Cross-hatched Surface Research Articles

Strain-balanced Si/SiGe short period superlattices: Disruption of the surface crosshatch

We have studied the effects of inserting Si/Si0.6Ge0.4 strain-balanced superlattices (SLs) into Si0.8Ge0.2 (001) virtual substrates. The SiGe SL layer thickness chosen was larger than the critical thickness for elastic relaxation and generated numerous hemicylindrical features oriented along the 〈100〉 directions. These features lead, when covered by Si0.8Ge0.2, to a disruption of the well-ordered surface crosshatch along the 〈110〉 directions, and to a significant lowering of the surface roughness. There is also evidence for some filtering of the threading dislocations by the SL.

Surface characterization of III–V heteroepitaxial systems by laser light scattering

With the aim of a better understanding of in situ laser light scattering measurements, we report a detailed study of some typical surface morphologies which appear during molecular beam epitaxy growth of III–V compounds by means of ex situ angle-resolved light scattering characterization. The power spectral density of a cross-hatched surface is reconstructed from scattered intensity and the periodicity values obtained are in good agreement with atomic force microscopy data. The presence of quantum dots on the surface is analyzed and limitation for quantum dots detection due to oval defects is considered. We also consider possible in situ configurations in order to avoid this restriction.

Optical on wafer measurement of Ge content of virtual SiGe-substrates

We developed a procedure for a fast and non-destructive determination of the Ge-content in virtual Si 1− x Ge x -substrates in the range from x=0.2–1. The virtual substrates were grown in our MBE-system. The reflectivity of SiGe substrates depends on the Ge content x and on the crosshatch surface corrugations. The extremes in relative reflectance values are rather insensitive to surface corrugations contrary to the absolute values which depend significantly on the surface morphology. The reflectance of films was measured ex situ with a commercial measuring system (NanoSpec). This system measures relatively to a measurement standard wafer (Silicon-wafer) over wavelength range from 370 to 800 nm. The reflection graphs have maxima in the range from 410 to 590 nm depending on the Ge-content. Results obtained by these measurements with the NanoSpec measuring system were in a good agreement with SIMS-analysis and XRD data.

STM study of step graded Si 1− xGe x/Si(001) buffers

We report on the surface structure of step graded Si 1− x Ge x /Si(001) buffers, as obtained by scanning tunneling microscopy (STM). The samples in this study were grown by means of rf magnetron sputter epitaxy (MSE). The study was performed by varying two essential growth parameters: the substrate temperature and the final Ge concentration x f of the buffer. The emergence of the typical cross-hatched surface was monitored by investigating the surface at x f=0.15, 0.20 and 0.30. The influence of the growth temperature was studied in the range between 370 and 570°C. Even at the lowest temperatures, the strain was found to be partially relieved as evidenced by the formation of a cross-hatch.

High minority-carrier lifetimes in GaAs grown on low-defect-density Ge/GeSi/Si substrates

A high bulk minority-carrier lifetime in GaAs grown on Si-based substrates is demonstrated. This was achieved by utilizing a step-graded Ge/GeSi buffer (threading dislocation density 2×106 cm−2) grown on an offcut (001) Si wafer, coupled with monolayer-scale control of the GaAs nucleation to suppress antiphase domains. Bulk minority-carrier lifetimes (τp) were measured using room-temperature time-resolved photoluminescence applied to a series of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double-heterojunction structures doped n=1.1×1017 cm−3 with GaAs thicknesses of 0.5, 1.0, and 1.5 μm. A lifetime τp=7.7 ns was determined for GaAs grown on Si. The extracted interface recombination velocity of 3.9×103 cm/s is comparable to recombination velocities found for Al0.3Ga0.7As/GaAs interfaces grown on both GaAs and Ge wafers, indicating that the crosshatch surface morphology characteristic of strain-relaxed Ge/GeSi surfaces does not impede the formation of high-electronic-quality interfaces. These results hold great promise for future integration of III–V minority-carrier devices with Si wafer technologies.

Low-Energy Plasma Enhanced Chemical Vapor Deposition

AbstractWe discuss a new method for plasma enhanced chemical vapor deposition, applied to the epitaxial growth of Si and of Si-Ge heterostructures. Growth rates up to 5 nm/s become possible at substrate temperatures below 600°C, by utilizing very intense but low energy plasmas to crack the reactive gases, SiH4 and GeH4, and to speed up the surface kinetics. The method is applied to the synthesis of step-graded Si-Ge buffer layers, exhibiting the well known cross-hatched surface morphology.

