Ge Films Research Articles

Visible color camouflage and infrared, laser band stealth, compatible with dual-band radiative heat dissipation

Infrared stealth technology garners significant attention due to its value in military applications. With the rise of multi-band detection technologies, the infrared stealth target is easily detected. Therefore, it is urgent to study multi-band camouflage devices. Here, this paper proposes a multi-band camouflage device composed of ZnS, Ge, TiO2, crystalline Ge2Sb2Te5 (cGST) and Ag multilayer films, which can realize multi-band camouflage across visible, mid-wave infrared (MWIR, ε3∼5 μm = 0.22), long-wave infrared (LWIR, ε8∼14 μm = 0.16), and laser (R1.06 μm = 0.01) spectra. In addition, effective radiative heat dissipation was realized in two non-atmospheric transparent windows (ε2.5–3 μm = 0.51 and ε5∼8 μm = 0.65). Among them, visible camouflage in different background environments can be realized independently by adjusting the thickness of the ZnS film to produce different structural colors. Notably, this emissivity spectrum remains robust even as the incident angle increases to 60°. These results can guarantee the realization of multi-band infrared camouflage, laser camouflage and radiation heat dissipation. This study not only addresses high-efficiency infrared camouflage and thermal management but also achieves outstanding visible and laser camouflage performance, offering a comprehensive guidance for advancing multi-band camouflage technology.

Amorphous to Polycrystalline Phase Transition of In Situ Annealed Ge Films: Structural, Morphological, Optical, and Electrical Characteristics

AbstractRadio Frequency (RF) sputtered germanium (Ge) thin films were deposited on glass substrate at in situ annealing temperatures of 400, 450, 500, 550, and 600 °C. X‐ray diffraction (XRD) and Raman spectroscopy revealed that the Ge films were initially amorphous but transformed to polycrystalline after annealing at 450 °C. Further annealing up to 600 °C resulted in improved crystallinity. Strain studies of the films using XRD and Raman showed compressive (out‐of‐plane) and tensile (in‐plane) strains, respectively, throughout all the annealing temperatures. The surface morphology of the films was recorded using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM), which showed grain growth with annealing temperatures. The optical energy gap evaluated from absorption spectra for crystalline films decreased with annealing temperature, indicating improvement in crystallinity of films. The resistivity of the films deduced using current‐voltage measurements was found to decrease with annealing temperature, which can be attributed to enhancement in grain size. This study establishes a correlation between strain, grain size, optical energy gap, and electrical resistivity with temperature for Ge films deposited on glass substrates. Thus, the present study provides insights into variation in physical and electrical properties of films with in situ annealing temperature for their applicability in optoelectronic devices.

Electrical properties of completely compensated single-crystal Ge-on-GaAs films with two-dimension Coulomb disorder

We present electrical properties of heavily doped and completely compensated Ge films grown on semiinsulating GaAs(100) substrates by vacuum evaporation. The thin (∼100 nm) Ge films are single-crystal and characterized using temperature-dependent transport measurements, with anomalously large activation energy up to half the Ge bandgap, anisotropy of the transverse magnetoresistance, high resistivity (up to 140 Ω cm), low free charge carrier mobility (∼50 cm2/V·s), and concentration (∼1014–1015 cm−3). This behaviour is attributed to a completely compensated semiconductors arising from Ga and As impurity incorporation and large-scale potential fluctuations. Analysis suggests a two-dimensional percolative transport mechanism in Ge-on-GaAs heterostructures.

Dual-dielectric Fabry-Perot film for visible-infrared compatible stealth and radiative heat dissipation

Multiband camouflage of space satellite to against advanced detection has attracted rising attention. However, the combination of compatible camouflage and radiative heat dissipation for space satellite still remains challenging, which requires low visibility and selective thermal emission in a dark and deep cold background. In this work, a dual dielectric Ge/SiO2 layer embedded Fabry-Perot multilayer film (W/Ge/SiO2/W/SiO2) is proposed for space stealth and heat dissipation. Ge and SiO2 films with appropriate thickness are designed to realize high visible absorption and spectra selective IR emission, simultaneously. The optimized multilayer film achieves a high absorbance (α = 0.983) in visible light (0.38–0.80 μm) for visible stealth, low emissivity (ε3–5 μm = 0.038; ε8–14 μm = 0.200) in atmospheric windows for infrared stealth, and relative high emissivity outside the atmospheric windows (ε5–8 μm = 0.487) for radiative heat dissipation. The simulation results indicate that the coupling of tunneling effect in ultrathin metallic W film and Fabry-Perot resonant is the main contribution to the excellent absorptivity and emissivity tunability. The proposed dual spacers embedded Fabry-Perot multilayer film paves a new way for multispectral camouflage of space satellite and other applications such as solar-thermal conversion, thermal management, energy saving, and detective technologies.

