Ge Nanoislands Research Articles

Selective etching of Si, SiGe, Ge and its usage for increasing the efficiency of silicon solar cells

Dependences of the etch rates for KOH and HF:H2O2:CH3COOH solutions on SiGe layer composition were investigated. The obtained results has been proposed to use for formation of the submicron relief on the silicon surface via selective etching of the structures with Ge(Si) self-assembled nanoislands. In the framework of the proposed approach the Ge(Si) nanoislands serve as a mask for selective etching of Si in a mixture of an aqueous solution of KOH with isopropyl alcohol, followed by the islands removal from the surface by the selective etching in HF:H2O2:CH3COOH. It was demonstrated experimentally that such approach allows to produce the submicron relief on a silicon surface, which leads to the significant decrease of the reflectivity in a wide spectral range. It is believed that the proposed method of surface relief formation can be used to improve the efficiency of the thin-film solar cells based on the crystalline silicon.

Effect of Interstitials Embedded in Pre‐Patterned Si Substrate on Location of Ge Nanoislands

Large‐area periodical pattern on Si(100) was fabricated using combination of nanoimprint lithography and ion irradiation through mask. Ordered structures with grooves and ridges were prepared by the selective etching of regions amorphized by ion irradiation. Laterally ordered chains of Ge nanoislands were grown by molecular beam epitaxy of Ge on the pre‐patterned Si substrates. It was shown, that the location of subsequently grown Ge nanoislands depends upon the sidewall inclination in grooves. The effect is ascribed to strain induced by Ge interstitials under the groove's bottom. Our proposal was confirmed by molecular dynamics calculations that allowed the determination of strain in grooves as dependent on the groove's shape.

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.

Electroluminescence from MIS silicon-based light emitters with arrays of self-assembled Ge(Si) nanoislands

We report on the electroluminescence from silicon-based metal–insulator–semiconductor (MIS) diodes with arrays of self-assembled Ge(Si) nanoislands. Aluminum oxide (Al2O3) is used as an insulator material in the MIS contact. Variations in the electroluminescence spectra caused by changing the metal work function are examined. The intense electroluminescence from Ge(Si) nanoislands localized at a distance of 50 nm from the insulator–semiconductor interface is observed at room temperature. The emission spectrum is found to be controlled by choosing the design of the semiconductor structure and the barrier height for injected carriers.

Spatially arranged chains of Ge quantum dots grown on Si substrate prepatterned by ion‐beam‐assisted nanoimprint lithography

AbstractJoint experimental and theoretical study of Ge nanoislands growth on groove‐patterned Si(001) substrate prepared by ion‐beam‐assisted nanoimprint lithography is carried out. Prepatterning procedure includes ion irradiation of Si substrate through imprinted resist mask with linear mounds and subsequent multistep oxidation/etching of irradiated Si domains. It is shown that the temperature of subsequent heteroepitaxy on the groovepatterned substrate affects the location of Ge nanoislands. The effect is attributed to additional surface tensile strain formed inside grooves by residual irradiationinduced defects. The interplay of strain and surface curvature makes the critical size of 3D nanoisland larger at the concave regions in grooves than between them. At lower temperatures this results in nanoisland nucleation at the top of mounds. At higher temperatures due to the intensive migration of Ge inside grooves the formation of large nanoislands in grooves occurs. The effect of temperature on the location and size of nanoislands is described by Monte Carlo simulations. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

(Invited) Silicon Nano-Tip Pattern Approach for Surface and Interface Engineering of Fully Coherent, High Ge Content Nanostructure Arrays for High Performance Photodetection

Defects in form of dislocations at the interface and threading arms in the film as well as SiGe intermixing are the main mechanisms to release the crystallographic misfit strain in Germanium (Ge) heterostructures grown on the mature Silicon (Si) wafer platform, severely deteriorating thereby the superior optoelectronic properties of Ge for device applications. We demonstrate here a novel technique, enabling fully coherent, high Ge content islands on nano-tip patterned Si (001) substrates by the suppression of plastic relaxation as well as SiGe intermixing. The Si-tip patterned substrate, fabricated by complementary metal-oxide-semiconductor (CMOS) compatible nanotechnology, features ~50 nm wide Si tips emerging from a SiO2 matrix and arranged in an ordered lattice. Molecular beam epitaxy (MBE) growths result in Ge nano-islands with high selectivity and having homogeneous shape and size. The ~850°C growth temperature required for ensuring selective growth has been shown to lead to the formation of Ge islands of high crystalline quality without extensive Si intermixing (with 91 at.% Ge). Nano-tip patterned wafers result in geometric, kinetic diffusion barrier intermixing hindrance (in particular surface diffusion suppression) confining the major intermixing to the pedestal region of Ge islands where kinetic diffusion barriers are however high. Theoretical calculations suggest that the self-assembled thin SiGe layer at the interface plays nevertheless a significant role in realizing fully coherent Ge nano-islands free from extended defects especially dislocations. Finally, single layer graphene (SLG)/Ge/Si-tip Schottky junctions were fabricated and thanks to the absence of extended defects and SiGe intermixing in the Ge islands, high performance photodetection characteristics with responsivity and Ion/Ioff ratio of ~45 mA/W and ~103, respectively, are demonstrated.

