R.O. Dusane

Photoluminescent, wide-bandgap a-SiC:H alloy films deposited by Cat-CVD using acetylene

Hydrogenated amorphous silicon/carbon films (a-Si-C:H) are deposited from a silane and acetylene gas mixture by the catalytic chemical vapour deposition (Cat-CVD) technique. It is observed that under certain conditions of total gas pressure and filament temperature (TF), the optical bandgap varies non-linearly with the acetylene to silane (C2H2/SiH4) ratio, having a maximum value of 3.6 eV for a C2H2/SiH4 ratio ≥0.8. However, the deposition rate drastically reduces with an increase in acetylene fraction. FTIR spectra indicate that the total hydrogen content is lower compared to samples deposited by PECVD using similar gas mixtures, with hydrogen being preferentially attached to carbon rather than silicon atoms. The photoluminescence (PL) spectra of these films show PL in the visible spectral region at room temperature. The films with larger bandgap (>2.5 eV) exhibit PL at room temperature, with the emission having peak energy in the range 2.0–2.3 eV.

Revisiting the B-factor variation in a-SiC:H deposited by HWCVD

In order to understand material properties in a better way, it is always desirable to come up with new variables that might be related to the film properties. The B-parameter is such a variable, which relates to the quality of a-SiC:H films both in terms of electronic and optical properties. B (scaling factor) is essentially the slope of the straight-line part of the (αE)1/2–E (Tauc plot). Due to dependence on a large number of parameters and no detailed research, many previous authors have surmised that B has an ambiguous correlation with carbon content. We have made an attempt to establish the relation between the B-parameter as a quality-indicating factor of a-SiC:H films in both carbon- and silicon-rich material. For this we studied a-SiC:H films deposited by the HWCVD method with broad deposition parameters of substrate temperature (Ts), filament temperature (TF) and C2H2 fraction. Our results indicate that the B-parameter varies considerably with process conditions such as TF, total gas pressure and carbon content. An attempt is made to correlate the B-parameter with an opto-electronic parameter, such as the mobility edge, which has relevance to the device-quality aspects of a-SiC:H films prepared by HWCVD.

Low temperature silicon nitride deposited by Cat-CVD for deep sub- micron metal-oxide-semiconductor devices

Silicon nitride as a gate dielectric can improve the performance of ULSI CMOS devices by decreasing the gate leakage currents. In this paper we report a a-SiN:H gate dielectric fabricated using Cat-CVD at a relatively low substrate temperature of ∼250°C, using silane and ammonia as the source gases. The films were deposited at various gas pressures, (NH3/SiH4) flow rate ratios and at different filament temperatures (TF). The deposition parameters, i.e. total gas pressure and gas composition (silane+ammonia) were optimized to deposit insulating and transparent films with high breakdown strength. The structural properties of these films were studied by Fourier transform infrared (FTIR) spectroscopy and ultraviolet-visible (UV-vis) spectroscopy. Films with bandgap as high as 5.5 eV were obtained. The optimized conditions were used to deposit ultrathin films of the order of 8 nm thickness for deep-submicron CMOS technology. Electrical properties such as C–V and I–V measurements were studied on metal–nitride–semiconductor (MNS) capacitor structures. These characterization results on MNS capacitors show breakdown fields of the order of 10 MV cm−1 and good interface properties.

p-i interface engineering and i-layer control of hot-wire a-Si:H based p-i-n solar cells using in-situ ellipsometry

Abstract In this paper we report on the effect of monitoring the i-layer region near the p-i interface with the help of in-situ kinetic and spectroscopic ellipsometry on the performance of hot-wire deposited hydrogenated amorphous silicon p-i-n solar cells. It is very clearly observed that the microstructure at the p-i interface region in terms of the SiSi bond packing density and surface roughness significantly affects the cell performance. The filament temperature, T Fil , was the main parameter varied to control the above mentioned two properties near the p-i interface as well as in the bulk i-layer. In order to achieve significant enhancement in the cell performance we extended the idea of the “soft start”, earlier employed for the glow discharge deposited solar cells, to the hot-wire deposited i-layer. We were able to control the i-layer properties at the p-i interface and in the bulk independently and correlate these to the cell performance. It is shown that a major increase in cell performance can be achieved by improving the microstructure of the growing film directly at the p-i interface. Most interestingly, no significant deterioration in cell efficiency has been observed if only the p-i interface was properly controlled but the i-layer was of lower quality. These results are also shown to be consistent with model calculations of a numerical simulation. Our results therefore provide a clue to prepare hot-wire a-Si:H based solar cells with high efficiency and in the whole at high growth rates, which is needed for a more economic a-Si:H solar cell production.

