Epitaxial Ge Films Research Articles

High-quality Ge epitaxial film based on dislocation trapping mechanism in patterned Si substrate

Threading dislocation density (TDD) in a Ge heteroepitaxial film on Si is reduced based on a dislocation trapping mechanism in a patterned substrate. An array of V-shaped grooves on the submicron scale is patterned in the [110] direction on a (001) Si substrate. Epitaxial growth of Ge with a thickness of 1 μm by CVD realizes a reasonable flat surface despite the non-flat starting surface. The TDD in Ge for a V-shaped groove pattern of 0.5 μm in width with an inter-groove distance of 0.3 μm is obtained as low as 4 × 107 cm–2, which is lower than about 6 × 107 cm–2 for the rectangular one with the same groove width and inter-groove distance and about 22 × 107 cm–2 for the unpatterned one. The reduction is attributed to the dislocation trapping at the groove regions, as observed by cross-sectional transmission electron microscopy.

Reduced Threading Dislocation Density in a Ge Epitaxial Film on a Submicron-Patterned Si Substrate Grown by Chemical Vapor Deposition

Reduced Threading Dislocation Density in a Ge Epitaxial Film on a Submicron-Patterned Si Substrate Grown by Chemical Vapor Deposition

Reduced threading dislocation density in a germanium epitaxial film coalesced on an arrayed silicon-on-insulator strip

This paper reports a reduction in the threading dislocation density (TDD) of a Ge epitaxial film on a Si-on-insulator (SOI) wafer in terms of the Si-photonics device application. An array of submicron SOI strips is prepared as a patterned substrate, on which Ge is epitaxially grown by chemical vapor deposition. A continuous Ge film is formed by a coalescence of the adjacent Ge crystals on the arrayed SOI strip, while leaving semicylindrical voids on the exposed surface of the buried SiO2 (BOX) layer between the strips. The TDD of the coalesced Ge film is reduced to 1.0 ± 0.1 × 108 cm–2, which is approximately a half of 2.2 ± 0.2 × 108 cm–2 for a Ge film on an unpatterned SOI. A transmission electron microscope observation reveals that the TDD reduction is derived from a downward bending of the dislocation toward the void. An accumulation of the dislocations at the strip sidewalls also contributes to the TDD reduction.

Enhanced Reliability, Switching Speed and Uniformity for Ferroelectric HfZrO x on Epitaxial Ge Film by Post Deposition Annealing for Oxygen Vacancy Control

Instead of employing post metal annealing (PMA), post deposition annealing (PDA) was proposed to crystalize HfZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> (HZO) into the ferroelectric phase on an epitaxial Ge film with higher reliability. Due to the absence of top electrode/HZO reaction during annealing, PDA-processed HZO intrinsically possesses a better capability to control the amount of oxygen vacancies ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{o}$ </tex-math></inline-formula> ). It is physically and electrically confirmed that the amount of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{o}$ </tex-math></inline-formula> for the PDA-HZO is suppressed by 10.3%. Due to fewer <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{o}$ </tex-math></inline-formula> , the PDA-based device shows a higher switching speed than the counterpart by a factor of 12. By integration with an AlON interfacial layer, the PDA-based device reveals superior reliability performance to that by PMA in terms of robust endurance of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> cycles, stable retention up to ten years, and smaller imprint. In addition, the PDA process also leads to enhanced remanent polarization ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula> ) uniformity among devices by 61.3% due to reduced grain size. Furthermore, additional thermal annealing after metal deposition hardly affects the devices’ performance, implying that the PDA process can be integrated with a subsequent dopant activation annealing to implement ferroelectric field-effect transistors (FeFETs) and pave a viable way to advance the development of high-reliability Ge-based FeFET memory.

Dopant Redistribution in High-Temperature-Grown Sb-Doped Ge Epitaxial Films

Heavily doped n-type Ge crystals are essential for Ge-based electronics and optical applications. In this report, we describe the mechanism of dopant redistribution via out-diffusion and re-evaporation processes in Sb-doped Ge epitaxial films grown by molecular beam epitaxy (MBE). The temperature-modulated depositions yielded a uniformly distributed and high Sb concentration (1019 to 1021 cm–3) on highly ordered crystalline Ge films. Here, the deposition temperatures (Td) control the transition of amorphous to epitaxial growth, as well as the Sb concentration profiles. A significant difference in the dopant levels is attributed to Sb out-diffusion that correlated with the diffusion parameters, followed by the re-evaporation from the Ge crystal in high-temperature conditions. Meanwhile, the substituted Sb atoms act as donors that form the n-type Ge film. This understanding of dopant behavior in the epitaxially grown Ge film opens a path to achieve well-defined n+-Ge thin films for Ge-based devices with improved electrical properties.

