Armando Somintac

University of the Philippines | UPD · National Institute of Physics

We investigate strain effects on the ultrafast carrier dynamics and transport of gallium arsenide films on silicon (GaAs/Si) and magnesium oxide (GaAs/MgO) substrates using temperature-dependent photoluminescence (PL) and terahertz time-domain spectroscopy (THz-TDS) from 11 K - 300 K. The PL shows that GaAs/Si and GaAs/MgO samples are under tensile...

We study the transport of photogenerated carriers in molecular beam epitaxy (MBE)-grown gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) coupled (CDQW) and uncoupled (UDQW) double quantum wells. Photoluminescence (PL) spectroscopy was used to investigate the optical properties and establish differences in the tunneling properties between th...

We employ micro-Raman spectroscopy to optically infer the stress experienced by the legs of a bridge-type microelectromechanical systems (MEMS) used in high contrast gratings tunable vertical cavity surface emitting lasers (VCSELs). We then employ micro-photoluminescence (PL) spectroscopy to indirectly measure the air cavity displacement of the sam...

We report on the enhancement of the terahertz (THz) emission characteristics of gallium arsenide (GaAs)-based photoconductive antennas (PCA) upon coating with silicon nanowires (SiNWs). The SiNWs were fabricated using metal-assisted chemical etching and were dropcasted onto PCAs with dipole antenna patterns. The THz emission of the SiNW-coated PCA...

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (reference). In this study, we invest...

Spintronic THz emission from Ni/Pt bilayer grown on MgO is reported based on the novel THz emitter using metallic structures. The Ni metal was deposited first on a MgO substrate and capped with a thin Pt metal via electron beam deposition. The THz emission data was obtained using a standard terahertz time-domain spectroscopy setup using a Ti: sapph...

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates ("reference"). In this study, we inves...

Terahertz (THz) emission increase is observed for GaAs thin films that exhibit structural defects. The GaAs epilayers are grown by molecular beam epitaxy on exactly oriented Si (100) substrates at three different temperatures (T s = 320°C, 520°C and 630°C). The growth method involves the deposition of two low-temperature-grown (LTG)-GaAs buffers wi...

Terahertz (THz) emission increase is observed for GaAs thin films that exhibit structural defects. The GaAs epilayers are grown by molecular beam epitaxy on exactly oriented Si (100) substrates at three different temperatures ( T s = 320ºC, 520ºC and 630ºC). The growth method involves the deposition of two low-temperature-grown (LTG)-GaAs buffers w...

We report on the continuing efforts of the National Institute of Physics, University of the Philippines, in the design, growth, and fabrication of novel structures for THz photoconductive antennas (PCA’s) [1-3]. In particular, the talk presents recent works on the molecular beam epitaxial growth of gallium arsenide-based semiconductor trilayer film...

We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We comp...

We present threefold enhancement of terahertz emission from silicon nanowire (SiNW)-coated gallium-arsenide photoconductive antenna over its uncoated counterpart. The enhancement is attributed to the increased photoabsorption, and possibly additional photoconductive pathways induced by the SiNWs.

We report on the influence of chamber design on the gas sensing performance of a graphene field-effect-transistor (GFET). A conventional chamber (V = 400 ml) and a cap chamber (V = 1 ml), were used to perform dynamic measurements on a GFET. To gain a-priori knowledge on the gas flow in the chambers, Naiver–Stokes and convection-diffusion equations...

We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk characteristics using first-principles methods with experimental verifications. We found that the AsGa in the third-layer mimics the geometry, partial charge density and more importantly, the density of states of AsGa in bulk GaAs. Notably, the mid-gap state induc...

Carrier transport in semiconductors with different dimensionalities, i.e., 3D (bulk), 2D (QW), and 0D (QD), were investigated via temperature-dependent terahertz time-domain spectroscopy (THz-TDS). The optical properties and recombination dynamics in the samples were probed via photoluminescence spectroscopy. The temperature-dependence of the THz e...

