SiGe Nanocrystals Research Articles

Growth of Ge QDs-Decorated SiGe Nanocrystals: Toward Integration of Quantum Dots and Mie Resonators in Ultrathin Film for Photodetection and Energy Harvesting

Growth of Ge QDs-Decorated SiGe Nanocrystals: Toward Integration of Quantum Dots and Mie Resonators in Ultrathin Film for Photodetection and Energy Harvesting

Nanocrystallized Ge-Rich SiGe-HfO2 Highly Photosensitive in Short-Wave Infrared.

Group IV nanocrystals (NCs), in particular from the Si–Ge system, are of high interest for Si photonics applications. Ge-rich SiGe NCs embedded in nanocrystallized HfO2 were obtained by magnetron sputtering deposition followed by rapid thermal annealing at 600 °C for nanostructuring. The complex characterization of morphology and crystalline structure by X-ray diffraction, μ-Raman spectroscopy, and cross-section transmission electron microscopy evidenced the formation of Ge-rich SiGe NCs (3–7 nm diameter) in a matrix of nanocrystallized HfO2. For avoiding the fast diffusion of Ge, the layer containing SiGe NCs was cladded by very thin top and bottom pure HfO2 layers. Nanocrystallized HfO2 with tetragonal/orthorhombic structure was revealed beside the monoclinic phase in both buffer HfO2 and SiGe NCs–HfO2 layers. In the top part, the film is mainly crystallized in the monoclinic phase. High efficiency of the photocurrent was obtained in a broad spectral range of curves of 600–2000 nm at low temperatures. The high-quality SiGe NC/HfO2 matrix interface together with the strain induced in SiGe NCs by nanocrystallization of both HfO2 matrix and SiGe nanoparticles explain the unexpectedly extended photoelectric sensitivity in short-wave infrared up to about 2000 nm that is more than the sensitivity limit for Ge, in spite of the increase of bandgap by well-known quantum confinement effect in SiGe NCs.

Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals

We present a detailed study regarding the bandgap dependence on diameter and composition of spherical Ge-rich GexSi1−x nanocrystals (NCs). For this, we conducted a series of atomistic density functional theory (DFT) calculations on H-passivated NCs of Ge-rich GeSi random alloys, with Ge atomic concentration varied from 50 to 100% and diameters ranging from 1 to 4 nm. As a result of the dominant confinement effect in the DFT computations, a composition invariance of the line shape of the bandgap diameter dependence was found for the entire computation range, the curves being shifted for different Ge concentrations by ΔE(eV) = 0.651(1 − x). The shape of the dependence of NCs bandgap on the diameter is well described by a power function 4.58/d1.25 for 2–4 nm diameter range, while for smaller diameters, there is a tendency to limit the bandgap to a finite value. By H-passivation of the NC surface, the effect of surface states near the band edges is excluded aiming to accurately determine the NC bandgap. The number of H atoms necessary to fully passivate the spherical GexSi1−x NC surface reaches the total number atoms of the Ge + Si core for smallest NCs and still remains about 25% from total number of atoms for bigger NC diameters of 4 nm. The findings are in line with existing theoretical and experimental published data on pure Ge NCs and allow the evaluation of the GeSi NCs behavior required by desired optical sensor applications for which there is a lack of DFT simulation data in literature.

Формирование нанокристаллов германия в пленках GeO[SiO-=SUB=-2-=/SUB=-] и GeO[SiO

Films of nonstoichiometric germanosilicate glasses GeO0.5[SiO2]0.5 and GeO0.5[SiO]0.5 were obtained by co-evaporation of GeO2 and SiO2 or GeO2 and SiO powders, respectively, and by deposition on cold Si (100) and fused quartz substrates in a high vacuum. Then the films were annealed at temperatures up to 900 oC. The presence and phase composition of germanium nanoclusters in the films was investigated using Raman spectroscopy technique. The transformation after annealing of the GeSixOy matrix surrounding the nanoclusters was investigated using Fourier transform infrared spectroscopy. Analysis of the Raman spectra showed that annealing at a temperature of 800 oC led to the formation of germanium nanocrystals (NC-Ge), but the content of the amorphous germanium phase remains up to half of the volume. Annealing at a temperature of 900 oC led to almost complete crystallization of amorphous nanoclusters in all films. The sizes of the NC-Ge depended on the annealing temperature, the composition of the films, and on the substrate; in this case, the formation of Ge-Si nanocrystals was not observed.

