Ge Intermixing Research Articles

Underlying principles of Ge-induced early cluster-to-layer transition for ultrathin Ag layer formation on oxides

The fabrication of continuous Ag layers with thicknesses of <10 nm are challenging. Moreover, information on the crucial factors responsible for early cluster-to-layer transition is limited. Hence, this study focuses on the effects of thin Ge interlayers in forming Ag layers on SiOx substrates. Experimental and numerical analyses demonstrate the active migration and intermixing of atomic Ge in the Ag and SiOx matrices during the early Ag layering stages. The incorporation of Ge into Ag matrices facilitates a faster cluster-to-layer transition than those with Al and Cu. Ge-mediated Ag clustering reduces the rearrangement momenta of extremely small Ag clusters densely concentrated on SiOx substrates. This is attributed to the Ge-induced reduction in the diffusion of atomic Ag in the surfaces of volatile Ag clusters and consequently, the suppression of cluster coalescence. These findings provide insights into the unconventional role of Ge in Ag clustering dynamics. Thus, this study presents a crucial framework for optimizing Ag wetting on oxide substrates with ultralow optical and electrical losses via extreme dissipation of the residual amounts of metalloid wetting-inducing elements.

Concurrent Characterization of Surface Diffusion and Intermixing of Ge on Si: A Classical Molecular Dynamics Study

AbstractThe surface diffusion and intermixing of Ge ad‐atoms over Si (001) 2 × 1 substrates using classical molecular dynamics (CMD) simulations are characterized here. Several interatomic potentials, parametrizations, and parameter mixing rules are contemplated. A novel simulation scheme is devised to characterize the effective frequency of surface diffusion and intermixing events overcoming the inherent difficulties related to their interdependency in heteroepitaxial systems. The effective energy barriers of these events encompass different atomistic mechanisms weighted by their occurrence probabilities. The overall description of surface diffusion and intermixing based on Stillinger–Weber (SW) potential is in agreement with ab initio calculations and experimental observations, though some atomistic details differ. This study is extended to Si(001) substrates with stressed Ge monolayers grown on top. It is found that Ge ad‐atom dynamics is accelerated with respect to the case of the pure Si substrate and that diffusion across dimer rows is mainly mediated by the atomic exchange of the Ge ad‐atom with a Ge atom on the surface.

Pixel device based on a quantum well: Preliminary results on gate dielectrics

The development of pixel arrays close to the interaction point in large detector systems used in high energy physics require high radiation hardness for the pixels and their readout. A pixel device based on a quantum well, called the DoTPiX uses a sensing n-channel MOS device with a control gate. A buried Ge layer acts as a current modulation gate, which localize holes generated by impinging particles. The DotPIX buried Ge gate is obtained by low temperature epitaxial growth of Ge on Si. We have started to study the different ways to achieve these prerequisites: the need for a low temperature budget to reduce the Ge and Si intermixing, which may be detrimental to the DoTPiX operation. The use of Si thermal oxide is investigated together with that of deposited oxide (Hafnium Oxide for example), which differs from the Silicon Dioxide. The possibility of a combination of thermal silicon dioxide and deposited oxides opens another possibility in this study.

500-period epitaxial Ge/Si0.18Ge0.82 multi-quantum wells on silicon

Ge/SiGe multi-quantum well heterostructures are highly sought-after for silicon-integrated optoelectronic devices operating in the broad range of the electromagnetic spectrum covering infrared to terahertz wavelengths. However, the epitaxial growth of these heterostructures at a thickness of a few micrometers has been a challenging task due to the lattice mismatch and its associated instabilities resulting from the formation of growth defects. To elucidate these limits, we outline herein a process for the strain-balanced growth on silicon of 11.1/21.5 nm Ge/Si0.18Ge0.82 superlattices (SLs) with a total thickness of 16 μm corresponding to 500 periods. Composition, thickness, and interface width are preserved across the entire SL heterostructure, which is an indication of limited Si–Ge intermixing. High crystallinity and low defect density are obtained in the Ge/Si0.18Ge0.82 layers; however, the dislocation pileup at the interface with the growth substrate induces micrometer-long cracks on the surface. This eventually leads to significant layer tilt in the strain-balanced SL and in the formation of millimeter-long, free-standing flakes. These results confirm the local uniformity of structural properties and highlight the critical importance of threading dislocations in shaping the wafer-level stability of thick multi-quantum well heterostructures required to implement effective silicon-compatible Ge/SiGe photonic devices.

