Anisotropic thermal conductivity of Ge quantum-dot and symmetrically strained Si/Ge superlattices

We report the first experimental results on the temperature dependent in-plane and cross-plane thermal conductivities of a symmetrically strained Si/Ge superlattice and a Ge quantum-dot superlattice measured by the two-wire 3 omega method. The measured thermal conductivity values are highly anisotropic and are significantly reduced compared to the bulk thermal conductivity of the structures. The results can be explained by using heat transport models based on the Boltzmann transport equation with partially diffusive scattering of the phonons at the superlattice interfaces.     

Investigation of thermal stability of hydrogenated amorphous Si/Ge multilayers

Thermal stability of hydrogenated amorphous Si/Ge multilayers has been investigated by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Small-Angle X-Ray Diffraction (SAXRD) techniques. Amorphous H-Si/Ge multilayers were prepared by RF sputtering with 1.5 and 6 ml/min H₂ flow-rate. It is shown by Elastic Recoil Detection Analysis (ERDA) that the hydrogen concentration increased by increasing the H₂ flow-rate. Annealing of the samples was carried out at 400 and 450 °C for several hours. It has been observed that samples prepared with 6 ml/min flow-rate at both annealing temperatures underwent significant structural changes: the surface of the samples was visibly roughened, gas bubbles were formed and craters were created. The decay of the periodic structure of Si and Ge layers in these types of multilayers was faster than in non-hydrogenated samples. Samples prepared with 1.5 ml/min flow-rate have similar behaviour at 450 °C, but at 400 °C the decay of the first order SAXRD peaks was slower than in case of the non-hydrogenated multilayers. Qualitatively the observed behaviour can be explained by the fast desorption of the saturated hydrogen, leading to the formation of bubbles and craters at 450 °C, as well as, at 400 °C in the sample with lower H-content, by the possible passivation of the dangling bonds resulting in a slowing down of the diffusion intermixing.     

离子注入诱导硅材料发光的研究进展

阐述了采用离子注入技术在硅晶体(Si)中形成缺陷的发光机制,综述了离子注入诱导硅材料发光的研究进展,包括稀土离子掺入硅晶体形成稀土杂质中心发光,Ⅳ族元素C、Ge离子注入Si/SiO2形成等离子缺陷中心发光,以及B、S、P等常规离子注入硅晶体的缺陷发光,其中重点介绍了Si离子自注入诱导硅材料发光的研究现状,最后展望了硅基材料发光的未来发展。     

Ge/Si量子点生长的研究进展

回顾了近年来在Ge/Si量子点生长方面的研究进展.主要讨论了为了提高量子点空间分布有序性、增大量子点的密度、减小量子点的尺寸及改善其分布均匀性而采取的各种方法,如图形衬底辅助生长、表面原子掺杂及利用超薄SiO2层辅助生长等,以及Ge量子点的演变及组分变化.     

Hydrogen in amorphous Si and Ge during solid phase epitaxy

Studies into the effect of hydrogen on the kinetics of solid phase epitaxy (SPE) in amorphous Si (a-Si) and Ge (a-Ge) are presented. During SPE, H diffuses into surface amorphous layers from the surface and segregates at the crystalline-amorphous interface. Some of the H crosses the interface and diffuses into the crystalline material where it either leaves the sample or is trapped by defects. H segregation at concentrations up to 2.3 × 10²⁰ H/cm³ is observed in buried pha-Si layers with the SPE rate decreasing by up to 20%. H also results in a reduction of dopant-enhanced SPE rates and is used to explain the asymmetry effects between the SPE velocity profile and the dopant concentration profile observed with shallow dopant implants. Conversely, H diffusion is enhanced by dopants in a-Si. These studies suggest that H diffusion and SPE may be mediated by the same defect. The extent of H in-diffusion into a-Ge surface layers during SPE is about one order of magnitude less that that observed for a-Si layers. This is thought to be due to the lack of a stable surface oxide on a-Ge. However, a considerably greater retarding effect on the SPE rate in a-Ge of up to 70% is observed. A single unifying model is applied to both dopant-enhanced SPE and H diffusion processes.     

