Ge Layer Research Articles

Lateral PIN Photodiode with Germanium and Silicon Layer on SOI Wafers

It has been verified through numerical simulations calibrated to experimental data the changes that the insertion of a germanium layer can bring to the electrical power generation of a silicon solar cell. The insertion of a germanium layer on top or below a silicon PIN diode designed in SOI technology has been considered. Results showed that different semiconductor characteristics (bandgap, mobility, and absorption coefficients) result in a general improvement in the solar cell performance, being able to reach a power 136% greater than the device without the heterogeneous layer. In the evaluated device the average power was improved from 9.43 nW to 14.92 nW with the Ge layer insertion. Besides that, the analysis has allowed for a better understanding of the phenomena that occur in the photogeneration of a cell with a heterojunction between germanium and silicon.

(Electronics and Photonics Division Award) Epitaxy of Group-IV Semiconductors for Nanoelectronics and Optoelectronics

Epitaxy stands for the deposition of a perfectly ordered layer on a monocrystalline substrate, which is typically, for group-IV semiconductors, bulk Si or Silicon-On-Insulator (SOI). If the layer is chemically different from the substrate and lattice parameters are not the same, epitaxy can be of two types. If the atomic columns of the substrate extend into the layer with the same in-plane lattice parameter, the layer will be pseudomorphic. The built-in strain will be compressive if the layer has a larger lattice parameter than that of the substrate or tensile if it is lower. If the layer thickness exceeds the so-called critical thickness for plastic relaxation, it will relax, with the formation of misfit dislocations to accomodate the lattice parameter mimatch.Epitaxy of group-IV semiconductors is key for numerous Si-based nanoelectronics and optoelectronics devices. Although they crystallize in the same diamond structure, C, Si, Ge and Sn bandgaps (5.47, 1.12, 0.66 and -0.41 eV), lattice parameters (3.57, 5.43, 5.66 and 6.49 Å) and fusion temperatures (> 4000°C, 1414°C, 938°C and 232°C) are very different. This can be an advantage (bandgap and mobility engineering in devices) or a difficulty (when growing stacks). Finally, although Si and Ge are miscible in all proportions, solid solubilities of C in Si and Sn in Ge are exceedingly low (less than 10-4 % and 0.8%, respectively). Low growth temperatures and high growth rates will then be mandatory in order to have high quality alloys.In that presentation, I will show how epitaxy can be put to good use in order to boost the properties of devices. Layers or stacks were grown on 200 mm and 300 mm diameter wafers in industrial Reduced-Pressure – Chemical Vapour Deposition tools with temperatures in the 300°C-900°C range and pressures in the 20-100 Torr range, typically. Precursors were mono-methysilane (for C), silane, dichlorosilane or disilane (for Si), germane or digermane (for Ge) and tin tetrachloride (for Sn). Finally, diborane and phosphine were the P-type and N-type dopant precursors, respectively.The electrical properties of p-type Metal Oxyde Semiconductor – Field Effect Transistors (MOSFETs) can be improved by the insertion of a thin pseudomorphic SiGe layer beneath the gate of transistors. The compressive strain in it will boost by a factor of ~2 the hole mobility in pMOSFETs. The use of SiGe:B in the sources and drains regions on each side of such devices will help in minimizing the contact resistivity and increasing the compressive strain in the channel. Ramping-up, at high temperatures, the Ge concentration in microns thick layers grown on Si and capping them with constant composition layers will result in SiGe virtual substrates which are almost fully relaxed (e.g. with a lattice parameter close to that of bulk SiGe), slightly defective (Threading Dislocations Density (TDD) around 105 cm-2) and with surface cross-hatches. Growing thin tensily-strained Si layers on top and transferring them onto oxydized Si will result in strained SOI substrates, with an increase by a factor of two of the electron mobility in n-type MOSFETs built on top. Using low contact resistance tensily-strained SiP sources and drains will further boost nMOSFET performances.Microns-thick Ge layers grown directly on Si thanks to a low temperature/high temperature approach can be used as templates for III-V epitaxy. The surface root mean square roughness will be less than 1 nm and the TDD around 107 cm-2 with some short duration thermal cycling. Ge can also be grown selectively in trenches at the end of Si waveguides and near infra-red photo-detectors processed. Microns-thick SiGe layers (capped with Si) and pure Ge layers (capped with SiGe) can otherwise be used to fabricate mid and long Infra-Red waveguides. Mid-IR supercontinuum light was generated when sending ultrashort light pulses in such waveguides. Finally, germanium can be transformed into a direct bandgap semiconductor thanks to high amounts of tensile strain or some alloying with more than 8% of tin. Optically pumped lasing at room temperature was recently achieved in bonded or pillared micro-disks made of thick, plastically relaxed GeSn 16-17% layers.Finally, the feasability of selectively etching SiGe versus Si can be used to fabricate stacked nanosheet MOSFETs, with superior ION/IOFF tradeoffs for a given footprint. The core structures of such devices are epitaxial Si/SiGe superlattices, with the alternance of typically ten nm thick Si and SiGe layers. SiGe layers are later on selectively etched. The resulting voids are conformally filled with gate dielectrics and metals. We recently succeeded in stacking seven Si nanosheets, with a record nMOSFET saturation current of 2.8mA/µm for a 1V drain voltage. Figure 1

