Ge-buffered Si Research Articles

High-performance GeSi/Ge multi-quantum well photodetector on a Ge-buffered Si substrate.

This work demonstrates a high-performance photodetector with a 4-cycle Ge0.86Si0.14/Ge multi-quantum well (MQW) structure grown by reduced pressure chemical vapor deposition techniques on a Ge-buffered Si (100) substrate. At -1 V bias, the dark current density of the fabricated PIN mesa devices is as low as 3 mA/cm2, and the optical responsivities are 0.51 and 0.17 A/W at 1310 and 1550 nm, respectively, corresponding to the cutoff wavelength of 1620 nm. At the same time, the device has good high-power performance and continuous repeatable light response. On the other hand, the temperature coefficient of resistance (TCR) of the device is as high as -5.18%/K, surpassing all commercial thermal detectors. These results indicate that the CMOS-compatible and low-cost Ge0.86Si0.14/Ge multilayer structure is promising for short-wave infrared and uncooled infrared imaging.

Synthesis of High Sn Content Ge1-x-ySixSny (0.1 < y < 0.22) Semiconductors on Si for MWIR Direct Band Gap Applications.

A systematic effort has been described to grow ternary Ge1-x-ySixSny semiconductors on silicon with high Sn concentrations spanning the 9.5-21.2% range. The ultimate goal is not only to produce direct band gap materials well into the infrared region of the spectrum but also to approach a critical concentration (yc) for which further additions of Si would decrease─rather than increase─the band gap. This counterintuitive behavior is expected as a result of the giant bowing parameter in the compositional dependence of the band gap associated with the presence of Si-Sn pairs. The growth approach in this study was based on a chemical vacuum deposition method that uses Si4H10, Ge3H8, and SnD4 or SnH4 as the sources of Si, Ge, and Sn, respectively. A fixed Si concentration near x = 0.05-0.07 was chosen to focus the exploration of the compositional space. A first family of samples was grown of Ge-buffered Si substrates. For Sn concentrations y < 0.12, it was found that the samples relaxed their mismatch strain in situ during growth, resulting in high Sn content films that had relatively low levels of strain and exhibited photoluminescence signals that demonstrated direct band gap behavior for the first time. The device potential of these materials was also demonstrated by fabricating a prototype photodiode with low dark currents. The optical studies suggest that the above-mentioned critical concentration is close to yc = 0.2. As the growth temperature was lowered in an effort to reach such values, Sn concentrations as high as y = 0.15 were obtained, but the films grew fully strained with compressive levels as high as 1.7%. To increase the Sn concentration beyond y = 0.15, a new strategy was adopted, in which the Ge buffer layer was eliminated, and the ternary alloy was grown directly on Si. The much higher lattice mismatch between the Ge1-x-ySixSny layer and the Si substrate caused strain relaxation right at the film/substrate interface, and the subsequent films grew with much lower levels of strain. This made it possible to lower the growth temperatures even further and achieve a comprehensive series of strained relaxed samples with tunable Sn concentrations as high as y = 0.21 (and beyond). The latter represent the highest Sn contents in crystalline Ge1-x-ySixSny attained to date and reach the desired yc = 0.2 range. The synthesized films exhibited significant thickness, allowing a thorough determination of composition, crystallinity, morphology, and bonding properties, indicating the formation of single-phase single-crystal alloys with random cubic structures. Further work will focus on optimizing the latter samples to explore the optical and electronic properties.

Harvesting strong photoluminescence of physical vapor deposited GeSn with record high deposition temperature

In this work, strong photoluminescence (PL) from physical vapor deposited GeSn on Ge-buffered Si is harvested by pursuing high deposition temperature. High-quality Ge0.938Sn0.062 films are obtained through sputtering epitaxy at a record high temperature of 405 °C. The PL peak intensity of the sputtering-grown GeSn is enhanced by 21 times compared to that of GeSn with similar Sn content grown by molecular beam epitaxy at 150 °C. The PL intensity ratio between the sputtering-grown GeSn and Ge virtual substrate reaches a value of 18. Power-dependent and temperature-dependent PL characterizations demonstrate that band-to-band recombination dominates in the sputtering-grown GeSn film. The results indicate that high-temperature sputtering epitaxy, which has the merit of low cost and high-productivity potential, is promising for preparing high-quality GeSn films for optoelectronic applications.

