Terahertz Emission of Gallium Arsenide on Textured p-type Silicon (100) Substrates Grown via Molecular Beam Epitaxy

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. Reflectivity measurements for the textured p-Si(100) substrates indicated that the overall texturing increases with longer etching times. Raman spectroscopy of the post-growth samples confirmed GaAs growth. The THz emission intensities were the same order of magnitude. The GaAs grown on p-Si(100) textured for 60 minutes exhibited the most intense THz emission attributed to the increased absorption from a larger surface-to-volume ratio due to surface texturing. All GaAs on textured p-Si(100) samples had frequency bandwidth of ~2.5 THz

The effect of arsenic on MBE-grown modulation-doped GaAs/AlGaAs heterostructures

In this paper, the effect of As on the mobility and junction electric field of modulation-doped GaAs/AlGaAs heterostructures were investigated by varying the As flux during the MBE growth of the samples. Hall measurements using the van der Pauw configuration determined the carrier concentration and Hall mobility. The carrier concentration was observed to increase with As flux. The room temperature mobilities of the samples tend to decrease, while the 77 K mobilities increase with increasing As flux. Photoreflectance spectroscopy was utilized to derive the junction electric field. These values were observed to be higher than the electric field values calculated from Hall measurements for the samples grown at relatively lower As fluxes. This may be the effect of electron traps due to As vacancies coupled to C impurities. A remarkable improvement in the 77 K mobility of the sample grown after baking the substrate holder was observed. The 10 K mobility however was low compared to the benchmark set by other groups

Enhanced terahertz emission and Raman signal from silicon nanopyramids

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 potassium hydroxide (KOH) solution. The formation of nanopyramidal structures was verified using Scanning Electron Microscopy (SEM). Enhanced Raman and THz signals were observed from SiNPys compared to un etched silicon surface. The enhancement of Raman signal in SiNPys is ascribed to the enhanced photon absorption from efficient light trapping effect of the nanopyramids. Moreover, broadening of the Raman peaks was observed indicating an amorphous-like structure with prolonged etching. The enhancement of THz signal in SiNPys is ascribed to increased transient current on the nanopyramids\u27 surface. The maximum enhancement for Raman and THz signals was found for SiNPys formed after 30 mins etching. Further etching beyond 30 mins resulted in weaker Raman and THz signals. Results suggest strong correlation between the THz emission and Raman scattering of SiNPy\u27s. This correlation may be understood from the vibrational mode dependence of both Raman scattering and THz emission

Growth of anatase titanium dioxide nanotubes via anodization

In this work, titanium dioxide nanotubes were grown via anodization of sputtered titanium thin films using different anodization parameters in order to formulate a method of producing long anatase titanium dioxide nanotubes intended for solar cell applications. The morphological features of the nanotubes grown via anodization were explored using a Philips XL30 Field Emission Scanning Electron Microscope. Furthermore, the grown nanotubes were also subjected to X-ray diffraction and Raman spectroscopy in order to investigate the effect of the predominant crystal orientation of the parent titanium thin film on the crystal phase of the nanotubes. After optimizing the anodization parameters, nanotubes with anatase TiO2 crystal phase and tube length more than 2 microns was produced from parent titanium thin films with predominant Ti(010) crystal orientation and using ammonium fluoride in ethylene glycol as an electrolyte with a working voltage equal to 60V during 1-hour anodization runs

Optimization of anodized aluminum oxide pore morphology for GaAs nanowire growth

Anodic Aluminum oxide films were produced by anodization of sputtered Aluminum thin films on Silicon substrates. The effects of anodization voltage and aqueous oxalic acid solution on the pore diameter and interpore distance were studied. Parameters were sequentially varied to optimize the pore uniformity. Pore morphology was most uniform at 40V anodization voltage and 0.3M solution concentration. Average pore diameter and interpore distance for these parameters are 26.14nm ± 13% and 74.62 ± 8%, respectively. Pore diameter uniformity was further improved by etching with phosphoric acid solution. The AAO films were also successfully used to pattern gold nanoparticle catalysts for the synthesis of semiconductor nanowires

Terahertz emission from CuO nanowires synthesized through thermal oxidation of Cu foils

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 grains at 300 and 350 °C to the formation of NWs at 400 and 450 °C. The NWs were seen to have densities and dimensions that vary depending on the oxidation temperature. X-ray diffractometry showed that the grains that form at 300 and 350 °C were composed of a mixed phase of Cu2O and CuO, while the CuO NWs were observed to crystallize at temperatures greater than 400 °C. THz time domain spectroscopy (THz-TDS) showed that the foils containing CuO NWs were observed to emit THz radiation. It was further shown that increasing the density and dimensions of the NWs enhances the transient photocurrent generated throughout the length of the NWs, resulting in a stronger THz emission

Interruption-assisted epitaxy of faceted p-InAs on buffered GaSb for terahertz emitters

We demonstrate molecular beam epitaxy growth of p-InAs layers on GaAs-buffered GaSb that may be suitable for terahertz applications. GaAs buffer deposition is initiated by applying growth interruption. Reflection high-energy electron diffraction shows that GaAs growth proceeds to a quasi-two-dimensional growth mode. The scheme allows growth of a p-InAs layer 600nm to 1.0 μm thick. Growth performed without GaAs and growth interruption resulted in decomposition of the p-InAs. When the scheme is used, the ensuing p-InAs first follows quasi-two-dimensional growth before favoring faceted islanding. Under 800-nm-wavelength femtosecond laser excitation, the p-InAs layer generates terahertz signals 70% of that of bulk p-InAs. © 2015 The Japan Society of Applied Physics

Photocarrier transport and carrier recombination efficiency in vertically aligned nanowire arrays synthesized via metal-assisted chemical etching

The carrier dynamics and recombination characteristics of vertically aligned silicon nanowires are investigated using terahertz emission and photoluminescence spectroscopy, respectively. It is observed that the presence of pores on the walls in two-step-synthesized silicon nanowires greatly affects the carrier dynamics, compared with nanowires synthesized using a one-step process. These pores become efficient carrier recombination sites wherein carriers are collected upon photoexcitation. Additionally, pores effectively diminish the surface electric field thereby inhibiting the terahertz emission. Finally, nanowire-length-dependent terahertz emission is observed only for the one-step-synthesized nanowires whereas the two-step-synthesized nanowire samples exhibited length dependence of their photoluminescence intensity

An electronic nose using a single graphene FET and machine learning for water, methanol, and ethanol.

The poor gas selectivity problem has been a long-standing issue for miniaturized chemical-resistor gas sensors. The electronic nose (e-nose) was proposed in the 1980s to tackle the selectivity issue, but it required top-down chemical functionalization processes to deposit multiple functional materials. Here, we report a novel gas-sensing scheme using a single graphene field-effect transistor (GFET) and machine learning to realize gas selectivity under particular conditions by combining the unique properties of the GFET and e-nose concept. Instead of using multiple functional materials, the gas-sensing conductivity profiles of a GFET are recorded and decoupled into four distinctive physical properties and projected onto a feature space as 4D output vectors and classified to differentiated target gases by using machine-learning analyses. Our single-GFET approach coupled with trained pattern recognition algorithms was able to classify water, methanol, and ethanol vapors with high accuracy quantitatively when they were tested individually. Furthermore, the gas-sensing patterns of methanol were qualitatively distinguished from those of water vapor in a binary mixture condition, suggesting that the proposed scheme is capable of differentiating a gas from the realistic scenario of an ambient environment with background humidity. As such, this work offers a new class of gas-sensing schemes using a single GFET without multiple functional materials toward miniaturized e-noses