K. S. R. Koteswara Rao

Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications

As an alternative to the gold standard TiO2 photocatalyst, the use of zinc oxide (ZnO) as a robust candidate for wastewater treatment is widespread due to its similarity in charge carrier dynamics upon bandgap excitation and the generation of reactive oxygen species in aqueous suspensions with TiO2. However, the large bandgap of ZnO, the massive charge carrier recombination, and the photoinduced corrosion–dissolution at extreme pH conditions, together with the formation of inert Zn(OH)2 during photocatalytic reactions act as barriers for its extensive applicability. To this end, research has been intensified to improve the performance of ZnO by tailoring its surface-bulk structure and by altering its photogenerated charge transfer pathways with an intention to inhibit the surface-bulk charge carrier recombination. For the first time, the several strategies, such as tailoring the intrinsic defects, surface modification with organic compounds, doping with foreign ions, noble metal deposition, heterostructuring with other semiconductors and modification with carbon nanostructures, which have been successfully employed to improve the photoactivity and stability of ZnO are critically reviewed. Such modifications enhance the charge separation and facilitate the generation of reactive oxygenated free radicals, and also the interaction with the pollutant molecules. The synthetic route to obtain hierarchical nanostructured morphologies and study their impact on the photocatalytic performance is explained by considering the morphological influence and the defect-rich chemistry of ZnO. Finally, the crystal facet engineering of polar and non-polar facets and their relevance in photocatalysis is outlined. It is with this intention that the present review directs the further design, tailoring and tuning of the physico-chemical and optoelectronic properties of ZnO for better applications, ranging from photocatalysis to photovoltaics.

Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects

Among the armoury of photovoltaic materials, thin film heterojunction photovoltaics continue to be a promising candidate for solar energy conversion delivering a vast scope in terms of device design and fabrication. Their production does not require expensive semiconductor substrates and high temperature device processing, which allows reduced cost per unit area while maintaining reasonable efficiency. In this regard, superstrate CdTe/CdS solar cells are extensively investigated because of their suitable bandgap alignments, cost effective methods of production at large scales and stability against proton/electron irradiation. The conversion efficiencies in the range of 6–20% are achieved by structuring the device by varying the absorber/window layer thickness, junction activation/annealing steps, with more suitable front/back contacts, preparation techniques, doping with foreign ions, etc. This review focuses on fundamental and critical aspects like: (a) choice of CdS window layer and CdTe absorber layer; (b) drawbacks associated with the device including environmental problems, optical absorption losses and back contact barriers; (c) structural dynamics at CdS–CdTe interface; (d) influence of junction activation process by CdCl2 or HCF2Cl treatment; (e) interface and grain boundary passivation effects; (f) device degradation due to impurity diffusion and stress; (g) fabrication with suitable front and back contacts; (h) chemical processes occurring at various interfaces; (i) strategies and modifications developed to improve their efficiency. The complexity involved in understanding the multiple aspects of tuning the solar cell efficiency is reviewed in detail by considering the individual contribution from each component of the device. It is expected that this review article will enrich the materials aspects of CdTe/CdS devices for solar energy conversion and stimulate further innovative research interest on this intriguing topic.

Highly reliable spin-coated titanium dioxide dielectric

Dielectric degradation as low as 0.3 % has been observed for a highly reliable Titanium dioxide (TiO2) film after constant voltage stressing (CVS) with – 4 V for 105 second at room temperature (300 K). The film was fabricated by sol –gel spin – coating method on a lightly doped p-Si (~1015 cm−3) substrate. The equivalent oxide thickness (EOT) is 7 nm with a dielectric constant 33 (at 1 MHz). Metal – Oxide – Semiconductor (MOS) capacitors have been fabricated with an optimum annealing temperature of 800°C for one hour in a preheated furnace. The dielectric degradation is annealing temperature dependent. A degradation of 1.4 %, 1.2 % and 1.1 % has been observed for 400°C, 600°C and 1000°C temperature annealed MOS respectively. The dielectric degradation increases below or above the optimum temperature of annealing.

Zirconium doped TiO2 thin films: A promising dielectric layer

In the present work, we have fabricated the zirconium doped TiO2 thin (ZTO) films from a facile spin - coating method. The addition of Zirconium in TiO2 offers conduction band offset to Si and consequently decreased the leakage current density by approximately two orders as compared to pure TiO2 thin (TO) films. The ZTO thin film shows a high dielectric constant 27 with a very low leakage current density similar to 10(-8) A/cm(2). The oxide capacitate, flat band voltage and change in flat band voltage are 172 pF, -1.19 V and 54 mV. The AFM analysis confirmed the compact and pore free flat surface. The RMS surface roughness is found to be 1.5 angstrom. The ellipsometry analysis also verified the fact with a high refractive index 2.21.

High-κ TiO2 thin film prepared by sol-gel spin-coating method

High-k TiO2 thin film on p-type silicon substrate was fabricated by a combined sol-gel and spin coating method. Thus deposited titania film had anatase phase with a small grain size of 16 nm and surface roughness of ≅ 0.6 nm. The oxide capacitance (Cox), flat band capacitance (CFB), flat band voltage (VFB), oxide trapped charge (Qot), calculated from the high frequency (1 MHz) C-V curve were 0.47 nF, 0.16 nF, − 0.91 V, 4.7x10−12 C, respectively. As compared to the previous reports, a high dielectric constant of 94 at 1 MHz frequency was observed in the devices investigated here and an equivalent oxide thickness (EOT) was 4.1 nm. Dispersion in accumulation capacitance shows a linear relationship with AC frequencies. Leakage current density was found in acceptable limits (2.1e-5 A/cm2 for −1 V and 5.7e-7 A/cm2 for +1 V) for CMOS applications.

