Germanium Films Research Articles

Magnetic order in nanogranular iron germanium (Fe0.53Ge0.47) films

We study the effect of strain on the magnetic properties and magnetization configurations in nanogranular FexGe1-xfilms (x=0.53±0.05) with and without B20 FeGe nanocrystals surrounded by an amorphous structure. Relaxed films on amorphous silicon nitride membranes reveal a disordered skyrmion phase while films near and on top of a rigid substrate favor ferromagnetism and an anisotropic hybridization of Fedlevels and spin-polarized Gespband states. The weakly coupled topological states emerge at room temperature and become more abundant at cryogenic temperatures without showing indications of pinning at defects or confinement to individual grains. These results demonstrate the possibility to control magnetic exchange and topological magnetism by strain and inform magnetoelasticity-mediated voltage control of topological phases in amorphous quantum materials.

Nanometric Ge Films for Ultrafast Modulation of THz Waves with Flexible Metasurface

AbstractHarnessing confined light on a subwavelength scale within Fano metasurfaces enhances light‐matter interaction on a flexible, low‐loss substrate. This approach enables the integration of ultra‐thin semiconducting films for designing low‐power, ultrafast switchable terahertz, and optical meta devices. Here, an ultra‐thin, ∼λ/6000 germanium overlayers of 25nm or 50nm is thermally evaporated onto a flexible Fano metallic metasurface to achieve ultrafast optical modulation of terahertz radiation. A remarkable 85% modulation depth with an ultrafast speed of 400 GHz is obtained in the resonant transmission. These ultrathin, flexible, and ultrafast functional metasurfaces pave the way for developing flexible terahertz active meta devices based on nanometric scale germanium films, offering the additional advantage of compatibility with complementary metal‐oxide‐semiconductor (CMOS) technology in microelectronics.

Preparation and loss analysis of Ge on Si SWIR optical strip waveguides

On-chip integrated photonics with operating wavelength at short-wave infrared region is becoming popular for its potential advantages in extending the telecommunication bandwidth and the application in gas sensing. As an important component of integrated optoelectronic chips, high-performance waveguides have attracted widespread attention. In this work, high-performance Ge waveguides were designed and fabricated on the Ge-on-Si platform, in which germanium films were epitaxially grown by the chemical vapor deposition process. The propagation loss for the waveguide was measured to be 2.34 dB/cm at the wavelength of 2 μm through the cut-back method. The quantitative relationship between the waveguide propagation loss and sidewall roughness as well as material defect density was established by introducing an empirical coefficient m. The m value was calculated to be 3.8 × 10−4 dB for the waveguide with a fixed Ge thickness of 1.5 μm by fitting the experimental propagation loss data at an operating wavelength of 2 μm. This formula provides a significant reference for future design of waveguide dimension structures, where defect induced absorption loss is non-negligible.

Low-frequency electrodynamics in the mixed state of superconducting NbN and a-MoGe films using two-coil mutual inductance technique

We investigate the low-frequency electrodynamics in the vortex state of two type-II superconducting films, namely, a moderate-to-strongly pinned Niobium Nitride (NbN) and a very weakly pinned amorphous Molybdenum Germanium (a-MoGe) film. We employ a two-coil mutual inductance technique to extract the complex penetration depth, λ~ . The sample response is studied through the temperature variation of λ~ in the mixed state, where we employ a model developed by Coffey and Clem (the CC model) to extract different vortex lattice (VL) parameters, such as the restoring pinning force constant (Labusch parameter), VL drag coefficient and pinning potential barrier. We observe that a consistent description of the inductive and dissipative part of the response is only possible when we take the viscous drag on the vortices to be several orders of magnitude larger than the viscous drag estimated from the Bardeen–Stephen model.

Strain mapping in amorphous germanium thin films with scanning reflectance anisotropy microscopy

Strain imaging is a critical aspect in the design and characterization of opto-electronics, microelectronics, flexible electronics, and on-chip photonics. However, strain mapping techniques are often material specific and strain measurements in amorphous materials remain a challenge. Here, we demonstrate strain mapping and optical characterization of an amorphous semiconductor using scanning reflectance anisotropy microscopy. Using reflection anisotropy spectroscopy and finite element simulations on evaporated amorphous germanium films, we showcase the strain sensitivity of the ellipsometric parameters. We demonstrate nondestructive mapping for simple and complex strain states in amorphous systems. The sub-degree phase and amplitude sensitivity of the microscope is able to determine strain states on the order of 10−3.