Semiconductor interfaces are the backbone of modern optoelectronic devices. In terahertz (THz) science, the narrow region of an interface is crucial in the emission process. However, reports on the direct correlation of THz emission with local interface properties remain scarce owing to the inherent difficulty of using the same sample for nanoscale...

The photocarrier dynamics in molecular beam epitaxy (MBE)-grown single- (SLQD) and multi-layered InAs/GaAs quantum dots (MLQD) were studied. Photoluminescence (PL) spectroscopy have shown that the MLQD has more uniform QD size distribution as compared to the bimodal SLQD. Correlation between PL and THz-TDS have shown that photocarrier transport is...

We investigated the effects of different annealing temperatures on cupric oxide (CuO) films as supercapacitor electrode. CuO films were grown on stainless steel substrates via spray pyrolysis and annealed for two hours at different temperatures of 160 °C, 250 °C, and 400 °C. X-ray diffractometry (XRD) and micro-Raman spectroscopy (MRS) confirmed th...

Carrier dynamics and photoconductivity in epitaxial-grown low-temperature GaAs on nominal and vicinal Si(1 0 0) substrates ('LT-GaAs/Si') were studied to predict their actual performance as THz photoconductive antenna (PCA) detectors. An optical-pump terahertz-probe technique was used to obtain the transmittance, carrier lifetime and photoconductiv...

In this work, we demonstrate the NIP’s all in-house development of a vertical cavity surface emitting laser structure. The VCSEL structure grown via MBE consists of an AlAs/AlGaAs distributed Bragg reflector and an AlGaAs/GaAs quantum well designed to issue at the 850 nm region. Reflectance spectroscopy showed that the stop band is centered around...

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due...

The carrier lifetimes and electron mobility values were estimated for 2-μm-thick GaAs films grown on Si (100) substrates by means of optical pump terahertz probe (OPTP) technique. The GaAs/Si films measured were epitaxial grown at different substrate temperatures (TS = 520˚C or TS = 630˚C). From X-ray diffraction measurements and Raman spectroscopy...

This study presents the terahertz (THz) emission of molecular beam epitaxy (MBE)-grown Gallium Arsenide (GaAs) on surface textured p-type Silicon (p- Si) (100) substrates. Surface texturing was achieved by anisotropic wet chemical etching using 5% wt Potassium Hydroxide (KOH): Isopropyl alcohol (IPA) (50:1) solution for 15, 30, 45, and 60 minutes....

The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-GaAs grown at a much lower temperature (< \(300\,^{\circ }\)C), where the defect density is high, without compromising the spectral bandwidth and carrier lifetimes necessary for ultrafast THz detection. The LT-GaAs grown at \(220\) and \(270\,^{\circ }\)C...

The influence of crystal thickness of metal-coated <100>-cut GaAs (M-G-M) on Cherenkov-phase-matched terahertz (THz) pulse detection was studied. The M-G-M detectors were utilized in conjunction with a metallic tapered parallel-plate waveguide (TPPWG). Polarization-sensitive measurements were carried out to exemplify the efficacy of GaAs in detecti...

Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstrated. Results showed that high quality GaAs grown by molecular beam epitaxy with a thin n-doped buffer can rival existing intense bare semiconductor THz surface emitters. The incorporation of a 0.2 μm n-GaAs buffer proved effective in enhancing the THz e...

Photocarrier dynamics in GaAs/AlGaAs modulation-doped heterostructures (MDH) having i-AlGaAs spacer layers of different thicknesses were investigated using temperature-dependent terahertz time-domain spectroscopy (THz-TDS) and photoreflectance spectroscopy. In particular, results are discussed in the framework of the temperature dependence of the h...

We investigate the hydrothermal growth of zinc oxide (ZnO) nanorods to realize their potential scintillator applications. Vertically aligned ZnO nanorods were successfully fabricated on seeded silicon (Si) substrates at a relatively low temperature of 85 °C using different hexamethylenetetramine [(CH2)6N4, HMTA] and zinc acetate dihydrate [Zn(CH3CO...

