Elektronische Struktur und Kristallgittereigenschaften von metastabilen III-(N,V)-Halbleitersystemen
In III-V-Halbleitern stellt das Stickstoffatom aufgrund seiner geringen Größe und hohen Elektronegativität eine starke Störung der elektronischen Struktur und der Kristallgittereigenschaften dar. Die vorliegende Arbeit gibt für Ga-V-Halbleiter (V = P, As, Sb) einen Überblick über diese Einflüsse. An...
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Table of Contents: In III-V semiconductors, the nitrogen atom gives rise to strong perturbations of the electronic structure and of the crystal lattice, due to its small size and high electronegativity. This thesis gives an overview of these influences for Ga-V semiconductors (V = P, As, Sb). Lattice vibrations of the ternary alloys Ga(N,P), Ga(N,As) und Ga(N,Sb) are studied and analyzed with respect to the local binding of the N atoms in the host lattices. For the first time, pressure coefficients of the extended host phonons as well as of the N local vibrational modes in Ga(N,As) und Ga(N,P) are determined by Raman spectroscopy under hydrostatic pressure. It is shown that the two materials behave similarly in this respect. In both cases the influence of N incorporation on the host phonon modes is small. The pressure coefficients of the N local modes in both alloys are considerably higher than the ones of the phonons in binary GaN crystals. The relationship between the force constant of the Ga-N bond and the bond length is determined. It turns out that it is nearly the same for Ga(N,P) and Ga(N,As). This result is a proof for the strong localization of the mode and for the little influence of the host matrix on the mechanical properties of the Ga-N bonds. Studies of the vibrational modes in the quaternary (Al,Ga)(N,As) exemplarily show the effects of a disordered nearest-neighbour shell of the N atoms. A central aspect of the thesis is the concentration dependence of optical transitions in Ga(N,P) and Ga(N,As), studied by spectroscopic methods. A comparison between both materials is interesting because they behave oppositely with respect to the energetic ordering of their host conduction bands and their N defect states. The impurity levels in both materials are determined by the spatial statistics of the N atoms. The defect states in Ga(N,As) are mainly resonant to the conduction band. Due to the large difference in atomic diameter between N and As, a strong interaction between conduction-band minimum E- and localized states is observed. This leads to a strong coupling of the N states and to a high N-related density of states at the energy E+. A repulsive interaction occurs between the E+ band and the alloy conduction-band minimum E-. This behaviour can be well parameterized by a phenomenological band anticrossing (BAC) model. In Ga(N,P), where the defect states are located in the host band-gap and where the interaction is weaker due to the smaller size difference between N and P, such a strong build-up of optical oscillator strength is not observed. Instead, the N defect states stay distributed over a large energy interval at N concentrations up to the percent range. The two-level BAC model is not able to describe such a behaviour. Additionally, the level repulsion is considerably weaker than in Ga(N,As). The observed red shift of the luminescence of Ga(N,P) with increasing N content is therefore mainly governed by the formation of close NN pairs and only to a lesser extent by level repulsion. These experimental results concerning the conduction band structure of Ga(N,P) confirm the LCINS model ("Linear Combination of Isolated Nitrogen States") for dilute nitride semiconductors. Furthermore, this thesis shows strong correlations between the local N environment and the global alloy band-structure. For example, chemical bonds of the N atoms in these materials can be manipulated by post-growth implantation of hydrogen atoms. It leads to a partial restorage of the electronic structure of the N-free crystals. This effect is very strong in Ga(N,As), and it is accompanied by a nearly complete disappearance of the N local mode in the Raman spectrum. In contrast, the N atoms in (Al,Ga)(N,As) are more stable against reactions with H when bound in certain Al-rich configurations than they are when being bound in other complexes. Similarly, in Ga(N,P) the electronic passivation by H is less complete than in Ga(N,As) and dependent on the spatial configuration of the defects. The results obtained in this thesis can provide valuable information for the design of new dilute-nitride based optoelectronic devices in the future. : A. Lindsay and E. P. O’Reilly, Phys. Rev. Lett. 93, 196402 (2004).