Reduced Dislocation of GaAs Layer Grown on Ge-Buffered Si (001) Substrate Using Dislocation Filter Layers for an O-Band InAs/GaAs Quantum Dot Narrow-Ridge Laser.

The development of the low dislocation density of the Si-based GaAs buffer is considered the key technical route for realizing InAs/GaAs quantum dot lasers for photonic integrated circuits. To prepare the high-quality GaAs layer on the Si substrate, we employed an engineered Ge-buffer on Si, used thermal cycle annealing, and introduced filtering layers, e.g., strained-layer superlattices, to control/reduce the threading dislocation density in the active part of the laser. In this way, a low defect density of 2.9 × 107 cm−2 could be achieved in the GaAs layer with a surface roughness of 1.01 nm. Transmission electron microscopy has been applied to study the effect of cycling, annealing, and filtering layers for blocking or bending threading-dislocation into the InAs QDs active region of the laser. In addition, the dependence of optical properties of InAs QDs on the growth temperature was also investigated. The results show that a density of 3.4 × 1010 InAs quantum dots could be grown at 450 °C, and the photoluminescence exhibits emission wavelengths of 1274 nm with a fullwidth at half-maximum (FWHM) equal to 32 nm at room temperature. The laser structure demonstrates a peak at 1.27 μm with an FWHM equal to 2.6 nm under a continuous-wave operation with a threshold current density of ∼158 A/cm2 for a 4-μm narrow-ridge width InAs QD device. This work, therefore, paves the path for a monolithic solution for photonic integrated circuits when III−V light sources (which is required for Si photonics) are grown on a Ge-platform (engineered Ge-buffer on Si) for the integration of the CMOS part with other photonic devices on the same chip in near future.

Room temperature emission spectroscopy of GeSn waveguides under optical pumping

Waveguides were fabricated from highly n-type doped GeSn layers with Sn content at 5.4%–6.2% and grown on Ge-buffered Si substrates. The waveguides were optically pumped using a 976 nm continuous-wave laser, and the waveguide emission spectrum was collected and analyzed. The results indicate a non-linear power increase via higher injection-level at room temperature. Comprehensive theoretical models for the waveguide emission power dependence were developed to reproduce experimental data and provide an understanding of the nonlinear power dependence.

Composition uniformity and large degree of strain relaxation in MBE-grown thick GeSn epitaxial layers, containing 16% Sn

We systematically investigate the compositional uniformity, degree of strain relaxation (DSR), defect structure and surface morphology of GeSn epitaxial layers with 16% Sn, grown by low temperature molecular beam epitaxy (MBE) on Ge-buffered Si(001) substrates. Combining atom probe tomography, reciprocal space mapping, cross-sectional transmission electron microscopy, and atomic force microscopy analyses, we demonstrate that for a layer thickness of , a high DSR (∼70%) can be achieved, while maintaining compositional uniformity at the atomic scale. We find no evidence of Sn clustering in the bulk, or Sn segregation to the surface, for at least this value of . The observed compositional uniformity contrasts the well-established phenomenon of strain-relaxation enhancement of Sn content in chemical vapour deposition (CVD) growth of GeSn. The defect structure leading to strain relaxation in these MBE-grown GeSn epitaxial layers is also distinctly different from that observed in CVD growth of the alloy. We observe the co-existence of highly strain-relaxed and pseudomorphically strained regions in the grown epilayers, tentatively explained by bunching of threading dislocations. Considering that MBE growth of GeSn epitaxial layers, with such high-Sn content and layer thickness, has not been reported before, our results are encouraging for future improvements in design and fabrication of group-IV-based mid-infrared photonic devices.

O-Band Emitting InAs Quantum Dots Grown By MOCVD On A 300 mm Ge-Buffered Si (001) Substrate.

