Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

Nano-islands on a composite substrate with misfit dislocations

Spatial arrangements of nano-islands (quantum dots) on the free surface of a composite two-layer substrate containing misfit dislocations of edge type are theoretically examined. It is shown that the elastic interaction between misfit dislocations and nano-islands is capable of causing coagulation of nano-islands. The coagulation of nano-islands is shown to be favourable when the upper-layer thickness is smaller than a critical thickness, H₀. An analytical form of H₀ is presented for the partial case with four-to-one correspondence between nano-islands and cells of the misfit dislocation network. Received: 5 December 2000 / Accepted: 29 March 2001 / Published online: 20 June 2001     

TEM image contrast from antiphase domains in GaAs: Ge(001) grown by MBE

Antiphase domains occur in thin GaAs epitaxial films grown on Ge(001) by molecular beam epitaxy. The domains have been imaged by transmission electron microscopy using 200-type reflections in dark field. The carefully chosen imaging conditions with convergent illumination ensure that doubly diffracted beams from a pair of first order Laue zone discs contribute to the singly diffracted 200-type beams. The three Bragg reflections may add in or out of phase to give domains in either light or dark contrast depending on their polarity.     

Raman spectroscopy of macroscopic defects of GaAs grown by molecular beam epitaxy

Macroscopic defects of the GaAs surface grown by molecular beam epitaxy (MBE) have been investigated by using a micro-probing method of Raman spectroscopy. Especially, the oval defects, the most common macroscopic defects in MBE GaAs, were focused in this study. In Raman spectroscopy for the oval defect on the (100)GaAs surface, TO phonon mode of the 269 cm^–1 peak was observed. This indicates that the oval defects can include the (111) growth direction or the amorphized surface. The TO/LO intensity ratios for the defects are in the range from 0.3 to 1.0. In the sample grown under the condition that the substrate temperature is 580° C with the As/Ga ratio of 20, the density of the oval defects is about 200 cm^–2 at a growth thickness of 5 m. With increasing thickness of the epilayer, the density and the size of the -tye oval defect increased, while the TO/LO ratio decreased. From the spatial measurement by Raman spectroscopy for the -type oval defect, it is supposed that the -type oval defect remains in a rather good crystalline state and its orientation along the (100) growth direction is much closer to the (111) direction, but the growth direction of the defect might tend toward the (100) direction with a thicker layer.     

Trench formation in surfactant mediated epitaxial film growth of Ge on Si(100)

We have used STM to study the surface morphology of thin epitaxial Ge films grown on Si(001) in the presence of the surfactant As. The surfactant forces layer-by-layer growth up to 12 ML Ge coverage which could partly be explained by the geometrical surface arrangement of the growing film. Beyond 12 ML coverage we observed a network of trenches which decorate the earlier described V-shaped defects inside the film. Overgrowth of such defects is studied and a mechanism discussed.     

Effect of Si in reactively sputtered Ti-Si-N films on structure and diffusion barrier performance

34 Si₂₃N₄₃ (b3) and Ti₃₅Si₁₃N₅₂ (c3), are synthesized by reactively sputtering a Ti₅Si₃ or a Ti₃Si target, respectively. The silicon-lean film (c3) has a columnar structure closely resembling that of TiN. As a diffusion barrier between a shallow Si n⁺p junction diode and a Cu overlayer, this material is effective up to 700 °C for 30 min annealing in vacuum, a performance similar to that for TiN. The silicon-rich (b3) film contains nanocrystals of TiN, randomly oriented and embedded in an amorphous matrix. A film of (b3) maintains the stability of the same diode structure up to 850 °C for 30 min in vacuum. This film (b3) is clearly superior to TiN or to (c3). Similar experiments performed with Al instead of Cu overlayers highlight the importance of the thermodynamic stability of a barrier layer and demonstrate convincingly that for stable barriers the microstructure is a parameter that directly determines the barrier performance. Received: 18 November 1996/Accepted: 22 January 1997     

Impact of misfit dislocations on wavefront distortion in Si/SiGe/Si optical waveguides

Silicon-rich SiGe alloys represent a promising platform for the development of large-area single-mode optical waveguides to be integrated in silicon-based optical circuits. We find that SiGe layers epitaxially grown on Si successfully guide radiation with a 1.55 μm wavelength, but, beyond a critical core thickness, their optical properties are strongly affected by the clustering of misfit dislocations at the interface between Si and SiGe, leading to a significant perturbation of the local refractive index. Transmission electron microscopy and micro-Raman spectroscopy, together with finite-element simulations, provide a complete analysis of the impact of dislocations on optical propagation.     

