Exploring the band structure of Wurtzite InAs nanowires using photocurrent spectroscopy

We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This information is vital to the development of crystal phase engineering of this important III-V semiconductor.ER

Morphology and chemical composition of inxGa1-xAs NWs Au-assisted grown at low growth temperature using MOCVD

Cylindrical InxGa1-xAs NWs have been successfully grown at low growth temperature using MOCVD. Field Emission-Scanning Electron Microscopy (FE-SEM) characterization and Energy Dispersive X-ray (EDX) analysis have been used to investigate the morphology and chemical composition of NWs, respectively. Both characterization results consistently reinforce that the NWs growth were via direct impinging mechanism and NW have relatively uniform chemical composition

Intercarrier interference (ICI) analysis using correlative coding OFDM system

The orthogonal frequency division multiplexing (OFDM) technique is an attractive wireless communication system. In many applications, however, OFDM is very sensitive to frequency errors caused by two destructive effects. One effect is a reduction of signal amplitude; the other is the introduction of intercarrier interference (ICI) from other carriers caused by the loss of orthogonality between subchannels. In order to minimize ICI in OFDM systems, the correlative coding method was proposed. A normalized frequency offset was introduced in order to analyze the performance of the system where subcarrier frequency offset (SCFO) was used to measure the ICI in the system using Matlab software. The result shows that correlative coding OFDM (CCOFDM) can improve the level of the ICI signal compared to OFDM without correlative coding (OFDMWOCC) by about 6.9 dB

AFM, HR-XRD and PL characterization of stacked structures In 0.5Ga0.5As/GaAs quantum dots

In0.5Ga0.5As quantum dots (QDs) stacked structure were studied using atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD) and photoluminescence (PL) characterization. Evolution in the dots size and dots density in the stacked structures is strongly influenced by the dot formation in the under-layer and the structure of the spacer layers. AFM results revealed that the dots formation on the top can be changed by increasing the number of stacked QDs. However, the dots formation is not vertically aligned since HR-XRD measurement gave different satellite peak on n-stacked QD structures. Room-temperature PL measurements show variation in the PL spectra, where blue-shifted PL peak positions are observed when the number of stack is increased. Variation in the HR-XRD and PL measurement is also attributed to the size, composition and density of the dots in the stacked structures. © 2010 World Scientific Publishing Company

Supporting information for InP-InxGa1-x as core-multi-shell nanowire quantum wells with tunable emission in the 1.3 – 1.55 μm wavelength range

Section 1: Experimental details pertaining to the growth of InP- InxGa1-xAs QWs InP nanowire core: The InP nanowire cores were seeded by 50 nm colloidal Au particles. The Au particle-deposited InP (111)B substrates were heated to the growth temperature of 450˚C. Substrates were not annealed prior to growth, but TMIn was pre-flown for 15 s before initiating the growth. This pre-flow step reduces the non-vertical nanowire growths that arise from lack of alloying when the pre-growth annealing step is avoided. The TMIn and PH3 flows were 1.62 × 10-5 and 5 × 10-3 mol/min, respectively. Nanowire core growth was carried out for 30 min at 100 mbar reactor pressure. InxGa1-xAs QWs: After the core growth, the temperature was ramped up to the shell growth temperature of 550˚C and the reactor pressure was ramped to 180 mbar which was the pressure normally used in the current MOVPE system for InP-related planar vapour-solid epitaxial growth. The nanowire core was annealed for 3 min before depositing a thin InP buffer layer on the nanowire side facets in order to ensure a high quality surface for the subsequent QW growth. After a growth interruption of 5 s the QW growth was initiated. The TMIn, TMGa and AsH3 flows used for the study of the effect of QW thickness variation were 6.75 × 10-6, 5.51 × 10-6 and 1.34 × 10-3 mol/min, respectively, giving a vapour phase In molar fraction Xv = [TMIn]/([TMIn]+[TMGa]) of 0.55. The QW growth time was varied between 20 to 180 s depending on the targeted QW thickness. For the study of QW composition variation, the TMIn flow was kept constant at 6.75 × 10-6 mol/min while varying the TMGa flow to achieve compositions between GaAs and InAs, except in the case of GaAs, where the TMIn source was turned off. The growth time of these composition-varied QWs were also scaled accordingly in order to achieve a nominal thickness of 7 nm. Another 5 s growth interruption was included after the QW growth with AsH3 left on in order to prevent As desorption from the thin QW 1. Lastly, an InP barrier shell was grown for 12 min. For the growth of MQW structure, the QW, interruption and barrier growth steps were repeated two times more

Exploring the band structure of Wurtzite InAs nanowires using photocurrent spectroscopy

We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This information is vital to the development of crystal phase engineering of this important III-V semiconductor

InP-In<inf>x</inf>Ga<inf>1-x</inf>As core-multi-shell nanowire quantum wells with tunable emission in the 1.3-1.55 μm wavelength range

© 2017 The Royal Society of Chemistry. The usability and tunability of the essential InP-InGaAs material combination in nanowire-based quantum wells (QWs) are assessed. The wurtzite phase core-multi-shell InP-InGaAs-InP nanowire QWs are characterised using cross-section transmission electron microscopy and photoluminescence measurements. The InP-InGaAs direct interface is found to be sharp while the InGaAs-InP inverted interface is more diffused, in agreement with their planar counterpart. Bright emission is observed from the single nanowires containing the QWs at room temperature, with no emission from the InP core or outer barrier. The tunability of the QW emission wavelength in the 1.3-1.55 μm communication wavelength range is demonstrated by varying the QW thickness and in the 1.3 μm range by varying the composition. The experiments are supported by simulation of the emission wavelength of the wurtzite phase InP-InGaAs QWs in the thickness range considered. The radial heterostructure is further extended to design multiple QWs with bright emission, therefore establishing the capability of this material system for nanowire based optical devices for communication applications

InP-InxGa1-xAs core-multi-shell nanowire quantum wells with tunable emission in the 1.3-1.55 µm wavelength range

The usability and tunability of the essential InP–InGaAs material combination in nanowire-based quantum wells (QWs) are assessed. The wurtzite phase core-multi-shell InP–InGaAs–InP nanowire QWs are characterised using cross-section transmission electron microscopy and photoluminescence measurements. The InP–InGaAs direct interface is found to be sharp while the InGaAs–InP inverted interface is more diffused, in agreement with their planar counterpart. Bright emission is observed from the single nanowires containing the QWs at room temperature, with no emission from the InP core or outer barrier. The tunability of the QW emission wavelength in the 1.3–1.55 μm communication wavelength range is demonstrated by varying the QW thickness and in the 1.3 μm range by varying the composition. The experiments are supported by simulation of the emission wavelength of the wurtzite phase InP–InGaAs QWs in the thickness range considered. The radial heterostructure is further extended to design multiple QWs with bright emission, therefore establishing the capability of this material system for nanowire based optical devices for communication applications

Critical Temperature for the Conversion from Wurtzite to Zincblende of the Optical Emission of InAs Nanowires

One hour annealing at 300 °C changes the optical emission characteristics of InAs nanowires (NWs) from the wurtzite (WZ) phase into that of zincblende (ZB). These results are accounted for by the conversion of a small fraction of the NW WZ metastable structure into the stable ZB structure. Several paths toward the polytype transformation in the configuration space are also demonstrated using first-principles calculations. For lower annealing temperatures, emission which is likely related to WZ polytypes is observed at energies that agree with theoretical predictions. These results demonstrate severe constraints on thermal processes to which devices made from InAs WZ NWs can be exposed