Substitutional Sn Concentration Research Articles

Growth of High Sn Concentration Germanium-Tin Films on Insulators by Microsecond Laser Annealing

1. IntroductionThanks to their high carrier mobility, germanium based group IV semiconductors are promising materials to realize next generation electronics [1]. Especially, germanium tin alloys (GeSn) having high Sn concentration (> 6 at. %) are attracting much attention due to their direct band structure [2]. To apply such significant advantage to thin-film devices such as thin-film transistors (TFTs) or three dimensional large-scale integrated circuits (3D-LSIs), various groups are working on to crystallize these materials on insulating substrates by various approaches such as germanium condensation [3], solid phase crystallization [4], and so on.Recently, we have established a technique to grow highly strained germanium based materials on insulating substrates by microsecond annealing using novel high speed continuous wave laser annealing (CWLA) system [5]. Here, in this presentation, we will introduce our recent achievements in GeSn crystal growth by the microsecond annealing technique.2. Experimental ProcedureIn the experiment, amorphous GeSn films with Sn concentration of 15% (thickness: 100 nm) were firstly deposited on quartz substrates by molecular beam deposition. Then the samples were annealed by the high speed CWLA system, which can scan laser light (focused to 20 μm diameter) on samples with very high speed up to 15 m/s. The laser irradiation time (tirr) can be varied from 1.5 to 6.0 μs by controlling the scanning speed. Here, Nd:YVO4 solid state laser with wavelength of 532 nm was used as a light source, and laser power was controlled from 350 to 1000 mW. After the laser annealing, the samples were characterized by microscopic Raman spectroscopy and electron back scattered diffraction (EBSD) method.3. Results and DiscussionsFig. 1(a) shows Nomarski microscopic image after laser annealing (500 mW, tirr: 1.5 μs), which shows bright contrast at annealed region. Raman signals measured at surface of GeSn thin film before and after LA (tirr: 1.5 μs) are shown in Fig. 1(b). We can see clear peak around 300 cm-1 due to Ge-Ge atomic bond at the annealed region, which supports that GeSn film has been successfully crystallized by the annealing.To discuss about the growth feature of GeSn films, EBSD measurements were carried out. Mapping images of Ge and Sn phase signals at surfaces of samples are summarized in Fig. 2 as a function of tirr. Here, we can clearly see that the area of Sn phase signal is expanding by increasing tirr. These results (Raman and EBSD) suggest that by increasing tirr, Sn precipitation toward surface of GeSn occurs after crystallization of GeSn films, due to excess annealing. As a result, by increasing tirr, substitutional Sn concentration in the films decreases after LA [Fig. 3], showing that short time annealing is essential to prevent the precipitations. Fig. 4 summarizes substitutional Sn concentration in GeSn films grown with shortest irradiation time (1.5 μs) as a function of laser power. By minimizing the irradiation time and tuning laser power to minimum value for crystallization, we could realize GeSn thin films with a high substitutional Sn concentration (13 %) on insulating substrates. These are very attracting results to realize next generation electronics.Detailed physics and further tin concentration improvement methods will be discussed in the presentation.[References][1] J.R. Haynes, W. Shockley, Phys. Rev. 81 835–843 (1951).[2] S. Gupta, K. C. Saraswat, et.al., J. Appl. Phys. 113 073707 (2013).[3] S. Nakaharai, S. Takagi, et. al., J. Appl. Phys. 105 024515 (2009).[4] K. Toko, et. al., Sci. Rep., 7, 16981 (2017)[5] R. Matsumura, N. Fukata, ECS J. Solid State Sci. Technol, 9 063002 (2020). Figure 1

Growth of High Sn Concentration Germanium-Tin Films on Insulators by Microsecond Laser Annealing

Germanium-tin (GeSn) is attractive group-IV material with great optical properties as light emitting and receiving element. To apply such significant properties on photonic devices, the growth of GeSn films with high substitutional Sn concentration on insulating substrates is essential. In this work, we have investigated continuous wave laser annealing of amorphous germanium (a-Ge) and a-GeSn films (initial Sn concentration: 20 at.%) on quartz substrates with microsecond annealing time. As a result, large shifting of Raman peak was observed for GeSn samples after annealing, while there was little shifting of that for Ge samples. We speculate that these results are due to incorporation of Sn atoms into the grown films.