We report the molecular beam epitaxy growth of high-quality p-InAs thin films evaluated in the context of 1.55 µm femtosecond laser-excited THz emission efficiency. The presence of p-InAs is confirmed via scanning electron microscopy and x-ray diffraction. Using a GaAs buffer layer, the epitaxial growth of p-InAs layers was successfully achieved. I...

Simultaneous molecular beam epitaxial growth of GaAs/AlGaAs multiple quantum wells on two different substrates (one on GaAs (100) and another on a GaAs substrate misoriented by 4° in the (111) direction) resulted in samples of similar structure, but having different defect profiles. The on-axis sample had a higher defect density and more types of e...

Two asymmetric double quantum wells of different coupling strengths (barrier widths) were grown via molecular beam epitaxy, both samples allowing tunneling. Photoluminescence was measured at 10 and 300 K to provide evidence of tunneling, barrier dependence, and structural uniformity. Carrier dynamics at room temperature was investigated by optical...

Transparent and conducting fluorine doped tin oxide (FTO) films were deposited on glass substrates at 300°C using nebulized spray pyrolysis. The fluorine concentration in the precursor solution was varied from 0 to 40 at.%. UV-Visible spectroscopy, Hall effect measurement, EDX, SEM and ToF-SIMS were employed to investigate the optoelectronic proper...

We have demonstrated experimental evidence of non-resonant tunneling at room temperature in GaAs/AlGaAs asymmetric coupled double quantum wells (ACDQW’s) using time resolved photoluminescence (TRPL) spectroscopy at 300 K. Two ACDQW samples (A and B) with a barrier thickness of 25Å were grown via molecular beam epitaxy. The energy separation (ΔE) be...

A unique and novel thin film of fluorine-doped tin oxide (SnO2:F) nanoflakes on ZnOnanorods were fabricated using nebulized spraypyrolysis technique (NSPT) and hydrothermal growth method, respectively, for ammonia and humidity sensing applications. XRD studies confirm the growth of a hexagonal wurtzite ZnO and a tetragonal SnO2:F. SEM images of the...

Porous silicon was fabricated through electrochemicaletching and is used as an optical template for liquid sensing application. Using reflectance spectroscopy, change in optical properties such as refractive index and reflectivity upon liquid introduction were obtained. Chromatic dispersion of porous silicon upon detection of transparent liquids su...

We report increased radiation in the visible and terahertz (THz) regimes in silicon(Si)-based nanostructures. The nanostructures, Si nanowires (SiNWs) and porous Si (PSi), were synthesized via electroless and electrochemical surface modification, respectively. In particular, picosecond (ps) radiative lifetimes in the order of 250 ps were obtained f...

The Raman scattering and Terahertz emission of silicon nanopyramids (SiNPys) formed at different etching times were investigated. Additionally, photoluminescence spectroscopy measurements were performed to investigate the recombination properties of SiNPys. The SiNPys were fabricated via wet chemical etching of heavily doped p-type silicon (100) in...

We demonstrate terahertz (THz) emission from cupric oxide nanowires (CuO NWs) synthesized through thermal oxidation of Cu foils in ambient air by heating the foils in a hotplate for two hours at 300, 350, 400, and 450 °C. Scanning electron microscopy revealed the changes in the morphology of the foils; from the formation of a film composed of grain...

We investigate the effects of an externally applied magnetic field on the terahertz (THz) emission of Gallium Manganese Arsenide (GaMnAs) films grown via molecular beam epitaxy (MBE). Results show that low Mn-doping in GaMnAs resulted to increased THz emission as compared with a SI-GaAs substrate. Further increase in Mn-doping content resulted to a...

Silicon nanowires (SiNWs) were etched on Si (100), (110), and (111) substrates. Slanted nanowires with respect to the surface normal were produced on the Si (110) and (111) substrates, and vertical nanowires were produced on the Si (100) substrate. Photoluminescence spectroscopy exhibited luminescence lines attributed to oxide defects from the nano...

We present the use of a “double optical pump” technique in terahertz time-domain emission spectroscopy as an alternative method to investigate the lifetime of photo-excited carriers in semiconductors. Compared to the commonly employed optical pump-probe transient photo-reflectance, this non-contact and room temperature characterization technique al...

