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

Temperature and deposition time dependence of the geometrical properties of tin oxide nanostructures

Tin oxide nanomaterial was synthesized using the horizontal vapor phase growth (HVPG) technique. The study investigated the optimum growth parameters by varying the growth temperature from 900 C to 1200 C and growth time of 1 hour to 5 hours. The SnO2 bulk powder with purity rate of 99.99% were placed in a sealed quartz tube with a vacuum pressure of≈ 10-5 Torr and baked with the desired growth parameters. The resulting nanocrystals displayed different structures ranging from nanobelts to nanorods as confirmed by the SEM. Results from EDX and DTA showed that indeed the grown samples were congruent based on the atomic composition and thermal property of the nanomaterials. The XRD also verified that the crystal structure was rutile but with low indexed peaks. Using the same growth technique, samples were grown on Silicon (100) substrate and exhibited nanorods and nanobelts. The SnO2 nanomaterial also displayed fluorescence and photoluminescence signals. The photoluminescence spectrum has a broad emission in the visible region with peaks at 558 nm and 666 nm. The visible light emission was known to be related to defect levels within the band gap of SnO2, associated with O vacancies or Sn interstitials that have formed during the synthesis process

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

Fabrication and characterization of zinc oxide ultraviolet filter for solar cell applications

Zinc oxide (ZnO) thin films are grown and fabricated on cover slips using spray pyrolysis method with a concentration of 0.1M zinc acetate. Three samples were post- annealed at 400°C, 450°C and 500°C. An improved efficiency and a better responsivity were measured from silicon solar cells under concentrated light with a 0.1M ZnO UV filter compared to a non-filtered solar cell under one-sun reference light. The spectral response data of the Si solar cell provide solid evidence that energies greater than 3.26 eV (UV) are indeed absorbed by the ZnO UV filter and prevented these high energy light from reaching the solar cell. The optimal parameter for a UV filter is determined to be 0.1M 400°C ZnO film based from the Si solar cell higher efficiency of 0.38% compared to an unfiltered and non-concentrated Si solar cell

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