SiGe Islands Research Articles

Evolution of GeSi islands in epitaxial Ge-on-Si during annealing

Epitaxial growth of Ge on Si has been widely studied for photonic and electronic applications. However, GeSi interdiffusion during the growth or annealing deteriorates the performance of the devices. This paper explores the evolution of the interface and GeSi interdiffusion upon post-annealing. We found the GeSi interdiffusion islands with concentration plateaus gradually disappeared upon annealing and the interdiffusion became more pronounced in the Ge layer. Moreover, by using a thin layer of SiO2 as a reference, we directly visualized the Kirkendall phenomenon, i.e., the migration of the Ge/Si interface towards the Si substrate after the annealing. Effective interdiffusivity at 900 °C was extracted using Boltzmann-Matano analysis, which indicated that the GeSi interdiffusion at the interface was dominated by dislocation-mediated interdiffusion.

Light-emitting diodes with Ge(Si) nanoislands embedded in photonic crystals

Room temperature lateral p+–i–n+ light-emitting diodes (LEDs) with photonic crystals embedded in the i-region were fabricated on structures with Ge(Si) self-assembled islands and their optical properties were investigated. The use of preliminary amorphization and solid phase epitaxy of the implanted p+ and n+ contact regions made it possible to reduce the impurity activation temperature from 800 °С–1100 °С to 600 °С, which corresponds to the growth temperature of Ge(Si) islands. This resulted in a significant reduction of the detrimental effect of the high-temperature annealing used for diode formation on the intensity and spectral position of the luminescence signal from the islands. It was shown that significant enhancement (more than an order of magnitude) of room temperature electroluminescence of Ge(Si) islands in the spectral range of 1.3–1.55 μm can be achieved due to their interaction with different modes of the photonic crystals. The measured radiation power of the obtained diodes in the spectral range of 1.3–1.55 μm exceeds 50 pW at a pump current of 8 mA, which is an order of magnitude higher than the previously achieved values for micro-LEDs with Ge(Si) nanoislands. The obtained results open up new possibilities for the realization of silicon-based light emitting devices operating at telecommunication wavelengths.

Interaction of Ge(Si) Self-Assembled Nanoislands with Different Modes of Two-Dimensional Photonic Crystal.

The interaction of Ge(Si)/SOI self-assembled nanoislands with modes of photonic crystal slabs (PCS) with a hexagonal lattice is studied in detail. Appropriate selection of the PCS parameters and conditions for collecting the photoluminescence (PL) signal allowed to distinguish the PCS modes of different physical nature, particularly the radiative modes and modes associated to the bound states in the continuum (BIC). It is shown that the radiative modes with relatively low Q-factors could provide a increase greater than an order of magnitude in the integrated PL intensity in the wavelength range of 1.3–1.55 µm compared to the area outside of PCS at room temperature. At the same time, the interaction of Ge(Si) islands emission with the BIC-related modes provides the peak PL intensity increase of more than two orders of magnitude. The experimentally measured Q-factor of the PL line associated with the symmetry-protected BIC mode reaches the value of 2600.

Linear and nonlinear optical properties of dewetted SiGe islands

We propose to exploit the natural mechanical instability of thin solid films to form regular patterns of monocrystalline atomically smooth silicon and germanium nanostructures that cannot be realized with conventional methods. The solid-state dewetting dynamics is guided by pre-patterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final Si1-xGex structures. Here we describe our progress in the spectroscopic investigation of individual dewetted Si1-xGex nanoislands: in the linear regime, bright Mie-type localized resonances are detected in the visible spectral range, with a spectral position that can be tuned by modifying the size of the nanoparticles. In the non-linear regime, instead, sizable third-harmonic generation is observed at the level of single islands. We believe that these results will be pivotal to a novel approach in spectral filtering, sensing and structural color with all-dielectric photonic devices.

Luminescence of Spatially Ordered Self-Assembled Solitary Ge(Si) Nanoislands and their Groups Incorporated into Photonic Crystals

The luminescence properties of arrays of spatially ordered self-assembled solitary Ge(Si) nanoislands and their groups, including those embedded in two-dimensional photonic crystals, are studied. It is shown that the incorporation of an array of ordered solitary Ge(Si) islands and their groups into photonic crystals results in an increase in the intensity of their photoluminescence signal at liquid-nitrogen temperature. The maximum increase in the intensity (by a factor of up to ~30) is observed for an ordered array of solitary Ge(Si) islands. The increase in the intensity is attributed to the interaction of emission from islands with photonic-crystal radiative modes. This interaction is more efficient in the case of an array of solitary islands. Due to such interaction the luminescence signal from ordered solitary Ge(Si) islands incorporated into photonic crystals is observed at up to room temperature.

