Ge Spacer Research Articles

Realization of a cost-effective thermochromic solar absorber with a high emittance change based on VO2 and an infrared transparent intermediate layer

In this study, we present a thermochromic solar absorber coating that reaches a high thermal emittance change by using a thin, optically switching VOx film located on an infrared transparent interlayer (spacer) of Si or Ge with an optical thickness of λ/4 (for λ = 7 μm). Using this so-called “lambda/4-concept,” temperature-dependent reflection measurements in the spectral range between 2500 and 50 000 nm from an absorber with a 450 nm Si spacer and a VOx film oxidized from a 30-nm-thick V display an overall increase in emittance from ε(25 °C) = 12.2% to ε(150 °C) = 55.1%, resulting in a change of Δε = 42.9%. In addition, using an absorber with a 400 nm Ge spacer in combination with a VOx film oxidized from 17.5-nm-thick V, an increase in emittance from ε(25 °C) = 8.2% to ε(150 °C) = 49.2% with a change of Δε = 41.0% was achieved. In addition, the optical properties of Ge and Si thin films over a wide spectral range of 250–38 000 nm were determined using spectroscopic ellipsometry. Using this optical data and a simple optical model of the VOx film, reflectance simulations could be performed by using the ellipsometry analysis software WVASE©. It was shown by x-ray diffraction measurements that the optically switching VOx films oxidized from V in a belt furnace consist of the VO2 and the V2O5 phases. Using scanning electron microscopy images, the surface morphology of optically switching VOx films was compared with over-oxidized VOx films. A correlation between the surface morphology and the crystalline phases was revealed and applied to search for the optimal furnace parameters. This method could significantly reduce the time cost to achieve optically switching VOx coatings using an oxidation process.

Magnetic Materials: 2D‐Berry‐Curvature‐Driven Large Anomalous Hall Effect in Layered Topological Nodal‐Line MnAlGe (Adv. Mater. 21/2021)

In article number 2006301, Satya N. Guin, Claudia Felser, and co-workers report a layered topological nodal-line magnet MnAlGe, which is composed of magnetic Mn square-net layers that are separated by nonmagnetic Al–Ge spacers. The material exhibits a large anomalous Hall conductivity driven by the unique 2D Berry curvature distribution. From this work the search for new layered topological magnets and exploration of magnetism and topological properties are enabled.

2D-Berry-Curvature-Driven Large Anomalous Hall Effect in Layered Topological Nodal-Line MnAlGe.

Topological magnets comprising 2D magnetic layers with Curie temperatures (TC ) exceeding room temperature are key for dissipationless quantum transport devices. However, the identification of a material with 2D ferromagnetic planes that exhibits an out-of-plane-magnetization remains a challenge. This study reports a ferromagnetic, topological, nodal-line, and semimetal MnAlGe composed of square-net Mn layers that are separated by nonmagnetic Al-Ge spacers. The 2D ferromagnetic Mn layers exhibit an out-of-plane magnetization below TC ≈503K. Density functional calculations demonstrate that 2D arrays of Mn atoms control the electrical, magnetic, and therefore topological properties in MnAlGe. The unique 2D distribution of the Berry curvature resembles the 2D Fermi surface of the bands that form the topological nodal line near the Fermi energy. A large anomalous Hall conductivity of ≈700 Scm-1 is obtained at 2 K and related to this nodal-line-induced 2D Berry curvature distribution. The high transition temperature, large anisotropic out-of-plane magnetism, and natural heterostructure-type atomic arrangements consisting of magnetic Mn and nonmagnetic Al/Ge elements render nodal-line MnAlGe one of the few, unique, and layered topological ferromagnets that have ever been observed.

Thin-film perfect infrared absorbers over single- and dual-band atmospheric windows.

A thin-film perfect electromagnetic absorber with a tunable response in the infrared (IR) region is proposed using a metal-dielectric-metal configuration, which consists of a Ti top layer and a Ge spacer layer on a Ti substrate. The thin-film structure simplifies the absorber design by tuning the thicknesses of the two layers, which is suitable for large-scale fabrication by matured deposition technologies. The absorber supports perfect IR absorption with tunability from 3 µm to over 15 µm. Furthermore, the total thickness is much smaller than the wavelength, and the absorption has small iridescence. Based on this design, we demonstrated two samples with one supporting single-band absorption in the atmospheric absorption window (5-8 µm) and the other one supporting dual-band absorption in the two atmospheric transmission windows (3-5 and 8-13 µm). These absorption signatures can find applications in IR invisibility and radiative cooling.

