S. C. Saha

Development of highly conductive n‐type μc‐Si:H films at low power for device applications

Highly conductive phosphorus‐doped n‐type hydrogenated microcrystalline silicon (μc‐Si:H) films have been prepared by the usual (13.56 MHz) radio‐frequency glow discharge of silane (SiH4), phosphine (PH3), and hydrogen (H2) in an ultrahigh‐vacuum deposition system. The highest conductivity of the films obtained in this study is 100 S cm−1 after optimizing the hydrogen dilution ratio, chamber pressure, substrate temperature, and doping concentration of phosphorus. The formation of microcrystallinity in the material has been studied by transmission electron microscopy, x‐ray‐diffraction studies, and Raman spectroscopy. The volume fraction of microcrystallinity in these amorphous‐microcrystalline mixed‐phase materials has been estimated from Raman spectra. Sizes of the crystallites and volume fraction of microcrystallinity vary with hydrogen dilution, chamber pressure, and substrate temperature. The variations in the properties with deposition parameters have been explained in terms of the growth kinetics. T...

Effects of substrate temperature on structural properties of undoped silicon thin films

Undoped silicon thin films were deposited by a radio frequency plasma enhanced chemical vapor deposition technique over a wide range of substrate temperatures (170–370 °C) using a mixture of silane and hydrogen gas. A low power density (35 mW cm−2) was chosen. The effects of substrate temperature on the structural properties of the films was studied. A distinct transition from amorphous to microcrystalline phase is observed with an increase in the substrate temperature (Ts). Raman spectroscopy shows the variation of amorphous and crystalline volume fractions in the silicon films. The amorphous matrix seems to be composed mainly of monohydrides in contrast to the usual dominance of polyhydrides. At the onset of crystallinity, the films have tiny crystallites and the grain size (δ) increases with Ts (at Ts∼370 °C, δ∼350 A). The deposition rate increases with Ts and attains its maximum (28.2 A min−1) at the amorphous to microcrystalline transition region. All the microcrystalline films with their differing c...

The role of hydrogen dilution and radio frequency power in the formation of microcrystallinity of n‐type Si:H thin film

Phosphorus doped n‐type hydrogenated microcrystalline silicon (μc‐Si:H) films have been prepared at low power densities suitable for application in solar cells by the usual radio‐frequency plasma enhanced chemical vapor deposition method (rf‐PECVD, 13.56 MHz). For this purpose hydrogen (H2) dilution in the silane (SiH4) and phosphine (PH3) gas mixture and rf power densities have been varied carefully to produce a plasma condition conducive to the growth of microcrystallinity. The structural properties of the films have been studied by Raman spectroscopy, x‐ray diffractometry, transmission electron microscopy, and infrared vibrational spectroscopy. The electrical and optical characterizations have been done by dark conductivity, dark conductivity activation energy, and optical absorption measurements, respectively. Effects of variations of hydrogen dilution and rf power density on the electrical and structural properties of the films have been investigated thoroughly. Film with highest conductivity (32.6 S...

Properties of polycrystalline silicon films prepared from fluorinated precursors

Abstract Polycrystalline silicon films have been prepared at low temperature on glass substrate from fluorinated precursors by PECVD technique varying the hydrogen dilution and gas flow rate. Undoped film with dark-conductivity 1.05×10 −2 S cm −1 has been obtained. For n-type poly-Si film the highest conductivity achieved is 2.8 S cm −1 . Grain size observed from SEM varies from 4 to 6 μm for undoped and 2 to 3 μm for phosphorous doped films. The main crystalline peak is 〈111〉 whereas the crystallite size calculated from XRD is 350 A. The optical absorptions and hydrogen contents in the films deposited under different conditions have been studied. Growth kinetics are dominated by the precursors SiF n H m ( m + n ≤3) and concentrations of F and H on the growth surface.

