast switching time within 10 ps. By adding current-bias, the pump threshold to modulate the metasurface is greatly reduced from 1600 to 200 μJ cm−2. Compared with single laser control, the terahertz amplitude modulation is increased by 56.3% under dual-stimulus function.

Improved PECVD processed hydrogenated germanium films through temperature induced densification

Amorphous and nano-crystalline germanium is of potential interest for a wide range of electronic, optical, opto-electronic and photovoltaic applications. In this work the influence of deposition temperature on hydrogenated germanium (Ge:H) films was characterized, using over 200 Ge:H and over 70 SiGe:H films. The demonstrated temperature-induced densification of Ge:H films resulted in more stable films with a lower bandgap energy and dark conductivity and higher activation energy.

Diffusion processes in germanium and silicon films grown on Si3N4 substrates

In this article, the results of investigation of processes occurring during the molecular-beam deposition of germanium layers on Si$_3$N$_4$ dielectric substrates within a wide range of the Ge film growth temperatures (30 to 600{\deg}C) are presented. The intensity of the IR absorption bands related to the vibrations of the N$-$H and Si$-$N bonds are established to decrease with the increase of the Ge deposition temperature. It appears that this phenomenon cannot be explained only as a thermally activated process. Simultaneously, the peak corresponding to the Ge$-$N vibration bonds emerges in the X-ray photoelectron spectra. We suppose that the deposition of germanium layers on Si3N4 dielectric substrates containing hydrogen atoms causes the diffusion of hydrogen atoms from the dielectric layer into the growing film. The experimental results may be interpreted in terms of a model, according to which the migration of hydrogen atoms from the Si$_3$N$_4$ layer into the growing germanium film is due to the difference in chemical potentials of hydrogen atoms in the dielectric layer and the germanium film. This process initiates the diffusion of germanium atoms in the opposite direction, into the Si$_3$N$_4$ layer, where they connect to free bonds of nitrogen atoms arising due to the escape of hydrogen atoms. The analogous processes occur during the deposition of silicon layers on Si$_3$N$_4$ substrates.

Low microwave loss in deposited Si and Ge thin-film dielectrics at single-photon power and low temperatures

In this study, we show that deposited Ge and Si dielectric thin-films can exhibit low microwave losses at single-photon powers and sub-Kelvin temperatures (≈40 mK). This low loss enables their use in a wide range of devices, including coplanar, microstrip, and stripline resonators, as well as layers for device isolation, interwiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication. We use coplanar microwave resonator structures with narrow trace widths and minimal over-etch to maximize the sensitivity of loss tangent measurements to the interface and properties of the deposited dielectrics, rather than to optimize the quality factor. In this configuration, thermally evaporated ≈1 µm thick amorphous germanium (a-Ge) films deposited on Si (100) have effective single-photon loss tangents of 4–5 × 10−6 and 9 μm-thick chemical vapor deposited homoepitaxial single-crystal Si has effective single-photon loss tangents of 4–14 × 10−6. Material characterization suggests that interface contamination could be the limiting factor for the loss.

Design and simulation of a piezoelectric transducer by finite element

One of the problems encountered in the manufacture of microstructures (MEMS) is the presence of residual stress caused by high temperatures during the manufacturing process. Residual stress generates deformation (ε) in materials, which consists of fractional changes in their dimensions (linear, surface or volume). The application of amorphous materials in the manufacture of MEMS is carried out at a lower temperature than those used with Polysilicon, so it is an alternative to reduce residual stress. The objective of the work is to analyze the mechanical behavior of thin films of amorphous Silicon germanium (a-SiGe). The analysis is carried out through the behavior of springboard type resonant structures and using the simulation of the finite element method (FEM). The resonance frequency of a trampoline-type structure depends only on its geometric dimensions, its density, and its Young's modulus. The simulation results show that the behavior of the resonance frequency and the Young's Modulus of thin films of a-SiGe, placed on a trampoline-type structure changes linearly whit Germanium content.

Explosive crystallization of sputter-deposited amorphous germanium films by irradiation with an electron beam of SEM-level energies

The crystallization of sputter-deposited substrate-free films of amorphous germanium was induced by electron irradiation at SEM-level energies of less than 20 keV at ambient temperature using an electron probe microanalyzer. Instantaneous crystallization, referred to as explosive crystallization, occurred consistently at 2–20 keV; the threshold of electron fluxes is 1015–1016 m−2 s−1, which is five to six orders of magnitude lower than those at 100 keV reported previously. This process is expected to be advantageous in the production of polycrystalline Ge films since it is rapid, requires little energy, and results in negligible damage to the substrate.

