GeSn Epilayers Research Articles

Temperature-dependent morphology and composition of ultra-thin GeSn epilayers prepared by remote plasma enhanced chemical vapor deposition

Two distinct ultra-thin Ge1−xSnx (x ≤ 0.1) epilayers were deposited on (001) Si substrates at 457 and 313 °C through remote plasma-enhanced chemical vapor deposition. These films are considered potential initiation layers for synthesizing thick epitaxial GeSn films. The GeSn film deposited at 313 °C has a thickness of 10 nm and exhibits a highly epitaxial continuous structure with its lattice being compressed along the interface plane to coherently match Si without mismatch dislocations. The GeSn film deposited at 457 °C exhibits a discrete epitaxial island-like morphology with a peak height of ∼30 nm and full-width half maximum (FWHM) varying from 20 to 100 nm. GeSn islands with an FWHM smaller than 20 nm are defect free, whereas those exceeding 25 nm encompass nanotwins and/or stacking faults. The GeSn islands form two-dimensional modulated superlattice structures at the interface with Si. The GeSn film deposited at 457 °C possesses a lower Sn content compared to the one deposited at lower temperature. The potential impact of using these two distinct ultra-thin layers as initiation layers for the direct growth of thicker GeSn epitaxial films on (001) Si substrates is discussed.

The transition of growth behaviors of moderate Sn fraction Ge1-xSnx (8 % < x < 15 %) epilayers with low temperature molecular beam epitaxy

Epitaxial breakdown phenomenon for Ge1-xSnx thin film with Sn fraction of less than 6 % during low-temperature molecular beam epitaxy has been reported [J. Appl. Phys. 97, 044904, 2005]. However more Sn incorporation is required for direct bandgap transition of GeSn alloy, which has much attractive for GeSn optoelectronic devices. In this work, moderate Sn fraction Ge1-xSnx (8 %∼15.3 %), as well as composition-stepped multiple GeSn layers, grown on Ge (001) substrate by molecular beam epitaxy at 120–150 ℃ are investigated. Epitaxy breakdown occurs for the Ge1-xSnx epilayer with Sn fraction of about 8 % and 10 %, even with low Sn fraction buffer layers, in which the film delaminates into a fully compressively strained bottom layer with dislocation free and a fully relaxed top layer spontaneously. Once the epitaxy breakdown occurs, the amorphous columnar structures extend into the following deposited GeSn epilayer, regardless of Sn fraction. While, the strain in the whole Ge1-xSnx epilayer with Sn fraction of 15.3 % is directly released during growth by generation of misfit and threading dislocations or agglomeration of islands. Those results indicate that there are three growth modes including fully compressive strained thin GeSn epilayer, epitaxial breakdown induced self-delamination of GeSn layer with moderate Sn fraction, and strain relaxed Ge1-xSnx epilayer with high Sn fraction of above 15 %. The complex behavior can be attributed to the competition between kinetic roughening and strain- driven GeSn agglomeration or generation of dislocations. The understanding of the growth behavior is helpful for controlling crystal quality of GeSn thin films for their application in optoelectronic devices.

Thickness-dependent behavior of strain relaxation and Sn segregation of GeSn epilayer during rapid thermal annealing

The behavior of strain relaxation and Sn segregation of GeSn epilayers during growth and thermal annealing is very complex depending on the growth method, thickness and Sn content of the GeSn epilayer. Herein, we report on the thickness-dependent behavior of fully strained Ge1-xSnx epilayers (x = 0.097) on Ge (100) substrate grown by molecular beam epitaxy during rapid thermal annealing (RTA). It is found that when the GeSn epilayer is thinner than the critical thickness at Sn segregation temperature (hc(Ts)), GeSn epilayers would hardly be relaxed before Sn segregation during RTA. While for GeSn epilayer thicker than the hc(Ts), GeSn epilayers will undergo the process of strain relaxation before Sn segregation. The results indicate that the competition between Sn segregation and strain relaxation by generation of misfit dislocations strongly depends on the thickness of GeSn epilayers. A semi-quantitative model is proposed to explain the priority of Sn segregation or strain relaxation during RTA in terms of temperature-dependent critical thickness, which can be used to guide the design of strain-relaxed GeSn epilayer without Sn segregation for optoelectronic device application.