Preliminary Study on the Magnetic Properties of GeMn Nanocolumn/Ge Multilayers

Ge0.94Mn0.06 nanocolumn thin film is a unique phase of GeMn diluted magnetic semiconductors (DMS) which exhibit Curie temperature (TC) > 400 K. The multilayers of Ge0.94Mn0.06 nanocolumns separated by nano-scaled spacers represent great interests for spintronic applications, such as spin valves or giant magneto-resistance (GMR) multilayers. In this article, we present the results obtained from the preliminary study on the exchange coupling in two types of GeMn nanocolumn/Ge multilayers. All the samples have been grown using molecular beam epitaxy (MBE). The superconducting quantum interference device (SQUID) magnetometer has been used to determine the magnetic properties of the samples. In the multilayer system Ge/[Ge0.94Mn0.06(40 nm)/Ge(d nm)]9/Ge0.94Mn0.06(40 nm)/Ge, no exchange coupling can be observed. Inversely, exchange coupling between the layers exists and depends on the thickness of the Ge spacers for the GeMn nanocolumns/Ge multilayer spin valve systems. The exchange coupling in the nanocolumns multilayer systems has been shown to be complex due to the leakage field induced by neighboring nanocolumns and the magnetic anisotropy of nanocolumns.

Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals.

In this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 μm to 7.3 μm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design.

Photoluminescence of phosphorus atomic layer doped Ge grown on Si

Improvement of the photoluminescence (PL) of Phosphorus (P) doped Ge by P atomic layer doping (ALD) is investigated. Fifty P delta layers of 8 × 1013 cm−2 separated by 4 nm Ge spacer are selectively deposited at 300 °C on a 700 nm thick P-doped Ge buffer layer of 1.4 × 1019 cm−3 on SiO2 structured Si (100) substrate. A high P concentration region of 1.6 × 1020 cm−3 with abrupt P delta profiles is formed by the P-ALD process. Compared to the P-doped Ge buffer layer, a reduced PL intensity is observed, which might be caused by a higher density of point defects in the P delta doped Ge layer. The peak position is shifted by ∼0.1 eV towards lower energy, indicating an increased active carrier concentration in the P-delta doped Ge layer. By introducing annealing at 400 °C to 500 °C after each Ge spacer deposition, P desorption and diffusion is observed resulting in relatively uniform P profiles of ∼2 × 1019 cm−3. Increased PL intensity and red shift of the PL peak are observed due to improved crystallinity and higher active P concentration.

Investigation of the thermal stability of metastable GeSn epitaxial layers

A stack of five elastically strained metastable GeSn layers with a thickness of 200 nm each separated by Ge spacer layers with a thickness of 20 nm is grown on a (001) Si/Ge virtual substrate. The molar fraction of Sn in the GeSn layers is 0.005, 0.034, 0.047, 0.072, and 0.10. After growth the structure is subjected to thermal annealing for 2 min at a temperature of 400°C. It is demonstrated that during the course of annealing the GeSn alloy, along with plastic relaxation, undergoes phase separation; this phase separation begins before the end of plastic relaxation. The structural degradation of the GeSn layers increases with increasing concentration of Sn accumulated on the structure surface in the form of an amorphous layer.

Fabrication of GeSn-multiple quantum wells by overgrowth of Sn on Ge by using molecular beam epitaxy

We report on the fabrication and structural characterization of epitaxially grown ultra-thin layers of Sn on Ge virtual substrates (Si buffer layer overgrown by a 50 nm thick Ge epilayer followed by an annealing step). Samples with 1 to 5 monolayers of Sn on Ge virtual substrates were grown using solid source molecular beam epitaxy and characterized by atomic force microscopy. We determined the critical thickness at which the transition from two-dimensional to three-dimensional growth occurs. This transition is due to the large lattice mismatch between Ge and Sn (≈14.7%). By depositing Ge on top of Sn layers, which have thicknesses at or just below the critical thickness, we were able to fabricate ultra-narrow GeSn multi-quantum-well structures that are fully embedded in Ge. We report results on samples with one and ten GeSn wells separated by 5 and 10 nm thick Ge spacer layers that were characterized by high resolution transmission electron microscopy and X-ray diffraction. We discuss the structure and material intermixing observed in the samples.