Au-induced crystallization of amorphous Ge thin films: An indication of the existence of a recrystallization process during growth at deep subeutectic temperature

The influence of Au catalyst microstructure on the crystallization behavior of Ge thin films obtained by Au-induced crystallization method is studied. By improving the Au catalyst crystallinity by pre-annealing, the hole mobility of Ge thin film crystallized at 200°C is improved to nearly 150 cm2/Vs thanks to the increase in Ge grain size, which is fairly high for a Ge film as thin as 10 nm. It is found that the increase in Ge grain size originates from the recrystallization process at deep subeutectic temperature. This is attributed to the existence of a metastable AuGe alloy phase surrounding crystalline Ge, which is kinetically stabilized by a continuous flow of Ge atoms through the alloy phase and realizes a solution-like growth. The process using a kinetically stabilized metastable phase will open up the possibility to realize a novel cost-effective and low-temperature process to fabricate high-performance semiconductor devices on plastic substrates.

Experimental analysis and ANFIS modeling of optical properties and band gap engineering in Se–Te–Ge film composition and insights into its photonic applications

Experimental analysis and ANFIS modeling of optical properties and band gap engineering in Se–Te–Ge film composition and insights into its photonic applications

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.

Pit-formation in germanium homoepitaxial layers

With increasing importance of germanium (Ge) for semiconductor quantum technologies, the necessity of growing defect-free Ge films has become crucial. Here, Ge homoepitaxial growth experiments were conducted using molecular beam epitaxy (MBE). This requires a smooth (σrms≤10 Å) and contamination-free substrate surface. We have shown that common ex-situ wet chemical cleaning methods are not sufficient. Foreign atoms like carbon stick to the substrate surface and tend to form clusters which results in macroscopic growth defects which are referred here as pits. The density of such pits is in the range of 106 to 109 cm−2. The formation and characteristics of the pits have been investigated. Pits emerge after a growth at 370 °C with a thickness of above 5 nm . At growth temperatures lower than 300 °C, the density of pits increases, while it decreases at temperatures higher than 400 °C. Pits can be faceted with the main facets being {1 1 3} and {3 15 23}.Furthermore, a procedure is presented involving a Ge buffer growth at room temperature with a high growth rate (0.02 nm/s). This leads to a coverage of present contamination and the formation of a new substrate surface which prevent pit formation.

Separation of wafer bonding interface from heterogenous mismatched interface achieved high quality bonded Ge-Si heterojunction

In the realm of optoelectronic integration, silicon-based Ge bonding has attracted great attention due to its advantages in short-wave infrared response and low cost. However, owing to the inherent lattice mismatch and thermal mismatch between Ge and Si, the bonded Ge-Si heterogenous interfaces encountered problems of thermal instability and high interface states. Herein, an approach of separating the bonding interface from the heterogenous interface is proposed through inserting a Ge epitaxial layer (GeEL) between the Si substrate and Ge film. This separation modifies the bonding interface to a homogenous one, alleviating the massive mismatch stress and achieving film relaxation, enhancing bonding stability in all aspects. Moreover, during 850 °C annealing, GeEL is doped via diffusion hence realizes electric filed modulation, inhibiting the carrier recombination leakage current and trap assisted tunneling in this defect-rich layer. A Ge-Si heterogenous PIN diode prepared by this bonding method has achieved a remarkable low dark current density (1.33 mA/cm2), a low ideality factor (1.11), and a high on–off ratio (106), confirming the outstanding quality of the bonded heterojunction. This work provides great prospects for higher performance, larger scale and multi-functional Si-based heterogenous material device applications.

Complementary absorption effect to improve the optical efficiency of dual-absorption-layered PSCs

By taking advantage of the complementary absorption effect of germanium (Ge) and perovskite, a novel perovskite solar cell (PSC) was proposed by stacking Ge and perovskite thin films to serve as the dual absorption layer. Its absorption spectra and photocurrent were calculated and optimized as a function of the thickness of Ge film by finite element method (FEM). Then, the optical efficiency was further boosted by embedding an array of Ge semi-ellipsoidal nanoparticles within the perovskite film. The predicted maximum photocurrent is revealed to reach 45.72 mA/cm2, which is 2.55 times larger than that of planar referenced PSC. Besides the complementary absorption effect, the localized surface plasmon resonance (LSPR) of Ge nanoparticles and the grating effects of Ge nanoparticle array are revealed to contribute to the obtained Jsc improvement. The present work is insightful for future design and applications of PSCs by introducing Ge thin film and Ge nanoparticles.

