Single-crystalline GeSn Research Articles

Growth of single crystalline GeSn alloy epilayer on Gd2O3/Si (111) engineered insulating substrate using RF sputtering and solid phase epitaxy

The article showcases a low-cost, low-temperature deposition andHVMtechnique to develop single crystalline GeSn alloy epilayers on Gd2O3/Si (111) substrate. First, GeSn alloy amorphous layer is deposited on the insulating substrates using an Radio Frequency (RF) sputtering apparatus. Subsequently, an inductively coupled plasma-assisted chemical vapor deposition (ICP-CVD) process is used to deposit a SiO2 capping layer to protect against Sn out-diffusion during heat treatment. The samples are then subjected to solid phase epitaxy (SPE) at 450 °C, 550 °C, and 650 °C. Sample processed for SPE at 450 °C has weak crystallinity and only shows Type-A stacking. Those processed for SPE at 550 °C and 650 °C, on the other hand, have revealed formation of the single-crystalline GeSn alloy epilayer with Type-A and Type-B stacking. However, SPE at 650 °C revealed tin out-diffusion and segregation effects. This work is significant for enabling the preparation of high-Sn-content GeSn alloy epilayers on insulating Gd2O3/Si (111) substrates, as it requires the initial deposition of a GeSn amorphous alloy epilayer using RF sputtering. This advancement promises benefits which includes advantages such as lower operating voltage, reduced leakage current, and minimized parasitic and short-channel effects, making it ideal for advancing RF technology.

Growth of single-crystalline GeSn films with high-Sn content on InP substrates by sputtering and rapid thermal annealing

In this work, the polycrystalline GeSn film with Sn content of 25.3 % on InP is obtained by sputtering at room temperature and then rapid thermal annealing (RTA) at 300 ℃. The single-crystalline GeSn film with Sn content of 24.7 % on InP is obtained by sputtering at 200 ℃ and then RTA at 300 ℃. This indicates that the growth temperature is one of the key factors affecting the formation of single-crystalline films. The effects of the sputtering power and film thickness on the crystallization of the GeSn films are investigated. The competition between the spontaneous and substrate-induced crystallization of the films limits the preparation of single-crystalline GeSn films. The strain relaxation of the film (R = 69.3 %) is achieved by annealing. The device fabricated by the GeSn film exhibits a high response of 18.1 A/W @-1 V at room temperature with the 1550-nm-line excitation. These suggest that sputtering epitaxy can be an effective method for growing single-crystalline GeSn films with high-Sn content on InP substrates, which is of great significance for developing high-performance mid-infrared devices.

Photoluminescence characterization of GeSn prepared by rapid melting growth method

In this work, single crystalline GeSn on insulator (GSOI) were grown by rapid melting growth (RMG). The Sn content distribution along the strip was investigated, reaching to 7.8% near the end of the strip. The GeSn strip had high quality with no threading dislocations, as revealed by cross-sectional transmission electron microscopy (XTEM). Photoluminescence (PL) measurements along the strip were performed to check the indirect-direct bandgap transition Sn content of GeSn alloys. It was found the integrated PL intensity increased about 6.6 times for Sn content 0.86%–7.8%. For GeSn with Sn content 6.1%, the temperature-dependent PL shows that the energy difference between direct Γ-Γ valley and indirect L-Γ valley decreases due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. The temperature-dependent PL spectra have only one peak for GeSn with Sn content 7.8%, suggesting the direct bandgap property. These results indicate that GeSn grown by RMG is very promising for fabricating efficient Si based light sources.

Flexible Titanium Nitride/Germanium-Tin Photodetectors Based on Sub-Bandgap Absorption.

