Samadhan B. Patil

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.

Hybrid magnetoresistive∕microelectromechanical devices for static field modulation and sensor 1∕f noise cancellation

Low frequency 1∕f noise in magnetoresistive (MR) spin valve sensors was suppressed by modulating an external dc magnetic field at high frequency microelectromechanical system using a (MEMS) microcantilever structure with an integrated magnetic flux guide. With this hybrid MR∕MEMS device, direct detection of dc magnetic fields in the sensor high frequency thermal noise regime was achieved. The microcantilever was actuated using a gate electrode by applying an ac voltage with frequency f, causing it to oscillate at 2f. Measurements show detection of a dc magnetic field at 2f frequency (400kHz), where sensor 1∕f noise is two orders of magnitude lower than dc.

Quantitative sub-surface and non-contact imaging using scanning microwave microscopy

The capability of scanning microwave microscopy for calibrated sub-surface and non-contact capacitance imaging of silicon (Si) samples is quantitatively studied at broadband frequencies ranging from 1 to 20 GHz. Calibrated capacitance images of flat Si test samples with varying dopant density (10(15)-10(19) atoms cm(-3)) and covered with dielectric thin films of SiO2 (100-400 nm thickness) are measured to demonstrate the sensitivity of scanning microwave microscopy (SMM) for sub-surface imaging. Using standard SMM imaging conditions the dopant areas could still be sensed under a 400 nm thick oxide layer. Non-contact SMM imaging in lift-mode and constant height mode is quantitatively demonstrated on a 50 nm thick SiO2 test pad. The differences between non-contact and contact mode capacitances are studied with respect to the main parameters influencing the imaging contrast, namely the probe tip diameter and the tip-sample distance. Finite element modelling was used to further analyse the influence of the tip radius and the tip-sample distance on the SMM sensitivity. The understanding of how the two key parameters determine the SMM sensitivity and quantitative capacitances represents an important step towards its routine application for non-contact and sub-surface imaging.

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.

Hybrid Magnetic Tunnel Junction-MEMS High Frequency Field Modulator for 1/f Noise Suppression

A dc to ac magnetic field transformer was developed using a magnetic tunnel junction (MTJ)/microelectromechanical system (MEMS) mixed device. A MEMS torsionator was fabricated with an incorporated magnetic flux guide, that when actuated by a gate electrode, modulates an external dc field into the same frequency of the micro-torsionator oscillation. Attached to it a MgO based MTJ was fabricated, performing the detection of the generated ac magnetic field. This dc to ac field transformation enables the detection of dc magnetic fields in the high frequency thermal noise regime, where the 1/f noise is typically two orders of magnitude lower. Measurements show the detection of a modulated field at 460 kHz, coming from the MEMS torsionator designed to exhibit high modulation efficiency. This efficiency was of 11%, i.e., 11% of the external dc field is modulated. As a result, from a MTJ exhibiting a noise of SV MTJ=4.4 nT/Hz1/2 at 460 kHz, this hybrid device enables a dc magnetic field detection of 40 nT/Hz1/2. This work acts as a major improvement to a previous one , mainly through the optimization of the MEMS magnetic field modulation efficiency.

On-chip magnetoresistive detection of resonance in microcantilevers

Magnetoresistive spin-valve sensors were used to provide on-chip detection of the mechanical resonance of a thin silicon microelectromechanical systems cantilever. The spin-valve sensor was placed underneath the free end of the cantilever. A CoCrPt thin-film permanent magnet was placed on top of the amorphous silicon/Al cantilever. The cantilever was electrostatically actuated and its deflection creates a change in the magnetic field that can be sensed by the spin-valve sensor. The resonance frequency of the structure in the megahertz range is detected by the measurement of the spin-valve sensor output. Minimum deflection detection limit is determined to be 0.06 A/Hz1/2.

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.

Comparison of the mechanical and resonance properties of thin film silicon MEMS fabricated at 110 and 250 °C

Microbridges with a thin film hydrogenated amorphous silicon structural layer deposited at 100 °C and 250 °C were fabricated by surface micromachining on glass substrates using aluminum as the sacrificial layer. These microbridges were electrostatically actuated and their frequency response was measured optically. The resonance frequency and quality factors of the microbridges were measured and used to characterize their mechanical properties. A comparison is made to microbridges with the structural layer deposited at 100 °C using a photoresist as a sacrificial layer. Microbridges fabricated using an Al sacrificial layer process show the presence of measurable axial residual stresses and have higher quality factors.

Hot-wire CVD growth simulation for thickness uniformity

Obtaining thickness uniformity over a large substrate area seems to be a bottleneck as far as the industrial applications of the hot-wire CVD (Cat-CVD) process is concerned. In order to address the different issues in this respect, we have simulated the hot-wire CVD growth process and proposed a proper filament geometry for maximum thickness uniformity. The hot filament was assumed as a one-dimensional assembly of point sources. Five types of commonly used filament geometries were considered for their performance to identify the best filament geometry for maximum thickness uniformity. Here, the chamber pressure was assumed to be low enough so that the Knudsen number Kn>1. Based on our results, we propose a parallel filament geometry for maximum thickness uniformity over large substrate areas. By applying the model further to the parallel filament geometry, the relations between substrate–filament distance and minimum filament length, as well as the number of parallel filaments and the separation between them, which are necessary for the required thickness uniformity over the given substrate area, were determined. The validity of the model was checked using the ‘Matched-Pair t-test’. The effect of chamber pressure on thickness uniformity and growth rate, when it is sufficiently high to make the Knudsen number Kn<1, was also simulated. The thickness uniformity was observed to increase with an increase in chamber pressure.