Growth of Ge islands on Si substrates

We present a study of Ge islands formation on Si(100) substrates using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Samples were prepared by magnetron sputtering of a 5 nm thick Ge layer in a very high vacuum on Si(100) substrate held at different temperatures. The vertical cut (perpendicular to the surface) of the experimental 2D GISAXS pattern has been fitted using a Guinier approximation. The optimum temperature for the islands formation was 650 °C. At this temperature, islands grow in conical shape with very similar dimensions; however, inter-island distances varied significantly.     

硅、锗材料在Si(100)表面的生长研究

利用扫描隧道显微镜和超高真空实验装置系统进行了Si(10 0 )表面生长Si,Ge的实验研究。分析了所生成表面的形貌、结构等物理性质。研究表明 :Si在Si(10 0 )表面的同质生长可以形成纳米结构薄膜。Ge在Si(10 0 )表面生长形成规则的三维小岛。而在Si/Ge/Si(10 0 )多层膜上生长则形成大小二种三维岛。研究表明大岛具有Ge/Si/Ge的壳层结构     

Sb mediated formation of Ge/Si quantum dots: Growth and properties

The phenomenon of surfactant (Sb) mediated formation of Ge/Si(100) islands (quantum dots) by means of molecular beam epitaxy is discussed. The limited diffusivity of Si and Ge adatoms caused by the Sb layer leads to a reduction of the size of Ge islands, the increase in the island density, and the sharpening of the interfaces of Ge islands. Thereby, a thin Sb layer is considered to be a powerful tool that provides more freedom in designing Ge quantum dot features. Ge quantum dots, grown via a thin Sb layer and embedded coherently in a Si p-n junction, are revealed to be the origin of the intense photo- and electroluminescence in the spectral range of about 1.5 μm at room temperature.     

Effects of gamma irradiation on electroluminescence spectra from Au/amorphous Si/SiO2 superlattices/p-Si structures

Amorphous-Si/SiO₂ superlattices have been grown using the two-target alternation magnetron sputtering technique. The thickness of the SiO₂ layers in all the superlattices was 2.0 nm and that of the Si layers in nine types of the superlattice ranged from 0.6 to 3.8 nm in steps of 0.4 nm. Visible electroluminescence (EL) spectra of Au/amorphous-Si/SiO₂ superlattices/p-Si structures with Si layers of nine different thicknesses showed a peak located at 650 nm. After gamma irradiation, the EL peak increased 2.5 times in intensity. Moreover, a strong new 470 nm blue peak emerged from the EL spectra in all the Au/amorphous-Si/SiO₂ superlattices/p-Si structures. The experimental results indicate that the EL recombination process mainly originates from luminescence centers in the SiO₂ layers rather than from the Si layers in the superlattices.     

Si_3N_4/Si表面Ge生长过程的STM研究

利用STM和LEED分析了Ge在Si3 N4 /Si(111)和Si3 N4 /Si(10 0 )表面生长过程的结构演变。在生长早期 ,Ge在两种衬底表面上都形成高密度的三维纳米团簇 ,这些团簇的大小均在几个纳米范围内 ,并在高温退火时体积增大。当生长继续时 ,Ge的晶体小面开始显现。在晶态的Si3 N4 (0 0 0 1) /Si(111)表面 ,Ge的 (111)晶向的小面生长比其他方向优先。最后在大范围内形成以 (111)方向为主的晶面。相反 ,在非晶的Si3 N4 表面 ,即Si3 N4 /Si(10 0 ) ,Ge晶体的高指数侧面生长较顶面快 ,最终形成金字塔形的岛结构。对这样的表面生长过程进行了探讨并给出了合理的物理解释     

Ge/SiGe superlattices for thermoelectric energy conversion devices

Ge-rich multiple quantum well heterostructures have been investigated as engineered material for efficient thermoelectric generators monolithically integrated on silicon substrates. Thick Ge/SiGe multilayers on Si substrates designed for lateral thermoelectric devices have been grown and characterized in which electrical and thermal conduction occur parallel to the heterostructure interfaces. In this study, an overview of the investigated structures is presented together with results from X-ray scattering and transmission electron microscopy experiments. These analyses confirm the high quality of the material and the uniformity of the structure over the whole deposited thickness. Important parameters in terms of the optimization of the material quality which could affect thermoelectric properties, such as the interfaces roughness and the threading dislocation density, have also been evaluated. Preliminary electrical and Seebeck coefficient measurements indicate the viability of this material for the realization of thermoelectric devices.     