Magnetron Sputter Heteroepitaxy of Si1-x.Gex/Si(001): The Evolution of the Cross-Hatched Surface

AbstractThe surface step structure of step graded Si1-xGex buffers was investigated by scanning tunneling microscopy (STM). On pseudomorphically grown samples, single atomic steps were found, with the 2×8 reconstruction indicating Ge segregation. Upon strain relaxation, surface slips and the development of the cross-hatched surface takes place. Here, close to the surface slips, the step structure was found to consist of double height DA steps. This feature is not simply kinematically driven step bunching but can be attributed to the anisotropic strain field on the surface and to the Ge segregation. Analysis of STM cross-sections were taken to estimate the relaxation of our samples. The obtained values agree well with x-ray measurements. The relaxation is found to depend exponentially on the temperature, as theoretically expected.

Molecular beam epitaxy of heterostructures on metamorphic AlxGayIn1 − x − yAs buffer layers

Ternary Al x In 1 − x As Ga y In 1 − y As heterostructures with a lattice mismatch up to 4% are grown on GaAs by molecular beam epitaxy. Two buffer layer concepts to compensate the lattice misfit between the Al x In 1 − x As Ga y In 1 − y As layers and the GaAs substrate using the quaternary Al x Ga y In 1 − y As in a linear graded and two-step graded fashion, respectively, are presented. The Al and Ga content of the ternary layers were chosen to be x = 0.48 and y = 0.47, respectively, in order to obtain the same heterostructures identical to those grown lattice matched on InP as a reference. The surface morphology and the transport properties of Al 0.48 In 0.52 As Ga 0.47 In 0.53 As high-electron mobility transistor structures were studied by atomic force microscopy and Hall measurements, respectively. Optical properties were investigated by low-temperature photoluminescence on quantum well structures. The use of the two step graded buffer layers resulted in three-dimensional layer growth and inferior layer quality. In contrast, the linear graded buffer approach was found to result in superior heterostructure properties due to the two-dimensional growth mode during the whole growth process resulting in the typical cross-hatched surface morphology.

Strain field imaging on Si/SiGe(001)-(2×1) surfaces by low-energy electron microscopy and scanning tunneling microscopy

We show that ultrahigh-vacuum low-energy electron microscopy and scanning tunneling microscopy can be used to image residual uniaxial strain fields on (001) surfaces of SiGe heterostructures. We find that the surface crosshatch morphology on these films is highly correlated with large spatial variations in the residual uniaxial strain fields, confirming the importance of local strain fields in the formation of crosshatch.

Partial characterization and cloning of the genome of PvSNPV (= BP-type virus) pathogenic for Penaeus vannamei

DAO Diseases of Aquatic Organisms Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials DAO 23:59-66 (1995) - doi:10.3354/dao023059 Partial characterization and cloning of the genome of PvSNPV (= BP-type virus) pathogenic for Penaeus vannamei Bonami JR, Bruce LD, Poulos BT, Mari J, Lightner DV PvSNPV isolated from the shrimp Penaeus vannamei was characterized after purification. In negative staining, the rod-shaped, enveloped particles were 312 to 320 nm in length and 75 to 87 nm in diameter. The nucleocapsids, 306 to 312 nm in length and 62 to 68 nm in diameter, showed a cross-hatched surface arranged in a helical pattern and a trilaminar structure capping both extremities. Purified occlusion body subunits (polyhedrin subunits) were 17 to 19 nm in diameter and, in SDS-PAGE, demonstrated 1 major polypeptide of 52 kDa. BamHI digested viral genome revealed at least 7 different fragments. Cloning of these DNA fragments, and the resulting study of them, revealed the presence of 2 more fragments which had co-migrated in gel electrophoresis. The cloned fragments, which represent about 40% of the estimated size of the genome, were characterized by their size and by the position of restriction sites. When the cloned fragments were labeled and used as probes, no homology was found among the different inserts. These gene probes reacted with different BP-type strains, but not with uninfected shrimp tissue. The gene probes also did not react with shrimp tissues infected with other shrimp viruses, indicating their specificity for the BP-type viruses. BP . PvSNPV . Penaeid shrimp . Genome cloning . Gene probes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in DAO Vol. 23, No. 1. Publication date: September 14, 1995 Print ISSN:0177-5103; Online ISSN:1616-1580 Copyright © 1995 Inter-Research.

