GeSn Layers Research Articles

Comprehensive study on epitaxial growth of GeSn(111) layers with high Sn content on Si(111) featuring Ge buffer layer

The study comprehensively discussed the crystalline properties of GeSn(111) epitaxial layers on Si(111) using Ge double buffer layers toward the formation of the direct transition GeSn(111) with a Sn content around 10%. We found that the introduction of Ge double buffer layers is rather effective to improve the crystallinity of GeSn(111) than the single buffer case. However, thicker Ge buffer layer degrades the crystallinity of GeSn(111) because the strain relaxation of GeSn layer is likely to occur. In this study, through optimizing the thicknesses of Ge buffer layers formed at low and high temperatures (LT-Ge and HT-Ge, respectively), it was clarified that 100 and 500 nm for LT- and HT-Ge thicknesses, respectively, are suited to realize a high quality GeSn(111). The grown GeSn(111) epitaxial layer includes a Sn content of ~8–11% with a compressive strain of ~1–2%, which is contained within the region of direct transition GeSn(111).

Infrared optical properties of SiGeSn and GeSn layers grown by molecular beam epitaxy

The infrared optical properties of thick GeSn films (>500 nm) having 10% Sn concentration and of SiGeSn layers, utilized for the growth of strain-relieved, direct-gap GeSn films by molecular beam epitaxy, are investigated. Two growth methods are used: a graded-growth structure and a stepped-growth structure that help us to illustrate the properties of the GeSn and SiGeSn layers. Interestingly, there can be strong absorption in SiGeSn films throughout the infrared. We observe an increase in infrared absorption with increasing Sn concentration up to 21% Sn and in films, where the Sn is held constant at 18%, with increasing Si concentration up to 30%. Cavity effects in the infrared transmission measurement of stepped-growth structures are observed and associated with reflections at growth interfaces. Si–Si bond formation is proposed to occur at high Si concentrations in SiGeSn films, and the bandgap in SiGeSn films appears to decrease with increasing Si and Sn concentrations.

Development of GeSn epitaxial films with strong direct bandgap luminescence in the mid-wave infrared region using a commercial chemical vapor deposition reactor

Germanium tin (GeSn) is a material of interest for electronic and photonic device applications, but its development and commercialization have been limited by material quality issues and lack of availability from epitaxy suppliers. In this paper, we report initial studies in optimizing GeSn films deposited on a Ge buffer layer grown on 200-mm diameter silicon (Si) substrates with an ASM Epsilon 2000 chemical vapor deposition reactor designed for commercial production. Using a single-step growth process, a Sn content up to 22% near the surface of a GeSn film was achieved due to the increase in Sn incorporation via strain relaxation. A two-step growth process resulted in a bilayer structure with a nearly 100% relaxation on the first layer, followed by a higher quality GeSn layer with 18% Sn as evident by a high photoluminescence intensity emitting in the mid-wave infrared region at 3.2 μm at 20 K.

Comparison of GeSn alloy films prepared by ion implantation and remote plasma-enhanced chemical vapor deposition methods

Thin films of germanium-tin (GeSn) alloy with Sn content well above its equilibrium solubility limit in Ge are produced using both remote plasma-enhanced chemical vapor deposition (RPECVD) directly on silicon substrates and ion implantation of Sn into Ge. For RPECVD, the growth temperature of 302 °C resulted in fully relaxed GeSn alloys with high defect density, principally threading dislocations related to the large lattice mismatch between Si and GeSn. For the implantation case, pulsed laser melting was used to melt and crystallize the GeSn layer on a time scale of a few tens of nanoseconds. The resulting GeSn layers were also relaxed and defective, presumably again as a result of lattice mismatch with the underlying Ge lattice. However, the nature of the defects was quite different to the RPECVD method, whereby the line defects were not threading dislocations but stackinglike defects, which developed into arrays of these defects in the high Sn content region close to the surface. For the purpose of comparing RPECVD and ion-implantation methods, alloy films of similar thickness (400–450 nm) and Sn content (4.5–6.5 at. %) were examined. Film parameters (thickness, Sn content, Sn solubility, and segregation), as well as film quality and defect structures, were examined for both fabrication methods using several analytical techniques. This comparison provided us with a better physical understanding of our GeSn films and will help inform future growth/fabrication strategies targeted at minimizing defects formed in the GeSn films for the realization of optoelectronic devices.