Features of SOI substrates heating in MBE growth process obtained by low-coherence tandem interferometry

Differences in heating of silicon and silicon-on-insulator (SOI) substrates in molecular beam epitaxy were revealed by low-coherence tandem interferometry. Using this technique the interference effects which impede the correct evaluation of SOI substrate temperature by infrared pyrometers can be eliminated and so the reliable temperature readout can be achieved. It was shown that at the same thermocouple and heater power settings the real temperature of SOI substrates is higher than of silicon ones and the difference may be as high as 40–50°C at temperatures close to 600°C. It is supposed that such effect is caused by the additional absorption of heater radiation by the buried oxide layer in the mid-infrared range. Independent proof of this effect was obtained by growing on both types of substrates a series of structures with self-assembled Ge nanoislands whose parameters are known to be very temperature sensitive. The proposed low-coherence interferometry technique provides precise real-time control of the growth temperature and so allows formation of SiGe nanostructures with desired parameters.

Composition and stress of SiGe nanostructures on curved substrates

Recent experimental studies of Ge nanoislands on silicon-on-insulator (SOI) substrates have provided a defect-free strain relaxation mechanism through the bending of the substrate. Here, using atomistic Monte Carlo simulations and analytical modeling, we couple this relaxation mechanism with interdiffusion and alloying and observe composition profiles that are completely different from those observed in flat nanoislands. Moreover, for comparable SOI and island thicknesses, intermixing can be greatly reduced and Ge content in the islands is highly preserved.

Dislocation-free Ge Nano-crystals via Pattern Independent Selective Ge Heteroepitaxy on Si Nano-Tip Wafers.

The integration of dislocation-free Ge nano-islands was realized via selective molecular beam epitaxy on Si nano-tip patterned substrates. The Si-tip wafers feature a rectangular array of nanometer sized Si tips with (001) facet exposed among a SiO2 matrix. These wafers were fabricated by complementary metal-oxide-semiconductor (CMOS) compatible nanotechnology. Calculations based on nucleation theory predict that the selective growth occurs close to thermodynamic equilibrium, where condensation of Ge adatoms on SiO2 is disfavored due to the extremely short re-evaporation time and diffusion length. The growth selectivity is ensured by the desorption-limited growth regime leading to the observed pattern independence, i.e. the absence of loading effect commonly encountered in chemical vapor deposition. The growth condition of high temperature and low deposition rate is responsible for the observed high crystalline quality of the Ge islands which is also associated with negligible Si-Ge intermixing owing to geometric hindrance by the Si nano-tip approach. Single island as well as area-averaged characterization methods demonstrate that Ge islands are dislocation-free and heteroepitaxial strain is fully relaxed. Such well-ordered high quality Ge islands present a step towards the achievement of materials suitable for optical applications.

Photodetection in Hybrid Single-Layer Graphene/Fully Coherent Germanium Island Nanostructures Selectively Grown on Silicon Nanotip Patterns.

Dislocation networks are one of the most principle sources deteriorating the performances of devices based on lattice-mismatched heteroepitaxial systems. We demonstrate here a technique enabling fully coherent germanium (Ge) islands selectively grown on nanotip-patterned Si(001) substrates. The silicon (Si)-tip-patterned substrate, fabricated by complementary metal oxide semiconductor compatible nanotechnology, features ∼50-nm-wide Si areas emerging from a SiO2 matrix and arranged in an ordered lattice. Molecular beam epitaxy growths result in Ge nanoislands with high selectivity and having homogeneous shape and size. The ∼850 °C growth temperature required for ensuring selective growth has been shown to lead to the formation of Ge islands of high crystalline quality without extensive Si intermixing (with 91 atom % Ge). Nanotip-patterned wafers result in geometric, kinetic-diffusion-barrier intermixing hindrance, confining the major intermixing to the pedestal region of Ge islands, where kinetic diffusion barriers are, however, high. Theoretical calculations suggest that the thin Si/Ge layer at the interface plays, nevertheless, a significant role in realizing our fully coherent Ge nanoislands free from extended defects especially dislocations. Single-layer graphene/Ge/Si-tip Schottky junctions were fabricated, and thanks to the absence of extended defects in Ge islands, they demonstrate high-performance photodetection characteristics with responsivity of ∼45 mA W(-1) and an Ion/Ioff ratio of ∼10(3).

