Premila Mohan

Fabrication of InP∕InAs∕InP core-multishell heterostructure nanowires by selective area metalorganic vapor phase epitaxy

We report the growth of InP∕InAs∕InP core-multishell nanowire arrays by selective area metalorganic vapor phase epitaxy. The core-multishell nanowires were designed to accommodate a strained InAs quantum well layer in a higher band gap InP nanowire. The precise control over nanowire growth direction and heterojunction formation enabled the successful fabrication of the nanostructure in which all three layers were epitaxially grown without the assistance of any catalyst. The grown nanowires were highly uniform, vertically oriented, and periodically aligned with controllable dimensions. 4K photoluminescence measurements confirmed the formation of strained InAs quantum well on InP (110) sidewalls and the well widths corresponding to the photoluminescence peaks were in good agreement with calculated values.

Realization of conductive InAs nanotubes based on lattice-mismatched InP∕InAs core-shell nanowires

We report the realization of ordered arrays of single-crystalline InAs nanotubes by a simple pure-eptiaxial approach. The process involved the fabrication of lattice-mismatched InP∕InAs core-shell nanowires using selective area metalorganic vapor phase epitaxy on InP (111)A substrates. The subsequent removal of the InP core resulted in vertically aligned InAs nanotubes which were highly uniform with well-defined features and controllable dimensions. Transmission electron microscopy studies confirmed that the nanotubes were single-crystalline with wurtzite crystal structure and temperature-dependent transport measurements revealed that they were conductive without any intentional doping. The realization of such conductive InAs nanotubes opens up new possibilities for both fundamental studies and future device applications.

Type-II behavior in wurtzite InP/InAs/InP core-multishell nanowires

We study optical transitions from a periodic array of InP/InAs/InP core-multishell nanowires (CMNs) having a wurtzite crystal structure by using photoluminescence (PL) and PL excitation (PLE) spectroscopy. Observing a large Stokes shift between PL and PLE spectra, a blueshift of the PL peak with a cube-root dependence on the excitation power and a slow and nonexponential decay of PL with an effective decay time of 16 ns suggest a type-II band alignment. Band-offset calculation based on the “model-solid theory” of Van de Walle [Phys. Rev. B 39, 1871 (1989)] supports type-II band lineup if the InAs layer in the wurtzite CMNs is assumed to sustain compressive strain in all directions.

Fabrication of semiconductor Kagome lattice structure by selective area metalorganic vapor phase epitaxy

Artificial two-dimensional semiconductor Kagome lattice structures formed by quantum wires can show ferromagnetism when the flatband is half filled, even though it does not have any magnetic elements. Experimental realization of such a Kagome lattice structure is reported. The structure, with different pattern periods, was formed with GaAs quantum wires by selective area metalorganic vapor phase epitaxy on GaAs (111)B substrates. To overcome the lateral overgrowth and to improve the shape of smaller period pattern, flow rate modulation epitaxy was employed and a GaAs Kagome lattice structure with 1 μm period was effectively grown.

Fabrication of Axial and Radial Heterostructures for Semiconductor Nanowires by Using Selective-Area Metal-Organic Vapor-Phase Epitaxy

The fabrication of GaAs- and InP-based III-V semiconductor nanowires with axial/radial heterostructures by using selective-area metal-organic vapor-phase epitaxy is reviewed. Nanowires, with a diameter of 50–300 nm and with a length of up to 10 μm, have been grown along the 〈111〉B or 〈111〉A crystallographic orientation from lithography-defined SiO2 mask openings on a group III-V semiconductor substrate surface. An InGaAs quantum well (QW) in GaAs/InGaAs nanowires and a GaAs QW in GaAs/AlGaAs or GaAs/GaAsP nanowires have been fabricated for the axial heterostructures to investigate photoluminescence spectra from QWs with various thicknesses. Transmission electron microscopy combined with energy dispersive X-ray spectroscopy measurements have been used to analyze the crystal structure and the atomic composition profile for the nanowires. GaAs/AlGaAs, InP/InAs/InP, and GaAs/GaAsP core-shell structures have been found to be effective for the radial heterostructures to increase photoluminescence intensity and have enabled laser emissions from a single GaAs/GaAsP nanowire waveguide. The results have indicated that the core-shell structure is indispensable for surface passivation and practical use of nanowire optoelectronics devices.

