Si Spacer Research Articles

Multi-stacks of epitaxial GeSn self-assembled dots in Si: Structural analysis

We report on the growth and structural and morphologic characterization of stacked layers of self-assembled GeSn dots grown on Si (100) substrates by molecular beam epitaxy at low substrate temperature T = 350 °C. Samples consist of layers (from 1 up to 10) of Ge0.96Sn0.04 self-assembled dots separated by Si spacer layers, 10 nm thick. Their structural analysis was performed based on transmission electron microscopy, atomic force microscopy, and Raman scattering. We found that up to 4 stacks of dots could be grown with good dot layer homogeneity, making the GeSn dots interesting candidates for optoelectronic device applications.

The Study of Morphology and Photoelectric Sensitivity of Low-Temperature Ge/Si Heterostructures with Ge Quantum Dots

The morphology of low-temperature (growth temperature 360 °C) multilayer Ge/Si heterostructures with Ge quantum dots was investigated by means of transmitting electron microscopy, scanning tunneling microscopy and X-ray optics. The possibility to determine integral morphological characteristics of heterostructures’ layers using X-ray optics method is shown. The results of X-ray optics are in a good agreement with TEM measurements and we discover the cross-correlation between Ge layers in heterostructures containing thin Si spacers. The spectral sensitive region of structures under investigation is estimated by measuring photo-emf spectra at the temperature of 78 K and 300 K. The possibility of controlled forming of spectral sensitive region of Ge/Si heterostructures in the range of 1.5 – 2.8 μm is verified.

Optimization of radiation hardness and charge collection of edgeless silicon pixel sensors for photon science

Recent progress in active-edge technology of silicon sensors enables the development of large-area tiled silicon pixel detectors with small dead space between modules by utilizing edgeless sensors. Such technology has been proven in successful productions of ATLAS and Medipix-based silicon pixel sensors by a few foundries. However, the drawbacks of edgeless sensors are poor radiation hardness for ionizing radiation and non-uniform charge collection by edge pixels. In this work, the radiation hardness of edgeless sensors with different polarities has been investigated using Synopsys TCAD with X-ray radiation-damage parameters implemented. Results show that if no conventional guard ring is present, none of the current designs are able to achieve a high breakdown voltage (typically < 30 V) after irradiation to a dose of ∼ 10 MGy. In addition, a charge-collection model has been developed and was used to calculate the charges collected by the edge pixels of edgeless sensors when illuminated with X-rays. The model takes into account the electric field distribution inside the pixel sensor, the absorption of X-rays, drift and diffusion of electrons and holes, charge sharing effects, and threshold settings in ASICs. It is found that the non-uniform charge collection of edge pixels is caused by the strong bending of the electric field and the non-uniformity depends on bias voltage, sensor thickness and distance from active edge to the last pixel (``edge space"). In particular, the last few pixels close to the active edge of the sensor are not sensitive to low-energy X-rays ( < 10 keV), especially for sensors with thicker Si and smaller edge space. The results from the model calculation have been compared to measurements and good agreement was obtained. The model can be used to optimize the edge design. From the edge optimization, it is found that in order to guarantee the sensitivity of the last few pixels to low-energy X-rays, the edge space should be kept at least 50% of the sensor thickness.

Al‐Si‐Cu 주조합금의 이차 수지상 간격 및 공정 Si이 기계적 특성에 미치는 영향

The present study aims at investigating the effects of eutectic Si and Secondary dendrite arm spacing (SDAS) on mechanical properties of Al-Si-Cu alloy. Heat treatment and controlling of solidification rate affect to microstructure of Al-Si-Cu alloy. Al-Si-Cu alloy was dissolved in an electric furnace. The alloy cast in STD61 mold which had been pre-heated to 95 °C and 200 °C. Eutectic Si and SDAS were finer as cooling rate increased. Image analysis technique has been utilized to examine the microstructure. Microstructure observation results showed that T6 heat treatment has a strong influence eutectic Si particle morphology. The mechanical properties, such as tensile strength, yield strength, elongation, were improved by ASTM E8 standard. Tensile properties of the Al-Si-Cu alloys prepared by different cooling rates were the same as each other by T6 heat treatment. †(Received December 19, 2013)

