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Nanowires (NWs) based on ternary GaAsP and quaternary GaNAsP alloys are considered as very promising materials for optoelectronic applications, including in multi-junction and intermediate band solar cells. The efficiency of such devices is expected to be largely controlled by grown-in defects. In this work we use the optically detected magnetic resonance (ODMR) technique combined with photoluminescence measurements to investigate the origin of point defects in Ga(N)AsP NWs grown by molecular beam epitaxy on Si substrates. We identify gallium vacancies, which act as non-radiative recombination centers, as common defects in ternary and quaternary Ga(N)AsP NWs. Furthermore, we show that the presence of N is not strictly necessary for, but promotes, the formation of gallium vacancies in these NWs. ; Funding: Linkoping University; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2019-04312]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

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 photoexcited 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 nonradiative 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. ; Funding Agencies|Swedish Energy Agency [P40119-1]; Swedish Research Council [2015-0.5532]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; National Science Foundation [DMR-1106369]

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. ; Funding Agencies|Linkoping University; Swedish Research Council [2016-05091]; Swedish Energy Agency [P40119-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swiss National Foundation through the NCCR QSIT