The epitaxy of III-V semiconductors on silicon substrates remains challenging because of lattice parameter and material polarity differences. In this work, we report on the Metal Organic Chemical Vapor Deposition (MOCVD) and characterization of InAs/GaAs Quantum Dots (QDs) epitaxially grown on quasi-nominal 300 mm Ge/Si(001) and GaAs(001) substrates. QD properties were studied by Atomic Force Microscopy (AFM) and Photoluminescence (PL) spectroscopy. A wafer level µPL mapping of the entire 300 mm Ge/Si substrate shows the homogeneity of the three-stacked InAs QDs emitting at 1.30 ± 0.04 µm at room temperature. The correlation between PL spectroscopy and numerical modeling revealed, in accordance with transmission electron microscopy images, that buried QDs had a truncated pyramidal shape with base sides and heights around 29 and 4 nm, respectively. InAs QDs on Ge/Si substrate had the same shape as QDs on GaAs substrates, with a slightly increased size and reduced luminescence intensity. Our results suggest that 1.3 μm emitting InAs QDs quantum dots can be successfully grown on CMOS compatible Ge/Si substrates.

(G03 Best Paper Award Winner) Analysis of Sn Behavior During Ni/GeSn Solid-State Reaction by Correlated X-ray Diffraction, Atomic Force Microscopy, and Ex-situ/In-situ Transmission Electron Microscopy

GeSn material has been envisioned as an alternative to the complex integration of III-V lasers on Silicon (Si) for photonic applications [1]. GeSn heterostructures with 16 at.% of Sn were demonstrated [2]. Increasing the Sn at.% is of high interest in these devices. Additionally, high quality contacts are needed to achieve electrical pumping, for which Nickel intermetallics were proposed. Indeed, Ni(GeSn) exhibit low temperatures formation and low resistances values. Currently, it is not entirely known how Sn incorporates in Ni-GeSn phases. Sn might: (i) be soluble in the phase, (ii) segregate, and/or (iii) form Ni-Sn alloys [3,4]. The Sn content is otherwise expected to impact different aspects of the phase sequence [5], as well as morphological and electrical properties.We propose a comprehensive study of the Ni-GeSn intermetallics focusing on Sn segregation and its impact on phase stability. 60 nm-thick Ge and Ge1-xSnx layers with 6, 10 and 15 at.% of Sn, were epitaxially grown on Ge-buffered Si (100) substrates. Surface preparation was performed with 1% diluted HF. Then, 10 nm of Ni were deposited by physical vapor deposition and capped with 7 nm of TiN. The Ni-Ge1-xSnx phase sequence and crystalline evolution were monitored by in-situ X-ray diffraction (XRD). Cross-section transmission electron microscopy (XTEM) measurements coupled with energy-dispersive X-ray spectrometry (EDS) and electron energy-loss spectroscopy (EELS) atomic maps were acquired to follow Sn distribution and segregation. Figure 1 shows the phase sequence of the Ni-Ge0.90Sn0.10 system. The growth was sequential. The first phase appearing was the Ni5(GeSn)3 phase. It was followed by the growth of the Ni(GeSn) phase [5 – 7]. EDS profiles confirmed that Sn was incorporated in both the Ni-rich and Ni(GeSn) phases. Similar phase sequences were obtained for all systems, from a straightforward comparison of in-situ XRD patterns. The threshold temperature above which the Ni-rich phase disappeared (and the less resistive Ni(GeSn) phase appeared) increased with Sn content: from 180 °C up to 235 °C and 245 °C for 0, 6 and 10 at.% of Sn, respectively (Figure 2). A dependence of the phase transition on the Sn content in the layers was evidenced. This trend was not observed for the Ni-Ge0.85Sn0.15 system due to Sn segregation at lower temperatures. XTEM analyses coupled with EDS and EELS (Figure 3) enabled us to track Sn segregation. Flat and continuous layers were obtained from the as-deposited state up to 350 °C. At 350 °C, Sn segregation started. It happened first around grain boundaries (GB) and then near the surface. At 400 °C, the Ni(GeSn) layers lost their continuity due to grain boundary grooving and agglomeration (confirmed by atomic force microscopy (AFM)). Grains were mainly composed of Ni and Ge atoms. Sn was predominantly localized at GB and close to the surface. At 550 °C, Sn coming from the intermetallic and from the GeSn thick layers underneath, completely migrated towards the surface. Figure 4 presents a Sn behavior schematic summary. Sn distribution and segregation impacted the Ni-Ge(Sn) phase sequences. Due to the role GB have on atomic transport, Sn segregation in those highly energetic areas might have reduced atoms mobility. Indeed, GB normally offer preferential paths, with enhanced atomic mobility [8]. GBs modified/saturated by Sn, can reduce the diffusion rate for phase formation. We thus believe that Sn acts as a “stuffed barrier”, hampering two-way atomic traffic. The higher the Sn content, the greater the hampering effect is, which slowed down Ni diffusion and delayed phase formation.We showed that Sn segregation in Ni-GeSn layers resulted in higher thermal budgets needed to obtain the less resistive phase (Ni(GeSn)) and have proper electrical contacts. Those thermal budgets could be detrimental for layers’ stability. In-situ TEM-EDS, AFM and sheet resistance results will also be presented, together with alternative solutions to delay Sn segregation. Acknowledgements: We would like to thank Virginie Loup for the wet surface treatment. This work was supported by the French National Research Agency (ANR) under the “Investissements d’avenir” programs: ANR-10-AIRT-0005 (IRT-NANOELEC), ANR-10-EQPX-0030 (EQUIPEX-FDSOI-11) and by the CEA DSM-DRT Phare project “Photonics”.

