Structure and optoelectronic properties of Si/O superlattice

We have carried out structural study of the Si/O semiconductor atomic superlattices (SAS) with up to 18 Si/O layers fabricated by molecular beam epitaxy and in situ oxygen exposure on both Sb-doped and undoped Si buffer layers, and correlated the results with our photoluminescence, electroluminescence (EL) and I–V data. The Si/O SAS is a new type of superlattice, where monolayers of oxygen are sandwiched between the Si layers. High-resolution cross-sectional transmission electron microscopy (TEM) study has confirmed the presence of the superlattice and shown epitaxy in the Si/O superlattices. The high structural quality of the layers grown on the undoped Si buffer layers with low density of stacking faults—less than 10⁷/cm²—was established by TEM. Although structure perfection is very important allowing this new class of superlattices to be extended to other systems, it is important to point out that a 9-period SAS-based EL device with emission of light in green has been life-tested with stable output for over 1 year of continuous operation. The Si/O superlattice also serves as an epitaxially grown insulating layer as possible replacement of silicon-on-insulator. Together with the tailor-made effective band gap, this epitaxially grown superlattice may serve as future silicon-based three-dimensional integrated circuits.     

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

Infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices

We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.     

GaAs/SiO_2超晶格的制备和Raman光谱研究

我们采用射频磁控溅射方法在 p- Si衬底上成功地制备出四周期的非晶 Ga As/Si O2超晶格 ,并取得其高分辨率电镜像。以 80 0℃快速退火方法使超晶格中非晶的 Ga As层局部晶化 ,利用 Raman散射谱研究了其结构变化。     

Mechanisms of self-organization of Ge/Si(0 0 1) quantum dots

The effect of vertical ordering in superlattices of self-assembled Ge/Si(0 0 1) quantum dots was investigated by a combination of structural and optical characterizations via in situ reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy and photoluminescence spectroscopy. We show that the vertical ordering observed in quantum-dot superlattices is characterized not only by the alignment of islands along the growth direction but also by a reduction of the critical thickness. The better the vertical ordering is, the more pronounced the reduction of the critical thickness will be. Such an evolution of the critical thickness could be explained by elastic strain fields induced by buried islands and propagated through the spacer layers. An important result issued from this work is the realization of superlattices in which dots can have equal size in all layers. On the other hand, experiments performed on the transformation of the island shape versus the spacer layer thickness suggest that preferential nucleation induced by surface roughness may be the main mechanism responsible for the vertical ordering observed in quantum-dot superlattices.     

非晶Si/SiO2超晶格的制备及其光谱研究

用磁控溅射方法在玻璃基底上制备了非晶Si/SiO₂超晶格.利用透射电子显微镜 (TEM) 和X射线衍射技术对其结构进行了分析,结果表明,超晶格中Si层大部分区域为非晶相,局域微区呈现有序结构,其厚度由1.8—3.2nm变化,SiO₂层厚度为4.0nm.并采用多种光谱测量技术,如吸收光谱、光致发光光谱和Raman光谱技术,对该结构的光学性质进行了系统研究.结果表明,随纳米Si层厚度的减小,光学吸收边以及光致荧光峰发生明显蓝移,Raman峰发生展宽,即观测到明显的量 关键词：     

GexSi1-x/Si应变层超晶格的分子束外延生长及其特性研究

在Si(100)衬底上用分子束外延在不同的温度下生长了不同组份的Ge_xSi_1-x/Si应变层超晶格。用反射式高能电子衍射、X射线双晶衍射、卢瑟福背散射、透射电子显微镜以及Raman。散射等测试方法研究了Ge_xSi_1-x/Si超晶格的生长及其结构特性。结果表明,对不同合金组份的超晶格,其最佳生长温度不同。x值小,生长温度高;反之,则要求生长温度低。对于x为0.1—0.6,在400—600℃的生长温度范围能够长成界面平整、 关键词：     

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si（001） substrate

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films.In this paper,Co films with different thickness of Co Si2buffer layers were grown on Si(001)substrates.In order to investigate morphology,structure,and magnetic properties of films,scanning tunneling microscope(STM),low energy electron diffraction(LEED),high resolution transmission electron microscopy(HRTEM),and surface magneto-optical Kerr effect(SMOKE)were used.The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of Co Si2buffer layers.Few Co Si2monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality.Furthermore,the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process,which is the typical phenomenon in cubic(001)films.     

