Crystalline GeSn Research Articles

Growth of single crystalline GeSn alloy epilayer on Gd2O3/Si (111) engineered insulating substrate using RF sputtering and solid phase epitaxy

The article showcases a low-cost, low-temperature deposition andHVMtechnique to develop single crystalline GeSn alloy epilayers on Gd2O3/Si (111) substrate. First, GeSn alloy amorphous layer is deposited on the insulating substrates using an Radio Frequency (RF) sputtering apparatus. Subsequently, an inductively coupled plasma-assisted chemical vapor deposition (ICP-CVD) process is used to deposit a SiO2 capping layer to protect against Sn out-diffusion during heat treatment. The samples are then subjected to solid phase epitaxy (SPE) at 450 °C, 550 °C, and 650 °C. Sample processed for SPE at 450 °C has weak crystallinity and only shows Type-A stacking. Those processed for SPE at 550 °C and 650 °C, on the other hand, have revealed formation of the single-crystalline GeSn alloy epilayer with Type-A and Type-B stacking. However, SPE at 650 °C revealed tin out-diffusion and segregation effects. This work is significant for enabling the preparation of high-Sn-content GeSn alloy epilayers on insulating Gd2O3/Si (111) substrates, as it requires the initial deposition of a GeSn amorphous alloy epilayer using RF sputtering. This advancement promises benefits which includes advantages such as lower operating voltage, reduced leakage current, and minimized parasitic and short-channel effects, making it ideal for advancing RF technology.

Photoluminescence characterization of GeSn prepared by rapid melting growth method

In this work, single crystalline GeSn on insulator (GSOI) were grown by rapid melting growth (RMG). The Sn content distribution along the strip was investigated, reaching to 7.8% near the end of the strip. The GeSn strip had high quality with no threading dislocations, as revealed by cross-sectional transmission electron microscopy (XTEM). Photoluminescence (PL) measurements along the strip were performed to check the indirect-direct bandgap transition Sn content of GeSn alloys. It was found the integrated PL intensity increased about 6.6 times for Sn content 0.86%–7.8%. For GeSn with Sn content 6.1%, the temperature-dependent PL shows that the energy difference between direct Γ-Γ valley and indirect L-Γ valley decreases due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. The temperature-dependent PL spectra have only one peak for GeSn with Sn content 7.8%, suggesting the direct bandgap property. These results indicate that GeSn grown by RMG is very promising for fabricating efficient Si based light sources.

GeSn/SiO2 Multilayers by Magnetron Sputtering Deposition for Short-Wave Infrared Photonics.

The development of short-wave infrared (SWIR) photonics based on GeSn alloys is of high technological interest for many application fields, such as the Internet of things or pollution monitoring. The manufacture of crystalline GeSn is a major challenge, mainly because of the low miscibility of Ge and Sn. The use of embedded GeSn nanocrystals (NCs) by magnetron sputtering is a cost-effective and efficient method to relax the growth conditions. We report on the use of GeSn/SiO2 multilayer deposition as a way to control the NC size and their insulation. The in situ prenucleation of NCs during deposition was followed by ex situ rapid thermal annealing. The nanocrystallization of 20×(11nm_Ge0.865Sn0.135/1.5nm_SiO2) multilayers leads to formation of GeSn NCs with ∼16% Sn concentration and ∼9 nm size. Formation of GeSn domes that are vertically correlated contributes to the nanocrystallization process. The absorption limit of ∼0.4 eV in SWIR found by ellipsometry is in agreement with the spectral photosensitivity. The ITO/20×(GeSn NC/SiO2)/p-Si/Al diodes show a maximum value of the SWIR photosensitivity at a reverse voltage of 0.5 V, with extended sensitivity to wavelengths longer than 2200 nm. The multilayer diodes have higher photocurrent efficiency compared to diodes based on a thick monolayer of GeSn NCs.

Spontaneously Conversion from Film to High Crystalline Quality Stripe during Molecular Beam Epitaxy for High Sn Content GeSn

Two series of Ge0.8Sn0.2 samples were grown on Ge buffered Si substrate by molecular beam epitaxy (MBE) to investigate the influence of growth temperature and film thickness towards the evolution of surface morphology. A novel phenomena was observed that the Ge0.8Sn0.2 film was segregated and relaxed by the formation of GeSn stripes on the film. Under specific growth condition, the stripes can cover nearly the whole surface. XRD, TEM, AFM, PL and TEM results indicated that the stripes are high quality single crystalline GeSn with Sn content around 5%. The formation of GeSn stripes proposes an effective strategy to fabricate high crystalline quality GeSn stripe on Si, where the Ge0.8Sn0.2 film serves as precursor and the segregated Sn works as catalyst droplets. This technique has great potential for future optoelectronic and microelectronic applications.