Growth of Tensile Strained Poly Germanium Thin Film on Glass Substrates by High Speed Continuous Wave Laser Annealing, and its Application to Germanium-Tin

To realize next generation electronics, liquid phase crystallization of germanium based materials on quartz substrates is investigated by continuous wave laser annealing system which enables microsecond annealing. As a result, large grain polycrystalline germanium thin film with ∼0.6% tensile strain has successfully grown on quartz substrates. Moreover, we have applied this method on germanium-tin thin film growth and realized germanium-tin crystals with high substitutional tin concentration (up to ∼13%). By using these films, modulation phenomena of tin concentration in the crystals depending on annealing time were observed. From the phenomena, we could also closely discuss about strong correlation of substitutional Sn concentration on Raman signals. These findings and discussions will facilitate the study of germanium-tin thin film growth and characterization.

Low-Temperature (∼250°C) Gold-Induced Lateral Growth of Sn-Doped Ge on Insulator Enhanced by Layer-Exchange Reaction

Low-temperature (≤250°C) formation of Sn-doped Ge (GeSn) (substitutional Sn concentration: 2–3%) on insulator is desired to realize flexible electronics, where high-speed thin-film transistors are integrated on flexible plastic-substrates (softening temperature: ∼300°C). To achieve this, gold-induced lateral crystallization of amorphous GeSn (a-GeSn) is investigated. Here, a-GeSn (Sn concentration: 0%–20%) on insulator structures, having Au-island patterns, were annealed. For initial Sn concentrations of 0%–5%, high-speed lateral crystallization proceeds around Au-patterns, and large grown regions (∼5–20 μm) are obtained by low-temperature short-time annealing (150°C–250°C, 10 min). However, bottom regions of GeSn are not crystallized at 150°C–200°C, while the whole films including bottom regions are crystallized at 250°C. These phenomena are caused by the following temperature-dependent growth mechanisms. At 150°C–200°C, Au atoms are supplied into surface-regions of a-GeSn films from Au-islands by surface-diffusion, resulting in crystallization of only surface-regions. On the other hand, at 250°C, layer-exchange of Au/GeSn occurs, which increases supply channel of Au for diffusion. This results in complete crystallization of GeSn to the bottom. Substitutional Sn concentrations in grown layers are increased with increasing annealing temperature. As a result, GeSn films (substitutional Sn concentration: ∼3%) are obtained at ∼250°C.

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.

Low-temperature (<200 oC) solid-phase crystallization of high substitutional Sn concentration (∼10%) GeSn on insulator enhanced by weak laser irradiation

Low temperature (<200 oC) crystallization of GeSn (substitutional Sn concentration: >8%) on insulating substrates is essential to realize next generation flexible electronics. To achieve this, a growth method of high quality GeSn films on insulating substrates by combination of laser irradiation and subsequent thermal annealing is developed. Here, the laser fluence is chosen as weak, which is below the critical fluence for crystallization of GeSn. It is clarified that for samples irradiated with weak laser fluence, complete crystallization of GeSn films is achieved by subsequent thermal annealing at ∼170 oC without incubation time. In addition, the quality of GeSn films obtained by this method is higher compared with conventional growth techniques such as melting growth by pulsed laser annealing or solid-phase crystallization (SPC) without pre-laser irradiation. Substitutional Sn concentrations in the grown layers estimated by Raman spectroscopy measurements are 8-10%, which far exceed thermal equilibrium solid-solubility of Sn in Ge (∼2%). These phenomena are explained by generation of a limited number of nuclei by weak laser irradiation and lateral SPC by subsequent thermal annealing. This method will facilitate realization of next-generation high performance devices on flexible insulating substrates.

Cooling Rate Dependent High Substitutional Sn Concentration (>10%) in GeSn Crystals on Insulator by Pulsed Laser-Annealing

