Crystalline Ge Research Articles

Phase Transformation in Germanium-on-Insulator (GeOI) Films Using Raman Spectroscopy and Nanoindentation

Germanium-on-insulator (GeOI) films fabricated using the SMART CUT wafer bonding and film exfoliation technology were investigated for the mechanical properties and induced phase transformations by using nanoindentation and Raman spectroscopy experiments. The hardness and modulus results of the GeOI films are significantly different from the published SOI literature and bulk germanium results obtained by Liu et al., 2020 [1] and SOI films results obtained by Miller et al., 2007 [2]. From our measured hardness results of 8 to 10 GPa for very thin single crystal Ge(100) films compared to 13.1 GPa for Bulk Ge (100), one can conclude that the thinning of crystalline Ge films leads to a considerable softening of the crystalline Ge resulting in less brittle robust films. Similarly, our measured modulus of 130 to 140 GPa for thin Ge (100) films compared to the reported bulk Ge (100) value of 157 GPa corroborates these research findings. The GeOI films are softer and more flexible as compared to bulk Ge hardness and stiffness properties. The Raman spectroscopy of the spherical indents indicate bands of metastable Ge phases @ 220 cm-1, 195 cm-1 and 184 cm-1 wavenumbers. Our results demonstrate that a spherical indenter impacted a wider area of contact and produced GeOI indented surfaces free of cracks and fracture. The spherical indenter tip kept the Ge top layer intact when compared to the Berkovich indenter tip during penetration. In contrast, the Berkovich indenter tip developed excessive fracture that resulted in displacing the Ge top layer sideways and exposed the Si substrate underneath revealing Raman spectra bands of metastable Si phases @ 350 cm-1, 399 cm-1 and 430 cm-1.Apart from the well documented electrical advantages of GeOI, the reported research on the nanomechanical properties reveals considerable softening and flexibility of the very thin exfoliated single crystal Ge films that promise added benefits and further implications for enhancing potential device application of GeOI wafers for process integration.1.] N. Liu, X-J Yang, Z. Yu, and L. Zhao, Trans. Nonferrous Met. Soc. China, 30, 181 (2020).2.] N. Miller, K. Tapily, H. Baumgart, G.K. Celler, F. Brunier, and A.A. Elmustafa, Mater. Res. Soc. Symp. Proc. Vol. 1021 © 2007 Materials Research Society, 1021-HH05-24. Boston, Massachusetts. (2007).

Underlayer Selection to Improve the Performance of Polycrystalline Ge Thin Film Transistors

