Optical characterization of dilute nitride semiconductors and related quantum structures

Sammanfattning: Dilute nitrides (i.e. nitrogen-containing, anion-mixed III-V ternary and quaternary alloys) have recently attracted much attention due to their unusual fundamental properties promising for device applications in optoelectronics and photonics, such as highly efficient and low cost near infrared lasers, efficient visible light emitting diodes (LEDs), multi-junction solar cells, as well as heterojunction bipolar transistors (HBTs). In order to fully explore the potential of these new materials, detailed knowledge on their fundamental and material-related properties is required.The work presented in this thesis focuses on optical studies of the electronic structure, radiative recombination processes and also characterization of material properties of several dilute nitrides systems that are relevant to device applications, such as GaNP/GaP, GaNP/Si, GaNAs/GaAs and GaInNP/GaAs. The thesis is divided into two parts. The first part includes five chapters that give a general introduction to the research field and also describes experimental methods utilized in the research work. The second part contains seven original scientific papers.Papers I and II report detailed studies of effects of post-growth hydrogen incorporation on the electronic structure of GaNAs and GaNP alloys by using photoluminescence (PL), PL excitation (PLE), and Raman spectroscopies, as well as high resolution X-ray diffraction (HRXRD) measurements. Introduction of hydrogen in the alloys was found to cause passivation of N-related localized states. Additionally, profound and rather astonishing changes in the band structure upon H incorporation were observed, such as a recovery of the bandgap energies of the parental GaAs and GaP, i.e. deactivation of the N-induced bandgap bowing. In GaNP, this was accompanied by a reduction in the N-induced coupling between the conduction band states. Raman spectroscopy has showed that these effects are related to hydrogeninduced breaking of the Ga-N bond. Raman and HRXRD measurements have also shown that the hydrogenation caused a strong expansion of the GaNP lattice, which changes the sign of strain from tensile strain in the as-grown GaNP epilayers to compressive strain after hydrogenation, due to formation of complexes between N and H.Paper III-IV discuss optical quality and defect properties of GaNP/Si and GaNP /GaP alloys, as well as effects of rapid thermal annealing (RTA). By employing a variety of optical characterization techniques including cathodoluminescence (CL), cw- and time-resolved PL, PLE, and optically detected magnetic resonance (ODMR), high optical quality of the GaN0.018P0.982 epilayers lattice matched to Si substrates was demonstrated and was shown to be comparable to that of the “state-of-the-art" GaNP alloys grown on GaP substrates. The growth of GaNP on Si is, however, found to facilitate the formation of several point defects, including complexes involving Ga interstitials (Gai). A reduction and removal of competing non-radiative point defects by RTA has been concluded to be responsible for a substantial increase in radiative efficiency of the GaNP epilayers subjected to the post-growth annealing, evident from reduced thermal quenching of the PL intensity as well as from a substantial increase in carrier lifetime at room temperature.Papers V-VII are devoted to detailed studies of Ga0.46In0.54NxP1-x alloys lattice matched to GaAs, by using cw- and time-resolved PL, PLE, and optically detected cyclotron resonance (ODCR) measurements. The type-II band alignment at the Ga0.46In0.54NxP1-x/GaAs interface was concluded with x ≥ 0.5% based on (i) highly efficient photoluminescence upconversion (PLU) observed in the N containing alloys and (ii) appearance of a near-infrared PL emission attributed to the spatially indirect type-II transitions. Compositional dependence of the conduction band offset at the GayIn1-yNxP1-x/GaAs interface was also estimated. Origin of the PLU process was determined as being due to two-step two-photon absorption (TS-TPA). Different from other direct band gap dilute nitrides, the low temperature PL emission was shown to largely arise from radiative transitions involving spatially separated localized electronhole pairs. The observed charge separation was tentatively attributed to the long range CuPt ordering promoted by the presence of nitrogen.