Unusually-high growth rate (∼2.8 μm/s) of germania nanowires and its hierarchical structures by an in-situ continuous precursor supply

Low-dimensional nanostructured semiconductors are becoming the promising materials for high-performance nanophotonics, nanoelectronics, and quantum devices. To enable these applications, it requires an efficient methodology to control the dimension of these materials during synthesis processes, and to achieve mass production of these materials with high reproducibility, perfect crystallinity, and low production cost. In this study, an ultra-fast, facile synthesis strategy is presented for reproducible monocrystalline hexagonal germania (GeO2) nanowires (NWs) and hierarchical structures. These GeO2 nanostructures were grown by one-step-annealing of the Ni film-covered GeSn epilayers in a rapid thermal annealing (RTA) system without any gaseous or liquid Ge sources. It was found that after short annealing for 60 s at 675 °C, the long GeO2 NWs of more than 170 μm are obtained, indicating that the growth rate is several magnitude orders higher than that of the common chemical vapor deposition (CVD) methods. The mechanism of the growth was studied by changing the growth temperature, catalyst type, and surface oxidation. The results indicate that this record-fast growth (>2.8 μm/s) of NW is due to the continuously generated in-situ GeO vapors from the Ni-catalyst decomposition of supersaturated GeSn epilayer. This work presents a rapid, and low-cost method to synthesis high-density GeO2 NW and its hierarchical structures which have the potential applications for optoelectronic communication/detection, superhydrophobic surfaces, photocatalyst, and sensing.

Effects of high-temperature thermal annealing on GeSn thin-film material and photodetector operating at 2 µm

Here, we explore the thermal stability of GeSn epilayers with varying Sn contents (3–10%) at an annealing temperature ranging from 300° to 750°C. It is found that ordered nanopatterns are formed on the surface of GeSn with Sn content of 8% without excessive Sn precipitation after thermal annealing at 700 °C. Despite being annealed at high temperatures, the GeSn maintains its crystal structure, which is confirmed by the X-ray Diffraction (XRD), Raman spectrum, and secondary-ion mass spectrometry (SIMS). The corresponding photocurrents of the photodetectors at the wavelength of 2 µm also indicate the crystal quality of the GeSn alloys does not deteriorate significantly after high-temperature annealing (675–700 °C). Meanwhile, the decrease of dark current with the enhancement of Ip/Id ratio (on-off ratio) indicates the improvement of detectivity of the photodetector due to the annealing process. Furthermore, the annealing temperature is optimized to 550 °C to achieve 200% enhancement of photocurrents of the GeSn photodetectors operated at 2 µm.

Quantitative Correlation Study of Dislocation Generation, Strain Relief, and Sn Outdiffusion in Thermally Annealed GeSn Epilayers

The instability during the growth and processing of epitaxial GeSn layers with high Sn molar fraction and high compressive strain is still to be fully studied. In this work, the relationship among ...

Insights into the Origins of Guided Microtrenches and Microholes/rings from Sn Segregation in Germanium–Tin Epilayers

We demonstrate the self-assembly synthesis of millimeter-long guided trenches and microholes/rings in the supersaturated GeSn epilayers through two approaches: epitaxial growth engineering and ther...

Structural Property Study for GeSn Thin Films.

The structural properties of GeSn thin films with different Sn concentrations and thicknesses grown on Ge (001) by molecular beam epitaxy (MBE) and on Ge-buffered Si (001) wafers by chemical vapor deposition (CVD) were analyzed through high resolution X-ray diffraction and cross-sectional transmission electron microscopy. Two-dimensional reciprocal space maps around the asymmetric (224) reflection were collected by X-ray diffraction for both the whole structures and the GeSn epilayers. The broadenings of the features of the GeSn epilayers with different relaxations in the ω direction, along the ω-2θ direction and parallel to the surface were investigated. The dislocations were identified by transmission electron microscopy. Threading dislocations were found in MBE grown GeSn layers, but not in the CVD grown ones. The point defects and dislocations were two possible reasons for the poor optical properties in the GeSn alloys grown by MBE.