1. Introduction To improve the performance of Ge-thin film transistors (TFTs), it is essential not only to enhance the crystallinity of poly-Ge but also comprehensively study the relationship between its electrical properties and TFT characteristics. We recently advanced conventional solid-phase crystallization (SPC) method, significantly enlarged the grain size of poly-Ge and recorded the highest Hall effect hole mobility (μ Hall) of 620 cm2/Vs [1,2]. We fabricate poly-Ge TFTs using SPC-Ge and discuss the relationship between the film properties and TFT characteristics [3]. In the present study, we discuss the Ge thickness effects on the poly-Ge TFTs. To further improve the TFT performance, we enhanced the Ge crystallinity at thin film region (< 100 nm) by inserting Al2O3 underlayer. 2. Experimental Procedure The a-Ge layers were deposited on SiO2 glass substrates by vacuum deposition while heating at 125 °C. The initial film thickness of the a-Ge layer (t i) ranged from 25 to 200 nm. The samples were also prepared in which the insulating underlayer (Al2O3, GeO2) was sputtered at 10-50 nm for Ge with t i = 50-300 nm. The samples were then loaded into a conventional tube furnace in an N2 atmosphere annealed at 450 °C for 5 h to induce SPC.We fabricated accumulation-mode metal source/drain (S/D) p-channel TFTs using SPC-Ge. Figure 1 shows the process and structure. Pt and TiN were sequentially formed as a metal S/D and a capping layer, respectively. We used Al/SiO2/Al2O3/SiO2/GeO2 for the gate stack. The channel width and length (W/L) were 55 and 10 μm, respectively. All process including SPC were conducted below 450 °C. 3. Results and Discussion Figures 2(a)‒2(e) show the grain size dramatically varies with t i. All samples showed p-type conduction because the defects in Ge provides shallow acceptor levels that generate holes at room temperature. Therefore, larger grain sizes provide lower hole concentration p, as shown in Figs. 2(f) and 2(g). Figure 2(g) also shows that μ Hall peaks at t i = 200 nm, whereas the grain size peaks at t i = 100 nm. This behavior is likely attributed to the carrier scattering near the Ge/SiO2 interface [2].Figure 2(h) shows that on-current I on increases with increasing t i, which reflects μ Hall. Off-current I off is determined by the relationship between t i and the maximum depletion layer width (d max), which can be estimated from p. From the relationship between d max and t i, we found that the lower t i provides the higher occupation of the depletion layer in the whole SPC-Ge layer, which decreases I off. Reflecting I on, I off, and the on/off current ratio I on/I off reaches the maximum at t i = 50 nm. Figure 2(i) shows that field-effect mobility (μ FE) is consistent with the trend of μ Hall, while μ FE is much lower than μ Hall. This is likely because not only carrier scattering at the MOS interface, but also large I off causes underestimation of transconductance and μ FE.We thinned the t i = 100 nm sample to 55 nm using chemical-mechanical polishing. The film keeps low p enough to get full-depleted, and was processed into TFT as before. Figures 3(a) and 3(b) show the typical p-channel transistor operation, high I on/I off (102) and μ FE (170 cm2/Vs) because of both the high I on due to the high μ Hall and low I off. Therefore, to improve the TFT performance, a Ge layer compatible with high μ Hall and a thin film is desirable.Because the nucleation in a thin film occurs at the substrate interface, we modulated the interface by preparing Al2O3 and GeO2 underlayers. Figures 4(a)‒4(f) show the grain size is almost doubled by inserting underlayers for both t i = 50 nm and 300 nm, while the t i = 50 nm Ge layer with GeO2 did not crystallize completely due to the slow growth rate. Compared to the samples without underalyers, Fig. 4(g) shows the underlayers reduce p reflecting the large grain size. Figures 4(h) shows the GeO2 underlayer provides high μ Hall for t i = 300 nm, while low μ Hall for t i < 300 nm. This behavior is likely due to the carrier scattering near the Ge/GeO2 interface. Conversely, the Al2O3 underlayer provides high μ Hall for t i < 300 nm. In particular, the t i = 50 nm Ge layer exhibits relatively low p and high μ Hall. These results suggest that the Al2O3 underlayer has the potential to further improve the performance of poly-Ge TFTs.

Effect of Ion Irradiation on Amorphous and Crystalline Ge–Se and Their Application as Phase Change Temperature Sensor

Research on phase change materials is predominantly focused on their application as memory devices or for temperature control which requires low phase change temperature. The Ge–Se binary chalcogenide glass system with its wide glass‐forming region is a potential candidate for high‐temperature and high‐radiation phase change applications. Herein, the concept of employing Ge x Se100−x glasses to monitor high temperature (450–528 °C) using the phase change effect, is reported. Materials selection, device structure, and performance of prototype sensors are analyzed. In addition, the effect of heavy ion irradiation by Xe ions with energies of 200, 600, and 1000 keV (fluence ≈1014 cm−2) on the Ge x Se100−x (x = 30, 33, 40) thin films and phase change devices is studied. The irradiation effect on the amorphous and crystalline structure of the thin films is evaluated by Raman spectroscopy and X‐ray diffraction (XRD). Although the changes in the structural units of amorphous films are negligible, in crystalline films orthorhombic‐GeSe2 crystals are found to be most affected by irradiation and a new phase, orthorhombic GeSe is found in the thin films after irradiation. The performance of a sensor with an active film of Ge40Se60 is also shown as an example.