Surface ripples, crosshatch pattern, and dislocation formation: Cooperating mechanisms in lattice mismatch relaxation

We study the interplay of elastic and plastic strain relaxation of SiGe/Si(001). We show that the formation of crosshatch patterns is the result of a strain relaxation process that essentially consists of four subsequent stages: (i) elastic strain relaxation by surface ripple formation; (ii) nucleation of dislocations at the rim of the substrate followed by dislocation glide and deposition of a misfit dislocation at the interface; (iii) a locally enhanced growth rate at the strain relaxed surface above the misfit dislocations that results in ridge formation. These ridges then form a crosshatch pattern that relax strain elastically. (iv) Preferred nucleation and multiplication of dislocations in the troughs of the crosshatch pattern due to strain concentration. The preferred formation of dislocations again results in locally enhanced growth rates in the trough and thus leads to smoothing of the growth surface.

Gas Source Molecular Beam Epitaxy of ZnSe on (In,Ga)P

ABSTRACTThe wide bandgap semiconductor ZnSe has been nucleated on epitaxial (In,Ga)P buffer layers (on GaAs substrates) having various In compositions, and hence various lattice constants. The III-V ternary alloy offers a wide range of lattice constants for the heteroepitaxy of a multitude of potential II-VI light emitting devices, such as blue pn injection lasers composed of the (Zn,Mg)(S,Se) material system. Since the II-VI and III-V layers are grown using gas source molecular beam epitaxy in separate dedicated reactors, the technique of amorphous As deposition is employed to passivate the (In,Ga)P surface prior to the ex situ transfer. High resolution double crystal x-ray diffraction measurements on the ZnSe/(In,Ga)P/GaAs heterostructures indicate that for In compositions of 50-52%, the buffer layers with a thickness of 4 μm were only partially relaxed on the GaAs substrates, with the residual mismatch remaining at the ZnSe/III-V heterointerface. The critical thickness of (In,Ga)P, with In concentrations near 52-56%, on GaAs greatly exceeds the predicted critical thickness from either the energy balancing or force balancing model. For an In composition of 56% (and a film thickness of 4 μm), the buffer layers contain an in-plane lattice constant equal to that of ZnSe, and therefore represent the lattice-matched condition, even though the film is not fully relaxed. For (In,Ga)P buffer layers lattice-matched to ZnSe, but mismatched to GaAs, the surface exhibits the expected cross-hatched surface morphology. The occurrence of the cross-hatched surface is significantly alleviated by the addition of a pseudomorphic layer of GaAs positioned between the ZnSe and (In,Ga)P layer.

Roughness Analysis of Si1-χGeχ Films

The morphology of Si 1-χ Ge χ films is studied using atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Depending on growth temperature and Ge content, growth proceeds in either a layer-by-layer mode or by me growth of 3-dimensional coherent islands. In the former case strain relaxation occurs by formation and multiplication of misfit dislo-cations, leading to a cross-hatched surface morphology. In the latter case the rough morphology leads to a higher density of nucleation sites, and reduced glide efficiency, for the dislocations. A Fourier transform based method for analyzing the morphology is introduced, and is compared with other, existing roughness analysis methods.

Cross-hatched surface morphology in InGaAs/GaAs superlattices

Cross-hatched surface morphology in InGaAs/GaAs superlattices

Characteristics of heavily carbon-doped GaAs by LPMOCVD and critical layer thickness

The carbon-doping characteristics of GaAs epilayers have been investigated by varying the growth parameters - V/III ratio and growth temperature. We have obtained the highest 1.4 × 10 20 cm -3 hole concentration with hole mobility 42 cm 2/V·s without using intentional doping source gases. The carbon incorporation increased with increasing growth temperature up to the critical temperature and above this temperature the carbon incorporation decreased. This critical temperature depends on input TMG flow rate. The peak energy of the PL intensity distribution was shifted to lower energy (red shift) and the full width at half maximum (FWHM) increased with increasing hole concentration. The (400) diffraction peak of the carbon-doped GaAs epilayer was split from the substrate GaAs peak. The split angle was 191.25 arc for the carbon-doped GaAs with 9 × 10 19 cm -3 hole concentration. We also have discussed the relationship between carbon-doping concentration and critical layer thickness ( L C ) by an excess stress model. The excess stress at which partial strain relief became observable was σ exc / μ = 0.0021. Thus we identified this excess stress as the critical excess stress for carbon-doped GaAs epilayers. If the excess stress was greater than this, the epilayers showed the surface cross-hatching.

