Crystalline Ge Research Articles

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.

Self-Diffusion of Ge in Amorphous Ge x Si1-x Films Studied In Situ by Neutron Reflectometry.

Ge x Si1-x alloys are gaining renewed interest for many applications in electronics and optics, especially for miniaturized devices showing quantum size effects. Point defects and atomic diffusion play a crucial role in miniaturized and metastable systems. In the present work, Ge self-diffusion in sputter deposited amorphous Ge x Si1-x alloys is studied in situ as a function of Ge content x = 0.13, 0.43, 0.8, and 1.0 by neutron reflectometry. The determined Ge self-diffusivities obey the Arrhenius law in the investigated temperature ranges. The higher the Ge content x, the higher the Ge self-diffusivity at the same temperature. The activation enthalpy decreases with x from 4.4 eV for self-diffusion in pure silicon films to about 2 eV self-diffusion in Ge0.8Si0.2 and Ge. The decrease of the activation enthalpy for amorphous Ge x Si1-x is similar to the case of crystalline Ge x Si1-x . Possible explanations are discussed.

Impact of post-ion implantation annealing on Se-hyperdoped Ge

Hyperdoped germanium (Ge) has demonstrated increased sub-bandgap absorption, offering potential applications in the short-wavelength-infrared spectrum (1.0–3.0 μm). This study employs ion implantation to introduce a high concentration of selenium (Se) into Ge and investigates the effects of post-implantation annealing techniques on the recovery of implantation damage and alterations in optical properties. We identify optimal conditions for two distinct annealing techniques: rapid thermal annealing (RTA) at a temperature of 650 °C and ultrafast laser heating (ULH) at a fluence of 6 mJ/cm2. The optimized ULH process outperforms the RTA method in preserving high doping profiles and achieving a fourfold increase in sub-bandgap absorption. However, RTA leads to regrowth of single crystalline Ge, while ULH most likely leads to polycrystalline Ge. The study offers valuable insights into the hyperdoping processes in Ge for the development of advanced optoelectronic devices.

Ge-on-insulator fabrication based on Ge-on-nothing technology

Ge-on-Insulator (GOI) is considered to be a necessary structure for novel Ge-based devices. This paper proposes an alternative approach for fabricating GOI based on the Ge-on-Nothing (GeON) template. In this approach, a regular macropore array is formed by lithography and dry etching. These pores close and merge upon annealing, forming a suspended monocrystalline Ge membrane on one buried void. GOI is fabricated by direct bonding of GeON on Si carrier substrates, using an oxide bonding interface, and subsequent detachment. The fabricated GOI shows uniform physical properties as demonstrated using micro-photoluminescence measurements. Its electrical characteristics and cross-sectional structure are superior to those of Smart-CutTM GOI. To demonstrate its application potential, back-gate GOI capacitors and MOSFETs are fabricated. Their characteristics nicely agree with the theoretically calculated one and show typical MOSFET operations, respectively, which indicates promising Ge crystallinity. This method, therefore, shows the potential to provide high-quality GOI for advanced Ge application devices.

Controlled formation of three-dimensional cavities during lateral epitaxial growth

Epitaxial growth is a fundamental step required to create devices for the semiconductor industry, enabling different materials to be combined in layers with precise control of strain and defect structure. Patterning the growth substrate with a mask before performing epitaxial growth offers additional degrees of freedom to engineer the structure and hence function of the semiconductor device. Here, we demonstrate that conditions exist where such epitaxial lateral overgrowth can produce complex, three-dimensional structures that incorporate cavities of deterministic size. We grow germanium on silicon substrates patterned with a dielectric mask and show that fully-enclosed cavities can be created through an unexpected self-assembly process that is controlled by surface diffusion and surface energy minimization. The result is confined cavities enclosed by single crystalline Ge, with size and position tunable through the initial mask pattern. We present a model to account for the observed cavity symmetry, pinch-off and subsequent evolution, reflecting the dominant role of surface energy. Since dielectric mask patterning and epitaxial growth are compatible with conventional device processing steps, we suggest that this mechanism provides a strategy for developing electronic and photonic functionalities.