Measurement of Ehrlich-Schwoebel barrier contribution to the self-organized formation of ordered surface patterns on Ge(001)

Normal incidence 1 keV Ar$^+$ ion bombardment leads to amorphization and ultrasmoothing of Ge at room temperature, but at elevated temperatures the Ge surface remains crystalline and is unstable to the formation of self-organized nanoscale patterns of ordered pyramid-shaped pits. The physical phenomenon distinguishing the high temperature patterning from room temperature ultrasmoothing is believed to be a surface instability due to the Ehrlich-Schwoebel barrier for diffusing vacancies and adatoms, which is not present on the amorphous material. This real-time GISAXS study compares smoothing of a pre-patterned Ge sample at room temperature with patterning of an initially flat Ge sample at an elevated temperature. In both experiments, when the nanoscale structures are relatively small in height, the average kinetics can be explained by a linear theory. The linear theory coefficients, indicating surface stability or instability, were extracted for both experiments. A comparison between the two measurements allows estimation of the contribution of the Ehrlich-Schwoebel barrier to the self-organized formation of ordered nanoscale patterns on crystalline Ge surfaces.

Effect of Process Temperature on the Growth Kinetic and Structure of Ge Nanowires Formed by Galvanostatic Electrodeposition Using in Nanoparticles

In this work, germanium nanowires (GeNWs) were fabricated by galvanostatic electrodeposition using In nanoparticles from water solutions at different temperatures. It was found that in the temperature range from 10°C to 60°C there was no significant change in the structure of GeNWs, and the average diameter was about 40 nm. The growth time of GeNWs increases linearly with increasing temperature of the electrolyte solution. However, the structure of GeNW obtained at a solution temperature of 90°C has changed. It was shown that these GeNWs have a core-shell structure: the core is a crystalline Ge phase containing In atoms, and the shell is Ge oxides (hydroxides).

Nanopatterning of the (001) surface of crystalline Ge by ion irradiation at off-normal incidence: Experiment and simulation

Intricate topographical patterns can form on the surface of crystalline Ge(001) subject to low-energy ion irradiation in the reverse epitaxy regime, i.e., at elevated temperatures which enable dynamic recrystallization. We compare such nanoscale patterns produced by irradiation from varied polar and azimuthal ion incidence angles with corresponding calculated surface topographies. To this end, we propose a continuum equation including both anisotropic erosive and anisotropic diffusive effects. Molecular dynamics simulations provide the coefficients of angle-dependent sputter erosion for the calculations. By merely changing these coefficients accordingly, the experimentally observed surface morphologies can be reproduced, except for extreme ion incidence angles. Angle-dependent sputter erosion is thereby identified as a dominant mechanism in ion-induced pattern formation on crystalline surfaces under irradiation from off-normal incidence angles.

Layer-exchange crystallization for low-temperature (∼450 °C) formation of n-type tensile-strained Ge on insulator

Layer-exchange crystallization of Ge using a group-V element has been investigated to develop a low-temperature (&amp;lt;500 °C) formation technique of n-type tensile-strained crystalline Ge on insulator. Here, the Sb of a group-V element is employed as a catalyst. Annealing (450 °C) of a-Ge (100 nm)/Sb (100 nm) bi-layer stacked structures generates layer-exchange crystallization. Namely, Ge and Sb layers exchange their positions, and Ge layers are crystallized on insulator substrates. However, Ge evaporation occurs during annealing, and a high concentration of Sb (∼20%) remains at the Ge/insulator interface. To solve these problems, the thickness reduction of Sb films and introduction of a-Ge thin under-layers are examined. By annealing (450 °C) a-Ge (100 nm)/Sb (50 nm)/a-Ge (5 nm) tri-layer structures, layer-exchange crystallization of Ge layers on insulator without Ge evaporation or Sb residue has been achieved. This enables formation of n-type tensile-strained (∼0.3%) Ge layers (free electron concentration: ∼5 × 1017 cm−3). Moreover, crystal orientation control of grown Ge films through the introduction of the diffusion barrier is examined. These results demonstrate the possibility of layer-exchange crystallization induced by a group-V element to realize functional thin-film devices for advanced electronics and photonics.

