2.8-µm polarization-maintaining Er fiber laser mode-locked by a GaSb-based SESAM.

A GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM) enables continuous-wave picosecond mode-locked operation with excellent stability of a polarization-maintaining mid-infrared Er:ZBLAN fiber laser. The GaSb-based SESAM mode-locked fiber laser delivers an average output power of 190 mW at 2.76 µm at a repetition rate of 32.07 MHz (corresponding to a pulse energy of ∼6 nJ) and exhibits a high signal-to-noise ratio of ∼80 dB. The polarization extinction ratio is more than 23 dB. By employing an intracavity diffraction grating, the laser wavelength is continuously tunable across 2.706-2.816 µm. Passively Q-switched operation of this laser is also demonstrated.

Influence of the Bismuth Content on the Optical Properties and Photoluminescence Decay Time in GaSbBi Films.

We report the optical properties of GaSbBi layers grown on GaSb (100) substrates with different bismuth contents of 5.8 and 8.0% Bi. Fourier-transform photoluminescence spectra were determined to identify the band gaps of the studied materials. Further temperature- and power-dependent photoluminescence measurements indicated the presence of localized states connected to bismuth clustering. Finally, time-resolved photoluminescence measurements based on single-photon counting allowed the determination of characteristic photoluminescence decay time constants. Because of the increasing bismuth content and clustering effects, an increase in the time constant was observed.

Widely tunable 2 µm hybrid laser using GaSb semiconductor optical amplifiers and a Si3N4 photonics integrated reflector.

Tunable lasers emitting in the 2-3 µm wavelength range that are compatible with photonic integration platforms are of great interest for sensing applications. To this end, combining GaSb-based semiconductor gain chips with Si3N4 photonic integrated circuits offers an attractive platform. Herein, we utilize the low-loss features of Si3N4 waveguides and demonstrate a hybrid laser comprising a GaSb gain chip with an integrated tunable Si3N4 Vernier mirror. At room temperature, the laser exhibited a maximum output power of 15 mW and a tuning range of ∼90 nm (1937-2026 nm). The low-loss performance of several fundamental Si3N4 building blocks for photonic integrated circuits is also validated. More specifically, the single-mode waveguide exhibits a transmission loss as low as 0.15 dB/cm, the 90° bend has 0.008 dB loss, and the 50/50 Y-branch has an insertion loss of 0.075 dB.

Hybrid silicon photonics DBR laser based on flip-chip integration of GaSb amplifiers and µm-scale SOI waveguides.

The development of integrated photonics experiences an unprecedented growth dynamic, owing to accelerated penetration to new applications. This leads to new requirements in terms of functionality, with the most obvious feature being the increased need for wavelength versatility. To this end, we demonstrate for the first time the flip-chip integration of a GaSb semiconductor optical amplifier with a silicon photonic circuit, addressing the transition of photonic integration technology towards mid-IR wavelengths. In particular, an on-chip hybrid DBR laser emitting in the 2 µm region with an output power of 6 mW at room temperature is demonstrated. Wavelength locking was achieved employing a grating realized using 3 µm thick silicon-on-insulator (SOI) technology. The SOI waveguides exhibit strong mode confinement and low losses, as well as excellent mode matching with GaSb optoelectronic chips ensuring low loss coupling. These narrow line-width laser diodes with an on-chip extended cavity can generate a continuous-wave output power of more than 1 mW even when operated at an elevated temperature of 45°C. The demonstration opens an attractive perspective for the on-chip silicon photonics integration of GaSb gain chips, enabling the development of PICs in a broad spectral range extending from 1.8 µm to beyond 3 µm.

Cathodoluminescence mapping of electron concentration in MBE-grown GaAs:Te nanowires.

Cathodoluminescence mapping is used as a contactless method to probe the electron concentration gradient of Te-doped GaAs nanowires. The room temperature and low temperature (10 K) cathodoluminescence analysis method previously developed for GaAs:Si is first validated on five GaAs:Te thin film samples, before extending it to the two GaAs:Te NW samples. We evidence an electron concentration gradient ranging from below 1 × 1018cm-3to 3.3 ×1018cm-3along the axis of a GaAs:Te nanowire grown at 640 °C, and a homogeneous electron concentration of around 6-8 × 1017cm-3along the axis of a GaAs:Te nanowire grown at 620 °C. The differences in the electron concentration levels and gradients between the two nanowires is attributed to different Te incorporation efficiencies by vapor-solid and vapor-liquid-solid processes.