Intermediate morphology in the patterning of the crystalline Ge(001) surface induced by ion irradiation

We investigate the morphologies of the Ge(001) surface that are produced by bombardment with a normally incident, broad argon ion beam at sample temperatures above the recrystallization temperature. Two previously observed kinds of topographies are seen, i.e., patterns consisting of upright and inverted rectangular pyramids, as well as patterns composed of shallow, isotropic basins. In addition, we observe the formation of an unexpected third type of pattern for intermediate values of the temperature, ion energy, and ion flux. In this type of intermediate morphology, isolated peaks with rectangular cross-sections stand above a landscape of shallow, rounded basins. We also extend past theoretical work to include a second-order correction term that comes from the curvature dependence of the sputter yield. For a range of parameter values, the resulting continuum model of the surface dynamics produces patterns that are remarkably similar to the intermediate morphologies we observe in our experiments. The formation of the isolated peaks is the result of a term that is not ordinarily included in the equation of motion, a second-order correction to the curvature dependence of the sputter yield. Published by the American Physical Society 2024

Controllable synthesis of crystalline germanium nanorods as anode for lithium-ion batteries with high cycling stability

Germanium (Ge) nanomaterials have emerged as promising anode materials for lithium-ion batteries (LIBs) due to their higher capacity compared to commercial graphite. However, their practical application has been limited by the high cost associated with harsh preparation conditions and the poor electrode cycling stability in charging and diacharging. In this study, we successfully synthesized crystalline Ge nanorods through the reaction of intermetallic compound CaGe and ZnCl2. Ge nanorods with different morphologies and crystallinity can be obtained through precisely controlling the reaction temperature. When employed as electrodes for LIBs, the Ge nanorods demonstrate exceptional long-term cyclic stability. Even after 1000 cycles at a high rate of 2C (1C = 1600 mA g−1), it exhibits a remarkable reversible capacity of around 1000 mAh/g. Furthermore, such Ge electrode displays excellent cycling performance across a wide temperature range. And it could achieve reversible capacities of 1267, 832, and 690 mAh/g, with the rate of 1C, at temperatures of 20, 0, and −20 °C, respectively. Above all, our study offers a cost-effective approach for the synthesis of crystalline Ge nanorods, addressing the concerns associated with high production costs. And the application of Ge nanorods as anode materials in LIBs over a wide temperature range opens up new possibilities for the development of advanced energy storage systems.

(Invited) Sequential 3D Integration of Ge Transistors on Si CMOS

In order to keep the scaling progress going is to go 3D. This paper outlines some technology challenges and solutions to integrate Ge p-type MOSFETs sequentially on Si CMOS. Such a solution addresses the grand challenge to enable increased device density. However, the device itself does not have to scale but at the same time innovative solutions are suggested for low supply voltage operation enabling energy efficient integrated circuits (ICs) that will not be dominated by energy consumption in interconnects.By stacking the transistors on top of each other, and connect them with inter-tier via, the density of transistors per unit area increases. This approach demands that transistors are fabricated at a lower temperature compared to today’s Si CMOS technology. Therefore, we have focused on Ge based transistors, which has an inherently lower process temperature compared to Si transistors.In this paper several technological and design breakthroughs towards realizing Ge based sequential 3D circuits will be shown. We will present: A process to realize thin single crystalline Ge layers on planarized wafers with metal layers in lower tiers.A gate dielectric stack (Ge/Si/TmSiO/Tm2O3/HfO2/TiN) on Ge that enables adequately low defect densities at the dielectric/Ge interface allowing predictable and reliable Ge transistorsFully depleted Ge pFET devices fabricated at a low temperature compatible with sequential 3D. The devices exhibits 60% higher mobility compared to reference Si devices.3D digital circuits with pFETs on top of nFETs can enable area reduction by 30-50% depending on cell type.3D standard cells with lower parasitic capacitance (~30%) compared to 2D cells, enabling lower dynamic power consumption and more energy efficient integrated circuits.

Polymer-Embedding Germanium Nanostrip Waveguide of High Polarization Extinction.