Introduction Crystalline GeSn thin-films on insulator with high substitutional Sn concentration (>10%) are deseired to realize high-speed thin film transistors. However, thermal equilibrium solid-solubility of Sn in Ge is only 1%. To solve this problem, we examined non-thermal equilibrium crystallization of amorphous-GeSn (a-GeSn) by pulsed laser-annealing (PLA). In the present study, we focus on the influence of pulse number on crystallization of a-GeSn by PLA. Experimental Procedure In the experiment, a-Ge1-xSnx (0≦x≦0.2) films (thickness: 100 nm) were prepared on quartz substrates by a molecular beam deposition technique. These samples were irradiated with a pulsed laser (wavelength: 248 nm, pulse duration: 80 ns, repetition rate: 100 Hz, fluence: ~140 mJ/cm2, pulse number: 1-200 shot). The laser-annealed regions were analyzed by Nomarski microscopy, Raman scattering spectroscopy, and transmission electron microscopy (TEM). Cooling rate after PLA was estimated by in-situ optical reflectivity measurements. Results and Discussion Figure 1(a) shows Raman spectra of Ge0.95Sn0.05 samples after PLA (laser flunence: 140 mJ/cm2) with various pulse numbers. In all spectra, sharp peaks due to Ge-Ge bonding in crystalline GeSn are observed. Thus, GeSn has been crystallized for all pulse numbers, which was confirmed by TEM observation. Substitutional Sn concentration in the grown layers is investigated from the difference of Ge-Ge Raman peak positions between GeSn samples and a crystalline Ge substrate [1]. Pulse number dependence of substitutional Sn concentration is shown in Fig. 1(b). Substitutional Sn concentration increases with decreasing pulse number, and GeSn crystals with very high substitutional Sn concentration (~13%) has been realized for the sample (initial Sn concentration: 20%) with a single shot. These results suggest that non-thermal equilibrium conditions are enhanced by lower pulse number. To explore non-thermal equilibrium growth by modulating pulse number, cooling rate after PLA is estimated by in-situ optical reflectivity measurements, as shown in Fig. 2(a). Reflectivity at 10th and 100th shots as a function of time is shown in the inset of Fig. 2(b). From the decreasing curve of reflectivity, time constants of cooling processes are estimated, which are summarized in Fig. 2(b). Interestingly, substitutional Sn concentration increases with decreasing time constant. The detailed physics will be discussed in the presentation.

Low-Temperature Formation of Sn-Doped Ge on Insulating Substrates by Metal-Induced Crystallization

Low-temperature formation of Sn-doped Ge on insulator is desired to realize next generation flexible electronics. To achieve this, metal-induced crystallization (MIC) of a-GeSn is investigated. By MIC of a-GeSn with initial Sn concentration of 5%, Sn-doped Ge is obtained at low temperatures (<250℃). The Sn concentration (0.5-2.0%) in the grown layers can be controlled by modulating the annealing temperatures in the range of 150-250℃. Introduction A technique for low-temperature (≤250°C) formation of high-quality crystalline Ge films on insulator is expected to realize advanced high performance thin film transistors (TFT) on flexible plastic substrates (softening temperature: ~300oC). To improve quality of Ge crystals, Sn-doping (<4%) is very effective, because it enables passivation of point defects in Ge [1]. This triggered the recent research of solid-phase crystallization of Sn-doped Ge (Sn concentration: 2%) on SiO2layers [2]. However, the growth temperature (425°C) reported to date is higher than the softening temperatures of plastic substrates. To decrease the growth temperature of Sn-doped Ge, in the present study, we investigate metal-induced crystallization (MIC) of a-GeSn. This successfully decreases the growth temperature below 250°C. Experimental Procedure Fig.1 schematically shows the procedure of sample preparation. Amorphous Ge1-xSnx layers (x: 0-0.2, thickness: 100 nm) were deposited on quartz substrates at room temperature. Then, Au layer patterns were formed on the amorphous GeSn layers. The samples were annealed at 150-250oC for 10-60 min to induce lateral growth in a N2ambient. The growth characteristics were analyzed by Nomarski optical microscopy and microprobe Raman spectroscopy. Results and Discussion Figs. 2(a) and 2(b) show Nomarski optical micrographs of Ge1-xSnx (x: 0.05 and 0.2, respectively) samples after annealing at 250oC for 10 min. Au pattern regions are located in the left side areas of the micrographs. In the case of the low Sn concentration (5%), different contrast regions are observed around the Au patterns, as indicated by the broken lines. On the other hand, the sample with the high Sn concentration (20%) shows no change around the Au pattern after annealing. The Raman spectra obtained from these samples are summarized in Fig. 2(c). In Raman spectra #1 for the low Sn concentration (5%), a sharp peak due to Ge-Ge bonding in c-Ge is clearly observed. On the other hand, no peaks are detected in spectra #3 for the sample with the high Sn concentration (20%). These results indicate that lateral growth is generated from the Au patterns for samples with the low Sn concentration (5%), while it is not generated for the high Sn concentration (20%).The lateral growth lengths at 150 and 250oC for samples with various Sn concentrations were estimated using theNomarski optical microscopy combined with line-scanning of microprobe Raman measurements. The results are summarized as a function of the annealing time in Fig. 3. In the case of low Sn concentrations (<5%), lateral growth lengths increase in a short annealing time (10 min). In addition, the growth lengths increase with increasing annealing temperature and exceed 20 μm at 250°C, which is sufficient for device fabrication.The substitutional Sn concentrations in grown layers for samples with the initial Sn concentration of 5% are evaluated by peak shift of Raman spectra. Substitutional Sn concentrations of 0.5−2.0% are obtained in grown layers, which is useful to passivate point defects in Ge [1]. The substitutional Sn concentration depended on the annealing temperature. This indicates that the substitutional Sn concentration can be controlled by modulating annealing temperature. This technique will be useful to realize next generation flexible electronics.

