Ge Virtual Substrates Research Articles

Noise Analysis of Group IV Material-Based Heterojunction Phototransistor for Fiber-Optic Telecommunication Networks

This paper presents an analytical study of the noise behavior and spectral responsivity of Ge1− x Sn x alloy-based heterojunction phototransistors (HPTs) for different Sn compositions and base doping concentrations. The structure consists of n-Ge/p+ − Ge1− x Sn x /n− − Ge1− x Sn x layers pseudomorphically grown on Si substrate via Ge virtual substrate, compatible with complementary metal-oxide-semiconductor technology. The high absorption coefficient of the alloy in base significantly improves signal-to-noise ratio in the high-frequency region, leading to efficient photodetection in the $C$ - and $L$ -bands of fiber-optic telecommunication window. A comparison is made between the theoretical findings for GeSn HPTs and the available data for currently employed group III–V compound-based HPTs, to examine if GeSn-based HPT can be a good alternative.

Growth and characterization of SiGeSn pseudomorphic layers on 200 mm Ge virtual substrates

In this contribution, we report on the growth of pseudomorphic SiGeSn layers on 2.5 μm thick Ge virtual substrates on Si(001). A single wafer reduced pressure chemical vapor deposition reactor was used for the 100 Torr epitaxy of those layers, with digermane (Ge2H6), tin-tetrachloride (SnCl4) and disilane (Si2H6) as precursors. Detailed analyses regarding the growth kinetics, layer composition and surface morphology as function of the growth temperature and Si2H6 flow are presented. As the temperature increases we have, for the same precursor flows, a Si content increase and a Sn content decrease. The activation energy associated with the exponential increase of the SiGeSn growth rate with the temperature, 9.5 kCal.mol−1, is close to that of GeSn (10.6 kCal.mol−1). Furthermore, we show that the addition of Si2H6 to the gaseous mixture results (besides the expected Si content increase) in a Sn content increase and a Ge content decrease. This study yields a better understanding of the growth of SiGeSn compound semiconductors.

Epitaxial GeSn: impact of process conditions on material quality

The electrical and optical material properties of epitaxial Ge1−xSnx and SiyGe1−x−ySnx are of high interest for novel device applications. However, the limited Sn solubility in Ge makes the epitaxial growth of Ge1−xSnx and SiyGe1−x−ySnx challenging. Most of the literature describing the epitaxial growth is for Ge2H6 and SnCl4 as Ge and Sn precursors, respectively. A more recent publication deals with the epitaxial growth of high-quality Ge1−xSnx with the more conventional GeH4. In this manuscript, we compare the structural and optical material quality of Ge1−xSnx, epitaxially grown on Ge virtual substrates as a function of growth pressure, growth temperature, the choice of the carrier gas (H2 or N2) and the choice of the Ge precursor (GeH4 versus Ge2H6). The best material quality in terms of surface morphology and photoluminescence characteristics is obtained if GeH4 is used as a Ge precursor. For Ge1−xSnx grown with Ge2H6 and at atmospheric pressure, pyramidical defects can be seen and there is a risk for uncontrolled local Sn agglomeration. The pyramidical defects are not observed on Ge1−xSnx layers grown at reduced pressure, but the highest achievable substitutional Sn concentration is lower. No pyramidical defects are found for Ge1−xSnx layers grown with GeH4 and the issue of uncontrolled local Sn agglomeration does not appear.

MBE-Grown SixGe1-x- ySny Diode and Quantum Well Diode Structures with High Sn Content for Optical Applications

