Si Strained-layer Superlattices Research Articles

Asymmetric Mosaic Spread During Relaxation in SiGe/ Si Strained Layer Superlattices Grown on Miscut Substrates

ABSTRACTThe evolution of defects in SiGe/Si strained layer superlattices (SLS)-with thickness and composition near the critical thickness -was investigated. The structures were grown on 2° miscut (001) substrates by ultrahigh vacuum chemical vapor deposition. The samples were then annealed between 700 °C and 900 °C. After annealing, the satellite peak intensity from double axis diffraction decreased and triple axis diffraction showed that this decreased intensity was due to increased mosaic structure. Interestingly, for some of the annealed samples, the (004) reciprocal space maps showed an asymmetric mosaic spread, indicating a preferential tilt. This result stems from a preferential propagation of certain types of misfit dislocations due to the substrate miscut.

Effects of elastic relaxation on large-angle convergent-beam electron diffraction from cross-sectional specimens of GexSi1−x/Si strained-layer superlattices

Abstract A large-angle convergent-beam electron diffraction pattern with fine diffraction lines from a cross-sectional specimen of Gexsi1−x/Si strained-layer superlattices can give much information on local strain and misfit stress relaxation. The diffraction lines in the GeSi layers are shifted from those in the Si layers by the misfit strain and the strain relaxation. In this paper we shall demonstrate that the effects of elastic relaxation and the residual strains can be separated.

Quantum confinement effects of Si/SiGe strained-layer superlattices grown by an ultrahigh vacuum/chemical vapor deposition technique

Well-resolved band-edge luminescence is observed for Si0.86Ge0.14/Si strained-layer superlattices grown by an ultrahigh vacuum/chemical vapour deposition technique at 550 ‡C. High-resolution double-crystal X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) were used to determine the strain and other parameters for these strained-layer superlattices. Quantum confinement is observed for a SiGe well as thin as 1.3 nm. The blue shift of the emission peaks with decreasing well width is found to be in good agreement with theoretical calculation.

Heavy ion RBS analysis of strained layer superlattices

The extension of Rutherford backscattering spectrometry (RBS) to heavier mass projectiles has several advantages over conventional RBS. To understand the interaction of heavy ions with solids properly, a systematic study of the energy straggling of MeV heavy ions has led to an empirical formula for this term. This formula, along with the stopping power, lateral spread and multiple scattering, predicts the energy width for a wide range of energies, projectiles and targets, allowing the development of realistic computer simulation for the energy spectra. This model has been verified by measurements of the depth resolution of 4 and 6 MeV C projectiles using Si/Si 1− x Ge x /Si strained layer superlattices (SLSs) samples.

Ellipsometric study of Si0.5Ge0.5/Si strained-layer superlattices

We present an ellipsometric study of two Si0.5Ge0.5/Si strained-layer superlattices grown by MBE at low temperature (500 °C), and compare our results with x-ray diffraction (XRD) estimates. Excellent agreement is obtained between target values, XRD, and ellipsometry when one of two available SixGe1−x databases is used. We show that ellipsometry can be used to nondestructively determine the number of superlattice periods, layer thicknesses, SixGe1−x composition, and oxide thickness without resorting to additional sources of information. We also note that we do not observe any strain effect on the E1 critical point.

Kinematical study of GexSi1 – x/Si strained-layer superlattice by a double crystal X-ray diffraction method

Abstract Two samples of nominal 20-period Ge0.20Si0.80(5nm)/Si(25nm) and Ge0.5Si0-5(5nm)/Si(25nm) strained-layer superlattices (SLSs) were studied by the double-crystal X-ray diffraction method. It is convenient to define the perpendicular strains relative to the average crystal. Computer simulations of the rocking curves were performed using a kinematical step model. An excellent agreement between the measured and simulated satellite patterns is achieved. The dependence of the sensitivity of the rocking curves to the structural parameters of the SLS, such as the alloying concentration x and the layer thicknesses and the L component of the reflection g = (HKL), are clearly demonstrated.