Broadband optical spin dependent reflection in self-assembled GaAs-based nanowires asymmetrically hybridized with Au.

Hybridization of semiconductor nanostructures with asymmetric metallic layers offers new paths to circular polarization control and chiral properties. Here we study, both experimentally and numerically, chiral properties of GaAs-based nanowires (NWs) which have two out of six sidewalls covered by Au. Sparse ensembles of vertical, free-standing NWs were fabricated by means of lithography-free self-assembled technique on Si substrates and subsequently covered by Au using tilted evaporation. We report on optical spin-dependent specular reflection in the 680-1000 nm spectral range when the orientation of the golden layers follows the rule of extrinsic chirality. The analysis shows reflection peaks of the chiral medium whose intensity is dependent on the light handedness. We further propose a novel, time-efficient numerical method that enables a better insight into the far-field intensity and distribution of the scattered light from a sparse NW ensembles. The measurements done on three different samples in various orientations show good agreement with theoretical predictions over a broad wavelength range.

GaAs surface passivation for InAs/GaAs quantum dot based nanophotonic devices.

Several passivation techniques are developed and compared in terms of their ability to preserve the optical properties of close-to-surface InAs/GaAs quantum dots (QDs). In particular, the influence of N-passivation by hydrazine chemical treatment, N-passivation by hydrazine followed by atomic layer deposition (ALD) of AlO x and use of AlN x deposited by plasma-enhanced ALD are reported. The effectiveness of the passivation is benchmarked by measuring the emission linewidths and decay rates of photo-carriers for the near-surface QDs. All three passivation mechanisms resulted in reducing the oxidation of Ga and As atoms at the GaAs surface and consequently in enhancing the room-temperature photoluminescence (PL) intensity. However, long-term stability of the passivation effect is exhibited only by the hydrazine + AlO x process and more significantly by the AlN x method. Moreover, in contrast to the results obtained from hydrazine-based methods, the AlN x passivation strongly reduces the spectral diffusion of the QD exciton lines caused by charge fluctuations at the GaAs surface. The AlN x passivation is found to reduce the surface recombination velocity by three orders of magnitude (corresponding to an increase of room-temperature PL signal by ∼1030 times). The reduction of surface recombination velocity is demonstrated on surface-sensitive GaAs (100) and the passivating effect is stable for more than one year. This effective method of passivation, coupled with its stability in time, is extremely promising for practical device applications such as quantum light sources based on InAs/GaAs QDs positioned in small-volume photonic cavities and hence in the proximity of GaAs-air interface.

The role of As species in self-catalyzed growth of GaAs and GaAsSb nanowires.

Precise control and broad tunability of the growth parameters are essential in engineering the optical and electrical properties of semiconductor nanowires (NWs) to make them suitable for practical applications. To this end, we report the effect of As species, namely As2 and As4, on the growth of self-catalyzed GaAs based NWs. The role of As species is further studied in the presence of Te as n-type dopant in GaAs NWs and Sb as an additional group V element to form GaAsSb NWs. Using As4 enhances the growth of NWs in the axial direction over a wide range of growth parameters and diminishes the tendency of Te and Sb to reduce the NW aspect ratio. By extending the axial growth parameter window, As4 allows growth of GaAsSb NWs with up to 47% in Sb composition. On the other hand, As2 favors sidewall growth which enhances the growth in the radial direction. Thus, the selection of As species is critical for tuning not only the NW dimensions, but also the incorporation mechanisms of dopants and ternary elements. Moreover, the commonly observed dependence of twinning on Te and Sb remains unaffected by the As species. By exploiting the extended growth window associated with the use of As4, enhanced Sb incorporation and optical emission up to 1400 nm wavelength range is demonstrated. This wavelength corresponds to the telecom E-band, which opens new prospects for this NW material system in future telecom applications while simultaneously enabling their integration to the silicon photonics platform.

Circular Dichroism in the Second Harmonic Field Evidenced by Asymmetric Au Coated GaAs Nanowires.

