O.S. Panwar

Diamond-like carbon films with extremely low stress

Abstract We in this paper report different ways to realise thick diamond-like carbon (DLC) films with stress values lower than 0.5 GPa, Thick DLC films grown by conventional r.f. self bias technique often delaminate from the substrates due to the presence of high compressive stresses of the order of 4–7 GPa. We have made an in-depth study of the delamination problem of DLC films at NPL and found that only for substrates kept away from the plasma (plume) it is possible to grow thick DLC films. This goes to show the heating of the substrates, when m contact with the plasma, appears to be one of the most important factors giving rise to the high stress values. Techniques that have produced consistently low stress values (0.2–0.5 GPa) in this laboratory are pulse plasma PECVD and the one using dc saddle field fast atom beam source. Electronic properties of the materials so produced have been estimated by evaluating Urbach energy using photothermal deflection spectroscopy (PDS) measurements. A correlation between the unbound hydrogen in these films, as measured by a nuclear technique (ERDA), and the stress levels has been found. Deposition rate, room temperature conductivity, optical bandgap and refractive index have also been measured for these films.

Dependence of field-emission threshold in diamond-like carbon films grown by various techniques

In this paper, we report field-emission measurements from ∼0.5-μm-thick hydrogenated amorphous carbon (diamond-like carbon (DLC)) films. These films were grown by a variety of easily implementable plasma-enhanced chemical vapor deposition (PECVD) based techniques and also by a method that uses a saddle-field fast atom beam source. Field-emission behavior in these materials has been discussed in light of residual stress, hardness, optical band gap, and characteristic energy of band tails (Urbach energy). Onset emission-fields as low as ∼6 V/μm, together with low residual stress of 0.25 GPa, hardness of 17.5 GPa, optical band gap of 1.5 eV, and Urbach energy of 165 meV, have been obtained in DLC films grown by pulsed-PECVD at 13.56 MHz. DLC films of comparable quality could also be grown using a saddle-field fast atom beam source, which operates on modest dc power supply and with no heated filaments or magnets.

Effect of high substrate bias and hydrogen and nitrogen incorporation on density of states and field-emission threshold in tetrahedral amorphous carbon films

This article reports the influence of substrate bias during growth and of hydrogen and nitrogen incorporation on density of states [N (EF)] and field-emission threshold (Eturn-on) in tetrahedral amorphous carbon (ta-C) films, deposited using an S-bend filtered cathodic vacuum arc process. The variation in negative substrate bias from −20 to −200 V was found to initially lead to a small decrease in N (EF) and Eturn-on, and a small increase in the emission current density (J) at 12.5 V/μm in the case of as-grown ta-C films; beyond −200 V substrate bias there is a reversal in the trend. The values of N (EF)=1.3×1017 cm−3 eV−1, Eturn-on=8.3 V/μm, and J=6.19 mA/cm2 were observed at −200 V substrate bias. However at −300 V the properties were not very different from those at −200 V substrate bias and so with a view to use the higher energy, hydrogen and nitrogen incorporation studies were carried out in this condition. It was observed that there was further enhancement in properties with hydrogen and nitrogen i...

Characterization of Boron- and Phosphorous-Incorporated Tetrahedral Amorphous Carbon Films Deposited by the Filtered Cathodic Vacuum Arc Process

This paper reports the X-ray photoelectron spectroscopy (XPS), X-ray-induced Auger electron spectroscopy (XAES), and Raman studies of boron- and phosphorous-incorporated tetrahedral amorphous carbon (ta-C) films deposited by the filtered cathodic vacuum arc process. A systematic study of the influence of varying boron (B) and phosphorous (P) content on the properties of the as-grown ta-C films deposited at high negative substrate bias (-300 V) is reported by analyzing the C 1s, B 1s, and P 1s core levels using photoelectron spectroscopy. The sp3 and sp2 contents in the films were determined by measuring the width of the X-ray-induced Auger peaks. B incorporation in ta-C films up to 2.0 at. % increases the sp2 content and decreases the sp3 content by 3.6%, whereas P incorporation up to 2.0 at. % results in an increase of sp2 content and decrease of sp3 content by ~ 30%. The valence band spectra show changes in the Fermi level as B and P are incorporated into the ta-C films. The characteristic Raman spectra confirm the high sp3 content in the deposited films. Thus, the study demonstrates, in the case of high negative substrate bias films, that a pronounced decrease in sp3 fraction or the diamond-like nature of the ta-C films occurs upon P incorporation in comparison to that upon B incorporation.

Correlation of residual stress with optical absorption edge in diamond-like carbon films

Abstract A correlation has been observed between the residual stress and the optical absorption edge of diamond-like carbon films and on the basis of this correlation, an empirical relation has been established between the Urbach energy ( E 0 ) and the residual stress ( S ) in the films given by E 0 = E 00 + mS , where E 00 =140 meV and m =37 meV/GPa. The residual stress and the optical absorption edges of diamond-like carbon films, grown by different techniques, are then discussed in terms of amount of disorder in the network.

