Direct Optical Observation of miniband states in Super-Networks GaAs / AlGaAs

Author(s)

Ricardo Faveron de Oliveira

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2005-05-09

Examining board members

Andre Bohomoletz Henriques; José Alzamir Pereira da Costa; Helena Maria Petrilli; Flávio Orlando Plentz Filho; Hercilio Rodolfo Rechenberg

• Advisor: Andre Bohomoletz Henriques

Abstract

Semiconductor superlattices consist of semiconductor materials with different gaps, arranged periodically. The electrons of the narrow gap layer, coupled by tunneling through the wider gap layer, form an energy miniband. The width of a miniband is equal to the difference in energy between its two edges, the bottom and the top. A Van Hove singularity is associated with each edge of the miniband. The direct detection of a miniband demands experimental techniques sensitive to these singularities. One example of such a technique is the Shubnikov-de Haas effect (SdH). Another technique is the absorption spectroscopy associated with transitions between the valence and the conduction bands. In this case, excitonic peaks associated with the two singularities in the density of states are observed. However, to estimate the miniband width from the absorption excitonic spectrum, it would be necessary to know accurately the binding energies of both excitons involved, the difference between which values is of the same order of magnitude as the miniband energy width. It should be possible to avoid the formation of excitons by doping the superlattice, however, in superlattices doped conventionally, the photoluminescence (PL) is completely dominated by transitions between Tamm states, which precludes the observation of extended minibancl states. In this work we investigate the possibility of avoiding both the formation of excitonic states, as well as the formation of Tamm states, by tayloring the modulation doping profile, and of detecting directly the interband transitions associated to extended rniniband states by PL measurements. By solving the Schrodinger and Poisson equations numerically, it was verified that a modulation doping profile that avoids the formation of Tamm states consists in a superlattice doped at the center of the wider gap layer, and also doped in the external layers, whereby the concentration of doping atoms in the external layers is equal to half of the value used in the internal ones. Such superlattices were investigated experimentally, and it was confirmed, from SdH measurements at oblique angles, that Tamm states were not present in the structures. The PL spectrum is characterized Ly an emission band whose width is approximately equal to the Fermi energy. The PL band is situated at photon energies greater than the energy bandgap of the confining layer. These characteristics suggest that the observed spectrum is associated to extended electronic miniband states. To confirm this interpretation, we detected and analyzed the PL spectrum in an external magnetic field. A theoretical model for the lineshape of the PL as function of the magnetic field intensity was developed. Using this model, it was possible to estimate all characteristics parameters for the superlattice: the energy width of the minibanel, the reduced mass of the electron-hole pair, and the renormalized energy gap. The latter parameter is not accessible by the experimental techniques used previously, and it was measured for the first time in this work. The access to this parameter opens a new perspective for the study of many body effects in structures, in which the dimensionality of the electronic system can be controlled artificially. This perspective is explored in this work. (AU)