High extinction ratio TE-Pass polarizer using the effect of superstrate layer in Ge-Clad optical waveguide

Attenuation characteristics of germanium (Ge) film clad planar optical waveguide are theoretically studied at the spectral wavelength of 0.6328 μm. Thin films of Ge are found to possess greater attenuation values than the other recognized materials. The existence of these greater attenuations is caused by lossy modes that are sustained by these films. It has been shown that the attenuation of the TM mode can be greatly increased by adding a high-index buffer layer between the Ge layer and the dielectric guide. It occurred because of the resonant coupling between the lossy modes supported by the Ge layer and the waveguide modes. Moreover, it is claimed that by adding a dielectric superstrate layer above the Ge thin film, the strength of the TM mode in the proposed structure can be further attenuated to an ultra-high value. In this study, a TE pass polarizer is proposed having a high extinction ratio of 30582.15 dB. Attenuation results are presented, after optimization of several parameters like thicknesses of Ge, buffer, and superstrate layers along with environmental stability of the suggested structure. The generated field distribution profiles are also in good agreement with the obtained attenuation results.

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.

Tailoring polycarbonate surfaces for improved Ge film adhesion: The role of plasma treatments in 50:50 O2/Ar atmospheres

Polycarbonate (PC) substrates were exposed to plasma environments consisting of a 50:50 mixture of oxygen and argon, and were subsequently coated with germanium (Ge) films grown via sputtering. The hydrophilicity of the PC surfaces was tailored by ion bombardment with energy levels ranging from 50 to 300 eV, which led to a reduction in water contact angles from a native 80° to a superhydrophilic state. Analyses of chemical composition and structure of the PC were performed using X-ray Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy in Attenuated Total Reflectance geometry (FTIR-ATR), and then correlated to the adhesion of the Ge thin films. Optimal adhesion of the Ge films was achieved by bombarding the PC with ions at energies between 100 and 200 eV, activating the polymer surface while avoiding photodegradation as confirmed by chemical analysis. We report an efficient method for achieving superhydrophilicity of PC within a short treatment time of 60 s that can be effectively integrated in diverse vacuum applications.

Spin-related negative magnetoresistance in germanium films

Herein, we report the transport properties of Ge films. The variable-range hopping transport at low temperatures (T≲50K) and thermal activation transport at high temperatures (T≳50K) are observed in our Ge films. In the different temperature regimes, the anomalous magnetotransport properties are observed. In the low-temperature regime (T≲15K), the negative magnetoresistance (MR) at low field and positive MR at high field can be seen. In the moderate-temperature regime (15K≲T≲100K), the positive MR curve gradually evolves from a linear curve to a parabolic curve with increasing temperature, and the MR magnitude appears to be insensitive to temperature. In the high-temperature regime (T≳100K), the positive MR value increases with increasing temperature. By considering the angular-dependent MR, we can determine that the negative MR comes from the spin-related mechanism, and the positive MR is caused by the orbital-related mechanism. However, further study is required to determine the exact mechanisms behind the anomalous magnetotransport properties.

Ultralow thermal conductivity of amorphous silicon–germanium thin films for alloy and disorder scattering determined by 3ω method and nanoindentation

Abstract The ultralow thermal conductivity (1.3 W/(m∙K)) of amorphous silicon–germanium films for alloy and disorder scattering was investigated using the 3ω method and nanoindentation. The films exhibited the lowest phonon mean free path (MFP) of 0.5 nm compared to that of amorphous silicon (1.1 nm) and germanium (0.9 nm) films, owing to alloy scattering in the silicon–germanium films. Based on Matthiessen’s rule, the phonon MFPs of the amorphous silicon–germanium films contributing to alloy and disorder scattering were calculated to be 1.0 nm for both. Therefore, alloy and disorder scattering contribute equally to the reduction in the phonon MFP.

Flexible top-illuminated organic photodetector using an ultrathin-metal-based transparent electrode

Top-illuminated structure facilitates the integration of organic photodetectors (OPDs) into high-resolution flexible wearable light detection systems by allowing the OPDs to be deposited on the bottom readout circuit. However, constructing this structure poses a challenge as it demands metallic electrodes with both high optical transparency and high electrical conductivity. But to achieve practical sheet resistances, most semitransparent metallic electrodes tend to reflect a large portion of incident light instead of allowing it to be absorbed by the photoactive layer of the OPDs. This, in turn, results in reduced photocurrent generation. To address this issue, a semiconducting germanium (Ge) film is introduced into a sliver (Ag) film, effectively reducing its reflectivity by lessening scattering. The Ge film also changes how the Ag film grows, further reducing its absorption by lowering the critical thickness needed for forming a continuous film. This approach yields a 10 nm metallic electrode with a transmittance of 70%, a reflectivity of 12%, and a sheet resistance of 35.5 Ω/□. Using this metallic electrode, flexible OPDs exhibit a high photo-to-dark current ratio of 2.9×104 and improved mechanical properties. This finding highlights the benefits of the top-illuminated structure, which effectively reduces losses caused by waveguided modes of the incident light.