Electroplating on Unconventional Ultra-Compliant Substrates for Travelling Wave Tube Amplifiers

We report gold electroplating on ultra-compliant substrates comprising helical slow wave structures (SWSs) for traveling wave tube amplifiers (TWTAs) [1]. The novel ultra-compliant substrates are composed of edge-tethered tri-layer metal ribbons with a helical geometry of microscale diameter. After electroplating with gold, we obtain overall thicknesses of a few um. We discuss different controllable electroplating conditions that influence thickness, uniformity, roughness, and related properties of deposited gold films on helical ribbons. In addition to increasing conductance of the electroplated helical ribbons, electroplating stabilizes the helix at its equilibrium diameter, with the pitch predicted by Prakash et al.[1].Our method of fabrication of ultra-compliant helical ribbons starts with defining strips of width 5 µm to 10 µm by optical lithography, metal evaporation, and lift-off, deposited on Si substrates coated with a sacrificial layer of Ge or GeOx,. We use Cr/Au/Cr tri-layers to create an inherent stress gradient that causes the strip to self-assemble into a helix, after etching with XeF2 for selective removal of a sacrificial layer. Diameter and pitch of the released helices are controlled by varying the thickness, the elastic modulus, the residual stress, and the in-plane geometry of the deposited tri-layer metal strips. The gold electroplating process uses a sulphate-based gold solution in a two-electrode electrochemical setup with the helix as the cathode and a platinized mesh as the anode [2]. We deposit gold to a thickness of a few um, using a pulsed current source with variable parameters. Direct current can also be used with smaller deposition times. Our results demonstrate the application of electroplating to unconventional ultra-compliant helix of nanoscale dimensions. Reference [1] Divya J. Prakash,., Matthew M. Dwyer, Marcos Martinez Argudo, Mengistie L. Debasu, Hassan Dibaji, Max G. Lagally, Daniel W. van der Weide, and Francesca Cavallo. 2020. “Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes.” ACS Nano, 2021, 15, 1229-1239. doi:10.1021/acsnano.0c08296.[2] Max G. Lagally, Anjali Chaudhary, Daniel van der Weide, Divya J. Prakash, and Francesca Cavallo, Improved Self-assembly of Helices via Electrodeposition on Freestanding Nanoribbons for TWT Application, provisional US patent application (P220249US01)Work supported by U.S. AFOSR-Award No FA9550-22-1-0086.