Photo detection and modulation from 1,550 to 2,000 nm realized by a GeSn/Ge multiple-quantum-well photodiode on a 300-mm Si substrate.

A GeSn/Ge multiple-quantum-well (MQW) p-i-n photodiode structure was proposed for simultaneously realizing high detectivity photo detection with low dark current and effective optical modulation based on the quantum confined Stark (QCSE) effect. The MQW stacks were grown on a 300-mm Ge-buffered Si substrate using reduced pressure chemical vapor deposition (RPCVD). GeSn/Ge MQW p-i-n photodiodes with varying mesa diameters were fabricated and characterized. An ultralow dark current density of 16.3 mA/cm2 at -1 V was achieved as expected due to the low threading dislocation density (TDD) in pseudomorphic GeSn layer. Owing to the ultralow dark current density and high responsivity of 0.307 A/W, a high specific detectivity of 1.37×1010 cm·Hz1/2/W was accomplished at 1,550 nm, which is comparable with commercial Ge and extended-InGaAs photodetectors. Meanwhile, the bias voltage-dependent photo response was investigated from 1,700 to 2,200 nm. The extracted effective absorption coefficient of GeSn/Ge MQW shows a QCSE behavior with electric field-dependent exciton peaks from 0.688 to 0.690 eV. An absorption ratio of 1.81 under -2 V was achieved at 2 μm, which shows early promise for effective optical modulation. The high frequency response was calculated theoretically, and the predicted 3-dB bandwidth for the photodiode with a mesa diameter of 30 μm could reach 12 GHz at -2 V.

Low Contact Resistivity to Ge Using In-Situ B and Sn Incorporation by Chemical Vapor Deposition

The low contact resistivity with the median value of <inline-formula> <tex-math notation="LaTeX">$3.1\times 10^{-{9}}\,\,\Omega \cdot \text {cm}^{{2}}$ </tex-math></inline-formula> (the lowest value of <inline-formula> <tex-math notation="LaTeX">$1.1\times 10^{-{9}}\,\,\Omega \cdot \text {cm}^{{2}}$ </tex-math></inline-formula>) is achieved by Ti metal contact to <i>in-situ</i> B-doped GeSn with B segregation at Ti/GeSn:B interface. The newly developed two-sheet-resistance model is used to extract the contact resistivity, considering the different sheet resistance of GeSn:B in the gap regions and under the metal layers. Sn incorporation into Ge lowers the Schottky barrier height of holes, and the heavy B doping at the Ti/GeSn:B interface reduces the hole tunneling distance. Fully compressively strained GeSn:B epitaxial layer with the bulk active [B] of <inline-formula> <tex-math notation="LaTeX">$1.9\times 10^{{20}}$ </tex-math></inline-formula> cm^&#x2212;3 and [Sn] of 4.7&#x0025; on the Ge-buffered Si substrate is successfully grown by chemical vapor deposition. The effects of B₂H₆ precursor on growth rate and [Sn] are investigated. Low contact resistivity is obtained using conventional B-doping in GeSn grown at a temperature as low as 305 &#x00B0;C and submitted to a post epitaxy thermal budget of 400 &#x00B0;C for 30 s.

