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We report on the pitch-induced bandgap tuning in GaAsSb (axial) and GaAs/GaAsSb (core–shell) patterned vertical nanowire (NW) arrays grown by Ga-assisted molecular beam epitaxy on Si(111). Red shifts in the range of 40–50 meV in the 4 K micro-photoluminescence (μ-PL) spectral peaks have been observed, for NW arrays with pitch length variation from 200 to 1200 nm, in the axial and core–shell configurations. The variation in the PL peak intensity closely follows the optical absorption dependency on the pitch length of the NW array computed using finite dimension time domain simulation. A semiempirical mass conservation of the growth-species-based-model has been proposed encompassing different material pathways. The secondary fluxes re-emitted from the side facets of the neighboring NWs contribute substantially toward the growth for smaller pitch lengths, while those from the oxide surface dominate at larger pitch lengths for high V/III beam equivalent pressure ratios. Excellent agreement between the experim...

GaAs/GaAsSb nanowire (NW) arrays are ideally suited to meet the demands of the next generation infrared (IR) photodetectors with potential for improving detection. NWs in a core–shell geometry have the advantage of providing axial direction for a long optical path for enhanced optical absorption and a short radial path for charge diffusion and collection. For the Ga-assisted molecular beam epitaxial growth of vertical, dense and uniform GaAs core NWs on Si (111), the effects of substrate surface preparation in combination with growth parameter variation were examined. On the epiready substrate without any surface preparation, both initial Ga shutter opening duration and V/III beam equivalent pressure ratio play a vital role in achieving almost all vertical NWs with moderate density ~107 cm−2. Also the spatial uniformity of the NWs was poor. Substrate surface preparation by chemical cleaning followed by oxidation in air led to highly vertical and uniform NWs with high density (8 × 108 cm−2). The GaAsSb shell was then successfully grown around the highly dense and vertical core GaAs NWs at growth temperatures ranging from 550°C to 590°C. It was found that growth temperature has a strong influence on Sb incorporation in the NWs and, hence, the NW morphology and 4K photoluminescence (PL) spectra. The presence of x-ray diffraction peaks corresponding to (111) reflection only and its higher-order reflections attest to the vertical alignment of NWs. Strain in the NWs as estimated using the Williamson–Hall isotropic strain model increases with Sb incorporation, which results in bending of the NWs with increasing Sb. Structural properties of these NWs using scanning transmission electron microscopy (STEM) are also presented. The temperature dependence PL of the NWs exhibited “S-curve” behavior, which is a well-known signature of localized excitons and a room temperature band edge PL emission occurring at ~1.3 μm.

We report on the site-selective growth of >90% vertical GaAs nanowires (NWs) on Si (111) using self-assisted molecular beam epitaxy. The influences of growth parameters (pre-growth Ga opening time, V/III flux ratio) and processing conditions (reactive ion etching (RIE) and HF etching time) are investigated for different pitch lengths (200- 1000 nm) to achieve vertical NWs. The processing variables determine the removal of the native oxide layer and the contact angle of Ga-droplet inside the patterned hole that are critical to the vertical orientation of the NWs. Pre-growth Ga-opening time is found to be a crucial factor determining the size of the droplet in the patterned hole, while the V/III beam equivalent pressure (BEP) ratio influenced the occupancy of the holes due to the axial growth of NWs being group-V limited.

Ga assisted GaAs/GaAsSb core-shell structured nanowires were successfully grown on chemically etched p-type Si(111) substrate by molecular beam epitaxy (MBE). The morphology, structural and optical properties of the nanowires are found to be strongly influenced by the shell growth temperature and Sb% in the nanowires. The nanowires exhibit planar defects like twins and stacking faults, with more stacking faults and micro-twins found at the top section. Optical characteristics of the nanowires as measured by 4K photoluminescence (PL) exhibit a red shift to 1.2 eV with increasing Sb incorporation up to 12%. The Raman spectra of reference GaAs nanowires show TO and LO modes representative of the zinc blende structure at 291 cm -1 and 267.8 cm -1 , respectively. Red shifts of both modes in conjunction with corresponding asymmetrical peak broadening observed in X-ray diffraction with increasing Sb incorporation are attributed to enhanced strain and disorder within the nanostructures. Nanowires of similar Sb composition but grown at different shell temperatures reveal straight nanowires with better microstructural and optical quality when grown at higher growth temperatures. The presence of GaAs passivation layer significantly enhanced the PL intensity such that PL was observed even at room temperature.

