Rolled-Up Nanotech: Illumination-Controlled Hydrofluoric Acid Etching of AlAs Sacrificial Layers

<p>Abstract</p> <p>The effect of illumination on the hydrofluoric acid etching of AlAs sacrificial layers with systematically varied thicknesses in order to release and roll up InGaAs/GaAs bilayers was studied. For thicknesses of AlAs below 10 nm, there were two etching regimes for the area under illumination: one at low illumination intensities, in which the etching and releasing proceeds as expected and one at higher intensities in which the etching and any releasing are completely suppressed. The &#8220;etch suppression&#8221; area is well defined by the illumination spot, a feature that can be used to create heterogeneously etched regions with a high degree of control, shown here on patterned samples. Together with the studied self-limitation effect, the technique offers a way to determine the position of rolled-up micro- and nanotubes independently from the predefined lithographic pattern.</p

Thickness dependent electrical properties of CdO thin films prepared by spray pyrolysis method

A large number of thin films of cadmium oxide have been prepared on glass substrates by spray pyrolysis method. The prepared films have uniform thickness varying from 200-600 nm and good adherence to the glass substrate. A systematic study has been made on the influence of thickness on resistivity, sheet resistance, carrier concentration and mobility of the films. The resistivity, sheet resistance, carrier concentration and mobility values varied from 1.56-5.72 à 10-3 Ω-cm, 128-189 Ω/â¡, 1.6-3.9 à 1021 cm-3 and 0.3-3 cm2/Vs, respectively for varying film thicknesses. A systematic increase in mobility with grain size clearly indicates the reduction of overall scattering of charge carriers at the grain boundaries. The large concentration of charge carriers and low mobility values have been attributed to the presence of Cd as an impurity in CdO microcrystallites. Using the optical transmission data, the band gap was estimated and found to vary from 2.20-2.42 eV. These films have transmittance around 77 and average reflectance is below 2.6 in the spectral range 350-850 nm. The films are n-type and polycrystalline in nature. SEM micrographs of the CdO films were taken and the films exhibit clear grains and grain boundary formation at a substrate temperature as low as 523 K

Temperature and excitation power dependence of bi-excitonic luminescence in InGaAs/GaAs multiple quantum wells

A six period InGaAs/GaAs multiple quantum wells were grown with a fixed indium mole fraction of 0.1 during growth at different temperatures (480 - 600°C) using MBE. Thus grown samples were studied using Fourier transform photoluminescence spectroscopy. Five luminescence bands were observed at 1.404 eV, 1.411 eV, 1.440 eV, and 1.476 eV. The transitions at 1.440 eV and 1.476 eV found to have some dynamic behaviour with respect to the excitation intensity. Their relative intensities were observed as a function of excitation power and found to have linear dependence, while their associated transitions have almost constant behaviour. These transitions were related to excitonic and bi-excitonic molecules. The appearance of bi-excitons in only two quantum wells indicates the uniform distribution of indium and smooth interfaces in two quantum wells grown at relatively higher temperatures (>560°C). From these results it was inferred that the alloy disorder and uneven interfaces could forbid the bi-excitonic transitions that indicate a perfect crystal growth