Low-Temperature Plasma-Assisted Growth of Core–Shell GeSn Nanowires with 30% Sn

We report on the growth of Sn-catalyzed GeSn nanowires (NWs) having a GeSn core and a c-Ge shell in the presence of germane plasma at substrate temperatures (TS) below the GeSn eutectic temperature (TE), containing an exceptional Sn concentration of 30 at. % in their core. The differences between the NWs produced at TS above and below TE of the GeSn alloy are highlighted. Two types of NW growth process are identified: the previously reported in-plane solid–liquid–solid (IPSLS) process for TS ≥ TE and a plasma-assisted IPSLS (PA-IPSLS) method taking place at TS < TE; the crucial role of plasma in providing the energy necessary to melt the Sn catalyst at substrate temperatures lower than TE is discussed. The thermal activation window for each method is determined. The PA-IPSLS process is shown to provide an efficient strategy for the growth of crystalline GeSn NWs with a high Sn incorporation in a growth duration of less than 3 min.

Low-temperature formation of GeSn nanodots by Sn mediation

GeSn is expected as a light-emitting material in Si photonics because a direct-bandgap nature appears with Sn composition above 10%. However, the emission wavelength of GeSn is in the mid-IR range. Thus, GeSn nanodots (NDs) are needed for telecom-wavelength emission. In this paper, we propose a formation method for GeSn NDs with Sn mediation. This process consists of low-temperature deposition of Sn and GeSn by vacuum evaporation and subsequent low-temperature annealing. Crystalline GeSn NDs with high density and high Sn content (more than 10%) have been successfully formed without metallic Sn precipitation or segregation of Sn on the dot surface. The possible formation mechanism of Sn-mediated GeSn NDs is also discussed.

Band gap renormalization in n-type GeSn alloys made by ion implantation and flash lamp annealing

The last missing piece of the puzzle for the full functionalization of group IV optoelectronic devices is a direct bandgap semiconductor made by CMOS compatible technology. Here, we report on the fabrication of GeSn alloys with Sn concentrations up to 4.5% using ion implantation followed by millisecond-range explosive solid phase epitaxy. The n-type single crystalline GeSn alloys are realized by co-implantation of Sn and P into Ge. Both the activation of P and the formation of GeSn are performed during a single-step flash lamp annealing for 3 ms. The bandgap engineering in GeSn as a function of the doping level and Sn concentration is theoretically predicted by density functional theory and experimentally verified using ellipsometric spectroscopy. We demonstrate that both the diffusion and the segregation of Sn and P atoms in Ge are fully suppressed by millisecond-range nonequilibrium thermal processing.

GeSn Nanocrystals in GeSnSiO2 by Magnetron Sputtering for Short-Wave Infrared Detection

Detection in short-wave infrared (SWIR) has become a very stringent technology requirement for developing fields like hyperspectral imaging or climate changes. In a market dominated by III–V materials, GeSn, a Si compatible semiconductor, has the advantage of cost efficiency and inerrability by using the mature Si technology. Despite the recent progress in material growth, the easy fabrication of crystalline GeSn still remains a major challenge, and different methods are under investigation. We present the formation of GeSn nanocrystals (NCs) embedded in oxide matrix and their SWIR characterization. The simple and cost-effective fabrication method is based on thermal treatment of amorphous (Ge1–xSnx)y(SiO2)1–y layers deposited by magnetron sputtering. The nanocrystallization for Ge1–xSnx with 9–22 at. % Sn composition in SiO2 matrix with 9% to 15% mole percent was studied under low thermal budget annealing in the 350–450 °C temperature range. While the Sn at.% content is the main parameter influencing the band-structure of the NCs, the SWIR sensitivity can be optimized by SiO2 content and H2 gas component in the deposition atmosphere. Their role is not only changing the crystallization parameters but also to reduce the carrier recombination by passivation of NCs defects. The experiments indicate a limited composition dependent temperature range for GeSn NCs formation before β-Sn phase segregation occurs. NCs with an average size of 6 nm are uniformly distributed in the film, except the surface region where larger GeSn NCs are formed. Spectral photovoltaic current measured on SiO2 embedded GeSn NCs deposited on p-Si substrate shows extended SWIR sensitivity up to 2.4 μm for 15 at. % Sn in GeSn NCs. The large extension of the SWIR detection is a result of many factors related to the growth parameters and also to the in situ or ex situ annealing procedures that influence the uniformity and size distribution of NCs.