Epitaxy of Group-IV Semiconductors for Quantum Electronics

Epitaxy of group-IV semiconductors is a key enabler for quantum devices. Low temperature epitaxy can be used to deposit Si:B layers with boron concentrations so high that they are superconductive (ECS Transactions 98(5), 203 (2020)). Tensily strained Si layers sandwiched between relaxed Si0.7Ge0.3 layers behave as quantum wells for electrons, enabling electron spin quantum bit (qubit) fabrication. Purified 28Si without deleterious 29Si isotopes (with a nuclear spin) are ideal as the core of fully-depleted, multiple gate transistors for qubits. Compressively-strained Ge layers sandwiched between relaxed Si0.2Ge0.8 layers can confine a two-dimensional hole gas (2DHG) offering an emerging pathway to hole-spin qubits.In the following, we will focus on the latter two subjects. We will show how we succeeded in growing 28Si layers with the following concentrations: 28Si isotopes > 99,992%, 29Si isotopes < 0.006% and 30Si isotopes < 0.002% (Journal of Crystal Growth 509, 1 (2019)). Such values can instructively be compared to those in natural Si: 28Si: 92.223%, 29Si: 4.678% and 30Si: 3.092%. The availability and cost of isotopically enriched 28SiH4 is a major difficulty, however. We thus quantified the impact of growth temperature and HCl mass-flow on the Si growth rate. At high temperature, above 850°C, we reached a silane supply limited regime with a good decomposition efficiency, high growth rates (> 100 nm min.-1 for the SiH4 mass-flow selected) and almost no impact of the HCl flow. There was otherwise, below 850°C, a H- and Cl-surface desorption limited regime, with a lesser decomposition efficiency and Si growth rates which dropped as the temperature decreased and/or the HCl mass-flow increased. Thick 28Si layers should be grown at high temperature, while low temperature epitaxy should be limited to the deposition of thin 28Si layers on top of SiGe sacrificial layers (28SOI fabrication with a bonding-etch back approach) or the thickening of SOI substrates (to avoid elastic or plastic relaxation/dewetting).We otherwise fabricated c-Ge/SiGe heterostructures for hole spin qubits. We first grew at 850°C, 20 Torr and with a SiH2Cl2 + GeH4 chemistry, SiGe virtual substrates (VS), with a gradual ramping-up of the Ge concentration (to confine misfit dislocations) and a capping with 3 µm thick constant composition layers. Reciprocal Space Maps around the (004) and (224) X-Ray Diffraction (XRD) orders gave us the Ge concentration in those SiGe caps (73.8% and 78.7%) and their macroscopic degrees of strain relaxation (102.0 and 102.5%). The surface cross-hatch, e.g. the regular array of undulations with a 1-2 µm spatial wavelength because of a periodic strain field in VS, was suppressed using Chemical Mechanical Polishing (CMP). We then grew on top of the polished SiGe VS, at 500°C, 20 Torr and with a SiH2Cl2 + GeH4 chemistry, {100 nm thick SiGe 74% or 79% / 16 nm thick compressively-strained Ge / variable thickness SiGe 74% or 79% overlayer / Si 2nm cap} stacks. The parameter that changed was the SiGe overlayer, with 22, 33, 44 or 55 nm thicknesses probed.Compared to polished surfaces, a slight surface roughening was observed for the SiGe stacks, larger for the SiGe 74%/c-Ge than for 79%/c-Ge stacks. Thicker SiGe overlayers yielded smoother surfaces. We ascribe these surface undulations, with a ~ 100 nm wavelength, to an elastic relaxation of the compressive strain in the c-Ge layers. XRD showed that those stacks were pseudomorphic, with the same in-plane lattice parameter for the c-Ge layers than that of the SiGe VS underneath. Energy Dispersive X-ray spectroscopy (EDX) mapping of the whole structure showed that the Ge grading was rather linear with, as intended, a 10% Ge/µm grading. Cross-sectional Transmission Electron Microscopy (TEM) showed the presence of numerous misfit dislocations in the graded layer and none in the thick Si0.21Ge0.79 / c-Ge stack on top. A slight Ge concentration increase, by a few %, was measured by EDX at the CMP location, with a perfect crystallinity in the stack grown on top. The 16 nm thick c-Ge layer itself was perfectly monocrystalline, with a 1-2 bi-atomic layer roughness at c-Ge/SiGe interfaces, no in-plane deformation compared to the surrounding SiGe and an out-of-plane deformation of 1.5% from Precession Electron Diffraction.Magnetotransport measurements in Hall-bar devices were performed at 4.2 K to assess the electrical properties of the 2DHG in the grown SiGe/c-Ge heterostructures. At low magnetic field, a hole mobility of 1.2 x 105 cm2 V-1 s-1 was obtained for a hole density of n2DHG = 3.7x1011 cm-2 in the c-Ge/SiGe 79% 55nm sample, whereas quantum Hall effect plateaus and Shubnikov-De-Haas oscillations were observed at higher fields. Figure 1