Iron silicide formation at different layers of (Fe/Si)3 multilayered structures determined by conversion electron Mössbauer spectroscopy

The morphology and the quantitative composition of the Fe-Si interface layer forming at each Fe layer of a (Fe/Si)3 multilayer have been determined by means of conversion electron Mössbauer spectroscopy (CEMS) and high-resolution transmission electron microscopy (HRTEM). For the CEMS measurements, each layer was selected by depositing the Mössbauer active 57Fe isotope with 95% enrichment. Samples with Fe layers of nominal thickness dFe = 2.6 nm and Si spacers of dSi = 1.5 nm were prepared by thermal evaporation onto a GaAs(001) substrate with an intermediate Ag(001) buffer layer. HRTEM images showed that Si layers grow amorphous and the epitaxial growth of the Fe is good only for the first deposited layer. The CEMS spectra show that at all Fe/Si and Si/Fe interfaces a paramagnetic c-Fe1−xSi phase is formed, which contains 16% of the nominal Fe deposited in the Fe layer. The bottom Fe layer, which is in contact with the Ag buffer, also contains α-Fe and an Fe1−xSix alloy that cannot be attributed to a single phase. In contrast, the other two layers only comprise an Fe1−xSix alloy with a Si concentration of ≃0.15, but no α-Fe.

Thermomagnetic behaviour and compositional irreversibility on (Fe/Si)3 multilayer films

This work presents the correlation between the morphology and magnetic properties of (Fe/Si)3 multilayers with different Fe layer thicknesses and fixed Si spacer thickness in a broad temperature range (5<T<800K). Films were prepared by thermal evaporation under ultrahigh vacuum onto a buffer layer of Fe/Ag deposited on a GaAs(001) substrate. Transmission electron microscopy reveals good epitaxial growth and phase transformations in the c-FeSi phase formed during deposition as well as upon subsequent annealing of the sample up to 800K. Remanence to saturation magnetization MR/MS ratios and saturation fields are related to several types of interlayer exchange coupling. 90°-coupling and a superposition of 90° and antiferromagnetic interlayer exchange coupling are found depending on the Fe layer thickness. Magnetization curves were investigated as a function of temperature by in situ annealing. They show an irreversible thermal process as temperature increases from 300 to 450K that is correlated to the formation of a ferromagnetic silicide phase. At higher temperature this phase transforms into a paramagnetic Fe–Si phase.

Optical and structural investigations of self-assembled Ge/Si bi-layer containing Ge QDs

We report the influence of Si spacer thickness variation (10–40nm) on structural and optical properties of Ge quantum dots (QDs) in Ge/Si(100) bi-layer grown by radio frequency magnetron sputtering. AFM images reveal the spacer dependent width, height, root mean square roughness and number density of QDs vary in the range of ~12–25nm, ~2–6nm, ~1.95–1.05nm and ~0.55×1011–2.1×1011cm−2, respectively. XRD patterns exhibit the presence of poly-oriented structures of Ge with preferred growth along (111) direction accompanied by a reduction in strain from 4.9% to 1.2% (estimated from Williamson–Hall plot) due to bi-layering. The room temperature luminescence displays strong blue–violet peak associated with a blue shift as much as 0.05eV upon increasing the thickness of Si spacer. This shift is attributed to the quantum size effect, the material intermixing and the strain mediation. Raman spectra for both mono and bi-layer samples show intense Ge–Ge optical phonon mode that is shifted towards higher frequency. Furthermore, the first order features of Raman spectra affirm the occurrence of interfacial intermixing and phase formation during deposition. The excellent features of the results suggest that our systematic method may constitute a basis for the tunable growth of Ge QDs suitable in nanophotonics.

