In this work we carry out a comprehensive investigation of structural and optical effects in GaNAsP nanowires (NWs), which are novel materials promising for advanced photovoltaic applications. Despite a significant mismatch in electronegativity between N and As/P atoms, we show that incorporation of nitrogen does not degrade structural quality of the nanowires and the fabricated NW arrays have excellent compositional uniformity among individual wires. From temperature-dependent photoluminescence (PL) measurements, statistical fluctuations of the alloy composition are shown to lead to localization of photo-excited carriers at low temperatures but do not affect material properties at room temperature. According to time-resolved PL measurements, the room-temperature carrier lifetime increases in the GaNAsP NWs as compared with the GaAsP NWs, which indicates reduced non-radiative recombination. Moreover, in spite of the very low N content in the studied NWs (up to 0.16 %), their bandgap energy can be tuned by more than 100 meV. This is accompanied by about 30% reduction in the temperature dependence of the bandgap energy. The presented results demonstrate that alloying of GaAsP with nitrogen provides an additional means of design optimization, beneficial for, e.g., NW-based intermediate band solar cells that are highly dependent on the optimum bandgap structure.
The authors report epitaxial growth of dilute nitride GaNAsP nanowires (NWs) and GaAsP/GaNAsP core–shell nanowires on Si (111) by a self-catalyzed method. Different sizes of GaNAsP NWs were observed with various N content. Room-temperature photoluminescence (PL) is observed from the GaNAsP NWs. The spectral peak of the temperature-dependent PL shows an S-shape and indicates defect states in the bandgap. The room-temperature PL intensity increased after rapid thermal annealing. A growth model explains the observed diameters of GaAsP/GaNAsP core–shell.
Cathodoluminescence (CL) and micro-photoluminescence spectroscopies are employed to investigate effects of structural defects on carrier recombination in GaNAsP nanowires (NWs) grown by molecular beam epitaxy on Si substrates. In the NWs with a low N content of 0.08%, these defects are found to promote non-radiative (NR) recombination, which causes spatial variation of the CL peak position and its intensity. Unexpectedly, these detrimental effects can be suppressed even by a small increase in the nitrogen composition from 0.08% to 0.12%. This is attributed to more efficient trapping of excited carriers/excitons to the localized states promoted by N-induced localization and also the presence of other NR channels. At room temperature, the structural defects no longer dominate in carrier recombination even in the NWs with the lower nitrogen content, likely due to increasing importance of other recombination channels. Our work underlines the need in eliminating important thermally activated NR defects, other than the structural defects, for future optoelectronic applications of these NWs.
The paper addresses the role of ambient elements (H, C, N and O) in the wide-bandgap semiconductor compounds. Their prevalence in the atmosphere imposes limitations not only on the purity of the materials under processing but, also, on the detection and measurement of the content of these species. Specifically, a review of electrical and optical properties, based on the available literature, is presented for: hydrogen in GaN, ZnO and SiC, carbon in GaN and ZnO, nitrogen in ZnO and SiC and oxygen in GaN and SiC. Further, the refinements of the SIMS (Secondary Ions Mass Spectrometry) analytical technique, aiming to improve the sensitivity and detection limit of atmospheric elements, are described in detail. These include the choice of primary beam type and current, type of secondary single or cluster ions, geometry and vacuum conditions. Finally, the evaluated so-called RSF parameters (Relative Sensitivity Factors) are given for each atom-semiconductor pair, converting raw data to the absolute value of concentration.
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