Dual-Band Laser Stealth Based on Quasi Photonic Crystals

A quasi photonic crystal (QPC) dual-band absorber for laser stealth is designed and numerically studied. It consists of a defective two-dimensional photonic crystal on a thick Ni film. The defective photonic crystal is a continuous Ge layer with air holes, but some of the holes are periodically removed. Under a normal incidence that is perpendicular to the plane of the defects, the absorptivity can achieve 92.8% at the 1.064 μm wavelength and 93.2% at the 1.55 μm wavelength. Within large incident angles (&lt;45 degrees), the dual-band absorptivity is still &gt;80%. Additionally, the emissivity is as low as 5.8%~20.0% and 2.8%~5.8% in the 3–5 μm and 8–14 μm atmospheric windows. It is found that the introduced defects can couple the incidence into the structure and lead to spectral peaks (electromagnetic resonances) even without the bottom Ni film. With the help of the high-loss Ni film, the resonances are transformed into magnetic or/and electric modes of different orders. As a result, the QPC generates four absorption peaks. They are superimposed in pairs, resulting in enhanced absorption of the two laser wavelengths.

A multilayer film based on thin-film interference and impedance matching for dual-laser and infrared stealth as well as thermal management

Multispectral compatible stealth encounters higher challenges and requirements with the combined application of multiple detection technologies. This paper proposes a multilayer film based on thin-film interference and impedance matching to achieve 1.06 µm and 1.54 µm dual-laser stealth and mid-wavelength infrared (MWIR, 3–5 µm) and long-wavelength infrared (LWIR, 8–14 µm) stealth, as well as thermal management by the non-atmospheric window (5–8 µm). The multilayer film consists of Al2O3, Fe3O4, ZnS, Ge and Ag layer (Al2O3/ZnS/Ge/Ag/Fe3O4/Ag) and the optical performance of multilayer film is calculated using the finite-difference time-domain (FDTD) method. The numerical results indicated that the spectral emissivity of the multilayer film is lower than 27 % in the MWIR and 14 % in the LWIR and the spectral absorptances are both over 98 % at the laser wavelength 1.06 µm and 1.54 µm, respectively, as well as in the non-atmospheric window. Moreover, the optical performance of the multilayer film could be influenced with incident angle whether it is TM or TE wave but independent of the polarization angle. This work provides theoretical guidance for design and optimization of multilayer film that processes the compatibility of infrared stealth in the MWIR and LWIR and dual-laser stealth with thermal management in the non-atmospheric windows.

Strain relaxation from annealing of SiGe heterostructures for qubits

The misfit dislocation formation related to plastic strain relaxation in Si or Ge quantum well layers in SiGe heterostructures for spin qubits tends to negatively affect the qubit behaviors. Therefore, it is essential to understand and then suppress the misfit dislocation formation in the quantum well layers in order to achieve high-performance qubits. In this work, we studied the misfit dislocation propagation kinetics and interactions by annealing the strained Si or Ge layers grown by molecular beam epitaxy. The annealing temperatures are from 500 to 600 °C for Si layers and from 300 to 400 °C for Ge layers. The misfit dislocations were investigated by electron channeling contrast imaging. Our results show that the misfit dislocation propagation is a thermally activated process. Alongside, the blocking and unblocking interactions during misfit dislocations were also observed. The blocking interactions will reduce the strain relaxation according to theoretical calculation. These observations imply that it is possible to suppress the misfit dislocation formation kinetically by reducing the temperatures during the SiGe heterostructure epitaxy and post-epitaxy processes for developing well-functional SiGe-based spin qubits.

