Above: Multi frequency capacitance
Nanocrystalline Si:H is a fascinating new electronic material. The Si grains are small, ~10-20 nm, and the grain-boundaries are coated with H and very thin a-Si:H. The H distribution in the material can be estimated using Molecular Dynamic simulations, and is shown in Fig. 1. The simulations clearly show that most of the hydrogen is at the grain boundaries, with nothing inside the nano-grain. The presence of H and a-Si:H tissue at the grain boundaries passivates these boundaries, and allows for efficient transfer of minority and majority carriers across the grain. As a result, hole lifetimes of the order of microsecond can be obtained even in this nanocrystallione material. Electron mobilities are of the order of 30 cm2/V-sec and hole mobilities ~ 10 cm2/V-sec in as-grown materials, orders of magnitude higher than in amorphous materials. The hole diffusion lengths can be in excess of 1 micrometer. See Fig. 2. The nanocrystalline Si material is fundamentally stable, as opposed to amorphous materials. These outstanding properties allow one to make useful devices like solar cells and thin film transistors on plastic substrates in this material.
In addition to Si, one can also grow nano (Si,Ge):H, nano (Si,C):H and nano-(Ge,C):H materials. This versatility allows one to visualize many different classes of electo-optic devices in this new material system.
The materials are grown using plasma enhanced CVD in a reactive plasma system. Both VHF and ECR plasmas are used. Typical Raman spectra of the materials are shown in Fig.3, and x-ray diffraction in Fig. 4. Solar cell and TFT devices are being fabricated in these material systems.
Fig. 1 Hydrogen distribution in nanocrystalline Si:H modeled using Molecular Dynamic simulations
Fig. 2 Diffusion length measurement in nanocrystalline Si:H solar cells
Fig. 3 Raman spectrum of nanocrystalline Si film deposited using low pressure VHF plasma
Fig. 4 x-ray diffraction spectrum of nanocrystalline Si:H deposited on stainless steel substrate, showing the <111> peak dominating. The sharp peak at 45° is due to iron from the substrate.
Spectroscopic gas sensors have greater sensitivity and stability than conventional electrochemical sensors. Spectroscopic sensors of toxic gases rely on the fact that each gas has a unique absorption line in the infrared portion of the spectrum, arising from the molecular stretching or rotational modes. For example CO has a sharp absorption at 4.7 m, NO absorbs at 5.3 m whereas nerve gases and toxic serin have absorption features near 10 m. Spectroscopic infrared gas sensors offer very high sensitivity for conclusive detection of individual species since each gas has unique absorption lines in the infrared spectrum. Spectroscopic sensors are lightweight, battery-powered, low-maintenance and low-cost- essential attributes for counter-terrorism applications.
Faculty
Rana Biswas, Vikram Dalal
Staff
Max Noack, Ruth Shinar
Students
Durga Panda, Dan Pates, Puneet Sharma, Dan Stieler, Nanlin Wang
Papers
Funding Agencies
Above: ECR Plasma Reactor