Ultrafast Infrared Laser Crystallization of Amorphous Ge Films on Glass Substrates.

Amorphous germanium films on nonrefractory glass substrates were annealed by ultrashort near-infrared (1030 nm, 1.4 ps) and mid-infrared (1500 nm, 70 fs) laser pulses. Crystallization of germanium irradiated at a laser energy density (fluence) range from 25 to 400 mJ/cm2 under single-shot and multishot conditions was investigated using Raman spectroscopy. The dependence of the fraction of the crystalline phase on the fluence was obtained for picosecond and femtosecond laser annealing. The regimes of almost complete crystallization of germanium films over the entire thickness were obtained (from the analysis of Raman spectra with excitation of 785 nm laser). The possibility of scanning laser processing is shown, which can be used to create films of micro- and nanocrystalline germanium on flexible substrates.

Growth of Germanium Thin Films on Sapphire Using Molecular Beam Epitaxy

Germanium films were grown on c-plane sapphire with a 10 nm AlAs buffer layer using molecular beam epitaxy. The effects of Ge film thickness on the surface morphology and crystal structure were investigated using ex situ characterization techniques. The nucleation of Ge proceeds by forming (111) oriented three-dimensional islands with two rotational twin domains about the growth axis. The boundaries between the twin grains are the origin of the 0.2% strain and tilt grains. The transition to a single-grain orientation reduces the strain and results in a better-quality Ge buffer. Understanding the role of thickness on material quality during the Ge(111)/Al2O3(0001) epitaxy is vital for achieving device quality when using group IV material on the sapphire platform.

Electrodeposition of crystalline germanium thin films by the electrochemical liquid-liquid-solid method

The direct electrodeposition of crystalline germanium thin films in aqueous solutions at low temperatures represents a promising and cost-effective method for synthesising semiconductor materials. This study investigates the influence of the deposition potential and time on the characteristics of crystalline germanium thin films derived from germanium oxide precursors in aqueous solution through electrochemical liquid–liquid-solid (ec-LLS) process at low temperature. Additionally, it establishes a Ge/Cu Schottky junction by heterogeneous growth of germanium films on copper (Cu) metal via electrochemical liquid–liquid-solid–solid (ec-LLSS) process.X-ray diffraction (XRD) analysis revealed that an increase in the deposition potential to −1.7 V resulted in a shift in the crystal orientation of the germanium film from (220) to (111), while excessive deposition potential (>−2.0 V) led to a reduction in the crystallinity of the deposited samples. At a deposition temperature of 70 °C and a deposition potential of −1.8 V, the crystal cluster size and the crystal quality increased with the increasing deposition time. However, at −2.0 V, the crystallinity of the germanium film reached an optimal state after 30 min and then gradually declined with time.Charactersation techniques such as EDS, XPS and Raman spectroscopy indicated that the use of liquid electrode Ga(l) as a doping source during the electrodeposition process allowed for heterogeneous growth of Ga-doped p-type germanium films on a Cu substrate.The I-V characteristic curves demonstrated that the Ge/Cu Schottky junctions exhibited significant rectification characteristics, rendering them suitable for potential applications in solar cells.

Superconductivity in hyperdoped Ge by molecular beam epitaxy

Superconducting germanium films are an intriguing material for possible applications in fields such as cryogenic electronics and quantum bits. Recently, there has been a great deal of progress in hyperdoping of Ga doped Ge using ion implantation. Thin film growth of such a material would be advantageous, allowing homoepitaxy of doped and undoped Ge films and opening possibilities for vertical Josephson junctions. Here, we present our studies on the growth of one layer of hyperdoped superconducting germanium thin film via molecular beam epitaxy. We observe a fragile superconducting phase, which is extremely sensitive to processing conditions and can easily phase-segregate, forming a percolated network of pure gallium metal. By suppressing phase segregation through temperature control, we find a superconducting phase that is unique and appears coherent to the underlying Ge substrate.