Impact of ex-situ annealing on strain and composition of MBE grown GeSn

The application of GeSn is extended to semiconductor lasers thanks to its band engineering via Sn composition and strain manipulation. As one of the strain engineering methods, thermal annealing, however, is not yet being widely adopted by the majority due to the thermal instability it induces. The thermal stability of GeSn is highly sensitive to initial material conditions, consequently thorough investigations are still demanded with different purposes. A detailed investigation on the thermal annealing effects of thick GeSn layers with a nominal 8% Sn grown on Ge-buffered Si (001) substrate by molecular beam epitaxy is presented here. Atomic force microscopy and high-resolution x-ray diffraction were used to trace the change of GeSn surface morphology and the strain relaxation after annealing. It is confirmed that the tetragonal compressive strain in GeSn, which is a proven detriment to the realisation of direct-bandgap material, can be relaxed by 90% while improving crystal quality, e.g. reduced surface roughness by appropriate annealing conditions. These findings reveal the potential of annealed GeSn to serve as a much thinner (750 nm), better lattice-matched to GeSn active layer and highly strain-relaxed platform to grow GeSn on compared to the thick Ge or the compositional-graded (Si)GeSn buffer layers, which are complicated and time-consuming in growth procedures and also securing an easier approach.

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.

Temperature-Dependent Photoluminescence Studies of Ge1−ySny (y = 4.3%–9.0%) Grown on Ge-Buffered Si: Evidence for a Direct Bandgap Cross-Over Point

The temperature (T)-dependent photoluminescence (PL) from Ge1−ySny (y = 4.3%–9.0%) alloys grown on Ge-buffered Si substrates was studied as a function of the Sn content. The PL from Ge1−ySny alloys with high Sn contents (≥7.0%) exhibited the typical characteristics of direct bandgap semiconductors, such as an increase in the PL intensity with decreasing T and a single PL peak corresponding to a transition from the direct bandgap (Γ-valley) to the valence band at all temperatures from 10 to 300 K. For the Ge1−ySny alloys with low Sn contents (≤6.2%), the PL emission peaks corresponding to both the direct bandgap (ED) and the indirect bandgap (EID) PL appeared at most temperatures and as T was increased, the integrated PL intensities of ED initially increased, then decreased, and finally increased again. The unstrained ED and EID energies estimated from the PL spectra at 75 and 125 K were plotted as functions of the Sn concentration, and the cross-over point for unstrained Ge1−ySny was found to be about 6.4%–6.7% Sn by using linear fits to the data in the range of Sn contents from 0% to 9.0%. Based on the results at 75 and 125 K, the cross-over Sn concentration of unstrained Ge1−ySny should be about 6.4%–6.7% Sn content at room temperature. The ED energies of the Ge0.925Sn0.075 alloys were estimated from the T-dependent photoreflectance spectra, and the ED values was consistent with those obtained from PL spectra.

UHV-CVD growth of high quality GeSn using SnCl4: from material growth development to prototype devices

The persistent interest of the epitaxy of group IV alloy GeSn is mainly driven by the demand for an efficient light source that could be monolithically integrated on Si for mid-infrared Si photonics. For chemical vapor deposition of GeSn, the exploration of the growth window is difficult from the beginning due to the metastable nature of the material requiring non-equilibrium growth condition. In this work, we demonstrated an effective pathway to achieve high quality GeSn with high levels of Sn incorporation. The GeSn films were grown on Ge-buffered Si via ultra-high vacuum chemical vapor deposition using GeH4 and SnCl4 as the precursor. The influence of both SnCl4 flow fraction and growth temperature on the Sn incorporation and material quality were investigated. Different growth regimes were explored leading to an optimized regime at low temperature which suppressed the Sn precipitation allowing for increased Sn incorporation. The prototype GeSn photoconductors were fabricated with typical samples, which show the promising device applications.