Waveguide-Integrated Ge/Si Avalanche Photodiode with Vertical Multiplication Region for 1310 nm Detection

Ge/Si separate absorption, charge, and multiplication avalanche photodiodes (SACM APDs) coupled with waveguides have shown significant potential as high-sensitivity, low-noise, and high-speed photodetectors for optical communications. In this study, we present a waveguide-integrated Ge/Si SACM APD fabricated on an eight-inch silicon photonics platform. The device exhibits a primary responsivity of 0.68 A/W at the unit gain voltage of 6 V for the O-band (1310 nm) wavelength, with a 10 μm-long and 1 μm-wide Ge layer. Additionally, the device demonstrates a 3 dB bandwidth of 25.7 GHz, with an input optical power of −16.8 dBm. The largest gain bandwidth product (GBP) is 247 GHz at a gain of 9.64 and a bias voltage of 15.7 V. The eye diagram is open at the bias voltage of 16 V, with a capacity to receive 28 Gbps of data. This APD shows potential for application in high-speed data transmission systems.

Metal-induced lateral crystallization of germanium thin films

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

Broadband long-wave infrared metamaterial absorbers based on germanium resonators

The broadband metamaterial perfect absorbers over various incident angles are highly significant in long-wave infrared (LWIR) detection. Previous research on LWIR metamaterial absorbers has mainly focused on metallic resonators. Here, we propose two broadband LWIR metamaterial absorbers that use dielectric resonators instead. The germanium (Ge) resonators on the top layer excite resonance modes that control the equivalent impedance of the structure, which acts on the high-lossy Si3N4-Ti reflecting layer to assume the absorption. Firstly, we designed a three-layer structure of Ge-Si3N4-Ti, which achieved a broadband absorption with an average absorptivity of 93.1 % in the 8–12 μm range. The absorption ratio of metal can be effectively reduced by replacing the metal. Then, to further enhance the absorptivity, we inserted a Si3N4 layer into the Ge layer, increasing the 90 % absorption bandwidth to 7.96–14.16 μm and the average absorptivity in 8–14 μm to 96.5 %. Compared to metamaterial absorbers based on metallic resonators, dielectric resonators with a large feature size make fabricating easier. Our proposed metamaterial absorber provides ultra-wideband absorption covering the LWIR range. In addition, it is insensitive to the incident angle, making it a potential candidate for thermal imaging and broadband thermal emission applications.

Three-Layered Thin Films for Simultaneous Infrared Camouflage and Radiative Cooling.

With the rapid advancements in aerospace technology and infrared detection technology, there are increasing needs for materials with simultaneous infrared camouflage and radiative cooling capabilities. In this study, a three-layered Ge/Ag/Si thin film structure on a titanium alloy TC4 substrate (a widely used skin material for spacecraft) is designed and optimized to achieve such spectral compatibility by combining the transfer matrix method and the genetic algorithm. The structure exhibits a low average emissivity of 0.11 in the atmospheric windows of 3-5 μm and 8-14 μm for infrared camouflage and a high average emissivity of 0.69 in 5-8 μm for radiative cooling. Furthermore, the designed metasurface shows a high degree of robustness regarding the polarization and incidence angle of the incoming electromagnetic wave. The underlying mechanisms allowing for the spectral compatibility of the metasurface can be elucidated as follows: the top Ge layer selectively transmits electromagnetic waves ranging from 5-8 μm while it reflects those in the ranges of 3-5 μm and 8-14 μm. The transmitted electromagnetic waves from the Ge layer are first absorbed by the Ag layer and then localized in the Fabry-Perot resonance cavity formed by Ag layer, Si layer and TC4 substrate. Ag and TC4 make further intrinsic absorptions during the multiple reflections of the localized electromagnetic waves.

A comparison of Ge, Sb and Te thermal diffusion through Ge2Sb2Te5

Chalcogenide Ge-Sb-Te alloys are used in a new generation of nonvolatile memories named Phase Change Memories which can be thermally switched from low resistivity to high resistivity states by feeding the memory cells with appropriate electrical pulses. During their lifetime, the heterogeneous redistribution of the elements within the active region of the cells is at the origin of the drift of their electrical characteristics and, ultimately, of their failure. Up to now, little is known about the relative diffusivities of the atomic elements that compose the alloys, what hampers the modeling and optimization of the materials and devices. Here, we report an experimental study of the thermal diffusion of Ge, Sb and Te through Ge2Sb2Te5. For this purpose, specific structures consisting of a layer of Ge, Sb or Te sandwiched between two Ge2Sb2Te5 layers have been fabricated and annealed at various temperatures and times. Using transmission electron microscopy based techniques, we evidence that Te diffusion can be activated from about 160 °C while Ge and Sb needs much higher temperatures, 220 °C and 350 °C, respectively. These results are in disagreement with previous theoretical simulations predicting that Ge is the most diffusive element while Te is the least.