Cooling Rate Dependent High Substitutional Sn Concentration (>10%) in GeSn Crystals on Insulator by Pulsed Laser-Annealing

Crystallization of Ge1-xSnx (0.1≤x≤0.2) on quartz substrate by pulsed laser annealing was investigated. Substitutional Sn concentration increase with decreasing pulse number. As a result, GeSn crystals with very high substitutional Sn concentration (~12%) has been realized for the sample (initial Sn concentration: 15%) with a single shot. These results were attributed to the non-thermal equilibrium growth with higher cooling rate by decreasing pulse number. This technique is expected for application to high-speed thin-film-transistors and high-efficiency optical devices.

Pulse number controlled laser annealing for GeSn on insulator structure with high substitutional Sn concentration

Crystalline GeSn-on-insulator structures with high Sn concentration (&amp;gt;8%), which exceeds thermal equilibrium solid-solubility (∼2%) of Sn in Ge, are essential to achieve high-speed thin film transistors and high-efficiency optical devices. We investigate non-thermal equilibrium growth of Ge1−xSnx (0 ≤ x ≤ 0.2) on quartz substrates by using pulsed laser annealing (PLA). The window of laser fluence enabling complete crystallization without film ablation is drastically expanded (∼5 times) by Sn doping above 5% into Ge. Substitutional Sn concentration in grown layers is found to be increased with decreasing irradiation pulse number. This phenomenon can be explained on the basis of significant thermal non-equilibrium growth achieved by higher cooling rate after PLA with a lower pulse number. As a result, GeSn crystals with substitutional Sn concentration of ∼12% are realized at pulse irradiation of single shot for the samples with the initial Sn concentration of 15%. Raman spectroscopy and electron microscopy measurements reveal the high quality of the grown layer. This technique will be useful to fabricate high-speed thin film transistors and high-efficiency optical devices on insulating substrates.

Low-temperature (∼180 °C) position-controlled lateral solid-phase crystallization of GeSn with laser-anneal seeding

To realize next-generation flexible thin-film devices, solid-phase crystallization (SPC) of amorphous germanium tin (GeSn) films on insulating substrates combined with seeds formed by laser annealing (LA) has been investigated. This technique enables the crystallization of GeSn at controlled positions at low temperature (∼180 °C) due to the determination of the starting points of crystallization by LA seeding and Sn-induced SPC enhancement. The GeSn crystals grown by SPC from LA seeds showed abnormal lateral profiles of substitutional Sn concentration. These lateral profiles are caused by the annealing time after crystallization being a function of distance from the LA seeds. This observation of a post-annealing effect also indicates that GeSn with a substitutional Sn concentration of up to ∼10% possesses high thermal stability. These results will facilitate the fabrication of next-generation thin-film devices on flexible plastic substrates with low softening temperatures (∼250 °C).

High Sn-concentration (~ 8%) GeSn by low-temperature (~ 150 °C) solid-phase epitaxy of a-GeSn/c-Ge

GeSn with a high substitutional Sn concentration (>7%) is an attractive direct band gap material for high-efficiency photodevices that can be merged with large-scale integrated circuits (LSIs). To achieve GeSn with high Sn concentration, low-temperature solid-phase epitaxy using amorphous-GeSn (a-GeSn) (Sn concentration: 10%–36%)/crystal-Ge (c-Ge) stacked structures was investigated. Solid-phase growth of GeSn was enhanced as Sn concentration was increased, which enabled epitaxial growth at very low temperature (150–200°C). Interestingly, concentrations of substitutional Sn increased with decreasing growth temperature. As a result, epitaxial growth of GeSn with substitutional Sn concentrations of ~8% was achieved by decreasing the growth temperature to 150°C using a-GeSn (Sn concentration: 36%)/c-Ge stacked structures. This technique is expected to be useful to realize multi-function LSIs, where high-efficiency photodevices are integrated with transistors.