Bandgap reduction of 10% by incorporation of a dilute amount of N is reported for the first time, in axial GaAsSb nanowires (NWs) grown on Si (111) via Ga-assisted molecular beam epitaxy. Impact of N incorporation on the surface morphology, NW growth kinetics, and their structural and optical properties were examined. Dilute nitride NWs with Sb composition of 7 at% did not exhibit any noticeable planar defects, as revealed by the absence of satellite twin peaks in the selected-area diffraction pattern and high-resolution transmission electron microscopy imaging. Point defects were also minimal in as-grown dilute nitride NWs, as ascertained from the comparison of low-temperature photoluminescence spectra as well as the shape and shift of Raman modes, with in situ annealed NWs in different ambients. Evidence of enhanced incorporation of N was found in the NWs in situ annealed in N ambient, but with deteriorated optical quality due to simultaneous creation of N-induced defects. The lack of any noticeable defects in the as-grown GaAsSbN NWs demonstrates the advantage of the vapor–liquid–solid mechanism responsible for growth of axial configuration over the vapor–solid growth mechanism for core–shell NWs as well as their thin film counterpart, which commonly exhibit N-induced point defects.

In this work we present a comprehensive study on the effects of Sb incorporation on the composition modulation, structural and optical properties of self-assisted axial GaAs1−xSbx nanowires of 2–6 μm in length grown on (111) Si substrate by molecular beam epitaxy. The Sb composition in the GaAs1−xSbx axial nanowire (NW) was varied from 2.8–16 at.%, as determined from energy dispersive x-ray spectroscopy. Lower Sb composition leads to thinner nanowires and inhomogeneous Sb composition distribution radially with a depleted Sb surface region inducing weak type-II optical emission, the presence of an additional peak at higher Bragg angle in the x-ray diffraction spectra and an electric-field-induced strong Raman LO mode. Higher Sb composition of 16 at.% leads to a more uniform Sb compositional distribution radially leading to type-I optical transitions exhibiting the lowest PL peak energy occurring at 1.13 eV. In addition, the high quality of these nanowires exhibiting pure zinc blende crystal structure, largely free of any planar defects, is borne out by high resolution transmission electron microscopy and selected area diffraction patterns. The shift and broadening of the Raman LO and TO modes reveal evidence of increased Sb incorporation in the nanowires. Significant improvement in optical characteristics was achieved by the incorporation of a Al0.2Ga0.8As passivating shell. The results are very promising and reveal the potential to further red shift the optical emission wavelength by fine tuning of the fluxes during growth.

We report on in situ Te-doping in GaAs1−xSbx nanowires (NWs) grown via self-assisted molecular beam epitaxy. Enhanced Te incorporation in the NW at higher Te cell temperature was attested by the broadening of the x-ray diffraction peak and the presence of a strong coupled-LO phonon mode in the Raman spectra. Te-doping was estimated from the shift in the coupled-LO phonon mode to be ~2.0 × 1018/cm3. The surfactant nature of the Te modulated the growth kinetics, which was manifested in an enhanced radial growth rate with improved photoluminescence (PL) characteristics at both room temperature (RT) and 4 K. No noticeable planar defects were observed as ascertained from the high-resolution transmission electron microscopy images and selected-area electron diffraction patterns. Finally, we demonstrate the experimental realization of a GaAs1−xSbx axial p-type/intrinsic/n-type (p-i-n) structure on a Si substrate with Te as the n-type dopant. The GaAs1−xSbx p-i-n NW structures exhibited rectifying current–voltage (I–V) behavior. The dopant concentration and the transport parameters estimated from the PL spectra and I–V curve were found to be in good agreement.

Effective implementation of doped nanowires (NWs) in nanoscaled devices requires controlled and effective dopant incorporation. The one dimensional configuration of NWs poses a challenge for efficient doping due to the large number of surface states pinning the Fermi level close to the middle of the band gap and thus creating a large depletion layer at the surface. This effectively reduces the effective volume for doping. However, the flexibility of different architectures offered by the NWs, in particular, the core–shell configuration along with different growth mechanisms associated with the core and shell can be strategically used for efficient doping. In this work, the authors report on a catalyst free Ga-assisted approach for the growth of Be-doped GaAs NWs by molecular beam epitaxy. A systematic and a comprehensive study is reported using a variety of characterization techniques to determine the impact of NW configuration, Be cell temperature, and V/III beam equivalent pressure (BEP) ratio individua...