One-Step Cost-Effective Growth of High-Quality Epitaxial Ge Films on Si (100) Using a Simplified PECVD Reactor

Heteroepitaxial growth of Ge films on Si is necessary for the progress of integrated Si photonics technology. In this work, an in-house assembled plasma enhanced chemical vapor deposition reactor was used to grow high quality epitaxial Ge films on Si (100) substrates. Low economic and thermal budget were accomplished by the avoidance of ultra-high vacuum conditions or high temperature substrate pre-deposition bake for the process. Films were deposited with and without plasma assistance using germane (GeH4) precursor in a single step at process temperatures of 350–385 °C and chamber pressures of 1–10 Torr at various precursor flow rates. Film growth was realized at high ambient chamber pressures (&gt;10−6 Torr) by utilizing a rigorous ex situ substrate cleaning process, closely controlling substrate loading times, chamber pumping and the dead-time prior to the initiation of film growth. Plasma allowed for higher film deposition rates at lower processing temperatures. An epitaxial growth was confirmed by X-Ray diffraction studies, while crystalline quality of the films was verified by X-ray rocking curve, Raman spectroscopy, transmission electron microscopy and infra-red spectroscopy.

Study of Si–Ge Interdiffusion in Laser Recrystallization of Ge Epitaxial Film on Si Substrate

Direct epitaxial growth of germanium (Ge) film on silicon (Si) substrate (GOSS) holds great potential in micro-electronics and optoelectronics. However, due to the 4.2% lattice mismatch between Si and Ge, it is difficult to directly obtain high quality Ge by epitaxy on Si substrate. Laser recrystallization technology provides a simple, efficient and low-cost way to improve the crystal quality of epitaxial Ge film grown on Si substrate. This technology is essentially a process of thermally induced phase transformation. By controlling the laser process parameters, epitaxial film of a certain thickness is melted, so that lattice rearrangement and recrystallization are achieved, and high-quality thin Ge/Si can be prepared. Laser recrystallization is a high temperature thermal process, and Si–Ge interdiffusion may detrimentally occur. In this paper, the mechanism of Si–Ge interdiffusion is discussed. Based on Fick's law of diffusion, a numerical model for Si–Ge interdiffusion of GOSS is established. On this basis, the process simulation of thermal annealing and laser recrystallization Si–Ge interdiffusion is carried out by Sentaurus Process simulation. The results show that compared with the traditional thermal annealing, the Si–Ge interdiffusion of Ge on Si almost does not occur in the process of laser recrystallization. By reasonably controlling the process parameters of laser recrystallization, the thin Ge film near the Si–Ge interface does not melt, which can not only improve the crystal quality of Ge epitaxial layer, but also effectively avoid the Si–Ge interdiffusion in the process of laser recrystallization. Through this research, we have aimed at predicting and control the Si–Ge interdiffusion, providing an important technical reference for the preparation of high quality GOSS by laser recrystallization technology.

Thermoelectric Si1−xGex and Ge epitaxial films on Si(001) with controlled composition and strain for group IV element-based thermoelectric generators

This study presents the material design of Si1−xGex epitaxial films/Si for thin film thermoelectric generators (TFTEGs) by investigating their thermoelectric properties. The thermoelectric films composed of group-IV elements are advantageous due to their compatibility with the Si process. We fabricated Si1−xGex epitaxial films with various controlled x values and strains using various growth methods. Ge epitaxial films without strains exhibited the highest thermoelectric power factor (∼47 μW cm−1 K−2) among various strain-controlled Si1−xGex (x ≠ 1) epitaxial films, which is higher at room temperature than SiGe alloy-based bulks ever reported. On the other hand, strained Si1−xGex epitaxial films showed an ultralow thermal conductivity of ∼2 W m−1 K−1, which is close to the value for amorphous Si. In addition to strained SiGe films with the ultralow thermal conductivity, unstrained Ge films with a high thermoelectric power factor can also be used for future TFTEGs by applying a nanostructuring technique. A preliminary TFTEG of Ge epitaxial films was realized, which generated a maximum power of ∼0.10 μW cm−2 under a temperature difference of 20 K. This demonstrates that epitaxial films composed of group-IV semiconductors are promising materials for TFTEG applications.