Impact of Ge deposition temperature on parameters of c-Si solar cells with surface texture formed by etching of Si using SiGe islands as a mask

In this paper, the Ge self-assembled islands grown at different temperatures were used as an etching mask for selective anisotropic etching of crystalline silicon (c-Si) in order to form a textured Si surface. The effects of Ge deposition temperature on the texture morphology as well as the parameters of c-Si solar cells fabricated on such structures were studied with the range of temperature from 500 °C to 800 °C. The textures on Si surface with low (<1 μm) etching margin were successfully fabricated for all investigated structures. It was revealed that usage of rapid annealing of Si wafer at 800 °C prior to epitaxial growth along with the low Ge deposition temperature of 500 °C allowed to achieve the relatively high effective carrier lifetime as compared to structures formed at higher (≥700 °C) temperatures thanks to the reduced degradation of Si wafer characteristics. Furthermore, the structure obtained using the lowest deposition temperature of 500 °C exhibited the highest light absorption among all studied samples thanks to the most homogeneous and most pyramidal texture morphology. As a result, the calculated short circuit current density JSC for the sample in which Ge islands were grown at 500 °C was as high as 35.89 mA/cm2. The proposed approach of surface texturing meets the requirements for thin c-Si solar cells due to small etching margin and the obtained results pave the way for increase the efficiency of thin c-Si solar cells by engineering the surface morphology to improve light absorption.

Raman microscopy and infrared optical properties of SiGe Mie resonators formed on SiO2 via Ge condensation and solid state dewetting

All-dielectric photonics is a rapidly developing field of optics and material science. The main interest at visible and near-infrared frequencies is light management using high-refractive-index Mie-resonant dielectric particles. Most work in this area of research focuses on exploiting Si-based particles. Here, we study monocrystalline Mie-resonant particles made of Ge-rich SiGe alloys with refractive index higher than that of Si. These islands are formed via solid state dewetting of SiGe flat layers by using two different processes: (i) dewetting of monocrystalline SiGe layers (60%–80% Ge content) obtained via Ge condensation of SiGe on silicon on insulator; and (ii) dewetting of a SiGe layer deposited via molecular beam epitaxy on silicon on insulator and ex situ Ge condensation, forming a Ge-rich shell surrounding a SiGe-core. Using high-spatial-resolution Raman microscopy we monitor Ge content x and strain ϵ of flat layers and SiGe-islands. We observe strain relaxation associated with formation of trading dislocations in the SiGe islands compared to the starting SiGe layers, as confirmed by TEM images. For initial high Ge concentration in the flat layers, the corresponding Ge content in the dewetted islands is lower, owing to diffusion of Si atoms from Si or SiO2 into SiGe islands. The Ge content also varies from particle to particle on the same sample. Size and shape of the dewetted particles depend on the fabrication process: thicker initial SiGe layers lead to larger particles. Samples with narrow island size distribution display rather sharp Mie resonances in the 1000–2500 nm spectral range. Larger islands display Mie resonances at longer wavelength. Positions of the resonances are in agreement with the theoretical calculations in the discrete dipole approximation.

Liquid phase epitaxy SiGe films on a CVD-grown SiGe/Si (0 0 1) graded film

Abstract SiGe films were grown on a chemical vapor deposition (CVD)-grown Si0.88Ge0.12/Si (0 0 1) graded film with different growth solution compositions using a temperature interval of 950-940 °C by liquid phase epitaxy (LPE). The LPE films grown on the CVD-grown SiGe/Si (0 0 1) graded film are ≥86% relaxed, much more relaxed than those on bare Si (Wang and Quitoriano, 2019) since the existing threading dislocations in the CVD-grown SiGe/Si (0 0 1) graded film glide to create misfit dislocations without needing to nucleate new ones. The threading dislocation density of the LPE films is slightly lower than that of the CVD-grown SiGe films and on the same order of magnitude, ~106 cm−2. Since the CVD-grown SiGe/Si (0 0 1) graded film was annealed for 4 h before the LPE growth occurred, the properties of the CVD-grown SiGe/Si (0 0 1) graded film were studied and we found that the CVD-grown SiGe/Si (0 0 1) graded film have SiGe islands appear all over the surface aligned along the 〈1 1 0〉 crosshatch morphology and become more relaxed after annealing.