(Invited) Narrow Linewidth, Highly Efficient, and Integrated Light Emitting Diodes Based on Ge Quantum Dots in Optical Microcavities

Ge self-assembled quantum dots (QDs) grown on Si are promising solution for Si-based light sources due to their emission wavelength in the telecommunication band, capability of electrical injection and CMOS process compatibility. However, as indirect band gap material, their light emission efficiency is still rather low. By embedding Ge QDs into optical microcavities with large Q-factor and small mode volume, the spontaneous emission rate can be enhanced due to the Purcell effect and the light extraction efficiency can also be improved due to the modified cavity mode fields. We have demonstrated strong and sharp resonant photoluminescence peaks from Ge QDs in photonic crystal (PhC) nanocavities and microdisk resonators [1]. For practical application, light emission by electrical injection is desirable. In this abstract, we will review some of our results on electrical injected light emitting diodes (LEDs) based on Ge QDs in microcavities. Electrical injected LEDs were firstly demonstrated by integrating vertical PIN diodes with H3-type PhC cavities [2]. Room-temperature electroluminescence (EL) was successfully observed, but with relatively broad emission peaks. This is due to the large cavity loss induced by the electrical structure. By using lateral PIN diode structure, in which the P+ and N+ regions are located laterally on the PhC slab, while the cavity region is left intrinsic, we achieved intense EL peaks around 1.3 µm with Q-factor of about 260, corresponding to a linewidth of 5 nm [3]. After optimization of PhC nanocavities and electrical structures, the Q-factor of emission peaks increased to 800-1500. The required current for occurrence of resonant peaks was reduced by one order, to 50 µA [4]. The schematic diagram and EL spectra of the device are shown in Fig. 1(a) and 1(b), respectively. Moreover, at injected current of 3 mA, we obtained a measurable output power on the pW order. We also confirmed that the output power was limited by the fabrication process and could be increased further. In order to realize monolithic integration with other photonic devices, waveguide-coupled LEDs are necessary. Waveguide-coupled microdisks containing Ge QDs were fabricated with vertical PIN diodes structures [5], as shown in Fig. 1(c). When there was current injection, light emission from the microdisk was coupled into the waveguide. Sharp resonant peaks around 1.55 µm with Q-factor over 5000 were obtained in the EL spectra collected through the waveguide, as shown in Fig. 1(d). The coupling between the waveguide and microdisk was confirmed by comparing the EL and optical transmission spectra along the waveguide. The LEDs thus can be easily integrated with other waveguide devices. Interestingly, the devices also can be operated as resonant-cavity enhanced photodetectors under reverse bias. Wavelength selective photo-response, with high responsivity and ultra-low dark current around 1.55 µm was also realized [6]. The light emission efficiency of Ge QDs themselves is fundamental to obtain higher intensity. Due to the type-II band lineup of Si/Ge heterostructure, only holes can be well confined in Ge QDs, and there is very weak confinement for electrons. By using n-type doping, extrinsic electrons can be provided and the radiative recombination is expected to be enhanced. In the talk, we will also present some results about light emission enhancement by phosphorus delta-doping between Ge QDs and Si spacers.

Thermal transport through short-period SiGe nanodot superlattices

The cross-plane thermal conductivity κ of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far κ can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of κ is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane κ of Ge/Si superlattices with period lengths down to 1.5 nm. Although κ tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for κ. To assess the role played by the nanodots in the reduction of κ we studied Ge/Si superlattices with nanodot densities varying from 0 to ∼8×1010 cm−2 and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with ‘‘pyramid’’-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1−xGex (with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the κ by ∼20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics.

Strain-induced self-assembly of Ge nanodashes, nanodumbbells, and dot chains on Si(001)

We investigate the growth of self-assembled Ge nanostructures on top of embedded Ge nanowires on Si(001) substrates. Ge nanostructures, such as nanodashes, nanodumbbells, and dot chains are observed simply by tuning the growth temperature and thickness of the Si spacer between the Ge layers. The self-assembly process is governed by the surface strain fields generated by the embedded Ge nanowires and is well-described by our theoretical calculations. The catalyst-free and horizontal growth of such Ge nanostructures directly on Si(001) is attractive for investigating exotic transport properties through Si/Ge-based quantum devices.

