Amplitude Of Excitation Signal Research Articles

General Theory of Microscopic Dynamical Response in Surface Probe Microscopy: From Imaging to Dissipation

We present a general theory of atomistic dynamical response in surface probe microscopy when two solid surfaces move with respect to each other in close proximity, when atomic instabilities are likely to occur. These instabilities result in a bistable potential energy surface, leading to temperature dependent atomic scale topography and damping (dissipation) images. The theory is illustrated on noncontact atomic force microscopy and enables us to calculate, on the same footing, both the frequency shift and the excitation signal amplitude for tip oscillations. We show, using atomistic simulations, how dissipation occurs through reversible jumps of a surface atom between the minima when a tip is close to the surface, resulting in dissipated energies of 1.6 eV. We also demonstrate that atomic instabilities lead to jumps in the frequency shift that are smoothed out with increasing temperature.

Hyrogenated Amorphous Silicon with High Growth Rate, Gas Utilization and Homogeneous Deposition by Amplitude Modulated Vhf-Pecvd for Solar Cell Application

AbstractHydrogenated amorphous silicon samples have been deposited by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD), using a square-wave amplitudemodulated radio-frequency excitation. It is observed that the gas-utilization efficiency improves by a considerable amount when amplitude modulation is combined with a reduction in the gas flows. Using a conventional continuous wave (cw) 50 MHz plasma with SiH4 and H2 gas flows of 30 sccm each at a pressure of 0.2 mbar, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s; which is twice as large as compared to the deposition rate of a cw deposition. Device-quality opto-electronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value around 0.02. The materials exhibited a low defect density (CPM) which is in the order of 3-8x1015 per cubic centimeter and photo-to-dark-conductivity ratio of 4-6x106. N-i-p solar cells of size 0.16 cm2 deposited on 10cmx10cm stainless steel (SS) substrate in the configuration SS/n-a-Si:H/i-a- Si:H/buffer/p-μc-Si/ITO/Ag grid (without back reflector) using amorphous silicon i-layer made by amplitude-modulated VHF plasma CVD showed an efficiency of 6.5%. This is a similar efficiency to the cell with standard device-quality cw a-Si:H in the same n-i-p structure, but at a high growth of 0.55 nm/s and gas utilization of ∼50%.

An Optical Method for Measuring Drop Flight Stability in a Continuous Ink Jet

This paper describes an optical method for improved drop velocity stability measurement to be used in continuous ink jet printing applications. Stable drop formation is demanded in continuous ink jet printing and it is normally achieved by introducing mechanical vibrations from a piezoelectric crystal onto the jet emerging from a nozzle. The method in use today to obtain information about the stability of drop velocity is to view the drops in stroboscopic light. This method does not provide quantified information about the level of drop velocity stability and the roughness of the method makes comparison between different levels of stability subjective and hence difficult. In our method we illuminate the drop train with a continuous HeNe-laser to create a shadow image of the drops. This image is magnified through a microscope and projected onto the light sensitive area of a PIN photodiode-based detector the output of which is sampled by a digitizing oscilloscope. The sampled data is used to calculate the standard deviation of time between drops and this value is used as a measure of drop velocity stability. Our method is primarily developed to measure the stability of drop velocity of drops with a diameter of 15 mm at crystal excitation frequencies in the interval of 800 to 1400 kHz. However, the set-up can easily measure drop velocity stability for different sizes of drops by simply changing the magnification of the microscope. Measurements with our method show that an increased excitation signal amplitude will result in a higher level of stability. The drop velocity is to a great extent decreased by air resistance as the drops travel. The presence of good and poor stimulation frequencies for nozzle systems is shown, and the frequencies are indicated by low and high levels of standard deviation for time between drops.