Simulation of radiation-defect formation processes in heterostructures with self-assembled Ge(Si)/Si(001) nanoislands under neutron irradiation

TRISQD software is developed for the computer simulation of processes in which radiation defects are formed under the corpuscular irradiation of semiconductor heterostructures with lenticular nanoinclusions of various shapes. The computer program is used to study defect-formation processes in p-i-n diodes with the i region having a built-in 20-period lattice of self-assembled Ge(Si) nanoislands formed under irradiation with high-energy neutrons. It is found that the fraction of Ge(Si) nanoislands in which point radiation defects are formed under the impact of atomic-displacement cascades is ≤3% of their total number in the lattice. More than 94% of the defects are localized in the bulk of the p, n, and i regions of the diode and in silicon layers that separate sheets of Ge(Si) nanoislands.

Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100)

In this paper, the study of the recombination of non-equilibrium charge carriers and determination of recombination mechanisms in Ge/Si heterostructures with nanoislands have been presented. The effects of long-term photoconductivity decay in Ge/Si heterostructures with Ge nanoislands have been found as caused by variations of the electrostatic potential in the near-surface region of Si(100) substrate and spatial separation of electron-hole pairs between localized states of Ge nanoislands and states of wetting layer and Si. It has been shown that the photoconductivity decay depends on the excitation energy and temperature, while Ge nanoislands are Shockley-Read recombination centers with a higher recombination rate as compared with Si.

Nucleation and growth of ordered groups of SiGe quantum dots

An approach for the formation of ordered groups of Ge nanoislands (quantum dots, QDs) upon epitaxial growth on the surface of a heterostructure constituted by a Si (100) substrate having preliminarily formed seeds in the form of disk-like SiGe nanomounds is developed. It is found that the observed arrangement of QDs within a group is due to the anisotropic elastic-strain energy distribution on the surface of a SiGe nanomound, namely, to the existence of four local energy minima arranged in an ordered manner along the [100] and [010] directions with respect to the seed center. Multilayer structures with vertically aligned QD groups are grown using the suggested approach. The crystal structure and the elemental composition of the spatially ordered nanostructures are examined by transmission electron microscopy, X-ray diffraction analysis, and Raman spectroscopy.

Ge Nanoislands Grown by Radio Frequency Magnetron Sputtering: Comprehensive Investigation of Surface Morphology and Optical Properties

The comprehensive investigation of the effect of growth parameters on structural and optical properties of Si‐based single layer Ge nanoislands grown via Stranski‐Krastanov mechanism employing radio frequency magnetron sputtering due to its high deposition rate, easy procedure, economical cost, and safety is carried out. The estimated width and height of Ge nanoislands produced by this technique are in the range of ∼8 to ∼30 and ∼2 to 8 nm, respectively. Varieties parameters are manipulated to optimize the surface morphology and structural and optical behavior of Ge nanoislands. The resulted nanoislands are analyzed using various analytical techniques including atomic force microscope, X‐ray diffraction, energy dispersive X‐ray spectroscopy, room temperature photoluminescence, and Raman spectroscopy. The optimum parameters for growing high quality samples having high number density and homogenous and small size distribution are found to be 400°C for substrate temperature, 300 sec for deposition time, 10 sccm for Ar flow, and 100 W for radio frequency power. The excellent features of the results suggest that our systematic investigation on the organized growth factors and their effects on surface parameters and photoluminescence emission energy may constitute a basis for the tunable growth of Ge nanoislands (100) nanoislands suitable in nanophotonics.