Recent years have seen an increasing interest in Group IV photonics approach for monolithic integration of optoelectronic devices on the Si platform. While passive and active devices such as photodetectors [1,2], modulators [3–5], waveguides and splitters have been successfully realized on-chip, light sources are the last missing key component. Ever since the optically pumped Ge1-zSnz laser was demonstrated experimentally [6], this binary alloy presented itself as a direct Group IV semiconductor at high Sn concentrations. However, the growth of Ge1-zSnz is challenging due to the lattice mismatch to Si and Ge, which limits the layer thickness as well as the crystal quality. Furthermore, solid solubility of Sn in Ge is limited, making it difficult to achieve high Sn concentrations. The ternary alloy SixGe1-x-ySny is particularly interesting in this context because it allows to decouple its bandgap from its lattice constant, which enables e.g. the lattice matched growth on Ge at a given Si to Sn ratio. Moreover, SixGe1-x-ySny can have a larger bandgap than Ge, which makes it interesting as cladding material. The ternary alloy, therefore, is interesting for the growth e.g. of double hetero (DHS) as well as quantum well structures (QW) with Ge1-zSnz as the well material. We have recently investigated the molecular beam epitaxy (MBE) based growth of SixGe1-x-ySny bulk alloys lattice matched to Ge with very high Sn content up to 12,5 % [7,8] Here, we will highlight the opportunities and difficulties of lattice matched growth of Sn-rich SixGe1-x-ySny layers by MBE for SixGe1-x-ySny pin diodes. We doped SixGe1-x-ySny with B as well as Sb, achieving a total layer thickness of up to d = 900nm on a Ge virtual substrate (VS). Furthermore, we developed a growth strategy for SixGe1-x-ySny QW structures, where doped SixGe1-x-ySny layers work as cladding for a Sn-rich Ge1-zSnz quantum well, with the aim of combining the advantages of lattice matched growth on Ge with the direct semiconductor behaviour of Ge1-zSnz. We discuss material and device characterization results of a series of three SixGe1-x-ySny pin diodes with variable intrinsic zone and a total layer thickness of 900 nm (Fig. 1 (a)). The growth process takes place in a 6” MBE system, where Si, Ge and Sn are used as matrix materials and B and Sb as dopants respectively. The growth rate of the SixGe1-x-ySny layers is calibrated and kept at R = 1 Å/s. The condition of lattice matched growth of SixGe1-x-ySny (i.e. x/y = 3.67) requires highly precise calibration of the alloy compound fluxes, which is done in a complex multi-step method. The most critical growth parameter is the growth temperature, which is therefore measured and controlled in-situ by a thermocouple as well as two pyrometers. While a lower temperature allows better suppression of the Sn segregation, it can also result in a reduced crystal quality. For our samples, we chose to keep the substrate temperature at TSub = 200 °C for the growth of the ternary alloy, which is mainly based on our experience from previous SixGe1-x-ySny growth studies. In our pin diode device layers, the intrinsic zone is sandwiched between p- and n-doped SixGe1-x-ySny layers. While simple co-evaporation of B was used for p-type doping, the n-doped top contact layer was grown as follows: after the growth of the intrinsic zone with a thickness of 300 nm the growth was interrupted to deposit a pre-built up of Sb (≈ 1012 cm-2) for the following n-SixGe1-x-ySny contact in order to counterbalance the segregation of Sb [9,10]. In order to obtain detailed information of the composition as well as the crystal quality, the samples were analysed by X-Ray diffraction (XRD) and Raman spectroscopy. The reciprocal space map (RSM) of sample A (Fig. 1 (b)) shows that very good lattice matching was achieved for the SixGe1-x-ySny layers. After MBE growth the samples were processed to devices in a CMOS compatible single mesa process. Our samples clearly show diode behaviour with dark current densities as low as jD = 0.1 A/cm2 for sample A (Fig. 1 (c)). We present detailed results on the electrical and optical characterization of our devices. Our results show that MBE based growth not only enables the fabrication of SixGe1-x-ySny bulk alloys with high Sn content, it can also be used to dope the ternary alloy as a prerequisite for the fabrication of SixGe1-x-ySny diodes that contain single or multiple quantum well structures with high-Sn content Ge1-zSnz. We discuss possible improvements on growth strategies as well as future steps for the investigation of MBE-grown Sn-based light emitters on Si. Figure 1

Investigation of the Growth of Si-Ge-Sn Pseudomorphic Layers on 200 mm Ge Virtual Substrates: Impact of Growth Pressure, HCl and Si2H6 Flows

The implementation of new materials on Si substrates has been a subject of intensive research since decades. Recently, group IV germanium tin (GeSn) and silicon germanium tin (SiGeSn) have triggered huge scientific interest due to the unique features of these materials [1,2]. However, their epitaxial growth is a complex task due to the low solid solubility of Sn in Ge and the limited thermal stability of such alloys. The aim of this work was first to set up a relatively fast and non-destructive experimental protocol to characterize SiGeSn ternary layers by combining X-ray diffraction (XRD) with wavelength dispersive X-ray fluorescence (WDXRF). The surface morphology of the grown layers was otherwise assessed through atomic force microscopy (AFM). Afterwards, different growth conditions were tested in order to optimize growth conditions. Binary and ternary alloys were grown on 2.5 µm thick strain relaxed Ge buffers on 200 mm Si(001) substrates in a reduced pressure chemical vapor deposition reactor from Applied Materials. Close to 30 nm thick GeSn and GeSi pseudomorphic layers were characterized first through XRD. Figure 1 shows the omega-2theta scans around the symmetric (004) reflections. Through these scans, the layer thickness and composition could be calculated. Afterwards, WDXRF measurements were carried out on the same samples and the intensity of the specific element drawn as function of the layer thickness multiplied by this element composition in the layer, as shown in Figure 2. The fitting lines served afterwards to find the composition of each element in the SiGeSn alloys. Ternary alloys were then grown at 349°C and 100 Torr using Si2H6, SnCl4 and Ge2H6 precursors and H2 as a carrier gas. In that first set of samples, we investigated the influence of the Si2H6 flow. Thus, we varied the F(Si2H6 )/F(H2) mass-flow ratio from 0 up to 5.2×10-3. Figure 3 shows the omega-2theta scans around the (004) XRD order for the various SiGeSn/Ge/Si(001) heterostructures grown. Interestingly, the SiGeSn XRD peak did not significantly shift even though the Si2H6 flow varied considerably. Using Figure 2 XRF calibration curves and the layer thickness extracted from XRD scans, the concentration of Si and Sn in the layers could be calculated. Resulting values are shown as functions of the F(Si2H6)/F(H2) mass-flow ratio in Figure 4. As expected, the increase of the Si2H6 flow leads to an increase of the Si content in the layers (from 0% up to nearly 21%). However, this increase is accompanied by an increase of the Sn content in the layer, from ~ 5% up to ~ 11% (Figure 4). This likely explains the stability of the SiGeSn peak’s angular position in XRD profiles. The initial compressive strain in the layer (for Si2H6 = 0 sccm i.e. for the GeSn layer) is kept despite Si2H6 flux variations. Figure 5 shows the growth rate as a function of the F(Si2H6)/F(H2) mass-flow ratio. There is a clear drop of the growth rate when the Si2H6 flux increases (from ~ 32 down to ~ 20 nm min.-1). The surface morphology was otherwise assessed using AFM. Results are provided in Figure 6 and Figure 7. The AFM images highlight that the surface roughness increases when the Si2H6 flux increases. Above a certain F(Si2H6)/F(H2) value, a catastrophic degradation of the surface morphology occurred which could be related to Sn segregation (cf. Figure 6f). In order to better understand the growth of ternary SiGeSn pseudomorphic layers, we have explored a wide range of growth conditions. First, the growth temperature was varied from 313°C to 409°C in 12°C steps, all the other parameters being fixed (pressure (100 Torr), fluxes…). This study allowed us to determine the optimal growth temperatures. Afterwards, three growth temperatures were selected (349°C, 337°C and 325°C) and the Si2H6 flow varied, while keeping constant the SnCl4 and Ge2H6 flows. The layers grown under these conditions are currently under investigations and detailed results will be presented during the conference. [1] S. Wirths et al, Nat. Photonics 9 (2015) 88. [2] S. Wirths et al, Thin Solid Films 557 (2014) 183. Figure 1