Non-Thermal Equilibrium Formation of Ge1-xSnx (0≤x≤0.2) Crystals on Insulator by Pulsed Laser Annealing

To realize high-speed thin film transistors, we investigated formation of Ge1-xSnx (0≤x≤0.2) crystals on quartz by pulsed laser annealing. The process-window necessary for crystallization was found to be significantly expanded by introducing Sn, i.e., 16 mJ/cm2 for Ge, ~130 mJ/cm2 for GeSn. Moreover, Ge0.8Sn0.2 samples had high substitutional Sn concentration (~7%) after the growth. These results are expected to be useful to realize high-speed thin film transistors and multi-functional devices.

Ultra-Low Temperature (~180°C) Solid-Phase Crystallization of GeSn on Insulator Triggered by Laser-Anneal Seeding

Solid-phase crystallization (SPC) of amorphous-GeSn (a-GeSn) films on insulating substrates has been developed combining with laser-annealed seeding, to realize next generation thin-film devices. By this technique, we have realized crystallization of GeSn (>10%) at low temperatures (~180oC), which is applicable for flexible thin-film devices on low cost plastic substrates with low softening temperatures (~200oC). In addition, the starting point of crystallization can be controlled by seeding, which is big advantage in device designing. Introduction In order to realize next generation flexible devices, novel functional materials have to be crystallized at controlled positions on insulating substrates below the softening temperature of low cost plastic substrates (~200oC). GeSn is attracting material for this purpose, since GeSn has higher carrier mobility than Si or Ge, and moreover, GeSn shows direct band structure by introducing substitutional Sn concentration more than 8%. Under such background, we have investigated solid-phase crystallization (SPC) of GeSn [1] and realized growth of GeSn at 180oC. However, to initiate the SPC at controlled positions, the samples have to be annealed above the melting point of Sn (~232oC) to form seeding regions, which is not applicable on flexible plastic substrates. In line with this, we propose an idea to use laser-annealing (LA) to form crystal seeds. By forming seeds by LA, lateral SPC is expected at process temperature below 200°C. Moreover, by decreasing the process temperature, we can expect that GeSn crystals with high substitutional Sn concentration should be obtained. Experimental Procedure In the experiment, amorphous Ge0.8Sn0.2 layers (100 nm thickness) were deposited on quartz substrates by a molecular beam technique. The samples were irradiated with a KrF excimer laser (wavelength: 248 nm, energy: 160 mJ/cm2) with 100 pulses in 1 sec to form seed regions, followed by thermal annealing (180 - 220 oC) for lateral SPC as shown in Fig. 1. Results and Discussion Nomarski images of samples as LA and after annealing at 180oC for 48h are shown in Figs. 2(a) and 2(b), respectively. The laser annealed seed region is formed as shown in Fig. 2(a). From the seed, a bright region spreads after 180oC annealing. The Raman spectra obtained at points A, B, and C in Figs. 2(a) and 2(b) are shown in Fig. 2(c). From these spectra, we can clearly say that lateral SPC is initiated from only the seed and no spontaneous nucleation occurs. From the length of lateral SPC region, growth rates of GeSn at various annealing temperatures (180, 200, 220 oC) are evaluated. It is found that the growth rate is about 1 μm/h at 180 oC, which is nearly 106 time as large as that of Ge [2]. The activation energy of the growth rate of Ge0.8Sn0.2 was estimated to be ~1.5 eV, which was smaller than that of Ge (~2.0 eV). This is attributed to the weakening atomic bonds by Sn atoms. This is a very useful technique to realize next generation flexible thin-film devices. Substitutional Sn concentration in grown layer obtained after annealing at 180oC for 48h is evaluated by Raman measurement, which is shown in Fig. 3. Substitutional Sn concentration in SPC region shows higher value (>10%) than that of LA region (~7.5%), which is attributed to difference of non-equilibrium conditions between SPC and LA. Interestingly, substitutional Sn concentration shows very high values exceeding 15% near the growth edge. The Sn concentration decreases with increasing distance from the edge. Decrease in the Sn concentration from >15% to ~10% is attributed to the post annealing effect of grown regions. Detailed physics will be discussed at the presentation.