Transferable GeSn ribbon photodetectors for high-speed short-wave infrared photonic applications

We experimentally demonstrate a low-cost transfer process of GeSn ribbons to insulating substrates for short-wave infrared (SWIR) sensing/imaging applications. By releasing the original compressive GeSn layer to nearly fully relaxed state GeSn ribbons, the room-temperature spectral response of the photodetector is further extended to 3.2 μm, which can cover the entire SWIR range. Compared with the as-grown GeSn reference photodetectors, the fabricated GeSn ribbon photodetectors have a fivefold improvement in the light-to-dark current ratio, which can improve the detectivity for high-performance photodetection. The transient performance of a GeSn ribbon photodetector is investigated with a rise time of about 40 μs, which exceeds the response time of most GeSn (Ge)-related devices. In addition, this transfer process can be applied on various substrates, making it a versatile technology that can be used for various applications ranging from optoelectronics to large-area electronics. These results provide insightful guidance for the development of low-cost and high-speed SWIR photodetectors based on Sn-containing group IV low-dimensional structures.

Lateral GeSn p-i-n photodetectors on insulator prepared by the rapid melting growth method.

In this work, GeSn lateral p-i-n photodetectors (PDs) on insulator were fabricated with an active GeSn layer grown by the rapid melting growth (RMG) method. Taking advantages of the defect-free GeSn strips, GeSn PDs with 5.3% Sn content have low dark current and high responsivities, which are about 0.48, 0.47, and 0.24 A/W for wavelengths of 1550, 1630, and 2000 nm, respectively. The radio frequency of the lateral GeSn PDs was also studied and a 3 dB bandwidth of about 3.8 GHz was achieved. These results indicate that the GeSn grown by the rapid melting growth method is capable of fabricating high-performance Si-based optoelectronic devices.

In situ TEM thermal study of MBE and CVD GeSn layers: Cross-section and plan-view geometries

In situ TEM thermal study of MBE and CVD GeSn layers: Cross-section and plan-view geometries

Sn component gradient GeSn photodetector with 3 dB bandwidth over 50 GHz for extending L band telecommunication.

In this work, high-performance GeSn photodetectors with a Sn content gradient GeSn layer were fabricated on SOI substrate by CMOS-compatible process for C and L band telecommunication. The active GeSn layer has a Sn component increased from 9 to 10.7% with the controlled relaxation degree up to 84%. The responsivities of GeSn detectors at 1550 nm and 1630 nm are 0.47 A/W and 0.32 A/W under -4 V bias, respectively. Over 50 GHz 3 dB bandwidth with the eye pattern about 70 Gb/s was also evidenced at 1630 nm. These results indicate that the GeSn photodetectors have a promising application for extending the silicon photonics from C band to L band.

Role of dislocations on Sn diffusion during low temperature annealing of GeSn layers

A study of the mechanism of Sn out-diffusion was performed by annealing Ge0.905Sn0.095 layers at 300 °C. The changes in Sn composition and strain state were confirmed by x-ray diffraction and photoluminescence spectroscopy. Surface defects, appearing as Sn particles, with the highest density of 3.5 × 108 cm−2 were detected by atomic force microscopy after annealing for 2 h. The strain in the GeSn layer stabilized for more prolonged annealing, while the density of particles decreased and their size increased. Annealing results are discussed in terms of Sn segregation and subsequent diffusion along dislocation lines, enhanced out-diffusion by dislocations migration, and surface particle coalescence.

