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Silicon-based heterostructures have come a long way from the discovery of strain as a new and essential parameter for band structure engineering to the present state of electron and hole mobilities, which surpass those achieved in the traditional Si/SiO 2 material combination by more than an order of magnitude and rapidly close up to the best III–V heteromaterials. Besides the inherent performance enhancement, undoubtedly the main attraction of high-mobility Si/SiGe and Si/SiC heterostructures is their basic compatibility with standard Si technologies. This unique combination of properties makes these materials the natural choice for future high performance VLSI circuits based on the contemporary MOSFET. It is the purpose of this contribution to critically review the most recent developments and the performance levels achieved to date in these material systems, to address problems related with device fabrication and process integration, and to discuss alternative routes that could circumvent the use of strain-adjusting epilayers, which are presently the bottleneck for an introduction of these promising materials into a production environment.

We studied the substitutional carbon incorporation in Si 1-y C y layers grown by molecular beam epitaxy (MBE) on Si(100). We grew a series of pseudomorphic Si 1-y C y /Si superlattices (SL) with carbon concentrations between 0 and 2% and growth temperatures varying between 400 and 650°C. From a comparison between measured and simulated X-ray rocking curves of our samples, we determined the substitutional carbon content as a function of the growth temperature. We observed a decrease of the substitutional carbon incorporation with increasing growth temperature and increasing carbon flux. Reflection high energy electron diffraction (RHEED) showed a 3D pattern of those layers, which had not incorporated all the offered C atoms substitutionally. This result is confirmed by atomic force microscopy investigations of Si 1-y C y epilayers grown at different temperatures. To check the crystal quality we performed photoluminescence (PL) investigations of a Si/Si 1-y C y SL and of a 1100 A Si 0.99 C 0.01 epilayer. Before and after annealing the epilayer was analyzed by PL and X-ray diffraction. We observed a blueshift of the NP and TO line, which is caused by a homogenization of potential fluctuations in the band edge energies.

In this work we observed strain relaxation at 550 °C following the interaction of palladium with strained Si 1− x Ge x epilayers. The germanium concentrations x in the Si 1− x Ge x films were 0.09 and 0.21 with thicknesses of 3500 A and 2300 A respectively, which are below the critical thickness h c . An overlayer of palladium, 1300 A thick, was deposited onto the Si 1− x Ge x epilayers, and the samples were annealed at 250 and 550 °C in a vacuum furnace for compound formation. Low temperature anneals (250 °C) were characterized by the formation of a uniform, highly textured ternary compound (Pd 2 Si 1− y Ge y with y x ) on top of the unreacted Si 1− x Ge x layer, and no substantial change in the state of strain was observed in the underlying unreacted Si 1− x Ge x layer. On the contrary, a higher temperature anneal (550 °C) of metallized samples was characterized by the formation of a double-layer structure, with a highly defected germanium-rich Si 1− z Ge z layer (0.37 z 1− x Ge x . In these samples a considerable decrease in the vertical lattice parameter and correspondingly strain relaxation in the unreacted Si 1− x Ge x layer was observed.

MBE-grown single quantum wells of pseudomorphic Si1 − xGex/Si(100), with x = 21%, 24% and 36% and well widths Lz from 1.2 nm to 250 nm were studied by photoluminescence using Fourier transform spectroscopy. Observations showed very low thermal deactivation energies of the luminescence, power-dependent luminescence intensities which vary with Lz in a characteristic way, and power-dependent luminescence energy shifts, again showing characteristic Lz dependence. Moreover, the TO-phonon sidebands were decomposed into their parent SiSi, SiGe and GeGe vibrations. These components exhibit relative strengths depending strongly on the optical power. All these data conspire to suggest Type II staggered band line-up in the Si1 − xGex/Si heterojunction system for x ⩽ 36%, whereas there is no easy explanation of the data by invoking a Type I band alignment.

The electroluminescence (EL) and photoluminescence (PL) of ultrametastable Si 1− x Ge x layers grown by molecular beam epitaxy (MBE) on Si(100) substrates has been studied. Two layers 150 and 175 nm thick (samples B2005 and C0997 respectively) were pseudomorphically grown on silicon in two different MBE chambers at 450 °C with x ≈ 0.25 and x = ≈ 0.23 respectively. At liquid-helium temperature both samples show luminescence spectra from the Si 1− x Ge x layers consisting of resolved band edge recombination and a broad low energy band peaking at about 810 meV. In one sample (B2005) the broad band is dominant; in the other sample (C0997) the band edge PL and EL signals are dominant. Variation of the exciting current density or laser power over a wide range shows a different power dependence for the low energy band, the Si 1− x Ge x band edge luminescence and the silicon substrate luminescence. In addition, we have studied the EL of fully strained Si 1− x Ge x single quantum wells. At constant excitation current density the EL signal increases with decreasing layer thickness, while the peak energy shifts to higher energy owing to the quantum size effect. For thin quantum wells, two lines at 969 and 897 meV are observed, known as the G line and the E line respectively, which are associated with C s SiC s defect.