Structure and Optoelectronic Properties of Atomically Random Sn-Rich Gesn Semiconductors

The monolithic integration of group IV optoelectronic devices on silicon platform is a long-sought-for paradigm holding the key to a wealth of opportunities in ultrafast data transfer, low-power electronics, energy conversion, and sensing, to name a few. This all-group IV integrated photonics has recently gained a renewed interest with the re-emergence of direct band gap germanium-tin (GeSn) semiconductors. GeSn layers with Sn contents above 10% are commonly grown on a Ge virtual substrate (VS) on Si wafers. In recent years, digermane (Ge2H6) has been the canonical precursor to accomplish the growth of various Sn-rich GeSn layers and heterostructures.[1,2] However, Monogermane (GeH4) has recently been proposed as an alternative precursor for large-scale, CMOS-compatible growth of GeSn semiconductor lasers.[3-4] Here, we present the the growth and properties of metastable Sn-rich GeSn layers from the wafer-level down to the atomic scale.[5] The chemical vapor deposition (CVD) growth of GeSn multi-layer structures with Sn content in the 7-18% range was performed on a ~600 nm-thick Ge layer on 4-inch silicon wafers using Monogermane (GeH4) and tin-tetrachloride (SnCl4) precursors. The Sn incorporation is controlled by the change in temperature, ranging from 320°C, 300 °C, and 280 °C for the bottom (BL), middle (ML), and top (TL) GeSn layers, respectively. This leads to a BL/ML/TL stacking in Fig. 1a with Sn contents of 8.2%/11-13.5%/18%. Using transmission electron microscopy (TEM) measurements a TL/ML/BL GeSn layers stacking with a total thickness of ~380 nm grown on a ~600 nm-thick Ge-VS is observed. No threading dislocations propagating toward the GeSn TL are observed, while misfit and edge dislocations are confined at the Ge-GeSn interface and within the first 60-70 nm of the GeSn stacking. Herein, the lower Sn content BL and ML seem to minimize or even suppress the propagation of dislocations towards the defect-free 18% Sn TL. Atom probe tomography (APT) measurements are performed to establish an atomistic-level understanding of the evolution of Sn content across the grown layer. The APT profile for the Sn incorporation in Fig. 1b reveals the presence of abrupt interfaces between the different GeSn layers, with the highest incorporation of 18% in the TL. To provide details regarding the short-range atomic distribution on the scale of few lattice constants a series of four different statistical APT analyses will be discussed: frequency distribution analysis; partial radial distribution function; nearest-neighbor analysis; and iso-concentration surfaces.[5-6] The APT analysis demonstrates the growth of a GeSn random alloy, with a perfect random distribution of Sn atoms within the grown layers, and without the presence of short-range ordering effects and Sn precipitates. Interestingly, the defect-free highest incorporation of Sn is achieved at a growth rate (~1.3 nm/min) that is one order of magnitude lower than other studies where either GeH4 or Ge2H6 precursors were used.[1-3] It is clear that the use of GeH4 allows for additional set of growth parameters were a high incorporation of Sn still be achieved with a drastic decrease reduction (one order of magnitude) in the growth rate. The presence of multiple experimental growth conditions with extremely different growth rates suggests that the growth rate is a marginal factor in the Sn incorporation process, whereas the strain relaxation using lower Sn content buffered layers (BL/ML) will be discussed as a key factor in enhancing the incorporation of Sn. We therefore highlight the importance of reducing strain at the growth surface while keeping a low growth temperature, which can enhance the incorporation of Sn. This behavior will facilitate the growth of GeSn multilayer structures enabling more advanced strain and band structure engineering. The sharp interfaces across GeSn layers pave the way for achieving a high degree of control of the growth of GeSn-based multi-quantum wells heterostructures for mid-IR sources and detectors. Furthermore, by combining theoretical calculations and experiments, the presentation will also discuss the influence of structure and composition on the optoelectronic properties of the as-grown GeSn layers. Acknowledgements The authors thank J. Bouchard for the technical support, and NSERC Canada (Discovery, SPG, and CRD Grants), Canada Research Chair, Canada Foundation for Innovation, Mitacs, FQRNT, and MRIF Québec for support.