Scaling behavior of fully spin-coated TFT

We studied channel scaling behavior of fully spin coated, low temperature solution processed thin film transistor (TFT) fabricated on p(++) - Si (similar to 10(21) cm(-3)) as bottom gate. The solution processed, spin coated 40 um thick amorphous Indium Gallium Zinc Oxide (a-IGZO) and 50 nni thick amorphous zirconium di-oxide (a-ZnO2) has been used as channel and low leakage dielectric at 350 degrees C respectively. The channel scaling effect of the TFT with different width/length ratio (W/L= 2.5, 5 and 15) for same channel length (L = 10 mu m) has been demonstrated. The lowest threshold voltage (V-th) is 6.25 V tbr the W/L=50/10. The maximum field effect mobility (mu(FE)) has been found to be 0.123 cm(2)/Vs from WiL of 50/10 with the drain to source voltage (V-D) of 10V and 20V gate to source voltage (V-G). We also demonstrated that there is no contact resistance effect on the mobility of the fully sol-gel spin coated TFT

Electrical study of Al/HfO2/p-Si (100) gate stack

Low leakage current density and high relative permittivity (dielectric constant) are the key factors in order to replace the SiO2 from Si based technology towards its further down scaling. HfO2 thin films received significant attention due to its excellent optoelectronic properties. In this work, ultra - thin (17 nm) HfO2 films on Si substrate are fabricated by RF sputtering. As deposited films are amorphous in nature and in order to get the reasonable high dielectric constant the films are annealed (700 degrees C, 30 min) in nitrogen environment. A high refractive index (2.08) and small grain size (similar to 10) nm was extracted from ellipsometry and XRD, respectively. The AFM study revealed a small RMS surface roughness 9 angstrom. Towards electrical exploration, the films are integrated in Metal-Insulator-Semiconductor (MIS) capacitors structure. The oxide capacitance (C-ox), flat band capacitance (C-FB), flat band voltage (V-FB), and oxide trapped charges (Q(ot)) calculated from high frequency (1 MHz) C-V curve are 490 pF, 183 pF, 1.33 V and 1.61 x 10(-10) C, respectively. The dielectric constant calculated from accumulation capacitance is 17. The films show a very low leakage current density 4.3 x 10(-8) A/cm(2) at +/- 1 V.

Evaluation of Rice Varieties against Multiple Diseases Under Middle IGP of Bihar

An investigation was carried out to evaluate the susceptibility of rice cultivation to major field diseases like brown spot, sheath blight and bacterial leaf blight under middle Indo Gangetic Plain. The disease incidence and disease severity were observed at three different growth stages namely flowering, milk and maturity in boro seasons during 2015-16. The incidence of brown spot ranged from 8.93 to 17.83%, 16.67 to 25.67% and 18.33 to 28.33% at flowering, milk and maturity stages, respectively. The severity of brown spot in grade (0-9 scale) ranged from 5.13 to 10.33, 9.33 to 20.00 and 11.57 to 22.67, respectively at flowering, milk and maturity stages. The incidence of sheath blight of paddy ranged from 7. 03 to 16.50%, 13.13 to 22.67% and 19.67 to 35.67% at flowering, milk and maturity stages, respectively. The severity of sheath blight in grade (0-9 scale) ranged from 4.00 to 8.60, 8.33 to 13.93 and 18.33 to 34.33, respectively at flowering, milk and maturity stages. The incidence of bacterial leaf blight ranged from 5.00 to 14.00%, 11.43 to 21.00% and 17.00 to 34.00% at flowering, milk and maturity stages, respectively. The severity of bacterial leaf blight in grade (0-9 scale) ranged from 2.00 to 8.00%, 6.33 to 12.05% and 14.47 to 20.17%, respectively at flowering, milk and maturity stages. Among the varieties, the highest incidence and severity of Brown spot was recorded on Rajendra Bhagawati whereas it was lowest on 27P31 at all growth stages. In the case of Sheath blight, highest incidence and severity was recorded on Sambha Mahsuri whereas it was the lowest on CRL 193. Beside this in the case of Bacterial leaf blight, the highest incidence and severity was recorded on Kranti whereas the lowest was recorded on CRL 193. In general, it was observed that the incidence and severity of diseases increased gradually from flowering to maturity stage and the genotypes with the minimum incidence and severity of diseases gave the maximum yield.

Band alignment and electrical investigations of ultra-thin Al2O3 on Si by E-beam evaporation

The continuous downscaling leads the search of high-gate dielectrics. The films amorphous in nature offered good mechanical flexibility, smooth surfaces and better uniformity associated with low leakage current density. In this work, 16 nm thick amorphous Al2O3 films on silicon substrate are fabricated by E-beam evaporation. The high value of refractive index (1.76) extracted from ellipsometry analysis directs the deposition of compact film. The AFM analysis reveal a flat surface with small RMS surface roughness 1.5 angstrom. The band gap is extracted from O-1s electron loss spectra and was found 6.7 eV and band alignment of Al2O3/Si is derived from the UPS measurements. The films are incorporated in Metal Insulator -Semiconductor (MIS) capacitor to perform the electrical measurement. The flat band voltage (V-FB), dielectric constant () and oxide trapped charges (Q(ot)) extracted from high frequency (1 MHz) C-V curve are - 0.4 V, 8.4 and 2 x 10(11) cm(-2), respectively. The small flat band voltage - 0.4 V, narrow hysteresis and very little frequency dispersion suggest an exceptional good Al2O3/Si interface with small quantity of trapped charges in the oxide. The leakage current density was 4.27 x 10(-8) A/cm(2) at 1 V. The moderate dielectric constant and low leakage current density with ultra-smooth surface is quite useful towards its application in future CMOS and memory devices.