Thermal Stability of Ge/GeSn Nanostructures Grown by MBE on (001) Si/Ge Virtual Wafers

A stack of five metastable 200-nm-thick elastically strained GeSn epitaxial layers separated by 20-nm-thick Ge spacers was grown on (001) Si/Ge virtual substrate by MBE. The molar fraction of Sn in different layers varied from 0.005 to 0.10, increasing with the layer distance from the Ge buffer. The phase separation of the GeSn alloy during postgrowth annealing takes place along with plastic relaxation. The phase separation begins well before the completion of the plastic relaxation process. The degree of phase separation at a given annealing temperature depends strongly on the Sn content in the GeSn alloy. The Sn released from the decomposed GeSn alloy predominantly accumulates as an amorphous layer on the surface of the sample.

Dependence of magnetic properties on the growth temperature of Mn0.04Ge0.96 grown on Si (001)

The structural and magnetic properties of Mn 0.04 Ge 0.96 thin films grown on Si (001) substrates at different growth temperatures were investigated. The films were grown by sequential deposition of MnGe and Ge multilayers using molecular beam epitaxy . It was found that the magnetic ordering and Curie temperature could be controlled by adjusting the distance between the Mn ions in the Ge spacer layer depending on growth temperature. We found that the samples grown below 130 °C contained highly disordered ferromagnetic Mn rich domains. Both structural and magnetic characterizations revealed that Mn 5 Ge 3 precipitates were formed in Mn clusters free Ge matrix when the samples were grown at 150 °C. Both the Mn rich domains and Mn 5 Ge 3 precipitates showed out of the plane magnetic anisotropy and similar ESR line shapes. • We investigated the effects of growth temperature on spin systems in Mn 0.04 Ge 0.96 . • The degree of ferromagnetic ordering increases with temperature. • Ferromagnetism occurs due to interaction of Mn 2+ ions in Mn rich clusters. • ESR reveals that the formation of magnetic phases depends on growth temperature.

Franz-Keldysh Effect in GeSn Detectors

The fundament for silicon based photonics is Ge grown on Si and SOI. A simple photonic integrated circuit (PIC) consist of an integrated light source, a waveguide, a modulator and a detector. Due to the fact that Ge is an indirect semiconductors, the light source is the main challenge for PICs. One concept is to add Sn into the Ge matrix. Theoretical calculations predict that Ge1-xSnx becomes a direct semiconductor for a Sn content of x = 10 % and therefore is an interesting material mixture for photonic devices integrated on Si. Recent publications have shown that detectors using GeSn as active material excess 40 GHz bandwidth. For PICs, also the modulation of the light is needed. Beside Mach-Zehnder modulators, the modulator can be realized using the Franz-Keldysh (FK) effect. Due to the FK effect, the absorption in the material can be changed by varying the applied electric field.In this paper we report the FK effect in GeSn PIN diodes with Sn concentrations up to 4.2 %. In order to analyze the absorption of GeSn, vertical PIN diodes with different Sn content were produced. First, the epitaxial layers were grown with a solid source molecular beam epitaxy system [1] on B-doped Si (100) substrates with a high specific resistance ρ > 1000 Ωcm. Fig. 1(a) shows the vertical layer sequence. The layer stack consists of a vertical PIN structure. Starting with a 400 nm B-doped Ge contact layer (1020 cm-3), the active layer with 300 nm intrinsic Ge1-xSnx and a 10 nm Ge spacer follow. The top contact consist of 100 nm Sb-doped Ge (1020 cm-3) and 100 nm Sb-doped Si (1020 cm-3). The three investigated samples have a Sn content of x = 0 %, 2 % and 4.2 %, respectively. The GeSn PIN heterojunction photodiodes are produced in a quasi-planar technology. The device processing by double mesa etching, passivation and metallization is similar to earlier reported GeSn pin photodiodes [2]. A schematic high frequency device structure of the GeSn pin photodetector with the typical ground signal ground (GSG) probe configuration with a pitch of 100 µm is shown in Fig 1(b).The electrical and optical characterization of the diodes is done with on-wafer measurement of the current-voltage characteristics. As a light source, a super-continuum laser with a tunable acousto-optical wavelength filter is used. The light couples into the diode through a lensed glass fiber. From measuring the dark current and photo current, the responsivity of the GeSn detectors were determined by dividing the photo current by the optical power.Fig. 2 shows the responsivity of the GeSn PIN diodes with Sn concentrations of x = 0 %, 2 % and 4.2 %. With increasing Sn content, the bandgap shifts to higher wavelengths and therefore, the responsivity increases for higher wavelengths. A voltage dependence of the optical responsivity is only shown near the direct bandgap Eg,Γ. This is due to the FK effect.From the measurement of the responsivity and reflectivity, the internal quantum efficiency can be calculated and further the absorption is determined. The electro-optical modulation from the FK effect was clearly observed in GeSn material. The maximum absorption ratio Δα/α0 for the 2 % Sn sample at room temperature was 1.5 at a wavelength of 1806 nm and a voltage swing of 3 V.