Control of Dip Shape in Photonic Nanostructures by Maskless Wet-Etching Process and Its Impact on Optical Properties

We demonstrate that maskless wet etching of self-assembled Ge quantum dot (QD) multilayers permits us to realize large-area photonic nanostructures, and their dip shape can be tuned by controlling structural parameters in the Ge QD multilayers. It is found that the reduction of Si spacer thickness brings about an increase in the dip depth and causes a reduction of reflectance at all the wavelengths. Furthermore, the presence of photonic nanostructures was shown to lead to the enhancement of photoluminescence intensity from Ge QDs. These results demonstrate that photonic nanostructures formed by this technique can enhance optical absorption in Ge QD multilayers.

Enhancement of the thermoelectric power factor of MnSi1.7 film by modulation doping of Al and Cu

It is well known that aluminum (Al) and copper (Cu) are acceptor impurities with shallow- and deep-energy levels in silicon (Si), respectively. The thermoelectric power factor of Al and Cu codoped Si film is larger than that of only Al-doped Si film. In this report, the Al and Cu codoped Si layer, Si: (Al + Cu), is used as a barrier layer, while a higher manganese silicide (HMS, MnSi1.7) layer is used as a well layer to enhance the power factor of MnSi1.7 film. It is found that the Al and Cu modulation doped MnSi1.7 film, Si: (Al + Cu)/MnSi1.7, has a power factor almost two times larger than that of only Al modulation doped MnSi1.7 film, Si: Al/MnSi1.7. It is also found that an undoped Si spacer layer between the Si: (Al + Cu) barrier layer and the MnSi1.7 well layer can enhance the power factor further. Finally, it is demonstrated that the MnSi1.7 film with double Si barrier layers, Si: (Al + Cu)/MnSi1.7/Si: (Al + Cu), has the highest power factor, 1423×10−6 W/m K2 at 783 K, which is very close to that of MnSi1.7 bulk material.

The location and doping effect of boron in Si nanocrystals embedded silicon oxide film

Electrically activated doping of boron (B) atoms into the Si-nanocrystals (Si-NCs) embedded silicon oxide film is achieved by co-sputtering technique following with the annealing treatment. The evolution of the size, the shape, and the density of Si-NCs with the doping of B atoms is investigated. The observation of x-ray photoelectron spectroscopy of Si 2p and B 1s and the decrease in lattice spacing of Si (111) plane suggest that B atoms are doped into Si-NCs. The activated doping is confirmed by the Fano effect of the micro-Raman spectra for Si-NCs and the drastic decrease of the sheet resistance.

Theoretical study of the maximum power point of n-type and p-type crystalline silicon space solar cells

The performance of crystalline silicon n+/p/p+ and n+/n/p+ solar cells under AM0 spectrum, irradiated with 1 MeV electrons at fluences below 1017 cm−2 has been analyzed by means of computer simulation. The software used, fully developed by the authors, solves numerically in one dimension under steady-state conditions, the Poisson and continuity equations self-consistently. The influence of constructive characteristics and different levels of hazardous environmental work conditions on the maximum power point of over 150 devices has been investigated. The study has allowed the authors to propose a useful analytical model related to the constructive characteristics of the device such as polarity, base resistivity and total thickness, with the aim of examining the electrical performance of Si space solar cells. Results presented in this paper are important in order to contribute to the design of radiation-hardened devices.

Formation and Characterization of Multilayer GeSi Nanowires on Miscut Si (001) Substrates

A multilayer of GeSi nanowires separated with Si spacers was readily self-assembled on miscut Si (001) substrates with 8 degrees off toward (110). The nanowires oriented along the miscut direction were very small and compactly arranged on the vicinal surface. Systematic photoluminescence (PL) spectroscopy studies were carried out on the GeSi nanowires. With increasing excitation power, a sublinear power dependent PL intensity and a blue-shift of -7 meV/decade with nearly constant full width of half maximum (FWHM) of PL peaks from multilayer GeSi nanowires was observed. These results indicated a typical type-III band alignment of GeSi nanowires/Si. The blue-shift was attributed to band bending effect with the increase of photon-generated carriers. The nearly independent FWHM of PL peaks with the excitation power was explained in terms of the formation of mini-band due to strong coupling of holes in closely neighboring nanowires. An activation energy of -12 meV was extracted from the temperature-dependent intensity of PL peaks of the nanowires, which was assigned to be the exciton binding energy around the nanowires. Based on Raman spectra, the Ge content in GeSi nanowires was estimated to be -61%.