Decoupling Strain and Composition Effects on Ge1-YSny Lattice Vibrations

Probing the behavior of vibrational modes in semiconductors is central to understand their basic structural, optical, and phononic properties. This is particularly true for the emerging GeSn and SiGeSn. This family of group-IV semiconductors is of growing interest for optoelectronic applications, notably due to the possibility of tuning the bandgap to cover a broad range from the short-wave infrared to the mid-infrared. An indirect to direct transition in GeSn alloys is expected for Sn incorporation higher than 9%, which can lead to the development of high efficiency light emission and detection devices. The growth of such alloys has however proven challenging due to the low solubility of Sn in Ge (≈1%). In addition, any residual strain means more tin incorporation is required to obtain a direct-bandgap [1]. The alloying process enables tuning the electronic and optoelectronic properties, but it also comes with enhanced lattice disorder thus affecting the lattice vibrational modes. Raman spectroscopy is commonly used to assess the role of composition y and strain ε in shaping the lattice vibrations. Curiously enough, previous works on GeSn mainly focused on analyzing the behavior of Ge-Ge mode. This is due to the broad use of 488 nm [2] or 532 nm [3], [4] excitation lines, which yield a very weak signal of the Ge-Sn mode. Alternatively, the use of a 633 nm excitation enables a clear detection of all Raman modes in GeSn layers independently of their composition. This is plausibly attributed to the fact that this wavelength might be close to resonance with the alloy’s E1 gap [5]. There are very few reports on the identification of the Ge-Sn mode [6], but quantitative analyses of the effects of composition and strain on this mode remain conspicuously missing in literature. With this perspective, this work presents a detailed study of Raman vibrational modes in Sn-rich (7-18 at.%) GeSn semiconductors. Samples were grown in a low-pressure chemical vapor deposition (LP-CVD) reactor using monogermane and tin-tetrachloride precursors. The samples consist of one or two GeSn layer(s) on a Ge virtual substrate (VS) on a Si wafer. Using X-ray diffraction (XRD), reciprocal space mappings were performed on all samples to retrieve the composition and the strain. Raman measurements have been carried out on an InVia Microscope from Renishaw with a 633 nm laser. While Voigt or Lorentzian functions are commonly used for fitting peaks, they cannot reproduce the asymmetric broadening typical to GeSn Raman peaks (Fig. 1). This asymmetry is due to alloying as substitutional Sn atoms break the translational symmetry and lead to a relaxation of the momentum selection rule [7]. To perform the fits, we are therefore using exponentially modified gaussian (EMG) functions, which can better reproduce the line shape of the Raman modes. A typical Raman spectrum appears in Fig. 1. Two-dimensional linear regressions are then performed to decouple the influence of strain and composition on the Raman shift of both modes. The resulting functions are illustrated in Fig. 2 and the coefficients appear in table 1. The planar fits accurately represent the distributions, as confirmed by the relatively small error on both a and b parameters (<10%) and the coefficients of determination near 0.99. Furthermore, the calculated Raman shift of the Ge-Ge mode in the limit of a pure and completely relaxed Ge layer is equal to the value obtained for bulk germanium, and the a and b parameters are comparable to those found in earlier studies for the Ge-Ge mode. Based on these detailed Raman studies, an exhaustive discussion of the influence of lattice strain and Sn content on GeSn vibrational modes will be presented. Acknowledgements The authors thank J. Bouchard for the technical support, and NSERC Canada (Discovery, SPG, and CRD Grants), Canada Research Chair, Canada Foundation for Innovation, Mitacs, FRQNT, Institut de l’énergie Trottier and MRIF Québec for support.

Mapping Strain and Composition Effects on Gesn Band Structure Using Spectroscopic Ellipsometry

One of the most interesting research areas in silicon photonics has been the development of silicon-compatible electro-optical devices including detectors, modulators, and light sources, to name a few. IN this regard, the binary alloy Ge1-xSn x has been of particular importance due its bandgap directness in contrast to silicon and germanium.[1-2] As a matter of fact, great efforts have been expended in recent years towards the development of the epitaxial growth of high-quality GeSn crystals on silicon platform and their introduction in design and fabrication of optoelectronic devices.[3] This interest has been nurtured by the ability to manipulate the GeSn band structure by controlling Sn composition and strain. These two degrees of freedom provide flexibility to tune the optoelectronic properties relevant to a variety of devices. For instance, GeSn is attractive material system for photodetectors with absorption edge extending over a broad wavelength range from the short wavelength infrared (SWIR, 1.6-2 µm) to the mid-infrared (MIR, 2-5 µm). In this perspective, this work reports detailed investigations of the influence of both strain and composition on the band structure above 1 eV of GeSn ternary alloys through a spectroscopic ellipsometry study. Understanding the individual influence of each parameter is highly critical to establish and optimize the properties of GeSn device layers. GeSn thin films investigated in this work were grown using a low-pressure Chemical Vapor Deposition (LP-CVD) at different Sn compositions in the 7-18 at.% range.[4] A graded growth process on Ge-virtual substrates was used. Figure A shows a typical example of the investigated samples. The figure exhibits a schematic representation where the bottom layer (BL- 6.3% of Sn) and the top layer (TL- 12.5% of Sn) have different Sn composition. Furthermore, High-Resolution X-Ray Diffraction (HR-XRD) was used to characterize the compressive strain present in each layer. In the sample shown in Figure A, the strain was found to be respectively equal to -0.65% and -1.503%, for the BL and TL. Next, a rotating-analyzer spectroscopic ellipsometry system measures the following parameters (Ψ and Δ) for different incidence angles from 45 to 70° as shown in Figure B. These parameters are then coupled with an optical model to allow for an accurate determination of the complex dielectric (ε=ε1+iε2) of the sample. The developed optical model is presented in Figure A where the different constituting layers are shown: GeSn(BL/ML)/ Ge(VS)/Si. Furthermore, two additional surface layers were introduced to simulate the surface roughness as well as the presence of the native GeO2 oxide. The optical properties of the GeO2 oxide were used in their tabulated form from Reference [5]. The optical model is shown as a dashed line in Figure B, and the accuracy is measured by a very low MSE of 0.598. Consequently, the dielectric constant can be extracted. Having extracted the dielectric constant from the ellipsometry measurement, it becomes now possible to quantify the contributions from the E1, E1 + Δ1, E0’, E2, and E1’ critical points in the joint density of electronic states which they will be enhanced by computing numerical second derivatives of the already measured dielectric function. The numerical second derivative is often coupled with the Savitsky-Golay smoothing filter to reduce noise while maintaining the shape and the height of waveform peaks. The resulting lineshapes were fitted with model expressions from which the critical point energies Ej, amplitudes, broadenings Aj, and phases ϕj were determined. The model lineshapes have been well established in literature. [6] In Figure C, a lineshape fit for the E2 critical point energy for the Bottom layer (BL) was undertaken. The accuracy of the fit was confirmed with a coefficient of determination (R2) higher than 0.97. The Levenberg-Marquardt fit gave an E2 energy of 4.06±0.20 eV for the BL, whereas for the Top layer (TL), E2 was equal to 4.10±0.30 eV. After finding the energy for each sample, it becomes possible to map the effect of strain on the bang gap energies. Based on these systematic studies, this presentation will describe the individual influence of strain and composition on the optical properties of Sn-rich GeSn semiconductors.