Structural changes due to reannealing of Ge thin films prepared by solid phase epitaxial growth on Si (111) surface

We investigate effects of annealing and reannealing temperatures on the Ge film with thickness of 3 bilayer prepared by the solid phase epitaxial growth on Si(111) substrate. Intermixing of Ge and Si is suppressed up to 400°C, but is clearly observed at 425°C. The wetting layer with thickness of 2 bilayer formed during the annealing at 400°C has many defects due to stacking faults in the Si(111) 7×7 structure on the substrate. By reannealing at 600°C, the defects can be removed while suppressing intermixing. The recrystallization mechanism is discussed from the morphological and structural changes of the Ge film by reannealing at temperatures up to 600°C.

The formation of intermediate layers in covered Ge/Si heterostructures with low-temperature quantum dots: a study using high-resolution transmission electron microscopy and Raman spectroscopy

The method of software analysis of high-resolution TEM images using the peak pairs algorithm [Galindo P L et al 2007 Ultramicroscopy 107 1186–93] in combination with Raman spectroscopy was employed to study lattice deformations in Ge/Si(001) structures with low-temperature Ge quantum dots. It was found that the stresses do not spread in a thick Si layer above quantum dots, but completely relax via the formation of a thin boundary layer of mixed composition. However, intermixing of Ge and Si is absent beneath the Ge layer in samples with a Ge coverage of 10 Å. Besides intermixing was not observed at all, both beneath and above the Ge layer, in samples with a Ge coverage of 6 Å or less. This may be due to the predominance of Ge diffusion into the Si matrix from the {105} facets of Ge huts, not from the Ge wetting layer, at low temperatures of the Ge/Si structure deposition. The critical thickness of Si coverage at which the intense stress-induced diffusion takes place is determined to lie in the range from 5 to 8 nm.

Synthesis, theoretical and experimental characterisation of thin film Cu2Sn1-xGexS3 ternary alloys (x = 0 to 1): Homogeneous intermixing of Sn and Ge

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x = 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x = 0 to x = 1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 1018 cm−3. These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure.

Interfacial sharpness and intermixing in a Ge-SiGe multiple quantum well structure

A Ge-SiGe multiple quantum well structure created by low energy plasma enhanced chemical vapour deposition, with nominal well thickness of 5.4 nm separated by 3.6 nm SiGe spacers, is analysed quantitatively using scanning transmission electron microscopy. Both high angle annular dark field imaging and electron energy loss spectroscopy show that the interfaces are not completely sharp, suggesting that there is some intermixing of Si and Ge at each interface. Two methods are compared for the quantification of the spectroscopy datasets: a self-consistent approach that calculates binary substitutional trends without requiring experimental or computational k-factors from elsewhere and a standards-based cross sectional calculation. Whilst the cross section approach is shown to be ultimately more reliable, the self-consistent approach provides surprisingly good results. It is found that the Ge quantum wells are actually about 95% Ge and that the spacers, whilst apparently peaking at about 35% Si, contain significant interdiffused Ge at each side. This result is shown to be not just an artefact of electron beam spreading in the sample, but mostly arising from a real chemical interdiffusion resulting from the growth. Similar results are found by use of X-ray diffraction from a similar area of the sample. Putting the results together suggests a real interdiffusion with a standard deviation of about 0.87 nm, or put another way—a true width defined from 10%–90% of the compositional gradient of about 2.9 nm. This suggests an intrinsic limit on how sharp such interfaces can be grown by this method and, whilst 95% Ge quantum wells (QWs) still behave well enough to have good properties, any attempt to grow thinner QWs would require modifications to the growth procedure to reduce this interdiffusion, in order to maintain a composition of ≥95% Ge.

