F. A. Thiel

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. Thes...

Extremely high electron mobility in Si/GexSi1−x structures grown by molecular beam epitaxy

A modulation‐doped Si/GexSi1−x structure was fabricated in which a thin Si layer is employed as the conduction channel for the two‐dimensional electron gas. The strained heterostructure is fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The mobility of the two‐dimensional electron gas as determined from Hall measurements was 1600 cm2/V s at 300 K and 96 000 cm2/V s at 4.2 K. Recently, a 4.2 K mobility of 125 000 cm2/V s was observed from a similar sample.

Reproducibility studies of lattice matched GaInAsP on (100) InP grown by molecular beam epitaxy using solid phosphorus

Growth of lattice matched GaInAsP on (100) InP was achieved using all solid source molecular beam epitaxy (MBE). Two valved cracking cells, one for phosphorus and the other for arsenic, were employed to supply the column V fluxes. The ability to obtain lattice matched conditions to InP was found to be highly reproducible and readily achievable using two valved cracking cells. X‐ray diffraction and photoluminescence measurements showed run‐to‐run variations in the arsenic/phosphorus mole fractions of less than 1%. Lattice matched Ga0.30In0.70As0.68P0.32 layers displayed 300 K luminescence full width at half maximums as low as 40.6 meV at λ∼1.43 μm. The results suggest all solid source MBE offers a viable alternative to existing heterojunction growth technologies.

Scanning tunneling microscopy of decagonal and icosahedral quasicrystals

The scanning tunneling microscope is used under ultrahigh vacuum conditions to image the clean surfaces of decagonal Al65Co20Cu15 and icosahedral Al65Cu20Fe15. The tunneling images show nonperiodic surfaces whose features are adequately described by pentagonal quasilattice for the case of the decagonal material, suggesting that a tiling model accounts for the surface features. The scanning tunneling microscope measurements of the scale of surface features is in good agreement with high resolution x‐ray diffraction measurements of the bulk.

YbBa2Cu3O7 epitaxial films grown by a Ag-enhanced liquid gas solidification process

Effects of Ag addition on the epitaxially grown YbBa2Cu3O7 superconductor on a SrTiO3 (100) substrate by the liquid gas solidification process have been investigated. Ag acts as flux and does not incorporate in the oxide formation. Its addition, however, enhances the transport properties resulting from a reduction in film‐substrate interaction. Values of Tc(R=0)=90 K and Jc(77 K, H=0)=3×104 A/cm2 can be achieved reproducibly.

Structure of Decagonal Quasicrystals

Recent discoveries made in synthesizing stable quasicrystals allows us to apply conventional techniques in growing macroscopic single grains and use conventional probes to study the complex structure of this novel form of matter. The decagonal quasicrystals in particular appears to have a far less complicated phase diagram and an extreme phase stability compared to their three dimensional icosahedral counterparts. We have grown several millimeter size single grains and studied the structure of stable A165Co20Cu15 Decagonal quasicrystals, using High Resolution X-ray Diffraction (HRXD) and Scanning Tunneling Microscopy (STM). Unlike diffraction probes which provide large volume reciprocal space information, STM is a local probe and provides direct real-space information with atomic resolution. The two techniques are therefore complimentary and when combined becomes very powerful in structure determinations of complex systems like quasicrystals. We have been able to prepare clean single grain surfaces in UHV and resolve individual atoms in quasiperiodic ordering with STM. Images of 10-fold quasiperiodic surfaces exhibit very well defined five sets of mass density lines, separated by multiples of 72 degree rotations. Near perfect quasiperiodic order seem to extend beyond thousands of angstroms, and can be modeled by a Penrose lattice. All atomic layers observed have identical local structure unlike the two dissimilar layers of the approximant Al13Fe4 phase. High resolution x-ray scattering from single grains finds 2000 angstroms size defect free quasicrystalline layers stacked periodically.