Development of depressions and voids during LPE growth of GaAs

Depressions and voids, morphological defects which are frequently encountered in LPE-layers of III–V compounds, are described. Their development is discussed for GaAs LPE layers exhibiting facet growth as well as terrace growth. Experimental conditions for the growth of depression- and void-free layers are specified.     

The structure and defect structure of high-T c superconducting materials in the system Tl-Ba-Ca-Cu-O

Electron diffraction and high resolution electron microscopy have been applied to study Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀. The structures only differ in the thickness of the perovskite lamellae and have tetragonal basic unit cells witha=0.385,c=2.93 nm anda=0.385,c=3.58 nm, respectively. A modulation with wave vectors in (100)^* and (010)^* planes occurs in both compounds. This modulation of which the intensity is somewhat sample dependent is much weaker than in the Bi compounds and disappears irreversibly upon heating of the specimen. Intergrowth of lamellae of different phases does occur, although not as frequently as in the Bi compounds.Work performed in the framework of The Institute for Materials Science, I.M.S. (Antwerp) with financial support of I.U.A.P.     

Vibrational property of the slow N 2 + ions deposited SiNx films

We have investigated vibrational properties of silicon-nitride films, SiN_x (0.3×1.33), produced by a non-thermal method using high-resolution electron energy loss spectroscopy. The results, based on a continuous random network model assuming a planar XY₃ vibrational bond unit, show that the Si-N bonds in the films closely resemble those in typical thermal silicon nitride although nitrogens occupy some metastable binding sites. We estimate force constants of the restoring forces for a Si₃N bond unit, which tend to increase gradually with increasing nitrogen content x. In particular, the central force constant k₁ for the in-plane stretching mode of silicon atoms varies with x in the range 297k331 N/M, larger than the theoretical value for a nitrogen atom imbedded in a pure Si crystal.     

X-ray studies of dislocations in multi-layer Cu films

Vacuum-prepared multi-layer Cu films have been studied by x-ray diffraction to detect the averaged dislocation density and distribution. There are very few stacking faults and a very small degree of texture in the films.     

Formation of antisite defects by gliding dislocations in sphalerite-structure crystals

Conditions are discussed under which an interstitial atom absorbed in the core of a gliding 60° dislocation belonging to the glide set can be transformed into an antisite defect. The mechanism considered may be responsible for an increase in the concentration of As_Ga defects observed in GaAs single crystals after their plastic deformation.     

An investigation of point defects in silicon carbide

Positron lifetime measurement on bulk samples of single crystal SiC wafers have shown that both 6H and 4H polytypes exhibit a bulk lifetime of 150±2ps. All samples contained a second, defect-related lifetime component, ranging in value from 250 to about 300 ps with rather low intensities. The defect structure exhibited by the nanocrystalline samples is, not unexpectedly, much more complex.Positron beam experiments on Electron Cyclotron Resonance Chemical Vapour Deposited (ECR-CVD) SiC thin films showed that the positrons are very sensitive to changes in important film parameters as a function of the deposition conditions. It was found that the film density is lower than expected, probably due to hydrogen incorporation; variations in compositions among different films were detected through variations in theS parameters, and differences were observed in the electric field at the film-substrate interface due to hydrogen passivation of dangling bonds and different substrate resistivities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

HREM investigation of twinning in very high dose phosphorus ion-implanted silicon

Results are discussed of a study by means of high-resolution electron microscopy (HREM), electron diffraction, optical diffraction and image simulation of twinning in very high dose phosphorus ion-implanted (011) silicon wafers. Except for twins on the (111) planes (i. e.,3 boundaries) and9 boundaries, also regions showing, in the high-resolution image, a threefold periodicity are frequently observed. It is demonstrated that the diffraction pattern and the image of such regions can be explained by the overlap of twinned grains. Interpretation by the presence of polytypes of silicon is excluded. The possibility to image twins on inclined (111) planes is discussed.     

Determination of in-depth thermal strain distribution in Molecular Beam Epitaxy GaAs on Si

In-depth stress distribution GaAs layers grown by Molecular Beam Epitaxy (MBE) on Si (001) has been studied by X-ray diffraction, photoluminescence and Raman spectroscopy. In order to determine the stress state at different distances to the interface GaAs/Si, layers of different thickness were prepared by chemical etching of the grown samples. We observe a non-uniform residual strain distribution through the GaAs on Si epilayer. Residual strain of thermal origin is larger in the highly defective region ( 0.4 m) near the GaAs/Si interface where we have found a non-elastic relation between measured in-plane (a ) and in growth direction (a ) lattice parameters. However, thermal strain is partially relaxed by formation of 10⁷ cm^–2 dislocations in the region of better crystalline quality near the external surface.     