Germanium (Ge) nanostrip was embedded in a polymer and studied as a waveguide. The measurements reveal that this new type of semiconductor/polymer heterogeneous waveguide exhibits strong absorption for the TE mode from 1500 nm to 2004 nm, while the propagation loss for the TM mode declines from 20.56 dB/cm at 1500 nm to 4.89 dB/cm at 2004 nm. The transmission characteristics serve as an essential tool for verifying the optical parameters (n-κ, refractive index, and extinction coefficient) of the strip, addressing the ambiguity raised by spectroscopic ellipsometry regarding highly absorbing materials. Furthermore, the observed strong absorption for the TE mode at 2004 nm is well beyond the cut-off wavelength of the crystalline bulk Ge (~1850 nm at room temperature). This redshift is modeled to manifest the narrowing of the Tauc-fitted bandgap due to the grain order effect in the amorphous Ge layer. The accurate measurement of the nanometer-scale light-absorbing strips in a waveguide form is a crucial step toward the accurate design of integrated photonic devices that utilize such components.

Self-Oscillated Growth Formation of Standing Ultrathin Nanosheets out of Uniform Ge/Si Superlattice Nanowires

Self-oscillation is an intriguing and omnipresent phenomenon that governs a broad range of growth dynamics and formation of nanoscale periodic and delicate heterostructures. A self-oscillating growth phenomenon of catalyst droplets, consuming surface-coating a-Si/a-Ge bilayer, is exploited to accomplish a high-frequency alternating growth of ultrathin crystalline Si and Ge (c-Si/c-Ge) nano-slates, with Ge-rich layer thickness of 14–19 nm, embedded within a superlattice nanowire structure, with pre-known position and uniform channel diameter. A subsequent selective etching of the Ge-rich segments leaves a chain of ultrafine standing c-Si nanosheets down to ∼ 6 nm thick, without the use of any expensive high-resolution lithography and growth modulation control. A ternary-phase-competition model has been established to explain the underlying formation mechanism of this nanoscale self-oscillating growth dynamics. It is also suggested that these ultrathin nanosheets could help to produce ultrathin fin-channels for advanced electronics, or provide size-specified trapping sites to capture and position hetero nanoparticle for high-precision labelling or light emission.

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.

High hole mobility and non-localized states in amorphous germanium

Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to occur through the hopping mechanism via these localized states. The carrier mobility of these materials is usually very low, in the order of ∼10−3–10−2 cm2/Vs at room temperature. In this study, we show that pure high-density amorphous Ge has exceptionally high carrier mobility, in the order of ∼100 cm2/Vs, and a high hole concentration of ∼1018 cm−3. The temperature-dependent conductivity of the material is also very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results provide direct evidence for a largely preserved band structure and non-localized states within the valence band in high-density amorphous Ge, as previously suggested by Tauc et al. from optical characterization alone.

Enhanced Catalytic Palladium Embedded Inside Porous Silicon for Improved Hydrogen Gas Sensing

In this work, we reported on room temperature porous silicon (PS) and embedding PS using simple and economical techniques of electrochemical etching and thermal evaporation. The PS substrate was prepared using the technique of electrochemically etching the n-type Si (100) wafer at a constant current density of 10 mA/cm2 for 10 min under the illumination of incandescent white light. After PS formation, Ge pieces were thermally evaporated onto the two PS substrates in a vacuum condition. This was then followed by the deposition of the ZnO layer onto the Ge/PS substrate by the same method using commercial 99.9% pure ZnO powders. The three samples were identified as PS, Ge/PS and ZnO/Ge/PS samples, respectively. Pd finger contacts were deposited on the PS and embedding PS (Ge/PS and ZnO/Ge/PS) to form Pd on PS hydrogen sensors using RF magnetron sputtering. SEM and EDX suggested the presence of substantial Ge and ZnO inside the uniform circular pores for Ge/PS and ZnO/Ge/PS samples, respectively. Raman spectra showed that good crystalline Ge and ZnO nanostructures embedded inside the pores were obtained. For hydrogen sensing, Pd on ZnO/Ge/PS Schottky diode exhibited a dramatic change of current after exposure to H2 as compared to PS and Ge/PS devices. It is observed that the sensitivity increased exponentially with the hydrogen flow rate for all the sensors. The ZnO/Ge/PS showed more sensitivity towards H2 than that of PS and Ge/PS especially at high flow rate of H2 with higher current gain (69.11) and shorter response (180 s) and recovery times (30 s).

