Semiconductor nanowires are currently attracting great interest due to their unique optical and electrical properties with great prospects for potential optoelectronic device applications at nanoscale. The GaAsSb materials system, in particular, is promising in the telecommunication wavelength range of 1.3-1.55 ?m. This work describes the synthesis of self-catalyzed GaAs/GaAsSb nanowires (NWs) on Si (111) substrates using molecular beam epitaxy (MBE), bandgap tuning to achieve 1.3 ?m by varying Sb content in the NWs and optimizing the growth condition for enhanced Be doping in core-shell GaAs NWs. We investigate the substrate preparation prior to the growth of NWs which is critical for achieving vertical and dense NWs. Growth temperature, catalyst droplet and V/III ratio plays an important role in it. We achieve highly vertical NWs after etching of the Si substrate and oxidizing it in the ambient atmosphere for appropriate time. This provides better control on the thickness of the SiO2 layer with appropriate micropore opening for the nucleation of Ga droplet initiating the NW growth. Further we study the effect of Sb on the optical and structural properties of GaAs/GaAsSb axial NWs. The Sb composition is varied upto 8.5 at.% in the 3?m GaAsSb segment in the NW. The NWs exhibit a zinc blende (ZB) crystal structure in the GaAsSb segment and the high quality of the NWs is attested by the lack of any defects in the segment. Although mixed wurtzite and zinc blende phases are observed towards the top of the NW which is from the GaAs cap. An increase in the Sb content lead to strain accumulation and surface deformation which is observed by the curving of the NWs. Broadening of the TO peak in the Raman spectra attested the same. A significant redshift in the ?-PL peak energy upto 1.07 eV (1.15?m) is seen in the NWs for Sb composition of 8.5at.%. Type II transitions are dominant in the NWs with large blueshift ~ 60 meV which is attributed to band filling and band bending effect. For achieving better structure and further tailoring the bandgap similar work is performed with core-shell GaAs/GaAsSb NWs with compostion of Sb from 1 to 26 at.%. The presence of multiple PL peaks for Sb compositions ?12 at.% and degradation in the nanowire quality is attested by broadening of Raman and x-ray diffraction peaks reveal compositional instability in the nanowires. Although the NWs exhibited planar defects, we could successfully attain 1.3?m from the core-shell structure. For making a p-i-n heterostructured NW photodetector, Be doping of the core shell GaAs NW is also examined. Be incorporation in the NW is strongly influenced by the Be cell temperature and V/III flux ratio for the shell. Photoluminescence emission with a broader full with half maxima of 92 meV show higher Be incorporation. A low and broaden LO intensity in the Raman spectra and a much symmetric I-V curve with more linearity and higher current for a given voltage indicate better p type doping in the NWs.
Dr. Ojha received his Master of Science in Physics with photonics specialization from Sathya Sai Institute of Higher Learning located in Puttaparthi, Andhra Pradesh. He has completed his Ph.D. from the Department of Electrical and Computer Engineering, North Carolina A&T State University (United States), with a focus on GaAsSb/GaAs semiconductor nanowires through molecular beam epitaxy for its application as Near Infrared Photodetectors application. He is currently a postdoctoral fellow at the Department of Electrical and Computer Engineering, North Carolina A&T State University.