Terahertz (THz) wave detection and emission via Cherenkov-phase-matched nonlinear optical effects at 1.55-μm optical wavelength were demonstrated using a GaAs with metal-coating (M-G-M) and bare GaAs as a reference sample in conjunction with a metallic tapered parallel-plate waveguide (TPPWG). The metal-coated GaAs is superior to the bare wafer bot...

A one-order-of-magnitude terahertz (THz) emission enhancement in the transmission geometry, over a 0.7-THz broadband range, was observed in semi-insulating gallium arsenide (SI-GaAs) integrated with a subwavelength one-dimensional metal line array (1DMLA). THz emission of the 1DMLA samples showed an intensity increase and exhibited a full-width-at-...

Porosity dependent terahertz emission of porous silicon (PSi) was studied. The PSi samples were fabricated via electrochemical etching of boron-doped (100) silicon in a solution containing 48% hydrofluoric acid, deionized water and absolute ethanol in a 1:3:4 volumetric ratio. The porosity was controlled by varying the supplied anodic current for e...

ZnO microrods are fabricated by a simple hydrothermal growth route using zinc acetate dihydrate [Zn(CH3COO)2·2H2O] and hexamethylenetetramine [(CH2)6N4] aqueous solutions. The as-prepared microrods exhibit uniform dimensions, well-faceted surfaces, and hexagonal crystal structure. The microrods also have an intense ultraviolet (UV) emission at 392...

The resistance-based pH sensing capability of ZnO nanorods was presented in this study. Interdigitated finger structures of nickel/gold (Ni/Au) electrodes were fabricated on the substrates prior to the sensing material. The effect of varying electrode widths was also considered. Zinc oxide (ZnO) film, as seed layer, was deposited via spray pyrolysi...

The application of a p–n homojunction based on zinc oxide (ZnO) nanorods as photodetector is presented in this study. The homojunctions were grown via chemical bath deposition for 6, 9, and 12 hours per layer of the junction. X-ray diffraction and scanning electron micrographs confirmed the material composition, structure, and morphology of the gro...

This study have investigated the structural and optical properties of porous silicon on silicon substrate and of free-standing porous silicon layers. Porous silicon samples were fabricated through electrochemical etching of boron doped silicon wafer in 12% HF solution. For pore formation, current densities of 1.875 mA/cm², 5 mA/cm², and 15 mA/cm²we...

GaAs/Al 0.1 Ga 0.9 As core-shell nanowires (CSNWs), with average lateral size of 125 nm, were grown on gold nanoparticle-activated Si (100) and Si (111) substrates via molecular beam epitaxy. Room temperature-photoluminescence (RT-PL) from the samples showed bulk-like GaAs and Al 0.1 Ga 0.9 As bandgap emission peaks at 1.43 and 1.56 eV, respectivel...

We report on the THz emission characteristics of an aluminum-doped zinc oxide (AZO)-nGaAs heterostructure observed via reflection-geometry THz time-domain spectroscopy. Our results show that upon deposition of AZO on nGaAs, the THz emission is stronger compared to the bare substrate. This increase is attributed to the stronger electric field at the...

The Cardona equation describing the temperature behavior of a bulk semiconductor is utilized to fit experimental photoluminescence data for both strained and unstrained quantum wells (QWs). To account for confinement energy and strain energy, the Cardona equation was modified to include the respective energy offsets. Results of modeling experimenta...

Increased terahertz (THz) emission of surface modified semi-insulating gallium arsenide (SI-GaAs) is reported. A metal line array with sub-wavelength spacing was fabricated via e-beam deposition and THz measurements were done in the transmission excitation geometry. Results show an order of magnitude THz broadband emission enhancement. Pump-power d...

We present experimental demonstration of photocarrier dynamics in InAs quantum dots (QDs) via terahertz (THz) time-domain spectroscopy (TDS) using two excitation wavelengths and observing the magnetic field polarity characteristics of the THz signal. The InAs QDs was grown using standard Stranski-Krastanow technique on semi-insulating GaAs substrat...