GeSn Thin Films Research Articles

Polycrystalline GeSn thin films on Si formed by alloy evaporation

Polycrystalline GeSn thin films on Si substrates with a Sn composition up to 4.5% have been fabricated and characterized. The crystalline structure, surface morphology, and infrared (IR) absorption coefficient of the annealed GeSn thin films were carefully investigated. It was found that the GeSn thin films with a Sn composition of 4.5% annealed at 450 °C possessed a desirable polycrystalline structure according to X-ray diffraction (XRD) analyses and Raman spectroscopy analyses. In addition, the absorption coefficient of the polycrystalline GeSn thin films in the IR region was significantly better than that of the single crystalline bulk Ge.

70 °C synthesis of high-Sn content (25%) GeSn on insulator by Sn-induced crystallization of amorphous Ge

Polycrystalline GeSn thin films are fabricated on insulating substrates at low temperatures by using Sn-induced crystallization of amorphous Ge (a-Ge). The Sn layer stacked on the a-Ge layer (100-nm thickness each) had two roles: lowering the crystallization temperature of a-Ge and composing GeSn. Slow annealing at an extremely low temperature of 70 °C allowed for a large-grained (350 nm) GeSn layer with a lattice constant of 0.590 nm, corresponding to a Sn composition exceeding 25%. The present investigation paves the way for advanced electronic optical devices integrated on a flexible plastic substrate as well as on a Si platform.

Shortwave-infrared photoluminescence from Ge1−xSnx thin films on silicon

Ge1−xSnx thin films with Sn composition up to 7% were epitaxially grown by chemical vapor deposition on silicon. Temperature-dependent photoluminescence was investigated and the peaks corresponding to the direct and indirect transitions were observed in a wavelength range from 1.6 to 2.2 μm. The exact peak positions obtained from Gaussian fitting were fitted with an empirical temperature dependent band-gap equation (Varshni relationship). The separation between direct and indirect peaks was equal to 0.012 eV for GeSn thin film with 7% Sn content at room temperature. This observation indicates that the indirect-to-direct crossover would take place at slightly higher Sn compositions.

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

Defect formation and propagation in Ge1-xSnx/Ge thin films were investigated by using X-ray diffraction, transmission electron microscopy and scanning electron microscopy techniques. Samples with 0.9 to 7 % Sn mole fraction were deposited on a Si substrate with a Ge buffer layer using a chemical vapor deposition technique. The X-ray diffraction results show that the Ge1-xSnxfilms are 46-100 % compressively strained. Transmission electron microscopy images show that symmetrical and asymmetrical Lomer dislocations are formed at GeSn/Ge interface, which stops propagation of defects parallel to the growth direction. Average threading dislocation density (TDD) in the Ge buffer layer is measured to be 9.71×108 cm-2. Adding Sn to Ge made the Ge1-xSnx films to be more flexible, which traps 80 % of TDD propagating in Ge buffer layer. Reduction of TDD to 3×107cm-2 indicates achievement of higher quality of Ge1-xSnx film.

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.

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

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

Hole Mobility Enhancement in Compressively Strained ${\rm Ge}_{0.93}{\rm Sn}_{0.07}$ pMOSFETs

Germanium tin (GeSn) pMOSFETs with channel Sn composition of 7% are fabricated using a low thermal budget process. GeSn pMOSFETs show enhancement in hole mobility over control Ge devices by 85% in high inversion charge density regime. Hole mobility improvement observed in GeSn channel pMOSFETs compared with Ge control is due to the biaxial compressive strain in GeSn resulting from epitaxial growth of GeSn thin films on relaxed Ge buffer layers.

Characterization of Sputtered Ge-Sn Thin Films by High Resolution Methods

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Formation of nanoscale trenches and wires as a pathway to phase-separation in strained epitaxial Ge-Sn alloys

We report on the molecular beam epitaxial growth of Ge 0.93Sn 0.07 alloys on Ge (100). A strained planar Ge 0.93Sn 0.07 film forms at the initial stages, as observed and confirmed by AFM and X-ray analysis. Continued deposition leads to formation of 3D Sn islands, which initiates phase-separation in the strained GeSn thin film through a kinetic pathway that leaves behind a pattern of nanoscale trenches and wires that run along 〈100〉directions. This morphological transition only happens in a certain range of temperature, thickness and composition. The morphological features observed in Ge 0.93Sn 0.07 films potentially offer a unique pathway for fabrication of nanostructures.

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.