GeSn Films Research Articles

Strained Germanium–Tin (GeSn) P-Channel Metal-Oxide-Semiconductor Field-Effect Transistors Featuring High Effective Hole Mobility

Compressively strained \(\hbox {Ge}_{0.973}\hbox {Sn}_{0.027}\) and \(\hbox {Ge}_{0.925}\hbox {Sn}_{0.075}\) p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) are fabricated with low-temperature \(\hbox {Si}_{2}\hbox {H}_{6}\) surface passivation. High crystallinity GeSn films epitaxially grown on a Ge(001) substrate are used for the device fabrication. The impacts of the Sn composition on the subthreshold swing \(S\), threshold voltage \(V_\mathrm{TH}\), on-state current \(I_\mathrm{ON}\), and effective hole mobility \(\mu _\mathrm{eff}\) of the devices are investigated. GeSn pMOSFETs with different Sn compositions show a similar \(S\), indicating almost the same midgap density of interface states \(D_\mathrm{it}\). A positive shift of \(V_\mathrm{TH}\) with an increase of the Sn composition is observed. A \(\hbox {Ge}_{0.925}\hbox {Sn}_{0.075}\) pMOSFET exhibits a significant improvement in \(I_\mathrm{ON}\) as compared to a device with a lower Sn composition, which is due to the superior hole mobility in a device with a higher Sn composition. \(\hbox {Ge}_{0.925}\hbox {Sn}_{0.075}\) pMOSFETs achieve a peak effective hole mobility \(\mu _\mathrm{eff}\) of \(500\,\hbox {cm}^{2}{\cdot }\hbox {V}^{-1}{\cdot } \hbox {s}^{-1}\), which is much higher than that of \(\hbox {Ge}_{0.973}\hbox {Sn}_{0.027}\) devices. The enhancement of the compressive strain and chemical effect in the channel region with increased Sn composition leads to an improvement of \(\mu _\mathrm{eff}\).

Observation of heavy- and light-hole split direct bandgap photoluminescence from tensile-strained GeSn (0.03% Sn)

Temperature- (T-) and laser power-dependent photoluminescence (PL) measurements have been made for the tensile-strained, undoped GeSn (0.03% Sn) film grown on Si substrate. The PL results show not only clear strain-split direct bandgap transitions to the light-hole (LH) and heavy-hole (HH) bands at energies of 0.827 and 0.851 eV at 10 K, respectively, but also clearly show both strong direct and indirect bandgap related PL emissions at almost all temperatures, which are rarely observed. This split of PL emissions can be directly observed only at low T and moderate laser power, and the two PL peaks merge into one broad PL peak at room temperature, which is mainly due to the HH PL emission rather than LH transition. The evolution of T-dependent PL results also clearly show the competitive nature between the direct and indirect bandgap related PL transitions as T changes. The PL analysis also indicates that the energy gap reduction in Γ valley could be larger, whereas the bandgap reduction in L valley could be smaller than the theory predicted. As a result, the separation energy between Γ and L valleys (∼86 meV at 300 K) is smaller than theory predicted (125 meV) for this Ge-like sample, which is mainly due to the tensile strain. This finding strongly suggests that the indirect-to-direct bandgap transition of Ge1−ySny could be achieved at much lower Sn concentration than originally anticipated if one utilizes the tensile strain properly. Thus, Ge1−ySny alloys could be attractive materials for the fabrication of direct bandgap Si-based light emitting devices.