Optical Pumped Gesn Waveguides Room Temperature Amplified Spontaneous Emission Observation

The successful epitaxial growth of Ge-on-Si has opened a new gateway for Si-based laser research. Ge is not a direct band gap material, however it possible to induce Ge towards direct band emission due to the small band gap energy differences between the direct and indirect. Recently lasing cavities have been successfully engineered by introducing tensile strain to the Ge lattice or through Sn alloying. The two methods have similar effects, lowering both direct and indirect band gaps of Ge with different rate. For a critical or higher Sn content, the direct band can become the lowest gap. This research is motivated by GeSn alloys with high potential to overcome many difficulties such as integration scale limits, excessive cost, extreme operating condition, etc in terms of engineering Si-based lasers. In this work, we explore the properties of optical emission from optically-pumped waveguides fabricated from GeSn alloys with Sn concentrations below the direct band gap transition. The fabrication process of the GeSn waveguides starts with growing undoped Ge buffer layers on Si (100) substrates using gas source molecular epitaxy with Ge4H10 and the wafers were annealed in situ at 650 °C for 3 min. After aqueous HF solution Ge virtual substrates cleaning, flows 5% Ge2H6 in H2 at 30 mTorr for 5 min for additional surface cleaning. Then the mixtures of Ge3H8, SnD4, and P(GeH3)3 (resulting in a concentration of 4.4% Sn in this research study) grows n-type doped GeSn films around 285-335 °C. On the GeSn surface, strip waveguides with width 3 μm and length 4 mm were BCl3 reactive ion etched by using standard UV contact photolithography. With plasma-enhanced chemical vapor deposition, a thickness of 180nm SiO2 was deposited for surface passivation after downright Ge buffer layer to Si interface etching process. For the experimental procedures, the waveguide with both end mirror polished, and one end is Al deposited to form the cavity. The other open end is aligned with a lens fiber as the collection prosses of the output. A 976 nm laser beam is shaped into a line and focused onto the top surface of the waveguide. The signal is collected by an optical fiber and sent through a grating spectrometer with an extended InGaAs detector for spectrum analysis or direct into the detector for power measurements. For spectral analysis each data points are generated by wavelength corresponded grating reflection angle and its amplitude within still pump level and sets of data are measured at different pump levels. The power curve measurements signal amplitude is mapped directly from different pump levels. Room temperature is maintained by using active cooling for all pumping operations. At 4.4% Sn, the material still has indirect bandgap as the lowest band and direct band energy is slightly above. This results in emission curves due to the indirect band gap and the direct band gap at lower output power from the spectrum analysis. It also shows that with increasing the pump level, the ratio between the emission power of the two processes increases and the two emission peaks separate further as the pump power is increased. Meanwhile the emission power from our waveguide versus pump power shows an exponential dependence, indicating the population inversion and optical gain at targeted spectrum reign. In this research the amplified spontaneous emission and its spectrum from optically pumped GeSn waveguide on Si at room temperature has been successfully observed. GeSn with 4.4% Sn content provides promising results as a gain material at mid-IR range.

Investigation of the Growth of Si-Ge-Sn Pseudomorphic Layers on 200 mm Ge Virtual Substrates: Impact of Growth Pressure, HCl and Si2H6 Flows

We have quantified the impact of various process parameters on the growth of thin, pseudomorphic SiyGe1-x-ySnx layers on 2.5 µm Ge buffers (themselves on Si(001) substrates). For GeSn layers, we found that 100 Torr was appropriate for the growth of high crystalline quality layers. The impact of the HCl mass-flow on the growth kinetics of thin Ge1-xSnx layers was also evaluated. Adding HCl retained Ge in the gaseous phase, resulting at 325°C in a growth rate decrease and a Sn content increase. Moreover, the growth of Ge1-xSnx was shown to be selective against SiO2 and SiN-covered Si substrates, even without HCl. Finally, various Si2H6 flows were added to the gas mixture to grow pseudomorphic SiGeSn layers. The quality of these layers was assessed by X-ray diffraction (XRD), wavelength dispersive X-ray fluorescence (WDXRF) and atomic force microscopy (AFM).

