Self-assembled SiGe Research Articles

Влияние условий оптического возбуждения на спектральные и временные характеристики излучения двумерных фотонных кристаллов с Ge(Si)-наноостровками

The effect of optical excitation conditions on the spectral and temporal characteristics of the radiation from two-dimensional photonic crystals based on the structures with self-assembled Ge(Si) nanoislands has been studied. It was shown that one of the main factors effecting the spectral position and shape of photoluminescence lines, as well as the photoluminescence kinetics of photonic crystals with Ge(Si) nanoislands, along with the mode structure of the photonic crystal, are the local heating of the samples and the concentration of nonequilibrium charge carriers created by the absorption of the pumping radiation.

Strain-induced ordered Ge(Si) hut wires on patterned Si (001) substrates.

Ge/Si nanowires are predicted to be a promising platform for spin and even topological qubits. While for large-scale integration of these devices, nanowires with fully controlled positions and arrangements are a prerequisite. Here, we have reported ordered Ge hut wires by multilayer heteroepitaxy on patterned Si (001) substrates. Self-assembled GeSi hut wire arrays are orderly grown inside patterned trenches with post growth surface flatness. Such embedded GeSi wires induce tensile strain on the Si surface, which results in preferential nucleation of Ge nanostructures. Ordered Ge nano-dashes, disconnected wires and continuous wires are obtained correspondingly by tuning the growth conditions. These site-controlled Ge nanowires on a flattened surface lead to the ease of fabrication and large-scale integration of nanowire quantum devices.

One-Stage Formation of Two-Dimensional Photonic Crystal and Spatially Ordered Arrays of Self-Assembled Ge(Si) Nanoislandson Pit-Patterned Silicon-On-Insulator Substrate.

A new approach to improve the light-emitting efficiency of Ge(Si) quantum dots (QDs) by the formation of an ordered array of QDs on a pit-patterned silicon-on-insulator (SOI) substrate is presented. This approach makes it possible to use the same pre-patterned substrate both for the growth of spatially ordered QDs and for the formation of photonic crystal (PhC) in which QDs are embedded. The periodic array of deep pits on the SOI substrate simultaneously serves as a template for spatially ordering of QDs and the basis for two-dimensional PhCs. As a result of theoretical and experimental studies, the main regularities of the QD nucleation on the pre-patterned surface with deep pits were revealed. The parameters of the pit-patterned substrate (the period of the location of the pits, the pit shape, and depth) providing a significant increase of the QD luminescence intensity due to the effective interaction of QD emission with the PhC modes are found.

Kinetics of the Luminescence Response of Self-Assembled Ge(Si) Nanoislands Embedded in Two-Dimensional Photonic Crystals

The results of studies of the spectral and kinetic characteristics of the photoluminescence of photonic crystals formed on the basis of structures with self-assembled Ge(Si) nanoislands are reported. The experimentally observed enhancement of the photoluminescence-signal intensity of the nanoislands in the spectral range 1.1–1.6 μm due to interaction with the radiative modes of photonic crystals in the vicinity of the Γ point of the Brillouin zone and the effect of such interaction on the probability of radiative recombination in Ge(Si) nanoislands are considered.

Enhancement of the Luminescence Signal from Self-Assembled Ge(Si) Nanoislands due to Interaction with the Modes of Two-Dimensional Photonic Crystals

The results of studies of the luminescence properties of two-dimensional photonic crystals formed on the basis of silicon structures with self-assembled Ge(Si) nanoislands are reported. The possibilities of substantially enhancing the luminescence response of the active medium (Ge(Si) nanoislands) in the wavelength range 1.2–1.6 μm are shown for such structures. The specific features of the luminescence response of a photonic crystal in the vicinity of the Γ point of its Brillouin zone are studied. It is shown that, along with the broadband response characteristic of the radiative modes of photonic crystals, high-Q resonances, for which the Q factor exceeds 103, can also be observed in such structures. The last-mentioned resonances are observed in a certain range of lattice parameters of photonic crystals.

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.