Investigation on the Formation and Propagation of Defects in GeSn Thin Films

Silicon photonics is rapidly growing because of the recent introduction of GeSn alloys as a group IV direct bandgap material. Viability of GeSn alloys for complementary metal-oxide-semiconductor technology, as demonstrated recently, is another step towards commercializing this material for integrated circuits applications [1]. However, ability to grow high quality materials is imperative for device applications. In order to compensate the high lattice mismatch between the Si substrate and the GeSn film different strain relaxed buffer layers, such as Ge, are used. Relaxation of Ge buffer layer introduces a network of misfit dislocations. This can result in high threading dislocation density (TDD) in GeSn degrading the performance of the final device. In this paper, we report on defect formation and propagation mechanism in Ge1-xSnx films.Plastic relaxation of higher lattice constant materials (SiGe, Ge, GeSn) on top of Si forces the layers to form 60o misfit dislocations. These dislocations are caused by slip of (110) and (111) planes in the diamond lattice structure, which have a screw component that threads through the grown film. Fig. 1 (a) schematically shows how defects form and propagate through the film. A network of misfit dislocations results in a cross-hatch pattern on the surface of the film. Fig. 1 (b) shows the plan view transmission electron microscopy (TEM) image of Ge.93Sn.07 on top of Ge buffer layer. The cross-hatch patterns formed at GeSn/Ge interface have overlapped with those of Ge/Si. The denser pattern is attributed to the Ge/Si interface due to higher lattice mismatch. Fig. 1 (c) delineates scanning electron microscopy image of decorated threading dislocations at the surface by using cold Schimmel solution (CrO3+HF) for 10 minutes. Density of pits (1.1×107 cm-2) is in good agreement with TDD measured from TEM micrographs (3×107 cm2)[1]. It also confirms that the quality of GeSn material is higher than Ge buffer layer (TDD: 9.71×108 cm-2) by an order of magnitude.Formation of Lomer misfit dislocaltion prevents propagation of threadig dislocation in the top layer.The main reasons behind this phenomenon is that the Burgers vector in Lomer misfit dislocation, for a (001) substrate, lies parallel to the interface [2]. Therefore, the substrate and film lattices can match while no prependicular component exists. In this case the film is grown relaxed without introducing structural imperfection such as threading dislocations, which degrades the material quality. Different models of Lomer misfit dislocation are proposed including a five-atom ring and seven-atom ring without dangling bonds [3]. Available models show that Lomer dislocations can combine two lattices through symmetric and asymmetric loops [4]. Fig. 1 (d-g) shows formation of the symmetric and asymmetric loops at Ge/GeSn interface films using high resolution TEM images.Our investigations show that formation of symmetric and asymmetric Lomer dislocation at the interface of Ge/GeSn film has reduced the TDD in GeSn film. Results from TEM images and etch pit density measurement verifies this conclusion by showing that 80% of the threading dislocations has not passed through the GeSn film. The current results show achievement of high quality GeSn film as well as indicating that GeSn is a unique candidate to be used as a universal compliant layer.

Virtual Substrate Technology for Ge1-XSnX Heteroepitaxy on Si Substrates

The fabrication of GeSn layers is one of the worldwide current areas of research in material science. The driving force behind it, is the possibility to realize a group IV direct bandgap semiconductor. Theoretical calculations show values between 10 % for relaxed GeSn crystals and 6 % for tensile-strained GeSn crystals. However, only its synthesis on Si substrates is of interest for the photonic components applications. For the manufacture of GeSn based components on Si substrates, a specific Ge VS (virtual substrate) has to be first realized in order to overcome the large lattice mismatch (≥ 4.2 %). This method has been developed by Kasper et al. for SiGe heterostructures on Si and its here used to provide, without epitaxial break, the desired lattice constant for the grow of Sn based alloys. The use of such VS minimize the number of the threading dislocations as required for high performance optical devices.The significant progress made in recent years allowed the demonstration of GeSn based electro-optical components integrated on Si.A compressively strained heterostructure can by relaxed by various methods. These are, for example, special annealing steps, increasing the layer thickness over the critical thickness tcrit or by means of point defects introduced during low temperature growth. If a Ge or GeSn film is deposited directly on Si it relaxes very quickly since the critical thickness is only a few nanometer thick. At the same time the threading dislocation density is extremely high. An alternative is an ultra-thin Ge VS developed in the last few years. The threading dislocation density is significantly reduced by high temperature annealing steps close to the melting point of Ge. This technology is used here as a basis for GeSn structure growth.The optical properties of the GeSn layers with 2 % and 4 % Sn content are addressed via pin photodetectors fabrication (see Fig. 1). As a reference the GeSn pin detectors were manufactured without VS directly on Si. An overview of the layer structures are shown in figure 2.The strain statuses of the individual samples were analyzed by RSM (reciprocal space mapping). These show that the samples on Si are 100 % strain relaxed and the samples on Ge VS are still under 50 % rest compressive strain.The dark current of the individual samples are shown in the figure 3. The GeSn samples grown directly on Si show a much higher dark current due to the significantly higher threading dislocation densities compared to the samples on Ge-VS. At a bias of -1 V, the dark current is increased by more than one order of magnitude. In Figure 4, the optical responsivity of each sample is shown as a function of wavelength.