Numerical Analysis of Si and GaAs Solar Cells Exposed to Space Radiation

In this paper, we present a study about Si and GaAs space solar cells by means of numerical simulations. We have investigated the variations of the short-circuit current, open circuit voltage and maximum power point, before and after the devices were irradiated with 10 MeV protons and fluences between 109 and 1013 p+/cm2. The simulation results allow to obtain the optimum solar cell with specific requirements for space applications.

Effect of doping on the photoluminescence of multilayer Ge quantum dots deposited on Si(001) substrate

Four-bilayer Ge quantum dots (QDs) with Si spacers were epitaxially grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition. In two samples, Ge QDs were in situ doped with phosphorus or boron, separately. Surface morphology and room temperature photoluminescence (PL) of multilayer Ge/Si QDs wer studied. Compared with the undoped Ge QDs, phosphorus-doping did not change the morphology of Ge QDs, enhanced PL wer observed from the phosphorus-doped Ge QDs. But reduction of Ge QDs density and PL intensity wer observed from the boron-doped Ge QDs. The intensity enhancement of PL could be attributed to the sufficient supply of electrons in Ge QDs for radiative recombination.

Nanostructure and Strain Field in Vertically Aligned Nano-Islands for Si/Ge 2D Photonic Nanocrystals.

AbstractThe local strain field and the intermixing of a Ge nano-islands (NIs)/Si spacer stacked structure in a novel solar cell with a p-i-n type Si single crystal with two-dimensional photonic nanocrystals connecting to the vertically aligned NIs were analyzed using electron microscopy. High-angle annular dark field-scanning transmission electron microscope (HAADF-STEM) images show intermixing between Ge and Si clearly and reveal that the surface segregation of Ge becomes advanced. The average composition of the NIs is Ge0.42Si0.58, which is almost constant in a row of vertically aligned NIs. The local strain analysis results obtained from the high-resolution transmission electron microscope (HRTEM) images show that the strain state is partially relaxed after the elastic relaxation of NIs.

Factors influencing epitaxial growth of three-dimensional Ge quantum dot crystals on pit-patterned Si substrate

We investigated the molecular beam epitaxy growth of three-dimensional (3D) Ge quantum dot crystals (QDCs) on periodically pit-patterned Si substrates. A series of factors influencing the growth of QDCs were investigated in detail and the optimized growth conditions were found. The growth of the Si buffer layer and the first quantum dot (QD) layer play a key role in the growth of QDCs. The pit facet inclination angle decreased with increasing buffer layer thickness, and its optimized value was found to be around 21°, ensuring that all the QDs in the first layer nucleate within the pits. A large Ge deposition amount in the first QD layer favors strain build-up by QDs, size uniformity of QDs and hence periodicity of the strain distribution; a thin Si spacer layer favors strain correlation along the growth direction; both effects contribute to the vertical ordering of the QDCs. Results obtained by atomic force microscopy and cross-sectional transmission electron microscopy showed that 3D ordering was achieved in the Ge QDCs with the highest ever areal dot density of 1.2 × 1010 cm−2, and that the lateral and the vertical interdot spacing were ∼10 and ∼2.5 nm, respectively.

Effect of growth rate on rod spacing and undercooling of Bridgman-grown Si–TaSi2 eutectic in situ composite

Directionally solidified Si–6wt.%TaSi2 eutectic alloy has been studied to characterize the variation of inter-rod spacing at different growth rates (V=6–200μm/s) with a constant temperature gradient (210K/cm). Statistical distributions of inter-rod spacings at each solidification rate are determined. It is confirmed that the relationship between inter-rod spacing and solidification rate obeys the Magnin–Kurz model, and the theoretical value of λa2V=5435.6μm3/s (λa is the average inter-rod spacing) approximately equals to the experimental value of λa2V1.06=5343.6μm3.06/s. For the Si–TaSi2 system, the operating parameter ϕ is a function of the solidification rate due to the transition of the TaSi2 phase from facet to nonfacet, and as the solidification rate increases the kinetic undercooling cannot be ignored.