L. Ferrari

Optical properties of stepped InxGa1−xAs/GaAs quantum wells

The presence of a ternary alloy as well material in InxGa1−xAs/GaAs(001) strained quantum wells introduces a disorder mechanism by which the optical selection rules for symmetric wells may be relaxed and forbidden transitions usually appear in optical spectra. Strain and alloy disorder are studied as a function of well thickness and indium concentration in noninteracting double quantum wells of InxGa1−xAs/GaAs(001). Optical spectra are compared with an accurate Wannier exciton model. The agreement between theory and experiments points out that the optical selection rule for symmetric wells is restored in “high quality” and rather thin quantum wells. Finally, the presence of forbidden transitions in optical spectra is used as a fingerprint of nonhomogeneous indium concentration in thick quantum wells. This property is promising in order to study indium composition for well thicknesses in the range of quasi-two-dimensional behavior of the Wannier exciton.

Second Harmonic Generation in Stepped InAsGaAs/GaAs Quantum Wells

A systematic study in stepped quantum wells of InxGa1—xAs/InyGa1—yAs/GaAs(001) with indium concentration x = 14.9% andy = 6.5% and thicknesses in the range of quasi-two-dimensional behavior of Wannier excitons is performed. Experimental optical spectra are compared with model calculations. For well widths in the range of the pseudo-two-dimensional transition, where the oscillator strength of the exciton odd states is maximized, a large second harmonic generation signal is experimentally observed.

Electronic states of moiré modulated Cu films

We examined by low-energy electron diffraction and scanning tunneling microscopy the surface of thin Cu films on Pt(111). The Cu/Pt lattice mismatch induces a moiré modulation for films from 3 to about 10 ML thickness. We used angle-resolved photoemission spectroscopy to examine the effects of this structural modulation on the electronic states of the system. A series of hexagonal- and trigonal-like constant energy contours is found in the proximity of the Cu(111) zone boundaries. These electronic patterns are generated by Cu sp-quantum well state replicas, originating from multiple points of the reciprocal lattice associated with the moiré superstructure. Layer-dependent strain relaxation and hybridization with the substrate bands concur to determine the dispersion and energy position of the Cu Shockley surface state.

Optical and spectroscopic characterization of GaAs passivated surfaces

The importance of the surface treatment for GaAs surfaces has been well established. The most relevant source of recombination centers lies just on the surface. GaAs-based devices are obvious subjects where surface treatments, involving both oxide removal and surface passivation, are so important for this rapidly evolving application. We have extensively tested some methods for treating the GaAs surfaces. A simple commercial GaAs:Zn 2.7 x 10 18 cm -3 p-doped wafer has undergone different procedures: simple acetone/methyl-alcohol degreasing, a method that uses an anodic cell with (NH 4 ) 2 S and a passivating P 2 S 5 /(NH 4 ) 2 S solution. Hydrochloric acid has been used as well, because of the well-known utility for GaAs oxide removal. Energy positions of E F and filled and empty surface density of states have been measured by scanning tunneling microscope spectroscopy. Photo-induced luminescence has been also used to study the role of the surface trap states after different surface treatments.

Electronic states on Si(1 0 0)2 × 1-Sb: Existence of two semiconducting phases

Abstract Two different Sb-induced phases have been found upon annealing on freshly evaporated Si(1 0 0)2 × 1 surfaces. The surface electronic structures of the two phases (1 × 1-Sb and 2 × 1-Sb) have been studied with surface differential reflectivity (SDR) and angle-resolved photoelectron spectroscopy (ARUPS). Both techniques show the existence of a gap of approximately 1.6 and 1.4 eV for the two phases, with the empty state located near the Fermi level.

Surface proximity effects in III–V quantum wells investigated by photoreflectance

Abstract Photoreflectance spectroscopy (PR) of In 0.1 Ga 0.9 As GaAs and Al0.3Ga0.7 As/GaAs near-surface quantum wells has been investigated to determine the influence of the quantum well on the surface potential. In the regime of carriers tunneling from the quantum well to surface states, the PR signal from the well decreases, until vanishing when tunneling dominates over radiative recombination in the well. This result suggests that a quantum well next to the surface has the effect of creating a nearly flat band condition in a region comprehensive of the surface and the well itself.

Energy-momentum mapping of d-derived Au(111) states in a thin film

The quantum well states of a film can be used to sample the electronic structure of the parent bulk material and determine its band parameters. We highlight the benefits of two-dimensional film band mapping, with respect to complex bulk analysis, in an angle-resolved photoemission spectroscopy study of the

Observation of luminescence and Franz-Keldish effect on cleaved CdTe(110) surfaces

5d

Si(100)1×1-Sb and Si(100)2×1-Sb surfaces studied with angle-resolved photoemission and surface differential reflectivity

states of Au(111). Discrete

Quantum size effects arising from incompatible point-group symmetries : Angle-resolved photoemission study

5d