Influence of SiGe Nanocrystallization on Short-Wave Infrared Sensitivity of SiGe–TiO2 Films and Multilayers

Continuous development of Si photonics requires ecological and cost-effective materials. In this work, SiGe nanocrystals (NCs) embedded in TiO2 are investigated as a photosensitive material for vis...

SiGe nanocrystals in SiO2 with high photosensitivity from visible to short-wave infrared

Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.

Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing

Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.

Silicothermic reduction reaction for fabricating interconnected Si–Ge nanocrystals with fast and stable Li-storage

An innovative silicothermic reduction is developed to prepare Si–Ge nanocrystals, which exhibit impressive rate capability and long cycling stability.

Tight-binding calculations of SiGe alloy nanocrystals in SiO2 matrix

In the empirical tight-binding approach we study the electronic states in spherical SiGe nanocrystals embedded in SiO2 matrix. For the SiGe alloy and the matrix we use the virtual crystal approximation. The energy and valley structure of electron states is obtained as a function of Ge composition and nanocrystal size. Calculations show that the mixing of hot electrons in the nanocrystal with the electrons in wide band gap matrix is possible and this mixing strongly depends on the Ge composition in the nanocrystal.

Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing

Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.

SiGe layer thickness effect on the structural and optical properties of well-organized SiGe/SiO2 multilayers

In this work, we report on the production of regular (SiGe/SiO2)20 multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO2 thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (tSiGe) from 3.5–2 nm results in a transition from continuous SiGe crystalline layer (tSiGe ∼ 3.5 nm) to layers consisting of isolated nanocrystals (tSiGe ∼ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ∼3–8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for tSiGe ∼ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO2 nanostructures show an emission band located between 0.7–0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.

Ge nanocrystals formed by furnace annealing of Ge(x)[SiO2](1−x) films: structure and optical properties

Ge(x)[SiO2](1−x) (0.1 ⩽ x ⩽ 0.4) films were deposited onto Si(0 0 1) or fused quartz substrates using co-evaporation of both Ge and SiO2 in high vacuum. Germanium nanocrystals were synthesized in the SiO2 matrix by furnace annealing of Gex[SiO2](1−x) films with x ⩾ 0.2. According to electron microscopy and Raman spectroscopy data, the average size of the nanocrystals depends weakly on the annealing temperature (700, 800, or 900 °C) and on the Ge concentration in the films. Neither amorphous Ge clusters nor Ge nanocrystals were observed in as-deposited and annealed Ge0.1[SiO2]0.9 films. Infrared absorption spectroscopy measurements show that the studied films do not contain a noticeable amount of GeOx clusters. After annealing at 900 °C intermixing of germanium and silicon atoms was still negligible thus preventing the formation of GeSi nanocrystals. For annealed samples, we report the observation of infrared photoluminescence at low temperatures, which can be explained by exciton recombination in Ge nanocrystals. Moreover, we report strong photoluminescence in the visible range at room temperature, which is certainly due to Ge-related defect-induced radiative transitions.

Resonant Tunneling through Monolayer Si Colloidal Quantum Dots and Ge Nanocrystals