Amplified Spontaneous Emission from Gesn Waveguides at Room Temperature

Silicon is the basis for a multibillion dollar industry and has been the preferred platform for integrated electronics for decades. Unfortunately, the band structure of Si has limited its use in infrared applications. The 1.12 eV band gap restricts the range of Si photodetectors to wavelengths of 1100 nm or lower, and the indirect nature of the band gap has prevented the realization of a Si-based laser. Although there are devices available that cover these wavelength ranges, they are typically made from exotic materials such as InGaAs or HgCdTe. These materials are incompatible with Si processing, so the device cannot be integrated with other Si-based electronics on the same chip. Furthermore, many of the devices require cryogenic temperature operation. These unfortunate conditions mean that the devices and systems that use them are prohibitively expensive, which prevents their widespread adoption for both military and commercial applications. Si-compatible materials that have desirable optical properties beyond the band gap of silicon could produce inexpensive devices and systems with applications in telecommunications, low-light imaging, targeting and acquisition, photonic integrated circuit. A number of approaches have been attempted for producing infrared devices on silicon, but they have achieved limited success. The integration of III-V materials on Si has been an active research area for a number of years, and it has yielded limited success. The flip chip wafer bonding and packaging process still requires a separate facility for production of the III-V material, so the costs cannot be brought down to the level that would be achieved with a Si-compatible material. Another approach has been to use other Group IV materials that are grown epitaxially on Si. SiGe alloys were explored for this purpose, but for the concentrations of Ge that can readily be achieved, the band gap still does not extend far enough into the infrared. Furthermore, SiGe is still an indirect band gap material, so it is not suitable for producing a laser. Pure Ge grown epitaxially on Si has also been a recent area of research, and this has resulted in some success. Detectors operating at 1550 nm have been achieved by using tensile strain to lower the direct band gap to enhance absorption at that wavelength, but this approach cannot be extended to further wavelengths due to the limits of achievable strain. Tensile-strained Ge has also been used to demonstrate optically- and electrically-pumped lasers. A new class of Group IV materials that are grown on Si or Ge-buffered Si is the GeSn alloy system. Pure Ge is an indirect band gap semiconductor, but the difference between the conduction band minima at the L- and Γ-points is only 140 meV. The diamond cubic structure of Sn, α-Sn, is a semimetal. When Sn is substituted for Ge in the Ge lattice, a hybrid band structure is formed, yielding a material with a tunable band gap. For Sn concentrations around 10% or lower, the band structure is still Ge-like with the lowest conduction band minimum at the L-point. However, the energy difference between the L- and Γ-minima reduces with increasing Sn concentration. For higher Sn concentrations, the minimum at the Γ-point becomes the lowest minimum, thereby creating a direct band gap semiconductor. In this talk, I will discuss our efforts to enhance optical emission in GeSn through the use of waveguide architectures and the introduction of n-type dopants. Our results obtained from optically-pumped waveguides indicate that we have achieved amplified spontaneous emission, where we see an exponential dependence of output power versus input power, but we do not see a narrowing of the optical emission spectrum. Instead, the entire spontaneous emission spectrum is amplified. This does produces some evolution in the spectrum, and we attempt to understand the emission spectrum through mathematical and physical modeling.

Comparative Analysis of Ni- and Ni0.9Pt0.1-Ge0.9Sn0.1 Solid-State Reaction by Combined Characterizations Methods