Crystalline GeSn growth by plasma enhanced chemical vapor deposition

Single crystalline GeSn growth on Si substrate was successfully demonstrated by using plasma enhanced chemical vapor deposition (PE-CVD) with commercially available GeH4 and SnCl4 precursors. Using the plasma enhancement technique, low temperature growth at 350°C for GeSn epitaxy on Si substrate was achieved with the growth rate of 51.4 nm/min and Sn content up to 6%. The relaxed GeSn films with 1 µm thickness were able to be grown despite of the huge lattice mismatch between GeSn and Si. Structural and optical characterizations were conducted to study the film properties. The demonstrated plasma enhancement growth showed its effectiveness to enhance the Sn incorporation of the crystalline GeSn at low temperature and to maintain the high growth rate.

Characterization of crystalline GeSn layer on tensile-strained Ge buffer deposited by magnetron sputtering

High-quality low Sn-content GeSn film has been grown on tensile strained Ge/Si (100) substrate using a physical vapor deposition reactor and no subsequent annealing was carried out after the deposition of the film. High Resolution X-ray diffraction (HR-XRD) measurement was performed to estimate the crystallinity and the residual strain in the Ge layer. Secondary-ion mass spectrometry (SIMS) and X-ray photoelectron spectrometer (XPS) show Sn incorporation of the film is approximately 3%. High-resolution transmission electron microscopy (TEM) and atomic force microscopy (AFM) results show that the crystallinity of the film is quite good and it shows a smooth surface (RMS = 0.744 nm) in 5 µm × 5 µm scan. Raman spectroscopy and photoluminescence (PL) measurements were carried out to study the structural and optical properties of the film. Room-temperature PL of the film was observed and it results in a decrease in the difference between Γ and L valleys of tensile strained Ge (0.21%) from 87 meV to 76 meV, which makes this method promising for obtaining Pseudo-direct bandgap group IV films.

Growth of high-Sn content (28%) GeSn alloy films by sputtering epitaxy

Crystalline GeSn thin films with Sn content up to 0.28 were deposited on Sn graded GeSn buffer on a Ge substrate at low temperatures by sputtering epitaxy. The structural properties of the high-Sn content GeSn alloy films were characterized by high resolution transmission electron microscopy and X-ray diffraction. The effect of annealing on the segregation of Sn in the high-Sn content GeSn film was investigated, and both the Ge0.72Sn0.28 and the Ge0.8Sn0.2 films were found to be stable after annealing at temperatures below 400 °C, which meets the needs of thermal budget for future photonic devices fabrication. The present results indicate that sputtering epitaxy is cost-effective method for growing high-Sn GeSn films.

Formation of high-Sn content polycrystalline GeSn films by pulsed laser annealing on co-sputtered amorphous GeSn on Ge substrate**Project supported by the National Natural Science Foundation of China (Grant No. 61474094) and the National Basic Research Program of China (GrantNo. 2013CB632103).

Polycrystalline Ge1−x (poly-Ge1−xSnx) alloy thin films with high Sn content (> 10%) were fabricated by cosputtering amorphous GeSn (a-GeSn) on Ge (100) wafers and subsequently pulsed laser annealing with laser energy density in the range of 250 mJ/cm2 to 550 mJ/cm2. High quality poly-crystal Ge0.90Sn0.10 and Ge0.82Sn0.18 films with average grain sizes of 94 nm and 54 nm were obtained, respectively. Sn segregation at the grain boundaries makes Sn content in the poly-GeSn alloys slightly less than that in the corresponding primary a-GeSn. The crystalline grain size is reduced with the increase of the laser energy density or higher Sn content in the primary a-GeSn films due to the booming of nucleation numbers. The Raman peak shift of Ge–Ge mode in the poly crystalline GeSn can be attributed to Sn substitution, strain, and disorder. The dependence of Raman peak shift of the Ge–Ge mode caused by strain and disorder in GeSn films on full-width at half-maximum (FWHM) is well quantified by a linear relationship, which provides an effective method to evaluate the quality of poly-Ge1−xSnx by Raman spectra.

Low-temperature synthesis of crystalline GeSn with high Sn concentration by electron excitation effect

The low-temperature synthesis of high-Sn-concentration GeSn is challenging in realizing flexible thin-film transistors and solar cells. Because of athermal processes, irradiation with energetic particles is anticipated to significantly reduce the processing temperature for device fabrication. Here, we demonstrated that polycrystalline Ge with ∼30 at. % Sn can be realized at room temperature by the electron-beam-induced recrystallization of amorphous GeSn. We found that inelastic electronic stopping, the so-called electron excitation effect, plays an important role in the recrystallization of amorphous GeSn.