Epitaxial growth of Ge1–xSnx films with x up to 0.14 grown on Ge (00l) at low temperature

We characterize the structures of Ge1–xSnx films with x up to 0.14 grown on Ge (00l) by molecular-beam epitaxy at low temperature. The results show that Ge1–xSnx films are fully strained even at high Sn composition. The in-plane lattice parameters remain exactly the same as that of the substrate. Depth sensitivity analysis of the lattice parameters indicates that the strains of the epitaxial films are all in homogeneity. The films are fully strained. Poisson ratios, the force constants for the bonds between Ge and Sn are estimated and discussed in the present paper. Raman results show Ge—Ge, Ge—Sn, Sn—Sn vibrational modes. The Sn—Sn bond aggregation may respond to the high quality of our films. The fully strained epitaxy films with high content of Sn may be useful in designing the high quality GeSn films.

Electrical characteristics of Ni Ohmic contact on n-type GeSn

We report an investigation of the electrical and material characteristics of Ni on an n-type GeSn film under thermal annealing. The current-voltage traces measured with the transmission line method are linear for a wide range of annealing temperatures. The specific contact resistivity was found to decrease with increasing annealing temperature, followed by an increase as the annealing temperature further increased, with a minimum value at an annealing temperature of 350 °C. The material characteristics at the interface layer were measured by energy-dispersive spectrometer, showing that an atomic ratio of (Ni)/(GeSn) = 1:1 yields the lowest specific contact resistivity.

Atomic insight into Ge1−xSnx using atom probe tomography

Ge(1-x)Sn(x) is receiving a growing interest in the scientific community, as it has important applications in opto-electronic devices, ( as stressor) Source/Drain materials for Ge and SiGe MOSFETS. It is predicted that at 10% Sn concentration or even lower, unstrained Ge(1-x)Sn(x) will exhibit a direct band gap. Moreover, in strained Ge(1-x)Sn(x) the expected concentration of Sn for this cross-over is even lower. As the theoretical Sn incorporation in Ge(1-x)Sn(x) is less than 1%, and Ge(1-x)Sn(x) is prone to relaxation, routes towards the growth of metastable strained films has been extensively explored. Although Ge(1-x)Sn(x) films (with x up to 10%) have been grown using various methods like molecular beam epitaxy, CVD growth etc. there remain issues with tendency of these layers to relax. Detailed studies on the relaxation mechanisms and effects on the Sn-atoms require suitable characterization techniques. Various techniques have been used to study the surface of the film, crystallography or concentration of Sn in the film but none of them provides information at the atomic scale as they average over many layers and atoms. Atom probe tomography (APT) analysis, on the other hand, is one such method that can provide atomic scale resolutions (∼0.3 nm) due to its ability to perform atom by atom analysis. In this paper we explore the use of APT for characterizing Ge(1-x)Sn(x) layers. We comment on the difference of field evaporation values of Ge and Sn in Ge(1-x)Sn(x) layer by taking a closer look at the co-evaporation of the two elements and comment on the accuracy of depth reconstruction of APT for Ge(1-x)Sn(x) layer. Comparing the Sn-distributions and their local surroundings we saw a tendency for the Sn to locally enrich forming Sn clusters. Higher order clusters were observed for the relaxed sample.

Material characterization of high Sn-content, compressively-strained GeSn epitaxial films after rapid thermal processing

We report on the characterization of high Sn-content (∼10% Sn) GeSn films grown on (001) Ge/Si substrates using reduced-pressure chemical vapor deposition. Pseudomorphic 30nm GeSn films were grown on relaxed Ge buffers, exhibit a smooth surface, and display strong photoluminescence (PL) with cavity-resonance fringes from the Ge buffer. Additional luminescence studies confirm that the measured PL originates from the GeSn film. A study on the effects of rapid thermal annealing is presented along with evidence of Sn surface segregation and formation of surface nanodots for anneals at 450°C and above. Anneals at 400°C for up to 500s showed little change in PL intensity or material properties, which suggest that a critical temperature exists for post-growth thermal-processing of high Sn-content, compressively-strained GeSn films.

Investigation of Ge1-xSnx/Ge with high Sn composition grown at low-temperature

We report on experimental investigations of the growth of Ge1-xSnx film with thickness above the critical thickness using Molecular Beam Epitaxy. A series of Ge1-xSnx films with various Sn compositions up to 14% are deposited on a Ge buffer layer for growth at low temperatures close to the melting point of Sn. Analysis of various measurements shows that the Ge1-xSnx film is defect free in the XTEM image and that Sn is distributed almost uniformly in the film for Sn compositions up to 9.3%. The Sn composition of the films is higher than the Sn composition that is theoretically predicted to cause the energy band of Ge to change from an indirect to a direct bandgap; thus, the present investigation provides a method for growing direct bandgap GeSn film, which is desired for use in applications involving optoelectronic devices.

