L.M.R. Scolfaro

STRUCTURAL PROPERTIES AND RAMAN MODES OF ZINC BLENDE INN EPITAXIAL LAYERS

We report on x-ray diffraction and micro-Raman scattering studies on zinc blende InN epitaxial films. The samples were grown by molecular beam epitaxy on GaAs(001) substrates using a InAs layer as a buffer. The transverse-optical (TO) and longitudinal-optical phonon frequencies at Γ of c-InN are determined and compared to the corresponding values for c-GaN. Ab initio self-consistent calculations are carried out for the c-InN and c-GaN lattice parameters and TO phonon frequencies. A good agreement between theory and experiment is found.

Hole confinement effects on multiple Si δ doping in GaAs

The observation of quantum‐confined optical transitions in multiple δ doping in GaAs, grown by molecular beam epitaxy, is reported. Doping efficiency and carrier confinement are investigated by Hall and photoluminescence measurements. Hall measurement results for multiple δ‐doped samples show a dramatic enhancement of carrier concentrations compared to the uniform doping case. From photoluminescence spectra we observed that the cutoff energy is significantly affected by the spacing between the dopant sheets. The strong localization of confined photoexcited holes in the spacing layers of these structures plays a fundamental role in the interpretation of the optical data.

Valence band structure of cubic AlGaN/GaN superlattices

The influence of different material parameters, spin-orbit interaction, and strain effects on the valence band structure of cubic AlGaN/GaN superlattices is investigated. One-particle hole state calculations are carried out within the k⋅p theory by means of a full six-band Luttinger-Kohn Hamiltonian in a plane-wave representation. It is shown that the use of distinct values for the Luttinger parameters for the barrier and well regions leads to significant changes in the hole levels, particularly for large Al content. Spin-orbit interaction effects are responsible for strong nonparabolicities due to the light- and split-off-hole bands mixing. Besides, the hole levels are very sensitive to strain effects. Thus, it is demonstrated that these effects cannot be neglected in a realistic description of the valence band structure of these materials.

Exchange-correlation effects on the hole miniband structure and confinement potential in zinc-blende AlxGa1-xN/GaN superlattices

We present valence band-structure calculations for undoped and p-doped cubic AlxGa1-xN/GaN superlattices (SLs), in which the coupling between the heavy-hole, light-hole, and spin-orbit-split-hole bands and strain effects due to lattice mismatch are taken into account. The calculations are performed within a self-consistent approach to the kp theory by means of a full six-band Luttinger-Kohn Hamiltonian. Exchange-correlation effects within the two-dimensional hole gas are included in the calculations in the local density approximation. Results for hole minibands and potential profiles are shown as functions of the SL period. It is shown that exchange and correlation play an important role in the correct description of the systems.

First-principles calculations of the effective mass parameters of Al xGa1-xN and Zn xCd1-xTe alloys

First-principles calculations of electronic band structures of the ordered cubic alloys AlxGa1-xN and CdxZn1-xTe are carried out. The band structures are used to provide e ective masses and Luttinger parameters which are useful in the parametrization of theories based on e ective hamiltonians.

Miniband structures and effective masses of n-type -doping superlattices in GaN

Self-consistent electronic structure calculations of n-type -doping superlattices (SLs) in GaN are performed using effective-mass theory and local density functional theory. The electron bulk mass of cubic GaN is obtained from ab initio full potential linear augmented plane wave (FLAPW) band-structure calculations. For the SLs, self-consistent potentials, subband levels, miniband dispersions and Fermi-level positions are calculated, assuming sheet donor concentrations up to and periods between 50 A and 400 A which cover the whole range between an isolated-well and SL regime. The miniband effective masses exhibit strong dependence on the SL wavevector. The confinement effects in GaN n-type -doping SLs are strong enough to be seen experimentally in transport and optical properties. A comparison of the results obtained for GaN with -doped layers in other III-V compounds, such as GaAs, is made.

Ab initio calculation of the (100) and (110) surface phonon dispersion of GaAs and GaN

1) reconstruction presentsinteresting features which is closely related to the reduced mass of the compound. All the studied cases show thepresence of a Rayleigh mode. The calculated properties for the (110) surfaces agree very well with the availableexperimental data from HEELS and inelastic He-atom scattering as well as with other theoretical calculations.

Photoreflectance investigations of semiconductor device structures

The application of photoreflectance (PR) spectroscopy to metal-semiconductor field effect transistor and high electron mobility transistor structures is demonstrated. The line shape analysis relies on a new method for calculating PR spectra and electric field profiles of heterostructures with weakly inhomogeneous layers. The method allows inhomogeneous built-in electric field distributions below the surface to be detected. It can be used to determine characteristic parameters of device structures and to detect quantum states confined to potential wells. Differences between PR spectra from nominally identical or similar structures are attributed to different interface qualities.

Comparative studies of photoluminescence from n and p δ doping wells in GaAs

Abstract Luminescence spectra for n and p δ doping wells in GaAs are calculated. The stationary electron and hole states of the wells are obtained self-consistently by means of the multiband effective mass theory. The overlap integrals of the electron and hole envelope functions are crucial for the luminescence intensities. Those for light holes are larger than those for heavy holes, and those for p δ doping wells exceed those for n δ doping wells by almost two orders of magnitude. This explains the experimental findings on luminescence from such wells.

Stability of native defects in cubic boron nitride

Abstract The electronic structure and formation energies of the native defects boron and nitrogen vacancies and nitrogen antisite defect in cubic BN have been calculated with the use of the full-potential linear augmented-plane-wave method within the local density and large unit cell approximations. We show that the nitrogen vacancy and the nitrogen antisite defect have high formation energies in B-rich or N-rich and p-type or n-type material. These centers are unlikely to occur even at the high temperature conditions assumed for the samples growth. For the nitrogen vacancy to be ascribed to the high resistivity n-type effect and to the dominant electron paramagnetic line observed in c-BN samples, a pertubation is required, for example by an impurity, to lower the formation energy of the defect. Besides, the calculations show that the deep acceptor level, with activation energy at about 1 eV, observed in n-type or intrinsic c-BN crystals, can be ascribed to the negatively charged boron vacancy.