Eero Koivusalo

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.

Optical Energy Transfer and Loss Mechanisms in Coupled Intracavity Light Emitters

Despite the near-unity internal quantum efficiencies (IQEs) demonstrated for GaAs-based light emitters, laser cooling of the ubiquitous III-V semiconductors has not been feasible. The key challenges for III-V optical cooling are the reduced absorption of optical excitation at photon energies well below the bandgap and the strong confinement of light in the high refractive index semiconductors. Here, we investigate the possibility to eliminate the need for light extraction and to eventually relax the requirements of the IQE. This is done using electroluminescence and optical energy transfer within intracavity devices consisting of an AlGaAs/GaAs double heterojunction light emitting diodes and a GaAs p-n-homojunction photodiode enclosed within a single semiconductor cavity. We measure the intracavity energy transfer, i.e., the coupling quantum efficiency (CQE) between the two diodes and estimate loss mechanisms by simultaneously measuring the IV characteristics of the emitter diode and the photocurrent of the absorber diode. The measured CQE of our devices is below 60% due to the mirror, light extraction, nonradiative, and detection losses. While this is far below the state-of-the-art efficiencies, our results suggest that it will be possible to substantially improve the efficiency by adopting the fabrication and design principles used for the best performing photoluminescent emitters.

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 β of up to 112 at anode-cathode distance d = 350 nm. A linear dependence of β 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.

GaSb superluminescent diodes with broadband emission at 2.55 μm

We report the development of superluminescent diodes (SLDs) emitting mW-level output power in a broad spectrum centered at a wavelength of 2.55 μm. The emitting structure consists of two compressively strained GaInAsSb/GaSb-quantum wells placed within a lattice-matched AlGaAsSb waveguide. An average output power of more than 3 mW and a peak power of 38 mW are demonstrated at room temperature under pulsed operation. A cavity suppression element is used to prevent lasing at high current injection allowing emission in a broad spectrum with a full width at half maximum (FWHM) of 124 nm. The measured far-field of the SLD confirms a good beam quality at different currents. These devices open further development possibilities in the field of spectroscopy, enabling, for example, detection of complex molecules and mixtures of gases that manifest a complex absorption spectrum over a broad spectral range.

High power GaSb superluminescent diodes with broadband emission around 2.55 µm (Conference Presentation)

Most of the environmental gases like H2S, C2H2, CH4, CO(2), N2O and H2O have strong absorption lines within 2-3 µm wavelength range. Detection of these gases requires a spectrally broad, compact, efficient, cost-effective, and high output power light source. Such choice of parameters can be offered by high brightness and broadband superluminescent diodes (SLD). Here we report the development of GaSb-based high power broadband superluminescent diodes (SLDs) emitting around 2.55 μm. The active region consists of two GaInAsSb/GaSb quantum wells. To enable high gain and high output power we adopted a long ridge waveguide (RWG) geometry, i.e. 2.5 mm. The width (5µm) and etching depth (1800 nm) of waveguide was chosen to operate device in single transverse mode. Lasing inside the cavity was suppressed by tilting the waveguide 8° with respect to cavity facets. Recently developed cavity suppression element [1] was employed to further suppress the spectral modulations and smoothen out the spectrum. For operation at long wavelengths, we employed a pulsed driving scheme with sub-µs pulse injection to address temperature dependent non-radiative Auger recombination. Devices have demonstrated an average output power of more than 3 mW and peak power over 15 mW at room temperature (RT). The maximum full-width at half-maximum (FWHM) of spectrum was ~124 nm, corresponding to 1200 mA drive current. This is the highest power reported to date for 2.55 µm SLDs. For comparison, SLD at 1.90µm emitted a continuous wave (CW) output power up to 60 mW and FWHM of ~ 60 nm [1]. Integration of this high brightness, broadband light sources with SOI-waveguides enables realization of a compact multiple gas sensor in this wavelength range [2]. [1] N. Zia, J. Viheriala, R. Koskinen, A. Aho, S. Suomalainen, and M. Guina, “High power (60 mW) GaSb-based 1.9 μm superluminescent diode with cavity suppression element,” Appl. Phys. Lett., vol. 109, no. 23, p. 231102, 2016. [2] P. Karioja, T. Alajoki et al, “Multi-wavelength mid-IR light source for gas sensing”, Proc. SPIE 10110, 2017.

GaAs-based nanowires partially covered with gold give rise to optical circular dichroism

Hybridized nanostructures composed by metals and dielectrics, semiconductors or organics offer new opportunities achieving new functionalities in nonlinear optics, plasmonics, sensing [1-4]. In particular GaAs-AlGaAs-GaAs core-shell-supershell nanowires (NWs) fabricated by self-catalyzed growth on Si substrates were partially covered with gold, thus producing a symmetry breaking in the sample geometry that induces an “extrinsic” chiral response.

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.