The thickness dependences of the electronic and structural properties of n-type microcrystalline silicon films deposited under various powers

Highly conductive n-type hydrogenated microcrystalline silicon (c-Si:H) films have been prepared by a radio-frequency plasma-enhanced chemical vapour deposition technique in an ultra-high-vacuum deposition system, using a mixture of hydrogen, silane and phosphine gases under a low RF power. The electrical, optical and structural properties of films of various thicknesses have been studied and correlated. Comparisons between n-type Si:H films deposited under high (120 mW ) and low (30 mW ) powers have been made. A conductivity as high as 7.5 S has been achieved for 180 ? thick films. Their crystallinity was confirmed by transmission electron microscopy and Raman spectroscopy. Changes in optical absorption at various thicknesses have been studied using photothermal deflection spectroscopy.

Widegap a-Si:H films prepared at low substrate temperature

Abstract Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E g ∼ 1.90 eV and σ ph ≥ 10 −4 Scm −1 has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.

Substrate temperature and hydrogen dilution: parameters for amorphous to microcrystalline phase transition in silicon thin films

Amorphous to microcrystalline phase transition in hydrogenated silicon (Si:H) is realized separately with the variations of substrate temperature and hydrogen dilution. The Raman spectroscopy reveals structural transformations and marks the transition. It occurs at ∼450°C with 10% silane concentration, whereas that is noted at 250°C with a silane concentration of 4.5%. The material evolved in the transition region is a well-developed amorphous matrix containing a small fraction (∼12%) of crystallites. A uniform distribution of small (∼100 A) crystallites in the films is observed by transmission electron microscopy. The transition material is photosensitive.

Polycrystalline silicon thin films prepared by plasma enhanced chemical vapour deposition at C using fluorinated source gas

Polycrystalline silicon thin films have been developed using plasma enhanced chemical vapour deposition at a low substrate temperature C on glass substrates. An innovative approach has been introduced to deposit polycrystalline silicon film at such a low temperature. Usually a high deposition temperature is used to develop crystalline thin films. For the formation of crystallites a considerable amount of energy transfer is required at the surface of the growing film, which can be done by high-temperature processing. In the present work a high energy supply to the growing surface has been achieved by the increasing plasma power. Plasma power has been enhanced by the proper choice of deposition parameters, namely the nature of the source gas, the flow rate of diluent gas, the chamber pressure and the radiofrequency power density. Dark and photoconductivities achieved for the polycrystalline silicon thin films are and respectively. The average grain size of such film is as seen by scanning electron microscopy, while the estimated size of crystallites from the x-ray diffraction spectra is 323 ?.

Role of Substrate Temperature on the Properties of Microcrystalline Silicon Thin Films

Hydrogenated silicon (Si:H) thin films were deposited in an ultrahigh vacuum system varying substrate temperature (Ts) from 200°C to 570°C, decomposing a gas mixture of monosilane-hydrogen, by a conventional (13.56 MHz) radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) technique. A compact microcrystalline (µc) Si:H film has been developed at a deposition rate (Rd) of 25.1 A min-1. The structural and bonding configurations were characterized by X-ray diffractometry and infrared (IR) vibrational spectroscopy. Growth processes are different for films deposited at low (Ts≈250°C) and high (Ts≥350°C) temperatures. In the films of low Ts, the planes (111) and (220), and polyhydrides are formed whereas, the growth of crystallites only along and the monohydride formations are observed for the films deposited with high Ts. The dependence of both structural and optoelectronic properties on deposition temperature is discussed in terms of the growth kinetics and the properties are correlated.

Effect of hydrogen dilution in silane on light induced degradation of hydrogenated amorphous silicon films for solar photovoltaic applications

Abstract Hydrogenated amorphous silicon films (a-Si:H) have been deposited by r.f. glow discharge of silane (SiH 4 ) and hydrogen (H 2 ) gas mixture. The hydrogen concentration in the gas mixture and the applied r.f. power density have been varied. The initial photoconductivity ( σ p ), photosensitivity ( σ p σ d ), activation energy ( E a ), optical band gap ( E g ), hydrogen concentration, sub band gap absorption, Urbach energy ( E 0 ) and defect density of the films have been measured, and the degradation, i.e., the reduction of photoconductivity under AM-I (100 mW/cm 2 ) white light, has been studied. Films prepared with 95% hydrogen dilution and 6O mW/cm 2 r.f power density show the lowest degradation.