Si-W/Si/SiO2/Si(100)的横截面电子显微镜研究

本文用横截面电子显微镜法分析了Si-W/Si/SiO₂/Si(100)在440—1000℃退火后的晶化过程,以及各个界面的变化情况.发现Si-W合金膜中,WSi₂并未优先在表面、界面处形成晶核.当退火温度不高于700℃时,反应在合金膜内发生,表面、界面起伏和缓.退火温度高达800—1000℃时,界面、表面出现原子扩散,造成剧烈的界面起伏;表面则出现小的热沟槽,Si/SiO₂界面也出现高分辨电子显微镜才能观察到的起伏.表面、界面的原子迁移的动力来源于晶界与表面、界面张力.由于SiO₂中Si—O键很稳定,不易发生Si和O在界面处的互扩散,所以Si/SiO₂界面起伏很小. 关键词：     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

Investigation of the structure and luminescence properties of CdF2-CaF2 : Eu superlattices on Si(111)

A complex investigation of CdF₂-CaF₂ : Eu superlattices with different bilayer thicknesses (2.0–17.5 nm) grown by molecular beam epitaxy on Si(111) has been carried out. The structural perfection of the layers and interfaces of the superlattices have been estimated from the X-ray diffractometry and reflectometry data. The possibility of producing short-period pseudomorphic superlattices with a period of approximately 2 nm has been established. It has been shown that these superlattices are characterized by a larger root-mean-square roughness amplitude of the interfaces as compared to the long-period superlattices. The specific features of cathodoluminescence spectra have been analyzed as a function of the superlattice period. It has been revealed that, with a decrease in the superlattice period, the intrinsic luminescence intensity of fluorides increases in comparison with the intensity of the luminescence associated with the emission of Eu²⁺ impurity ions; in this case, several Eu³⁺ luminescence bands appear in the spectrum. The possibilities of electron probe microanalysis for determining the ratio of thicknesses of individual layers in short-period superlattices have been demonstrated.     

Initial stage of Pd adsorption on Si(111)7 × 7 surface studied by AES and EELS

The initial stage of adsorption of Pd on a Si(111)7 × 7 surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and surface work-function change. For Pd deposition at room temperature (RT) the Si(LVV) Auger signal intensity decays in a broken linear line. The structure factor, defined as the intensity ratio of the subpeak to the main one in Si(LVV) Auger spectra, increases up to a maximum around one monolayer coverages. In EELS spectra two peaks, characteristics of Pd, appear at the completion of the first Pd layer. Pd atoms deposited on Si(111) at RT form initially flat layers of a few monolayers height without mixing with substrate Si atoms. For Pd deposition at a moderately high temperature (MT) of about 300°C, however, the structure factor for Si(LVV) Auger spectra does not change. EELS peaks, characteristic of Si substrate, remain clearly even beyond one monolayer coverage. Pd atoms deposited at MT are unstable and easily diffuse into the bulk. We present evidences to support the “screening” model for the bond-breaking mechanism at the Pd/Si interface.     

Photoluminescence and the structure of heterointerfaces of (311)A-and (311)B-oriented GaAs/AlAs superlattices

GaAs/AlAs superlattices grown simultaneously on GaAs substrates with the (311)A and (311)B orientations have been studied by photoluminescence and high-resolution transmission electron microscopy with a Fourier analysis of images. A periodic interface corrugation is observed for (311)B superlattices. A comparison of the structure of (311)A and (311)B superlattices indicates that the corrugation occurs in both cases and its period along the $[01\overline 1 ]$ direction is equal to 3.2 nm. The corrugation is less pronounced in (311)B superlattices, wherein it exhibits an additional modulation (long-wavelength disorder) with the characteristic lateral size exceeding 10 nm. The vertical correlation of regions rich in GaAs and AlAs, which is well observed in (311)A superlattices, is weak in (311)B superlattices due to the occurrence of long-wavelength disorder. The optical properties of (311)B superlattices are similar to those of (100) ones and differ radically from those of (311)A superlattices. As distinct from (311)B, strong photoluminescence polarization anisotropy is observed for (311)A superlattices. It is shown that it is the interface corrugation rather than the crystallographic (311) surface orientation that determines the optical properties of (311)A corrugated superlattices with thin GaAs and AlAs layers.     

Ab initio calculations of interfacial magnetism in Fe/Mo superlattices

Ab initio calculations have been performed on Fe/Mo(1 0 0) superlattices in order to study the interfacial magnetic properties and layer thickness effect on the magnetic moments. In most cases, the magnetic moments of interfacial Fe monolayers are always smaller than those of the inner layers, and the induced magnetic moments of interfacial Mo monolayers oriented in the opposite direction. Calculation results show that the Fe layers are ferromagnetic when n = 3. As the thickness of the Mo layers increases, the influence of the Mo layer increases and the magnetic state of the Fe layer gradually changes into an antiferromagnetic or non-magnetic state. The change of magnetic moments of Fe/Mo superlattices is in agreement with the experimentally observed oscillation periods.     

Growth of high quality CoSi2/Si - superstructures on Si (111)

Using a newly developed solid phase epitaxy technique (SPE) it is shown that ultrathin essentially pinhole-free CoSi₂ layers can be grown epitaxially on Si (111). These form the basis of a number of short period metal/semiconductor superlattices that have been grown by combining SPE-grown CoSi₂ with MBE-grown Si. Substrate temperatures for Si-MBE have to be chosen very low (≈ 350 °C) in order to avoid a roughening of the layers.     