Non-Thermal Equilibrium Formation of Ge1-xSnx (0≤x≤0.2) Crystals on Insulator by Pulsed Laser Annealing

Introduction Formation of crystalline GeSn film on insulator with high Sn concentration exceeding thermal equilibrium solid-solubility is strongly expected to realize high-speed thin film transistors. To explore non-thermal equilibrium processing [1], we examined crystallization of amorphous-GeSn (a-GeSn) with various modulated Sn concentration by pulsed laser annealing (PLA). In the present study, we focus on the influence of the Sn concentration on crystallization of a-GeSn by PLA. Experimental Procedure Schematic sample structure and PLA conditions are shown in Fig.1. A-Ge1-xSnx (0≦x≦0.2) films (thickness: 100 nm) were deposited on quartz substrates by a molecular beam technique. These samples were irradiated with a pulsed laser (energy : 10 - 230 mJ/cm2, pulse period : 1 sec, pulse number : 100 shot). The laser annealed regions were analyzed by Nomarski microscopy and Raman scattering spectroscopy. Results and Discussion Nomarski micrograph and Raman spectra of Ge0.8Sn0.2 samples before and after PLA with various energy are summarized in Fig. 2. In the samples after PLA, the contrast-changed regions were observed, compared with the as-deposited sample. Here, the contrast becomes stronger with increasing energy from 50 mJ/cm2 to 170 mJ/cm2, and finally the GeSn layer was damaged for energy of 230 mJ/cm2. Raman spectra reveales that Ge0.8Sn0.2 is crystallized on insulating substrates by 50 - 170 mJ/cm2 PLA. The process-window to achieve crystalline GeSn is summarized in Fig. 3(a), where the energy for starting crystallization and the energy just before damaging are indicated as Ecryst. and Edamage , respectively. Substitutional Sn concentration estimated from shift of Ge-Ge peak obtained by Raman measurements is shown in Fig. 3(b). The process window necessary for crystallization is significantly expanded by introducing Sn, i.e. 16 mJ/cm2 for Ge, ~ 130 mJ/cm2 for GeSn. High Sn concentration exceeding thermal equilibrium solid-solubility is obtained by PLA, for all GeSn samples. Interestingly, for samples with initial Sn concentration of 5 % and 10 %, almost all Sn atoms are incorporated into substitutional site, while for 20 % sample, only half of Sn atoms are incorporated into substitutional site. The detailed physics will be discussed in the presentation.

Structural and Optical Properties of Amorphous and Crystalline GeSn Layers on Si

We have investigated the structural and optical properties of metastable amorphous and crystalline GeSn layers on Si substrates. The as-deposited amorphous layers crystallize during annealing at 500°C. This transition leads to a significant change in the local environment of the Sn atoms and in the optical properties. The Ge-Sn bond length is decreased after crystallization. The as-deposited GeSn layers, with nominal 4.5% and 11.3% Sn content, do not show Sn segregation. For the crystallized GeSn with nominal 4.5%, the Sn appears to be substitutional, as no Sn clustering was observed. However, for the crystallized GeSn with nominal 11.3% Sn, Sn segration and the presence of β-Sn is observed by EXAFS. A method to suppress Sn segregation and increase the substitutional Sn concentration is discussed. Furthermore, we determined the optical properties of amorphous and crystalline GeSn with nominal 4.5% Sn. The bandgap energy decreases significantly from 0.89 eV (1392 nm) ± 0.05 eV for the amorphous layer to 0.52 eV (2383 nm) ± 0.05 eV for crystalline GeSn, leading to significant reduction in penetration depth.

Laser synthesis of germanium tin alloys on virtual germanium

Synthesis of heteroepitaxial germanium tin (GeSn) alloys using excimer laser processing of a thin 4 nm Sn layer on Ge has been demonstrated and studied. Laser induced rapid heating, subsequent melting, and re-solidification processes at extremely high cooling rates have been experimentally achieved and also simulated numerically to optimize the processing parameters. “In situ” measured sample reflectivity with nanosecond time resolution was used as feedback for the simulations and directly correlated to alloy composition. Detailed characterization of the GeSn alloys after the optimization of the processing conditions indicated substitutional Sn concentration of up to 1% in the Ge matrix.