Morphologie innerer Grenzflächen in verdünnt stickstoffhaltigen III/V-Materialsystemen

This study aims to clarify structure formation processes in dilute N-containing III/V-based material systems, using highly selective etching methods and subsequent atomic force microscopy (AFM) to expose and analyse interior interfaces. Due to their metastable nature, the material systems (GaIn)(NAs) and Ga(NAsP) show very complex structure formation processes which strongly affect the deposition of devices, e.g. lasers and solar cells, by metal organic vapour phase epitaxy (MOVPE). A previously established etching method was used to investigate the influence of Sb on the structure formation in GaAs-based (GaIn)(NAs). This material is characterized by a structural degradation (“structural phase transition”) which emerges as soon as a critical N-content or a critical growth interruption time (GI) is exceeded. In the first part of this study it was directly proved for the first time that adding Sb during growth interruption inhibits the GI-induced structural phase transition and reduces the diffusivity on GaAs and (GaIn)(NAs) surfaces. The dynamics of the structure formation process depend on the growth temperature of the material as well as the Sb content of the gas phase. Quantitative investigations using secondary ion mass spectrometry (SIMS) indicate that, during this process, Sb segregates and desorbs to a large extent. However, applying Sb during GI does not affect the driving force of the structural phase transition. Therefore a fundamental analysis about the incorporation of Sb into GaAs, Ga(NAs) and (GaIn)(NAs) was carried out in the second part of the study. Using a combination of high resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM) and SIMS measurements, it was verified that incorporating Sb into (GaIn)(NAs) causes an increase of the In content and a decrease of the N content. The increased In content results from an Sb-induced reduction of the effective Ga partial pressure in the gas phase and leads to an enhanced N-desorption. However, it could be shown that this effect is not sufficient by itself to explain the drastic reduction of the N content: there is an additional Sb-induced N-desorption which obeys a Langmuir mechanism. After these processes have been identified, for the first time the possibility arises to investigate the Sb incorporation into (GaIn)(NAs) while preserving the original composition. This is important in so far as the incorporation of Sb using molecular beam epitaxy (MBE) results in a considerably enhanced (GaIn)(NAs) growth window. In the third part of the study, novel etching methods for the GaP-based material system Ga(NAsP) are introduced which provide the opportunity to analyse structure formation processes on interior interfaces in this material system by AFM. Comprehensive investigations of functionality and chemical selectivity lead to the result that GaP barrier interfaces are accessible by removing an AlP/(GaIn)P cap with 30% hydrochloric acid (HCl method) or by removing a Ga(NAsP) cap with ammoniacal hydrogen peroxide solution (H2O2 method). Interior interfaces of quaternary Ga(NAsP) material can also be exposed by the HCl method, using AlP/(GaInP) or GaP as cap material. Consequently, interior quantum well and barrier interfaces can be exposed consecutively by combining the HCl and H2O2 methods. Using defects to mark the position, this procedure allows, for the first time, a direct correlation of two interior interfaces with an accuracy of less than 20 nm. The structure formation processes of the interior interfaces during growth interruption depend on the material and the gas phase composition: the structure formation of lattice matched Ga(NAsP) solar cell material with high N content is governed by local, N-induced strain fields, whereas the integral strain acts as a predominant driving force in compressively strained Ga(NAsP) laser material with low N content.