Structural and magnetic properties of Ni nanofilms on Ge(001) by molecular beam epitaxy

Abstract Ni films of 20 nm nominal thickness were grown on Ge(001) substrates by molecular beam epitaxy at several different temperatures from room temperature up to 400 °C. X-ray diffraction and X-ray photoelectron spectroscopy reveal the nucleation of Ni-Ge compounds (NiGe, Ni 2 Ge, Ni 5 Ge 2 ) as well as a departure from the fcc Ni structure exhibited by the films at and beyond a temperature of 100 °C. The binding energy of the Ni 2p peak increases from the RT value (852.7 eV) by 0.5–1.1 eV for the Ni/Ge(001) samples, while the Ge 2p binding energy changes by 0.6–0.7 eV after Ni growth compared to a clean Ge(001) substrate (there is only a ±0.15 eV shift among the samples grown on substrates at higher temperatures). By increasing substrate temperature, we obtained higher intermixing of Ni and Ge, but rather than both Ni and Ge interdiffusing, we find that Ni diffuses further into the germanium with higher substrate temperature, forming increasingly Ni-rich Ni-Ge compounds diluted into the Ge matrix. Based on Magneto-optic Kerr Effect measurements, Ni/Ge(001) grown on substrates at 100 and 200 °C does not exhibit a hysteresis loop, while the samples on 300 and 400 °C substrates show magnetic behavior, which we attribute to the magnetic character of hexagonal Ni 5 Ge 2 (which is determined here for the first time to be a ferromagnetic phase).

Role of the substrate temperature on the growth of Mn5Ge3 thin films by co-deposition of Mn and Ge on Ge(001) substrates by magnetron sputtering

Mn5Ge3 thin films grown on Ge(001) substrates were obtained by co-deposition of pure Mn and Ge elements using rf-magnetron sputtering at different substrate temperatures, Ts (250°C≤Ts≤550°C) in order to analyze the influence of the substrate temperature on the structural and magnetic properties of the samples. These films were thoroughly studied by reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and SQUID magnetometry, evidencing the important role of the substrate temperature on the microstructure of the films. For temperatures higher or equal to 350°C, a crystalline Mn11Ge8 phase also appears in addition of the Mn5Ge3 one, evidenced by the diffraction and magnetic measurements. Additionally, spin-glass-like magnetic behavior is also observed in all the samples (more evident when the Mn11Ge8 phase is small), due to the existence of a layer with spin-glass properties at the Ge/Mn5Ge3 interface caused by the intermixing of Ge and Mn at that interface.

Formation of large-grain crystalline germanium on single layer graphene on insulator by rapid melting growth

We demonstrate the crystallization of thermally deposited amorphous germanium (Ge) microstrips on single layer graphene (SLG) by rapid melting growth. Lateral growth of large grain crystalline Ge was successfully obtained over entire microstrip structure. SLG has shown its capability to suppress the spontaneous nucleation in the melting Ge, where no or less intermixing of C and Ge atoms has been detected. The interaction of C atoms from the graphene and Ge atoms at the interface is the possible reason for the observation of large compressive strain generated in the Ge strip grown on SLG. This technique provides an innovative breakthrough towards the realization of single-crystalline Ge-on-insulator (GOI) structure on SLG to facilitate the next-generation ultra-large-scale integrated circuits (ULSIs) with multifunctionalities.

Formation of lateral nanowires by Ge deposition on Si(111) at high temperatures

The lattice strain is the main factor which governs the surface morphology formation during the Si/Ge heteroepitaxial growth. Its influence becomes significantly weakened at high growth temperatures due to strong Si–Ge intermixing, giving an advantage of other factors which may produce unusual effects. We observed the formation of lateral SiGe nanowires (NWs) after Ge deposition on Si(111) at 830–860°С using solid-source molecular beam epitaxy. An additional factor to the strain minimization is associated with an energy barrier for the misfit dislocation network introduction at the NW/substrate interface, which causes the NWs to be straight. However, the requirement to attain a certain SiGe composition provides the formation of winding NWs, reflecting the subtle aspects of the growth process.