Formation and characteristics of 100-nm scale GaAs quantum wires by selective area MOVPE

We fabricated quantum wires (QWRs) with sub-micron wire width using GaAs/AlGaAs selectively doped structures grown by selective area metalorganic vapor phase epitaxy (SA-MOVPE) on (0 0 1) masked GaAs substrates partially covered by SiON. From the measurement of a two-terminal conductance as a function of geometrical wire width, QWRs with effective channel width <100 nm are formed without application of any gate bias. The magnetoresistance measurement at 1.7 K also suggests the formation of narrow QWRs, although it also indicates a presence of potential fluctuation along the QWRs. The effective channel width of present QWRs are much narrower than the previously reported values (∼300 nm) of those formed by SA-MOVPE.

Realization of InAs-based two-dimensional artificial lattice by selective area metalorganic vapor phase epitaxy

The experimental realization of two-dimensional semiconductor artificial lattice based on InAs quantum wires is reported here. Artificial Kagome lattice fabricated using InAs quantum wires of unit cell size 0.7 μm has been theoretically proved to show ferromagnetism. Fabrication of such a structure with InAs quantum wires was attempted by selective area metalorganic vapor phase epitaxy using GaAs (111)A substrates. Temperature-dependent growth mode change was observed and Volmer-Weber growth mode at high temperature inhibited the formation of uniform structure. Low temperature and low AsH3 partial pressure resulted in the successful fabrication of 0.7 μm period InAs-based Kagome lattice structure.

One- and Two-Dimensional Spectral Diffusions in InP/InAs/InP Core–Multishell Nanowires

We report on the photoluminescence and time-resolved photoluminescence spectroscopy of wurtzite InP/InAs/InP core–multishell nanowires. Multiple peaks appeared in the photoluminescence spectra owing to the monolayer variation of InAs layers. Each photoluminescence peak has a broad photoluminescence band coming from the inhomogeneous broadening in a core–multishell single nanowire. Inhomogeneous broadening caused time-dependent red shift (spectral diffusion) at a constant energy-loss rate. The long-time-span (~10 ns) time-resolved photoluminescence measurement clarified that spectral diffusion takes place in two stages (initially at a fast rate and later at a slow rate). The fast one is ascribed to spectral diffusion on the side of the nanowire and the slow one is ascribed to spectral diffusion at the corner of the nanowire. This suggests that photoexcited excitons in a core–multishell nanowire move from the side of the nanowire toward the corner of the nanowire.

InP/InAs core-multishell heterostructure nanowires grown by metalorganic vapor phase epitaxy

We report the growth and characteristics of InP nanowires and InP/InAs/InP core-multishell nanowire arrays by selective area metalorganic vapor phase epitaxy. InP hexagonal nanowires with (110) sidewall facets were successfully formed on (111)B InP substrates. The core-multishell nanowires were also formed composed of InAs tube-like layer buried in a higher bandgap InP nanowire. The precise control over nanowire growth direction and heterojunction formation enabled the successful fabrication of the nanostructure in which all the three layers were epitaxially grown without the assistance of any catalyst. The periodically aligned nanowires and core-multishell nanowires were highly uniform, and Wulzaite structures. 4 K photoluminescence measurements confirmed the formation of strained InAs quantum well on InP (110) sidewalls and the well widths corresponding to the PL peaks were in good agreement with calculated values.

Time- and spectrally-resolved PL study of a regular array of InP/InAs/InP core-multishell nanowires

Time- and spectrally-resolved PL from a periodic array of InP/InAs/InP core-multishell nanowires is presented. InAs layer shows multipeak PL spectra. PL decay is nonexponential and very slow, with decay rate depending on energy.