Paving the way to dislocation reduction in Ge/Si(001) heteroepitaxy using C-based strained layer superlattices

Epitaxial Ge films were grown on Si(001) substrates by molecular beam epitaxy. During epitaxial growth, two carbon interlayers were deposited at varying substrate temperatures (140−620°C) and with varying C quantity (0−1.5monolayers). The influence of the second carbon interlayer on in-plane strain was investigated using high-resolution x-ray diffraction and transmission electron microscopy (TEM). All samples exhibited compressive strain, which was attributed to substitutional incorporation of carbon atoms. In-plane strain decreases with increasing substrate temperature during carbon deposition, indicating that enhanced surface mobility of carbon adatoms leads to formation of carbon clusters. This was confirmed by cross-sectional TEM investigations. Variation of C quantity at 180°C reveals maximum strain at an intermediate quantity of 0.8 monolayers. Omission of the second C interlayer results in much lower strain, indicating a mismatch between the two Ge layers separated by a C interlayer. This could be used to enforce dislocation filtering following the principle of strained layer superlattices. An upper estimate of 1×10−3 was found for the mismatch strain, resulting in a critical thickness for dislocation filtering of hc=153nm. A sample just exceeding hc exhibited a clear dislocation reduction effect as shown by TEM.

Epitaxial Ge thin film Growth on Si Using a Cost-Effective Process in Simplified CVD Reactor

Germanium on silicon has the potential to enable growth of high quality group IV semiconductors for the field of silicon photonics by being used as virtual buffers. In addition Ge layers can be used in the fabrication of passive components such as waveguides and resonators. A cost-effective technology for the heteroepitaxial growth of Ge on Si will be highly beneficial. In this work, a simplified chemical vapor deposition reactor was assembled in-house to deposit Ge films. Epitaxial Ge film growth on Si (100) substrates was accomplished without requiring ultra-high vacuum conditions or a high temperature substrate pre-deposition bake thereby minimizing the economic and thermal budget of the process. Films were deposited using digermane (Ge2H6) precursor in a single step at a process temperature of 350 °C and chamber pressures of 1–10 Torr. Films were achieved at high chamber background pressures (>10−6 Torr) by implementing a thorough ex situ substrate cleaning process and optimizing times for wafer loading, chamber pumping and initiation of film deposition. Transmission electron microscopy and X-ray diffraction were used to confirm the epitaxial growth and crystalline quality of the obtained films. Optical properties of the films were characterized using Raman spectroscopy, Photoluminescence and Infra-red spectroscopy.

Growth of high quality Ge-on-Si layer by using an ultra-thin LT-Si buffer in RPCVD

Ge epitaxial layer on Si substrate becomes more attractive for Si-based monolithic photonic integrated circuit (PIC). Low threading dislocation density and low surface roughness play a vital role in realizing the fabrication of the Ge optoelectronic devices. In this study, an ultra-thin LT-Si buffer for the deposition of Ge layer was investigated with regard to Ge layer’s crystallinity, symmetrical characteristic, threading dislocation density, and surface roughness. As a result, Ge epitaxial film has low threading dislocation density (TDD=5×106cm−2) and smooth surface (RMS = 0.68nm) in 5µm×5µm scan field with the Ge epitaxial layer thickness of about 1µm. What’s more, it was grown in a short time by using an ultra-thin LT-Si buffer layer without using chemical mechanical polishing (CMP), high temperature annealing, H2 annealing or cyclic annealing. In addition, room temperature direct gap photoluminescence was observed from intrinsic tensile-strained (0.21%) Ge-on-Si layer and bandgap narrowing in Ge epitaxial layer is 85meV.

High concentration phosphorus doping in Ge for CMOS-integrated laser applications

Germanium is a promising material for the laser that can be monolithically integrated on Si complementary metal-oxide-semiconductor platform and has emission wavelength of 1550 nm for optical interconnect. To obtain significant optical gain, it is necessary to achieve high n-type doping concentration level, while avoiding the damage to Ge crystalline quality. In this paper, we report an ex-situ phosphorus diffusion doping of Ge film, based on low-temperature phosphosilicate glass (PSG) pre-deposition process such as spin-on-glass and sub-atmospheric chemical vapor deposition (SACVD) methods. Closely related to optical gain properties of Ge for Ge-on-Si laser application, the photoluminescence characteristics of Ge epitaxial film after P diffusion doping were investigated. In particular, SACVD-processed PSG deposited directly on Ge film without any Si capping layer successfully led to high phosphorus doping concentration of ∼1019 cm−3 deep inside Ge and dramatically enhanced photoluminescence intensity by more than 10 times compared to intrinsic Ge film. By using the SACVD-PSG based P doping process, we developed an inverted-rib Ge waveguide structure for more effective optical gain. In the inverted-rib Ge structure, the mode will be positioned upward and stay relatively away from the Ge-Si interface where many dislocations are located and, as a result, we can expect less optical loss due to scattering and the overall higher mode gain. As a very promising preliminary result, from optical-pumping of the inverted-rib Ge, a threshold-like behavior starting at 18 kW/cm2 and amplified spontaneous emission around 1570 nm were demonstrated.