We report an enhanced performance of flexible titanium nitride/germanium-tin (TiN/GeSn) photodetectors (PDs) with an extended photodetection range based on sub-bandgap absorption. Single-crystalline GeSn membranes transfer-printed on poly(ethylene terephthalate) are integrated with plasmonic TiN to form a TiN/GeSn heterojunction. Formation of the heterojunction creates a Schottky contact between the TiN and GeSn. A Schottky barrier height of 0.49 eV extends the photodetection wavelength to 2530 nm and further enhances the light absorption capability within the detection range. In addition, finite-difference time-domain simulation proves that the integration of TiN and GeSn could enhance average absorption from 0.13 to 0.33 in the near-infrared (NIR) region (e.g., 1400-2000 nm) and more than 70% of light is absorbed in TiN. The responsivity of the fabricated TiN/GeSn PDs is increased from 30 to 148.5 mA W-1 at 1550 nm. There is also an ∼180 nm extension in the optical absorption wavelength of the flexible TiN/GeSn PD. The enhanced performance of the device is attributed to the absorption and separation of plasmonic hot carriers via TiN and the TiN/GeSn junction, respectively. The effect of external uniaxial strain is also investigated. A tensile strain of 0.3% could further increase the responsivity from 148.5 to 218 mA W-1, while it is decreased to 102 mA W-1 by 0.25% compressive strain. In addition, the devices maintain stable performance after multiple and long bending cycles. Our results provide a robust and cost-effective method to extend the NIR photodetection capability of flexible group IV PDs.

Monolithic Germanium-Tin Pedestal Waveguide for Mid-Infrared Applications

Germanium-tin (GeSn) is a CMOS-compatible group-IV material. Its growth, however, is plagued by the tendency of Sn segregation and the generation of defects within the GeSn layer when it is grown on the lattice-mismatched substrate. Thus far, thin GeSn has been reported for use in a direct-band gap for near-mid infrared light source and photodetector. In this communication, we report the growth of high quality single-crystalline GeSn (∼ 1 μm) with low compressive stress (−0.3%) and low defects (3 × 107 /cm2) on Ge buffer on Si substrate. The as-grown GeSn is then fabricated into pedestal waveguide of width 1.25 μm. An estimated propagation loss of 1.81 dB/cm and bending loss of 0.19 dB/ bend are measured at 3.74 μm. In the absence of Ge-O absorption peaks at 820 and 550 cm−1, under optimal fabrication and measurement condition, the proposed GeSn waveguide might possibly support light propagation for wavelength beyond 25 μm.

Spontaneously Conversion from Film to High Crystalline Quality Stripe during Molecular Beam Epitaxy for High Sn Content GeSn

Two series of Ge0.8Sn0.2 samples were grown on Ge buffered Si substrate by molecular beam epitaxy (MBE) to investigate the influence of growth temperature and film thickness towards the evolution of surface morphology. A novel phenomena was observed that the Ge0.8Sn0.2 film was segregated and relaxed by the formation of GeSn stripes on the film. Under specific growth condition, the stripes can cover nearly the whole surface. XRD, TEM, AFM, PL and TEM results indicated that the stripes are high quality single crystalline GeSn with Sn content around 5%. The formation of GeSn stripes proposes an effective strategy to fabricate high crystalline quality GeSn stripe on Si, where the Ge0.8Sn0.2 film serves as precursor and the segregated Sn works as catalyst droplets. This technique has great potential for future optoelectronic and microelectronic applications.

Band gap renormalization in n-type GeSn alloys made by ion implantation and flash lamp annealing

The last missing piece of the puzzle for the full functionalization of group IV optoelectronic devices is a direct bandgap semiconductor made by CMOS compatible technology. Here, we report on the fabrication of GeSn alloys with Sn concentrations up to 4.5% using ion implantation followed by millisecond-range explosive solid phase epitaxy. The n-type single crystalline GeSn alloys are realized by co-implantation of Sn and P into Ge. Both the activation of P and the formation of GeSn are performed during a single-step flash lamp annealing for 3 ms. The bandgap engineering in GeSn as a function of the doping level and Sn concentration is theoretically predicted by density functional theory and experimentally verified using ellipsometric spectroscopy. We demonstrate that both the diffusion and the segregation of Sn and P atoms in Ge are fully suppressed by millisecond-range nonequilibrium thermal processing.

