K. Deneffe

Franz–Keldysh oscillations originating from a well‐controlled electric field in the GaAs depletion region

The Franz–Keldysh oscillations induced by the electric field in the depleted zone below the GaAs surface are studied by photoreflectance spectroscopy. The electric field is precisely controlled by a molecular beam epitaxy grown buried highly doped layer and the pinned position of the Fermi level at the surface. It is shown that the electric field value as derived from theory is in disagreement with the value derived from electrostatic calculations. Consequently a determination of the Fermi level pinning is only possible from a measurement of both n‐ and p‐doped samples.

Band-gap narrowing in highly doped n- and p-type GaAs studied by photoluminescence spectroscopy

Band‐gap narrowing of GaAs as a function of doping concentration has been measured using photoluminescence spectroscopy on samples grown by molecular beam epitaxy. Both n‐ (Si) and p‐ (Be) doped samples with concentrations varying from 3×1017 to 3×1018 cm−3 have been measured. The experimental results obtained from a line‐shape analysis of the spectra taking tailing effects into account are in good agreement with recent theoretical calculations. A simple expression for the band‐gap narrowing as a function of concentration for both n‐and p‐doped GaAs is given.

Strain effects and band offsets in GaAs/InGaAs strained layered quantum structures

Strained single quantum wells composed of GaAs/InGaAs/GaAs were grown by molecular beam epitaxy and characterized at room temperature by photoreflectance and at 6 and 77 K by photoluminescence spectroscopy. For the InGaAs/GaAs heterojunction, utilizing a band offset ratio of 85:15 (conduction band:valence band) for the intrinsic (nonstrained) interface and a contribution of the hydrostatic compression to the valence band movement corresponding to the pressure sensitivity of the spin orbit band, excellent agreement is found between calculated excitonic transition energies and those found by experiment at all temperatures studied. Our analysis indicates that material parameters and the combined strain components used to calculate band structure are not temperature dependent to our degree of sensitivity. An empirical equation, which differs slightly from that for bulk InGaAs crystals, describing the nonstrained band‐gap energy as a function of In fraction at 77 K is presented. The difference between band off...

Low-lying Jπ=0+ states in 190,192Pb populated in the α-decay of 194,196Po

Abstract Further experimental evidence for a low-lying J π =0 + excited state in the neutron-deficient lead isotopes is obtained. This was done via a study of the α-decay of mass-separated 194,196 Po. The excited 0 + state in 192 Pb at 769 keV is confirmed and an excited 0 + state in 190 Pb at 669 keV has been found.

LIGISOL: The Leuven ion guide isotope separator on-line

Abstract The development of a new ion-guide facility for mass separation operating on-line with a heavy-ion accelerator is described. Different aspects related to the ionisation principle for reaction products with high recoil energies are discussed. It is proven that this ion-guide technique can be developed further to become one of the most general separation techniques, independent from physical (half-life, recoil energy,…) or chemical (refractoriness,…) characteristics.

Intruder states in odd-odd Tl nuclei: The completion of a unique set of intruder systematics

Abstract The α-decay of mass-separated Bi isotopes is studied to characterize shell-model intruder states in odd-odd Tl nuclei. By studying both experimentally and theoretically the members of the π 1 h 9 2 intruder -ν li 13 2 multiplet, the position of the intruder-based multiplet could be extracted. The scaling of intruder-state energies is discussed.

Optical characterization of stress in narrow GaAs stripes on patterned Si substrates

Photoluminescence (PL) and spatially resolved cathodoluminescence (CL) techniques are used to characterize the stress in narrow GaAs stripes grown on Si platforms. Since no overgrowth occurs over the recess edges, the GaAs stripes are grown unconstrained along one dimension. A duplication of the optical transitions is found in the PL spectrum from a region containing embedded GaAs and stripes. The peaks in the high‐energy shoulders of the PL spectrum are identified by CL measurements with high spatial resolution as the luminescence contribution of the GaAs stripes. They are submitted to lower internal stress values. A study on the geometrical dependence of the strain regime shows that a nonuniform biaxial strain field with a dominant longitudinal component is present in the stripes. The uniform biaxial strain, found in GaAs on Si (001), is present at stripe intersections.

PRODUCTION AND MASS SEPARATION OF SHORT-LIVING NEUTRON-DEFICIENT ACTINIDES

Abstract Light-ion reactions on radioactive as well as stable targets, in combination with the ion-guide technique represent a new and promising approach to the problem of the production and mass separation of short-living neutron-deficient actinides.

β+/electron-capture decay of192,194,196,198,200Bi: Experimental evidence for low lying0+states

Levels in the /sup 192, 194, 196, 198, 200/Pb isotopes were studied via the ..beta../sup +//electron-capture decay of mass separated Bi isotopes. Multiscaled spectra of ..gamma.. rays, x rays, and conversion electrons, and ..gamma..-..gamma.. and ..gamma..-e/sup -/ coincidences were measured. The existence of 0/sup +/ states in the /sup 192-200/Pb nuclei is shown and evidence for the beginning of a collective band built on top of the 0/sub 2//sup +/ states is seen in /sup 192, 194, 196/Pb. A discussion in terms of characteristic properties of intruder states in the even-even nuclei and in terms of a one-broken-neutron-pair formalism is given. This shows clearly the presence of both neutron and proton (..pi..(2p-2h)) based states in the semimagic Pb nuclei at low excitation energy.

Structural characterization of embedded gallium arsenide on silicon by molecular beam epitaxy

A gallium arsenide film is grown embedded in masked pre‐etched wells in silicon vicinal (001) substrates by molecular beam epitaxy. The morphology and crystallinity of the embedded GaAs on Si layers are identical to those of large area GaAs on Si grown on the same wafer, as indicated by the comparison of Nomarski contrast photomicrographs and electron channeling patterns. High‐resolution electron microscope images reveal the epitaxial relation of the GaAs/Si interface on the bottom and the sidewall of the wells. On the slope most of the dislocations are restricted to a narrow region near the interface. The same photoluminescent behavior is found for the GaAs deposit in the different silicon environments. This embedded growth technique is suitable for realization of a coplanar GaAs on Si surface.