Magnetic and eletronic transport properties in n-type diluted magnetic semiconductor Ge&lt;sub&gt;0.96–x&lt;/sub&gt;Bi&lt;sub&gt;x&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te film

The epitaxial thin films of Ge&lt;sub&gt;0.96−&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Bi&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te are deposited on BaF&lt;sub&gt;2&lt;/sub&gt; substrates by using pulsed laser deposition technique. The thin films with three different compositions i.e. Ge&lt;sub&gt;0.8&lt;/sub&gt;Bi&lt;sub&gt;0.2&lt;/sub&gt;Te, Ge&lt;sub&gt;0.76&lt;/sub&gt;Bi&lt;sub&gt;0.2&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te, and Ge&lt;sub&gt;0.64&lt;/sub&gt;Bi&lt;sub&gt;0.32&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te are prepared in this wok. Their high-quality epitaxy and crystallinity are confirmed by X-ray diffraction and atomic force microscopy. According to the measurements of Hall effect variation, we find that each of all curves exhibits a negative slope for the different films as the temperature varies from low temperature to room temperature, indicating that Ge&lt;sub&gt;0.96−&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Bi&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te films are n-type material because the substitution of Bi for Ge makes the carriers change from holes into electrons. Temperature dependence of resistivity confirms that the electronic transport behavior for each of Ge&lt;sub&gt;0.96−&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Bi&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te thin films exhibits a typical semiconductor characteristic. From the measurements of temperature dependence of electronic transport under various external magnetic fields, we find that the Ge&lt;sub&gt;0.64&lt;/sub&gt;Bi&lt;sub&gt;0.32&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te thin film shows some magnetoresistive effect while other composition films do not possess such a property. Based on the linear fitting of temperature dependence of magnetic susceptibility in high temperature and low temperature region, the magnetic property of Ge&lt;sub&gt;0.64&lt;/sub&gt;Bi&lt;sub&gt;0.32&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te thin film changes from 253 K. Together with the study of magnetic susceptibility curve in the paramagnetic region, the Curie-Weiss temperature is determined to be 102 K. At a low temperature of 10.0 K, we observe an obvious ferromagnetic hystersis loop in Ge&lt;sub&gt;0.64&lt;/sub&gt;Bi&lt;sub&gt;0.32&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te instead of in Ge&lt;sub&gt;0.76&lt;/sub&gt;Bi&lt;sub&gt;0.2&lt;/sub&gt;Fe&lt;sub&gt;0.04&lt;/sub&gt;Te thin film. These results imply that the increase of Bi dopant is main reason for the establishment of ferromagnetic ordering state. The carrier concentration increases and thus promotes the carriers transporting the Ruderman-Kittel-Kasuya-Yoshida interaction, thereby leading to the separated Fe ions producing the magnetic interaction and forming an n-type diluted magnetic semiconductor.

Flexible GaAs solar cells on roll-to-roll processed epitaxial Ge films on metal foils: a route towards low-cost and high-performance III–V photovoltaics

Single-junction GaAs solar cells were developed on metal foils, laying the foundation for scalable manufacturing of low-cost, lightweight and high-efficiency flexible III–V solar cells.

Thermoelectric properties of epitaxial Ge thin films on Si(001) with strong crystallinity dependence

The thermoelectric properties of epitaxial Ge films on Si(001) substrates were investigated, where the film crystallinity was controlled using various growth techniques established in large-scale integration research. The films exhibited substantially lower thermal conductivities than that of bulk Ge. Unlike carrier mobility with strong crystallinity dependence, the Seebeck coefficient was similar to the high bulk value and exhibited less crystallinity dependence. This demonstrates that inherently high-carrier-mobility Ge films with high crystallinity can exhibit reduced thermal conductivity and a high thermoelectric power factor simultaneously, leading to the realization of thermoelectric films on Si for reusing low-temperature waste heat from Si-based electronic devices.