Demonstration of mm long nearly intrinsic GeSn single-crystalline wires on quartz substrate fabricated by nucleation-controlled liquid-phase crystallization

Millimeter long GeSn wires were fabricated on quartz substrates by using an advanced rapid melt growth method, named “nucleation-controlled liquid-phase crystallization” (NCLPC). Local melting of amorphous GeSn wires surrounded by SiO2 capping layers led to the propagation of single crystal growth from the solid/liquid interface without any crystal seeds and catalysts. Slow cooling just after rapid melting was beneficial in NCLPC for prolonged and stable lateral growth, and 1.7 mm long single-crystalline GeSn wires were successfully demonstrated. Physical characterizations revealed that, since excessive Sn atoms were swept out toward the growth direction, a mostly uniform GeSn single crystal, in terms of Sn content and crystallinity, was formed along the wires. Electrical measurements of top-gate MOSFETs with NCLPC-GeSn channels revealed the formation of nearly intrinsic GeSn single crystal, indicating a significant reduction in acceptor-like point defects achieved with the slow-cooling procedure.

Crystalline GeSn growth by plasma enhanced chemical vapor deposition

Single crystalline GeSn growth on Si substrate was successfully demonstrated by using plasma enhanced chemical vapor deposition (PE-CVD) with commercially available GeH4 and SnCl4 precursors. Using the plasma enhancement technique, low temperature growth at 350°C for GeSn epitaxy on Si substrate was achieved with the growth rate of 51.4 nm/min and Sn content up to 6%. The relaxed GeSn films with 1 µm thickness were able to be grown despite of the huge lattice mismatch between GeSn and Si. Structural and optical characterizations were conducted to study the film properties. The demonstrated plasma enhancement growth showed its effectiveness to enhance the Sn incorporation of the crystalline GeSn at low temperature and to maintain the high growth rate.

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.

Lightly doped n-type tensile-strained single-crystalline GeSn-on-insulator structures formed by lateral liquid-phase crystallization

A tensile-strained single-crystalline n-type GeSn-on-insulator structure was demonstrated by lateral liquid-phase crystallization of an Sb-doped GeSn layer, and the diffusion and activation behaviors of Sb atoms in a liquid-phase-grown GeSn wire were investigated. A photoluminescence-based study revealed that a substantial amount of Sb was swept out during the liquid-phase growth of the GeSn wire. A thin-film transistor based on an Sb-doped GeSn wire on a quartz substrate exhibited n-channel accumulation-mode operation, and the temperature dependence of field-effect mobility revealed that the single-crystalline GeSn wire is a lightly doped n-type (∼1017 cm−3). The combination of Sb-doped n-type and undoped p-type GeSn wires would be an ideal platform for GeSn-based optoelectronic integration.

Fabrication of tensile-strained single-crystalline GeSn on transparent substrate by nucleation-controlled liquid-phase crystallization

We developed a method of forming single-crystalline germanium-tin (GeSn) alloy on transparent substrates that is based on liquid-phase crystallization. By controlling and designing nucleation during the melting growth process, a highly tensile-strained single-crystalline GeSn layer was grown on a quartz substrate without using any crystal-seeds or catalysts. The peak field-effect hole mobility of 423 cm2/V s was obtained for a top-gate single-crystalline GeSn MOSFET on a quartz substrate with a Sn content of 2.6%, indicating excellent crystal quality and mobility enhancement due to Sn incorporation and tensile strain.

Low Temperature Geometrically Confined Growth of Pseudo Single Crystalline GeSn on Amorphous Layers for Advanced Optoelectronics

Highly textured Ge0.91Sn0.09 is obtained on both amorphous SiO2/Si and glass substrates at low temperatures <475 oC, which shows grain sizes up to tens of microns. Strikingly, the nucleation center spacing ranges from 0.1 to 1 mm, orders of magnitude larger than common solid state crystallization. This observation indicates an exceedingly high grain growth rate vs. a low nucleation rate. Therefore, we can control nucleation sites and fabricate geometrically confined pseudo single crystalline GeSn grain using patterning techniques, including surface Sn dots/patches, local laser annealing, and nanotaper patterns. Another remarkable result is that ~9 at.% Sn is incorporated substitutionally into Ge, far exceeding the equilibrium solubility limit of ~1 at.%. The high Sn composition, together with ~0.24% thermally induced tensile strain in the film, shifts the GeSn direct band gap to ~0.5 eV (2500nm) and converts it into a direct band gap semiconductor with significantly enhanced optoelectronic properties.