High-quality GeSn thin-film resonant cavities for short-wave infrared applications

High-quality infrared (IR) devices made of group IV materials are highly promising to replace traditional III–V semiconductor-based IR optoelectronics due primarily to their compatibility with mature silicon-based technologies and much lower costs. In this regard, germanium tin (GeSn) has emerged as the only direct bandgap material in the group IV family exhibiting superior electrical and optical characteristics. In the past years, GeSn IR optoelectronics including lasers and photodetectors have been realized, although novel device architectures are still needed to enhance their device performance. Here, we experimentally demonstrate high-performance, large-area (mm2) GeSn thin-film resonant cavities (film thickness resonance operating at short-wave IR wavelengths by employing membrane transfer techniques). The fabricated asymmetric air/GeSn/metal structures exhibit high absorptance (>90%) at designed resonance wavelengths, which are easily tuned by tailoring the GeSn layer thickness. The measured resonance absorption reveals excellent agreement with numerical simulations, which further elucidates the mode characteristics of the observed cavity resonances. The demonstrated thin-film device architectures could greatly facilitate the development of novel GeSn photonic devices with tunable wavelengths and enhanced performance enabled by strain engineering, and could allow for the integration of GeSn into many existing group IV-based devices for next-generation IR optoelectronics where high performance, small footprint, and low cost are all required.

High-electron mobility P-doped polycrystalline GeSn layers formed on insulators at low temperatures

Despite its long history, synthesizing n-type polycrystalline Ge layers with high-electron mobility on insulating substrates has been difficult. Based on our recently developed solid-phase crystallization technology, here, we have demonstrated the highest recorded electron mobility (450 cm2 V−1 s−1) for Ge-based polycrystalline thin films on insulating substrates. The underlayer type and small amount of Sn addition were the key parameters controlling both the density and barrier height of the grain boundaries in the P-doped polycrystalline Ge layers. The low growth temperature (≤400 °C) allowed us to develop a GeSn layer on a heat-resistant polyimide film, which exhibited the highest electron mobility (200 cm2 V−1 s−1), as a semiconductor thin film synthesized directly on a flexible substrate. These achievements herald the development of high-performance polycrystalline Ge-based devices on inexpensive glass and flexible plastic substrates.

Nanosecond laser annealing of pseudomorphic GeSn layers: Impact of Sn content

Interactions between nanosecond laser pulses and various Sn content pseudomorphic GeSn layers were investigated. The aim was to evaluate the suitability of Ultraviolet Nanosecond Laser Annealing (UV-NLA), with an excimer laser emitting at 308 nm and a pulse duration of 160 ns, for the fabrication of performant contacts in GeSn devices such as electrically pumped lasers. Atomic Force Microscopy (AFM) and X-Ray Diffraction (XRD) revealed similar melt regimes for GeSn on Ge and SiGe on Si. At the melt threshold, surface structures formed where small areas melted. We were then in the so-called surface melt regime. The surface structures’ shape changed with the Sn content. A similar trend was recently evidenced when submitting SiGe layers on Si to UV-NLA. The shape change was more drastic in GeSn than in SiGe. A larger built-in compressive strain because of a much larger size difference between Ge and Sn than Si and Ge might be the reason why. Time Resolved Reflectivity maps showed a more reflective plateau after the melt peak at an Energy Density (ED) of around 1.00 Jcm−2, stemming from the presence of a smooth Sn rich surface layer, as revealed by AFM images at the same ED. Stacked XRD maps outlined that this ED corresponded to the formation of rather high Sn content layers with Sn contents of up to 6.3%, i.e., concentrations significantly above the solid solubility limit, which is below 1%, at variance with conventional annealing processes at 450 °C. UV-NLA has thus opened a new processing window that might be useful for contact formation.

Fabrication of GeSn Nanowire MOSFETs by Utilizing Highly Selective Etching Techniques

Germanium–tin (GeSn) epitaxy layer was prepared on an 8-in SOI wafer with a Ge buffer layer. The etching rates of different solutions for the GeSn layer were investigated. The ammonia peroxide mixture can remove the Ge buffer layer with high efficiency and selectivity to the GeSn layer. Heated ammonia solution is able to etch the Si layer without damaging the GeSn layer significantly. The two-step etching process developed in this study is conducive to achieving GeSn nanowires (NWs) by selectively etching the Ge buffer and Si bottom layers. GeSn NWFETs were fabricated and measured. The strain of the GeSn NW channels is preserved with the optimized fabrication process proposed in this study.