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.

High-reliability infrared broadband thin-film polarizing beam splitter with ZnSe compensation layers.

Thin-film polarizing beam splitters (PBSs) fulfill a pivotal role in laser beam splitting, modulation, shaping and isolation. In this study, a high-reliability infrared broadband thin-film PBS was developed. To correct for tensile stress in Ge/YbF3 multilayer coatings, ZnSe compensation layers were incorporated in the multilayer design. The effects of different symmetrical periods on the spectral properties of the infrared PBS were systematically discussed. The infrared PBS operated at 45° and in the long-wave infrared (LWIR) band. Using the percent of optical extrema monitoring (POEM) strategy combined with the high-temperature optical constants (HTOC) of Ge film, the infrared PBS was precisely fabricated on ZnSe substrates. Subsequently, the spectral performance and film reliability of the infrared PBS were carefully characterized. Specifically, the transmittance of p-polarization surpassed 96%, while the extinction ratio exceeded 100:1 within the 10.6 ± 0.15 µm band. The infrared PBS demonstrated commendable environmental reliability, in addition to exhibiting excellent spectral characteristics.

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.

Development of nanostructured Ge/C anodes with a multistacking layer fabricated via Ar high-pressure sputtering for high-capacity Li+-ion batteries

To realize high-capacity Ge anodes for next-generation Li+-ion batteries, a multilayer anode with a C(top)/Ge(middle)/C(bottom) structure was developed, where nanostructured amorphous Ge (a-Ge) and amorphous-like carbon films with a grain size of 10–20 nm were deposited sequentially by high-pressure Ar sputtering at 500 mTorr. Compared with the a-Ge anode, the C(top)/a-Ge(middle)/C(bottom) multistacking layer anode showed improved capacity degradation for repeated lithiation/delithiation reactions and achieved a high capacity of 910 mAh g−1 with no capacity fading after 90 cycles at a C-rate of 0.1.

Infinite lateral growth of (001) single crystal strip in Ge film on SiO2 by micro-chevron laser scanning method

A single crystal strip of Ge, exhibiting a predominant crystal orientation of (001), was successfully grown within a 60 nm thick sputtered Ge film using the micro-chevron laser scanning method. The continuous lateral growth of the Ge strip was achieved through the strategic implementation of a thick cap layer and a SiO2 interlayer between the Ge layer and cap layer. The thick cap layer was pivotal as a heat sink, effectively extending the period during which the Ge film remained molten, preventing unwanted nucleation. Yaw rotation of (001) crystal orientations, which typically trigger the formation of grain boundaries in Si, was found to be negligible in Ge. This observation offers compelling evidence for the potential for infinite lateral growth of (001)-oriented Ge strips. The Raman shift of the single crystal strip was measured at 297.4 cm−1, indicating that the film exhibited tensile stress.

Suppression of phase segregations in Ge–Fe–Co–Ni–Mn films by high-entropy effect

Fe–Co–Ni–Mn films doped with different concentrations of Ge were prepared on the Si substrates by using radio frequency magnetron sputtering. Transmission electron microscopy (with an energy dispersive x-ray spectrometer) and an x-ray diffractometer were used to systematically study the microstructure evolution of the Fe–Co–Ni–Mn–Ge films. The results indicate that the Fe–Co–Ni–Mn films doped with a large amount of Ge show significant element segregation after rapid high-temperature annealing. However, with the decrease in the doping amount of Ge to approximately equal molar ratio with magnetic elements, Ge and magnetic elements achieve perfect mutual dissolution at the same annealing conditions, forming single-phase solid solution. Electrical transport tests suggest that its electrical property is close to semiconductors. The mechanism of enhanced mutual solubility between semiconductor elements and magnetic elements is discussed in detail.