Atom probe tomography of hyper-doped Ge layers synthesized by Sb in-diffusion by pulsed laser melting

Hyper-doped Ge layers synthesized by Sb in-diffusion by pulsed laser melting have been analyzed by Atom Probe Tomography (APT). The 1D concentration profiles obtained by APT under low field conditions are consistent with those obtained by secondary ion mass spectrometry. However, the 3D distribution of Sb indicates the occurrence of Sb segregation along line defects, outside of which Sb is randomly distributed. The APT measurements indicate a dependence of the measured composition on the electric field. By analyzing the mass spectra and the multiple detection events, we show that low-field conditions should be adopted in order to obtain an accurate composition measurement.

Carrier Recombination Dynamics of Surface-Passivated Epitaxial (100)Ge, (110)Ge, and (111)Ge Layers by Atomic Layer Deposited Al2O3

Germanium (Ge) and its heterostructures with compound semiconductors offer a unique optoelectronic functionality due to its pseudo-bandgap nature, that can be transformed to a direct bandgap material by providing strain and/or mixing with tin. Moreover, two crystal surfaces, (100)Ge and (110)Ge, that are technologically important for ultralow power fin or nanosheet transistors, could offer unprecedented properties with reduced surface defects after passivating these surfaces by atomic layer deposited (ALD) dielectrics. In this work, the crystallographically oriented epitaxial Ge/AlAs heterostructures were grown and passivated with ALD Al2O3 dielectrics, and the microwave photoconductive decay (μ-PCD) technique was employed to evaluate carrier lifetimes at room temperature. The X-ray photoelectron spectroscopy analysis reveals no role of orientation effect in the quality of the ALD Al2O3 dielectric on oriented Ge layers. The carrier lifetimes measured using the μ-PCD technique were benchmarked against unpassivated Ge/AlAs heterostructures. Excitation wavelengths of 1500 and 1800 nm with an estimated injection level of ∼1013 cm–3 were selected to measure the orientation-specific carrier lifetimes. The carrier lifetime was increased from 390 ns to 565 ns for (100)Ge and from 260 ns to 440 ns for (110)Ge orientations with passivation, whereas the carrier lifetime is almost unchanged for (111)Ge after passivation. This behavior indicates a strong dependence of the measured lifetime on surface orientation and surface passivation. The observed increase (>1.5×) in lifetime with Al2O3-passivated (100)Ge and (110)Ge surfaces is due to the lower surface recombination velocity compared to unpassivated Ge/AlAs heterostructures. The enhancement of carrier lifetime from passivated Ge/AlAs heterostructures with (100)Ge and (110)Ge surface orientations offers a path for the development of nanoscale transistors due to the reduced interface state density.

High-quality Ge/Si hetero-bonding by sputtered microcrystalline Ge interlayer

We report a high-quality germanium (Ge) and silicon (Si) wafer bonding based on an intermediate layer of 5 nm-thick microcrystalline Ge. The bubble evolution, crystalline state evolution, and oxide layer evolution at the wafer-bonded interface are investigated. The mechanisms of bubble elimination and oxide layer disappearance are systematically analyzed. The sputtered microcrystalline Ge film consists of an amorphous Ge (a-Ge) layer and a low-quality single-crystal Ge (c-Ge) layer. The bubbles can be eliminated by the loose structure of the low-quality c-Ge at the bonded interface. Thus, the bubble-free bonded interface can be obtained. Besides, the bonding strength decreases with the increase of the sputtering temperature. The pressure on the bonded wafers before post-annealing can eliminate large bubbles and improve bonding strength. It is worth mentioning that the oxide layer and the threading dislocations are not observed at the bonded interface. This study can give a guidance for the subsequent manufacture of high-performance Si-based Ge optoelectronic devices.