Investigation of Si and Ge growth on Si3N4/Si

The growth of Si and Ge on silicon nitride surfaces has been investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectrometer (AES). In the early stages, Si or Ge nanoclusters appeared irrespective of the different substrates. When annealed, the Si clusters were more stable against coalescence than those of Ge. As these clusters continued to grow, crystalline facets started to form. Both Si and Ge islands grew predominantly with (111)-oriented top facets on the crystalline Si₃N₄(0001)/Si(111). By contrast, they both grew in random orientation on the amorphous Si₃N₄ surface. Low-index facets such as (111) and (001) coexisted with high-index facets such as (113).     

One-step Ge/Si epitaxial growth

Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Ge films on silicon wafer with a Ge(x)Si(1-x) (0 < x < 1) graded buffer layer was demonstrated through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO(2) powder under hydrogen atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diffraction analysis shows that the Ge film with an epitaxial relationship is along the in-plane direction of Si. The successful growth of epitaxial Ge films on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.     

Real-time studies of Ge growth on nanostructured Si substrates

Nanostructured substrates are an interesting path towards the production of quantum dots devoted to microelectronic applications. In this work we report results on the effects of different nanopatterning methods to obtain lateral ordering of Ge islands grown on Si. By using Scanning Tunneling Microscopy we have studied in real-time the wetting layer growth and islands formation on nanopatterned Si substrates at 500 °C. We compare results obtained on Si substrates nanopatterned by using two different techniques: STM lithography and natural patterning induced by surface instabilities such as step bunching. Different issues on both cases have been addressed: substrate preparation, Ge dots placement and growth mode. We have observed that, on Si(001), the Ge islands nucleate near the holes and on Si(111) step bunching can guide the growth of aligned rows of islands.     

SiO2/Si(111)表面Ge量子点的生长研究

Si衬底用化学方法清洗后,表面大约残余1.0 nm厚SiO2薄膜.利用原子力显微镜(AFM)和反射高能电子衍射(RHEED)来研究温度和Ge蒸发厚度对在SiO2薄膜表面生长的Ge量子点的影响.实验结果表明,当衬底温度超过500 ℃时,SiO2开始与Ge原子发生化学反应,并形成与Si(111)表面直接外延的Ge量子点.在650 ℃时,只有Ge的厚度达到0.5nm时,Ge量子点才开始形成.     

The structure and electrical characteristics of Si/Ge heterojunctions II: The electrical characteristics of Si/Ge heterojunctions

The I–V characteristics of p-Ge/nn⁺ -Si and p-Ge_xSi_1?x/nn⁺ -Si heterojunctions produced by the vacuum epitaxy of germanium onto nn⁺ -Si were determined. The mechanisms of the d.c. and reverse current were investigated.     

Temperature-dependent and pump-probe ellipsometric studies of TlInSe2

Temperature-dependent phase-modulated spectroscopic ellipsometry over the photon energies 1.5-5.5 eV and temperatures 300-400 K, as well as pump-probe single wavelength rotating-analyzer laser ellipsometry at photon energy 1.61 eV and pumping power-density up to 160 W/cm² has been carried out on TlInSe₂ that has a quasi one-dimensional crystalline structure and apparently undergoes an incommensurate phase transition with temperature. Refractive indices and extinction coefficients, as well as their variation with temperature and pumping power-density have been determined. The change of the surface profile under pumping has been disclosed. The reversibility of the effect has been confirmed by the dynamic reflectance examination. Obtained results are discussed in terms of thermal effects and incommensurate phase transition in TlInSe₂. It is proposed that a giant thermo-elastic effect and a soliton wave formation are behind the observed reversible change of the surface profile.