Resonant tunneling through a 4 nm nanocrystal Ge (nc‐Ge) layer and a 2.4 nm monolayer of Si colloidal quantum dots (QD) is achieved with 0.7 nm amorphous Al2O3 (a‐Al2O3) barriers. The nc‐Ge resonant tunneling diode (RTD) demonstrates a peak‐to‐valley current ratio (PVCR) of 8 and a full width at half maximum (FWHM) of 30 mV at 300 K, the best performance among RTDs based on annealed nanocrystals. The Si QD RTD is first achieved with PVCRs up to 47 and FWHMs as small as 10 mV at room temperature, confirming theoretically expected excellences of 3D carrier confinements. The high performances are partially due to the smooth profile of nc‐Ge layer and the uniform distribution of Si QDs, which reduce the adverse influences of many‐body effects. More importantly, carrier decoherence is avoided in the 0.7 nm a‐Al2O3 barriers thinner than the phase coherence length (≈1.5 nm). Ultrathin a‐Al2O3 also passivates well materials and suppresses leakage currents. Additionally, the interfacial bandgap of ultrathin a‐Al2O3 is found to be similar to the bulk, forming deep potential wells to sharpen transmission curves. This work can be easily extended to other materials, which may enable resonant tunneling in various nanosystems for diverse purposes.

Impurity and Defect Monitoring in Hexagonal Si and SiGe Nanocrystals

Silicon and Germanium naturally occur in a cubic crystal structure and have an indirect bandgap1 making them unsuitable materials for light emission. When synthesized in a hexagonal crystal structure Germanium and Silicon-Germanium are however predicted to have a direct bandgap2 which makes these crystal phases candidate materials for integrating optical functionality into silicon based CMOS. We have recently demonstrated the synthesis of hexagonal silicon3 using Gallium-phosphide nanowires with a hexagonal crystal lattice4as a virtual substrate. Based on this approach we have succeeded in growing hexagonal Germanium and Silicon-Germanium both as shells around and branches on Gallium-phosphide nanowires. We are now aiming to grow optically active hexagonal Germanium and Silicon-Germanium with a direct bandgap. However, in order to create efficient light sources, we will have to suppress non-radiative recombination channels in the material and hence grow defect and impurity free hexagonal Germanium and Silicon-Germanium. This makes it necessary to measure the defect and impurity level inside the hexagonal shells and branches. Unfortunately, the size and the geometry of the structures make these measurements challenging as typical bulk characterization methods like Secondary Ion Mass Spectroscopy or X-Ray Diffraction are not applicable. Here we will show how we use Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) to monitor the crystal structure, crystal defect density, impurity level, interface roughness and matrix concentration in this complex material system. TEM allows us to investigate the defect density and defect formation during the growth of the hexagonal lattices making it possible to work towards the growth of defect free structures. APT allows us to create three-dimensional images of the elemental distribution inside the structures and hence makes it possible to supervise the impurity level, interface roughness and matrix concentration in the structures. Combining the two methods thus enables us to optimize the growth parameters, minimizing both the defect and the impurity density. [1] M. Cardona et al. Phys Rev., 142:530, 1966. [2] C.Raffy et al. Phys. Rev. B, 66:075201, 2002. [3] H. I. T. Hauge et al. Nano Lett., 15(9):5855, 2015. [4] S. Assali et al. Nano Lett., 13(4):1559, 2013. Figure 1: Defects and impurities in a hexagonal Si shell (a-d) and hexagonal Ge branches (e,f) grown on GaP nanowires. Side-view imaging with TEM (a,c,f) enables us to identify defects. In combination with cross-section TEM images (b) we can confirm the epitaxial growth of Si on the hexagonal GaP lattice. APT analyses of the Si shells (d) and the Ge branches (e) allow us to map impurity atoms in the hexagonal crystals and reveal the diffusion of Ga and P into the Si and the Ge respectively. Figure 1

Impurity and Defect Monitoring in Hexagonal Si and SiGe Nanocrystals

Silicon-Germanium in a hexagonal crystal-structure is a candidate material for a direct band-gap group IV semiconductor that can be integrated into the CMOS process. It has recently been synthesized as a crystalline shell grown epitaxial around a nanowire core of hexagonal Gallium-Phosphide. In order to study the optical properties of this newly generated material and evaluate its potential for building optical devices it is necessary to grow defect and impurity free hexagonal Silicon-Germanium. Impurity detection and mapping in nano-structures is however challenging as most bulk and thin film characterization methods cannot be used. Here we show that Atom Probe Tomography can be used to map the impurities in hexagonal shells of Silicon-Germanium and Silicon. This will allow to optimize growth of hexagonal Silicon-Germanium nanocrystals towards impurity free, optically active crystals.

