Jhuma Gope | National Physical Laboratory (original) (raw)

Papers by Jhuma Gope

Physical chemistry chemical physics : PCCP, Jan 21, 2014

Thermal ALD deposited Al2O3 films on silicon show a marked difference in surface passivation qual... more Thermal ALD deposited Al2O3 films on silicon show a marked difference in surface passivation quality as a function of annealing time (using a rapid thermal process). An effective and quality passivation is realized in short anneal duration (∼100 s) in nitrogen ambient which is reflected in the low surface recombination velocity (SRV <10 cm s(-1)). The deduced values are close to the best reported SRV obtained by the high thermal budget process (with annealing time between 10-30 min), conventionally used for improved surface passivation. Both as-deposited and low thermal budget annealed films show the presence of positive fixed charges and this is never been reported in the literature before. The role of field and chemical passivation is investigated in terms of fixed charge and interface defect densities. Further, the importance of the annealing step sequence in the MIS structure fabrication protocol is also investigated from the view point of its effect on the nature of fixed ch...

Thermal ALD deposited Al2O3 films on silicon show marked difference in surface passivation qualit... more Thermal ALD deposited Al2O3 films on silicon show marked difference in surface passivation quality as a function of annealing time (using rapid thermal process). An effective and quality passivation is realized in short anneal duration (~100s) which is reflected in the low surface recombination velocity (SRV <10 cm/s). The deduced values are close to the best reported SRV obtained by high thermal budget process (with annealing time between 10-30 min) conventionally used for improved surface passivation. Both as-deposited and low thermal budget annealed films show the presence of positive fixed charges and has never been reported in the literature before. The role of field and chemical passivation is also investigated in terms of fixed charge and interface defect densities. Further, the importance of anneal step sequence in MIS structure fabrication protocol is also investigated from the view point of its effect on nature of fixed changes.

Journal of Materials Science: Materials in Electronics

Materials Science in Semiconductor Processing, 2015

ABSTRACT Mixed phase amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H) thin films were d... more ABSTRACT Mixed phase amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H) thin films were deposited by VHF-PECVD (60 MHz) using Argon (Ar) as the diluent of silane. These amorphous and crystalline silicon thin films were deposited by varying the argon dilution (fAr) from 10 to 97.5% while keeping other process parameters constant. The effects of argon dilution on deposition rate, structural and optical properties of micro/nanocrystalline silicon thin films are studied. It has been observed that the films deposited from fAr 10–70% showed the deposition rate >20 Å/s with the highest deposition rate achieved of ~25 Å/s. Structural characterization has been performed by micro-Raman analysis and Atomic force microscopy. Raman shift towards higher wave number (515 cm−1) with increase of fAr indicates variation in crystallinity of silicon films. HRTEM studies revealed the distribution of grain size and the degree of crystallinity. Optical absorption spectroscopy confirmed the increase in band gap of the materials from 1.5 to 2.1 eV.

AIP Advances, 2015

The effect of deposition temperature (T dep) and subsequent annealing time (t anl) of atomic laye... more The effect of deposition temperature (T dep) and subsequent annealing time (t anl) of atomic layer deposited aluminum oxide (Al 2 O3) films on silicon surface passivation (in terms of surface recombination velocity, SRV) is investigated. The pristine samples (as-deposited) show presence of positive fixed charges, Q F. The interface defect density (D it) decreases with increase in T dep which further decreases with t anl up to 100s. An effective surface passivation (SRV<8 cm/s) is realized for T dep ≥ 200 • C. The present investigation suggests that low thermal budget processing provides the same quality of passivation as realized by high thermal budget process (t anl between 10 to 30 min).

Journal of Alloys and Compounds, 2015

ABSTRACT Boron doped hydrogenated micro/nanocrystalline silicon (μc/nc-Si:H) thin films have been... more ABSTRACT Boron doped hydrogenated micro/nanocrystalline silicon (μc/nc-Si:H) thin films have been deposited by plasma enhanced chemical vapor deposition technique (PECVD) using silane (SiH4) diluted in argon. Diborane (B2H6) was used as the dopant gas and deposition was carried out at substrate temperature of 200°C. The diborane flow (FB) varied in the range 0.00-0.30. Here, we report the effects of B2H6 doping on electronic, optical and structural properties of hydrogenated micro/nanocrystalline silicon films. The structural properties were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD). The doped micro/nano crystalline silicon films presented a crystallographic orientation preferentially in the (111) and (2 2 0) plane. We resolve the deposition parameters that lead to the formation of p-type micro/nano crystalline silicon thin films with very high value of conductivity and lower optical band gap. Correlations between structural and electrical properties were also studied. Based on temperature dependent conductivity measurements, it has been observed that the room temperature dark conductivity varies in the range 1.45 × 10−4 Ω-1 cm−1 to 2.02 Ω-1 cm−1 for the B-doped films. Meanwhile, the corresponding value of activation energies decreased to 0.06 eV for the B-doped films, which indicates the doped μc/nc-Si films with high conductivity can be achieved and these films prove to be a very good candidate for application in amorphous and micro/nano crystalline silicon solar cells as a p-type window layer