Люминесценция пространственно упорядоченных одиночных и групп самоформирующихся Ge(Si) наноостровков, встроенных в фотонные кристаллы

Photoluminescence (PL) properties of arrays of spatially ordered single Ge(Si) self-assembled islands, including embedded in two-dimensional photonic crystals (PhC), was studied. Incorporation of an array of ordered single islands and their groups into PhC significantly increases the intensity of their PL signal. The most pronounced increase in intensity (up to 30 times at nitrogen temperatures) is observed for an ordered array of single islands. The increase in PL intensity is associated with interaction of island emission with the radiation modes of the PhC, which is more efficient for an array of spatially ordered single islands. As result PL from single ordered Ge(Si) islands embedded in PhC was observed up to room temperature.

Self-assemble SiGe island structures formed by nanosecond laser irradiation on SiGe virtual film

In the present work, laser-induced directed dewetting of smooth SiGe thin films generates SiGe islands with high aspect ratio of 0.9 on the surface. The laser-induced island is fully relaxed, and the thermal effect accelerates the relaxation of strain, which finally forms disorder surface islands. Furthermore, it is interesting to note that these surface SiGe islands can be served as masks, and uniformly orientated degree (30°) Si-based nanowires can be fabricated by Ar IBE. The proposed laser programming method features high precision, tunability and scalability with relatively limited patterning effort and opens a new way for fabricating efficiency light-emitting Si-based nano-Ge/SiGe heterostructures and Si integrated-circuit technology.

Kelvin force and Raman microscopies of flat SiGe structures with different compositions grown on Si(111) at high temperatures

The Ge deposition on Si(111) at the very high temperature of 900 °C is accompanied by an intense Si-Ge interdiffusion and leads to the formation of three-dimensional (3D) structures, such as flat islands and lateral nanowires located on wide atomically flat (111) terraces with high atomic steps at their edges. The use of Raman spectroscopy with high spatial resolution shows that the surface areas with different 3D structures have different Ge contents from about 0.04–0.10. The Si substrate under the SiGe surface layers is weakly strained, while the substrate areas around SiGe island edges display a relatively strong compression. The areas with different Ge contents form type II heterostructures in the surface plane. The Kelvin force microscopy (KFM) data reveal that the surface potential was maximal and, hence, the Ge content was minimal in the terrace areas located near 3D SiGe structures, indicating the presence of the solid-state dewetting effect. The spatial positions of maximal KFM potentials coincide with the heterojunction positions. The results demonstrate the correlation between the Ge concentration and the KFM potential that allows mapping the composition with a high KFM spatial resolution.

Self-assembled antireflection coatings for light trapping based on SiGe random metasurfaces

We demonstrate a simple self-assembly method based on solid state dewetting of ultrathin silicon films and germanium deposition for the fabrication of efficient antireflection coatings on silicon for light trapping. We fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows one to reduce the reflectance to low values in a broad spectral range (from 500 nm to 2500 nm) and a broad angle (up to ${55}^{\ensuremath{\circ}})$ and to trap within the wafer a large portion of the impinging light $(\ensuremath{\sim}40%)$ also below the band gap, where the Si substrate is nonabsorbing. Theoretical simulations agree with the experimental results, showing that the efficient light coupling into the substrate is mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin ${\mathrm{SiO}}_{2}$ layers and its duration only depends on the sample thickness and on the annealing temperature.

Crystallization of silicon–germanium by aluminum-induced layer exchange

We have studied the crystallization of amorphous silicon–germanium (a-SiGe) by aluminum (Al)-induced layer exchange (ALILE) with a starting structure of glass/Al/Al oxide/a-SiGe. We examined ALILE at 450 °C, which is slightly higher than the eutectic temperature of Ge and Al, in order to shorten the ALILE time. We successfully produced c-SiGe films oriented in the (111) direction for 16 h without significant alloying. The thickness of Al layers should be 2800 Å or more to complete the ALILE for the a-SiGe layers of 2000–2800 Å thickness. When the Al layer is as thick as the a-SiGe layer, almost uniform c-SiGe is formed on the glass substrate. On the other hand, the islands of c-SiGe are formed on the glass substrate when the Al layer is thicker than the a-SiGe layer. The islands become smaller with thicker Al layers because more excess Al remains between the SiGe islands. The results indicate that the configuration of c-SiGe can be altered from a uniform structure to island structures of various sizes by changing the ratio of a-SiGe thickness to Al thickness.