(Invited) Solid Phase Epitaxy of GeSn Alloys on Silicon and Integration in MOSFET Devices

We investigate the crystallization of amorphous Ge alloy layers on silicon substrates for optical and electronic applications. Amorphous GeSn layers are obtained by limiting the adatom surface mobility during deposition. Subsequent annealing transforms the amorphous layer into single-crystalline GeSn by solid phase epitaxy. Structural investigation shows excellent structural quality for layers with up to 6.1% Sn. The surface and interface are smooth thanks to the low thermal budget. The GeSn layers show a significant increase in optical absorption with respect to Ge. We demonstrate tensile-strained GeSn p-type pMOSFET devices on Si(111) substrates using solid phase epitaxy of amorphous GeSn layer, with 4.5% Sn and +0.33% tensile strain. Structural investigation showed the presence of twin defects in the GeSn layers after crystallization. In this work we present a method to effectively suppress the formation of twin defects. Furthermore we demonstrate the crystallization of multiple amorphous layers in a 1 step anneal.

Ni(SiGeSn) Metal Contact Formation on Low Bandgap Strained (Si)Ge(Sn) Semiconductors

GeSn has gained a lot of attention as alternative high mobility and low bandgap material to boost the performance of MOSFETs and Tunnel-FETs. Self-aligned contacts with low sheet resistance and good thermal stability are desired for GeSn FETs. Preliminary works on Ni-GeSn contacts indicate a low thermal stability. However, to date only GeSn with low Sn concentrations below 6 % have been reported. Here we investigate the formation of metallic contacts for high mobility Sn-based group IV semiconductors. The formation of ternary NiGeSn alloys for a large range of Sn compositions up to 12 at.% and quaternary NiSiGeSn alloys, formed on SiGeSn ternaries with Si/Sn compositions ranging from 20%/3% to 3%/12% is presented.(Si)GeSn layers with thicknesses of about 30-60 nm were pseudomorphically grown on high quality Ge virtual substrates (VS) on Si(100) by RP-CVD. The growth temperature was varied between 350°C and 475°C to adjust the Sn and Si concentration within the epilayers.For the layers under investigation Sn and Si concentrations between 3 – 12 at.% and 3 – 18 at.% have been obtained. All GeSn layers exhibit high compressive strain while for SiGeSn ternaries biaxial compressive and biaxial tensile strain is observed according to the Si/Sn ratio. For the metallization studies 10 nm Ni were evaporated followed by a forming gas annealing step for 10 or 30 s. Here, the temperature was varied between 250°C and 400°C.Specular XRD, taken in off-orientation relative to the Si substrate was performed for phase identification of differently annealed (275°C – 350°C) Ni-Ge0.91Sn0.09 samples (Fig. 1a). The temperature dependence of the stanogermanide phase formation shows that the Ni5X3 phase (x = Ge0.88Sn0.12, Ge0.91Sn0.09 Ge0.94Sn0.06) appears already at 275°C. The epitaxial Ni5X3 phase transforms into a [Ni1X1 + Ni5X3] multi-phase at 300°C while at 325°C only poly crystalline grains of the Ni1X1 phase are observed. The morphology of the stanogermanides was investigated by TEM. For all investigated Sn concentrations at a formation temperature of 300°C, the formed metallic layers are continuous with a mean thickness of about 25 nm. High resolution TEM images taken on NiGe0.94Sn0.06 and NiGe0.88Sn0.12 are shown in Fig. 1b,c. The rough Ni1X1/GeSn interface is typical for poly-crystalline layers containing large and non-substrate oriented grains. The TEM micrograph also proves that the grains are crystalline with well-defined grain boundaries and an unconsumed single crystalline GeSn layer. The well-oriented Ni5X3 phase possess a flat Ni5X3/GeSn interface (not shown here).Compared to the stanogermanides, we obtained an extended thermal stability window for the Ni5(SiGeSn)3 phase. This epitaxial phase, demonstrated by pole figures, is observed for all Si/Sn ratios, from high Si content (20% Si and 3% Sn) to high Sn content (3% Si and 12% Sn) alloys. The sheet resistance of NiSiGeSn alloys versus temperature is shown in Fig. 1d for different compositions. For Sn concentrations up to 8 at.% low sheet resistances (< 10 W/sq) are obtained for the complete temperature range. The thermal stability for 12 at.% Sn is very limited.