Mössbauer spectroscopy investigation of body centered cubic Co in Co/Fe superlattices prepared with MBE

A new spectrum component is observed in Mössbauer spectra of thin body centered cubic Co layers prepared in Fe/Co superlattices doped with⁵⁷Co. It is characterized by a large magnetic hyperfine field (31.2 T) and an isomer shift nearly equal to that of -Fe. The decrease of the isomer shift in bcc Co with respect to hcp Co is consistent with smaller s to d charge transfer in bcc Co as compared to hcp Co. The cubic structure of the CoFe superlattices is evidenced with X-ray diffraction and ion-channeling measurements. The Fe/Co interface is investigated with conversion electron Mössbauer spectroscopy. It is shown that the interface alloy thickness is about six monolayers for growth temperatures up to 450 K and that increasing alloying occurs for higher growth temperatures.     

DX centers in selectively Si-doped GaAs---AlAs superlattices

DX center has been characterized in four GaAs---AlAs superlattices grown by MBE at 580°C. The structures are uniformly Si-doped or selectively Si-doped in the AlAs layers or in the middle of the GaAs layers or on both sides of the interfaces. Deep level transient spectroscopy measurements (DLTS) put in evidence one dominant electron trap, with an activation energy for thermal emission of about 0.42eV for all the superlattices. This defect shows a thermally activated capture cross section and a large concentration except for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy of 0.36-0.37 eV, which allows us to locate the DX level nearly 60 meV below the conduction miniband. From these results, we show that the observed DX center is related to silicon in the AlAs layers. For the case when the AlAs barriers are not doped, the DX level is due to the Si diffusion from the middle of the wells towards the barriers, the Si atoms having diffused during the growth.     

Si/Gen/Si(001)异质结薄膜的掠入射荧光X射线吸收精细结构研究

利用掠入射荧光X射线吸收精细结构(XAFS)方法研究了在400℃的温度下分子束外延生长的Si/Ge_n/Si(001)异质结薄膜(n=1，2，4和8个原子层)中Ge原子的局域环境结构.结果表明，在1至2个Ge原子层(ML)生长厚度的异质结薄膜中，Ge原子的第一近邻配位主要是Si原子.随着Ge原子层厚度增加到4ML，Ge原子的最近邻配位壳层中的Ge-Ge配位的平均配位数增加到1.3.当Ge原子层厚度增加到8ML时，第一配位壳层中的Ge-Ge配位占的比例只有55%.这表明在400℃的生长条件下，Ge原子有很强的迁移到Si覆盖层的能力.随着Ge层厚度从1 增加到2，4和8ML，Ge原子迁移到Si覆盖层的量由0.5ML分别增加到1.5，2.0和3.0ML.认为在覆盖Si过程中Ge原子的迁移主要是通过产生Ge原子表面偏析来降低表面能和Ge层的应变能. 关键词： XAFS n/Si(001)异质膜')" href="#">Si/Ge_n/Si(001)异质膜 迁移效应     

Comparison between atomic layer epitaxy grown and molecular beam epitaxy grown CdTe/MnTe superlattices: a structural and optical study

Atomic layer epitaxy (ALE) is investigated for the growth of CdTe/MnTe superlattices. A systematic structural characterization (X-ray diffraction, transmission electron microscopy), together with a magneto-optical study (reflectivity and photoluminescence), demonstrate that: for MnTe ALE, all deposited Mn atoms are incorporated, so that no autoregulated growth mode can be obtained, in contrast with CdTe ALE, atomic layer epitaxy allows well-controlled CdTe/MnTe superlattices to be achieved but does not prevent the exchange between Cd and Mn atoms which occurs at the interfaces between CdTe and MnTe, as observed in CdTe/MnTe superlattices grown by conventional molecular beam epitaxy.     

First principles studies of Fe/Co superlattices and multilayers with bcc (0 0 1) and (1 1 0) orientations

The layer resolved magnetic moments and magnetic anisotropy energy of Fe/Co superlattices and multilayers with bcc (0 0 1) and (1 1 0) orientations obtained from first principles simulations are reported here. The magnetic moment of Fe atoms are found to depend on the geometry, coordination number and proximity to Co atoms, whereas that of Co remains almost constant in the superlattices and multilayers. Mixing of atoms at the interface resulted in enhanced Fe magnetic moment while that of Co is unaffected. The magnetic anisotropy energy in superlattices and multilayers are found to be larger than the corresponding values of bulk counterparts. Calculated easy axis of magnetization is in the plane for all superlattice compositions considered in the study, while that in multilayers, changes with crystalline orientation and thickness of Co layers.