Alka Kumbhar

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.

Preliminary results on a-SiC:H based thin film light emitting diode by hot wire CVD

Preliminary results on the first hot wire deposited a-SiC:H based thin film light emitting p–i–n diode having the structure glass/TCO(SnO2:F)/p-a-SiC:H/i-SiC:H/n-a-SiC:H/Al are reported. The paper discusses the results of our attempts to optimize the p-, i- and the n-layers for the desired electrical and optical properties. The optimized p-layers have a bandgap Eg∼2 eV and conductivity a little lower than 10−5 (Ω cm)−1. On the other hand, the optimized n-type a-SiC:H show a conductivity of ∼10−4 (Ω cm)−1 with bandgap 2.06 eV. The highest bandgap of the intrinsic layer is approximately 3.4 eV and shows room temperature photoluminescence peak at approximately 2.21 eV. Thin film p–i–n diodes having i-layers with Eg from 2.7 to 3.4 eV show white light emission at room temperature under forward bias of >5 V. However, the 50-nm thick devices show appreciable reverse leakage current and a low emission intensity, which we attribute to the contamination across the p–i interface since these devices are made in a single chamber with the same filament.

Highly conducting doped poly-Si deposited by hot wire CVD and its applicability as gate material for CMOS devices

Highly conducting p- and n-type poly-Si:H films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH4+H2+B2H6 and SiH4+H2+PH3 gas mixtures, respectively. Conductivity of 1.2×102 (Ω cm)−1 for the p-type films and 2.25×102 (Ω cm)−1 for the n-type films was obtained. These are the highest values obtained so far by this technique. The increase in conductivity with substrate temperature (Ts) is attributed to the increase in grain size as reflected in the atomic force microscopy results. Interestingly conductivity of n-type films is higher than the p-type films deposited at the same Ts. To test the applicability of these films as gate contact Al/poly-Si/SiO2/Si capacitor structures with oxide thickness of 4 nm were fabricated on n-type c-Si wafers. Sputter etching of the poly-Si was optimized in order to fabricate the devices. The performance of the HWCVD poly-Si as gate material was monitored using C–V measurements on a MOS test device at different frequencies. The results reveal that as deposited poly-Si without annealing shows low series resistance.

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.

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.

a-Si:H passivation layer growth by HWCVD for Si heterojunction solar cells: Critical dependence on substrate temperature

We investigate the passivation properties of the hot-wire CVD deposited thin intrinsic a-Si:H layers grown at different substrate temperatures, Tsub, ranging from 150°C - 250°C, on n type crystalline Si (c-Si) substrates. Highest effective minority carrier lifetime of 288 µs at 1015 cm-3 injection level was obtained on symmetrically passivated c-Si wafers with a-Si:H layers (∼ 9 nm) grown at Tsub = 200°C whereas for the greater or lower Tsub, the respective values of τeff drop. The cross section TEM results revealed mixed phase epitaxy for Tsub = 250°C that apparently indicates its detrimental effect on passivation. The defect densities and Urbach energy values within respective bulk a-Si:H films were also compared which lead us to conclude that there exists a balance between having an electronically superior a-Si:H film and reduced defect densities at the a-Si:H/c-Si interface for effective passivation. Finally, prototype hetero-junction solar cells having structure: ITO (95 nm)/ (p) a-Si:H (10 nm)/ (i) a-Si:H (9 nm)/c-Si (280µm) / (i) a-Si:H (9 nm)/ (n+) Si:H (30 nm)/Al were fabricated. A comparably higher Voc of 0.57 V was found for solar cells grown around Tsub of 175°C and the reasons for the same are discussed.

Synthesis and Characterization of SiNW-MnO2 Core-Shell Structure

A simple and low temperature approach has been used for the deposition of manganese oxide (MnO2) film and nanoparticles on silicon nanowires (SiNWs). Firstly, SiNWs were grown using hot wire chemical vapour process (HWCVP) technique via Vapor-Liquid-Solid (VLS) mechanism using Sn as a catalyst and then electrophoretic deposition method (EPD) was used to deposit MnO2 on them. Since SiNWs have good electron transportation and high aspect ratio, the role of SiNWs is thus for improving the electrical conduction and the surface area for MnO2 for its application in a desired form. First the deposition parameters were optimized on a transparent conductive oxide (TCO) coated glass substrate to control the thickness of the MnO2 film and then it was synthesized on SiNWs. The deposition of MnO2 has been confirmed by FEG-SEM and EDX. This structure of MnO2-SiNWs could be useful for various applications.

Gas Phase Chemistry Study During Deposition of a-Si:H and μc-Si:H Films by HWCVD using Quadrupole Mass Spectrometry

Amorphous and microcrystalline silicon films were deposited by HWCVD under different deposition conditions and the gas phase chemistry was studied by in situ Quadrupole Mass Spectrometry. Attempt is made to correlate the properties of the films with the gas phase chemistry during deposition. Interestingly, unlike in PECVD, partial pressure of H 2 is higher than any other species during deposition of a-Si:H as well as μc-Si:H. Effect of hydrogen dilution on film properties and on concentration of various chemical species in the gas phase is studied. For low hydrogen dilution [H2]/ [SiH4] from 0 to 1 (where [SiH4] is 10 sccm), all films deposited are amorphous with photoconductivity gain of ∼ 10 6 . During deposition of these amorphous films SiH 2 was dominant in gas phase next to [H 2 ]. Interestingly [Si]/[SiH 2 ] ratio increases from 0.4 to 0.5 as dilution increased from 0 to 1, and further to more than 1 for higher hydrogen dilution leading to [Si] dominance. At hydrogen dilution ratio 20, consequently films deposited were microcrystalline.