Hyperfine fields and anisotropy of the orbital moment in epitaxial Mn5Ge3 films studied by Mn55 NMR

55 Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn 5 Ge 3 /Ge(111) epitaxial films with thicknesses between 9 and 300 nm. The NMR spectra have been recorded as a function of strong external magnetic field applied in the film plane and perpendicular to it. Two 55 Mn NMR resonances have been observed, corresponding to the two manganese sites 4d and 6g, in the hexagonal D8 8 structure; in zero field their frequency is centered around 207.5 and 428 MHz, respectively. The anisotropy of 55 Mn hyperfine fields between the hexagonal c direction and the c plane at both Mn sites was evidenced and attributed to the anisotropic term due to the unquenched Mn orbital momentum. The anisotropy of the orbital contribution to hyperfine fields was determined as 1.52 T in the 4d site and up to 2.77 T in the 6g site. The 4d site reveals a quadrupolar interaction due to the strong electric field gradient: V zz = 5.3 × 10 19 V/m 2 in this site, which is shown to be oriented along the hexagonal c axis.

Van der Waals epitaxy of Ge films on mica

To date, many materials have been successfully grown on substrates through van der Waals epitaxy without adhering to the constraint of lattice matching as is required for traditional chemical epitaxy. However, for elemental semiconductors such as Ge, this has been challenging and therefore it has not been achieved thus far. In this paper, we report the observation of Ge epitaxially grown on mica at a narrow substrate temperature range around 425 °C. Despite the large lattice mismatch (23%) and the lack of high in-plane symmetry in the mica surface, an epitaxial Ge film with [111] out-of-plane orientation is observed. Crystallinity and electrical properties degrade upon deviation from the ideal growth temperature, as shown by Raman spectroscopy, X-ray diffraction, and Hall effect measurements. X-ray pole figure analysis reveals that there exist multiple rotational domains in the epitaxial Ge film with dominant in-plane orientations between Ge1¯10 and mica[100] of (20n)°, where n = 0, 1, 2, 3, 4, 5. A superlattice area mismatch model was used to account for the likelihood of the in-plane orientation formation and was found to be qualitatively consistent with the observed dominant orientations. Our observation of Ge epitaxy with one out-of-plane growth direction through van der Waals forces is a step toward the growth of single crystal Ge films without the constraint in the lattice and symmetry matches with the substrates.

Local anisotropic strain evaluation in thin Ge epitaxial film using SiGe stressor template grown on Ge substrate by selective ion implantation

Uniaxial strain states in a Ge epitaxial film were evaluated by oil-immersion Raman spectroscopy. The Ge epitaxial film used in this study was fabricated using a SiGe stressor template grown on a Ge substrate by selective ion implantation. The one-dimensional strain distribution in the Ge epitaxial film was evaluated by Raman spectroscopy, and large compressive strains and strain relaxations in implanted and unimplanted areas were confirmed, respectively. For further analysis of the strain states in the Ge epitaxial film in the unimplanted area, we obtained transversal optical phonon (TO1 and TO2) peaks by liquid-immersion Raman spectroscopy with a high-numerical-aperture objective lens. The stress states calculated from these peaks were clearly anisotropic. A uniaxially strained Ge epitaxial film, which is an appropriate high-performance transistor channel material, was achieved by using a SiGe template epitaxially grown on a selectively implanted Ge substrate.

Emergence of Dirac-like bands in the monolayer limit of epitaxial Ge films on Au(1 1 1)

After the discovery of Dirac fermions in graphene, it has become a natural question to ask whether it is possible to realize Dirac fermions in other two-dimensional (2D) materials as well. In this work, we report the discovery of multiple Dirac-like electronic bands in ultrathin Ge films grown on Au(1 1 1) by angle-resolved photoelectron spectroscopy. By tuning the thickness of the films, we are able to observe the evolution of their electronic structure when passing through the monolayer limit. Our discovery may signify the synthesis of germanene, a 2D honeycomb structure made of Ge, which is a promising platform for exploring exotic topological phenomena and enabling potential applications.

Revealing the role of the native oxide and the surface roughness in the visible luminescence of Ge epitaxial films

We report here the first observation of a visible luminescence from planar, relaxed germanium films epitaxially grown on silicon (001) wafers. Even the as-grown films exhibit a bright blue-green (2.4eV) emission at room temperature. The Ge surface was subsequently etched in RF SF6 remote plasma in order to form Ge nanodots. The photoluminescence measurements of the etched surface reveal an additional emission at 2.8eV. We combine morphological characterizations with photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) analysis to explain the origin of the observed visible luminescence. The measured emissions did not show an excitonic behavior expected for quantum confined carriers in the nanostructures. It is found that the defects in the GeO2 layer natively formed on the Ge surface are responsible for the blue-green emission of the films before and after the etching process. For the etched surface, the role of the interface Ge/GeO2 defects in the blue emission (2.8eV) will also be discussed.