Sukti Hazra

Fabrication technology of a Si nanowire memory transistor using an inorganic electron beam resist process

We report on a novel fabrication technology of a Si nanowire memory transistor using an inorganic SiO2 electron beam (EB) resist process. The inorganic EB resist process technique was put to practical use in Si nanodevice fabrication for the first time. We have successfully demonstrated the 15-nm-wide and 20-nm-thick Si nanowire memory transistor with a Si nanodot less than 10 nm in diameter. In the fabricated Si nanowire nanodot memory transistor, we have observed a large electron memory effect, i.e., a threshold voltage shift ΔVth of 2.2 V at room temperature. It is experimentally shown that the inorganic EB resist process is promising for fabricating various Si nanodevices.

Formation of nanocrystallites governed by the initial stress in the ultrathin hydrogenated amorphous silicon films

Spectroscopic ellipsometry identified E1 transitions at a lower energy than that for c-Si (3.38 eV). These transitions are generated from the Si paracrystallites or disordered crystallites in the ultrathin silicon films (2–10 nm) deposited by thermal chemical vapor deposition. During the growth of the film, paracrystallites expand gradually; disorder in the paracrystallites increases. Finally, a completely disordered Si network, i.e., the amorphous network, is generated. The presence of disorder crystallites in the ultrathin Si films acts as a constraint for the crystallization of the ultrathin films by rapid thermal annealing.

Spectroscopic Ellipsometry Studies on Ultrathin Hydrogenated Amorphous Silicon Films Prepared by Thermal Chemical Vapor Deposition

Ultrathin amorphous silicon films prepared by the thermal chemical vapor deposition (CVD) have been investigated using a spectroscopic ellipsometer. The analysis of ultraviolet-visible (UV-VIS) spectroscopic ellipsometric data reveals the morphology of the ultrathin films. To determine the optical functions of such films from ellipsometric data, a new parameterization, i.e., the Sellmeier law with four Lorentz oscillators, has been successfully introduced. A direct correlation has been made between the new parameters and the change of optical functions with the thickness of ultrathin a-Si:H films. By the analysis of ellipsometric data, it has been shown that the formation of dense Si matrices with low structural disorder is possible when the thickness of a-Si:H is more than about 8 nm, and the film with a thickness of less than around 3.5 nm develops voids.

Photovoltaic Application of Nanomorph Silicon Thin Films Prepared by Plasma Enhanced Chemical Vapor Deposition

Nanomorph silicon (nanoa-Si) thin films have been developed from the SiH4/H2 plasma in high plasma power regime of plasma enhanced chemical vapor deposition (PECVD). Dark and photoconductivity of nanoa-Si films are of the order of 10-12 Scm-1 and 10-6 Scm-1 respectively. The average size of nanocrystallites, embedded in the amorphous matrix, is ~10 nm. Optical gap of such films is ≥1.80 eV. The photoluminescence has been observed with a wide peak around 1.6 eV. Unlike nanocrystalline silicon, these films are photosensitive. This nanoa-Si thin film may be a good alternative to common wide bandgap a-SiC:H which is an active layer of the top cell of multijunction solar cell.

Development of High Quality 1.36 eV Amorphous SiGe:H Alloy by RF Glow Discharge under Helium Dilution

The use of 1.35 eV amorphous silicon-germanium (a-SiGe:H) alloy as the second/third intrinsic layer along with 1.85 eV front layer in double/triple tandem solar cells is believed to be the best combination for the maximum power output for multijunction cells. In this study high quality low-band-gap (1.36 eV) a-SiGe:H alloy has been developed by RF glow discharge optimizing the deposition parameters and helium dilution of source gases. It has been observed that the structural, electronic properties and defect densities of alloy films developed under the deposition condition which is the transition from low-discharge-power to high-discharge-power regime, become optimum. In the present case this deposition condition is a combination of chamber pressure 0.8 Torr and RF power 60 mW/cm2. The properties of the alloy films developed under helium dilution improve and defect density decreases with the increase of deposition rate up to 120 ?/min. The 1.36 eV alloy film prepared under this condition has very low defect density ( 3.2?1016 cm-3 eV-1). The analysis of spectral response of Pd/a-SiGe:H Schottky barrier solar cells reveals that the hole transport properties improve due to increase of RF power from 15 to 60 mW/cm2 and also due to increase of growth rate from 51 to 120 ?/min.

Morphology and electronic transport of polycrystalline silicon films deposited by SiF4/H2 at a substrate temperature of 200 °C

Undoped and phosphorous doped polycrystalline silicon (poly-Si) films were deposited using a SiF4/H2 gas mixture at a substrate temperature of 200 °C by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD). Fourier transform infrared (FTIR) spectroscopy and x-ray diffraction (XRD) experiments reveal that the present poly-Si films are equivalent to the poly-Si films deposited at high temperature (>600 °C). XRD and scanning electron microscope observations show that the crystalline quality of slightly P-doped film is better compared to that of undoped poly-Si films. Phosphorus atom concentration in the slightly P-doped poly-Si film is 5.0×1016 atoms/cm3. Association of a few phosphorous atoms in the silicon matrix enhances crystallization as eutectic-forming metals do. Dark conductivity of slightly P-doped film is 4 orders of magnitude higher, although mobility–lifetime product (ημτ) is 2 orders of magnitude lower than that of undoped film. The presence of higher density of strained Si–Si bo...

Spectroscopic Ellipsometry for the Characterization of the Morphology of Ultra-thin Thermal CVD Amorphous and Nanocrystalline Silicon Thin Films

Ultra-thin hydrogenated amorphous silicon thin films have been deposited by thermal chemical vapor deposition (CVD) to prepare smooth top surface of the films avoiding the ion bombardment. Rapid thermal oxidation of thermal CVD a-Si:H results in nanocrystalline dots in the ultra-thin silicon films. Spectroscopic ellipsometry (SE) and high resolution transmission electron microscopy (TEM) have been used to investigate the optical and structural properties of both ultra-thin a-Si:H and nanocrystalline silicon films. To analyze the ellipsometric data of ultra-thin a-Si:H films, a new parameterization i.e., the combination of Sellmeier law and four Lorentz peaks, has been successfully introduced. Width of the Lorentz peaks are directly related with the change of optical functions with the thickness of a-Si:H films. It has been certified that the dense Si matrix with smaller degree of disorder is formed when the thickness exceeds 8nm and the films with the thickness of less than 3.8 nm becomes voided. To interpret the ellipsometric data for nanocrystalline silicon films, three layer model (SiO 2 , poly-Si+a-Si+void and SiO 2 ) has been adapted. It is inferred from SE and TEM analyses that the size and the density of nanocrystalline dots can be controlled by the morphology of initial ultra-thin a-Si:H films and RTO conditions.

Low bandgap a-Si:H film with better stability prepared by RF PECVD method using helium dilution

Low bandgap a-Si:H films have been prepared by RF PECVD method (13.56 MHz) using helium as diluent to silane gas. Helium dilution and chamber pressure play an important role in reducing the optical gap at substrate temperature /spl sim/210/spl deg/C. This highly photosensitive low bandgap material shows less light induced degradation compared to that for normal bandgap a-Si:H material. Single junction solar cells having efficiency 7.2% (1 cm/sup 2/ area) and spectral response upto 850 nm have been fabricated using 1.6 eV a-Si:H film.