Secondary epitaxy of high Sn fraction GeSn layer on strain-relaxed GeSn virtue substrate by molecular beam epitaxy

Strain relaxation is critical for GeSn alloys transforming from indirect to direct bandgap nature with Sn fraction above 6.5%, but difficult for them grown by molecular beam epitaxy (MBE), in which low temperature has to be set up to avoid Sn segregation. In this work, compressively strained Ge0.935Sn0.065 thin films grown on Si with a Ge buffer layer by MBE are firstly treated by ex-situ rapid thermal annealing, rendering partially strain relaxation in the Ge0.935Sn0.065 by generation of misfit dislocation networks without Sn segregation. Then, secondary epitaxy of Ge0.905Sn0.095 layer is carried out on the thermally annealed Ge0.935Sn0.065 virtual substrate. The secondary epitaxial GeSn layers exhibit partial strain relaxation and strong photoluminescence with red-shift of peak position, compared to that of fully compressive strained GeSn thin films with the same structure grown primary epitaxially. Those results manifest that secondary epitaxy, combining with ex-situ rapid thermal annealing for strain relaxed GeSn virtual substrate, is a practical way to achieve strain relaxed GeSn thin films with direct bandgap nature by MBE.

Time-Resolved Photoluminescence in GeSn Film by New Infrared Streak Camera Attachment Based on a Broadband Light Upconversion

GeSn coatings on commercial Si substrates have gained increased interest for application in infrared detectors and lasers. The characterization of these materials is crucial for their quality assessment and in describing device performance for commercialization. The time-resolved photoluminescence is an efficient method for contactless, time-resolved characterization of GeSn optoelectronic properties. For this purpose, in this work, we developed an infrared streak camera attachment based on the broadband upconversion of infrared photoluminescence by using powerful nanosecond 1064 nm pulses. The attachment achieved picosecond time resolution, being limited by the laser pulse duration, jitter, and temporal resolution of the streak camera. The spectral range for time-resolved photoluminescence in the 1100–2400 nm interval was obtained, outperforming the range of commercial infrared InGaAs streak cameras. The developed setup was applied for excitation-dependent time-resolved photoluminescence decay measurements in a GeSn and compared to the conventional upconversion technique with the optically delayed picosecond gate pulses. The new setup provided 2D spectro-temporal images for analysis. The photoluminescence decay times in the 30–80 ps range were obtained in the GeSn layer depending on the excitation pulse energy and spectral emission wavelength. Carrier thermalization was observed as a redshift of the photoluminescence spectra with time.

Sn distribution in Ge/GeSn heterostructures formed by sputter epitaxy method

Ge/GeSn heterostructure was formed on Ge and Ge virtual substrates using the sputter epitaxy method, and the Sn profiles at the Ge/GeSn heterostructure were investigated using secondary ion mass spectrometry (SIMS). The growth temperature of the Ge/GeSn heterostructure was about 215 and 250 °C, and the thicknesses of the Ge and GeSn layers were about 90 and 70 nm, respectively. The growth rates of Ge and GeSn were relatively high and were about 0.36 and 0.4 nm/sec, respectively. The Sn content of the formed GeSn layers was estimated using an X-ray diffraction (XRD) two-dimensional reciprocal space map and was about 9 %. From the results of the atomic force microscopy and the transmission electron microscopy, the interface of the Ge/GeSn heterostructure was sharp and the surface was flat. At the Ge/GeSn interface in the sample formed at 215 °C, the Sn concentration was decreased at 3.4 nm/dec, and the 1/e decay length was about 15Å, which were smaller than 8 nm/dec and 35 Å in the previous report. The Sn diffusion depended on the growth temperature and the underlayer. The gradient of the Sn content at the 390-nm-thick GeSn layer was estimated using SIMS and was about 0.55 %/μm in the sputter epitaxy method, which was smaller than 6.2 %/μm in the previous reports. From the XRD measurements, the 390-nm-thick GeSn layer had no strain relaxation. It is considered that a high-growth rate and no strain relaxation contribute to limiting the Sn diffusion, and uniform Sn content GeSn layers were obtained.