Editors' Choice—Epitaxial CVD Growth of Ultra-Thin Si Passivation Layers on Strained Ge Fin Structures

Epitaxially grown ultra-thin Si layers are often used to passivate Ge surfaces in the high-k gate module of (strained) Ge FinFET and Gate All Around devices. We use Si4H10 as Si precursor as it enables epitaxial Si growth at temperatures down to 330°C. C-V characteristics of blanket capacitors made on Ge virtual substrates point to the presence of an optimal Si thickness. In case of compressively strained Ge fin structures, the Si growth results in non-uniform and high strain levels in the strained Ge fin. These strain levels have been calculated for different shapes of the Ge fin and in function of the grown Si thickness. The high strain is the driving force for potential (unwanted) Ge surface reflow during Si deposition. The Ge surface reflow is strongly affected by the strength of the H-passivation during Si-capping and can be avoided by carefully selected process conditions.

In situ phosphorus-doped Ge1−xSnx layers grown using low-temperature metal-organic chemical vapor deposition

We report the metal-organic chemical vapor deposition method for epitaxy of n+-Ge and Ge1−xSnx layer with an electron concentration as high as 2 × 1019 cm−3 by in situ phosphorus (P) doping. In this study, we examined MO precursors of tertiary-butyl-germane (t-BGe), tri-butyl-vinyl-tin (TBVSn), and two kinds of P precursors of tri-ethyl-phosphine (TEP) and tertiary-butyl-phosphine (t-BP). We have investigated crystalline and electrical characteristics of P-doped Ge and Ge1−xSnx layers. In the case of using TEP, the P-doped Ge0.98Sn0.02 epitaxial layer grown at 320 °C on virtual Ge substrate was demonstrated, in which the chemical P-incorporation as high as 1 × 1019 cm−3 and the full electrical activation of P. By using t-BP instead of TEP, the chemical P-incorporation as high as 8.1 × 1019 cm−3 was achieved for Ge epitaxial layer grown at 300 °C. It is found that the activation energy of the growth rate of Ge and P with t-BGe, TEP, and t-BP was estimated to be 1.0−1.2, 2.1, and 1.1 eV, respectively. The precursor combination of t-BGe and t-BP has a wider temperature window for a lower growth temperature. In situ P-doping with t-BP enables us to grow the Ge0.975Sn0.025 epitaxial layer with an electron concentration as high as 1.7 × 1019 cm−3 at the temperature as low as 300 °C. For both cases of TEP and t-BP, we observed that increasing Sn content significantly reduces the P-incorporation, which is not common phenomenon in a conventional CVD growth.

Control of Ge1−x−ySixSny layer lattice constant for energy band alignment in Ge1−xSnx/Ge1−x−ySixSny heterostructures

The energy band alignment of Ge1−xSnx/Ge1−x−ySixSny heterostructures was investigated, and control of the valence band offset at the Ge1−xSnx/Ge1−x−ySixSny heterointerface was achieved by controlling the Si and Sn contents in the Ge1−x−ySixSny layer. The valence band offset in the Ge0.902Sn0.098/Ge0.41Si0.50Sn0.09 heterostructure was evaluated to be as high as 330 meV, and its conduction band offset was estimated to be 150 meV by considering the energy bandgap calculated from the theoretical prediction. In addition, the formation of the strain-relaxed Ge1−x−ySixSny layer was examined and the crystalline structure was characterized. The epitaxial growth of a strain-relaxed Ge0.64Si0.21Sn0.15 layer with the degree of strain relaxation of 55% was examined using a virtual Ge substrate. Moreover, enhancement of the strain relaxation was demonstrated by post-deposition annealing, where a degree of strain relaxation of 70% was achieved after annealing at 400 °C. These results indicate the possibility for enhancing the indirect-direct crossover with a strained and high-Sn-content Ge1−xSnx layer on a strain-relaxed Ge1−x−ySixSny layer, realizing preferable carrier confinement by type-I energy band alignment with high conduction and valence band offsets.