Influence of atomic hydrogen on the growth kinetics of a-Si : H films and on the properties of silicon substrates

We present two different investigations showing the influence of hydrogen in hot-wire chemical vapor deposition (HWCVD). First, dissimilarities in the growth kinetics of hydrogenated amorphous silicon (a-Si:H) films by the plasma enhanced chemical vapor deposition (PECVD) and HWCVD are discussed. A series of a-Si:H films with varying substrate temperature (TS) was deposited by HWCVD and PECVD. In comparing the initial growth, which was measured by in-situ kinetic ellipsometry for both deposition methods, we conclude that, in the PECVD process, a faster coalescence takes place as a result of a larger surface mobility of the adsorbed precursor radicals at the growing surface. This dissimilarity can be explained by different silane dissociation processes which yield to a higher ratio of atomic hydrogen to silicon radicals and a lower hydrogen coverage of the film growing surface in the case of HWCVD. Therefore, dense high quality a-Si:H films deposited by HWCVD are formed at higher substrate temperatures and lower hydrogen dilutions compared to PECVD. The second investigation deals with the effect of atomic hydrogen on polished and textured silicon wafers. The hydrogen treatment can decrease the surface defect density without deteriorating the properties of the bulk material. Therefore, the fill factor and the open circuit voltage of (n)a-Si:H/(p)c-Si heterojunction solar cells increase and the intrinsic conversion efficiency reaches up to 15.2%.

Ultra-thin silicon nitride by hot wire chemical vapor deposition (HWCVD) for deep sub-micron CMOS technologies

Silicon nitride is considered a promising candidate to replace thermal oxide dielectrics, as the latter is reaching its scaling limits due to the excessive increase in the gate tunneling leakage current. The novel hot wire chemical vapor deposition (HWCVD) technique shows promise for gate quality silicon nitride film yields at 250 °C while maintaining their primary advantage of a higher dielectric constant of 7.1. In this paper we report the results of our efforts towards developing ultra-thin HWCVD silicon nitride as an advanced gate dielectric for the replacement of thermal gate oxides in future generations of ultra large scale integration (ULSI) devices.

Effect of local structural order on the doping in hydrogenated amorphous silicon (a - Si : H)

Doping in a-Si:H has been studied in the light of local structural order present in these films and the consequent modification in this upon dopant incorporation. It is seen that the local structural order is a very important parameter to understand the doping mechanism and variation in this correlates well with the variation in doping efficiency with increasing dopant concentration and the increase in the defect density. Interesting results on the compensated samples are also reported and discussed.

Photovoltaic, I-V and C-V Characteristics of SnO2/SiO2/a-Si:H/mc-Si:H Structures

Structures of the type SnO2/SiO2/a-Si:H/mc-Si:H/Al and SnO2/SiO2/a-Si:H/mc-Si:H/a-Si:H/mc-Si:H/ were fabricated and studied for their I-V, C-V and photovoltaic characteristics. Results indicate that the structures behave like two diodes formed in the back-to-bak geometry. Furthermore, from the I-V studies under illumination, it is observed that a sufficient drift field exists in the structure without intentional doping of Si-based layers to yield a gain in current under illumination of about 104.

Nitrogen dilution effects on structural and electrical properties of hot-wire-deposited a-SiN:H films for deep-sub-micron CMOS technologies

Hot-wire chemical vapor-deposited silicon nitride is a potential dielectric material compared to glow-discharge-deposited material due to its lower hydrogen content. In several earlier publications we have demonstrated these aspects of the HWCVD nitride. However, to replace SiO2 with a-SiN:H as the gate dielectric, this material needs further improvement. In this paper we report the results of our efforts to achieve this through nitrogen dilution of the SiH4+NH3 gas mixture used for deposition. To understand the electrical behavior of these nitride films, we characterized the films by high-frequency capacitance–voltage (HFCV) and DC J–E measurements. We attempted to evolve a correlation between the breakdown strength, as determined from the J–E curves, and aspects such as the bond density, etching rate, deposition rate and refractive index. From these correlations, we infer that nitrogen dilution of the source gas mixture has a beneficial effect on the physical and electrical properties of the hot-wire a-SiN:H films. For the highest dilution, we obtained a breakdown voltage of 12 MV cm−1.

Effect of RF power on the structure and related gap states in hydrogenated amorphous silicon

Abstract The intricate relationship between the structure and processing parameters in a-Si:H is not well understood. The effect of variation in rf power on the structural, optical and electronic properties of glow discharge deposited hydrogenated amorphous silicon (a-Si:H) has been studied using deep level transient spectroscopy (DLTS), photoluminescence (PL), spectroscopic ellipsometry and Raman spectroscopy. The DLTS results show a predominant increase in the gap state density around 0.5 eV below the conduction band, which is also supported by the decrease in the intensity of the PL peak around 0.8 eV with rf power. Ellipsometry results show an overall decrease in the value of e 2 , while the energy corresponding to the e 2 peak position remains constant. Significant modifications of spectral features are observed in the Raman spectra of film deposited at higher rf powers. These results suggest that the a-Si:H films grown at higher rf power are less dense and have larger vacancy or void concentration. It is proposed that such structural defects lead to the observed energy levels in the gap and associated physical properties of the a-Si:H films.