We review recent Z-contrast imaging studies of Si-Ge ultrathin superlattices, alloys, and buried Ge layers. It is found that whenever Si is deposited onto a Ge (2 × 1) surface, Ge is pumped into the growing Si layer, and this is accompanied by interfacial ordering. This is explained by a novel Ge atom pump mechanism which occurs during MBE growth. Codeposition and alloy growth results in long range (111) ordering as a consequence of lateral segregation during nonequilibrium growth.

P-I-N diodes whose intrinsic region consists of strained layer superlattices (SLS), separated by 40 nm Si spacers, have been grown by MBE on Si substrates with <100>, <110>, and <111> orientations. These structures have been characterized by x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). The dual periodicities in these structures produced unique XRD effects and the quality was highly dependent on substrate orientation. The <100> sample was in general free of defects, whereas the <110> and <111> specimens contained significant numbers of twins and dislocations.

We compare and contrast GexSi1−x alloys grown on Si (100), (110) and (111) surfaces. The geometry of interfacial misfit dislocations are observed to be different on these three surfaces, as the intersections of available ♣111} glide planes are of different symmetries for the different interfaces. In addition, angular factors resolving the applied and line tension stresses onto misfit dislocations vary over the different surfaces, producing different effective stresses for identical layer thicknesses, compositions and microstructures. Finally, markedly different dislocation microstructures are observed on the different surfaces, as geometrical considerations show that partial dislocations may separately propagate on the (110) and (111) surfaces, in contrast to the (100) surface.

Interface mixing between the Ge and Si layers in symmetrically strained SimGem superlattices occurs during post growth thermal anneals. Interdiffusion coefficients were obtained from intensity changes in the low angle superlattice x-ray satellites on samples with nominal periodicities between 1.4nm and 5.6nm. A common activation energy of 3.0±0.1 eV was found. The bulk interdiffusion coefficients for SimGem were derived since measurements were made on samples with different layer thicknesses. Intermixing appears to occur by diffusion of Si atoms into the Ge layers via a vacancy mechanism. Raman scattering measurements support this process as well as the formation of Si1−xGex, alloy layers during the anneals.

High resolution and analytical transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize short-period strained-layer Sim-Gen superlattices ( m monolayers Si, n monolayers Ge, total number of periods N≤ 145, total thickness ≃ 200 nm). The superlattices were grown by low-temperature molecular beam epitaxy (T = 300–400°C) on different SiGe alloy buffer layers on Si (100)substrates. The combination of these two methods shows that detailed informations can be obtained about superlattice periodicity, interface roughness, strain, and average composition.

Strain relaxation of GexSi1−x layers is studied as a function of growth temperature. Extremely thick coherently strained layers whose thicknesses exceed more than fifty times of the critical thicknesses predicted by Matthews and Blakeslee's model were successfully grown by MBE. There exits a narrow temperature window from 310 °C to 350 °C for growing this kind of high quality thick strained layers. Below this temperature window, the layers are poor in quality as indicated from RHEED patterns. Above this window, the strain of the layers relaxes very fast accompanied with a high density of misfit dislocations as the growth temperature increases. Moreover, for samples grown in this temperature window, the strain relaxation shows a dependence of the residual gas pressure, which has never been reported before.

The valence band discontinuity ΔEV in the coherently strained GexSi1−x/Si heterostruc-ture is determined using I-V-T measurement. The electrical measurements of the band discontinuity of the pseudomorphic layers are difficult due to the thin layer imposed by the strain. Recently, low temperature growth of thick layer (>100 nm) of coherently strained GexSi1−x on Si has been achieved and thus made it possible for an accurate electrical measurement of band offset. The results obtained are in good agreement with the theoretical calculations by pseudopotential method.

A series of relaxed Si1−xGex alloy layers with germanium contents up to 70% has been deposited on silicon. Although direct deposition ot these highly mismatched layers on silicon gave dislocation densities of 109-1010cm2 and poor morphology, it was found that the use of a linear grade enabled completely relaxed Si.3Ge.7 layers with defect densities of ∼3.105cm−2 to be obtained. However, if the grading was too rapid the dislocation density was much higher. The role of dislocation nucleation and propagation in determining the required thickness of graded layer is discussed.