Defect free strain relaxation of microcrystals on mesoporous patterned silicon

A perfectly compliant substrate would allow the monolithic integration of high-quality semiconductor materials such as Ge and III-V on Silicon (Si) substrate, enabling novel functionalities on the well-established low-cost Si technology platform. Here, we demonstrate a compliant Si substrate allowing defect-free epitaxial growth of lattice mismatched materials. The method is based on the deep patterning of the Si substrate to form micrometer-scale pillars and subsequent electrochemical porosification. The investigation of the epitaxial Ge crystalline quality by X-ray diffraction, transmission electron microscopy and etch-pits counting demonstrates the full elastic relaxation of defect-free microcrystals. The achievement of dislocation free heteroepitaxy relies on the interplay between elastic deformation of the porous micropillars, set under stress by the lattice mismatch between Ge and Si, and on the diffusion of Ge into the mesoporous patterned substrate attenuating the mismatch strain at the Ge/Si interface.

Spectro-ellipsometric probing of wetting, nucleation, and dot/island formation during photo-excited chemical vapor deposition of Ge on SiO2 substrate

The morphological evolution of Ge layers growing on the SiO2/Si(100) substrate by photo-excited chemical vapor deposition was traced through an analysis of pseudodielectric functions measured by real-time spectroscopic ellipsometry. Simulation and fitting were carried out on multiple samples with various Ge film thicknesses as well as on sequential optical spectra from a sample with an incremental buildup of Ge atoms on one substrate. Single- and two-layer models involving crystalline Ge (c-Ge), amorphous Ge (a-Ge), and void components were employed under the Bruggeman effective medium approximation to represent wetting of the SiO2 surface, nucleation of Ge seeds for the subsequent dot/island formation, and steady-state dot/island growth. A combination of c-Ge and a-Ge represents intermediate crystallinity, and void represents vacant space between dots/islands. A single-layer model with a mixture of c-Ge, a-Ge, and void components was used for crude estimation of the composition from which the time evolution of the volume fraction of the components was derived. However, fitting in the early growth stage resulted in an unrealistic structure, indicating that the dielectric function of the thin hydrogenated Ge network layer was very different from those of c-Ge and a-Ge. The optical spectra of dots/islands at the intermediate growth stage could be reproduced by a two-layer model consisting of a (a-Ge + void) layer overlaid on a (c-Ge + void) base layer. The real-time Ψ–Δ trajectories of ellipsometric angles monitored at a photon energy of 3.4 eV consisted of three branches. They could be reproduced by assuming the growth of an outer layer with an appropriate composition. After wetting on SiO2 (branch 1), the Ge seed layer nucleates while the volume fraction of Ge rapidly decreases from 70% to 25% with proceeding growth (branch 2). Then, the volume fraction of Ge continuously increases up to 65%, eventually reaching steady-state dots/island growth (branch 3)

Metal-induced crystallization of amorphous semiconductor films: Nucleation phenomena in Ag-Ge films

The initial stages of interfacial interactions between Ag nanoparticles and amorphous Ge (a-Ge) films, in particular the mechanism and kinetics of metal-induced crystallization (MIC) of a-Ge films are investigated herein. Unsupported Ag-Ge films formed by PVD were used as a model. In situ high-resolution (S)TEM and low-loss monochromated STEM-EELS techniques were used for real-time observation of the nucleation and growth of crystalline Ge and intermediate phases, as well as mapping of chemical elements at the reaction zone. We show that crystallization is activated at the metal–semiconductor interface around ≈200 °C, followed by lateral crystallization of a-Ge film at temperatures above ≈400 °C. It was revealed that the MIC process occurs through intermediate phases. Metastable Ag0.8Ge0.2 hcp phase was identified at the reaction front at elevated temperatures, thus revealing the non-equilibrium melting of a eutectic alloy as the most probable mechanism of Ag-induced crystallization of a-Ge film. We show that the MIC process is activated only if the size of Ag nanoparticles exceeds a critical value, which is ≈8 nm in our case and equal to the thickness of the a-Ge film.