Invited; Solid Phase Epitaxy of GeSn Alloys on Silicon and Integration in MOSFET Devices

Single crystalline Ge compounds (SiGe, GeSn) have interesting electrical and optical properties [1], rendering them well-suited to a wide variety of applications. GeSn has been predicted to exhibit carrier mobilities exceeding that of Ge [2], which makes this material suitable as a channel material for high-speed complementary metal-oxide semiconductor (CMOS) technology [3]. In addition to increased carrier mobility, the GeSn material system is interesting for its optical properties. GeSn exhibits a direct band gap between 0.61 and 0.35 eV for Sn concentrations between 6 and 15 % [4]. This makes it much better fit for optoelectronic applications than Si and Ge, especially at long wavelengths.Epitaxial growth of GeSn on Si substrates poses several challenges including a limited solubility of Sn in Ge (0.5%), compositional fluctuations, Sn segregation and a large lattice mismatch (>4%). It is critical to suppress these effects while simultaneously optimizing the GeSn layer characteristics for the basis of high performance devices.Recently, we demonstrated the growth of single crystalline GeSn layers with excellent structural quality grown on Si(111) substrates, by solid phase epitaxy (SPE) of amorphous GeSn alloys[5]. This technique has an advantage in the realization of thin GeSn layers directly on Si. We do not use surfactants, a capping layer, nor a buffer layer. Amorphous GeSn layers are obtained by the co-evaporation of Ge and Sn atoms, with beam equivalent pressures of 1.33 x 10-7 Torr for Ge and 9 x 10-9 Torr for Sn, while exposing the substrate to a N2 flux with a beam equivalent pressure of about 1 x 10-5 Torr, at a deposition temperature around RT. Subsequent annealing transforms the amorphous GeSn into a single crystalline layer via solid phase epitaxy. Excellent structural quality is demonstrated for layers with up to 6.1 % of Sn. The GeSn layers show tensile strain (up to +0.34%), which lowers the difference between the direct and indirect band transition and makes this method promising for obtaining direct band gap group IV layers. GeSn with 4.5% Sn shows increased optical absorption compared to Ge and an optical band gap of 0.52 eV. Furthermore, the surface and interface are smooth (RMS roughness < 1 nm), despite the relaxation of the layers.Additional structural investigation by XRD and TEM showed the presence of twin defects in the GeSn layers after crystallization. These defects can have a significant impact on the carrier mobility and it is therefore important to suppress the formation of twin defects during crystallization. The formation of twin defects is likely related to the presence of contaminants, in particular oxygen, at the interface between the crystalline substrate and the amorphous layer leading to the roughening of the amorphous-crystalline interface during SPE growth. We have developed a 2-step deposition process which successfully suppresses the formation of such twin defects.Additionally, we demonstrate excellent electrical properties of the GeSn layers by the fabrication of depletion-mode GeSn pMOSFETs on Si(111) using SPE of amorphous GeSn layers, TaN/Al2O3 metal-gate/high-k gate stacks, and Ni-based metal S/D contacts. The GeSn MOSFET devices show +100% improvement in hole mobility with respect to bulk Si and good transfer characteristics with On/Off ratio of ~100 for ultrathin (<10 nm) GeSn layers on Si [6].Finally, we also demonstrate successful crystallization of amorphous SiGe layers, deposited by plasma enhanced chemical vapor deposition (PECVD) on Si substrates and annealing in N2 atmosphere. The SiGe layers show excellent structural quality for compositions ranging from Ge- to Si-rich.[1] J. Kouvetakis et al., Ann. Rev. Mater. Res. 36, 497 (2006)[2] J. D. Sau and M. L. Cohen, Phys. Rev. B 75, 045208 (2007)[3] C. Merckling et al., Appl. Phys. Lett. 11, 192110 (2011)[4] G. He and H. A. Atwater, Phys. Rev. Lett. 79, 1937 (1997)[5] R. R. Lieten et al., Appl. Phys. Lett. 102, 052106 (2013)[6] R. R. Lieten et al., Appl. Phys. Express 6, 101301 (2013)