Optical circular dichroism (CD) is an important phenomenon in nanophotonics, that addresses top level applications such as circular polarized photon generation in optics, enantiomeric recognition in biophotonics and so on. Chiral nanostructures can lead to high CD, but the fabrication process usually requires a large effort, and extrinsic chiral samples can be produced by simpler techniques. Glancing angle deposition of gold on GaAs nanowires can (NWs) induces a symmetry breaking that leads to an optical CD response that mimics chiral behavior. The GaAs NWs have been fabricated by a self-catalyzed, bottom-up approach, leading to large surfaces and high-quality samples at a relatively low cost. Here, we investigate the second harmonic generation circular dichroism (SHG-CD) signal on GaAs nanowires partially covered with Au. SHG is a nonlinear process of even order, and thus extremely sensitive to symmetry breaking. Therefore, the visibility of the signal is very high when the fabricated samples present resonances at first and second harmonic frequencies (i.e., 800 and 400 nm, in our case).

Optimization of Ohmic Contacts to p-GaAs Nanowires.

The performance of Ohmic contacts applied to semiconductor nanowires (NWs) is an important aspect for enabling their use in electronic or optoelectronic devices. Due to the small dimensions and specific surface orientation of NWs, the standard processing technology widely developed for planar heterostructures cannot be directly applied. Here, we report on the fabrication and optimization of Pt/Ti/Pt/Au Ohmic contacts for p-type GaAs nanowires grown by molecular beam epitaxy. The devices were characterized by current-voltage (IV) measurements. The linearity of the IV characteristics curves of individual nanowires was optimized by adjusting the layout of the contact metal layers, the surface treatment prior to metal evaporation, and post-processing thermal annealing. Our results reveal that the contact resistance is remarkably decreased when a Pt layer is deposited on the GaAs nanowire prior to the traditional Ti/Pt/Au multilayer layout used for p-type planar GaAs. These findings are explained by an improved quality of the metal-GaAs interface, which was evidenced by grazing incidence X-ray diffraction measurements in similar metallic thin films deposited on GaAs (110) substrates. In particular, we show that Ti exhibits low degree of crystallinity when deposited on GaAs (110) surface which directly affects the contact resistance of the NW devices. The deposition of a thin Pt layer on the NWs prior to Ti/Pt/Au results in a 95% decrease in the total electrical resistance of Be-doped GaAs NWs which is associated to the higher degree of crystallinity of Pt than Ti when deposited directly on GaAs (110).

Gradients of Be-dopant concentration in self-catalyzed GaAs nanowires.

Effective and controllable doping is instrumental for enabling the use of III-V semiconductor nanowires (NWs) in practical electronics and optoelectronics applications. To this end, dopants are incorporated during self-catalyzed growth via vapor-liquid-solid mechanism through the catalyst droplet or by vapor-solid mechanism of the sidewall growth. The interplay of these mechanisms together with the competition between axial elongation and radial growth of NWs can result in dopant concentration gradients along the NW axis. Here, we report an investigation of Be-doped p-type GaAs NWs grown by the self-catalyzed method on lithography-free Si/SiO x templates. The influence of dopant incorporation on the structural properties of the NWs is analyzed by scanning and transmission electron microscopy. By combining spatially resolved Raman spectroscopy and transport characterization, we are able to estimate the carrier concentration, mobility and resistivity on single-NW level. We show that Be dopants are incorporated predominantly by vapor-solid mechanism for low Be flux, while the relative contribution of vapor-liquid-solid incorporation is increased for higher Be flux, resulting in axial dopant gradients that depend on the nominal doping level.

Deterministic Switching of the Growth Direction of Self-Catalyzed GaAs Nanowires.

The typical vapor-liquid-solid growth of nanowires is restricted to a vertical one-dimensional geometry, while there is a broad interest for more complex structures in the context of electronics and photonics applications. Controllable switching of the nanowire growth direction opens up new horizons in the bottom-up engineering of self-assembled nanostructures, for example, to fabricate interconnected nanowires used for quantum transport measurements. In this work, we demonstrate a robust and highly controllable method for deterministic switching of the growth direction of self-catalyzed GaAs nanowires. The method is based on the modification of the droplet-nanowire interface in the annealing stage without any fluxes and subsequent growth in the horizontal direction by a twin-mediated mechanism with indications of a novel type of interface oscillations. A 100% yield of switching the nanowire growth direction from vertical to horizontal is achieved by systematically optimizing the growth parameters. A kinetic model describing the competition of different interface structures is introduced to explain the switching mechanism and the related nanowire geometries. The model also predicts that the growth of similar structures is possible for all vapor-liquid-solid nanowires with commonly observed truncated facets at the growth interface.