Crystallization properties of melt-quenched Ge-rich GeSbTe thin films for phase change memory applications

The crystallization process of melt quenched Ge-rich GeSbTe films, with composition optimized for memory applications, has been studied by optical reflectance measurements. The optical properties have been related to the structure and composition by means of the effective medium approximation. The compositional variations have been investigated by transmission electron microscopy and electron energy loss spectroscopy. Amorphous materials prepared by melt-quenching with different laser energy densities have been studied. For the energy density of 1.5 J cm−2, a uniform amorphous layer, with embedded Ge crystalline grains, is obtained. The film exhibits a crystallization temperature of 275 °C and no relevant phase separation during crystallization. For a lower energy density of 1 J cm−2, only half of the film thickness is quenched to the amorphous phase, with Ge depletion. The crystallization temperature of the Ge depleted film is 245 °C, and a partial phase separation occurs.

Photon absorption and scattering of Ge nanocrystals embedded in SiO2 prepared by co-sputtering

Ge nanocrystals (GeNCs) embedded in SiO2 matrix were prepared by co-sputtering method and heat treatment. The formation of the GeNCs were confirmed by X-ray diffraction and Raman spectroscopy. Photon absorbance recordedly increased with the increased Ge content, being in consistent with the results shown in the transient induced absorption measurements. The direct transitions between Γ and L points in the Brillouin zone, assigned as E1, were established at approximately 2 eV and relatively independent from the GeNC sizes. Raman scattering peaks characterised for crystalline Ge at approximately 300 cm−1 were observed to shift with the increasing laser excitation flux. While the displacement of the Raman peak (ΔRs) increased with the Ge content, the ratios of the displacement rates and optical coefficients (a), ΔRs/a, were found to be the same for all the samples, suggesting an intrinsic thermal characteristic of the GeNCs.

Temperature-dependent Raman scattering of the Ge + GeOx system and its potential as an optical thermometer

The phonon behavior of various Ge-based samples has been investigated in great detail. The study comprised Ge films (amorphous and partially crystallized and alloyed with oxygen), crystalline Ge wafers (pristine and alloyed with oxygen), and GeO2 powder. The oxygen-containing samples obtained after thermal annealing the Ge film and Ge wafer correspond to sub-stoichiometric GeO2, or the so-called Ge + GeOx system, in which case [O] ∼40 at.%. A thorough examination of this Ge + GeOx system – as provided by temperature-dependent Raman scattering, their analysis according to the existing theoretical models (involving anharmonic phonon coupling and thermal expansion processes), and a critical review of the literature – indicates the coexistence of practically independent Ge–Ge- and Ge–O-related structures and confirms the presence of both structural and chemical (dis)order. Moreover, the main features regarding the phonon frequencies (ω), line widths (γ), and scattering intensities (IS) of the Ge + GeOx system suggest their suitability in temperature sensing applications. Accordingly, experimentally determined reference curves (i.e., ωT, γT, and IST data described in terms of simple mathematical functions) gave rise to temperature accuracies on the order of 20 K. Everything considered, in its current form, this work presents the first comprehensive analysis-discussion regarding the Ge + GeOx system that, additionally, is being proposed as an effective medium for optical thermometric applications.

Blade-Type Reaction Front in Micrometer-Sized Germanium Particles during Lithiation.

To investigate the lithium transport mechanism in micrometer-sized germanium (Ge) particles, in situ focused ion beam-scanning electron microscopy was used to monitor the structural evolution of individual Ge particles during lithiation. Our results show that there are two types of reaction fronts during lithiation, representing the differences of reactions on the surface and in bulk. The cross-sectional SEM images and transmission electron microscopy characterizations show that the interface between amorphous LixGe and Ge has a wedge shape because of the higher Li transport rate on the surface of the particle. The blade-type reaction front is formed at the interface of the amorphous LixGe and crystalline Ge and is attributed to the large strain at the interface.

Four-step heating process for solid-phase crystallization of Ge leading to high carrier mobility

Solid-phase crystallization is rapidly developing as a method for obtaining high-quality polycrystalline Ge layers. We propose a four-step heating process, focusing on post-deposition annealing (PDA), which is performed between the film deposition and crystallization. PDA at an appropriate temperature (500 °C) reduces the grain size while improving the crystallinity of Ge (150 nm thickness), which leads to the reduction of the grain boundary barrier height (5 meV). The resulting hole mobility (530 cm2 V−1 s−1) is higher than that of any other semiconductor thin films (<300 nm) and even single-crystal Si wafers. These findings will further improve Ge thin-film transistors for advanced devices.