Characterisation of a saddle field fast atom beam source and its application to the growth of diamond-like carbon films

Abstract The use of saddle field fast atom beam (FAB) sources to grow diamond-like carbon (DLC) films is attractive due to its dc and rf attributes. This paper reports the detailed analysis of the beam coming out from the FAB source using argon (Ar), methane (CH 4 ) and acetylene (C 2 H 2 ) as source gases. Three operation modes of the FAB source are found, viz. glow discharge mode , transition mode and oscillation mode . A new process parameter, applied power, is defined for characterisation of the FAB source. Energy distribution analysis shows that the mean energy of the radicals coming out of the FAB source lies within 50% of the applied voltage to the FAB source. The neutralisation coefficient of the beam was estimated to be more than 90% and was found to be almost independent of the discharge current of the FAB source. It will be seen through this characterisation study that a completely novel technique has been found to deposit DLC films, simultaneously by neutral and ionic radicals. The mechanical and opto-electronic properties of these DLC films are discussed. The role of ion assistance is seen in hardening the films. DLC films deposited by ionic radicals show better opto-electronic properties as measured by photothermal deflection spectroscopy (PDS).

Infrared studies of hard hydrogenated amorphous carbon (aC:H) film and the effect of argon plasma treatment

Abstract The hard hydrogenated amorphous carbon (aC:H) films were deposited on both sides of well polished germanium (Ge) substrate by RF plasma decomposition of benzene vapours. Infrared spectroscopy was used to measure the transmission and Sp3/Sp2 ratio of diamond-like carbon (DLC) film as a function of plasma treatments. It has been demonstrated that the partial etching of aC:H film by argon plasma treatment may be a suitable process (1) to improve the film quality and also (2) to tailor at required wavelengths the infrared transmission of aC:H coated Ge optics.

Comparative study of large grains and high-performance TFTs in low-temperature crystallized LPCVD and APCVD amorphous silicon films

The crystallization of undoped amorphous silicon films deposited by low-pressure and atmospheric-pressure chemical vapour deposition (LPCVD and APCVD) at temperatures ranging between 510 and 650 °C and subsequently annealed at temperatures between 510 and 700 °C, for different durations, have been studied by transmission electron microscopy (TEM). It is found that the grain size in these films is influenced by the deposition and annealing temperatures and also by the deposition rate and film thickness. Maximum grain size of approximately 6400 A has been obtained in LPCVD silicon films 2000 A thick deposited at 540 °C and annealed at 550 °C for 144 h, whereas APCVD silicon films deposited at 590 °C and annealed at 610 °C for 72 h produced a grain size of approximately 4100 A. At higher and lower deposition temperatures the grain size was found to be smaller. Self-aligned silicon-gate thin-film transistors (TFTs) have been made using these crystallized amorphous/polycrystalline silicon films deposited at different temperatures by LPCVD and APCVD techniques, using SiO2 grown thermally at 850 °C as a gate dielectric. Device properties in these TFTs depend upon the deposition temperature, and a mobility value of 26–28 cm2 V−1 s−1 has been observed in LPCVD silicon TFTs at a deposition temperature of 540 °C, whereas APCVD silicon TFTs deposited at 590 °C show a mobility value of only 10.5 cm2 V−1 s−1.

Studies of subgap absorption and related parameters by the constant photocurrent method of high rate deposited hydrogenated amorphous silicon films

Abstract The constant photocurrent method (CPM) has been used to measure subgap- related parameters such as absorption coefficient α, characteristic energy E 0 of tail states and density of subgap defect states together with an estimate of the band gap of hydrogenated amorphous silicon (a-Si:H) films prepared at various deposition rates. A higher deposition rate in the home-made plasma chemical vapour deposition system was obtained by an improved design of powered electrode and an earthed shield coupled with optimization of process parameters. The results of our measurement show that with increase in deposition rate of a-Si:H films from 6.7 to 18 A s -1 , the values of the characteristic energy E 0 , defect density N s and band gap E g increases from 55.9 to 65.2 meV, from 3.3 × 10 16 to 9.8 × 10 16 cm −3 and from 1.73 to 1.80 eV respectively. CPM measurements were further extended on light-soaked and annealed samples and it was found that E 0 and the defect density increase after light soaking and the effect is reversed after annealing the sample at 160 °C for 1 h.

Investigations on hydrogenated amorphous silicon films grown at high rate in a UHV plasma CVD system

Abstract In this paper we analyze the results of an extensive characterisation study involving scanning electron microscopy, spectroscopic ellipsometry, Laser Raman spectroscopy, optical band gap, dark and photoconductivity measurements as a function of temperature and light intensity, current-voltage characteristics and capacitance versus frequency measurements on Schottky diodes of a-Si:H films, prepared in a UHV plasma CVD system at varying deposition rates upto ≈ 10.0 A/s. We find that the values of imaginary part of the dielectric constant ( ϵ 2max ), dark and photoconductivity, activation energy, conductivity pre-exponential factor, exponent Λ of photoconductivity decrease, whereas the values of diode quality factor n and density of defect states increase and the values of the width of TO-peak in Raman spectra, photosensitivity and optical band gap do not change significantly with the increasing rate of deposition. The less severe deterioration of opto-electronic properties observed in these films, even when grown at high rates in our plasma CVD reactors, is attributed to the effective control of secondary plasma reactions in the interelectrode space when kept very low (10–12 mm). Implications of obtaining high growth rate for i-layer, in a p-i-n solar cell structure, is discussed with the available results of modelling analysis.