Low temperature chemical vapor deposition of Si-based compounds via SiH3SiH2SiH3: Metastable SiSn∕GeSn∕Si(100) heteroepitaxial structures

Growth of Si1−xSnx alloys on Ge1−ySny-buffered Si(100) was achieved via reactions of SnD4 and SiH3SiH2SiH3 at 275°C. Kinetic studies indicate that unprecedented low growth temperatures are made possible by the highly reactive SiH2 groups. The authors obtain supersaturated metastable compositions (y∼25%) near the indirect to direct band gap crossover predicted by first principles simulations. Extensive characterizations of composition, structure, and morphology show that the SiSn∕GeSn films grow lattice matched via a “compositional pinning” mechanism. The initial Raman observations of Si–Sn bond vibrations in a condensed phase are discussed in the context of simulated bond distributions in the alloys.

Sn-based Group-IV Semiconductors on Si: New Infrared Materials and New Templates for Mismatched Epitaxy

ABSTRACTWe report growth and properties of GeSn and SiGeSn alloys on Si (100). These materials are prepared using a novel CVD approach based on reactions of Si-Ge hydrides and SnD4. High quality GeSn films with Sn contents up to 20%, and strain free microstructures have been obtained. The lattice mismatch between the films and Si is relieved by Lomer edge dislocations located at the interface. This material is of interest due to the predicted cross-over to a direct gap semiconductor for moderate Sn concentrations. We find that the direct band gap, and, consequently, the main absorption edge, shifts monotonically to lower energies as the Sn concentration is increased. The compositional dependence of the direct band gap shows a strong bowing, such that the direct band gap is reduced to 0.4 eV (from 0.8 eV for pure Ge) for a concentration of 14% Sn. The ternary SiGeSn alloy has been grown for the first time on GeSn buffer layers. This material opens up entirely new opportunities for strain and band gap engineering using group-IV materials via decoupling of strain and composition. Our SiGeSn layers have lattice constants above and below that of pure Ge, and depending on the thickness and composition of the underlying buffer layer they can be grown relaxed, with compressive, or with tensile strain. In addition to acting as a buffer layer for the growth of SiGeSn, we have found that GeSn can act as a template for the subsequent growth of a variety of materials, including III-V semiconductors.

Germanium supersaturation during the crystallization of amorphous Te–Ge–Sn thin films

The structure and phase relations of Te–Ge–Sn thin films are examined with application to erasable optical storage media. Free energy data from the literature predict that the region of the Te–Ge–Sn phase diagram between Ge, Sn, and the TeGe–TeSn pseudobinary consists of one two-phase field [α–Ge and Te50 (GexSn1−x)50] and one three-phase field (α–Ge, β–Sn, and TeSn). Electron diffraction from five different Te–Ge–Sn films annealed at 623 K experimentally confirms this prediction. One composition from the two-phase field is deposited as a tri-layer film with the structure 150 nm SiO2/75 nm Te36.3Ge47.4Sn16.3/150 nm SiO2 on a grooved disk substrate, and the microstructure resulting from low-power (12 mW) CW and higher-power (∽50 mW) pulsed laser exposure is studied by transmission electron microscopy and electron diffraction. Of particular significance is that laser-induced crystallization produces a single-phase structure consisting of the Te–Ge–Sn compound phase which is supersaturated with respect to the excess Ge. This supersaturation leads to a disordering of the equilibrium NaCl-type structure of this phase. Crystallization of a micron-sized amorphous spot on a ∼200 ns time scale occurs by a diffusionless process. The fast erase times required by a phase-change optical recording application can thus be achieved in off-stoichiometric compound compositions by way of a nonequilibrium crystallization process.

Photo-induced potential barrier in As-Se-Ge films

Photo-excited effects in AsSeGe and AsSeGeSn amorphous films have been studied under illumination of different light sources. AsSeGe system exhibited rectifying characteristics under illumination of the light with hν > E g, while AsSeGeSn film did not show such phenomena. The illumination of the IR light along with the light of hν > E g weakened the rectification behavior. The photovoltage and I-V characteristics results suggest the existence of “photo-induced” potential barrier in AsSeGe system, which is considered to concern the creation and destruction of neutral defect states D°.

Pseudogap of Energy in Amorphous Ge-Sn Films

Pseudogap of Energy in Amorphous Ge-Sn Films