On the Influence of Pulsed Gamma-Neutron Irradiation on the Morphology of Self-Assembled GeSi/Si(001) Nanoislands

The influence of pulsed gamma-neutron irradiation on the surface morphology of GeSi/Si(001) heterostructures with surface self-assembled GeSi nanoislands grown by the combined sublimation molecular-beam epitaxy of Si and the low-pressure vapor-phase epitaxy of Ge is experimentally studied. Gamma-neutron irradiation is found to cause a change in the shapes of self-assembled GeSi nanoislands, in particular, a reduction in the aspect ratio of the nanoislands. The effect is related to elastic-strain relaxation in the GeSi nanoislands under their exposure to elastic waves generated in the course of gamma-neutron irradiation.

Резистивное переключение в структурах металл-оксид-полупроводник с наноостровками GeSi на подложке кремния

The self-assembled GeSi nanoislands built into the semiconductor-insulator interface of the MOS-structures based on Si(001) with SiOx and ZrO2(Y) oxide layers deposited by magnetron sputtering have been shown to initiate bipolar resistive switching without preliminary electroforming. The current-voltage curves and electrical parameters of the MOS-structures in the high-resistance state and in the low-resistance state have been studied. A change in the built-in charge in the dielectric near the insulator-semiconductor interface during resistive switching is established and associated with the formation and destruction of conductive filaments. The light-stimulated resistive switching of MOS-structures with ZrO2(Y) layer from the high-resistance to the low-resistance state is observed, which is associated with an increase in the conductivity of the space-charge region in the Si substrate due to interband optical absorption in Si, which causes a voltage redistribution between Si and ZrO2(Y) layer. A difference in the shape of the small signal photo-voltage spectra of MOS-structures is found in the spectral region of intrinsic photosensitivity of Si in the high and low resistance states due to the leakage of photo-excited charge carriers from Si to the metal electrode through filaments.

Ordered Arrays of Ge(Si) Quantum Dots Incorporated into Two-Dimensional Photonic Crystals

Two different approaches to the integration of self-assembled Ge(Si) quantum dots into two-dimensional photonic crystals are considered. One approach includes the synthesis of an ordered array of Ge(Si) quantum dots on the textured surface of a substrate followed by the formation of a photonic crystal on this array. In the other approach, the photonic crystal itself serves as a template for the ordered growth of quantum dots. It is shown that, by varying the diameter of holes of photonic crystals in the second approach, it is possible to implement the growth of quantum dots in two modes, in which quantum dots are formed inside or outside the holes of the photonic crystal. For structures with ordered quantum dots incorporated into a photonic crystal, an increase in the photoluminescence signal intensity is detected at room temperature in the spectral range 0.9–1.2 eV. This increase is attributed to the interaction of emission from the structure with radiation modes of the photonic crystal.

Light emission from Ge(Si)/SOI self-assembled nanoislands embedded in photonic crystal slabs of various periods with and without cavities

Luminescent properties of self-assembled Ge(Si)/SOI nanoislands embedded in two-dimensional photonic crystal (PhC) slabs with and without L3 cavities were studied with PhC period a varied between 350 and 600 nm. For small periods (a ≤ 450 nm), the nanoisland luminescence, which spans over the wavelength range from 1.2 to 1.6 μm, overlaps with the PhC bandgap resulting in a coupling with the localized modes of an L3 cavity. It is shown that for larger periods (a > 450 nm), nanoisland emission couples to the radiative modes above the bandgap located in the vicinity of the Г-point of the photonic crystal Brillouin zone and is characterized by the low group velocity. In this case, a significant (up to 35-fold) increase in the PL intensity was observed in a number of PhCs without a cavity. From a technological point of view, the latter result makes such types of photonic crystal structures particularly promising for the realization of Si-based light emitters operating in the telecommunication wavelength range because, firstly, their manufacture does not require a precise cavity formation and, secondly, they provide a much larger area for the radiating region, as compared with PhC cavities.