Disorder and cavity evolution in single-crystalline Ge during implantation of Sb ions monitored in-situ by spectroscopic ellipsometry

Ion implantation has been a key technology for the controlled surface modification of materials in microelectronics and generally, for tribology, biocompatibility, corrosion resistance and many more. To form shallow junctions in Ge is a challenging task. In this work the formation and accumulation of shallow damage profiles was studied by in-situ spectroscopic ellipsometry (SE) for the accurate tracking and evaluation of void and damage fractions in crystalline Ge during implantation of 200-keV Sb + ions with a total fluence up to 1016 cm−2 and an ion flux of 2.1 × 1012 cm−2s−1. The consecutive stages of damage accumulation were identified using optical multi-layer models with quantitative parameters of the thickness of modified layers as well as the volume fractions of amorphized material and voids. The effective size of damaged zones formed from ion tracks initiated by individual bombarding ions can be estimated by numerical simulation compared with the dynamics of damage profiles measured by ion beam analysis and ellipsometry. According to our observations, the formation of initial partial disorder was followed by complete amorphization and void formation occurring at the fluence of about 1 × 1015 cm−2, leading to a high volume fraction of voids and a modified layer thickness of ≈200 nm by the end of the irradiation process. This agrees with the results of numerical simulations and complementary scanning electron microscopy (SEM) measurements. In addition, we found a quasi-periodic time dependent behavior of amorphization and void formation represented by alternating accelerations and decelerations of different reorganization processes, respectively. For the understanding and prevention of adverse void formation and for controlled evolution of subsurface nanocavities or cellular surface texture the in-situ monitoring of the dynamics of structural damage accumulation by the developed SE method is essential.

Impact of substrate heating during Al deposition and post annealing on surface morphology, Al crystallinity, and Ge segregation in Al/Ge(111) structure

We have studied the impact of Ge substrate heating during ∼25 nm thick Al deposition and post annealing in N2 ambient on the surface flatness of an Al/Ge(111) structure, the crystallographic structure of the deposited Al layer, and formation of a Ge segregated layer. Surface segregation of Ge atoms on a flat metal surface is an effective means of growing two-dimensional Ge crystals as well as an ultrathin Ge crystalline layer. The surface morphology of the Al/Ge(111) structure becomes flat by substrate heating during Al deposition. The crystallinity of the Al layer on Ge(111) can be improved by both substrate heating and post annealing. Ge segregation on a flat Al(111) surface also occurred by post annealing.

Epitaxial Growth of Planar Hutwires on Silicon‐on‐Insulator Substrates

Quantum devices based on holes confined in crystalline Ge nanostructures have emerged as a promising platform in the field of quantum technology. Epitaxial Ge hutwires (HWs) successfully used in pioneering quantum bit (qubit) experiments are the logical next choice for long-distance coherent spin–spin coupling experiments. However, leakage currents are currently limiting the device performance of HWs on bulk Si substrates. This drawback can be mitigated by the HW growth on silicon-on-insulator (SOI) substrates. Herein, molecular beam epitaxy (MBE) HW growth developed on technologically relevant SOI substrates with different device layer thicknesses is shown. Using atomic force microscopy characterization, the fabrication of HWs on SOI with lengths of up to 600 nm is demonstrated. In addition, the very narrow window of growth parameters for which successful HW growth can be achieved and the distinct differences of HW growth on SOI versus bulk Si substrates are addressed.

(De)Lithiation and Strain Mechanism in Crystalline Ge Nanoparticles.

Germanium is a promising active material for high energy density anodes in Li-ion batteries thanks to its good Li-ion conduction and mechanical properties. However, a deep understanding of the (de)lithiation mechanism of Ge requires advanced characterizations to correlate structural and chemical evolution during charge and discharge. Here we report a combined operando X-ray diffraction (XRD) and ex situ 7Li solid-state NMR investigation performed on crystalline germanium nanoparticles (c-Ge Nps) based anodes during partial and complete cycling at C/10 versus Li metal. High-resolution XRD data, acquired along three successive partial cycles, revealed the formation process of crystalline core-amorphous shell particles and their associated strain behavior, demonstrating the reversibility of the c-Ge lattice strain, unlike what is observed in the crystalline silicon nanoparticles. Moreover, the crystalline and amorphous lithiated phases formed during a complete lithiation cycle are identified. Amorphous Li7Ge3 and Li7Ge2 are formed successively, followed by the appearance of crystalline Li15Ge4 (c-Li15Ge4) at the end of lithiation. These results highlight the enhanced mechanical properties of germanium compared to silicon, which can mitigate pulverization and increase structural stability, in the perspective for developing high-performance anodes.