Epitaxial growth of high-quality Ge layers on Si with Ge2H6 under UHV-CVD conditions

Epitaxial growth of Ge films on Si(100) substrates has been studied under ultra-high vacuum chemical vapor deposition (CVD) conditions by using digermane (Ge2H6) as the precursor. It was found out that high quality layers with thicknesses beyond 500 nm could be produced at complementary metal–oxide–semiconductor compatible conditions, demonstrating low defect density, sharp and narrow x-ray diffraction peaks, as well as room temperature photoluminescence around 1550 nm. The surface roughness values are comparable to prior reduced pressure CVD results at similar growth temperatures. By employing higher growth temperatures, growth rates are significantly enhanced, resulting in much thicker layers beyond 2000 nm. Smoother sample surface could also be obtained, yielding a state-of-the-art surface root-mean-square roughness value of 0.34 nm for the as-grown sample. At the same time, after being annealed at 750 °C for 20 min, the full width at half maximum (FWHM) of x-ray diffraction 004 rocking curve spectrum of the Ge layer is as low as 88 arcseconds, which stands the best among all Ge/Si samples. The current work has provided important reference for Ge/Si growth with Ge2H6 in low pressure regime and solidified material grounding for Ge-based optoelectronics and Si photonics.

Sub-10 μm-Thick Ge Thin Film Fabrication from Bulk-Ge Substrates via a Wet Etching Method.

Low-defect density Ge thin films are crucial for studying the impact of defect density on the performance limits of Ge-based optical devices (optical detectors, LEDs, and lasers). Ge thinning is also important for Ge-based multijunction solar cells. In this work, Ge wet etching using three acidic H2O2 solutions (HF, HCl, and H2SO4) was studied in terms of etching rate, surface morphology, and surface roughness. HCl-H2O2-H2O (1:1:5) was demonstrated to wet-etch 535 μm-thick bulk-Ge substrates to 4.1 μm with a corresponding RMS surface roughness of 10 nm, which was the thinnest Ge film from bulk-Ge via a wet etching method to the best of our knowledge. The good quality of pre-etched bulk-Ge was preserved, and the low threading dislocation density of 6000-7000 cm-2 was maintained after the etching process. This approach provides an inexpensive and convenient way for accurate Ge substrate thinning in applications such as multijunction solar cells and sub-10 μm-thick Ge thin film preparation, which enables future studies of low-defect density Ge-based devices such as photodetectors, LEDs, and lasers.

Improving the selector characteristics of ovonic threshold switch via UV treatment process

In this study, we investigated the influence of ultraviolet (UV) treatment on the ovonic threshold switch (OTS) to improve its selector properties. Our findings demonstrate that iteratively applying UV treatment during the film deposition phase considerably improves device characteristics compared to a single UV treatment. Consequently, this process provided a significant decrease in the forming voltage, maintaining outstanding switching features, with an off-state current of approximately 2 nA. Furthermore, the refined UV treatment process resulted in an impressive 45% improvement in threshold voltage drift characteristics and facilitated excellent switching uniformity. X-ray photoelectron spectroscopy analysis revealed alterations in the bonding structure of the Si–Te–As–Ge film after UV exposure. Specifically, a transition was observed from unstable homopolar bonds, such as As-As or Te–Te, to their more stable heteropolar equivalents, such as As–Te. These results highlight the potential of UV treatment as an effective method for enhancing the OTS performance.

The value of charge of Fe single to multiple atoms doped in Ge: Combined experimental and density functional theory study

With the aim to find out the value of an electronic charge that the Fe atoms acquire when they are doped in Ge bulk, a set of experimental and density functional theory (DFT) studies have been carried out of the model systems consisting of intermixed Fe–Ge films. Such films were prepared by electron and thermal evaporation in ultra-high vacuum (UHV) on the surface of Mo(110) substrate by initial formation of the Ge film of a mean thickness of 7.5 nm, onto which the Fe films were subsequently deposited, maintaining certain Fe to Ge concentration ratio, namely, 0.2, 0.4, 0.7 and 1.0. Annealing of such double Fe–Ge films results in notable interdiffusion of the components with quite uniform distribution of the elements throughout the intermixed layer. The details of formation of such layers were in-situ controlled by Auger electron spectroscopy (AES), low-energy ion scattering spectroscopy (LEIS), low-energy electron diffraction (LEED) and work function measurements, in combination with Ar ion depth profiling. By analyzing the Fe Auger LMM-triplet, the absolute values of the charge that Fe atoms acquire when doped in Ge, were estimated for four different Fe–Ge intermixed layers with above-mentioned different Fe to Ge concentration ratio. The corresponding plot of the charge versus Fe to Ge concentration allows to estimate the charge of a single doped Fe atom, which equals to +0.34e (electron charge units) and gradually decreases to 0.07e for high Fe concentration. To verify the experimental values of the Fe charge the calculations were done by periodic DFT as implemented in full-potential FHI-aims code. Among the used algorithms of Mulliken, Hirshfeld, and Bader's atoms-in-molecules, the latter gives good correlation between charges of Fe dopant and its concentration.