GeSn-based alloys present a novel pathway towards the monolithic integration of light sources in mid infrared CMOS-compatible Si photonic circuitry [1]. Semiconductor optoelectronic devices need efficient metallic contacts to receive and deliver power and signal. Ni-stanogermanides (Ni-GeSn) have been proposed as a promising candidate material to reach both low specific contact (Rc) and sheet resistance (Rsh) [2,3] to contact efficiently GeSn-based photonic or electronic devices. However, a low thermal stability of these layers has been evidenced, together with compound agglomeration and segregation [4]. As a solution to these problems Wang et al. showed that the co-sputtering of Ni and Pt extended the thermal stability of the stanogermanides to higher temperatures. Thus leading to a broader window for stanogermanidation and suppressing, to some extent, agglomeration and degradation [4]. In this work, we propose a comparative and comprehensive study of the solid-state reaction of Ni and Ni0.9Pt0.1 films with compressively-strained, 60 nm-thick Ge0.9Sn0.1 layers, epitaxially grown on Ge-buffered Si (100) substrates. Prior to metallization, surface preparation was performed with a 1 min dip in 1% diluted HF sequentially rinsed in deionized water and dried with a N2 gun. Then, 10 nm of Ni or Ni0.9Pt0.1 were deposited by physical vapor deposition and capped with 7 nm of TiN. The Ni and Ni0.9Pt0.1-Ge0.9Sn0.1 phase sequence and crystalline evolution were monitored by in-situ x-ray diffraction (XRD) using an Empyrean PANalytical X-ray diffractometer equipped with a copper (Cu Kα) source, a linear detector (PIXcel1D) and a HTK 1200 Anton Paar furnace working under secondary vacuum. Profiles were acquired from 50 to 600 °C, with 5 °C steps and a scanning time of 7 min per temperature step. The phase analysis was complemented by ex-situ in-plane reciprocal space map with a Rigaku SmartLab X-ray diffractometer. Additionally, samples metallized with the same procedure were ex-situ annealed for 30 s at temperatures ranging from 150 up to 550 °C (with 50 °C steps), in a rapid thermal annealing (RTA) treatment in a N2 environment. The evolution of the surface morphology and the electrical properties were measured by atomic force microscopy (AFM) in Tapping mode with a Bruker FastScan equipment and sheet resistance (Rsh) with a four-point probe meter. AFM and Rsh evidenced that Pt addition improves both morphological and electrical properties of the layers at high temperature. The addition of Pt prevents the surface Root-mean-square (RMS) roughness from increasing with temperature (Figure 1) and postpones morphological surface degradation. Concerning the electrical properties (Figure 2), a plateau where sheet resistance remains stable and low is obtained with Pt addition between 350 and 450 °C. Meanwhile, a sudden increase of Rsh values for Ni-system is observed, this immediately after obtaining the minimum values (9.1 Ω/sq at 350 °C). XRD results for the Ni-Ge0.9Sn0.1 system (Figure 3) reveal a sequential growth in which the first phase appearing corresponds to a Ni-rich phase, i.e. Ni5(Ge0.9Sn0.1)3. Then, at 245 °C, the mono-stanogermanide phase Ni(Ge0.9Sn0.1) is observed; this latter is stable up to 600 °C. The addition of 10 at.% of Pt in Ni thin films (Figure 4) modifies the reaction kinetics, evidenced by the delay in Ni consumption together with a snowplow phenomenon. Additionally, it strongly affects the phase formation sequence promoting PtSnx compound formation and β-Sn segregation as well as a preferential out-of-plane orientation of the mono-stanogermanide phase. Further, the Ni0.9Pt0.1 mono-stanogermanide is unstable and a destabilization of the system is observed to the benefit of the Ni mono-stanogermanide phase, around the 345 – 355 °C temperature range. A summary of the phase formation sequence for both systems is given in Figure 5. The potential impact on device integration of these observations will be discussed. Even though the addition of Pt complicates the solid-state reaction, it was observed that its beneficial impact on surface morphology has an important role for maintaining the integrity of the electrical properties.

IR-Photodetector Fabrication on Suspended Gesn Thin Layers

The energy bandgap (Eg) of Ge effectively shifts toward lower energies by alloying with Sn atoms. Strain-relaxed (SR) GeSn layers with more than 8% Sn content assumes a direct bandgap and can be potentially used for highly-sensitive photodetectors for the short-wavelength infrared (SWIR) region (Eg<0.6 eV). However, the epitaxial growth of SR GeSn alloys with high crystalline quality is challenging due to the large lattice mismatch with both Si and SR Ge [1]. One method for fabricating SR GeSn is based on the growth of compressively strained GeSn layers on SR Ge and releasing the strain by selective Ge under-etching [2]. In this study, a suspended IR-photodetector was fabricated using strain-relaxed Ge0.9Sn0.1 layer through microstructuring and Ge under-etching. The GeSn layers were grown on a Ge-buffered Si wafer [3] in a reduced pressure chemical vapor deposition tool using digermane and tin tetrachloride as Ge and Sn precursors. Two batches of detectors were fabricated; one based on GeSn on Ge and the other one based on suspended GeSn layers. Photolithography and ICP dry etching were used for patterning the detectors and the Ge underneath were removed by anisotropic selective etching using CF4 gas. XRD and RBS measurements indicate Sn contents of more than 10% without any Sn out diffusion. Raman spectroscopy shows the strain relaxation through Ge under-etching and layer release. The temperature dependence dependent photoluminescence spectra shown in Fig. 1 is in accordance with a direct band gap transitions of the GeSn layers around 0.5 eV. The effects of layer suspension on the absorption spectra and the photo response of the detectors will be discussed in the presentation. Fig. 1) Temperature dependence photoluminescence spectra of GeSn layer with 12% Sn content Acknowledgements: This work was supported by the Swedish Foundation for Strategic Research (SSF) and Vetenskapsrådet (VR).