Applied Surface Science, 2015

Silicon thin films are deposited using plasma enhanced chemical vapor deposition (PECVD) of silan... more Silicon thin films are deposited using plasma enhanced chemical vapor deposition (PECVD) of silane,
argon, hydrogen mixture at various pressures in the range of 2–8 Torr. Raman scattering shows these
to be amorphous in the pressure range 6–8 Torr, and nanocrystalline in the range 2–4 Torr. The volume
fraction of nanocrystals is estimated by fitting the Raman data to three peaks and is found to be 75% for
the films deposited at low pressure, density of states of these films was measured. It is observed that the
electrical conduction in these films depends on the crystalline volume fraction (q), estimated from the
laser Raman Spectroscopy. Temperature dependence electrical conductivity shows that at lower temper-
atures thermionic emission dominates for the films with lower q, whereas, hopping is the main conduc-
tion mechanism for the films having high q. The density of states is estimated from the space charge
limited currents (SCLC) observed at high fields. Photoconductivity at room temperature is also measured.
The amorphous films are found to be more photosensitive than the nanocrystalline one. In the context of
these findings, changes in the properties of silicon from amorphous to nanocrystalline are described.

Journal of Non-crystalline Solids, 2010

Hydrogenated amorphous and micro/nanocrystalline silicon films have been deposited using RF (13.5... more Hydrogenated amorphous and micro/nanocrystalline silicon films have been deposited using RF (13.56 MHz) PECVD technique in the RF power density range of 177 to 885 mW/cm 2 exploiting SiH 4 + H 2 + Ar gaseous mixture. Predominant crystalline phase was observed in the films, as revealed from Raman spectroscopic study, deposited at power densities of 531 and 708 mW/cm 2 and amorphous phase at 177, 354 and 885 mW/ cm 2 . Amorphous to microcrystalline phase transition occurred near RF power density of 531 mW/cm 2 . The high crystalline phase (~80%) was found in the film deposited at 708 mW/cm 2 . Lower values of bond angle distortion of 6.7 and 5°were found for the films deposited at 531 and 708 mW/cm 2 as compared to bond angle distortion of 8.4, 9.3 and 9.1°for the films deposited at 177, 354 and 885 mW/cm 2 , respectively. Plasma parameters extracted from V/I probe and properties of the films revealed contribution of ions to be beneficial for the enhancement of deposition rate of the films. The increased ion energy (sheath field) and ion flux together with SiH x (x = 1,2,3) radicals on growing film surface might help in the enhancement of crystallinity at 708 mW/cm 2 .

Solar Energy Materials and Solar Cells, 2010

The effect of argon concentration (66–87%) in total gaseous mixture (SiH4+H2+Ar) on growth and pr... more The effect of argon concentration (66–87%) in total gaseous mixture (SiH4+H2+Ar) on growth and properties of hydrogenated nanocrystalline silicon films deposited by RF (13.56 MHz) PECVD technique was investigated. Raman and XRD measurements revealed increasing argon fraction favored enhancement of crystallinity, enlargement of crystallites and relaxation of strained bonds. Photoluminescence spectra of nc-Si:H films exhibited two radiative transitions in the photon energy ranges of 2.8–3.1 eV and 1.6–2.1 eV. The high energy PL peaks are attributed to surface effect of the films whereas peaks in the range of 1.6–2.1 eV are due to nanocrystallinity in the films. Argon dilution also helped enhancement of deposition rate and conductivity of the films. A film deposited at 81% of argon fraction possesses high crystallinity (75%), conductivity in the order of 10−5 (Ω cm)−1, size of the crystallite (Raman=12 nm, XRD=18 nm), and low residual stress (125 MPa).

Modern Physics Letters B, 2011

Nanocluster carbon thin films (NC) were deposited at room temperature by cathodic arc process. Th... more Nanocluster carbon thin films (NC) were deposited at room temperature by cathodic arc process. These films were clustered, amorphous and disordered in nature which is verified using Raman spectroscopy. Density of defect states (DOS), which influences electronic and optical properties, were determined from the capacitance-voltage (C-V) characteristic of Al/NC/c-Si metal-insulator-semiconductor (MIS) structure near the Fermi level of undoped samples. Dielectric constant of the films was also estimated using the same technique. The DOS were found to be varying from 5.68 × 1016 to 4.9 × 1019 cm-3. The dielectric constant varied between 2.76 to 11.8.