We have studied interfacial mixing of Si/Ge strained-layer superlattices during Si molecular beam epitaxy. The mixing has been shown to be primarily due to the surface segregation of Ge atoms during Si overlayer growth. It has been found that only the Ge atoms on the topmost Ge layer dominantly segregate to the growing surface. It has also been found that the surface segregation of Ge is effectively suppressed by depositing Sb atoms on the Ge layers. It has been demonstrated that Si/Ge superlattices with abrupt Si/Ge interfaces can be grown by depositing Sb. The two state exchange model is used to discuss the surface segregation of Ge and the suppression of the segregation by Sb deposition.

Germanium incorporated oxide films formed by wet oxidation of SiGe substrates have been studied using Electron Paramagnetic Resonance. SiGe layers 400 nm thick were prepared by molecular beam epitaxy and oxidized in a steam ambient at 900° C. After a 10 Mrad x-ray irradiation, an oxide defect with a zero crossing at g=1.995 is observed. Comparison of the spectrum with that obtained from Ge-doped silica suggests that the defect observed in the thin film is an oxygen vacancy related defect, a Ge E' center.

Misfit dislocation glide velocities in Si.94Ge.06 layers doped to 2.1018cm−2 with boron were measured over the temperature range 500 – 600°C and found to be up to two times slower than that of undoped material. Arsenic doped Si.932Ge.0ee layers on silicon were also studied, and the glide velocity was found to be enhanced by a factor of 80 at the lowest temperature (500°C) and highest doping level (9.1018cm−2) consistent with a decrease in the activation energy for glide from approximately 2eV to 1.3eV as the doping level was increased.

We have grown linearly compositionally graded GexSi1−x structures at high temperatures (700–900°C) on Si substrates to form a surface which resembles a GexSi1−x substrate. We have obtained completely relaxed structures with x≤0.50 and threading dislocation densities in the 105cm−2 - 106cm−2 range. Because of the very low threading dislocation densities, the structures appear dislocation free in conventional transmission electron microscopy (TEM) cross-section and plan view. Employing the electron beam induced current technique (EBIC), we were able to consistently measure these low threading dislocation densities. A direct comparison of two x=0.35 films, one graded in Ge content and one uniform in Ge content, shows that compositional grading decreases the dislocation density by a factor of 100–1000. These. higher quality graded buffers have been used as templates for the subsequent growth of InGaP light emitting diodes (LED) and GexSi1−x/Si two-dimensional electron gas (2DEG) structures. Room temperature operation of orange-red LEDs were obtained at current densities of =600A/cm, and mobilities as high as 96,000 cm2/V-s were achieved at 4.2K in the 2DEG structures.

Kinetics of Ge segregation during molecular beam epitaxial growth is described. It is shown that the Ge segregation is self-limited in Si epitaxial overlayers due to a high concentration effect when the Ge concentration exceeds 0.01 monolayer (ML). As a result, segregation profiles of Ge are found to decay non-exponentially in the growth direction. This unusual Ge segregation was found to be suppressed with an adlayer of strong segregant, Sb, during the kinetic MBE growth. We develop a novel scheme to realize sharp Si/Ge interfaces with strong segregante. Lower limit of the effective amount of Sb for this was found to be 0.75 ML.

A candidate GeSi buffer layer structure suitable for strain-symmetrised GeSi/Si superlattice overgrowth is grown by molecular-beam epitaxy and its strain state characterised using microdiffraction in a dedicated scanning transmission electron microscope. The structure consists of five alloy layers of increasing Ge concentration grown on a Si (100) substrate, with all but the last annealed at high temperature. Analysis of higher order Laue zone deficit lines in the microdiffraction patterns acquired from each layer indicate that relaxation is complete in all the layers. Images of the structure in the transmission electron microscope show good crystallinity of the final layer with low concentrations of threading dislocations.