The intersubband absorption in the conduction band of strain-symmetrized Si/Si 1− x Ge x multiple quantum wells has been measured. Modulation- and well-doped samples with different well widths and carrier concentrations are studied in a temperature range from 8 K to room temperature. In good agreement with self-consistent subband calculations the observed absorption lines are attributed to intersubband transitions from the ground to the first excited state or from the first excited to the second excited state.

Patterned multilayers are deposited by means of differential silicon MBE. Simultaneously monocrystalline islands are formed in a polycrystalline embedment. The structure is investigated with respect to the poly/mono-boundary and the polysilicon. The poly-embedment can work as an isolation or as an interconnection for a mono-island. Several devices have been realized, namely differential diodes, a differential three-dimensional IMPATT, and a differential resonant tunnel diode and these demonstrate the feasibility of the differential multilayer silicon MBE.

n-type modulation-doped Si/Si 0.5 Ge 0.5 strained layer multiple quantum well structures have been grown on (100)Si substrates by molecular beam epitaxy. The concept of strain symmetrization has been applied by inserting a strain-relaxed SiGe buffer layer. The samples have been investigated by Hall measurements supplemented by Shubnikov-de Haas and Rutherfored backscattering analysis. We present mobility results on the two-dimensional electron gas in the silicon wells as a function of electron concentration, quantum well width, and spacer thickness. The influence of the sample parameters on the electron mobility is discussed. With an electron sheet concentration of 2.4×10 12 cm −2 per well, a well width of 10 nm, and a spacer thickness of 10 nm a Hall mobility at room temperature of 1280 cm 2 (V s) -1 has been achieved.

Gallium doping of silicon molecular beam epitaxial layers is investigated in the temperature range between 550 and 300°C. Down to about 450°C, the incorporation behaviour can be described by a simple kinetic model. Deviations from an activated behaviour are indicative of kinetic limitations. At lower temperatures a complicated incorporation dynamics with two distinct regimes is observed. Doping by secondary implantation is investigated as a function of ion energy. A significant implantation yield is observed at low energies, while at higher energies sputter desorption of the gallium adlayer becomes dominant. Implications of the results for more efficient gallium doping techniques are discussed.

Electrical properties of gallium- and antimony-doped solid phase epitaxial (SPE) layers, which are fabricated by epitaxial growth of ultrahigh vacuum deposited silicon layers at 575 °C, are discussed. Though high crystalline quality of the layers can be demonstrated, electrical shortcomings are revealed: high gallium-doped SPE samples show a reduced mobility when compared with boron-doped bulk material. Antimony-doped layers are still in a metastable phase after crystallization; the low electrical activation of those layers can be improved by an additional post annealing process.

A review is given of the photoluminescence observed from silicon grown by molecular beam epitaxy (MBE), with particular emphasis on recent work on material grown at temperatures carried out by the authors. The areas addressed are distinguishing between luminescence originating in the epitaxial layer and in the substrate, complexes formed by the incorporation of carbon and nitrogen, the suppression of bound-exciton luminescence from electrically active impurities and the effects of gettering by dislocations, and dislocation-related luminescence and the effects of transition metal contamination. It is shown that the suppression of bound-exciton luminescence from electrically active impurities is not an inherent property of MBE silicon grown at low temperatures and must be due to the presence of contaminants. The nature of the contaminants responsible and the mechanism of luminescence quenching has not been identified. It is also shown that dislocation-related D-band luminescence is associated with dislocations decorated by transition metals as distinct from undecorated dislocations.

Si 1− x Ge x layers with overcritical thicknesses grown on (100 Si by molecular beam epitaxy (MBE) were characterized by Rutherford backscattering/channeling (RBS), transmission electron microscopu (TEM) and Nomarski differential interference contrast (NDIC) microscopy. The influence of growth temperature and subsequent furnace annealing or rapid thermal annealing (RTA) on crystal quality and surface morphology was investigated. Microstructural investigations by TEM of plane-view and cross-section specimens show an array of misfit dislocations at the interface with the substrate as well as threading dislocations which extend from the interface to the surface. RTA up to 1150°C led to a significant reduction of the threading dislocation density. The minimum yield ξ min of a sample grown at 450°C decreased from 6.7 to 3.9% after RTA at 1100°C for 30s. NDIC microscopy shows that the surface remains smooth after RTA in contrast to in-situ thermal annealing and ex situ furnace annealing. Similar experiments were carried out with pure germanium layers on (100) Si. RTA up to 600°C leads to an improvement of crystal quality. Higher temperatures result in the formation of square-shaped holes.

In temperature ramped Si(100) molecular beam epitaxy films the crystalline /disorder transition seen at very low temperatures shifts to lower temperatures when the deposition rate is lowered. At deposition rates less than 0.025 nm s−1 no crystalline/disorder transition could be detected down to 120 °C. Cross-section transmission electron microscopy reveals the appearance of cone-like defects prior to the crystalline/disorder transition which are accompanied by twins. The results are described by a kinetic model which predicts the existence of a finite epitaxy width ΔE in the very low temperature growth regime.