Effect of Pulsed Gamma-Neutron Irradiation on the Photosensitivity of Si-Based Photodiodes with GeSi Nanoislands and Ge Epitaxial Layers

A comparative study of the effect of pulsed γ-neutron irradiation on the photosensitivity spectra of Si p–n photodiodes with active layers based on self-assembled GeSi nanoisland arrays and Ge epitaxial layers is performed. The irradiation of photodiodes with GeSi nanoislands is found to not lead to photosensitivity degradation in the spectral region of interband optical absorption in nanoislands (wavelength range of 1.1–1.7 μm). At the same time, a steady decrease in the intrinsic photosensitivity of Si and the photosensitivity of photodiodes based on Ge epitaxial layers with an increase in irradiation dose is observed. This effect is attributed to the accumulation of radiation-induced defects in the Si matrix and deep in Ge epitaxial layers, respectively.

Влияние импульсного гамма-нейтронного облучения на фоточувствительность фотодиодов на базе Si с наноостровками GeSi и эпитаксиальными слоями Ge

AbstractA comparative study of the effect of pulsed γ-neutron irradiation on the photosensitivity spectra of Si p – n photodiodes with active layers based on self-assembled GeSi nanoisland arrays and Ge epitaxial layers is performed. The irradiation of photodiodes with GeSi nanoislands is found to not lead to photosensitivity degradation in the spectral region of interband optical absorption in nanoislands (wavelength range of 1.1–1.7 μm). At the same time, a steady decrease in the intrinsic photosensitivity of Si and the photosensitivity of photodiodes based on Ge epitaxial layers with an increase in irradiation dose is observed. This effect is attributed to the accumulation of radiation-induced defects in the Si matrix and deep in Ge epitaxial layers, respectively.

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.

(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.

A random telegraph signal in tunneling silicon p–n junctions with GeSi nanoislands

We have experimentally discovered random telegraph signal generation in tunneling silicon p+–n+ junctions with embedded self-assembled GeSi nanoislands. The observed phenomenon is related to blocking of the electron tunneling via individual GeSi nanoislands due to the generation of holes in, and their thermal emission from, the nanoislands.

Towards promising modification of GeSi nanostructures via self-assembly on miscut Si(001) substrates

Self-assembled GeSi nanostructures on miscut Si(001) substrates are studied systematically with regard to the miscut angle and azimuth, the amount of Ge and the growth temperature. The comprehensive dependence of the spatial arrangement, which can exhibit one- and two-dimensional (1D and 2D) ordering, as well as the shape and density, of GeSi nanostructures on the miscut angle is observed. The orientation and side-walls of the 1D ordered in-plane GeSi nanowires on miscut Si(001) substrates are intimately associated with the miscut azimuth towards the 〈110〉 or 〈010〉 directions. Furthermore, the unique evolution of the GeSi nanostructures with the amount of Ge and the growth temperature on miscut Si (001) substrates towards the 〈010〉 direction is discovered. Such promising features of self-assembled GeSi nanostructures on miscut Si (001) substrates are explained in terms of the thermodynamics and growth kinetics, which are both affected significantly by the substrate vicinality. These results demonstrate that the miscut substrates offer a promising degree of freedom for the feasible modification of self-assembled nanostructures.

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.

Delayed plastic relaxation limit in SiGe islands grown by Ge diffusion from a local source

The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by the surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices.

Growth and Characterization of Epitaxial In-plane SiGe Alloy Nanowires

We study the growth of in-plane SiGe alloy nanowires by molecular beam epitaxy on Si substrates using the vapor-liquid-solid mechanism. These horizontal nanowires grow from liquid AuSi seeds which are obtained by annealing Au layers deposited on clean Si(001) surfaces. By continuously supplying Ge to the liquid droplets, an epitaxial process takes place, where wires crawl along <110> directions of the Si substrate, giving rise to the reproducible achievement of self-assembled in-plane SiGe nanowires. The morphology of the obtained nanostructures is characterized by scanning electronic and atomic force microscopies, and their composition and strain status is evaluated by micro-Raman imaging.