Semiconductor nanowires have been identified as a viable technology for next-generation infrared (IR) photodetectors with improved detectivity and detection across a range of energies as well as for novel single-photon detection in quantum networking. The GaAsSb materials system is especially promising in the 1.3–1.55 μm spectral range. In this work we present band-gap tuning up to 1.3 μm in GaAs/GaAsSb core–shell nanowires, by varying the Sb content using Ga-assisted molecular beam epitaxy. An increase in Sb content leads to strain accumulation in shell manifesting in rough surface morphology, multifaceted growths, curved nanowires, and deterioration in the microstructural and optical quality of the nanowires. The presence of multiple PL peaks for Sb compositions ≥12 at.% and degradation in the nanowire quality as attested by broadening of Raman and x-ray diffraction peaks reveal compositional instability in the nanowires. Transmission electron microscope (TEM) images show the presence of stacking faults and twins. Based on photoluminescence (PL) peak energies and their excitation power dependence behavior, an energy-band diagram for GaAs/GaAsSb core–shell nanowires is proposed. Optical transitions are dominated by type II transitions at lower Sb compositions and a combination of type I and type II transitions for compositions ≥12 at.%. Type I optical transitions as low as 0.93 eV (1.3 μm) from the GaAsSb for Sb composition of 26 at.% have been observed. The PL spectrum of a single nanowire is replicated in the ensemble nanowires, demonstrating good compositional homogeneity of the latter. A double-shell configuration for passivation of deleterious surface states leads to significant enhancement in the PL intensity resulting in the observation of room temperature emission, which provides significant potential for further improvement with important implications for nanostructured optoelectronic devices operating in the near-infrared regime.

Bandgap tuning up to 1.3 μm in GaAsSb based nanowires by incorporation of dilute amount of N is reported. Highly vertical GaAs/GaAsSbN/GaAs core-shell configured nanowires were grown for different N contents on Si (111) substrates using plasma assisted molecular beam epitaxy. X-ray diffraction analysis revealed close lattice matching of GaAsSbN with GaAs. Micro-photoluminescence (μ-PL) revealed red shift as well as broadening of the spectra attesting to N incorporation in the nanowires. Replication of the 4K PL spectra for several different single nanowires compared to the corresponding nanowire array suggests good compositional homogeneity amongst the nanowires. A large red shift of the Raman spectrum and associated symmetric line shape in these nanowires have been attributed to phonon localization at point defects. Transmission electron microscopy reveals the dominance of stacking faults and twins in these nanowires. The lower strain present in these dilute nitride nanowires, as opposed to GaAsSb nanowires having the same PL emission wavelength, and the observation of room temperature PL demonstrate the advantage of the dilute nitride system offers in the nanowire configuration, providing a pathway for realizing nanoscale optoelectronic devices in the telecommunication wavelength region.

The effects of ex-situ annealing in a N2 ambient on the properties of GaAs/GaAsSbN/GaAs core-multi-shell nanowires on Si (111) substrate grown by self-catalyzed molecular beam epitaxy (MBE) are reported. As-grown nanowires exhibit band edge emission at ~0.99 eV with a shoulder peak at ~0.85 eV, identified to arise from band tail states. A large red shift of 7 cm(-1) and broadened Raman spectra of as-grown nanowires compared to that of non-nitride nanowires confirmed phonon localization at N-induced localized defects. On annealing nanowires to 750 °C, there was no change in the planar defects in the nanowire with respect to the as-grown nanowire; however, vanishing of the photoluminescence (PL) peak corresponding to band tail states along with enhanced band edge PL intensity, recovery of the Raman shift and increase in the Schottky barrier height from 0.1 to 0.4 eV clearly point to the efficient annihilation of point defects in these GaAsSbN nanowires. A significant reduction in the temperature-induced energy shift in the annealed nanowires is attributed to annihilation of band tail states and weak temperature dependence of N-related localized states. The observation of room temperature PL signal in the 1.3 μm region shows that the strategy of adding small amounts of N to GaAsSb is a promising route to realization of efficient nanoscale light emitters with reduced temperature sensitivity in the telecommunication wavelength region.