X-ray diffraction study of strain relaxation, spontaneous compositional gradient, and dislocation density in GeSn/Ge/Si(100) heterostructures

The strain relaxation, depth profiles of composition and density of dislocations in GeSn epilayers were studied by using the x-ray diffraction. Regions with uniform and graded composition, different levels of strain relaxation, and dislocation density were found in the GeSn layers grown at fixed growth conditions. At the initial stage of growth, the GeSn layer is under ∼8 × 10–3 compressive strain, and the Sn composition is close to the target value of 6.6%. With increasing layer thickness, the Sn content is enhanced by 0.85 ± 0.12% due to the relief of compressive strain down to ∼5.0 × 10–4. The plastic strain relaxation is dominated by 60° misfit dislocations at the GeSn/Ge interface, where their density is 3 × 105 cm−1. The relaxed GeSn layer serves as a buffer for the high-quality GeSn overlayer with uniform and enhanced Sn composition. The laboratory-based x-ray diffraction is shown as a non-destructive reliable technique for strain, composition, and defects characterization, as an alternative to secondary ion mass spectrometry and transmission electron microscope techniques.

Wafer-scale mono-crystalline GeSn alloy on Ge by sputtering and solid phase epitaxy

Germanium-tin (GeSn) alloys are of great interest for electronic as well as photonic applications, and their integration on a group-IV (Si or Ge) platform. This letter describes a low deposition temperature, process integration-friendly, high throughput and low cost approach for growing device-quality GeSn epilayers on Ge substrates. Mono-crystalline, wafer-scale GeSn alloy with 3.4% Sn content was grown on Ge (100) substrates by sputtering an amorphous GeSn layer followed by solid phase epitaxy (SPE) at . Deposition of amorphous layers at room temperature enables incorporation of high bulk Sn content in GeSn epilayers. SPE of the amorphous layer at leads to a strained, high quality GeSn epilayer having a low full width half maximum of , root mean square surface roughness of 0.3 nm and a good GeSn/Ge interface as confirmed through extensive x-ray diffraction measurements, atomic force microscopy and transmission electron microscopy imaging. Additionally, an SiO2 capping layer employed during SPE is shown to prevent Sn out-diffusion and segregation at the surface. To examine electrical properties for potential applications of the GeSn/n-Ge heterostructure, metal/GeSn/n-Ge vertical contact diodes and metal/GeSn circular transfer length method contact structures were fabricated and electrically characterized.

Experimental Calibration of Sn‐Related Varshni Parameters for High Sn Content GeSn Layers

AbstractFrom temperature‐dependent photoluminescence of a single Ge0.84Sn0.16 sample, Sn‐related Varshni parameters are extracted and used as input parameters in an 8‐band k·p model, and predict direct bandgap energies of high Sn content GeSn alloys with concentration varying from 8% to 16%. Then, exhaustively compared are the calculated direct Γ‐valley bandgap energies to photoluminescence results of 13% and 16% Sn content alloys and to direct bandgap energies found in literature. The agreement between k·p modeling and experimental datasets is close to 3% for different strain conditions of GeSn epilayers. The outcome of this study is an 8‐band k·p model with a fixed set of parameters, the model being self‐sufficient to describe the direct bandgap of Ge1−xSnx layers with x varying from 8% to 16% for large ranges of strain and temperature.

Engineering strain relaxation of GeSn epilayers on Ge/Si(001) substrates

A mechanism of controlling the degree of strain relaxation in GeSn epilayers, grown by molecular beam epitaxy on Ge/Si(001) substrates, is reported in this work. It is demonstrated that by suitably controlling the thickness and the growth recipe of the underlying Ge buffer layer, both fully-strained and highly-relaxed GeSn epilayers can be obtained, without significant Sn segregation. The strain relaxation of the GeSn epilayer is mediated by threading dislocations of the Ge buffer layer, propagating across the Ge-GeSn interface. Systematic estimation of the threading dislocation density in both the alloy epilayer and the Ge buffer layer, by the approach developed by Benediktovich et al. [A. Benediktovitch, A. Zhylik, T. Ulyanenkova, M. Myronov, A. Ulyanenkov (2015), J. Appl. Cryst. 48, 655–665] supports this observation, and also reveals that no additional dislocations are generated at the Ge-GeSn interface. Together with recently reported techniques to arrest dislocation propagation in GeSn epilayers, these results bode extremely well for realization of highly-relaxed GeSn epilayers, much coveted for development of GeSn-based emitters.