(Invited) Epitaxial Growth of Ge/III-V Films and Hetero-Layer Lift-off for Ultra-Thin GeOI Fabrication

Ge have been considered as an alternative channel material to Si as well as a promising base material for optical components, adding new functions into current Si-LSI. It is necessary to form high quality Ge layers onto Si substrate for realization of Si/Ge monolithic 3D (M3D) devices. Combing high quality hetero-epitaxial growth of Ge/III-V semiconductors, low-temperature bonding, and an advanced lift-off technique, we have achieved Ge layer transfer on Si substrate, possessing excellent Ge crystallinity and electrical properties. The advanced HEtero-Layer-Lift-Off (HELLO) technique utilizing SiGe-based hetero-epitaxy and dozens digital etching (DDE) processes are demonstrated to realize high quality and ultra-thin Ge-on-insulator (GeOI) structures for ultimately scaled CMOS devices on Si platform.

Effect of gold layer thickness on the low-temperature crystallization process of germanium thin films by gold-induced crystallization

The effect of Au layer thickness (1, 2.5, 5, 7.5 and 10 nm) on the low-temperature crystallization of Ge thin films (30 nm) was examined. It is found that the best Ge crystallinity is achieved at an Au layer thickness of 2.5 nm. This finding will open up the possibility to reduce the amount of Au consumption in the low-temperature crystallization process of Ge thin films.

Sensitivity of germanium content on growth conditions of silicon-germanium nanoparticles prepared in nonthermal capacitively-coupled plasmas

We report on the synthesis of Si1−x Ge x alloy nanocrystals by very-high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique at different silane to germane gas flow ratio (R) in a mixture of (H2+Ar) dilution gas and H2 dilution gas alone. TEM, SAED, EDS studies and HAADF-STEM mapping of the samples were done to investigate the NCs' size, crystallinity and distribution of Si and Ge in the Si1−x Ge x alloy NCs. The average estimated size of the NCs in all the samples are in the order of exciton Bohr radius of Ge (24.3 nm), thereby indicating the probability of good quantum confinement. The alloy nature of NCs was confirmed in Raman study. The content of Ge in SiGe NCs was evaluated from Raman spectra which show a direct correlation with the fraction of hydrogen flow in the dilution gas mixture.

Growth of Ge epilayers using iso-butylgermane (IBGe) and its memory effect in an III-V chemical beam epitaxy reactor

Employing group IV semiconductors as germanium (Ge) and III-V compounds as heterostructures it enables efficient devices such as triple junction solar cells, thermophotovoltaic and infrared detection devices. In this work, we studied the growth of Ge with iso-butylgermane (IBGe) as germanium source and the Ge memory effect in our VG Semicon V90F 4-inch chemical beam epitaxy (CBE) reactor for III-V materials. High crystalline quality Ge epilayers were grown on GaAs substrates, while a Ge background doping in the range of 5 × 1018 cm−3 has been found in the GaAs epilayers. The low temperature photoluminescence (LTPL) spectra showed that the Ge background leads to a blueshift and broadening of the exitonic transition in the grown GaAs layers. To eliminate the Ge memory effect several approaches of chamber bake have been investigated, and a reduction of more than one order of magnitude have been demonstrated. Hall effect and secondary ion mass spectroscopy (SIMS) measurements corroborate the reduction in Ge background doping in GaAs epilayers down to 1 × 1017 cm−3.