Pure, single crystal Ge nanodots formed using a sandwich structure via pulsed UV excimer laser annealing

In this paper, a sandwich structure comprising a SiO2 capping layer, amorphous Germanium (a-Ge) nanodots (NDs), and a pit-patterned Silicon (Si) substrate is developed, which is then annealed by utilizing a pulsed ultraviolet excimer laser in order to fabricate an array of pure, single crystal Ge NDs at room temperature. A wide bandgap SiO2 capping layer is used as a transparent thermally isolated layer to prevent thermal loss and Si–Ge intermixing. The two-dimensional pit-patterned Si substrate is designed to confine the absorbed laser energy, reduce the melting point, and block the surface migration of the Ge. After optimizing the laser radiation parameters such that the laser energy density is 200 mJ cm−2, the laser annealing period is 10 s, and the number of laser shots is 10, pure, single crystal Ge NDs that have both a regular arrangement and a uniform size distribution are obtained in the pits of the Si substrates. The Raman spectrum shows a highly symmetric Ge transversal optical peak with a full width at half maximum of 4.2 cm−1 at 300.7 cm−1, which is close to that of the original Ge wafer. In addition, the high-resolution transmission electron microscopy image for the Ge NDs and the corresponding selected area electron diffraction pattern shows a clear single crystalline structure without any impurities.

Electronic properties of single Ge/Si quantum dot grown by ion beam sputtering deposition

The dependence of the electronic properties of a single Ge/Si quantum dot (QD) grown by the ion-beam sputtering deposition technique on growth temperature and QD diameter is investigated by conductive atomic force microscopy (CAFM). The Si–Ge intermixing effect is demonstrated to be important for the current distribution of single QDs. The current staircase induced by the Coulomb blockade effect is observed at higher growth temperatures (>700 °C) due to the formation of an additional barrier between dislocated QDs and Si substrate for the resonant tunneling of holes. According to the proposed single-hole-tunneling model, the fact that the intermixing effect is observed to increase as the incoherent QD size decreases may explain the increase in the starting voltage of the current staircase and the decrease in the current step width.

Characterization of Hetero-Epitaxial Ge Films on Si Using Multiwavelength Micro-Raman Spectroscopy

To meet various physical property requirements of materials for advanced applications for specific devices, combinations of Si/Ge, Ge/Si, Si1-xGex/Si, are frequently introduced in the device fabrication process. Epitaxy, condensation and annealing processes are commonly used. Since a small variation in composition, strain and crystallinity can result in reduced device performance or failure, the composition, strain and crystallinity must be carefully monitored and controlled throughout the manufacturing process. We have studied the dependence of Ge and Si intermixing on annealing temperature and Raman excitation wavelength. Using multiwavelength Raman spectroscopy, we found Ge and Si intermixing in epitaxially grown Ge(100)/Si(100) after successive thermal anneals. Very strong dependence of signal-to-noise (S/N) ratio measurements on excitation wavelength and film structure was observed. Suitable excitation wavelengths must be chosen to properly characterize Si and Ge-based heteroepitaxial layers based on the thickness, stacking order and composition of epitaxial films having different optical properties at different wavelengths.

Detection of Ge and Si Intermixing in Ge/Si Using Multiwavelength Micro-Raman Spectroscopy

To meet various physical property requirements of materials for advanced application, for specific devices, combinations of Si/Ge, Ge/Si, Si1-xGex/Si, are frequently introduced in the device fabrication process. Epitaxy, condensation and annealing processes are commonly used. Since a small variation in composition, strain and crystallinity can result in reduced device performance or failure, the composition, strain and crystallinity must be carefully monitored and controlled throughout the manufacturing process. We report the detection of Ge and Si intermixing in epitaxially grown Ge/Si after successive thermal anneals using multiwavelength Raman spectroscopy. We have studied the dependence of Ge and Si intermixing on annealing temperature and Raman excitation wavelength. Very strong dependence of signal-to-noise (S/N) ratio measurements on excitation wavelength and film structure was observed. Suitable excitation wavelengths must be chosen to properly detect and characterize Si and Ge intermixing, based on the stacking order of epitaxial films and thicknesses.