Si1−xGex photodiode with segregated Ge nanocrystals

P-N junction diodes with excellent rectifying characteristics were prepared by segregating the Ge nanocrystals of Si1−xGex thin films deposited by co-sputtering. The current-voltage characteristics in darkness and under illumination were studied. The correlation between the p-n junction performance and the microstructure of the films is discussed, and the rectifying property became stronger as the fraction of Ge in the Si1−xGex films increased. The optical bandgap can be tuned by controlling the grain size in Ge and SiGe nanocrystals. The graded structure of the Si1−xGex photodiode is proposed to widen the light absorption region. The concept can be used to design high-efficiency photodiodes.

Structure and infrared photoluminescence of GeSi nanocrystals formed by high temperature annealing of GeOx/SiO2 multilayers

Germanium and GeSi nanocrystals were synthesized in SiGeO2 glass by high temperature annealings of GeOx(5 nm)/SiO2(5 nm) multilayers. According to electron microscopy data, the size distribution and stoichiometry of the nanocrystals depend on the annealing temperature (700, 800, or 900 °C). Spatial redistribution of Ge with the formation of large faceted nanocrystals located near the Si substrate and GeSi intermixing at the substrate/film interface were observed. In the case of the 900 °C annealed sample, we note that some nanocrystals have a pyramid-like shape. Infrared absorption spectroscopy demonstrates that intermixing takes place between the GeOx and SiO2 layers leading to the formation of SiGeO2 glass. Raman spectroscopy confirms the formation of Ge nanocrystals after annealing at 700 °C and GeSi nanocrystals after annealing at 800 and 900 °C. For all annealed samples, we report the observation of infrared photoluminescence (PL) at low temperatures in the spectral range 1300–2100 nm. The observation of PL at wavelengths close to 2000 nm may be due to defect-induced radiative transitions in the nanocrystals.

Infrared photoluminescence from GeSi nanocrystals embedded in a germanium–silicate matrix

We investigate the structural and optical properties of GeO/SiO2 multilayers obtained by evaporation of GeO2 and SiO2 powders under ultrahigh vacuum conditions on Si(001) substrates. Both Raman and infrared absorption spectroscopy measurements indicate the formation of GeSi nanocrystals after postgrowth annealing at 800°C. High-resolution transmission electron microscopy characterizations show that the average size of the nanocrystals is about 5 nm. For samples containing GeSi nanocrystals, photoluminescence is observed at 14 K in the spectral range 1500–1600 nm. The temperature dependence of the photoluminescence is studied.

Multilayer Ge nanocrystals embedded within Al2O3 matrix for high performance floating gate memory devices

Metal-insulator-silicon devices with Ge nanocrystals dispersed in Al2O3 have been studied with a view to exploit them for floating gate memory applications. Multilayer devices comprising of five layers Ge nanocrystals have exhibited superior memory characteristics over the single layer Ge and multilayer Si nanocrystals reported in literature. The effect of interface traps on the memory behavior using frequency dependent capacitance- and conductance-voltage measurements has been investigated. This study has demonstrated an enhanced memory window with superior retention characteristics, owing to the Coulomb blockade effect, due to the introduction of multi-layer nanocrystals in the floating gate.

Boron- and phosphorus-doped silicon germanium alloy nanocrystals—Nonthermal plasma synthesis and gas-phase thin film deposition

Alloyed silicon-germanium (SiGe) nanostructures are the topic of renewed research due to applications in modern optoelectronics and high-temperature thermoelectric materials. However, common techniques for producing nanostructured SiGe focus on bulk processing; therefore little is known of the physical properties of SiGe nanocrystals (NCs) synthesized from molecular precursors. In this letter, we synthesize and deposit thin films of doped SiGe NCs using a single, flow-through nonthermal plasma reactor and inertial impaction. Using x-ray and vibrational analysis, we show that the SiGe NC structure appears truly alloyed for Si1−xGex for 0.16 < x < 0.24, and quantify the atomic dopant incorporation within the SiGe NC films.