(Keynote) Lasing from SiGeSn/GeSn Multiple Quantum Well Structures

The amount of data collected, transferred and processed increased over the last decade with a substantial annual rate. With the internet of things in sight, this trend will be continued. Already today more than 10% of the electrical power produced in the western world is consumed by IT products, underlining the demand to drastically reduce the power consumption. In fact, energy consumption will be the main driver of development in information technology. Silicon photonics is considered to be an attractive pathway to drastically reduce power consumption in information technology based on classical Si microelectronics /1/. Moreover Si photonics are envisioned also as a promising pathway towards quantum computing/2/. Group IV alloys, namely Ge and SiGe on Si, are heavily investigated for the integration of modulators and detectors on Si. However, higher integration levels of optical data transfer on the chip will require the integration of low power laser. The latter is approached by a wide range of technologies to deposit III/V structures on Si as well as by bonding III/V lasers Si. Recently, a direct bandgap as well as optically pumped lasing was demonstrated for GeSn alloys deposited on Ge virtual substrates on Si (100) 8” wafers. Here, efforts are reported of this ongoing endeavor towards an electrically pumped group IV laser. The group IV alloy GeSn provide a direct band gap for Sn concentration above ~8%, which is far beyond the solid solubility limit of ~1%. Recently high quality GeSn alloys with Sn concentrations up to 14.5% could be grown at low temperatures and high growth rates by means reactive gas source epitaxy. These GeSn have a direct band gap, in the range of 0.48-0.63 eV dependening on the Sn concentration. Phosphorous and Boron doping has been realized to fabricate p-i-n junction in SiGeSn/GeSn structures. Thus these materials pave the road for the integration of optoelectronic circuitry on Si (100) substrates. Optically pumped laser in the Fabry Perot geometry as well as microdisc lasers have been fabricated from GeSn films, SiGeSn double heterostructures and SiGeSn/GeSn multiple quantum wells (MQW) on Ge virtual substrates. The threshold required to achieve lasing dropped from ~300 kW/cm2 for a thick GeSn film to about 30 kW/cm2 for a MQW structure. This can be attributed to reduced optical losses due to surface scattering as well as due to the reduced number of states in the multiple quantum wells, which allows carrier inversion at smaller pumping powers. However, due to the very small effective mass of electrons in the Г valley compared to large mass in the L valley, the quantum wells have to be designed carefully. For thin quantum wells the confinement shift of the subbands of the Г valley is larger than that for the L valley reducing the directness and may even turn it into an indirect material. Moreover, a (Si)GeSn buffer layer above critical thickness is required to relax the strain before the growth of the active region of the laser. The strain relaxation occurs predominantly via 90° step edge dislocations at the Ge/GeSn interface, within the hetero- and MQW structures no dislocations were detected by cross sectional TEM. However, besides defect engineering, care has to be taken in the design of the structures also to avoid losses due to absorption in GeSn and SiGeSn cladding layers, since the limited band offsets give rather stringent design rules. To achieve electrically pumped lasing double hetero- and MQW-structures have been grown using SiGeSn cladding and barrier layers. Parts of the SiGeSn cladding layers have been doped by P and B to achieve p-i-n junctions. First devices have been fabricated showing a superior electroluminescence efficiency of MQW structures compared to hetero- of homojunction devices. /1/ R. Soref, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 12, 1678 (2006) /2/ T. Rudolph APL PHOTONICS 2, 030901 (2017)

(Invited) Epitaxial CVD Growth of Ultra-Thin Si Passivation Layers on Strained Ge Fin Structures

The continuous scaling of CMOS devices requires new process developments because of the strong reduction of the allowable thermal budget for device processing. This is especially the case for narrow FinFET structures and vertically stacked MOSFET devices. New epitaxial growth schemes using higher order precursors are being assessed to enable epitaxial growth at the required reduced growth temperatures. An example is the epitaxial Si growth on narrow Ge fins to passivate (strained) Ge surfaces in the high-k gate module [1-3]. The benefit of the Si passivation layer over GeOx-based gate stacks is its potential to improve Bias Temperature Instability (BTI) reliability. During the epitaxial Si growth Ge surface segregation needs to be avoided as it leads to an increase of the interfacial trap density and distribution in the final gate stack. On the other hand, the Si passivation layer has to be sufficiently thin to approach an Equivalent Oxide Thickness (EOT) close to 1 nm and below as implemented in the current 14 nm-node FinFET. If the Si passivation layer is grown on strained Ge FinFET structures, there is a risk for Ge surface reflow during the Si deposition. This in turn would lead to a (partial) relaxation of the strained Ge layer [1,3]. The requirement for extremely low process temperatures during epitaxial Si growth (£ 450 °C) sets the need to use higher order Si-precursors such as Si3H8 or Si4H10. Despite the use of these higher order precursors it is challenging to avoid the surface reflow of compressively strained Ge, especially for narrow fins [1,3]. In this contribution, we will discuss epitaxial Si growth by means of CVD at temperatures down to 330 °C and using tetrasilane (Si4H10) as Si precursor. Replacing Si3H8 by Si4H10results in ~40% higher growth rates. The growth rate also depends on the choice of the carrier gas and is affected by the underlying virtual substrate [2]. The Si growth characteristics are discussed in view of the use for Ge surface passivation in the high-k gate module [4-6]. We use relaxed Ge fins for nFINFET devices and compressively strained Ge epitaxially grown on SiGe virtual substrates for pFINFET devices [1,3]. The deposition of the Si passivation layer on relaxed or compressively strained Ge FinFET structures is expected to be conformal as the extracted growth rate is very similar for (001) and (110) surfaces. For the given Si growth conditions, Ge segregation into the Si epi layer is supressed as confirmed for Si growth on blanket virtual substrates. CV characteristics of blanket capacitors made on such Ge virtual substrates point to the presence of an optimal Si thickness. In case of strained Ge fin structures, the Si growth results in non-uniform and high strain levels in the strained Ge fin. The non-uniform strain is caused by the extremely large lattice mismatch between the Si passivation layer and the Ge fin. Using atomistic modelling, the strain levels in the compressively strained Ge have been calculated for different shapes of the Ge fin and in function of the grown Si thickness. The high strain is the driving force for eventual Ge surface reflow during the Si deposition. The unwanted Ge surface reflow is reflected in a reduction of the compressive strain as measured by HR-XRD. The Ge surface reflow is strongly affected by the strength of the H-passivation during Si-capping and can be avoided by carefully selected process conditions. Acknowledgements: The imec core CMOS program members, European Commission, local authorities and the imec pilot line are acknowledged for their support. Epi layers are grown in EpsilonTM3200 and IntrepidTMXP systems from ASM. Air Liquide Advanced Materials is acknowledged for providing advanced precursor gases and Bruker Semiconductor Division for their kind support in XRD characterization of high mobility materials implemented in complex device architectures.