X-ray and transmission electron microscopy analysis of imperfect Ge x Si1−x /Si strained-layer superlattices

X-ray and transmission electron microscopy analysis of imperfect Ge x Si1−x /Si strained-layer superlattices

Measurement of elastic relaxation in cross-sectional transmission electron microscopy of GexSi1−x/Si strained-layer superlattices

Two types of relaxation occur in the cross-sectional transmission electron microscopy samples of the GexSi1−x/Si strained-layer superlattices (SLS) by large-angle convergent-beam electron diffraction (LACBED) and imaging technique which gives a good LACBED pattern superimposed on a high spatial resolution shadow image of the SLS. One type of relaxation occurs between the Si and the GeSi layers. It is negligible in the convergent-beam electron diffraction (CBED) case for the larger value of the ratio of the sample thickness to the SLS wavelength. Another type occurs between the superlattice as an average crystal and the Si substrate. The relaxation of the SLS can be measured by the shift of the Kikuchi line in the SLS from that in the Si substrate.

High-resolution shadow image superimposed on LACBED patterns: a method demonstrated on Ge xSi 1− x/Si superlattice

Large-angle convergent-beam electron diffraction patterns superimposed on a high-spatial-resolution shadow image of a cross-sectional specimen of the Ge x Si 1− x /Si strained-layer superlattice has been obtained by using an electron beam with a rather small spot size in a Philips EM420 electron microscope working at 100 kV and room temperature. As an example, such a CBED shadow image can provide information about the changes in the lattice parameters, localized variations in strain, and the surface relaxation effect.

GeSi [sbnd]Si strained layer superlattices grown by low-pressure CVD

SiGe Si and Si Ge strained-layer superlattices were grown by a low-pressure CVD system (LP-CVD) using Si 2H 6 (or Si 3H 8) and GeH 4. Their characteristics were studied using TEM observations, X-ray diffraction, AES, RBS, and Raman scattering measurements. It was found that for producing SiGe Si SLS's, the superlattice periodicity could be controlled in the range of several to several hundredångströms. It is concluded that interfaces between the SiGe and Si layers are very sharp. The thickness of the interface region, where the Ge content changes, is less than 10Å. On the other hand, for producing Si Ge SLS's, intermixing of Si and Ge atoms was an inevitable problem. It was, however, found that decreasing the growth temperature reduces effectively the intermixing.

Electroluminescence and photoluminescence from Si1−xGex alloys grown on (100) silicon by molecular beam epitaxy

Abstract Photoluminescence has been observed from Si 1− x Ge x alloy layers, superlattices and non-periodic multilayers where x was varied from 0 to 0.6 A p-type Si 0.82 Ge 0.18 alloy layer 200 nm thick, grown by molecular beam epitaxy (MBE), has been fabricated into a mesa diode which operated as a light-emitting diode, emitting at 1.4 μm at temperatures up to 80 K. This diode was selected from a series of heterostructures which exhibited intense photoluminescence with internal quantum efficiencies in the range 1%–10% at low temperatures. Photoluminescence in the wavelength range 1.2–1.7 μm has been observed from thick (100–200 nm) Si 1− x Ge x alloys and Si 1− x −Si strained layer superlattices with a range of dimensions which was large compared with the unit cell; i.e. where Brillouin zone folding effects were negligible. The intense Si 1− x Ge x alloy photoluminescence peak had a halfwidth of about 80 meV and the peak energy was found to shift consistently and predictably with the germanium fraction. Photoluminescence peak energies at 4.2 K varied from 620 to 990 meV for germanium fractions where 0.53 > x > 0.06. The photoluminescence and electroluminescence peaks were consistently about 120 meV below the established bandgap for tetragonally strained Si 1− x Ge x . In general, Si 1− x Ge x strained layers grown at low temperatures typical of MBE (below 500°C) exhibited low photoluminescence intensity. However, post-growth annealing in the 500–700°C temperature range enhanced luminescence efficiency by up to two orders of magnitude. A comparison is also made of the broad intense photoluminescence observed from MBE material with the weak near-band-edge spectra obtained from Si 1− x Ge x -Si grown by low temperature chemical vapour deposition.