Field Emission from Self-Catalyzed GaAs Nanowires.

We report observations of field emission from self-catalyzed GaAs nanowires grown on Si (111). The measurements were taken inside a scanning electron microscope chamber with a nano-controlled tungsten tip functioning as anode. Experimental data were analyzed in the framework of the Fowler-Nordheim theory. We demonstrate stable current up to 10-7 A emitted from the tip of single nanowire, with a field enhancement factor ß of up to 112 at anode-cathode distance d = 350 nm. A linear dependence of ß on the anode-cathode distance was found. We also show that the presence of a Ga catalyst droplet suppresses the emission of current from the nanowire tip. This allowed for the detection of field emission from the nanowire sidewalls, which occurred with a reduced field enhancement factor and stability. This study further extends GaAs technology to vacuum electronics applications.

Sub-Poissonian Narrowing of Length Distributions Realized in Ga-Catalyzed GaAs Nanowires.

Herein, we present experimental data on the record length uniformity within the ensembles of semiconductor nanowires. The length distributions of Ga-catalyzed GaAs nanowires obtained by cost-effective lithography-free technique on silicon substrates systematically feature a pronounced sub-Poissonian character. For example, nanowires with the mean length ⟨L⟩ of 2480 nm show a length distribution variance of only 367 nm2, which is more than twice smaller than the Poisson variance h⟨L⟩ of 808 nm2 for this mean length (with h = 0.326 nm as the height of GaAs monolayer). For 5125 nm mean length, the measured variance is 1200 nm2 against 1671 nm2 for Poisson distribution. A supporting model to explain the experimental findings is proposed. We speculate that the fluctuation-induced broadening of the length distribution is suppressed by nucleation antibunching, the effect which is commonly observed in individual vapor-liquid-solid nanowires but has never been seen for their ensembles. Without kinetic fluctuations, the two remaining effects contributing to the length distribution width are the nucleation randomness for nanowires emerging from the substrate and the shadowing effect on long enough nanowires. This explains an interesting time evolution of the variance that saturates after a short incubation stage but then starts increasing again due to shadowing, remaining, however, smaller than the Poisson value for a sufficiently long time.

Photo-acoustic spectroscopy revealing resonant absorption of self-assembled GaAs-based nanowires.

III-V semiconductors nanowires (NW) have recently attracted a significant interest for their potential application in the development of high efficiency, highly-integrated photonic devices and in particular for the possibility to integrate direct bandgap materials with silicon-based devices. Here we report the absorbance properties of GaAs-AlGaAs-GaAs core-shell-supershell NWs using photo-acoustic spectroscopy (PAS) measurements in the spectral range from 300 nm to 1100 nm wavelengths. The NWs were fabricated by self-catalyzed growth on Si substrates and their dimensions (length ~5 µm, diameter ~140-150 nm) allow for the coupling of the incident light to the guided modes in near-infrared (IR) part of the spectrum. This coupling results in resonant absorption peaks in the visible and near IR clearly evidenced by PAS. The analysis reveal broadening of the resonant absorption peaks arising from the NW size distribution and the interaction with other NWs. The results show that the PAS technique, directly providing scattering independent absorption spectra, is a very useful tool for the characterization and investigation of vertical NWs as well as for the design of NW ensembles for photonic applications, such as Si-integrated light sources, solar cells, and wavelength dependent photodetectors.

Structural Investigation of Uniform Ensembles of Self-Catalyzed GaAs Nanowires Fabricated by a Lithography-Free Technique.

Structural analysis of self-catalyzed GaAs nanowires (NWs) grown on lithography-free oxide patterns is described with insight on their growth kinetics. Statistical analysis of templates and NWs in different phases of the growth reveals extremely high-dimensional uniformity due to a combination of uniform nucleation sites, lack of secondary nucleation of NWs, and self-regulated growth under the effect of nucleation antibunching. Consequently, we observed the first evidence of sub-Poissonian GaAs NW length distributions. The high phase purity of the NWs is demonstrated using complementary transmission electron microscopy (TEM) and high-resolution X-ray diffractometry (HR-XRD). It is also shown that, while NWs are to a large extent defect-free with up to 2-µm-long twin-free zincblende segments, low-temperature micro-photoluminescence spectroscopy reveals that the proportion of structurally disordered sections can be detected from their spectral properties.