Fabrication of Germanium-on-insulator in a Ge wafer with a crystalline Ge top layer and buried GeO2 layer by oxygen ion implantation

The paper reports fabrication of Germanium-on-Insulator (GeOI) wafer by Oxygen ion implantation of an undoped Ge wafer of orientation (100). O+ ions (energy 200 keV) were implanted to a fluence of 1.9 × 1018 ions-cm−2 and the implanted wafer was subjected to Rapid Thermal Annealing. The resulting wafer has a top crystalline Ge layer of ~ 220 nm thickness and resistivity ≈32 Ohm-cm and a buried Oxide layer (BOX) of crystalline GeO2 (thickness ≈0.62 μm). The crystalline GeO2 layer has hexagonal crystal structure with lattice constants close to the standard values. Raman Spectroscopy and cross-sectional electron microscopy established that the top Ge layer was recrystallized during annealing with a residual tensile strain of around +0.4% and an estimated dislocation density of 2.7 × 107cm−2. The crystallinity and electrical characteristics of the top layer and the quality of the BOX layer are such that it can be utilized for device fabrication.

New insights into thermomechanical behavior of GeTe thin films during crystallization

In this work, we reexamine Ge rejection in Ge-rich GeTe thin films with a slight deviation from stoichiometry using a unique combination of in situ measurements: curvature and x-ray diffraction as well as electrical resistance and x-ray diffraction and reflectivity during annealing. This unique combination of several experiments performed simultaneously on a synchrotron beamline allows to monitor in situ, during the crystallization and phase transformation, the microstructure, the strain and the stress changes, as well as electrical properties of GeTe films. Structural, electrical and thermomechanical evolutions of the GeTe thin films upon annealing are shown to follow three different steps. Stage I, before crystallization, is characterized by a tensile stress variation and a small decrease of the mass density. Stage II corresponds to the rhombohedral αGeTe phase crystallization leading to an abrupt tensile stress jump (+72 MPa), a mass density increase, and followed by a slight compressive stress evolution. During stage III, Ge crystallization is observed leading to a compressive stress jump (−54 MPa), an abrupt increase in αGeTe lattice spacing and diffracted intensity, whereas αGeTe diffraction peak widths decrease. During cooling a thermoelastic behavior is observed. A detailed analysis of stage III (Ge precipitation and crystallization) is performed and discussed regarding structural, stress, microstrain, electrical and thermomechanical properties. In particular, this study reveals that crystalline Ge precipitation results in important changes (volume of the unit cell, homogeneity of lattice spacing, average stress …) in the surrounding GeTe matrix. Different scenarios are proposed to understand these results.

Lattice Boltzmann simulation of metal-induced crystallization of amorphous semiconductor films

Interface interactions in metal-semiconductor couples are a subject of current interest in electronics, optics, catalysis, energy, and biotechnology. The process of interaction is unknown in many aspects and might be accompanied by a mass transfer on large scales as in the case of the metal-induced crystallization of amorphous semiconductor films. We simulated the kinetics of interaction of liquid Au nanoparticles on the amorphous Ge substrate by means of lattice Boltzmann method for the first time. We supposed that (i) crystalline Ge randomly nucleated and grew under the Au nanoparticle, and (ii) amorphous Ge has a higher surface wettability with Au than the crystalline one. The advancement of the Au nanoparticle on the Ge substrate has been observed and the conditions of activation and maintenance of the movement have been analyzed. Nucleation and growth rates of the crystalline phase, as well as the difference in wetting conditions, were the primary factors that initiated and controlled the nanoparticle movement. The simulations results have been compared with the experimental high-resolution TEM observations.

An infrared transmission study of Ge:Mn thick films prepared by ion implantation and post-annealing

Ge:Mn thick films (t≈3 μm) with low average Mn concentration (&amp;lt;0.3%) were prepared by ion implantation at 77 K followed by either conventional or flashlamp annealing. The films were characterized by x-ray diffraction, secondary ion mass spectrometry, magnetometry, and infrared transmission (100–6500 cm−1). Post-annealing at a high enough temperature recrystallizes the amorphous Ge:Mn films without significant migration of Mn to the surface, while solid phase epitaxy does not occur, resulting in polycrystalline films. Annealing causes an estimated 50%–80% of the implanted Mn to migrate to Mn-rich clusters or form Mn5Ge3, while the remainder enters the Ge lattice substitutionally creating free holes. Evidence for free holes comes from the structure in the mid-infrared absorption coefficient that is similar to previous observations in p-type Ge. The data suggest that the maximum solubility of Mn in the Ge crystalline lattice has an upper limit of &amp;lt;0.08%.