Wafer-scale all-epitaxial GeSn-on-insulator on Si(1 1 1) by molecular beam epitaxy

In this letter, fabrication of all-epitaxial GeSn-on-insulator (GeSnOI) heterostructures is investigated, wherein both the GeSn epilayer and the Gd2O3 insulator are grown on Si(1 1 1) substrates by conventional molecular beam epitaxy. Analysis of the crystal and surface quality by high-resolution x-ray diffraction, cross-sectional transmission electron microscopy, and atomic force microscopy reveals the formation of a continuous and fully-relaxed single-crystalline GeSn epilayer (with a root-mean-square surface roughness of 3.5 nm), albeit GeSn epitaxy on Gd2O3 initiates in the Volmer–Weber growth mode. The defect structure of the GeSn epilayers is dominated by stacking faults and reflection microtwins, which are formed during the coalescence of the initially-formed islands. The concentration and mobility of holes, introduced by un-intentional p-type doping of the GeSn epilayers, were estimated to cm−3 and cm−2 V−1 s−1, respectively. In metal–semiconductor–metal Schottky diodes, fabricated with these GeSnOI heterostructures, the dark current was observed to be lower by a decade, when compared to similar diodes fabricated with GeSn/Ge/Si(0 0 1) heterostructures. The results presented here are thus promising for the development of these engineered substrates for (opto-)electronic applications.

The Low Temperature Epitaxy of Strained GeSn Layers Using RTCVD System

We have investigated the low temperature (LT) growth of GeSn–Ge–Si structures using rapid thermal chemical vapor deposition system utilizing Ge2H6 and SnCl4 as the reactive precursors. Due to inappropriate phenomena, such as, Ge etch and Sn segregation, it was hard to achieve high quality GeSn epitaxy at the temperature > 350 °C. On the contrary, we found that the SnCl4 promoted the reaction of Ge2H6 precursors in a certain process condition of LT, 240–360 °C. In return, we could perform the growth of GeSn epi layer with 7.7% of Sn and its remaining compressive strain of 71.7%. The surface propagated defects were increased with increasing the Sn content in the GeSn layer confirmed by TEM analysis. And we could calculate the activation energies at lower GeSn growth temperature regime using by Ge2H6 and SnCl4 precursors about 0.43 eV.

Correlation of Bandgap Reduction with Inversion Response in (Si)GeSn/High-k/Metal Stacks.

The bandgap tunability of (Si)GeSn group IV semiconductors opens a new era in Si-technology. Depending on the Si/Sn contents, direct and indirect bandgaps in the range of 0.4-0.8 eV can be obtained, offering a broad spectrum of both photonic and low power electronic applications. In this work, we systematically studied capacitance-voltage characteristics of high-k/metal gate stacks formed on GeSn and SiGeSn alloys with Sn-contents ranging from 0 to 14 at. % and Si-contents from 0 to 10 at. % particularly focusing on the minority carrier inversion response. A clear correlation between the Sn-induced shrinkage of the bandgap energy and enhanced minority carrier response was confirmed using temperature and frequency dependent capacitance voltage-measurements, in good agreement with k.p theory predictions and photoluminescence measurements of the analyzed epilayers as reported earlier. The enhanced minority generation rate for higher Sn-contents can be firmly linked to the bandgap reduction in the GeSn epilayer without significant influence of substrate/interface effects. It thus offers a unique possibility to analyze intrinsic defects in (Si)GeSn epilayers. The extracted dominant defect level for minority carrier inversion lies approximately 0.4 eV above the valence band edge in the studied Sn-content range (0-12.5 at. %). This finding is of critical importance since it shows that the presence of Sn by itself does not impair the minority carrier lifetime. Therefore, the continuous improvement of (Si)GeSn material quality should yield longer nonradiative recombination times which are required for the fabrication of efficient light detectors and to obtain room temperature lasing action.