Ni-based metallization of GeSn layers: A review and recent advances

In this paper, we review the last 10 years studies related to Ni-based metallization of GeSn layers. We also introduce the recent achievements obtained at CEA-Leti.First, we present and discuss the solid-state reaction between a Ni thin film and GeSn layers. At low temperature, a Ni-rich phase, namely the hexagonal ∊-Ni5(GeSn)3 metastable phase, is obtained. This phase is then consumed to the benefit of the mono-stanogermanide phase Ni(GeSn). The Ni/GeSn system exhibits a poor thermal stability mainly governed by 2 phenomena: Ni(GeSn) agglomeration and Sn segregation.We then describe the behavior of Sn during the solid-state reaction and its subsequent impact. Sn tends to segregate first around grain boundaries (GBs) and then toward the sample surface. The presence of Sn around GBs can hamper diffusion paths and impact the solid-state reaction.The last part of this study is dedicated to the technological solutions implemented to overcome the low thermal stability of this system. We review literature solutions and present two options: pre-amorphization by implantation (PAI) prior to metallization and the addition of an alloying element in Ni-based metallization. These solutions enable to extent the thermal stability of the Ni/GeSn system by delaying the Ni(GeSn) agglomeration and Sn segregation phenomena.

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.

Gesn Alloys: From Optical to Electrical Pumped Lasers

Internet of Things applications, including medical diagnostics, imaging and industrial process controls, pollution monitoring or real-time motion control, push the extension of the Si photonics towards mid-infrared range.In recent years, group IV SiGeSn-based optoelectronic devices including light emitting, detectors, modulators, and CMOS devices have been developed. The main advantage of SiGeSn is the bandgap tunability in the range between 1.5 to 5 µm via Si and Sn compositions. Remarkably, the GeSn binary alloy becomes a direct bandgap semiconductor at a relative low Sn content of about 7-8 at.% opening the quest for the realization of monolithically integrated laser sources on Si. In this direction, from the first laser emission proof in 2015 remarkable progress has been done. Supported by epitaxial growth success, processing technology, and cavity design, the laser emission of optically pumped was continuously increased from 80K to 270K. The main role here was played by increasing the Sn content up a concentration of 17 at.%.The next major improvement came from layer transfer technology allowing defect engineering and heat management. Their combination made possible room temperature lasing. However, the real life application requires low laser threshold. Here the use of multiple quantum wells based on SiGeSn/GeSn heterostructures and strain engineering allow a remarkable threshold reduction to values of about 1 kA/cm2. Interestingly, modeling shows that the very epitaxial challenging of having high Sn contents can be reduced by band structure engineering via introducing tensile strain and, in addition, larger gains are possible compared with high-Sn strain free layers. The optical laser spectrum at 0 °C for a GeSn layer under 1 % biaxial tensile strain is shown in Fig 1a.The next step is the realization of an electrical pumped GeSn laser. The first data indicate that both waveguide and microdisk cavities can be electrically pumped leading to low temperature laser emission. The effect of the geometry and strain relaxation of under-etched cavities on the laser emission is under study. Figure 1b shows an exemplary spectrum and current-in light-out (LI) curve for a device with a SiGeSn/GeSn heterostructure taken at 50 K. Figure 1

GeSn resonance cavity enhanced photodetector with gold bottom reflector for the L band optical communication.

In this work, GeSn resonant cavity enhanced (RCE) p-i-n photodetectors (PDs) with 3.7% Sn content in a GeSn layer were fabricated on a silicon on insulator (SOI) substrate. The gold (Au) layer and the deposited SiO2 layer constitute the bottom reflector and top reflector of the RCE detectors, respectively. The GeSn RCE PD has three resonant peaks and its responsivity is improved about 4.5 times at 1630 nm, compared with GeSn PDs without a gold bottom mirror. The cutoff wavelength of GeSn RCE PDs is up to 1820 nm, while it is only 1730 nm for GeSn PDs without a gold reflector. The responsivity of RCE PDs at 1630 nm reaches 0.126 A/W and 3-dB bandwidth at about 36 GHz is achieved. These results indicate that the RCE structure is an effective approach for enhancing the GeSn PD performance operated at the L band.