Formation of black silicon using SiGe self-assembled islands as a mask for selective anisotropic etching of silicon

Simple technique of formation of “black silicon” using wet chemical etching of crystalline Si wafers with SiGe self-assembled islands is proposed. The main idea consists of the utilization of SiGe islands as a mask for wet anisotropic etching of Si in alkali-based solution at the first etching stage and further removal of SiGe residuals by etching in a HF:H2O2:CH3COOH mixture at the second etching stage. Initial samples were the crystalline Si wafers with SiGe islands formed on them by deposition of 2.5–14nm of Ge at 800°C. After the two above-mentioned etching steps a submicron relief on a Si surface was formed. Investigation of optical properties of fabricated structures revealed significant decrease of reflection (AM 1.5G weighted reflection ~2–3%) and increase of absorption in the wavelength range of 500–1200nm. Due to the very small amount of Si removal (<0.5µm), utilization of standard chemicals for Si-based solar cell technology and potential suitability for usage in large-scale manufacturing the proposed technique is promising for increasing of efficiency of thin wafers crystalline Si solar cell.

Nanosecond laser induce size-controllable SiGe islands with high Ge composition, large aspect ratio and defect-free characteristics

In the present work, laser-induced directed dewetting (LIDD) of amorphous germanium thin films generates single crystalline SiGe islands on Si substrate. Furthermore, SiGe islands with a high aspect ratio of 1.06 are prepared by pulse laser irradiation. Variation of the nanosecond heating laser’s pulse fluence enables modulation of the nanoscale island size, and the diameter of islands can be selected in the range of 40nm–150nm.

Submicron- and micron-sized SiGe island formation on Si(100) by dewetting

The SiGe dewetting from Si(100) is found to take place during Ge deposition at 750–950°C. The dewetting provides the formation of compact islands with lateral sizes from about 0.1 to 3μm, large contact angles (θ) and aspect ratios up to 0.8 depending on the deposited Ge amount (d) and the Ge deposition rate. We derived an expression for the total energy of the structures with islands in the sphere segment shape that is typical at dewetting. Its analysis revealed that the use of such island shapes is valid if the strain energy at the island/substrate interface is less than the surface energy of the bare substrate areas. The energy as a function of θ for a given d allowed obtaining the dependence of θ on d, when the dependence on d was introduced via the strain energy, using the experimental data for the SiGe island composition measured for different d. The minimum is shallow for the relatively large d that explains the large scattering of θ observed in the experiment. The comparison of the obtained dependence of θ on d with experimental results allowed estimating the effective thickness of the strained layer around the island/substrate interface.

Study of low dimensional SiGe island on Si for potential visible Metal-Semiconductor-Metal photodetector

In this paper, an investigation of design and simulation of silicon germanium (SiGe) islands on silicon (Si) was presented for potential visible metal semiconductor metal (MSM) photodetector. The characterization of the performances in term of the structural, optical and electrical properties of the structures was analyzed from the simulation results. The project involves simulation using SILVACO Technology Computer Aided Design (TCAD) tools. The different structures of the silicon germanium (SiGe) island on silicon substrate were created, which were large SiGe, small SiGe, combination SiGe and bulk Ge. All the structures were tested for potential Metal Semiconductor Metal (MSM) photodetector. The extracted data such as current versus voltage characteristic, current gain and spectral response were obtained using ATLAS SILVACO tools. The performance of SiGe island structures and bulk Ge on Si substrate as (MSM) photodetector was evaluated by photo and dark current-voltage (I-V) characteristics. It was found that SiGe islands exhibited higher energy band gap compared to bulk Ge. The SiGe islands current-voltage characteristics showed improved current gain compared to bulk Ge. Specifically the enhancement of the islands gain was contributed by the enhanced photo currents and lower dark currents. The spectral responses of the SiGe islands showed peak response at 590 nm (yellow) which is at the visible wavelength. This shows the feasibility of the SiGe islands to be utilized for visible photodetections.