Ultrafast all-optical terahertz modulation based on an inverse-designed metasurface

Metasurface plays a key role in various terahertz metadevices, while the designed terahertz metasurface still lacks flexibility and variety. On the other hand, inverse design has drawn plenty of attention due to its flexibility and robustness in the application of photonics. This provides an excellent opportunity for metasurface design as well as the development of multifunctional, high-performance terahertz devices. In this work, we demonstrate that, for the first time, a terahertz metasurface supported by the electromagnetically induced transparency (EIT) effect can be constructed by inverse design, which combines the particle swarm optimization algorithm with the finite-difference time-domain method. Incorporating germanium (Ge) film with inverse-designed metasurface, an ultrafast EIT modulation on the picosecond scale has been experimentally verified. The experimental results suggest a feasibility to build the terahertz EIT effect in the metasurface through an optimization algorithm of inverse design. Furthermore, this method can be further utilized to design multifunctional and high-performance terahertz devices, which is hard to accomplish in a traditional metamaterial structure. In a word, our method not only provides a novel way to design an ultrafast all-optical terahertz modulator based on artificial metamaterials but also shows the potential applications of inverse design on the terahertz devices.

Growth of High Sn Concentration Germanium-Tin Films on Insulators by Microsecond Laser Annealing

Germanium-tin (GeSn) is attractive group-IV material with great optical properties as light emitting and receiving element. To apply such significant properties on photonic devices, the growth of GeSn films with high substitutional Sn concentration on insulating substrates is essential. In this work, we have investigated continuous wave laser annealing of amorphous germanium (a-Ge) and a-GeSn films (initial Sn concentration: 20 at.%) on quartz substrates with microsecond annealing time. As a result, large shifting of Raman peak was observed for GeSn samples after annealing, while there was little shifting of that for Ge samples. We speculate that these results are due to incorporation of Sn atoms into the grown films.

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.

Atmospheric pressure metal organic chemical vapor deposition of thin germanium films

The deposition of thin germanium films by atmospheric pressure metal organic chemical vapor deposition at temperatures below 400 °C on substrates such as silicon wafers, float glass, and polyimide (Kapton®) using the diorganogermanes GeH2Cp4M2 and GeH2Cp*2 as molecular precursors is described. The deposition rates and thus the layer thicknesses can be varied by temperature and time to give layers with a thickness in the nanometer range. The homogeneity and roughness of the deposited films were analyzed by means of atomic force microscopy measurements showing the formation of smooth and uniform surfaces with roughnesses of the films in the range of (1 ± 0.15) nm to (4.5 ± 1.5) nm. Films with thicknesses between 50 and 750 nm were deposited and analyzed by Raman spectroscopy, vis–NIR spectroscopy, electron microscopy, energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The as-deposited films are composed of amorphous germanium containing approximately 10% of carbon. Using Kapton® as a substrate highly flexible films were obtained.

Self-organized 〈1 0 0〉 direction growth of germanium film on insulator obtained by high speed continuous wave laser annealing

To realize next-generation electronics, germanium crystal growth on insulating substrates is attracting attention due to its high carrier mobility, and further improvement of carrier mobility is anticipated by applying tensile strain to germanium. We have previously realized highly tensile-strained germanium films on insulating substrates using a high speed continuous wave laser annealing process. This letter reports how we characterized the grown films using electron diffraction backscatter measurements and discovered an interesting self-organized 〈100〉 direction type of growth that results from scanning the annealing laser at high speed. Dominant {110} oriented films were obtained in this way. This phenomenon is discussed below from the viewpoint of surface energy.

Study of Si–Ge Interdiffusion in Laser Recrystallization of Ge Epitaxial Film on Si Substrate

Direct epitaxial growth of germanium (Ge) film on silicon (Si) substrate (GOSS) holds great potential in micro-electronics and optoelectronics. However, due to the 4.2% lattice mismatch between Si and Ge, it is difficult to directly obtain high quality Ge by epitaxy on Si substrate. Laser recrystallization technology provides a simple, efficient and low-cost way to improve the crystal quality of epitaxial Ge film grown on Si substrate. This technology is essentially a process of thermally induced phase transformation. By controlling the laser process parameters, epitaxial film of a certain thickness is melted, so that lattice rearrangement and recrystallization are achieved, and high-quality thin Ge/Si can be prepared. Laser recrystallization is a high temperature thermal process, and Si–Ge interdiffusion may detrimentally occur. In this paper, the mechanism of Si–Ge interdiffusion is discussed. Based on Fick's law of diffusion, a numerical model for Si–Ge interdiffusion of GOSS is established. On this basis, the process simulation of thermal annealing and laser recrystallization Si–Ge interdiffusion is carried out by Sentaurus Process simulation. The results show that compared with the traditional thermal annealing, the Si–Ge interdiffusion of Ge on Si almost does not occur in the process of laser recrystallization. By reasonably controlling the process parameters of laser recrystallization, the thin Ge film near the Si–Ge interface does not melt, which can not only improve the crystal quality of Ge epitaxial layer, but also effectively avoid the Si–Ge interdiffusion in the process of laser recrystallization. Through this research, we have aimed at predicting and control the Si–Ge interdiffusion, providing an important technical reference for the preparation of high quality GOSS by laser recrystallization technology.

