Hans von Känel

Scaling Hetero-Epitaxy from Layers to Three-Dimensional Crystals

Laying It on Thick The growth of one layered material onto a second lies at the heart of many electronic devices. However, if there is a lattice mismatch between the two materials, strains develop in the overgrowth material leading to bowing and cracking. Falub et al. (p. 1330; see the cover) patterned Si substrates into a series of pillars onto which they grew a germanium layer. The germanium initially coated the top of each silicon pillar but then widened as the layer thickened, leading to thick, crack-free germanium films. A space-filling array of self-limited three-dimensional epitaxial crystals averts wafer bowing, layer cracking, and dislocation propagation. Quantum structures made from epitaxial semiconductor layers have revolutionized our understanding of low-dimensional systems and are used for ultrafast transistors, semiconductor lasers, and detectors. Strain induced by different lattice parameters and thermal properties offers additional degrees of freedom for tailoring materials, but often at the expense of dislocation generation, wafer bowing, and cracks. We eliminated these drawbacks by fast, low-temperature epitaxial growth of Ge and SiGe crystals onto micrometer-scale tall pillars etched into Si(001) substrates. Faceted crystals were shown to be strain- and defect-free by x-ray diffraction, electron microscopy, and defect etching. They formed space-filling arrays up to tens of micrometers in height by a mechanism of self-limited lateral growth. The mechanism is explained by reduced surface diffusion and flux shielding by nearest-neighbor crystals.

Very high hole mobilities in modulation-doped Ge quantum wells grown by low-energy plasma enhanced chemical vapor deposition

We report on the fabrication of modulation-doped compressively strained Ge quantum wells by low-energy plasma enhanced chemical vapor deposition. A virtual substrate consisting of a thick linearly graded SiGe buffer layer and a cap layer of constant composition is first grown at a high rate (>5 nm/s). The active layer stack, grown at a reduced rate, contains strain compensating cladding layers with modulation doping above the channel. Mobilities of up to 3000 cm2/V s and 87 000 cm2/V s have been achieved at room temperature and liquid He temperature, respectively.

Surface study of thin epitaxial CoSi2/Si(100) layers by scanning tunneling microscopy and reflection high-energy electron diffraction

Epitaxial single-domain CoSi2(100) layers have been grown on Si(100) by the use of a template technique. In situ scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) have been used for a detailed surface study. The dependence of the surface step structures on the stoichiometry is described. The transformation of the Co-rich C-surface to the Si-rich S-surface at about 500°C is related to the formation of pinholes. The (2 × 2)R45 reconstruction of the C-surface and the (32 × 2)R45 as well as a newly discovered (2 × 2)R45 of the S-surface have been resolved in real space and are described in detail. Different structural models are discussed. A (2 × 2) reconstruction is related to the transition from the C-surface to the S-surface. The occurence of misoriented CoSi2(011) grains and the dependence of the critical thickness for strain relaxation on the detailed template recipe have been analyzed by STM.

Unexpected Dominance of Vertical Dislocations in High-Misfit Ge/Si(001) Films and Their Elimination by Deep Substrate Patterning

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts.

Effective mass in remotely doped Ge quantum wells

We report on the dependence of the effective masses on hole density in remotely doped strained Ge layers on relaxed Si0.3Ge0.7 buffers with sheet densities from 2.9×1011 cm−2 to 1.9×1012 cm−2. The masses have been determined using temperature dependent Shubnikov–de Haas oscillations. No noticeable dependence of the mass on the magnetic field has been found. The extrapolated Γ point effective mass has been found to be 0.080 times the free electron mass. From the measured data the variation of the mass with kinetic energy and the shape of the topmost heavy hole subband have been calculated. The results are in good agreement with theoretical predictions.

Observation of misfit dislocations in epitaxial CoSi2/Si (111) layers by scanning tunneling microscopy

The surfaces of epitaxial CoSi2 layers grown on Si(111) have been examined by scanning tunneling microscopy (STM) in ultrahigh vacuum. The onset of strain relaxation above the critical thickness of about 40 A has been monitored by STM for the first time. This relaxation takes place by the formation of a honeycomb network of partial dislocations lying in the interface plane. An associated network of protruding lines has been detected in STM topographs for film thicknesses up to 104 A. The topographic cross sections perpendicular to the lines are found to have a Lorentzian shape with a height of 0.6 A and a half‐width equal to the layer thickness. Our analysis suggests that similar effects should be observable for a wider class of heteroepitaxial systems.

Perfect crystals grown from imperfect interfaces

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.

An experimental and theoretical investigation of a magnetically confined dc plasma discharge

A magnetically confined dc plasma discharge sustained by a thermionic source was investigated using a combined experimental and theoretical approach. The discharge originates in an arc plasma source and is expanded in a cylindrical chamber, where it is stabilized by an annular anode. The plasma expansion is contained by an axial magnetic field generated by coils positioned at the top and the bottom of the reactor. The plasma reactor design allows control of the energy of ions impinging on the substrate and thus a high electron density of about 1017 m−3 at 1 Pa can be reached. The plasma is studied using a model composed of the Poisson and of the charged species continuity equations, solved in the flow and temperature fields determined by solving the Navier–Stokes and Fourier equations. The model equations are integrated using the finite element method in a two-dimensional axial symmetric domain. Ionization rates are either assumed constant or determined by solving the Boltzmann transport equation in the l...

Strain-induced (2 × 1) reconstruction on epitaxial CoSi2/Si(111) observed by scanning tunneling microscopy: structure model and electrical properties

The formation of a (2 × 1) reconstruction on ultrathin (d<45 A), strained layers of CoSi2 grown epitaxially on Si(111) is reported. Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) show that the stable low-temperature (T<450 K) structure of the Si-rich CoSi2 surface consists of chains perpendicular to the three 〈211〉 directions. A structure model for the atomic rearrangement of this surface is presented which agrees with the results of a detailed analysis of high-resolution STM topographs, including point defects, different domain boundaries and I-U tunneling spectroscopy. The results are compared with the π -bonded chain model for the Si(111)2 × 1 reconstruction.

Sub-100 nm Gate Technologies for Si/SiGe-Buried-Channel RF Devices

A novel fabrication process of sub-100 nm self-aligned T-gates for heterostructure field-effect transistors (HFETs) using optical contact lithography is presented. A 500-nm-wide polyimide fin is used as an implantation mask, shrunk by dry etching and subsequently replaced by a gate metal. A low-resistive gate head to form a T-shape is independently defined by wet chemical etching. Using this method, Si/SiGe modulation-doped field-effect transistors (MODFETs) have been prepared, having a gate length of 90 nm. The self-alignment enables the realization of very small source/gate and gate/drain spacings of 200 nm. This yields, together with an optimized salicide (self-aligned silicide) ohmic contact, a much lower access resistance compared to conventional gates defined by e-beam lithography. A record transit frequency fT of 90 GHz and a very high transconductance of 570 mS/mm have been achieved for MODFETs.