Solar Energy Materials and Solar Cells, 2008

The characteristics of 13.56-MHz discharged SiH 4 +Ar+H 2 plasma at high pressure (2-8 Torr), use... more The characteristics of 13.56-MHz discharged SiH 4 +Ar+H 2 plasma at high pressure (2-8 Torr), used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films in a capacitively coupled symmetric PECVD system, has been investigated. Plasma parameters such as average electron density, sheath field and bulk field are extracted from equivalent circuit model of the plasma using outputs (current, voltage and phase) of RF V-I probe under different pressure conditions. The conditions of growth in terms of plasma parameters are correlated with properties of the hydrogenated nanocrystalline silicon films characterized by Raman, AFM and dc conductivity. The film deposited at 4 Torr of pressure, where relatively low sheath/bulk field ratio is observed, exhibits high crystallinity and conductivity. The crystalline volume fraction of the films estimated from the Raman spectra is found to vary from 23% to 79%, and the trend of variation is similar to the RF real plasma impedance data.

Journal of Physics-condensed Matter, 2008

... For the optical bandgap estimation, Tauc&amp;#x27;s [23] and Cody&amp;#x27;s ... more ... For the optical bandgap estimation, Tauc&amp;#x27;s [23] and Cody&amp;#x27;s [24] methods are generally employed. Tauc&amp;#x27;s gap shows higher values (as shown in figure 2(b)) of bandgap as compared to the value obtained by Cody (not shown here). ... Status Solidi 15 627 [24] Cody GD 1981 Phys. ...

Silicon, 2012

ABSTRACT A mixed-phase of amorphous (a-Si:H) and ultra nanocrystalline silicon thin films (ultra ... more ABSTRACT A mixed-phase of amorphous (a-Si:H) and ultra nanocrystalline silicon thin films (ultra nc-Si:H) were grown in a low deposition pressure regime (0.10–0.62 Torr) using a very high frequency (60 MHz) plasma enhanced chemical vapor deposition (VHF PECVD) technique. The deposition rate, stress, hydrogen configuration and morphology varying with the deposition pressure were systematically studied. The maximum deposition rate was found to be 8.3 Å/s at a pressure of 0.47 Torr. The stress of these films decreases from 669.7 MPa to 285 MPa with the increase of deposition pressure from 0.10 to 0.62 Torr. The change in deposition pressure showed the variation of the microstructure and hydrogen bonding configuration of the nc-Si:H films. The ultra small crystallites (size ~ 2 nm) were formed in the films which were confirmed by the X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements. An attempt was made to understand their growth by analysis of the bonding environment.

ISRN Nanomaterials, 2012

The mechanical properties of hydrogenated silicon thin films deposited using high-frequency PECVD... more The mechanical properties of hydrogenated silicon thin films deposited using high-frequency PECVD process were studied, which certainly have importance for optoelectronic devices particularly for getting stability and long operating lifetime in harsh conditions. Nanoindentation technique was used to measure the load versus displacement curves, hardness (H), elastic modulus (E), plastic resistance parameter (H/E), elastic recovery (ER), and plastic deformation energy (U r ), while laser scanning stress measurement setup was used to measure the intrinsic stress of these films. The concentration of bonded hydrogen in these films was found in the range of 3.6 to 6.5 at. % which was estimated using integrated intensity of IR absorption peak near 640 cm −1 . Dependence of mechanical properties of these films on hydrogen content and bonding environment has been investigated. The film containing minimum hydrogen content (3.6%) shows the maximum elastic recovery (52.76%) and minimum plastic deformation energy (3.95 × 10 −10 J). Surface roughness measured by AFM was found to decrease with the increase in hydrogen content in the film. The dependency of stress on the plasma frequency and applied power has also been discussed.

Materials Chemistry and Physics, 2013

ABSTRACT Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow r... more ABSTRACT Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow rates have been investigated. The silane flow rate was varied from 5 sccm to 30 sccm, maintaining all other parameters constant. The electrical, structural and optical properties of these films were systematically studied as a function of silane flow rate. These films were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and UV-visible (UV-Vis) spectroscopy. Different crystalline volume fraction (22%-60%) and band gap (similar to 1.58 eV-similar to 1.96 eV) were achieved for silicon thin films by varying the silane concentration. A transition from amorphous to nanocrystalline silicon has been confirmed by Raman and FTIR analysis. The film grown at this transition region shows the high conductivity in the order of 10(-4) Omega(-1) cm(-1).

16th International Workshop on Physics of Semiconductor Devices, 2012

ABSTRACT Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited using very high... more ABSTRACT Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited using very high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD) technique at 60 MHz. Films were grown at different power from 5 W to 40 W. The maximum deposition rate was found to be 6.1 A0/sec at 20 W power. These films were characterized by XRD which revealed that the size of the Si nanocrystals are in the range of 15.7 to 19.6 A0. The optical band gap was found in the range between 1.64 to 1.74 eV.

Environmental Science and Engineering, 2013