Relaxed GexSi1−x structures for III–V integration with Si and high mobility two-dimensional electron gases in Si

To obtain a large lattice constant on Si, we have grown compositionally graded GexSi1−x on Si. These buffer layers have been characterized with electron-beam-induced current, transmission electron microscopy, scanning electron microscopy, x-ray diffraction, and photoluminescence to determine the extent of relaxation, the threading dislocation density, the surface morphology, and the optical properties. We have observed that it is possible to obtain completely relaxed GexSi1−x layers with 0.1<x<1, threading dislocation densities of 105–5 × 106 cm−2, and with bulk GexSi1−x optical properties. Calculations show that gradually graded layers grown at relatively high temperatures can remain in equilibrium throughout growth, thereby avoiding strain buildup and the introduction of more threading dislocations through dislocation nucleation. It is also shown that the degree of surface crosshatch is related to inhomogeneous strain fields in the epilayer and to the thickness at which dislocations are introduced. These relaxed buffers have been used for the fabrication of very high mobility two-dimensional electron gases in Si and also as templates for lattice-matched III–V growth. We have obtained electron gases in Si with mobilities as high as 170 000 cm2/V s and red-emitting InGaP light emitting diodes on Si with less than 20 μA reverse current at −6 V bias.

Crosshatched surface morphology in strained III-V semiconductor films

The correlation between the surface crosshatched morphology and the interfacial misfit dislocations in strained III-V semiconductor heteroepitaxy has been studied. The surface pattern is clearly seen on samples grown at high temperature (520 °C) and those with small lattice-mismatched (f<2%) systems. A poorly defined crosshatched morphology was found on layers grown at relatively low temperature (400 °C). As the lattice mismatch of the strained layer becomes larger than 2%, a roughly textured surface morphology is commonly observed in place of actual cross-hatching. Few threading dislocations are observed in the strained layer when the crosshatched pattern develops. It is also noted that the surface crosshatched pattern develops after the majority of the interfacial misfit dislocations are generated. The result suggests that the surface crosshatch pattern is directly related to the generation of interfacial misfit dislocations through glide processes.

Surface Cross-Hatched Morphology on Strained III-V Semiconductor Heterostructures

AbstractThe correlation between surface cross-hatched morphology and interfacial misfit dislocations in strained III-V semiconductor heteroepitaxy has been studied. The surface pattern is clearly seen on samples grown at high temperature (520°C) and with lattice mismatch f < 2%. A poorly defined cross-hatched morphology is found on layers grown at low temperature (400°C). For f > 2%, a rough textured surface morphology is observed in place of cross hatching. Few threading dislocations are observed in the strained layer when cross hatch develops. Cross hatch occurs after most interfacial misfit dislocations are generated. The results suggest that surface cross hatch is directly related to the generation and glide of interfacial misfit dislocations.

Pseudomorphic GaAs/InGaAs single quantum wells by atmospheric pressure organometallic chemical vapor deposition

High-quality pseudomorphic GaAs/In0.12Ga0.88As single quantum wells (QW’s) were prepared by atmospheric-pressure organometallic chemical vapor deposition. Photoluminesence spectra measured at 2.5 and 78 K exhibit intense, sharp peaks [full width at half-maximum (FWHM)=2.6 meV for a 17-Å well at 78 K] from the quantized energy transitions of the QW’s. Peak positions agree well with a square well calculation that includes the strain-induced band-gap shift in the In0.12Ga0.88As. Quite unlike previous work with QW’s in which the FWHM was found to exponentially increase with decreasing well width, we observed a narrowing of the QW signals as the well width went below ∼30 Å. In larger well samples (300 Å), the onset of surface crosshatch patterns was observed, which is expected from critical thickness theory.

Vibratory modal structure of a cantilevered elliptic plate

The measured amplitude and phase of the acceleration of the surface of a cantilevered elliptic plate are shown using an interative computer graphics environment. The data are shown as a cross-hatched surface, which slowly oscillates with the measured amplitude and phase, as it is rotated, providing insight into the nature of the mode shapes which cannot be practically obtained by other methods. A short color film will be used to demonstrate the effectiveness of this technique.