Photoluminescence and electroreflectance of GeSi strained layer superlattices

The photoluminescence and electroreflectance of GeSi strained layer superlattices were investigated in detail for several superlattice structures. It was found that superlattice samples of Ge n Si 3 n ( n = 4, 5 and 6) showed strong signals typical of semiconductors with a direct bandgap. Pseudopotential band calculations taking account of the perpendicular lattice strain dependence suggested the possibility of a direct bandgap even for GeSi superlattices on Si(001). The experimental results and theoretical considerations together demonstrated the possibility of a direct bandgap in the Ge n Si 3 n samples.

Plan-view diffraction contrast imaging of surface-relaxation effects from strained-layer superlattices

Two-beam electron diffraction contrast imaging techniques have been used to study surface-stress-relaxation effects from Ge x Si 1- x /Si strained-layer superlattices (SLS) imaged in plan-view. Strong, “anomalous” black/white strain-field contrast is observed due to lattice plane bending where coherently strained layers emerge at the free surface of a back-thinned foil. In addition, differential thinning-induced steps can give rise to superimposed fringe-contrast effects in very thin foil regions; the latter effect is shown to make a much smaller contribution to the total diffraction contrast. The results presented here indicate the importance of understanding stress-relaxation effects in plan-view electron diffraction where strained layers emerge at a free surface.

X-ray studies of heat treated SiGe/Si strained-layer superlattices

An x-ray study of annealing effects in Si0.65 Ge0.35 /Si strained-layer superlattices grown by molecular-beam epitaxy (MBE) at 500 °C is reported. The samples were subjected to thermal treatments between 600 and 698 °C, and the superlattice peaks and Si(004) peak were monitored at both low and high angles using powder and double crystal diffractometries. The interdiffusion coefficient of the average composition, obtained from the decay in intensity of the first superlattice peak about (000), was found to be similar to the results obtained from satellites about (004). This leads to the important conclusion that the diffusion coefficient can be obtained using the high-angle technique, which is generally faster and less susceptible to experimental errors. The comparison between the two groups of measurements further emphasizes the fact that the critical slopes of the intensity plots should be excluded from the calculation of the diffusion coefficient.

Heterogeneous nucleation sources in molecular beam epitaxy-grown Ge xSi 1− x/Si strained layer superlattices

In the past few years, a number of studies have indicated the importance of heterogeneous sources in understanding defect nucleation mechanisms. Nonetheless, the majority of critical thickness research is still based on homogeneous misfit dislocation nucleation theory to predict the onset of plastic relaxation at strained layer interfaces. It will be shown that heterogeneous sources are plentiful even in molecular beam epitaxy (MBE)-grown material wherein the generation of defects depends on the oxygen/carbon contamination levels. Several metastable Ge x Si 1− x /Si strained layer superlattices have been grown under slightly different conditions in order to vary the concentration of oxygen/carbon at the substrate-buffer interface. Subsequent annealing treatments were carried out in order to accentuate any characteristic differences in the plastic relaxation behaviour of these metastable structures. Chemical defect etching, plan-view and cross-sectional transmission electron microscopy have been used extensively to characterize misfit dislocation nucleation behaviour at the strained layer interfaces based on differences in the number and type of heterogeneous sources.

Optical properties of strained GeSi superlattices grown on (001)Ge

Modulation spectroscopy has been used to study optical transitions in GeSi strained layer superlattices with unit cell periodicity of ten atomic monolayers in the growth direction. The samples in this report were grown on (001)Ge producing a 4.5% compressive strain along the superlattice growth direction. Spectra taken at 40 K show additional sharp structures near 0.94 eV comparable in magnitude with that measured for bona fide direct optical transitions in bulk germanium. These results are discussed in terms of structurally induced optical transitions in GeSi-ordered superlattices and are compared with recent theoretical and experimental results on similar structures.