Interfacial properties of all-epitaxial Fe–Ge/Ge heterostructures on Ge(111)

The molecular-beam-epitaxy growth of Fe–Ge/Ge/Fe–Ge trilayers on Ge(111) wafers has been investigated as a function of three parameters: the Ge spacer coverage, the substrate temperature (TD) and the dynamic atomic hydrogen (H) exposure during the Ge spacer deposition. Morphology and crystal structure have been characterized in situ by means of scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron diffraction and ex situ with high-resolution transmission microscopy. Whatever the H flux, epitaxial growth of Ge spacer requires deposition temperature above ~220°C. At the earliest stages of Ge deposition, the surface periodicity of the bottom Fe1.9Ge epilayer changes from p(2×2) to 3×3R30° for deposition temperature above ~220°C, whatever the H dosing. It results from severe intermixing that modifies the stoichiometry of the whole Fe–Ge layer. However, this layer preserves its hexagonal B82 crystal structure and no additional Fe–Ge compounds formed at the interface. We found also that the H flux drastically modifies the Ge spacer growth mode for deposition temperature above ~220°C. In particular, the Ge morphology evolves from 3D islands without H supply to flat and continuous film for H partial pressure above 10–3Pa. Finally, the top Fe–Ge electrode crystallizes in the same structure as the bottom electrode, and the planar relations of the trilayer are: (111)Ge-wafer||(0001)bottom–Fe1.9Ge||(111)Ge-spacer||(0001)top–Fe1.9Ge and [−110]Ge-wafer||[11–20]bottom–Fe1.9Ge||[1–21]Ge-spacer||[11–20]top–Fe1.9Ge. These fully epitaxial Fe–Ge/germanium hybrid heterostructures with single crystal Ge layer of diamond structure appear therefore as promising candidates for semiconductor spintronics.

Ferromagnetism of Mn/Ge Multilayers Grown by Molecular Beam Epitaxy

We report on novel ferromagnetic Mn/Ge multilayers for spintronics applications investigated both experimentally and theoretically. Two Mn/Ge multilayers are grown on GaAs (001) substrates by molecular beam epitaxy. The period of each multilayer consists of an Mn layer of varying thickness (0.6 and 5 A) and a 10 A thick Ge spacer layer. From temperature-dependent magnetization and hysteresis loop measurements, the Mn (0.6 A)/Ge (10 A) multilayer showed very weak ferromagnetic ordering, which is persistent up to 260 K, whereas the Mn (5 A)/Ge (10 A) multilayer exhibited strong ferromagnetism up to 305 K. The coercive field of the Mn (5 A)/Ge (10 A) multilayer was 277 Oe at 200 K. Density functional electronic band structure calculations on a number of Mn/Ge (001) multilayers determined them to be ferromagnetic, and estimates of their critical temperatures are reported.