High-electron mobility P-doped polycrystalline GeSn layers formed on insulators at low temperatures

Despite its long history, synthesizing n-type polycrystalline Ge layers with high-electron mobility on insulating substrates has been difficult. Based on our recently developed solid-phase crystallization technology, here, we have demonstrated the highest recorded electron mobility (450 cm2 V−1 s−1) for Ge-based polycrystalline thin films on insulating substrates. The underlayer type and small amount of Sn addition were the key parameters controlling both the density and barrier height of the grain boundaries in the P-doped polycrystalline Ge layers. The low growth temperature (≤400 °C) allowed us to develop a GeSn layer on a heat-resistant polyimide film, which exhibited the highest electron mobility (200 cm2 V−1 s−1), as a semiconductor thin film synthesized directly on a flexible substrate. These achievements herald the development of high-performance polycrystalline Ge-based devices on inexpensive glass and flexible plastic substrates.

Mechanical properties of multi-scale germanium specimens from space solar cells under electron irradiation

During long-term service in space, Gallium Arsenide (GaAs) solar cells are directly exposed to electron irradiation which usually causes a dramatic decrease in their performance. In the multilayer structure of solar cells, the germanium (Ge) layer occupies the majority of the thickness as the substrate. Due to the intrinsic brittleness of semiconductor material, there exist various defects during the preparation and assembly of solar cells, the influences of which tend to be intensified by the irradiation effect. In this work, first, Ge specimens for mechanical tests were prepared at scales from microscopic to macroscopic. Then, after different doses of electron irradiation, the mechanical properties of the Ge specimens were investigated. The experimental results demonstrate that electron irradiation has an obvious effect on the mechanical property variation of Ge in diverse scales. The four-point bending test indicates that the elastic modulus, fracture strength, and maximum displacement of the Ge specimens all increase, and reach the maximum value at the irradiation dose of 1 × 1015 e/cm2. The micrometer scale cantilever and nanoindentation tests present similar trends for Ge specimens after irradiation. Atomic Force Microscope (AFM) also observed the change in surface roughness. Finally, a fitting model was established to characterize the relation between modulus change and electron irradiation dose.

Germanium metasurface assisted broadband detectors

Abstract The demand on broadband near-infrared photodetections with high responsivity is becoming increasingly eminent; however its realization remains a significant technological challenge. Here we design, fabricate, and characterize a broadband Ge photodetector (1000–1600 nm), composed of densely packed 230-nm-thick Ge disks of different diameters (255 nm, 320 nm, and 500 nm), placed on top of a 105-nm-thin Ge layer. Using experimentally measured and calculated transmission and absorption spectra, we demonstrate that the absorption and detector responsivity are increased by nearly 2 orders of magnitude, compared to the unstructured Ge photodetector, due to the excitation of magnetic dipole resonances in Ge disks, while preserving a relatively low dark current. Our approach is simple and can be easily adapted to other semiconductor material platforms and operation wavelengths to enable performance improvements of broadband photodetector devices.

Ultrafast Infrared Laser Crystallization of Amorphous Si/Ge Multilayer Structures.

Silicon-germanium multilayer structures consisting of alternating Si and Ge amorphous nanolayers were annealed by ultrashort laser pulses at near-infrared (1030 nm) and mid-infrared (1500 nm) wavelengths. In this paper, we investigate the effects of the type of substrate (Si or glass), and the number of laser pulses (single-shot and multi-shot regimes) on the crystallization of the layers. Based on structural Raman spectroscopy analysis, several annealing regimes were revealed depending on laser fluence, including partial or complete crystallization of the components and formation of solid Si-Ge alloys. Conditions for selective crystallization of germanium when Si remains amorphous and there is no intermixing between the Si and Ge layers were found. Femtosecond mid-IR laser annealing appeared to be particularly favorable for such selective crystallization. Similar crystallization regimes were observed for both single-shot and multi-shot conditions, although at lower fluences and with a lower selectivity in the latter case. A theoretical analysis was carried out based on the laser energy absorption mechanisms, thermal stresses, and non-thermal effects.

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

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