The strain dependence of Ge1 − xsnx (x = 0.083) Raman shift

We report an investigation of strain-dependent Raman frequency shift in Ge0.917Sn0.083 epilayer where the Sn composition is near the indirect-to-direct band gap transition point. The strain is modulated by ex-situ rapid thermal annealing and the amount of strain relaxation is determined by X-ray diffraction measurement. The results show that the Raman shift of Ge–Ge longitudinal optical phonon is linearly dependent on the in-plane strain of the GeSn layers with a coefficient b=−299.3cm−1. Such a relationship enables characterization of GeSn epilayers using the readily accessible Raman spectroscopy.

Growth and Characterization of Epitaxial Ge1-XSnx Alloys and Heterostructures Using a Commercial CVD System

Group IV alloys incorporating Sn have several possible applications ranging from optoelectronics to stress engineering in FinFET based CMOS designs. From an optoelectronics perspective the benefits of incorporating Sn into Ge is realized in the reduction of the difference between the direct and indirect band gap transitions resulting in more efficient luminescence/ absorption. Furthermore, alloys of Ge1-xSnx are theoretically expected to become a direct band gap material for Sn contents of approximately 10 at. % [1]. With regards to logic applications of Sn containing alloys the possibilities include the passive function as a common strain relieved buffer (SRB) for both NMOS and PMOS applications as well as PMOS S/D stressor for an active GeSn channel FinFET devices [2]. Given these potential application one of the key issues lies in the lack of viable growth process on a commercial deposition system.The Ge1-xSnx alloys and Ge buffer layers presented here were grown in an ASM Epsilon® 2000 Plus chemical vapor deposition (CVD) system This system is ideally suited for the low deposition temperatures required to form these metastable alloys. A specialized growth approach has been adopted to grow the Ge1-xSnx layers at temperatures of less than 450°C.Material and optical characterization of these alloys indicate that the materials are of high crystal quality. Figure 1 shows a series symmetrical (004) 2θ-ω XRD scans for Ge1-xSnx alloys with Sn contents ranging from 0.5 to 10 at. % in which the perpendicular lattice constant increases with Sn content. Rutherford Backscattering Spectroscopy (RBS) in combination with ion channeling was used to verify the thickness and composition of the alloys as well as to determine substitutionality of the Sn within the Ge lattice (Fig.1). The thickness and composition of these alloys was in close agreement with those obtained from SIMS. Further structural characterization of the Ge1-xSnx alloys was obtained by cross-sectional Transmission Electron Microscopy (XTEM) analysis. Figure 3 shows a bright field XTEM image of a Ge0.93Sn0.07/Ge/Si heterostructure in which no threading defects are observed propagating through the GeSn epilayer. Room temperature photoluminescence studies of the as-deposited alloys show a strong well-defined peak which shifts to lower energy with increasing Sn contents. Growth of heterostructures with different alloy compositions have also been demonstrated in which abrupt compositional transitions are observed. Optical devices based on these alloys have been fabricated.

Strain relaxation and Sn segregation in GeSn epilayers under thermal treatment

We report the effects of thermal annealing on the characteristics of GeSn epilayers grown on Ge-buffered Si wafers with a high Sn content near a threshold value that affords a direct bandgap. On annealing at temperatures below 400 °C, the characteristics of the epilayer remain unchanged, compared to those of the as-grown samples. On annealing the samples at a temperature in the range of 440–540 °C, strain relaxation in the epilayer is observed, accompanied by generation of misfit dislocations at the GeSn/Ge interface. A further increase in annealing temperature beyond 580 °C causes not only a relaxation in strain but also a change in the microstructure of the epilayer. In addition, Sn forms clusters and segregates to the surface, resulting in a reduction in the Sn content of the epilayer. The present investigation shows changes in the characteristics of the film under thermal treatment, providing an insight into the physical properties of such devices.

Deep-Level Transient Spectroscopy of MOS Capacitors on GeSn Epitaxial Layers

Deep levels present in MOS capacitors, fabricated on GeSn epitaxial layers on Ge-on-Si substrates have been studied by Deep-Level Transient Spectroscopy (DLTS). The gate dielectric is composed of 9 nm Al2O3 deposited by Molecular Beam Epitaxy (MBE) on two different types of Interfacial Oxide Layers (IOL). It is shown that the density of interface traps (Dit) near the valence band edge is significantly reduced for GeSn epilayers compared with the same gate stack on a Ge cap. At the same time, several deep-level traps in the germanium depletion region have been observed, whereof the origin is discussed.