Modeling and Analysis of ZnSe–Ge HBTs

A Gummel-Poon model is developed for ZnSe-Ge-GaAs heterojunction bipolar transistors (HBTs). In this structure, undoped Ge spacers are placed at the emitter-base and collector-base junctions. Injected current components as well as bulk, spacer, and space charge recombination current components are modeled. Early voltage and bandgap narrowing effects are included in the model. The device performance was simulated and compared with the experimental results. The paper shows a good agreement between our model and the experimental results. The paper shows also that using spacers would improve the device performance. The advantages of this model is that it is analytical, compact, and can be easily implemented in CAD tool programs to simulate single or double HBTs with similar or dissimilar materials structure for the emitter and collector.

Magnetoresistance measurements on Fe/Si and Fe/Ge multilayer thin films

The magnetoresistance and magnetization of multilayer Fe/Si and Fe/Ge films on GaAs were measured simultaneously as a function of magnetic field along the in-plane [110] and [11¯0] directions of the Fe. The field induced changes in the magnetoresistance and magnetization for each film are discussed in terms of the exchange coupling between the iron layers, and the thickness of the semiconductor spacer layer. The data show that there is exchange coupling across Ge spacer layers for layer thickness less than 1.2nm.

Orientation and layer thickness dependence on the longitudinal magnetization andtransverse magnetization hysteresis loops of sputtered multilayer Fe/Si and Fe/Ge thinfilms

The transverse magnetization and longitudinal magnetization hysteresis loops of sputteredthin films of Fe/Ge and Fe/Si multilayers on GaAs(001) substrates have been studied. Thedependence of the film magnetic anisotropy on the bilayer period and semiconductorcomposition was investigated using a MOKE magnetometer. The hysteresis loops weremeasured as a function of the angle between the applied magnetic field and the hard–hardaxis of the film. For the longitudinal magnetization, the Fe/Si film loops had lowerremanent magnetization compared to the Fe/Ge film with the same spacer thickness. Thusthe Fe/Si film had stronger exchange coupling across the spacer layer compared tothe Fe/Ge film. For the thicker Ge spacer layer film, no exchange coupling wasmeasured. For the transverse magnetization, the Fe/Ge multilayer films loops hadonly one Barkhausen jump. For the Fe/Si multilayer films loops, some containedone Barkhausen jump while others had two jumps, due to the cubic anisotropycontribution. These results are interpreted in terms of anisotropy and exchange energies.

Perpendicular transport in Fe/Ge model heterostructures

Based on the Kubo-Greenwood equation as formulated for layered systems, an approach is discussed that allows us to separate the resistance of the current leads from that of the region whose resistance we wish to calculate for current perpendicular to the plane of the layers. By applying this approach to Fe/Ge/Fe model structures related to the parent lattice of bcc Fe we find that at least nine layers of the magnetic electrodes should be considered as being part of the calculation in order to perform such a separation. With different structures in the Ge spacer, we find that the concentration of vacancies plays a crucial role for the existence of a sizeable magnetoresistance (MR), while the actual structure in the spacer seems to be of less importance. Depending on the type of structure and the number of spacer layers (in a typical regime of 6--21 layers) the MR for ordered structure varies between 35% and 45%. Vacancy concentrations of more than 10%, however, wipe out the MR completely. Interdiffusion at the Fe/Ge interfaces produces very similar effects.

Unusual Surface Undulations Observed in Ge/Si(100) Multilayers

AbstractMultilayer Ge/Si quantum wells grown on Si(100) at lower temperatures (300 C for Ge and 435 C for Si) exhibit unexpected large surface undulations. These groove-like features along <110> directions are correlated to the underlying dislocation networks. Studies on the dependence of surface roughness versus Ge spacer thickness exclude kinetic limitation as the cause of the observed troughs. Further comparison of the surfaces for multilayer and single-layer samples indicates a combination of the multilayer structure and the inhomogeneous surface strain field greatly enhances the undulations.

Heat Induced Antiferromagnetic Coupling in Multilayers with Ge Spacers

Heat Induced Antiferromagnetic Coupling in Multilayers with Ge Spacers