Investigation of hydrogen inductively coupled plasma treatment effect for Ge0.938Sn0.062/Ge/Si film using photoreflectance spectroscopy

Temperature-dependent photoreflectance (PR) measurements for Ge0.938Sn0.062/Ge/Si films treated with a hydrogen inductively coupled plasma (H2-ICP) have been performed. The Ge0.938Sn0.062 film is grown on Ge-buffered Si substrate by ultra-high vacuum chemical vapor deposition method, and the H2-ICP treatment was carried out for 5min for the Ge0.938Sn0.062 epitaxial layer. The high-resolution X-ray diffraction results show that the compressive strain of the Ge0.938Sn0.062 layer decreases and the tensile strain of the Ge buffer layer increases after H2-ICP treatment. The PR spectrum of as-grown Ge0.938Sn0.062/Ge/Si film at 300K consists of three signals at 0.603, 0.782 and 0.814eV, which are assigned to the direct transitions from conduction Γ valley to the valence bands related to the surface of Ge0.938Sn0.062, the Ge/Si and GeSn/Ge interfaces, respectively. After H2-ICP treatment, two PR signals of 0.604 and 0.781eV were obtained at 300K and they are attributed to the direct transition energies of the Ge0.938Sn0.062 and the Ge buffer layer, respectively. As temperature decreases, new weak PR signals appear in the lower energy regions of both PR signals from the H2-ICP treated GeSn and Ge layers at 210K and 130K, respectively, due to the increase of tensile strain in Ge layer while no new signal appears for the as-grown sample. The PR spectrum of the H2-ICP treated sample at 10K shows four signals, and these signals are assigned to the surface of GeSn and GeSn at the interface between GeSn and Ge buffer layers, the Ge at the interface between GeSn and Ge buffer layers, and the Ge at the interface between Ge buffer layer and Si substrate, respectively. These PR results indicate that H2-ICP treatment significantly affects the strain and defects near the interface between GeSn and Ge layers, and thus optical properties of GeSn layer are also altered by H2-ICP treatment.

Study of material and optical properties of Si_xGe_1-x-ySn_y alloys for Si-based optoelectronic device applications

A series of SiGeSn alloy samples with various Si and Sn compositions and thicknesses were grown on Ge-buffered Si substrates. The growth was conducted by using low-cost commercially available silane and germane precursors in a standard industrial reduced pressure chemical vapor deposition reactor. Si and Sn compositional- and film thickness-dependent material and optical properties have been characterized using X-ray diffraction (XRD), Raman, photoluminescence (PL), and ellipsometry spectroscopies. Moreover, thermal stability in harsh growth environment, such as in subsequent III-V growth, was studied for future multi-junction solar cell applications. In situ rapid thermal annealing at 650°C was conducted to investigate the enhanced material quality and direct bandgap emission, which were confirmed by XRD, transmission electron microscopy, Raman, and PL measurements.

Sn-based waveguide p-i-n photodetector with strained GeSn/Ge multiple-quantum-well active layer.

We report on Sn-based p-i-n waveguide photodetectors (WGPD) with a pseudomorphic GeSn/Ge multiple-quantum-well (MQW) active layer on a Ge-buffered Si substrate. A reduced dark-current density of 59 mA/cm2 was obtained at a reverse bias of 1V due to the suppressed strain relaxation in the GeSn/Ge active layer. Responsivity experiments revealed an extended photodetection range covering the O, E, S, C, and L telecommunication bands completely due to the bandgap reduction resulting from Sn-alloying. Band structure analysis of the pseudomorphic GeSn/Ge quantum well structures indicated that, despite the stronger quantum confinement, the absorption edge can be shifted to longer wavelengths by increasing the Sn content, thereby enabling efficient photodetection in the infrared region. These results demonstrate the feasibility of using GeSn/Ge MQW planar photodetectors as building blocks of electronic-photonic integrated circuits for telecommunication and optical interconnection applications.