Detection of Ge and Si Intermixing in Ge/Si Using Multiwavelength Micro-Raman Spectroscopy

To meet various physical property requirements of materials for advanced application specific devices, combinations of Si/Ge, Ge/Si, Si1-xGex/Si are frequently introduced in the device fabrication process. Epitaxy, condensation and annealing processes are commonly used. Since a small variation in composition, strain and crystallinity can result in reduced device performance or failure, the composition, strain and crystallinity must be carefully monitored and controlled throughout the manufacturing process.In this study, we report the detection of Ge and Si intermixing in epitaxially grown Ge/Si after successive thermal anneals using multiwavelength Raman spectroscopy. We have studied dependence of Ge and Si intermixing on annealing temperature and Raman on excitation wavelength. Very strong excitation wavelength and film structure dependence of signal-to-noise (S/N) ratio of measurements was observed. Suitable excitation wavelengths must be chosen to properly detect and characterize Si and Ge intermixing, based on the stacking order of epitaxial films and thicknesses.Figure 1 shows 514.5nm excited Raman spectra from reference Si, reference Ge and epitaxially grown Ge/Si wafers before and after thermal annealing. As grown Ge/Si wafers showed very distinct Ge and Si Raman peaks. Si and Ge intermixing was found in one annealing condition. Ge epitaxial was converted to Si1-xGexafter annealing.S/N ratio of measurements and their dependence on excitation wavelength and film structure dependence will be discussed. Excitation wavelength selection guidelines will be proposed.

Monitoring the kinetic evolution of self-assembled SiGe islands grown by Ge surface thermal diffusion from a local source

In this paper we experimentally study the growth of self-assembled SiGe islands formed on Si(001) by exploiting the thermally activated surface diffusion of Ge atoms from a local Ge source stripe in the temperature range 600–700 °C. This new growth strategy allows us to vary continuously the Ge coverage from 8 to 0 monolayers as the distance from the source increases, and thus enables the investigation of the island growth over a wide range of dynamical regimes at the same time, providing a unique birds eye view of the factors governing the growth process and the dominant mechanism for the mass collection by a critical nucleus. Our results give experimental evidence that the nucleation process evolves within a diffusion limited regime. At a given annealing temperature, we find that the nucleation density depends only on the kinetics of the Ge surface diffusion resulting in a universal scaling distribution depending only on the Ge coverage. An analytical model is able to reproduce quantitatively the trend of the island density. Following the nucleation, the growth process appears to be driven mainly by short-range interactions between an island and the atoms diffusing within its vicinities. The islands volume distribution is, in fact, well described in the whole range of parameters by the Mulheran’s capture zone model. The complex growth mechanism leads to a strong intermixing of Si and Ge within the island volume. Our growth strategy allows us to directly investigate the correlation between the Si incorporation and the Ge coverage in the same experimental conditions: higher intermixing is found for lower Ge coverage. This confirms that, besides the Ge gathering from the surface, also the Si incorporation from the substrate is driven by the diffusion kinetics, thus imposing a strict constraint on the initial Ge coverage, its diffusion properties and the final island volume

Surface reconstruction and optical absorption changes for Ge nanoclusters grown on chemically oxidized Si(1 0 0) surfaces

Germanium (Ge) nanoclusters are grown by a molecular-beam epitaxy technique on the chemically oxidized Si(1 0 0) surface at 700 °C. X-ray diffraction and photocurrent spectroscopy demonstrate that the nanoclusters have the local structure of body-centred-tetragonal Ge, exhibiting an optical adsorption edge at 0.48 eV at 50 K. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters' core becomes tetragonal SiGe alloy. The intrinsic absorption edge is shifted to 0.73 eV due to Si–Ge intermixing. Possible mechanisms for nanoclusters growth are discussed.

The microstructural and optical properties of Ge/Si heterostructures grown by low-temperature molecular beam epitaxy

Abstract