Electroluminescence of structures with self-assembled Ge(Si) nanoislands confined between strained Si layers

The electroluminescence of structures with self-assembled Ge(Si) nanoislands grown on relaxed SiGe/Si(001) buffer layers and confined between strained Si layers is studied for the first time. The electroluminescence signal from the structures is observed in the wavelength range from 1.6 to 2.0 μm, i.e., at longer wavelengths compared to those in the case of structures with Ge(Si) islands formed on Si (001) substrates. This give grounds to consider the structures with Ge(Si) islands confined between strained Si layers as candidates for the production of Si-based sources of emission at wavelengths of >1.55 μm.

Critical conditions for SiGe island formation during Ge deposition on Si(100) at high temperatures

The strain relaxation during the Ge growth on Si(100) occurs vikia surface diffusion and Si-Ge intermixing at temperatures below 800°C. The Ge diffusion into the Si substrate is an additional process at higher temperatures. We found that, if its rate is higher than the Ge deposition rate, the island formation is not realized. We determined the critical Ge deposition rate as a function of the temperature in the range of 840–960°C, at which the dynamic equilibrium between the growth of islands and their decay through the diffusion takes place. The islands grown in the conditions close to the dynamic equilibrium are ordered with a distance between them of about 1µm and they form a smoothed surface morphology. These are indicative of the surface layer strain uniformity. The islands have a SiGe composition which, in the direction parallel to the sample surface, is more uniform in comparison with the islands grown at lower temperatures. The results show that the use of high temperatures essentially improves the conditions for the heterostructure self-organization.

(Invited) Modeling Extended Defects in Semiconductor Devices

We perform analysis of several key aspects of the extended defect formation and their impact on silicon and GaN transistors. Heterogeneous epitaxy has been an enabling technique in transistor design since 90 nm technology node. The most widely used stress engineering technique is based on introducing crystal lattice mismatch stress by growing SiGe on Si. Depending on Ge content and on the size and shape of the selectively grown SiGe islands, the lattice mismatch stress can be high enough to introduce edge dislocations and stacking faults. Once the dislocations are created, they can move, driven by stress gradients towards more energetically favorable locations. An example of creating geometrical features to attract dislocations towards certain area has been demonstrated experimentally for epitaxial SiGe layers exceeding critical thickness on non-planar Si surfaces [1]. We reproduce such behavior by numerical minimization of elastic strain energy of an ensemble of {111} stacking faults (Fig. 1). Whenever dislocations manage to escape such herding and encroach into sensitive transistor areas, we perform quantum transport analysis to estimate the impact of a dislocation on electron scattering, on-state and off-state currents for Si nanowires with 5nm design rules. Another area where extended defect monitoring is essential is the GaN power switches. To achieve low buffer leakage in AlGaN/GaN HEMT for high voltage applications, it has been proposed to intentionally dope the GaN buffer with carbon [2]. This creates acceptor traps in the buffer to pin the Fermi Level at ~0.86 eV above the valence band [3]. A p-type buffer is thus formed and isolated from the Two-Dimensional Electron Gas (2DEG), which aggravates current collapse [3][4]. However, experimental results show that the magnitude of current collapse is less than suggested by modeling which can be attributed to the buffer being connected to the 2DEG through threading dislocations [3]. In this work, we perform 3D TCAD analysis of the impact of threading dislocations on the buffer trapping/de-trapping and current collapse. [1] R. Gatti, F. Boioli, M. Grydlik, M. Brehm, H. Groiss et al., Appl. Phys. Lett., 98, 121908 (2011) [2] E. Bahat-Treidel, F. Brunner, O. Hilt et al., IEEE Trans. Electron Devices, 57(11), 3050–3058 (2010) [3] M. J. Uren, M. Silvestri, M. Cäsar, G. A. M. Hurkx, J. A. Croon, J. Šonský, and M. Kuball, IEEE Electron Device Lett. 35(3), 327–329 (2014) [4] M. J. Uren, J. Möreke, and M. Kuball, IEEE Trans. Electron Devices 59 (12), 3327–3333 (2012) Fig. 1. Dislocations in epitaxial SiGe layer move towards the bottom of a V-shaped trench in Si wafer (a). Each additional dislocation reduces the total elastic strain energy of the structure (b). Figure 1