A step toward calculating the uncertainties in combined GIXRF‐XRR

AbstractThe combination of X‐ray reflectivity (XRR) and grazing incidence X‐ray fluorescence (GIXRF) is a surface sensitive analytical method, which can be used for the characterization of thin films and multilayered materials. Both of these techniques are implemented on the same experimental setup and make use of similar mechanical processes and the same fundamental physical concept required for a combined data analysis. The combination of these techniques removes ambiguous results for the characterization of nanometer layers, as well as nanometer depth profiles, resulting in more accurate characterization of thickness, roughness, density, and elemental composition. Due to the vast number of fitting parameters, the estimation of the thin film sample structure is a challenging task. In this paper, we propose a recursive method for estimating the uncertainties of data from GIXRF‐XRR analysis, based on a Bootstrap statistical method. This approach relies on re‐sampling a dataset to estimate statistics on a population by applying random weights. We applied this method on an as‐deposited chalcogenide germanium, antimony, and tellurium (GST) thin film with a carbon‐capping layer. We found good agreement between the experimental and the theoretical XRR‐GIXRF values for a sample structure model, of which the parameters were determined within a confidence interval using the bootstrap method. We also propose an approach for calculating the uncertainty on the solid angle of detection based on Monte Carlo simulations.

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.

Electrical, optical, and magnetic properties of amorphous yttrium iron oxide thin films and consequences for non-local resistance measurements

We present magnetic characterization, charge resistivity, and optical photoluminescence measurements on amorphous yttrium iron oxide thin films (a-Y–Fe–O), with supporting comparisons to amorphous germanium (a-Ge) films. We measured magnetic properties with both SQUID magnetometry and polarized neutron reflectometry. These results not only confirm that a-Y–Fe–O is a disordered magnetic material with strong predominantly antiferromagnetic exchange interactions and a high degree of frustration, but also that it is best understood electrically as a disordered semiconductor. As with amorphous germanium, a-Y–Fe–O obeys expectations for variable-range hopping through localized electron states over a wide range of temperature. We also clarify the consequences of charge transport through such a semiconducting medium for non-local voltage measurements intended to probe spin transport in nominally insulating magnetic materials. We further compare non-local resistance measurements made with “quasi-dc” automated current reversal to ac measurements made with a lock-in amplifier. These show that the “quasi-dc” measurement has an effective ac current excitation with frequency up to approximately 22 Hz, and that this effective ac excitation can cause artifacts in these measurements including incorrect sign of the non-local resistance. This comprehensive investigation of non-local resistance measurements in a-Y–Fe–O shows no evidence of spin transport on micrometer length scales, which is contrary to our original work, and in line with more recent investigations by other groups.

Reactive Force Field Molecular Dynamics Study of the Effects of Gaseous Species on the Composition and Crystallinity of Silicon–Germanium Thin Films

We simulated the growth of a silicon–germanium (SiGe) film using reactive force field molecular dynamics (ReaxFF MD) in combinations of SiH3, SiH2, GeH3, and GeH2 radicals to evaluate the effects of gaseous species on thin-film composition and crystallinity and to understand the growth mechanisms. The film compositions could be estimated in these combinations because of the linear increase in the Ge content of the films. The average crystallinity grown by SiH3 was higher than that by SiH2 radicals. The crystallinity of the film grown by SiH3 radicals tends to be drastically decreased by GeH2 radicals. The growth mechanisms for XH3 and XH2 (X = Si or Ge) radicals were compared. XH3 radicals abstracted surface H atoms, and then more XH3 radicals chemisorbed onto the formed dangling bonds, resulting in film growth through a two-step reaction known as the Eley–Rideal-type (ER-type) mechanism. The ER-type mechanism grows the film with a low hydrogen content and high crystallinity. In contrast, XH2 radicals displayed not only the ER-type mechanism but also a one-step reaction, the H-capturing mechanism, which incorporates surface H atoms into the gaseous species. The H-capturing mechanism results in film growth with high hydrogen content and low crystallinity. The growth mechanisms are influenced by high/low H-coverage. The surface H atoms thermally move around the bonded atoms and give their kinetic energy to the diffusing gaseous species. Excess surface H atoms promote desorption. Our results from the ReaxFF MD suggested experimental settings and conditions that would enable the growth of high-quality films. Our results also suggested that SiH3 and GeH3 radicals should be mainly generated in the gas phase for high-quality SiGe film growth.

Improving transmittance of long-wave infrared guided-mode resonant filters

The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.