Raman spectra of c-Si1-xGex alloys.

We observe several weak features between 420 and 470 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in addition to the normally observed Si-Si (\ensuremath{\sim}500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$), Si-Ge (\ensuremath{\sim}400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$), and Ge-Ge (\ensuremath{\sim}300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) optic modes in the Raman spectra of ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ (0.28\ensuremath{\le}x\ensuremath{\le}0.77) single crystal layers grown by liquid-phase epitaxy (LPE). The quasiequilibrium LPE-growth process rules out the type of long-range ordering recently observed in ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$/Si strained-layer superlattices as the origin for these peaks. Calculations of the first-order Raman spectra of random 216-atom c-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ alloys reproduce these weak features, which proves that they are not due to second-order Raman processes. Normal-mode analysis shows that they are due to localized Si-Si motion in the neighborhood of one or more Ge atoms. Implications for strain-induced long-range ordering in c-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ alloys are discussed.

The Kinetics of Si1-xGex/Si Relaxation using Large Area Dislocation Imaging Techniques

AbstractAnnealing experiments in the 450-1000 °C temperature range on various MBE grown Si1-xGex/(100)Si heterostructures have revealed an abrupt thermally activated transition from coherently strained to partially relaxed microstructures. Misfit dislocations at densities 1 to 104 cm-2 have been monitored using Nomarski interference microscopy of defect etched surfaces and charge collection microscopy (EBIC) of Si1-xGex/Si heterojunctions in the scanning electron microscope. The kinetics of misfit relaxation by dislocation glide can be characterized by a relaxation temperature which for 30 minute furnace anneals was ~500 °C for an uncapped Si0.83Ge0.17 stramed layer, and ~600 °C for a similar buried strained layer typical of heterojunction bipolar transistor geometries. Rapid thermal anneals (5s) delayed the onset of relaxation to higher temperatures (~ 200 °C increase) and allowed misfit dislocation velocities to be determined for the temperature range 500-900 °C. The activation energy for dislocation glide was found to be 2.2±0.1eV for a Si0.9Ge0.1/Si strained layer superlattice, 1.6±0.2eV for a Si capped Si0.88Ge0.12 layer and 1.6±0.2eV for a Si0.83Ge0.17 uncapped strained layer.

Kinematical simulation of high-resolution x-ray diffraction curves of GexSi1−x/Si strained-layer superlattices: A structural assessment

High-resolution x-ray diffraction (HRXRD) measurements of GexSi1−x/Si strained-layer superlattices are carried out using a four-crystal monochromator. A wide asymmetric range of extremely sharp higher order x-ray satellite peaks is observed indicating a well-defined strained-layer superlattice with abrupt interfaces. This is further confirmed by cross-section transmission electron microscopy. Using a kinematical diffraction step model which assumes ideally sharp interfaces, the thickness, strain, and composition of the GexSi1−x well could be extracted. Excellent agreement between measured and simulated x-ray satellite patterns is achieved. These results show that HRXRD together with kinematical simulation provides a powerful tool to evaluate the structural perfection of GexSi1−x/Si strained-layer superlattices.

Dependence of critical thickness on growth temperature in GexSi1−x/Si superlattices

We present direct evidence for the dependence of critical thickness on growth temperature in a lattice-mismatched epitaxial system. Ge0.5Si0.5/Si strained-layer superlattices have been grown by molecular beam epitaxy on (100) Si substrates at temperatures between 330 and 530 °C. The extent to which lattice mismatch is accommodated by elastic strain has been determined through x-ray diffraction, channeled Rutherford backscattering spectroscopy, and transmission electron microscopy. Lattice mismatch is found to be accommodated purely elastically in a structure grown at 365 °C. Samples grown at higher temperatures are seen to display increasingly high densities of misfit-accommodating dislocations. This growth-temperature dependence may account for apparent inconsistencies in critical thickness data reported in the literature. Our results clearly demonstrate the need to account adequately for the kinetics of defect formation in the prediction of critical thicknesses.