(Invited) Epitaxial CVD Growth of Ultra-Thin Si Passivation Layers on Strained Ge Fin Structures

Epitaxially grown ultra-thin Si layers are often used to passivate Ge surfaces in the high-k gate module of (strained) Ge FinFET devices. We use Si4H10 as Si precursor as it enables epitaxial Si growth at temperatures down to 330 °C. C-V characteristics of blanket capacitors made on Ge virtual substrates point to the presence of an optimal Si thickness. In case of compressively strained Ge fin structures, the Si growth results in non-uniform and high strain levels in the strained Ge fin. These strain levels have been calculated for different shapes of the Ge fin and in function of the grown Si thickness. The high strain is the driving force for potential (unwanted) Ge surface reflow during the Si deposition. The Ge surface reflow is strongly affected by the strength of the H-passivation during Si-capping and can be avoided by carefully selected process conditions.

Monolithic integration of AlGaAs distributed Bragg reflectors on virtual Ge substrates via aspect ratio trapping

High quality AlxGa1-xAs distributed Bragg reflectors (DBRs) were successfully monolithically grown on on-axis Si (100) substrates via a Ge layer formed by aspect ratio trapping (ART) technique. The GaAs/ART-Ge/Si-based DBRs have reflectivity spectra comparable to those grown on conventional bulk off-cut GaAs substrates and have smooth morphology and reasonable periodicity and uniformity. Antiphase domain formation is significantly reduced in GaAs on ART-Ge/Si substrates, and etch pit density of the GaAs base layer on the ART-Ge substrates ranges from 105 to 6 × 106 cm−2. These results paved the way for future VCSEL growth and fabrication on these ART-Ge substrates and also confirm that virtual Ge substrates via ART technique are effective Si platforms for optoelectronic integrated circuits.

Simulated characteristics of a heterojunction phototransistor with Ge1-xSnx alloy as base

Groups III-V compound semiconductors and their alloys are the main photodetecting elements for the entire fiber optic telecommunication band. However, the recent successful growth of Ge1-xSnx alloy on Ge virtual substrates on Si platform makes the group IV alloys a potential competitor. Ge1-xSnx alloy shows direct band gap and has an absorption coefficient almost 10 times higher than that of Ge. The photonic devices are complementary metal–oxide–semiconductor compatible. We have considered an n-Ge/p+-Ge1-xSnx/n-Ge1-xSnx heterojunction phototransistor (HPT) and studied the variations of terminal currents by considering the Gummel Poon model of HPT, and values of optical and current gains, photocurrent, and responsivity have been obtained. The performance of the device as a photodetector at fiber optic communication wavelengths seems quite encouraging to justify the use of GeSn-based HPTs as a replacement of III-IV semiconductor-based photodetectors.

Design and Modeling of GeSn-Based Heterojunction Phototransistors for Communication Applications

We propose the use of Ge $_{1-x}$ Sn $_x$ heterojunction phototransistors (HPTs) as efficient optical receivers on Si substrates and analyze their performance. Our designs use n-Ge/p-Ge $_{1-x}$ Sn $_x$ /n-Ge $_{1-x}$ Sn $_x$ layers pseudomorphically grown on Si wafers via a Ge virtual substrate, which offers compatibility with complementary metal–oxide–semiconductor (CMOS) technology. By incorporating Sn into the Ge photon-absorbing layer to shrink the bandgap, the photodetection range can be significantly extended to the mid-infrared (MIR) region with a considerably enhanced optical response. The use of HPT structures provides optical conversion gain to further enhance the optical responsivity, thereby enabling efficient photodetection in the short-wave infrared region. We develop theoretical models to calculate the composition-dependent band alignments, the band structures (by taking into account the nonparabolic effect), the absorption coefficient, and the optical responsivity for the proposed GeSn HPTs. As the Sn content increases, the conduction band nonparabolicity becomes increasingly significant and considerably impacts the optical absorption coefficient. Moreover, analysis of the spectral response for the Ge $_{1-x}$ Sn $_x$ HPTs shows that efficient photodetection covering the entirety of the fiber-optic telecommunication bands, as well as the emerging 2- $\mathbf {\mu }$ m MIR communication band, can be achieved. These results indicate that the proposed Ge $_{1-x}$ Sn $_x$ HPTs are attractive for use as high-responsivity CMOS-compatible photodetectors in communication applications.

Room Temperature Electroluminescence from Tensile-Strained Si0.13Ge0.87/Ge Multiple Quantum Wells on a Ge Virtual Substrate.

Direct band electroluminescence (EL) from tensile-strained Si0.13Ge0.87/Ge multiple quantum wells (MQWs) on a Ge virtual substrate (VS) at room temperature is reported herein. Due to the competitive result of quantum confinement Stark effect and bandgap narrowing induced by tensile strain in Ge wells, electroluminescence from Γ1-HH1 transition in 12-nm Ge wells was observed at around 1550 nm. As injection current density increases, additional emission shoulders from Γ2-HH2 transition in Ge wells and Ge VS appeared at around 1300–1400 nm and 1600–1700 nm, respectively. The peak energy of EL shifted to the lower energy side superquadratically with an increase of injection current density as a result of the Joule heating effect. During the elevation of environmental temperature, EL intensity increased due to a reduction of energy between L and Γ valleys of Ge. Empirical fitting of the relationship between the integrated intensity of EL (L) and injection current density (J) with L~Jm shows that the m factor increased with injection current density, suggesting higher light emitting efficiency of the diode at larger injection current densities, which can be attributed to larger carrier occupations in the Γ valley and the heavy hole (HH) valance band at higher temperatures.