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.

Formation of nanocrystalline GeSn thin film on Si substrate by sputtering and rapid thermal annealing

Nanocrystalline Ge1-xSnx thin films have been formed after rapid thermal annealing of sputtered GeSn layers. The alloy films were deposited onto the Silicon (100) substrate via low cost radio frequency magnetron sputtering. Then, the films were annealed by rapid thermal annealing at 350 °C, 400 °C, and 450 °C for 10 s. The morphological, structural, and optical properties of the layers were investigated with field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and high-resolution X-ray diffraction (HR-XRD). The Raman analysis showed that the only observed phonon mode is attributed to Ge-Ge vibrations. Raman phonon intensities of GeSn thin films were enhanced with increasing the annealing temperature. The results clearly revealed that by increasing the annealing temperature the crystalline quality of the films were improved. The XRD measurements revealed the nanocrystalline phase formation in the annealed films with (111) preferred orientation. The results showed the potentiality of using the sputtering technique and rapid thermal anneal to produce crystalline GeSn layer.

Raman scattering study of amorphous GeSn films and their crystallization on Si substrates

The dependence of Raman scattering spectra on Sn composition in the amorphous Ge1-xSnx films deposited on Si(001) substrates by magnetron sputtering is investigated. Two broad phonon scattering peaks are clearly observed, which are attributed to the disordered SnSn and GeGe bonds, shifting to lower wavenumbers with the increase of Sn content. The ratio of integrated intensity of SnSn to GeGe modes increases linearly with Sn content in the range of 0.06 to 0.2. With rapid thermal annealing, although the critical crystallization temperature becomes lower for higher Sn content amorphous GeSn, severe Sn segregation occurs for all of the samples. Relative lower crystallization temperature is proved to be beneficial for tailoring high Sn content crystalline GeSn films. Polycrystalline GeSn alloys with maximum substitutional Sn content of 0.07 are achieved with an annealing temperature of 400°C.

Seeding Effects of Sn/a-Ge Island Structures for Low-Temperature Lateral-Growth of a-GeSn on Insulator

Position control of low-temperature solid-phase crystallization in amorphous-GeSn (a-GeSn) films is investigated using a-GeSn films covering Sn/a-Ge stacked islands on insulator. Two-steps annealing has been performed, i.e., first-step annealing (∼250°C) to crystallize Sn/a-Ge islands for crystalline-seed formation and second-step annealing (≤250°C) to proceed lateral-growth from the crystalline-seed. It is found that Sn thickness exceeding ∼70% of Ge thickness is the essential condition to achieve crystalline GeSn islands (grain size >10 nm) with the diamond structure. This acts as crystalline-seed enabling the lateral-growth of a-GeSn over 80 μm. The present process below the softening-temperature (∼300°C) of plastic substrates facilitates realization of advanced flexible electronics.

Parametrized dielectric functions of amorphous GeSn alloys

We obtained the complex dielectric function of amorphous Ge1−xSnx (0 ≤ x ≤ 0.07) alloys using spectroscopic ellipsometry from 0.4 to 4.5 eV. Amorphous GeSn films were formed by room-temperature implantation of phosphorus into crystalline GeSn alloys grown by molecular beam epitaxy. The optical response of amorphous GeSn alloys is similar to amorphous Ge and can be parametrized using a Kramers-Kronig consistent Cody-Lorentz dispersion model. The parametric model was extended to account for the dielectric functions of amorphous Ge0.75Sn0.25 and Ge0.50Sn0.50 alloys from literature. The compositional dependence of band gap energy Eg and parameters associated with the Lorentzian oscillator have been determined. The behavior of these parameters with varying x can be understood in terms of the alloying effect of Sn on Ge.

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.

Ultra-high-speed lateral solid phase crystallization of GeSn on insulator combined with Sn-melting-induced seeding

To produce high-performance devices on flexible plastic substrates, it is essential to form Ge-based group IV semiconductors on insulating substrates at low temperatures (≤250 °C). We have developed a technique for solid phase crystallization of amorphous GeSn (≤220 °C) enhanced by Sn doping, and combined with a seeding technique induced by Sn melting (∼250 °C). This combination produces lateral crystallization of amorphous GeSn from seed arrays with no incubation time. As a result, extremely high growth velocities at 220 °C, depending on Sn concentration, e.g., 0.13 μm/h (14% Sn) and 1100 μm/h (23% Sn), are achieved. These velocities are 104–108 times higher than that of pure Ge. This technique enables growth of crystalline GeSn island arrays (diameters: 50–150 μm) at low temperatures (≤250 °C) at controlled positions on insulating substrates.