Interplay of surface morphology, strain relief, and surface stress during surfactant mediated epitaxy of Ge on Si

Lattice-mismatch-induced surface or film stress has significant influence on the morphology of heteroepitaxial films. This is demonstrated using Sb surfactant-mediated epitaxy of Ge on Si(111). The surfactant forces a two-dimensional growth of a continous Ge film instead of islanding. Two qualitatively different growth regimes are observed. Elastic relaxation: Prior to the generation of strain-relieving defects the Ge film grows pseudomorphically with the Si lattice constant and is under strong compressive stress. The Ge film relieves strain by forming a rough surface on a nm scale which allows partial elastic relaxation towards the Ge bulk lattice constant. The unfavorable increase of surface area is outbalanced by the large decrease of strain energy. The change of film stress and surface morphology is monitored in situ during deposition at elevated temperature with surface stress-induced optical deflection and high-resolution spot profile analysis low-energy electron diffraction. Plastic relaxation: After a critical thickness the generation of dislocations is initiated. The rough phase acts as a nucleation center for dislocations. On Si(111) those misfit dislocations are arranged in a threefold quasi periodic array at the interface that accommodate exactly the different lattice constants of Ge and Si. Received: 1 April 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

AlSb/GaAs(001)失配位错的高分辨电子显微学研究

用200kV六硼化镧光源的高分辨透射电子显微镜观察了AlSb/GaAs(001)外延薄膜的失配位错,结合解卷处理方法把[110]高分辨电子显微像转换为试样的结构投影图,其分辨率接近电子显微镜的信息极限.根据赝弱相位物体近似像衬理论,通过分析AlSb薄膜完整区解卷像的衬度随试样厚度的变化,确定了哑铃原子对中Al和Sb原子的位置.在此基础上构建出失配位错的结构模型,再结合模拟像与实验像的匹配,确定了AlAs型界面以及Lomer和60°两类失配全位错的核心结构.     

Atomic-resolution imaging and spectroscopy of semiconductor interfaces

Incoherent Z-contrast imaging uses a high-angle annular detector to collect only highly local, incoherently generated scattering with the result that images become dependent on intensities, not phases. No model structures are required for a first-order structure determination, and the images remain intuitively interpretable even at interfaces. Under suitable conditions, incoherently generated inelastic scattering may be collected simultaneously with a large-aperture axial spectrometer, and, by using the Z-contrast image to locate the scanning transmission electron microscope (STEM) probe over selected atomic columns, can provide an atomic-resolution chemical analysis. This is demonstrated with reference to an epitaxial CoSi₂/Si(100) interface, achieving a 2.7 Å spatial resolution. Recent insights into the growth and relaxation of strained Si-Ge epitaxial films are described, highlighting the role of stress concentrations, and contrasting the case of a free surface with that of a surface constrained by an oxide layer.     

X-ray line profile analysis of dislocations and stacking faults in deformed copper

Dislocation configurations and stacking faults in commercial copper rods after 20% and 70% cold-rolling, fatigue and unidirectional tension here studied by x-ray line profile analysis. The analysis of dislocation is based on the Stokes method, the Warren-Averbach analysis and the Wilkens theory for Gaussian or mixed type of strain broadening profiles. For the Cauchy type the range of stress field of dislocations is small, and a model of regular distribution of dislocation dipoles is proposed instead of the Wilkens model of a restrictedly random distribution of dislocations. Due to the obvious texture in all four kinds of deformed samples, the possible glide systems are obtained by using a biaxial stress system. The analysis of stacking faults is based on theories of Patterson, Warren, and Wagner by measuring profile peak shifts, asymmetry and broadening. The broadening due to perfect dislocations and stacking faults can easily be separated. The configuration parameters and density of dislocations, the probabilities of intrinsic, extrinsic and twin stacking faults were deduced in all cases.     

Si DX centers in GaAs at large hydrostatic pressures

A number of experimental and theoretical studies indicate that DX centers in GaAs, its alloys and other III–V semiconductors have negative U properties. Using far infrared localized vibrational mode (LVM) spectroscopy of Si donors in GaAs under large hydrostatic pressure in a diamond anvil cell we have discovered an LVM of the Si DX center. From the ratio of the LVM absorption lines of Si_Ga and Si_DX and the compensation in our GaAs samples, we show unambiguously that two electrons are trapped when the ionized shallow Si donors transform into negatively charged DX centers, in full agreement with the negative U model.Dedicated to H.-J. Queisser on the occasion of his 60th birthday