X-ray diffraction is applied for the assessment of structural data such as lattice mismatch and layer thickness of MBE grown Si/Si1−xGex heterobipolar transistor and double-barrier resonant-tunneling structures. Rocking curves from the former structure show distinct features which are obviously correlated to the individual layer parameters. The diffraction profile of the resonant tunneling structure is not only more complicated because of the larger number of parameters but also due to the strong interference effects resulting from the layer set-up. For a determination of the structural parameters a comparison of the experimental diffraction pattern with simulated rocking curves is performed.

The glancing angle x-ray diffraction spectra of GexSi1−x/Si superlattices grown by MBE under different temperatures are investigated. Three different types of the intensity distributions of diffraction peaks are observed, which are believed to be corresponded with different situations of interfaces. If all the interfaces in superlattice structure are highly flat, up to 17 orders diffraction peaks are identified with their intensities modulated by a periodical envelope function, otherwise the distribution of diffraction intensities follows simply a decaying function. A quantitative analysis using computer simulation based on the modified Bragg's law and the optical multilayer theory is used to derive the structural parameters including the thicknesses of Si and GexSi1−x layers, superlattice period, Ge content x and the degree of interfacial roughness.

For obtaining good structural perfection, the molecular beam epitaxial (MBE) growth of GexSi1−x on Si substrate should not only be kept in the pseudomorphic form but also in layer-by-layer growth stage. We found that the two dimensional layer-by-layer growth of GexSi1−x on Si could persist to a certain deposition thickness, beyond that the transition to islanding growth occurs. The transition thickness is significantly dependent on the growth temperature and germanium content, and is always smaller than the critical thickness of pseudomorphic growth. In order to obtain good crystalline quality in growing GexSi1−x superlattices on Si substrates, the thickness of GexSi1−x layers should be controlled below the transition thickness and lower growth temperature is favorable.

Si1−xGex layers containing 2×1020 oxygen atoms/cm3 exhibit an enhancement in thermal stability when compared to similar films (comparable Ge content and thickness) with 2 orders of magnitude less oxygen. X-ray measurements of the lattice constants in the strained films indicate that the oxygen does not substantially change the amount of strain in the layers. A prediction of the effect of oxygen based on solid solution strengthening theory is shown to be consistent with experimental annealing results. In addition, experimental measurements of slower misfit dislocation velocities in the layers with high oxygen content compared to those measured in films with low oxygen content, support the idea of solid solution strengthening. It is therefore likely that oxygen impedes the kinetics of dislocation formation.

We illustrate the usefulness of the Extended X-ray Absorption Fine Structure (EXAFS) technique to determine the amount of interface mixing and strain condition in the study of (SimGen)p short-period superlattices. It is found that for n < 4, the number of Ge and Si nearest neighbours to Ge atoms is consistent with ∼25% interfacial mixing and that the Ge-Ge bond length corresponds to that of coherently strained Ge. The Si-Ge bond length is shorter, close to that of a strained Si0.25Ge0.75 alloy. For n > 4, the Ge-Ge bond length and the number of Si-Ge nearest neighbours increase significantly consistent with partial relaxation and interdiffusion. Raman scattering spectroscopy and x-ray reflectometry measurements are also presented and are consistent with the conclusions of the EXAFS analysis.

The thermal oxidation of epitaxial Ge.36Si.64 on (100)Si is investigated experimentally for a wet ambient at 700°C and 1000°C. A pure silicon dioxide layer with pile-up of Ge behind the oxide is formed at 1000°C. At 700°C, however, both Ge and Si are oxidized. The Ge is included uniformly into the oxide layer without changing the initial Ge/Si ratio. The result at 1000°C follows the thermodynamical picture which predicts the same result at 700°C also, contrary to observation. The different result at 700°C is due to kinetic constraints which can be explained by different activation energies for the rate of the oxidation reaction and for the Ge or Si diffusivities in GeSi.