Low Temperature Geometrically Confined Growth of Pseudo Single Crystalline GeSn on Amorphous Layers for Advanced Optoelectronics

The geometrically confined growth of highly crystalline semiconductor materials like Ge on amorphous layers (without epitaxial template) has been a challenge in materials science. The success of this technology, especially at low fabrication temperature can enable 3D Si photonics by building photonic devices on metal/interconnect layers, and enable inexpensive, high-throughput fabrication of devices such as tandem solar cell and thin-film transistor.Here we report that, by incorporating Sn into Ge, we are able to obtain highly crystalline GeSn thin film on amorphous substrates at low crystallization temperatures ranging from 340 to 464 ºC through enhanced crystallization from the Ge-Sn euetectic system, and are able to transform the material toward a direct band-gap material through band structure modification. A highly textured Ge0.91Sn0.09 is obtained from the sample with 9.5 at. % as-deposited Sn composition, which shows grain sizes up to tens of microns, as revealed by electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and Raman spectroscopy. EBSD confirms that most grain boundaries are either twin or low angle boundaries, which are benign to optoelectronic properties. Strikingly, the nucleation center distances range from 0.1 to 1 mm, orders of magnitude larger than common solid state crystallization, which indicates an exceedingly high grain growth rate vs. a low nucleation rate. As a result of this feature, we are able to fabricate geometrically confined pseudo single crystalline GeSn grain using patterning techniques. Another remarkable result is that 9 at.% Sn is incorporated substitutionally into Ge, far exceeding the equilibrium solubility limit of ~1%. The high Sn composition, with the addition of ~0.24% tensile strain in the film, shifts the direct band gap to 0.5 eV (2500nm), right on the verge of indirect-to-direct band gap transition. This transition will greatly enhance the material’s optoelectronic performance as a photonic device, and will also broaden the absorption spectrum for a tandem solar cell.

Single Crystalline GeSn on Silicon by Solid Phase Crystallization

We investigate the crystallization of amorphous GeSn layers on silicon substrates. Amorphous GeSn layers are deposited on silicon substrates kept at approximately room temperature by molecular beam epitaxy. Thermal annealing transforms these layers into single crystalline GeSn. Structural investigation shows excellent structural quality for layers with 4.5% Sn. Furthermore the surface and interface are smooth, especially when considering the relaxation of the layers. The surface root mean square roughness is smaller than 1 nm. Finally, we show an increased optical absorption for the GeSn layers with respect to Ge, which is promising for integration of optical devices on silicon substrates.

Strained germanium–tin (GeSn) p-channel metal-oxide-semiconductor field-effect-transistors (p-MOSFETs) with ammonium sulfide passivation

High-mobility strained Ge0.958Sn0.042 p-channel metal-oxide-semiconductor field-effect-transistors (p-MOSFETs) with ammonium sulfide [(NH4)2S] surface passivation were demonstrated. A ∼10nm thick fully-strained single crystalline GeSn layer was epitaxially grown on Ge (100) substrate as the channel layer. (NH4)2S surface passivation was performed for the GeSn surface, followed by gate stack formation. Ge0.958Sn0.042 p-MOSFETs with (NH4)2S passivation show decent electrical characteristics and a peak effective mobility of 509cm2/Vs, which is the highest reported peak mobility obtained for GeSn channel p-MOSFETs so far.

Tensile strained GeSn on Si by solid phase epitaxy

We demonstrate single crystalline GeSn with tensile strain on silicon substrates. Amorphous GeSn layers are obtained by limiting the adatom surface mobility during deposition. Subsequent annealing transforms the amorphous layer into single crystalline GeSn by solid phase epitaxy. Excellent structural quality is demonstrated for layers with up to 6.1% of Sn. The GeSn layers show tensile strain (up to +0.34%), which lowers the difference between direct and indirect band transition and makes this method promising for obtaining direct band gap group IV layers. GeSn with 4.5% Sn shows increased optical absorption compared to Ge and an optical band gap of 0.52 eV.