Measurement of Volume Changes and Associated Stresses in Ge Electrodes Due to Na/Na+ Redox Reactions

In situ electrochemical cells were assembled with an amorphous germanium (a-Ge) film as working electrode and sodium foil as reference and counter electrode. The stresses generated in a-Ge electrodes due to electrochemical reaction with sodium were measured in real-time during the galvanostatic cycling. A specially designed patterned a-Ge electrode was cycled against sodium and the corresponding volume changes were measured using an AFM; it was observed that sodiation/desodiation of a-Ge results in more than 300% volume change, consistent with literature. The potential and stress response showed that the a-Ge film undergoes irreversible changes during the first sodiation process, but the subsequent desodiation/sodiation cycles are reversible. The stress response of the film reached steady-state after the initial sodiation and is qualitatively similar to the response of Ge during lithiation, i.e., initial linear elastic response followed by extensive plastic deformation of the film to accommodate large volume changes. However, despite being bigger ion, sodiation of Ge generated lower stress levels compared to lithiation. Consequently, the mechanical dissipation losses associated with plastic deformation are lower during sodiation process than it is for lithiation.

Updated Model for Thermal Conductivity Calculation of Thin Films of Silicon and Germanium

A model for calculation of thermal conductivity of thin films of silicon and germanium is presented on the basis of solving the Boltzmann equation in the time relaxation approximation. The unique feature of this model is the application of recent achievements in determination of the time of phonon–phonon interactions obtained from “first principles” and also the use of real dispersion curves and the propagation velocities of phonons with different polarization. The proposed approach contains no averaging and fitting parameters, which already for more than half a century and until now have been common in calculations of thermal conductivity of macroscopic bodies and nanostructures. A detailed qualitative and quantitative comparison of results of calculations by the “classical” and “updated” models is carried out. Problems in the methods for calculating thermal conductivity for simulation of new structures in advanced semiconductor devices and ways of their further development are set forth.

The impact of processing conditions and post-deposition oxidation on the opto-electrical properties of hydrogenated amorphous and nano-crystalline Germanium films

Low-cost multijunction photovoltaic devices are the next step in the solar energy revolution. Adding a bottom junction with a low bandgap energy material through plasma enhanced chemical vapor deposition (PECVD) processing could potentially provide a low-cost boost in conversion efficiency. A logical candidate for this low bandgap material is germanium. In this work we investigate the growth of PECVD processed hydrogenated amorphous/nano-crystalline germanium (a/nc-Ge:H), by characterizing over 100 samples, processed with a wide range of deposition pressures, powers, temperatures and GeH4 dilution in hydrogen, using elemental analysis, vibrational analysis and analysis of the opto-electrical properties. We have identified a small processing window in which nc-Ge:H films are processed reproducibly. We also report on the strong correlation between the refractive index of the films and the presence- and extent of post-deposition oxidation. Notably, the oxidation generally increased the photoresponse of the films, as it results in a decrease of room temperature σd by 1-3 orders of magnitude. However, oxidation results in an increase of the bandgap energy and therefore impedes the development of a low bandgap material. The lowest E04 we report is about 1.1eV, with an ETauc of 0.9eV and an σph/σd of 3.4.

Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition.

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, i Pr2 P(C6 H4 )BCy2 (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH2 }] and [PB{GeH2 }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH2 }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2 }] was also prepared, and shown to release poly(dimethylsilane) [SiMe2 ]n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Frustrated Lewis Pair Chelation as a Vehicle for Low‐Temperature Semiconductor Element and Polymer Deposition

AbstractThe stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, iPr2P(C6H4)BCy2 (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH2}] and [PB{GeH2}] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine‐borane ligand (PB) after element deposition, and 2) the single‐source precursor [PB{SiH2}] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2}] was also prepared, and shown to release poly(dimethylsilane) [SiMe2]n upon heating. Overall, this study introduces a “closed loop” deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.