Unique surface structure formations on a Ge-covered Si(110)-16×2 surface

Si–Ge structures forming new shapes on a Si(110)-16×2 reconstructed surface were investigated via scanning tunneling microscopy. Pyramidal-shaped Si–Ge nanoislands lying along the <111> directions were formed on the striped structure at Ge coverage between 3 and 6 monolayers. However, when a single monolayer of Ge was deposited on the Si(110)-16×2 surface, single-domain of 16×2 striped structure disappeared, and a new double-domain striped structure was formed over the surface along directions that differed from the <112> directions. This structure represents a new Si–Ge striped structure that forms by the mixing of Ge and Si due to high temperature annealing. These results indicate that the surface structure changes specifically with trace amounts of Ge.

Formation of germanium nanoislands on pit-patterned silicon substrates by means of the molecular dynamics method

The molecular dynamics method is used to study the formation of Ge nanoislands on pit-patterned Si(100) substrates. For substrates with overlapping pits and pits in the shape of truncated inverted pyramids, the energy surface is calculated. On the basis of its analysis, the mechanism of nuclear surface diffusion on the pit-patterned surface is described. The specific energy of Ge/Si heterostructures with different morphology of nanoislands in pits is calculated. It is shown that the configuration with multiple nanoislands in a pit can be thermodynamically favorable.

Self-assembled Ge/Si nanoislands: effect of argon flow and radio frequency power

The growth of a germanium–silicon (Ge/Si) heterostructure using radio frequency (rf) magnetron sputtering under optimized conditions is performed. High-density self-assembled Ge nanoislands ∼14 nm in size are obtained directly on Si(100) under 10 sccm argon (Ar) flow and 100 W rf power. The surface roughness and number density of nanoislands revealed by atomic force microscopy are found to be in the range of 3.7–0.6 nm and 45 × 102–20 × 102 μm−2, respectively. An increase in Ar flow and rf power leads to an enhancement of defects, anti-sputtering of the deposited islands and a decrease in the sample quality. X-ray diffraction is used to measure the strain, which is found to be close to the lattice misfit of Ge and Si. It shows the carefully controlled growth condition with minimum density of dislocation. Room temperature photoluminescence exhibits a very strong blue–violet peak at 3.2 eV, which is attributed to the transition from excited triplet level to grand-state singlet level.

Large-scale DFT simulations with a linear-scaling DFT code CONQUEST on K-computer

In oder to realize the DFT simulations on large-scale complex systems, we have been developing a linear-scaling DFT code Conquest. In this paper, we report the parallel efficiency of the code on K-computer and show that it has almost ideal parallel efficiency even when we use more than 200,000 cores. Using the code on such large-scale parallel computers, we are now ready to do actual DFT study on million-atom systems. By showing our current study on the nucleation of Ge dimers on three-dimensional Ge nano-islands on Si(001), we demonstrate that accurate, efficient and robust structure relaxation based on the DFT is possible in the actual scientific research on complex nano-structured materials.

Germanium nanoislands grown by radio frequency magnetron sputtering: Annealing time dependent surface morphology and photoluminescence

Structural and optical properties of ∼ 20 nm Ge nanoislands grown on Si(100) by radio frequency (rf) magnetron sputtering under varying annealing conditions are reported. Rapid thermal annealing at a temperature of 600°C for 30 s, 90 s, and 120 s are performed to examine the influence of annealing time on the surface morphology and photoluminescence properties. X-ray diffraction spectra reveal prominent Ge and GeO2 peaks highly sensitive to the annealing time. Atomic force microscope micrographs of the as-grown sample show pyramidal nanoislands with relatively high-density 1011 cm−2)). The nanoislands become dome-shaped upon annealing through a coarsening process mediated by Oswald ripening. The room temperature photoluminescence peaks for both as-grown 3.29 eV) and annealed 3.19 eV) samples consist of high intensity and broad emission, attributed to the effect of quantum confinement. The red shift (∼0.10 eV) of the emission peak is attributed to the change in the size of the Ge nanoislands caused by annealing. Our easy fabrication method may contribute to the development of Ge nanostructure-based optoelectronics.

Formation of ordered groups of quantum dots during Ge/Si heteroepitaxy

A new approach to the creation of circularly ordered Ge nanoislands by epitaxy on the surface of a heterophase structure consisting of a Si(100) substrate with premade seeds in the form of SiGe nanodisks or SiGe nanorings is developed. It is shown that the spatial configuration of islands in the group is due to the nucleation of the islands in the area of local minima of the elastic energy density on the surface of a circular seed. On the basis of this approach, a number of multilayer structures with vertically aligned ring groups of quantum dots were grown. The elemental composition and luminescent properties of the ordered structures are studied.